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)(2)L-Et](infinity) and bicyclic molecule [Cu(hfac)(2)L-Et](2)-I (L-R = pyrazolylsubstituted tert-butylnitroxide; 1-R-5-(tert-butyl-oxylamino)pyrazole, R = Et, Pr) were found to undergo spontaneous transformation into the bicyclic molecule [Cu(hfac)(2)L-Et](2) -II. The single-crystal to single-crystal (SC-SC) transformation of [Cu(hfac)(2)L-Et](2)-I into [Cu(hfac)(2)L-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)(2)L-Et](2)-I -> [Cu(hfac)(2)L-Et](2)-II SC-SC transformation proceeded for 12-18 h. The [Cu(hfac)(2)L-Et](infinity) -> [Cu(hfac)(2)L-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)(2)L-Et](infinity) -> [Cu(hfac)(2)L-Et](2)-II occurred comFletely within 4 h at room temperature. Under normal conditions, [Cu(hfac)(2)L-Pr](2)-I also undergoes transformation into [Cu(hfac)(2)L-Pr](2). At the macro level, the transformation [Cu(hac)(2)L-Pr](2)-I -> [Cu(hfac)(2)L-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)(2)L-Pr](2)-II -> [Cu(hfac)(2)L-Pr](2)-I occurs when [Cu(hfac)(2)L-Pr](2)-II is cooled below 225 K. When [Cu(hfac)(2)L-Pr](2)-II was heated above 300 K, the irreversible SC-SC phase transformation [Cu(hfac)(2)L-Pr](2)-II -> [Cu(hfac)(2)L-Pr](infinity) was observed, which caused a pronounced change in the color of the crystals from dark green to orange. Heat treatment of the [Cu(hfac)(2)L-Pr](infinity) 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)(2)L*(Pr)](infinity) 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-S-yl)propan-1-one) and L-PPr (2,2-dimethyl-1-(1propyl-1H-pyrazol-S-yl)propan-1-one), we obtained the complexes [Cu(hfac)(2)L-PEt](infinity), [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 polymoThic transformations are also uncharacteristic of complexes of other metals with the acyclic nitroxides under study ([Zn(hfac)(2)L-Et](2), [Zn(hfac)(2)L-Pr](2), [Mn(hfac)(2)L-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 stereodiemical nonrigidity and multiple phase transformations in the compounds of Cu(hfac)(2) with acyclic nitroxides.

Automated summary

The heterospin solid phases of the chain polymer [Cu(hfac)(2)L-Et](infinity) and bicyclic molecule [Cu(hfac)(2)L-Et](2)-I were found to undergo spontaneous transformation into the bicyclic molecule [Cu(hfac)(2)L-Et](2)-II. The transformation was observed through X-ray diffraction analysis and was accompanied by changes in crystal shape, mechanical displacements of crystals, and color. Similar transformations were observed in other complexes as well.