Nickel(II)- and Palladium(II)-NHC Complexes from Hydroxypyridine Functionalized C,O Chelate Type Ligands: Synthesis, Structure, and Catalytic Activity toward Kumada–Tamao–Corriu Reaction

Abstract

Hydroxypyridine functionalized imidazolium salts (2a−c) have been prepared in a one pot neat reaction between alkyl/aryl imidazoles and 2-chloro-3-hydroxypyridine. The imidazolium salts were used as proligands for the synthesis of new Ni(II) (3a, 3c, and 3c′) and Pd(II) NHC (4a−4c) complexes. Complexes, 3a, 3c, and 4a−4c are four coordinated with square planar geometry around the metal center and feature the C,O chelation of the heterobidentate NHC ligands, using carbene atoms and the O atom of the hydroxypyridine arm. Depending upon the steric bulk of the alkyl/aryl substituents on the ligand, either cis (3a and 4a) or trans (3c, 4b, and 4c) complexes are obtained. For the nickel complex with the 2c ligand, two different isomeric forms (3c and 3c′) were observed. In 3c, both the NHC ligands are bound via C2 and O atoms of the hydroxypyridine arm, whereas, in 3c′, one of the two ligands binds via C4 and N atoms of the hydroxypyridine side arm. 3c′ represents a unique square planar complex with the central metal atom bound simultaneously to both normal and abnormal carbenes. All the five complexes, except 3c′, were evaluated as catalysts for the Kumada−Tamao−Corriu cross-coupling reaction between phenylmagnesium bromide and different aryl chlorides at room temperature.

Efficient Synthesis of Cu(II)‐N‐Heterocyclic Carbene Complexes in Water and Their Activity Towards Aerobic Alcohol Oxidation

“…Unlike nickel complexes reported earlier, [12d] all the Cu(II) complexes are four coordinate [Cu(IRpyO) 2 ; where R=Me, Mes, Dipp] with the Cu‐atom bound by two heterobidentate C n , O chelation leaving pyridine nitrogen atom free. The central Cu‐atom is coordinated to two carbene carbons and two oxygen atoms of the pyridinoxy side‐arm in a trans fashion (Figure 2).…”

“…Cu(OAc) 2 , were purchased and used as received. Imidazolium chlorides 1 a – c were prepared according to the already reported procedure [12d] . Melting points were taken in a capillary (sealed tubes) and the values reported are uncorrected.…”

“…Our laboratory has been working on the functionalized NHCs and the catalytic aspects of their metal complexes [12] . In continuation, we have recently synthesized a series of new hydroxypyridine functionalized imidazolium salts ( 1 a – c ) as precursors for the synthesis of Ni(II)‐ and Pd(II)‐NHC complexes and evaluated their catalytic properties [12d] . In the ligand, the OH functionality helps (a) to increase the water solubility of the imidazolium salts, (b) to act as an in‐situ base in an otherwise (external) base assisted catalysis like alcohol oxidation, and (c) to bind to the hard acid centers (like high oxidation state metal ions).…”

“…In the ligand, the OH functionality helps (a) to increase the water solubility of the imidazolium salts, (b) to act as an in‐situ base in an otherwise (external) base assisted catalysis like alcohol oxidation, and (c) to bind to the hard acid centers (like high oxidation state metal ions). In addition, the presence of hemilabile pyridine nitrogen makes it possible for the ligand: (d) to bind to the metal center in different coordination modes [12d] and (e) to coordinate to the metal ion, in lieu of oxygen during the catalytic process and stabilize the metal complexes during the catalytic cycle. Several examples of copper(II) ions containing different coordination geometries, like octahedral, tetrahedral, and square planar, are known with O,O/N,N/N,O/ C n , O chelates, and some of them are found to be very active in alcohol oxidation [13] .…”

Status of Organometallics and Catalysis Research in India: N–Heterocyclic Carbene (NHC) Ligands for Organometallic Catalysis

“…Ghosh at IITM has combined organometallic fragments with borane constructs (De et al, 2018;Anju et al, 2014;Saha et al, 2017). Anantharaman (IITK) studies metal-NHC compounds and their catalytic activity towards cross-coupling reactions (Karthik et al, 2014;Bhat et al, 2019;Karthik et al, 2013).…”