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Complexes [Fe(Hhbi)(2)(NO(3))].2EtOH (1.2EtOH) and [Fe(2)(mu-OH)(2)(Hhbi)(4)].2H(2)O.8EtOH (2.2H(2)O.8EtOH) crystallize in the orthorhombic Fdd2 and P4(2)2(1)2 space groups, respectively (Hhbi(-) = the monoanion of 2-(2'-hydroxyphenyl benzimidazole). Complex 1 exhibits paramagnetic relaxation as evidenced by Mossbauer spectroscopy, and significant axial zero-field splitting (1.5 cm(1)

A family of spin-crossover (SC) complexes, [Fe(II)H(3)L(Me)]Cl.X (X(-) = PF(6) (-), AsF(6) (-), SbF(6) (-), CF(3)SO(3) (-)), 1-4, has been synthesized, in which H(3)L(Me) denotes the hexadentate N(6) tripod-like ligand tris[2-{[(2-methylimidazol-4-yl)methylidene]amino}ethyl]amine, containing three imidazole groups, with a view to establishing the effect of the counter anion on the SC behavior. These complexes have been found to crystallize in the same monoclinic crystal system with similar cell dimensions. The general crystal structure consists of a two-dimensional (2D) extended network constructed by NH...Cl- hydrogen bonds between Cl- and the imidazole NH groups of three neighboring [Fe(II)H(3)L(Me)]2+ ions, while the anion X exists as an isolated counter anion and occupies the space between the 2D sheets. Magnetic susceptibilities and Mössbauer spectra have revealed a variety of SC behaviors depending on the counter anion, including a one-step HS<==>(HS + LS)/2 (1, X = PF(6) (-)), a two-step HS<==>(HS + LS)/2<==>LS with a slow thermal relaxation (2, X = AsF(6) (-)), a gradual one-step HS<==>LS (3, X = SbF(6) (-)), and a steep one-step HS<==>LS with hysteresis (4, X = CF(3)SO(3) (-)). The complexes assume the space group P2(1)/n in the HS state, P2(1) in the HS + LS state, and P2(1)/n in the LS state. The Fe-N bond lengths and the N-Fe-N bond angles are indicative of the HS, HS + LS, and LS states. The molecular volumes, V, of the counter anions have been evaluated by quantum-chemical calculations as follows: 53.4 A(3) (BF(4) (-)), 54.4 A(3) (ClO(4) (-)), 73.0 A(3) (PF(6) (-)), 78.5 A(3) (AsF(6) (-)), 88.7 A(3) (SbF(6) (-)), and 86.9 A(3) (CF(3)SO(3) (-)). The size and shape of the counter anion affects the flexible 2D network structure constructed by the hydrogen bonds, leading to modifications of the SC behavior. These estimated relative sizes of the counter anions correlate well with the observed SC behaviors.

Three FeII complexes, [Fe(HLR)2](BPh4)2.solvent (R=H, Me, Ph), were synthesized, where BPh4-=tetraphenylborate and HLR=2-substituted-imidazol-4-yl-methylideneamino-2-ethylpyridine. The magnetic susceptibility measurements in 5-300 K revealed that [Fe(HLH)2](BPh4)2.H2O, [Fe(HLMe)2](BPh4)2.2CH3CN, and [Fe(HLPh)2](BPh4)2.CH3CN are low-spin (LS), spin-crossover (SC), and high-spin (HS) FeII complexes, respectively, indicating that the spin state can be effectively tuned by the bulkiness of the substituent. Complex shows a steep SC around 250 K, where it assumes a cyclic structure of {[Fe(HLMe)2]BPh4}2 constructed by four NH/pi bonds between the imidazole group and the phenyl ring of BPh4- in the HS state and a deformed structure with NH/pi bonds and linear CH3CN...HN hydrogen bonds at the terminals in the LS state.

The tetranuclear complexes...have been prepared, and their structure and magnetic properties have been studied (pypentOH = 1,5-bis[(2-pyridylmethyl)amino]pentan-3-ol, pymH = 2-pyridylmethanol). The X-ray diffraction analysis of...showed a Fe4O4 cubane-like arrangement of four iron(II) atoms, four 73-O bridging ligands, one (1) or two (2) synmsyn bridging acetates. The X-ray diffraction analysis...showed a zigzag bis-dinuclear arrangement of four iron(II) cations, two 72-O bridging pypentO ligands, four 72-N-cyanato bridging ligands (3) or four end-on azido bridging ligands (4):u they are the first examples of cyanato and azido bridged discrete polynuclear ferrous compounds, respectively. The Mossbauer spectra of 1 are consistent with four different high-spin iron(II) sites in the Fe4O4 cubane-type structure. The Mossbauer spectra of 3 are consistent with two high-spin iron(II) sites (N5O and N4O). Below 190 K, the Mossbauer spectra of 4 are consistent with one N5O and two N4O high-spin iron(II) sites. The temperature dependence of the magnetic susceptibility was fitted with...The differences in sign among the J1, J2, and J3 super-exchange interactions indicate that the faces including only 73-OR bridges exhibit ferromagnetic interactions. The nature of the ground state in 1m3 is confirmed by simulation of the magnetization curves at 2 and 5 K. In the bis-dinuclear iron(II) compounds 3 and 4, the J2 interaction resulting from the bridging of two Fe2(pypentO)X3 units through two pseudo-halide anions is ferromagnetic in 3 (X = 72-N-cyanato) and may be either ferro- or antiferromagnetic in 4 (X = end-on azido). The J1 interaction through the central Oalkoxo and pseudo-halide bridges inside the dinuclear units is ferromagnetic in 3 (X = 72-N-cyanato) and antiferromagnetic in 4 (X = end-on azido). In agreement with the symmetry of the two FeII sites in complexes 3 and 4, D (pentacoordinated sites) is larger than D' (octahedral sites).