Enclosure of a Keggin-type heteropolyoxometalate into a tubular π-space viahydrogen bonds with a nonplanar Mo(v)-porphyrin complex forming a supramolecular assembly

Abstract

A 2 : 1 supramolecular assembly composed of a non-planar Mo(V)-porphyrin, [Mo(DPP)(O)(H₂O)]⁺ (1) (DPP²⁺; dodecaphenylporphyrin), and a Keggin-type heteropolyoxometalate (POM), α-[(n-butyl)₄N]₂[SW₁₂O₄₀] (2), was formed via hydrogen bonds. The crystal structure was determined by X-ray crystallography to clarify that the POM was enclosed into a π-space of a supramolecular porphyrin nanotube by virtue of a hydrogen-bond network. In contrast to the formation of the 2 : 1 assembly ([{Mo(DPP)(O)(H₂O)}₂(SW₁₂O₄₀)] (3)) between 1 and [SW₁₂O₄₀]²⁻ in the crystal, it was revealed that those two components form a 1 : 1 assembly in solution, in light of the results of MALDI-TOF-MS measurements in PhCN. Variable-temperature UV-vis spectroscopic titration allowed us to determine the thermodynamic parameters for the formation of the 1 : 1 supramolecular assembly in solution, the heat of formation (ΔH) and the entropy change (ΔS). These results provide the first thermodynamic data set to elucidate the formation process of supramolecuar structures emerged by hydrogen bonding between metalloporphyrin complexes and POMs, indicating that the formation of the assembly is an entropy-controlled process rather than an enthalpy-controlled one. Comparisons of the thermodynamic parameters with those of a planar Mo(V)-porphyrin complex also highlighted high Lewis acidity of the Mo(V) centre in the distorted porphyrin.