Synthesis and Reactivity of Remarkably Stable and Nucleophilic Hydroxide-Bridged Dimetallic Nickel NHC Complexes
journal contribution
posted on 2020-09-16, 09:03 authored by Simone Bertini, Martin AlbrechtThe novel class of dicationic homodimetallic
nickel(II) NHC complexes
[(NHC)2Ni(μ-OH)2Ni(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)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) 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)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, the monomeric species
[Ni(X)2(trz)2] with X = I, OAc or (X)2 = CO3. All complexes were inert toward Lewis and Brønsted
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.
