Copper(II) Complexes with CF₃-Substituted Spin-Labeled Pyrazoles

Abstract—CF₃-substituted pyrazolyl nitroxides ${{{\text{L}}}^{{{{\text{R}} \mathord{\left/ {\vphantom {{\text{R}} {{\text{C}}{{{\text{F}}}_{{\text{3}}}}}}} \right. \kern-0em} {{\text{C}}{{{\text{F}}}_{{\text{3}}}}}}}}}$ and their Cu(II) complexes were synthesized. The molecular and crystal structures of the products were studied by X-ray diffraction (CCDC no. 2180506–2180521). It was found that the introduction of the electron-withdrawing CF₃ group into the pyrazole ring decreases the donor ability of the N atom, and nitronyl nitroxides ${{{\text{L}}}^{{{{\text{R}} \mathord{\left/ {\vphantom {{\text{R}} {{\text{C}}{{{\text{F}}}_{{\text{3}}}}}}} \right. \kern-0em} {{\text{C}}{{{\text{F}}}_{{\text{3}}}}}}}}}$ are coordinated in heterospin complexes only by the O atoms of the nitronyl nitroxide moieties. Magnetochemical studies of polymer chain complexes [Cu(Hfac)₂${{{\text{L}}}^{{{{\text{R}} \mathord{\left/ {\vphantom {{\text{R}} {{\text{C}}{{{\text{F}}}_{{\text{3}}}}}}} \right. \kern-0em} {{\text{C}}{{{\text{F}}}_{{\text{3}}}}}}}}}$]_n (Hfac = hexafluoroacetylacetonate anion) in the 2–300 K range revealed ferromagnetic ordering at temperatures below 5 K. Thermally induced magnetic structural phase transitions were detected in two polymorphs of the molecular complex, α-[Cu(Hfac)₂${{\left( {{{{\text{L}}}^{{{{{\text{Me}}} \mathord{\left/ {\vphantom {{{\text{Me}}} {{\text{C}}{{{\text{F}}}_{{\text{3}}}}}}} \right. \kern-0em} {{\text{C}}{{{\text{F}}}_{{\text{3}}}}}}}}}} \right)}_{2}}$] and β-[Cu(Hfac)₂${{\left( {{{{\text{L}}}^{{{{{\text{Me}}} \mathord{\left/ {\vphantom {{{\text{Me}}} {{\text{C}}{{{\text{F}}}_{{\text{3}}}}}}} \right. \kern-0em} {{\text{C}}{{{\text{F}}}_{{\text{3}}}}}}}}}} \right)}_{2}}$]. These polymorphs are new examples of molecular heterospin complexes that undergo thermally induced magnetic structural phase transitions without crystal degradation.