Abstract

The ligand 2-(1H-pyrazol-1-yl)pyrazine (pypyr) was prepared by reaction of 2-bromopyrazine with N-lithium pyrazolate with 92.5% yield, while the anthracenyl derivatives 2-(anthracen-9-yl)-5-(1H-pyrazol-1-yl)pyrazine (pypyr-anthra) and 9,10-bis(5-(1H-pyrazol-1-yl)pyrazin-2-yl)anthracene (pypyr-anthra-pypyr) were prepared by reaction of the 9-anthraceneboronic or bis-boronic acid pinacol ester with 2-bromo-5-(1H-pyrazol-1-yl)pyrazine with medium yields (49.6 and 31.5 % respectively). The respective ReI(CO)3Br complexes: [(pypyr)Re (CO)(3)Br], [(pypyr-anthra)Re(CO)(3)Br] and [Br(CO)3Re(pypyr-anthra-pypyr)Re(CO)3Br] were prepared in high yields (94, 84 and 78 % respectively) by reaction with bromotricarbonyl(tetrahydrofuran)rhenium(I) dimer. DFT modelling suggests the anthracenyl moiety is not coplanar with the pyrazolyl-pyrazine fragment, defining a dihedral angle of 65 degrees and 67 degrees for [(pypyr-anthra)Re(CO)(3)Br] and [Br(CO)(3)Re(pypyr-anthra-pypyr)Re (CO)(3)Br] respectively. Each one of the three rhenium(I) molecules shows a quasi-reversible reduction wave around - 1.10 V, assigned by comparison with the uncoordinated ligand, the pypyr fragment, while additional reductions waves related to this core or the anthryl arms are present. UV-Vis spectra show absorption bands and/ or shoulders around 400-450 nm for the series of compounds in solution. The respective extinction coefficients (similar to 4 x 10(3) M(-1)cm(-1)), the sensitivity to solvent polarity and DFT modelling suggest they corresponds to MLCT Re-d pi -> pi((pyr))* transitions. Despite their high emissivity, the uncoordinated ligands completely lose their emission upon coordinated to ReI(CO)(3)Br. The rhenium(I) complexes emit around 650 nm upon excitation, although [(pypyr)Re(CO)(3)Br] is a moderate emissive molecule, while [(pypyr-anthra)Re(CO)(3)Br] and [Br(CO)(3)Re (pypyr-anthra-pypyr)Re(CO)(3)Br] are very weak ones. In contrast these two last molecules showed a remarkably high yield as singlet oxygen sensitizers.