Modeling Anhydrous and Aqua Copper(II) Amino Acid Complexes:  A New Molecular Mechanics Force Field Parametrization Based on Quantum Chemical Studies and Experimental Crystal Data

Abstract

This paper presents the vacuum structures of aquacopper(II) bis(amino acid) complexes with glycine, sarcosine, N,N-dimethylglycine, and N-tert-butyl-N-methylglycine estimated using the B3LYP method. The differences between the B3LYP vacuum structures and experimental crystal structures suggested considerable influence of crystal lattice packing effects on the changes in the complexes' geometries. A previously developed molecular mechanics force field for modeling anhydrous copper(II) amino acidates was reoptimized to simulate these changes and predict the properties of both trans and cis anhydrous and aqua copper(II) amino acid complexes. The modeling included experimental molecular and crystal structures of 13 anhydrous and 10 aqua copper(II) amino acidates with the same atom types (Cu(II), C, H, N, and O) but various copper(II) coordination polyhedron geometries, crystal symmetries, and intermolecular interactions. The empirical parameters of the selected potential energy functions were optimized on the B3LYP vacuum copper(II) coordination geometries of three anhydrous copper(II) amino acidates and on experimental crystalline internal coordinates and unit cell dimensions of six anhydrous and six aqua copper(II) amino acid complexes. The respective equilibrium structures were calculated in vacuo and in simulated crystalline environment. The efficacy of the final force field, FFW, was examined. The total root-mean-square deviations between the experimental and theoretical crystal values were 0.018 A in the bond lengths, 2.2 degrees in the valence angles, 5.5 degrees in the torsion angles, and 0.395 A in the unit cell lengths. FFW reproduced the unit cell volumes in the range from -8.1 to 9.6%. The means of Cu to axial water oxygen distances were 2.4 +/- 0.1 A (experiment) and 2.6 +/- 0.1 A (FFW). This paper describes the ability of the molecular mechanics model and FFW force field to simulate the flexibility of the metal coordination polyhedron. The new force field proved effective in predicting the most stable molecular conformation of copper(II) amino acidato systems in vacuo.

Hydration of Copper(II): New Insights from Density Functional Theory and the COSMO Solvation Model

“…A key step was to locate the low-energy conformers for each coordination number (4-6) and cluster size (n ) [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] hydrogen bond between water molecules on the periphery of a Cu(II)-water complex is stronger than the energy of water binding to the vacant axial sites on Cu 2+ . This much less favorable interaction of water molecules in the axial directions is due to the double occupation of the Cu 2+ 3d z 2 orbital effectively screening the positive charge on the metal ion.…”

“…[1][2][3][4][5] Detailed information on ligands arrangement around the Cu(II) ion is only available in the solid state [6][7][8][9][10][11][12][13] whereas the structural information in the aqueous phase [13][14][15][16][17][18][19][20] is less definitive and available only for few Cu 2+ complexes. Although static [20][21][22][23][24][25][26][27][28][29][30] and dynamic [31][32][33][34][35][36][37] electronic structure calculations can, in principle, provide such information, realistic modeling of Cu(II) complexes in aqueous solution with highly flexible coordination environment still remains a challenge.…”

“…Another direct consequence of the Jahn-Teller effect is the flexibility (plasticity) of the coordination geometry of Cu(II) which can adopt a variety of coordination geometries in the crystalline phase. [6][7][8][9][10][11][12][13] For example, crystal structures of Cu(II)-amino acids complexes with four-, five-, and six-coordinate geometries are all common, suggesting that energetic differences between them are small. Consistent with the Jahn-Teller effect, many of these structures include irregular or distorted coordination geometries.…”

Computational Study of Copper(II) Complexation and Hydrolysis in Aqueous Solutions Using Mixed Cluster/Continuum Models

“…We checked the accuracy of the pseudopotential treatment of Cu by comparing with the all-electron calculations employing the TZVP basis set and the diffuse function (R d ) 0.07) on Cu. 16 ] showed that the use of the ECP approximation leads to only small changes in bond lengths (root mean squared deviation of 0.004 and 0.070 Å for equatorial and axial bonds, respectively) and relative energies (0.06-0.26 kcal/mol) without affecting the metal coordination structure. The overall good agreement thus justifies the subsequent use of the LACV3P ECP for Cu.…”

“…Distortion of the octahedral geometry occurs along one of the three 4-fold axes. One of the important consequences of the Jahn-Teller effect is a variability of the coordination geometry of Cu 2+ in the solid state [14][15][16][17][18][19][20][21] and the fast first-shell ligand exchange dynamics in the aqueous phase. 22,23 Surprisingly, the coordination environment even for the simplest hydrated Cu(II) ion has been the subject of extensive debate in recent years.…”