Ion pairing of trinuclear macrocation cluster (known as basic carboxylate), [ $$\hbox {M}_{3}(\upmu _{3}$$ M 3 ( μ 3 -O) $$(\hbox {ClCH}_{2}\hbox {COO})_{6}(\hbox {H}_{2}\hbox {O})_{3}]^{1+}$$ ( ClCH 2 COO ) 6 ( H 2 O ) 3 ] 1 + and a Keggin type polyoxometalate cluster anion $$[\hbox {SiW}_{12}\hbox {O}_{40}]^{4-}$$ [ SiW 12 O 40 ] 4 - is stabilized with a number of crystal water molecules in composite type compounds [ $$\hbox {M}_{3}(\upmu _{3}$$ M 3 ( μ 3 -O) $$(\hbox {ClCH}_{2}\hbox {COO})_{6}(\hbox {H}_{2}\hbox {O})_{3}] _{4}$$ ( ClCH 2 COO ) 6 ( H 2 O ) 3 ] 4 [ $$\hbox {SiW}_{12}\hbox {O}_{40}$$ SiW 12 O 40 ] $$\cdot \hbox {xH}_{2}\hbox {O}\, \cdot \hbox {2ClCH}_{2}\hbox {COOH}$$ · xH 2 O · 2ClCH 2 COOH [ $$\hbox {M}=\hbox {Fe}^{3+}, \,\,\hbox {x} = 18 (\mathbf{1}); \,\,\hbox {M} = \hbox {Cr}^{3+} \,\,\hbox {x} = 14 (\mathbf{2})$$ M = Fe 3 + , x = 18 ( 1 ) ; M = Cr 3 + x = 14 ( 2 ) ]. When the crystals of 1 are heated at 85 $${^{\circ }}$$ ∘ C and 135 $${^{\circ }}$$ ∘ C for 3.5 hours in an open atmospheric condition, it goes to [ $$\hbox {Fe}_{3}(\upmu _{3}$$ Fe 3 ( μ 3 -O) $$(\hbox {ClCH}_{2}\hbox {COO})_{6}(\hbox {H}_{2}\hbox {O}) _{3}]_{4}$$ ( ClCH 2 COO ) 6 ( H 2 O ) 3 ] 4 [ $$\hbox {SiW}_{12}\hbox {O}_{40}$$ SiW 12 O 40 ] $$\cdot 10\hbox {H}_{2}$$ · 10 H 2 O $$\cdot \hbox {2ClCH}_{2}$$ · 2ClCH 2 COOH (dehydrated1- $${\mathbf{85}}^{\mathbf{o}}\equiv $$ 85 o ≡ $$\mathbf{1}^{\prime }$$ 1 ′ ), and [ $$\hbox {Fe}_{3}(\upmu _{3}$$ Fe 3 ( μ 3 -O)( $$\hbox {ClCH}_{2}\hbox {COO})_{6}$$ ClCH 2 COO ) 6 ( $$\hbox {H}_{2}\hbox {O}) _{3}]_{4}$$ H 2 O ) 3 ] 4 [ $$\hbox {SiW}_{12}\hbox {O}_{40}$$ SiW 12 O 40 ] $$\cdot \,\, 8\hbox {H}_{2}$$ · 8 H 2 O $$\cdot \,\,2\hbox {ClCH}_{2}$$ · 2 ClCH 2 COOH (dehydrated 1- $$\mathbf{135}^{\mathbf{o}}\equiv $$ 135 o ≡ $$\mathbf{1}^{\prime \prime }$$ 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 $${^{\circ }}$$ ∘ C or 135 $${^{\circ }}$$ ∘ C for 3.5 hours, undergo ‘crystal-to-crystal transformation’ to the single crystals of [ $$\hbox {Cr}_{3}(\upmu _{3}$$ Cr 3 ( μ 3 -O)( $$\hbox {ClCH}_{2}\hbox {COO})_{6} (\hbox {H}_{2}\hbox {O})_{3}]_{4}$$ ClCH 2 COO ) 6 ( H 2 O ) 3 ] 4 [ $$\hbox {SiW}_{12}\hbox {O}_{40}$$ SiW 12 O 40 ] $$\cdot 8\hbox {H}_{2}\hbox {O}\cdot 2\hbox {ClCH}_{2}$$ · 8 H 2 O · 2 ClCH 2 COOH ( $$\mathbf{dehydrated\, 2}\equiv \mathbf{2}^{\prime }$$ dehydrated 2 ≡ 2 ′ ). 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 ( $$\mathbf{1}^{\prime }, \mathbf{1}^{\prime \prime }$$ 1 ′ , 1 ″ and $$\mathbf{2}^{\prime }$$ 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 Å $$^{3})$$ 3 ) of [ $$\hbox {Fe}_{3}(\upmu _{3}$$ Fe 3 ( μ 3 -O)( $$\hbox {ClCH}_{2}\hbox {COO})_{6}(\hbox {H}_{2}\hbox {O})_{3}] _{4}[\hbox {SiW}_{12}\hbox {O}_{40}$$ ClCH 2 COO ) 6 ( H 2 O ) 3 ] 4 [ SiW 12 O 40 ] $$\cdot 18\hbox {H}_{2}\hbox {O}\cdot \,\,2\hbox {ClCH}_{2}$$ · 18 H 2 O · 2 ClCH 2 COOH (1) shrinks on heating 1 at 85 $${^{\circ }}$$ ∘ C to produce the reduced unit cell volume (15084 Å $$^{3}$$ 3 ) of [ $$\hbox {Fe}_{3}(\upmu _{3}$$ Fe 3 ( μ 3 -O)( $$\hbox {ClCH}_{2}\hbox {COO})_{6}(\hbox {H}_{2}\hbox {O})_{3}] _{4}[\hbox {SiW}_{12}\hbox {O}_{40}$$ ClCH 2 COO ) 6 ( H 2 O ) 3 ] 4 [ SiW 12 O 40 ] $$\cdot 10\hbox {H}_{2}\hbox {O}\cdot \,\,2\hbox {ClCH}_{2}$$ · 10 H 2 O · 2 ClCH 2 COOH (dehydrated 1- $${\mathbf{85}}^{\mathbf{o}}$$ 85 o ), represented as (a) $$\rightarrow $$ → (b). This can be reversed, (b) $$\rightarrow $$ → (a), in the solid state on re-hydration of dehydrated 1- $${\mathbf{85}}^{\mathbf{o}}$$ 85 o resulting in the formation of parent 1.