Abnormal binding in a carbene complex formed from an imidazolium salt and a metal hydride complex

Abstract

2-Pyridylmethylimidazolium salts and IrH5(PPh3)2 give an [(N-C)IrH2(PPh3)2]+ species with the imidazole ring bound in the 'wrong way': at C-5, not at the expected C-2.

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

“…Only recently, coordination via C4 has been identified as another bonding mode of such carbenes. 7 This bonding mode provides access to a new type of NHC ligands in which the heteroatoms are not located in a,a 0 position but in a,b 0 position with respect to the metal-bound carbon. As a consequence, no canonical resonance form of the carbene can be drawn without introducing additional charges.…”

Palladium nanoparticle catalysts in ionic liquids: synthesis, characterisation and selective partial hydrogenation of alkynes to Z-alkenes

“…3a) were observed, it cannot be discarded that classical and abnormal N-heterocyclic carbenes (NHCs) are formed in situ and these species might act as ligand. 54,55 Therefore, Pd(II)carbene complexes may be formed, most likely by reaction of the imidazolium cation with Pd(0) species. 25,[56][57][58][59] Fig .…”

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)

“…Moreover, they can bind to the metal in a normal (I, III) or abnormal (II) fashion (Scheme 1) with the possibility of both normal and abnormal carbenes arising in a remote position. We have extended Crabtree's terminology 6,7 by using the term abnormal whenever it is not possible to illustrate the carbene structure without charge separation (in contrast to Ia and IIIa). 18 Remote denotes a carbene that contains no stabilizing heteroatom directly adjacent to the carbene carbon.…”

“…[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.…”