Steric Effect of a Capping Ligand on the Formation of Supramolecular Coordination Networks of Ni(II): Solid-State Entrapment of Cyclic Water Dimer

Abstract

Supramolecular dimer of water is the simplest of the small water clusters [(H 2 O) n , n = 2–10]. During the course of our work on supramolecular coordination networks of three-component systems (divalent metal ion, tridentate capping ligand, and ditopic carboxylate linker), a cyclic water dimer is found to be entrapped in the network of [Ni 2 (6-Mebpta) 2 (adc) 2 ]·2H 2 O ( 1 ) (6-Mebpta = 2-methyl- N -((6-methylpyridin-2-yl)methyl)- N -(pyridin-2-ylmethyl)propan-2-amine and adc = acetylenedicarboxylate). Based on the single-crystal structure of 1 , the water dimer plays an important role in connecting the bis(adc) bridged dinickel synthons to form a one-dimensional (1D) supramolecular network. To emphasize the role of 6-Mebpta in the judicious choice of components for 1 , one simple modification to it by having another methyl group in the second pendant pyridyl group to make 6,6′-Me 2 bpta (2-methyl- N , N -bis((6-methylpyridin-2-yl)methyl)propan-2-amine) did not allow the formation of any water cluster in [Ni(6,6′-Me 2 bpta)(adc)(H 2 O)]·H 2 O ( 2 ), where a different coordination environment around Ni(II) is also observed. Further quantification of the difference in supramolecular interactions observed in 1 and 2 has been assessed by Hirshfeld surface analysis. Both 1 and 2 are obtained in good yields at room temperature (methanol as solvent) and are further characterized by elemental analysis, Fourier transform infrared (FTIR) and Raman spectroscopy, powder X-ray diffraction, and thermogravimetric analysis.

Assessing the Experimental Hydrogen Bonding Energy of the Cyclic Water Dimer Transition State with a Cyclooctatetraene-Based Molecular Balance

“…Although cyclic arrangements of molecules interacting via hydrogen bonding are a common sight, , side-on OH hydrogen bonding interactions, where the oxygen and hydrogen atoms resemble vertices of a trapezoid (Figure , bottom), are far less common. Although such interactions have been observed in organic crystals, − few examples are known, where said interactions alone provide stabilization of a molecular aggregate. This is due to the lower HBE ,,, of the interaction in comparison to a trans-linear hydrogen bond; hence, an ensemble of water molecules interacting via hydrogen bonding will preferably arrange in a way so that the amount of trans-linear interactions is maximized.…”