Tris(2-aminoethyl) borate, B(OCH2CH2NH2)3, and the analogous N-methylamino and N,N-dimethylamino compounds have been prepared by the transesterification of methyl borate with the appropriate 2-aminoethanol. The nuclear magnetic resonance spectra show that extensive association of the terminal amino groups occurs in pure tris(N-methylaminoethyl) borate and that this association can be broken apart by heating to 160° or by dissolution in polar organic solvents such as triethylamine or acetonitrile. In order to determine whether the association results from hydrogen bonding or internal B-N coordination, several model systems have been investigated. The spectra of NH2C5H5-BF3, piperidine-BF3, and (C6H5)BO-CH2CH2NH2 in acetonitrile contain very complicated NCH2 peaks and broad NH peaks which appear at low applied magnetic field (r 5.26-5.56). The broadening of the NH peak in the boron-nitrogen adducts is attributed to the effect of the N14 quadrupole, whereas the complex splitting of the NCH2 multiplet is attributed to coupling with B11 in the dative bond with nitrogen and possibly with the nitrogen protons. The spectra of B(OCH2CH2NH2)3 and B(OCH2CH2NHCH3)3, on the other hand, show sharp NH peaks at somewhat higher applied magnetic fields (r 7.84, 6.78) and contain two sharp triplets attributed to the two sets of methylene protons in the -O-CH2-CH2-N˂ units (toch2 6.26-6.55; tnch2 7.23-7.57). The N-H peak of B(OCH2CH2NHCH3)3 appears as a sharp singlet in acetonitrile solutions owing to rapid exchange of the amine proton. Evidence is presented that the exchange is catalyzed by a trace (˂8.7 ppm) of water in the hygroscopic solvent, acetonitrile, even after rigorous drying. All of the data indicate that intermolecular hydrogen bonding strongly predominates over N→B dative bonding in the association of B(OCH2CH2NHCH3)3.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry