Lanthanide Complexes of the Hexadentate N-Donor Podand Tris[3-(2-pyridyl)pyrazolyl]hydroborate:  Solid-State and Solution Properties

Abstract

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 lanthanide(III) complexes were prepared: [M(Tp Py )(MeOH) 2 F][PF 6 ] (series A ), [M(Tp Py )(NO 3 ) 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 )(H 2 O) q ](NO 3 ) 2; the relaxivity of [Gd(Tp Py )(NO 3 ) 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 )(NO 3 ) 2 ] (M = Eu, Tb) in H 2 O/D 2 O and CH 3 OH/CD 3 OD 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.