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

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