Diseño, síntesis y caracterización de materiales moleculares multifuncionales conmutables

Abstract

This thesis dissertation collects the synthesis and characterization of a new generation of switchable molecular materials that displays the well-known spin crossover (SCO) phenomena. The main goal of this Doctoral thesis has been focused on the rational design of novel multifunctional Fe(II) materials leading to excellent bi-stable molecular-based platforms that has allowed an accurate study of SCO behaviors synchronized with others newfangled physico-chemical properties, as can be host-guest chemistry, solid-liquid phase transitions, chemical doping,…The interplay between SCO and other intimately interlaced property has been monitored, mainly, by crystallographic and magnetic studies, manifesting key magneto-structural relationships that boost the use of this promising materials in new practical applications. Chapter one of this thesis briefly reviews the most relevant theoretical concepts of the SCO phenomena. This singular behavior is related to the reversible electronic switching between a paramagnetic high spin state (HS) and diamagnetic low spin state (LS) showed by some first-row transition-metal complexes (mostly Fe(II) compounds) under an external physical perturbation, as can be a variation of temperature, pressure or irradiating with a light. The spin state change in this very sensitive class of materials is always accompanied by modifications of many physical properties, i.e. magnetic response, color, volume of the material, dielectric constant…, which strongly depends on their local electronic structure. At the end of this chapter the motivational purpose in the development of this doctoral thesis together the general experimental procedure stablished for synthesize and full characterize the different herein presented compounds are introduced. Chapter two of this thesis introduces the synthesis and characterization of new series of two- (2D) and three-dimensional (3D) Hofmann-like spin crossover (SCO) coordination polymers based on self-assembling iron(II) ions, 2-fluoropyrazine (Fpz), and [MII(CN)4]2- (MII = Ni, Pd, Pt) or [AuI(CN)2]- building blocks, which display strong cooperative magnetic, calorimetric, and optical properties. The iron(II) ions, lying on inversion centers, define elongated octahedrons equatorially surrounded by four equivalent centrosymmetric μ4-[MII(CN)4]2- bridging groups. The axial positions are occupied by two terminal Fpz ligands affording significantly corrugated 2D layers {Fe(Fpz)2[MII(CN)4]} (FpzM). The FpzPt and FpzPd derivates undergo thermal- and light-induced SCO characterized by T1/2 temperatures centered at 155.5 and 116 K and hysteresis loops 22 K wide, while the FpzNi derivate is high spin at all temperatures, even at pressures of 0.7 GPa. The great stability of the high-spin state in the FpzNi derivate has tentatively been ascribed to the tight packing of the layers, which contrasts with that of FpzPt and FpzPd derivates in the high- and low-spin states. The 3D frameworks formulated as {Fe(Fpz)[Pt(CN)4]}·1/2H2O (FpzPt3D) and {Fe(Fpz)[Au(CN)2]2} (FpzAu), where Fpz acts as bridging ligand, are also characterized in detail. The former is high spin at all temperatures, while the latter displays very strong cooperative SCO centered at 243 K accompanied by a hysteresis loop 42.5 K wide. The crystal structures and SCO properties are compared with those of related complexes derived from pyrazine, 3-fluoropyridine, and pyridine. Chapter three of this thesis presents the synthesis of ligands 1,3,5-tris(pyridin-4-ylethynyl)benzene (LN3) and 1,2,4,5-tetrakis(pyridin-4-ylethynyl)benzene (LN4), and their use as basic building units in novel SCO Hofmann-like coordination polymers formulated as {Fe(LN3)[MI(CN)2]2}·Guest [MI = Ag (1·Guest) , Au (2·Guest); Guest = nitrobenzene (PhNO2), benzonitrile (PhCN), o-dichlorobenzene (o-PhCl2)] and {Fe(LN4)[Ag2(CN)3][Ag(CN)2]·H2O (3·H2O), respectively. The specie LN3 acts as a bis-monodentate ligand in the double interpenetrated tridimensional networks 1·Guest and 2·Guest, defining small cavities where small aromatic molecules are located. The isostructural networks 1·Guest and 2·Guest undergo first order thermal- and photo-induced SCO behavior, which critical temperatures (T1/2 and TLIESST) are characteristic of each guest molecule hosted in the pores. Favored by the “in situ” generated Ag2(CN)3- anionic units, the LN4 ligand acts as a tetrakis-monodentate in compound 3·H2O, leading to a poorly porous triply intrincated tridimensional coordination polymer. This uncommon network displays a gradual SCO behavior with an incomplete photo-population of the HS* metastable state at low temperatures. Chapter four of this thesis congregates the synthesis, structural characterization and magnetic properties of two new isostructural porous 3D compounds with general formula {FeII(pina)[MI(CN)2]2}·xMeOH (x = 0–5; pina = N-(pyridin-4-yl)isonicotinamide; MI = AgI and x ~ 5 (1·xMeOH); MI = AuI and x ~ 5 (2·xMeOH)). The single-crystal X-ray diffraction analyses have revealed that the structure of 1·xMeOH (or 2·xMeOH) presents two equivalent doubly interpenetrated 3D frameworks stabilized by both argentophilic (or aurophilic) interactions and interligand C=O···HC H-bonds. Despite the interpenetration of the networks, these compounds display accessible void volume capable of hosting up to five molecules of methanol which interact with the host pina ligand and establish an infinite lattice of hydrogen bonds along the structural channels. Interestingly, the magnetic studies have shown that the solvated complexes 1·xMeOH and 2·xMeOH display two- and four-step hysteretic thermally driven SCO behavior, respectively. However, when these compounds lose the methanol molecules, the magnetic behavior changes drastically giving place to gradual spin conversions evidencing the relevant influence of the guest molecules on the spin-crossover properties. Importantly, since the solvent desorption takes place following a single-crystal-to-single-crystal transformation, empty structures 1 and 2 (x = 0) could be also determined allowing us to evaluate the correlation between the structural changes and the modification of the magnetic properties triggered by the loss of methanol molecules. Chapter five of this thesis describes the extraordinary kinetic stability of new discovered thermo- and photo-induced phases of [Fe(nBu-im)3tren](PF6)2 ((nBu-im)3tren = n-butylimidazoltris(2-ethylamino)amine) compound, reveling the mechanism that leads to this stabilization. In a previous work of our group was discussed the two well-differentiated SCO behaviors characterized by large hysteresis loops of this mononuclear complex, which critically depend on the sweeping rate of temperature. For scan-rates higher than 2 K min–1 the SCO is characterized by an average critical temperature Tcav = 122 K with a hysteresis loop 14 K wide (channel A). In contrast, for rates below 0.1 K min–1 the SCO takes play at Tcav = 156 K and the hysteresis loop widens up to a value of 41 K (channel B). This behavior is governed by competition between two crystallographically independent phases that manifest in the LS state: a kinetic phase (LS1), which is isostructural to the HS state, and a thermodynamic phase (LS2). The LS phases differ in the disposition of the butyl tails and the organization of the PF6- groups. Interestingly, the intrinsic structural disorder characteristic of the aliphatic tails also plays a crucial role in the kinetic HS*-to-LS1 relaxation properties of the thermal and light generated metastable HS* phases at low temperatures. More precisely, the HS*-to-LS1 relaxation shows an unusual long relaxation time of 20 h after light-induced excited spin state trapping (LIESST) when irradiating at 80 K. This is more longer than when irradiating in the interval 10 – 70 K. Optical absorption spectroscopy and X-ray diffraction using synchrotron radiation as well as magnetic measurements were used to characterize and compare the LIESST behavior of this compound after irradiation in the temperature interval 10 K and 100 K. Depending on the temperature at which the compound is irradiated, the generated metastable HS* LIESST state differ in the arrangement of the butyl chains of the ligands. Hence, the structural reorganization of the butyl chains occurring during the relaxation to adopt the structure of the thermodynamically stable LS1 phase seems to be responsible for these differences. Chapter six of this thesis introduces an efficient synthetic strategy to separate the synchronous coupled crystallographic phase transition and SCO behavior of previously studied compound [Fe(nBu-im)3(tren)](PF6)2 (100P). This strategy is based on the preparation of molecular alloys generically formulated as [Fe1-xMx(nBu-im)3(tren)](P1-yAsyF6)2 (M = ZnII, NiII). By controlling the composition of this isomorphous series, two cooperative thermally-induced SCO events featuring distinct critical temperatures (Tc) and hysteresis width (∆Tc, memory), can be selected at will. In fact, precise control of AsF6–  PF6– substitution selectively selects the memory channel B of 100P when [x = 0, y ≈ 0.7], stabilizing the arrangement of butyl groups and PF6-/AsF6- anions of LS2 phase. In contrast, substitution of active FeII centers with ZnII or NiII [x ≈ 0.2, y = 0] favors the arrangement of butyl groups and PF6-/AsF6- anions of LS1 phase, characteristic of the low temperature memory channel A of 100P. Analogously to 100P, pure 100As (x = 0, y = 1) derivative displays a hysteretic SCO behavior coupled with a structural phase transition, both located at higher temperatures due to the higher molecular volume of AsF6-. The internal chemical pressure induced by AsF6- anions provokes an increase of relaxation kinetics which avoid the appearance of two independent switching channels. These effects are also reported when 100P compound is stressed by an external hydrostatic pressure close to 0.1 GPa. Additionally, complex 100P displays a two-step SCO behavior, as 100As shows, when is perturbed by an external pressure, evidencing that a similar effect on the SCO properties of the material can be reached via application internal/external pressure. Chapter seven of this thesis presents the synthesis and characterization on a series of charge neutral [FeII(Ln)2] meltable complexes based on Schiff bases Ln = pm2-n or pyH-n derived from condensation of 2-pyrimidine ethyl ketone or pyridine aldehyde, respectively, with benzohydrazide functionalized with three aliphatic chains CnH2n+1 (being n the number of carbon atoms that defines the substituted alkyl chains, ranging between 4 – 14). In the solid-state [FeII(pm2-n)2] complexes show two different crystal packing motifs depending on alkyl chain length (n). However, upon heating, the compounds melt into an isotropic phase and, due to releasing the solid-state effects, reveal identical SCO behavior for all the compounds with the SCO transition midpoint temperature T1/2 ≈ 354 K. For short chain compounds (n = 4, 6, 8), T1/2 is far above 400 K in solid state, therefore they show an abrupt jump of the susceptibility on passing to the liquid phase. Cooling back shapes regular LS-to-HS (“forward”) spin transition hysteresis loop with the center shifting down on n growth. In contrast, the long chain compounds (n = 10, 12, 14) display a gradual SCO behavior in the solid state with characteristic critical temperatures T1/2 centered around ~ 275 K. However, at temperatures above ~350 K, where are essentially HS, compounds on going to liquid phase and show entropically prohibited HS-to-LS (“reverse”) SCO behavior of up to 50% of FeII ions, while the center of the shaped hysteresis shifts up with increasing n. This finding is unprecedented in Fe(II) SCO complexes and provides a method of guiding the spin transition direction and its location in temperature taking in advantage the conformational instability of aliphatic chains. Chapter eight assembled the general conclusions of this doctoral thesis. Finally it is enlisted an appendix gathering all peer reviewed scientific articles that have led to this Doctoral Thesis.