HILL GROUP'S PRESENT RESEARCH
Classical and Emerging Challenges in Inorganic Chemistry
Structurally or Dynamically Complex Inorganic Molecules and Nanosystems
Specific areas of research:• Nanosize and enantiopure cluster molecules
• Realizing “impossible” inorganic structures (late-transition-metal oxo complexes and other long proposed structures)
• Multifunctional nanomaterials (sensing, catalytic arrays and microporous materials)
• Important processes: oxo transfer, O2 activation, C-H functionalization, H2O oxidation
• Functional nanosize clusters in diagnostics and therapeutics
General overview: The group designs, prepares and investigates complex large molecules, including polyoxometalates (POMs), and nanomaterials with specific structural, electronic and/or dynamic features that define them to be of potential value in catalysis, sensing, medicine and nanoscience. We also use such systems to investigate long-standing and emerging challenges in mechanisms and reactivity.
• Enantiopure nanosized cluster molecules
We are preparing robust nanosized inorganic clusters that are not only chiral but also enantiopure. Since these clusters have tunable reduction potentials, the ability to be reduced by many electrons, and other attractive features, they are of considerable interest as oxidatively stable catalysts for asymmetric oxidations, sensors and possible selective therapeutic or diagnostic agents.
The approach involves binding a chiral and enantiopure natural product, such as tartrate (as in complex 1 below) to a large inorganic cluster (polytungstates thus far) via Zr(IV) or other polyvalent large metal ion bridges. The chirality in the natural product is transferred to and amplified in the giant robust tunable (and thus potentially useful) inorganic nanomolecule. Both enantiomers, D and L, of 1 have been made optically pure starting with D- or L-tartrate.
(Slide coming soon)
• Giant (multi-nanometer) magnetic cluster molecules
We are preparing nanosized metal oxide clusters that have large numbers of delocalized electrons (up to 100 or more). At several nanometers in diameter, these structures are the largest and most complex non-polymeric inorganic molecules in the literature. Unlike the very common “nanoclusters” prepared by materials scientists and experimental physical chemists, our materials are actual molecules. This means that they are atomically pure and every atom can be located by X-ray crystallography. In addition, their molecular properties can be determined far more rigorously and precisely.
These nanosized molecules have several features of fundamental and applied interest:
(1) unusual magnetic properties, including quantum effects as seen for example in cluster 2 below, and marked proton relaxivity for magnetic resonance imaging (MRI)
(2) ion transport across inorganic membranes (this is studied using the roughly spherical and hollow nanosized clusters (e.g. 2, 3, 4 below); work in collaboration with Achim Müller in Germany)
(3) possible quantum computing applications
(4) catalytic oxidation activity
We seek to develop new levels of control over the self assembly of highly complex functional inorganic molecules. One example below (5) is the most complex inorganic molecule characterized by X-ray crystallography to date. It is a giant chiral polyanion comprised of two separate structural components, a nanoring and a nanosphere, that are linked by two K+ ions (purple). Both the nanoring and nanosphere parts of the molecule have delocalized electrons in particular positions. Such molecules maybe candidates for quantum computing studies.
• Realizing “impossible” inorganic structures (late-transition-metal oxo complexes and other long proposed structures)
The terminal metal-oxo group, (L)nM=O (L = ligand), plays a central role in oxygen chemistry. This inorganic functional group is involved in myriad oxidations, both in Nature and in industry, and is very likely also involved in O2 activation by various biological and abiological systems. Fe-oxo (ferryl) species appear to be most dominant potent oxidizing agents in biology, and Mn-oxo species are likely key intermediates in photosynthetic oxygen evolution and other systems. The metal-oxo group gets progressively less stable as one moves from left to right across the periodic table and more d electrons are present in the metal center. Thus while d0-d2 terminal metal-oxo units are very stable and ubiquitous for several elements in the earth’s crust and elsewhere, d3 and d4 metal-oxo units are quite reactive, and metal-oxo units with electron counts above d5 are very rare indeed (only 2 examples prior to our work).
The interaction air O2/air with the “work-horse” catalytic elements (Pt, Pd, Au, etc.) has global technological and economic impact (Pt and Pd supported on metal oxides are used in virtually all catalytic converters, Pt is the key element in fuel cell electrodes, supported Pd/Au nanoclusters and films are the catalysts in several air-based industrial oxidations, supported Ag is the catalyst for ethylene oxide production, etc.). However, no metal-oxo units involving these elements had been made despite 40 years of interest and effort because the high-d-electron counts rendered all such species very unstable (all d electrons above d2 are generally antibonding in a metal-oxo unit).
By using metal-oxide clusters that mimic the structural, acid-base, and other properties of the metal oxide supports used in all the above technologies, we have been able to prepare and isolate terminal metal-oxo compounds of Pt, Pd, Au, Ir (to date). These complexes have been characterized by X-ray and neutron diffraction, XAS and EXAFS, ultralow-temperature spectroscopy, vibrational spectroscopy, NMR, etc. (14 techniques in some cases thus far).
Features of a Pt-oxo unit, the first isolated late-transition metal-oxo (LTMO) compound, 6, are shown below.
In collaboration with five other groups at Argonne National Laboratory, Stanford University, U. of New Mexico, the Emerson Center, the DOE Ames Laboratory and Toho University, Japan, we are mapping out the atomic and unique electronic structures of these new compounds, elucidating their reactivities (oxo transfer and O2 activation) and assessing their utility in various applications.
• Multifunctional nanomaterials (sensing, catalytic arrays and microporous materials)
There is considerable interest in porous materials, including metal-organic frameworks, for gas storage, catalysis, sensing and possibly other areas. Despite the all the recent publications and discussion (these materials represent one of the 2 or 3 most cited areas in inorganic chemistry in the last 5 years), applications beyond gas sorption have been limited.
Materials that would trap, detect and catalytically transform target molecules (pollutants, chemical warfare agents, other toxics) would have global markets yet no metal-organic framework or other porous materials have such capabilities.
We have generated two prototype materials that trap, sense and destroy a range of molecules by air-based oxidation. The first such material is a tri-functional metal organic framework (see 7 below), and the second is a tri-functional gelating nanoarray of metal oxide clusters (POMs) (see 8 below).
• Important processes: oxo transfer, O2 activation, C-H functionalization, H2O oxidation
The development of molecules and materials that are so reactive they can catalyze organic oxidation reactions rapidly using only the ambient air (O2 at room temperature and pressure) are a focus of ongoing work. These materials require no heat, light, additives or other input to be functional. The most reactive catalysts have proprietary formulations and are in development by companies as self-decontaminating fabrics, coatings and other functional materials to improve human and other environments.
The most environmentally and economically attractive oxidant and solvent, respectively, are O2/air and H2O. We are developing systems that catalyze air-based oxidations in H2O. Dynamic self-repairing catalyst systems, such as that we reported for conversion of wood or wood pulp to paper using only air and water are of broad interest (see below).
• Functional nanosize clusters in diagnostics and therapeutics
We are developing polytungstates and other anionic inorganic clusters as therapeutic agents, detectors of in situ species of importance, and diagnostic agents (contrast agents in MRI). The size, shape and surface distribution of the negative charge can be controlled rendering appropriate polytungstates highly selective in recognizing and binding to key proteins including HIV-1 protease.
Our work is funded by the National Science Foundation, the Department of Energy, the Department of Defense, the National lnstitutes of Health and industry.