QM/MM Approach to Isomerization of Ruthenium(II) Sulfur Dioxide Complex in Crystal; Comparison with Solution and Gas Phases

The theoretical study of chemical reactions in molecular crystals is a challenging research target. To optimize the transition state in a crystal, we have improved the crystal model and modified the quantum mechanics/molecular mechanics (QM/MM) method based on the periodic MM crystal model. We applied this method to the ruthenium­(II) sulfur dioxide complex [RuII(NH3)4(SO2)Py]2+ 1Py which is potentially useful for optical data storage systems in crystal because this Ru complex has two isolable metastable structures, η1-O-bound MS1 and η2-S,O-bound MS2, in addition to η1-S-bound ground state GS in crystal. MS2 and MS1 are formed from GS by photoirradiation at low temperature (10–100 K) and thermal isomerization occurs from MS2 to MS1 and from MS2 to GS by raising the temperature. For comparison, the thermal isomerization of [RuII(NH3)4(SO2)L]n+ (L = Cl–, W, and Py) was investigated in gas and aqueous phases, where the three-regions 3D reference site interaction model self-consistent field (RISM-SCF) method was employed to incorporate solvation effects in the aqueous phase. In the gas phase, MS2 is more stable than MS1 for L = Cl– but less stable for L = W and Py. Thermal isomerization occurs with similar changes in geometry and charge distribution among L = Cl–, W, and Py, whereas the energy profile of L = Cl– differs very much from those of L = W and Py. In the aqueous phase, MS1 becomes much less stable than MS2 and the RuII complex becomes lower in energy following the order MS1 > MS2 > GS for L = Cl–, W, and Py because of the small solvation free energy for MS1. In crystal, 1Py also becomes lower in energy following the order MS1 > MS2 > GS. The MS1 → MS2 isomerization occurs with much smaller activation energy than the MS2 → GS in crystal phase, which is consistent with the experimental observations. In crystal, the short-range steric repulsion between the SO2 of the target QM molecule and the neighboring Ru complex and the long-range ES interaction between the target molecule and the MM crystal play crucially important roles in determining the relative stabilities of MS1 and MS2 and the energy barrier of the thermal isomerization