Redox Chemistry, Acid Reactivity, and Hydrogenation Reactions of Two-Electron Mixed Valence Diiridium and Dirhodium Complexes

The syntheses and reaction chemistry of two electron mixed-valence diphosphazane-bridged dirhodium and diiridium complexes M20,II(tfepma)2(CNtBu)2Cl2 [M = Rh (1), Ir (2); tfepma = MeN[P(OCH2CF3)2]2, CNtBu = tert-butyl isocyanide] are described. 1 and 2 undergo addition and two-electron oxidation and reduction chemistries. In the presence of CNtBu, the addition product with the stoichiometry M20,II(tfepma)2(CNtBu)3Cl2 [M = Rh (3), Ir (3)] is generated; in the presence of 1 equiv of CNtBu and 2 equiv of bis(pentamethyl-cyclopentadienyl)cobalt(II), 1 and 2 are reduced to furnish M20,0(tfepma)2(CNtBu)3 [M = Rh (5), Ir (6)], which feature both four- and five-coordinate M0 centers. Complexes 1, 2, 5, and 6 all possess coordinatively unsaturated square planar M0 centers that are reactive: (1) 2 reacts with PhICl2 to produce Ir2II,II(tfepma)2(CNtBu)2Cl4 (7); (2) protonation of 2 with HX yields Ir2II,II(tfepma)2(CNtBu)2Cl2HX [X = Cl– (8), OTs– (9)]; (3) protonation of 5 with HOTs produces [Rh2I,I(tfepma)2(CNtBu)3(μ-H)](OTs); and (4) the reversible hydrogenation of 2 proceeds smoothly, furnishing the cis-dihydride complex Ir2II,II(tfepma)2(CNtBu)2(H)2Cl2 (11). Substitution of tfepma in 2 with bis(diphenylphsophino)methane (dppm) yields the orthometalated complex Ir2II,II(dppm)(PPh(o-C6H4)CH2PPh2)(CNtBu)2Cl2H (12). The X-ray crystal structures of 11 compounds are presented and discussed, and spectroscopic characterization by multinuclear and variable temperature NMR provides details about solution structures and in some cases the formation of isomeric products. The electronic spectra of the new complexes are also described briefly, with absorption and emission features derived from the bimetallic core.