Characterization, Redox Properties and Pulse-Radiolysis Study of Dichloro-(Tetraazacyclotetradecane)Manganese(III) Complexes, and X-Ray Crystal-Structure of the meso-5,7,7,12,14,14-Hexamethyl-1,4,8,11-Tetraazacyclotetradecane Complex

Abstract

The complexes [Mn ( mac )Cl₂]⁺ [ mac = 1,4,8,1l-tetraazacyclotetradecane (cyclam), meso and rac-5,7,7,12,14,14-hexamethylcyclam ( teta and tetb respectively)] have been prepared. The trans-[ Mn ( teta )Cl₂] Cl.3H₂O complex crystallizes in the monoclinic space group P2₁ /c, Z 2, a 8.4006(8), b 12.465(2), c 11 .659(2) Å, β 103.20(1)º. The geometry about the manganese(III) is close to octahedral with the macrocycle coordinated to four equatorial sites ( Mn -N 2.043(2), 2.074(2) Å) and chlorides in trans axial sites (Mn -Cl 2.549(1) Å. Since electronic spectra of the cyclam and teta complexes are similar, the ions should exist in similar geometries; however, the tet b analogue is spectroscopically unique, and the complex may adopt a different geometry. All complexes in water exhibit quasi-reversible one-electron couples near O V at a mercury electrode; irreversible reductions of manganese(II) are also observed before the solvent limit near -1.5 V for the teta and tet b complexes. No metal-centred oxidation waves are seen before the solvent limit (c.+ 1 V) at a platinum electrode. However, pulse radiolytic oxidation (with OH) of all complexes is observed with formation k_f > 10⁹ dm³mol^-1 s^-1; apparently d³ manganese(IV) monomers with electronic spectra qualitatively similar to d³ chromium(III) complexes are formed. Decay of these intermediates is first-order, with all k_d ≥ 5×10³ s^-1, but varying with the macrocyclic ligand present. Alkyl substitution on the cyclam stabilizes the manganese(IV) oxidation state, the intermediate cyclam complex decaying 25 times faster than the teta analogue. The final decay product appears to be MnO₂, and rate-determining ligand loss is presumed.