Crystal habit modification of Cu(ii) isonicotinate–N-oxide complexes using gel phase crystallisation

We report the crystallisation of three forms of the copper(II) isonicotinate–N-oxide complex and their phase interconversion via solvent-mediated crystal-to-crystal transformation. The different forms of the copper complex have been isolated and characterised by single crystal X-ray diffraction. Gel phase crystallisation performed in hydrogels, low molecular weight gels and gels of a tailored gelator showed crystal habit modification. Crystallisation in aqueous ethanol resulted in the concomitant formation of blue (form-I) and green (form-II/IV) crystals while the use of a low molecular weight gel resulted in the selective crystallization of the blue form-I under identical conditions. Comparison of the gel phase and the solution state crystallisation in various solvent compositions reveals that the blue form-I is the thermodynamically stable form under ambient conditions