The EPR spectra of a series of mononuclear complexes [CpFe(dppe)SR]PF₆ R = C₆H₅ (1), C₃H₇ (2), p-C₆H₄Br (3) and the binuclear [CpFe(dppe)-S-CH₂CH₂-S-CpFe(dppe)](PF ₆)₂ (4) were measured. All the paramagnetic complexes exhibit intense signals in both solid state as well as in CH₂Cl₂ solution. Analysis of the spectra indicates a pseudooctahedral structure for the complexes except for complex (2) for which a tetragonal distortion in solid state was observed. Some degree of delocalization of the unpaired electron for (3) and (4) was suggested. The stabilization of the radicals by the thiolate ligands is discussed.

KEY WORDS: Iron-sulfur complexes, EPR, organometallic radical, thiolate compounds.

Se midieron los espectros EPR de una serie de complejos mononucleares [CpFe(dppe)SR]PF₆ R = C₆H₅ (1), C₃H₇ (2), p-C₆H₄Br (3) y del complejo binuclear [CpFe(dppe)-S-CH₂CH₂-S-CpFe(dppe)](PF ₆)₂ (4). Todos los compuestos paramagnéticos muestran intensas señales tanto en estado sólido como en solución de CH₂Cl₂. El análisis de sus espectros indica una estructura pseudooctaédrica para los complejos, excepto para el compuesto (2) en estado sólido el cual muestra una distorsión tetragonal. Un cierto grado de deslocalización del electrón desapareado para los compuestos (3) y (4) es sugerido. Se discute la estabilización de los radicales por ligante tiolato.

PALABRAS CLAVES: Complejos hierro-azufre, EPR, radicales organometálicos, compuestos tiolatos.

Although organotransition metal chemistry has long been dominated by complexes of obeying the so-called the 18-electron rule¹⁾, several examples of compounds having 17 ²⁾ or 19 ³⁾ electron have appeared in the literature. In spite of that 19 ⁴⁾ or 17 e ⁵⁾ complexes are usually unstable and hence they are isolated only in solution or detected as transient species in the last time numerous stable compounds of these type have been reported⁶⁾.

Organometallic 17e complexes of iron(III) are particularly scarce, although coordination iron 17e compounds are common.

It appears to be that the stabilization of organometallic iron 17e complexes require two conditions:

i) The presence of ligands around the iron atom, having bulky substituents.

ii) Ligands s or p donor, to compensate the +3 formal oxidation state of the iron.

Consistently with this, species [CpFe(CO)₂R]^·+ are believed to be transient intermediate generally too unstable to be detected^2,7). On the other hand substitution of CO by a bulky diphosphine as Ph₂P(CH₂)₂PPh ₂, (dppe) as well as most s donor ligand and substitution of Cp by Cp* (C₅Me₅) leads to stable paramagnetic [Cp*Fe(dppe)R]^·+ compounds⁵⁾. We have previously reported the formation of stable Fe(III)-thiolate 17e complexes from the oxidative addition of dithioethers as well as on thiols to the fragment CpFe(dppe)⁺ (scheme 1)^8-13). Here we report a EPR characterization of such organometallic radicals. The first example of stable containing the unsubstituted C₅H₅ ring.

We have selected from the series [CpFe(dppe)SR]PF₆^10-12) one representative of each type: compound (1) having a phenyl substituent linked to the sulfur atom complex (2) has an aliphatic substituent bond to the sulfur atom, compound (3) has a para-substituent heavy atom in the phenyl ring and complex (4) a binuclear Fe(III)-Fe(III) bridged thiolate.

Room temperature CH₂Cl₂ solutions of complexes (1) and (2) have symmetric one single peak. In the EPR spectra these features are similar to that observed for the solution EPR spectra for similar [CpFe(diphos)R]⁺ complexes diphos = diphosphine, R = alquil, C = CR^6c).

The EPR spectrum of (1) is shown in Figure 1 and values are displayed in Table I. As expected the EPR lines in solution are sharp indicting an average anisotropies in both, the g tensor and the nuclear coupling due to the rapid tumbling in solution¹⁴⁾. On the contrary the EPR spectra of (3) and (4) in CH₂Cl₂ solution appears most broad as is shown in Figure 1 for (3). In the case of (3) the broadening can be due to an unresolved hyperfine coupling of the bromine atom¹⁵⁾.

TABLE I. ESR spectra data^a for [CpFe(dppe)SR]PF₆ and {[CpFe(dppe)]₂µ-S(CH₂) ₂S}(PF₆)₂.
 

^aAt room temperature; ^bCH₂Cl₂; ^cHalf linewidth values in parenthesis.

On the other hand, the some broad signal observed for 4 can be due to the electron exchange between the two iron atoms in the binuclear complex. Similar broadening of the EPR signal was observed in the spectrum of [CpFe(dppe)-S-C₆H₄N-Fe(Cp((dppe)](PF ₆)₂¹⁶⁾, and also int he spectrum of the binuclear complex [Cp*Fe(dppe)-CC-C₆H₄-CC-Fe(dppe)Cp ^*][PF₆]₂¹⁷⁾. The g values are some higher than those of the free electron value(g=2.0023) as usually observed for 17 electron iron(III) compounds having a single occupied HOMO with a predominant metallic character^5c).

FIG. 1. EPR spectra of complexes (1) and (3) in CH₂Cl₂ solution at room temperature.

Owing to the high stability of the complexes (1)-(4) in solid state we have measured the EPR spectra directly of the solid samples. The spectra of complexes (1), (3) and (4) exhibit a single one signal, sharp for (1) and broad for (3) and (4), at g values similar to that observed in solution (see Figure 2). Lacking hyperfine splitting, the spectra of these compounds cannot delineate the precise orbital makeup of the SOMO (semioccupied MO) but they do not indicate that the radical retains the approximate octahedral symmetry¹⁸⁾, since lower symmetry would lead to a splitting of the g-tensor.

On the other hand for (2) the expected two g-tensor component for an axially symmetric was observed (Figure 2).

No phosphorus hyperfine splitting in any of the g components of the spectrum is observed, indicating no interaction of the unpaired electron with the phosphorus atom around the metal center. The g values are greater than 2, which suggest that the unpaired electron is located in a predominantly metal (iron(III)), low spin d⁵ orbital.

This in consistently with MO calculation for the similar organometallic radicals [Cp*Fe(dppe)R]⁺ where the single occupied HOMO is 52%, localized on the metal^5c). This is also in agreement with the low chemical reactivity exhibited by these radicals. Similarly to data in solution the solid spectra of (3) exhibits a most broad signal which suggest some probable unresolved splitting hyperfine through the bromine atom. Consistently with this the hyperfine coupling constant for bromine in Ru-Br complexes is 120 G ¹⁹⁾, similar to the half linewidth for (3), see Table I. For complex (4) as mentioned previously, the broadening can be due to an exchange interaction of the unpaired electron between the two centers in the binuclear complex (4) see Figure 3.

FIG. 2. EPR spectra of complexes (1), (2) and (3) in solid state at room temperature.

FIG. 3. EPR spectra complex (4) in solution (A) and in solid (B).

Our early electrochemical studies on the complexes [CpFe(dppe)-SR]PF₆¹³⁾ showing that the reduction Fe(III) Æ Fe(II) is an iron centered process also confirm the present EPR results.

It is interesting to note the similar g values for the complexes (1)-(4) with those observed for other d⁵ low spin metal radical^19-21). Values for these species are shown for comparison in Table II.

TABLE II. Comparison of EPR parameters with other d⁵ low spin metal complexes.
 

To our best knowledge this EPR characterization constitutes the only one EPR study of stable low spin d⁵, Fe(III) complexes containing the unsubstituted cyclopentadienyl ring²²⁾. We believe that the stability of the CpFe(dppe)SR⁺ radical complexes compared with those of [Cp*Fe(dppe)R]^± can be due to soft and polarizable thiolate ligand which stabilizes the formation of the Fe(III) complex compensating the high positive charge of the iron.It is also interesting to note the easily of the EPR measurement in solid state which will be useful in the study of future intercalation solid state reactions containing these radical species.

[CpFe(dppe)SR]PF₆ complexes were prepared as described previously^8-10). Solution freshly prepared by dissolving the solid sample in CH₂Cl₂ and degassed with nitrogen were used. Its EPR spectrum was immediately recorded using a Brucker ECS 106 spectrometer using a rectangular mode cavity with a 50 Hz field modulation. The measurements were made with the microwave band X (9.79 GHz). For the solid sample the crushed powder was used.

We are grateful to DID U. de Chile (Project 98-018) and FONDECYT (Project 198-1082) for financial support.

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14. W. Gordy. "Theory and Applications of Electron Spin Resonance". John Wiley and Sons, New York.

15. Unresolved hyperfine coupling to the phosphorus atom is less probably because it has not been observed in all our CpFe(dppe) compounds studied. Thus hyperfine splitting due to naturally abundant ⁵⁷Fe(2.2%) has been rarely observed. Thus the ^79.81Br hyperfine coupling for Ru-Br complexes is 120G in frozen solution see later ref. 19.

16. C. Díaz and A. Arancibia. Inorganic Chemistry manuscript in revision.

17. T. Weylan, K. Costuas, A. Mari, J.-F. Halet and C. Lapinte. Organometallics, 17, 5569 (1998).

18. The Cp ligand is topologically equivalent to three positions of an octahedral geometry. In the structure of CpFe(dppe) L complexes, the iron atom shows a pseudooctahedral coordination commonly referred as a three-legged piano-stool configuration with the centroid of cyclopentadienyl ligand occupying the center of three octahedral sites, see ref. 5a.

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22. The only knowledged EPR example of CpFe(III) species arises from the EPR detection of the in situ generated radical CpFe(PPh₃)₂⁺, see reference 5b.