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In this third-year undergraduate experiment, a coordination complex [Fe(NH2trz)3]Br2⋅H2O (NH2trz is 4-amino-1,2,4-triazole, C2H4N4) was synthesized and characterized by IR and UV-visible spectroscopies. This compound exhibits a spectacular reversible purple-white thermochromic transition upon heating--cooling with a large hysteresis centered at room temperature. The thermochromic transition is related to the spin crossover (SCO) from the low-spin state (LS, purple, S = 0, diamagnetic) to the high-spin state (HS, white, S = 2, paramagnetic). During this experiment, students discovered synthetic methods, such as anion metathesis and Ostwald ripening, and physical properties, such as magnetic properties, thermochromism, and bistability phenomenon, altogether leading to possible applications in optical-data storage and display. [ABSTRACT FROM AUTHOR]

ABSTRACT We describe herein the synthesis of ( rac)- or enantiopure ( S)-(−)-(2-MeBu)N(Pr)2MeI ammonium salts. These racemic and enantiopure ammonium salts were used as cationic templates to obtain new two-dimensional (2D) ferromagnets [( rac)-(2-MeBu)N(Pr)2Me][MnCr(C2O4)3] and [( S)-(−)-(2-MeBu)N(Pr)2Me][ ΔMn Λ nCr(C2O4)3]. The absolute configuration of the hexacoordinated Cr(III) metallic ion in the enantiopure 2D network was determined by a circular dichroism measurement. The structure of [(2-MeBu)N(Pr)2Me][MnCr(C2O4)3], established by single crystal X-ray diffraction, belongs to the chiral P63 space group. According to direct current (dc) magnetic measurements, these compounds are ferrromagnets with a temperature Tc = 6°K. Chirality 25:444-448, 2013. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Tris(bipyridine)ruthenium(II) is used as a templating agent to insert palladium(II) into three-dimensional oxalate-based networks. The templated-assembly of [Ru(bpy)3]-[Pd2(ox)3] (Pd2) and [Ru(bpy)3]-[PdMn(ox)3] (PdMn) is described. The latter compound is structurally characterized by powder X-ray diffraction and X-ray absorption spectroscopy. These techniques reveal an unusual 6-fold oxygen environment around the Pd(II) atoms with two short (2.02 Å) and four long (2.17 Å) Pd-O distances. As stated by magnetometry, this environment is associated with a triplet ground state (S = 1) of the palladium-(II) ion: when the temperature is decreased, the χMT product shows a monotonous decrease from 5.54 cm³ K mol-1 at 300 K, a value which is slightly lower than the one expected for independent paramagnetic Pd(II) (S = 1, g = 2) and Mn(II) (S = 5/2, g = 2) ions. This thermal variation is due to antiferromagnetic exchange interactions between the two spin bearers. Nevertheless, no long-range magnetic order is detected down to 2 K. These results are confirmed by an analysis of the [MIIC2O4)3]4- (M = Ni, Pd, Pt) complex and of a [PdII{μ-(C2O4)MnII(OH2)4}3]2+ tetranuclear model using density functional theory. [ABSTRACT FROM AUTHOR]

Abstract: The two enantiomers of [Ru(bpy)3][Mn2(ox)3] (bpy=2,2′-bipyridine, ox=oxalate), namely [(Δ)-Ru(bpy)3][(Δ)-Mn2(ox)3], ( Δ -1) and [(Λ)-Ru(bpy)3][(Λ)-Mn2(ox)3], ( Λ -1), were obtained as single crystals using [(Δ)-Ru(bpy)3]2+ and [(Λ)-Ru(bpy)3]2+, respectively, as a chiral templating cation. Their structures were determined by single-crystal X-ray diffraction. The compounds crystallise in the enantiomeric chiral cubic space groups, P4332 ( Δ -1) and P4132 ( Λ -1), with a =15.492(2) and 15.507(2)Å, respectively (Z =4). Both structures include a three-dimensional 10-gon 3-connected (10,3) anionic network wrapped around the [Ru(bpy)3]2+ cations. In both crystalline enantiomers, the resolved ruthenium template cation imposes both the topology and the absolute configuration of all the metal centres. The thermal variation of the magnetic susceptibility, measured on Δ -1 and Λ -1 crystals, reveals an antiferromagnetic coupling between the oxalate-bridged manganese ions in the paramagnetic region characterised by a negative Weiss constant Θ =−35K. Below T N =13K, Δ -1 and Λ -1 exhibit a canted antiferromagnetic order. [Copyright &y& Elsevier]

Here we report on the Magneto‐Chiral Dichroism (MChD) detected through visible and near‐infrared light absorption of two enantiomeric pairs of ErIII and TmIII chiral complexes featuring a propeller‐like molecular structure. The magnetic properties show typical features of isolated paramagnetic ions associated with 4I15/2 and 3H6 ground state terms. MChD spectroscopy shows high gMChD dissymmetry factors of ca. 0.12 T−1 and 0.05 T−1 (T=4.0 K and B=1.0 T) for ErIII and TmIII, respectively, associated with the magnetic‐dipole allowed 4I13/2 ← 4I15/2 and 3H5 ← 3H6 transitions. MChD signals of the two complexes were detected up to room temperature and under magnetic fields up to 5.0 T. For the first time, the MChD results are discussed in the context of the Richardson theory of lanthanide optical activity and provide clear indications on the strongest MChD‐active electronic transitions of lanthanide complexes. [ABSTRACT FROM AUTHOR]

1-Propyl-2-phenyl-3-methylamidinium (A) and 1-hexyl-2-phenyl-3-methylamidinium (B) cations are used to template the formation of two-dimensional (2D) oxalate-based bimetallic compounds of formula A [MnIICrIII(C 2 O 4) 3 ] (1) and B [MnIICrIII(C 2 O 4) 3 ] (2). The single-crystal X-rays diffraction analysis revealed that both compounds crystallize in noncentrosymmetric space groups. In both compounds, the bimetallic coordination network is made of slightly corrugated 2D (6,3) bimetallic honeycomb layers arranged in an eclipsed manner. The monocations are located between the layers without any preferential interaction with the anionic coordination network. The magnetic susceptibility measurements can be fitted using a Curie–Weiss law with Curie–Weiss temperatures of 9.9 K for 1 and 9.0 K for 2 , revealing that the dominating exchange interaction is ferromagnetic. This favors the onset of a ferromagnetic long-range ordering at 6.0 K for 1 and 5.8 K for 2. [ABSTRACT FROM AUTHOR]

The combination of physical properties sensitive to molecular chirality in a single system allows the observation of fascinating phenomena such as magneto‐chiral dichroism (MChD) and circularly polarized luminescence (CPL) having potential applications for optical data readout and display technology. Homochiral monodimensional coordination polymers of YbIII were designed from a 2,15‐bis‐ethynyl‐hexahelicenic scaffold decorated with two terminal 4‐pyridyl units. Thanks to the coordination of the chiral organic chromophore to Yb(hfac)3 units (hfac−=1,1,1,5,5,5‐hexafluoroacetylaconate), efficient NIR‐CPL activity is observed. Moreover, the specific crystal field around the YbIII induces a strong magnetic anisotropy which leads to a single‐molecule magnet (SMM) behaviour and a remarkable room temperature MChD. The MChD‐structural correlation is supported by computational investigations. [ABSTRACT FROM AUTHOR]

Abstract: The synthesis of two oxoverdazyls based compounds, the 1,5-dimethyl-3-(2′-hydroxyphenyl)-6-oxoverdazyl 1 and the perchlorate salt of 1,5-dimethyl-3-(2′-imidazolium)-6-oxoverdazyl 2 are reported. The structural analysis of 1 reveals that radicals are closely packed in regular columns by way of π-stacking. In contrast, the packing in 2 is definitely influenced by the protonation state of the radical substituent and shows strong lateral staggering of the organic radicals. The organisation in the solid state strongly influences the intermolecular exchange interaction between π-stacked radicals which goes from J =−113cm−1 (H =−J∑SiSi +1) in 1 to J =−1.0cm−1 and J′=−0.67cm−1 in 2. [Copyright &y& Elsevier]

Abstract: We have contacted Na-saponite with aqueous solutions containing a well-defined iron polycation with structuring ligands [Fe8(μ 3-O)2(μ 2-OH2)12(tacn)6]8+, which is known as a promising candidate for molecular magnets. When the resulting suspensions were activated with either microwaves or ultrasound, macroscopic-level characterization of the solid phases obtained after centrifugation (elemental analysis, surface area, XRD) suggested the intercalation of stoichiometrically intact polycations in the interlayer space by cation exchange. Furthermore, 57Fe Mössbauer spectroscopy and Fe K-edge EXAFS are compatible with a conservation of the structure of the polycations inorganic core: connectivity seems to be maintained, while the polycations must be somewhat flattened by strain due to intercalation. Magnetization curves also appear compatible with a conservation of the polycation nuclearity, although experiments with a magnetically clean clay sample are necessary to confirm this point. [Copyright &y& Elsevier]

Chiral ammonium cations of the type [R1R2R3R4N]+, new templates for a new family of bimetallic oxalate bridged networks. This work deals with the obtention of a new family of bimetallic [R1R2R3R4N][MnCr(C2O4)3] oxalate bridged 2D networks templated by chiral ammonium cations of the type [R1R2R3R4N]+, in which R stands for n-alkyl groups from methyl to dodecyl. To cite this article: M. Gruselle et al., C. R. Chimie 6 (2003). [Copyright &y& Elsevier]

The reactivity of 1,5-diisopropyl-3-(4′-carboxyphenyl)-6-oxoverdazyl H I i -Pr toward divalent metal ions is dominated by the coordination of the carboxylate moiety onto the metal ions. The formula of the compound obtained by the reaction with the earth-alkaline magnesium (II) is [Mg(I i -Pr) 2 ·2H 2 O] n ·0.35H 2 O (I i -Pr –Mg) whereas the reaction with 3d divalent metal ions yields compounds of general formula [M(I i -Pr) 2 (CH 3 OH) 4 ] (M = Mn (I i -Pr –Mn), Co (I i -Pr –Co), Ni (I i -Pr –Ni), Zn (I i -Pr –Zn)). The single-crystal X-ray diffraction analysis of I i -Pr –Mg reveals that the magnesium (II) adduct forms a one-dimensional (1D) coordination polymer with a terminal radical in a bidentate κ(O,O′) coordination mode and a bridging one in a μ − 1κ(O); 1κ (O′) syn – anti mode. The coordination sphere of the magnesium (II) is completed by two water molecules that established H-bonds with the carboxylate moiety of the terminal radicals that further reinforce the 1D coordination polymer. Adjacent chains interact through π-stacking of head-to-tail radicals. The single-crystal X-ray diffraction analysis of I i -Pr –Mn and I i -Pr –Ni reveals that the two compounds are isostructural. They are composed of mononuclear complexes where the divalent metal is surrounded by two radicals coordinated by monodentate carboxylate and four methanol molecules in the equatorial plane. Two methanol ligands act as H-bond donors toward the second oxygen atom of the carboxylate groups. The complexes interact through π-stacking of head-to-tail radicals and H-bonds between the two remaining methanol ligands and the oxygen atom of the oxoverdazyl moieties. In all of the compounds, the exchange interaction is dominated by the weak antiferromagnetic coupling related to the π-stacking of the radicals. [ABSTRACT FROM AUTHOR]

Two polycarboxylic acids, bis( p -carboxyphenyl)diphenylsilane and bis(3,4-dicarboxyphenyl)dimethylsilane, were prepared according to published procedures and characterized, besides elemental and spectral analysis, for the first time by X-ray single crystal diffraction. Their copper(II) complexes were obtained in the presence of 1,10-phenantroline as co-ligand, the crystallographic data revealing the formation of 2D structures through hydrogen bonds. The hydrogen bond dynamics of the copper complexes was studied by FTIR-ATR spectrometry. The thermal stability of the acids and derived copper complexes was evaluated by thermogravimetrical analysis. The moisture uptake capacity and porosity of the complexes were estimated on the basis of the water vapour sorption measurements in dynamic regime. Magnetic measurements performed on the two metal complexes correspond to not interacting copper(II) with S = 1/2 and g factor equal to 2.14 and 2.19 being consistent with pentacoordinated Cu(II) paramagnetic centres. [ABSTRACT FROM AUTHOR]

Abstract: The reaction of Ni(xbsms) (H2xbsms=1,2-bis(4-mercapto-3,3-dimethyl-2-thiabutyl)benzene) with [Ru(CO)3Cl2(thf)] yields green crystals of [NiCl(xbsms)Ru(CO)3Cl]. The structure of this structural model of the active site of NiFe hydrogenase reveals a pentacoordinated nickel(II) center with bound chloride anion. It therefore adopts a paramagnetic (S=1) electronic configuration as shown by magnetic susceptibility measurements. In DMF, this compound is converted into a red ionic-salt [NiL(xbsms)Ru(CO)3Cl]Cl (L=water or DMF) that catalyzes hydrogen electro-evolution from Et3NHCl at −1.52V vs. Ag/AgCl (−2.05V vs. Fc0/+). [Copyright &y& Elsevier]

We discuss the different structural arrangements of NBu4[FeIICrIII(C2O4)3] layered compounds in their racemic and enantiomeric forms and related magnetic properties. For [MnIIFeIII(C2O4)3] networks of dimensionalities 2 and 3 Mössbauer spectroscopy was applied to study the FeIII sublattice magnetization. Unusual magnetic relaxation phenomena below TN were observed for both 2D and 3D networks. [Copyright &y& Elsevier]

Abstract: We experimentally show that the magnetocaloric properties of molecule-based Prussian blue analogues can be adjusted by controlling during the synthesis the amount of intrinsic vacancies. For , we find indeed that the ferromagnetic phase transition induces significantly large magnetic entropy changes, whose maxima shift from to by varying the number of vacancies, offering a unique tunability of the magnetocaloric effect in this complex. [Copyright &y& Elsevier]

Magnetic materials are widely used for many technologies in energy, health, transportation, computation, and data storage. For the latter, the readout of the magnetic state of a medium is crucial. Optical readout based on the magneto-optical Faraday effect was commercialized but soon abandoned because of the need for a complex circular polarization-sensitive readout. Combining chirality with magnetism can remove this obstacle, as chiral magnetic materials exhibit magneto-chiral dichroism, a differential absorption of unpolarized light dependent on their magnetic state. Molecular chemistry allows the rational introduction of chirality into single-molecule magnets (SMMs), ultimate nanoobjects capable of retaining magnetization. Here, we report the first experimental demonstration of optical detection of the magnetic state of an SMM using unpolarized light on a novel air-stable Dy-based chiral SMM featuring a strong single-ion magnetic anisotropy. These findings might represent a paradigm shift in the field of optical data readout technologies.

Here we report magneto-chiral dichroism (MChD) detected through visible and near-infrared light absorption of a chiral dysprosium(III) coordination polymer. The two enantiomers of [Dy III (H6(py) 2 )(hfac) 3 ] n [H6(py) 2 = 2,15-bis(4-pyridyl)ethynylcarbo[6]helicene; hfac - = 1,1,1,5,5,5-hexafluoroacetylacetonate], where the chirality is provided by a functionalized helicene ligand, were structurally, spectroscopically, and magnetically investigated. Magnetic measurements reveal a slow relaxation of the magnetization, with differences between enantiopure and racemic systems rationalized on the basis of theoretical calculations. When the enantiopure complexes are irradiated with unpolarized light in a magnetic field, they exhibit multiple MChD signals associated with the f-f electronic transitions of Dy III , thus providing the coexistence of MChD-active absorptions and single-molecule-magnet (SMM) behavior. These findings clearly show the potential that rationally designed chiral SMMs have in enabling the optical readout of magnetic memory through MChD.

Because the combination of chiral and magnetic properties is becoming more and more attractive for magneto-chiral phenomena, we here aim at exploring the induction of chirality to achiral magnetic molecules as a strategy for the preparation of magneto-chiral objects. To this end, we have associated free base- and metallo-porphyrins with silica nano helices, using a variety of elaboration methods, and have studied them mainly by electronic natural circular dichroism (NCD) and magnetic circular dichroism (MCD) spectroscopies. While electrostatic or covalent surface grafting uniformly yielded very low induced CD (ICD) for the four assayed porphyrins, a moderate response was observed when the porphyrins were incorporated into the interior of the double-walled helices, likely due to the association of the molecules with the chirally-organized gemini surfactant. A generally stronger, but more variable, ICD was observed when the molecules were drop casted onto the helices immobilised on a quartz plate, likely due to the different capacities of the porphyrins to aggregate into chiral assemblies. Electronic spectroscopy, electron microscopy and IR spectroscopy were used to interpret the patterns of aggregation and their influence on ICD and MCD. No enhancement of MCD was observed as a result of association with the nanohelices except in the case of the free base, 5,10,15,20-tetra-(4-sulfonatophenyl)porphyrin (TPPS). This nanocomposite demonstrated a large ICD in the Soret region and a large MCD in the Q-region due to J-aggregation. However, no induced MChD was observed, possibly due to the spectral mismatch between the ICD and MCD peaks.

1,5-Dimethyl-3-(3'-(hydroxymethyl)-2'-pyridine)-6-oxotetrazane (H 3 vdpyCH 2 OH) or its oxidized verdazyl form (vdpyCH 2 OH) reacted with transition metal and/or lanthanide acetates to yield [(vdpyCH 2 O) 2 Co 2 Ln 2 (acO) 8 ] (Ln = Y(III): I Co,Y ; Gd(III): I Co,Gd ), [(vdpyCH 2 O) 2 M 3 (acO) 4 ] (M = Zn(II): II Zn ; Co(II): II Co ) and [(vdpyCH 2 OH)Zn(acO) 2 ] ( III Zn ) through self-assembly implying a complex- as -ligand intermediate. Single-crystal diffraction reveals that I MT,Ln are composed of 2p-3d-4f centrosymmetric clusters with verdazyl radicals at the two ends coordinated to the transition-metal ion in a tridentate mode and to the {Ln 2 (acO) 4 } lanthanide central core in a monodentate mode through its alkoxo moiety. In I Co,Gd , the transition-metal ions adopt an irregular octahedral environment, and the {Ln 2 (acO) 4 } core adopts a paddlewheel motif, whereas in I Co,Y , the transition metal is pentacoordinated, and the central core contains only two acetate bridges. Going from I Co,Y to II Co , the central {Y 2 (acO) 4 } core is replaced by an axially compressed octahedral cobalt(II) center, whereas the outer parts of the molecule remain still. The dc magnetic studies revealed that the alternate π-stacking of the verdazyl radicals in II Zn led to the formation of alternate antiferromagnetically coupled 1D chains with J vd-vd = -8.2(1) cm -1 and J vd-vd ' = -7.6(1) cm -1 (-2 J convention). In I Co,Y , a complex fitting procedure allowed us to retrieve a complete set of magnetic parameters to take into account both the magnetic anisotropy of the cobalt(II) centers and intra- and inter-molecular exchange effects. For I Co,Y , it led to g Co = 2.13(4), D Co = 100(2) cm -1 , E Co = 19.9(5) cm -1 , J Co-vd = +26.5(4) cm -1 , and J vd-vd = -7.95(4) cm -1 . ac magnetic susceptibility of I Co,Y , I Co,Gd and II Co did not reveal any slow relaxation of the magnetization even when a dc external magnetic field up to 2000 Oe was applied.

Here, we report the molecular self-assembly of hydroxido-bridged {Ln 5 Ni 6 } ((Ln 3+ = Dy 3+ , Y 3+ ) metal clusters by the reaction of enantiopure chiral ligands, namely, ( R / S )-(2-hydroxy-3-methoxybenzyl)-serine), with Ni II and Ln III precursors. Single-crystal diffraction analysis reveals that these compounds are isostructural sandwich-like 3d-4f heterometallic clusters showing helical chirality. Direct current magnetic measurements on {Dy 5 Ni 6 } indicates ferromagnetic coupling between Dy III and Ni II centers, whereas those on {Y 5 Ni 6 } denote that the Ni II centers are antiferromagnetically coupled and/or magnetically anisotropic. Magneto-chiral dichroism (MChD) measurements on {Dy 5 Ni 6 } and its comparison to that of {Y 5 Ni 6 } provide the first experimental observation of intense multimetal site MChD signals in the visible-near-infrared region. Moreover, the comparison of MChD with natural and magnetic circular dichroism spectra unambiguously demonstrate for the first time that the MChD signals associated with the Ni II d-d transitions are mostly driven by natural optical activity and those associated with the Dy III f-f transitions are driven by magnetic optical activity.

The interplay between chirality and magnetic fields gives rise to a cross effect referred to as magneto-chiral anisotropy (MChA), which can manifest itself in different physical properties of chiral magnetized materials. The first experimental demonstration of MChA was by optical means with visible light. Further optical manifestations of MChA have been evidenced across most of the electromagnetic spectrum, from terahertz to X-rays. Moreover, exploiting the versatility of molecular chemistry toward chiral magnetic systems, many efforts have been made to identify the microscopic origins of optical MChA, necessary to advance the effect toward technological applications. In parallel, the replacement of light by electric current has allowed the observation of nonreciprocal electrical charge transport in both molecular and inorganic conductors as a result of electrical MChA (eMChA). MChA in other domains such as sound propagation, photochemistry, and electrochemistry are still in their infancy, with only a few experimental demonstrations, and offer wide perspectives for further studies with potentially large impact, like the understanding of the homochirality of life. After a general introduction to MChA, we give a complete review of all these phenomena, particularly during the last decade., (© 2021 Wiley Periodicals LLC.)

Here we report the first experimental observation of magneto-chiral dichroism (MChD) detected through light absorption in an enantiopure lanthanide complex. The P and M enantiomers of [Yb III (( X )- L )( hfac ) 3 ] (X = P , M ; L = 3-(2-pyridyl)-4-aza[6]-helicene; hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate), where the chirality is held by the helicene-based ligand, were studied in the near-infrared spectral window. When irradiated with unpolarized light in a magnetic field, these chiral complexes exhibit a strong MChD signal ( g MChD ca. 0.12 T -1 ) associated with the 2 F 5/2 ← 2 F 7/2 electronic transition of Yb III . The low temperature absorption and MChD spectra reveal a fine structure associated with crystal field splitting and vibronic coupling. The temperature dependence of the main dichroic signal detected up to 150 K allowed, for the first time, the disentanglement of the two main microscopic contributions to the dichroic signal predicted by the MChD theory. These findings pave the way toward probing MChD in chiral lanthanide-based single-molecule magnets.

A new iron(iii)-centred ferric wheel Fe⊂{Fe 6 } of the formula [Fe 7 (H 2 L) 6 (NCS) 6 ](ClO 4 ) 3 ·10H 2 O, where H 4 L = N,N'-bis(3-carboxylsalicylidene)-1,3-bis(3-aminopropyl)tetramethyldisiloxane, was synthesised and fully characterised. 57 Fe Mössbauer spectra indicate the presence of high spin (S = 5/2) Fe 3+ cations adopting a slightly different coordination environment in agreement with the X-ray diffraction structure. There are competing antiferromagnetic exchange interactions along the rim (J 1 = -1.00 cm -1 ) and the radius (J 2 = -1.46 cm -1 ) of the wheel.

We describe herein the synthesis of (rac)- or enantiopure (S)-(-)-(2-MeBu)N(Pr)₂MeI ammonium salts. These racemic and enantiopure ammonium salts were used as cationic templates to obtain new two-dimensional (2D) ferromagnets [(rac)-(2-MeBu)N(Pr)₂Me][MnCr(C₂O₄)₃] and [(S)-(-)-(2-MeBu)N(Pr)₂Me][ΔMnΛ nCr(C₂O₄)₃]. The absolute configuration of the hexacoordinated Cr(III) metallic ion in the enantiopure 2D network was determined by a circular dichroism measurement. The structure of [(2-MeBu)N(Pr)₂Me][MnCr(C₂O₄)₃], established by single crystal X-ray diffraction, belongs to the chiral P63 space group. According to direct current (dc) magnetic measurements, these compounds are ferrromagnets with a temperature Tc = 6°K., (Copyright © 2013 Wiley Periodicals, Inc.)

The oxidation of 1,5-dimethyl-3-(2'-pyridyl)-6-thiooxotetrazane (SvdH(3) py) by benzoquinone leads to a 1:1 adduct of 1,5-dimethyl-3-(2'-pyridyl)-6-thiooxoverdazyl radical (Svdpy) with hydroquinone (hq). The single-crystal X-ray diffraction of this adduct at room temperature (RT) shows that the radicals exhibit a slight curvature that leads to the formation of alternating head-to-tail (antiparallel) stacked 1D chains. Moreover, temperature-dependent X-ray measurements at 100, 200, and 303 K reveal that the lateral slippages between the radicals of the stacks |δ(1) | and |δ(2) | vary from 0.64 to 0.78 Å and 0.54 to 0.40 Å between 100 and 303 K. Despite the alternation of the inter-radical distances and lateral slippages, the magnetic susceptibility data can be fitted with excellent agreement using a regular one-dimensional antiferromagnetic chain model with J=-5.9 cm(-1). Wavefunction-based calculations indicate an alternation of the magnetic interaction parameters correlated with the structural analysis at RT. Moreover, they demonstrate that the thermal slippage of the radicals induces a switching of the physical behavior, since the exchange interaction changes from antiferromagnetic (-0.9 cm(-1)) at 100 K to ferromagnetic (1.4 cm(-1)) at 303 K. The theoretical approach thus reveals a much richer magnetic behavior than the analysis of the magnetic susceptibility data and ultimately questions the relevance of a spin-coupled picture based on temperature-independent parameters., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The cyano-substituted metallocenes [M(C5H4CN)2] (M=Fe, 1; Co, 2; Ni 3) and [M(C5Me5)(C5H4CN)] (M=Fe, 4; Co, 5; Ni, 6) were synthesized in yields up to 58 % by treating K(C5H4CN) or Tl(C5H4CN) with suitable transition-metal precursors. Cyclic voltammetry indicated that the oxidation and reduction potentials of all the cyanometallocenes were shifted to positive values by up to 0.8 V. Single-crystal X-ray structure analysis showed that 1 had eclipsed ligands, formed planes in the lattice, and--unlike usual metallocenes--lined up in stacks perpendicular to these planes. Powder X-ray studies established that 1 and 2 are isotypic. The 1H and 13C NMR spectra were recorded for all the new compounds. Signal shifts of up to delta=1500 ppm were recorded for the paramagnetic molecules 2 and 3 and were, at a given temperature, strikingly different for solution and solid-state spectra. These results pointed to antiferromagnetic interactions as a consequence of molecular ordering in the lattice, as confirmed by magnetic measurements. The temperature-dependent susceptibilities were reproduced by Heisenberg spin-chain models (H=-J sum n- 1 i=1 SiSi+1), thus yielding J=-28.3 and -10.3 cm(-1) for 2 and 3, respectively, whereas J=-11.8 cm(-1) was obtained for 3 from the Ising spin-chain model. In accordance with molecular orbital (MO) considerations, much spin density was found to be delocalized not only on the cyclopentadienyl ligand but also the cyano substituents. The magnetic interaction was interpreted as a Heitler-London spin exchange and was analyzed based on how the interaction depends on the singly occupied MOs and the shift of parallel metallocenes relative to each other.