A reaction of a 2-(imidazol-1-yl)methyl-6-(pyrazol-3-yl)pyridine with [RuCl2 (PPh3 )3 ] resulted in tautomerization of the imidazole unit to afford the unsymmetrical pincer-type ruthenium complex 2 containing a protic pyrazole and N-heterocyclic carbene (NHC) arms. Deprotonation of 2 with one equivalent of a base led to the formation of the NHC-pyrazolato complex 3, indicating that the protic NHC arm is less acidic. When 2 was treated with two equivalents of a base under H2 or in 2-propanol, the hydrido complex 4 containing protic NHC and pyrazolato groups was obtained through metal-ligand cooperation.

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 [RuCl (PPh ) ]. N-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.

The reaction of the 1,3-bis(pyrazol-3-yl)benzenes 1 with [RuCl(OAc)(PPh 3 ) 3 ] resulted in selective C−H cleavage at the 2-position of 1 to give the protic NCN pincer-type ruthenium(II) complexes [RuCl(R-NCN-LH 2 )-(PPh 3 ) 2 ] (2; R-NCN-LH 2 = 4-R-2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)phenyl). Similar cyclometalation with iridium trichloride followed by addition of triphenylphosphine led to the formation of the iridium(III) analogue [IrCl(Bu t -NCN-LH 2 )(PPh 3 )]Cl (5). Treatment of the ruthenium complexes 2 with carbon monoxide afforded the carbonyl complexes [Ru(CO)(R-NCN-LH 2 )(PPh 3 ) 2 ]Cl (4). On the other hand, the pyridine analogue of 1, 2,6-bis(5-tert-butyl-1H-pyrazol-3yl)pyridine (NNN-LH 2 ), reacted with iridium trichloride to yield the protic NNN pincer-type complex [IrCl 3 (NNN-LH 3 )] ( 7). The stronger σ donation of the NCN pincer-type ligand in comparison with the analogous NNN ligand was suggested by the CO stretching frequencies of the ruthenium carbonyl complexes 4 as well as the M−Cl distances. The catalytic activity of the ruthenium complexes 2a,b toward transfer hydrogenation of a ketone was also evaluated.

NNN and NCN pincer-type ruthenium(II) complexes featuring two protic pyrazol-3-yl arms with at rifluoromethyl (CF 3 )g roup at the 5-position weres ynthesized and structurally characterized to evaluate the impacto ft he substitution on the properties and catalysis. The increased Brønsted acidity by the highly electron-withdrawing CF 3 pendants was demonstrated by protonation-deprotonation experiments.B yc ontrast, the IR spectra of the carbonyl derivatives as well as the cyclic voltammogram indicated that the electron density of the ruthenium atom is negligibly influenced by the CF 3 group.C atalysis of these complexes in the decomposition of formic acid to dihydrogen and carbon dioxide was also examined. The NNNp incer-type complex 1a with the CF 3 group exhibited ah igherc atalytic activity than the tBu-substituted analogue 1b.I na ddition, the bis(CF 3 -pyrazolato) ammine derivative 4 catalyzed the reaction even in the absence of base additives.

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