New complexes of chromium(III) containing organic π-radical ligands: an experimental and density functional theory study.

The electronic structures of a series of chromium complexes 1-7 have been experimentally investigated using a combination of X-ray crystallography, magneto- and electrochemistry, and Cr K-edge X-ray absorption and UV-vis spectroscopies. Reaction of the dimer [Cr(II)2(μ-CH3CO2)4](0) with 2,2'-bipyridine (bpy(0)) produced the complex [Cr(III)(bpy(0))(bpy(•))(CH3CO2)2](0) (S = 1) (1), but in the presence of isopropylamine ((i)PrNH2) [Cr(III)(bpy(•))((i)PrNH2)2(CH3CO2)2](0) (S = 1) (2) was obtained. Both 1 and 2 contain a Cr(III) ion and a single (bpy(•))(1-) ligand, so are not low-spin Cr(II) species. One-electron oxidation of 1 and 2 yielded [Cr(III)(bpy(0))2(CH3CO2)2]PF6 (S = 3/2) (3) in both cases. In addition, the new neutral species [Cr(III)(DAD(•))3](0) (S = 0) (4) and [Cr(III)(CF3AP(•))3](0) (S = 0) (5) have been synthesized. Both complexes contain three π-radical anion ligands, which derive from one electron reduction of 1,4-bis(cyclohexyl)-1,4-diaza-1,3-butadiene and one electron oxidation of 2-(2-trifluoromethyl)-anilino-4,6-di-tert-butylphenolate, respectively. Intramolecular antiferromagnetic coupling to d(3) Cr(III) gives the observed singlet ground states. Reaction of [Cr(II)(CH3CN)6](PF6)2 with 2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine (PDI(0)) under anaerobic conditions affords dark brown microcrystals of [Cr(III)(PDI(0))(PDI(•))](PF6)2 (S = 1) (6). This complex is shown to be a member of the electron transfer series [Cr(III)(PDI)2](3+/2+/1+/0), in which all one-electron transfer processes are ligand-based. By X-ray crystallography, it was shown that 6 possesses a localized electronic structure, such that one ligand is neutral (PDI(0)) and the other is a π-radical monoanion (PDI(•))(1-). Again, it should be highlighted that 6 is not a Cr(II) species. Lastly, the structure of [Cr(III)((Me)bpy(•))3](0) (S = 0) (7, (Me)bpy = 4,4'-dimethyl-2,2'-bipyridine) has been established by high resolution X-ray crystallography and clearly shows that three ((Me)bpy(•))(1-) radical anions are present. To further validate our electronic structure assignments, complexes 1-6 were investigated computationally using density functional theory (DFT) and found in all cases to contain a Cr(III) ion. This oxidation state assignment was experimentally confirmed for complexes 2, 4, 5, and 6 by Cr K-edge X-ray absorption spectroscopy.