Oxidative Decarbonylation of <i>m</i>-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 <i>m</i>-terphenyl isocyanide ligand CNAr<sup>Dipp2</sup> (Ar<sup>Dipp2</sup> = 2,6-(2,6-(<i>i</i>-Pr)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>). These studies represent an effort to access halide or pseudohalide M/CNAr<sup>Dipp2</sup> 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 <i>trans-</i>W(CO)<sub>4</sub>(CNAr<sup>Dipp2</sup>)<sub>2</sub> and <i>trans-</i>W(NCMe)(CO)<sub>3</sub>(CNAr<sup>Dipp2</sup>)<sub>2</sub> are reported. The acetonitrile adducts <i>trans-</i>M(NCMe)(CO)<sub>3</sub>(CNAr<sup>Dipp2</sup>)<sub>2</sub> (M = Mo, W) react with I<sub>2</sub> 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<sub>2</sub>(CO)<sub>2</sub>(CNAr<sup>Dipp2</sup>)<sub>2</sub> is generated, which has an <i>S</i> = 1 ground state in solution. Paramagnetic group 6 MX<sub>2</sub>L<sub>4</sub> complexes are rare, and the structure of MoI<sub>2</sub>(CO)<sub>2</sub>(CNAr<sup>Dipp2</sup>)<sub>2</sub> is discussed in relation to other diamagnetic and <i>C</i><sub>2<i>v</i></sub>-distorted MX<sub>2</sub>L<sub>4</sub> complexes. Diiodide MoI<sub>2</sub>(CO)<sub>2</sub>(CNAr<sup>Dipp2</sup>)<sub>2</sub> reacts further with I<sub>2</sub> to effect complete decarbonylation, producing the paramagnetic tetraiodide complex <i>trans</i>-MoI<sub>4</sub>(CNAr<sup>Dipp2</sup>)<sub>2</sub>. The reactivity of the <i>trans</i>-M(NCMe)(CO)<sub>3</sub>(CNAr<sup>Dipp2</sup>)<sub>2</sub> (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 <i>trans</i>-MoI<sub>4</sub>(CNAr<sup>Dipp2</sup>)<sub>2</sub> toward Mg metal and sodium amalgam is studied. In benzene solution under N<sub>2</sub>, <i>trans</i>-MoI<sub>4</sub>(CNAr<sup>Dipp2</sup>)<sub>2</sub> is reduced by Na/Hg to the η<sup>6</sup>-arene-dinitrogen complex, (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)Mo(N<sub>2</sub>)(CNAr<sup>Dipp2</sup>)<sub>2</sub>. The diiodide-η<sup>6</sup>-benzene complex (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)MoI<sub>2</sub>(CNAr<sup>Dipp2</sup>)<sub>2</sub> 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.