Ruthenium compounds, Ruthenium-Isotopes, Nickel compounds, Chemistry-Data processing, Nicotinamide, Isomerization, Computational chemistry, eta2 complexes
In 1992, Creutz, Brunschwig, and Sutin reported that when ruthenium(III) pentaammine, initially η¹-amide bound to nicotinamide or isonicotinamide, was reduced to ruthenium(II), it rapidly isomerized to the η¹-pyridyl form at a rate of 9.6 s‾¹.¹ They determined that the rate was too rapid for the reaction to occur via an intermolecular mechanism at experimental concentrations between 0.05 and 1 mM, and too slow to be explained by solvent cage collapse. However, the rate is similar to the rates of N-to-O and O-to-S linkage isomerizations by ruthenium(II) pentaammine on glycylglycine and DMSO, respectively. Creutz et al. inferred that the experimentally observed isomerization must occur via an intramolecular mechanism. They proposed that ruthenium(II) "walks" around the ring, passing through several η¹-arene intermediates in which ruthenium sits on top of an arene bond and coordinates with two atoms simultaneously. Their conclusion is particularly fascinating because while some metals such as nickel(0) and osmium(II) form relatively stable η² complexes with arenes and double and triple bonds, such behavior is not as well established in ruthenium(II), and Creutz et al. characterize ruthenium(II) η² complexes as "metastable". In an attempt to identify and understand the factors dictating the strength of η²- arene interactions, a survey of nickel(0)(PMe2H)2 η²-arene complexes was conducted. A frontier orbital argument, electron donating and withdrawing group effects, and Hirshfeld charges were considered as potential indicators of the stability of η² complex formation. While no quantitative relationship was identified, it is clear that low electron density on the arene promotes η² interactions. Following nickel studies, ruthenium(II) pentaammine η² complexes with small molecules as well as arenes were investigated with the intention of determining whether the isomerization mechanism proposed by Creutz et al. is reasonable. A transition state between adjacent η² complexes on benzene indicates that the η²-benzene complexes are fluxional, and ruthenium effectively "gallops" around the benzene ring. Transition states on pyridine reveal a general decrease in energy as ruthenium(II) approaches the η¹- pyridyl isomer, a trend which, in all probability, is mirrored in nicotinamide and isonicotinamide. These results clearly indicate that ruthenium does in fact gallop the nicotinamide ring, and that the η¹-pyridyl isomer is its destination.
Spear, Eliza Kate, "Ruthenium gallops the nicotinamide ring" (2012). Honors Project, Smith College, Northampton, MA.
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