Solid State Sciences 2 (2000) 711–715

Synthesis of potassium cyanamide, and crystal structuredetermination by pareto optimisation of the cost functions

‘lattice energy’ and ‘powder intensities’

Michael Becker, Martin Jansen *Max-Planck-Institut fur Festkorperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany

Received 8 June 2000; accepted 25 July 2000

Abstract

The synthesis of K2CN2 was carried out by reaction of potassium hydrogen cyanamide (KHCN2) with potassium amide(KNH2) in liquid ammonia. The crystal structure has been determined by global optimisation techniques using the latticeenergy and the difference between observed and calculated X-ray intensities as cost functions, simultaneously. The structurerefinement has been performed by conventional Rietveld profile fitting of X-ray powder data (monoclinic, C 2/m,a=5.7877(1), b=5.7030(1), c=5.7857(2) A, ; b=109.016(1) °; Z=2). The CN2

2− units are linear exhibiting a C–N bondlength of 1.234(2) A, , while potassium is coordinated by five nitrogen atoms forming a square pyramid. The structuralrelationships to aristotypic K2NiO2 are pointed out. © 2000 Editions scientifiques et medicales Elsevier SAS. All rightsreserved.

Keywords: Potassium; Cyanamide; Synthesis; Crystal structure; Solid state chemistry; Solid state NMR; Computer chemistry

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1. Introduction

Cyanamide (H2CN2) and its salts are well knowncompounds with a wide range of applications. Whilethe crystal structures of the alkaline–earth metalcyanamides, except beryllium cyanamide, have beendetermined reliably [1], out of the alkaline metalcyanamides, only the lithium [2] and sodium [3]cyanamides are known and characterised struc-turally. Lithium cyanamide forms at reacting lithiumacetylide and lithium nitride in molten lithium at600°C. Sodium cyanamide can be generated by the

reaction of sodium hydrogen cyanamide (NaHCN2)and an excess of sodium at 300°C.

All attempts to prepare potassium cyanamide in apure state have failed, so far. Either the syntheticapproach implied the evolution of water as a byproduct [4], which inevitably lead to hydrolysis ofthe desired product, or the deprotonation of KHCN2

was not complete, and K5H(CN2)3 [5] was obtainedas an intermediate. In this paper we report on thesynthesis of pure potassium cyanamide using a novelprocedure.

K2CN2 has not been available as single crystals, sofar, and the crystal structure had to be solved bypowder techniques. Here we have used a novel ap-proach for generating a raw structure at an accuracythat allows to use it as a starting model for conven-tional Rietveld refinements.

* Correspondence and reprints: Tel.: +49-711-6891500; fax:+49-711-6891502.

E-mail address: martin@jansen.mpi-stuttgart.mpg.de (J. Mar-tin).

1293-2558/00/$ - see front matter © 2000 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

PII: S1293-2558(00)01090-6

B. Michael, J. Martin / Solid State Sciences 2 (2000) 711–715712

Table 1Crystallographic data for K2CN2

MonoclinicCrystal systemC 2/m (Nr.12)Space group (No.)a=5.7877(1) A,Unit cell dimensionsb=5.7030(1) A,c=5.7857(1) A,b=109.016(1) °

Cell volume [A, 3] 180.549(6)Z 2Density [g cm−3] 4.301

(calculated)Cu�Ka1Radiation(Germanium�monochromator)

Datapoints 78451052u5902u-range15Number of parameters

refined

Rp=4.82Final R values [16]Rwp=6.28GOF=1.2x2=1.35

temperature. The resulting product was a colourless,microcrystalline and moisture sensitive powder.

Potassium amide, used as a starting material, wasprepared by reacting twice distilled potassium withliquid ammonia at −50°C, applying a platinumcatalyst. The purity of the product was checked byX-ray diffractometry.

Potassium hydrogen cyanamide was synthesisedby reaction of cyanamide (H2CN2) with potassiumt-butoxide (KOtBu) in t-butyl alcohol [6]. The purityof the product was checked by X-ray diffractometryand vibrational spectroscopy.

2.2. Analytical characterisation

Thermal analyses were carried out using a DTA/TG device (STA 409, Netzsch, Selb) coupled with aquadrupole mass spectrometer(QMG 421, Balzers,Wetzlar).

Infrared spectra were recorded applying a FTIRspectrometer (IFS 113v, Bruker, Karlsruhe) measur-ing a range from 500 to 4000 cm−1 on a KBr pellet(500 mg KBr/0.5 mg K2CN2), prepared in a glove-box under dry argon atmosphere.

Solid state nuclear resonance experiments werecarried out on a DSX-400-NMR device (Bruker,Karlsruhe, Germany).

For X-ray data collection, K2CN2 was sealed un-der dry argon in a glass capillary (diameter: 0.3 mm).Data collection was performed on a diffractometer(Stadi P, Stoe, Darmstadt, Germany; Germaniummonochromator; position sensitive detector). The X-ray diffractogram could be indexed in an or-thorhombic crystal system. The programENDEAVOUR [7] was used to generate a suitablestarting model for Rietveld refinement. The basicidea for crystal structure solution from powder pat-terns, using ENDEAVOUR [7], consists of a com-bined global optimisation (Pareto optimisation [8])of the difference between the calculated and themeasured diffraction patterns and of the potential

2. Experimental details

2.1. Synthesis of potassium cyanamide

K2CN2 was prepared by reaction of potassiumhydrogen cyanamide with potassium amide (ratio1:1) in liquid ammonia. In a H-shaped reactionvessel, the thighs of which were connected by a glasssieve (porosity 3), 62.25 mg potassium amide and90.5 mg potassium hydrogen cyanamide were pro-pounded separately, and at −78°C, 30 mL of am-monia were condensed in each thigh. The solutionswere warmed to −50°C, at which temperaturepotassium amide was dissolved completely, andunified. The acid/base-reaction occurredspontaneously.

KHCN2+KNH2 �liquid NH3

K2CN2+NH3

After completion of the reaction all ammonia wasevaporated by warming the reaction vessel to room

Table 2Positional and isotropic displacement parameters for K2CN2 (standard deviations in parentheses)

yX BisoWyckoff-positionAtom z

K 0.8271(3) 0.5 0.7145(3) 1.42(2)4I4I 2.45(1)N 0.2097(3)0.50.6467(5)

0.84(4)00.50.52aC

B. Michael, J. Martin / Solid State Sciences 2 (2000) 711–715 713

Fig. 1. X-ray powder pattern after Rietveld refinement (measured (dotted) and calculated (continuous line) intensity) and difference plotof K2CN2.

energy of the system. The combined cost function C[9] is calculated as

C=a ·Epot+ (1−a)RB (1)

were Epot is the potential energy of the atomic arrange-ment, with the empirically parameterised two-bodypotential

Epot=12

%i, j

(e2/4po0)(qiqj/dij)+ (dijmin/dij)12 (2)

RB is given as

RB=100%2u

�Iexp(2u)−Icalc(2u)�/%2u

Iexp(2u) (3)

CN22− groups were defined as rigid bodies. As

starting values for the calculation, lattice parametersand cell content, but no symmetry information, wereprovided. The symmetries found did not lead to anyorthorhombic space group. A reasonable crystallo-graphic description was only possible when assumingmonoclinic symmetry (space group: C 2/m, latticeconstants and atomic parameters as given in Tables1 and 2, respectively). Splittings of part of the reflec-tions which should occur on lowering the latticesymmetry to monoclinic were not resolved. However,

half width analysis showed significant broadening ofthe respective reflections. The positions as well as theunit cell found could be refined using the programFullprof [10]. The refinement converged, with applica-tion of isotropic temperature factors, to Rp=4.82%,Rwp=6.28%, x2 =1.35. Final plots of observed andcalculated X-ray powder patterns and of the differenceare shown in Fig. 1. Further details on data collectionand the results of the X-ray structure determination1

are given in Table 1. Selected bond distances and bondangles are shown in Table 3.

Table 3Selected bond distances [A, ] and bond angles [°] of K2CN2 (stan-dard deviations in parenthesis)

K�K 2.890(2)3.257(2) K�C3.2838(8)K�C 2x2.763(2)K�N

2.8815(3)K�N 2x C�N 1.234(2)2.9148(3) N�C�NK�N 180.0(2)

163.7(2)2.932(3)K�N N�K�N3.354(3) K�N�C 97.8(1)K�N

1 Further details of the crystal structure investigation are availablefrom the Fachinformationszentrum Karlsruhe, D-76344 Eggen-stein-Leopoldshafen (Germany), on quoting the depository numberCSD-411094, the name of the authors, and citation of the paper..

B. Michael, J. Martin / Solid State Sciences 2 (2000) 711–715714

3. Results

Potassium cyanamide was synthesised by reactionof potassium hydrogen cyanamide and potassiumamide, for the first time. The title compound precip-itates as a colourless, microcrystalline powder that issensitive to air and moisture.

According to an evaluation of powder data, potas-sium cyanamide crystallises monoclinic with the lat-tice constants a=5.7877(1), b=5.7030(1),c=5.7857(1) A, ; b=109.016(1)° (Table 1). The crys-tal structure was solved by global optimisation andrefined by the Rietveld technique.

The crystal structure of K2CN2 is composed oflinear CN2

2− anions and KN5/5 (notation accordingto P. Niggli [11]) square pyramids (Fig. 2), which areconnected via common edges forming layers runningalong the a/b-plane. These layers are linked by theCN2

2− rods. Nitrogen is coordinated by potassiumand carbon according to NK5/5C1/2. The CN2

2−

groups are inclined with respect to the K/N-layers(Fig. 3). The cyanamide anion is characterised by aC–N bond distance of 1.234(2) A, and an N–C–Nbond angle of 180.0(2)°, thus showing D�h pointgroup symmetry. The carbon atoms are coordinated

Fig. 3. Perspective view of the crystal structure of K2CN2. Coordi-nation of potassium drawn as polyhedra.

by potassium cations, in addition to nitrogen, CN2/

1K6/3. Overall, K2CN2 is structurally closely relatedto K2NiO2 [12], which can be addressed as thearistotype [13] of this family. As the main differencethe NiO2-dumb bells are perpendicular to the layersformed by the square pyramids. The structural simi-larities can be expressed by group–subgroup rela-tions as depicted in Fig. 4.

The vibrational spectra of potassium cyanamideshow the typical features of linear three-atomic ar-rangements with D� symmetry. As expected by theselection principle for molecules containing an inver-sion centre, the asymmetric stretching and the defor-mation vibration of the CN2

2− group are IR active,whereas the symmetric stretching mode is Ramanactive [14]. Previous predictions for the vibrationfrequencies of K2CN2 could not be confirmed [15].The vibrational frequencies are shown in TTable 4.

Up to now, no nuclear magnetic resonance experi-ments were realised with proton free metalcyanamides. The 100 MHz-13C-MAS-NMR spectra(magnetic field: 9.4 Tesla) showed only one signal (d :142.15 ppm) which confirms the result of the X-rayinvestigation according to which there is only onecrystallographically independent carbon atom.Fig. 2. Coordination sphere of K+ in K2CN2.

B. Michael, J. Martin / Solid State Sciences 2 (2000) 711–715 715

Fig. 4. Group-subgroup relations between K2CN2 and K2NiO2.

Table 4Observed infrared frequencies of K2CN2 [cm−1]

Wavenumber Assignment

3285 nas+2d

3170 nas+ns

1945 nas

1340 2d

670 d

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Differential thermal analysis coupled with massspectroscopy indicated decomposition, evolution ofnitrogen, to occur at 810°C.