Abstract
The Pd-3(dppm*)(3)(CO)(n+) enantiomers (n = 2 (2), 1 (3)) were prepared either from (R, R)-or (S, S)-P-chirogenic bis(phenyl-m-xylylphosphino) methane (dppm*; 1) and Pd(OAc) 2 in the presence of CF3CO2H, CO and water (n = 2), and then by reductive electrolysis (n = 1). The stable enantiomeric [Pd-3((S,S)-dppm*)(3)(CO)](+center dot) (3), is the first C-3-symmetry radical-cation M-M bonded cluster, therefore the odd electron is delocalized onto the Pd-3 frame within this symmetry. The novel chiral species have been characterized by circular dichroism (CD) of both enantiomers of the Pd-3(dppm*)(3)(CO)(2+) clusters (2) and by EPR spectroscopy for the Pd-3((S, S)-dppm*)(3)(CO)(+center dot) paramagnetic compounds (3, g = 2.041). Evidence for reduced symmetry with respect to the achiral cluster was also obvious from the hyperfine splittings of the EPR signal which display three different hyperfine coupling values: 3 A(31P) = 83.9 10(-4) cm(-1), 3 X A(P-31) = 69.7 10(-4) cm(-1), 3 A(Pd-105) = 12.5 10(-4) cm(-1). In the absence of an X-ray structure for the paramagnetic clusters, DFT computations were performed to address the geometry. The optimized geometry of the Pd-3((S, S)-dppm*)(3)(CO)(+center dot) radicals (3) exhibits three phosphorus atoms placed well above the Pd-3 plane, while the three others are located below the trimetallic frame within C-3-symmetry due to intramolecular steric hindrance. This makes them chemically different with respect to the carbonyl group and explains the experimental EPR spectrum well. Consequently this C-3-symmetry deformation also induces a change in the shape of the SOMO (semi-occupied molecular orbital) towards this same symmetry compared to the corresponding achiral C-3v species.
