Ligand effects on structural, protophilic and reductive features of stannylated dinuclear iron dithiolato complexes

Abstract

The synthesis and characterization of Fe₂(CO)₅(L){μ-(SCH₂)₂SnMe₂} (L = PPh₃ (2) and P(OMe)₃ (3)) derived from the parent hexacarbonyl complex Fe₂(CO)₆{μ-(SCH₂)₂}SnMe₂ (1) are reported. Whereas 1 exhibits a unique planar structure, X-ray crystallography showed that the apical orientation of L in complexes 2 and 3 results in a chair/boat conformation of the Fe₂S₂C₂Sn fused six-membered rings, which is typical for diiron dithiolato complexes. In solution, NMR and FTIR spectroscopic techniques provide evidence for a dynamic process of apical–basal site exchange of the ligand L in 2 and 3. Protonation experiments on 2 and 3 in MeCN using CF₃CO₂H, HCl or HBF₄·Et₂O suggest enhanced protophilicity of the Fe–Fe bond due to the presence of the electron donor ligands L as well as the stannylation effect. While the carbonyl ligands in 2 stretch at lower wavenumbers ν(CO) than those in 3, the cyclic voltammetric reduction of 2 unpredictably occurs at less negative potential than that of 3. In contrast to 1, the presence of PPh₃ and P(OMe)₃ in 2 and 3, respectively, allows protonation prior to reduction as shown by FTIR spectroscopy and cyclic voltammetry.