Cubane-like Structure Research Articles

The Role of a Confined Space on the Reactivity and Emission Properties of Copper(I) Clusters.

Metal clusters have gained a lot of interest for their remarkable photoluminescence and catalytic properties. However, a major drawback of such materials is their poor stability in air and humidity conditions. Herein we describe a versatile method to synthesize luminescent Cu(I) clusters inside the pores of zeolites, using a sublimation technique with the help of high vacuum and high temperature. The porous materials play an essential role as a protecting media against the undesirable and easy oxidation of Cu(I). The obtained clusters show fascinating luminescence properties, and their reactivity can be triggered by insertion in the pores of organic monodentate ligands such as pyridine or triphenylphosphine. The coordinating ligands can lead to the formation of Cu(I) complexes with completely different emission properties. In the case of pyridine, the final compound was characterized and identified as a cubane-like structure. A thermochromism effect is also observed, featuring, for instance, a hypsochromic effect for a phosphine derivative at 77K. The stability of the encapsulated systems in zeolites is rather enthralling: they are stable and emissive even after several months in the air.

A new series of Schiff base Ni(II)4 cubanes: Evaluation of magnetic coupling via carboxylate bridges

The Schiff base ligand H2L has been synthesized by condensation of 2-aminophenol with 3-methyl-1-phenyl-4-formylpyrazol-5-one and reaction between H2L and Ni(OOCR3)2, R = H in 1, –CH3 in 2, Cl in 3, yielded three new tetranuclear Ni(II) complexes. The complexes have been characterized by elemental analysis, IR- and ES-MS spectroscopy. Their structures as well as structure of ligand were determined by single-crystal X-ray diffraction. Compounds 1–3 possesses tetranuclear cubane-like structures containing [Ni4L4(R3COO)2]2− complex anions, which are charge balanced by two triethyl ammonium cations. Furthermore, the crystal structure of the Ni(II) cubane compound containing trichloroacetate bridging ligands, is reported for the first time. Variable temperature magnetic susceptibility measurements revealed interplay between ferromagnetic and antiferromagnetic exchange in the tetranuclear cubane-like compounds 1–3, in which ferromagnetic interactions were enhanced by introducing carboxylate bridging groups. DFT calculations supported the analysis of magnetic data.

Determining the Activation Energy of Photoassembly of the Oxygen‐evolving Complex Using an Alternative Redox Mediator

The oxygen‐evolving complex (OEC) is a well‐researched Mn4CaO5 cluster with a cubane‐like structure that is responsible for oxidizing water to fuel photosynthetic electron transfer. It is a cofactor of photosystem II (PSII), which catalyzes light‐induced charge separation and oxidation of the OEC. PSII is comprised of pigment molecules and cofactors that transport electrons to a mobile plastoquinone that is located at the QB site. These reducing equivalents can eventually be used to reduce CO2 into simple sugars. In nature, the OEC is constantly degraded as a result of PSII photodamage and must be reassembled through a light‐dependent process after the affected PSII is repaired. OEC assembly occurs in the presence of light in vivo, but if conducted in situ is significantly enhanced by including a dark incubation after the first light dependent step. This dark‐dependent step is the rate‐limiting step of OEC assembly. Determining the activation energy will provide insight into the mechanism of OEC formation and provide clues on how artificial systems could be engineered.To conduct these studies, subcellular thylakoid membranes were isolated and enriched in PSII and the Mn4CaO5 cluster was removed via an alkaline incubation to generate apo‐PSII. The apo‐PSII enriched membranes were then used to measure the efficiency of OEC assembly in situ at temperatures ranging from 5 – 45 °C using Clark type oximetry. However, when OEC assembly occurs in apo‐PSII‐enriched membranes the rate‐determining step is electron shuttling from the QB site to the terminal electron acceptor {K3[Fe(CN)6]} via the native plastoquinones, thus preventing an accurate determination of the activation energy. This issue was remedied by supplementing with an alternative redox mediator know as phenyl‐p‐benzoquinone (PPBQ), which has been shown to increase the rate of electron shuttling from the QB site to a terminal electron acceptor. OEC photoassembly was successful in the presence of PPBQ and the oxygen production at varying temperatures was used to produce an Arrhenius plot to calculate the activation energy of OEC assembly.Currently, the activation energies have been determined for photoassembled PSII‐enriched membranes in the presence of PPBQ (18 kJ/mol) and native plastoquinones (49 kJ/mol). From these results, we speculate that OEC photoassembly in apo‐PSII membranes with PPBQ allows us to measure the true rate‐limiting step.Support or Funding InformationE.N. was supported by a fellowship from the SURE Program funded through the WV Research Challenge Fund, and administered by the WV HEPC, Division of Science and Research, Award Number DSR.17.09. E.N. had additional support from the WV NASA Space Grant Consortium’s NASA WVSG Undergraduate Fellowship Program and from the DOW Undergraduate Research Fellowship.D.K. and A.S. were supported by the National Science Foundation under Cooperative Agreement No. OIA‐1458952. Amanda Smythers was also funded by the NASA West Virginia Space Grant Consortium, Grant #NNX15AK74A and Training Grant #NNX15AI01H.J.B. had additional support from the WV NASA Space Grant Consortium’s NASA WVSG Undergraduate Fellowship Program and from the DOW Undergraduate Research Fellowship.

А New Series of Schiff Base Ni(II)4 Cubanes: Evaluation of Magnetic Coupling Via Carboxylate Bridges

The Schiff base ligand H 2 L has been synthesized by condensation of 2-aminophenol with 3-methyl-1-phenyl-4-formylpyrazol-5-one and reaction with Ni(OOCR 3 ) 2 , R = H in 1 , -CH 3 in 2 , Cl in 3 , to yield three new tetranuclear Ni(II) complexes The ligands and the complexes have been characterized by elemental analysis, IR- and NMR- spectroscopy. Their structures were determined by single-crystal X-ray diffraction. Compounds 1 - 3 possesses tetranuclear cubane-like structures containing [Ni 4 L 4 (R 3 COO) 2 ] 2- complex as anions, which are charge balanced by two triethyl ammonium cations. Furthermore, the crystal structure of the Ni(II) cubane compound containing trichloroacetate bridging ligands, is reported for the first time,. Variable temperature magnetic susceptibility measurements revealed interplay between ferromagnetic and antiferromagnetic exchange in the tetranuclear cubane-like compounds 1-3 , in which ferromagnetic interactions were enhanced by introducing carboxylate bridging groups. DFT calculations supported the analysis of magnetic data.

Synthesis, Magnetic and High-Field EPR Investigation of Two Tetranuclear NiII-Based Complexes.

Two tetranuclear compounds with a cubane-like structure were synthesized from a one-pot reaction between NiII and 2,2,6,6-tetramethyl-3,5-heptanedione (Hdpm) for 1 or 4,4,4-trifluoro-1-phenyl-1,3-butanedione (Hbta) for 2 in the presence of sodium methoxide. The crystal structures of both compounds have been determined by single-crystal X-ray diffraction, and their magnetic properties have been studied by SQUID magnetometry as well as by high-field electron paramagnetic resonance (HFEPR) spectroscopy. For 1, the temperature dependence of the magnetic susceptibility can be fitted by taking into account Ni···Ni ferromagnetic interactions, which leads to an S = 4 ground-state spin. For 2, both antiferromagnetic and ferromagnetic interactions are present. However, the latter are dominant, which also leads to an S = 4 ground-state spin, in good agreement with the HFEPR study.

Architecture of Biomimetic Water Oxidation Catalyst with Mn4CaO5 Clusterlike Structure Unit.

Mn4CaO5 cluster in green plant is considered as the ideal structure for water oxidation catalysis. However, this structure is difficult to be constructed in heterogeneous catalyst because of its distorted spatial structure and unique electronic state. Herein, we report the synthesis of two-dimensional biomimetic Ca-Mn-O catalyst with Mn4CaO5 clusterlike structure through ultrasonic-assisted reduction treatment toward Ca-birnessite. The synergistic effect between ultrasonic and reduction successfully reduced the Mn oxidation state in Ca-birnessite without breaking the structure of MnO2 monolayers, forming a regular two-dimensional structure with Mn4CaO5 cubanelike structure unit for the first time. The biomimetic catalyst shows a superior water oxidation activity (turnover frequency = 3.43 s-1), which is the best in manganese-based heterogeneous catalyst to date. This work provides a new strategy for the precise synthesis of specific structure and exhibits a great prospect of biomimic in heterogeneous catalyst.

Dinuclear and tetranuclear copper(I) iodide complexes with P and P^N donor ligands: Structural and photoluminescence studies

In order to investigate photophysical properties and crystallization medium effects, three Cu complexes, [CuI(PCy3)]2 (1), [Cu(I)(PPh2Me)]4 (2), and [Cu4(I)4(PPh2py)2] (3) were prepared and characterized using single crystal X-ray crystallography, solid state NMR, UV–Vis, and IR spectroscopy. In 1, as previously reported, Cu2I2 core is a square dimer with alternating copper and iodides positioned in the corners. Each copper atom is surrounded by two bridging iodides and a terminal tricyclohexylphosphine ligand. The cluster 2 adopts a cubane-like Cu4I4 core, while in cluster 3 (as thf solvate) a Cu4I2 distorted octahedral core is decorated with two bridging iodides and two chelating P^N ligands. The solid state 31P NMR analysis showed the presence in the powder of several polymorphs, the most abundant of which being, in the case of 2, the cubane-like structure resolved by XRD methods, and in the case of 3, a butterfly structure, different from that found for 3·thf. The solid state blue photoluminescence intensity in 1 is dependent upon its poly- or single-crystallinity. While dimer 1 in polycrystalline form is a stronger blue light emitter under 254 nm light irradiation than in 365 nm, in single crystalline phase it shows a similar higher intensity blue luminescence under lower energy light (365 nm) irradiation. On the other hand, clusters 2 and 3 do not show phase-dependent luminescence intensity variation. Time-dependent Density Functional Theory (TD-DFT) showed that the emission in 1 and 3 is assigned to a mixed metal to ligand and halide to ligand charge transfer, (X + M)LCT. In 2, a dual emission band at low temperature was observed which was attributed to 3CC (LE band) and 3XLCT (HE band). Thermochromism and mechanochromism of complexes 1–3 were also investigated.

Synthesis, Characterization and Luminescent Properties of Copper(I) Halide Complexes Containing 1-(Diphenylphosphino)naphthalene

A series of neutral luminescent tetranuclear and mononuclear copper(I) halide complexes, [Cu4I4(dpna)4] (1) and [CuX(dpna)2] dpna = 1-(diphenylphosphino)naphthalene, X = Br (2), Cl (3), were synthesized, and their molecular structures and photophysical properties were investigated. Complex 1 has a cubane-like structure with four Cu atoms pseudo-tetrahedral and four I atoms serving as μ3-bridges. The coordination geometry of the copper centers in 2–3 are trigonal-planar. In the solid state, complexes 1–3 display blue photoluminescence (λmax = 423–463 nm) at 298 K. The emission of the complexes originate from the (σ+X)→π* transition. All three complexes displayed good thermal stability.

Cooperative Crystallization of Heterometallic Indium-Chromium Metal-Organic Polyhedra and Their Fast Proton Conductivity.

Metal-organic polyhedra (MOPs) or frameworks (MOFs) based on Cr(3+) are notoriously difficult to synthesize, especially as crystals large enough to be suitable for characterization of the structure or properties. It is now shown that the co-existence of In(3+) and Cr(3+) induces a rapid crystal growth of large single crystals of heterometallic In-Cr-MOPs with the [M8L12] (M=In/Cr, L=dinegative 4,5-imidazole-dicarboxylate) cubane-like structure. With a high concentration of protons from 12 carboxyl groups decorating every edge of the cube and an extensive H-bonded network between cubes and surrounding H2O molecules, the newly synthesized In-Cr-MOPs exhibit an exceptionally high proton conductivity (up to 5.8×10(-2) S cm(-1) at 22.5 °C and 98% relative humidity, single crystal).

Binuclear and polynuclear cymantrenecarboxylate complexes of heavy lanthanides

New binuclear cymantrenecarboxylate complexes of rare-earth metals, [Ln2(μ-O,η2-O2CCym)2(μ2-O,O′-O2CCym)2(η2-O2CCym)2(DMSO)4] (Ln = Tb (I), Dy (II); Cym = (η5-C5H4)Mn(CO)3) and [Ln2(μ2-O,O′-O2CCym)4(η2-NO3)2(DMSO)4] (Ln = Tb (III), Dy (IV)), are synthesized and characterized by X-ray diffraction analysis. The carboxylate clusters containing the Mn2+ ion, which is formed due to the destruction of the cymantrenenecarboxylate anion, [Tb4(μ3-OH)4(μ2-O,O′-O2CCym)6(H2O)3(THF)4][MnCl4] · 4CH2Cl2 · 6THF (V) with the cubane-like structure, and [Er2Mn(μ2-O2CCym)6(η2-O2CCym)2((MeO)3PO)4] · 2MePh (VI) with linear structure, are also obtained. The magnetism of complexes I, II, V, and VI is studied in a direct magnetic field. The magnetic properties of complexes II and VI are studied in direct and alternating magnetic fields. Complex II exhibits the properties of a single-molecule magnet. The thermal decomposition of complexes I–IV is studied by differential scanning calorimetry and thermogravimetric analysis. According to the X-ray diffraction analysis data, the final thermolysis products of complexes III and IV in air are multiferroics LnMn2O5.

Hetero-octanuclear cubane-like and one-dimensional cyano complexes with the N,N-dimethylethylenediamine ligand

A series of complexes containing dmen (dmen=N,N-dimethylethylenediamine) units and cyano groups, namely bimetallic octanuclear and binuclear [Zn4(dmen)8Ni4(CN)16] (1), [Cd4(dmen)8Ni4(CN)16] (2), [Zn(dmen)2Pd(μ-CN)2(CN)2] (3) and [Cd(dmen)2Pd(μ-CN)2(CN)2] (4), have been synthesized and characterized by elemental, spectral (FT-IR and Raman) and thermal analyses techniques. The molecular structures of the complexes have been determined by the single crystal X-ray diffraction technique. The X-ray analyses of 1 and 2 show heterometallic octanuclear cubane-like structures in which Zn(II) or Cd(II) and Ni(II) ions reside in alternate corners, whereas complexes 3 and 4 display one-dimensional chain structures.

Synthesis, crystal structure, and photophysical properties of a double open cubane-like Cd(II) complex based on 2-substituted-8-hydroxyquinoline

A 2-substituted-8-hydroxyquinoline (E)-2-[2-(3-thienyl)ethenyl]-8-quinolinol (HL) was synthesized and characterized by ESI-MS, NMR spectroscopy, and elemental analysis. Using solvothermal method, a tetranuclear complex [Cd4L6Br2]·6DMF (1) was fabricated by assembly of Cd(II) with HL. X-ray structural analysis shows that 1 exhibits a double open cubane-like core structure, which is bridged by six 8-hydroxyquinolinate-based ligands. The supramolecular structure of 1 features a 3-D porous solid constructed by aromatic stacking interactions, C–H···π interactions and C–H···O hydrogen bonds. The assembly of cadmium salts and HL in solution was investigated by UV–vis and photoluminescence. We also studied the thermal stability and photophysical properties (fluorescent emission, lifetime, and quantum yield) of 1. The results show that 1 emits yellow luminescence in the solid state.

Photosystem II: its function, structure, and implications for artificial photosynthesis.

Somewhere in the region of 3 billion years ago an enzyme emerged which would dramatically change the chemical composition of our planet and set in motion an unprecedented explosion in biological activity. This enzyme used solar energy to power the thermodynamically and chemically demanding reaction of water splitting. In so doing it provided biology with an unlimited supply of hydrogen equivalents needed to convert carbon dioxide into the organic molecules of life. The enzyme, which facilitates this reaction and therefore underpins virtually all life on our planet, is known as Photosystem II (PSII). It is a multisubunit enzyme embedded in the lipid environment of the thylakoid membranes of plants, algae, and cyanobacteria. Over the past 10 years, crystal structures of a 700kDa cyanobacterial dimeric PSII complex have been reported with ever increasing improvement in resolution with the latest being at 1.9 Å. Thus the organizational and structural details of its many subunits and cofactors are now well understood. The water splitting site was revealed as a cluster of four Mn ions and a Ca ion surrounded by amino acid side chains, of which seven provide ligands to the metals. The metal cluster is organized as a cubane-like structure composed of three Mn ions and the Ca2+ linked by oxo-bonds with the fourth Mn attached to the cubane via one of its bridging oxygens together with another oxo bridge to a Mn ion of the cubane. The overall structure of the catalytic site is providing a framework on which to develop a mechanistic scheme for the water splitting process and gives a blue print and confidence for the development of catalysts for mimicking the reaction in an artificial photo-electrochemical system to generate solar fuels.

Syntheses, characterization, and structural studies of copper(I) complexes containing 1,1'-bis(di-tert-butylphosphino)ferrocene (dtbpf) and their application in palladium-catalyzed Sonogashira coupling of aryl halides.

Four copper(I) complexes [Cu4(μ3-Cl)4(μ-dtbpf)2] (1), [Cu6(μ3-Br)4(μ2-Br)2(μ-dtbpf)3] (2), [Cu4(μ2-I)2(μ3-I)2(μ-dtbpf)2] (2) and [Cu2(μ1-CN)2(κ(2)-dtbpf)2] (4) were prepared using CuX (X = Cl, Br, I, CN) and 1,1'-bis(di-tert-butylphosphino)ferrocene (dtbpf) in 2 : 1 molar ratio in DCM-MeOH (50 : 50 V/V) at room temperature. These complexes were characterized by elemental analyses, IR, (1)H and (31)P NMR, ESI(+)-MS and electronic absorption spectroscopy. Molecular structures of the complexes 1,2 and 3 were determined crystallographically. Complexes 1 and 3 are tetrameric Cu(I) pseudo-cubane like structures with a Cu4Cl4 core, and a stepped cubane-like structure with a Cu4I4 core, respectively, whereas complex 2 shows a trimeric copper(I) framework containing two [Cu3(μ3-Br)2(μ2-Br)] units that are bridged by three bidentate μ-dtbpf ligands. Each [Cu3(μ3-Br)2(μ2-Br)] unit forms a pyramid with one copper atom at the apex and one of the triangular faces capped by one bromine atom. All the complexes were found to be efficient catalysts for the Sonogashira reaction. The coupling products were obtained in moderate to good yields (58-99%) using Pd loadings of 0.2 mol% as well as complex loading of 0.1 mol%. The (31)P NMR studies show that all the complexes are unstable during the course of tandem catalytic reaction and the dtbpf ligand migrates from copper(I) to palladium(II), which promotes palladium Sonogashira cross-coupling of activated and non-activated aryl halides.

New iodocuprates(I) with N-heterocyclic molecules as the cations

Abstract Under the hydrothermal conditions, the reactions between CuI, KI and bp/bpp (bp=4,4′-bipiperidine, bpp=1,3-bis(4-piperidyl)propane) in an acidic alcohol solution produced three new organically templated iodocuprates(I) as [H2bp]2[Cu2I6] 1, [tmbp][Cu2I4] 2 and [tmbpp] 2 [Cu4I8]·2H2O 3 (tmbp2+=N,N,N′,N′-tetramethyl-4,4′-bipiperidinium; tmbpp2+=N,N,N′,N′-tetramethyl-1,3-bis(4-piperidyl)propane dication). X-ray analysis revealed that (i) tmbp2+ and tmbpp2+ in compounds 2 and 3 originated from the complete N-alklation of bp/bpp with CH3OH; (ii) templated by H2bp2+, the inorganic anion [Cu2I6]2− of 1 possesses a dinuclear structure, whereas templated by tmbp2+, the inorganic anion [Cu2I4]2− of 2 exhibits a one-dimensional (1D) chain structure; (iii) templated by tmbpp2+, the inorganic anion [Cu4I8]4− of 3 shows a cubane-like structure modified by four terminal I− ions. The photoluminescence analysis indicates that compounds 1 and 2 emit blue light, while compound 3 emits green light.

A thiocyanato-bridged copper(i) cubane complex and its application in palladium-catalyzed Sonogashira coupling of aryl halides

Reaction of copper(I) thiocyanate with 1,1'-bis(di-tert-butylphosphino) ferrocene (dtbpf) in a 2:1 molar ratio in DCM-MeOH (50:50 V/V) afforded a tetranuclear copper(I) complex [Cu4(μ3-SCN)4(κ(1)-P,P-dtbpf)2] (1) with a cubane-like structure. Complex 1 was shown to be an efficient catalyst in comparison to CuI in the Sonogashira reaction. The coupling products were obtained in high yields by using Pd loadings of 0.2 mol% as well as complex-1 of 0.1 mol%.

Methylene-bis[(aminomethyl)phosphinic acids]: synthesis, acid–base and coordination properties

Three symmetrical methylene-bis[(aminomethyl)phosphinic acids] bearing different substituents on the central carbon atom, (NH(2)CH(2))PO(2)H-C(R(1))(R(2))-PO(2)H(CH(2)NH(2)) where R(1) = OH, R(2) = Me (H(2)L(1)), R(1) = OH, R(2) = Ph (H(2)L(2)) and R(1),R(2) = H (H(2)L(3)), were synthesized. Acid-base and complexing properties of the ligands were studied in solution as well as in the solid state. The ligands show unusually high basicity of the nitrogen atoms (log K(1) = 9.5-10, log K(2) = 8.5-9) if compared with simple (aminomethyl)phosphinic acids and, consequently, high stability constants of the complexes with studied divalent metal ions. The study showed the important role of the hydroxo group attached to the central carbon atom of the geminal bis(phosphinate) moiety. Deprotonation of the hydroxo group yields the alcoholate anion which tends to play the role of a bridging ligand and induces formation of polynuclear complexes. Solid-state structures of complexes [H(2)N=C(NH(2))(2)][Cu(2)(H(-1)L(2))(2)]CO(3)·10H(2)O and Li(2)[Co(4)(H(-1)L(1))(3)(OH)]·17.5H(2)O were determined by X-ray diffraction. The complexes show unexpected geometries forming dinuclear and cubane-like structures, respectively. The dinuclear copper(II) complex contains a bridging μ(2)-alcoholate group with the (-)O-P(=O)-CH(2)-NH(2) fragments of each ligand molecule chelated to the different central ion. In the cubane cobalt(II) complex, one μ(3)-hydroxide and three μ(3)-alcoholate anions are located in the cube vertices and both phosphinate groups of one ligand molecule are chelating the same cobalt(II) ion while each of its amino groups are bound to different neighbouring metal ions. All such three metal ions are bridged by the alcoholate group of a given ligand.

On the Controversial Fitting of Susceptibility Curves of Ferromagnetic CuII Cubanes: Insights from Theoretical Calculations

This paper reports a theoretical analysis of the electronic structure and magnetic properties of a tetranuclear Cu(II) complex, [Cu(4) (HL)(4)], which has a 4+2 cubane-like structure (H(3) L=N,N'-(2-hydroxypropane-1,3-diyl)bis(acetylacetoneimine)). These theoretical calculations indicate a quintet (S=2) ground state; the energy-level distribution of the magnetic states confirm Heisenberg behaviour and correspond to an S(4) spin-spin interaction model. The dominant interaction is the ferromagnetic coupling between the pseudo-dimeric units (J(1) =22.2 cm(-1)), whilst a weak and ferromagnetic interaction is found within the pseudo-dimeric units (J(2) =1.4 cm(-1)). The amplitude and sign of these interactions are consistent with the structure and arrangement of the magnetic Cu 3d orbitals; they accurately simulate the thermal dependence of magnetic susceptibility, but do not agree with the reported J values (J(1) =38.4 cm(-1), J(2) =-18.0 cm(-1)) that result from the experimental fitting. This result is not an isolated case; many other polynuclear systems, in particular 4+2 Cu(II) cubanes, have been reported in which the fitted magnetic terms are not consistent with the geometrical features of the system. In this context, theoretical evaluation can be considered as a valuable tool in the interpretation of the macroscopic behaviour, thus providing clues for a rational and directed design of new materials with specific properties.

Condensation of a Nickel Tetranuclear Cubane into a Heptanuclear Single-Molecule Magnet

A tetranuclear complex, [Ni(4)], with a cubane-like structure synthesized from hexafluoroacetylacetone gives, after drying at high temperature and treatment with pyridine, a heptanuclear nickel(II) complex, [Ni(7)]. The crystal structures of both compounds have been determined by single-crystal X-ray diffraction. Their magnetic properties have been studied by SQUID and μ-SQUID magnetometry as well as by high-frequency EPR spectroscopy (HF-EPR). For [Ni(4)], the temperature dependence of the magnetic susceptibility can be fitted by taking into account strong Ni···Ni ferromagnetic interactions which lead to an S = 4 ground-state spin, in good agreement with the HF-EPR study. For [Ni(7)], the temperature dependence of the magnetic susceptibility shows that the Ni···Ni ferromagnetic interactions are kept within the metal core. However, it was not possible to fit this with a clear set of parameters, and the ground-state spin was undetermined. The field dependence of the magnetization indicates an S = 7 ground-state spin at high field. In contrast, the temperature dependence of the magnetic susceptibility indicates a ground-state spin of S = 6 or even S = 5. These results agree with complicated high-frequency EPR spectra which have been ascribed to the superposition of signals from the ground spin multiplet and from an excited spin multiplet very close in energy, with the excited state having a larger S value than the ground state. Very low temperature studies show that only the heptanuclear complex behaves as a single-molecule magnet.

Synthesis, structure, and catalytic activity of a new zinc complex derived from a chiral hydroxyazetidine ligand

Abstract A new chiral tetrameric zinc complex [(1)ZnEt]4 (2) has been readily prepared via alkane elimination between ZnEt2 and a chiral azetidine derivative, (2R,3R)-1-[(1S)-1-(4-methoxyphenyl)ethyl]-2-phenyl-3-hydroxyazetidine (1H). Complex 2 has been characterized by various spectroscopic techniques, elemental analyses, and X-ray diffraction analysis. Complex 2 has a cubane-like Zn4O4 core structure, and is an active catalyst for the polymerization of rac-lactide, leading to the heterotactic-rich polylactides.