Inorg. Chem. 2004, 43, 802−811
Synthesis of Linear and Cyclic Carbophosphazenes via an Oxidative Chlorination Strategy Eric Rivard,† Alan J. Lough,† Tristram Chivers,‡ and Ian Manners*,† Departments of Chemistry, UniVersity of Toronto, Toronto, Ontario, Canada, M5S 3H6, and UniVersity of Calgary, Calgary, Alberta, Canada T2N 1N4 Received August 9, 2003
The use of a mild, oxidative chlorination route for the synthesis of linear and cyclic carbophosphazenes is described. For example, chlorination of the linear PNCN chain Ph2P−NdC(Ph)−N(SiMe3)2 (1) with C2Cl6 led to the clean formation of the previously known 8- and 6-membered rings [Ph2PNC(Ph)N]2 (2) and [Ph2PNC(Ph)NP(Ph)2N] (3), respectively. In a similar fashion, the N-alkyl-substituted PNCN derivatives, Ph2P−NdC(Ph)−N(tBu)SiMe3 (4) and Ph2P−NdC(Ph)−NiPr2 (7) were readily converted by C2Cl6 into the halogenated derivatives ClPh2PdN−C(Ph)dNtBu (5) and [ClPh2PdNdC(Ph)−NiPr2]Cl (8), respectively. Protonation of 5 was accomplished using HCl and gave the carbophosphazenium salt [ClPh2PdN−C(Ph)dN(tBu)H]Cl (6). In addition, the isolation of a rare 8-membered P2N4C2 heterocycle [(Cl3PdN)ClPNC(Ph)NP(Cl)2NC(Ph)N] (9) from the reaction of PCl5 and Li[PhC(NSiMe3)2] is reported. Treatment of 9 with one equivalent of GaCl3 led to the discovery of an unusual Lewis acid-induced ring contraction reaction whereby the (PNCN)2 ring in 9 is converted into the novel 6-membered P2N3C heterocyclic adduct [(Cl3PdN)ClPNP(Cl)2NC(Ph)N]‚GaCl3 (10) with concomitant release of PhCN. Structural characterization of compounds 1, 5, 6, and 8−10 by single-crystal X-ray diffraction is also provided.
Introduction N-Silylated phosphoranimines, R3PdNSiMe3, have been used as precursors to main group and transition metal phosphoraniminato complexes which adopt a variety of structural motifs,1 and often display unusual reactivity.2 In addition, phosphoraniminato complexes have recently been explored as a new class of non-metallocene olefin polymerization catalysts.3 With a judicious choice of substituents * Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: (+1) 416-978-2450. † University of Toronto. ‡ University of Calgary. (1) (a) Dehnicke, K.; Stra¨hle, J. Polyhedron 1989, 8, 707. (b) Dehnicke, K.; Krieger, M.; Massa, W. Coord. Chem. ReV. 1999, 182, 19. (c) Lichtenhan, J. D.; Ziller, J. W.; Doherty, N. M. Inorg. Chem. 1992, 31, 2893. (d) Courtenay, S.; Mutus, J. Y.; Schurko, R. W.; Stephan, D. W. Angew. Chem., Int. Ed. 2002, 41, 498. (e) Ja¨schke, B.; Jansen, M. Z. Naturforsch. 2002, 57B, 1237. (f) Armstrong, A.; Chivers, T.; Krahn, M.; Parvez, M.; Schatte, G. Chem. Commun. 2002, 2332. (g) Honeyman, C. H.; Lough, A. J.; Manners, I. Inorg. Chem. 1994, 33, 2988. (2) (a) Kickham, J. E.; Gue´rin, F.; Stephan, D. W. J. Am. Chem. Soc. 2002, 124, 11486. (b) Bell, S. A.; Geib, S. J.; Meyer, T. Y. Chem. Commun. 2000, 1375. (c) Rivard, E.; Honeyman, C. H.; McWilliams, A. R.; Lough, A. J.; Manners, I. Inorg. Chem. 2001, 40, 1489. (3) (a) LePichon, L.; Stephan, D. W.; Gao, X.; Wang, Q. Organometallics 2002, 21, 1362. (b) Stephan, D. W.; Gue´rin, F.; Spence, R. E. v. H.; Koch, L.; Gao, X.; Brown, S. J.; Swabey, J. W.; Wang, Q.; Xu, W.; Zoricak, P.; Harrison, D. G. Organometallics 1999, 18, 2046.
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and reaction conditions, N-silylated phosphoranimines can be used as suitable monomers for the preparation of high molecular weight polyphosphazenes via condensation polymerization (whereby a volatile silane is released during the course of the reaction). In this context, a number of elegant thermal routes to aryl/alkyl-,4 alkoxy-,5 and halogensubstituted6 polyphosphazenes have been developed. Recently, we showed that the chlorinated phosphoranimine, Cl3PdNSiMe3, could be polymerized under ambient conditions using PCl5 as a catalyst to give poly(dichlorophosphazene) with controlled molecular weights and narrow polydispersities due to the living nature of the polymerization (Scheme 1).7 Bearing in mind the wealth of interesting chemistry associated with phosphoranimines, we have now targeted the (4) (a) Neilson, R. H.; Wisian-Neilson, P. Chem. ReV. 1988, 88, 541. (b) Gruneich, J. A.; Wisian-Neilson, P. Macromolecules 1996, 29, 5511. (c) Wisian-Neilson, P.; Neilson, R. H. Inorg. Synth. 1989, 25, 69. (5) Montague, R. A.; Matyjaszewski, K. J. Am. Chem. Soc. 1990, 112, 6721. (6) D’Halluin, G.; De Jaeger, R.; Chambrette, J. P.; Potin, Ph. Macromolecules 1992, 25, 1254. (7) (a) Honeyman, C. H.; Manners, I.; Morrissey, C. T.; Allcock, H. R. J. Am. Chem. Soc. 1995, 117, 7035. (b) Honeyman, C. H. M.Sc. Thesis, University of Toronto, 1993. (c) Allcock, H. R.; Crane, C. A.; Morrissey, C. T.; Nelson, J. M.; Reeves, S. D.; Honeyman, C. H.; Manners, I. Macromolecules 1996, 29, 7740.
10.1021/ic034954c CCC: $27.50
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Linear and Cyclic Carbophosphazenes Scheme 1
Scheme 2
synthesis of related heterophosphazene chains with the aim of investigating their suitability as monomers for polymerization.8 Specifically, we focused on monomers with similar reactive terminal groups as the phosphoranimine Cl3Pd NSiMe3 but with their separation by a longer spacer group. As an initial synthetic goal, we chose the linear carbophosphazene ClR2PdN-C(R′)dNSiMe3. We anticipated that polymerization of this monomer might yield the novel material [R2PNC(R′)N]n, which would be expected to possess interesting properties.9,10 This paper reports full details of our initial explorations in this area. Results and Discussion Attempted Synthesis of SiMe3-Terminated Linear Carbophosphazenes. Prior to initiating this work, to our knowledge, no examples of the desired carbophosphazene monomers ClR2PdN-C(R′)dNSiMe3 had been prepared. However, it has been shown that SO2Cl2 cleanly converts the chlorophosphine Cl2P-N(SiMe3)2 to the phosphoranimine Cl3PdNSiMe3 with SO2 and ClSiMe3 as byproducts (Scheme 2).11,12 As a consequence, a similar chlorination strategy was explored for the synthesis of the desired PNCN heterophosphazene chains. When the linear precursor Ph2PNdC(Ph)-N(SiMe3)2 (1), prepared from in situ generated Li[PhC(NSiMe3)2] and Ph2PCl,13,14 was reacted with one equivalent of SO2Cl2 in Et2O, we obtained a complex mixture of products (by 31P NMR spectroscopy) from which no single phosphorus-containing species could be isolated. Analysis (8) Chunechom, V.; Vidal, T. E.; Adams, H.; Turner, M. L. Angew. Chem., Int. Ed. 1998, 37, 1928. (9) Polycarbophosphazenes containing CNPNPN sequences have been previously synthesized by the ring-opening polymerization of the halogenated precursor [NCCl(NPCl2)2]: (a) Manners, I.; Allcock, H. R.; Renner, G.; Nuyken, O. J. Am. Chem. Soc. 1989, 111, 5478. (b) Allcock, H. R.; Coley, S. M.; Manners, I.; Nuyken, O.; Renner, G. Macromolecules 1991, 24, 2024. (10) Manners, I. Angew. Chem., Int. Ed. Engl. 1996, 35, 1602. (11) Wang, B.; Rivard, E.; Manners, I. Inorg. Chem. 2002, 41, 1690. (12) For a related synthesis of phosphoranimines via chlorination or bromination of phosphorus(III) amides, see: (a) Wisian-Neilson, P.; Neilson, R. H. J. Am. Chem. Soc. 1980, 102, 2848. (b) Klaehn, J. R.; Neilson, R. H. Inorg. Chem. 2002, 41, 5859. (c) Samuel, R. C.; Kashyap, R. P.; Krawiec, M.; Watson, W. K.; Neilson, R. H. Inorg. Chem. 2002, 41, 7113. (d) Roesky, H. W.; Lucas, J.; Noltemeyer, M.; Sheldrick, G. M. Chem. Ber. 1984, 117, 1583. (13) Initial synthesis: Scholz, U.; Noltemeyer, M.; Roesky, H. W. Z. Naturforsch. 1988, 43B, 937. (14) Chandrasekhar, V.; Chivers, T.; Kumaravel, S. S.; Meetsma, A.; van de Grampel, J. C. Inorg. Chem. 1991, 30, 3402.
of the mixture by 1H NMR spectroscopy suggested that an almost quantitative elimination of the SiMe3 groups derived from 1 had occurred as only very small signals for trimethylsilyl groups were detected. One possible reason for the generation of multiple products during the chlorination is that an undesired condensation reaction occurred between the S-Cl bonds of SO2Cl2 and the silylated nitrogen atoms of either 1 or an oxidatively chlorinated intermediate (e.g., ClPh2PdN-C(Ph)dNSiMe3). To circumvent any possible side reactions during the chlorination, we therefore explored the reaction of 1 with the milder chlorinating agent hexachloroethane, C2Cl6. Rather than producing a complex mixture, two predominant products were obtained which were identified as the known 8- and 6-membered cyclocarbophosphazenes 2 and 3 (Scheme 3). Heterocycles 2 and 3 were previously obtained by Chivers et al. from the reaction of Ph2PCl3 with Li[PhC(NSiMe3)2].14 The formation of the high oxidation state, phosphorus(V)containing heterophosphazenes 2 and 3 suggested that the oxidative chlorination of 1 was successful. However, it appears that the targeted carbophosphazene, ClPh2PdNC(Ph)dNSiMe3, is unstable at ambient temperatures and subsequently undergoes a self-condensation reaction to give the dimer 2 and 3 (presumably via the loss of PhCN from 2).15 When the reaction was repeated at -78 °C, an intermediate species with a 31P NMR resonance at 29 ppm was observed. However, when the reaction mixture was warmed to -10 °C, this intermediate, whose possible structure we will return to later, was rapidly ( 2σ[I])a Rw (%)b a
1
5
6
C25H33N2PSi2 448.68 monoclinic P21/c 150(1) 13.0491(3) 11.6549(3) 17.0704(4) 90 96.5890(10) 90 2579.02(11) 4 0.214 2.72-27.52 19724 5898/0.074 272 4.85 13.38
C23H24ClN2P 394.86 triclinic P1h 150(1) 9.6700(2) 10.5020(2) 10.6120(2) 98.5040(12) 91.0030(12) 106.5820(11) 1019.49(3) 2 0.276 2.59-27.59 15494 4693/0.0542 245 3.72 10.82
C23H25Cl2N2P 431.32 orthorhombic Pbca 150(1) 22.47100(10) 8.7260(4) 22.9941(5) 90 90 90 4508.7(2) 8 0.370 2.66-27.50 36451 5178/0.1330 258 5.49 14.82
R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2.
Figure 1. Molecular structure of 1. All hydrogen atoms have been omitted for clarity. Selected bond lengths and angles are presented in Table 3.
for a typical P-N single bond (1.69 to 1.73 Å).19 The length of the adjacent C-N bond, [C(1)-N(1)], was determined to be 1.277(3) Å and was much shorter than the neighboring C(1)-N(2) bond [1.427(3) Å]; these bond lengths lie within the values generally observed for double and single bonds, respectively, and are consistent with the canonical representation of the bonding within 1 depicted in Scheme 3. Reflecting the presence of a lone pair at phosphorus, a pyramidal geometry was observed with bond angles substantially narrowed from those of an ideal tetrahedron [range from 98.97(9)° to 100.14(9)°]. An X-ray diffraction study of the tBu-terminated carbophosphazene, ClPh2PdNC(Ph)dNtBu (5), revealed that this (19) Burrow, R. A.; Farrar, D. H.; Honeyman, C. H. Acta Crystallogr. 1994, 50C, 681.
Linear and Cyclic Carbophosphazenes Table 2. Relevant Crystallographic Data for Compounds 8-10
chemical formula formula weight crystal system space group temp, K a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z µ(Mo KR), mm-1 θ range/deg no. of reflns collcd no. of indp reflns/Rint no. of refined params R (%) (I > 2σ[I])a Rw (%)b a
8‚CH2Cl2
9
10
C26H31Cl4N2P 544.30 monoclinic P21/c 150(1) 11.1470(3) 10.0030(3) 24.7400(6) 90 90.3240(17) 90 2758.55(13) 4 0.505 2.62-27.50 17475 6298/0.0489 299 4.78 12.48
C14H10Cl6N5P3 553.88 monoclinic P21/c 150(1) 12.5680(4) 8.6020(3) 19.7070(5) 90 93.5260(17) 90 2126.49(11) 4 1.046 2.55-27.48 12270 4846/0.0386 254 4.07 11.01
C7H5Cl9GaN4P3 626.83 triclinic P1h 150(1) 9.1460(2) 10.0440(3) 12.4390(4) 103.6300(9) 109.1460(10) 91.4420(14) 1042.52(5) 2 2.704 2.99-27.50 13106 4743/0.0599 218 3.88 9.92
Figure 2. Molecular structure of 5. All hydrogen atoms have been omitted for clarity. Selected bond lengths and angles are presented in Table 3.
R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for Compounds 1, 5, 6, and 8 with Estimated Standard Deviations in Parentheses
P-N C-N (internal) (internal) (terminal)
1
5
1.7271(18) 1.277(3)
1.5417(12) 1.4005(18) 1.2788(19) 1.4829(18)
1.564(2) 1.332(3) 1.315(3) 1.494(3)
1.5785(19) 1.330(3) 1.322(3) 1.495(3) 1.495(3)
2.0801(5)
2.0259(8)
2.0103(8)
132.73(10) 126.68(13) 123.71(13)
138.38(18) 121.6(2) 126.5(2)
132.55(17) 119.6(2) 122.08(19) 122.26(19)
1.427(3)
P-Cl P-N-C (internal) N-C-N (internal) C-N-C (internal)
122.17(15) 126.61(19)
C-N-Si
117.76(13) 117.69(14)
6
8
species exists as a monomer in the solid state (Figure 2) with a significantly shorter P-N distance of 1.5417(12) Å when compared to that of the phosphorus(III)-containing PNCN chain 1. Furthermore, the observed P-N bond length within 5 is similar to those found within the bis(trichlorophosphine)iminium cation [Cl3PdNdPCl3]+ (1.5-1.6 Å)20 and suggests the presence of significant multiple bond character. A localized bonding arrangement is present within the adjacent C-N bonds of the chain, with an internal (imine) C-N bond length [C(1)-N(2)] of 1.4005(18) Å and a shortened terminal C-N distance [C(1)-N(1)] of 1.2788(19) Å. For comparison, the remaining C-N single bond involving the quaternary carbon atom of the tBu group [C(8)-N(1)] was 1.4829(18) Å. The P-Cl bond length within 5 was 2.0801(5) Å and is typical for a phosphorus(V)-chlorine bond.20 As compound 5 was anticipated to be a strong base, reaction with HCl to form the protonated derivative, [ClPh2PdN-C(Ph)dN(tBu)H]Cl (6), was expected to be facile. Indeed, when 5 was allowed to react with one equivalent of HCl, quantitative formation of a new product with a downfield-shifted 31P NMR resonance at 32.9 ppm was detected. A white solid was subsequently isolated in (20) Rivard, E.; McWilliams, A. R.; Lough, A. J.; Manners, I. Acta Crystallogr. 2002, 58C, i114 and references therein.
Figure 3. Molecular structure of 6. All hydrogen atoms bound to carbon have been omitted for clarity. Selected bond lengths and angles are presented in Table 3.
67% yield which gave a 1H NMR spectrum similar to that of 5 except for an additional broad signal at 11.6 ppm, consistent with the presence of an iminium proton. The resonance associated with the tBu group within 6 was located at 1.66 ppm and, as a result of the increase in cationic character within the PNCN unit, lies considerably downfield with respect to the tBu resonance in the neutral PNCN chain 5 [δ 1.47]. The spectroscopic data, coupled with a singlecrystal X-ray diffraction study (Figure 3), conclusively identified the product as the terminally protonated carbophosphazene [ClPh2PdN-C(Ph)dN(tBu)H]Cl (6). Inspection of the metrical parameters of 6 indicates the presence of a delocalized bonding scheme within the PNCN chain, with similar internal C-N bond lengths of 1.315(3) and 1.332(3) Å, and a P-N distance of 1.564(2) Å. These bond lengths illustrate that, to a significant extent, the cationic charge is delocalized throughout the entire PNCN framework. As a consequence of increased cationic character at phosphorus, a contracted P-Cl bond length [2.0259(8) Å] is observed when compared to the P-Cl bond within the neutral analogue 5 [2.0801(5) Å]. A weak intramolecular hydrogen bonding interaction is also present involving the iminium proton bound to N(1) and the counterion Cl(2) [2.04(4) Å]. No intermolecular hydrogen bonding was present between the N-H moiety and the P-Cl bond of the cation. Preparation of Ph2P-NdC(Ph)-NiPr2 (7) and the Carbophosphazenium Salt [ClPh2PdNdC(Ph)-NiPr2]Cl (8): Insight into the Oxidative Chlorination Mechanism. With the goal of gaining a better understanding of the initial steps involved in the chlorination of 1, we decided to explore Inorganic Chemistry, Vol. 43, No. 2, 2004
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Rivard et al.
Figure 4. Molecular structure of 8. All hydrogen atoms have been omitted for clarity. Selected bond lengths and angles are presented in Table 3. Scheme 5. Synthesis of the Carbophosphazenium Salt 8
the chlorination of a PNCN chain where the elimination of a silyl halide was not possible. To facilitate the isolation of an amido-terminated (-NR2) linear carbophosphazene, the reactive silyl groups at the terminal nitrogen atom of the PNCN precursor were replaced by sterically similar, yet elimination-resistant isopropyl groups. Following the established procedure for the construction of PNCN chains, the isopropyl-substituted derivative Ph2P-NdC(Ph)-NiPr2 (7) was synthesized (yellow solid, 79% yield) from the reaction of Ph2PCl with in situ generated Li[NdC(Ph)-NiPr2]. Upon the addition of Cl3C-CCl3 in Et2O, a pale yellow precipitate was obtained within a few minutes. This product was readily soluble in chlorinated solvents and gave a 31P NMR signal at 37.1 ppm in CDCl3. The location of this chemical shift was similar to the observed resonances for the cationic linear carbophosphazene 6 (δ 32.9), suggesting that the isolated species was the carbophosphazenium salt [ClPh2PdNdC(Ph)-NiPr2]Cl (8) (Scheme 5), rather than the constitutional isomer Cl2Ph2P-NdC(Ph)-NiPr2.21 The 1H and 13C{1H} NMR spectra of 8 revealed that there was restricted rotation about the terminal C-N bond in solution, as indicated by the presence of two distinct sets of resonances due to diastereotopically positioned iPr groups; a similar situation was also noted for the precursor 7. Final confirmation for the formation of 8 was obtained from single-crystal X-ray crystallography (Figure 4). (21) Although the structure of 8 was confirmed to be a salt in the solid state, we could not immediately rule out the possibility that 8 existed as the pentacoordinate species Cl2Ph2P-NdC(Ph)-NiPr2 in solution. To probe the structure of 8 in solution, we reacted 8 with one equivalent of the halide abstractor, Ag[BF4], in dichloromethane. Upon the addition of Ag[BF4] an immediate reaction was observed (as evinced by the formation of a white precipitate, presumably AgCl). The 31P NMR spectra remained unchanged throughout the reaction [δ ) 36.9 (s) ppm] while 19F NMR spectroscopy revealed the presence of the BF4- ion [δ -152.7 (s)]. These results suggest that a simple anion exchange had occurred (BF4- for Cl-) to give [ClPh2PdNdC(Ph)NiPr2]BF4, and therefore, 8 is also ionized in solution.
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Compound 8 crystallized in the P21/c space group with one equivalent of solvate CH2Cl2 as part of the crystal lattice (Table 2). The PNCN unit within 8 is formally positively charged with no close contacts (
