Novel Organophosphorus Cage Compound Produced by an Unexpected Oxidative Coupling of 1,2,4-Triphosphole: Crystal and Molecular Structures of Two Isomers of Formula P6C4Bu t4 CHSiMe3

Communication

Novel Organophosphorus Cage Compound Produced by an Unexpected Oxidative Coupling of 1,2,4–Triphosphole: Crystal and Molecular Structures of Two Isomers of Formula P₆C₄Bu^t₄ CHSiMe₃

Maria Helena Araujo,^a Daniel A. Rajão,^a Antonio C. Doriguetto,^b Javier Ellena,^b Eduardo E. Castellano,^b Peter B. Hitchcock^c and Vinicius Caliman ^{* ,a}

^a Departamento de Química, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte - MG, Brazil

^b Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos - SP, Brazil

^c School of Chemistry, Physics and Environmental Sciences, University of Sussex, Brighton, UK BN1 9QJ

Li(P₃C₂Bu^t₂) reage com BrCH(SiMe₃)₂, via adição oxidativa, formando um novo composto organofosforado de fórmula molecular P₆C₄Bu^t₄ CHSiMe₃ (1). O composto 1 foi caracterizado espectroscopicamente e sua estrutura molecular determinada através de difração de raios X de monocristais. Juntamente com 1, foi caracterizado outro composto (2) com a mesma fórmula molecular, mas apresentando uma estrutura completamente saturada.

Reaction of Li(P₃C₂Bu^t₂ ) with BrCH(SiMe₃)₂ results in a novel organophosphorus compound with the formula, P₆C₄Bu^t₄ CHSiMe₃ (1). Compound 1 was fully characterised spectroscopically and its unique molecular structure determined by single crystal X-ray diffraction. Another isomer (2) with a saturated structure was also fully characterised.

Keywords: organophosphorus compounds, multinuclear NMR, X-ray molecular structure

Introduction

The chemistry of organophosphorus compounds is in constant development,¹ and it was the synthesis of P≡CBu^t in 1981,² the first stable phosphaalkyne at room temperature, that marked the beginning of this fascinating field of chemistry. This terbutylphosphaalkyne can undergo cyclo oligomerisation reactions to produce rings and cages. There are reports in the literature of compounds containing from 2 to 6 P≡CBu^t units with structures similar to those of the hydrocarbon analogues.³ The anionic rings (P₃C₂Bu^t₂ )^- (3) and (P₂C₃Bu^t₃ )^- (4) have also been used as starting materials for the syntheses of several cages compounds containing phosphorus, e.g. P₆C₆Bu^t₆ , P₄C₆Bu^t₆ H₂, usually by oxidative coupling reactions using metal complexes³ and protonating reagents.⁴

Using phosphaalkyne, P≡CBu^t, and an iron complex, [Fe(η⁶-C₆H₆)(η⁴-C ₁₀H₁₀)], Zenneck and co-workers⁵ have prepared several organophosphorus compounds including P₆C₄Bu^t₄ CHBu^t (5) which contains two P₃C₂Bu^t₂ rings bonded together, and a CHBu^t group bridging two phosphorus atoms. Very recently, Avent et al.⁶ reported the synthesis of P₆C₄Bu^t₄ GeI₂ (6), whose structure is similar to that of compound 5 but with a GeI₂ fragment at the bridging position.

Results and Discussion

We report herein another oxidative coupling reaction involving the anionic ring 3 to give a new unsaturated structure of formula P₆C₄Bu^t₄ CHSiMe₃ (1). This new cage compound has an open structure with two P=C double bonds and a CHSiMe₃ group bridging two phosphorus atoms. Another isomer of this species, compound 2, was isolated from the same reaction mixture. This compound has a completely saturated structure, similar to that observed for compounds 5 and 6, but with a CHSiMe₃ fragment at the bridging position, instead of CHBu^t and GeI₂, respectively.

Compounds 1 and 2 were synthesised from the reaction of equimolar amounts of 3 with a bulky group⁷ BrCH(SiMe₃)₂, in DME.⁸ The mechanism of formation of both isomers is not clear, but one can speculate that it could take place via the well known 1,2,4-triphosphole (7)⁹ which is also identified in the reaction mixture and probably dimerises to give 1 and 2 with loss of CH(SiMe₃)₃, which was not isolated. Compound 2 was also prepared in low yield (6.4%) by Nixon and co-workers¹⁰ using a cobalt complex to promote the coupling.

Compounds 1 and 2, both air and moisture stable, were characterised by mass spectrometry, ¹H, ¹³C and ³¹P NMR spectroscopy and had their structures determined by single crystal X-ray diffraction studies.

The mass spectrum of 1 exhibits a molecular ion peak at m/z 548, and the ³¹P{¹H} NMR spectrum shows six resonances, two typical of sp² hybridised phosphorus with their chemical shifts in the high frequency region (δ 346.7 and 294.7) and only small couplings indicating that no direct P-P bonds are present.¹¹ The remaining resonances are all typical of sp³ phosphorus (δ 74.6, 45.6, 30.0 and -18.1). The resonance at δ 75.0 shows two large coupling constants (¹J_P(1)P(2) 268.2 and ¹J_P(1)P(5) 317.5 Hz) giving evidence that this phosphorus atom is bonded to other two in a P-P-P network.

The new tetracyclic structure of 1, presented in Figure 1, was elucidated by single crystal X-ray analysis¹² and reveals three phosphorus atoms bonded together, P(2)-P(1)-P(5), and two P=C doubled bonds. The distance between P(2)-P(1) and P(1)-P(5) (2.250(1) and 2.191(2) Å, respectively) are in the expected range for a P-P single bond.¹³ The two doubled bonds P(3)-C(3) and P(4)-C(5) (1.668(4) and 1.676(4) Å, respectively) are the shortest ones in this cage framework and lie in the expected range previously reported (1.64 - 1.69 Å).¹⁴ The C(3) and C(5) carbons show bond angles typical of sp²-hybridised atoms.

Isomer 2 with the same MS molecular ion m/z 548 was characterised by ³¹P{¹H} NMR spectroscopy showing six resonances all typical of sp³ hybridised phosphorus. The two resonances at δ 196.9 and 178.4 with small P-P couplings (in the 15-30 Hz range) indicate the presence of two phosphorus bonded only to carbon atoms.¹⁵ Large P-P couplings for the remaining four resonances (¹J_P(1)P(2) 228.4 and ¹J_P(4)P(5) 237.2 Hz) suggest four phosphorus atoms bonded together two by two.

The molecular structure of 2 is shown in Figure 2.¹⁶ It presents the two five membered rings joined side by side by a CHSiMe₃ bridge forming a cage with four five-membered and three four-membered rings. The average C-C and P-C distances are 1.54(2) and 1.88(1) Å, respectively, which are in the expected range.^13,14 The P(2)-P(1) and P(4)-P(5) bond distances are 2.201(1) and 2.190(1) Å, respectively, in agreement with the triad, P-P-P, found in structure 1.

Acknowledgements

This work has been supported by Brazilian agencies CNPq, FAPEMIG and FAPESP (Projeto Temático No. 98/12151-1). J.E. and A.C.D. thank FAPESP and CNPq, respectively, for the postdoctoral fellowship. We also thank Prof. J. F. Nixon (UK) for his constant encouragement.

supplementary Material

Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication numbers CCDC 173911 and 173912. Copies of the data can be obtained, free of charge, on apllication to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:[phone] or e-mail: [e-mail]).

11. General procedure for the preparation of P₆C₄Bu^t₄ CHSiMe₃1 and 2: HCBr(SiMe₃)₂ (0.6 mL, 2.31 mmol) in DME (10mL), was added slowly to a solution of Li(P₃C₂Bu^t₂ ) (2.31 mmol) in 15 mL of DME, at -30 ºC and the resulting mixture was stirred for 20 h. The LiBr formed during the reaction was filtered off and the solvent evaporated under vacuum. The residue was purified by column chromatography (kieselgel/hexane) to give two yellow bands. The first was characterised as compound 1 (250 mg, 20%), and the second as compound 2 (200 mg, 16%). Recrystalisation from hexane at room temperature afforded yellow crystals suitable for X-ray diffraction study for both compounds.

14. Compound 1: ³¹P{¹H} NMR (121.44 MHz, C₆D₆, H₃PO₄ ext. standard, 25 °C): δ 346.7 [P(3)], 294.7 [P(4)], 74.6 [P(1)], 45.6 [P(5)], 30.0 [P(6)], -18.1 [P(2)]; ¹J_P(1)P(5) 317.5 Hz, ¹J_P(1)P(2) 268.2 Hz. ¹³C {¹H} NMR (75.43 MHz, C₆D₆, 25 °C): δ 204.2 (m, PCP); 200.52 (m, PCP); 91.89 (m, P=CP); 75.44 (m, P=CP); 43.06 (m, C(CH₃)₃); 42.21 (m, C(CH₃)₃); 40.65 (m, C(CH₃)₃); 39.23 (m, C(CH₃)₃); 36.80 (m, CH); 32.70 (m, C(CH₃)₃); 30.39 (m, C(CH₃)₃); 29.13 (m, C(CH₃)₃); 0.03 (s, Si(CH₃)₃). ¹H NMR (300.00 MHz, C₆D₆): δ 2.46 (m, CH); 1.53 (s, 18H, 2C(CH₃)₃); 1.36 (d, 9H, C(CH₃)₃, ⁴J_(HP) 2.14 Hz); 1.30 (d, 9H, C(CH₃)₃, ⁴J_(HP) 1.4 Hz); 0.22 (s, 9H, Si(CH₃)₃). EI-MS m/z (%): 548 (30) [P₆C₄Bu^t₄ CHSiMe₃]⁺, 533 (12) [P₆C₄Bu^t₄ CHSiMe₂]⁺, 491 (11) [P₆C₄Bu^t₃ CHSiMe₃]⁺, 476 (13) [P₆C₄Bu^t₄ CH]⁺.

15. Crystal data for 1: P₆C₂₄H₄₆Si, M = 548.51, monoclinic P2₁/n (N^o 14), a = 11.768(3), b = 15.249(6), c = 17.033(5) Å, b = 93.35(2), V = 3051(2) ų, T = 173(2) K, Z = 4, μ = 0.405 mm^-1, λ = 0.71073 Å, 5608 reflections collected, 5339 independent (R_int = 0.030), 3901 reflections with I > 2σ(I) R(F) = 0.049, wR(F²) = 0.108 for I>2σI, R(F) = 0.078, wR(F²) = 0.123 for all data.

22. Compound 2: ³¹P{¹H} NMR (161.97 MHz, CDCl₃, H₃PO₄ ext. standard, 25 °C): δ 196.9 [P(3)], 178.4 [P(6)], 148.5 [P(2)], 139.3 [P(5)], 122.9 [P(1)], 120.5 [P(4)]; ¹J_P(4)P(5) 237.2 Hz, ¹J_P(1)P(2) 228.4 Hz. ¹³C {¹H} NMR (100.61 MHz, CDCl₃, 25 °C): δ 41.5 (s, CMe₃); 41.1 (s, 2(CMe₃)); 40.6 (s, CMe₃); 37.1 (m, CBu^t); 35.1 (m, CBu^t); 30.3 (m, CBu^t); 29.0 (m, CBu^t); 26.7 (m, CSiMe₃); 1.1 (s, CH₃). ¹H NMR (400.13 MHz, CDCl₃): δ 1.62 (m, CH); 1.27 (s, 9H, (CH₃)₃); 1.23 (s, 9H, (CH₃)₃); 1.17 (s, 9H, (CH₃)₃); 1.16 (s, 9H, (CH₃)₃); 0.26 (s, 9H, Si(CH₃)₃). EI-MS m/z (%): 548 (100) [P₆C₄Bu^t₄CHSiMe₃] ⁺, 491 (35) [P₆C₄Bu^t₃ CHSiMe₃]⁺, 169 (17) [CBu^t]⁺, 73 (70) [SiMe₃]⁺.

23. Crystal data for 2: P₆C₂₄H₄₆Si, M = 548.51, monoclinic P2₁/c (standard N^o 14), a = 12.9500(2), b = 19.4710(3), c = 12.0920(2) Å, b = 95.625(1)°, V = 3034.31(8) ų, T = 173(2), Z = 4, μ = 0.405 mm^-1, λ = 0.71070 Å, 60382 measured reflections, 5334 independent reflections (R_int = 0.0858), 4488 reflections with I > 2σ(I), R(F) = 0.0446, wR(F²) = 0.1366 for I>2σI, R(F) = 0.0654, wR(F²) = 0.1662 for all data.

Received: February 22, 2002

Published on the web: August 6, 2002

FAPESP helped in meeting the publication costs of this article.

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Publication Dates

• Publication in this collection
  08 Oct 2008
• Date of issue
  Sept 2002

History

• Accepted
  06 Aug 2002
• Received
  22 Feb 2002