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]

Abstract

The reaction of 2-bromo-5-(1H-pyrazol-1-yl)pyrazine (L_I) 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 Re^I center, a fac-correlation for carbonyls, with the pyrazine and pyrazolyl rings being highly coplanar. I experiences one irreversible oxidation at E_p,a = 1.067 V and two irreversible reductions at E_p,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 (Re^I/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.