Crystal-to-Crystal Isomerization via Drastic Intramolecular Ligand Exchange: Vapochromism of a Bis(arenethiolato)cobalt(II) Complex Containing Bulky Acylamino Groups

Crystal-to-crystal transitions normally avoid drastic configurational, changes that result in loss of crystallinity. A cobalt(II) complex, containing two unsymmetrically disubstituted arenethiolato ligands with bulky acylamino (Ar3CCONH, Ar = 4-t-BuC6H4) and t-BuCONH groups, showed crystal-to-crystal configurational isomerization accompanied by vapochromism. Recrystallization using THF/n-hexane afforded green crystals (isomer G) containing one n-hexane molecule per asymmetric unit. In contrast, blue crystals (isomer B), containing two toluene molecules per unit, were obtained using a toluene solution. Isomer G contained S,O-chelates involving Ar3CCO carbonyl groups, while isomer B contained chelates involving t-BuCO groups. Upon exposure to toluene vapor, crystal isomer G was gradually converted to crystal isomer B, maintaining its crystallinity despite the drastic intramolecular ligand exchange. Although the blue coloration was due to an unfavorable, distorted, and stretched chemical structure, formation of strong intermolecular NH center dot center dot center dot O=C hydrogen bond chains counteracted this disadvantage. Therefore, blue crystal formation as a whole was thermodynamically favorable. The release of n-hexane from the green crystals initiated the isomerization and resulted in tightly packed blue crystals. The tense conformation of the blue crystals was facilely relaxed by cleaving the hydrogen bond chains through grinding and resulted in a green powder that maintained the original coordination of isomer B, which reverted easily to blue crystals upon exposure to toluene vapor.