Potential of Ru(III/II) Couple Increases with Increased Sterics of Scorpionate Ligands

http://www.sciencedirect.com/science/article/pii/S0020169313000674.

Volume 405, 24 August 2013, Pages 470–476

Synthesis and electrochemical characterization of [Ru(NCCH₃)₆]²⁺, tris(acetonitrile) tris(pyrazolyl)borate, and tris(acetonitrile) tris(pyrazolyl)methane ruthenium(II) complexes

• Christopher C. Underwood,
• Bradley S. Stadelman,
• Mark L. Sleeper,
• Julia L. Brumaghim^,

http://dx.doi.org/10.1016/j.ica.2013.02.027

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Abstract

Tris(acetonitrile) tris(pyrazolyl)borato- and tris(pyrazolyl)methano ruthenium(II) complexes would make good synthons in ruthenium chemistry for synthesis of catalysts and DNA binding drugs. However, these complexes are not widely used as starting materials due to the long reaction times and multiple synthetic steps required or the lack of their successful synthesis. We have developed a new synthesis for the ruthenium(II) acetonitrile complex [Ru(NCCH₃)₆]²⁺ with noncoordinating BF₄⁻ or OTf (OTf = trifluoromethanesulfonate) counterions. Using this [Ru(NCCH₃)₆]²⁺ complex, the previously reported tris(acetonitrile) tris(pyrazolyl)borato ruthenium(II) complexes [Tp^RRu(NCCH₃)₃]⁺ (Tp^R = tris(pyrazolyl)borate; R = H, Me) and the unreported tris(acetonitrile) tris(pyrazolyl)borato ruthenium(II) complex (R = Ph) have been synthesized using an improved synthetic pathway that reduces the number of required steps by up to six and the average synthesis times by up to 45 h. Novel tris(acetonitrile) tris(pyrazolyl)methano ruthenium(II) complexes of the formula [Tpm^RRu(NCCH₃)₃]²⁺ (Tpm^R = tris(pyrazolyl)methane; R = Me, Ph) have also been synthesized in one step in 12 h using this method. Cyclic voltammetry studies of the synthesized complexes show that Ru^2+/3+ redox potentials generally increase with increasing steric bulk of the Tp^R or Tpm^R ligand. The ability to sterically tune Ru^2+/3+ redox potentials may be used to promote catalysis development and in the development of ruthenium-based drugs.

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Graphical abstract

We report improved syntheses for [Ru(NCCH₃)₆]²⁺ and complexes of the formula [Tp^RRu(NCCH₃)₃]⁺ (Tp^R = tris(pyrazolyl)borate; R = H, Me, Ph), as well as syntheses for unreported [Tpm^RRu(NCCH₃)₃]²⁺ (Tpm^R = tris(pyrazolyl)methane; R = Me, Ph) complexes. Electrochemical studies indicate that the Ru^2+/3+ redox potentials of these complexes vary by up to 427 mV, a trait that may be useful for ruthenium-based drug and catalysis development.