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Synthesis and reactivity of bis(dialkylphosphino)methane complexes of palladium Pamplin, Craig Bruce

Abstract

The coordination chemistry of Pd complexes containing bis(dialkylphosphino)- methane (P-P) ligands was examined; these ligands have the general formula R₂PCH₂PR₂ where R = OEt (deopm), Me (dmpm), Et (depm), Ph (dppm), z'-Pr (dippm) or Cy (dcpm). Several Pd(I) chloro-complexes containing a reactive Pd-Pd bond were synthesized via conproportionation of PdCl₂(PhCN)₂ and Pd₂(dba)₃CHCl₃ (dba = dibenzylideneacetone) in the presence of the appropriate P-P ligand. The new complexes Pd₂Cl₂(μ-depm)₂ (2a) and Pd₂Cl₂(μ-dippm)₂ (3a) were isolated in 87 and 8% yield, respectively, and the latter was structurally characterized. When more bulky phosphines were used, the corresponding Pd(I) complexes 3a or Pd₂Cl₂ (μ-dcpm) ₂ (4a) were formed along with the Pd(II) chelated complexes PdCl₂(P-P) (P-P = dippm (13a), dcpm (14a)); no Pd(H) complexes with chelating dmpm were identified; The known complex Pd₂Cl₂(μ-dmpm)₂ (la) was best prepared from the reaction of Pd₂Cl₂(μ-dmpm)₂ with dmpm in benzene. Halide metathesis of the Pd(I) and Pd(II) complexes with NaBr or Nal generally provided the analogous bromo- and iodocomplexes in good yield; however, an improved, high yield synthesis of PD₂l₂(μ-dmpm)₂ (lc) from Pd₂Cl₂(μ-dmpm)₂ via phosphine metathesis avoids the undesirable formation of Pd₂Cl₂(μ-dmpm)₂ (μ-dmpm)₂ (9) from lc in the presence of excess iodide. The Pd(I) complexes are fluxional in solution, and the dynamics of an intramolecular process that exchanges the Mesignals of the dmpm ligand in Pd₂X₂(μ-dmpm)₂ (la-c) are discussed on the basis of variabletemperature NMR data. The activation barriers to this exchange process are ΔG[sub C] = 37.9 ± 0.9 (la) < 39.0 ± 0.9 (lb) < 43.2 ± 0.7 (lc) kJ mol⁻¹.

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