Spin Density Maps in the Triplet Ground State of [Cu₂(t-Bupy)₄(N₃)₂](ClO₄)₂ (t-Bupy = p-tert-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.

Introduction to the density functional formalism and some illustrative applications to magnetism

“…Table 3 bridge. This is in excellent agreement with other reports available in the litterature and compares nicely with data of Polarized Neutron Diffraction (PND) [95,96,109]. The sign alternation of the spin density at the N 1 and N 2 atoms of the azido-bridge is consistent with spin polarization by the bridging nitrogen atoms and it highlights the superexchange pathway between the two magnetic cations.…”

Ferromagnetic Interaction in an Asymmetric End‐to‐End Azido Double‐Bridged Copper(II) Dinuclear Complex: A Combined Structure, Magnetic, Polarized Neutron Diffraction and Theoretical Study

“…[11,22,23] In the early studies, [3][4][5][6][17][18][19][20] the symmetric EO coordination mode behavior seemed intrinsically ferromagnetic and independent of any structural parameters. This is why most previous theoretical studies focused on EO systems to explain such peculiar behavior, [24,[26][27][28][29][30] which was initially attributed to spin polarization effects. [28,30] However, a polarized neutron diffraction study has shown that spin delocalization is indeed the main effect and that spin polarization occurs only within the azido bridge.…”

A Polyoxometalate Containing the {Ni2N3} Fragment: Ferromagnetic Coupling in a NiII μ‐1,1 Azido Complex with a Large Bridging Angle

“…For an end-on bridging mode, simultaneous pairing of the two paramagnetic centers would lead to ferromagnetic coupling, irrespectively of the q value. Nevertheless, based on polarized neutron diffraction studies, [13] Aebersold et al showed later that a spin-delocalization mechanism in the active-electron approximation must be considered. The coupling would then be ferromagnetic (or antiferromagnetic) under a given q AO value and antiferromagnetic (or ferromagnetic) above, where q AO is the angle for accidental orthogonality.…”

“…a ferromagnetic coupling occurring in a dimeric m-1,1 azido complex with a q value of 129.38) provides empirical evidence for a positive answer to the question posed by Kahn et al "Can the 1,1-azido group be considered as an almost universal ferromagnetic coupler?". [13] At least, we can say that this assumption must be true for nickel(ii) complexes; nevertheless, MO calculations must be performed to verify that no countercomplementarity effect arises from the long O-P-O and O-W-O bridges connecting the divalent centers. Owing to the lack of available compounds, no clear correlation between q and J for Ni II azido bridged compounds could be found to date.…”