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

Abstract

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 λ_max 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.