Three half-sandwiched iron(II) monocarbonyl complexes with PNP ligands: Their chemistry upon reduction and catalysis on proton reduction

Abstract

Three half-sandwiched iron(II) monocarbonyl cationic complexes (1+−3+) containing a azadiphoshine ligand (PNP, L1–3) and an iodide as the counter anion, [Fe(II)Cp(CO)(L1–3)]Ι (Cp = η5-cyclopentadienyl; L1 = Ph2PN(cyclohexyl)PPh2, 1+; L2 = Ph2PN(phenyl)PPh2, 2+; L3 = Ph2PN(benzyl)PPh2, 3+) were reported. All the complexes were fully characterized, of which complexes 2+ and 3+ were crystallographically analysed to confirm their “piano stool” geometry. Electrochemically, complexes 1+−3+ exhibited two major reduction events at about −1.9 V and −2.3 V, respectively, assigned to Fe(II) → Fe(I) and Fe(I) → Fe(0), respectively. The first reduction produces a 19e-species, [Fe(I)Cp(CO)(L1–3)] (1, 2, and 3) whereas the second event is not assigned to the further reduction of the 19e-species but a 17e-intermediate ([Fe(I)Cp(CO)(m-L1–3)]; 1′, 2′ and 3′) which is chemically generated from the 19e-species by cleaving one of the two Fe−P bonds. However, the 19e-species 3 is considerably more stable than the other two analogues (1 and 2) due to H−π interaction between the ortho-H atoms from one of the four phenyl rings on the phosphine atom and the phenyl ring of the benzyl group as revealed by theoretic calculations. Therefore, species 3 is stable enough for the reduction of 3 → 3− to be electrochemically detected right before the second major reduction. To further shed some light on the electrochemical mechanism, both bulk electrolysis and chemical reduction of complexes 1+−3+ with cobaltocene as the reducing agent were conducted. Under N2 atmosphere, the neutral 17e-species, [Fe(I)Cp(CO)(m-L1–3)] (1′-3′) was spectroscopically detected whereas under CO atmosphere, CO-adducts, [Fe(I)Cp(CO)2(m-L1–3)], were generated. The formation of CO-adduct explains the loss of reversibility of the first reduction (3+) under CO atmosphere in its electrochemistry. With acetic acid as the proton source, only the second major reduction of the complexes exhibited significant catalytic activity, of which complex 1+ possessed the best performance. Significant proton-coupled electron transfer (PCET) effect was observed for complex 1+ due to the strong electron-donating capability of the ligand (L1).