‘Ionic crystals’ consisting of trinuclear macrocations and polyoxometalate anions exhibiting single crystal to single crystal transformation: breathing of crystals

Abstract

Ion pairing of trinuclear macrocation cluster (known as basic carboxylate), [M 3(μ 3-O)(ClCH2COO)6(H2O)3]1+ and a Keggin type polyoxometalate cluster anion [SiW12O40]4- is stabilized with a number of crystal water molecules in composite type compounds [M 3(μ 3-O)(ClCH2COO)6(H2O)3]4 [SiW 12O 40] ·xH2O·2ClCH2COOH [M=Fe3+,x=18(1);M=Cr3+x=14(2)]. When the crystals of 1 are heated at 85∘C and 135∘C for 3.5 hours in an open atmospheric condition, it goes to [Fe 3(μ 3-O)(ClCH2COO)6(H2O)3]4[SiW 12O 40] · 10 H 2O · 2ClCH 2COOH (dehydrated1-85o≡ 1′), and [Fe 3(μ 3-O)(ClCH 2COO) 6(H2O)3]4[SiW 12O 40] ·8H2O ·2ClCH2COOH (dehydrated 1-135o≡ 1″) respectively with the loss of considerable amount of lattice water molecules retaining their single crystallinity. On the other hand, the single crystals of compound 2, upon heating at 85∘C or 135∘C for 3.5 hours, undergo ‘crystal-to-crystal transformation’ to the single crystals of [Cr 3(μ 3-O)(ClCH2COO)6(H2O)3]4[SiW 12O 40]· 8 H 2O · 2 ClCH 2COOH (dehydrated2. Crystal structure analyses show that the parent compounds 1 and 2 undergo molecular rearrangement (molecular motion in the solid state) in respective dehydrated compounds. Remarkably, these dehydrated crystals (1′, 1″ and 2′), upon exposure to water vapor at an ambient condition, regenerate the crystals of parent compounds 1 and 2, respectively. Graphical Abstract:: Breathing of crystal: The unit cell volume (17106 Å3) of [Fe 3(μ 3-O)(ClCH2COO)6(H2O)3]4[SiW12O40]·18H2O·2ClCH2COOH (1) shrinks on heating 1 at 85∘C to produce the reduced unit cell volume (15084 Å3) of [Fe 3(μ 3-O)(ClCH2COO)6(H2O)3]4[SiW12O40]·10H2O·2ClCH2COOH (dehydrated 1-85o), represented as (a) → (b). This can be reversed, (b) → (a), in the solid state on re-hydration of dehydrated 1-85o resulting in the formation of parent 1. [Figure not available: see fulltext.].