(α-diimine)chromium complexes:: Molecular and electronic structures;: A combined experimental and density functional theoretical study

Dark brown crystals of [Cr(L-1)(2)] (1) were obtained from the reaction of [Cr-III(acac)(3)] (acac(-) = 2,4-pentanedionate) with 2 equiv of 2-methyl-1,4-bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene (L-1) and 3 equiv of sodium in tetrahydrofuran (thf) under an Ar atmosphere. Complex 1 possesses an S = 1 ground state, which is attained via intramolecular antiferromagnetic coupling between a high-spin Cr-II ion (S-Cr = 2) and two anionic alpha-diiminato(1-) ligand pi radicals. The molecular structure of (L-1(center dot))(1-) exhibits a distorted tetrahedral, nearly square-planar geometry. The average C-N-imine bond length at 1.346 angstrom is characteristic for the pi radical anion (L-1(center dot))(1-), and therefore, the electronic structure of 1 is best described as [Cr-II(L-1(center dot))(2)]. This has been confirmed by broken symmetry density functional theoretical calculations BS(4,2) (DFT) at the B3LYP level. The reaction of [Cr-III(acac)(3)] with 1 equiv of 2,3-dimethyl-1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (L-2) and 1 equiv of Na in thf under Ar yields red-brown crystals of [Cr-III(L-2(center dot))(acac)(2)] (2) (S = 1). The oxidation of 2 with 1 equiv of Fc(PF6) (Fc(+) = ferrocenium) in CH2Cl2 affords crystals of [Cr-III(L-2(ox))(acac)(2)](PF6) (3) (S = 3/2). The crystal structure determinations of 2 and 3 revealed that 2 contains a neutral, octahedral Cr-III species [Cr-III(L-2(center dot))(acac)(2)], whereas in 3 the ligand is oxidized, yielding an octahedral monocation [Cr-III(L-2(ox))(acac)(2)](+). These electronic structures have been confirmed by DFT calculations.