Crystal and molecular structures of three trimethylamine-boron halide adducts: (CH₃)₃NBCl₃, (CH₃)₃NBBr₃, and (CH₃)₃NBI₃

Abstract

X-ray diffraction data from single crystals of the trimethylamine complexes of the three boron halides, BCl₃, BBr₃, and BI₃, lead to aP2₁/m monoclinic cell containing two molecules for each complex. The unit cell dimensions area = 6·68(1),b = 10·247(3),c = 6·502(6) Å, β =116·2(1)° (chloro);a = 6·86(1),b = 10·612(4),c = 6·737(6) Å, β = 115·8(1)° (bromo);a = 6·92(2),b = 10·86(1),c = 7·147(6) Å, β = 93·9(1)° (iodo). The structures were solved by three-dimensional sharpened Patterson functions and show only the chloro and bromo compounds to be isomorphous. Refinement of 662,718 and 954 observed reflexions for the chloro, bromo and iodo complexes, respectively, using anisotropic thermal parameters yielded conventionalR factors of 0·045, 0·087 and 0·054.

The molecules are shown to possess a B—N dative bond, a staggered conformation, and effective 3m (C _3v) symmetry. Average C—N bond lengths are 1·52(1) Å for all three complexes. Boronhalogen bond lengths average 1·864(4), 2·04(2) and 2·28(2) Å, while B—N bond distances are 1·609(6), 1·60(2) and 1·58(3) Å, respectively, for the chloro through iodo compounds. Bond angles are approximately tetrahedral with the C—N—C angle decreasing by several degrees in the Cl → Br → I series.