Bromine nuclear quadrupole resonance studies of some hexabromostannates: X-ray crystal structure of pyridinium hexabromostannate(IV), [Hpy]2[SnBr6]

Abstract

Bromine n.q.r. measurements have shown that the anion in [NHMe₃]₂[SnBr₆] has a regular octahedral structure in the temperature range 77–300 K, and that [SMe₃]₂[SnBr₆] may well have a similar structure at room temperature, whereas [NMe₄]₂[SnBr₆] undergoes a phase transition to a less regular structure between 178 and 77 K. No signals were obtained from the hexa-bromostannates(IV) of the cations [NEt₄]⁺, [NPrⁿ₄]⁺, or [IPh₂]⁺. A variable-temperature study of the corresponding pyridinium compound in the range 77–335 K has shown that one of the three bromine signals (⁷⁹Br or ⁸¹Br) is at a much lower frequency than the other two, probably due to hydrogen bonding. The only evidence of a possible phase transition was a cusp at 296 ± 1 K in the temperature-dependence plot for one of the ⁸¹Br resonances. The crystals of this compound are monoclinic at room temperature, with a= 13.02(1), b= 8.66(1), c= 8.25(1)Å, β= 96(1)°, Z= 2, and space group C2. The structure has been refined to R= 0.041 for 926 diffractometer-measured intensities for which F > 4σ(F). Three significantly different Sn–Br distances of 2.624(5), 2.582(1), and 2.559(6)Å were found, in agreement with the n.q.r. results where the three signals at 300 K are in a 3:2:1 intensity ratio. The relationship between Sn–Br bond lengths and bromine n.q.r. frequencies in this and other hexabromostannates is discussed.