Theoretical Investigations on Reactions of a Series of Stable Dialkyl

Jan 6, 2010 - Michael M. Linden , Hans Peter Reisenauer , Dennis Gerbig , Miriam Karni , Annemarie Schäfer , Thomas Müller , Yitzhak Apeloig , Peter...
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Organometallics 2010, 29, 527–535 DOI: 10.1021/om900594z

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Theoretical Investigations on Reactions of a Series of Stable Dialkyl-Substituted Silicon-Chalcogen Doubly Bonded Compounds Jeng-Horng Sheu and Ming-Der Su* Department of Applied Chemistry, National Chiayi University, Chiayi 60004, Taiwan, Republic of China Received July 9, 2009

The potential energy surfaces for the formation, addition, and cycloaddition reactions of dialkylsilanechalcogenone (2) species have been studied using three levels of theories, i.e., B3LYP/ LANL2DZdp, CCSD theory, and Gibbs free energy (at the B3LYP level). Four dialkylsilanechalcogenone species with a SidX bond, where X = O, S, Se, and Te, have been chosen as model reactants in this work. Also, both MeOH addition and isoprene cycloaddition have been used to study the chemical reactivities of these species (2). The present theoretical investigations suggest that the relative reactivity of 2 increases in the order X = O < S < Se , Te. That is, the species with a less electronegative and a heavier chalcogen atom will have a smaller ΔEst, which facilitates its addition with MeOH and its cycloaddition reaction to isoprene. Furthermore, the singlet-triplet energy splitting, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict the reactivity of species such as 2. The results obtained are consistent with the available experimental observations and allow a number of predictions to be made.

*To whom correspondence should be addressed. E-mail: midesu@ mail.ncyu.edu.tw. (1) For recent reviews on silicon-chalcogen doubly bonded compounds, see: (a) Tokitoh, N.; Okazaki, R. In The Chemistry of Organosilicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998; Vol. 2, Chapter 17, pp 1063-1103. (b) Okazaki, R.; Tokitoh, N. Acc. Chem. Res. 2000, 37, 625–630. (c) Tokitoh, N.; Okazaki, R. Adv. Organomet. Chem. 2001, 47, 121–166. (2) Arya, P.; Boyer, J.; Carre, F.; Corriu, R.; Lanneau, G.; Lapasset, J.; Perrot, M.; Priou, C. Angew. Chem., Int. Ed. Engl. 1989, 28, 1016. (3) (a) Suzuki, H.; Tokitoh, N.; Nagase, S.; Okazaki, R. J. Am. Chem. Soc. 1994, 116, 11578. (b) Suzuki, H.; Tokitoh, N.; Okazaki, R.; Nagase, S.; Goto, M. J. Am. Chem. Soc. 1998, 120, 11096. (c) Tokitoh, N.; Sadahiro, T.; Hatano, K.; Sasaki, T.; Takeda, N.; Okazaki, R. Chem. Lett. 2002, 34. (4) For stable germanium-chalcogen doubly bonded compounds, see: (a) Tokitoh, N.; Matsumoto, T.; Manmaru, k.; Okazaki, R. J. Am. Chem. Soc. 1993, 115, 8855. (b) Matsumoto, T.; Tokitoh, N.; Okazaki, R. Angew. Chem., Int. Ed. Engl. 1994, 33, 2316. (c) Tokitoh, N.; Matsumoto, T.; Okazaki, R. J. Am. Chem. Soc. 1997, 119, 2337. (d) Tokitoh, N.; Matsumoto, T.; Okazaki, R. Bull. Chem. Soc. Jpn. 1999, 72, 1665. (e) Matsumoto, T.; Tokitoh, N.; Okazaki, R. J. Am. Chem. Soc. 1999, 121, 8811. For stable tin-chalcogen doubly bonded compounds, see: (f) Tokitoh, N.; Saito, M.; Okazaki, R. J. Am. Chem. Soc. 1993, 115, 2065. (g) Saito, M.; Tokitoh, N.; Okazaki, R. Organometallics 1996, 15, 4531. (h) Saito, M.; Tokitoh, N.; Okazaki, R. J. Am. Chem. Soc. 1997, 119, 11124. (i) Okazaki, R.; Saito, M.; Tokitoh, N. Phosphorus, Sulfur Silicon Relat. Elem. 1997, 124-125, 363. (j) Saito, M.; Tokitoh, N.; Okazaki, R. J. Am. Chem. Soc. 2004, 126, 15572. (5) Iwamoto, T.; Sato, K.; Ishida, S.; Kabuto, C.; Kira, M. J. Am. Chem. Soc. 2006, 128, 16914.

Corriu,2 Tokitoh,3,4 Okazaki,3,4 Kira,5 and many co-workers, nowadays several metallanechalcogenone molecules can be isolated and characterized to allow their analysis in the solid state. Although there have been a number of reports concerning the synthesis and chemical as well as physical properties of such metallanechalcogenone species,1-5 to the best of our knowledge, neither experimental nor theoretical work has yet been devoted to a systematic study of their reactivities. Recently, through the elegant research performed by Kira and co-workers,5 some reactions of a series of the first dialkylsilanechalcogenones have been investigated and their product structures have also been determined by X-ray crystallography. This allows the first discussion of the systematic structural changes dependent on the chalcogen atoms. For instance, silanethione is synthesized as colorless crystals by the reaction of a stable dialkylsilylene with trimethylphosphine sulfide.5 Also, dialkyl-substituted silaneselone and silanetellone are prepared by similar chemical reactions.5 On the other hand, silanechalcogenones (X = S, Se, and Te) react with methanol to give the corresponding adducts in high yields.5 Furthermore, silanechalcogenones (X = Se and Te) react with isoprene in toluene at 100 C to give the corresponding [2 þ 4] cycloadducts in a regiospecific manner. Our goal in this work is to obtain detailed mechanistic knowledge in order to exercise greater control over the above reactions. In fact, a detailed understanding of metallanechalcogenone reactivity is of interest not only for the advancement of basic science but also for the continued development of their applications. We therefore present the first density functional theory (DFT) study of the reactions in Scheme 1.

r 2010 American Chemical Society

Published on Web 01/06/2010

I. Introduction The search for stable group-14 element-group-16 element doubly bonded compounds, metallanechalcogenones (R2Ed X; E = Si, Ge, Sn; X = S, Se, Te), that are formally analogous to ketones has aroused increasing interest in recent years because such compounds can show great potential as ligands in synthetic and coordination chemistry.1 Thanks to

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Organometallics, Vol. 29, No. 3, 2010 Scheme 1

In this work, we consider theoretically the reaction paths of three kinds of model reactions involving dialkylsilylenes of type 1, where four Me3Si groups are replaced by four H3Si groups, and of a series of dialkylsilanechalcogenones (X = O, S, Se, and Te) of type 2.6 These reactions have been chosen because they represent the various kinds of dialkylsilanechalcogenone reactions for which experimental results have been reported by Kira and co-workers.5 As a result, through this theoretical study, we hope (i) to clarify the reaction mechanism and to determine the possible transition-state structures and the relative energetics for eqs A, B, and C, (ii) to investigate the thermodynamics of the dialkylsilanechalcogenone reactions with various substrate molecules, and (iii) to establish general trends and predictions for the chemical reactions of metallanechalcogenones. We anticipate that the results obtained in this work will allow predictions to be made concerning the reaction pathways of some known and/or as yet unknown systems, and will shed some light on optimal designs for further related synthesis and catalytic processes.

Sheu and Su theory, the results of which have been shown to be closer to the QCISD(T) reference than the MP2 values.10 The LANL2DZdp basis sets for Si, O, S, Se, and Te were obtained from the Extensible Computational Chemistry Environment Basis Set Database (http://www.emsl.pnl.gov/forms/basisform.html) and distributed by the Molecular Science Computing Facility, Environmental and Molecular Sciences Laboratory. Accordingly, we denote our B3LYP calculations by B3LYP/ LANL2DZdp.11 The spin-unrestricted (UB3LYP) formalism was used for the open-shell (triplet) species. The S2 expectation values of the triplet state for the biradical reactants all showed an ideal value (2.00) after spin annihilation, so that their geometries and energetics are reliable for this study. Frequency calculations were performed on all structures to confirm that the reactants and products had no imaginary frequencies and that transition states possessed only one imaginary frequency. The relative energies were thus corrected for vibrational zero-point energies (ZPE, not scaled). Thermodynamic corrections to 298 K, ZPE corrections, heat capacity corrections, and entropy corrections (ΔS) obtained were applied at the B3LYP/ LANL2DZdp level. Thus, the relative free energy (ΔG) at 298 K was also calculated at the same level of theory. For better energetics, single-point energies were also calculated at CCSD(FC)/LANL2DZdp//B3LYP/LANL2DZdp þ ZPE (B3LYP/LANL2DZdp) (hereafter designated CCSD),12 to improve the treatment of electron correlation. All of the theoretical calculations were performed using the GAUSSIAN 03 package of programs.13

III. Results and Discussion The model reactions that we adopt in this paper are already shown in eq A (metallanechalcogenone formation), eq B (metallanechalcogenone addition), and eq C ([2 þ 4] cycloaddition). Despite its simplicity, when comparing our model with the realistic systems, we believe that the reactivities of the various substituted metallanechalcogenones can be reproduced by these model systems. In this section the results for the above three reactions will be discussed. The fully optimized geometries for the stationary points

II. Theoretical Methods All geometries were fully optimized without imposing any symmetry constraints, although in some instances the resulting structure showed various elements of symmetry. For our DFT calculations, we used the Becke three-parameter hybrid functional7 combined with the Lee-Yang-Parr8,9 correlation functional, abbreviated as the B3LYP level of density functional (6) As pointed out by a reviewer, it was suggested to use the SiMe3 substituent groups in this work in order to represent the realistic system. It should be noted here that our study using the smaller SiH3 substituent groups to explore reactions of the dialkylsilylene of type 1 and 2 systems does not imply that the effect of larger bulky substituents is not important. It means only that we acknowledge that the size of the systems currently studied prevents highly accurate optimizations in the quantum theoretical calculations. In particular, the cost and available computational facilities make them impractical. We thus choose to treat such species at the smaller SiH3 groups. Highly accurate relative energies between the stationary points can be considered later, and it may or may not affect the chemical reactions studied here. Nevertheless, in view of recent dramatic developments in silicon and chalcogen chemistry, it is therefore hoped that our theoretical study will provide a crucial starting point for developing silicon and chalcogen doubly bonded molecules and for opening up new academic and synthetic areas. For example, see ref 1. (7) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Becke, A. D. J. Chem. Phys. 1992, 96, 215. (c) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (8) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (9) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623.

(10) (a) Schleyer, P. v. R.; Allinger, N. L.; Clark. T.; Gaseiger, J.; Kollman, P. A.; Schafer, H. F., III; Schreiner, P. R. Encyclopeida of Computational Chemistry, Vol. 3; Wiley: New York, 1998. (b) Jursic, S. J. Mol. Struct. 1997, 401, 45. (c) Su, M.-D. J. Phys. Chem. A 2004, 108, 823. (d) Su, M.-D. Inorg. Chem. 2004, 43, 4846. (e) Su, M.-D. Eur. J. Chem. 2005, 10, 5877. (11) (a) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976; pp 1-28. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (c) Hay, P. J.; Wadt. W. R. J. Chem. Phys. 1985, 82, 284. (d) Hay, P. J.; Wadt. W. R. J. Chem. Phys. 1985, 82, 299. (e) Check, C. E.; Faust, T. O.; Bailey, J. M.; Wright, B. J.; Gilbert, T. M.; Sunderlin, L. S. J. Phys. Chem. A 2001, 105, 8111. (12) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. J. Chem. Phys. 1987, 87, 5968. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.GAUSSIAN 03; Gaussian, Inc.: Wallingford, CT, 2003.

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Figure 1. B3LYP/LANL2DZdp-optimized geometries (in A˚ and deg) of the reactant (singlet) dialkylsilanechalcogenones 2 (X = O, S, Se, and Te). The experimental values (see ref 5) are in parentheses. For relative energies for each species see Table 1. Hydrogens are omitted for clarity.

calculated at the B3LYP/LANL2DZdp level are given in Figures 1-5, respectively, where they are compared with some available experimental data.5 The relative energies obtained by B3LYP, CCSD, and Gibbs free energy calculations (at the B3LYP level) are summarized in Tables 1-4, respectively. Their Cartesian coordinates are included in the Supporting Information. A. Formation of Dialkyl-Substituted Silanechalcogenones. Before discussing the potential energy surfaces for the formation reactions of dialkyl-substituted silanechalcogenones 2 (eq A), we shall first discuss the geometries of 2 compared with the available experimental data.5 Unfortunately, as mentioned in the Introduction, only three stable compounds of type 2, with a SidS, SidSe, and SidTe double bond, respectively, have been isolated and characterized unequivocally. Selected geometric parameters for these experimentally observed molecules are given in Figure 1, along with the corresponding values calculated for the dialkylsilanechalcogenone 2 model compounds. As one can see in Figure 1, in principle, the DFT results for the structure of 2S (SidS), 2Se (SidSe), and 2Te (SidTe), with four H3Si groups, are in good agreement with the available experimental data for that of 2S0 , 2Se0 , and 2Te0 ,

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Figure 2. Calculated frontier molecular orbital for the dialkylsilanechalcogenones 2 (X = O, S, Se, and Te) species. For more information see the text.

with four Me3Si groups, respectively.6 For instance, the SidS, SidSe, and SidTe bond lengths determined by X-ray diffraction for 2S0 , 2Se0 , and 2Te0 are 1.958, 2.093, and 2.321 A˚,5 while our predicted B3LYP bond lengths for 2S, 2Se, and 2Te are 1.966, 2.101, and 2.307 A˚, respectively. That is, the agreement between both SidX bond lengths in 2S0 , 2Se0 , 2Te0 and 2S, 2Se, 2Te is quite good, with agreement to within 0.014 A˚. In addition, the experimental Si-C1 and Si-C2 bond lengths in 2S0 (1.853-1.855 A˚), 2Se0 (1.853-1.855 A˚), and 2Te0 (1.861-1.863 A˚) are somewhat shorter than those in the 2S (1.875 A˚), 2Se (1.877 A˚), and 2Te (1.882 A˚) structures, bearing in mind that the synthesized molecules contain bulkier substituents. Similarly, Figure 1 shows a wider bond angle ( — C1-Si-C2) of 102.1 in 2S0 , 102.2 in 2Se0 , and 101.6 in 2Te0 ,5 whereas narrow bond angles of 99.37, 99.23, and 98.85 are found in 2S, 2Se, and 2Te. The fact that the — C1-Si-C2 angle in 2 is smaller than that of 20 may be attributed to the substantially lower steric crowding of the model ligands used in 2.14 In any event, the good agreement between our computational results and the available experimental data is encouraging. We therefore believe that (14) The reason for the reactivity of the SidX bonded system can be understood easily in terms of the SidX bond polarity. Because the electronegativities of Si (y = 1.7), O (y = 3.5), S (y = 2.4), Se (y = 2.5), and Te (y = 2.0) elements are different, the SidX bond in dialkylsilylenes is expected to be polarized. The chalcogen atom is negatively charged, and the silicon atom carries a positive charge, i.e., SiδþdXδ-.

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Figure 3. B3LYP/LANL2DZdp-optimized geometries (in A˚) of the stationary points for the process, dialkylsilylene (1) þ Me3PdX (X = O, S, Se, Te) f precursor complex (1-PC) f transition state (1-TS) f dialkylsilanechalcogenone (2). For relative energies for each species see Table 1. The heavy arrows indicate the main atomic motions in the transition-state eigenvector. Hydrogens are omitted for clarity.

the present models with the current methods (B3LYP/ LANL2DZdp and CCSD) employed in this study should provide reliable information for the discussion of the reaction mechanism, for which experimental data are still not available. In order to gain more insight into the nature of the chemical bonding in the series of dialkylsilanechalcogenones 2, the valence molecular orbitals based on the B3LYP/ LANL2DZdp calculations are represented in Figure 2. Basically, all the type 2 species with a SidX (X = O, S, Se, and Te) bond have in common a three atomic orbital system containing two π orbitals and a nonbonding orbital. That is, the lowest unoccupied molecular orbital (LUMO) corresponds to the Si-X π* antibonding orbital, whereas the highest occupied molecular orbital (HOMO) is dominated by the nonbonding lone-pair molecular orbital centered on the X atom. The HOMO-1 is largely a Si-X π-bonding orbital. The valence orbital diagram in Figure 2 is in agreement with that found in Kira’s work.5 It is apparent from Figure 2 that in the triplet state one electron is situated in the LUMO (π*), in which antibonding interactions exist between the central silicon and chalcogen atoms. As a result,

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Figure 4. B3LYP/LANL2DZdp-optimized geometries (in A˚) of the stationary points for the process, 2 þ CH3OH f precursor complex (PC-MeOH) f transition state 1 (TS1MeOH) f intermediate (Int-MeOH) f transition state 2 (TS2-MeOH) f products (Pro-MeOH). For relative energies for each species see Table 2. The heavy arrows indicate the main atomic motions in the transition-state eigenvector. Hydrogens are omitted for clarity.

the bond distance r(SidX) is expected to be longer for the triplet compared to the singlet state. This prediction agrees well with our B3LYP/LANL2DZdp results for all cases, as given in the Supporting Information. For instance, our B3LYP computations demonstrate that singlet 2O (1.530 A˚) < triplet 2O (1.638 A˚), singlet 2S (1.966 A˚) < triplet 2S (2.156 A˚), singlet 2Se (2.101 A˚) < triplet 2Se (2.308 A˚), and singlet 2Te (2.307 A˚) < triplet 2Te (2.529 A˚). Furthermore, the most striking results are for the singlettriplet energy separations of the type 2 species. In Figure 2, it can be seen that the HOMO-LUMO energy gap of 2 decreases as the atomic number of the chalcogen atom (X) is increased. This strongly implies that the corresponding singlet-triplet energy splitting should also decrease as the X atom gets heavier. Indeed, this prediction is in agreement with what we observe in the present theoretical calculations. As will be discussed below, our theoretical findings indicate that the singlet-triplet energy separation of 2 decreases as the atomic number of the chalcogen atom (X) increases. We shall use the above results to explain the origin of barrier heights for the addition and [2 þ 4] cycloaddition reactions in a later section.

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Table 1. Relative Energies for Singlet and Triplet Me3PdX and for the Process, Dialkylsilylene (1) þ Me3PdX (X = O, S, Se, Te) f Precursor Complex f Transition State f Dialkylsilanechalcogenone (2)a,b system Me3PdO Me3PdS Me3PdSe Me3PdTe

ΔEstc (kcal/mol)

ΔHcpxd (kcal/mol)

ΔE qe (kcal/mol)

ΔH f (kcal/mol)

þ94.69 (þ94.08) þ60.57 (þ61.10) þ46.80 (þ48.97) þ30.76 (þ35.01)

-26.38 (-17.90) -14.06 (-15.92) -16.57 (-13.87) -16.03 (-12.35)

þ8.789 (þ7.826) -4.165 (-2.408) -9.383 (-8.035) -13.77 (-11.61)

-25.27 (-25.46) -32.12 (-30.19) -37.45 (-30.64) -41.67 (-34.17)

a At the CCSD(FC)/LANL2DZdp//B3LYP/LANL2DZ and B3LYP/ LANL2DZdp (in parentheses) levels of theory. For B3LYP-optimized structures of the stationary points see Figure 2. b Energy differences have been zero-point corrected. See the text. c Energy relative to the corresponding singlet state. A positive value means the singlet is the ground state. d Energy of the precursor complex, relative to the corresponding reactants. e Enthalpy of the transition state, relative to the corresponding reactants. f Enthalpy of the abstraction product, relative to the corresponding reactants.

Table 2. Relative Energies for Singlet and Triplet Dialkylsilanechalcogenones (2; X = O, S, Se, Te) and for the Process, 2 þ CH3OH f Precursor Complex (PC-MeOH) f Transition State 1 (TS1-MeOH) f Intermediate (Int-MeOH) f Transition State 2 (TS2-MeOH) f Products (Pro-MeOH)a,b ΔEstc ΔHcpxd ΔE1qd ΔHintd ΔE2qd ΔHprod system (kcal/mol) (kcal/mol) (kcal/mol) (kcal/mol) (kcal/mol) (kcal/mol) þ59.59 [þ62.33] -17.82] (þ60.09) (-16.80) þ48.62 þ48.17 -8.236] (þ48.97) (-7.516) þ43.04 þ41.95 -6.125] (þ43.04) (-5.320) þ33.99 þ33.12 -4.335] (þ34.57) (-3.214)

2O

Figure 5. B3LYP/LANL2DZdp-optimized geometries of the stationary points for the two processes, i.e., path I, 2 þ isoprene f 3-TS f cycloadduct 3, and path II, 2 þ isoprene f 4-TS f cycloadduct 4. Selected bond distances (A˚) for the stationary points are given in Table 3. For relative energies for each species see Table 4. The heavy arrows indicate the main atomic motions in the transition-state eigenvector. Hydrogens are omitted for clarity.

Selected geometrical parameters for the stationary point structures along the pathway given in eq A and calculated at the B3LYP/LANL2DZdp level are shown in Figure 3 for X = O, S, Se, and Te, respectively. The relative energies obtained at both B3LYP and CCSD levels are collected in Table 1. Cartesian coordinates for these stationary points are included in the Supporting Information. Several noteworthy features from Figure 3 and Table 1 are apparent. (1) When the potential energy surfaces for chalcogen abstraction for dialkylsilylene 1 with Me3PdX were searched for transition structures, an initial decrease in the total energy as compared with the isolated molecules at large separation was observed. As one can see in Figure 3, the abstraction of Me3PdX by 1 initiates the formation of a precursor complex. All four precursor complexes (i.e., 1O-PC, 1S-PC, 1Se-PC, and 1Te-PC) adopt very similar Me3PdX-1 bonding characteristics. Our DFT calculations give Si-O, Si-S, Si-Se, and Si-Te bond lengths of 1.945, 2.510, 2.671, and 2.840 A˚, respectively. This finding can be explained in terms of the size of the group 16 atom X, which increases from O to Te. Attempts to locate molecular complexes at much longer Si-X distances failed. Furthermore, as can be seen from Table 1, our theoretical results suggest that stabilization energies for

2S

2Se

2Te

þ11.91 þ2.492 þ11.99 [-6.838] -13.49] [-12.23] (-5.788) (-8.651) (-8.400) þ9.935 þ7.300 þ9.403 [-4.573] [-8.633] [-7.035] (-2.932) (-3.601) (-2.788) þ9.086 þ9.170 þ9.978 [-3.248] [-7.179] [-5.501] (-2.139) (-2.435) (-1.829) þ8.260 þ5.226 þ3.161 [-2.993] [-6.162] [-4.231] (-1.374) (-1.705) (-0.429)

-41.87 [-59.86] (-53.55) -22.49 [-41.35] (-34.42) -19.75 [-37.35] (-31.33) -16.54 [-35.41] (-29.91)

a At the Gibbs free energy (B3LYP/LANL2DZdp), CCSD(FC)/ LANL2DZdp//B3LYP/LANL2DZ (in square brackets), and B3LYP/ LANL2DZdp (in parentheses) levels of theory. For B3LYP-optimized structures of the stationary points see Figure 3. b Energy differences have been zero-point corrected. See the text. c Energy relative to the corresponding singlet state. A positive value means the singlet is the ground state. d Energy relative to the corresponding reactants.

Table 3. Selected Bond Distances (A˚) for the Process, 2 þ Isoprene f 3-TS f Cycloadduct 3 and 2 þ Isoprene f 4-TS f Cycloadduct 4a system 1 2 3 4 5 6 7 8 9 10 11 12 a

O

S

Se

Te

3.448 2.750 1.566 3.350 2.748 1.566 2.215 1.920 1.664 2.192 1.897 1.665

3.358 2.634 1.999 3.252 2.652 2.000 2.023 1.912 2.178 1.998 1.889 2.167

3.284 2.530 2.135 3.180 2.582 2.138 1.880 1.906 2.322 1.860 1.886 2.309

2.630 2.177 2.345 2.630 2.177 2.350 1.425 1.885 2.535 1.426 1.885 2.552

See Figure 5. At the B3LYP/LANL2DZdp level of theory.

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Table 4. Relative Energies (kcal/mol) for Singlet and Triplet R2SidX and for the Process, Dialkylsilanechalcogenone (2) þ Isoprene f Transition State f Cycloaddition Producta,b system 2O 2S 2Se 2Te

ΔEstc

ΔE qd (path I)

ΔE qd (path II)

ΔHd (path I)

ΔHd (path II)

þ59.59 [þ62.33] (þ60.09) þ48.62 þ48.17 (þ48.97) þ43.04 þ41.95 (þ43.04) þ33.99 þ33.12 (þ34.57)

þ21.41 [þ1.899] (þ7.922) þ20.94 [þ1.186] (þ7.807) þ20.87 [þ1.183] (þ5.203) þ18.05 [þ1.096] (þ3.976)

þ22.57 [þ3.227] (þ8.164) þ22.13 [þ2.458] (þ8.117) þ21.91 [þ2.201] (þ6.954) þ19.01 [þ1.896] (þ5.270)

-8.457 [-23.50] (-36.00) -8.961 [-23.91] (-36.27) -9.664 [-24.48] (-38.33) -22.66 [-36.83] (-47.06)

-4.886 [-21.19] (-19.14) -5.894 [-22.54] (-20.28) -8.495 [-23.37] (-22.60) -21.63 [-34.77] (-35.62)

a At the Gibbs free energy (B3LYP/LANL2DZdp), CCSD(FC)/ LANL2DZdp//B3LYP/LANL2DZ (in square brackets), and B3LYP/ LANL2DZdp (in parentheses) levels of theory. For B3LYP-optimized structures of the stationary points see Figure 2. b Energy differences have been zero-point corrected. See the text. c Energy relative to the corresponding singlet state. A positive value means the singlet is the ground state. d Energy relative to the corresponding reactants.

these four precursor complexes lie within the range of -26 to -16 kcal/mol. According to these theoretical results, it appears that the precursor complexes can be isolated experimentally at room temperature because their stabilization energies are not low. However, to the best of our knowledge, no experimental detection of 1-Me3PdX complexes formed during the reactions has been reported yet.5 (2) We have calculated the transition states for the chalcogen (X) abstraction process (eq A). All the transition states at the B3LYP level of theory are confirmed by calculation of the energy Hessian, which shows only one imaginary vibrational frequency: 362i cm-1 (1O-TS), 176i cm-1 (1S-TS), 197i cm-1 (1Se-TS), and 156i cm-1 (1Te-TS). It should be noted that the primary similarity among these transition states is a two-center pattern involving the central silicon and chalcogen (X) atoms. Additionally, as shown in Table 1, the CCSD results suggest that the activation energy for such a chalcogen abstraction decreases in the order (in kcal/mol) 35 (1O-TS) > 9.9 (1S-TS) > 7.2 (1Se-TS) > 2.3 (1Te-TS) kcal/ mol. Namely, the greater the atomic weight of the chalcogen atom X, the smaller the barrier height and the easier the 1,2-chalcogen migration. As mentioned earlier, the relatively small distance (1.662 A˚) between the central silicon and oxygen atoms in the 1O-TS structure may lead to steric crowding of the large alkyl groups during the 1,2-shift reaction. This would result in a larger than expected activation barrier for the 1O system. On the other hand, a large distance between central Si and Te atoms (2.600 A˚ in 1TeTS) would reduce the crowding and, in turn, result in a lower barrier height. Furthermore, for the predicted transitionstate structures (see Figure 3), DFT calculations indicate that the PdX bond is stretched by 23%, 13%, 16%, and 12% for 1O-TS, 1S-TS, 1Se-TS, and 1Te-TS, respectively, relative to its value in the corresponding Me3PdX species. Also, it should be emphasized that the forming SidX bond in 1OTS, 1S-TS, 1Se-TS, and 1Te-TS is longer by 8.6%, 14%, 9.7%, and 13%, respectively, relative to that in the corresponding product. According to Hammond’s postulate,15 these features suggest that Me3PdTe reaches the transition (15) Hammond, G. S. J. Am. Chem. Soc. 1954, 77, 334.

state relatively early, whereas Me3PdO arrives relatively late. Namely, the barrier for the forward process is encountered earlier the heavier the chalcogen atom X. One may thus anticipate a lower activation barrier for 1 þ Me3PdTe than for 1 þ Me3PdO, which is confirmed by our B3LYP calculations as shown in Table 1. (3) The equilibrium geometries for the abstraction products 2 (i.e., 2O, 2S, 2Se, and 2Te) are presented in Figures 1 and 3, respectively. The reaction enthalpies at both B3LYP and CCSD levels of theory are presented in Table 1. The theoretical results given in Figures 1 and 3 show that all the abstraction products 2 adopt a SidX conformation at the silicon center. As discussed earlier, DFT calculations on the model compounds, 2, performed at the B3LYP/ LANL2DZdp level gave structural parameters that are in good agreement with the available experimental values.5 Furthermore, it is apparent that all the dialkylsilylene abstractions are thermodynamically exothermic. Moreover, as shown in Table 1, these results are consistent with the prediction that the activation barrier should be correlated with the exothermicity of the dialkylsilylene abstraction.15 That is, the order of enthalpy follows a similar trend to the activation energy: 2O (-25 kcal/mol) > 2S (-32 kcal/mol) > 2Se (-38 kcal/mol) > 2Te (-42 kcal/mol). Consequently, considering both the calculated activation barriers and reaction enthalpies, we conclude that for the abstraction reaction of dialkylsilylene with Me3PdX, the order of reactivity is Me3PdO , Me3PdS < Me3PdSe < Me3PdTe. This may be a reflection of phosphorus-chalcogen bond strength. According to our CCSD results, the bond dissociation energies between Me3P and chalcogen (i.e., Me3PdX f Me3P þ X) are 158 (O), 99.1 (S), 83.3 (Se), and 62.8 (Te) kcal/mol, respectively, which are in line with above observations. That is to say, the heavier the chalcogen atom X, the lower the barrier height and the larger the exothermicity of its dialkylsilylene abstraction reaction (eq A). (4) Our computational results can be rationalized on the basis of a configuration mixing (CM) model, which was developed by Pross and Shaik.16,17 According to this model the barrier height (ΔEq), as well as the reaction enthalpy (ΔH), can be expressed in terms of the singlet-triplet splitting ΔEst (= Etriplet - Esinglet) of the reactants. Namely, the stabilization of the transition state of an abstraction reaction depends on the singlet-triplet splitting ΔEst (= Etriplet Esinglet) of the reactant Me3PdX.16,17 Accordingly, the smaller the value of ΔEst of Me3PdX, the lower its barrier height, the greater its exothermicity, and the faster the abstraction reaction with dialkylsilylene will occur. In other words, a strong correlation between ΔEst and the activation energy as well as the reaction enthalpy is expected.16,17 For the CCSD calculations on the aforementioned four systems from Table 1, we obtain the following correlations (units in kcal/ mol; r2 is the correction coefficient):

ΔE q ¼ 0:524ΔE st -16:9 ðr2 ¼ 0:939Þ

ð1Þ

ΔH ¼ 0:258ΔEst -49:2 ðr2 ¼ 0:983Þ

ð2Þ

(16) For details, see: (a) Shaik, S.; Schlegel, H. B.; Wolfe, S. In Theoretical Aspects of Physical Organic Chemistry; John Wiley & Sons Inc.: New York, 1992. (b) Pross, A. In Theoretical and Physical Principles of Organic Reactivity; John Wiley & Sons Inc.: New York, 1995. (c) Shaik, S. Prog. Phys. Org. Chem. 1985, 15, 197. (17) (a) The first paper that originated the CM model: Shaik, S. J. Am. Chem. Soc. 1981, 103, 3692. (b) About the most updated review of the CM model: Shaik, S.; Shurki, A. Angew. Chem., Int. Ed. 1999, 38, 586.

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As one can see in eqs 1 and 2, a linear correlation exists between ΔEst and ΔEq (the abstraction barrier) as well as ΔH (the reaction enthalpy). Consequently, our model calculations provide strong evidence that electronic factors resulting from the group 16 element will play a decisive role in determining the reactivity of the Me3PdX species. B. Geometries and Energetics of Dialkylsilanechalcogenone Additions. Next, let us consider the addition reactions, which proceed via eq B, focusing on the intermediates, the transition states, and the addition products themselves. That is, the addition reactions studied in this work follow the general reaction path, reactants (2 þ MeOH) f precursor complex (PC-MeOH) f transition state 1 (TS1-MeOH) f intermediate (Int-MeOH) f transition state 2 (TS2-MeOH) f products (Pro-MeOH). The optimized geometries calculated at the B3LYP/LANL2DZdp level of theory are collected in Figure 4, with some selected geometrical parameters. To simplify the comparisons and to emphasize the trends, we have also given the energies relative to the two reactant molecules, i.e., 2 þ MeOH, which are summarized in Table 2. Cartesian coordinates calculated for the stationary points at the B3LYP level are available as Supporting Information. There are several important conclusions from these results to which attention should be drawn.18 (1) When the potential energy surface of the addition of dialkylsilanechalcogenone (2) with MeOH was searched for transition structures, an initial decrease in the total energy as compared with the isolated molecules at large separation was observed. That is to say, the addition of MeOH by 2 initiates the formation of a precursor complex (i.e., OPC-MeOH, SPC-MeOH, SePC-MeOH, and TePC-MeOH). These species are described as complexes because there are only minor differences between their bond lengths and those of the free reactants. It is not surprising that such long bond distances are reflected in the calculated complexation energies. As given in Table 2, the energy of the precursor complex relative to its corresponding reactants is less than 18 kcal/mol at the CCSD level of theory. As a consequence, our theoretical calculations show that these complexes are weakly bound and fall in shallow minima at large distances on the reaction surfaces. (2) We have been able to locate the first transition state (TS1-MeOH) for each dialkylsilanechalcogenone addition reaction and have confirmed that they are true transition states on the basis of frequency analysis. Examination of the single imaginary frequency for each transition state (62.9i cm-1 for OTS1-MeOH, 61.6i cm-1 for STS1-MeOH, 69.7i cm-1 for SeTS1-MeOH, and 60.8i cm-1 for TeTS1-MeOH) provides an excellent confirmation of the concept of the combination process. That is, the reactants approach each other with their molecular planes perpendicular, and one new bond between silicon and oxygen atoms is formed simultaneously. However, our B3LYP calculations show that the forming Si-O bond distance is 2.73, 2.76, 2.58, and 2.77 A˚ for OTS1-MeOH, STS1-MeOH, SeTS1-MeOH, and TeTS1-MeOH, respectively, nearly the same for all reactants. Moreover, as seen in Table 2, the first activation barrier relative to the corresponding precursor complex (18) As suggested by a reviewer, in eq B, it could be possible to produce such a Te-O bond formation product, i.e., formation of the isomer opposite the one shown in Scheme 1. We tried to obtain such reaction mechanisms using the present DFT method. However, they always failed. We thus believe that the Si-X bond formation product is the only product for the alcohol addition reaction.

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(PC-MeOH) is 11, 3.7, 1.5, and 0.34 kcal/mol, respectively, based on the CCSD level calculations. On the other hand, we also find a second transition state (TS2-MeOH) on the addition energy surface. The optimized transition-state structures (OTS2-MeOH, STS2-MeOH, SeTS2-MeOH, and TeTS2-MeOH) together with heavy arrows indicating the main atomic motions in the transition-state eigenvector are shown in Figure 4. These four transition-state structures show a similar four-center pattern involving chalcogen, silicon, oxygen, and hydrogen atoms. The B3LYP frequency calculations for the transition states OTS2-MeOH, STS2MeOH, SeTS2-MeOH, and TeTS2-MeOH predict the unique imaginary frequency values of 858i, 1181i, 1202i, and 1240i cm-1, respectively. The CCSD calculations estimate that their energies relative to the corresponding reactants decrease in the order OTS2-MeOH (-12 kcal/mol) < STS2MeOH (-7.0 kcal/mol) < SeTS2-MeOH (-5.5 kcal/mol) < TeTS2-MeOH (-4.2 kcal/mol). As will be shown below, our theoretical model suggests that the heavier the X atom involved in the dialkylsilanechalcogenone skeleton, the lower the barrier height and the smaller its MeOH addition enthalpy. (3) The structures of the final addition product (OProMeOH, SPro-MeOH, SePro-MeOH, and TePro-MeOH) generated at the B3LYP level of theory are illustrated in Figure 4. Additionally, the calculated reaction enthalpies for such combination-migrations are collected in Table 2. As Figure 4 shows, the order of the final addition enthalpy follows the same trend as the atomic weight of atom X: OPro-MeOH (-60 kcal/mol) 9.1 kcal/mol (SeTS1-MeOH) > 8.3 kcal/mol (TeTS1-MeOH). This implies that the chemical reactivity of 2 is in the order O < S < Se < Te, although the barrier energy differences between them are not large. The Gibbs free energy for the reaction enthalpy increases in the order OPro-MeOH (-42 kcal/mol) < SPro-MeOH (-22 kcal/mol) < SePro-MeOH (-19 kcal/mol) < TeProMeOH (-17 kcal/mol). In other words, our theoretical findings suggest that the MeOH addition of dialkylsilanechalcogenone bearing a heavier atomic weight chalcogen atom (X) is kinetically more facile than that bearing a lighter chalcogen atom. More specifically, the computational results suggest that dialkylsilaneselone and dialkylsilanetellone can readily insert into ROH to produce the kinetically stable product, whereas dialkylsilaneoxyone reacts with ROH to yield the thermodynamically stable product. This

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finding correlates well with the available experimental observations.5 C. Geometries and Energetics of Dialkylsilanechalcogenone [2 þ 4] Cycloadditions. The reaction of a 1,4-butadiene, such as isoprene, has also been studied in this work. Selected geometrical parameters of the reactants (2 þ isoprene), transition state, and cycloaddition products for eq C are collected in Figure 5. Two reaction pathways (path I and path II), which lead to regioisomers 3 and 4, respectively, have been considered in the present work. Barriers and reaction energies for the two reaction pathways appear in Table 4. The selected geometrical parameters are given in Table 3. Several interesting results can be drawn from Tables 3 and 4 and Figure 5 as follows. (1) Our DFT-based scans of the potential energy surfaces between reactants and products suggest that [2 þ 4] cycloadditions in the 2 þ isoprene systems are concerted. Also, the data given in Table 3 and Figure 5 show that the transition states and the bicyclic product isomers (3 and 4) are structurally similar. Moreover, the transition-state vectors represented by the heavy arrows in the complexes 3-TS and 4-TS are all consistent with the cyclization process, primarily the X-C and Si-C bond stretching in order to form the bicyclic product. In addition, the DFT calculations suggest that, for instance, the X-C1 bonds are stretched by 56%, 52%, 75%, and 85% in 3O-TS, 3S-TS, 3Se-TS, and 3Te-TS, respectively, relative to their corresponding cycloaddition products 3. The transition structures for 3O-TS and 3S-TS therefore take on a more product-like character than those in the 3SeTS and 3Te-TS systems. This is consistent with the smaller barrier and the greater exothermicity of the latter two reactions (see below). (2) As shown in Table 4, our theoretical investigations indicate that, for 2O 2S, 2Se, and 2Te systems, the cycloaddition reactions with isoprene for path I and path II are found to be kinetically and thermodynamically competitive. Nevertheless, as stated in the previous work,5 only one kind of regioisomer 3 has been found in the experimental observations. No regioisomers 4 are detected in the reaction mixtures. However, our Gibbs free energy (at the B3LYP level), CCSD energy, and B3LYP energy all indicate that the energies of the transition states for path I are slightly lower than those for path II. The same is found to hold true for the reaction enthalpies. That is, our theoretical calculations imply that the formation of 3 should be more facile than the formation of 4, which is in line with the available experimental observations.5 The reason for this finding can be ascribed to the larger energy of steric repulsion between the methyl group of isoprene and the trihdrosilyl (SiH3) groups in 2. In particular, comparing 3-TS (path I) and 4-TS (path II) in Figure 5, one can readily appreciate that the cycloaddition reaction in the 3-TS case has fewer steric interactions than a 4-TS approach since the latter closes to more crowded substituents than the former. As a result, the activation barrier of the 3-TS approach is lower in energy than that of the 4-TS addition by about 1.8-0.24 kcal/mol for the dialkylsilanechalcogenone cases at the three levels of calculations. This evidently accounts for the exclusive formation of 3 experimentally. Additionally, for computational convenience, we used trihdrosilyl (SiH3) groups, instead of trimethylsilyl (SiMe3) groups, in the model compound 2 in the present study. If we had used the latter groups in 2, presumably the energy difference between path I and path II would have been much larger that found in Table 4.6

Sheu and Su

(3) Considering both the activation barrier and the exothermicity based on the model calculations presented here, we conclude that for the cycloaddition of 2 with isoprene the order of reactivity is 2O < 2S < 2Se , 2Te, which is in agreement with the values of ΔEst calculated for 2. Namely, the smaller its ΔEst, the lower the activation barrier and the larger the exothermicity of its cycloaddition to an isoprene. In other words, the ease of cycloaddition reaction with isoprene increases with increasing atomic size of the chalcogen atom. Because the XdO bond distance is shorter in 2O than in 2Te, the reaction of 2O with isoprene will encounter more severe steric strain at the cyclic transition state than will 2Te. Thus it is expected that the reaction of 2Te will have a lower barrier height and larger exothermicity. Indeed, so far only 3Se-Pro and 3Te-Pro regioisomers have been obtained in the laboratory.5

IV. Conclusions Taking all four dialkylsilanechalcogenone (2) systems and related chemical reactions studied in this paper together, one can draw the following conclusions: (1) The theoretical findings suggest that the addition reactions of dialkylsilanechalcogenones with methanol proceed in multiple steps, forming intermediates that could be detected in the laboratory. (2) On the other hand, the [4 þ 2] cycloadduct resulting from the reaction of the dialkylsilanechalcogenone species and isoprene undergoes a one-step process leading to the final [4 þ 2] cycloadduct in a concerted manner. Namely, these [4 þ 2] cycloadditions will proceed stereospecifically, leading to cycloproducts with retained stereochemistry. (3) For the cycloaddition reaction, two reaction paths (path I and path II) producing regioisomers 3 and 4, respectively, have been investigated in this work. Despite the energy difference between the two paths being small, it is found that path I is favored over path II due to the latter meeting more severe steric effects. (4) The magnitudes of the barriers to dialkylsilanechalcogenone addition and cycloaddition are not high. This strongly implies that both addition and cycloaddition are facile processes at room temperature. This is consistent with the available experimental findings.5 (5) This investigation presents the first theoretical evidence that the chemical reactivity of dialkylsilanechalcogenone species increases in the order 2O (SidO) < 2S (SidS) < 2Se (SidSe) , 2Te (SidTe). (6) It is found that knowledge of the singlet-triplet splitting of Me3PdX as well as of the dialkylsilanechalcogenone species is of great importance in understanding the reactivity since it can affect the driving force for various chemical reactions. (7) In principle, the smaller the value of ΔEst of Me3PdX, the lower its barrier height, the larger its exothermicity, and the faster the abstraction reaction with dialkylsilylene (1) occurs. (8) The less electronegative and the heavier atom X in the SidX bond of the dialkylsilanechalcogenone species, the smaller its ΔEst, the lower the activation barrier, and the greater the exothermicity of its addition and cycloaddition reactions. (9) Electronic as well as steric factors should play an important role in determining the chemical reactivity of the

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dialkylsilanechalcogenone species from both kinetic and thermodynamic viewpoints. It is hoped that this study will be helpful for further developments in group-14 element-group-16 element doubly bonded compound chemistry.

Acknowledgment. We are grateful to the National Center for High-Performance Computing of Taiwan

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for generous amounts of computing time. We also thank the National Science Council of Taiwan for their financial support. Special thanks are also due to the reviewers for very helpful suggestions and comments. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.