Structure and Dynamic Behavior of (η3-Allyl)bromodicarbonylmolybdenum(II) Complexes Containing Polydentate 2-Pyridylphosphanes or Their Oxides as Chelating Ligands: Occurrence of Three Fluxional Processes

Abstract

The pseudo-octahedral complexes [Mo(η 3 -allyl)Br(CO) 2 (PPy n Ph 3-n -P,N)] (Py = 2-pyridyl; n = 2, 3) and [Mo(η 3 -allyl)Br(CO) 2 (OPPy m Ph 3-m -O,N)] (m = 1, 2, 3) undergo three different dynamic processes in solution, depending on the chelating 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 com-

Fluxional Processes in Diamagnetic and Paramagnetic Allyl Dicarbonyl and 2-Methylallyl Dicarbonyl Molybdenum Histidinato Complexes as Revealed by Spectroscopic Data and Density Functional Calculations

“…The stereochemical implications of fluxional processes in [M(h-allyl)(CO) 2 L 2 X] have been discussed, but all complexes investigated so far were either achiral or racemic mixtures. [35] In the chiral complex 1, which is easily obtained enantiomerically pure, three different orientations of the His ligand with respect to the (allyl)(CO) 2 plane are possible. Furthermore, very little is known about the presence of rotational isomers (either allyl rotation or trigonal twist) in related one-electron-oxidized species, [36,37] let alone about the parameters that might cause such fluxionality in paramagnetic compounds [Mo-(h-allyl)(CO) 2 L 2 X] .…”

“…This has been observed previously for related pseudooctahedral compounds with the Mo(allyl)(CO) 2 moiety. [34,35,39,40] Also, the activation barriers of interconversion are in the range reported for trigonal twists of other pseudooctahedral Mo(allyl)(CO) 2 compounds. [39,40] Unlike all previous examples with a higher symmetry, three different positions of the allyl ligand with respect to the Mo(His) plane are possible in 1 ± 4.…”

“…In the parent allyl derivative 1 , at least one isomer (b) is similar in energy and this fact is also reflected by more complicated EPR spectra. Previous work on [Mo(allyl)(CO) 2 ] complexes with additional bipyridyl and chelating phosphine ligands [35] shows that these compounds were fluxional in solution as shown by NMR spectroscopy, but only one isomer was observed in the oxidized form. In contrast, we were able to investigate the fluxionality, relative energies and the mechanisms of interconversion also of the paramagnetic 17-electron species in this study.…”

Synthesis and Structure of Dicarbonyldichloro{2-(Diphenylphosphineoxide)pyridine}ruthenium(ii)

“…Similar spectroscopic behaviour is found in related octahedral molybdenum complexes containing the 2-(diphenylphosphineoxid e)pyridine ligand. 8 The 1 H NMR spectrum of the complex shows a broad multiplet centred at δ: 7.8 ppm, assigned to the phenyl and 4,5,6-pyridine protons, and a doublet resonance at δ: 9.23 ppm (J = 5.3 Hz) assigned to H 3 proton of the pyridine moiety. These resonances were assigned of the basis of reported data.…”

“…These resonances were assigned of the basis of reported data. 8 The In order to determine the molecular structure in the solid state of the complex, an X-ray diffraction study was undertaken. Suitable crystals for structural determination were obtained from a slow diffusion of n-pentane into a chloroform solution of the complex.…”