Molecular dynamics of four-coordinate carbene-Cu(I) complexes employing tris(pyrazolyl)borate ligands

Abstract

A series of three- and four-coordinate Cu(I) complexes was prepared employing a N-heterocyclic carbene and various scorpionate trispyrazolylborate (Tp) ligands. The NMR spectra of the four-coordinated complexes suggest C3 symmetry for the Tp ligands despite the asymmetric scorpionate η3-binding fashion observed by single crystal X-ray diffraction. Variable temperature (VT) 1H and 13C NMR spectroscopy was used to elucidate the dynamic process that equilibrates the different structural isomers. We report that the rapid exchange of the pyrazoles in the Tp ligand was hampered but not fully inhibited at −88 °C. Lastly, we show how the electrophilicity of the carbene and the hapticity of the Tp moiety both control the photophysics: more electrophilic carbenes red-shift the luminescence and bulkier Tp suppress modes of nonradiative decay (knr).