The Structure of Water in p‐Sulfonatocalix[4]arene

Abstract

Tetrasodium p-sulfonatocalix[4]arene exists as a hydrate with approximately 14 water molecules and has three polymorphic modifications, all of which contain a water molecule in the molecular cavity that is engaged in OH···π interactions. Single-crystal neutron structures are reported for two of these three forms and reveal a "compressed" water molecule with short OH bonds. Partial atomic charges and hardness analysis (PACHA) calculations based on the neutron coordinates give an OH···π interaction energy of 6.9-7.5 kJ mol(-1). The PACHA analysis also reveals the dominance of the charge-assisted hydrogen bonds from the Na(+)-coordinated water molecules. The instability of the crystal towards dehydration can be traced to an uncoordinated lattice water site. The remarkable calixarene-Na(+)-hydrate motif is conserved almost unchanged across all three polymorphs. A single-crystal neutron structure is also reported for pentasodium p-sulfonatocalix[4]arene·12H(2)O, which exhibits an intracavity water molecule that is engaged in both OH···π and OH···O hydrogen bonding. The shorter covalent bond to the hydrogen atom that forms the interaction with the aromatic ring is again apparent.

The “True” Affinities of Metal Cations to p‐Sulfonatocalix[4]arene: A Thermodynamic Study at Neutral pH Reveals a Pitfall Due to Salt Effects in Microcalorimetry

“…[44] This displacement, and also the stoichiometric cation binding, is best rationalized through the involvement of an inclusion complex between the inorganic cations and SC4, as depicted in Scheme 2. In contrast to solid-state structures, [36,45] the precise position of the bound cation in solution (near the sulfonato rim, near the phenoxyl rim, or inside the cavity) cannot be deduced from the ITC experiments reported herein. Nevertheless, in view of recent experimental investigations by NMR spectroscopy for Cs + and Tl + [27,28] and the fact that the investigated alkali and earth alkaline metal cations also displace neutral guest molecules, which can be demonstrated by NMR spectroscopy to be positioned inside the SC4 cavity, [22] binding inside the cavity is likely to be the dominant binding mode for the investigated inorganic cations in solution.…”

“…The concentration ranges to perform the experiments were determined by taking into account the results from this work for the complexation of the counterion Na + by SC4. From the presumed binding constant and the salt content at neutral pH (5 Na + per SC4 to neutralize all sulfo groups and the hydroxyl group), [5,36] it is possible to calculate the degrees of complexation (see Figure 1).…”

Many Simple Molecular Cavitands Are Intrinsically Porous (Zero-Dimensional Pore) Materials

“…The deep position of the water molecule within the cavity-O⋅⋅⋅C(arene) intermolecular contact distances as low as 3.64 Å, and O⋅⋅⋅arene(centroid) distances as low as 3.59 Å-is suggestive of O-H⋅⋅⋅arene(π) hydrogen bonds such as those observed in related, water-occupied ca-8 lixarenes. 98 Similar behaviour has been observed for Me,H,SiMe 2 , which purportedly crystallizes from CH 2 Cl 2 as xH 2 O@Me,H,SiMe 2 , though the quality of the structure determination is poor (R 1 > 9%) and the water molecule is modelled as being disordered over two sites. 99 The above observations are mostly consistent with Cram's pioneering work concerning the solution phase binding properties of H,Me,SiR 2 cavitands.…”

The Hydrophobic Effect Revisited—Studies with Supramolecular Complexes Imply High‐Energy Water as a Noncovalent Driving Force

“…Recent structural evidence in both the solid state40 and the gas phase41 revealed the presence of only a single water molecule, bound through two OH⋅⋅⋅π hydrogen‐bonding interactions with two aryl rings, inside the bowl‐shaped cavity of CX4. These two hydrogen bonds contribute up to 15 kJ mol −1 to the complex stability, or about one half of the total binding energy of the single water molecule (37 kJ mol −1 ) 40. The remainder can be attributed to dispersive interactions with the electron‐rich cavity.…”