Abnormal carbene coordination

Note The chelate carbene complex with normal carbene coordination (C ) has a smaller bite angle and is therefore more strained than the chelate carbene complex with abnormal carbene coordination (C )... 

Guo T, Dechert S, Meyer F. Dinuclear paUadium complexes of pyrazole-bridged bis(NHC) Hgands a delicate balance between normal and abnormal carbene coordination. Organometallics. 2014 33 5145-5155. 

Scheme 4.5 Base-dependent normal and abnormal carbene coordination (R = Et, n-Bu, Mes).

Note The chelate carbene complexes with normal carbene coordination (O) do exist. They have even bulkier wingtip groups (tert-butyl instead of isopropyl) and a more crowded equatorial plane since the abnormally coordinated (C ) carbene chelate ligands are faced with sterically uniquely undemanding hydride ligands. 

Adduct 18 may be considered as model intermediate for reactions of the palladium complex with other electrophiles. For example, strong Bronsted acids like HCl, which reacted with the abnormal complex 15 but not with its normal counterpart, induced Ccarbene Pd bond cleavage to yield the imidazo-lium salt 20. However, the rates of this acidolysis were too fast to allow the detection of an intermediate similar to 18. Notably, the coordination of two abnormal carbenes inverts the electronic properties of the palladium centre. While palladium(II) is generally considered to be electrophilic, it is obviously nucleophilic in complex 15 and related species. This fact offers a plethora of new synthetic possibilities and unprecedented reactivity schemes, which cannot be accessed by using classical carbene or phosphine ligands. 

As a versatile alternative to the generation of free carbenes, Hashmi and co-workers developed a methodology for the synthesis of abnormal NHCs where the carbene is constructed directly at the metal center. Thus, a [3 + 2] dipolar cycloaddition of azomethine ylides and gold isonitriles afforded the abnormal saturated imidazolinylidene gold eomplexes 9 [eqn (3.3)]. This methodology circumvents the regioseleetivity issue of C4 vs. C5 eoordination because the carbenic carbon is pre-coordinated to the metal center in the isonitrile precursor. Moreover, this procedure provided aeeess to a new subclass of abnormal carbenes and yielded the first abnormal earbene complexes featuring a saturated heteroeyele, unlike all previous abnormal imidazolylidene complexes, derived from unsaturated heteroeyeles. 

Other interesting examples arise from the tautomerization of normal NHCs to form abnormal NHCs, a process that is generally assumed to be driven by the release of steric strain. For example, Dagorne and co-workers observed the tautomerization of complex 10 to form abnormal carbene complex 11, which contains only one bulky t-Bu group adjacent to the carbenic carbon (Scheme 3.2). Complex 11 constitutes a rare example of an abnormal NHC main group complex (see Chapter 5 for further examples). While the mechanism of this tautomerization remains elusive, the authors noted that the process is fast in a coordinating solvent such as THF (minutes), and slower in toluene [ca. 20 hours), which hints at a dissociation of the free carbene and Cnhc A1 bond cleavage. 

Direct C-H activation at abnormal carbene positions was achieved by heteroatom-directed cyclometalation reactions using pyridyl-functionalized azolium salts and [IrCp Cl2]2 (Cp = pentamethylcyclopentadienyl Scheme 3.4). Cyclometalation proceeded well with both triazolium salt 20 and imidazolium salt 22 and yielded iridium complexes 21 and 23 in good yields. Transmetalation from silver was not efficient in either case. The imidazolium precursor underwent activation and oxidation of the exocyclic C2-bound CH3 group, whereas triazolium salt 20 formed a mixture of compounds upon reaction with Ag20, probably due to poor selectivity and competitive coordination of the pyridyl nitrogen. 

By convention, the carbenes displayed in Fig. 1.7 are normal NHCs as they are coordinated to the metal centre throngh the atom. By contrast, abnormal (also named non-classical or unnsnal) are those bound through the atom. Abnormal is also a term used for NHCs having a valence representation requiring additional charges. Remote is a term nsed to describe a carbene which does not have any heteroatom on the a-position to the carbenic carbon (Fig. 1.8) [57]. 

In addition to complexes of type 44 with a C2 coordinated NHC ligand, complexes of type 45 with abnormal C4 or C5 bound carbene ligands have recently been described (Fig. 16) [143, 144]. The carbene carbon atom in these complexes is stabilized by only one nitrogen atom. A similar situation has been observed for the cyclic (alkyl)(amino)carbene (type 16, Fig. 6) [38, 39]. 

Four years after Grundemann et al. s original observation [49], Peris and coworkers [40] proved that steric strain is not the decisive factor in the abnormal coordination of the carbene in these particular iridium(ni) hydride complexes. We may well suspect that differences in... 

The corresponding reactions with iridium(I) precursors again behave differently. When the phosphino functionalised imidazolium salt is reacted with [IrlcodlCl], the phosphane adduct with a pendant imidazolium moiety is formed. A similar reaction with the more reactive [Ir(cod)( Li-H)(p,-Cl)2]2 yields a five coordinate iridium(I) complex that might be described as having square pyramidal geometry with the bromide in apical position and the carbene in abnormal coordination mode [47-49] (see Figure 3.97). 

Within the same triad, iron has the smaller ionic radius compared with ruthenium [20], although electronically and structurally the two elements should form the same complexes. They essentially do, but when a second pincer carbene ligand is coordinated, it coordinates with one NHC moiety in abnormal coordination mode [476] as opposed to ruthenium, where both pincer carbene ligands are coordinated normally [468], In the absence of coordinating anions (BPh instead of bromide) octahedral cationic complexes are formed instead of square pyramidal neutral ones (see Figure 3.158). The additional n-donor ligand compensates the positive charge electronically. 

The chemistry of NHC and of complexes bearing NHC continues to fascinate and surprise. The synthesis of abnormal coordinated carbenes (C5 coordination) in a process explored for more than 8 years is just one example that encourages chemists to examine closely the exact nature of active species involved in catalytic cycles. There is no doubt that recent and future developments in this field of chemistry will separate NHC ligands from the phosphine mimic label placed on them at the early development stages and will confer on them a distinct role in modern catalytic chemistry [102]. 

The first discussed example was reported by Bertrand and coworkers. By using different iminium salts they reported the coordination of an array of different abnormal NHC ligands, so-called CAAC ligands, to an AuQ fragment. Here, depending on the bulkiness of the free carbene used, the formation of mono- or biscarbene gold complexes is observed [12]. The synthesis of the most known representative of this class of Au(I) complexes in gold catalysis is shown in Scheme 9.3. The unique thermal stabiUty of this NHC-Au(I) complex is discussed in more detail in Section 9.3. 

A similar complex bearing a normal and an abnormal coordinated carbene moiety was reported by Zuo and Braunstein its catalytic activity in the transfer dehydrogenation of cyclooctene was however markedly lower than that of the complexes by Chianese/ Remarkably, CCC pincer complexes of iridium(III) developed by Chianese and by the gyoup of Herrmann and Kuhn " turned out to be active catalysts also for another C—H functionalization reaction, namely theborylation of arenes. Activities were however low compared to iridium(III) complexes with other ligands which were extensively optimized over the years. 

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