Solution characterization, conformation analysis, and dynamics of the ligand 1,1′-bis(diphenylphosphino)ferrocene (dppf) in Re2(μ-OMe)2(μ-dppf)(CO)6

Abstract

The dynamic process involving the diphenyls (φ, φ′) in 1,1′-bis(diphenylphosphino)ferrocene (dppf) in [Re2(μ-OMe)2(μ-dppf)(CO)6] at 228 K is characterized by conformational analysis. At this temperature, the fluxionality of the dppf ligand ceases whereas the dynamics of the phenyl rings about the P-Cipso(φ,φ′) (the "ipso" in the phenyl rings) was rapid on the NMR time scale down to -85°C. The fluxional barrier (ΔG*exp = 46.4 ± 1.0 kJ/mol) for the interconversion of the R and the 5 forms was determined by variable-temperature 1H NMR spectroscopic studies. The range of the Cp proton chemical shifts varies from δ(1H) = 3.00 to 5.68 ppm and is the largest reported. The [Re2(μ-OMe)2] core was found to be static throughout the temperature-lowering process indicating that fluxionality is thus confined to the dppf fragment in the molecule. Ring current effects similar to those observed for the internal protons (Hi) in cyclophane molecules are attributed to be responsible for the anomalous proton shifts. Molecular modeling calculations indicates complete ring flipping of the diphenyls about their respective P-Cipso bonds was not possible because of the steric congestion about the ferrocene. The torsional twist of the diphenyls about their P-Cipso bonds was correlated, restricted, and in opposite directions. The ranges for the torsional angles φt′ and φt were determined respectively to be 92.6° ≤ φt′ ≤ 112.4° and 147.6° ≤ φt ≤ 177.8°. © 1994 American Chemical Society.