The Pd3(dppm)3(CO)n clusters (n = 1-,2-); rare cases of anionic palladium species

Two novel anionic palladium clusters, Pd(3)(dppm)(3)(CO)(n-) (Pd(3)(n); n = 1-,2-) were electrochemically generated from the dicationic cluster Pd(3)(2+) in 0.2 M THF/Bu(4)NPF(6)via two first consecutive reversible 1-electron reductions (Pd(3)(2+) + 1 e(-) ⇌ Pd(3)(+), -0.210, and Pd(3)(+) + 1 e(-) ⇌ Pd(3)(0), -0.470 V vs. SCE) followed by two others at -2.350 (Pd(3)(0) + 1 e(-) ⇌ Pd(3)(1-), reversible) and at -2.690 V vs. SCE (Pd(3)(1-) + 1 e(-) ⇌ Pd(3)(2-), irreversible). The chemical stability and instability, respectively, of the Pd(3)(dppm)(3)(CO)(n-) clusters (Pd(3)(n); n = 1-,2-) at the time scale of the electrochemical experiments were addressed by DFT computations. Indeed, geometry optimisations (B3LYP) indicate expected Pd-Pd, Pd-P, Pd-C bond length variations, but severe structure distortions are noted for the anions Pd(3)(1-) and Pd(3)(2-), including large deviations from the planarity of the Pd(3)P(6) core and for the triangular frame of the Pd(3) center. Space filling models indicate that this skeleton distortion places the phenyl-dppm groups above the unsaturated site of the M(3) frame hence protecting it from any interactions with substrates, and hence explaining the stability of the Pd(3)(1-) species. The computed gas phase total energy shows a decrease going from Pd(3)(2+) to Pd(3)(1+) to Pd(3)(0) and to Pd(3)(1-), but increases going to Pd(3)(2-) hence corroborating the relative stability of these species and the observed chemical reversibility of the CV waves. Large steps in energy stabilisation going from Pd(3)(2+) to Pd(3)(1+) to Pd(3)(0) is totally consistent with the low reduction potentials associated with these species, but the much smaller steps going from Pd(3)(0) to Pd(3)(1-) and to Pd(3)(2-) corroborates their much larger reduction potentials. The host-guest behaviour of Pd(3)(1-) and Pd(3)(2-) in the presence of the neutral substrate EtO(2)C-CC-CO(2)Et (L) and CF(3)CO(2)(-) (X(-)) was examined by CV. From the shifts of the reduction waves, it was possible to demonstrate that Pd(3)(2+) and Pd(3)(+) act as host for X(-) but not Pd(3)(0), Pd(3)(1-) and Pd(3)(2-), whereas Pd(3)(2+), Pd(3)(+), Pd(3)(0), bind L but Pd(3)(1-) and Pd(3)(2-) do not, corroborating evidence for stability but non-reactivity at the same time, particularly for the Pd(3)(1-) cluster. All in all, these anionic clusters exhibit the lowest oxidation state for palladium species ever investigated.