|
The Mountford Group |
|
Home
|
Research
Interests |
Publications |
Group |
Joining the
Group |
Philip Mountford
Laboratory Facilities |
Visitor Information |
Oxford
Chemistry |
|
Research Interests Our Group is currently interested in three main topics which are described below. All of these areas involved organometallic synthesis and then either stoichiometric or catalytic reactivity, along with studies of reaction mechanisms. We also collaborate closely with computational and experimental groups both within Oxford and also overseas to maximise the impact and understanding of our research (see below). Our current and recent work has been funded by a range of organisations including the UK research councils, charitable organisations and industry. Synthesis, bonding and small molecule activation reactions of transition metal hydrazide complexes Studies of the synthesis and reactivity of transition metal hydrazides, (L)M=NNR2, have been of sustained interest and activity for over 25 years. Historically, this is because of their relevance to the biological conversion (fixation) of N2 to ammonia. For this reason, much of the early work was based around Group 6 systems for which M=N-NR2 group reactivity is characterised only by reactions of the N atom and/or N-N reductive cleavage under very forcing conditions. Although initial reactivity reports of Group 4 hydrazides appeared some time ago, there has recently been a surge in activity in this area. We have been at the forefront of this drive and some of our new titanium hydrazide complexes are shown below:
Figure: New titanium hydrazide compounds from our group. These all have the Ti=NNR2 functional group in a new ligand environment A large number of new reactions of the M=NNR2 functional group have been discovered in the last 3-4 years. These include the cycloaddition of alkynes and heteroalkynes to the M=Nmultiple bond, and N-N bond insertion reactions with several substrates, including alkynes and isocyanides, and single and double substrate insertion reactions into M=N. Here are some specific new transformations:
Figure: Single and double substrate insertion into the Ti=N bond.
Figure: Hydrazide to hydrazidium transformation then 2+2 cycloaddtion.
Figure: Addition of a C-P triple bond to Ti=NNPh2.
Figure: Insertion of alkynes into the N-N bond and discovery of the catalytic alkyne diamination reaction. Detailed mechanistic studies in conjunction with theoretical studies (Dr Eric Clot, Montpellier) have shown that the remarkable N-N bond cleavage/insertion reactions with isonitriles (RNC) and alkynes (RCCR') proceed via an initial [2+2] addition to Ti=Na followed by Nb coordination and then finally N-N bond rupture:
Figure: Summary of a combined experimental and DFT study of the N-N insertion reactions of titanium hydrazides with alkynes We are actively continuing to develop the chemistry of this type of (L)M=NNR2 complex for a number of different metals. Our aim is to develop further our control over this chemistry and identify catalytic applications of these systems. Selected references: "A remarkable switch from a diamination to a hydrohydrazination catalyst and observation of an unprecedented catalyst resting state.” A. D. Schwarz, C. S. Onn and P. Mountford, Angew. Chem. Int. Ed., 2012, in the press. [link to journal] "Si-H and Si-Cl bond activation of titanium hydrazides with silanes and subsequent Ti-H / E-H (E = Si or H) sigma bond metathesis". P. J. Tiong, A. Nova, E. Clot and P. Mountford, Chem. Commun., 2011, 47, 3147-3149. [link to journal] "Titanium alkoxyimido (Ti=N-OR) complexes: reductive N-O bond cleavage at the boundary between hydrazide and peroxide ligands". A. D. Schwarz, A. Nova, E. Clot and P. Mountford, Chem. Commun., 2011, 47, 4926–4928. [link to journal]. "M=N cycloaddition and N-N insertion in the reactions of titanium hydrazido compounds with alkynes: a combined experimental and computational study". A. D. Schofield, A. Nova, J. D. Selby, C. D. Manley, A. D. Schwarz, E. Clot and P. Mountford, J. Am. Chem. Soc., 2010, 132, 10484-10497. [link to journal]. "Single and double substrate insertion into the Ti=N bonds of terminal titanium hydrazides". P.-J. Tiong, A. D. Schofield, J. D. Selby, A. Nova, E. Clot and P. Mountford, Chem. Commun. 2010, 46, 85-87. [link to journal]. "Cycloaddition reactions of transition metal hydrazides with alkynes and heteroalkynes: coupling of Ti=NNPh2 with PhCCMe, PCCH, MeCN and tBuCP." J. D. Selby, A. D. Schwarz, C. Schulten, E. Clot, C. Jones and P. Mountford, Chem. Commun., 2008, 5101-5103. Designated a "hot article" by the Editor. [link to journal]. "New ligand platforms for developing the chemistry of the Ti=N-NR2 functional group and insertion of alkynes into the N-N bond of a Ti=N-NPh2 ligand". J. D. Selby, C. D. Manley, M. Feliz, A. D. Schwarz, E. Clot and P. Mountford, Chem. Commun., 2007, 4937-4939. [link to journal]. New ring-opening polymerisation catalysts for biodegradable and biocompatible polymers There is a great deal of current interest in the controlled ring-opening polymerization (ROP) of cyclic esters such as -caprolactone (-CL) or lactide (LA) to form biocompatible or biodegradable thermoplastic materials, from both a molecular catalysis and materials chemistry point of view. Polylactides and polycaproactones, for example, are both FDA-approved for medical use. Lactide is in principle infinitely renewable, being derived from corn and is thus an non-oil-derived, renewable resource. The key challenges are the controlled ROP of these cyclic esters allowing control of polymer molecular weight, tacticity (lactide has either R or S stereocentres). We have been developing three new approaches to polyesters using metal-borohydride catalysts, sulfonamide-supported catalysts or amine-co-oinitiated cationic or zwitterionic catalysts.
Figure: -caprolactone and rac-lactide and examples of polylactide-based products We were the first group to use lanthanide-borohydride complexes as alternative initiators for the ROP of rac-lactide. Using bulky phenolate supporting ligands good control of molecular weight and stereochemistry was achieved, with the highly Lewis acidic lanthanide centres providing for high activities. In the following study a detailed study of the ROP of -caprolactone and rac-lactide by a series of samarium borohydride compounds was reported. Computational (with Prof Laurent Maron, Toulouse) and detailed spectrometric studies revealed a new two-stage ring-opening process by Sm–BH4 leading to –CH(Me)CH2OH or –CH(Me)CHO end groups:
Figure: Use of lanthanide borohydride complexes for the efficient ROP of rac-lactide While poly(phenolate) supporting ligands have been widely used in ROP catalysis (see above), N-donor ligands have been much less widely used. We have developed novel families of ROP initiators using poly(sulfonamide) ligands where the electron-deficient -NSO2R donors act as "phenolate mimics". Extremely good control of molecular weight and molecular weight distributions were found using these systems:
Figure: Sulfonamide-supported Group 4 and aluminium initiators (top) developed in our group along with well behaved first order kinetic plots (bottom left) and living polymerisation characteristics (bottom right) In a third area of ROP catalysis we have recently developed well-defined zwitterionic (see Figure below) and cationic catalysts for the amine-co-initiated ROP of lactide. Using amines, which are readily available commercially, in this manner allows facile access to new polymer architectures through branched polyamine starting materials, while the high rates of conversion and excellent molecular weight and tacticity control lead to well-defined materials:
Figure: Zwitterionic and yttrium compounds act as catalysts for the primary or secondary amine-initiated immortal ROP of rac-lactide forming amine-terminated, highly heterotactic poly(rac-lactides) with narrow polydispersities Current work in the group is focussed on extending these three areas while also developing new classes of initiators for -caprolactone and rac-lactide and other important cyclic and non-cyclic monomers. Selected references: "Synthesis and rac-lactide ring-opening polymerisation studies of new alkaline earth tetrahydroborate complexes.” R. A. Collins, J. Unruangsri and P. Mountford, Dalton Trans., in the press. Invited contribution for a Special Issue on metal borane and borohydride chemistry. [link to journal]. "Cationic and charge-neutral calcium tetrahydroborate complexes and their use in the controlled ring-opening polymerisation of rac-lactide." M. G. Cushion and P. Mountford, Chem. Commun., 2011, 47, 2276-2278. [link to journal]. "Ligand variations in new sulfonamide-supported Group 4 ring-opening polymerization catalysts." A. D. Schwarz, K. R. Herbert, C. Paniagua and P. Mountford. Organometallics, 2010, 29, 4171-4188. [link to journal]. "Ring-opening polymerization of rac-lactide by bis(phenolate)amine-supported samarium borohydride complexes: an experimental and DFT study." H. E. Dyer, S. Huijser, N. Susperregui, F. Bonnet, A. D. Schwarz, R. Duchateau, L. Maron and P. Mountford. Organometallics, 2010, 29, 3602-3621. [link to journal]. "Low-coordinate rare earth complexes of the asymmetric 2,4-di-tert-butylphenolate ligand prepared by redox transmetallation/protonolysis reactions and their reactivity towards ring-opening polymerisation." L. Clark, G. B. Deacon, C. M. Forsyth, P. C. Junk, P. Mountford and J. P. Townley, Dalton Trans., 2010, 39, 6693-6704. Invited contribution for a Special Issue on "Organo f-element chemistry". [link to journal]. "Dicationic and zwitterionic catalysts for the amine-initiated, immortal ring-opening polymerisation of rac-lactide: facile synthesis of amine-terminated, highly heterotactic PLA". L. Clark, M. G. Cushion, H. E. Dyer, A. D. Schwarz, R. Duchateau and P. Mountford, Chem. Commun. 2010, 46, 273-275. [link to journal]. Fundamental and applied studies of new olefin polymerisation and oligomerisation catalysts Another very important current area of organometallic chemistry is Ziegler-Natta alkene polymerisation catalysis. Inspired by the early successes of Group 4 metallocene catalysts Cp2MX2 (X = alkyl or halide), a huge amount of academic and industrial effort is being spent world-wide on developing new transition metal catalysts that combine very high productivites with good control of polymer molecular weight, as well as also understanding the underlying fundamental chemistry. The catalytically active species in this chemistry are alkyl cations [(L)M-R]+. In a joint venture between our group, industrial sponsors in the field of catalysis and the UK research councils we are developing new families of new "post-metallocene" catalysts of the type I and II below by combining dianionic imido ligands 'NR2-' with 6 electron donor face-capping neutral ligands TACN (in I below) and TPM (in II):
Figure: New "post-metallocene" catalysts with the neutral face-capping ligands Well-defined alkyl cations (the active species in polymerisation catalysis) can be observed (e.g. III - which is stabilised by a β-Si-C--Ti agostic interaction). These have been studied extensively with regard to their reactions with unsaturated molecules and other substrates:
Figure: Reactions of the novel 14 valence electron cation [Ti(NtBu)(TACN)Me]+ The catalyst families Ti(NR)(TACN)Cl2 (I) and Ti(NR)(TPM)Cl2 (II) are the most active in their class for imido (NR), TACN or TPM ligands. As illustrated in Figure 3 above, they show significant differences in their polymerisation activities depending on both the NR group and the nature of the neutral face-capping ligand (TACN or TPM):
Figure: Ethylene polymerisation activities Ti(NR)(TACN)Cl2 (I) and Ti(NR)(TPM)Cl2 (II). Note how activity varies as a function of Ti=NR group differently depending on the N3 donor used (TACN or TPM) We have more recently been collaborating with DSM Elastomers on new titanium-amidinate complexes for the copolymerisation of ethylene and propylene. These catalysts are entering commercial production and we are working towards a better understanding of how to improve these systems and to also understand the underlying organometallic chemistry. For example, reaction of Cp*Ti{NC(ArF2)NiPr2}Me2 with [Ph3C][B(C6F5)4] gave the base-free dication [Cp*2Ti2{NC(ArF2)NiPr2}2(μ-Me)2][B(C6F5)4]2 containing two doubly α-agostic bridging methyl groups which is a highly effective ethylene–propylene polymerization catalyst. This type of complex has never been observed before in olefin polymerisation systems:
Figure: Unique dimeric dications as resting precursors to remarkably active olefin copolymerisation catalysts Selected references: "Synthesis, solid state and DFT structure and olefin polymerization capability of a unique base-free dimeric methyl titanium dication". E. G. Ijpeij, B. Coussens, M. A. Zuideveld, G. H. J. van Doremaele, P. Mountford, M. Lutz and A. L. Spek, Chem. Commun. 2010, 46, 3339-3341. [link to journal]. "Ti=NR vs Ti=NR' functional group selectivity in imido titanium alkyl cations from an experimental perspective." P. D. Bolton, M. Feliz, A. R. Cowley, E. Clot and P. Mountford, Organometallics, 2008, 27, 6096-6110. [link to journal]. "Synthesis, DFT studies and reactions of scandium and yttrium dialkyl cations containing neutral fac-N3 and fac-S3 donor ligands". C. S. Tredget, E. Clot and P. Mountford, Organometallics, 2008, 27, 3458-3473. [link to journal]. "AlMe3 and ZnMe2 adducts of a titanium imido methyl cation: a combined crystallographic, spectroscopic and DFT study". P.D. Bolton, E. Clot, A. R. Cowley and P. Mountford, J. Am. Chem. Soc., 2006, 128, 15005-15018. [link to journal]. Here are some details of our current and recent collaborators. Prof Simon Aldridge (Oxford) and Prof Cameron Jones (Monash University, Australia). Transition and main group boryl and gallyl chemistry. Dr Eric Clot (University of Montpellier, France). Computational studies of mechanism and electronic structure (in particular metal-ligand multiple bonds). Profs Glen Deacon and Peter Junk (Monash University, Australia). Lanthanide and Group 2 phenolate complexes for the living and immortal ring-opening polymerisation of cyclic esters. Prof Nik Kaltsoyannis (University College London). Computational studies of mechanism and electronic structure (in particular metal-boryl, silylene and metal-metal bonding). Prof Laurent Maron (University of Toulouse, France). Computational studies of mechanism with particular regard to polymerisation chemistry of the lanathanides. Prof Georgii Nikonov (Brock University, Canada). Reactions of transition metal imido and related compounds with silanes; non-classical bonding interactions in silyl-hydride and silane complexes. Our work has been funded by grants from the EPSRC, Leverhulme Trust, Nuffield Foundation, China Scholarship Council, EC (Marie Curie), University of Oxford, British Council, Royal Society and the Royal Society of Chemistry. Applied aspects of our research have been supported by DSM Research, DSM Elastomers, Sabic Europe, Cambridge Material Science and Millenium Pharamaceuticals.
|
