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.

Eight new crystalline α-pyrrolidinonate-bridged homo- and mixed-valence cis-diammineplatinum dimers and tetramers, HT-[Pt(2.0+)2(NH3)4(μ-C4H6NO)2](ClO4)2 (2), HH-[Pt(2.25+)2(NH3)4(μ-C4H6NO)2]2(ClO4)5 (4), HH-[Pt(2.25+)2(NH3)4(μ-C4H6NO)2]2(PF6)3(NO3)2 (5), {HH-[Pt(2.25+)2(NH3)4(μ-C4H6NO)2]2}{HH-[Pt(2.5+)2(NH3)4(μ-C4H6NO)2(NO3)]2}(PF6)2(NO3)7·6H2O (7), HH-[Pt(2.5+)4(NH3)8(μ-C4H6NO)4(Cl)](ClO4)3Cl2 (9), HH-[Pt(3.0+)2(NH3)4(μ-C4H6NO)2(Cl)2](NO3)2 (11), HH-[Pt(3.0+)2(NH3)4(μ-C4H6NO)2(Cl)(NO3)](NO3)2·H2O (12), and HT-[(H2O)(H3N)2Pt(3.0+)(μ-C4H6NO)2Pt(3.0+)(NH3)(μ-OH)]2(NO3)6·4H2O (14), have been isolated and characterized by X-ray diffraction, where HH and HT correspond to the head-to-head and head-to-tail isomers, respectively. The first spontaneous resolution has been observed for the chiral HT-Pt(2.0+)2 molecules (2). 7 has turned out to be the first 1:1 “stoichiometric” mixture of blue and tan (Pt(2.25+)4 and Pt(2.5+)4). The structural evidence supporting the axial coordination behaviors of the Pt(2.5+)4 tetramer has been obtained (7 and 9). A very unique quasi-one-dimensional halide-bridged system having a chain definition of [Pt(2.5+)4−Cl···]∞ has been characterized (9). The HT-Pt(3.0+)2 dimer has been found to release an NH3 group in aqueous media to form a dimer of dimers linked by μ-OH bridges (14). Relationships between the structural parameters and the average Pt oxidation state have been examined in detail to better understand the structure-transformation properties of the platinum-blue family. Remarkable structural differences observed between the pentacyclic α-pyrrolidinonate and the hexacyclic α-pyridonate systems are rationally interpreted in terms of differences in both the N−C−O bite angle and the Pt−N−C angle around the bridging geometry. In addition, spectrophotometric studies reveal that an outer-sphere electron-transfer process between two Pt(2.25+)4 molecules takes place to afford a Pt(2.0+)4 and a Pt(2.5+)4 molecule. General consideration for the solution properties of this family is also described. The synthesis of an α-pyridonate Pt(2.5+)4 complex is also reported.

The phenomenon of spin-crossover between the low-spin (LS) and high-spin (HS) states, [1] which is observed in some octahedral 3d n (4 n 7) metal complexes and is induced by an external perturbation, such as temperature, pressure, or light irradiation, is one of the most spectacular examples of molecular bistability. [2, 3] Such phenomena can be utilized in new electronic devices, such as molecular memories and switches that are controlled by many different physical channels. [4,5] It is now established that cooperative spincrossover, the interaction between spin-crossover sites, is the predominant factor governing bistability. [6,7] We now report the first homochiral mixed-valence spin-crossover compound, in which three independent properties are united, namely, mixed valence states of iron metal centers, spin-crossover of the Fe II

The [Fe(II)(H(3)L)](BF(4))(2).3H(2)O (1) complex was synthesized, where H(3)L (tris[[2-[(imidazole-4-yl)methylidene]amino]ethyl]amine) is a tripodal ligand obtained by condensation of tris(2-aminoethyl)amine and 4-formylimidazole (fim) in a 1:3 molar ratio. Starting from 1, a series of complexes, [Fe(II)(H(1.5)L)](BF(4))(0.5) (2) (=[Fe(II)(H(3)L)][Fe(II)(L)]BF(4)), [Fe(H(1.5)L)]BF(4) (3) (=[Fe(II)(H(3)L)][Fe(III)(L)](BF(4))(2)), [Fe(III)(H(3)L)](BF(4))(3).fim.H(2)O (4), and [Fe(III)(L)].2.5H(2)O (5), has been synthesized and characterized. The single-crystal X-ray structure of each complex has been determined. The Fe(II) compound, 2, and a mixed valence Fe(II)-Fe(III) compound, 3, involve formally hemi-deprotonated ligands, H(1.5)L. The structure of 3 consists of a homochiral two-dimensional assembled sheet, arising from the intermolecular hydrogen bonds between [Fe(II)(H(3)L)](2+) and [Fe(III)(L)](0) (3). All but 5 exhibit spin crossover between low-spin (LS) and high-spin (HS) states. This is a rare case where both Fe(II) and Fe(III) complexes containing the same ligand exhibit spin-crossover behavior. Magnetic susceptibility and Mössbauer studies showed that 3 has three accessible electronic states: LS Fe(II)-LS Fe(III), HS Fe(II)-LS Fe(III), and HS Fe(II)-HS Fe(III). Compounds 1-3 show the light-induced excited spin-state trapping effect at the Fe(II) sites upon irradiation with green light. The solution magnetic properties, electronic spectra, and electrochemical properties of 1, 4, and 5 were also studied.