Direct Transformation of SiH₄ to a Molecular L(H)₂Co═Si═Co(H)₂L Silicide Complex

Abstract

The synthesis of bimetallic molecular silicide complexes is reported, based on the use of multiple Si–H bond activations in SiH4 at the metal centers of 14-electron LCoI fragments (L = Tp″, HB(3,5-diisopropylpyrazolyl)3 –; [BP2 tBuPz], PhB(CH2P t Bu2)2(pyrazolyl)). Upon exposure of (Tp″Co)2(μ-N2) (1) to SiH4, a mixture of (Tp″Co)2(μ-H) (2) and (Tp″Co)2(μ-H)2 (3) was formed and no evidence for Si–H oxidative addition products was observed. In contrast, [BP2 tBuPz]-supported Co complexes led to Si–H oxidative additions with the generation of silylene and silicide complexes as products. Notably, the reaction of ([BP2 tBuPz]Co)2(μ-N2) (5) with SiH4 gave the dicobalt silicide complex [BP2 tBuPz](H)2CoSiCo(H)2[BP2 tBuPz] (8) in high yield, representing the first direct route to a symmetrical bimetallic silicide. The effect of the [BP2 tBuPz] ligand on Co–Si bonding in 7 and 8 was explored by analysis of solid-state molecular structures and density functional theory (DFT) investigations. Upon exposure to CO or DMAP (DMAP = 4-dimethylaminopyridine), 8 converted to the corresponding [BP2 tBuPz]Co(L)x adducts (L = CO, x = 2; L = DMAP, x = 1) with concomitant loss of SiH4, despite the lack of significant Si–H interactions in the starting complex. On heating to 60 °C, 8 underwent reaction with MeCl to produce small quantities of Me x SiH4–x (x = 1–3), demonstrating functionalization of the μ-silicon atom in a molecular silicide to form organosilanes.

A [CoSiH2] Silylene Synthon Provides Modular Access to Homo- and Heterobimetallic [Co═Si═M] (M = Co, Fe) Silicide Complexes

“…A darkening of the solution's color was immediately apparent, and 1 H and 31 ), was generated in an analogous fashion by treatment of 1 with 0.5 equiv of (Tp″Co) 2 (μ-N 2 ). 17 A color change of the reaction mixture from dark brown to blue was observed upon addition of (Tp″Co) 2 (μ-N 2 ), and multinuclear NMR spectroscopy ( 1 H, 31 P{ 1 H}) indicated formation of a single diamagnetic species (in addition to DMAP) possessing features consistent with [BP 3 iPr ](H) 2 Co�Si�Co(H) 2 Tp″ (4; Scheme 4). Complex 4 was isolated as an analytically pure dark-blue powder, following precipitation from a concentrated diethyl ether solution.…”

“…For example, reaction of (Tp″Co) 2 (μ-N 2 ) with SiH 4 was found to generate mixtures containing (Tp″Co) 2 (μ-H) 1,2 as opposed to a silicide product. 17 This result implies that the presence of a bulky [BP 19 or a mixture of 2 and 5 (both in ∼20% yield) 39 is produced as the major siliconcontaining product, respectively. Thus, 1 avoids degradation pathways involving the [BP 3 iPr ] ligand, such as −CH 2 P i Pr 2 side-arm migration and coupling to generate [H 2 SiCH 2 P i Pr 2 ] fragments, as seen in 2.…”

“…These 29 Si chemical shifts span a well-defined range that is diagnostic for this family of 3d-metal silicides and is consistent with structurally related, previously reported silicides. 17,19 Notably, the 4d-and 5d-metal silicides in the Tp*(OC) 2 M�Si−M(CO) 2 (L)Tp* class (M = Mo, W) generally possess lower-field 29 Si resonances (δ 396− 439). 16,18,27 Comparison of the solid-state molecular structures of 3, 5, and 7 reveal similar geometries for the [(H) 2 M�Si�M′(H) 2 ] cores (M = Co, M′ = Co, Fe).…”

“…16 In addition, our laboratories reported the formation of [M�Si�M] tBu Pz] = PhB(CH 2 P t Bu 2 ) 2 (pyrazolyl) − ). 17 The [MSiM] cores of these silicides react in unprecedented ways; for example, the μsilicon center of Tp*(OC) 2 Mo�Si−Mo(CO) 2 (PMe 3 )Tp* binds alkynes to generate adducts incorporating planar, tetracoordinate silicon in [(RCCR')SiMo 2 ] units. 18 In addition, reaction of [BP 2…”

“…tBu Pz] with MeCl produced small quantities of functionalized silane products Me x SiH 4−x (x = 1−3). 17 Despite these recent advances, molecular silicide chemistry remains largely unexplored, mainly due to the absence of synthetic methods that provide control over stoichiometry and structural features (e.g., symmetry, coordination numbers, metal identities, etc.). The results reported here demonstrate the versatile, modular assembly of complex silicide structures by way of a synthon for the simplest terminal silylene complex, the adduct [BP − ; DMAP = 4-dimethylaminopyridine).…”