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

The preparation of
osmium­(II) complexes containing a CC6H4,CBzIm,CBzIm,CC6H4-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(PiPr3)2 (1) reacts with 1,1′-diphenyl-3,3′-ethylenedibenzimidazolium
bromide in dimethylformamide to afford OsBr­{κ3-C,C,C-(C6H4-BzIm-CH2CH2-BzIm-Ph)}­(CO)­(PiPr3)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{κ3-C,C,C-(C6H4-BzIm-CH2CH2-BzIm-Ph)}­(CO)­(PiPr3)2 (3), which evolves
into Os­{κ4-C,C,C,C-(C6H4-BzIm-CH2CH2-BzIm-C6H4)}­(CO)­(PiPr3)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{κ4-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{κ4-C,C,C,C-(C6H4-BzIm-CH2CH2-BzIm-C6H4)}­L2 (L2 = 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{κ2-C,C-(MeBzIm-C6H4)}2(dppbz)
(13) and Os­{κ2-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.