Figure 6: DFT simulations of the bond-breaking pulling forces in solution environments.
(a) Schematic illustrations of the target bond-breaking systems. Three initial configurations examined in the two oxidation states are: (1) [Os(terpy)2] is osmium in octahedral ligand environment provided by the nitrogen atoms (blue) of two terpy molecules. (2) [Os(terpy)2(H2O)] is coordinated to a second terpy molecule through two nitrogen atoms. The non-bonding group is rotated by about 90°, allowing coordination of a water molecule. (3) [Os(terpy)2(H2O)2] is osmium coordinated to a single nitrogen atom of the second terpy ligand, with two water molecules occupying the coordination sites. The middle pyridine unit of terpy was systematically moved by 0.2 Å increments. At each increment, the Os–N distance (of the middle pyridine unit) was kept fixed and all other degrees of freedom allowed to be relaxed. A notable side reaction was the transfer of a proton from a water ligand to a non-coordinated nitrogen of terpy. (b) Potential energy changes of the simulated bond-breaking systems in aqueous solution. Solid lines are linear best fits to the calculated data for Os2+ (black lines) and Os3+ (red lines), respectively. The slopes were obtained from these linear fits, and the values are listed in the right panel of b. It is clear that the Os2+ state favours full binding to terpy, but as water occupies coordination sites the terpy–Os3+ bond becomes the stronger. It is noted that the energy starts to level off when the Os-to-terpy distance is beyond 1.0 Å.
