# Marcetta Y. Darensbourg

Marcetta York Darensbourg is an American inorganic chemist. She is a Distinguished Professor of Chemistry at Texas A&M University. Her current work focuses on iron hydrogenases and iron nitrosyl complexes.

## Early life

Marcetta Bernice (York) Darensbourg was born May 4, 1942, in Artemus, Kentucky. She is daughter to school teachers, Atlas H. York, and Elsie Walton York. She has an older sister named Mary Lucille York, and a younger brother named Larry Hercules York. Darensbourg attended a local high school named Knox Central High School in Barbourville, Kentucky. In high school, she was a studious pupil and was a member of the band, choir, and cheerleading team. This is where Darensbourg met her role model, Mrs. Bolton. Mrs. Bolton taught biology, physics, and chemistry which interested Darensbourg. One of the reasons Darensbourg wanted to go into science and teach was from the great influence that Mrs. Bolton left on her.

## Education

Darensbourg received a B.S. in chemistry from Union College in 1963, and a Ph.D. in inorganic chemistry from the University of Illinois under the guidance of Theodore L. Brown in 1967. Her doctoral work focused on the kinetic studies of organolithium reactions.

## Career

Darensbourg was an assistant professor at Vassar College from 1967to 1969. From 1971 to 1982, she taught at Tulane University, attaining the rank of professor. In 1982, Marcetta Darensbourg was appointed professor at Texas A&M University together with Donald J. Darensbourg. She was subsequently awarded the title of Distinguished Professor in 2010. Her research interests include bimetallic hydrogenase enzymes containing CO and CN ligands.
Darensbourg is a member of the board of Inorganic Syntheses, where she also served as the editor-in-chief of volume 32. In 2011, she was elected fellow of the American Academy of Arts and Sciences.

## Overview of Research Career of Marcetta York Darensbourg

Transition metals in the center of the periodic table, most notably iron, cobalt, nickel, are available on earth in various minerals and within biology as single metals trapped in evolutionarily designed biomolecular binding sites as in proteins. Just how they get naturally removed from the extended structures of minerals, transported, selectively taken up into those sites, and for what purpose(s) they exist in them, are the ultimate inspirations for the research program of Marcetta Darensbourg. The fundamental chemical characteristics of such molecules and mechanisms of their interchange has fueled her excitement for chemistry over a long and continuing research career. During that time she has produced upwards of 287 publications, mentored 54 Ph.D. graduates and ca. 15 M.S. students. Approximately 20 post-doctoral Fellows have contributed to her program. n particular, mineral deposits composed of iron sulfide are posited as the origins of the organometallic catalysts for hydrogen metabolism and for the formation of carbon-carbon bonds in the reducing atmosphere that existed on earth prior to the Great Oxygenation Event. That there is an evolutionarily developed molecular memory is seen in the biocatalytic active site structures, buried within the protein folds of Hydrogenases and AcetylcoASynthase enzymes. The central molecular constructs of those enzymes have specific geometries and spectroscopic signals that may be interpreted by the knowledge that has evolved over the past 40 years.
Marcetta’s early independent work with molecules containing transition metals surrounded by diatomic ligands such as carbon monoxide, nitric oxide, and cyanide as stabilizing ligands in specific coordination spheres comprised fundamental work published in the first 60 publications of her > 285 publications. This pre-era of her research performed largely at Tulane University was the foundation for her subsequent recognition of their roles in a new branch of chemistry which she called "Bioorganometallic Chemistry"—as a subset of Bioinorganic Chemistry. The move to Texas A&M in 1982 opened the door to new possibilities for a larger graduate student cohort and scores of undergraduate coworkers over the decades that followed. The following description of her research contributions during this time speaks to the number of major projects for which she gratefully received funding largely from the National Science Foundation and the R.A. Welch Foundation. These publications are described in the numbered list from her Website. Overall the patterns found in these works lead through the need to understand the reactivity of transition metal hydrides in industrial catalysis, expanded to their possibilities as intermediates within the intricate mechanisms of hydrogenase enzymes—in nature.
Marcetta’s engagement with the inorganic chemistry community was early demonstrated in a symposium she and Andrea Wayda organized for the American Chemical Society in 1985. "Remember Chicago" took on new meaning as an unexpectedly huge audience overfilled a conference room with inorganic chemists who were interested in the topic of "Experimental Organometallic Chemistry: A Practicum for Synthesis and Characterization". The most forefront experimentalists presented how they did their work, not just the results. This symposium resulted in a monograph of the same name and confirmed Marcetta’s role in Synthetic Inorganic Chemistry.
The special qualities of carbon monoxide as a ligand to transition metals permit electron density to build up on even the small first row transition metals (vanadium, chromium, manganese, iron, cobalt, nickel) producing formal oxidation states as low as -4! The famous metal to CO ligand π-back bonding stabilizes these negative charges. Marcetta noted the geometrical stereospecificity of ion-pairing of counter-cations with oxygen of the metal-bound CO ligand. From this an orientational effect on C-C coupling reactions was confirmed, developing from addition of organic halides to metal carbonyl anions (M-44). From this period a very short intellectual step took her to metal carbonyl hydrides; a functionality critical to the understanding of mechanisms of the hydrogenases, including requirements for H removal from a metal hydride (M-47).
The rich chemistries of anionic metal carbonyl hydrides, including possibilities of their use as hydride–transfer agents in the heterolytic (H$^−$/H$^+$) H$_2$ reduction of olefins, was the next plateau on her hydrogen research landscape.
Collaborators played key roles in Marcetta’s research journey. Most notable in the past two decades is the role of Mike Hall, computational chemist par excellent, whose insight into structure and bonding enriched many, many publications of his colleagues, including those of Marcetta. Mechanistic design for hydrogen atom transferals benefitted by the expertise of Martin Newcomb and Don Darensbourg. In her later decades, no collaborator was more crucial than TAMU’s scientific staff, especially Joe Reibenspies, world renown X-ray crystallographer.
In the late 1980s, early 1990’s Marcetta’s group delved more and more into synthesis that linked thiolates (sp?) as ligands to metal hydrides and metal carbonyls. With students such as Charlie Riordan and Wen-Feng Liaw, the publications delved more and more deeply into metal carbonyl clusters, hydrides, and thiolates. The influence of colleague Arthur Martell and Paul Lindahl, and the perseverance of graduate student Dan Mills, was great towards her shift to a new career phase. Arthur encouraged her interest in "classical" bioinorganic chemistry, and the obvious chemical promiscuity problems of Paul’s ACS enzyme active site (a dissymmetric, sulfur bridged Ni—M site) led her group to an amazing tetradentate S - - N - - N - - S ligand motief that wrapped around metals as did cysteine-glycine-cysteine tripeptide. The ability of tetra-dentate N$_2$S$_2$ ligands to bind Ni, Fe, Co and their heavier congeners broke the bank in terms of structures and publications. This prolific body of work commanded attention as the nucleophilicity of the cis-dithiolate sulfurs held promise for binding exogeneous electrophiles such as oxygen or metals. Thus was established a new paradigm for the synthesis of S – bridged hetero bi, -tri, -tetra - - metallics in well-defined structures and led to an exhaustive review, published in 2015. Marcetta’s insight into the construction of the ACS active site, and its further connection to an iron-sulfur cluster was the preparation for her usefulness to the Hydrogenase Community of Scientists.
In the late 1990’s a true gamechanger in structural bioinorganic chemistry appeared. Scientists at Los Alamos saw suspicious spectral features in the infrared spectrum that might be interpreted as diatomic ligands, CO and CN. Synchronously, crystallographers in France saw strange elongated electron densities attached to iron. We chemists in College Station recognized the connection between these observations and the possibility that Nature had developed the [NiFe]-H$_2$ase active site to employ carbon monoxide and cyanide ligands, rather than the typical hard donor bases. For synthetic inorganic chemists this meant that small molecule models of iron might be targeted to match or verify spectral features to the enzyme active site as determined by biophysical chemists. It should be mentioned that the redox-active hydrogenases and acetylcoAsynthase, the spectroscopic method of choice had been the less common electron spin resonance. Now that diatomic ligands were verified, a host of synthetic inorganic/organometallic chemists could enter the field as biomimetic chemists.
It was an easy step from Nickel-Iron Hydrogenase to the Diiron Hydrogenase. Marcetta and two respected competitors stepped into the literature with a dithiolate-bridged diiron hexacarbonyl that had certain properties matching the latter enzyme, with hardly any modification, and publishing within a few months of its structure determination. It was an exciting time, as a large following of the [FeFe]-H$_2$ase story was engaged by the possibility of using its principles to construct abundant metal electrocatalysts for proton reduction to hydrogen.
Currently, Marcetta has branched out from her hydrogenase inspiration into an application of synthetic skills to design molecules that delocalize electron density as well as electron spin. A key component of the latter is another diatomic ligand, nitric oxide. During Covid, her group developed examples of NO transfer reagents including iron nitrosyl. It turns out that the same N2S2 binding site for iron that is seen in another enzyme active site, Nitrile Hydratase, becomes a superb vehicle for NO binding and NO transfer of importance to human physiology. The group has established principles of binding and electron spin coupling through long distances.

## Research projects
### Organolithium chemistry

Darensbourg investigated certain kinetic aspects of organolithium compounds. During the course of these studies, the kinetics of the rate-determining step of tert-butyllithium dissociation from tetramer to a dimer were analyzed. Using 7Li nuclear magnetic resonance (NMR), the study delineates the rate-determining step of the equilibrium of the tert-butyllithium mixture, revealing that the dissociation from tetramer to dimer is key. Notably, the dissociation rate was found to be significantly affected by the solvent used, and the dissociation rate of toluene was significantly faster than cyclopentane. The findings also highlight the role of stereoconfiguration in these reactions, where tert-butyllithium exhibits a uniquely slow intermolecular exchange rate compared to other alkyl lithium compounds due to its larger size. It has been observed that the presence of even a small number of bases like triethylamine greatly accelerates the exchange rate. Using mass spectroscopy, the existence of cross-association with other organolithium species in the vapor phase could also be observed.

### Metal carbonyl chemistry

Darensbourg's interest in charge distribution molecules that could be probed with reactivity led to her work on mapping nucleophilic attack on metal carbonyls. Infrared, nuclear magnetic resonance and electronic spectroscopy of some carbene pentacarbonyl complexes of chromium(0) and tungsten(0) indicated that carbene ligands are better sigma donors than a carbonyl ligand, while simultaneously behaving as strong pi acceptors. Substitutions of iron and cobalt sites were made to see how the CO strength force constants affected the nucleophilic attacks. The substitutions illustrated that the nucleophilic attacks always occurred at the CO group with the greater force constant when there is a choice of carbonyl groups present in a molecule.

### Hydrogenase mimics

Darensbourg has pioneered the development of synthetic mimics of hydrogenase enzymes. These include synthetic complexes featuring Fe-based organometallics species, which serve as precursor for producing iron only Hydrogenase enzyme active site. These enzymes are capable of carry out reaction even in the absence of the protein-based active site organization or carry out the proton production with high efficiencies. However, these hydrogenase enzymes were found to be highly sensitive with oxygen (O$_2$), which can over oxidize and inactivate them. Even after the oxygen was removed, they do not regain catalytic activity immediately, requiring multiple steps to do so.
In 2020, Darensbourg et al. reported a variety of characterizations of Ni-Fe based hydrogenase species, which eventually encounter oxygen damage during their lifetime. Although some hydrogenase catalysts remain tolerant to oxygen damage, a majority of such catalysts typically undergo irreversible damage upon exposure. Darensbourg et al. reported an overview of sustainable water splitting technologies in which the hydrogenase species can be reductively repaired. Modifications of single atoms within hydrogenase active sites allowed for customizable activities, oxygen tolerance, and structures of the catalysts, permitting practical applications of enzymes and fragile biomimetrics of the active sites. Studies of a [NiFeSe]-H$_2$ase active site presented new applications for selenium in hydrogenase enzymes, as the complex exhibited a high hydrogen-processing catalytic ability and a relatively quick recovery from oxygen damage.

### Metallodithiolates chemistry

In the beginning of 2017, Darensbourg shifted her focus to studying the metallodithiolates ligands, which act as building blocks for the synthesis of various bimetallic enzyme active sites. The ligands can act as a catalyst to carry out different reactions, depending on which transition metal being at the center.
Darensbourg et al. reported that metallodithiolates ligands with nickel centers can increase the electron density of bonds such as Fe-S, allowing them to be cleaved easily. Darensbourg et al. also determined that this nickel center complex associated with a lead atom also plays an important role in the addition of CO and ethylene in the Suzuki-Miyaura reaction, which couples the organic compounds of boron and the halides, along alkyl halides and alkylboranes. Furthermore, with the cCobalt center, the metallodithiolates ligands can catalyze the transfer of NO and nitrosylate moieties, which allows the glycosidase conjugation of dinitrosyl iron complexes. With this conjugation, other carbohydrates can achieve higher potential in attaching for drug delivery.

#### Molecular Magnetism

In 2023, Darensbourg began exploring metallodithiolates in the field of molecular magnetism. Seeing that few publications had reported analyses of metal-based linkers with sulfur bridge ligands, Darensbourg et al. characterized a paramagnetic nitrosylated iron complex with N$_2$S$_2$ ligands.
In the complex, the [Fe(NO)]$^{2+}$ unit lies centered above the N$_2$S$_2$ field, exhibiting strong antiferromagnetic coupling to triplet NO$^−$. Density Functional Theory (DFT) computations indicate that the Fe spin stabilizes by delocalizing onto the surrounding dithiolate sulfurs. In expectation of spin delocalization of bimetallic derivatives upon interactions with sulfur, Darensbourg et al. performed syntheses of various sulfur-bridged multimetallic complexes.
Darensbourg et al. reported that reactions of the paramagnetic (NO)Fe(N$_2$S$_2$) with [M(CH$_3$CN$_n$][BF$_4$]$_2$ salts forms a stairstep bond arrangement with square planar MS$_4$ conformations. Reactions of the nitrosylated iron complex were conducted with metal salts composed of Ni$^{II}$, Pd$^{II}$, and Pt$^{II}$. Darensbourg et al. reported that each tri-metallic complex demonstrated similar nitrosyl stretching values in IR spectroscopy despite differences in magnetic properties. Magnetic susceptibility and DFT calculations additionally showed that each of the {Fe(NO)}$^7$ units exhibited antiferromatic coupling and that each N$_2$S$_2$ ligand engaged in a superexchange interaction with the bimetallic derivatives. The interactions presented by each metal ion displayed a trend of increasing covalency in the order of Ni$^{II}$ << Pd$^{II}$ << Pt$^{II}$. Upon comparisons of the coupling strengths of each Nickel-sulfur-bridged multimetallic complex, Darensbourg et al. concluded that the antiferromatic coupling of each Fe(NO) spin center was facilitated by an intricate d-orbital overlap with the NI$_2$S$_2$ plane.
Darensbourg et al. explained that the antiferromatic coupling of Fe(NO) presented new strategies for obtaining strong magnetic exchange within metallodithiolate complex through $^4$d and $^5$d orbital interactions. In place of steric effects, differences in the metal ion identity play roles in the electronic effects of each metal-sulfur magnetic interaction. Through combinations of various paramagnetic metallodithiolate donors and metal receivers, a vast collection of thiolate-bridged multimetallic complexes can be prepared with different magnetic communication strengths.
The wide variety of possible sulfur-bridged multimetallic complexes presents many opportunities for bioinorganic chemistry. Darensbourg et al. indicated potential for the development of $^n$d-$^4$f complexes, of which some can be used as single-molecule magnets. Interactions between orbitals with even higher energies allows for the customization of modern biocatalysts in evolutionary biology. The improved tunability of such biocatalysts enables the synthesis of catalysts exhibiting long-term sustainability.

## Awards

Most recently, Darensbourg has been awarded with the American Chemical Society Willard Gibbs Medal Award, a highly prestigious award recognizing the contributions of a chemist to the field. In 2018, Darensbourg was recognized as the SEC professor of the year. Darensbourg was also awarded the American Chemical Society Award in Organometallic Chemistry in 2017 for her application of organometallic chemistry to hydrogenase enzyme active sites and synthetic analogues. In 2016, Darensbourg received awards for her teaching and mentoring abilities at both Texas A&M University and UCLA. Darensbourg was the recipient of the 2018 Kosolapoff Award from the Department of Chemistry and Biochemistry in the College of Sciences and Mathematics at Auburn University. In 2024 Darensbourg was honored by the Texas A&M Aggie Women Network as the recipient of its 2024 Eminent Scholar Award.