C4-bound imidazolylidenes: from curiosities to high-impact carbene ligands

Abstract

This feature article summarizes the progress achieved thus far in using C4-bound imidazolylidenes as a new class of ligands for transition metals. Since the discovery of this unusual carbene bonding mode in 2001, various rational routes towards complexes containing C4-bound carbenes have evolved. These advances allowed for studying the impact of this new type of ligand on the transition metal center, both from a fundamental point of view as well as from a more applied perspective, in particular for catalytic applications. The promising results accomplished in this relatively short period of time demonstrate the potential of C4-bound imidazolylidenes as unique carbene ligands for inducing catalytic activity and for mediating unprecedented transformations.

Pyridine-Derived N-Heterocyclic Carbenes: An Experimental and Theoretical Evaluation of the Bonding in and Reactivity of Selected Normal and Abnormal Complexes of Nickel(II) and Palladium(II)

“…However, the 13 Computational studies support such a conclusion, since the σ and π orbital contributions in pyridyl-metal and pyridylidene-metal bonds have been calculated to be remarkably similar. 24 Consistent with our conclusions from crystallographic analyses, the observed influence of the metal fragment on the heterocycle might be rationalized by non-negligible M→L π-backdonation.…”

“…Simultaneously, a secondary process apparently increases the electron density at the carbon atoms that are remote to the coordination site. Owing to this effect, the 13 to the mutual trans arrangement of nitrogen and carbene carbon atoms, the polarization in the complex is sufficiently large to render the carbons in ortho and meta position almost magnetically equivalent. This result lends further support to the carbene character of C4-pyridylidene complexes (VIe, cf.…”

“…[1][2][3][4][5] Only recently it has been shown that an abnormal binding mode via the C4 (or C5) position of imidazolylidenes yields extraordinary electron rich metal centers. [6][7][8][9][10][11][12][13][14][15][16][17] These discoveries led to a considerably increased interest in "non-classical" NHCs, i.e. carbenes that are not stabilized by two adjacent heteroatoms.…”

Pyrazolium-sulfonates. Mesomeric betaines possessing iminium-sulfonate partial structures

“…The molecule crystallizes with one molecule of methanol. The C-S bond length was determined to be 177.1 pm, which is slightly shorter than the sum of the covalent radii of C and S [179 pm], 2 and considerably shorter than in the compounds mentioned above. The N-N bond length of pyrazole was found to be 137.3 pm.…”

“…Imidazol-2-ylidene is an NHC, whereas imidazol-4-ylidene -which cannot be represented by an electron sextet structureis a member of the class of aNHC. 2 Pyrazol-3-ylidene has been isolated as metal complexes, 3 trapped with heterocumulenes, 4 examined mass-spectrometrically, 5 and rearranged to 4-aminoquinolines. 6 It belongs to the class of NHCs whereas its isomer, pyrazol-4-ylidene has been described as remote N-heterocyclic carbene, 7 cyclic allene 8 or aromatic zwitterion.…”