One-Step Assembly of Visible and Near-Infrared Emitting Metallacrown Dimers Using a Bifunctional Linker.

A family of dimeric LnIII [12-MCGa(III)N(shi) -4] metallacrowns (MCs) (LnIII =Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) was synthesized using the isophthalate group (ip2- ) as a linker. The [LnGa4 ]2 complexes exhibit remarkable photophysical properties, with large molar absorptivities of ≈4×104 m-1 cm-1 , high quantum yields and long luminescence lifetimes with values of (i) 31.2(2)% and 1.410(1) ms, respectively for the visible-emitting [TbGa4 ]2 complex and (ii) 2.43(6)% and 30.5(1) µs for the near-infrared (NIR) emitting [YbGa4 ]2 in the solid state. The NIR emission was obtained not only from Yb, Nd, and Er complexes but also from the less frequently observed emitters such as Pr and Ho. In addition, emission in both visible and NIR domains could be detected for Dy and Sm MCs. ESI-MS and UV/Vis data revealed that the complexes are highly stable in dimethylsulfoxide (DMSO) solution with the 1 H- and COSY-NMR spectra of the diamagnetic [YGa4 ]2 analogue providing evidence for long-term solution stability. This new approach allows one to construct a basis for highly luminescent MCs that may be further modified to be adapted for applications such as optical imaging.

The Nature of the Bridging Anion Controls the Single-Molecule Magnetic Properties of DyX4M 12-Metallacrown-4 Complexes.

A family of DyX4M(12-MCMnIII(N)shi-4) compounds were synthesized and magnetically characterized (X = salicylate, acetate, benzoate, trimethylacetate, M = NaI or KI). The bridging ligands were systematically varied while keeping the remainder of the MC-geometry constant. The type of monovalent cation, necessary for charge balance, was also altered. The dc magnetization and susceptibility of all compounds were similar across the series. Regardless of the identity of the countercation, the Dy(Hsal)4M 12-MC-4 compounds were the only compounds to show frequency-dependent ac magnetic susceptibility, a hallmark of single-molecule magnetism. This indicates that the nature of the bridging ligand in the 12-MCMnIII(N)shi-4 compounds strongly affects the out-of-phase magnetic properties. The SMM behavior appears to correlate with the pKa of the bridging ligand.

Synthesis and Magnetic Characterization of Fe(III)-Based 9-Metallacrown-3 Complexes Which Exhibit Magnetorefrigerant Properties.

The structural characterization and magnetic properties of three related 9-metallacrown-3 (9-MC-3) structures are reported. Each of these iron complexes is shown to exhibit significant magnetic refrigerant properties. FeIII(acetate)3[9-MCFeIIIN(shi)-3](MeOH)3·MeOH·7H2O (1-OAc) and FeIII(benzoate)3[9-MCFeIIIN(shi)-3](MeOH)3·MeOH·4H2O (1-OBz) are structurally analogous tetranuclear iron(III) clusters which exhibit drastically different magnetic properties, due to differences in intermolecular and intramolecular π interactions which affect superexchange. 1-OAc displays a magnetocaloric effect with a maximum entropy change of -ΔSm = 15.4 J kg-1 K-1 at T = 3 K and an applied field change of µoΔH = 7 T, whereas 1-OBz exhibits a maximum -ΔSm = 7.4 J kg-1 K-1 at T = 7 K and µoΔH = 7 T and displays an inverse magnetocaloric effect at lower temperatures and field changes. 1-OAc has -ΔSm values comparable to those of other Fe-based MCE materials and displays a significant MCE at lower applied fields, with -ΔSm = 11.2 J kg-1 K-1 at 3 K and µoΔH = 3 T. The tetranuclear core of 1 may be linked with isophthalate to form an octanuclear FeIII2(isophthalate)3[9-MCFeIIIN(shi)-3]2 dimer (2) that crystallizes in a honeycomb packing arrangement and exhibits solvation-dependent magnetic properties. The MCE for this molecule ranges from -ΔSm = 9.9 J kg-1 K-1 at T = 5 K and µoΔH = 7 T, when the pores of the material are highly occupied with solvent, to -ΔSm = 5.4 J kg-1 K-1, when the system is fully desolvated.

Design of 2D Porous Coordination Polymers Based on Metallacrown Units.

A 12-metallacrown-4 (MC) complex was designed and employed as the building block in the synthesis of coordination polymers, one of which is the first permanently porous MC architecture. The connection of the four-fold symmetric MC subunits by Cu(II) nodes led to the formation of 2D layers of metallacrowns. Channels are present in the crystalline architecture, which exhibits permanent porosity manifested in N2 and CO2 uptake capacity.

Ga(3+)/Ln(3+) Metallacrowns: A Promising Family of Highly Luminescent Lanthanide Complexes That Covers Visible and Near-Infrared Domains.

Luminescent lanthanide(III)-based molecular scaffolds hold great promises for materials science and for biological applications. Their fascinating photophysical properties enable spectral discrimination of emission bands that range from the visible to the near-infrared (NIR) regions. In addition, their strong resistance to photobleaching makes them suitable for long duration or repeated biological experiments using a broad range of sources of excitation including intense and focalized systems such as lasers (e.g., confocal microscopy). A main challenge in the creation of luminescent lanthanide(III) complexes lies in the design of a ligand framework that combines two main features: (i) it must include a chromophoric moiety that possesses a large molar absorptivity and is able to sensitize several different lanthanide(III) ions emitting in the visible and/or in the near-infrared, and (ii) it must protect the Ln(3+) cation by minimizing nonradiative deactivation pathways due to the presence of -OH, -NH and -CH vibrations. Herein, a new family of luminescent Ga(3+)/Ln(3+) metallacrown (MC) complexes is reported. The MCs with the general composition [LnGa4(shi)4(C6H5CO2)4(C5H5N) (CH3OH)] (Ln-1, Ln = Sm(3+)-Yb(3+)) were synthesized in a one pot reaction using salicylhydroxamic acid (H3shi) with Ga(3+) and Ln(3+) nitrates as reagents. The molecular structure of [DyGa4(shi)4(C6H5CO2)4(C5H5N) (CH3OH)] was obtained by X-ray analysis of single crystals and shows that the complex is formed as a [12-MCGa(III)shi-4] core with four benzoate molecules bridging the central Dy(3+) ion to the Ga(3+) ring metals. The powder X-ray diffraction analysis demonstrates that all other isolated complexes are isostructural. The extended analysis of the luminescence properties of these complexes, excited by the electronic states of the chromophoric ligands, showed the presence of characteristic, sharp f-f transitions that can be generated not only in the NIR (Sm, Dy, Ho, Er, Yb) but also in the visible (Sm, Eu, Tb, Dy, Tm). All Ln-1 complexes possess very high quantum yield values with respect to other literature compounds, indicating a good sensitization efficiency of the [12-MCGa(III)shi-4] scaffold. Especially, as of today, the Yb-1 complex exhibits the highest NIR quantum yield reported for a lanthanide(III) complex containing C-H bonds with a value of 5.88(2)% in the solid state. This work is a significant step forward toward versatile, easily prepared luminescent lanthanide(III) complexes suitable for a variety of applications including highly in demand biological imaging, especially in the NIR domain.

Assessing the exchange coupling in binuclear lanthanide(iii) complexes and the slow relaxation of the magnetization in the antiferromagnetically coupled Dy2 derivative.

We report here the synthesis and the investigation of the magnetic properties of a series of binuclear lanthanide complexes belonging to the metallacrown family. The isostructural complexes have a core structure with the general formula [Ga4Ln2(shi3-)4(Hshi2-)2(H2shi-)2(C5H5N)4(CH3OH) x (H2O) x ]·xC5H5N·xCH3OH·xH2O (where H3shi = salicylhydroxamic acid and Ln = GdIII1; TbIII2; DyIII3; ErIII4; YIII5; YIII0.9DyIII0.16). Apart from the Er-containing complex, all complexes exhibit an antiferromagnetic exchange coupling leading to a diamagnetic ground state. Magnetic studies, below 2 K, on a single crystal of 3 using a micro-squid array reveal an opening of the magnetic hysteresis cycle at zero field. The dynamic susceptibility studies of 3 and of the diluted DyY 6 complexes reveal the presence of two relaxation processes for 3 that are due to the excited ferromagnetic state and to the uncoupled DyIII ions. The antiferromagnetic coupling in 3 was shown to be mainly due to an exchange mechanism, which accounts for about 2/3 of the energy gap between the antiferro- and the ferromagnetic states. The overlap integrals between the Natural Spin Orbitals (NSOs) of the mononuclear fragments, which are related to the magnitude of the antiferromagnetic exchange, are one order of magnitude larger for the Dy2 than for the Er2 complex.

Correction: Assessing the exchange coupling in binuclear lanthanide(iii) complexes and the slow relaxation of the magnetization in the antiferromagnetically coupled Dy2 derivative.

[This corrects the article DOI: 10.1039/C5SC01029B.].