ELECTROCHEMICAL PROPERTIES OF RHENIUM CARBONYL
COMPLEXES CONTAINING TERPYRIDINE LIGAND DERIVATIVES

S.A. MOYA^*, R. PASTENE, H. LE BOZEC¹, M. FERNANDEZ², A.J. PARDEY³, P. BARICELLI³ Y R. LOPEZ

Departamento de Química Aplicada, Facultad de Química y Biología, Universidad de
Santiago de Chile, Santiago, Chile.
¹Laboratoire de Chimie de Coordination et Catalyse, UMR6509, CNRS. Université de Renne
I, Rennes, France.
²Escuela de Química, Facultad de Ciencias, Universidad Central de Venezuela, Caracas,
Venezuela.
³Centro de Investigaciones Químicas, Facultad de Ingeniería, Universidd de Carabobo,
Valencia, Venezuela
(Recibido: Mayo 20, 1999 - Aceptado: Julio 30, 1999)

ABSTRACT

Correlations between the oxidation potentials and the stretching frequencies of the carbonyl groups in the fac-tricarbonylbromo(polypyridyl)rhenium(I) complexes with respect to the BrRe(CO)₅ precursor are discussed.

KEY WORDS: Cyclic voltammetry, ligand effect, rhenium(I) complexes, heterocyclic nitrogen ligands.

RESUMEN

Correlaciones entre los potenciales de oxidación y las frecuencias de estiramiento de los grupos carbonilos son discutidas en complejos tricarbonilbromo(polipiridilo) de renio(I), respecto al precursor BrRe(CO)₅.

PALABRAS CLAVES: Voltametría cíclica, efecto ligando, complejos de renio(I), ligandos nitrogenados heterocíclicos.

*To whom correspondence should be addressed. Casilla 40-33, Fax: 56-2-6812108, e-mail: smoya@lauca.usach.cl, Santiago, Chile.

Electrochemical potential measurements are extremely sensible to any variation of the electronic density about the metal center in a transition metal complex. This sensibility can be used to monitoring the electronic effects provided by the replacement of one ligand by another or by the use substituted or unsubstituted ligands on many properties of the complex¹⁾. The oxidation potential of the metal (E_p^OX) in a coordination compound can be related with the electronic density about the metal center or more specifically can be correlated with the changes on the HOMO energies. Several molecular parameters such as vibrational and electronic transitions and even the chemical reactivities can be severely affected by modification of the redox potentials²⁾. In carbonyl complexes the E_p^OX and the stretching frequencies of the carbonyl groups (n_C-O) can be correlated properly by using specific ligands to modificate directly the strength of the p backbonding between the metal and the carbonyl group³⁾.

We report here correlations between the E_p^OX and the Dn_C-O obtained from a number of BrRe(CO)₃(L) complexes where L is a substituted terpyridine ligand (Figure 1). The heterocyclic nitrogen ligands and the rhenium(I) complexes were prepared by published procedures⁴⁾. Cyclic voltammetry was performed with a Bank-Wenking POS 73 potentiostat, a Gould OS 4100 oscilloscope and a Graphtec XY recorder WX 4301. The three-electrode configuration contained a platinum button working electrode, platinum wire as auxiliary electrode and saturated calomel electrode (SCE) as reference. 0.1 mol L^-1 of tetrabutylammonium perchlorate (TBAP) dry solution, oxygen-free acetonitrile was used as electrolyte. IR spectra were recorded on a Bruker IFS-66V FT spectrometer in KBr disk and CHCl₃ solutions.

As can be seen in Table I the reduction potential values decrease with the number of substituents on the terpyridine ligand which provides a fine tuning to have control on the electron donor/acceptor capabilities assigned to the heterocyclic nitrogen ligand.

Cathodic behaviour. The cyclic voltammograms of these systems in the region of 0.0 to 2.2 V vs Fc⁺/Fc show a quasi-reversible one-electron reduction wave, with i_pc/i_pa ratio close to one and peak separations (DEp = E_pc - E_pa) greater than 60 mV. The reduction wave is assigned to a reduction occurring on the heterocyclic nitrogen ligand (Eq. 1). The chemical process may be visualized as an addition of one electron to the LUMO (a p* molecular orbital mostly centered on the nitrogen atom of the heterocyclic nitrogen ligand)⁵⁾.

BrRe(CO)₃(L) + e^- ® [BrRe(CO)₃(L^·-)] 00000000000000000(1)

Anodic behaviour. All the complexes show two oxidation processes between 0.4 and 1.5 V vs Fc⁺/Fc range, except for the BrRe(CO)₃(py-dppy) complex. The first irreversible one-electron peak should be associated with the oxidation Re(II)/Re(I) pair⁴⁾. The most positive oxidation peak is difficult to assign to a specific process due to irreversibility of the initial oxidation⁶⁾.

FIG. 2. Correlation between EpcOX and DnC-O for the series BrRe(CO)3(L).

Traditionally the terpyridine ligand coordinates as a tridentate chelate. The compounds studied in this work show an IR spectra where three strong bands in the n_C-O stretching region which namely corresponds to the normal vibration modes n_C-O of terminal carbonyl groups in molecules with C_s symmetry⁷⁾. The above implies that the heterocyclic nitrogen ligand acts as an asymmetric bidentate chelate and non as usually terpyridine ligand does. The coordination of the heterocyclic nitrogen ligands increases the backbonding on the carbonyl groups. By comparison of the stretching frequency of the carbonyl groups in the polypyridyl complex with respect to the precursor BrRe(CO)₅ complex, Dn_C-O = n_C-O^precursor - n_C-O^complex (Table I) we can estimate the distribution excess of negative charge on the carbonyl groups. The movement of this excess of charge provided by the heterocyclic nitrogen ligands on the metal center and distributed on the carbonyl groups can be monitored by analyzing the redox potential. Thus it is possible to establish a correlation between the high frequency (n_C-O¹) of the carbonyl stretching vibration and the oxidation potential of the Re^II/Re^I pair. A lineal relation should be found for the studied complexes (Eq. 2):

Dn_C-O = A + B (E_1/2)00000000000000000(2)

^aredox potential in V vs Fc⁺/Fc, in CH₃CN 0.1 mol L^-1 of TBAP, n = 200 mV s^-1; ^birreversible; ^c DE_p = E_pc - E_pa in mV, where E_pc and E_pa are cathodic and anodic peak potentials, respectively; ^d Dn_C-O = n_C-O^precursor - n_C-O^complex

However as can be observed in Figure 2 the measurements show several points which are far from a lineal relation slope. A polynomial of high order containing terms derived from E_p^OX larger than 1 can be rationalized other effects that can appear for high or low potential values (Eq. 3):

Dn_C-O = A + B (E_1/2) + C (E_1/2)² + ...00000000000000000(3)

The above relation allows to estimate or predict the metal oxidation potential from the differences abserved for the Dn_C-O.

In accordance with the values contained in Table I, the polynomial regression with Dn_C-O in cm^-1 and E_p^OX,1 in volts (Eq. 4) becomes:

Dn_C-O = 29.87 + 188.75 (E_p^{OX, 1}) (r = 0.83714)00000000000000000(4)

ACKNOWLEDGEMENTS

This work was supported by DICYT/USACH, FONDECYT (1980374) and Prorject CNRS/CONICYT (Chile-France). We thank to S. Fernández for his assistance with computer calculations. A.J.P. thanks to CDCH-UCV (AI 4232.98).

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