Two [Ln4] molecular rings folded as compact tetrahedra

Please do not adjust margins a. Departament de Química Inorgànica i Orgànica, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain. E-mail: guillem.aromi@qi.ub.es. b. Institut of Nanoscience and Nanotechnology of the University of Barcelona (IN2UB), Barcelona, Spain. c. Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC and Universidad de Zaragoza, Plaza San Francisco s/n, 50009, Zaragoza, Spain † Footnotes relating to the title and/or authors should appear here. Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x Received 00th January 20xx, Accepted 00th January 20xx


Introduction
Lanthanide coordination chemistry is of relevance to a large number of scientific areas and applications ranging lightemitting diodes, 1 molecular information storage, 2-5 magnetic refrigeration, 6-8 magnetic resonance imaging 9 or quantum computing. [10][11][12][13][14][15] On the topic of molecular magnetism, a Dy(III) organometallic complex has emerged as an outstanding single molecule magnet (SMM), with a record blocking temperature above liquid nitrogen. 16 Indeed, compounds of this metal are known to exhibit very often slow relaxation of the molecular magnetic moment. 3,17 Dy(III) is therefore, an excellent ingredient in the search of magnetic molecular materials capable of switching or modulating their properties reversibly by an external stimulus, such as light irradiation. 18 In the attempt to obtain molecular assemblies made of Dy(III) single ion magnets, with photo-switchable properties, we have designed and prepared a new bridging ligand, 1,2-bis-(5-(N'-(pyridine-2-yl-methylene)-carbohydrazide)-3-methyl-thien-3yl)-cyclopentene (H 2L), incorporating a dithienylethene spacer, which undergoes a reversible light-triggered process of cyclization (Fig. 1). 19 The latter is a highly convenient photochromic unit because of its resistance to fatigue and the fact that both isomers are thermally stable. Also, because the switching takes place with a different light frequency (UV or visible) for each direction. 20 Dithienylethenes have been exploited before to design molecular species with photoswitchable magnetic properties. [21][22][23][24][25] Here, we have explored the reactivity of deprotonated H 2L with Dy(III) or Tb(III) chloride in an attempt to prepare Ln-based SMMs, with tuneable properties via photo switching processes. 26 The coordination metallocycles H 2O@[Ln4L4Cl4(H2O)4] (Ln=Dy, 1; Tb, 2) were obtained, highly folded to yield a compact structure with the metals disposed at the vertices of a tetrahedron. The magnetic slow relaxation properties of 1 and 2 have been studied, and the photochromic properties of all the new compounds have been explored. The ligand H2L is synthesized by a Schiff base reaction between a bis-carbohydrazide containing a dithienylethene a spacer (1,2-bis-(5-carbohydrazide-2-methyl-thiophen-3-yl)cyclopentene) and 2-pyridinaldehyde (Scheme S1). The analogous ligand made with salicylaldehyde (instead of pyridinaldehyde) together its reactivity with Cu(II) was recently reported. 27 this yielded coordination complexes exhibiting reversible photochromic activity. Ligand H 2L was fully characterized (see below) and its photo-switching properties in solution were studied in ethanol. Thus, irradiation of a colorless solution of H 2L with UV light causes a color change to deep violet that is completely reversed upon illumination with visible light. This photochromic behavior was monitored through electronic spectroscopy (Fig. 2). The spectrum of H 2L is dominated by an intense band at 308 nm, featuring a small shoulder at higher energies. Smaller bands at 208 and 396 nm, respectively are also present. UV irradiation causes a drastic decrease of absorbance of the 308 nm band together with the fast growth of a very broad and intense band at 564 nm, characteristic of the conjugated system formed upon photocyclization ( Fig. 1), which gives the strong coloration to the closed form of the photoactive moiety. 28 A photostationary state (PPS) is reached in 70 seconds. The reverse process caused by visible light also occurs very fast, leading to the restoration of the original spectrum at the PPS in about 80 seconds (Fig. 2). Small differences with respect to the initial system may have to do with the potential interference by cis/trans isomerization processes of the hydrazone, susceptible to occur under light irradiation. 29   (Table  S1). The asymmetric unit is composed by one Ln(III) ion, one deprotonated ligand L 2-, one terminal Cl − , one coordinated molecule of H2O, one fourth of another molecule of H2O and, four/three lattice pyridine molecules (for 1/2). Compound 2 additionally, includes half molecule of MeOH in this unit. The coordination complex (Fig. 3) is formed by four Ln(III) ions, each chelated by two (O,N,N) coordination pockets of two L 2ligands, which, in this manner bridge them to two other metals of the tetranuclear cluster. Eight coordination around each lanthanide ion is completed by one Cland one H2O terminal ligand, in mutual cis configuration, and mutually disordered.  Table S2. Thus, the aggregate consists of a [Ln 4L4] molecular ring, which is, by virtue of the large flexibility of the ligands, highly folded onto itself. The result is a compact structure, featuring the four metals at the vertices of a flattened tetrahedron (Fig. 3, S2 and S3), with two long edges, 10.726/10.784 Å, and four shorter ones, 9.613/9.645 Å (in the 1/2 format). For each ligand in this structure, one hydrazone and its adjacent thienyl ring point towards the interior of the cage and the other analogous group is oriented outwards (Fig. 3). A molecule of water is allocated the centre of the assembly, establishing hydrogen bonds to the non-coordinated N-atoms of the internal hydrazone moieties, clearly contributing to the stability of the folded structure (Table S3). Static or dynamic disorder cause a symmetric relationship with each of the four N-atoms, possible acceptors of the two hydrogen bonds per molecule (O···N distance of 2.906/2.887 Å). Reinforcing the compact structure, each ligand establishes one set of intramolecular C-H···π interactions through the methyl group of one thienyl ring with its other thienyl ring and the pyridyl of an adjacent ligand (Fig. S4). The solid-state configuration of the H 2O@[Ln4L4Cl4(H2O)4] units causes the methylthiophenyl groups within each L 2ligand to adopt the so-called "parallel" conformation, i.e. with both methyl groups oriented towards the same side of the idealized plane of the photochromic unit. This prevents the photocyclization process upon UV light irradiation, 30 as verified experimentally here. Each molecule of 1 or 2 is surrounded in the lattice by four first neighbours interacting via S···S contacts (with distances of 3.401/3.397 Å), organizing the clusters in sheets perpendicular to the crystallographic c axis (Fig. S5). Coordination clusters of four Ln ions in form of a tetrahedron are quite common. One category is that of compact cages with monoatomic bridges between the metals, 31-33 sometimes incorporating a central μ 4−O 2− ion linking all of them. [34][35][36][37] Another type is made by coordination assemblies ensembled by extended polytopic ligands spanning either the vertices [38][39][40][41] or the faces 42-46 of the tetrahedron. In this latter category, the metals at kept much further apart, therefore the cages feature large cavities, with the capacity to host various types of guests, 47-50 including molecules of water. 51 Interestingly, none of the tetrahedric [Ln 4] cages reported to the Cambridge Structure Data Center (CCDC) consist of a molecular ring assembled by ditopic ligands, folded into itself as a tetrahedron via intramolecular weak contacts as in 1 and 2.

Magnetic Properties
As mentioned in the introduction, lanthanide complexes have acquired increased relevance in the area of molecular magnetism. In relation to compounds 1 and 2, Dy(III) and Tb(III) have become part of some of the most outstanding SMMs known in the literature. 16 ). For both compounds, χT declines upon decreasing the temperature, with increasing rates as the latter approaches 2 K, where χT reaches 46.56/27.10 cm 3 Kmol -1 . The decrease of the χT product with cooling is caused by depopulation of Stark levels and is more significant in 2. Most of the decrease of χT below 10 K is however an effect of the applied dc field, since the equilibrium susceptibility derived from zero-field ac measurements reflects a much-reduced decline of χT below that temperature (Fig.   S6). Besides indicating the absence of any significant exchange interaction, this observation points at the presence of slow dynamics of the magnetization, which was further investigated through ac susceptibility measurements. A significant out-ofphase component of the susceptibility (χ") is indeed detected at 2 K and zero dc field for the [Dy 4] cluster, which is frequency dependent. However, a maximum in χ" vs. frequency, which defines the characteristic relaxation time, could not be observed. The application of even small dc fields is nevertheless enough to bring the frequency of this maximum to within the experimental window (Figs. S7-S8, see below).
This was not the case for the [Tb 4] complex, for which no χ" signal could be detected at zero dc field (as measured up to ≈1kHz), while the application a dc magnetic field up to 0.2 T was not sufficient to have a maximum of χ" emerge in variable-temperature measurements down to 2 K (Fig. S7). Thus, only the slow relaxation of the magnetization of the Dy complex was studied in more detail. Variable frequency (0.1 to 10 kHz) isothermal measurements on 1 were first performed at increasing applied dc fields at 2 K, and the characteristic relaxation times τ was extracted with the generalized Debye model (see SI, Fig. S8). τ is found to rapidly increase at low fields and already reaches ca. 1.5 ms at 750 Oe (Fig. 5). In the low field range studied, up to 0.1 T, the increase of τ is ≈aB 2 , considering the estimated error, which becomes very large as soon as frequency maximum is localized closer to the limit or out of the experimental frequency window. This fielddependence, depicted in Fig. 5 also as a log-log plot, is expected for a relaxation mechanism based on the spin-spin and spin-nuclei coupling. It is also consistent with the fact that the direct process of relaxation is not yet significant at these low fields. The temperature dependence of τ was then studied at 400 and 750 Oe, as well as in zero-field (Figs. S9-11). In zerofield, the relaxation time remains approximately constant at 1-2 10 -6 s, in agreement with a fast tunnelling relaxation. Application of a dc field cancels out this fast relaxation process as observed, at least at sufficiently low temperatures. Thermal activation is clearly present already above 5 K, especially at 750 Oe. At these relatively low temperatures, this is more likely due to Raman spin-phonon processes than to the Orbach mechanism, as discussed elsewhere. 53  The fast relaxation properties depicted here are understandable considering the absence of adequate local symmetry at the Dy centers in the structure of 1, in particular, of the strong axiality necessary for SMMs with exceptionally large energy barriers to relaxation. 5,16,56 While H 2L facilitates the assembly of clusters with slow relaxation of the magnetization, the metals promote a supramolecular arrangement locking effectively its photochromic activity. Future efforts will be directed at including co-ligands to the reaction mixture to isolate open assemblies. Avoiding the folding of the structure should prevent the inhibition of the photoswitching behaviour of the dithyenylethene moieties.

Experimental Synthesis
All chemicals were purchased from Aldrich and used without further purification. All coordination chemistry reactions were performed under aerobic conditions. The precursor 1,2-bis-(5carbohydrazide-2-methyl-thiophen-3-yl)-cyclopentene was synthesized according to a previously reported procedure. 27

Single Crystal X-ray Diffraction (SCXRD).
Data for compound 1 were collected at 150 K using an Oxford Diffraction Excalibur diffractometer with enhanced Mo Kα radiation (λ=0.71073 Å) at the X-ray diffraction and Fluorescence Analysis Service of the University of Zaragoza, on a yellow lath of dimensions 0.25 x 0.08 x 0.04 mm 3 . Cell refinement, data reduction, and absorption corrections were performed with the CrysAlisPro suite. 57 Data for compound 2 were obtained at 100 K on a Bruker APEX II CCD diffractometer at the Advanced Light Source beam-line 11.3.1 at Lawrence Berkeley National Laboratory, from a silicon 111 monochromator (λ = 0.77490 Å), on a yellow lath of dimensions 0.40 x 0.14 x 0.10 mm 3 . Data reduction and Please do not adjust margins Please do not adjust margins absorption corrections were performed with SAINT and SADABS, respectively. 58 Both structures were solved by intrinsic phasing with SHELXT 59 and refined on F 2 with SHELXL. 60 In both cases, crystals appeared to damage upon removal from their mother liquor and exposure to air. Even with fast manipulation, this likely results in some areas of the cell remaining at the end of the refinement with only weak electron density peaks, that could not be modelled satisfactorily as solvent molecules. The corresponding void spaces were thus analysed and taken into account with PLATON/SQUEEZE. 61 The derived electron content/void volume are reasonable for respectively 12 and 2 additional diffuse pyridine molecules per [Ln 4], which was reflected in the formula. All details can be found in CCDC 1983839-1983840 (1-2) that contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via https://www.ccdc.cam.ac.uk/structures/. Crystallographic and refinement parameters are summarized in Table S1. Selected bond lengths and hydrogen bonding details are given in Tables  S2 and S3.

Magnetic Properties.
Variable-temperature magnetic susceptibility data were obtained with a MPMS-XL SQUID magnetometer at the Unitat de Mesures Magnètiques of the Universitat de Barcelona. ac susceptibility measurements were performed with a commercial MPMS-XL SQUID magnetometer and the ACMS option of a commercial Physical Properties Measurement System (PPMS), both hosted by the Physical Measurements Unit of the Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza. Samples were crystals freshly recovered from their crystallization vials. To avoid orientation effects of larger crystallites, these were gently crushed and then constrained in between the two parts of the gelatin capsule used as sample holder. The data were corrected for the contribution of the capsule sample holder, determined empirically, and the sample diamagnetic contributions to the susceptibility using Pascal's constant tables. Isothermal alternating current (ac) data were collected with a 4 Oe field oscillating at different frequencies in the range 100 ≤ ν ≤ 10.000 Hz.

Conclusions
The newly designed ligand 1,2-bis-(5-(N'-(pyridine-2-ylmethylene)-carbohydrazide)-3-methyl-thien-3-yl)cyclopentene (H 2L), with a photochromic dithyenylethene spacer, undergoes reversible photocyclization in solution. The flexibility of H 2L causes the folding of the tetranuclear molecular rings H 2O@[Ln4L4Cl4(H2O)4] (Ln=Tb,Dy), formed upon reaction with the corresponding LnCl 3 salts. The folding is driven by several intramolecular C−H···π interactions, leading to a compact tetrahedral cage. This causes the inhibition of the photochromic activity of H 2L, by forcing it to adopt its inactive conformation. The Dy cluster exhibits slow relaxation of the magnetization, enhanced under an applied magnetic field, which quenches the quantum tunnelling of the magnetization, unveiling a combination of Raman and direct mechanisms of relaxation.

Conflicts of interest
There are no conflicts to declare.