Thorsten Langer

Structural characterization of the lithium silicides Li15Si4, Li13Si4, and Li7Si3 using solid state NMR.

Local environments and lithium ion dynamics in the binary lithium silicides Li(15)Si(4), Li(13)Si(4), and Li(7)Si(3) have been characterized by detailed variable temperature static and magic-angle spinning (MAS) NMR spectroscopic experiments. In the (6)Li MAS-NMR spectra, individual lithium sites are generally well-resolved at temperatures below 200 K, whereas at higher temperatures partial or complete site averaging is observed on the ms timescale. The NMR spectra also serve to monitor the phase transitions occurring in Li(7)Si(3) and Li(13)Si(4) at 235 K and 146 K, respectively. The observed lithium isotropic shift ranges of up to approximately 50 ppm indicate a significant amount of electronic charge stored on the lithium species, consistent with the expectation of the extended Zintl-Klemm-Busmann concept for the electronic structure of these materials. The (29)Si MAS-NMR spectra obtained on isotopically enriched samples, aided by double-quantum spectroscopy, are well suited for differentiating between the individual types of silicon sites within the silicon frameworks, and in Li(13)Si(4) their identification aids in the assignment of individual lithium sites via(29)Si{(7)Li} cross-polarization/heteronuclear correlation NMR. Variable temperature static (7)Li NMR spectra reveal motional narrowing effects, illustrating high lithium ionic mobilities in all of these compounds. Differences in the mobilities of individual lithium sites can be resolved by temperature dependent (6)Li MAS-NMR as well as (6)Li{(7)Li} rotational echo double resonance (REDOR) spectroscopy. For the compound Li(15)Si(4) the lithium mobility appears to be strongly geometrically restricted, which may result in a significant impediment for the use of Li-Si anodes for high-performance batteries. A comparison of all the (6)Li and (7)Li NMR spectroscopic data obtained for the three different lithium silicides and of Li(12)Si(7) previously studied suggests that lithium ions in the vicinity of silicon clusters or dimers have generally higher mobilities than those interacting with monomeric silicon atoms.

Stannylene or Metallastanna(IV)ocane: A Matter of Formalism

The s basicity of electron-rich transition metals (TMs) plays a crucial role in Brønsted acid–base reactions of TM complexes, such as [H2Fe(CO)4] and [HCo(CO)4] (strong acids, poor s-basicity of the corresponding conjugate bases) and was shown to increase upon coordination of good donor ligands L, such as phosphines; that is, lowered acidity of [H2Fe(CO)3(PPh3)] or [HCo(CO)3(PPh3)]. [2] Thus, P and/or S donors bearing electron-rich TM centers have been shown to support s donation towards other main-group-element (E) Lewis acidic centers, for example in the so-called metallaboratranes I and II and Be, Al, and Ga compounds of type III (Scheme 1). Very recently, we have described compounds IV–VII comprising {L5TM(d )} moieties that exhibit s donation towards electronically saturated Lewis acidic centers E, that is, Si and Sn. Gabba et al. have reported similar intermetallic interactions in the heterobimetallic complexes VIII–X (Scheme 1), which comprise d TM donor sites with an almost square-planar coordination sphere. Whereas compounds IV–X were obtained by a straightforward route starting from sources that comprise TM and E in the desired oxidation states, herein we present a (formal) redox approach, which involves a reaction sequence starting from a stannylene (SnCl2) and yielding hypercoordinate tin compounds that can be regarded as palladastanna(IV)ocanes. In a convenient one-pot synthesis, [PdCl2(PPh3)2] was treated with the potassium salt of 1-methyl-2-mercaptoimidazole (methimazole, Hmt) and [SnCl2(dioxane)] (Scheme 2) to afford compound 1. Substitution of the tin-bound chlorine atoms with a dianionic tridentate ligand afforded compound 2, which comprises a hexacoordinate tin atom (Scheme 2). Reference compounds 3 and 4 (comprising Sn and Sn, respectively, and the same tridentate ONN ligand as 2) were prepared as references for spectroscopic properties. The molecular structures of 1–4 were confirmed crystallographically (see Figure 1 and the Supporting Information). Scheme 1. Selected examples of TM–base complexes with electrophilic main-group-element sites (“Z-type ligands”). Cy = cyclohexyl.

Structural and dynamic characterization of Li12Si7 and Li12Ge7 using solid state NMR

Local environments and lithium ion dynamics in the binary lithium silicide Li(12)Si(7), and the analogous germanium compound have been characterized by detailed (6)Li, (7)Li, and (29)Si variable temperature static and magic-angle spinning (MAS) NMR experiments. In the MAS-NMR spectra, individual lithium sites are generally well-resolved at temperatures below 200K, whereas at higher temperatures partial site averaging is observed on the kHz timescale. The observed lithium chemical shift ranges of up to 60 ppm indicate a significant amount of electronic charge stored on the lithium species, consistent with the expectation of the extended Zintl-Klemm-Bussmann concept used for the theoretical description of lithium silicides. Furthermore the strongly diamagnetic chemical shifts observed for the lithium ions situated directly above the five-membered Si(5) rings suggest the possibility of aromatic ring currents in these structural elements. This assignment is confirmed further by (29)Si{(7)Li} CPMAS-heteronuclear correlation experiments. The (29)Si MAS-NMR spectra of Li(12)Si(7), aided by 2-D J-resolved spectroscopy, are well suited for differentiating between the individual sites within the silicon framework, while further detailed connectivity information is available on the basis of 2-D INADEQUATE and radio frequency driven recoupling (RFDR) spectra. Variable temperature static (7)Li NMR spectra reveal the onset of strong motional narrowing effects, illustrating high lithium ionic mobilities in both of these compounds.

Ylenes in the MII→SiIV (M=Si, Ge, Sn) Coordination Mode

The reaction of the methimazolyl (mt, i.e., 2-mercapto-1-methylimidazolide) substituted silane Si(mt)(4) with SnCl(2) and GeCl(2) in dioxane affords the paddlewheel-shaped complexes [ClSi(μ-mt)(4)MCl] (M=Sn (1) and Ge (2), respectively). These compounds represent the first crystallographically characterized hexacoordinate silicon complexes comprising a Sn or Ge atom in the Si coordination sphere. An attempt to synthesize the related silicon compound 3 [ClSi(μ-mt)(4)SiCl] instead afforded the trisilane [ClSi(μ-mt)(4)Si-SiCl(3)] (3a), which provides the first crystallographic evidence for the feasibility of oligosilanes with adjacent hexacoordinate Si atoms. One of the hexacoordinate Si atoms of 3a features the unprecedented (Si(2)S(4))Si skeleton. Natural bonding orbital (NBO) analyses of compounds 1, 2, 3a (and the target compound 3) revealed characteristics of M(II)→Si(IV) (for 2 and 3) or M(I)→Si(IV) (for 3a) dative bonding in the systems with M=Si and Ge, whereas compound 1 exhibits a covalent Sn(III)-Si(III) bond.

Unraveling the electronic structures of low-valent naphthalene and anthracene iron complexes: X-ray, spectroscopic, and density functional theory studies.

Naphthalene and anthracene transition metalates are potent reagents, but their electronic structures have remained poorly explored. A study of four Cp*-substituted iron complexes (Cp* = pentamethylcyclopentadienyl) now gives rare insight into the bonding features of such species. The highly oxygen- and water-sensitive compounds [K(18-crown-6){Cp*Fe(η(4)-C(10)H(8))}] (K1), [K(18-crown-6){Cp*Fe(η(4)-C(14)H(10))}] (K2), [Cp*Fe(η(4)-C(10)H(8))] (1), and [Cp*Fe(η(4)-C(14)H(10))] (2) were synthesized and characterized by NMR, UV-vis, and (57)Fe Mössbauer spectroscopy. The paramagnetic complexes 1 and 2 were additionally characterized by electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements. The molecular structures of complexes K1, K2, and 2 were determined by single-crystal X-ray crystallography. Cyclic voltammetry of 1 and 2 and spectroelectrochemical experiments revealed the redox properties of these complexes, which are reversibly reduced to the monoanions [Cp*Fe(η(4)-C(10)H(8))](-) (1(-)) and [Cp*Fe(η(4)-C(14)H(10))](-) (2(-)) and reversibly oxidized to the cations [Cp*Fe(η(6)-C(10)H(8))](+) (1(+)) and [Cp*Fe(η(6)-C(14)H(10))](+) (2(+)). Reduced orbital charges and spin densities of the naphthalene complexes 1(-/0/+) and the anthracene derivatives 2(-/0/+) were obtained by density functional theory (DFT) methods. Analysis of these data suggests that the electronic structures of the anions 1(-) and 2(-) are best represented by low-spin Fe(II) ions coordinated by anionic Cp* and dianionic naphthalene and anthracene ligands. The electronic structures of the neutral complexes 1 and 2 may be described by a superposition of two resonance configurations which, on the one hand, involve a low-spin Fe(I) ion coordinated by the neutral naphthalene or anthracene ligand L, and, on the other hand, a low-spin Fe(II) ion coordinated to a ligand radical L(•-). Our study thus reveals the redox noninnocent character of the naphthalene and anthracene ligands, which effectively stabilize the iron atoms in a low formal, but significantly higher spectroscopic oxidation state.

LiBC – Synthesis, Electrochemical and Solid-state NMR Investigations

LiBC was synthesized from the elements in a sealed niobium ampoule. It represents a totally intercalated heterographite with a structural relationship to graphite, the most commonly used anode material for lithium ion batteries. Since LiBC could accommodate three times as much lithium as graphite, its electrochemical properties in the anode and the cathode voltage range were investigated. However, LiBC did show poor performance both as an anode and as a cathode material. The unfavorable characteristics of LiBC with respect to electrochemical de-lithiation and re-insertion can be rationalized on the basis of nuclear magnetic resonance results. 7Li and 6Li isotropic chemical shifts are consistent with complete ionization of the lithium species. Variable-temperature static 7Li NMR lineshapes indicate that the mobility of the lithium ions is rather restricted, even at temperatures up to 500 K. The 11B and 13C NMR parameters are consistent with those measured in sp2-hybridized boron/carbon networks and also support the ionic bonding model. Graphical Abstract LiBC – Synthesis, Electrochemical and Solid-state NMR Investigations

Ionic Pathways in Li13Si4 investigated by 6Li and 7Li solid state NMR experiments

Local environments and dynamics of lithium ions in the binary lithium silicide Li13Si4 have been studied by (6)Li MAS-NMR, (7)Li spin-lattice relaxation time and site-resolved (7)Li 2D exchange NMR measurements as a function of mixing time. Variable temperature experiments result in distinct differences in activation energies characterizing the transfer rates between the different lithium sites. Based on this information, a comprehensive picture of the preferred ionic transfer pathways in this silicide has been developed. With respect to local mobility, the results of the present study suggests the ordering Li6/Li7>Li5>Li1>Li4 >Li2/Li3. Mobility within the z=0.5 plane is distinctly higher than within the z=0 plane, and the ionic transfer between the planes is most facile via Li1/Li5 exchange. The lithium ionic mobility can be rationalized on the basis of the type of the coordinating silicide anions and the lithium-lithium distances within the structure. Lithium ions strongly interacting with the isolated Si(4-) anions have distinctly lower mobility than those the coordination of which is dominated by Si2(6-) dumbbells.

Refined Crystal Structure and Idealized Structure of Mixed-Valent Eu4F5(CN2)2: Transition Possible

The new europium fluoride carbodiimide Eu(4)F(5)(CN(2))(2) was synthesized by solid state reaction from mixtures of EuF(3) and Li(2)(CN(2)) at 700 °C. The crystal structure as refined by single crystal X-ray diffraction (P ̅42(1)c, no. 114, a = 16.053(1) Å, c = 6.5150(6) Å, Z = 8) reveals three crystallographically distinct [N═C═N](2-) ions in the structure of mixed-valent Eu(4)F(5)(CN(2))(2). The presence of one Eu(3+) and three Eu(2+) per formula unit Eu(4)F(5)(CN(2))(2) is confirmed by magnetic measurements and (151)Eu-Mössbauer spectroscopy. The arrangement of Eu ions and gravity centers of [NCN](2-) ions in the structure of Eu(4)F(5)(CN(2))(2) follow the motif formed by atoms in the CuAl(2)-type structure. A possible high-symmetry structure of Eu(4)F(5)(CN(2))(2) is discussed on the basis of a group-subgroup scheme.

Solid Solutions RE16Rh11–xZx (RE = La, Ce, Pr, Nd, Sm; Z = Ga, Zn, Cd, In, Sn, Sb, Pb, Bi) – Centrosymmetric n = 2 Variants of Parthé’s Homologous Series A5n+6B3n+5

Several samples of solid solutions RE16Rh11-xZx (RE = La, Ce, Pr, Nd, Sm; Z = Ga, Zn, Cd,In, Sn, Sb, Pb, Bi) were synthesized by high-frequency melting of the elements in sealed tantalum ampoules. The samples were characterized by powder X-ray diffraction, and the structures of eight compounds were refined on the basis of single-crystal X-ray diffractometer data. The compounds crystallize with a centrosymmetric variant (space group P4⁄mbm) of the Ca16Sb11 type (P4̄ 21m). The relation between both structure types is discussed on the basis of a group-subgroup scheme. Only for La16Rh8Sn3 we observed full rhodium-tin ordering. The striking structural motif is a chain of face-sharing square prisms (filled with tin) and anti-prisms (filled with rhodium). The La16Rh8Sn3 structure is closely related to the structure types W5Si3, Ca16Sb11, Y3Rh2, Sm26Co11Ga6, Pu31Pt20, and Yb36Sn23 and is the centrosymmetric n = 2 member of Parthé’s A5n+6B3n+5 series. 119Sn Mössbauer spectra resolved the two crystallographically independent tin sites of La16Rh8Sn3, while a Pr16Rh9Sb2 sample shows only a singlet in its 121Sb Mössbauer spectrum. Graphical Abstract Solid Solutions RE16Rh11–xZx (RE = La, Ce, Pr, Nd, Sm; Z = Ga, Zn, Cd, In, Sn, Sb, Pb, Bi) – Centrosymmetric n = 2 Variants of Parthé’s Homologous Series A5n+6B3n+5

A Modified Solid State Synthesis of LiFePO4/C as Active Material for Lithium-ion Batteries

Abstract Solid state chemistry is already established as a conventional procedure for obtaining well-crystallized particles of LiFePO4 with ordered structure. The disadvantages of this method still are the uncontrollable particle growth and agglomeration, long reaction time and moreover the cost of production. The advantages of using 1-Hexadecanol as an additive to create a reducing atmosphere and additionally as a carbon source for the solid state method the existence of iron in the oxidation state Fe3+ can be prevented. The carbon coating can be controlled by the amount of the additive and the reaction conditions such as reaction time or reaction temperature. This modified synthesis cannot reduce the cost of energy but represents a simple variant to obtain both carbon coated LiFePO4 with a specific conductivity up to 10-2 S cm-1 and pure LiFePO4. The latter does not show any impurities as confirmed by Mös sbauer spectroscopy. The reaction time only takes three hours without any time-consuming preparations.