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

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.