β‐Protic Pyrazole and N‐Heterocyclic Carbene Complexes: Synthesis, Properties, and Metal–Ligand Cooperative Bifunctional Catalysis

Abstract

This Minireview provides an overview of the chemistry of pyrazole and N-heterocyclic carbene (NHC) complexes bearing an NH group at the β-position to the metal. The synthesis and structures as well as the Brønsted acidic nature of the β-NH group are described in detail. These complexes are attractive candidates for novel metal-ligand cooperative bifunctional catalysts, which would benefit highly effective molecular and energy transformations.

Unsymmetrical Pincer‐Type Ruthenium Complex Containing β‐Protic Pyrazole and N‐Heterocyclic Carbene Arms: Comparison of Brønsted Acidity of NH Groups in Second Coordination Sphere

“…The β‐nitrogen atom in the NHC unit lies close to the chlorido ligand (N3⋅⋅⋅Cl1 3.203 Å, mean of two crystallographically independent molecules), whereas that in the pyrazole unit is more separated (N1⋅⋅⋅Cl1 3.887 Å, mean) in agreement with the deprotonation of the pyrazole arm, as well as the presence of an intermolecular hydrogen bond between the protic NHC and chlorido ligand. The decreased N2‐N1‐C1 angle in the pyrazole ring [106.7° (mean) vs. 111.8(3)° in 2 ] is also diagnostic of the NH deprotonation 4. In contrast, the metrical parameters in the NHC ring of 2 and 3 are almost identical as summarized in Table 1.…”

“…An unprecedented protic NHC–pyrazole hybrid 2 was obtained by chelation‐assisted tautomerization of the imidazole group4, 17–20, 22 in a 2‐(imidazol‐1‐yl)methyl‐6‐(pyrazol‐3‐yl)pyridine as shown in Equation (1). …”

Reactivity studies of pincer bis-protic N-heterocyclic carbene complexes of platinum and palladium under basic conditions

“…Less common are protic imidazol-2-ylidene (PNHC) ligands with a hydrogen atom on one or both of the stabilizing nitrogens. The synthesis of PNHC complexes has proven to be a challenge, which has limited studies of their reactivity [4–8]. …”

Synthesis, Structures, and Transfer Hydrogenation Catalysis of Bifunctional Iridium Complexes Bearing a C–N Chelate Oxime Ligand

“…[5,6] The cyclometalation methodology is also applicable to N-heteroaromatics, which leads to the formation of the structurally diverse bifunctional N-heterocyclic carbene [7] and pyrazole [8] complexes bearing an NH group in the β-position to the metal. [9] We have further explored such β-protic bifunctional catalysts [10] derived from the cyclometalation of appropriately designed ligand precursors. In this paper, we report cyclometalated ketoxime complexes as new β-OH bifunctional catalysts for the transfer hydrogenation of ketones.…”