Rhodium(III) complexes of the ambidentate macrocycle 6,13-diamino-6,13-dimethyl-1,4,8,11-tetra-azacyclotetradecane. Crystal structure of the sexidentate complex

Abstract

The diamino-substituted macrocyclic polyamine 6,13-diamino-6,13-dimethyl-1,4,8,11-tetra-azacyclotetradecane (L1) may act as a quadri-, quinque-, or sexi-dentate ligand to rhodium(III), with trans-[Rh(H2L1)Cl2]3+, cis-[Rh(HL1)Cl]3+, and [RhL1]3+ ions all isolated as perchlorate salts and characterized spectroscopically. Unlike the cobalt(III) analogue, which forms a trans isomer with the ligand L1 quinquedentate, only the cis isomer was observed with rhodium(III). The sexidentate complex adopts the common arrangement with the pendant primary amine groups in trans dispositions, confirmed by an X-ray crystal structure analysis: [RhL1][ClO4]3 crystallizes in the space group P21/c, with a= 16.700(3), b= 8.776(2), c= 17.896(3)A, and β= 114.58(1)°. The equatorial Rh–N bonds, 2.033–2.055 A, are the shortest reported for a saturated hexa-amine complex of rhodium(III); axial bonds are only slightly longer (2.055 and 2.058 A). This bond ‘compression’ is reflected in physical properties, with electronic maxima shifted to higher energy, and E½ for RhIII–RhII observed at a very negative potential. All complexes display irreversible one-electron reductions of the rhodium(III) centre in solution. Metal-centred reduction of the sexidentate complex with the aquated electron revealed a decay process which is rapid (kd 4 × 104 s–1), consistent with the voltammetric observations.