Variable Coordination Mode of Chloranilic Acid. Synthesis, Structure, and Electrochemical Properties of Some Osmium Complexes

Reaction of chloranilic acid (H2ca) with [Os(bpy)2Br2] (bpy = 2,2‘-bipyridine) affords a dinuclear complex of type [{Os(bpy)2}2(ca)]2+, isolated as the perchlorate salt. A similar reaction of H2ca with [Os(PPh3)2(pap)Br2] (pap = 2-(phenylazo)pyridine) affords a dinuclear complex of type [{Os(PPh3)2(pap)}2(ca)]2+ (isolated as the perchlorate salt) and a mononuclear complex of type [Os(PPh3)2(pap)(ca)]. Reaction of H2ca with [Os(PPh3)2(CO)2(HCOO)2] gives a dinuclear complex of type [{Os(PPh3)2(CO)2}2(r-ca)], where r-ca is the two electron reduced form of the chloranilate ligand. The structures of the [{Os(PPh3)2(pap)}2(ca)](ClO4)2, [Os(PPh3)2(pap)(ca)], and [{Os(PPh3)2(CO)2}2(r-ca)] complexes have been determined by X-ray crystallography. In the [{Os(bpy)2}2(ca)]2+ and [{Os(PPh3)2(pap)}2(ca)]2+ complexes, the chloranilate dianion is serving as a tetradentate bridging ligand. In the [Os(PPh3)2(pap)(ca)] complex, the chloranilate dianion is serving as a bidentate chelating ligand. In the [{Os(PPh3)2(CO)2}2(r-ca)] complex, the reduced form of the chloranilate ligand (r-ca4-) is serving as a tetradentate bridging ligand. All the four complexes are diamagnetic and show intense metal-to-ligand charge-transfer transitions in the visible region. The [Os(PPh3)2(pap)(ca)] complex shows an Os(II)−Os(III) oxidation, followed by an Os(III)−Os(IV) oxidation on the positive side of a standard calomel electrode. The three dinuclear complexes show two successive oxidations on the positive side of SCE. The mixed-valent OsII−OsIII species have been generated in the case of the two chloranilate-bridged complexes by coulometric oxidation of the homovalent OsII−OsII species. The mixed-valent OsII−OsIII species show intense intervalence charge-transfer transitions in the near-IR region.