Synthesis and Magnetic Properties of Heterometal Cyclic Tetranuclear Complexes [Cu^IILM^II(hfac)]₂ (M^II = Zn, Cu, Ni, Co, Fe, Mn; H₃L = 1-(2-Hydroxybenzamido)-2-((2-hydroxy-3-methoxybenzylidene)amino)ethane; Hhfac = Hexafluoroacetylacetone)

A series of heterometal cyclic tetranuclear complexes [CuIILMII(hfac)]2 (MII = Zn (1), Cu (2), Ni (3), Co (4), Fe(5), and Mn (6)) have been synthesized by the assembly reaction of K[CuL] and [MII(hfac)2(H2O)2] with a 1:1 mole ratio in methanol, where H3L = 1-(2-hydroxybenzamido)-2-((2-hydroxy-3-methoxybenzylidene)amino)ethane and Hhfac = hexafluoroacetylacetone. The crystal structures of 2, 4, and [CuIILMnII(acac)]2 (6a) (Hacac = acetylacetone) were determined by single-crystal X-ray analyses. Each complex has a cyclic tetranuclear CuII2MII2 structure, in which the CuII complex functions as a “bridging ligand complex”, and the CuII and MII ions are alternately arrayed. One side of the planar CuII complex coordinates to one MII ion at the two phenoxo and the methoxy oxygen atoms, and the opposite side of the CuII complex coordinates to another MII ion at the amido oxygen atom. The temperature-dependent magnetic susceptibilities revealed spin states of SM = 0, 1/2, 1, 3/2, 2, and 5/2 for the ZnII, CuII, NiII, CoII, FeII, and MnII ions, respectively. Satisfactory fittings to the observed magnetic susceptibility data were obtained by assuming a rectangular arrangement with two different g-factors for the CuII and MII ions, two different isotropic magnetic exchange interactions, J1 and J2, between the CuII and MII ions, and a zero-field splitting term for the MII ion. In all cases, the antiferromagnetic coupling constants were found for both exchange interactions suggesting nonzero spin ground states with ST = 2|SM − SCu|, which were confirmed by the analysis of the field-dependent magnetization measurements

Interlayer Interaction of Two-Dimensional Layered Spin Crossover Complexes [Fe^IIH₃L^Me][Fe^IIL^Me]X (X^- = ClO₄^-, BF₄^-, PF₆^-, AsF₆^-, and SbF₆^-; H₃L^Me = Tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine)

A series of two-dimensional (2D) spin crossover complexes, [FeIIH3LMe][FeIILMe]X (X- = ClO4-, BF4-, PF6-, AsF6-, SbF6-) 1−5, have been synthesized, where H3LMe denotes an hexadentate N6 tripodlike ligand containing three imidazole groups, tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine. Compounds 1−5 exhibit a two-step (HS-[FeIIH3LMe]2+ + HS-[FeIILMe]-) ↔ (HS-[FeIIH3LMe]2+ + LS-[FeIILMe]-) ↔ (LS-[FeIIH3LMe]2+ + LS-[FeIILMe]-) spin-transition. The crystal structure of [FeIIH3LMe][FeIILMe]PF6 (3) was determined at 295, 200, and 100 K. The structure consists of homochiral extended 2D puckered sheets, in which the complementary [FeIIH3LMe]2+ and [FeIILMe]- capped tripodlike components, linked together by imidazole−imidazolate hydrogen bonds, are alternately arrayed in an up-and-down mode. The Fe−N bond distances and angles revealed that the FeII sites of both constituting units are in the high-spin (HS) state at 295 K; at 200 K, the FeII sites of [FeIIH3LMe]2+ and [FeIILMe]- are in the HS and low-spin (LS) states, respectively. The FeII sites of both constituting units are in the LS state at 100 K. The size of the counteranion affects significantly the intra- and interlayer interactions leading to modifications of the spin crossover behavior. The onset of the second spin-transition of the ClO4- (1) and BF4- (2) salts adjoins the first spin-transition, while a mixed (HS-[FeIIH3LMe]2+ + LS-[FeIILMe]-) spin-state spans a temperature range as wide as 70 K for salts 3−5 with larger counteranions, PF6-, AsF6-, and SbF6-, respectively. Compounds 1 and 2 showed remarkable LIESST (light induced excited spin state trapping) and reverse-LIESST effects, whereas 3−5 showed no remarkable LIESST effect. The interlayer interaction due to the size of the counteranion is an important factor governing the spin crossover behavior and LIESST effect

A New Family of Spin Crossover Complexes with a Tripod Ligand Containing Three Imidazoles: Synthesis, Characterization, and Magnetic Properties of [Fe^IIH₃L^Me](NO₃)₂·1.5H₂O, [Fe^IIIL^Me]·3.5H₂O, [Fe^IIH₃L^Me][Fe^IIL^Me]NO₃, and [Fe^IIH₃L^Me][Fe^IIIL^Me](NO₃)₂ (H₃L^Me = Tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine)

A new family of spin crossover complexes, [FeIIH3LMe](NO3)2·1.5H2O (1), [FeIIILMe]·3.5H2O (2), [FeIIH3LMe][FeIILMe]NO3 (3), and [FeIIH3LMe][FeIIILMe](NO3)2 (4), has been synthesized and characterized, where H3LMe denotes a hexadentate N6 tripod ligand containing three imidazole groups, tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine. It was found that the spin and oxidation states of the iron complexes with this tripod ligand are tuned by the degree of deprotonation of the imidazole groups and by the 2-methyl imidazole substituent. Magnetic susceptibility and Mössbauer studies revealed that 1 is an HS-FeII complex, 2 exhibits a spin equilibrium between HS and LS-FeIII, 3 exhibits a two-step spin transition, where the component [FeIILMe]- with the deprotonated ligand participates in the spin transition process in the higher temperature range and the component [FeIIH3LMe]2+ with the neutral ligand participates in the spin transition process in the lower temperature range, and 4 exhibits spin transition of both the FeII and FeIII sites. The crystal structure of 3 consists of homochiral extended 2D puckered sheets, in which the capped tripodlike components [FeIIH3LMe]2+ and [FeIILMe]- are alternately arrayed in an up-and-down mode and are linked by the imidazole−imidazolate hydrogen bonds. Furthermore, the adjacent 2D homochiral sheets are stacked in the crystal lattice yielding a conglomerate as confirmed by the enantiomeric circular dichorism spectra. Compounds 3 and 4 showed the LIESST (light induced excited spin state trapping) and reverse-LIESST effects upon irradiation with green and red light, respectively

Magnetic Interactions in Cu^II−Ln^III Cyclic Tetranuclear Complexes: Is It Possible to Explain the Occurrence of SMM Behavior in Cu^II−Tb^III and Cu^II−Dy^III Complexes?

An extensive series of tetranuclear CuII2LnIII2 complexes [CuIILLnIII(hfac)2]2 (with LnIII being all lanthanide(III) ions except for the radioactive PmIII) has been prepared in order to investigate the nature of the CuII−LnIII magnetic interactions and to try to answer the following question:  What makes the CuII2TbIII2 and CuII2DyIII2 complexes single molecule magnets while the other complexes are not? All the complexes within this series possess a similar cyclic tetranuclear structure, in which the CuII and LnIII ions are arrayed alternately via bridges of ligand complex (CuIIL). Regular SQUID magnetometry measurements have been performed on the series. The temperature-dependent magnetic susceptibilities from 2 to 300 K and the field-dependent magnetizations from 0 to 5 T at 2 K have been measured for the CuII2LnIII2 and NiII2LnIII2 complexes, with the NiII2LnIII2 complex containing diamagnetic NiII ions being used as a reference for the evaluation of the CuII−LnIII magnetic interactions. These measurements have revealed that the interactions between CuII and LnIII ions are very weakly antiferromagnetic if Ln = Ce, Nd, Sm, Yb, ferromagnetic if Ln = Gd, Tb, Dy, Ho, Er, Tm, and negligible if Ln = La, Eu, Pr, Lu. With the same goal of better understanding the evolution of the intramolecular magnetic interactions, X-ray magnetic circular dichroism (XMCD) has also been measured on CuII2TbIII2, CuII2DyIII2, and NiII2TbIII2 complexes, either at the L- and M-edges of the metal ions or at the K-edge of the N and O atoms. Last, the CuII2TbIII2 complex exhibiting SMM behavior has received a closer examination of its low temperature magnetic properties down to 0.1 K. These particular measurements have revealed the unusual very slow setting-up of a 3D order below 0.6 K

Magnetic Interactions in Cu^II−Ln^III Cyclic Tetranuclear Complexes: Is It Possible to Explain the Occurrence of SMM Behavior in Cu^II−Tb^III and Cu^II−Dy^III Complexes?

An extensive series of tetranuclear CuII2LnIII2 complexes [CuIILLnIII(hfac)2]2 (with LnIII being all lanthanide(III) ions except for the radioactive PmIII) has been prepared in order to investigate the nature of the CuII−LnIII magnetic interactions and to try to answer the following question:  What makes the CuII2TbIII2 and CuII2DyIII2 complexes single molecule magnets while the other complexes are not? All the complexes within this series possess a similar cyclic tetranuclear structure, in which the CuII and LnIII ions are arrayed alternately via bridges of ligand complex (CuIIL). Regular SQUID magnetometry measurements have been performed on the series. The temperature-dependent magnetic susceptibilities from 2 to 300 K and the field-dependent magnetizations from 0 to 5 T at 2 K have been measured for the CuII2LnIII2 and NiII2LnIII2 complexes, with the NiII2LnIII2 complex containing diamagnetic NiII ions being used as a reference for the evaluation of the CuII−LnIII magnetic interactions. These measurements have revealed that the interactions between CuII and LnIII ions are very weakly antiferromagnetic if Ln = Ce, Nd, Sm, Yb, ferromagnetic if Ln = Gd, Tb, Dy, Ho, Er, Tm, and negligible if Ln = La, Eu, Pr, Lu. With the same goal of better understanding the evolution of the intramolecular magnetic interactions, X-ray magnetic circular dichroism (XMCD) has also been measured on CuII2TbIII2, CuII2DyIII2, and NiII2TbIII2 complexes, either at the L- and M-edges of the metal ions or at the K-edge of the N and O atoms. Last, the CuII2TbIII2 complex exhibiting SMM behavior has received a closer examination of its low temperature magnetic properties down to 0.1 K. These particular measurements have revealed the unusual very slow setting-up of a 3D order below 0.6 K