Abstracts

ARTICLE

Novel zwitterionic oxorhenium(V) complexes: synthesis, characterization and crystal structure of [ReOX₂(Hdhp)(PPh₃)] (X = Cl, Br; H₂dhp = 2,3-dihydroxypyridine)

Elizeu J. de Souza^I; André G. de A. Fernandes^I; Victor M. Deflon^* * e-mail: deflon@unb.br ^{, I}; Karl E. Bessler^I; Sebastião S. Lemos^I; Alzir A. Batista^II; Javier Ellena^III; Eduardo E. Castellano^III; Ulrich Abram^IV; Adelheid Hagenbach^IV

^IInstituto de Química, Universidade de Brasília, 70919-970 Brasília – DF, Brazil

^IIDepartamento de Química, Universidade Federal de São Carlos, 13565-905 São Carlos – SP, Brazil

^IIIInstituto de Física de São Carlos, Universidade de São Paulo, 13560-970 São Carlos – SP, Brazil

^IVInstitut für Chemie, Freie Universität Berlin, D-14195 Berlin, Germany

ABSTRACT

Two novel zwitterionic oxorhenium(V) complexes, [ReOCl₂(Hdhp)(PPh₃)] (1) and [ReOBr₂(Hdhp)(PPh₃)] (2) (H₂dhp = 2,3-dihydroxypyridine), were synthesized and characterized by infrared spectroscopy, ¹H and ³¹P nuclear magnetic resonance, elemental analysis and crystal and molecular structure determination by X-ray diffraction on single crystals. Both complexes show distorted octahedral coordination geometry, with the halide ligands arranged in equatorial cis positions, the triphenylphosphine ligand in a trans position to one of the halides and the Hdhp^– ligand coordinated in a bidentate form through its oxygen atoms, one in trans position to the oxo-ligand and the other in trans position to the second halide. The nitrogen atom of this ligand is protonated. Compounds 1 and 2 show quite different crystal packing, both influenced by hydrogen bonds of the types N–H×××X (X = Cl, Br) and N–H×××O.

Keywords: oxorhenium(V), zwitterionic complexes, 2,3-dihydroxypyridine

RESUMO

Dois novos complexos zwitteriônicos de oxorrênio(V), [ReOCl₂(Hdhp)(PPh₃)] (1) e [ReOBr₂(Hdhp)(PPh₃)] (2) (H₂dhp = 2,3-dihidroxipiridina), foram sintetizados e caracterizados por espectroscopia de absorção no infravermelho, ressonância magnética nuclear de ¹H e ³¹P, análise elementar e determinação da estrutura cristalina e molecular por difração de raios X em monocristais. Os complexos apresentam geometria de coordenação octaédrica bastante distorcida, com os dois ligantes haletos arranjados em posições cis equatoriais, o ligante trifenilfosfina em posição trans a um dos haletos e o ligante Hdhp^– coordenado de forma bidentada através de seus átomos de oxigênio, sendo um em posição trans ao ligante oxo e o outro em posição trans com relação ao outro haleto. Este ligante tem seu átomo de nitrogênio protonado. Os compostos 1 e 2 apresentam empacotamento cristalino bastante diferente, influenciado em ambos os casos por ligações de hidrogênio intermoleculares dos tipos N–H×××X (X = Cl, Br) e N–H×××O.

Introduction

The ability of 2,3-dihydroxypyridine (H₂dhp) to act as a chelating ligand is well known. Previous X-ray diffraction studies showed a bidentate coordination via the oxygen atoms, as Hdhp^–, in complexes with the trivalent ions Al³⁺, Cr³⁺ and Fe³⁺. The pyridine nitrogen atoms in these examples remain protonated, resulting in zwitterionic structures.^1,2 To the best of our knowledge no rhenium complex was reported so far involving the Hdhp^– anion as a ligand. In a recent work we described new zwitterionic rhenium complexes containing 2-hydroxypyridine as ligand.³

This work describes the synthesis, characterization and the X-ray crystal structure of novel rhenium complexes with Hdhp^– ligands. The zwitterionic [ReOX₂(Hdhp) (PPh₃)] (X = Cl, Br) complexes contain monoanionic, chelating Hdhp^- ligands, which are coordinated via their oxygen atoms. They formally possess a positive charge at the protonated nitrogen atom and a negative charge at the rhenium(V) center.

Experimental

Reagents and apparatus

The rhenium starting complexes [ReOX₃(PPh₃)₂] (X = Cl, Br) were prepared as previously described.⁴ 2,3-Dihydroxypyridine was used as purchased (Aldrich) without further purification. Solvents of analytical grade were degassed by purging with argon for about 15 minutes prior to use. Melting points were measured on a Melt-Temp II apparatus. Microanalytical data for C, H and N were obtained on a CHNS analyzer, FISONS model EA 1108. IR spectra were recorded on a BOMEM MICHELSON FT BM 102 spectrophotometer, using CsI pellets, within the 4000-200 cm^-1 range. NMR spectra were recorded at room temperature, in CD₂Cl₂ solutions, on a Varian Mercury Plus spectrometer, 7.05 T, operating at 300.07 MHz for ¹H and 121.47 MHz for ³¹P. The ¹H spectra were internally referenced to TMS and ³¹P{¹H} spectra were externally referenced to H₃PO₄ (85%, d 0). The data collections for crystal structure determinations were performed on a NONIUS KAPPA CCD diffractometer, equipped with Mo-K_a radiation (71.073 pm) and graphite monochromator, applying standard procedures.

Preparation of [ReOCl₂(Hdhp)(PPh₃)] (1)

H₂dhp (33.3 mg, 0.30 mmol) was added to a suspension of [ReOCl₃(PPh₃)₂] (249.9 mg, 0.30 mmol) in degassed dichloromethane (25 mL). The mixture was stirred and heated to reflux for 3 h. After cooling, the green precipitate was filtered off and washed with dichloromethane followed by hexane. Yield: 92% (176 mg, 0.27 mmol). Melting point 178-181 °C (dec.). Anal. Calc. for C₂₃H₁₉NO₃Cl₂ PRe (645.50 g mol^-1): C, 42.80; H, 2.97; N, 2.17%. Found: C, 43.09; H, 3.16; N, 2.59%.

IR (CsI) n_max/cm^-1: 3266 n(N-H), 1627 n(C=O), 1599 and 1542 n(C=C + C=N), 1186 n(C-O), 1482 and 1435 n(C=C, PPh₃), 1096 n(P-C), 692 g(ring C₆H₅, PPh₃), 978 n(Re=O), 335 n(Re-Cl). ¹H NMR: d 6.166 (t, J 7,5 Hz, 1H, Hdhp^–), 6.599 (d, J 7.5 Hz, 1H, Hdhp^–), 6.765 (m, 1H, Hdhp^–), 7.35 – 7.73 (m, 15H, PPh₃), and 10.7 (br, 1H, NH). ³¹P{¹H} NMR (ppm, CD₂Cl₂): -21.06 (s, Re-PPh₃).

Preparation of [ReOBr₂(Hdhp)(PPh₃)] (2)

The reaction was conducted as described for 1, but using [ReOBr₃(PPh₃)₂] (298.8 mg, 0.30 mmol). After the reflux time, the partially precipitated product was separated by filtration and washed with dichloromethane and hexane. Addition of more hexane (10 mL) and storing overnight at -15 °C resulted in more product. Overall yield: 80% (176.6 mg, 0.24 mmol). Melting point 169-172 °C (dec.). Anal. Calc. for C₂₃H₁₉NO₃Br₂ PRe (734.40 g mol^-1): C, 37.62; H, 2.61; N, 1.91%. Found: C, 38.67; H, 2.74; N, 1.90%.

IR (CsI) n_max/cm^-1: 3224 n(N-H), 1627 n(C=O), 1605 and 1546 n(C=C + C=N), 1197 n(C-O), 1482 and 1435 n(C=C, PPh₃), 1097 n(P-C), 693 g(ring C₆H₅, PPh₃), 978 n(Re=O), 273 n(Re-Br). ¹H NMR: d, 6.219 (t, J 7.5 Hz, 1H, Hdhp^–), 6.651 (d, J 7.5 Hz, 1H, Hdhp^–), 6.900 (m, 1H, Hdhp^–), 7.36 – 7.90 (m, 15H, PPh₃), and 11.1 (br, 1H, NH). ³¹P{¹H} NMR: d -20.60 (s, Re-PPh₃).

Crystal structure determinations

Suitable single crystals of 1 and 2 were obtained in the form of green needles by crystallization from dichloromethane. The cell constants were calculated from 33906 reflections for 1 and 77905 reflections for 2. Direct methods⁵ were used for the solution of the structures. Except for the hydrogen atoms, which were calculated at idealized positions, all other atoms were refined with anisotropic displacement parameters.⁶ Complex 1 crystallizes in the trigonal crystal system, space group , with one complex molecule in the asymmetric unit. Complex 2 crystallizes in the triclinic system, space group , with two complexes in the asymmetric unit. This compound presented radiation damage which prevented the acquisition of an ideally complete data set. Additional information on the crystal structure analyses is given in Table 1.

Results and Discussion

Synthesis of the complexes

Complexes 1 and 2 are formed in good yields by reactions starting from [ReOX₃(PPh₃)₂] (X = Cl, Br) and H₂dhp, with elimination of HX and PPh₃, after 3 h of stirring under reflux.

[ReOX₃(PPh₃)₂] + H₂dhp ® [ReOX₂(Hdhp)(PPh₃)] + HX + PPh₃ (1)

The complexes were isolated as green precipitates of good purity and could be obtained also in the crystalline form by crystallization from their dichloromethane solutions. The air stable compounds are moderately soluble in dichloromethane, but only slightly soluble in chloroform.

Spectroscopic characterization

The IR spectra of 1 and 2 are very similar in the range between 4000 and 400 cm^-1, which excludes the absorption bands related to n(Re-X) (X = Cl in 1 and Br in 2), indicating that they have an analogous coordination sphere. The n(Re=O) bands appear at 978 cm^-1 for both complexes. The O,O-coordination mode of the Hdhp^– ligand is indicated by shifts of the n(C=O) (1627 cm^-1 for 1 and 2) and n(C-O) bands (1186 cm^-1 for 1 and 1197 cm^-1 for 2) with respect to the values found in uncoordinated H₂dhp (1663 cm^-1 and 1188 cm^-1). The n(C=C + C=N) bands are found at 1599 cm^-1 and 1542 cm^-1 in 1 and at 1605 and 1546 in 2, respectively. The n(N-H) bands at 3266 cm^-1 for 1 and at 3224 cm^-1 for 2 are consistent with the zwitterionic structure of the complexes. The presence of the triphenylphosphine ligands in the complexes is evidenced by the presence of characteristic bands at 1482 cm^-1 and 1435 cm^-1 (n(C=C)), and 1096 cm^-1 and 1097 cm^-1 (n(P-C)) for 1 and 2, respectively. The n(Re-Cl) band appears at 335 cm^-1 for 1, while a band at 273 cm^-1 was tentatively assigned for n(Re-Br) for 2.

The ¹H-NMR spectra of 1 and 2 (for details see the Experimental Section) are consistent with the structures determined in the solid state. The ³¹P{¹H}-NMR spectra show predictable singlet peaks for the phosphorus donor atoms at -21.06 ppm and -20.60 ppm for 1 and 2, respectively.

Crystal structures of 1 and 2

Complexes 1 and 2 possess a similar arrangement of the ligands around the hexacoordinate rhenium(V) centers. The coordination spheres of the rhenium atoms are distorted octahedra, each one formed by one oxo ligand, two halides (Cl^– for 1 and Br^– for 2), a triphenylphosphine and two oxygen atoms of the Hdhp^- ligand, which coordinates as a O,O-chelate. The pyridine nitrogen atom of the chelating ligands is protonated. The halides have a cis arrangement to each other. The triphenylphosphine ligand is in trans position to one of the halides. One of the oxygen donor atoms from Hdhp^– is located trans to the oxo ligand, while the other is trans to the second halide. The molecular structure of 1 is shown in Figure 1. The molecular structures of the two independent molecules of 2 are virtually identical and essentially the same as 1, and are therefore not shown here. Selected bond lengths and angles for both compounds are given in Table 2. The molecular labeling scheme of 1 has also been adopted for the bromo complex.

Two crystal structures of complexes with ReO₃Cl₂P coordination spheres, in similar modes as observed in 1, have been reported previously, namely [ReOCl₂(acac)(PPh₃)]⁷ (3) and [ReOCl₂(benzac)(PPh₃)]⁸ (4) (acac^– = acetylacetonate, benzac^– = benzoylacetonate).

As in 1, in 4 the chlorine atoms are disposed in cis positions, while the structure of 3 presents these ligands in trans positions. No crystallographic data were found in the literature for a complex with a ReO₃Br₂P coordination sphere, as observed in 2.

As observed in 4, the Re-Cl bond in 2 is longer for the chloro ligand trans to PPh₃. This results from the trans-labilizing effect of the phosphine ligand. The Re-O bonds trans to the oxo ligands are shorter than those trans to the chloro ligands in 1 and in 4. This phenomenon is also observed in other rhenium complexes with similar coordination spheres and is assigned to delocalization of p-electron density from the oxo bonds to the trans-situated Re–O bonds.^9-11 The Re–O and Re–P bond lengths in 2 are similar to the values observed in 1. The distances between the rhenium atoms and the oxo ligands correspond to values normally found for Re=O double bonds in oxorhenium(V) complexes.⁹

The bond lengths in the Hdhp^- ligands in 1 and 2 do not differ significantly from the values found for the uncoordinated H₂dhp, which is also protonated at the nitrogen atom.¹ The following resonance structures (a, b and c) should best represent the structure of the Hdhp^– ligands in 1 and 2.

The C(1)-O(1) bond is expectedly shorter than C(2)-O(2), showing a larger double bond character. This results from the contribution of the resonance structure a. The N-C(1) and N-C(5) distances, however, are similar and have considerable double bond character, showing that the zwitterionic resonance structures b and c also give significant contributions to the resonance hybrid.

Crystal packings in 1 and 2 are quite different, being apparently influenced by the type of intermolecular H-bonds involved in each structure. The trigonal structure of 1 contains channels formed by groups of six molecules, as shown in Figure 2, which are arranged in a cyclic form by N–H×××Cl hydrogen bonds. No solvent molecules were found in these voids. In the triclinic structure of 2, N–H×××Br and N–H×××O H-bonds give rise to a network of molecules, as seen in Figure 3.

Conclusions

The zwitterionic complexes 1 and 2 can be prepared in good yields and purities following a relatively simple procedure. A distorted octahedral ReO₃X₂P coordination (X = Cl in 1 and Br in 2) is observed for them. The crystal structures of the compounds show different kinds of complex molecular packing, influenced by intermolecular hydrogen bonds.

Acknowledgments

This work was supported by CNPq, FAPESP and FINEP.

Supplementary Information

Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary material (deposition numbers CCDC 610190 and 610191 for 1 and 2, respectively). Copies of the data can be obtained, free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK ; fax: +44 1223 336033 ; or e-mail: deposit@ccdc.cam.ac.uk).

The ellipsoid representation of 2 with the thermal ellipsoids representing 50% probability is available free of charge at http://jbcs.sbq.org.br, as PDF file.

Received: June 12, 2006

Published on the web: November 14, 2006

FAPESP helped in meeting the publication costs of this article.

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