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-(CF 3 ) 2 Pz) 3 ] - and [HB(3-(CF 3 )Pz) 3 ] - (where Pz = pyrazolyl) were synthesized by treating BrMn(CO) 5 with [HB(3,5-(CF 3 ) 2 Pz) 3 ]Ag(THF) or [HB(3-(CF 3 )Pz) 3 ]Na(THF). The reaction of [HB(3-(CF 3 )Pz) 3 ]Na(THF) with copper(I) trifluoromethanesulfonate under CO afforded [HB(3-(CF 3 )Pz) 3 ]CuCO. Compounds [HB(3,5-(CF 3 ) 2 Pz) 3 ]Mn(CO) 3 ( 5 ), [HB(3-(CF 3 )Pz) 3 ]Mn(CO) 3 ( 6 ), and [HB(3-(CF 3 )Pz) 3 ]CuCO ( 7 ) were characterized by 1 H NMR, 19 F NMR, and IR spectroscopy and by X-ray crystallography. They have relatively high C−O stretching frequencies. However, the ν CO values are much lower than that in free CO and the relative lowering is higher for manganese adducts. The CF 3 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.