Wide bite angle phosphine ligands in catalysis: from molecular complexes to nanoparticles

In the last two decades we have explored the effect of diphosphine ligands in a number of catalytic reactions.1 Initially the Xantphos ligands were designed for their use in rhodium catalyzed hydroformylation, in which indeed... [ view full abstract ]

In the last two decades we have explored the effect of diphosphine ligands in a number of catalytic reactions.¹ Initially the Xantphos ligands were designed for their use in rhodium catalyzed hydroformylation, in which indeed the wide-bite angle catalysts give highly linear products.² Xantphos ligands turned out to be the first phosphines leading to active hydrocyanation catalysts using nickel.³ Palladium-catalyzed cross-coupling reactions gave optimal results when Xantphos and DPEphos were used, rather than the ligands with still wider bite angles. The most important use of Xantphos lies in cross coupling chemistry.⁴ Wider bite angles, such as that in SPANphos lead to trans complexes and bimetallic complexes. SPANphos gives rise to the fastest phosphine system for the rhodium catalyzed carbonylation of methanol.⁵ The monodentate analogs found a very useful application in butadiene dimerization.⁶

We considered that ligands with wide bite angles might also be good ligands for nanoparticle ligation.⁷ Even ligands that cannot form simple molecular complexes might fit well on metal nanoparticles. There are now ample examples that ligands in nanoparticle catalysis play a role similar to their role in homogeneous catalysis, i.e. they give stability, modify electronic properties, and influence the steric environment of the catalytic site. The latter phenomenon is difficult to control in such a multi-site species. We found that electron-rich RuNPs containing alkylphosphines are faster catalysts for the hydrogenation of aromatics than electron-poor systems. Secondary phosphine oxide ligands are peculiar ligands in homogeneous catalysis, and we found that also in AuNP catalysis the SPO ligand participates in heterolytic hydrogen cleavage.⁸

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8 I. Cano, et al., J. Am. Chem. Soc., 2015, 137, 7718.