Cu(hfac)₂ Complexes with Acyclic Nitroxide Prone to Single-Crystal to Single-Crystal Transformation and Showing Mechanical Activity

The heterospin solid phases of the chain polymer [Cu­(hfac)₂L^Et]_∞ and bicyclic molecule [Cu­(hfac)₂L^Et]₂-I (L^R = pyrazolyl-substituted tert-butylnitroxide; 1-R-5-(tert-butyl-oxylamino)­pyrazole, R = Et, Pr) were found to undergo spontaneous transformation into the bicyclic molecule [Cu­(hfac)₂L^Et]₂-II. The single-crystal to single-crystal (SC–SC) transformation of [Cu­(hfac)₂L^Et]₂-I into [Cu­(hfac)₂L^Et]₂-II was recorded by X-ray diffraction analysis of the crystal as a function of time. At 255–277 K, the [Cu­(hfac)₂L^Et]₂-I → [Cu­(hfac)₂L^Et]₂-II SC–SC transformation proceeded for 12–18 h. The [Cu­(hfac)₂L^Et]_∞ → [Cu­(hfac)₂L^Et]₂-II SC–SC phase transformation was accompanied by a change in the crystal shape, spontaneous mechanical displacements of crystals, and a change in color from orange to dark green. This process started, to a certain extent, already in the crystals lying under the layer of the mother solution. After the crystals were separated from the solution, the SC–SC transformation [Cu­(hfac)₂L^Et]_∞ → [Cu­(hfac)₂L^Et]₂-II occurred completely within 4 h at room temperature. Under normal conditions, [Cu­(hfac)₂L^Pr]₂-I also undergoes transformation into [Cu­(hfac)₂L^Pr]₂-II. At the macro level, the transformation [Cu­(hfac)₂L^Pr]₂-I → [Cu­(hfac)₂L^Pr]₂-II is accompanied by spontaneous fragmentation of crystals, visualized as a scatter of small particles of the formed phase in different directions. The reverse transformation [Cu­(hfac)₂L^Pr]₂-II → [Cu­(hfac)₂L^Pr]₂-I occurs when [Cu­(hfac)₂L^Pr]₂-II is cooled below 225 K. When [Cu­(hfac)₂L^Pr]₂-II was heated above 300 K, the irreversible SC–SC phase transformation [Cu­(hfac)₂L^Pr]₂-II → [Cu­(hfac)₂L^Pr]_∞ was observed, which caused a pronounced change in the color of the crystals from dark green to orange. Heat treatment of the [Cu­(hfac)₂L^Pr]_∞ single crystal at 303 K on a diffractometer for 1 day or more caused partial melting of the starting crystal, disappearance of X-ray diffraction reflections from the sample under study, and appearance of reflections corresponding to the formation of the new polymer complex [Cu­(hfac)₂L^*Pr]_∞, where L^*Pr is the product of transformation of the radical including the oxidation of L^Pr and migration of the nitroxide O atom to the heterocycle, leading to the formation of 5-(tert-butylimino)-1-propyl-1,5-dihydro-4H-pyrazol-4-one (L^*Pr). The results of the X-ray diffraction study of the phase transformations completely agreed with the data of magnetochemical measurements for the complexes. Having replaced the acyclic nitroxides L^Et and L^Pr by their diamagnetic structural analogues L^PEt (2,2-dimethyl-1-(1-ethyl-1H-pyrazol-5-yl)­propan-1-one) and L^PPr (2,2-dimethyl-1-(1-propyl-1H-pyrazol-5-yl)­propan-1-one), we obtained the complexes [Cu­(hfac)₂L^PEt]_∞, [Cu­(hfac)₂(L^PPr)₂], and [(Cu­(hfac)₂)₃(L^PPr)₂], for which the transformations are absolutely not characteristic. It was also found that polymorphic transformations are also uncharacteristic of complexes of other metals with the acyclic nitroxides under study ([Zn­(hfac)₂L^Et]₂, [Zn­(hfac)₂L^Pr]₂, [Mn­(hfac)₂L^Et]₂). Thus, it was shown that the presence of both the Cu­(II) ion and coordinated O–N group of acyclic nitroxide in the solid phase are favorable conditions for the emergence of stereochemical nonrigidity and multiple phase transformations in the compounds of Cu­(hfac)₂ with acyclic nitroxides.