Magnetic superexchange switching by transformation of trigonal bipyramid into square pyramid in Cu(II)-nitroxide complex

The reaction of Cu(hfac)₂ and imino nitroxide 2-(pyridin-4-yl)-4,4,5,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl (IN) gives the complex [Cu₂(hfac)₄(IN)]_n (1), which has the structure of a chain-polymer with side branches. IN, acting as a μ₃-O,N,N-tridentate ligand, bridges the intrachain Cu atoms through the O_NO atoms of the nitroxide groups and the N_Im atoms of the imidazoline ring of IN, while the N_Py atom of the pyridine cycle binds to the branching Cu atom. The specific disorder of the imino nitroxide fragment in IN and the intrachain Cu(hfac)₂ moieties makes the structure of 1 almost similar to that of the complex [Cu₂(hfac)₄(NN)]_n (2) with nitronyl nitroxide NN, an analog of IN. Thermally induced hysteretic change in the χT value (T↓ = 109 K and T↑ = 119 K) and transformations of EPR spectra were found for the complex 1. Applying pressure above 1.9 kbar not only completly suppresses this magnetic anomaly, but also manifests significant differences in the magnetic behavior of the low-temperature and pressure-stabilized modifications at χT(T) curve. Comparing magnetostructural correlations for 1 at atmospheric and hydrostatic pressure with those for an analogue of nitronyl nitroxide 2 using quantum chemical calculations made it possible to clarify the nature of the magnetic anomaly. It was established that the magnetic transition of 1 is related to the transformation of the trigonal-bipyramidal environment of the branched Cu atom into a square-pyramidal one with a change in the position of the N_Py atom of the pyridine ring from equatorial to axial. EPR spectroscopy clearly indicates the changes of g-tensor components of the branching Cu²⁺ ion, while the g_iso parameter remaining almost unchanged. Magnetic anomaly in 1 revealed by SQUID magnetometry is a unique case of switching the indirect magnetic superexchange {Cu²⁺...IN} through the pyridine fragment during the rearengement of peripheral fragments of the structure.