Functionalized isocyanides as ligands. Part 5. Syntheses and reactions of 3-(benzylphosphonio)indolin-2-ylidene complexes of platinum(II). X-Ray crystal structure of trans-[Pt{o-CN(H)C6H4C[P(CH2Ph)2Ph]}X(PPh3)2]BF4·2C2H4Cl2(X = Cl or Br)

Abstract

Reactions of the isocyanide o-ClCH₂C₆H₄NC with benzylphosphines, PR₃= P(CH₂Ph)Ph₂, P(CH₂Ph)₂Ph, or P(CH₂Ph)₃, in acetone at room temperature in the presence of excess of LiBr, yield the benzylphosphonium-substituted isocyanides [o-R₃PCH₂C₆H₄NC]Br, which are converted to the less hydroscopic tetrafluoroborate salts [o-R₃PCH₂C₆H₄NC]BF₄(L)[PR₃= P(CH₂Ph)Ph₂(L¹), P(CH₂Ph)₂Ph (L²), or P (CH₂Ph)₃(L³)] upon reaction with excess NaBF₄ in acetone. The ligands, L, react with a mixture of cis- and trans-[PtCl₂(PPh₃)₂] and NaBF₄ to form the cationic complexes trans-[Pt(L)Cl(PPh₃)₂] BF₄[(1a)–(3a) for L¹–L³]. Treatment of complexes (1a)–(3a) with a 10-fold excess of NEt₃ in CH₂Cl₂ at room temperature leads to the cyclization reaction of the isocyanide ligands via phosphorus-ylide intermediates with the formation of 3-(benzylphosphonio)indolin-2-ylidene derivatives trans-[Pt{o-[graphic omitted](PR₃)}Cl(PPh₃)₂]BF₄[PR₃= P(CH₂Ph)Ph₂(1b), P(CH₂Ph)₂Ph (2b), or P(CH₂Ph)₃(3b)]. Complexes (1a)–(3a) and (1b)–(3b) were characterized by their elemental analyses, i.r., ¹H, and ³¹P n.m.r. spectra. The structure of trans-[Pt{o-[graphic omitted][P(CH₂Ph)₂Ph]}X(PPh₃)₂]BF₄·2C₂H₄Cl₂(X = Cl or Br) was determined by X-ray diffraction: space group C2, a= 21.946(1), b= 14.097(3), c= 19.801(1)Å, β= 96.95(1)°, Z= 4, R= 0.037 (R′= 0.041) for 4 802 independent reflections. The co-ordination geometry around the Pt^II atom is square planar with the indole ligand perpendicular to the plane. The Pt–C(sp²) distance is 2.036(8)Å. The bond lengths within the condensed system indicate extensive electronic delocalization. Complex (1b) reacts with aqueous KOH in acetone at room temperature to give, in high yield, the platinaheterocycle [[graphic omitted]HPh)Ph₂]}Cl(PPh₃)](1c). Chloride abstraction by AgBF₄ from (1c) and reaction of the cationic intermediate with OH^–, CO, and PPh₃ afford the corresponding derivatives (1d), (1e), and (1f), respectively.