Our program is broadly
grounded in synthetic inorganic and organometallic chemistry,
with a focus on transition-metal systems containing multiply-bonded
ligands. Of particular interest to us are organoimido transition
metal complexes, of the form [LnMºN-R].
Like their isoelectronic oxo counterparts, organoimido ligands
are excellent pi-donors capable of stabilizing high oxidation state
metals in a variety of coordination environments. Unlike the situation for
oxo ligands, however, organoimido systems provide an appealing route for
the introduction of desirable physical and chemical properties
through manipulation of the imido substituent. A sampling of our
current interests in organoimido chemistry is provided below.
I. Imido derivatives
of polyoxometalate clusters.
The truly large class of anionic polyoxometalate clusters [MxOyE]n-
(where M is typically high-valent Mo, W, V, Nb or Ta, and E can be a range
of main group elements) presents an astonishing variety of unusual structural,
catalytic, magnetic and biological properties. We are studying ways to replace
the oxo ligands in several such systems with organoimido groups, in an effort
to further extend the utility of this class of nano-scale clusters. For example,
we have demonstrated that the superoctahedral "hexamolybdate" cluster
[Mo6O19]2- can be functionalized sequentially to replace from one to six
terminal oxo ligands with arylimido ligands – Figure 1 below shows
the molecular structures of the [Mo6O19]2- parent system along with its hexakis(imido)
derivative [Mo6(NAr)6O13H]-. Related highly functionalized systems are very
attractive building blocks for the rational construction of tunable, redox-active,
porous three-dimensional molecular materials. We have acquired a thorough
understanding of this family of compounds by using a combination of X-ray
crystallography, electronic and vibrational spectroscopy, cyclic voltammetry,
and multinuclear (95Mo, 17O, 14N, 1H) NMR techniques.
Structures of [Mo6O19]2-(left) and its hexakis(NAr) derivative
II. Organoimido ligands
incorporating remote functionality.
The flexibility inherent in an [N-R]2- ligand can be exploited through judicious
choice of the imido substituent R. For example, if R includes a functionality
capable of serving as a donor ligand, then specifically targeted multimetallic
systems should be easily accessible. We've demonstrated the feasibility of
this concept through the synthesis of a 4-pyridylimido complex of vanadium,
[(N3N)VºN-py], shown in Figure 2. The exposed nitrogen atom of the pyridylimido
ligand is a competent donor toward both low-valent (e.g., RhI) and high-valent
(e.g., WVI) metal centers producing unprecedented types of conjugated heterobimetallic
complexes. We also described the first example of an electroactive imido
ligand in the form of a ferrocenylimido-hexamolybdate complex, [Mo6O18(NFc)]2-,
whose structure is also shown in Fig. 2. This complex displays a prominent
Fe(II)–[Mo6] charge transfer absorption at 536 nm, suggesting the possibility
to create bi-stable materials in which magnetic and optical changes are triggered
by absorption of visible light. There are many further opportunities for
synthesizing useful and unusual molecular materials by varying the functionality
present in an organoimido ligand..
Functional imido ligands. Structures of the metalloligand [(N3N)VºNpy]
(left) and the covalent donor-acceptor ferrocenylimido system
III. Organic - Inorganic
Hybrid Materials: Imido Complexes as Polymer Pendants.
We've begun a program to prepare new types of hybrid materials in which exploitable
transition metal complexes are present within conventional organic polymers
as covalently attached backbone substituents. Our method, which is both simple
and general, consists of preparing an organoimido complex in which the imido
ligand substituent incorporates a polymerizable functionality (such as a vinyl
group), followed by co-polymerization into a particular organic matrix. An
example which provides polyoxometalate substituents within a polystyrene matrix
is illustrated in Figure 3: the styrylimido hexamolybdate (shown on the left)
undergoes free radical-induced co-polymerizations with styrene derivatives
to afford soluble hybrid materials ( a segment of one such species is shown
on the right). We have made several such polymerizable imido complexes of various
transition metals, and these systems offer many opportunities for incorporating
useful properties such as luminescence, photo- and electrochromism, and electron
and ion conduction into conventional polymeric environments.
Imido-metal complexes in hybrid polymeric materials. Structure
of the styrylimido hexamolybdate [Mo6O18(NC6H4CH=CH2)]2– (left)
and its methylstyrene copolymer (right).
IV. Homogeneous modeling
of ammoxidation catalysis.
Approximately 10 billion pounds of acrylonitrile are produced worldwide each
year in the remarkable heterogeneous allylic oxidation of propylene in the
presence of ammonia known as ammoxidation. Various molybdenum oxides catalyze
this reaction, but the precise details are not known. Our interest in this
chemistry stems from the proposed involvement of various multiply-bonded
nitrogenous ligands at the Mo surface sites, many of which were unknown in
solution chemistry. We have prepared a range of soluble complexes bearing
such catalytically relevant ligands and have studied their reaction chemistry.
We have been able to mimic certain aspects of the purported surface chemistry
and are constantly refining our models to bring them into closer congruence
with the structure and reactivity of the likely active site. Nitrogenous
derivatives of the [Mo6O19]2- cluster are a current focus of our work in
this area since the Mo coordination environment in this soluble anion provides
a striking similarity to that of the MoO3 catalyst component. A particularly
exciting recent result which suggests that we are getting very close to a
functional ammoxidation mimic is our observation of the production of benzonitrile
from the benzylimido complex [Mo6O18(NCH2Ph)]2-.
T. R.; Yap, G. P. A.; Rheingold, A. L.; Maatta, E. A. "An
Organoimido Derivative of the Hexatungstate Cluster: Preparation
and Structure of [W6O18(NAr)]2- (Ar = 2,6-(i-Pr)2C6H3)", Inorg.
Chem. 1995, 34, 9.
2. Hill, P. L.;
Yap, G. P. A.; Rheingold, A. L.; Maatta, E. A. "Organoimido
Ligands with Remote Functionality: A p-Pyridylimido Complex of
Vanadium(V) and Its Use as a Metalloligand", J. Chem. Soc.,
Chem. Commun. 1995, 737.
3. Stark, J. L.;
Rheingold, A. L.; Maatta, E. A. "Polyoxometalate Clusters
as Building Blocks: Preparation and Structure of Bis(hexamolybdate)
Complexes Covalently Bridged by Organodi-imido Ligands", J.
Chem. Soc., Chem. Commun. 1995, 1165.
4. Stark, J. L.;
Young, V. G., Jr.; Maatta, E. A. "A Functionalized Polyoxometalate
Bearing a Ferrocenylimido Ligand: Preparation and Structure of
[(FcN)Mo6O18]2-", Angew. Chem. 1995, 107, 2751; Angew. Chem.
Int. Ed. Engl. 1995, 38, 2547.
5. Strong, J. B.;
Haggerty, B. S.; Rheingold, A. L.; Maatta, E. A. "A Superoctahedral
Complex Derived From a Polyoxometalate: The Hexakis(arylimido)hexamolybdate
Anion [Mo6(NAr)6O13H]–", J. Chem. Soc., Chem. Commun.
6. Mohs, T. R.;
Plashko, B.; Du, Y.; Maatta, E. A. "Homogeneous Modeling of
Ammoxidation Chemistry: Nitrile Formation From a Soluble Analogue
of MoO3", J. Chem. Soc., Chem. Commun. 1997, 1707.
7. Wheeler, D.
E.; Wu, J.-F.; Maatta, E. A. "Organoimido and Organodi-imido
Vanadium Complexes", Polyhedron 1998, 17, 969 (honoring
Prof. Donald C. Bradley).
8. Kwen, H.; Young,
V. G., Jr.; Maatta, E. A. "A Diazoalkane Derivative of a Polyoxometalate:
Preparation and Structure of [Mo6O18(NNC(C6H4OCH3)CH3)]2–",
Angew. Chem. Int. Ed. Engl. 1999, 38, 1145.
9. Strong, J. B.;
Yap, G. P. A.; Ostrander, R.; Liable-Sands, L. M.; Rheingold, A.
L.; Thouvenot, R.; Gouzerh, P.; Maatta, E. A. "A New Class
of Functionalized Polyoxometalates: Synthetic, Structural, Spectroscopic
and Electrochemical Studies of Organoimido Derivatives of [Mo6O19]2–",
J. Am. Chem. Soc. 2000, 122, 639.
10. Moore, A. R.; Kwen, H.;
Beatty, A. M.; Maatta, E. A. "Organoimido-Polyoxometalates
as Polymer Pendants", Chem. Commun. 2000, 1793.