Ozerov Group Research
The projects in our group are diverse but often share the same philosophy. In general, the ideal-case research strategy is to: 1) conceive of or discover a new reaction or a ligand environment; 2) demonstrate unusual reactivity, structural, or electronic novelty; 3) apply the findings to develop or discover a new catalytic process. We strive to use the diversity of the pursued goals to our advantage through synergy and exposure of students to a breadth of ideas.
Ongoing Projects
Ligand design. Ligand design is an important part of our
research, yet it is merely a tool and not a goal in and of itself. We are interested in tri- and tetradentate
ligands that possess a rigid structure and impose certain geometry at the metal
center. Besides increasing the stability
of transition metal complexes, rigid ligand construction, to borrow synthetic
organic terminology, decreases the complexity of what may happen at the
metal center. We are developing a series
of ligands with systematically varied properties. The variation is not only in the common sense
of using substituent effects to control sterics and electronics, but also in
modifying trans-influence, charge, p-donor/p-acid properties in the
context of the analogous, enforced geometries.
Our approaches also lend themselves to adapting the ligands for
recyclability and introduction of chirality.
New catalysts for coupling
reactions. We have
recently discovered that the (PNP)Rh and (PNP)Ir fragments readily undergo a
variety of oxidative addition and reductive elimination reactions while
maintaining the integrity of the (PNP)M fragment (e.g., JACS 2005,
127, 16772; JACS 2006, 128, 2808). We are targeting developing new catalytic
reactions, in particular for coupling of aryl halides and aryl organosulfonates
with carbon- and heteroatom-based nucleophiles.
Other processes include, for instance, highly regioselective alkyne
dimerization (Chem. Commun. 2006, 197). This project should offer new avenues for
catalysis, different from the classical Pd0/PdII
chemistry. In addition, elaboration of
the pincer ligands should allow recyclability through either supporting the
catalysts on solids or enabling fluorous solubility. This work is currently supported by the NSF.
Catalysis with manganese? Much of the
classical organometallic chemistry involves the heavier transition elements,
those of the 5th and 6th period (“4d” and “5d”
metals). Utilization of 3d metals as
catalysts offers obvious advantages of lower cost, greater biocompatibility,
and lower environmental hazard.
Catalysis with Ti, Cr, Fe, Co, Ni, Co, and Zn is either well-known or at
least heavily pursued. We note that
manganese remains underutilized and would like to develop
practical Mn catalysis chemistry. Mn is
a challenging metal to work with in part because of the prevalence of the
high-spin d5 Mn(II) oxidation state with
the reactivity akin to Ca2+ or Zn2+ rather than a typical
mid-transition series metal. We are
working to “tame” Mn by using designer rigid ligands that would stabilize
“organometallically-relevant” electronic states. We target developing Mn catalysts for simpler
processes, such as olefin hydrogenation, at first. This is a relatively high-risk project that
will eye enantioselective transformations and NIH funding if substantial
progress is made.
Hypercoordinate main group compounds. The rigidity
of PNP ligands offers interesting possibilities for synthesis of
hypercoordinate main group compounds.
The key idea is that the rigid backbone of the pincer ligand may help
enforce binding of both phosphine arms to the central main group element and
create bonding situations that are otherwise unattainable. Some examples of potential and accomplished
targets are shown (e.g., Mend. Comm. 2007, 17, 63). This work is currently funded by
ACS-PRF.
Early metal-carbon multiple
bonds. We have been
using the PNP ligands to stabilize unusual examples of early metal-carbon
multiple bonds. The enforcement of
phosphine binding (even to hard, early metals!) leads to the increase in steric
congestion at the metal which in the case of polyalkyl complexes often leads to
a-abstraction reactions and formation of metal alkylidenes and
alkylidynes. We have so far shown the
viability of this approach through preparation of Zr alkylidenes (OM 2004,
23, 4700) and a Ta bis(methylidene) complex (OM 2007, 26,
4866), the first known M(=CH2)2 complex for any
metal. The Mindiola group at Indiana U
is also pursuing similar goals; we collaborate and divide the spheres of
attention, as necessary. In my group,
this project has been mainly moved by undergraduates in the recent years. We are also collaborating with the Kiplinger
group at LANL on the studies of PNP complexes of lanthanides. Lanthanide alkylidenes, as of yet unknown,
are an exciting target!
Lewis acid catalysis. We are interested in developing a family of
mid- to late-metal, pincer-supported Lewis acid catalysts for organic
reactions. We have recently shown that
(PNP)NiOTf is an efficient catalyst for coupling of
nitriles and aldehydes (Chem. Commun. 2005, 4450). (P2C=)Ru(H)(OTf)
is also an active catalyst. Work is
underway to include coupling reactions of aldehydes, ketones, and imines on one
hand with nitriles, nitroalkanes, and esters on the other. There is much potential here for developing
new enantioselective reactions through application of chiral pincer
ligands. This is part of the research
currently supported by an NSF grant.
Electrophilic C-F bond
activation. Activation of
carbon-fluorine bonds is both a formidable fundamental challenge and a problem
of relevance to environmental problems.
Perfluoroalkanes are extremely potent and long-lived greenhouse gases
while chlorofluoroalkanes (freons) are a threat to the ozone layer. Our group has achieved a breakthrough in
catalytic C-F activation by using highly electrophilic silylium cation-like
catalysts (JACS 2005, 127, 2852). The choice of the anion to supported
the highly electrophilic species is critical.
We have recently embraced the extremely enduring carborane anions ([HCB11X11]-,
X = H or halogen) and that now allows us to activate even the least reactive
C-F bonds in perfluoroalkyl moieties with turnover numbers in the thousands at
room temperature. The simplest catalytic
process is the replacement of F by H (hydrodefluorination or HDF), but we are
making progress towards replacement of F by hydrocarbyl groups, as well. This work is supported by a DOE grant.
Emerging directions
O-O bond forming reactions. The chemistry of the O-O bond formation and,
in general, of the transformations of OnHm ligands in well-defined
systems is critical for the understanding and development of water oxidation
catalysts. We are interested in
discovering new O-O bond forming reactions, particularly using late transition
metals.
Catalytic utilization of O2. O2
is the most benign and abundant oxidant.
We seek new concepts in the involvement of O2 in catalytic
cycles. Our interests are in developing
well-defined transition metal systems that combine and O2-derived
ligand and an available empty coordination site for selective transformations.
Hydrogenation of biomass
molecules. Hydrogenation
of biomass is an attractive way of producing usable fuels from renewable
resources and may also be viewed as a part of the hydrogen storage
solution. Interactions of biomass
molecules (fats, sugars, peptides) with transition metal reagents remain poorly
understood at best. Biomolecules may
present special challenges, different from small organic analogs, in part
having to do with the high density of organic functionalities. We are interested in discovering new
homogeneous catalysts for hydrogenation of biomass molecules as well as in
investigating the fundamental aspects of the interaction of common biomolecules
with reactive transition metal fragments.
For a full publication list, see: http://people.brandeis.edu/~ozerov/Pages/Pubs.htm
Selected Publications
Gerber, L. C. H.; Watson, L. A.; Parkin, S.; Weng, W.; Foxman, B. M.; Ozerov, O. V. "A Bis(methylidene) Complex of Tantalum Supported by a PNP Ligand", Organometallics, 2007, 26, 4866.
Fafard, C. M.; Adhikari, D.; Foxman, B. M.; Mindiola, D. J.; Ozerov, O. V. "Addition of Ammonia, Water, and Dihydrogen Across a Single Pd-Pd Bond", J. Am. Chem. Soc. 2007, 129, 10318.
Gatard, S.; Çelenligil-Çetin, R.; Guo, C.; Foxman, B. M.; Ozerov, O. V. "Carbon-Halide Oxidative Addition and Carbon-Carbon Reductive Elimination at a (PNP)Rh Center", J. Am. Chem. Soc. 2006, 128, 2808.
Weng, W.; Guo, C.; Çelenligil-Çetin, R.; Foxman, B. M.; Ozerov, O. V. "Skeletal Change In The PNP Pincer Ligand Leads To A Highly Regioselective Alkyne Dimerization Catalyst", Chem. Commun., 2006, 197.
Fan. L.; Parkin, S.; Ozerov, O. V. "Halobenzenes and Ir(I): Kinetic C-H Oxidative Addition and Thermodynamic C-Hal Oxidative Addition", J. Am. Chem. Soc. 2005, 127, 16772-16773.
Fan, L.; Ozerov, O. V. "Efficient Nickel Catalyst For Coupling Of Acetonitrile With Aldehydes", Chem. Commun. 2005, 4450.
Scott, V. J.; Çelenligil-Çetin, R.; Ozerov, O. V. "Room-Temperature Catalytic Hydrodefluorination of Aliphatic C-F Bonds", J. Am. Chem. Soc. 2005, 127, 2852-2853. (See a highlight in the March 14th, 2005 issue of Chemical and Engineering News 2005, 83(11), 33).
Ozerov, O. V.; Guo, C.; Papkov, V. A.; Foxman, B. M. "Facile Oxidative Addition of N-C and N-H Bonds to Monovalent Rhodium and Iridium", J. Am. Chem. Soc. 2004, 126, 4792-4793.
Fan, L.; Foxman, B. M.; Ozerov, O. V. "N-H Cleavage as a Route to Palladium Complexes of a New PNP Pincer Ligand", Organometallics 2004, 23, 326-328.