Scan Rate Dependent Spin Crossover Iron(II) Complex with Two Different Relaxations and Thermal Hysteresis fac-[Fe(II)(HL(n-Pr))3]Cl·PF6 (HL(n-Pr) = 2-Methylimidazol-4-yl-methylideneamino-n-propyl).

Solvent-free spin crossover Fe(II) complex fac-[Fe(II)(HL(n-Pr))3]Cl·PF6 was prepared, where HL(n-Pr) denotes 2-methylimidazol-4-yl-methylideneamino-n-propyl. The magnetic susceptibility measurements at scan rate of 0.5 K min(-1) showed two successive spin transition processes consisting of the first spin transition T1 centered at 122 K (T1↑ = 127.1 K, T1↓ = 115.8 K) and the second spin transition T2 centered at ca. 105 K (T2↑ = 115.8 K, T2↓ = 97.2 K). The magnetic susceptibility measurements at the scan rate of 2.0, 1.0, 0.5, 0.25, and 0.1 K min(-1) showed two scan speed dependent spin transitions, while the Mössbauer spectra detected only the first spin transition T1. The crystal structures were determined at 160, 143, 120, 110, 95 K in the cooling mode, and 110, 120, and 130 K in the warming mode so as to follow the spin transition process of high-spin HS → HS(T1) → HS(T2) → low-spin LS → LS(T2) → LS(T1) → HS. The crystal structures at all temperatures have a triclinic space group P1̅ with Z = 2. The complex-cation has an octahedral N6 coordination geometry with three bidentate ligands and assume a facial-isomer with Δ- and Λ-enantimorphs. Three imidazole groups of fac-[Fe(II)(HL(n-Pr))3](2+) are hydrogen-bonded to three Cl(-) ions. The 3:3 NH(imidazole)···Cl(-) hydrogen-bonds form a stepwise ladder assembly structure, which is maintained during the spin transition process. The spin transition process is related to the structural changes of the FeN6 coordination environment, the order-disorder of PF6(-) anion, and the conformation change of n-propyl groups. The Fe-N bond distance in the HS state is longer by 0.2 Å than that in the LS state. Disorder of PF6(-) anion is not observed in the LS state but in the HS state. The conformational changes of n-propyl groups are found in the spin transition processes except for HS → HS(T1) → HS(T2).

Syntheses, structures, and magnetic properties of acetato- and diphenolato-bridged 3d-4f binuclear complexes [M(3-MeOsaltn)(MeOH)x(ac)Ln(hfac)2] (M = Zn(II), Cu(II), Ni(II), Co(II); Ln = La(III), Gd(III), Tb(III), Dy(III); 3-MeOsaltn = N,N'-bis(3-methoxy-2-oxybenzylidene)-1,3-propanediaminato; ac = acetato; hfac = hexafluoroacetylacetonato; x = 0 or 1).

A series of 3d-4f binuclear complexes, [M(3-MeOsaltn)(MeOH)x(ac)Ln(hfac)2] (x = 0 for M = Cu(II), Zn(II); x = 1 for M = Co(II), Ni(II); Ln = Gd(III), Tb(III), Dy(III), La(III)), have been synthesized and characterized, where 3-MeOsaltn, ac, and hfac denote N,N'-bis(3-methoxy-2-oxybenzylidene)-1,3-propanediaminato, acetato, and hexafluoroacetylacetonato, respectively. The X-ray analyses demonstrated that all the complexes have an acetato- and diphenolato-bridged M(II)-Ln(III) binuclear structure. The Cu(II)-Ln(III) and Zn(II)-Ln(III) complexes are crystallized in an isomorphous triclinic space group P1, where the Cu(II) or Zn(II) ion has square pyramidal coordination geometry with N2O2 donor atoms of 3-MeOsaltn at the equatorial coordination sites and one oxygen atom of the bridging acetato ion at the axial site. The Co(II)-Ln(III) and Ni(II)-Ln(III) complexes are crystallized in an isomorphous monoclinic space group P2(1)/c, where the Co(II) or Ni(II) ion at the high-spin state has an octahedral coordination environment with N2O2 donor atoms of 3-MeOsaltn at the equatorial sites, and one oxygen atom of the bridged acetato and a methanol oxygen atom at the two axial sites. Each Ln(III) ion for all the complexes is coordinated by four oxygen atoms of two phenolato and two methoxy oxygen atoms of "ligand-complex" M(3-MeOsaltn), four oxygen atoms of two hfac(-), and one oxygen atom of the bridging acetato ion; thus, the coordination number is nine. The temperature dependent magnetic susceptibilities from 1.9 to 300 K and the field-dependent magnetization up to 5 T at 1.9 K were measured. Due to the important orbital contributions of the Ln(III) (Tb(III), Dy(III)) and to a lesser extent the M(II) (Ni(II), Co(II)) components, the magnetic interaction between M(II) and Ln(III) ions were investigated by an empirical approach based on a comparison of the magnetic properties of the M(II)-Ln(III), Zn(II)-Ln(III), and M(II)-La(III) complexes. The differences of χ(M)T and M(H) values for the M(II)-Ln(III), Zn(II)-Ln(III) and those for the M(II)-La(III) complexes, that is, Δ(T) = (χ(M)T)(MLn) - (χ(M)T)(ZnLn) - (χ(M)T)(MLa) = J(MLn)(T) and Δ(H) = M(MLn)(H) - M(ZnLn)(H) - M(MLa)(H) = J(MLn)(H), give the information of 3d-4f magnetic interaction. The magnetic interactions are ferromagnetic if M(II) = (Cu(II), Ni(II), and Co(II)) and Ln = (Gd(III), Tb(III), and Dy(III)). The magnitudes of the ferromagnetic interaction, J(MLn)(T) and J(MLn)(H), are in the order Cu(II)-Gd(III) > Cu(II)-Dy(III) > Cu(II)-Tb(III), while those are in the order of M(II)-Gd(III) ≈ M(II)-Tb(III) > M(II)-Dy(III) for M(II) = Ni(II) and Co(II). Alternating current (ac) susceptibility measurements demonstrated that the Ni(II)-Tb(III) and Co(II)-Tb(III) complexes showed out-of-phase signal with frequency-dependence and the Ni(II)-Dy(III) and Co(II)-Dy(III) complexes showed small frequency-dependence. The energy barrier for the spin flipping was estimated from the Arrhenius plot to be 14.9(6) and 17.0(4) K for the Ni(II)-Tb(III) and Co(II)-Tb(III) complexes, respectively, under a dc bias field of 1000 Oe.

Carbonato-bridged Ni(II)2Ln(III)2 (Ln(III) = Gd(III), Tb(III), Dy(III)) complexes generated by atmospheric CO2 fixation and their single-molecule-magnet behavior: [(µ4-CO3)2{Ni(II)(3-MeOsaltn)(MeOH or H2O)Ln(III)(NO3)}2]·solvent [3-MeOsaltn = N,N'-bis(3-methoxy-2-oxybenzylidene)-1,3-propanediaminato].

Atmospheric CO2 fixation of [Ni(II)(3-MeOsaltn)(H2O)2]·2.5H2O [3-MeOsaltn = N,N'-bis(3-methoxy-2-oxybenzylidene)-1,3-propanediaminato], Ln(III)(NO3)3·6H2O, and triethylamine occurred in methanol/acetone, giving a first series of carbonato-bridged Ni(II)2Ln(III)2 complexes [(µ4-CO3)2{Ni(II)(3-MeOsaltn)(MeOH)Ln(III)(NO3)}2] (1Gd, 1Tb, and 1Dy). When the reaction was carried out in acetonitrile/water, it gave a second series of complexes [(µ4-CO3)2{Ni(II)(3-MeOsaltn)(H2O)Ln(III)(NO3)}2]·2CH3CN·2H2O (2Gd, 2Tb, and 2Dy). For both series, each Ni(II)2Ln(III)2 structure can be described as two di-µ-phenoxo-bridged Ni(II)Ln(III) binuclear units bridged by two carbonato CO3(2-) units to form a carbonato-bridged (µ4-CO3)2{Ni(II)2Ln(III)2} structure. The high-spin Ni(II) ion has octahedral coordination geometry, and the Ln(III) ion is coordinated by O9 donor atoms from Ni(II)(3-MeOsaltn), bidentate NO3(-), and one and two oxygen atoms of two CO3(2-) ions. The NO3(-) ion for the first series roughly lie on Ln-O(methoxy) bonds and are tilted toward the outside, while for the second series, the two oxygen atoms roughly lie on one of the Ln-O(phenoxy) bonds due to the intramolecular hydrogen bond. The temperature-dependent magnetic susceptibilities indicated a ferromagnetic interaction between the Ni(II) and Ln(III) ions (Ln(III) = Gd(III), Tb(III), Dy(III)) for all of the complexes, with a distinctly different magnetic behavior between the two series in the lowest-temperature region due to the Ln(III)-Ln(III) magnetic interaction and/or different magnetic anisotropies of the Tb(III) or Dy(III) ion. Alternating-current susceptibility measurements under the 0 and 1000 Oe direct-current (dc) bias fields showed no magnetic relaxation for the Ni(II)2Gd(III)2 complexes but exhibited an out-of-phase signal for Ni(II)2Tb(III)2 and Ni(II)2Dy(III)2, indicative of slow relaxation of magnetization. The energy barriers, Δ/kB, for the spin flipping were estimated from the Arrhenius plot to be 12.2(7) and 6.1(3) K for 1Tb and 2Tb, respectively, and 18.1(6) and 14.5(4) K for 1Dy and 2Dy, respectively, under a dc bias field of 1000 Oe. Compound 1Dy showed relatively slow relaxation of magnetization reorientation even at zero dc applied field with Δ/kB = 6.6(4) K.

A hydrogen bond motif giving a variety of supramolecular assembly structures and spin-crossover behaviors.

A series of spin-crossover (SCO) iron(II) compounds, fac-[Fe(II)(HL(R))(3)]Cl·PF(6) [R = methyl (Me, 1), ethyl (Et, 2), n-propyl (n-Pr, 3), n-butyl (n-Bu, 4), and n-pentyl (n-Pen, 5)], were synthesized, where HL(R) denotes a series of [(2-methylimidazol-4-yl)methylidene]monoalkylamines. The cations fac-[Fe(II)(HL(R))(3)](2+) and chloride anions associate through 3:3 imidazole···chloride hydrogen bonding. This hydrogen-bonding motif gives rise to a variety of assembly structures consisting of a one-dimensional ladder for 3 and 4, two kinds of two-dimensional networks for 1 and 2, and a cubane-like structure for 5. The compounds exhibit various types of SCO transitions between high-spin (S = 2) and low-spin (S = 0) states as a result of their intermolecular interactions.

1D and 2D assembly structures by imidazole···chloride hydrogen bonds of iron(II) complexes [Fe(II)(HL(n-Pr))3]Cl·Y (HL(n-Pr) = 2-methylimidazol-4-yl-methylideneamino-n-propyl; Y = AsF6, BF4) and their spin states.

Two Fe(II) complexes fac-[Fe(II)(HL(n-Pr))(3)]Cl·Y (Y = AsF(6) (1) and BF(4) (2)) were synthesized, where HL(n-Pr) is 2-methylimidazole-4-yl-methylideneamino-n-propyl. Each complex-cation has the same octahedral N(6) geometry coordinated by three bidentate ligands and assumes facial-isomerism, fac-[Fe(II)(HL(n-Pr))(3)](2+) with Δ- and Λ-enantiomorphs. Three imidazole groups per Δ- or Λ-fac-[Fe(II)(HL(n-Pr))(3)](2+) are hydrogen-bonded to three Cl(-) ions or, from the viewpoint of the Cl(-) ion, one Cl(-) ion is hydrogen-bonded to three neighbouring fac-[Fe(II)(HL(n-Pr))(3)](2+) cations. The 3 : 3 NH···Cl(-) hydrogen bonds between Δ- or Λ-fac-[Fe(II)(HL(n-Pr))(3)](2+) and Cl(-) generate two kinds of assembly structures. The directions of the 3 : 3 NH···Cl(-) hydrogen bonds and hence the resulting assembly structures are determined by the size of the anion Y, though Y is not involved into the network structure and just accommodated in the cavity. Compound 1 has a 1D ladder structure giving a larger cavity, in which the Δ- and Λ-fac-[Fe(II)(HL(n-Pr))(3)](2+) enantiomorphs are bridged by two NH···Cl(-) hydrogen bonds. Compound 2 has a 2D network structure with a net unit of a cyclic trimer of {fac-[Fe(II)(HL(n-Pr))(3)](2+)···Cl(-)}(3) giving a smaller cavity, in which Δ- or Λ-fac-[Fe(II)(HL(n-Pr))(3)](2+) species with the same chirality are linked by NH···Cl(-) hydrogen bonds to give a homochiral 2D network structure. Magnetic susceptibility and Mössbauer spectral measurements demonstrated that compound 1 showed an abrupt one-step spin crossover with 4.0 K thermal hysteresis of T(c↓) = 125.5 K and T(c↑) = 129.5 K and compound 2 showed no spin transition and stayed in the high-spin state over the 5-300 K temperature range.

One-dimensional spin-crossover Iron(II) complexes bridged by intermolecular imidazole-pyridine NH...N hydrogen bonds, [Fe(HL(Me))(3)]X(2) (HL(Me) = (2-methylimidazol-4-yl-methylideneamino-2-ethylpyridine; X = PF(6), ClO(4), BF(4)).

2-Methylimidazol-4-yl-methylideneamino-2-ethylpyridine (abbreviated as HL(Me)) is the 1:1 condensation product of 2-methyl-4-formylimidazole and 2-aminoethylpyridine and functions as a bidentate ligand to the iron(II) ion to produce the 3:1 complexes together with anions, [Fe(HL(Me))(3)]X(2) (X = PF(6) (1), ClO(4) (2), BF(4) (3)). The magnetic susceptibilities, differential scanning calorimetric measurements, and Mossbauer spectral measurements demonstrated that complexes 1, 2, and 3 showed a steep one-step spin crossover (SCO) between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states with small thermal hysteresis. Three complexes have an isomorphous structure and are crystallized in the same monoclinic space group, C2/c, both in the HS and LS states. The iron(II) ion has the octahedral coordination geometry of a facial isomer with N(6) donor atoms of three bidentate ligands, in which an imidazole and an imine nitrogen atom per ligand participate in the formation of the coordination bond, but the pyridine nitrogen is free from coordination. The complex cation fac-[Fe(HL(Me))(3)](2+) is a chiral species with a Delta or Lambda isomer, and the adjacent Delta and Lambda isomers are linked alternately by an intermolecular imidazole-pyridine NH...N hydrogen bond to produce an achiral 1D chain. The two remaining imidazole moieties per complex are hydrogen-bonded to the anions that occupy the space among the chains. The SCO profile becomes steeper with the decrease of the anion size (73.0 A(3) for PF(6)(-), 54.4 A(3) for ClO(4)(-), and 53.4 A(3) for BF(4)(-)). The SCO transition temperature T(1/2) of the PF(6) (1), ClO(4) (2), and BF(4) (3) salts estimated from the magnetic susceptibility measurements are (T( downward arrow) = 151.8 K, T( upward arrow) = 155.3 K), (T( downward arrow) = 184.5 K, T( upward arrow) = 186.0 K), and (T( downward arrow) = 146.4 K, T( upward arrow) = 148.2 K), respectively, indicating that the T(1/2) value is not in accord with the anion size.

One-step and two-step spin-crossover iron(II) complexes of ((2-methylimidazol-4-yl)methylidene)histamine.

A tridentate ligand ((2-methylimidazol-4-yl)methylidene)histamine (abbreviated as H(2)L(2-Me)), that is, the 1:1 condensation product of 2-methyl-4-formylimidazole and histamine, was used for the syntheses of a new family of iron(II) spin-crossover (SCO) complexes with the general chemical formulas [Fe(H(2)L(2-Me))(2)]X(2) x solvent (X = Cl, ClO(4), and BPh(4); solvent = 2-PrOH and CH(3)CN) and [Fe(H(2)L(2-Me))(2)]X x Y x solvent (X = Cl and Br; Y = ClO(4), BF(4), PF(6), and AsF(6); solvent = EtOH and 2-PrOH). The complex cation [Fe(H(2)L(2-Me))(2)](2+) is a chiral species due to an octahedral coordination of two unsymmetrical tridentate ligands, has a ligand field strength around the spin-crossover point, and is hydrogen-bonded to anions to form a variety of network structures. The dichloride complexes [Fe(H(2)L(2-Me))(2)]Cl(2) x 2-PrOH x 0.5 H(2)O (1) and [Fe(H(2)L(2-Me))(2)]Cl(2) x 2-PrOH x H(2)O (1') have a one-dimensional (1D) structure, in which adjacent two chiral complex-cations are doubly bridged by two Cl(-) ions through NH(histamine)...Cl(-)...HN(2-methyl-4-formylimidazole) hydrogen bonds to give a chiral 1D rod. The chiral rods with the same chirality are stacked in the crystal lattices to give a conglomerate, 1, and those with the opposite chiralities are stacked to give a racemic compound, 1'. The enantiomeric circular dichromism spectra of 1 gave definitive evidence of the conglomerate. Compound 1 showed a two-step SCO, while the desolvated sample showed a steep one-step SCO at T(1/2) = 180 K. A series of complexes, [Fe(H(2)L(2-Me))(2)]Cl x X x EtOH (X = ClO(4) (2a), BF(4) (2b), PF(6) (2c), and AsF(6) (2d)), [Fe(H(2)L(2-Me))(2)]Cl x ClO(4) x 0.5(1-PrOH) x 1.5 H(2)O (2a'), and [Fe(H(2)L(2-Me))(2)]Br x ClO(4) x 0.5 EtOH (2a''), display an isomorphous two-dimensional (2D) network at room temperature (296 K), in which the structure is constructed by the NH...Cl(-) (or Br(-)) hydrogen bonds between the imidazole NH groups of [Fe(H(2)L(2-Me))(2)](2+) and the Cl(-) (or Br(-)) ion as a connector. The complexes showed a variety of SCO properties depending on the anion, solvent molecule, and the kind of bridging halogen ion. The complexes of ClO(4)(-) (2a, 2a', 2a'') and BF(4)(-) (2b) with smaller anions showed a two-step SCO with a wide temperature region of the intermediate state of (high-spin (HS) + low-spin (LS))/2 state, their values of (T(1/2,1), T(1/2,2)) being (75, 255 K), (100, 220 K), (110, 220 K), and (100, 260 K), respectively, where the crystal changes from monoclinic P2(1)/n in the HS state to triclinic P1 in the intermediate state. The complexes of PF(6)(-) (2c) and AsF(6)(-) (2d) with larger anions showed a one-step SCO at T(1/2) = 198 and 173 K, respectively, in which the crystal system and space group showed no change during the spin transition. The crystal solvent and halide ion also affected the completeness of the SCO in the lower-temperature region and the steepness of the SCO profile. The experimental results were correlated to the theoretical results based on an Ising-like model. [Fe(H(2)L(2-Me))(2)](BPh(4))(2) x CH(3)CN (3) has no network structure. [Fe(H(2)L(2-Me))(2)](ClO(4))(2) (4) assumes a chiral 3D network structure constructed by the hydrogen bonds between the imidazole groups of one enantiomorph [Fe(H(2)L(2-Me))(2)](2+) and the bridging ClO(4)(-) ion. Compounds 3 and 4 in the solid states are in the HS state, demonstrating that the formation of imidazole-Cl(-) or Br-hydrogen bonds can give SCO properties, but the hydrogen bond of imidazole-ClO(4)(-) cannot give SCO.