Structural studies of bis(o-sulfobenzimidato)praseodymium(III) chloride hexahydrate

Journal of Molecular Structure 606 (2002) 77±86

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Structural studies of bis(o-sulfobenzimidato)praseodymium(III) chloride hexahydrate Hasan Icbudak a,*, PancÏe Naumov b, Mirjana Ristova b, Gligor Jovanovski b a

Department of Chemistry, Faculty of Arts and Sciences, Ondokuz Mayis University, 55139 Kurupelit-Samsun, Turkey Institute of Chemistry, Faculty of Science, Sv. Kiril i Metodij University, P.O. Box 162, MK-1001 Skopje, Macedonia

b

Received 8 May 2001; accepted 17 July 2001

Abstract A novel rare-earth metal(III) saccharinate, namely bis(o-sulfobenzimidato)praseodymium(III) chloride hexahydrate, is synthesized and its solid-state structure is suggested from the FT IR and UV/vis spectroscopic, thermoanalytical, elemental and magnetic measurements. The OH, CO, SO2 and OD stretchings (the latter isolated in H2O matrix) in the FT mid-infrared spectra of the frozen protiated and partially deuterated analogues of the complex spectra-structural correlations in other saccharinates to predict the structure of the o-sulfobenzimidate residues and the hydration water. The structure features a single crystallographic type of O-coordinated or ionic saccharinato residues, with at least three structural types of water of hydration, and chloride counter-ions in the outer-sphere. q 2002 Elsevier Science B.V. All rights reserved. Keywords: FT IR; Praseodymium(III) complex; Rare-earth complexes; Saccharinates; Spectroscopy; Thermal analysis

1. Introduction The primary interest in the chemistry of the common arti®cial sweetener saccharin (o-sulfobenzimide) can be owed to the controversies regarding its carcinogenic properties [1]. The continuing medicinal studies are not entirely conclusive [2,3] and it is still unclear whether the chemical imposes an increased cancer risk to humans. In vitro studies ascertained enhanced activity of some N-substituted saccharinates [4±6], protease-inhibiting properties of the ®rst-row transition metals, dioxouranium(IV), dioxovanadium(IV) and cerium(IV) saccharinates [7,8], as well as superoxide dismutase-like activity for several metal(II) saccharinates [9]. Apart from the biological signi®cance, the variety * Corresponding author. E-mail address: [email protected] (H. Icbudak).

of bonding patterns of N-deprotonated o-sulfobenzimide attributed to the proximity of three functionalities (CO, NH, SO2) and the good crystallinity of its metal compounds have turned much of the attention lately to fundamental structural research in this ®eld. The potentials stemming from the molecular rigidity and the competitive proton/metal attracting properties of the three vicinal groups have now been recognized for employment of the entity in both model and crystal engineering purposes, resulting in an amount of individual and comprehensive systematic vibrationalstructural reports on saccharinates [10±15], including the preferences in coordination or the geometrical strains, in¯uenced by the metal±sac bond types and the counter-cation [14]. Relations between the ionic radii and sul®mide geometry [12], metal±N distances, unit cell parameters [14,16±18] and thermal properties [19] have been also demonstrated. Vibrational properties are well understood [10,11] and correlated

0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(01)00815-8

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H. Icbudak et al. / Journal of Molecular Structure 606 (2002) 77±86

Scheme 1.

with bond distances, bond orders and coordination type [15,20]. A number of theoretical semiempirical and ab initio geometry optimizations and vibrational analyses were performed at various levels [13,21,22]. Bonding patterns I±V (Scheme 1) have all been documented by X-ray diffraction, and often, o-sulfobenzimide appears in combination of these modes. Alkali [23,24], ammonium [25] and a number of transition metal saccharinates [26±28] are purely ionic (I), and o-sulfobenzimidate ions were demonstrated for several other salts [13,29,30]. Exclusively, osulfobenzimidato±N ligands II exist in the mercury(II) complexes [31±34], triphenyl saccharinates [35,36] and a number of transition metal complexes [16,18,37±39]. Several complexes comprise only 1,2-benzisothiazolyl-3-olato 1,1dioxide form III [18], whereas the deprotonated saccharin in the polymeric silver complex serves as a tridentate ligand V [40]. Recently, molecular saccharin was embedded in two ionic saccharinates [30] and a de®nite inclusion of saccharin molecules has been con®rmed by X-ray diffraction [41], yielding a novel binding mode (VIII). Noteworthy among the coordination motifs I±VIII is the amidato-bridging bidentate ligand IV observed in two imidazole saccharinates [42,43], lead(II) saccharinate [44] and pyridine adduct of tetrasaccharinatedichromium(II) [45], as it shows the potentials of the complex entity as a corner unit in crystal engineering purposes. Contrary to the other saccharinates, data on rareearth compounds (and on trivalent metals in general)

are very scarce and at present consist of only four reports. In their systematic investigation, Nabar and Khosla have reported the main IR bands of the saccharinates of yttrium(III), lanthanum(III) and the lanthanides from cerium(III) to ytterbium(III) [46]. Bidentate chelating saccharinate mode VI was suggested for large ionic radii in rare-earth complexes, e.g. europium(III) [47,48]. This mode has been exclusively reported in the structure of lead(II) saccharinate, comprising N and carbonyl O atoms from the same saccharinate ion coordinated to the same lead(II) cation. The carbonyl group in the only known structure, neodymium(III) saccharinate, interacts with the metal ion via coordinated water molecule, VII [49]. In the attempt to examine the bonding attributes of saccharin in the solid state further, we approached synthesis of novel rare-earth saccharinates, and the current paper focuses on the physicochemical characterization of the ®rst mixed saccharinato±halide complex of a rare-earth metal cation.

2. Experimental 2.1. Synthesis The complex was prepared as a light-greenish powder upon treatment of 10 mmol PrCl3´6H2O (Aldrich) in 50 ml water with 20 mmol sodium saccharinate (Aldrich) in 50 ml water upon stirring at 808C, until the solid formed. After cooling the reaction mixture to room temperature, the solid was

H. Icbudak et al. / Journal of Molecular Structure 606 (2002) 77±86

®ltered and subsequently washed with cold distilled water, alcohol and acetone, and dried in vacuum. Analytical data (calculated on Pr(sac)2Cl´6H2O/ found): C 39.26/39.14%; H 4.00/4.08%; N 10.18/ 9.76%. The product exhibited positive chloride test. The complex dissolves well in water, DMSO and DMF; it is fairly soluble in ethanol and methanol, but practically insoluble in diethyl ether, chloroform, acetone, benzene and hexane. Our continuing efforts to obtain single crystals of the complex suitable for diffraction analysis have so far ended without success. 2.2. Analytical measurements Elemental analyses were performed by standard methods at TUBITAK (The Turkish Scienti®c Research Centre) Marmara Research Centre (Gebze). Magnetic susceptibility measurement was carried out at room temperature using a Sherwood Scienti®c MXI model Gouy magnetic balance according to the Evans method. The UV/vis spectrum was obtained on Unicam UV2 spectrometer. The FT IR spectra in the 4000±400 cm 21 frequency range were recorded with a System 2000 interferometer (Perkin±Elmer) from pressed KBr pellets by averaging 32 background and 64 sample spectra at a resolution of 4 cm 21. A P/N 21525 (Graseby Specac) variable-temperature cell, equipped with KBr windows, was used for both RT and LNT measurements. A Rigaku TG 8110 thermal analyser combined with a TAS 100 thermogravimetric analyser was used to record simultaneous TG, DTG and DTA curves. The experiments were performed both in static air and nitrogen atmospheres with a heating rate of 108C min 21 from room temperature up to 10008C, in platinum crucibles (DTG sensitivity 0.05 mg s 21). Approximately 12 mg of the samples were weighed and highly sintered a-Al2O3 was used as a reference material.

79

formula supported by elemental and other analyses (discussed below). The positive chloride test con®rmed the all-outer-sphere coordination of the chloride ions. From mixed halide±saccharinates, the only examples known to the date are the covalent chloromercury(II) saccharinates HgCl(sac) [32] and [HgCl(py)(sac)]2 (py Ð pyridine) [34] prepared by partial ligand substitution in mercury(II) chloride. Due to the large ionic radii associated with the rare earth cations, formation of other mixed-ligand complexes can also be expected, and even in case of counter-ions with poor coordination ability such as is nitrate in Eu2(sac)3(NO3)3´12H2O [47,48]. As for the praseodymium(III) cation, the reaction of the more basic carbonate with saccharin results in complete substitution and Pr(sac)3´10H2O [46], a clearly distinct compound from the one reported here. This latter example illustrates the potentials in employing rare earth counter-cations for studies of the saccharin structural chemistry. The magnetic susceptibility of the title complex of 3.4 BM agrees well with the usual range 3.4±3.6 BM and the calculated value of 3.58 BM for the Pr 31 with [Xe]f 2 structure. Absorption bands appearing in the 444±589 nm interval of the UV/vis spectrum (Fig. 1) correspond to the f±f transitions of Pr 31(f 2). The pronounced shielding of the f orbitals by the 5s 2 and 5p 6 subshells disfavors any strong overlap with the

3. Results and discussion 3.1. General discussion The double-ion exchange reaction between praseodymium(III) chloride and sodium saccharinate led to incomplete substitution of the chloride ions, affording mixed chloride±saccharinate Pr(sac)2Cl´6H2O, a

Fig. 1. The UV/vis spectrum of bis(o-sulfobenzimidato)praseodymium(III) chloride hexahydrate.

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H. Icbudak et al. / Journal of Molecular Structure 606 (2002) 77±86

Fig. 2. The 4000±370 cm 21 region in the (A) room-temperature and (B) 77 K FT infrared spectra of bis(o-sulfobenzimidato)praseodymium(III) chloride hexahydrate.

ligand orbitals; hence, f orbitals interact only weakly, being nearly independent form the ligand orbitals [50±52], absorption bands of the f±f transitions are fairly sharp (Fig. 1) and remind those of the free ions, unlike broad bands, characteristic for d±d transitions. The absorption bands at 444, 468, 482 and 589 nm can be ascribed to the 3H4 ! 3P2, 3H4 ! 3P1, 3H4 ! 3P0 and 3H4 ! 1D2 transitions, respectively. 3.2. Infrared spectra Owing to the possibility of extensive vibrational interactions and overlap with the CH fundamentals and second-order modes, the broad and strong OH stretching absorption in the 3700±3000 cm 21 interval (Fig. 2), with a maximum at about 3370 cm 21, is not very informative about the structure of the title compound, solely insinuating a complex water structure. The range of absorption indicates hydrogen bonding of the hydration water. The temperature behavior of the minimum at 3405 cm 21 seems to conceive transmission window effects of the H2O stretchings, with lower frequency mode(s) on the

broad background absorption [53]. In this respect, much more valuable structural information is gained from the spectra of isotopically isolated water molecules, which are considered effectively mechanically inter- and intra-molecularly decoupled and are thus favorable as structural probe. Spectrum of the HDO impurity, isotopically isolated in crystalline H2O matrix (,0.7% deuterium) of the compound at 77 K, presents ®ve bands of various intensities (Fig. 3). From the curve-®tted native contour line, integrated intensity ratio of 1.82:2.58:5.29:2.53:1 was obtained, close to 2:2.5:5:2.5:1, supporting the presumptions about the enhanced complexity of the hydrogen bonding network. Assuming statistical distribution of deuterons and absence of disordered protons, a rough qualitative inspection would simply imply only ®ve crystallographic OD oscillator types, the band at 2501 cm 21 (middle) apparently resulting from the accidental degeneracy of a pair of equivalent or geometrically identical water stretching oscillators. Larger halfwidth (or, integrated intensity) of the two lowest-frequency bands is normally expected from the stronger hydrogen bonding of the respective water

H. Icbudak et al. / Journal of Molecular Structure 606 (2002) 77±86

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Fig. 3. Curve ®tting of the OD stretching region in the 77 K spectrum of slightly deuterated (,0.7% D) bis(o-sulfobenzimidato)praseodymium(III) chloride hexahydrate (A Ð experimental spectrum, dotted line Ð ®tted spectrum; other bands are the ®tted components).

molecules, associated with the temperature-sensitivity of the spectrum, below 750 cm 21 (Fig. 2). Otherwise, the band positions (2542, 2530, 2501, 2474, 2434 cm 21 at 77 K) prove that all water molecules are moderately hydrogen bonded. Assuming only hydrogen bonds of type O±H´ ´ ´O (which is not necessarily true, as the nitranionic nitrogen center of Table 1 The O´´´O distances predicted from the empirical correlations Frequency (cm 21)

2542 2530 2501 2474 2434

Ê )a Predicted d(O´´´O) (A Eq. A

Eq. B

Eq. C

2.892 2.876 2.839 2.809 2.769

2.892 2.873 2.830 2.795 2.750

2.918 2.899 2.858 2.825 2.781

a The correlation expressions have the following form (where n is  ˆ the frequency of the isotopically isolated oscillators): (A) d …A†  ˆ …21=3:73† ln[(2727 2 n (cm 21))/…8:97 £ 106 †] [58], (B) d …A†  ˆ …21=3:23† ln[(2727 2 n (cm 21))/(2:11 £ 106 )] [59], (C) d …A† 4:47128 2 0:29754 ln…2727 2 n …cm21 †† [60].

unbound saccharinato ion may also serve as protonacceptor), the O´ ´ ´O distances predicted by three different empirical correlation functions belong to Ê (Table 1). the narrow interval 2.7±2.9 A Despite the fact that in presence of enough theoretical grounds [10,11], and particularly in case of the simple spectral appearance as the present case, a complete assignment of the saccharinato mid-infrared active vibrations is quite feasible, and such a task would not be very bene®cial in course of structural information. 1 The spectral simplicity of the o-sulfobenzimidate modes taken as a whole, merely implies pronounced similarity, if not outright equivalence, among all saccharinato residues in the structure. On the other hand, the ab initio Hartree±Fock and correlated harmonic vibrational analyses of the groundstate isolated saccharinato ion have justi®ed [13,21] the earlier assumptions [10,11] about the potentials of the carbonyl stretching for structural predictions [15]. 1

Due to the pronounced coupling of the `internal' skeletal osulfobenzimidato vibrations, the only bene®t would be the assignment itself.

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Fig. 4. The 1800±910 cm 21 region in the (A) room-temperature and (B) 77 K FT infrared spectra of bis(o-sulfobenzimidato)praseodymium(III) chloride hexahydrate.

Fairly independent among the heavily coupled vibrations of o-sulfobenzimide moiety, with contribution of the dominating internal coordinate of about 70% from the PED matrix, the mode accurately re¯ects the structural environment of the carbonyl group, which holds a dominant role in structural preferences of the ligand/ion, in the solid state. Two of the factors known to direct the carbonyl stretching frequency are the type of metal±saccharinato bonding [13,15,21] and the coordination type [20]: increase in the ionic character of the metal, in the metal±saccharinato bonds, results in a stronger red-shift of the n (CO) mode in the spectrum. Practical dif®culties in assignment of the respective bands due to overlap with either intrinsic (saccharinato phenylene ring stretchings) or extrinsic (water bending) modes have prompted us to de®ne a set of useful empirical criteria for reliable assignment of this mode. At least three overlapped bands (1663, 1639, 1617 cm 21) resolved in the low temperature spectrum of the studied compound (Fig. 4) were candidates for the n (CO) assignment; the additional sharp tempera-

ture insensitive band at 1586 cm 21 is identi®ed as the benzenoid stretching mode of relatively constant position [13]. However, the two higher-frequency modes decreased in the spectra of higher deuterates (Fig. 5) and can therefore be safely ascribed to the water bending, 2 leaving the band at 1617 cm 21 (1619 cm 21 at RT) for the n (CO) assignment. The band falls within the range of fourteen ionic rareearth saccharinates (1615±1618 cm 21 [46]), as con®rmed by X-ray diffraction for [Nd(o-sulfobenzimidate-O)(H2O)8] (sac)2´H2O [49], and corresponds to either ionic or more probably O-coordinated saccharinato entities in the structure of the praseodymium(III) compound. Correspondingly, it is red-shifted from the analogous modes of Zn(sac)2(im)2 (1669, 1662 cm 21), the ionic Mg(sac)2´7H2O (1660, 1627 cm 21), Sr(sac)2´4H2O (1635 cm 21 [29]), Na3(sac)3´2H2O (1635 cm 21) [13] and saccharinato ion in diluted 2

The small peaks at 1654 and 1636 cm 21 are very similar to incompensations seen from the higher-frequency side of the n (CO) band and are of the same nature.

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21

DMSO solution (1638 cm [21]). Due to the large ionic radius, the praseodymium ion might be coordinated by several saccharinato ligands as well as water molecules; nevertheless, the above comparison of the n (CO) bending frequencies with the values found in the metal saccharinates which were studied earlier, strongly suggests that the metal±saccharinato bonds in the studied compound are ionic in character. Inclusion of the n (CO) frequency value in the empirical spectra-structural correlation equation [15] yields a Ê for the C±O distance in value of 1.244 A Pr(sac)2Cl´6H2O. It is worth noting that, while the d (HOH) and n (CO) frequencies in the present case differ from those of the other rare-earth saccharinates, they are surprisingly similar to the mixed saccharinate±nitrate complex Eu2(sac)3(NO3)3 12H2O (1660, 1620 cm 21 [48]), pointing out that the higherfrequency band in the spectrum of the latter might also originate from water molecules and further studies are essential to ®rmly assign this. For the mixed europium complex, bidentate chelating mode ±N±C±O± of saccharin was postulated [47,48]. However, we consider that the internal strain inherent in the saccharinato sulfobenzimide structure and rather apparent from the deviant internal angles from their normal values 3 is unlikely to permit true coordination, even in case of large metal ionic radius. Bidentate coordination involving a small bridging intermediate, e.g. a coordinated water molecule as in the [Nd(o-sulfobenzimidate±O) (H2O)8](sac)2´H2O complex [49], therefore, would be a much more feasible pattern. The respective n (CO) frequency shifts can be solely explained by changing the ligand atom of the monodentate o-sulfobenzimide moiety or due to rather tentative empirical assignments. The ab initio theoretical studies of the saccharinato ion at various levels [13,21] have recommended the `outer-ring' SO2 stretchings, and particularly the antisymmetric mode [13] as highly pure vibrations, for correlation purposes. Customarily, a pair of strongest and relatively broad bands in the 1350±1100 cm 21 region of the spectra of saccharinates has been empirically assigned to the pair of n (SO2) modes. External (e.g. temperature) and internal (e.g. eventual partici3

This is especially the case with the internal angle at sulfur atom averaging 96.34(1.08)8 [12], a clearly distinct value from the normal one at 109.58 for tetrahedral coordination geometry.

Fig. 5. The carbonyl stretching region in the FT infrared spectra of (A) protiated and (B) deuterated (,50% deuterons/hydrons) analogues of bis(o-sulfobenzimidato)praseodymium(III) chloride hexahydrate at room temperature.

pation of the sulfonyl oxygen atoms in hydrogen bonds and/or in the metal coordination sphere) factors have been reported to control over the position of these modes [54]. The presence of a single pair of strong, deuteration-insensitive bands attributable to the n (SO2) modes in the IR spectrum of Pr(sac)2Cl´6H2O at 1251 and 1171 cm 21 (Fig. 4), supports the n (CO) assignment, indicating high similarity or probably, equivalence among the sulfonyl groups. The remaining sharp bands of medium intensity in this region could be individually assigned to benzenoid CC stretches located within the phenylene ring. Here, it may be noted that the sulfonyl

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Fig. 6. Thermoanalytical curves of bis(o-sulfobenzimidato)praseodymium(III) chloride hexahydrate in (A) air and (B) nitrogen atmosphere.

stretching region of the title compound resembles those [54] of the isomorphous ®rst-transition row saccharinates [M(sac)2(H2O)4]´2H2O from vanadium(II) to zinc(II) and cadmium(II), which include single-type trans-octahedrally arranged saccharinato ligands [37]. The small split between the antisymmetric and symmetric mode (80 cm 21) suggests that, compared to the structures of the previously studied metal saccharinates, the O±S±O angle in the structure

of the studied compound is rather small [54] and amounts around 112 to 1148. 3.3. Thermoanalytical measurements The thermal decomposition pathway of the complex presented in Fig. 6 (the respective data listed in Table 2) is well-de®ned, consisting of three stages. During a one-step dehydration of the complex in the

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Table 2 Thermoanalytical results (TG, DTG, DTA) of bis(o-sulfobenzimidato)praseodymium(III) chloride hexahydrate in air atmosphere Stage

1 2 3 a

Temperature range (8C)

DTGmax a (8C)

Removed group

Mass loss (%)

Total mass loss (%)

Found

Calcd

Found

98±189 383±535 536±632

145(1) 499(1) 588(2)

6H2O SO2 ?

16.50 23.35 27.30

16.66 ± ±

67.15

Calcd

70.36

Solid decomp. product

Color

[Pr(sac)2Cl] ± PrOCl

Yellow-green Yellow-green

Exothermic and endothermic processes are denoted by (2) and (1), respectively.

temperature interval 98±1898C, both the crystallization and coordinated water are released. The process is expectedly endothermic (DTA maximum at 1458C) and the dehydration mechanism is supported by the mass loss values. In this stage, the complex behaves similar to the isomorphous series of hexahydrated ®rst row transition metal saccharinates [M(sac)2 (H2O)4]´2H2O [19], but appears to be of greater thermal stability. The order of the dehydration process was found to be 0.5 and the respective activation energy amounts to 21 kJ mol 21, the latter value exceeding those of the isomorphous complexes. The resulting dehydrated structure is rather distorted and relaxes via a solid-state phase transition at 3788C, which was clearly observable in inert nitrogen atmosphere (Fig. 6(B)). The anhydrous complex is stable in air up to 3838C and afterwards adopts the characteristic two-step decomposition pattern of the saccharinato ligands; the respective intervals in the present case are 383±535 and 536±6328C. As concluded from earlier studies [55±57], preferred decomposition of the saccharinato residues in either oxidizing or inert atmosphere proceeds by initial release of SO2 and subsequent strong exothermic process leading to metal oxides or metals. The release of SO2 in the early stage of [Pr(sac)2Cl] decomposition was in situ followed by the decrease in the SO2 stretching intensity, in the vibrational spectrum. The strong exothermic peak at 5928C is associated with subsequent burning of the organic residue, ®nally leading to PrOCl at 6328C. Acknowledgements One of the authors (H.I.) would like to thank the Research Fund of Ondokuz Mayis University

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