A Nuclear Magnetic Resonance Study of Hydrogen Bonding in Tris(2-N-methylaminoethyl) Borate and Similar Compounds

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.