Dichloro(1,4,8,11-tetraazacyclotetradecane)manganese(III) chloride: cis–trans isomerisation evidenced by infrared and electrochemical studies

Abstract

The isomers cis- and trans-dichloro(1,4,8,11-tetraazacyclotetradecane)manganese(III) chloride have been synthesized and characterised. An isomerisation of the type cis-Mn^III → trans-Mn^III was observed by IR spectroscopy. Recrystallisation of the cis species gave systematically crystals of the trans derivative. The crystal structure of the latter complex has been determined. An electrochemical study of the trans-manganese(III) complex in dimethyl sulfoxide revealed a rare trans → cis isomerisation reaction which proceeds through an electrochemical step–chemical step mechanism. The overall first-order rate constant, k_iso, for this isomerisation was determined by simulation of the chronoamperometric data. Supplementary electrochemical information for the trans-manganese(III) isomer was obtained through simulation of the cyclic voltammograms over a large range of scan rates. No influence of chloride ion concentration was observed as determined by cyclic voltammetry using an internal chloride standard, [N(PPh₃)₂]Cl. The cyclic voltammogram of the cis-dichloro(1,4,8,11-tetraazacyclotetradecane)manganese(II) generated in situ displays the usual cis → trans isomerisation which also occurs via an electrochemical step–chemical step mechanism.