The Organometallic Chemistry of N-heterocyclic Carbenes

Han Vinh Huynh

• Publisher: Wiley
• Year: 2017
• Print ISBN:
  9781118593776
• eBook ISBN:
  9781118698792
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Inorganic Chemistry

The Organometallic Chemistry of N-heterocyclic Carbenes describes various aspects of N-heterocyclic Carbenes (NHCs) and their transition metal complexes at an entry level suitable for advanced undergraduate students and above.

The book starts with a historical overview on the quest for carbenes and their complexes. Subsequently, unique properties, reactivities and nomenclature of the four classical NHCs derived from imidazoline, imidazole, benzimidazole and 1,2,4-triazole are elaborated. General and historically relevant synthetic aspects for NHCs, their precursors and complexes are then explained. The book continues with coverage on the preparation and characteristics of selected NHC complexes containing the most common metals in this area, i.e. Ni, Pd, Pt, Ag, Cu, Au, Ru, Rh and Ir. The book concludes with an overview and outlook on the development of various non-classical NHCs beyond the four classical types.

Topics covered include:

• Stabilization, dimerization and decomposition of NHCs
• Stereoelectronic properties of NHCs and their evaluation
• Diversity of NHCs
• Isomers of NHC complexes and their identification
• NMR spectroscopic signatures of NHC complexes
• normal, abnormal and mesoionic NHCs

The Organometallic Chemistry of N-heterocyclic Carbenes is an essential resource for all students and researchers interested in this increasingly important and popular field of research.

1
General Introduction

1.1 Definition of Carbenes

According to the International Union of Pure and Applied Chemistry (IUPAC) a carbene [1] is “the electrically neutral species H₂C: and its derivatives, in which the carbon is covalently bonded to two univalent groups of any kind or a divalent group and bears two nonbonding electrons, which may be spin‐paired (singlet state) or spin‐non‐paired (triplet state).” In general terms, carbenes are therefore neutral compounds R₂C: derived from the parent methylene (H₂C:) that feature a divalent carbon atom with only six valence electrons, which result from four bonding electrons in the two R–C bonds and two nonbonding electrons remaining at the carbene center. The geometry at the carbene carbon can be either linear or bent, depending on the degree of hybridization [2]. The linear geometry is based on an sp‐hybridized carbene center with two nonbonding, energetically degenerate p orbitals (p_x and p_y). On the other hand, the bent geometry is adopted when the carbene carbon atom is sp²‐hybridized. On transition from the sp‐ to sp²‐hybridization, the energy of one p orbital, usually called p_π, remains almost unchanged, while the newly formed sp²‐hybrid orbital, normally called σ, is energetically stabilized as it acquires partial s character (Figure 1.1). However, the linear geometry is rarely observed, and most carbenes contain a sp²‐hybridized carbene center and are therefore bent.

Figure 1.1 Relationship between the carbene bond angle, the nature of the frontier orbitals and singlet–triplet separation.

For the simple linear case and without considering π contributions of the R‐substituents, only the p_x¹p_y¹ electronic configuration is feasible according to Hund’s first rule [3], due to the degeneracy of the p_x and p_y orbitals. The two unpaired electrons are both “spin up” (m_s = ½) giving rise to a total spin of S = 1, which in turn results in a spin multiplicity of M = 3 (Equation 1.1). Therefore, linear carbenes are generally in a triplet state. On the other hand, two common electronic configurations are possible for the carbene carbon in bent species. The two nonbonding electrons can singly occupy the two different σ and p_π orbitals with parallel spins (σ¹p_π¹), which also leads to a triplet ground state (³B₁). Alternatively, the two nonbonding electrons can also be spin‐paired in the energetically more favorable σ orbital (σ²p_π⁰) leading to a singlet ground state (¹A₁).

In addition to the two ground states in bent carbenes, two less favorable electronic configurations are conceivable (not depicted) that give rise to singlet states. The first has two spin‐paired electrons in the p_π orbital (σ⁰p_π², ¹A₁), and the second has two electrons singly occupying the σ and p_π orbitals, but with opposite spins (σ¹p_π¹), giving rise to an excited singlet state (¹B₁) [4]. The latter two electronic configurations and their states have little significance for the discussion in this work.

The properties and reactivities of bent carbenes are primarily determined by their ground state spin multiplicity [5]. The two singly occupied orbitals in triplet carbenes are unsaturated (open‐shell) and can accommodate one more electrons of opposite spin each. Thus, it is intuitive to assign an electrophilic or diradical character to the carbene carbon (Figure 1.1). Singlet carbenes, on the other hand, contain a fully occupied σ orbital (closed‐shell, nucleophilic) and an empty p_π orbital (electrophilic). The presence of both electrophilic and nucleophilic sites makes singlet carbenes formally ambiphilic.

Equation 1.1
Definition of spin multiplicity for the determination of singlet and triplet state.

Whether a bent carbene adopts the singlet or triplet ground state is determined by the relative energies of the σ and p_π orbitals, which in turn is influenced by the direct substituents R at the carbene carbon. A large energy gap of at least 2 eV (~193 kJ/mol) between the σ orbital and the p_π orbital is required to stabilize a singlet ground state, whereas an energy difference of less than 1.5 eV (~145 kJ/mol) leads to a triplet ground state [6].

The relative energies of σ and p_π orbitals can also be influenced by the steric and electronic effects of the...

Table of contents

• Cover
• Title Page
• Table of Contents
• Foreword
• Preface
• List of Abbreviations and Definitions
• 1 General Introduction
• 2 General Properties of Classical NHCs and Their Complexes
• 3 Synthetic Aspects
• 4 Group 10 Metal(0)‐NHC Complexes
• 5 Group 10 Metal(II)‐NHC Complexes
• 6 Group 11 Metal‐NHC Complexes
• 7 Ruthenium, Rhodium, and Iridium Metal‐NHC Complexes
• 8 Beyond Classical N‐heterocyclic Carbenes I
• 9 Beyond Classical N‐heterocyclic Carbenes II
• Index
• End User License Agreement