Ligand strain and conformations in a family of Fe(II) spin crossover hexadentate complexes involving the 2-pyridylmethyl-amino moiety: DFT modelling.

DFT calculations of the mononuclear Fe(II) spin crossover complexes [Fe(L)](2+) (L = ({bis[N-(2-pyridylmethyl)-3-aminopropyl](2-pyridylmethyl)amine})), ({[N-(2-pyridylmethyl)-3-aminopropyl][N-(2-pyridylmethyl)-2-aminoethyl](2-pyridylmethyl)amine}) and ({bis[N-(2-pyridylmethyl)-2-aminoethyl](2-pyridylmethyl)amine}) abbreviated as (66), (56) and (55) have been performed in order to explain the observed spin transition temperature differences. The complexes differ in the size of two chelate rings, revealing two six-membered, one six-membered and one five-membered, and two five membered rings for (66), (56) and (55), respectively. Calculations of the electronic energy differences ΔEel = Eel(HS) - Eel(LS) with the use of the basis set TZVP with B3LYP*, PBE, TPSS and TPSSh functionals reproduced the experimentally observed trends. The best reproduction of bond distances is obtained using the TPSSh functional. The Continuous Shape Measure (CShM) analysis of the optimised structures of all six spin isomers revealed the most significant distortion from the trigonal prism for the low-spin (66) system, which has the lowest spin transition temperature. The corresponding trigonal twist is proposed to be the main cause of releasing strain that is induced by the size of two fused chelate rings. Different conformers of low-spin and high-spin (66) systems were modelled using the TPSSh/TZVP method, including the calculations of transition states of conformational rearrangements in both spin isomers. A normal co-ordinate analysis was performed for all six spin isomers. This allows the assignment of previously reported Raman marker bands to specific modes of the (66) system. The estimate of the vibrational contribution to the spin transition entropy revealed values of 50-60 J K(-1) mol at room temperature for all three complexes.

Spin crossover and polymorphism in a family of 1,2-bis(4-pyridyl)ethene-bridged binuclear iron(II) complexes. A key role of structural distortions.

Two polymorphic modifications 1 and 3 of binuclear compound [{Fe(dpia)(NCS)(2)}(2)(bpe)] and pseudo-polymorphic modification [{Fe(dpia)(NCS)(2)}(2)(bpe)]·2CH(3)OH (2), where dpia = di-(2-picolyl)amine, bpe = 1,2-bis(4-pyridyl)ethene, were synthesized, and their structures, magnetic properties, and Mössbauer spectra were studied. Variable-temperature magnetic susceptibility measurements of three binuclear compounds show different types of magnetic behaviour. The complex 1 exhibits a gradual two-step spin crossover (SCO) suggesting the occurrence of the mixed [HS-LS] (HS: high spin, LS: low spin) pair at the plateau temperature (182 K), at which about 50% of the complexes undergoes a thermal spin conversion. The complex 2 displays an abrupt full one-step spin transition without hysteresis, centred at about 159 K. The complex 3 is paramagnetic over the temperature range 20-290 K. The single-crystal X-ray studies show that all three compounds are built up from the bpe-bridged binuclear molecules. The structure of 1 was solved for three spin isomers [HS-HS], [HS-LS], and [LS-LS] at three temperatures 300 K, 183 K, and 90 K. The crystal structures for 2 and 3 were determined for the [HS-HS] complexes at room temperature. The analysis of correlations between the structural characteristics and different types of magnetic behaviour for new 1-3 binuclear complexes, as well as for previously reported binuclear compounds, revealed that the SCO process (occurrence of full one-step, two-step, or partial (50%) SCO) is specified by the degree of distortion of the octahedral geometry of the [FeN(6)] core, caused by both packing and strain effects arising from terminal and/or bridging ligands. The comparison of the magnetic properties and the networks of intra- and inter-molecular interactions in the crystal lattice for the family of related SCO binuclear compounds suggests that the intermolecular interactions play a predominant role in the cooperativeness of the spin transition relative to the intramolecular interactions through the bridging ligand.

Ligand strain and the nature of spin crossover in binuclear complexes: two-step spin crossover in a 4,4'-bipyridine-bridged iron(II) complex [{Fe(dpia)(NCS)(2)}(2)(4,4'-bpy)] (dpia = di(2-picolyl)amine; 4,4'-bpy = 4,4'-bipyridine).

The synthesis and detailed characterization of the new spin crossover (SCO) binuclear complex [{Fe(dpia)(NCS)(2)}(2)(4,4'-bpy)] (1; dpia=di(2-picolyl)amine, 4,4'-bpy = 4,4'-bipyridine) are reported. Variable-temperature magnetic susceptibility measurements show a relatively cooperative two-step spin transition suggesting the occurrence of three spin-state isomers: [HS-HS], [HS-LS], and [LS-LS] (HS: high spin, LS: low spin). A short plateau at 204 K separates the two steps and conforms with about 50% of the complexes having undergone a thermal spin conversion. Routine Mössbauer spectroscopy without applying a magnetic field clearly separates four iron(II) one-center spin states in three [HS-HS], [HS-LS], and [LS-LS] pairs and unambiguously confirms that the spin transition at the plateau temperature goes through the intermediate [HS-LS] state. The single-crystal X-ray structure was solved for three spin isomers at 293, 208, and 120 K. The structural study at the plateau temperature was unable to resolve the HS and LS sites in the [HS-LS] pair and only an average Fe-N bond length was obtained, which suggests that there is an intermediate [HS-LS] phase. The structural analysis at three temperatures revealed a dense three-dimensional network of both intra- and intermolecular interactions. The relative energies of the three spin-state isomers were evaluated by quantum-chemical DFT calculations. Comparison of compound 1 with previously known analogues, as well as the overall analysis of structural data for numerous binuclear complexes, allowed a conclusion to be reached on the crucial role of ligand strain effects in the SCO behavior of binuclear complexes. The suggested intramolecular mechanism explains the different types of SCO observed in binuclear complexes: one-step, two-step, and partial (50%) transitions.

Spin crossover in a family of iron(II) complexes with hexadentate ligands: ligand strain as a factor determining the transition temperature.

This paper reports the synthesis of a family of mononuclear complexes [Fe(L)]X(2) (X=BF(4), PF(6), ClO(4)) with hexadentate ligands L=Hpy-DAPP ({bis[N-(2-pyridylmethyl)-3-aminopropyl](2-pyridylmethyl)amine}), Hpy-EPPA ({[N-(2-pyridylmethyl)-3-aminopropyl][N-(2-pyridylmethyl)-2-aminoethyl](2-pyridylmethyl)amine}) and Hpy-DEPA ({bis[N-(2-pyridylmethyl)-2-aminoethyl](2-pyridylmethyl)amine}). The systematic change of the length of amino-aliphatic chains in these ligands results in chelate rings of different size: two six-membered rings for Hpy-DAPP, one five- and one six-membered rings for Hpy-EPPA, and two five-membered rings for Hpy-DEPA. The X-ray analysis of three low-spin complexes [Fe(L)](BF(4))(2) revealed similarities in their molecular and crystal structures. The magnetic measurements have shown that all synthesized complexes display spin-crossover behavior. The spin-transition temperature increases upon the change from six-membered to five-membered chelate rings, clearly demonstrating the role of the ligand strain. This effect does not depend on the nature of the counter ion. We discuss the structural features accountable for the strain effect on the spin-transition temperature.

First dicyanamide-bridged spin-crossover coordination polymer: synthesis, structural, magnetic, and spectroscopic studies.

We report here on the synthesis and characterisation of a first iron(II) spin-crossover coordination polymer with the dca spacer ligand, having the formula [Fe(aqin)2(dca)]ClO4.MeOH (aqin=8-aminoquinoline, dca=dicyanamide), which displays a two-step complete spin transition. Variable-temperature magnetic susceptibility measurements and Mössbauer spectroscopy have revealed that the two relatively gradual steps are centred at 215 and 186 K and are separated by an inflection point at about 201 K, at which 50 % of the complex molecules undergo a spin transition. The two steps are related to the existence of two crystallographically inequivalent metal sites, as confirmed by the structural and Mössbauer studies. The crystal structure was resolved at 293 K (HS form) and 130 K (LS form). Both spin-state structures belong to the triclinic P1 space group (Z=2). The complex assumes a linear chain structure, in which the active iron(II) sites are linked to each other by anionic dicyanamide ligands acting as chemical bridges. The Fe-Fe distances through the dca ligand are 8.119(1) and 7.835(1) A in the high-spin and low-spin structures, respectively. The polymeric chains extend along a (1, 0, -1) axis and are packed in sheets, between which the perchlorate anions and methanol molecules are inserted. The complex molecules are linked together by pi-stacking interactions and H-bonding between the H-donor aqin ligands and the perchlorate ions. These structural features provide a basis for cooperative interactions in the crystal lattice. Analysis of the two-step spin-crossover character in this compound suggests that covalent interactions through the spacer ligand do not provide the main mechanism of cooperativity.

Spin crossover phenomenon accompanying order-disorder phase transition in the ligand of [FeII(DAPP)(abpt)](ClO4)2 compound (DAPP = bis(3-aminopropyl)(2-pyridylmethyl)amine, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) and its successive self-grinding effect.

The spin crossover phenomenon of the recently described spin crossover complex [FeII(DAPP)(abpt)](ClO4)2 [DAPP = bis(3-aminopropyl)(2-pyridylmethyl)amine, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole] accompanying an order-disorder phase transition of the ligand was investigated by adiabatic heat capacity calorimetry, far-IR, IR, and Raman spectroscopies, and normal vibrational mode calculation. A large heat capacity peak due to the spin crossover transition was observed at T(trs) = 185.61 K. The transition enthalpy and entropy amounted to Delta(trs)H = 15.44 kJ mol-1 and Delta(trs)S = 83.74 J K-1 mol-1, respectively. The transition entropy is larger than the expected value 60.66 J K-1 mol-1, which is contributed from the spin multiplicity (R ln 5; R: the gas constant), disordering of the carbon atom of the six-membered metallocycle in the DAPP ligand, and one of the two perchlorate anions (2R ln 2), and change of the normal vibrational modes between the high-spin (HS) and low-spin (LS) states (35.75 J K-1 mol-1). The remaining entropy would be ascribed to changes of the lattice vibrations and molecular librations between the HS and LS states. Furthermore, [Fe(DAPP)(abpt)](ClO4)2 crystals disintegrated and became smaller crystallites whenever they experienced the phase transition. This may be regarded as a successive self-grinding effect, evidenced by adiabatic calorimetry, DSC, magnetic susceptibility, and microscope observation. The relationship between the crystal size and the physical quantities is discussed.

Spin crossover behavior in a family of iron(II) zigzag chain coordination polymers.

The paper reports the synthesis and detailed characterization of two new Fe(II) compounds: [Fe(pyim)(2)(bpen)](ClO(4))(2).2C(2)H(5)OH (2) and [Fe(pyim)(2)(bpe)](ClO(4))(2).C(2)H(5)OH (3) (pyim = 2-(2-pyridyl)imidazole, bpen = 1,2-bis(4-pyridyl)ethane, and bpe = 1,2-bis(4-pyridyl)ethene). Both compounds and the earlier synthesized [Fe(pyim)(2)(bpy)](ClO(4))(2).2C(2)H(5)OH (1) (bpy = 4,4'-bipyridine) form a family of one-dimensional spin crossover coordination polymers. Variable-temperature magnetic susceptibility measurements and Mössbauer spectroscopy have revealed rather gradual spin transitions centered at 176 and 198 K for 2 and 3, respectively. The fitting of magnetic properties with the regular solution model leads to the enthalpy and entropy of spin transitions and the cooperativity parameter equal to DeltaH = 12.3 kJ mol(-1), DeltaS = 68.5 J mol(-1) K(-1), Gamma = 1.80 kJ mol(-1) for 2 and DeltaH = 13.6 kJ mol(-1), DeltaS = 68.1 J mol(-1) K(-1), Gamma = 2.05 kJ mol(-1) for 3. The crystal structures of 2 and 3, resolved by X-ray diffraction at 293 K, belong to the monoclinic space group C2/c (Z = 4). Both compounds display a one-dimensional infinite zigzag-chain structure. The polymer chains are stacked into two-dimensional sheets through intermolecular pi-interactions. The crystal packing of both compounds encloses two kinds of channels in which the counter ions and ethanol molecules are inserted. The DFT calculations of binuclear fragments extracted from three polymers resulted in the energy gaps between the LS and HS states being ordered as the observed transition temperatures. The influence of bridging ligands in the studied family of compounds was found in the modulation of the energy gap between the LS and HS states, leading to different transition temperatures.

Quantum chemical study of three polymorphs of the mononuclear spin-transition complex [Fe(DPPA)(NCS)2].

The calculations of the high spin (HS) and low spin (LS) states of the [Fe(II)(DPPA)(NCS)(2)] complex have been performed at three experimentally observed geometries corresponding to three synthesized polymorphs with different spin-transition behavior. The structure optimization leads to a single molecular structure, suggesting that the existence of three geometries is not an intrinsic phenomenon but is induced by the crystal lattice. The structural difference between three forms can be reproduced by introducing the Madelung field of the crystal lattice. However, the calculations show that the differences in magnetic behavior of the three polymorphs cannot be attributed only to variations of the energy gap between two spin states.

Cooperative spin crossover and order-disorder phenomena in a mononuclear compound [Fe(DAPP)(abpt)](ClO(4))(2) [DAPP = [bis(3-aminopropyl)(2-pyridylmethyl)amine], abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole].

The synthesis and detailed characterization of the new spin crossover mononuclear complex [Fe(II)(DAPP)(abpt)](ClO(4))(2), where DAPP = [bis(3-aminopropyl)(2-pyridylmethyl)amine] and abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, are reported. Variable-temperature magnetic susceptibility measurements and Mössbauer spectroscopy have revealed the occurrence of an abrupt spin transition with a hysteresis loop. The hysteresis width derived from magnetic susceptibility measurements is 10 K, the transition being centered at T(c) downward arrow = 171 K for decreasing and T(c) upward arrow = 181 K for increasing temperatures. The crystal structure was resolved in the high-spin (293 and 183 K) and low-spin (123 K) states. Both spin-state structures belong to the monoclinic space group P2(1)/n (Z = 4). The thermal spin transition is accompanied by the shortening of the mean Fe-N distances by 0.177 A. The two main structural characteristics of [Fe(DAPP)(abpt)](ClO(4))(2) are a branched network of intermolecular links in the crystal lattice and the occurrence of two types of order-disorder transitions (in the DAPP ligand and in the perchlorate anions) accompanying the thermal spin change. These features are discussed relative to the magnetic properties of the complex. The electronic structure calculations show that the structural disorder in the DAPP ligand modulates the energy gap between the HS and LS states. In line with previous studies, the order-disorder phenomena and the spin transition in [Fe(DAPP)(abpt)](ClO(4))(2) are found to be interrelated.

Spin Transition in [Fe(DPEA)(NCS)(2)], a Compound with the New Tetradentate Ligand (2-Aminoethyl)bis(2-pyridylmethyl)amine (DPEA): Crystal Structure, Magnetic Properties, and Mössbauer Spectroscopy.

The new spin transition compound [Fe(II)(DPEA)(NCS)(2)], where DPEA [(2-aminoethyl)bis(2-pyridylmethyl)amine] is a new tetradentate ligand, has been synthesized, and its structure, magnetic properties, and Mössbauer spectra have been investigated. The crystal structure has been determined by X-ray diffraction at 298 K. The compound crystallizes in the monoclinic system, space group is P2(1)/c, with Z = 4,a = 9.358(1) Å, b = 11.812(2) Å, c = 17.135(2) Å, and beta = 94.5(4) degrees. The distorted [FeN(6)] octahedron is formed from four nitrogen atoms belonging to DPEA and two provided by the cis thiocyanate groups. The two pyridine rings of DPEA are in mer positions. Each molecule is linked to its neighbors by hydrogen-bonding interactions as well as by numerous van der Waals contacts supposed to be responsible for the cooperativity of the system. Variable-temperature magnetic susceptibility measurements (20-290 K) have evidenced a relatively abrupt S = 2 right harpoon over left harpoon S = 0 transition centered at T(1/2) = 138 K. The thermal variation of the high spin state fraction observed by Mössbauer spectroscopy is in agreement with that obtained from magnetic susceptibility measurements. The fitting of Mössbauer and magnetic data with the Ising-like model allowed us to determine the energy gap between the high-spin and low-spin states (Delta(eff) = 835 K) and to estimate the variation of the thermodynamic parameters upon spin transition. The calculated variations of enthalpy (DeltaH = 6.76 kJ mol(-)(1)) and entropy (DeltaS = 49 J mol(-)(1) K(-)(1)) associated with the spin transition are in agreement with those previously observed for iron(II) spin-crossover compounds. The spin conversion is found to be close to a first-order phenomenon.

Polymorphism in Spin Transition Systems. Crystal Structure, Magnetic Properties, and Mössbauer Spectroscopy of Three Polymorphic Modifications of [Fe(DPPA)(NCS)(2)] [DPPA = (3-Aminopropyl)bis(2-pyridylmethyl)amine].

Three polymorphic modifications A-C of [Fe(II)(DPPA)(NCS)(2)], where DPPA = (3-aminopropyl)bis(2-pyridylmethyl)amine is a new tetradentate ligand, have been synthesized, and their structures, magnetic properties, and Mössbauer spectra have been investigated. For polymorph A, variable-temperature magnetic susceptibility measurements as well as Mössbauer spectroscopy have revealed the occurrence of a rather gradual HS if LS transition without hysteresis, centered at about 176 K. The same methods have shown that polymorph B is paramagnetic over the temperature range 4.5-295 K, whereas polymorph C exhibits a very abrupt S = 2 if S = 0 transition with a hysteresis. The hysteresis width is 8 K, the transitions being centered at T(c) downward arrow = 112 K for decreasing and T(c) upward arrow = 120 K for increasing temperatures. The crystal structures of the three polymorphs have been solved by X-ray diffraction at 298 K. Polymorph A is triclinic, space group P&onemacr; with Z = 2, a = 8.710(2) Å, b = 15.645(2) Å, c = 7.985(1) Å, alpha = 101.57(1) degrees, beta = 112.59(2) degrees, and gamma = 82.68(2) degrees. Polymorph B is monoclinic, space group P2(1)/c with Z = 4, a = 8.936(2) Å, b = 16.855(4) Å, c = 13.645(3) Å, and beta = 97.78(2) degrees. Polymorph C is orthorhombic, space group Pbca with Z = 8, a = 8.449(2) Å, b = 14.239(2) Å, and c = 33.463(5) Å. In the three polymorphs, the asymmetric units are almost identical and consist of one chiral complex molecule with the same configuration and conformation. The distorted [FeN(6)] octahedron is formed by four nitrogen atoms belonging to DPPA and two provided by the cis thiocyanate groups. The two pyridine rings of DPPA are in fac positions. The main differences between the structures of the three polymorphs are found in their crystal packing. The stabilization of the high-spin ground state of polymorph B is tentatively explained by the presence of two centers of steric strain in the crystal lattice resulting in the elongation of the Fe-N(aromatic) distance. The observed hysteresis in polymorph C seems to be due to the existence of an array of intermolecular contacts in the crystal lattice making the spin transition more cooperative than in polymorph A.