Infrared Spectra and Density Functional Calculations for Singlet CH2

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Infrared Spectra and Density Functional Calculations for Singlet CH2SiX2 and Triplet HC−SiX3 and XC−SiX3 Intermediates in Reactions of Laser-Ablated Silicon Atoms with Di‑, Tri‑, and Tetrahalomethanes Han-Gook Cho†,‡ and Lester Andrews*,‡ †

Department of Chemistry, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 406-772, South Korea Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, Virginia 22904-4319, United States

‡

S Supporting Information *

ABSTRACT: Reactions of laser-ablated silicon atoms with di-, tri-, and tetrahalomethanes in excess argon were investigated, and the products were identified from the matrix infrared spectra, isotopic shifts, and density functional theory energy, bond length, and frequency calculations. Dihalomethanes produce planar singlet silenes (CH2SiX2), and tri- and tetrahalomethanes form triplet halosilyl carbenes (HC− SiX3 and XC−SiX3). The Si-bearing molecules identified are the most stable, lowestenergy product in the reaction systems. While the C−Si bond in the silene is a true double bond, the C−Si bond in the carbene is a shortened single bond enhanced by hyperconjugation of the two unpaired electrons on C to σ*(Si−X) orbitals, which contributes stabilization through a small amount of π-bonding and reduction of the HCSi or XCSi angles. The C−Si bond lengths in these carbenes (1.782 Å for HC− SiF3) are between the single-bond length in the unobserved first insertion intermediate (1.975 Å for CHF2−SiF) and the double-bond length in the silene (1.704 Å for CHFSiF2). The silicon s2p2 and titanium s2d2 electron configurations produce similar primary products, but the methylidyne with Ti has a bond to carbon stronger than that of the halosilyl carbene.

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CH3−SiX and CH2SiHX in reactions of Si with methyl halides and produced the silaethenes CH2SiH2 and CH2 SiCl2 from the vacuum flash pyrolysis of bicyclic precursors at 650 °C followed by trapping in argon at 10 K.10 Interestingly, these workers also found that the neat silaethene material dimerizes to 1,3-disilacyclobutene upon evaporation of the isolating argon matrix. Finally, the Michl group has prepared CH3−SiHCH2 by photolysis of (CH3)2Si(N3)2,11a and in addition, photochemical reactions of ozone with silane and methylsilanes have produced silanones with SiO double bonds.11b,c Reactions of transition metal atoms with small alkanes and halomethanes have been investigated in a series of recent studies.8,12−14 Insertion products were identified in the matrix spectra along with methylidenes and methylidynes. Only highoxidation-state methylidynes (HCMH3) were observed in the reactions of Re and Os,12 but preference for the highestoxidation-state products decreases on moving away from the third row Group 7 and 8 metals in the periodic table.13,14 Substitution of H with halogen tends to increase the stability of the higher-oxidation-state complexes. For example, Group 4 metals generate CH3−MH and CH2MH2 in reactions with CH4, but they produce CH2MX2 with CH2X2 reagents and

INTRODUCTION Silicon is the second most abundant element in the earth’s crust after oxygen, found largely as silicon oxides (sand), and silicon is the basic ingredient for the semiconductor industry.1 Alkyl silicon oxides and their polymers (silicones) are widely used as sealants and adhesives,2 and they are also developed as water repellants.3 This Group 14 element is a metalloid, readily donating or sharing its four valence electrons to form chemical bonds.4 While its ability to make four bonds allows the opportunity to combine with many other elements to form various compounds, unlike carbon it sometimes accepts additional electrons to form hypervalent products with five or six bonds.5 Reactions of Si with methane and methyl halides have been investigated previously. Maier et al. have conducted photochemical reactions of Si with CH4 and identified CH3−SiH and CH2SiH2 from the matrix infrared spectra6 and also examined this reaction path theoretically,7 which is in fact similar to that for reactions of transition metal atoms with methane.8 Schreiner et al. have observed HC−SiH3 from matrix IR and ESR spectra following the reaction of SiH4 with C(3P) and concluded from the hyperfine structures that the silyl carbene has a triplet ground state.9a These workers also observed infrared spectra of triplet HC−SiHCl2 following 313 nm irradiation into the absorption band of its CH2SiCl2 precursor.9b Maier et al. have also reported the formation of © XXXX American Chemical Society

Received: November 13, 2015

A

DOI: 10.1021/acs.inorgchem.5b02610 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Frequencies of Product Absorptions Observed from Reactions of CH2X2 with Si in Excess Argona S

CH2F2

CD2F2

1335.8 coveredb

933.5 1102.0

CH2FCl

CH2Cl2

13

CD2Cl2 857.4 1107.0, 1106.3, 1103.7

CH2Cl2

CH2Br2

848.7 1092.7

CH2 scis. C−Si str.

488.4

458.1

SiX2 as str.

716.7 609.4,c 604.1

1007.0, 1005.6 600.8,c 593.2, 590.6,d 587.9d 490.5

460.9

597.7,c 591.7, 589.4,d 586.5d 487.0

731.1 630.4

653.3 483.6

covered 616.7

969.9

876.4

773.8

730.8

544.3, 537.7d 717.9 584.9

description

1341.8 995.6

coveredb

980.3

CD2Br2

1309.6 991.7, 989.7

SiX2 s str. 598.4 466.4,c 463.4

CH2 rock CH2 wag

a All frequencies are in inverse centimeters. Stronger absorptions are shown in bold. Description gives major coordinate. S stands for the silylene product, which is also known as silaethene. bCovered by a precursor band. cMatrix site splitting. dChlorine isotopic splitting.

triplet carbenes (HC÷CX3 and XC÷MX3) with tri- and tetrahalomethanes.13b It is interesting to question whether Si, a Group 14 metalloid, can undergo similar reactions to provide higher-oxidation-state products. In this study, we report the reactions of Si with di-, tri-, and tetrahalomethanes. The products are identified from the matrix spectra on the basis of correlation with density functional theory (DFT)-calculated frequencies, isotopic substitution, and relative energies. The products are surprisingly similar to those of Group 4 metals with some interesting differences.8,13 Silicon makes up to four bonds in these systems unlike the transition metals, which can form more chemical bonds by accepting electrons into the lower-lying empty d orbitals.

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Table 2. Frequencies of Product Absorptions Observed from Reactions of CHX3 with Si in Excess Argona C

CHF3

CDF3

CHCl3

CDCl3

description

975.2 939.6 888.5

coveredb 928.2 879.1, 877.7

597.7, 594.2 597.7, 594.2 847.1, 845.5

598.4, 595.4 598.4, 595.4 828.5, 827.3

A″ SiX3 as str. A′ SiX3 as str. A′ C−Si str.

a

All frequencies are in inverse centimeters. Stronger absorptions are shown in bold. Description gives major coordinate. C stands for the triplet carbene product. bCovered by a strong precursor band.

Table 3. Frequencies of Product Absorptions Observed from Reactions of CX4 with Si in Excess Argona CF4 c

EXPERIMENTAL AND COMPUTATIONAL METHODS

Laser-ablated Si atoms (Johnson-Matthey) were reacted with CH2F2, CH2FCl, CF4, CF3Cl, CF2Cl2 (Dupont), CH2Cl2, CHCl3, CCl4 (Fisher), CD2Cl2, CDCl3, 13CH2Cl2, 13CCl4 (90% enriched) (MSD Isotopes), and CD2F2 (synthesized15) in excess argon during condensation at 10 K using a closed-cycle refrigerator (Air Products HC-2). The methods have been described in detail elsewhere.16 Concentrations of gas mixtures were typically 0.2−0.7% in argon. After reaction, infrared spectra were recorded at a resolution of 0.5 cm−1 using a Nicolet 550 spectrometer with an MCT-B detector. Samples were next irradiated by a mercury arc street lamp (175 W) with a combination of optical filters for 20 min, warmed, and recooled (annealed), and more spectra were recorded. Complementary DFT calculations were conducted using the Gaussian 09 package,17 the B3LYP density functional,18 and the 6311++G(3df,3pd) all-electron basis sets for H, C, F, Cl, Br, and Si19 to provide a consistent set of vibrational frequencies and energies for the reaction products. Geometries were fully relaxed during optimization, and the optimized geometry was confirmed by vibrational analysis. BPW9120 calculations were also performed to support the B3LYP results. The vibrational frequencies were calculated analytically. In the calculation of binding energy for a metal complex, the zero-point energy is included.

CF2Cl2

CFCl3

1367.5 997.9

969.3 825.4

980.6

937.0

600.3, 597.4

620.4, 617.6

622.9, 615.6

870.8 649.9

897.9

CCl4

13

CCl4

614.9, 611.5 595.9, 593.3

614.6, 611.1 595.8, 593.0

531.4

530.9

description C-X str. A′ SiX3 as str. A″ SiX3 as str. SiX3 s str. C−Si str.

a

All frequencies are in inverse centimeters. Stronger absorptions are shown in bold. Description gives major coordinate. c stands for the triplet carbene product. Stronger matrix site splitting shown in bold.

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RESULTS AND ASSIGNMENTS Reactions of laser-ablated Si atoms with di-, tri-, and tetrahalomethane isotopomers in condensing argon have been investigated. The observed product vibrational characteristics and their variations upon photolysis and annealing as well as reaction profiles and product structures are listed in Tables 1−3, Figures 1−13 , and Figures S1 and S2. Vibrational modes and isotopic shifts are correlated with those for related molecules. Observed frequencies are compared with computed harmonic frequencies in Tables S1−S41. This work allows

Figure 1. IR spectra in the product absorption regions for laser-ablated Si atoms codeposited with CH2F2 in excess argon at 10 K and their variation on photolysis and annealing. (a) Si with 0.5% CH2F2 in Ar codeposited for 1 h. (b−d) Like (a) after photolysis with λ > 420 nm and 240 nm < λ < 380 nm and annealing to 28 K. s and p stand for silene product and CH2F2 precursor absorptions, respectively.

comparisons between calculated and observed vibrational frequencies in novel silicon-bearing halocarbon intermediates, B


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Inorganic Chemistry which substantiates the importance of each method of investigation. Furthermore, potential energy surface calculations verify the intuitive reaction path for Si that was observed earlier for Ti with these halomethanes.8,13 Si + CH2X2. Figures 1−4 and Figure S1 illustrate the IR spectra from reactions of laser-ablated Si atoms with CH2F2,

Figure 4. IR spectra in the product absorption regions for laser-ablated Si atoms codeposited with CH2Br2 isotopomers in excess argon at 10 K and their variation upon photolysis and annealing. (a) Si + 0.5% CH2Br2 in Ar codeposited for 1 h. (b−d) Like (a) after photolysis with λ > 420 nm and 240 nm < λ < 380 nm and annealing to 28 K. (e) Si + 0.5% CD2Br2 in Ar codeposited for 1 h. (f−h) Like (e) after photolysis with λ > 420 nm and 240 nm < λ < 380 nm and annealing to 28 K. s and p stand for silene product and CH2Br2 precursor absorptions, respectively.

Figure 2. IR spectra in the product absorption regions for laser-ablated Si atoms codeposited with CH2FCl in excess argon at 10 K and their variation upon photolysis and annealing. (a) Si + 0.5% CH2FCl in Ar codeposited for 1 h. (b−d) Like (a) after photolysis with λ > 420 nm and 240 nm < λ < 380 nm and annealing to 30 K. s and p stand for silene product and CH2FCl precursor absorptions, respectively.

metal work13) are observed in each spectrum, and they increase upon UV (240 nm < λ < 380 nm) photolysis. These product absorption intensities vary in concert during photolysis and annealing, thus indicating that only one primary product is generated in reactions of Si atoms with the methylene halides mentioned above. The observed frequencies in product spectra are listed in Table 1 and compared with DFT-computed values in Tables S2, S5, S8, and S11, showing mostly good correlation with the predicted vibrational characteristics for the silenes (CH2SiX2). The observed absorptions all arise from SiX2 symmetric and antisymmetric stretching, C−Si stretching, CH2 scissoring, rocking, and/or wagging modes of CH2SiX2, which are the strongest infrared absorptions for these silenes. Note that the observed frequencies are 0.93−1.02 times the B3LYP-calculated values. The large differences observed between the antisymmetric and symmetric SiX2 stretching frequencies [980.3 and 773.8 cm−1 for CH2SiF2, 593.2 and 490.5 cm−1 for CH2 SiCl2, and 488.4 and 349.8 (calculated) cm−1 for CH2SiBr2] are due to mixing with the CH2 rocking and scissoring modes. Comparing the 969.9 and 730.8 cm−1 Si−F2 stretching modes for CD2SiF2 also reveals this mode mixing. These mode separations can be compared with the much smaller differences in simple silicon dihalides (855 and 843 cm−1 for SiF2, 502.0 and 512.7 cm−1 for SiCl2, and 402.6 and 399.5 cm−1 for SiBr2).21 The SiH2Cl2 molecule exhibits slightly higherfrequency Si−Cl stretching modes at 584, 581 and 527, 523 cm−1 than does SiCl2, and these absorptions show appropriate chlorine isotopic splittings for a molecule with two equivalent chlorine atoms. Likewise for the photolysis product SiCl2, expanded scale spectra show well-resolved peaks at 512.7, 510.3 and 502.0, 498.7 cm−1, which are appropriate for the Si35Cl2 and Si35Cl37Cl isotopic pair for a molecule with two equivalent chlorine atoms.21c

Figure 3. IR spectra in the product absorption regions for laser-ablated Si atoms codeposited with CH2Cl2 isotopomers in excess argon at 10 K and their variation upon photolysis and annealing. (a) Si + 0.5% CH2Cl2 in Ar codeposited for 1 h. (b−d) Like (a−i) after photolysis with λ > 420 nm and 240 nm < λ < 380 nm and annealing to 28 K. (e) Si + 0.5% CD2Cl2 in Ar codeposited for 1 h. (f−h) Like (e) after photolysis with λ > 420 nm and 240 nm < λ < 380 nm and annealing to 28 K. (i) Si + 0.5% 13CH2Cl2 in Ar codeposited for 1 h. (j−l) Like (i) after annealing to 30 K, photolysis with λ > 220 nm, and annealing to 35 K. s and p stand for silene product and CH2Cl2 precursor absorptions, respectively.

CH2FCl, CH2Cl2, and CH2Br2 isotopomers. The product absorptions marked “s” (s for silene, also called silaethene,10a which corresponds to the methylidene from earlier transition C


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Inorganic Chemistry

CH2F−SiF originates from the stability of Si−F bonds over C− F bonds: the C−H, C−F, Si−H, and Si−F bond energies are 414, 439, 293, and 540 kJ/mol, respectively.22 Similarly, CH2F−SiCl, CH2Cl−SiF, CH2SiFCl, and triplet CH−SiHFCl are 342, 409, 505, and 347 kJ/mol lower in energy than Si(3P) + CH2FCl, respectively. CH2Cl−SiCl, CH2SiCl2, and triplet CH−SiHCl2 are 372, 471, and 299 kJ/ mol more stable than the reactants, respectively, again using B3LYP/6-311++G(3df,3pd). Previous workers found CH− SiHCl2 to be 168 kJ/mol higher in energy than its CH2SiCl2 precursor at the CCSD(T)/cc-pVTZ level of theory, which is near our B3LYP value of 172 kJ/mol.9b Nevertheless, they observed the higher-energy isomer to form under monochromatic 313 nm irradiation into the absorption band for CH2SiCl2, but our experiments employed 240−380 nm photolysis, which increased CH2SiCl2 absorptions without any detectable amount of CH−SiHCl2, based on the reported spectrum.6a CH2Br−SiBr, CH2SiBr2, and triplet HC−SiHBr2 are 378, 457, and 281 kJ/mol lower in energy than the reactants, respectively. The C−Cl, C−Br, Si−Cl, and Si−Br bond energies are 330, 276, 360, and 289 kJ/mol, respectively.22 Si + CHX3. Product infrared spectra from the reactions of Si with trifluoro- and trichloromethane isotopic modifications are illustrated in Figures 5 and 6. Similar to the methylene halide

On the other hand, the Si−F and Si−Cl stretching frequencies, 876.4 and 544.3 cm−1 for the lower-symmetry CH2SiFCl product, are less affected by the CH2 bending modes than those of dihalosilenes. The 544.3 cm−1 product band with CH2FCl has a 537.7 cm−1 satellite peak with approximately one-third of the intensity of the stronger band, which characterizes the vibration of a single chlorine atom. Our calculations predict these bands at 537.2 and 530.5 cm−1 for the chlorine isotopic species CH2SiF35Cl and CH2SiF37Cl, respectively, in excellent agreement with the observed bands in position and chlorine isotopic shift. Finally, the analogous CH2SiHCl product has been reported with 544(m) and 537(w) cm−1 bands,6a which are experimentally the same as our observations for the vibration of the single chlorine atom in CH2SiFCl. The stronger band for CH2SiCl2 at 593.2 cm−1 exhibits splittings at 590.6 and 587.9 cm−1, and the approximately 9:6:1 intensity pattern again characterizes the vibration of two equivalent chlorine atoms with natural abundance chlorine isotopes. Accordingly, these bands are assigned to CH2 Si35Cl2, CH2Si35Cl37Cl, and CH2Si37Cl2. A similar pattern was observed for the 13CH2SiCl2 isotopic species red-shifted just 1.5 cm−1 (Table 1), which indicates a small amount of coupling with a carbon bearing mode. Notice that the full 35Cl to 37Cl shift for this antisymmetric Si−Cl stretching mode is 5.3 cm−1, which is smaller than the 6.6 cm−1 shift observed for the symmetric Si−Cl stretching mode in CH2SiFCl. Weak absorptions of SiF2 were observed in the Si + CF4 spectra, but the heavier silicon dihalides were not found. The 630.4 cm−1 CH2 wagging mode for CH2SiCl2 shifts to 483.6 cm−1 for CD2SiCl2 with a 1.304 frequency ratio, which is in the range for anharmonic modes of this type. The bromine counterpart CH2SiBr2 has a similar mode at 604.1 cm−1 with CD2SiBr2 at 463.4 cm−1 and a 1.303 ratio. The silene frequencies observed here compare favorably with previous observations.10a The strongest three absorptions for CH2SiCl2 were reported as 1008, 732, and 593 cm−1 in solid argon from flash pyrolysis and trapping,10a which agree with our bands at 1007.0, 731.1, and 593.2 cm−1 from the reaction of laser-ablated Si with CH2Cl2: we observed two additional weaker bands at 630.4 and 490.5 cm−1 as well as the CD2 SiCl2 and 13CH2SiCl2 counterparts. The latter is the symmetric Si−Cl2 stretching mode, and the average of the symmetric and antisymmetric modes at 593.2 cm−1 for CH2 SiCl2 is 542 cm−1, which is very close to the value of 544 cm−1 reported for the Si−Cl stretching mode of CH2SiHCl.6a We observed the symmetric Si−Cl2 stretching mode of CD2SiCl2 shifted to 460.9 cm−1, which verifies its mixing with a vibration involving the CD2 subunit. Finally, the average of our two Si−F stretching modes for CH2SiF2 is 877 cm−1, which is slightly higher than the value of 839 cm−1 for CH2SiHF.10b The single silene product (CH2SiX2) observed from each Si and CH2X2 reaction is also the most stable among the plausible products. Conversion of CH2SiX2 to undetected HC−SiHX2 would decrease the number of chemical bonds in the product from six to five. The insertion product CH2F−SiF, silenes CF2SiH2, CHFSiHF, CH2SiF2, and singlet and triplet carbenes HC−SiHF2 are 363, 154, 300, 531, 291, and 387 kJ/mol more stable than Si(3P) + CH2F2, respectively, at the B3LYP level of theory. The Si carbenes examined in this study all have triplet ground states, while the insertion and silene products have singlet ground states, in line with the previous reports.6,7,9−11 The preference of CH2SiF2 over

Figure 5. IR spectra in the product absorption regions for laser-ablated Si atoms codeposited with CHF3 and CDF3 in excess argon at 10 K and their variation upon photolysis and annealing. (a) Si + 0.5% CHF3 in Ar codeposited for 1 h. (b−d) Like (a) after photolysis with λ > 420 nm and 240 nm < λ < 380 nm and annealing to 30 K. (e) Si + 0.5% CDF3 in Ar codeposited for 1 h. (f−h) Like (e) after photolysis with λ > 420 nm and 240 nm < λ < 380 nm and annealing to 30 K. c and p stand for carbene product and CHF3 precursor absorptions, respectively.

cases, only one group of product absorptions were all marked “c” (c to denote a carbene, HC−SiX3: also called methyl silylene or halosilyl carbene, 6a which is analogous to methylidynes in our transition metal work13 ). These absorptions increase dramatically (particularly in the Si + CHCl3 spectra) in concert upon UV irradiation. The product frequencies correlate best with the computed frequencies for triplet HC−SiX3 (Table 2 and Table S16). The c absorptions at 975.2, 939.6, and 888.5 cm−1 in the Si + CHF3 spectra are designated to the A″ and A′ SiF3 antisymmetric stretching and A′ C−Si stretching modes of triplet HC−SiF3, respectively. They are the strongest bands of the Si carbene, and the observed frequencies are 0.95−1.01 times the predicted values D


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Inorganic Chemistry

Figure 6. IR spectra in the product absorption regions for laser-ablated Si atoms codeposited with CHCl3 and 13CDCl3 in excess argon at 10 K and their variation upon photolysis and annealing. (a) Si + 0.5% CHCl3 in Ar codeposited for 1 h. (b−d) Like (a) after photolysis with λ > 420 nm and 240 nm < λ < 380 nm and annealing to 28 K. (e) Si + 0.5% CDCl3 in Ar codeposited for 1 h. (f and g) Like (e) after photolysis with 240 nm < λ < 380 nm and annealing to 28 K. c and p stand for carbene product and CHCl3 precursor absorptions, respectively. A CHCl2−Cl absorption is also designated (see ref 28).

Figure 7. IR spectra in the product absorption regions for laser-ablated Si atoms codeposited with CCl4 and 13CCl4 in excess argon at 10 K and their variation upon photolysis and annealing. (a) Si + 0.5% CCl4 in Ar codeposited for 1 h. (b−e) Like (a) after photolysis with λ > 420 nm, 240 nm < λ < 380 nm, and λ > 220 nm and annealing to 28 K. (f) Si + 0.5% 13CCl4 in Ar codeposited for 1 h. (g−i) Like (f) after photolysis with λ > 420 nm and 240 nm < λ < 380 nm and annealing to 28 K. c stands for carbene product absorption. CCl3−Cl absorption is also indicated.

(Table S16). Corresponding bands for the two lowest modes of DC−SiF3 were observed at 928.2 and 877.7 cm−1. Finally, the two antisymmetric Si−F stretching modes for HC−SiF3 mentioned above bracket the 954 cm−1 value for the SiF3 radical.23a Absorptions in the Si + CHCl3 spectra at 847.1 and 597.7 cm−1 with split absorptions at 845.5 and 594.2 cm−1 are assigned to the C−Si stretching and SiX3 antisymmetric stretching modes of triplet HC−SiCl3, respectively, which are again the strongest bands and show good agreement with the computed values (Table S20). The observed SiX3 antisymmetric stretching absorption pair consist of a strong one at 597.7 cm−1 and a weaker one (one-third as strong) at 594.2 cm−1, while a frequency difference of 4.9 cm−1 is predicted for the A″ and A′ SiX3 antisymmetric stretching modes of its Cs structure (Figure 10) with similar intensities (Table S20). It is not clear whether the A″ and A′ vibrational bands are too close to resolve so that they are overlapped, but the weaker band is probably due to a mixed chlorine isotope absorption. The strong antisymmetric stretching absorption for the SiCl3 radical has been observed slightly lower at 584 cm−1 in solid argon.23b The trifluorosilyl carbene observed here is considerably lower in energy than the Si insertion and silene products: CHF2−SiF, CHFSiF2, and triplet CH−SiF3 are 323, 416, and 548 kJ/mol more stable than Si(3P) + CHF3, respectively. On the other hand, the silene and trichlorosilyl carbene are energetically comparable: the insertion product silylene CHCl2−SiCl, the silene CHClSiCl2, and triplet HC−SiCl3 are 386, 470, and 464 kJ/mol more stable than the reactants, respectively. However, the observed vibrational characteristics correlate much better with those predicted for the triplet carbene as shown in Tables S18 and S20. Among the plausible products, only triplet HC−SiCl3 is predicted to have a C−Si stretching band near 850 cm−1, and this band is observed at 847.1 cm−1. Si + CX4. Figure S2 and Figure 7 show product spectra from reactions of Si with CF4, CF2Cl2, CFCl3, CCl4, and 13CCl4. The product absorptions marked “c” increase in concert upon UV

photolysis and sharpen in the early stage of annealing, and they are all assigned to silyl carbenes (XC−SiX3) on the basis of good correlation with the predicted vibrational characteristics (Table 3 and Tables S24, S28, S31, and S36). The observed product absorptions are the C−X and C−Si stretching bands. Those at 1367.5, 997.9, 980.6, 870.8, and 649.9 cm−1 in the Si + CF4 spectra are assigned to the strong C−F stretching, A′ and A″ SiF3 antisymmetric stretching, A′ SiF3 symmetric stretching, and C−Si stretching modes of triplet FC−SiF3, respectively. The c absorptions at 969.3, 937.0, 825.4, and 620.4 cm−1 in the Si + CF2Cl2 spectra are assigned to the C−Cl stretching, SiF2 antisymmetric and symmetric stretching absorptions, and C−Si stretching mode of ClC−SiF2Cl, respectively. Product absorptions at 897.9, 615.6, and 600.3 cm−1 in the Si + CFCl3 spectra are due to the Si−F stretching, C−Si stretching, and SiCl 2 antisymmetric stretching modes of ClC−SiFCl 2 , respectively. The c absorptions at 614.9, 595.9, and 531.4 cm−1 in the Si + CCl4 spectra are the A′ and A″ SiCl3 antisymmetric stretching and Cl−C−Si symmetric stretching bands of triplet ClC−SiCl3, respectively. These modes all have small (0.1−0.5 cm−1) 13C shifts, which are in agreement with calculations for the triplet and not the singlet states for ClC− SiCl3 (Tables S35 and S36). The two bands observed at 614.9 and 595.9 cm−1 arise from the symmetry being lowered to Cs, which splits the antisymmetric stretching mode for the SiCl3 subunit (Figure 11). Note that these bands are slightly higher than the 584 cm−1 SiCl3 radical value.23b It should be mentioned here that the vibrational characteristics predicted for the singlet and triplet states of these chlorine-containing carbenes are in fact quite similar to each other as shown in Tables S28, S31, S35, and S36, and the triplet states of ClC− SiF2Cl, ClC−SiFCl2, and ClC−SiCl3 are marginally more stable E


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Inorganic Chemistry

Figure 8. Energies of the plausible products and transition states relative to the reactants [Si(3P) + CH2F2] calculated at the B3LYP/6-311+ +G(3df,3pd) level of theory. Each transition state is linked to its reactant and product by intrinsic reaction coordinate (IRC) calculations. The activation energy must be provided through laser ablation or UV irradiation to overcome TS1, and reaction exothermicity overcomes TS2 to form the stable silene product; however, the high TS3 and carbene energy prevent its formation, which is consistent with observation of only the silene product in the spectra (Figure 1).

and form additional bonds is too demanding energetically for that to occur in these reactions. Because the reaction of Si with CH2F2 forms the silene product CH2SiF2, it is straightforward to conclude that this reaction first goes through the silylene insertion product CH2F−SiF and then sustains a second α-F transfer to Si yielding the more stable final CH 2 SiF 2 product as demonstrated for the analogous Ti reactions.8,13b,c To confirm this mechanism, the full reaction potential energy surface (PES)17 has been calculated for the Si and CH2F2, CH2Cl2, and CH2Br2 reactions, and the PES for the methylene fluoride reaction is illustrated in Figure 8. Each transition state is linked to its reactant and product by intrinsic reaction coordinate (IRC) calculations.24 The transition state for the first insertion TS1 is 40 kJ/mol higher than the reagent energy, and this activation energy is provided by the ablation laser during the deposition process for product formation (see Figure 1a) or on subsequent UV irradiation of the matrix sample (see Figure 1c): because of TS1, there is no product growth upon annealing to 28 K (compare spectra in parts c and d of Figure 1). However, the silylene insertion intermediate is not detected here as the TS2 energy is well below the reagent total energy, and the exothermicity for the insertion reaction overcomes, with or without the assistance of mercury arc UV irradiation, TS2 on the way to the much lower-energy triplet CH2−SiF2 intermediate species, which undergoes collisional relaxation and intersystem crossing to the lower-energy singlet silene CH2 SiF2. The TS3 and carbene are higher in energy than the silene, and the reaction ends with the lower-energy silene product (reaction 1). Note, however, that the CH2Cl2, and CH2Br2 reactions show product growth (∼20%) on the final 28 K annealing cycles (Figures 3 and 4), which suggests that their initial insertion reactions are barrierless, and there are no obvious TS1’s in these PES calculations (Figures S3 and S4). Titanium appears to be less reactive than Si as CH2TiCl2 does not increase

than the singlet states (49, 44, and 38 kJ/mol more stable, respectively). These Si tetrahalocarbenes are again the most stable among the plausible products. CF3−SiF, CF2SiF2, and singlet and triplet FC−SiF3 are 334, 392, 468, and 501 kJ/mol lower in energy than Si(3P) + CF4, respectively. The large Si−F bond energy again leads to the stability of the observed product with more Si−F bonds. CF2Cl−SiCl, CFCl2−SiF, CF2SiCl2, CFClSiFCl, CCl2SiF2, triplet FC−SiFCl2, and triplet ClC−SiF2Cl are 367, 388, 411, 438, 485, 480, and 575 kJ/ mol more stable than Si(3P) + CF2Cl2, respectively. Singlet FC−SiFCl2 and ClC−SiF2Cl are 480 and 526 kJ/mol more stable than the reactants, respectively. CFCl2−SiCl, CCl3−SiF, CFClSiCl2, CCl2SiFCl, triplet FC−SiCl3, and triplet ClC− SiFCl2 are 382, 414, 444, 491, 464, and 557 kJ/mol lower in energy than Si(3P) + CFCl3, respectively. Singlet FC−SiCl3 and ClC−SiFCl2 are 448 and 513 kJ/mol more stable than the reactants, respectively. CCl3−SiCl, CCl2SiCl2, and singlet and triplet ClC−SiCl3 are 402, 492, 495, and 533 kJ/mol more stable than the reactants, respectively. Reaction Mechanisms. The Si reaction products identified here show that the four valence electrons in the s2p2 (Group 14) and s2d2 (Group 4) orbitals lead to analogous reaction products, for example, CH2SiX2 and CH2TiX2 with dihalomethanes.8,13 In contrast, Group 3 transition metals produced bridge-bonded methylidene intermediates [HC(X)− ScX2 and XC(X)−ScX2] instead of methylidynes in reactions with tri- and tetrahalomethanes. Group 5 metals formed anionic methylidynes (HCMH3− and HCMH2X−) in reactions with methane and methyl halides along with products similar to those of Group 4 metals. Group 6 metals produce stable methylidyne molecules (HCMH3 and HCMX3).8,13 Although hypervalent Si compounds have been reported,5 Si accommodates only four bonds in reactions with halomethanes. Utilizing the empty 3d orbitals of Si to accept electron density F


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Inorganic Chemistry upon annealing like Si.13 Because of TS3 and the higher carbene energy, further migration of H or X in the silene product is energetically unfavorable.2,4,5 In support of this conclusion, geometry optimization of HC−SiHCl2(S) converges to the structure of CH2SiCl2(S). Si* + CH 2X 2 → [CH 2X−SiX]* → CH 2SiX 2

(1)

Si* + CHX3 → [CHX 2−SiX]* → [CHXSiX 2]* → HC−SiX3

(2)

Silicon reactions with fluoroform and chloroform provide one more α-halogen transfer to form the more stable trihalosilyl carbenes, and these PES diagrams are shown in Figures S5 and S6. The plot for CHF3 reactions shows a TS1 barrier of 62 kJ/ mol, and this reaction requires activation as did the CH2F2 reactions; however, the final carbene product is lower in energy, and as a result, only the carbene is observed. In contrast, the chloroform insertion reaction is also without a TS1 barrier (Figure S6), and the reaction proceeds upon annealing (20% growth) to successively lower-energy products ending with the trichlorosilyl carbene as summarized by reaction 2. Transition-state structures are shown for the Si(CH2F2) and Si(CHF3) reactions in Figure S7. Analogous TS2 and TS3 structures were found for the Si(CH2Cl2), Si(CH2Br2), and Si(CHCl3) reactions. The product with more Si−F bonds is considerably more stable in fluorochloromethane reactions than those with more Si−Cl bonds because of the higher Si−F bond energy (540 kJ/ mol) in comparison with Si−Cl and Si−Br bond energies (360 and 289 kJ/mol, respectively).22 A similar preference for the products with more M−F bonds has been observed in early transition metal systems,13 whereas in the late transition metal systems, the products with more M−Cl bonds are more stable because of the smaller difference in the bond energies of M−F and M−Cl bonds compared to the difference in the bond energies of C−F and C−Cl bonds (439 and 330 kJ/mol, respectively).14 Structures and Bonding. Calculated structures of the most plausible products from reactions of Si with di-, tri-, and tetrahalomethanes are shown in Figures 9-11. The silylene insertion, silene, and halosilyl carbene intermediates from halomethanes are all stable conformations, but only the most stable product (singlet silene from dihalomethane and triplet carbene from tri- and tetrahalomethane) is isolated and identified in this study as documented above. In our experience, halogen substitution stabilizes the higher-oxidation-state products formed through α-halogen transfer from carbon to the transition metal atom,8,13 and we also expect these reactions to occur with Si as the attacking reagent.9b It is noteworthy that structures of the insertion intermediates with Cl or Br but not F bonded to C yield configurations with Cl or Br in a bridging position [∠(X)CSi < 90°], which are similar to those found in reactions of Group 3 transition metal atoms with halomethanes.13 The bonding between the bridging halogen and Si, however, is weaker than that found in the scandium complexes. While ∠XCSi angles in the Si insertion complexes are slightly smaller than 90°, the distortion is much more significant with scandium; e.g., ∠FCSc for CH2(F)−ScF is 67.3°.13 In addition, unlike in the transition metal analogues, the dihedral angle of XCSiX′ (X = Cl or Br, and X′ = F, Cl, or

Figure 9. B3LYP structures of the most plausible (insertion, silene, and triplet carbene) products from reactions of Si with CH2F2, CH2FCl, CH2Cl2, and CH2Br2. The 6-311++G(3df,3pd) basis sets are used for C, H, F, Cl, Br, and Si. The bond lengths and angles are in angstroms and degrees, respectively.

Figure 10. B3LYP structures of the most plausible (insertion, silene, and triplet carbene) products from reactions of Si with CHF3 and CHCl3. The 6-311++G(3df,3pd) basis sets are used for C, H, F, Cl, and Si. The bond lengths and angles are in angstroms and degrees, respectively.

Br) in the Si complex is also close to 90°; e.g., for CH2(Cl)− SiCl, Φ(ClCSiCl) = 87.8° and r(Cl)−Si = 2.611 Å. To learn more about this Cl or Br in a bridging position between carbon and silicon in the silylenes, we have performed more calculations with B3LYP/6-311++G(3df,3pd) and the molecules constrained to a Cs structure with a cis arrangement of the two halogen atoms. The cis conformations of CH2(Cl)− SiF, CH2(Cl)−SiCl, and CH2(Br)−SiBr are 6.2, 14.4, and 8.9 kJ/mol higher in energy, respectively, than the bridged structures in Figure 9. We have also tried various initial structures for geometry optimization of the insertion products, G


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Inorganic Chemistry

Si insertion, silene, and triplet carbene complexes are nominal single, double, and single bonds, respectively: the natural C−Si bond orders25 of CH2(Cl)−SiCl, CH2SiCl2, and triplet HC− SiHCl2 are 0.93, 1.89, and 0.95, respectively. The unpaired electrons of the carbene stay mostly on C (e.g., the Mulliken atomic spin density of C in HC−SiHCl2 is 1.93). The C−Si bond of a singlet Si carbene is also a single bond (e.g., C−Si natural bond order of 0.96 for HC−SiHF2), and the HCSi and XCSi moieties are even more bent (e.g., 114.7° for HC− SiHF2). Apparently the unpaired electrons on C are distributed in part to other atoms in the silyl carbene products, leading to larger XCSi angles (133−165° in Figures 9−11) than in the above triplet methylenes. Our NBO results indicate that part of the unpaired electrons on C are donated not only to the σ*(Si−X) orbitals (X = H, F, or Cl) but also to Rydberg orbitals of Si that contain substantial 3d character, and this interaction energy is estimated to be 10−30 kJ/mol. In comparison, small Re and Os transition metal carbynes, XC MX3, in which nonbonding d electrons on the metal center are donated to C, also own bent XCM moieties.12 Evidence of this transfer to σ*(Si−X) orbitals is found in their NBO occupation numbers (Table S37), which are 0.073, 0.107, and 0.084 for X = H, F, and Cl, respectively, in HC−SiHFCl for example. The Mulliken atomic spin densities for these carbenes given in Table S38 also show a distribution of carbon unpaired electron density into the SiHFCl subunit. Also notice that the C−Si bond is slightly shorter (Figure 10, Table 4) for HC−SiF3 than Figure 11. B3LYP structures of the most plausible (insertion, silene, and triplet carbene) products from reactions of Si with CF4, CF2Cl2, CFCl3, and CCl4. The 6-311++G(3df,3pd) basis sets are used for C, F, Cl, and Si. The bond lengths and angles are in angstroms and degrees, respectively.

Table 4. Comparison of Calculated [B3LYP/6-311+ +G(3df,3pd)] C−Si Bond Lengths (angstroms) for the Silylene, Silene, and Carbene Products from Reactions of Si with the Reagent

and they all end up with their bridged structures. Calculations with B3LYP/6-31G* for the insertion compounds gave almost identical bridged structures. These calculations strongly suggest that the bridged conformation is real and most stable for the initial insertion product. NBO analysis25 shows distinct bonding between the halogen in the bridging position and Si with a bond order of 0.93 for the (Cl)···Si bond of CH2(Cl)−SiCl. Si contributes 59.8% p and 32.4% d character to the bond, while Cl contributes mostly p character (95.6%). The high p character of Si is consistent with a dihedral angle of ∼90°. No such dihedral angle or bridged structure is observed with the Si products containing a C−F bond. For example, CH2F−SiF owns a Cs structure with ∠FCSi = 110.1° and Φ(FCSiF) = 180°, indicating that donation of an electron from the smaller and more electronegative fluorine to Si is more difficult. This is compared with the recently reported highly distorted structures of Group 3 metal complexes with bridging F atoms.13 Evidently, donation of an electron to the d orbitals of the early transition metals is more efficient than to the p orbitals of Si in SiX bonds. While the CH2SiX2 silenes all have planar or near planar structures, the HCSi and XCSi subunits of the silyl carbene products are all noticeably bent, leading to Cs or C1 structures. The bent HCSi and XCSi moieties remind us of the bent structures of triplet CH2 and CX2 methylenes due to the unpaired electrons on C (e.g., calculated angles of 135.2°, 119.7°, and 128.9° for triplet CH 2 , CF 2 , and CCl 2 , respectively). The previously reported triplet HC−SiH3 also contains a bent HCSi moiety (152.6°).9 The C−Si bonds of the

a

reagent

silylene

silene

average

carbene

differencea

CH4 CH3F CH2F2 CH2FCl CH2Cl2 CH2Br2 CHF3 CHCl3 CF2Cl2 CFCl3 CCl4

1.907 1.895 1.925 1.927 1.925 1.920 1.975 1.946 1.967 1.965 1.969

1.702 1.686 1.676 1.680 1.686 1.690 1.704 1.699 1.692 1.700 1.709

1.804 1.790 1.800 1.803 1.805 1.805 1.839 1.822 1.829 1.832 1.839

1.808 1.804 1.795 1.795 1.795 1.792 1.782 1.786 1.799 1.802 1.806

0.103 0.091 0.130 0.132 0.130 0.128 0.193 0.160 0.168 0.163 0.163

Difference = silylene minus carbene C−Si bond distance.

for HC-SiCl3, and this follows higher Mulliken atomic charges on Si for HC−SiF3 (Table S39), which will contract the Si 3d orbitals and enhance their participation in bonding. Notice also that the atomic charges on Si are larger in the carbenes than in the trihalosilanes. We notice that the C−Si bond of the insertion complex is much longer than that of the silene, and the C−Si bond of the triplet carbene is intermediate between the former two. The methane and methyl fluoride systems were investigated by the Maier group,6,7,10 and we have performed an analogous theoretical analysis of these products for comparison to our new results. First, structures were computed at the B3LYP/6311++G(3df,3pd) level of theory for each system, and they are illustrated in Figure S8. We find that CH3−SiH, CH2SiH2, and HC−SiH3(T) are 153, 171, and −20 kJ/mol more stable H


Article

Inorganic Chemistry than CH4 + Si(3P0), respectively, which may be compared with values of 143, 154, and −39 kJ/mol, respectively, using B3LYP and the smaller B3LYP/6-311++G** basis set.7 We are interested in comparing the C−Si bond lengths in these three silicon-bearing transient species, which are related by [1,2] X-shifts, and these bond lengths from the earlier report are 1.910, 1.708, and 1.817 Å, respectively.7 Our values from Figure S8 are 1.907, 1.702, and 1.808 Å, respectively. Likewise for the CH3−SiF, CH2SiHF, and HC−SiH2F(T) series, the C−Si bonds are 1.895, 1.686, and 1.804 Å, respectively. Hence, the triplet carbenes contain C−Si bonds that are near the average of the silylene insertion and singlet silene for these first two systems. Table 4 lists the bond length data calculated here and gives the average of the silylene insertion and silene C−Si bond lengths. Notice that the C−Si bond lengths in the first two columns change little as the number of halogen substituents increases from 0 to 4 (CF4 is excluded because CF2SiF2 is weakened by the inductive effect of four fluorine substituents). The last two columns list the carbene bond length and the difference between the nominal single and carbene bond lengths, which is a measure of the decrease in the carbene C−Si bond length relative to a single bond. Notice that this bond shortening effect generally increases with increasing halogen substitution, and our HC−SiF3 exhibits the strongest enhancement. This leads us to propose that hyperconjugation of the unpaired electrons on C feeds electron density into σ*(Si−X) orbitals, causing a shortening of the C−Si bond and very small elongation of the Si−X bonds. We illustrate this with several examples: the Si−H bonds computed for HC−SiH3, 1.483, 1.483, and 1.485 Å, are longer than for SiH4, 1.480 Å. One Si−F bond in HC−SiF3, 1.583 Å, is longer than in HSiF3 as a reference, 1.582 Å. The Si−Cl bond lengths calculated for HC−SiCl3, 2.046, 2.046, and 2.048 Å (Figure 12), are longer than that computed for HSiCl3, 2.042 Å. With Cl−C−SiCl3, two bonds are longer, 2.047 Å, and the 2.037 Å bond in the molecular plane is shorter than the Si−Cl bond in HSiCl3. For these differences in C−Si bond lengths, it is reasonable that slightly shorter than average single and double C−Si bond lengths characterize some additional π-bonding in the carbene, which we believe arises from hyperconjugation of the two unpaired electrons in C 2px and 2py orbitals with antibonding Si−X orbitals leading to two additional weak partial π-bonds. Notice that the two single-electron-occupied C px and py orbitals look a lot like π-bonds in HC−SiH3 as well as HC− SiHF2 molecular orbitals in Figures S9 and 13, thus indicating that F atoms do not change the shapes of the p orbitals much and Si is a major player. Also examine the π-orbital of CH2 SiF2 in Figure 13: It looks like the py orbital of C, but of course, it has much more overlap. NBO25 analysis for CH2SiCl2 shows that Si contributes 47% s and 52% p character to the σ-bond and 0% s and 99% p character to the π-bond. In contrast, Zr contributes mostly d character to the C−Zr double bond of CH2ZrCl2: Zr provides 16% s, 17% p, and 67% d character to the σ-bond and 0% s, 19% p, and 81% d character to the π-bond. The C−M bond of Group 4 metal triplet carbenes (HC÷MX3 or XC÷MX3) is treated as a true double bond in the NBO calculations, reflecting the fact that the two unpaired electrons are shared mostly by p orbitals of C and d orbitals of the transition metal; C provides 100% p character for the π-bonds of HC÷ZrCl3 and Zr 13% p and 87% d character. It is important to notice that agostic distortion26 is not observed in these and previously reported Si silenes,6,7,9−11 because it arises

Figure 12. CASSCF structure and orbitals plotted with an isodensity of 0.04 e/Å of CH2SiF2. The active space consists of 12 electrons and 10 orbitals [CASSCF(12,10)] for the two C−H bonds, two Si−F bonds, C−Si σ- and π-bonds, C−Si σ*- and π*-bonds, and two C−H σ*-bonds. The occupation numbers are in parentheses, and the 6-311+ +G(3df,3pd) basis sets are used for C, H, F, and Si. The bond lengths and angles are in angstroms and degrees, respectively. The C−Si bond is a true double bond.

from donation of electron density from the C−H bond to the empty d orbitals of a nearby metal center. A Complete Active Space Multiconfiguration SCF (CASSCF) calculation27 has also been performed for CH2 SiF2 using the 6-311++G(3df,3pd) basis sets (the largest number of electrons we can handle) to verify the NBO results and to show that the C−Si bonds of the silenes investigated in this study are full double bonds; e.g., the natural bond order of the C−Si bond in CH2SiF2 is 1.92. Our active space consists of 12 electrons and 10 orbitals [CASSCF(12,10)] for the two C−H bonds, two Si−F bonds, C−Si σ- and π-bonds, C−Si σ*and π*-bonds, and two C−H σ*-bonds. The resulting CASSCF structure and orbitals are shown in Figure 12, and the C−Si bond order is also 1.92, which is the same as that from NBO analysis; the C−Si bond length is 1.693 Å, which compares favorably with the value of 1.676 Å from our B3LYP calculation. A similar CASSCF calculation has also been done for triplet HC−SiHF2, and its structure and orbitals are shown in Figure 13. The active space comprises 12 electrons and 10 orbitals [CASSCF(12,10)] for the C−H σ-bond, two Si−F bonds, the Si−H σ-bond, the C−Si σ-bond, two 2p orbitals of C, the C−H σ*-bond, the Si−H σ*-bond, and C−Si σ*-bonds. The C−Si bond order is 0.98, which is compared with the natural bond order of 0.96, and the C−Si bond length is 1.824 Å, near the B3LYP value of 1.795 Å. It is notable that the two 2p orbitals of C containing the unpaired electrons of the triplet carbene are I


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Inorganic Chemistry

reactions.13,20 The relative Si bond energies (Si−F, Si−Cl, Si− Br, and Si−H) mainly determine the observed primary product. Potential energy surface calculations show that these reactions proceed through C−X bond insertion and [1,2] C−X bond migration to the lowest-energy products. The C−Si bond lengths and the bonding in these three novel silicon-bearing transient species have been compared. Structures of the Si products reveal that this Group 14 metalloid uses its four valence electrons and s and p orbitals and forms up to four bonds in these systems. The (X)CSiX′ dihedral angle of the bridged insertion silylenes with a C−Cl or C−Br bond [CH2(X)−SiX′] is close to 90° because of the interaction between X and the p orbital of Si. The C−Si bond in the silaethenes is a true double bond, and the HCSi or XCSi moiety of the Si carbenes is bent because of the two unpaired electrons on C in the triplet ground states. The two unpaired electrons on carbon contribute to C−Si bond stabilization, reducing its length to the average between the single bonded first insertion product silylene and the double bonded silene, which we believe arises from hyperconjugation of the two unpaired electrons in C 2px and 2py orbitals with σ*(Si−X) orbitals (X = H, F, or Cl), leading to two additional weak partial π-bonds. The last column in Table 4 compares this effect in a series of silyl carbenes. In the case of Ti, the d orbitals participate more effectively and provide a considerable increase in bond strength for the nominally (C) σ2π1π1(Ti) bonding in HC÷TiF3.13b Although NBO25 treats the C−Ti bond as a double bond [one σ-bond (0.96) and two half-πbonds (0.46 and 0.48)], the spin density is concentrated on C (1.78) and more so for these silyl carbenes, C (1.93 for HC− SiHCl2).

Figure 13. CASSCF structure and orbitals plotted with an isodensity of 0.04 e/Å of triplet HC−SiHF2. The active space comprises 12 electrons and 10 orbitals [CASSCF(12,10)] for the C−H σ-bond, two Si−F bonds, the Si−H σ-bond, the C−Si σ-bond, two 2p orbitals of C, the C−H σ*-bond, the Si−H σ*-bond, and C−Si σ*-bonds. The occupation numbers are in parentheses, and the 6-311++G(3df,3pd) basis sets are used for C, H, F, and Si. The bond lengths and angles are in angstroms and degrees, respectively. Notice that the unpaired electrons on C contribute to the C−Si bond.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02610. Tables S1 to S39 of calculated vibrational frequencies, intensities, Mulliken atomic spin densities and charges, and Cartesian coordinates of the observed products. Figures S1 and S2 of Si + CD2Cl2 and CX4 spectra, Figures S3 to S6 of product energy profiles, Figure S7 of transition state structures, Figure S8 of structures from Si reactions with CH4 and CH3F, and Figure S9 of CASSCF structure and molecular orbitals for triplet HC-SiH3 (PDF)

considerably shifted toward Si, indicating a contribution to the C−Si bond. Though the unpaired electrons do not form a regular bond with Si because of its filled valence shell, they still strengthen the C−Si bond, resulting in a C−Si bond, 1.782 Å, shorter than that in the insertion product CH2F−SiF, 1.925 Å. At first, we thought that the fluorine substituent on silicon might assist in hyperconjugation, so we performed a similar CASSCF calculation on triplet HC−SiH3; the molecular orbitals are shown in Figure S9. The C px and py orbitals appear to touch the Si center as much as they did in the HC− SiHF2 case, so it follows that the F substituents are not responsible for shortening the C−Si bond to the median of CH2F−SiF and CH2SiF2 values.

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AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected]. Telephone: 434-924-6844.

CONCLUSIONS Products in reactions of laser-ablated Si atoms with di-, tri-, and tetrahalomethanes were identified from the matrix infrared spectra on the basis of isotopic shifts, correlation with B3LYP computed frequencies, and product energies. Although the first insertion silylenes are not detected here, the singlet silenes (CH2SiX2) are observed in reactions of dihalomethanes, and triplet halosilyl carbenes (HC−SiX3 and XC−SiX3) in reactions of tri- and tetrahalomethanes. The latter Si intermediates are the lowest-energy (most stable) products among the plausible products in B3LYP calculations and are similar to the primary products in the previous studies of Group 4 and Th metal

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Korea Research Foundation (KRF) funded by a Korean government grant (NRF-2013R1A1A2060088) and the KISTI supercomputing center, and retirement funds from TIAA for L.A.

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REFERENCES

(1) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heinemann: Amsterdam, 1997.

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Inorganic Chemistry (2) Pawlenko, S. Organosilicon Chemistry; Walter de Gruyter, Inc.: New York, 1986. (3) (a) Liu, H.; Zhang, P.; Liu, M.; Wang, S.; Jiang, L. Adv. Mater. 2013, 25, 4477−4481. (b) Gilman, H.; Dunn, G. E. Chem. Rev. 1953, 52, 77−115. (4) (a) Haiduc, I.; Zuckerman, J. J. Basic Organometallic Chemistry; Walter de Gruyter, Inc.: New York, 1985. (b) Spessard, G. O.; Miessler, G. L. Organometallic Chemistry; Oxford University Press: New York, 2010. (5) (a) Rendler, S.; Oestreich, M. Synthesis 2005, 2005, 1727−1747. (b) Dilman, A. D.; Ioffe, S. L. Chem. Rev. 2003, 103, 733−772. (c) Kano, N.; Miyake, H.; Kawashima, T. Chem. Lett. 2007, 36, 1260− 1261. (hypervalent Si compounds). (6) (a) Reisenauer, H. P.; Mihm, G.; Maier, G. Angew. Chem., Int. Ed. Engl. 1982, 21, 854−855. (b) Maier, G.; Mihm, G.; Reisenauer, H. P. Chem. Ber. 1984, 117, 2351−2368. (c) Maier, G.; Mihm, G.; Reisenauer, H. P.; Littmann, D. Chem. Ber. 1984, 117, 2369−2381. (CH3−SiH and CH2SiH2). (7) Maier, G.; Reisenauer, H. P.; Glatthaar, J. Chem. - Eur. J. 2002, 8, 4383−7391. (8) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040. (review article). (9) (a) Schreiner, P. R.; Reisenauer, H. P.; Sattelmeyer, K. W.; Allen, W. D. J. Am. Chem. Soc. 2005, 127, 12156−12157. (HC−SiH3, EPR and IR matrix spectra). (b) Schreiner, P. R.; Reisenauer, H. P.; Allen, W. D.; Sattelmeyer, K. W. Org. Lett. 2004, 6, 1163−1167. (triplet HC−SiHCl2). (10) (a) Maier, G.; Mihm, G.; Reisenauer, H. P. Angew. Chem., Int. Ed. Engl. 1981, 20, 597−598. (CH2SiCl2). (b) Maier, G.; Glatthaar, J.; Reisenauer, H. P. J. Organomet. Chem. 2003, 686, 341−362. (Si + CH3X → CH2SiHX). (11) (a) Raabe, G.; Vančik, H.; West, R.; Michl, J. J. Am. Chem. Soc. 1986, 108, 671−677. (CH3SiHCH2). (b) Withnall, R.; Andrews, L. J. Phys. Chem. 1985, 89, 3261−3268. (SiH4 + O). (c) Withnall, R.; Andrews, L. J. Am. Chem. Soc. 1986, 108, 8118−8119. (12) (a) Cho, H.-G.; Andrews, L. Organometallics 2008, 27, 1786− 1796. (Os + CH4). (b) Cho, H.-G.; Andrews, L. Organometallics 2007, 26, 4098−4101. (Re + CH4). (c) Lyon, J. T.; Cho, H.-G.; Andrews, L.; Hu, H.-S.; Li, J. Inorg. Chem. 2007, 46, 8728−8738. (d) Cho, H.-G.; Andrews, L. Dalton Trans. 2009, 5858−5866. (13) (a) Cho, H.-G.; Andrews, L. Organometallics 2015, 34, 3390− 3399. (Sc + CX4). (b) Lyon, J. T.; Andrews, L. Inorg. Chem. 2007, 46, 4799−4808. (Ti, Zr, and Hf + CH2F2 and CHF3). (c) Lyon, J. T.; Andrews, L. Organometallics 2007, 26, 2519−2527. (Group 4 + CX4). (d) Lyon, J. T.; Andrews, L. Organometallics 2007, 26, 332−339. (Group 4 + CH2X2 and CHX3). (e) Lyon, J. T.; Andrews, L. Eur. J. Inorg. Chem. 2008, 2008, 1047−1058. (Th + CX4). (f) Cho, H.-G.; Andrews, L. Eur. J. Inorg. Chem. 2016, 2016, 380−392. (Y, La + CX4). (14) (a) Cho, H.-G.; Andrews, L. Dalton Trans. 2010, 39, 5478− 5489. (Ir + CX4). (b) Cho, H.-G.; Andrews, L. J. Am. Chem. Soc. 2008, 130, 15836−15841. (Pt + CX4). (c) Cho, H.-G.; Andrews, L. Organometallics 2013, 32, 2753−2759. (Au, Pt, Pd, Ni + CH3X). (15) (a) Andrews, L.; Willner, H.; Prochaska, F. T. J. Fluorine Chem. 1979, 13, 273−278. Isotopic modifications synthesized. (b) Prochaska, F. T.; Andrews, L. J. Chem. Phys. 1978, 68, 5577−5586. (16) (a) Andrews, L.; Citra, A. Chem. Rev. 2002, 102, 885−911. and references therein. (b) Andrews, L. Chem. Soc. Rev. 2004, 33, 123−132 and references therein. (17) 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.; et al. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (18) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee, C.; Yang, Y.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (19) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265−3269. (20) (a) Becke, A. D. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (b) Burke, K.; Perdew, J. P.; Wang, Y. In Electronic Density

Functional Theory: Recent Progress and New Directions; Dobson, J. F., Vignale, G., Das, M. P., Eds.; Plenum Press: New York, 1998. (21) (a) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1968, 49, 4269− 4276. (SiF2). (b) Hastie, J. W.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. Soc. 1969, 91, 2536−2538. (SiF2). (c) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1968, 49, 1938−1943. (SiCl2). (d) Maass, G.; Hauge, R. H.; Margrave, J. L. Z. Anorg. Allg. Chem. 1972, 392, 295−302. (SiBr2 and SiCl2). (22) Kotz, J. C.; Purcell, K. F. Chemistry and Chemical Reactivity, 2nd ed.; Saunders College Publishing: San Francisco, 1991. (23) (a) Milligan, D. E.; Jacox, M. E.; Guillory, W. A. J. Chem. Phys. 1968, 49, 5330−5335. (b) Miller, J. H.; Andrews, L. J. Mol. Struct. 1981, 77, 65−73 and references therein. (24) Fukui, K. Acc. Chem. Res. 1981, 14, 363−368. (25) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899−926 and references therein. (26) (a) Scherer, W.; McGrady, G. S. Angew. Chem., Int. Ed. 2004, 43, 1782. (b) Clot, E.; Eisenstein, O. Agostic Interactions from a Computational Perspective. In Structure and Bonding, Computational Inorganic Chemistry; Kaltzoyannis, N., McGrady, E., Eds.; SpringerVerlag: Heidelberg, Germany, 2004; pp 1−36. (27) Yamamoto, N.; Vreven, T.; Robb, M. A.; Frisch, M. J.; Schlegel, H. B. Chem. Phys. Lett. 1996, 250, 373−378. (28) Cho, H.-G.; Andrews, L. Bull. Korean Chem. Soc. 2015, 36, 1580−1585.

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