Synthetic Solid State Chemistry

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Synthetic Solid State Chemistry Guest Editor Duncan Gregory University of Glasgow, UK

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Published in issue 26, 2010 of Dalton Transactions

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Articles in the issue include: PERSPECTIVES: Syntheses and characterization of some solid-state actinide (Th, U, Np) compounds Daniel E. Bugaris and James A. Ibers, Dalton Trans., 2010, DOI: 10.1039/b927026d Hybrid materials through linkage of chalcogenide tetrahedral clusters Paz Vaqueiro, Dalton Trans., 2010, DOI: 10.1039/c000130a COMMUNICATIONS: Increasing the dimensionality of hybrid vanadium oxyfluorides using ionothermal synthesis Farida Himeur, Phoebe K. Allan, Simon J. Teat, Richard J. Goff, Russell E. Morris and Philip Lightfoot, Dalton Trans., 2010, DOI: 10.1039/c000318b One-step synthesis of high-purity fluorous-capped inorganic nanoparticles Rakesh Voggu, Ajmala Shireen and C. N. R. Rao, Dalton Trans., 2010, DOI: 10.1039/b927355g Visit the Dalton Transactions website for more cutting-edge inorganic and solid-state research www.rsc.org/dalton

PERSPECTIVE

www.rsc.org/dalton | Dalton Transactions

Solid state metathesis reactions as a conceptual tool in the synthesis of new materials


H.-Jürgen Meyer* Received 18th January 2010, Accepted 24th March 2010 First published as an Advance Article on the web 22nd April 2010 DOI: 10.1039/c001031f Solid state metathesis reactions can be used in the syntheses of inorganic solids and for strategic design of novel, eventually thermally labile materials. An explorative study of solid state metathesis reactions is presented for a number of examples, including syntheses of nitridoborates, carbodiimides, tetracyanoborates, tetracyanamidosilicates, carbon-nitride materials, and a number of other exciting compounds. This unique type of reaction is very efficient because it uses the intrinsic energy of reaction partners being involved. Desired compositions are achieved by appropriate starting materials and their relative amounts being combined into a solid state metathesis reaction. Reactions can be controlled through the heating-up procedure and by using a reactive flux, which may lower the ignition temperature of a reaction mixture and promote crystal growth of products.

Introduction The search for functional inorganic solids is a great temptation for chemists. However, the repertoire of preparative methods in inorganic chemistry is not well enough developed, especially when compared to organic chemistry. Classical solid state reactions following the ceramic method can be considered as a universal tool being commonly used for preparations of inorganic solids in research laboratories and in technical manufacturing. Until now, only this method can manage a great deal of reactions between compounds being composed of individual elements from an entire periodic table. Solid state reactions usually involve thermodynamically controlled reactions of solids or precursors, being reacted at sufficiently high temperatures and durations until a stable equilibrium Abteilung für Festkörperchemie und Theoretische Anorganische Chemie, Institut für Anorganische Chemie, Universität Tübingen, Ob dem Himmelreich 7, D-72074, Tübingen, Germany. E-mail: [email protected]; Fax: +49 (0)7071-29-5702; Tel: +49 (0)7071-29-76226

H.-Jürgen Meyer received his PhD at the Technische Universität of Berlin in 1988. After postdoctoral positions at the Ames Laboratory (Ames/IA), Cornell University (Ithaca; NY), and the University of Hannover, he completed his Habilitation in 1993. Since 1996 he has been professor for solid state and theoretical chemistry at the University of Tübingen. His widespread research interests involve syntheses H.-Jürgen Meyer of new solid state materials and functionalities of materials such as luminescence, magnetism, and lithium ionic conductivity. This journal is © The Royal Society of Chemistry 2010

state is reached. In the course of this type of reaction it is desired to transform a well crunched and homogenized powder via diffusion controlled reorganizations of its constituents into a new product. In order to speed up the intrinsically low diffusion rates in the solid state it may be necessary to raise the reaction temperature, even beyond two-thirds of the melting temperature of the starting material1 or to use a reactive flux. Following this route, high yield syntheses of oxide materials (such as the industrial production of YAG) and of refractory materials (such as metal borides, -carbides, or -nitrides) usually require very high heating temperatures up to the melting points of the constituents. However, the use of high temperatures brings an unfavourable drawback, namely the fact that thermally labile compounds are not being discovered. Thus, the general use of high temperature reactions is a strong limitation when we assume that thermally labile compounds may belong to the largest group of undiscovered compounds. Remarkable developments in the application of solid state metathesis (SSM) reactions have been reported on the preparation of known refractories in the fields of metal borides,2,3 nitrides,4-6 carbides,7 and other compounds8-12 by the groups of Parkin (London) und Kaner (Los Angeles).13,14 A solid state metathesis (SSM) reaction takes advantage of the intrinsically available energy of reaction partners, and promotes an exchange of atoms or atom groups between compounds to yield a desired product. A typical SSM reaction may depart from a carefully homogenized mixture of a metal chloride (MCl3 ) and a lithium salt such as Li3 N in a so-called salt balanced reaction, defining a balanced number of reaction equivalents of constituting ions (atoms) to participate in the reaction and to form a desired product and a coproduced salt, as described in the sample reaction: MCl3 + Li3 N → MN + 3 LiCl

(1)

A reaction like this is conducted by providing a sufficient amount of external energy, usually being introduced as heat, by gradually heating up a reaction mixture, or by filament Dalton Trans., 2010, 39, 5973–5982 | 5973


lamp initiated reactions. When heating up a reaction mixture gradually an ignition of the sample mixture is suddenly obtained at a certain external heating temperature, which we refer as the ignition temperature (T i ), which can be read off from the corresponding exothermic signal of a DTA (differential thermal analysis) measurement. Once the ignition occurs, the actual temperature inside a sample itself may be already much higher, reaching temperatures up to 1000 ◦ C or higher. Reactions of this type proceed fast, exothermically, and usually without a notable attack of the wall of the reaction container. In fact, reactions can proceed so quickly (within less than a few seconds) so that local temperatures of a sample can hardly be accurately observed. Caution has to be taken with respect to highly exothermic SSM reactions which are initiated by external heat, like the well known thermite reaction, or even without external heating, once mixtures of solid compounds are ground with each other. The ignition and reaction stage of a highly exothermic SSM reaction mixture is displayed in Fig. 1. The given mixture ignites spontaneously with a thermal flash when being ground under inert gas conditions at room temperature, or when being heated externally. It can be assumed the ignition takes place in some part of the sample and then propagates throughout the whole sample mixture very quickly.15 In the case of our NbCl5 /Li3 N mixture most of the reaction performs within a time interval of about 0.2 s, as shown in Fig. 1. A reaction like this can be designed to synthesize NbN or Li7 NbN4 ,16 indicating that the course of a SSM reaction can be controlled stoichiometrically just through the relative proportions of starting materials used. The efficiency of stoichiometric control has been already demonstrated for reactions between rare earth trichloride (RECl3 ) and lithium nitride (Li3 N) performed in molar compositions of 1 : 1 and 2 : 1 to yield REN and RE2 NCl3 , respectively,16 and for reactions and structure families presented here. Fortunately, most SSM reactions perform less exothermically than the higher transition metal halides like NbCl5 with Li3 N, allowing for a better control and handling of reactions. As an extreme, a SSM reaction may even proceed very slowly when being performed below the ignition temperature under reactive flux conditions. SSM reactions have been previously reported as a successful instrument for syntheses of several already known compounds. In this study, a number of explorative SSM reactions are presented

for syntheses of compounds that were new at the time, including nitridoborates, carbodiimides, tetracyanoborates, tetracyanamidosilicates, carbon-nitride, and others, whose crystal structures have been investigated by means of X-ray diffraction (XRD) techniques.

Applications of solid state metathesis reactions 1)

Rare earth nitridoborates

Rare earth nitridoborates may be compared with rare earth carbides, which are prepared through high temperature reactions of rare earth elements with elemental carbon (graphite). Corresponding reactions between rare earth elements and a-BN also afford very high temperatures (> 1400 ◦ C), often leading to product mixtures including binary phases. This problem was bypassed in the preparation of RE nitridoborates by using SSM reactions between rare earth chloride and lithium dinitridoborate (Li3 [N=B=N]). SSM reactions between various proportions of RECl3 and Li3 (BN2 ) have led to the development of a whole family of rare earth nitridoborates.17 Reaction enthalpies of these components bring along exothermic reactions with ignition temperatures below 600 ◦ C, like in the following reaction involving a trimerization of [N=B=N]3- ions to yield cyclic [B3 N6 ]9- ions, with the thermogram of the reaction given in Fig. 2. Coproduced LiCl is removed by washing the reaction product with water or methanol. 3 RECl3 + 3 Li3 (BN2 ) → RE3 (B3 N6 ) + 9 LiCl

(2)

Experiments have shown that SSM reactions allow a rational synthesis approach to aim for various compositions when combining appropriately balanced amounts of starting materials via stoichiometric control. As a result, various rare earth nitridoborate compounds were characterized being composed of ions like (BN)n- , (BN2 )3- , (BN3 )6- , (B2 N4 )8- , (B3 N6 )9- , and mixed-anion combinations thereof. Reactions like this also enable the simultaneous employment of three reaction partners in multilateral SSM reactions. Thus, a combined reaction of RECl3 , Li3 (BN2 ), and Li3 N leads to the formation of nitridoborate-nitrides. In contrast to this combination of reaction partners, the employment of elemental metal (Li) in such a reaction initiates reductive SSM reactions to synthesize metal-rich nitridoborate (or nitridoborate-nitride) compounds.

Fig. 1 A sequence of photographs showing the ignition and reaction stage of a SSM reaction between NbCl5 and Li3 N, covering a time span of about 2 tenths of a second, with pictures taken every 0.04 s. The reaction was initiated through external heating by using a hot plate.

5974 | Dalton Trans., 2010, 39, 5973–5982

This journal is © The Royal Society of Chemistry 2010


earth trihalides (REX3 ) and lithium carbodiimide (Li2 (CN2 )). Most rare earth cyanamide or carbodiimide compounds known to date can be considered to contain nearly linear and nearly symmetrical anions. Deviations from the ideal D•h symmetry of the [N=C=N]2- ions are due to non-equivalent chemical environments of the two terminal nitrogen atoms, or otherwise due to interactions with soft cations,27 and can be observed from structural data or from infrared measurements.28 Departing from the two distinct bond length within the cyanamide molecule (H2 N– C≡N) of roughly 131 pm and 115 pm, we usually note less distinct or even equivalent C–N distances within [N=C=N]2- ions of rare earth compounds, typically amounting to values around 123 pm, as present in the structure of lithium carbodiimide. Fig. 2 Thermogram of a 1 : 1 molar mixture of LaCl3 and Li3 (BN2 ) showing the exothermic ignition temperature near 560 ◦ C (solid line) yielding La3 (B3 N6 ) and LiCl (heating and cooling rate: 5 ◦ C min-1 .). The endothermic melting of coproduced LiCl is obtained slightly above 600 ◦ C, and the crystallization of the mixture on cooling (dotted line) is at about 580 ◦ C.

Altogether, SSM reactions in the field of rare earth nitridoborates include syntheses of insulating, metallic, and superconducting compounds. In addition, mixed-anion compounds like RE6 (BN3 )O6 (RE = La, Pr, Nd)18 were also synthesized via multilateral metathesis reactions: 6 REOCl + Li3 N + Li3 (BN2 ) → RE6 (BN3 )O6 + 6 LiCl

(3)

Thus, SSM reactions have been successfully performed to synthesize and to develop the family of rare earth nitridoborates— but can they be successfully employed for syntheses of other systems, too? 2)

Rare earth carbodiimides

Rare earth (RE) dioxide monocarbodiimides were reported in 1994, presumably synthesized under fortuitous conditions with the aid of a graphite vessel serving as a carbon source for the carbodiimde ion in RE2 O2 (CN2 ).19 Two distinct structural patterns have been described for RE2 O2 (CN2 ) compounds. Tetragonal La2 O2 (CN2 ) is reported to contain (a,b-) axially-parallel disordered [N=C=N]2- ions,20 whereas trigonal RE2 O2 (CN2 ) of the heavier examples of RE elements contains [N=C=N]2- ions being strictly aligned along the threefold (c-) axis.21,22 Leaning to the original synthesis of RE2 O2 (CN2 ) compounds from RE2 O3 , graphite, and ammonia,21 these compounds can be also prepared by an ammonolysis reaction of RE2 (CO3 )3 ·8H2 O. The thermal decomposition of RE2 (CO3 )3 ·8H2 O takes course over several steps. Unfortunately, the dioxide monocarbonate La2 O2 (CO3 )·3H2 O23 is already formed below 150 ◦ C to yield La2 O2 (CO3 )24 near 400 ◦ C.25 An ammonolysis reaction performed between 500 and 700 ◦ C yields La2 O2 (CN2 ). The same route can be performed for other rare earth examples, too. However, binary rare earth carbodiimides are not likely to be accessible by this route because the intermediate rare earth dioxide carbonate occurs at very low temperatures already. Since 2004 an increasing number of rare earth carbodiimide [N=C=N]2- or infrequently cyanamide [N≡C–N]2- compounds26 were reported to be synthesized via SSM reactions between rare This journal is © The Royal Society of Chemistry 2010

Carbodiimide (cyanamide) sources for SSM reactions Li2 (CN2 ) can be considered as a good carbodiimide source even in high temperature environments because it can be recrystallized in a sealed tantalum capsule at temperatures as high as 800 ◦ C. X-ray pure Li2 (CN2 ) is synthesized via an ammonolysis reaction of Li2 CO3 29 or by reaction of Li3 N with melamine (C3 N3 (NH2 )3 ).30 Due to the high efficiency of Li2 (CN2 ) in syntheses of carbodiimide compounds, it was used in favour of other sources such as Ag2 (CN2 ),31 Ca(CN2 ),32 and Zn(CN2 ),33 respectively. Ag2 (CN2 ) is easily accessible34 but it contains the cyanamide ion having [N– C≡N]2- distances of 126.6(5) and 119.4(6) pm, respectively, and has been shown to behave thermally unstable. Ca(CN2 ) appears also useful in reactions, although it was often charged to carry some oxygen contamination when prepared from CaCO3 and NH3 , due to the decomposition of CaCO3 phases into CaO at temperatures around 600 ◦ C.35 The use of Zn(CN2 )36 may provide an advantage in SSM reactions because the coproduced ZnCl2 can be washed away with an organic solvent, or sublimed off from a reaction product. This requirement was, however, not important in syntheses of the most compounds being reported herein, which appear at least moderately stable in air and water, thus allowing a washing procedure to remove coproduced LiCl from a reaction product.

Syntheses of rare earth carbodiimides SSM reactions between rare earth trihalides and lithium carbodiimide were performed by using carefully balanced amounts of high-purity starting materials. Substances were loaded into silica tubes under argon (glove box) and fused therein. When rare earth fluorides were employed as starting materials, fused copper ampoules (sealed in silica) were used as reaction containers. Mixtures of rare earth metal halides and lithium carbodiimide were heated slowly with increasing temperature. SSM reactions reveal their characteristics with a relatively soft ignition coming in a temperature region around 450 to 550 ◦ C when double exchange of ions takes place. A flux (eutectic LiCl/KCl mixture, m.p. ª 354 ◦ C37 ) can be useful in reactions to lower the reaction temperature and to improve the crystal growth of products. The efficiency of carbodiimide syntheses with Li2 (CN2 ) can be demonstrated even in reactions with highly stable compounds such as calcium fluoride, leading to the formation of calcium carbodiimide at 650 ◦ C (50 h):38 Dalton Trans., 2010, 39, 5973–5982 | 5975


CaF2 + Li2 (CN2 ) → Ca(CN2 ) + 2 LiF

(4)

Syntheses of rare earth carbodiimide compounds were performed by reacting appropriate mixtures of REX3 (X = Cl, F) and Li2 (CN2 ) according to the following reaction equations. The mixtures were heated up to temperatures around 500 ◦ C and remained at this temperature for a few days before they were cooled down to room temperature. We note that the stoichiometry of starting material predefines the composition of the product, which can be obtained in a well-crystalline state. Crystal structures of products from the following reactions were refined by using single-crystal XRD: 2 REX3 + 3 Li2 (CN2 ) → RE2 (CN2 )3 + 6 LiX

(5)

REX3 + 2 Li2 (CN2 ) → LiRE(CN2 )2 + 3 LiX

(6)

REX3 + Li2 (CN2 ) → REX(CN2 ) + 2 LiX

(7)

2 REX3 + 2 Li2 (CN2 ) → LiRE2 X3 (CN2 )2 + 3 LiX

(8)

Syntheses of rare earth sesquicarbodiimide compounds having the general formula RE2 (CN2 )3 were performed straightforwardly. In addition, the most simple example of a lithium containing carbodiimide was synthesized as LiRE(CN2 )2 . The most simple halide-carbodiimide composition was established as REX(CN2 ) with X = Cl, F.39 Compounds having the composition LiRE2 X3 (CN2 )2 can be considered as adducts “(REX(CN2 ))2 ·LiCl”. Moreover, additional anions such as N3- , O2- , SiO4 4- or others may be included into multilateral SSM reactions to yield mixedanion compounds, according to the following reaction examples: 2 REX3 + Li2 (CN2 ) + Li3 N → RE2 X(CN2 )N + 5 LiX

(9)

3 REX3 + 3 Li2 (CN2 ) + Li3 N → RE3 (CN2 )3 N + 9 LiX

(10)

REX3 + 2 Li2 (CN2 ) + REOX → RE2 O(CN2 )2 + 4 LiX

(11)

Li2 (CN2 ) + 2 REOX → RE2 O2 (CN2 ) + 2 LiX

(12)

2 REX3 + Li2 (CN2 ) + Li4 SiO4 → RE2 (CN2 )(SiO4 ) + 6 LiX (13) These reactions demonstrate that even three reaction partners can be combined in a SSM reaction. SSM reactions can be recorded thermoanalytically, on heating a reaction mixture under inert gas in a DTA apparatus using a cyclic heating procedure (Fig. 3). A typical feature of a thermogram is the exothermic effect, indicating the ignition temperature (T i ) of a reaction. An endothermic effect indicates the melting point of the coproduced salt (near 600 ◦ C for LiCl). Another exothermic effect is obtained when cooling down the reaction mixture, representing the recrystallization of LiCl or other products. It may be suspected that starting materials that are to be combined into a multilateral SSM reaction should exhibit comparable thermal stabilities40 and should have individual reactivities with each other within a restricted temperature range. For example, the ignition temperature of a reaction between LaCl3 and Li2 (CN2 ) (in 1 : 1 molar ratio) yielding LaCl(CN2 ) was observed slightly below 500 ◦ C, and the ignition temperature of a reaction between LaCl3 and Li3 N (in 1 : 1 molar ratio) yielding LaN was reported slightly 5976 | Dalton Trans., 2010, 39, 5973–5982

Fig. 3 Thermogram of a typical SSM reaction showing the exothermic ignition temperature of a 2 : 1 : 1 molar LaCl3 , Li2 (CN2 ), Li3 N mixture near 475 ◦ C (solid line) yielding La2 Cl(CN2 )N and LiCl (heating and cooling rate: 5 ◦ C min-1 ). The melting of LiCl is obtained near 600 ◦ C, and the recrystallization of the mixture on cooling (dotted line) at about 575 ◦ C.

above 500 ◦ C. The combined reaction of a mixture containing LaCl3 , Li2 (CN2 ), and Li3 N in a 2 : 1 : 1 molar ratio revealed an ignition temperature of T i ª 470 ◦ C, with the corresponding thermogram shown in Fig. 3. It is interesting to note, that only one single exothermic effect was obtained for the La2 Cl(CN2 )N formation, resulting from a simultaneous SSM reaction of all three reaction partners. When LaBr3 was used in a corresponding reaction instead of LaCl3 , the reaction already proceeds at T i ª 440 ◦ C with an endothermic (melting) effect of LiBr being observed at 550 ◦ C. Most products obtained from SSM reactions occurred as single-phases according to X-ray powder diffraction, besides the coproduced LiCl. A eutectic LiCl/KCl flux was frequently used in reactions to improve single-crystal growth. Preliminary investigations on the effect of a flux being employed in SSM reactions of RE2 (CN2 )3 compounds have suggested that an increasing amount of flux lowers the ignition temperature from about T i ª 500 ◦ C downwards to T i ª 350 ◦ C. Thereby, the exothermicity of the reaction at T i was increasingly consumed by the endothermic melting of the flux. But more careful investigations in these and other systems will be necessary. Crystalline powders of rare earth carbodiimide compounds are quite stable in air. When exposed to water, the surfaces of carbodiimide particles may slowly begin to transform into carbonate as can be shown by infrared spectra, rather than by XRD patterns. This reaction of carbodiimide with water represents the inverse of the well known ammonolysis reaction of carbonate to yield carbodiimide (i.e. Li2 (CO3 ) to Li2 (CN2 )) which may be described by the equilibrium: (CO3 )2- + 2 NH3 (CN2 )2- + 3 H2 O

(14)

Compounds and structures The series of quasi-binary carbodiimide RE2 (CN2 )3 compounds41,42 containing trivalent rare earth ions (RE = Y–Lu, except for La,43 and Pm44 ) are fundamental examples of the rare earth carbodiimide chemistry (Fig. 4). SSM reactions for This journal is © The Royal Society of Chemistry 2010


Fig. 4 Volumes (V/Z) of RE2 (CN2 )3 compounds forming monoclinic structures for RE = Ce–Tm, and rhombohedral structures for RE = Tm–Lu.

the synthesis of these compounds typically ignite when reaction mixtures are heated to temperatures around 450 to 500 ◦ C. Within the series of RE2 (CN2 )3 compounds we note two distinct structure types having monoclinic structures for RE = Y, Ce– Tm, and rhombohedral structures for RE = Tm–Lu (Fig. 4). Tm2 (CN2 )3 has been shown to crystallize dimorphic with a monoclinic (I) and a rhombohedral (II) modification. Structures of both modifications of RE2 (CN2 )3 compounds are constructed from alternating layers of RE atoms and [N=C=N] units. Opposite to the trend of the lanthanide contraction, the volume of the di˚ 3 per formula unit morphic thulium compound jumps from 135.1 A 3 ˚ Tm2 (CN2 )3 -I to 171.6 A per formula unit Tm2 (CN2 )3 -II (Fig. 4). A pressure induced phase transformation from rhombohedral Tm2 (CN2 )3 -II into monoclinic Tm2 (CN2 )3 -I involves a volume

Fig. 5

decrease in the order of 20% which is predominantly due to the shrinkage within the layers of the structure. In the course of the pressure transformation, the linear [N=C=N] units are readjusted. The [N=C=N] units are alternately tilted away from each other within layers of rhombohedral Tm2 (CN2 )3 -II and become aligned nearly parallel to the stacking direction in layers of monoclinic Tm2 (CN2 )3 -I (Fig. 5). Thereby, the coordination number of RE ions increases from six to seven. As it may be regarded for europium, a divalent (Eu(CN2 ))45 and a trivalent carbodiimide (Eu2 (CN2 )3 )48 have been discovered. In addition, a mixed-valent europium carbodiimide fluoride was obtained as Eu4 F5 (CN2 )2 .46 More multinary carbodiimide compounds are represented by the formulae REX(CN2 ) (X = Cl, F),47,48 LiRE(CN2 )2 (RE = La, Ce),49 RE3 (CN2 )3 N (RE = La, Ce),49 RE2 X(CN2 )N (X = Cl, Br, I),50,51 and LiRE2 F3 (CN2 )2 (RE = Ce, Pr, Nd, Sm, Eu, Gd).48 All these compounds were obtained from SSM reactions, departing from different compositions of REX3 and Li2 (CN2 ) mixtures, accounting for the rich potential of this method of synthesis and for the rapid growth of this new family of rare earth carbodiimide compounds. Among these, the PbFCl-type related LaCl(CN2 ) structure is remarkably stable and transforms into La2 Cl(CN2 )N (Fig. 6) on heating at temperatures around 750 ◦ C. Similarly, a thermal treatment of LiRE(CN2 )2 above 650 ◦ C yields RE3 (CN2 )3 N, being so far established for RE = La, Ce. An extension in the application of SSM reactions towards higher complexity comes about when three or even more reaction partners are involved. The introduction of oxide was explored with various oxygen sources in order to develop a high yield synthesis of Y2 O2 (CN2 ):Eu.52 Another mixed anion composition is represented by RE2 (CN2 )(SiO4 )53 forming three distinct crystal structures with RE = Y, La, and Pr, with all of them being related

Sections from layered structures of monoclinic RE2 (CN2 )3 -I (left) and rhombohedral RE2 (CN2 )3 -II (right).

Fig. 6

Sections of crystal structures of LaCl(CN2 ) (left), and La2 Cl(CN2 )N (right).


Dalton Trans., 2010, 39, 5973–5982 | 5977


to the NaCl-type structure. The obvious relationship between carbodiimide and chalcogenide anions by charge and thus by composition of compounds have led to the assignment of the [CN2 ]2- ion as a pseudo-chalcogenide.54 In addition, some crystal structures of chalcogenides and carbodiimides are closely related, as emphasized in Fig. 7 for Y2 O2 S and Y2 O2 (CN2 ), which both ¯ crystallize with the space group P3m1.

Fig. 8 A section of the band structure (along the chain direction) and the DOS of La2 Cl(CN2 )N. Orbital contributions of the [NCN]2- ion to the total DOS are projected in gray. The fermi energy (ef ) is shown as a dashed line. Fig. 7 Comparison of crystal structures of A-La2 O3 , Y2 O2 S, and Y2 O2 (CN2 ) (from left to right).

Many rare earth carbodiimide compounds behave stable up to temperatures around 600 ◦ C when heated under argon. When they are heated to even higher temperatures, carbodiimide compounds start to decompose into carbodiimide-nitrides and other products. This moderate thermal stability of most rare earth and transition metal carbodiimide compounds is the reason why classical solid state syntheses will usually fail. However, there are exceptions as exemplified by the synthesis of RE2 O2 (CN2 ) obtained by ammonolysis reaction of rare earth oxide and graphite at 950 ◦ C, and for the preparation of Sr6 N[CoN2 ][CN2 ]2 by direct solid state reaction from a mixture of Sr, Sr2 N, NaN3 , Co, and C (graphite) at 1100 ◦ C.55

Luminescence properties of rare earth carbodiimides During the past few years, nitridic rare earth materials have received considerable attention due to their outstanding properties in luminescence.56 In this context rare earth carbodiimide compounds were studied as host and energy transfer materials for incorporated lanthanide ions. The white body colour of rare earth carbodiimide compounds results from the relatively large (the pg –pu *, HOMO–LUMO) band gap of the [N=C=N]2- ion, displayed in Fig. 8, which is related with a strong absorption band around 250 nm and represents a window for f–f or f–d transitions of incorporated lanthanide ion dopants. The photoluminescence behaviour of doped carbodiimide oxide compounds having the general formula RE2 O2 (CN2 ):Ln57-59 has been already reported some years ago. Moreover, the luminescence of Y2 O2 (CN2 ):Eu has shown to be quite similar with that of the commercially used red light emitter Y2 O2 S : Eu.60 Also RE2 (CN2 )3 :Ln compounds with combinations of RE = Y, Gd, Lu and Ln = Ce, Eu, Tb have shown interesting luminescence properties, with the emission spectrum of the cerium-doped compound Gd2 (CN2 )3 :Ce displayed in Fig. 9.42 The luminescence of Gd2 (CN2 )3 :Ce is due to the transition between the ground state levels (2 F5/2 and 2 F7/2 ) of the 4f1 configuration and the lowest crystal field component of the [Xe]5d1 configuration of the Ce3+ dopant. The energy of the crystal 5978 | Dalton Trans., 2010, 39, 5973–5982

Fig. 9 Emission spectrum of Gd2 (CN2 )3 :Ce on excitation at 415 nm.

field component is sensitive to the chemical bonding between d-orbitals and surrounding ligands, principally allowing Ce3+ doped compounds to show UV, blue, green, or red luminescence. The maximum emission band of Gd2 (CN2 )3 :Ce at 575 nm shows an amber luminescence colour and is slightly red shifted (by 15 nm) compared to the most widely used light-emitting diode material Y3 Al5 O12 : Ce (YAG:Ce). 3)

Transition metal carbodiimides

Carbodiimide compounds of the 3d transition metals (M) occur divalent, having the composition M(CN2 ) with M = Mn,61 Fe,62 Co,63 Ni,63 Cu.64 The crystal structure of Mn(CN2 ) is isotypic to the Ca(CN2 ) structure. Very similar to this appears the structure of Cu(CN2 ) which suffers from an apparent Jahn–Teller effect. Compounds with M = Fe, Co, Ni crystallize with structures being closely related with the NiAs structure type, in which the carbon atom in M(CN2 ) substitutes for arsenic in the NiAs type. In addition, a trivalent example is known for Cr2 (CN2 )3 crystallizing isotypically with RE2 (CN2 )3 -II.65 The compounds Mn(CN2 ) and Cr2 (CN2 )3 were synthesized by SSM reactions. Low thermal stabilities with decomposition temperatures in the range 250–550 ◦ C are a great temptation for making all these compounds by SSM reactions and to obtain well crystallized M(CN2 ) products. SSM reactions studied between some other transition metal halides and Li2 (CN2 ) have been diagnosed to run through intermediate stages with increasing heating temperatures, thereby This journal is © The Royal Society of Chemistry 2010


unfolding the reducing nature of the carbodiimde ion, to result in nitride or carbide containing compounds of remarkable nature. Tungsten hexachloride is easily reduced under formation of LiWCl6 66 and WCl4 , which can further react with Li2 (CN2 ) to yield the carbon-centred cluster compound Li[W6 CCl18 ].67 When Na2 (CN2 ) is used instead of Li2 (CN2 ), the isotypic carbon and nitrogen centred clusters Na[W6 CCl18 ]68 and Na[W6 NCl18 ]69 were obtained, having significantly different atomic distances and lattice parameters. With niobium pentachloride and Li2 (CN2 ) the condensed cluster compound Nb3 NCl11 70 was formed with the reaction passing through at least one as yet non-interpreted intermediate product stage. 4)

Tetracyanoborates and tetracyanamidosilicates

An extension of SSM reactions towards higher complexity is already established by multilateral reactions leading to the assembly of compounds containing complex anions of different geometry (e.g. La5 (B3 N6 )(BN3 )), and by synthesis of mixed-anion compounds (e.g. La2 O(CN2 )2 , La2 (CN2 )(SiO4 )). An attractive perspective concerning the application of SSM reactions is the assembly of complex anions from smaller fragments. An impressive example is the synthesis of Li[B(CN)4 ] containing the tetracyanoborate ion, being assembled in a KCN/LiCl melt from [BF4 ]- and [CN]- when removing fluoride ions in the temperature region 280–340 ◦ C:71 K[BF4 ] + 4 KCN + 5 LiCl → Li[B(CN)4 ] + 5 KCl + 4 LiF (15) Multilateral SSM reactions for the assembly of complex anions are designed under consideration of appropriate polarities of atoms or ions that are to be combined (e.g. B3+ and (CN)- ). A similar reaction strategy was successfully employed for the synthesis of compounds containing the tetracyanamidosilicate ion, when reaction mixtures with [SiF6 ]2- and (CN2 )2- were heated in silica tubes to temperatures around 550 ◦ C:72 RECl3 + A2 [SiF6 ] + 4 Li2 (CN2 ) → ARE[Si(CN2 )4 ] + 3 LiCl + 5 LiF + AF (16) A large number of examples of ARE[Si(CN2 )4 ] compounds, indicated in Fig. 10, has been discovered by this type of reaction and structurally characterized. The structures of [B(CN)4 ]- and [Si(CN2 )4 ]4- ions may be derived from their orthoborate and orthosilicate ions, respectively, having essentially a tetrahedral shape (Fig. 11). However, the Si–(N–CN) angles clearly deviate from linearity, as a result of the packing arrangement in ARE[Si(CN2 )4 ] structures.

Fig. 10 Unit cell volumes of ARE[Si(CN2 )4 ] compounds with alkali (A) and rare earth (RE) elements. Compounds with A = Cs (squares), Rb (circles) crystallize tetragonally, and compounds with A = K (triangles) and La–Gd crystallize orthorhombically.

Structures of ARE[Si(CN2 )4 ] compounds can be derived from a cubic closest packing (ccp) arrangement of tetracyanamidosilcate ions, with all octahedral interstices being occupied by RE, and half of tetrahedral interstices being occupied by A ions. Even though the [Si(CN2 )4 ]4- anion appears as a novel ion, it may be considered as a fragment of silicon dicarbodiimide Si(CN2 )2 73 whose cubic structure has been refined from powder XRD, although a reproduction of the synthesis has not yet been achieved.74 ARE[Si(CN2 )4 ] compounds with RE being Ce and Tb exhibit photoluminescence properties even without any doping because the RE ions are well separated in structures by voluminous [Si(CN2 )4 ]4- ions so that energy quenching between adjacent RE ions is reduced. Better luminescent properties are reported for ARE[Si(CN2 )4 ]:Ln compounds with RE = Y, La, or Gd doped with Ln = Ce, Eu, or Tb.75

5)

Carbon nitride materials

Carbon nitride is the only missing binary compound within the combinatory B–C–N system being attempted by several syntheses routes.76 SSM reactions can be useful in syntheses even of nonmetallic compounds when the previously used metal halide is substituted by a non-metal halide. A straight forward SSM reaction for the synthesis of C3 N4 follows a reaction of cyanuric chloride with lithium nitride under highly exothermic conditions:77 C3 N3 Cl3 + Li3 N → C3 N4 + 3 LiCl

(17)

Fig. 11 Lewis formulae of tetracyanoborate and tetracyanamidosilicate ions and the distorted tetracyanamidosilicate ion from the structure of RbLa[Si(CN2 )4 ] (left to right).


Dalton Trans., 2010, 39, 5973–5982 | 5979

Following another rational synthesis approach, the reaction of cyanuric chloride can be performed under more moderate reaction conditions when lithium carbodiimide is employed in a fused silica tube:78


2 C3 N3 Cl3 + 3 Li2 (CN2 ) → 3 C3 N4 + 6 LiCl

(18)

During the heating process with Li2 (CN2 ) a melt of cyanuric chloride is obtained near 150 ◦ C, allowing for a slow progress of the reaction. On further heating the melt becomes more viscose and turns yellow in colour until N(C3 N3 Cl2 )3 (1) and then higher condensed reaction products (C6 N7 Cl3 (2) and N(C6 N7 Cl2 )3 (3), displayed in Fig. 12) were obtained as intermediate products when reactions were suspended at certain temperatures, well below the ignition temperature near 500 ◦ C. Tris-(4,6-dichloro-s-triazin-2yl)-amin (1) was identified to serve as a template in the formation of micro- and nano-tubes of carbon nitride. The template leaves its hexagonal fingerprint inside the tube (Fig. 12) and becomes volatile during the final heating stage approaching 500 ◦ C.79

LaCl3 + 3/2 Li2 C2 → LaC2 + 3 LiCl + C

An excess of Li2 C2 employed in this reductive SSM reaction allows a salt balanced reaction when including the formation of carbon as a reaction product. Thus, the formation of carbon is expected in this reaction but not verified analytically or by powder XRD. Of course a reductive metathesis reaction using lithium metal, as already performed in the synthesis of metal-rich nitridoborates,17 would be a useful alternative. Binary carbonates of rare earth elements could parallel the chemistry of rare earth carbodiimides, but they remain unknown to date. However, ternary carbonates have been reported to be synthesized by combining A2 (CO3 ) and RE2 (C2 O4 )3 ·xH2 O in autoclave reactions at high CO2 pressures (2000 bar) and elevated temperatures (500 ◦ C).82 In the meantime structures of several ARE(CO3 )2 compounds have been determined83 but a convenient synthesis approach is still missing. Recently, a successful attempt was undertaken by us to synthesize LiLa(CO3 )2 via a SSM reaction, by heating up a mixture of LaCl3 and Li2 (CO3 ) in a sealed silica tube to 400 ◦ C. SSM reactions have to be heated carefully in this case, because decomposition temperatures of LiRE(CO3 )2 compounds are reported somewhat above 400 ◦ C.82 The crystalline reaction product was identified as LiLa(CO3 )2 by single crystal XRD:81 LaCl3 + 2 Li2 CO3 → LiLa(CO3 )2 + 3 LiCl

Fig. 12 Intermediate compounds during SSM reaction of cyanuric chloride with lithium carbodiimide (left) and crystals of 1 serving as a template in the formation of C3 N4 tubes (right).

All reactions being reported in an attempt to synthesize C3 N4 have so far produced only X-ray amorphous carbon nitride materials80 which have been characterized by combinations of various techniques. Miscellaneous examples

There are many ways to employ SSM reactions in making new compounds based on strategic design and to discover new systems. Having this in mind, several fields can be considered which have not yet been explored in great detail. But syntheses of already known compounds via SSM reactions may be useful when compared to established syntheses routes.7 Rare earth dicarbides have been reported by a diverse structure family, with LaC2 being probably the most simple compound. Rare earth dicarbides are usually synthesized by reactions from elements at high temperatures (e.g. La + graphite, ª 1700 ◦ C). Recently, we have undertaken a successful attempt to synthesize LaC2 via the SSM reaction, departing from LaCl3 and Li2 C2 in a sealed silica tube at 500 ◦ C:81 5980 | Dalton Trans., 2010, 39, 5973–5982

(20)

It may be noted that no over-pressure appeared in the silica tube when it was opened after being cooled down to room temperature. Preliminary experiments have also demonstrated that this method can be useful also for preparations of 3d element carbonates. A new generation of iron pnictide superconductors being constructed of [FeAs]- layers was discovered in 2008, having critical temperatures above 50 K.84 Most simple examples of these materials include LiFeAs or NaFeAs, which are usually synthesized by reacting alkaline metal with FeAs in a sealed metal ampoule.85 Control of electron or hole doping affords precise control of compositions, which appears delicate when employing reaction conditions were evaporation losses of the alkaline metal must be regarded. In a preliminary experiment, a mixture of iron dichloride and lithium arsenide was combined in a SSM reaction and heated to 600 ◦ C, according to the following eqn (21). The reaction revealed an exothermic effect near 400 ◦ C, according to DTA. FeCl2 + Li3 As → LiFeAs + 2 LiCl

6)

(19)

(21)

The reaction product yielded crystalline LiFeAs powder (besides LiCl) exhibiting superconducting properties, with T c ª 15 K.85 These examples of SSM reactions presented on a carbide, carbonate, and pnictide compound are just some preliminary results regarding syntheses in these systems. There is surely more to be discovered.

Conclusion An explorative expedition on the application of SSM reactions has been presented for syntheses of compounds containing new anions, mixed-anions, and complex anions. Due to the limited thermal stability, many of these compounds can not be synthesized This journal is © The Royal Society of Chemistry 2010


by classical solid state reactions. SSM reactions reported for preparations of nitridoborates, carbodiimides, tetracyanamidosilicates ignite when the starting materials are heated above temperatures between 400 and 550 ◦ C to form mostly air stable compounds that are purified from the coproduced salt (LiCl) by washing with water. SSM reactions were often reported to yield nanocrystalline products, after being initiated by externally heating a sample within a furnace or by a hot wire. However, when samples are further heated in the presence of the coproduced salt or by an appropriate flux, nice single crystals can be observed. The choice and the high purity of starting materials are most important in SSM reactions. Covalently bound anions can remain intact during reactions and may even transform to yield dimeric or trimeric anions. In addition, ions may be combined to form mixed-anion compounds, and to form larger anionic units for a new solid state complex chemistry. A broad chemistry of nitridoborates involves several different anions ((BN)n- , (BN2 )3- , (BN3 )6- , (B2 N4 )8- , (B3 N6 )9- ) and combinations thereof being combined as mixed-anions in structures. Carbodiimide compounds were successfully developed during the past few years but reactions designed to synthesize higher carbon nitride anions such as the expected carbonate analogue (CN3 )5- or the oxalate analogue (C2 N4 )6- have failed until now. SSM reactions can be controlled by the choice of starting materials and their relative amounts being used in a reaction in order to reach an intended composition of a material. The course of a reaction can be studied analytically by monitoring thermal effects such as the exothermic effect related to the ignition temperature and the melting temperature of the coproduced salt. However, these parameters may be considerably altered when a flux is being used. In addition, it can be regarded that reactions may proceed even below the ignition temperature with expectedly slower speed. In the case of the formation of a carbon nitride material a slow reaction progress even allowed the isolation of intermediate products which were formed during the successive condensation reaction below the ignition temperature (T i ). Syntheses of several new compounds can be developed empirically by rational SSM reactions, but some important challenges still remain to be studied in more detail. These would require more thermodynamic information, calculations, and calorimetric measurements. Also a kinetic impact to SSM reactions has to be taken into consideration, which appears quite feasible. The employment of highly fluid media will be of interest to achieve even lower reaction temperatures for the synthesis of new compounds (LiLa(CO3 ) is synthesized at 400 ◦ C).

Acknowledgements Thanks to Dr Radhakrishnan Srinivasan, Dr Sonja Tragl, Dr ¨ Sindlinger, Dr Michael Neukirch, Dipl.Martina Weisser, Jorg Chem. Leonid Unverfehrt for their dedication and their enthusiasm in carrying out experimental work. Special thanks to Dr ¨ Jochen Glaser and Dr Markus Strobele for their commitment in this research and for carrying out structure refinements. The Deutsche Forschungsgemeinschaft has generously supported this research. This journal is © The Royal Society of Chemistry 2010

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