η⁶-Arene complexes of Ni(II), efficient catalysts for 1,3-butadiene and styrene polymerization

Abstract

A simple procedure for the preparation of cationic arene complexes of Ni(II) of composition [Ni(η⁶-ArX)(η³-C₃H₅)]⁺[BAr′₄]⁻ (X = OH, H) is reported. These compounds are shown to behave as highly active catalysts for the polymerization of 1,3-butadiene and styrene.

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In spite of the importance of transition metal arenes, the occurrence of Ni(II)–arene systems is rare.¹ Apart from the early works of Klabunde and co-workers,^2,3 only sporadic contributions have dealt with this kind of compounds.⁴

We⁵ and others⁶ have found recently that by placing ortho-substituents on aryloxide ligands their otherwise uncommon π-coordination to Ni(II) and Pd(II) centers becomes favoured over the usual sigma O-bonding. Thus, instead of a conventional dimeric formulation of the type [Ni(η³-C₃H₅)(μ-OAr)]₂, the sterically demanding 2,6-di-tert-butyl-4-methylphenolate gives rise to the monomeric structure Ni(η³-allyl)(η⁵-OAr) (1). In this paper we wish to report the facile formation of cationic Ni(II) organometallics that contain Ni(η⁶-ArOH) or Ni(η⁶-ArH) units (compounds 2 and 3; see Scheme 1) and describe their use as active catalysts for 1,3-butadiene and styrene polymerization reactions.

Scheme 1

As anticipated,⁷ complex 1 reacts with the acid H(OEt₂)₂(BAr’₄) [Ar′ = (C₆H₃(CF₃)₂-3,5]⁸ to produce compound 2 which contains a neutral phenol ligand (Scheme 1(a)). Alternatively 2 may be obtained directly from [Ni(η³-C₃H₅)(μ-Cl)]₂ and NaBAr′₄ in the presence of 2,6-di-tert-butyl-4-methylphenol. This easier and more convenient route allows extension to the 1,3,5-tris(tert-butyl)benzene compound, 3 (Scheme 1(b)).

The Ni–arenes 2 and 3 can be readily characterized by NMR spectroscopy.⁹ Using the former as an example, and leaving aside the characteristic and easily identifiable signals of the Ni(η³-C₃H₅) fragment and ^tBu and Me substituents of the phenol, this compound exhibits ¹H resonances (CD₂Cl₂, 20 °C) at δ 6.94 (2H) and 5.84 (1H). The latter is due to the phenol OH group, which is also responsible for an IR absorption at 3600 cm⁻¹, due to ν(O–H). In the ¹³C{¹H} NMR spectrum, four aromatic resonances are identified at δ 140.2 (COH), 126.5 (C–^tBu), 109.2 (C–Me) and 109.0 (C–H).

The structure of compound 2 has been additionally confirmed by an X-ray investigation (Fig. 1).‡ The Ni(II)–arene interaction is relatively strong (Ni to ring centre distance of 1.663(2) Å, the shortest hitherto observed for Ni(II)–arene complexes^2c,3c,4) and is fairly symmetrical with the Ni–C distances varying between 2.126(4) and 2.248(4) Å. No significant C–C bond alternation is observed in the aromatic ring which is almost perfectly planar. The arene and allyl ligands have a relative orientation similar to that in complex 1,^5a with the allyl back-bone aligned approximately parallel to the axis defined by O, C(4), C(7) and C(14).

Fig. 1 ORTEP perspective of the cationic part of 2.

Since the Ni(η³-allyl)L₂⁺ and [Ni(η³-allyl)(μ-X)]₂ compounds are good catalysts for the polymerization of 1,3-dienes¹⁰ and styrene,¹¹ we have tested the catalytic activity of 1–3 in these reactions. While 1 is inactive in both cases, 2 and 3 exhibit remarkable activity toward 1,3-butadiene (Table 1) yielding stereoregular polymers of high molecular weight. ¹H and ¹³C NMR spectroscopy reveals that they consist mostly of cis-1,4-polybutadiene (93%), accompanied by minor amounts of trans-1,4- and 1,2-units (5% and 2%, respectively).

Table 1 1,3-Butadiene and styrene polymerization with 2 and 3

Entry	Catalyst	Monomer^a/mol	Solvent^b/mL	Yield (%)	TOF^c (× 10^−4)	Stereospecifity of polymers^d	M w	M n	M w/Mn
Polymerization conditions: 60 °C, 60 min., 3.45 × 10^−6 mol of Ni complex.a BN: 1,3-butadiene; ST: styrene.b TOL: toluene; HEX: hexane; DCM: dichloromethane.c h^−1.d Determined from the ^13C NMR spectra.e cis/trans/1,2.f mm, mr, rr, approximate distribution from integration of the Cipso signals.
1	2	BN (0.44)	TOL (100)	63.0	8.03	93.5/4.3/2.2^e	87,062	34,307	2.5
2	2	BN (0.24)	TOL (30)	98.7	6.78	93.6/4.7/1.7^e	149,602	32,187	4.6
3	3	BN (0.24)	TOL (30)	76.4	5.56	94.0/4.2/1.8^e	114,720	52,920	2.2
4	2	BN (0.24)	HEX (30)	48.2	3.35	92.9/4.9/2.2^e	64,000	25,400	2.5
5	2	BN (0.24)	DCM (30)	22.3	1.55	92.4/5.5/2.1^e	33,800	16,450	2.2
6	2	ST (0.12)	TOL (11)	56.3	1.95	44, 29, 27^f	4,400	2,140	2.1
7	2	ST (0.12)	HEX (11)	66.4	2.31	41, 30, 29^f	2,770	1,140	2.4
8	2	ST (0.12)	DCM (11)	76.9	2.67	40, 30, 30^f	2,300	1,090	2.1

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Complex 2 also polymerizes styrene (entries 5–7). The molecular weights of the polymers are considerably lower than those observed for 1,3-butadiene. The polystyrenes display broad ¹H and complex ¹³C NMR spectra,¹² which are consistent with a predominantly atactic structure. In addition, the carbon spectra indicate strict head-to-tail polymerization, since they lack methylene resonances within the region δ 25–35 (corresponding to CHPhCH₂CH₂CHPh fragments) or methyne signals in the proximity of δ 48 (PhCHCHPh groups).¹³ It is interesting that the catalytic activity of 2 is not affected by the presence of free 2,6-di-tert-butyl-4-methylphenol. Phenols, including the latter, are well known radical inhibitors,¹⁴ and therefore a radical polymerization mechanism appears unlikely in this case.

The well known lability of η⁶-arene–Ni(II) complexes¹⁵ suggests that the polymerization reaction could be triggered by displacement of the η⁶-arene by butadiene, to give the reactive intermediate [Ni(η³-C₃H₅)(η⁴-C₄H₆)]⁺, able to undergo consecutive butadiene insertions and thereby following the stereoselective path described by Taube and Tobisch.^16,17 Notwithstanding this, the ready formation of compounds 2 and 3 also suggests that aromatic molecules exert a positive influence on the catalysis by stabilizing the Ni(II)–allyl species responsible for the polymerization process. This might be true particulary in aromatic solvents which are frequently used in this type of polymerization reactions,¹⁰ and, moreover could account for the catalytic activities and molecular weights observed in toluene in the butadiene polymerization (Table 1, entries 2, 4, 5). On these grounds, the lower sensitivity of the styrene polymerization rates to the solvent effect (Table 1, entries 6–8) is not surprising, since this monomer already incorporates an aromatic fragment.

In summary, we have shown that [Ni(η³-C₃H₅)(η⁶-arene)]⁺[BAr’₄]⁻ complexes, 2 and 3, can be prepared by a very simple procedure, using readily available materials. They are single-component catalysts for the polymerization of butadiene and styrene. Their activity is comparable to those of the best Ni catalysts hitherto reported.^10a,11 Even if π-arene complexes of Ni have been known for many years, the ready complexation of the Ni(η³-C₃H₅)⁺ fragment by aromatic molecules has been unrecognized so far. We are currently examining the influence of aromatic solvents on these polymerization reactions, and studying the use of the new catalysts in the (co)polymerization of 1,3-butadiene, styrene and other monomers.

Financial support from DGESIC (Projects BQU2000-1169) and EU (RTN-CT2000-00010) are gratefully acknowledged. MMC thanks the Consejo Superior de Investigaciones Científicas for a postdoctoral contract.

Notes and references

1. (a) M. J. Chetcuti, in, Comprehensive Organometallic Chemistry II, ed. G. Wilkinson, F. G. A. Stone and E. W. Abel, Elsevier Science Ltd, Oxford, 1995. vol. 9, pp. 167–191; (b) P. W. Jolly, in, Comprehensive Organometallic Chemistry, ed. G. Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon Press Ltd., Oxford, 1982, vol. 6, pp. 229–231.
2. (a) K. J. Klabunde, B. B. Anderson, M. A. Bader and L. J. Radonovich, J. Am. Chem. Soc., 1978, 100, 1314; (b) R. G. Gastinger, B. B. Anderson and K. J. Klabunde, J. Am. Chem. Soc., 1980, 102, 4959; (c) L. J. Radonovich, K. J. Klabunde, D. P. McCollor and B. B. Anderson, Inorg. Chem., 1980, 19, 1221.
3. (a) S. T. Lin, R. N. Narske and K. J. Klabunde, Organometallics, 1985, 4, 571; (b) S. B. Choe and K. J. Klabunde, J. Organomet. Chem., 1989, 359, 409; (c) L. J. Radonovich, F. J. Koch and T. A. Albright, Inorg. Chem., 1980, 19, 3373.
4. J. L. Priego, L. H. Doerrer, L. H. Rees and M. L. H. Green, Chem. Commun., 2000, 779.
5. (a) J. Cámpora, M. L. Reyes, T. Hackl, A. Monge and C. Ruiz, Organometallics, 2000, 19, 2950; (b) J. Cámpora, M. L. Reyes and K. Mereiter, Organometallics, 2002, 21, 1014.
6. A. Klein and A. Dogan, Organometallics, 2002, 21, 2340.
7. (a) D. A. Herebian, C. S. Schmidt and C. van Wüllen, Eur. J. Inorg. Chem., 1988, 1991; (b) J. Le Bras, H. El Amouri and J. Vaissermann, Organometallics, 1996, 15, 5706; (c) U. Koelle, M. H. Wang and G. Raabe, Organometallics, 1991, 10, 2573.
8. M. Brookhart, B. Grant and A. F. Volpe Jr., Organometallics, 1992, 11, 3920–3922.
9. Selected spectroscopic data: 2: ¹H NMR (CD₂Cl₂, 20 °C): δ 6.94 (s, 2H, CH arom.), 5.85 (m, 1 H, allyl H_c), 5.84 (s, 1H, OH), 3.61 (d, ³J_HH 6.6 Hz, 2H, allyl H_syn), 2.46 (d, ³J_HH 12.4 Hz, 2H, allyl H_anti). ¹³C NMR (CD₂Cl₂, 20 °C): δ 140.2 (C_arom–OH), 126.5 (C_arom–^tBu), 109.2 (C_arom–Me), 109.0 (C_arom–H), 106.5 (allyl CH), 58.7 (allyl CH₂). IR (Nujol mull): 3600 cm⁻¹ (ν(OH)). 3: ¹H NMR (CD₂Cl₂, 20 °C): δ 7.00 (s, 3H, CH arom.), 5.76 (m, 1 H, allyl H_C), 3.83 (d, ³J_HH 6.5 Hz, 2H, allyl H_syn), 2.40 (d, ³J_HH 12.4 Hz, 2H, allyl H_anti). ¹³C NMR (CD₂Cl₂, 20 °C), δ 138.2 (C_arom–^tBu), 105.3 ( C_arom–H), 102.6 (allyl CH), 58.3 (allyl CH₂).
10. (a) R. Taube and G. Sylvester, in, Applied Homogeneous Catalysis with Organometallic Compounds, ed. B. Cornils and W. A. Herrmann, VCH, Weinheim, 1996, vol. 1, pp. 280–318; (b) P. W. Jolly and G. Wilke, The Organic Chemistry of Nickel, Academic Press, New York and London, 1975, vol II, pp. 213–245; (c) P. W. Jolly, in, Comprehensive Organometallic Chemistry, ed. G. Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon Press Ltd., Oxford, 1982, vol. 8, pp. 671–709.
11. J. R. Ascenso, A. R. Dias, P. T. Gomes, C. C. Romão, I. Tkatchenko, A. Revillon and Q. T. Pham, Macromolecules, 1996, 29, 4172.
12. A. J. Brandolini and D. D. Hills, NMR Spectra of Polymers and Polymer Additives, Marcel Dekker, New York and Basel, 2000.
13. D. R. Hensley, S. D. Goodrich, A. Y. Huckstep, H. J. Harwood and P. L. Rinaldi, Macromolecules, 1995, 28, 1586 , and references therein.
14. A. Ravve, Principles of Polymer Chemistry, Kluwer Academic/Plenum Press, New York, 2nd edn., 2000.
15. Both 2 and 3 undergo ready arene displacement and exchange reactions. See also ref. 2b.
16. (a) R. Taube and S. Wache, J. Organomet. Chem., 1992, 428, 431; (b) R. Taube, S. Wache, J. Sieler and R. Kempe, J. Organomet. Chem., 1993, 456, 131; (c) R. Taube, S. Wache and K. Kehlen, J. Mol. Catal. A., 1995, 97, 21.
17. (a) S. Tobisch and R. Taube, Organometallics, 1999, 18, 5204; (b) S. Tobisch, Acc. Chem. Res., 2002, 35, 96.

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Footnotes

† Electronic supplementary information (ESI) available: synthetic procedures and analytical data for compounds 2 and 3. ¹H and ¹³C NMR spectra for polybutadiene and polystyrene. See http://www.rsc.org/suppdata/cc/b2/b209838e/

‡ Crystal data for 2 (as dichloromethane solvate): C₅₀H₄₁BF₂₄NiO (2·1.5CH₂Cl₂), M = 1310.74, triclinic, P1̄ (no. 2), a = 13.807(9), b = 13.981(9), c = 17.466(11) Å, α = 95.84(2), β = 101.41(2), γ = 118.19(2)°, V = 2838(3) ų, Z = 2, D_c = 1.534 g cm⁻³, T = 213(2) K, total reflections = 31705, unique reflections = 9971 (R_int = 0,043). Final R₁ = 0.0931, wR₂ = 0.1514 (all data). CCDC 195226. See http://www.rsc.org/suppdata/cc/b2/b209838e/ for crystallographic data in CIF or other electronic format.

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