Reductive Coupling of CO Templated by Iron Bound to the Tris(pyrrolide)ethane Scaffold

The reactivity of the high-spin (S = 2) [(Mestpe)Fe(THF)][Li(THF)4] (1) complex (Mestpe = tris(mesitylpyrrolide)ethane) with isocyanide and CO substrates is explored. Reaction of 1 with excess tBuNC forms a low-spin (S = 0), six-coordinate iron(II) species with three tBuNC ligands bound to iron, producing a notable tautomerization of one of the pyrrolide units from N- to C-ligation to iron. Reaction of 1 with an atmosphere of CO also produces a new diamagnetic complex, wherein two molecules of CO are consecutively reductively coupled, driven by the two-electron oxidation and fragmentation of the tris(pyrrolide)ethane ligand. The product features a six-coordinate Fe(II) species bound to a dipyrromethene ligand (resulting from oxidative fragmentation of the Mestpe ligand), an oxalyl-imino pyrrole fragment from pyrrole coupling to two molecules of CO. The reactions of 1 with tBuNC and CO provide insight into how tautomerization of the tris(pyrrolide) ligand upon substrate binding initiates the contiguous reductive coupling of CO.