Towards polymorphism control in coordination networks and metallo-organic salts

[{(4,4′-bipy)ZnCl₂}_n] (4) is known in three different polymorphs, in the space groups C2/c (4a), Pnma (4b) and Pban (4c). Solution synthesis produces a mixture of 4a and 4b, but solid-state synthesis, either by grinding together 4,4′-bipy and ZnCl₂ or by heating [4,4′-H₂bipy][ZnCl₄] 3 to thermally eliminate HCl, generates only the 4b form, demonstrating that solid-state synthesis can exert control over polymorph formation; other solid-state preparations of 4 such as reaction of basic zinc carbonate with [4,4′-H₂bipy]Cl ₂ or reaction of [4,4′-H₂bipy][ZnCl₄] 3 with KOH also increase the amount of 4b generated relative to the solution synthesis. It is also possible to form the mixed-metal phases [{(4,4′-bipy)Co_1-xZn_xCl₂}_n] 4_x as effectively homogeneous solid-solutions by the same techniques, and when x < 0.5 (i.e. there is more cobalt present than zinc) then only the Pban polymorph of the solid-solution is observed. When x > 0.5, the amount of this polymorph is still greater than would be expected for a predominantly zinc-containing phase.