Correlating the Activity of Rhodium(I)-Phosphite-Lariat Ether Styrene Hydroformylation Catalysts with Alkali Metal Cation Binding through NMR Spectroscopic Titration Methods

Academic Article

Abstract

  • © 2016 American Chemical Society. Alkali metal salts can affect both the activities and regioselectivities of alkene hydroformylation catalysts containing polyether-functionalized phosphorus-donor ligands; however, it is unclear whether these effects arise from direct alkali metal cation binding to the active catalysts. To gain more insight into these effects, a series of phosphite-lariat ether ligands derived from the alkali metal cation binding agents 2-hydroxymethyl-12-crown-4 and 2-hydroxymethyl-15-crown-5 have been prepared. Rhodium(I) complexes of these ligands have been evaluated as styrene hydroformylation catalysts in the absence and presence of a variety of alkali metal salts. The activities of catalysts containing phosphites derived from 2,2′-biphenol or 1,1′-binaphthol increased significantly (up to 92%) in the presence of alkali metal cations that are "moderately oversized" for archetypal binding to the crown cavity. When this criterion are not met, a decrease in the catalytic activity is observed upon addition of an alkali metal salt. NMR titrations (31P{1H} and 1H) of two model cis-Mo(CO)4(phosphite-lariat)2 complexes in which the phosphite was derived from 2,2′-biphenol were carried out to gain insight into the manner in which the alkali metal cations interact with the ligands. Both model complexes bind Li+ through a 2:1 two-site binding mechanism, and the model complex with the larger crown ether also binds Na+ in this fashion. In contrast, 1:1 complexes are formed upon Na+ and K+ binding to the model complex containing the smaller crown ether and upon K+ binding to the model complex containing the larger crown ether. Correlation between increases in catalyst activity and binding mode in complexes containing cations "moderately oversized" for archetypal binding to the crown cavity strongly suggests that the increases are due to a specific type of alkali metal cation binding by the lariat ether groups in these catalysts.
  • Published In

  • Organometallics  Journal
  • Digital Object Identifier (doi)

    Author List

  • Martin JR; Cagle EC; Lucius AL; Gray GM
  • Start Page

  • 2609
  • End Page

  • 2620
  • Volume

  • 35
  • Issue

  • 16