The spectroscopic study of cationic complexes of dithiomalonamide with Ni(II), Pd(II) and Pt(II)

SPECTROCHIMICA ACTA PART A

ELSEVIER

Spectrochimica Acta Part A 51 (1995) 1617-1633

The spectroscopic study of cationic complexes of dithiomalonamide with Ni(II), Pd(II) and Pt(II) Sabrina H.J. De Beukeleer *, Herman O. Desseyn Laboratorium Anorganische Scheikunde, Universitair Centrum Antwerpen, Groenenborgerlaan 17l, B-2020 Antwerp, Belgium

Received 2 December 1994; accepted 10 January 1995

Abstract

In acid medium dithiomalonamide forms stable, cationic complexes of general formula: M(H2A)2X2 (M is Ni(II), Pd(II), Pt(II); H2A is H2NCSCH2CSNH2; X is CI, Br, I, C104). A full vibrational analysis (infrared and Raman spectroscopy) is presented and the influenceof the central metal ion and the counterion is investigated. Furthermore some solid state exchange reactions are studied and the thermal behaviour of the complexes is described. Finally the compounds are compared with analogous complexes of dithiooxamide. It seems that the presence of the methylene function between the thioamide groups in dithiomalonamide creates a series of interesting features.

1. Introduction

For several years our group has been engaged in the systematic study of dithiooxamides and their complexes with transition metals (mainly group VIII, Ib and IIb) [1-8]. This work is now extended to the characterization of some metal complexes of dithiomalonamides (H2NCSCH2CSNH2, hereafter called H2A). In this bidentate ligand a CH2 group is introduced between two thioamide functions which, as will be shown in this paper, dramatically changes the behaviour of this ligand compared with dithiooxamide. Dithiomalonamide complexes have already been investigated by several workers. In 1973 Peyronel and co-workers [9] described Ni(II) and Pd(II) complexes of H2A, prepared in acid and neutral media, and proposed S,N coordination for the cationic species and S,S coordination for the neutral complexes. Martin [10] reported on M(HA)2 with M = Ni(II), Pd(II) and Pt(II). Also other metals such as Zn(II) [11], Cu(I) [12] and Sb(III) [13] have been discussed. Ray and Sathyarayana [14] proposed S,S coordination for the cationic species Ni(H2A)2CI2 in analogy with dithiobiureto complexes. Crystal structures of Ni(NN'diPhdtm)2 • 2DMF [15] and Cu(NN'HdiPhdtm)2I [16] finally showed that both in deprotonated and in protonated form dithiomalonamide coordinates through both sulphur atoms. However until now the vibrational analysis was always very vague, as only some fundamentals were tabulated without any further discussion. Therefore this article contributes to a detailed vibrational study (infrared and Raman spectroscopy) of the cationic complexes M(H2A)2X2 (M is Ni, Pd, Pt; X is CI, Br, I, CIO4) in combination

* Corresponding author. 0584-8539/95/$09.50 © 1995 ElsevierScience B.V. All rights reserved SSDI 0584-8539(95)01404-7

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with results from UV/visible spectroscopy, magnetic measurements and thermal analyses. In the first part, Ni(H2A)2C12 is considered as an example and then the influence of the central metal and the counterion is examined. Secondly, the thermal analysis of the compounds is discussed and finally a detailed comparison is made between the dithiomalonamide complexes and the oxa-analogues.

2. Experimental The ligand dithiomalonamide has been prepared according to literature data [17]. The metal salts and solvents were used as received. The cationic complexes can be prepared by adding a solution of the appropriate metal salt MX2 in EtOH and HX to a hot, stirred solution of H2A in AcOH and HX in a M:L = 1:2.2 molar ratio. Immediate colour change and precipitation occur. The complexes are then isolated by filtration, washed with small amounts of AcOH, EtOH and Et20 and dried in vacuo. The deuterated analogues have been prepared in a similar way using deuterated solvents. Elemental analysis affirmed the proposed formula (Table 1). For the characterization of the compounds, several techniques have been used. The IR spectra have been recorded on a Bruker IFS 113V Fourier transform spectrometer equipped with a Globar source and a liquid nitrogen cooled MCT detector for the mid IR region and a DTGS detector for the far IR measurements: the sample is pressed into a KX or polyethylene matrix respectively. The Raman data have been collected on a Bruker IFS 66 V equipped with a FRA 106 FT-Raman accessory, a CW Nd:YAG laser source and a liquid nitrogen-cooled Ge detector. The thermal analyses have been performed on a Seiko 200 TG/DTA for thermogravimetry (TG) and differential thermal analysis (DTA), a Dupont R90 instrument equipped with a 951 thermogravimetric analyser has been used for the isothermal measurements. The UV/visible spectra have been recorded on a Beckman DU 640 instrument equipped with a deuterium and tungsten lamp and a photodiode detector. Magnetic susceptibilities have been measured at room temperature by the Faraday method using a Cahn-Ventron RM-2 balance standardized with HgCo(NCS)4.

3. Results and discussion The cationic complexes all exhibit similar features. The observed diamagnetism suggests a square planar configuration around the central metal ion. The results from the electronic spectra of the different compounds are gathered in Table 2. The d - d spectra of Ni(H2A)2X 2 exhibit a single band in the 18 700-17 900 cm -~ region and confirm the presence of a square planar NiS4 unit [18]. For the Pd(II) and Pt(II) Table I Analytical data and colours of compounds Compound

RMM

%M

%C

%H

%N

%S

Colour

Ni(H2A)2CI 2

398.12 486.93

Ni(H2A)212

580.93

Pd(H2A)2CI 2

445.82

Pt(H2A)2CI 2

534.51

18.77 (18.10) 15.39 (14.80) 13.13 (12.41) 16.52 (16.16) 14.16 (13.48)

3.16 (3.04) 2.55 (2.48) 2.09 (2.08) 3.43 (2.71) 2.71 (2.26)

14.34 (14.07) 11.76 (11.51) 9.62 (9.64) 13.38 (12.57) 10.71 (10.48)

32.86 (32.21) 27.73 (26.34) 22.70 (22.07) 29.10 (28.76) 24.88 (23.99)

Red-brown

Ni(H2A)2Br2

14.23 14.74 10.89 (12.06) 8.57 (10.10)

Brown Green-brown Dark brown Yellow - brown

S.H.J. De Beukeleer, H.O. Desseyn/Spectrochimica Acta Part A 51 (1995) 1617-1633

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Table 2 Electronic spectral data (in l0 s cm-~) on dithiomalonamide and M(H2A)_,X2compounds (measured in a KX matrix) HzA

Ni(H2A)2CI2

Ni(H2A)2Br2

Ni(H2A),I:

Pd(H2A)2CI2

Pt(H2A)2CI2

35.09 28.01

40.32 31.65 26.60 18.69

31.55 26.88 18.28

31.35 26.60 17.92

32.89 27.47

34.48 27.78

Assignment 7r---,~r* n~Tr* d-d

analogues these bands are expected to shift to higher energy [19]. Therefore the weak d - d transitions cannot be observed for these metals, as the corresponding bands are obscured by the more intense M - L and L - L bands. In strong acidic media the thioamide function occurs in the neutral form and can only coordinate through sulphur. Considering the high polar character of this group (Fig. l) and the strong delocalization of the rc electrons in the conjugated system [20], in comparison with amides for instance, we can ignore the possibility of N-coordination. The S,S-coordination of dithiomalonamide was previously proven by the X-ray structure determination of an analogous product by Battaglia et al. [16], namely [Cu(NN'HdiPhdtm)2]X. Furthermore the softness of Pd(II) and Pt(II) automatically favours S-coordination. The exact position of the counterions has not yet been determined, despite several attempts to crystallize the compounds. F r o m the IR spectra however we can conclude that they are involved in intense associations with NH2 groups leading to intermolecular hydrogen bonds and to partial a m m o n i u m character for the N H 2 groups. This type of interaction has already been reported in similar studies with primary thioamides [1,21,22] and indicate N + - H . . . X - bonds. From these data we can propose the molecular geometry shown in Formula 1. 2+ H H--N

\

/ C~S

N--H

S~C

H,,,C/

\

/

\ ..H

H< \

/M\

/C,,H

/ H--N

C~S

S~C

\

\ / H

H

• 2x

N--H

Formula 1. 3. I. Vibrational analysis o f M ( H e A ) , X ,

As the I R spectra of the different M(H2A)2X 2 compounds show clear similarities, the vibrational analysis will be based on one representative complex, Ni(H2A)2CI 2. For the assignments of the fundamentals we relied on the IR (mid and far region) and R a m a n

H

I

H

I

Fig. 1. The two most important resonance forms for a thioamide functionality.

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Table 3 Vibrational analysis (cm -I) of Ni(H2A)2C12 IR

Raman

Assignment

IR

v(H20 ) v(NH2) v(NH2) va~(CH2) v~(CH2) 6(NH2) v(CN) v(CN) 6(CH2) p(NH2) p(NH2) co(CH2) z(CH2) p(CH2) v~s(CCC ) %(CCC) to(NH2)

3416 3338 3281 3179 3009

758 mw 747 mw 712 mw 689 m 607 mw 429 s 387 w 335 m 233 m 192 w 156 vs 140 vs 133 vs 109 vs

Assignment

NiW2AbC%

Ni(H2.4)2CI2 3411 wbr 3190 mbr 2989 vsbr 2864 vw 1653 m 1516m 1477 s 1403 w 1333 s 1318m 1278 vw 1162 vw 1037 w 1005 m 930 vw 816 mbr

Raman

2941 [2] 2859 [3]

1508 [4l 1478 1406 1337 1319 1283 1159 1040 1001 930

[9] [2] [3] [3] [1] [4] [1] [2] [5]

768 [3] 715 689 616 449 430

[1] [l] [10] [31 [6]

328 276 235 190 145

[4] [61 [4] [15]

r(NH2) 6 (CN) b b u v(NiS) b v(NiS) v~

[12]

114 [15]

¢

vw vw vw wbr wbr

2864 vw 2425 m 2334 vw 2227 s

1629 mw 1551 mw 1512 vs 1480 m 1404 vw 1333 mw 1319mw 1239 m 1063 w 1036 w 808 790 746 717 693 667 631 614

mw wsh vw VW w m w w

579 w 539 vw 403 mw 368 W 326 mw 257 m 229 m 193 w 154 S 128 vs 106 s

v(H20)

2938 2865 2423 2336 2242 2203 2136 2099

[2] [1] [3] [1] [2] [3] [1] [1]

a v(ND2) v(ND2) v(ND2) v(ND2) v(CD2) v(CD2)

1553 [1] 1510 [10] [2] 1436 [1]

v(CN) v(CN) v(CN)

1244 [1] 1064 [1]

6(ND2) 6 (CD2)

829 [3] 803 [2]

p(CD2)

695 [1] 669 [1] 610 [3] 595 [3] 586 [4] 446 402 370 316 265 227 185

[3] [3] [2] [2] [5] [4] [12]

w(ND2) 6(CN)

r(ND2) b b b v(NiS) b v(NiS) v~

134 [14] 107 [12]

Due to partially deuterated NH 2 groups, b Ring deformations. ~ Several lattice modes.

spectra o f the normal and the deuterated products, as well as on a low temperature IR study o f Ni(H2A)2CI2 to learn more about the nature o f the hydrogen bonding. The results are all gathered in Table 3. In Fig. 2 the full IR and Raman spectra o f Ni(H2A)2C12 are shown. The results for the other compounds are given later on in the article, when the influence o f the metal and the counterions is discussed. The 3500-2800 cm -j region in the IR spectrum o f Ni(HzA)2CI2 is dominated by a strong band at 2989 cm-1 with several shoulders on the low and high frequency side. Cooling the sample causes this broad band to split and a clear sharpening to occur. N o bands can be observed in the same region in Raman, except two medium weak ones at

S.H.J. De Beukeleer, H.O. Desseyn/Spectrochimica Acta Part A 51 (1995) 16/7-1633

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2941 cm -~ and 2859cm -1 which can be assigned to Vas(CH2) and vs(CH2) respectively. These observations lead us to the conclusion that the NH2 groups are involved in strong intermolecular associations, probably with the available counterions and that they acquire some ammonium-type character. Moreover the presence of overtones and combinations and their Fermi resonance with the v(NH2) fundamentals explain the broad profile of the observed bands. Similar features have previously been encountered in the study of dithiooxamide complexes [1]. After deuteration new bands occur in the 2450-2100 cm 1 region which can be assigned to v(ND2) and v(CD2). When the v(NH)/v(ND) ratio is calculated, values between 1.34 and 1.23 are obtained. Although strong O H • • • O, N H • • • X and other A H • • • B associations often exhibit very low v(AH)/v(AD) values [23], the very low positions of the v(NH2) bands in these compounds should be accompanied by very low isotopic ratios. This however is not the case. As N a k a m o t o et al. [24] pointed out, the position of v(NH) in a N - H • • - X system not only depends on the effective strength of the association but also on the charge and nature of X and on the electron configuration around the nitrogen atom. These factors make it rather complicated to compare our results with other similar but not identical systems. At first sight the low v(NH2) positions in our spectra seem to indicate extremely strong hydrogen bonding when compared with the free v(NH) at about 3500 cm 1. But due to the a m m o n i u m character of the bands it is better to take the v ( N + H ) band at about 3100 cm-~ as a reference. In this case the frequency shift is not so large and we can state that the interactions are not of the strong type. In the 1700-1400cm -1 region of the I R spectrum two important fundamentals, ~(NH2) and v(CN), appear as rather intense bands at 1653 cm ~ and 1516-1477 cm respectively. In R a m a n the g(NH2) mode cannot be observed, but the v(CN) band exhibits a very high intensity which makes it a good diagnostic band for the thioamide functionality. The rather high position and the broadness of the ~5(NH2) band confirm the N-H..-X type interaction, in analogy with dithiooxamide complexes [1]. U p o n deuteration, the 6(NH2) mode disappears and a new band appears at 1240 cm i in the IR spectrum. In R a m a n ~(ND2) is again too weak to be observed.

%T

/ I

II I

4OO0

I

3000

~

1

I

I

I

2000

1500

1000

500

Fig. 2. IR and Raman spectra of Ni(H,A)2CI2.

I

100 (cmd)

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S.H.J. De Beukeleer, H.O. Desseyn/Spectrochirnica Acta Part A 51 (1995) 1617-1633

I-I

I

I-I

!\ I

Fig. 3. The pile-up effect due to hydrogen bonding and the spill-over effect due to M-L a-coordination (both donor-acceptor complexes) leading to an increase of v(CN). The v(CN) band shifts to 1510cm -1 in the spectrum of the deuterated compound. This increase in wavenumber of 30 c m - ~ can only be related to alternating coupling with neighbouring fundamentals. Indeed strong coupling occurs between ~(NH2) and v(CN) while in the deuterated product no such interaction exists. Clearly this 1510cm ~ band has a higher v(CN) contribution and gives a better indication of the actual strength of the C - N bond. As shown in Fig. 3, we can expect an increase in C - N bond strength for two reasons: the pile-up effect due to hydrogen bonding and the spill-over effect which results from the metal-ligand a-coordination, both leading to a very high position of the v(CN) band. The p(NH2) mode is medium intense in IR and R a m a n and appears as a sharp split band at 1340-1320 cm -~. For the deuterated compound p(ND2) can be observed as a weak band in the 1000 c m - J region of the IR and R a m a n spectra. The v(CC) fundamentals appear in the 1000-900 cm-~ region and give a nice example of the complementarity of IR and R a m a n on the subject of intensity. The IR spectrum in this region shows a medium intense band at 1005 cm - t and a weak band at 930 cm -~ which can be assigned to vas(CCC) and vs(CCC) respectively. These two bands can also be observed in the R a m a n spectrum at the very same positions but the intensity ratio is completely inverted: vas(CCC) exhibits weak intensity while vs(CCC) is rather intense. The out-of-plane deformations of the N H 2 groups can be observed in the 8 2 0 - 7 0 0 c m -~ region. Also the v(CS) band is expected in this region: this vibration however is strongly coupled and no band with high v(CS) contribution could be assigned, as expected from literature data [25,26]. The to(NH2) mode can be observed as a medium weak broad band at 820 cm J in IR. In R a m a n this fundamental, as well as the r(NH2), is too weak to be detected. In IR r(NH2) appears at 712cm -1. For Ni(D2A)2CI 2 the e)(ND2) and r(ND2) bands can be observed as weak bands at 631 and 539cm -~ respectively. The medium intense IR band at 610 cm -~ is very intense in Raman, is not influenced by deuteration or low temperature and can be assigned to the d(CN) mode. In the low frequency region we expect several ring vibrations, which are usually strongly coupled and are therefore not explicitly mentioned any further in the analysis, but also m e t a l - l i g a n d vibrations occur in this region. They can provide us with useful information about the strength of complexation. The v(NiS) fundamentals are to be expected in the 400-200 cm ~ region and are dependent on the mass of the complexing agents, on the coordination number and on the electronic configuration of the central metal ion [27]. In comparison with the spectra of the free ligand we can assign two bands at 330 cm ' and 230 cm -~ in the spectrum of Ni(H2A)2C12 to v(NiS), both in IR and in Raman. Nickel isotopic substitution gave no additional information as the bands were very broad. In the R a m a n spectrum of Ni(H2A)2CI 2 a very intense band appears at 190 c m which shifts to 185 cm - j upon deuteration. This band can tentatively be assigned to the vibration of the hydrogen bond v ( A - H . • . X ) [28]. Influence o f the central metal ion N i / P d / P t The three metal ions studied in this paper, belong to the same group and have a d 8 electronic configuration. The spectra of M(H2A)2CI2 all exhibit similar patterns, therefore we expect them to have a similar molecular geometry. Through this series the hydrogen bonding and of course the metal-ligand bond strength are to be compared.

S.H.J. De Beukeleer, H.O. Desseyn/Spectrochimica Acta Part A 51 (1995) 1617-1633

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First of all we observe a remarkable change in the position and also in the profile of v(NH2) in this metal series (see Fig. 4). While for the Ni compound v(NH2) can be assigned to the bands at 3190cm -~ and 2989 cm -~, the spectrum of Pd(H2A)2C12 and the Pt analogue exhibit intense, broad bands at 3220 cm -t and 3089cm -~, and at 3257 cm ~ and 3111 cm ~ respectively. Moreover the changes in the profiles of these bands indicate an increasing contribution of the classical v~,s(NH,) and vs(NH2) features, at the expense of the ammonium character. These variations are confirmed by the NH2 deformations, as these bands all shift to lower wavenumbers and become more narrow in the series N i - P d - P t . The out-of-plane modes of the NH2 groups undergo an even larger shift compared with 6(NH2) and p(NH2), as these vibrations are less coupled with other fundamentals. The ~o(NH2) band can be observed at 816 cm-~ for the Ni compound, at 769 cm-~ for the Pd compound and at 708 cm ~ for the Pt compound, whereas r(NH2) can be assigned to the band at 712cm -~, to a shoulder on the low frequency side of the 710cm -t band, and to the 674 cm ~ band respectively. From these results we can state that the nature of the N - H bond and the electron configuration around the nitrogen atom are clearly affected by the metal ion and by the M - L interaction. Concerning these M - L bonding characteristics, we mainly have to concentrate on the behaviour of the v(MS) and v(CN) fundamentals, as the v(CS) vibration is too delocalized among several bands in the 1000-600 cm-~ region. According to Ferraro [27] the v(PdS) and v(PtS) stretching vibrations are consistently found at higher wavenumbers than those for the Ni analogues. For the dithiolate complexes for example the corresponding metal ligand stretching vibrations can be observed in the 435-333 c m - ' region for Ni, in the 401-352 cm -~ region for Pd and at 405-310 cm ~ for Pt. The Ni, Pd and Pt complexes under investigation in this paper exhibit v(MS) modes at 335 and 232cm J, at 360 and 250cm ~ and at 325 and 224cm ' respectively. These high positions reveal that strong coordination indeed exists between the metal ions and the sulphur donor atoms, but to compare them and to deduce information about the absolute strength of the M - L bond is not evident because, besides the force constant, many other factors such as size, mass, and electron density of the ion play an important role. The v(CN) band is situated at 1516-1477 cm -~ for Ni(HzA)2Ch_, at 1501-1479 c m - ' for the Pd analogue and at 1498-1453 cm-~ for the Pt analogue. As we expect stronger metal-ligand interactions for the more soft metal ions, v(CN) should be situated at the %T

\ I

3500

I

i

3000

2500 (ern-1)

Fig. 4. The v(NH 2) region for the M(H2A)2C1: compounds: M is Ni (top), Pd (middle) and Pt (bottom).

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Table 4 Influence of the halogenide ion on the NH 2 deformation modes of M(H2A),X 2 (IR/cm - I ) X

6(NH2) p(NH2) to(NH,) r(NH2)

CI

Br

I

1653 mbr 1333 s 1318 m 816 mbr 714 mw

1641 s 1325 ms 1313m 776 m 666 mw

1630 vs 1313 m 1307 m 724 ms 637 w

highest position for Pt. Yet we observe the opposite trend in our metal series. Indeed we have to take into account the high contribution of the O(NH2) mode to this band. The 6(NH2) mode can be observed at the lowest wavenumber for the Pt complex. At the same time we expect v(CN) to appear at a higher position compared with the other metals. As a result of this decreased energy difference between these two fundamentals and consequently a more intense interaction with each other, the v(CN) band is shifted back down towards lower wavenumbers. No such vibrational mixing occurs in the deuterated compounds and we can observe v(CN) now at slightly higher positions for Ni < P d < P t , in agreement with our expectations. Nevertheless some reserves should be made as it is impossible to deuterate the products all to the same extent. In a short summary, we can state that the platinum complex exhibits the highest v(NH2) and v(CN) modes while v(MS) is situated at nearly the same position for the three metal complexes, clearly indicating that the metal-ligand interaction is the strongest for M = Pt.

Influence of the counterion Cl/Br/I Not only does the central metal ion have an effect on the spectra, but also the counterion exercises an important influence on the profile of the fundamentals. As the halide ion is involved in hydrogen bonding with the NH2 group, the vibrations of this group will exhibit the largest dependence on the nature of the halogenide. These interactions lead for all the compounds Ni(H2A)2X 2 with X = C1, Br, I to the characteristic broad, intense ammonium-type bands for the v(NH2) vibrations. The nature of the halogenide clearly has a great influence on the position of these fundamentals. For the chloride complex v(NH2) can be observed at 3190-2989cm -I, while for the Br and I analogues these modes are situated in the 3263-3040 cm-~ and 3306-3050 cm J regions respectively. Clearly the proton affinity of the anions decreases from C1- to I , as expected. This effect can also be seen from the positions of the N H 2 deformations, which demand less energy as the hydrogen bond strength decreases. The 6(NH2) can be observed at 1653 cm-J, 1641 cm -~ and 1630 cm -~ respectively for the three compounds. Also p(NH2), ~(NH2) and r(NH2) undergo considerable shifts towards lower wavenumber ( - 2 0 c m -~ to - 8 2 c m -~) between C1- and I - (see Table 4). These variations can also be observed from Fig. 5. Not only the positions of the band changes with the halogenide, but also the profile is clearly dependent on the strength of the interaction. The bands that can be assigned to N H 2 vibrations, both stretching and deformation modes, in the spectrum of Ni(H2A)2C12 are significantly broader than the Br and I analogues. The relationship between stronger hydrogen bonds and increasing bandwidth of ( A - H ) modes, was already observed by several authors [28-31] and is very well applicable to these dithiomalonamide complexes. Besides the above mentioned properties of the NH2 vibrations, the isotopic ratio and the temperature dependence give valuable information about the actual strength of the hydrogen bonds in which the NH2 groups are involved. Saitoh et al. [23] described the

S.H.J. De Beukeleer, H.O. Desseyn/Spectrochimica Acta Part A 51 (1995) 1617-1633

%T

1625

%T

b(NH2)

A 1700

,

1650

,

1600

I

1550

900

800

700

~" (era-l)

600

~ (era -1)

Fig. 5. The 6(NH2) region (A) and the out-of-plane (NH2) (B) region for Ni(HeA)2X 2 where X is CI (top), Br (middle), I (bottom).

deuterium isotope effect as an anomaly in the isotopic frequency ratio (v(OH)/v(OD)) caused by an expansion (positive isotope effect) or a decrease (negative isotope effect) of the R ( O . . "O) distance upon deuteration, which is dependent on the shape of the potential well of the hydrogen bond. Several experiments indicate that shorter (O - • - O ) distances are accompanied by lower isotopic ratios for (OH) stretching vibrations and higher isotopic ratios for (OH) deformations. Shorter (O • - • O ) distances imply stronger hydrogen bonds. These relationships exist not only for O - H - . . O systems, but also other A - H ' " B compounds that have been studied, such as ( N - H . . . O ) and ( N + - H .. . X - ) and give similar results, although the latter, even for the strongest hydrogen bonds, show a relatively small isotope effect. This suggests that these kinds of interactions are not very strong and that in all cases the proton remains firmly attached to the nitrogen atom and no delocalization occurs. The calculated isotopic ratios for the different NH2 vibrations in the complexes studied in this work are scheduled in Table 5 Table 5 Isotopic ratios and mean temperature dependence for the NH 2 fundamentals of Ni(H2A)2X 2

CI

Isotopic ratios Jor the different NH 2 vibrations v(NH2)/v(ND2) 1.319 6(NH2)/d(ND2) 1.334 p(NH2)/p (ND 2) 1.318 o)(N H 2)/~o(N D2 ) 1.293 r(NH2)/'r(ND 2) 1.321

Br

1

1.331 1.323 1.330 1.268 1.264

1.341 1.317 1.330 1.199 1.264

Mean temperature dependence Jbr the deformations (cm- i) + 3.26

+ 1.95

+0.85

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S.H.J. De Beukeleer, H.O. Desseyn/Spectrochimica Acta Part .4 51 (1995) 1617-1633

and reveal a systematic weaker interaction in the halogenide series C1 > Br > I. The ratios for the stretching vibrations clearly increase (from 1.3195 to 1.341) and the ratios for the deformations decrease (from 1.293 to 1.199 for the wagging mode for instance). Finally the effect of low temperature has been investigated, in order to find one more affirmation for the dependence of the hydrogen bond strength on the nature of the counterion. Lowering the temperature of the sample causes the intermolecular distances to decrease and the intermolecular interactions to increase. Moreover it is well known [32,33] that strong hydrogen bonds exhibit large shifts to lower wavenumbers for stretching vibrations and to higher wavenumbers for deformations. In the experiments we have performed on the Ni(HzA)zX2 compounds the NH2 deformations indeed undergo a larger mean shift for Ni(H2A)2C12 compared with the bromide and iodide analogue (see Table 4), again confirming weaker N - H . " X - interactions in the series C1 > Br > I. Solid state exchange reactions in different halogenide matrices

To investigate the stability of the halogenide associations in terms of mutual exchange reactions [22], we have performed several experiments in which a complex Ni(H2A)2X z is pressed into a KY matrix. Influence of time, pressure and temperature on the exchange rate was studied. The detailed analysis of the evolution occurring in the spectra confirms that indeed halogenide exchange does take place and new compounds are indeed formed. By pressing Ni(H2A)2C12 into KBr or KI it was possible to observe structural changes in band positions and profiles caused by the formation of Ni(H2A)2Br 2 or Ni(H2A)2I 2 and thus the replacement of C1 for Y - . The v(NH2) band exhibits a maximum intensity at 2989 cm J in the spectrum of Ni(H2A)2C12 when the compound is pressed into KC1. In KBr or KI this maximum is shifted towards 3024 c m - ~ or 3044 cm ~ respectively. A close examination of the profile of these bands reveals not only the position but also the shape has changed, which can be compared very well now with the same region of the Ni(HzA)2Br2 or Ni(H2A)212 spectra. The 6 (NH2) mode as well undergoes a dramatic change: the broad band at 1653 cm-1 becomes a neat sharp one at 1640 c m - t in KBr or even at 1630 cm I in KI, again in full agreement with the spectra of the corresponding complexes. Fundamentals which are not immediately involved in the hydrogen bonding itself remain practically unchanged, despite some that suffer from different coupling or mass effects. As we know that the out-of-plane vibrations of the NH2 groups are quite pure and thus very sensitive towards any variation in the associations with the halide ions, the 9 0 0 - 6 0 0 c m -~ region undergoes a spectacular change: where og(NH~) at 815 cm ~ gradually disappears, a new band at 758cm -~ (Br) or 723cm -~ (I) occurs and the characteristic patterns of Ni(H2A)2Br 2 or Ni(H2A)2I 2 become recognizable. The physical parameters, time, pressure and temperature, all exercise a different influence on the rate and extent of halogenide exchange. The mechanism of exchange is probably controlled by diffusion as a result of a 100-fold excess of a foreign halide ion. The most efficient way to speed up the reaction rate is by increasing the pressure on the pellet. Keeping the pellet under constant pressure of 10 ton during 15 min already has a great effect on the spectra while after only 1 h at 80 °C some minor changes start to appear. Thirty minutes under 10 ton gives almost 100% exchange when during this time the pellet is regularly re-pressed to spread a local high concentration of expelled halogenide X - into the whole matrix and thus again increase the diffusion rate. Analogous experiments under equal conditions, that is pressing of the pellet during 10min under 10 ton, reveal that the exchange of chloride in a KI matrix proceeds considerably faster than in a KBr matrix. Secondly the replacement of I for CI- is negligible under these circumstances. From the vibrational analysis of these complexes we have concluded that the N - H . . . C1- interaction is the strongest interaction. So one could expect this system to be the most stable and to resist most firmly an exchange which results in a less stable situation. Still the CI ion is the ion which is replaced at the highest rate, by the I - ion which creates the complex, the least stabilized by hydrogen bonding. Clearly other parameters govern this diffusion. Undoubtedly the lattice energy

S.H.J. De Beukeleer, H.O. Desseyn/Spectrochimica Acta Part A 51 (1995) 1617-1633

1627

of the ionic salts K + X - plays a major role. The lattice energy U0 provides us with a useful tool for the estimation of the stability of the ionic salts. For KC1 the calculated values vary between 700 and 6 9 0 k J m o l -~, for KBr U0 lies between 700 and 660 kJ mol ~ and for KI U0 is close to 640-620 kJ m o l - i. From these data, we can state that the lattice energy, which is an interesting measure of the stability of the salt, decreases with increasing covalent character in the KX bonding. So we believe that the formation of the most stable ionic salt is the most important determining factor in the exchange rate. An extra confirmation of the halide exchange is given by the spectra of Ni(H2A)2(CIO4)2 when pressed into KC1, KBr and KI. The spectra of this complex, when recorded immediately after short pressing are all identical and exhibit a different profile compared with the previously mentioned Ni(H2A)zX 2 with X = CI, Br, I. We suggest that the complex is still intact and no exchange has yet occurred. The IR spectra of the C l O 4 complex without exchange and the Raman spectrum of the pure sample do coincide, indicating that there is no inversion centre in the molecule. In Fig. 6(a) the IR and Raman spectra of Ni(H2A)2(CIO4) 2 in KC1 are displayed. Table 6 gives the assignments for the fundamentals of the perchlorate complex when no exchange has yet occurred. The two intense bands in the 3400-3200 cm-~ region can be assigned to v~,~(NH2) and v~(NH2). In IR these fundamentals are medium intense, in Raman they have rather weak intensity but are nevertheless observable, which is not the case for the v(NH 2) vibrations in the C1, Br or I compounds where these modes show more ammonium-type character. The interactions with the perchlorate are significantly different to those of the halide ions. There exist too many additional factors, such as crystal effects, linearity of the hydrogen bond, charge, basicity of the donor atoms, which affect the hydrogen bonding characteristics, in order to be able to compare the strengths of the hydrogen bonds with the halide ions on the one hand and with the perchlorate on the other, from the positions of the NH 2 fundamentals only. The high v(NH2) and low out-of-plane (NH2) might indicate weaker interactions for C 1 0 4 but the isotopic ratios do not confirm this sequence. Moreover the influence of temperature is at its greatest for this ion, which is in contradiction to previous results. The (5(NH 2) band is situated at 1657cm -~. This band is again very sharp in IR and has little intensity in Raman. The v(CN) band can be observed in the 1513-1480cm region in IR and in Raman. Both this position and the profile resemble the v(CN) band in the halogenide complexes, indicating similar metal-ligand interactions. The p(NH2) band appears as a medium intense band at 1318cm -~. In the 9 0 0 - 6 0 0 c m ~ region where we expect the co(NH 2) and r ( N H 2) modes, the profile is quite different from the other compounds as a consequence of the different N - H . . . X interactions. The oJ(NH2) mode can be assigned to the broad band at 710 cm J while the r ( N H 2) mode cannot be observed due to the overlap with the strong fundamental at 624 cm In the far 1R region we can assign the v(NiS) modes to the medium intense bands at 333 cm t and 230 cm- ~, which affirms that the nature of the metal-ligand coordination is not affected by the presence of a different counterion. The vibrations of the C102 ion [34,35] can be located in the IR spectrum at 1090 cm J as a very intense, broad band, at 624 c m - l as a medium strong, sharp band and at 451 cm ~ in the far IR region. In Raman the same bands appear, but with opposite intensities. The 1100 cm - ~ band exhibits only very weak intensity while the 625 cm - ~ and 451 c m - t bands are medium intense. The strong band at 935 cm ~ can also be assigned to a perchlorate vibration mode which is not active in IR. The most fascinating thing about this complex is the fact that after pressing Ni(H2A)2(CI04) 2 into KX (X = C1, Br, l) during a longer time, 20 rain or more, the bands belonging to the thioamide function gradually change towards the corresponding Ni(H2A)2X, fundamentals. Indeed we can observe the characteristic patterns of the Ni(H2A)2C12 (Fig. 6b), Ni(H2A)2Br 2 and Ni(H2A)zI2 complexes, as obtained when these compounds are pressed into KC1, KBr, KI respectively. This is the ultimate proof of the existence of halogenide exchange. Secondly the rate of reaction seems to be at its highest

1628

S,H.J. De Beukeleer, H.O. Desseyn/Spectrochimica .4cta Part .4 51 (1995) 1617-1633 %T

i

4000

I

'

3000

I

I

I

I

2000

1500

1000

500

I

100 (cm-1)

Fig. 6(a). IR and Raman spectra of Ni(H2A)2(CIO4) 2, pressed into KCI, recorded before halogenide exchange took place. %T

'

4000

I

3000

'

I

I

2000

1500

1000

500

{cm-1) Fig. 6(b). IR spectrum of Ni(H2A)2(CIO4) 2, pressed into KCI, recorded after halogenide exchange.

in the K! matrix. Although in all three matrices KC104 is formed and thus the formation of a more stable ionic salt cannot be the driving force, it is the least stable KI salt which expels the C104 ion the fastest from the hydrogen bonded complex. Then what happens to the CIO~- ion? In the spectra of the compound after the exchange, the bands which can be assigned to C10;- change as a result of a different symmetry environment. The intense band at 1090 cm-~ becomes a triplet (1146, I 116 and 1088 cm-~), as well as the 625 c m - ' band (637, 630 and 626 c m - ' ) , and the previously IR inactive mode, which is expected to appear at 941 cm-~ becomes active. These facts suggest that in the original C104 complex the perchlorate ion exists in a highly symmetric tetrahedral environment, and all four oxygen atoms are involved in weak associations with NH2 groups. Whenever pressed into KX, the X - ions replace the oxygen atoms and form N - H - • • X - hydrogen bonds. The C102- ion experiences a lower symmetry, more like C2v which explains the observed changes.

S.H.J. De Beukeleer, H.O. Desseyn/Spectrochimica Acta Part A 51 (1995) 1617-1633

1629

Finally, to round up the solid state experiments, the above mentioned complexes have been pressed into NaF. When Ni(H2A)2CI2 or Ni(H2A)2(CIO4)2 are pressed into NaF, some dramatic changes occur in the IR spectrum. Clearly another complex is formed. To be more precise the characteristic bands of the deprotonated compound Ni(HA)2 appear as displayed in Fig. 7. No decent spectrum could be obtained from the bromide or the iodide compounds in NaF. According to Desseyn [22], pressing in NaF leads to an halogenide exchange and automatically to deprotonation because the F - . - . H - N interaction is too strong. The HF is then captured by NaF molecules to form NaHF2 which exhibits a very diagnostic band in the 2500-2000cm -~ region. The above mentioned experiments also indicate deprotonation but no NaHF2 could be observed. The reason for this is still unknown!

3.2. Thermal analysis of M(H2A)2X2 The different M(H=A)2X 2 compounds have been subjected to a detailed thermal analysis. TG and DTA spectra have been recorded to learn more about the thermal stability of these compounds. Isothermal methods create possibilities of isolating any potential stable intermediate. The data are all collected in Table 7. Fig. 8 exhibits the TG and DTA curves for Ni(H2A)2X2 and Ni(HA)2. Table 6 Full vibrational analysis (cm-~) of Ni(H2A)2(CIOa)2 IR

Raman

Ni(H2A)2(CIO4)2 3329 ms 3328 3206 ms 3217 2964 2893 w 2892 1657 s 1658 1513 mw 1513 1481 ms 1486 1397 mw 1399 1317 m 1319 1286 1158 1089 vs 1101 1017 vs 988 m 990 935 917 w 919 762 709 mbr 715 624 ms 625 607 511 w 507 497 vw 467 mw 468 451 m 451 439 w 441 423 m 423 384 mw 333 m 322 270 230 S 231 185 106

Assignment

IR

v~s(NH2) vs(NH2) va~(CH2) v~(CH2) 6(NH2) v(CN) v(CN) 6(CH2) p(NH2) ~o(CH2) r(CH2) v3(ClO4) p(CH2) vas(CCC) vI (CIO4) v~(CCC)

3329 3222 2544 2505 2392 2332

Raman

Assignment

Ni(D2A)2(C1Oa) 2 [1] [2] [2] [3] [1] [4] [10] [3] [2] [1] [2] [2] [1] [9] [4] [2] [1] [4] [9] [2] [3] [3] [3] [4] [3] [5] [4] [1 I] [13]

w(NH2) v4(C104) 6(CN) 6(CCC) b v2(ClO4) b b b b v(NiS) b v(NiS) v, ¢

wbr wbr mw mw m m

1712 vw 1645 w 1550 w 1510 s 1244 ms 1173 w 1130 ms 1085 vs 932 w 871 vw 831 w 807 w 716 w 694 W 669 mw 636 W 624 m 601 W 555 mw

2546 2506 2403 2336 2219 2147

[2] [2] [2] l 1] [2] [1]

1550 [1] 1512 [9] 1245 [1]

v~s(ND2) v~s(ND2) vs(ND 2) v~(ND2) va~(CD2) v~(CD2)

v(CN) v(CN) 6(ND2) v~(CIO4)

1067 932 869 828 807

[2] [10] [1] [3] [3]

668 [1] 635 [2] 597 557 454 396 362 315 258 220 178 112

[6] [1] [4] [4] [1] [3] [5] [3] [10] [13]

~"~(CIO4) v'~(C104) v(CC) v(CC) p(CD 2)

v~(ClO4) v](ClO 4) 6(CN) ~o(ND 2) v2(CIO4) b v(NiS) b v(NiS) v.

" Due to partially deuterated N H 2 groups, b Ring deformations. ¢ Several lattice modes. $A(A) 51:10-0

S.H.J. De Beukeleer,, H.O. Desseyn¢Spectrochimica .4cta Part A 51 (1995) 1617-1633

1630

%T

%T

3500

! 3O00

.

A 25OO

1700

I

!

1600

1500

1400

(cma)

(cm'l)

Fig. 7. Deprotonation occurring when Ni(H2A)2CI 2 is pressed into NaF as shown by the v(NH 2) (A) and (~(NH2) and v(CN)) (B) regions: top, Ni(H2A)2CI2 in KCI; middle, Ni(H2A)2CI 2 in NaF; bottom, Ni(HA)2 in KCI.

The perchlorate complex is not discussed any further: above 200 °C the product is blown out of the sample holder due to some kind of explosion. The other compounds all behave in a more or less similar way to each other. The degradation consists of several consecutive steps without formation of any stable intermediate. In each case the reaction starts with a similar loss of weight in an endothermic process, and at the end of the reaction at about 550-600 °C the metal sulphide is formed. To characterize the first step, isothermal measurements at 150-155°C have been performed. For all the studied compounds a stable intermediate can be isolated, which has been identified by IR. The evolved gases have been checked for the presence of HX and these tests gave affirmative results indicating that deprotonation occurred during the experiment. The IR spectrum of the so-called new compound clearly exhibits the characteristic bands of M(HA)a, which can be prepared in neutral medium and will be discussed in a following paper. The bromide and iodide analogues behave slightly different as the isolated intermediate is not very stable and further decomposition starts immediately. This is confirmed by the presence of nitrile bands in the corresponding IR spectra of these intermediates. The TG spectra of Ni(H2A)2CI 2 and Pd(H2A)2C12 reveal that the formation of M(HA)2 is completed before this deprotonated compound starts to decompose: further degradation patterns are indeed quite similar as can be seen from Fig. 8 for Ni(H2A)2CI2. Table 7 Thermal analysis data on M(H2A)2X 2 M,X

Ti a I ° step b (calc.) b rest ¢ (calc.) ~

Ni, CI

Ni, Br

Ni, i

Pd, CI

Pt, C1

171.3 19.5 (18.35) 22.20 (22.70)

223.6 39.9 (33.23) 18.40 (18.64)

217.4 32.6 (44.04) 16.50 (15.62)

206.2 13.13 (16.38) 33.50 (31.06)

220.4 13.30 (13.66) 48.70 (42.50)

T~ in °C, initial temperature for the first loss of weight, b Experimental and calculated percentage weight loss of two molecules of HX. ¢ Experimental and calculated percentage weights of metal sulphide.

S.H.J. De Beukeleer, H.O. Desseyn/Spectrochimica Acta Part A 51 (1995) 1617-1633

1631

-100

-2HCI

60 0

0

I

"o

I

I

I

I

100

R"

\ ",. -10 -

60 ,

\ "..,

20 -20

B 0

/

200

I

400

600

Temperature (*C) Fig. 8, T G and D T A spectra for Ni(H2A)2CI 2 (A) and Ni(HA)2 (B).

Ni(H2A)2Br2, Ni(H2A)2I 2 and Pt(H2A)2CI 2 however follow different reaction paths as deprotonation occurs at a temperature at which M(HA)2 is not stable anymore. As decomposition of the complexes starts by releasing two molecules of HX, the temperature of this step will depend on the strength of the N - H • • • X - interaction and the nature of the N - H bond. Indeed the Ni(H2A)2CI2 complex exhibits the strongest hydrogen bonding and is stable up to 170 °C. In the Pd and Pt analogues this interaction gradually weakens, explaining the higher initial temperature T~ for the release of HCI (ca. 200 °C for Pd and ca. 220 °C for Pt). In the series Ni(H2A)2X2 with X = CI, Br, I other factors such as weight and volality of the HX gases also play a role in the deprotonation process. The chloride complex decomposes at 170 °C while the bromide and the iodide compounds are stable up to about 220 °C. The exact mechanism for deprotonation is not very clear as the deprotonation initially occurs at the nitrogen atom, while the intermediate exhibits the characteristics of both NH2 and CH groups, indicating that the methylene group is deprotonated. We believe that hypothetical polymeric compounds are sterically not stable and the fact that C-deprotonation which leads to the formation of a pseudo-aromatic system is the driving forces for a quick proton migration.

3.3. Comparison with the dithiooxamide (DTO) ligand Dithiooxamide forms cationic complexes M(DTO)2X2 with Ni(II), Pd(II), Pt(II) and other metals, and also in these compounds the halogenide acts as a counterion and is hydrogen bonded to the NH2 groups [1,8]. Consequently the NH2 fundamentals, especially clear in the 3500-2500 c m - ' region, exhibit strong ammonium character. The DTO complexes show a considerable deviation from planarity around the metal ion as the protons of the neighbouring thioamide groups cause severe steric hindrance. In dithiomalonamide (DTM) the methylene function prevents this tension and planarity of the MS 4 unit can be maintained. Ni(DTO)212 could not be obtained by the usual synthetic methods, nor by exchange reactions in a KI pellet, as DTO immediately decomposes and H2S is released even before precipitation occurs. As complexation and hydrogen bonding show certain similarities, we can make a comparison between the two ligands. The v(NH2) vibrations can be observed at

1632

S.H.J. De Beukeleer, H.O. Desseyn/Spectrochimica Acta Part A 51 (1995) 1617-1633

significantly lower wavenumbers for DTO compared with DTM although the exact position of this mode is quite obscured by the presence of overtones and combinations. Nevertheless the maximum in the band is shifted from 2989 cm - ~ for DTM to 2925 c m for DTO. In full agreement with this result we can observe the 6(NH2) mode at slightly higher wavenumbers for DTO, i.e. 1653cm -~ and 1685cm-' respectively. The other deformations obey this sequence as well except for the p(NH2) mode which is found to shift from the 1320cm -~ region (DTM) to 1280cm -~ (DTO). The reason for this difference is given by the presence of the CHz fundamentals which cause an upward shift for this band which can predominantly be assigned to p(NH2) but several other modes also contribute to the ultimate intensity. These weaker N - H bonds in the dithiooxamide complexes may indicate stronger hydrogen bonds (higher N + - H character) and/or an increased p character in the hybridization of the nitrogen atoms [24] (weaker M - L bonding). Information about the metal-ligand bond strength is provided by the positions of the v(MS) and v(CN) modes. The far IR region of the Ni(DTM)2C12 complex exhibits two bands of medium intensity at 335 and 233 cm-~ which can be assigned to (Ni-S) modes while these vibrations are observed at 376 and 244 cm - ~ for the dithiooxamide analogue, indicating stronger metal-ligand interaction for the latter. Due to this stronger M - L bonding we expect the v(CN) mode to appear at higher wavenumbers. Indeed, the v(CN) mode is found at 1477 cm -t for DTM and at 1485 cm -~ for DTO, confirming the previous conclusions. From the increased double bonding character of C - N for DTO we expect the v(N -H ) modes at higher wavenumbers, in analogy with C - H vibrations, as the increased s character in the hybridized N orbitals gives a better overlap with the hydrogen atomic orbital [24]. However as mentioned before, the opposite sequence is observed for the v(NH2) modes and the following statements can be made. Comparison of the analogous Ni(LH2)2C12 compounds for LH2 = DTO and DTM, reveals that the dithiooxamide ligand binds more firmly to the metal ion than dithiomalonamide does, leading to higher v(CN) and v(MS) modes for DTO. The positions of the (NH2) fundamentals indicate also that the interactions between the NH2 groups and the X - ion are significantly weaker in the DTM compounds. No systematic variations can be observed for the fundamental vibrations of M(DTO):C12 with M = Ni, Pd and Pt while DTM does show systematic differences when the central metal ion is changed. Concerning thermal analysis, DTO [2] and DTM exhibit completely different behaviour. Pd(DTO)2CI2 and Pt(DTO)2CI2 do not form any stable intermediates when heated up to 600 °C and gradually lose weight as a result of the thermal breakdown of the ligand. These complexes are stable up to about 100°C. Ni(DTO)2CI2 and Ni(DTO)2Br2 however behave differently from the previous ones as stable polymers [Ni(DTO)],, can be isolated. During this reaction HX and one molecule of ligand are released according to the reaction (LH 2 = DTO): nNi(LH2)2X2 ~ (LH)(NiL),,_ t(LH) + 2nHX T + (n - 1)LH2 T These polymeric compounds can also be prepared starting from Ni(II) and DTO in an alkaline medium. It should be very clear that DTM complexes react in a different way with a similar heating process. From the usual T G measurements one might conclude that no intermediates are formed: nevertheless an obvious inflection point during decomposition suggests that a new compound can be isolated, as confirmed by isothermal analysis. The first loss of weight is always caused by the release of HX. The weaker N - H • • • X - interactions in the DTM compounds explain the increased stability of these compounds as the DTO complexes are stable up to about 100°C while for DTM degradation starts above 170 °C. The presence of the CH 2 group now unequivocally creates the possibility of the formation of a stable, pseudo-aromatic, deprotonated complex at this stage after a relatively easy proton migration, while for DTO further

S.H.J. De Beukeleer, H.O. Desseyn/Spectrochimica Acta Part A 51 (1995) 1617-1633

1633

reactions have to proceed before the polymer can be formed, i.e. a conversion from S,N to S,S coordination, which is not so self-evident [36]. As Pd and Pt are very soft, this does not occur and the complex further breaks down into bits and pieces, whereas the nature of the nickel ion allows for this transition and consequently the corresponding polymer can be isolated.

Acknowledgements Sabrina De Beukeleer thanks N.F.W.O. for financial support. The authors also express their gratitude to Ing. J. Janssens for the thermal measurements, Dr. S.P. Perlepes for the determination of the magnetic properties, Prof. Dr. A. Fabretti for the elemental analyses and Greta Thijs for technical assistance. The N.F.W.O. is thanked for the financial support towards the spectroscopic equipment used in this study.

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