Solvent-Induced Polymorphism of Iron(II) Spin Crossover Complexes

Spin crossover of a Fe(II) mononuclear complex induced by intermolecular factors involving chloride and solvent ordering

Two new salts of a mononuclear tripodal Fe(II) complex were prepared, using ClO4- and Cl-. The ClO4- sample (1) remained HS at low temperatures, similar to the previously reported BF4- analogue. Crystallising with the Cl- anion (2) led to a markedly different crystal packing arrangement, and engendered SCO activity. This has been correlated to the lower crystal packing density in 2 and the coordination complex conformational differences arising due to the packing motifs of 1 and 2. Further, solvent ordering effects have been proposed to facilitate spin transition behaviour in 2.

Above room temperature spin crossover in mononuclear iron(II) complexes featuring pyridyl-benzimidazole bidentate ligands adorned with aliphatic chains

Two bidentate ligands (L1 = 1-pentyl-2-(pyridin-2-yl)-1H-benzimidazole and L2 = 1-heptyl-2-(pyridin-2-yl)-1H-benzimidazole) were employed for the synthesis of five mononuclear Fe(II) coordination compounds 1-5 containing perchlorate, tetrafluoroborate and triflate counterions. Single-crystal X-ray diffraction analysis confirmed the expected molecular structures of all the reported compounds, revealing a moderately distorted octahedral geometry of {FeN6} coordination chromophores. All five compounds exhibit thermal spin crossover with T1/2 temperatures allocated above 400 K. The theoretical calculations supported the experimental magnetic investigation and helped to explain the electronic structures of the reported complexes with respect to the occurrence of thermal spin state switching. In addition, compound 4 was employed for the preparation of Langmuir-Blodgett films and fabrication of molecular films using the method of spontaneous evaporation of the subphase. While the formation of Langmuir-Blodgett films was unsuccessful due to the instability of the compound at the water/air interface, the latter technique allowed the formation of molecular films of 4 with well-defined thickness and homogeneity.

Solvatomorphism, polymorphism and spin crossover in bis[hydrotris(1,2,3-triazol-1-yl)borate]iron(ii)

We describe a detailed characterization of the spin crossover bis[hydrotris(1,2,3-triazol-1-yl)borate]iron(ii) complex ([Fe(HB(1,2,3-tz)3)2]) and solvatomorphs obtained by recrystallisation or single crystal to single crystal transformation.

Manipulating the spin crossover behaviour in a series of cyanide-bridged {FeIII2FeII2} molecular squares through NCE- co-ligands

Manipulating the transition temperature (T1/2) of spin-crossover (SCO) complexes capable of fulfilling practical criteria through different synthetic strategies is one of the main focuses in the field of molecular magnetism....

The effect of tether groups on the spin states of iron(II)/bis[2,6-di(pyrazol-1-yl)pyridine] complexes

The synthesis of six 2,6-di(pyrazol-1-yl)pyridine derivatives bearing dithiolane or carboxylic acid tether groups is described: [2,6-di(pyrazol-1-yl)pyrid-4-yl]methyl (R)-lipoate (L1), 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxamido]ethyl (R)-lipoate (L2), 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxy]ethyl (R)-lipoate (L3), N-([2,6-di(pyrazol-1-yl)pyrid-4-ylsulfanyl]-2-aminoethyl (R)-lipoamide (L4), 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxamido]acetic acid (L5) and 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxamido]propionic acid (L6). The iron(ii) perchlorate complexes of all the new ligands exhibit gradual thermal spin-crossover (SCO) in the solid state above room temperature, except L4 whose complex remains predominantly high-spin. Crystalline [Fe(L6)2][ClO4]2·2MeCN contains three unique cation sites which alternate within hydrogen-bonded chains, and undergo gradual SCO at different temperatures upon warming. The SCO midpoint temperature (T1/2) of the complexes in CD3CN solution ranges between 208-274 K, depending on the functional group linking the tether groups to the pyridyl ring. This could be useful for predicting how these complexes might behave when deposited on gold or silica surfaces.

The flexibility of long chain substituents influences spin-crossover in isomorphous lipid bilayer crystals

These two compounds are isomorphous but show different spin-state behaviour on cooling. This may be influenced by the packing of alkyl vs. alkynyl chains in the solid state.

Investigation of SCO property–structural relationships in a family of mononuclear Fe(ii) complexes

Six complexes with the formula [FeL(NCE)₂] (E = S, Se, BH₃) have been reported. A survey of the magneto-structural relationship has been conducted.

Competition between coordination bonds and hydrogen bonding interactions in solvatomorphs of copper(II), cadmium(II) and cobalt(II) complexes with 2,2′-bipyridyl and acetate

The delicate balance among conformation, coordination bonds and hydrogen bonding has been probed in solvatomorphs of known metal-organic molecules synthesised from copper(II), cadmium(II) and cobalt(II) with acetate (OAc) and 2,2′-bipyridine (bipy). The Cu(OAc)2(bipy) complex, isolated as a pentahydrate, has the acetate ligands oriented to opposite sides of the coordination square plane. DFT calculations show the energy difference between this structure and a syn form amount to approximately 16 kJ/mol. The presence of lattice water enables the formation of O–H···O hydrogen bonds with the acetate ligands. Different coordination numbers and energies are found as a function of the number of water molecules co-crystallising in the Cd(OAc)2(bipy)(OH2)·3H2O and [Co(OAc)(bipy)2](OAc)·3H2O complexes.

Spin crossover Fe(ii) complexes of a cross-bridged cyclam derivative

A cross-bridged cyclam derivative containing two 2-pyridylmethyl pendant arms (L = 4,11-bis((pyridin-2-yl)methyl)-1,4,8,11-tetraaza-bicyclo[6.6.2]hexadecane) was synthesized by dialkylation of the cross-bridged cyclam with 2-chloromethylpyridine. A series of Fe(ii) complexes with L and different counter-anions of the formulas [Fe(L)][FeCl4]·H2O (1·H2O), [Fe(L)]Cl2·4H2O (2·4H2O), [Fe(L)](BF4)2·0.5CH3CN (3·0.5CH3CN) and [Fe(L)](BPh4)2·CH3OH (4·CH3OH) was prepared and thoroughly characterized. In all the cases, the [Fe(L)]2+ cation adopts a cis-V configuration with a distorted octahedral geometry, and with the FeN6 donor set. The magnetic measurements within the temperature interval of 5-400 K revealed the spin crossover (SCO) behaviour of all the complexes with the transition temperature T1/2 increasing with the counter anion in the order BF4- (3) < [FeCl4]2- (1) < BPh4- (4). However, the SCO process was complete in the case of compound 3 only, with T1/2 = 177 K, which proceeded after removal of co-crystallized CH3CN molecules accompanied by a change of the crystallographic phase. The SCO behaviour of 3 was also confirmed by a single crystal X-ray analysis providing the average Fe-N distances of 2.086 Å at 120 K and 2.197 Å at 293 K typical of low-spin, and high-spin FeII complexes, respectively. The obtained results clearly showed that the nature of the counter anion and the presence/absence of co-crystallized solvent molecule(s) significantly affected the temperature as well as the abruptness of the spin transition. This is the first report of SCO behaviour observed for iron complexes containing a cross-bridged cyclam derivative.

Evidence of crystal packing effects in stabilizing high or low spin states of iron(ii) complexes with functionalized 2,6-bis(pyrazol-1-yl)pyridine ligands

The molecular structures and magnetic properties of homoleptic iron(ii) compounds [Fe(bpp-COOMe)₂](ClO₄)₂ (1) and [Fe(bpp-triolH₃)₂](ClO₄)₂ (2) have been investigated to ascertain their spin crossover (SCO) behaviour. In these hexacoordinated complexes, the bpp (2,6-bis(pyrazol-1-yl)pyridine) ligands adopt a mer-mer coordination mode and carry COOMe or C(O)NHC(CH₂OH)₃para substituents, respectively, on the central pyridyl ring. In spite of the almost equal donor power of the ligands to the iron(ii) centre, the two compounds feature different spin state configurations at room temperature. Compound 1 displays a highly-distorted octahedral environment around the iron(ii) centre, which adopts a high spin (HS) state at all temperatures, even under an external applied pressure up to 1.0 GPa. By contrast, 2 is characterized by a more regular octahedral coordination around the metal ion and exhibits a low spin (LS) configuration at or below room temperature. However, it shows a thermally-induced SCO behaviour at T > 400 K, along with Light-Induced Excited Spin State Trapping (LIESST) at low temperature, with T_LIESST = 38 K. Since DFT (U)M06/6-311+G(d) geometry optimizations in vacuo indicate that both complexes should adopt a HS state and a highly-distorted coordination geometry, the stabilization of a LS configuration in 2 is ultimately ascribed to the effect of intermolecular hydrogen bonds, which align the [Fe(bpp-triolH₃)₂]²⁺ cations in 1D chains and impart profound differences in the geometric arrangement of the ligands.

Packing polymorphism in 3-amino-2-pyrazinecarboxylate based tin(ii) complexes and their catalytic activity towards cyanosilylation of aldehydes

3-Amino-2-pyrazinecarboxylic acid is used to synthesize two new mononuclear interconvertible packing polymorphs of tin(ii) which act as heterogeneous catalysts for the cyanosilylation of aldehydes with trimethylsilyl cyanide.

Two polymorphic Co(ii) field-induced single-ion magnets with enormous angular distortion from the ideal octahedron

A mononuclear complex [Co(neo)(PhCOO)₂] was prepared in two polymorphic forms, which both possess large magnetic anisotropy with different degrees of rhombicity. Furthermore, both polymorphs behave as field-induced single-ion magnets.

Polymorphism of a widely used building block for halogen-bonded assemblies: 1,3,5-trifluoro-2,4,6-triiodobenzene

After reporting the structure of a new polymorph of 1,3,5-trifluoro-2,4,6-triiodobenzene (denoted BzF3I3), C₆F₃I₃, (I), which crystallized in the space group P2₁/c, we perform a comparative analysis with the already reported P2₁/n polymorph, (II) [Reddy et al. (2006). Chem. Eur. J. 12, 2222-2234]. In polymorph (II), type-II I...I halogen bonds and I...π interactions connect molecules in such a way that a three-dimensional structure is formed; however, the way in which molecules are connected in polymorph (I), through type-II I...I halogen bonds and π-π interactions, gives rise to an exfoldable lamellar structure, which looks less tightly bound than that of (II). In agreement with this structural observation, both the melting point and the melting enthalpy of (I) are lower than those of (II).

Synthesis of [Fe(Leq)(Lax)] n coordination polymer nanoparticles using blockcopolymer micelles

Spin-crossover compounds are a class of materials that can change their spin state from high spin (HS) to low spin (LS) by external stimuli such as light, pressure or temperature. Applications demand compounds with defined properties concerning the size and switchability that are maintained when the compound is integrated into composite materials. Here, we report the synthesis of [Fe(L_eq)(L_ax)]_n coordination polymer (CP) nanoparticles using self-assembled polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer (BCP) micelles as template. Variation of the solvent (THF and toluene) and the rigidity of the axial ligand L_ax (L_ax = 1,2-di(pyridin-4-yl)ethane) (bpea), trans-1,2-di(pyridin-4-yl)ethene (bpee), and 1,2-di(pyridin-4-yl)ethyne) (bpey); L_eq = 1,2-phenylenebis(iminomethylidyne)-bis(2,4-pentanedionato)(2-)) allowed the determination of the preconditions for the selective formation of nanoparticles. A low solubility of the CP in the used solvent and a high stability of the Fe-L bond with regard to ligand exchange are necessary for the formation of composite nanoparticles where the BCP micelle is filled with the CP, as in the case of the [FeL_eq(bpey)] _n @BCP. Otherwise, in the case of more flexible ligands or ligands that lead to high spin complexes, the formation of microcrystals next to the CP-BCP nanoparticles is observed above a certain concentration of [Fe(L_eq)(L_ax)] _n . The core of the nanoparticles is about 45 nm in diameter due to the templating effect of the BCP micelle, independent of the used iron complex and [Fe(L_eq)(L_ax)] _n concentration. The spin-crossover properties of the composite material are similar to those of the bulk for FeL_eq(bpea)] _n @BCP while pronounced differences are observed in the case of [FeL_eq(bpey)] _n @BCP nanoparticles.