title: Protic N-Heterocyclic Carbene Versus Pyrazole: Rigorous Comparison of Proton- and Electron-Donating Abilities in a Pincer-Type Framework.
abstract: Evaluation of the acidity of proton-responsive ligands such as protic N-heterocyclic carbenes (NHCs) bearing an NH-wingtip provides a key to understanding the metal-ligand cooperation in enzymatic and artificial catalysis. Here, we design a CNN pincer-type ruthenium complex 2 bearing protic NHC and isoelectronic pyrazole units in a symmetrical skeleton, to compare their acidities and electron-donating abilities. The synthesis is achieved by direct C-H metalation of 2-(imidazol-1-yl)-6-(pyrazol-3-yl)pyridine with [RuCl2(PPh3)3].15N-Labeling experiments confirm that deprotonation of 2 occurs first at the pyrazole side, indicating clearly that the protic pyrazole is more acidic than the NHC group. The electrochemical measurements as well as derivatization to carbonyl complexes demonstrate that the protic NHC is more electron-donating than pyrazole in both protonated and deprotonated forms.
authors.affiliation: Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2 12-1 E4-1 O-okayama, Meguro-ku, Tokyo, 152-8552, Japan.
authors.forename: Tatsuro
authors.initials: T
authors.lastname: Toda
authors.affiliation: Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2 12-1 E4-1 O-okayama, Meguro-ku, Tokyo, 152-8552, Japan.
authors.forename: Akihiro
authors.initials: A
authors.lastname: Yoshinari
authors.affiliation: Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2 12-1 E4-1 O-okayama, Meguro-ku, Tokyo, 152-8552, Japan. tikariya@apc.titech.ac.jp.
authors.forename: Takao
authors.initials: T
authors.lastname: Ikariya
authors.affiliation: Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2 12-1 E4-1 O-okayama, Meguro-ku, Tokyo, 152-8552, Japan. skuwata@apc.titech.ac.jp.|PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan. skuwata@apc.titech.ac.jp.
authors.forename: Shigeki
authors.identifier: http://orcid.org/0000-0002-3165-9882
authors.initials: S
authors.lastname: Kuwata
keywords: N ligands; carbene ligands; carbonyl ligands; ruthenium; tridentate ligands