J. Chil. Chem. Soc, 53, N° 1 (2008)

A NEW LINDQVIST-TYPE HEXAMOLYBDATE CLUSTER FUNCTIONALIZED WITH THE jt-DONOR LIGAND 4-BROMO-2,6-DIMETHYLPHENYLIMIDO. SPECTROSCOPIC, ELECTROCHEMICAL AND STRUCTURAL STUDIES

MARCELA CORTÉS^a, MAURICIO FUENTEALBA^b, CAROLINA MANZUR^a, DAVID CARRILLO^a,*

^aLaboratorio de Química Inorgánica, Instituto de Química,Pontificia Universidad Católica de Valparaíso, Avenida Brasil 2950, Valparaíso, Chile,
e-mail: dcarrill@ucv.cl

^bLaboratorio de Cristalografía, Departamento de Física, Universidad de Chile, Blanco Encalada 2008, Santiago, Chile

ABSTRACT

A new ionic organic-inorganic hybrid complex of formula («-Bu₄N)₂[Mo_(¡0₁₈(NR)]¹/2Me₂CO, R = -C₆H₂-2,6-Me₂-4-Br, (w-Bu₄N)₂[l]-i4Me₂CO, has been prepared in acetonitrile by reacting tetrabutylammonium a-octamolybdate, («-Bu₄N)₄[0t-Mo₈O₂₆], with 2,6-dimethyl-4-bromoaniline hydrochloride, 2,6-Me₂-4-Br-C₆H₂NH₂HCl, using N,N'-dicyclohexylcarbodiimide, (C₆H_U)N=C=N(C₆H₁₁), as dehydrating agent. This complex, formulated as («-Bu₄N)₂[l]¹/2Me₂CO, contains a C-Br group which can be functionahzed for constructing novel hybrid materials. The complex was fully characterized by IR, UV-Vis, 'H- and "C-NMR spectroscopies, and authenticated by single crystal X-ray diffraction analysis. The asymmetric unit contains two crystallographically independent anions, [l]²", differing by the orientation of the phenyl ring relative to the hexamolybdate skeleton, and one molecule of acetone. Both molecules differ in the angles Mo(l)-N(l)-C(l)= 172.2(6) and Mo(7)-N(2)-C(9)= 175.6(6)°. These angles, near to 180°, indicate the presence of a Mo=N triple bond.

Keywords: areneimido-derivative, phenylimido-derivative, Lindqvist-type hexamolybdate, polyoxometalate, crystal structure, conformational isomers.

INTRODUCTION

Since the pioneering paper of Maatta et al.¹ describing the synthesis and X-ray structural determination of the first Lindqvist-type organoimido-derivative of hexamolybdate formulated as [Mo₆0₁₈(NC₆H₄Me-p)]²", a number of new organoimido-derivatives of hexamolybdate,²"¹⁹ hexatungstate,²⁰ and pentatu ngstenmolybdate,²¹ have been synthesized, spectroscopically characterized and, the majority, structurally authenticated by X-ray diffraction analysis. Very recently, the stability, bonding character, electronic properties, first hyperpolarizabilities of this type of complexes have also been studied by Density Functional Theory (DFT).²²"²⁴ Likewise, several topics concerning the progresses in polyoxometalate chemistry have been published²⁵" and, particularly, that dealing with derivatized polyoxometalates including organic and organometallic entities.^25b Among these complexes, the chloro-¹², bromo-¹²¹⁸ and iodoareneimido hexamolybdates⁷"¹⁰¹³"¹⁵ and iodoareneimido pentatungstenmolybdate²¹ seems to be of great importance considering their potential applications as precursors for the synthesis of new organic-inorganic hybrids containing additional organic^7,8101415 or organometallic¹³ groups, or two similar clusters linked by an extended ii-conjugated organic spacer.⁸"¹⁰¹⁵ These halogenated precursors have successfully been applied for the synthesis of organoimido-hexamolybdates as polymer pendants⁵ and main-chain polymers.⁹¹⁵ In this paper we report the synthesis, spectroscopic characterization, electrochemical studies and the X-ray crystal and molecular structure of the new organic-inorganic hybrid bearing the electron-withdrawal bromo group in a remote position: («-Bu₄N)₂[Mo₆0₁₈(NC₆H₂-2,6-Me₂-4-Br)]¹/₂Me₂CO, («-Bu₄N)₂[l]¹/2Me₂CO. The X-ray structure of a similar hybrid compound of formula («-Bu₄N)₂[Mo₆0₁₈(NC₆H₄-4-Br)], has previously been described in the literature¹⁸.

EXPERIMENTAL SECTION

General remarks

Solvents were dried and distilled under dinitrogen by standard methods prior to use. Reagents were purchased from commercial suppliers and used without further purification. («-Bu₄N)₄[a-Mo₈0₂₆] was synthesized according to published procedures.²⁶ Microanalytical data were obtained on a Perkin Elmer model 2400 elemental analyzer. IR spectra were obtained as KBr disks on a Perkin Elmer model 1600 FT-IR spectrophotometer, in the range of 4000-450 cm"¹. Electronic spectra were recorded in MeCN and DMSO solutions with a Spectronic, Genesys 2, spectrophotometer. ¹H- and "C-NMR spectra were acquired at 297 K on a multinuclear Bruker AC 400 spectrometer in acetone-d . All NMR spectra are reported in ppm (8) relative to tetramethylsilane, with the residual solvent proton resonances and carbon resonances used as internal standards. Coupling constants (J) are reported in Hertz (Hz), and integrations are reported as number of protons. The following abbreviations are used to describe peak patterns: br= broad, s=singlet, d=doublet, t=triplet, m=multiplet. Electrochemical measurements were performed using a Radiometer Analytical model PGZ 100 all-in-one potentiostat, using a standard three-electrode setup with a platinum electrode, platinum wire auxiliary electrode and Ag/AgCl as the reference electrode. DMF solutions were 1.0 mM in the compound under study and 0.1 M in the supporting electrolyte «-Bu₄N⁺PF₆" with the voltage scan rate = 100 mV s"¹. Melting point was determined in evacuated capillary and was not corrected.

Synthesis of (n-Bu₄N)₂[l]¹/₂Me₂CO

A mixture of 6.45 g (3.0 mmol) of («-Bu₄N)₄[a-Mo₈0₂J, 1.42 g (6.0 mmol) of 2,6-dimethyl-4-bromoaniline hydrochloride, 2,6-Me₂-4-Br-C₆H₂NH₂HC1, and 2.11 g (10.2 mmol) of N,N'-dicyclohexylcarbodiimide, (C₆H_n)N=C=N(C₆H_n)⁷, were dissolved in 50 niL of acetonitrile and refluxed for 12 h under N . Then, the reaction mixture was allowed to stand at room temperature. The white solid, corresponding to the dicyclohexylurea, was filtered off. The filtrate was concentrated under vacuum and the red colloidal residue was dissolved in acetone and then layered with ethanol and allow to stand at -18°C A red crystalline solid was deposited, which was filtered off. washed with diethyl ether, and dried under vacuum. Suitable single crystals were chosen of this crop for an X-ray diffraction structure determination. Yield 3.34 g (72% based on Mo), m. p. 236°C Anal. Caled CH.RrMo.NO,Mr= 3151.32 gmol"¹: caled C, 31.63; H, 5.31; N, 2.67. Found C, 31.54; H, 5.26; N, 2.54. UV/Vis (Mei~~ ): %_m% (log E/LmoUcm"¹) = 254 (4.56), 274sh (4.47), 358 (4.37). IR (KBr): ^V = 2961s, 2920m, 2874m (C-H); 975m (Mo-N); 950vs (Mo-O_t); 790vs (Mo-O-Mo) cm"¹. ^:H NMR (400 MHz, CD₃COCD₃): 8 = 0.97 (t, CH₃, 24H); 1.44(m,Cf/₂, 16H); 1.80(m,C#₂, 16H); 2.17(s, CH_y 6H); 3.43 (t, N-Cf/₂, 16H); 7.23 (s, ArH, 2H). ¹³C NMR (400 MHz, CD₃COCD₃): 6 = 13.92 (CH₃); 18.38 (CH₂); 20.38 (CH₂); 24.52 (Me₂-2,6); 59.36 (CH₂); 120.56 (C-¥); 130.39 (C-5); 140.34 (C-2); 153.18 (C_¡pJ.

Structure determination of (n-Bu₄N)₂[l] !4Me₂CO

Complete details of the crystal, X-ray data collection, and structure solution are provided as Supporting Information. Semi-empirical corrections, via \|/-scans, were applied for absorption. Intensity data were collected on a Bruker Smart Apex diffractometer equipped with a bidimensional CCD detector using graphite monochromated Mo-Ka radiation (X=0.71073 Á). The diffraction frames were integrated using the SAINT package²⁷, and corrected for absorption with SADABS²⁸. The structure was solved using XS in SHELXTL-PC²⁹ by direct methods and completed (non-H atoms) by difference Fourier techniques. Refinement was performed by the full-matrix least-squares method based on P. Anisotropic thermal parameters were refined for all non-hydrogen atoms. Hydrogen atoms were placed in their calculated positions, assigned fixed isotropic thermal parameters and allowed to ride on their respective parent atoms.

RESULT AND DISCUSSION

Synthesis and spectroscopic characterization

It is well known that the synthesis of organoimido-derivatives of polyoxometalates in reasonable yield using anilines containing an electron- withdrawing group in the phenyl ring is very difficult. The presence of this type of group, v. gr., chloro or bromo, provokes a deactivation of the amine group and, consequently, a diminution of its nucleophilic ability. Fortunately, Peng et al. have discovered that the addition of the dehydrating agent dicyclohexylcarbodiimide (DCC), facilitates the reaction of hexamolybdate with anilines, giving very good yields.⁷ Recently, this group has described two new organoimido-derivatives of hexamolybdate using 2-, 3- and 4-chloro-¹² and 4-bromoanilines.¹²¹⁸ The reaction of 2,6-Me₂-4-Br-aniline as hydrochloride with tetra-butylammonium a-octamolybdate, («-Bu₄N)₄[0t-Mo₈O₂₆], and DCC in refluxing acetonitrile, has effectively allowed the synthesis of («-Bu₄N)₂[l]¹/2Me₂CO in 72% yield. The role of the proton in the enhancement of the electrophilic ability of DCC toward the terminal oxo atom of the Mo=0 group and in the conversion of the [o-Mo₈OJ⁴' into [Mo₆0₁₉]²" through a degradation and re-assembly process, has recently been discussed.^9,18

Complex («-Bu₄N)₂[l]¹/2Me₂CO was obtained as red and air stable crystalline solid, and was fully characterized by 'H- and ¹³C-NMR, IR and UV-vis spectroscopy (see Experimental Section). The crystalline and molecular structure of this complex was determined by single crystal X-ray diffraction analysis (vide infra). The more salient features of the 'H-NMR spectrum of this compound are (i) the singlet resonance in the at 2.17 ppm corresponding to the proton resonances of the methyl groups occupying the symmetrical 2,6-positions and the singlet at 7.23 ppm corresponding to the resonances of protons placed in the symmetrical 3,5-positions of the phenyl ring and (ii) the four groups of signals corresponding to the proton resonances of the two «-Bu₄N⁺ cations. Likewise, consistent with the proposed structure, the proton-decoupled "C-NMR spectrum of complex («- Bu₄N)₂[l]¹/2Me₂CO exhibits the expected characteristic sharp resonances of the phenyl ring carbon atoms and the resonances of the methyl carbon atoms placed in the 2,6-positions (see Experimental Section). On the other hand, the IR spectrum of («- Bu₄N)₂[l]¹/2Me₂CO is characterized by the very strong absorption bands at 950 and 790 cm"¹ attributed to the Mo=0 and Mo-O-Mo stretching modes,¹¹¹³¹⁹ respectively. A strong shoulder near 975 cm"¹, attributed to the Mo=N stretching mode,¹¹¹"^1319,30 proves this complex to be the mono-organoimido substituted derivative. Finally, Figure 1 shows the UV-vis absorption spectrum of («-Bu₄N)₂[l]¹/2Me₂CO in acetonitrile (a) and, for the sake of comparison, the spectrum of («-Bu₄N)₂[Mo₆0₁₉] in the same solvent (b). The lowest-energy electronic transition at 325 nm in the precursor [Mo₆0₁₉]²" has been attributed to a ligand-to-metal charge-transfer transition associated with the terminal oxygen non-bonding ii-type HOMO to the molybdenum ii-type LUMO.⁷¹⁸ In complex [l]²" the ligand-to-metal charge transfer transition band, observed at 358 nm, exhibits two important features relative to the spectrum of («-Bu₄N)₂[Mo₆0₁₉]: (i) its intensity becomes nearly three times, and (ii) its wavelength is bathochromically shifted by 33 nm. These observations indicate that the N-Mo ii-bonding is strongly delocalized through the organic conjugated backbone.⁷¹³¹⁸ The spectrum of («-Bu₄N)₂[l]¹/2Me₂CO registered in DMSO does not exhibits hipso- or bathochromic effects.

Crystal structure

A summary of X-ray crystal data for («-Bu₄N)₂[l]¹/2Me₂CO is presented in Table 1. Table 2 contains selected bond lengths and angles. The cluster crystallizes in the triclinic space group P-l. The asymmetric unit contains two conformational isomers A and B, generated by a different orientation of the phenyl ring (see Figure 2). From the structural point of view, both crystallographically independent isomers differ by the rotation of the phenyl ring by 12.1(2)° relative to the plane including the Mo(l)-0(2)-Mo(2)-0(15> Mo(6)-0(17)-Mo(4)-0(4) atoms in the case of the isomer A, and by 34.5(2)° relative to the plane including the Mo(7)-0(21)-Mo(9)-0(34)-Mo(12)-0(32> Mo(ll)-0(19) atoms in the case of the isomer B (see Figure 3). Similar examples have been described for the isoelectronic imido-hexamolybdate clusters [Mo₆0₁₈(NC₆H₂-2,6-Me₂-4-I)]²" and [Mo₆0₁₈(NC₆H₂-2,6-Me₂-4-C=CH)]²".⁷ Although the majority of the molecular parameters of structures A and B are nearly the same, both structures differ in the angles Mo(l)-N(l)-C(l) and Mo(7)-N(2)-C(9) by 3.4°. These values, near to 180°, and the short Mo-N bond distances of 1.720(6) and 1.723(6) A for the anions A and B, respectively, are in good accord with the presence of a triple bond in the Mo=N fragment.³¹ On the other hand, the C-Br bond lengths of isomer A and B, 1.889(10) and 1.888(9) Á, respectively, are similar to the average bond length, 1.897 Á, calculated in aromatic bromine compounds. As distinguished from [Mo₆0₁₈(NC₆H₄-4-Br]²",¹⁸ complex [l]²" does not exhibits dimerization.

Finally, as it has been observed in arylimido-derivatives of hexamolybdate, when the arylimido group substitutes an oxygen atom of the [Mo₆0₁₉]²" precursor the bond lengths of Mo-O do not change significantly, however, the Mo(l)-O_b are lengthened and that of Mo(l)-O_c²² is shortened. On the other hand, the Mo(l)-0 is always shorter than that of Mo(6)-0 . In the case of complex (n-Bu₄K)₂[iy/Me₂CO, the Mo(l)-0(13) bond length, in the conformational isomer A, is shorter than that of Mo(6)-0(13) by 0.155 , while in the conformational isomer B this difference is 0.135 (see Tables 2). Table 3 compares the structural features of arylimido-derivatives of hexamolybdate. The influence of the substituent groups on the phenyl rings seems to be weak, so that a meaningful structural correlation between the bond lengths and the Hammett constants, o -X, could not be established.

Electrochemistry

In DMF solution, Cyclic Voltammetry of («-Bu₄N)₂[l]-!/2Me₂CO exhibits two irreversible one-electron reduction processes at Ep = -0.785 and -0.890 V (ys Ag/AgCl). Corresponding values for [Mo₆0₁₈(NC₆H₄-4-Me)]²- are -0.52 and-1.09 V, whereas for the [Mo₆0₁₉]²" are -0.39 and-1.12 V.¹ The first process can be attributed to the reduction of the hexamolybdate fragment of the [l]²" complex to afford the reduced anionic species [l]³". This potential is more negative than those of [Mo₆0₁₈(NC₆H₄-4-Me)]²- and [Mo₆0₁₉]²" which indicates that the electron-accepting ability of the bromo group has been compensated by the two methyl groups. Under the same experimental conditions, ferrocene exhibits a reversible process atE'A = +0.566 V (AE = 68 mV).

CONCLUDING REMARKS

We have successfully prepared and characterized by spectroscopic methods and by X-ray diffraction analysis the new functionalized Lindqvist-type hexamolybdate cluster containing an arylimido moiety bearing two methyl substituent groups in the 2,6-positions and an electron-withdrawing bromo group in the remote 4-position of the phenyl group, (ra-Bu₄N)₂[Mo₆0₁₈(NC₆H₂-2,6-Me₂-4-Br)]. From the synthetic point of view, the C-Br moiety of this new precursor may be useful as an additional reaction site in this type of cluster for a further functionalization through a carbon-carbon coupling reaction,⁸"¹⁰¹³ and, potentially, with nucleophilic species to afford new molecular or polymeric hybrid materials._

From the structural point of view, both conformational isomers A and B observed in the asymmetric unit of (w-Bu N) [l]-i4Me CO differ only by (i) the different rotation of the phenyl rings relative to the hexamolybdate skeleton, and (ii) the angles Mo(l)-N(l)-C(l)= 172.2(6) and Mo(7)-N(2)-C(9)= 175.6(6), which prove the presence of a Mo=N triple bond.

Supplementary materials

Crystallographic data (excluding structure factors) for the structure reported in this paper, (w-Bu N) [l]-i4Me CO, have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-631809. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).

ACKNOWLEDGEMENTS

Financial support from the Fondo Nacional de Desarrollo Científico y Tecnológico, FONDECYT, Chile, grant no. 1060490 (D. C. and C. M.) and the Pontificia Universidad Católica de Valparaíso, are gratefully acknowledged.

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(Received: 4 October 2007 - Accepted: 18 January 2008)