Oxidative decarbonylation of m-terphenyl isocyanide complexes of molybdenum and tungsten: Precursors to low-coordinate isocyanide complexes

摘要

Synthetic studies are presented addressing the oxidative decarbonylation of molybdenum and tungsten complexes supported by the encumbering m-terphenyl isocyanide ligand CNAr~(Dipp2) (Ar~(Dipp2) = 2,6-(2,6-(i-Pr)_2C_6H_3)_2C _6H_3). These studies represent an effort to access halide or pseudohalide M/CNAr~(Dipp2) species (M = Mo, W) for use as precursors to low-coordinate, low-valent group 6 isocyanide complexes. The synthesis and structural chemistry of the tetra- and tricarbonyl tungsten complexes trans-W(CO)_4(CNAr~(Dipp2))_2 and trans-W(NCMe)(CO)_3(CNAr~(Dipp2))_2 are reported. The acetonitrile adducts trans-M(NCMe)(CO)_3(CNAr~(Dipp2)) _2 (M = Mo, W) react with I_2 to form divalent, diiodide complexes in which the extent of decarbonylation differs between Mo and W. In the molybdenum example, the diiodide, dicarbonyl complex MoI_2(CO) _2(CNAr~(Dipp2))_2 is generated, which has an S = 1 ground state in solution. Paramagnetic group 6 MX_2L_4 complexes are rare, and the structure of MoI_2(CO) _2(CNAr~(Dipp2))_2 is discussed in relation to other diamagnetic and C_(2v)-distorted MX_2L_4 complexes. Diiodide MoI_2(CO)_2(CNAr~(Dipp2))_2 reacts further with I_2 to effect complete decarbonylation, producing the paramagnetic tetraiodide complex trans-MoI_4(CNAr ~(Dipp2))_2. The reactivity of the trans-M(NCMe)(CO) _3(CNAr~(Dipp2))_2 (M = Mo, W) complexes toward benzoyl peroxide is also surveyed, and it is shown that dicarboxylate complexes can be obtained by oxidative or salt-elimination routes. The reduction behavior of the tetraiodide complex trans-MoI_4(CNAr~(Dipp2)) _2 toward Mg metal and sodium amalgam is studied. In benzene solution under N_2, trans-MoI_4(CNAr~(Dipp2))_2 is reduced by Na/Hg to the η~6-arene-dinitrogen complex, (η~6-C_6H_6)Mo(N_2)(CNAr ~(Dipp2))_2. The diiodide-η~6-benzene complex (η~6-C_6H_6)MoI_2(CNAr ~(Dipp2))_2 is an isolable intermediate in this reduction reaction, and its formation and structure are discussed in context of putative low-coordinate, low-valent molybdenum isocyanide complexes.