Dynamics of intramolecular hydrogen exchange in cyclic organohydroborate–zirconocene complexes

Abstract

ABSTRACT: Proton and 11B NMR investigations of two zirconocene boracycloalkane derivatives A and B, as a function of temperature, identified two hydrogens bridged between boron and zirconium and a third attached only to zirconium. Observation of the 11B decoupled proton NMR lineshape of A and B (toluene-d8 solution) revealed a fast intramolecular mutual exchange of the two bridged hydrogens and a slower process which exchanges each of the latter with the single hydrogen bonded to zirconium alone. Comparison of the observed 11B decoupled proton NMR spectra with calculated lineshapes which take account of the exchange processes yielded the corresponding rates and activation parameters. Proton NMR data for A and B (with 11B coupled) indicate spin coupling between 11B and the bridge hydrogens of ca. 55 Hz, subject to averaging by 11B nuclear electric quadrupole induced relaxation; its rate increases with decreasing temperature. Thus, the coupling is averaged at the lower temperatures due to 11B relaxation and on warming becomes obscured as a result of hydrogen exchange. Analysis of these latter lineshapes taking account of exchange dynamics, 11B relaxation and the 11B proton coupling constant provided dynamic parameters closely similar to those obtained from the 11B decoupled proton NMR data. At elevated temperature (50°C), broadening of the Cp proton resonance indicates that these protons undergo slow exchange with the three hydrido hydrogens. © 1998 John Wiley & Sons, Ltd.