A computational study of solution equilibria of platinum-based ethylene hydroamination catalytic species including solvation and counterion effects: Proper treatment of the free energy of solvation

A DFT/B3LYP study of the effect of the explicit inclusion of the Me4P+ cation (as a model of nBu(4)P(+)) on the calculation of solution equilibria involving anionic Pt-II complexes is reported. The calculated complexes are models of species that potentially participate in the low-energy portion of the catalytic cycle of the ethylene hydroamination by aniline catalyzed by the PtBr2/(nBu(4)P)Br system. namely (nBu(4)P)(2)[PtBr4] (1), (nBu(4)P)(2)[Pt2Br6] (1'). (nBu(4)P)[PtBr3(C2H4)] (2), (nBu(4)P)[PtBr3(PhNH2)] (3), trans-[PtBr2(C2H4)(PhNH2)] (4), cis-[PtBr2(C2H4)(PhNH2)] (5), cis-[PtBr2(PhNH2)(2)] (6), trans-[PtBr2(C2H4)(2)] (7), and cis-[PtBr2(C2H4)(2)] (8). The relative energies are based on gas-phase geometry optimizations followed by C-PCM calculations of the solvation effects in dichloromethane and aniline at 25 degrees C and 150 degrees C. Three different approaches have been considered to describe the relative energies in solution: Delta E-CPCM (Delta E-el(gas) + Delta Delta G(solv)), Delta G(V)(CPCM) (Delta E-el(gas) + Delta E-ZPVE(gas) Delta E-V(gas) T Delta S-V(gas) + Delta Delta G(solv)) and Delta G(CPCM) (Delta H-gas - T Delta S-gas + Delta Delta G(solv)). where Delta E-V(gas) and Delta S-V(gas) includes only the vibrational contribution and Delta Delta G(solv) for each compound is the solvation free energy resulting from the C-PCM calculation. The cation-anion association was found favourable in both solvents at the Delta E-CPCM and Delta G(V)(CPCM) levels, but nearly neutral at the Delta G(CPCM) level. Consideration of the associated salts has a drastic effect on the energy scheme but significantly changes the relative energies only when doubly charged complexes are involved. The energy changes for equilibria that involve only neutral of singly charged species are not greatly affected by the cation inclusion. The Delta G(CPCM) approach provides results in greater agreement with the available equilibrium data. (C) 2010 Elsevier B.V. All rights reserved.