Heterodinuclear nickel(ii)-iron(ii) azadithiolates as structural and functional models for the active site of [NiFe]-hydrogenases

Abstract

To develop the biomimetic chemistry of [NiFe]-H₂ases, the first azadithiolato-bridged NiFe model complexes [CpNi{(μ-SCH₂)₂NR}Fe(CO)(diphos)]BF₄ (5, R = Ph, diphos = dppv; 6, 4-ClC₆H₄, dppv; 7, 4-MeC₆H₄, dppv; 8, CO₂CH₂Ph, dppe; 9, H, dppe) have been synthesized via well-designed synthetic routes. Thus, treatment of RN[CH₂S(O)CMe]₂ with t-BuONa followed by reaction of the resulting intermediates RN(CH₂SNa)₂ with (dppv)Fe(CO)₂Cl₂ or (dppe)Fe(CO)₂Cl₂ gave the N-substituted azadithiolato-chelated Fe complexes [RN(CH₂S)₂]Fe(CO)₂(diphos) (1, R = Ph, diphos = dppv; 2, 4-ClC₆H₄, dppv; 3, 4-MeC₆H₄, dppv; 4, CO₂CH₂Ph, dppe). Further treatment of 1-4 with nickelocene in the presence of HBF₄·Et₂O afforded the corresponding N-substituted azadithiolato-bridged NiFe model complexes 5-8, while treatment of 8 with HBF₄·Et₂O resulted in formation of the parent azadithiolato-bridged model complex 9. While all the new complexes 1-9 were characterized by elemental analysis and spectroscopy, the molecular structures of model complexes 6-8 were confirmed by X-ray crystallographic study. In addition, model complexes 7 and 9 were found to be catalysts for H₂ production with moderate i _cat/i _p and overpotential values from TFA under CV conditions.

Fig. 1. Active site of [NiFe]-H₂ases.

Fig. 2. Models of [NiFe]-H₂ases reported in this article.

Scheme 1. Synthesis of precursor complexes 1–4.

Scheme 2. Synthesis of targeted models 5–8.

Scheme 3. Synthesis of targeted model 9.

Fig. 3. Molecular structure of model complex 6 with ellipsoids drawn at a 50% probability level. All hydrogen atoms and the anion BF₄⁻ are omitted for clarity.

Fig. 4. Molecular structure of model complex 7 with ellipsoids drawn at a 50% probability level. All hydrogen atoms and the anion BF₄⁻ are omitted for clarity.

Fig. 5. Molecular structure of model complex 8 with ellipsoids drawn at a 50% probability level. All hydrogen atoms and the anion BF₄⁻ are omitted for clarity.

Fig. 6. Cyclic voltammograms of 7 and 9 (1.0 mM) in 0.1 M n-Bu₄NPF₆/MeCN at a scan rate of 0.1 V s⁻¹. Arrows indicate the starting potential and scan direction.

Fig. 7. Cyclic voltammograms of 7 (1.0 mM) with TFA (0–10 mM) in 0.1 M n-Bu₄NPF₆/MeCN at a scan rate of 0.1 V s⁻¹.

Fig. 8. Cyclic voltammograms of 9 (1.0 mM) with TFA (0–10 mM) in 0.1 M n-Bu₄NPF₆/MeCN at a scan rate of 0.1 V s⁻¹.