Ligand dependence of binding to three-coordinate Fe(II) complexes

A series of three- and four-coordinate iron(II) complexes with nitrogen, chlorine, oxygen, and sulfur ligands is presented. The electronic variation is explored by measuring the association constant of the neutral ligands and the reduction potential of the iron(II) complexes. Varying the neutral ligand gives large changes in K(eq), which decrease in the order CN(t)Bu > pyridine >2-picoline > DMF > MeCN > THF > PPh(3). These differences can be attributed to a mixture of steric effects and electronic effects (both sigma-donation and pi-backbonding). The binding constants and the reduction potentials are surprisingly insensitive to changes in an anionic spectator ligand. This suggests that three-coordinate iron(II) complexes may have similar binding trends as proposed three-coordinate iron(II) intermediates in the FeMoco of nitrogenase, even though the anionic spectator ligands in the synthetic complexes differ from the sulfides in the FeMoco.