Electrochemically controlling ligand binding affinity for transition metals via RHLs: The importance of electrostatic effects

A series of redox-switchable hemilabile ligands (RHLs) has been synthesized that incorporates ferrocene as the redox group and phosphine ether or phosphine thioether moieties as binding groups. These ligands, which complex to Rh(I) and Pd(II), yield electrochemical control over ligand binding affinity for transition metals in complexes of the following type: [M(η⁴-(η⁵-C₅H₄XCH₂CH₂PR₂)₂Fe)](y+) (5: M = Rh, X = O, R = Ph (phenyl), y = 1; 6: M = Rh, X = O, R = Cy (cyclohexyl), y = 1; 9: M = Rh, X = S, R = Ph, y = 1; 10: M = Pd, X = O, R = Ph, y = 2; 11: M = Pd, X = O, R = Cy, y = 2). In the case of 11, ligand based oxidation decreases the ligand to metal binding constant by nearly ten orders of magnitude. An examination of the crystal structures of 5, 9, 10, and 11 and the electrochemical behavior of a series of RHL-complexes and isoelectronic model complexes reveals that electrostatic effects play a significant role in the charge dependent behaviors of these complexes. Additionally, there is a correlation between the phosphine substituents and RHL-complex stability. As a general rule cyclohexyl groups stabilize the complexes in their oxidized states over phenyl groups. In this study, RHLs are shown to provide a viable means of electrochemically controlling ligand binding affinity and thus the steric and electronic environment of bound transition metals.