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} 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.