Synthesis, characterization and molecular structures of the pyridinium trans-Bis(pyridine)tetrachlororuthenate(III) and pyridinium trans-(carbonyl)(pyridine)tetrachlororuthenate(III)

Article

Synthesis, Characterization and Molecular Structures of the Pyridinium trans-Bis(pyridine)tetrachlororuthenate(III) and Pyridinium trans-(carbonyl)(pyridine)tetrachlororuthenate(III)

Alzir A. Batistaª ^* , Sandra A. Onofreª, Salete L. Queirozª, Glaucius Oliva^b, Marcos R.M. Fontes^b,c, and Otaciro R. Nascimento^b

^aDepartamento de Química, Universidade Federal de São Carlos, C.P. 676, 13565-905 São Carlos - SP, Brazil

^bInstituto de Física de São Carlos, Universidade de São Paulo, C.P. 369,13560-970 São Carlos - SP, Brazil

^cDepartamento de Física e Biofísica, IB-UNESP, C.P. 510,18618-000 Botucatu - SP, Brazil

Received: April 7, 1997

Os complexos pyHtrans-RuCl₄(py)₂(1) and pyHtrans-RuCl₄(CO)(py)(2) foram sintetizados e cristalizam-se no grupo espacial P2₁/n, Z = 4 com a = 8.080(7), b = 22.503(7), c = 10.125(6) Å, b = 93.19(6)° para (1) e a = 7.821(1), b = 10.337(3), c = 19.763(3) Å, b = 93.07(1)° para (2). As estruturas foram resolvidas pelas técnicas de Patterson e diferenciais de Fourier e refinadas para R = 0.062 para (1) e R = 0.038 para (2). Em ambos os casos o Ru(III) encontra-se octaédricamente coordenado a quatro átomos de cloro, ao nitrogênio do anel piridínico ou ao carbono do monóxido de carbono. Outro grupo piridínico protonado, o qual forma o cátion, completa as estruturas cristalinas. Os espectros de absorção na região do UV-Vis apresentam três bandas: (1) 360 (e = 1180 M^-1 cm^-1), 441 (e = 3200 M^-1 cm^-1) e 532 nm (e = 400 M^-1 cm^-1); (2) 315(e = 1150 M^-1 cm^-1), 442 (e = 3170 M^-1 cm^-1) e 530 nm (e = 390 M^-1 cm^-1). As duas bandas de energias mais altas foram associadas com transições de transferência de ligante metal e a terceira banda, de mais baixa energia foi atribuída à transição d-d. Espectro de RPE a baixa temperatura confirmou a presença de Ru(III) paramagnéticamente ativo e é consistente com uma simetria axial para os complexos. A posição da banda de estiramento CO no complexo (2) é discutida em termos da retrodoação metal-CO.

The pyHtrans-RuCl₄(py)₂(1) and pyHtrans-RuCl₄(CO)(py)(2) complexes were synthesized and found to crystallize in space group P2₁/n, Z = 4 with a = 8.080(7), b = 22.503(7), c = 10.125(6) Å, b = 93.19(6)° for (1) and a = 7.821(1), b = 10.337(3), c = 19.763(3) Å, b = 93.07(1)° for (2). The structures were solved by Patterson and difference Fourier techniques and refined to R = 0.062 for (1) and R = 0.038 for (2). In both cases the Ru(III) ion is octahedrally coordinated to four co-planar chlorine atoms, the nitrogen of the pyridine rings or carbon from the carbon monoxide. Another protonated pyridine group, which forms the counter-cation completes the crystal structures. The UV-Vis absorption spectra show three bands: (1) 360 (e = 1180 M^-1 cm^-1), 441 (e = 3200 M^-1 cm^-1) and 532 nm (e = 400 M^-1 cm^-1); (2) 315(e = 1150 M^-1 cm^-1), 442 (e = 3170 M^-1 cm^-1) and 530 nm (e = 390 M^-1 cm^-1). The two higher energy bands were associated with ligand-to-metal charge transfer transitions and a third band at lower energy was assigned to a d-d transition. Low temperature EPR data confirmed the presence of the paramagnetically active Ru(III) and it is consistent with axial symmetry of the complexes. The position of the stretching CO band in complex (2) is discussed in terms of metal-CO backbonding.

Keywords: ruthenium, tetrachlororuthenate(III), X-ray structures

Introduction

During the last 30 years, since cis-diaminedichloroplatinum(II) was discovered as a potent tumor-inhibiting agent, intense research activity has been devoted to the field of antitumor-active metal complexes¹. Most of these efforts were concentrated on platinum as the central metal. Many platinum complexes were synthesized and more than a thousand were investigated in preclinical tests for antitumor activity. Furthermore it was clearly demonstrated that the relatively close pharmacological and toxicological behavior of most of these new derivatives compared well with the original cisplatin. Owing to these facts, it is necessary to search for non-platinum complexes, which may also exhibit tumor-inhibiting properties. So the development of complexes as an alternative to platinum metal inhibitors is of special interest and ruthenium compounds have been studied extensively for this purpose^2,3. A few years ago the ImHtrans-RuCl₄(Im)₂ was reported in the literature and its tumor-inhibiting properties were described⁴. This Ru(III) imidazole complex is active against colorectal tumors, reducing the tumor volume to about 20-10% and it is currently undergoing preclinical pharmacological tests^4,5. It has been reported that it binds to DNA and blocks its template-primer properties in DNA-polymerase catalyzed duplication⁶. Due to the increasing interest in this class of ruthenium(III) complexes , we started recently a research program to search for simple methods of synthesis of this kind of compounds. Consequently, the synthesis and characterization of several new complexes were undertaken⁷. In the present paper we report on the synthesis and characterization of pyHtrans-RuCl₄(py)₂ and pyHtrans-RuCl₄ (CO)(py).

Experimental

Solvents for the preparation of the complexes or measurements were chemically pure grade and were dried prior to use.

Preparations

Complex (1): pyHtrans-RuCl₄(py)₂

Commercial (Degussa) hydrated ruthenium trichloride (0.3 g 1.2 mmol), was dissolved in ethanol (10 mL) and a stream of carbon monoxide was passed through it, for 12 h, at room temperature. The red solution was allowed to stand in contact with air for 24 h and pyridine (7.6 mmol, Merck) dissolved in 3 mL of ethanol/1mL of 8 N HCl was added. The solution was stirred for ca. 3 h and the solution was allowed to stand for 2 days at room temperature. The volume of the solution was reduced to about 2 mL and diethyl ether was added to precipitate a red powder (yield 70%) which was filtered off, washed with ethanol and dried in a dessicator over CaCl₂. The red powder was recrystalized from dichloromethane/ether affording bright red orange crystals. Calc. for C₁₅H₁₆N₃Cl₄Ru: C, 37.44; H, 3.35; N, 8.73%. Found; C, 37.62; H, 3.28, N, 9.00%

Complex (2): pyHtrans-RuCl₄(CO)(py)

This complex was prepared according to the method previously described in the literature⁸ with some modifications. The red solution was obtained as described above. The pyridine (7.6 mmol) dissolved in 3 mL of ethanol was added and the mixture was refluxed for ca. 2 h. Addition of diethyl ether gave the complex in the form of an orange powder (yield 80%) which was filtered off, washed with ethanol and dried in dessicator over CaCl₂. The powder was recrystalized from dichloromethane/ether affording the product as bright orange crystals. Calc. for C₁₁H₁₁N₂O₁ Cl₄Ru: C, 30.72; H, 2.58; N, 6.51%. Found: C, 30.70; H, 2.80; N, 6.71%.

X-ray diffraction data

Complete data sets were collected on an Enraf-Nonius CAD-4 four cycle diffractometer, from flat prismatic crystals. Experimental details are given in Table 1. Cell dimensions and the orientation matrices were calculated by least-squares from 25 centered reflections in the range 10 < q < 19 for (1) and 7 < q < 19° for (2). Difraction intensities were measured by the w - 2q scan technique using a variable scan speed of 0.8-5.5° min^-1 for (1) and 1.7-5.5° min^-1 for (2) determined by a pre-scan at 5.5° min^-1. The intensity of one standard reflection was essentially constant over the duration of both experiments. Data were corrected for Lorentz, polarization and absorption effects, following the procedure of Walker and Stuart⁹.

Crystal structure determination and refinement

The determination and refinement of the structures were performed with the SHELX76¹⁰ system of programs. The structures were solved by standard Patterson and difference Fourier techniques and refined by full-matrix least-squares methods with anisotropic thermal parameters for the non-hydrogen atoms, with the exception of the pyH ring of structure (2) which, due to disorder, was refined as a rigid group with isotropic temperature factors for individual atoms. All hydrogen atoms were located on stereochemical grounds and included as fixed contributors with a common isotropic temperature factor of 0.08 Ų, with the exception of the H-atoms of the mobile pyH of structure (2) which were not included. Bonded H-atom scattering factors¹¹ and complex scattering factors¹² were employed for the remaining atoms. Figure 1a and 1b were drawn with the ORTEP program¹³ with all non-H atoms represented with 50% ellipsoids for the anisotropic thermal parameters.

List of H-atom positions, anisotropic thermal parameters and structure factors are available on request from the authors.

Spectroscopic measurements

IR spectra

Pellets were prepared from crystalline powder samples diluted in CsI. Measurements were performed on a Bomem-Michelson 102 spectrometer in the region 4000-200 cm^-1.

UV/Vis spectra

The electronic spectra were measured in CH₂Cl₂ solution (8 x 10^-5 mol/L) on a Varian DMS-100 spectrophotometer.

EPR measurements

EPR spectra were obtained from a polycrystalline powder sample, using a quartz tube, on a Varian E-109 spectrometer equipped with X band bridge at -150 °C Modulation amplitude: 9 gauss. Microwave power: 10 mW.

Magnetic susceptibility

Solution magnetic susceptibilities were measured by the Evans NMR method¹⁴, with a 200 MHz Bruker instrument, using CD₂Cl₂ solution at room temperature.

Electrical conductivity

Solution electrical conductivities were measured in nitromethane at 25 °C under anaerobic conditions using a Micronal conductivity bridge.

Results and Discussion

Selected interatomic bond distances are in Table 2 and interatomic angles are in Table 3. Final atomic parameters for non-H atoms are given in Table 4 and Table 5. Figure 1a and 1b are drawings of the complex pyHtrans-RuCl₄(py)₂ (1) and trans-pyHtrans-RuCl₄(CO)(py)(2), respectively. In complex (1) the Ru(III) ion is octahedrally coordinated to four coplanar chlorine atoms and two nitrogen atoms of the pyridine rings trans to each other. In complex (2) the Ru(III) ion is also octahedrally coordinated to four coplanar chlorine atoms and to the carbon atom of a CO group trans to a nitrogen of the pyridine ring. In both cases a protonated pyridine molecule is electrostatically bonded to the negatively charged Ru(III) complex, completing the crystal structures of the compounds.

The Ru-Cl and Ru-N bond lengths in the complexes (1) 2.353(2) and 2.082(8) Å and (2) 2.342(4) and 2.14(1) Å, respectively are comparable to those found for similar complexes: ImHtrans-RuCl₄(Im)₂ complex 2.349(1) and 2.079(3) Å⁴; 4-M-ImHtrans-RuCl₄(4-M-Im)₂ 2.364(2) and 2.087(6) Å⁴ and M-ImHtrans-RuCl₄(CO) (M-Im) (M-Im = 1-methylimidazole) 2.348(2) and 2.126(6) Å⁷. The Ru-N distances present in the above mentioned carbonyl complexes reflect the backbonding of the Ru-CO interactions, making the Ru-N length slightly longer when compared with those found for the related pyHtrans-RuCl₄(py)₂, ImHtrans-RuCl₄(Im)₂ and 4-M-ImHtrans-RuCl₄(4-M-Im)₂ compounds. The Ru-C distance for the complex (2) 1.87(2) Å can be appropriately compared with those of M-ImHtrans-RuCl₄(CO)(M-Im) 1.869(9) Å⁷. It is interesting to note that the species 4-NO₂ImRuCl₄(5-NO₂Im)₂ was reported to show similar bond distances found for the crystal structures of the Ru(III) complexes mentioned above¹⁵.

The strong absorption band in the IR spectrum at 2044 cm^-1 corresponding to the n_(CO) stretch, is shifted with respect to the free CO absorption band at 2143 cm^{-1 16}, indicating a small amount of metal-CO backbonding. The same behavior was found for the M- ImHtrans-RuCl₄ (CO)(M-Im) ⁷ complex where n_(CO) is 2037 cm^-1. The Ru-Cl stretching frequencies were also observed, at 316 cm^-1 and 328 cm^-1 for the complexes (1) and (2), respectively.

The electronic absorption spectra for the complexes with the pyridine ligands are similar to the complex with the methylimidazole ligand⁷ and show three bands:(1) 360 (e = 1180 M^-1 cm^-1), 441 (e = 3200 M^-1 cm^-1) and 532 nm (e = 400 M^-1 cm^-1); (2) 315(e = 1150 M^-1 cm^-1), 442 (e = 3170 M^-1 cm^-1) and 530 nm(e = 390 M^-1 cm^-1). The two higher energy transitions for these complexes can be assigned as ligand-to-metal charge transfer (LMCT) transitions. On the basis of the observed C_4v symmetry of the complexes (1) and (2), the bands at lower energies can be tentatively assigned to the d-d transitions ¹A_l ¹B₂, according to the energy-levels proposed by Bacci and co-workers¹⁷.

The EPR spectra of the complexes measured at X band and -150 °C show axial symmetries in the structures with g_^ = 2.2591(3) and 2.2585(3) and g_|| =1.9431(3) and 1.9433(3), for compound (1) and (2), respectively.

The solution m_eff values for the complexes (1) and (2) yielded spin-only values close to 1.7 m_B being consistent with one unpaired electron per atom of ruthenium. The molar conductivity data obtained for these complexes in nitromethane, close to 70 W^-1 cm² mol^-1, is consistent with 1:1 electrolytes¹⁸.

Acknowledgments

We thank FAPESP, CAPES, CNPq and FINEP for financial support of this work.

FAPESP helped in meeting the publication costs of this article

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Publication Dates

• Publication in this collection
  23 July 2010
• Date of issue
  1997

History

• Received
  07 Apr 1997