Alkyne adducts of paramagnetic and diamagnetic tripalladium clusters supported by dppm ligands

The unsaturated redox-active cluster [Pd3(dppm)3(CO)]n+ (n = 0,1, or 2) reacts with ethylene dicarboxylic esters and a variety of terminal acetylenes (but not PhC≡CPh) in all three charge states. In particular, the radical cations [Pd3(dppm)3(CO)(RC≡CR′)].+ can be produced by several routes: one-electron electrochemical reduction of the dication adducts, comproportionation of the neutral and dicationic adducts, as well as the direct complexation of alkyne to the known radical cation [Pd3(dppm)3(CO)].+. Compared with the latter, each of the adducts have significantly extended lifetimes and classify as persistent radicals in solution. The alkyne adducts have been characterized by voltammetry (cyclic and rotating disk electrode), by UV – vis titrations and by MALDI-TOF mass spectra from dithranol matrices. Isotropic solution EPR spectra of the adducts with R = R′ = MeOC(O) and EtOC(O), as well as those with R′ = H and R = C6H5, FC6H4, HC≡CC6H4, and EtO have been obtained. Full line shape fitting simulations demonstrate that all display coupling to six different 31P (I = 1/2) and to one to three 106Pd (I = 5/2) nuclei. The A(31P) values range from a low of 3.0 × 104 to a high of 180.5 × 104 cm–1; this variation is caused by changes in the contribution of P s orbitals to the SOMO resulting from structural distortions of the Pd3P6 core upon alkyne coordination.Key words: electrochemistry, redox state, electron paramagnetic resonance, catalytic model, palladium, bridging phosphine.

Thermal and Electrochemically Assisted Pd−Cl Bond Cleavage in the d9−d9 Pd2(dppm)2Cl2 Complex by Pd3(dppm)3(CO)n+ Clusters (n = 2, 1, 0)

A new aspect of reactivity of the cluster [Pd3(dppm)3(micro3-CO)]n+, ([Pd3]n+, n = 2, 1, 0) with the low-valent metal-metal-bonded Pd2(dppm)2Cl2 dimer (Pd2Cl2) was observed using electrochemical techniques. The direct reaction between [Pd3]2+ and Pd2Cl2 in THF at room temperature leads to the known [Pd3(dppm)3(micro3-CO)(Cl)]+ ([Pd3(Cl)]+) adduct and the monocationic species Pd2(dppm)2Cl+ (very likely as Pd2(dppm)2(Cl)(THF)+, [Pd2Cl]+) as unambiguously demonstrated by UV-vis and 31P NMR spectroscopy. In this case, [Pd3]2+ acts as a strong Lewis acid toward the labile Cl- ion, which weakly dissociates from Pd2Cl2 (i.e., dissociative mechanism). Host-guest interactions between [Pd3]2+ and Pd2Cl2 seem unlikely on the basis of computer modeling because of the strong screening of the Pd-Cl fragment by the Ph-dppm groups in Pd2Cl2. The electrogenerated clusters [Pd3]+ and [Pd3]0 also react with Pd2Cl2 to unexpectedly form the same oxidized adduct, [Pd3(Cl)]+, despite the known very low affinity of [Pd3]+ and [Pd3]0 toward Cl- ions. The reduced biproduct in this case is the highly reactive zerovalent species "Pd2(dppm)2" or "Pd(dppm)" as demonstrated by quenching with CDCl3 (forming the well-known complex Pd(dppm)Cl2) or in presence of dppm (forming the known Pd2(dppm)3 d10-d10 dimer). To bring these halide-electron exchange reactions to completion for [Pd3]+ and [Pd3]0, 0.5 and 1.0 equiv of Pd2Cl2 are necessary, respectively, accounting perfectly for the number of exchanged electrons. The presence of a partial dissociation of Pd2Cl2 into the Cl- ion and the monocation [Pd2Cl]+, which is easier to reduce than Pd2Cl2, is suggested to explain the overall electrochemical results. It is possible to regulate the nature of the species formed from Pd2Cl2 by changing the state of charge of the title cluster.