Synthesis, electronic and photophysical investigations of ruthenium(ii)-centred heterometallic Kuratowski complexes which feature redox-active metal centres

We present the synthesis and comprehensive characterization of a series of complexes belonging to the Kuratowski (K-3,K-3) family. These are pentanuclear {(RuM4)-M-II} complexes (M = Co2+, Ni2+, Zn2+) which were prepared by employing a directed two-step synthesis facilitated by the recently published [Ru-II(Me(2)bta)(2)(Me(2)btaH)(4)] precursor complex (Me(2)btaH = 5,6-dimethyl-1,2,3-benzotriazole). The pentanuclear Kuratowski complexes showcase a unique combination of photo-active ruthenium with redox-active metal centres. The mu(3)-bridging 1,2,3-triazolate ligands in these complexes facilitate electronic coupling between the metal centers, as revealed through electrochemical and photophysical studies. Comparisons with {(RuZn4)-Zn-II} and {(RuCu4)-Cu-II} Kuratowski compounds reveal that Co(II) significantly influences both the Ru(II/III) redox step and the position of the MLCT (metal-to-ligand charge transfer) band, whereas Cu(II) and Ni(II) exhibit minimal influence. Photophysical investigations reveal the {(RuZn4)-Zn-II} compound as the only phosphorescent species, displaying an emission band extending into the near-infrared region. This emission originates from a triplet (MLCT)-M-3 state and features an exceptionally large Stokes shift, with a long lifetime of the excited-state of about 3.3 mu s in powdered form at room temperature.