An unprecedented asymmetric end-on azido-bridged copper(II) imino nitroxide complex: Structure, magnetic properties, and density functional theory analysis

The dinuclear copper(II) complex [Cu-2(mu(1,1)-N-3)(2)(im-2py)(2)(N-3)(2)] [im-2py = 2-(2-pyridyl)-4,4,5,5-tetramethylimidazolinyl-1-oxy] has been prepared and structurally characterized. The crystal structure consists of a dinuclear unit in which the Cu-II ions are bridged by two azido ions in a end-on asymmetric fashion and the imino nitroxide radicals are chelating by the two imino N atoms. Accordingly, the magnetic susceptibility data were analyzed considering a linear spin-coupling scheme rad(1)-Cu(2)-Cu(3)-rad(4) (with Si = (1)/(2), i = 1-4), where the Heisenberg spin Hamiltonian assumes the general form -2 Sigma(i < j)(S) over cap (i)(S) over cap (j). Considering only first-neighbor spin-coupling constants (J(13) = J(24) = J(14) = 0), magnetic susceptibility measurements show that the copper(II) imino nitroxide rad -Cu-(Cu-rad)(rad-Cu)-Cu-rad exchange coupling is ferromagnetic and large (J(12) = J(34) = J(1) > +190 cm(-1)), as is expected for copper imino nitroxide species, and the copper-copper (rad)-Cu-Cu-(rad) coupling through the asymmetric double end-on azide bridges appeared antiferromagnetic and rather large [J(23) = J(2) = -43(2) cm(-1)]. By contrast, a density functional theory analysis of the system through the computation of broken-symmetry-state energies resulted in J(2) approximate to 0 cm(-1). This apparent paradox is resolved by introducing a second-neighbor rad-(Cu)-Cu(rad)(rad)-Cu-(Cu)-rad spin-coupling constant J(13) = J(24) = J(3), which turns out to be antiferromagnetic both experimentally (when J(2) is set equal to zero) and computationally.