Selective signalling of alcohols by a molecular lattice and mechanism of single-crystal-to-single-crystal transformations

Single-crystal-to-single-crystal (SCSC) transformations of molecular materials involving exchange of lattice molecules are becoming commonplace and very relevant in areas like chemical sensing or the pharmaceutical sector. Spin crossover (SCO) complexes could be great candidates to act as molecular chemical sensors using spin switching to signal detection. We describe here the capacity of the Fe(ii) molecular material [Fe(bpp)(H2L)](ClO4)(2)center dot C3H6O (bpp and H2L are 2,6-bis-(pyrazol-3-yl)-pyridine type ligands) to have its lattice acetone molecules replaced by certain selected alcohols from the gas phase (MeOH, EtOH or(n)PrOH but not(i)PrOH), signalling the process by a spin transition that also changes the colour of the crystals. The magnetic response of the signalling complex depends on the chain length of the alcohol, allowing selective detection. As these molecular exchanges are SCSC processes, the structures of the alcoholates obtained have been determined by single crystal X-ray diffraction (SCXRD). The removal ofn-propanol from its host lattice has been quenchedin operandoat various intermediate stages and studied by SCXRD to unveil crucial details of the mechanism of this SCSC transformation.