Iron Complexes with Macrocyclic Ligands Containing the α-Diimine Functional unit and Its Position-Specific Formation under the Influence of the Iron Atom

The low-spin iron(II) complex [Fe([14]aneN₄)(CH₃CN)₂](BF₄)₂, where [14]aneN₄ is the tetradentate macrocyclic ligand meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, is found to undergo oxidative dehydrogenation in acetonitrile solution. The product initially isolated is a cyclic triimine having an α-diimine functional unit. In basic acetone the coordinated triimine can be converted rapidly to a tetraimine ligand containing two α-diimine units. The isolated iron(II) complexes all have the general formula [Fe(MAC)(X)₂]ⁿ⁺, where MAC is the macrocycle and X is CH₃CN, imidazole, SCN^-, NO₂^-, CN^-, CNH, CNCH₃, and CL and n= 0 or 2. The complexes containing the triimine ligand are air sensitive and are further dehydrogenated by molecular oxygen to the corresponding complexes of the tetraimine macrocycle. The bound cyanide ligands of the dicyano complex with the tetraimine macrocycle are sensitive to electrophilic reagents and can be protonated in a stepwise manner or alkylated. All of the complexes isolated, with the exception of the two trans-dichloroiron(III) derivatives, display charge-transfer bands in the visible region of their electronic spectra. The positions of these bands have been discussed in terms of the π-bonding abilities of the axial ligands.