Bis(imino)pyridine Iron Dinitrogen Compounds Revisited: Differences in Electronic Structure Between Four- and Five-Coordinate Derivatives.

The electronic structures of the four- and five-coordinate aryl-substituted bis­(imino)­pyridine iron dinitrogen complexes, (^iPrPDI)­FeN₂ and (^iPrPDI)­Fe­(N₂)₂ (^iPrPDI = 2,6-(2,6-ⁱPr₂–C₆H₃–N=CMe)₂C₅H₃N), 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)­FeN₂ (^iPrRPDI = 2,6-(2,6-ⁱPr₂–C₆H₃–N=CR)₂C₅H₃N; 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)­FeN₂ compounds are intermediate spin ferrous derivatives (<i>S</i>_Fe = 1) antiferromagnetically coupled to bis­(imino)­pyridine triplet diradical dianions (<i>S</i>_PDI = 1). While this ground state description is identical to that previously reported for (^iPrPDI)­Fe­(DMAP) (DMAP = 4-<i>N</i>,<i>N</i>-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)­FeN₂ family, one of the iron singly occupied molecular orbitals (SOMOs) is essentially d_<i>z</i>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)­FeN₂ and (^iPrPDI)­Fe­(DMAP) differ from (^iPrPDI)­Fe­(N₂)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