Control of Fe(3+) coordination by excess Cl(−) in alcohol solutions

We spectroscopically investigated coordination state of Fe(3+) in methanol (MeOH) and ethanol (EtOH) solutions against Cl(−) concentration ([Cl(−)]). In both the system, we observed characteristic absorption bands due to the FeCl(4) complex at high-[Cl(−)] region. In the MeOH system, the proportion (r) of [FeCl(4)](−) exhibits a stationary value of 0.2–0.3 in the intermediate region of 10 mM < [Cl(−)] < 50 mM, which is interpretted in terms of [FeCl(n)L(6−n)](3−n) (n = 1 and 2). In the EtOH system, r steeply increases from 0.1 at [Cl(−)] = 1.5 mM to 0.7 at [Cl(−)] = 3.5 mM, indicating direct transformation from [FeL(6)](3+) to [FeCl(4)](−). We further found that the coordination change significantly decreases the redox potential of Fe(2+)/Fe(3+).