Novel Organometallic Building Blocks for Crystal Engineering. Synthesis and Structural Characterization of the Dicarboxylic Acid [Cr0(η6-C6H5COOH)2], of Two Polymorphs of Its Oxidation Derivative [CrI(η6-C6H5COOH)2]+[PF6]-, and of the Zwitterionic Form [CrI(η6-C6H5COOH)(η6-C6H5COO)]

The synthesis and structural characterization of the neutral diamagnetic organometallic dicarboxylic acid [Cr0(η6-C6H5COOH)2] (1) is reported. Oxidation of 1 by oxygen leads to formation of the paramagnetic dicarboxylic acid cation [CrI(η6-C6H5COOH)2]+ (2), which has been isolated as the [PF6]- salt in two crystalline polymorphic modifications, a monoclinic form α and a triclinic form β. The neutral zwitterionic form [CrI(η6-C6H5COOH)(η6-C6H5COO)] (3) has been obtained in a cocrystal with [NH4][PF6] upon treatment of the acid salt [CrI(η6-C6H5COOH)2]+[PF6]-with ammonia. The hydrogen bonding interactions established by the neutral and cationic acids are discussed and compared with those observed in the zwitterionic form and in the related complexes [FeII(η5-C5H4COOH)2] and [CoIII(η5-C5H4COOH)2]+[PF6]-.