Synthesis of Complexes with Protic NH,NH-NHC Ligands via Oxidative Addition of 2‑Halogenoazoles to Zero-Valent Transition Metals

A versatile,
one-pot synthesis for the preparation of transition
metal complexes bearing protic NH,NH-NHC ligands is disclosed. The
reaction of unsubstituted 2-halogenoazoles with zerovalent metal complexes
of the type [M­(PPh3)4] (M = Pd, Pt) in the presence
of NH4BF4 proceeds by oxidative addition of
the C2–X (X = Cl, Br, I) bond to the transition metal followed
by protonation at the ring nitrogen atom, leading to complexes of
the type trans-[MX­(PPh3)2(NH,NH-NHC)]
(M = PdII, PtII; NHC = benzimidazolylidene or
imidazolylidene). The trans-complexes have been obtained
in all cases. The time needed to complete the reaction depends on
the halogen present in the azole, and the rate of the oxidative addition
follows the order C2–I > C2–Br > C2–Cl.
The N–H
groups of the coordinated NH,NH-NHC have been deprotonated followed
by reaction with MeI to give the NMe,NMe-substituted classical NHC
ligands. The N–H groups of the protic NHCs are hydrogen bond
donors. The formation of N–H···O hydrogen bonds
has been observed via 1H NMR spectroscopy upon titration
of the complexes bearing protic NHCs with DMPU.