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High-Yield Synthesis of PPh3-Ligated Decanuclear Tl–Pd Cluster, Pd9[Tl(acac)](CO)9(PPh3)6: Comparative Analysis of Tl(I)–Pd(0) Bonding Connectivities with Known Tl–Pd Clusters and Resulting Insight Concerning Their Dissimilar Dynamic Solution Behavior

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journal contribution
posted on 2016-02-21, 15:14 authored by Evgueni G. Mednikov, Nicky Vo, Charles G. Fry, Lawrence F. Dahl
The new Tl­(I)–Pd(0) cluster Pd93/3-Tl­(acac)]­(μ2-CO)63-CO)3(PPh3)6 (1) was prepared in high yields (over 90%), both by reaction of Pd10(CO)12(PPh3)6 (4), PPh3, and TlPF6 in THF in the presence of acetylacetone (Hacac) and base (NEt3) and by direct reaction of Pd10(CO)12(PPh3)6 with PPh3 and Tl­(acac). The composition and molecular structure of 1 were unambiguously established from 100 K CCD X-ray diffractometry studies of two solvated crystals, 1·1.5Hacac·0.5THF (1A) and 1·0.3THF (1B), which showed essentially identical geometries for the entire Pd9Tl­(CO)9P6 fragment of pseudo-C3v symmetry; its composition is in agreement with X-ray Tl/Pd field-emission microanalysis with a scanning electron microscope for crystals of 1B. This cluster can be viewed as a markedly deformed Pd6 octahedron (oct) with the three Pd­(oct) atoms of one of its eight triangular faces connected both by three edge-bridging wingtip (wt) Pd­(μ2-CO)2PPh3 fragments and by a symmetrical capping Tl­(I). Three triply bridging carbonyl ligands asymmetrically cap the lower alternate 3-fold-related triangular faces of the Pd6 octahedron, and the three other PPh3 ligands are each coordinated to Pd atoms in the geometrically opposite staggered Pd­(oct)3 face. The 6s25d10 Tl­(I) is also equivalently attached to both chelating O atoms of a bidentate acetylacetonate (acac) monoanion. Although the C2 axis of the pseudo-C2v planar Tl­(acac) fragment is approximately parallel to the pseudo-C3 axis of the TlPd9 core, the orientation of the Tl­(acac) plane relative to the octahedral-based Pd9 geometry is considerably different for each of the three independent nondisordered molecules of 1 in 1A and 1B; these different planar Tl­(acac) orientations may be mainly attributed to anisotropic crystal-packing effects. Coordination of the Tl­(I) atom to the three Pd­(oct) atoms of the Pd9 core presumably occurs via its so-called “inert” 6s2 electron pair with resulting three short Tl–Pd­(oct) connectivities of mean distance 2.83 Å; these connectivities together with three longer Tl–Pd­(wt) ones of mean distance 3.15 Å give rise to a (crown-like)­Pd6 sextuple (μ3/3-Tl) coordination mode. Of particular stereochemical interest is a comparison of solution behavior of 1 with that for the known structurally related analogue, Pd93-TlCo­(CO)3L]­(μ2-CO)63-CO)3L6 (2) (with L = PEt3 instead of PPh3). In 2 the Tl­(I) is alternatively attached to a trigonal-bipyramidal Co­(CO)3L monoanion and primarily coordinated to the three inner Pd­(oct) atoms of a similar PR3/CO-ligated octahedron; corresponding Tl–Pd­(oct) and Tl–Pd­(wt) mean distances for two independent molecules in 2 are 2.77 and 3.31 Å, respectively. Variable-temperature 31P­{1H} NMR solution data of 1 indicate the occurrence of presumed fast wobbling-like motion of the [μ3/3-Tl­(acac)] entity about the pseudo-C3 axis of the Pd92-CO)63-CO)3P6 fragment without Pd–Tl detachment (i.e., the entire cluster of 1 remains intact). In direct contrast, corresponding temperature-dependent 31P and 13C NMR data of 2 instead are consistent with rapid, reversible dissociation/association of the entire3-TlCo­(CO)3L] ligand from the analogous Pd92-CO)63-CO)3P6 fragment of 2. This highly dissimilar dynamic solution behavior that points to a stronger Tl­(I) attachment to the Pd9 core in 1 than that in 2 may be attributed from the above crystallographic evidence to greater involvement of the outer three wingtip Pd­(wt) atoms in bonding connectivities to the Tl­(I) in 1 compared to predominant bonding connectivities of only the three inner Pd­(oct) atoms to the Tl­(I) in 2. 1H NMR solution spectra of 1 also suggest significant covalent character in the bidentate Tl–O­(acac) bonding in 1 based upon the observation of H­(acac)–Tl coupling; this premise is consistent with its Tl–O distances of 2.35 Å (av) being ca. 0.2 Å shorter than those of 2.52 Å (av) found in crystalline Tl­(acac), which with no observed H–Tl NMR coupling in solution implies ionicity of its bidentate Tl–O bonding. Both 1 and 2 conform to an 86 CVE count expected for an octahedral metal polyhedron based upon the Tl­(I) and each wingtip Pd­(μ2-CO)2L fragment contributing 2 and 4 CVEs, respectively.

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