INOR 18 |
| Romero and Piers recently reported a mechanistic study in which they detailed the intramolecular exchange of adjacent methylene groups of a ruthenacyclobutane and the intermolecular degenerate exchange of free ethylene with a second-generation Grubbs-type catalyst, a 14-electron ruthenacyclobutane, (NHC)Cl2Ru(CH2CH2CH2), where NHC is an N-heterocyclic carbene. These authors measured activation parameters for the intramolecular and intermolecular exchange processes (ΔG‡ = 12.3 kcal mol-1 and ΔH‡ = 13.2(5) kcal mol-1 and ΔS‡ = 15(2) eu, respectively) and called for “further investigations, particularly computational studies” on their mechanistic proposals. The current study applies density functional theory calculations to address their proposed exchange mechanisms. For intramolecular exchange, the formation of a structure with coordinated ethylene proceeds from the cyclobutane species via a direct rotational bond-breaking transition state, which produces an ethylene which is essentially perpendicular to the remaining methylene unit; and the computed free energy of activation including solvation for the pathway with trans-disposed chlorides (ΔG‡, -50 °CCH2Cl2 = 14.4 kcal mol–1) is close to the observed free energy of activation. This type of mechanism is operational regardless of whether the ruthenacyclobutane has cis- or trans-disposed chloride ligands; however, the species with trans-disposed chlorides is lower in energy than the one with cis-disposed chlorides and the pathway with trans-disposed chlorides has a lower barrier. For intermolecular exchange, ethylene binds to the metal of the ruthenacyclobutane complex, a rotational bond-breaking transition state produces a bis-ethylene, methylene intermediate, from which the ethylene, formally of the metallocyclobutane, dissociates. For intermolecular exchange, the computed free energy of activation including solvation for the pathway with trans-disposed chlorides ( ΔG‡, -50 °CCH2Cl2 = 18.6 kcal mol–1) is close to the observed free energy of activation (ΔG‡, -50 °CCD2Cl2 = 16.9 kcal mol–1).
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Theoretical and Computational Chemistry
9:00 AM-11:20 AM, Sunday, August 19, 2007 BCEC -- 203, Oral
Division of Inorganic Chemistry |
