Crystal structures of complexes between alkali-metal salts and cyclic polyethers. 12. The complex between sodium bromide and 2,3-dimethoxybenzo-15-crown-5 {bromo(2,3-dimethoxy-6,7,9,10,12,13,15,16-octahydrobenzo[1,4,7,10,13]pentaoxacyclopentadecene)sodium}