Phthalocyanine as a Chemically Inert, Redox-Active Ligand: Structural and Electronic Properties of a Nb(IV)-Oxo Complex Incorporating a Highly Reduced Phthalocyanine(4-) Anion

摘要

This report describes the reduction of a niobium(V) phthalocyanine complex and investigation of the electronic structure of the resulting products. The reduction of PcNbCl3 (Pc = phthalocyanine dianion) with 5.5 equiv of potassium graphite in 1,2-dimethoxyethane (DME) resulted in the isolation of K2PcNbO.5DME (1a). Addition of 18-crown-6 to 1a gave [K(18-crown-6)](2)(mu-DME)PoNbO (1b). Both la and lb were structurally characterized by single-crystal X-ray diffraction analysis. In both complexes, the niobium center adopts a square pyramidal geometry and is coordinated by four basal Pc nitrogen atoms and an apical oxo ligand. Notably, the Pc ligand in la is saddle-shaped, with significant bond length alternation, rather than flat with delocalized bonding. The production of ethylene during the reduction of PcNbCl3, detected by gas chromatography/mass spectrometry (GC/MS), suggests that the oxo ligand likely results from double C-O bond activation of DME solvent. A combination of spectroscopic techniques and density functional theory (DFT) calculations were used to establish the electronic structure of la. The close correspondence of the electronic absorption spectrum of la to that of [PcZn](2-) with a di-reduced PC4- ligand, indicates a similar electronic structure for the two complexes. Evaluation of the electronic transitions for la and [PcZn](2-) by time-dependent OFT calculations further suggests a similar electronic structure for both complexes, indicating that differences in symmetry between la and [PcZn](2-) do not significantly affect the nature of the electronic transitions. Electron paramagnetic resonance (EPR) spectroscopy of la in solution at room temperature gave a 10-line spectrum, while frozen-solution X- and Q-band EPR spectra are consistent with powder-pattern spectra defined by uniaxial g and Nb-93 hyperfine tensors: these imply the presence of a d(1) Nb(IV) metal center. EPR and electron nuclear double resonance spectroscopy suggests that the spin density in la is centered almost completely on the niobium, in agreement with the DFT calculations. These results illustrate the value of Pc as a chemically inert, redox-active ligand for stabilizing reactive metal centers.