The electronic structures of the four- and five-coordinate aryl-substituted bis(imino)pyridine iron dinitrogen complexes, ( iPrPDI)FeN 2 and ( iPrPDI)Fe(N 2) 2 ( iPrPDI = 2,6-(2,6- iPr 2-C 6H 3-N=CMe) 2C 5H 3N), have been investigated by a combination of spectroscopic techniques (NMR, Mössbauer, X-ray Absorption, and X-ray Emission) and DFT calculations. Homologation of the imine methyl backbone to ethyl or isopropyl groups resulted in the preparation of the new bis(imino)pyridine iron dinitrogen complexes, ( iPrRPDI) FeN 2 ( iPrRPDI = 2,6-(2,6- iPr 2-C 6H 3-N=CR) 2C 5H 3N; R = Et, iPr), that are exclusively four coordinate both in the solid state and in solution. The spectroscopic and computational data establish that the ( iPrRPDI)FeN 2 compounds are intermediate spin ferrous derivatives (S Fe = 1) antiferromagnetically coupled to bis(imino)pyridine triplet diradical dianions (S PDI = 1). While this ground state description is identical to that previously reported for ( iPrPDI)Fe(DMAP) (DMAP = 4-N,N-dimethylaminopyridine) and other four-coordinate iron compounds with principally σ-donating ligands, the d-orbital energetics determine the degree of coupling of the metal-chelate magnetic orbitals resulting in different NMR spectroscopic behavior. For ( iPrRPDI)Fe(DMAP) and related compounds, this coupling is strong and results in temperature independent paramagnetism where a triplet excited state mixes with the singlet ground state via spin orbit coupling. In the ( iPrRPDI)FeN 2 family, one of the iron singly occupied molecular orbitals (SOMOs) is essentially dz 2 in character resulting in poor overlap with the magnetic orbitals of the chelate, leading to thermal population of the triplet state and hence temperature dependent NMR behavior. The electronic structures of ( iPrRPDI)FeN 2 and ( iPrPDI)Fe(DMAP) differ from ( iPrPDI)Fe(N 2) 2, a highly covalent molecule with a redox noninnocent chelate that is best described as a resonance hybrid between iron(0) and iron(II) canonical forms as originally proposed in 2004.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Inorganic Chemistry