II International Symposium on Carbon for Catalysis ABSTRACTS

PP-<strong>II</strong>-17
THE ROLE OF 14-ELECTRON INTERMEDIATES IN THE RUTHENIUM
ALKYLIDENE-CATALYZED POLYMERIZATION OF NORBORN-2-ENE
Naumov S., Buchmeiser M.R.
Leibniz Institute of Surface Modification, Permoserstrasse 15, D-04303 Leipzig, Germany
e-mail: sergej.naumov@iom-leipzig.de
Olefin metathesis is a fundamental reaction for the formation of carbon-carbon double bonds.
The activity of the "first-generation" ruthenium-based metathesis catalysts
RuX 2 (PCy 3 ) 2 (=CH 2 ) (type I catalysts) was significantly improved with the "secondgeneration"
catalysts RuX 2 (PCy 3 )(H 2 IMes)(=CH 2 ) (type <strong>II</strong> catalysts), where an
N-heterocyclic carbene (NHC) replaces one phosphane group 1 . It was concluded 2 that the
greatly increased activity of type <strong>II</strong> catalysts in ring-opening metathesis polymerization
(ROMP) origins from the increased reactivity of the four-coordinate 14-electron intermediate
RuX 2 (L)(=CH 2 ) produced by phosphane dissociation from type I and <strong>II</strong> catalysts.
Additionally it was found 3 by comparing the gas-phase reactivity of 14-electron reactive
intermediates with experiments that type <strong>II</strong> catalysts show hundred-fold higher activity
despite slower phosphane dissociation because of a much more-favourable partitioning of the
14-electron active species between entry into the catalytic cycle and return to the precatalyst
(by rebinding of phosphane). The gas-phase study 4 findings in ROMP of norborn-2-ene
(NBE) are that type I catalysts display higher barriers for the conversion of the π-complex
into the metallacyclobutane than for phosphane dissociation. In contrast, type <strong>II</strong> catalysts
behave as if there was not any significant barrier at all. Moreover, it has been shown by
quantum chemical calculations 3 that there are only small differences between the energy
surface of first and second-generation catalysts. Thus, there is still no definite answer on the
paradox finding that a slower activation step (dissociation of one phosphane ligand) is
overcompensated by an improved partitioning into product direction in type <strong>II</strong> catalysts
clearing such different activity of type I and <strong>II</strong> catalysts. The aim of the present work was to
find the rate-limiting critical step in the ROMP of NBE, which could explain the discrepancy
between slower phosphane dissociation and higher activity of second-generation catalysts.
Using DFT B3LYP/LANL2DZ (Gaussian03) method, the first step of complex formation
between the transition-metal carbene and NBE as well as properties of the active 14-electron
intermediate were modelled in dependence on the ligands L used (L=IMes, H 2 IMes,
tetrahydropyrimidin-2-ylidenes 5,6 and PCy 3 ) and X (F, Cl, Br, I, and OCOCF 3 ). In agreement
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