Challenges in Engineering Spin Crossover: Structures and Magnetic Properties of Six Alcohol Solvates of Iron(II) Tris(2-picolylamine) Dichloride

Solvent-induced transformation of single crystals of a spin-crossover (SCO) compound to single crystals with two distinct SCO centers

The long-sought-after crystal structure of Fe(tpa)(NCS)(2) (1, tpa = tris(2-pyridylmethyl)amine), an otherwise well-studied spin-crossover (SCO) complex, has been obtained, and its one-step, incomplete spin transition was correlated to its solid-state structures at different temperatures. Upon exposure to methanol vapor, single-crystal-to-single-crystal transformation of 1 to a new SCO compound, 2, formulated as {[Fe(tpa)(NCS)(2)] x [Fe(tpa)(NCS)(2) x CH(3)OH]}, occurs with a dramatic color change from yellow to red. Crystallographic studies revealed that the asymmetric unit of the structure of 2 contains two independent Fe(II) centers. Studies by magnetic measurements and Mossbauer spectroscopy revealed a two-step complete spin transition for compound 2, between LS-LS and HS-HS, via an unambiguous intermediate LS-HS phase; the two SCO centers of disparate spin states were resolved crystallographically. That a significant portion of the original crystal structure is maintained indicates that the present approach is a more subtle means of altering the properties associated with SCO phenomenon than by changing counteranions or crystallization using different solvents. Furthermore, the dramatic changes in crystal structure and SCO behaviors triggered by mere solvent sorption suggest that this approach is rather efficient in modifying and hopefully fine-tuning and optimizing properties of SCO compounds. Coupled with the aforementioned gentleness and subtlety, the present approach of heterogeneously introducing perturbations to pre-existing supramolecular arrays of SCO units is more conducive to systematic studies aiming at the discovery of new SCO systems and phenomenon toward their ultimate materials applications.

Thermally induced magnetic anomalies in solvates of the bis(hexafluoroacetylacetonate)copper(II) complex with pyrazolyl-substituted nitronyl nitroxide

We succeeded in synthesizing of a whole family of isostructural solvates of the copper(II) hexafluoroacetylacetonate complex with pyrazolyl-substituted nitronyl nitroxide (L): Cu(hfac)2L x 0.Solv. The main feature inherent in nature of Cu(hfac)2L x 0.5 Solv single crystals is their incredible mechanical stability and ability to undergo reversible structural rearrangements with temperature variation, accompanied by anomalies on the mu(eff(T)) dependence. Structural investigation of the complexes over a wide temperature range before and after the structural transition and the ensuing magnetic phase transition showed that the spatial peculiarities of the solvent molecules incorporated into the solid govern the character of the mu(eff(T)) dependence and the temperature region of the magnetic anomaly. Thus, doping of crystals with definite solvent molecules could be used as an efficient method of control over the magnetic anomaly temperature (T(a)). The investigation of this special series of crystals has revealed the relationship between the chemical step and the magnetic properties. It was shown that "mild" modification of T(a) for Cu(hfac)2L x 0.5 Solv required a much smaller structural step than the typical change of one -CH2- fragment in a homologous series in organic chemistry. Quantum-chemical calculations with the use of X-ray diffraction data allowed us to trace the character of changes in the exchange interaction parameters in the range of the phase transition. In the temperature range of the phase transition, the exchange parameter changes substantially. The gradual decrease in the magnetic moment, observed in most experiments during sample cooling to T(a), is the result of the gradual increase in the fraction of the low-temperature phase in the high-temperature phase.

Modification of Spin Crossover Behavior through Solvent Assisted Formation and Solvent Inclusion in a Triply Interpenetrating Three-Dimensional Network

The 3D coordination polymer [Fe(4ditz)3](PF6)2.solv consists of three interpenetrating infinite networks. There are cavities between iron atoms of different networks, which are partly filled with solvent molecules. With a change of the solvent used during synthesis from methanol to ethanol, the magnetic behavior of the materials changes. Both show an abrupt two-step spin crossover from low spin (S = 0) to high spin (S = 2) with the methanolate curve lying 7 K higher and showing a small hysteresis. Single crystal and powder diffraction studies show that they both have the same structure, but in powder form, the methanolate slowly loses methanol to finally leave about 0.075 MeOH/Fe. In comparison, the bigger ethanol remains at 0.25 EtOH/Fe. These results, in conjunction with thermodynamic data, strongly suggest that the differences in magnetic behavior are largely entropic in nature. Possible reasons for this are discussed.

Spin Crossover in a Series of Iron(II) Complexes of 2-(2-Alkyl-2H-tetrazol-5-yl)-1,10-phenanthroline: Effects of Alkyl Side Chain, Solvent, and Anion

2-(2H-Tetrazol-5-yl)-1,10-phenanthroline (HL0), its alkyl-substituted derivatives (Ln, where n = 1-8, 10, 12, 14, and 16, denoting the carbon atom number of the alkyl chain) at the 2H position of the tetrazole ring, and their iron(II) complexes (a for [Fe(L0)2], na for [Fe(Ln)2](ClO4)2, and nb for [Fe(Ln)2](BF4)2) were synthesized and characterized. The crystal structures of a, a.CH3OH, 1a.CH3OH, 1b.CH3OH.CH3CN, 2a.H2O, 2b.H2O, 4b.CH3OH, 5a.H2O, 5b.H2O, 6a, 6b, 7a, 7b, and 16a are described, along with thermal analyses. a undergoes an abrupt spin crossover (SCO) at 255 K with a hysteresis loop of 6 K. a.CH3OH, 2a.H2O, and 2b.H2O exhibit irreversible SCO behaviors due to the loss of solvent molecules upon heating. 3a, 3b, 4a, and 5a.H2O show simple spin transitions above 350 K. The desolvated samples of 4b.CH3OH and 5b.H2O undergo two-step spin transitions. 16a exhibits a two-step SCO behavior between 100 and 300 K, corresponding to sequential phase transitions from the low-spin (LS) phase to the intermediate phase and then to the high-spin phase, respectively, proved by crystal structure analysis and 57Fe Mössbauer spectroscopy. 1a.CH3OH, 10a, 10b, 12a, 12b, 14a, 14b, and 16b show gradual and incomplete SCO behaviors after cooling down from 400 K. 1b.CH3OH.CH3CN, 6a, 6b, 7a, 7b, 8a, and 8b remain in the LS state even at 400 K. This proves that the alkyl side chains, together with the solvent molecules and anions, play a crucial role in the complicated SCO behaviors in this system.

Structural determination of a short-lived excited iron(II) complex by picosecond x-ray absorption spectroscopy

Structural changes of the iron(II)-tris-bipyridine ([Fe(II)(bpy)(3)](2+)) complex induced by ultrashort pulse excitation and population of its short-lived (< or =0.6 ns) quintet high spin state have been detected by picosecond x-ray absorption spectroscopy. The structural relaxation from the high spin to the low spin state was followed over the entire lifetime of the excited state. A combined analysis of the x-ray-absorption near-edge structure and extended x-ray-absorption fine structure spectroscopy features delivers an Fe-N bond elongation of 0.2 A in the quintet state compared to the singlet ground state.

Iron(ii) complexes with a terpyridine embrace packing motif show remarkably consistent cooperative spin-transitions

Six structurally related iron(II) complexes show remarkably similar abrupt thermal spin-transitions.

Anion doping as a probe of cooperativity in the molecular spin-crossover compound [FeL2][BF4]2(L = 2,6-di{pyrazol-1-yl}pyridine)

The complex dications in the cooperative spin-crossover compound [FeL(2)][BF(4)](2) (2,6-di(pyrazol-1-yl)pyridine) pack through pi-pi interactions into a 2-D layered structure (a "terpyridine embrace" motif). The effects of doping the larger ClO(4)(-) ion into this lattice have been investigated. The bulk solids [FeL(2)][ClO(4)](x)[BF(4)](2-x) are isostructural with [FeL(2)][BF(4)](2) when x = 0.30 and 0.98, and isostructural with (structurally distinct) [FeL(2)][ClO(4)](2) when x = 1.89. When x = 1.68, powder samples are a mixture of both phases, but crystalline material adopts purely the ClO(4)(-) structure. Increasing the perchlorate content in the lattice for 0 < or =x< or = 1.68 causes a small decrease in T(1/2) and a narrowing of hysteresis in their spin-crossover, but with no significant reduction in cooperativity. It also leads to more pronounced decreases in DeltaH [by up to 3.2(5) kJ mol(-1)] and DeltaS [by up to 10(2) J mol(-1) K(-1)] for the transition by DSC. Single crystals of formula [FeL(2)][ClO(4)](y)[BF(4)](2-y) (y = 0.44 and 1.13) are isostructural with the pure BF(4)(-) salt. While their molecular structures are indistinguishable, the distances between cations in the lattice increase in the doped materials. Weakening of intermolecular pi-pi interactions between cations is the likely reason for the reduced enthalpy of spin-crossover as x increases. However, the biggest stuctural change is an increase in the spacing between the 2-D layers with increased ClO(4)(-). These results suggest that cooperativity in this material is transmitted within the terpyridine embrace layers.

Lattice-solvent controlled spin transitions in iron(ii) complexes

A series of spin transition (ST) iron(II) compounds of the type [FeII2](X)2.{S}2 (where is 4'-(4'''-cyanophenyl)-1,2':6'1''-bispyrazolylpyridine, X=ClO4- or BF4-, and S is acetonitrile) was synthesized and magnetically investigated. The effects of the removal of the lattice-solvent molecules and of their different positions relative to the iron(II) cations on the ST process were investigated. Crystallization yields orange block (A.{S}2) crystals of the composition [FeII()2](ClO4)2.{S}2, and two polymorphic compounds of the stoichiometry [FeII()2](BF4)2.{S}2 as red coffin (B.{S}2) and orange block (C.{S}2) crystals. The Fe-N bond distances of A.{S}2 (from 1.921(9) to 1.992(3) A; at 150 K), B.{S}2 (from 1.943(2) to 2.017(2) A; at 180 K) and C.{S}2 (from 1.883(3) to 1.962(3) A; at 180 K) indicate low spin (LS) states of the respective iron(II) ions. Notably, the observed small difference in the Fe-N distances at 180 K for the two polymorphs B.{S}2and C.{S}2 are due to different positions of the acetonitrile molecules in the crystal lattices and illustrate the sensitivity of the spin transition properties on lattice-solvent effects. Variable-temperature single crystal X-ray studies display single-crystal thermochroism (red (LS)<-->orange (HS)) for A.{S}2 and B.{S}2 and ca. 3.6% decrease in the unit cell volume of A.{S}2 from 4403 A3 at 300 K to 4278 A3 at 150 K. The temperature dependent magnetic susceptibilities of A.{S}2 and B.{S}2 demonstrate systematic increase of the spin transition temperatures (T1/2) and continuous decreases of the hysteresis loop width (DeltaT1/2) upon slow lattice-solvent exclusion.

Interplay of Spin Conversion and Structural Phase Transformations: Re-Entrant Phase Transitions in the 2-Propanol Solvate of Tris(2-picolylamine)iron(II) Dichloride

Crystal structures, magnetic and thermodynamic properties of the spin-crossover compound tris(2-picolylamine)iron(II) dichloride (with 2-propanol solvent molecules) have been measured in the temperature range from 15 to 293 K. X-ray diffraction, SQUID, and calorimetric experiments all showed two first-order phase transitions with hysteresis loops, a narrow one at T(1) approximately 196 K and a broad, triangular one covering the range 153<T(2)<166 K. Crystal structures were analysed at fourteen temperatures in a cooling cycle and at seven temperatures in a heating cycle. They reveal a complex, temperature-dependent ordering behaviour of both the complex cations and the alcohol molecules. A phenomenological model accounting for spin conversion, solvent ordering and coupling between the two processes describes the observed phase transitions and ordering phenomena reasonably well. Similarities and differences in the behaviour of the corresponding ethanol solvate with the same crystal architecture are discussed. It is concluded that spin-crossover behaviour depends as much on molecular properties as it does on intermolecular interactions, both of the spin active and the spin inactive components.

Influence of the Counterion and the Solvent Molecules in the Spin Crossover System [Co(4-terpyridone)2]Xp·nH2O

A series of new complexes belonging to the [Co(4-terpyridone)2]X(p) x nS family (4-terpyridone = 2,6-bis(2-pyridyl)-4(1H)-pyridone) have been synthesized and characterized, using X-ray single crystal determination and magnetic susceptibility studies, to be X = [BF4]- (p = 2) and S = H2O for polymorphs 1 and 2, X = [BF4]- (p = 1) and [SiF6]2- (p = 0.5) and S = CH(3)OH for 3, X = [SiF6]2- (p = 1) and S = 3CH3OH and H2O for 4, X = [Co(NCS)4]2- (p = 1) and S = 0.5CH3OH for 5, X = I- (p = 2) and S = 5H2O for 6, X = [PF6]- (p = 1) for 7, and X = [NO3]- (p = 2) for 8. Compounds 1-7 can be grouped in three sets according to the space group in which they crystallize: (i) P1 triclinic (1, 3), (ii) P2(1) monoclinic (2), and (iii) P2(1)/c monoclinic (4-7). The tridentate 4-terpyridone ligands coordinate the Co(II) ions in a mer fashion defining essentially tetragonally compressed [CoN6] octahedrons. The Co-N axial bonds involving the pyridone rings are markedly shorter than the Co-N equatorial bonds collectively denoted as Co-N(central) and Co-N(distal), respectively. The differences in the average Co-N(central) or Co-N(distal) distances observed for 1-7 reflect the different spin states of Co(II). Complexes 7 and 4' are fully high spin (HS), while 5 and 6 are low spin (LS). However, the counterion [Co(NCS)4]2- in complex 5 is high spin. Complexes 1, 2, 3, and 8 exhibit spin-crossover behavior in the 400-100 K temperature region. Compounds 1 and 2 are polymorphs, and interestingly, 1 irreversibly transforms into 2 above 340 K because of a crystallographic phase transition which involves a drastic modification of the crystal packing. The relevant thermodynamic parameters associated with the spin transition of polymorph 2 have been estimated using the regular solution theory leading to DeltaH = 3.04 kJ mol(-1), DeltaS = 20 J K(-1) mol(-1), and Gamma = 0.95 kJ mol(-1).

Both Spacer Length and Parity Influence the Thermal and Light-Induced Properties of Iron(II) α,ω-Bis(tetrazole-1-yl)alkane Coordination Polymers

A new series of [mu-tris-{1,n-bis(tetrazol-1-yl)alkane-N4,N4'}iron(II)] bis(perchlorate) spin-crossover coordination polymers ([Fe(nditz)3](ClO4)2]; n = 4-9) has been synthesised and characterised. The ditetrazole bridging ligands provide octahedral symmetry at the iron(II) centres while allowing the distance between iron(II) centres to be varied. These polymers have therefore been investigated to determine the effects of spacer length on their thermal and light-induced spin-transition behaviour. An increase in the number of carbon atoms in the spacer (n) raises the thermal spin-crossover temperature, while decreasing the stability of the light-induced metastable state generated through the light-induced excited spin state trapping (LIESST) effect by irradiating the sample at 530 nm. Remarkably, however, the parity of the spacer also has an effect, enabling the series of complexes to be divided into two sub-series depending on whether the bridging ligand possesses an even or an odd number of carbon atoms. An explanation at the molecular level using the single configurational coordinate (SCC) model is presented.

Thermal, pressure and light switchable spin-crossover materials

This article reviews the most relevant chemical and structural aspects that influence the spin-crossover phenomenon (SCO). Special attention is focussed on the recent development of SCO coordination polymers. The different approaches currently being explored in order to achieve multifunctionality in SCO materials are discussed.

Formation of Third Generation Poly(pyrazolyl)borate Ligands from Alkyne Coupling Reactions of Fe[(p-IC6H4)B(3-Rpz)3]2 (R = H, Me; pz = Pyrazolyl): Pathways toward Controlling an Iron(II) Electronic Spin-State Crossover

Sonogashira coupling reactions of terminal alkynes with Fe[(p-IC6H4)B(3-Mepz)3]2 (pz = pyrazolyl ring) yield Fe[(p-PhC2C6H4)B(3-Mepz)3]2 (2), Fe[(p-Me3SiC2C6H4)B(3-Rpz)3]2 (R = H, 3a, R = Me, 3b), and Fe[(p-HC2C6H4)B(3-Mepz)3]2 (R = H, 4a, R = Me, 4b), a series of new complexes containing "third generation" poly(pyrazolyl)borate ligands. Complex 2 undergoes a fairly gradual iron(II) electronic spin-state crossover with a 30 K hysteresis, whereas complex 3b is an unusual example of a complex with equivalent iron(II) sites in the high-spin form that shows an abrupt 50% spin crossover. For complex 4b, 50% of the iron(II) sites undergo a gradual spin-state transition between 185 and 350 K with an activation energy of 1590 +/- 30 cm(-1) and a T(1/2) = 280 K and, for the remaining iron(II) sites, an abrupt cooperative spin-state crossover between 106 and 114 K. The crystal structures of 4b obtained for each of the three distinct electronic spin states reveal two crystallographically different iron(II) sites, and analysis of the molecular/supramolecular structures indicates that the difference in the degree of pyrazolyl ring tilting in the ligands between the two sites, rather than the strength of the intermolecular forces, play a prominent role in determining the temperature of the spin-state crossover.

A structural, magnetic and Mössbauer spectroscopic study of an unusual angular Jahn–Teller distortion in a series of high-spin iron(ii) complexes

Single crystal X-ray structures and susceptibility data are described for six homoleptic iron(II) complex salts, of 2,6-di(pyrazol-1-yl)pyridine or a 3,3"-disubstituted derivative of it. Zero field Mossbauer spectroscopic data for four of the complexes, and one previously reported analogue, are also discussed. Four of these compounds exhibit an unusual angular Jahn-Teller distortion towards C(2) symmetry to differing degrees, while the other two exhibit structures close to the "ideal" D(2d) symmetry for this ligand set. This structural distortion has two components: a twisting of the plane of one ligand relative to the other about the N{pyridine}-Fe-N{pyridine} vector, so that the two ligands are no longer perpendicular; and a rotation of one ligand about the Fe ion, so that the N{pyridine}-Fe-N{pyridine} angle < 180 degrees. Susceptibility data show that all the complexes are fully high-spin between 5 and 300 K, but yield an unusually wide range of zero-field splitting parameters for the different compounds of between 2.6 and 13.4 cm(-1). Magnetostructural correlations suggest that a low value of |D| is diagnostic for a high degree of "rotation" distortion. The Mossbauer spectra imply that an increased quadrupole splitting might also be diagnostic for the presence of the angular distortion.