Halogen Bond Mediated Self-Assembly of Mononuclear Lanthanide Complexes: Perception of Supramolecular Interactions, Slow Magnetic Relaxation, and Photoluminescence Properties

Abstract

Five new mononuclear lanthanide complexes, [LnL 2 ][Et 3 NH]•THF/H 2 O (Ln = Nd, Tb, Dy) (H 2 L Cl = 2-bis(2hydroxy-3,5-dichloro benzyl)aminomethyl]pyridine), Ln = Nd (1), Tb (2), and Dy (3), and (H 2 L Br = 2-bis(2-hydroxy-3,5-dibromo benzyl)aminomethyl]pyridine), Ln = Nd (4, H 2 O) and Tb ( 5), were synthesized and structurally characterized by single-crystal Xray diffraction analyses. Being isostructural in all the five cases, the metal center is octa-coordinated with a triangular dodecahedron (D 2d symmetry) geometry, and it is independent of the halogen substitution (Cl/Br). This close similarity is due to the composite interplay of hydrogen/halogen bond interactions that control the overall crystal packing, yet notable differences in association patterns among the individual ones arise from the subtle stereo-electronic requirement of individual molecules in the threedimensional (3D) architecture. Hirshfeld surface and density functional theory (DFT) calculations clearly vouch for the importance of the hydrogen bond and halogen bond interactions observed in the structure. Detailed magnetic measurements using direct-current and alternating-current susceptibility measurements show slow magnetic relaxation in 3, a characteristic feature of the singlemolecule magnets (SMMs), which is not shown by 1 and 2. Steady-state and time-resolved photoluminescence of Tb(III) complexes shows a strong ligand-to-metal energy transfer that can be modulated by changing the substitution on phenolic ligands. The results from these analyses indicate that it may be advantageous to consider the subtle role of hydrogen bond (HB)/halogen bond (XB) intermolecular interactions judiciously for the design of SMMs and luminescent materials based on halogen-substituted ligands.

The importance of second sphere interactions on single molecule magnet performance

“…Where H 2 L5 = 2-[N,N-bis(2-hydroxy-3,5-dichlorobenzyl)aminomethyl]pyridine and H 2 L6 = 2-[N,N-bis(2-hydroxy-3,5-dibromobenzyl)aminomethyl]pyridine. 90 The complexes are essentially isostructural however, only the dysprosium containing complex ( 49 ) showed SMM behaviour with U eff of 13.45 K. A notable trend of this series is that the overall coordination environment of the magnetic centre was invariant to the halogen used. This is a result of the hydrogen/halogen bonding interplay, which controls the overall crystal packing, being largely independent of the halogen present.…”

“…For example, secondary interactions are the dominant forces behind crystal packing and the formation of supramolecular architectures, where control of these could prove beneficial in the development of future SMMs. 88,89 Exploring this, Sushila 90 The complexes are essentially isostructural however, only the dysprosium containing complex (49) showed SMM behaviour with U eff of 13.45 K. A notable trend of this series is that the overall coordination environment of the magnetic centre was invariant to the halogen used. This is a result of the hydrogen/halogen bonding interplay, which controls the overall crystal packing, being largely independent of the halogen present.…”

Tuning the optical and magnetic properties of lanthanide single-ion magnets using nitro-functionalized trispyrazolylborate ligands

“…We are aware of only two reports, by Sushila et al and Gálico et al, on the influence of halo-substitution on the optical and magnetic properties of lanthanide amino-bisphenol and β-diketonate complexes respectively, that addresses the impact of substituent (halogen) effects on optical and magnetic properties of Ln-materials. 42,43 Given the variety of lanthanide molecular magnets with pyrazolylborates and our lab's recent work with selective functionalization of trispyrazolylborate (Tp − ) ligands, 29,44 we postulate that asymmetrically functionalized Tp − ligands (Fig. 1) are an ideal platform for establishing how aromatic ring substituents influence the properties of lanthanide molecular magnets.…”

Tuning the Optical and Magnetic Properties of Lanthanide Single-Ion Magnets using Nitro-functionalized Trispyrazolylborate Ligands

“…We are aware of only a single report, by Goswami, Venugopalan, Patra and coworkers on the influence of halo-substitution on the optical and magnetic properties of lanthanide amino-bisphenol complexes that addresses the impact of substituent (halogen) effects on magnetic properties of Ln-materials. 42 Given the variety of lanthanide molecular magnets with pyrazolylborates and our lab's recent work with selective functionalization of trispyrazolylborate (Tp -) ligands, 29,43 we postulate that asymmetrically functionalized Tpligands (Figure 1) are an ideal platform for establishing how aromatic ring substituents influence the properties of lanthanide molecular magnets.…”