Stoichiometrically Controlled Assembly of Lanthanide Molecular Complexes of the Heteroditopic Divergent Ligand 4′-(4-Pyridyl)-2,2′:6′,2″-terpyridine N-Oxide in Hypodentate or Bridging Coordination Modes. Structural, Magnetic, and Photoluminescence Studies

Abstract

Mononuclear rare-earth tris-β-diketonato complexes RE­(tta)3dme [RE = Y (1), La (2), Dy (3), or Eu (4); Htta = 2-thenoylacetone; dme = 1,2-dimethoxyethane] react cleanly at room temperature in a 1:1 molar ratio with the heteroditopic divergent ligand 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine N-oxide (pyterpyNO) to yield RE2(tta)6(pyterpyNO) n , where n = 2 for RE = Y (5), Dy (6), or Eu (7) and n = 3 for RE = La (8). The crystal structure of 5 revealed a dinuclear compound with two pyterpyNO’s bridging through the oxygen atom in a hypodentate mode leaving the terpyridine moieties uncoordinated. Using a metal:pyterpyNO molar ratio of 2 for RE = Y (9), Dy (10), or Eu (11), it was possible to isolate the molecular complexes RE4(tta)12(pyterpyNO)2, while using a 5:3 molar ratio, the product La5(tta)12(pyterpyNO)3 (12) can be obtained. 89Y nuclear magnetic resonance spectroscopy revealed two different yttrium centers at room temperature for 9. An X-ray diffraction study of 10 showed a symmetrical tetranuclear structure resulting from the coordination of two Dy­(tta)3 fragments to the two hypodentate terpyridines of the dinuclear unit and presenting two different coordination sites for metals with coordination numbers of 8 and 9. Magnetic studies of 6 and 10 revealed the presence of an antiferromagnetic interaction between the two Dy­(III) atoms bound by the NO bridges. These compounds displayed a slow relaxing magnetization through Orbach (6) and Raman (10) processes in the absence of an applied magnetic field; the rate increased upon application of a 1 kOe field. 7 and 11 showed a bright red emission typical of Eu3+. The two complexes have similar emission properties mainly determined by the employed β-diketonato ligands.

Size Selectivity in Heterolanthanide Molecular Complexes with a Ditopic Ligand

“…The dynamics of the magnetization of compound 5 are reported in Figure S9 as temperature dependence of the inphase and out-of-phase magnetic susceptibilities measured without static field applied for eleven sweeping frequencies of the oscillating field. As previously observed for the homometallic Dy 4 (tta) 12 (pyterpyNO) 2 analogue of 5, [70] two sets of peaks appear in the out-of-phase susceptibility, which move to higher temperature upon increase in sweeping frequency, highlighting a Single Molecule Magnet behaviour. A fitting procedure of these peaks using an extended Debye model has been carried out (Figure S9); however, due to the high uncertainty in the determination of the magnetic relaxation times of the faster process, only the relaxation times of the slower one have been taken in consideration in the following discussion, and their temperature dependence is reported in Figure S10.…”

“…First, we assume that, for both molecules, the slow relaxing magnetic moment arises from the dinuclear Dy 2 core, featuring Dy 3 + ions with an antiferromagnetic exchange channelled through the two bridging oxygen atoms, and not from the nona-coordinated Dy 3 + ions located in position 2. Such an assumption is supported by the slight decrease in the magnetic relaxation time upon passing from zero to 1 kOe static field applied (Figure S11), a phenomenon typical of coupled dysprosium(III) Single Molecule Magnets and, as such, observed also for Dy 4 , [70] while it is unusual for uncoupled Dy 3 + Single Ion Magnets. Moreover, this treatment relies on the assumption that the dipolar field acting on the slow relaxing Dy 2 core is the same for 5 and Dy 4 .…”

“…Chemical shifts at À 74.7 and À 75.1 ppm can be attributed to tta ligands of O-coordinated lanthanum or yttrium, respectively, while signals at À 75.3 and À 75.5 ppm can be attributed to Ncoordinated lanthanum and yttrium, since they closely resemble the homo-metallic shifts (Figure 2). [70] Taking into account an estimated error of � 10 % on the integration of the signals, the values obtained can be used for an estimate of the positional 19 F NMR spectrum of 4 compared with 19 F NMR spectra of the homometallic compounds of yttrium (Y 4 , in blue) and lanthanum (La 5 , in green). [70] Chemistry-A European Journal disorder in solution.…”

“…In addition, Dy 3 + ions in position 1 are known to interact antiferromagnetically, [70] further differentiating their magnetic properties from the ones of the outer, non-interacting, ions in position 2. In order to obtain information about the positional disorder between the Dy 3 + and La 3 + ions in compound 5, the temperature dependence of the χ M T of compound 5 has been fitted using a linear combination of the χ M T plot of compounds Dy 2 and [Dy 4 (tta) 12 (pyterpyNO) 2 ] (Dy 4 ) 70 (Figure 4), according to Equation (1):…”

“…Figure 2 19. F NMR spectrum of 4 compared with19 F NMR spectra of the homometallic compounds of yttrium (Y 4 , in blue) and lanthanum (La 5 , in green) [70]. …”