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43124-B3
Magnetic Studies and Room Temperature Carbon-Carbon Bond Cleavage in Ruthenium Carboxylate Clusters

Laura E. Pence, University of Hartford

Our determination of a general strategy for the synthesis and purification of oxo-bridged basic triruthenium carboxylate clusters has resulted in the preparation and characterization of two butyrate species, [Ru₃O(O₂CCH(CH₃)₂)₆(py)₃] ¹ and [Ru₃O(O₂CCH₂CH₂CH₃)₆(py)₃] ² The overall synthetic strategy is seen below in Figure 1. 

            Figure 1.  General strategy for the synthesis of basic ruthenium carboxylates

Our efforts to optimize the synthesis have correlated with the report by Toma and coworkers,³ 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.

The extensive redox behavior of the {Ru₃O(O₂CR)₆}ⁿ⁺ clusters has long been a source of the interest in these systems since the extensive electron delocalization results in the stability of multiple different charges for the cluster.  With our isolation and characterization of two new carboxylate clusters in this series, the effect of the change in carboxylate ligand from the acetate to a longer chain may be observed.  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 2.  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 2.  Cyclic voltammetry for [Ru₃O(O₂CCH(CH₃)₂)₆(py)₃](PF₆)

This grant award has had substantial impact on my students and I. It has formed the foundation of my ability to carry out laboratory research involving undergraduates, which is an important focus for the faculty members in my department.   Of the two students who worked on the project supported by this grant in the past two years, Jennifer Schlough '08 used the foundation of her results obtained in summer 2007 to present a complete picture of the synthesis and characterization of the triruthenium isobutyrate complex in the undergraduate research symposium at the spring 2008 ACS meeting in New Orleans, and she is a coauthor on the recently submitted manuscript describing the work.  Contributing to these public products strengthened Jennifer's critical thinking abilities, deepened her understanding of her project, and exposed her to the standards required for professional communication of results.  Megan Burak, '10 who was supported by the grant in summer 2008, made similar strides in connecting her classroom knowledge to the uncertainties and challenges of a research project.  The PRF support for her summer work allowed her to form the basis for the Honors Thesis that she will write with me in another year.  At the University of Hartford, promotion to Full Professor is based on sustained and distinguished performance in both teaching and scholarship as well as adequate performance in the area of service.  This PRF grant award, combined with the manuscript submitted to Dalton Transactions acknowledging the support of PRF has directly contributed to demonstrating my sustained and distinguished performance in the traditional scholarship of discovery.  I have submitted my application for promotion to Full Professor as of September 2008, and if it is approved, my success will be in part due to the support of PRF.

References

1.  C. C. Pink,* N. L. Saad,* J. M. Schlough,* J. L. Eglin, and L. E. Pence, submitted to Dalton Trans.

2.  C. C. Pink,* N. L. Saad,* Mugge, A. M.*; Svenson, A.*; Pence, L. E., manuscript in preparation.

3.  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.

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