η3-Allyl carbonyl complexes of group 6 metals: Structural aspects, isomerism, dynamic behaviour and reactivity

Abstract

Contents 1. Introduction 2. Complexes [CpM(CO)2(η 3 -allyl)] and related compounds 2.1. Synthetic strategies 2.2. Exo ⇌ endo isomerism 2.3. 95 Mo and 183 W NMR spectroscopy 2.4. Oxidized cationic complexes -reactivity, fluxionality, structural features 2.5. Mixed nitrosyl carbonyl complexes [CpM(CO)(NO)(η 3 -allyl)] + (M = Mo, W) 3. Complexes [M(CO)2(η 3 -allyl)(α-diimine)X] (X = anionic monodentate ligand) and related compounds 3.1. Synthetic strategies 3.2. Mechanistic features, structural aspects, and dynamic behaviour 3.3. Monodentate ligands bound to M(CO)2(η 3 -allyl)(L⏜L): reactivity and intermediacy in allylic alkylations 3.4. Redox Properties and Electrocatalysis 4. Amidinato and pyrazolato complexes related to [M(CO)2(η 3 -allyl)(α-diimine)X] 4.1. Synthesis and dynamic behaviour 4.2. Coordinatively unsaturated species and their reactivity 5. Summary and outlook Acknowledgement References 2 ABSTRACTTransition metal complexes with π-allylic ligands remain an attractive topic in organometallic chemistry, given the numerous reports of a wide variety of synthetic routes, dynamic behaviour and reactivity, structural (including isomerism), spectroscopic and redox properties, and applications in organic synthesis and catalysis. Surprisingly, despite the considerable interest in the rich and varied chemistry of this family of organometallic compounds, there is no recent review. This review is focused on π-allylic representatives of low-cost Group-6 metals bearing one or more carbonyl ligand, the coordination sphere being complemented with η 5cyclopentadienyl (Section 2), chelating ligands, including redox active α-diimines and various complementary diphosphine (Section 3) and novel anionic amidinate or pyrazolate (Section 4) ligands. In Section 1 particular attention is paid to rearrangements of the π-allylic ligand, namely exo and endo isomerism, interconversion mechanisms, fluxionality, and agostic interactions. In addition, the application of multinuclear NMR spectroscopy to the elucidation of such isomerism, and the effect of the metal centre oxidation state on the bonding, dynamic behaviour and reactivity of the π-allylic ligand are described. The detailed mechanistic description of the synthetic routes and dynamic behaviour of selected representatives of αdiimine complexes in Section 2 is followed by a description of the [M(CO)2(η 3 -allyl-H,R)(αdiimine)] 0/+ fragment as a convenient scaffold for diverse monodentate ligands participating in a range of substitution, insertion, intramolecular migration and C-C coupling reactionsfrequently involving also the π-allylic ligand, such as allylic alkylation. Special attention is devoted to selected examples of redox and acid-base reactivity of the α-diimine complexes with emphasis on prospects in electrocatalysis. The amidinate (and related pyrazolate) ligands treated in Section 4 may directly replace the π-allylic ligand in some cyclopentadienyl complexes (Section 2) or the α-diimine ligand in some dicarbonyl π-allylic complexes (Section 3). The brief description...

Solvent and Ligand Substitution Effects on the Electrocatalytic Reduction of CO2 with [Mo(CO)4(x,x′‐dimethyl‐2,2′‐bipyridine)] (x=4–6) Enhanced at a Gold Cathodic Surface

“…has proven a robust platform from which to analyze electrocatalytic effects of a range of non‐innocent α‐diimine ligands . Beside the emerging tetracarbonyl α‐diimine catalyst precursors discussed hereinafter, complexes of the type [Mo(η 3 ‐allyl)(CO) 2 (α‐diimine)X] (X=(pseudo)‐halide) are another notable family among Group‐6 carbonyls, which also involve catalytically active species along their reduction path ,. Complexes [M(CO) 4 (bipy)] exhibit a relatively simple cathodic pathway (Scheme ).…”

Molybdenum and rhenium carbonyl complexes containing thiolato ligands

“…The allyl group is oriented with its open face pointing toward the carbonyl ligands, as is common to most known [MoX(h 3 -allyl)(CO) 2 (N-N)] complexes. [49,50] The Mo1-S1 bond distance, of 2.460(2) Å is slightly shorter than that found in the only {Mo(h 3 -allyl)(CO) 2 } thiolato complex characterized by X-ray diffraction, a hydrosulfide complex with a Mo-S bond length of 2.506(1) Å. [51] Only a small number of monomeric Mo(II) thiolato compounds are known, a situation that may be due to the strong tendency of thiolato ligands to coordinate as bridges, and therefore to form polymetallic structures.…”

Group 6 Metal Complexes as Electrocatalysts of CO2 Reduction: Strong Substituent Control of the Reduction Path of [Mo(η3-allyl)(CO)2(x,x′-dimethyl-2,2′-bipyridine)(NCS)] (x = 4–6)

“…Inspired by the promising performance of the Group 6 tetracarbonyl catalysts, the related complex, [Mo­(η 3 -allyl)­(CO) 2 (bipy)­X] (X = (pseudo)­halide), was identified as a precursor to the catalytically active 5-coordinate anion, [Mo­(η 3 -allyl)­(CO) 2 (bipy)] − . , In an analogous fashion to the well-studied catalyst precursor [Mn­(CO) 3 (bipy)­Br], the radical anion [Mo­(η 3 -allyl)­(CO) 2 (bipy)­(NCS)] • – , formed by the initial one-electron reduction ( E pc = −1.99 V vs Fc/Fc + ), is very unstable at room temperature. The concomitant dissociation of the NCS – ligand yields the 5-coordinate radical [Mo­(η 3 -allyl)­(CO) 2 ((bipy)] that can be reduced at the applied electrode potential of the parent complex to [Mo­(η 3 -allyl)­(CO) 2 (bipy)] − .…”