Air-stable platinum and palladium complexes featuring bis[2,4-bis(trifluoromethyl)phenyl]phosphinous acid ligands

Secondary phosphane oxides, R(2)P(O)H, are commonly used as preligands for transition-metal complexes of phosphinous acids, R(2)P-OH (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))(2)P-OH, 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))(2)P(O)H, reacts smoothly with solid platinum(II) dichloride yielding the trans-configured phosphinous acid platinum complex trans-[PtCl(2)({2,4-(CF(3))(2)C(6)H(3)}(2)POH)(2)]. The deprotonation of one phosphinous acid ligand with an appropriate base leads to the cis-configured monoanion complex cis-[PtCl(2)({2,4-(CF(3))(2)C(6)H(3)}(2)PO)(2)H](-), featuring the quasi-chelating phosphinous acid phosphinito unit, (R(f))(2)P-O-H···O=P(R(f))(2), which exhibits a strong hydrogen bridge substantiated by an O···O distance of 245.1(4) pm. The second deprotonation step is accompanied by a rearrangement to afford the trans-configured dianion trans-[PtCl(2)({2,4-(CF(3))(2)C(6)H(3)}(2)PO)(2)](2-). The reaction of (R(f))(2)P(O)H with solid palladium(II) dichloride initially yields a mononuclear palladium complex [PdCl(2)({2,4-(CF(3))(2)C(6)H(3)}(2)POH)(2)], which condenses under liberation of HCl to the neutral dinuclear palladium complex [Pd(2)(μ-Cl)(2){({2,4-(CF(3))(2)C(6)H(3)}(2)PO)(2)H}(2)]. The equilibrium between the mononuclear [PdCl(2)({2,4-(CF(3))(2)C(6)H(3)}(2)POH)(2)] and dinuclear [Pd(2)(μ-Cl)(2){({2,4-(CF(3))(2)C(6)H(3)}(2)PO)(2)H}(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(F(6)acac)(2)], with a slight excess of (R(f))(2)P(O)H yields the complex [Pd(F(6)acac)({2,4-(CF(3))(2)C(6)H(3)}(2)PO)(2)H]. The quasi-chelating phosphinous acid phosphinito unit, which is formed by the liberation of HF(6)acac, is characterized by a O···O 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.