Experimental and Computational Study of Steric and Electronic Effects on the Coordination of Bulky, Water-Soluble Alkylphosphines to Palladium under Reducing Conditions: Correlation to Catalytic Activity
The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Sterically demanding, water-soluble alkylphosphine ligands 2-(di-tert-butylphosphino)- ethyltrimethylammonium chloride (t-Bu-Amphos) and 4-(di-tert-butylphosphino)-N,N-dimethylpiperidinium chloride (t-Bu-Pip-phos) in combination with palladium salts provided active catalysts for the cross-coupling of aryl halides under mild conditions in aqueous solvents, whereas 4-(dicyclohexylphosphino)-N,N-dimethylpiperidinium chloride (Cy-Pipphos) gave a less active catalyst. Catalyst activity increased with increasing cone angle of the ligands, but the ø electronic parameter determined from the symmetric C-O stretching frequency of LNi(CO)₃ did not correlate with catalyst activity. Catalyst activity correlated with other calculated electronic parameters, such as the HOMO-LUMO energy gap of the ligand and the HOMO energy level of the LPd(0) species. Multinuclear NMR spectroscopic studies showed that t-Bu-Amphos and t-Bu-Pip-phos rapidly form L₂Pd(0) (L= t-Bu-Amphos or t-Bu-Pip-phos) complexes when reacted with Pd(OAc)₂ under reducing conditions over a range of L:Pd ratios. In contrast, the coordination chemistry of Cy-Pip-phos depended on the Cy-Pip-phos:Pd ratio. At ≤1:1 Cy-Pip-phos:Pd ratio, rapid formation of L₂Pd(0) occurred. At higher L:Pd ratios, initial formation of trans-(Cy-Pip-phos)₂PdCl₂ was observed followed by slow reduction to the Pd(0) complex.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 921860
- Journal Information:
- Organometallics, 24(5):962-971, Vol. 24, Issue 5; ISSN 0276-7333
- Country of Publication:
- United States
- Language:
- English
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