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vol.45 número4NEUTRAL COMPLEXES OF SILVER HALIDES AND TETRAMETHYLTHIURAM MONOSULFIDESÍNTESIS Y CARACTERIZACIÓN DE NUEVOS COMPLEJOS DE Ni(II) CON BASES DE SCHIFF DERIVADAS DE n-OCTILAMINA, n-DODECILAMINA Y n-OCTADECILAMINA índice de autoresíndice de assuntospesquisa de artigos
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Boletín de la Sociedad Chilena de Química

versão impressa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.45 n.4 Concepción dez. 2000

http://dx.doi.org/10.4067/S0366-16442000000400010 

Synthesis and Characterization of New Ruthenium
Complexes With 11-carboxy-dipyrido
(3,2-a:2’,3’-c)phenazine as Ligand

Alejandra Arancibia1, Ana María Leiva and Bárbara Loeb.

Facultad de Química, Pontificia Universidad Católica de Chile,
Casilla 306, Santiago 22, Chile.
(Received: May 11, 2000 - Accepted: August 7, 2000)

Abstract

A general method for the synthesis of Ruthenium(II) complexes of the type [(HOOC-dppz)2RuX2] and [(HOOC-dppz)RuL2](PF6)2 (X = Cl- or SCN-, L = 2,2'-dipyridine or 4,4'-dimethyl-2,2'-dipyridine and HOOC-dppz = 11-carboxy-dipyrido(3,2-a:2',3'-c)phenazine), is reported. The complexes were characterized by conventional technics as UV-Visible, IR and 1H-NMR spectroscopies and cyclic voltammetry.

Key words : carboxy-dppz, synthesis, Ruthenium complexes, acid group.

Resumen

En el presente trabajo se reporta un método general para la síntesis de complejos de Rutenio del tipo [(HOOC-dppz)2RuX2] y [(HOOC-dppz)RuL2](PF6)2 (X = Cl- ó SCN- , L = 2,2'-dipiridina ó 4,4'-dimetil-2,2'-dipiridina y HOOC-dppz = 11-carboxi-dipirido(3,2-a:2',3'-c)fenacina. Los complejos fueron caracterizados por métodos convencionales, como espectroscopías UV-Visible, IR y 1H-NMR y Voltametría Cíclica.

Palabras Claves : carboxi-dppz, síntesis, complejos de Rutenio, grupo ácido.

INTRODUCTION

In the last years the interest in searching new polypyridinic Ruthenium complexes has increased mainly because of their potential applications, e.g. as dyes in nanocristaline TiO2 solar cell electrodes.1-3 Ruthenium has many characteristics that justify its election : i) its octahedral geometry, that permits the introduction of specific ligands in a controlled way, ii) the photophysical, photochemical and electrochemical properties of its complexes that can be directed in a predictable way, iii) its capacity to form stable complexes in different oxidation states, and iv) polypyridinic complexes of Ruthenium absorb visible light where the solar radiation has its maximum intensity. Regarding the ligand, complexes with the dipyrido(3,2-a:2’,3’-c)phenazine fragment (dppz) have been vastly reported in literature, mainly because of their possibility of intercalation in DNA.4 Its structure can be considered as originating from two fragments: 2,2-bipyridine (bpy) and phenazine (phz), or 1,10-phenanthroline (phen) and quinoxaline (quin). In either case, dppz seems to retain some of the physical or chemical properties of the fragments.5-9

Moreover, if a complex is intended to be used as a sensibilizing dye in solar cell electrodes, at least one of its ligands must posses an interlocking moiety between it and a semiconductor as TiO2. The COOH group has been proven to be suitable for this, particulary when it is acting as substituent in bpy. Promising results have been obtained for the Ru(bpy-COOH)2(SCN)2 complex reported by Graetzel et.al.1b

With the aim of looking for new and more efficient solar cell dyes, in this paper, the synthesis and characterization of a series of Ruthenium complexes with the dppz-COOH ligand is reported. Specifically, neutral complexes as (dppz-COOH)2RuX2 (X= Cl- and SCN-) or charged complexes as [(HOOC-dppz)RuL2](PF6)2 (L = 2,2’-bipyridine (bpy) and 4,4’-dimethyl-2,2’-bipyridine (Mebpy)) were studied.

Experimental

Materials

All solid chemicals were reagent grade and used as received. 2,2’-bipyridine, 4,4’-dimethyl-2,2’-bipyridine, 3,4-diamine benzoic acid, RuCl3·3H2O, NH4PF6 and KNCS were obtained from Aldrich. The solvents were Merck p.a. and were dried by conventional methods.10

Physical measurements

UV-VIS spectra were recorded on a Shimadzu UV-3101PC spectrophotometer with a 1 cm quartz cell. 1H-NMR spectra were recorded on a Bruker AC/200, 200 MHz spectrometer with TMS as reference. IR spectra were recorded in KBr mulls in a Bruker Vector 22 FTIR spectrometer. Cyclic voltammetry was performed with a Bas CV-5OW 2.3 MF-9093 equipment. Elemental Analysis were performed with a Fisons Instruments EA 1108/CHNS-O equipment, at the P. Universidad Católica de Chile.

Synthesis

11-carboxy-dipyrido(3,2-a:2’,3’-c)phenazine 0.5 g (2.38 mmol) of 1,10-phenanthroline-5,6-dione11 and 0.54 g (3.55 mmol) of 3,4-diaminobenzoic acid were dissolved in 50 ml of ethanol and the mixture refluxed under nitrogen for 2 h. After cooling to room temperature a light brown, highly insoluble, solid precipitated. Yield: 86%. Anal. Calc. for C19H10N4O2·2H2O : C, 66.27; H, 2.93; N, 16.27. Found: C, 65.62; H, 2.60; N, 16.05.

[(HOOC-dppz)2RuCl2]. 0.07 g (0.34 mmol) of RuCl3 x 2H2O and 0.20g (0.61 mmol) of HOOC-dppz were dissolved in 15 ml of DMF and the mixture refluxed under nitrogen for 8 h. After cooling to room temperature, the solution was concentrated to one half in the rotovap. With the addition of ether and after 24 h in the freezer, a dark-red highly insoluble solid precipitated. The solid was filtered off, washed with ethyl ether and dried. Yield: 78%. Anal. Calc for C38H20N8O4·1H2O·1DMF: C, 53.77; H, 3.53; N, 13.78. Found: C, 53.98; H, 3.21; N, 12.98.

[(HOOC-dppz)RuL2](PF6)2 (L= bpy and Mebpy) 0.03 g (0.04 mmol) of [(HOOC-dppz)2RuCl2] and 0.02 g of L (0.13 bpy mmol or 0.11 mmol Mebpy) were dissolved in 30 ml of MeOH/H2O (3:1) and the mixture refluxed under nitrogen for 8 h. The colour changed from dark brown to red. The solution was cooled to room temperature, and concentrated to one half in the rotovap. After the addition of NH4PF6 and stirring the solution for 30 min, 5 ml ethyl ether was added and the solution left in the freezer for 24 h. A red solid precipitated, that was filtered off, washed with ethyl ether and dried. Yield: 73% (bpy), 76% (Me-bpy) Anal. Calc. for C39H26N8O2F12P2Ru (L=bpy): C, 45.49; H, 2.55; N, 10.88. Found: C, 45.89; H, 2.44; N, 10.82. C43H34N8O2F12P2Ru (L=Mebpy): C, 47.56; H, 3.16; N, 10.32. Found: C, 46.01; H, 3.40; N, 11.10.

[(HOOC-dppz)2Ru(NCS)2]. 0.05 g (0.06 mmol) of [(HOOC-dppz)2RuCl2] were suspended in a minimum amount of water and the suspension neutralized to pH 7 by dropwise addition of sodium hydroxide, until all the solid was dissolved originating a red-violet solution. The solvent was eliminated to one half in the rotovap, and 15 ml of methanol and 0.4 g (4.12 mmol) of KNCS were added. The solution was refluxed for 3 h in the dark under nitrogen and then evapored to dryness. The solid was redissolved in water and the complex precipitated at pH 2 by addition of 1 M HCl. The dark product was separated by filtration and dried. Yield: 90%. Anal. Calc. for C40H22N10O5S2Ru·H2O : C, 54.11; H, 2.50; N, 15,78. Found: C, 53.97; H, 2.66; N, 15.26.

RESULTS AND DISCUSSION

The dppz-COOH ligand was synthesized in a similar way as described for unsubstituted dppz and for X2-dppz derivatives:12

The free ligand showed to be highly insoluble in most common solvents. Therefore, the corresponding 1H-NMR spectrum was obtained in DMSO-d6 with low resolution, and only after treating the sample with concentred HCl. For this reason, the assignment of the chemical shifts was difficult. Nevertheless, the chemical shifts are in the range of those reported for free and coordinated dppz 8,12. The 1H-NMR spectral resolution obtained for the Ru complexes with the dppz-COOH ligand was also low. However, the main features of the pattern observed in the free ligand spectra remained. As a general trend, the signals are shifted to higher fields by coordination, Figure 1, behaviour that is also coincident with literature reports for similar species.12 The simplicity of the 1H-NMR spectra for a complex as [(HOOC-dppz)Ru(bpy)2](PF6)2 , shown in Figure 1, can be understood in the same manner as reported for similar complexes.14 The fenazine fragment nitrogens act as a barrier and therefore the behaviour of the dppz type ligand is similar to the bpy ones.


The coordination of the dppz-COOH ligand to the metal is clearly evident by comparing the free and coordinated spectra. The phenanthroline fragment protons are considerably displaced to higher field. Specifically, the ortho protons (a, a’) shift 1.2 ppm, reflecting the metal retrodonation that accompanies coordination. An effect on the meta (b, b’) and para (c, c’) protons is also observed, but in a lower extent: 0.6 and 0.2 ppm, respectively. Apparently, the quinoxaline fragment protons (d, d’ y e’) are not affected by coordination. Their chemical shifts are unaffected (9.0 and 8.6 ppm) reflecting again the barrier effect of the phenazine nitrogens, that avoids the electronic flux towards the acceptor COOH group. Regarding bpy, its coordination to Ruthenium shows the expected pattern. The 6,6’ protons are displaced from 8.58 ppm in the free ligand to 9.0 ppm in the complex, due to the change in coordination number of the nitrogen and the consequent loss in anisotropy.15 The magnetic inequivalence of the bpy pyridine rings shows up by the decoupling of the corresponding proton signals (7.7 ppm H5 and 7.5 ppm H5’).

For L = 4,4’-dimethyl-2,2’-bipyridine, the 1H-NMR pattern is the same, the main difference being the disappearance of the multiplet at 8.23 ppm, which corresponds to the protons at the 4,4’ position in bpy, that were substituted by methyl. This latter group also shows magnetic inequivalency in the spectra, giving two groups of signals at 2.64 and 2.59 ppm.

The intensity for the bpy and dppz-COOH protons in the NMR spectra for both, L = bpy and L = Me-bpy, is in a 2:1 ratio, indicating the coordination of two bpy and one dppz-COOH. Since the starting material for the synthesis of the [(HOOC-dppz)RuL2](PF6)2 complexes was [(HOOC-dppz) 2RuCl2], the large excess of bpy (Me-bpy) used in the synthetic process resulted in the replacement of one dppz-COOH ligand.

In the IR spectra the band corresponding to the C=O group in COOH appears at 1714 cm-1 in the free ligand. This band is displaced in the corresponding complexes. The coordination of SCN is evidenced by the nCN bands2 at 2060 and 2110 cm-1.

In the complexes with bpy and Mebpy, in addition to the ligand’s characteristic signals, the valence P-F vibrations to 844 and 834 cm-1 of PF6 are observed. In this way, the cationic nature of the complexes is corroborated, Table 1.


The UV-Visible absorption spectrum of the dppz-COOH complex, Fig. 2, exhibits bands at 366 and 384 nm which are similar in shape and energy to the bands of the free dppz ligand and other monosubtituted dipyridophenazine ruthenium (II) complexes.13 These bands are assigned as dppz(p) ® dppz(p*) transitions. The low-energy bands at 444-504 nm are assigned as MLCT Ru(dp) ® dppz(p*) transitions on the basis of relative intensity (*) and position in regard to the p ® p* bands, Table 1. Moreover analogous assignments have been done for similar complexes reported in literature16.


Table 1 also shows the electrochemical information for the complexes studied. The data for the free ligand and the complexes correlate well with data reported in literature for similar complexes.5,6 The first ligand reduction is notably displaced to more negative potentials in the complexes in regard to the free ligand. This effect reflects the electron density gain of the dppz-COOH ligand because of coordination to the metal and the consequent retrodonation. This is in agreement with the NMR results described above. On the other hand, the oxidation of Ru - which appears approximately at 1 V for all complexes - remains practically unchanged in all the series studied. The constancy of the oxidation potentials in Ru polypirydil complexes is a well known behaviour5,8,13b,14,17. It suggest that the energy of the metal centered orbital involved in the oxidation is not affected by changing the X or L ligand.

CONCLUSIONS

The synthetic route to prepare the complexes is rather simple, and permits the desired products to be obtained in good yield and purity. The complexes reported posses most of the characteristics for a solar cell dye. Work is in progress to check this prediction.

1*To whom correspondence should be addressed.

(*) The low solubility of the compounds precluded the determination of reliable e values.

ACKNOWLEDGEMENT

Support from Fondecyt 3980029 (A.A.) and Fondecyt Líneas Complementarias 8980007 is greatfully acknowledged.

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