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

摘要

Secondary phosphane oxides, R _2P(O)H, are commonly used as preligands for transition-metal complexes of phosphinous acids, R _2P-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) _2P-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) _2P(O)H, reacts smoothly with solid platinum(II) dichloride yielding the trans-configured phosphinous acid platinum complex trans-[PtCl _2({2,4-(CF _3) _2C _6H _3} _2POH) _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) _2C _6H _3} _2- PO) _2H] ~-, featuring the quasi-chelating phosphinous acid phosphinito unit, (R _f) _2P-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) _2C _6H _3} _2PO) _2] ~(2-). The reaction of (R _f) _2P(O)H with solid palladium(II) dichloride initially yields a mononuclear palladium complex [PdCl _2({2,4-(CF _3) _2C _6H _3} _2POH) _2], which condenses under liberation of HCl to the neutral dinuclear palladium complex [Pd _2(μ-Cl) _2{({2,4-(CF _3) _2C _6H _3} _2PO) _2H} _2]. The equilibrium between the mononuclear [PdCl _2({2,4-(CF _3) _2C _6H _3} _2POH) _2] and dinuclear [Pd _2(μ-Cl) _2{({2,4-(CF _3) _2C _6H _3} _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(F _6acac) _2], with a slight excess of (R _f) _2P(O)H yields the complex [Pd(F _6acac)({2,4- (CF _3) _2C _6H _3} _2- PO) _2H]. The quasi-chelating phosphinous acid phosphinito unit, which is formed by the liberation of HF _6acac, 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.