Structure and Dynamic Behavior of (η~3-Allyl) bromodicarbonylmolybdenum (II) Complexes Containing Polydentate 2-Pyridylphosphanes or Their Oxides as Chelating Ligands: Occurrence of Three Fluxional Processes

摘要

The pseudo-octahedral complexes [Mo(η~3-allyl)Br(CO))_2(PPy_nPh_(3-n)-P,N)] (Py = 2-pyridyl; n = 2, 3) and Mo(η~3-allyl)Br(CO))_2(OPPy_mPh_(3-m)-O,N)] (m = 1, 2, 3) undergo three different dynamic processes in solution, depending on the clelating ligand. The complexes containing PPhPy_2 and PPy_3 as chelating P,N-donors undergo a novel "pivoted double switch" mechanism which scrambles two equatorial coordination sites with racemization, while maintaining the identity of the phosphorus atom trans to the allyl ligand. The complexes containing 2-pyridylphosphane oxides as chelating O,N-donors undergo a nondissociative intramolecular trigonal-twist (or turnstile) rearrangement which maintains the identity of the phosphane oxide oxygen atom coordinated in an equatorial position. All the complexes containing uncoordinated pyridyls undergo a slow dissociative exchange of pyridyls without coordination site scrambling. The structure of the complex [Mo(Mo(η~3-allyl)Br(CO)_2(OPPy_3-O,N)] has been determined by X-ray diffraction.