Steric Bulkiness-Dependent Structural Diversity in Nickel(II) Complexes of N-Heterocyclic Carbenes: Synthesis and Structural Characterization of Tetra-, Penta-, and Hexacoordinate Nickel Complexes

Abstract

A family of tetra-, penta-, and hexacoordinate nickel complexes of functionalized N-heterocyclic carbene (NHC) ligands have been prepared via carbene transfer reactions of Ni(PPh3)2Cl2 and silver-NHC complexes in situ generated from corresponding imidazolium salts and Ag2O. These nickel(II) complexes have been characterized by 1H NMR, 13CNMR, and ESI spectroscopy and X-ray diffraction analysis. Structural analysis revealed that [Ni(L1)3](PF6)2 (L1 = 3-butyl-1-(5,7-dimethyl-1,8-naphthyridin-2-yl)imidazolylidene, 3) is a rare example of a nickel complex with trigonal-bipyramidal geometry. [Ni(L4)2](PF6)2 (L4 = 1-butyl-3-((7-methyl-1,8-naphthyridin-2-yl)methyl)imidazolylidene, 5) and [Ni(L20)2](PF6)2 (L20 = 3-(pyridin-2-ylmethyl)-1-(pyrimidin-2-yl)imidazolylidene, 21) are normal square-planar. [Ni(L8)3](PF6)2 (L8 = 3-methyl-1-(pyrimidin-2-yl)imidazolylidene, 9), [Ni(L10)2(CH3CN)](PF6)2 (L10 = 3-ethyl-1-(pyrimidin-2-yl)imidazol-ylidene, 11), [Ni(L12)2(CH3CN)](PF6)2 (L12 = 3-butyl-1-(pyrimidin-2-yl)imidazolylidene, 13), [Ni(L14)2(CH3CN)](PF6)2 (L14 = 3-benzyl-1-(pyrimidin-2-yl)imidazolylidene, 15), and [Ni(L16)2(CH3CN)](PF6)2 (L16 = 3-isopropyl-1-(pyrimidin-2-yl)imidazolylidene, 17) are pentacoordinate, displaying unusual square-pyramidal geometry. [Ni(L12)3](PF6)2 (L9 = 3-butyl-1-(pyrimidin-2-yl)imidazolylid-ene, 19), [Ni(L22)2(CH3CN)2](PF6)2 (L22 = 3-(2,6-diisopropylphenyl)-1-(pyrimidin-2-yl)imidazolylidene, 23), and [Ni(L24)(CH3CN)(H2O)] (L22 = 3-(2,4,6-trimethylphenyl)-1-(pyrimidin-2-yl)imidazolylidene, 25) are octahedral complexes. The structural diversity of these nickel(II) complexes originating from steric pressure of the ligands is discussed.

Nickel N-heterocyclic carbene complexes and their utility in homogeneous catalysis

“…The first approach utilizes the transmetallation route (Scheme 13) for preparing the (665), [201] (666−667), [202] (675), [203] (676), [161] (677−678), [204] (679), [203] (682), [205] (683), [206] (684−685), [207] (686−687), [208] (689), [209] (690), [210] (691), [211] (692−695), [203] (697−702), [203] (703−706), [212] (707), [213] (708) [214] and (709−710) [215] complexes from the respective Ag(I)−NHC complexes by the treatment with various Ni(II) precursors namely, NiCl 2 (PPh 3 ) 2 , (DME)NiX 2 (X = Cl, Br) and…”

“…In the first approach, the transmetallation method (Scheme 13) was successfully employed for preparing several complexes like (729−732), [230] (733−734), [231] (735), [194] (740), [203] (741), [211] (742) [232] and (743−754) [233] In the second approach, the direct method (Scheme 14) yielded the following complexes (736−737), [194] (756), [234] (757−760), [235] (761) [236] and (768−769) [217] from the reactions of the free or the in situ generated NHC ligands with the Ni(II) precursors namely, Ni(OAc) 2 , (DME)NiX 2 , (X = Cl, Br) NiBr 2 and NiCl 2 (PMe 3 ) 2 .…”

“…Towards this ultimate objective, the Ni−NHC complexes have been reported to successfully generate hydrogen by electro catalytic method (Scheme 46). In particular, the complexes (200) and (203) have been reported for the electro catalytic dehydrogenation reaction of tetrahydroquinaldine [91].…”

“…(200),[91] (201),[92] (202),[93] (203),[91] (204−206),[92] (207) [94] and(208−209) [95] were synthesized by this method from the reaction of nickelocene derivatives and the azolium salts (Scheme 14 andFigure 16).…”

Synthesis, structural characterization of benzimidazole-functionalized Ni(II) and Hg(II) N-heterocyclic carbene complexes and their applications as efficient catalysts for Friedel–Crafts alkylations

“…Chen reported the rigidity and steric bulkiness of ligand significantly affected the structure of corresponding complex, the coordination numbers getting higher when the ligands become bulkier [80]. The nickel(II) ion tends to form stable square-planar complexes with NHC ligands.…”