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

Abstract

Two novel anionic palladium clusters, Pd₃(dppm)₃(CO)ⁿ⁻ (Pd₃ⁿ; n = 1−,2−) were electrochemically generated from the dicationic cluster Pd₃²⁺ in 0.2M THF/Bu₄NPF₆via two first consecutive reversible 1-electron reductions (Pd₃²⁺ + 1 e⁻ ⇌ Pd₃⁺, −0.210, and Pd₃⁺ + 1 e⁻ ⇌ Pd₃⁰, −0.470 V vs. SCE) followed by two others at −2.350 (Pd₃⁰ + 1 e⁻ ⇌ Pd₃¹⁻, reversible) and at −2.690 V vs. SCE (Pd₃¹⁻ + 1 e⁻ ⇌ Pd₃²⁻, irreversible). The chemical stability and instability, respectively, of the Pd₃(dppm)₃(CO)ⁿ⁻ clusters (Pd₃ⁿ; 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 Pd₃¹⁻ and Pd₃²⁻, including large deviations from the planarity of the Pd₃P₆ core and for the triangular frame of the Pd₃ center. Space filling models indicate that this skeleton distortion places the phenyl-dppm groups above the unsaturated site of the M₃ frame hence protecting it from any interactions with substrates, and hence explaining the stability of the Pd₃¹⁻ species. The computed gas phase total energy shows a decrease going from Pd₃²⁺ to Pd₃¹⁺ to Pd₃⁰ and to Pd₃¹⁻, but increases going to Pd₃²⁻ hence corroborating the relative stability of these species and the observed chemical reversibility of the CV waves. Large steps in energy stabilisation going from Pd₃²⁺ to Pd₃¹⁺ to Pd₃⁰ is totally consistent with the low reduction potentials associated with these species, but the much smaller steps going from Pd₃⁰ to Pd₃¹⁻ and to Pd₃²⁻ corroborates their much larger reduction potentials. The host–guest behaviour of Pd₃¹⁻ and Pd₃²⁻ in the presence of the neutral substrate EtO₂C–CC–CO₂Et (L) and CF₃CO₂⁻ (X⁻) was examined by CV. From the shifts of the reduction waves, it was possible to demonstrate that Pd₃²⁺ and Pd₃⁺ act as host for X⁻ but not Pd₃⁰, Pd₃¹⁻ and Pd₃²⁻, whereas Pd₃²⁺, Pd₃⁺, Pd₃⁰, bind L but Pd₃¹⁻ and Pd₃²⁻ do not, corroborating evidence for stability but non-reactivity at the same time, particularly for the Pd₃¹⁻ cluster. All in all, these anionic clusters exhibit the lowest oxidation state for palladium species ever investigated.