(α-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 lll (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" ion (Scr = 2) and two anionic α-diiminato(1-) ligand π 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 π radical anion ( 1 L•) 1- , and therefore, the electronic structure of 1 is best described as [Cr ll ( 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 lll (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 lll ( 2 L•)(acac) 2 ] (2) (S = 1). The oxidation of 2 with 1 equiv of Fc(PF 6 ) (Fc + = ferrocenium) in CH 2 Cl 2 affords crystals of [Cr lll (2L°x)(acac)2](PF6) (3) (S = 3/2). The crystal structure determinations of 2 and 3 revealed that 2 contains a neutral, octahedral Cr lll species [Cr lll ( 2 L•)(acac) 2 ], whereas in 3 the ligand is oxidized, yielding an octahedral monocation [Crlll(2L°x)(acac)2] + . These electronic structures have been confirmed by DFT calculations.