Fundamentals of Chemistry that Guide Formation of Sulfur-bridged Bi- and Multi-metallic Molecular Units

With the support of the Chemical Synthesis program in the Division of Chemistry, Professor Marcetta Y. Darensbourg and coworkers of Texas A&M University will study molecules that inform chemists about Nature’s choice of sulfur to connect metals in structures that perform as catalysts in biology. In contrast to the heavy, precious/expensive metals such as platinum and rhodium that one might find in the catalytic converter of an automobile, Nature must rely on the natural abundance distribution of metals on earth.

Consequently, earth-abundant metals such as iron, nickel, and manganese are taken up in molecular motifs that utilize cooperativity between metals. In addition, their strategic positioning within a molecular environment, and timing of delivery of reactants and release of products, renders incredible reactions that are facile at ordinary temperatures and pressures.

The importance of connections is vital to these molecular-binding motifs. Sulfur is a key ingredient in metalloenzymes that perform some of the most fundamental reactions of life. Examples include catalysts that facilitate hydrogen production, carbon dioxide reduction, and nitrogen conversion.

How these molecular catalysts, imbedded in protein folds, work remains largely a mystery that inspires chemists to search for clues in small, well characterized molecules. This work will target connections between nickel and iron in new synthetic molecules outfitted with “reporter” molecules that indicate reactive centers. The Darensbourg team will analyze their structures and properties, monitoring changes as electrons are added or removed one by one.

They will search for and characterize products of catalysis of proton reduction or synthesis. These fundamental studies are ideal for training graduate and undergraduate students and coworkers to recognize and appreciate sciences that connect biology and chemistry.

With the support of the Chemical Synthesis program in the Division of Chemistry, Professor Marcetta Y. Darensbourg of Texas A&M University will establish a synthetic chemistry plan that targets an improved understanding of bimetallic constructs prevalent in redox-processing enzymes including diiron and nickel-iron hydrogenases. The overall goal is to understand how two (or multiple) abundant first-row metals share the burden of 2-electron transformations, whereby obviating the need for noble metals in natural catalysis and organic synthesis.

Questions arise from the ubiquitous mediation by sulfur as a bridge between metal complexes that have redox activity either at the metal or the ligands, and how substrates are positioned on the reactive centers. Two principal classic “non-innocent” redox-active ligands will be engaged for this work, namely nitric oxide (NO) and dithiolenes. The Darensbourg group is developing NO-containing metallodithiolate ligands as bidentate S-donors to a transition metal Lewis acid receiver for examination of properties of the redox levels in a diiron trinitrosyl.

The team proposes to monitor the electronic and vibrational coupling between two S-bridged iron as affected by redox level of the diiron complex using N-15 isotopic labelling and two-dimensional infrared spectroscopy. In addition, the effect of redox level on the observed scrambling of the NO ligand between the two irons is expected to inform our understanding of communications between metals in the sulfur-bridged bimetallics.

The concerted effect of two different types of delocalization ligands with a nickel-dithiolene as receiver to the metallodithiolates will offer an opportunity to explore inter- and/or intramolecular magnetic quenching and electronic coupling. Studies of the properties of the new bimetallics will improve our understanding of multimetallic bioinorganic enzyme active sites.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.