(Alpha-diimine)chromium complexes: molecular and electronic structures; a combined experimental and density functional theoretical study

Dark brown crystals of Cr( (1)L) 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 ( (1)L) 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 ( (1)L (*)) (1-). The molecular structure of 1 exhibits a distorted tetrahedral, nearly square-planar geometry. The average C-N imine bond length at 1.346 A is characteristic for the pi radical anion ( (1)L (*)) (1-), and therefore, the electronic structure of 1 is best described as Cr (II)( (1)L (*)) 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 ( (2)L) and 1 equiv of Na in thf under Ar yields red-brown crystals of Cr (III)( (2)L (*))(acac) 2] ( 2) ( S = 1). The oxidation of 2 with 1 equiv of Fc(PF 6) (Fc (+) = ferrocenium) in CH 2Cl 2 affords crystals of Cr (III)( (2)L (ox))(acac) 2](PF 6) ( 3) ( S = (3)/ 2). The crystal structure determinations of 2 and 3 revealed that 2 contains a neutral, octahedral Cr (III) species Cr (III)( (2)L (*))(acac) 2], whereas in 3 the ligand is oxidized, yielding an octahedral monocation Cr (III)( (2)L (ox))(acac) 2] (+). These electronic structures have been confirmed by DFT calculations.