Hydrophilic quaternary ammonium-group-containing [FeFe]H2ase models prepared by quaternization of the pyridyl N atoms in pyridylazadiphosphine- and pyridylmethylazadiphosphine-bridged diiron complexes with various electrophiles

Abstract

The first aromatic quaternary ammonium-group-containing [FeFe]H₂ase models have been prepared by a simple and convenient two-step method in high yields. Thus, on the basis of preparation of the N-pyridylazadiphosphine-bridged diiron complex (μ-PDT)Fe₂(CO)₄[μ-3-(Ph₂P)₂NC₅H₄N] (A) by CO substitution of parent complex (μ-PDT)Fe₂(CO)₆ with N-pyridylazadiphosphine 3-(Ph₂P)₂NC₅H₄N in refluxing xylene, further quaternization of the pyridyl N atom in complex A with electrophile 1,3-propanesultone, 1,3,2-dioxathiolane-2,2-dioxide, or 4-bromobutyric acid in refluxing MeCN afforded the pyridyl quaternary ammonium-group-containing models (μ-PDT)Fe₂(CO)₄[μ-3-(Ph₂P)₂NC₅H₄NR] (1, R = (CH₂)₃SO₃; 2, R = (CH₂)₂OSO₃) and (μ-PDT)Fe₂(CO)₄[μ-3-(Ph₂P)₂NC₅H₄N(CH₂)₃CO₂H]Br (3). Similarly, the N-pyridylmethylazadiphosphine-bridged diiron complex (μ-PDT)Fe₂(CO)₄[μ-3-(Ph₂P)₂NCH₂C₅H₄N] (B) could be prepared by CO substitution of parent complex (μ-PDT)Fe₂(CO)₆ with N-pyridylmethylazadiphosphine 3-(Ph₂P)₂NCH₂C₅H₄N in refluxing xylene, while further quaternization of the pyridylmethyl N atom in complex B with 1,3-propanesultone and 3-bromo-1-propanol in MeCN at reflux resulted in formation of the pyridylmethyl quaternary ammonium-group-containing models (μ-PDT)Fe₂(CO)₄[μ-3-(Ph₂P)₂NCH₂C₅H₄N(CH₂)₃SO₃] (4) and (μ-PDT)Fe₂(CO)₄[μ-3-(Ph₂P)₂NCH₂C₅H₄N(CH₂)₃OH]Br (5), respectively. All new complexes A, B, and 1–5 were characterized by elemental analysis and various spectroscopies, while the molecular structures of complexes A, B, 2 and 5 were further confirmed by X-ray crystallography. The electrochemical study on hydrophilic models 1 and 3 in MeCN and the MeCN/H₂O mixed solvent indicated that the reduction potentials were shifted to less-negative potentials as the water content increased; such an observation implies that both 1 and 3 are easily reduced in the mixed MeCN/H₂O solvent than in MeCN. In addition, the electrocatalytic study demonstrated that both 1 and 3 can serve as electrocatalysts for H₂ production from acetic acid with higher i_cat/i_p and TONs in MeCN/H₂O than in MeCN.