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The oxo-bridged basic triruthenium carboxylate species, [Ru₃O(O₂CR)₆(L)₃]ⁿ⁺, are of general interest for their extensive redox properties which are characterized by multiple electron transfer processes that leave the structure of the metal core intact.^{1, 2}  Although these complexes have been explored for applications in modified electrodes and extended linked systems, the building block of choice for all these experiments has been the acetate system.^3-10  Although reports of analogous triruthenium carboxylate complexes exist, the challenge of separating the products from the similarly charged byproducts is laborious and time-consuming, thus limiting their utility.  Our research group has established a general procedure for the synthesis and purification of the [Ru₃O(O₂CR)₆(L)₃]⁺ species for O₂CR = isobutyrate and n-butyrate.¹¹

The cyclic voltammetry of the species revealed the expected reversible one electron processes.  Both the isobutyrate and n-butyrate clusters display three widely spaced reversible one-electron couples and a fourth poorly resolved process as seen in Figure 1.  Compared to the acetate data collected under similar conditions, the potentials of the reductions from an overall cluster charge of +1 -> 0 (-0.23 V) and 0 -> -1 (-1.50 V) are noticeably more negative, corresponding to the increased pK_a of the butyric acids.  In contrast, the reduction potential for the +2 -> +1 cluster charges is consistently around 0.95 V for all of the clusters, indicating that the pK_a effect is negligible for electron transfer between the higher oxidation states.

            Figure 1.  Cyclic voltammetry for [Ru₃O(O₂CCH(CH₃)₂)₆(py)₃](PF₆)

The paramagnetism of the metal core does not preclude characterization of the complex by ¹H NMR, but it does result in several unusual features of the spectra for both the n-butyrate and isobutyrate systems.  Closer proximity of the protons to the ruthenium atoms correlates with a relaxation of the proton signal so that the splitting patterns become increasingly less distinct.  Specifically for the ortho proton on the axial pyridine ligands, the signal is not only relaxed, but it is also shifted dramatically upfield to approximately 1 ppm.  The assignment of this resonance to the ortho pyridine proton was confirmed by COSY NMR, as seen in Figure 2.

            Figure 2.  COSY ¹H NMR [Ru₃O(O₂CCH(CH₃)₂)₆(py)₃](PF₆)

Our efforts to optimize the synthesis have correlated with the report by Toma and coworkers,¹¹ that this apparently simple system is actually extremely sensitive to pH, even when carried out in organic solvents.  By using a generous amount of acid in the first step and by minimizing the pyridine in the second step, the desired blue complexes may be produced reliably.  Our experiments have determined that in the presence of less acid in the first step and more base in the second, that a green product, hypothesized to be the neutral complex, [Ru₃O(O₂CCH₂CH₂CH₃)₆(OH)(py)₂], results from the deprotonation of one axial water ligand.  The resulting axial hydroxide blocks ligand exchange with pyridine.  We are currently refining the purification to isolate solid samples of this product for complete characterization.

Over the course of this grant award, six undergraduate students have been supported and have greatly benefited from the experience of doing research.  Of these students, one has gone to medical school, one earned a Masters of Public Health degree, one earned a Masters degree in Forensic Science, and one is entering graduate school in chemistry this fall.  Although no published manuscripts have resulted from this research, six undergraduate research presentations have given the students a forum to hone their speaking skills, weekly group meetings have encouraged the students to think on their feet, and writing skills have been improved through semester end reports and one honors thesis.  As the PI of the project, the award has had significant impact on my own career since it was an important component of my promotion to the rank of full professor in 2009.  

References

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2.  Abe, M.; Sasaki, Y.; Yamaguchi, T.; Ito, T., Bull. Chem. Soc. Jpn., 1992, 65, 1585.

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9.  S. H. Toma and H. E. Toma, Electrochem. Comm., 2006, 8, 1628-1632.

10.  C. C. Pink,* N. L. Saad,* Mugge, A. M.*; J. M. Schlough,* Svenson, A.*; J. L. Eglin, Pence, L. E., manuscript in preparation.

11.  Nunes, G. S.; Alexiou, A. D. P.; Araki, K.; Formiga, A. L. B.; Rocha, R. C.; Toma, H. E. Eur. J. Inorg. Chem. 2006, 1487-1495.