(α-Diimine)chromium Complexes: Molecular and Electronic Structures; A Combined Experimental and Density Functional Theoretical Study

Dark brown crystals of [Cr(1L)2] (1) were obtained from the reaction of [CrIII(acac)3] (acac− = 2,4-pentanedionate) with 2 equiv of 2-methyl-1,4-bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene (1L) 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 CrII ion (SCr = 2) and two anionic α-diiminato(1-) ligand π radicals (1L•)1−. The molecular structure of 1 exhibits a distorted tetrahedral, nearly square-planar geometry. The average C−Nimine bond length at 1.346 Å is characteristic for the π radical anion (1L•)1−, and therefore, the electronic structure of 1 is best described as [CrII(1L•)2]. This has been confirmed by broken symmetry density functional theoretical calculations BS(4,2) (DFT) at the B3LYP level. The reaction of [CrIII(acac)3] with 1 equiv of 2,3-dimethyl-1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (2L) and 1 equiv of Na in thf under Ar yields red-brown crystals of [CrIII(2L•)(acac)2] (2) (S = 1). The oxidation of 2 with 1 equiv of Fc(PF6) (Fc+ = ferrocenium) in CH2Cl2 affords crystals of [CrIII(2Lox)(acac)2](PF6) (3) (S = 3/2). The crystal structure determinations of 2 and 3 revealed that 2 contains a neutral, octahedral CrIII species [CrIII(2L•)(acac)2], whereas in 3 the ligand is oxidized, yielding an octahedral monocation [CrIII(2Lox)(acac)2]+. These electronic structures have been confirmed by DFT calculations.