**2.2 NMR spectroscopy: σ-donor and π-acceptor abilities of NHCs**

NMR spectroscopy, especially 13C and <sup>1</sup> H NMR, are the most efficient and reliable techniques to understand the properties of NHCs, the NHC precursors, and


**Table 1.** *TEP values for selected NHCs.* *Imidazolium-Based N-Heterocyclic Carbenes (NHCs) and Metal-Mediated Catalysis DOI: http://dx.doi.org/10.5772/intechopen.102561*

related NHC-metal complexes. In general, imidazolium proton (-N-C(H)-N-) in NHCs is acidic due to its connection to the two adjacent electron-withdrawing nitrogen atoms. Hence, this proton appears in the deshielded region of proton NMR, around 8–12 ppm. However, upon deprotonation, the imidazolium proton signal disappears in the proton NMR. The 13C{1 H} NMR spectra of NHCs exhibit the carbene (-N-C-N-) carbon peak close to the highly deshielded region of around 200–250 ppm for free carbene, whereas it is approximately 130–160 ppm for its respective precursor imidazolium salt. Due to the increased shielding effect, the highly shielded carbene carbon upon metal complexation, will undergo an upfield shift. Apart from a diagnostic tool, the NMR technique is helpful to explore various properties of NHCs such as (a) the steric properties; (b) σ-donor ability from ligand to the metal center; and (c) π-back bonding from metal to carbene. Understanding these parameters of NHCs is crucial for controlling the catalyst selectivity, reactivity, and efficiency. Earlier, the Tolman electronic parameters (TEP) obtained from the CO stretching frequencies of metal complexes such as [Ni(CO)3L] and [MCl(CO)2L] (M = Rh or Ir) were used to assess the electronic properties. However, this method has limitations due to inherent inaccuracies in IR spectroscopy measurements of metal complexes. Also, the necessity of preparing these complexes under highly inert conditions, the requirement of expensive metals, and the usage of highly toxic CO gas are the important drawbacks of the TEP method. Moreover, the TEP only gives information about the overall electron density around the metal center but not independent σ-donor or π-back bonding abilities.

Huynh et al. reported the unique use of NMR for studying the donor properties of NHCs [28]. This method utilizes the 13C-NMR technique, where the *trans*-[PdBr2( *i* Pr2-bimy)L]n ( *i* Pr2-bimy = 1, 3-diisopropylbenzimidazolin-2 ylidene; L = NHC ligand under investigation) complex was the spectroscopic probe. In this complex, the presence of ligands (L) with the good σ-donor ability induces a downfield shift of the carbene carbon peak of the NHC (i.e., *<sup>i</sup>* Pr2-bimy). Thus, it enabled easy comparison of NHCs with other significant ligands, such as phosphines, amines, and isocyanides. Moreover, this method utilizes easily preparable spectroscopic probes. Based on such carbene carbon chemical shift values, the NHC ligands are arranged on a unique σ-donor scale as shown in **Figure 5**.

Betrand et al. in 2013 reported the determination of π-acceptor ability of NHC ligands using the 31P chemical shift value of carbene-phosphine adducts, which exist in two different resonance forms "I" and "II" (**Figure 6**) [29]. The phosphaalkene form (I) has a P=C bond, and the form (II) has P-C dative bond with P having two lone pairs of electrons. The upfield phosphorus NMR chemical shift associated with P atom in the carbene-phosphine adducts agrees with the electron-rich resonance form (II). It suggests that the carbene carbon prefers a dative bond with phosphorus rather than a π bond. If the carbenes have π-accepting property, it should have reflected through a downfield shift in the 31P NMR.

Therefore, the NMR-based method provides an independent way to determine the π-accepting ability of NHC ligand, independent of its σ-donor abilities, which was one of the limitations of Tolman's method for determining the electronic properties of NHCs. The arrangement of a few known NHCs according to their π-accepting ability, based on the 31P NMR chemical shift values, are provided in **Figure 6**.

Ganter et al. reported a similar technique in which the 77Se chemical shift of carbene-selenium adducts were used to assess the π-acceptor properties of the NHC ligands [30]. The carbene-selenium adduct exists in two different resonance forms

#### **Figure 5.**

*Arrangement of NHCs based on their σ-donor ability as per the 13C NMR data, using the spectroscopic probe-*trans*-[PdBr2(*<sup>i</sup> *Pr2-bimy)L]<sup>n</sup> [28].*

**Figure 6.**

*Arrangement of NHCs according to their π-accepting ability based on the 31P NMR chemical shift values [29].*

(III) and (IV) (**Figure 7**), similar to that of carbene-phosphine adduct. The increase in the π-accepting property of the NHC is reflected through an increase in the chemical shift value of 77Se NMR. The arrangement of common NHCs according to their π-accepting ability was established based on the 77Se NMR chemical shifts (**Figure 7**). In addition to the π-acceptor ability, the σ-donor abilities of NHCs can also be found using carbon-selenium coupling constant values [31].

*Imidazolium-Based N-Heterocyclic Carbenes (NHCs) and Metal-Mediated Catalysis DOI: http://dx.doi.org/10.5772/intechopen.102561*

**Figure 7.**

*Arrangement of NHCs according to their π-accepting ability based on the 77Se NMR chemical shift values [30, 31].*

Szostak et al. [32] in 2019 reported a simple and straightforward method for the determination of the σ-donor properties of NHCs based on the Ccarbene-H coupling constant (*JC-H*). The significance of this method in comparison to the earlier reported NMR techniques and TEP determination method is that there is no need for the synthesis of any metal complexes or adducts for the analysis. The <sup>1</sup> H NMR is used for the characterization. The magnetic moment of a 13C nuclei couples to a bonded proton through the intervening bonding electrons, and it is represented by the carbon-proton coupling constant (*JC-H*). This value relies on the probability of finding bonding electrons at the two nuclei involved, that is, C and H. The likelihood of finding an electron at the nucleus of a pure *p*-orbital of carbon is zero, while it has a finite value in the case of an *s*-orbital. Based on this principle, an empirical relationship (i.e., *JC-H* = 500 x *s*) between the C-H coupling constant (*JC-H*) and the *s*-character of carbon atom (*s*-value = 0.25 (for *sp*<sup>3</sup> -C), 0.33 (for *sp*2 -C), and 0.50 (for *sp*-C)) was derived, for a 500 MHz <sup>1</sup> H NMR instrument. Therefore, carbon with increased "*s*" character is expected to have a large (*JC-H*) coupling constant.

Thus, a large carbene carbon and proton coupling constant (*JC(carbene)-H*) value corresponds to a weaker σ-donor property of the NHC ligand. The coupling constants (*JC(carbene)-H*) were obtained from the 13C satellite peaks of <sup>1</sup> H NMR and the <sup>1</sup> H coupled 13C NMR spectra. Based on the *JC-H*, coupling constant values, NHCs have been arranged according to their σ-donor properties (**Figure 8**).

### **2.3 NMR and UV-Vis spectroscopy:** *p***Ka of NHC precursors**

Basicity and catalytic reactivity of NHC precursors (azolium salts) are intertwined. Therefore, the *p*Ka studies of NHCs are crucial for understanding the properties of the carbenes. Imidazolium salts upon deprotonation form NHCs (**Figure 9**). The *p*Ka values reflect the ease of imidazolium salts to form NHC precursors. It also provides information about the nucleophilicity of NHCs. Often

#### **Figure 8.**

*Arrangement of NHCs according to their σ-donor ability based on the* JC(carbene)-H *coupling constant values [32].*

#### **Figure 9.**

*Deprotonation of imidazolium salts to form N-heterocyclic carbenes.*

the NHCs are formed *in situ* and in many NHC-catalyzed reactions, nucleophilic addition is the first and most crucial step. Hence, the details related to nucleophilicity of NHCs have gained considerable interest [33]. Moreover, it will be helpful to draw the correlation between the substituents, nature of the azolium ring, and its catalytic activity. This section highlights the use of NMR and UV-Visible spectroscopy tools for studying the *p*Ka values of NHC precursors, that is, imidazolium salts.

Alder et al. reported the *p*Ka value of 1,3-diisopropyl-4,5-dimethylimidazolium cation to be 24.0 in DMSO-*d6* using <sup>1</sup> H-NMR spectroscopy [34]. This study uses the NMR technique to assess the deprotonation ability of NHCs from acidic hydrocarbons of known *p*Ka values (e.g., indene, 9-phenylxanthene, triphenylmethane, etc.) (**Figure 10**). If the NHC deprotonates the hydrocarbon (V), it gives a different proton signal for its corresponding anion (VI). Therefore, based on the proton integration and the equilibrium ratio of (V) and (VI), the *p*Ka of NHCs can be determined.

**Figure 10.** *Deprotonation of hydrocarbon by NHC to form hydrocarbon anion [34].*

*Imidazolium-Based N-Heterocyclic Carbenes (NHCs) and Metal-Mediated Catalysis DOI: http://dx.doi.org/10.5772/intechopen.102561*

**Figure 11.**

*Arrangement of imidazolium salts based on their* p*Ka values in DMSO as determined by Harper et al. using bracketing indicator method [33].*

Streitwieser et al. reported that the anomalies in the determination of *p*Ka of imidazolium salts, is due to the interaction of acidic solvent (DMSO) with carbene. It can be eliminated through the use of inert solvents such as THF by using bracketing/overlapping indicator method [35]. In this process, the deprotonated form of the fluorene-based indicator (of known *p*Ka value) shows a significant change in the UV-Vis absorption value upon adding the NHC precursor salt. It helps to monitor the equilibrium and thereby the *p*Ka can be determined. Chu et al. extended this method for various alkyl imidazolium salts [36]. Harper et al., applied this method to determine the *p*Ka values of different types of electronically, sterically diverse alkyl and aromatic imidazolium salts (**Figure 11**) [33]. Later the *p*Ka values of a few imidazolium salts in aqueous medium were also reported by Amyes et al. [37] using deuterium exchange studies. Similar exchange studies by O'Donoghue et al. [38] on triazolium salts facilitated a detailed understanding of the correlation between the NHC precursor salt's cationic structures and its acidity.

#### **2.4 Electrochemical technique: redox potentials and basicity of NHCs**

The architecture of NHC allows facile structural modifications, and hence, it provides an opportunity to fine-tune the properties. The reactivity of NHCs is controllable through the structural variations on its cyclic carbon backbone or the introduction of different heteroatoms such as S or O in the heterocyclic ring. The nature of counter anions in the precursor (i.e., imidazolium salts) will also affect the reactivity. Several external factors also contribute to the overall property of NHCs, such as the solvent effects, type of reagents in the reaction mixture, and temperature, etc. Therefore, considering all these factors, the electrochemical techniques will also be helpful for the easy identification of ideal conditions for optimizing the reactivity and efficient catalysis of NHCs. During the electrochemical reduction of imidazolium salt, the imidazolium proton (i.e., C2-H group of the cationic part) NHCH<sup>+</sup> is reduced to NHC. In this case, the reactivity mainly depends on the NHC generated from the conjugated acid, that is, NHCH<sup>+</sup> .

The electrochemical studies of NHCs were first reported by Enders and Simonet et al. using triaryl-triazol-ylidene [39]. The cyclic voltammetry (CV) of imidazolium salts exhibited a reversible reduction, which can be due to singleelectron reduction of NHCH<sup>+</sup> to NHC (i.e., imidazolium salt to NHC) (**Figure 12**). Recently, Boydston et al. have also reported a systematic study on the redox behavior of a series of azolium salts, including benzothiazolium, thiazolinium, thiazolium, triazolium, imidazolium, and imidazolinium salts [40]. The study demonstrated that N-aryl moiety would be helpful to tune the reduction potential of the

**Figure 12.** *Schematic CV representation of oxidation and reduction peaks of a model NHC [39–41].*

imidazolium ring [41]. The carbene generation usually depends upon the acidity of the NHCH<sup>+</sup> ; hence, with an increase in the reduction potential, the acidity of the NHCH<sup>+</sup> also increases [42]. Therefore, based on the electrochemical method, it is possible to choose appropriate NHC precursors [43]. A number of electrochemical studies related to the NHC metal complexes were also reported [44–48], the data of few selective NHC precursors (i.e., azolium salts) and its corresponding reduction potential are summarized from the literature (**Table 2**).


**Table 2.** *Reduction potentials for azolium salts, in V* vs. *SCE [40–43].* *Imidazolium-Based N-Heterocyclic Carbenes (NHCs) and Metal-Mediated Catalysis DOI: http://dx.doi.org/10.5772/intechopen.102561*

The overall electrochemical reduction potential will reflect the acidic nature of the cationic part of the NHC and its nucleophilicity (availability of lone pair). Therefore, the reduction potential data of the azolium salts will be helpful to gain insight into the nucleophilicity of different NHCs and identify the structural moieties that were crucial for a specific function or property. The catalytic efficiency of the NHC ligand primarily depends upon the nucleophilicity of carbene.

## **3. Steric properties of NHCs**

Apart from the electronic properties, it is significant to understand the steric parameters of NHCs, since it plays a crucial role in the reductive elimination step of a catalytic cycle. In case of chiral NHCs, the steric parameters control the enantioselectivity of a catalytic reaction (**Figure 13**).

The earlier attempts to define the steric properties of NHCs were based on the Tolman cone angle [49], which is the solid angle made by an imaginary cone with metal atom at its apex, and ligands are at the outer edges (**Figure 14a**). The cone angle

**Figure 13.** *Effect of carbon backbone and N-substituent modifications on enantioselectivity.*

**Figure 14.**

*(a) Tolman cone angle of NHCs; (b) percentage buried volume of NHC (%Vbur); and (c) %Vbur for some selected NHCs [50].*

can be computationally calculated [51] or obtained from the single-crystal XRD data [52]. In the case of tertiary phosphine ligands, the cone angle was a well-defined concept; however, not in the case of NHCs, where it lacks a predominant three-dimensional symmetry. Thus, the determination of steric parameters, through the Tolman cone angle technique, was inappropriate for NHCs [49]. In order to overcome this, the concept of percentage of buried volume (%Vbur) was introduced by Nolan et al. [53], which is the percentage of total volume occupied by the ligand in an imaginary sphere of a well-defined radius and the metal atom residing at the center of the imaginary sphere (**Figure 14b**). This parameter can be determined either from the single-crystal XRD data or can be calculated computationally (**Figure 14c**). The nature of backbone, N-substituents, and the ring size are the various factors that influence the percentage of buried volume (% Vbur).

Several advancements in the steric parameter determination of NHCs came up with the introduction of Salerno molecular buried volume calculation (Samb*V*ca) by Cavallo et al. [54]. The online tool developed for this method utilizes the CIF file of NHC or metal-coordinated NHC as the input file to generate a two-dimensional color-coded contour map around the carbene, from which the catalytical active pockets of the complex or the sterics around the catalytic active sites can be visualized. A detailed steric map study of various types of NHC complexes was well explored by Nolan et al. [50].
