63Cu NMR Study of Copper(I) Carbonyl Complexes with Various Hydrotris(pyrazolyl)borates: Correlation between 63Cu Chemical Shifts and CO Stretching Vibrations

Copper(I) carbonyl complexes with a series of hindered LR1,R2 ligands (L:  hydrotris(pyrazolyl)borate, R1 and R2 are substituents at the 3- and 5-positions of the pyrazole ring, respectively), LR1,R2CuCO [R1, R2 = Me, Me (1), i-Pr, i-Pr (2), t-Bu, Me (3), t-Bu, i-Pr (4), Ph, i-Pr (5), Ph, Ph (6)] have been synthesized and characterized by 1H NMR and IR spectroscopy and elemental analysis. The molecular structures of 3 and 6 have been determined by X-ray crystallography. The electronic structures of copper(I) sites are characterized by means of 63Cu NMR spectroscopy and by the C⋮O stretching vibration. The sharp 63Cu NMR signals are observed for LR1,R2CuCO complexes in toluene at room temperature. The 63Cu NMR signals of copper(I) complexes with alkyl-substituted ligands (1−4) are observed in lower field than those of the phenyl derivatives (5, 6) correlating with the electron-density at the copper center. This argument is supported by the good correlation between the δ(63Cu) value and C⋮O stretching vibra...