A study of the behaviour of Cu6(TePh)6(PPh2Et)5 and related phosphine (PPh2Et) and phosphine oxide (Ph2EtPO) adsorbed in mesoporous molecular sieves

We have examined the adsorption of the phosphine stabilized cluster Cu6(TePh)6(PPh2Et)5 as well as the free ligands ethyldiphenylphosphine (EDPP) and ethyldiphenylphosphine oxide (EDPPO) in several MCM-41 types of mesoporous materials by 31P solid-state NMR spectroscopy. The results indicate that upon loading into siliceous MCM-41 (Si-MCM-41), the phosphine shell of the copper cluster dissociates from the metal core. After dissociation, some EDPP molecules exist as physisorbed species and a significant amount of phosphine is oxidized to the corresponding phosphine oxide, occluded in the channels of Si-MCM-41. The reaction seems to be catalyzed by the Cu(I) species since direct loading of EDPP under the same conditions does not yield any phosphine oxide in the framework of Si-MCM-41. The 31P NMR spectra of EDPP and EDPPO adsorbed in Si-MCM-41 suggest that neither Brønsted nor Lewis acid sites exist on the channel surface of Si-MCM-41. The adsorption of EDPP and EDPPO in the acidic form of aluminium-containing MCM-41 (Al-MCM-41), however, does indicate the interactions between the guest species and three Brønsted acid sites with different strengths. No Lewis acid site was observed in Al-MCM-41.