Regulation of Multistep Spin Crossover Across Multiple Stimuli in a 2-D Framework Material.

We investigate the effects of a broad array of external stimuli on the structural, spin-crossover (SCO) properties and nature of the elastic interaction within the two-dimensional Hofmann framework material [Fe(cintrz)2Pd(CN)4]·guest (cintrz = N-cinnamalidene 4-amino-1,2,4-triazole; A·guest; guest = 3H2O, 2H2O, and Ø). This framework exhibits a delicate balance between ferro- and antiferro-elastic interaction characters; we show that manipulation of the pore contents across guests = 3H2O, 2H2O, and Ø can be exploited to regulate this balance. In A·3H2O, the dominant antiferroelastic interaction character between neighboring FeII sites sees the low-temperature persistence of the mixed spin-state species {HS-LS} for {Fe1-Fe2} (HS = high spin, LS = low spin). Elastic interaction strain is responsible for stabilizing the {HS-LS} state and can be overcome by three mechanisms: (1) partial (2H2O) or complete (Ø) guest removal, (2) irradiation via the reverse light-induced excited spin-state trapping (LIESST) effect (λ = 830 nm), and (3) the application of external hydrostatic pressure. Combining experimental data with elastic models presents a clear interpretation that while guest molecules cause a negative chemical pressure, they also have consequences for the elastic interactions between metals beyond the simple chemical pressure picture typically proposed.

Guest Removal and External Pressure Variation Induce Spin Crossover in Halogen-Functionalized 2-D Hofmann Frameworks.

The effect of halogen functionalization on the spin crossover (SCO) properties of a family of 2-D Hofmann framework materials, [FeIIPd(CN)4(thioX)2]·2H2O (X = Cl and Br; thioCl = (E)-1-(5-chlorothiophen-2-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine) and thioBr = (E)-1-(5-bromothiophen-2-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine)), is reported. Inclusion of both the chloro- and bromo-functionalized ligands into the Hofmann-type frameworks (1Cl·2H2O and 2Br·2H2O) results in a blocking of spin-state transitions due to internal chemical pressure effects derived by the collective steric bulk of the halogen atoms and guest molecules. Cooperative one-step SCO transitions are revealed by either guest removal or the application of external physical pressure. Notably, removal of solvent water reveals a robust framework scaffold with only marginal variation between the solvated and desolvated structures (as investigated by powder and single crystal X-ray diffraction). Yet, one-step complete SCO transitions are revealed in 1Cl and 2Br with a transition temperature shift between the analogues due to various steric, structural, and electronic considerations. SCO can also be induced in the solvated species, 1Cl·2H2O and 2Br·2H2O, with the application of physical pressure, revealing a complete one-step SCO transition above 0.62 GPa (as investigated by magnetic susceptibility and single crystal X-ray diffraction measurements).

Single-Crystal X-Ray Diffraction Study of Pressure and Temperature-Induced Spin Trapping in a Bistable Iron(II) Hofmann Framework.

High-pressure single-crystal X-ray diffraction has been used to trap both the low-spin (LS) and high-spin (HS) states of the iron(II) Hofmann spin crossover framework, [FeII (pdm)(H2 O)[Ag(CN)2 ]2 ⋅H2 O, under identical experimental conditions, allowing the structural changes arising from the spin-transition to be deconvoluted from previously reported thermal effects.

Switchable Spin-Crossover Hofmann-Type 3D Coordination Polymers Based on Tri- and Tetratopic Ligands.

FeII spin-crossover (SCO) coordination polymers of the Hofmann type have become an archetypal class of responsive materials. Almost invariably, the construction of their architectures has been based on the use of monotopic and linear ditopic pyridine-like ligands. In the search for new Hofmann-type architectures with SCO properties, here we analyze the possibilities of bridging ligands with higher connectivity degree. More precisely, the synthesis and structure of {FeII(LN3)[MI(CN)2]2}·(Guest) (Guest = nitrobenzene, benzonitrile, o-dichlorobenzene; MI = Ag, Au) and {FeII(LN4)[Ag2(CN)3][Ag(CN)2]}·H2O are described, where LN3 and LN4 are the tritopic and tetratopic ligands 1,3,5-tris(pyridin-4-ylethynyl)benzene and 1,2,4,5-tetrakis(pyridin-4-ylethynyl)benzene. This new series of Hofmann clathrates displays thermo- and photoinduced SCO behaviors.

Long-range magnetic order in the porous metal-organic framework Ni(pyrazine)[Pt(CN)4].

A combined study involving DFT calculations, neutron scattering, heat capacity and magnetic measurements at very low temperatures demonstrates the long-range magnetic ordering of Ni(pyrazine)[Pt(CN)4] below 1.9 K, describing its antiferromagnetic spin arrangement. This compound belongs to the family of porous coordination polymers M(pyrazine)[Pt(CN)4] (M = divalent metal), renowned for showing interesting combinations of porosity and magnetic properties. The possibility of including long-range magnetic ordering, one of the most pursued functional properties, opens new perspectives for the multifunctionality of this class of compounds.

Electronic Structure Modulation in an Exceptionally Stable Non-Heme Nitrosyl Iron(II) Spin-Crossover Complex.

The highly stable nitrosyl iron(II) mononuclear complex [Fe(bztpen)(NO)](PF6 )2 (bztpen=N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethylenediamine) displays an S=1/2↔S=3/2 spin crossover (SCO) behavior (T1/2 =370 K, ΔH=12.48 kJ mol(-1) , ΔS=33 J K(-1) mol(-1) ) stemming from strong magnetic coupling between the NO radical (S=1/2) and thermally interconverted (S=0↔S=2) ferrous spin states. The crystal structure of this robust complex has been investigated in the temperature range 120-420 K affording a detailed picture of how the electronic distribution of the t2g -eg orbitals modulates the structure of the {FeNO}(7) bond, providing valuable magneto-structural and spectroscopic correlations and DFT analysis.

Exploiting Pressure To Induce a "Guest-Blocked" Spin Transition in a Framework Material.

A new functionalized 1,2,4-triazole ligand, 4-[(E)-2-(5-methyl-2-thienyl)vinyl]-1,2,4-triazole (thiome), was prepared to assess the broad applicability of strategically producing multistep spin transitions in two-dimensional Hofmann-type materials of the type [FeIIPd(CN)4(R-1,2,4-trz)2]·nH2O (R-1,2,4-trz = a 4-functionalized 1,2,4-triazole ligand). A variety of structural and magnetic investigations on the resultant framework material [FeIIPd(CN)4(thiome)2]·2H2O (A·2H2O) reveal that a high-spin (HS) to low-spin (LS) transition is inhibited in A·2H2O due to a combination of guest and ligand steric bulk effects. The water molecules can be reversibly removed with retention of the porous host framework and result in the emergence of an abrupt and hysteretic one-step spin transition due to the removal of guest internal pressure. A spin transition can, furthermore, be induced in A·2H2O (0-0.68 GPa) under hydrostatic pressure, as evidenced by variable-pressure structure and magnetic studies, resulting in a two-step spin transition at ambient temperatures at 0.68 GPa. The presence of a two-step spin crossover (SCO) in A·2H2O under hydrostatic pressure compared to a one-step SCO in A at ambient pressure is discussed in terms of the relative ability of each phase to accommodate mixed HS/LS states according to differing lattice flexibilities.

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 T1/2 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 T1/2 decreases as the Hammet σp 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/H2 O.

Synthesis of Nanocrystals and Particle Size Effects Studies on the Thermally Induced Spin Transition of the Model Spin Crossover Compound [Fe(phen)2(NCS)2].

Surfactant-free nanocrystals of the model spin-crossover compound [Fe(phen)2(NCS)2] (phen: 1,10-phenanthroline) have been synthesized applying the reverse micelle technique. The morphology of the nanocrystals, characterized by scanning electronic microscopy, corresponds to rhombohedric platelets with dimensions ranging from 203 × 203 × 106 nm to 142 × 142 × 74 nm. Variation of the concentration of the Fe(BF4)2·6H2O salt in the synthesis has been found to have little influence on the crystallite size. In contrast, the solvent-surfactant ratio (ω) is critical for a good particle growth. The spin transition of the nanocrystals has been characterized by magnetic susceptibility measurements and Mössbauer spectroscopy. The nanocrystals undergo an abrupt and more cooperative spin transition in comparison with the bulk compound. The spin transition is centered in the interval of temperature of 175-185 K and is accompanied by 8 K of thermal hysteresis width. The crystallite quality more than the crystallite size is responsible for the higher cooperativity. The magnetic properties of the nanocrystals embedded in organic polymers such as polyethylene glycol, nujol, glycerol, and triton have been studied as well. The spin transition in the nanocrystals is affected by the polymer coating. The abrupt and first-order spin transition transforms into a more continuous spin transition as a result of the chemical pressure asserted by the organic polymers on the Fe(II) centers.

Spin crossover behavior in a series of iron(III) alkoxide complexes.

The synthesis, crystal structures, magnetic behavior, and electron paramagnetic resonance studies of five new Fe(III) spin crossover (SCO) complexes are reported. The [Fe(III)N5O] coordination core is constituted of the pentadentate ligand bztpen (N5) and a series of alkoxide anions (ethoxide, propoxide, n-butoxide, isobutoxide, and ethylene glycoxide). The methoxide derivative previously reported by us is also reinvestigated. The six complexes crystallize in the orthorhombic Pbca space group and show similar molecular structures and crystal packing. The coordination octahedron is strongly distorted in both the high- and low-temperature structures. The structural changes upon spin conversion are consistent with those previously observed for [Fe(III)N4O2] SCO complexes of the Schiff base type, except for the Fe-O(alkoxide) bond distance, which shortens significantly in the high-spin state. Application of the Slichter-Drickamer thermodynamic model to the experimental SCO curves afforded reasonably good simulations with typical enthalpy and entropy variations ranging in the intervals ΔH = 6-13 kJ mol(-1) and ΔS = 40-50 J mol(-1) K(-1), respectively. The estimated values of the cooperativity parameter Γ, found in the interval 0-2.2 kJ mol(-1), were consistent with the nature of the SCO. Electron paramagnetic resonance spectroscopy confirmed the transformation between the high-spin and low-spin states, characterized by signals at g ≈ 4.47 and 2.10, respectively. Electrochemical analysis demonstrated the instability of the Fe(II) alkoxide derivatives in solution.

Meltable Spin Transition Molecular Materials with Tunable Tc and Hysteresis Loop Width.

Herein, we report a way to achieve abrupt high-spin to low-spin transition with controllable transition temperature and hysteresis width, relying not on solid-state cooperative interactions, but utilizing coherency between phase and spin transitions in neutral Fe(II) meltable complexes.

Spin crossover star-shaped metallomesogens of iron(II).

Three new types of spin crossover (SCO) metallomesogens of Fe(II) based on symmetric tripod ligands and their magnetic and structural properties are reported here. These were obtained by condensation of tris(2-aminoethyl)amin (tren) with the aldehyde derived from 3-alkoxy-6-methylpyridine (mpyN, N (number of carbon atoms in n-alkyl chains) = 8, 18), 1-alkyl-1H-imidazole (imN, N = 4, 16, 18, 20, 22), or 1-alkyl-1H-benzimidazole (bimN, N = 6, 14, 16, 18, 20). A complex derived from 1-octadecyl-1H-naphtho[2,3-d]imidazole (nim18) retains the high spin state at any temperature. Single crystals of the short-chain complexes were investigated by a combination of X-ray crystallography, magnetic measurements and Mössbauer spectroscopy. Generally, in comparison with the short-chain complexes the long-chain complexes display more gradual SCO and undergo a phase transition crystal-liquid crystal that is reflected in their magnetic properties. Characterization by X-ray powder diffractometry and differential calorimetry reveal formation of a smectic mesophase upon melting.

Reversible chemisorption of sulfur dioxide in a spin crossover porous coordination polymer.

The chemisorption of sulfur dioxide (SO2) on the Hofmann-like spin crossover porous coordination polymer (SCO-PCP) {Fe(pz)[Pt(CN)4]} has been investigated at room temperature. Thermal analysis and adsorption-desorption isotherms showed that ca. 1 mol of SO2 per mol of {Fe(pz)[Pt(CN)4]} was retained in the pores. Nevertheless, the SO2 was loosely attached to the walls of the host network and completely released in 24 h at 298 K. Single crystals of {Fe(pz)[Pt(CN)4]}·nSO2 (n ≈ 0.25) were grown in water solutions saturated with SO2, and its crystal structure was analyzed at 120 K. The SO2 molecule is coordinated to the Pt(II) ion through the sulfur atom ion, Pt-S = 2.585(4) Å. This coordination slightly stabilizes the low-spin state of the Fe(II) ions shifting the critical temperatures of the spin transition by 8-12 K. DFT calculations have been performed to rationalize these observations.

A switchable molecular rotator: neutron spectroscopy study on a polymeric spin-crossover compound.

A quasielastic neutron scattering and solid-state (2)H NMR spectroscopy study of the polymeric spin-crossover compound {Fe(pyrazine)[Pt(CN)(4)]} shows that the switching of the rotation of a molecular fragment--the pyrazine ligand--occurs in association with the change of spin state. The rotation switching was examined on a wide time scale (10(-13)-10(-3) s) by both techniques, which clearly demonstrated the combination between molecular rotation and spin-crossover transition under external stimuli (temperature and chemical). The pyrazine rings are seen to perform a 4-fold jump motion about the coordinating nitrogen axis in the high-spin state. In the low-spin state, however, the motion is suppressed, while when the system incorporates benzene guest molecules, the movements of the system are even more restricted.

Heterobimetallic MOFs containing tetrathiocyanometallate building blocks: pressure-induced spin crossover in the porous {Fe(II)(pz)[Pd(II)(SCN)4]} 3D coordination polymer.

Here we describe the synthesis, structure, and magnetic properties of two related coordination polymers made up of self-assembling Fe(II) ions, pyrazine (pz), and the tetrathiocyanopalladate anion. Compound {Fe(MeOH)(2)[Pd(SCN)(4)]}·pz (1a) is a two-dimensional coordination polymer where the Fe(II) ions are equatorially coordinated by the nitrogen atoms of four [Pd(SCN)(4)](2-) anions, each of which connects four Fe(II) ions, forming corrugated layers {Fe[Pd(SCN)(4)]}(∞). The coordination sphere of Fe(II) is completed by the oxygen atoms of two CH(3)OH molecules. The layers stack one on top of each other in such a way that the included pz molecule establishes strong hydrogen bonds with the coordinated methanol molecules of adjacent layers. Compound {Fe(pz)[Pd(SCN)(4)]} (2) is a three-dimensional porous coordination polymer formed by flat {Fe[Pd(SCN)(4)]}(∞) layers pillared by the pz ligand. Thermal analysis of 1a shows a clear desorption of the two coordinated CH(3)OH molecules giving a rather stable phase (1b), which presumably is a polymorphic form of 2. The magnetic properties of the three derivatives are typical of the high-spin Fe(II) compounds. However, compounds 1b and 2, with coordination sphere [FeN(6)], show thermal spin crossover behavior at pressures higher than ambient pressure (10(5) MPa).

Precise control and consecutive modulation of spin transition temperature using chemical migration in porous coordination polymers.

Precise control of spin transition temperature (T(c)) is one of the most important challenges in molecular magnetism. A Hofmann-type porous coordination polymer {Fe(pz)[Pt(II)(CN)(4)]} (1; pz = pyrazine) exhibited cooperative spin transition near room temperature (T(c)(up) = 304 K and T(c)(down) = 284 K) and its iodine adduct {Fe(pz)[Pt(II/IV)(CN)(4)(I)]} (1-I), prepared by oxidative addition of iodine to the open metal sites of Pt(II), raised the T(c) by 100 K. DSC and microscopic Raman spectra of a solid mixture of 1-I and 1 revealed that iodine migrated from 1-I to 1 through the grain boundary after heating above 398 K. We have succeeded in precisely controlling the iodine content of {Fe(pz)[Pt(CN)(4)(I)(n)]} (1-In; n = 0.0-1.0), which resulted in consecutive modulation of T(c) in the range 300-400 K while maintaining the hysteresis width. Furthermore, it was demonstrated that iodine migration in the solid mixture was triggered by the spin transition of 1-I. The magnetically bistable porous framework decorating guest interactive open-metal-site in the pore surface makes it possible to modulate T(c) ad arbitrium through unique postsynthetic method using iodine migration.

Spin-crossover behavior in cyanide-bridged iron(II)-copper(I) bimetallic 1-3D metal-organic frameworks.

The synthesis and characterization of a series of 1-3D cyanide-bridged iron(II)-copper(I) bimetallic coordination polymers formulated as {Fe(3-Xpy)(2)[Cu(3-Xpy)(z)(CN)(2)](2)}, where 3-Xpy is a 3-halogenpyridine ligand with X = F (z = 1.5, 1), Cl (z = 1, 2 and 3), Br (z = 1, 4), and I (z = 1, 5), are reported. In all derivatives, the Fe(II) ion lies in pseudoctahedral [FeN(6)] sites defined by four in situ formed [Cu(3-Xpy)(z)(CN)(2)](-) bridging ligands and two 3-Xpy terminal ligands occupying the equatorial and axial positions, respectively. 1 consists of stacks of corrugated grids whose square windows are defined by pseudotrigonal and pseudotetrahedral [Cu(3-Fpy)(CN)(2)](-) and [Cu(3-Fpy)(2)(CN)(2)](-) units, respectively. 2 is a 3D coordination polymer with the topology of the open-framework CdSO(4). The [Cu(3-Clpy)(CN)(2)](-) rods connecting the pseudooctahedral Fe(II) sites are arranged in such a way that interpenetration is avoided. 3, an architectural isomer of 2, is defined by arrays of linear chains. 4 and 5 are isostructural to 3. Polymer 1 is essentially a low-spin (LS) compound with ca. 19% of residual Fe(II) ions in the high-spin (HS) state at 293 K. It undergoes an irreversible spin transition at T(c) = 356 K. Subsequent cooling-warming cycles give a new spin-crossover behavior characterized by T(c) = 187 K. The structural analysis at 130 and 293 K and at 293 K after irreversible transformation (293 K*) reveals a large unit cell volume variation of 67 A(3) per Fe atom. In addition to the volume change associated with the spin-state conversion, remarkable bond and angle modifications around the Cu(I) sites account for the high flexible nature of the crystal. 2 displays a complete not well-resolved two-step spin conversion, T(c1) = 169 K and T(c2) = 210 K, reflecting the occurrence of two distinct crystallographically Fe(II) sites. The large unit cell volume variation per Fe atom in 2, 59 A(3), has been rationalized in terms similar to those for 1. 1D polymers 3-5 are HS compounds.

Polynuclear spin crossover complexes: synthesis, structure, and magnetic behavior of [Fe4(mu-CN)4(phen)4(L)2)]4+ squares.

Three new tetranuclear compounds of formula [Fe(4)(mu-CN)(4)(phen)(4)(L)(2))](PF(6))(4) x G where L = tris(pyridin-2-ylmethyl)amine (TPMA) [G = 0] (1), (6-methylpyrid-2-ylmethyl)-bis(pyrid-2-ylmethyl)amine (MeTPMA) [G = 0] (2), or bis(6-methylpyrid-2-ylmethyl)-(pyrid-2-ylmethyl)amine (Me(2)TPMA) [G = NH(4)PF(6)] (3) and phen = 1,10-phenanthroline) have been synthesized and characterized. The three compounds crystallize in the C2/c space group and consist of [Fe(4)(mu-CN)(4)(phen)(4)(L)(2))](4+) square shaped cations with two distinct iron(II) sites. The sites, associated with [Fe(phen)(2)(CN)(2)] and [Fe(L)(NC)(2)] moieties, are connected by cyanide bridging ligands and reside in different [FeN(4)C(2)] and [FeN(6)] ligand field strength environments. For 1, the structural features of both sites at 100 and 293 K are those of an iron(II) atom in the low-spin state, according to the magnetic properties. At 370 K the structure of the [FeN(6)] site is consistent with a quite complete change of spin state from the low-spin state to the high-spin state, a behavior confirmed by the magnetic study. Introduction of a methyl substituent in the sixth position of one or two pyridine groups to get MeTPMA or Me(2)TPMA derivatives, respectively, induce in 2 and 3 notable steric constraint in the [FeN(6)] site making longer the average Fe-N bond distances thereby weakening the ligand field strength and stabilizing the high-spin state. The [FeN(4)C(2)] site remains in the low-spin state in the three compounds.

Synthesis and X-ray structures of a series of heteroleptic selenites of copper.

The reactions of [Cu(R-pySe)], R = H, 3-CF(3), with 2,2'-bipyridine or 1,10-phenanthroline in acetonitrile yielded the one-dimensional complexes {[Cu(SeO(3))(phen)] x 2 (H(2)O)}(n), 1, {[Cu(SeO(3))(bipy)] x 2 (H(2)O)}(n), 2. Recrystallization of 1 and 2 from a 1:1 methanol/water mixture led to the formation of the dinuclear complexes [Cu(SeO(3))(phen)(H(2)O)](2), 3, and [Cu(SeO(3))(bipy)(H(2)O)](2), 4. Single crystal X-ray diffraction analysis revealed that 1 and 2 have a polymeric structure in which each oxygen atom of the selenite group coordinates to a copper atom. The two other coordination positions are occupied by two nitrogen atoms of the organic ligand, and this defines an almost square pyramidal geometry around the metal. Both compounds show a very weak antiferromagnetic coupling (J = -0.9 for 1 and -0.6 cm(-1) for 2). Compounds 3 and 4 consist of dinuclear species with the copper atoms lying in a square pyramidal geometry in which the selenite group acts as a bridge between two copper atoms and one oxygen atom is uncoordinated. The two copper ions in 3 show antiferromagnetic coupling with J = -22.1 cm(-1).

Bidirectional chemo-switching of spin state in a microporous framework.

The ins and outs of spin: Using the microporous coordination polymer {Fe(pz)[Pt(CN)(4)]} (1, pz=pyrazine), incorporating spin-crossover subunits, two-directional magnetic chemo-switching is achieved at room temperature. In situ magnetic measurements following guest vapor injection show that most guest molecules transform 1 from the low-spin (LS) state to the high-spin (HS) state, whereas CS(2) uniquely causes the reverse HS-to-LS transition.