Hemilabile and reversible carbon monoxide binding properties of iron(ii), cobalt(ii) and nickel(ii) complexes containing a new tridentate P–S–N ligand

Abstract

Several first-row transition metal complexes of the formulation [M(1)₂](X)₂ {where 1 = Ph₂PCH₂CH₂S(2-C₆H₄NH₂); M = Fe^II, X = BF₄⁻ (2); M = Co^II, X = BF₄⁻ (3), Ni^II, X = ClO₄⁻ (4)} have been prepared by reaction of two equivalents of the new P–S–N ligand Ph₂PCH₂CH₂S(2-C₆H₄NH₂) 1 with one equivalent of the appropriate [M(OH₂)₆](X)₂ precursor in acetonitrile. In the solid state, complexes 2–4 exist as distorted centrosymmetric octahedral structures featuring facially capping ligands in an all-trans arrangement. Reaction of 2 and 3 with a stream of carbon monoxide (1 atm.) for 5 min in acetonitrile generates iron(II) monocarbonyl species of formulation [Fe(CO)(1)₂](BF₄)₂2a, and a cobalt(II) dicarbonyl complex, [Co(CO)₂(1)₂](BF₄)₂3a, which can be isolated in the solid state. Complete removal of CO is achieved by either heating to reflux samples of 2a in acetonitrile for 5 min or by heating solid samples of 3a at 120 °C in vacuo over a period of 4 h. The binding of carbon monoxide is fully reversible for 2 and 3 and can be repeated over multiple cycles. When the same trapping reactions were carried out with very low radiochemical ¹¹CO concentrations, metal carbonyl species were no longer formed. It is likely that the kinetics of ¹¹CO adduct formation are too slow to allow for effective trapping under the applied radiochemical conditions.