Redox Reactions of [FeFe]-Hydrogenase Models Containing an Internal Amine and a Pendant Phosphine

A diiron dithiolate complex with a pendant phosphine coordinated to one of the iron centers, [(μ-SCH₂)₂N­(CH₂C₆H₄-<i>o</i>-PPh₂)­{Fe₂(CO)₅}] (<b>1</b>), was prepared and structurally characterized. The pendant phosphine is dissociated together with a CO ligand in the presence of excess PMe₃, to afford [(μ-SCH₂)₂N­(CH₂C₆H₄-<i>o</i>-PPh₂)­{Fe­(CO)₂(PMe₃)}₂] (<b>2</b>). Redox reactions of <b>2</b> and related complexes were studied in detail by in situ IR spectroscopy. A series of new Fe^IIFe^I ([<b>3</b>]⁺ and [<b>6</b>]⁺), Fe^IIFe^II ([<b>4</b>]²⁺), and Fe^IFe^I (<b>5</b>) complexes relevant to H_ox, H_ox^CO, and H_red states of the [FeFe]-hydrogenase active site were detected. Among these complexes, the molecular structures of the diferrous complex [<b>4</b>]²⁺ with the internal amine and the pendant phosphine co-coordinated to the same iron center and the triphosphine diiron complex <b>5</b> were determined by X-ray crystallography. To make a comparison, the redox reactions of an analogous complex, [(μ-SCH₂)₂N­(CH₂C₆H₅)­{Fe­(CO)₂(PMe₃)}₂] (<b>7</b>), were also investigated by in situ IR spectroscopy in the absence or presence of extrinsic PPh₃, which has no influence on the oxidation reaction of <b>7</b>. The pendant phosphine in the second coordination sphere makes the redox reaction of <b>2</b> different from that of its analogue <b>7