Facile Hydrolysis of Nickel(II) Complexes with N‑Heterocyclic Carbene Ligands

Metal complexes with N-heterocyclic
carbene ligands (NHC) are ubiquitously
used in catalysis, where the stability of the metal–ligand
framework is a key issue. Our study shows that Ni-NHC complexes may
undergo facile decomposition due to the presence of water in organic
solvents (hydrolysis). The ability to hydrolyze Ni­(NHC)2X2 complexes decreases in the order of NHC = 1,2,4-triazolium
> benzimidazolium ≈ imidazolium. Depending on the ligand
and
substituents, the half reaction time of the complex decomposition
may change from several minutes to hours. The nature of the halogen
is also an important factor, and the ability for decomposition of
the studied complexes decreases in the order of Cl > Br > I.
NMR and
MS monitoring revealed that Ni-NHC complexes in the presence of water
undergo hydrolysis with Ni–Ccarbene bond cleavage,
affording the corresponding N,N′-dialkylated
azolium salts and nickel­(II) hydroxide. These findings are of great
importance for designing efficient and recyclable catalytic systems,
because trace water is a common contaminant in routine synthetic applications.