Investigation of the Electronic and Geometric Effects of Trifluoromethyl Substituents on Tris(pyrazolyl)borate Ligands Using Manganese(I) and Copper(I) Complexes

Manganese tricarbonyl complexes of fluorinated tris(pyrazolyl)borate ligands [HB(3,5-(CF3)2Pz)3]- and [HB(3-(CF3)Pz)3]- (where Pz = pyrazolyl) were synthesized by treating BrMn(CO)5 with [HB(3,5-(CF3)2Pz)3]Ag(THF) or [HB(3-(CF3)Pz)3]Na(THF). The reaction of [HB(3-(CF3)Pz)3]Na(THF) with copper(I) trifluoromethanesulfonate under CO afforded [HB(3-(CF3)Pz)3]CuCO. Compounds [HB(3,5-(CF3)2Pz)3]Mn(CO)3 (5), [HB(3-(CF3)Pz)3]Mn(CO)3 (6), and [HB(3-(CF3)Pz)3]CuCO (7) were characterized by 1H NMR, 19F NMR, and IR spectroscopy and by X-ray crystallography. They have relatively high C−O stretching frequencies. However, the νCO values are much lower than that in free CO and the relative lowering is higher for manganese adducts. The CF3 substitution on the 5-position has a significant effect on the metal center. This is readily apparent from the infrared spectroscopic data. Theoretical calculations of the geometries of [tris(pyrazolyl)borato]manganese(I) and -copper(I) carbonyls are in excellent agreement with the experimental results. Increasing the degree of fluorination lengthens the metal−C and metal−N bonds and shortens the C−O distance. The metal−Pz bonding is essentially a pure σ-interaction. Cyclic voltammetry data for the copper complexes show high oxidation potentials for the fluorinated analogs. IR spectroscopic data of the [tris(pyrazolyl)borato]manganese and -copper carbonyls have also been compared to those of cyclopentadienyl analogs.