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Matrix Isolation Infrared Spectroscopic and Density Functional Theoretical Study of the Reactions of Scandium and Yttrium Monoxides with Monochloromethane Yongfei Huang,1,† Yanying Zhao,1,† Xuming Zheng,1,*,† and Mingfei Zhou2,*,‡ 1
Department of Chemistry and State Key Laboratory of ATMMT, Zhejiang Sci-Tech UniVersity, Hanzhou, People’s Republic of China and 2 Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and InnoVatiVe Materials, AdVanced Materials Laboratory, Fudan UniVersity, Shanghai 200433, People’s Republic of China ReceiVed: October 26, 2009; ReVised Manuscript ReceiVed: January 8, 2010
Reactions of scandium and yttrium monoxide molecules (ScO and YO) with monochloromethane have been studied in solid argon by infrared absorption spectroscopy and density functional theoretical calculations. The metal monoxide molecules were prepared by laser-evaporation of bulk metal oxide targets. The results show that the ground state scandium and yttrium monoxide molecules reacted with CH3Cl to form two MO(CH3Cl) (M ) Sc, Y) complex isomers spontaneously on annealing. Broad-band UV-visible irradiation initiated the addition of the Cl-C bond to the MdO bond to form the CH3OMCl molecule and the addition of the C-H bond to the MdO bond to give the CH2ClMOH isomer, both of which are more stable than the MO(CH3Cl) complex structures. The CH2ClMOH molecule was predicted to involve agnostic interaction between the chlorine atom and the metal atom. Introduction Chloromethane is a kind of volatile organic compound, which plays an important role in the industrial chemistry. It is also a major source of contamination of the environment. The catalytic oxidation of chloromethane is one of the most effective and promising methods of the waste chloromethane disposal. The reactions of transition metal centers with chloromethane may serve as the simplest model for understanding the intrinsic mechanism of the catalytic oxidation processes. The reactions of transition metal atoms with halomethane have been intensively studied in solid noble gas matrixes, which have provided a wealth of insight concerning the reactivity of bare metal atoms toward halomethane.1–4 In contrast, the reactions between transition metal oxides and halomethane have received much less attention, in part because of the experimental difficulty in preparing “pure” transition metal oxide molecules for reaction study. Pulsed laser evaporation is a conventional method in preparing neutral transition metal atoms for matrix reaction study.5–7 However, the preparation of transition metal oxides is more difficult than that of transition metal atoms. Most transition metals exhibit several oxidation states, and hence, the species evaporated from the bulk metal oxide target usually is a mixture composed of oxides with different oxidation states. We were able to prepare relatively “pure” transition metal monoxides by laser evaporation of selected bulk metal oxide targets with controlled laser energy for some transition metals. As a result, the reactions of transition metal oxides with small molecules such as H2, N2, H2O, and CH4 have been investigated using matrix isolation infrared spectroscopy.8–15 Recent investigations on the reactions of transition metal monoxide neutrals * To whom correspondence should be addressed. E-mail: mfzhou@ fudan.edu.cn. † Department of Chemistry and State Key Laboratory of ATMMT, Zhejiang Sci-Tech University. ‡ Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Advanced Materials Laboratory, Fudan University.
with methane have shown that transition metal monoxide molecules are effective in C-H bond activation.12–15 Reactions of late transition metal monoxides with methane gave the methyl metal hydroxide CH3MOH intermediate, whereas high oxidation state metallo-acetaldehyde CH3M(O)H was formed in the early transition metal monoxide reactions.12–15 In this work, we report a combined matrix isolation infrared spectroscopic and theoretical study of the reactions of scandium and yttrium monoxides with monochloromethane. Experimental Section The experimental setup for pulsed laser evaporation and matrix isolation Fourier transform infrared (FTIR) spectroscopic investigation has been described in detail previously.15 Briefly, the 1064 nm Nd:YAG laser fundamental (Spectra Physics, DCR 150, 20 Hz repetition rate and 8 ns pulse width) was focused onto the rotating bulk Sc2O3 or Y2O3 targets, which were prepared by sintered metal oxide powders. The laser-evaporated species were codeposited with chloromethane in excess argon onto a CsI window cooled normally to 6 K by means of a closed-cycle helium refrigerator (ARS, 202N). The matrix samples were deposited for one to two hours at a rate of ∼4 mmol/h. The CH3Cl/Ar samples were prepared in a stainless steel vacuum line using standard manometric technique. The CH3Cl sample was subjected to several freeze-pump-thaw cycles at 77 K before use. Isotopic-labeled 13CH3Cl and CD3Cl (ISOTEC, 99%) were used without further purification. Infrared spectra were recorded on a Bruker IFS 66v/s spectrometer with 0.5 cm-1 resolution and 0.1 cm-1 accuracy between 4000 and 450 cm-1 using a liquid nitrogen cooled HgCdTe (MCT) detector. Samples were annealed to different temperatures and cooled back to 6 K for spectral acquisition, and selected samples were subjected to broadband irradiation using a high-pressure mercury arc lamp with glass filters. Quantum chemical calculations were performed by using the Gaussian 03 program.16 The three-parameter hybrid functional according to Becke with additional correlation corrections from
10.1021/jp9101948 2010 American Chemical Society Published on Web 02/03/2010
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Figure 2. Difference spectra in the1250-650 cm-1 region from codeposition of laser-evaporated scandium oxides with isotopicsubstituted chloromethane in excess argon. (Spectrum taken after 15 min of broadband irradiation minus spectrum taken after 25 K annealing). (a) 0.5% CH3Cl, (b) 0.5% 13CH3Cl, and (c) 0.5% CD3Cl.
Figure 1. Infrared spectra in the 3800-3760, 1200-850, and 650-550 cm-1 regions from codeposition of laser-evaporated scandium oxides with 0.5% CH3Cl in argon. (a) 1 h of sample deposition at 6 K, (b) after 25 K annealing, (c) after 15 min of broadband irradiation (250 < λ 580 nm), and (d) after 25 K annealing.
Lee, Yang, and Parr (B3LYP) was utilized.17 The 6-311++G** basis set was used for the hydrogen, carbon, oxygen, and chlorine atoms, the all-electron basis set of Wachters-Hay as modified by Gaussian was used for the Sc atom, and the scalarrelativistic SDD pseudopotential and basis set was used for the Y atom.18,19 The geometries were fully optimized; the harmonic vibrational frequencies were calculated, and zero-point vibrational energies were derived. Transition-state optimizations were performed with the Berny geometry optimization algorithm at the B3LYP level. Results and Discussions ScO + CH3Cl. The scandium monoxide molecules were prepared by pulsed laser evaporation of bulk Sc2O3 target. As has been reported previously,20 pulsed laser evaporation of bulk Sc2O3 target followed by condensation with pure argon formed ScO (954.8 cm-1) as the major product with minor ScO2- (722.5 cm-1).21,22 Higher oxide species were not produced under controlled laser energy.23 Distinct new product absorptions were observed when the laser-evaporated scandium oxide molecules were codeposited with CH3Cl in excess argon. The spectra in selected regions are shown in Figure 1, while the band positions of the product absorptions are listed in Table 1. Besides the scandium oxide absorptions, weak absorptions at 898.4 and 919.1 cm-1 were observed on sample deposition (Figure 1a). Both absorptions increased on 25 K annealing (Figure 1b) at the expense of the ScO absorption. The 919.1 cm-1 absorption decreased whereas the 898.4 cm-1 absorption disappeared on subsequent broadband irradiation using the high-pressure mercury lamp (250 < λ < 580 nm), during which new absorptions
Figure 3. Difference spectra in the 3850-3600 and 2900-2700 cm-1 regions from codeposition of laser-evaporated scandium oxides with isotopic-substituted chloromethane in excess argon. (Spectrum taken after 15 min of broadband irradiation minus spectrum taken after 25 K annealing). (a) 0.5% CH3Cl, (b) 0.5% 13CH3Cl, and (c) 0.5% CD3Cl.
at 3775.0, 1171.5, and 565.6 cm-1 were produced (Figure 1c). The [ScO(Ar)5]+ absorption (976.4 cm-1) was also produced under broadband irradiation.20 No absorptions due to the Sc + CH3Cl reaction were observed in the present experiments.3 The experiments were repeated by using the isotopic-substituted 13 CH3Cl and CD3Cl samples. The isotope frequencies are also listed in Table 1. The difference spectra in selected regions using different isotopic samples are shown in Figures 2 and 3, respectively. YO + CH3Cl. The yttrium monoxide molecules were prepared by pulsed laser evaporation of bulk Y2O3 target. Co-
TABLE 1: Infrared Absorptions (cm-1) from the Reaction of ScO and CH3Cl in Solid Argon CH3Cl
13
CD3Cl
CH3Cl
molecule
assignment
obsd
calcd
obsd
calcd
obsd
calcd
ScO(CH3Cl) (Cs) ScO(CH3Cl) (C3V) CH3OScCl
ν(ScdO) ν(ScdO) ν(C-O) ν(Sc-O) ν(O-H) ν(Sc-O)
898.4 919.1 1171.5 565.6 3775.0 738.4
954.1 993.1 1161.2 563.2 3971.2 684.0
897.3 918.9 1174.2 532.4 2786.9 713.0
951.9 993.1 1163.2 539.1 2894.4 668.6
898.4 919.1 1155.4 552.8 3775.0 738.3
954.1 993.1 1145.5 555.9 3971.2 683.9
CH2ClScOH
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Figure 4. Infrared spectra in 3800-3760 and 1200-565 cm-1 regions from codeposition of laser-evaporated yttrium oxides with 0.5% CH3Cl in argon. (a) 1 h of sample deposition at 6 K, (b) after 25 K annealing, (c) after 15 min of broadband irradiation, and (d) after 25 K annealing.
deposition of the species from laser evaporation of the Y2O3 target with pure argon at 6 K resulted in strong yttrium monoxide absorptions at 843.1, 839.1, and 835.3 cm-1 in different trapping sites.24 Weak absorption at 872.0 cm-1 was also observed, which was initially attributed to the YO+ cation, but has been reassigned to the [YO(Ar)5]+ cation.25 The spectra from codeposition of laser-evaporated yttrium oxide species with 0.5% CH3Cl in argon are shown in Figure 4. Weak new absorptions at 1050.9, 805.9, and 783.5 cm-1 appeared on sample deposition and increased on 25 K annealing. The 1050.9 and 783.5 cm-1 absorptions disappeared upon broadband irradiation, whereas new absorptions at 3774.2, 1149.2, 627.6, and 490.9 cm-1 were produced. Similar experiments were performed with the isotopic-substituted 13CH3Cl and CD3Cl samples. The difference spectra in selected regions using different isotopic samples are shown in Figures 5 and 6. The band positions of the newly observed product absorptions are listed in Table 2.
Huang et al.
Figure 6. Difference spectra in the 3900-3600 and 3000-2600 cm-1 regions from codeposition of laser-evaporated yttrium oxides with isotopic-substituted chloromethane in excess argon. (Spectrum taken after 15 min of broadband irradiation minus spectrum taken after 25 K annealing). (a) 0.5% CH3Cl, (b) 0.5% CD3Cl, and (c) 0.5% 13CH3Cl.
MO(CH3Cl). The 919.1 and 898.4 cm-1 absorptions in the scandium experiments increased on annealing at the expense of the ScO absorption. Both absorptions exhibited very small isotopic shifts with CD3Cl, and no shift with 13CH3Cl, suggesting that these absorptions should be due to ScdO stretching vibrations. The band positions are about 35.7 and 56.4 cm-1 red-shifted from the ScO absorption observed at 954.8 cm-1 in solid argon, and lie in the spectral region expected for the ScO complexes. Accordingly, we assign the 919.1 and 898.4 cm-1 absorptions to the ScdO stretching mode of the ScO(CH3Cl) complexes. To validate the experimental assignment, density functional theory calculations were performed. Starting with various initial structures, we have performed geometry optimization on the ScO(CH3Cl) complex. Two stable local minima were found on the doublet potential surface, as shown in Figure 7. The complex of isomer A has a five-membered cyclic structure with Cs symmetry. The O atom of ScO is coordinated
Figure 5. Difference spectra in the 1250-550 cm-1 region from codeposition of laser-evaporated yttrium oxides with isotopic-substituted chloromethane in excess argon. (Spectrum taken after 15 min of broadband irradiation minus spectrum taken after 25 K annealing). (a) 0.5% CH3Cl, (b) 0.5% CD3Cl, and (c) 0.5%13 CH3Cl.
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TABLE 2: Infrared Absorptions (cm-1) from the Reaction of YO and CH3Cl in Solid Argon CH3Cl
13
CD3Cl
CH3Cl
molecule
assignment
obsd
calcd
obsd
calcd
obsd
calcd
YO(CH3Cl) (Cs)
ν(YdO) δ(CH3) ν(YdO) ν(C-O) ν(Y-O) ν(O-H) ν(Y-O)
783.5 1050.9 805.9 1149.2 490.9 3774.2 627.6
826.7 1066.9 856.0 1160.1 479.9 3973.4 621.4
782.5 792.4 805.4 1156.9
826.2 797.2 856.0 1163.7 456.3 2896.2 605.8
783.2 1045.4 805.6 1131.9 478.1 3774.0 627.4
826.7 1061.7 856.0 1142.9 472.4 3973.4 621.4
YO(CH3Cl) (C3V) CH3OYCl CH2ClYOH
2785.4 609.6
TABLE 3: Calculated Total Energies (In Hartree, After Zero Point Energy Corrections), Vibrational Frequencies (cm-1) and Intensities (km/mol) of the Species Mentioned in the Text (Only the Vibrations above 400 cm-1 are Listed) energy
frequency (intensity)
CH3Cl (1A1)
molecule
-500.11422
ScO (2Σ+) ScO(CH3Cl) (Cs)
-835.96297 -1336.09036
ScO(CH3Cl) (C3V)
-1336.08070
CH3OScCl (Cs)
-1336.17930
CH2ClScOH (C1)
-1336.11360
YO (2Σ+) YO(CH3Cl) (Cs)
-113.56546 -613.69378
YO(CH3Cl) (C3V)
-613.68468
CH3OYCl (C1)
-613.79366
CH2ClYOH (C1)
-613.72506
3169.9 (6 × 2), 3072.0 (26), 1478.9 (7 × 2), 1388.2 (16), 1030.7 (4 × 2), 710.4 (30) 999.5(211) 3199.5 (1), 3182.9 (11), 3062.9 (44), 1488.2 (4), 1463.9 (6), 1373.0 (1), 1058.7 (18), 1034.5 (0), 954.1 (229), 635.0 (37) 3195.8 (4 × 2), 3093.6 (32), 1472.4 (5 × 2), 1366.1 (10), 1013.8 (3 × 2), 993.1 (268), 686.1 (72) 3062.4 (31), 3060.8 (32), 2997.5 (111), 1493.6 (4), 1492.9 (6), 1473.6 (7), 1173.1 (127), 1169.7 (2), 1161.2 (369), 563.2 (102) 3971.2 (196), 3146.7 (2), 3072.4 (8), 1408.2 (8), 1083.8 (3), 1024.2 (0), 684.0 (240), 503.2 (27), 444.4 (38), 435.9 (18) 857.7 (190) 3196.6 (2), 3165.3 (14), 3036.1 (85), 1493.0 (3), 1462.3 (5), 1375.8 (0), 1066.9 (20), 1037.0 (0), 826.7 (179), 634.2 (37) 3202.7 (3 × 2), 3099.0 (31), 1470.3 (5 × 2), 1357.7 (10), 1008.5 (3 × 2), 856.0 (249), 675.4 (92) 3053.6 (36), 3052.8 (38), 2992.2 (124), 1495.6 (4), 1494.1 (5), 1474.5 (10), 1172.9 (40), 1171.2 (10), 1160.1 (372), 479.9 (53) 3973.4 (177), 3136.8 (2), 3064.8 (12), 1410.5 (9), 1086.2 (1), 1038.7 (0), 621.4 (154), 504.1 (32), 437.4 (95), 435.3 (45), 424.0 (1), 406.3 (91)
Figure 7. Optimized structures (bond lengths in Å and bond angles in deg) and relative energies (in kJ/mol) of the species involved in the ScO + CH3Cl reaction.
to one H atom of CH3Cl, and the Sc atom of ScO is coordinated to the Cl atom of CH3Cl. The O---H distance was predicted to be 2.254 Å, slightly longer than those of the OH-OB(OH) and OH-H2O complexes characterized in solid argon.26,27 The Sc---Cl distance was predicted to be 2.762 Å. The ScdO bond length of the complex increases by about 0.02 Å with respect to that of free ScO calculated at the same level of theory. The ScO(CH3Cl) complex of structure B is formed by coordinating the O atom of ScO to all three hydrogen atoms of CH3Cl with C3V symmetry. The O---H distance was predicted to be 2.952 Å, suggesting very weak interaction between ScO and CH3Cl. The ScdO bond of the ScO(CH3Cl) complex is elongated by only 0.004 Å with respect to that of free ScO. The Cs structure was predicted to be 25.5 kJ/mol more stable than the C3V isomer. The harmonic ScdO stretching vibrational frequencies of the two ScO(CH3Cl) complexes were predicted to be 954.1 and 993.1 cm-1, respectively, which are about 45.4 and 6.4 cm-1 red-shifted with respect to that of ScO calculated at the same level of theory. On the basis of the calculated ScdO stretching vibrational frequency differences, we assign the 898.4 cm-1 absorption to the ScO(CH3Cl) complex with structure A, and the 919.1 cm-1 absorption to the ScO(CH3Cl) complex with structure B. For both structures, the ScdO stretching vibrational mode was predicted to have the largest IR intensity; the other vibrational modes were predicted to have much lower IR intensities than the ScdO stretching mode (Table 3), and therefore, are unable to be experimentally observed due to weakness. Similar absorptions at 783.5 and 805.9 cm-1 in the yttrium experiments are assigned to the YdO stretching vibrations of
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the YO(CH3Cl) complex isomers. Besides the YdO stretching vibration, another absorption at 1050.9 cm-1 was observed to track with the 783.5 cm-1 absorption. On the basis of the observed isotopic frequency shifts, the 1050.9 cm-1 absorption is assigned to the CH3 deformation mode of the YO(CH3Cl) complex with Cs symmetry (Structure A, see Table 2). CH3OMCl. The absorptions at 1171.5 and 565.6 cm-1 appeared under broadband irradiation at the expense of the ScO(CH3Cl) absorptions. This observation suggests that the absorber of the 1171.5 and 565.6 cm-1 absorptions is due to a structural isomer of ScO(CH3Cl). The 1171.5 cm-1 absorption is slightly blue-shifted to 1174.2 cm-1 when the CD3Cl sample was used, whereas it shifted to 1155.4 cm-1 with the 13 CH3Cl sample. The 12C/13C isotopic ratio of 1.0139 is indicative of a C-O stretching vibration. As a reference point, the C-O stretching mode of the CH3OScH molecule was observed at 1158.5 cm-1 with a 12C/13C isotopic frequency ratio of 1.0136.28 The 565.6 cm-1 absorption shifted to 532.4 cm-1 with CD3Cl and to 552.8 cm-1 with 13CH3Cl. This absorption is assigned to the Sc-O stretching vibration. Accordingly, the 1171.5 and 565.6 cm-1 absorptions are assigned to the CH3OScCl molecule. The assignment is supported by the DFT calculations. As shown in Figure 7, the CH3OScCl molecule was predicted to have a doublet ground state without symmetry. The two experimentally observed vibrational modes were computed at 1161.2 and 563.2 cm-1, which are very close to the experimental values. As listed in Table 1, the calculated isotopic frequency shifts are in good agreement with the experimental values. Besides the above-mentioned modes, the CH3 deformation and C-H stretching modes were predicted to have appreciable intensities (Table 3), however, these modes were not observed experimentally. We suggest that density functional theoretical calculations overestimate the relative intensities of these modes. There are many examples where density functional theoretical calculations do not provide very reliable IR intensity predictions. The relative IR intensities of the vibrations such as C-H stretching are frequently overestimated.8(a),29 Similar absorptions at 1149.2 and 490.9 cm-1 in the yttrium experiments are assigned to the CH3OYCl molecule, which was calculated to have a doublet ground state without symmetry (Figure 8). The C-O and Y-O stretching modes were computed at 1160.1 and 479.9 cm-1 with isotopic shifts in good agreement with the observed values (Table 2). CH2ClMOH. The 3775.0 cm-1 absorption in the scandium experiments was also produced under broadband irradiation at the expense of the ScO(CH3Cl) absorptions. It showed no shift when the 13CH3Cl sample was used, but shifted to 2786.9 cm-1 in the experiment with the CD3Cl sample. The band position and H/D isotopic frequency ratio (1.3546) imply that the 3775.0 cm-1 absorption is due to an O-H stretching vibration. This absorption is tentatively assigned to the CH2ClScOH molecule. The O-H stretching modes of various scandium hydroxide complexes were characterized to be located in the range of 3750-3800 cm-1.30 Quantum chemical calculations at the B3LYP/6-311++G** level of theory predicted that the CH2ClScOH molecule has a 2 A′ ground state with Cs symmetry (Figure 7). The O-H and Sc-O stretching modes of CH2ClScOH were predicted at 3971.2 and 684.0 cm-1, which have the largest IR intensities. The Sc-O stretching mode cannot be clearly observed in the experiments. This mode may be overlapped by the strong CH3Cl absorption around 700 cm-1. In the difference spectrum shown in Figure 2, a weak absorption at 738.4 cm-1 seems to track
Huang et al.
Figure 8. Optimized structures (bond lengths in Å and bond angles in deg) and relative energies (in kJ/mol) of the species involved in the YO + CH3Cl reaction.
with the 3775.0 cm-1 absorption and is tentatively assigned to the Sc-O stretching mode of CH2ClScOH. The same mode of the previously characterized HScOH molecule was observed at 713.0 cm-1 in solid argon.31 It should be pointed out that hydroxide species can also be formed via highly mobile H atoms during UV photolysis. We also calculated the CH3Sc(OH)Cl compound which may be formed via highly mobile H atoms during UV photolysis. It was found that the O-H stretching vibration of singlet ground state CH3Sc(OH)Cl is very similar to that of CH2ClScOH. The Sc-C bond length of CH2ClScOH was calculated to be 2.224 Å, comparable to those of the methyl scandium hydride and scandium methylidene complexes recently characterized.3,32 The Sc-C bond is basically a single bond. It is quite interesting that the CH2ClScOH molecule involves interaction between the chlorine atom and the scandium atom. As shown in Figure 7, the CH2Cl group is distorted with the chlorine atom located close to the Sc atom: ∠ClCSc ) 77.1° and rCl---Sc ) 2.598 Å. Such interaction is quite similar to the agnostic interactions generally defined to characterize the distortion of an organometallic moiety which brings an appended C-H bond into close proximity with the metal center.33 Agostic distortion is common in the structures of alkylidene complexes of early transition metals and even more obvious in the structures of the small methylidene complexes of group 4-6 metals,4 in which agostic interactions are observed between the metal atom and one of the R-hydrogen atoms. However, the group 3 metal methylidene complexes show no agostic interaction.3 Similar absorptions at 3774.2 and 627.6 cm-1 in the yttrium experiments are assigned to the CH2ClYOH molecule. The 3774.2 cm-1 absorption shifted to 2785.4 cm-1 with CD3Cl. The band position and isotopic H/D ratio imply that this absorption is due to an O-H stretching vibration. The 627.6 cm-1 absorption showed very small shift with 13CH3Cl, but shifted to 609.6 cm-1 with CD3Cl, which is appropriate for the
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Y-O stretching vibration. The CH2ClYOH molecule was calculated to have a doublet ground state without symmetry (Figure 8). The O-H and Y-O stretching modes were computed at 3973.4 and 621.4 cm-1, with isotopic shifts in good agreement with the observed values (Table 2). The CH2ClYOH molecule also shows weak interaction between the chlorine atom and the yttrium atom. The Cl---Y distance was predicted to be 2.821 Å, and the YCCl bond angle was computed to be 81.4°. Reaction Mechanism. Laser evaporation of the Sc2O3 and Y2O3 targets produced the scandium and yttrium monoxide molecules as the major products, which were trapped in solid argon. The MO(CH3Cl) complexes were formed via the reactions of metal monoxides with CH3Cl on sample annealing, reactions 1-4. +
ScO(2
∑) +
CH3Cl(1A1) f ScO(CH3Cl)(2A′,Cs)∆E ) -34.9 kJ/mol
(1)
+
ScO(2
∑) +
CH3Cl(1A1) f ScO(CH3Cl)(2A1,C3V)∆E ) -9.4 kJ/mol
(2)
+
YO(2
∑ ) + CH3Cl(1A1) f YO(CH3Cl)(2A′,Cs)∆E ) - 37.0 kJ/mol
(3) Figure 9. Potential-energy profiles for the MO + CH3Cl (MdSc, above; MdY, under) reactions calculated at the B3LYP/6-311++G(d, p) level of theory (values are given in kJ/mol).
+
2
YO(
∑) +
CH3Cl( A1) f YO(CH3Cl)( A1,C3V)∆E ) 1
2
- 13.0 kJ/mol
(4)
Two MO(CH3Cl) isomers were formed in both the scandium and yttrium experiments. On the basis of DFT calculations, the complex with Cs structure is more stable than the complex with C3V structure by 25.5 (Sc) or 24.0 (Y) kJ/mol. The binding energies of the ScO(CH3Cl) complexes are significantly higher than that of the ScO(CH4) complex calculated at the same level of theory (4.6 kJ/mol).28 The MO(CH3Cl) complex absorptions decreased on broadband irradiation, during which the CH3OMCl and CH2ClMOH absorptions were produced. This observation suggests that the CH3OMCl and CH2ClMOH molecules were generated from the MO(CH3Cl) complexes via photoinduced isomerization reactions 5 and 6. Filtered experiment with high pressure mercury arc lamp indicates that only the light in the wavelength range of 250-300 nm is responsible for the observed photochemistry in solid argon. Both the Cs and C3V complex absorptions decreased upon UV irradiation, therefore, it is not able to determine whether the geometry of the complex determines the final photoproducts. The CH3OMCl molecule can be regarded as being formed via the addition of the Cl-C bond to the MdO bond, whereas the CH2ClMOH isomer is formed via the addition of the C-H bond to the MdO bond. According to the DFT calculations, the CH3OScCl structure is more stable than the CH2ClScOH isomer by about 173.0 kJ/mol. Both CH3OScCl and CH2ClScOH are more stable than the ScO(CH3Cl) complex isomers. The CH3OScCl isomer is about 233.9 kJ/mol more stable than the ScO(CH3Cl) complex with Cs symmetry. The observation of CH3OMCl and CH2ClMOH only under broadband irradiation indicates that the formation of
CH3OMCl and CH2ClMOH from MO(CH3Cl) requires activation, and some excited states may be involved.
MO(CH3Cl) + hν f CH3OMCl
(5)
MO(CH3Cl) + hν f CH2ClMOH
(6)
The potential energy surface for the reaction of MO with CH3Cl was calculated and is shown in Figure 9. The initial step of the ScO + CH3Cl reaction is the formation of the ScO(CH3Cl) complex. The addition of the Cl-C bond to the ScdO bond to form the CH3OScCl molecule from the complex was predicted to proceed via a transition state with an energy barrier of 17.7 kJ/mol, while the addition of the C-H bond to the ScdO bond to form the CH2ClScOH isomer also proceeded via a transition state with a much higher energy barrier of 160.1 kJ/mol. The CH3OScCl structure is 173.0 kJ/mol more stable than the CH2ClScOH isomer. The potential energy profile of the ScO + CH3Cl reaction is energetically different from that of the ScO + CH4 reaction. Early calculations at the DFT/B3LYP level showed that scandium monoxide is unable to form a stable molecular complex with methane, instead, the reaction proceeds by insertion of ScO into the C-H bond to form the CH3ScOH intermediate via a transition state.34 More recent calculations at the B3LYP/6-311++G(d,p) level indicated that the initial step of the ScO + CH4 reaction is the formation of a very weakly bound ScO(CH4) complex. The reaction path with the formation of CH3ScOH via hydrogen atom migration is
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energetically more favorable than another path to form the CH3OScH intermediate. The CH3ScOH molecule was predicted to be 82.8 kJ/mol more stable than the CH3OScH isomer. Conclusions Reactions of scandium and yttrium monoxide molecules with monochloromethane were carried out, and the resulting matrix infrared absorption spectra of the reaction intermediates and products were reported. The results showed that the ground state scandium and yttrium monoxide molecules reacted with CH3Cl to form two MO(CH3Cl) (M ) Sc, Y) complex isomers spontaneously on annealing. One complex structure was predicted to have C3V symmetry with the O atom of MO coordinated to all three H atoms of CH3Cl. Another isomer was characterized to have Cs symmetry with the Cl atom and one of H atom of CH3Cl coordinated to the O and M atoms of MO, respectively, which was predicted to be more stable than the C3V isomer. Broad-band irradiation initiated either the addition of the Cl-C bond to the MdO bond to form the CH3OMCl molecules or the addition of the C-H bond to the MdO bond to give the CH2ClMOH isomer. The CH2ClMOH molecules were calculated to involve interaction between the chlorine atom and the metal atom, which is quite similar to the agnostic interactions generally defined to characterize the distortion of an organometallic moiety which brings an appended C-H bond into close proximity with the metal center. Acknowledgment. We gratefully acknowledge finical support from National Natural Science Foundation of China (Grant Nos. 20803066 and 20773030), Zhejiang Provincial Natural Science Foundation (Grant No. Y4090161) and National Basic Research Program of China (2007CB815203 and 2010CB732306). References and Notes (1) Andrews, L.; Cho, H. G. Organometallics 2006, 25, 4040. (2) Cho, H. G.; Andrews, L. J. Phys. Chem. A 2006, 110, 10063. (3) (a) Cho, H. G.; Andrews, L. J. Phys. Chem. A 2007, 111, 2480. (b) Cho, H. G.; Andrews, L. Inorg. Chim. Acta 2008, 361, 551. (4) (a) Cho, H. G.; Lyon, J. T.; Andrews, L. Organometallics 2008, 27, 5241. (b) Cho, H. G.; Andrews, L. Inorg. Chem. 2008, 47, 1653. (c) Cho, H. G.; Andrews, L. Organometallics 2009, 28, 1358. (5) Bondybey, V. E.; Smith, A. M.; Agreiter, J. Chem. ReV. 1996, 96, 2113. (6) Zhou, M. F.; Andrews, L.; Bauschlicher, C. W., Jr. Chem. ReV. 2001, 101, 1931. (7) Gong, Y.; Zhou, M. F.; Andrews, L. Chem. ReV. 2009, 109, 6765. (8) (a) Zhou, M. F.; Zhang, L. N.; Shao, L. M.; Wang, W. N.; Fan, K. N.; Qin, Q. Z. J. Phys. Chem. A 2001, 105, 10747. (b) Zhang, L. N.; Zhou, M. F.; Shao, L. M.; Wang, W. N.; Fan, K. N.; Qin, Q. Z. J. Phys. Chem. A 2001, 105, 6998. (9) (a) Zhou, M. F.; Zhang, L. N.; Qin, Q. Z. J. Phys. Chem. A 2001, 105, 6407. (b) Zhou, M. F.; Wang, G. J.; Zhao, Y. Y.; Chen, M. H.; Ding, C. F. J. Phys. Chem. A 2005, 109, 5079. (c) Chen, M. H.; Wang, G. J.; Zhou, M. F. Chem. Phys. Lett. 2005, 409, 70. (10) Shao, L. M.; Zhang, L. N.; Chen, M. H.; Lu, H.; Zhou, M. F. Chem. Phys. Lett. 2001, 343, 178.
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