Synthesis of novel carbene-stabilized silylenes and 1,3,2-diazaborole-derived carbene complexes of boron
Hickox, Hunter Patrick
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The investigation of novel silylene (:SiR2) species is presented herein. 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 (72). Further reaction of 72 with an additional equivalent of Fe(CO)5 at raised temperatures affords the carbene-stabilized silylene-iron carbonyl cluster, Si[μ-Fe2(CO)6](μ-CO)Si (73), through the insertion of a CO and a Fe2(CO)6 unit into the Si=Si double bond. Each silylene center in 73 is covalently bonded to one Fe(CO)3 center, and datively bonded to the other Fe(CO)3 using the silicon-based electron lone pair. Notably, compound 73 represents the first example of direct cleavage of a Si=Si double bond by a transition metal species. Subsequently, the reactivity of complex 72 was studied. The reaction of 72 with HCl·NC5H5 yields a “push-pull” stabilized parent monochlorosilylene [:Si(H)Cl] (78), with addition of an HCl unit to each silicon atom, and both silicon-based electron lone pairs coordinating a central Fe(CO)3. Importantly, compound 78 is the first stabilized parent monochlorosilylene isolated at ambient conditions. In addition, the reaction of carbene-stabilized diiodo-bis-silylene with an imidazole-based thiolate ligand in toluene or THF gives a five-membered (82) and four-membered (83) cyclic silylene via unexpected C–H and C–N bond activation, respectively. Compounds 82 and 83 represent the first cyclic silylenes containing a silicon-silicon bond, and compound 83 is the first example of silicon(I)-mediated C–N bond cleavage of N-heterocyclic carbenes (NHCs). Recently, the synthesis of 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 (87) and BI3 (88) complexes in quantitative yield (by 1H NMR). Interestingly, both 87 and 88 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 87 favors the formation of the reactants, while the equilibrium of 88 favors the formation of the products, which could be ascribed to the relative Lewis acidity of BBr3 when compared to BI3.