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 H2L2-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(H2L2-Me)(2)]X-2 center dot solvent (X = Cl, ClO4, and BPh4; solvent = 2-PrOH and CH3CN) and [Fe(H2L2-Me)(2)] X center dot Y center dot solvent (X = Cl and Br; Y = ClO4, BF4, PF6, and AsF6; solvent = EtOH and 2-PrOH). The complex cation [Fe(H2L2-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(H2L2-Me)(2)]Cl-2 center dot 2-PrOH center dot 0.5H(2)O (1) and [Fe(H2L2-Me)(2)]Cl-2 center dot 2-PrOH center dot H2O (1') have a one-dimensional (1D) structure, in which adjacent two chiral complex-cations are doubly bridged by two Cl- ions through NH(histamine) center dot center dot center dot Cl- center dot center dot center dot 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(H2L2-Me)(2)]Cl center dot X center dot EtOH(X = ClO4 (2a), BF4 (2b), PF6 (2c), and AsF6 (2d)), [Fe(H2L2-Me)(2)]Cl center dot ClO4 center dot 0.5(1-PrOH) center dot 1.5H(2)O (2a'), and [Fe(H2L2-Me)(2)]Br center dot ClO4 center dot 0.5EtOH (2a ''), display an isomorphous two-dimensional (2D) network at room temperature (296 K), in which the structure is constructed by the NH center dot center dot center dot Cl- (or Br-) hydrogen bonds between the imidazole NH groups of [Fe(H2L2-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 ClO4- (2a, 2a', 2a '') and BF4- (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,T-1, T-1/2,T-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 P (1) over bar in the intermediate stale. The complexes of PF6- (2c) and AsF6- (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 solven 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(H2L2-Me)(2)](BPh4)(2) center dot CH3CN (3) has no network structure. [Fe(H2L2-Me)(2)](ClO4)(2) (4) assumes a chiral 3D network structure constructed by the hydrogen bonds between the imidazole groups of one enantiomorph [Fe(H2L2-Me)(2)](2+) and the bridging ClO4- 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-ClO4- cannot give SCO.