Structure, magnetic susceptibility and EPR spectra of a dimeric macrocyclic chromium(III) complex bridged by a single carbonate ligand

Abstract

The orange dimeric carbonate bridged cation trans-[Cr(NH3)(cyclam)O]2CO4+ [cy-clam = 1,4,8,11-teraazacyclotetradecane] has been isolated as the chloride salt from reaction mixtures of [Cr(cyclam)Cl2]Cl and liquid ammonia slightly contaminated with carbon dioxide. This dimeric cation is unusually robust and resists depolymer-ization even in hot 6 M hydrochloric acid. The iodide salt {trans-[Cr(NH3)(cyclam) O]2CO}I4·2H2O crystallizes in the space group P21/n, with a = 9.368(5), b = 21.977 (5), c = 19.341(4) Å, β = 96.45(2)° and Z = 4. Both independent 'cyclam' ligands are coordinated in the trans configuration with the RRSS chirality of the coordinated nitrogen atoms, and the five-and six-membered chelate rings in gauche and chair conformations, respectively. The temperature dependence of the magnetic susceptibility is in agreement with an antiferromagnetic coupling between the two chromium(III) centres with J = 5.2(2) cm-1. The EPR spectra have been recorded on frozen glasses at temperatures down to 3.8 K. The dominant part of the spectrum is that of the quintet state, which has been computer-simulated using the mononuclear zero field splitting parameters D = 0.120(5) cm-1 and E =-0.060(5) cm-1, and the dinuclear exchange parameters De =-0.0090(1) cm-1 and Ee =-0.0038(1) cm-1.