Triazole‐Based One‐Dimensional Spin‐Crossover Coordination Polymers

One‐dimensional coordination Fe^II polymers constructed through triple N¹,N²‐1,2,4‐triazole bridges form a unique class of spin‐crossover materials, the synthetic versatility of which allows tuning the spin‐crossover properties, the design of gels, films, liquid crystals, and nanoparticles and single‐particle addressing. This Minireview provides the first complete overview of these very attractive switchable materials and their most recent developments.     

Tuning the Spin‐Crossover Behaviour of a Hydrogen‐Accepting Porous Coordination Polymer by Hydrogen‐Donating Guests

A Hoffman‐like coordination polymer with appreciable porosity and uncoordinated pyridyl groups, namely, [Fe(2,5‐bpp){Au(CN)₂}₂] ? x Solv (2,5‐bpp=2,5‐bis(pyrid‐4‐yl)pyridine; Solv=solvent), was synthesised and characterised. A series of fascinating spin‐crossover behaviours with abrupt, stepwise and hysteretic features were obtained by exchange with a range of protic solvents (ethanol, n‐propanol, isopropyl alcohol, sec‐butanol and isobutanol). Guest–host hydrogen‐bonding interactions involving the H‐accepting site of the framework are primarily responsible for the pronounced cooperativity of these spin‐crossover behaviours. Meanwhile, the tunable critical temperatures over a range of about 130 K are presumably attributable to a certain degree of competition between internal pressure and local electronic influences of solvents.     

Synthesis of Spin‐Crossover Nano‐ and Micro‐objects in Homogeneous Media

New methods are proposed for the synthesis of spin‐crossover nano‐ and micro‐objects. Several nano‐objects that are based upon the spin‐crossover complex [Fe(hptrz)₃](OTs)₂ (hptrz=4‐heptyl‐1,2,4‐triazole, Ts=para‐toluenesulfonyl) were prepared in homogeneous media. The use of various reagents (Triton X‐100, PVP, TOPO, and PEGs of different molecular weights) as stabilizing agents yielded materials of different size (6 nm–2 μm) and morphology (nanorods, nanoplates, small spherical particles, and nano‐ and micro‐crystals). In particular, when Triton X‐100 was used, a variation in the morphology from nanorods to nanoplates was observed by changing the nature of the solvent. Interestingly, the preparation of the nanorods and nanoplates was always accompanied by the formation of small spherical particles. Alternatively, when PEG was used, 200–400 nm crystals of the complex were obtained. In addition, a very promising polymer‐free synthetic method is discussed that was based on the preparation of relatively stable Fe^II–triazole oligomers in CHCl₃. Their specific treatment led to micro‐crystals, small nanoparticles, or gels. The size and morphology of all of these objects were characterized by TEM and by dynamic light scattering (DLS) where possible. Their spin‐crossover behavior was studied by optical and magnetic measurements. The spin‐transition features for large particles (>100 nm) were very similar to that of the bulk material, that is, close to room temperature with a hysteresis width of up to 8 K. The effects of the matrix and/or size‐reduction led to modification of the transition temperature and an abruptness of the spin transition for oligomeric solutions and small nanoparticles of 6 nm in size.     

Synthesis and Characterisation of a New Series of Bistable Iron(II) Spin‐Crossover 2D Metal–Organic Frameworks

Twelve coordination polymers with formula {Fe(3‐Xpy)₂[M^II(CN)₄]} (M^II: Ni, Pd, Pt; X: F, Cl, Br, I; py: pyridine) have been synthesised, and their crystal structures have been determined by single‐crystal or powder X‐ray analysis. All of the fluoro and iodo compounds, as well as the chloro derivative in which M^II is Pt, crystallise in the monoclinic C2/m space group, whereas the rest of the chloro and all of the bromo derivatives crystallise in the orthorhombic Pnc2 space group. In all cases, the iron(II) atom resides in a pseudo‐octahedral [FeN₆] coordination core, with similar bond lengths and angles in the various derivatives. The major difference between the two kinds of structure arises from the stacking of consecutive two‐dimensional {Fe(3‐Xpy)₂[M^II(CN)₄]}_∞ layers, which allows different dispositions of the X atoms. The fluoro and chloro derivatives undergo cooperative spin crossover (SCO) with significant hysteretic behaviour, whereas the rest are paramagnetic. The thermal hysteresis, if X is F, shifts toward room temperature without changing the cooperativity as the pressure increases in the interval 10⁵ Pa–0.5 GPa. At ambient pressure, the SCO phenomenon has been structurally characterised at different significant temperatures, and the corresponding thermodynamic parameters were obtained from DSC calorimetric measurements. Compound {Fe(3‐Clpy)₂[Pd(CN)₄]} represents a new example of a “re‐entrant” two‐step spin transition by showing the Pnma space group in the intermediate phase (IP) and the Pnc2 space group in the low‐spin (LS) and high‐spin (HS) phases.     

Two‐Step Thermal Spin Transition and LIESST Relaxation of the Polymeric Spin‐Crossover Compounds Fe(X‐py)2[Ag(CN)2]2 (X=H, 3‐methyl, 4‐methyl, 3,4‐dimethyl, 3‐Cl)

In the series of polymeric spin‐crossover compounds Fe(X‐py)₂[Ag(CN)₂)]₂ (py=pyridine, X=H, 3‐Cl, 3‐methyl, 4‐methyl, 3,4‐dimethyl), magnetic and calorimetric measurements have revealed that the conversion from the high‐spin (HS) to the low‐spin (LS) state occurs by two‐step transitions for three out of five members of the family (X=H, 4‐methyl, and X=3,4‐dimethyl). The two other compounds (X=3‐Cl and 3‐methyl) show respectively an incomplete spin transition and no transition at all, the latter remaining in the HS state in the whole temperature range. The spin‐crossover behaviour of the compound undergoing two‐step transitions is well described by a thermodynamic model that considers both steps. Calculations with this model show low cooperativity in this type of systems. Reflectivity and photomagnetic experiments reveal that all of the compounds except that with X=3‐methyl undergo light‐induced excited spin state trapping (LIESST) at low temperatures. Isothermal HS‐to‐LS relaxation curves at different temperatures support the low‐cooperativity character by following an exponential decay law, although in the thermally activated regime and for aX=H and X=3,4‐dimethyl the behaviour is well described by a double exponential function in accordance with the two‐step thermal spin transition. The thermodynamic parameters determined from this isothermal analysis were used for simulation of thermal relaxation curves, which nicely reproduce the experimental data.     

Two‐Step Spin Transition in a 1D FeII 1,2,4‐Triazole Chain Compound

A thermochromic 1D spin crossover coordination (SCO) polymer [Fe(βAlatrz)₃](BF₄)₂ ? 2 H₂O ( 1? 2 H₂O), whose precursor βAlatrz, (1,2,4‐triazol‐4‐yl‐propionate) has been tailored from a β‐amino acid ester is investigated in detail by a set of superconducting quantum interference device (SQUID), ⁵⁷Fe Mössbauer, differential scanning calorimetry, infrared, and Raman measurements. An hysteretic abrupt two‐step spin crossover (T_1/2^↓=230 K and T_1/2^↑=235 K, and T_1/2^↓=172 K and T_1/2^↑=188 K, respectively) is registered for the first time for a 1,2,4‐triazole‐based Fe^II 1D coordination polymer. The two‐step SCO configuration is observed in a 1:2 ratio of low‐spin/high‐spin in the intermediate phase for a 1D chain. The origin of the stepwise transition was attributed to a distribution of chains of different lengths in 1? 2 H₂O after First Order Reversal Curves (FORC) analyses. A detailed DFT analysis allowed us to propose the normal mode assignment of the Raman peaks in the low‐spin and high‐spin states of 1? 2 H₂O. Vibrational spectra of 1? 2 H₂O reveal that the BF₄^? anions and water molecules play no significant role on the vibrational properties of the [Fe(βAlatrz)₃]²⁺ polymeric chains, although non‐coordinated water molecules have a dramatic influence on the emergence of a step in the spin transition curve. The dehydrated material [Fe(βAlatrz)₃](BF₄)₂ ( 1 ) reveals indeed a significantly different magnetic behavior with a one‐step SCO which was also investigated.     

Thermal‐ and Light‐Induced Spin Crossover in a Guest‐Dependent Dinuclear Iron(II) System

We previously reported the dinuclear material [Fe^II₂(ddpp)₂(NCS)₄] ? 4 CH₂Cl₂ ( 1? 4 CH₂Cl₂; ddpp=2,5‐di(2′,2′′‐dipyridylamino)pyridine) and its partially desolvated analogue ( 1? CH₂Cl₂), which undergo two‐ and one‐step spin‐crossover (SCO) transitions, respectively. Here, we manipulate the type and degree of solvation in this system and find that either a one‐ or two‐step spin transition can be specifically targeted. The chloroform clathrate 1? 4 CHCl₃ undergoes a relatively abrupt one‐step SCO, in which the two equivalent Fe^II sites within the dinuclear molecule crossover simultaneously. Partial desolvation of 1? 4 CHCl₃ to form 1? 3 CHCl₃ and 1? CHCl₃ occurs through single‐crystal‐to‐single‐crystal processes (monoclinic C2/c to P2₁/n to P2₁/n) in which the two equivalent Fe^II sites become inequivalent sites within the dinuclear molecule of each phase. Both 1? 3 CHCl₃ and 1? CHCl₃ undergo one‐step spin transitions, with the former having a significantly higher SCO temperature than 1? 4 CHCl₃ and the latter, and each has a broader SCO transition than 1? 4 CHCl₃, attributable to the overlap of two SCO steps in each case. Further magnetic manipulation can be carried out on these materials through reversibly resolvating the partially desolvated material with chloroform to produce the original one‐step SCO, or with dichloromethane to produce a two‐step SCO reminiscent of that seen for 1? 4 CH₂Cl₂. Furthermore, we investigate the light‐induced excited spin state trapping (LIESST) effect on 1? 4 CH₂Cl₂ and 1? CH₂Cl₂ and observe partial LIESST activity for the former and no activity for the latter.     

Nanoporosity,Inclusion Chemistry,and Spin Crossover in Orthogonally Interlocked Two‐Dimensional Metal–Organic Frameworks

[Fe(tvp)₂(NCS)₂] ( 1 ) (tvp=trans‐(4,4′‐vinylenedipyridine)) consists of two independent perpendicular stacks of mutually interpenetrated two‐dimensional grids. This uncommon supramolecular conformation defines square‐sectional nanochannels (diagonal≈2.2 nm) in which inclusion molecules are located. The guest‐loaded framework 1@guest displays complete thermal spin‐crossover (SCO) behavior with the characteristic temperature T_1/2 dependent on the guest molecule, whereas the guest‐free species 1 is paramagnetic whatever the temperature. For the benzene–guest derivatives, the characteristic SCO temperature T_1/2 decreases as the Hammet σ_p parameter increases. In general, the 1@guest series shows large entropy variations associated with the SCO and conformational changes of the interpenetrated grids that leads to a crystallographic‐phase transition when the guest is benzonitrile or acetonitrile/H₂O.     

[Fe(TPT)2/3{MI(CN)2}2]⋅nSolv (MI=Ag,Au): New Bimetallic Porous Coordination Polymers with Spin‐Crossover Properties

Two new heterobimetallic porous coordination polymers with the formula [Fe(TPT)_2/3{M^I(CN)₂}₂] ? nSolv (TPT=[(2,4,6‐tris(4‐pyridyl)‐1,3,5‐triazine]; M^I=Ag (nSolv=0, 1 MeOH, 2 CH₂Cl₂), Au (nSolv=0, 2 CH₂Cl₂)) have been synthesized and their crystal structures were determined at 120 K and 293 K by single‐crystal X‐ray analysis. These structures crystallized in the trigonal R‐3m space group. The Fe^II ion resides at an inversion centre that defines a [FeN₆] coordination core. Four dicyanometallate groups coordinate at the equatorial positions, whilst the axial positions are occupied by the TPT ligand. Each TPT ligand is centred in a ternary axis and bridges three crystallographically equivalent Fe^II ions, whilst each dicyanometallate group bridges two crystallographically equivalent Fe^II ions that define a 3D network with the topology of NbO. There are two such networks, which interpenetrate each other, thereby giving rise to large spaces in which very labile solvent molecules are included (CH₂Cl₂ or MeOH). Crystallographic analysis confirmed the reversible structural changes that were associated with the occurrence of spin‐crossover behaviour at the Fe^II ions, the most significant structural variation being the change in unit‐cell volume (about 59 ų per Fe^II ion). The spin‐crossover behaviour has been monitored by means of thermal dependence of the magnetic properties, Mössbauer spectroscopy, and calorimetry.     

Light‐Driven Coordination‐Induced Spin‐State Switching: Rational Design of Photodissociable Ligands

The bistability of spin states (e.g., spin crossover) in bulk materials is well investigated and understood. We recently extended spin‐state switching to isolated molecules at room temperature (light‐driven coordination‐induced spin‐state switching, or LD‐CISSS). Whereas bistability and hysteresis in conventional spin‐crossover materials are caused by cooperative effects in the crystal lattice, spin switching in LD‐CISSS is achieved by reversibly changing the coordination number of a metal complex by means of a photochromic ligand that binds in one configuration but dissociates in the other form. We present mathematical proof that the maximum efficiency in property switching by such a photodissociable ligand (PDL) is only dependent on the ratio of the association constants of both configurations. Rational design by using DFT calculations was applied to develop a photoswitchable ligand with a high switching efficiency. The starting point was a nickel–porphyrin as the transition‐metal complex and 3‐phenylazopyridine as the photodissociable ligand. Calculations and experiments were performed in two iterative steps to find a substitution pattern at the phenylazopyridine ligand that provided optimum performance. Following this strategy, we synthesized an improved photodissociable ligand that binds to the Ni–porphyrin with an association constant that is 5.36 times higher in its trans form than in the cis form. The switching efficiency between the diamagnetic and paramagnetic state is efficient as well (72 % paramagnetic Ni–porphyrin after irradiation at 365 nm, 32 % paramagnetic species after irradiation at 440 nm). Potential applications arise from the fact that the LD‐CISSS approach for the first time allows reversible switching of the magnetic susceptibility of a homogeneous solution. Photoswitchable contrast agents for magnetic resonance imaging and light‐controlled magnetic levitation are conceivable applications.     

Ultrafast Deactivation Mechanism of the Excited Singlet in the Light‐Induced Spin Crossover of [Fe(2,2′‐bipyridine)3]2+

The mechanism of the light‐induced spin crossover of the [Fe(bpy)₃]²⁺ complex (bpy=2,2′‐bipyridine) has been studied by combining accurate electronic‐structure calculations and time‐dependent approaches to calculate intersystem‐crossing rates. We investigate how the initially excited metal‐to‐ligand charge transfer (MLCT) singlet state deactivates to the final metastable high‐spin state. Although ultrafast X‐ray free‐electron spectroscopy has established that the total timescale of this process is on the order of a few tenths of a picosecond, the details of the mechanisms still remain unclear. We determine all the intermediate electronic states along the pathway from low spin to high spin and give estimates for the deactivation times of the different stages. The calculations result in a total deactivation time on the same order of magnitude as the experimentally determined rate and indicate that the complex can reach the final high‐spin state by means of different deactivation channels. The optically populated excited singlet state rapidly decays to a triplet state with an Fe d⁶(${{\rm t}{{5\hfill \atop {\rm 2g}\hfill}}}$ ${{\rm e}{{1\hfill \atop {\rm g}\hfill}}}$ ) configuration either directly or by means of a triplet MLCT state. This triplet ligand‐field state could in principle decay directly to the final quintet state, but a much faster channel is provided by internal conversion to a lower‐lying triplet state and subsequent intersystem crossing to the high‐spin state. The deactivation rate to the low‐spin ground state is much smaller, which is in line with the large quantum yield reported for the process.     

Reversible Guest Binding in a Non‐Porous FeII Coordination Polymer Host Toggles Spin Crossover

Formation of either a dimetallic compound or a 1 D coordination polymer of adiponitrile adducts of [Fe(bpte)]²⁺ (bpte=[1,2‐bis(pyridin‐2‐ylmethyl)thio]ethane) can be controlled by the choice of counteranion. The iron(II) atoms of the bis(adiponitrile)‐bridged dimeric complex [Fe₂(bpte)₂(μ₂‐(NC(CH₂)₄CN)₂](SbF₆)₄ ( 2 ) are low spin at room temperature, as are those in the polymeric adiponitrile‐linked acetone solvate polymer {[Fe(bpte)(μ₂‐NC(CH₂)₄CN)](BPh₄)₂ ? Me₂CO} ( 3? Me₂CO). On heating 3? Me₂CO to 80 °C, the acetone is abruptly removed with an accompanying purple to dull lavender colour change corresponding to a conversion to a high‐spin compound. Cooling reveals that the desolvate 3 shows hysteretic and abrupt spin crossover (SCO) S=0?S=2 behaviour centred at 205 K. Non‐porous 3 can reversibly absorb one equivalent of acetone per iron centre to regenerate the same crystalline phase of 3? Me₂CO concurrently reinstating a low‐spin state.     

FeIII Quinolylsalicylaldimine Complexes: A Rare Mixed‐Spin‐State Complex and Abrupt Spin Crossover

A new synthesis of (8‐quinolyl)‐5‐methoxysalicylaldimine (Hqsal‐5‐OMe) is reported and its crystal structure is presented. Two Fe^III complexes, [Fe(qsal‐5‐OMe)₂]Cl ? solvent (solvent=2 MeOH ? 0.5 H₂O ( 1 ) and MeCN ? H₂O ( 2 )) have been prepared and their structural, electronic and magnetic properties studied. [Fe(qsal‐5‐OMe)₂] Cl ? 2 MeOH ? 0.5 H₂O ( 1 ) exhibits rare crystallographically independent high‐spin and low‐spin Fe^III centres at 150 K, whereas [Fe(qsal‐5‐OMe)₂]Cl ? MeCN ? H₂O ( 2 ) is low spin at 100 K. In both structures there are extensive π–π and C? H???π interactions. SQUID magnetometry of 2 reveals an unusual abrupt stepped‐spin crossover with T_1/2=245 K and 275 K for steps 1 and 2, respectively, with a slight hysteresis of 5 K in the first step and a plateau of 15 K between the steps. In contrast, 1 is found to undergo an abrupt half‐spin crossover also with a hysteresis of 10 K. The two compounds are the first Fe^III complexes of a substituted qsal ligand to exhibit abrupt spin crossover. These conclusions are supported by ⁵⁷Fe Mössbauer spectroscopy. Both complexes exhibit reversible reduction to Fe^II at ?0.18 V and irreversible oxidation of the coordinated qsal‐5‐OMe ligand at +1.10 V.