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 Cs (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 C3v symmetry in 1 and trigonal antiprismatic D3d (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 A in 1 for the six sandwich Tl(I)–Pd distances (to the Pd(A)s and Pd(D)s) being 0.1 A shorter than that of 2.91 A in 2; and (b) the intertriangular distance of 4.50 A between the Pd(A) and Pd(D) centroids in 1 being 0.4 A less than that of 4.90 A 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.