Role of an electron‐transfer chain reaction in the unusual photochemical formation of five‐coordinated anions [Mn(CO)3(α‐diimine)]− from fac‐[Mn(X)(CO)3(α‐diimine)] (X = halide) at low temperatures

Abstract

AbstractThe 16e− radicals [Mn(CO)3(α‐diimine)] are the key transients in the complex mechanism of formation of five‐coordinated anions [Mn(CO)3(α‐diimine)]− in 2‐MeTHF at 135K by visible excitation of the complexes fac‐[Mn(X)(CO)3(α‐diimine)] [X = halide; α‐diimine = 2,2′‐ bipyridine (bpy), pyridine‐2‐carbaldehyde N‐isopropylimine (iPr‐PyCa), and pyridine‐2‐carbaldehyde N‐p‐tolylimine (pTol‐PyCa)] into their low‐energy MLCT/XLCT transitions. This article describes the as yet unresolved electron‐transfer step in the mechanism which converts the radicals [Mn(CO)3(α‐diimine)] into the corresponding anions. Cyclic voltammetry and IR spectroelectro‐chemistry in optically transparent thin‐layer electrochemical cells were employed at variable temperatures in order to study the temperature‐dependent stability of the radicals [Mn(CO)3(α‐diimine)], their redox properties and their interaction with a coordinating solvent (Sv) to give the 18e− adducts fac‐[Mn(Sv)(CO)3(α‐diimine)]. The latter radicals are strong reductors capable of electron transfer to their five‐coordinated precursors which are quite stable at temperatures below 220K. This behaviour was demonstrated in the complex fac‐[Mn(Cl)(CO)3(bpy)] and the related complexes fac‐[Mn(Br)(CO)3(iPr‐DAB)] and fac‐[Mn(PrCN)(CO)3(iPr‐DAB)]+(PrCN = n‐ butyronitrile; iPr‐DAB = N, N′‐diisopropyl‐1,4‐diaza‐1,3‐butadieneChem. Abstracts name: N, N′‐diisopropyl‐ethane‐1,2 diimine (iPr‐ED) which were used as reference. The (spectro)electrochemical results have revealed that the formation of [Mn(CO)3(α‐diimine)]− in the course of the above photoreaction is most probably accompanied by parallel generation of the cations fac‐[Mn(Sv)(CO)3(α‐diimine)]+. The cations in turn react back with the free halide to give the starting complex fac‐[Mn(X)(CO)3(α‐diimine)], thereby closing the catalytic cycle. The overall mechanism of the formation of [Mn(CO)3(α‐diimine)]− from the neutral Mn(halide) complexes can therefore be viewed as a photo‐assisted/ETC reaction. No anions [Mn(CO)3(α‐diimine)]− are formed in the presence of excess PR3 which is reasonably ascribed to a complete conversion of the 16e− transients [Mn(CO)3(α‐diimine)] into the stable 18e− adducts fac‐[Mn(PR3)(CO)3(α‐diimine)], the final photoproducts observed in this case.