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 Pt3(μ-PBut2)3X3 (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 Pt3(μ-PBut2)3(CO)2X with the corresponding dihalogen X2. Their X-ray structures are close to the D3h 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 Pt3(μ-PBut2)3I3, the redox potentials
indicated that I2 can only perform the first monoelectronic
oxidation but is unsuited for the second one. Accordingly, the 43e– intermediate [Pt3(μ-PBut2)3(CO)2I]+ was ascertained to play a key role. Another piece of information
is that, together with the fully oxidized product Pt3(μ-PBut2)3I3, the
transient 44e– species [Pt3(μ-PBut2)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 Pt3 clusters can be attributed to the simultaneous presence of three
terminal halides.