Bol. Soc. Chil. Quím., 47, 213-220 (2002) ISSN 0366-1644

NEW ENTRY TO PIANO -STOOL ELECTRON RICH
(PENTAMETHYL CYCLOPENTADIENYL) IRON COMPLEXES

C. DIAZ* ,N. CABEZAS ,F. MENDIZABAL

Departamento de Quimica, Facultad de Ciencias, Universidad de Chile
Casilla 653,Santiago Chile

(Received: October 29, 2001 - Accepted: April 8, 2002)

ABSTRACT

A new route to the electron rich pentamethylcyclopetadienyl iron complexes (n⁵-C₅Me₅)=Cp*) starting from the commercially available dimer [Cp*Fe(CO)₂]₂ ,is presented. Reaction of the dimer with I₂ in CH₂Cl_2, affords the mononuclear carbonyl complex Cp*Fe(CO)₂I which in turn reacts with bis(diphenylphosphinoethane) in toluene under Uv irradiation to give Cp*Fe(dppe)I. Treatment of Cp*Fe(dppe)I with the neutral ligands L (L = CH₃CN, PPh₃, SEt₂) in the presence of TlPF₆ affords the cationic derivatives [Cp*Fe(dppe)L]PF₆ ,while that the reaction with S₂(CH₂C₆H₅ )₂ in CH₃OH yields the thiolate complex [Cp*Fe(dppe)S-CH₂C₆H₅ ] PF₆ .The unusual magnetic properties of these complexes are discussed. Extended Hückel OM calculation confirmed the most electron rich character of the Cp*Fe(dppe)⁺ derivatives than their unsustituted CpFe(dppe)⁺ .Electrochemical as well as Mösbauer data are in agree with this.

Keywords: pentamethylcyclopentadienyl iron, electron rich fragments, OM calculation, organometallic

RESUMEN

Se presenta una nueva ruta de síntesis para compuestos complejos pentametilociclopentadienilo-hierro(II) usando como precursor el dímero [Cp*Fe(CO)₂]₂.

La reacción de este dímero con I₂ in CH₂Cl_2, produce el compuesto Cp*Fe(CO)₂I el cual a su vez reacciona con bis(difenifosfinoetano) en tolueno para dar el complejo Cp*Fe(dppe)I.

La reacción de éste con ligandos neutros L (L = CH₃CN, PPh₃, SEt₂) en presencia de TlPF₆ produce los derivados cationicos [Cp*Fe(dppe)L]PF₆ mientras que la reacción con S₂(CH₂C₆H₅ )₂ en CH₃OH genera el complejo thiolato [Cp*Fe(dppe)S-CH₂C₆H₅ ] PF₆. Se discuten las propiedades inusuales paramagnéticas de los complejos. Cálculos de orbitales moleculares Hückel extendido confirman la más alta densidad electrónica de los derivados Cp*Fe(dppe)⁺ respecto de los correspondientes no sustituidos CpFe(dppe)⁺ .Estudios electroquimicos y de espectometria Mösbauer están de acuerdo con estos resultados.

Palabras Claves: pentametilciclopentadienilo-hierro(II), fragmentos con alta densidad electrónica, cálculos de OM ,organometalico

INTRODUCTION

Whereas the cyclopentadienyl iron dicarbonyl series is one of the most widely studied organometallic families, [1] pentamethyl cyclopetadienyl (n⁵-C₅Me₅)=Cp*) homologues have been less studied [2] Green et al. reported the first synthetic route to this series by metal vapor synthesis [2c]. Subsequently Lehmkuhl [1f] et al. proposed another entry to bis(monophosphine) iron complexes of the type Cp* Fe [(P(CH₃))_n(C₆H ₅)_3-n] ₂Cl n = 0-3. More recently Astrucs [2a,b] and Bercaws [2e] proposed a new route to pentamethyl cyclopentadienyl complexes starting from CpFe(acac), which can be obtained from Fe(acac)₂ and Cp*Li [2d]. The last reported method to the series Cp*Fe(dppe)X involves the reaction of Fe (dppe)Cl₂ with Cp*Li [2a] or with C₅Me₅H [2g] to give Cp*Fe(dppe)Cl. Here we report a new synthetic entry to iron pentamethylcyclopentadienyl compounds and a general discussion about the influence of the methyl substitution ring using experimental and OM calculations .

RESULTS AND DISCUSSION

We have previously reported [3] the synthesis of CpFe(dppe)I from CpFe(CO)₂I and dppe under Uv - radiation in toluene as solvent and also of their cationic [CpFe(dppe) L]PF₆ from treatment with L in CH₂Cl_2, in presence of TlPF₆ [4]. An analogous treatment of Cp* Fe(CO)₂I (1) with dppe in toluene under Uv - irradiation affords Cp*Fe(dppe)I [2a] (2) which was conveniently isolated in 72 %, see Scheme 1.

The precursor (2) in spite of that has been used previously as starting reactive for to introduce the Cp*Fe(CO)₂ fragment [5], surprisingly its has been poorly characterized [6]. The original cite of their synthesis by Moro- Oko Akita et al. [6] does not include an adequate characterization of this complex. Treatment of the commercially available [Cp*Fe(CO)₂]₂ with a excess of I₂ in CH₂Cl₂ at room temperature followed by elimination of excess of I₂ with Na₂S₂O₃ affords (2) as a black - red powder, which was recrystallized from a n-hexane/CH₂Cl₂ solution as dark red crystals in a 91%. As expected two carbonyl bands were observed in their Ir spectrum, at 2000 and 1953 cm^-1. The ¹³C-NMR exhibit the Cp* signal at 97.9 ppm while that the carbonyl signal was clearly observed at 217 ppm. The signal methyl group of Cp* ligand signal appears at 11.15 ppm.

The complex Cp*Fe(dppe)I react with neutral ligand L (L = CH₃CN, PPh₃, SEt₂) in CH₂Cl₂ and in presence of Tl PF₆ to give the series of complexes [Cp*Fe(dppe)L]PF₆. The complex [Cp*Fe(dppe) NCCH₃] PF₆ have been previously prepared by reaction of [Cp*Fe (CH₃CN)₃]⁺ with dppe [2b] and starting from Cp*Fe(dppe)Cl [2a].

On the other hand the reaction of Cp*Fe(dppe)I with S₂(CH₂C₆H₅ )₂ in CH₃OH and in presence of NH₄PF₆ yield the new iron(III)-thiolate [Cp*Fe (dppe) S-CH₂C₆H₅] PF₆.

The new complexes [Cp*Fe(dppe)SEt₂]PF₆ , [Cp*Fe(dppe) PPh₃]PF₆ are paramagnetic and were characterized by elemental analysis, Ir, Uv-visible and magnetic method. ³¹P and ¹H NMR spectra of the complexes give rise to broad (almost missing in some cases) and/or shifted signal. As example in figure 1

Figure 1.	1H - NMR spectrum of [Cp*Fe(dppe)SEt2]PF6 in the aromatic region in CDCl3 solution. The asterisk denote the signal of CDCl3 and of the CH2Cl2 of crystallization.

in shown the ¹H-NMR spectrum of [CpFe(dppe) (SEt₂)]PF₆. On the other hand their ³¹P-NMR spectrum exhibits the dppe signal shifted at 28.95 ppm respect to the normal position for Cp*Fe dppe X or [Cp*Fe(dppe) L]PF₆ around 90 ppm [2a]. The signal of PF₆ appears very weak (due to broadening probably) at - 142 ppm The rare paramagnetism exhibit by 18 electron [Cp*Fe(dppe)L]⁺ species in solution have been attributed to the equilibrium.

[Cp*Fe (dppe) L]+ [Cp*Fe(dppe)]+ + L

(1)

to generate the 16e specie Cp*Fe(dppe)⁺ which is known to be paramagnetic [8]. Recent theoretical calculation [9] found a tripet ground state for Cp*Fe(dppe)⁺ species. However the solid state paramagnetism exhibit by the 18e complexes [Cp*Fe(dppe)SEt₂]PF₆ and [Cp*Fe(dppe)PPh₃]PF₆, neither the previously reported [Cp*Fe(dppe)OCMe₂]PF₆ and Cp*Fe(dppe)OSO₂CF₃ can not be explained by the theoretical results. The paramagnetic behavior exhibit by the complexes [Cp*Fe(dppe)PPh₃]PF₆ and [Cp*Fe(dppe)SEt₂]PF₆ in solution can be explained by the equilibrium (eq.1); however the origin of the paramagnetism in solid state is not clear. The paramagnetism exhibited by these species it appears to be associated in same unclear manner to the presence of pentamethylcyclopentadienyl groups because [CpFe(dppe)L]PF₆ complexes are generally diamagnetic. Consistently with this, the complex [CpFe(dippe)NCCH₃]PF₆ is diamagnetic while the complex [Cp*Fe(dippe)NCCH₃]⁺ is paramagnetic dippe ¾ 1,2 Bis (diisopropylphosphine) ethane.

The high magnetic moment exhibits by the complex [Cp*Fe(dppe)SCH₂C₆H₅ ]PF₆ can be due to a ground state of Fe(III) high spin situation. In contrast the complexes [Cp Fe(dppe)SR]PF₆ are paramagnetic (m =1,6-2.4BM) with a low spin ground state Fe(III). Most detailed temperature variable susceptibility measurements for all the paramagnetic complexes are in course.

Uv-visible spectra of the complexes (2)-(7) exhibit a similar absorption patterns to that Cp*Fe(PMe₃)₂CH₃ and Cp*Fe(PMe₃)₂Cl previously assigned [2e], however a less defined maximal were observed.The band around 450nm and some unresolved band in the range 450-600nm were observed.

Comparison of the CpFe(dppe) and Cp*Fe(dppe) fragments.

Several experimental evidences have pointed the most electron donor character of the C₅Me₅ ring than the C₅H₅, leading to most electron rich character to C₅Me₅Fe(dppe)⁺ fragment than C₅H₅Fe(dppe)⁺. In fact as is shows in Table I the half wave potential oxidation is lower for the C₅Me₅ series than the unsubstituted C₅H₅, indicating an iron with greatest electron density.

T A B L E I

Comparison of Oxidation Potential for CpFe(dppe)⁺ and their Methylated
Analogue Cp*Fe(dppe)⁺

On the other, hand Mössbauer data also indicate a most high electron density on the iron atom for the Cp* derivative compared with their Cp counter parts. As shown in Table II, the Cp* derivative produces a most lower isomeric shifts which can be interpreted as a most high electron density around the iron atom.

T A B L E II

For to confirm this we have performed extended Hückel (EH) calculation on the models Cp*Fe(PH₃ )₂⁺ and CpFe(PH₃)₂⁺. As in shown in figure 2 the main changes on going from Cp to Cp* iron derivatives are the increasing in the HOMO - LUMO level, the HOMO-LUMO gap as well as the increasing of the electron density from - 0.5 to - 1.4.

Figure 2. OM diagram and atomic charges for the fragment models CpFe(PH₃)₂ and Cp*Fe(PH₃)₂+

EXPERIMENTAL

All reactions were carried out under purified N₂ or Ar using standard Schlenk techniques and the solvent used (methanol, dichloromethane, diethyl ether and n-hexane) were appropriately distilled and dried before use. [Cp*Fe(CO)₂]₂ (Strem), I₂, [NBu₄]PF₆, NH₄PF₆ and dppe (Aldrich) were used as received. TlPF₆ (caution! Thallous salts are very poisonous and should be handled with precaution) was prepared from Tl₂CO₃ and HPF₆. IR spectra were recorded as KBr pellets on a FT-Bruker 66V spectrometer. NMR spectra were run in CD₂Cl₂ or (CD₂)₂C0 solution at room temperature on a Bruker AMX 300 spectrometer with TMS(H) d = 0.0 ppm as internal standard or 85% H₃PO₄ and downfield positive to the reference as external standard for the ³¹P measurements. UV-Visible spectra were run on a Varian DMS-9 spectrophotometer with 1 cm optical path cuvettes.. Magnetic measurements were carried out by a Gouy`s method at room temperature (25ºC) using a Johnson Matthey Balance, using mercury tetrathiocyanate cobaltate(II) as calibrant.

Reaction of [Cp*Fe(CO)₂]₂ with I₂ in CH₂Cl₂.

A mixture of 0.5 g (1.0 mmol) of [Cp*Fe(CO)₂]₂ and I₂ 1.0 g (3.9 mmol) in 50 ml dichloromethane was stirred at room temperature for 1h. The solution was filtered and washed three twice with an aqueous solution of Na₂S₂O₃ (2.5g in 50ml H₂O) and the organic phase was treated with anhydrous Na₂SO₄ and filtered through celite.The red solution was then evaporated to dried under reduced pressure. The resulting solid was recrystallized from of a n-hexane-diethyl ether 1:1 mixture to give (2) as red-dark crystal . Yield 0.69 g; 91%. Anal. Found. : C 37.42, H 3.93. Calc.for C₁₂H₂₁O₂IFe C 38.52, H 4.01.

Ir(Kbr,pellets): n(CO):2000,1953

Reaction of Cp*Fe(CO)₂I with dppe in Toluene.

A solution of Cp*Fe(CO)₂I 0.5 g (1.33mmol) and dppe 0.6 g (0.15mmol) were irradiated in toluene(50ml) for 8hr.After this, the solution was filtered through celite and the solution evaporated to dried in a rotavapor. The resulting red dark solid was redisolved in a n-hexane-diethyl ether 1:1 mixture and placed in the freezer for overnitgh to give (3) as red-brown solid. Yield 0.62 g, 72%.The complex Cp*Fe(dppe)I was characterized by Ir and NMR spectroscopy by comparison with an authentically sample [2a].

Reaction of Cp*Fe(dppe)I with CH₃CN in presence of NH₄PF₆.

Cp*Fe(dppe)I 0.08 g (011 mmol) was stirred with 10ml of a mixture CH₃CN/CH₂Cl₂ in presence of NH₄PF₆ 0.04 g (0.2 mmol) were stirred for 22h. at room temperature. The solution was evaporated under vacuum and the solid residue extracted with dichloromethane and filtered through Celite. Then the solution was evaporated and the red solid washed with diethyl ether. and dried under reduced pressure. Yield 0.05g 63%.The complex [Cp*Fe(dppe)(CH₃CN)]PF₆ was characterized by Ir and NMR spectroscopy by comparison with an authentically sample [2a,b].

Reaction of Cp*Fe(dppe)I with CH₃CN in presence of TlPF₆.

Cp*Fe(dppe)I 0.08 g (011 mmol) was stirred with 10ml of CH₃CN in presence of TlPF₆ 0.08g (0.22 mmol) were stirred for 27h. at room temperature. The solution was filtered through Celite and the solvent was evaporated to dried .The red solid washed with diethyl ether. and dried under reduced pressure. Yield 0.05g 63%.The complex [Cp*Fe(dppe)(CH₃CN)]PF₆ was characterized by Ir and NMR spectroscopy by comparison with an authentically sample [2a,b]

Reaction of Cp*Fe(dppe)I with PPh₃ in presence of NH₄PF₆.

Cp*Fe(dppe)I 0.11g (0.15 mmol) and 0.08g (0.3mmol) PPh₃ in presence of NH₄PF₆ 0.02 g (0.24 mmol) in CH₃OH (20 ml) were stirred for 21 h. The solvent was evaporated under vacuum and the solid residue extracted with CH₂Cl₂ and filtered through celite. The solvent was eliminated and the solid residue washed with ether and dried under reduced pressure give (5) as a red _brown solid. Yield 0.08g 53% m_eff=1.58 BM . Anal. Found. : C 62.72, H 4.96. Calc. for C₆₁H₅₄F₆P₄ Fe•3CH₂Cl₂ C 61.41, H 4.79.

Ir(KBr, pellets): 1117 d(CH)_ip : 695 d(CH)_op : 843 (PF₆).

Reaction of Cp*Fe(dppe)I with PPh₃ in presence of TlPF₆.

Cp*Fe(dppe)I 0.14g (0.19 mmol) and 0.08g (0.3mmol) PPh₃ in presence of TlPF₆ 0.14 g (0.4 mmol) in CH₂Cl₂ (20 ml) were stirred for 19 h.The solution was filtered through celite and the solvent was evaporated under vacuum and the solid residue washed with ether and dried under reduced pressure. Yield 0.08g, 53%. Spectroscopic data were similar to that obtained for the complex [Cp*Fe(dppe)PPh₃][PF₆] obtained by the above method.

Reaction of Cp*Fe(dppe)I with S(C₂H₅)₂ in presence of TlPF₆.

Cp*Fe(dppe)I 0.10g (013mmol) and 0.08g (0.93mmol) S(C₂H₅)₂ in presence of TlPF₆ 0.14 g (0.4 mmol) in CH₂Cl₂ (20 ml) were stirred for 22 h.The red solution was filtered through celite and the solvent was evaporated under vacuum and the solid residue washed with ether and dried under reduced pressure to,give (6) as brown solid. Yield 0.05g 56%. m_eff=1.34 BM . Anal. Found. : C 50.57, H4.51. Calc. for C₄₀H₄₉F₆S P₃Fe • 2CH₂Cl₂ C 50.72, H 4.93.

Ir(KBr, pellets): 1120 d(CH)_ip , 695 d(CH)_op , 843 (PF₆).

Reaction of Cp*Fe(dppe)I with S₂(CH₂-C₆H₅ )₂ in presence of TlPF₆.

. Cp*Fe(dppe)I 0.1g (0.14 mmol) and 0.05g (0.2 mmol) S₂(CH₂-C₆H₅ )₂ in presence of TlPF₆ 0.02 g (0.28 mmol) in CH₂Cl₂ (20 ml) were stirred for 22 h at room temperature. The solution was filtered through celite, and the solvent eliminated in vacuum. The solid residue was washed with ether and dried under reduced pressure, to give (6) as a gray-brown solid. Yield 0.09 g, 75%.m_eff= 6.36 BM Ir(Kbr, pellets) 1123 d(CH)_ip , 695 d(CH)_op , 843 (PF₆).

Calculations. EH. MO calculations were carried out using the modified Wolfsberg -Helmholz formula [13]. The atomic parameters for the elements involved in our calculations have been previously reported [14]. The bonding distances used in the models CpFe(PH₃)_{2 3}⁺ and Cp*Fe(PH₃)₂⁺ were similar to those for the crystal structure of [CpFe(dppe)L]⁺ as well as [Cp*Fe(dppe)]⁺ derivatives [8,9]. For the two fragments the basic parameters were: bond distances Fe-P 2.17 Å, Fe-C 2.08 Å, bond angle P-Fe-P 98.9°. MO drawing were generated with the use of the program CACAO [15].

ACKNOWLEDGEMENTS

The authors thanks to FONDECYT(1000672) the financial support.

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