ISSN 0012�5008, Doklady Chemistry, 2010, Vol. 431, Part 1, pp. 65–70. © Pleiades Publishing, Ltd., 2010.Original Russian Text © V.I. Sokolov, L.A. Bulygina, V.N. Khrustalev, Z.A. Starikova, L.N. Nikitin, A.R. Khokhlov, 2010, published in Doklady Akademii Nauk, 2010, Vol. 431,No. 1, pp. 52–57.

65

Supercritical carbon dioxide (sc�CO2) can dis�solve many organic and inorganic metal�containingcompounds and deliver them by impregnation intoporous materials and, with the use of sorption mech�anisms, into nonporous polymeric materials [1–3].However, only a few examples are known in whichsc�CO2 is directly involved in chemical reactions as areagent [4–6].

When studying the effect of sc�CO2 on organome�tallic compounds, we found that decreasing pressureand temperature of an acetylferrocene solution in asupercritical medium leads to the deposition of well�formed crystals of this compound [7].

The present paper deals with the verification of theability of sc�CO2 to interact, jointly with active chem�ical groups of a Nafion® polymeric membrane, withmetallocenylcarbinols (C5H5)M(C5H4)CH2OH, whereM = Fe (1a) or Ru (1b).

We found that ferrocenylmethanol (1a) andruthenocenylmethanol (1b) are readily soluble in sc�CO2 at a pressure of at least 8 MPa and temperature ofat least 35°C and, after solvent removal, form longneedle�shaped crystals shown in Fig. 1. In acidicmedia, these compounds form metallocenylcarbe�

nium ions Мс–С (2а and 2b, respectively), whichare stable in solution due to the fact that the metallo�

H2+

CHEMISTRY

Supercritical Carbon Dioxide as a Solvent for Crystallization and a Reaction Medium

for Metallocene DerivativesV. I. Sokolov, L. A. Bulygina, V. N. Khrustalev,

Z. A. Starikova, L. N. Nikitin, and Academician A. R. Khokhlov

Received September 7, 2009

DOI: 10.1134/S001250081003002X

Nesmeyanov Institute of Organoelement Chemistry, Russian Academy of Sciences, ul. Vavilova 28, Moscow, 119991 Russia

Fig. 1. Photomicrographs of single crystals of (C5H5)M(C5H4)CH2OH for (a) M = Fe (1a, light yellow) and (b) M = Ru (1b,white).

50 μm 50 мкм(a) (b)

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SOKOLOV et al.

cenyl group is involved as the electron donor in delo�calization of the (+) charge of the α�carbon atom overthe entire molecule [8].

We were interested in checking whether such a cat�ion can form with the participation of the Nafion®

polymeric film containing strong acidic moieties,sulfo groups bound to perfluoroalkyl radicals. To dothis, a small piece of the film was placed into a high�pressure cell in order to infuse substrates 1a and 1b dis�solved in sc�CO2 into the film where they could con�vert into cations 2a and 2b under the action ofRfSO3H, which in this case would be the counterion.The resulting film after the experiment with 1a at 80°Сabsorbed light in the λmax range of 600–630 nm, whichis typical of ferricinium cations [С5Н5FeC5H4X]+ (3а),where X = H, СН3, or СН2ОН; after the analogous

experiment with 1b, the film absorbed at λmax ~ 500 nm(Fig. 2). Such a behavior is expected to 2a and 2b.After the experiments at 35°С, the absorption typicalof ferricinium ions (~600 nm) was not observed. It isworth noting that, in the control experiment, a neutralpolyethylene film did not change its color.

On exposure to air, the Nafion® film impregnatedwith FcCH2OH (1a) did not change its color; however,within 30–60 min, light yellow needle�shaped singlecrystals appeared on the smooth film surface (Fig. 3).X�ray crystallography showed that these crystals arethose of the Fc–CH2–O–CH2–Fc ether (4a), whichwas studied previously [9]. The process can be inter�preted in the following way. Carbocation 2a inside thefilm reacts with water penetrating into the film toform, due to water deficiency the ether rather than theinitial alcohol:

Two pathways are possible: either the end product4a diffuses to the film surface where it crystallizes, orthe final stage of formation of ether 4a occurs at thesurface.

Analogously, ruthenocenylmethanol 1b gave ether4b. It is currently difficult to decide between these twopossibilities: whether ethers form inside the film or onits surface. The increase in the film weight (see below)

Fc–CH2–OH Fc–CH2+

Fc–CH2–O–CH2–Fc H+

,+ +1a 2a

2Fc–CH2+

H2O+ Fc–CH2–O–CH2–Fc 2H+

.+4a2a

1

5000

700 900 1100λ, nm

1

2

3

2

3

4

Abs

orba

nce

Fig. 2. Visible absorption spectra of (1) the initial Nafion® film, and (2, 3) the films treated with (2) FcCH2OH and(3) RcCH2OH in sc�CO2 at 20 MPa and 80°C.

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SUPERCRITICAL CARBON DIOXIDE 67

is presumably evidence of the involvement of exter�nal water in the reaction. The absorption at ~600 nmis likely due to ferricinium salt 3a, the minor prod�uct of a side reaction. The same sulfo acid anionscan be counterions. In the case of compounds 1, theintense ferricinium color prevents the observation

of the color of ferrocenyl cation 2a in the film.Owing to the known difference between ferroceneand ruthenocene, the latter is not able to lose onlyone electron in oxidation; therefore, there is no col�ored species analogous to ferricinium forruthenocene.

Table 1. Crystal data for ruthenium compounds 1b and 4b

Parameter 1b 4b

Empirical formula C11H12ORu C22H22ORu2

FW 261.28 504.54

T, K 120(2) 100(2)

Crystal size, mm 0.30 × 0.03 × 0.03 0.24 × 0.12 × 0.02

Symmetry system Monoclinic Monoclinic

Space group C2/c P2/n

a, Å 44.870(5) 12.7652(12)

b, Å 5.9408(7) 5.6624(5)

c, Å 25.126(3) 25.065(2)

α, deg 90 90

β, deg 122.121(2) 102.080(2)

γ, deg 90 90

V, Å3 5672.5(11) 1771.6(3)

Z 24 4

dc, g cm–3 1.836 1.892

F(000) 3120 1000

μ, mm–1 1.610 1.711

2θmax, deg 60 60

Number of measured reflections 31466 22003

Number of unique reflections 8237 (Rint = 0.062) 5166 (Rint = 0.067)

Number of observed reflections with I > 2σ(I) 5752 3924

Number of refined parameters 352 227

R1; wR2 (I > 2σ(I)) 0.0522; 0.1119 0.0359; 0.0614

R1; wR2 (all data) 0.0843; 0.1282 0.0592; 0.0670

GOF 1.014 1.004

Tmin; Tmax 0.644; 0.953 0.684; 0.966

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To elucidate the conditions for the formation ofions in the Nafion® film, we compared the UV spectraof the films after the experiments at 20 MPa and 80°C(Fig. 2). It was found that decreasing temperaturerules out the formation of the ferricinium ion (λmax ~600 nm) and that the absorption maximum of car�bocation 2a (about 500 nm) is, as expected, close tothat of ruthenium analogue 2b. This proves that for�mation of the ferricinium ion is a side reaction path�way rather than the result of intramolecular electrontransfer in carbenium ion 2a. Indeed, the experimentwith ferrocene at 80°C showed the ferricinium ionresulted from oxidation in the system (Fig. 2).

The structures of 1b and 4b were studied by X�raycrystallography (Fig. 4). The structure of 1b is mono�clinic, space group C2/c. The unit cell contains threeindependent molecules. The general features of thesethree molecules in the structure of 1b are almost thesame as those of ferrocenyl analogue 1a [9] and arepresumably determined by the formation of helicalchains through intermolecular hydrogen bonds in thecrystal. The difference is that the crystal structure of 1acontains only left�handed H�bonded helical chains,which leads to the chiral space group C2, whereas thecrystal structure of 1b contains both left� and right�handed helices.

The structure of 4b is monoclinic, spacegroup P2/n, and is isostructural with ferrocenyl ana�logue 4a [9].

All experiments were carried out in a stainless steelhigh�pressure with a volume of 10 cm3. Solid metallo�cenylcarbinol was placed into the cell and kept at apressure of 20 MPa and temperature of 35 or 80°C for2 h. A CO2 cylinder, a pressure generator (working vol�ume, 90 mL; High Pressure Equipment, UnitedStates), and the cell were connected to pressure gauges

to control the pressure by means of a system of valvesand capillaries that let CO2 in and out. The specifiedtemperature was maintained with an accuracy of±0.2°. The pressure in the cell was maintained with anaccuracy of ±0.2 MPa. The CO2 pressure release wasperformed through a fine control valve for 10 min.After crystallization, single crystals were obtained aslight yellow or white needles (Figs. 1a and 1b). Theywere examined under a Carl Zeiss Axiolab Pol polariz�ing microscope between crossed and uncrossed polar�izer and analyzer. The appearance of a fine phase canbe evidence of the good solubility of metallocenyl�carbinols in sc�CO2 under the experimental condi�tions used.

Under the same conditions, FcCH2OH 1a (12 mg)and the Nafion film (60 mg) were placed in the high�pressure cell and kept at 20 MPa and 35 or 80°Cfor 2 h. Then, the pressure was released and the cellwas opened. Within 30 min after the experiment, thefilm weight increased to 64.5 mg (7.5 wt %) due to theformation of crystals of 4a.

The unit cell parameters and reflection intensitiesfor compounds 1b and 4b were measured, respectively,on Bruker SMART 1000 CCD and SMART APEX IICCD automated diffractometers (λMoKα, graphitemonochromator, ϕ and ω scan mode). The experi�mental intensity array was corrected for X�ray radia�tion absorption with the SADABS program. SelectedCrystal data are presented in the table.

The structures of both compounds were solved bydirect methods and refined by the full�matrix least�squares method in the anisotropic approximation forthe non�hydrogen atoms. The hydrogen atoms of thehydroxyl groups in the structure of 1b were locatedfrom difference Fourier syntheses and refined in theisotropic approximation with fixed positional andthermal (Uiso(H) = 1.2Ueq(O)) parameters. The otherhydrogen atoms were introduced in the geometricallycalculated positions and refined isotropically withfixed positional (the rider model) and thermal param�eters (Uiso(H) = 1.5Ueq(C) for CH3 groups andUiso(H) = 1.2Ueq(C) for the other groups). All calcula�tions were performed with the SHELXTL software[10]. The atomic coordinates, bond lengths, bond andtorsion angles, and anisotropic thermal parameters forcompounds 1b and 4b were deposited with the Cam�bridge Structural Database (collection codes CCDC718014 and CCDC 718015).

Thus, we showed for the first time that, althoughsc�CO2 is not involved in the process as a reagent, itpromotes the formation of the ether from metalloce�nylcarbinol, which is a new process in the chemistry ofmetallocenes.

50 μm

Fig. 3. Surface of the Nafion® film after the experiment.There are crystals of 4a on the surface.

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SUPERCRITICAL CARBON DIOXIDE 69

ACKNOWLEDGMENTS

We are grateful to A.V. Kaznacheev for performingthe microscopic studies.

This work was supported by the Russian Founda�tion for Basic Research (project nos. 08–03–00169,08–03–00294, 08–03–90012, and 09–03–91227),the Russian Academy of Sciences (Program of theDivision of Chemistry and Materials Science of theRAS “Design of New Metallic, Ceramic, Glassy,Polymeric, and Composite Materials”), and the Pre�sidium of the RAS (program P27 “Fundamentals of

Basic Research of Nanotechnologies and Nanomate�rials”).

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C31

C32 C28

C29

C30

Ru3

C27

C26

C25C24

C23 C33

O3

O1

C11

C2C1

C3

C4

C5Ru1

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C10C8

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C7A

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(b)

(а)

Fig. 4. Molecular structures of (a) 1b and (b) 4b. For 1b, three crystallographically independent molecules are shown; dashed linesdenote hydrogen bonds. For 4b, one of the two crystallographically independent molecules in special positions on twofold axes isshown.

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