Novel photo- and/or hydrochromic organometallics derived from methyl viologen cations

Abstract

By the reaction of TM(NO3)2 (TM = Co, Ni, Cd) with 1,1′-bis(methyl)-4,4′-bipyridinium dibromide (MV·2Br) and 1,4-benzenedicarboxylic acid (p-H2BDC), three novel organometallic compounds have been obtained, (MV)[Co(H2O)6](p-BDC)2·4H2O (1), (MV)[Ni(H2O)6](p-BDC)2·4H2O (2), {(MV)[Cd(p-BDC)(Br)2]}n (3). Compounds 1 and 2 are isostructural ionic salts made up of one MV2+ cation, one [TM(H2O)6]2+ cation, two uncoordinated p-BDC2− anions, and four crystallization water molecules, and linked by extensive hydrogen bonds to afford the 3D networks; Compound 3 is constituted by MV2+ cations and zigzag chains of Cd(II) ions bridged by the deprotonated p-BDC2- species through four carboxylate oxygen atoms to form a 1D coordination polymer. Compound 3 exhibits electron-transfer photochromism similar to other viologen-based materials, while 1 and 2 are photoinert. However, the thermal stimulation not only makes compounds 1 and 2 exhibit hydrochromic behaviors by means of dehydration and rehydration of the crystals, but also can induce a phase transition from photochemically inert to active states, demonstrating a positive role in improving photoinduced electron transfer.