Supported Pd(I) dimers on polymeric materials for catalysis: Evaluation of plausible strategies and synthetic routes

Abstract

In this work, plausible strategies and synthetic routes for supported Pd(I) dimers have been evaluated. The use of Pd(I) dimers of the type [Pd2(μ-allyl)(μ-X)LL’] supported in vinylic addition polynorbornenes (VA-PNB) could avoid the problem of Pd leaching in catalysis. These precatalysts could carry out catalytic processes releasing active Pd(0) species that after the process would comproportionate again regenerating the precatalyst and allowing its recycling and reuse. Two strategies for the synthesis of those complexes have been studied. The first one consists of the anchor of the Pd(I) dimer by a bridging thiolate ligand. To do so, the oxidative addition of an allyl alkyl sulfide to a Pd(0) was attempted with no good results. The ligand exchange of Cl- in [Pd2(μ-allyl)(μ-Cl)(PCy3)2] for -S(pentyl) using the sodium salt was neither successful. However, the use of other salts may bring better results. The second strategy consists of the use of a phosphino-thiolato chelating ligand. The synthesis of the complex [Pd2(μ-allyl)(κ2-μ-SPhNPPh2)(PPh3)] seems possible although further work must be done in this aspect. Theoretical methods were used to study the variations of the ΔG of the comproportionation reaction of Pd(I) dimers changing either the bridging ligand or the solvent. In the first case, the comproportionation is more favourable for a thiolate ligand than for a chelating ligand, and for alkyl thiolates more favourable than for phenyl thiolates. Finally, the changes of the comproportionation constant when the solvent is changed, allows the modification of it by using different solvents or a mixture of them.