Formation of thallium(i) sandwich M3TlM3 clusters, [(μ6-Tl)M6(μ2-CO)6(PEt3)6]+ (M = Pt, Pd), with two unconnected triangular M3(μ2-CO)3(PEt3)3 units: implications of comparative analysis of isostructural 5d106s2 Tl(i)–(M3)2 sandwiches (M = Pt, Pd) with known 5d10 Au(i)–(Pt3)2 sandwich

Abstract

The preparation, isolation, and structural/spectroscopic IR, ³¹P{¹H} NMR characterization of two new isostructural 5d¹⁰6s² Tl(I) sandwich clusters, [(μ₆-Tl)Pt₆(μ₂-CO)₆(PEt₃)₆]⁺ (1) and [(μ₆-Tl)Pd₆(μ₂-CO)₆(PEt₃)₆]⁺ (2) as [PF₆]⁻ salts are presented. Each of these closed-subshell M₃TlM₃ sandwiches (M = d¹⁰ Pt (1), Pd (2)) containing two unconnected triangular M₃(μ₂-CO)₃(PR₃)₃ units is held together solely by delocalized/electrostatic M₃–Tl–M₃ bonding. 1 and 2 were synthesized in ca. 90% yields by reactions (under markedly different boundary conditions) of M₄(μ₂-CO)₅(PEt₃)₄ (M = Pt (3), Pd (4)) with TlPF₆. Their isostructural geometries and stoichiometries were unequivocally established from low-temperature CCD X-ray crystallographic determinations. Both 1 and 2 (without their P-attached ethyl substituents) closely conform to a centrosymmetric trigonal-antiprismatic architecture of trigonal D_3d symmetry. A comparison of their well-refined isomorphous crystal structures reveals that the Tl–Pd mean of 2.91 Å in 2 is 0.05 Å smaller than the Tl–Pt mean of 2.96 Å in 1. In solution, 2 is much more kinetically labile than 1 and (unlike 1) readily converts under N₂ into the recently reported [Tl₂Pd₁₂(μ₂-CO)₆(μ₃-CO)₃(PEt₃)₉]²⁺ (5) as the [PF₆]⁻ salt, which was isolated in ca. 90% yield from the same reactants (viz., Pd₄(μ₂-CO)₅(PEt₃)₄ and TlPF₆). In fact, obtaining crystalline material of 2 from recrystallization procedures was greatly hampered by its facile transformation into large quantities of co-crystallized 5. A comparative analysis of the molecular parameters and relative stabilities of the closed-subshell 5d¹⁰6s² Tl(I)–(M₃)₂ sandwiches (M = d¹⁰ Pt (1), Pd (2)) with the corresponding known closed-subshell 5d¹⁰ Au(I)–(Pt₃)₂ sandwich together with an examination of relative shifts of corresponding bridging carbonyl IR frequencies for selected pairs of related clusters provide compelling evidence that the so-called “inert” 6s² electron-pair on the Tl(I) exerts an overall destabilizing influence: namely, that the highly electrophilic 5d¹⁰ Au(I) forms considerably stronger delocalized sandwich Pt₃–Au–Pt₃ bonding (due to its empty, relativistically stabilized 6s acceptor AO) which is presumed to have considerable covalent character. The Tl(I)–Pt(0) distances in 1 are similar to the Tl(I)–Au(I) distances found for another recently reported class of two electronically equivalent closed-subshell Au₃TlAu₃ sandwich units (i.e., 5d¹⁰ Au(I) vs. 5d¹⁰ Pt(0)) formed by intercalation of Tl⁺ between two electron-rich intramolecular, weakly bonded (aurophilic) Au₃ triangles in trinuclear cyclic gold(I) benzylimidazolate and carbeniate molecules; the Au₃TlAu₃ sandwich units stack into linear chains with intermolecular aurophilic Au(I)–Au(I) interactions between four of the six Au(I) atoms in adjacent units. Of particular interest is that the Tl(I)–Au(I) distances (means, 3.02 and 3.09 Å) in the distorted trigonal-prismatic (μ₆-Tl)Au₆ sandwich units of the geometrically related Tl⁺-intercalated TR(bzim) and TR(carb) complexes are 0.2–0.3 Å longer than the Ag(I)–Au(I) distances (mean, 2.81 Å) in the initially known Au₃AgAu₃ sandwich unit of the Ag⁺-intercalated TR(bzim) analogue; it is similarly proposed that this parallel (M′–Au) bond-length difference may likewise be attributed to the considerably smaller electrophilic character of the central 5d¹⁰6s² Tl(I) vs. that of the 4d¹⁰ Ag(I) due to the overall destabilizing effect of the thallium(I) 6s² electron-pair.