Article pubs.acs.org/Organometallics
Theoretical Study for the Reactions of (Silyl)(silylene)tungsten and -molybdenum Complexes with Ethylene Sulfide Yoshitomo Ishiguro,† Takako Kudo,*,† Takako Muraoka,‡ and Keiji Ueno‡ †
Division of Pure and Applied Science, Graduate School of Science and Technology, Gunma University, Kiryu 376-8515, Japan Division of Molecular Science, Graduate School of Science and Technology, Gunma University, Kiryu 376-8515, Japan
‡
S Supporting Information *
ABSTRACT: The reaction mechanisms of the (silyl)(silylene)tungsten and -molybdenum complexes Cp*(OC)2M(SiMe3)(SiMes2) (M = W (1), Mo (2); Cp* = η5-C5Me5; Mes = 2,4,6-Me3C6H2) with the sulfur donor reagent ethylene sulfide 3 have been explored by ab initio molecular orbital and DFT (B3LYP) methods. As a result, the metal− ligand interactions were found to be stronger in the tungsten complexes than in the molybdenum complexes. Therefore, in the early stages which are common for both tungsten and molybdenum complexes, the energy barriers of the reactions of the tungsten tend to be higher than those of the molybdenum. The insertion of a sulfur atom into the metal−ligand bond takes place more easily in the molybdenum complex in comparison to that in the tungsten complex. This is the plausible reason parts of the reaction mechanisms and the final products are different between the W and Mo complexes.
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INTRODUCTION Reactions of transition-metal carbene complexes LnMCR2 (LnM = transition-metal fragment) with sulfur donors, such as S8 and alkene sulfides, have been extensively investigated which afforded SCR2 coordinated complexes as the initial products via addition of sulfur atom to the MC bonds.1−3 In contrast to carbene complexes, reactions of the heavier group 14 congeners of carbene complexes, i.e., silylene and germylene complexes LnMER2 (E = Si, Ge), with sulfur donors have been scarcely demonstrated.4,5 In addition to Ueno’s paper discussed below,4 only three examples have been reported so far.5 Germylene Pd complexes P2PdGe{N(SiMe3)2}2 (P2 = 2PEt3, 2PPh3, Ph2P(CH2)2PPh2) reacted with carbonyl sulfide COS to form germathione complexes in a manner similar to that for the carbene complexes. 5a In contrast to the germathione complexes, silanethione complexes have not yet been isolated. The reaction of a cationic silylene osmium complex with S8 resulted in the elimination of the silylene ligand to give a disulfur-bridged dimetallic complex.5b A hydride(silylene)ruthenium complex was converted into a silathiolato complex upon treatment with MesNCS (Mes = 2,4,6-Me3C6H2).5c Ueno et al. have recently found that treatment of the (silyl)(silylene)tungsten or -molybdenum complex Cp*(OC)2M(SiMe3)(SiMes2) (M = W (1), Mo (2); Cp* = η5-C5Me5) with S8 or ethylene sulfide 3 afforded the novel cyclic complex 4 or 5, as shown in Scheme 1.4 These reactions are quite unique since, during the reaction, not only the silylene ligand was incorporated into a metallacyclic skeleton with a sulfur atom but also the two CO ligands were transformed into the CC(O)O− moiety via CO bond fission.6 The © XXXX American Chemical Society
Scheme 1
silyl group on the M also migrated to the carbene carbon atom of the CC(O)O− moiety.7 The reaction mechanisms for these unprecedented reactions have unfortunately remained unknown, since no experimental evidence such as spectroscopic detections of the intermediates has been obtained at all. Furthermore, the reason for the formation of different types of products for the tungsten and molybdenum complexes is also unclear.8 Thus, theoretical investigations on the reaction mechanisms were conducted using the simplified model complexes Cp(OC)2M(SiH3)(SiPh2) (M = W (Reactant(W)), Mo (Reactant(Mo)), Cp = η5-C5H5) and ab initio molecular orbital and DFT methods. Here we report the details of the theoretical study. We found that (1) both reactions proceed via the formation of silanethione-coordinated comReceived: November 12, 2013
A
dx.doi.org/10.1021/om401084f | Organometallics XXXX, XXX, XXX−XXX
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Figure 1. Optimized geometries of the reactants of the metal complexes Cp(OC)2M(SiH3)(SiPh2) (M = W (Reactant(W)), Mo (Reactant(Mo)); Cp = C5H5). The values in parentheses are the experimental data of 1 and 2.14,15
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plexes as intermediates, (2) the bond fission of CO is induced by the nucleophilic attack of the oxygen to π* of the silanethione R2SiS and R3Si migration to carbon, and (3) the sulfur atom of the Mo−S−C three-membered ring is incorporated not by addition to the MoC double bond in the molybdenum analogue of carbene tungsten complex 4 but via insertion into the Mo−C bond in an intermediate generated prior to the formation of the Mo−S−Si−O−C−C sixmembered ring.
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RESULTS AND DISCUSSION Reactants and Products. Before we discuss the reaction mechanisms, it may be interesting to compare the optimized geometries of the reactants, Cp(OC)2M(SiH3)(SiPh2) (M = W (Reactant(W)), Mo (Reactant(Mo))), with the geometries 1 and 2 obtained from the experimental data14,15 to evaluate the present theoretical level. As Figure 1 shows, the calculated geometries are in very good agreement with those obtained from the experimental analyses. The same is true for the products (see Figure S1 in the Supporting Information).4 Furthermore, we tried to optimize the reactant and the product of the tungsten complex in the real form at two different levels of theory (Figures S2 and S3, Supporting Information). However, there are no meaningful differences between the model and the real system, as seen from the figures. Therefore, the calculational levels and models are considered to be appropriate for the present purpose. Figure 2 shows orbital pictures with energy levels of the HOMO and LUMO of Reactant(W). The HOMO consists of dxy (|ml| = 2) orbitals of the metal and π* of carbonyl ligands, while the LUMO consists of the empty p (Si) and π (Ph) orbitals of SiPh2 and the dxy (|ml| = 2) orbital of the metal. Thus, the nucleophilic attack of sulfur is expected to occur at Si of the silylene ligand in these compounds. The orbital character is basically the same as that in the real system (Figure S4, Supporting Information) and even in Reactant(Mo) (Figure S5, Supporting Information). Reaction of Sulfur with the Tungsten Complex. Both ethylene sulfide 3 and S8 (only for the W complexes) were
COMPUTATIONAL METHODS
Geometry optimizations were performed with density functional theory (DFT) methods in which the B3LYP functional9 was adopted. In addition, the Hartree−Fock (HF) level of calculations was used to obtain orbital energies. The LANL2DZ basis set10 was employed for W and Mo, while the 6-31G(d,p) basis set11 was used for the other atoms. For the obtained structures, vibrational frequency calculations were carried out to investigate the feature (equilibrium vs transitionstate structure). Then, intrinsic reaction coordinate (IRC) calculations were performed to confirm the connection among the stationary points on the potential energy surfaces. Furthermore, natural bond analysis (NBO)12 was applied to investigate the bond character and net atomic charges. We employed here Cp(OC)2 M(SiH3 )(SiPh 2 ) (M = W (Reactant(W)), Mo (Reactant(Mo))) as the models of the reactants in the experiment, Cp*(OC)2M(SiMe3)(SiMes2) (M = W (1), Mo (2)). The Gaussian09 program package13 was used for all computations. B
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depicted in Figure 3 and Figures S6 and S7 (Supporting Information), respectively. The first step is the addition of a sulfur atom of ethylene sulfide 3 (Reactant(W)−Inter-1(W)). The sulfur donor reagent 3 has a three-membered-ring structure with C2v symmetry, and the S−C and C−C distances are 1.837 and 1.481 Å, respectively. As expected, compound 3 attacks the silylene ligand accompanied by the elimination of an ethylene molecule to form the intermediate Inter-1(W), where the sulfur bridges over the W−Si bond. This process has a small energy barrier (5.6 kcal/mol) and is considerably exothermic. The geometry of the C2H4S part in TS-1(W) significantly changes from that in free 3, i.e., longer S−C distances (average 2.174 Å) and a shorter C−C distance (1.397 Å), implying that the dissociation of ethylene is under way. As shown in Table 2, total net charges for the S and C2H4 parts in TS-1(W) are slightly positive (+0.116), suggesting the nucleophilic attack of 3 to the empty p orbital of SiPh2 as expected. Finally, however, the sulfur atom obtains some electrons from the metal side in Inter-1(W). The addition of a sulfur atom is found to weaken the W− SiPh2 bond, as the distance in Inter-1(W) (2.667 Å) is longer than that in Reactant(W) (2.432 Å). It is noteworthy that the Si(1)−S(1) bond length (2.080 Å) in Inter-1(W) is much shorter than those of the common Si−S single bonds.16 Furthermore, the sum of the three bond angles around Si(1) is 347.1°, suggesting Si−S double-bond character. Therefore, Inter-1(W) may be considered as the η2-silanethionecoordinated complex (RR′SiS−). Reactions of silylene complexes with sulfur donor reagents have been expected to proceed via the formations of silanethione-coordinated complexes as intermediates, though no example has been isolated experimentally so far, as described in the Introduc-
Figure 2. Orbital pictures of the HOMO and LUMO of Reactant(W) with the orbital energies obtained at the HF/LANL2DZ, 631G(d,p)//B3LYP/LANL2DZ, and 6-31G(d,p) level. In addition, the directions of the three orthogonal axes are indicated.
utilized as sulfur donors in the experiments.4 However, the comparison of the W and Mo compounds is one of our main concerns in the present study. In addition, the consideration of the decomposition of S8 makes the reaction mechanism much more confusing. Therefore, we have decided to apply compound 3 in the present calculations. The geometrical parameters from TS-1 to Product(W) are summarized in Table 1, and the structures and relative energies of the stationary points from Reactant(W) to Product(W) are
Table 1. Geometrical Parameters (in Å) of the Stationary Points for the Reaction of the Tungsten Complex Cp(OC)2W(SiH3)(SiPh2) (Reactant(W)) with Ethylene Sulfide 3 TS-1(W)
Inter-1(W)
TS-2(W)
lnter-2(W)
W−Si(1) = 2.517 W−S(1) = 3.291 Si(1)−S(1) = 2.237 W−C(1) = 1.966 C(1)−O(1) = 1.172 S(1)−C(3) = 2.219 S(1)−C(4) = 2.129 C(3)−C(4) = 1.397 TS-3(W)
W−Si(1) = 2.667 W−S(1) = 2.622 Si(1)−S(1) = 2.080 W−C(1) = 1.961 C(1)−O(1) = 1.175
W−S(1) = 2.656 Si(1)−S(1) = 2.070 W−C(1) = 1.888 C(1)−O(1) = 1.222
W−C(1) = 1.845 C(l)−O(1) = 1.266 Si(1)−O(1) = 1.833 W−Si(2) = 2.611 Si(1)−S(2) = 2.124
lnter-3(W)
TS-4(W)
lnter-4(W)
W−C(1) = 1.862 C(1)−O(1) = 1.294 W−Si(2) = 2.796 C(1)−Si(2) = 2.125
W−O(1) = 2.167 W−C(1) = 1.852 O(1)−C(1) = 1.949
W−C(1) = 1.801 W−O(1) = 1.997 W−C(2) = 2.062 C(2)−O(2) = 1.148
TS-5(W)
W−C(1) = 1.988 C(1)−O(1) = 1.329 C(1)−Si(2) = 1.836 Si(2)−H(1) = 1.607 W−H(1) = 1.906 Inter-5(W)
TS-6(W)
Inter-6(W)
W−C(1) = 1.848 W−C(2) = 2.012 C(1)−C(2) = 1.733 C(2)−O(2) = 1.178 TS-7(W)
W−C(1) = 2.138 C(1)−C(2) = 1.313 C(2)−O(2) = 1.179 W−O(1) = 1.874 lnter-7(W)
W−S(2) = 3.918 S(2)−C(5) = 1.844 S(2)−C(6) = 1.842 C(5)−C(6) = 1.477 TS-8(W)
W−S(2) = 2.560 S(2)−C(5) = 1.864 S(2)−C(6) = 1.845 C(5)−C(6) = 1.476 Product(W)
W−S(2) = 2.339 S(2)−C(5) = 2.262 S(2)−C(6) = 2.142 C(5)−C(6) = 1.393
W−S(2) = 2.155 W−O(1) = 1.999 W−C(1) = 2.146
W−O(1) = 2.184 O(1)−C(2) = 1.836 W−C(1) = 2.036
C
W−O(1) = 3.542 O(1)−C(2) = 1.362 W−C(1) = 1.921
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higher than that of the W−SiPh2 bond. As a result, the antibonding orbital of the W−S(1) bond hardly obtains electrons; therefore, the effective bond order of W−S(1) is relatively larger than that of W−SiPh2. After the W−SiPh2 bond cleavage, nucleophilic attack of the O(1) in a C(1)O(1) ligand to the π* orbital of the silanethione Ph2Si(1)S(1) immediately takes place, as the π* orbital of the Ph2Si(1)S(1) is oriented to O(1).18 The resultant intermediate with a five-membered ring, Inter-2(W), is less stable than Inter-1(W) by 13.8 kcal/mol. The C(1)− O(1) distance is lengthened to 1.266 Å, while the W−C(1) distance is shortened to 1.845 Å in comparison with the corresponding bond distances in Inter-1(W) (W−C(1) = 1.961 Å, C(1)−O(1) = 1.175 Å). This means the electronic structure of the W−C−O bond sequence has changed from W−CO in Inter-1(W) to WCO− in Inter-2(W).19 The Si(1)−S(1) distance and the sum of the three bond angles around Si(1) vary from Inter-1(W) to Inter-2(W) as follows: 2.080 Å and 347.1° (Inter-1(W)), 2.070 Å and 348.6° (TS-2(W)), and 2.124 Å and 344.2° (Inter-2(W)). Therefore, the Si(1)−S(1) double-bond character is strongest in TS-2(W) but is weakened again in Inter-2(W). If TS-2(W) were to be more destabilized, the silanethione intermediate (Inter-1(W)) could be obtained. For this purpose, the interaction that is stabilizing TS-2(W), i.e., between the carbonyl ligand and the π* orbital of Ph2Si(1)S(1), should be eliminated. Therefore, the replacement of the carbonyl ligand with another ligand which does not contain nucleophilic atoms such as PMe3 might be one of the ways to isolate the silanethione complex from a theoretical point of view. The SiH3 ligand transfers from W to C(1) in the next step (Inter-2(W)−Inter-3(W)). The positive charge on the Si of the silyl group is as large as +0.725 in Inter-2(W). Therefore, a considerably high electron density on the W−C(1) bond in Inter-2(W) seems to enhance the 1,2-SiH3 shift and it takes place easily. The stabilities of the two intermediates are similar, but Inter-3(W) is slightly more stable than Inter-2(W). The agostic interaction between H(1) of the Si(2) and W in Inter3(W) may be one of the reasons (see Table 1). The energy barrier of this reaction is only 8 kcal/mol, suggesting that the SiH3 fragment can readily move from W to C(1) over the short W−C(1) bond. Incidentally, the silyl migration to C(2) in another carbonyl group has not been observed. As a SiMe3 group migrates instead of a SiH3 in the real system, we have also investigated the 1,2-SiMe3 shift using a more realistic model in which the SiH3 is replaced with a SiMe3 group for these complexes in order to estimate the effect of the methyl group for the reaction. The relative energies of the three complexes are as follows: Inter-2(W,SiMe3) (−11.8 kcal/mol), TS-3(W,SiMe3) (−6.2 kcal/mol), and Inter-3(W,SiMe3) (−11.7 kcal/mol). Therefore, the silyl transfer is expected to take place more easily. In addition, the stabilities of Inter2(W,SiMe3) and Inter-3(W,SiMe3) are almost the same, probably because an agostic interaction does not exist in Inter3(W,SiMe3). The C(1)−O(1) distance in Inter-3(W) is significantly elongated (1.329 Å) after the 1,2-SiH3 transfer, and it is finally broken, as seen in the following reaction from Inter-3(W) to Inter-4(W). C−O bond cleavage is very rare except for the reactions under reduction conditions, such as Fischer−Tropsch synthesis.6 However, the C−O bond of the five-membered ring is no longer an original triple bond but is weakened via several reaction steps in the present case.19 This is the plausible reason
Figure 3. Structures and relative energies of the stationary points for the reaction of the tungsten complex Cp(OC)2W(SiH3)(=SiPh2) (Reactant(W)) with ethylene sulfide 3 (Reactant(W)−Product(W)).
Table 2. Change of the Net Atomic Charge of the Fragments from Reactant(W) to Inter-1(W)
Reactant(W) TS-1(W) lnter-1(W)
W
CpW(CO) SiH3
SiPh2
S
C2H4
−1.564 −1.438 −1.230
−0.809 −0.727 −0.382
+0.809 +0.611 +0.715
−0.084 −0.333
+0.200
tion.4,5b,c,17 Thus, studies on stabilization and isolation of the silanethione complex are now underway. The next stages are the cleavage of the W−SiPh2 bond and a new Si−O bond formation between the −SSiPh2 ligand and the oxygen of one of the carbonyl fragments with an energy barrier of 22.7 kcal/mol (Inter-1(W)−Inter-2(W)). The reason the W−S(1) bond breaking does not take place in this case can be explained by an NBO analysis, as seen in Table 3. The interaction between the more electronegative sulfur and W is stronger than that between the silicon in SiPh2 and W, which makes the antibonding orbital energy of the W−S(1) bond Table 3. NBO Analyses for the W−Si(1) and W−S(1) Bonds in Inter-1(W)a
W−Si(l) W−S(l)
electron occupancy of BD (A)
electron occupancy of BD* (B)
effective bond order (eBO)
1.555 1.827
0.443 0.145
0.556 0.841
a Definitions: BD, bonding orbital; BD*, antibonding orbital, eBO = (A − B)/2.
D
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7(W). In contrast, W−S(2) in Inter-7(W) is expected to have double-bond character, as judged from the relatively short distance (2.155 Å). Furthermore, S(2) and O(1) are in positions trans to each other. It does not seem strange that the stronger interaction between the W and S(2) reduces that between the W and O(1) in the trans position. In this regard, the W−O(1) bond cleavage is induced by a kind of trans effect of S(2).20 In spite of the bond scission of W−O(1), Product(W) is more stable than Inter-7(W) by ca. 10 kcal/ mol probably because of the release of the ring strain caused by the ring expansion from four-membered to six-membered rings as well as the new C(2)−O(1) bond formation. Reaction of Sulfur with the Molybdenum Complex. Until addition of the second ethylene sulfide 3 (Reactant(Mo)−Inter-6(Mo)), the mechanism of the reaction of the (silylene)molybdenum complex with sulfur is basically the same as that of the (silylene)tungsten complex (see Figures S9 and S10 and Table S1 in the Supporting Information). However, we must note that a η2-silanethione-coodinated complex (Inter1(Mo)) was observed in the molybdenum case as well. The parameters of the coordinated silanethione (Si(1)−S(1) bond distance (2.072 Å) and the sum of the three bond angles around the Si(1) (348.1°)) are very similar to those in the tungsten analogue Inter-1(W). Now we explain the reaction pathways after Inter-6(Mo) (see Figure 4 and Figure S11 (Supporting Information) and
why C−O bond scission takes place, even though the energy barrier is relatively high. In contrast, the C(1)−O(1) bond breaking brings about the shortening of the W−C(1) bond to near that of a triple bond and a linear W−C(1)--Si(2) bond angle. In Inter-4(W), the 1,2-transfer of the remaining carbonyl ligand to the carbyne C(1) of W−C(SiH3) has been observed. This is caused by the same reason for the previous 1,2-SiH3 migration. In addition to that, another reason is the elongation of the W−C(2) bond distance (2.062 Å) in comparison to that for Reactant(W) (1.975 Å), as seen from Table 4. This Table 4. Changes of W−C(2) and C(2)−O(2) Distances (Å) and the Net Atomic Charge on W from Reactant(W) to Inter-4(W) r(W−C(2))
r(C(2)−O(2))
net atomic charge on W
1.975 1.978 1.999 2.004 2.018 1.982 2.003 2.008 2.062
1.169 1.170 1.161 1.160 1.157 1.163 1.158 1.156 1.148
−1.564 −1.439 −1.230 −1.002 −0.913 −0.570 −0.732 −0.230 +0.020
Reactant(W) TS-1(W) Inter-1(W) TS-2(W) Inter-2(W) TS-3(W) Inter-3(W) TS-4(W) lnter-4(W)
elongation is caused by the decrease in the π back-donation from W because of the decrease of the net atomic charge as the reaction proceeds. The CO migration takes place easily with a small energy barrier (5.6 kcal/mol), and the stabilities of the reactant (Inter-4(W)) and the product (Inter-5(W)) are similar, as in the case of the previous silyl transfer. After that, the addition of the second ethylene sulfide 3 to W of Inter-5(W) followed by the elimination of ethylene (TS7(W)) results in an incorporation of another sulfur atom to afford the intermediate Inter-7(W). In this case, the second 3 attacks W instead of the Si of SiPh2 because a large component of the LUMO is on the metal. In contrast to the addition of the first 3, the addition product of the second 3 can be located as an intermediate (Inter-6(W)) in this case. This can be explained by more room around W and a positive net atomic charge on W in Inter-5(W). The slightly larger energy barrier of ca. 9 kcal/mol in comparison to that in the first sulfur addition (5.6 kcal/mol) is apparently reflected by the stability of Inter-6(W). The sulfur donates some amount of electrons to W and accepts a part of those from W as in the first case (see Table 5). Finally, Inter-7(W) turns into Product(W) after the W− O(1) bond cleavage followed by C(2)−O(1) bond formation. The reason for the cleavage can be explained by the release of ring strain and a weak W−O(1) bond. The W−O(1) bond lengthens from 1.874 Å in Inter-5(W) to 1.999 Å in Inter-
Figure 4. Structures and relative energies of the stationary points for the latter half of the reaction of the molybdenum complex Cp(OC)2Mo(SiH3)(SiPh2) (Reactant(Mo)) with ethylene sulfide 3 (Inter-5(Mo)−Product(Mo)).
Table 5. Changes of the Net Atomic Charge on W and S(2) from Inter-5(W) to Inter-7(W)
Inter-5(W) TS-6(W) Inter-6(W) TS-7(W) Inter-7(W)
W
S(2)
+0.619 +0.512 −0.002 −0.041 +0.031
+0.077 +0.501 +0.421 +0.231
Table 6). As seen from the structure of TS-6(Mo) in Figure 4, approach of the second sulfur S(2) to C(1) results in an increase of the Mo−C(1) distance by ca. 0.2 Å from Inter6(Mo) as the ethylene part is leaving. The elongation of the M−C(1) bond is not observed in the tungsten case (TS7(W)). As a result, the second sulfur S(2) is inserted into the Mo−C(1) bond in Inter-7(Mo) instead of just attaching on Mo as in the tungsten complex (Inter-7(W)).21 The energy barrier of the reaction is relatively high (19.1 kcal/mol) as E
dx.doi.org/10.1021/om401084f | Organometallics XXXX, XXX, XXX−XXX
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Table 6. Geometrical Parameters (Å) of the Stationary Points for the Latter Half of the Reaction for the Molybdenum Complex Cp(OC)2Mo(SiH3)(SiPh2) (Reactant(Mo)) with Ethylene Sulfide 3 (Inter-6(Mo)−Product(Mo)) Inter-6(Mo)
TS-6(Mo)
lnter-7(Mo)
TS-7(Mo)
Mo−S(2) = 2.599 S(2)−C(5) = 1.857 S(2)−C(6) = 1.845 C(5)−C(6) = 1.476 Mo−C(1) = 2.140
Mo−S(2) = 2.394 S(2)−C(5) = 2.299 S(2)−C(6) = 2.217 C(5)−C(6) = 1.380 Mo−C(1) = 2.335 S(2)−C(1) = 2.331 lnter-9(Mo)
Mo−S(2) = 2.362 S(2)−C(1) = 1.822 Mo−O(1) = 1.896
Mo−C(1) = 2.176 Mo−O(1) = 2.196 C(1)−C(2) = 1.414 O(1)−C(2) = 2.002
TS-9(Mo)
Product(Mo)
Mo−S(3) = 2.591 S(3)−C(7) = 1.849 S(3)−C(8) = 1.837 C(7)−C(8) = 1.481 Mo−O(1) = 2.308
Mo−S(3) = 2.298 S(3)−C(7) = 2.204 S(3)−C(8) = 2.138 C(7)−C(8) = 1.396 Mo−O(1) = 3.678
Mo−S(3) = 2.127 Mo−O(1) = 3.818
lnter-8(Mo) Mo−C(1) = 2.172 Mo−O(1) = 2.310 C(1)−C(2) = 1.499 O(1)−C(2) = 1.430
accompanying the Mo−C(1) bond cleavage. According to NBO analyses for both Inter-6(M) species (M = W, Mo), the energy level of the Mo−C(1) antibonding orbital (0.138 eV) is much lower than that (0.488 eV) of the W−C(1) orbital, which means a higher electron-accepting ability of the Mo−C(1) bond from the sulfur in comparison to that of the W−C(1) bond. In addition, the Wiberg bond index (WBI)22 of the Mo− C(1) bond (0.909) is slightly smaller than that of the W−C(1) bond (0.933) in Inter-6(M) (M = W, Mo). These are the reasons the sulfur readily inserts into the Mo−C(1) bond. Next process is the formation of a six-membered ring associated with the Mo−O(1) bond cleavage and O(1)−C(2) bond formation (Inter-7(Mo)−Inter-8(Mo)), as seen in the final stage of the reaction of tungsten. In this process, negatively charged O(1) attacks nucleophilically the positively charged C(1) of the carbonyl group. The energy barrier for the transformation from Inter-7(Mo) to Inter-8(Mo) is calculated to be 16.2 kcal/mol. In TS-7(Mo), the three-membered ring including the molybdenum is formed by the regeneration of the Mo−C(1) bond. However, the Mo−O(1) bond does not seem to be completely broken, as the quasi-four-membered ring consisting of Mo−S(1)−Si(1)−O(1) still remains in TS7(Mo) and even in Inter-8(Mo). Therefore, the six-membered ring has a significantly folded structure. These unfavorable geometrical requests to keep the interactions between the Mo and ligands may explain the relatively high energy barrier in the molybdenum complexes. The addition of the third 3 to Inter-8(Mo) forms Inter9(Mo), which is transformed into the final product, Product(Mo), after the elimination of an ethylene molecule. The situation of the transformation from Inter-9(Mo) to Product(Mo) resembles that of the final step in the tungsten case (Inter-7(W)−Product(W)). Obviously, the elimination of an ethylene molecule strengthens the Mo−S(3) bond, which is considered to be the trigger of the cancellation of the interaction between the Mo and O(1) and unfavorable folded six-membered-ring structure. Therefore, a kind of trans effect is also seen in this process.20 Here, it may be interesting to examine whether the tungsten complex could be transferred into a product similar to that for molybdenum, Product-H(W), though such a conversion was not observed experimentally.8 Therefore, we have tried to bring the third 3 close to C(1) of Product(W), as shown in Figure 5 and Figure S12 (Supporting Information). The geometrical parameters are shown in Table S2 (see the Supporting Information). As a result, two kinds of transition states (TS-
Figure 5. Structures and relative energies of the stationary points for the addition reaction of ethylene sulfide 3 to Product(W).
H(W) and TS2-H(W)), an intermediate derived from addition of 3 (Inter-H(W)), and the product similar to that for molybdenum (Product-H(W)) were obtained with an energy barrier of ca. 25 kcal/mol. TS-H(W) is the transition state for the addition of 3 to C(1) of Product(W), while TS2-H(W) is for the elimination of an ethylene molecule accompanied by the formation of a W−S(3)−C(1) three-membered ring. The latter is slightly less stable than the former in energy, but the relative energies are almost the same as those of TS-H(W) and InterH(W); thus, the intermediate is kinetically unstable. The virtual Product-H(W) is 11 kcal/mol more stable than Product(W); therefore, this species seems to have the potential to exist once the addition of 3 takes place. In fact, the stability relative to Reactant(W) (−60.1 kcal/mol) is rather larger than that of Product(Mo) relative to Reactant(Mo) (−51.0 kcal/mol).23 However, in the real system, the addition of the third 3 is expected to be inhibited by serious steric congestion around C(1) caused by the methyl groups of SiMe3 and Cp*. In order to confirm this expectation, we tried to locate TS2-H(W) in the real form, TS2-H(W,SiMe3,Cp*,SiMes2), and estimate the energy barrier in the real system. As a result, the approach of the third 3 was found to demand a serious deformation of the structure of Product(W) in the real form and the energy barrier for the addition of 3 is as high as 42.4 kcal/mol (see Figure S13, Supporting Information). This may be the reason ProductH(W) is not observed in experiments. Comparison of the Reaction Mechanisms: Tungsten and Molybdenum Complexes. The potential energy surfaces of the reaction of 3 with tungsten and molybdenum complexes are depicted in Figures 6 and 7, respectively. In both F
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proceeds in some cases. Therefore, we compare here the energy levels of all metal−ligand bonds of Reactant(M) (M = W, Mo), in which the all ligands are the same in the both metal complexes, as shown in Figure 8. The energy levels of bonding
Figure 6. Potential energy surface of the reaction of the tungsten complex Cp(OC)2W(SiH3)(SiPh2) (Reactant(W)) with ethylene sulfide 3.
Figure 8. Comparison of the energy levels (au) of the bonding and antibonding orbitals of the metal−ligand bonds in Reactant(W) and Reactant(Mo) on NBO analyses. In this graph, BD denotes bonding orbitals while BD* denotes antibonding orbitals. The existence of π orbitals of the W−SiPh2 bond was not suggested, probably because two atomic orbitals of W and Si are localized on each atom.
orbitals are very similar to each other, but those of the antibonding orbitals are lower in the molybdenum complex than in the tungsten complex. This suggests that the energies for the M−L bond cleavages in the molybdenum complexes are smaller than those of the tungsten complexes because the antibonding orbitals in the molybdenum complexes can accept electrons more easily in comparison to those in the tungsten complexes. This is one of the explanations for the difference in reactivity of both metal complexes. However, for the C(1)−O(1) bond cleavage of the fivemembered ring and the formation of the M−O(1) bond (Inter-3−Inter-4), the trend seems to be reversed. According to NBO analyses again in Table 7, the effective bond order (eBO) of the C(1)−O(1) bond in Inter-3 is almost the same for each metal complex. However, the newly formed M−O(1) bond is shorter and stronger in the tungsten complex than in the molybdenum complex (TS-4 and Inter-4). The M−O(1) bond strength is expected to contribute to the relative stabilities of TS-4 and Inter-4.24 Therefore, the stronger M−L interactions in the tungsten complexes are considered to effect the reverse of the barrier heights and the stability of Inter-4 in both complexes.
Figure 7. Potential energy surface for the reaction of the molybdenum complex Cp(OC)2Mo(SiH3)(SiPh2) (Reactant(Mo)) with ethylene sulfide 3.
complexes, energies of the all stationary points relative to Reactant(M), except for a few cases, are lower than 0 and the overall reactions are considerably exothermic. These results are quite consistent with the experimental observations that, once the reaction starts, any stable intermediates except for the final products are not detected even at room temperature for both metal complexes. Roughly speaking, the energy barriers are lower in the molybdenum case than in the tungsten case, though the landscapes of the potential energy surfaces from Reactant to Inter-6 resemble each other. As mentioned in the preceding sections, the reaction mechanisms are different between the tungsten and molybdenum complexes after the addition of the second 3. Then, in the previous section we have explained the reason for the difference at the branching point from the difference of the energy level of antibonding orbital of the M−C(1) bond (M = W, Mo). Then, we tried to confirm whether the trend is also seen in the other intermediates. However, a similar comparison is not easy, since the intermediates are not always common for both metal complexes. In addition, the ligands on the metals lose the original form or the style of coordination as the reaction
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CONCLUDING REMARKS We have succeeded in exploring the series of reaction mechanisms of (silyl)(silylene)tungsten and -molybdenum complexes 1 and 2 with a sulfur donor reagent, ethylene sulfide (3), by the DFT method with the use of model compounds. It is noteworthy that (1) the reactions proceed via the formation of the η2-silanethione-coordinated complexes as intermediates and (2) the CO bond is gradually weakened in a stepwise manner and cleaved by nucleophilic attack of oxygen to π* of the silanethione ligand R2SiS and migration of SiH3 to carbon. The reaction mechanisms have been investigated in detail with comparison of both metal complexes. As a result, it was found that the metal−ligand interactions are stronger in the G
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Table 7. Changes of C(1)−O(1) and M−O(1) Distances (Å) and the Effective Bond Order (eBO) from Inter-3(M) to Inter4(M) (M = W, Mo) r(C(1)−O(1))
eBO of C(1)−O(1)
r(M−O(1))
1.329 1.949 2.906 1.319 2.008 2.897
0.974
3.107 2.167 1.997 3.102 2.190 2.030
Inter-3(W) TS-4(W) Inter-4(W) Inter-3(Mo) TS-4(Mo) Inter-4(Mo)
0.977
ASSOCIATED CONTENT
S Supporting Information *
Additional calculational results (Figures S1−S14 and Tables S1−S3) and an xyz file giving Cartesian coordinates of calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for T.K.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research (Nos. 23550066 and 24750053), a Grant-in-Aid for Scientific Research on Innovative Areas “Stimuli-responsive Chemical Species for the Creation of Functional Molecules” (No. 25109509), and the “Element Innovation” Project from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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0.732 0.871 0.720 0.842
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tungsten complex than in the molybdenum complex. Therefore, the energy barriers of the reactions of the tungsten complexes are higher than those of the molybdenum analogues in many cases and the insertion of a sulfur atom into the metal−ligand bond takes place more easily in the molybdenum complex in comparison to that in the tungsten complex. This is the reason parts of the reaction mechanisms and final products are different between the metal complexes. Through the present study, we have learned that metal− ligand interactions are stronger in the tungsten complexes than in the molybdenum complexes and the difference in the interactions can explain the different experimental results for the metal complexes. This is very interesting to us; therefore, we have started an investigation into the reactions of other tungsten and molybdenum complexes to confirm the present findings.
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eBO of M−O(1)
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