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 [Cu2(μ1,1-N3)2(im-2py)2(N3)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 CuII 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∑i<jŜiŜj. Considering only first-neighbor spin-coupling constants (J13 =
J24 = J14 = 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 (J12 = J34 = J1 > +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 [J23 = J2 = −43(2) cm-1]. By contrast,
a density functional theory analysis of the system through the computation of broken-symmetry-state energies
resulted in J2 ≈ 0 cm-1. This apparent paradox is resolved by introducing a second-neighbor rad−(Cu)−Cu−(rad)⋮(rad)−Cu−(Cu)−rad spin-coupling constant J13 = J24 = J3, which turns out to be antiferromagnetic both
experimentally (when J2 is set equal to zero) and computationally.