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Boletín de la Sociedad Chilena de Química

versión impresa ISSN 0366-1644

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

http://dx.doi.org/10.4067/S0366-16442000000400003 

Dendrimers based on cyclophosphazene unit and
containing iron (III)

C. Díaz* and M.L. Valenzuela

Departamento de Química.Facultad de Ciencias, U. de Chile
Las Palmeras 3425, Casilla 653
Santiago - Chile
(Received: May 24, 2000 - Accepted: July 24, 2000)

Abstract

The complexes [N3P3(OC6H5)5OC6H4CH2CN.FeCl2]PF6 (1) and [N3P3(OC6H4CH2CN)6. (FeCl2)6](PF6)6 (2) have been syntethyzed by reaction of the corresponding cyclophosphazene ligands: N3P3(OC6H5)5OC6H4CH2CN (3) and N3P3(OC6H4CH2CN)6 (4) with FeCl3 in CH3OH as solvent and in presence of NH4PF6. The new compounds were characterized by elemental analysis and IR, UV-Visible and EPR spectroscopy as well as electrochemical methods.

The hexametalladendrimers exhibits a six-electron reduction while that the monometalladendrimers exhibit a single one-electron reduction. The paramagnetic complexes exhibit strong EPR signals in solid as well as in CH2Cl2 solution.

KEY WORDS: Dendrimers,cyclophosphazene,iron,nitrile complexes

Resumen

Se han preparado los complejos [N3P3(OC6H5)5OC6H4CH2CN.FeCl2]PF6 (1) y [N3P3(OC6H4CH2CN)6. (FeCl2)6](PF6)6 (2) por reacción de los correspondientes ligantes ciclofosfacenos N3P3(OC6H5)5OC6H4CH2CN (3) y N3P3(OC6H4CH2CN)6 (4) con FeCl3 en CH3OH como solvente y en presencia de NH4PF6. Los nuevos compuestos fueron caracterizados por análisis elemental y espectroscopia Ir ,Uv-visisble y EPR así como también métodos electroquimicos .

Los hexametalodendrimeros exhiben una reducción de seis electrones mientras que los monodendrimeros muestran una reducción simple de un electrón. Los complejos paramagnéticos exhiben intensas señales EPR en sólido así como también en solución de CH2 Cl2.

PALABRAS CLAVES: Dendrimeros,ciclofosfacenos ,hierro.complejos nitrilo

Introduction

Dendrimers chemistry is a rapid developing field of research [1]. Dendrimers offer the prospect of novel material with advantageous properties allowing them to serve, as soluble catalyses [2], dendritic boxes [3], is other supramolecular dendritic arrangements. Since the initial report on this class of molecules by Vögtle in 1978, [4], and many different structural classes of dendritic macromolecules have been reported. Two main classes of dendrimers can be distingly: the classical organic dendrimers [1] and the emergent inorganic dendrimers [5].

The incorporation of transition-metal complexes in both organic and inorganic dendrimers lead to new and interesting material with specific properties such as the capability to absorb visible light, to give luminescence and to undergo reversible multielectron redox processes [6]. Such species; in fact could find applications as component in molecular electronic and as a photochemical molecular devices for solar energy conversion and information storage [6]. Metal-containing dendrimers can be classified in to three categories:

(a) Dendrimers with coordination centers on the periphery,
(b) Dendrimers with coordination centers though all layers and
(c) Dendrimers with coordination centers at the core.

Although dendrimers with coordination centers on the periphery have been reported [1,2,6] for: those with the dendrimer - core being organic, few examples of inorganic dendrimers with metal derivatives on the surface have been reported [5,8]. This communication reports the synthesis and characterization of new inorganic dendrimers based on cyclophosphazene containing single metal complexes on the surface. We have recently reported some organometallic derivatives of partially branched cyclophosphazene derivatives [9,10]

Experimental

Experimental procedures and IR, UV-Visible, ESR and electrochemical measurements were performed as previously reported [9,10]. The ligands N3P3(OC6H5)5OC6H4CH2CN (3) and N3P3(OC6H4CH2CN)6 (4) were prepared as described previously [9,10]. FeCl3. 6H2O and NH4PF6 were used as received.

Preparation of the complexes:

(1). The ligand N3P3(OC6H5)5OC6H4CH2CN 0.13 g (0.1775 mmol) and FeCl3 0.0617 g (0.2282 mmol) were stirred in a Schlenk in CH3OH 15 ml and in presence of the NH4PF6 0.0719 g (0.4411 mmol), for 24h at room temperature. From the orange solution the solvent was removed by rotary evaporation. The orange solid was dissolved in CH2Cl2 and filtered though celite. The solvent was removed under vacuum and the oil solid was breaked by washing several times with a n-hexano/diethylether mixture. The resulting orange solid was vacuum dried.

Yield:.0.32g,62%
Analysis: Found C 43.52; H 3.50; N 3.80

Calcd for C38H31F6O6N4P4Cl2Fe.CH2Cl2 C 42.98; H 3.03; N 5.1

(2). The ligand N3P3(OC6H4CH2CN)6 0.14 g (0.151 mmol) and FeCl3 0.245 g (0.9063 mmol) were stirred together NH4PF6 0.295 g (1.8098 mmol) in CH3OH 15 ml at room temperature for 24 h. The yellow-orange solution was treated similarly to separation and purification of (1) to give 0,25g of (2) as a yellow-orange powder .Yield:71%

The compound was found to have high water content which can be due to some water arising from the methanol as well as to the hygroscopic behavior of the solid.

Analyses: Found C 15.65; H 3.26; N 5.71

Calcd.for C48H36F6O6N9P9Cl12Fe6.70H2O C 15.24; H 4.65; N 3.33

Results and discussion

As expected the synthesis of the dendrimers (3) and (4) was made by the divergent strategy [1] by reaction of N3P3Cl6 with HO-C6H4CH2CN in acetone and in presence of the CaCO3 [9,10]. The metallic derivatives (1) and (2) were obtained by reaction of the respective dendrimer ligands with FeCl3 in methanol as solvent (see scheme 2).

Complexes (1) and (2) were characterized by elemental analyses, IR, UV-Visible and EPR spectroscopy as well cyclic voltammetry. Paramagnetism of the complexes precludes then a NMR characterization. Complex (2) is highly hygroscopic as revealed by its elemental analyses as well as their IR spectrum (see experimental part).

The IR spectra of complexes (1) and (2) exhibit the expected bands due to n(N=P) and n(CN) of the ligand as well as the n(PF6) band [11]. Characteristic splitting of the n(N=P) bands due to coordination was observed (fig 1). This IR data are in good agreement with the IR of fully 1H-31P and 13C multinuclear NMR characterized diamagnetic complexes of Zn (II) and Cu (I) [10 b]. Two intense and broad bands around 3479 and 3159 cm-1 corresponding to n(OH2)as and n(OH2)s respectively, as well as a medium intensity band around 1658 cm-1 corresponding to d(OH2) are consistent with the presence of a high degree of hydration for complex (2).The IR spectra are collected in Table 1



Cyclic voltammetry studies.

There are few reports of electrochemical studies on any cyclophosphazene metal complexes [8 b, c]. The cyclic voltammogram recorded for complex (1) and (2) are shown in figure 2.


Analysis of the cyclic voltammetric responses for these complexes with scan rate varying from 50 to 20 mVs-1 showed an irreversible one-electron transfer for the Fe(III)/Fe(II) couple which is located around 0.50 and 0.44 V for complexes (1) and (2) respectively. These data are consistent with the one reduction wave at 0.47 V for the complex [Cl3Fe(µ-NC)Mn(CO)(dppm)2] [12a]. N3P3(OC6H5)6 and N3P3Cl(OC6H5)5 does not exhibit redox process in the range of potentials -2.0 ® +2.0 V [8 b]. Similar substituted phosphazenes with CN groups also exhibit redox reduction activity below -1.8 V [8 b].

The presence of one single reduction peak, (although irreversible) indicate that the six iron are reduced to the same potential which suggests that the six redox iron centers are non-interacting. Considering that solution have a concentration of 1.04*10-3 M for (1) and 7.76*10-4 M for (2) and assuming that the complex (1) is a monomer unit and the complex (2) is their polymer (n=6), containing six non-interacting redox unit, then the electron number transferred in the reduction of (2) respect to the electron transferred in (1) (one electron) can be estimated by the equation [12 b]:

Where ip and im are the current intensity for the polymers and the monomer respectively, Mp and Mm are the molecular weigh. Equation 1 gives a value np»6 for complex (2)

UV-Visible:

As found for FeCl3 and other iron-chloride derivatives [13]; the UV-Visible spectra of the complexes (1) and (2) exhibit intense ligand-metal charge transfer bands around 360 and 400 nm, which to obscure completely the very weak; spin-forbidden d-d bands. The complexes do not exhibit absorption above 500 nm as is shown for complex (1) in figure 3. Data are shown in table 2. Intensity and position for the charge transfer transition are similar those found for similar chloro-iron complexes.



EPR spectra:

As expected, the paramagnetic complexes (1) and (2) exhibit the typical EPR signal of Fe (III) complexes [14]. The Fe (III) ion is a ground state 6S (6A1g in the complexes) and is not split within an octahedral or even a lower symmetry ligand field [15]. In addition, there can been no splitting by spin-orbital interaction. Then a single isotropic resonance line at g = 2 should be always observed. Figure 4 shows the EPR spectrum of complex (2) in CH2Cl2 solution and values are displayed in table 2. Hyperfine interaction of the unpaired electron with the 14N nucleus (I =1) of the cyanide group was not observed, although the broad, signal in solid state could contain some unresolved splitting. On the other hands, the g values (table 2) near 2 for complexes (1) and (2) correspond to high spin Fe (III) complexes because low spin Fe (III) complexes exhibit higher g values [16-18].


Conclusion

The dendrimers (3) and (4) are able to coordinate one and six iron atoms in the periphery. Formation of metallodendrimers (1) and (2) open interesant possible applications for to solubilize conventionally insoluble inorganic compounds (as MXn salts) in organic solvents. Although the redox properties of the iron are irreversible, they are reduced at the same potential which suggest a possible application as molecular battery. Experiments to coordinate other transition metals with possible reversible redox properties are in progress. To our knowledge complexes (3) and (4) are the first iron (III) containing a cyclophosphazene dendrimers [19].

*To whom correspondence should be addressed, e-mail: cdiaz@uchile.cl

ACKNOWLEDGEMENTS

Financial support from FONDECYT, project 1000672 is gratefully acknowledged

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