A Copper–Nitroxide Adduct Exhibiting Separate Single Crystal-to-Single Crystal Polymerization–Depolymerization and Spin Crossover Transitions

A complex cascade of solid-state processes initiated
by variation of temperature was found for the heterospin complex [Cu­(hfac)2LMe/Et] formed in the reaction of copper­(II) hexafluoroacetylacetonate
[Cu­(hfac)2] with stable nitronyl nitroxide 2-(1-methyl-3-ethyl-1H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LMe/Et). The cooling
of the compound below 260 K initiated a solid-state chemical reaction,
which led to a depolymerization of chains and formation of a pair
heterospin complex [Cu­(hfac)2LMe/Et2]­[[Cu­(hfac)2]3LMe/Et2]. Further decrease in temperature below 144 K led to a spin transition
accompanied by a drastic decrease in the effective magnetic moment
from 2.52 to 2.24 μB. When the compound was heated,
the order of effects was reversed: at first, the magnetic moment abruptly
increased, and then the molecular fragments of the pair cluster united
into polymer chains. Two hysteresis loops correspond to this cascade
of temperature-induced structural transformations on the experimental
dependence μeff(T): one at high
(T↑ = 283 K and T↓
= 260 K) and the other at low (T↑ = 161 K, T↓ = 144 K) temperature. The spin transitions were
also recorded for the [[Cu­(hfac)2]3LBu/Et2] and [[Cu­(hfac)2]5LBu/Et4] molecular complexes, which are models of the trinuclear
fragment of the {[Cu­(hfac)2]3LMe/Et2} pair cluster.