Five-Spin Copper(II) Nitroxide Complex with Apparently Compressed Octahedral Geometry: Design, Synthesis, and Magnetostructural Studies

Abstract

Multispin Cu(II)-nitroxide compounds have attracted significant interest in the field of molecular magnetism; however, complexes involving more than three spins in a single molecule have rarely been reported. In this work we describe the synthesis and detailed physicochemical study of the solvated six-coordinated five-spin complex [CuL2] (L = 2,2′-(1H-pyrrole-2,5-diyl)bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl)). This complex features a tridentate meridional coordination of the biradical L, resulting in an exchange-coupled cluster of four radical and one Cu(II) spin. The structural and magnetic properties of [CuL2] have been thoroughly investigated using single-crystal X-ray diffraction (XRD), SQUID magnetometry, electron paramagnetic resonance (EPR), Fourier-transform infrared (FTIR) spectroscopy, and quantum chemical calculations (QCC). An apparently compressed octahedral coordination geometry has been found by XRD, which, however, stems from the superposition of two isomeric structures of elongated octahedral CuN2O4 units. Neither X-ray nor FTIR has revealed any structural transitions in [CuL2] at 8–300 K, whereas the magnetic behavior studied by magnetometry and QCC is governed by temperature-dependent populations of high-spin multiplets. EPR spectroscopy revealed a drastic temperature dependence of the line width in [CuL2], which changes by almost 2 orders of magnitude within 4–300 K. A comparison with EPR and QCC data for close-structure four- and three-spin analogues has revealed an interplay of the temperature-dependent population of high-spin multiplets and intermolecular exchange interactions, which govern the line width in such systems.