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

Abstract

The hexadentate N6-donor podand tris[3-(2-pyridyl)pyrazolyl]hydroborate (TpPy) contains 2-pyridyl fragments attached to the pyrazolyl C3-positions such that each arm is a bidentate chelate. Three series of lanthanide(III) complexes were prepared: [M(TpPy)(MeOH)2F][PF6] (series A), [M(TpPy)(NO3)2] (series B), and [M(TpPy)2][BPh4] (series C). Crystallographic studies showed that series A and B have a 1:1 metal:TpPy 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:TpPy 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(TpPy)(H2O) q ](NO3)2; the relaxivity of [Gd(TpPy)(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(TpPy)(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(TpPy)2][BPh4] also has q = 2.6 in methanol, suggesting that partial ligand dissociation allows access of solvent molecules to the metal coordination sphere.

New Polar Pyrazolylborate Ligands and Their Basic Zinc Complex Chemistry

“…We have attached 3-pyridyl substituents to Tp ligands and studied their zinc complex chemistry, [5][6][7] and Ward did the same with 2-pyridyl groups. [8,9] Carrano reported Tp ligands with carboxylic ester and carboxamide substituents and a number of zinc complexes thereof; [10,11] unfortunately this work could not be reproduced by us. [12] In our eyes the most promising approach has been reported recently by Trofimenko and involved pyrrolidinocarbonyl substituents, [13] which have not yet been exploited in zinc complex chemistry.…”

Effect of a halogenide substituent on the stability and photophysical properties of lanthanide triple-stranded helicates with ditopic ligands derived from bis(benzimidazolyl)pyridine †

“…preorganised 10 and predisposed 11 macrocyclic ligands or multidentate podands. 12 However, self-assembly processes 13 appear to be best suited to achieve the required ultra fine tuning of the lanthanide() co-ordination sphere.…”

A C3-symmetric chiral hexadentate podand ligand based on a tris(pyrazolyl)borate core

“…This synthesis is summarised in Scheme 2; full experimental details will be described elsewhere. 13 Conversion of the acetyl group of A to a pyrazole is straightforward, 9 Fig. 2 is a view down the Tl ؒ ؒ ؒ B axis which emphasises the C 3 symmetry.…”