Lanthanide complexes of the tetradentate N-donor ligand dihydrobis[3-(2-pyridyl)pyrazolyl]borate and the terdentate N-donor ligand 2,6-bis(1H-pyrazol-3-yl)pyridine: Syntheses, crystal structures and solution structures based on luminescence lifetime studies

Lanthanide complexes of two polydentate N-donor ligands containing a mixture of pyridyl and pyrazolyl donors have been prepared. Dihydrobis[3-(2-pyridyl)pyrazolyl]borate (L-1)(-) is a tetradentate ligand with two bidentate chelating pyridyl/pyrazolyl arms linked by an apical BH2 group; 2,6-bis(1H-pyrazol-3-yl)pyridine (L-2) is a terdentate chelating ligand reminiscent of terpyridine. Reaction of L-1 with lanthanide salts gave complexes of the type [M(L-1)(2)X](n+) the crystal structures of [Eu(L-1)(2)(dmf)][ClO4]. 2.5CH(2)Cl(2), [Tb(L-1)(2)(NO3)]. 2CH(2)Cl(2) and [Tb(L-1)(2)(H2O)][L-1]. H2O . 0.5CH(2)Cl(2) were determined and all contain two tetradentate ligands L-1 and an ancillary ligand X [dimethylformamide (dmf), nitrate or water] whose nature depends on the reaction/recrystallisation conditions to complete the co-ordination sphere. Luminescence studies of [Tb(L-1)(2)(NO3)] in water or D2O and MeOH or CD3OD showed that in methanol the solvation number q is approximate to 1.8, consistent with displacement of nitrate by the solvent; however in wafer g approximate to 4.5, indicating additional displacement of some of the N-donor heterocyclic rings of L-1 by co-ordinating water molecules. Reaction of L-2 with lanthanide salts afforded [M(L-2)(3)](3+), all isolated as their hexafluorophosphate salts. The crystal structures of three of these (M = Eu, Gd or Ho) showed that they are isostructural and isomorphous, with tricapped trigonal-prismatic nine-co-ordinate geometries similar to that of [M(terpy)(3)](3+) (terpy = 2,2': 6',2 ''-terpyridine); Luminescence studies of [Tb(L-2)(3)][PE6](3) gave a solvation number 4 of 0.6 in methanol, which is small enough to be accounted for by second-sphere solvation effects alone and therefore suggests that the nine-co-ordinate structure is retained in methanol solution. However in water, q is again approximate to 4.5, due to displacement of some of the donor groups of the L-2 ligands by water.