An Air-Stable Heterobimetallic Si2 M2 Tetrahedral Cluster.

Air- and moisture-stable heterobimetallic tetrahedral clusters [Cp(CO)2 MSiR]2 (M=Mo or W; R=SitBu3 ) were isolated from the reaction of N-heterocyclic carbene (NHC) stabilized silyl(silylidene) metal complexes Cp(CO)2 M=Si(SitBu3 )NHC with a mild Lewis acid (BPh3 ). Alternatively, treatment of the NHC-stabilized silylidene complex Cp(CO)2 W=Si(SitBu3 )NHC with stronger Lewis acids such as AlCl3 or B(C6 F5 )3 resulted in the reversible coordination of the Lewis acid to one of the carbonyl ligands. Computational investigations revealed that the dimerization of the intermediate metal silylidyne (M≡Si) complex to a tetrahedral cluster instead of a planar four-membered ring is due to steric bulk.

NHC-Stabilized Silyl-Substituted Chlorosilylene.

The first N-heterocyclic carbene (NHC) stabilized silyl-substituted chlorosilylene (1) was isolated via selective dehydrochlorination by NHC from silyl-based Si(IV) precursor tBu3SiSiHCl2. Compound 1 can form an iron chlorosilylene complex (2) with an iron carbonyl dimer and undergoes chloride/hydride metathesis to yield a stable NHC-silylene hydride borane adduct (3). Upon treatment with additional NHC, chlorosilylene 1 was converted into silyl-substituted silyliumylidene ions (4).

Reactivity of an NHC-stabilized pyramidal hydrosilylene with electrophilic boron sources.

Silylenes have become an indispensable tool for molecular bond activation. Their use for the construction of silicon-boron bonds is uncommon in comparison to the numerous studies on silylene-derived silicon-element bond formations. Herein we investigate the reactivity of the pyramidal NHC-coordinated hydrosilylene tBu3SiSi(H)LMe4 (1; NHC = N-heterocyclic carbene, LMe4 = 1,3,4,5-tetramethylimidazolin-2-ylidene) with various boron-centered electrophiles. The reaction of 1 with THF·BH3 or H3N→BH3 afforded the silylene complex 1→BH3 or the product of insertion of the silicon(ii) atom into an N-H bond with concomitant dehydrogenation along the HN-BH moiety (2). The respective conversion of 1 with BPh3 yields 1→BPh3 which readily reacts with excess LMe4 to form the more stable complex LMe4→BPh3 with release of 1. Treatment of 1 with the haloboranes Et2O→BF3, BCl3, BBr3 and Me2S→BBr3 resulted in the formation of the Lewis acid base adducts 1→BX3 (X = F, Cl, Br) and an equilibrium with their auto-ionization products [12BX2]+[BX4]- slowly develops. The ratio of 1→BX3 significantly increases with rising atomic number of the halide, thus 1→BF3 majorly transforms within hours while 1→BBr3 is near-quantitatively retained over time. Accordingly, the complex 1→BPhBr2 was isolated after conversion of 1 with PhBBr2.

Systematic Study of N-Heterocyclic Carbene Coordinate Hydrosilylene Transition-Metal Complexes.

An in-depth study of the synthesis and structures of N-heterocyclic carbene (NHC)-stabilized silylene transition-metal complexes is reported. An iron hydrosilylene complex, [tBu3Si(NHC)(H)Si:→Fe(CO)4] (2), was synthesized starting from the corresponding hydrosilylene [tBu3Si(NHC)(H)Si:] (1). Complex 2 was fully characterized, including X-ray diffraction analysis, which showed an unusual long Si-Fe bond length. A very long bond length was also observed in the novel hydrosilylene tungsten complex [tBu3Si(NHC)(H)Si:→W(CO)5] (3). A series of NHC-stabilized silylene iron complexes ([R2(NHC)Si:→Fe(CO)4], where R = Cl (4), H (5), and Me (6)) were synthesized and fully characterized to investigate the influence of different substituents. The dihydrosilylene iron complex [H2(NHC)Si:→Fe(CO)4] (5) represents a new example of a donor-acceptor-stabilized parent silylene (H2Si:). Density functional theory calculations were utilized to understand the influence of the electronic and steric effects of the silylene unit and its substituents on the Si-Fe bond in these iron complexes, in particular to rationalize the long Si-Fe bond in 2.