The reactions of the third transition series metal hexafluorides, MF 6, M = Pt, Ir, Os, Re and W, with carbon monoxide in superacid media

Abstract

The reactions of the third transition (5d) series metal hexafluorides PtF 6, IrF 6, OsF 6, ReF 6 and WF 6 with CO are studied in the Lewis superacid SbF 5, the Brønsted superacid HF and the conjugate Brønsted–Lewis superacid HF–SbF 5, in order to evaluate their potential as reactants in reductive carbonylations. These reactions are aimed at the generation of homoleptic metal carbonyl cations of the type [M(CO) n ] m+ , M = Pt, Ir, Os or Re, n = 4 or 6 and m = 1, 2 or 3. Under very mild conditions, PtF 6 is cleanly converted in liquid SbF 5 to give [Pt(CO) 4][Sb 2F 11] 2 in quantitative yield. In anhydrous hydrogen fluoride, PtF 6 is reduced and partly carbonylated to the mixed valency product [Pt(CO) 4][PtF 6], the first carbonyl fluoride of platinum. With IrF 6 in SbF 5, [Ir(CO) 6][Sb 2F 11] 3 is obtained, which contains with [Ir(CO) 6] 3+ the first and so far only known homoleptic tripositive metal carbonyl cation. The reaction of OsF 6 with CO in HF–SbF 5 produces single crystals of [Os(CO) 6][Sb 2F 11] 2, which are suitable for a molecular structure determination. ReF 6 reacts in SbF 5 very slowly with CO, and the previously known [Re(CO) 6] + is only obtained in low yield. Finally WF 6 does not react with CO in any of the superacid media under comparable conditions. As by-product in the reduction of MF 6, M = Pt, Ir and Os, only carbonyl fluoride, COF 2, forms, which allows very facile product isolation. The observed order of reactivity: PtF 6 > IrF 6 > OsF 6 > ReF 6 > > WF 6 follows a previously established order of oxidizing ability and electron affinity of the metal hexafluorides. The mixed valency compound [Pt(CO) 4][PtF 6] is studied by vibrational spectroscopy and is found to contain a square planar [Pt(CO) 4] 2+ cation and a [PtF 6] 2− anion with a trigonally distorted octahedral geometry of pointgroup D 3d. A vibrational assignment of [Ir(CO) 6] 3+ is presented, aided by 13C substitution and supported by ab initio calculations.