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.

Incorporation of a Lewis acidic BMes 2 (Mes = mesityl) moiety at the backbone of the imidazole ring was achieved by metal−halogen exchange procedure. Among them, two isomeric boron-phosphine functionalized imidazoles (3 and 6), monoboron-functionalized imidazoles (4 and 5), and its corresponding imidazolium salts were synthesized and thoroughly characterized. The solid-state structure of 3 reveals a dimeric B−N adduct that possesses six-membered [C−B− N] 2 ring, and 5 crystallizes as tetrameric B−N adduct that forms an interesting 16membered macrocycle, whereas 4 and 6 were obtained as monomeric BMes 2substituted imidazoles. 6 behaves as a P^N-type ligand upon the coordination with CuI to afford luminescent L 2 Cu 4 I 4 -type metal complexes (10 and 11) whose photophysical properties were also studied. The presence (in 10) and the absence (in 11) of BMes 2 made a remarkable impact on fluorescence emission causing shift from the green (10) to orange (11) region. The fluoride sensing properties of BMes 2 -containing imidazoles (4 to 9) were studied using UV−vis and fluorescence spectroscopy.

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The majority of the copper N-heterocyclic carbene (NHC) complexes contain Cu in a + 1 oxidation state. The Cu(II)-NHC complexes reported so far usually require strict inert atmosphere conditions. In this work, we report a very facile and quantitative preparation of a series of new Cu(II)-NHC complexes ( 2a-c) starting from the inexpensive and readily available Cu(II) salts like Cu(OAc) 2 and CuCl 2 • 2H 2 O with imidazolium salts (1 a-c), wherein 2 b and 2 c were exclusively synthesized in water under aerobic condition. It should be emphasized that this is the first example where Cu(II)-NHC complexes have been prepared in the aqueous medium. All the complexes have manifested with a heterobidentate C, O chelate type of coordination to Cu(II) centers forming trans-[Cu(IRpyO) 2 ] (where R = Me, Mes, Dipp) type of complexes where 2 b forms a perfect square planar Cu(II)-NHC complex. Complexes 2 a-c were evaluated as catalysts for the aerobic oxidation of alcohols.

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