Matrix Infrared Spectroscopic and Theoretical Studies for Products

Jun 27, 2019 - Matrix Infrared Spectroscopic and Theoretical Studies for Products Provided in Reactions of Sn with Ethane and Halomethanes ...
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Article Cite This: J. Phys. Chem. A 2019, 123, 6259−6268

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Matrix Infrared Spectroscopic and Theoretical Studies for Products Provided in Reactions of Sn with Ethane and Halomethanes Han-Gook Cho*,†,‡ and Lester Andrews‡ †

Department of Chemistry, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 22012, South Korea Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, Virginia 22904-4319, United States

‡

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S Supporting Information *

ABSTRACT: Tin insertion products (oxidation state 2+) were observed in reactions of laser-ablated Sn atoms with ethane, and halomethanes in excess argon, parallel to the Pb reactions. The CSnX bond angles of the observed Sn complexes are close to right angles, and natural bond orbital calculations show that Sn also utilizes mostly its p-orbitals to make chemical bonds. Bridged Sn complexes [CX2(X)−SnX] were also provided in reactions of tetrahalomethanes via photo-isomerization of the insertion products, showing that the porbitals of Sn are more accessible than those of Pb. These products were identified from the matrix infrared spectra on the basis of isotopic shifts and density functional theory frequencies. Considering the previously reported high-oxidation-state products of the lighter group 14 elements and the Pb products with primarily oxidation state 2+ because of the relativistic effects, the observed Sn complexes show a trend that the high-oxidation-state complexes are less favored with increasing atomic mass in group 14, which is opposite to that observed in transition-metal columns.

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INTRODUCTION The chemistry of group 14 elements varies significantly with atomic mass. The lighter elements readily produce high +4 oxidation-state products. Si generates CX2SiX2 and CX− SiX3 (X = H or halogen) along with the insertion complexes (CX3−SiX) in reactions with halomethanes. SiH4 and atomic carbon provided CH−SiH3 in a triplet ground state.1−7 On the other hand, Pb yielded only insertion complexes (oxidation state +2) in reactions with small alkanes and halomethanes.8,9 The strong preference of Pb for the low oxidation state is traced to the relativistic effects in the heavy element, leading to stronger contraction of the 6s orbital than the 6p orbitals, making promotion between the orbitals require more energy.9,10 The main group elements CX2MX2 and CX−MX3 are analogous to the high oxidation-state complexes of transition metals (CX2MX2 and XCMX3), resembling similar structures at the metal with larger ligands.11,12 The small transition-metal complexes often exhibit interesting structures, such as agostic and Jahn−Teller distortions and photoisomerizations. Theoretical investigations have been performed to illuminate their electronic structures and reaction paths.13−16 The previous investigations have shown that unlike transition metals, a main group element forms a smaller number of chemical bonds because the d-orbitals are not accessible. Subsequently, the C−Si bond of CX−SiX3 is essentially a single bond, and Pb forms basically only two chemical bonds using its 6p orbitals.7−9 Halogenation generally stabilizes higher oxidation-state products.11,12 Tin has been used broadly since the Bronze Age for flatware, various alloys, plating, solder, and transparent conducting © 2019 American Chemical Society

electrodes. Tin displays a chemical similarity to its neighbors in group 14, Ge and Pb, and has two main oxidation states, +2 and +4 (e.g., SnX2 and SnX4, X = halogen).17−19 Organotin compounds, some of which are highly toxic and have been used as biocides, are normally tetravalent. Divalent organotin compounds are less common, but still more common than organogermanium and organosilicon compounds.20,21 In our previous study, CH3−SnH (oxidation state +2) was generated in direct reactions of CH4 with no trace of higher oxidationstate products.8 In this paper, we report spectroscopic identification of the products in reactions of laser-ablated Sn atoms with ethane and halomethanes. The observed tin compounds including the interesting bridged complexes indicate that the electronic promotion from 5s to 5p required for formation of higher oxidation-state products is still difficult although easier than that from 6s to 6p for Pb.9 The structures of organotin compounds reveal their electronic properties. Experimental and Computational Methods. Laserablated Sn atoms (Johnson Matthey) were reacted with C2H6, CH3F (Matheson), CD3F, CDF3 (synthesized22), CH2F2, CH2FCl, CHF3 (PCR Research), CF2Cl2, CFCl3 (Dupont), CCl4 (Fisher), C2D6, 13CH3F (99%), and 13CCl4 (90% enriched) (MSD Isotopes) in excess argon during condensation at 4 or 10 K using a closed-cycle refrigerator (Sumitomo or Air Products, Displex). The experimental details have been previously reported elsewhere.23,24 Concentrations of gas Received: May 3, 2019 Revised: June 26, 2019 Published: June 27, 2019 6259

DOI: 10.1021/acs.jpca.9b04177 J. Phys. Chem. A 2019, 123, 6259−6268

Article

The Journal of Physical Chemistry A

Table 1. Frequencies of Product Absorptions Observed from Reactions of C2H6 and CH3F with Sn in Excess Argona C2H5−SnH

C2D5−SnD

1633 (1643.5)

1173 (1167.8)

CH3−SnF

CD3−SnF

CH3−SnF

670.8 (641.4) 560.1, 554.2, 529.1 (551.7) 487.2 (481.9)

511 (491.9) 560.2, 554.5 (553.5) 445.1 (436.3)

13

667.6 (636.9) 559.6, 554.2 (551.5) 473.9 (468.7)

description Sn−H str. CH3 rock Sn−F str. C−Sn str.

All frequencies are in cm−1. Stronger absorptions are in bold. The numbers in parentheses are the calculated frequencies with M06/aug-cc-pVTZ/ aug-cc-pVTZ-pp. The description gives the major coordinates. a

mixtures were in the range of 0.2−0.7% and typically 0.5% in argon. After co-deposition normally for an hour, infrared spectra were recorded at a resolution of 0.5 cm−1 using a Nicolet iS50 or a Nicolet 550 spectrometer equipped with a liquid nitrogen-cooled MCT-A or -B detector. The deposited samples were next irradiated by a mercury arc street lamp (175 W) with a combination of optical filters for 10 min, were then annealed (warmed and re-cooled), and more spectra were recorded. Density functional theory (DFT) calculations were performed for the plausible products using the Gaussian 09 and 16 packages,25,26 the M06 density functional,27 and the aug-cc-pVTZ basis sets for H, C, F, Cl, and the aug-cc-pVTZpp pseudopotential and basis set (28-electron core and 22 valence electrons)28−30 were used to provide a consistent set of vibrational frequencies and reaction energies. Geometries were fully relaxed during optimization, and the optimized geometry was confirmed by vibrational analysis. B3LYP31,32 calculations with the 6-311++G(3df,3pd) basis sets33 for H, C, F, Cl, and the SDD pseudopotential and basis set for Sn34,35 imbedded in the Gaussian package (46-electron core and four valence electrons) were also done to compare with the previous results and to support the M06 results. Open shell calculations were spin-unrestricted (UM06 and UB3LYP). BPW91,36 MP2,37 and coupled cluster singles and doubles (CCSD)38 calculations were also done for the bridged products in their singlet ground states to examine their structures and to test the theoretical methods. Only valence electrons were correlated in the MP2 and CCSD calculations. The vibrational frequencies were calculated analytically. The binding energy for a metal complex is a M06/aug-cc-pVTZ/aug-cc-pVTZ-pp value, and the zeropoint energy is included [ΔE 0 = E elec (product) − Eelec(reactants) + Σ(ZE)]. The spin−orbit interaction energy, which stabilizes the Sn atom by 28 kJ/mol (102 kJ/mol for the Pb atom),39 was counted in the calculation of Eelec(reactants) [reactants = Sn(3P0) + precursor]. These calculation results were compared with the observed vibrational characteristics for identification of the reaction products.

Table 2. Frequencies of Product Absorptions Observed from Reactions of CH2X2 with Sn in Excess Argona CH2F−SnF 967.0, 960.1 (1042.1) 563.3, 559.8 (556.2)

CD2F−SnF 963.2 (990.8) 560.0 (556.1)

CH2F−SnCl 972.0, 963.5, 956.6 (1044.3)

description C−F str. Sn−F str.

All frequencies are in cm−1. Stronger absorptions are in bold. The numbers in parentheses are the calculated frequencies with M06/augcc-pVTZ/aug-cc-pVTZ-pp. The description gives the major coordinates. a

Table 3. Frequencies of Product Absorptions Observed from Reactions of CHF3 with Sn in Excess Argona CHF2−SnF

CDF2−SnF

description

1224.0 (1239.0) 1046.1, 1034.7 (1092.2) 986.1, 987.8 (1033.2) 569.1 (563.3)

1021.3 (1054.6) 1054.2 (1094.0) 933.5 (953.9) 568.7 (562.9)

A′ C−H ip bend A′ CF2 s str. A″ CF2 as str. A′ Sn−F str.

All frequencies are in cm−1. Stronger absorptions are in bold. The numbers in parentheses are the calculated frequencies with M06/augcc-pVTZ/aug-cc-pVTZ-pp. The description gives the major coordinates. a

halved on photolysis with λ > 220 nm, and further decreased on annealing to 35 K. These product absorptions were weaker and broader than those observed in reactions with Pb.9 The i absorption at 1633 cm−1 shifted to 1173 cm−1 on deuteration (H/D ratio 1.392), and it is assigned to the Sn−H stretching mode of C2H5−SnH on the basis of its frequency, proper H/D ratio for a heavy metal H/D vibration, and correlation with the M06 computed harmonic values (1643.5 and 1167.8 cm−1, Table S1). The CH3Sn−H molecule’s absorption is 1627.3 and H/D ratio is the same 1.392, and there is no 13C shift.8 The computed harmonic Sn−H/Sn−D frequency ratio 1643.5/ 1167.8 = 1.407 is higher than the above observed value that is the anharmonic ratio, which reveals the effect of anharmonicity on the relative H and D stretching frequencies where Sn−H is higher in the potential well and more anharmonic than Sn−D. (The CH3Sn−F stretching mode is quite different in this regard: next section). This Sn−H stretching frequency is compared with 2004, 1798.6, 1627.3, 1505.0, and 1513.4 cm−1 for the group 14 family of terminal hydrides CH3−SiH,40 CH3−GeH,41 CH3− SnH,8 CH3−PbH,8 and C2H5−PbH,9 respectively, showing a trend of decreasing M−H stretching frequency with increasing atomic mass. The observed Sn−H stretching frequencies are also compared with 1657.3 and 1654.8 cm−1 for SnH2 and 1190.8 and 1188.2 cm−1 for SnD2 (H/D ratios 1.392 and 1.393).42

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RESULTS AND ASSIGNMENTS The observed frequencies from laser-ablated tin atom reaction products with ethane and halomethanes in an argon matrix are listed in Tables 1−4 and compared with calculated values in Tables S1−S12. The spectral variations from reactions of Sn and the precursors upon photolysis and annealing are shown in Figures 1−7, and the calculated product structures are shown in Figures 8−10. The calculated reaction energies of Sn with ethane and halomethanes are summarized in Table S13. Sn + C2H6. Figure 1 shows the IR spectra from reactions of laser-ablated Sn atoms with ethane isotopes (C2H6 and C2D6). The broad product absorptions marked “i” (i for insertion) were observed in the original deposition spectrum, increased on annealing to 20 K, slightly decreased on annealing to 30 K, 6260

DOI: 10.1021/acs.jpca.9b04177 J. Phys. Chem. A 2019, 123, 6259−6268

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The Journal of Physical Chemistry A Table 4. Frequencies of Product Absorptions Observed from Reactions of CX4 with Sn in Excess Argona CF2Cl−SnCl

CF2(Cl)−SnCl

CFCl2−SnCl

1037.4 (1154.4) 998.2 (1115.1)

1036.5 (1192.0) 724.3 (780.2)

729.6 (748.9)

626.7 (600.1)

CFCl(Cl)−SnCl

CCl3−SnCl

CCl3−SnCl

13

CCl2(Cl)−SnCl

13

CCl2(Cl)−SnCl2

735.7 (711.2) 603.0 (694.4)

CX3 as str.

712.0 (688.9) 585.0 (671.5)

CX3 as str.

610.7b (623.0) 1301.3 (1344.6) 1237.8 (1291.5)

CX2 wag CX2 s str.

1279.2 (1273.8) 864.3, 858.4 (878.1)

839.4, 833.4 (850.1)

578.6b (483.3) a

description CX3 s str.

CX2 as str. C−(X) str.

−1

All frequencies are in cm . Stronger absorptions are in bold. The numbers in parentheses are the calculated frequencies with M06/aug-cc-pVTZ/ aug-cc-pVTZ-pp. The description gives the major coordinates. bTentative assignment.

Figure 1. IR spectra in the product absorption regions for laserablated Sn atoms co-deposited with C2H6 and C2D6 in excess argon at 4 K and their variation. (a) Sn + 0.5% C2H6 in Ar co-deposited for 1 h. (b−e) As (a) after annealing to 20 and 30 K, photolysis with λ > 220 nm, and annealing to 35 K. (f) Sn + 0.5% C2D6 in Ar codeposited for 1 h. (g−j) As (f) after annealing to 20 and 30 K, photolysis with λ > 220 nm, and annealing at 35 K. i stands for the insertion product, and w and p for water residue and precursor absorption.

Figure 2. IR spectra in the product absorption regions for laserablated Sn atoms co-deposited with CH3F isotopomers in excess argon at 4 K and their variation. (a) Sn + 0.5% CH3F in Ar codeposited for 1 h. (b−d) As (a) after annealing to 20 K, photolysis with λ > 220 nm, and annealing to 35 K. (e) Sn + CD3F 0.5% in Ar co-deposited for 1 h. (f−i) As (e) after annealing to 20 and 30 K, photolysis with λ > 220 nm, and annealing at 35 K. (j) Sn + 0.5% 13 CH3F in Ar co-deposited for 1 h. (k−n) As (j) after annealing to 20 and 32 K, photolysis with λ > 220 nm, and annealing at 35 K. i and * stand for insertion and high-order products.

Ethylidene (CH 3 CHSnH 2 ), metallacyclopropane [(CH2)2−SnH2], and ethylidyne (CH3C−SnH3) complexes, common products in previous studies of many transition-metal reactions with ethane,11,12 have not been observed in this study, parallel to the Pb + C2H6 case.9 The Sn insertion complex (oxidation state +2) is the most stable among the plausible products: CH3CH2−SnH, (CH2)2SnH2, CH3CH SnH2, and CH3C−SnH3 are −50, −1, 163, and 250 kJ/mol relative to the reactants [Sn(3P0) + C2H6] (Table S13). The Sn insertion complex was generated in the process of annealing as shown in Figure 1, and it dissociates during photolysis (>220 nm). CH3−SnH, which was −54 kJ/mol with respect to its reactants, was also produced and dissociated in annealing and photolysis in a previous study.8 Parallel to the previous studies,11,12 the C−C insertion product (CH3−Sn−CH3), though it is the most stable (−93 kJ/mol relative to the reactants), has not been observed probably because the C−C bond is more difficult to approach and its low absorption constants make it difficult to detect. The transition state between Sn(3P0) + C2H6 and C2H5− SnH(T) is 180 kJ/mol higher in energy than the reactants (B3LYP, an imaginary frequency 733.2 cm−1), and intrinsic reaction coordinate (IRC)43 calculations (Figure S1) show that C−H bond insertion by Sn(3P0) is supposed to be smooth (Figure S1). Reaction (1) summarizes these reactions. The

triplet−singlet crossing is believed to occur after production of the insertion complex, where the singlet state becomes more stable. annealing

Sn(3P0) + C2H6 HoooooooooI CH3CH 2−SnH λ> 220 nm

(1)

The metallacyclopropane complex [(CH2)2SnH2] is 49 kJ/ mol higher in energy than the insertion product, and its transition state between C2H5−SnH and (CH2)2SnH2 on the singlet potential surface is 77 kJ/mol higher than C2H5−SnH (B3LYP, an imaginary frequency 872.2 cm−1), giving a possible condition for its generation particularly in photolysis (Figure S2). Previous studies have shown conversion of the insertion complex to higher oxidation-state products.11,12 However, the SnH 2 symmetric and antisymmetric stretching modes predicted at 1684.0 and 1668.7 cm−1, which were close to the Sn−H stretching band of C2H5−SnH, were not observed in this study. 6261

DOI: 10.1021/acs.jpca.9b04177 J. Phys. Chem. A 2019, 123, 6259−6268

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The Journal of Physical Chemistry A

Figure 5. IR spectra in the product absorption regions for laserablated Sn atoms co-deposited with CF2Cl2 in excess argon at 10 K and their variation. (a) Sn + 0.5% CF2Cl2 in Ar co-deposited for 1 h. (b−e) As (a) after photolysis with λ > 420 nm and 240 < λ < 380 nm, and annealing to 20 and 28 K. i, b, and p stand for insertion and bridged products and precursor absorptions.

Figure 3. IR spectra in the product absorption regions for laserablated Sn atoms co-deposited with CH2F2 and CH2FCl in excess argon at 10 K and their variation. (a) Sn + 0.5% CH2F2 in Ar codeposited for 1 h. (b−d) As (a) after photolysis with λ > 420 nm and 240 < λ < 380 nm, and annealing to 28 K. (e) Sn + 0.5% CH2FCl in Ar co-deposited for 1 h. (f−h) As (e) after photolysis with λ > 420 nm and 240 < λ < 380 nm, and annealing at 28 K. i stands for the insertion product. p denotes high-order product absorption.

Figure 6. IR spectrum in the product absorption region for laserablated Sn atoms co-deposited with CFCl3 in excess argon at 10 K and its variation. (a) Sn + 0.5% CFCl3 in Ar co-deposited for 1 h. (b− d) As (a) after photolysis with λ > 420, 240 < λ < 380, and λ > 220 nm and (e,f) annealing to 20 and 28 K. i, b, and p stand for insertion and bridged products and precursor absorptions. Figure 4. IR spectra in the product absorption regions for laserablated Sn atoms co-deposited with CHF3 in excess argon at 4 K and their variation. (a) Sn + 0.5% CHF3in Ar co-deposited for 1 h. (b−e) As (a) after annealing to 20 K, photolysis with λ > 220 nm, photolysis with λ > 220 nm at 20 K, and annealing to 30 K. (f) Sn + 0.5% CDF3 in Ar co-deposited for 1 h. (g−j) As (f) after annealing to 20 K, photolysis with λ > 220 nm, and photolysis with λ > 220 nm at 20 K, and annealing to 30 K. i and p stand for insertion product and precursor absorptions.

The stronger i absorption at 554.2 cm−1 (with matrix site absorptions at 560.1 and 529.1 cm−1) showed small isotopic shifts on deuteration and 13C substitution. In the case of the strongest Sn−F stretching mode (calculated frequency of 551.7 cm−1), the observed D shift is +0.3 cm−1 and the harmonic calculated shift is +2.2 cm−1. These differences are due to mode coupling and not anharmonicity. We assign it to the Sn− F stretching mode. A weak i absorption at 670.8 cm−1 exhibited D and 13C counterparts at 511 and 667.6 cm−1 (H/D and 12/13 ratios 1.313 and 1.005, respectively) and was designated to the CH3 rocking mode (calculated values 641.4, 491.9, and 636.9 cm−1). This C−F insertion complex is also the most stable among the plausible products, parallel to the previously studied Pb system. CH3−SnF, CH2F−SnH, CH2SnHF, and CH− SnH2F(T) are −265, −54, −37, and +149 kJ/mol relative to the reactants, respectively (Table S13). Tin, like lead, primarily prefers oxidation state +2, forming only the insertion complex.9 Sn + CH2FCl, CH2F2. In Figure 3, the product absorptions marked “i” from reactions of Sn with CH2F2 and CH2FCl increased on photolysis with 240 < λ < 380 nm. The i absorption at 967.0 cm−1 with a site absorption at 960.1 cm−1 shifted on deuteration to 963.2 cm−1 (H/D ratio 1.007) and

Sn + CH3F. The IR spectra shown in Figure 2 are from reactions of laser-ablated Sn atoms and methyl fluoride isotopomers. The product absorptions marked “i” sharpened in the early process of annealing and increased on photolysis. Another product absorption marked “*”, which increased during annealing and photolysis, probably originates from a higher-order product. The i absorption at 487.2 cm−1 shifts to 445.1 cm−1 on deuteration (H/D ratio 1.095) and to 473.9 cm−1 on 13C substitution (12/13 ratio 1.028). It is assigned to the C−Sn stretching mode of CH3−SnF on the basis of the large 13C isotopic shifts, good correlation with the predicted values (481.9, 436.3, and 468.7 cm−1), and comparison with previous similar Pb results.9 6262

DOI: 10.1021/acs.jpca.9b04177 J. Phys. Chem. A 2019, 123, 6259−6268

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The Journal of Physical Chemistry A

Figure 7. IR spectra in the product absorption regions for laserablated Sn atoms co-deposited with CCl4 and 13CCl4 in excess argon at 4 K and their variation. (a) Sn + 0.5% CCl4 in Ar co-deposited for 1 h. (b−e) As (a) after annealing to 20 K, photolysis with λ > 220 nm, photolysis with λ > 220 nm at 20 K, annealing to 30 K. (f) Sn + 0.5% 13 CCl4 in Ar co-deposited for 1 h. (g−j) As (f) after annealing to 20 K, photolysis with λ > 220 nm, photolysis with λ > 220 nm at 20 K, and annealing to 30 K. i, b, and * stand for insertion, bridged, and high-order product absorptions. p denotes precursor absorption.

Figure 9. M06 structures of the most plausible products from reactions of Sn with CH2F2, CH2FCl, and CHF3. The aug-cc-pVTZ basis sets are used for C, H, F, and Cl and the aug-cc-pVTZ-pp pseudopotential and basis set for Sn. The bond lengths and angles are in Å and degree. The chemical formula, relative energy in kJ/mol to the insertion product, molecular symmetry, and electronic state are shown below the structure.

CH2F−SnF, CHF2−SnH, CH2−SnF2, CHF−SnHF, and CH−SnHF2 are −208, −43, −117, +6, and +124 kJ/mol relative to the reactants [Sn(3P0) + CH2F2], respectively. The product with the lowest oxidation state (+2) of Sn is again the most stable among the plausible ones. CH2F−SnCl, CH2Cl− SnF, CH2−SnFCl, and CH−SnHFCl are −261, −270, −175, and +72 kJ/mol with respect to the reactants [Sn(3P0) + CH2FCl]. CH2Cl−SnF, which was the most stable, was not detected in this study. The strongest Sn−F stretching band was expected in the noisy near 500 cm−1 region and the much weaker C−F stretching band at ∼600 cm−1, making observation of them difficult. Generation of CH2F−SnCl (Figure 3) indicates that the metal atom is effectively attracted to the larger electron-rich Cl atom, providing the C−Cl insertion products during deposition and photolysis. Sn + CHF3. Figure 4 shows the product absorption regions after reactions of Sn with fluoroform isotopomers, where the product absorptions marked i increased on photolysis and sharpened on annealing. The strongest absorption at 986.1 cm−1 shifted to 933.5 cm−1 on deuteration (H/D ratio 1.056) and is assigned to the mainly CF2 antisymmetric stretching mode of CHF2−SnF (calculated frequencies 1033.2 and 953.9 cm−1). Its C−H in-plane bending, CF2 symmetric stretching, and Sn−F stretching bands were observed at 1224.0, 1046.1, and 569.1 cm−1 along with their D counterparts at 1021.3, 1054.2, and 568.7 cm−1 (H/D ratios 1.198, 0.992, and 1.001). Deuteration increased the CF2 symmetric stretching frequency by 8.1 cm−1 because of coupling with the C−H in-plane bending mode (+2.0 and +9.0 cm−1 increases predicted by M06 and B3LYP, Tables 3 and S5). The observed absorptions are the strongest bands of the insertion complex and the other ones are too weak to observe (Table S5). The other plausible products are energetically too high to produce: CHF2−SnF, CF3−SnH, CHF−SnF2, and CH− SnF3(T) are −168, −52, −64, and +24 kJ/mol with respect to the reactants [Sn(3P0) + CHF3]. Similarly, CHCl2−SnCl and CHCl−SnCl2 are −322 and −247 kJ/mol relative to the

Figure 8. M06 structures of the most plausible (insertion and methylidene) products from reactions of Sn with C2H6 and CH3F. The aug-cc-pVTZ basis sets are used for C, H, and F and the aug-ccpVTZ-pp pseudopotential and basis set for Sn. The bond lengths and angles are in Å and degree. The chemical formula, relative energy in kJ/mol to the insertion product, molecular symmetry, and electronic state are shown below the structure.

was assigned to the C−F stretching mode of CH2F−SnF on the basis of its frequency, small isotopic shift, and reasonable correlation with the calculated values (particularly with the B3LYP frequencies 981.3 and 976.9 cm−1, Table S3). Another i absorption at 559.8 cm−1 (with site absorption at 563.3 cm−1) carries its D counterpart at 560.0 cm−1, and it is designated to the Sn−F stretching mode on the basis of its frequency and negligible D shift (556.2 and 556.1 cm−1, M06). In the CH2FCl spectra, the i absorption at 972.0 cm−1 with site absorptions at 963.5 and 956.6 cm−1 showing similar increase on UV irradiation is assigned to the C−F stretching mode of CH2F−SnCl (M06 and B3LYP values 1044.3 and 980.4 cm−1). 6263

DOI: 10.1021/acs.jpca.9b04177 J. Phys. Chem. A 2019, 123, 6259−6268

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The Journal of Physical Chemistry A

CF2Cl−SnCl (Table 4) on the basis of its frequencies and reasonable correlation with the M06 values of 1154.4 and 1115.1 cm−1 and B3LYP values of 1093.4 and 1042.0 cm−1 (Table S6). Another i absorption at 729.6 cm−1 is designated to the C−Cl stretching mode of the insertion product (748.9 cm−1, M06). These observed i absorptions are the strongest bands of CF2Cl−SnCl as shown in Table S6. CF2Cl−SnCl, CFCl2−SnF, CCl2−SnF2, CF−SnFCl2, CCl− SnF2Cl(T) energies are −287, −243, −188, −138, and −99 kJ/mol with respect to the reactants [Sn(3P0) + CF2Cl2]. Our geometry optimization for CF2−SnCl2 ended up with a bridged complex CF2(Cl)−SnCl, which is energetically more stable than the insertion complex (−310 kJ/mol relative to the reactants). Moreover, its CF2 symmetric and antisymmetric stretching frequencies (M06 frequencies 1344.6 and 1291.5 cm−1) are in good correlation with the frequencies of the b absorptions (1301.3 and 1237.8 cm−1) (Table S7). The observed b absorptions in the C−F stretching region do not match with the vibrational characteristics of other plausible products. None of them gives a pair of strong C−F stretching bands in the high-frequency region. We, therefore, assign the strong b absorptions to the CF2 symmetric and antisymmetric stretching modes of the bridged complex [CF2(Cl)−SnCl] on the basis of their relatively high frequencies and good correlation with the calculated values by several theoretical methods (Table S7). These two bands are the strongest ones of CF2(Cl)−SnCl and the other bands are either too weak to observe or out of our observation range. Our previous studies have shown that the C−X frequencies are in general higher for the higher oxidation-state complexes (CX2MX2 and CXMX3, X = H or halogen).11,12 The structure of the bridged Sn complex [CF2(Cl)−SnCl] varies substantially with the theoretical methods, and so do the CF2 stretching frequencies (Figure S3 and Table S7). The M06, MP2, and CCSD frequencies are in good correlation with the observed values, whereas the B3LYP and BPW92 frequencies are too low. Employment of correlation-consistent basis sets and pseudopotential (aug-cc-pVTZ for C, F, and Cl and aug-cc-pVTZ-pp for Sn)28−30 results in better correlation in frequencies (Tables S7 and S9) and favors the structures with bridging Cl (Figures S3−S5). The shorter C−(Cl) bonds calculated by the B3LYP and BPW91 methods, which gives structures close to the insertion complex, lead to lower frequencies, whereas the longer C−(Cl) bonds resulted from the CCSD methods lead to structures more like a methylidene complex (CF2−SnCl2) and higher frequencies. Tin also yielded two sets of product absorptions (marked “i” and “b”) in reactions with CFCl3 (Figure 6). The product absorptions were weaker. Both the i and b absorptions were invisible in the original deposition spectrum; they emerged in a series of photolysis, particularly on full arc photolysis (λ > 220 nm). Whereas the i absorptions sharpened in the process of annealing, the b absorptions gradually decreased. The i absorptions at 1036.5, 724.3, and 626.7 cm−1 are assigned to the C−F stretching and CCl2 symmetric and antisymmetric stretching modes of CFCl2−SnCl, on the basis of their frequencies and correlation with the predicted values (M06 frequencies of 1192.0, 780.2, and 600.1 cm−1 and B3LYP frequencies of 1092.5, 737.9, and 655.4 cm−1) as shown in Table S8. CFCl2−SnCl is a stable product in the reaction of Sn(3P0) with CFCl3; CFCl2−SnCl, CCl3−SnF, CCl2−SnFCl, CF−SnCl3, and CCl−SnFCl2 are −323, −273, −266, −217, and −175 kJ/mol with respect to the reactants.

Figure 10. M06 structures of the most plausible products from reactions of Sn with CF2Cl2, CFCl3, and CCl4. The aug-cc-pVTZ basis sets are used for C, F, and Cl and the aug-cc-pVTZ-pp pseudopotential and basis set for Sn. The bond lengths and angles are in Å and degree. The chemical formula, relative energy in kJ/mol to the insertion product, molecular symmetry, and electronic state are shown below the structure.

reactants, but no products have been identified in a similar experiment with CHCl3 probably because of their lower absorption intensities (the strongest CCl2 antisymmetric stretching band of CHCl2−SnCl would have only 74 km/ mol intensity). The previous investigation of Pb reactions with CHCl3 also showed barely visible product absorptions.9 These results are compared with the earlier report that the Si + CHX3 reactions primarily produce CH−SiX3 and many transition metals including actinides CHMX3.7,11,12 Clearly Sn, similar to Pb, prefers a low oxidation state in generation of organotin compounds with small alkanes and halomethanes. Sn + Tetrahalomethanes. In contrast to the reactions with small alkanes and mono-, di-, and trithalomethanes, two sets of product absorptions from reactions of Sn with CF2Cl2 were observed as shown in Figure 5. The product absorptions marked “i” were barely visible in the deposition spectrum, remained almost unchanged on photolysis with λ > 420 nm, but more than tripled on photolysis with 240 < λ < 380 nm. Another set of stronger product absorptions marked “b” appeared on photolysis with 240 < λ < 380 nm and gradually decreased on annealing. The i absorptions at 1037.0 and 998.2 cm−1 are assigned to the CF2 symmetric and antisymmetric stretching modes of 6264

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CCl2 antisymmetric stretching band of CCl2(Cl)−PbCl (M06 frequency 893.3 cm−1). Although Sn highly prefers the 2+ oxidation state like Pb in reaction with small alkanes and halomethanes, generating mostly the insertion compounds in reactions with small alkanes and halomethanes, Sn is evidently more tolerable to the higher oxidation state [(2 + α)+] than Pb,8,9 producing the bridged complexes in reactions with fluorochloromethanes as shown above. This indicates that the energy gap between the s and p orbitals is smaller for Sn, which is in between those for Ge and Pb. As a result, the high oxidation-state complex becomes less favored with increasing atomic mass among the main group elements, an opposite tendency observed previously in transition-metal systems because of easier promotion between the valence s to d orbitals in heavier metals and the stronger C−M bonds.11,12 Structures and Bonding. These tin compounds all have singlet ground states other than several methylidynes (XC− SnX3, X = H or halogen) as denoted in Figures 8−10 and Table S13. The M06 structures of the most plausible products in reactions of tin with ethane and methyl fluoride are illustrated in Figure 8, which are similar to those of the Pb analogues.9 Sn employs mostly p-orbitals in bonding for the observed products. The C−Sn−H and C−Sn−F bond angles of the insertion complexes (C2H5−SnH and CH3−SnF) are 91.2 and 92.6° comparable with those of the Pb equivalents (92.1 and 93.1°).9 These bond angles near right angles suggest that the Sn atom also employs mostly p-orbitals for the bonds like the Pb atom. Natural bond orbital (NBO)44 analyses show that Sn contributes 93% p- and 7% s-characters to the C−Sn and Sn−F bonds of CH3−SnF and 80% p- and 20% scharacters to the C−Sn bond of C2H5−SnH and 92% p- and 8% s-characters to the Sn−H bond (1−2% less p-characters than those of the Pb analogues).9 The metallacyclopropane [(CH2)2−MH2] (49 kJ/mol higher than CH3CH2−SnH) was not observed in this study, though it was a common product with reasonably strong C−M bonds in reactions of transition metals and ethane.11,12 It is a weakly bound complex of SnH2 and ethylene with the two long C−Sn bonds and the structure of the ethylene moiety close to that of ethylene (the C−C bond length of 1.347 Å is comparable to 1.319 Å for ethylene). The C−C bond is still longer than that of the Pb analogue (1.337 Å),9 indicating that Sn interacts more strongly with the π orbitals of ethylene. The ethylidyne (CH3C−SnH3) in its triplet ground state is also a stable configuration, where the bond angles of the SnH3 group and four chemical bonds to Sn suggest sp3 hybridization, leading to its high energy (305 kJ/mol higher than CH3CH2− SnH). Sn contributes 75−76% p- and 24−27% s-characters to the C−Sn and Sn−H bonds according to NBO44 analysis. The unpaired electrons stay mostly on the carbon atom bonded to Sn, resulting in its Mulliken spin density45 of 1.99 (0.02 for Sn) and the CCSn bond angle of 148.3°. The CSnF bond angle of CH3−SnF (91.6°) is close to a right angle; Sn contributes 93% p-character to the C−Sn and Sn−F bond. Undetected CH2SnHF and triplet CH−SnH2F (228 and 414 kJ/mol higher in energy than CH3−SnF) are also stable configurations, where the natural bond orders44 of the C−Sn bonds are 1.92 and 0.96. CH2SnHF is near planar [(HCSnH) = 49.0°] but not agostic unlike the early transitionmetal analogues,11,12 reconfirming that the d-orbitals of a main group metal are difficult to utilize. The HCSn moiety of CH− SnH2F (oxidation state +4) is bent (148.4°) and C carries high

The bridged complex CFCl(Cl)−SnCl (−313 kJ/mol relative to the reactants) is again as stable as the insertion product. The b absorptions at 1279.2 cm−1 are designated to the C−F stretching mode of the bridged complex on the basis of its relatively high frequency and correlation with the predicted values (e.g., M06 frequency 1273.8 cm−1, Table S9). The weak b absorption at 578.6 cm−1 is tentatively assigned to the CFCl bending mode. Similarly, shown in Figure 7 are the two sets of product absorptions from reactions of laser-ablated Sn atoms with CCl4 isotopomers. The product absorptions were not visible in the original spectra, the b absorptions appeared on photolysis with λ > 220 nm, and they disappeared on photolysis with λ > 220 nm at 20 K while the i absorptions appeared. The i absorptions increased further on annealing at 30 K, and the b absorptions did not reappear. 13C substitution shifted the i absorptions at 735.7 and 603.0 cm−1 to 712.0 and 585.0 cm−1 (12/13 ratios 1.033 and 1.031), and the b absorption at 864.3 to 839.4 cm−1 (12/13 ratio 1.030). We assign the i absorptions to the A″ and A′ CCl3 antisymmetric stretching modes of CCl 3 −SnCl (M06 frequencies 711.2 and 688.9 cm−1). The i absorptions are actually the only observable bands of the insertion product, and the other bands are either too weak or out of our observation range. CCl3−SnCl, CCl2−SnCl2, and CCl−SnCl3 are −335, −318, and −249 kJ/mol with respect to the reactants. The bridged product [CCl2(Cl)−SnCl], which is −325 kJ/mol in energy relative to the reactants, is again energetically comparable to the insertion complex. The relatively high frequency of the b absorption (864.3 cm−1) is in good agreement with the predicted values for its CCl2 antisymmetric stretching mode (878.1 cm−1, M06) as shown in Table S11. The weaker CCl2 symmetric stretching band of the bridged complex expected at ∼800 cm−1 was probably covered by a strong precursor absorption in the region, and the other bands were too low in frequency to observe. The present results show that the bridged and insertion complexes [CX2(Cl)−SnCl and CX3−SnCl] are the two most stable products among the plausible ones. The bridged complexes give the relatively high CX2 stretching frequencies that match with the observed b absorptions in reactions with tetrahalomethanes although their structures and frequencies vary considerably with the calculation methods. These bridged structures via electron-rich Cl provide a testing ground for theoretical methods. The M06, MP2, and CCSD frequencies in good agreement with the observed values of the bridged complexes contrast with the B3LYP and BPW91 calculated frequencies, which are considerably lower than the measured values. The correlation basis sets and pseudopotential (aug-ccpVTZ and aug-cc-pVTZ-pp)28−30 gave better agreements for the vibrational characteristics of the Sn-bridged products. In the previous Pb + CX4 study, only the insertion complexes were produced in reactions of small alkanes and halomethanes (including fluorochloromethanes); however, in reactions of CCl4, an additional set of absorptions marked “m” were observed and designated to a methylidene product CCl2− PbCl2, which is 29 kJ/mol higher in energy.9 The present results suggest that they too probably arise from a Pb-bridged product considering the similarities in chemistry of the two elements. Our M06 calculation shows that the bridged complex is 14 kJ/mol higher in energy than the insertion complex, and the observed m absorptions at 874.7 cm−1 correlate better with the vibrational characteristics of the 6265

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The Journal of Physical Chemistry A atomic spin density (Mulliken spin density45 on C 2.11 and 0.05 for Sn). NBO44 analysis shows that the unpaired electrons of C are donated substantially to the SnH2F moiety, leading to the relatively short C−Sn bond (2.083 Å). The promotion energy of Sn from s2p2 to sp3, although it is smaller than that of Pb (205 vs 353 kJ/mol), is still considerably higher than those of transition metals (e.g., 145, 96, and 126 kJ/mol for Ti, Zr, and Hf)39 and it makes a smaller number of chemical bonds (four at most), leading to the high energies for the tin methylidynes. The M06 structures of most plausible products in reactions with CH2F2 and CHF3 are shown in Figure 9. The C−Sn−X bond angles of the insertion products (89.4 and 91.8° for CH2F−SnF and CHF2−SnF) are again close to right angles. The methylidenes (CH2SnF2 and CHFSnF2) are nonplanar [Φ(HCSnF) = 56.0 and 62.8°] and show no agostic distortion. The natural bond orders44 of the C−Sn bonds are 1.85 and 1.52. Sn contributes 27% s- and 73% p-characters to the Sn−F bonds of CH2SnF2, and similar amounts to those of CHFSnF2. These methylidenes are 91 and 104 kJ/mol higher in energy than the corresponding insertion complexes. The unobserved methylidynes in their triplet ground states are 332 and 200 kJ/mol higher in energy, where the natural bond orders of the C−M bonds are 0.95 and 0.94. Figure 10 shows the structures of most plausible products in reactions of Sn with CF2Cl2, CFCl3, and CCl4. The CSnCl angles of the insertion complexes are again close to right angles, but slightly larger than those shown in Figures 8 and 9 because of repulsion between the halogen atoms. Our attempts at geometry optimization of CF2−SnCl2 and CFCl−SnCl2 in their singlet ground states lead to the bridged complexes as described above. On the other hand, CCl2−SnCl2 is a stable energy minimum, similar to the previously studied Pb analogue.9 NBO analysis indicates that its C−Sn bond is a single bond, and single point calculation with 90° rotation along the C−Sn bond gives an increase of 10 kJ/mol. The structures of the bridged complexes with M06 are shown in Figure 10, and those calculated with other methods are in Figures S3−S5. They show that the empty 5p-orbital of the heavy group 14 metal can accept electrons from the bridging Cl atom like the empty d-orbitals of transition metals causing agostic and other distortions. The CSnCl angle of bridged complexes are slightly smaller than right angle, where Sn contributes mostly p-characters (>95%) to the C−Sn and Sn−Cl bonds. The bridged complexes are halfway between the insertion and methylidene complexes.11,12 These observed bridged products demonstrate that the p-orbitals of Sn are more accessible than those of Pb although they do not allow generation of the higher oxidation-state products. The undetected tetrahalo methylidynes (CX−SnX3) are again stable configurations although energetically much higher than the observed products; CF−SnFCl2(T), CF−SnCl3(S), and CCl−SnCl3(T) are 149, 106, and 86 kJ/mol higher in energy than the corresponding insertion products, respectively. The singlet and triplet methylidynes are often energetically comparable to one another as compared in Table S13; for example, triplet CCl−SnCl3 is only −4 kJ/mol with respect to the singlet state. The higher oxidation-state complex becomes energetically more feasible with the increasing number of Cl atoms; the C−F, C−Cl, Sn−F, and Sn−Cl bond energies are 547, 297, 495, and 359 kJ/mol for CF4, CCl4, SnF2, and SnCl2, respectively.46,47 The undetected tin methylidynes show that the energy required for promotion from 5s25p2 to 5s15p3

cannot be compensated by the larger Sn−F and Sn−Cl bond energies in reactions with small alkanes and halomethanes.

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CONCLUSIONS Direct reactions of laser-ablated Sn atoms with ethane and halomethanes have been carried out and the products identified from the matrix IR spectra on the basis of the previous results and DFT calculations. Ethane and mono-, di-, and tri-halomethanes generated one set of product absorptions showing the same behavior in the photolysis and annealing, which were assigned to the insertion products (oxidation state +2), parallel to the previously investigated Pb case.8,9 Tetrahalomethanes, on the other hand, exhibited two sets of product bands designated to the insertion and bridged complexes on the basis of the calculation results. The observed tin bridged complexes, which are halfway between the insertion and methylidene products, indicate that the higher oxidation state is more feasible for tin than lead. The C−Sn−X (X = H or halogen) moieties of the insertion and bridged complexes are highly bent (close to right angles), demonstrating that Sn uses two p-orbitals to form the new bonds, similar to Pb.8,9 NBO41 analysis also illustrates that group 14 provides mostly p-characters to the bonds. The two products are energetically comparable and most stable among the plausible products. The structure and CX2 frequencies of the bridged complexes vary substantially with theoretical methods. The undetected high oxidation-state products, which are energetically much higher, reveal that electronic promotion from 5s25p2 to 5s15p3 for Sn is almost as difficult as for Pb, but the observed Sn-bridged product p-orbitals are more accessible than for the heaviest group 14 metal Pb.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.9b04177.

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Calculated vibrational frequencies and intensities, reaction energies of Sn with ethane and halomethanes, interconversions between C2H6 + Sn and C2H5−SnH and between C2H5−SnH and (CH2)2−SnH2 and the structures of tetrahalo-bridged complexes, and Cartesian coordinates of the observed products (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-32-835-8236. ORCID

Han-Gook Cho: 0000-0003-0579-376X Lester Andrews: 0000-0001-6306-0340 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by a research grant of Incheon National University (2018-0392). We thank David A. Dixon for helpful discussions.

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REFERENCES

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