Lanthanide complexes of the hexadentate N-donor podand tris[3-(2-pyridyl)pyrazolyl]hydroborate: Solid-state and solution properties

The hexadentate N-6-donor podand tris[3-(2-pyridyl)pyrazolyl]hydroborate (Tp(Py)) contains 2-pyridyl fragments attached to the pyrazolyl C-3-positions such that each arm is a bidentate chelate. Three series of lantharide(III) complexes were prepared: [M(Tp(Py))(MeOH)(2)F][PF6] (series A), [M(Tp(Py))(NO3)(2)] (series B), and [M(Tp(Py))(2)]-[BPh(4)] (series C). Crystallographic studies showed that series A and B have a 1:1 metal:Tp(Py) ratio, with the metal ion lying within the podand cavity and the remaining coordination sites occupied by solvent molecules and/or counterions to give 9-coordination (A, with one fluoride and two methanol ligands) or 10-coordination (B, with two bidentate nitrate ligands). The C complexes were prepared in the absence of any coordinating anions and have a 1:2 metal:Tp(Py) ratio with an unusual icosahedral geometry arising from coordination of the 12 nitrogen donors from two interleaved podands. Conductivity studies on the B complexes show that in water the nitrates dissociate to give [M(Tp(Py))(H2O)(q)](NO3)(2); the relaxivity of [Gd(Tp(Py))(NO3)(2)] in water is 4.4 s(-1) mM(-1), a value comparable to those of clinically useful MRI contrast enhancement agents. Comparison of emission lifetimes of [M(Tp(Py))(NO3)(2)] (M = Eu, Tb) in H2O/D2O and CH3OH/CD3OD give values for q, the number of coordinated solvent molecules, of 3.6 (water) and 2.6 (methanol). The C complex [Tb(Tp(Py))(2)][BPh(4)] also has q = 2.6 in methanol, suggesting that partial ligand dissociation allows access of solvent molecules to the metal coordination sphere.