Dr Simon Aldridge

Inorganic Chemistry Laboratory

Email Address:Simon.Aldridge@chem.ox.ac.uk

Telephone: 01865 2 85201

Research Group Website

Academic History

Jan 2007 -                        University Lecturer in Inorganic Chemistry and Fellow of the Queen’s College, Oxford.

Aug 1998  -    Dec 2006     School of Chemistry, Cardiff University. Senior lecturer from 2004.

Dec 1997  -    Jul 1998      Post-doctoral research associate, Imperial College London (with Prof DMP Mingos FRS).

Mar 1996  -    Nov 1997     Fulbright Scholar and post-doctoral research associate, University of Notre Dame, Indiana, USA (with Prof TP Fehlner).

Oct 1992  -     Feb 1996     DPhil, Inorganic Chemistry Laboratory, University of Oxford (with Prof AJ Downs).

Research Interests / Highlights

Main Group and Transition Metal Organometallic Chemistry

Our research interests span the broad fields of main group and transition metal organometallic chemistry. The main thrust of much of our work involves the synthesis of novel organometallic complexes, investigation of their underlying electronic structure and delineation of fundamental patterns of reactivity. Recent work within the Aldridge group is focussed in three main areas:

(i)    The synthetic, structural and reaction chemistry of transition metal complexes containing multiple bonds to group 13 elements;

(ii)   Structure/bonding studies of transition metal boryl complexes and investigation of their implication in C-H activation chemistry; and

(iii)  The design and synthesis of novel Lewis acids with applications in catalysis and sensors.

Although much of the work has its roots in low coordinate and Lewis acidic main group chemistry, recent expansion has explored more diverse areas of transition metal based reactivity and chemical recognition. The research ranges from fundamental studies (e.g. structure and bonding of new classes of compound) to more applied sensor technology work, and is supported by a range of sources including the EPRSC, DSTL/MoD and industrial partners.

Recent highlights in these areas include the first cationic group 13 diyl complex and Fe=B double bond (J. Am. Chem. Soc., 2003, 125, 6356), the first cationic two-coordinate gallium ligand system (Chem. Commun., 2004, 1732), the first examples of metathesis chemistry involving M=B bonds (Angew. Chem., Int. Ed., 2005, 44, 7457), and novel Lewis acid sensors for F^- and HF (Angew. Chem., Int. Ed., 2005, 44, 3606). Details of current and proposed work in these and related areas are given overleaf.

The Aldridge group specialises in a range of techniques for the synthesis and manipulation of air-sensitive compounds, and uses a variety of approaches (multinuclear NMR, X-ray crystallography, computational chemistry) to characterize new molecules. Active collaborations exist with David Willock and Ian Fallis (both Cardiff), Cameron Jones (Monash, Australia) and the Defence Science and Technology Laboratory (Porton Down).

Recent highlighted work on M=B double bonds (left) and chemical sensors (right)

MULTIPLE BONDING:

Transition metal complexes containing multiply-bonded ligands from groups 14 and 15 are well known. Thus, for example, metal complexes containing carbene (M=C), carbyne (MºC) or imido (MºN) ligands are now found in many undergraduate textbooks. The electronic structures and modes of reactivity of these systems are well understood, and in addition to such fundamental issues, their applications in synthesis have been widely exploited. Hence the Nobel Prize in Chemistry was recently awarded for metathesis chemistry involving M=C and C=C bonds.

Analysis of compounds containing M=B multiple bonds by crystallography (left) and quantum chemistry (right).

By contrast, multiple bonding involving the group 13 elements (B – Tl) is a much more poorly understood and sometimes controversial area. We have been working recently to develop synthetic routes to compounds containing multiple bonds between group 13 elements (chiefly B and Ga) and transition metals. In addition, we have been using a range of techniques (spectroscopic, crystallographic and computational methods) to probe the nature of M=B and M=Ga bonds, which are found to have similarities with classical Fischer carbenes (M=C), vinylidenes (M=C=C) and carbonyls. A further goal of this project is to elucidate fundamental patterns of reactivity for these new ligand systems. We have recently demonstrated metathesis chemistry for M=B bonds (which proceeds via a different mechanism to metathesis involving M=C systems), together with the first examples of [4+1] cycloaddition, insertion and hydride transfer reactions for such compounds (see scheme).

Ongoing/future synthetic targets for this project include chemically more robust M=B systems e.g. containing more electron-rich phosphine based metal/ligand frameworks, and the synthesis of isolated M=Ga and MºB bonds. In addition the exploitation of borylene transfer chemistry towards unsaturated organic substrates will be probed.

Recent references:

1. Cationic terminal borylenes by halide abstraction: synthesis, spectroscopic and structural characterization of an Fe=B double bond. D.L. Coombs, S. Aldridge, C. Jones and D.J. Willock. J. Am. Chem. Soc., 2003, 125, 6356-6357.
2. Cationic terminal borylene complexes: a B/N vinylidene analogue and M=B metathesis chemistry. D.L. Kays (née Coombs), S. Aldridge, J.K. Day and L.-L. Ooi. Angew. Chem., Int Ed., 2005, 44, 7457-7460. (highlighted in Science: Science, 2005, 310, 747).
3. Cationic terminal borylene complexes: inter-conversion of amino- and alkoxy-borylenes via an unprecedented Meerwein-Ponndorf hydride transfer. D.L. Kays (née Coombs), J.K. Day, S. Aldridge, R.W. Harrington and W. Clegg. Angew. Chem., Int. Ed., 2006, 45, 3513-3516.
4. Cationic terminal borylene complexes: structure/bonding analysis and [4+1] cycloaddition reactivity of a BN vinylidene analogue^.. S. Aldridge, C. Jones, T. Gans-Eichler, A. Stasch, D.L. Kays (née Coombs), N.D. Coombs and D.J. Willock. Angew. Chem., Int. Ed., 2006, 45, 6118-6122.

TRANSITION METAL BORYL COMPLEXES - STRUCTURE BONDING AND C-H ACTIVATION STUDIES:

Transition metal boryl complexes (L_nMBX₂) are highly topical compounds, most notably due to their implication in the catalytic functionalization of C-H bonds in arenes and alkanes - one of the ‘Holy Grails’ of organometallic chemistry. Thus, catalytic systems involving boryl complexes as intermediates, will selectively react at the terminal C-H bonds of alkanes such as octane to give useful organic products containing C-B bonds.

We have been involved in this area principally with the aim of better understanding the fundamental issues of electronic structure which underpin this unusual reactivity. Thus, novel synthetic, spectroscopic and computational approaches have been use to investigate the role of the formally vacant boron-centred p-orbital in structural and reaction chemistry. Recent work has revealed an alternative mechanism for attack at both intra- and intermolecular CH bonds by perfluoroaryl metal boryl systems

C-H insertion chemistry for metal boryl complexes driven by coordination of an external ligand (CO)

Ongoing/future work in this area targets (i) late transition metal catalyzed dehydrocoupling of boranes R₂BH to synthetically (and commercially) valuable diboron(4) reagents; and (ii) analogies between isoelectronic N-heterocyclic carbene and anionic diaminoboryl ligand systems, both from perspectives of both structure and reactivity; and (iii) the synthesis and reactivity of amine-donor stabilized boryl complexes and their implication in hydrogen storage and the dehydrogenative coupling of amine boranes R₂NH^.BHX₂.

Recent references:

1. Migratory insertion of [B(C₆F₅)₂] into C-H bonds: CO promoted transfer of the boryl fragment. S. Aldridge, D.L. Kays (née Coombs), A. Al-Fawaz, P.N. Horton, M.B Hursthouse, R.W. Harrington and W. Clegg. Chem. Commun., 2006, 2578-2580.
2. Reactivity of the bis(pentafluorophenyl)boranes ClB(C₆F₅)₂ and [HB(C₆F₅)₂]_n towards late transition metal reagents. A. Al-Fawaz, S. Aldridge, D.L. Coombs, A.A. Dickinson, D.J. Willock, L. Ooi, M.E. Light, S.J. Coles, and M.B. Hursthouse. Dalton Trans., 2004, 4030-4037.
3. Transition metal boryl and borylene complexes: substitution and abstraction chemistry. S. Aldridge and D.L. Coombs. Coord. Chem. Rev., 2004, 248, 535-559.

ANION AND NEUTRAL MOLECULE SENSORS:

The binding of anions by receptor molecules is an area of enormous recent research interest, which is not only relevant to biological systems, but has widespread applications, for example in catalysis and sensor systems. From the viewpoint of sensor design, key features are selectivity (i.e. the recognition of the target anion over possible contaminants) and signalling (i.e. the triggering of a measurable response on anion binding). A wide variety of chemical strategies have been employed to selectively bind anions, and we have been using group 13 based Lewis acids in this area – with the selectivity for given anions based, for example, on the strength of the donor/acceptor bond formed (e.g. for fluoride, F^-) or on the complementary geometry of the binding sites and target anion (e.g. for CN^-or [CH₃CO₂]^-)

A colorimetric sensor for fluoride based on a redox-active (ferrocene) core and anion binding by boronic ester groups

Ongoing work is centred on the design of novel multifunctional Lewis acids and mixed Lewis acid/base systems for the selective detection of fluoride and HF, and the exploitation of such receptors for the sensing of the fluorinated chemical warfare agents (CWAs). Key receptor design principles are (i) the known selectivity of fluoride binding by boronic esters; and (ii) electrochemical, colorimetric or fluorescence based reporting (e.g. utilizing ferrocene units, Fc). A key target of ongoing research is the selective sensing of cyanide (or hydrogen cyanide) in the presence of potentially competitive anions (e.g. halides); such systems are targeting a colorimetric, electrochemical or fluorescence based output.

A key future target in this area, ultimately aimed at improving device sensitivity, is the development of catalytic sensors. The aim is to identify host/guest complexes formed between receptor and target analyte which will catalyze an orthogonal reaction. Our approach utilizes electron transfer chemistry as the basis for catalysis, e.g. of a dye bleaching reaction.

Recent references:

1. Selective electrochemical detection of hydrogen fluoride by ambiphilic ferrocene derivatives. C. Bresner, S. Aldridge, I.A. Fallis, C.Jones and L.-L. Ooi. Angew. Chem. Int. Ed., 2005, 44, 3606-3609.
2. Fluoride anion binding by cyclic boronic esters: influence of backbone chelate on receptor integrity. C. Bresner, J.K. Day, N.D. Coombs, I.A. Fallis, S. Aldridge, S.J. Coles and M.B. Hursthouse. Dalton Trans., 2006, 3660-3667.
3. The chemistry of boryl and borate substituted metallocenes. S. Aldridge and C. Bresner. Coord. Chem. Rev., 2003, 244, 71-92.