Stuart Macgregor BSc, PhD, FRSC

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• Head of Research Institute
• Programme Director, Chemistry with a Year in Australia
• Institute Management Group
  

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, [L_nM-(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.

1. 'Catalytic Hydrodefluorination of Pentafluorobenzene by [Ru(NHC)-(PPh₃)₂(CO)H₂]: A Nucleophilic Attack by a Metal-Bound Hydride Ligand Explains an Unusual ortho-Regioselectivity', J. A. Panetier, S. A. Macgregor and M. K. Whittlesey, Angew. Chem. Int. Ed., 2011, 50, 2783.
2. 'Experimental and Computational Investigation of C-N Bond Activation in Ruthenium N-Heterocyclic Carbene Complexes', L. J. L. Häller, M. J. Page, S. Erhardt, S. A. Macgregor, M. F. Mahon, M. A. Naser, A. Velez and M. K. Whittlesey, J. Am. Chem. Soc., 2010, 132, 18408.
3. 'A Highly Reactive Rhodium(I) Boryl Complex as a Useful Tool for C-H Bond Activation and Catalytic C-F Bond Borylation', M. Teltewskoi, J. A. Panetier, S. A. Macgregor and T. Braun, Angew. Chem. Int. Ed., 2010, 49, 3947.
4. 'Activation of an Alkyl C-H Bond Geminal to an Agostic Interaction: An Unusual Mode of Base-Induced C-H Activation', L. J. L. Häller, M. Page, S. A. Macgregor, M. F. Mahon and M. K. Whittlesey, J. Am. Chem. Soc., 2009, 131, 4604.
5. 'Mechanisms of C-H bond activation: rich synergy between computation and experiment', Y. Boutadla, D. L. Davies, S. A. Macgregor and A. I. Poblador-Bahamonde, Dalton Trans., 2009, 5820.

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