Reports: DNI150206-DNI1: Development of New Homogeneous Catalysts for the Activation of N2O

Jennifer M. Schomaker, PhD, University of Wisconsin (Madison)

Activation of N2O by New Homogeneous Catalysts

Introduction. The majority of oxidation methods available for organic hydrocarbon feedstocks, including crude oil and natural gas, require oxidants that are not particularly environmentally friendly or generate copious amounts of waste. While O2 is an attractive alternative, it can lead to overoxidation and requires the reaction to be stopped at very low conversions in order to prevent significant by-product formation. Nitrous oxide (N2O), a potent greenhouse gas, has received attention as a benign oxidant that yields only N2 as a waste product. In addition, the fact that only one oxygen atom is available for reaction in N2O makes these oxidations much more selective than the corresponding transformations employing O2as the terminal oxidant.

The major drawback to using N2O as an oxidant is its high kinetic stability. Although nitrous oxide reductase can decompose N2O at ambient temperatures and pressures, homogeneous catalysts that have been reported to activate nitrous oxide require high temperatures and pressures.  If homogenous catalysts capable of N2O activation at near-ambient temperatures and pressures could be designed, this would represent a major step forward in the utilization of this potent and selective oxidant.

Previous studies. In the previous funding period, we focused on trying to understand the mechanistic details of N2O activation promoted by high-valent Ru oxo complexes supported by a sterically bulky porphyrin ring. We prepared a new Ru(VI)(por)O2 complex that was supported by a very bulky D4-porphyrinato{5,10,15,20-tetrakis[1,2,3,4,5,6,7,8-octahydro-1,4:5,8–dimethanoanthracen–9-yl]porphyrin} to shut down any competing dimerization of monomeric Ru(IV)complexes into inactive Ru(IV) µ-oxo dimers. We found that several reaction parameters had an impact on the epoxidation of a particular alkene substrate, cholesteryl acetate. Coordinating solvents, such as tetrahydrofuran and dichloromethane, inhibited the reaction by binding to the Ru center, but benzene and chlorobenzene were effective for the oxidation with N2O. There was a clear dependence on the pressure of the nitrous oxide, as the yield of the epoxidation reaction increased significantly when the pressure was raised from 1 to 12 atmospheres, although further increases in pressure did not improve the reaction outcome. In terms of temperature, we found that the reaction did not proceed at a reasonable rate at temperatures lower than 80 °C, even at 12 atm N2O. As expected, the reaction rate and yield increased with increasing concentrations of Ru(VI)(por)(O)2. Interestingly, the alkene concentration had the largest effect on the reaction outcome. If the reaction was sufficiently dilute, the epoxidation of cholesteryl acetate could be promoted at temperatures lower than 80 °C and at lower pressures of nitrous oxide than those previously reported. This suggests that substrate inhibition is the major issue with the use of high-valent oxo complexes in the epoxidation of alkenes with N2O. Thus, one potential way to improve the oxidation of these substrates with N2O is to design ligand sets that can prevent or minimize substrate inhibition. Another valuable insight that was provided by our mechanistic and kinetic studies was the likelihood that the regeneration of the Ru(VI)(por)(O)is the rate-determining step in the reaction.

Design of new bi- and trimetallic catalysts. Our mechanistic and kinetic studies indicated that a di- or trimetallic Ru catalyst, where the distance between the metal centers could be exquisitely tuned, might be able to speed up the rate-determining step, which we hypothesize is regeneration of the active oxidant, Ru(VI)(por)(O)2, from Ru(IV)(por)(O).  With this idea in mind, we have proposed the synthesis of a series of bimetallic porphyrins with buttressing groups that can be easily varied to fine-tune the distance between the two metal centers. The same strategy can be applied to the preparation of trimetallic systems.  These new catalysts, designed to support two or three high-valent Ru metal centers, will be investigated for their efficacy in the oxidation of various alkenes with N2O.  The way in which nitrous oxide binds to these di- and trinuclear metal complexes can also be probed using spectroscopic methods such as REACT-IR or high-pressure NMR. Depending on the ligand set, the oxidant may be capable of binding at either the axial position of the metal or approach in a side-on fashion to generate a different form of the active oxidizing species. This information can be used to design improved systems where the detrimental binding of the substrate to a Ru(II)(por)L2or Ru(IV)(por)O can be minimized.

Other catalyst systems. The difficulty of synthesizing porphyrin-based systems prompted us to examine a number of other transition metal catalysts and supporting ligands for the oxidation of alkenes and alcohols using N2O as the terminal oxidant. These included Ru supported by macrocyclic nitrogen ligands and Cu catalysts stabilized with simple amine ligands. We also investigated the use of Co catalysts supported by thiohydantoin ligands that were reported in the literature to oxidize triphenylphosphine to triphenylphosphine oxide and promote the epoxidation of norbornene using N2O at ambient temperatures and pressures. Unfortunately, in our hands, these systems did not behave as advertised and we strongly suspect that adventitious oxygen was responsible for the oxidation observed in the authors' studies. Finally, catalysts based on Pt and Pd were also explored. Unfortunately, none of these systems proved to be more effective than high-valent Ru complexes supported by bulky porphyrins. Future directions include the use of metal organic frameworks or other self-assembling systems to support Fe or Ru metals in a more controlled and convenient manner.