Nickel NHC complexes

For example, complex 37 with an imidazolin-2-ylidene and a methyl ligand in cis-position to each other decomposes to yield the 1,2,3-trimethylimidazolium salt 38, Pd°, and cod (Fig. 13) [124], Additional examples for the reductive elimination of 2-alkyl and 2-aryl substituted azohum salts from palladium or nickel NHC complexes have been reported [125, 126]. Today, reductive elimination reactions have been established as one important reaction pathway for the deactivation of catalytically active metal NHC complexes [126, 127]. 

Nickel/NHC complexes have been examined as catalysts for the Heck coupling as well. Inamoto and coworkers have discovered that a variety of aryl bromides and iodides could be coupled with acrylates using 5 mol % Ni(acac)2 and 5 mol % of the appropriate NHC salt in the presence of Na2CC>3 (Eq. 23) [57]. While the majority of aryl halides could be coupled using the... 

Figure 13.20 NegIshI couplings using nIckel-NHC complexes.

The first reported use of a metal-NHC complex in a Heck reaction was by Herrmann et al. using a palladium complex [47]. However, the use of nickel has not been investigated extensively. Work by Inamoto et al. showed the efficiency of Heck reactions catalyzed by nickel-NHC complexes formed in situ (Figure 13.21) [48]. This protocol allows for good yields with electron-rich aromatic systems and provides moderate yields with deactivated substrates. Further developments from Inamoto et al. utilized a pyridine bis-NHC complex shown in Figure 13.5 [49], which expanded the scope of substrates to include a variety of electronic variations of the aryl iodide. 

The synthesis of thioethers has been developed using various nickel-NHC complexes. The first report by Ying and coworkers [57] highlighted the abilitjr of a nickel catalyst to couple thiophenol with a variety of aryl bromides and iodides, producing high yields of diaryl thioethers. A recyclable nickel-NHC catalyst was also used in the formation of carbon-sulfur bonds, but was still restricted to the synthesis of diaryl thioethers [58]. Well-defined allyl nickel complexes were shown to be efficient for both the formation of carbon-sulfur and carbon-... 

HALF-SANDWICH IRON AND NICKEL NHC COMPLEXES AS CATALYSTS FOR REDUCTIONS... 

Addition of carbon monoxide to this complex results in the isolation of a tetrahedral cationic dicarbonyl complex [Ni(GO)2(trz)2][OTf]2. Due to their ionic character, these nickel NHC complexes are extremely soluble in polar solvents, including water, promising catalytic applications in aqueous media. 

A special type of reaction is observed with the platinum(IV) complex [PtI(Me)3] which cleaves the Af,N,Af, A -tetraphenyltetraaminoethylene under reduction to form the dimeric cyclometallated mono(NHC) complex of platinum(II) iodide [Eq. (31)]. Cyclometallation with the same ligand is also observed for ruthe-nium. Additional cyclometallations with various substituents of NHCs have been reported for ruthenium(II), rhodium(III), iridium(I), palladium(II), " and platinum(II). In the case of iridium, alkyl groups can be activated twice. In rare cases like for nickel(II) /x-bridging NHCs have been obtained. ... 

A variation of the thermal elimination of an alcohol from the neutral 2-alkoxy-1,2-dihydro-l//-imidazole is the preformation of a chelate vic-bisamine complex which is subsequently attacked by an ort/io-ester to form the desired NHC complex. This principle has been shown with nickel and platinum [Eq. 

For nickel(O) complexes prepared from Ni(r -cod)2 and an excess of the free NHC, it was shown that they exhibit outstanding catalytic activity in the Kumada-Corriu reaction at room temperature toward unreactive substrates like aryl chlorides and even aryl fluorides.Again, an essential element of these catalysts is the need for sterically demanding NHC ligands as observed for the palladium catalysts. 

While the reductive elimination is a major pathway for the deactivation of catalytically active NHC complexes [127, 128], it can also be utilized for selective transformations. Cavell et al. [135] described an interesting combination of oxidative addition and reductive elimination for the preparation of C2-alkylated imida-zohum salts. The in situ generated nickel catalyst [Ni(PPh3)2] oxidatively added the C2-H bond of an imidazolium salt to form a Ni hydrido complex. This complex reacts under alkene insertion into the Ni-H bond followed by reductive elimination of the 2-alkylimidazolium salt 39 (Fig. 14). Treatment of N-alkenyl functionalized azolium salts with [NiL2] (L = carbene or phosphine) resulted in the formation of five- and six-membered ring-fused azolium (type 40) and thiazolium salts [136, 137]. 

Density functional theory studies arene chromium tricarbonyls, 5, 255 beryllium monocyclopentadienyls, 2, 75 chromium carbonyls, 5, 228 in computational chemistry, 1, 663 Cp-amido titanium complexes, 4, 464—465 diiron carbonyl complexes, 6, 222 manganese carbonyls, 5, 763 molybdenum hexacarbonyl, 5, 392 and multiconfiguration techniques, 1, 649 neutral, cationic, anionic chromium carbonyls, 5, 203-204 nickel rj2-alkene complexes, 8, 134—135 palladium NHC complexes, 8, 234 Deoxygenative coupling, carbonyls to olefins, 11, 40 (+)-4,5-Deoxyneodolabelline, via ring-closing diene metathesis, 11, 219... 

Several systematic experimental and computational studies have compared the sigma-donating abilities of NHCs and tertiary phosphines for a variety of transition-metal complexes [8-17]. As illustrative examples, analyses of the nickel-carbonyl complex 1 and iridium carbonyl complex 2 (Fig. 1) re-... 

Table 1 DFT-calculated M-NHC bond dissociation energies (kcalmol 1) and °/oVbut f°r the carbene ligands in nickel-carbonyl complexes ...

The known crystal structures of these complexes show no undue strain upon the geometry inflicted by the tether length. Apparently, an ethyl bridge between the indenyl (flu-orenyl) part of the ligand and the NHC unit is sufficiently long. In a comparative study between a tethered [135] and an untethered [194] nickel(II) complex (see Figure 4.64), no significant differences in the steric parameters were reported. However, a downfield shift of A8 = 4.1 ppm (from 166.8 to 170.9 ppm) in the C-NMR spectrum for the tethered complex was considered by the authors to be due to a chelate effect [135]. 

Figure 4.64 A tethered and untethered nickel(ll) complex bearing a Cp and a NHC ligand.

Interestingly, when Ni(CO)4 was treated with two equivalents of IDM, the biscarbene complex (47) was formed. Several other NHC-nickel carbonyl complexes have been synthesized following the same route, as also complexes (48)-(50) containing IMes, IPr, SIMes, SIPr, and ICy, respectively, which have been obtained and characterized. Importantly, when more sterically demanding PBu and I Ad were used, dissociation of two CO molecules occurred, yielding a rare unsaturated three-coordinated [(NHC)Ni(CO)2] (51) and (52). These results show that the behavior difference between the various NHC ligands is more a consequence of their steric congestion than of their electronic properties. 

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