Synthesis and Crystal Structure of KLa[C(CN)3]4·H2O containing a ShieldedPotassium Ion

Radhakrishnan Srinivasan, Sonja Tragl, and H.-Jürgen Meyer*

Tübingen, Abteilung für Festkörperchemie und Theoretische Anorganische Chemie am IAC der Universität

Received November 24th, 2004.

Professor Rüdiger Kniep zum 60. Geburtstag gewidmet

Abstract. KLa[C(CN)3]4·H2O was crystallized from an aqueoussolution containing La(NO3)3·6H2O and K[C(CN)3]. The crystalstructure was solved and refined from X-ray single crystal data(P4/n, Z � 2, a � b � 12.379(1) A, c � 6.5695(8) A, V �

1006.7(2) A3). Solid KLa[C(CN)3]4·H2O forms a framework struc-ture with bridging tricyanomethanide ions, each of them con-necting two La3� ions and one K� ion. Potassium is apparently

IntroductionPolycyano anions, such as dicyanamide [N(CN)2]� or tri-cyanomethanide [C(CN)3]� anions exhibit capabilities asmetal bridging ligands and for the formation of frameworkstructures. These structures can exhibit magnetic orderingwhen their conjugated π resonance structures provide coup-ling pathways for magnetic interactions between paramag-netic metal centers [1, 2]. For example, compoundsM[N(CN)2]2 with divalent M � Cr, Mn, Co, Ni, Cu [3, 4,5] have shown ferromagnetic properties with ordering tem-peratures between 9 and 47 K [1]. Tricyanomethanide (tcm)compounds M[C(CN)3]2 have been reported with M � V,Cr, Mn, Fe, Co, Ni, Cu [6, 7, 8, 9] to form interpenetratingnetwork structures, with antiferromagnetic coupling be-tween metal centers, due to spin-frustrations related withtrigonal metal bridging of the tcm ions in these structures[1, 9].

To date no lanthanide containing compounds are knownwith tricyanomethanide (tcm) anions. However, a numberof (NCN)2� containing lanthanide compounds such asEu(CN2) [10], LiEu2(CN2)I3, LiEu4(CN2)3I3 [11],LnCl(CN2) with Ln � La, Ce, Pr [12], Ln2Cl(CN2)N withLn � La, Ce [13], and La2O(CN2)2 [14] were recentlyreported after La2O2(CN2) [15] is already known since awhile.

* Prof. Dr. H.-Jürgen MeyerUniversität TübingenAbteilung für Festkörperchemie und Theoretische AnorganischeChemieInstitut für Anorganische ChemieAuf der Morgenstelle 18D-72076 TübingenTel.: 07071-29-76226Fax. 07071-29-5702e-mail: juergen.meyer@uni-tuebingen.deWeb-Seite: http://www.uni-tuebingen.de/AK-Meyer

Z. Anorg. Allg. Chem. 2005, 631, 1075�1078 DOI: 10.1002/zaac.200400500 © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1075

situated in a tetrahedral environment with four K�N distances of2.741(3) A, shielded by four other tricyanomethanide ions. EachLa3� has a square antiprismatic coordination of nitrogen atomsfrom eight [C(CN)3]� ions, and this antiprism is mono-capped byone water molecule.

Keywords: Tricyanomethanide; Lanthanum; Synthesis; Structure

Following the solution chemistry route adapted from theabove-mentioned transition metal tcm compounds, we herereport on the synthesis and crystal structure ofKLa[C(CN)3]4·H2O as an example of a lanthanide contain-ing tcm compound.

Experimental Section

Preparation Strategies

In attempt to find ways to synthesize (C,N)-containing lanthanidecompounds, we have investigated several synthesis routes and start-ing materials. Solid state reactions, typically performed at 500 °C orabove, have yielded (NCN)2� containing compounds. For example,LaCl(CN2) was obtained from solid state reactions of LaCl3 withreaction partners such as Li2(CN2), NaCN, or even K4[Fe(CN)6].Solid state reactions of LaCl3 with Na[N(CN)2] or K[C(CN)3] in aLiCl/KCl flux at 500 °C again yielded LaCl(CN2). At more elevatedtemperatures (around 750 °C) La2Cl(CN2)N is obtained insteadof LaCl(CN2).

To date no reactive or low-melting reaction medium has been foundto preserve the unity of the [C(CN)3]� ion and to allow solid statemetathesis reactions [16] in a similar way as successfully performedfor the (CN2)2� ion:

LaCl3 � Li2(CN2) � LaCl(CN2) � 2 LiCl (1)

In contrast, the transition metal tcm compounds M[C(CN)3]2 withM � V, Cr, Mn, Fe, Co, Ni, Cu were simply precipitated fromaqueous solutions containing a metal salt and K[C(CN)3].

Synthesis of KLa[C(CN)3]4·H2O

Following the reaction procedure described for transition metaltcm compounds, we performed a straight forward reaction of lan-thanum nitrate and potassium tcm. A solution containing 433 mgof La(NO3)3·6H2O (Merck, 99 %) in 25 ml water was mixed witha yellow solution of 388 mg K[C(CN)3] (Alfa Aesar, X-ray pure) in

R. Srinivasan, S. Tragl, H.-J. Meyer

Table 1 Selected crystal data and measuring conditions.

Space group (no.), Z P4/n (85), 2 Range: θ /° 3.10 to 30.52Lattice constants /A a � b � 12.379(1), c � 6.5695(8) Index range �17 � h � 17, �17 � k � 17, �9 � l � 9Cell volume /A3 1006.7(2) μ /mm�1 2.365Density calc. /g·cm�3 1.832 Collected reflections 17574Molecular weight /g·mol�1 556.31 Unique reflections 1535Crystal appearance transparent light yellow rods Parameters refined 72Crystal size /mm3 0.22 � 0.06 � 0.05 R indices (all data) R1

a) � 0.0383, wR2b) � 0.0755

Diffractometer STOE, IPDS Final R indices [I > 2σ(Io)] R1a) � 0.0325, wR2

b) � 0.0740Radiation, temperature Mo-Kα (λ � 0.71073 A), GooF (all reflections) 1.118

graphite monochromator, 293(2) K Res. peak: max.; min. /e·A�3 1.63; �0.5

a) R1 � Σ�Fo���Fc�/Σ�Fo�; b) wR2 � [Σw(Fo2�Fc

2)2 / [Σw(Fo2)2]]1/2

15 ml water under constant stirring, targeting on the hypotheticalcomposition La[C(CN)3]3. The mixture was light yellow in colourwithout any precipitation or cloudiness. On heating the solutionand after the evaporation of some water, a light yellow powder wasobtained along with KNO3. The presence of KNO3 and the ab-sence of La(NO3)3·6H2O in the product was indicated by the X-ray powder diffraction pattern of the product. Single crystals ofwhat later turned out to be KLa[C(CN)3]4·H2O were recrystallizedfrom water. The reaction equation can be written as follows:

La(NO3)3·6H2O � 4 K[C(CN)3] � KLa[C(CN)3]4·H2O (2)� 3 K(NO3) � 5 H2O

Powder diffraction and crystal structure determination

All phase analyses were done by X-ray powder diffraction tech-niques. The powder patterns were recorded on a StadiP dif-fractometer (STOE), using germanium monochromated Cu-Kα1

radiation (λ � 1.540598 A) and a position sensitive X-ray detector(opening angle: 2θ � 6°). The powdered samples of the reactionproducts were placed in between two mylar foils for the X-ray pow-der pattern measurements in the 2θ range between 8 and 60°. Thereflections were indexed using Louer’s algorithm (DICVOL) for atetragonal cell with a � b � 12.388(3) A, c � 6.569(3) A, V �

1008.1(6) A3 for KLa[C(CN)3]4·H2O. The number of single indexedlines was 27 from a total of 37 indexed peaks. In addition, someunidentified diffraction lines were detected in the powder patterns.Well developed light yellow, rod-like crystals of KLa[C(CN)3]4·H2Owere recrystallized from water, selected under a microscope andmounted on the tips of glass fibers for X-ray diffraction studies.Single crystal measurements were performed on an IPDS (STOE)using graphite monochromated Mo-Kα radiation (λ � 0.71073 A).The structure was solved by direct methods in the tetragonal spacegroup P4/n. Selected crystal data and measuring conditions areprovided in Table 1. All atoms except for hydrogen were refinedanisotropically. The hydrogen atom position belonging to the watermolecule was not determined. The atomic positions along with theisotropic-equivalent displacement parameters are given in Table 2.Selected bond lengths and the C-C-C bond angles are shown inTable 3. Details on the crystal structure investigations can be ob-tained from the Fachinformationszentrum Karlsruhe, Germany, D-76344 Eggenstein-Leopoldshafen (fax: (�49)7247-808-666; e-mail:crysdata@fiz-karlsruhe.de) on quoting the depository number414534 for KLa[C(CN)3]4·H2O.

Results and Discussion

The tricyanomethanide (tcm) anion is an efficient com-plexing agent and bridging ligand to form coordination

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2005, 631, 1075�10781076

Table 2 Atomic coordinates and isotropic-equivalent displace-ment parameters (in A2) for KLa[C(CN)3]4·H2O.

Atom Multiplicity and x y z Ueqa

Wyckoff letter

La 2 c 0.25 0.25 0.04365(5) 0.0164(1)K 2 a 0.75 0.25 0 0.0374(3)O 2 c 0.25 0.25 0.4349(7) 0.038(1)N1 8 g 0.9250(2) 0.8392(3) 0.8084(5) 0.0391(7)N2 8 g 0.1803(2) 0.0917(2) 0.8071(5) 0.0375(6)N3 8 g 0.1439(3) 0.8669(3) 0.2926(6) 0.0513(9)C1 8 g 0.9995(2) 0.8803(3) 0.7391(5) 0.0291(6)C2 8 g 0.1394(2) 0.0178(3) 0.7371(5) 0.0271(5)C3 8 g 0.1207(3) 0.8942(3) 0.4526(5) 0.0330(6)C4 8 g 0.0896(2) 0.9281(2) 0.6491(5) 0.0277(6)

a Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.

Table 3 Bond distances (in A) and bond angles (in °) forKLa[C(CN)3]4·H2O.

La�O 2.570(4) � 1 C1�C4�C2 121.0(3)La�N1 2.619(3) � 4 C1�C4�C3 118.7(3)La�N2 2.646(3) � 4 C2�C4�C3 119.7(3)K�N3 2.741(3) � 4C1�N1 1.147(4) � 1C1�C4 1.394(4) � 1C2�N2 1.141(4) � 1C2�C4 1.396(4) � 1C3�N3 1.141(5) � 1C3�C4 1.411(5) � 1

polymers. For example, in structures of transition metalcompounds like M(tcm)2 (M � V, Cr) [7], the tcm ion con-nects three metal atoms in a trigonal array.

The tetragonal crystal structure of KLa[C(CN)3]4·H2Oshows a flower-like structure pattern with tcm ions bridgingboth types of cations, La3� and K� (Figures 1 and 2). Allthe cations are arranged in layers near z � 0, with La3�

being situated on a fourfold rotation axis, and K� on afourfold inversion axis. The lanthanum ions are situated inthe a,b-plane, similar to a 44 network arrangement. In thisarray, each lanthanum ion is bridged with four La3� neigh-bours by four pairs of tcm ions (Figure 2). The bridgingtcm ions act as bidentate ligands with respect to La3�

through their N atoms (N1 and N2), and thereby generatea two-dimensional structure. The third nitrogen atom (N3)of each tcm interpenetrates into an adjacent layer, pointingtowards a potassium ion, and thereby connecting adjacentlayers through La�tcm�K bridges. The bridging func-tionality of tcm is shown in Figure 3.

Synthesis and Crystal Structure of KLa[C(CN)3]4·H2O

Fig. 1 Perspective view of the crystal structure ofKLa[C(CN)3]4·H2O along the c axis.Atom types: K large black, La large light grey, C white, N mediumgrey, and O of H2O dark grey.

Fig. 2 Partial structure with the layer like arrangement of[La(C(CN)3)4]�.

The lanthanum ion is nine-coordinated, situated in asquare antiprism of nitrogen atoms, which is monocappedby one water molecule. The coordination environment ofLa3� with two crystallographically distinct nitrogen atoms(N1 and N2) from the eight surrounding tcm ions, and withone oxygen atom from the water molecule is shown in Fig-ure 4.

The potassium ion is surrounded by four nearest nitrogenatoms belonging to four tcm ions, generating an approxi-

Z. Anorg. Allg. Chem. 2005, 631, 1075�1078 zaac.wiley-vch.de © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1077

Fig. 3 Bridging mode of the [C(CN)3]� ion.

Fig. 4 Coordination environment around the La3� ion.

mately tetrahedral environment (D2d-symmetry) with fourequally short K�N distances of 2.74 A. Both, the low coor-dination number and the four short K�N distances areanomalous for a potassium ion. For example, the averageK�N distances in K[C(CN)3] [17] amounts to 2.89 A, andto approximately 2.90 A in potassium ferrocyanides [18]with coordination numbers of seven and more.

In the structure of KLa[C(CN)3]4·H2O, the potassiumion is in fact surrounded by eight tcm ions. In addition tothe four tcm ions already mentioned, there are four ad-ditional tcm ions that shield the potassium ion in an um-brella-like fashion at K�C distances of 3.76 A with the cen-tral tcm carbon atoms (C4), as shown in Figure 5. If thearrangement of the central tcm carbon atoms (C4) is con-sidered, both groups of tcm ions follow the motif of dis-torted tetrahedra.

The C-C-C angles of the tcm ion deviate only little from120° (Table 3). Considering a 4σ precision for the estimatedstandard deviation of the calculated bond angles (inTable 3), the ideal D3h symmetry of [C(CN)3]� is retained.This is consistent with the observation of one, althoughbroad, C-N vibration at 2199 cm�1 in the recorded infraredspectrum (Perkin Elmer FT-IR spectrometer, KBr pellet)of KLa[C(CN)3]4·H2O.

R. Srinivasan, S. Tragl, H.-J. Meyer

Fig. 5 Environment around the K� ion with 4 � 4 tcm ions, shown as a ball and stick (left) and as a space-filling model (right).

Outlook

Structures with [C(CN)3]� ions show promise in the designof magnetic materials because of their conjugated π reson-ance structures, which may provide coupling pathways formagnetic interactions between paramagnetic lanthanide oreven mixed lanthanide�transition metal centers.

Acknowledgements. This research was supported by DeutscheForschungsgemeinschaft (Bonn) through the project “Nitridocar-bonate“. One of the authors (R. Srinivasan) is thankful for thescholarship provided by the Landesgraduiertenförderung Baden-Württemberg.

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