Reversible C-C Bond Formation between Redox-Active Pyridine Ligands in Iron Complexes

This manuscript describes the formally iron(I) complexes (LFe)-Fe-Me(Py-R)(2) (L-Me = bulky beta-diketiminate; R = H, 4-tBu), in which the basal pyridine ligands preferentially accept significant unpaired spin density. Structural, spectroscopic, and computational studies on the complex with 4-tert-butylpyridine ((tBu)py) indicate that the S = 3/2 species is a resonance hybrid between descriptions as (a) high-spin iron(II) with antiferromagnetic coupling to a pyridine anion radical and (b) high-spin iron(I). When the pyridine lacks the protection of the tert-butyl group, it rapidly and reversibly undergoes radical coupling reactions that form new C-C bonds. In one reaction, the coordinated pyridine couples to triphenylmethyl radical, and in another, it dimerizes to give a pyridine-derived dianion that bridges two iron(II) ions. The rapid, reversible C-C bond formation in the dimer stores electrons from the formally reduced metal as a C-C bond in the ligands, as demonstrated by using the coupled diiron(II) complex to generate products that are known to come from iron(I) precursors.