First high-nuclearity thallium–palladium carbonyl phosphine cluster, [Tl2Pd12(CO)9(PEt3)9]2+, and its initial mistaken identity as the unknown Au2Pd12 analogue: structure-to-synthesis approach concerning its formation

Abstract

Our exploratory research objective to obtain new high-nuclearity Au–Pd carbonyl phosphine clusters from reactions in DMF of preformed Pd₁₀(CO)₁₂(PEt₃)₆ with Au(PPh₃)Cl in the presence of TlPF₆ (a frequently utilized chloride-scavenger) has given rise unexpectedly in 40% yield to the first example of a heterometallic Tl–Pd carbonyl phosphine cluster, [Tl₂Pd₁₂(CO)₉(PEt₃)₉]²⁺ (1-Et), as the [PF₆]⁻ salt. Its initial incorrect formulation as the unknown Au₂Pd₁₂ cluster, obtained from a well-refined low-temperature CCD X-ray diffraction analysis of its crystal structure, was primarily based upon its related molecular geometry to that of the previously reported [Au₂Pd₁₄(CO)₉(PMe₃)₁₁]²⁺ (as the [PF₆]⁻ salt) prepared from an analogous reaction of Pd₈(CO)₈(PMe₃)₇ and Au(PCy₃)Cl in the presence of TlPF₆. (Because X-ray scattering occurs via the electrons of atoms, an assignment in the crystal-structure determination of 1-Et of the two independent “heavy” atoms as either Tl (at. no. 81) or Au (at. no. 79) would result in non-distinguishable refinements). 1-Et was originally characterized by IR and ³¹P{¹H} NMR; attempted MALDI-ToF mass-spectrometric measurements were unsuccessful. The geometrically unprecedented pseudo-C_3h core of 1-Et may now be described as edge-fusions of three trigonal bipyramidal Pd₅ fragments to a central trigonal bipyramidal Tl₂Pd₃ kernel. Its formation was originally viewed as the condensation product of three partially ligated butterfly Pd₄(CO)₃(PEt₃)₃ fragments that are also linked to and stabilized by two capping naked Au⁺ cations. This proposed “structure-to-synthesis” approach led to the isolation of 1-Et in ca. 90% yield from the reaction in DMF of the butterfly Pd₄(CO)₅(PEt₃)₄ with the phosphine-scavenger Au(SMe₂)Cl together with TlPF₆. Our later realization and resulting conclusive evidence that its metal-core stoichiometry is Tl₂Pd₁₂ instead of Au₂Pd₁₂ was a consequence of: (1) our bothersome inability based upon a presumed Au₂Pd₁₂ core-geometry to interpret its complex ³¹P{¹H} NMR spectrum despite ³¹P{¹H} COSY experiments clearly showing couplings between the seven major resonances that are consistent with intramolecular processes involving only one species; (2) our subsequent direct preparation of the same Tl₂Pd₁₂ cluster (90% yield) from the reaction in THF of Pd₄(CO)₅(PEt₃)₄ with TlPF₆ (mol. ratio, 3/2), and the ensuing low-temperature CCD X-ray determination revealing a virtually identical solid-state structure (as expected) but with ³¹P{¹H} NMR measurements displaying an analogous complex spectrum that now can be interpreted; and (3) an elemental analysis (Tl, Au, Pd, P), which had been delayed because of the misleading confidence concerning our initially assigned stoichiometry, that ascertained its present formulation; noteworthy is that an elemental analysis of a sample of this compound would not disclose its true identity unless directly tested for Tl (and the absence of Au). Gradient-corrected DFT calculations performed on the PH₃-model of the crystallographically known butterfly Pd₄(CO)₅(PPh₃)₄ and on its hypothetical Tl⁺, Au⁺, and [Au(PH₃)]⁺ adducts (where the optimized geometries consisted of a trigonal bipyramidal MPd₄ core with an equatorial M = Tl⁺, Au⁺, or [Au(PH₃)]⁺) revealed: (a) that the monocationic Tl⁺ charge is primarily localized on thallium in contrast to the monocationic Au⁺ charge being much more delocalized over the entire molecule with charge density having been withdrawn mainly from CO ligands (relative to that of the neutral Pd₄(CO)₅(PH₃)₄); (b) that the interactions of Tl⁺, Au⁺, or [Au(PH₃)]⁺ adducts with a stable butterfly Pd₄(CO)₅(PH₃)₄ model are energetically favorable processes, with Au⁺ bonding being stronger than Tl⁺ bonding to Pd₄(CO)₅(PH₃)₄; and (c) that the presence of an additional PH₃ ligand on the Au⁺ significantly weakens the Au–Pd bonding interactions such that its bonding energy is comparable with that of the Tl–Pd interactions.