Zirconium(IV) sandwich complexes of porphyrins and tetraazaporphyrins: Synthesis, structure, and nonlinear optical properties

A family of bis(porphyrin) zirconium sandwich complexes containing octaethylporphyrin (OEP) and octaethyltetraazaporphyrin (OETAP) was synthesized and characterized by W-vis and NMR spectroscopies. The two ligands are structural analogues yet have dramatically different redox properties (the redox potentials of OETAP complexes are much more positive than those of the corresponding OEP complexes). Cyclic voltammetry results indicate that Zr(OETAP)(2) is about 600 mV harder to oxidize and about 600 mV easier to reduce than Zr(OEP)2, while the mixed sandwich Zr(OEP)(OETAP) exhibits intermediate redox potentials. The structures of Zr(OEP)(OETAP) and Zr(OETAP)2 were determined by X-ray crystallography and compared to Zr(OEP)(2). The solid state structures were very similar, indicating that OEP and OETAP have similar steric parameters and that observed spectroscopic and electrochemical differences are due primarily to electronic factors. The one electron oxidized porphyrin sandwiches were also synthesized. Characterization by W-vis, near-IR, EPR, and IR spectroscopies confirm the it radical nature of these complexes. IR spectra indicate that the cation is delocalized over the entire complex in both [Zr(OEP)(OETAP)](+)[SbCl6](-) and [Zr(OETAP)(2)](+)[SbCl6](-), consistent with a strongly coupled pi system. Additional evidence for strong coupling was obtained from measurement of the nonlinear optical properties of the mixed complex; the molecular hyperpolarizability is negative, indicating that the dipole moment reverses direction upon electronic excitation.