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Influence of protonation on crystal packing and thermal behaviour of tert-butylammonium decavanadates
Polyhedron Vol. 15, No. 24, pp. 4555 4564, 1996
~
Pergamon S0277-5387 (96)00186-6
Copyright 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0277 5387,'96 $15.00+0.00
I N F L U E N C E OF P R O T O N A T I O N O N CRYSTAL P A C K I N G A N D T H E R M A L B E H A V I O U R OF TERTBUTYLAMMONIUM DECAVANADATES ANA S. J. WERY, JUAN M. GUTIERREZ-ZORRILA,* ANTONIO LUQUE
and PASCUAL ROMAN* Departamento de Quimica Inorgfinica, Universidad del Pa/s Vasco, Apartado 644, 48080 Bilbao, Spain
and MARTIN MARTINEZ-RIPOLL Departamento de Cristalograffa, Instituto de Qufmica-Ffsica Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain
(Received 24 January 1996 ; accepted 4 April 1996) Abstract Two tert-butylammonium decavanadate(V) salts with the formulae [(CH3)3CNH316[VI0028] • 8H20 (1) and [(CH3)3CNH314[H2VI0028] (2), have been synthesized and their crystal structures have been determined by means of single-crystal X-ray diffraction. The crystal structure of compound 1 is stabilized by electrostatic forces and an extensive network of hydrogen contacts involving anions, cations and water molecules. The anions and cations of this compound are arranged in layers perpendicular to the [010] direction following the sequence, cation anion-cation. In the crystal structure of compound (2), each dihydrogen decavanadate(V) anion is joined to three adjacent polyanions by means of O(6)--H...O(4) hydrogen contacts forming layers parallel to the plane (101) and the hydrophobic groups of the cations are disposed in layers parallel to the anionic sheets. The thermal behaviour of both compounds has been studied. Compound 1 is an octahydrate and its thermal decomposition begins at 70°C with the loss of water of crystallization, while compound 2 is anhydrous and is consequently more stable, with decomposition starting at 200°C. Copyright © 1996 Elsevier Science Ltd
The solution and solid-state chemistries of some early transition metals (V, Mo, W, Nb and Ta) are both characterized by a tremendous variety of polyoxoanion species, ranging from monomers to high nuclearity clusters which are formed by the assembly of MOx units. ~ Owing to the structural richness, the varied physical properties and the successful uses of these inorganic metal-oxygen clusters, polyoxometalate chemistry is an active and renewed area of research, which not only is of interest in organic chemistry but also has numerous
* Authors to whom correspondence should be addressed.
applications in field such as catalysis, biology, medicine and materials science. 2 The understanding of the driving force for the formulation of the oxometalate cages is still a challenge, but it is well known that in solution there are quick and complex equilibria between polyoxo species with different nuclearity which are strongly dependent on the media conditions such as pH value, ionic strength, temperature and nature of the counter ion. 3'4 One of the best-studied groups in polyoxometalate chemistry is the oxovanadium(V) compounds.1 3.5 Acidification of metavanadate solutions to PH ~<6 leads to orange solutions containing the decavanadate anion [V1002g]6 . The
4555
4556
A. S. J. WERY et al.
equilibria between metavanadate~decavanadate are established very slowly and there are almost certainly other intermediate species, one of which, a cyclic hexamer has been detected by 5~V NMR. 6 The structure of the decavanadate anion has been well established by several X-ray studies 7 and consists of an arrangement of ten edge-shared VO6 octahedra with approximate D2h (mmm) symmetry. The decavanadate anion can be found in different protonation states. The sites of protonation have been determined in several studies. 8 The doublelinked and triple-linked oxygen atoms are those which are most susceptible of protonation owing to the basicity of these atoms compared with the terminal oxygen atoms. In the diprotonated anions, the hydrogen atom are located, generally, in a double-linked oxygen atom that joins vanadium atoms from the equatorial plane with vanadium atoms from the upper plane or the lower one. 8 Recently, Romfin et al. 9 have isolated and characterized a dihydrogen decavanadate salt of formula (C6H~6N)4[H2V~oO28], in which the hydrogen atoms are attached for the first time to two triplelinked oxygen atoms which reinforce theoretical calculations of relative basicities of the oxygen site atoms in decavanadate anions. On the other hand, the first tetrahydrogen decavanadate anions has been isolated from an alcoholic solution by Wang et al. ~° In this case, the hydrogen atoms are joined to two double-linked oxygen atoms and to two triple-linked oxygen atoms. As a part of a study on polyoxometalates we have isolated and chemically and structurally characterized two compounds of formula [(CH3)3 CNH316[V,oO28]'8H20 (1) and [(CH3)3CNH3]4[H 2 V10028] (2). This work has allowed us to study the influence of protonation on the crystal packing and thermal behaviour of polyoxovanadate salts. EXPERIMENTAL
General and apparatus All chemicals were procured commercially and used without further purification. Microanalyses of carbon, nitrogen and hydrogen were performed on a Perkin-Elmer 240 C-, H-, N- analyser and vanadium was determined thermogravitationally as V205 after thermal decomposition of the samples in an argon-oxygen atmosphere. The densities were measured by flotation in CHBr3/CC14. IR spectra were recorded in the range 4000-400 cm -1 on a Nicolet 740 FT-IR spectrometer, and the solid compounds were mixed with potassium bromide (Fluka, FT-IR spectroscopy grade) into trans-
parent disks. A Setaram T A G 24S 16 thermobalance was used to obtain the differential thermal analysis (DTA) and thermogravimetric analysis (TGA) curves, simultaneously, in an argon and oxygen atmosphere (4 : 1) with a heating rate of 5°C min-~.
Synthesis of compounds Hexakis( tert-butylammonium) decavanadate( VI) octahydrate,[(CH3)3CNH3]6[V~o028]" 8H20 (1). To a stirred aqueous suspension (30 cm 3) of V205 (1.00 g, 5.5 mmol) was added t-butylamine (0.75 cm3; 7.0 mmol) and the mixture heated under reflux for 2 h. The resulting orange solution (pH = 5.5) was filtered off and allowed to stand at room temperature. Owing to the high solubility of this compound in water, single crystals were obtained by diffusion of acetone into the aqueous solution. This compound is soluble in water and D M F and insoluble in acetone and ethanol. Crystals are stable to the air and light but they are decomposed slowly by Xray exposure. Found: C, 18.7; H, 5.8; N, 5.5; V (V205), 57.1. Calc. for C24H88N6036VI0 : C, 18.7; H, 5.7 ; N, 5.5 ; V (V205), 58.8%. Tetrakis(tert-butylammonium)dihydrogendecavanadate(VI), [(CH3) 3CNH314[H2VjoO28] (2). This compound can be prepared by the following two methods. (a) Heating with reflux stoichiometric amounts of vanadium pentoxide (2.13 g, 11.7 mmol) and tbutylamine (1.0 cm 3, 9.4 mmol) in 100 cm 3 of water. The resulting solution (pH = 3.5) was allowed to stand at room temperature. Crystals of compound 2 were filtered off from the solution two weeks later. (b) Single crystals suitable for X-ray diffraction were obtained by adding a solution of Na2HPO4 (0.40 g, 2.84 retool) in 10 cm 3 H20 to an orange solution of compound 1. Hydrochloric acid (2.5 M) was added up to pH 3. Two weeks later, prismatic orange crystals were filtered off from the solution. Crystals were found to be stable to air, light and Xray exposure. Found: C, 15.5; H, 4.0; N, 4.3; V (V205), 72.4. Calc. for CI6HsoNaO28VI0 : C, 15.3; H, 4.0 ; N, 4.5 ; V (V205), 72.0%.
Crystallographic data collection refinement
and structure
Single crystals of compounds 1 and 2 were obtained as described above. Table 1 summarizes the crystal data and collection procedures for both compounds. Single crystals of compound 1 were not stable to X-ray exposure and a significant decay of the intensity of two reference reflections was
Effect of protonation on t-butylammonium decavanadates
4557
Table 1. Crystallographic data for the compounds 1 and 2 Compound
1
2
Formula Mr System Space group a (A) b (A) c (fit) /3 C) V (fit3) Z F(000) Do (Mg m -~) Dx /~ [Mo-K,] (cm ') Form Dimensions (ram) Colour Diffractometer Temperature (K) Radiation (fit) Monochromator Scan-mode 0 Range
(C4HI2N)6[V,oO28]"8 H 2 0
(C4H,2N)4[H2V10028]
1546.4 Monoclinic P2,/a (No. 14) 10.642(9) 26.35(1) 11.711(7) 115.15(5) 2973(4) 2 1580 1.72(1) 1.727 15.386 Prism 0.10 x 0.15 × 0.22 Orange Enraf-Nonius CAD4 295(1) 0.71069 Oriented graphite o9/20 1-30 0-~ 1 4 ; 0 ~ 3 7 ; -16--+ 16 Intensity and orientation ~ 71~ ; ~ ~ ~ 7200 s; 100 reflect. 8572 3740 [1>12or(/)] 283 2.52 (0.85 fit del O(13)) DIFABS Tram= 0.803, Tm~x= 1.117 0.148 0.178
1256.0 Monoclinic P21/n (No. 14) 16.244(1) 11.336(1) 11.871(1) 108.625(3) 2071.6(3) 2 1252 2.012(5) 2.010 21.701 Prism 0.10 x 0.12 x 0.20 Red-orange Philips PW 1100 295(1) 0.71069 Oriented graphite o9/20 240 0--+28;0--+20; - 2 1 --+21 Intensity and orientation 3 36;36 3 5400 s 12848 9201 [I~>3cr(/)] 325 0.81 DIFABS Tram= 0.814, Tmax= 1.074 0.047 0.047
hkl Controls - - Reflections -Periodicity
Indep. reflections Observ. reflect. No of variables (Ap)(e fit 3) Absorption correction R Rw
observed, so, a linear decay correction ranging from 1.00 to 1.30 was applied. In both cases the intensity data were also corrected for Lorentz and polarization effects, and an empirical absorption correction ~ based on the isotropically refined structures was applied. Both structures were solved by a combination of direct methods 12and difference Fourier syntheses and refined by full-matrix leastsquares methods on F. 13 The crystal structure of compound 1 was refined with anisotropic thermal parameters for all the nonhydrogen atoms except the carbon atoms from the tert-butylammonium cations, which were included as isotropic contributors. The refinement resulted in a rather high reliability factor (R = 14.8%) owing to the X-ray instability, the modest quality
and size of the single crystals which led to low intensity reflections, as can be observed in the supplementary Fo/Fc list. The crystal structure of compound 2 was refined with anisotropic thermal parameters for all the nonhydrogen atoms. A convenient weighting scheme 14 of type oo=~oj~oz with o91=kl/(a+b[Fo[+ c lFo [2)2 and ~o2 = k2/[d+e(sinO/2)] was used so as to give no trends in (eo A2F) vs (Fo> and vs (sin0/2>. Hydrogen a t o m positions from cations were discernible from electron density difference maps except for those belonging to the methyl groups C(23), C(24) and C(25) which were generated at geometrically fixed positions. Besides, the hydrogen atom belonging to the anion was also found in an electron density difference map.
A. S. J. WERY et al.
4558 R E S U L T S AND D I S C U S S I O N
Thermal analyses Table 2 lists steps, initial and final temperature ( C ) , partial and total weight loss, enthalpy (endothermic or exothermic) and the maximum peak for each step in the thermal decomposition of both compounds in an argon-oxygen atmosphere. Thermal decomposition of compound 1 starts at 70°C with two almost overlapping steps that correspond to the loss of the crystallization water molecules (70 110'C) and part of the organic cations (110-130°C). These steps are followed by a progressive weight loss with no clear peaks in the D T A curve. After this progressive weight loss, an endothermic step (195-240°C) and another exothermic one (24(~260 C) take place yielding a weight loss higher than that expected for V205. This residue could be a mixture of mixed-valence vanadium oxides V3OT/V6013 based on calculations. This fact has also been observed in the thermal decomposition of diethylentriammonium decavanadate tetrahydrate. 15 Some authors have suggested that the formation of these mixed-valence vanadium oxides is a result of the reductive action
Table 2. Steps, initial and final temperature ('C), partial and total weight loss, enthalpy (endothermic or exothermic) and maximum peak for each step in the thermal decomposition of compounds 1 and 2 under an argon oxygen atmosphere Step
TI-T~
Compound 1 1 2 3" 4 5 6
70 1l0 110-130 13(~195 195240 240-260 32~360
Compound 2 1 2 3 4"
200-245 245-265 315-355 355-550
Tm
AH
%Am
95 115
Endo Endo
225 255 335
Endo Exo Exo Z%Am Calc.
9.55 6.62 1.80 13.86 10.40 - 1.21 41.02 41.15
235 250 330
Endo Exo Exo
11.64 15.11 - 0.30 1.02
Z%Am Calc.
27.47 27.56
a Progressive weight loss without clear peaks in the DTG or DTA curves.
of NH3 during the thermal decomposition of the base.16 These oxides are oxidized to give vanadium pentoxide in the last exothermic step once the organic base has been totally decomposed (Fig. 1a). C o m p o u n d 2 is anhydrous and stable up to 200"C. Its thermal decomposition starts with two successive steps, the first one endothermic and the second one exothermic, yielding an intermediate stable compound up to 315"C. Then, another exothermic step, similar to the one observed for compound 1 takes place with a weight increase of 0.30%, corresponding to the reoxidation of vanadium(IV). The process finishes with a progressive weight loss without clear peaks in the D T G and D T A curves to give V2Os (Fig. l b).
Infrared spectroscopy Figure 2 shows the I R spectra for both anions. In the spectrum of compound 2, there are two bands characteristic of protonated decavanadate anions and placed at 985 and 615 cm-1. The first one can be assigned to V - - O r bonds in V O s O - - H octahedra, which are shortened owing to the elongation of the V--Ob--H bonds. The second band is assigned to the V--Ob--H bonds. The bands between 990 and 900 c m - ' are attributable to the V - - O r stretching vibration while the ones corresponding to the V - - O b stretching vibration are observed in the range 840-440 cm ~. The signal at 420-400 c m - 1 is attributed to the deformation vibration of the groups V--Oh.
Description of the crystal structures The X-ray structural analysis reveals that the asymmetric unit of compound 1 is composed of one half of a centrosymmetric decavanadate anion, [ V i 0 0 2 8 ] 6 , three tert-butylammonium cations and four water molecules, while the asymmetric unit of compound 2 contains one half of a centrosymmetric dihydrogendecavanadate anion [H2V10028]4- and two tert-butylammonium cations. Figure 3 shows both decavanadate anions together with the numbering scheme. The decavanadate anion can be described as a section of cubic close-packed oxygen atoms with vanadium atoms in the octahedral holes. The vanadium atoms are not in the geometric centre of the octahedra but they are closer to the vertex occupied by the terminal oxygen atoms. The V - - O bond distances for compounds 1 and 2 are listed in Tables 3 and 4. Taking into account the ideal symmetry of the decavanadate anion D2h (mmm), there are three types of VO6 octahedra according to the position in the anion, each of these types
Effect of protonation on t-butylammonium decavanadates 10
*S
70
(1)
o 0
4559
18 (2)
TG
o
-10
~o ~
-10
$ <
-20
2
-30
so
~
~o ~
-20 -30 -40
DTA
,,
-50
100
200
300
10 ~__ ATD,,
400 500 600 TEMPERATURE (ac)
-40
-2
r,l,i,
i,
1O0
i,i,J,i,
200
300
I,~,l.llj
400 500 600 TEMPERATURE (oC)
-10
Fig. 1. TG-DTA curves for the thermal decomposition of compounds 1 and 2 in argon-oxygen atmosphere.
I
I 1)V-O I
1200
1000
i
I ~)asV-Ob
I
II ~)siV-Ob
I
I
800
600
I ~V*O b 0(2) 400
'0 (cm "1)
V(2)
0(13)
Fig. 2. Infrared spectra for both anions.
shows a particular distribution of the V - - O distance. (a) Octahedra type I : those located above and below the equatorial plane that present a short distance (1.60/k) ; two medium ones (ca 1.80-1.85 A) ; two long ones (ca 2.00-2.05/k) and a very long one (ca 2.30-2.35 A). (b) Octahedra type II: those located on the equatorial plane with one terminal oxygen atom that have a short distance (ca 1.60 A); three medium ones (ca 1.80-1.90/k); a long one (ca 2.00-2.05 A) and a very long one (ca 2.25 A). (c) Octahedra type III: central octahedra on the equatorial plane that possess two short distances (ca 1.65-1.70 ,&) ; two medium ones (ca 1.901.95 A) and two long ones (ca 2.10-2.15/k). The hydrogen atom for the centrosymmetric dihydrogendecavanadate anion of compound 2 was located from an electron density difference map, and it is bonded to the bridged oxygen atom O(6),
N oc1,)
~-- ~ v(4)'~
0(4) ,
Fig. 3. ORTEP views for [ V i 0 0 2 8 ] 6 - anion (top) and [HzVioOz8] 4 anion (bottom) with atom labelling.
which joins the V(1) atom from the equatorial plane with the V(2) atom from the upper level. Besides, the protonation site was also confirmed by means of the empirical bond length/bond number calculation, s=(R/1.79) -5J, ( R = V - - O distance; s = bond number). 17Table 5 lists the bond number
4560
A.S.J. WERY et al.
Table 3. Bond lengths V--O (A), O." .O distances (A) (inferior part of the diagonal) and O--V--O angles (°) (superior part of the diagonal) for compound 1
O(1) 0(6) 0(7) 0(8) 0(9) O(14)
0(2) 0(6) 0(10) 0(12') 0(13) 0(14)
0(3) 0(7) O(10) 0(5') O(11) O(14)
0(4) 0(8) O(11) O(13') O(12) O(14)
0(5) 0(9) 0(13) 0(14') 0(12) 0(14)
V(1)
0(1)
1.61(2) 1.89(1) 1.84(1) 1.88(1) 2.04(1) 2.30(1)
2.74(3) 2.69(2) 2.71(2) 2.82(2)
V(2)
0(2)
1.62(1) 1.84(2) 1.80(1) 2.02(1) 1.98(1) 2.25(1)
2.72(2) 2.68(2) 2.77(2) 2.76(2)
V(3)
0(3)
1.58(2) 1.84(2) 1.88(1) 2.02(2) 1.88(1) 2.38(1)
2.71(2) 2.70(2) 2.79(2) 2.71(2)
V(4)
0(4)
1.59(1) 1.79(2) 1.83(1) 2.02(1) 2.00(1) 2.20(1)
2.65(2) 2.67(2) 2.75(2) 2.75(2)
V(5)
0(5)
1.68(1) 1.71(2) 1.91(1) 2.16(1) 1.90(1) 2.07(1)
2.75(2) 2.68(1) 2.66(2) 2.71(2)
0(6)
0(7)
0(8)
0(9)
102.9(7)
102.3(8) 90.7(6)
101.5(8) 154.2(6) 92.1 (6)
100.9(7) 83.9(6) 156.8(6) 83.5(5)
2.65(2) 2.68(2) 2.63(2) 2.64(1)*
2.77(2)*
2.61(2) 2.64(2)*
0(10)
0(12')
0(13)
103.9(6)
103.1(7) 94.5(7)
98.7(7) 155.0(5) 90.8(6)
99.8(7) 89.7(6) 155.0(5) 75.8(5)
2.73(2) 2.69(2) 2.64(1)*
2.68(2)*
2.46(2) 2.66(2)
O(10)
O(5')
O(11)
104.5(7)
102.6(7) 90.0(6)
101.1(7) 154.5(6) 83.9(6)
103.1(7) 91.8(6) 152.9(6) 82.9(6)
2.61(2) 2.67(2) 2.77(2)*
2.68(2)*
2.59(2) 2.66(2)
O(11)
O(13')
O(12)
103.1(7)
102.5(7) 95.9(6)
98.7(7) 155.7(5) 89.8(6)
99.6(6) 90.4(6) 155.0(6) 75.3(5)
2.72(2) 2.69(2) 2.64(2)* 0(9) 108.4(6)
2.68(2)* 0(13) 98.2(6) 99.5(5)
2.76(2)
2.46(2) 2.58(1)
2.58(2)*
0(12)
86.8(6) 164.8(5) 78.7(5)
98.2(6) 95.3(6) 154.6(5) 81.6(5)
2.66(2) 2.64(2)
O(14) 174.7(7) 80.8(6) 76.8(5) 73.7(4) 76.8(5) O(14) 172.2(6) 82.0(6) 82.7(5) 75.3(4) 74.3(5)
2.54(2)*
0(14')
2.58(2) 2.67(2) 2.62(2)*
173.5(6) 79.8(5) 81.8(5) 76.8(5) 74.7(4)
2.68(2)*
0(8)
2.69(2)
0(14)
2.58(2)*
0(7)
2.63(2)
174.7(7) 77.5(5) 83.0(5) 77.3(5) 73.8(5)
2.62(2)*
0(6)
2.67(2)
0(14)
0(14) 164.4(6) 87.3(5) 80.8(5) 77.6(5) 79.5(5)
2.54(2)*
* Shared edge.
for all the oxygen atoms considering the contribution of the adjacent hydrogen atoms from O . . . H - - N ~8and O - - . H - - O ~9interactions. A comparison of the V - - O bond lengths in both anions shows that the attachment of the hydrogen atom on the 0(6) atom produces an elongation of the V--O(6) bond and consequently a significant
shortening of the V - - O bonds involving the oxygen atoms in trans positions with respect to the 0(6) atom in the V(1) and V(2) octahedra. Both anions have a Ci point symmetry but they are close to the D2h ideal point symmetry. However, the disposition of the planes in the dihydrogendecavanadate anion shows a stronger deviation
Effect of protonation on t-butylammonium decavanadates
4561
Table 4. Bond lengths V--O (A), O...O distances (A) (inferior part of the diagonal) and O--V--O angles () (superior part of the diagonal) for the compound 2 V(1) 0(1) 0(6) 0(7) 0(8) 0(9) 0(14)
1.598(2) 1.992(2) 1.853(2) 1.790(1) 2.029(2) 2.307(1)
V(2) 0(2) 0(6) O(10) O(12') O(13) O(14)
1.614(2) 1.974(2) 1.753(2) 1.894(1) 2.047(2) 2.209(1) V(3)
0(3) 0(7) O(10) 0(5') O(11) O(14)
1.594(2) 1.824(2) 1.932(2) 2.018(2) 1.876(1) 2.341(2)
V(4) 0(4) 0(8) 0(11) 0(13') 0(12) 0(14)
1.611(2) 1.887(2) 1.808(2) 1.917(2) 2.013(2) 2.260(1) V(5)
0(5) 0(9) O(13) O(14') O(12) O(14)
1.692(2) 1.680(2) 1.906(1) 2.162(1) 1.941(1) 2.107(2)
0(1) 2.771(2) 2.703(3) 2.685(2) 2.813(2)
0(2)
0(6)
0(7)
0(8)
0(9)
100.4(1)
102.9(I) 87.5(1)
104.7(1) 153.9(1) 93.7(1)
101.0(1) 80.9(1) 155.0(1) 87.4(1)
2.662(3) 2.657(2) 2.608(2) 2.632(2)*
0(6) 98.6(1)
2.731(2) 2.635(2) 2.738(2) 2.778(2) 0(3) 2.696(3) 2.736(3) 2.816(2) 2.721 (2)
0(4) 2.700(2) 2.670(2) 2.747(2) 2.794(2) 0(5)
2.645(2) 2.647(2)* 2.648(2)*
0(10)
0(12')
0(13)
103.0(1) 92.8(1)
102.3(1) 154.1(1) 92.2(1)
98.0(1) 84.1(1) 159.0(1) 78.0(1)
2.702(3) 2.737(2) 2.694(2) 2.632(2)*
2.670(2)*
2.484(2) 2.637(2)
0(10)
0(5')
0(11)
103.9(1)
101.3(1) 90.0(1)
101.8(1) 154.2(1) 83.0(1)
103.0(1) 91.6(1) 154.5(1) 84.5(1)
2.658(2) 2.619(2) 2.652(2) 2.714(2)*
2.670(2)*
2.620(2) 2.636(2)
0(11)
O(lY)
0(12)
100.8(1)
102.5(1) 92.7(1)
101.9(1) 154.5(1) 93.7(1)
102.3(1) 86.2(1) 156.9(1) 78.3(1)
2.720(2) 2.665(2) 2.647(2)* 0(9)
2.692(2)* 0(13) 97.7(1) 98.9(1)
2.729(2)
2.484(2) 2.635(2)
2.599(2)*
0(12)
85.4(1) 165.8(1) 80.5(1)
96.0(1 96.4(1) 154.9(1) 78.8(1)
2.637(2) 2.685(2)
172.5(1) 77.8(1) 83.9(1) 79.6(1) 75.2(1) 0(14)
175.5(1) 80.3(1) 76.7(1) 74.0(1) 78.5(1) 0(14) 175.4(1) 78.8(1) 82.0(1) 77.7(1) 75.1(1)
2.613(2)*
0(14')
2.634(2) 2.706(2) 2.648(2)*
0(14)
2.692(2)*
0(8)
2.674(2)
174.3(1) 75.1(1) 80.6(1) 79.4(1) 75.0(1)
2.599(2)*
0(7)
108.7(1) 2.740(2) 2.712(2) 2.636(2) 2.706(2)
2.714(2)*
0(14)
0(14) 163.3(1) 88.011) 80.6(1) 77.9(1) 80.3(1)
2.613(2)*
* Shared edge.
from the D2hideal situation. The distortion may be explained by the structural modifications produced by the protonation of the anion. Figure 4a shows the projection on to the (001) plane of the crystal structure of compound 1. The polyanions and cations, with their nitrogen atoms directed towards the oxygen atoms from the polyanions and water molecules, are located in layers perpendicular to the [010] direction following the
sequence, cation-anion-cation. The hydrophobic groups of the cations are directed to the regions y = 1/4 and y = 3/4. These cations, together with the water molecules, connect the polyanions from the same y level (Fig. 4b). These hydrophobic regions are the exfoliation areas of the crystal. The strong hydrogen bond interactions between the water molecules and the cations explain the fact that the first two steps of the thermal decompo-
4562
A, S. J. WERY et al. Table 5. Bond number calculations for all the oxygen atoms in the anion [H2V10028]4Atom O(1) 0(2) 0(3) 0(4) 0(5) 0(6) 0(7)
Zsi
Zs~*
1.79(1) 1.70(1) 1.81(1) 1.72(1) 1.88(1) 1.190(6) 1.75(1)
1.85(1) 1.91(1) 1.81(1) 1.84(2) 1.88(1) 1.190(6) 1.98(1)
Atom
Z~i
0(8) 0(9) O(10) O(11) O(12) O(13) O(14)
Zsi*
1.769(7) 2.021(8) 1 . 9 2 ( 1 ) 1.92(1) 1.80(1) 1.93(1) 1.743(8) 1.957(9) 1.967(7) 1.967(7) 1.941(9) 1.941(9) 2.053(8) 2.053(8)
* Contribution of adjacent hydrogen atoms from hydrogen bondings are included.
o
O
O O
O
O
O ©
q
%
o
@ ® ®
O ® ®
o
(a)
(b)
Fig. 4. Compound 1. (a) Projection of the crystal structure along the c-axis showing the layer packing. (b) Arrangement of polyanions belonging to the same layer.
sition, corresponding to loss of the water molecules and organic cation, are overlapped. Table 6 shows the possible hydrogen contacts in the structure of compound 1. The crystal structure of the compound 2 consists of corrugated layers of polyanions located on the (TO1) plane and the hydrophobic groups of the cations are located following the same corrugated disposition in parallel layers with a sequence, cationanion-cation along the direction [T01] (Fig. 5a). Each dihydrogendecavanadate anion is joined to the four neighbouring polyanions by means of hydrogen contacts 0(6)--.0(4). 20 On the other
hand, each cation is joined to three polyanions by means of hydrogen contacts N - - H . • "O, reinforcing the joint of the anions of the same layer (Fig. 5b). The hydrogen contacts are listed in the Table 7. CONCLUSIONS The tert-butylammonium cation, which has three hydrophobic methyl groups on the opposite side of an NH~- hydrophillic group, tends to pack with its methyl groups forming hydrophobic regions. 21 Both compounds described in this work show a crystal packing in layers of anions and cations, the
Effect of protonation on t-butylammonium decavanadates
4563
Table 6. Possible hydrogen contacts (A) in the crystal structure of compound 1 Bond type
N(I I)...O(I 5)w N(I 1)...0(4)' N(I I)...O(12) ~ N(I I)...O(16)w 2 N(21)...O(17)w N(21)..-0(5) 3 N(21)...O(11) 4 N(21)...O(I 3) 3 N(21)...O(I 8)w5 C(23)...0(5) 3
X..-O (A) Bond type
2.90(2) 3.36(2) 2.80(2) 2.78(3) 3.08(3) 3.17(2) 2.97(2) 2.82(2) 2.87(4) 3.23(3)
C(24)...O(16)w 2 N(31)...O(I) 6 N(31)-..O(17)w 7 N(31)-..O(I 8)w5 C(34)...0(9) 6
C(35)...O(17), 7 O(15),...O(1) 8 O(15)w...O(4)' O(I 5)w.- .0(7) 8 O(16)w.- .0(4) 4
X...O (A) Bond type
3.49(4) 3.03(3) 3.09(4) 3.26(4) 3.46(3) 3.44(4) 3.34(2) 2.80(2) 2.85(3) 3.31(2)
O(16),. O(16),' O(17)w O(I 7)w' O(17)w. O(17)w. O(18)w. O(18),,; O(I 8)w. O(I 8)w.
•0(8) 6 •O(11) 4 •O(15)w •0(6) 3 •0(9) 3 -O(13) 3 •O(16)w ~° •O(1) 9 -0(2) ~° •0(4)"
X...O (A) 2.76(2) 2.80(2) 2.86(2) 2.76(2) 3.06(2) 3.40(2) 3.27(4) 2.99(4) 3.05(4) 3.35(3)
Symmetry codes : 1. - x , - y , --z; 2. x, y, z - 1 ; 3. - - x + 1, - - y , - z + 1 ; 4. x + 1, y, z + 1 ; 5. - - x + 1/2, y - 1/2, - z + 1 ; 6. - x , - y , - z + 1 ; 7. x - - l , y , z ; 8. x + l , y , z ; 9 , x + l / 2 , --y+ 1/2, z; 10. - x + 1/2, y + 1/2, - - z + 1 ; 11. - x - 1/2, y + 1/2, - z .
(a)
(b)
Fig. 5. Compound 2. (a) Projection of the crystal structure showing the layer packing. (b) Arrangement of polyanions belonging to the same layer showing the anion-anion hydrogen contacts (dotted lines).
distance between layers of anions being 13.2 and 10.8 /~ for c o m p o u n d s 1 and 2, respectively. In c o m p o u n d 1, the d e c a v a n a d a t e anions are arranged in layers perfectly perpendicular to the [010] direction and consequently single crystals o f this comp o u n d are quite easily exfoliable. However, d i h y d r o g e n d e c a v a n d a t e polyanions in c o m p o u n d 2 are located forming corrugated layers parallel to
the (101) plane. In c o m p o u n d 1, the polyanions belonging to the same layer are held together by means o f the water molecules and the cations, while in c o m p o u n d 2, the polyanions f r o m the same layer interact by means of hydrogen contacts a n i o n anion [O(6)--H...0(4)] and cation-anion ( N - - H . . . O ) . So, it is possible to conclude that water molecules in c o m p o u n d 1 play a similar role
A. S. J. WERY et al.
4564
Table 7. Hydrogen contacts (A, °) in the crystal structure of compound 2 Bond type
X--H
O ( 6 ) - - H ( I ) . . "O(4) l
0.785(1) 2.769(2) 0.90(3) 2.750(2) 0.95(4) 2.843(3) 0.87(4) 2.794(2) 0.88(4) 2.827(3) 0.86(7) 3.282(4) 0.84(5) 3.037(3) 1.04(6) 3.363(6)
N(11)--H(111). .O(8) z N ( l l ) - H ( I I 2 ) . •0(2) 3 N(11)-H(113). •0(7) 4 N(21)--H(211). •O(11) 2 N(21)--H(212). •O(1) 4 N(21)--H(213). •O(10) 3 C(15)--H(152)...0(6) 3
X...O
Symmetry codes: 1. x - l / 2 , - y - 1 / 2 , - z + 1/2; 3. - x , - y , - z ; 4. x + 1/2, - y -
to h y d r o g e n atoms from the polyanion in comp o u n d 2. Acknowledgements--This work was supported by UPV/EHU (Grant 169-310EA 134/95). A.S.J.W. acknowledges financial support from Departamento de Educaci6n (Gobierno Vasco) (Grant No. BFI94.180 Modalidad EK).
REFERENCES 1. M. T. Pope and A. Miiller, Anyew. Chem., Int. Ed. En9l. 1991, 30, 34. 2. M. T. Pope and A. M~iller (Eds), Polyoxometalates : from Platonic Solids to Antiretroviral Activity, Kluwer, Dordrecht, The Netherlands (1994). 3. M. T. Pope, in Encyclopedia o f Inorganic Chemistry (edited by R. B. King), Vol. 6, pp. 3361-3371, John Wiley, New York (1994). 4. L. Pettersson, B. Hedman, I. Andersson and N. lngri, Chem. Scr. 1983, 22, 254 ; L. Pettersson, I. Andersson and B. Hedman, Chem. Scr. 1985, 25, 309 ; E. Heath and O. W. Howarth, J. Chem. Soc., Dalton Trans. 1981, 1105 ; N. Etxebarria, Ph.D. thesis, Universidad del Pais Vasco, Bilbao (1993). 5. J. Livage, Chem. Mater. 1991, 3, 578; M. T. Pope, Heteropoly and lsopoly Oxometalates, SpringerVerlag, New York (1983); M. T. Pope, in Comprehensive Coordination Chemistry (edited by G. Wilkinson, R. D. Gillard and J. A. McCleverty), Pergamon Press, New York (1987). 6. S. E. O'Donnell and M. T. Pope, J. Chem. Soc., Dalton Trans. 1976, 2290. 7. H. T. Evans, lnor9. Chem. 1966, 5, 967; P. Roman, A. Aranzabe, A. Luque and J. M. Guti6rrez-Zorrilla, Mat. Res. Bull. 1991, 26, 731; J. M. Arrieta, Polyhedron 1992, 11, 3045; M. Hahimoto, M. Misono, M. Hisino, J. Oi and T. Kudo, Polyhedron 1993, 12, 2745. 8. W. G. Klemperer and W. Shum, J. Am. Chem. Soc.
9.
10. 11. 12.
13.
14.
15. 16.
17. 18.
H...O
2.012(2) 1.85(3) 1.92(4) 1.93(4) 1.96(4) 2.45(7) 2.24(5) 2.42(7)
z - 1 / 2 ; 2. - x + l / 2 , 1/2, z + 1/2.
162.2(1) 176(3) 162(3) 178(4) 171(4) 163(6) 160(5) 149(4) y+1/2,
1977, 99, 3544; H. T. Evans and M. T. Pope, lnor9. Chem. 1984, 23, 501 ; V. W. Day, W. G. Klemperer and D. J. Maltbie, J. Am. Chem. Soc. 1987, 109, 2991 ; M. I. Khan, J. Zubieta and P. Toscano, Inorg. Chim. Acta 1992, 193, 17; J.-Y. Kempf, M.-M. Rohmer, J. M. Poblet, C. Bo and M. B6nard, J. Am. Chem. Soc. 1992, 114, 1136. P. Romfin, A. Aranzabe, A. Luque, J. M. Guti6rrezZorrilla and M. Martinez-Ripoll, J. Chem. Soc., Dalton Trans. 1995, 2225. W. Wang, F.-L. Zeng, X. Wang and M.-Y. Tan, Polyhedron 1996, 15, 265. N. Walker and D. Stuart, Acta Cryst. 1983, A39, 158. P. T. Beurkens, G. Admiraal, G. Beurkens, W. P. Bosman, S. Garcfa-Granda, R. O. Gould, J. M. M. Smits and C. Smykalla. The DIRDIF Program System. Technical Report of the Crystallography Laboratory, University of Nijmegen, The Netherlands (1992). J. M. Stewart, P. A. Machin, C. W. Dickinson, H. L. Ammon, H. Heck and H. Flack, The XRA Y 76 System, Technical report TR-446, Computer Science Center, University of Maryland, College Park, Maryland, USA (1976). M. Martinez-Ripoll and F. H. Cano, PESOS Program, Instituto Rocasolano, CSIC, Madrid, Spain (1975). P. Roman, R. Macias, A. Luque and C. Guzm~inMiralles, Thermochim. Acta 1992, 209, 189. J. Trau, J. Therm. Anal. 1974, 6, 355 ; F. Stander and C. P. J. Van Vuuren, Thermochim. Acta 1990, 157, 347. I. D. Brown and K. K. Wu, Acta Cryst., 1976, B32, 1957. G. Donnay and R. Allmann, Am. Miner. 1970, 55,
1003. 19. I. D. Brown, Chem. Soc. Rev. 1987, 7, 359. 20. P. Gilli, V. Bertolasi, V. Ferreti and G. Gilli, J. Am. Chem. Soc. 1994, 116, 909. 21. A. S. J. W~ry, Ph.D. thesis, Universidad del Pals Vasco, Bilbao, Spain (1995).
~
Pergamon S0277-5387 (96)00186-6
Copyright 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0277 5387,'96 $15.00+0.00
I N F L U E N C E OF P R O T O N A T I O N O N CRYSTAL P A C K I N G A N D T H E R M A L B E H A V I O U R OF TERTBUTYLAMMONIUM DECAVANADATES ANA S. J. WERY, JUAN M. GUTIERREZ-ZORRILA,* ANTONIO LUQUE
and PASCUAL ROMAN* Departamento de Quimica Inorgfinica, Universidad del Pa/s Vasco, Apartado 644, 48080 Bilbao, Spain
and MARTIN MARTINEZ-RIPOLL Departamento de Cristalograffa, Instituto de Qufmica-Ffsica Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain
(Received 24 January 1996 ; accepted 4 April 1996) Abstract Two tert-butylammonium decavanadate(V) salts with the formulae [(CH3)3CNH316[VI0028] • 8H20 (1) and [(CH3)3CNH314[H2VI0028] (2), have been synthesized and their crystal structures have been determined by means of single-crystal X-ray diffraction. The crystal structure of compound 1 is stabilized by electrostatic forces and an extensive network of hydrogen contacts involving anions, cations and water molecules. The anions and cations of this compound are arranged in layers perpendicular to the [010] direction following the sequence, cation anion-cation. In the crystal structure of compound (2), each dihydrogen decavanadate(V) anion is joined to three adjacent polyanions by means of O(6)--H...O(4) hydrogen contacts forming layers parallel to the plane (101) and the hydrophobic groups of the cations are disposed in layers parallel to the anionic sheets. The thermal behaviour of both compounds has been studied. Compound 1 is an octahydrate and its thermal decomposition begins at 70°C with the loss of water of crystallization, while compound 2 is anhydrous and is consequently more stable, with decomposition starting at 200°C. Copyright © 1996 Elsevier Science Ltd
The solution and solid-state chemistries of some early transition metals (V, Mo, W, Nb and Ta) are both characterized by a tremendous variety of polyoxoanion species, ranging from monomers to high nuclearity clusters which are formed by the assembly of MOx units. ~ Owing to the structural richness, the varied physical properties and the successful uses of these inorganic metal-oxygen clusters, polyoxometalate chemistry is an active and renewed area of research, which not only is of interest in organic chemistry but also has numerous
* Authors to whom correspondence should be addressed.
applications in field such as catalysis, biology, medicine and materials science. 2 The understanding of the driving force for the formulation of the oxometalate cages is still a challenge, but it is well known that in solution there are quick and complex equilibria between polyoxo species with different nuclearity which are strongly dependent on the media conditions such as pH value, ionic strength, temperature and nature of the counter ion. 3'4 One of the best-studied groups in polyoxometalate chemistry is the oxovanadium(V) compounds.1 3.5 Acidification of metavanadate solutions to PH ~<6 leads to orange solutions containing the decavanadate anion [V1002g]6 . The
4555
4556
A. S. J. WERY et al.
equilibria between metavanadate~decavanadate are established very slowly and there are almost certainly other intermediate species, one of which, a cyclic hexamer has been detected by 5~V NMR. 6 The structure of the decavanadate anion has been well established by several X-ray studies 7 and consists of an arrangement of ten edge-shared VO6 octahedra with approximate D2h (mmm) symmetry. The decavanadate anion can be found in different protonation states. The sites of protonation have been determined in several studies. 8 The doublelinked and triple-linked oxygen atoms are those which are most susceptible of protonation owing to the basicity of these atoms compared with the terminal oxygen atoms. In the diprotonated anions, the hydrogen atom are located, generally, in a double-linked oxygen atom that joins vanadium atoms from the equatorial plane with vanadium atoms from the upper plane or the lower one. 8 Recently, Romfin et al. 9 have isolated and characterized a dihydrogen decavanadate salt of formula (C6H~6N)4[H2V~oO28], in which the hydrogen atoms are attached for the first time to two triplelinked oxygen atoms which reinforce theoretical calculations of relative basicities of the oxygen site atoms in decavanadate anions. On the other hand, the first tetrahydrogen decavanadate anions has been isolated from an alcoholic solution by Wang et al. ~° In this case, the hydrogen atoms are joined to two double-linked oxygen atoms and to two triple-linked oxygen atoms. As a part of a study on polyoxometalates we have isolated and chemically and structurally characterized two compounds of formula [(CH3)3 CNH316[V,oO28]'8H20 (1) and [(CH3)3CNH3]4[H 2 V10028] (2). This work has allowed us to study the influence of protonation on the crystal packing and thermal behaviour of polyoxovanadate salts. EXPERIMENTAL
General and apparatus All chemicals were procured commercially and used without further purification. Microanalyses of carbon, nitrogen and hydrogen were performed on a Perkin-Elmer 240 C-, H-, N- analyser and vanadium was determined thermogravitationally as V205 after thermal decomposition of the samples in an argon-oxygen atmosphere. The densities were measured by flotation in CHBr3/CC14. IR spectra were recorded in the range 4000-400 cm -1 on a Nicolet 740 FT-IR spectrometer, and the solid compounds were mixed with potassium bromide (Fluka, FT-IR spectroscopy grade) into trans-
parent disks. A Setaram T A G 24S 16 thermobalance was used to obtain the differential thermal analysis (DTA) and thermogravimetric analysis (TGA) curves, simultaneously, in an argon and oxygen atmosphere (4 : 1) with a heating rate of 5°C min-~.
Synthesis of compounds Hexakis( tert-butylammonium) decavanadate( VI) octahydrate,[(CH3)3CNH3]6[V~o028]" 8H20 (1). To a stirred aqueous suspension (30 cm 3) of V205 (1.00 g, 5.5 mmol) was added t-butylamine (0.75 cm3; 7.0 mmol) and the mixture heated under reflux for 2 h. The resulting orange solution (pH = 5.5) was filtered off and allowed to stand at room temperature. Owing to the high solubility of this compound in water, single crystals were obtained by diffusion of acetone into the aqueous solution. This compound is soluble in water and D M F and insoluble in acetone and ethanol. Crystals are stable to the air and light but they are decomposed slowly by Xray exposure. Found: C, 18.7; H, 5.8; N, 5.5; V (V205), 57.1. Calc. for C24H88N6036VI0 : C, 18.7; H, 5.7 ; N, 5.5 ; V (V205), 58.8%. Tetrakis(tert-butylammonium)dihydrogendecavanadate(VI), [(CH3) 3CNH314[H2VjoO28] (2). This compound can be prepared by the following two methods. (a) Heating with reflux stoichiometric amounts of vanadium pentoxide (2.13 g, 11.7 mmol) and tbutylamine (1.0 cm 3, 9.4 mmol) in 100 cm 3 of water. The resulting solution (pH = 3.5) was allowed to stand at room temperature. Crystals of compound 2 were filtered off from the solution two weeks later. (b) Single crystals suitable for X-ray diffraction were obtained by adding a solution of Na2HPO4 (0.40 g, 2.84 retool) in 10 cm 3 H20 to an orange solution of compound 1. Hydrochloric acid (2.5 M) was added up to pH 3. Two weeks later, prismatic orange crystals were filtered off from the solution. Crystals were found to be stable to air, light and Xray exposure. Found: C, 15.5; H, 4.0; N, 4.3; V (V205), 72.4. Calc. for CI6HsoNaO28VI0 : C, 15.3; H, 4.0 ; N, 4.5 ; V (V205), 72.0%.
Crystallographic data collection refinement
and structure
Single crystals of compounds 1 and 2 were obtained as described above. Table 1 summarizes the crystal data and collection procedures for both compounds. Single crystals of compound 1 were not stable to X-ray exposure and a significant decay of the intensity of two reference reflections was
Effect of protonation on t-butylammonium decavanadates
4557
Table 1. Crystallographic data for the compounds 1 and 2 Compound
1
2
Formula Mr System Space group a (A) b (A) c (fit) /3 C) V (fit3) Z F(000) Do (Mg m -~) Dx /~ [Mo-K,] (cm ') Form Dimensions (ram) Colour Diffractometer Temperature (K) Radiation (fit) Monochromator Scan-mode 0 Range
(C4HI2N)6[V,oO28]"8 H 2 0
(C4H,2N)4[H2V10028]
1546.4 Monoclinic P2,/a (No. 14) 10.642(9) 26.35(1) 11.711(7) 115.15(5) 2973(4) 2 1580 1.72(1) 1.727 15.386 Prism 0.10 x 0.15 × 0.22 Orange Enraf-Nonius CAD4 295(1) 0.71069 Oriented graphite o9/20 1-30 0-~ 1 4 ; 0 ~ 3 7 ; -16--+ 16 Intensity and orientation ~ 71~ ; ~ ~ ~ 7200 s; 100 reflect. 8572 3740 [1>12or(/)] 283 2.52 (0.85 fit del O(13)) DIFABS Tram= 0.803, Tm~x= 1.117 0.148 0.178
1256.0 Monoclinic P21/n (No. 14) 16.244(1) 11.336(1) 11.871(1) 108.625(3) 2071.6(3) 2 1252 2.012(5) 2.010 21.701 Prism 0.10 x 0.12 x 0.20 Red-orange Philips PW 1100 295(1) 0.71069 Oriented graphite o9/20 240 0--+28;0--+20; - 2 1 --+21 Intensity and orientation 3 36;36 3 5400 s 12848 9201 [I~>3cr(/)] 325 0.81 DIFABS Tram= 0.814, Tmax= 1.074 0.047 0.047
hkl Controls - - Reflections -Periodicity
Indep. reflections Observ. reflect. No of variables (Ap)(e fit 3) Absorption correction R Rw
observed, so, a linear decay correction ranging from 1.00 to 1.30 was applied. In both cases the intensity data were also corrected for Lorentz and polarization effects, and an empirical absorption correction ~ based on the isotropically refined structures was applied. Both structures were solved by a combination of direct methods 12and difference Fourier syntheses and refined by full-matrix leastsquares methods on F. 13 The crystal structure of compound 1 was refined with anisotropic thermal parameters for all the nonhydrogen atoms except the carbon atoms from the tert-butylammonium cations, which were included as isotropic contributors. The refinement resulted in a rather high reliability factor (R = 14.8%) owing to the X-ray instability, the modest quality
and size of the single crystals which led to low intensity reflections, as can be observed in the supplementary Fo/Fc list. The crystal structure of compound 2 was refined with anisotropic thermal parameters for all the nonhydrogen atoms. A convenient weighting scheme 14 of type oo=~oj~oz with o91=kl/(a+b[Fo[+ c lFo [2)2 and ~o2 = k2/[d+e(sinO/2)] was used so as to give no trends in (eo A2F) vs (Fo> and vs (sin0/2>. Hydrogen a t o m positions from cations were discernible from electron density difference maps except for those belonging to the methyl groups C(23), C(24) and C(25) which were generated at geometrically fixed positions. Besides, the hydrogen atom belonging to the anion was also found in an electron density difference map.
A. S. J. WERY et al.
4558 R E S U L T S AND D I S C U S S I O N
Thermal analyses Table 2 lists steps, initial and final temperature ( C ) , partial and total weight loss, enthalpy (endothermic or exothermic) and the maximum peak for each step in the thermal decomposition of both compounds in an argon-oxygen atmosphere. Thermal decomposition of compound 1 starts at 70°C with two almost overlapping steps that correspond to the loss of the crystallization water molecules (70 110'C) and part of the organic cations (110-130°C). These steps are followed by a progressive weight loss with no clear peaks in the D T A curve. After this progressive weight loss, an endothermic step (195-240°C) and another exothermic one (24(~260 C) take place yielding a weight loss higher than that expected for V205. This residue could be a mixture of mixed-valence vanadium oxides V3OT/V6013 based on calculations. This fact has also been observed in the thermal decomposition of diethylentriammonium decavanadate tetrahydrate. 15 Some authors have suggested that the formation of these mixed-valence vanadium oxides is a result of the reductive action
Table 2. Steps, initial and final temperature ('C), partial and total weight loss, enthalpy (endothermic or exothermic) and maximum peak for each step in the thermal decomposition of compounds 1 and 2 under an argon oxygen atmosphere Step
TI-T~
Compound 1 1 2 3" 4 5 6
70 1l0 110-130 13(~195 195240 240-260 32~360
Compound 2 1 2 3 4"
200-245 245-265 315-355 355-550
Tm
AH
%Am
95 115
Endo Endo
225 255 335
Endo Exo Exo Z%Am Calc.
9.55 6.62 1.80 13.86 10.40 - 1.21 41.02 41.15
235 250 330
Endo Exo Exo
11.64 15.11 - 0.30 1.02
Z%Am Calc.
27.47 27.56
a Progressive weight loss without clear peaks in the DTG or DTA curves.
of NH3 during the thermal decomposition of the base.16 These oxides are oxidized to give vanadium pentoxide in the last exothermic step once the organic base has been totally decomposed (Fig. 1a). C o m p o u n d 2 is anhydrous and stable up to 200"C. Its thermal decomposition starts with two successive steps, the first one endothermic and the second one exothermic, yielding an intermediate stable compound up to 315"C. Then, another exothermic step, similar to the one observed for compound 1 takes place with a weight increase of 0.30%, corresponding to the reoxidation of vanadium(IV). The process finishes with a progressive weight loss without clear peaks in the D T G and D T A curves to give V2Os (Fig. l b).
Infrared spectroscopy Figure 2 shows the I R spectra for both anions. In the spectrum of compound 2, there are two bands characteristic of protonated decavanadate anions and placed at 985 and 615 cm-1. The first one can be assigned to V - - O r bonds in V O s O - - H octahedra, which are shortened owing to the elongation of the V--Ob--H bonds. The second band is assigned to the V--Ob--H bonds. The bands between 990 and 900 c m - ' are attributable to the V - - O r stretching vibration while the ones corresponding to the V - - O b stretching vibration are observed in the range 840-440 cm ~. The signal at 420-400 c m - 1 is attributed to the deformation vibration of the groups V--Oh.
Description of the crystal structures The X-ray structural analysis reveals that the asymmetric unit of compound 1 is composed of one half of a centrosymmetric decavanadate anion, [ V i 0 0 2 8 ] 6 , three tert-butylammonium cations and four water molecules, while the asymmetric unit of compound 2 contains one half of a centrosymmetric dihydrogendecavanadate anion [H2V10028]4- and two tert-butylammonium cations. Figure 3 shows both decavanadate anions together with the numbering scheme. The decavanadate anion can be described as a section of cubic close-packed oxygen atoms with vanadium atoms in the octahedral holes. The vanadium atoms are not in the geometric centre of the octahedra but they are closer to the vertex occupied by the terminal oxygen atoms. The V - - O bond distances for compounds 1 and 2 are listed in Tables 3 and 4. Taking into account the ideal symmetry of the decavanadate anion D2h (mmm), there are three types of VO6 octahedra according to the position in the anion, each of these types
Effect of protonation on t-butylammonium decavanadates 10
*S
70
(1)
o 0
4559
18 (2)
TG
o
-10
~o ~
-10
$ <
-20
2
-30
so
~
~o ~
-20 -30 -40
DTA
,,
-50
100
200
300
10 ~__ ATD,,
400 500 600 TEMPERATURE (ac)
-40
-2
r,l,i,
i,
1O0
i,i,J,i,
200
300
I,~,l.llj
400 500 600 TEMPERATURE (oC)
-10
Fig. 1. TG-DTA curves for the thermal decomposition of compounds 1 and 2 in argon-oxygen atmosphere.
I
I 1)V-O I
1200
1000
i
I ~)asV-Ob
I
II ~)siV-Ob
I
I
800
600
I ~V*O b 0(2) 400
'0 (cm "1)
V(2)
0(13)
Fig. 2. Infrared spectra for both anions.
shows a particular distribution of the V - - O distance. (a) Octahedra type I : those located above and below the equatorial plane that present a short distance (1.60/k) ; two medium ones (ca 1.80-1.85 A) ; two long ones (ca 2.00-2.05/k) and a very long one (ca 2.30-2.35 A). (b) Octahedra type II: those located on the equatorial plane with one terminal oxygen atom that have a short distance (ca 1.60 A); three medium ones (ca 1.80-1.90/k); a long one (ca 2.00-2.05 A) and a very long one (ca 2.25 A). (c) Octahedra type III: central octahedra on the equatorial plane that possess two short distances (ca 1.65-1.70 ,&) ; two medium ones (ca 1.901.95 A) and two long ones (ca 2.10-2.15/k). The hydrogen atom for the centrosymmetric dihydrogendecavanadate anion of compound 2 was located from an electron density difference map, and it is bonded to the bridged oxygen atom O(6),
N oc1,)
~-- ~ v(4)'~
0(4) ,
Fig. 3. ORTEP views for [ V i 0 0 2 8 ] 6 - anion (top) and [HzVioOz8] 4 anion (bottom) with atom labelling.
which joins the V(1) atom from the equatorial plane with the V(2) atom from the upper level. Besides, the protonation site was also confirmed by means of the empirical bond length/bond number calculation, s=(R/1.79) -5J, ( R = V - - O distance; s = bond number). 17Table 5 lists the bond number
4560
A.S.J. WERY et al.
Table 3. Bond lengths V--O (A), O." .O distances (A) (inferior part of the diagonal) and O--V--O angles (°) (superior part of the diagonal) for compound 1
O(1) 0(6) 0(7) 0(8) 0(9) O(14)
0(2) 0(6) 0(10) 0(12') 0(13) 0(14)
0(3) 0(7) O(10) 0(5') O(11) O(14)
0(4) 0(8) O(11) O(13') O(12) O(14)
0(5) 0(9) 0(13) 0(14') 0(12) 0(14)
V(1)
0(1)
1.61(2) 1.89(1) 1.84(1) 1.88(1) 2.04(1) 2.30(1)
2.74(3) 2.69(2) 2.71(2) 2.82(2)
V(2)
0(2)
1.62(1) 1.84(2) 1.80(1) 2.02(1) 1.98(1) 2.25(1)
2.72(2) 2.68(2) 2.77(2) 2.76(2)
V(3)
0(3)
1.58(2) 1.84(2) 1.88(1) 2.02(2) 1.88(1) 2.38(1)
2.71(2) 2.70(2) 2.79(2) 2.71(2)
V(4)
0(4)
1.59(1) 1.79(2) 1.83(1) 2.02(1) 2.00(1) 2.20(1)
2.65(2) 2.67(2) 2.75(2) 2.75(2)
V(5)
0(5)
1.68(1) 1.71(2) 1.91(1) 2.16(1) 1.90(1) 2.07(1)
2.75(2) 2.68(1) 2.66(2) 2.71(2)
0(6)
0(7)
0(8)
0(9)
102.9(7)
102.3(8) 90.7(6)
101.5(8) 154.2(6) 92.1 (6)
100.9(7) 83.9(6) 156.8(6) 83.5(5)
2.65(2) 2.68(2) 2.63(2) 2.64(1)*
2.77(2)*
2.61(2) 2.64(2)*
0(10)
0(12')
0(13)
103.9(6)
103.1(7) 94.5(7)
98.7(7) 155.0(5) 90.8(6)
99.8(7) 89.7(6) 155.0(5) 75.8(5)
2.73(2) 2.69(2) 2.64(1)*
2.68(2)*
2.46(2) 2.66(2)
O(10)
O(5')
O(11)
104.5(7)
102.6(7) 90.0(6)
101.1(7) 154.5(6) 83.9(6)
103.1(7) 91.8(6) 152.9(6) 82.9(6)
2.61(2) 2.67(2) 2.77(2)*
2.68(2)*
2.59(2) 2.66(2)
O(11)
O(13')
O(12)
103.1(7)
102.5(7) 95.9(6)
98.7(7) 155.7(5) 89.8(6)
99.6(6) 90.4(6) 155.0(6) 75.3(5)
2.72(2) 2.69(2) 2.64(2)* 0(9) 108.4(6)
2.68(2)* 0(13) 98.2(6) 99.5(5)
2.76(2)
2.46(2) 2.58(1)
2.58(2)*
0(12)
86.8(6) 164.8(5) 78.7(5)
98.2(6) 95.3(6) 154.6(5) 81.6(5)
2.66(2) 2.64(2)
O(14) 174.7(7) 80.8(6) 76.8(5) 73.7(4) 76.8(5) O(14) 172.2(6) 82.0(6) 82.7(5) 75.3(4) 74.3(5)
2.54(2)*
0(14')
2.58(2) 2.67(2) 2.62(2)*
173.5(6) 79.8(5) 81.8(5) 76.8(5) 74.7(4)
2.68(2)*
0(8)
2.69(2)
0(14)
2.58(2)*
0(7)
2.63(2)
174.7(7) 77.5(5) 83.0(5) 77.3(5) 73.8(5)
2.62(2)*
0(6)
2.67(2)
0(14)
0(14) 164.4(6) 87.3(5) 80.8(5) 77.6(5) 79.5(5)
2.54(2)*
* Shared edge.
for all the oxygen atoms considering the contribution of the adjacent hydrogen atoms from O . . . H - - N ~8and O - - . H - - O ~9interactions. A comparison of the V - - O bond lengths in both anions shows that the attachment of the hydrogen atom on the 0(6) atom produces an elongation of the V--O(6) bond and consequently a significant
shortening of the V - - O bonds involving the oxygen atoms in trans positions with respect to the 0(6) atom in the V(1) and V(2) octahedra. Both anions have a Ci point symmetry but they are close to the D2h ideal point symmetry. However, the disposition of the planes in the dihydrogendecavanadate anion shows a stronger deviation
Effect of protonation on t-butylammonium decavanadates
4561
Table 4. Bond lengths V--O (A), O...O distances (A) (inferior part of the diagonal) and O--V--O angles () (superior part of the diagonal) for the compound 2 V(1) 0(1) 0(6) 0(7) 0(8) 0(9) 0(14)
1.598(2) 1.992(2) 1.853(2) 1.790(1) 2.029(2) 2.307(1)
V(2) 0(2) 0(6) O(10) O(12') O(13) O(14)
1.614(2) 1.974(2) 1.753(2) 1.894(1) 2.047(2) 2.209(1) V(3)
0(3) 0(7) O(10) 0(5') O(11) O(14)
1.594(2) 1.824(2) 1.932(2) 2.018(2) 1.876(1) 2.341(2)
V(4) 0(4) 0(8) 0(11) 0(13') 0(12) 0(14)
1.611(2) 1.887(2) 1.808(2) 1.917(2) 2.013(2) 2.260(1) V(5)
0(5) 0(9) O(13) O(14') O(12) O(14)
1.692(2) 1.680(2) 1.906(1) 2.162(1) 1.941(1) 2.107(2)
0(1) 2.771(2) 2.703(3) 2.685(2) 2.813(2)
0(2)
0(6)
0(7)
0(8)
0(9)
100.4(1)
102.9(I) 87.5(1)
104.7(1) 153.9(1) 93.7(1)
101.0(1) 80.9(1) 155.0(1) 87.4(1)
2.662(3) 2.657(2) 2.608(2) 2.632(2)*
0(6) 98.6(1)
2.731(2) 2.635(2) 2.738(2) 2.778(2) 0(3) 2.696(3) 2.736(3) 2.816(2) 2.721 (2)
0(4) 2.700(2) 2.670(2) 2.747(2) 2.794(2) 0(5)
2.645(2) 2.647(2)* 2.648(2)*
0(10)
0(12')
0(13)
103.0(1) 92.8(1)
102.3(1) 154.1(1) 92.2(1)
98.0(1) 84.1(1) 159.0(1) 78.0(1)
2.702(3) 2.737(2) 2.694(2) 2.632(2)*
2.670(2)*
2.484(2) 2.637(2)
0(10)
0(5')
0(11)
103.9(1)
101.3(1) 90.0(1)
101.8(1) 154.2(1) 83.0(1)
103.0(1) 91.6(1) 154.5(1) 84.5(1)
2.658(2) 2.619(2) 2.652(2) 2.714(2)*
2.670(2)*
2.620(2) 2.636(2)
0(11)
O(lY)
0(12)
100.8(1)
102.5(1) 92.7(1)
101.9(1) 154.5(1) 93.7(1)
102.3(1) 86.2(1) 156.9(1) 78.3(1)
2.720(2) 2.665(2) 2.647(2)* 0(9)
2.692(2)* 0(13) 97.7(1) 98.9(1)
2.729(2)
2.484(2) 2.635(2)
2.599(2)*
0(12)
85.4(1) 165.8(1) 80.5(1)
96.0(1 96.4(1) 154.9(1) 78.8(1)
2.637(2) 2.685(2)
172.5(1) 77.8(1) 83.9(1) 79.6(1) 75.2(1) 0(14)
175.5(1) 80.3(1) 76.7(1) 74.0(1) 78.5(1) 0(14) 175.4(1) 78.8(1) 82.0(1) 77.7(1) 75.1(1)
2.613(2)*
0(14')
2.634(2) 2.706(2) 2.648(2)*
0(14)
2.692(2)*
0(8)
2.674(2)
174.3(1) 75.1(1) 80.6(1) 79.4(1) 75.0(1)
2.599(2)*
0(7)
108.7(1) 2.740(2) 2.712(2) 2.636(2) 2.706(2)
2.714(2)*
0(14)
0(14) 163.3(1) 88.011) 80.6(1) 77.9(1) 80.3(1)
2.613(2)*
* Shared edge.
from the D2hideal situation. The distortion may be explained by the structural modifications produced by the protonation of the anion. Figure 4a shows the projection on to the (001) plane of the crystal structure of compound 1. The polyanions and cations, with their nitrogen atoms directed towards the oxygen atoms from the polyanions and water molecules, are located in layers perpendicular to the [010] direction following the
sequence, cation-anion-cation. The hydrophobic groups of the cations are directed to the regions y = 1/4 and y = 3/4. These cations, together with the water molecules, connect the polyanions from the same y level (Fig. 4b). These hydrophobic regions are the exfoliation areas of the crystal. The strong hydrogen bond interactions between the water molecules and the cations explain the fact that the first two steps of the thermal decompo-
4562
A, S. J. WERY et al. Table 5. Bond number calculations for all the oxygen atoms in the anion [H2V10028]4Atom O(1) 0(2) 0(3) 0(4) 0(5) 0(6) 0(7)
Zsi
Zs~*
1.79(1) 1.70(1) 1.81(1) 1.72(1) 1.88(1) 1.190(6) 1.75(1)
1.85(1) 1.91(1) 1.81(1) 1.84(2) 1.88(1) 1.190(6) 1.98(1)
Atom
Z~i
0(8) 0(9) O(10) O(11) O(12) O(13) O(14)
Zsi*
1.769(7) 2.021(8) 1 . 9 2 ( 1 ) 1.92(1) 1.80(1) 1.93(1) 1.743(8) 1.957(9) 1.967(7) 1.967(7) 1.941(9) 1.941(9) 2.053(8) 2.053(8)
* Contribution of adjacent hydrogen atoms from hydrogen bondings are included.
o
O
O O
O
O
O ©
q
%
o
@ ® ®
O ® ®
o
(a)
(b)
Fig. 4. Compound 1. (a) Projection of the crystal structure along the c-axis showing the layer packing. (b) Arrangement of polyanions belonging to the same layer.
sition, corresponding to loss of the water molecules and organic cation, are overlapped. Table 6 shows the possible hydrogen contacts in the structure of compound 1. The crystal structure of the compound 2 consists of corrugated layers of polyanions located on the (TO1) plane and the hydrophobic groups of the cations are located following the same corrugated disposition in parallel layers with a sequence, cationanion-cation along the direction [T01] (Fig. 5a). Each dihydrogendecavanadate anion is joined to the four neighbouring polyanions by means of hydrogen contacts 0(6)--.0(4). 20 On the other
hand, each cation is joined to three polyanions by means of hydrogen contacts N - - H . • "O, reinforcing the joint of the anions of the same layer (Fig. 5b). The hydrogen contacts are listed in the Table 7. CONCLUSIONS The tert-butylammonium cation, which has three hydrophobic methyl groups on the opposite side of an NH~- hydrophillic group, tends to pack with its methyl groups forming hydrophobic regions. 21 Both compounds described in this work show a crystal packing in layers of anions and cations, the
Effect of protonation on t-butylammonium decavanadates
4563
Table 6. Possible hydrogen contacts (A) in the crystal structure of compound 1 Bond type
N(I I)...O(I 5)w N(I 1)...0(4)' N(I I)...O(12) ~ N(I I)...O(16)w 2 N(21)...O(17)w N(21)..-0(5) 3 N(21)...O(11) 4 N(21)...O(I 3) 3 N(21)...O(I 8)w5 C(23)...0(5) 3
X..-O (A) Bond type
2.90(2) 3.36(2) 2.80(2) 2.78(3) 3.08(3) 3.17(2) 2.97(2) 2.82(2) 2.87(4) 3.23(3)
C(24)...O(16)w 2 N(31)...O(I) 6 N(31)-..O(17)w 7 N(31)-..O(I 8)w5 C(34)...0(9) 6
C(35)...O(17), 7 O(15),...O(1) 8 O(15)w...O(4)' O(I 5)w.- .0(7) 8 O(16)w.- .0(4) 4
X...O (A) Bond type
3.49(4) 3.03(3) 3.09(4) 3.26(4) 3.46(3) 3.44(4) 3.34(2) 2.80(2) 2.85(3) 3.31(2)
O(16),. O(16),' O(17)w O(I 7)w' O(17)w. O(17)w. O(18)w. O(18),,; O(I 8)w. O(I 8)w.
•0(8) 6 •O(11) 4 •O(15)w •0(6) 3 •0(9) 3 -O(13) 3 •O(16)w ~° •O(1) 9 -0(2) ~° •0(4)"
X...O (A) 2.76(2) 2.80(2) 2.86(2) 2.76(2) 3.06(2) 3.40(2) 3.27(4) 2.99(4) 3.05(4) 3.35(3)
Symmetry codes : 1. - x , - y , --z; 2. x, y, z - 1 ; 3. - - x + 1, - - y , - z + 1 ; 4. x + 1, y, z + 1 ; 5. - - x + 1/2, y - 1/2, - z + 1 ; 6. - x , - y , - z + 1 ; 7. x - - l , y , z ; 8. x + l , y , z ; 9 , x + l / 2 , --y+ 1/2, z; 10. - x + 1/2, y + 1/2, - - z + 1 ; 11. - x - 1/2, y + 1/2, - z .
(a)
(b)
Fig. 5. Compound 2. (a) Projection of the crystal structure showing the layer packing. (b) Arrangement of polyanions belonging to the same layer showing the anion-anion hydrogen contacts (dotted lines).
distance between layers of anions being 13.2 and 10.8 /~ for c o m p o u n d s 1 and 2, respectively. In c o m p o u n d 1, the d e c a v a n a d a t e anions are arranged in layers perfectly perpendicular to the [010] direction and consequently single crystals o f this comp o u n d are quite easily exfoliable. However, d i h y d r o g e n d e c a v a n d a t e polyanions in c o m p o u n d 2 are located forming corrugated layers parallel to
the (101) plane. In c o m p o u n d 1, the polyanions belonging to the same layer are held together by means o f the water molecules and the cations, while in c o m p o u n d 2, the polyanions f r o m the same layer interact by means of hydrogen contacts a n i o n anion [O(6)--H...0(4)] and cation-anion ( N - - H . . . O ) . So, it is possible to conclude that water molecules in c o m p o u n d 1 play a similar role
A. S. J. WERY et al.
4564
Table 7. Hydrogen contacts (A, °) in the crystal structure of compound 2 Bond type
X--H
O ( 6 ) - - H ( I ) . . "O(4) l
0.785(1) 2.769(2) 0.90(3) 2.750(2) 0.95(4) 2.843(3) 0.87(4) 2.794(2) 0.88(4) 2.827(3) 0.86(7) 3.282(4) 0.84(5) 3.037(3) 1.04(6) 3.363(6)
N(11)--H(111). .O(8) z N ( l l ) - H ( I I 2 ) . •0(2) 3 N(11)-H(113). •0(7) 4 N(21)--H(211). •O(11) 2 N(21)--H(212). •O(1) 4 N(21)--H(213). •O(10) 3 C(15)--H(152)...0(6) 3
X...O
Symmetry codes: 1. x - l / 2 , - y - 1 / 2 , - z + 1/2; 3. - x , - y , - z ; 4. x + 1/2, - y -
to h y d r o g e n atoms from the polyanion in comp o u n d 2. Acknowledgements--This work was supported by UPV/EHU (Grant 169-310EA 134/95). A.S.J.W. acknowledges financial support from Departamento de Educaci6n (Gobierno Vasco) (Grant No. BFI94.180 Modalidad EK).
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10. 11. 12.
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14.
15. 16.
17. 18.
H...O
2.012(2) 1.85(3) 1.92(4) 1.93(4) 1.96(4) 2.45(7) 2.24(5) 2.42(7)
z - 1 / 2 ; 2. - x + l / 2 , 1/2, z + 1/2.
162.2(1) 176(3) 162(3) 178(4) 171(4) 163(6) 160(5) 149(4) y+1/2,
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1003. 19. I. D. Brown, Chem. Soc. Rev. 1987, 7, 359. 20. P. Gilli, V. Bertolasi, V. Ferreti and G. Gilli, J. Am. Chem. Soc. 1994, 116, 909. 21. A. S. J. W~ry, Ph.D. thesis, Universidad del Pals Vasco, Bilbao, Spain (1995).