Chemical Implications of Incompatible Ligand versus Metal Coordination Geometry Preferences

Binding
an electron deficient pincer ligand which strongly dictates planar, mer stereochemistry, to a metal which prefers tetrahedral
structure, e.g., d10 CuCl, is explored for possible intramolecular
redox chemistry. Experiment shows that the pincer ligand 2,2′-bis-tetrazinyl pyridine, btzp, forms a complex (btzp)­CuCl which is
a chloride-bridged polymer in the solid state, hence with 20 valence
electrons around copper. DFT calculations show that even the monomer
has nonplanar copper with the tetrazinyl nitrogen lone pairs somewhat
misdirected away from copper, with long Cu/N bonds, in a singlet ground
state; 13.9 kcal/mol less stable is a triplet, whose electronic structure
shows one electron from the ground state Cu­(I) has been transferred
to a pincer π* orbital. Outer sphere electron transfer to (btzp)­CuCl
yields (btzp)Cu where the added electron has gone into the pincer,
to leave a ligand-centered radical, characterized by EPR, chemical
reactivity, and X-ray photoelectron spectroscopy.