Archives

Graduate Student

Alice Hui

Biography

I am originally from Concord, North Carolina, and received my bachelor of science degree in chemistry from the University of North Carolina at Chapel Hill in 2009. As an undergrad, I worked for Dr. Royce Murray, synthesizing various ligands and metal complexes in preparation for electrochemical studies.

Research Projects

The ultimate goal of my project is to install a new type of non-innocent ligand on unsaturated metals thus creating a new type of unsaturated metal complexes to explore reactivity with small substrates like N2, CO, CO2,and H2.

1,2,4,5-s-tetrazines have been demonstrated as redox active ligands that can store up to 2 electrons. A new type of redox active ligand is the bis(tetrazinyl)pyridine ligand (bTzP) which incorporatestwo tetrazines.

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Preliminary results of reaction of bTzP and Cu powder have shown the oxidation of Cu(0) powder to Cu(II) via Electron Paramagnetic Resonance Spectroscopy (EPR) indicating that the bTzP ligand can indeed oxidize metallic copper. The question that still remains is whether there are two ligands per Cu or only one?

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Currently, the redox properties of this pincer ligand are being explored with Ni metal. Group 10 metals are known for their abilities to activate substrates in catalytic reactions. A motivation to study nickel catalysis chemistry is simply that nickel, in comparison to other metals in the same group (Ni, Pd, Pt), is cheaper and more abundant.

A synthetic route was explored via NiCl2·THF1.5and bTzP. The product crystallized in dichloromethane by slow diffusion of pentane to yield bright orange crystals with an empirical formula of Ni2(bTzP)2Cl4, which exists as a centrosymmetric chloride-bridged dimer as seen below.

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Ni(bTzP)Cl2(MeCN), (shown below) can be synthesized by refluxing NiCl2·THF1.5and bTzP ligand in MeCN. The product was crystallized by slow evaporation in MeCN to yield bright orange crystals.

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These two Ni species can then act as a platform from which we will make, by reaction with reducing agents, unsaturated Ni complexes with electrons stored in the bTzP ligand.

Graduate Student

René Buell

Biography

I am originally from Kansas City, MO, and I received my B.S. in chemistry at Truman State University in Kirksville, MO in 2009. As an undergrad I worked under Dr. James M. McCormick on oxidation of Ru and Cr complexes using ozone. I also worked with Dr. H. David Wohlers on the development of a tactile periodic table for use as an educational tool for the visually impaired.

Research Projects

A new class of potentially redox non-innocent ligands includes molecules containing both pyridyl and pyrrolide moieties. One such ligand is 2-pyridylpyrrolide.

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Transition metal complexes containing these ligands have the potential to store electrons and then transfer them to substrates.

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Addition of reducing equivalents to divalent iron, cobalt, nickel, and copper bis-pyridylpyrrolide complexes results metal-based reduction followed by ligand rearrangement!

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Cyclic voltammetry shows two oxidative waves for NiL2.

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However, upon addition of chemical oxidizing equivalents to either CuL2or NiL2complexes a ligand-based oxidation occurs, followed by further reactivity of the presumed radical species to form free protonated ligand and a secondary organic product. I am currently working on modification of the ligand to inhibit degradation of the radical cationic species.

Installation of only one ligand onto the metal center will result in a more unsaturated (reactive) species, so work is currently underway to synthesize three or four coordinate divalent metal complexes containing only one pyridylpyrrolide equivalent. For example:

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In addition, a ligand containing one pyridyl and two pyrrolide moieties has been developed by Dr. Nobuyuki Komine (a). I am working to install this dianionic ligand onto divalent late transition metals to form neutral complexes whose redox properties will be investigated.

An analogous ligand containing one pyrrolide and two pyridyl moieties has been previously prepared, though application of redox non-innocent properties has yet to be investigated (b).

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I am working to compare the redox properties of these two ligands, which can then be applied for use in chemical transformations.

Lab Member

Kuntal Pal

Biography

Kuntal Pal was born in Howrah, India in 1978. He got his early chemistry education of Bachelor and Master’s Degrees from the University of Calcutta. Soon after, he joined as a NET-CSIR junior research fellow at IIT Kanpur, India to pursue a Ph.D. under the supervision of Professor Sabyasachi Sarkar in the field of bioinorganic chemistry. He was inspired by the biomimicing model studies of a Mo-containing enzyme and worked on the project of synthesis and reactivity studies of model complexes in relevance to the native enzymes of sulfite oxidase and nitrate reductase. Kuntal synthesized several metal complexes of dithiolene ligands which are structurally very similar to the active site of enzyme. He learned x-ray crystallography to analyze the structure of the synthesized complex. For functionality, his synthesized model complexes were subjected to reactivity studies with native substrate followed by comparisons of their kinetics with that of the native enzyme. Kuntal performed DFT computational work on the synthesized metal complexes to understand the chemistry involved and the mechanistic aspects of the enzymatic reaction. He was awarded the Ph.D. degree in May, 2008.At the end of Kuntal’s dissertation work, he was very much interested in the activation of small molecules which are toxic in nature (carbon monoxide, nitric oxide) using model complexes associated with metal-organic framework. To enhance his research skill at the international level, he moved to Japan as a Japan Society for the Promotion of Science (JSPS) post-doctoral fellow in Osaka University under the guidance of Professor Kazushi Mashima. Kuntal developed a series of bimetallic Mo/Cu containing tri and tetranuclear clusters systems as precursor catalysts for the activation and removal of toxicity of carbon monoxide.After completing the JSPS program, Kuntal joined on a joint collaboration project under Professor Caulton and Professor Baik in the Department of Chemistry at Indiana University. Kuntal’s current research is focused on electronic structure calculations and the finding of true mechanistic pathway on newly developed variety of catalytic reactions here at Indiana. Currently, he is working on the mechanistic studies of the transition metal (Cu(I) & Ag(I)) catalyzed C-H bond activation of alkanes on a pyridylpyrrolide-based ligand platform. Together with another Caulton group postdoc, Atanu Das, he is working on redox-active ligands having impact on interaction of metal centers with O2.

Research Projects

For this project, Kuntal is doing DFT calculations to understand the mechanism of a catalytic alkane C-H bond functionalization.  Initial experimental observations here at Indiana by Jaime Flores found the species [LAg] as an active catalyst, from a trimeric precursor.  [LAg] can interact with HC(CO2Et)(N2) to form two possible intermediates; N2 bound diazoalkane and N2-free carbene; both are thermodynamically equally accessible.  Kuntal is now on the way to finding out how the C-H bond of an alkane can be cleaved.  Does this reaction proceed via an even-electron or a radical mechanism?

DFT Studies on the Mechanistic Aspects of Alkane C-H Activation by Coinage metal-carbene complexes (M = Cu, Ag)

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Graduate Student

Keith F. Searles

Biography

Keith Searles was born and raised in Richmond, VA. In the city if Richmond he attended Atlee High School for three years and graduated from Hermitage High School in the spring of 2005. Keith made the decision to remain close to his hometown by attending Randolph-Macon College, located approximately 15 miles north of Richmond, VA. At Randolph-Macon College he obtained a B.S. in chemistry while researching the theory of “ion exclusion” in thin ice films under the guidance of Dr. Rebecca Michelsen. After graduation from Randolph-Macon College in the spring of 2009, Keith became an employee of the college while working as a research assistant for Dr. Serge Schreiner. Under the advisement of Dr. Schreiner, Keith’s research focused on the synthesis of rhodium and iridium complexes, stabilized by bis(dicyclohexylphosphino) methane, for use as potential catalysts. In the fall of 2010, Keith began pursuing a doctorate in chemistry at Indiana University. He continues his research focus in organometallic chemistry under the advisement of Professor Daniel J. Mindiola and Professor Kenneth G. Caulton.

Research Projects

The Caulton group is currently interested in the design and application of redox noninnocent ligands. Not only are we interested in the synthesis and full characterization of these metal complexes, but we also want to investigate the application of these complexes in catalysis. We envision using a single ligand, which we can electronically tune to favor the reduction or oxidation of various substrates.

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The ligand class we are employing is pyridylpyrrolide, which has a high degree of electronic tunability. The synthesis of the ligand allows for the substitution of the 3,5-positions on the pyrrolide with either electron withdrawing trifluoromethyl or electron donating tert-butyl substituents, which have a direct effect on the redox behavior of the ligand itself. The electron donating tert-butyl substituents favor the oxidation of the pyrrolide π-system, while the low-lying π* of the pyridyl in combination with elctron withdrawing trifluoromethyl groups favor reduction. This allows for tunable redox behavior in pyridylpyrrolide complexes, where the redox processes do not have to be 100% metal centric. Thus, pyridylpyrrolide can serve as either an electron reservoir or electron hole to aid the metal center.

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I am currently investigating the redox properties and reactivity of Co(L)2complexes. Having synthesized two varieties, one with tert-butyl groups and the other with trifluoromethyl groups, I am able to directly compare the reactivity and redox properties of the two complexes, which possess drastically different electronic properties.

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R = –tBu

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R = -CF3

After fully characterizing these complexes, my future work will focus on fine-tuning the electronic properties of these complexes and using redox chemistry to induce further reactivity in catalysis.

Graduate Student

Atanu Kumar Das

Biography

Atanu Kumar Das was born in 1982 in West Bengal, India. He completed his Bachelor of Science (BSc) major in chemistry and minor in physics and mathematics, at Ramakrishna Mission Residential College, Narendrapur affiliated under Calcutta University in 2003. In 2005, he received his Master of Science (MSc) in chemistry (specialization in Inorganic chemistry) working on “Synthesis of 21-thiacalix(4)phyrin with two meso sp3 carbons” from Indian Institute of Technology, Bombay, India. He completed his Ph.D. in 2009 under the supervision of Professor Wolfgang Kaim, Institute for Inorganic Chemistry, University of Stuttgart, Germany. His doctoral thesis focused on “Structural, electron transfer and spectroscopic studies of transition metal complexes with redox-active ligands”. Atanu spent one year (2009–2010) as a postdoctoral fellow with Assistant Professor Hong Soon Hyeok at Nanyang Technological University, Singapore working on “Gold-abnormal carbene catalyzed organic transformation”. Atanu joined Professor Kenneth G. Caulton’s group in July 2010 pursuing his second postdoctoral research.

Research Projects

Redox-active Ligand Mediated Bond Making/Breaking Transformations

Metal complexes with redox-active ligands have gained much attention not only for their unique and interesting electron transfer behavior but also the prospective role in either stoichiometric or catalytic molecular transformation. Redox-active ligands have more energetically accessible levels that allow redox reactions to change their charge state. Pyridylpyrrolide ligands (scheme 1) have seen considerable recent research from the synthetic perspective, they have not been considered as redox active. We are exploring the 2-pyridylpyrrolide, (Ln)–class of ligands as possible redox active and to expand the redox activity of their metal complexes beyond simple-metal-centered redox chemistry.

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Reactivities of [(L0)Ir(Cp*)](BArF4)

A. Activation of oxygen: Electron Buffering from Ligand

The coordination and activation of dioxygen by transition metal complexes continues to be the focus of considerable interest in attempts to develop new, atom economical oxidation catalysts. However activation by a d6metal [Ir(III) or Ru(II)] complex isn’t well explored so far. A redox non-innocent ligand 2.2’-pyridylpyrrolide that has the potential to store the redox equivalent, both in oxidative and reductive direction, was installed to synthesize a mononuclear [(L0)IrIII(Cp*)Cl] (L0= 2.2’-pyridylpyrrolide) complex to test the reactivity with O2. I find the 18e–complex [(L0)IrIII(Cp*)Cl] is inert to O2, indicating outer sphere electron transfer isn’t viable and O2isn’t strong enough to replace the chloride. However, the unsaturated 16e–species [(L0)IrIII(Cp*)]+obtained from reaction of [(L0)IrIII(Cp*)Cl] with NaBArF4, reacts with O2in the time of mixing (Scheme 2) to form a [(L0)IrIII(Cp*)(O2)]+adduct. The remarkable fact is that the process is irreversible and redox change for the adduct formation occurs at the pyridylpyrrolide. To the best of our knowledge, this is the first example where d6Ir(III)-complex acts as a reducing agent to activate O2. DFT calculations (fig. 1) were conducted by Dr. Kuntal Pal to rationalize the electronic structure of [(L0)IrIII(Cp*)(O2)]+. Different trapping experiments help me to understand the answer to the question, “How many oxygen atom(s) can effectively be transferred to oxidize the substrate?”

Schematic representation of O2activation by pyridylpyrrolide-Ir(III) and oxidation of substrate

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B. Heterolytic Cleavage of H2

I am interested in extending the redox participation of the pyridylpyrrolide ligand to other redox reagents, a significant one being H2. Questions of interest here are whether H2is a strong enough oxidant to oxidize iridium(III), and what the product of this reaction will be 2H+or dihydride, coordinated dihydrogen, or heterolytic splitting of H2to H–and H+. I find that reaction of dihydrogen (1 atm) with Cp*L0Ir+in CH2Cl2is complete in time of mixing at RT, with rapid color change from green to orange. The iridium containing product is exclusively (Cp*Ir)2(m-H)3+, identified by spectroscopic comparison to an authentic sample. Also produced stoichiometrically (one mole per one Ir) is HL0, the neutral ligand where an H2-derived proton is on the pyrrole nitrogen. Finally, one proton is produced for every two-reagent complexes (eq. 1). This reaction is thus iridium-promotedheterolyticsplitting of three moles of H2. We are using DFT calculation to understand the reaction mechanism.

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C. Electrocatalytic CO2Reduction

CO2is the planet’s most important source of carbon and one of the most important atmospheric gases contributing to the greenhouse effect. These are the reasons that its electrochemical reduction into fuels attracts sustained attention. Being motivated by our newly discovered“redox-storage”property of pyridylpyrrolide ligand which serve as“electron-buffer”in small molecule activation e.g., hydrogen oxidation, oxygen activation, we are interested in exploring further the reductive reactivity of M(pyridylpyrrolide)2type of complex.

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Fe(L2)2(THF) (fig. 2) was synthesized and employed to reduce CO2electrochemically. I have established that Fe(L2)2shows catalytic current flow at -2.42 V (vs. Ag/AgCl) (fig 3) when CV is

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run under 1 atm CO2. Analysis of the colorless precipitate formed concurrently in the electrochemical cell indicates the co-product to be [N(n-bu)4]CO3, hence the product of capture of O2-by CO2in eq 2; [N(n-bu)4]+is the cation in my supporting electrolyte. My current and future plans are to search the liquid phase for other CO2reduction products, particularly more reduced (e.g., alkoxides) and C2species.

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Lab Member

Dr. Skye Fortier

Biography

Skye Fortier is a native of the Southwest, born and raised in the far West Texas town of El Paso, which is directly located on the US/Mexico border. As an undergraduate, Skye attended the University of Texas at El Paso (UTEP) where he received a MARC (Minority Access to Research Careers) scholarship. As a MARC scholar, Skye worked under the direction of Prof. Keith Pannell investigating the photochemically induced formation of carbon-silicon bonds utilizing ‘Fp’ precursors (Fp = CpFe(CO)2). After graduating in 2005, Skye worked for one year as a high school science teacher at Irvin High School in El Paso teaching freshmen and junior classes.In 2006, Skye entered the graduate program at the University of California, Santa Barbara (UCSB). At UCSB, Skye worked in Prof. Trevor Hayton’s laboratory investigating the organometallic chemistry of uranium. In particular, his studies in the Hayton laboratory focused on the synthesis of high-valent homoleptic molecules and uranium complexes featuring metal-ligand multiple bonds. Graduating from UCSB with his Ph.D. in Fall 2011, Skye traded in the West Coast for the Midwest in order to work as a postdoctoral researcher under the joint supervision of Profs. Mindiola and Caulton. At IU, he is investigating the synthesis, reactivity, and redox chemistry of metal complexes supported by non-innocent, redox-active frameworks. Skye is a recipient of an NSF American Competitiveness in Chemistry Postdoctoral Fellowship.

Research Projects

My research is broadly targeted towards investigating the synthesis, reactivity, and redox chemistry of low-valent, first-row transition metal complexes supported by non-innocent, redox-active frameworks. The ultimate goal of this work is to develop electron-rich complexes, incorporating inexpensive earth abundant metals, for use in catalysis and group transfer chemistry.

Currently, my project is centered on examining the use of a brand new redox-active ligand platform, namely the indigo derivedN,N’-diaryldiimines (best known as ‘Nindigo’) recently developed by the Hicks’ Group at the University of Victoria. Nindigo is novel in the sense that it is binucleating, consisting of two nacnac-type fragments. Moreover, Nindigo is a modular system; the sterics of the imine substituents can be tuned depending on the aniline used during the condensation reaction with indigo. Finally, Nindigo is highly conjugated which is clearly evinced by itsintensedeep purple color – staining all that it touches!

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It has been established that free Nindigo can readily undergo a two-electron oxidation process to generate neutral, stable dehydroNindigo species. In principle, this means that Nindigo possesses the capacity to act as an electronic reservoir capable of releasing electrons. When complexed to a metal, it is anticipated that a synergetic metal-ligand effect will occur affording multi-electron reactivity that extends beyond the capabilities of the metal center itself.

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My research has revealed that Nindigo can be readily deprotonated using silylamide bases. For instance, treatment of H2dmp2Nindigo (dmp = 2,6-dimethylphenyl) with 2 equivalents of Li[N(SiMe3)2] readily affords dmp2Nindigo[Li(THF)2]2in good yields.

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Reaction of dmp2Nindigo[Li(THF)2]2with metal halides, namely CoCl2and FeCl2, affords the tetrahedral metal ‘-ate’ complexes dmp2Nindigo[MCl{Li(THF)3Cl}]2(M = Co, Fe).

Additionally, reaction of H2dipp2Nindigo (dipp = 2,6-diisopropylphenyl) with Co(NR2)2(R = SiMe3) yields the tetrahedral Co(II) complex dipp2Nindigo[Co(NR2)(THF)]2. In a similar fashion, I intend to use M(NR2)2with Nindigo to generate other Nindigo[M(NR2)]2species.

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These complexes will serve as the platform for my investigation into the reactivity of first-row transition metal Nindigo systems.

Finally, it should be noted that given Nindigo is both highly conjugated and binucleating, the potential for intramolecular metal-metal communication exists which could allow for interesting magnetic and electronic properties. Indeed, I have found that analysis of paramagnetic dipp2Nindigo[Co(NR2)(THF)]2by SQUID magnetometry reveals antiferromagnetic coupling between the Co(II) centers at low temperatures.

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Lab Member

Zhenyu ‘Justin’ Wu

Biography

I was born and raised in Anhui province, southern China, and received my Bachelor of Science degree in Applied Chemistry from Beijing University of Chemical Technology in 2011. When I was an undergraduate student, I worked with Dr. Haijun Hao on the study of synthesis and reactivity of Zinc Hydride compound [HC(CMeNAr)2}Zn(μ-H)]2 (Ar = 2,6-Me2C6H3)] and complexes with Zn-Zn single bond.

Research Projects

Use of carbon-based fuels has caused an increasing concentration of carbon dioxide in the atmosphere that has been continuously rising. Carbon dioxide is the most abundant green house gas, but its utilization is confronting several technical barriers. Therefore, exploring catalysts that could effectively capture CO2and convert it into valuable products has become an exciting topic in the past decades. Interestingly, nature uses Rubsico – the world’s most abundant enzyme – to capture CO2from the air and converts it into the carbon ultimately contained in our food, the fuels we use, and clothes we wear.

My project is focused on the study of the synthesis and reactivity of redox active ligands. We have already synthesized the iron and cobalt complexes with this newo-imino-semiquinonate ligand.

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Treated with appropriate reducing agent, the chlorine in the complex above could be removed from the center metal to form a new planar compound and the ligand in this product is redox non-innocent.

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I believe that such a ligand and metal system can be used as Rubsico mimics. The CO2can be captured by the amine hanging out the system and further stabilized by coordination to the metal center. Therefore I can convert the CO2in the air into more interesting chemicals.

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