# Pyridylidene transition metal complexes – An underexplored class of organometallic carbenes

Salwa Simona Jamil, Jamal Moussa

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Salwa Simona Jamil, Jamal Moussa. Pyridylidene transition metal complexes – An underexplored class of organometallic carbenes. Coordination Chemistry Reviews, 2025, 538, pp.216600. 10.1016/j.ccr.2025.216600. hal-05047026

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# Pyridylidene transition metal complexes – An underexplored class of organometallic carbenes

Salwa Simona Jamil, Jamal Moussa*

Sorbonne Université, Institut Parisien de Chimie Moléculaire, UMR 8232, 4 Place Jussieu, 75252, Paris Cedex 05, France

## ARTICLE INFO

### Keywords:

- NHCs
- Carbenes
- Pyridylidenes
- Organometallics

## ABSTRACT

Pyridylidene compounds, carbene isomers of related pyridines, remain a fascinating class of intermediates in organic synthesis. Although of high interest as ligands in organometallic and coordination chemistry, this aspect is by far underexplored because of obvious lack of general and efficient synthetic methodologies. This review summarizes the relatively few reported examples of organometallic compounds with pyridylidene ligands. Several important reported examples recently open the way to a promising development of this class of carbenes in organometallic catalysis, biology or materials sciences.

## 1. General and historical aspects: metal-free carbenes

Carbene compounds are neutral organic species featuring a divalent carbon atom with two free electrons. These elusive species have been used for more than a century as key highly reactive intermediates or synthetic tools in organic chemistry. The historically first report about the use of such species dates back to the end of XIXth and beginning of the XXth century by Curtius and Staudinger, they described the use of diazo and ketene compounds to generate free carbenes, the authors rapidly concluded that these type of compounds were highly reactive and certainly not isolable as such [1]. We can note that much earlier in 1835 Dumas and Peligot attempted to prepare the simplest carbene "CH₂" by dehydration of methanol, which seemed quite reasonable at that time prior to tetravalence of carbon was established [2].

In 1862 Geuther et al. suggested involvement of dichlorocarbene as a reactive intermediate, they in fact considered chloroform as "CH₂·HCl" [3]. Hine reinvestigated this description in 1950 [4]. Just after Doering et al. reported the addition of dichlorocarbene to olefins in 1954 proving definitely its generation from chloroform [5].

Decarboxylation reactions seem to be at the origin of history of what would be called much later N-heterocyclic carbenes (NHCs), this can be traced back to the end of the XIXth century. It was found between 1881–1909 that iminocarboxylic acids decarboxylate very quickly [6].

In 1924, the oxidative deamination of amino-acids suggested the transient carbene intermediate that is immediately protonated (Scheme 1) [7].

This route has, in particular, been explored with hetarenum-

carboxylates to lead to different types of NHCs (Scheme 2), among which one can find the seminal example of the so-called “Hammick intermediate” that evolves from spontaneous decarboxylation of α-picolinic acid upon heating (Scheme 3) [8].

In the 1960s, Wanzlick and Breslow found that presence of amino substituents on the carbene provided significant stabilization, however they could only isolate the dimerization products of the so-postulated intermediates [9]. During this period emerged also the great discovery of organometallic carbene complexes by the pioneering groups of Fischer, Ofèle, Lappert, Stone and Wanzlick [10]. (see next section)

It is only in 1988 that the group of Bertrand et al. isolated for the first time a stable metal free carbene at room temperature, in this compound the carbene is stabilized by phosphorus and silicon substituents [11].

Three years later Arduengo et al. described in a landmark paper the preparation and kinetic stabilization of the first metal-free diaminocarbene what will be called later as Arduengo's carbenes [12]. This seminal work by Arduengo et al. has led to an explosion in the use and applications of imidazolylidene ligands in organic/organometallic catalysis [13], in materials chemistry [14] as well as medicinal chemistry [15]. In stark contrast, when expanding the five-membered ring to pyridine, the related carbenes so-called *pyridylidenes* become much less obvious to generate and hence to handle. This is due to the chemical reactivity of pyridine/pyridinium compounds in particular toward nucleophiles/bases that hampers the isolation of these highly reactive species.

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: jamal.moussa@sorbonne-universite.fr (J. Moussa).</footnote>
<figure><img src="image_2.png" /><figcaption>**Scheme 1.** Oxidative deamination of amino-acids leading to the transient carbene intermediate.</figcaption></figure>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Scheme 2.** Decarboxylation of hetarenium-carboxylates leading to different types of NHCs.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Scheme 3.** Hammick intermediate generation from 2-picolinic acid.</figcaption></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Scheme 4.** Decarboxylation of N-methylpyridinium-2-carboxylate and the related elusive intermediate N-methyl-pyridin-2-ylidene trapped with various electrophiles.</figcaption></figure>

## 2. General and historical aspects: case of pyridylidenes

As mentioned above, the Hammick equilibrium leads to pyridine-2-ylidene upon heating of α-picolinic acid that undergoes facile decarboxylation. This pyridylidene is about 50 Kcal/mol less stable than related pyridine isomer. The carbene function in pyridylidenes can also be generated at the positions 3- and 4- of the pyridyl ring.

Existence of this elusive intermediate was proven by combined computational and mass spectrometry investigations in 1996 [16]. This decarboxylation process is also observed in the case of N-methylpyridinium-2-carboxylate at 60 °C to generate the corresponding elusive intermediate N-methyl-pyridin-2-ylidene (2); the latter could be trapped with various electrophiles such as elemental sulfur and carbon disulfide [17], protons [18], or arylaldehydes (Scheme 4) [19].

Although the chemistry of carbenes of the NHCs family has been extremely developed in recent years, despite the numerous nitrogenous N-heterocycles which can be classified as carbenes, research has essentially focused on carbenes of the imidazolylidene type, known as Arduengo's carbenes. It is only recently that imidazolylidene carbenes with an unusual bonding mode, via the C4 or C5 carbon atoms have been characterized. Their σ-donating power was found to be stronger than that of the carbenes coordinated via the C2 carbon [20]. This discovery subsequently raised interest in "non-classical" NHCs, carbenes that are not stabilized by two adjacent heteroatoms, such as pyridylidenes in particular. Indeed, the latter are carbenes formally generated from a pyridinium unit, therefore comprising only one nitrogen atom at the α or at γ positions. Theoretical studies have shown that the free 2- and 4-pyridylidene isomers are in a stable singlet state and exhibit a more markedly nucleophilic, basic, σ-donor and π-acceptor character than that of the imidazole-2-ylidenes [21].

Unlike the so-called Arduengo's carbenes, pyridylidenes are difficult to isolate and hence to handle, or even non-isolable, they have been poorly described in the literature and have always been observed coordinated to a metal. Only a recent work describes the possibility of observing a very specific free pyridylidene in solution by NMR spectroscopy at low temperature. The use of a strong base to generate these compounds from the corresponding pyridinium salts is tricky due to the sensitivity of pyridinium to thermodynamically and kinetically more favourable nucleophilic attacks. Pyridylidenes are very reactive species for which stabilization pathways are not yet established.

## 3. General and historical aspects: theoretical investigations

Recent DFT theoretical calculations at the B3LYP/6-31+G* and B3LYP/6-311+G** by Nyulázi et al. were reported to study the possible stabilization of the Hammick intermediate upon N-alkylation by adamantyl and by dialkylamino groups on the pyridyl ring. It was found that the most suitable targets for the most stabilized structure according to this study to be even more stable than 1,3-dimethylimidazol-2-ylidene. At the same period, DFT calculations using the PBE functional on the TZ2P basis set on 2- and 4-isomers of Pyridylidenes predict better σ-donor ability than the related five membered ring carbenes [22a].

Frison et al. reported an interesting theoretical investigation comparing various type of divalent carbon compounds. In this report, 2-, 3- and 4-pyridylidenes are compared to cyclic and acyclic carbene molecules. [22c] The authors evaluated the σ- and π-electronic properties of the studied carbenes, they found that the σ-basicity is highly influenced by the charge and the nature of the neighbouring atoms of the carbene centre. Determination of the proton affinity allowed to classify the pyridylidene structures a-c in Fig. 1 among the strongest σ-donating ligands based on the correlation demonstrated by Frenking et al. The π-acidity was also investigated based on the analysis of the π-electron density using Hückel model and the second proton affinity values versus the Kohn-Sham eigenvalues of the π-MO, example of compound c in Fig. 1 shows this class of carbenes to be strongly π-acidic.
<figure><img src="image_10.png" /><figcaption>**Fig. 1.** Theoretical comparative investigation of the electronic properties of different type of carbenes to pyridylidenes.</figcaption></figure>

Nechaev et al. recently reported an important DFT theoretical investigation of pyridylidene metal complexes. This study describes the pyridylidenes as singlet in the ground state, as stronger bases than classical imidazolylidenes but lower than bent allenes. This work analyses also the three different mesoionic and remote isomers of the so-called normal pyridin-2-ylidenes. The free pyridylidenes are found to strongly activate intramolecular C—H bonds leading to cyclization reactions. The study describes the exceptional coordination properties of pyridylidenes (and related diazines) using the model [ClRh(CO)₂(NHC)] complexes showing high metal-ligand binding energies which make them very attracting to stabilize coordinatively unsaturated species for instance. Theoretical and experimental ¹³C NMR data exploitation indicates a high degree of aromaticity in the coordinated ligands in contrast to the diene character in the related free pyridylidene. Analysis

<figure><img src="image_11.png" /><figcaption>**Scheme 5.** Attempted stabilization and isolation of a metal-free pyridin-2-ylidene.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Scheme 6.** Hypothetic dihydrogen activation by the elusive pyridylidene 6.</figcaption></figure>

of ligand dissociation energies indicates strength comparable to imidazolylidenes for normal pyridylidenes whereas mesoionic and remote isomers exhibit values superior than those found for abnormal or bent-allene derivatives. This trend is confirmed by the TEP (Tolman electronic parameter) values. The steric properties were estimated by the buried volume parameter (Vbur); High Vbur values (24.1 % to 25.7 %) were found for pyridylidenes in comparison to the five membered rings (20.5 % to 21.4 %); this greater steric shielding in the six membered ring is obviously the result of the increase in the valence angle of the ylidene centre (from 104 to 105° to 115–117°) leading to a closer proximity of the substituents toward the metal centre. In this work, the author anticipates interesting photoactive properties such as NIR emission in pyridylidene complexes of iridium and also some potential use in enantioselective organometallic catalysis [22d].

Itami and Segawa et al. attempted to stabilize a pyridin-2-ylidene in order to isolate the first metal-free compound of this fascinating family of carbenes. The authors demonstrated that substitution of the N-mesitylpyridinium with xylyl groups in positions 3 and 5 is not sufficient to achieve this goal. Beside they demonstrated that LiHMDS in THF allowed to generate the pyridin-2-ylidene (3) that was trapped by a chlorogold(I) moiety (4) or elemental sulfur (5). The authors were unable to isolate the free carbene 3, while attempting to characterize and isolate this elusive species, an intramolecular cyclization occurred followed by subsequent elimination of dihydrogen to provide the indole derivative (Scheme 5) [23].

Pfaltz and Maulteon et al. recently reported a pyridylidene-mediated dihydrogen activation coupled with catalytic imine reduction but the paper had been corrected later since the reaction mechanism was unclear and some experiments not reproducible. The authors suggested finally two independent parallel reactions namely a base mediated reduction of pyridinium salt via a pyridylidene intermediate 6 and a purely base-induced hydrogenation of imines (Scheme 6) [24].

As already mentioned, some metal complexes of pyridylidenes have been characterized but in comparison to imidazolylidenes type carbene metal compounds, these examples are relatively rare; at the time of writing the present review we could only find less than 50 reports describing pyridylidene metal complexes. Pyridylidenes can serve as strongly coordinating ligands with a high π-acidic feature in contrast to imidazolylidene type ligands. We present an overview of the transition metal complexes of pyridylidenes reported in the literature.

## 4. General and historical aspects: metal carbene complexes including pyridylidenes

Historically, the first metalated carbene complex dates back to the preparation by Chugaev et al. in 1915 [25] of a platinum complex but characterized only in 1970 so that this work is often omitted in most citations of early NHCs metal complexes [26]. In the 1960s Wanzlick, one of the pioneers in the coordination chemistry of this family of compounds, synthesized the first mercury(II) complex carrying an N-heterocyclic carbene [27]. These ligands have found important catalytic applications. Very recently, metal complexes with imidazol-2-ylidene based polydentate ligands have proven to be excellent candidates for various applications [28–31].

In comparison, pyridylidenes remain scarcely investigated for obvious synthetic challenges to be overcome - Pyridylidenes are isomers of pyridines with one carbene centre in either position 2- or 4- of the pyridyl ring - they have been shown to be even better σ-donors than imidazolylidenes, moreover pyridylidenes also behave as good π-acceptors in contrast to Arduengo's type NHCs which makes them attractive ligands to promote emission in transition metal complexes [32]. Ofèle et al. explored decomposition reactions of ionic complexes of chromium using chromium hydrides to reduce pyridinium ions by hydrogen transfer. However, when attempting to extend this methodology to imidazolium salts, they observed deprotonation of ring carbon 1 instead of the expected hydride transfer (Scheme 7). [10b] In the early
<figure><img src="image_13.png" /><figcaption>**Scheme 7.** The first imidazol-2-ylidene as a chromium(0) complex; attempt with pyridinium resulted in reduction of the ring and π-complexation to chromium.</figcaption></figure>

1970s, Stone et al. and Lappert et al. investigated the coordination chemistry of these carbene derived from platinum(II), palladium(II) and ruthenium(II) [33–36].

Obtaining organometallic complexes from free pyridylidenes similarly to the work of Arduengo et al. still constitutes a difficult preparation strategy due to the peculiar reactivity of the related pyridinium proligands. Pyridylidenes coordinated to transition metal centres can be generated through different synthetic strategies summarized in Scheme 8 including:

(i) The oxidative addition of C-X bonds (X = Cl, Br) of pyridinium halides to transition metal complexes, this strategy is obviously limited to metal centres capable of such a reactivity.

(ii) Direct electrophilic C-H bond activation of pyridinium salts. This strategy can sometimes be assisted by a base initially present on the metallic precursor as shown by Bercaw et al. they observed the cycloplatination of a pyridinium salt on a platinum(II) precursor possessing methyl ligands, [(CH₃)₂Pt(µ-SMe₂)]₂. This method results in a release of methane and has the advantage of proceeding at room temperature [37].

(iii) This strategy is based on the decarboxylation of pyridinium carboxylates as shown by Conejero et al. They have recently developed an original approach aiming at systematizing the complexation of pyridylidenes to different transition metals. Complexation of 2-carboxylate pyridinium derivatives has been applied to various iridium(I), rhodium(I), platinum(II) and gold(I) precursors.

(iv) Transmetallation is a method that has been developed relatively little or not at all for these ligands compared to imidazole-2-ylidenes. To our knowledge, there is only one very specific example in the literature described by Raubenheimer et al. of transfer of pyridylidene from pentacarbonylchromium(0) or tungsten(0) complexes to gold(I) and rhodium(I).

(v) Deprotonation followed by complexation or trapping with electrophilic reagents in situ as demonstrated by Segawa et al.

Despite the efforts accomplished in this field, the chemistry in general and the coordination chemistry in particular of pyridylidenes

<figure><img src="image_14.png" /><figcaption>**Scheme 8.** Different strategies leading to transition metal complexes of pyridylidenes.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Scheme 9.** Preparation of gold(I) pyridylidene complexes through sequential transmetallation and N-alkylation/protonation.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Scheme 10.** First and unique example of pyridylidene transmetallation.</figcaption></figure>

remains surprisingly poorly explored today compared to what exists around the carbenes of Arduengo's type. We present here an exhaustive overview of the reported examples of transition metal complexes with pyridylidene based ligands.

**5. Metal pyridylidene complexes: the d¹⁰ family**

Careful examination of literature shows only a handful d¹⁰ pyridylidene complexes. The first structurally reported pyridylidene gold(I) complex was described by Raubenheimer et al. in 1994. Treatment of 2-lithiopyridine with one or two equivalents of [ClAu(THT)] affords pyridylidene complexes **8** and **9**, upon N-protonation or methylation in a second step. Compound **8** was structurally characterized by a single-crystal X-Ray diffraction analysis, it confirms the proposed formula from spectroscopic data, it exhibits a gold(I) centre linearly coordinated to two carbene centres, the nitrogen atoms are protonated. The carbon gold(I) bond distances in this complex are in the lower values of those observed in related bis-carbene complexes (Scheme 9) [38].

In 2009 Raubenheimer et al. reported as far as we are aware the first and only pyridylidene transmetallation reaction from pentacarbonyl-chromium(0) and pentacarbonyltungsten(0) complexes to gold(I) ions according to Scheme 10 [39]. The authors followed a first strategy based on the oxidative addition of the CCl bond of 4-chloro-N-methypyridinium triflate to chromium in $Na_2[Cr(CO)_5]$ to obtain the pyridylidene

complex **10** that undergoes efficient transmetallation to gold(I) upon reaction with [ClAu(THT)] to provide compound **11** (Scheme 10). In a second approach, the authors prepared the pyridylidene pentacarbonylchromium(0) **12a** ($M=Cr$) or tungsten(0) **12b** ($M=W$) complexes by converting Fischer carbenes to $\alpha,\beta$-unsaturated compounds [M(C(OMe)CH=C(R₁)NHR₂)(CO)₅] ($M=Cr, W$) upon treatment with n-BuLi and N-methylacetonitrile salt. Ring closure is then performed by reacting [M(C(OMe)CH=C(R₁)NHR₂)(CO)₅] ($M=Cr, W$) with the suitable alkyne RCCH. Finally the suitable pyridylidene complex [M(C(OMe)CH=C(R₁)NHR₂)(CO)₅] ($M=Cr, W, 12b$) is treated with [ClAu(THT)] ($M=W$) or [ClAu(PPh₃)] ($M=Cr$) to afford the target compounds **13** and **14** respectively. The gold(I) pyridylidene complexes prepared were structurally characterized. In all the reported complexes the gold(I) atom is linearly coordinated, the Au—C bond distances lie in the range of the shortest carbon-gold(I) observed in neutral chlorogold(I) complexes which indicates a strong C—Au interaction.

The year later, Conejero et al. described the application of an old strategy based on the decarboxylation of N-methyl-2-pyridinecarboxylates. In particular they could trap the in-situ generated pyridylidene ligand by [ClAu(SMe₂)] to obtain complexes **15a-b**. Compound **15a** was crystallographically characterized showing that gold(I) is in a linear geometry, the carbene-gold(I) bond distance is 2.015 Å, slightly longer than values in NHCs related complexes (Scheme 11) [40].

An interesting attempt to isolate a free sterically hindered
<figure><img src="image_17.png" /><figcaption>**Scheme 11.** Gold(I) complexes obtained by trapping the in-situ generated pyridylidene from the pyridinium carboxylate.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>**Scheme 12.** Trapping of the elusive pyridylidene generated in-situ by elemental sulfur or by gold(I) chloride.</figcaption></figure>

pyridylidene 3 species was reported by Itami and Segawa et al. in 2012 as already mentioned in the introduction (Scheme 12). The authors attempted a kinetic stabilization by introducing xylyl bulky groups in positions 3 and 5 of the pyridinium ring, moreover the nitrogen atom was arylated by a mesityl group. Though, this relevant strategy did not allow to isolate the free pyridylidene in pure form, it was generated in-situ upon treatment of the pyridinium salt by LiHMDS or NaH and trapped by elemental sulfur or [ClAu(SMe₂)]. The gold complex was structurally characterized, the carbene-gold(I) bond distance is among the shortest reported Au—C values, the gold(I) chloride bond distance is significantly long indicating the strong trans-influence of the pyridylidene centre. Of note during their attempt to isolate and/or characterize the free pyridylidene ligand, a deprotonation/oxidative cyclization sequence was observed. This confirms the high reactivity of pyridyldenes and their subsequent isolation in pure form (Scheme 13) [41]. Later this group demonstrated the ability of this pyridylidene ligand coordinated to gold(I) to facilitate the oxidative C—H arylation of heterocycles. They proved that the superior σ-donating capacity of pyridyldenes regarding classical Arduengo's type carbenes favours the oxidative addition process to gold(I) in the pyridylidene complex 4 [42].

A peculiar example of pyridylidene silver(I) complexes was reported by Liu et al. that are key intermediates in the catalyzed oxidative

<figure><img src="image_19.png" /><figcaption>**Scheme 13.** Deprotonation/oxidative cyclization sequence during the attempt to isolate pyridylidene 3.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Scheme 14.** Silver(I) pyridylidene complexes formed through metal mediated cyclization.</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>**Scheme 15.** First and unique pyridylidene copper(I) complex ever reported in the literature. (R = $^{1}$Bu, Mes).</figcaption></figure>

fluorination of isoquinolinium salts. The authors put in evidence fast pyridylidene exchange and the selectivity of the mono-pyridylidene versus the bis-pyridylidene silver(I) complex was possible by adjusting the stoiechiometry of a Pyox coligand that acts as silver(I) scavenger (Scheme 14) [43].

So far, only one example of structurally characterized copper(I) pyridylidene complexes has been reported to date by Brown et al. inspired by the previously described work of Itami and Segawa, (Scheme 15). The so-called complexes were prepared following an adapted procedure used by Itami and Segawa. Treatment of the related pyridinium salts by CuCl in the presence of KOT-Bu provided the target complexes 18a-b. These complexes were found to act as efficient co-catalysts in Pd/Cu-catalyzed heteroarylboration of alkenylheteroarenes [44].

Overall this demonstrates that only a handful of d¹⁰ pyridylidene complexes were described in the literature. This reveals the challenging need of novel general synthetic procedures to be set-up in order to explore the great potential of these type of copper(I), silver(I) and gold(I) complexes in catalysis, biology or materials science.

## 6. Metal pyridylidene complexes: the d⁸ family

The first d⁸ metal-pyridylidene complexes were described in 1974 by the group of Stone (Scheme 16). They described the preparation of rhodium(I) and iridium(I) with N-methyl-2-pyridylidene complexes 19a-b by treatment of the corresponding anionic complex trans-[bis (carbonyltriphenylphosphine)M] (M = Rh, Ir).

Later in the 1980's Crociani et al. described palladium(II) and platinum(II) metal complexes containing a 2-pyridylidene ligand. The synthetic strategy relies on a first oxidative addition of C—Cl bond of 2-chloropyridine to Pd(0) or Pt(0) precursors followed by the N-methylation of the coordinated pyridine moiety providing the pyridyl-2-ylidene complexes 20 and 22 or 21 and 23 upon protonation instead of methylation (Scheme 17) [45]. The authors found that this pyridin-2-ylidene ligand showed a comparable trans-influence to tertiary phosphines. The platinum analogues exhibited a similar reactivity. The authors determined that the dissociation constant increased from palladium to platinum complexes, probably because of the increased d electron back-donation contributing to a carbene-like structure of the C₂ atom. To shed on light the π-acceptor character of these pyridylidene ligands, Crociani et al. prepared the 6-chloro and 6-pyridone analogues of the previous complexes; for all compounds, ¹H NMR data confirmed the restricted rotation of the M-C₆₃pyridylidene bond and subsequently a double bond character. The electrophilic character of C₆ atom was suggested to be related to a steric factor, derived from the phosphine in trans position, and to the d electron back-donation from the metal,
<figure><img src="image_22.png" /><figcaption>**Scheme 16.** First pyridylidene metal complexes of rhodium(I) and iridium(I).</figcaption></figure>

already been recognized by the group. Therefore, in the synthesis of the pyridone complexes the reaction rate decreased when the phosphine ligand is the bulkier PPh₃ and when the metal is changed from palladium to platinum which favours an increased d-backdonation and thus a higher contribution of the carbene-like structure of the C₂ atom (Scheme 18). Crociani and coworkers devoted then their studies to the σ-donor character of 2-pyridylidene ligands and for this purpose they prepared some Pd(II)-C₂ compounds, in which the number of the N atom in alpha position is changed. Interestingly, the 2-pyridylidene analogue show a stronger σ-donor character compared with other heterocycle, demonstrating the increased nucleophilic character of the C₂ atom when the number of nitrogen atoms in the heterocyclic ring is decreased.

At the same period the group of Wimmer et al. reported several palladium(II) and platinum(II) cyclometalated complexes with N-alkylbipyridyl derivatives obtained by electrophilic C—H bond activation providing pyridin-3-ylidene complexes 26 and 27 (Scheme 19)[46].

Reaction of N-methyl-2-chloropyridinium with the anionic [Cp*Rh(H)(SiMe₃)₂]*⁻ complex provides a very peculiar pentamethylcyclopentadienylrhodium(I) as bis-pyridin-2-ylidene complex 28 (Scheme 20) [47].

An important contribution was added by Bercaw et al.. The enhanced σ-donor and π-acceptor characters of pyridin-2-ylidene ligands led the group to investigate new complexes based on these intriguing ligands for C—H activation. For this purpose, they prepared by cyclometallation the first platinum(II) complexes based on pyridin-2-ylidene chelating

<figure><img src="image_23.png" /><figcaption>**Scheme 18.** Pyridone derived pyridylidene Pd(II) and Pt(II) complexes.</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>**Scheme 19.** Cyclometalated palladium(II) and platinum(II) pyridyl-3-ylidene complexes.</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>**Scheme 17.** Palladium(II) and platinum(II) pyridyl-2-ylidene complexes obtained by successive oxidative addition and N-alkylation or –protonation.</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>**Scheme 20.** Rhodium(I) bis-pyridyl-2-idene complex.</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>**Scheme 21.** Room-temperature cyclometallation of pyridyl-pyridinium ligands by a dimethylplatinum(II) reagent.</figcaption></figure>

ligands **29a-d** and **30a-d** (Scheme 21) [37].

The ¹³C NMR data showed the carbene signals at 160–175 ppm, which was recognized by the satellites due to the large ¹⁹⁵Pt–¹³C con-stant coupling, thus suggesting a high Pt—C bond order. The X-ray crystal structure shows significantly shortened platinum-carbon dis-tance compared with a classical Csp₂-Pt bond. These data demonstrate the strong π-acidity of the carbene ligand, increased with the electron-donor nature of the substituent on the pyridinium moiety. Further-more, in all cases the dimethyl sulphide and sulfoxide ligands take place in trans position with respect of the carbene moiety. To investigate the *trans*-effect and the *trans*-influence of these ligands, Bercaw et al. measured the degenerate DMSO exchange kinetics, in which the *trans*-influence can be determined by the bond lengths of the trans ligand to the pyridin-2-ylidene moiety. In comparison to the related classical bipyridine and phenylpyridine chelates, platinum-sulfur bond distances in complexes **30a-d** are significantly longer, this clearly demonstrates that the neutral N-heterocyclic carbene and the anionic sp²-hybridized phenyl carbon exhibit similar *trans*-influence. Nevertheless, the DMSO exchange rates showed a second-order rate constant for the pyridin-2-ylidene complex of three orders of magnitude faster than the 2-phenyl-pyridine analogue, which makes these N-heterocyclic pyridin-2-ylidene well suited for accelerating ligand substitution than formally anionic analogues. Bercaw and co-workers extended the study to the cyclo-metallated complexes **31a-c** with acac coligand and varying the group 10 metals (Ni, Pd, Pt) in Scheme 22.

As found in their previous studies, analysis of the X-ray structure showed a strong *trans*-influence of the carbene donor moiety which derived from the different M-O bond lengths values. The ratio of the metal-oxygen bond lengths suggests that the *trans*-influence increase with Pt = Pd > Ni, suggesting a stronger M-C bond. Furthermore, the complete cyclometallation in the case of palladium(II) and platinum(II) and protonolysis experiments showed the preference of these pyridin-2-ylidene for the second and third row transition metals, which better

support the large ligand field of the carbene donor moiety. Bercaw and co-workers investigated then the *trans*-effect of the pyridin-2-ylidene moiety, the rate constants of dimethylsulfide ligand by DMSO were found 32 to 1800 faster than related processes in the charge neutral analogous cyclometalated complex of 2-phenylpyridine. This important acceleration in the dimethylsulfide ligand substitution rate highlights the strong π-acceptor ability and *trans*-influence of the carbene donor.

Albrecht et al. described a palladium(II) pyridin-2-ylidene complex obtained by acetate assisted electrophilic C—H bond cyclometallation (Scheme 23). The titled complex was found to be catalytically active in Suzuki cross coupling of aryle iodides and bromides with phenylboronic acids [48].

An interesting report in 2007 described the preparation of a series of rhodium(I) based pyridin-2-ylidenes using isoquinolinium scaffolds (Scheme 24). The authors described the rhodiation of isoquinolin-2-ylidene derivatives by [ClRh(COD)]₂. The isoquinolinium derivatives were treated with KHDMS or KOTBu resulting in nucleophilic addition to

<figure><img src="image_28.png" /><figcaption>**Scheme 23.** Palladium(II) cyclometalated complex as precatalyst in Suzuki cross coupling of aryliodides and bromides with phenyboronic acids.</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>**Scheme 24.** Rhodium(I) pyridylidene complexes obtained by deprotonation of quinolinium and subsequent trapping in-situ.</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>**Scheme 22.** Cyclometalated pyridin-2-ylidene complexes of Ni(II), Pd(II) and Pt(II).</figcaption></figure>
the C=N bond, these adducts are presumably in equilibrium with the isoquinolin-2-ylidene form that can be trapped in situ with elemental sulfur or selenium and chlororhodium(I) ions. The rhodium(I) isoquinolin-2-ylidene complexes **35** described in this work exhibited a moderate catalytic activity in hydrosilylation of benzophenone. Interestingly enantiopure versions were also described [49].

Albrecht et al. reported the synthesis of palladium(II) pyridylidene complexes obtained upon N-protonation of the coordinated pyridyl moiety (Scheme 25). In a first step, 2-bromo-3-alkylpyridine derivatives undergo oxidative addition to [Pd(db a)₂] to provide dimeric species, then N-protonation affords the pyridin-2-ylidene compounds **36a-c**, of note N-alkylation resulted in decoordination of the dimethylamino or thioether chelating arm [50].

In 2009, some pyridin-2-ylidene palladium(II) complexes **37-39** were described by Jones et al. upon oxidative addition of a aldehyde functionalized 2-chloro-methylpyridinium salt (37), further condensation of the obtained complex with chiral amines provided chelating C=N bidentate pyridin-2-ylidene ligands coordinated to the palladium(II) centre (38a-b) [51]. With p-anisidine, it was possible to first functionalize the aldehyde moiety of the 3-chloropyridinium salt and then metalation with [Pd(db a)₂] occurs via oxidative addition. The chloride ligands in compounds **38a-b** and **39** can be readily abstracted by Me₃SiOTf to provide the related dicationic solvent complexes (Scheme 26).

A very important combined theoretical and experimental contribution in 2010 described various nickel(II) and palladium(II) complexes **40-43** of monodentate 2-, 3- and 4-pyridylidenes obtained by oxidative addition of the corresponding chloropyridinium salts (Scheme 27) [52]. This study allowed for a systematic comparison of the different pyridylidene isomers in particular by structural and ¹³C NMR analyses. The mesoionic pyridylidene complex requires longer reaction times (3 days)

with regard to the normal 2- and 4-pyridylidene counterparts (ca 17/18 h). In the case of the 2,4-dichloropyridinium salt selective metalation occurs with both nickel and palladium at the remote C4 position maybe because of the shielding effect of the methyl group although rather small according to the authors. Introduction of amino groups at the C2 or the C4 position did not result in effective electronic back donation from the amino moiety.

<figure><img src="image_31.png" /><figcaption>**Scheme 27.** Pyridylidene nickel(II), palladium(II) and platinum(II) complexes 40a-c/43a-c</figcaption></figure>

<figure><img src="image_32.png" /><figcaption>**Scheme 25.** Cyclometalated pyridin-2-ylidene palladium(II) complexes.</figcaption></figure>

<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /><figcaption>**Scheme 26.** Palladium(II) pyridin-2-ylidene complexes 37, 38a-b and 39</figcaption></figure>
<figure><img src="image_35.png" /><figcaption>(44)</figcaption></figure>

<figure><img src="image_36.png" /><figcaption>(46)</figcaption></figure>

<caption>(45)</caption>

<caption>**Scheme 28.** Rare C^C bis-pyridylidene palladium(II) complex.</caption>

In 2013 Segawa et al. described very peculiar palladium(II) complexes with a chelating bis-pyridylidene ligand (Scheme 28). Treatment of the bis-pyridinium salt with [Pd(PPh₃)₄] results in a first step in the oxidative addition of one C—Cl bond to provide the mono-pyridylidene complex 44, then a C—H bond activation takes place promoted by carbonates to afford complex 45, the latter compound could be isolated also directly by treatment of the bis-pyridinium salt with [Pd(PPh₃)₄] in presence of Cs₂CO₃. Addition of acetic acid to the carbonate compound 46 resulted in formation of a dimeric structure with two palladium-dipyridylidene units doubly bridged by acetate ligand. The charge neutral chelating bis-pyridylidene ligand in this bimetallic structure could be seen as a bipyridine or phenanthroline analogue though with much more σ-donating coordinating carbene centers as demonstrated by electrochemical measurements; compound 45 exhibits two oxidation waves at +0.51 V and +0.97 V (vs. Fc⁺/Fc) whereas phenanthroline nor bipyridine analogues do not show any oxidation wave up to +2.0 V. This suggests that the ligand would stabilize high oxidation states [53].

A gold(III) pyridylidene complex has been reported obtained from a metal mediated intramolecular cyclization (Scheme 29) [54].

Our group initiated a research program in last years to develop synthetic routes to prepare various transition metal complexes of pyridylidenes. In particular, we have started our investigations with exploration of the chemistry and the photophysical properties of the [Pt(CH₃)(C^N)]⁺ moiety of complex 29a with various monodentate phosphine and phosphinine ligands. The reported compounds were the first luminescent platinum(II) complexes based on a 2-pyridylidene ligand. The reaction of complex 29a in dichloromethane at room temperature with one equivalent of triphenylphosphine or 2,4,6-triphenylphosphine afforded 48a and 48b (Scheme 30).

<figure><img src="image_37.png" /><figcaption>(47)</figcaption></figure>

<figure><img src="image_38.png" /><figcaption>(29a)</figcaption></figure>

<figure><img src="image_39.png" /><figcaption>**Scheme 30.** First luminescent pyridylidene platinum(II) complexes.</figcaption></figure>

We have further set up a synthetic method to protodemethylate the methyl ligand coordinated to platinum(II) in [Pt(CH₃)(C^N)(SMe₂)]⁺ upon simple treatment with aqueous hydrochloric acid in methanol to provide the cornerstone compound [Pt(Cl)₂(C^N)] (49), remarkably the C—Pt bond is unaffected by the acidic conditions perhaps helped by precipitation of the target complex (Scheme 31).

This compound is an important precursor to the synthesis of complexes of the type [Pt(L^X)(C^N)]⁺ (L^X = acac, Q and Pic) (50–52) after chloride abstraction by silver(I) salts [55]. We have extended this approach to other C^N pyridylidene-heterarely bidentate ligands.

One year later our first report of luminescent platinum(II) pyridylidene complex, Chi et al. developed an appealing new class of intensely luminescent platinum(II) complexes bearing a pyridylidene-imidazolydene chelating ligand. These compounds were prepared by the deprotonation of a pyridinium-imidazolium precursor with NaHCO₃ followed by in situ cyclometallation with [Pt(Cl)₂(DMSO)₂]; then, treatment with bidentate pyrazole-based ligands in presence of sodium acetate at 110 °C affords the square-planar platinum(II) complexes 53–55 (Scheme 32) [56].

For the protonated-2-pyridylidene compounds, ¹H NMR in DMSO-d₆ solution showed a strongly downfield signal at 15.80 (53), 15.86 (54) and 15.91 (55), corresponding to the proton linked to the N-pyridylidene atom unit as well as a unique set of the alkyl group. X-ray crystal structures revealed a comparable Pt—C and Pt—N distances with some classical Pt(II)-NHC-based complexes, longer where the alkyl substituent is bulkier. Furthermore, it was found a shorter Pt—C pyridylidene distance of 1.948 Å than of Pt—C imidazolylidene (1.994 Å). This highlight the stronger σ-donating ability of pyridylidenes in comparison to classical Arduengo's type carbenes. Stronger *trans*-influence of the pyridylidene moiety is also manifested by the longer Pt—N distance trans to the pyridylidene than that of the imidazolylidene. The UV–Vis. absorption and emission of these novel compounds revealed important photoluminescence properties in the solid state. In particular aggregation phenomena imparted by π-π and Pt(II)···Pt(II) interactions were found to be responsible of MMLCT centred transitions. Associated theoretical studies showed that HOMOs are mainly localized on the pyrazolate chelating ligand with a small platinum(II) contribution, while the LUMOs are mainly developed on the pyridylidene moiety with a small platinum(II) contribution, thus regarding the transitions as a mixed MLMLCT.

This exhaustive list of examples of pyridylidene complexes of d⁸ transition metal shows how still is difficult to find general procedures to prepare pyridylidene complexes with useful chemical and physical properties.
<figure><img src="image_40.png" /></figure>

<figure><img src="image_41.png" /><figcaption>Scheme 31. Luminescent pyridylidene platinum(II) complexes.</figcaption></figure>

<figure><img src="image_42.png" /><figcaption>Scheme 32. Luminescent platinum(II) complexes.</figcaption></figure>

## 7. Metal pyridylidene complexes: the d⁶ family

Although transition metals of the d⁶ family such as Fe(II), Ru(II), Os (II), Rh(III), Ir(III) or zerovalent chromium family play a prominent role in organometallic chemistry, pyridylidene examples with these transition metals are rather limited. The first example of a d⁶ transition metal complex with a pyridylidene ligand was reported by the group of Stone in 1974, it is an iridium(III) complex 56 obtained from complex 19 upon treatment by excess of LiCl in refluxing methanol for 72 h (Scheme 33) [57].

In 1981 Serpone et al. reported an unusual coordination of bipyridine in complex 57 proved by an X-Ray diffraction study as depicted in Figure 2 [58]. This compound exhibits two classically coordinated bipyridyl bidentate ligands but one unusually bidentate third bipyridyl ligand that is coordinated through one of the nitrogen atoms but in a roll-over fashion through the C3 of the other pyridine ring, the nitrogen atom of the latter is protonated as suggested by the X-Ray diffraction study.

<figure><img src="image_43.png" /><figcaption>Scheme 33. First iridium(III) pyridylidene complex.</figcaption></figure>

First examples of the synthesis of 2-pyridylidene complexes through tautomerisation of pyridines and quinolines were described in middle 2000s. Carmona et al. studied the influence of substitution at the 2-position in pyridine on its coordination properties with the tris(pyrazolyl) borate iridium(III) complex [($Tp^{Me2}Ir(C_6H_5)_2(N_2)$)] containing the labile dinitrogen ligand. Treatment of complex [($Tp^{Me2}Ir(C_6H_5)_2(N_2)$)] with 2-substituted pyridines leads to replacement of the dinitrogen ligand by the related pyridine (Scheme 34) [59].
<figure><img src="image_44.png" /><figcaption>**Fig. 2.** Iridium(III) complex exhibiting a roll-over coordinated bipyridine ligand (pyridyl-3-idene).</figcaption></figure>

<figure><img src="image_45.png" /><figcaption>**Scheme 34.** Tautomerization of pyridine to pyridyl-2-ylidene.</figcaption></figure>

It was shown that the process involves a 1,2-H shift from the carbon to nitrogen of the pyridine which, in the case of bulky 2-pyridine, is in part due to its decreased Lewis basicity that destabilized the N-adduct by F-strain, while the carbene isomer is not concerned by this kind of steric repulsion. In the case of a smaller R substituent, Carmona and coworkers showed that a N-bound adduct is formed and in presence of water the side-product **58** is favoured instead of **59b**, but heating at 90 °C the complex **58** is totally converted to the corresponding carbene. X-ray crystal structure showed in all cases a normal Ir—C carbene bond length about 1.98, while the Ir—N is 0.15 Å longer than the covalent N—C sp² bond, thus confirming the favourable thermodynamic for the carbenic complexes. Studying the reactivity of complex **59a-b** with different saturated and unsaturated hydrocarbons, Carmona and coworkers reported the formation of iridium(III)-metallacyclic pyridylidene complexes, in which the reaction pathway was studied either with experimental and computational methods. In the case of 2-methylpyridylidene complex **59a**, the reactions with ethylene in benzene at high temperature gives a metallacyclic compound in high yield while under propene atmosphere, the reaction of the 2-phenylpyridylidene complex **59b** under the same reaction conditions leads to a regioselective product, in which the formation of the C—N bond takes place on the more substituted carbon atom. In both ways, the reaction pathway is in agreement with the generation of an unsaturated iridium(III) complex due to the loss of a benzene ligand.

In 2008, Li et al. described a series of iridium(III) hydride cyclometalated complexes **60a** and **60b** of a chelating pyridylidene ligand obtained by oxidative addition of C—H bonds of pyridine-tethered pyridinium halides to chlorocyclooctadieneiridium(I) dimer (Scheme 35). The compounds were structurally characterized. The mutual cis-orientation of the hydride and halide ligands was observed in the solid state for all complexes. No β-hydride insertion into the C—C bond of the cyclooctadiene ligand was observed, this has been rationalized by theoretical calculations that showed this step as thermodynamically

<figure><img src="image_46.png" /><figcaption>**Scheme 35.** Iridium(III) complexes **60a-b** obtained by oxidative addition of the C—H bond of the pyridinium derivatives.</figcaption></figure>

unfavoured. These iridium(III) pyridylidene complexes showed high catalytic activity in ketone transfer hydrogenation [60]. In this work the authors reported DFT calculations of the CO stretching frequencies on dicarbonyl model rhodium(I) complexes to show that normal and remote pyridylidene ligands in complexes **61** and **62** are more σ-donating than the normal and abnormal imidazolylidene ligands in comparable complexes (Scheme 36).

Esteruelas et al. described in 2007 pyridylidene derivatives of benzo [h]quinoline with ruthenium(II) and osmium(II) **64a-b** that undergo cyclometallation upon treatment with triethylamine (Scheme 37) [61].

In 2010, 1,9-phenanthroline was reported to undergo cyclometallation with [ClIr(COD)]₂ to provide complex **65** in which the non-coordinated nitrogen is protonated thus giving a carbene nature to the cyclometalated carbon (66) (Scheme 38). The N-bound proton interacts with the chloride counterion. Anion metathesis of the chloride by the lipophilic tetrapentafluorophenylborate anion results in loss of this hydrogen bond. The anion metathesis induces a 1,3-hydrogen migration equilibrated exchange. Temperature-dependant NMR studies revealed that the free energy difference is negligible between the two species (ΔG°²⁹⁸ = -0.09 kcal.mol⁻¹) with a small enthalpy variation (ΔH°²⁹⁸ = -1.3 kcal.mol⁻¹). This equilibrium was found to be sensitive to the nature of the phosphine [62]. DFT calculations have shown that the cyclometallation occurs via C—H oxidative addition as initial step providing a pentacoordinated iridium(III) hydride intermediate (66). This step is followed by two plausible pathways for the subsequent 1,3-migration of the iridium-bound hydrogen to the free nitrogen atom. One path involves a β-H insertion step and the second path more energetically favoured implies a water-assisted proton transfer [63].

In 2010 Albrecht et al. described the preparation of 3-pyridylidene-triazolylidene complexes **67a-e** from the related pyridinium-triazolium

<figure><img src="image_47.png" /><figcaption>**Scheme 36.** Calculated CO stretching frequencies of pyridylidene rhodium(I) complexes **61** and **62**.</figcaption></figure>

<figure><img src="image_48.png" /><figcaption>**Scheme 37.** Proton tautomerization in benzo[h]quinoline complexes of ruthenium(II) **64a** and osmium(II) **64b**.</figcaption></figure>
<figure><img src="image_49.png" /><figcaption>**Scheme 38.** 1,3-migration of the iridium-bound hydrogen to the free nitrogen atom in complexes 66–66'.</figcaption></figure>

<figure><img src="image_50.png" /><figcaption>**Scheme 39.** C<sub>sp2</sub>-H bond activation leading to 3-pyridylidene complex. If R = Bn, cyclometallation of C<sub>sp3</sub>-H of the methyl of the pyridinium occurs.</figcaption></figure>

<figure><img src="image_51.png" /><figcaption>**Scheme 40.** Cyclometallation of a pyridinium-imidazolylidene with rhodium(III).</figcaption></figure>

<figure><img src="image_52.png" /><figcaption>**Scheme 41.** Cyclometallation of a pyridinium-imidazolylidene with rhodium (III) and iridium(III).</figcaption></figure>

salts of the proligands upon reaction with [Cp\*Ir(Cl)₂]₂ in the presence of silver oxide (Scheme 39). The coordination is initiated by the triazolylidene metalation with silver(I) followed by transmetalation to the iridium(III) centre. This intermediate undergoes subsequent C<sub>sp2</sub>-H bond activation to provide the C\*C-chelated 3-pyridylidene complex 67a-e. In the case of R = Bn a complex with cyclometallation of C<sub>sp3</sub>-H of the methyl of the pyridinium moiety occurs providing a minor by-product. If

the benzylic substituent contains electro-poor moieties, C<sub>sp2</sub>-H cyclometallation is favoured. DFT calculations supported an electrophilic C—H bond activation mechanism [64].

Colbran et al. described the cyclometallation of a pyridinium-imidazolylidene proligand to provide complexes 68a-d in which the pyridinium moiety has been coordinated to the rhodium centre through C2 as a 2-pyridylidene ligand (Scheme 40). The reaction proceeds by reacting the proligand with [ClRh(COD)]₂ in the presence of various carboxylates to afford the pyridylidene-imidazolylidene chelated to rhodium(III). A plausible mechanism is the first coordination of the imidazolylidene part followed by oxidative addition of the C—H bond of the pyridinium moiety [65].

Similarly a series of rhodium(III) and iridium(III) with an imidazolylidene-pyridylidene ligand was reported by Peris et al. (Scheme 41). Reaction at high temperatures of the related proligands with [CIM(COD)]₂ (M = Rh, Ir) affords compounds 69a-b in which the ligand is chelated to the metal centre through the normal imidazolylidene and the remote C4 carbon of the pyridylidene moiety in contrast to complexes 68a-b with the nicotinamide ligand. Interestingly when [ClRh(NBD)]₂ (NBD = norbornadiene) is used instead of [ClRh(COD)]₂ the reaction under similar reaction conditions otherwise provides results in C2 metalation of the pyridylidene part rather than the C4 metalation. This selectivity is certainly due to steric factors [66].

Similarly reaction of the pyridinium salt in Scheme 42 with an imidazolium substituent in the 4 position with [CIM(COD)]₂ (M = Rh, Ir) provides the mesoionic complexes 70a-b. Experimental data showed that cyclooctadiene does not act as hydrogen acceptor ligand. Then a plausible mechanism perhaps involves the C<sub>imi</sub>-H oxidative addition followed by the C<sub>pyr</sub>-H bond activation by the hydride intermediate with release of dihydrogen. The Rh-N<sub>MeCN</sub> bond distance *trans* to the pyridylidene in compound 70a is considerably longer than the Rh-N<sub>MeCN</sub>

<figure><img src="image_53.png" /><figcaption>**Scheme 42.** Cyclometallation of a pyridinium-imidazolylidene.</figcaption></figure>
<figure><img src="image_54.png" /><figcaption>Scheme 43. Cyclometallation of a pyridinium-imidazolylidene with iridium(III).</figcaption></figure>

bond distance trans to the imidazolylidene ligand. This important difference marks the larger *trans*-influence of pyridylidene ligands compared to Arduengo's type carbenes.[65]

Peris et al. also reported a series of pyridylidene complexes of Cp\*Ir and Cp\*Rh metal centres and discovered unconventional reactivity. Treatment of the imidazolium-pyridinium proligands in Scheme 43 with [Cp\*Ir(Cl)₂]₂ in the presence of caesium carbonate and potassium iodide affords compounds 71 and 72 [67].

Both proligands give complexes with the pyridylidene linked through the remote C4 carbon but the coordination mode of imidazolylidene moiety differs between complex 71 and 72. In compound 72 the imidazolylidene ligand is coordinated by the normal C1 carbon while in complex 71 this moiety is coordinated through an exocyclic carbon of the initial methyl substituent. When [Cp\*Rh(Cl)₂]₂ is used instead of [Cp\*Ir(Cl)₂]₂ the coordination modes are more rich. For proligand with R = Me the exocyclic C—H bond activation of the methyl substituent on the imidazole ring is again observed as in the iridium congener, the difference remains in the fact that one of the methyl groups of the Cp\* has been coupled with a C6 carbon of the pyridinium ring as a consequence of a 3,6-cycloaddition of Cp\*Rh with dearomatization. When using proligand with R = H the authors isolated three different compounds 73, 74 and 74' (Scheme 44). In all three compounds the imidazolylidene ligand is coordinated through the normal mode by the C1 atom, the rhodium centre is moreover chelated by the pyridylidene part at the C2 position in compound 73 and in the C4 position in compound 74. In compound 74', one methyl substituent of the Cp\* ligand is coupled to the C2 carbon of the pyridinium ring perhaps due to the extreme basicity of pyridylidene function.

Some peculiar examples of ruthenium-pyridylidene type complexes 75a-c and 76a-c were described by Cabeza et al. Reaction of [Ru₃(CO)₁₂] N-methylpyridinium salts in scheme 45 in the presence of a strong base KHMDS affords the related pyridylidene ligands coordinated to three ruthenium atoms of the cluster in a face-capping mode in a bridging mode [68].

A specific unselective oxidative cyclization occurs by treatment of the inner-methylated N-fused porphyrin with [Re₂(CO)₁₀] to afford

<figure><img src="image_55.png" /></figure>

<figure><img src="image_56.png" /></figure>

<figure><img src="image_57.png" /><figcaption>Scheme 44. Cyclometallation of a pyridinium-imidazolylidene with rhodium(III).</figcaption></figure>

<figure><img src="image_58.png" /></figure>

<figure><img src="image_59.png" /><figcaption>Scheme 45. Bridging pyridylidene ligands.</figcaption></figure>

complexes 77 and 78 (Scheme 46) [69].

Another paper describing pyridylidene ruthenium complexes appeared in 2015 (Scheme 47). Cyclometalated 1,5, 1,6 and 1,7-phenanthroline complexes of bis-bibyridylruthenium(II) can be protonated at

<figure><img src="image_60.png" /><figcaption>Scheme 46. Oxidative cyclization leading to a N-fused pyridylidene ligand bound to rhenium.</figcaption></figure>

<figure><img src="image_61.png" /></figure>

<figure><img src="image_62.png" /></figure>

<figure><img src="image_63.png" /><figcaption>Scheme 47. Ruthenium(II) pyridylidene complexes.</figcaption></figure>
<figure><img src="image_64.png" /><figcaption>**Scheme 48.** Cyclometalated C=N pyridylidene complex of iridium(III) as switchable hydrogenation catalyst.</figcaption></figure>

<figure><img src="image_65.png" /><figcaption>**Scheme 49.** Ruthenium(II) complexes with a tridentate pyridylidene ligand.</figcaption></figure>

<figure><img src="image_66.png" /><figcaption>**Scheme 50.** Cyclometalated ruthenium(II) complexes with a tridentate C=N ligand.</figcaption></figure>

the free exo nitrogen atom providing pyridylidene complexes of ruthenium(II) **79–81** [70].

In 2016 interesting iridium(III) benzimidazole-pyridylidene complexes **82** and **83** were reported. These compounds were obtained by cyclometallation of the pyridinium proligand in **Scheme 48** with [Cp\*Ir(Cl)₂]₂ in the presence of bases such as acetates or carbonates. The remote NH moiety of the coordinated ligand in compound **82** undergoes a reversible deprotonation to provide the neutral complex **83**. This provides an interesting catalytic system that can be controlled by the pH input [71].

In 2003, an interesting pyridylidene derivative of bis-terpyridylruthenium(II) was described by Tanaka et al. they showed that treatment of trichoroterpyridylruthenium(III) by proligands 4'-methylthio-N''-methyl-2,2':6',4''-terpyridinium hexafluorophosphate and 4'-methylthio-N''-methyl-2,2':6',3''-terpyridinium hexafluorophosphate in ethanol in the presence of triethylamine provided compounds **84** and **85** in which the proligands were cyclometalated via base assisted electrophilic C—H activation (**Scheme 49**). Thus, pyridylidene derivatives **84** and **85** were described and characterized structurally as acetone solvates. In both cases the Ru—N bond distances trans to the C<sub>pyridylidene</sub> ligand are markedly longer than the other Ru—N bond distances, this highlights the strong trans-influence of pyridylidene ligands [72].

The same group reported complexes **86** and **87** using the previous proligands but with a ruthenium(II) brick containing 1,8-naphthyridine as bidentate ligand (**Scheme 50**). The cyclometallation provides compounds **86** and **87**. These compounds were found to be active catalysts in the electrochemical reduction of carbon dioxide [73].

This group also described pyridylidene derivatives of the bipyridine

<figure><img src="image_67.png" /><figcaption>**Scheme 51.** Pyridylidene ruthenium(II) derivatives of a bipyridine ligand.</figcaption></figure>

ligand. Treatment of N-methyl-2,3'-bipyridinium or N-methyl-2,4'-bipyridinium hexafluorophosphate salts with [(Cl)₂Ru(bpy)₂] in the presence of triethylamine provides complexes **88** and **89**. (**Scheme 51**) Electrochemical data supported a high donating property of the pyridylidene ligand in comparison to the bipyridine analogue [74].

A report in 2012 demonstrated the deactivation of a ruthenium(II) catalyst [CpRu(PPh₃)(py)₂]⁺ in the C—H bond activation of pyridine by terminal alkynes leads to a pyridylidene complexes **90a-b** that were isolated and structurally characterized (**Scheme 52**).

<figure><img src="image_68.png" /><figcaption>**Scheme 52.** C—H bond functionalization of pyridine by terminal alkynes through pyridylidene intermediates.</figcaption></figure>
<figure><img src="image_69.png" /><figcaption>**Scheme 53.** First luminescent bis-cyclometalated iridium(III) complexes of pyridylidenes.</figcaption></figure>

In 2015 Coe et al. described an interesting type of iridium(III) complexes bis-cyclometalated by a pyridyl-pyridylidene bidentate ligand (Scheme 53). Treatment of 1-methyl-3-(2"-pyridyl)pyridinium hexafluorophosphate with iridium trichloride at elevated temperatures provides the dimer **91** that upon reaction with bipyridyl derivatives provides compounds **92a-c**. Compounds **92a-c** were found to be intensely phosphorescent in the blue region. Anion metathesis of the hexafluorophosphates by chlorides provided water soluble compounds that are compatible with bioimaging media [75]. These compounds were used as photosensitizers in light driven biocatalytic reduction of α,β-unsaturated compounds by ene reductases [76]. The authors then studied the effect of the position of the quaternised nitrogen atom on the luminescence properties [77].

Compounds **92a-c** reported by Coe et al. were derived to prepare photocytotoxic agents [78] and DNA photo-cleaving species [79].

In 2017, a novel class of cyclometalated iridium(III) complexes containing a chelating pyridyl-pyridylidene ligand were reported and exhibited photoredox activity in oxidative hydrothiolation of alkenes and carbon dioxide reduction to carbon monoxide (Scheme 54). The target compounds **93a-e** were obtained at elevated temperatures via electrophilic C—H bond activation of the corresponding pyridinium salt. Some of the compounds were found to be highly phosphorescent [80].

Recently, Meggers et al. reported a highly important class of chiral-at-ruthenium bis-cyclometalated complexes of pyridylidenes (Scheme 55). Moreover, these compounds showed high catalytic activity in C—H bond amidation or Curtius rearrangement. The so-called compounds were obtained by cyclometallation of the phenanthrolinium salt by ruthenium trichloride at elevated temperatures to afford the chlorobridged dimer **rac-94**. Resolution of this racemic compound was performed by treatment with an enantiopure sulfinamide bidentate ligand that allows quantitative resolution of the Δ and Λ enantiomers of the ruthenium(II) complex as diastereoisomers. Replacement of the chiral auxiliary sulfinamide ligand by nitrile solvent molecules provides enantiomers of the complex [81].

<figure><img src="image_70.png" /><figcaption>**Scheme 55.** Chiral ruthenium(II) cyclometalated pyridylidene complexes.</figcaption></figure>

## 8. Conclusions and future perspectives

In summary, we have described exhaustively the few described examples of pyridylidene complexes of transition metals. This poorly documented field suffers from lack of general synthetic methodologies that hampers the development of such interesting ligands. In all the reported examples the metal-pyridylidene interaction is highly robust with intense trans-influence. In some cases, important catalytic/photo-catalytic and luminescent properties were shed light on. The development of general methodologies to access pyridylidene complexes is still highly challenging. Discovery of powerful synthetic methodologies will profoundly impact the organometallic chemistry in various fields such as catalysis, bioorganometallic chemistry or materials chemistry as demonstrated by the several recent examples using pyridylidene metal complexes with useful applications.

<figure><img src="image_71.png" /><figcaption>**Scheme 54.** Cyclometalated pyridylidene iridium(III) complexes as photoredox active catalysts.</figcaption></figure>
Declaration of competing interest

The authors declare the following financial interests/personal re-
lationships which may be considered as potential competing interests:
Jamal Moussa reports was provided by Sorbonne University - Pierre and
Marie Curie Campus. If there are other authors, they declare that they
have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.

Data availability

No data was used for the research described in the article.

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