Professor P. Mountford
Telephone: 44 (0)1865 285140
We are working on the synthesis and applications of new organometallic
compounds of the transition metals and lanthanides. We are
particularly interested in the following areas. Please visit our research
group web pages for more details and information about our
group.
(1) The synthesis and reactions of compounds with metal-nitrogen
multiple bonds, especially with regard to novel N-C and C-C bond-forming
reactions.
Early transition metal compounds of the type (L)M=NR possess polar,
unsaturated metal-nitrogen multiple bonds which undergo unusual
and interesting reactions with a wide range of organic substrates.
In addition to being intrinsically novel, these fundamental N-C
and C-C coupling reactions can also be made catalytic in certain
instances. A particularly important example of this occurs in the
titanium-catalysed addition of amines RNH2 across the carbon-carbon
multiple bonds of alkynes and allenes to give new imine or ene-amine
products (this is the "hydroamination" reaction which
is of enormous current interest in organic synthesis).
Figure 1: New reactions of Ti=NR
multiple bonds showing single and multiple N-C and C-C forming
reactions leading to new heterocyclic fragments. Some of the reaction
also involve highly selective C-H bond activation steps.
We have been advancing the fundamental chemistry of early transition
metal imido compounds of this type, and we have discovered
a wide range of novel N-C. C-C and other bond-forming reactions.
Figure 1 shows some of our recent titanium chemistry using a diamide-pyridine
supporting ligand set. More recently we used the same system for
the anti-Markovnikov selective stoichiometric and catalytic hydroamination
of terminal alkynes.
Figure 2: X-ray structure of the
unique pseudo two-coordinate imido compound Ti(NtBu)(C8H8) recently
prepared in the group, along with the HOMO (metal-ligand π bonding)
and LUMO (metal dz2) orbitals that govern the reaction chemistry
of this remarkable compound.
In addition to developing the M=NR bond reaction chemistry of
imido complexes we are also interested in determining and undestanding
their electronic structures using Density Functional Theory (which
gives an accurate view of the bonding and also mechanisms of reaction)
and physical methods such as X-ray diffraction, NMR techiques and
gas-phase photoelectron spectroscopy. The computational work is
carried in collaboration with Prof
J C Green (Oxford University) and Dr
E Clot (Montpellier University, France). Figure 2 shows the
crystal structure of a unique "pogo stick" imido complex
that we recently prepared, along with representations of two of
the most important molecular orbitals.
(2) The development and applications of new cationic Group
3 and Group 4 organometallic compounds, and in particular their
use as alkene polymerisation catalysts.
Another very important current area of organometallic chemistry
is Ziegler-Natta alkene polymerisation catalysis. Inspired by the
early successes in this area 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 and also understanding the underlying
fundamental chemistry. The catalytically active species in this
chemistry are alkyl cations [(L)M-R]+. A very recent development
in this area has been the recognition of the importance of analogous
cationic Group 3 and lanthanide compounds. We are developing new
cationic polymerisation catalysts in both Groups 3 and 4 using
as-yet underexploited neutral N3 donor ligands.
In a joint venture between our group, industrial sponsors in the
field of catalysis and the UK research councils we are developing
two families (see Figure 3) of new "post-metallocene" catalysts I and II by
combining dianionic imido ligands 'NR2-' with 6 electron donor
face-capping neutral ligands TACN (in I) and TPM
(in II). The compounds are isolobal analogues
of the important Group 4 metallocene catalysts Cp2MX2. Well-defined
alkyl cations (the active species in polymerisation catalysis)
can be observed (e.g. III - which is stabilied
by a β-Si-C--Ti agostic interaction as established by DFT
electronic structure calculations and 29Si NMR spectroscopy).
Figure 3: New"post-metallocene" catalysts
with the neutral donor face-capping neutral ligands TACN (in I)
and TPM (in II) and the β-Si-C--Ti agostic
stabilised cation III (structure as predicted
by DFT calculations in collaboration with Dr E Clot (Montpellier
University, France)
The methyl cation [Ti(NtBu)(TACN)Me]+ (IV) can
also be prepared and it has a rich reaction chemistry towards a
range of substrates as shown in Figure 4. Compound V (X-ray
structure also shown) was the first structurally characterised
AlMe3 adduct of an alkyl cation (such species are key resting states
in most industrially-relevant polymerisation processes because
of the nature of the aluminium co-catalysts used).
Figure 4: Selected reactions of
the novel 14 valence electron cation [Ti(NtBu)(TACN)Me]+ (IV)
with unsaturated substrates and with AlMe3 to form V (X-ray
structure shown)
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
5, 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). This leads to the possibility of tuning catalyst
performance in a very subtle way.
Figure 5: Ethylene polymerisation
activities of the new catalyst families Ti(NR)(TACN)Cl2 (I)
and Ti(NR)(TPM)Cl2 (II) showing how activity varies
as a function of Ti=NR group
(3) The development of novel "post-metallocene" early
transition metal and lanthanide compounds with new supporting
ligands and their applications in catalytic chemistry, incuding
the synthesis of biodegradable, environmentally-friendly polymers.
Over the last 10-15 years there has been a move away from Cp-type
ligand environments so as to discover new patterns of stoichiometric
and catalytic reactivity. Anionic polydentate N- and O-donor ligands
have been particularly successful in this context. We have been
developing the new organometallic stoichiometric and catalytic
reaction chemistry of diamido-amine supported transition metal
and lanthanide complexes. A selection of the new classes of compounds
are shown in Figure 6. In particular we have recently developed
new zirconium diamide-amine catalysts for the polymerisation of
ethylene and lanthanide diamide-diamine borohydride compounds the
polymerisation of polar monomers such as methyl methacrylate, e-caprolactone
and rac-lactide (these give environmentally friendly /
bio-degradable plastics).
Figure 6: Some of the new early
transition metal and lanthanide compounds being developed in the
group.
Further details
Our group is based in the new Chemistry
Research Laboratory which opened in January 2004 (click here
for a virtual tour). We occupy excellent state-of-the-art
laboratory space on the 2nd floor in which every researcher has
their own 2m fume hood with all of the necessary equipment (glove
boxes, office and computer area, NMR and other spectroscopic/analytical
equipment) close at hand.
Our academic research has been funded by grants from the EPSRC,
Leverhulme Trust, Nuffield Foundation, China Scholar Council, EC
(Marie Curie), University of Oxford, British Council, Royal Society,
Royal Society of Chemistry, and NATO. Applied aspects of
our work have been supported by DSM Research and Elastomers, Sabic
EuroPetrochemicals, Cambridge Material Science and Millenium Pharamaceuticals.
We have a number of ongoing collaborations with other research
groups both here in Oxford and at other universities, and also
with industrial labs.
Selected publications
The synthesis and reaction chemistry of compounds with
metal-nitrogen multiple bonds
"Reactions and applications of titanium imido complexes".
N Hazari and P. Mountford, Acc. Chem. Res., 2006, 39,
in press (ASAP).
"Reactions of neutral and cationic diamide-supported imido
complexes with CO2 and other heterocumulenes: issues of site selectivity".
B. D. Ward, G. Orde, E. Clot, A. R. Cowley, L. H. Gade and P. Mountford, Organometallics,
2005, 24, 2368-2385.
"Pendant arm functionalised benzamidinate titanium imido
compounds: experimental and computational studies of their reactions
with CO2". C. L. Boyd, E. Clot, A. E. Guiducci and P. Mountford, Organometallics.,
2005, 24, 2347-2367.
"Titanium imido complexes of cyclooctatetraenenyl ligands".
A. J. Blake, A. R. Cowley, S. C. Dunn, J. C. Green, N. Hazari,
N. M. Jones, A. G. Moody and P. Mountford. Chem Eur J.,
2005, 11, 2111-2124.
"Synthesis, reactivity and computational studies of the cationic
tungsten methyl complex [W(NPh)(N2Npy)Me]+ and related compounds
(N2Npy = MeC(2-C5H4N)(CH2NSiMe3)2)". B. D. Ward, G. Orde,
E. Clot, A. R. Cowley, L. H. Gade and P Mountford, Organometallics,
2004, 23, 4444-4461.
"Synthesis and Structural Characterization of an Azatitanacyclobutene:
The Key Intermediate in the Catalytic Anti-Markovnikov Addition
of Primary Amines to a-Alkynes". B. D. Ward, A. Maisse-Francois,
P. Mountford and L. H. Gade, Chem. Commun., 2004, 704-705.
The development and applications of new cationic Group
3 and Group 4 organometallic compounds
"The first rare earth organometallic complex of 1,4,7-trithiacyclononane:
a precursor to unique cationic ethylene and a-olefin polymerisation
catalysts supported by an all-sulfur donor ligand". C. S.
Tredget, F. Bonnet, A. R. Cowley and P. Mountford, Chem. Commun.,
2005, 3301-3303.
"Well-defined imidotitanium alkyl cations: agostic interactions,
migratory insertion vs [2+2] cycloaddition, and the first
structurally authenticated AlMe3 adduct of any transition metal
alkyl cation". P. D. Bolton, E. Clot, A. R. Cowley and P.
Mountford, Chem. Commun., 2005, 3313-3315.
"Discovery of highly active imidotitanium ethylene polymerisation
catalysts and their evaluation using high throughput catalyst screening".
N. Adams, H. J. Arts, P. D. Bolton, D. Cowell, S. R. Dubberley,
N. Friederichs, C. Grant, M. Kranenburg, B. Wang, P. J. Wilson,
A. R. Cowley, P. Mountford, and M. Schroder, Chem. Commun.,
2004, 434-435.
"Highly efficient ethylene polymerisation by scandium alkyls
supported by simple fac-k3 coordinated N3 donor ligands".
S. C. Lawrence, B. D. Ward, S. R. Dubberley, C. M. Kozak and P.
Mountford, Chem. Commun., 2003, 2880-2881.
The development of novel "post-metallocene" early
transition metal and lanthanide compounds with new supporting
ligands
"Lanthanide Borohydride Complexes Supported by Diamino-bis(phenoxide)
Ligands for the Polymerization of e-Caprolactone and L- and rac-Lactide". F.
Bonnet, A. R. Cowley and P. Mountford, Inorg. Chem., 2005, 44,
in press (ASAP).
" New Group 4 organometallic and imido compounds of diamide-diamine
and related dianionic O2N2-donor ligands". M. E. G.
Skinner, T. Toupance, D. A. Cowhig, B. R. Tyrrell and P. Mountford, Organometallics,
2005, 24, 5586-5603.
Lanthanide mono(borohydride) complexes of diamide-diamine donor
ligands: novel single site catalysts for the polymerisation of
methyl methacrylate". F. Bonnet, A. C. Hillier, A. Collins,
S. R. Dubberley and P. Mountford, Dalton Trans., 2005,
421-423 (communication).
"Coordination, organometallic and related chemistry of tris(pyrazolyl)methane
ligands". H. R. Bigmore, S. C. Lawrence, P. Mountford and
C. S. Tredget, Dalton Trans., 2005, 635-651 (Perspective
Article review).
"A new diamido-amine ligand based on three-carbon atom "arms":
synthesis, structures and polymerisation capability of zirconium
derivatives of MeN(CH2CH2CH2NSiMe3)2". T. A. Lowes, B. D.
Ward, R. A. Whannel, S. R. Dubberley and P. Mountford, Chem.
Commun., 2005, 113-115.
Other recent projects and reviews
"Transition metal imido compounds as Ziegler-Natta olefin
polymerisation catalysts". P. D. Bolton and P. Mountford, Adv.
Synth. Catal., 2005, 347, 355-366 (review).
"Revelations in dinitrogen activation and functionalisation
by metal complexes", C. M. Kozak and P. Mountford, Angew.
Chem. Int. Ed., 2004, 43,
1186-1189 (Highlight).
"A monomeric organolithium compound containing a free pyramidal
carbanion in solution and the solid state". F. Breher,
J. Grunenberg, S. C. Lawrence, P. Mountford and H. Rüegger, Angew.
Chem. Int. Ed.,2004, 43,
2521-2524.
"Unexpected features of stretched Si-H···Mo
b-agostic interactions". S. K. Ignatov,N. H. Rees, S.
R. Dubberley, A. G. Razuvaev, P. Mountford and G. I. Nikonov, Chem.
Commun., 2004, 952-953.
"Are J(Si-H) NMR coupling constants really a probe
for the existence of non-classical H-Si interactions?". S.
R. Dubberley, S. K. Ignatov, N. H. Rees, A. G. Razuvaev, P. Mountford
and G. I. Nikonov, J. Am. Chem. Soc., 2003, 125,
642-643. |
