In a series of donor-acceptor adducts of amines to triorganylboroxines (Tables 1,2) it has been shown by 'H and "B NMR spectroscopy that in solution the amine undergoes a temperature dependent fluctuation between the boron atoms of the boroxine ring. In the solid state, as determined by X-ray structural analysis of two selected 3 : 2 and 1 : 2 adducts (3 and 6) (boron-nitrogen ratios of 1 : 1 and 3 : 1, respectively), only one boron atom of each boroxine ring is involved in adduct formation.
Die Struktur von Amin-Addukten von Triorganylboroxinen
In einer Reihe von Donor-Acceptor-Addukten von Aminen mit Triorganylboroxinen (Tabb. 1, 2) wird gezeigt, da8 in Losung die Aminmolekule zwischen den drei Boratomen des Boroxin-Rings fluktuieren. Im festen Zustand, wie es aus den Rontgenstrukturanalysen von zwei ausgewahlten 3 : 2und 1 : I-Addukten (3 und 6) (mit Bor-Stickstoff-Verhaltnissen von 1 : 1 bzw. 3 : 1) hervorgeht, ist nur ein Bor-Atom des Boroxin-Rings an der Adduktbildung beteiligt.
Simple and cyclic esters of monoorganylboronic acids act as weak Lewis acids and usually do not form donor-acceptor complexes with amines. We have previously shown that the boron atoms in 1 act as strong acceptors for amines and that adducts with up to four amines per molecule of 1, i.e., a boron to nitrogen ratio of 1 : 1, can be formed'). This complex forming ability is similar to that of the triorganylboroxines which also readily form adducts having the general formula (RBO), . NR; (2). In all but one of these the amine to boroxine ratio is 1 : 3 and may formally be regarded as being uniformly bonded to the three boron atoms of the boroxine ring. The only exception is