Linear chain complexes linked by hydrogen bonded O2H3– and O3H5– units: structural and magnetic comparison of trans-dihydroxo-, -diaqua-, and -aquahydroxo-chromium(iii) complexes

Abstract

The syntheses and characterization of a series of chromium(III) complexes of the general types trans-[(A)4Cr(OH)(H2O)]2+, trans-[(A)4Cr(OH)2]+, and trans-[(A)4Cr(H2O)2]3+ are described. The ligands (A)4 used include the tetradentate ligand 1,4,8,11-tetraazacyclotetradecane (cyclam), its hexamethylated derivative C-meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (cyca), and the bidentate ligand 2-aminomethylpyridine (pico). The cobalt(III) aquahydroxo complexes with cyclam and cyca are also reported. The trans-diaqua and trans-dihydroxo complexes are monomeric, but the trans-aquahydroxo complexes form linear chains. The complexes trans-[Cr(cyclam)(OH)(H2O)](CF3SO3)2 1 and trans- [Co(cyclam)(OH)(H2O)](CF3SO3)2 3 and their corresponding perchlorates (2 and 4, respectively) have been isolated and the structures of the chromium triflate salt 1 and the cobalt perchlorate salt 4 have been determined. Both of these complexes 1 and 4 form infinite chains in the solid state, adjacent metal centers being bridged by hydrogen bonds between hydroxo and aqua ligands. The Cr⋯Cr separation in 1 is 6.085 Å whereas the Co⋯Co separation in 4 is 6.078 Å. The complex trans-[(pico)2Cr(OH2)2](NO3)3·H2O 8 (where pico is picolylamine, 2-aminomethylpyridine) crystallizes with only one monomeric formula in a unit cell. In contrast to the structures of 1 and 4 (and, presumably, of 2 and 3 also), the two complexes trans-[Cr(cyclam)(OH)2]ClO4·H2O 5 and 8 are monomeric in the solid state. The chromium complex trans-[Cr(cyca)(OH)(H2O)](ClO4)2·H2O 6 crystallizes with two monomeric formula units in a unit cell and the cobalt analogue 7 is isomorphous with the chromium complex. The structures of these complexes are also linear chains, but in this case there is an intervening water molecule between the aqua and hydroxo ligands on adjacent chromium centers. EPR spectroscopy of the chromium complex 1 doped in the diamagnetic cobalt host 3 reveals the presence of weak magnetic interactions.