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Systematics and Future Projections Concerning Redox-Noninnocent Amide/Imine Ligands



An analysis of the conjugative possibilities of dienes carrying two terminal heteroatom donors shows some generalizations of these as possible redox-active (redox “noninnocent”) ligands. If the two E=C functionalities (E = O,S, NR, CHR) are connected by an odd number of sp2-hybridized atoms, full conjugation between the two terminal E donors is not possible, a situation which is found both in 2,6-bis(imino)pyridine (BIMPY) and beta-diketiminate ligands; the character of their redox noninnocence is thus altered, compared to 1,4-diheterodienes. Some insight into the BIMPY ligand comes from the analog where one imino arm is absent. The traditional stereochemical relationships which govern conjugation in arenes are analyzed for their implications for different bis(imino)pyridine isomers, and also for 2,6-dipyrimidyl- and dipyridazinyl-pyridines, as generalizations of the classic terpyridyl ligands. Factors which favor oxidized vs. reduced ligand forms are discussed.