Osmium(II) Complexes Containing a Dianionic CCCC-Donor Tetradentate Ligand

The preparation of osmium(II) complexes containing a C-C6H4,C-BzIm,C-BzIm,C-C6H4-tetradentate ligand with a-CH2CH2- link between the benzimidazolidene (BzIm) groups is described, and the influence of the link on their structures and emissive properties is analyzed. The hexahydride complex OsH6((PPr3)-Pr-i)(2) (1) reacts with 1,1'-diphenyl-3,3'-ethylenedibenzimidazolium bromide in dimethylformamide to afford OsBr{kappa(3)-C,C,C-(C6H4-BzIm-CH2CH2-BzIm-Ph)}(CO)((PPr3)-Pr-i)(2) (2), as a result of the direct metalation of both benzimidazolium moieties of the salt, the o-CH bond activation of a phenyl substituent, and the carbonylation of the metal center by action of the solvent. Treatment of 2 with Na[BH4] leads to the hydride derivative OsH{kappa(3)-C,C,C-(C6H4-BzIm-CH2CH2-BzIm-Ph)}(CO)((PPr3)-Pr-i)(2) (3), which evolves into Os{kappa(4)-C,C,C,C-(C6H4-BzIm-CH2CH2-BzIm-C6H4)}(CO)((PPr3)-Pr-i)(2) (4) as a consequence of the assisted C-H bond activation of the second phenyl substituent. The use of dimethyl sulfoxide instead of dimethylformamide allows the generation of the tetradentate ligand in one pot. Stirring of dimethyl sulfoxide solutions of 1 leads to Os{kappa(4)-C,C,C,C-(C6H4-BzIm-CH2CH2-BzIm-C6H4)}(DMSO)(2) (5). The solvent molecules of 5 can be displaced by 1,2-bis(diphenylphosphino)ethylene (bdppe), 1,2-bis(diphenylphosphino)benzene (dppbz), and tetrafluorobenzobarrelene (TFB) to yield the [4 + 2] derivatives Os{kappa(4)-C,C,C,C(C6H4-BzIm-CH2CH2-BzIm-C6H4)}L-2 (L-2 = bdppe (6), dppbz (7), TFB (8)). The X-ray structures of 4, 6, and 7 reveal that, in these compounds, the tetradentate ligand adopts a mer three-planar disposition of a phenyl and two benzimidazolidene groups with the other phenyl group situated in a perpendicular direction disposed trans to L. The reason for this preference is electronic. Thus, the stereochemistry of 7 is the same as that found in the related complexes Os{kappa(2)-C,C-(MeBzIm-C6H4)}(2)(dppbz) (13) and Os{kappa(2)-C,C-(MeBzIm*-C6H4)}2(dppbz) (14) containing two free ortho-metalated N-phenylbenzimidazolidene ligands. Complexes 7, 13, and 14 are phosphorescent. The first of them shows emissions narrower and bluer than those of 13 and 14.