Oxidative Decarbonylation of m-Terphenyl Isocyanide Complexes of Molybdenum and Tungsten: Precursors to Low-Coordinate Isocyanide Complexes

Synthetic studies are presented addressing the oxidative decarbonylation of molybdenum and tungsten complexes supported by the encumbering m-terphenyl isocyanide ligand CNArDipp2 (ArDipp2 = 2,6-(2,6-(i-Pr)2C6H3)2C6H3). These studies represent an effort to access halide or pseudohalide M/CNArDipp2 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(CNArDipp2)2 and trans-W(NCMe)(CO)3(CNArDipp2)2 are reported. The acetonitrile adducts trans-M(NCMe)(CO)3(CNArDipp2)2 (M = Mo, W) react with I2 to form divalent, diiodide complexes in which the extent of decarbonylation differs between Mo and W. In the molybdenum example, the diiodide, dicarbonyl complex MoI2(CO)2(CNArDipp2)2 is generated, which has an S = 1 ground state in solution. Paramagnetic group 6 MX2L4 complexes are rare, and the structure of MoI2(CO)2(CNArDipp2)2 is discussed in relation to other diamagnetic and C2v-distorted MX2L4 complexes. Diiodide MoI2(CO)2(CNArDipp2)2 reacts further with I2 to effect complete decarbonylation, producing the paramagnetic tetraiodide complex trans-MoI4(CNArDipp2)2. The reactivity of the trans-M(NCMe)(CO)3(CNArDipp2)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-MoI4(CNArDipp2)2 toward Mg metal and sodium amalgam is studied. In benzene solution under N2, trans-MoI4(CNArDipp2)2 is reduced by Na/Hg to the η6-arene-dinitrogen complex, (η6-C6H6)Mo(N2)(CNArDipp2)2. The diiodide-η6-benzene complex (η6-C6H6)MoI2(CNArDipp2)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.