Synthesis, electronic and photophysical investigations of ruthenium(<scp>ii</scp>)-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_3,3) family. These are pentanuclear {Ru^IIM₄} complexes (M = Co²⁺, Ni²⁺, Zn²⁺) which were prepared by employing a directed two-step synthesis facilitated by the recently published [Ru^II(Me₂bta)₂(Me₂btaH)₄] precursor complex (Me₂btaH = 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 μ₃-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 {Ru^IIZn₄} and {Ru^IICu₄} 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 {Ru^IIZn₄} compound as the only phosphorescent species, displaying an emission band extending into the near-infrared region. This emission originates from a triplet ³MLCT state and features an exceptionally large Stokes shift, with a long lifetime of the excited-state of about 3.3 μs in powdered form at room temperature.