Absorption of CO₂ and CS₂ into the Hofmann-type porous coordination polymer: Electrostatic versus dispersion interactions

Absorption of CO₂ and CS₂ molecules into the Hofmann-type three-dimensional porous coordination polymer (PCP) {Fe(Pz)[Pt(CN)₄]}_n (Pz = pyrazine) was theoretically explored with the ONIOM(MP2.5 or SCS-MP2:DFT) method, where the M06-2X functional was employed in the DFT calculations. The binding energies of CS ₂ and CO₂ were evaluated to be -17.3 and -5.2 kcal mol^-1, respectively, at the ONIOM(MP2.5:M06-2X) level and -16.9 and -4.4 kcal mol^-1 at the ONIOM(SCS-MP2:M06-2X) level. It is concluded that CS₂ is strongly absorbed in this PCP but CO₂ is only weakly absorbed. The absorption positions of these two molecules are completely different: CO₂ is located between two Pt atoms, whereas one S atom of CS₂ is located between two Pz ligands and the other S atom is between two Pt atoms. The optimized position of CS₂ agrees with the experimentally reported X-ray structure. To elucidate the reasons for these differences, we performed an energy decomposition analysis and found that (i) both the large binding energy and the absorption position of CS₂ arise from a large dispersion interaction between CS₂ and the PCP, (ii) the absorption position of CO₂ is mainly determined by the electrostatic interaction between CO₂ and the Pt moiety, and (iii) the small binding energy of CO₂ comes from the weak dispersion interaction between CO₂ and the PCP. Important molecular properties relating to the dispersion and electrostatic interactions, which are useful for understanding and predicting gas absorption into PCPs, are discussed in detail.