Spin Density Maps in the Triplet Ground State of [Cu₂(<i>t</i>-Bupy)₄(N₃)₂](ClO₄)₂ (<i>t</i>-Bupy = <i>p</i>-<i>tert</i>-butylpyridine):  A Polarized Neutron Diffraction Study

Abstract

This paper is devoted to the determination of the spin distribution in the spin triplet ground state of [Cu2(t-Bupy)4(N3)2](ClO4)2, with t-Bupy = p-tert-butylpyridine. The crystal structure, previously solved at room temperature from X-ray diffraction, has been redetermined at 18 K from unpolarized neutron diffraction. The structure consists of binuclear cations in which Cu2+ ions are doubly bridged by azido groups in the 1,1-fashion, and noncoordinated perchlorate anions. The experimental spin distribution has been determined from polarized neutron diffraction (PND) at 1.6 K under 50 kOe. The spin populations have been found to be strongly positive on the Cu2+ ions, weakly positive on the terminal and bridging nitrogen atoms of the azido groups as well as on the nitrogen atoms of the t-Bupy ligands, and weakly negative on the central nitrogen atoms of the N3- bridges. The PND results have been discussed. The spin distribution in [Cu2(t-Bupy)4(N3)2](ClO4)2 has been analyzed as resulting from a spin delocalization from the Cu2+ ions toward the azido bridges, to which a spin polarization effect within the azido π orbitals is superimposed. The experimental data have been compared to the results of DFT calculations. The spin density map is qualitatively reproduced; however, the DFT calculations overestimate the spin delocalization from the Cu2+ ions toward the peripheral and bridging ligands.