Article pubs.acs.org/Organometallics
Reactions of Silanone(silyl)tungsten and -molybdenum Complexes with MesCNO, (Me2SiO)3, MeOH, and H2O: Experimental and Theoretical Studies Takako Muraoka,† Haruhiko Kimura,† Gama Trigagema,† Masayuki Nakagaki,‡ Shigeyoshi Sakaki,*,‡ and Keiji Ueno*,† †
Division of Molecular Science, Graduate School of Science and Technology, Gunma University, Kiryu 376-8515, Japan Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103, Japan
‡
S Supporting Information *
ABSTRACT: Reactions of silanone(silyl)tungsten and -molybdenum complexes Cp*(OC)2M{OSiMes2(DMAP)}(SiMe3) (M = W (1a), Mo (1b), Cp* = η5-C5Me5, Mes = 2,4,6-Me3C6H2, DMAP = 4(Me2N)C5H4N) with MesCNO, (Me2SiO)3, MeOH, and H2O were investigated. No silanone trapping product was detected in the reaction of 1 with MesCNO and (Me2SiO)3, while reactions of 1 with MeOH afforded the addition product Mes2Si(OMe)OH (5). The hydrolysis of molybdenum complex 1b afforded silanediol Mes2Si(OH)2 (8) as a main product with a trace amount of Mes2Si(OH)OSiMe3 (9), while that of the tungsten analogue 1a gave siloxysilanol 9 as the sole product, indicating that product distribution in the hydrolysis strikingly depends on the metal fragments. A theoretical investigation revealed that the product distribution difference between tungsten and molybdenum complexes arises from the difference in electrophilicity of the silicon atom in the silanone ligand and that of the lability for oxidative addition of water to the MII−L fragment.
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INTRODUCTION Ketones R2CO have been investigated as important building blocks for organic compounds, as versatile solvents, and as components for polymers. Ketones are stable from the standpoint of polymerization of the CO bond because both σ and π bonds are sufficiently strong. In contrast to ketones, silanones R2SiO, a heavier congener of ketones, are reactive due to an intrinsically weak π bond (Scheme 1a) and
Scheme 2. Stabilized Silanone: (a) Kinetically Stabilized Silanone by Bulky Substituents and (b) Thermodynamically Stabilized Silanone by Lewis Acid and Base
Scheme 1. (a) Silanone and Its Zwitterionic Form and (b) Formation of Polysiloxane via Oligomerization of Silanone Scheme 3. Schematic Representation of SilanoneCoordinated Transition-Metal Complex
readily oligomerize to give stable polysiloxanes (R2SiO)n (Scheme 1b).1 Silanones have been investigated mainly by spectroscopic methods in inert matrices at low temperature;2 however, a few examples have been recently isolated utilizing kinetic stabilization by bulky substituents on Si and thermodynamic stabilization by Lewis base and/or Lewis acid coordination (Scheme 2).3−6 Coordination of silanones to transition-metal fragments to form silanone complexes is a reliable method for stabilization of a silanone (Scheme 3).7−9 The first silanone-coordinated zinc complex was synthesized by the reaction of the isolated © XXXX American Chemical Society
silanone with ZnMe2.7 We reported the first silanone transitionmetal complex 1a, its molybdenum analogue 1b, and 2, in which the η1-Mes2SiO ligand (Mes = 2,4,6-Me3C6H2) is stabilized by transition-metal fragments and Lewis bases Received: December 28, 2016
A
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Organometallics (Scheme 4).8 Very recently, an anionic silanone complex with an η2-silanone ligand was synthesized by Tobita, Hashimoto et al.9
were purchased from Wako Pure Chemical Industries (toluene, THF, MeOH, benzene-d6, and CDCl3) or Kanto Chemical ((Me2SiO)3) and purified as follows. Toluene and THF were dried by refluxing over sodium benzophenone ketyl followed by distillation under a nitrogen atmosphere before use. (Me2SiO)3 and CDCl3 were dried over molecular sieves 4A. MeOH was dried over CaH2 followed by distillation under a nitrogen atmosphere and stored over molecular sieves 4A. Benzene-d6 was distilled from a potassium mirror under vacuum and stored over molecular sieves 4A. MeOSiMe3 (6)15 and Mes2Si(OH)2 (8)16 are known compounds, and the spectroscopic data of 6 and 8 are identical with those reported in the literature. Column chromatography was performed using a glass column (1.7 cm i.d., 52 cm length) filled with silica gel 60 (Merck, particle size 0.063−0.200 mm). NMR spectra were recorded on a JEOL JNM-AL300, a JNMECS300, a JNM-ECS400, a JNM-AL500, or a JNM-ECS600 Fourier transform spectrometer at room temperature. IR spectra were recorded on a JASCO FT/IR-600 Plus spectrometer at room temperature. Elemental analysis was performed by the Microanalytical Center, Gunma University. NMR Monitoring of Reactions between Cp*(OC)2M{O SiMes2(DMAP)}(SiMe3) (M = W (1a), Mo (1b)) and MesCNO, (Me2SiO)3, MeOH, and H2O. A typical procedure is as follows; MesCNO (3 mg, 2 × 10−5 mol) was added to a C6D6 (0.5 mL) solution of Cp*(OC)2Mo{OSiMes2(DMAP)}(SiMe3) (1b, Cp* = η5-C5Me5, DMAP = 4-(Me2N)C5H4N) (5 mg, 7 × 10−6 mol) in a Pyrex NMR tube (5 mm o.d.) equipped with a Teflon stop valve. The reaction was periodically monitored by 1H NMR spectroscopy. Isolation of Reaction Products: Mes2Si(OMe)OH (5). MeOH (48 μL, 1.2 × 10−3 mol) was added to a toluene solution (15 mL) of Cp*(OC)2W{OSiMes2(DMAP)}(SiMe3) (1a) (480 mg, 5.6 × 10−4 mol) in a 50 mL Schlenk flask with vigorous stirring. The reaction mixture was stirred at 50 °C for 36 h and concentrated to 1 mL under reduced pressure at ambient temperature. The residue was purified by column chromatography on silica gel (eluent hexane/CH3CO2Et 50/ 1). The fraction with Rf = 0.14 was collected and concentrated under reduced pressure to afford colorless solids of Mes2Si(OMe)OH (5) (34 mg, 1.1 × 10−4 mol) in 20% yield. 1H NMR (300 MHz, C6D6): δ/ ppm 6.74 (s, 4H, m-H), 3.35 (s, 3H, OMe), 2.51 (s, 12H, o-Me), 2.10 (s, 6H, p-Me). (The OH signal was not detected because of severe broadening.) 13C{1H} NMR (125.7 MHz, CDCl3): δ/ppm 145 (Ar), 140 (Ar), 131 (Ar), 129 (Ar), 49.8 (OMe), 23.5 (o-Me), 21.2 (p-Me). 29 Si{1H} NMR (99.3 MHz, CDCl3): δ/ppm −23.0. IR (C6D6, cm−1): 3433 (w, νOH), 1048 (s, νSiO). Anal. Calcd for 5 (C19H26O2Si): C, 72.56; H, 8.33. Found: C, 72.35; H, 8.40. Mes2Si(OH)OSiMe3 (9). A THF solution (1 mL) of H2O (4.4 μL, 2.5 × 10−4 mol) was added to a THF solution (8 mL) of Cp*(OC)2W{OSiMes2(DMAP)}(SiMe3) (1a) (210 mg, 2.4 × 10−4 mol) in a 50 mL Schlenk flask with vigorous stirring. The reaction mixture was stirred for 30 min and then concentrated to 1 mL under reduced pressure. The residue was purified by column chromatography on silica gel (eluent hexane/CH3CO2Et 20/1). The fraction with Rf = 0.2 was collected and concentrated under reduced pressure to afford a colorless liquid of Mes2Si(OH)OSiMe3 (9) (49 mg, 1.3 × 10−4 mol) in 52% yield. 1H NMR (300 MHz, C6D6): δ/ppm 6.73 (s, 4H, m-H), 2.50 (s, 12H, o-Me), 2.10 (s, 6H, p-Me), 0.12 (s, 9H, SiMe3). (The OH signal was not detected because of severe broadening.) 13C{1H} NMR (125.7 MHz, CDCl3): δ/ppm 144 (Ar), 139 (Ar), 133 (Ar), 129 (Ar), 23.6 (o-Me), 21.2 (p-Me), 1.8 (SiMe3). 29Si{1H} NMR (99.3 MHz, CDCl3): δ/ppm 9.6 (SiMe3), −33.1 (SiMes2). IR (C6D6, cm−1): 3611 (w, νOH), 1073, 1039 (s, δSi−O−Si, νSiO). Anal. Calcd for 9 (C21H32O2Si2): C, 67.69; H, 8.66. Found: C, 67.40; H, 8.79. Computational Details and Models. Geometry optimization was carried out by the DFT method with the B3PW91 functional,17 using the LANL2DZ basis sets with effective core potentials (ECPs)18 for Mo and W atoms and 6-31G(d) basis sets for other atoms. Energy changes were evaluated at the SCS-MP2 level, using (311111/22111/ 411) basis sets with effective core potentials by the Stuttgart− Dresden−Bonn group19 for Mo and W atoms and 6-311G(d) basis
Scheme 4. Silanone Complexes 1 and 2 Reported by Us
Silanones with small substituents, H and Me, have been known to react with hexamethylcyclotrisiloxane ((Me2SiO)3),10 MeOH,11 and H2O,11 furnishing the cyclotetrasiloxane, Me2Si(OMe)OH, and Me2Si(OH)2, respectively (Scheme 5). Scheme 5. Trapping Reactions of Silanones with (Me2SiO)3, MesCNO, MeOH, and H2O
Relatively bulky silanones PhR1SiO10g (R1 = 8-(dimethylaminomethyl)-1-naphthyl) and Tbt(Mes)SiO12 (Tbt = 2,4,6{CH(SiMe3)2}3C6H2) were also trapped by (Me2SiO)3 and MesCNO to afford cyclotetrasiloxane and the [2 + 3] cyclization product 3, respectively (Scheme 5). Although the reactivities of silanones have been extensively explored,13 those of the silanone complexes have remained unexplored except for the thermal decomposition and oxygenation of complex 1 and thermal decomposition of 2 in the presence and absence of PMe3 reported by us.8 In this paper, we report the reactions of silanone(silyl)tungsten and -molybdenum complex 1 with MesCNO, (Me2SiO)3, MeOH, and H2O. A theoretical investigation was also performed to clarify the mechanism for the hydrolysis reaction.
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EXPERIMENTAL SECTION
All manipulations were performed using standard Schlenk tube techniques under nitrogen, vacuum line techniques, or a drybox under nitrogen. The synthesis of silanone complex 1 was reported previously.8 MesCNO (Mes = 2,4,6-Me3C6H2) was prepared according to the literature.14 The other chemicals except for H2O B
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Organometallics sets20 for the other atoms. Solvation effects (benzene) were evaluated by the PCM method.21 The Gaussian 09 program was used for all of these calculations.22 Because the real complex is too large for SCS-MP2 calculations, model complexes were employed which were constructed by substituting methyl groups for mesityl groups on the Si atom and substituting Cp (η5-C5H5) for Cp* (η5-C5Me5). Other moieties were not modified. This model seems reasonable because the optimized geometry agrees with the experimental geometry, as will be discussed below.
MeOSiMe315 (6, 70−79%), [Cp*M(CO)2]223 (M = W (7a), Mo (7b)), and 4 (in the case of 1b) (eq 3).24 Yields of 5 and 6
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RESULTS AND DISCUSSION Reactions of 1 with Aprotic Silanone Trapping Reagents: MesCNO and (Me2SiO)3. As mentioned in previous communications, silanone(silyl)molybdenum complex 1b gradually decomposed in C6D6 at 25 °C for 2 days to give Cp*(OC)2Mo(DMAP)(SiMe3) (4, Cp* = η5-C5Me5, DMAP = 4-(Me2N)C5H4N) in 48% NMR yield, while silanone(silyl) tungsten complex 1a was stable in C6D6 at 25 °C for at least 7 days and decomposed to give a complex mixture at 80 °C for 2 h (eq 1).8 The formation of complex 4 suggests the generation
in W complex 1a were comparable to those observed in Mo complex 1b. Compounds 5 and 6 are formed via addition of MeOH to the SiO bond and methanolysis of the M−SiMe3 bond,25 respectively. Since free silanone Mes2SiO is not generated under the reaction condition as mentioned in the previous section, MeOH probably attacked the silanone ligand remaining on the metal centers. Reactions of 1 and H2O also occurred rapidly, but the main product is different between 1a and 1b. Treatment of silanone(silyl)molybdenum complex 1b with 1 equiv of H2O in C6D6 at 25 °C resulted in the complete consumption of 1b for 0.5 h to afford H2O addition product Mes2Si(OH)216 (8, 86%) and a trace amount of Mes2Si(OH)(OSiMe3) (9, 7%). Hydrolysis of silanone(silyl)tungsten complex 1a proceeded rapidly, but the main product was 9 (82%) and 8 was not detected at all (eq 4). The product distribution remained unchanged in the reactions of 1a/1b with 2 equiv of H2O. Compound 8 was formed by addition of H2O to the SiO bond of Mes2SiO, while compound 9 seems to be formed by addition of an OH group to the silicon center followed by migration of the SiMe3 ligand to the oxygen atom of the siloxy fragment Mes2Si(OH)O. Another formation path including initial formation of 8 followed by oxidative addition of the OH bond of 8 to the 16-electron complex Cp*(OC)2M(SiMe3) and reductive elimination of 9 can be excluded, since treatment of Cp*(OC)2W(pyridine)(SiMeEt2),26 which is known to generate a 16e silyltungsten intermediate readily, or of complex 4
of free silanone Mes2SiO during the reaction. Thus, we conducted reactions of complex 1 with aprotic silanone trapping reagents, MesCNO12 and (Me2SiO)3.10g However, trapping products were not detected at all, though complex 1 was consumed completely (eq 2). These results clearly demonstrate that free silanone Mes2SiO is not generated from complex 1 under the reaction conditions. Reactions of 1 with Protic Silanone Trapping Reagents: MeOH and H2O. Although aprotic silanone trapping reagents did not react with complex 1, reactions of protic reagents MeOH and H2O proceeded smoothly to afford the corresponding silanone trapping products. Thus, treatment of silanone(silyl) complexes 1 with 2 equiv of MeOH in C6D6 at 25 °C for 8−10 h afforded Mes2Si(OMe)OH (5, 79−84%), C
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Organometallics Scheme 6. Reaction Course for the Formation of Me2Si(OH)2 (Prd1) in the Reaction of CpM(CO)2(SiMe3){O SiMe2(DMAP)} (M = W (Reactant(W)), Mo (Reactant(Mo))) with Two Water Molecules (SiO Reaction Path)
Figure 1. Geometry and energy changes in the reaction of two water molecules with CpMo(CO)2(SiMe3){OSiMe2(DMAP)} (Reactant(Mo)) (SiO reaction path). Values in parentheses represent the Gibbs energy change in kcal/mol.
with 8 in C6D6 at 25−50 °C for 1 day afforded no siloxysilanol Mes2Si(OH)(OSiMeR2) (R = Et, Me (9)). To elucidate the formation mechanism of 9 and the dependence of the product distribution of 8 and 9 on the metal fragments, we performed a theoretical investigation of the geometry and electronic structure of the model complexes Cp(OC)2M{OSiMe2(DMAP)}(SiMe3) (Cp = η5-C5H5, M = W (Reactant(W)), Mo (Reactant(Mo))) and their reactions with H2O in C6H6. Geometry and Electronic Structure of the Model Complexes Cp(OC)2M{OSiMe2(DMAP)}(SiMe3) (M = W (Reactant(W)), Mo (Reactant(Mo))). The optimized structure of the model complex Cp(OC)2M{OSiMe2(DMAP)}(SiMe3) (M = W (Reactant(W)), Mo (Reactant(Mo))) agrees with the experimental structure of 1;8 important structural parameters are compared between experimental and optimized values in Figure S1 of the Supporting Information. This suggests that the use of Cp instead of Cp* seems reasonable. In Cp(OC)2M(OSiMe2)(SiMe3) without DMAP (M = W, Mo), the silanone coordinates with the M through the O atom in an η1 fashion, while the Si atom of the SiO double bond does not coordinate with the M, as shown in Figure S2 of the Supporting Information. The SiO π and π* MOs are calculated at −8.10 and −1.06 eV, respectively, in free silanone (OSiMe2), whereas they are shifted to −9.74 and −2.08 eV in the W complex and −9.56 and −2.00 eV in the Mo complex. Because the π and π* MOs become lower in energy by the coordination with the M atom, it is likely that the σ donation of silanone is stronger than the π back-donation in the silanone coordinate bond; if the π back-donation is strong, the π* MO becomes higher in energy by coordination with the M atom.
The weak π back-donation is consistent with the coordination structure in which the O atom of silanone interacts with the metal but the Si atom does not. The π and π* MO energies are lower in the W complex than in the Mo complex, which suggests that the σ donation is stronger in the former than in the latter. On comparison with the π* MO (−0.57 eV) of acetone, the π* MO of silanone in Cp(OC)2M(OSiMe2)(SiMe3) (M = W, Mo) exists at much lower energy. These features suggest that the Si center is strongly electrophilic, as expected. Actually, DMAP interacts strongly with the Si atom of silanone coordinating with the W and Mo centers, which yields a considerably large stabilization energy (42.0 kcal/mol for M = W and 40.5 kcal/mol for M = Mo). The Si−O distance is moderately elongated by 0.029 and 0.027 Å in M = W, Mo, respectively. Consistent with the large stabilization energy, the electron population of DMAP considerably decreases by 0.25e and the electron population of the silanone moiety somewhat increases by +0.15e for M = W and +0.17e for M = Mo. The metal atomic population changes little (by −0.02e for M = W, Mo). The electron populations of CO and SiMe3 moieties also change little, while that of Cp somewhat increases. As a result of the DMAP coordination, the π* MO of the silanone disappears and thereby the Si center becomes much less reactive for a nucleophile such as water. The HOMO of Reactant(M) mainly consists of the metal d orbital, and the LUMO also mainly consists of the metal d orbital; the HOMO and LUMO of Reactant(M) are shown in Figure S3 of the Supporting Information. These features suggest that both the O atom of silanone and the metal centers are reactive with a water molecule. D
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Figure 2. Gibbs energy changes in the reaction of CpM(CO)2(SiMe3){OSiMe2(DMAP)} (Reactant(M)) with the water molecule.28
Scheme 7. Reaction Course for Formation of Me2Si(OH)OSiMe3 (Prd2) in the Reaction of CpM(CO)2(SiMe3){O SiMe2(DMAP)} (M = W (Reactant(W)), Mo (Reactant(Mo))) with Water Molecules (Metal Reaction Path)
Reaction of Water with Silanone Ligand, Leading to the Formation of Me2Si(OH)2 (Prd1) (SiO Reaction Path). In this section, the reaction of a water molecule with the OSiMe2(DMAP) ligand in Reactant(M) is investigated; since a methyl group was employed as a substituent in the calculation instead of a mesityl group to save CPU time, Me2Si(OH)2 and Me2Si(OH)OSiMe3 are products of the model reaction instead of 8 and 9 and are denoted as Prd1 and Prd2, respectively, hereafter. Because water molecules usually
form aggregates via H···O hydrogen bonds in a nonpolar solvent27a and also participation of two water molecules in the reaction has been theoretically reported,27b we calculated the reaction with two water molecules as well as with one water molecule for comparison. These results of calculations clearly showed that the reaction with two water molecules proceeds more easily than that with one water molecule. The results obtained for the reaction with two water molecules are described here (see Figures S4 and S5 of the Supporting E
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Figure 3. Geometry and energy changes in the H−OH oxidative addition (two water molecules) to CpMo(CO)2(SiMe3){OSiMe2(DMAP)} (Reactant(Mo)) in the metal reaction path. Values in parentheses represent the Gibbs energy change in kcal/mol.
H−OH bond reacts with the M−silanone moiety, as shown in Scheme 7. The DFT calculations show that, when one or two water molecules are approaching the Mo and Si centers, the transition state TS2b (shown in Figure S6 of the Supporting Information) or TS2b′ (shown in Figure 3) is located along with dissociation of DMAP from the Si center. Hereafter, we investigated the reaction of two water molecules with Reactant because the reaction of two water molecules occurs more easily than that of one water molecule; actually, the reaction with one water molecule needs a Gibbs activation energy greater than that of two water molecules, as shown in the Supporting Information for the geometry and energy changes of the reaction via one water molecule (Figures S6 and S7 (the upper part)). In the transition state TS2b′ (M = Mo), the H atom of one water molecule (w1) is approaching the metal center (Mo−Hw1 = 2.158 Å), the OH group of another water molecule (w2) is approaching the Si atom (Si−Ow2 = 1.726 Å), and the SiO distance is moderately elongated to 1.622 Å; see Scheme 7 and Figure 3 for w1, w2, Hw1, etc. The Ow1−Hw1 distance is considerably elongated to 1.350 Å. The Ow2−Hw2 distance is also considerably elongated to 1.654 Å. These geometrical features indicate that the Mo−H and Si−OH bond formations are in progress concomitantly with the H−OH bond breaking. To compensate for those bond elongations, Ow1 is approaching Hw2 (Ow1−Hw2 = 1.008 Å) to form an O−H bond between w1 and w2, which contributes greatly to the stabilization of the transition state. The ΔG°⧧ value is 14.8 kcal/mol in the reaction with two water molecules. Then, the intermediate CpMo(H)(CO)2{OSiMe2(OH·H2O)} (Int2b′) is produced, in which a H (hydride) is bound with the metal and an OH group is bound with the Si atom; in other words, the silanone is converted to a silanolate anion. These changes indicate that the oxidation state of the metal center is increased by +2 in the reaction. This reaction is understood to be oxidative addition to an M−L moiety (L = neutral species).29 In the tungsten complex, the reaction occurs via essentially the same transition state, as shown in Figure S7 of the Supporting Information (the lower part). The ΔG°⧧ value is 14.0 kcal/mol in the presence of two water molecules. The oxidative addition to the W complex occurs more easily than that to the Mo complex in the reaction with two water
Information (the upper part) for detailed geometry changes of the reaction with one water molecule). It is likely that two water molecules can react with the silanone moiety from a different side of DMAP to avoid steric repulsion with DMAP, as shown by Scheme 6. In this reaction course, water dimer approaches the SiO bond with one H atom ahead in the transition state, whereas the O of water is a bit distant from the Si atom of silanone, as shown in Figure 1. This geometry of the transition state is reasonable because the silanone-DMAP moiety does not have a good LUMO for forming an interaction with the water lone pair, as mentioned above. It is noted that DMAP starts to dissociate from the Si center; the Si−N distance is elongated by about 0.15 Å. The DMAP dissociation leads to the formation of the LUMO (π* MO of silanone) on the Si center, which starts to interact with the O atom of water. In Int1b′ (M = Mo), Me2Si(OH)2 coordinates with the metal center through one OH group and also interacts with one water molecule through the hydrogen bonds. DMAP is still interacting with the Si center, suggesting that the Si center has five-coordinated hypervalency. The Gibbs activation energy (ΔG°⧧) is 15.2 kcal/mol for the Mo complex and 16.1 kcal/ mol for the W complex. Figure S5 (the lower part) shows detailed geometries and energy changes in the reaction of the W complex. It should be noted that this reaction of the Mo complex occurs with a smaller ΔG°⧧ value than that of the W complex; see Figure 2 (left-hand side). Prd1 is released as a product from Int1′ along with formation of “Cp(OC)2M(SiMe3)(DMAP)”. Reactions of Water with Metal and Metal−Silanone Moiety: Leading to the Formation of Me2Si(OH)(OSiMe3) (Prd2) (Metal Reaction Path). Not only the O SiMe2(DMAP) ligand but also the metal center is reactive with the water molecule. Here, oxidative addition of the H− OH bond to the metal center was investigated. Though we tried to locate the transition state for such concerted oxidative addition, we failed, probably because the metal center is considerably congested; the metal center has a sevencoordinated structure and DMAP interacting with the Si atom of silanone takes a position between two CO ligands, as shown in Figure S1 of the Supporting Information. Instead of direct oxidative addition to the metal center, we found that the F
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Figure 4. Geometry and energy changes from Int2b to Int6b (Mo complex) in the metal reaction path. Values in parentheses represents the Gibbs energy change in kcal/mol.28
(CO)2(H-SiMe3){OSi(OH)Me2} (Int4b). In this intermediate, trimethylsilane forms a σ complex with the metal center through the coordination of the Si−H bond. This is not surprising because the Si−H σ-bonding MO exists at high energy. Similar σ-silane complexes have been investigated experimentally and theoretically as intermediates for oxidative addition/reductive elimination of H−Si bonds on the transition-metal center.32−34 The oxidative addition of HSiMe3 occurs through the transition state TS5b to afford CpMo(CO)2(H)(SiMe3){OSi(OH)Me2} (Int5b), in which the positions of SiMe3 and H are exchanged with each other from Int2b. As a result, the SiMe3 group takes a position close to the OSi(OH)Me2 group. Starting from Int5b, the reductive coupling between SiMe3 and OSi(OH)Me2 groups occurs easily because the Si atom wants to bind with the O atom. In the product CpMo(CO)2(H){Me3SiOSi(OH)Me2} (Int6b), the silyl ether is bound with the metal center because the O atom is negatively charged and has a lone pair orbital. Prd2 is released from the metal center with the concomitant formation of “Cp(OC)2MH”. The energy changes along these elementary steps are shown in Figure 2 (right-hand side). In the Mo complex, the first step (oxidative addition of water molecule to the Mo center) needs a ΔG°⧧ value of 14.8 kcal/mol and the step from Int3 to TS4 needs the ΔG°⧧ value of 9.7 kcal/mol relative to Int3. However, to complete the reaction yielding Prd2, the reaction system needs to pass the highest energy
molecules. It is likely that the oxidative addition to the 5d metal occurs more easily than that to the 4d metal because the 5d orbital energy is higher than the 4d energy.30 This understanding is consistent with the present result that oxidative addition to the W complex occurs more easily than that to the Mo complex. In Int2b′, DMAP almost dissociates from the Si center and the additional water molecule also weakly interacts with the OH group on the Si atom through a hydrogen bond. Therefore, we investigated the further reaction from Int2 to Prd2 without DMAP and an extra water molecule to save CPU time. Though Me2Si(OH)OSiMe3 (Prd2) is formed by the reductive elimination of the SiMe3 and OSi(OH)Me2 groups, such reductive elimination is difficult from Int2b because the SiMe3 group is very distant from the OSi(OH)Me2 group.31 This means that the SiMe3 group must migrate to the position cis to the OSiMe2(OH) group prior to the reductive elimination. However, direct position exchange of the H and SiMe3 ligands was calculated to be difficult because the metal center is congested; it is seven-coordinated in a formal sense, as mentioned above. Instead, they change positions via the silane-coordinated intermediate Int4b, as shown in Figure 4 (for the W complex, see Figure S8 of the Supporting Information). The H ligand takes a position close to the SiMe3 group in Int3b, and then the H−Si reductive elimination occurs through the transition state TS4b to afford CpMoG
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Organometallics
respectively. The HOMO of the W complex is higher in energy than that of the Mo complex, and the LUMO of the W complex is lower in energy than that of the Mo complex. As a result, the donation of silanone to the metal center is stronger in the W complex than in the Mo complex, which leads to the presence of the silanone π MO at lower energy in the W complex than in the Mo complex. Therefore, the reaction of a water molecule with the silanone occurs more easily in the Mo complex than in the W complex. TS2′ is the transition state of the reaction between the H− OH bond and the M−silanone moiety. This step is understood to be the oxidative addition of the H−OH bond to the M− silanone moiety, as discussed above. Because the HOMO, mainly consisting of the d orbital, is calculated at higher energy in the W complex than in the Mo complex as shown in Figure S3 of the Supporting Information, the W complex is more reactive for the oxidative addition in comparison to the Mo complex. This trend seems reasonable because the 5d orbital energy is higher than the 4d energy.30 In the Mo complex, however, the highest energy transition state leading to Prd2 is not TS2′ but TS4, which is the transition state for the Si−H reductive elimination. This result is seemingly unreasonable because the reductive elimination is generally easier in the 4d metal than in the 5d metal. It is noted that the activation energy of this step from Int3 to TS4 is almost the same between the Mo and W complexes (9.7 kcal/mol): i.e., the highest energy TS4 in the Mo complex arises from high-energy Int3. The less stable Int3 should be attributed to the fact that the oxidative addition to the Mo complex is less exothermic than that to the W complex. All of the above results indicate that Prd1 is more easily formed in the reaction of the Mo complex through the electrophilic attack of the H atom of water to the O atom of silanone-DMAP. On the other hand, Prd2 is preferably formed in the W complex via oxidative addition of the H−OH bond of water to the W−silanone moiety, Si−H reductive elimination and oxidative addition on the W center, and reductive elimination of the Si−O bond. At the end of this section, we wish to discuss briefly the reason Prd1 is produced in methanol (MeOH). In comparison to H2O, MeOH is electron donating because of the presence of the Me group. It is likely that the oxidative addition of MeOH is more difficult than that of H2O but the attack of MeO moiety of MeOH to the SiO moiety would occur more easily than that of the OH moiety of H2O because of the electron-donating nature of the Me group. As a result, Prd1 is formed more easily in the reaction with MeOH than with H2O.
transition state TS4b. Because Int3 is less stable in energy than Reactant, the ΔG°⧧ value to complete the reaction is the energy difference between Reactant and TS4b, which is 15.3 kcal/mol. In the W complex, the rate-determining step is the oxidative addition of a water molecule to the W center (TS2a′), the ΔG°⧧ value of which is 14.0 kcal/mol. It is concluded that this reaction occurs more easily in the W complex than in the Mo complex. Difference in Product Distribution between W and Mo. As clearly shown in Figure 2, Prd2 is a preferable product in the case of the W complex, because TS1′ is higher in energy than TS2′, which is the rate-determining step leading to the formation of Prd2. These computational results of the W complex agree with the experimental observation that only product 9 is produced and 8 is not produced at all; remember that 8 and 9 correspond to Prd1 and Prd2, respectively. In the Mo complex, on the other hand, TS1 is slightly more stable than TS4, indicating that Prd1 is produced slightly more than Prd2. These computational results of the Mo complex are consistent with the experimental finding that silanediol 8 is the main product and 9 is a minor product. It is noted that, though the difference in energy between TS1 and TS4 is very small, the difference of the Mo complex from the W complex is correctly discussed because only Prd2 is produced in the W complex. It is of considerable importance to elucidate the reasons TS1′ of the W reaction system is higher in energy than that of the Mo reaction system. In this transition state, the O−H and Si− OH bond formations and the H−OH bond breaking are in progress and the oxidation state of the metal center does not change at all. As mentioned above, the approach of the H atom of water to the O atom of the OSiMe2(DMAP) moiety occurs prior to the approach of the OH group to the Si atom. Though the atomic charges of the O atom of the silanone moiety differ little between the W and the Mo complexes (oxygen atomic population −1.037e (W) and −1.042e (Mo)), the SiO π MO energy (−8.86 eV) of the Mo complex is higher than that (−8.98 eV) of the W complex. Thus, the Mo complex is more favorable for the electrophilic attack of the proton-like H atom of water molecule in comparison to the W complex. To find the reason for the SiO π MO energy difference, we investigated the frontier orbitals of CpM(CO)2(SiMe3), where the geometry was taken to be the same as the corresponding part of the silanone complex CpM(SiMe3)(CO)2(OSiMe2). As shown in Figure 5, the HOMO and the LUMO are the dπ orbital and the dσ orbital,
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CONCLUSIONS We investigated the reactions of the silanone-coordinated tungsten and molybdenum complexes Cp*(OC)2M{O SiMes2(DMAP)}(SiMe3) (M = W (1a), Mo (1b)) with aprotic (MesCNO and (Me2SiO)3) and protic (MeOH, H2O) silanone trapping reagents. No trapping product was obtained from the reactions with MesCNO and (Me2SiO)3, indicating that free silanone Mes2SiO is not generated from 1 under the reaction conditions. Reactions of 1 with methanol afforded the addition product Mes2Si(OMe)(OH) (5). The hydrolysis of molybdenum complex 1b afforded silanediol Mes2Si(OH)2 (8) as the main product with a trace amount of Mes2Si(OH)OSiMe3 (9), while that of tungsten analogue 1a gave siloxysilanol 9 as a sole product, indicating that product distribution in the hydrolysis strikingly depends on the metal fragments. A theoretical
Figure 5. HOMO and LUMO of [CpM(CO)2(SiMe3)] (M = W, Mo). H
DOI: 10.1021/acs.organomet.6b00958 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Hirakawa, F.; Kosai, T.; Sato, K.; Kira, M.; Iwamoto, T. Chem. - Eur. J. 2015, 21, 15100. (3) (a) Yao, S.; Xiong, Y.; Brym, M.; Driess, M. J. Am. Chem. Soc. 2007, 129, 7268. (b) Yao, S.; Brym, M.; Wullen, C.; Driess, M. Angew. Chem., Int. Ed. 2007, 46, 4159. (c) Xiong, Y.; Yao, S.; Driess, M. J. Am. Chem. Soc. 2009, 131, 7562. (d) Yao, S.; Xiong, Y.; Driess, M. Chem. Eur. J. 2010, 16, 1281. (e) Xiong, Y.; Yao, S.; Muller, R.; Kaupp, M.; Driess, M. Nat. Chem. 2010, 2, 577. (f) Xiong, Y.; Yao, S.; Muller, R.; Kaupp, M.; Driess, M. J. Am. Chem. Soc. 2010, 132, 6912. (g) Xiong, Y.; Yao, S.; Driess, M. Angew. Chem., Int. Ed. 2010, 49, 6642. (h) Xiong, Y.; Yao, S.; Irran, E.; Driess, M. Chem. - Eur. J. 2011, 17, 11274. (4) (a) Ghadwal, R. S.; Azhakar, R.; Roesky, H. W.; Propper, K.; Dittrich, B.; Klein, S.; Frenking, G. J. Am. Chem. Soc. 2011, 133, 17552. (b) Ghadwal, R. S.; Azhakar, R.; Roesky, H. W.; Propper, K.; Dittrich, B.; Goedecke, C.; Frenking, G. Chem. Commun. 2012, 48, 8186. (5) (a) Rodriguez, R.; Troadec, T.; Gau, D.; Saffon-Merceron, N.; Hashizume, D.; Miqueu, K.; Sotiropoulos, J.-M.; Baceiredo, A.; Kato, T. Angew. Chem., Int. Ed. 2013, 52, 4426. (b) Rodriguez, R.; Gau, D.; Troadec, T.; Saffon-Merceron, N.; Branchadell, V.; Baceiredo, A.; Kato, T. Angew. Chem., Int. Ed. 2013, 52, 8980. (6) Filippou, A. C.; Baars, B.; Chernov, O.; Lebedev, Y. N.; Schnakenburg, G. Angew. Chem., Int. Ed. 2014, 53, 565. (7) Xiong, Y.; Yao, S.; Driess, M. Dalton Trans. 2010, 39, 9282. (8) (a) Muraoka, T.; Abe, K.; Haga, Y.; Nakamura, T.; Ueno, K. J. Am. Chem. Soc. 2011, 133, 15365. (b) Muraoka, T.; Abe, K.; Kimura, H.; Haga, Y.; Ueno, K.; Sunada, Y. Dalton Trans. 2014, 43, 16610. (9) Fukuda, T.; Hashimoto, H.; Sakaki, S.; Tobita, H. Angew. Chem., Int. Ed. 2016, 55, 188. (10) (a) Golino, C. M.; Bush, R. D.; Sommer, L. H. J. Am. Chem. Soc. 1975, 97, 7371. (b) Dilanjan Soysa, H. S.; Okinoshima, H.; Weber, W. P. J. Organomet. Chem. 1977, 133, C17. (c) Seyferth, D.; Lim, T. F. O.; Duncan, D. P. J. Am. Chem. Soc. 1978, 100, 1626. (d) Okinoshima, H.; Weber, W. P. J. Organomet. Chem. 1978, 149, 279. (e) Okinoshima, H.; Weber, W. P. J. Organomet. Chem. 1978, 155, 165. (f) Sekiguchi, A.; Ando, W. J. Am. Chem. Soc. 1984, 106, 1486. (g) Arya, P.; Boyer, J.; Corriu, R. J. P.; Lanneau, G. F.; Perrot, M. J. Organomet. Chem. 1988, 346, C11. (11) (a) Davidson, I. M. T.; Thompson, J. F. J. Chem. Soc., Faraday Trans. 1 1975, 71, 2260. (b) Davidson, I. M. T.; Fenton, A. Organometallics 1985, 4, 2060. For another report on the trapping of Me2SiO by Me3SiCl, see: (c) Barton, T. J.; Wulff, W. D. J. Am. Chem. Soc. 1979, 101, 2735. (12) Takeda, N.; Tokitoh, N.; Okazaki, R. Chem. Lett. 2000, 29, 244. (13) An isolated silanone also reacted with H2O. See ref 6. (14) (a) Grundmann, C.; Dean, J. M. J. Org. Chem. 1965, 30, 2809. (b) Barybin, M. V.; Diaconescu, P. L.; Cummins, C. C. Inorg. Chem. 2001, 40, 2892. (15) Wakabayashi, R.; Sugiura, Y.; Shibue, T.; Kuroda, K. Angew. Chem., Int. Ed. 2011, 50, 10708. (16) Tran, N. T.; Min, T.; Franz, A. K. Chem. - Eur. J. 2011, 17, 9897. (17) (a) Becke, A. D. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098. (b) Perdew, J. P.; Wang, Y. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 45, 13244. (18) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (19) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chem. Acc. 1990, 77, 123. (20) (a) Raghavachari, K.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (b) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639. (21) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999. (22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
investigation revealed that the siloxysilanol is produced via multiple steps including oxidative addition of water to the M− silanone moiety and the generation of a silane σ-complex intermediate. The product distribution difference between tungsten and molybdenum complexes arises from the difference in electrophilicity of the silicon atom in the silanone ligand and that of the lability for oxidative addition of water to the MII−L fragment.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00958. Experimental details in the reactions of 1 with 1 and 2 equiv of MeOH and H2O, optimized geometries of Reactant and selected structural parameters for Reactant and 1, optimized geometries of Cp(OC)2M(OSiMe2)(SiMe3) (M = W, Mo), HOMO and LUMO and their energy levels in Reactant, geometry and energy changes in the reaction of one water molecule with Reactant(Mo) and one and two water molecules with Reactant(W) in the SiO reaction path, geometry and energy changes in the reaction of one water molecule with Reactant(Mo) and one and two water molecules with Reactant(W) in the metal reaction path, and optimized bond distances and bond angles of tungsten and molybdenum complexes (PDF) Computed Cartesian coordinates of all complexes in this study (XYZ)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for S.S.:
[email protected]. *E-mail for K.U.:
[email protected]. ORCID
Keiji Ueno: 0000-0003-0567-7697 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Numbers JP24750053, JP25109509, JP26410066, JP15H00916, and JP15K05446 and the “Element Innovation” Project from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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REFERENCES
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DOI: 10.1021/acs.organomet.6b00958 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
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DOI: 10.1021/acs.organomet.6b00958 Organometallics XXXX, XXX, XXX−XXX
