Crystal engineering of lattice metrics of perhalometallate salts and MOFs

The synthesis of the salt 3 and metallo-organic framework (MOF) [{(4,4^′-bipy)CoBr₂}_n] 4 by a range of solid state (mechanochemical and thermochemical) and solution methods is reported; they are isostructural with their respective chloride analogues 1 and 2. 3 and 4 can be interconverted by means of HBr elimination and absorption. Single phases of controlled composition and general formula [4,4^′-H₂bipy][CoBr_4-xCl_x] 5_x may be prepared from 2 and 4 by solid—gas reactions involving HBr or HCl respectively. Crystalline single phase samples of 5_x and [{(4,4^′-bipy)CoBr_2-xCl_x}_n] 6_x were prepared by solid-state mechanochemical routes, allowing fine control over the composition and unit cell volume of the product. Collectively these methods enable continuous variation of the unit cell dimensions of the salts [4,4^′-H₂bipy][CoBr_4-xCl_x] (5_x) and the MOFs [{(4,4^′-bipy)CoBr_2-xCl_x}_n] (6_x) by varying the bromide to chloride ratio and establish a means of controlling MOF composition and the lattice metrics, and so the physical and chemical properties that derive from it.