Photomagnetic Switching of the Complex [Nd(dmf)4(H2O)3(μ‐CN)Fe(CN)5]⋅H2O Analyzed by Single‐Crystal X‐Ray Diffraction

X‐ray vision : Single‐crystal XRD experiments (see picture) reveal the excited‐state structure of the photomagnetic heterobimetallic title complex. The system shows a decrease in all the iron–ligand bond lengths, suggesting that photoexcitation involves a ligand‐to‐metal charge transfer or a change in the superexchange coupling between the metal centers.

     

Photomagnetic Switching of Heterometallic Complexes [M(dmf)4(H2O)3(μ‐CN)Fe(CN)5]⋅H2O (M=Nd,La, Gd,Y) Analyzed by Single‐Crystal X‐ray Diffraction and Ab Initio Theory

Single‐crystal X‐ray diffraction measurements have been carried out on [Nd(dmf)₄(H₂O)₃(μ‐CN)Fe(CN)₅]?H₂O ( 1 ; dmf=dimethylformamide), [Nd(dmf)₄(H₂O)₃(μ‐CN)Co(CN)₅]?H₂O ( 2 ), [La(dmf)₄(H₂O)₃(μ‐CN)Fe(CN)₅]?H₂O ( 3 ), [Gd(dmf)₄(H₂O)₃(μ‐CN)Fe(CN)₅]?H₂O ( 4 ), and [Y(dmf)₄(H₂O)₃(μ‐CN)Fe(CN)₅]?H₂O ( 5 ), at 15(2) K with and without UV illumination of the crystals. Significant changes in unit‐cell parameters were observed for all the iron‐containing complexes, whereas 2 showed no response to UV illumination. Photoexcited crystal structures have been determined for 1 , 3 , and 4 based on refinements of two‐conformer models, and excited‐state occupancies of 78.6(1), 84(6), and 86.6(7) % were reached, respectively. Significant bond‐length changes were observed for the Fe–ligand bonds (up to 0.19 Å), the cyano bonds (up to 0.09 Å), and the lanthanide–ligand bonds (up to 0.10 Å). Ab initio theoretical calculations were carried out for the experimental ground‐state geometry of 1 to understand the electronic structure changes upon UV illumination. The calculations suggest that UV illumination gives a charge transfer from the cyano groups on the iron atom to the lanthanide ion moiety, {Nd(dmf)₄(H₂O)₃}, with a distance of approximately 6 Å from the iron atom. The charge transfer is accompanied by a reorganization of the spin state on the {Fe(CN)₆} complex, and a change in geometry that produces a metastable charge‐transfer state with an increased number of unpaired electrons, thus accounting for the observed photomagnetic effect.     

Complex Interrupted Tetrahedral Frameworks in the Nitridosilicates M7Si6N15 (M=La,Ce, Pr)

A new structure type of nitridosilicates with an interrupted framework has been identified for M₇Si₆N₁₅ with M=La, Ce, and Pr. The materials have been synthesized in a radio‐frequency furnace at temperatures between 1550–1625 °C, starting from the respective metals, metal nitrides, and silicon diimide. The crystal structure of Ce₇Si₆N₁₅ has been determined by using single‐crystal X‐ray diffraction. Besides ordered crystals 1 with a complicated triclinic superstructure and multiple twinning (P , no. 2; a=13.009(3), b=25.483(5), c=25.508(10) Å; α=117.35(3), β=99.59(3), γ=99.63(3)°; V=7114(2) ų; Z=18; R1=0.0411), disordered crystals 2 with identical composition exhibiting a trigonal average structure (R , no. 148) have also been observed (a=43.420(6), c=6.506(2) Å; V=10 623(3) ų; Z=27; R1=0.0309). Pr₇Si₆N₁₅ ( 3 ) and La₇Si₆N₁₅ ( 4 ) are isostructural with 1 as evidenced by twinned single‐crystal data for 3 (P , no. 2; a=12.966(3), b=25.449(10), c=25.459(10) Å; α=117.28(3), β=99.70(4), γ=99.60(4)°; V=7068(4) ų; Z=18; R1=0.0526) and powder diffraction data for 4 (P , no. 2; a=13.109(9), b=25.606(18), c=25.609(18) Å; V=7223(12) ų; Z=18; R_P=0.0194; R_F=0.0936). The crystal structure of M₇Si₆N₁₅ (M=La, Ce, Pr) is built up exclusively of corner‐sharing tetrahedrons that appear as Q²‐, Q³‐, and Q⁴‐type tetrahedrons forming different ring sizes within a less condensed three‐dimensional network. Among the characteristic structural motifs are saw‐blade‐shaped 12‐rings and finite chains consisting of four corner‐sharing SiN₄ tetrahedrons. High‐resolution transmission electron micrographs indicate both ordered and disordered crystallites. In the diffraction patterns of disordered rhombohedral crystals, diffuse maxima appear in reciprocal space at those positions in which sharp superstructure reflections are found in the case of the respective ordered crystallites. Magnetic susceptibility measurements of Ce₇Si₆N₁₅ show paramagnetic behavior with an experimental magnetic moment of 2.29 μ_B per Ce, thereby corroborating the existence of Ce³⁺.     

Developing a Hetero‐Alkali‐Metal Chemistry of 2,2,6,6‐Tetramethylpiperidide (TMP): Stoichiometric and Structural Diversity within a Series of Lithium/Sodium,Lithium/Potassium and Sodium/Potassium TMP Compounds

Studied extensively in solution and in the solid state, Li(TMP) (TMP=2,2,6,6‐tetramethylpiperidide) is an important utility reagent popular as a strongly basic, weakly nucleophilic tool for C? H metallation. Recently, there has been a surge in interest in mixed metal derivatives containing the bulky TMP anion. Herein, we start to develop hetero (alkali metal) TMP chemistry by reporting the N,N,N′,N′‐tetramethylethylenediamine (TMEDA)‐hemisolvated sodium–lithium cycloheterodimer [(tmeda)Na(μ‐tmp)₂Li], and its TMEDA‐free variant [{Na(μ‐tmp)Li(μ‐tmp)}_∞], which provides a rare example of a crystallographically authenticated polymeric alkali metal amide. Experimental observations suggest that the former is a kinetic intermediate en route to the latter thermodynamic product. Furthermore, a third modification, the mixed potassium–lithium‐rich cycloheterotrimer [(tmeda)K(μ‐tmp)Li(μ‐tmp)Li(μ‐tmp)], has also been synthesised and crystallographically characterised. On moving to the bulkier tridentate donor N,N,N′,N′′,N′′‐pentamethyldiethylenediamine (PMDETA), the additional ligation forces the sodium–lithium and potassium–dilithium ring species to open giving the acyclic arc‐shaped complexes [(pmdeta)Na(μ‐tmp)Li(tmp)] and [(pmdeta)K(μ‐tmp)Li(μ‐tmp)Li(tmp)], respectively. Completing the series, the potassium–lithium and potassium–sodium derivatives [(pmdeta)K(μ‐tmp)₂M] (M=Li, Na) have also been isolated as closed structures with a distinctly asymmetric central MN₂K ring. Collectively, these seven new bimetallic compounds display five distinct structural motifs, four of which have never hitherto been witnessed in TMP chemistry and three of which are unprecedented in the vast structural library of alkali metal amide chemistry.     

Decanuclear Ln10 Wheels and Vertex‐Shared Spirocyclic Ln5 Cores: Synthesis,Structure, SMM Behavior,and MCE Properties

The reaction of a Schiff base ligand (LH₃) with lanthanide salts, pivalic acid and triethylamine in 1:1:1:3 and 4:5:8:20 stoichiometric ratios results in the formation of decanuclear Ln₁₀ (Ln=Dy( 1 ), Tb( 2 ), and Gd ( 3 )) and pentanuclear Ln₅ complexes (Ln=Gd ( 4 ), Tb ( 5 ), and Dy ( 6 )), respectively. The formation of Ln₁₀ and Ln₅ complexes are fully governed by the stoichiometry of the reagents used. Detailed magnetic studies on these complexes ( 1 – 6 ) have been carried out. Complex 1 shows a SMM behavior with an effective energy barrier for the reversal of the magnetization (U_eff)=16.12(8) K and relaxation time (τ_o)=3.3×10^?5 s under 4000 Oe direct current (dc) field. Complex 6 shows the frequency dependent maxima in the out‐of‐phase signal under zero dc field, without achieving maxima above 2 K. Complexes 3 and 4 show a large magnetocaloric effect with the following characteristic values: ?ΔS_m=26.6 J kg^?1 K^?1 at T=2.2 K for 3 and ?ΔS_m=27.1 J kg^?1 K^?1 at T=2.4 K for 4 , both for an applied field change of 7 T.     

XAS and XMCD investigation of Mn12 monolayers on gold

The deposition of Mn(12) single molecule magnets on gold surfaces was studied for the first time using combined X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) methods at low temperature. The ability of the proposed approach to probe the electronic structure and magnetism of Mn(12) complexes without significant sample damage was successfully checked on bulk samples. Detailed information on the oxidation state and magnetic polarization of manganese ions in the adsorbates was obtained from XAS and XMCD spectra, respectively. Partial reduction of metal ions to Mn(II) was clearly observed upon deposition on Au(111) of two different Mn(12) derivatives bearing 16-acetylthio-hexadecanoate and 4-(methylthio)benzoate ligands. The average oxidation state, as well as the relative proportions of Mn(II), Mn(III) and Mn(IV) species, are strongly influenced by the deposition protocol. Furthermore, the local magnetic polarizations are significantly decreased as compared with bulk Mn(12) samples. The results highlight an utmost redox instability of Mn(12) complexes at gold surfaces, presumably accompanied by structural rearrangements, which cannot be easily revealed by standard surface analysis based on X-ray photoelectron spectroscopy and scanning tunnelling microscopy.     

Molecular dynamics (MD) simulations and large-angle X-ray scattering (LAXS) studies of the solid-state structure and assembly of isotactic (R)-poly(2,2'-dioxy-1,1'-binaphthyl-)phosphazene in the bulk state and in the cast film

The intrachain conformation, molecular structure and interchain assembly of isotactic (R)-poly(2,2'-dioxy-1,1'-binaphthyl)phosphazene (P-DBNP) both in the bulk state (I) and in the cast film (II) were studied by molecular dynamics (MD) simulations of models, as implemented by a bias potential for the analysis of the radial distribution function (RDF) obtained from large-angle X-ray scattering (LAXS) data. The microscopic structure and order extension of the polymer changed from I to II, as qualitatively shown in the shapes of their experimentally measured RDF curves. With the use of a bias potential, the MD simulations provided a much more accurate analysis of the models, as seen in the reproduction of the RDFs. The chiral P-DBNP chain was found to be consistent with helix conformations in both the I and the II samples. The predominant interchain clustering motif was best reproduced with a seven-chain model. In the case of I, the maximum chain length was 18 monomeric -R(2)NP- units, while in the case of the cast film II the chain was more elongated, up to distances of approximately 100 A, equivalent to over 48 monomeric -R(2)NP- units. The seven-chain assembly was accounted for in terms of nonbonded interactions favouring the minimum voids area between the seven tubular structures of the material. The results validate our earlier finding that MD analysis with implementation of a biasing potential for the RDFs can provide quantitative information on the structural and conformational features of amorphous solids. The combined theoretical and experimental approach was found to be a useful tool to detect, locate and evaluate the intra- and intermolecular modifications of materials subsequent to their phase transformation and, as in the present case, changes in their microscopic structures or preparation methods.     

氰根桥联三核异金属配合物的合成、晶体结构及磁性研究

采用[(Tp)Fe(CN)₃]-(Tp=hydrotris(pyrazolyl)borate)与Mn(Ac)₂·4H₂O反应，合成了氰根桥联的异金属三核配合物[Mn(phen)₂][(Tp)Fe(CN)₃]₂·5H₂O (1)(phen=1，10-phenanthroline)，并对其结构和磁性进行了研究。晶体结构分析结果表明该化合物晶体属于三斜晶系，P1空间群。在该配合物中，Mn(Ⅱ)与2个phen分子及2个[(Tp)Fe(CN)₃]^-配位，形成一种弯曲的三核结构。磁性测量结果表明，Mn(Ⅱ)和Fe(Ⅲ)之间通过氰根桥联产生弱的反铁磁相互作用。     

High-spin- and low-spin-state structures of [Fe(chloroethyltetrazole)6](ClO4)2 from synchrotron powder diffraction data

The spin-crossover complex [Fe(teec)(6)](ClO(4))(2) (teec = chloroethyltetrazole) exhibits a 50 % incomplete spin crossover in the temperature range 300-30 K. Time-resolved synchrotron powder diffraction experiments have been carried out to elucidate its structural behavior. We report crystal structure models of this material at 300 K (high spin) and 90 K (low spin), as solved from synchrotron powder diffraction data by using Genetic Algorithm and Parallel Tempering techniques and refined with Rietveld refinement. During short synchrotron powder diffraction experiments (five minutes duration) two distinguishable lattices were observed the quantities of which vary with temperature. The implication of this phenomenon, that is interpreted as a structural phase transition associated with the high-to-low spin crossover, and the structural characteristics of the high-spin and low-spin models are discussed in relation to other compounds showing a similar type of spin-crossover behavior.