$NO_x$ conversion at a nickel center supported by a pincer-type PNP ligand

Abstract

$NO_x$ conversion involving multi-step reduction processes is recently receiving much attention due to environmental and social issues. According to the substantial use in industry and agriculture, nitrates ($NO_3^−$) has become a critical water pollutant and the removal is in trouble because of its harsh reaction conditions to be reduced. As a main detrimental components of exhaust gas, $NO_2$, NO and $N_2O$ also should be converted effectively under the flue gas conditions. The current consecration catalyst mainly utilizes noble metals such as Pt and Rh embedded on a ceramic support but the relative low activity concerned as a main problem. In nature, such conversion known as denitrification occurs via a multi-step process to reduce $NO_3^−$ to $N_2$ as a part of biological nitrogen cycle. Although the efficient enzymatic reactions attractively operate under mild conditions, four different metalloenzymes are needed to convert a series of intermediate $NO_x$ species. In other words, their applications to the $NO_x$ conversion catalyst are limited due to the complications of the reaction path that comes from the inherent biological purpose. Even though each step of $NO_x$ reduction with a transition metal complex is well established through invaluable investigations, there is no example which can accomplish the overall $NO_x$ reduction by a single complex. To achieve an efficient catalyst, simplification of the system is one of the essential factors. In this context, stepwise NOx conversion and its further application with a nickel complex supported by a pincer type PNP ligand will be presented.

In chapter 1, general information of nitrogen oxides will be discussed. Especially, transition metal complexes of $NO_x$, reaction mechanisms of denitrification and $NO_x$ conversion by transition metal complexes are summarized. In chapter 2, a systematic introduction of electronic and geometric changes to a metal nitrosyl complex will be described. Nickel nitrosyl complexes supported by a pincer type PEP (PNP, $PP^{Ph}P$, $PhSiP_2$) ligand were synthesized and the different characters of the complexes were analyzed especially on the Ni-NO moiety. The existence of ${Ni(NO)}^{11}$ species was also gleaned by cyclic voltammetric experiments. In chapter 3, a stepwise transformation of $NO_3–$ to $N_2$ by employing a single nickel ion will be presented. A sequential conversion of a nickel nitrate species to a nitrosyl species occurs via stepwise oxygen atom transfer with CO. Further treatment of 2 equiv. NO to nickel nitrosyl species cleanly produces nickel nitrite complex with nitrous oxide. Additional deoxygenation reaction of $N_2O$ with nitrosyl complex generates $N_2$ which is a relatively slow reaction with undesired side products. Up to our knowledge, this is a unique example to convert nitrate to dinitrogen occurring at a single metal center. In chapter 4, a novel nitrosyl group transfer reaction to organohalides from a nickel nitrosyl complex will be discussed. Especially, the synthetic cycle of generating cyclohexanone oxime from iodocyclohexane trough nitrosyl group transfer was constructed. Up to our knowledge, this is the first example of nitrosyl transfer to organic materials. It is also suggest a new method to produce oxime species.