“Coordination caps” of graded electron-donor capacity

Abstract An efficient synthesis of the novel {6-[1,1-di(pyridin-2-yl)ethyl]pyridine-2-yl}2-methyl-1,3-propanediamine ( 2 ) is reported, as well as a reliable large-scale synthesis (of the order of 100 g) of previously known 2,2’-[1-(6-chloropyridin-2-yl)ethane-1,1-diyl]dipyridine ( 4 ); the latter is the starting material for the preparation of the former, as well as a multitude of other polypodal polyamine/polyimine ligands. Both materials, as well as the intermediates in their multi-step syntheses, have been fully characterised. Ligand 2 , in conjunction with ligands 2,2’-(pyridine-2,6-diyl)bis(2-methylpropane-1,3-diamine) ( 1 ) and 2,6-bis(1,1-di(pyridin-2-yl)ethyl)pyridine ( 3 ), establishes a series of tetrapodal pentadentate N 5 ligands L of like scaffold, and thus coordination geometry, but graded primary amine/imine donor atom ratios. The iron(II) complexes [Fe( L )NCCH 3 ](OTf) 2 ( L = 1 : A ; L = 2 : B ; L = 3 : C ; OTf = triflate) have been prepared and fully characterised, including X-ray single-crystal structure analyses. The metal-centred one-electron oxidation/reduction potential (Fe II /Fe III ) depends sensitively on the electron donor capacity of the capping ligand used, as demonstrated by cyclic voltammetry. Whereas the acetonitrile ligand in A and C is readily exchanged for a methanol ligand in methanol solution, the resulting complexes showing variable-temperature spin crossover (SCO) in solution, B has been found to be inert to this type of ligand exchange.