Supramolecular triruthenium cluster-based benzene hydrogenation catalysis: Fact or fiction?

The question is addressed of whether the triruthenium cluster cation [Ru-3(mu(2)-H)(3)(eta(6)C(6)H(6))(eta(6)-C6Me6)(2)(mu(3)-O)](+), 1, is a supramolecular, outer-sphere benzene hydrogenation catalyst or is 1 a precatalyst to well-known Ru(0)(n) catalysis of benzene hydrogenation. This question of "is it homogeneous or heterogeneous catalysis?" is especially important in the present case since if 1 is a supramolecular, homogeneous catalyst as postulated in the literature that is, if 1 can in fact accomplish catalysis of reactions as difficult as benzene reduction with no inner-sphere, d-orbital-mediated ligand dissociation, oxidative addition, migratory insertion, or reductive elimination-then that finding holds promise of rewriting the rules of organometallic-based catalysis. The identity of the true catalyst derived from 1 is, therefore, addressed by a collaborative effort between research groups at the Universite de Neuchatel and Colorado State University. The methodology employed is that worked out previously for addressing the historically vexing question of "is it homogeneous or heterogeneous catalysis?" (Lin, Y.; Finke, R. G. Inorg. Chem. 1994, 33, 489 1). A combination of the following classes of experiments have been employed: (i) Ru metal product studies; (ii) kinetic studies; (iii) Hg(0) and quantitative poisoning experiments, (iv) NMR studies of H/D exchange rates; (v) other data, plus (vi) the principle that the correct mechanism will explain all of the data. The results provide a compelling case that 1 is not the true benzene hydrogenation catalyst as previously believed; instead, all our evidence is consistent with, and supportive of, trace Ru(0) derived from 1 under the reaction conditions as the true, active catalyst. Nine additional conclusions are also presented as part of the summary and take-home messages, as well as a citation of "Halpern's rules" for catalysis.