Air-Stable Platinum and Palladium Complexes Featuring Bis[2,4-bis(trifluoromethyl)phenyl]phosphinous Acid Ligands

摘要

Secondary phosphane oxides, R₂P(O)H, are commonly used as preligands for transition-metal complexes of phosphinous acids, R₂POH (R=alkyl, aryl), which are relevant as efficient catalysts in cross-coupling processes. In contrast to previous work by other groups, we are interested in the ligating properties of an electron-deficient phosphinous acid, (R_f)₂POH, bearing the strongly electron-withdrawing and sterically demanding 2,4-bis(trifluoromethyl)phenyl group towards catalysis-relevant metals, such as palladium and platinum. The preligand bis[2,4-bis(trifluoromethyl)phenyl]phosphane oxide, (R_f)₂P(O)H, reacts smoothly with solid platinum(II) dichloride yielding the trans-configured phosphinous acid platinum complex trans-[PtCl₂({2,4-(CF₃)₂C₆H₃}₂POH)₂]. The deprotonation of one phosphinous acid ligand with an appropriate base leads to the cis-configured monoanion complex cis-[PtCl₂({2,4-(CF₃)₂C₆H₃}₂- PO)₂H]−, featuring the quasi-chelating phosphinous acid phosphinito unit, (R_f)₂POHOP(R_f)₂, which exhibits a strong hydrogen bridge substantiated by an OO distance of 245.1(4) pm. The second deprotonation step is accompanied by a rearrangement to afford the trans-configured dianion trans-[PtCl₂({2,4-(CF₃)₂C₆H₃}₂PO)₂]2−. The reaction of (R_f)₂P(O)H with solid palladium(II) dichloride initially yields a mononuclear palladium complex [PdCl₂({2,4-(CF₃)₂C₆H₃}₂POH)₂], which condenses under liberation of HCl to the neutral dinuclear palladium complex [Pd₂(μ-Cl)₂{({2,4-(CF₃)₂C₆H₃}₂PO)₂H}₂]. The equilibrium between the mononuclear [PdCl₂({2,4-(CF₃)₂C₆H₃}₂POH)₂] and dinuclear [Pd₂(μ-Cl)₂{({2,4-(CF₃)₂C₆H₃}₂PO)₂H}₂] palladium complexes is reversible and can be shifted in each direction by the addition of base or HCl, respectively. Treatment of palladium(II) hexafluoroacetylacetonate, [Pd(F₆acac)₂], with a slight excess of (R_f)₂P(O)H yields the complex [Pd(F₆acac)({2,4-(CF₃)₂C₆H₃}₂- PO)₂H]. The quasi-chelating phosphinous acid phosphinito unit, which is formed by the liberation of HF₆acac, is characterized by a OO distance of 244.1(3) pm. These transition metal complexes are stable towards air and moisture and can be stored for months without any evidence of decomposition.