Anion Effects on Spin Crossover Systems: From Supramolecular Chemistry to Magnetism.

The field of anion supramolecular chemistry has received more and more attention in recent years. Anions with diverse types and geometries have been widely used for the synthesis of ionic spin crossover (SCO) complexes. This review is devoted to anion effects on the molecular, supramolecular structures and magnetic properties of discrete SCO compounds. Firstly, typical anions used in the synthesis of these compounds are briefly summarized according to their various geometries. This is followed by a collection of representative examples of anion-based SCO compounds, whose SCO properties are analyzed in terms of supramolecular interactions, geometry and charge of anions. In the third part, anion effects on SCO complexes of different kinds of metal centers and ligands are outlined and finally remarks on the synthesis new type of ionic SCO complexes in the future are described.

The dynamic interplay between intramolecular and intermolecular interactions in mononuclear manganese(III) SCO complexes.

The effect of counter anions on thermally induced manganese(iii)-based SCO within the [Mn(5-F-sal-N-1,5,8,12)]Y family has been investigated. All the complexes are crystallized without any lattice solvents. Crystal packing and intermolecular forces influence the spin-state stabilization and spin-transition profiles. Magnetic measurements indicate that salts with octahedral anions, [Mn(5-F-sal-N-1,5,8,12)]PF6 (1), [Mn(5-F-sal-N-1,5,8,12)]AsF6 (2) and [Mn(5-F-sal-N-1,5,8,12)]SbF6 (3), show HS electronic configurations between 2 and 300 K, and there exist π-π stackings between the phenyl groups from the neighboring [Mn(5-F-sal-N-1,5,8,12)]+ cations. As for the tetrahedral anions, complex [Mn(5-F-sal-N-1,5,8,12)]BF4 (4) exhibits a gradual and incomplete spin conversion. Complex [Mn(5-F-sal-N-1,5,8,12)] ClO4 (5) shows a nearly complete SCO with T1/2 = 100 K. The remaining salts with spherical anions form Nam-HX (X = Cl, Br, I) hydrogen bonds between Mn(iii) cations and counterions. Complexes [Mn(5-F-sal-N-1,5,8,12)]Cl (6) and [Mn(5-F-sal-N-1,5,8,12)] Cl0.28Br0.72 (7) feature gradual SCO behaviors with T1/2 = 220 K and 235 K, respectively. Complex [Mn(5-F-sal-N-1,5,8,12)]I (8) exhibits a more gradual spin conversion and is far from a complete HS state with a χMT value of 1.89 cm3 mol-1 K at 400 K.

Anion-driven supramolecular modulation of spin-crossover properties in mononuclear iron(III) Schiff-base complexes.

A series of Fe(III) complexes [Fe(5-F-sal-N-1,4,7,10)]Y (Y = PF6- for 1, Y = ClO4- for 2, Y = I- for 3 and Y = NO3- for 4) have been prepared. Single-crystal X-ray crystallographic studies show that complex 1 crystallizes in the orthorhombic Pna21 space group and complexes 2-4 have an isomorphous structure and crystallize in the same monoclinic space group, P21/n. Complexes 2-4 have two independent molecules (Fe1 and Fe2) in the unit cell. Magnetic susceptibility measurements demonstrated that complexes 1 and 3 showed a gradual one-step SCO behavior (T1/2 for 1 = 177 K and for 3 = 227 K) without thermal hysteresis. The magnetic behavior of 2 shows an incomplete two-step SCO process at T1/2 = 114 K and 170 K, respectively, while 4 is in a high-spin state at all measured temperatures. A careful evaluation of the supramolecular structures of these complexes revealed correlation between the supramolecular packing forces and their SCO behavior. The crystal structure of 1 consists of a three-dimensional (3D) extended network constructed from N-H⋯F and C-H⋯F hydrogen bonds, and C-H⋯π and C⋯C short contacts. In compounds 2-4, the crystal packing is governed by C⋯C, C-H⋯π and p-π interactions for the Fe1 centers and by C-H⋯π/O interactions for the Fe2 centers, which form 1D chains. Additional interactions (C-H⋯F and N-H⋯O/I) connect the neighboring chains and planes to form a complex supramolecular network. The anion⋯π interactions in 4 provide a means for preventing SCO occurring at low temperatures. This suggests that the supramolecular connectivity of the anions influences the magnetic properties.

Structure-function correlations in mononuclear manganese(iii) spin crossover systems with a big conjugated hexadentate Schiff-base ligand.

This paper reports the syntheses, crystal structures and magnetic properties of spin crossover (SCO) salts of formulae [Mn(naphth-sal-N-1,5,8,12)]SbF6 (1), [Mn(naphth-sal-N-1,5,8,12)]AsF6 (2), [Mn(naphth-sal-N-1,5,8,12)]PF6·1/2CH3OH (3) and [Mn(naphth-sal-N-1,5,8,12)]ClO4 (4), where (naphth-sal-N-1,5,8,12)2- (2,2'-((1E,14E)-2,6,10,14-tetraazapentadeca-1,14-diene-1,15-diyl)diphenolate) is a big conjugated hexadentate Schiff-base ligand. Magnetic susceptibility measurements demonstrated that complexes 1 and 2 showed a gradual one-step SCO between the high-spin (HS, S = 2) and low-spin (LS, S = 1) states without thermal hysteresis. The transition temperatures T1/2 of the SbF6 (1) and AsF6 (2) salts estimated from the magnetic susceptibility measurements are 164 and 171 K, respectively. The existence of the crystal solvent of complex 3 changes the supramolecular packing, leading to close ππ stacking interactions between the phenyl groups of the ligands. These close stacking interactions hinder the flexibility of the whole ligand, precluding the spin transformation of complex 3 and leading to its stabilization in the HS state in the temperature range of 2-300 K. For 4, crystal structure analysis indicates that the reduction in the anion size leads to close contacts between the naphthalene rings. These C-Hπ interactions provide a means for preventing the spin crossover occurring at low temperatures.

Isolation of two bis(silyl)nickel complexes with Si-Si bond formation in a single-crystal-to-single-crystal fashion.

A unique penta-coordinate nickel complex [{1,2-C6H4(SiMe2)(SiH)}2Ni(PEt3)(depe)] (6) that generates two new Si-Si single bonds has been prepared exclusively by the reaction of a chelating silyl hydride 1-(dimethylsilyl)-2-silylbenzene (4) and Ni(depe)(PEt3)2 (depe = Et2PCH2CH2PEt2) in a 2 : 1 ratio. Complex 6 is the first example of mononuclear silyl nickel complexes containing a Si-Si single bond formed in situ. Interestingly, framework 6 exhibits reversible single-crystal-to-single-crystal transformations upon the removal and rebinding of the coordinating PEt3 molecule, which give rise to a PEt3-free complex [{1,2-C6H4(SiMe2)(SiH)}2Ni(depe)] (6'), concomitant with the alteration of the coordination geometry of central metal atoms.