Product Kinetic Energy Release Distributions as a Probe of the Energetics and Mechanisms of Organometallic Reactions Involving the Formation of Metallacyclobutanes in the Gas Phase

Article abstract of DOI:10.1021/ja00188a007

Product kinetic energy release distributions and collision-induced dissociation studies are used to probe the energetics and mechanisms of several gas-phase organometallic reactions involving the formation of metallacyclobutanes.Reaction of atomic cobalt ions with 1-pentene yields Co(C2H4)⁺.Loss of C3H6 in this process exhibits a bimodal kinetic energy release distribution.The low-energy portion can be modeled using statistical phase space theory by assuming that propylene is eliminated.The high-energy portion of the distribution is similar to that observed for the decarbonylation of cyclobutanone by Co⁺ to yield Co(CO)⁺.It is inferred for both systems that cyclopropane elimination is being observed with a tight transition state and a reverse activation energy.The characteristically broad kinetic energy release distributions cannot be described by statistical theories.Similar results are observed with Fe⁺ as a reactant.In this case, however, the reaction with 1-pentene leads to a broadened rather than a bimodal distribution.These arguments are substantiated using product distributions measured in collision-induced dissociation studies of various adducts which might have structures analogous to those invoked for the reactions of Co⁺ and Fe⁺ with cyclobutanone.Metastable loss of CO is also observed in these reactions.Fitting the statistical phase space theory to the measured distribution yields a heat of formation for the cobaltacyclobutane ion of 274+/- 5kcal/mol.The heat of formation (0 K) of the ferracyclobutane ion is less well determined but is approximately 268 kcal/mol.These are substantially higher (by 27 and 16 kcal/mol, respectively) than those for the corresponding isomeric propylene complexes.From these measurements, we estimate strain energies of cobaltacyclobutane and ferracyclobutane to be 22 and 18 kcal/mol, respectively, to be compared with 26 kcal/mol for cyclobutane.