Isostructural Series of Nine-Coordinate Chiral Lanthanide Complexes Based on Triazacyclononane James W. Walton, † Rachel Carr, † Nicholas H. Evans, † Alexander M. Funk, † Alan M. Kenwright, † David Parker,* ,† Dmitry S. Yufit, † Mauro Botta, ‡ Sara De Pinto, ‡ and Ka-Leung Wong § † Department of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K. ‡ Dipartimento di Scienze e Innovazione Tecnologica, Universita ̀ del Piemonte Orientale, “Amedeo Avogadro” Viale Teresa Michel 11, 15121 Alessandria, Italy § Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong * S Supporting Information ABSTRACT: Nonadentate ligands based on triazacyclononane incorporating pyridyl-2-phosphinate groups form an isostructural series of complexes with Ln ions in the solid state and in solution. The Ln ion is effectively shielded from the solvent environment. Crystal structures reveal a rigid C 3 -symmetric tricapped trigonal-prismatic coordination geometry that is maintained in solution for the methyl and phenylphosphinate series, as shown by multinuclear NMR analysis. Variable- temperature measurements of the field dependence of the water proton relaxivity in gadolinium complexes indicate that these systems exclude solvent from the primary coordination environment and minimize the second sphere of solvation. The electronic relaxation time for the gadolinium methylphosphinate complex has been estimated to be 550 (±150) ps by EPR and NMR methods, compared to values of around 0.30-0.05 ps for the terbium-ytterbium series, deduced by analyzing the field dependence (4.7-16.5 T) of the 31 P NMR longitudinal relaxation times. Values are compared with analogous azacarboxylate ligand complexes, supporting a key role for donor atom polarizability in determining the electronic relaxation. Spectral emission behavior in solution of samarium, europium, terbium, and dysprosium complexes is compared, and the resolved RRR-Λ and SSS- Δ complexes show strong circularly polarized luminescence. The molecular quadratic hyperpolarizability ⟨β HLS ⟩ has been measured in solution using hyper-Raleigh light-scattering methods, for the whole series of lanthanide complexes of one ligand. The values of ⟨β HLS ⟩ reach a maximum around the center of the series and are not simply dependent on the number of f electrons, suggesting a dominant contribution from the octupolar rather than the dipolar term. ■ INTRODUCTION Nonadentate ligands forming an isostructural series of complexes with Ln III ions are not common, compared to the very large number of octadentate ligands forming well-defined 1:1 complexes. The identification of such an isostructural series allows various comparative physicochemical studies to be undertaken, examining behavior across the series. In this work, the first set of studies relates to applications in photo- and electroluminescence, where stability with respect to deactivation of the long-lived lanthanide excited state is sought, typically by shielding the coordinated ion from intermolecular quenching processes, such as vibrational deactivation by the solvent. 1 This aspect is a key, underpinning requirement in the development of new emissive tags for labeling purposes, allowing their exploitation in time-resolved bioassays, for example. These systems are also of interest as purely “outer- sphere” magnetic resonance (MR) probes for macromolecular structural analysis, taking advantage of the high paramagnetism of the central Ln III ions. Such nonadentate ligands may have C 3 symmetry, allowing a comparison of their spectral properties to be made with “classical” ML 3 complexes of certain tridentate ligands, such as oxydiacetate and dipicolinate (pyridine-2,6- dicarboxylate) and their analogues and derivatives. 2 Received: January 19, 2012 Published: July 19, 2012 Article pubs.acs.org/IC © 2012 American Chemical Society 8042 dx.doi.org/10.1021/ic300147p | Inorg. Chem. 2012, 51, 8042-8056