Investigating di-n-butylcalcium

Gabriel Duneş, Matthieu Hédouin, Cléo Bastien, Samia Kahlal, Marie Cordier, William Erb, Florence Mongin, Jean-yves Saillard, Hassan Oulyadi, Yann Sarazin

► To cite this version:

Gabriel Duneş, Matthieu Hédouin, Cléo Bastien, Samia Kahlal, Marie Cordier, et al. Investigating di-n-butylcalcium. ChemistryEurope, 2025, 10.1002/ceur.202500196. hal-05210274

HAL Id: hal-05210274

https://hal.science/hal-05210274v1

Submitted on 14 Aug 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Investigating di-*n*-butylcalcium

Gabriel Duneş,[a]† Matthieu Hédouin,[b]† Cléo Bastien,[a] Samia Kahlal,[a] Marie Cordier,[a] William Erb,[a] Florence Mongin,[a] Jean-Yves Saillard,[a] Hassan Oulyadi,[b] and Yann Sarazin*[a]

[a] Univ Rennes, CNRS, ISCR-UMR 6226, 35000 Rennes, France.

[b] Institut CARMEN UMR 6064, INC3M FR 3038, Univ Rouen Normandie, INSA Rouen Normandie, CNRS, Normandie Univ, 76821 Mont Saint Aignan, France.

* Corresponding author. E-mail address:
yann.sarazin@univ-rennes.fr
hassan.oulyadi@univ-rouen.fr

† These two co-authors have contributed equally to the work, and must both be considered as first co-authors.

**Keywords:** calcium – DFT calculations – lithium – main-group reagents – NMR spectroscopy – organometallic chemistry

**Electronic Supporting Information (ESI) available:**

Experimental details, spectroscopic characterizations, crystallographic and computational data. CCDC 2447662-2447664 contain the supplementary crystallographic data for this paper.
**Abstract**

The synthesis and detailed characterization by NMR spectroscopy of di-*n*-butylcalcium are reported. The reaction of [Ca{N(SiMe₃)₂}₂.(thf)₂] with equimolar "BuLi in thf at -83 °C generates [Ca("Bu){N(SiMe₃)₂}.(thf)₃] (6), a complex which evolves towards [Ca{N(SiMe₃)(SiMe₂-μ-CH₂)}.(thf)₃]₂ (7) upon warming to -35 °C. With two equivalents of the lithium reagent vs. [Ca{N(SiMe₃)₂}₂.(thf)₂], NMR investigations attest to the formation of [Ca("Bu)₂.(thf)₄] (9). Both 6 and 9 are stable in solution at low temperatures. DFT calculations corroborate the stability of the two compounds. In petroleum ether, the reaction of the solvent-free [Ca{N(SiMe₃)₂}₂]₂ with 1, 1.5 or 2 equivalents of "BuLi vs. Ca reproducibly yields the heterobimetallic aggregate [Ca₃(μ-"Bu)₄{μ'-N(SiMe₃)₂}₄Li₂] (5). The molecular solid-state structures of 5 and 7 have been confirmed by XRD analysis. Unlike the reactions with [Ca{N(SiMe₃)₂}₂.(thf)₂], we cannot assert that reaction of crystalline [CaI₂.(thf)₄] with two molar equivalents of "BuLi in thf generates 9, even if our spectroscopic and reactivity data suggest that with equimolar "BuLi, the formation of [Ca("Bu)(I).(thf)₂]₂ is clean and quantitative. The protocols described herein for the production of discrete calcium/*n*-butyl species only require easily accessible calcium precursors. They are readily implantable, and may hence constitute a convenient entry point for broad uptake of alkylcalcium reagents in organic synthesis.
**Introduction**

Calcium bis[(hexamethyl)disilazide] has been a formidable asset in molecular calcium chemistry, either as the dimeric [Ca{N(SiMe₃)₂}₂]₂(1) [1] or as a solvated monomer, e.g. [Ca{N(SiMe₃)₂}₂.(thf)₂] (1').[2] It has proved effective in calcium-promoted catalysis,[³⁻⁷] and is a convenient synthetic precursor, granting access to homoleptic[⁸⁻¹¹] and heteroleptic calcium complexes supported by bulky ancillary ligands.[³] It is synthesized on multi-gram scales, and the high solubility of 1' in aprotic organic solvents is most handy. Access to 1 and 1' has been optimized[¹²,¹³] since the seminal preparation with activated calcium,[¹,²] up to Westerhausen's recent in-situ Grignard metalations.[¹⁴] Besides, a variety of donors (D) has provided 1.(D)ₙ complexes with enhanced stability and/or solubility.[¹⁵,¹⁶] Yet, the rather low basicity of the amide (pKₐ = 25.8 in thf[¹⁷]) can impede wider implantation of 1 and its derivatives in catalytic or sub-catalytic reactions. The (Me₃Si)₂N⁻ amide cannot easily deprotonate weak acids, e.g. 1° and 2° amines, or C₅₃⁻H moieties where the ensuing negative charge on the carbanion is not attenuated by delocalization or other means. Salt metathesis between CaI₂ and potassium salts offers an alternative for synthetic purposes, but it can be contaminated by potassium calciates tedious to remove from the product.[¹⁸⁻²⁰] For most purposes, dialkylcalcium species should be more versatile starting materials.

Yet, the story is not straightforward. Because the Ca–C bond is more polar and reactive than, for instance, the Mg–C bond in Grignard or dialkylmagnesium reagents, organocalcium chemistry is uncharted compared to its lighter congener. Among key achievements, Westerhausen’s input with aryl- and alkylcalcium halides has been spectacular.[²¹⁻³¹] Calcium homoleptic benzyl[³²⁻³⁷] or di/tri-phenylmethanide[³⁸⁻⁴⁰] species stabilized by SiMe₃ groups or Lewis bases are known. The diallyl [Ca{η³-C₃(SiMe₃)₂H₃}₂.(thf)₂] was obtained in 1999.[⁴¹] Okuda prepared [Ca{η³-C₃H₅}₂]∞ and [Ca{η³-C₃H₅}₂.(triglyme-κ⁴] in 2009.[⁴²] Nonetheless, dialkylcalcium complexes *per se* are seldom (Fig. 1). Lappert disclosed [Ca{CH(SiMe₃)₂}₂.(1,4-dioxane)₂] (A) in 1991,[⁴³] and the bulkier, non-solvated [Ca{C(SiMe₃)₃}₂] (B) was described in 1997.[⁴⁴] Hill reported the molecular structure of [Ca{CH(SiMe₃)₂}₂.(thf)₂] (C) in 2008.[⁴⁵] Sadow synthesized [Ca{C(SiHMe₂)₃}₂.(thf)₂] (D), a complex stabilized by β-anagostic Ca···H–Si interactions but not prone to β-hydride elimination.[⁴⁶] The related [Ca{C(SiMe₃)₂(SiMe₂hpp)}₂] (E) was also reported (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine).[⁴⁷] In these cases, the carbanion is stabilized by α-silicon groups, effectively taming its reactivity through delocalization of the negative charge in σ*<sub>Si-C</sub> orbitals. Another strategy relying on alanes afforded the heterobimetallic peralkylated [Ca{(μ-R)₂(AlR₂)}₂]∞ (R = Me, F; Et, G) and the solvated [Ca{(μ-Et)₂(AlEt₂)}₂.(thf)₂] (H) upon reaction of 1 with excess AlR₃.[⁴⁸] The reaction of 1 with Al(CH₂SiMe₃)₃ produced the calcium aluminate [{(Me₃Si)₂N}Ca(μ-CH₂SiMe₃)₂Al(CH₂SiMe₃)₂]₂.[⁴⁹] The synthesis of [CaMe₂]∞ (I) was a breakthrough only achieved in 2018, by treatment of 1 with halide-free methyllithium.[⁵⁰] Overall, so far only C has been turned into a versatile starting material, primarily for the syntheses of molecular heteroleptic alkylcalcium catalysts.[⁵¹⁻⁵⁵]

In this list of dialkylcalcium compounds, one notable absentee is [Ca("Bu)₂], considering the widespread use of *n*-butyllithium,[⁵⁶,⁵⁷] *n*-butylmagnesium halides and di(butyl)magnesium.[⁵⁸] One
known parent is the heteroleptic [{BDI}Ca(µ-"Bu)]₂ (BDI = CH{C(CH₃)NDipp}₂, Dipp = 2,6-diisopropylphenyl), constructed by insertion of but-1-ene in the Ca-H bonds of [{BDI}Ca(µ-H)]₂.[⁵⁹] This Ca-hydride has also been used to generate [{BDI}Ca(µ-Me)]₂ upon reaction with ZnMe₂.[⁶⁰]

<figure><img src="image_2.png" /><figcaption>Fig. 1. Known dialkylcalcium complexes (A-I).[⁴³⁻⁴⁸,⁵⁰]</figcaption></figure>

We describe herein the preparation of Ca("Bu)₂ by reaction of **1** or **1'** with "BuLi. In the course of this work, we have also isolated heterobimetallic calcium-alkali amides. Few such compounds have been structurally authenticated (Fig. 2), e.g. [(thf)Li{µ-N(SiMe₃)₂}₂Ca{N(SiMe₃)₂}] (J)⁶¹ and [Li{µ-N(SiMe₃)₂}₂Ca{N(SiMe₃)₂}]∞ (L) was produced in the presence of thf,⁶³ while [K{µ-N(SiMe₃)₂}₂Ca{N(SiMe₃)₂}]∞ (M) was isolated by ball-milling of CaI₂ with [KN(SiMe₃)₂].⁶⁴ [K₂Ca{N(Ph)Me}₄]∞ is a three-dimensional network, whereas the solvated [{(thf)₂K(µ-N(Ph)')Pr}₂]₂Ca (N) and [K₂Ca(NPh₂)₄.(thf)₃]∞ form respectively a monomer and a monodimensional polymer.²⁰ Beyond amides, calcium-alkali salts are known for other bases, e.g. [Ca{(µ-CH₂Ph)₂Li.(tmeda)}₂]⁶⁵ or the enolate [K₂Ca₂{OC(Mes)=CH₂}₆.(thf)₂].⁶³

<figure><img src="image_3.png" /><figcaption>Fig. 2. Known heterobimetallic alkali/Ae amides.[²⁰,⁶¹⁻⁶⁴]</figcaption></figure>

Combinations of "BuLi and **1'** have been shown to polymerize 1,4-butadiene.[⁶⁶] Although it was not specifically addressed, catalyst speciation is important, as **1'** releases [LiN(SiMe₃)₂]ₙ upon
reaction with, for instance, MeLi[50] or [Ph₂CHLi·tmeda].[40] To get our own insight, we have probed the reactivity of mixtures of "BuLi and calcium sources (1' and [CaI₂.(thf)₄][⁶⁷]) in nucleophilic additions, halogen/metal exchanges, and deprotometalation reactions. Confronted with unexpected behavior, we have investigated the nature of these mixtures. Our experimental findings, including thorough solution NMR analysis, toward the formation of [Ca("Bu)₂] are detailed herein. The molecular structures of [Ca₃(µ-"Bu)₄{µ'-N(SiMe₃)₂}₄Li₂] and [Ca{N(SiMe₃)(SiMe₂-µ-CH₂)}.(thf)₃]₂ have been obtained and are discussed. Theoretical calculations are provided to shed light on the bonding patterns and on the relative stability of these species.

## Results and Discussion

*On the modified reactivity of n-butyllithium in the presence of calcium disilazide or calcium iodide*

The behavior of [Ca{N(SiMe₃)₂}₂.(thf)₂] (1') or [CaI₂.(thf)₄] in the presence of "BuLi was assessed in a range of transformations performed in thf (for the nucleophilic addition onto 4-anisaldehyde leading to 1-(4-Anisyl)pentanol (2), and other reactions, see Scheme S1 and below in the Supporting Information).

2-Bromothiophene is a versatile substrate to learn about the reactivity of the species arising from mixing 1' or [CaI₂.(thf)₄] with "BuLi (Table 1, top, and Scheme S2 in SI). On this substrate, the 1:2 mixture of 1' (or [CaI₂.(thf)₄]) and "BuLi at -80 °C behaved similarly to "BuLi,[68] providing the alcohol 3 resulting from halogen/lithium exchange and subsequent trapping with 4-anisaldehyde in 88% yield (entry 1). With the 1:1 mixture of 1' and "BuLi, the yield of 3 remained good (60%; entry 2). But when the 1:1 mixture of 1' and "BuLi was warmed to -30 °C prior to reaction with 2-bromothiophene, 3 was obtained in a low, non-reproducible yield, and (4-anisyl)(2-bromo-5-thienyl)methanol (3') was also formed (entry 3). As this alcohol 3' results from a deprotometalation-trapping sequence usually observed using hindered lithium dialkylamides,[69] these data suggest that, if a [Ca]-"Bu species is formed at -80 °C from equimolar 1' and "BuLi, it decomposes slowly when warmed to -30 °C.

Compared with 2-bromothiophene, benzothiophene is a more suitable substrate for deprotometalation-trapping (Table 1, bottom, and Scheme S3 in SI). The 1:2 mixture of 1' and "BuLi, prepared and rapidly used at -80 °C, converted it to the alcohol 4 in good yield (entry 4), as does "BuLi. In contrast, for the 1:1 mixture, the yield fell to 21% (entry 5), indicating a change in base reactivity in the presence of the calcium source. This study shows that the reactivity of "BuLi is modified in the presence of 1' (or [CaI₂.(thf)₄]), with a change occurring between -80 and -35°C. We hence set out to investigate the identity of the species formed in solution by reacting 1' or [CaI₂.(thf)₄] with "BuLi. Halogen/calcium exchange and deprotocalciation reactions of aromatic compounds are exceptional,[21,23,69,70] and generating a C-[Ca] bond directly could have a considerable impact.
<figure><img src="image_4.png" /><figcaption>**Table 1.** Probing reagents obtained from either [Ca{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>(thf)<sub>2</sub>] (1') or [CaI<sub>2</sub>(thf)<sub>4</sub>] and <sup>n</sup>BuLi towards 2-bromothiophene and benzothiophene.<sup>[a]</sup></figcaption></figure>

<table><thead><tr><th>Entry</th><th>Ratio, temperature, contact time</th><th>Reaction conditions</th><th>Product(s), Yield(s) (%)<sup>[b]</sup></th></tr></thead><tbody><tr><td>1</td><td>1:2 ; -80 °C, 10 min</td><td>-80 °C, 1 h</td><td><b>3</b>, 88</td></tr><tr><td>2</td><td>1:1 ; -80 °C, 10 min</td><td>-80 °C, 1 h</td><td><b>3</b>, 60</td></tr><tr><td>3</td><td>1:1 ; -80 to -35 °C, 1 h</td><td>-35 °C, 1 h</td><td><b>3</b>, 10-58; <b>3'</b>, 0-28</td></tr><tr><td>4</td><td>1:2 ; -80 °C, 10 min</td><td>-80 °C, 1 h</td><td><b>4</b>, 68<sup>[c]</sup></td></tr><tr><td>5</td><td>1:1 ; -80 °C, 10 min</td><td>-80 °C, 1 h</td><td><b>4</b>, 21</td></tr></tbody></table>

[a] Reactions performed in thf. [b] Estimated from the ¹H NMR spectrum of the crude mixtures. [c] 86% yield using <sup>n</sup>BuLi (1 equiv) at -80 °C for 1 h.

*On the monosubstitution of calcium hexamethyldisilazide*

The reaction of the non-solvated **1** with one equivalent of <sup>n</sup>BuLi was examined first. Its outcome is solvent-dependent. In petroleum ether, the reaction mixture formed a suspension at -78 °C. The solid dissolved upon warming to -10 °C, to give a clear solution. Following storage at -35 °C, colorless single-crystals of the heterobimetallic [Ca<sub>3</sub>(μ-<sup>n</sup>Bu)<sub>4</sub>{μ'-N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>4</sub>Li<sub>2</sub>] (5) reproducibly formed within 24 h (isolated yield 49% vs. Ca). Its identity was established by XRD analysis, and was corroborated by NMR spectroscopy. Further crystallization did not afford another crop of 5, but provided instead crystals of the known trimer [LiN(SiMe<sub>3</sub>)<sub>2</sub>]<sub>3</sub>.<sup>[71]</sup> We could not identify other Ca- or Li-containing species (Scheme 1). Complex 5 can be obtained in greater yields (55% and 67% vs. Ca, respectively) using 1.5 or 2.0 equivalents of <sup>n</sup>BuLi vs. **1**. In contrast, with half equivalents of <sup>n</sup>BuLi, X-ray quality crystals of [Li{μ-N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>Ca{N(SiMe<sub>3</sub>)<sub>2</sub>}]<sup>[62]</sup> were the sole isolated product (53%); the fate of the *n*-butyl species was unknown. Last, the attempt to sequester lithium disilazide with a four-fold excess of 12-crown-4 returned crystalline [(12-cr-4)LiN(SiMe<sub>3</sub>)<sub>2</sub>]<sup>[72]</sup> almost quantitatively. No trace of a putative [Ca(<sup>n</sup>Bu)<sub>2</sub>]<sub>n</sub> could be detected. Any calcium and *n*-butyl species were removed from the isolated lithium product during work-up.
<figure><img src="image_5.png" /><figcaption>**Scheme 1.** Synthesis of $[Ca_3(\mu^{-}Bu)_4\{\mu'\text{-}N(SiMe}_3)_2\}_4Li_2]$ (5) in petroleum ether. The bimetallic $[Li\{\mu-N(SiMe}_3)_2\}_2Ca\{N(SiMe}_3)_2\}]$ is the sole isolated product with 0.5 equiv of $^{n}$BuLi.</figcaption></figure>

The molecular solid-state structure of 5 forms a pentametallic core with a near-linear Li1-Ca1-Ca2-Ca3-Li2 arrangement (Figure 3). The Ca²⁺ ions are sandwiched between two capping Li⁺ ones. We are unaware of similar structural motif involving these ions.[73] The central Ca2 is surrounded by four µ₂⁻'Bu bridging groups, whereas Ca1 and Ca3 are bound to two µ₂⁻'Bu groups and two µ₂-N(SiMe₃)₂ disilazides. The coordination sphere of each calcium is saturated by Ca···H-C agostic interactions with α- or β-carbon atoms of the n-butyl chains. The interatomic Ca–C distances to the carbanions (2.543(4)-2.564(12) Å for Ca1; 2.499(16)-2.550(4) Å for Ca2; 2.545(4)-2.586(5) Å for Ca3) match those in heteroleptic dimers bearing alkyl groups, e.g. [{BDI<sup>DiPP</sup>}Ca''Bu]₂ and its congeners.[59,74]

The lithium ions are coordinated by two bridging µ₂-silazides with standard Li–N interatomic distances, in the range 1.970(7)-1.989(7) Å. Their coordination sphere is completed by Li···H–C agostic interactions involving SiCH₃ groups. The Li–C interatomic distances to the pertaining C atoms are relatively short, in the range 2.325(13)-2.501(9) Å. The ¹H NMR spectrum of 5 in benzene-d₆ displays a diagnostic triplet at δ₁H -0.17 ppm for the CaCH₂CH₂CH₂CH₃ methylene groups in α position to the calcium centers. The ²⁹Si NMR spectrum displays two major resonances, at δ₂₉Si -10.71 and -13.01 ppm. In thf-d₈, 5 dissociates into the homometallic solvates [LiN(SiMe₃)₂.(thf)₄] and [Ca("Bu){N(SiMe₃)₂}.(thf)₃] (6), *vide infra*.
<figure><img src="image_6.png" /><figcaption>**Figure 3.** ORTEP drawing of the molecular solid-state structure of [Ca₃(µ-"Bu)₄{µ'-N(SiMe₃)₂}₄Li₂] (5). Ellipsoids at the 30% probability level. Only the main components of the disordered butyl and trimethylsilyl groups are depicted. H atoms (except for those attached to the α- and β-carbons and those on close SiMe₃ groups) are omitted. Selected interatomic distances (Å): Ca1–C4 = 2.901(5), Ca1–C10 = 2.976(5), Ca1–C13 = 2.543(4), Ca1–C17A = 2.564(12), Ca1–N1 = 2.479(3), Ca1–N2 = 2.448(3), Ca2–C13 = 2.550(4), Ca2–C14 = 2.812(4), Ca2–C17A = 2.499(16), Ca2–C18A = 2.980(7), Ca2–C21 = 2.539(4), Ca2–C22 = 2.853(4), Ca2–C25 = 2.523(5), Ca2–C26 = 2.898(4), Ca3–C21 = 2.545(4), Ca3–C25 = 2.586(5), Ca3–C29 = 2.976(5), Ca3–C35 = 2.895(5), Ca3–N3 = 2.468(3), Ca3–N4 = 2.467(3); Li1–C1C = 2.325(13), Li1–C7A = 2.40(3), Li1–N1 = 1.975(8), Li1–N2 = 1.989(7), Li2–C32 = 2.439(8), Li2–C38 = 2.501(9), Li2–N3 = 1.970(7), Li2–N4 = 1.980(7).</figcaption></figure>

The reaction in thf follows a course different from that in petroleum ether. Treatment of the thf-solvated 1' with equimolar "BuLi at –78 °C in thf for 2 h, followed by layering with petroleum ether and storage at –35 °C, provided a large crop of crystals of [Ca{N(SiMe₃)(SiMe₂-µ-CH₂)}.(thf)₃]₂ (7), in 55% yield, as authenticated by NMR and XRD (Scheme 2). Another load of different crystals formed after further storage. The identity of this second compound, [LiN(SiMe₃)₂.(thf)₂], was revealed by NMR and SC-XRD analysis (yield 57%).<sup>[75]</sup> Except for small amounts of the known, non-solvated [Li{µ-N(SiMe₃)₂}₂Ca{N(SiMe₃)₂}] (8) detected in the NMR of the crude mixture,<sup>[62]</sup> we were unable to identify any other Ca-based species in this mixture. The reproducibility of this reaction is excellent. Complex 7 results from deprotonation of a methyl group in the silazide, a phenomenon documented for other oxophilic metals, e.g. with magnesium,<sup>[76]</sup> lanthanides<sup>[77]</sup> or zirconium.<sup>[78]</sup> It is the first time it is observed with calcium, and results from self-deprotonation in a transient species subsequently identified as [Ca("Bu){N(SiMe₃)₂}.(thf)₃] (6) by NMR analysis (vide infra).
<figure><img src="image_7.png" /><figcaption>**Scheme 2.** Formation of $[Ca\{N(SiMe_3)(SiMe_2-\mu-CH_2)\}\cdot(thf)_2]_2$ (7) in thf.</figcaption></figure>

The ¹H NMR spectrum of complex 7 in thf-d₈ features a diagnostic singlet at δ₁H -1.81 ppm for the CaCH₂Si methylene unit. The corresponding ¹³C resonance appears at δ₁³C 16.72 ppm, a surprisingly low-field chemical shift, in the ¹³C{¹H} spectrum. The molecular solid-state structure of the complex shows it forms a centrosymmetric four-member Ca₂C₂ metallacycle that dimerizes with bridging CH₂⁻ methylene groups and terminal amides (Figure 4). The geometry around the Ca²⁺ ions is octahedral. The dimer adopts a Ca₂N₂Si₂C₂ ladder-like arrangement, with a large distortion of the strained geometry around the trigonal nitrogens (compare Ca1–N1–Si1, 138.17(8)°, and Ca1–N1–Si2, 95.18(6)°). The Si2–C4 interatomic distance to the bridging methylene group (1.8526(17) Å) is marginally smaller than the distances to the Si-bound methyl groups (Si2–C5 = 1.8933(18) Å, Si2–C6 = 1.8923(18) Å). The Ca1–C4 and Ca1–C4' bond lengths (2.6094(17) and 2.6140(17) Å) are noticeably longer than the related Ca–C distances in complex 5 (2.499(16)-2.586(5) Å), which presumably reflects lower electronic density on the carbanion in 7 owing to the stabilizing α-silicon effect.

Compound 7 is reminiscent of Mulvey's [LiMg(TMP){CH₂Si(Me)₂N(SiMe₃)}]₂, a magnesium-lithium heterobimetallic complex synthesized in hydrocarbons (TMP = 2,2,6,6-tetramethylpiperidide).[⁷⁶] In our hands, treatment of 1 with [K(TMP)]ₙ with a Ca:K ratio of 1:1 in toluene only afforded the donor-free [K{μ-N(SiMe₃)₂}₂Ca{N(SiMe₃)₂}]∞[⁶⁴] as the sole identifiable product (88% yield). By contrast, in petroleum ether at -35 °C, the reaction quantitatively returned the deprotonated, mixed aggregate [Ca{μ-N(SiMe₃)(SiMe₂-μ'-CH₂)}{μ''-N(SiMe₃)₂}K]₂. All pertaining data for this heterobimetallic aggregate, including the X-ray structure of the benzene solvate, are provided in the Supporting Information. In thf, it reacts to generate 7 upon removal of potassium disilazide.
<figure><img src="image_8.png" /></figure>

**Figure 4.** ORTEP drawing of the molecular solid-state structure of [Ca{N(SiMe₃)(SiMe₂-μ-CH₂)}.(thf)₃]₂ (7). Ellipsoids at the 30% probability level. Only the main components of the disordered thf molecules and trimethylsilyl groups are depicted, and H atoms (except for those attached to the α-carbons, which were located through Fourier difference maps analysis) are omitted. Selected interatomic distances (Å) and angles (°): Ca1-C4 = 2.6094(17), Ca1-C4' = 2.6140(17), Ca1-N1 = 2.3564(14), Ca1-O1 = 2.410(4), Ca1-O2 = 2.437(7), Ca1-O3 = 2.4556(12), N1-Si1 = 1.6740(14), N1-Si2 = 1.7055(15), Si2-C4 = 1.8526(17), Si2-C5 = 1.8933(18), Si2-C6 = 1.8923(18); Ca1-C4-Ca1' = 84.46(5), Ca1-N1-Si1 = 138.17(8), Ca1-N1-Si2 = 95.18(6), N1-Ca1-C4 = 70.57(5), N1-Si2-C4 = 107.78(7), N1-Si2-C5 = 112.65(8), N1-Si2-C6 = 112.39(8), Ca1-C4-Si2 = 83.74(6), Ca1'-C4-Si2 = 163.38(9). Symmetry transformations: -x, -y, -z.

### NMR analysis of the monosubstitution of Ca{N(SiMe₃)₂}₂.(thf)₂ in thf-d₈

The details of the reaction between the calcium precursor **1'** and one equivalent on "BuLi were examined by NMR spectroscopy in **thf-d**₈. The sample was prepared directly in the NMR tube by adding one equivalent of "BuLi vs. Ca to a 0.15 M solution of **1'** in **thf-d**₈ at -78 °C. Note that in this solvent, the solvent-free dimer **1** dissociates instantly to generate the **thf-adduct 1'**, and hence all NMR investigations in **thf-d**₈ were conducted with the solvated **1'**, easier to access and to handle. The ¹H (Figure 5) and ⁷Li (Figure S15) one-dimensional NMR spectra, recorded at -83 °C, showed progressive fading of the root resonance for SiMe₃ groups in **1'** at δ₁H -0.12 ppm. We monitored gradual disappearance of the precursor, until its complete consumption, by ¹H and ²⁹Si NMR spectroscopy (δ₁H -0.12 ppm and δ₂₉Si - 14.93 ppm).
<figure><img src="image_9.png" /><figcaption>**Figure 5.** 1D ¹H NMR spectra (500 MHz, thf-d₈, –83 °C) of: (a) [Ca{N(SiMe₃)₂}₂.(thf)₂] (1'), (b) "BuLi (0.4 M), (c) the mixture of 1' (0.15 M) and 0.66 equiv of "BuLi vs. Ca, and (d) the mixture of 1' (0.15 M) and 1.0 equiv of "BuLi.</figcaption></figure>

The ¹H NMR spectrum revealed that during the progressive addition of one equivalent of "BuLi, the intensity of the ¹H resonance corresponding to 1' (labelled H1'a in Figure 5) gradually decreased, while three new signals corresponding to hydrogens of SiMe₃ groups emerged. Additionally, a new multiplet was observed at –0.92 ppm, which did not correspond to the CH₂Li methylene group of "BuLi. It was attributed to the CH₂Ca resonance H⁶ᵃ from compound 6. Notably, the signal at –0.08 ppm increased with the addition of the first 0.5 equivalent of "BuLi before decreasing and ultimately disappearing after the addition of the first full equivalent of the lithium reagent (Figure S16). The 2D ¹H-²⁹Si HMBC NMR spectrum (Figure S17) allowed us to assign the various SiMe₃ ¹H resonances to their corresponding SiMe₃ ²⁹Si signals. The evolution of the signals on the 1D ²⁹Si NMR spectra (Figure 6) are consistent with those observed in the ¹H NMR spectra, which reinforces the reliability of our assignments. Based on this information, we concluded that the reaction of 1' with "BuLi resulted in the formation of two new compounds, labelled as [{LiN(SiMe₃)₂}₂.(thf)₃] (with 1 ≤ x ≤ 2; the mono- and disolvated species are in equilibrium in solution in thf;[79] in our experimental conditions, we observed by ¹H DOSY NMR spectroscopy, *vide infra*, that x is closer to 1) and 6. The 2D NMR spectra, ¹H-¹H COSY (Figure 7) and ¹H-¹H NOESY (Figure S18), were recorded, allowing a complete attribution of the ¹H signals of the compound 6.
<figure><img src="image_10.png" /><figcaption>**Figure 6.** 1D $^{29}$Si DEPT NMR spectra (99.4 MHz, thf-d$_{8}$, 190K) of: (a) [Ca{N(SiMe$_{3}$)$_{2}$}$_{2}$.thalf] (1'), (b) the mixture of 1' (0.15 M) and 0.66 equivalent of "BuLi, and (c) the mixture of 1' (0.15 M) and 1.0 equivalent of "BuLi.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 7.** 2D $^{1}$H-$^{1}$H COSY spectrum (500 MHz, thf-d$_{8}$, 190K) of [Ca{N(SiMe$_{3}$)$_{2}$}.(thf)$_{2}$] (1') after addition of 1.0 equivalent of "BuLi.</figcaption></figure>
To determine the structure and solvation of 6 in thf solution, a ¹H DOSY experiment was carried out, using Stalke's external calibration method for the molecular weight calculation (See Figure 8 and Table S1).[80] A calculated mass of 502 g.mol⁻¹ was estimated, which was assigned to the trisolvated, monosubstituted monomer [Ca("Bu){N(SiMe₃)₂}.(thf)₃] (6). This finding was corroborated by integration of the corresponding signals in the 1D ¹H NMR spectrum (Figure S19). All identified compounds and intermediates are described in Scheme 3.

<figure><img src="image_12.png" /><figcaption>**Figure 8.** ¹H DOSY spectrum (500 MHz, thf-d₈, 190K) of the mixture of [Ca{N(SiMe₃)₂}₂.(thf)₂] (1') with 1.0 equivalent of "BuLi.</figcaption></figure>

Furthermore, by utilizing the same ¹H DOSY experiment (Figure 8 and Table S1) and in line with the ¹H, ⁷Li, and ²⁹Si resonances discussed above, the singlet observed at -0.26 ppm, which increased continuously upon addition of "BuLi (see Figure 5), was assigned to [LiN(SiMe₃)₂.(thf)₂]₂ (0.5 ≤ x ≤ 1).[81,82] The emergence of solvated lithium disilazide in the medium was expected, as this salt is the co-product of the transmetalation between "BuLi and the calcium precursor. The properties of this lithium aggregate, along with all related spectroscopic data, were consistent with existing literature.[81,82]

The analysis was completed by examining the last remaining signals at -0.08 ppm and -16.15 ppm on the ¹H and ²⁹Si spectra, respectively. The ¹H resonance at -0.08 ppm correlates with the ²⁹Si resonance at -16.15 ppm and shows no spatial proximity or scalar coupling with *n*-butyl chain signals, in accordance with the formulation for the mixed aggregate between calcium and lithium amides 8'. To support this hypothesis, we mixed pristine samples of 1' and [LiN(SiMe₃)₂]n in thf-d₈ in near-equal proportions. The 1D ¹H, ⁷Li, and ²⁹Si NMR spectra of this mixture (Figure S20) revealed resonances corresponding to the mixed aggregate [CaLi{μ₂-N(SiMe₃)₂}₂{N(SiMe₃)₂}.(thf)] (8'), i.e. the solvated analogue of 8. The diffusion coefficient, extracted from the ¹H DOSY map (see Figure 8 and Table S1), is consistent with a mixed dimer solvated by one molecule of thf. Such aggregates have been previously reported in the literature, yielding similar findings.[61,62]

In summary (Scheme 3), upon reaction of [Ca{N(SiMe₃)₂}₂.(thf)₂] (1') with one equivalent of "BuLi in thf-d₈ at -78 °C, three compounds are formed: the heteroleptic (*n*-butyl)calcium amide
[Ca($^{n}$Bu){N(SiMe$_{3}$)$_{2}$}.(thf)$_{3}$] (6) found as a trisolvated complex, and two co-products, the lithium aggregate [LiN(SiMe$_{3}$)$_{2}$. (thf)$_{x}$]$_{2}$ (an equilibrium between the mono- and disolvated dimers, for x = 0.5 and 1), and the mixed aggregate [CaLi{μ$_{2}$-N(SiMe$_{3}$)$_{2}$}$_{2}$ {N(SiMe$_{3}$)$_{2}$}.(thf)] (8'). The overall mass balance is displayed in Equation 1.

<figure><img src="image_13.png" /><figcaption>**Scheme 3.** Monitoring of the reaction of [Ca{N(SiMe$_{3}$)$_{2}$}$_{2}$. (thf)$_{2}$] (1') with 1.0 equiv of $^{n}$BuLi in thf-d$_{8}$.</figcaption></figure>

$$4 [\text{Ca}\{\text{N(SiMe}_3)_2\}_2 \cdot \text{(thf)}_2] + 3 {}^n\text{BuLi} \xrightarrow[\text{(excess)}]{\text{thf}} 3 [\text{Ca}(^n\text{Bu})\{\text{N(SiMe}_3)_2\} \cdot \text{(thf)}_3] + [\text{LiN(SiMe}_3)_2 \cdot \text{(thf)}_x]_2 \quad \text{(Eq. 1)} \\ + [\text{CaLi}\{\text{N(SiMe}_3)_2\}_3 \cdot \text{(thf)}]$$

The stability of compound 6 under the conditions used for reactivity studies was further investigated. The NMR sample in thf-d$_{8}$ was gradually warmed up to -35 °C, and the resulting degradation was monitored by 1D $^{1}$H and $^{29}$Si NMR every 30 minutes. The resulting 1D $^{1}$H and $^{29}$Si NMR spectra displayed progressive fading of the resonances for compound 6, concomitant with the emergence of three new $^{1}$H resonances at -0.23, -0.27 and -1.92 ppm, and two new $^{29}$Si signals at -14.69 and -21.67 ppm (Figures 9 and S21). The analysis of the 2D $^{1}$H-$^{29}$Si HMBC and $^{1}$H-$^{1}$H COSY spectra demonstrates that these new signals originate from the same compound. The structure and solvation of this new compound were further investigated using $^{1}$H DOSY NMR (Figure S24). Compound 1', whose $^{1}$H resonance in the $^{1}$H NMR spectrum can be easily identified, was employed as an internal reference. The resulting diffusion coefficients, reported in Table S2, yielded an apparent molecular weight of 532 g.mol$^{-1}$. This value, in conjunction with the spectroscopic data presented above, supports the formation of a disolvated dimer of composition [Ca{N(SiMe$_{3}$)(SiMe$_{2}$-μ-CH$_{2}$)}$_{2}$. (thf)]$_{2}$ (7'). This dimer results from deprotonation of a methyl group in one of the SiMe$_{3}$ substituent in 6, followed by dimerization through bridging methylene units (see Scheme 3). Full assignment of the above $^{1}$H and $^{29}$Si NMR data was consistent with this formulation (Figures 9 and S21).
<figure><img src="image_14.png" /><figcaption>**Figure 9.** 1D ¹H spectra (500 MHz, thf-d₈) of: (a) [Ca{N(SiMe₃)₂}₂.(thf)₂] (1') with 1.0 equiv of "BuLi at -83 °C, (b) **1** with 1.0 equiv of "BuLi after 1 h at -35 °C, (c) enlargement of ¹H spectrum of the mixture of **1** with 0.66 equiv "BuLi at -35 °C and (d) the mixture of **1** with 1.0 equiv of "BuLi after 1 h at -35 °C.</figcaption></figure>

*On the disubstitution of [Ca{N(SiMe₃)₂}₂.(thf)₂] (1') in thf-d₈*

Compared to the previous scenario of monosubstitution, and in agreement with the reported synthesis of dimethylcalcium,[50] the addition of two equivalents of "BuLi to the calcium precursor **1'** was expected to trigger an extra nucleophilic substitution on calcium, hence generating a di-*n*-butylcalcium species accompanied by the release of a second equivalent of lithium disilazide. Monitoring of the reaction in thf-d₈ at -83 °C proved insightful. As anticipated, upon gradual addition of "BuLi, the resulting one-dimensional ¹H NMR spectra (Figure 10) features a slow disappearance of the resonances assigned to the monosubstituted complex **6** at -0.16 and -0.92 ppm with concomitant increase in the intensity of a new resonance at -1.06 ppm. The full addition of two equivalents of "BuLi, relative to **1'**, led to the complete disappearance of **6** and to the emergence of resonance at *ca.* -1.16 ppm, which corresponds to CH₂Li units in *α* position to the metal in "BuLi. These observations were corroborated by the 1D ²⁹Si NMR spectrum (Figure S25), which showed the disappearance of the ²⁹Si resonance for compound **6** at -16.82 ppm, accompanied by the rise of the peak corresponding to [LiN(SiMe₃)₂]₂.(thf)₂ at -18.65 ppm. Accordingly, the corresponding one-dimensional ⁷Li NMR spectrum (Figure S26c) predominantly displayed resonances assigned to [{LiN(SiMe₃)₂]₂}.thf] dimers at 0.16 ppm, along with a minor signal at 1.60 ppm belonging to "BuLi. Furthermore, the 2D ¹H-¹H NOESY spectrum (Figure S27) revealed a correlation between the new ¹H resonance at -1.06 ppm and two other resonances at 1.08 and 1.39 ppm. Additionally, a 2D ¹H-¹H COSY spectrum (Figure S28) demonstrated a scalar coupling correlation between the signals at -1.06 and 1.39 ppm. Taken collectively, these data are consistent with the
expected formation of a di-*n*-butylcalcium species, [Ca($^{n}$Bu)$_{2}$.($_{thf}$)$_{x}$], resulting from the second nucleophilic attack of $^{n}$BuLi on **6** (Scheme 4).

<figure><img src="image_15.png" /><figcaption>**Figure 10.** 1D $^{1}$H NMR spectra (500 MHz, thf-$d_{8}$, -83 °C): (a) 1D $^{1}$H of [Ca${\{N(SiMe_{3})_{2}\}}_{2}.({_{thf}}_{2})$ (1') with 1.0 equiv of $^{n}$BuLi, (b) 1D $^{1}$H of 1' with 2.0 equiv of $^{n}$BuLi, (c) 1D $^{1}$H PSYCHE of 1' with 2.0 equiv of $^{n}$BuLi, and (d) 1D $^{1}$H of $^{n}$BuLi solution (0.3 M).</figcaption></figure>

Going further in the analysis of the ¹H NMR data was achieved using a broadband homodecoupling experiment (¹H PSYCHE), which suppresses the effects of homonuclear scalar coupling interactions, thereby transforming multiplets into singlets (Figures 10c and S29).[83] The resulting 1D ¹H PSYCHE spectrum exhibits two unresolved singlet signals at -1.06 ppm. Deconvolution of the signals carried out using the Dmfit software[84] indicated a relative proportion of 4:1. However, this relative ratio was found to depend on sample preparation (Figure S30), suggesting that these two resonances do not originate from the same compound. The structure and solvation of the major species was finally assessed by ¹H DOSY NMR in thf-$d_{8}$ (similar analysis could not be achieved with the minor species, because the intensity of its resonance was significantly weaker; its decay in response to increasing gradient strength was very rapid, resulting in insufficient data points to accurately determine the diffusion coefficient). Analysis of the corresponding diffusion coefficients indicated an apparent molecular weight of 408 g.mol⁻¹ (Figure S31, Table S3). This result, in link with the other spectroscopic data above, is consistent with formation of a tetrasolvated monomer of overall composition [Ca($^{n}$Bu)$_{2}$.($_{thf}$)$_{4}$ (9) (Scheme 4). The formulation is identical to that of the phenyl derivative, [Ca(Ph)$_{2}$.($_{thf}$)$_{4}$], whose molecular solid-state structure was established by XRD crystallography.[85] Our attempts to grow X-ray quality crystals of **9** proved unsuccessful. Note that comparative NMR data for
[Mg($^{n}$Bu)$_{2}$]$_{x}$ have been recorded, and are provided in Supporting Information (Figure S35). Its ¹H NMR spectrum shows substantial differences with the spectrum of 9, in particular for the CH$_{2}$ hydrogens in α position to the metal, at -1.06 ppm for Ca and -0.5/-0.6 ppm for Mg. This was to be expected, as the density of negative charge on the α carbon atom is greater for Ca than for Mg.

<figure><img src="image_16.png" /><figcaption>**Scheme 4.** Summary of the reaction of [Ca{N(SiMe$_{3}$)$_{2}$}$_{2}$·(thf)$_{2}$] (1') with $^{n}$BuLi in thf-$d_{8}$ at -83 °C.</figcaption></figure>

*On the substitution of [CaI$_{2}$. (thf)$_{4}$]*

Based on the intriguing reactivity results, we investigated CaI$_{2}$ as the calcium source to generate Ca/$^{n}$Bu species. Compared to 1', the utilization of CaI$_{2}$ appeared to slightly inhibit the reactivity of the resulting species in thf. We surmised that this inhibition may stem from the presence of LiI in the medium, which could lead to the undesirable formation of mixed aggregates in the presence of 6 or 9.[50]

We initiated our investigation by progressively adding $^{n}$BuLi to calcium iodide (Figure 11). The sample was prepared directly in a J-Young's NMR tube, by introducing 1.0 equivalent of $^{n}$BuLi into a 0.15 M solution of crystalline [CaI$_{2}$. (thf)$_{4}$] (10) in thf-$d_{8}$ at -78 °C. The resulting 1D ¹H NMR spectrum (Figure 11c) revealed the appearance of two new resonances, a prevailing one at $\delta_{1H}$ -0.92 ppm, and a minor one at -0.79 ppm, in a ratio of 73:27. They were assumed to correspond to a monosubstituted product of general formulation [Ca("Bu)I.(thf)$_{x}$], as they did not match the resonances diagnostic of $^{n}$BuLi.[86,87] This hypothesis was corroborated by the 1D ⁷Li spectrum (Figure S32b), which did not exhibit any resonance for $^{n}$BuLi, but instead showed a single signal at $\delta_{7Li}$ 0.05 ppm assigned to the aggregate [LiI]$_{n}$.[88,89] The analysis was completed with the acquisition of a 2D ¹H-¹H COSY map (Figure 12) that confirmed the coupling correlation between the resonance at $\delta_{1H}$ -0.92 ppm and two others at 1.43 and 1.02 ppm. It was further supported by the 2D ¹H-¹H NOESY spectrum (Figure S33) that showed the presence of NOE correlation between these three resonances.

The structure and solvation of the two components of the mixture were determined using a ¹H COSY NMR experiment (Figure S34, Table S4). In relation with the above data, molecular weight determination analysis based on the diffusion coefficients indicated that the minor species was the dimeric aggregate [Ca("Bu)(I).(thf)$_{2}$]$_{2}$. A similar dimeric aggregate was previously reported for calcium-methyl species.[50] Besides, the major compound in the mixture is proposed to correspond to the tetrasolvated monomer, i.e. [Ca("Bu)(I).(thf)$_{4}$] (11).

From there, the transfer of a second equivalent of $^{n}$BuLi onto the initial solution was expected to induce an additional substitution onto calcium and to generate [Ca("Bu)$_{2}$. (thf)$_{4}$] (9). Yet, despite our
best efforts, the 1D ¹H NMR spectrum of the solution (Figure 11d) proved difficult to analyze reliably
due to significant signal overlap. Note however that the corresponding 1D ⁷Li spectrum (Figure S32c)
exhibited only a single resonance, at the frequency diagnostic of [LiI]n, without indication of residual
"BuLi, thereby suggesting full consumption of the lithium reagent with, presumably, occurrence of the
second substitution at calcium. Our analytical data did not allow us to validate this assumption.

<figure><img src="image_17.png" /><figcaption>**Figure 11.** 1D ¹H NMR spectra (500 MHz, thf-d₈, -83 °C) of: (a) CaI₂, (b) "BuLi, (c) CaI₂ with 1.0 equivalent of "BuLi, and (d) CaI₂ with 2.0 equivalents of "BuLi.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Figure 12.** 2D ¹H-¹H COSY spectrum (500 MHz, thf-d₈, -83 °C) of the mixture generated upon reaction of CaI₂ with 1.0 equivalent of nBuLi.</figcaption></figure>

*Computational bonding analyses*

To gain insights into their stability, structure and bonding, DFT calculations were performed on the most
crucial species uncovered in this study. The optimized geometry of [Ca₃(µ-"Bu)₄{µ'-N(SiMe₃)₂}₄Li₂] (5) is overall in excellent agreement with its SC-XRD counterpart (Figure S36 and Table S5). The small values of the Wiberg bond indices (WBIs), the natural and Bader atomic charges and the bond critical point (bcp) indicators determined from a quantum theory of atoms in molecules (QTAIM) analysis (Tables S5-S7)[⁹⁰] are consistent with a weakly polar covalent, strongly ionic character of all the bonds around the Ca²⁺ and Li⁺ ions. Among several Ca···H agostic contacts, only one gives rise to a bcp (distance 2.573 Å) (Table S7). Three Ca···C(Me) bcp's of similar nature and strength were also found (distances 2.880, 2.913 and 2.904 Å).

The optimized geometry of [Ca{N(SiMe₃)(SiMe₂-µ-CH₂)}.(thf)₂] (7) also agrees well with the X-ray structure (Figure S36 and Table S8.), with computed bonding features similar to those in 5 (Tables S8-S10). Its disolvated parent [Ca{N(SiMe₃)(SiMe₂-µ-CH₂)}.(thf)₂] (7') has a non-planar optimized geometry caused by the tetrahedralization of the now tetracoordinated calcium atoms (Figure S36). Its computed ¹H NMR chemical shifts are close to those found for 7 and in good agreement with their experimental counterparts recorded in thf (Table S11). The bonds in the Ca coordination sphere in 7' are ca. 3% shorter and, hence, slightly stronger than in 7 (Tables S12-S14). The intermediate [Ca("Bu){N(SiMe₃)₂}.(thf)₃] (6, see Scheme 2) was also computed. Two almost isoenergetic isomers, 6a and 6b (ΔG₁₉₅K in thf = 2.0 kcal mol⁻¹, 6a being the most stable) were found, both adopting a distorted
trigonal bipyramidal structure (Figure 13). Both have their amido and alkyl substituents lying in the equatorial plane but differ in the orientation of the *n*-butyl C₄ chain, approximately perpendicular to and within the equatorial plane, in **6a** (N-C-C = 126°) and **6b** (N-C-C = 150°), respectively. Both isomers show very similar bonding features (Tables S15-S17). It is of note that their Ca-C("Bu) bcp descriptors (Table S17) have larger absolute values than those found for **5** (in particular |V|/G), indicating a stronger (polar) covalent character for this bond in **6**. The computed ¹H chemical shifts of **6a** and **6b** are not significantly different, and agree well with those measured experimentally (Table S18). The free energy of the reaction of formation of **7** and *n*-butane from **6** (see Scheme 2) was computed to be ΔG₁₉₅K in **thf** = -15.9 kcal mol⁻¹ (for one mole of **6**), whereas that for the formation of **6** and [LiN(SiMe₃)₂(thf)]₂ from **1'** and ["BuLi(thf)]₄ is ΔG₁₉₅K in **thf** = +1.1 kcal mol⁻¹ (for one mole of **6**). These data are in line with the experimental findings.

<figure><img src="image_19.png" /><figcaption>**Figure 13.** The DFT-optimized geometries of compounds **7', 6**, and **9** with isomer relative energy (ΔE) and free energy (ΔG) in vacuum. Hydrogens omitted for clarity in **6a** and **6b**.</figcaption></figure>

The di(*n*-butyl) species [Ca("Bu)₂.(thf)₄] (**9**; see Scheme 4) was also investigated. Two structures, the *trans* (**9a**; two "Bu groups in apical positions) and *cis* (**9b**; two "Bu groups in *cis* equatorial positions) isomers of an octahedral [Ca("Bu)₂.(thf)₄] complex, were found (Figure 13), **9a** being slightly more stable (ΔG₁₉₅K = 1.5 kcal mol⁻¹ in **thf**). Both structures have similar bonding features (Tables S19-
S21). We note that the *trans* structure matches that of related isolated [CaR<sub>2</sub>.(thf)<sub>4</sub>] complexes in which Ca is bonded to sp<sup>2</sup> or sp<sup>3</sup> carbon atoms.[23,85,91] The computed <sup>1</sup>H chemical shifts of both isomers are not significantly different at our computational level, and are in line with experimental NMR measurements (Table S22). In order to get an energetic overview of the bonding of the *n*-butyl anion to Ca<sup>2+</sup> in **9a**, an energy decomposition analysis (EDA) according to the Morokuma-Ziegler procedure[92-94] of the interaction between the ["Bu]<sup>-</sup> and [Ca("Bu)(thf)<sub>4</sub>]<sup>+</sup> frozen (unrelaxed) molecular fragments of the *trans* isomer was performed. The total bonding energy (TBE) between the two fragments is expressed as the sum of four components, the Pauli repulsion (*E*<sub>Pauli</sub>), the electrostatic interaction energy (*E*<sub>elstat</sub>), the orbital interaction energy (*E*<sub>orb</sub>) and the component resulting from the dispersion forces (*E*<sub>disp</sub>). Their values are given in Table 2. With an *E*<sub>elstat</sub> component more than four times larger than the *E*<sub>orb</sub> one, these results are fully consistent with a dominant ionic nature of the Ca-C bond. Similar values were obtained for the Ca-C bond in **6a** (Table 2), for which one should note that a comparable TBE value of -5.87 eV was found for the silazide ligand. A comparison of the bonding in **9a** and that in the related [*trans*-Ca(*p*'-Bu-benzyl)<sub>2</sub>(thf)<sub>4</sub>], for which a crystal structure is known,[91] was also performed. With a TBE value of -5.17 eV and quite similar energy components (Table 2), the bonding in this isolated complex does not differ significantly from that in **9a**. Moreover, an EDA analysis of these two [CaR<sub>2</sub>(thf)<sub>4</sub>] complexes considering this time the decomposition into the [R<sub>2</sub>]<sup>2-</sup> and [Ca(thf)<sub>4</sub>]<sup>+</sup> fragments (Table 2) indicated even stronger bonding for "Bu (TBE = 2 × -9.10 eV) than for *p*'-Bu-benzyl (TBE = 2 × -7.97 eV).

<table><caption>**Table 2.** Morokuma-Ziegler energy decomposition analysis in **9a**, **6a** and [Ca(*p*'-Bu-benzyl)<sub>2</sub>(thf)<sub>4</sub>].<sup>a</sup></caption><thead><tr><th rowspan="2"></th><th colspan="2">9a</th><th colspan="2">6a</th><th colspan="2">[Ca[(CH<sub>2</sub>C<sub>6</sub>H<sub>4</sub>'Bu)<sub>2</sub>(thf)<sub>4</sub>]<sup>91</sup></th></tr><tr><th>[Ca[("Bu)(thf)<sub>4</sub>]<sup>+</sup><br/>+<br/>["Bu]<sup>-</sup></th><th>[Ca(thf)<sub>4</sub>]<sup>2+</sup><br/>+<br/>["Bu)<sub>2</sub>]<sup>2-</sup></th><th>[Ca{N(SiMe<sub>3</sub>)}(thf)<sub>3</sub>]<sup>+</sup><br/>+<br/>["Bu]<sup>-</sup></th><th>[Ca[(CH<sub>2</sub>C<sub>6</sub>H<sub>4</sub>'Bu)(thf)<sub>4</sub>]<sup>+</sup><br/>+<br/>[CH<sub>2</sub>C<sub>6</sub>H<sub>4</sub>'Bu]<sup>-</sup></th><th>[Ca(thf)<sub>4</sub>]<sup>2+</sup><br/>+<br/>[(CH<sub>2</sub>C<sub>6</sub>H<sub>4</sub>'Bu)<sub>2</sub>]<sup>2-</sup></th></tr></thead><tbody><tr><td><i>E</i><sub>Pauli</sub></td><td>3.24</td><td>2 × 2.47</td><td>3.65</td><td>2.40</td><td>2 × 1.96</td></tr><tr><td><i>E</i><sub>elstat</sub></td><td>-7.00</td><td>2 × -9.12</td><td>-7.96</td><td>-5.53</td><td>2 × -7.59</td></tr><tr><td><i>E</i><sub>orb</sub></td><td>-1.73</td><td>2 × -2.16</td><td>-1.84</td><td>-1.53</td><td>2 × -1.86</td></tr><tr><td><i>E</i><sub>disp</sub></td><td>-0.27</td><td>2 × -0.28</td><td>-0.30</td><td>-0.51</td><td>2 × -0.48</td></tr><tr><td><b>TBE<sup>a</sup></b></td><td>-5.77</td><td>2 × -9.10</td><td>-6.45</td><td>-5.17</td><td>2 × -7.97</td></tr></tbody></table>

## Concluding remarks

Through a combination of synthetic, crystallographic and NMR investigations, we have shown that the common calcium reagents [Ca{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub> (1) or [Ca{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>.(thf)<sub>2</sub> (1') react with "BuLi to generate [Ca]-"Bu species at low temperatures. In petroleum ether, 1 generates the heterobimetallic aggregate [Ca<sub>3</sub>(μ-"Bu)<sub>4</sub>{μ'-N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>4</sub>Li<sub>2</sub>] (5) upon reaction with 1.0-2.0 equivalents of "BuLi. This insight will be useful for those interested in reactivity studies in non-coordinating solvents, as often

<footnote><sup>a</sup> Total bonding energy (TBE) = *E*<sub>Pauli</sub> + *E*<sub>elstat</sub> + *E*<sub>orb</sub> + *E*<sub>disp</sub>. All values given in eV.</footnote>
required, for instance, for catalysis. By contrast, in thf, NMR spectroscopy has revealed that a first equivalent of "BuLi is consumed by 1' to produce the heteroleptic solvate [Ca("Bu){N(SiMe₃)₂}.(thf)₃] (6) at -83 °C. Upon warming to -35 °C, this complex evolves towards the alkylcalcium [Ca{N(SiMe₃)(SiMe₂-μ-CH₂)}.thf]₂ (7'), which crystallized as [Ca{N(SiMe₃)(SiMe₂-μ-CH₂)}.(thf)₃]₂ (7). It should be noted that 6 was able to selectively perform the halogen/calcium exchange of 2-bromothiophene, while 7 was not (Table 1, entries 2 and 3). NMR analysis indicates that further reaction of 6 with another equivalent of "BuLi at -83 °C (or the reaction of 1' with two equivalents of "BuLi) generates [Ca("Bu)₂.(thf)₄] (9) which could not be crystallized. DFT calculations confirm the existence of compounds 6 and 9. They indicate in particular that 9 should be (at least) equally stable as [Ca(p'-Bu-benzyl)₂(thf)₄].[91] We hence show here for the first time that these calcium derivatives of the ever-popular "BuLi reagent can be obtained if good control of the experimental conditions is maintained. Note that 9 not only reacted with 2-bromothiophene to afford the halogen/metal exchange product in high yield, but it also achieved the deprotocalciation of benzothiophene (Table 1, entries 1 and 4).

Although it can be complicated by the limited solubility of calcium iodide, we are confident that the reaction of crystalline [CaI₂.(thf)₄] (10) with equimolar "BuLi in thf at -83 °C yields the heteroleptic [Ca("Bu)(I).(thf)₂]₂ (11). However, for the time being, we cannot conclude as to the nature of the product formed with two equivalents of "BuLi, even if NMR data confirm the lithiated reagent is fully consumed. With regard to reactivity, 6 was found inert towards 2-bromothiophene in thf at low temperature, while the product formed with an additional equivalent of "BuLi proved reactive towards both 2-bromothiophene (Scheme S2 in ESI) and benzothiophene (Scheme S3 in ESI).

Considering the all-important role of structurally simple main group alkylmetal reagents, these findings should be particularly helpful to those with an interest in calcium chemistry, and in calcium-mediated catalyzed or functionalization reactions. The necessity to elucidate the precise nature of the species formed in solution cannot be emphasized enough, as highlighted in our preliminary reactivity foray. This initial proof of concept has allowed us to build an understanding of the chemistry of di-n-butylcalcium that can be exportable to broader purposes in synthetic organometallic and organic chemistry, and the simplicity of the protocol makes it usable by all. With this information in hand, we are now looking at optimizing our reactivity protocols, and at implementing this knowledge in enantioselective calcium-promoted reactions. Moreover, we are also assessing how this chemistry can be extrapolated to the heavier alkaline earth, barium. Preliminary results indicate that while access and handling of [Ca]-"butyl species are, as shown herein, reasonably easy, the barium analogues exhibit much greater reactivity and sensitivity. These results will be detailed in forthcoming reports.

## Data availability

All data related to this article can be found in the ESI document published with this article.†
**Author contributions**

G.D. synthesized all complexes, recorded and interpreted their analytical data. M.H. and H.O. proceeded with the NMR analyses. C.B. and W.E. performed the reactivity studies. M.C. carried out the XRD analyses. S.K. and J.-Y.S. performed the DFT calculations. Y.S. planned experimental work and performed data analysis. J.-Y.S, F.M., H.O., and Y.S. secured the funding and wrote the manuscript.

**Conflicts of interest**

There are no conflicts to declare.

**Acknowledgements**

The authors thank the French Agence Nationale de la Recherche for the provision of research grants to C.B., G.D. and M.H. (project *BiMeDep*, ANR-21-CE07-0045). This work has been partially supported by Université Rouen Normandie, INSA Rouen Normandie, Centre National de la Recherche Scientifique (CNRS), European Regional Development Fund (ERDF), Région Normandie, Labex SynOrg (ANR-11-LABX-0029), Carnot Institute I2C, and the graduate school for research XL-Chem (ANR-18-EUR-0020 XL CHEM).

**References**

[1] P. B. Hitchcock, M. F. Lappert, G. A. Lawless, B. Royo, *J. Chem. Soc. Chem. Commun.* **1990**, 1141–1142.

[2] M. Westerhausen, *Inorg. Chem.* **1991**, *30*, 96–101.

[3] S. Harder, Ed., *Early Main Group Metal Catalysis: Concepts and Reactions*, Wiley-VCH, Weinheim, Germany, **2020**.

[4] S. Harder, *Chem. Rev.* **2010**, *110*, 3852–3876.

[5] M. Arrowsmith, M. S. Hill, in *Compr. Inorg. Chem. II*, Elsevier, **2013**, pp. 1189–1216.

[6] M. S. Hill, D. J. Liptrot, C. Weetman, *Chem. Soc. Rev.* **2016**, *45*, 972–988.

[7] Y. Sarazin, P. M. Chapple, in *Compr. Organomet. Chem. IV* (Eds.: G. Parkin, K. Meyer, D. O’hare), Elsevier, Oxford, **2022**, pp. 104–192.

[8] P. S. Tanner, D. J. Burkey, T. P. Hanusa, *Polyhedron* **1995**, *14*, 331–333.

[9] M. R. Crimmin, A. G. M. Barrett, M. S. Hill, P. B. Hitchcock, P. A. Procopiou, *Inorg. Chem.* **2007**, *46*, 10410–10415.

[10] Y. Sarazin, D. Roșca, V. Poirier, T. Roisnel, A. Silvestru, L. Maron, J.-F. Carpentier, *Organometallics* **2010**, *29*, 6569–6577.

[11] W. D. Buchanan, M. A. Guino-o, K. Ruhlandt-Senge, *Inorg. Chem.* **2010**, *49*, 7144–7155.

[12] J. S. Alexander, K. Ruhlandt-Senge, *Eur. J. Inorg. Chem.* **2002**, *2002*, 2761–2774.

[13] A. Torvisco, A. Y. O’Brien, K. Ruhlandt-Senge, *Coord. Chem. Rev.* **2011**, *255*, 1268–1292.

[14] S. Krieck, P. Schüler, J. Peschel, M. Westerhausen, *Synthesis* **2019**, *51*, 1115–1122.

[15] S. Krieck, P. Schüler, H. Görls, M. Westerhausen, *Inorg. Chem.* **2018**, *57*, 13937–13943.

[16] P. Schüler, H. Görls, M. Westerhausen, S. Krieck, *Dalton Trans.* **2019**, *48*, 8966–8975.

[17] R. R. Fraser, T. S. Mansour, S. Savard, *J. Org. Chem.* **1985**, *50*, 3232–3234.

[18] A. M. Johns, S. C. Chmely, T. P. Hanusa, *Inorg. Chem.* **2009**, *48*, 1380–1384.

[19] S. Harder, C. Ruspic, *Eur. J. Inorg. Chem.* **2015**, *2015*, 5743–5750.

[20] C. Glock, H. Görls, M. Westerhausen, *Inorg. Chem.* **2009**, *48*, 394–399.

[21] M. Westerhausen, A. Koch, H. Görls, S. Krieck, *Chem. Eur. J.* **2017**, *23*, 1456–1483.

[22] M. Gärtner, H. Görls, M. Westerhausen, *Dalton Trans.* **2008**, 1574.
[23] A. Koch, M. Wirgenings, S. Krieck, H. Görls, G. Pohnert, M. Westerhausen, *Organometallics* **2017**, *36*, 3981–3986.

[24] M. Köhler, J. Langer, H. Görls, M. Westerhausen, *Organometallics* **2012**, *31*, 8647–8653.

[25] J. Langer, H. Görls, M. Westerhausen, *Organometallics* **2010**, *29*, 2034–2039.

[26] J. Langer, M. Köhler, R. Fischer, F. Dündar, H. Görls, M. Westerhausen, *Organometallics* **2012**, *31*, 6172–6182.

[27] J. Langer, S. Krieck, R. Fischer, H. Görls, M. Westerhausen, *Z. Anorg. Allg. Chem.* **2010**, *636*, 1190–1198.

[28] C. Loh, S. Seupel, H. Görls, S. Krieck, M. Westerhausen, *Organometallics* **2014**, *33*, 1480–1491.

[29] R. Fischer, M. Gärtner, H. Görls, M. Westerhausen, *Angew. Chem. Int. Ed.* **2006**, *45*, 609–612.

[30] P. Schüler, S. Sengupta, A. Koch, H. Görls, S. Krieck, M. Westerhausen, *Chem. Eur. J.* **2022**, *28*, e202201897.

[31] J. Langer, M. Köhler, J. Hildebrand, R. Fischer, H. Görls, M. Westerhausen, *Angew. Chem. Int. Ed.* **2013**, *52*, 3507–3510.

[32] F. Feil, S. Harder, *Organometallics* **2000**, *19*, 5010–5015.

[33] S. Harder, F. Feil, A. Weeber, *Organometallics* **2001**, *20*, 1044–1046.

[34] S. Harder, F. Feil, K. Knoll, *Angew. Chem. Int. Ed.* **2001**, *40*, 4261–4264.

[35] F. Feil, C. Müller, S. Harder, *J. Organomet. Chem.* **2003**, *683*, 56–63.

[36] K. Izod, P. G. Waddell, *Organometallics* **2015**, *34*, 2726–2730.

[37] B. Rösch, J. Martin, J. Eyselein, J. Langer, M. Wiesinger, S. Harder, *Organometallics* **2021**, *40*, 1395–1401.

[38] J. S. Alexander, K. Ruhlandt-Senge, *Angew. Chem. Int. Ed.* **2001**, *40*, 2658–2660.

[39] J. S. Alexander, K. Ruhlandt-Senge, *Chem. Eur. J.* **2004**, *10*, 1274–1280.

[40] E. Le Coz, H. Roueindeji, V. Dorcet, T. Roisnel, J.-F. Carpentier, Y. Sarazin, *Dalton Trans.* **2019**, *48*, 5500–5504.

[41] M. J. Harvey, T. P. Hanusa, V. G. Young, Jr., *Angew. Chem. Int. Ed.* **1999**, *38*, 217–219.

[42] P. Jochmann, T. S. Dols, T. P. Spaniol, L. Perrin, L. Maron, J. Okuda, *Angew. Chem. Int. Ed.* **2009**, *48*, 5715–5719.

[43] F. G. N. Cloke, P. B. Hitchcock, M. F. Lappert, G. A. Lawless, B. Royo, *J. Chem. Soc., Chem. Commun.* **1991**, 724–726.

[44] C. Eaborn, P. B. Hitchcock, *Chem. Commun.* **1997**, 1961–1962.

[45] M. R. Crimmin, A. G. M. Barrett, M. S. Hill, D. J. MacDougall, M. F. Mahon, P. A. Procopiou, *Chem. Eur. J.* **2008**, *14*, 11292–11295.

[46] K. Yan, B. M. Upton, A. Ellern, A. D. Sadow, *J. Am. Chem. Soc.* **2009**, *131*, 15110–15111.

[47] M. P. Coles, S. E. Sözerli, J. D. Smith, P. B. Hitchcock, I. J. Day, *Organometallics* **2009**, *28*, 1579–1581.

[48] O. Michel, C. Meermann, K. W. Törnroos, R. Anwander, *Organometallics* **2009**, *28*, 4783–4790.

[49] M. Westerhausen, C. Birg, H. Nöth, J. Knizek, T. Seifert, *Eur. J. Inorg. Chem.* **1999**, 2209–2214.

[50] B. M. Wolf, C. Stuhl, C. Maichle-Mössmer, R. Anwander, *J. Am. Chem. Soc.* **2018**, *140*, 2373–2383.

[51] I. V. Basalov, B. Liu, T. Roisnel, A. V. Cherkasov, G. K. Fukin, J.-F. Carpentier, Y. Sarazin, A. A. Trifonov, *Organometallics* **2016**, *35*, 3261–3271.

[52] X. Shi, C. Hou, C. Zhou, Y. Song, J. Cheng, *Angew. Chem. Int. Ed.* **2017**, *56*, 16650–16653.

[53] X. Shi, G. Qin, Y. Wang, L. Zhao, Z. Liu, J. Cheng, *Angew. Chem. Int. Ed.* **2019**, *58*, 4356–4360.

[54] P. M. Chapple, M. Cordier, V. Dorcet, T. Roisnel, J.-F. Carpentier, Y. Sarazin, *Dalton Trans.* **2020**, *49*, 11878–11889.

[55] N. Li, B.-T. Guan, *Eur. J. Inorg. Chem.* **2019**, 2231–2235.

[56] D. Seyferth, *Organometallics* **2006**, *25*, 2–24.

[57] D. Seyferth, *Organometallics* **2009**, *28*, 2–33.

[58] D. Seyferth, *Organometallics* **2009**, *28*, 1598–1605.

[59] A. S. S. Wilson, M. S. Hill, M. F. Mahon, C. Dinoi, L. Maron, *Science* **2017**, *358*, 1168–1171.

[60] K. G. Pearce, S. E. Neale, C. L. McMullin, M. F. Mahon, M. S. Hill, *Chem. Commun.* **2024**, 7882–7885.

[61] R. P. Davies, *Inorg. Chem. Commun.* **2000**, *3*, 13–15.
[62] A. R. Kennedy, R. E. Mulvey, R. B. Rowlings, *J. Organomet. Chem.* **2002**, *648*, 288-292.

[63] X. He, B. C. Noll, A. Beatty, R. E. Mulvey, K. W. Henderson, *J. Am. Chem. Soc.* **2004**, *126*, 7444-7445.

[64] I. R. Speight, S. C. Chmely, T. P. Hanusa, A. L. Rheingold, *Chem. Commun.* **2019**, *55*, 2202-2205.

[65] M. A. Guino-o, C. F. Campana, K. Ruhlandt-Senge, *Chem. Commun.* **2008**, 1692-1694.

[66] T. La Porta, R. Perrin, M. Ray, B. Gadenne, C. Dire, S. Carlotti, *Macromolecules* **2024**, *57*, 2376-2384.

[67] K. F. Tesh, D. J. Burkey, T. P. Hanusa, *J. Am. Chem. Soc.* **1994**, *116*, 2409-2417.

[68] C. Pampillón, J. Claffey, M. Hogan, M. Tacke, *Biometals* **2008**, *21*, 197-204.

[69] R. Fischer, M. Gärtner, H. Görls, L. Yu, M. Reiher, M. Westerhausen, *Angew. Chem. Int. Ed.* **2007**, *46*, 1618-1623.

[70] A. Koch, S. Krieck, H. Görls, M. Westerhausen, *Organometallics* **2017**, *36*, 2811-2817.

[71] R. D. Rogers, J. L. Atwood, R. Grüning, *J. Organomet. Chem.* **1978**, *157*, 229-237.

[72] P. P. Power, X. Xiaojie, *J. Chem. Soc., Chem. Commun.* **1984**, 358-359.

[73] B. Wei, L. Liu, W. Zhang, Z. Xi, *Angew. Chem. Int. Ed.* **2017**, *56*, 9188-9192.

[74] A. S. S. Wilson, M. S. Hill, M. F. Mahon, *Organometallics* **2019**, *38*, 351-360.

[75] H. Mack, G. Frenzen, M. Bendikov, M. S. Eisen, *J. Organomet. Chem.* **1997**, *549*, 39-43.

[76] L. Barr, A. R. Kennedy, J. G. MacLellan, J. H. Moir, R. E. Mulvey, P. J. A. Rodger, *Chem. Commun.* **2000**, 1757-1758.

[77] A. B. Chung, C. R. Stennett, W. N. G. Moore, M. Fang, J. W. Ziller, W. J. Evans, *Inorg. Chem.* **2023**, *62*, 5854-5862.

[78] R. P. Planalp, R. A. Andersen, *Organometallics* **1983**, *2*, 1675-1680.

[79] D. B. Collum, *Acc. Chem. Res.* **1993**, *26*, 227-234.

[80] R. Neufeld, D. Stalke, *Chem. Sci.* **2015**, *6*, 3354-3364.

[81] F. E. Romesberg, M. P. Bernstein, J. H. Gilchrist, A. T. Harrison, D. J. Fuller, D. B. Collum, *J. Am. Chem. Soc.* **1993**, *115*, 3475-3483.

[82] I. Fernández, E. Martínez-Viviente, F. Breher, P. S. Pregosin, *Chem. Eur. J.* **2005**, *11*, 1495-1506.

[83] M. Foroozandeh, R. W. Adams, M. Nilsson, G. A. Morris, *J. Am. Chem. Soc.* **2014**, *136*, 11867-11869.

[84] D. Massiot, F. Fayon, M. Capron, I. King, S. Le Calvé, B. Alonso, J.-O. Durand, B. Bujoli, Z. Gan, G. Hoatson, *Magn. Reson. Chem.* **2002**, *40*, 70-76.

[85] J. Langer, S. Krieck, H. Görls, M. Westerhausen, *Angew. Chem. Int. Ed.* **2009**, *48*, 5741-5744.

[86] J. F. McGarrity, C. A. Ogle, *J. Am. Chem. Soc.* **1985**, *107*, 1805-1810.

[87] I. Keresztes, P. G. Williard, *J. Am. Chem. Soc.* **2000**, *122*, 10228-10229.

[88] O. Eppers, H. Günther, *Helv. Chim. Acta* **1990**, *73*, 2071-2082.

[89] H. J. Reich, J. P. Borst, R. R. Dykstra, P. D. Green, *J. Am. Chem. Soc.* **1993**, *115*, 8728-8741.

[90] R. F. W. Bader, *Atoms in Molecules: A Quantum Theory*, Oxford University Press, **1990**.

[91] S. Harder, S. Müller, E. Hübner, *Organometallics* **2004**, *23*, 178-183.

[92] F. M. Bickelhaupt, E. J. Baerends, in *Rev. Comput. Chem.* (Eds.: K.B. Lipkowitz, D.B. Boyd), Wiley, **2000**, pp. 1-86.

[93] K. Morokuma, *J. Chem. Phys.* **1971**, *55*, 1236-1244.

[94] T. Ziegler, A. Rauk, *Inorg. Chem.* **1979**, *18*, 1558-1565.