Cooperative Hydrogen-Bonding Effects in a Water Square: A Single-Crystal Neutron and Partial Atomic Charges and Hardness Analysis Study

Four isomorphous complexes of formula [M(L)4(H2O)2]SO4·2H2O (M = Co, 1a; Ni, 1b; Cu, 1c;
Zn, 1d) have been isolated and characterized by single-crystal X-ray diffraction and neutron diffraction
using the quasi-Laue diffractometer VIVALDI at the Institut Laue-Langevin as well as by thermogravimetric
analysis. The structures contain a discrete, strongly hydrogen-bonded water tetramer which causes a
significant distortion of the metal coordination sphere in each case. Partial atomic charges and hardness
analysis (PACHA) calculations reveal that the shortest hydrogen bonds are not the strongest in this
constrained, cyclic solid-state structure and show that the distortion at the metal center is caused by the
drive to maintain the integrity of the water tetramer. The system undergoes a disorder−order transition on
slow cooling that provides insight into the nature of communication between water squares.