Cover Picture: A Spin‐Crossover Cluster of Iron(II) Exhibiting a Mixed‐Spin Structure and Synergy between Spin Transition and Magnetic Interaction (Angew. Chem. Int. Ed. 8/2009)

A spin‐crossover cluster with the {Fe^II₄O₄} core structure is presented by D. Y. Wu, O. Sato et al. in their Communication on page 1475 ff. The cluster is synthesized by self‐assembly and shows an abrupt spin transition, giving two high‐spin and two low‐spin states. It exhibits complete light‐induced excited spin‐state trapping effects. Importantly, synergy effects between the magnetic interaction and spin transition operate in the cluster.

     

Inside Cover: Bidirectional Chemo‐Switching of Spin State in a Microporous Framework (Angew. Chem. Int. Ed. 26/2009)

Bidirectional chemo‐switching of magnetism occurs in a microporous coordination polymer containing spin‐crossover subunits, as described by M. Ohba, J. A. Real, S. Kitagawa, and co‐workers in their Communication on page 4767 ff. In situ magnetic measurements reveal that most guest molecules transform the framework spin state from diamagnetic low spin (red) to paramagnetic high spin (yellow), whereas the guest CS₂ stabilizes the low‐spin state. These induced spin states are retained as a memory effect after the release of the guest.

     

A Spin‐Crossover Cluster of Iron(II) Exhibiting a Mixed‐Spin Structure and Synergy between Spin Transition and Magnetic Interaction

How low can you go? An Fe^II₄ square was prepared by self‐assembly and exhibits both thermally induced and photoinduced spin crossover from a system with four high‐spin (HS) centers to one with two high‐spin and two low‐spin (LS) centers. The spin‐crossover sites are located on the same side of the square, and the spin transition and magnetic interactions (see picture) are synergistically coupled.

     

Inside Cover: A Nanoscale Molecular Switch Triggered by Thermal,Light, and Guest Perturbation (Angew. Chem. Int. Ed. 14/2009)

Continuous switching between high‐spin and low‐spin magnetic states can be accomplished by irradiation with red and green laser light, respectively. In their Communication on page 2549 ff., S. R. Batten and co‐workers report a metallo‐supramolecule that undergoes spin crossover (SCO) switching induced by temperature change or light irradiation. The SCO behavior is also dependent on the presence and nature of intercalated solvent molecules within the porous crystal structure.

     

Hysteretic Spin Crossover above Room Temperature and Magnetic Coupling in Trinuclear Transition‐Metal Complexes with Anionic 1,2,4‐Triazole Ligands

The reaction of 4‐(1,2,4‐triazol‐4‐yl)ethanesulfonate ( L ) with Zn²⁺, Cu²⁺, Ni²⁺, Co²⁺, and Fe²⁺ gave a series of analogous neutral trinuclear complexes with the formula [M₃(μ‐ L )₆(H₂O)₆] ( 1 – 5 ). These compounds were characterized by single‐crystal X‐ray diffraction, thermogravimetry, and elemental analysis. The magnetic properties of compounds 2 – 5 were studied. Complexes 2 – 4 show weak antiferromagnetic superexchange, with J values of ?0.33 ( 2 ), ?9.56 ( 3 ), and ?4.50 cm^?1 ( 4 ) (exchange Hamiltonian H=?2 J (S₁S₂+S₂S₃)). Compound 5 shows two additional crystallographic phases ( 5 b and 5 c ) that can be obtained by dehydration and/or thermal treatment. These three phases exhibit distinct magnetic behavior. The Fe²⁺ centers in 5 are in high‐spin (HS) configuration at room temperature, with the central one exhibiting a non‐cooperative gradual spin transition below 250 K with T_1/2=150 K. In 5 b , the central Fe²⁺ stays in its low‐spin (LS) state at room temperature, and cooperative spin transition occurs at higher temperatures and with the appearance of memory effect (T_1/2↑=357 K and T_1/2↓=343 K). In the case of 5 c , all iron centers remain in their HS configuration down to very low temperatures, with weak antiferromagnetic coupling (J=?1.16 cm^?1). Compound 5 b exhibits spin transition with memory effect at the highest temperature reported, which matches the remarkable features of coordination polymers.