# Toward Transition-Metal-Templated Construction of Arylated B-4 Chains by Dihydroborane Dehydrocoupling

Carsten Lenczyk, Dipak Kumar Roy, Kai Oberdorf, Joern Nitsch, Rian D.

Dewhurst, Krzysztof Radacki, Jean-François Halet, Todd B. Marder, Matthias

Bickelhaupt, Holger Braunschweig

► To cite this version:

Carsten Lenczyk, Dipak Kumar Roy, Kai Oberdorf, Joern Nitsch, Rian D. Dewhurst, et al., Toward Transition-Metal-Templated Construction of Arylated B-4 Chains by Dihydroborane Dehydrocoupling. Chemistry - A European Journal, 2019, 25 (72), pp.16544-16549. 10.1002/chem.201904772. hal-02438529

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Main Group Chemistry

# Toward Transition-Metal-Templated Construction of Arylated B₄ Chains by Dihydroborane Dehydrocoupling

Carsten Lenczyk,[a] Dipak Kumar Roy,[a, b] Kai Oberdorf,[a] Jörn Nitsch,[a] Rian D. Dewhurst,[a] Krzysztof Radacki,[a] Jean-François Halet,[*c] Todd B. Marder,[*a] Matthias Bickelhaupt,[*d] and Holger Braunschweig*[a]

**Abstract:** The reactivity of a diruthenium tetrahydride complex towards three selected dihydroboranes was investigated. The use of [DurBH₂] (Dur = 2,3,5,6-Me₄C₆H) and [(Me₃Si)₂NBH₂] led to the formation of bridging borylene complexes of the form [[Cp*RuH)₂BR] (Cp* = C₅Me₅; **1 a:** R = Dur; **1 b:** R = N(SiMe₃)₂ through oxidative addition of the B–H bonds with concomitant hydrogen liberation. Employing the more electron-deficient dihydroborane [3,5-(CF₃)₂-C₆H₃BH₂] led to the formation of an anionic complex bearing a tetraarylated chain of four boron atoms, namely Li(THF)₄[(Cp*Ru)₂B₄H₅(3,5-(CF₃)₂C₆H₃)₄] (4), through an unusual, incomplete threefold dehydrocoupling process. A comparative theoretical investigation of the bonding in a simplified model of **4** and the analogous complex *nido*-[1,2(Cp*Ru)₂(µ-H)B₄H₉] (I) indicates that there appear to be no classical σ-bonds between the boron atoms in complex I, whereas in the case of **4** the B₄ chain better resembles a network of three B–B σ bonds, the central bond being significantly weaker than the other two.

Diborane(4) compounds are highly useful synthetic modules for organic synthesis,[1] in particular in catalytic diboration[2] and other borylation reactions.[1,3] As such, diborane(4) compounds represent the simplest—and by far the best-known—examples of boron-chain molecules. Despite their ubiquitous use throughout organic chemistry, only a handful of diborane compounds are commercially available.[1,4] Longer chains of

boron molecules are practically non-existent. This phenomenon was elegantly summarized by Boldyrev in a 2012 computational study of BₙHₙ+₂ molecules, which showed that, in the absence of electronic or steric perturbations, the stability of linear chains—that is, those with sp²-hybridized boron atoms and electron-precise B–B bonds—quickly diminishes relative to cluster structures as *n* increases.[5] For some time, our groups have been involved in the search for techniques to form electron-precise B–B bonds selectively, with the ultimate goal of preparing long chains of hypovalent boron atoms. This challenge has resulted in the development of new metal-free and metal-mediated B–B bond formation processes, although the synthesis of boron chains remains difficult.[4, 6, 7]

One promising strategy for the preparation of linear boron chains is their construction on a metal template, followed by demetallation. The construction of linear boron chains as part of multinuclear transition metal clusters, although rare in comparison with more complex clusters of boron atoms, is known in the literature. Typically, these reactions involve the combination and thermolysis of a metal-halide source, a borohydride salt and/or a BH₃ adduct, with some reactions also involving a second organometallic fragment.[8-10] This technique, pioneered by the group of Fehlner and since built upon significantly by Ghosh and co-workers, generally provides products in which the boron atoms are bound exclusively to hydride and metal groups. Examples of clusters containing functionalized B₄ chains exist in the literature, namely [(Cp*Mo)₂B₂H₅(BER)₂(µ-η¹-ER)] (E = S, R = 2,6-(tBu)₂-C₆H₂OH; E = Se, R = Ph; Cp* = η⁵-Me₂C₅) and *nido*-[(Cp*Ru)₂B₄H₉(SePh)], reported by Ghosh and co-workers, resulting from the application of dichalcogenides

<figure><img src="image_2.png" /></figure>

Supporting information and the ORCID identification number(s) for the

<figure><img src="image_3.png" /></figure>

author(s) of this article can be found under:

https://doi.org/10.1002/chem.201904772.

<figure><img src="image_4.png" /></figure>

© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons At-
tribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.

<footnote>[a] Dr. C. Lenczyk, Dr. D. K. Roy, K. Oberdorf, Dr. J. Nitsch, Dr. R. D. Dewhurst, Dr. K. Radacki, Prof. Dr. T. B. Marder, Prof. Dr. H. Braunschweig
Institute for Inorganic Chemistry, Julius-Maximilians-Universität Würzburg
Am Hubland, 97074 Würzburg (Germany), and Institute for
Sustainable Chemistry & Catalysis with Boron
Julius-Maximilians-Universität Würzburg, Am Hubland
97074 Würzburg (Germany)
E-mail: todd.marder@uni-wuerzburg.de
h.braunschweig@uni-wuerzburg.de</footnote>

<footnote>[b] Dr. D. K. Roy
Discipline of Chemistry, Indian Institute of Technology Indore
Khandwa Road, Simrol, Indore 453552, M.P. (India)</footnote>

<footnote>[c] Prof. Dr. J.-F. Halet
Univ Rennes CNRS, Institut des Sciences Chimiques de Rennes
UMR 6226, 35000 Rennes (France)
E-mail: halet@univ-rennes1.fr</footnote>

<footnote>[d] Prof. Dr. M. Bickelhaupt
Department of Theoretical Chemistry and Amsterdam Center for
Multiscale Modeling (ACMM), Vrije Universiteit Amsterdam
De Boelelaan 1083, 1081 HV Amsterdam (The Netherlands)
and Institute for Molecules and Materials (IMM), Radboud University
Heyendaalseweg 135, 6525 AJ Nijmegen (The Netherlands)
E-mail: f.m.bickelhaupt@vu.nl</footnote>
or the phenylselenolate-functionalized salt Li[H₃B(SePh)] in place of a borohydride salt.[11] However, despite the presence of a boron-bound SePh group in the precursor, only one of the boron atoms in the final product was ultimately found to bear this group in the latter case.

Almost all longer linear chains of boron atoms are metal-free and include π-donor groups (such as dialkylamino) bound to boron; indeed their presence is likely useful for their formation.[12] However, the reliance on such groups is expected to reduce the π-acceptor character of the boron atoms drastically and thus dampen their potentially interesting reactivity and electronic properties. In our attempts to find synthetic strategies to construct chains of boron atoms, we have thus mainly focused on the use of non-π-donor substituents at boron.[13]

This work describes our efforts to apply the known metal-
mediated hydroborane dehydrocoupling process[6] to the con-
struction of chains of hypovalent boron atoms bearing sub-
stituents other than hydrogen. We establish herein the excep-
tional ability of the diruthenium tetrahydrido complex
[(Cp*Ru)₂(μ-H)₄] to mediate the dehydrogenation of functional-
ized dihydroboranes, leading either to bridging borylene com-
plexes or the synthesis of complexes bearing B–B-bonded li-
gands. The latter includes the synthesis of a complex bearing a
unit containing four connected boron atoms, representing the
first example of a perarylated boron chain.

The high electron density of metal hydride clusters makes them potentially very active towards oxidative addition of substrates.[14] We thus reasoned that the polyhydride complex [(Cp*Ru)₂(μ-H)₄] may favor the activation of functionalized dihydroboranes to produce borylene complexes, in contrast to the mononuclear polyhydride ruthenium complexes [(R₃P)₂RuH₂(H₂)₂], which, in our hands, have proven reluctant to dehydrogenate dihydroboranes fully, instead providing bis(σ)-boranes.[15]

Addition of slight excesses of dihydroboranes (DurBH₂ and (Me₃Si)₂NBH₂) to THF solutions of [(Cp*Ru)₂(μ-H)₄], and heating of the reaction mixtures at 60 °C, led to color changes of the solutions to yellow, and subsequent isolation of the bridging borylene complexes [(Cp*Ru)₂(μ-H)₂(μ-BR)] (1a: R=Dur, 11% yield; 1b: R=N(SiMe₃)₂, 10% yield, Scheme 1). These complexes were identified by NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction analyses. Compounds 1a and 1b show single broad ¹¹B NMR signals at δ=127.4 and 91.5 ppm, respectively, both far downfield from that of the

(DurBH₂)₂ dimer in benzene (δ = 22.4 ppm) and (Me₃Si)₂NBH₂
(δ = 46.3 ppm), and consistent with the signals of other bridg-
ing borylene complexes.[¹⁶a,b] Broad singlets were observed
(1a: -12.28; 1b: -13.17 ppm) for the bridging Ru–H–Ru hy-
drides (integral approx. 2H relative to the Cp* ligand) in the
¹H NMR spectra of 1a and 1b. Crystallization at room tempera-
ture allowed single-crystal X-ray diffraction studies of the com-
plexes, confirming the presence of a borylene (BR) unit bridg-
ing the [(Cp*Ru)₂(μ-H)₂] fragments (Figure 1). Interestingly, the

short Ru–Ru distance (2.463(1) Å) observed in [(Cp*Ru)₂(μ-H)₄] is further shortened in the borylene complexes **1a** (2.4220(5)) and **1b** (2.434(1) Å). The Ru–B bond lengths (**1a**: 2.049(4), 2.047(4); **1b**: 2.110(7), 2.095(7) Å) are in the range typical of known transition-metal-bridging borylene complexes of di- and trinuclear carbonyl-bridged[¹⁶c] or trinuclear hydride-bridged[¹⁶d] complexes.

Given the isolation of borylene complexes **1a** and **1b**, we sought to determine the intermediate(s) formed during the course of the reaction. Recording ¹¹B NMR spectra directly after the addition of duryldihydroborane to [(Cp*Ru)₂(μ-H)₄], which is accompanied by immediate gas evolution, indicated the formation of two new compounds (¹¹B NMR signals at δ = 62 and 25 ppm) which were later crystallographically identified as hydroborate complexes [(Cp*Ru)₂(μ-H)(μ-κ³-H,H,H-H₃BDur)] (**2a**) and [Cp*Ru{κ³-H,H,H-(H₃BDur)}] (**3a**). Further heating and evaporation of the initial mixture under vacuum resulted in the formation of the bridging borylene complex **1a**. A small sample of red crystals of **2a** were isolated, and their solid-state structure was determined to consist of a DurBH₃ ligand bridging two Ru centers, with an additional hydride bridging the two Ru centers (Figure 2), structurally analogous to the reported dirhodium complex [((DiPPE)Rh)₂(μ-H){μ-η²-H₂BH(CMe₂iPr)}] (DiPPE = 1,2-bis(diisopropylphosphino)ethane).[17] The Ru–Ru distance (2.7307(13) Å) in **2a** is significantly longer than that in **1a** (2.4220(5) Å) and [(Cp*Ru)₂(μ-H)₄] (2.463(1) Å). The distance in the former corresponds to that generally

<figure><img src="image_5.png" /><figcaption>**Figure 1.** Crystallographically derived structures of bridging borylene complexes **1a** (left) and **1b** (right). Ellipsoids shown at the 50% probability level. All hydrogen atoms bound to carbon atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°) for **1a**: Ru1–B 2.049(4), Ru2–B 2.047(4), Ru1–Ru2 2.4220(5), B–C1 1.538(5); Ru1–B-Ru2 53.8(1). For **1b**: Ru1–B 2.110(7), Ru2–B 2.095(7), Ru1–Ru2 2.434(1), B–N 1.421(8); Ru1–B-Ru2 54.3(2).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Scheme 1.** Double B–H activation of a dihydroborane by a dinuclear ruthenium complex, leading to bridging borylene complexes **1a,b**.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 2.** Crystallographically derived structures of bridging trihydroborate complexes **2a** (left) and **3a** (right). Ellipsoids shown at the 50% probability level. All hydrogen atoms bound to carbon atoms have been omitted for clarity. Selected bond lengths (Å) for **2a**: Ru1–B 2.254(4), Ru2–B 2.184(4), Ru1–Ru2 2.7307(13), Ru1–H1 1.72(4), Ru2–H2 1.66(4), B–H1 1.26(4), B–H2 1.27(4), B–H3 1.45(4). For **3a**: Ru–B 1.948(4), Ru–H1 1.71(4), Ru–H2 1.79(4), Ru–H3 1.81(4), B–H1 1.29(3), B–H2 1.26(4), B–H3 1.30(4).</figcaption></figure>

observed in complexes with single bonds between ruthenium centers. The Ru–B distances of **2a** are slightly longer than those of **1a**.

Interestingly, in addition to **2a**, yellow crystals of **3a** (see Scheme S22, Supporting Information) were also isolated from the slow evaporation of the pentane extract of the above-mentioned reaction. The solid-state structure of **3a** (Figure 2) similarly exhibits a DurBH₃ ligand, which in this case is bound through all three hydrogens to a single Ru center in a κ³ fashion. The Ru–hydrogen distances are similar to those of reported κ³-bound σ-borate complexes of ruthenium.[18] Although we were able to isolate **2a** and **3a** in the case of duryldihydroborane, we were unable to isolate a complex analogous to **2a** from the reaction with [(Me₃Si)₂NBH₂] (see the Supporting Information for **3b**, a complex analogous to **3a**).

Promisingly, these reactions demonstrate that the use of bulky and/or electron-rich dihydroboranes can lead to full dehydrogenation of dihydroboranes and borylene formation. However, no signs of B–B bond formation were observed. In order to perturb the system further, we turned to an aryldihydroborane bearing an electron-poor aryl group, namely [3,5-(CF₃)₂C₆H₃BH₂]. Although this unstable borane requires in situ generation and use, its combination with [[Cp*Ru*]₂(μ-H)₄] provided a 15% yield of orange crystals that were determined to consist of an anionic diruthenium complex containing a tetraarylated U-shaped B₄ unit, namely Li(THF)₄[(Cp*Ru*)(μ-B₄H₅){3,5-(CF₃)₂C₆H₃}₄] (4), a result of incomplete threefold dehydrocoupling (Scheme 2). The ¹¹B NMR spectrum of 4 showed two broad signals at δ = −31.1 and 28.7 ppm, and three signals were observed in the upfield region of its ¹H NMR spectrum (δ = −9.64, −8.31, and −6.69) in a 2:2:1 intensity ratio. The solid-state structure of 4, as shown in Figure 3, revealed a tetraarylated B₄ unit stabilized by two ruthenium centers. The B1–B2 (1.709(5) Å) and B3–B4

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 3.** Crystallographically derived structures of **4** (top) and **5** (bottom). Ellipsoids shown at the 50% probability level. The [Li(THF)₄]⁺ counterion of **4**, and all hydrogen atoms bound to carbon have been omitted for clarity. Selected bond lengths (Å) and angles (°) for **4**: Ru1–B1 2.360(4), Ru1–B2 2.394(4), Ru2–B3 2.375(4), Ru2–B4 2.365(4), B1–B2 1.709(5), B2–B3 1.798(6), B3–B4 1.698(5); B1–B2–B3 112.1(3), B2–B3–B4 110.6(3). For **5**: Ru1–B1 2.148(4), Ru1–B2 2.175(4), Ru2–B3 2.171(4), Ru2–B4 2.135(4), B1–B2 1.731(5), B3–B4 1.723(6).</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Scheme 2.** In situ generation and threefold dehydrocoupling of a dihydroborane.</figcaption></figure>
(1.698(5) Å) distances are significantly shorter than the B2–B3
distance (1.798(6) Å). The Ru–B distances are similar to those
of reported ruthenaborane clusters[8c,19] and are in the typical
range of single bonds, whereas the two ruthenium centers are
too distant from each other (3.676 Å) for there to be any inter-
action. The core formula of **4** is analogous to Fehlner’s *nido*-
[1,2(Cp\*Ru)₂(μ-H)B₄H₉] (I),[8c] and its derivative *nido*-
[1,2(Cp\*Ru)₂(μ-H)B₄H₇Cl₂],[8e] which adopt a *nido* pentagonal bi-
pyramidal geometry with one metal atom occupying one of
the two axial vertices and the second metal atom and boron
atoms the five equatorial vertices. This contrasts with **4**, the
core of which has a *nido* geometry based on a pentagonal bi-
pyramidal deltathedron with metal atoms occupying the axial
vertices and boron atoms four of the five equatorial vertices.
Note that I and **4** are isoelectronic (48 cluster valence electrons
(cve)) and adopt different isomeric geometries with a metal–
metal bond in the former and no metal–metal bond in the
latter. It is also worth mentioning that compound **4** strongly
resembles Fehlner’s 48-cve diruthenacarborane *nido*-[1,7-
(Cp\*Ru)₂-4,5-Me₂-4,5-C₂B₂H₆].[20]

Interestingly, in addition to **4**, a small amount of yellow crys-
tals of a minor product, **5** (see Scheme 2), were also isolated
from the reaction mixture. A single-crystal X-ray diffraction
study revealed that in **5** two diborane(4) units are stabilized by
two ruthenium centers, best represented by the formula
[(Cp\*Ru)₂(μ-η²:η²-B₂H₅{3,5-(CF₃)₂C₆H₃}₂)₂] (Figure 3). The two B–B
units are parallel to each other and perpendicular to the Ru–
Ru axis. The B–B bond distances in **5** (1.731(5) and 1.723(6) Å)
match those of complexes in which a metal-ligand fragment
binds to the two H atoms of a planar H–B(sp³)–B(sp³)–H unit,
namely those of Kodama and Shimoi[21] and Himmel.[22]

Density functional theory (DFT) calculations were performed using ADF[23] at the ZORA-BLYP-D3(BJ)/TZ2P level of theory on the model compound [(Cp\*Ru)₂B₄H₉]⁻ (**4-H**), in which the four 3,5-(CF₃)₂C₆H₃ units of **4** were replaced with four hydrogen atoms (see the Supporting Information, Figure S23), in order to

compare its bonding properties with those of the previously
reported related (non-arylated) compounds [(Cp\*₂Ru₂)B₄H₁₀] (I)
(48 cves),[8c] [(Cp\*₂Cr₂)B₄H₈] (II) (42 cves),[8d] and [(Cp\*₂Re₂)B₄H₈]
(III) (44 cves), which were also calculated (Figure 4).[8e]

The calculated B–B distances of **4-H** and **I** show excellent agreement with the distances obtained experimentally for **4** and **I**, respectively (Table S1, Supporting Information). A HOMO–LUMO gap of 1.58 eV was computed for **4-H**, indicating that **4** possesses good thermodynamic stability. Interestingly, **4-H** can be considered to contain two B₂H₄²⁻ (=B₄H₈⁴⁻ in total) moieties that are weakly bound to each other, interacting with two [Cp\*Ru]⁺ fragments and a proton. B–B σ-bonds are present in each B₂H₄²⁻ unit. Indeed, quantitative Kohn–Sham MO analyses reveal that the HOMO and HOMO–1 calculated for **4-H** comprise σ-bonding interactions between the two outer B–B atoms in the B₄H₈⁴⁻ unit (i.e. the B1–B2 and B3–B4 bonds), although their contribution to the total MO is relatively small (19 and 17%, respectively). The largest contribution to these orbitals is formed by d-orbitals of the Ru atoms, which interact in an antibonding fashion with the filled B–B σ-bonds (Figure S26, Supporting Information). In agreement with the presence of B–B σ-bonds, the Mayer bond orders (MBOs) of the outer (B1–B2 and B3–B4) bonds are close to unity (0.87, 0.88), whereas the internal B–B bond (B2–B3) has an MBO of 0.43. This agrees well with the presence of two B₂H₄²⁻ moieties that are connected by a weaker B–B bond. (Figures S25 and S26, Supporting Information). A closer inspection of the frontier orbitals reveals metal-to-ligand backbonding (HOMO–5), resulting from donation of electron density from filled metal orbitals to π\*(BB) orbitals. The acceptor orbital is formed by the antibonding combination of two p orbitals of B2 and B3 (Figure 4), thereby weakening the interaction between the two B₂H₄²⁻ moieties. The resulting B₄H₈⁴⁻ unit interacts with the two [Cp\*Ru]⁺ fragments, the single Ru-bound hydrogen of which bridges the internal BB bond.

<figure><img src="image_11.png" /><figcaption>**Figure 4.** Distances (black type) and MBOs (blue type) of **4-H** (top left), **I-III** (bottom), and important molecular orbitals of **4-H** (top right, schematic, only the B₄H₉³⁻ subunit is shown).</figcaption></figure>
The Mayer bond orders and orbitals of the model complex 4-H were additionally compared with those arising from a single-point calculation on complex 4. The results are displayed in Figure S25 (Supporting Information) and indicate that replacing the four 3,5-(CF₃)₂C₆H₃ units of 4 with four hydrogen atoms in 4-H has almost no effect on the bonding situation of the subunit. In marked contrast, there are no σ-bonds between boron atoms in the complexes I and II. Instead, for complex I, HOMO through HOMO-3 are solely formed by non-bonding metal d-orbitals. This agrees well with the MBOs of the B-B bonds, which are between 0.34-0.36 (Figure S25, Supporting Information). Interestingly, in the Group 6 complex [(Cp\*₂Cr₂)B₄H₈] (II), the LUMO and LUMO+1 correspond to the HOMO and HOMO-1 of complex 4-H, that is, they comprise bonding σ interactions between the boron atoms. Consequently, the MBOs are smaller (0.58) and the B-B distances are longer (1.803 Å) than those of the outer B-B bonds in 4-H. Similarly, the LUMO and HOMO-1 of Group 7 metal complex III [([(Cp\*₂Re₂)B₄H₈]] correspond to the HOMO and HOMO-1 of complex 4-H. This is accompanied by a shortening of the terminal B-B bonds from 1.803 (II) to 1.778 Å (III), whereas the MBOs do not change significantly (0.53). Therefore, only in [(Cp\*Ru₂)₂B₄H₉]⁻ (4-H) are both outer B-B bonds fully formed, giving high B-B bond orders and suggesting the presence of σ-bonds in each B₂H₄²⁻ unit.

We have herein demonstrated the exceptional ability of the diruthenium tetrahydrido complex [(Cp\*Ru)₂(µ-H)₄] to mediate the dehydrogenation of functionalized dihydroboranes, leading to bridging borylene complexes in the case of bulky and/or electron-rich dihydroboranes. However, with the electron-poor aryldihydroborane [3,5-(CF₃)₂C₆H₃BH₂], we isolated a complex bearing a unit comprising four connected boron atoms, representing the first example of a perarylated boron chain, albeit one with a significantly weaker central B-B bond. Computational results suggest that the bonding in the B₄ unit of this complex more closely resembles a network of classical σ-bonds than previously reported clusters with B₄ networks, thus making it the closest we have come to the construction of a B₄ chain through B-B dehydrocoupling.

## Acknowledgements

Financial support from the Deutsche Forschungsgemeinschaft, the Julius-Maximilians-Universität Würzburg and the Netherlands Organisation for Scientific Research (NWO) is gratefully acknowledged. D.K.R. is grateful for a postdoctoral fellowship from the Science and Engineering Research Board (Award no. SB/OS/PDF-052/2016-17) of India.

## Conflict of interest

The authors declare no conflict of interest.

**Keywords:** B-H activation • boron • dehydrocoupling • ruthenium • transition metal

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Manuscript received: October 18, 2019

Accepted manuscript online: October 30, 2019

Version of record online: December 9, 2019