Presentation on theme: "1 Density Functional Theory Examination of the C-H Activation Step of the Catalytica Methane Activation Reaction: Pyrimidine Ligand Protonation, Chloride."— Presentation transcript:
1 Density Functional Theory Examination of the C-H Activation Step of the Catalytica Methane Activation Reaction: Pyrimidine Ligand Protonation, Chloride vs. Bisulfate ligands, and Metathesis vs. Oxidative Addition Thomas M. Gilbert 1 and Tom Ziegler Department of Chemistry, University of Calgary 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 1 On sabbatical from Department of Chemistry and Biochemistry, Northern Illinois University, Dekalb, Illinois, USA 60115
2 Introduction It has proven difficult to derivatize methane specifically and in high yield. Recent work has focussed on sulfonating methane with fuming sulfuric acid. This is motivated by the facts that: one, the presence of the highly oxidized sulfonate group lessens the chance of further oxidation at carbon; two, the ready hydrolysis of the sulfonate group provides the useful commodity chemical, methanol; and three, the SO 2 by-product readily oxidizes to SO 3, providing a plausible catalytic cycle.
3 In this regard, the recent report from Periana and coworkers at Catalytica provides great promise. They found that methane reacts with fuming sulfuric acid solvent in the presence of (bipyrimidine)PtCl 2 to give only methyl bisulfate, with no products of oxidation or further substitution. Remarkably, the catalyst shows impressive stability at the required reaction temperatures of >180 °C. Even more remarkably, yields of methanol via methyl bisulfate in excess of 70% have been obtained. R. A. Periana, D. J. Taube, S. Gamble, H. Taube, T. Satoh, H. Fujii Science 1998, 280, 560 - 564 The Catalytica Methane-to-Methyl Bisulfate Process
4 Periana et al propose the following mechanism based on isotopic labeling studies, the use of other nitrogenous ligands, and the use of Pt (IV) compounds instead of Pt (II) catalysts:
5 A number of computational studies have appeared addressing various aspects of this mechanism. Hush and coworkers compared model energies for several sulfuric-acid- solvated and -free molecules, finding that (N) 2 Pt(OSO 3 H)- (H 2 SO 4 ) + appeared a reasonable candidate for the actual catalyst. 1 However, they did not explore transition state barriers, and used NH 3 as a model for the bipyrimidine ligand. Swang et al focussed on the related (tmeda)PtCH 3 + + CH 4 reaction, showing that this appears to utilize an oxidative addition pathway. 2 This contrasts with Hush’s work and that of Siegbahn and Crabtree 3 on (H 2 O) 2 PtCl 2 + CH 4, both of which implicate a -metathesis pathway. 1 Mylvaganam, K.; Bacskay, G. B.; Hush, N. S. J. Am. Chem. Soc. 1999, 121, 4633- 4639, ibd 2000,122, 2041-2052 2 Heiberg, H.; Swang, O.; Ryan, O. B.; Gropen, O. J. Phys. Chem. A. 1999, 103, 10004-10008. 3 Siegbahn, P. E. M.; Crabtree, R. H. J. Am. Chem. Soc. 1996, 118, 4442-4450.
6 We felt it necessary to expand on these studies. We chose to focus on the C-H bond breaking (methane activation) step, and examined the following key issues: Is modeling the pyrimidine ligand necessary, or can one get by with NH 3 or another nitrogenous ligand ? Which is the more likely X ligand, Cl or OSO 3 H ? Which is the more likely C-H bond breaking mechanism, metathesis or oxidative addition ? Since the reaction is done in fuming sulfuric acid, what impact does protonating the external pyrmidine nitrogen atoms (thereby increasing the molecular charge) have ? Is the need to run the reaction at 220 °C related to the C-H activation step ?
7 Computational Methods All DFT calculations were carried out using the Amsterdam Density Functional (ADF 2.3.3) program developed by Baerends et al and vectorized by Ravenek. The numerical integration scheme applied for the calculations was developed by te Velde et al; the geometry optimization procedure was based on the method of Versluis and Ziegler. Geometry optimizations were carried out and energy differences determined using the local density approximation of Vosko, Wilk, and Nusair (LDA VWN) augmented with the nonlocal gradient correction PW91 from Perdew and Wang. Relativistic corrections were added using a Pauli spin-orbit Hamiltonian. The electronic configurations of the molecular systems were described by a triple- basis set for all atoms. The inner shells of all non-hydrogen atoms were treated within the frozen core approximation. A set of auxiliary s, p, d, and f functions, centered on all nuclei, was used to fit the molecular density and represent Coulomb and exchange potentials accurately in each SCF cycle. Transition states were located in linear transit fashion. The reaction coordinate was kept fixed while optimizing all other degrees of freedom. The value of the constrained parameter was varied until the force acting on it proved smaller than 0.0015 au.
8 Calculations for the Catalyst (bipyrimidine)PtCl + Catalyst 1a can react with CH 4 either by metathesis or by oxidative addition. In each case, the methane coordinates to the metal through a C-H bond, lowering the energy of the system by 13 - 16 kcal/mole. The C-H bond then stretches substantially until the transition state is attained, with an energy demand of 10 - 12 kcal/mole. The C-H bond then breaks, forming either the methyl/HCl complex 1mm (metathesis mechanism) or the Pt (IV) complex 1o (oxidative addition mechanism). In neither case is the product complex comparable in stability to the -bond complex. As the barriers to return to the -bond complex are small compared to the energy required to form the product methyl cation 1m + free HCl, it appears unlikely that 1a is the actual catalyst for the methane activation process. If it is the catalyst, then the oxidative addition pathway is slightly favored based on its smaller activation barrier to C-H bond breaking.
10 Calculations for the Catalyst (bipyrimidineH 2 )PtCl 3 + Catalyst 2a exhibits substantial energetic differences from the analogous 1a. The larger positive charge increases the Lewis acidity of the platinum center, resulting in greater exothermicity for the formation of the -bond complexes 2o and 2m . At this stage, the two pathways diverge. Attaining the transition state 2mts for the metathesis process is far easier than for 2ots in the oxidative addition process. Furthermore, forming the methyl/HCl complex 2mm is more exothermic than forming the Pt (IV) cation 2o. Thus, protonating the bipyrimidine ligand enhances the metathesis pathway compared to the oxidative addition pathway. However, the overall reaction is still rather endothermic, although far less so than for the unprotonated case. It therefore appears unlikely that the actual catalyst for the process contains a chloride ligand.
11 Methane Activation by (pyrimidineH 2 )PtCl 3+
12 Calculations for the Catalyst (bipyrimidine)Pt(OSO 3 H) + Save the presence of bidentate 3cy along the potential surface, the oxidative addition pathway for the bisulfate complex resembles that of the chloride analogue. The metathesis pathway, however, differs starkly from that of the chloride. The -complex 3ms is bound exclusively through the methane H atom; the Pt-H-C angle is linear. Thus the extra transition state 3m ts, where the C-H bond coordinates to Pt, lies between 3ms and 3mtsi. From 3m ts, the molecule can climb the 11 kcal barrier to 3mtsi, then fall to the expected methyl/H 2 SO 4 complex 3mmi. However, a lower energy path is available, wherein the activated H atom of 3m ts transfers directly from the Pt to a peripheral bisulfate O atom, forming the more stable 3mme. We find no barrier for this process, and thus it appears likely if the reaction follows a metathesis path. The overall reaction is still slightly endothermic under these conditions (substantially endothermic if 3cy is considered). However, the energy is now small enough that solvation of the products might provide sufficient energy to make the reaction viable.
13 Methane Activation by (pyrimidine)Pt(OSO 3 H) +
14 Calculations for the Catalyst (bipyrimidineH 2 )Pt(OSO 3 H) 3+ As in the chloride case, the additional positive charge on the catalyst engendered by protonating the bipyrimidine ring has little effect on the oxidative addition pathway, but stabilizes the metathesis pathway substantially. The barrier between the - complex 4ms and the metathesis transition states decreases notably, substantially for the trek to 4mtsi. The activated products 4mme and 4mmi lie in deep energy wells; in fact, both are more stable than the bidentate complex 4cy ! Again no barrier appears between 4msts and 4mme, suggesting that this is the favored path, but the barrier presented by 4mtsi is sufficiently small that this provides an acceptable alternative. The overall reaction is essentially thermoneutral. Thus 4a appears the most likely catalyst for the Catalytica process of those we examined.
16 The C-H Activation Step: Conclusions The OSO 3 H group is most likely the X ligand in the actual catalyst. Though its presence means that the reaction must be run at high temperature to shift the equilibrium from the bidentate form to the monodentate form, the stronger O-H bond formed (as compared to Cl-H) makes the other steps easier. The external nitrogen atoms of the pyrimidine ligand are probably protonated in the catalyst. The added positive charge makes the elementary steps more exothermic. The oxidative addition and metathesis processes are both plausible mechanisms for the reaction, although metathesis appears to have an energetic advantage, particularly when OSO 3 H is the X ligand. During the metathesis process, the methane hydrogen atom likely transfers to a peripheral oxygen atom of the coordinated bisulfate ligand, not to the Pt-bound oxygen.
17 Thanks to: The Ziegler GroupUseful Insights Dr. Mary ChanProfessor Tris Chivers, UC Dr. Liqun Deng Dr. Roy Periana, Catalytica and USC Dr. Cory Pye Torben Rasmussen Jana Khandogin Funding NSERC Canada Council (Killam Fellowship to T.Z.) Northern Illinois University (Sabbatical leave to T.M.G.)