Synthesis and Reactivity of Remarkably Stable and Nucleophilic Hydroxide-Bridged Dimetallic Nickel NHC Complexes

Abstract

A novel class of dicationic homodimetallic nickel(II) NHC complexes [(NHC)2Ni(µ-OH)2Ni(NHC)2] 2+ was synthesized starting from nickel acetate as metal precursor. Symmetrically substituted N-alkyl (methyl, isopropyl and isobutyl) imidazolylidene (imi) as well as N1-methyl, phenyl, mesityl and butyl substituted triazolylidene (trz) ligands were coordinated to the metal center through NaH-mediated metalation. Reaction of these bimetallic complexes with CH3 + as electrophile (MeOTf) induced alkylation of the bridging hydroxid ligands and afforded new alkoxy-bridged complexes [(NHC)2Ni(µ-OMe)2Ni(NHC)2] 2+ . In contrast, reactions of [(imi)2Ni(µ-OH)2Ni(imi)2] 2+ with H + as electrophile (mild acids with pKa >6) lead to cleavage of the dimeric structure and formation of mononuclear complexes [Ni(X)2(imi)2]. Conversely, no reaction occurs for the triazolylidene analogues [(trz)2Ni(µ-OH)2Ni(trz)2] 2+ , indicating different robustness of the dimetallic core to acidic media. Only exposure to stronger acids (pKa <5) induced dimer cleavage for the trz complexes and gave either the corresponding triazolium salt, or in the presence of a coordinating anion, monomeric species [Ni(X)2(trz)2] with X = I, OAc or (X)2 = CO3. All complexes were inert towards Lewis and Brønsted bases such as NEt3 or NaOMe. These results reveal a remarkable robustness of the acidic Ni centers and the OH protons towards bases, and contrast the distinct reactivity of the Lewis basic oxygen donor sites, which are more reactive than the carbene towards electrophiles.

Abnormal N‐heterocyclic Carbene Based Ni(II) π‐allyl Complex towards Molecular Oxygen Activation

“…Crystallographic interpretation of 5 unveils a hydroxo‐bridged dimer with a cradle like bridging structure displaying a Ni−Ni distance of 2.75 Å (Figure 2a–b) which is consistent with the value reported earlier [41] . Ni1−O1, Ni1−C1 and Ni1−C5 bond distances were determined as 1.8863, 1.869 and 1.899 Å.…”

Pincer‐Type Mesoionic Carbene Nickel(II) Complexes: Synthesis, Properties, Reactions, and Catalytic Application to the Suzuki–Miyaura Coupling Reaction of Aryl Bromides

“…In particular, 1,2,3‐triazol‐5‐ylidene (triazolylidene) is the most attractive ligand motif and has been well established because it can be easily synthesized via the [3+2] Huisgen reaction between alkynes and azides [17–18] . Although several reports have been published on the palladium chemistry of MIC complexes, [19–38] examples of MICs bound to nickel centers are rare, as shown in Figure 1 [39–45] . Among these complexes, there are several examples of complexes prepared using monodentate and bidentate MIC ligands, [39–42] but only a few complexes in which the MIC moiety is incorporated into a tridentate ligand [43–45] …”

“…Although several reports have been published on the palladium chemistry of MIC complexes, [19–38] examples of MICs bound to nickel centers are rare, as shown in Figure 1 [39–45] . Among these complexes, there are several examples of complexes prepared using monodentate and bidentate MIC ligands, [39–42] but only a few complexes in which the MIC moiety is incorporated into a tridentate ligand [43–45] …”

“…Different chemoselectivities, ketones and imines for the imidazolylidene complexes and aldehydes for the triazolylidene complexes, have been performed depending on the NHC ligand used. Bertini and Albrecht have indicated that the nucleophilicity of the μ‐hydroxo ligand in the bis(triazolylidene)‐ligated nickel(II) dimer was significantly lower than that of its imidazolylidene analogue [60] . However, the origin of these differences has not been clarified to date.…”

“…[17][18] Although several reports have been published on the palladium chemistry of MIC complexes, [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] examples of MICs bound to nickel centers are rare, as shown in Figure 1. [39][40][41][42][43][44][45] Among these complexes, there are several examples of complexes prepared using monodentate and bidentate MIC ligands, [39][40][41][42] but only a few complexes in which the MIC moiety is incorporated into a tridentate ligand. [43][44][45] Limited numbers of MIC-ligated nickel complexes have been used in catalytic reactions.…”