OSAMUYAMAMOTO AND KIKUKO HAYAMIZU
822
Proton Magnetic Resonance Studies of Mixed Aluminum Alkyls. I. Aluminum Trialkyls’ by Osamu Yamamoto and Kikuko Hayamizu Government Chemical Industrial Research Institute, Tokyo, Japan
(Received June 25, 1987)
The systems AlMes-AlEta, AlMe3-Al(n-Pr)a, and AIMea-Al(i-Bu)awere studied by means of pmr spectroscopy at room temperature as well as at low temperature. A bridge-bonding factor, defined as a measure of the tendency of an alkyl group to occupy the bridge position in the mixed dimer formed in the aluminum trialkyl mixture, was experimentally determined for ethyl, n-propyl, and isobutyl groups in the trialkylaluminum dimers as compared with methyl group and was found to be l/6, I/,, apd respectively. It depends upon the bulkiness of the alkyl group. In terms of the bridge-bonding factor, the chemical shifts of the a-methyl and the a-methylene groups of the mixed dimers in the mixtures were calculated and compared with the experimental values. The equilibrium between the various dimers in each system was also discussed.
Introduction It is well known that the lower members of aluminum tri-n-alkyls associate into dimers with an alkyl bridge structure such as2
R
R
\/\/ AI /\
R
R
R
496
Me
/’\
\ /Me A1 A1 / \ / \ Me
The Journal of Physical Chemistry
i-Bu
Me
i-BU
\ / \ /
R
In the pmr spectra of these compounds, for example trimethyl aluminum, although there are two nonequivalent methyl groups in this structure, only one methyl signal is observed at room temperature. At sufficiently low temperature, however, two distinct signals are obtained, which may be assigned to the terminal and the bridge methyl groups, respectively. This fact was first observed by Muller and Pritcharda and was explained as a rapid exchange between the terminal and the bridge methyl groups. It has been shown also that the same is true for other aluminum alkyls. On the other hand, the stability of aluminum trialkyl dimers would be different from one to the other, and it is expected from the results of the vapor-pressure studies by Laubengayer and Gilliama that trimethylaluminum, the lowest member of aluminum trialkyls, is the most stable one. Hoff mann’ found out from a cryoscopic study that a mixture of triisobutylaluminum and trimethylaluminum forms mixed dimer, such as i-BU
and
i-BU
This is very interesting because, in general, branched aluminum alkyls have little tendency to form dimers.’ Hoffmann further studied the system by means of nmr spectroscopy* and reported that the resultant mixed aluminum trialkyls are strongly associated by virtue of their methyl groups and the ratio of the bridge and the terminal methyl groups is dependent on the total concentration of methyl groups. He suggested from his nmr studies at room tepperature that, in contrast to his earlier cryoscopic investigation and in spite of the rather weak bonding across an isobutyl group, a system of dimers with mixed-bonded ring, >AlA1
