The metal-carbon stretching frequencies in methyl complexes of Rh, Ir, Ga and In with porphyrins and a tetradentate pyridine-amide ligand

Interactions of trans-[Rh(bpb)(CH3)(H2O)] (1) [bpb=1,2-bis(2-pyridinecarboxamido)benzene] with Lewis bases L afford the respective adducts trans-[Rh(bpb)(CH3)(L)], where L=4-substituted pyridine 4-Xpy (X=H (2),tBu (3), NMe2(4) CN (5)), PMe2Ph (6) or benzimidazole (7). The structures of complexes 3 and 6 have been established by X-ray crystallography. The Rh-C distance in complex 6 (2.095(6) Å) is longer than that in complex 3 (2.02(1) Å), indicating that PMe2Ph has a stronger trans influence than 4-tBupy. The metal-carbon stretching frequencies for trans-[Rh(bpb)(CH3)(L)] and [M(TTP)(CH3)] [TTP=5,10,15,20-tetrakis(4-methylphenyl)porphyrin dianion; M=Rh, Ir, Ga, In] have been determined by near IR FT-Raman spectroscopy. Complex 1 exhibits ν(Rh-C) at 562 cm-1, which downshifts to 532 cm-1upon deuteriation of the axial methyl group. Replacement of the aquo ligand in complex 1 with nitrogen ligands or phosphine resulted in downshift in ν(Rh-C). The ν(Rh-C) for trans-[Rh(bpb)(CH3)(L)] was found to decrease in the order L: PMe2Ph≫4-Xpy~BzIm>H2O, consistent the order of trans influence of L. For [M(TTP)(CH3)] (M=Co, Rh, Ir, Ga, or In) the M-C force constant was found to decrease in the orders Ir>Rh>Co and Ga>In, consistent with the trends of metal-carbon bond strength for these metals. For trans-[Rh(bpb)(CH3)(4-Xpy)] and trans-[Rh(TTP)(CH3)(4-Xpy)], the ν(Rh-C) were found to be not very sensitive to the nature of X, suggesting that the electronic factors of the axial pyridine ligand do not have a significant effect on the Rh-C bonds for these rhodium alkyl complexes.