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

The dinuclear copper(ll) complex [Cu 2 (μ 1,1 -N3)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" 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 S i = 1/2, i = 1-4), where the Heisenberg spin Hamiltonian assumes the general form -2Σ i +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 ≈ 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.