Carbene binding, abnormal

Bidentate NHC-Pd complexes have been tested as hydrogenation catalysts of cyclooctene under mild conditions (room temperature, 1 atm, ethanol). The complex 22 (Fig. 2.5), featuring abnormal carbene binding from the O carbon of the imidazole heterocycles, has stronger Pd-C jj, bonds and more nucleophilic metal centre than the bound normal carbene chelate 21. The different ligand properties are reflected in the superior activity of 22 in the hydrogenation of cyclooctene at 1-2 mol% loadings under mild conditions. The exact reasons for the reactivity difference in terms of elementary reaction steps are not clearly understood [19]. 

The abnormal metallation is also favored when the conjugated pyridine-imid-azolylidene precursor (with a smaller bite angle) is used. For these precursors, an abnormal binding is produced even with a small wingtip group, as shown in Scheme 3.12. Under the same conditions, the pyridine-benzimidazolium analogue afforded the C2 carbene complex (Scheme 3.12) [27]. 

Treatment of the yttrium(III) adduct 60 with potassium naphthalenide in dme-diethyl ether mixture results in deprotonation of the C4 carbon and migration to afford the abnormal carbene complex 63 (Fig. 13).72 The C2 binding carbon migrates from the yttrium(III) centre to the incorporated potassium(I) cation. The C4 carbanion forms a short bond with the yttrium(III) centre in the solid state (2.447(2) A) and exhibits a large jYc coupling constant of 62 Hz in solution. Complex 63 may be quenched with a variety of electrophiles. For example, reaction with Me3SiCl silylates the NHC backbone to afford 64. 

Carbene complexes have been synthesized by various CH activation routes. For example, amines and ethers can undergo double geminal CH activation to yield a Fischer-type (heteroatom substituted) carbene and an imidazolium salt can yield iV-heterocyclic carbenes directly in the same way. In the latter case, abnormal binding via C-4 can occur as well as the usual C-2 binding. 

The increased donor properties of non-classical carbenes relative to their classical analogues were demonstrated both theoretically and experimentally. Complexes 14 and 15 were analysed by X-ray photoelectron spectroscopy. Both the palladium 3d and 3p electron binding energies in the abnormal complex 15 were lower by 0.5 eV than in the normal complex 14, which reflected the stronger donor capabilities of the abnormal carbene ligand. Furthermore, X-ray diffraction and infrared spectroscopic studies were used to demonstrate the larger trans influence of abnormal carbenes when compared to their normal analogues. ... 

Grundemann S, Kovacevic A, Albrecht M, FaUer JW, Crabtree RH. Abnormal binding in a carbene complex formed from an imidazolium salt and a metal hydride complex. Chem Commun. 2001 21 2274-2275. 

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