Unprecedented Tris-Phosphido-Bridged Triangular Clusters with 42 Valence Electrons. Chemical, Electrochemical and Computational Studies of their Formation and Stability

This paper presents the synthesis and structural characterization of the unprecedented tris-phosphido-bridged compounds Pt-3(mu-PBu2t)(3)X-3 (X = Cl, Br, I), having only 42 valence electrons, while up to now analogous clusters typically have 44e(-). The new species were obtained by an apparent bielectronic oxidation of the 44e(-) monohalides Pt-3(mu-PBu2t)(3)(CO)(2)X with the corresponding dihalogen X-2. Their X-ray structures are close to the D-3h symmetry, similarly to the 44e(-) analogues with three terminal carbonyl ligands. The products were also obtained by electrochemical oxidation of the same monohalides in the presence of the corresponding halide. In a detailed study on the formation of Pt-3(mu-PBu2t)(3)I-3, the redox potentials indicated that I-2 can only perform the first monoelectronic oxidation but is unsuited for the second one. Accordingly, the 43e(-) intermediate [Pt-3(mu-PBu2t)(3)(CO)(2)Ir](+) was ascertained to play a key role. Another piece of information is that, together with the fully oxidized product Pt-3(mu-PBu2t)(3)I-3, the transient 44e(-) species [Pt-3(mu-PBu2t)(3)(CO)(3)](+) is formed in the early steps of the reaction. In order to extract detailed information on the formation pathway, involving both terminal ligand substitutions and electron transfer processes, a DFT investigation has been performed and all the possible intermediates have been defined together with their associated energy costs. The profile highlights many important aspects, such as the formation of an appropriate couple of 43e(-) intermediates having different sets of terminal coligands, and suitable redox potentials for the transfer of one electron. Optimizations of 45e(-) associative intermediates in the ligand substitution reactions indicate their possible involvement in the redox process with reduction of the overall energy cost. Finally, according to MO arguments, the unique stability of the 42e(-) phosphido-bridged Pt-3 clusters can be attributed to the simultaneous presence of three terminal halides.