Novel Disilaplatinacyclopentenes Bearing Dialkylsulfide Ligands

Dec 13, 2010 - SMe2), 7.21r7.28 (m, 4H, C6H4), 7.35r7.41 (m, 2H, C6H4), 7.76r. 7.81 (m, 2H, C6H4);13C NMR (toluene-d8, room temperature) δ5.7...
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Organometallics 2011, 30, 68–76 DOI: 10.1021/om100867f

Novel Disilaplatinacyclopentenes Bearing Dialkylsulfide Ligands: Preparation, Characterization, and Mechanistic Consideration of Hydrosilane Reduction of Carboxamides by Bifunctional Organohydrosilanes Hironori Tsutsumi,‡ Yusuke Sunada,† and Hideo Nagashima*,†,‡ †

Institute for Materials Chemistry and Engineering and ‡Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Received September 7, 2010

Three new disilaplatinacyclopentane complexes, (Me2S)Pt[(SiMe2)2C6H4]2 (1), (MeSCH2CH2CH2SMe)Pt[(SiMe2)2C6H4]2 (2), and (MeSCH2CH2CH2SMe)Pt[(SiMe2)2C6H4] (3), were prepared and characterized by the reaction of (PtMe2)2(μ-SMe2)2 with 1,2-bis(dimethylsilyl)benzene in the presence of dialkylsulfide ligands. Catalytic hydrosilane reduction of carboxamides with 1,2-bis(dimethylsilyl)benzene was investigated in the presence of these new complexes, and the possible involvement of disilaplatinacycle dihydride species, “Pt(Si)2(H)2(SR2)”, stabilized by dialkylsulfides in catalysis was discussed.

Introduction Transition metal-catalyzed hydrosilylation of unsaturated organic molecules is widely recognized as a good synthetic tool for organosilicon compounds by way of addition of a Si-H bond of organohydrosilanes across carbon-carbon, carbon-oxygen, or carbon-nitrogen multiple bonds. Among various transition metal catalysts, it is well-known that platinum compounds are essentially important for production of various organosilicon compounds on both laboratory and industrial scales.1 In sharp contrast to the facile hydrosilylation of carbon-carbon multiple bonds, it is curious that platinum catalysts are not good at reducing carbon-oxygen double bonds and carbon-nitrogen multiple bonds, and only a few have been reported in the literature.2 We have recently discovered a breakthrough for this problem, in which certain hydrosilanes having two proximate SiH groups are a good reducing reagent for reduction of carboxamides in the presence *To whom correspondence should be addressed. E-mail: nagasima@ cm.kyushu-u.ac.jp. (1) (a) Marciniec, B. Comprehensive Handbook on Hydrosilylation; Pergamon Press: Oxford, U.K., 1992. (b) Ojima, I. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; John Wiley & Sons: New York, 1989; Part 2, Chapter 25, pp 1479. (c) Brook, M. A. Silicon in Organic, Organometallic, and Polymer Chemistry; John Wiley & Sons: New York, 2000. (d) Marciniec, B. In Applied Homogeneous Catalysis with Organometallic Compounds; Cornils, B., Herrmann, W. A., Eds.; Wiley-VCH: Weinheim, Germany, 1996; Vol. 1, Chapter 2. (2) (a) Barlow, A. P.; Boag, N. M.; Stone, F. G. A. J. Organomet. Chem. 1980, 191, 39. (b) Yamamoto, K.; Hayashi, T.; Kumada, M. J. Organomet. Chem. 1972, 46, C65. (c) Hayashi, T.; Yamamoto, K.; Kumada, M. J. Organomet. Chem. 1976, 112, 253. (d) Cullen, W. R.; Evans, S. V.; Han, N.-F.; Trotter, J. Inorg. Chem. 1987, 26, 514. (e) Zuev, V. V.; de Vekki, D. A. Phosphrus, Sulfer Silicon Relat. Elem. 2005, 180, 2071. (f) Igarashi, M.; Fuchikami, T. Tetrahedron Lett. 2001, 42, 1945. (3) (a) Hanada, S.; Motoyama, Y.; Nagashima, H. Tetrahedron Lett. 2006, 47, 6173. (b) Hanada, S.; Tsutsumi, E.; Motoyama, Y.; Nagashima, H. J. Am. Chem. Soc. 2009, 131, 15032. pubs.acs.org/Organometallics

Published on Web 12/13/2010

of common platinum catalysts widely used for the hydrosilylation of alkenes and alkynes such as H2PtCl6 3 6H2O and Karstedt’s catalyst.3 In particular, 1,1,3,3-tetramethyldisiloxane (TMDS) and 1,2-bis(dimethylsilyl)benzene (BDSB) shown in Chart 1 take part in smooth reduction of various carboxamides under mild conditions. Like the successful reduction of carboxamides with TMDS and BDSB by platinum catalysts described above, certain organohydrosilanes having dual SiH groups close together have now recognized as unique reducing reagents for carbonyl compounds by catalysis of transition metal compounds.4 In particular, the proximate Si-H effect now be applied to the reduction of ketones by Rh4a-d and Ru4e,f catalysts and that of carboxamides by Ru,4g,h Ir,4i and Fe4j catalysts, which facilely realizes the reduction that cannot be achieved by conventional trialkylhydrosilanes having one Si-H moiety in the molecule. One of the most studied examples of the dual Si-H effect is the RhCl(PPh3)3-catalyzed hydrosilylation reactions of ketones, in which significant rate acceleration by two proximate Si-H groups was discovered.4a-d,5,6 Mechanisms of the dual Si-H effect in the RhCl(PPh3)3catalyzed hydrosilylation of ketones are investigated by isolation and characterization of possible intermediates.4a In particular, our recent study of the oxidative addition of (4) (a) Sunada, Y.; Fujimura, Y.; Nagashima, H. Organometallics 2008, 27, 3502. (b) Nagashima, H.; Tatebe, K.; Ishibashi, I.; Sakakibara, J.; Itoh, K. Organometallics 1989, 8, 2495. (c) Nagashima, H.; Tatebe, K.; Itoh, K. J. Chem. Soc., Perkin Trans. 1 1989, 1707. (d) Nagashima, H.; Tatebe, K.; Ishibashi, I.; Nakaoka, A.; Sakakibara, J.; Itoh, K. Organometallics 1995, 14, 2868. (e) Nagashima, H.; Suzuki, A.; Iura, T.; Ryu, K.; Matsubara, K. Organometallics 2000, 19, 3579. (f) Matsubara, K.; Iura, T.; Maki, T.; Nagashima, H. J. Org. Chem. 2002, 67, 4985. (g) Motoyama, Y.; Mitsui, K.; Ishida, T.; Nagashima, H. J. Am. Chem. Soc. 2005, 127, 13150. (h) Hanada, S.; Ishida, T.; Motoyama, Y.; Nagashima, H. J. Org. Chem. 2007, 72, 7551. (i) Motoyama, Y.; Aoki, M.; Takaoka, N.; Aoto, R.; Nagashima, H. Chem. Commun. 2009, 1574. (j) Sunada, Y.; Kawakami, H.; Imaoka, T.; Motoyama, Y.; Nagashima, H. Angew. Chem., Int. Ed. 2009, 48, 9511. r 2010 American Chemical Society

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Chart 1

Chart 2

bis(dimethylsilyl)benzene (BDSB) to RhCl(PPh3)3 or RhH(PPh3)4 led to detection of disilarhodacyclopentanes II-IV (Chart 1), suggesting that the rate acceleration by the dual Si-H effect results from double oxidative addition of two Si-H groups to the metal center. In this sense, the platinum-catalyzed reduction of carboxamides with TMDS and BDSB described above is likely to involve disilaplatinacyclopentene intermediates formed via double oxidative addition of two Si-H groups to the platinum center. Disilaplatinacyclopentenes are one of the most investigated examples of disilametallacycles, and the first one was reported by Eaborn in 1973 by the reaction of BDSB with Pt(CH2dCH2)(PPh3)2.7 The reaction involves the double oxidative addition of two Si-H groups in BDSB followed by elimination of H2 to form a Pt(II) complex, [C6H4(SiMe2)2]Pt(PPh3)2. The chemistry of disilaplatinacyclopentenes was later expanded by Tanaka and co-workers, who isolated a variety of disilaplatinacyclic complexes stabilized by phosphine ligands as shown in Chart 2.8,9 Under the circumstances, we first conducted the mechanistic studies on the platinum-catalyzed reduction of carboxamides with BDSB (5) Corriu and co-workers briefly reported that treatment of N,Ndiethyl-2-phenylacetamide with BDSB resulted in the formation of the corresponding enamine by catalysis of RhCl(PPh3)3, but nothing was mentioned about the possibility that dual Si-H group may accelerate the reaction.6 We have recently found that IrCl(CO)(PPh3)2 exhibited extremely high catalytic activity toward the enamine formation.4i The reaction occurs only when TMDS is used as the hydrosilane; the dual Si-H effect is evident. (6) Corriu, R. P. J.; Moreau, J. J. E.; Pataud-Sat, M. J. Organomet. Chem. 1982, 228, 301. (7) Eaborn, C.; Metham, T. N.; Pidcock, A. J. Organomet. Chem. 1973, 63, 107. (8) (a) Shimada, S.; Tanaka, M. Coord. Chem. Rev. 2006, 250, 991. (b) Shimada, S.; Li, Y.-H.; Rao, M. L. N.; Tanaka, M. Organometallics 2006, 25, 3796. (c) Shimada, S.; Rao, M. L. N.; Li, Y.-H.; Tanaka, M. Organometallics 2005, 24, 6029. (d) Shimada, S.; Tanaka, M.; Honda, K. J. Am. Chem. Soc. 1995, 117, 8289. (e) Li, Y.-H.; Zhang, Y.; Shimada, S. J. Organomet. Chem. 2010, 695, 2057. (9) (a) Tanaka, M.; Uchimaru, Y.; Lautenschlager, H.-J. J. Organomet. Chem. 1992, 428, 1. (b) Tanaka, M.; Uchimaru, Y.; Lautenschlager, H.-J. Organometallics 1991, 10, 16. (c) Shimada, S.; Tanaka, M.; Honda, M. Inorg. Chim. Acta 1997, 265, 1. (d) Uchimaru, Y.; Brandi, P.; Tanaka, M.; Goto, M. J. Chem. Soc., Chem. Commun. 1993, 744. (e) Shimada, S.; Uchimaru, Y.; Tanaka, M. Chem. Lett. 1995, 223. (f) Tanaka, M.; Uchimaru, Y. Bull. Soc. Chim. Fr. 1992, 129, 667.

Scheme 1

by [C6H4(SiMe2)2]Pt(PPh3)2; however, this was hampered by the fact that addition of phosphines to the reaction medium completely inhibited the reduction (vide infra). A search for the appropriate ligand, which can effectively capture the platinum species related to the catalysis and does not inhibit the catalytic reaction, has shown that dialkylsulfides are suitable for the purpose. In this paper, we report novel disilaplatina(II)- and disilaplatina(IV)cyclopentenes stabilized by SR2, which are active toward the reduction of carboxamides. From the catalytic reactions using the newly isolated platinum complexes, mechanistic considerations are made on dual Si-H effects in the platinum-catalyzed reduction of carboxamides.

Results and Discussion A Five-Coordinate Disilaplatina(IV)cyclopentene. A Pt(II) complex, (PtMe2)2(μ-SMe2)2, was found to be a good precursor for a disilaplatinacyclopentene stabilized by a dialkylsulfide ligand in the reaction of BDSB. Reaction of (PtMe2)2(μ-SMe2)2 with BDSB was monitored by 1H NMR spectroscopy; this leads to the observation of a single platinumsilyl species in solution, which is not a disilaplatina(II)cyclopentene deduced from the Eaborn’s result but a Pt(IV) complex 1 formed from two molecules of BDSB and one molecule of a Pt species. Because 1 is highly soluble in common organic solvents, it is difficult to isolate 1 in high yields. Treatment of (PtMe2)2(μ-SMe2)2 with 4 equiv of BDSB resulted in the complete consumption of (PtMe2)2(μ-SMe2)2 and quantitative formation of 1 at room temperature for 14 h in benzene; the product was isolated by crystallization from hexane in 40% yield (Scheme 1). The reaction is formally explained by

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Figure 1. Molecular structure of 1 with 50% probability ellipsoids. Hydrogen atoms were omitted for the sake of clarity. Table 1. Representative Bond Lengths and Angles for 1-3 1

2

3

Bond Lengths (A˚) Pt-Si(1) Pt-Si(2) Pt-Si(3) Pt-Si(4) Pt-S(1) Pt-S(2)

2.4448(11) 2.3316(13) 2.4560(12) 2.3119(14) 2.445(2) -

2.477(2) [2.475(2)] 2.3746(15) [2.3622(14)] 2.3692(14) [2.3679(13)] 2.4784(16) [2.481(2)] 2.5107(12) [2.483(2)] 2.4979(13) [2.5008(12)]

2.3234(14) 2.3198(14) 2.4087(13) 2.4029(13)

Bond Angles (deg) Si(1)-Pt-Si(2) Si(3)-Pt-Si(4) S(1)-Pt-S(2) Si(1)-Pt-Si(4)

81.10(4) 79.63(4) 101.65(5)

83.38(5) [84.08(5)] 84.57(5) [83.76(4)] 94.31(4) [89.03(4)] 170.18(4) [168. 30(4)]

82.81(4) 97.94(4) -

double oxidative addition of the first molecule of BDSB to coordinatively unsaturated “PtMe2(SMe2)” species formed from (PtMe2)2(μ-SMe2)2; this was followed by elimination of 2 mol of methane. Double oxidative addition of the second molecule of BDSB and the subsequent elimination of H2 from the resulting Pt(II) disilametallacyclic species lead to the formation of 1. Although the first oxidative addition could afford a disilaplatina(II)cyclopentene similar to complex 3 described later, the second oxidative addition is too fast to detect it under the conditions. Single crystals of 1 suitable for X-ray diffraction analysis were obtained by slow evaporation of the concentrated hexane solution of 1 at room temperature. The molecular structure of 1 is depicted in Figure 1, and selected bond distances and angles are summarized in Table 1. Complex 1 consists of two disilaplatinacyclopentene skeletons with one dimethylsulfide ligand. The platinum center adopts a distorted trigonal bipyramidal coordination geometry, in which two silicon atoms are at the apical position, whereas the other two silicon atoms and one sulfur atom are at the basal position. The Si(1)-Pt-Si(3) group is arranged almost linearly, and the sum of the equatorial angles around the platinum atom is ∼360°. Two trans-arranged Pt-Si bond distances [2.4448(11) and 2.4560(12) A˚] are significantly longer than those of other Pt-Si bonds [2.3316(13) and 2.3119(14) A˚, respectively], which reflect the elongation of the bond distance induced by the strong trans influence of the Si atom. Spectroscopic data of 1 are reasonably explained by the molecular structure determined by X-ray analysis in considering reversible site exchange of the SMe2 ligand as shown in Chart 3. Dynamic behavior was observed in 1H NMR spectrum; at -90 °C, there were three broad singlets at

δ 0.31, 0.72, and 0.97 in an integral ratio of 2:1:1, and these can be assigned to the signals caused by the four SiMe2 moieties in the molecular structure of 1. The three signals were coalesced at -40 °C and turned to a slightly broad singlet at δ 0.49 at room temperature. Although the crystal structure indicates the existence of eight magnetically inequivalent methyl signals, the rapid and reversible site exchange shown in Chart 3 provides a symmetry of the molecule giving four inequivalent signals. The signals in a ratio of 2:1:1 suggest one of the two peaks is accidentally overlapped. The signal due to the SMe2 group appeared as a singlet at 1.99 ppm in the 1H NMR spectrum, whereas one carbon resonance was found to be at δ 5.7 in the 13C NMR spectrum at room temperature. The 29Si NMR spectrum of 1 gave two signals at δ -20.3 and 0.7 at -90 °C. Six-Coordinate Disilaplatina(IV)cyclopentenes. As described above, complex 1 is the first example of coordinatively unsaturated Pt(IV) compounds having two disilaplatinacyclopentene moieties derived from BDSB. The examples of coordinatively unsaturated five-coordinate Pt(IV) complexes are extremely rare, and there have been few reports on the preparation of them.10 Goldberg et al.10a-d have described the synthesis of the (nacnac)PtR3 (R = Me, H, or SiR0 3) complex, while Templeton and co-workers10e reported the preparation of cationic [(κ2-TpMe2)Pt(SiR3)(H)2]þ (R3 = Et3, Ph3, or Ph2H). It is interesting that the molecular structure of 1 apparently has a vacant site for coordination of another molecule of SMe2 to form a coordinatively saturated Pt(IV) complex 10 with the octahedral coordination geometry (Scheme 2). Low-temperature NMR suggests the reversible formation of 10 in the reaction of 1 with 1 equiv of SMe2 in toluene-d8. At room temperature, the mixture gave a 1H resonance due to the SiMe2 moiety as a broad singlet at δ 0.49 and that due to the methyl signals of SMe2 at δ 1.85. The integral ratio of the two signals was 24:12. In contrast, four slightly broad singlets at δ 0.44, 0.48, 0.68, and 0.95 and one singlet at δ 1.65 appeared due to the coordinated SiMe2 groups and the SMe2 moiety, respectively, at -90 °C. The integral ratio of these five signals is 6:6:6:6:12. These spectral features indicate the presence of four magnetically inequivalent SiMe2 moieties and two magnetically equivalent SMe2 groups. This implies that coordination of another SMe2 molecule to the vacant site of 1 gives 10 having a six-coordinate octahedral geometry with C2v symmetry. Although the six-coordinate species is detected by NMR spectroscopy, attempted isolation of 10 by removal of the solvent in vacuo resulted in quantitative regeneration of five-coordinate complex 1. This is presumably due to reversible coordination of SMe2 to 1: it is favorable for the equilibrium to give 1 once volatile SMe2 dissociated from 10 was removed from the solution during evaporation of the solvent. Reversible coordination of SMe2 to 1 is accompanied by exchange of the SMe2 ligand in 1 with uncoordinated SMe2. This is evidenced by reaction of 1 with SEt2, which resulted in partial exchange of the coordinated SMe2 in 1 by SEt2. Treatment of a C6D6 solution of 1 with 2 equiv of SEt2 at room (10) (a) West, N. M.; White, P. S.; Templeton, J. L.; Nixon, J. F. Organometallics 2009, 28, 1425. (b) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123, 6423. (c) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125, 15286. (d) Kloek, S. M.; Goldberg, K. I. J. Am. Chem. Soc. 2007, 129, 3460. (e) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am. Chem. Soc. 2001, 123, 6425. (f) Zhao, S.-B.; Wu, G.; Wang, S. Organometallics 2008, 27, 1030. (g) Luedtke, A. T.; Goldberg, K. I. Inorg. Chem. 2007, 46, 8496. (h) Sangtrirutnugul, P.; Tilley, T. D. Organometallics 2008, 27, 2223. (i) Karshtedt, D.; McBee, J. L.; Bell, A. T.; Tilley, T. D. Organometallics 2006, 25, 1801.

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Table 2. Crystallographic Data for 1-3 1 empirical formula C22H38SSi4Pt formula weight 642.03 crystal system monoclinic lattice type primitive space group P21 (No. 4) a (A˚) 8.6106(13) b (A˚) 16.235(2) c (A˚) 9.8012(13) R (deg) 90 β (deg) 92.359(2) γ (deg) 90 1369.0(3) volume (A˚3) Z 2 3 1.496 Dcalc (g/cm ) F(000) 640.00 53.627 μ(Mo KR) (cm-1) crystal color, habit dark orange, plate crystal dimensions (mm) 0.15  0.05  0.02 no. of observations (all reflections) 5941 no. of variables 292 reflection:parameter ratio 20.35 R (all reflections) 0.0282 a 0.0276 R1 [I > 2.00σ(I)] 0.0759 wR2 (all reflections)b) goodness of fit 1.031 flack parameter -0.006(5) Friedel pairs 2775 maximum shift/error in final cycle 0.008 2.84 maximum peak in final differential map (e-/A˚3) -1.52 minimum peak in final differential map (e-/A˚3) P P P P a R1 = |Fo| - |Fc|/ |Fo|. b) wR2 = { [w(Fo2 - Fc2)2]/ [w(Fo2)2]}1/2.

2

3

C31H56PtSi4S2 800.34 monoclinic primitive P21 (No. 4) 10.045(4) 33.746(12) 10.429(4) 90 98.600(5) 90 3496(2) 4 1.521 1632.00 42.745 pale yellow, plate 0.10  0.03  0.02 15076 694 21.72 0.0298 0.0266 0.0656 1.012 0.098(3) 7000 0.000 3.04 -1.62

C15H28PtSi2S2 523.77 monoclinic primitive P21/n (No. 14) 8.787(2) 17.074(4) 12.779(3) 90 93.668(5) 90 1913.1(9) 4 1.818 1024.00 76.384 colorless, plate 0.10  0.08  0.05 4367 209 20.89 0.0479 0.0464 0.1569 1.005 0.000 3.90 -1.49

Chart 3

Scheme 2

Scheme 3

temperature for 12 h followed by evaporation under vacuum afforded the mixture of 1 and its SEt2 analogue in an integral ratio of 1:20 (see the Supporting Information).

Figure 2. Molecular structure of 2 in the crystal. Ellipsoids represent 50% probability. Hydrogen atoms and solvate molecules were omitted for the sake of clarity.

A bidentate dialkylsulfide ligand was found to be effective for the isolation of a six-coordinate disilaplatina(IV)cyclopentene. Treatment of 1 equiv of 2,6-dithiaheptane with 1 in benzene at room temperature overnight led to exchange of the SMe2 ligand in 1 with MeS(CH2)3SMe to give six-coordinate Pt(IV) complex 2 in 95% yield (Scheme 3). Single crystals of 2 were obtained by recrystallization from benzene and hexane at room temperature. The molecular structure of 2 is depicted in Figure 2, and the selected bond distances and angles are listed in Table 1. The unit cell contains two independent molecules of 2, and one of the two molecules is shown in Figure 2. The bond distances and angles of the second molecule are given in brackets in Table 1. The coordination geometry around the Pt center is best described as a slightly distorted octahedral arrangement, formed by two sulfur and two silicon moieties in the basal plane, and the remaining two silicon [Si(1) and Si(4)] atoms at the apical position. The four Pt-Si and two Pt-S bond distances are ca. 0.03-0.05 A˚ longer compared with those of 1 presumably because of steric repulsion.

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Scheme 5

Figure 3. Molecular structure of 3 showing 50% probability ellipsoids. Hydrogen atoms were omitted for the sake of clarity.

Complex 2 also exhibited dynamic behavior in solution. At -90 °C, the 1H NMR resonance of 2 due to the SiMe2 moieties showed three slightly broad singlets at δ 0.39, 0.64, and 1.02 with an integral ratio of 1:2:1. The signals appeared as a broad signal at δ 0.55 at room temperature. In the 13C NMR spectrum of 2 at room temperature, the resonance due to the SiMe2 carbons appeared at δ 6.2, and the signal derived from the phenyl groups of the silylbenzene moiety appeared as singlets at δ 128.0, 130.9, and 155.2. The signals due to the 2,6-dithiaheptane moiety appeared at δ 1.33 (quint, SCH2CH2), 1.85 (s, SMe), and 2.41 (t, SCH2CH2) in the 1H NMR spectrum, whereas three carbon resonances were found to be at δ 24.6, 31.6, and 32.9 in the 13C NMR spectrum. In the 29Si NMR spectrum of 2, a singlet appeared at δ -20.3 at -90 °C. Four-Coordinate Disilaplatina(II)cyclopentene and Pt(II)Pt(IV) Interconversion. As described above, the reaction of (PtMe2)2(μ-SMe2)2 with BDSB gave five-coordinate Pt(IV) complex 1, whereas treatment of 1 with MeS(CH2)3SMe afforded Pt(IV) analogues 2 with six-coordinate geometry. Interestingly, attempted preparation of 2 from a 1:2:2 (PtMe2)2(μ-SMe2)2/BDSB/2,6-ditiaheptane mixture resulted in the unprecedented formation of a disilaplatina(II)cyclopentene complex 3 in 70% yield (Scheme 4). The four-coordinate square planar geometry is confirmed by X-ray diffraction analysis of 3, of which the ORTEP drawing is shown in Figure 3, and selected bond distances and angles are summarized in Table 1. The molecular structure of 3 shows that the platinum center adopts a square planar coordination geometry with the two silicon atoms and two sulfur atoms in one plane, and the sum of the equatorial angles around platinum atom is 360.0°. Complex 3 has a five-member planar disilaplatinacylopentene skeleton in which the sum of the internal angles is 538.8°, whereas the conformation of the dithiaplatinacyclohexane adopts a slightly distorted chair conformation. The Pt-Si bond distances of 3 [2.3234(14) and 2.3198(14) A˚] are comparable to those of previously reported disilaplatina(II)cyclopentene complexes.11 In contrast, the Pt-S distances of 2.4087(13) and 2.4029(13) A˚ are ∼0.1 A˚ longer than those found in other Pt-SMe2 complexes such as (SMe2)2PtCl2;12 this reflects the (11) Pfeiffer, J.; Kickelbick, G.; Schubert, U. Organometallics 2000, 19, 62. (12) (a) Horn, G. W.; Kumar, R.; Maverick, A. W.; Fronczek, F. R.; Watkins, S. F. Acta Crystallogr. 1990, C46, 135. (b) Hansson, C.; Carlson, S.; Giveen, D.; Johansson, M.; Yong, S.; Oskarsson, A. Acta Crystallogr. 2006, B62, 474.

strong trans influence of the SiMe2 moiety. This Pt-S bond in 3 is considerably shorter than those of 1 and 2 because the Pt(IV) centers of 1 and 2 are more sterically crowded than that of 3. 1 H NMR spectrum of 3 is consistent with the molecular structure giving a singlet at δ 0.82 due to the four magnetically equivalent Si-Me groups. Satellite signals of the Pt(IV) complexes, 1, 10 , and 2, were difficult to observe. In sharp contrast, a satellite signal due to the coupling with 195Pt (JPt-H = 30.4 Hz) was clearly visible in the 1H NMR spectrum of 3, of which the coupling constant is comparable to those found for previously reported disilaplatina(II)cyclopentene complexes.11,13 A singlet that appeared at δ 1.93 with a satellite signal (JPt-H = 22.0 Hz) can be assigned to the methyl group of the 2,6dithiaheptane group. Four 13C resonances are seen at δ 5.2 (JPt-C = 67.9 Hz, SiMe2), 24.1 (JPt-C = 14.5 Hz, SMe2), 21.8 (MeSCH2CH2), and 33.3 (MeSCH2CH2). These spectral features clearly demonstrate the C2v symmetry of complex 3. Disilaplatina(II)cyclopentene 3 was found to undergo further oxidative addition of BDSB and subsequent elimination of H2 to form disilaplatina(IV)cyclopentene 2. Thus, the reaction of isolated 3 with 1 equiv of BDSB in C6D6 at 50 °C for 4 h afforded complex 2 quantitatively. The reverse reaction from 2 to 3 took place under a hydrogen atmosphere for 14 h at 50 °C; an equimolar amount of BDSB to 3 was formed (Scheme 5). The reaction from 3 to 2 takes place by way of the oxidative addition of BDSB to the Pt(II) center of 3, whereas 3 is formed through the oxidative addition of H2 to the Pt(IV) center of 2. Reversible oxidative addition of one of the Si-H bonds in 1,2-disilylbenzene to a square planar disilaplatina(II)cyclopentene stabilized by phosphine ligands was reported by Tanaka and co-workers (Scheme 6).8d There are striking differences between our results and Tanaka’s reaction; the reaction of 3 to 2 is furnished by double oxidative addition of two Si-H moieties in BDSB to the square planar disilaplatina(II)cyclopentene accompanied by formation of H2, whereas the reaction from 2 to 3 is accomplished by hydrogenolysis of two Pt-Si bonds. In both of the reactions, one plausible explanation is the involvement of a Pt(VI)-tetrasilyldihydride intermediate; however, attempted detection of (13) (a) Fink, W. Helv. Chim. Acta 1976, 59, 606. (b) Pfeiffer, J.; Kickelbick, G.; Schubert, U. Organometallics 2000, 19, 957.

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Scheme 6

Scheme 7

hydride signals due to the Pt(VI)H2 species by low-temperature NMR failed. Summary of the Coordination Chemistry of Pt(II) and Pt(IV) Disilaplatinacyclopentenes Stabilized by Dialkylsulfides. In our previous paper,4a we have investigated three rhodium compounds, Rh(III)(Si)2(Si;SiH)(PPh3) (A), Rh(III)(Si)2(H)(PPh3)2 (B), and Rh(V)(Si)2(H)3(PPh3)2 (C), shown in Scheme 7; A and B were characterized by crystallography, whereas C was detected by NMR. All three complexes were formed by the reaction of RhCl(PPh3)3 with BDSB. The initial exchange of a Rh-Cl group with a Rh-H group is followed by dissociation of one PPh3 ligand, and the double oxidative addition of BDSB to the resulting “Rh(H)(PPh3)2” forms C. Two of the three hydrides in C are eliminated as H2 to form B. Complex A is formed from B by oxidative addition of one Si-H moiety in BDSB to B followed by elimination of H2 and dissociation of PPh3. Either the reaction from C to B or that of B to A is reversible. Complex C is a sevencoordinate complex and coordinatively saturated, whereas A has a square pyramidal geometry with a 16-electron configuration. There are some analogies between these disilarhodacyclopentenes and disilaplatinacyclopentenes presented in this paper (A0 -C0 in Scheme 7). If we assume that a “Rh(H)” species is isoelectronic to Pt, both B and B0 would be similar in that they have two organosilyl and two-electron donating ligands (PPh3 or SR2) with lower oxidation states [Rh(III) vs Pt(II)]. Although the coordination geometry is different, both of them are coordinatively unsaturated. The agostic interaction of a Si-H bond with the Rh center is considered to be a prior stage of the oxidative addition. If oxidative addition of the agostic Si-H bond in A takes place, the formed Rh(V)(H)(Si)4 species may be comparable to Pt(IV)(Si)4 complex A0 . As described in detail later, our earlier study indicated Rh(V)(Si)2(H)3 species C is the closest species among the three disilarhodacyclopentenes to a plausible intermediate in the catalytic cycle. In this sense, Pt(IV)(Si)2(H)2 species like C0

(cannot be detected at present) may exist, and it may be an important species with respect to the reaction mechanisms of platinum-catalyzed reduction of carboxamides with BDSB. Catalytic Reduction of a Carboxamide Catalyzed by Pt(II) and Pt(IV) Disilaplatinacyclopentenes Stabilized by Dialkylsulfides. To understand the mechanisms of platinum-catalyzed reduction of carboxamides with BDSB, we conducted reduction of N,N-dimethyldihydrocinnamamide in the presence of 1 mol % platinum catalysts in THF at 50 °C. The reaction profiles are illustrated in Figures 4-6. The reactions catalyzed by H2PtCl6 3 6H2O or Pt(dba)2 proceeded smoothly to give the corresponding amine as a single product. It is evident that two proximate Si-H groups in BDSB are essentially important for acceleration of the reaction; no consumption of the amide was seen in the reduction with PhMe2SiH (Figure 4). The additive effect was investigated by PPh3 and SMe2 as shown in Figure 5. Addition of PPh3 (1 equiv to Pt) completely inhibited the reaction. In contrast, no retardation was observed when SMe2 (1-5 equiv to Pt) was added to the reaction medium. Catalyst deactivation by PPh3 and a lack of retardation by dialkylsulfides were also evidenced by a series of experiments in which Eaborn’s complex [C6H4(SiMe2)2]Pt(PPh3)2 and three disilaplatinacyclopentenes presented in this paper (1-3) were used as the catalyst. As shown in Figure 6, [C6H4(SiMe2)2]Pt(PPh3)2 did not exhibit any catalytic activity, whereas other Pt complexes stabilized by dialkylsulfides exhibited catalytic activities comparable to that of Pt(dba)2 regardless of the oxidation state of Pt and the monodentate or bidentate dialkylsulfide ligand. Mechanistic Considerations. In sharp contrast to the fact that PPh3 poisoned the catalytically active species, the catalytic reactions described above clearly demonstrate that SMe2 and MeS(CH2)3SMe did not affect the reaction rate. Facile exchange of the dialkylsulfide ligand was seen in the conversion from 1 to 2 or preparation of 3 from (PtMe2)2(μ-SMe2)2, BDSB, and 2,6-dithiaheptane. These indicate that dialkylsulfides behave like solvents in the usual catalytic reactions, which

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Figure 4. Decrease in the level of N,N-dimethyldihydrocinnamamide during the hydrosilane reduction catalyzed by Pt catalysts with PhMe2SiH (left) and 1,2-bis(dimethylsilyl)benzene (right).

Figure 5. Decrease in the level of N,N-dimethyldihydrocinnamamide during the hydrosilane reduction catalyzed by Pt(dba)2 in the presence of additional PPh3 (left) or SMe2 (right).

effectively stabilize the metallic species existing in the catalytic reactions. An important feature of dialkylsulfides in this study is the fact that they are able to stabilize the platinum species strongly related to net catalytic species in the reduction of carboxamides, taking part in their isolation and characterization. It is noteworthy that the oxidative adduct expectedly formed by the reaction of BDSB with either Pt(0) or Pt(II) precursors should contain hydride ligands; however, no hydride is present in all of the isolated platinum complexes, 1-3. As reported earlier, we observed interconversion between Rh(Si)2(H)3(PPh3)2 (C) and Rh(Si)2(H)(PPh3)2 (B) with H2; facile elimination of H2 from C resulted in isolation of B as a stable complex. In contrast, addition of H2 to Rh(Si)3 complex A gives B. Because a small amount of H2 is catalytically formed by reaction of BDSB with a trace amount of moisture, interconversion among A-C is likely to take place in a solution of real catalytic reaction mixtures. Important is the fact that the proposed catalytic cycle for the rhodium-catalyzed hydrosilylation of ketones involves a species derived from C. In other words, all three species (A-C) possibly exist in solution but are involved as precursors for net catalytic species formed in contact with H2. We consider that similar reactions would occur in the platinum-catalyzed reduction of carboxamides; Pt(IV)(Si)4 species A0 is in equilibrium with Pt(II)(Si)2 species B0 in the presence of a small amount of H2 formed by platinum-catalyzed reaction of BDSB with moisture. The closest species to the net catalytic intermediate would be C0 , which can be generated either by double oxidative addition of BDSB to a Pt(0) precursor in the presence of SMe2 or by hydrogenation of B0 . Dissociation of one of the two coordinated SMe2 ligands provides a vacant site for coordination of a carboxamide that initiates the catalytic cycle. In the catalytic

Figure 6. Decrease in the level of N,N-dimethyldihydrocinnamamide during the hydrosilane reduction catalyzed by Pt(dba)2, complexes 1-3, and Eaborn’s complex.

reactions in the absence of SMe2, solvents like THF, dioxane, and toluene would behave as its substitute. The solvent would solvate the active platinum species to facilitate the catalysis but would not stabilize the platinum species related to the catalysis enough for isolation as a stable complex. The result that may support this is formation of a dinuclear platinum complex 4, which was isolated in low yield by treatment of Pt(dba)2 with 2 equiv of BDSB in toluene at room temperature overnight.14 Formation of 4 can be considered as follows. A (14) The formation and molecular structure of 4 were confirmed by X-ray diffraction analysis. However, because of poor crystal quality, a full set of data could not be collected for 4; hence, comprehensive structural data are not presented here.

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Scheme 8

very unstable disilaplatina(IV)pentane species, Pt[(Me2Si)2C6H4](H)2(solvent), is formed as a primary product in the reaction of Pt(0) with BDSB. In the absence of SMe2, this unstable species rapidly dimerizes to form 4 by way of oxidative addition of BDSB accompanied by elimination of methane. The Pt[(Me2Si)2C6H4](H)2(solvent) species also explain the catalytic cycle; coordination of a carboxamide to the Pt center is followed by insertion between either the Pt-Si bond or the Pt-H bond and subsequent reductive elimination. The rate acceleration observed in the reduction of carboxamide with BDSB, which was not seen in the reaction with PhMe2SiH, is due to the high reactivity of the Pt(IV)(Si)2(H)2 intermediate, which is facilely formed from BDSB but not from PhMe2SiH. A possible mechanism is illustrated in Scheme 8.

Conclusion We describe here the preparation and characterization of several new disilaplatinapentanes. The reaction of (PtMe2)2(μ-SMe2)2 with 1,2-bis(dimethylsilyl)benzene (BDSB) afforded a five-coordinate Pt(IV) complex 1, which is the first example of a coordinatively unsaturated Pt(IV) complex having a disilaplatinacyclopentene skeleton. Additional SMe2 reversibly coordinates to 1, resulting in interconversion of five-coordinate and six-coordinate geometries. The complex with a sixcoordinate geometry is actually isolated as 2 by the reaction of 1 with 2,6-dithiaheptane. Hydrogenolysis of Pt-Si bonds of 2 followed by elimination of BDSB resulted in formation of a disilaplatina(II)cyclopentene complex 3. The reverse reaction from 3 to 2 occurred by treatment with BDSB. The possible involvement of these disilaplatinacyclopentenes in the catalytic hydrosilane reduction of carboxamides with BDSB, in which the weak coordination of the dialkylsulfide to the Pt center stabilizes the active species, is discussed. In other words, the Pt(Si)2(H)2 species stabilized by SR2, which has not been detectable but is easily deduced from the results presented in this paper, would be a key intermediate in explaining the proximity effect of two Si-H bonds in accelerating the

platinum-catalyzed reduction of carboxamides. The results described in this paper provide a new aspect for designing highly active catalysts toward hydrosilane reduction of various organic substrates based on the synergistic effects of a dual Si-H group.

Experimental Section General. Manipulation of air and moisture sensitive organometallic compounds was conducted under a dry argon atmosphere using standard Schlenk tube techniques associated with a high-vacuum line. Alternatively, the experiments were performed in a glovebox filled with dry nitrogen. All solvents were distilled over Ph2CO/Na prior to use. 1H, 13C, and 29Si NMR spectra were recorded on a JEOL Lambda 600 or a Lambda 400 spectrometer at ambient temperature unless otherwise noted. 1H and 13C NMR chemical shifts (δ values) are given in parts per million relative to the solvent signal (1H and 13C) or standard resonances (29Si; external SiMe4). Elemental analyses were performed with a PerkinElmer 2400II/CHN analyzer. 1,2-Bis(dimethylsilyl)benzene4a and (PtMe2)2(μ-SMe2)215 were synthesized by a literature method. Preparation of (Me2S)Pt[(SiMe2)2C6H4]2 (1). In a glovebox, (PtMe2)2(μ-SMe2)2 (300 mg, 0.522 mmol) was dissolved in benzene (20 mL), and 1,2-bis(dimethylsilyl)benzene (450 μL, 2.08 mmol) was added to this solution. The resulting mixture was stirred at room temperature for 14 h. The color of the solution was gradually changed from yellow to dark orange. The solvent was evaporated and the residue extracted with hexane (5 10 mL). Insoluble material was removed by centrifugation, and concentration of the resulting solution afforded 1 as orange crystals (267 mg, 40%): 1H NMR (toluene-d8, room temperature) δ 0.49 (br s, 24H, SiMe2), 1.99 (s, 6H, SMe2), 7.21-7.23 (m, 4H, C6H4), 7.48-7.50 (m, 4H, C6H4); 1H NMR (toluene-d8, -90 °C) δ 0.31 (br s, 12H, SiMe2), 0.72 (br s, 6H, SiMe2), 0.97 (br s, 6H, SiMe2), 1.62 (s, 6H, SMe2), 7.21-7.28 (m, 4H, C6H4), 7.35-7.41 (m, 2H, C6H4), 7.767.81 (m, 2H, C6H4); 13C NMR (toluene-d8, room temperature) δ 5.7 (br s, SiMe2), 23.6 (br s, SMe2), 128.5 (s, C6H4), 131.3 (s, C6H4), 155.3 (s, C6H4); 29Si{1H} NMR (toluene-d8, -90 °C) δ -20.3 (15) (a) Hill, G. S.; Irwin, M. J.; Levy, C. J.; Rendina, L. M.; Puddephatt, R. D. Inorg. Synth. 1998, 32, 149. (b) Scott, J. D.; Puddephatt, R. D. Organometallics 1983, 2, 1643.

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(s, SiMe2), 0.70 (s, SiMe2). Anal. Calcd for C20H39SSi4Pt: C, 41.16; H, 5.97. Found: C, 41.30; H, 6.25. Preparation of (MeSCH2CH2CH2SMe)Pt[(SiMe2)2C6H4]2 (2). In a glovebox, 1 (220 mg, 0.343 mmol) was dissolved in benzene (5 mL). To this solution was added 2,6-dithiaheptane (47 μL, 0.348 mmol). The resulting mixture was stirred at room temperature for 14 h. The solvent was removed in vacuo, and the residue was washed with hexane (5 mL). The resulting solid materials were extracted with benzene. After insoluble materials had been removed via centrifugation, the resulting solution was concentrated in vacuo. The solid was washed with hexane to give 2 as a pale yellow powder (233 mg, 95%). Single crystals of 2 were obtained from a benzene/hexane solution at room temperature: 1H NMR (toluene-d8, room temperature) δ 0.55 (br s, 24H, SiMe2), 1.33 (quint, 2H, JH-H = 6.1 Hz, MeSCH2CH2), 1.85 (s, 6H, SMe2), 2.41 (t, 4H, JH-H = 6.1 Hz, MeSCH2CH2), 7.23 (m, 4H, C6H4), 7.54 (m, 4H, C6H4); 1H NMR (toluene-d8, -90 °C) δ 0.39 (br s, 6H, SiMe2), 0.64 (br s, 12H, SiMe2), 0.84 (br s, 2H, MeSCH2CH2), 1.02 (br s, 6H, SiMe2), 1.47 (s, 6H, SMe2), 1.86 (br s, 4H, MeSCH2CH2), 7.23-7.29 (m, 2H, C6H4), 7.297.36 (m, 3H, C6H4), 7.38-7.44 (m, 2H, C6H4), 7.63-7.68 (m, 1H, C6H4), 7.88-7.96 (m, 2H, C6H4); 13C NMR (toluene-d8, room temperature) δ 6.2 (br s, SiMe2), 24.6 (s, MeSCH2CH2), 31.6 (s, SMe2), 32.9 (s, MeSCH2CH2), 128.0 (s, C6H4), 130.9 (s, with a satellite signal due to the coupling with 195Pt, JPt-C = 20.2 Hz, C6H4), 155.2 (s, C6H4); 29Si{1H} NMR (toluene-d8, -90 °C) δ -20.3 (s, SiMe2). Anal. Calcd for C25H44S2Si4Pt: C, 41.93; H, 6.19. Found: C, 41.86; H, 6.19. Preparation of (MeSCH2CH2CH2SMe)Pt[(SiMe2)2C6H4] (3). In a glovebox, (PtMe2)2(μ-SMe2)2 (100 mg, 0.174 mmol) was dissolved in benzene (10 mL). To this solution were added 2,6dithiaheptane (47 μL, 0.348 mmol) and 1,2-bis(dimethylsilyl)benzene (75 μL, 0.348 mmol) at room temperature. The resulting mixture was stirred at room temperature for 14 h. The color of the solution changed from yellow to orange. The solvent was removed in vacuo, and the residue was washed with hexane (2 5 mL). The remaining solid was dissolved in benzene, and the insoluble materials were removed by centrifugation. Removal of volatiles in vacuo was followed by washing of the solid residue with hexane. A desired complex 3 was obtained as white powder (128 mg, 70%). Single crystals of 3 were grown from a saturated benzene solution at room temperature: 1H NMR (C6D6, room temperature) δ 0.82 (s with a satellite signal due to the coupling with 195Pt, 12H, JPt-H = 30.4 Hz, SiMe2), 1.32 (quint, 2H, JH-H = 4.8 Hz, MeSCH2CH2), 1.87 (t, 4H, JH-H = 4.8 Hz, MeSCH2CH2), 1.93 (s with a satellite signal due to the coupling with 195Pt, 6H, JPt-H = 22.0 Hz, SMe), 7.36-7.41 (m, 4H, C6H4), 7.83-7.87 (m, 4H, C6H4); 13C NMR (C6D6, room temperature) δ 5.2 (s with a satellite signal due to the coupling with 195Pt, JPt-C = 67.9 Hz, (16) (a) Sheldrick, G. M. SHELX97; 1997. (b) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Guagliardi, A.; Moliterni, A.; Polidori, G.; Spagna, R. SIR97; 1999.

Tsutsumi et al. SiMe2), 21.8 (s, MeSCH2CH2), 24.1 (s with a satellite signal due to the coupling with 195Pt, 6H, JPt-C =14.5 Hz, SMe), 33.3 (s, MeSCH2CH2), 128.2 (s, C6H4), 131.6 (s with a satellite signal due to the coupling with 195Pt, JPt-C = 46.2 Hz, C6H4), 160.0 (s with a satellite signal due to the coupling with 195Pt, JPt-C = 187.8 Hz, C6H4); 29Si{1H} NMR (C6D6, room temperature) δ 14.3 (s with a satellite signal due to the coupling with 195Pt, JPt-Si = 1535 Hz). Anal. Calcd for C15H28S2Si2Pt: C, 34.40; H, 5.39. Found: C, 34.31; H, 5.29. X-ray Data Collection and Reduction. X-ray crystallography was performed on a Rigaku Saturn CCD area detector with graphite monochromatized Mo KR radiation (λ = 0.71070 A˚). The data were collected at 123(2) K using an ω scan in θ ranges of 3.1-27.5° (1), 3.0-27.5° (2), 3.1-27.5° (3). The data obtained were processed using Crystal-Clear (Rigaku) on a Pentium computer and were corrected for Lorentz and polarization effects. The structure was determined by direct methods14 for 1-3 and expanded using Fourier techniques.16 The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were refined using the riding model. The final cycle of full-matrix least-squares refinement on F2 was based on 5941 observed reflections and 292 variable parameters for 1, 15076 observed reflections and 694 variable parameters for 2, and 4367 observed reflections and 209 variable parameters for 3. Neutral atom scattering factors were taken from Cromer and Waber.17 All calculations were performed using the CrystalStructure18-20 crystallographic software package. Details of final refinement as well as the bond distances and angles are summarized in the Supporting Information, and the numbering scheme employed is also shown in the Supporting Information (figures of which were drawn with ORTEP at 50% probability ellipsoids).

Acknowledgment. This work was supported by Grantin-Aid for Science Research on Priority Areas (18064014, Synergy of Elements) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Supporting Information Available: Molecular structures of 1-3, details of crystallographic studies (1-3), and actual 1H, 13 C, and 29Si NMR data of 1-3. This material is available free of charge via the Internet at http://pubs.acs.org. (17) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; de Gelder, R.; Israel, R.; Smits, J. M. M. DIRDIF99. The DIRDIF-99 program system; Technical Report of the Crystallography Laboratory; University of Nijmegen: Nijmegen, The Netherlands, 1999. (18) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystallography; Kynoch Press: Birmingham, U.K., 1974; Vol. 4. (19) CrystalStructure 3.8.0: Crystal Structure Analysis Package; Rigaku and Rigaku/MSC: The Woodlands, TX, 2000-2006. (20) Carruthers, J. R.; Rollett, J. S.; Betteridge, P. W.; Kinna, D.; Pearce, L.; Larsen, A.; Gabe, E. CRYSTALS Issue 11; Chemical Crystallography Laboratory: Oxford, U.K., 1999.