*3.3.2 NHC metal adducts*

Another important NHC precursor is silver (I) complexes of NHC. There have been several types of such complexes like: Imidazolin-2-ylidene involving imidazole ring with substituents at the nitrogen atoms, Benzimidazol-2-ylidenes having a benzene ring fused with the imidazole moiety, imidazolidin-2-ylidenesand related heterocycles. The first Ag (I)–NHC adduct was reported by Arduengo in the early 90s by the reaction of Ag(I) salt with a free NHC [80]. Later, the uses of Ag(OAc) and Ag2O as silver base were reported to synthesize various Ag(I)– NHCs. These silver bases are used for the deprotonation of azolium salts, and generation of Ag(I)–NHCs in situ. These Ag(I)-NHC complexes easily decompose under thermolysis to provide the free carbenes for various applications. One limitation to this approach is the use of silver metal in stoichiometric amounts. The different synthetic routes as well the structural diversity and the applications of such precursors have been well established in the carbene literature [81].

#### *3.3.3 Other NHC precursors*

In addition to the above mentioned adducts there are reports of similar complexes of NHC namely the chloroform and pentafluorobenzene adducts of 1,3 dimesitylimidazolidin-2-ylidene (SIMes). They are stable at room temperature and afford the corresponding NHC on thermolysis [82, 83]. One such example is SIMes (H)(O-t-Bu) which can produce the corresponding NHC at room temperature. The alcohol adducts of triazolin-5-ylidene and imidazolidin-2-ylidene also proved to be excellent NHC precursors [84, 85] (**Figure 17**).

## **4. Photophysical studies of N-heterocyclic carbene**

New efficient light-emitting materials related to iridium(III) and platinum(II) complexes have attracted research area and wide range of applications in OLED and WOLED technologies. The suitable ligand based iridium(III) and platinum(II)

**Figure 17.** *Various strategies for generation of carbene.*

complexes allow tailoring the emission properties for specific application in organic light emitting devices (OLEDs) and white organic light emitting devices (WOLEDs).

In particular, extensive investigations have been carried out on iridium(III) and platinum(II) complexes as triplet emitters in OLEDs. In OLEDs, significant progress has been achieved for making highly efficient and stable green and red emitters. But further advancement in recent progress of solid state full-colored OLED displays and WOLEDs appliances is also required in the research area of blue and white light emitting iridium(III) and platinum(II) complexes.

#### **4.1 Photophysical studies of N-heterocyclic carbene platinum(II) complexes**

Here N-heterocyclic carbene platinum(II) complexes are selected to investigated their photophysical properties (**Figure 18**). Selected N-heterocyclic carbene platinum(II) complexes have shown distinct absorption bands in 325–405 nm region with higher extinction coefficients (order of 10<sup>3</sup> or 10<sup>4</sup> ). These complexes are known as either blue, bluish green, or green emitters depending on emission bands within the 430–530 nm region with large Stokes shifts.

The [PtII(C^N^C)Cl][PF6] complex shows strong absorption bands at 272 nm and moderately intense bands at364 nm with higher extinction coefficients (<sup>ε</sup> � <sup>10</sup><sup>3</sup> –104 ) in acetonitrile (**Figure 19**). The high-energy intense absorption band (277–291 nm) is assigned as π ! π\* transitions (IL: Intra ligand) of the C � CR and C^N^C pincer ligands, whereas the low-energy absorption band (band (383–471 nm)) is observed due to presence of the dπ(Pt) ! π\* (C^N^C)] transitions, and [π(C � CR) ! π\* (C^N^C)] transition, considered as metal-to-ligand charge-transfer (MLCT), ligand-to-ligand charge-transfer (LLCT), mixed with the π ! π\* transitions (IL) of the C^N^C pincer ligands [75–79].

A blue-shifted absorption band (383 nm) of Complex **1** with the alkylalkynyl ligand is observed compared to Complex 3 (appeared at 405 nm) with the phenylalkynyl because of weak π-donating ability of alkylalkynyl ligand. The

*Recent Development of Carbenes: Synthesis, Structure, Photophysical Properties… DOI: http://dx.doi.org/10.5772/intechopen.101413*

**Figure 18.** *Structures of pyridine-based N-Heterocyclic Carbene Platinum(II) Complexes (1–6).*

**Figure 19.**

*UV–Vis spectra of pyridine-based N-Heterocyclic Carbene Platinum(II) Complexes 1–6 in ACN at 298 K (Reprinted with permission from Chem. Eur. J. 2013, 19, 10,360–10,369. Copyright@ 2013 Wiley-VCH).*

absorption band is redshifted due to increasing the π-electron-donating property of arylalkynyl ligand is assigned as MLCT/ LLCT transition.

The MLCT/LLCT absorption band is sensitive towards polarity of the solvents and shows a blue shifted absorption band from DCM (382 nm) to ACN (376 nm). A negative solvatochromism is observed in Pt (II)–polypyridine [82, 83], and Pt (II)– bzimpy complexes [76, 79] because of decreasing dipole moment during electronic transition.

Non-emissive nature of [PtII(C^N^C)Cl][PF6] complex in ACN is can be explained on the basis of low-energy d-d ligand field (LF) states, and effective quenched <sup>3</sup> MLCT/3 IL state [86]. In contrast to the [PtII(C^N^C)Cl][PF6] complex, the tridentate pyridine-based N-heterocyclic carbene ligand based alkynylplatinum(II) complexes 1–5 (**Figure 18**) exhibit strong luminescence in solution with gaussian shaped emission bands (range: 497–631 nm) (**Figure 20**). Only alkynylplatinum(II) complex 6 (**Figure 18**) shows non-emissive character in solution. Interestingly, all alkynylplatinum(II) complexes 1–6 (**Figure 18**) have shown emissive character at low temperature in solid state and glass matrices at 77 K.

The large Stokes shifts and lifetimes in the microsecond are originated from triplet energy state. The emission bands are appeared from an predominantly 3 MLCT excited state of [dπ(Pt) ! π\* (C^N^C)] transition, along with 3 LLCT [π(C � CR) ! π\*(C^N^C)] transition (**Figure 20**). Moreover, CT band and emission band of these metal complexes is altered depending on the nature of the substituted phenyl ring of alkynyl ligands in solution.

The intense luminescence from green to yellow and high PL quantum yield of alkynylplatinum(II) complexes 1–5 can be readily achieved by alternation of alkynyl ligands. The electron-rich moiety quenches the luminescence from 3 MLCT excited state via photoinduced electron transfer (PET) process is responsible for non-emissive property of complex 6 in solution [82, 83]. Depending upon increasing the polarity of the solvents, excited state (<sup>3</sup> MLCT/<sup>3</sup> LLCT) is lesser stabilized compared to its ground state, leading to a blue shift of absorption spectra in solution, shows negative solvatochromism for alkynylplatinum(II) complex 2. A red shift of the emission band state (559–640 nm) has also been observed for complexes 2–4 in solid at room temperature. The low-energy emission band (559–640 nm) is originated from triplet states (due to presence of metal to-ligand charge transfer (MMLCT) character) of alkynylplatinum(II) complexes 2–4 due to presence of significant contribution from Pt���Pt interaction in solid state [86].

#### **Figure 20.**

*Normalized emission spectra of pyridine-based N-Heterocyclic Carbene Platinum(II) Complexes 1–5 in ACN at 298 K (Reprinted with permission from Chem. Eur. J. 2013, 19, 10,360–10,369. Copyright@ 2013 Wiley-VCH).*

*Recent Development of Carbenes: Synthesis, Structure, Photophysical Properties… DOI: http://dx.doi.org/10.5772/intechopen.101413*

### **4.2 N-heterocyclic carbene Ir (III) complexes and their applications to deep-blue phosphorescent organic light-emitting diodes**

Absorption and emission spectra of N-heterocyclic carbene Ir (III) complexes 1–3 (**Figure 21**) are measured in DCM (**Figure 22**). The absorption band at around 320 nm is due to overlap of the π ! π\* transition of triazolate chelate, the benzyl (carbene) and pyridyl (triazolate fragment) and considered as LLCT transition. Furthermore, spin-orbit coupling is enhanced by iridium and plays significant role on triplet absorption cross section.

N-heterocyclic carbene Ir (III) complexes 1–3 (**Figure 21**) show emission band at 461, 460, and 458 nm, respectively in DCM. The weak phosphorescence intensity of complex 1 is observed at 392 nm and also indicated by its low quantum efficiency (QE) (only 5.0 � <sup>10</sup>�<sup>4</sup> ). It is observed that the fluorescence quantum yield of complex 2 and 3, is much higher than that of complex 1. The radiative lifetimes of N-heterocyclic carbene Ir (III) complexes 1–3 confirm their phosphorescent nature. The nonradiative decay rate constants (knr) are found to be 1.2 � <sup>10</sup><sup>9</sup> , 3.5 � <sup>10</sup><sup>6</sup> and 7.0 � <sup>10</sup><sup>5</sup> <sup>s</sup> �<sup>1</sup> with large differences in quantum efficiency for complex 1–3 respectively.

## **5. Applications**

The carbene chemistry became more popular for their applications as organocatalysts [87, 88] and transition metal catalysts in the synthesis of complex

**Figure 21.** *Structures of N-heterocyclic carbene irridium (III) complexes (1–3).*

#### **Figure 22.**

*Absorption and fluorescence spectra of 1 (——), 2 ( … ..) and 3 (—) in CH2Cl2 at 298 K (Reprinted with permission from Angew. Chem. Int. Ed. 2008, 47, 4542. Copyright@ 2008 Wiley-VCH).*

**Figure 23.** *Types of oxidation reactions catalyzed by NHC-metal complexes.*

molecules [89, 90]. The N-heterocyclic carbene (NHCs) is widely used in organometallic chemistry during the last few years [47]. A brief summary of the applications include:

#### **5.1 Oxidation reactions catalyzed by NHC-metal complexes**

They can be employed for various types of oxidation reactions like (a) Opppenauer-Type Alcohol Oxidation [91] where smaller R groups show catalytic activity, (b) Palladium-Catalyzed Aerobic Alcohol Oxidation [92] or (c) Wacker-Type Oxidation [93] as shown in **Figure 23**.

In addition, NHC metal complexes have been extensively used for **oxidative cleavage of alkenes** [94] as well as **oxidation of methane** [33]. In this regard, it has been observed that electron deficient alkenes react slower than electron rich alkenes. The NHC-Ru complex remains stable throughout the course of oxidation reaction (**Figure 24**).

*Recent Development of Carbenes: Synthesis, Structure, Photophysical Properties… DOI: http://dx.doi.org/10.5772/intechopen.101413*

**Figure 24.** *NHC-Ru complex catalyzed oxidation of alkene.*

#### **5.2 Palladium catalyzed reactions**

NHCs are also frequently used in **Palladium catalyzed reactions** forming C-C bonds. Mori et al. reported the use of NHC ligand in allylic alkylation with excellent yield [95] (**Figure 25**).

Another important application has been the α-arylation of carbonyl compounds at moderate temperature and short reaction time [96]. The same strategy has been applied for esters and amides [60, 97] (**Figure 26**).

Besides these, NHC act as great ligands for Pd catalyzed various coupling reactions like Heck reaction [98], Negishi reaction [99], Sonagashira reaction [100], Suzuki-Miyaura reaction [101], Stille coupling [102], and Buchwald-Hartwig reaction [103].

The NHC ligands act as good catalysts for tandem coupling reactions as well [104]. The reaction proceeds via amination route (**Figure 27**).

## **5.3 NHC Complexes in Olefin Metathesis**

After the development of Grubbs I catalyst several modifications were carried out to develop more efficient catalysts. In this regard, NHC-Ruthenium complex

**Figure 25.** *Use of NHC ligands in allylic alkylation.*

**Figure 27.** *NHC-metal complex for tandem coupling reactions.*

was synthesized for RCM [105]. But the initial design did not show marked difference in activity compared to Grubbs I catalyst, Later on, some combination catalysts were developed which showed greater activity and selectivity [106–109]. Nowadays, these second-generation Grubbs' olefin metathesis catalysts are widely used for metathesis reactions (**Figure 28**).
