Single-Crystal-to-Single-Crystal Cluster Transformation in a Microporous Molybdoarsenate(V)-Metalorganic Framework

Crystal-to-crystal polymerisation of monosubstituted [PW11O39Cu(H2O)]5- Keggin-type anions

The reaction between neutral bis(picolinate)copper(II) complexes and copper(II)-monosubstituted Keggin-type phosphotungstate anions formed in situ leads to the formation of the hybrid [C(NH2)3]10[{PW11O39Cu(H2O)}2{Cu(pic)2}]·10H2O compound (1, pic = picolinate) in the presence of structure-directing guanidinium cations. Single-crystal X-ray diffraction studies demonstrate that 1 contains dimeric {PW11O39Cu(H2O)}2{Cu(pic)2} molecular species constituted by two Keggin-type anions linked by one {Cu(pic)2} octahedral complex through axial coordination to their terminal oxygen atoms. The extensive hydrogen-bonding network established by guanidium cations and Keggin clusters plays a key role in retaining the crystallinity of the system throughout dehydration to allow a single-crystal-to-single-crystal (SCSC) transformation into the anhydrous [C(NH2)3]10[{PW11O39Cu}2{Cu(pic)2}] (2a) at 170 °C. Structural modifications involve the re-orientation, shifting in ca. 1.5 Å and condensation of all the {PW11O39Cu} units to result in {PW11O39Cu}n chains in an unprecedented solid-state polymerisation. This phase transition also implies the cleavage of Cu-O bonds induced by the rotation and translation of Keggin-type anions, in such a way that hybrid dimeric units in 1 are dismantled and {Cu(pic)2} complexes become square planar. The irreversibility of the phase transition has been confirmed by combined thermal and diffractometric analyses, which evidence that the anhydrous phase adsorbs only one water molecule per cluster to become the [C(NH2)3]10[{PW11O39Cu}2{Cu(pic)2}]·2H2O (2h) hydrated derivative without any significant alteration in its cell parameters, nor in its crystalline structure. Phase transformations have been monitored by electron paramagnetic resonance spectroscopy.

Consecutive Single-Crystal-to-Single-Crystal Isomerization of Novel Octamolybdate Anions within a Microporous Hybrid Framework with Robust Water Sorption Properties

The 3D hybrid framework [{Cu(cyclam)}3 (κ-Mo8 O27 )] ⋅ 14H2 O (1) (cyclam=1,4,8,11-tetraazacyclotetradecane) undergoes sequential single-crystal-to-single-crystal transformations upon heating to afford two different anhydrous phases (2 a and 3 a). These transitions modify the framework dimensionality and enable the isomerization of κ-octamolybdate (κ-Mo8 ) anions into λ (2 a) and μ (3 a) forms through metal migration. Hydration of 3 a involves condensation of one water molecule to the cluster to afford the γ-Mo8 isomer in 4, which dehydrates back into 3 a through the 6 a intermediate. In contrast, 2 a reversibly hydrates to form 5, exhibiting the same Mo8 cluster as that of 1. It is remarkable that three of the Mo8 clusters (κ, λ and μ) are new and that up to three different microporous phases can be isolated from 1 (2 a, 3 a, and 6 a). Water vapor sorption analyses show high recyclability and the highest uptake values for POM-based systems. The isotherms display an abrupt step at low humidity level desirable for humidity control devices or water harvesting in drylands.