Bis-diimidazolylidine complexes of nickel: Investigations into nickel catalyzed coupling reactions

Abstract

Air and moisture stable homoleptic bis(diimidazolylidine)nickel(II) complexes, ([(diNHC)(2)Ni](2+)) 3a,b and their corresponding silver(I) 4a,b and palladium(II) 5a,b complexes were synthesized and characterized by NMR and single crystal X-ray analysis. The catalytic potential of complex 3a was assessed in Mizoroki-Heck and Suzuki-Miyaura coupling reactions. In the Suzuki-Miyaura coupling reaction, nickel precatalyst 3a was active for the coupling of aryl chlorides as well as aryl fluorides. The analogously synthesized Pd(II) complexes resulted in formation of (diNHC)PdCl(2) species which were not active for the coupling of aryl fluorides. For the Mizoroki-Heck reaction, it was found that aryl iodides could be activated in the absence of nickel or palladium precatalysts when using Na(2)CO(3) or NEt(3) as base while aryl iodides and aryl bromides could be activated in the Suzuki-Miyaura reaction sans precatalyst when K(3)PO(4) was used as base.

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

“…Specifically, the complex (666) [202] displayed activity towards the Heck coupling of bromide and iodide substrates while the complex (756) [234] carried out the Heck coupling of the chloride, bromide and iodide substrates.…”

“…Of particular mention is the complex (10) that catalyzed the Suzuki coupling of the notso-common and extremely challenging aryl fluoride substrates, namely the perfluoroarenes, in the presence [156] as well as in the absence of a base [248]. A methylene bridged bis−NHC complex (666) [202] catalyzed the Suzuki coupling of the fluoride, chloride and bromide substrates. Quite interestingly, the complexes (10), [148,154] (605−608) [186] and (714−716) [219] have been reported for the Suzuki coupling of the challenging chloride substrates.…”

“…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…”

“…The second approach is the direct method (Scheme 14), using which the following complexes (666−667), [202] (668−671), [216] (672−673), [217] (682), [205] (711), [205] (712), [218] (713), [168] (714−717), [219] (718), [220] (719−720), [221] (721), [222] (723), [223] (724) [224] and (725) [225] were obtained from the reactions of various azolium salts with the Ni(II) precursors like, Ni(OAc) 2 The next route involves the oxidation method (Scheme 15) that was employed for preparing the complexes, (679−681), [226] (685), [227] (688), [227] (691), [226] (696), [226] (722) [226] and (727) [226] from the reaction of the azolium salts with Raney Ni powder. Other variations of the oxidation method were reported for the preparations of the (685) and (688) [227] complexes, which were obtained from the reaction of the imidazolium salts with nickel powder and Ag 2 O and for the synthesis of the (680) [228] complex, which too was obtained from the reaction of a Cu(I)−NHC complex with Raney nickel.…”

Synthesis of palladium(0) and -(ii) complexes with chelating bis(N-heterocyclic carbene) ligands and their application in semihydrogenation

“…Bis-NHCs have been coordinated to many transition metals: ruthenium, 4 rhodium, 3,5 iridium, 6 palladium, 7 platinum, 8,9 and more recently also the first row metals such as nickel [10][11][12] and iron. 13,14 We are interested in the properties and reactivity of low-valent late transition metal complexes, notably electron rich zero-valent palladium NHC complexes, as these often play a vital role in the catalytic cycles of many reactions.…”