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 [CrII2(μ-CH3CO2)4]0 with 2,2′-bipyridine (bpy0) produced the complex [CrIII(bpy0)­(bpy•)­(CH3CO2)2]0 (S = 1) (1), but in the presence of isopropylamine (iPrNH2) [CrIII(bpy•)­(iPrNH2)2(CH3CO2)2]0 (S = 1) (2) was obtained. Both 1 and 2 contain a CrIII ion and a single (bpy•)1– ligand, so are not low-spin CrII species. One-electron oxidation of 1 and 2 yielded [CrIII(bpy0)2(CH3CO2)2]­PF6 (S = 3/2) (3) in both cases. In addition, the new neutral species [CrIII(DAD•)3]0 (S = 0) (4) and [CrIII(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 d3 CrIII gives the observed singlet ground states. Reaction of [CrII(CH3CN)6]­(PF6)2 with 2,6-bis­[1-(4-methoxyphenylimino)­ethyl]­pyridine (PDI0) under anaerobic conditions affords dark brown microcrystals of [CrIII(PDI0)­(PDI•)]­(PF6)2 (S = 1) (6). This complex is shown to be a member of the electron transfer series [CrIII(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 π-radical monoanion (PDI•)1–. Again, it should be highlighted that 6 is not a CrII species. Lastly, the structure of [CrIII(Mebpy•)3]0 (S = 0) (7, Mebpy = 4,4′-dimethyl-2,2′-bipyridine) has been established by high resolution X-ray crystallography and clearly shows that three (Mebpy•)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 CrIII ion. This oxidation state assignment was experimentally confirmed for complexes 2, 4, 5, and 6 by Cr K-edge X-ray absorption spectroscopy.