Dihydroazaborolyl complexes. 21. Syntheses and investigations of bis(1-tert-butyldihydro-2-methyl-1H-azaborolyl)beryllium and (1-tert-butyldihydro-2-methyl-1H-azaborolyl)cyclopentadienyl beryllium: a contribution to the beryllocene problem.
(1-tert-Butyldihydro-2-methyl-1H-azaborolyl)lithium (AbLi) and BeCl2 react to give the beryllocene analog Ab2Be. This complex has been characterized by a single-crystal x-ray diffraction study at -150 Deg. The mol. shows one h1- and one h5-coordinated Ab ring and so far agrees with the low-temp. structure of Cp2Be (Cp = cyclopentadienyl), but without a disorder of the Be atom. In soln. Ab2Be gives only NMR signals for h5-coordinated rings. The prochirality of Ab causes two diastereoisomers of Ab2Be with equiv. or different ring sites coordinated to Be. The isomers can be distinguished by 1H NMR spectroscopy. On dissolving one isomer in benzene, the other one is formed again within a few days. This suggests a dissocn. of one ring enabling the flip. More details on the structural and fluxional behavior of Be sandwiches can be learned from the mixed-complex AbCpBe that is easily prepd. from Ab2Be and Cp2Be. AbCpBe is, in contrast to Ab2Be and Cp2Be, stable in THF soln. The soln. shows a remarkable cond. in accord with the ring flip, obsd. in Ab2Be soln. As can be seen from NMR measurements, the h5-coordinated Cp ring dissocs., whereas the h1-bonded Ab ring remains bonded to Be. Temp.-dependent 1H NMR studies show that with increasing temps. the Ab ring becomes more and more h5-coordinated; i.e., the fluxionality becomes similar to that in Ab2Be at room temp.
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