Strong Hydrogen Bonds of Coordinated Ammonia Molecules

Abstract

The hydrogen bonds of noncoordinated (NH/O) and coordinated ammonia (MLNH/O) with water molecules were studied by analyzing data in the Cambridge Structural Database (CSD) and by DFT calculations. The data from the CSD on the distribution of hydrogen bond dHO distances of the coordinated ammonia show a peak in the range of 2.0−2.2 Å with a significant number of hydrogen bonds in the range of 1.8−2.0 Å. Analysis of Hirshfeld surfaces showed that coordinated NH3 molecules are involved in numerous noncovalent contacts. The DFT calculations were performed on linear complexes of silver(I), square-planar complexes of platinum(II), tetrahedral complexes of zinc(II), and octahedral complexes of cobalt(III) by varying the charge of the complexes. The calculated data show that coordinated ammonia has stronger hydrogen bonds than noncoordinated ammonia, even for neutral complexes. The hydrogen bond energy of noncoordinated ammonia is −2.3 kcal/mol, while for coordinated ammonia, attractive interactions are in the range of −3.7 to −25.0 kcal/mol, depending on the metal ion and charge of the complex. The interaction energies for metal complexes from neutral to charged species are for the linear silver(I) complex from −6.0 to −10.7 kcal/mol, while for the square planar complex, interactions span from −5.9 to −19.9 kcal/mol. The tetrahedral zinc(II) complexes have interaction energy from −5.5 to −17.5 kcal/mol, while for the octahedral cobalt(III) complex, attractive interaction energies are from −3.7 to −25.0 kcal/mol. With the increasing charge of the metal complex, the hydrogen bond between coordinated ammonia and free water becomes stronger, and in accordance with that, the dHO distance becomes shorter. The bifurcated interaction is stronger than monofurcated for all complexes. The interaction energies correspond well with the electrostatic potential (Vs) values on interacting hydrogen atoms; the more positive Vs values on hydrogen atoms lead to stronger interaction. The hydrogen bond between ammonia and water molecules (−2.3 kcal/mol) is quite weak in comparison to the water/water hydrogen bond; it is 50% of the water/water hydrogen bond (−4.84 kcal/mol). Although the hydrogen bonds of coordinated ammonia are also weaker than hydrogen bonds of coordinated water molecules, the difference is smaller, indicating the importance of the coordination on the strength of hydrogen bonds.