Photophysical and biological investigation of phenol substituted rhenium tetrazolato complexes.

The synthesis, structural and photophysical characterisation of four tricarbonyl rhenium(i) complexes bound to 1,10-phenanthroline and a tetrazolato ancillary ligand are reported. The complexes are differentiated by the nature (hydroxy or methoxy) and position (meta or para) of the substituent attached to the phenyl ring in conjugation to the tetrazole ring. The complexes exhibit phosphorescence emission from triplet charge transfer excited states, with the maxima around 600 nm, excited state lifetime decays in the 200-300 ns range, and quantum yield values of 4-6% in degassed acetonitrile solutions. The nature and position of the substituent does not significantly affect the photophysical properties, which remain unchanged even after deprotonation of the hydroxide group on the phenol ring. The interpretation of the photophysical data was further validated by resonance Raman spectroscopy and time-dependent density functional theory calculations. All the complexes are internalised within cells, albeit to variable degrees. As highlighted by a combination of flow cytometry and confocal microscopy, the species display diffuse cytoplasmic localisation except for the complex with the hydroxy functional group at the para position, which reveals lower accumulation in cells and more pronounced punctate staining. Overall, the complexes displayed low levels of cytotoxicity.

Targeting divalent metal cations with Re(I) tetrazolato complexes.

In order to exploit their potential as versatile luminescent sensors, four new Re(I)-tetrazolato complexes with the general formula fac-[Re(CO)3(diim)(L)], where diim is 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen) and L(-) is either the anion 5-(2'-pyridyl)tetrazolato (2-PTZ(-)) or 5-(2'-quinolyl)tetrazolato (2-QTZ(-)), were prepared and fully characterized. In all cases, the regioselective coordination of the Re(I) center through the N2 atom of the tetrazolato ring was observed. This particular feature ensures the availability of the diiminic (N^N) site that was systematically incorporated into the structure of the 2-PTZ(-) and 2-QTZ(-) ligands for further coordination with metal cations. Such a diimine-type coordination mode was preliminarily tested by using the mononuclear Re(I) complexes as N^N ligands for the preparation of two [(N^N)Cu(POP)] cationic species, where POP is the chelating diphosphine bis[2-(diphenylphosphino)phenyl]ether. The X-ray structures of the resulting Re(I)-Cu(I) dyads revealed that the Re(I) mononuclear complexes effectively behaved as chelating N^N ligands with respect to the [Cu(POP)](+) fragment, the coordination of which also resulted in significant modification of the Re(I)-centered luminescence. With these data in hand, the luminescent sensing abilities of the four new Re(I) tetrazolato complexes were screened with respect to divalent metal ions of toxicological and biological importance such as Zn(II), Cd(II) and Cu(II). The interaction of the Re(I) complexes with Zn(II) and Cd(II) was witnessed by the evident blue shift (Δλmax = 22-36 nm) of the emission maxima, which was also accompanied by a significant elongation of the emission lifetimes. On the contrary, the addition of the cupric ion caused substantial quenching of the radiative processes originating from the Re(I) luminophores.