Activation of NH3 and H2: Mechanistic insight through bond activation reactions
Introduction to ammonia and its properties
Ammonia is one of the most highly produced chemicals in the world (around 150 million tons per year) and has widespread use in many sectors.[1-5] Being an important source of nitrogen for living systems it contributes significantly to the nutritional needs of organisms by serving as a precursor to food and fertilizers. It is directly or indirectly a building block for the synthesis of many nitrogen-containing pharmaceuticals, therefore it has large applications in the production of fertilizers, chemicals, precursors, cleaners, vehicle fuel as well as explosives. The activation of ammonia has therefore attracted increasing interest over the years.
According to its structure, ammonia consists of one nitrogen atom covalently bonded to three hydrogen atoms. The nitrogen atom in the molecule has a lone electron pair, which makes NH3 a moderate base (pKb § 9.2 (H20)). The lone pair of electrons on the nitrogen induces the 107° H…N…H bond angle and ensuing triangular pyramid shape gives a dipole moment to the molecule by making it polar. Ammonia is very weak acid (pKa § 38 (H20)) and the N-H bond enthalpy is calculated as 107 kcal/ mol.
These characterizations combined with ammonia’s toxicity, danger and corrosiveness mostly explain the paucity of mild route for direct ammonia transformation to added-value products. Chapter IV will report more detailed discussion for full catalytic transformation of ammonia to amine and will focus on the preliminary transition metal activation of ammonia.
Activation of ammonia N-H bond
The interaction of the ammonia N-H bond with transition metal centers as well as with main group element atoms is of great importance in many fields such as catalysis, surface science, and material synthesis. However the main hurdle for this activation by the transition metal is to generate Lewis acid-base adducts, like “Werner’s complexes” with ammonia due to its electrophilicity.
This chapter will start with the significant results and challenging studies on ammonia N-H bond activation in addition to the mechanistic insights in the literature and will continue with the description of our results concerning this bond cleavage.
Ammonia oxidative addition by transition metal complexes:
Transition-metal complexes may react with many other small molecules by inserting into generally unreactive X-H bonds. This process, termed oxidative addition, is useful for chemical synthesis by enabling the catalysis of reactions of H2 (hydrogenation), H-SiR3 (hydrosilation), H-BR2 (hydroboration), C-H (hydroarylation, alkane dehydrogenation) that yield products ranging from chemical feedstocks to pharmaceuticals. Oxidative addition of the ammonia N-H bond can similarly be regarded as a model reaction for the development of new catalytic reaction cycles which involve the N–H bond cleavage of NH3 by insertion of a transition metal as a key-step. Even though ammonia activation by transition metal complexes has difficulties, there are some well-defined examples that have been reported:
The study of Casalnuovo and Milstein in 1987 on ammonia activation over late-transition-metal iridium (I) complex [Ir(PEt3)2(C2H4)Cl] led to the bimetallic amido-hydride complex including isolation, structural characterization, and reactivity of the product as given in Scheme 1, while Eq. 3 describes the oxidative addition of ammonia N-H bond over starting Ir(PEt3)2Cl species.
This reaction was crucial as it described for the first time the ammonia activation by electron-rich, low-valent, sterically unhindered transition metal complex and cleaving the N-H bond of ammonia by oxidative addition.
Using the ligands with different size in this reaction showed that the influence was not only on the reaction rate, as is well known, but also on the direction of reactivity as surprisingly C-H activation took place rather than N-H, leading to the hydrido vinyl compound in case of R=Pri.
The next challenging result of the transition metal complexes for the development of ammonia activation came from Hartwig and his group who reported the first stable terminal amido complex (Scheme 3).
Insertion of an iridium center with a tridentate pincer ligand into the N-H bond rapidly cleaved ammonia at room temperature in a homolytic way and obtained mononuclear iridium hydrido amido by oxidative addition.
The oxidative addition tends to be favoured by increasing electron density at the metal center. Therefore, a pincer ligand with an aliphatic backbone was preferred for this study in order to donate more electron than an aromatic ligand.[16-18] The activation of ammonia N-H bond has been explained by using electron-rich iridium complex, which helped the coordinated ammonia to transfer the electron density over the metal center as well as the ʌ-bonding between the electron pair on nitrogen and the LUMO on the metal and form a stable monomeric amido hydride complex.
Studies are ongoing in this field while one of the most representative example has been reported by Turculet  showing the N-H bond oxidative addition for both ammonia and aniline by Iridium complexes supported by silyl pincer ligands (Eq.4).
The isolable and stable [Cy-PSiP]Ir(H)(NHR) (R=H/aryl; [Cy-PSiP]= [ț3-(2-Cy2PC6H4)2SiMe] ) ] complexes were formed at the end of the reaction. The key issue in the iridium chemistry for bond activation reactions is particularly the relative stability between the amido hydride complex and the amine complex. An appropriate pincer ligand is usually used to control the thermodynamics and kinetics of the chemical transformation between the two types of complexes.
Following the studies on ammonia activation by oxidative addition, Milstein’s group subsequently worked on binuclear Ir(I) centers capable to yield amido complexes by stepwise coordination of NH3 oxidatively (Scheme 4).
The dinuclear Ir complex reacts with excess NH3 at í50 °C to give not only mono– and dicationic complexes but also, the first amido-olefin complex. N-H activation under such exceptionally mild conditions is of interest for the catalytic functionalization of olefins with NH3.
There have been several examples with binuclear metal complexes in the literature to activate ammonia, while the first example of the double activation (trinuclear oxidative addition) was reported by Suzuki with a polyhydride ruthenium complex which formed 3- imido cluster via ammonia (Eq.5).
A new mode of ammonia activation was later described by Ozerov and co-workers by a dimeric Pd pincer complex via a binuclear oxidative addition to give amido-hydrido Pd monomers (Eq. 6).
This initial example showed the cooperation of two metal centers to split NH3 into terminal M-H and M-NH2 (for conversion of NH3 to bridging amido and imido ligands see reference ). The described system is elegant, because of comprising a well-defined, tunable ligand with a highly reactive metal–metal bond as central feature in the bimetallic complex.
Ammonia oxidative addition “without transition metal complex”:
Metal-free ammonia N–H activation using main-group based systems has recently enjoyed much attention and progress: Bertrand et al. demonstrated the first example of the NH3 (and H2) oxidative addition by cyclic (alkyl)(amino)carbenes.
The group has reported the homolytic cleavage of ammonia by nucleophilic activation under very mild conditions at a stable single carbene center (mono (amino) carbenes) (Eq. 7 and 8).
Because of the strong nucleophilic character of carbenes, no ‘‘Werner-like’’ adducts were formed, and hence N–H bond cleavage occurred smoothly.
Power and co-workers subsequently described the activation of ammonia by the heavier group 14 element (Poor metal/ carbene analogue) SnAr*2 [*Ar =C6H3-2,6(C6H2-2,4,6-Me3)2)].
Related to this study (Eq 9), two-coordinate distannyl complexes were exhibited similar reactivity under mild conditions, leading to the formation of dimeric bridging amido tin-species, concomitant with arene elimination.
The quantitative reaction of ammonia with the carbene analogue poor metal, germylene has been recently reported by the group of Roesky leading N-H bond cleavage at room temperature.
This reaction generated a terminal GeNH2 group at mild conditions and oxidative addition to the final complex which demonstrated an important example of sustainable chemistry. The oxidative addition of ammonia at the silicon (II) center of a silylene in order to form Si(H)NH2 has been recently published by the same group.
Ammonia activation by d0 transition metal complexes:
Even though most of the studies on ammonia activation are based on transition metals and/ or carbenes/ carbene analogues, one of the earliest examples for N-H bond cleavage has been reported by Bercaw in 1984 on d0 complexes. The purpose of the group at that time was to investigate the interactions between 4B transition metals (mainly Zr and Hf) in their high oxidation states and hard ligands in order to study their reactions for water and ammonia splitting.
Table of contents :
CHAPTER I. Introduction
II. PREPARATION OF WELL-DEFINED TANTALUM IMIDO AMIDO SURFACE ORGANOMETALLIC COMPLEX
II.1 The support
II.2 The preparation of well-defined tantalum imido amido complex
III. OVERVIEW OF CHAPTER I and OBJECTIVE OF MY THESIS
CHAPTER II. Activation of NH3 and H2: Mechanistic insight through bond activation reactions
I.1 Introduction to ammonia and its properties
I.2 Activation of ammonia N-H bond
I.2.1. Ammonia Oxidative Addition by transition metal complexes:
I.2.2. Ammonia Oxidative addition “without transition metal complex”:
I.2.3. Ammonia activation by d0 transition metal complexes:
I.3 Mechanistic understanding of ammonia activation
I.4 Overview of chapter II
II. RESULTS and DISCUSSIONS
II. 1. Reaction of [(SiO)2Ta(=NH)(NH2)] with dihydrogen :
II. 2. Reaction of [(SiO)2Ta(=NH)(NH2)] with ammonia :
II.2.1 Spectroscopic studies of NH3 activation on [(SiO)2Ta(=NH)(NH2)], 2
II.2.2 Computational Studies of NH3 activation on cluster model 2q
II. 3. Mechanism of [(SiO)2TaHx (x: 1,3)] reaction with ammonia :
II.3.1 Spectroscopic studies of NH3 activation on [(SiO)2TaHx (x: 1,3)], 1
II.3.2 Computational Studies of NH3 activation on [(SiO)2TaHx (x: 1,3)], 1
III. CONCLUSION and PERSPECTIVES
IV. EXPERIMENTAL PART
CHAPTER III. Mechanistic insight of dinitrogen activation reaction over silica supported tantalum hydrides
I.1 Introduction to dinitrogen and its properties
I.2 Mechanistic understanding of dinitrogen activation
I.3 Overview of chapter III
II. 1 Reaction of [(SiO)2TaHx (x: 1, 3)] with dinitrogen
II.1.1 Reaction of [(SiO)2TaH1]enriched 1 with N2
II.1.2 Reaction of [(SiO)2TaHx (x: 1, 3)] with 15N2
II.1.3 Reaction of [(SiO)2TaDx (x: 1, 3)] with N2
II.2 Reaction of [(SiO)2TaHx (x: 1, 3)] with hydrazine
II.2.1 Reaction of [(SiO)2TaHx (x: 1, 3)] with N2H4
II.2.2 Reaction of [(SiO)2TaHx] with 15N2H4
II.3. Attempts to monitor reaction of [(SiO)2TaHx (x: 1, 3)] with diazene
IV. CONCLUSION and PERSPECTIVES
V. EXPERIMENTAL PART
CHAPTER IV. Attempts to use well-defined silica supported tantalum imido amido complex for C-H activation reactions
II. RESULTS and DISCUSSION
II.1. Reactivity of silica-supported [(SiO)2Ta(=NH)(NH2)] and [(SiO)2TaHx (x:1, 3)] complexes with alkynes:
II.2. Well-defined [(SiO)2Ta(=NH)(NH2)] complex in C-H activation reactions:
III. CONCLUSION and PERSPECTIVES
IV. EXPERIMENTAL PART
CHAPTER V. General Conclusions