Electronic structure of bis(imino)pyridine iron dichloride, monochloride, and neutral ligand complexes: A combined structural, spectroscopic, and computational study

The electronic structure of a family of bis(imino)pyridine iron dihalide, monohalide, and neutral ligand compounds has been investigated by spectroscopic and computational methods. The metrical parameters combined with Mössbauer spectroscopic and magnetic data for (ⁱPrPDI)FeCl₂ ( ⁱPrPDI = 2,6-(2,6-ⁱPr₂C₆H ₃N=CMe)₂C₅H₃N) established a high-spin ferrous center ligated by a neutral bis(imino)pyridine ligand. Comparing these data to those for the single electron reduction product, ( ⁱPrPDI)FeCl, again demonstrated a high-spin ferrous ion, but in this case the S_Fe = 2 metal center is antiferromagnetically coupled to a ligand-centered radical (S_L = 1/2), accounting for the experimentally observed S = 3/2 ground state. Continued reduction to (ⁱPrPDI) FeL_n (L = N₂, n = 1,2; CO, n = 2; 4-(N,N-dimethylamino) pyridine, n = 1) resulted in a doubly reduced bis(imino)pyridine diradical, preserving the ferrous ion. Both the computational and the experimental data for the N,N-(dimethylamino)pyridine compound demonstrate nearly isoenergetic singlet (S_L = 0) and triplet (S_L = 1) forms of the bis(imino)pyridine dianion. In both spin states, the iron is intermediate spin (S_Fe = 1) ferrous. Experimentally, the compound has a spin singlet ground state (S = 0) due to antiferromagnetic coupling of iron and the ligand triplet state. Mixing of the singlet diradical excited state with the triplet ground state of the ligand via spin-orbit coupling results in temperature-independent paramagnetism and accounts for the large dispersion in ¹H NMR chemical shifts observed for the in-plane protons on the chelate. Overall, these studies establish that reduction of ( ⁱPrPDI)FeCl₂ with alkali metal or borohydride reagents results in sequential electron transfers to the conjugated π-system of the ligand rather than to the metal center.