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(η3‐allyl)Br(CO)2(PPynPh3–n‐P,N)] (Py = 2‐pyridyl; n = 2, 3) and [Mo(η3‐allyl)Br(CO)2(OPPymPh3–m‐O,N)] (m = 1, 2, 3) undergo three different dynamic processes in solution, depending on the chelating ligand. The complexes containing PPhPy2 and PPy3 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(η3‐allyl)Br(CO)2(OPPy3‐O,N)] has been determined by X‐ray diffraction.

An experimental and theoretical study of the coordination and donor properties of tris-2-pyridyl-phosphine ligands

“…The downfield resonance is consistent with P coordination, whereas the upfield resonance corresponds to the P,N ring, as seen in the only other example of tris-(2-pyridyl)-phosphine P,N coordination to molybdenum found in [MoBr(η 3 -C 3 H 5 )(CO) 2 {P(2-py) 3 -κ 2 P,N}] (δ P −10.0 ppm). 44 Similar observations were made in CDCl 3 (i.e. 67.5, 3.1 (J PP = 24 Hz); 46.0, 1.8 (J PP = 24 Hz)).…”

Variable temperature electrochemistry as a powerful method for conformational investigations on the fluxional organometallic complex Mo(His-Nɛ-C2H4CO2Me)(η-allyl)(CO)2 (His = Nδ, N,O-l-histidinate)

“…3,4 Since that time, fluxionality in diamagnetic complexes of the general formula M(h-allyl)(CO) 2 L 2 X (M = Cr, Mo, W) has been studied in some detail by NMR spectroscopy in solution. [5][6][7] For some derivatives, a good stability of the one-electron oxidized species was demonstrated, 8,9 but related fluxionality was never mentioned. There seems to be no obvious method for investigations of this kind on the paramagnetic species [M(h-allyl)-(CO) 2 L 2 X] + .…”

“…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. 7 Here, we report the synthesis of the enantiomerically pure methyl propionate derivative Mo(His-N e -C 2 H 4 CO 2 Me)(hallyl)(CO) 2 2 and its complete characterization. The presence of two isomers of 2 is shown by NMR and electrochemical investigations.…”

“…Such a conformation has been shown to be energetically favourable 10 and is observed in all solid-state structures of this type reported thus far. 6,7,[11][12][13] The allyl ligand is located trans relative to the histidine-N d atom [N( 8 In solution, the situation is more complex. Two sets of signals are observed in the 1 H NMR as well as in the 13 C NMR spectrum of 2, indicating the presence of two isomers (2a and 2b, see Scheme 1) in solution.…”

Tetranuclear organometallic complexes containing Mo2O42+ and allylmolybdenum(ii) moieties

“…NMR spectra were recorded in Bruker AC-300 or ARX-300 instruments in CDCl 3 , at room temperature unless otherwise stated. NMR spectra are referred to the internal residual solvent peak for 1 H and 13 C{ 1 H} NMR. Assignment of the 13 3.…”