CU-63 NMR-STUDY OF COPPER(I) CARBONYL-COMPLEXES WITH VARIOUS HYDROTRIS(PYRAZOLYL)BORATES - CORRELATION BETWEEN CU-63 CHEMICAL-SHIFTS AND COSTRETCHING VIBRATIONS

Copper(I) carbonyl complexes with a series of hindered L-R1,L-R2 ligan ds (L: hydrotris(pyrazolyl)borate, R1 and R2 are substituents at the 3 - and 5-positions of the pyrazole ring, respectively), (LCuCO)-Cu-R1,R 2 [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)1 have been synthesized and characterized by]H NMR and IR spectroscopy and elemental analysis. The molecular structu res of 3 and 6 have been determined by X-ray crystallography. The elec tronic structures of copper(I) sites are characterized by means of Cu- 63 NMR spectroscopy and by the C=O stretching vibration. The sharp Cu- 63 NMR signals are observed for (LCuCO)-Cu-R1,R2 complexes in toluene at room temperature. The Cu-63 NMR Signals of copper(I) complexes with alkyl-substituted ligands (1-4) are observed in lower field than thos e of the phenyl derivatives (5, 6) correlating with the electron-densi ty at the copper center. This argument is supported by the good correl ation between the delta((CU)-C-63) value and C=O stretching vibration which is a sensitive indicator of the extent of back-donation of the C u d electrons to the antibonding C=O orbitals.