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.