Crystal chemistry and electronic structure of the metallic lithium ion conductor, LiNiN

The layered ternary nitride LiNiN shows an interesting combination of fast Li+ ion diffusion and metallic behavior, properties which suggest potential applications as an electrode material in lithium ion batteries. A detailed investigation of the structure and properties of LiNiN using powder neutron diffraction, ab initio calculations, SQUID magnetometry, and solid-state NMR is described. Variable-temperature neutron diffraction demonstrates that LiNiN forms a variant of the parent Li3N structure in which Li+ ion vacancies are ordered within the [LiN] planes and with Ni exclusively occupying interlayer positions (at 280 K: hexagonal space group Pm2, a = 3.74304(5) A, c = 3.52542(6) A, Z = 1). Calculations suggest that LiNiN is a one-dimensional metal, as a result of the mixed pi- and sigma-bonding interactions between Ni and N along the c-axis. Solid-state 7Li NMR spectra are consistent with both fast Li+ motion and metallic behavior.     

Relationship between the electrochemical behavior and Li arrangement in Li(x)M(y)Mn(2-y)O4 (M = Co, Cr) with spinel structure

The relationship between the electrochemical behavior and the arrangement of lithium/vacancies has been investigated with electrochemical Li removal in Li(x)M(y)Mn(2-y)O4 (x < or = 1.0, 0.0 < or = y < or = 0.3, M = Co, Cr). It was shown that the electrochemical removal proceeds via two voltage regions: (1) approximately 3.9 V at x > or = approximately 0.5 and (2) approximately 4.2 V at x < or = approximately 0.5. To understand the stepwise behavior, entropy measurement of reaction, DeltaS(obs), was performed by using the electrochemical methods. The changes of the sign in deltaS(obs) from negative to positive at the composition x approximately 0.50 in Li(x)M(y)Mn(2-y)O4 indicated that the ordered arrangement of Li/vacancies was formed with electrochemical Li removal. Moreover, such an ordering was suppressed by the substitution of Co3+ and Cr3+ for Mn3+. To clarify the nature and origin of Li/vacancy ordering, the Monte Carlo simulation was performed in view of Coulombic interaction. The simulation reproduced the formation of a new phase arising from Li/vacancy ordering at x = 0.50 in Li(x)Mn2O4. In addition, the ordered arrangement of Li/vacancy at x = 0.5 was perturbed by the trivalent M3+ replacement in spinel structure due to the local clustering of Li+ around M3+. Consequently, the electrochemical behavior in spinel LiMn2O4 was deeply related to the Coulombic interactions, proved by the fact that experimentally observed changes in entropy agreed well with Monte Carlo simulation based on the Coulombic interaction.     

Synthesis and characterization of (smif)2M(n) (n = 0, M = V, Cr, Mn, Fe, Co, Ni, Ru; n = +1, M = Cr, Mn, Co, Rh, Ir; smif =1,3-di-(2-pyridyl)-2-azaallyl)

A series of Werner complexes featuring the tridentate ligand smif, that is, 1,3-di-(2-pyridyl)-2-azaallyl, have been prepared. Syntheses of (smif)(2)M (1-M; M = Cr, Fe) were accomplished via treatment of M(NSiMe(3))(2)(THF)(n) (M = Cr, n = 2; Fe, n = 1) with 2 equiv of (smif)H (1,3-di-(2-pyridyl)-2-azapropene); ortho-methylated ((o)Mesmif)(2)Fe (2-Fe) and ((o)Me(2)smif)(2)Fe (3-Fe) were similarly prepared. Metatheses of MX(2) variants with 2 equiv of Li(smif) or Na(smif) generated 1-M (M = Cr, Mn, Fe, Co, Ni, Zn, Ru). Metathesis of VCl(3)(THF)(3) with 2 Li(smif) with a reducing equiv of Na/Hg present afforded 1-V, while 2 Na(smif) and IrCl(3)(THF)(3) in the presence of NaBPh(4) gave [(smif)(2)Ir]BPh(4) (1(+)-Ir). Electrochemical experiments led to the oxidation of 1-M (M = Cr, Mn, Co) by AgOTf to produce [(smif)(2)M]OTf (1(+)-M), and treatment of Rh(2)(O(2)CCF(3))(4) with 4 equiv Na(smif) and 2 AgOTf gave 1(+)-Rh. Characterizations by NMR, EPR, and UV-vis spectroscopies, SQUID magnetometry, X-ray crystallography, and DFT calculations are presented. Intraligand (IL) transitions derived from promotion of electrons from the unique CNC(nb) (nonbonding) orbitals of the smif backbone to ligand π*-type orbitals are intense (ε ≈ 10,000-60,000 M(-1)cm(-1)), dominate the UV-visible spectra, and give crystals a metallic-looking appearance. High energy K-edge spectroscopy was used to show that the smif in 1-Cr is redox noninnocent, and its electron configuration is best described as (smif(-))(smif(2-))Cr(III); an unusual S = 1 EPR spectrum (X-band) was obtained for 1-Cr.     

Comparative structural analysis of LiMPO<Subscript>4</Subscript> and Li<Subscript>2</Subscript>MPO<Subscript>4</Subscript>F (M = Mn,Fe, Co,Ni) according to XRD,IR, and NMR spectroscopy data

A comparative structural study of LiMPO₄ (M = Mn, Fe, Co, Ni) orthophosphates and Li₂MPO₄F (M = Co, Ni) fluorophosphates obtained by mechanochemically assisted solid-state synthesis is performed using powder XRD, IR, and NMR spectroscopy methods. It is shown that all compounds crystallize in the orthorhombic symmetry (space group Pnma). Lattice parameters decrease on passing from Mn to Ni, which is due to the decrease in the ionic radius of the d metal. According to the IR spectroscopy data, in this series an increase in the covalency of the P–O bond is observed along with a decrease in the covalency of the M–O bond. On passing to fluorophosphates, the symmetry of PO₄ tetrahedra increases. ⁶Li and ³¹P NMR spectra of all compounds are characterized by the dependence of the contact shift on the nature of metal M and the degree of distortion of the MO₆ coordination polyhedron. ⁶Li MAS NMR line width is noticeably affected by the concentration of structural defects. Unlike orthophosphates with equivalent lithium ions, fluorophosphates contain lithium ions in three different positions.     

Compositional dependence of negative thermal expansion in the Prussian Blue analogues M(II)Pt(IV)(CN)6 (M = Mn, Fe, Co, Ni, Cu, Zn, Cd)

The effect of M(II) substitution on the magnitude of the negative thermal expansion (NTE) behavior within a series of Prussian Blue analogues, M(II)Pt(IV)(CN)(6) for M(II) = Mn, Fe, Co, Ni, Cu, Zn, Cd, has been investigated using variable-temperature powder X-ray diffraction (100-400 K). The NTE behavior varies widely with M(II) substitution, from near zero thermal expansion in NiPt(CN)(6) (alpha = dl/l dT = -1.02(11) x 10(-)(6) K(-)(1)) up to a maximum in CdPt(CN)(6) (alpha = -10.02(11) x 10(-)(6) K(-)(1)). The trend in the magnitude of the NTE behavior, with increasing atomic number (Z) of the M(II) ion, follows the order Mn(II) > Fe(II) > Co(II) > Ni(II) < Cu(II) < Zn(II) < Cd(II), which correlates with the trends for M(II) cation size, the lattice parameter, and structural flexibility as indicated by the temperature-dependent structural refinements and Raman spectroscopy. Analysis of the temperature dependence of the average structures suggests that the differences in the thermal expansion are due principally to the different strengths of the metal-cyanide binding interaction and, accordingly, the different energies of transverse vibration of the cyanide bridge, with enhanced NTE behavior for more flexible lattices.     

Square planar vs tetrahedral geometry in four coordinate iron(II) complexes

The geometric preferences of a family of four coordinate, iron(II) d6 complexes of the general form L2FeX2 have been systematically evaluated. Treatment of Fe2(Mes)4 (Mes = 2,4,6-Me3C6H2) with monodentate phosphine and phosphite ligands furnished square planar trans-P2Fe(Mes)2 derivatives. Identification of the geometry has been accomplished by a combination of solution and solid-state magnetometry and, in two cases (P = PMe3, PEt2Ph), X-ray diffraction. In contrast, both tetrahedral and square planar coordination has been observed upon complexation of chelating phosphine ligands. A combination of crystallographic and magnetic susceptibility data for (depe)Fe(Mes)2 (depe = 1,2-bis(diethylphosphino)ethane) established a tetrahedral molecular geometry whereas SQUID magnetometry and M?ssbauer spectroscopy on samples of (dppe)Fe(Mes)2 (dppe = 1,2-bis(diphenylphosphino)ethane) indicated a planar molecule. When dissolved in chlorinated solvents, the latter compound promotes chlorine atom abstraction, forming tetrahedral (dppe)Fe(Mes)Cl and (dppe)FeCl2. Ligand substitution reactions have been studied for both structural types and are rapid on the NMR time scale at ambient temperature.     

Sandia octahedral molecular sieves (SOMS): structural and property effects of charge-balancing the M(IV)-substituted (M = Ti, Zr) Niobate framework

Sandia octahedral molecular sieves (SOMS) is an isostructural, variable composition class of ion exchangers with the general formula Na(2)Nb(2-x)M(IV)(x)O (6-x)(OH)(x).H(2)O (M(IV) = Ti, Zr; x = 0.04-0.40) where up to 20% of the framework Nb(V) can be substituted with Ti(IV) or Zr(IV). This class of molecular sieves is easily converted to perovskite through low-temperature heat treatment (500-600 degrees C). This report provides a detailed account of how the charge imbalance of this Nb(V)-M(IV) substitution is compensated. X-ray powder diffraction with Rietveld refinement, infrared spectroscopy, thermogravimetric analysis, (23)Na MAS NMR, and (1)H MAS NMR were used to determine how the framework anionic charge is cation-balanced over a range of framework compositions. All spectroscopic evidence indicated a proton addition for each M(IV) substitution. Evidences for variable proton content included (1) increasing OH observed by (1)H MAS NMR with increasing M(IV) substitution, (2) increased infrared band broadening indicating increased H-bonding with increasing M(IV) substitution, (3) increased TGA weight loss (due to increased OH content) with increasing M(IV) substitution, (4) no variance in population on the sodium sites (indicated by Rietveld refinement) with variable composition, and (5) no change in the (23)Na MAS NMR spectra with variable composition. Also observed by infrared spectroscopy and (23)Na MAS NMR was increased disorder on the Nb(V)/M(IV) framework sites with increasing M(IV) substitution, evidenced by broadening of these spectral features. These spectroscopic studies, along with ion exchange experiments, also revealed the effect of the Nb(V)/M(IV) framework substitution on materials properties. Namely, the temperature of conversion to NaNb(1-x)M(IV)(x)O(3) (M = Ti, Zr) perovskite increased with increasing Ti in the framework and decreased with increasing Zr in the framework. This suggested that Ti stabilizes the SOMS framework and Zr destabilizes the SOMS framework. Finally, comparing ion exchange properties of a SOMS material with minimal (2%) Ti to a SOMS material with maximum (20%) Ti revealed the divalent cation selectivity of these materials which was reported previously is a function of the M(IV) substitution in the framework. A thorough investigation of this class of SOMS materials has revealed the importance of understanding the influence of heterovalent substitutions in microporous frameworks on material properties.     

A family of cyanide-bridged molecular squares: structural and magnetic properties of [{MIICl2}2{CoII(triphos)(CN)2}2].xCH2Cl2, M = Mn, Fe, Co, Ni, Zn

The syntheses, structures, and magnetic properties of a series of tetranuclear cyanide-bridged compounds are reported. This family of molecular squares, [{M(II)Cl2}2{Co(II)(triphos)(CN)2}2] (M = Mn ([CoMn]), Fe ([CoFe]), Co ([CoCo]), Ni ([CoNi]), and Zn ([CoZn]), triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane), has been synthesized by the reaction of Co(II)(triphos)(CN)2 and MCl2 (M = Mn, Co, Ni, Zn) or Fe4Cl8(THF)6 in a CH2Cl2/EtOH mixture. These complexes are isostructural and consist of two pentacoordinate Co(II) and two tetrahedral M(II) centers. The resulting molecular squares are characterized by antiferromagnetic coupling between metal centers that generally follows the spin-coupling model S total = SM(II)1 - SCo1 + SM(II)2 - SCo2. Magnetic parameters for all the complexes were measured using SQUID magnetometry. Additionally, [CoZn] and [CoMn] were studied by both conventional and high-frequency and high-field electron paramagnetic resonance.     

Two phase morphology limits lithium diffusion in TiO(2)(anatase): a (7)Li MAS NMR study.

7Li magic angle spinning solid-state nuclear magnetic resonance is applied to investigate the lithium local environment and lithium ion mobility in tetragonal anatase TiO(2) and orthorhombic lithium titanate Li(0.6)TiO(2). Upon lithium insertion, an increasing fraction of the material changes its crystallographic structure from anatase TiO(2) to lithium titanate Li(0.6)TiO(2). Phase separation occurs, and as a result, the Li-rich lithium titanate phase is coexisting with the Li-poor TiO(2) phase containing only small Li amounts approximately equal to 0.01. In both the anatase and the lithium titanate lattice, Li is found to be hopping over the available sites with activation energies of 0.2 and 0.09 eV, respectively. This leads to rapid microscopic diffusion rates at room temperature (D(micr) = 4.7 x 10(-12) cm(2)s(-1) in anatase and D(micr) = 1.3 x 10(-11) cm(2)s(-1) in lithium titanate). However, macroscopic intercalation data show activation energies of approximately 0.5 eV and smaller diffusion coefficients. We suggest that the diffusion through the phase boundary is determining the activation energy of the overall diffusion and the overall diffusion rate itself. The chemical shift of lithium in anatase is independent of temperature up to approximately 250 K but decreases at higher temperatures, reflecting a change in the 3d conduction electron densities. The Li mobility becomes prominent from this same temperature showing that such electronic effects possibly facilitate the mobility.     

Fast lithium ion diffusion in the ternary layered nitridometalate LiNiN

The structure, Li+ diffusion dynamics, and magnetic properties of the layered nitridonickelate(II), LiNiN, have been investigated by powder X-ray diffraction, 7Li solid-state NMR, and SQUID magnetometry and compared and contrasted with those of the Li+ fast ion conductor, Li3N. The replacement of Li+ by Ni2+ with concomitant generation of Li+ vacancies has profound effects on ionic diffusion and electronic properties. The nitridonickelate, akin to its binary parent, displays rapid Li+ ion diffusion but, by contrast, the diffusion process is confined only to the Li-N planes. Further, replacement of Li by Ni leads to a transition from semiconducting to metallic behavior, likely mediated through the creation of infinite, 1D Ni-N chains of increased covalency.     

Synthesis, structure, and photocatalysis in a new structural variant of the Aurivillius phase: LiBi4M3O14 (M = Nb, Ta)

Two new compounds, LiBi4Nb3O14 and LiBi4Ta3O14, have been synthesized by the solid-state method, using Li2CO3, Bi2O3, and M2O5 (M = Nb, Ta) in stoichiometric quantities. These compounds crystallize in the monoclinic C2/c space group with a = 13.035(3) A, b = 7.647(2) A, c = 12.217(3) A, beta = 101.512(4) degrees , V = 1193.4(5) A3 , and Z = 4 and a = 13.016(2) A, b = 7.583(1) A, c = 12.226(2) A, beta = 101.477(3) degrees , V = 1182.6(5) A3, and Z = 4, respectively. These are isostructural and the structure along the b axis consists of layers of [Bi2O2]2+ units separated by layers of LiO4 tetrahedra and NbO6 octahedra hence depicting an unusual variation in the Aurivillius phase isolated for the first time. The presence of lithium has been confirmed by 7Li NMR studies. ac impedance measurements and variable temperature (7)Li NMR studies indicate oxygen ion conductivity in these materials. The UV-visible spectra suggest a band gap of 3.0 eV for LiBi4Nb3O14 and 3.5 eV for LiBi4Ta3O14, respectively, and the associated studies on degradation of dyes and phenols render these materials suitable for photocatalysis.     

On the influence of the vacancy distribution on the structure and ionic conductivity of A-site-deficient Li(x)Sr(x)La(2/3-x)TiO3 perovskites

The crystal structure and dielectric properties of slowly cooled A-site-deficient perovskites Li(x)Sr(x)La(2/3-x)□(1/3-x)TiO(3) (0.04 ≤ x ≤ 0.33) have been investigated by powder X-ray diffraction (XRD), impedance spectroscopy, and (7)Li NMR techniques. In this series, nominal vacancies decrease with Li content, but the total amount of A-site vacancies, n(t) = Li + □, participating in conduction processes remains basically constant. Rietveld analysis of the XRD patterns showed a change of symmetry from orthorhombic to tetragonal when the lithium and strontium contents increased above x = 0.08 and from tetragonal to cubic above x = 0.16. Structural modifications are mainly due to the cation vacancy ordering along the c axis, which disappear gradually when the lithium content increases. In agreement with the structural information, two lithium signals with different quadrupole constants are detected in (7)Li NMR spectra of orthorhombic/tetragonal phases, which have been associated with lithium in two crystallographic z/c = 0 and 1/2 planes of perovskites. In cubic samples, only a single narrow component, indicative of mobile species, was detected. Lithium motion was thermally activated, with activation energies going from 0.35 to 0.38 eV. Evolution of the bulk dc-conductivity preexponential factors along the series showed a maximum that has been first related to the dependence of lithium hopping on the lithium and vacancy concentrations. Finally, changes in the vacancy ordering, produced along the series, affect the dimensionality of the conductivity, indicating that not only the amount of vacancies but also its distribution are relevant.     

Synthesis and structures of selected triazapentadienate of Li, Mn, Fe, Co, Ni, Cu(I), and Cu(II) using 2,4-N,N'-disubstituted 1,3,5-triazapentadienate anions as ancillary ligands: [N(Ar)C(NMe2)NC(NMe2)N(R)](-) (Ar = Ph, 2,6-(i)Pr2-C6H3; R = H, SiMe3)

Addition reaction of ArN(SiMe 3)M (Ar = Ph or 2,6 - (i) Pr 2-C 6H 3 (Dipp); M = Li or Na) to 2 equivalents of alpha-hydrogen-free nitrile RCN (R = dimethylamido) gave the dimeric [M{N(Ar)C(NMe 2)NC(NMe 2)N(SiMe 3)}] 2 ( 1a, Ar = Ph, M = Li; 1b, Ar = Ph, M = Na; 1c, Ar = Dipp, M = Li). 1d was obtained by hydrolysis of 1c at ambient temperature. Treatment of a double ratio of 1a or 1b with anhydrous MCl 2 (M = Mn, Fe, Co) yielded the 1,3,5-triazapentadienato complexes [M{N(Ph)C(NMe 2)NC(NMe 2)N(SiMe 3)} 2] (M = Mn, 2; Fe, 3; Co, 4) and with NiCl 2.6H 2O gave [M{N(Ph)C(NMe 2)NC(NMe 2)N(H)} 2] (M = Ni, 5). Treatment of an equiv of 1c with anhydrous CuCl in situ and in air led to complexes [{N(Dipp)C(NMe 2)NC(NMe 2)N(SiMe 3)}CuPPh 3] 6 and [Cu{N(Dipp)C(NMe 2)NC(NMe 2)N(H)} 2] 7, respectively. 1c, 1d, and 2- 7 were characterized by X-ray crystallography and microanalysis. 1c, 1d, 5, and 6 were well characterized by (1)H, (13)C NMR, 1c by (7)Li, and 6 by (31)P NMR as well. The structural features of these complexes were described in detail.     

(7)Li NMR and two-dimensional exchange study of lithium dynamics in monoclinic Li(3)V(2)(PO(4))(3)

High-resolution solid-state (7)Li NMR was used to characterize the structure and dynamics of lithium ion transport in monoclinic Li(3)V(2)(PO(4))(3). Under fast magic-angle spinning (MAS) conditions (25 kHz), three resonances are clearly resolved and assigned to the three unique crystallographic sites. This assignment is based on the Fermi-contact delocalization interaction between the unpaired d-electrons at the vanadium centers and the lithium ions. One-dimensional variable-temperature NMR and two-dimensional exchange spectroscopy (EXSY) are used to probe Li mobility between the three sites. Very fast exchange, on the microsecond time scale, was observed for the Li hopping processes. Activation energies are determined and correlated to structural properties including interatomic Li distances and Li-O bottleneck sizes.     

Highly saddle shaped (porphyrinato)iron(III) iodide with a pure intermediate spin state

Combined analyses using NMR, EPR and M?ssbauer spectroscopy as well as SQUID magnetometry have revealed that highly saddle shaped Fe(OETPP)I adopts an essentially pure intermediate spin state in spite of the coordination of an iodide ligand.     

Unusual syntheses, structures, and electronic properties of compounds containing ternary, T3-type supertetrahedral M/Sn/S anions [M5Sn(mu3-S)4(SnS4)4](10- (M = Zn, Co)

A recently discovered series of quaternary compounds of the general type [K(m)(ROH)(n)()][M(x)Sn(y)()Se(z)] (R = H, Me), containing ternary anions with [SnSe(4)](4-)-coordinated transition metal centers (M = Co, Mn, Zn, Cd, Hg) has now been extended by the synthesis and characterization of the two ortho-thiostannate-coordinated species, [Na(10)(H(2)O)(32)][M(5)Sn(mu(3)-S)(4)(SnS(4))(4)].2H(2)O (M = Zn (1), Co (2)). The central structural motifs of compounds 1 and 2 are highly charged [M(5)Sn(mu(3)-S)(4)(SnS(4))(4)](10-) anions, being the first T3-type supertetrahedral ternary anions reported to date. The exposure of single crystals of 2 to a dynamic vacuum for several hours resulted in the reversible formation of a partially dehydrated, but still monocrystalline material of the composition [Na(10)(H(2)O)(6)][Co(5)Sn(mu(3)-S)(4)(SnS(4))(4)] (3). The loss of 28 of the 34 water molecules only slightly affects the internal structure of the ternary anion in 3 and leads to a significant compacting of the crystal structure with closer linkage of the [Co(5)Sn(5)S(20)](10-) cluster units via the Na(+) cations. Magnetic measurements on 3 show that the ground state of the Co/Sn/S cluster is S = 1/2, indicating a significant antiferromagnetic coupling between the Co centers, which has also been rationalized by DFT investigations of the electronic situation in the ternary subunits of 1-3.     

Substitution of Lithium for Magnesium,Zinc, and Aluminum in Li15Si4: Crystal Structures,Thermodynamic Properties,as well as 6Li and 7Li NMR Spectroscopy of Li15Si4 and Li15−xMxSi4 (M=Mg,Zn, and Al)

An investigation into the substitution effects in Li₁₅Si₄, which is discussed as metastable phase that forms during electrochemical charging and discharging cycles in silicon anode materials, is presented. The novel partial substitution of lithium by magnesium and zinc is reported and the results are compared to those obtained for aluminum substitution. The new lithium silicides Li₁₄MgSi₄ ( 1 ) and Li_14.05Zn_0.95Si₄ ( 2 ) were synthesized by high‐temperature reactions and their crystal structures were determined from single‐crystal data. The magnetic properties and thermodynamic stabilities were investigated and compared with those of Li_14.25Al_0.75Si₄ ( 3 ). The substitution of a small amount of Li in metastable Li₁₅Si₄ for more electron‐rich metals, such as Mg, Zn, or Al, leads to a vast increase in the thermodynamic stability of the resulting ternary compounds_. The ^6,7Li NMR chemical shift and spin relaxation time T₁‐NMR spectroscopy behavior at low temperatures indicate an increasing contribution of the conduction electrons to these NMR spectroscopy parameters in the series for 1 – 3 . However, the increasing thermal stability of the new ternary phases is accompanied by a decrease in Li diffusivity, with 2 exhibiting the lowest activation energy for Li mobility with values of 56, 60, and 62 kJ mol^?1 for 2 , Li_14.25Al_0.75Si₁₄, and 1 , respectively. The influence of the metastable property of Li₁₅Si₄ on NMR spectroscopy experiments is highlighted.     

Synthesis and structure of cationic guanidinate-bridged bimetallic {Li7M} cubes (M = Mn, Co, Zn) with inverse crown counter anions

The reactions of the heteroleptic lithium amide [Li(3)(μ-hmds)(2)(μ,μ-hpp)] (1), where [hmds](-) = hexamethyldisilazide and [hpp](-) = hexahydropyrimidopyrimidide, with MnCl(2), CoCl(2) or ZnBr(2) result in the formation of the separated ion-pairs [MLi(7)(μ(8)-O)(μ,μ-hpp)(6)](+)[A](-), which each consist of a {MLi(7)} oxo-centred cube structural motif (M = Mn 2, Co 4, Zn 5), with each face of the cube being bridged by an [hpp](-) ligand. In the case of M = Mn and Co, the counter ion, [A](-), is the pentagonal anionic inverse crown [{Li(μ-hmds)}(5)(μ(5)-Cl)](-) (3), whereas the reaction with M = Zn produces the known tris-amido zincate [Zn(hmds)(3)](-) counter anion.     

Rational enantioselective design of chiral heterobimetallic single-chain magnets: synthesis, crystal structures and magnetic properties of oxamato-bridged M(II)Cu(II) chains (M=Mn, Co)

A new series of neutral oxamato-bridged M(II)Cu(II) chiral chains of general formula [MCuL(x)(S)(m)(H(2)O)(n)]·aS·bH(2)O [L(1)=(M)-1,1'-binaphthalene-2,2'-bis(oxamate) with M=Mn (1a) and Co (1b); L(2)=(P)-1,1'-binaphthalene-2,2'-bis(oxamate) with M=Mn (2a) and Co (2b)] and the analogous racemic chains of formula [MCuL(3)(S)(m)(H(2)O)(n)]·aS·bH(2)O [L(3)=1,1'-binaphthalene-2,2'-bis(oxamate) with M=Mn (3a) and Co (3b)] have been prepared by reaction of the corresponding dianionic oxamatocopper(II) complex [Cu(L(x))](2-) with Mn(2+) or Co(2+) cations in either dimethylformamide (DMF) or dimethyl sulfoxide (DMSO). Solid circular dichroism (CD) spectra of the bimetallic chain compounds were recorded to establish their chiral and enantiomeric nature. They exhibit maximum positive and negative Cotton effects, each pair of enantiomeric chains being non-superimposable mirror images. The crystal structures of the Mn(II)Cu(II) (1a-3a) and the Co(II)Cu(II) (1b and 2b) chain compounds were solved by single-crystal X-ray diffraction methods. Our attempts to obtain X-ray quality crystals of 3b were unsuccessful. The values of the shortest interchain Mn···Mn and Co···Co distances are indicative of a good isolation of neighbouring chains in the crystal lattice, which is caused by the bulky aromatic ligand. Although all the Mn(II)Cu(II) and Co(II)Cu(II) chains exhibit ferrimagnetic behaviour (-J(MnCu)=18.9-26.6 cm(-1) and -J(CoCu)=19.5-32.5 cm(-1)), only the enantiopure Co(II)Cu(II) chains (1b and 2b) show slow magnetic relaxation at low temperatures (T(B)=0.6-1.8 K), which is a characteristic of single-chain magnets (SCMs) and is related to the magnetic anisotropy of the high-spin Co(II) ion. Analysis of the SCM behaviour of 1b and 2b, based on Glauber's theory for an Ising one-dimensional system, shows a thermally activated mechanism for the magnetic relaxation (Arrhenius law dependence). The energy barriers (E(a)) to reverse the magnetisation direction are 8.2 (1b) and 8.1cm(-1) (2b), whereas the pre-exponential factor (τ(0)) is 1.9×10(-8) (1b) and 6.0×10(-9) s (2b). Interestingly, the racemic Co(II)Cu(II) chain analogue, 3b, showed no evidence of SCM behaviour.