Alkyl tetrazoles as diimine (“diim”) ligands for fac-[Re(diim)(CO)3(L)]-type complexes. Synthesis, characterization and preliminary studies of the interaction with bovine serum albumin

Herein, we report a new family of luminescent Re(I) complexes with general formula fac-[Re(diim)(CO)3L]0/+ in which the role of the diimine-type chelating ligand (diim) is played by alkylated tetrazoles. In particular, the design of the new complexes involved the choice of molecular scaffolds based on 2-pyridyl tetrazole (PTZ) and 2-quinolyl tetrazole (QTZ) which were decorated with various alkyl residues at N-2 position of the pentatomic ring, thereby endowing the resulting alkyl tetrazoles PTZ-R and QTZ-R with the proper “bpy-like” coordination attitude. As the “third” ligand (L), pyridine (pyr) or the 5-phenyl tetrazolato anion (Tph)− were selected, leading to cationic species such as fac-[Re(CO)3(PTZ-R)(pyr)]+, fac-[Re(CO)3(QTZ-R)(pyr)]+ and to the neutrally charged “fully tetrazole” complex fac-[Re(CO)3(QTZ-Me)(Tph)]. All the new complexes were identified by ESI-MS spectrometry and fully characterized by IR, 1H and 13C NMR spectroscopy. The findings that were suggested from the interpretation of the spectroscopic data were further confirmed by X-ray crystallography, with the analysis of the molecular structures of the cationic complexes fac-[Re(CO)3(PTZ-Me)(pyr)][PF6] and fac-[Re(CO)3(QTZ-Me)(pyr)][PF6]. Following the in-depth investigation of their photophysical properties, the new luminescent Re(I) tetrazole -based complexes were studied for any possible interaction with Bovine Serum Albumin (BSA). The results obtained from this preliminary screening highlighted that, along the series of the Re(I) tetrazole complexes, the QTZ-R based cationic derivatives fac-[Re(CO)3(QTZ-Me)(pyr)]+, fac-[Re(CO)3(QTZ-tBu)(pyr)]+, as well as the neutrally charged and “fully tetrazole” complex fac-[Re(CO)3(QTZ-Me)(Tph)], displayed the highest affinity to BSA.