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-2(II)(mu-CH3CO2)(4)](0) with 2,2'-bipyridine (bpy(0)) produced the complex [Cr-III(bpy(0))(bpy*)(CH3CO2)(2)] (S = 1) (1), but in the presence of isopropylamine ((PrNH2)-Pr-i) [Cr-III(bpy*)-((PrNH2)-Pr-i)(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)(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(CF(3)AP*)(3)](0) (S = 0) (5) have been synthesized. Both complexes contain three pi-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 (PDI0) under anaerobic conditions affords dark brown microcrystals of [Cr-III(PDI0)(PDI*)](PF6)(3) (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 (PDI0) and the other is a pi-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.