63Cu NMR spectroscopy of copper(I) complexes with various tridentate ligands: CO as a useful 63Cu NMR probe for sharpening 63Cu NMR signals and analyzing the electronic donor effect of a ligand.

63Cu NMR spectroscopic studies of copper(I) complexes with various N-donor tridentate ligands are reported. As has been previously reported for most copper(I) complexes, 63Cu NMR signals, when acetonitrile is coordinated to copper(I) complexes of these tridentate ligands, are broad or undetectable. However, when CO is bound to tridentate copper(I) complexes, the 63Cu NMR signals become much sharper and show a large downfield shift compared to those for the corresponding acetonitrile complexes. Temperature dependence of 63Cu NMR signals for these copper(I) complexes show that a quadrupole relaxation process is much more significant to their 63Cu NMR line widths than a ligand exchange process. Therefore, an electronic effect of the copper bound CO makes the 63Cu NMR signal sharp and easily detected. The large downfield shift for the copper(I) carbonyl complex can be explained by a paramagnetic shielding effect induced by the copper bound CO, which amplifies small structural and electronic changes that occur around the copper ion to be easily detected in their 63Cu NMR shifts. This is evidenced by the correlation between the 63Cu NMR shifts for the copper(I) carbonyl complexes and their nu(C[triple bond]O) values. Furthermore, the 63Cu NMR shifts for copper(I) carbonyl complexes with imino-type tridentate ligands show a different correlation line with those for amino-type tridentate ligands. On the other hand, 13C NMR shifts for the copper bound 13CO for these copper(I) carbonyl complexes do not correlate with the nu(C[triple bond]O) values. The X-ray crystal structures of these copper(I) carbonyl complexes do not show any evidence of a significant structural change around the Cu-CO moiety. The findings herein indicate that CO complexation makes 63Cu NMR spectroscopy much more useful for Cu(I) chemistry.