Phosphine-ligated induced formation of thallium(i) “full” Pt3TlPt3 sandwich versus “open-face” TlPt3 sandwich with triangular Pt3(µ2-CO)3(PR3)3 units: synthesis and structural/spectroscopic analysis of triphenylphosphine [(µ3-Tl)Pt3(µ2-CO)3(PPh3)3]+ and its (µ3-AuPPh3)Pt3 analogue

Abstract

This research constitutes an operational test to assess the influence of platinum-attached phosphine ligands in the formation process of “open-face” TlPt₃ or “full” Pt₃TlPt₃ sandwich clusters. Accordingly, the reaction of TlPF₆ with triphenylphosphine Pt₄(µ₂-CO)₅(PPh₃)₄, under essentially identical boundary conditions originally used to prepare (90% yield) the triethylphosphine “full” Pt₃TlPt₃ sandwich, [(µ₆-Tl)Pt₆(µ₂-CO)₆(PEt₃)₆]⁺ (3) ([PF₆]⁻ salt), from Pt₄(µ₂-CO)₅(PEt₃)₄ was carried out to see whether it would likewise afford the unknown triphenylphosphine Pt₃TlPt₃ sandwich analogue of 3 or whether the change of phosphine ligands from sterically smaller, more basic PEt₃ to PPh₃ would cause the product to be the corresponding unknown triphenylphosphine “open-face” TlPt₃ sandwich that would geometrically resemble the known bulky tricyclohexylphosphine [(µ₃-Tl)Pt₃(µ₂-CO)₃(PCy₃)₃]⁺ sandwich (2a). Both the structure and composition of the resulting “open-face” sandwich product, [(µ₃-Tl)Pt₃(µ₂-CO)₃(PPh₃)₃]⁺ (1a) ([PF₆]⁻ salt), were unequivocally established from a low-temperature CCD X-ray crystallographic determination. The calculated Pt/Tl atom ratio (3/1) of 75%/25% is in excellent agreement with that of 72(3)%/28(5)% obtained from energy-resolved measurements on a single crystal with a scanning electron microscope. Crystals (80% yield) of the orange–red 1a were characterized by solid-state/solution IR and variable temperature ²⁰⁵Tl and ³¹P{¹H} NMR spectra; the ³¹P{¹H} spectra provide convincing evidence that 1a is exhibiting dynamic behavior at room temperature in CDCl₃ solution. The corresponding new “open-face” (µ₃-AuPPh₃)Pt₃ sandwich, [(µ₃-AuPPh₃)Pt₃(µ₂-CO)₃(PPh₃)₃]⁺ (1b) ([PF₆]⁻ salt), was quantitatively obtained from 1a by reaction with AuPPh₃Cl and spectroscopically characterized by IR and ³¹P{¹H} NMR spectra. A comparative geometrical evaluation of the observed steric dispositions of the platinum-attached PR₃ ligands in the “open-face” (µ₃-Tl)Pt₃ sandwiches of 1a (with PPh₃) and the known 2a (with PCy₃) and in the known “full” Pt₃TlPt₃ sandwich of 3 (with PEt₃) along with the considerably different observed steric dispositions of the PR₃ ligands in the known “open-face” (µ₃-AuPCy₃)Pt₃ sandwich of 2b (with PCy₃) and in the known “full” Pt₃AuPt₃ sandwich of 4 (with PPh₃) has been performed. The results clearly indicate that, in contradistinction to the known triphenylphosphine Pt₃AuPt₃ sandwich of 4, PPh₃ and bulkier PCy₃ ligands of Pt₃(µ₂-CO)₃(PR₃)₃ units are sterically too large to form “full” Pt₃TlPt₃ sandwiches. In other words, the nature of the thallium(I) sandwich-product in these reactions is sterically controlled by size effects of the phosphine ligands. Comparative examination of bridging carbonyl IR frequencies of 1a and 1b with those of closely related “open-face” and “full” sandwiches provides better insight concerning the relative electrophilic capacities of Tl⁺, Au⁺, and [AuPR₃]⁺ components in forming sandwich adducts with Pt₃(µ₂-CO)₃(PR₃)₃ nucleophiles.