Synthesis of hydridotrifluoromethyl complexes of platinum(II). A spectroscopic investigation of the trans and cis influence of ligands in hydridoplatinum(II) compounds

Abstract

The starting complex trans-[PtH(CF₃)(PPh₃)₂] is prepared from trans-[PtCl(CF₃)(PPh₃)₂] by treatment of the derived labile solvento cathionic species trans-[Pt(CF₃)(PPh₃)₂(solv)]BF₄(solv = acetone or CH₂Cl₂) with NaBH₄ in EtOH at 0 °C. The hydridotrifluoromethyl complexes trans-[PtH(CF₃)L₂][L = PMePh₂, PMe₂Ph, PMe₃, P(CH₂Ph)Ph₂, P(CH₂Ph)₂Ph, P(CH₂Ph)₃, PEtPh₂, P(C₆H₁₁)₃, or P (C₆H₄Me-4)₃] are obtained by reaction of trans-[PtH(CF₃)(PPh₃)₂] with 2 equivalents of phosphine, L, in n-heptane at room temperature. Similar exchange reactions between trans-[PtH(CF₃)(PPh₃)₂] and equivalent amounts of diphosphine, L–L =cis-Ph₂PCHCHPPh₂, Ph₂PCH₂CH₂PPh₂, Me₂PCH₂CH₂PMe₂, or Ph₂PCH₂CH₂CH₂PPh₂, lead to the formation of the corresponding [PtH(CF₃)(L–L)] compounds. The hydridotrifluoromethyl complexes with L–L =cis-Ph₂PCHCHPPh₂ and Ph₂PCH₂CH₂PPh₂ can be prepared also by reaction of the parent chloro derivatives [PtCl(CF₃)(L–L)] with NaBH₄ in EtOH at room temperatue. The mixed isocyanide–phosphine complexes [PtH(CF₃)(PPh₃)(CNR)][R = 2,6-Me₂C₆H₃, 4-MeOC₆H₄, or Bu^t] are obtained by reaction of trans-[PtH(CF₃)(PPh₃)₂] with a three-fold excess of RNC in n-heptane at room temperature. All the hydridotrifluoromethyl complexes are air-stable in the sold state and in solution. They were characterized by elemental analyses and i.r., ¹H, ¹⁹F, and ³¹P n.m.r. spectra. The data obtained for ν(PtH) and ¹J(PtH) in trans-[PtH(CF₃)(PPh₃)₂] have been used to compare the trans influence of CF₃^– with other σ carbon donors,R, in trans-[PtH(R)(PPh₃)₂] derivatives. The n.m.r.-based trans influence order is CF₃^– > C₆H₅^– > C₆H₉^– > CH₂CH₂CN^– > CH₂CH₂CH₂CN^– > CH₃^– > CH₂CN^– > Cl^–, wheras the i.r.-based trans influence order is C₆H₉^– > CH₂CH₂CH₂CN^– > C₆H₅^–≈ CH₃^– > CF₃^– > Cl^–. The opposite position of CF₃^– in the two series of trans influence has been explained by the different mechanism operating on ¹J(PtH) and ν(PtH). The first depends predominantly on the s character of the platinum hybrid orbital used in the Pt–H bond, while the second is sensitive also to electrostatic effects induced by the electronegative fluorine atoms. The ¹J(PtP) data for the same series of complexes gives the following order of cis influence: Cl^– > CH₂CN^– > CF₃^–≈ CH₃^–≈ C₆H₅^–≈ CH₂CH₂CN^– > CH₂CH₂CH₂CN^–. From the spectra of trans-[PtH(X)L₂](X = CF₃ or Cl), the effects of replacing two PPh₃ ligands in trans-[PtH(CF₃)(PPh₃)₂] by L on ν(PtH) and ¹J(PtH) were measured. With the assumption that the cis effects are additive, the i.r. and n.m.r. parameters were correlated with the electronic χ and steric θ parameters of the phosphine ligands.