New rare earth metal complexes with nitrogen-rich ligands: 5,5 '-bitetrazolate and 1,3-bis(tetrazol-5-yl)triazenate - On the borderline between coordination and the formation of salt-like compounds

From the two nitrogen-rich ligands BT2- (BT = 5,5'-bitetrazole) and BTT3- (BTT=1,3-bis(1H-tetrazol-5-yl)triazene), a series of novel rare earth metal complexes were synthesised. For the BT ligand, a vast number of these complexes could be structurally characterised by single-crystal XRD, revealing structures ranging from discrete molecular aggregates to salt-like compounds. The isomorphous complexes [La-2(BT)(3)]center dot 14H(2)O (1) and [Ce-2(BT)(3)]center dot 14H(2)O (2) reveal discrete molecules in which one BT2- acts as a bridging ligand and two BT groups as chelating ligands. The complexes, [M(BT)(H2O)(7)](2)[BT]center dot x H2O (3-5), (M=Nd (3), Sm (4), and Eu (5)), are also isomorphous and consist of [M(BT)(H2O)(7)](+) ions in which only one BT2- acts as a chelate ligand for each metal centre. [Tb-(H2O)(8)](2)[BT](3)center dot xH(2)O (6) and [Er-(H2O)(8)](2)[BT](3)center dot xH(2)O (7) are salt-like compounds that do not exhibit any significant metal-nitrogen contacts. In the BTT-samarium compound 9, discrete molecules were found in which BTT3- acts as a tridentate ligand with three Sm-N bonds.