Definition and Properties of NHCs

Carbene is a divalent carbon centre with a six-electron valence shell, which can exhibit either linear or bent geometry (Figure 1.1.1, a).^{[19a, 19b]} In the first case A, the non-bonding electrons singly occupy two degenerate, mutually orthogonal p-orbitals (px, py), while binding to the substituents is achieved via a sp-hybridized carbon centre. This geometry implies a triplet state (³B1). The majority of carbenes, however, consists of a sp²-hybridized carbon atom with more energetically stabilized bent geometry (B and C, Figure 1.1.1, a).

Concerning the multiplicity of bent carbenes both singlet or triplet states are possible. Firstly, the non-bonding electrons can either occupy the two empty orbitals with a parallel spin leading to a triplet ground state (σ¹pπ1, ³B1, B). A more stable solution is the occupation of σ orbital with antiparallel spin orientation (σ²pπ0, ¹A1, C). The excited singlet states (σ¹pπ1, ¹B1) and (σ⁰pπ2, ¹A1) aren’t generally of high importance and therefore will be not discussed here.

The multiplicity of the ground state is an important feature for understanding the properties and reactivity of carbenes.^[21] Singlet carbenes with a lone pair have ambiphilic character due to existence of a filled and an empty orbital. Triplet carbenes, on the other hand, are nucleophilic,

behave themselves as diradicals and therefore have an increased propensity for dimerization.

Due to higher intrinsic stability of singlet carbenes, this compound class is predominantly applied in modern carbene chemistry.^[22]

Figure 1.1.1. a). Frontier orbitals and possible electron configurations of carbenes.^[19b] b). Ground state electronic structure of imidazol-2-ylidenes.^[20]

So how can the electronic state of carbenes be controlled since it is so crucial for the reactivity?

Similar to crystal field theory, the singlet ground state is favoured by larger σ-pπ separation.^[19a,

19b] According to DFT calculation an energy difference of at least 2 eV is necessary to induce a singlet ground state, a value below 1.5 eV leads to a triplet state.^[23] The stabilization of the singlet ground state can be achieved in different ways since the orbital energy separation is heavily dependent on steric and electronic properties of the substituents on the carbene carbon. On the one hand, placing σ-withdrawing, generally more electronegative substituents in the direct proximity of the carbene centre results in stabilization by pull-pull (inductive) ability lowering the relative energy of non-bonding σ-orbital.^[24] On the other hand, mesomeric effects play a crucial role and can be achieved by application of π-donors (X) or π-acceptors (Z) as α-substituents.[19a, 19b, 23, 25]

The linear or almost linear (Z,Z) and (X,Z) carbenes will be not discussed here; for examples and the information on their electronic structure an interested reader is advised to read some excellent review articles.[19a, 19b] The (X,X)-carbenes however are predicted to be bent molecules in singlet ground state.^[22] The donation of π-electrons into pπ-orbital of a carbene raises its relative energy, therefore leading to more stabilization by increasing the HOMO-LUMO gap. The most important singlet (X,X) carbenes with the double stabilization though σ-withdrawing and π-donating substituents are N-heterocyclic carbenes (NHCs), where this situation is achieved by a virtue of placing the carbene carbon between two nitrogens by simultaneously enforcing a bent geometry though incorporation into a cyclic structure (Figure 1.1.1, b)^[20]. By donation of the substituents’ lone pair into an empty py-orbital of carbene centre

the carbene−nitrogen bonds acquire partial double-bond character, which is reflected for imidazol-2-ylidenes by a C2−N bond distance falling between corresponding imidazolium salt and saturated NCH2N-analogue.^{[1, 22]}

Figure 1.1.2. Illustration of structural features which influence the properties of NHCs.^[20]

The influence on the electronic situation in NHCs is not only limited to heteroatoms, also the wingtips (N-Substituents) affect the electronics, e.g. by a greater degree of σ-donation.

Consequently, the NHCs with alkyl wingtip-substituents have higher nucleophilic properties as their aryl-substituted analogues.^[22] Moreover, the backbone substituents (C4 and C5 positions) have also high potential for electronic “fine-tuning”. For instance, Arduengo et al. were able to isolate an exceptionally stable carbene by replacing the backbone hydrogens with chlorine atoms, which contributed to the stability of NHC with additional σ-withdrawing and π-donating abilities.^[26] The backbone of NHCs is also important in another context: although the concept of aromaticity in carbenes is far less important than in their imidazolium precursors, due to their partial aromaticity imidazol-2-ylidenes are more thermodynamically stable by about 25 kcal∙mol^-1 than their saturated analogues, which are, therefore, more prone to dimerization.^[27]

Furthermore, the steric hindrance, especially produced by N-substituents, can kinetically stabilize many types of carbenes and prevent them e.g. from dimerization as well.^{[19a, 20]} A good summary on all factors and structural elements affecting the stability and reactivity of NHCs in given in Figure 1.1.2. For more in-depth understanding of the bonding situation in NHCs a comprehensive overview on quantification of the parameters and factors defining electronic and steric situation in these compounds can be found in literature.^[22]

Nowadays, due to numerous structural and electronic modifications the portfolio of selectable NHCs is huge and extremely versatile (for some important classes see Figure 1.1.3).[4, 8b, 19b, 22] Although the vast majority of NHCs is derived from five-membered heterocycles containing two nitrogen atoms, the so-called “ring-expanding” alternatives have been also reported, e.g.

based on triazine, tetrahydropyrimidine and others.[19b], [19a, 22] Also a substitution of nitrogen by various heteroatoms is accessible, for example though the use of oxygen, sulphur or

phosphorus.[19a, 19b, 22] Moreover, the requirement of at least two nitrogens for stabilization of the carbene is not obligatory as a series of cyclic alkyl (amino)carbenes containing only one nitrogen atom have been also intensively investigated.^[28] Additionally, there is also a possibility of generation of so-called mesoionic or “abnormal” carbenes, which are normally more nucleophilic than neutral NHCs.^{[28c, 29]} Also remote NHCs, where the carbene is not directly bonded to a heteroatom, are known.^[28c]

Figure 1.1.3. Molecular structures of some of the most common classes of NHCs.^[20]

In conclusion, due to their electronic structure NHCs are considered as nucleophilic compounds. Consequently, they have strong propensity to act as σ-donors and bind to a wide range of metals and non-metals.^[20] Their stability, generally strong binding mode as well as the possibility for “fine-tuning” of electronics and sterics make them very attractive tools in organometallic chemistry. Additionally, also one of the most attractive features of the carbenes is the relatively easy preparation of structurally diverse analogues, which can be studied in comparative investigations.^[20]