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Biography:
I am from Jasper Indiana and currently a sophomore at Indiana University studying for a B.S. in biochemistry. I am working in the Caulton group for the chance to develop the skills and chemical intuition I will need for any future endeavors for real life applications. It was Indiana University graduate student, Alyssa Cabelof, who inspired me to take an interest in chemical research.
Research Goals:
In collaboration with Alyssa Cabelof, our goal is to install a manganese carbonyl complex into the center of the pincer ligand, PNNH. Upon completion of this manganese complex insertion, we plan to install nitrate with the hopes of reducing to complete deoxygenation to form a nitride product.
In terms of goals for my undergraduate research, I hope to learn what sort of various techniques chemists use in their daily work. Following under Alyssa, I hope to develop a new perspective on how to look at complexes through use of NMR, IR, and EPR and other resources used outside of my classes in order to build an understanding of the materials we are creating and using in the lab.
Biography
Originally, I am from the ‘Land of Enchantment’, otherwise known as Albuquerque, New Mexico. I graduated from Butler University in Indianapolis, Indiana with my B.S. in chemistry. During my time at Butler, I was graced the opportunity to synthesize and characterize a variety of seven coordinate Group VI transition metal complexes. I performed this work under the guidance of Dr. Stacy O’Reilly, who was one of the most important influences that drove me towards inorganic chemistry during my undergraduate career. I was afforded the opportunity during a summer research project to characterize a variety of these tungsten and molybdenum transition metal complexes, which drove me towards a desire to learn more about the capabilities these transition metals contain. This passion is what drove me into Ken Caulton’s group.
Research Project
N-heterocyclic carbenes (NHC) are ligands that have been known for quite awhile to be very strong σ-donors, allowing them to act as robust ligands that form strong bonds to metal centers, with little tendency for dissociation. NHCs can carry sterically hindering substituents as well, which prevents ligand dimerization, leaving open metal coordinate sites for substrate binding. . NHCs have since been shown to have inherent properties that are analogous to ligands much like trialkylphosphines, allowing for their use in areas such as metal-based inorganic and organometallic coordination chemistry, as well as homogeneous catalysis. One such example of this catalysis is shown below, where an NHC ligand is a key ligand within the Suzuki coupling of aryl chlorides.
New variations of these ligands exploits their extreme versatility in both flexibility of the ligand, as well as their structural and electronic properties.
Figure 1. Structures of NHCs that demonstrate both flexibility and rigidity ligand can possess, along with the various structural and electronic properties ligand can possess.
I hypothesize that these ligands have the possibility of providing a metal center the stabilization necessary for undergoing a variety of chemical transformations which might not normally be accessible with a weaker σ-donating ligand. I propose that a ligand with two carbene sites and a rigid backbone would essentially force the ligand to bind to a metal in a planar fashion. This is favorable binding for certain group 10 metals, such as Ni2+, binding the metal in a pseudo square planar manner. This allows for two open binding sites on the opposite open face of the metal. Tuning of the steric and electronic properties on this bis(NHC) ligand opens the possibility of selectively tuning the reactivity of the chosen metal center towards a variety of substrates.
Figure 2. Structure utilizing the Bis(NHC) ligand for the bidentate binding properties to both group 9 and 10 metals
As shown in the above depiction, there is potential for unfavorable strain upon binding of the rigid NHC with one metal. However, it has been proven that the bidentate binding mode of the NHC ligand will be comparable to that of a 1,10 phenanthroline ligand, with respect to characteristics such as bite angle and bond distance.
I also hypothesize that if it is possible to bind one NHC ligand to one metal, then it is theoretically plausible to have the NHC bind to two metals, leaving open binding sites on two metals instead of one. This would be ideal since two metals could provide twice the activation power to a bound substrate, allowing for accessibility to previously unattainable intermediates due to labile reactivity. Shown below is one proposed situation for this dinuclear metal-metal system, in which the metal complex is bound to a nitrate substrate, allowing for open reactivity on the nitrate via the metal center.
Biography
I was born in My Tho, Vietnam, and lived there for 11 years before my family and I immigrated to the United States in 2008. I earned my B.S in Chemistry from the Texas A&M University in 2019, where I participated in undergraduate research in the group of Dr. Donald Darensbourg. When I’m not doing chemistry, I enjoy drawing, reading, and spending time with my friends. I chose Indiana University, beside its great graduate program and the friendly senior graduate students, because I want to push the edge of my comfort zone being far away from home. The areas of chemistry that interest me are organic and inorganic synthesis.
Research Project
My research involves the reactivity of high oxidation state iron complexes. Specifically, the reduction of iron nitrides using a boron transfer reagent, called bis(pinacolato)diboron (B2pin2). B2pin2 can be modified to include a “shuttle”, such as pyrazine, to facilitate the transfer of boron onto the terminal nitrogen of metal nitrides. I aim to understand the mechanism of this boryl transfer and identify new reaction pathways for metal nitride complexes which functionalize the nitrogen in these useful catalytic intermediates.
Biography
I am a visiting scholar from Germany, staying here in Bloomington from July till end of October. I was born in Frankfurt (Germany) and studied chemistry at the Georg-August-University in Göttingen. I obtained my M.Sc. in the group of Prof. Sven Schneider, working on the reactivity of Co-PNP-pincer-complexes. I subsequently started to pursue my PhD as a graduate student in the Schneider group.
My graduate research focuses on N2-splitting and functionalization utilizing functional pincer ligands. In this project I want to transform N2, which is a very unreactive molecule, into other N-containing molecules like ammonia or other organic molecules.
Research Projects
Nitrogen is beside carbon, oxygen and hydrogen one of the main elements found in organic molecules. Even though nitrogen makes with 80% the major part of our atmosphere most of these organic molecules are not made directly from N2 but from its reduced form, ammonia, which is also essential for agricultural industry since most fertilizers contain at least one nitrogen atom. On industrial scale this transformation of N2 into NH3 is done via the so-called Haber-Bosch-process in gigantic scales with over 100 million tons per year, which also shows its high importance.
Since the reaction conditions of this process are very harsh (150-350 bar at 400 – 500 °C) there are enormous efforts in designing biomimetic catalysts for this reaction. Many recent studies of such homogeneous systems have shown that metal-nitrides seem to be a key intermediate in the catalytic cycle. Since the mechanism of N2-splitting is nearly completely unknown, there’s a high interest in synthesizing complexes which form bonds with N2 and then look on the conditions that are needed to initiate N2-splitting. In this way the mechanism of N2-splitting as well as the requirements regarding the supporting ligand can be investigated, which helps designing more effective catalysts for the transformation of N2 into NH3.
Within a close collaboration the Schneider (U Göttingen, Germany) and Caulton groups study N2-splitting mediated by dinuclear-PNP-complexes, as recently reported for [{(PNP)ClMo}2{µ-N2}] (1)[1]. Splitting of the N2-bond in such complexes leads to the formation of metal-nitrides, which can be either used as a catalyst for ammonia formation or to form new C-N-bonds. In an ideal case such C-N-coupling reactions would open up the possibility to generate N-containing organic molecules directly from N2 without the use of ammonia, which would make their synthesis much more efficient and economic.
[1] G. Silantyev, M. Förster, B. Schluschaß, J. Abbenseth, C. Würtele, C. Volkmann, M.C. Holthausen, S. Schneider Angew. Chem. Int. Ed. 2017, 56, 5872-5876.
Biography
I was born and raised in Cincinnati, Ohio. I received my B.S. in chemistry from the University of Cincinnati in 2017, where I worked in the lab of Dr. Bill Connick. When I’m not in the lab, I like to hike with my dog, run, go to the ballet, read, and even snorkel if I can. I chose Indiana University because of the welcoming graduate students in the department and my strong interest in several different advisors’ research, as well as the beauty of the campus and hiking trails around town. I’m interested in organic and inorganic synthesis for the catalysis of small molecule conversions.
Research Projects
Nitrates and nitrites exist as a waste product in water that result mainly from fertilizer runoff and septic tank leakage. If not removed from runoff, nitrates cause excess algae growth in rivers and oceans, draining oxygen from the water and killing fish and other vital organisms. Eutrophication is estimated to cause $2.2 billion of damage each year.1
Existing nitrate treatment options such as ion exchange, reverse osmosis, and electrodialysis only include nitrate removal, which generates a harmful waste brine.2 Chemical nitrate reduction does not produce this hazardous waste stream and produces value-added products.
In my research, I am trying to develop a chromium-based electrocatalyst that can reduce nitrate to downstream reduction products in aqueous environments. Chromium is a relatively cheap, earth-abundant metal, which makes it attractive for this purpose. It has several oxidation states, which gives it the needed electron transfer chemistry. Cyclam is a tetradentate ligand which may provide structure for the chromium complex’s interaction with nitrate.3
I have shown that cis– and trans-[Cr(cyclam)Cl2]Cl are able to selectively (and quantitatively, in the case of the trans isomer) reduce nitrate to nitrite in aqueous conditions at a mercury pool electrode.
The isomers exhibit different electrochemical behavior at a mercury pool electrode, while sharing similar behavior at a glassy carbon electrode. The trans complex is reduced at a less negative potential due to a suspected adsorptive interaction between this isomer and the mercury surface, leading to a less negative onset potential for nitrate anion reduction than with the cis complex. The mechanism of this interaction is being explored experimentally and computationally, which should provide more insight.
- M. F. Chislock, E. Doster, R. A. Zitomer and A. Wilson, E., Nature Education, 2013, 4.
- V. B. Jensen, J. L. Darby, C. Seidel and C. Gorman, Drinking Water Treatment for Nitrate, University of California, Davis, 2012.
- N. A. Kane-Maguire, K. C. Wallace and D. B. Miller, Inorganic Chemistry, 1985, 24, 597-605.
Biography
I grew up in Seoul, Korea and moved to Richmond, Virginia when I was fifteen. I graduated with a BS in chemistry from Randolph Macon College. While at RMC, I was given opportunities to have three years of undergraduate research experience in Serge Schreiner’s organometallics research group including two summers of research. My undergraduate research focused on small molecules activation by the PNP pincer ligated transition metal complexes (groups 9 and 10 metals!) and CO2 reduction with the activated complexes. Outside of chemistry, I love doing music: listening, playing (I’ve played violin and piano since I was six). Other than that I like taking a nap and just slacking around a lot.
Research Projects
In my C500 project, reactivity of two low valent chromium (Cr(II) and Cr(III)) compounds stabilized by bis(pyrazolyl)pyridine (BPZP) towards nitrate activation will be explored with the goal of contributing to design of metal complexes that are capable of proton coupled electron transfer (PCET) to accomplish reductive deoxygenation of the oxoanions such as NO2– and NO3–.
NO3– + 2H+ + 2e– → NO2– + H2O
NO2– + 2H+ + 2e– → NO– + H2O
The redox active ligand, BPZP, used in this study is with R groups being proton (H2L) which may play a key role for the nitrate reduction. For instance, protons are bonded to N of the pyridine ring outwardly, thus, once NO3– is coordinated to the metal center, the prolific proton can possibly interact with the closest O atom of the nitrate by hydrogen bonding followed by N-O bond cleavage (*).
Prior to the activation of NO3–, as the previous studies in the group showed that the metal-ligand chlorides precursors have low solubility, I am currently working towards chloride substitution using TMS-triflate to improve the solubility.
Once compound 2 is prepared and fully characterized, nitrate coordination and reduction will be thoroughly studied. An analogous study with Cr(II) will be carried out.
Biography
I am from Michigan and I graduated from Grand Valley State University with a B.S. in chemistry. As an undergraduate at GVSU, I worked for Professor Richard Lord, doing computational chemistry, primarily analyzing the electronic structure of early transition metal complexes. I spent a summer in a synthetic lab at Wayne State University, working under Professor Stanislav Groysman designing and synthesizing low-coordinate transition metal alkoxide complexes for the activation of small molecules. That summer research experience was the driving force in my decision to continue synthesis in graduate school. This brought me to Indiana University to work in Ken Caulton’s group to continue exploring catalytic ways to activate small molecules and recycle them into more useful starting materials.
Research Projects
My research focuses on the design and synthesis of transition metal compounds that can bind and aide in subsequent nitrate reduction. Nitrates are harmful environmental pollutants that contain nitrogen in its highest oxidation state—N+5. I have successfully bound nitrate to both cobalt and nickel supported by a novel pincer ligand, PNNH, which is comprised of a central pyridine donor flanked by a phosphine and pyrazolyl arm. In general, our group has employed an unusual reductant, coined the “Mashima reagent” to reduce these high-oxidation state nitrogen oxyanions. My work with (PNN)Ni(NO3) and the Mashima reagent thus far has demonstrated complete deoxygenation of the nitrate ligand to give a transient nitride, which is then intramolecularly reduced by the phosphine arm in the pincer. The low coordination number of the resulting nickel leads to aggregation, which is depicted below. This demonstrates total reduction of the nitrogen from +5 to -3 in the product cluster.
My work now focuses on catalytic nitrate reduction, which will begin with a “protected” phosphine pincer, illustrated below. Once the vulnerability of the pincer is turned off, we can imagine that following complete deoxygenation, reaction of the transient nitride with an additional equivalent of the Mashima reagent will reductively silylate the high-energy intermediate, giving rise to the seemingly conventional product (PNNH)Ni(N(SiMe3)2. This proposed catalytic cycle is depicted below.
Biography
I was raised at the base of the Smoky Mountains in Sevierville, Tennessee, before attending Belmont University in Nashville, Tennessee to earn a B.S. in chemistry. While at Belmont, I had the opportunity to synthesize and characterize Schiff-base nickel(II) complexes under the mentorship of Dr. Justin Stace. I truly knew that graduate school was the place for me after an REU at the University of Cincinnati, where I worked in Dr. William Connick’s lab. When I’m not in the lab, I enjoy spending time hiking, running, playing piano, and watching sports. Indiana University offers a welcoming environment, excellent facilities, and a beautiful location to call home. I am motivated by a desire to engage in meaningful research at the forefront of the field, and the Caulton group pushes the boundaries of inorganic research involving CO2, NO3-1, and NO2-1 reduction – important research that will help me achieve my goal of making a difference through chemistry.
Research Projects
A pre-reduced pincer ligand, H4btzp, can be easily synthesized and has the ability to store 4 protons and 4 electrons, as presented in the half reaction in Scheme 1.
Scheme 1. Half reaction for the oxidation of H4btzp to btzp
We envision that H4btzp is an attractive ligand for nitrogen oxyanion reduction as it has the necessary electrons to reduce high oxidation state nitrogen oxyanions, such as nitrate and nitrite, while also having protons to sequester the O2- released from N-O bond cleavage (Scheme 2).
Scheme 2. Half reaction for the two electron reduction of nitrate to nitrite
With redox chemistry intended to happen at the ligand, we chose the redox-inert zinc as our starting metal. H4btzpZnCl2 can be synthesized from H4btzp and ZnCl2, and salt metathesis with two equivalents of AgNO3 yields H4btzpZn(NO3)2. When two equivalents of AgNO2 are used instead, there is complete oxidation of H4btzp to btzp, with two equivalents of H2O formed, as monitored by 1H NMR. The oxygens in H2O come from nitrite, indicating that reduction has occurred. We can confirm the presence of NO gas from this reaction through the use of an NO scavenger, paramagnetic Cr(N(SiMe3)2)3, which readily reacts with NO gas to give diamagnetic Cr(NO)(N(SiMe3)2)3. Therefore, H4btzp when bound to zinc selectively reduces nitrite by one electron to yield NO gas, and full redox balance is shown in Scheme 3.
Scheme 3. Redox balance for the reaction of H4btzpZnCl2 with two equivalents of AgNO2
In an effort to capture reduced nitrogen containing species, we moved from zinc to iron. H4btzpFeCl2(MeCN) can be synthesized from FeCl2 and H4btzp in CH3CN. When H4btzpFeCl2(MeCN) is reacted with two equivalents of AgNO3, salt metathesis proceeds as expected to give H4btzpFe(NO3)2. However, when H4btzpFeCl2(MeCN) is reacted with two equivalents of NaNO2, there is complete oxidation of the pincer ligand and formation of H2O observed by 1H NMR. The IR spectrum of the reaction mixture indicates the formation of a di-nitrosyl iron complex (DNIC), currently under study. Because our nitrite delivery source in this case cannot act as an oxidant, the formation of this DNIC indicates that all four protons and four electrons stored in H4btzp are delivered to substrate.
Biography
I am originally from the great state of Maine, born and raised in the small city of Waterville. I graduated from Williams College, seated in the beautiful Berkshires of western Massachusetts, receiving a Bachelor of Arts with Honors in Chemistry in 2015. During my time at Williams, I did undergraduate research under the mentorship of Chris Goh working towards the development of aluminum-based ring-opening polymerization catalysts for the synthesis of polylactide, a biodegradable polymer derived from a renewable source. After taking a gap year post-graduation, I came to Indiana University with a passion for catalysis and aspirations to contribute towards a greener future. Naturally, this lead me to the Caulton group. Outside of chemistry, my interests include spending time in the great outdoors, live music, and sports (particularly football and basketball).
Research Projects
The reduction and recycling of environmentally-detrimental small molecule pollutants is a critical, yet unresolved, challenge faced by the global community. Processes to convert these pollutants into value-added products typically require the input of electrons and this has generated wide interest in transition metal catalysts containing redox-active ligands able to store and transport electrons.
Quinones and tetrazines are well-known for their redox capabilities and represent an unutilized class of ligands applicable to these types of molecular transformations. Our work seeks to link the fundamentally interconnected fields of heterogeneous and homogeneous catalysis through the rational design of ligands that promote self-assembly with atomic metals to form one and two-dimensional on-surface coordination polymers that may store and transport electrons to targeted substrates.
Our current work primarily focused on tetra-aza-anthraquinone ligands which have to potential to be reduced, by deposited elemental metal, by up to four electrons. Deposition to a surface followed by addition of metal results in highly disordered chains which we hypothesize is the result of numerous coordination geometries being allowed by the ligands four equivalent binding sites. This has lead us to more carefully design ligands that have greater control of coordination environment and directionality.
My primary role as the molecular side of our collaboration is to design and synthesize new ligands to be used on the surface (see below). Additionally, I synthesize small-molecule analogues, e.g. for Ti, below, of our surface species so that we can better understand the redox and spin localization properties of our systems using traditional molecular characterization techniques.
Biography
Brian Jeffrey Cook was born in Trenton, New Jersey in 1989 and grew up in the neighboring suburb of Lawrenceville, NJ. An interest in chemistry grew out of proficiency in high school and led to Brian double majoring in Biochemistry and Chemistry/ACS Certification track at Seton Hall University (South Orange, NJ campus). While at Seton Hall, Brian received numerous commendations, including the Undergraduate Organic Chemistry Award from the American Chemical Society Divisions of Polymer Chemistry and Polymer and Materials Science, and the New Jersey Institute of Chemists award. At Seton Hall, Brian was involved in numerous research projects, for example, in the Murphy group, the synthesis of new ruthenium-ruthenium and ruthenium-rhenium tetrametallic dendrimers with previously under-utilized bridging ligands for investigating metal-metal communication.
Research Projects
Redox active (“non-innocent”) ligands are becoming increasingly important partners to transition metal centers in assisting with delivering multiple electrons to substrates. One particular instance in which these ligands may provide such assistance would be the idea of “electron storage”, in that the ligand could hold some of the required electrons needed for the reaction, lessening the burden on the metal. If an iron(II) center reacting with an oxidizing substrate only has to reach iron(III) and not iron(IV) or iron(V) this would obviously allow the reaction to be much more energetically favorable, and thus faster. One such redox active ligand has been identified by the Flood group here at Indiana University called 2,6-(bis)-tetrazinylpyridine (btzp). The tetrazine moieties offer unique optical and electronic properties, including an unusually low pi* orbital, giving rise to the overall redox non-innocence of btzp.
While proven to be redox active alone in solution and complexed to a metal in the form of Fe(btzp)2,I am currently working on developing, characterizing, and eventually applying 1:1 metal complexes with btzp in the form of M(btzp). M(btzp) is much more desirable because multiple empty metal orbitals are available for substrate binding, and we would predict high reactivity for these complexes. Currently, the metals under my investigation are Fe, Ru, and Zn. Fe and Zn are attractive choices because they allow exploration of the purely redox active character of the ligand with a redox inert metal (Zn) and the abundance in the earth’s crust of Fe makes it a hot target for catalyst synthesizers. One scheme for developing these complexes is shown below (scheme 1 where M = Zn, Ru). Ru is attractive because of its relative ease of handling and characterization (almost exclusively low-spin and diamagnetic), whilst being an electronic analog to Fe. Besides complete characterization, my plan for these complexes is the reduction of CO2and N2as we continue to discover more and more about the potential and hidden power stored in redox active ligands.
To-date I have synthesized and characterized (btzp)RuCl2(CO), incorporating the CO ligand as a reporter, which shows that btzp is the most electron withdrawing pincer ligand yet discovered, and thus with the highest CO stretching frequency observed.
In addition, I have developed the 3d metal chemistry of the 2,6 bis-pyrazolyl pyridine chemistry of Fe and Co. This ligand is highly proton-responsive and redox active, leading to rich possibilities in small molecule activation. I have established the general tendency of the deprotonated version of these pyrazolates to aggregate, including any available oxo ligands. I have also shown that hydride reagents convert the cobalt complex to an aggregated species of Co(I), in green, to give an unprecedented oxo (in red) complex of this four coordinate
monovalent cobalt. I have a low-valent but highly fragile reduced cobalt species which I am currently characterizing as a possible reagent to reduce N2. I have also shown that LCo(PEt3)2 reacts rapidly with N2O to oxidize all phosphine to liberate OPEt3, and L2Co2(m-OPEt3), the latter containing an unprecedented bridging phosphine oxide. I already have obtained LFe(DMAP)3, where DMAP is para-dimethylamino pyridine, and this reacts rapidly with CO2 to give a carbonate product.
