Bis(CBT)palladium(II) Derivatives (CBT = m-carborane-1-thiolate): Synthesis, Molecular Structure, and Physicochemical Properties of cis-[(bipy)Pd(CBT)2] and trans-[(py)2Pd(CBT)2]

The new synthesized PdII complex cis-[(bipy)Pd(CBT)2] (bipy = 2,2′-bipyridyl; CBT = m-carborane-1-thiolate anion), which is a potential BNCT (boron neutron capture therapy) agent and of structure elucidated by single-crystal X-ray work, has been studied by infrared (IR) and ultraviolet-visible light (UV-vis) spectra and its properties compared with those of the previously reported and also the structurally characterized analogue trans-[(py)2Pd(CBT)2]. This trans species, prepared via a direct method, was previously isolated from a pyridine solution, consequent to the occurring releasing of the external Pd(CBT)2 moieties of the porphyrazine macrocycle [{Pd(CBT)2}4LZn]·xH2O (L = tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazinato anion), which is an active photosensitizer in photodynamic therapy (PDT) and a potential bimodal PDT/BNCT agent. The UV-vis spectral behavior of both cis and trans species in CHCl3 solution and in the gas phase has been examined in detail by density functional theory (DFT) and time-dependent density functional theory (TDDFT) studies devoted to explain their distinct behavior observed in the region of 400-500 nm, as determined by the presence in the cis structure of a vicinal arrangement of the two CBT groups, an ensemble of results closely similar to those observed for the macrocycles [{Pd(CBT)2}4LM]·xH2O (M = MgII(H2O), ZnII, PdII). It has also been experimentally proved the tendency of the cis isomer in CHCl3/pyridine solution to be changed to the respective trans analogue, with conversion occurring in two steps, as interpreted by detailed DFT studies.