Solvent-controlled solid-state phase transitions of a heterospin Cu(ii) complex with imidazolyl-substituted nitronyl nitroxide

Abstract

Spontaneous solvent-controlled solid-state transformations were observed for a series of polymeric chain solvates [Cu(hfac)₂L^Pr]·0.5Solv (Solv = (CH₃)₂CO, THF, CH₂Cl₂, CH₂Br₂, CHCl₃) with 2-(1-propyl-1H-imidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L^Pr) under ambient conditions. The conversion of powdered polymeric chain [Cu(hfac)₂L^Pr]·0.5THF and [Cu(hfac)₂L^Pr]·0.5(CH₃)₂CO complexes, accompanied by the loss of solvent molecules, occurs completely to binuclear [Cu(hfac)₂L^Pr]₂ within a day. On the other hand, in the case of [Cu(hfac)₂L^Pr]·0.5Solv (Solv = CH₂Cl₂, CH₂Br₂, CHCl₃), the partial transformation into a desolvated 1D polymer [Cu(hfac)₂L^Pr] or its mixture with [Cu(hfac)₂L^Pr]₂ takes much longer time. The magnetic behavior of the isostructural solvates is highly sensitive to the included solvent molecules. The complexes with THF, CH₂Cl₂, CH₂Br₂, and CHCl₃ undergo a transition to a magnetically ordered state below 4 K, a phenomenon reported for the first time for the polymeric chain Cu(II) complexes with a “head-to-tail” motif. [Cu(hfac)₂L^Pr]·0.5THF and [Cu(hfac)₂L^Pr]·0.5(CH₃)₂CO undergo a spin transition at 190 K, which is induced by the transformation of the Cu atom environment. In the case of the THF solvate, the increase in the distances between the Cu and oxygen atoms leads to enhanced ferromagnetic exchange interactions, while in the acetone solvate, the nitroxide coordination type changes from axial to equatorial at certain coordination sites, resulting in the emergence of strong antiferromagnetic exchange.