Halide-Assisted Electrophilic C–H Activation in Aqueous Acid and Salt Solutions for the Synthesis of Ru(III)-Abnormal NHC Complexes

Synthesis
of Ru(III)-aNHC complexes 1a’ and 1b’ has been achieved in the aqueous acid medium at
ambient temperature. The abnormal binding mode of NHC through this
method was undoubtedly confirmed by the synthesis of Ru(III)-aNHC
complex 2a’ from the C(2)-methylated ligand precursor.
The conversion of abnormal to normal NHC complexes (1a and 1b) was observed in the reaction medium. A pyridine-rollover
mechanism for the conversion of abnormal to normal NHC has been proposed
and validated by suitable substitution at the ligand backbone. Complexes 3a (Ru-nNHC) and 3a’ (Ru-aNHC) were prepared
with the bidentate ligand precursor having a Me-substituted pyridine
to prohibit the “pyridine-rollover” pathway. The conversion
of abnormal NHC complex 3a’ to the normal NHC
complex 3a was found to be suppressed in aqueous solutions
at room temperature as the λmax for 3a’ remained the same even after 15 days, suggesting the role of C(3)-H
of pyridine in the process. An investigation of this synthetic protocol
in various aqueous acid/salt solutions indicates that Cl– ions are required to form the complexes, indicating a halide-assisted
C–H activation pathway. All complexes have been characterized
using various spectroscopic techniques. The molecular structures of
complexes 1a’, 1b, 2a’, and 3a have been determined using the single-crystal
X-ray diffraction technique.