eta(3)-Allyl carbonyl complexes of group 6 metals: Structural aspects, isomerism, dynamic behaviour and reactivity
Ryan, DE; Cardin, DJ; Hartl, F
Transition metal complexes with pi-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 pi-allylic representatives of low-cost Group-6 metals bearing one or more carbonyl ligand, the coordination sphere being complemented with eta(5)-cyclopentadienyl (Section 2), chelating ligands, including redox-active alpha-diimines and various complementary diphosphines (Section 3), and novel anionic amidinate or pyrazolate ligands (Section 4). In Section 1, particular attention is paid to rearrangements of the pi-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 pi-allylic ligand are described. The detailed mechanistic description of the synthetic routes and dynamic behaviour of selected representatives of alpha-diimine complexes in Section 2 is followed by a description of the [M(CO)(2)(eta(3)-allyl-H,R)(alpha-diimine)](0/+) fragment as a convenient scaffold for diverse monodentate ligands participating in a range of substitution, insertion, intramolecular migration and C-C coupling reactions - frequently involving also the pi-allylic ligand, such as allylic alkylation. Special attention is devoted to selected examples of redox and acid-base reactivity of the alpha-diimine complexes with emphasis on prospects in electrocatalysis. The amidinate (and related pyrazolate) ligands treated in Section 4 may directly replace the pi-allylic ligand in some cyclopentadienyl complexes (Section 2) or the alpha-diimine ligand in some dicarbonyl pi-allylic complexes (Section 3). The brief description of their synthetic routes is complemented by intriguing examples of fluxionality and characteristic reactivity encountered for these unusual four-electron donor ligands. Crown Copyright (C) 2016 Published by Elsevier B.V. All rights reserved.