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Keywords:

  • ligand effects;
  • palladium;
  • phosphane oxides;
  • phosphinous acid;
  • platinum

Abstract

Secondary phosphane oxides, R2P(O)H, are commonly used as preligands for transition-metal complexes of phosphinous acids, R2P[BOND]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, (Rf)2P[BOND]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, (Rf)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, (Rf)2P[BOND]O[BOND]H⋅⋅⋅O[DOUBLE BOND]P(Rf)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-[PtCl2({2,4-(CF3)2C6H3}2PO)2]2−. The reaction of (Rf)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 (Rf)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 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.