Carbon monoxide and ethylene complexes of copper and silver supported by a highly fluorinated tris(triazolyl)borate

Abstract

Copper and silver carbonyl and ethylene adducts supported by the highly fluorinated hydridotris(triazolyl)borate ligand [HB(3,5-(CF₃)₂Tz)₃]⁻ have been synthesized and characterized, and compared to the analogous complexes based on the hydridotris(pyrazolyl)borate ligand [HB(3,5-(CF₃)₂Pz)₃]⁻. ¹H and ¹³C NMR signals of the ethylene moiety of [HB(3,5-(CF₃)₂Tz)₃]Cu(C₂H₄) and [HB(3,5-(CF₃)₂Tz)₃]Ag(C₂H₄) appear at a relatively downfield position from the corresponding signals of the [HB(3,5-(CF₃)₂Pz)₃]Cu(C₂H₄) and [HB(3,5-(CF₃)₂Pz)₃]Ag(C₂H₄) complexes. This indicates the presence of relatively acidic metal sites in the [HB(3,5-(CF₃)₂Tz)₃]⁻ supported molecules. [HB(3,5-(CF₃)₂Tz)₃]⁻ is also a relatively weaker donor compared to the tris(pyrazolyl)borate version [HB(3,5-(CF₃)₂Pz)₃]⁻. However, the CO stretching frequency values of [HB(3,5-(CF₃)₂Tz)₃]CuCO (2138 cm⁻¹) and [HB(3,5-(CF₃)₂Pz)₃]CuCO (2137 cm⁻¹) are essentially identical, and did not reflect the differences in the acidity of the metal sites. The analogous silver carbonyl adducts [HB(3,5-(CF₃)₂Tz)₃]AgCO and [HB(3,5-(CF₃)₂Pz)₃]AgCO also show similar ν_CO values (2178 cm⁻¹). These results illustrate the limitation of using the ν_CO value of metal carbonyls as a sole indicator of the electronic structure of metal sites in some metal complexes, especially when the metal→CO π-back bonding is less important, and the ν_CO values of two adducts are unexpectedly similar and appear at a region closer to that of the free CO (2143 cm⁻¹).