Rhenium(I) bromo tricarbonyl complexes from anthracenyl derivatized ligands

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 Re^I(CO)₃Br complexes: [(pypyr)Re(CO)₃Br], [(pypyr-anthra)Re(CO)₃Br] 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° and 67° for [(pypyr-anthra)Re(CO)₃Br] and [Br(CO)₃Re(pypyr-anthra-pypyr)Re(CO)₃Br] 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 (∼4 × 10³ M⁻¹cm⁻¹), the sensitivity to solvent polarity and DFT modelling suggest they corresponds to MLCT Re_dπ → π_(pyr)* transitions. Despite their high emissivity, the uncoordinated ligands completely lose their emission upon coordinated to Re^I(CO)₃Br. The rhenium(I) complexes emit around 650 nm upon excitation, although [(pypyr)Re(CO)₃Br] is a moderate emissive molecule, while [(pypyr-anthra)Re(CO)₃Br] and [Br(CO)₃Re(pypyr-anthra-pypyr)Re(CO)₃Br] are very weak ones. In contrast these two last molecules showed a remarkably high yield as singlet oxygen sensitizers.