Polybromides of Transition Metal Chelates – Synthesis, Structure and Spectral Properties

Abstract

Five new polybromide compounds containing tris-chelates of 1,10-phenanthroline and 2,2-bipyridine as counter cations are described: [Mn(phen)3](Br3)2 (1), [Co(phen)3](Br3)3 (3) and [M(phen)3](Br3)(Br8)1/2 where, M=Fe (2), Cu(4) or Zn(5). All are obtained by self assembly in solution starting from divalent transition metal bromides and bromine along with the ligand. In the case of CoBr2, bromine acts as a tribromide precursor as well as oxidising agent. The Mn compound is isomorphous with the corresponding triodide and analogously contains one short and one normal trihalide ion. The Co compound contains one bent but symmetric Br3− ion. The three compounds containing the Z-shaped octabromide ion are isostructural. While the cation in 1 has a crystallographically imposed C3 symmetry, the tris chelates in the other four compounds have near trigonal symmetry. Unusually, the trigonally compressed octahedral cation in 4 has six almost equal Cu−N bond distances. Attempts to prepare bpy analogs of 1 resulted in a new dibromo-bridged compound [Mn2Br2(bpy)4](Br3)2 (6) and a dioxo-bridged compound [Mn2O2(bpy)4](Br3)3 (7). The mixed valence cation in 7 formed by bromine oxidation of MnBr2, followed by disproportionation, is well known with other anions. All the paramagnetic compounds (1, 4, 6, 7) give resolved EPR spectra since the bulky anions help to magnetically isolate the cations. The unsymmetric dibromo bridges in 6 produce a net, albeit weak, ferromagnetic coupling between the two [Mn(bpy)2]2+ fragments. Raman as well as electronic spectra are reported for all compounds. Raman bands, though significantly different even for isostructural compounds, may be understood in terms of Br3− ions and Br2 molecules. All compounds give luminescence from phen and bpy both in solid and solution states, modulated by the coordinated metal ions.