Tetra-2,3-pyrazinoporphyrazines with Peripherally Appended Pyridine Rings. 19. Pentanuclear Octa(2-pyridyl)tetrapyrazinoporphyrazines Carrying Externally Carboranthiolate Groups: Physicochemical Properties and Potentialities as Anticancer Drugs

New pentanuclear porphyrazine complexes of formula [{Pd(CBT)₂}₄LM]· xH₂O (L = tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazinato anion, CBT = m-carborane-1-thiolate, and M = Mg^II(H₂O), Zn^II, Pd^II) were prepared in good yield as dark green hydrated amorphous solids by reaction of the respective pentanuclear species [(PdCl₂)₄LM] with m-carboran-1-thiol in CH₃CN. Physicochemical characterization of the new species was carried out by elemental and thermogravimetric analysis along with IR and ¹H/¹³C NMR measurements. UV-vis spectral characterization performed in DMSO, DMF, and pyridine solution provided information about the stability of the new homo/heteropentanuclear species and their tendency to undergo detachment of the peripheral Pd(CBT)₂ groups. The data from NMR, UV-vis, and electrochemical experiments indicate that external coordination of the Pd(CBT)₂ units to the mononuclear [LM] species affects only slightly the π electron distribution within the internal macrocyclic choromophore. The Pd(CBT)₂ units are released in pyridine solution and in the case of the Zn^II complex [{Pd(CBT)₂}₄LZn] give rise to a finely crystalline light-yellow solid identified by single-crystal X-ray work as the trans isomer of the bispyridine adduct [py₂(CBT)₂Pd]. The new pentanuclear macrocyclic complexes behave in DMF solution as active photosensitizers for singlet oxygen production, ¹O₂, the cytotoxic agent in anticancer photodynamic therapy, and have larger quantum yield values (Φ_Δ = 0.6-0.7) than those found on average for the related tetrapyrazinoporphyrazine analogs (Φ_Δ = 0.4-0.6). The presence of the CBT groups in the currently investigated complexes opens up the possibility for their use in boron neutron capture therapy, leading potentially to new bimodal anticancer curative drugs.