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

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](center dot-), as they possess unpaired electron spins in orthogonal d(x2-y2)/d(z2) and pi* orbitals, are described. [Cu-II(DPyA)(DTBSQ)](ClO4) (DPyA = 2,2'-dipyridylamine) was reinvestigated and its singlet excited state is 282 cm(-1) [0.035 eV; J/k(B) = 406 K (H = -JS(a)center dot S-b)] above the triplet ground state. [Cu-II(bipy)(DBCat)(MeOH)] (bipy = 2,2-bipyridine; DBCat = 3,5-di-tert-butylcatecholate), [Cu-II(bipy)(DTBSQ)](BF4), [Cu-II(DPyA)(DTBSQ)-(THF)(2)](BF4) and [Ni-II(DPyA)(DTBSQ)(THF)(2)](BF4) have been structurally characterized and have high spin ground states whose low spin excited states lie 28 cm(-1) (0.0034 eV), 382 cm(-1) (0.047 eV), 335 cm(-1) (0.042 eV), and 108 cm(-1) (0.013 eV) higher, respectively. The triplet-singlet separation is 18% greater for [Cu-II(DPyA)(DTBSQ)(THF)(2)](+) than [Cu-II(DPyA)(DTBSQ)](+) and is ascribed to greater orthogonality of the Cu-II d(x2-y2)/d(z2) and [DTBSQ](center dot-) pi* 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)](+) > [Cu-II(DPyA)(DTBSQ)(THF)(2)](+) > [Ni(DPyA)(DTBSQ)(THF)(2)](+). The order of magnitude lower value for [Cu-II(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](center dot-) sites. (C) 2017 Elsevier Ltd. All rights reserved.