Kinetics and mechanism of N2 hydrogenation in bis(cyclopentadienyl) zirconium complexes and dinitrogen functionalization by 1,2-addition of a saturated C-H bond

Wesley H. Bernskoetter, Emil Lobkovsky, Paul J. Chirik

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Abstract

The rates of hydrogenation of the N2 ligand in the side-on bound dinitrogen compounds, [(η5-C5Me4H) 2Zr]222, η2-N2) and [(η5-C5Me 5)(η5-C5H2-1,2-Me 2-4-R)Zr]222, η2-N2) (R = Me, Ph), to afford the corresponding hydrido zirconocene diazenido complexes have been measured by electronic spectroscopy. Determination of the rate law for the hydrogenation of [(η5-C5Me5)(η5-C 5H2-1,2,4-Me3)Zr]22, η22-N2) establishes an overall second-order reaction, first order with respect to each reagent. These data, in combination with a normal, primary kinetic isotope effect of 2.2(1) for H 2 versus D2 addition, establish the first H2 addition as the rate-determining step in N2 hydrogenation. Kinetic isotope effects of similar direction and magnitude have also been measured for hydrogenation (deuteration) of the two other zirconocene dinitrogen complexes. Measuring the rate constants for the hydrogenation of [(η5-C 5Me5)(η5-C5H2-1,2,4- Me3)Zr]222, η2-N2) over a 40 °C temperature range provided activation parameters of ΔH = 8.4(8) kcal/mol and ΔS = -33(4) eu. The entropy of activation is consistent with an ordered four-centered transition structure, where H2 undergoes formal 1,2-addition to a zirconium-nitrogen bond with considerable multiple bond character. Support for this hypothesis stems from the observation of N2 functionalization by C-H activation of a cyclopentadienyl methyl substituent in the mixed ring dinitrogen complexes, [(η5- C5Me5)(η5-C5H 2-1,2-Me2-4-R)Zr]22, η22-N2) (R = Me, Ph), to afford cyclometalated zirconocene diazenido derivatives.

Original languageEnglish (US)
Pages (from-to)14051-14061
Number of pages11
JournalJournal of the American Chemical Society
Volume127
Issue number40
DOIs
StatePublished - Oct 12 2005
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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