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logs of carbenes in line with nitrenes, silylenes, or ger- mylenes due to the presence of two free electrons. Species of this kind are believed to be formed in many.
ISSN 1023-1935, Russian Journal of Electrochemistry, 2007, Vol. 43, No. 10, pp. 1151–1155. © Pleiades Publishing, Ltd. 2007. Original Russian Text © T.V. Gryaznova, Yu.G. Budnikova, O.G. Sinyashin, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 10, pp. 1214–1219.

Electrochemical Approaches to Generation of Phosphinidene Complexes of Tungsten Pentacarbonyl T. V. Gryaznova, Yu. G. Budnikovaz, and O. G. Sinyashin Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center Russian Academy of Sciences ul. Arbuzova 8, Kazan, 420088 Russia Received December 11, 2006

Abstract—An alternative approach to synthesis of electrophilic terminal phosphinidene complexes of tungsten pentacarbonyl with various substituents at the phosphorus atom has been developed. The approach is based on the reaction of the electrochemically generated tungsten pentacarbonyl ion with trivalent phosphorus acyl chlorides. Electrochemical reduction of ethyldichlorophosphines in the presence of α-diimines is a one-step process that forms 1,3,2-diazaphospholenes containing an exocyclic P–C bond. Key words: tungsten pentacarbonyl anion, electrosynthesis, R-dichlorophosphine, terminal electrophilic phosphinidene complex of tungsten pentacarbonyl DOI: 10.1134/S1023193507100072

INTRODUCTION The chemistry of low-coordinated compounds has long attracted the attention of organic chemists because many chemical processes are postulated to be reactions that involve these species as intermediates. Phosphinidenes occupy an important place among these compounds. Phosphinidene species (RP) are compounds where the phosphorus atom is formally univalent; they may be regarded as phosphorus analogs of carbenes in line with nitrenes, silylenes, or germylenes due to the presence of two free electrons. Species of this kind are believed to be formed in many organic reactions involving phosphorus compounds [1]. The phosphinidene species as intermediate was reported back in 1963 [2]. However, attempts to generate these compounds were not undertaken until the early 1980s. Phosphinidene species were generated by thermal decomposition of cyclophosphanes [3], reduction of dihalophosphanes with different metals [4], photolysis of diphosphenes [5], and other reactions. The phosphinidene species was first recorded in 1994 by low-temperature EPR in the course of irradiation of mesitylphosphirene [6]. No other publications have appeared since then. Therefore it is important to achieve stabilization of these short-lived species. A number of factors help to stabilize highly reactive species: steric factors, electronic effects of substituents, and transition metals lying in the coordination sphere. Using the latter factor seems to be the most effective method. Recently, we have witnessed rapid developz The corresponding author: [email protected] (Yu. G. Budnikova).

ment of the chemistry of organometal complexes with phosphinidene species as ligands. In terminal phosphinidene metallocomplexes, the phosphorus atom may be either nucleophile or electrophile, which depends on the nature of the metal and ligands bonded to it. Nucleophilic terminal phosphinidene complexes are well-defined [7], and their structure has been investigated by X-ray diffraction (XRD) methods [8], but they are not very active. Electrophilic terminal phosphinidene complexes are highly reactive, but stable complexes of this type have not yet been synthesized [9]. Their high reactivity makes them valuable building blocks in syntheses of a wide variety of organophosphorus compounds, both cyclic and openchain. On the other hand, their high chemical activity hinders synthesis, isolation, and characterization of these compounds. RESULTS AND DISCUSSION All chemical methods for generating electrophilic terminal phosphinidene complexes are based on thermal or catalytic decomposition of appropriate precursors [9] and are often multistep, labor-consuming procedures. An original approach to generation of electrophilic phosphinidene complexes 3 developed by F. Mathey et al. is thermal decomposition of 7-phosphabi-cyclo[2,2,1]hepta-2,5-diene complexes 2 formed in reactions of phosholes 1 with acetylenedicarboxylic ethers [10]. This is now the basic method used for syntheses of electrophilic terminal phosphinidene

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complexes with various substituents at the phosphorus atom.

R Me

Me MeOOCC≡CCOOMe

M P

Me

COOMe

P R

M

COOMe

Me

1

2 ∆ substrate

Product

R P M

.

δê 692.9 ppm, R = Ph 6, R = c-Hex2N, 7; δê 777.9 ppm, R = p-Pr2N, 8). All phosphinidene complexes obtained in these reactions were unstable compounds, which existed for short periods of time in solution. Attempts to isolate them in individual form failed because they were easily hydrolyzed to the corresponding acids. The only exception was the dicyclohexylaminophosphinidene complex (c-Hex2N) of tungsten pentacarbonyl 7, which was isolated as a crystalline compound; its 31P NMR spectrum contained a singlet with δ = 402.7 ppm. Analysis of the spectral and literature [13] data suggested that the product was diphosphene complex 9. Probably, the room-temperature reaction leads to dimerization of the initial product aminophosphinidene complex 7.

3

M = Cr, Mo, W(CO)5

2–

W(CO)5 + c-Hex2NPCl2

While being universal from the viewpoint of incorporating different substituents R, this method has certain limitations on bulky substituents that hinder phosphorus cycloaddition to acetylenecarboxylate. Another problem arises when R = NR2. The electrophilic reagent—acetylenedicarboxylate—can add at the P–N bond. These problems may be solved by electrochemical methods, which have never been employed for this purpose. We have developed a procedure for the synthesis of tungsten pentacarbonyl phosphinidene complexes by a reaction of the electrochemically generated tungsten pentacarbonyl ion 4 with aryldichlorophosphines [11]. The key stage of the process is electrolysis of tungsten hexacarbonyl W(CO)6 in acetonitrile. Tungsten hexacarbonyl accepts two electrons per molecule, and the reaction forms the highly nucleophilic tungsten pentacarbonyl anion 4. W(CO) 6

+ 2e – CO

c-Hex2NP 7

c-Hex2N

W(CO)5 P

P

(CO)5W

NHex-c2 . 9

To confirm that terminal electrophilic phosphinidene complexes of tungsten pentacarbonyl 5–8 formed in these reactions, the latter were conducted in the presence of the phenylacetylene trapping reagent by analogy with the reaction reported in [14]. The 31P NMR spectra of the isolated compounds contain signals in the range of negative values, which correspond to the chemical shifts of the phosphorus atom of tungsten pentacarbonyl phosphirene complexes.

2–

[W(CO) 5 ] .

R

ccccccc 4 All subsequent reactions were conducted in situ at room temperature by adding the phosphorus substrate to the cathode-generated [W(CO)5]2– ion. RPCl2 – –2Cl

[RP

W(CO)5 P

R P W(CO)5 + PhC CH

[W(CO)5]2–

W(CO)5

Ph 10–13 R = Mes (10), Ph (11), c-Hex2N (12), p-Pr2N (13).

W(CO)5]

R = Mes (5), Ph (6), c-Hex2N (7), p-Pr2N (8).

The reaction mixtures were analyzed by 31P NMR spectroscopy. We have succeeded for the first time in recording singlet signals in the low-field (600–800 ppm) range of spectra, which were attributed to the resonance of the phosphorus atom in the electrophilic phosphinidene complexes (δê 766.4 ppm, R = Mes 5;

Formation of the phosphirene complexes of tungsten pentacarbonyl 10–13 was confirmed by analyzing their 1ç, 13ë, and 31P NMR and IR spectra. Interaction of tungsten pentacarbonyl anion 4 with dicyclohexylaminodichlorophosphine in the presence of N,N'-di-tert-butylethylenediimine 14 leads to diazaphospholene complex 16. Reactions of phosphinidene complexes with α-diimines are not found in the literature except one of the early papers by F. Mathey et al.

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[15], which reports on the reaction of the methoxyphosphinidene complex of tungsten pentacarbonyl with 1-azadienes, leading to the [1,4]-cycloaddition product. The 31P NMR spectrum of the reaction mixture contains two signals with δê = –87.20 ppm and δê = 103.12 ppm. Possibly, the strong nucleophilic agent N,N'-di-tertbutylethylenediimine 14 attacks the electrophilic phosphorus atom of the initial product aminophosphinidene complex 11, leading to azaphosphirane complex 15. The possibility of [1,2]-cycloadditions of phosphinidene complexes at the C=N double bond was reported earlier [16]. However, azaphosphirane complexes are unstable compounds and undergo further transformations. In our case, the reaction probably occurs as a [1,3]-sigmatropic shift, forming diazaphospholene complex 16. A [1,3]-sigmatropic shift of this kind was described by K. Lammertsma and co-workers for 2-vinylphosphiranes [17], although calculations indicated that the [1,4]-adduct is preferable in this case [18].

Ph

2–

W(CO)5 + c-Hex2NPCl2

c-Hex2NP

1153 W(CO)5

+ t-BuN CHCH NBu-t

c-Hex2N

14

W(CO)5 P

HC

c-Hex2N

N Bu-t

W(CO)5 P

t-Bu N

HC

N Bu-t .

N Bu-t 15

16

Our second approach to generation of phosphinidene complexes of tungsten pentacarbonyl was electrochemical decomposition of 7-phosphanorbornadiene complex 2. Electrolysis of 2 was conducted in tetrahydrofuran by passing electricity (2 F/mol) in the presence of phenylacetylene. According to 31P NMR, electrochemical reduction of phosphanorbornadiene 2 occurs with cleavage of two P–C bonds and leads to phenylphospinidene complex 6; formation of the latter was confirmed by a reaction with phenylacetylene:

W(CO)5 P

Ph

Me

COOMe

+ 2e

PhP W(CO)5

W(CO)5 P

PhC≡CH

Ph

.

COOMe

Me 2

6

We have also attempted to perform electrochemical reduction of ethyldichlorophosphine in the presence of α-diimines, N,N'-di-tert-butylethylenediimines 14, and N,N'-dibutyldibutanediimine 17. As is known [19], electrochemical reduction of trivalent phosphorus acyl dihalides occurs with simultaneous elimination of two halogen atoms via the formation of phosphinidene species. Preparative one-stage electrolysis of ethyldichlo-

11

rophosphine in the presence of diimine 17 gave 2-ethyl1,3-dibutyl-4,5-dimethyl-1,3,2-diazaphosphol-4-ene 18 [20] (δê = 110.31 ppm). Electrolysis of ethyldichlorophosphine in the presence of diimine 14 leads to a mixture of products, probably because of the lower reactivity of this compound ensuing from sterically crowded nitrogen atoms.

CH3

CH3

Bu

NBu

N

C C

Et

+ 2e PCl2 – 2Cl–

Et

.. P:

BuN 17

Et

P N Bu 18

Thus electrochemical methods of generating phosphinidene complexes of tungsten pentacarbonyl suggested in this work have a number of advantages over RUSSIAN JOURNAL OF ELECTROCHEMISTRY

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CH3 CH3

.

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EXPERIMENTAL Preparative electrolysis was carried out in a 50-ml three-electrode cell using a B5-49 direct current source. The working electrode potential was recorded with a Shch-50-1 direct current voltmeter relative to the Ag/0.01 M AgNO3 reference electrode in an appropriate solvent. The working surface of the cylindrical platinum cathode used as a working electrode was 20.0 cm2. Paper was used for the diaphragm, a glassycarbon plate with a working surface of 4 cm2 served as anode, and a saturated acetonitrile solution of Et4NBF4 was anolyte. 1H NMR spectra were measured on a Bruker WM-250 Fourier NMR spectrometer at a frequency of 250.132 MHz relative to the signal of the residual protons of the deuterated solvent used as internal standard. 31P NMR spectra were obtained on a CXP-100 Bruker spectrometer (36.47 MHz) relative to 85% orthophosphoric acid as external standard. 13C NMR spectra were taken on a Bruker MSL-400 Fourier NMR spectrometer (100.6 MHz) relative to the carbon signal of the deuterated solvent as internal standard. IR spectra were recorded on a Vector-22 Bruker spectrometer (400–3600 cm–1) in Vaseline oil or in a thin layer. Acetonitrile was purified by three fractional distillations at first with a potassium permanganate addition and then over phosphoric anhydride. Benzene and THF were dehydrated by distillation over sodium; diethyl ether, by boiling it over P2é5 with further distillation. After purification the solvents were stored under dry argon. The base salt Et4NBF4 was obtained by mixing aqueous Et4NOH (30%) and HBF4 till neutral to the indicator. The (Et4NBF4) precipitate was filtered off, subjected to two recrystallizations from ethanol, and dried in a vacuum chamber at 100°ë for 2 days. Electrolysis of Tungsten Pentacarbonyl Anion 4 An electrochemical cell was charged with a solution of tungsten hexacarbonyl (0.704 g, 2 × 10–3 mol) in acetonitrile (30 ml) containing tetraethylammonium tetrafluoroborate (0.0325 g), and 2 F of electricity per mole of the starting carbonyl was passed through the electrolyte while stirring it with a magnetic stirrer under a constant flow of argon. General Procedure for the Synthesis of 1-R-2-Phenylphosphirene Tungsten Pentacarbonyl Dichlorophosphine (2 × 10–3 mol) was added dropwise to the freshly prepared acetonitrile solution of tungsten pentacarbonyl anion 4 at –30°ë. The reaction mixture was stirred for 30 min, and phenylacetylene (7 × 10–3 mol) was added dropwise to the solution. The reaction mixture was warmed to room temperature and then heated at 50°ë for 3 h. The solvent was removed,

and the crystalline residue extracted with benzene. Then benzene was distilled off from the resulting extract, and the crystalline residue washed with diethyl ether to give the products characterized below. 1-Mesityl-2-phenylphosphirene tungsten pentacarbonyl 10: 0.52 g (45.1%), mp 128-130°ë. 31P NMR spectrum (CH3CN, δ, ppm J/Hz): –120.06 (1JPW 272.52). 1H NMR spectrum (CDCl , δ, ppm, J/Hz): 2.28 (s, 3H, 3 p-CH3, Mes), 2.54 (s, 6H, m-CH3, Mes), 6.87 (br.s, 2H, H, Mes), 7.33 (m, 5H, Ph), 7.47 (d, 1H, =CH, 2JPH 8.05). 13ë NMR spectrum (CDCl , δ, ppm, J/Hz): 20.96 (s, 3 p-CH3), 21.17 (d, m-CH3, 3JPC 8.03), 121.64 (d, C-CH, 1J 2 PC 32.12), 125.85 (d, ipso-C, Ph, JPC 8.57), 127.73 2 (d, o-C, Mes, JPC 14.45), 127.99 (s, m-C, Ph), 128.17 (s, o-C, Ph), 128.32 (d, ipso-C, Mes, 1JPC 29.72), 128.47 (s, p-C, Mes), 129.90 (s, m-C, Mes), 131.75 (s, p-C, Ph), 142.32 (d, c-C–Ph, 1JPC 32.16), 199.36 (d, cis-CO, 2JPC 6.43), 201.92 (d, trans-CO, 2JPC 24.10). IR spectrum (Vaseline, ν, cm–1): 2067 (CO), 1905 (CO), 1655 (C=C), 1605 (C=C arom). Found, %: C 44.98, H 2.45, P 5.59. Calculated for C22H17O5PW, %: C 45.83, H 2.95, P 5.38. 1,2-Diphenylphosphirene tungsten pentacarbonyl 11: 0.42 g (39.3%), mp 85-87°C. 31P NMR spectrum (CH3CN, δ, ppm, J/Hz): –157.34 (1JPW 288.53). 1H NMR spectrum (CDCl , δ, ppm, J/Hz) 6.65 (m, 5H, 3 Ph), 7.48 (m, 5H, =CPh), 8.04 (d, 1H, =CH, 2JPH 10.45). 13C NMR spectrum (CDCl , δ, ppm, J/Hz): 121.36 3 (d, c-CH, 1JPC 31.51), 139.22 (d, c-C–Ph, 1JPC 32.16), 199.53 (d, cis-CO, 2JPC 7.04), 201.02 (d, trans-CO, 2J –1 PC 28.14). IR spectrum (Vaseline, ν, cm ): 2076 (CO), 1949 (CO), 1653 (C=C), 1602 (C=C arom). Found, %: C 42.18, H 2.01, P 5.35. Calculated for C19H11O5PW, %: C 42.69, H 2.03, P 5.80. 1-Cyclohexylamino-2-phenylphosphirene tungsten pentacarbonyl 12: 0.58 g (45.5%), mp 115-117°C. 31P NMR spectrum (CH CN, δ, ppm, J/Hz): –23.16 3 (1JPW 266.72). 1H NMR spectrum (CD3CN, δ, ppm, J/Hz) 1.81 (m, 12H, ëç2-cycl.), 1.96 (m, 8H, ëç2-cycl.), 3.44 (m, 2H, CH-cycl.), 7.15 (m, 5H, Ph), 8.65 (s, 1H, =CH). IR spectrum (ν, cm–1): 1594 (C=C arom.), 1680 (C=C), 1905, 2043 (CO), 3102 (H–C=). Found, %: C 46.65, H 4.04, N 2.32, P 4.99. Calculated for C25H28NO5PW, %: C 47.09, H 4.39, N 2.20, P 4.87. 1-Propylamino-2-phenylphosphirene tungsten pentacarbonyl 13: 0.46 g (41.3%), mp 62-64°C. 31P NMR spectrum (CH3CN, δ, ppm): –37.20 (1JPW 273.17). 1H NMR spectrum (CD CN, δ, ppm, J/Hz) 0.99 (t, 6H, 3 −ëç3, 3JHH = 7.2 Hz), 1.58 (sextet, 4H, β-ëç2), 2.84 (m, 4H, α-ëç2), 7.84 (s, 1H, =CH), 7.42 (m, 5H, =CPh). IR spectrum (ν, cm–1): 1591 (C=C arom.), 1668 (C=C), 1992 and 2069 (CO), 3177 (H–C=). Found, %: C 40.56, H 3.20, N 2.31, P 5.79. Calculated for C19H20NO5PW, %: C 40.93, H 3.59, N 2.51, P 5.56. 1-Dicyclohexylamino-2,3-di-tert-butyl-1,3,2-diazophospholene tungsten pentacarbonyl 16. Dicyclohexylaminodichlorophosphine (0.564 g, 2 × 10–3 mol) in acetonitrile (10 ml) was added dropwise to a freshly

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prepared solution of tungsten pentacarbonyl anion 4 in acetonitrile at –30°ë. The reaction mixture was stirred for 30 min, whereupon N,N'-di-tert-butylethylenediimine 14 (0.336 g, 2 × 10–3 mol) was added dropwise to the solution. The reaction mixture was warmed to room temperature. The crystalline precipitate was filtered off and washed with diethyl ether to give compound 16 (0.91 g, 64.7%), mp 240°ë with sublimation. 31P NMR spectrum (DMF-d6, δ, ppm, J/Hz): 103.12 (1JPW 321.57). 1H NMR spectrum (DMF-d6, δ, ppm) 1.30, 1.34 (two s, 9H, t-Bu), 1.78, 2.01 (m, 20H, ëç2-cycl.), 3.09 (m, 2H, CH-cycl.), 8.53 (br.s, 2H, =CH). IR spectrum (ν, cm–1): 1667 (C=C), 1828, 1931 (CO), 3154 (=C–H). Found, %: C 46.36, H 6.08, N 5.87, P 4.69. Calculated for C27H42N3O5PW, %: C 46.09, H 5.97, N 5.97, P 4.41. bis(tungsten Tetracyclohexyldiaminophosphene pentacarbonyl) 9. Dicyclohexylaminodichlorophosphine (0.564 g, 2 × 10–3 mol) was added dropwise to a freshly prepared solution of tungsten pentacarbonyl anion 4 in acetonitrile at room temperature. The reaction mixture was stirred for 2 h, then the white crystalline precipitate was filtered off and washed with diethyl ether to give compound 16 (0.32 g, 46.6%), mp 220°ë. 31P NMR spectrum (DMF-d , δ, ppm, J/Hz): 402.7 6 (1JPW 317.45). 1H NMR spectrum (DMF-d6, δ, ppm) 1.78, 2.01 (m, 20H, ëç2cycl.), 3.09 (m, 2H, CH cycl.). IR spectrum (ν, cm–1): 1969 and 2051 (CO), 3154 (=C–H). Found, %: C 37.56, H 3.88, N 2.38, P 5.96. Calculated for C34H44N2O10P2W2, %: C 38.13, H 4.11, N 2.62, P 5.80. 1-Ethyl-1,3-dibutyl-4,5-dimethyl-1,3,2-diazaphosphol-4-ene 18. An electrochemical cell was charged with ethyldichlorophosphine hexacarbonyl tungsten (0.76 g, 6 × 10–3 mol) in acetonitrile (40 ml) containing tetraethylammonium tetrafluoroborate (0.0325 g). Electricity (12 F per mole of the starting carbonyl) was let through the electrolyte while stirring the latter with a magnetic stirrer under a constant flow of argon. mol) was added dropα-Diimine 17 (1.2 g, 6 × wise during electrolysis. After electrolysis the reaction mixture was stirred for 1 h. Then the solution was concentrated, and benzene (100 ml) was added to it. The base salt precipitate was filtered off, the solvent removed, and the residue distilled in vacuum. This gave compound 18 (1.01 g, 66.4%), bp 131–132°ë (10 Torr), 20 n D 1.4822). 31P NMR spectrum (C6D6, δ, ppm): 110.31. 1H NMR spectrum (C6D6, δ, ppm, J/Hz): 1.02 (t, 6H, ëç3, 3Jçç 6.5), 1.21 (sextet, 4H, γ-ëç2, 3Jçç 6.5), 3.35 (t, 3H, 3Jçç 8.0), 1.44 (quintet, 4H, β-ëç2, 3Jçç 6.5), 2.01 (s, 6H, =CCH3), 3.21 (m, 6H, êëç2 and α-ëç2). IR spectrum (ν, cm–1): 1660 (C=C). 10–3

This work was supported by the Russian Foundation for Basic Research, grant nos. 04-03-32830, 05-0308039, 06-03-08019, and 07-03-0013, by the OKhNM program, grant nos. 1 and 8, and by the Scientific School Program, grant no. 5148.2006.3. RUSSIAN JOURNAL OF ELECTROCHEMISTRY

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