Solid-state transformations of zinc 1,4-benzenedicarboxylates mediated by hydrogen-bond-forming molecules.

The zinc 1,4-benzenedicarboxylates [Zn3(bdc)3(H2O)3] . 4 DMF (1; bdc = 1,4-benzenedicarboxylate), [Zn(bdc)(H2O)] . DMF (2), and [Zn(bdc)] DMF (3) crystallise at room temperature from mixtures of toluene/ dimethylformamide (DMF) under concentrated, dilute and dry conditions, respectively. The structure of phase 1 (monoclinic: P2(1)/c, a 13.065(1), b = 9.661(1), c = 18.456(1) A, beta = 106.868(2) degrees) consists of layers containing stacks of three zinc cations linked by mono- and bidentate bdc groups. Structure 1 converts to the known phase 2 by an irreversible, reconstructive phase transformation, whereas 2 and 3 interconvert reversibly upon the loss or addition of water. Removal of all solvent molecules included during crystallisation gives poorly crystalline [Zn(bdc)] (4), which is readily converted to highly crystalline solids upon contact with hydrogen-bond-forming molecules such as water, DMF and small alcohols. The crystal structures of the mono- and dihydrates [Zn(bdc)(H2O)] (6) and [Zn(bdc)-(H2O)2] (7) have been determined ab initio from powder X-ray diffraction data (compound 6, monoclinic: C2/c, a = 17.979(1), b = 6.352(1), c = 7.257(1) A, beta=91.477(1) compound 7, monoclinic: C2/c, a = 14.992(1), b = 5.0303(2), c = 12.098(1) A, beta = 103.82(1) degrees). The methanol adduct [Zn3(bdc)3] . 6CH3OH (5) is the same as that prepared previously by direct crystallisation. Comparison of these adduct structures with those prepared directly reveal that they are formed by in situ recrystallisations. Subsequent removal of included molecules gives amorphous [Zn(bdc)], which can be recrystallised again when placed in contact with hydrogen-bond-forming molecules.