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-iPr2C6H3N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 SFe = 2 metal center is antiferromagnetically
coupled to a ligand-centered radical (SL = 1/2), accounting for the experimentally observed S = 3/2 ground
state. Continued reduction to (iPrPDI)FeLn (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 (SL = 0) and triplet (SL = 1) forms of the bis(imino)pyridine dianion. In both spin states,
the iron is intermediate spin (SFe = 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.