Experimental and Theoretical Charge Density Distribution in Two Ternary Cobalt(III) Complexes of Aromatic Amino Acids

The experimental charge density distributions in two optically active isomers of a Co complex have been
determined. The complexes are Δ-α-[Co(R,R-picchxn)(R-trp)](ClO4)2·H2O) (1) and Λ-β1-[Co(R,R-picchxn)(R-trp)](CF3SO3)2) (2), where picchxn is N,N‘-bis(2-picolyl-1,2-diaminocyclohexane) and R-trp is the
R-tryptophane anion. The molecular geometries of 1 and 2 are distinguished by the presence in complex 1 of
intramolecular π···π stacking interactions and the presence in complex 2 of intramolecular hydrogen bonding.
This pair of isomers therefore serves as an excellent model for studying noncovalent interactions and their
effects on structure and electron density and the transferability of electron density properties between closely
related molecules. For complex 2, a combination of X-ray and neutron diffraction data created the basis for
a X−N charge density refinement. A topological analysis of the resulting density distribution using the atoms
in molecules methodology is presented along with d-orbital populations, showing that the metal−ligand bonds
are relatively unaltered by the geometry changes between 1 and 2. The experimental density has been
supplemented by quantum chemical calculations on the cobalt complex cations:  close agreement between
theory and experiment is found in all cases. The energetics of the weak interactions are analyzed using both
theory and experiment showing excellent quantitative agreement. In particular it is found that both methods
correctly predict the stability of 2 over 1. The transferability between isomers of the charge density and
derived parameters is investigated and found to be invalid for these structurally related systems.