Stabilization of thallium(I) Pd₉TlPd₃ sandwich monocation with octahedral-based [Pd₉(CO)₉(PMe₃)₆] and [Pd₃(CO)₃(PMe₃)₃] entities versus unstable Pd₃Tl(I)Pd₃ sandwich monocation with [Pd₃(CO)₃(PEt₃)₃] entities: Comparative computational implications

Abstract The first example of a homopalladium-stabilized sandwich of thallium (I), the {[Pd₉(CO)₉(PMe₃)₆]Tl[Pd₃(CO)₃(PMe₃)₃]}⁺ monocation (1; [PF₆]^- counterion), is reported with the Tl(I) encapsulated within a Pd₉ cage between two unconnected neutral entities, an octahedral-based [Pd₉(CO)₉(PMe₃)₆] and triangular [Pd₃(CO)₃(PMe₃)₃]. This sandwich cluster was obtained (yields > 70%) from the reaction of Pd₈(CO)₈(PMe₃)₇ with TlPF₆, and its solid-state structure was unambiguously determined from a 100 K CCD X-ray diffractometry study. This sandwich cluster may be described as a markedly deformed Pd(A)₃Pd(B)₃ octahedron connected on its triangular Pd(A)₃ face by three edge-bridged wingtip Pd(C) atoms and by a symmetrical capping Tl(I) atom, to which are attached the three triangular Pd(D) atoms of the other [Pd₃(CO)₃(PMe₃)₃] entity. In sharp contrast to its observed stability, an initially isolated Pd₃Tl(I)Pd₃ sandwich monocation, {Tl[Pd₃(CO)₃(PEt₃)₃]₂}⁺ (2; [PF₆]^- counterion), is an intermediate, which in polar solutions spontaneously converts (85% yield) into the stable [Tl₂Pd₁₂(CO)₉(PEt₃)₉]²⁺ dication (3; [PF₆]^- counterion); this instability was attributed to the destabilizing influence of its 6 s² Tl(I) electron-pair. For three previously reported stable Pd-Tl(I) clusters containing analogous octahedral-based [Pd₉(CO)₉L₆] entities {namely, [Pd₉(CO)₉L₆][TlCo(CO)₃L] (4; L = PEt₃), [Pd₉(CO)₉L₆][Tl(acac)] (5; L = PPh₃), and [Pd₉(CO)₉L₆][Tl(PF₆)] (6; L = PPh₃)}, it was then suggested (and still proposed) that the Tl(I) exerts a positive stabilizing influence as a two-electron 6s² donor. Because each of the three edge-bridged wingtip [Pd(μ₂-CO)₂PR₃] fragments (within a Pd₉ entity) in 4, 5, 6, and 1 is now assigned (by us) as a 2e donor (instead of 4e donor), each octahedral-based TlPd₉ polyhedron in these four clusters now has 80 total CVEs (instead of 86 CVEs given by the Wade-Mingos electron-count for a normal octahedral-based polyhedron). The different-sized sandwich-forming Pd(A)₃ and Pd(D)₃ triangles in 1 are oriented in an exact angular-eclipsed conformation (due to crystallographic molecular C_s (m) site symmetry) that differs by 11.7 from being parallel compared to 5.2 in 2 along with a twist-angle deviation of 8.7 in 2 from a regular staggered conformation of the identical-sized Pd₃ sandwich triangles; idealized parallel bis-triangular regular geometries would possess trigonal prismatic C_3v symmetry in 1 and trigonal antiprismatic D_3d (centrosymmetric) symmetry in 2. Gradient-corrected DFT calculations performed on model 1-H and 2-H analogues (with P-attached alkyl substituents replaced by H atoms) suggest from a natural population analysis (NPA) that the observed stability of 1 vs. that of 2 is achieved via a small increased Tl(I) 6s² electron-pair donation onto the Pd cluster manifolds coupled with significantly larger back-donation from the Pd cluster entities onto the empty Tl(I) 6p valence AOs. This indicated strengthening of the Tl(I)-Pd interactions, which is in accordance with the increase in total Wiberg bond index on thallium from 1.70 in 2-H to 2.77 in 1-H, is likewise manifested geometrically in: (a) the mean distance of 2.81 Å in 1 for the six sandwich Tl(I)-Pd distances (to the Pd(A)s and Pd(D)s) being 0.1 Å shorter than that of 2.91 Å in 2; and (b) the intertriangular distance of 4.50 Å between the Pd(A) and Pd(D) centroids in 1 being 0.4 Å less than that of 4.90 Å in 2 (despite the eclipsed conformation in 1 vs. staggered conformation in 2). Intensity ratios of the three types of PMe₃ ligands in the ³¹P{¹H} NMR solution spectrum of 1 are consistent with its solid-state structure being retained in CD₂Cl₂ solution at room temperature.