Magneto-structural studies and thermal analysis of the 4-aminopyridinium tetrabromocuprate(II) monohydrate

~ Pergamon 0277-5387(95)00386-X

Polyhedron Vol. 15, No. 8, pp. 1253 1262, 1996 Copyright © 1996 Elsevier Science Ltd

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MAGNETO-STRUCTURAL STUDIES AND THERMAL ANALYSIS OF THE 4-AMINOPYRIDINIUM

TETRABROMOCUPRATE(II) MONOHYDRATE

PASCUAL ROMAN,* JON SERTUCHA, ANTONIO LUQUE, LUIS LEZAMA and TEOFILO ROJO

Departamento de Quimica Inorg~inica, Universidad del Pals Vasco, Apartado 644, 48080 Bilbao, Spain

(Received 14 June 1995 ; accepted 17 August 1995)

Abstract--Dark violet crystals suitable for X-ray analysis of the complex (4-apyH)2[Cu- Br4]" H20, where 4-apyH ÷ is 4-aminopyridinium, were prepared by air evaporation of an acetonitrile solution of the aromatic amine base, HBr and CuBr2 in a molar ratio 2 : 2 : 1. The compound has been characterized by elemental analysis, IR, UV-vis and EPR spec- troscopies, thermal analysis and magnetic susceptibility measurements. The crystal structure consists of flattened [CuBr4] 2- tetrahedra of C2 symmetry, organoammonium cations and water of crystallization molecules which are held together by electrostatic forces, an extensive three-dimensional network ofO---H. • • Br, N--H. • • Br and N - - H . . . O hydrogen bonds and weak face-to-face stacking interactions between the n systems of the aromatic rings. The X-band EPR spectra show an isotropic signal (giso = 2.16) which is resolved in an axial symmetry operating at Q-band, with g, = 2.302 > g± = 2.084, characteristic of a flattened [CuBr4] 2- tetrahedral complex. The magnetic susceptibility measurements indicate the presence of weak ferromagnetic interactions below 30 K, presumably through hydrogen contacts which are established between the water molecule and two [CuBr4] 2- anions related by a two-fold axis.

Halide transition-metal complexes represent a growing class of compounds whose chemistry is currently a topic of intense research. One of the best studied groups, both in solution and the solid state, are the halocuprate complexes, t They have been the subject of recent reviews 2 which emphasize approaches to synthesis, stereochemistry, magneto- structural correlations, bonding and reaction chemistry. These compounds and their properties are of interest not only in inorganic chemistry but also in fields ranging from solid-state physics to bioinorganic chemistry. Among solid-state physicists and chemists, there is a great interest in the copper(II) halides owing to the plasticity of the metal coordination sphere which leads to a great variety of crystalline architectures with different coordination

*Author to whom correspondence should be addressed.

numbers, geometries and nuclearities, 3 and makes copper(II) systems excellent candidates for analysing correlations between structural parameters and magnetic properties. 4 In the biological field, recent studies have shown that several tetra- halocuprates(II) of substituted pyridinium cations exhibit potent gastroprotective activity 5 as well as anti-epileptic effects. 6 These pharmacological studies have also shown that bromocomplexes are sig- nificantly more active and show a lower cytotoxicity than the analogous chlorocomplexes.

In this paper, we report the preparation, crystal structure and solid-state properties of the compound (4-apyH)2[CuBr4]" H20 (1) (4-apyH ÷ is the 4-aminopyridinium cation). The choice of the 4- aminopyridine base was determined firstly by the presence of two amino groups capable of estab- lishing strong hydrogen bonds, which involve the bromine and water oxygen atoms as acceptors and stabilize the crystal structure of the compound. Sec-

1253

1254

ondly, solid-state structures of chemical species containing rc aromatic systems are stabilized by stacking interactions involving the rt systems. Hydrogen bonds v and non-covalent ~ interactions 8 control many molecular recognition and self assembly processes in solution and solid state, and exercise important effects on the organization and properties of many materials in areas such as biology, crystal engineering and materials science. 9

This work is a part of our research programme on solid-state characterization of first-row transition halocomplexes with aromatic amines, in which we are attempting to analyse the influence of the organic cation features (size, shape and nature and position of the substituents) on the packing interactions that govern the crystal organization and, as a consequence, the properties of this kind of complex. In the last few years, a particular aspect of the field of solid-state chemistry that has received growing attention is the way in which molecules and ions are organized in the solid state to form novel materials with improved electrical, optical, magnetic or catalytic properties. ~° In spite of the fact that much progress" has been made in this area, our understanding of the packing choices of organic and inorganic compounds is very limited, and we are still quite a way from the intentional design and synthesis of specific structural architectures in the solid state (crystal engineering) which would result in the development of novel materials with characteristic physical and chemical properties. In this respect, although there have been theoretical studies on packing interactions 12 in sim- ple organic systems, up to now, very few systematic reports have been published analysing correlations between the characteristics of the ionic particle components and the way in which a crystalline inorganic complex ~3 is constructed and organized. This research would be of particular importance not only in the field of coordination solid-state chemistry, in which it is common practice to base the crystallization of new compounds on empirical rules and 'local' recipes, 14 but also in many areas such as catalysis, bioinorganic and material chemistry, where the coordination complexes play an important role.

EXPER I MEN TA L

Reagents and techniques

P. ROM/~N et al.

of copper was determined using a Perkin-Elmer 360 atomic absorption/flame emission spectrometer. The density value was measured by flotation in a mixture of carbon tetrachloride-bromoform. Infrared absorption spectra were recorded on a Per- kin-Elmer 4200 IR spectrometer using KBr disks over the wavenumber range 4000-200 cm-l. UV- vis spectra were measured using a Shimadzu 260 spectrometer for ethanol-HBr (48%) (20: 1, v/v) solutions of the complex using 1-cm path length quartz cells in the region 190-900 nm (52,630- 12,500 cm-1). Thermal analyses were carried out on a Setaram TAG 24 S 16 TG/DTA simultaneous thermal analyser and a Mettler TA 4000 DSC instrument, using synthetic air and dry dinitrogen atmospheres (flow rate of 50 cm 3 min -~) and a heating rate of 5°C min -1. Similar results were obtained with the two instruments. X-ray powder diffraction patterns of the final products from the thermal decompositions were recorded at room temperature with a Philips PW 1710 instrument equipped with graphite-monochromated Cu-K~ radiation, and they were compared with those obtained from the ASTM powder diffraction files of the Joint Committee on Powder Diffraction Standards, JCPDS. 15 A Bruker ESP300 spectrometer, operating at X-(ca 9.5 GHz) and Q-band (ca 34 GHz), was used to study the EPR poly- crystalline spectra between 4.2 and 300 K. The temperature was stabilized by an Oxford Instrument (ITC4) regulator. The magnetic field was measured with a Bruker 200 gaussmeter, and the frequency inside the cavity was determined using a Hewlett- Packard 5352B microwave frequency counter. Magnetic susceptibility measurements were performed on microcrystalline powder with a pen- dulum type susceptometer/magnetometer (Manics DSM8) equipped with a helium continuous flow cryostat working in the range 4.2-300 K. An applied magnetic field of approximately 2000 G was used for all the measurements. The diamagnetic correction term was estimated from Pascal tables and found to be -285.45 x 10 -6 emu mo1-1. The value 60x 10 -6 e m u mo1-1 was used for temperature-independent paramagnetism of copper(II) ion. The effective magnetic moment was calculated from eq. (1)

#eff = 2 . 8 3 ( Z m T)1/2 (B.M.) (1)

Copper(II) bromide, hydrobromic acid and 4- aminopyridine were of reagent grade and used without further purification. Carbon, nitrogen and hydrogen analyses were performed on a Perkin- Elmer 240 C-, H-, N-analyser. The concentration

Synthesis of (4-apyH)2[CuBr4] • H20

The title compound was obtained as follows : 0.5 c m 3 (4.4 mmol) of concentrated hydrobromic acid (48%) was added to an acetonitrile solution (10 cm 3) of 4-aminopyridine (0.42 g, 4.4 mmol) with

Studies of 4-aminopyridinium tetrabromocuprate(II) monohydrate 1255

continuous stirring. The white precipitate that formed (the 4-aminopyridinium bromide) was redis- solved by heating to 60°C until the solution became clear. Then, copper(II) bromide (0.49 g, 2.2 mmol) dissolved in acetonitrile (20 cm 3) was added drop- wise to the warm solution. The mixture was heated under reflux with continuous stirring for 3 h. The resulting hot, dark purple solution was filtered to remove impurities and left overnight, after which time, small polyhedral, dark violet crystals suitable for X-ray crystallographic studies were obtained. Crystals were filtered off, washed with diethyl ether and dried in a stream of dry dinitrogen. Crystals were essentially opaque due to the absorption of light in the visible region (546 nm, e = 295 M -1 cm-1) and showed metallic lustre. They were found to be quite stable to light and X-ray exposure, but after three weeks they lost crystallinity and gradu- ally became a powder. Yield 1.106 g, 85% based on copper. Found: C, 20.4; H, 2.7; N, 9.4; Cu, 10.9. Calc. for CloHl6Br4CuN40 : C, 20.3 ; H, 2.7 ; N, 9.5 ; Cu, 10.7%. Main IR bands (KBr, cm -1) : 3500m br [v(O--H)] ; 3390s, 3305s, 3205s [v(N--H)] ; 3100m, 2980s [v(C--H)]; 2690w, 2650w [v(N--H+)]; 1645vs [ v ( ~ N + ) ] ; 1600s [v(C----C)]; t415w, 1370d [v(C--N)]; 1270m, l195vs, 1090m, 995vs [hip(C--H)] ; 815vs, 650w, 570w, 520w, 515m, 405m [fop(C--H)] ; 275w and 250w [v(Cu--Br)]. UV-vis [EtOH/HBr, nm (e, M -1 cm-l)] : 546 (295), 342 (1530), 277 (16,550), 236 (3480). All manipulations were carried out in an open atmosphere; exposure to air (Oz) is necessary for the synthesis of bromocuprate(II) complexes in HBr media, since preparation under anaerobic conditions led to the formation of mixed-valence Cu(II)/Cu(I) compounds. 16

Crystallographic data collection and structure refinement

A suitable deep violet, almost opaque, single crystal of dimensions 0.15 x 0.20 x 0.25 mm was picked after inspection with a polarizing micro- scope. X-ray data were collected at 293(1) K with an Enraf-Nonius CAD4 automatic four-circle diffractometer, using graphite-monochromated Mo-K~ radiation (2 = 0.71069/~). Accurate lattice parameters and crystal orientation matrix were obtained from least-squares refinement of the set- ting angles of 25 reflections in the range 15 ° < 20 < 30 °. Crystallographic data and details of the refinement are reported in Table 1.

A unique data set was measured within the range 1°~<0<~30 ° (0~<h~<12, 0~<k~<20, - 1 9 ~ < l <~ 19) in conventional 0~--20 scan mode. As a check on crystal and electronic stability, two reflections were monitored every 60 rain. The intensity of these standards remained constant within experimental error throughout data collection. Lorentz and pol- arization corrections were applied to the data. Scat- tering factors for neutral atoms and anomalous dispersion corrections for all non-hydrogen atoms were taken from the International Tables for X- ray Crystalloyraphy (1974). 17 The positions of non- hydrogen atoms were found by direct methods.18 At the end of an isotropic refinement cycle an empirical absorption correction ~9 was performed (relative transmission coefficients range 0.890-1.114 with an average value 1.018). The structure was refined by the full-matrix least-squares method with the X- RAY76 computer program. 2° Anisotropic thermal parameters were given to all the non-hydrogen atoms. A convenient weighting scheme was used to

Table 1. Crystal and refinement data for (4-apyh)2[CuBr4] • H20

Formula C10H16CuBr4N40 F(000) 1124 Formula weight 591.43 p (Mo-K~) (cm-') 97.719 Crystal system Monoclinic Number of measured reflections 2689 Space group C2/c Number of observed reflections 1382 [I > 2a(I)] a (A) 8.797(1) Number of parameters 108 b (&) 14.860(1) S ~ 1.29 c (/~) 14.216(1) (A/if) . . . . . . . . . 0.066, 0.010 fl(°) 96.94(2) Rb(Fo) 0.051 V (/~3) 1844.8 (3) RwC(Fo) 0.050 Z 4 Dx (g cm -3) 2.13 D0 (g cm -3) 2.13(1)

"s = [ X : w ( I F o l - 1~l )21(Nobs-Np . . . . ) ]1/2.

° R = Z l IFo I - - IFo l I IZ lFo l cRw = [ Z w ( I r o l - IFol)21ZwlFol2] '/2

1256 P. ROMAN et al.

give no trends in (wA2F) vs. (Fo) and vs. (sin 0/•). 21 All hydrogen atoms were located on a Fourier difference map, but only those of water molecules and N - - H groups were isotropically refined, the remaining ones were introduced as fixed contributors in the final structure-factors calculations. Final geometrical calculations were carried out with the PARST program 22 and the graphical manipulations using the ORTEP utility of the XTAL 3.0 system. 23 Most calculations were carried out on a Micro VAX II computer. Final atomic positional and thermal parameters, together with anisotropic thermal parameters for non-hydrogen atoms, full lists of bond distances and angles, dihedral angles, and a list of observed and calculated structure factors, have been deposited as supplementary material with the Cambridge Crys- tallographic Data Centre.

RESULTS AND DISCUSSION

Description of the crystal structure

The crystal structure of the compound is made up of discrete [CuBr4] 2- anions, with the copper atom situated on a crystallographic two-fold axis, 4-aminopyridinium cations and water of crystallization molecules. An extensive network of hydrogen bonds links these units, most of them being represented by broken lines in Fig. 1. Selected bond lengths and angles and hydrogen contacts are listed in Table 2.

As expected, the [CuBr4] 2- anion shows acom-

pressed tetrahedral geometry? 4 The Cu--Br(2) bond (2.387 A) is slightly longer than the Cu--Br(1) bond (2.374 A) owing to its extensive involve- ment in hydrogen bonding. As will be described below, all bromine atoms are involved in hydrogen bond interactions, but Br(2) is hydrogen bonded to the water molecule and the organic cation, whereas Br(1) only participates in a hydrogen bond to the cation (Table 2). The Br- -Cu--Br bond angles vary within the range 98.21(3)-132.57(3) °, showing that the geometry is intermediate between square planar (D4h) and regular tetrahedral (Ta). The deformation from the tetrahedral geometry can be expressed by the dihedral angle between the planes Br(1)--Cu--Br(2) i and Br(2)--Cu--Br(1) i of 63.8 °. This dihedral angle is indicative of a geometry nearer to tetrahedral if we take into account that this angle is 90 ° for tetrahedral geometry and 0 ° for square planar. A quantitative description of the distortion from tetrahedral geometry has been evaluated by application of Muetterties and Guggenberger's method 25 involving dihedral angles between the various faces of the polyhedron. The value obtained, A = 0.22, shows that the anion is not distorted much from regular tetrahedral geometry (A = 0 tetrahedral, A = 1 square planar) and it is in good accordance with the semi-empirical MO study of the Td-D4h equi- librium in tetrabromometal complexes, 26 which indicates that the [CuBr4] 2- anion has a tendency to display the Td configuration.

Distances and angles within the 4-aminopyridinium cation are normal. 27 The pyridinium

N•(1) 2) '...,,.,, Br(1)i .. ,v4"ba-,, , 0 .%

Br(2) '. Br(1)

°.

~o(1) Fig. 1. Illustration of a portion of the hydrogen bonding network in the structure of (4-apyH)2[Cu-

Br4]" H20.

Studies of 4-aminopyridinium tetrabromocuprate(II) monohydrate

Table 2. Selected bond distances (/~) and angles (°) and hydrogen bond distances and angles for (4-apyH)2[CuBr4]

Anion Cu--Br(1) 2.374(1) Cu--Br(2) 2.387(1)

Br(1)---Cu--Br(l) i 98.21(3) Br(1)--Cu--Br(2) 99.50(3) Br(1)---Cu--Br(2) i 132.57(3) Br(2)---Cu--Br(1) i 132.57(3) Br(2)--Cu--Br(2) i 100.01(3) Br(1)~---Cu--Br(2) ~ 99.50(3)

Cation N(1)--C(2) 1 . 3 3 ( 1 ) C(2)--N(1)---C(6) 120.8(9) N(1)--C(6) 1 . 3 5 ( 1 ) N(1)--C(2)---C(3) 121.9(9) C(2)-42(3) 1.35(1) C(2)--C(3)---C(4) 119.6(7) C(3)--C(4) 1 . 3 7 ( 1 ) C(3)--C(4)--N(7) 122.5(7) C(4)---C(5) 1 . 4 4 ( 1 ) C(3)--C(4)--C(5) 118.1(7) C(4)--N(7) 1 . 3 5 ( 1 ) C(5)--C(4)--N(7) 119.3(7) C(5)--C(6) 1 . 3 4 ( 1 ) C(4)--C(5)--C(6) 118.6(9)

N(1)--C(6)--C(5) 120.8(10) Hydrogen contacts X--H. . .Y X--H X.. .Y H. . .Y < X--H.. .Y N(1)--H(11)...O(1) ii 1.03(8) 2 . 8 5 ( 1 ) 1.84(9) 170(7) N(7)--H(71)...Br(1) ~i~ 0.88(9) 3.479(8) 2.61(9) 171(7) N(7)--H(72)...Br(2) ~v 0.98(8) 3.500(8) 2.56(9) 159(7) O(1)--H(1)...Br(2) i~ 0.87(7) 3.406(6) 2.58(7) 158(6)

Symmetry codes: (i) - x , y, - z + l / 2 ; (ii) - x + l , - y , - z + l ; (iii) x + l / 2 , - y + l / 2 , z+l /2 ; (iv) x, - y , z+l/2.

1257

ring is nearly perfectly planar, and the nitrogen atom of the non-protonated amino group lies on the plane.

From a preliminary viewpoint, the structure can be visualized (Fig. 2) as a series of parallel (levels y ~ 0, y ~ 0.5) mixed anion-water ribbons running along the a-axis and placed at two z levels, z = 1/4 and z = 3/4, each water molecule (lying on a two- fold axis) is sandwiched between two [CuBr4] 2- anions and linked to them through the formation of two symmetry-related O- -H. . .Br (2 ) interactions.

There is no other important interaction between tetrabromocuprate(II) anions, the nearest non- bonded C u ' " Br distance is 6.02 A, and the shortest B r ( 1 ) . . . B r ( 2 ) [ - x + 1/2, y + 1/2, - z + 1/2] contact in the structure (4.55 A), established between two anions from parallel ribbons lying on the same z level, is 0.65 A longer than the sum of the bromine ion van der Waals radii (3.90 A). Distances to other ribbons in different z levels are considerably longer because of the presence of the 4-aminopyridinium cations in the interstitial spaces. The planar organic cations are oriented nearly perpendicular to the H20-[CuBr4] 2- ribbons' z levels and form stacks running in the [1 1 0] and [1 i 0] directions. Two adjacent cations are strictly parallel, face-to-face stacked with an interplanar distance of 3.35/~ and

interatomic contacts ranging from 3.35 to 3.70 A. These cations are offset one from another (2.50 A) and show a staggered orientation, presumably to minimize electrostatic interactions and steric hin- drances between the ammonium and amino groups. Cations are linked to the anion-water ribbons by means of N - - H . . . X hydrogen contacts. The two hydrogen atoms of the exo-amino group each form a hydrogen bond to bromine atoms from anions belonging to neighbouring ribbons in the same z level. These hydrogen interactions are weaker than those described above between the water molecules and the bromocuprate anions. The ring N - - H group, on the opposite side of the cation, is hydrogen bonded to water molecules lying on the other z level (z+ 1/2 or z - 1/2). The shortest metal-metal separation in the crystal structure is 7.16 A.

Infrared and UV-visible spectroscopy

The infrared spectrum of the complex is consistent with the structural data presented above. It shows a broad medium absorption at 3500 cm -~ due to the presence of lattice water molecules. In the region 3400-3200 cm-t three strong bands appear corresponding to the substituent N - - H stretching vibrations and they are shifted with respect to those


Fig. 2. Stereoview packing of the (4-apyH)2[CuBr4] • H20 as viewed parallel to the b-axis (c-axis is vertical, a-axis is horizontal).

of the aromatic base, 28 probably due to the different hydrogen-bonding arrangement shown in the crystal structure between the aromatic cation and the [CuBr4] 2- anions. 29 A similar shift is observed for the N - - H + stretching vibrations, at 2690 and 2650 cm -~, as a consequence of the strong hydrogen bonds with the water molecules. The IR spectrum of the salt in the range 1700--400 cm-1 is dominated by both internal vibrations of the pyridinium ring and the aromatic C - - H bend peaks. The far-IR region shows two bands at 275 and 250 cm -~ due to the Cu- -Br stretching vibrations, which are similar to those observed for other tetrabromocuprate(II) salts. 3°

The UV-vis spectrum shows strong bands at 236.0 and 276.8 nm, assigned to the ~c ~ zt* and n re* transitions of the 4-aminopyridinium cation, 31 which are not affected by the complex anion. Two weak bands appears at 342 and 546 nm which, according to the literature, 32 are associated with ligand-to-metal charge-transfer transitions, since the d-d transitions are expected in the near- IR region in the case of the [CuBr4] 2- chromo- phore. 29"33 The broad band at 546 nm is responsible for the dark violet colour of the compound.

EPR spectroscopy and magnetic susceptibility

The X-band powder EPR spectra of the compound in the range 300-4.2 K show a broad and isotropic signal which is narrower at low tern-

peratures (Fig. 3a). At room temperature, the 9 tensor value is 2.16 and the peak width is 555 G, while at 4.2 K the peak width is 420 G and 9 = 2.15. However, the Q-band spectra (Fig. 3b) show an axial-type signal, that remain practically unchanged over the work range of temperature, with Y tensor values ofgll = 2.302, g± = 2.084, ( g ) = 2.16. This axial spectrum is consistent with the compressed tetrahedral stereochemistry of the [CuBr4] 2- chro- mophore and the relation g, >> g± > 2.0 indicates a b] (dxLyO ground s ta te . 33'34

The magnetic susceptibility data for the compound were studied over the temperature range 300-4.2 K. The experimental data, plotted as thermal variations of reciprocal susceptibility and the )~m T product are shown in Fig. 4.

The magnetic susceptibility data are well described by the Curie-Weiss law, )~ = C / ( T - ® ) for T > 30 K, with a positive temperature intercept (®) of about +2.3 K. The slope corresponds to a Curie constant of C = 0.43 cm 3 K mol-1. As the temperature is lowered, the product )~m T, pro- portional to the effective magnetic moment, is practically constant up to 30 K but it increases slightly upon cooling from 30 to 4.2 K (Pelt = 1.86 PB at room temperature and 2.08/~B at 4.2 K). This fact, together with the positive value calculated for 19, suggests the presence of weak ferromagnetic interactions. In the light of the structural data described above, it is reasonable to assume that the most probable magnetic exchange pathway involves the hydrogen bond system C u - - B r - - - H - - O - - H . - -


(a) I \ 295 K . . . . . 4.2K

~ li t /

~j I , I I I " I I I I I

2100 2700 3300 3900 4500 H (Gauss]

I I I I

9600 10400 11200 I I I

12000 12800 H (Gauss)

Fig. 3. (a) X-band powder EPR spectra at room temperature ( ) and at 4.2 K (---), (b) Q-band EPR spectra at room temperature.

Br - -Cu which holds adjacent anions together in the one-dimensional HzO-[CuBr4] 2- ribbons. Hydro- gen bonding has been extensively demonstrated to be a powerful tool for generating ferromagnetic interactions and for propagating them in several pre-established spatial directions in both inorganic 35 and organic solids? 6 On this basis, we used, as an appropriate magnetic model, a 1D s = 1/2 Heisenberg linear-chain model and eq. (2) 37 was used to fit the experimental data by a least-squares procedure and to calculate the exchange parameter, J.

~m - 4 k T

L l + A x + B x2 + C x3 +---D-~x4 -~-~-xS- j (2)

where x = 2J/kT, N and k are the Avogadro and

Boltzmann constants, fl is the Bohr magneton, A = 5.7980, B = 16.9027, C = 29.3769, D = 29.8330, E--14.0370, A ' = 2.7980, B ' = 7.0087, C' = 8.6539 and D' = 4.5743.

The resulting value J = 0.7 cm -~ is low, as expected for magnetic interactions through hydrogen bonds, and the 9 value of 2.14 determined from the magnetic susceptibility agrees reasonably well with the average g value of the EPR spectra.

Thermal analysis

The thermal behaviour of the compound has been deduced from its T G / D T A and DSC curves in both synthetic air and dinitrogen atmospheres. The data show the occurrence of four consecutive steps: dehydration, melting point, organic cation pyrolysis and inorganic residue evolution (Fig. 5). The dehydration and melting processes do not show

350 1

30O 0.8

250 0.6

156 ^'" ~,~, = . 0.4

106 o~ ~

50

0 ' 0 0 30 60 90 120 150

T (K)

Fig. 4. Thermal variation of the reciprocal susceptibility and ;(roT for the compound (4-apyH)E[CU- Br4]" H20.

1260 P. ROMAN e t al.

-20

D T G (~ /min )

-40

-60

-80

'rG

i i ~ i , I , P , I , I , I h I , I , , I , I ,

0 100 200 300 400 500 600

T (°C)

(a)

1000

80O

6O0

40O

2O0

0

-200

- T G

0 -

-20 i

-40

-60

_i000 I , t , I , i , I , I , I , I , i , i , i i I , I ,

200 400 600

(b)

- 600 D T G

(%/rain)

450

-_ 15o i _ _

0

-150

8 O O

T (°C)

Fig. 5. TG-DTG-DTA curves for the thermal decomposition of compound (4-apyH)2[CuBr4] • H20 in synthetic air (top) and dinitrogen (bottom) atmospheres.

significant dependence on the nature of the environ- mental gas. In both atmospheres, the cleavage of the water of crystallization molecule takes place during an endothermic process in the range 80- 115°C. The total heat of the transformation (AH) is 65 and 60 kJ mol- l in air and nitrogen atmospheres, respectively. These values are in good agreement with those calculated for the dehydration heat of crystal hydrates. 38 Although the crystals conserve the form, the X-ray diffraction analysis of anhydrous product shows that the structure has been destroyed after the dehydration, in agreement with the fact that the dehydration is not reversible. Crys- tals lose crystallinity and as they are mounted on a X-ray Weissenberg camera, the photographic film does not show diffraction spots. The anhydrous compound is thermally stable up to c a 190°C, at

which temperature an endothermic peak (60 kJ mol-1), corresponding to the melting point, is observed. No structural phase transitions were observed for the anhydrous compound from the dehydration process to its melting point. Immedi- ately after the fusion of the sample, the thermal degradation starts. The decomposition pathway of the liquid and the final residue are affected by the surrounding atmosphere. Under air, the compound decomposes endothermically with loss of the organic cation to give copper(II) bromide followed by an exothermic loss of bromine to give copper(II) oxide. In nitrogen atmosphere, all degradation processes are endothermic and they lead, without intermediate stable species, to powdered metallic copper as the final product above 700°C. Some authors have suggested that the thermal degradation of


organoammonium halometallate complexes in inert atmospheres is an excellent method of obtaining powdered metals of great reactivity. 39

Acknowledgements--This work was financially sup- ported by the Universidad del Pais Vasco/Euskal Herriko Unibertsitatea (UPV 169.310-EA189/94). J. S. thanks the Ministerio de Educaci6n y Ciencia of the Spanish Government for a doctoral fellowship (Grant No. AP94.42172547).

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Documents

Magneto-structural studies and thermal analysis of the 4-aminopyridinium tetrabromocuprate(II) monohydrate