Pyridine and pyrimidine functionalized half-sandwich Ru(II)-N heterocyclic carbene complexes: Synthesis, structures, spectra, electrochemistry and biological studies

•New set of Ru-N-Heterocyclic Carbene (Ru-NHC) complexes 1a and 2a were synthesized from NHC ligands.•X-RD structure reveals the half-sandwich geometry of the compounds.•Absorption spectra show the mixed-mode MLCT type transitions.•The compounds show irreversible oxidation in the cyclic voltammograms.•The compounds have exhibited very promising cytotoxic activity against a series of tumor cell lines with optimistic impressive IC50 values.•This promising observation may lead to new drug molecules in tumor-selective therapeutic strategies.