Adaptive Behavior of a Ditopic Phosphine Ligand

Synthetic, structural and computational studies have been performed to investigate ligand interchange in the fluxional chelate complex [RhCl3{Ph2PACH2C(OA)OEt‐κ2POA}{Ph2PBCH2C(OB)OEt‐κP}], which contains two hybrid phosphine‐ester ligands, one acting as P,O chelator, the other as a P‐monodentate ligand. The observed ligand exchange may occur according to two pathways which both involve four elementary movements: a) oxygen dissociation with formation of a lacunary octahedral RhCl3P2 intermediate; b) migration of the Cl atom trans to PA towards the position trans to PB; c) rotations of the phosphine moieties about the Rh–P bonds, these occurring either concomitantly with the Cl displacement or in a separate step; d) coordination of an oxygen atom of the second phosphine. The two pathways thus differ by conformational changes within two distinct steps. In each pathway the rate‐limiting step is the one involving a movement of the two phosphines, which generates steric frictions between the two PPh2 groups. The calculated theoretical energetic spans of both pathways (ΔG≠ ≈ 17 kcal mol–1) is close to the energy barrier obtained from a variable temperature NMR study carried out in C2D2Cl4 (ΔG≠ = 15.5 kcal mol–1). While one of the pathways leads to an isomer with a Rh‐bound ethoxy O atom, the other results in the isomer having the metal coordinated to the adjacent C=O group. Exchange between the two O atoms of the coordinated ester group occurs readily (ΔGTS = 12.5 kcal mol–1). Fight for a binding site: Phosphine hemilability in [RhCl3(Ph2PCH2CO2Et‐κ2PO)(Ph2PCH2CO2Et‐κP)] occurs “spontaneously” and is best interpreted in terms of a dissociative, intramolecular mechanism. Steric frictions play an important role in making the ligand interchange process spectroscopically detectable.