High spin ground state copper(II) and nickel(II) complexes possessing the 3,5-di-tert-butyl-1,2-semiquinonate radical anion

Abstract

Several high spin ground-state compounds based upon ferromagnetic coupling between a metal ion and the chelating 3,5-di-tert-butyl-1,2-semiquinonate radical anion, [DTBSQ]−, as they possess unpaired electron spins in orthogonal dx2−y2/dz2 and π∗ orbitals, are described. [CuII(DPyA)(DTBSQ)](ClO4) (DPyA=2,2′-dipyridylamine) was reinvestigated and its singlet excited state is 282cm−1 [0.035eV; J/kB=406K (H=−JSa·Sb)] above the triplet ground state. [CuII(bipy)(DBCat)(MeOH)] (bipy=2,2-bipyridine; DBCat=3,5-di-tert-butylcatecholate), [CuII(bipy)(DTBSQ)](BF4), [CuII(DPyA)(DTBSQ)-(THF)2](BF4) and [NiII(DPyA)(DTBSQ)(THF)2](BF4) have been structurally characterized and have high spin ground states whose low spin excited states lie 28cm−1 (0.0034eV), 382cm−1 (0.047eV), 335cm−1 (0.042eV), and 108cm−1 (0.013eV) higher, respectively. The triplet-singlet separation is 18% greater for [CuII(DPyA)(DTBSQ)(THF)2]+ than [CuII(DPyA)(DTBSQ)]+ and is ascribed to greater orthogonality of the CuII dx2−y2/dz2 and [DTBSQ]− π∗ orbitals for octahedral [Cu(DPyA)(DTBSQ)(THF)2]+ with respect to distorted square pyramidal structure of [Cu(DPyA)(DTBSQ)]+. This greater energy of the excited state correlates with the shorter than average M-ODTBSQ distance, i.e. [Cu(bipy)(DTBSQ)]+>[CuII(DPyA)(DTBSQ)(THF)2]+>[Ni(DPyA)(DTBSQ)(THF)2]+. The order of magnitude lower value for [CuII(bipy)(DBCat)(MeOH)] arises from a weak intradimer S=1/2 Cu(II) interaction, not via ferromagnetic coupling between the S=1/2 Cu(II) and [DTBSQ]− sites.