An isolable phosphaalumene(3) capable of small molecule activation via unique modes of reactivity

Domain: neuroscience
Published: 2026-02-05
Editorial outcome: major_revision

Abstract
The chemistry of phosphaalumenes [R–Al=P–R’] featuring multiple bonds between phosphorus and aluminum remains largely unexplored. Here we show the syntheses of a phosphaalumene(3) through either the reaction of DippNacNacM (DippNacNac=HC[(CMe)N(2,6-iPr2C6H3)]2, M=Al) with a bisphosphirane-fused anthracene(1) or with a phospha-Wittig reagent(2) under mild conditions. This phosphaalumene exhibits reactivities that stand in contrast to those of previously reported transient species II, especially in its ability to activate small molecules such as dihydrogen (H2), white phosphorus (P4), isocyanides, CO2, N2O, trimethylsilylazide (TMSN3), diphenyldiselenide (PhSeSePh), PhSiH3, PhNH2, styrene and 1-ethynyl-4-methylbenzene. It has been found in this research that the ligand environment can have a subtle but profound impact on the reactivity of phosphaalumenes.

Introduction
The construction of heterodiatomic multiple bonds between group 13 and 15 elements is a fascinating area of research because of the unusual bonding properties and reactivities of these compounds, not found with their carbon analogues1–6. The polarities of such bonds, arising from the electronegativity differences between E13 and E15 elements, render these species highly reactive and potentially useful in the activation of small molecules and inert bonds. While much of this chemistry was pioneered with kinetically and electronically stabilized iminoboranes (R–B−≡N+–R)7–9, examples of compounds bearing B=E (E =P10–22, As23,24) bonds or E15=N (E13 =Al25–31, Ga25,32,33, In32,34) bonds, systems in which one element has a principal quantum number (n) greater than 2, have also been well described. The construction of heavier E13=E15 multiple bonds (E13 =Al, Ga, In; E15 =P, As, Sb)35–38 is hindered by a number of complicating factors, including the inherent weaknesses of these π-bonds stemming from inefficient overlap of the pπ-pπ orbitals on descending the group, their strong tendency to undergo oligomerization, and the lack of reliable synthetic protocols and appropriate precursors.

A landmark discovery by Power and coworkers demonstrated that M=N (M=Al, Ga) multiple bonds can be directly accessed by employing group 13 carbenoids DippNacNacM (M=Al, Ga) and organoazides25. This was then followed by the synthesis of a range of phosphagallenes containing P=Ga double bonds39–47. By contrast, compounds with Al=P double bonds remain exceedingly rare, presumably due to their increased polarities and thus significantly enhanced reactivities compared to Ga=P double bonds48,49. A major breakthrough was reported by Hering-Junghans and Braunschweig in 2021, describing the synthesis of the first isolable phosphaalumene I, which was characterized by a six-coordinate Al center and a two-coordinate P center (Fig. 1a)50. The same groups went on to detail how the pronounced π(Al=P)-character of these species enabled the activation of C–C and B–N multiple bonds, as well as organoazides51–53. It was demonstrated that when 1 reacts with unsaturated C–C bonds, it forms a multitude of unique P, Al-heterocycles that cannot be directly produced by other means. Furthermore, the reaction between 1 and organoazides revealed reaction patterns that significantly differed from those documented for alkyne and 1,3-dipolar molecules, thereby offering new perspectives on chemical reactions. Despite these advances, the synthesis of low-coordinate Al=P bonds remains a formidable challenge. Pioneering studies from the groups of Hering-Junghans, Braunschweig, and Goicoechea aimed at developing phosphaalumene(3) species by integrating a two-coordinate Al(I) species, DippNacNacM (M=Al), with mono-coordinate phosphinidene surrogates. However, this endeavor succeeded only in yielding fleeting species that could not be isolated under ambient conditions48,49. Among these transient compounds, phosphanyl phosphaalumene(II) was found to be highly reactive in the activation of kinetically challenging substrates such as H2 via an FLP-type pathway through the cooperative action of the Al and the pendant P centers and could be captured as a Lewis base (tetrahydrofuran) adduct49. Inspired by the advantageous characteristics of these transient phosphaalumenes in bond activation, we sought to develop an isolable phosphaalumene(3) and explore its reactivity patterns with small molecules.

Preliminary theoretical studies suggested that the incorporation of a nitrogen atom bonded to the phosphorus in an Al=P complex, in contrast to other elements like B, C, O, and P, significantly narrows the energy gap between the frontier molecular orbitals (FMO) associated with P = Al double bonds (Fig. S163). This characteristic is a determining factor in promoting the activation of kinetically challenging substrates such as H2. As such, we sought to develop novel phosphaalumenes, kinetically stabilized by sterically bulky N-substituents. Herein, we report the synthesis of a phosphaalumene(3) supported by a NacNac ligand and an isoindoline moiety. Remarkably, this species shows both π(Al=P)-behavior and FLP-type character in reactions with H2. We also demonstrate how Al=P π- (and σ-) bond cleavage is instrumental in their reactions with P4, isocyanides, CO2, N2O, TMSN3, PhSeSePh, E-H bonds (E=Si, N), styrene, and 1-ethynyl-4-methylbenzene.

Results
Synthesis and characterization of 3
We have recently reported the synthesis of a bisphosphrane-fused anthracene 1 that functioned as a highly effective phosphinidene surrogate in small molecule activation54,55. This compound features a π-donating isoindoline group and a sterically congested phosphorus center, which prompted us to investigate its reaction with Al(I) as a route to Al=P double bonds. The reaction of 1 with two equivalents of DippNacNacM (M=Al) in benzene at 50°C led to the formation of a reddish-brown solution of 3 (Fig. 2a). The 31P NMR spectrum of the crude reaction mixture indicated the presence of several phosphorus-containing species, which after recrystallization gave 3 as dark-red crystals in 13% yield. Alternatively, when a benzene solution of 1 and two equivalents of DippNacNacM was subjected to UV light irradiation at 365 nm, compound 3 was obtained in 18% yield. Despite the successful synthesis and characterization, the purification of 3 remained a challenge, primarily due to the quantitative formation of anthracene during the fragmentation of 1. To our delight, when the phospha-Wittig reagent 2 was employed as a phosphinidene precursor it reacted stoichiometrically with DippNacNacM in benzene at ambient temperature to yield 3, which was isolated as a red-brown powder in 91% yield. The 31P NMR spectrum of 3 showed a sharp resonance at 9.0 ppm, which is downfield-shifted compared to those observed in 1 and 2 (−99.8 and −14.8 ppm, respectively), and also to those of I (Ar=Dipp: −203.9 ppm and Ar=Tipp: −208.9 ppm) (Table S3). Single crystals of 3 were obtained from a saturated n-hexane solution in a glove box. Single-crystal X-ray diffraction analysis (SC-XRD) revealed a two-coordinate P center and a three-coordinate Al center (Fig. 2b). The length of Al–P bond (2.2340(8) Å) is close to those reported by Hering-Junghans and Braunschweig (2.2113(6) and 2.2022(6) Å) (Table S3)49,50. Both the Al3 and N1 centers are trigonal-planar with sums of bond angles measuring 357.2° (Al3) and 355.0° (N1), respectively.

The electronic features of 3 were further examined by DFT calculations at the PBE0-D3 (BJ)/Def2-SVP level of theory. As shown in Fig. 2c. the highest occupied molecular orbital (HOMO) of 3 is dominated by the Al–P π-bonding orbital, while the HOMO−1 is comprised of the Al–P σ-bonding orbital with some contribution from the lone pairs of N1 and P2. The HOMO−2 predominantly reflects the π-bonding interactions along the N1–P2 bond and in the aryl substituents attached to the isoindoline moiety. Remarkably, the lowest unoccupied molecular orbital (LUMO) of 3 (Fig. S164) is localized on the β-diketiminate ligand backbone due to the contribution of its low-lying π* orbital. The LUMO+1 mainly corresponds to the P–Al σ* orbital, which resembles closely the LUMO of I. Natural bond orbital (NBO) analysis calculated at the PBE0-D3 (BJ)/Def2-TZVP level of theory reveals the presence of one Al–P σ bonding interaction and one Al–P π bonding interaction (14.77% Al, 85.23% P), along with a Wiberg bond index (WBI) of 1.41, which further corroborates the Al=P double bond character. Natural population analysis (NPA) calculates the charges on the Al3-P2 moiety to be +1.43 for Al3 and −0.29 for P2, indicative of a polarized π-bonding interaction. The UV-vis spectrum of 3 in toluene displays an absorption maximum at 414 nm, which is assigned to the HOMO−1 to LUMO+1 transition. The simulated spectrum by a time-dependent density functional theory (TD-DFT) identifies three further maxima at 605, 553, and 468 nm, attributed to S1, S2, and S3, respectively. The latter absorptions are not clearly observed in the experiment spectrum; their low oscillator strengths (Fig. S145 and Table S7) make them too faint to resolve, and they are likely obscured by band broadening or solvent effects.

Additionally, the Electron Localization Function (ELF)57 and the Laplacian of the electron density were analyzed in the NAlP plane to provide validation of the NBO findings. The ELF depiction in the NPAl2 plane of 3 (Fig. 2d, left) reveals a lone pair on the P atom with substantial s-character. This is complemented by the Laplacian plot (Fig. 2d, right), which shows valence shell charge concentrations (VSCCs) at the P atom and bond critical points shifted toward Al, consistent with a polarized P–Al bond. The P–Al bond displays an electron density of 0.072 e·bohr−3 at its bond critical point (BCP). This, along with a negative energy density (−0.028 a.u.) and a positive Laplacian of the electron density, suggests a polarized covalent bond. Furthermore, its substantial ellipticity (ε=0.481) decisively indicates considerable π-character, aligning with a double bond. In comparison, the P–N BCP analysis yields ρ(r)=0.151 e·bohr−3, a positive ∇²ρ(r), H(r)=−0.117 a.u., and ε=0.015, which are indicative of pronounced single bond character.

3 can be stored in a glovebox as a solid for weeks, but slowly decomposes to give a complex mixture when heated to 50°C in C6D6. The increased thermal sensitivity relative to I prompted us to investigate its reactivity with small molecules such as H2, P4, isocyanides, CO2, N2O, TMSN3, PhSeSePh, PhSiH3, PhNH2, styrene, and 1-ethynyl-4-methylbenzene, allowing for the evaluation of the substituent effect in the chemical behavior of 3 in comparison with I and II.

Reactions of 3 with H2 (D2): π(Al=P) Bonding and Al/N Frustrated Lewis Pair Character
First, we examined the reactivity of 3 with H2. Treatment of a red-brown solution of 3 in benzene with H2 (1 Bar) at 40°C led to the formation of an orange mixture within 12 h (Fig. 3). Interestingly, 31P NMR experiments indicated the presence of two hydrogenation products in a ratio of 1:0.54. Compounds 4 and 5 were obtained by recrystallization from a saturated n-hexane solution, resulting in colorless and orange crystals with yields of 22% and 14%, respectively. The 31P NMR spectrum of 4 displayed a doublet of doublet at −87.8 ppm (1JP-H =204.5 Hz, 2JP-H =18.6 Hz), while 5 revealed a doublet at 55.3 ppm (2JP-H =20.1 Hz), which both collapsed to singlets upon proton decoupling. In their solid-state IR spectra, 4 exhibited two vibrations characteristic of P–H and Al–H at 2244 cm−1 and 1834 cm−1, while 5 showed two distinct vibrations for N–H and Al–H at 3347 cm−1 and 1817 cm−1, respectively. A labelling study employing D2 smoothly yielded a mixture of [D2]-4 and [D2]-5 (Figs. S29 and S30). The structures of 4 and 5 were unambiguously determined by SC-XRD. The solid-state structure of 4 revealed the presence of an Al–P single bond (2.3664(10) Å), with the Al atom positioned in a tetrahedral geometry and the P atom in a trigonal-pyramidal arrangement. These results were indicative of the addition of H2 across the Al–P double bond in a 1,2 fashion. Notably, 3 also functions as seen in FLP systems in the reaction with H2, resulting in the 1,3-addition of H2 to the N1 and Al3 atoms, which is then followed by the integration of P2 into the NC ring to yield 549,58,59. In the solid-state structure of 5, the Al–P bond distance (2.4355(6) Å) is slightly longer than that found in 4. It is worth highlighting that the formation of 4 represents a rare example of H2 splitting over heavier E13=E15 multiple bonds28,60.

To gain insight into the various possible reaction mechanisms, the pathways for the formation of 4 and 5 were elucidated through theoretical calculations using the simplified model 3′ and H2 at the B3LYP-D3(BJ)/Def2-TZVP PD-SMD(benzene)//PBE0-D3(BJ)/Def2-SVP level of theory. A mechanism that is consistent with these investigations is shown in Fig. 4. The addition of H2 across the Al=P double bond of 3′ produces 4′ via TS1 with a total energy barrier of 24.98 kcal∙mol−1. In a competitive reaction to yield 5′, the initial step involves a FLP-like H2 activation over the Al center and the pendant isoindoline moiety of 3′, giving INT1 via TS1 with a barrier of 24.47 kcal∙mol−1. Subsequent ring expansion of the pendant isoindoline ligand furnishes INT2 via TS2 with a small energy barrier of 3.19 kcal∙mol−1. In the next step, ring contraction of INT2 via a 1,3-shift of the phosphorus atom gives INT3 via TS3 (26.63 kcal∙mol−1). In the last step, stereo-inversion and conformational change afford 5′. Overall, the formation of 4′ and 5′ are thermodynamically favored by 26.59 and 25.91 kcal∙mol−1, respectively. The rate determining step (INT2 to INT3) for the formation of 5′ (26.63 kcal∙mol−1) is energetically less favorable compared to that of 4′ (24.98 kcal∙mol−1). To ascertain the reversibility of H2 activation, we reacted 4 and 5 with 1 atm of D2 in benzene-d6. Nonetheless, neither H–D exchange nor any novel products were detected (Figs. S37 and S38). To assess the hydrogen transferability of compounds 4 and 5, their reactions with ketones were investigated. When a benzene-d6 solution of these compounds was mixed with fluorenone at room temperature, the color of the solution transformed from yellow to red after two days. 1H NMR and 31P NMR spectroscopy confirmed that compound 4 remained unreacted, while compound 5’s characteristic signals completely disappeared. (Figs. S39 and S40). Unfortunately, all attempts to isolate this red product were unsuccessful.

Reactions of 3 with P4, Isocyanides, CO2, N2O, TMSN3, and PhSeSePh: Al=P π- and σ-Bond Cleavage
The controlled activation of P4 with main-group homoatomic double bond such as diborenes61–63, dialumenes64, disilenes65–67, dithallene68, and distannynes69, has been a major focus of recent interest, as it is a versatile reagent in generating organophosphorus substances. As far as we know, there is only one example of P4 activation facilitated by a heteroatomic double bond, specifically the phosphasilenes developed by Driess, which degrade P4 through the cleavage of the π and the σ bonds of the P=Si linkage70. When 3 was treated with P4 at ambient conditions, an orange mixture was formed, from which a stoichiometric amount of 6 was obtained as an orange solid (Fig. 5). The 31P NMR spectrum of 6 shows five multiplet resonances at 94.5 (qd, 1JP-P =187.9 Hz, 2JP-P =51.8 Hz), −21.9 (dd, 1JP-P =236.5 Hz, 2JP-P =50.2 Hz), −81.0 (ddd, 1JP-P =267.3 Hz, 2JP-P =121.5 Hz, 3JP-P =16.2 Hz), −112.8 (tt, 1JP-P =124.8 Hz, 2JP-P =53.5 Hz), and −126.3 ppm (dddd, 1JP-P =265.7 Hz, 2JP-P =139.3 Hz, 2JP-P =124.8 Hz, 3JP-P =21.1 Hz), indicating the cleavage of P–P σ-bonds of P4 and the formation of a non-symmetrical structure. The solid structure of 6 was determined by SC-XRD analysis, revealing a complete cleavage of the Al=P double bond to form an AlP5 cage scaffold, tethered by an isoindoline and a NacNac ligands (Fig. 6). This reaction facilitates a two-electron oxidation of the Al=P double bond of 3, yielding 6 characterized by P(I) and Al(III) centers, which is in line with the reactivity reported for dialumenes and disilenes63,65. The formation of 6 provides a noteworthy instance of P4 activation accomplished via an E13=E15 double bond.

Isocyanides are important C1 building blocks in the context of multicomponent reactions and materials science. The reductive coupling of isocyanides has been well documented for low-valent aluminum systems such as ArNacNacM71, dialumane72, and the aluminyl anion73, but it is not known for phosphaalumenes. As such, we examined the reactivity of 3 with isocyanides, specifically tBuNC and PhNC, which resulted in the complete rupture of the Al=P bond. Upon reacting 3 with two equivalents of tBuNC, 7′ rapidly emerged, but then slowly isomerized at ambient temperature, yielding a 1:10 mixture of 7′ and 7. This gradual transformation of 7′ into 7 impeded efforts to obtain single crystals of 7′, thereby precluding the validation of its solid-structure (Figs. S61 and S62). Similarly, when 3 was treated with three equivalents of tBuNC, only a 1:10 mixture of 7′ and 7 resulted, with no additional products detected (Figs. S61–S64). In the 31P NMR spectra, the resonances for the phosphorus atoms in 7′ and 7 appeared as singlets at 119.3 ppm and 138.9 ppm, respectively. In a separate experiment, compound 8 was isolated in 62% yield when 3 was reacted with two equivalents of PhNC, and subsequent treatment of 8 with an additional equivalent of PhNC produced 9 in almost quantitative yield (Figs. S71 and S72). Alternatively, combining 3 with three equivalents of PhNC directly afforded 9 in 45% yield. The 31P NMR spectra of 8 and 9 showed signal at 46.3 and 112.0 ppm, respectively.

The structures of 7, 8, and 9 were determined by SC-XRD. The solid-state structure of 7 revealed the incorporation of two equivalents of tBuNC into the Al=P double bond, yielding a spirocyclic structure constructed of a four-membered AlNC2 ring and an AlNC2 heterocycle. The aluminum atom is tetra-coordinated with the DippNacNac and cyclic (alkyl)(imino) ligands, while the phosphorus atom adopts a two-coordinate geometry. Compound 8 is characterized by a bicyclic core comprising two interconnected rings. The PCN and AlNC2 rings, which are fused within 8, exhibit a slight twist, with their internal tetragon angle sums measuring 359.2° and 356.3°, correspondingly. The solid-state structure of 9 confirmed the integration of three equivalents of PhNC into 3, furnishing a fused bicyclic system. All five atoms of the AlNC3 ring are coplanar with the sum of internal pentagon angles of 539.74°, while the fused PNC2 ring adopts a slightly butterfly-like geometry with a P2-C3-C4-N5 torsion angle of −14.14°. Given that reactions of 7 with less sterically encumbered isocyanides may enable the synthesis of more complex heterocycles, such as aluminum phosphaguanidinates74, we studied the reaction of 7 with an excess of PhNC, which yielded a complex mixture that precluded further purification (Figs. S81 and S82).

To investigate the reactivity of 3 with unsaturated bonds, we subjected it to treatment with CO2. Exposure of a C6D6 solution of 3 to an atmosphere of CO2 at ambient temperature led instantaneously and cleanly to the formation of 10. The 31P NMR of 10 displays a singlet at 21.8 ppm, which is downfield-shifted with respect to that of 3. In the 13C{1H} NMR spectrum, the resonance for the carbonyl carbons appears as a doublet at 173.7 ppm (d, 1JP-C =21.0 Hz), which is comparable to that of a reported phosphine-dicarboxylate compound obtained from the reaction of phosphagallene (P=Ga) with CO2 41,44. The solid-state structure of 10 was determined by SC-XRD, revealing the cleavage of the Al=P bond and double CO2 addition, yielding a product featuring a boat-type six-membered PC2O2Al ring, which bears a resemblance to those found in reactions involving arsanyl-phosphagallene or gallyl-phosphagallene and CO2 41,44. It should be emphasized that when II reacts with CO2, it yields a 1,3-addition product, in contrast to the total rupture of the Al=P bond seen in 3. To further elucidate the reaction of compound 3 with CO₂, the corresponding mechanism was investigated by theoretical studies (Fig. S165).

3 was also able to activate N2O, enable the oxidation and rupture of the Al=P double bond, yielding 11 and 12 in a 2.8:1 NMR ratio (Figs. S89 and S90). Based on these results, it’s reasonable to envisage that reaction of 3 with N2O may start from the insertion of one oxygen atom into the Al=P double bond to afford a AlPO three-membered ring scaffold, valence isoelectronic analogue of oxirane. Following intramolecular [2+2] cycloaddition of the Al–P bond with the arene substituent lead to the formation of 11. On the other hand, when this oxirane intermediate was reacted with another equivalent of N2O, the AlPO2 four-membered heterocycle 12 was formed. To further investigate this deoxygenation reaction, 3 was treated with an equimolar amount of N2O, resulting exclusively in 11 in 66% isolation yield. Despite our efforts, we could not obtain 12 in its pure state, yielding only a small number of crystals. The 31P NMR spectrum of 11 displayed a doublet at 158.3 ppm (3JP-H =9.5 Hz), which collapsed to singlets upon proton decoupling. In the 31P NMR spectrum of 12, a singlet was observed at 149.7 ppm. The solid-structures of 11 and 12 were unambiguously determined by SC-XRD.

Very recently, the groups of Hering-Junghans and Braunschweig disclosed the reactivity of I towards organoazides, which enables the formation of various four-membered AlNP ring systems52. Surprisingly, when 3 was subjected to reaction with TMSN3, a five-membered AlN3P ring (13) was obtained, indicative of a distinct reaction pathway compared to those reported by Hering-Junghans and Braunschweig. The 31P NMR spectrum of 13 displayed a singlet at 132.0 ppm, which is significantly downfield shifted with respect to that of 3. The structure of 13 was determined by SC-XRD to be a spirocyclic system. The five-membered AlN2P ring in 13 is nearly planar, with the sum of internal pentagon angles of 537.9°. Notably, the phosphorus atom of 13 shows a trigonal-planar geometry (the sum of bond angles: 360.0°), which is reminiscent of those reported in the phosphorus(V) selenides75. The bond lengths of P1-N2 (1.534(3) Å), P1-N4 (1.568(3) Å), and N3-N4 (1.506(3) Å) fall within the typical range of P-N single and double bonds, and N-N single and double bonds, respectively, indicative of strong π-electron delocalization over the N2-P1-N4-N3 moiety.

The reactivity of 3 with homoatomic element-element bonds such as azobenzene, bis(catecholato)diboron (Bcat2), and PhSeSePh was also investigated. However, the reaction of 3 with azobenzene or Bcat2 afforded complex mixtures, and purification of these mixtures by recrystallization was unsuccessful. When 3 was treatment with two equivalents of PhSeSePh, 14 and 15 were isolated in 43% and 25% yields, respectively55.

Reactions of 3 with PhSiH3 and PhNH2: π(Al=P)-Behavior
By reacting 3 with PhSiH3 and PhNH2, the addition of E–H bonds (E=Si, N) across the Al=P double bond was accomplished, producing silaphosphine 16 and aminophosphine 17 in yields of 56% and 82%, respectively (Fig. 7). In contrast, intermediate II only exhibited 1,2-addition of the Si–H bond across the P=Al bond when reacted with PhSiH3, but functions like an FLP towards amines, yielding 1,3-addition products49. For comparison, the reaction mechanism of 3 with aniline has been studies (Fig. S166). In the 31P NMR spectra, a broad doublet is observed at −81.0 ppm (d, 2JP-H(Al) =55.4 Hz) for 16 and at −83.3 ppm (d, 1JP-H =210.9 Hz) for 17, with both signals collapsing to a broad singlet upon proton decoupling. The 1H NMR spectrum of 16 displays a doublet in conjunction with distinct 29Si satellite signals for the Si–H at 4.48 ppm (d, 2JP-H =20.0 Hz), while the Al–H resonance is not observed. For 17 the P–H resonance was observed as a doublet at 4.31 ppm (d, 1JP-H =210.9 Hz), while the N-H resonance was identified as a singlet at 2.62 ppm. In the solid-state IR spectrum of 16, the characteristic stretching bands for Al–H and Si–H were detected at 1871 cm−1 and 2130 cm−1, respectively. The molecular structures of 16 and 17 were further confirmed by SC-XRD. In the solid-state structure of 16, the Al1-P2 and P2-Si1 bond lengths (2.3924(6) and 2.2397(7) Å, respectively) are comparable to those found in a previously reported silaphosphine (Al–P: 2.37 Å, P–Si: 2.24 Å), both indicative of pronounced single bond character47. The Al1-P1 bond distance (2.3718(15) Å) in 17 is comparable to that found in 16, consistent with single bond character. These isolated products clearly speak to the π(Al=P)-bonding behavior of 3.

When 3 was reacted with styrene and 1-ethynyl-4-methylbenzene, [2+2] cycloaddition product 18 and 19 was formed, respectively, in addition to a small amount of C–H addition product 20 in the latter case, which are in line with those reported by Hering-Junghans and Braunschweig (Fig. 8a)51. However, phenylacetylene and 1-ethynyl-4-fluorobenzene, when reacted with 3, yielded only complex mixtures, likely due to the increased acidity of their terminal alkyne C–H bond. This elevated acidity promotes alternative deprotonation or C–H activation routes, resulting in the formation of complicated mixtures instead of the intended cycloadduct. Additionally, we were unable to obtain any desired products when compound 3 reacted with azobenzene or bis(catecholato)diboron (Bcat2), which might be due to uncontrolled reactions with nonpolar substrates. In the 31P NMR spectra, the resonances for the phosphorus atoms in 18 and 19 were observed as a doublet of doublet at 56.7 ppm (2JP-H =35.5 Hz, 3JP-H =14.7 Hz) and a doublet at 57.6 ppm (2JP-H =29.3 Hz), respectively. The 31P NMR spectrum of 20 displayed a doublet at −89.3 ppm (1JP-H =204.1 Hz), which collapsed to a singlet upon proton decoupling. The solid-state structures of 18 and 19 were determined by SC-XRD, in which all four atoms of the PAlC2 rings are nearly coplanar with the sums of internal tetragon angles of 358.2° and 360.0°, respectively (Fig. 8b). Unfortunately, we were unable to obtain suitable single crystals to validate the solid-structure of 20.

In summary, by employing π-donating DippNacNac and isoindoline supporting ligands, a phosphaalumene(3) was successfully generated. DFT studies indicated that this phosphaalumene features a polarized Al=P double bond, with its reactivity primarily exhibiting π-character. Compound 3 is capable of activating kinetically challenging substrates, such as H2, and reacts with P4, isocyanides, CO2, N2O, TMSN3, PhSeSePh, PhSiH3, PhNH2, styrene, and 1-ethynyl-4-methylbenzene under ambient conditions. Remarkably, 3 showcases both π(Al=P)-behavior and FLP-type reactivity with H2, allowing 1,2-addition of H2 across the Al=P double bond and activation of H2 between the aluminum and pendant isoindoline substituent, respectively. Additional DFT calculations reveal that the energy barriers for these two competing pathways are close, rationalizing the concurrent observation of both reaction modes. The reaction of 3 with P4, isocyanides, CO2, N2O, TMSN3, or PhSeSePh proceed via complete cleavage of the Al=P bond. In addition, 3 displays distinct π(Al=P)-character in the activations of PhSiH3, PhNH2, styrene, and 1-ethynyl-4-methylbenzene, resulting in the cleavage of Si–H and N–H bonds, as well as the C–C unsaturated bond, across the Al=P double bond. These findings clearly demonstrate the significant π(Al=P)-character of 3, which is notably different from the chemical behavior of II. This comparison underscores how the ligand environment can subtly yet significantly alter the reaction site selectivity of a P=Al bond. To gain a more profound understanding of this divergence, DFT calculations were performed. Although NBO analyses furnished valuable electronic structure insights, the NPA charges on their own did not fully explain the observed differences in reactivity, suggesting that steric effects might also play a role to some degree. Significantly, our mechanistic investigation into the reaction involving aniline uncovered distinct energy barriers for the two routes, which correlates well with our experimental findings. The crucial impact of the ligand framework provides opportunities for further research into developing reactive main-group species and ongoing investigations aim to further elucidate potential novel reactivities afforded by this unique bonding environment.

Methods
General considerations
All reactions were performed under an atmosphere of dry nitrogen or argon by using standard Schlenk or drybox techniques; solvents were dried over Na metal, K metal, or CaH2. 1H, 13C, 29Si, and 31P NMR spectra were obtained with a Bruker Avance 400 spectrometer at 298 K. NMR multiplicities are abbreviated as follows: s=singlet, d=doublet, t=triplet, sep=septet, dd=doublet of doublets, ddd=doublet of doublets of doublets, qd = quartet of doublets, m=multiplet, br=broad signal. Coupling constants J are given in Hz. HRMS spectra were obtained from a SCIEX X500R-QTOF (ESI). Fourier transform infrared (FT-IR) spectra were recorded on a BRUKER ALPHA II spectrometer.

Synthesis of 2
At −78°C, toluene was slowly added to a mixture of 2-Br (3.280×g, 4.52 mmol) and KC8 (1.220×g, 9.04 mmol), followed by a toluene solution of 5 eq. PMe3 (2.6 mL, 8.7 mmol∙mL−1). The suspension was slowly warmed to ambient temperature and stirred in the dark overnight. Then, the mixture was filtered, and the filtrate was dried under reduced pressure. The resulting solid was washed with cold n-hexane (15 mL) and dried under vacuum to afford 2 as a white powder (1.510×g, 52% yield). Colorless single crystals of 2 were obtained from a saturated n-hexane solution at −25°C. 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.46 (s, 6H, Ar-H), 7.23 (dd, J=5.8, 3.2 Hz, 4H, Ar-H), 6.94 (dd, J=5.8, 3.2 Hz, 2H, Ar-H), 6.83 (s, 4H, Ar-H), 2.17 (s, 24H, Ar-CH3), 0.60 (m, 9H, PP(CH3)3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ=149.2 (Ar-C), 147.0 (Ar-C), 145.4 (Ar-C), 135.6 (Ar-C), 128.9 (Ar-CH), 128.0 (Ar-CH), 126.7 (Ar-CH), 125.7 (Ar-CH), 84.4 (CNP), 21.4 (Ar-CH3), 15.9 (dd, 1JP-C =30.3 Hz, 2JP-C =20.2 Hz, PP(CH3)3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ=−14.7 (s), −14.9 (s). 31P NMR (162 MHz, C6D6, 25°C, ppm): δ=−14.7 (s), −14.9 (s). FT-IR ν [cm−1] 2913, 1593, 1454, 1274, 1165, 1031, 953, 847, 714, 662. HRMS (ESI): m/z calculated for [C43H50NP]+ (M+H)+: 642.3417; found: 642.3418.

Synthesis of 3 from 1
a): In a glove box, 20 mL of benzene was added to a mixture of DippNacNacM (102 mg, 0.229 mmol) and 1 (150 mg, 0.115 mmol) at ambient temperature. The reaction mixture was stirred at 50°C for 24 h, which was accompanied by a color change from red to reddish-brown. Then all volatiles were removed under reduced pressure and the resulting mixture was dissolved in n-hexane. Dark-red crystals of 3 were obtained by slow evaporation of a saturated n-hexane solution (21 mg, 13% yield) in a glove box. 31P NMR analysis indicated the formation of 3 in 42% NMR yield (PPh3 and 1,3,5-trimethoxybenzene were used as internal standards for NMR measurements) (See Fig. S7-S8).
b): In a glove box, 20 mL of benzene was added to a mixture of DippNacNacM (102 mg, 0.229 mmol) and 1 (150 mg, 0.115 mmol) at ambient temperature. The reaction mixture was subjected to UV light irradiation at 365 nm overnight, which was accompanied by a color change from red to reddish-brown. Then all volatiles were removed under reduced pressure and the resulting mixture was dissolved in n-hexane. Dark-red crystals of 3 were obtained by slow evaporation of a saturated n-hexane solution in a glove box (29 mg, 18% yield). 31P NMR analysis indicated the formation of 3 in 89% NMR yield. (PPh3 and 1,3,5-trimethoxybenzene were used as internal standards for NMR measurements) (See Fig. S9-S10).

Synthesis of 3 from 2
In a glove box, 2 (433 mg, 0.675 mmol) was added to a benzene solution (20 mL) of DippNacNacM (300 mg, 0.675 mmol). The mixture was stirred at room temperature for 2 h, which was accompanied by a color change from red to reddish-brown. Then all volatiles were removed under reduced pressure. The resulting solid was washed with n-hexane (10 mL×2) and dried under vacuum to give 3 (624 mg, 91% yield) as a red-brown powder. Dark-red single crystals of 3 were obtained by slow evaporation of a saturated n-hexane solution in a glove box. 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.11–7.01 (m, 12H, Ar-H), 6.91 (d, J=7.7 Hz, 4H, Ar-H), 6.82–6.74 (m, 6H, Ar-H), 4.80 (s, 1H, NCCH), 3.24 (sep, J=6.8 Hz, 4H, CHCH3), 2.16 (s, 24H, ArCH3), 1.24 (s, 6H, NCCH3), 1.23 (d, J=6.8 Hz, 12H, CHCH3), 1.09 (d, J=6.8 Hz, 12H, CHCH3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 171.0 (NCCH), 147.5 (Ar-C), 146.1 (Ar-C), 142.4 (Ar-C), 142.4 (d, 3JP-C =2.8 Hz, Ar-C), 140.8 (d, 3JP-C =3.8 Hz, Ar-C), 140.8 (Ar-C), 134.9 (Ar-C), 128.7 (d, 4JP-C =2.5 Hz, Ar-CH), 128.2 (Ar-CH), 127.9 (Ar-CH), 127.5 (Ar-CH), 127.3 (Ar-CH), 126.0 (Ar-CH), 125.4 (Ar-CH), 124.1 (Ar-CH), 100.0 (NCCH), 84.6 (CNP), 29.1 (CHCH3), 24.9 (CHCH3), 23.8 (CHCH3), 23.5 (NCCH3), 21.7 (ArCH3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ 9.0. 31P NMR (162 MHz, C6D6, 25°C, ppm): δ 9.0. FT-IR ν [cm−1] 2952, 1594, 1518, 1435, 1372, 1318, 1248, 1008, 934, 853, 791, 728, 713, 670. HRMS (ESI): m/z calculated for [C69H82AlNP]+ (M+H)+: 1010.6057; found: 1010.6047.

Synthesis of 4 and 5
A benzene (15 mL) solution of 3 (300 mg, 0.296 mmol) in a Young-cock flask was frozen and degassed and refilled with dihydrogen gas (H2, 1 bar). The mixture was heated at 40°C for 12 h, which was accompanied by a color change from reddish-brown to orange. Then all volatiles were removed under reduced pressure, and the residue was recrystallized from n-hexane to afford 4 (65 mg, 22% yield) as colorless crystals and 5 (43 mg, 14% yield) as orange crystals, respectively. Spectroscopic data of 4: 1H NMR (400 MHz, CDCl3, 25°C, ppm): δ 7.39 (s, 1H, Ar-H), 7.16–7.10 (m, 2H, Ar-H), 6.97-6.86 (m, 7H, Ar-H), 6.74 (s, 4H, Ar-H), 6.69–6.64 (m, 8H, Ar-H), 5.14 (s, 1H, NCCH), 3.45 (d, 1JP-H =204.5 Hz, 1H, PH), 2.98 (sep, J=6.8 Hz, 4H, CHCH3), 2.08 (s, 24H, Ar-CH3), 1.53 (s, 6H, NCCH3), 1.16 (d, J=6.8 Hz, 6H, CHCH3), 1.15 (d, J=6.8 Hz, 6H, CHCH3), 0.96 (d, J=6.8 Hz, 6H, CHCH3), 0.63 (d, J=6.8 Hz, 6H, CHCH3). 13C{1H} NMR (101 MHz, CDCl3, 25°C, ppm): δ 170.5 (NCCH), 147.0 (Ar-C), 145.1 (Ar-C), 143.8 (Ar-C), 143.1 (Ar-C), 141.4 (Ar-C), 135.6 (Ar-C), 128.8 (Ar-CH), 127.8 (Ar-CH), 126.8 (Ar-CH), 126.4 (Ar-CH), 124.7 (Ar-CH), 123.6 (Ar-CH), 98.0 (NCCH), 83.7 (CNP), 28.7 (CHCH3), 28.6 (CHCH3), 28.1 (NCCH3), 24.4 (CHCH3), 24.3 (CHCH3), 24.1 (CHCH3), 23.6 (CHCH3), 21.7 (ArCH3). 31P{1H} NMR (162 MHz, CDCl3, 25°C, ppm): δ −87.8. 31P NMR (162 MHz, CDCl3, 25°C, ppm): δ −87.8 (dd, 1JP-H =204.5 Hz, 2JP-H =18.6 Hz). FT-IR ν [cm−1] 2961, 2916, 2863, 2244, 1834, 1594, 1522, 1436, 1385, 1318, 1252, 1165, 1017, 939, 855, 760, 712, 677, 612. HRMS (ESI): m/z calculated for [C69H84AlNP]+ (M+H)+: 1012.6213; found: 1012.6221. Spectroscopic data of 5: 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.24 (s, 4H, Ar-H), 7.16 (s, 3H, Ar-H), 7.11 (d, J=7.9 Hz, 2H, Ar-H), 7.00 (d, J=7.8 Hz, 2H, Ar-H), 6.96–6.86 (m, 4H, Ar-H), 6.82 (s, 2H, Ar-H), 6.74–6.56 (m, 5H, Ar-H), 4.93 (s, 1H, NCCH), 3.32 (sep, J=6.8 Hz, 2H, CHCH3), 3.06 (sep, J=6.8 Hz, 2H, CHCH3), 2.28 (s, 12H, Ar-CH3), 2.11 (s, 12H, Ar-CH3), 1.84 (d, 2JP-H =5.3 Hz, 1H, N−H), 1.41 (s, 6H, NCCH3), 1.10 (d, J=6.8 Hz, 12H, CHCH3), 1.03 (d, J=6.8 Hz, 6H, CHCH3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 170.3 (NCCH), 149.4 (Ar-C), 148.2 (Ar-C), 146.9 (Ar-C), 143.3 (Ar-C), 142.2 (Ar-C), 142.0 (Ar-C), 140.6 (Ar-C), 135.5 (Ar-C), 135.4 (Ar-C), 133.1 (Ar-CH), 132.2 (Ar-CH), 130.1 (Ar-CH), 127.9 (Ar-CH), 127.5 (Ar-CH), 127.2 (Ar-CH), 127.1 (Ar-CH), 125.0 (Ar-CH), 124.2 (Ar-CH), 124.0 (Ar-CH), 97.8 (NCCH), 70.8 (d, 2JP-C =5.5 Hz, CNP), 57.70 (d, 1JP-C =38.5 Hz, CPN), 28.3 (CHCH3), 28.2 (CHCH3), 24.5 (CHCH3), 24.4 (CHCH3), 24.1 (CHCH3), 24.0 (CHCH3), 23.3 (NCCH3), 21.7 (ArCH3), 21.6 (ArCH3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ 55.3. 31P NMR (162 MHz, C6D6, 25°C, ppm): δ 55.3 (d, 2JP-H =20.1 Hz). FT-IR ν [cm−1] 3347, 2962, 2920, 2865, 1817, 1592, 1522, 1439, 1380, 1320, 1254, 1178, 1100, 1018, 934, 847, 798, 762, 698, 636. HRMS (ESI): m/z calculated for [C69H84AlNP]+ (M+H)+: 1012.6213; found: 1012.6221.

Synthesis of 6
In a glove box, P4 (12.4 mg, 0.10 mmol) was added to a benzene solution (10 mL) of 3 (101 mg, 0.10 mmol). Stirred at room temperature, the mixture gradually changed color from reddish-brown to orange over a 5-min period. The solution was dried under vacuum to afford 6 (112 mg, 99% yield) as orange powder. Orange crystals of 6 were obtained through diffusion of n-hexane into a saturated benzene solution at room temperature over a period of one week. 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.47 (s, 4H, Ar-H), 7.38–7.27 (m, 6H, Ar-H), 7.01 (td, J=8.0, 1.5 Hz, 3H, Ar-H), 6.96–6.92 (m, 2H, Ar-H), 6.87–6.83 (m, 4H, Ar-H), 6.74–6.69 (m, 3H, Ar-H), 4.90 (s, 1H, Ar-H), 3.84 (sep, J=6.7 Hz, 1H, CHCH3), 3.67 (sep, J=6.7 Hz, 1H, CHCH3), 3.28 (sep, J=6.7 Hz, 1H, CHCH3), 2.78 (sep, J=6.7 Hz, 1H, CHCH3), 2.22 (s, 12H, Ar-CH3), 2.12 (s, 12H, Ar-CH3), 1.74 (d, J=6.7 Hz, 3H, CHCH3), 1.59 (s, 3H, NCCH3), 1.57 (s, 3H, NCCH3), 1.35 (t, J=6.7 Hz, 6H, CHCH3), 1.19 (d, J=6.7 Hz, 3H, CHCH3), 1.09 (d, J=6.7 Hz, 3H, CHCH3), 1.04 (d, J=6.7 Hz, 3H, CHCH3), 0.97 (d, J=6.7 Hz, 3H, CHCH3), 0.91 (d, J=6.7 Hz, 3H, CHCH3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 172.5 (NCCH), 168.8 (NCCH), 148.9 (Ar-C), 146.4 (Ar-C), 145.7 (Ar-C), 144.5 (Ar-C), 143.7 (Ar-C), 142.8 (Ar-C), 139.0 (Ar-C), 137.6 (Ar-C), 136.2 (Ar-C), 135.9 (Ar-C), 129.9 (Ar-CH), 129.4 (Ar-CH), 129.2 (Ar-CH), 128.2 (Ar-CH), 127.8 (Ar-CH), 127.6 (Ar-CH), 127.1 (Ar-CH), 126.7 (Ar-CH), 125.0 (Ar-CH), 124.7 (Ar-CH), 124.6 (Ar-CH), 124.0 (Ar-CH), 123.7 (Ar-CH), 97.3 (NCCH), 83.8 (d, 2JP-C =6.1 Hz, PNC), 29.4 (CHCH3), 29.0 (CHCH3), 28.7 (CHCH3), 28.3 (CHCH3), 25.9 (CHCH3), 25.7 (CHCH3), 25.4 (CHCH3), 25.3 (CHCH3), 25.0 (CHCH3), 24.4 (NCCH3), 24.3 (NCCH3), 24.1 (CHCH3), 24.0 (CHCH3), 23.1 (CHCH3), 21.7 (Ar-CH3), 21.5 (Ar-CH3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ 94.5 (qd, 1JP-P =187.9 Hz, 2JP-P =51.8 Hz), −21.9 (dd, 1JP-P =236.5 Hz, 2JP-P =50.2 Hz), −81.0 (ddd, 1JP-P =267.3 Hz, 2JP-P =121.5 Hz, 3JP-P =16.2 Hz), −112.8 (tt, 1JP-P =124.8 Hz, 2JP-P =53.5 Hz), −126.3 (dddd, 1JP-P =265.7 Hz, 2JP-P =139.3 Hz, 2JP-P =124.8 Hz, 3JP-P =21.1 Hz). 31P NMR (162 MHz, C6D6, 25°C, ppm): δ 94.5 (qd, 1JP-P =187.9 Hz, 2JP-P =51.8 Hz), −21.9 (dd, 1JP-P =236.5 Hz, 2JP-P =50.2 Hz), −81.0 (ddd, 1JP-P =267.3 Hz, 2JP-P =121.5 Hz, 3JP-P =16.2 Hz), −112.8 (tt, 1JP-P =124.8 Hz, 2JP-P =53.5 Hz), −126.3 (dddd, 1JP-P =265.7 Hz, 2JP-P =139.3 Hz, 2JP-P =124.8 Hz, 3JP-P =21.1 Hz). FT-IR ν [cm−1] 2964, 2916, 2865, 1592, 1528, 1437, 1385, 1315, 1254, 1168, 1010, 935, 852, 792, 719, 677. HRMS (ESI): m/z calculated for [C69H82AlN3P5]+ (M+H)+: 1134.5007; found: 1134.5000.

Synthesis of 7′
In a glove box, tBuNC (13 mg, 0.20 mmol) was added to a benzene solution (10 mL) of 3 (101 mg, 0.10 mmol). Upon addition, the mixture rapidly changed color from reddish-brown to orange at room temperature. Then all volatiles were removed under reduced pressure, and the residue was washed with n-hexane (5 mL×2) to yield 7′ (89 mg, 75% yield) as an orange powder. Despite numerous attempts, obtaining single crystals of 7′ proved impossible, as 7′ underwent isomerization to 7 in solution at ambient temperature. 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.34 (s, 8H, Ar-H), 7.15–7.09 (m, 4H, Ar-H), 7.06–7.03 (m, 2H, Ar-H), 6.99 (d, J=1.8 Hz, 1H, Ar-H), 6.98 (d, J=1.8 Hz, 1H, Ar-H), 6.92–6.89 (m, 2H, Ar-H), 6.78 (s, 4H, Ar-H), 4.86 (s, 1H, NCCH), 3.05 (sep, J=6.7 Hz, 2H, CHCH3), 2.98 (sep, J=6.7 Hz, 2H, CHCH3), 2.20 (s, 24H, ArCH3), 1.38 (s, 6H, NCCH3), 1.32 (s, 9H, NC(CH3)3), 1.20 (d, J=6.7 Hz, 6H, CHCH3), 1.17 (d, J=6.7 Hz, 6H, CHCH3), 1.13 (d, J=6.7 Hz, 6H, CHCH3), 0.95 (d, J=6.7 Hz, 6H, CHCH3), 0.54 (s, 9H, AlNC(CH3)3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 178.9 (PCN), 177.0 (PCN), 172.4 (NCCH), 149.5 (Ar-C), 145.6 (Ar-C), 142.4 (Ar-C), 139.8 (Ar-C), 134.8 (Ar-C), 130.4 (Ar-CH), 127.56 (Ar-CH), 127.5 (Ar-CH), 126.0 (Ar-CH), 125.5 (Ar-CH), 125.4 (Ar-CH), 124.0 (Ar-CH), 101.1 (NCCH), 84.9 (d, 2JP-C =3.5 Hz, CNP), 55.3 (NC(CH3)3), 51.3 (NC(CH3)3), 31.0 (NC(CH3)3), 29.2 (CHCH3), 28.6 (CHCH3), 28.4 (CHCH3), 26.3 (NCCH3), 24.8 (NC(CH3)3), 24.1 (CHCH3), 23.7 (CHCH3), 21.6 (ArCH3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ 119.3. 31P NMR (162 MHz, C6D6, 25°C, ppm): δ 119.3. FT-IR ν [cm−1] 2964, 2916, 2865, 1592, 1533, 1436, 1376, 1279, 1017, 932, 850, 760, 716, 603. HRMS (ESI): m/z calculated for [C79H103AlN6P]+ (M+NH4)+: 1193.7792; found: 1193.7806.

Synthesis of 7
In a glove box, tBuNC (13 mg, 0.20 mmol) was added to a benzene solution (10 mL) of 3 (101 mg, 0.10 mmol). Upon addition, the color of the reaction mixture rapidly changed from reddish-brown to orange at room temperature. Then the mixture was heated at 60°C for 2 days, and the color gradually changed from orange to red. Subsequently, all volatiles were removed under reduced pressure and the residue was recrystallized from n-hexane to afford 7 (52 mg, 44% yield) as red crystals. Single crystals of 7 were obtained by recrystallization from a saturated n-hexane solution at ambient temperature. 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.37 (s, 8H, Ar-H), 7.23–7.19 (m, 2H, Ar-H), 7.11–7.05 (m, 4H, Ar-H), 7.00–6.98 (m, 2H, Ar-H), 6.93–6.91 (m, 2H, Ar-H), 6.78 (s, 4H, Ar-H), 5.03 (s, 1H, NCCH), 3.45 (sep, J=6.7 Hz, 2H, CHCH3), 2.93 (sep, J=6.7 Hz, 2H, CHCH3), 2.14 (s, 24H, ArCH3), 1.67 (s, 9H, NC(CH3)3), 1.37 (s, 6H, NCCH3), 1.20 (d, J=6.7 Hz, 6H, CHCH3), 1.12 (d, J=6.7 Hz, 6H, CHCH3), 1.05 (d, J=6.7 Hz, 6H, CHCH3), 0.94 (d, J=6.7 Hz, 6H, CHCH3), 0.59 (s, 9H, AlNC(CH3)3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 199.3 (d, 1JP-C =139.1 Hz, PC), 171.6 (NCCH), 148.2 (AlCN), 147.4 (Ar-C), 145.2 (Ar-C), 142.9 (Ar-C), 140.7 (Ar-C), 134.7 (Ar-C), 130.0 (Ar-CH), 129.9 (Ar-CH), 127.7 (Ar-CH), 127.6 (Ar-CH), 125.9 (Ar-CH), 125.7 (Ar-CH), 125.5 (Ar-CH), 124.3 (Ar-CH), 102.3 (NCCH), 84.8 (d, 2JP-C =3.1 Hz, CNP), 56.8 (CNC(CH3)3), 53.1 (AlNC(CH3)3), 29.8 (CHCH3), 28.8 (CHCH3), 28.5 (CHCH3), 25.6 (CHCH3), 25.0 (CHCH3), 24.8 (NCCH3), 24.8 (CHCH3), 24.3 (CHCH3), 21.6 (ArCH3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ 138.9. 31P NMR (162 MHz, C6D6, 25°C, ppm): δ 138.9. FT-IR ν [cm−1] 2962, 2921, 2865, 1594, 1526, 1461, 1376, 1311, 1242, 1199, 1163, 1107, 1012, 954, 850, 801, 709, 614. HRMS (ESI): m/z calculated for [C79H100AlN5P]+ (M+H)+: 1176.7527; found: 1176.7531.

Synthesis of 8
In a glove box, PhNC (20 mg, 0.20 mmol) was added to a benzene solution (10 mL) of 3 (101 mg, 0.10 mmol). This led to a rapid color change from reddish-brown to dark red at ambient temperature. The mixture was subjected to vacuum drying and then recrystallized from n-hexane to afford 8 (71 mg, 62% yield) as dark red crystals. Red crystals of 8 were obtained through diffusion of n-hexane into a saturated benzene solution at room temperature. 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.57 (s, 2H, Ar-H), 7.44 (s, 2H, Ar-H), 7.29 (s, 2H, Ar-H), 7.19 (s, 2H, Ar-H), 7.10 (s, 3H, Ar-H), 7.02–6.67 (m, 15H, Ar-H), 6.64–6.59 (m 1H, Ar-H), 6.45–6.35 (m, 3H, Ar-H), 5.96–5.93 (m, 2H, Ar-H), 4.92 (s, 1H, NCCH), 3.12 (s, 3H, CHCH3), 2.70 (s, 1H, CHCH3), 2.20 (s, 12H, Ar-CH3), 2.04 (s, 6H, Ar-CH3), 1.73 (s, 6H, Ar-CH3), 1.46 (s, 3H, NCCH3), 1.44 (s, 3H, CHCH3), 1.38 (s, 3H, CHCH3), 1.12 (s, 3H, CHCH3), 1.03 (s, 3H, NCCH3), 0.98 (s, 6H, CHCH3), 0.82 (s, 3H, CHCH3), 0.75 (s, 3H, CHCH3), 0.43 (s, 3H, CHCH3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 173.0 (NCCH), 170.7 (NCCH), 159.6 (PCN), 159.0 (AlCN), 150.3 (Ar-C), 148.4 (Ar-C), 147.9 (Ar-C), 145.8 (Ar-C), 145.7 (Ar-C), 145.4 (Ar-C), 144.9 (Ar-C), 144.0 (Ar-C), 143.0 (Ar-C), 142.8 (Ar-C), 141.3 (Ar-C), 139.3 (Ar-C), 138.7 (Ar-C), 137.0 (Ar-C), 136.5 (Ar-C), 136.1 (Ar-C), 135.0 (Ar-C), 129.7 (Ar-CH), 128.9 (Ar-CH), 128.8 (Ar-CH), 128.4 (Ar-CH), 127.3 (Ar-CH), 126.6 (Ar-CH), 125.7 (Ar-CH), 125.3 (Ar-CH), 125.1 (Ar-CH), 124.5 (Ar-CH), 123.6 (Ar-CH), 123.2 (Ar-CH), 118.6 (Ar-CH), 118.4 (Ar-CH), 115.1 (Ar-CH), 113.9 (Ar-CH), 98.3 (NCCH), 83.0 (CNP), 29.2 (NCCH3), 28.0 (CHCH3), 27.6 (CHCH3), 25.3 (CHCH3), 24.6 (CHCH3), 24.1 (CHCH3), 23.9 (CHCH3), 23.3 (CHCH3), 23.0 (CHCH3), 22.2 (CHCH3), 21.8 (ArCH3), 21.5 (ArCH3), 21.2 (ArCH3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ 46.3. 31P NMR (162 MHz, C6D6, 25°C, ppm): δ 46.3. FT-IR ν [cm−1] 2923, 2961, 2863, 1585, 1529, 1492, 1378, 1313, 1250, 1170, 1020, 1061, 939, 860, 801, 760, 690, 646. HRMS (ESI): m/z calculated for [C83H92AlN5P]+ (M+H)+: 1126.6901; found: 1126.6904.

Synthesis of 9
In a glove box, PhNC (31 mg, 0.30 mmol) was added to a benzene solution (5 mL) of 3 (101 mg, 0.10 mmol). Immediately upon addition, the color of the mixture rapidly changed from reddish-brown to red at room temperature. Then the mixture was dried under vacuum and recrystallized from n-hexane to afford 9 (59 mg, 45% yield) as red crystals. Red single crystals of 9 were obtained by recrystallization from a saturated n-hexane solution at ambient temperature. 1H NMR (400 MHz, CDCl3, 25°C, ppm): δ 7.36–7.27 (m, 2H, Ar-H), 7.246–7.20 (m, 3H, Ar-H), 7.17–7.10 (m, 7H, Ar-H), 7.03 (s, 2H, Ar-H), 6.98 (d, J=7.5 Hz, 1H, Ar-H), 6.93–6.88 (m, 6H, Ar-H), 6.59 (d, J=7.1 Hz, 1H, Ar-H), 6.53–6.46 (m, 6H, Ar-H), 6.43 (t, J=6.6 Hz, 2H, Ar-H), 6.38–6.34 (m, 1H, Ar-H), 6.25 (t, J=7.5 Hz, 2H, Ar-H), 5.73 (d, J=7.6 Hz, 2H, Ar-H), 5.64 (s, 1H, NCCH), 5.54 (d, J=7.6 Hz, 2H, Ar-H), 3.54 (sep, J=6.7 Hz, 1H, CHCH3), 3.07 (sep, J=6.7 Hz, 1H, CHCH3), 2.90 (sep, J=6.7 Hz, 1H, CHCH3), 2.66 (sep, J=6.7 Hz, 1H, CHCH3), 2.36 (s, 6H, Ar-CH3), 2.03 (s, 6H, Ar-CH3), 1.90 (s, 6H, Ar-CH3), 1.88 (s, 6H, NCCH3), 1.82 (s, 6H, ArCH3), 1.50 (d, J=6.7 Hz, 3H, CHCH3), 1.20 (d, J=6.7 Hz, 3H, CHCH3), 1.11 (d, J=6.7 Hz, 3H, CHCH3), 1.10 (d, J=6.7 Hz, 3H, CHCH3), 0.96 (d, J=6.7 Hz, 3H, CHCH3), 0.95 (d, J=6.7 Hz, 3H, CHCH3), 0.62 (d, J=6.7 Hz, 3H, CHCH3), 0.29 (d, J=6.7 Hz, 3H, CHCH3). 13C{1H} NMR (101 MHz, CDCl3, 25°C, ppm): δ 172.8 (NCCH), 171.4 (NCCH), 155.8 (d, 1JP-C =34.7 Hz, PCC), 153.8 (Ar-C), 149.5 (Ar-C), 147.3 (Ar-C), 146.9 (Ar-C), 146.4 (Ar-C), 145.9 (Ar-C), 145.0 (Ar-C), 144.8 (Ar-C), 144.6 (Ar-C), 144.5 (Ar-C), 144.4 (Ar-C), 143.8 (Ar-C), 140.2 (Ar-C), 139.6 (Ar-C), 136.6 (Ar-C), 136.1 (Ar-C), 135.6 (Ar-C), 130.4 (Ar-CH), 130.0 (Ar-CH), 129.7 (Ar-CH), 129.6 (Ar-CH), 129.4 (Ar-CH), 129.0 (Ar-CH), 128.5 (Ar-CH), 128.1 (Ar-CH), 127.8 (Ar-CH), 127.7 (Ar-CH), 127.5 (Ar-CH), 127.1 (Ar-CH), 126.8 (Ar-CH), 126.6 (Ar-CH), 126.4 (Ar-CH), 126.3 (Ar-CH), 125.5 (Ar-CH), 125.0 (Ar-CH), 124.3 (Ar-CH), 124.1 (Ar-CH), 123.9 (Ar-CH), 122.4 (Ar-CH), 120.2 (Ar-CH), 119.9 (Ar-CH), 117.6 (Ar-CH), 117.1 (Ar-CH), 116.4 (Ar-CH), 99.6 (NCCH), 84.9 (d, 2JP-C =28.4 Hz, CNP), 84.0 (d, 2JP-C =13.6 Hz, CNP), 29.9 (CHCH3), 29.0 (CHCH3), 27.9 (CHCH3), 27.6 (CHCH3), 25.9 (CHCH3), 25.8 (CHCH3), 25.5 (CHCH3), 25.0 (CHCH3), 24.6 (CHCH3), 24.5 (CHCH3), 24.4 (CHCH3), 24.2 (NCCH3), 23.9 (NCCH3), 23.6 (CHCH3), 23.3 (CHCH3), 22.0 (ArCH3), 21.9 (ArCH3), 21.4 (ArCH3), 21.1 (ArCH3). 31P{1H} NMR (162 MHz, CDCl3, 25°C, ppm): δ 112.0. 31P NMR (162 MHz, CDCl3, 25°C, ppm): δ 112.0. FT-IR ν [cm−1] 2960, 2920, 2867, 1681, 1701, 1382, 1590, 1539, 1467, 1314, 1252, 1195, 1019, 861, 800, 721, 674. HRMS (ESI): m/z calculated for [C90H100AlN7P]+ (M+NH4)+: 1336.7588; found: 1336.7581.

Synthesis of 10
A C6D6 (0.6 mL) solution of 3 (50 mg, 0.05 mmol) in a J-Young NMR tube was frozen, degassed, and charged with CO2 (1 atm). At room temperature, the reaction mixture was shaken several times, and a rapid color change from reddish-brown to pale yellow was observed. Then all volatiles were removed under reduced pressure to give 10 (55 mg, 99% yield) as a yellow solid. Colorless crystals of 10 were obtained by recrystallization from a saturated n-hexane solution at ambient temperature. 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.26 (d, J=1.6 Hz, 8H, Ar-H), 7.20 (t, J=7.7 Hz, 2H, Ar-H), 7.12–7.04 (m, 6H, Ar-H), 6.95 (m, 2H, Ar-H), 6.66 (s, 4H, Ar-H), 4.90 (s, 1H, NCCH), 2.99 (sep, J=6.8 Hz, 4H, CHCH3), 2.08 (s, 24H, Ar-CH3), 1.45 (s, 6H, NCCH3), 1.31 (d, J=6.8 Hz, 12H, CHCH3), 1.00 (d, J=6.8 Hz, 12H, CHCH3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 173.7 (d, 1JP-C =21.0 Hz, PCO), 173.0 (NCCH), 147.8 (Ar-C), 144.8 (Ar-C), 144.3 (Ar-C), 136.5 (Ar-C), 135.4 (Ar-C), 129.4 (Ar-CH), 128.5 (Ar-CH), 128.3 (Ar-CH), 126.3 (Ar-CH), 125.8 (Ar-CH), 124.7 (Ar-CH), 97.5 (NCCH), 83.0 (CNP), 28.3 (CHCH3), 24.9 (CHCH3), 24.6 (CHCH3), 22.9 (NCCH3), 21.4 (ArCH3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ 21.8. 31P NMR (162 MHz, C6D6, 25°C, ppm): δ 21.8. FT-IR ν [cm−1] 2960, 2922, 2869, 1701, 1677, 1594, 1533, 1441, 1382, 1316, 1252, 1195, 1086, 1027, 883, 798, 723, 696, 662. HRMS (ESI): m/z calculated for [C71H81AlO4N3PNa]+ (M+Na)+: 1120.5673; found: 1120.5689.

Synthesis of 11
A benzene (5 mL) solution of 3 (101 mg, 0.10 mmol) in a Young-cock flask was injected with 2.5 mL of N2O gas (25°C, 1 atm, 0.10 mmol) using a syringe. The reaction mixture rapidly shifted color from reddish-brown to yellow upon introduction of N2O gas. Then all volatiles were removed under reduced pressure and recrystallized from n-hexane to afford 11 (68 mg, 66% yield) as colorless crystals. 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.91–7.85 (m, 1H, Ar-H), 7.68–7.66 (m, 1H, Ar-H), 7.54 (s, 2H, Ar-H), 7.52 (s, 1H, Ar-H), 7.41 (s, 2H, Ar-H), 7.34 (t, J=7.7 Hz, 1H, Ar-H), 7.15–7.11 (m, 2H, Ar-H), 7.08–7.06 (m, 3H, Ar-H), 6.99–6.93 (m, 3H, Ar-H), 6.80 (s, 1H, Ar-H), 6.69 (s, 1H, Ar-H), 6.38 (s, 1H, Ar-H), 5.11 (m, 3JP-H =9.5 Hz, 1H, PCCH), 5.09 (s, 1H, MeCCH), 4.89 (s, 1H, NCCH), 3.74 (sep, J=6.8 Hz, 1H, CHCH3), 3.11 (sep, J=6.8 Hz, 2H, CHCH3), 2.82 (sep, J=6.8 Hz, 1H, CHCH3), 2.20 (s, 6H, Ar-CH3), 2.18 (s, 6H, Ar-CH3), 2.04 (d, J=6.8 Hz, 3H, CHCH3), 2.01 (s, 6H, Ar-CH3), 1.84 (s, 1H, AlCH), 1.58 (s, 3H, PCCCH3), 1.55 (s, 3H, AlCHCCH3), 1.35 (s, 6H, NCCH3), 1.30 (d, J=6.8 Hz, 3H, CHCH3), 1.25 (d, J=6.8 Hz, 3H, CHCH3), 1.23 (d, J=6.8 Hz, 3H, CHCH3), 1.02 (d, J=6.8 Hz, 3H, CHCH3), 0.95 (d, J=6.8 Hz, 3H, CHCH3), 0.93 (d, J=6.8 Hz, 3H, CHCH3), 0.88 (d, J=6.8 Hz, 3H, CHCH3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 172.1 (NCCH), 170.8 (NCCH), 148.9 (Ar-C), 147.2 (Ar-C), 147.0 (Ar-C), 146.4 (Ar-C), 145.7 (Ar-C), 144.8 (Ar-C), 143.2 (Ar-C), 142.5 (Ar-C), 141.6 (Ar-C), 141.6 (Ar-C), 140.9 (Ar-C), 140.8 (Ar-C), 140.6 (Ar-C), 140.2 (Ar-C), 136.0 (Ar-C), 135.6 (Ar-C), 134.9 (Ar-C), 129.4 (Ar-CH), 129.3 (Ar-CH), 129.0 (Ar-C), 129.0 (Ar-C), 128.3 (Ar-CH), 127.8 (Ar-CH), 127.6 (Ar-CH), 127.5 (Ar-CH), 127.5 (Ar-CH), 126.7 (Ar-CH), 126.4 (Ar-CH), 125.7 (Ar-CH), 125.6 (Ar-CH), 125.1 (Ar-CH), 124.6 (Ar-CH), 124.5 (Ar-CH), 124. (Ar-CH)1, 123.2 (Ar-CH), 120.5 (Ar-CH), 120.2 (Ar-CH), 116.9 (Ar-CH), 98.2 (NCCH), 89.0 (d, 2JP-C =4.4 Hz, PNC), 80.1 (d, 2JP-C =20.8 Hz, PCCN), 49.1 (d, 1JP-C =14.9 Hz, P-C), 29.3 (CHCH3), 28.4 (CHCH3), 28.3 (CHCH3), 27.7 (Al-CH), 26.8 (CHCH3), 26.0 (NCCH3), 25.8 (NCCH3), 25.2 (CHCH3), 25.0 (CHCH3), 24.9 (CHCH3), 24.4 (CHCH3), 24.3 (CHCH3), 24.2 (CHCH3), 24.1 (CHCH3), 24.1 (CHCH3), 23.8 (ArCH3), 21.7 (ArCH3), 21.5 (ArCH3), 21.5 (ArCH3), 21.2 (ArCH3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ 158.3. 31P NMR (162 MHz, C6D6, 25°C, ppm): δ 158.3 (d, 3JP-H =9.5 Hz). FT-IR ν [cm−1] 3017, 2926, 2962, 2869, 2363, 2161, 1590, 1536, 1434, 1388, 1313, 1250, 1170, 1025, 935, 843, 794, 755, 675. HRMS (ESI): m/z calculated for [C69H82AlNOP]+ (M+H)+: 1026.6006; found: 1026.6005.

Synthesis of 11 and 12
A benzene (10 mL) solution of 3 (202 mg, 0.2 mmol) in a Young-cock flask was frozen, degassed, and refilled with N2O gas (1 atm). As the reaction mixture equilibrated to room temperature, its color gradually changed from reddish-brown to orange. The reaction mixture was then dried under vacuum to afford a mixture of 11 and 12 as an orange powder. Colorless crystals of 11 and 12 were obtained from n-hexane and benzene solutions at ambient temperature, respectively. However, all attempts to separate the two compounds were unsuccessful. 1H and 31P NMR analysis indicated the formation of 11 and 12 in 52% and 19% NMR yield, respectively. (PPh3 and 1,3,5-Trimethoxybenzene were used as internal standards for NMR measurements) (Figs. S95–S96). Spectroscopic data of 12: 31P NMR (162 MHz, C6D6, 25°C, ppm): δ 149.7. HRMS (ESI): m/z calculated for [C69H82AlN2O2P]+ (M+H)+: 1042.5955; found: 1042.5954.

Synthesis of 13
In a glove box, 3 (101 mg, 0.10 mmol) was added to a benzene solution (10 mL) of TMSN3 (12 mg, 0.10 mmol). The mixture was stirred at room temperature overnight, with the color gradually changing from reddish-brown to orange. Following removal of all volatiles under reduced pressure, the mixture was recrystallized from n-hexane to afford 13 (38 mg, 34% yield) as orange crystals. Single crystals of 13 were obtained by recrystallization from a saturated n-hexane solution at ambient temperature. 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.29–7.24 (m, 8H, Ar-H), 7.14 (d, J=7.5 Hz, 2H, Ar-H), 7.08–7.01 (m, 4H, Ar-H), 6.88-6.85 (m, 2H, Ar-H), 6.79–6.73 (m, 6H, Ar-H), 4.96 (s, 1H, NCCH), 3.63 (sep, J=6.8 Hz, 2H, CHCH3), 3.12 (sep, J=6.8 Hz, 2H, CHCH3), 2.04 (s, 24H, ArCH3), 1.30 (s, 6H, NCCH3), 1.26 (d, J=6.8 Hz, 6H, CHCH3), 1.15 (d, J=6.8 Hz, 6H, CHCH3), 1.12 (d, J=6.8 Hz, 6H, CHCH3), 0.93 (d, J=6.8 Hz, 6H, CHCH3), 0.27 (s, 9H, Si(CH3)3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 170.1 (NCCH), 145.8 (Ar-C), 145.7 (Ar-C), 145.5 (Ar-C), 144.0 (Ar-C), 143.7 (Ar-C), 141.2 (Ar-C), 135.7 (Ar-C), 128.6 (Ar-CH), 128.5 (Ar-CH), 127.1 (Ar-CH), 127.1 (Ar-CH), 124.7 (Ar-CH), 124.1 (Ar-CH), 123.9 (Ar-CH), 100.6 (NCCH), 82.2 (d, 2JP-C =3.1 Hz, CNP), 28.3 (NCCH3), 27.6 (CHCH3), 25.3 (CHCH3), 24.1 (CHCH3), 23.6 (CHCH3), 23.5 (CHCH3), 21.5 (ArCH3), 1.2 (Si(CH3)3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ 132.0. 31P NMR (162 MHz, C6D6, 25°C, ppm): δ 132.0. FT-IR ν [cm−1] 3017, 2961, 2923, 2867, 1594, 1539, 1462, 1379, 1318, 1233, 1194, 1126, 1020, 966, 835, 799, 704, 666. HRMS (ESI): m/z calculated for [C72H91AlN6PSi]+ (M+H)+: 1125.6622; found: 1125.6625.

Synthesis of 14 and 15
In a glove box, 3 (101 mg, 0.10 mmol) was added to a benzene solution (10 mL) of PhSeSePh (61 mg, 0.20 mmol). The mixture was stirred at room temperature overnight, with the color gradually changing from reddish-brown to yellow. Then all volatiles were removed under reduced pressure, and the resulting mixture was recrystallized from n-hexane to afford 1455 (38 mg, 43% yield) and 15 (16 mg, 25% yield) as colourless crystals. Single crystals of 15 were obtained by recrystallization from a saturated n-hexane solution at ambient temperature. 31P and 1H NMR analysis confirmed the formation of 14 and 15 with NMR yields of 99% and 99%, respectively. (PPh3 and 1,3,5-trimethoxybenzene were used as internal standards for NMR measurements) (For NMR spectra of the reaction mixture, see Figs. S104 and 105). Spectroscopic data of 15: 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.24–7.17 (m, 5H, Ar-H), 7.14 (d, J=5.9 Hz, 5H, Ar-H), 6.92–6.81 (m, 6H, Ar-H), 5.07 (s, 1H, NCCH), 3.41 (sep, J=6.8 Hz, 4H, CHCH3), 1.49 (s, 6H, NCCH3), 1.44 (d, J=6.8 Hz, 12H, CHCH3), 1.00 (d, J=6.8 Hz, 12H, CHCH3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 171.7 (NCCH), 144.8 (Ar-C), 140.0 (Ar-C), 136.0 (Ar-C), 129.2 (Ar-CH), 128.1 (Ar-CH), 127.8 (Ar-CH), 124.8 (Ar-CH), 124.8 (Ar-CH), 99.5 (NCCH), 28.6 (CHCH3), 25.2 (CHCH3), 24.7 (CHCH3), 23.7 (NCCH3). FT-IR ν [cm-1] 3058, 2957, 2925, 2865, 1575, 1526, 1436, 1364, 1313, 1243, 1175, 1100, 1019, 934, 877, 797, 729, 685. HRMS (ESI): m/z calculated for [C41H52AlN2Se2]+ (M+H)+: 759.2271; found: 759.2270.

Synthesis of 16
PhSiH3 (11 mg, 0.10 mmol) was added to a benzene solution (10 mL) of 3 (101 mg, 0.10 mmol) at room temperature. Upon addition, the mixture rapidly changed color from reddish-brown to yellow. Following removal of all volatiles under reduced pressure, the resulting mixture was recrystallized from n-hexane to afford 16 (64 mg, 56% yield) as yellow crystals. Single crystals of 16 were obtained by recrystallization from a saturated n-hexane solution at ambient temperature. 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.77–7.74 (m, 2H, Ar-H), 7.47 (s, 4H, Ar-H), 7.14–7.07 (m, 14H, Ar-H), 6.94 (dd, J=5.9, 3.2 Hz, 2H, Ar-H), 6.86 (s, 2H, Ar-H), 6.72–6.69 (m, 4H, Ar-H), 4.48 (d, 2JP-H =20.0 Hz, 2H, SiH2), 4.11 (s, 1H, NCCH), 3.51 (sep, J=6.7 Hz, 2H, CHCH3), 3.16 (sep, J=6.7 Hz, 2H, CHCH3), 2.26 (s, 12H, Ar-CH3), 1.77 (s, 12H, Ar-CH3), 1.33 (d, J=6.7 Hz, 6H, CHCH3), 1.20 (d, J=6.7 Hz, 6H, CHCH3), 1.10 (s, 6H, NCCH3), 1.00 (d, J=6.7 Hz, 6H, CHCH3), 0.99 (d, J=6.7 Hz, 6H, CHCH3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 170.0 (NCCH), 147.4 (d, JP-C =3.8 Hz, Ar-C), 147.3 (Ar-C), 145.9 (Ar-C), 144.1 (Ar-C), 142.8 (Ar-C), 136.8 (Ar-C), 136.6 (d, JP-C =3.3 Hz, Ar-CH), 136.2 (Ar-C), 129.7 (Ar-CH), 128.8 (Ar-CH), 128.5 (Ar-CH), 127.8 (Ar-CH), 127.6 (Ar-CH), 127.2 (Ar-CH), 126.4 (Ar-CH), 125.3 (Ar-CH), 125.0 (Ar-CH), 124.9 (Ar-CH), 100.2 (NCCH), 84.7 (d, JP-C =2.2 Hz, CNP), 28.3 (CHCH3), 27.8 (d, JP-C =3.8 Hz, CHCH3), 26.6 (NCCH3), 24.9 (CHCH3), 24.9 (CHCH3), 24.8 (CHCH3), 24.6 (CHCH3), 21.2 (ArCH3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ −81.0. 31P NMR (162 MHz, C6D6, 25°C, ppm): δ −81.0 (d, 2JP-H =55.4 Hz). 29Si NMR (79 MHz, C6D6, 25°C, ppm) δ −35.60 (d, 1JP-H =42.0 Hz). FT-IR ν [cm−1] 2958, 2920, 2865, 2130, 1871, 1594, 1514, 1463, 1390, 1308, 1244, 1163, 1104, 1031, 961, 853, 800, 700. HRMS (ESI): m/z calculated for [C75H89AlNPSiNa]+ (M+Na)+: 1140.6271; found: 1140.6271.

Synthesis of 17
In a glove box, 3 (101 mg, 0.10 mmol) was added to a benzene solution (10 mL) of PhNH2 (9 mg, 0.10 mmol). Upon addition of the substrate, the mixture rapidly changed color from reddish-brown to yellow at room temperature. Then all volatiles were removed under reduced pressure and washed with n-hexane (5 mL×2) to yield 17 (91 mg, 82% yield) as a white powder. Colorless crystals of 17 were obtained by recrystallization from a mixture of n-hexane and benzene at ambient temperature. 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.28–7.23 (m, 2H, Ar-H), 7.12 (s, 8H, Ar-H), 7.09–7.07 (m, 2H, Ar-H), 6.94 (d, J=7.7 Hz, 4H, Ar-H), 6.84 (s, 4H, Ar-H), 6.76 (d, J=7.2 Hz, 1H, Ar-H), 6.70 (d, J=2.5 Hz, 3H, Ar-H), 6.63 (d, J=8.0 Hz, 2H, Ar-H), 6.35 (d, J=7.5 Hz, 1H, Ar-H), 5.19 (s, 1H, NCCH), 4.31 (d, 1JP-H =210.9 Hz, 1H, P-H), 3.26 (sep, J=6.7 Hz, 2H, CHCH3), 3.09 (sep, J=6.7 Hz, 2H, CHCH3), 2.62 (s, 1H, N-H), 2.11 (s, 24H, Ar-CH3), 1.40 (d, J=6.7 Hz, 6H, CHCH3), 1.30 (s, 6H, NCCH3), 1.03 (d, J=6.7 Hz, 6H, CHCH3), 1.01 (d, J=6.7 Hz, 6H, CHCH3), 0.91 (d, J=6.7 Hz, 6H, CHCH3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 170.4 (NCCH), 150.8 (Ar-C), 148.6 (d, 2JP-C =4.1 Hz, Ar-C), 145.2 (Ar-C), 144.4 (Ar-C), 143.5 (Ar-C), 142.0 (Ar-C), 135.7 (Ar-C), 129.2 (Ar-CH), 128.2 (Ar-CH), 128.1 (Ar-CH), 127.4 (Ar-CH), 126.4 (Ar-CH), 124.5 (Ar-CH), 124.1 (Ar-CH), 123.9 (Ar-CH), 118.6 (Ar-CH), 115.2 (Ar-CH), 100.8 (NCCH), 83.4 (d, 2JP-C =4.7 Hz, CNP), 29.1 (CHCH3), 28.9 (CHCH3), 27.2 (CHCH3), 25.5 (CHCH3), 24.9 (CHCH3), 24.5 (CHCH3), 24.2 (CHCH3), 23.8 (NCCH3), 21.8 (ArCH3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ −83.3. 31P NMR (162 MHz, C6D6, 25°C, ppm): δ −83.3 (d, 1JP-H =210.9 Hz). FT-IR ν [cm−1] 3345, 2963, 2921, 2863, 1596, 1531, 1437, 1369, 1318, 1274, 1172, 1018, 938, 855, 798, 719. HRMS (ESI): m/z calculated for [C75H89AlN6P]+ (M+H)+: 1103.6635; found: 1103.6639.

Synthesis of 18
In a glove box, 3 (100 mg, 0.10 mmol) was added to a benzene solution (10 mL) of styrene (11 mg, 0.10 mmol). The mixture was stirred at room temperature overnight, with the color gradually changing from reddish-brown to red. Then all volatiles were removed under reduced pressure and the residue was washed with n-hexane to afford 18 (69 mg, 61% yield) as an orange powder. Orange crystals of 18 were obtained by recrystallization from a saturated benzene solution at ambient temperature. 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.50–7.18 (m, 7H, Ar-H), 7.15–6.66 (m, 15H, Ar-H), 6.59–6.31 (m, 5H, Ar-H), 4.83 (s, 1H, NCCH), 3.45–3.25 (m, 3H, CHCH3), 3.04 (sep, J=6.8 Hz, 2H, CHCH3), 2.31 (s, 6H, Ar-CH3), 2.24 (s, 6H, Ar-CH3), 2.18–2.15 (m, 2H, AlCH2), 1.97 (s, 6H, Ar-CH3), 1.83 (s, 6H, Ar-CH3), 1.57 (s, 3H, NCCH3), 1.33 (s, 3H, NCCH3), 1.27 (d, J=6.8 Hz, 3H, CHCH3), 1.14 (d, J=6.8 Hz, 3H, CHCH3), 1.10–0.96 (m, 12H, CHCH3), 0.93 (d, J=6.8 Hz, 3H, CHCH3), 0.86 (d, J=6.8 Hz, 3H, CHCH3), 0.29 (t, J=14.6 Hz, 1H, PCH). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 171.9 (NCCH), 171.1 (NCCH), 153.9 (Ar-C), 153.7 (Ar-C), 148.8 (Ar-C), 148.2 (Ar-C), 147.4 (Ar-C), 146.8 (Ar-C), 145.4 (Ar-C), 144.2 (Ar-C), 143.9 (Ar-C), 142.9 (Ar-C), 142.1 (Ar-C), 141.6 (Ar-C), 136.6 (Ar-C), 135.8 (Ar-C), 135.0 (Ar-C), 130.5 (Ar-CH), 129.1 (Ar-CH), 128.2 (Ar-CH), 127.8 (Ar-CH), 127.6 (Ar-CH), 127.2 (Ar-CH), 126.6 (Ar-CH), 125.8 (Ar-CH), 125.1 (Ar-CH), 124.7 (Ar-CH), 123.5 (Ar-CH), 122.8 (Ar-CH), 98.5 (NCCH), 85.1 (CNP), 41.3 (d, 1JP-C =24.7 Hz, PCH), 29.7 (CHCH3), 29.2 (CHCH3), 28.7 (CHCH3), 28.1 (CHCH3), 25.8 (AlCH2), 25.5 (CHCH3), 25.2 (NCCH3), 25.1 (CHCH3), 24.9 (CHCH3), 24.6 (CHCH3), 24.4 (NCCH3), 24.0 (CHCH3), 22.6 (CHCH3), 21.4 (ArCH3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ 56.7. 31P NMR (162 MHz, C6D6, 25°C, ppm): δ 56.7 (dd, 2JP-H =35.5 Hz, 3JP-H =14.7 Hz). FT-IR ν [cm−1] 2961, 2918, 2867, 1592, 1524, 1439, 1386, 1310, 1250, 1170, 1092, 1019, 934, 850, 797, 762, 702, 675. HRMS (ESI): m/z calculated for [C77H90AlN3P]+ (M+H)+: 1114.668; found: 1114.660.

Synthesis of 19 and 20
In a glove box, 3 (202 mg, 0.20 mmol) was added to a benzene solution (20 mL) of 1-ethynyl-4-methylbenzene (23 mg, 0.20 mmol). The mixture was stirred at room temperature, accompanied by a rapid color change from reddish-brown to red. Following removal of all volatiles under reduced pressure, the mixture was recrystallized successively from a mixture of n-hexane and benzene to afford red crystals of 19 (85 mg, 38% yield) and a small number of colorless crystals of 20 (16 mg, 7% yield), respectively. Single crystals of 19 were obtained by recrystallization from a saturated n-hexane solution at ambient temperature. Regrettably, we were unable to obtain single crystals of 20 that were suitable for SC-XRD analysis. 31P and 1H NMR analysis confirmed the formation of 19 and 20 with NMR yields of 47% and 17%, respectively. (PPh3 and 1,3,5-trimethoxybenzene were used as internal standards for NMR measurements) (For NMR spectra of the reaction mixture, see Figs. S131 and S132). Spectroscopic data of 19: 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 8.16 (d, 2JP-H =29.3 Hz, 1H, PCH), 7.46 (d, J=7.7 Hz, 2H, Ar-H), 7.34 (s, 1H, Ar-H), 7.16 (s, 5H, Ar-H), 7.11 (d, J=8.1 Hz, 3H, Ar-H), 7.08–7.02 (m, 5H, Ar-H), 6.90–6.88 (m, 2H, Ar-H), 6.86–6.83 (m, H, Ar-H), 6.74–6.70 (m, 6H, Ar-H), 5.12 (s, 1H, NCCH), 3.04 (sep, J=6.8 Hz, 2H, CHCH3), 3.00 (sep, J=6.8 Hz, 2H, CHCH3), 2.19 (s, 3H, Ar-CH3), 2.16 (s, 24H, ArCH3), 1.34 (s, 6H, NCCH3), 1.13 (d, J=6.8 Hz, 6H, CHCH3), 1.10 (d, J=6.8 Hz, 6H, CHCH3), 0.98 (d, J=6.8 Hz, 6H, CHCH3), 0.88 (d, J=6.8 Hz, 3H, CHCH3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 170.5 (NCCH), 161.7 (d, 1JP-C =22.8 Hz, PCHC), 148.8 (Ar-C), 146.0 (Ar-C), 144.4 (Ar-C), 142.0 (Ar-C), 141.6 (Ar-C), 140.6 (d, 2JP-C =13.3 Hz, PCHC), 135.2 (Ar-C), 135.0 (Ar-C), 133.6 (Ar-C), 129.4 (Ar-CH), 128.4 (Ar-CH), 128.1 (Ar-CH), 126.9 (Ar-CH), 126.3 (Ar-CH), 124.8 (Ar-CH), 124.5 (Ar-CH), 123.7 (Ar-CH), 99.5 (NCCH), 83.5 (d, 2JP-C =4.3 Hz, CNP), 28.3 (NCCH3), 27.5 (CHCH3), 25.2 (CHCH3), 24.4 (CHCH3), 24.0 (CHCH3), 23.8 (CHCH3), 21.8 (ArCH3), 21.0 (ArCH3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ 57.6. 31P NMR (162 MHz, C6D6, 25°C, ppm): δ 57.6 (d, 2JP-H =29.3 Hz). FT-IR ν [cm−1] 3017, 2954, 2921, 2865, 1574, 1521, 1436, 1374, 1313, 1250, 1172, 1012, 937, 855, 813, 716, 643. HRMS (ESI): m/z calculated for [C78H90AlN3P]+ (M+H)+: 1126.6688; found: 1126.6692. Spectroscopic data of 20: 1H NMR (400 MHz, C6D6, 25°C, ppm): δ 7.55 (d, J=8.0 Hz, 2H, Ar-H), 7.11 (d, J=7.6 Hz, 2H, Ar-H), 7.08–7.02 (m, 10H, Ar-H), 6.98–6.91 (m, 4H, Ar-H), 6.85–6.79 (m, 6H, Ar-H), 6.72–6.69 (m, 2H, Ar-H), 4.94 (s, 1H, NCCH), 3.97 (d, 1JP-H =204.1 Hz, 1H, P-H), 3.80 (sep, J=6.8 Hz, 2H, CHCH3), 3.12 (sep, J=6.8 Hz, 2H, CHCH3), 2.13 (s, 24H, ArCH3), 2.06 (s, 3H, ArCH3), 1.35 (d, J=6.8 Hz, 6H, CHCH3), 1.32 (d, J=6.8 Hz, 6H, CHCH3), 1.29 (s, 6H, NCCH3), 1.03 (d, J=6.8 Hz, 6H, CHCH3), 0.98 (d, J=6.8 Hz, 6H, CHCH3). 13C{1H} NMR (101 MHz, C6D6, 25°C, ppm): δ 170.4 (NCCH), 148.1 (Ar-C), 148.0 (Ar-C), 145.2 (Ar-C), 144.3 (Ar-C), 143.3 (Ar-C), 142.0 (Ar-C), 136.5 (Ar-C), 135.9 (Ar-C), 131.5 (Ar-CH), 129.0 (Ar-CH), 128.8 (Ar-CH), 127.0 (Ar-CH), 126.4 (Ar-CH), 124.8 (Ar-CH), 123.9 (AlCCAr), 123.8 (Ar-CH), 123.7 (Ar-CH), 105.1 (AlCCAr), 100.2 (NCCH), 83.7 (d, 2JP-C =4.7 Hz, CNP), 28.6 (CHCH3), 28.5 (CHCH3), 28.4 (CHCH3), 26.5 (CHCH3), 24.9 (NCCH3), 24.8 (NCCH3), 24.2 (CHCH3), 24.0 (CHCH3), 23.7 (CHCH3), 21.9 (ArCH3), 21.0 (ArCH3). 31P{1H} NMR (162 MHz, C6D6, 25°C, ppm): δ −89.5. 31P NMR (162 MHz, C6D6, 25°C, ppm): δ −89.5 (d, 1JP-H =204.1 Hz). FT-IR ν [cm−1] 3015, 2961, 2918, 2863, 2246, 1594, 1533, 1437, 1368, 1318, 1254, 1179, 1100, 1019, 937, 852, 797, 716, 680, 646. HRMS (ESI): m/z calculated for [C78H90AlN3P]+ (M+H)+: 1126.6688; found: 1126.6724.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript by Lu and co-workers reports the synthesis of a Phosphaalumene (3), which is subsequently employed in the activation of several small molecules, including H₂, P₄, and CO₂. The authors’ efforts to probe both the stability and reactivity of Phosphaalumene (3) are commendable. However, after a detailed reading, the manuscript gives the impression that the study would benefit from additional reactivity investigations to provide a more comprehensive picture of this system. At present, the scope of the reactivity appears somewhat limited, and broadening it would significantly strengthen the work.\nComments:\n1. It would be valuable to examine the reactivity of compound (3) towards some of the following:\n• Alkenes (e.g., ethylene, 1,3-butadiene, styrene).\n• Alkynes bearing electron-donating and electron-withdrawing substituents.\n• Other homolytically cleavable bonds (N=N, Se-Se, B-B etc.)\n• Several azides are known for their reactivity with main group double bonds\nIncluding such studies would substantially improve the impact of the manuscript and provide deeper insight into the versatility of this phosphaalumene system.\n2. In the sentence “Braunschweig and Goicoechea aimed at developing phosphaalumene (3) species by integrating a two-coordinate Al(I) species”, (3) does not give a proper understanding. What does “phosphaalumene (3)” mean? Is it the same as mentioned in the title?\n3. The authors must highlight the importance of this unconventional bond in the introduction section in detail.\n4. In Figure 1, the complete structure should be there for “in this work”; even for the short structure, it is giving us the impression that the methyl group is attached adjacent to the N-atom in the isoindoline moiety, which is not correct. The Ar group attached to the N-atoms is also not specified.\n5. The authors must modify the sentence “The reaction of 1 with two equivalents of DippNacNacAl: in benzene at 50 °C afforded a reddish-brown solution (Figure 2)”. This sentence should refer to the formation of desired product 3 as a reddish-brown solution.\n6. On Page 14, lines 330–332, the authors state that the reaction of PhSiH₃ demonstrates π(Al=P) character, while the reaction with PhNH₂ indicates FLP-type reactivity. However, according to the literature [Chem. Eur. J. 2024, 30, e202401326; Angew. Chem. Int. Ed. 2021, 60, 22057–22061], a similar product was observed for the reaction of amines with related systems. Importantly, these reports do not describe such reactivity as FLP-type. The authors are therefore requested to carefully re-examine this assignment and revise the statement accordingly, as the current wording could be misleading.\n7. Did the authors try to see if H2 activation is reversible? Or can it be transferred to some substrates?\n8. The authors should include a table of Al-P bond length comparison with previous reports.\n9. The 29Si NMR spectrum is missing for compound 12.\n10. Several NMR spectra show a significant amount of impurities are present; the authors are advised to either mark the impurities or re-measure the NMR spectrum if possible.\n11. Important peaks must be marked in the FT-IR spectrum.\n12. As all the compounds are coloured, the authors are advised to investigate the UV-Vis spectrum and add a discussion on it.\n13. In the reactivity of complex 3 with CO2, please provide a theoretical insight for the formation of 1,3-addition product by phoshphaalumenes, since the activation of CO2 by multiple bonds may lead to the formation of many other products (Angew. Chem. Int. Ed. 2018, 57, 14575 –14579).\nMinor typos:\n1. The π sign in Figure 1 is not uniform.\n2. ppm is missing in the SI for 1H and 13C NMR description, while others are fine.\n3. oC is not uniform in the description of 1H for 3 in SI\n4. δ and other Greek alphabetical symbols (e.g. π, σ, n, etc) should be in italics throughout the manuscript.\n5. The picture quality for 8 is not the same as the others.\n6. Page No. 12, In Fig. 6, Reactivity of complex 3 with CO2, please mention the equivalency of CO2.\n7. In the supporting information, it will be better if the authors can add the ChemDraw of the respective complexes with the NMR spectrum, it will help to understand the peak positions of the nuclei.", "reviewer_2": "This manuscript by Lu and coworkers describes the synthesis of a thermally stable phosphaalumene with a 3-coordinate Al center, which the authors term phosphaalumene(3). By contrast to a previously reported phosphaalumene(3), title compound 3 is thermally stable in the solid state and was afforded in very high yield through the combination of an isoindoline-substituted phosphanylidenephosphorane with the classic DippNacnacAl. The initial route using the groups landmark bisphosphrane-fused anthracene derivative 1 only afforded 3 in poor yields as a result from problematic anthracene separation. In solution 3 was found to thermally labile at 50 °C, implying an increased reactivity when compared to the only known isolable phosphaalumene. H2 activation was achieved using 3, giving two distinct products in a 2:1 ratio, with the major product being identified as the 1,2-addition product across the P=Al bond. The minor species originates from initial 1,3-addition at Al and N, followed by a rearrangement, which was probed theoretically. P4 activation was likewise achieved, giving a product in which two of the P4 sigma bonds have been broken. With isonitriles coupling reactions were observed, while with CO2 twofold insertion into the P=Al bond was observed. With N2O two different oxidation products were afforded, of which compound 10 could be selectively synthesized when 1 equiv. of N2O was applied. Lastly, the Si-H and N-H bond activation was probed in the reactions with PhSiH3 and PhNH2, respectively. Both showing 1,2-addition across the P=Al axis with the P-atom acting as silyl- or proton-acceptor, respectively.\nOverall, I really enjoyed evaluating this exciting manuscript, which reports on a lot of new aspects in the chemistry of phosphaalumenes. It shows principal design features that need to be applied to render phosphaalumenes(3) and how these features impact the reactivity of the Al-P bond. I believe that this work of considerable interest for the diverse readership of Nat. Commun. and I recommend publication after some specific points have been addressed.\nSpecific Points:\n1) The introduction provides a great overview of the field and the state-of-the-art is mainly covered. When discussing phosphagallenes (ref. 39-41) four more papers should be cited: Chem. Sci., 2025, 16, 7397-7410 (now ref. 64); Inorg. Chem. 2025, 64, 7, 3485–3494; Dalton Trans., 2020,49, 15249-15255; Chem. Sci., 2023, 14, 3018-3023.\n2) Page 3, 2nd par.: “…employing group 13 carbenoids DippNacNacM(I) (M = Al. Ga)…” This should read (M = Al, Ga). Also, later in the text the authors use DippNacnacAl: to account for the carbenoid character of that compound. This should be unified and I recommend just using DippNacnacM throughout the manuscript.\n3) Page 3: “…the first isolable phosphaalumene I, which was characterized by a four-coordinate Al center…” Why is the Al-center in I four-ccordinate? This needs further elaboration, as two descriptions are possible. Either it is diccordinate, when accounting for Cp* as an X-type ligand, or if the eta5-coordination is considered it would be 6-coordinate with an additional interaction to the flanking Dipp-group. Please clarify! In this regard, the authors are encouraged to also add the Al-coordination number in II into Fig.1!\n4) Figure 1: The Ar-substituent in part b) needs to be defined. In the caption it should read: a, Reported examples of phosphaalumenes (I-II).\n5) Page 4, line 4: Please correct “…bonds. (Figure S94).” to read “…bonds (Figure S94).”\n6) Page 6, line 4: The 31P NMR shift of compound 3 should be compared to that of I and II! What is the reason for the marked deshielding when compared to I for example?\n7) Page 6, line 7: When comparing the Al-P distance to that in DippTer and TippTer derivatives of I the exact values should be given, rather than just using rounded values. Each comparative value should be marked with a citation (Dipp: 44; Tipp: 45).\n8) Figure 2: At first, Figure 2 to me classifies as a Scheme. In reaction a, benzene should appear under the arrow (also remove the comma after benzene). The wavelength of irradiation should be given (365 nm according to the text). In part a) the NMR-yield for route 1 is given as 58%, while it is 42% in the ESI, please check! Same holds for route b: 83 vs 89%.\n9) Page 8: When discussing NBO results, the authors should also include the NPA charges at the N atom, as well as the WBI of the Al-N bond. Is there a donor-acceptor interaction between the LP on N and the P atom? Additionally, the authors state that 3 forms as a dark-red crystalline solid. I wonder whether UV-Vis data has been recorded and whether the origin of the color can be traced to individual electronic transitions?\n10) Page 10, line 4: “…as it offers a sustainable means of generating organophosphorus substances.” Using P4 as a building block is more “sustainable” than using PCl3-derived P-compounds, but P4 in no way is a sustainable reagent considering its high-energy synthesis. This sentence should be reformulated: “…as it is a versatile reagent in generating organophosphorus substances.”\n11) Page 10/11, line 1: When discussing the structure of compound 6, the authors should be more diligent. The structure is indicative of two electron reduction of P4 and oxidation of the P and Al centers to P(III) and Al(III), respectively. This is in line with the reactivity observed for dialumenes and disilenes. Also, the authors are urged to discuss the 31P NMR data in more detail, while also providing a zoom-in in the ESI of the individual multiplets, as insets (Figure S36).\n12) Page 11: When discussing the reactivity of 3 towards isocyanides two different types of products are observed. With tBuNC twofold insertion and isonitrile couling is observed, while with PhNC three molecules are incorporated into 3. From the text it is not clear that the reaction with tBuNC was conducted with 2 equiv., while in the case of PhCN 3 equiv. were utilized. What happens if only 2 equiv. of PhNC are used? Likewise, does using 3 equiv. of tBuNC give a compound similar to 8? Compound 7 is not a “spiral structure” but rather a spirocyclic Al-species.\n13) The reactivity towards CO2 contrasts that of II and is related to that of gallyl- and arsanyl-substituted phosphagallenes. The comparison should be more precise and clearly state the different types of phosphagallenes for which this reactivity was observed. “…, , which resembles those found in compounds containing Ga=P double bonds.” This part of the sentence should be reconsidered as no 6-membered ring is found in species with a Ga=P double bond!\n14) When discussing the N2O reactions, the authors should clearly state that 11 was only isolated from the mixture by crystal-packing. The discussion as it stands now makes the reader believe that both compounds can be isolated in pure form which is not true. Please also edit 6, bottom right, accordingly. The isolated yield part is confusing and should be removed.\n15) Page 14, line 3: “In contrast, intermediate II only showed pi(Al=P) character…” This statement is misleading and should be corrected. What the authors want to say is that II only reacted with 1,2-addition across the P-Al bond with PhSiH3.\n16) The calculation part is a nice addition, but I wonder whether it would be better placed directly in the beginning when the reactivity of 3 towards H2 is discussed. Overall, the mechanism shown is in line with the observed reactivity, however, the barriers in my eyes are a bit too high for facile reactions at 40°C. But more importantly, judging from the ESI, the solvation effect has not been implemented correctly.\nThe solvation energy is the difference between the single point energy using the SMD-model and the single point energy in the gas-phase at the same level of theory. In this case I suggest that the authors carry out the single point calculations at the SMD(benzene)-B3LYP-D3/6-311G(2d,2p) level of theory and single point calculations at the same level without the SMD model. Esolv than equates to: Esolv = Etot(SMD(benzene)-B3LYP-D3/6-311G(2d,2p)) - Etot(B3LYP-D3/6-311G(2d,2p)). The free enthalpy than equates to: G = Etot(B3LYP-D3/6-311G(2d,2p)) + Esolv + GCorr(PBE0-D3/def2-SVP). I hope this is understandable.\nI know that in many studies this is done incorrectly, but this can be rather quickly rectified as only the single point calculations have to be redone. A paper in which this is nicely demonstrated is the following: JACS 2021, 143, 17366. A detailed comment on how to calculate the energies correctly is given on Page S32 of the ESI.\nThe ESI is very detailed and should allow reproduction of the results. There are some minor points that need attention though:\n1) Synthesis of 2: The concentration of the PMe3 solution in toluene should be given together with the volume that was used. In the equation the stoichiometry should be correctly given: 2 KC8! Also separate the reagents by a comma or place them vertically on top of each other.\n2) Synthesis of 3: Benzene should appear under the arrow. Again, separate temperature, time and hv by commas (this holds for all equations in the ESI). The wavelength should be given in the equation. Also put either – C14H10 or – anthracene. If possible, add UV-Vis characterization data, vide supra.\n3) Synthesis of 6: …P4 () was added… The crystallization procedure should be described as follows: Orange crystals of 6 were obtained through diffusion of n-hexane into a saturated benzene solution at (add temperature) over a period of (add time).\n4) The crystal structure data is generally fine, however, the wR2 values are rather high in most cases. However, this is most likely to the solvent molecules, which have been refined rather than squeezing them out. Therefore, I recommend no further changes here.\n5) As for the calculations, see my comments above.\nOverall, I think the manuscript would benefit from taking into account the points raised above and I am happy to re-evaluate a revised version of the manuscript.", "reviewer_3": "This report by Lu and co-workers details their investigation of a newly synthesised compound containing a rarely isolated Al=P bond. The compound in question in synthesised by reaction of an appropriate Al(I) synthon (DippBDIAl:) with a phosphinidene transfer reagent. In the first case a bisphosphrane-fused anthracene was used as the transfer agent, but resulted in low yields of the desired compound. Using an analogous phospha-Wittig reagent (this is a known method for forming P=Al bonds: J. Am. Chem. Soc. 2021, 143, 4106) with the same bulky isoindoline motif, highly improved and synthetically useful yields were obtained (> 0.5 g). The combination of high yields, substantial isolated product quantities, and the rarity of P=Al bonds, makes this synthesis particularly notable.\nThe authors characterise this new compound (3) by multinuclear NMR and IR spectroscopy, high-resolution mass spectrometry, and single-crystal X-ray diffraction. To gain further insight into the P–Al bonding, density functional theory calculations were performed. Overall, the results suggest that indeed 3 does contain a polarised P=Al double bond. Notably, the bonding seems to involve nitrogen-based atomic orbitals within the N–P=Al motif, which might be influencing the chemistry of 3.\nThe reaction of various small molecules with 3 was investigated, including H , P , isocyanides, CO and N O, as well as both hydridic and protic R–H bonds (R = SiPhH and NPhH). In general, 1,2-addition across the P=Al bond was observed in all cases. In some cases, complete scission of the P=Al bond was observed. The reaction with H additionally afforded the 1,3-addition product, suggesting some FLP-like reactivity. Consequently, the reactivity between 3 and H was further investigated computationally, revealing two pathways corresponding to the 1,2- and 1,3-addition products to both be thermally accessible, with the rate determining step of the minor product calculated to have a higher energy barrier as expected.\nOverall, the isolation of 3, as well as the diversity of reactivity reported with addition reactions across the P=Al double bond makes the results presented of interest to the community. The conclusions and data interpretation are valid, and somewhat robust and reliable, but there are several additional points (see below) that must be considered and addressed. In particular, an expanded analysis and discussion of the actual [N–P=Al] motif must be considered, as this clearly influences the observed stability and reactivity of the P=Al double bond. Beyond reporting the reaction products, more justification and analysis of these products needs to be included. If the points raised below are suitably addressed and a significantly updated version of the manuscript is submitted, then I believe this will be suitable for publication in Nature Communications.\nGeneral considerations:\n• The key point which must be addressed is the influence of the adjacent nitrogen atom to the P=Al bond. This seems to be important for both the stability and reactivity of compound 3, and potentially the key result within the report. This favourable [N–P=Al] motif is alluded to in the final paragraph of the introduction when some “preliminary theoretical studies” suggested nitrogen to be the best atom to be bonded to phosphorus, but this is not built upon in the formal analysis of the bonding. In general, the bonding analysis should be expanded to include discussion of the influence of this adjacent nitrogen. Even within the existing analysis, the authors identify HOMO–2 as “predominantly reflecting the π-bonding interactions along the N–P bond”. This is interesting because as drawn, there is “only” a σ-bond between these atoms. This suggests some delocalisation and/or resonance such as N–P=Al ←→ N=P–Al is involved in the bonding. In line with this argument, the observed P—Al bond length by SCXRD (2.234 Å) is intermittent between an idealised P–Al (single) and P=Al (double) bond, 2.37 and 2.15 Å, respectively, according to Pyykkö (Chem. Eur. J. 2009, 15, 186; Chem. Eur. J. 2009, 15, 12770), as well as a calculated WBI of 1.52.\n• Similar to compound II reported by Goicoechea (Chem. Eur. J. 2024, 30, e202401326), both systems show a mixture of 1,2- and 1,3-addition reactions in certain cases. The reaction between II and PhNH resulted in FLP-like 1,3-addition but this 2 was found to subsequently isomerise to generate a product equivalent to the direct 1,2-addition. Could the same process be operational in the reaction between 3 and PhNH but the isomerisation proceeds at room temperature? Are any intermediates observed if the reaction between 3 and PhNH is monitored at low temperature?\n2\no Analysis of this reactivity (and the reactivity between 3 and PhSiH ) could be further supported by examining the NPA 3 charges on nitrogen, as well as phosphorus and aluminium. This should give an indication of the most reactive site for each electrophilic and nucleophilic component of PhNH – i.e. why is the protic hydrogen of PhNH activated by nitrogen and not 2 2 by phosphorus? How does this compare with Goicoechea’s P–P=Al system?\no The penultimate sentence highlights that the reactivity observed for 3 is different for II, but I do not believe this has been definitively demonstrated. More data is required to show that, for instance, no FLP-like reactivity is observed for PhNH , and 2 if this reactivity is different, then some explanation is required. 3 and II are similar, with the same [P=AlBDIDipp] fragment in both. The key differentiator is the atom bonded to P – nitrogen vs. phosphorus. Differences between the two should be contextualised with this key difference in mind.\n• Finally, the discussion section is more of a summary than discussing any of the results, or contrasting any of the bonding or reactivity data. This section should be edited to either discuss the data, or the heading removed and for it to be included as a final summary of the results.\no If the discussion is expanded, a focus on P–P=Al vs. N–P=Al would be interesting. Clearly the interplay between 1,2- and 1,3-addition is subtle, with only a few kcal/mol differentiating the two pathways and consequently both are operational under thermally accessible conditions for 3. This appears to not be the case for Goicoechea’s P–P=Al system, where 1,3-addition is predominantly observed (except for PhSiH where the same reactivity for both 3 and II is observed).\n• Which revision of Gaussian16 was used? The reference cites A.03 but this needs to be stated explicitly within the methodology text.\n• Why is no Becke Johnson dampening added to the dispersion? I know in other papers e.g. (J. Am. Chem. Soc. 2021, 143, 4106; ChemPlusChem 2023, 88, e202300078; Chem. Eur. J. 2024, 30, e202401326) the authors use the notation “PBE0-D3” but explicitly state that PBE0-D3(BJ) was implemented. The keyword for this is empiricaldispersion=gd3bj.\n• The geometry optimisations were performed with a split valence basis set on all atoms, which is fine, but there was no analysis of whether this level of theory accurately reproduced the structural data obtained from the single-crystal X-ray structures and/or whether there is precedent for using this level of theory (functional and basis set combination) for similar compounds. Either level of theory testing or precedent should be carried out or stated. I would recommend for completeness, at least some functional screening should be undertaken to assess the best functional for these systems.\n• It is stated that frequency calculations were carried out, but were these used to determine whether intermediates and transition states had converged to true minima through analysis of the imaginary frequencies? This is not stated.\n• Were the intermediates either side of a transition state found to be connected via intrinsic reaction coordinate calculations? This ensures that the transition states located do indeed link reactants to products. This should at least be done for the key transition states e.g. TS1, TS1’ and TS3.\n• The molecules were simplified for the DFT optimisations, but is there a chemical or computational reason for this simplification? Were the calculations not converging with the full system? A justification for this should be given.\n• Why was the functional changed for the single point calculations? Is there a reason why slightly reducing the amount of incorporated Hartree-Fock makes the single point more accurate or meaningful? A justification should be given for this choice.\n• Similarly, what is the reasoning behind changing from the Ahlrich’s basis set family for optimisations to the older Pople basis sets for the single point calculations?\n• 6-311G(2d,2p) adds double polarisation to H and heavier atoms, but no diffuse functions. For systems with significant charge separation (and ionic components to the bonding) the incorporation of diffuse functions will likely be important and should be considered. I would recommend def2-TZVPPD for the single-point calculations.\nIn addition to the DFT methodology, some further DFT calculations are required to complete the analysis:\n• The NBO calculations were performed at the lower level of theory (PBE0-D3/def2-SVP), but this has implications for the orbital populations, which form the basis for the analysis of how polarised the P=Al bond is – this analysis should be done at a higher level of theory. Ideally the basis set should have additional polarisation and diffuse functions such as def2-TZVP(P)D or if the Pople basis sets are to be used, 6-311G+(2d,2p).\n• An analysis similar to Angew. Chem. Int. Ed. 2025, 64 e202506356 p. 8 could be done to check if any resonance contributes to the observed chemistry of the P=Al bond. In fact, this seems to be a clear differentiator between this reported P=Al bond and others, which all contain strictly single bonds to phosphorus. More formally, natural resonance theory calculations (within the NBO analysis) could be undertaken in an attempt to quantify the extent to which each resonance structure contributes to the stability of 3.\n• In general, the WBIs and smaller amount of charge separation suggest this P=Al bond is less polarised and more covalent than the previously isolated P=Al bond – could the potential resonance described above help explain these data as well as the observed stability of the P=Al bond?\n• If possible, some electron topological analysis should be carried out such as QTAIM analysis. This will further corroborate the π-bond character of the P=Al bond and will help gain insight into the polarisation of the bond. Polarisation arguments from NBO calculations are valid, however the orbital analysis is strictly within the NBO basis, whereas QTAIM analysis is derived directly from the wavefunction and can provide a more realistic picture of any bond polarisation.\n• It might be interesting to investigate what would happen to the stability of the P=Al bond if the nitrogen is computationally changed to a phosphorus to make a P–P=Al motif, does this alter the stability/chemistry (and might help to understand why the attempted compound to be synthesised by Goicoechea and co-workers was found to be so elusive). At a minimum, the authors should reference the preliminary calculations which looked at H P–P=AlBDIDipp vs. H N–P=AlBDIDipp, and include 2 2 this as part of their discussion.\nDFT computed mechanism for H activation:\n2\n• The computed reaction mechanism is reasonable and supports the experimentally observed product distribution between 4 and 5.\n• It would be beneficial if additional experiments are able to validate the computed mechanism. For instance, can you alter the product distribution between 4 and 5 if the reaction between 3 and H is performed at lower temperatures over a longer 2 period. Would this result in improved (or selective) yields of 4?\n• Could you comment on any reversibility between 3’ and INT1. According to the computed mechanism the reverse reaction has a lower energy barrier than the forward reaction. It is conceivable that although the reaction from 3’ to 4’ via TS1’ is irreversible, 3’ to INT1 via TS1 should be reversible. Does this also affect the product distribution of 4 and 5? Perhaps this could be probed experimentally by selective protonation or methylation of the isoindoline nitrogen, this may shut down the mechanism leading to 5.\nSingle-crystal X-ray analysis:\n• For the X-ray analysis you state that the hydrogens on phosphorus were freely refined, but I would suggest that the same refinement is carried out for the hydrogens on nitrogen and aluminium. An indication of whether the N–H and/or Al–H protons were identifiable in the electron difference map should also be stated.\n• X-ray diffraction studies provide key evidence for identifying the Al–H protons (as well as IR), and hence their refinement should be explicitly stated as this is important for determining the products from the reaction of 3 with H .\n• There are some inconsistencies in the NMR analysis which should be addressed, at least in the supplementary information. For instance, the 31P NMR of 4 resonates as a doublet-of-doublets due to 1J and 2J coupling (i.e. through PH PH one-bond and two-bond coupling via P–Al–H). On the other hand, in 5 the 31P NMR resonance is observed as a doublet due to 2J coupling (20.1 Hz) presumably due to the coupling to the Al–H as the corresponding 2J coupling of the N–H PH HP proton (observed in the 1H NMR spectrum only) is 5.3 Hz. Why is the phosphorus resonance in 5 not observed as a doublet-of-doublets? The smaller coupling (with N–H) should be observable as well?\nAdditional points:\n• The authors state the P=Al bond reported by Braunschweig and Hering-Junghans in J. Am. Chem. Soc. 2021, 143, 4106 possesses a four-coordinate aluminium, but I don’t see how this is the case. The aluminium centre in question is bonded to a Cp* (η5) ligand, phosphorus via a double bond and there is an additional short contact to the ipso carbon of the pendant Dipp group. My understanding is that this is a three-coordinate aluminium.", "reviewer_1_attached": "The manuscript by Lu and co-workers reports the synthesis of a Phosphaalumene (3), which is\nsubsequently employed in the activation of several small molecules, including H , P , and CO . The\n2 4 2\nauthors’ efforts to probe both the stability and reactivity of Phosphaalumene(3) are commendable.\nHowever, after a detailed reading, the manuscript gives the impression that the study would benefit\nfrom additional reactivity investigations to provide a more comprehensive picture of this system. At\npresent, the scope of the reactivity appears somewhat limited, and broadening it would significantly\nstrengthen the work.\n1) It would be valuable to examine the reactivity of compound (3) towards some of the following:\n• Alkenes (e.g., ethylene, 1,3-butadiene, styrene).\n• Alkynes bearing electron-donating and electron-withdrawing substituents.\n• Other homolytically cleavable bonds (N=N, Se-Se, B-B etc.)\n• Several azides are known for their reactivity with main group double bonds\nIncluding such studies would substantially improve the impact of the manuscript and provide deeper\ninsight into the versatility of this phosphaalumene system.\nResponses: Following the reviewer’s suggestion, we have investigated the reactivity of compound 3\nwith a range of reagents, including: ethylene, 1,3-butadiene, styrene, phenylacetylene, 4-\nfluorophenylacetylene, 1-ethynyl-4-methylbenzene, azobenzene, bis(catecholato)diboron (B cat ),\n2 2\ndiphenyl diselenide (PhSeSePh), and trimethylsilyl azide (TMSN ). Our findings revealed that\n3\nreactions of 3 with ethylene, 1,3-butadiene, phenylacetylene, 4-fluorophenylacetylene, azobenzene,\nand B cat in benzene under ambient conditions resulted in complex mixtures that resisted purification\n2 2\nby recrystallization. However, when 3 was reacted with styrene and 1-ethynyl-4-methylbenzene, it\nyielded [2+2] cycloaddition products 18 and 19, respectively, with a small amount of C-H addition\nproduct 20 also formed in the latter case, aligning with reports by Hering-Junghans and Braunschweig\n(Figure S125-S144). Interestingly, subjecting 3 to a reaction with TMSN unexpectedly produced a\n3\nfive-membered AlN P ring (13), indicating a reaction pathway distinct from those previously\n3\ndocumented by Hering-Junghans and Braunschweig (Figure S98-S103). Additionally, treating 3 with\ntwo equivalents of PhSeSePh successfully isolated products 14 and 15 in 43% and 25% yields,\nrespectively (Figure S104-S109).\nScheme R1. Profiles for the reactions of 3 with ethylene, 1,3-butadiene, styrene, phenylacetylene, 4-\nfluorophenylacetylene, 1-ethynyl-4-methylbenzene, azobenzene, B cat , PhSeSePh, and TMSN (Ar = 3,5-\n2 2 3\ndimethylphenyl, *indicates NMR yield).\n2) In the sentence “Braunschweig and Goicoechea aimed at developing phosphaalumene(3) species\nby integrating a two-coordinate Al(I) species”, (3) does not give a proper understanding. What does\n“phosphaalumene(3)” mean? Is it the same as mentioned in the title?\nResponses: Phosphaalumene(3) shows that its Al=P double bond contains three substituents. The\nmeaning of this word is consistent between the title and the sentence cited. These types of descriptions\nare widely used in various diborane systems. For example, diborane(4) features four substituents\nattached to the central B unit, whereas diborane(6) has six substituents. Citations for diborane(4) can\n2\nbe found in: Chem. Eur. J. 2023, 29, e202301286; Dalton Trans. 2023, 52, 3869-3876; Dalton Trans.\n2022, 51, 6197-6203; Chem. Commun. 2021, 57, 2645-2648; Angew. Chem. Int. Ed. 2020, 59, 15480-\n15486. For diborane(6), see: Inorg. Chem. 2020, 59, 1917–1927.\n3) The authors must highlight the importance of this unconventional bond in the introduction section\nin detail.\nResponses: We have included the following sentences into the main text:\n“It was demonstrated that when I reacts with unsaturated C-C bonds, it forms a multitude of unique P,\nAl-heterocycles that cannot be directly produced by other means. Furthermore, the reaction between I\nand organoazides revealed reaction patterns that significantly differed from those documented for\nalkyne and 1,3-dipolar molecules, thereby offering new perspectives on chemical reactions.”\n4) In Figure 1, the complete structure should be there for “in this work”; even for the short structure,\nit is giving us the impression that the methyl group is attached adjacent to the N-atom in the isoindoline\nmoiety, which is not correct. The Ar group attached to the N-atoms is also not specified.\nResponses: We have revised the structure and specified the Ar group accordingly.\n5) The authors must modify the sentence “The reaction of 1 with two equivalents of DippNacNacAl: in\nbenzene at 50 °C afforded a reddish-brown solution (Figure 2)”. This sentence should refer to the\nformation of desired product 3 as a reddish-brown solution.\nResponses: We have revised the sentence as follows:\n“The reaction of 1 with two equivalents of DippNacNacAl in benzene at 50 °C led to the formation of a\nreddish-brown solution of 3 (Scheme 1)”.\n6) On Page 14, lines 330–332, the authors state that the reaction of PhSiH₃ demonstrates π(Al=P)\ncharacter, while the reaction with PhNH₂ indicates FLP type reactivity. However, according to the\nliterature [Chem. Eur. J. 2024, 30, e202401326; Angew. Chem. Int. Ed. 2021, 60, 22057–22061], a\nsimilar product was observed for the reaction of amines with related systems. Importantly, these\nreports do not describe such reactivity as FLP-type. The authors are therefore requested to carefully\nre-examine this assignment and revise the statement accordingly, as the current wording could be\nmisleading.\nResponses: As documented in the literature (Chem. Eur. J. 2024, 30, e202401326), transient\nphosphaalumene II initially reacts with amines to form 1,3-addition products. Subsequently, 1,2-H\nmigration product was obtained by subjecting the 1,3-addition products to thermal treatment at 90 °C\nfor 11 days. It is noteworthy that the former exemplifies an FLP-type reaction, with the latter's creation\nbeing solely due to a 1,2-H migration originating from the former. A similar process involved the\nreaction of phosphanyl-phosphagallene (Angew. Chem. Int. Ed. 2021, 60, 22057–22061) with amines,\nwhich initially yielded 1,3-hydroelementation products through an FLP-type pathway; these products\nthen transformed into 1,2-migration compounds only after being heated at 80 °C for 48 hours. It has\nalso been concluded by the authors that the Ga/P FLP-type reactivity is able to capture and cleave the\nE−H bond in amines, phosphines and terminal alkynes. Therefore, it is evident that the reactions of II\nor phosphanyl-phosphagallene with amines proceeded through FLP-type mechanisms.\n7) Did the authors try to see if H activation is reversible? Or can it be transferred to some substrates?\n2 2\nbenzene-d . The mixture was stored at 25 °C for five days, then subjected to sequential heating at 40 °C,\n6\n60 °C, and 80 °C for two days each, with continuous monitoring by 1H NMR and 31P NMR\nspectroscopy. Nonetheless, neither H-D exchange nor any novel products were detected (Figure S37-\nS38). Hence, the H activation is concluded to be an irreversible process.\ninvestigated. When a benzene-d solution of these compounds was mixed with fluorenone at room\n6\ntemperature, the color of the solution transformed from yellow to red after two days. 1H NMP and 31P\nNMR spectroscopy confirmed that compound 4 remained unreacted, while compound 5’s\ncharacteristic signals completely disappeared (Figure S39-S40). Unfortunately, all attempts to isolate\nthis red product were unsuccessful.\nWe have incorporated the subsequent sentence into the main text.\n“To ascertain the reversibility of H activation, we reacted 4 and 5 with 1 atm of D in benzene-d .\n2 2 6\nNonetheless, neither H-D exchange nor any novel products were detected (Figure S37-S38). To assess\nthe hydrogen transfer ability of compounds 4 and 5, their reactions with ketones were investigated.\nWhen a benzene-d solution of these compounds was mixed with fluorenone at room temperature, the\n6\ncolor of the solution transformed from yellow to red after two days. 1H NMP and 31P NMR\nspectroscopy confirmed that compound 4 remained unreacted, while compound 5’s characteristic\nsignals completely disappeared (Figure S39-S40). Unfortunately, all attempts to isolate this red product\nwere unsuccessful.”\n8) The authors should include a table of Al-P bond length comparison with previous reports.\nResponses: We sincerely appreciate the reviewers’ suggestion. We have included a table in Table S3,\ndetailing bond lengths and 31P NMR chemical shifts for 3, I, and II.\n9) The 29Si NMR spectrum is missing for compound 12.\nResponses: The 29Si NMR spectrum of compound 12 was updated accordingly (it is now referred to\nas Compound 16, Figure S115).\n10) Several NMR spectra show a significant amount of impurities are present; the authors are advised\nto either mark the impurities or re-measure the NMR spectrum if possible.\nResponses: We appreciate the reviewer’s suggestion and have re-examined the NMR spectra,\nannotating every detectable impurity. We would like to point out that when NMR spectroscopy is used\nto monitor reactions, it is usually for the purpose of screening reactivity and investigating reaction\nmechanisms, hence the spectra were provided in their original state (Figure S7-S10, S29-S40).\n11) Important peaks must be marked in the FT-IR spectrum.\nResponses: All FT-IR spectra in the Supporting Information (SI) have been updated, and their\ncharacteristic peaks are now labeled.\n12) As all the compounds are coloured, the authors are advised to investigate the UV-Vis spectrum and\nadd a discussion on it.\nResponses: The UV-Vis spectrum of 3 in toluene was determined, and the accompanying sentences\nwere subsequently integrated into the manuscript.\n“The UV-vis spectrum of 3 in toluene displays an absorption maximum at 414 nm, which is assigned\nto the HOMO-1 to LUMO+1 transition. The simulated spectrum by a time-dependent density\nfunctional theory (TD-DFT) identifies three further maxima at 605, 553 and 468 nm, attributed to S1,\nS2 and S3, respectively. The latter absorptions are not clearly observed in the experiment spectrum;\ntheir low oscillator strengths (Figure S145 and Table S7) make them too faint to resolve, and they are\nlikely obscured by band broadening or solvent effects.”\nFigure R1. UV-vis spectra of 3 in toluene at room temperature.\nTable R1: Calculated UV-Vis spectra of 3 at the PBE0-D3(BJ)/Def2-TZVP//PBE0-D3(BJ)/Def2-SVP level of\ntheory.\nState Wavelength(nm) Oscillator strength (f) Main excitation\nS1 605.45 0.0072 HOMO→LUMO\nS2 553.46 0.0010 HOMO-1→LUMO\nS3 468.48 0.0180 HOMO→LUMO+1\nHOMO-1→LUMO+1\nS4 396.38 0.0688 HOMO-1→LUMO+1\n13) In the reactivity of complex 3 with CO , please provide a theoretical insight for the formation of\n2\n1,3-addition product by phoshphaalumenes, since the activation of CO by multiple bonds may lead to\n2\nthe formation of many other products (Angew. Chem. Int. Ed. 2018, 57, 14575 –14579).\nResponses: We appreciate the reviewer’s thoughtful suggestion regarding the mechanism. In response,\nwe performed theoretical calculations to investigate the reaction pathway between 3’ and CO₂. The\nresults reveal that a single CO₂ molecule undergoes a [2+2] cycloaddition with the P=Al double bond,\nleading to the formation of a four-membered ring intermediate (CO -INT). Subsequently, this\n2\nintermediate interacts with a second molecule of CO₂, which facilitates the cleavage of the P–Al bond\nand a ring expansion process, ultimately yielding the observed six-membered ring product. The\nupdated mechanistic scheme has been included in the Supporting Information as Figure S165. For\nconvenience, it is also presented here in our response (Figure R2).\nFigure R2. Free-energy profile (kcal∙mol‒1) for the proposed mechanism of CO activation by 3 calculated at the the\nMinor typos:\n14) The π sign in Figure 1 is not uniform.\nResponses: We have fixed the typos.\n15) ppm is missing in the SI for 1H and 13C NMR description, while others are fine.\nResponses: We have revised the errors.\n16) ℃ is not uniform in the description of 1H for 3 in SI.\nResponses: We have rectified the typos.\n17) δ and other Greek alphabetical symbols (e.g. π, σ, n, etc) should be in italics throughout the\nmanuscript.\nResponses: We have fixed the errors.\n18) The picture quality for 8 is not the same as the others.\nResponses: We have exchanged the spectral plots for 8 (it is now referred to as Compound 9).\n19) Page No. 12, In Fig. 6, Reactivity of complex 3 with CO , please mention the equivalency of CO .\n2 2\nResponses: An update has been made to the CO equivalency in scheme 2.\n2\n20) In the supporting information, it will be better if the authors can add the ChemDraw of the\nrespective complexes with the NMR spectrum, it will help to understand the peak positions of the nuclei.\nResponses: We are grateful for the reviewer’s recommendation. As advised, ChemDraw molecular\nstructures have been placed alongside all NMR spectral plots in the Supporting Information. The\nupdated SI provides the corrected NMR spectra.", "reviewer_author_response": "Point-by-point response to the reviewers’ comments\nFirst, we extend our sincere gratitude to all three reviewers for their time, insightful\ncomments, and helpful suggestions.\n****************\nReviewer #1:\nIt is truly appreciable that the authors made an effort to address the comments and the\ncorresponding work done. Upon examining all the comments addressed by the authors,\nit is found that the authors have revised the manuscript in accordance with the\nsuggestions/comments from the reviewers. They have attempted to address nearly all\nthe comments and improve the manuscripts. Therefore, it seems ready for publication.\nHowever, a minor correction may be required.\n(1) As it shows two types of product formation by reacting with 1-ethynyl-4-\nmethylbenzene, however, no conclusive product was isolated when reacting with\nethynylbenzene/1-ethynyl-4-fluorobenzene. There might be some role in the resulting\n[2+2] cycloaddition product when an electron-donating group is attached at the para\nposition; the authors must address this.\nResponses: Thank you for raising this insightful point regarding the substituent effect.\nAs noted in the mechanistic study for related phosphaalumene-alkyne reactions (Angew.\nChem. Int. Ed. 2023, 62, e202215838), the initial interaction involves a weak hydrogen\nbond between the phosphorus atom and the terminal C-H bond of the alkyne. This\nsuggests that the acidity of the alkyne C–H bond plays a crucial role in the initial\nengagement and likely influences the subsequent reaction pathway. In our case, when\nan electron-donating group (EDG, e.g., -CH ) is present at the para position, the\n3\nalkyne's C-H acidity is relatively lower. This favors the [2+2] cycloaddition pathway,\nallowing us to isolate the corresponding product. Conversely, electron-withdrawing\ngroups at the para position increase the C-H acidity. This enhanced acidity may\npromote a competitive, and perhaps faster, side reaction involving C–H bond insertion\nor other decomposition pathways.\nThe following sentence has been incorporated into the main text: “However,\nphenylacetylene and 1-ethynyl-4-fluorobenzene, when reacted with 3, yielded only\ncomplex mixtures, likely due to the increased acidity of their terminal alkyne C-H bond.\nThis elevated acidity promotes alternative deprotonation or C-H activation routes,\nresulting in the formation of complicated mixtures instead of the intended cycloadduct”.\n(2) The authors should discuss the problem faced in not getting conclusive results from\nactivating N-N/B-B bonds.\nResponses: We sincerely thank you for your suggestion. We have integrated the\nfollowing sentence into the main text: “Additionally, we were unable to obtain any\ndesired products when compound 3 reacted with azobenzene or bis(catecholato)diboron\n(B cat ), which might be due to uncontrolled reactions with nonpolar substrates”.\n2 2\n(3) What is “1H NMP” spectroscopy? This is mentioned in both the response to the\ncomments (reviewer 1, comment 7) and the main text. It seems like a typo, but if it isn't,\nauthors are suggested to write the full form.\nResponses: \"1H NMP\" was indeed a typo, and it has been corrected to \"1H NMR\".\n(4) Including a list of all abbreviations used in the manuscript would be greatly\nappreciated.\nResponses: We have added a list of abbreviations in the Additional Information section\nat the end of the manuscript.\n(5) As suggested by reviewer 2, comment 15. It should be a “spirocyclic structure”. The\nauthors have addressed that they have changed the “spiral structure” to spirocyclic\nstructure, but it is found that “spiralcyclic structure” is mentioned in the main text,\ninstead of spirocyclic structure. The authors are requested to specify the intention of\nwriting “spiralcyclic structure” if there is a different meaning.\nResponses: We have carefully checked the manuscript and confirmed that \"spiralcyclic\nstructure\" was a typographical error. It has now been rectified as \"spirocyclic structure\"\nthroughout the text.\n(6) 13C{1H} NMR spectrum of 8 is not clean (S66-S67). At least the aliphatic region\nmust be clean (as it is an isolated pure product). Otherwise, the authors are advised to\npaste the elongated version as well to clearly view the chemical shift values (the same\napplies to compound 9).\nResponses: As suggested by the reviewer, we have updated the 13C{1H} NMR spectra\nfor compounds 8 and 9 (Figures S66-S67 and S74-S75), and the elongated/expanded\nviews of the aliphatic regions have been included.\n7. On page 56 (Fig. S68, S69) and page 73. (Fig. S101 and S102.) The 31P NMR spectra\nof compounds 8 and 13 appear to have been repeated in both sets of figures.\nResponses: We have carefully examined Figures S68, S69, S101, and S102 and found\nno repetition. We would like to clarify that these figures represent coupled and proton-\ndecoupled 31P NMR spectra, respectively. For structures without observable P-H\ncoupling, the two spectra appear nearly identical.\nReviewer #2:\nThis revised manuscript by Lu and coworkers clearly addresses all concerns raised in\nthe initial reviewing stage and should be published in its current form. During the\nrevision the authors have added additional reactivity studies, which further strengthen\nthe work. My biggest concern was the rather supericial discussion of the impressive\nresults, but all this has been rectified, through significantly improving the strcuture of\nthe manuscript and the detail-level of the discussion. The added reactivity studies now\nalso include TMS-N , acetylene derivatives and diselenides, which overall makes this\n3\nis very exciting study. The ESI is very detailed, the experimental part should allow\nreproduction of the results and the theoretical studies have been carried out with great\ncare and the details given are up to the highest standards.\nSo again, my clear recommendation is publication of this work, which should appeal to\na general chemistry audience.\nResponses: We sincerely thank the reviewer for your initial constructive suggestions\nand for your highly positive final assessment. We are particularly grateful for your\nvaluable recommendation for publication.\nReviewer #3:\nThis revised manuscript by Lu and co-workers involves a greatly expanded discussion\nsection on the nature of the Al=P-N bonding, as well as an expanded reactivity scope.\nThese have been compared to previously reported systems also featuring a Al=P motif\nand have demonstrated the unique chemistry of this system. This expansion has\nincluded an improved computational study which involved several new calculations.\nAlthough there are some outstanding questions, such as the influence of ligand sterics\nand the role this plays in the observed reactivity, the authors acknowledge this and are\nactively pursuing this line of research. With the current changes my concerns have been\nappropriately addressed through additional data and discussion. I believe this\nmanuscript should now be accepted for publication after the following minor points are\naddressed.\n(1) There are a few typos which remain, so I would encourage the authors to carefully\nre-read everything to make sure all these final issues can be resolved. For instance, I\nhave found the following: page 3 line 65 (M = Al. Ga) should be (M = Al, Ga) with a\ncomma instead of a full stop.\nResponses: We have changed \"(M = Al. Ga)\" to", "reviewer_ga)": "nd have ensured that\nsimilar errors are absent throughout the manuscript.\n(2) page 5 figure 1 there is an additional close bracket in \"FLP(N, Al))\"\nResponses: We have revised Figure 1 as requested; the updated version is shown in\nFigure 1 of the manuscript.\n(3) page 11 figure 5 the \"2\" in [D2]-4 and [D2]-5 should be subscripted\nResponses: We have rectified these errors. The updated content is shown in Figure 3.\n(4) page 13 line 237 reads 1H NMP when it should be NMR. The authors have put a\ngreat deal of effort into revising this manuscript and this should be acknowledged,\ncongratulations.\nResponses: \"1H NMP\" was indeed a typo, and it has been rectified to \"1H NMR\"."}, "author_response": ""}, {"round": 2, "reviewer_comments": {"reviewer_1": "It is truly appreciable that the authors made an effort to address the comments and the corresponding work done. Upon examining all the comments addressed by the authors, it is found that the authors have revised the manuscript in accordance with the suggestions/comments from the reviewers. They have attempted to address nearly all the comments and improve the manuscripts. Therefore, it seems ready for publication. However, a minor correction may be required.\n1. As it shows two types of product formation by reacting with 1-ethynyl-4-methylbenzene, however, no conclusive product was isolated when reacting with ethynylbenzene/1-ethynyl-4-fluorobenzene. There might be some role in the resulting [2+2] cycloaddition product when an electron-donating group is attached at the para position; the authors must address this.\n2. The authors should discuss the problem faced in not getting conclusive results from activating N-N/B-B bonds.\n3. What is “1H NMP” spectroscopy? This is mentioned in both the response to the comments (reviewer 1, comment 7) and the main text. It seems like a typo, but if it isn't, authors are suggested to write the full form.\n4. Including a list of all abbreviations used in the manuscript would be greatly appreciated.\n5. As suggested by reviewer 2, comment 15. It should be a “spirocyclic structure”. The authors have addressed that they have changed the “spiral structure” to spirocyclic structure, but it is found that “spiralcyclic structure” is mentioned in the main text, instead of spirocyclic structure. The authors are requested to specify the intention of writing “spiralcyclic structure” if there is a different meaning.\n6. 13C{1H} NMR spectrum of 8 is not clean (S66-S67). At least the aliphatic region must be clean (as it is an isolated pure product). Otherwise, the authors are advised to paste the elongated version as well to clearly view the chemical shift values (the same applies to compound 9).\n7. On page 56 (Fig. S68, S69) and page 73. (Fig. S101 and S102.) The 31P NMR spectra of compounds 8 and 13 appear to have been repeated in both sets of figures.", "reviewer_2": "This revised manuscript by Lu and coworkers clearly addresses all concerns raised in the initial reviewing stage and should be published in its current form. During the revision the authors have added additional reactivity studies, which further strengthen the work. My biggest concern was the rather supericial discussion of the impressive results, but all this has been rectified, through significantly improving the strcuture of the manuscript and the detail-level of the discussion. The added reactivity studies now also include TMS-N3, acetylene derivatives and diselenides, which overall makes this is very exciting study. The ESI is very detailed, the experimental part should a allow reproduction of the results and the theoretical studies have been carried out with great care and the details given are up to the highest standards.\nSo again, my clear recommendation is publication of this work, which should appeal to a general chemistry audience.", "reviewer_3": "This revised manuscript by Lu and co-workers involves a greatly expanded discussion section on the nature of the Al=P-N bonding, as well as an expanded reactivity scope. These have been compared to previously reported systems also featuring a Al=P motif and have demonstrated the unique chemistry of this system. This expansion has included an improved computational study which involved several new calculations. Although there are some outstanding questions, such as the influence of ligand sterics and the role this plays in the observed reactivity, the authors acknowledge this and are actively pursuing this line of research. With the current changes my concerns have been appropriately addressed through additional data and discussion. I believe this manuscript should now be accepted for publication after the following minor points are addressed.\nThere are a few typos which remain, so I would encourage the authors to carefully re-read everything to make sure all these final issues can be resolved. For instance, I have found the following:\n- page 3 line 65 (M = Al. Ga) should be (M = Al, Ga) with a comma instead of a full stop\n– page 5 figure 1 there is an additional close bracket in \"FLP(N, Al))\"\n– page 11 figure 5 the \"2\" in [D2]-4 and [D2]-5 should be subscripted\n– page 13 line 237 reads 1H NMP when it should be NMR.\nThe authors have put a great deal of effort into revising this manuscript and this should be acknowledged, congratulations."}, "author_response": "Point-by-point response to the reviewers’ comments\nFirst, we extend our sincere gratitude to all three reviewers for their time, insightful\ncomments, and helpful suggestions.\n****************\nReviewer #1:\nIt is truly appreciable that the authors made an effort to address the comments and the\ncorresponding work done. Upon examining all the comments addressed by the authors,\nit is found that the authors have revised the manuscript in accordance with the\nsuggestions/comments from the reviewers. They have attempted to address nearly all\nthe comments and improve the manuscripts. Therefore, it seems ready for publication.\nHowever, a minor correction may be required.\n(1) As it shows two types of product formation by reacting with 1-ethynyl-4-\nmethylbenzene, however, no conclusive product was isolated when reacting with\nethynylbenzene/1-ethynyl-4-fluorobenzene. There might be some role in the resulting\n[2+2] cycloaddition product when an electron-donating group is attached at the para\nposition; the authors must address this.\nResponses: Thank you for raising this insightful point regarding the substituent effect.\nAs noted in the mechanistic study for related phosphaalumene-alkyne reactions (Angew.\nChem. Int. Ed. 2023, 62, e202215838), the initial interaction involves a weak hydrogen\nbond between the phosphorus atom and the terminal C-H bond of the alkyne. This\nsuggests that the acidity of the alkyne C–H bond plays a crucial role in the initial\nengagement and likely influences the subsequent reaction pathway. In our case, when\nan electron-donating group (EDG, e.g., -CH ) is present at the para position, the\n3\nalkyne's C-H acidity is relatively lower. This favors the [2+2] cycloaddition pathway,\nallowing us to isolate the corresponding product. Conversely, electron-withdrawing\ngroups at the para position increase the C-H acidity. This enhanced acidity may\npromote a competitive, and perhaps faster, side reaction involving C–H bond insertion\nor other decomposition pathways.\nThe following sentence has been incorporated into the main text: “However,\nphenylacetylene and 1-ethynyl-4-fluorobenzene, when reacted with 3, yielded only\ncomplex mixtures, likely due to the increased acidity of their terminal alkyne C-H bond.\nThis elevated acidity promotes alternative deprotonation or C-H activation routes,\nresulting in the formation of complicated mixtures instead of the intended cycloadduct”.\n(2) The authors should discuss the problem faced in not getting conclusive results from\nactivating N-N/B-B bonds.\nResponses: We sincerely thank you for your suggestion. We have integrated the\nfollowing sentence into the main text: “Additionally, we were unable to obtain any\ndesired products when compound 3 reacted with azobenzene or bis(catecholato)diboron\n(B cat ), which might be due to uncontrolled reactions with nonpolar substrates”.\n2 2\n(3) What is “1H NMP” spectroscopy? This is mentioned in both the response to the\ncomments (reviewer 1, comment 7) and the main text. It seems like a typo, but if it isn't,\nauthors are suggested to write the full form.\nResponses: \"1H NMP\" was indeed a typo, and it has been corrected to \"1H NMR\".\n(4) Including a list of all abbreviations used in the manuscript would be greatly\nappreciated.\nResponses: We have added a list of abbreviations in the Additional Information section\nat the end of the manuscript.\n(5) As suggested by reviewer 2, comment 15. It should be a “spirocyclic structure”. The\nauthors have addressed that they have changed the “spiral structure” to spirocyclic\nstructure, but it is found that “spiralcyclic structure” is mentioned in the main text,\ninstead of spirocyclic structure. The authors are requested to specify the intention of\nwriting “spiralcyclic structure” if there is a different meaning.\nResponses: We have carefully checked the manuscript and confirmed that \"spiralcyclic\nstructure\" was a typographical error. It has now been rectified as \"spirocyclic structure\"\nthroughout the text.\n(6) 13C{1H} NMR spectrum of 8 is not clean (S66-S67). At least the aliphatic region\nmust be clean (as it is an isolated pure product). Otherwise, the authors are advised to\npaste the elongated version as well to clearly view the chemical shift values (the same\napplies to compound 9).\nResponses: As suggested by the reviewer, we have updated the 13C{1H} NMR spectra\nfor compounds 8 and 9 (Figures S66-S67 and S74-S75), and the elongated/expanded\nviews of the aliphatic regions have been included.\n7. On page 56 (Fig. S68, S69) and page 73. (Fig. S101 and S102.) The 31P NMR spectra\nof compounds 8 and 13 appear to have been repeated in both sets of figures.\nResponses: We have carefully examined Figures S68, S69, S101, and S102 and found\nno repetition. We would like to clarify that these figures represent coupled and proton-\ndecoupled 31P NMR spectra, respectively. For structures without observable P-H\ncoupling, the two spectra appear nearly identical.\nReviewer #2:\nThis revised manuscript by Lu and coworkers clearly addresses all concerns raised in\nthe initial reviewing stage and should be published in its current form. During the\nrevision the authors have added additional reactivity studies, which further strengthen\nthe work. My biggest concern was the rather supericial discussion of the impressive\nresults, but all this has been rectified, through significantly improving the strcuture of\nthe manuscript and the detail-level of the discussion. The added reactivity studies now\nalso include TMS-N , acetylene derivatives and diselenides, which overall makes this\n3\nis very exciting study. The ESI is very detailed, the experimental part should allow\nreproduction of the results and the theoretical studies have been carried out with great\ncare and the details given are up to the highest standards.\nSo again, my clear recommendation is publication of this work, which should appeal to\na general chemistry audience.\nResponses: We sincerely thank the reviewer for your initial constructive suggestions\nand for your highly positive final assessment. We are particularly grateful for your\nvaluable recommendation for publication.\nReviewer #3:\nThis revised manuscript by Lu and co-workers involves a greatly expanded discussion\nsection on the nature of the Al=P-N bonding, as well as an expanded reactivity scope.\nThese have been compared to previously reported systems also featuring a Al=P motif\nand have demonstrated the unique chemistry of this system. This expansion has\nincluded an improved computational study which involved several new calculations.\nAlthough there are some outstanding questions, such as the influence of ligand sterics\nand the role this plays in the observed reactivity, the authors acknowledge this and are\nactively pursuing this line of research. With the current changes my concerns have been\nappropriately addressed through additional data and discussion. I believe this\nmanuscript should now be accepted for publication after the following minor points are\naddressed.\n(1) There are a few typos which remain, so I would encourage the authors to carefully\nre-read everything to make sure all these final issues can be resolved. For instance, I\nhave found the following: page 3 line 65 (M = Al. Ga) should be (M = Al, Ga) with a\ncomma instead of a full stop.\nResponses: We have changed \"(M = Al. Ga)\" to"}]

Decision letter