- Ph.D., 1954, University of Illinois
- Guggenheim Fellow 1981-1982.
Areas of Specialization
Supramolecular Species in Bioinorganic Chemistry
Green inorganic chemistry: transition metal coordination chemistry, bioinorganic chemistry focused on dioxygen, its reduction products and their interaction with metal complexes, environmentally benign oxidation catalysis by transition metal compounds, and supramolecular chemistry, especially new materials derived from advanced molecular topologies. Professor Busch's work is concerned with the design, synthesis, characterization and exploitation of coordination compounds, ranging from transition metal derivatives to purely organic complexes. Further the design, synthesis, and potential uses of the new chemistry are all directed at positive contributions to the global environment.
(1) The dynamics of tight-binding ligands: Tight-binding ligands are produced by decreasing their flexibility or by making them more interconnected (i.e. chelate<macrocycle<macrobicycle). As the strength of ligand binding to a metal ion increases, the rate at which the ligand binds and dissociates from the metal ion decreases. Among the interesting fundamental questions are : why does the correlation exist? What mechanistic steps cause the phenomenon? And what structural changes or additions might increase the rate processes without impacting the very high binding ability? Attempts are underway to overcome the correlation of slow rates with high affinities by changing the interconnectedness during the binding and dissociation processes. This has required synthesis of a new kind of ligand--the primary example being a linear chelating molecule that can react with itself to form a macrocycle.This is shown in Figure 1.
(2) Application of tight-binding ligands to transition metal ion control of the chemistry of O2 and its reduction products HO2 and H202:
(a) What makes O2 carriers work and how can they be exploited? This group has designed and studied oxygen carriers for many years, with the goals of structural control of O2 affinity and reactivity. Questions addressed include: what determines the kinetics of O2 binding and dissociation? What are the mechanisms of autoxidation of O2 carriers and what factors make the oxygen carriers vulnerable to destruction by O2 oxidation? Current interests have moved to the oxidation reactions catalyzed by the oxygen adducts of these metal complexes, with focus on the structural factors controlling the selectivities and reactivities of these oxidation catalysts and (b), more specifically on the basic issue -- what is the range of substrate reactivities that will be vulnerable to oxidation by these families of catalysts.
(b) Catalysts designed for selective oxidations and why they work. Example: “design a transiton metal oxidation catalyst that will oxidize stains on colored cloth without damaging the dyes or pigment or the cloth itself.”
Our research has answered this challenge by applying the principles of tight-binding ligand chemistry and basic inorganic chemistry to catalysis. Catalyst requirements: efficacy--the activated catalyst must be a strong enough to oxidize stains; selectivity--not much stronger than that; robustness—catalyst must survive under reaction conditions long enough to do its work; availability—catalyst must be subject to routine synthesis at reasonable cost; sustainability—catalyst system should present no substantial hazards. While nature suggests manganese, copper, and iron as metals for such catalysts, copper is ruled for environmental reasons and manganese is chosen because of its readily accessible medium and high oxidation states and the relative ease (as compared to iron) with which its compounds can be managed in the presence of oxygen and base. The category of ligands illustrated by Figure 2 were designed for reasons of robustness and likely catalysis power. Except for those based on fluoride donors, the most powerful manganese oxidants are associated with double-donating donor atoms that can stabilize high charge by electron density donation through both the sigma and pi systems (e.g., permanganate) or involve both some of those qualities plus redox-active (sometimes called non-innocent) ligands. The bridged cyclam ligands contain only sigma donors, tertiary amines. Therefore only intermediate manganese oxidations states are to be expected. Robustness is favored by tertiary amines, but the bridged cyclam bridge also has build in constraints that were predicted to lead to exceptionally slow dissociation of the ligand, making it stable even in hot aqueous strong base solutions. These compounds are remarkably stable, dissociating at least a million times slower than most complexes of manganese(II). Also the maximum oxidation state attained is Mn(IV) and, remarkably, the activated form of this catalyst, the Mn(IV) complex has been isolated and its structure determined by X-ray crystallography and solution studies.
With the activated catalyst form available as a pure compound, it has been possible to determine the source of the high selectivity of this significant new catalyst system. Our studies have identified three mechanistic pathways by which this catalyst oxidizes substrates. (1) Hydrogen abstraction or the stoichiometric equivalent proton/electron transfer—this reaction is limited to very easily oxidized substrates having bond dissociation energies less than about 82 kcal/mole; i.e., the ligand does not rip off hydrocarbon hydrogens. (2) The dominant mechanism for the oxidation of olefins to epoxides proceeds by the unexpected highly selective Lewis acid activation of hydrogen peroxide, rather than the expected rebound mechanism. (3) A much slower radical pathway has also been detected which is probably initiated by a minor tendency of the catalyst to participate in a rebound mechanism.The predominant oxidation mechanisms (1) and (2) are highly selective and responsible for the good qualities of the catalysts systems.
(c) Partnership of Pure O2 and media based on dense phases of CO2 mixed with organic solvents for safe green oxidations of great generality.
Collaborations with Professor Bala Subramaniam of the Department of Chemical and Petroleum Engineering led to the discovery that mixed solvents composed of an organic component with more or less comparable amounts of CO2 dissolved in them are especially beneficial from the standpoints of personnel and environmental safety during selective oxidation reactions. For many decades extremely large scale oxidations (e.g., intermediates for nylon and polyesters) have been conducted in industry using oxygen in the air as the preferred oxidant, despite two major limitations, reaction rates limited by low oxygen solubility and the danger of fire and explosion. A third issue is both a cost and environmental one; one compresses 5 times as much gas as will be used into huge reactors, at substantial expense, and gets back 80% if the gas, the nitrogen, in a contaminated condition that must be cleaned up before it can be released into the environment. N2 and CO2 are both inert toward oxidations, but the phase relationships of the two gas are very different. Under chemical processing conditions, nitrogen is a permanent gas whereas CO2 is readily compressed into the liquid state in the vicinity of room temperature, with critical temperature of 31oC and 74 atmospheres. Even more useful, CO2 dissolved in many organic solvents forms mixed liquids that can be of almost any composition between the pure coponents, again under moderate temperature and pressure conditions. These mixed solvents are called CXLs, carbon dioxide expaned liquids. The huge amount of inert carbon dioxide dominates a gas phase when pure O2 is used as the oxidant in these mixed media.
In the Center for Environmentally Beneficial Catalysis, a major commitment has been made to develop the partnership of oxygen and carbon dioxide expanded liquids for both large scale and small scale oxidations. Working closely with industrial partners (companies that have joined CEBC as Industrial Members) we are evaluating the opportunities for new green chemistries that can be economically advantageous to our partners. At the same time we are exploring methodologies that are expected to replace such undesirable oxidants as chromium(VI) and manganese(IV) in stoichiometric ractions by catalytic O2 oxidations in CXLs.
Guochuan Yin Andrew M. Danby, Victor Day, Suparna Baksi Roy, John Carter, William M. Scheper, Daryle H. Busch, , “Similarities and Differences in Properties and Behavior of Two H2O2 Activated Manganese Catalysts Having Structures Differing only by Methyl and Ethyl Substituents,” Journal of Coordination Chemistry 64(1), 4-17 (2011).
Xiaobin Zuo, Fenghui Niu, Kirk Snavely, Bala Subramaniam, Daryle H. Busch, “Liquid Phase Oxidation of p-Xylene to Terephthalic Acid at Medium–High Temperatures: Multiple benefits of CO2-Expanded Liquids,” Green Chemistry, (2010), 12, 260-267.
Joseph A. Heppert, Daryle H Busch, “Celebration of inorganic lives: Interview of Daryle H. Busch by Joseph A. Heppert,” Coordination Chemistry Reviews, 254 (15-16), 1593-1606 (2010).
S. Chattopadhyay, R. A. Geiger, G. Yin, D. H. Busch, T. A. Jackson, ”Oxo- and Hydroxo-manganese(IV) Adducts: A Comparative Spectroscopic and Computational Study”, Inorg. Chem. 49, 7530 (2010).
Hyun-Jin Lee, Madhav Ghanta, Daryle H Busch, Bala Subramaniam "Toward A CO2-Free Ethylene Epoxidation Process: Homogeneous Ethylene Epoxidation In Gas-Expanded Liquids,” Chemical Engineering Science, 65, 128-134 (2010).
Daryle H. Busch and Bala Subramaniam, “Catalytic Oxidation Reactions in Carbon Dioxide—Expanded Liquids Uising the Green Oxidants Oxygen and Hydrogen Peroxide,” in Gas-Expanded Liquids and Near-Critical Media” Edited by Keith W. Hutchenson, Aaron M. Scurto, and Bala Subramaniam, ACS Symposium Series 1006, American Chemical Society, Washington, DC, Oxford 2009.
Guochuan Yin, Daryle H. Busch, “Mechanistic Details Facilitate Applications of an Exceptional Catalyst, Methyltrioxorhenium: Encouraging Results from oxygen-18 Isotopic Probes,” Catalysis Letters, (2009), 130, 52-55.
Guochuan Yin, Andrew M. Danby, David Kitko, John D. Carter, William M. Scheper, Daryle H. Busch, “Oxidative Reactivity Difference among the Metal Oxo and Metal Hydroxo Moieties: pH Dependent Hydrogen Abstraction by a Manganese(IV) Complex Having Two Hydroxide Ligands.” Journal of the American Chemical Society (2008), 130 (48), 16245-16253.
Daryle H. Busch, Guochuan Yin, and Hyun-Jin Lee, “Lewis Acid Catalyzed Epoxidation of Olefins Using Hydrogen Peroxide: Growing Prominence and Expanding Range,” pp. 119-153 in Mechanisms in Homogeneous and Heterogeneous Catalytic Epoxidation,, Ed. by S. T. Oyama, Elsevier, (2008).
Bhuma Rajagopalan, Hu Cai, Daryle H. Busch, Bala Subramaniam, “The catalytic efficacy of Co(salen)(AL) in O2 oxidation reactions in CO2-expanded solvent media: axial ligand dependence and substrate selectivity,” Catalysis Letters, (2008), 123(1-2), 46-50.
Xiaobin Zuo, Bala Subramaniam, Daryle H. Busch “Liquid phase oxidation of toluene and p-toluic acid under mild conditions: synergistic effects of cobalt, zirconium, ketones and carbon dioxide,” Ind. Engr. Chem. Res. , 47, 546-552 (2008).