Structure, Bonding, and Stability of a Catalytica Platinum(II) Catalyst: A Computational Study

Abstract

Periana et al. [Science 1998, 280, 560] previously reported two catalysts for low-temperature methane activation to methanol: PtCl2(NH3)2 and PtCl2(bpym). It was shown that the ammine catalyst is much more active, but it decomposes rapidly in sulfuric acid to form a PtCl2 precipitate, while the bpym system is long-lived. To have a basis for developing new catalysts that would not decompose, we undertook a study of the structure, bonding, and stability of the PtCl2(NH3)2 and PtCl2(bpym) catalysts, using quantum mechanics (QM) [density functional theory (DFT) at the B3LYP/LACVP**(+) level] including solvation in sulfuric acid via the Poisson−Boltzmann continuum approximation. Critical results include the following: (1) The influence of a trans ligand Y on the Pt−X bond follows the order Cl- > NH3 (bpym) > OSO3H- > □ (empty site). Thus the Pt−N bond length is longer (up to 0.04 Å) and the Pt−N bond is weaker (up to 18 kcal/mol) when trans to a Cl- as compared to trans to OSO3H-. (2) The bpym ligand acts as both a σ-donor and a π-acceptor. As bpym is protonated, the Pt−N bond strength decreases (by up to 51 kcal/mol). Thus, ΔH(soln, 453 K) for Pt(OSO3H)2(bpym) (69.6) > [Pt(OSO3H)2(bpymH)]+ (49.4) > [Pt(OSO3H)2(bpymH2)]2+ (18.7). (3) In sulfuric acid replacing the ammine ligands with bisulfate ligands is thermodynamically favorable [by ΔG(soln, 453 K) = −23 kcal/mol], whereas replacement of bpym with OSO3H- is unfavorable [by ΔG(soln, 453 K) = +16 kcal/mol]. (4) Replacement of chloride ligands with bisulfate ligands is thermodynamically unfavorable [by ΔG(soln, 453 K) = ∼7 kcal/mol for ammine and ∼12 kcal/mol for bpym]. (5) Protonation of PtCl2(bpym) is thermodynamically favorable, leading to [PtCl2(bpymH)]+ as the stable species in sulfuric acid (by 8 kcal/mol). Thus we conclude that in hot concentrated sulfuric acid it is quite favorable for PtCl2(NH3)2 to lose its ammine ligands to form PtCl2, which in turn will dimerize and oligomerize, leading eventually to a (PtCl2)n precipitate and catalyst death. We find that PtCl2(bpym) is resistant to solvent attack, favoring retention of the bpym ligand in hot concentrated sulfuric acid. These results agree with experimental findings. The insights from these findings should help screen for stable new ligands in the design of new catalysts.