Probing Redox Noninnocence of Copper and Zinc Bis-pyridylpyrrolides

A series of complexes of divalent copper and zinc carrying two systematically substituted 2,2'-pyridylpyrrolide ligands, designated L-n to indicate the number of pyrrole CF3 substituents (n = 0, 1, 2), have been studied for their geometric and electronic structures. These reveal the pyridylpyrrolide to be a highly anisotropic nitrogen donor ligand which distorts ML2 complexes away from both planar and tetrahedral structures. Characterization includes CV and mass spectrometry, which show access to cations beyond conventional maximum metal oxidation states. EPR studies at multiple microwave frequencies of the series Cu(L-n)(2) gave insight into the substituent effect on frontier orbital composition of these complexes. While Cu(L-2)(2) exhibited EPR spectra that were roughly comparable to those for typical, tetragonally distorted Cu-II complexes with the SOMO having d(x2-y2) character, the other two complexes exhibited more unusual EPR spectra indicative of their distinct geometry, reminiscent of equatorially-vacant trigonal bipyramidal hybridization and fully consistent with the X-ray crystal structure determinations of all three Cu(L-n)(2). DFT calculations map both geometric and delocalization changes upon redox change, and show the relevance of oxidation at the pyrrolide donors rather than at the metal.