Two helical manganese complexes, [Mn₅(L)₃(μ₃-O)(NO₃)(OMe)₂(MeOH)](NO₃)·4MeOH·H₂O (1) and [Mn₈(L)₆(μ₃-Cl)₂]Cl₂·3C₂H₄Cl₂·60H₂O·2DMF (2, DMF = N,N-dimethylformamide), were obtained by the reaction of the multidentate polypyridine ligand 2,6-bis[5-(2-pyridinyl)-1H-pyrazole-3-yl]-pyridine (H₂L) with manganese nitrate and chloride, respectively. The compounds were characterized by elemental analyses and single-crystal X-ray structural analyses. The pentanuclear complex 1 is composed of three ligands and five manganese ions with μ₃-oxido bridges, whereas the octanuclear complex 2 is constructed from six ligands and eight manganese ions with two μ₃-chlorido bridges. Both compounds have helix-type structures. The manganese ions in 1 are present in a mixture of +2 and +3 valence states, whereas 2 is homovalent and contains only +2 ions. The magnetic properties of both compounds were investigated in the temperature range 1.8–300 K, and antiferromagnetic interactions between the manganese ions are dominant in both complexes.

Invited for the cover of this issue is the group of Hiroki Oshio at the University of Tsukuba, Japan. The cover image shows the metaphoric link between the synthesis of two new manganese clusters and the ancient Japanese art of origami. Origami literally means “folded paper” and allows elaborate shapes to be created from the simplest materials, just like the rigid, planar ligands that were employed to “fold” manganese ions into sophisticated helical superstructures.

A bulky bidentate ligand was used to stabilize a macrocyclic [Fe^III₈Co^II₆] cluster. Tuning the basicity of the ligand by derivatization with one or two methoxy groups led to the isolation of a homologous [Fe^III₈Co^II₆] species and a [Fe^III₆Fe^II₂Co^III₂Co^II₂] complex, respectively. Lowering the reaction temperatures allowed isolation of [Fe^III₆Fe^II₂Co^III₂Co^II₂] clusters with all three ligands. Temperature-dependent absorption data and corresponding experiments with iron/nickel systems indicated that the iron/cobalt self-assembly process was directed by the occurrence of solution-state electron-transfer-coupled spin transition (ETCST) and its influence on reaction intermediate lability.

A bulky bidentate ligand was used to stabilize a macrocyclic [Fe^III₈Co^II₆] cluster. Tuning the basicity of the ligand by derivatization with one or two methoxy groups led to the isolation of a homologous [Fe^III₈Co^II₆] species and a [Fe^III₆Fe^II₂Co^III₂Co^II₂] complex, respectively. Lowering the reaction temperatures allowed isolation of [Fe^III₆Fe^II₂Co^III₂Co^II₂] clusters with all three ligands. Temperature-dependent absorption data and corresponding experiments with iron/nickel systems indicated that the iron/cobalt self-assembly process was directed by the occurrence of solution-state electron-transfer-coupled spin transition (ETCST) and its influence on reaction intermediate lability.

Two dimerized iron(II) complexes, [{Fe(L)(HL)₂}{Fe(HL)₃}](PF₆)(BF₄)₂·5CH₃CN·4H₂O (1) and [{Fe(L)(HL)₂}{Fe(HL)₃}](PF₆)(FeCl₄)₂ (2) {HL = 3-(2-pyridyl)-5-[4-(diphenylamino)phenyl]-1H-pyrazole}, supported by triphenylamine-derivatized bidentate ligands and encapsulating hexafluorophosphate ions were synthesized. The complex iron ions have octahedral coordination geometries in which they interact with six nitrogen atoms from three bidentate ligands. Both compounds have capsule-like dimerized supramolecular structures, in which the arrangements of the bulky ligands of two discrete mononuclear complexes create a central cavity in which a single hexafluorophosphate ion is captured. The bidentate ligands interact with the anionic guest molecules through the hydrogen atoms of the ligand pyrazole moieties. Cryomagnetic studies reveal that both compounds show spin-crossover behavior. Complex 1 showed quasi-reversible spin-crossover around 200–300 K, hindered by desolvation effects, whereas 2 showed reversible gradual spin-crossover behavior. The electronic states of the iron ions resulting from the spin-crossover were confirmed by X-ray structure analyses and Mössbauer spectra.

Two decanuclear cyanide-bridged cage compounds were synthesized through the combination of chiral bidentate ligand-capped transition metal building blocks with hexacyanometalate ions, and their structural and magnetic properties were investigated.

Cyanide-bridged molecular squares offer a unique opportunity for study, as they represent a model of the simplest unit of Prussian blue and allow access to a wide range of physical properties that can be monitored, such as rich electrochemistry, valence- and spin-state control, and spin-crossover phenomena. This microreview presents recent findings in the field of cyanide squares with particular focus on their syntheses and physical properties.

The cover picture shows two examples of the diverse physical properties observed in cyanide-bridged molecular square complexes: multistep spin crossover and electron transfer coupled spin transition. Cyanide-bridged squares can be considered to be the building blocks of Prussian Blue, which forms the backdrop of the image, and their discrete nature can allow easy access to multistable functionalities. The Microreview by H. Oshio et al. on p. 3031 ff. discusses the syntheses and physical properties of cyanide-bridged molecular squares since the discovery of the first high-spin example in 1999.

Cyanide-bridged metal complexes of [Fe₈M₆(μ-CN)₁₄(CN)₁₀ (tp)₈(HL)₁₀(CH₃CN)₂][PF₆]₄⋅n CH₃CN⋅m H₂O (HL=3-(2-pyridyl)-5-[4-(diphenylamino)phenyl]-1H-pyrazole), tp⁻=hydrotris(pyrazolylborate), 1: M=Ni with n=11 and m=7, and 2: M=Co with n=14 and m=5) were prepared. Complexes 1 and 2 are isomorphous, and crystallized in the monoclinic space group P2₁/n. They have tetradecanuclear cores composed of eight low-spin (LS) Fe^III and six high-spin (HS) M^II ions (M=Ni and Co), all of which are bridged by cyanide ions, to form a crown-like core structure. Magnetic susceptibility measurements revealed that intramolecular ferro- and antiferromagnetic interactions are operative in 1 and in a fresh sample of 2, respectively. Ac magnetic susceptibility measurements of 1 showed frequency-dependent in- and out-of-phase signals, characteristic of single-molecule magnetism (SMM), while desolvated samples of 2 showed thermal- and photoinduced intramolecular electron-transfer-coupled spin transition (ETCST) between the [(LS-Fe^II)₃(LS-Fe^III)₅(HS-Co^II)₃(LS-Co^III)₃] and the [(LS-Fe^III)₈(HS-Co^II)₆] states.

Cobalt(II) radical complexes with centric and acentric counter anions, [CoCl(bisimpy)(MeOH)₂]X [bisimpy = 2,6-bis(4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1′H-imidazol-1′-oxyl-2′-yl)pyridine; X = PF₆ (1), and ClO₄ (2)], crystallize in the centrosymmetric and polar space groups, respectively. Compound 1 is paramagnetic, whereas spin canting in 2 leads to a metamagnetic transition with a critical field of 1300 Oe at 1.8 K. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The cover picture shows that cobalt(II) diradical complexes with centric and acentric counter anions, [CoCl(bisimpy)(MeOH)₂]X (X = PF₆ and ClO₄), crystallize in the centrosymmetric and polar space groups of P2₁c and Pna2₁, respectively. The PF₆^– salt is paramagnetic down to 1.8 K, whereas the ClO₄^– salt shows a weak ferromagnetic long-range order with metamagnetic behavior at 1.8 K. Spin canting can arise from single-ion magnetic anisotropy and/or antisymmetric exchange interaction (Dzyaloshinsky–Moriya interaction). The noncentrosymmetric space group of the latter complex is compatible with both mechanisms. Details are discussed in the Short Communication by H. Oshio et al. on p. 4851 ff.