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)Å, and β= 114.58(1)°. The equatorial Rh–N bonds, 2.033–2.055 Å, are the shortest reported for a saturated hexa-amine complex of rhodium(III); axial bonds are only slightly longer (2.055 and 2.058 Å). This bond ‘compression’ is reflected in physical properties, with electronic maxima shifted to higher energy, and E_½ for Rh^III–Rh^II 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 (k_d 4 × 10⁴ s^–1), consistent with the voltammetric observations.