Computational Organometallic Chemistry
Research uses computational chemistry to model reaction mechanisms in transition metal organometallic chemistry. Methods employed include density functional theory, hybrid QM/MM calculations and molecular dynamics. We aim to understand challenging bond activation processes (C-H and C-F bond cleavage), rationalise unusual reactivity patterns and model multi-step catalytic cycles. Research is usually carried out in close collaboration with experimental chemists.
1. Ambiphilic Metal-Ligand Assisted (AMLA) C-H Activation
We have developed the concept of Ambiphilic Metal-Ligand Assisted (AMLA) C-H activation. In this process an agostic interaction to an unsaturated metal centre combines with H-bonding to a basic co-ligand to facilitate C-H bond cleavage. With aromatic C-H activation at [Pd(OAc)2] this mechanism supersedes the long-proposed Wheland-type intermediate. AMLA can account for facile C-H bond cleavage of both e--deficient and e--rich aromatic substrates at a range of late transition metal centres.
Figure 1. Computed agostic Intermediate in the AMLA-6 C-H Activation of dimethylbenzylamine at [Pd(OAc)2].
2. Metallophosphoranes and Aromatic C-F Bond Activation
We have defined novel ligand-assisted mechanisms for breaking the strong C-F bond of fluoroaromatics. This process involves nucleophilic attack by an e--rich metal centre with addition of a C-F bond over the M-L moiety, where L can be PR3, SiR3 or BR2. For L= PR3 metallophosphoranes, [LnM-(PFR3)], are formed as intermediates or transition states. Metallophosphoranes also play a role in the unusual M-F/P-R exchange reactions, such as the interconversion of [RhF(PPh3)3] to [Rh(Ph)(PFPh2)(PPh3)2].
Figure 2. The central role of metallophosphoranes in phosphine-assisted C-F bond activation and F/R exchange processes.
3. Ruthenium N-Heterocyclic Carbene (NHC) Complexes in Catalysis
NHC ligands often confer enhanced reactivity on metal complexes. An example is the hydrodefluorination of C6F5H to give 1,2-C6F4H2 catalysed by [Ru(H)2(CO)(NHC)(PR3)2] species. Calculations show this unusual ortho-selectivity arises from a nucleophilic attack mechanism where the hydride ligand (and not the metal) acts as the reacting species. Calculations also aim to understand the stability of NHC ligands towards metal-based decomposition reactions such as C-H, C-C and C-N activation.
Figure 3. Nucleophilic attack of a hydride ligand at the ortho position of C6F5H.