New vanadyl hydrogenphosphate hydrates. Electronic spectra of the VO2+ ion in the VO(HxPO4)x·yH2O system

Mat. Res. B u l l . , Vol. 24, p p . 1347-1360, 1989. P r i n t e d in the USA. 0025-5408/89 $3.00 + .00 C o p y r i g h t (c) 1989 Pergamon P r e s s plc.

NEW VANADYL HYDROGENPHOSPHATE HYDRATES. ELECTRONIC SPECTRA OF THE VO2+ ION IN THE VO(HxPO4)x.YH20 SYSTEM.

Pedro Amor0s, Rafael Ib~lnez, Eduardo Martfnez-Tamayo, Aurelio Beltr~n-Porter and Daniel Beltr~n-Porter* UlBCM, Departament de Qufmica Inorg~nica, Facultad de Ciencias Quimicas. Universitat de Valencia. C/Doctor Moliner, 50. 46100 Burjassot (Valencia), Spain. Gerard Villeneuve. Laboratoire de Chimie du Solide du C.N.R.S. Universit~ de Bordeaux I. 351 Cours de la Libdration. 33405 Talence Cedex, France.

( R e c e i v e d June 19, 1989; R e f e r e e d )

ABSTRACT

A general synthetic procedure to obtain oxovanadium(IV) derivatives with the general stoichiometry VO(HxPO4)x-YH20 is presented. This method has enabled us to isolate for the first time four new members of this series (corresponding to x=land y=1,2(c(), 2(8) and 3). New structural types are evidenced. A ligand-field analysis employing the angular overlap model of the electronic d-d spectra of these materials is performed. MATERIALS INDEX: Vanadium, phosphates.

Introduction

The oxovanadium (V) phosphate dihydrate can be considered as a chemical precursor of a wide family of oxovanadium (IV or V) derivatives (1)(2).Among these, several oxovanadium (IV) compounds with the general stoichiometry VO(HxPO4)x.YH20 have been studied in detail owing to their catalytic relevance (3). Otherwise, these materials display a large variety of crystal structures suitable to yield low dimensional magnetic interactions (2). Both the availability of different vanadium oxidation states and the determinant role of the reaction conditions (T, P/V ratio, reductor agents etc.) result in a very rich chemistry. The structure of the VOPO4.2H20 can formally be thought of as derived from that of the V20 5 by insertion of tetragonal PO43- groups (1). The layered morphology of the dihydrate 1347

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P. AMOROS, et ~ .

Vol. 24, No. 11

makes it a very versatile host for intercalation chemistry (1,4,5,6). Thus, the VO(HxPO4)x.yH20 series might be considered as resulting from the redox intercalation of "protons" into the VOPO4.2H20 host lattice. Up to now, the hydrates VO(HPO4).0.5H20 (7,8), 13-VO(HPO4).2H20 (9), and VO(HPO4).4H20 (8), and the dihydrogenphosphate VO(H2PO4) 2 (10) have been characterized by x-ray diffraction. Although all of them are based on the link of [VO6] octahedra and [PO4] tetrahedra, the actual structures are very different. In fact, whereas the hemihydrate contains dimeric oxovanadium (IV) entities (7,8), the 8-dihydrate and the tetrahydrate can be viewed as simple (8-dihydrate) or double (tetrahydrate) chains of alternating [VO6] octahedra and [PO4] tetrahedra (8,9), and the dihydrogenphosphate is built up of chains of VO 6 octahedra (stretched along the fourfold axes) and isolated [PO2(OH)2 ] entities (10). In order to synthesize these materials, a great variety of unrelated preparative procedures has been reported in the literature (11). Because of the theoretical and applied interest of this system, we have approached a new unified synthetic procedure to rationalize its preparative chemistry. In this way, we have been able to isolate, besides those above mentioned, three new hydrates (namely, x= 1 and y= 1, 2(a) and 3) involving additional structural types. In all cases, the syntheses have been carried out in acetone.water media and using iodhydric acid both as reducing agent and donor of protons.* At the same time, the application of the a.o.m, ligand-field analysis (13)to the spectroscopic study of these materials allows us to progress in the understanding of the electronic structure of the ion VO 2+. A full structural and magnetic characterization of these compounds is in course.

Preparation of VO(HPO4).O.5H20. An amount of VOPO4-2H20 (4 g; 20.2 mmol), prepared as described by Ladwig (14), was suspended in a dried-acetone: conc. iodhydric acid (125 cc:2.8 cc) mixture, and refluxed with stirring for 45 h at 50°C. After cooling, the resulting microcrystalline blue solid was filtered, washed with CCI 4 and acetone, and dried in air. (Found: V, 29.7; P, 17.8; H20, 5.3. VPO5.sH2 requires V, 29.6; P, 18.0; H20, 5.2%). Preparation of VO(HPO4).YH20 01= 2( a), 2(8), 3 and 4) . All four hydrates were prepared by refluxing (at 50gC) mixtures of V205 and conc. H3PO 4 in acetone:water media. The reflux is maintained with stirring for 50 h, except in the case of 8-dihydrate which requires only 24 h. In this last case, a longer reaction time originates the evolution of the 13-dihydrate towards higher hydrates. The required amounts of the reactives are, respectively: y=2(8), V20 5 4.17 g(22.9 mmol), H3PO 4 24 cc, HI 4 cc, acetone 150 cc; y=2(a), V20 5 2.08 g(11.5 mmol), H3PO 4 12 cc, HI 2.5 cc, acetone 100 cc, water 6 cc; y=3, V20 5 2.08 g (11.5 mmol), H3PO 4 12 cc, HI 2.5 cc, acetone 100 cc, water 12 cc; y=4, V20 5 4.17 g

* Preliminary results on this subject were advanced in ref. 2. Afterwards, while writing this paper, it has been published a work of J. T. Wrobleski (12) enhancing the advantages of using aqueous HI in the synthesis of VO(HPO4).4H20. Notwithstanding, besides its comparative complexity, no lower hydrates result through Wrobleski's procedure. On the other hand, Wrobleski's magnetic results are consistent with the hypothesis advanced in ref.2.

Vol. 24, No. 11

PHOSPHATES

1349

(22.9 mmol), H3PO 4 24 cc, HI 4 cc, acetone 160 cc, water 100 cc. After cooling, the microcrystalline blue solids were filtered, washed with CCI 4 and acetone, and dried in air. (y=2(8), Found: V, 25.5; P, 14.4; H20, 18.2. VPO7H 5 requires V, 25.6;P, 15.6; H20, 18.1%. y=2(a), Found: V, 25.6; P, 14.5; H20, 18.3. VPO7H 5 requires V, 25.6; P, 15.6; H20, 18.1%. y=3, Found: V, 23.4; P, 14.2; H20, 24.9. VPO8H 9 requires V, 23.5; P, 14.3; H20, 24.9%. y=4, Found: V, 21.8; P, 13.0; H20, 30.5. VPOgH 9 requires V, 21.7; P, 13.2;H20, 30.6%). Preparation of VO(HPO4).H20. When o¢-VO(HPO4)-2H20 is subjected to heating at 150gC during 4 h, the blue-grey monohydrate, VO(HPO4).H20, results. It must be handled with caution, and stored over P205, because it rehydrates rapidly in wet air. (Found: V, 28.1; P, 17.1; H20, 10.1. VPO6H 3 requires V, 28.2; P, 17.1; H20, 9.9%). Preparation of VO(H2P04) 2 . An aqueous suspension containing V205 (4.17 g, 22.9 retool), conc. H3PO 4 (24 cc) and conc. HI (3.03 cc) was refluxed (50~C) for 3 h. Carbon tetrachloride was used to extract the 12 formed. Then, the remaining blue solution was concentrated by heating until apparition of precipitate. After cooling, which induces the formation of new amounts of solid, the resulting solution was filtered and the deep blue solid obtained was washed with water and acetone. The solid was then dried by heating at 150-°C for 24 h. (Found: V, 19.5; P, 23.6. VP2OgH 4 requires V, 19.5; P, 23.7%).

Vanadium content and oxidation state were determined as described in ref. 1. P was determined by atomic absorption (Perkin-Elmer Zeeman 5000). Water was determined thermogravimetrically. Thermal analysis were carried out using a Setaram B70 simultaneous TGA-DTA thermobalance. Crucibles containing ca. 60 mg of sample were heated at 150 ~C h-1 under a flowing N2 atmosphere. Calcined AI203 was used as reference. IR spectra (KBr pellets) were recorded on a FTIR Perkin-Elmer 1750 spectrophotometer. Room temperature (25.0_+0.1gC) electronic spectra (diffuse reflectance) were recorded (8000-30000 cm -1) using a Perkin-Elmer Lambda-9 UV/VIS/NIR spectrophotometer. X-ray powder diffraction patterns were obtained by means of a Siemens Kristalloflex 810 diffractometer using CuKa radiation and having the Pt peaks as standard. The apparatus is equiped with a variable temperature device working from room temperature to ca. 1200~C. Results 8nd discussion Synthesis. When compared with those reported in the literature, the unified procedure described here shortens the reaction times to a great extent, leading to single phases. The acetone:water ratio (R) in the reaction medium is the main variable determining the end product. Thus, the following "stability ranges" can be established as a function of R : R<8, tetrahydrate; 827, 8-dihydrate. For the evaluation of R, the water contents of conc. H3PO 4 and conc. HI must be taken into account. In any case, in the absence of acetone (i.e., using pure waler as reaction medium), the resulting stable phase is the dihydrogenphophate, VO(H2PO4) 2. X-Ray powder diffraction . The x-ray powder diffraction data of the new VO(HxPO4)x.YH20 isolated phases (x--l; y=l, 2(¢x), 2(B) and 3) are given in Table 1. Data for the remaining solids agree with those previously reported (7,8,9). As indicated in Table

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1, the a-di- and the trihydrate show the same diffraction pattern. This result may be understood by assuming the presence in the trihydrate of one zeolitic-type water molecule occupying structural voids, and we will return to this aspect later on. Indexation and lattice parameters determination (see Table 1) have been performed by least square fitting of the data in the range 8<2 6<60 using the INDEX (15a,b) and LSUCRE programs (15c).

TABLE 1 X-ray powder diffraction data and unit cell parameters of the new monohydrogenphosphates VO(HxPO4)x'yH20. x=l ; y=l dobs. 6.553 5.587 4.865 4.454 3.705 3.433 3.139 2.879 2.799 2.716 2.280 2.217 2.182 2.111 1.909 1.814

I/Io% dcalc" 90 60 23 26 10 6 100 8 7 3 3 2 2 1 4 7

6.520 5.599 4.885 4.460 3.689 3.433 3.139 2.885 2.799 2.723 2.278 2.219 2.179 2.111 1.908 1.810

a=6.546(7)A b=7.37(3)A c=9.44(5)A 8 = 9 5 . 1 1 ( 2 ) o-

x = l ; y= 2(8)

x = l ; y = 2(c¢) or 3

hkl

dobs.

I/Io% dcalc"

hkl

dobs.

I/Io% dcalc"

hkl

100 -101 -110 -111 020 021 003 013 -20-2 -113 -104 131 032 222 302 041

6.500 5.525 4.795 4.550 4.398 4.050 4.014 3.909 3.789 3.620 3.187 3.092 3.071 2.999 2.979 2.903 2.857 2.790 2.757 2.680 2.630 2.567 2.479

19 100 13 58 59 12 9 10 44 7 27 87 30 22 40 10 8 21 16 9 20 17 18

01 - 1 100 01-2 -110 110 -11-1 -112 111 020 021 -120 004 120 103 -122 121 01-4 02-3 -200 -211 -210 210 03-1

7.578 5.856 5.307 4.718 4.189 3.848 3.651 3.461 3.419 3.346 3.274 3.106 2.901 2.829 2.786 2.671 2.592 2.396 2.361 2.322 2.270 2.249 2.195 2.090 2.072

100 35 40 6 23 16 33 25 27 5 11 95 8 17 5 7 13 5 8 19 10 10 5 8 8

1 oo 011 110 002 10-2 102 11-2 021 112 120 01-1 211 013 202 12-2 122 22-1 031 004 30-2 131 014 03-2 320 230

6.474 5.532 4.797 4.552 4.392 4.050 4.012 3.908 3.793 3.623 3.185 3.090 3.071 2.999 2.980 2.903 2.858 2.790 2.757 2.680 2.629 2.567 2.479

a=5.665(3)A b=7.S95(6)A c=12.646(8)A ¢z=89.71 (4) -o 8=102.27(5) 0 y = 9 2 . 1 8 ( 3 ) -°

7.587 5.845 5.312 4.725 4.193 3.850 3.653 3.461 3.410 3.339 3.272 3.100 2.900 2.829 2.783 2.675 2.602 2.398 2.363 2.323 2.271 2.252 2.196 2.091 2.075

a=7.621 (6)A b=7.439(4)A c=9.493(8)A 8 = 9 5 . 4 4 ( 6 ) -o

Concerning the phases already characterized ( x = l ; y= 0.5, 2(I]), 4 ; x= 2, y-- 0), our results fit in well with the literature data. Thus, the pattern of VO(HPO4).O.5H20 can be

Vol. 24, No. 11

PHOSPHATES

1351

indexed to an orthorhombic cell having a=7.443(3)A, b=9.618(5)A and c=5.712(3)A (in ref. 8, a=7.434(2)A, b=9.620(2)A and c=5.699(1)A). The unit cell of the 1 3 - V O ( H P O 4 ) . 2 H 2 0 is triclinic with a=5.665(3)A, b=7.595(6)A, c=12.646(8)A, ~=89.71(4) °, 8=102.27(5) ° and 1,=92.18(3) o (in ref. 9, a=5.659(2)A, b=7.578(4)A, c=12.623(5)A, c¢=89.66(2) o, 13=102.14(2) o, 7=92.23(2)-°). The unit cell of the VO(HPO4).4H20 is triclinic and the calculated parameters are a=6.383(3)A, b=8.920(2)A, c=13.465(4)A, (z=79.93(2), 13=76.35(3) and -/=71.02(3) (in ref. 8 a=6.379(2)A, b=8.921(2)A, c=13.462(3)A, ~=79.95(2), 13=76.33(3) and ~,=71.03(3)). Finally, the pattern of the dihydrogenophosphate, VO(H2P04)2, can be indexed to a tetragonal cell having a=8.956(1)A and c=7.971(2)A (in ref. 10, a=8.953(2)A and c=7.965(2)A). Thermal behaviour. Variable temperature x-ray powder diffraction, DTA and TGA have been used to monitor the thermal evolution of these materials.

After complete dehydration, all the monohydrogenphosphates yield pyrophosphate, (VO)2P20 7, as final product of the pyrolysis. Therefore, dehydration involves both the actual hydration water molecules and those arising from the condensation of hydrogenphosphate groups. Besides the endothermic peak(s) clearly associable to dehydration, all DTA curves exhibit one new weak endothermic effect at higher temperature (ca. 430°C). A remarkable feature is that, in all cases, the weight loss observed (TGA) prior to this last effect corresponds to the removal of all hydration water molecules and a fraction of the condensation water. This fraction is the same (near 60% of the condensation water), regardless of the starting material. Thus, this endothermic effect always is associated to the elimination of ca. 0.2 water molecules. It may be pointed out that, when performing isothermal experiments on the dehydration of the VO(HPO4).0.5H20, Bordes et al (16). and Johnson et al. (4) have also found that, below 420~C, dehydration does not proceed to completion. They do not discuss this fact, but their results agree with the general observation now made. Indeed, these new results suggest the possible existence of a metastable intermediate in the course of the progressive joining of the (VOHPO4)=o sheets (r15), although the full understanding of this process will require a more detailed study. On the other hand, the experimental weight losses (TGA) are accountable in terms of the equations (1)-(6):

-

1

H20

VO(HPO4)-0.5H20

~-- 1/2 (VO)2P207

(1)

~- 1/2 (VO)2P207

(2)

~

(3)

100-460°C -3/2 H20 VO(HPO4).IH20 200-4600C -5/2 H20 13-VO(HPO4).2H20

1/2 (VO)2P207

100-500°C -1 H20 (z-VO(HPO4)-2H20

-3/2 H20 ~ VO(HPO4).I H20

115-180°C

~ 200-460°C

1/2 (VO)2P207

(4)

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P. A M O R 6 S , e t al.

-1 H20 VO(HPO4).3H2~-VO(HPO4)-2H20 70-1209C

Vol. 24, No. 11

-1 H20 -3/2 H20 ~ VO(HPO4).I H20 ~ 4./2 (VO)2P207 (5) 120-190gC 200-460~C

-2 H20

VO(HPO4).4H20

-5/2H20 ~ 8-VO(HPO4).2H20 ~1/2 74-1409C 140-500~C

(VO)2P207

(6)

As can be noted, the hemihydrate does not arise as dehydration intermediate in any case. Thus, the hemihydrate appears as a singular case in this series. Otherwise, VO(HPO4)-3H20 loses water step by step (i.e., the formation of a-VO(HPO4).2H20 and VO(HPO4)-IH20 is detected in the course of the dehydration). Figure la illustrates the temperature dependence of the X-ray powder diffraction pattern of the trihydrate. As stated above, the room temperature pattern coincides with that registered for the dihydrate. Besides this, the TGA-DTA data indicate that the evolution of the first water molecule of the VO(HPO4).3H20 begins at relatively low temperatures (ca. 70gC), which would support its zeolitic-type character. On heating, the progressive formation of VO(HPO4).IH20 (115°C
# Concerning the temperatures indicated here, it must be stressed that the samples are maintained at the stated value during, at least, 15 min. Of course, owing to the different geometry of the samples in each experiment, the temperature values in the X-ray device do not coincide exactly with those measured through the thermal study.

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PHOSPHATES

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link, which is in g o o d agreement with the known bond distance values (7,8,9,10). On the other hand, given that the PO 3 a s y m m e t r i c stretching vibrations originate more than two well defined b a n d s (around 1100 cm "1, see Table 2), it can be inferred that the highest possible local s y m m e t r y of the [ H P 0 4 ] groups is always C s (actually, it is C s in the hemi- (7,8) and C 1 in the 8-dihydrate and the tetrahydrate (8,9)).

TABLE 2 IR absorption wavenumbers (cm -1) for the VO(HxPO4)x.YH20.

X=I y= 0.5

y= 1

x=2

y= 2(B)

y= 2(~)

y= 3

y= 4

3500 sh

3420 vs

3590 vs

3522 vs

3520 vs

3560 sh

3371 vs

3200 sh

3446 s

3310 3181 2960 2845 2460

3310 3165 2950 2845 2350

3365 s 3130 s

2925 sh 2845 sh 2328 w

2340 m

2259 w

s s sh sh m

s s sh sh m

Assignment

y=0 3414 vs

~(PO-H) ~(O-H)

2900 sh 2430 m

2400 vw

2~(POH) or ~(POH)+I.)(PO3)as

2072 w

2078 w

2070 w

2080 vw

21)(PO3)s

1935 w

1960 vw

1950 vw 1960 vw

't)(PO3)+'O(P-OH )

1641 s

1640 s

1660 sh 1625 s

1660 s 1610 s

1646 s

8(HOH)

1385 w

1380 w

1384 w

1667 1609 1545 1385

1385 w

1385 w

1385 w

~(POH) in plane

1202 vs

1210 sh

1207 m

1202 s

1205 s

1220 sh

1295 w

l)(PO3)as

1136 m 1106 s 1045 s

1150 sh 1080s 1065 s 1030 sh

1140 sh 1085 s 1045 vw

1147 1085 1065 1022

1145 1085 1055 1030

1153 1083 1060 1024

1186 s 1105 s

1975 w

s s m w

s sh s sh

s sh s sh

s s m m

980 vs

990s

970 m

987 vs

991 vs

989 s

980 w

~)(v=o)

929 m

920 vs

899 s

919 vs

923 vs

910 m

947 m

13(PO3)s

818 m

820 m

892 m

909 s

8(POH)out of plane

715 598 553 506 470

715 595 540 510 470

710 sh 628 w 519 m

602 m

~(OPO)+~(OVO)

810 m 720vw 685w 644 m 530 m 483 m

710 600 530 495 475

vw w w w m

736 633 528 488 414

w m w m m

m m w w m

m m w w m

489 w 421 m

vs, very strong, s, strong, m, medium, w, weak. vw, very weak. sh, shoulder.

A comment on the asslgnement of the bands due to OH stretching vibrations is necessary. Thus, b a n d s at around 2700-3000 and 2300-2400 cm -1 have been frequently assigned to the OH stretching vibrations of the PO-H bonds (19a,b). However, we find that, in all cases, at least one band assignable to t)(PO-H) occurs above 3400 cm -1 (see Table 2). This result, which

is o b v i o u s

for

the

anhydrous

dihydrogenphosphate,

can

be

e x t e n d e d to the

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Vol. 24, No. I i

P. AMOR()S, et al.

e a

T=2S°C

T = 2e°c

i

A

i

"X-

i

2#

a

T=1200C

b T=IIO°C

la

e

v

I

I

a

T=180oC

b

4z T=12o°c

® ei

e-

2e

i

15

T= 460°C

13

1~I a 2 e •

16

T = 4eO~:

' 11' '

12

b

•

I FIG. 1 Evolution of the X-ray powder diffraction pattern with the temperature~ (a), a-VO(HPO4).2H20; (~),c¢-VO(HPO4).2H20;~,VO(HPO4).H20; II,(VO)2P207.(b), VO(HPO4)-4H20; ~ VO(HPO4).4H20; -~r, 8-VO(HPO4).2 H20; II, (VO)2P207 .

Infrared spectra. The main features of the IR spectra of these materials have been summarized in Table 2. When structural data are available, our study, and the consequent assignment of the bands, includes both the site group analysis and the factor group splitting effect (19,20). From these results we have made the assignment for the remaining solids, and some refinements to previous partial results in the literature can be pointed out. In all cases, the vanadyl stretching vibration gives rise to a sharp and intense band near 990 cm -1. This value reflects an essentially double bond character for the vanadium to oxygen

Vol. 24, No. 11

PHOSPHATES

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monohydrogenphosphate hydrate series by correlating the site- and factor-group analyses with the available structural data. Thus, the results of the factor-group analysis referred to the OH stretching vibrations in the compounds whose structure are known are summarized in Figure 2. Hence, four and two ~)(PO-H) bands are expected in the IR spectra of the dihydrogenphosphate and the different hydrates, respectively. VO(H|P04)=I HgPO 4 group

Molecular point group C2 v

SilelJrouup Ci

A1 ~

F|ctor-group D4h

2 A ~ - - ~ " - ~ /

B2

~

2Agu 2E u

VO(HPO4)-O.SHgO HPO 4 group

HgO

Poinl-group Sitegroup Faclor-group C3v C8 Dgh

Point-group Sile-group Faclor-group Cgv Cgv Dgh

Blu A1

AI

A1

Blu

B2 - -

Bgu

A'

m

B2 u

B2 -

-

VO(HPO4)'4HgO HPO 4 group

HaO

Point-group Site-group Factor-group C3v

C1

Poinl-group Site-group Paclorgroup

C~

C2~ 4

2 A1 - -

2 A~ - -

2 A.

C1

AI~

8 A1 - -

CI

8 Au

4 B2 8"VO(HPO4)'gHgO HPO 4 group

H20

Point-group Sitegroup Factorgroup C3v

2 A1 - -

C1

Ci

2 A,~ - -

2 Au

Point group C2 v

Site-group Faclor-group Ci

4 A1 --

Ci

4 Au

2

FIG. 2 Correlation chart for the symmetry species of hydrogenphosphate ions and water molecules. Only IR allowed OH stretching vibrations have been included.

In the VO(H2PO4) 2 case, these four bands overlap into a broad and strong absorption centered at 3414 cm -1. Although this value might seem anomalously high (20a,b), it is in good agreement with Pimentel's correlation (that predicts a ~(PO-H) frequency of c a . 3500 cm -1 for the unique PO-H...O hydrogen bond at do_H...O=2.82 A (10))(21). The structure of the VO(HPO4)-4H20 also involves only one PO-H...O hydrogen bond with do_H...O=2.855(8) (8). The band at around 3560 cm -1 (Table 2) would be consistent with this fact (21). Similarly, the bands above 3500 cm -1 in the spectrum of the hemihydrate (Table 2) would be consistent with the factor-group analysis and the hydrogen bond distance in its structure (the do_H...O distance in the only PO-H...O bond is 3.04 ,&, (8)). As can be noted, both these and the remaining solids show a band (similar in shape and intensity) around 2300-2400 cm -1, which could be assigned to an overtone or combination mode (20b).ln short, the consideration

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Vol. 24, No. 11

of the IR spectra, which appear complex in the high frequencies region due to the hydration water, suggests that the [HPO4] groups are involved in relatively weak hydrogen bonds (do-H...O > 2.8 A) in all this monohydrogenphosphate series. Previous assignements on related compounds, such as those of Chapman (20b), can be understood in the light of the posterior X-ray structural data (22). Indeed, the hydrogen bonding network is relatively strong even in the anhydrous Na2HPO 4, and the bands near 2800 and 2400 cm -1 are easily explained on the basis of the experimental do.H... O values (2.654 and 2.57 A, respectively) (22).

Angular Overlap Model Analysis. Deconvolution of the electronic spectra has been made using a modification of the Pitha and Jones program (23). Experimental data, as result from deconvolution, and assignment of the bands, are summarized in Table 3. The presence of three d-d bands in the spectrum of VO(H2PO4) 2 indicates a C2v symmetry, at least, for the [VO6]-chromophore (actually C4v (10)). All the remaining solids present the band due to the dxz,dyz.--~dxy transition split into two components. Accordingly, the symmetry of the [VO6] groups must be now lower than C2v (i.e., Cs or C1), a result which finds support in the known structural data (7,8,9). The bands assignement has been made taking into account the selection rules (24). Thus, in all cases, the more intense band (dxz,dyz--=-dxy) is that appearing at lower energy, whereas that located at higher energy has the lowest intensity (dz2--~dxy). An approximate quantitative analysis of these spectra can be approached through the a.o.m, treatment (25) under the hypothesis that the required simplifying assumption eG>e=x=exy>> ez (whose validity is widely accepted for linear ligating ligands)(13,25) keeps on. On this basis, the d-orbital energies can be expressed as functions of four a.o.m. parameters (i.e., e~eq, e~eq, e~ ax and e=ax) and the spherical coordinates, 8 and (p, of each equatorial ligand (i.e., 0i= O=V-Oi eq., (Pij= dihedral angle defined by the planes O=V-O i eq. and O=V-Oj eq.)(26). When structural data are available (i.e., the respective 9 and ~0 values are known) the analysis becomes relatively straightforward. This is the case for y=0.5, 2(8) and 4 (7,8,10), in which we have as many equations (from the four transition energies) as unknowns (a.o.m. parameters)$. Given that the equation'system derived from Schaffer's expressions (26) is not linear, we have developped an iterative calculation program leading to the e~, values which give the best fit to the experimental transition energies (see Tables 3 and 4). Because of the chromophores analogy, it seems reasonable the similarity between the respective ez values for each hydrate. The structure of VO(H2PO4)2(and, consequently 0 i and (Pij values) is also known (10). In this case the [VO6] local symmetry is C4v. Then, to determine the four a.o.m, unknowns, we

$ e values are those reported by Leonowicz in ref. 8 and Le Bail in ref. 9, whereas (p values have been derived from the structural results of Leonowicz by means of the XANADU program (27).

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1357

only have three equations (from the three d-d transitions). A simplifying hypothesis has to be assumed. It can be noted that the ratio e~eq/eGeq (p) is 0.36 for both the hemi- and the tetrahydrate. This value also is close to the previously calculated (by a statistical procedure) for [VO6]-C4v chromophores in the VO(acac)2 system (p=0.33) (13). Under the C4v symmetry, the assumption p=0.36 leads to a linear system (13) with a single analytical solution (see Tables 3 and 4). TABLE 3 Experimental and calculated band maxima (cm-1): Tl=Edyz-Edxy; Til=Edxz-Edxy; Tiil=Edx2.y2-Edxy; Tiv=Edz2-Edxy. Solid

TIa

x= 1 ; y = 0.5 11700 x= 1;y= 1 9385 x= 1 ; y = 2(8) 10782 x= 1 ; y = 2(0¢) 9316 x=l;y=3 9411 x= 1;y-- 4 10813 x=2;y=0 11909

Tib 11510 10478 c 10995 10435 c 10470 c 10669 11909

TIIa 11834 11572 11391 11554 11529 11398 11909

TIIb 12072 10478 c 11244 10435 c 10470 c 11582 11909

TIlla

TIllb

TIVa

TIVb

15558 15850 15876 15864 15701 15109 15976

15618 15850 15873 15864 15701 15205 15976

21561 20153 19652 19440 20159 21427 21299

21576 20153 19674 19440 20159 21429 21299

a) As resulting from deconvolution, b) Transition energies calculated by means the a.o.m. treatment, c) The calculation procedure leads to the mean value of T I and TII.

The structures of the remaining monohydrogenphosphates (y= 1, 2(e¢) and 3) have been not yet resolved. The application of the a.o.m, treatment in the way that we have recently described (13) results particularly adventageous in such a situation. Hence, we have used the

TABLE 4 A,O.M. parameters (cm"1). Solid x= x= x= x= x= x= x=

1 ; y = 0.5 1;y= 1 1 ; y = 2(8) 1; y= 2(0¢) 1;y=3 1; .It= 4 2; y= 0

eoeq 10635 10545 10732 10640 10519 10207 10970

e=eq 3838 3796 3850 3830 3787 3691 3949

eaax

e ax

26233 24822 24378 24154 24819 25958 261 55

17337 17056 17115 16856 16846 17173 17890

calculation procedure described in ref. 13 to estimate an average azimuthal e angle for each compound. The calculated average values of e are: y= 1, 6 =97.8-°; y= 2(¢), 0=98.6-o and

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Vol. 24, No. 11

y=3, 0 =98.5 g. From these e values, and assuming again p=0.36, it can be determined (13) the set of e~.values which reproduces the experimental d-d transition energies for each compound (see Tables 3 and 4). In short, it can be pointed out the rather good agreement among the sets of e~ values of the VO2+ ion for this series of phosphates (Table 4). Moreover, these values are close to those found for the VO2+ ion in bis(acetylacetonato)oxovanadium(IV) complexes (13). However, some reasonable tendencies become apparent. As might be expected, & the highest similarity is found for the equatorial parameters (eoreq, e~eq), whereas the eaax values are a little more dispersed. This variation must be due to differences in the co-ordination of the "sixth" ligand in the [VO6]-chromophores (i.e., the ligand trans to the V=O double bond). While it does not seem possible to rationalize the observed variation in the light of the available structural data, what is sure is that the eaax values now obtained are lower than that found for solid [ V O ( a c e t y l a c e t o n a t o ) 2 ] (13). This result is consistent with the presence of a [VO5]-chromophore (i.e., with a co-ordination void trans to the V=O bond) in the acetylacetonate derivative. On the other hand, the ~ character in the V=O double bond (as measured by e=ax) is practically constant. Given that the vanadium to "sixth"-Iigand bond distance is very long in all the vanadyl derivatives (7,8,9,10,28), it can reasonably be assumed that the contribution of this "sixth" ligand to the ~-bond network is almost negligible. Thus, both the "constancy" of the e=ax values and the fact that these values be only slightly lower than those found for the [VO(acac)2 ] system, are coherent results. Once again, it is verified the validity of Woolley's "sume ru/e" (29). Conclusions

Besides the apparent transferability of the e~eq values of the VO2+ ion among different oxygenated chromophores, some aspects of the results reported here must be commented. Thus, the pyrophosphate has been recognized as the active phase in the main calalityc processes involving the V-P-O system (3). As stated above, all the hydrogen phosphate hydrates yield pyrophosphate after pyrolysis. In the light of the structural relation between the pyrophosphate and the hemihydrate (7,8,10), it seems surprising that this last phase be no detected in the course of the dehydration of any higher hydrate. To achieve a better understanding of the mechanisms of the solid-satate transformations leading to the active phase, a neutron diffraction study on the subject is in progress. On the other hand, it can reasonablely be assumed that the structural dissimilarities among the hydrates govern the formation processes in heterogeneous phase. The water withdrawing effect of the acetone present in the reaction medium would induce structural rearrangements of the ionic entities (in fact, it avoids the formation of H2P04-, the anionic prevailing species in aqueous solution). The crystallization/redisolution subsequent heterogeneous processes, depending on the relative amount of acetone in the medium, would finally lead to the more unsolvable phase.

& In fact, all three compounds whose structure is known (7,8,9,10) contain [VO6]-chromophores in which the V-O i eq distances are practically constant (ca. 200 pm), whereas the azimuthal

0 i angle values vary in a relatively short range (94.5< 0i<100.59).

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Acknowledaements.

This work was supported by the E.E.C., under Grant ST 2J-0164-4-E(CD). P.A. thanks the Spanish Ministerio de Educaci6n y Ciencia for a FPI fellowship.

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