Group VIII coordination complexes of bidentate P^N ligands bearing π-extended quinoline or phenanthridine N-heterocycles

Abstract

A series of Fe and Ru coordination complexes supported by bidentate P^N ligands bearing π-extended phenanthridine (3,4-benzoquinoline; ^PhenL^R,R') or quinoline (^QuinL^H,Ph) N-heterocycles have been synthesized and fully characterized. Tetrahedral monomeric (P^N)FeBr₂ and trigonal bipyramidal dimeric ((P^N)FeBr)₂(μ-Br)₂ complexes were found to be high-spin paramagnets as solids (magnetic susceptibility balance measurements) and in solution (Evans method), while tris-ligated octahedral [(P^N)₃M]²⁺ cations (M = Fe, Ru) are low-spin diamagnets. The electronic environment about each Fe centre was further examined by Mössbauer spectroscopy. The impact of π-extension is evident when comparing the lowest energy absorption bands in the UV-Vis spectra. Contrary to conventional assumptions regarding π-extension, a small hypsochromic shift was observed for the more π-extended phenanthridinyl phosphine-supported Fe(II) complex compared to the analogous quinolinyl complex. Density functional theory (DFT), time-dependent DFT (TDDFT) and charge decomposition analysis (CDA) revealed benzannulation most significantly impacts the character of the lowest energy excitation, rather than appreciably affecting the energies of the frontier orbitals. The impact of electron-transfer on the electronic absorption properties of the π-extended pseudo-octahedral Fe complexes was further explored by spectroelectrochemistry.