Controlled Heterometallic Composition in Linear Trinuclear [LnCeLn] Lanthanide Molecular Assemblies.

The combination of two different β-diketone ligands facilitates the size-controlled assembly of pure heterometallic [LnLn'Ln] linear compounds thanks to two different coordination sites present in the molecular scaffold. [HoCeHo], [ErCeEr], and [YbCeYb] analogues are presented here and are characterized both in the solid state and in solution, demonstrating the selectivity of this unique method to produce heterometallic 4f molecular entities

Designed asymmetric coordination helicates with bis-β-diketonate ligands.

A new bis-(β-diketone) ligand featuring built-up structural asymmetry yields non-symmetric Fe(iii) and Ga(iii) dinuclear, triple-stranded helicates by design. Their structural properties have been studied, both in solid state and in solution, and compared with their corresponding symmetric analogues. The robustness observed shows the potential of this synthetic strategy to develop non-symmetric helicoidal motifs with specific functional groups

Crystal size dependence of dipolar ferromagnetic order between Mn6 molecular nanomagnets

We study how crystal size influences magnetic ordering in arrays of molecular nanomagnets coupled by dipolar interactions. Compressed fluid techniques have been applied to synthesize crystals of Mn6 molecules (spin S=12) with sizes ranging from 28μm down to 220 nm. The onset of ferromagnetic order and the spin thermalization rates have been studied by means of ac susceptibility measurements. We find that the ordered phase remains ferromagnetic, as in the bulk, but the critical temperature Tc decreases with crystal size. Simple magnetostatic energy calculations, supported by Monte Carlo simulations, account for the observed drop in Tc in terms of the minimum attainable energy for finite-sized magnetic domains limited by the crystal boundaries. Frequency-dependent susceptibility measurements give access to the spin dynamics. Although magnetic relaxation remains dominated by individual spin flips, the onset of magnetic order leads to very long spin thermalization time scales. The results show that size influences the magnetism of dipolar systems with as many as 1011 spins and are relevant for the interpretation of quantum simulations performed on finite lattices

Distributive Nd-to-Yb Energy Transfer within Pure [YbNdYb] Heterometallic Molecules

Facile access to site-selective hetero-lanthanide molecules will open new avenues in the search of novel photophysical phenomena based on Ln-to-Ln′ energy transfer (ET). This challenge demands strategies to segregate efficiently different Ln metal ions among different positions in a molecule. We report here the one-step synthesis and structure of a pure [YbNdYb] (1) coordination complex featuring short Yb···Nd distances, ideal to investigate a potential distributive (i.e., from one donor to two acceptors) intramolecular ET from one Nd3+ ion to two Yb3+ centers within a well-characterized molecule. The difference in ionic radius is the mechanism allowing to allocate selectively both types of metal ion within the molecular structure, exploited with the simultaneous use of two β-diketone-type ligands. To assist the photophysical investigation of this heterometallic species, the analogues [YbLaYb] (2) and [LuNdLu] (3) have also been prepared. Sensitization of Yb3+ and Nd3+ in the last two complexes, respectively, was observed, with remarkably long decay times, facilitating the determination of the Nd-to-Yb ET within the [YbNdYb] composite. This ET was demonstrated by comparing the emission of iso-absorbant solutions of 1, 2, and 3 and through lifetime determinations in solution and solid state. The comparatively high efficiency of this process corroborates the facilitating effect of having two acceptors for the nonradiative decay of Nd3+ created within the [YbNdYb] molecule

Thermodynamic stability of heterodimetallic [LnLn] complexes: synthesis and DFT studies

The solid-state and solution configurations of the heterodimetallic complexes (Hpy)[LaEr(HL)(3)(NO3)(py)(H2O)] (1), (Hpy)[CeEr(HL)(3)(NO3)(py)(H2O)] (2), (Hpy)[CeGd(HL)(3)(NO3)(py)(H2O)] (3), (Hpy)[PrSm(HL)(3)(NO3)(py)(H2O)] (4), and (Hpy)(2)[LaYb(HL)(3)(NO3)(H2O)](NO3) (5), in which H3L is 6-(3-oxo-3-(2-hydroxyphenyl)propionyl)pyridine-2-carboxylic acid and py is pyridine, were analyzed experimentally and by using DFT calculations. Complexes 3, 4, and 5 are described here for the first time, and were analyzed by using single-crystal X-ray diffraction and mass spectrometry. The theoretical study was also extended to the [LaCe] and [LaLu] analogues. The results are consistent with a remarkable selectivity of the metal distribution within the molecule in the solid state, enhanced by the size difference between the different ions. This selectivity was reduced in solution, particularly for ions with the most similar radii. This unique entry into 4f-4f heterometallic chemistry establishes for the first time the difference between the selectivity in solution and that in the solid state, as a result of changes to the coordination that follow the dissociation of terminal ligands upon dissolution of the complexes

Selective signalling of alcohols by a molecular lattice and mechanism of single-crystal-to-single-crystal transformations

Single-crystal-to-single-crystal (SCSC) transformations of molecular materials involving exchange of lattice molecules are becoming commonplace and very relevant in areas like chemical sensing or the pharmaceutical sector. Spin crossover (SCO) complexes could be great candidates to act as molecular chemical sensors using spin switching to signal detection. We describe here the capacity of the Fe(ii) molecular material [Fe(bpp)(H2L)](ClO4)2·C3H6O (bpp and H2L are 2,6-bis-(pyrazol-3-yl)-pyridine type ligands) to have its lattice acetone molecules replaced by certain selected alcohols from the gas phase (MeOH, EtOH or nPrOH but not iPrOH), signalling the process by a spin transition that also changes the colour of the crystals. The magnetic response of the signalling complex depends on the chain length of the alcohol, allowing selective detection. As these molecular exchanges are SCSC processes, the structures of the alcoholates obtained have been determined by single crystal X-ray diffraction (SCXRD). The removal of n-propanol from its host lattice has been quenched in operando at various intermediate stages and studied by SCXRD to unveil crucial details of the mechanism of this SCSC transformation