Synthesis of Carbene-Stabilized Silylenes and 1,3,2-Diazaborole-Derived Carbene Complexes

The investigation of novel silylene (:SiR2) species will be presented. The reaction of carbene-stabilized disilicon(0) with one equivalent of Fe(CO)5 at room temperature yields the carbene-stabilized Si2Fe(CO)4 complex (1). Further reaction of 1 with an additional equivalent of Fe(CO)5 at raised temperatures affords carbene-stabilized silylene-iron carbonyl cluster, Si[μ-Fe2(CO)6](μ-CO)Si (2), through the insertion of a CO and a Fe2(CO)6 unit into the Si=Si double bond. Notably, compound 2 represents the first example of direct cleavage of a Si=Si double bond by a transition metal species. Subsequently, the reactivity of complex 1 was studied. The reaction of 1 with HCl·NC5H5 yields a “push-pull” stabilized parent monochlorosilylene (:SiHCl) (3), with addition of an HCl unit to each silicon atom, and both silicon-based electron lone pairs coordinating a central Fe(CO)3 unit. Importantly, compound 3 is the first “push-pull” stabilized parent monochlorosilylene at ambient conditions. In addition, the reaction of carbene-stabilized diiodo-bis-silylene with an imidazole-based thiolate in toluene or THF gives a five-membered (4) and four-membered (5) cyclic silylene, formed via C–H and C–N bond activation, respectively. Compounds 4 and 5 represent the first cyclic silylenes that contain a silicon-silicon bond, and compound 5 is the first example of silicon(I)-mediated C–N bond cleavage of n-heterocyclic carbenes (NHCs). Recently, the synthesis of a 1,2-azaborole-derived cyclic (alkyl)(amino)carbene (CAAC)-borane adducts were reported, via a 1,2-hydrogen migration. In an effort to extend this chemistry to the diazocyclo-borole system, we reported the synthesis of 1,3,2-diazaborole-derived NHC-boron halide complexes, via a 1,2-hydrogen migration. A reaction of 2-bromo-1,3,2-diazaborole with excess BX3 (X = Br, I) affords the BBr3 (6) and BI3 (7) complexes in quantitative yield (by 1H NMR). Interestingly, both 6 and 7 are in equilibrium in solution, likely due to the weak electron-donating properties of the 1,3,2-diazaborole-derived carbene. However, the equilibrium of 6 favors the formation of the reactants, while the equilibrium of 7 favors the formation of the products, which could be ascribed to the relative Lewis acidity of BBr3 when compared to BI3.