R E S E A R C H
The Donahue Group - Overview of Topics
Research in the Donahue Group is primarily synthesis-driven. All of our research endeavors are either related to the preparation of new compounds or to the development of improved ways to access important types of molecules. This orientation towards synthesis is motivated by fundamental principles and potential applications. It occasions the use of a broad variety of methods to characterize and understand new things we make. Students in the Donahue group routinely use calculations, X-ray crystallography, electrochemical techniques such as cyclic voltammetry and differential pulse voltammetry, mass spectrometry, and spectroscopic methods such as NMR (1H, 13C, 31P), UV-vis, IR, resonance Raman, and XAS. While we seek to extend the limits of inorganic/organometallic synthesis into new territories, we generally do so with an eye toward a practical end. We sometimes make the effort to understand a new compound or set of compounds in great detail, but we do so hoping for insights that lead to uses. An elevated theme in our group is energy related research, particularly CO2 activation mediated by transition metal complexes and exploration of new catalysts for production of H2. The following is a series of vignettes describing research that is currently underway in the Donahue Group. If the purpose-driven nature of these projects has an appeal to you, we hope that you will join us.
Individual Projects - 2015
I) Exploration of new, more efficient and more general methods of synthesis for dithiolene ligands, including the preparation of sterically encumbered variants not previously described. The synthesis of dioxolene and diselenolene ligands are also of interest in this context.
Dichalogenolenes (dioxolenes, dithiolenes and diselenolenes) represent an important class of “redox non-innocent” organic ligands that form complexes with the transition metals that have many real or potential applications. Examples of such applications include use as conducting materials, sensing devices, and catalysts for the generation of H2. The 2004 special issue of Progress in Inorganic Chemistry devoted to metallodithiolene chemistry is commended to the reader interested in learning more. A current limitation to further developments in this area is a lack of access to certain ligand types, examples being diselenolenes of most types and dithiolenes with sterically encumbering substituents. We are actively exploring a new method of synthesis of dichalogenolenes (Scheme 1) that may admit of greater generality and simplicity than currently employed methods and allow access to variants of these ligands that have heretofore been elusive.
II) Synthesis of a small molecule analogue of the active site of Oligotropha carboxidovorans carbon monoxide dehydrogenase (CODH), an aerobic bacterial enzyme that catalyzes the equilibration of CO and CO2 according to the relation CO + H2O ↔ CO2 + 2H+ + 2e-
Carbon Monoxide Dehydrogenase (CODH) is enzyme found in the aerobic bacterium Oligotropha carboxidovorans that equilibrates CO and CO2 according to the expression CO + H2O ↔ CO2 + 2H+ + 2e-. As such, it is interesting in the context of CO2 activation. We are exploring two avenues of synthesis aimed at the preparation of heterodimetal Mo-Cu complexes that preserve the single chalcogenide, preferably sulfide, bridge between the metal ions that occurs at the enzyme active site (Figure 1). One approach involves the joining of separate copper and molybdenum fragments via a “silane-mediated coupling” reaction (Scheme 2, (a)), while a distinctly different approach proceeds by displacement of a labile ligand on a copper fragment by a nucleophilic terminal chalcogenide ligand on molybdenum ((Scheme 2, (b)). In the connection with this chemistry, we recently reported a synthesis of triptycyl thiolate and of several complexes of this hindered thiolate with Cu(I), one of which is shown in Figure 2 and was featured on the cover of Inorganic Chemistry (2012, 51, Issue 12).
III) Synthesis, structures, and properties of multimetal dithiolene complexes, particularly those with the Group 6 and Group 10 metals.
We have for some time been engaged in the synthesis and study of complex multimetal-containing dithiolene complexes motivated by the expectation that such species would amplify or permit the fine tuning of some of the potentially useful properties of metallodithiolene complexes, such as their ability to support multiple, reversible redox processes. Our synthesis approach might be described as a “bottom-up” approach. Shown in Figure 3 is a ditungsten dithiolene compound recently prepared in our group, along with images of its HOMO and LUMO from a geometry optimization. A related tetra-tungsten species, a so-called “trigonal prism of trigonal prisms,” is also targeted for study, as is the platinum molecular square shown in Figure 4. The platinum molecular square is of interest for its anticipated photochemical behavior involving dithiolene ligand to bridging ligand charge transfer transitions.
IV) Carbon dioxide reduction, specifically its reduction to CO. Low-valent tungsten(II) complexes are being explored and developed for their ability to mediate this reaction.
The accumulation of CO2 in the atmosphere due to human activity has created an imperative for the development of ways to use it as a chemical feedstock. The reduction of CO2 to CO is especially desirable, as CO is a starting point for a broad variety of important commodity chemicals via Fisher-Tropsch chemistry. We have been motivated to examine low-valent tungsten complexes that have the ability to oxidatively add CO2 to form corresponding oxo-carbonyl compounds. In initial work (cf. Dalton Trans. 2010, 39, 9662) we re-examined the [WIICl2(phosphine)4] system, the general reactivity of which was reported by Mayer et al (cf. J. Am. Chem. Soc. 1987, 109, 2826) and found that CO could be thermally displaced from [WIVO(CO)Cl2(phosphine)2] and that non-redox oxo-for-chloride exchange using HCl was facile for [WIVOCl2(phosphine)3]. These newly defined steps provided for a closed cycle of reactivity for the reduction of CO2 to CO, with the oxygen atom of CO2 ultimately removed as H2O (Scheme 3).
In continuing work, we retain our focus upon tungsten because of its natural abundance, which may be a decisive advantage in the larger picture. We are currently working with different supporting ligand systems, such as pyrazolylborate and pyrazolylmethane ligands, because the multi-dentate nature of such ligands may enable greater stability.
V) Redox-controlled property changes, such as coordination geometry changes and reversible ligand binding.
An electric switch is arguably the simplest and most powerful way to exert “on-off” control in a system. In terms of discrete transition metal complexes, such switching would mean redox chemistry, either oxidation or reduction. In working with some metallodithiolene systems, we have observed reversible property changes as a function of redox chemistry. An interesting example is the case of [W(S2C2Me2)(CO)4] which undergoes a coordination geometry change from trigonal prismatic to octahedral upon one-election reduction (Figure 5). A detailed analysis of this system was recently reported by us (Inorganic Chemistry 2012, 51, 346). In related work, we have sought to explain the electronic structure basis for [W(CO)2(tdt)(phen)] being trigonal prismatic and [W(NO)2(tdt)(phen)] being octahedral (tdt = toluene-3,4-dithiolate; phen = phenanthroline). Computational work has pointed toward certain [W(CO)2(dithiolate)(N2)] complexes as having the capacity to support reversible coordination geometry changes as function of redox chemistry.
VI) Synthesis and study of soluble metal sulfides for their potential application as H2-evolving catalysts.
A number of reports from the recent chemical literature suggest that well-defined soluble metal sulfide complexes may have the capacity to function as catalysts for the formation of H2 from H+ and electrons. This idea motivates the synthesis and scrutiny of a range of transition metal sulfide complexes, one of which is a molybdenum species reported some 30 years ago by Enemark and coworkers (Inorg. Chem. 1982, 21, 3795). We recently obtained a new, higher resolution structure (Figure 6) of the compound, which shows the two types of dithiolene ligands (bridging and terminal) to be at different redox levels. Currently, we are studying the electrochemistry of this species, which appears quite complex upon initial examination by cyclic voltammetry.