Decanuclear Ln10 wheels and vertex-shared spirocyclic Ln5 cores: synthesis, structure, SMM behavior, and MCE properties

et al.The reaction of a Schiff base ligand (LH3) with lanthanide salts, pivalic acid and triethylamine in 1:1:1:3 and 4:5:8:20 stoichiometric ratios results in the formation of decanuclear Ln10 (Ln=Dy(1), Tb(2), and Gd (3)) and pentanuclear Ln5 complexes (Ln=Gd (4), Tb (5), and Dy (6)), respectively. The formation of Ln10 and Ln5 complexes are fully governed by the stoichiometry of the reagents used. Detailed magnetic studies on these complexes (1–6) have been carried out. Complex 1 shows a SMM behavior with an effective energy barrier for the reversal of the magnetization (Ueff)=16.12(8) K and relaxation time (τo)=3.3×10−5 s under 4000 Oe direct current (dc) field. Complex 6 shows the frequency dependent maxima in the out-of-phase signal under zero dc field, without achieving maxima above 2 K. Complexes 3 and 4 show a large magnetocaloric effect with the following characteristic values: −ΔSm=26.6 J kg−1 K−1 at T=2.2 K for 3 and −ΔSm=27.1 J kg−1 K−1 at T=2.4 K for 4, both for an applied field change of 7 T.We are thankful to the Department of Science and Technology, New Delhi, for financial support. S.D., A.D., S.K., and S.B. thank CSIR, India for Senior Research Fellowship. V.C. is thankful to the Department of Science and Technology for a J.C. Bose National Fellowship. EC is thankful for financial support to Ministerio de Economía y Competitividad (MINECO) for Projects CTQ-2011-24478, CTQ2014-56312-P, the Junta de Andalucía (FQM-195 and the Project of excellence P11-FQM-7756), the University of Granada financial support. M.E. acknowledges financial support from MINECO through grant MAT2012-38318-C03-01. ST thanks the Junta de Andalucía for a postdoctoral contract.Peer Reviewe

Monitoring spin-crossover phenomena via Re(I) luminescence in hybrid Fe(II) silica coated nanoparticles

Financial support from Projects CTQ2014-56312-P and PGC2018-102052-B-C21 financed by MCIN/AEI/10.13039/501100011033/FEDER "Una manera de hacer Europa", the Junta de Andalucia (FQM-195), Feder project A-FQM-172UGR18 and the University of Granada is gratefully acknowledged. I.-F. Diaz-Ortega and J.-R Jimenez are also thankful to the Junta de Andalucia for Postdoctoral research fellowships.Bare (1) and silica coated (1@SiO2) spin crossover (SCO) nanoparticles based on the polymer {[Fe (NH2Trz)3](BF4)2}n have been prepared following a water-in-oil synthetic procedure. For 1, the critical temperatures of the spin transition are TC↓ = 214.6 K and TC↑ = 220.9 K. For 1@SiO2, the abruptness of the transition is enhanced and the critical temperatures are centred at room temperature (TC↓ = 292.1 K and TC↑ = 296.3 K). An inert Re(I) complex of formula [Re(phen)(CO)3(PETES)](PF6) (phen = 1, 10-phenanthroline; PETES = 2(4-pyridylethyl)triethoxysilane) (Re) was also synthesized yielding intense green emission centred at λem = 560 nm. The grafting of this complex on the silica shell of 1@SiO2 led to a bifunctional SCO-luminescence composite (1@SiO2/Re) whose luminescence properties were tuned by the spin state switching. Temperature-variable photophysical studies showed that luminescence and spin transition were synchronized through a radiative (trivial) energy transfer mechanism between the Re(I) and the Fe(II)-LS (LS, Low Spin) centres.FEDER "Una manera de hacer Europa" CTQ2014-56312-P PGC2018-102052-B-C21Junta de Andalucia FQM-195European Commission A-FQM-172UGR18University of GranadaJunta de Andaluci

Amending the anisotropy barrier and luminescence behavior of heterometallic trinuclear linear M-II-Ln(III)-M-II (Ln(III)=Gd, Tb, Dy; M-II=Mg/Zn) complexes by change from divalent paramagnetic to diamagnetic metal ions

The sequential reaction of a multisite coordinating compartmental ligand LH4 [2-(2-hydroxy-3-(hydroxymethyl)-5-methylbenzylideneamino)-2-methylpropane-1,3-diol] with appropriate lanthanide salts followed by the addition of Mg(NO3)2·6H2O or Zn(NO3)2·6H2O in a 4:1:2 stoichiometric ratio in the presence of triethylamine affords a series of isostructural heterometallic trinuclear complexes containing [Mg2Ln]3+ [Ln = Dy (1), Gd (2) and Tb (3)] and [Zn2Ln]3+ [Ln = Dy (4), Gd (5) and Tb (6)] cores. The formation of 1-6 is demonstrated by X-ray crystallography as well as ESI-MS spectra. All complexes are isostructural possessing a linear trimetallic core with a central lanthanide ion. In this article we have discussed the comprehensive studies, involving synthesis, structure, magnetism and photophysical properties on this family of trinuclear [Mg2Ln]3+ and [Zn2Ln]3+ heterometallic complexes. Complexes 1 and 4 show slow relaxation of the magnetization below 12 K under zero applied direct-current field, but without reaching a neat maximum which is due to the overlapping with a faster quantum tunnelling relaxation mediated through dipole-dipole and hyperfine interactions. Under a small applied direct-current field of 1000 Oe the quantum tunneling was almost suppressed and temperature and frequency dependent peaks were observed, thus confirming the SMM behavior of complexes 1 and 4. The fit of the high-temperature relaxation times to the Arrhenius equation affords an effective energy barrier for the reversal of the magnetization of Ueff =72(2) K with o = 8 x 10-9 s for the SR process and Ueff = 61(2) K with o = 4 x 10-7 s for the FR process for 1 whereas for 4, an effective energy barrier for the reversal of the magnetization Ueff = 67(3) K with o = 4.5 x 10-8 s. To rule out the involvement of intermolecular collaborative interactions in the dynamic of relaxation, we have performed ac susceptibility measurements on 1:10 Dy:Y magnetic diluted samples of of 1 and 4, named as 1' and 4'. Interestingly, the diluted compounds 1' and 4' exhibits SMM behavior under zero magnetic field, thus suggesting that their relaxation processes are single molecular in origin and arise from the M-Dy-M unit. Ab initio CASSCF+RASSI calculations carried out on 1 and 4 confirm that the magnetic anisotropy is axial along the M-Dy-M axis and that the relaxation process occurs through the first excited energy level. Furthermore, the chromophoric [LH3]2- ligand is able to act as an 'antenna' group which was found to be effective in the selective sensitization of the emissions of TbIII-based complexes 3 and 6. The emission quantum yields and the luminescence lifetimes at room temperature are 11.7 % and 0.606 ms for 3, 22.7 % and 0.799 ms for 6

Studies on bifunctional Fe(II)-triazole spin crossover nanoparticles: time-dependent luminescence, surface grafting and the effect of a silica shell and hydrostatic pressure on the magnetic properties

Pure and silica wrapped Fe(II)-triazole (FeHTrz) spin-crossover (SCO) nanoparticles have been prepared following a water-in-oil synthetic procedure. The size and shape can be tuned by controlling the Fe(II) and triazole concentrations in the aqueous phase. The magnetic properties of these nanoparticles are strongly affected by the presence of a silica shell embedding the nanostructured FeHTrz polymer. Whereas bare FeHTrz nanoparticles exhibit abrupt and cooperative spin transition with 24–35K-wide thermal hysteresis loops, for the silica derivates the hysteresis width increases up to 37–42 K. This probes the efficiency of the silica shell to promote interparticle interactions and enhance cooperativity effects. Tomographic studies of the FeHTrz@SiO2 nanoparticles reveal a core–shell structure with the pure FeHTrz polymer wrapped into a thin shell of pure silica. Taking advantage of the chemical properties of the silica shell, these hybrid nanoparticles were coated with a dansyl derivate fluorophore whose luminescence properties can be adjusted by the spin state of the SCO polymer. Time-dependent luminescence studies reveal the existence of a non-radiative energy transfer (Förster type) between the organic fluorophore and the Fe(II)-low spin ions. These nanoparticles have also been functionalized with thiol groups allowing them to be deposited onto a gold surface in a controlled manner

Pressure and Temperature Spin Crossover Sensors with Optical Detection

Iron(II) spin crossover molecular materials are made of coordination centres switchable between two states by temperature, pressure or a visible light irradiation. The relevant macroscopic parameter which monitors the magnetic state of a given solid is the high-spin (HS) fraction denoted nHS, i.e., the relative population of HS molecules. Each spin crossover material is distinguished by a transition temperature T1/2 where 50% of active molecules have switched to the low-spin (LS) state. In strongly interacting systems, the thermal spin switching occurs abruptly at T1/2. Applying pressure induces a shift from HS to LS states, which is the direct consequence of the lower volume for the LS molecule. Each material has thus a well defined pressure value P1/2. In both cases the spin state change is easily detectable by optical means thanks to a thermo/piezochromic effect that is often encountered in these materials. In this contribution, we discuss potential use of spin crossover molecular materials as temperature and pressure sensors with optical detection. The ones presenting smooth transitions behaviour, which have not been seriously considered for any application, are spotlighted as potential sensors which should stimulate a large interest on this well investigated class of materials

Iron(II) complexes of tridentate indazolylpyridine ligands: enhanced spin-crossover hysteresis and ligand-based fluorescence.

Reaction of 2,6-difluoropyridine with 2 equiv of indazole and NaH at room temperature affords a mixture of 2,6-bis(indazol-1-yl)pyridine (1-bip), 2-(indazol-1-yl)-6-(indazol-2-yl)pyridine (1,2-bip), and 2,6-bis(indazol-2-yl)pyridine (2-bip), which can be separated by solvent extraction. A two-step procedure using the same conditions also affords both 2-(indazol-1-yl)-6-(pyrazol-1-yl)pyridine (1-ipp) and 2-(indazol-2-yl)-6-(pyrazol-1-yl)pyridine (2-ipp). These are all annelated analogues of 2,6-di(pyrazol-1-yl)pyridine, an important ligand for spin-crossover complexes. Iron(II) complexes [Fe(1-bip)2](2+), [Fe(1,2-bip)2](2+), and [Fe(1-ipp)2](2+) are low-spin at room temperature, reflecting sterically imposed conformational rigidity of the 1-indazolyl ligands. In contrast, the 2-indazolyl complexes [Fe(2-bip)2](2+) and [Fe(2-ipp)2](2+) are high-spin in solution at room temperature, whereas salts of [Fe(2-bip)2](2+) exhibit thermal spin transitions in the solid state. Notably, [Fe(2-bip)2][BF4]2·2MeNO2 adopts a terpyridine embrace lattice structure and undergoes a spin transition near room temperature after annealing, resulting in thermal hysteresis that is wider than previously observed for this structure type (T1/2 = 266 K, ΔT = 16-20 K). This reflects enhanced mechanical coupling between the cations in the lattice through interdigitation of their ligand arms, which supports a previously proposed structure/function relationship for spin-crossover materials with this form of crystal packing. All of the compounds in this work exhibit blue fluorescence in solution under ambient conditions. In most cases, the ligand-based emission maxima are slightly red shifted upon complexation, but there is no detectable correlation between the emission maximum and the spin state of the iron centers

Nanomateriales moleculares basados en compuestos de coordinación: magnetismo y luminiscencia

Tesis Univ. Granada. Departamento de Química Inorgánic

Analysis of the Role of Peripheral Ligands Coordinated to Zn-II in Enhancing the Energy Barrier in Luminescent Linear Trinuclear Zn-Dy-Zn Single-Molecule Magnets

Three new Dy complexes have been prepared according to a complex-as-ligand strategy. Structural determinations indicate that the central Dy ion is surrounded by two LZn units (L2- is the di-deprotonated form of the N2O2 compartmental N,N-2,2-dimethylpropylenedi(3-methoxysalicylideneiminato) Schiff base. The Dy ions are nonacoordinate to eight oxygen atoms from the two L ligands and to a water molecule. The Zn ions are pentacoordinate in all cases, linked to the N2O2 atoms from L, and the apical position of the Zn coordination sphere is occupied by a water molecule or bromide or chloride ions. These resulting complexes, formulated (LZnX)-Dy-(LZnX), are tricationic with X=H2O and monocationic with X=Br or Cl. They behave as field-free single-molecule magnets (SMMs) with effective energy barriers (U-eff) for the reversal of the magnetization of 96.9(6)K with (0)=2.4x10(-7)s, 146.8(5)K with (0)=9.2x10(-8)s, and 146.1(10)K with (0)=9.9x10(-8)s for compounds with ZnOH2, ZnBr, and ZnCl motifs, respectively. The Cole-Cole plots exhibit semicircular shapes with parameters in the range of 0.19 to 0.29, which suggests multiple relaxation processes. Under a dc applied magnetic field of 1000Oe, the quantum tunneling of magnetization (QTM) is partly or fully suppressed and the energy barriers increase to U-eff=128.6(5)K and (0)=1.8x10(-8)s for 1, U-eff=214.7K and (0)=9.8x10(-9)s for 2, and U-eff=202.4K and (0)=1.5x10(-8)s for 3. The two pairs of largely negatively charged phenoxido oxygen atoms with short DyO bonds are positioned at opposite sides of the Dy3+ ion, which thus creates a strong crystal field that stabilizes the axial M-J=+/- 15/2 doublet as the ground Kramers doublet. Although the compound with the ZnOH2 motifs possesses the larger negative charges on the phenolate oxygen atoms, as confirmed by using DFT calculations, it exhibits the larger distortions of the DyO9 coordination polyhedron from ideal geometries and a smaller U-eff value. Ab initio calculations support the easy-axis anisotropy of the ground Kramers doublet and predict zero-field SMM behavior through Orbach and TA-QTM relaxations via the first excited Kramers doublet, which leads to large energy barriers. In accordance with the experimental results, ab initio calculations have also shown that, compared with water, the peripheral halide ligands coordinated to the Zn2+ ions increase the barrier height when the distortions of the DyO9 have a negative effect. All the complexes exhibit metal-centered luminescence after excitation into the UV -* absorption band of ligand L2- at =335nm, which results in the appearance of the characteristic Dy-III ((F9/2HJ/2)-F-4-H-6; J=15/2, 13/2) emission bands in the visible region

Effect of Ligand Substitution around the Dy-III on the SMM Properties of Dual-Luminescent Zn-Dy and Zn-Dy-Zn Complexes with Large Anisotropy Energy Barriers: A Combined Theoretical and Experimental Magnetostructural Study

The new dinuclear (ZnDyIII)-Dy-II and trinuclear (ZnDyIII)-Dy-II-Zn-II complexes of formula [(LZnBrDy(ovan) (NO3)(H2O)](H2O)center dot 0.5(MeOH) (1) and [((LZnBr)-Zn-1)(2)Dy(MeOH)(2)](ClO4) (3) (L and L-1 are the dideprotonated forms of the N,N'-2,2-dimethylpropylenedi(3-methoxysalicylideneiminato and 2-{(E)-[(3-{[(2E,3E)-3-(hydroxyimino)butan-2-ylidene ]amino}-2,2-dimethylpropyl)imino]methyl}-6-methoxyphenol Schiff base compartmental ligands, respectively) have been prepared and magnetostructurally characterized. The X-ray structure of 1 indicates that the Dy-III ion exhibits a DyO9 coordination sphere, which is made from four O atoms coming from the compartmental ligand (two methoxy terminal groups and two phenoxido bridging groups connecting Zn-II and Dy-III ions), other four atoms belonging to the chelating nitrato and ovanillin ligands, and the last one coming to the coordinated water molecule. The structure of 3 shows the central Dy-III ion surrounded by two L1Zn units, so that the Dy-III and Zn-II ions are linked by phenoxido/oximato bridging groups. The Dy ion is eight-coordinated by the six O atoms afforded by two L1 ligands and two O atoms coming from two methanol molecules. Alternating current (AC) dynamic magnetic measurements of 1, 3, and the previously reported dinuclear [LZnClDy(thd)(2)] (2) complex (where thd = 2,2,6,6-tetramethyl-3,5-heptanedionato ligand) indicate single molecule magnet (SMM) behavior for all these complexes with large thermal energy barriers for the reversal of the magnetization and butterfly-shaped hysteresis loops at 2 K. Ab initio calculations on 1-3 show a pure Ising ground state for all of them, which induces almost completely suppressed quantum tunnelling magnetization (QTM), and thermally assisted quantum tunnelling magnetization (TA-QTM) relaxations via the first excited Kramers doublet, leading to large energy barriers, thus supporting the observation of SMM behavior. The comparison between the experimental and theoretical magnetostructural data for 13 has allowed us to draw some conclusions about the influence of ligand substitution around the Dy-III on the SMM properties. Finally, these SMMs exhibit metal- and ligand-centered dual emissions in the visible region, and, therefore, they can be considered as magnetoluminescent bifunctional molecular materials