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

Single-molecule magnet behavior and magnetocaloric effect in ferromagnetically coupled LnIII-NiII-NiII-LnIII (LnIII = DyIII and GdIII) linear complexes

New types of linear tetranuclear LnIII-NiII-NiII-LnIII (LnIII = Dy (1), Gd (2)) complexes have been prepared using the multidentate ligand N,N′-bis(3-methoxysalicylidene)-1,3-diaminobenzene, which has two sets of NO and OO′ coordination pockets that are able to selectively accommodate NiII and LnIII ions, respectively. The X-ray structure analysis reveals that the NiII ions are bridged by phenylenediimine groups forming a 12-membered metallacycle in the central body of the complex, whereas the LnIII ions are located at both sides of the metallacycle and linked to the NiII ions by diphenoxo bridging groups. Phenylenediimine and diphenoxo bridging groups transmit ferromagnetic exchange interactions between the two NiII ions and between the NiII and the LnIII ions, respectively. Complex 1 shows slow relaxation of the magnetization at zero field and a thermal energy barrier Ueff = 7.4 K with HDC = 1000 Oe, whereas complex 2 exhibits an S = 9 ground state and significant magnetocaloric effect (-ΔSm = 18.5 J kg-1 K-1 at T = 3 K and ΔB = 5 T). © 2014 American Chemical Society.Financial support from Ministerio de Economía y Competitividad (MINECO) for Project Nos. CTQ-2011-24478 and MAT2012-38318-C03-01, the Junta de Andalucía (No. FQM-195 and Project of Excellence No. P11-FQM-7756), and the University of Granada are acknowledged. S.T. thanks to Junta de Andalucia for a post-doctoral contract.Peer Reviewe