The first unpaired electron placed inside a C3-symmetry P-chirogenic cluster

Abstract

The Pd₃(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 [Pd₃((S,S)-dppm*)₃(CO)]⁺˙ (3), is the first C₃-symmetry radical-cation M-M bonded cluster, therefore the odd electron is delocalized onto the Pd₃ frame within this symmetry. The novel chiral species have been characterized by circular dichroism (CD) of both enantiomers of the Pd₃(dppm*)₃(CO)²⁺ clusters (2) and by EPR spectroscopy for the Pd₃((S,S)-dppm*)₃(CO)⁺˙ 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 ×A(³¹P) = 69.7 × 10⁻⁴ cm⁻¹, 3 ×A(¹⁰⁵Pd) = 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 Pd₃((S,S)-dppm*)₃(CO)⁺˙ radicals (3) exhibits three phosphorus atoms placed well above the Pd₃ plane, while the three others are located below the trimetallic frame within C₃-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 C₃-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.