1IL and 3MLCT excited states modulated by H+: the structure and photophysical properties of [(2-bromo-5-(1H-pyrazol-1-yl)pyrazine)Re(CO)3Br]

The reaction of 2-bromo-5-(1H-pyrazol-1-yl)pyrazine (LI) and bromotricarbonyl(tetrahydrofuran)-rhenium(I) dimer leads to the monometallic complex [(2-bromo-5-(1H-pyrazol-1-yl)pyrazine)Re(CO)3Br] (I), which displays a non-regular octahedral geometry around the ReI center, a fac-correlation for carbonyls, with the pyrazine and pyrazolyl rings being highly coplanar. I experiences one irreversible oxidation at Ep,a = 1.067 V and two irreversible reductions at Ep,c = −1.445 V and −1.675 V. DFT supports that the oxidation is a metal-centered process but the first reduction wave should be a ligand-centered process. Comparison with related complexes suggests that the second reduction is metal centered (ReI/Re0). Three absorption bands were observed for I around 260, 315 and 400 nm, and the first two were assigned to the intraligand (IL) π → π* transitions while the band around 400 nm corresponds to a metal-to-ligand charge transfer (MLCT) transition. Excitation of I at 280 nm leads to two emission bands at 360 nm and 640 nm, and the first one is attributed to the 1IL transition and the second to 3MLCT. The 3MLCT emission decays with lifetimes of 17 and 44 ns in MeCN and DCM, respectively. Addition of trifluoroacetic acid to the solution leads to its reaction with the 3MLCT excited state, giving rise to a long-lived and very oxygen sensitive species, 3ILH+. This behaviour makes I a potential sensor for protons and possibly other cations in solution.