Organometallics 2010, 29, 1977–1980 DOI: 10.1021/om100099f
1977
Insertion of [Pt(PEt3)2] into a Strained Si-C Bond of Diphenylsila[1]molybdarenophane Clinton L. Lund,†,§ Bidraha Bagh,† J. Wilson Quail,‡ and Jens M€ uller*,† †
Department of Chemistry and ‡Saskatchewan Structural Sciences Centre, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan S7N 5C9, Canada. §Current address: Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario, M5S 3H6 Canada. Received February 8, 2010
Summary: Diphenylsila[1]molybdarenophane (1) reacted with [Pt(PEt3)3] to give the platinasila[2]molybdarenophane [{(Et3P)2Pt}(Ph2Si)(η6-C6H5)2Mo] (2) in an isolated yield of 52%. Species 2 was characterized by 1H and 13C NMR spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray determination (monoclinic P21/c; a = 11.8115(3) A˚; b = 17.6336(3) A˚; c = 19.7795(4) A˚; R = 90°; β = 122.1420(10)°; γ = 90°). The insertion of the [Pt(PEt3)2] moiety into the strained C-Si bond resulted in a reduction of the tilt angle R from 20.23(29)° (1) to 13.62(20)° (2). Species 1 reacted with Karstedt’s catalyst (6.9 mol %; toluene, ambient temperature) to be completely consumed within 3 h. As revealed by 1H NMR spectroscopy, new products were formed. These products could not be isolated or identified; however, small half-bandwidths of their peaks indicated that polymers had not been formed. Salt metathesis between bis(dilithiobenzene)molybdenum and R2SiCl2 (R = iPr, Me) resulted in new sila[1]molybdarenophanes with iPr2Si (3) and Me2Si bridging units (4), respectively. Neither species could be isolated in chemically pure form. However, 3 and 4 were unequivocally identified by 1H NMR spectroscopy, especially by a very distinctive pattern of the Mo-coordinated phenyl groups (δ (in C6D6): 5.52 ( p-H ), 5.05 (m-H ), and 3.68 (o-H ) for 3; 5.52 ( p-H), 5.05 (m-H ), and 3.69 (o-H ) for 4).
Introduction Ring-opening polymerization (ROP) of strained metallacyclophanes is an elegant method to obtain high-molecularweight organometallic polymers with transition metals in the backbone.1 To date, thermal, anionic, photocontrolled,2 and transition-metal-catalyzed ROP are the common methods applied.1a In the case of the transition-metal-catalyzed ROP, usually platinum(0) or palladium(0) catalysts are employed. In order to illuminate the mechanism of transition-metalcatalyzed ROP of a silacyclobutane, Tanaka et al. had shown that [Pt(PEt3)2] moieties insert into Si-C bonds to form an isolatable cyclic platinum complex.3 This result supported the hypothesis that transition-metal-catalyzed ROP is
homogeneous and proceeds initially by insertion of the metal into a strained Si-C bond. Similarly, it was shown that [2]metallacyclophanes result from insertion reactions of metal complex fragments with strained [1]metallacyclophanes and species with metal moieties such as [Pt(PEt3)2]4 and [Pt(cod)]5 had been isolated. Platina[2]metallacyclophanes containing a cod ligand can serve as precatalysts for ROP of strained species;4c,e,6 however, in the case of polymerization of dimethylsila[1]ferrocenophane, the ferrocene unit of the precatalyst does not incorporate into the resulting polymer.6 This fact together with results of mercury poisoning experiments led Manners et al. to conclude that colloidal platinum was the major catalytically active species and a mechanism for a heterogeneous catalysis was proposed.6 Still, it could not be ruled out that homogeneous catalysis occurred to some minor extent. Except for four heteroleptic cycloheptadienylcyclopentadienyl sandwich compounds,4e-g,7 all other known platina[2]metallacyclophanes are ferrocene derivatives. The knowledge about strained [1]metallacyclophanes of 4d metals is limited to a few articles, whereas strained [1]metallacyclophanes of 5d metals are unknown. The first strained 4d sandwich compounds were [1]ruthenocenophanes with SnMes2 or Zr(C5H4tBu)2 in bridging positions, reported in 2004.8 Recently, we added two more [1]ruthenocenophanes, Aland Ga-bridged species, to this short list.9 The only strained 4d metal bis(benzene) derivatives known to date are [1]molybdarenophanes exhibiting Al(Me2Ntsi), Ga(Me2Ntsi), or SiPh2 moieties, which we characterized very recently.10,11
*To whom correspondence should be addressed. E-mail: jens.mueller@ usask.ca. (1) (a) Bellas, V.; Rehahn, M. Angew. Chem., Int. Ed. 2007, 46, 5082– 5104. (b) Herbert, D. E.; Mayer, U. F. J.; Manners, I. Angew. Chem., Int. Ed. 2007, 46, 5060–5081. (2) (a) Tanabe, M.; Manners, I. J. Am. Chem. Soc. 2004, 126, 11434– 11435. (b) Tanabe, M.; Vandermeulen, G. W. M.; Chan, W. Y.; Cyr, P. W.; Vanderark, L.; Rider, D. A.; Manners, I. Nat. Mater. 2006, 5, 467–470. (3) Yamashita, H.; Tanaka, M.; Honda, K. J. Am. Chem. Soc. 1995, 117, 8873–8874.
(4) (a) Sheridan, J. B.; Lough, A. J.; Manners, I. Organometallics 1996, 15, 2195–2197. (b) Reddy, N. P.; Choi, N.; Shimada, S.; Tanaka, M. Chem. Lett. 1996, 649–650. (c) Temple, K.; Lough, A. J.; Sheridan, J. B.; Manners, I. J. Chem. Soc., Dalton Trans. 1998, 2799–2805. (d) Chan, W. Y.; Berenbaum, A.; Clendenning, S. B.; Lough, A. J.; Manners, I. Organometallics 2003, 22, 3796–3808. (e) Tamm, M.; Kunst, A.; Herdtweck, E. Chem. Commun. 2005, 1729–1731. (f) Bartole-Scott, A.; Braunschweig, H.; Kupfer, T.; Lutz, M.; Manners, I.; Nguyen, T.-l.; Radacki, K.; Seeler, F. Chem. Eur. J. 2006, 12, 1266–1273. (g) Tamm, M.; Kunst, A.; Bannenberg, T.; Randoll, S.; Jones, P. G. Organometallics 2007, 26, 417–424. (5) (a) Sheridan, J. B.; Temple, K.; Lough, A. J.; Manners, I. J. Chem. Soc., Dalton Trans. 1997, 711–713. (b) J€akle, F.; Rulkens, R.; Zech, G.; Foucher, D. A.; Lough, A. J.; Manners, I. Chem. Eur. J. 1998, 4, 2117–2128. (6) Temple, K.; J€akle, F.; Sheridan, J. B.; Manners, I. J. Am. Chem. Soc. 2001, 123, 1355–1364. (7) Tamm, M. Chem. Commun. 2008, 3089–3100. (8) Vogel, U.; Lough, A. J.; Manners, I. Angew. Chem., Int. Ed. 2004, 43, 3321–3325. (9) Schachner, J. A.; Tockner, S.; Lund, C. L.; Quail, J. W.; Rehahn, M.; M€ uller, J. Organometallics 2007, 26, 4658–4662. (10) Lund, C. L.; Schachner, J. A.; Quail, J. W.; M€ uller, J. J. Am. Chem. Soc. 2007, 129, 9313–9320. (11) Me2Ntsi stands for the intramolecularly coordinating trisyl derivative C(SiMe3)2SiMe2NMe2.
r 2010 American Chemical Society
Published on Web 03/30/2010
pubs.acs.org/Organometallics
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Organometallics, Vol. 29, No. 8, 2010
Lund et al. Table 1. Crystal and Structural Refinement Data for 2
Figure 1. Molecular structure of 2 with thermal ellipsoids at the 50% probability level. H atoms are omitted for clarity. Selected bond lengths (A˚) and angles (deg): Si1-C7 = 1.901(5), Si1-C13 = 1.901(5), Si1-C19 = 1.910(5), Si1-Pt1 = 2.3849(14), Pt1-C1 = 2.083(5), Pt1-P1 = 2.3095(14), Pt1-P2 = 2.4169(14); C7-Si1Pt1 = 117.56(16), C13-Si1-Pt1 = 116.28(18), C19-Si1-Pt1 = 110.57(17), C7-Si1-C19 = 105.1(2), C7-Si1-C13 = 98.9(2), C13-Si1-C19 = 107.2(2), C1-Pt1-Si1 = 85.71(14), C1-Pt1P2 = 82.75(14), C1-Pt1-P1 = 178.09(15), Si1-Pt1-P1 = 95.38(5), Si1-Pt1-P2 = 167.43(5), P1-Pt1-P2 = 96.30(5).
Within this paper, we report on our attempts at ringopening polymerization of diphenylsila[1]molybdarenophane (1).
Results and Discussion The sila[1]molybdarenophane 110 reacted with 1 equiv of [Pt(PEt3)3] to give the platinasila[2]molybdarenophane 2 in an isolated yield of 52% (eq 1). In the course of this reaction, the number of signals for coordinated phenyl groups increased from three for compound 1 (C2v symmetry)10 to five for 2 with an intensity ratio of 2:2:2:3:1. The signal of highest intensity is caused by partially overlapping multiplets, which could be assigned to meta (Pt side) and para protons (Pt or Si side). This signal pattern hints toward the Cs-symmetric product 2, which is consistent with all other signals as well. Two different sets of signals were found for ethyl groups, matching with the expectation of a square-planar coordinated Pt(II) atom.
Single crystals of compound 2 suitable for structural determination by X-ray crystallography were grown from solutions of benzene at 6 °C (Figure 1 and Table 1). The four angles around the Pt atom with respect to adjacent coordinated atoms are found with a range of 82.75(14)-96.30(5)° and are close to 90°, which would be expected for an idealized
empirical formula C36H50MoP2PtSi fw 863.82 0.15 0.08 0.05 cryst size/mm3 cryst syst, space group monoclinic, P21/c Z 4 a/A˚ 11.8115(3) b/A˚ 17.6336(3) c/A˚ 19.7795(4) R/deg 90 β/deg 122.1420(10) γ/deg 90 3488.25(13) vol/A˚3 1.645 Fcalcd/mg m-3 temp/K 173(2) -1 4.515 μcalcd/mm θ range/deg 2.69-26.02 no. of rflns collected 43 621 no. of indep/obsd rflns 6860/43 621 abs cor ψ scan no. of data/restraints/params 6860/0/376 1.038 goodness of fit on F2 0.0358 R1 (I > 2σ(I))a a 0.0716 wR2 (all data) -3 0.703 and -0.912 largest diff peak and hole, Δσelect/A˚ P P P a 2 2 R1 = [ ||Fo|-|Fc]/[ |Fo|] for Fo > 2σ(Fo ); wR2 = {[ w(Fo2 2 2 P 2 2 1/2 Fc ) ]/[ w(Fo ) ]} for all data.
Figure 2. Angles to illustrate distortions in [2]metallacylophanes. Values for 2 (deg): R = 13.62(20), δ = 169.5(2), β = 180 anglecentroid1-C1-Pt1 = 3.6(4), β0 = 180 - anglecentroid2-C7-Si1 = 11.8(4).12
square-planar coordination. The rms deviation from a plane of the five atoms Pt1, P1, P2, C1, and Si1 is 0.0599 A˚, with the largest individual deviation of 0.0717(14) A˚ found for P2, illustrating a planar coordination of the d8 metal cation. The Si atom is in a distorted tetrahedral environment, with angles spanning from 98.9(2)° (C7-Si1-C13) to 117.56(16)° (C7-Si1-Pt1). Species 2 is still strained, as can be deduced from the tilt angle R of 13.62(20)° (Figure 2), a more than 6° reduction compared to the starting [1]molybdarenophane 1 (R = 20.23(29)°).10 Similarly, the β angles found to be 40.6(6) and 40.2(7)° in species 110 are reduced to 3.6(4)° on the platinum side and to 11.8(4)° on the silicon side in product 2 (Figure 2).12 For comparison, the archetypical metallacyclophane, dimethylsila[1]ferrocenophane, showed similar reductions of R and β angles on [Pt(PEt3)2] insertion: R changed from 20.8(5)°13 to 11.6(3)°,4a whereas β was reduced from 37.0(6)°13 to 1.2(3)° (Pt side) and 12.8(3)° (Si side).4a The fact that the molybdarenophane 1 gave the insertion product 2 indicated that transition-metal-catalyzed ROP of 1 might be possible. To date, only a few examples of successful ROP of [1]metallarenophanes have been reported in the literature. Manners et al. explored thermal and anionic (12) Standard deviations for angles which involve the centroids of the phenyl ring were estimated by comparing the standard deviations of similar angles where a ring atom replaces the centroid. (13) Finckh, W.; Tang, B.-Z.; Foucher, D. A.; Zamble, D. B.; Ziembinski, R.; Lough, A.; Manners, I. Organometallics 1993, 12, 823–829.
Note
copolymerization of dimethylsila[1]chromarenophane with dimethylsila[1]ferrocenophane.14 A few years later, it was shown that dimethylsila[1]chromarenophane can be polymerized using Karstedt’s catalyst, resulting in polymers of poor solubility.15 Against this background, 1H NMR experiments of molybdarenophane 1 with Karstedt’s catalyst in toluene at ambient temperatures were conducted (see the Supporting Information for details). From 1H NMR spectroscopy we could only prove that species 1 was consumed within 3 h and new signals were observed (Figure S1). The most indicative signals are those of phenyl groups coordinated to molybdenum. Over the course of 3 h the three signals of the coordinated phenyl groups in 1 at δ 5.46, 4.98, and 4.0410 disappeared, while at the same time all new signals in this range appeared in the narrow area between δ 4.45 and 4.70. The fact that the new signals cover just a narrow range close to the signal of the parent bis(benzene)molybdenum (δ 4.58) shows that Karstedt’s catalyst acts on species 1 to give unstrained derivatives of bis(benzene)molybdenum. However, we could not identify a new product. All attempts to crystallize or precipitate a product were unsuccessful. We believe that the species formed were not polymeric in nature, as peaks in 1H NMR spectra have half-bandwidths similar to those of the starting compound 1. One can envision that a polymerization of 1 might be hindered by the sterically demanding SiPh2 moiety; however, electronic effects could also play a role. Therefore, we attempted the synthesis of the iPr2Si- and Me2Si-bridged molybdarenophanes 3 and 4, respectively (see the Supporting Information for details). For both species we followed the procedure that we used for the preparation of compound 1 before.10 Proton NMR spectroscopy of crude reaction mixtures clearly revealed that the targeted compounds 3 and 4, respectively, were formed, among other species (Figures S2 and S3). This interpretation is based on the fact that the Mo-coordinated phenyl groups of the molybdarenophane 1 give rise to a very distinctive pattern: a triplet at δ 5.46 (p-H), a triplet at δ 4.98 (m-H), and a doublet at δ 4.04 (o-H). The new molybdarenophanes 3 and 4 were unequivocally identified by similar patterns with similar chemical shifts (δ 5.52, 5.05, and 3.68 for 3; 5.52, 5.05, and 3.69 for 4; see the Supporting Information for details). However, all attempts to purify these compounds through washing procedures, crystallizations from various solvents, and sublimations failed. Independent of our work, Braunschweig et al. had attempted the synthesis of sila[1]molybdarenophanes using Ph2SiCl2, iPr2SiCl2, and Me2SiCl2 before but did not find any direct evidence for the formation of the targeted species.16 Only in the case of iPr2SiCl2 had a product been identified, the 1,10 -disubstituted bis(benzene)molybdenum derivative [{(CliPr2Si)C6H5}2Mo], and its synthesis was subsequently optimized. We had shown that species 1 can be obtained, with the main impurities presumably being 1,10 -disubstitued bis(benzene)molybdenum species.10 Even though this first silicon-bridged [1]molybdarenophane was characterized by single-crystal X-ray analysis, it could only be isolated in a chemical purity of approximately 90%.10 In the case of species 4, a comparison with published NMR (14) Hultzsch, K. C.; Nelson, J. M.; Lough, A. J.; Manners, I. Organometallics 1995, 14, 5496–5502. (15) Berenbaum, A.; Manners, I. Dalton Trans. 2004, 2057–2058. (16) Braunschweig, H.; Buggisch, N.; Englert, U.; Homberger, M.; Kupfer, T.; Leusser, D.; Lutz, M.; Radacki, K. J. Am. Chem. Soc. 2007, 129, 4840–4846.
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data16 clearly revealed that [{(CliPr2Si)C6H5}2Mo] is the main byproduct (see Figure S3). In case of the Me2Si-bridged species 3, we could not identify the main byproduct; however, new signals appeared in a narrow range around that of [(C6H6)2Mo], indicating the presence of 1,10 -disubstituted bis(benzene)molybdenum species (Figure S2). However, most of these signals are broad, and we can only speculate that this indicates the formation of oligomers. Green et al. had investigated the metathesis reaction of Me2SiCl2 and bis(lithiobenzene)molybdenum before, and a “highly sensitive green solid” exhibiting broad NMR peaks was isolated.17 This led to the conclusion that the product is a polymer of the general formula Cl(Me2Si-η6-C6H5Mo-η6C6H5)nSiMe2Cl; however, it was not further characterized.18 Bis(benzene)molybdenum derivatives are in general “extremely oxygen-sensitive”,19 and one can assume that the introduction of strain will even increase the sensitivity toward oxygen. To date, it seems that the air sensitivity coupled with the high solubility of 3 and 4 in common organic solvents prevented the isolation of these new sila[1]molybdarenophanes in chemically pure forms (see the Supporting Information).
Experimental Section General Procedures. Manipulations were done using standard Schlenk and glovebox techniques (O2 level
