Delineation of Shift Reagent-Substrate Equilibria

The equilibria between Ln(fod)3 (Ln = Pr, Eu, Ho, or Yb; fod = 1,1,1,2,2,3,3-heptafluoro-7,7-di-methyloctanedionate) and the basic substrate (π-C₅H₅)Fe(CO)₂(CN) have been studied by vapor pressure osmometry on benzene solutions. Dimers and, in some cases, higher aggregates of the shift reagents are found in pure Ln(fod)3 solutions. In general, the shift reagent:substrate systems form 1:1 and 1:2 complexes. In the case of Pr(fod)3, 1:3 complex formation is also observed. The relative successive formation constants for 1:2 adduct formation decrease from light to heavy lanthanide. This, as well as the 1:3 formation with Pr(fod)3, is thought to reflect greater steric crowding with the heavier members. The osmometry results are compared with nmr shifts determined under similar conditions, and the factors determining the shapes of plots of substrate shift vs. shift reagent to substrate ratio are discussed.