A new series of protic amines containing cyclic amidine substituents in their side arms were synthesised from iminodiacetonitrile and 1,2- and 1,3-diamines. These amidine-anchored protic amines were treated with MnBr(CO)5 resulting in the formation of cationic mononuclear Mn(I) complexes with a facial coordination mode. Crystallographic analysis revealed that the C–O bond lengths of the carbonyl ligands, located at the trans position to the amidine, were marginally longer compared to those to typical phosphine- and other N(sp2)-anchors. This observation suggests that the amidine side arm possesses a strong electron donating ability. In the catalytic application of transfer hydrogenation of acetophenone, a Mn complex featuring a six-membered ring amidine anchor showed relatively high activity, possibly due to a high electron density of the metal, as estimated from the NH signal in the proton nuclear magnetic resonance (1H NMR) spectroscopy. Furthermore, cyclic voltammetry measurements revealed that the redox properties varied depending on the structure of amidine. Specifically, a Mn complex with readily accessible electron acceptance exhibited a significant increase in current under CO2. This behaviour demonstrated catalytic activity in the electrochemical reduction of CO2.
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