COORDINATION OF THE SEXIDENTATE MACROCYCLE 6,13-DIMETHYL-1,4,8,11-TETRAAZACYCLOTETRADECANE-6,13-DIAMINE TO IRON(III)

The sexidentate polyamine macrocycle 6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine (diammac) reacts with iron(II) in aqueous solution to form the low-spin iron(III) complex Fe(diammac)3+. Crystals of [Fe(diammac)](ClO4)Cl2 are monoclinic, space group C2/c, with a = 15.6612 (8) angstrom, b = 7.4390 (7) angstrom, c = 18.061 (2) angstrom, and beta = 108.626 (7)-degrees. The Fe(diammac)3+ ion is stable indefinitely in aerated aqueous solution, and its electronic and electron paramagnetic resonance spectra are consistent with a low-spin d5 electronic ground state. In aqueous base, deprotonation (pK(a) 10.2 +/- 0.2) is accompanied by a color change to red, but the deprotonated species is not stable. The reversible Fe(III/II) redox couple occurs at a quite negative potential (-0.35 V vs Ag/AgCl; DELTA-E = 60 mV at 10-500 mV/s scan rate for cyclic voltammetry at glassy carbon) compared with other iron(III) complexes. Although the couple is reversible on the voltammetric time scale, monitoring the chemical reduction with zinc amalgam by NMR spectroscopy showed metal ion exchange occurs. Detailed analysis of the EPR and electronic spectra was possible; from the latter, along with comparisons with five other octahedral complexes of diammac, the empirical ligand spectroscopic parameter, g, for Fe(III) has been reassessed as 16.3 cm-1, and sexidentate diammac has been defined as the strongest purely sigma-bonded ligand extant.