Preparation and Characterization of the Agostic Bonding Molecules

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J. Phys. Chem. A 2010, 114, 5779–5786

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Preparation and Characterization of the Agostic Bonding Molecules between Metal and Chlorine from the Reactions of Niobium and Tantalum Monoxide and Dioxide Molecules with Monochloromethane in Solid Argon Yanying Zhao,† Yongfei Huang,† Xuming Zheng,† and Mingfei Zhou*,‡ Department of Chemistry and State Key Laboratory of ATMMT (MOE), Zhejiang Sci-Tech UniVersity, Hanzhou, 310018, China, and Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and InnoVatiVe Materials, Fudan UniVersity, Shanghai 200433, China ReceiVed: March 10, 2010; ReVised Manuscript ReceiVed: April 9, 2010

Reactions of niobium and tantalum monoxide and dioxide molecules with monochloromethane in solid argon have been investigated by infrared absorption spectroscopy and density functional theoretical calculations. The results show that the ground-state MOx (M ) Nb, Ta, x ) 1, 2) molecules react with CH3Cl to form the weakly bound MO(CH3Cl) and MO2(CH3Cl) complexes. The MO(CH3Cl) complexes rearrange to the more stable CH2ClM(O)H isomer upon visible light excitation, whereas the MO2(CH3Cl) complexes isomerize to the more stable CH2ClM(O)OH molecules under ultraviolet light irradiation. The CH2ClM(O)H and CH2ClM(O)OH molecules were predicted to involve agostic interactions between the chlorine atom and the metal center. Introduction The catalytic selective oxidation of methane to methanol under mild conditions is of great economic and scientific importance.1 Whereas the catalytic processes consist of a complicated sequence of interrelated reactions, the investigation of the MO+ + CH4 f M+ + CH3OH and MO + CH4 f M + CH3OH reactions and their reverse reactions can potentially provide quantitative information regarding the thermodynamics and mechanisms for the catalytic methane-to-methanol conversion processes. Such model reactions have been extensively studied both experimentally and theoretically.2-20 Investigations on the reactions of transition metal monoxide neutrals with methane have shown that transition metal monoxide molecules are effective in C-H bond activation.15-17 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.20 Methyl monohalides, being one of the simplest functional derivatives of methane, are potentially important intermediates in the production of methanol. Converting methane first to methyl monohalides and then to methanol offers an alterative effective route of methane-to-methanol conversion. The catalytic oxidation of halomethane is one of the most effective and promising methods of converting halomethane to methanol. Whereas the model reactions of transition metal atoms with halomethane have been intensively studied in solid noble gas matrixes,21-24 which have provided a wealth of insight into the reactivity of bare metal atoms toward halomethane, the reactions of transition metal oxides with halomethane have received much less attention. Recently, the reactions of scandium and yttrium monoxide molecules with monochloromethane have been studied in solid argon by infrared absorption spectroscopy and * To whom correspondence should be addressed. E-mail: mfzhou@ fudan.edu.cn. † Zhejiang Sci-Tech University. ‡ Fudan University.

density functional theoretical calculations.25 The results show that the ground-state scandium and yttrium monoxide molecules react with CH3Cl to form the weakly bound MO(CH3Cl) (M ) Sc, Y) complexes, which isomerize to the more stable CH3OMCl and CH2ClMOH isomers upon UV excitation. In this article, we report a combined matrix isolation infrared spectroscopic and theoretical study of the reactions of niobium and tantalum monoxide and dioxide molecules with monochloromethane. Experimental and Computational Methods The metal oxide reactants were prepared by pulsed laser evaporation of bulk metal oxide targets, which has proven to be an effective method in preparing relatively “pure” transition metal monoxides or dioxides in solid argon for matrix isolation spectroscopic studies.26-28 The experimental setup for pulsed laser evaporation and matrix isolation Fourier transform infrared (FTIR) spectroscopic investigation has been previously described in detail.20 In brief, 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 Nb2O5 or Ta2O5 targets, which were prepared by sintered metal oxide powders. The laser-evaporated species were codeposited with monochloromethane 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 1 to 2 h 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%) samples were used without further purification. Infrared spectra were recorded on a Bruker IFS 66 v/s spectrometer at 0.5 cm-1 resolution 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 visible or broadband irradiation using a tungsten or highpressure mercury arc lamp with glass filters.

10.1021/jp102199c  2010 American Chemical Society Published on Web 04/20/2010

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Figure 1. Infrared spectra in 1720-1630 and 1050-850 cm-1 regions from codeposition of laser-evaporated niobium oxides with 0.5% CH3Cl in argon using relatively high laser energy. (a) 1 h of sample deposition at 6 K, (b) after 25 K annealing, (c) after 15 min of visible light irradiation (λ > 500 nm), and (d) after 25 K annealing.

Quantum chemical calculations were performed to support the experimental assignment. The three-parameter hybrid functional according to Becke with additional correlation corrections from Lee, Yang, and Parr (B3LYP) was utilized.29 The 6-311++G(3df, p) basis set was used for the hydrogen, carbon, oxygen, and chlorine atoms, the DGDZVP basis set was used for the Nb atom, and the scalar-relativistic SDD pseudopotential and basis set was used for the Ta atom.30,31 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 same B3LYP level. All of these calculations were performed using the Gaussian 03 program.32 Results and Discussion The niobium and tantalum monoxide and dioxide reactants were prepared by pulsed laser evaporation of bulk Nb2O5 and Ta2O5 targets. Laser evaporation of the Nb2O5 and Ta2O5 targets, followed by condensation with pure argon at 6 K, formed the metal monoxides (NbO, 963.9 cm-1 with site absorptions at 965.2, 967.4, and 970.6 cm-1; TaO, 1014.3 cm-1) and dioxides (NbO2, 931.2/933.5 and 869.9/875.9 cm-1; TaO2, 907.0 and 965.3 cm-1) as the major products.33,34 Recent investigation indicates that the NbO2 molecule trapped in solid argon is coordinated by two argon atoms and therefore should be regarded as the NbO2(Ar)2 complex.34 Weak absorptions due to NbO4(Ar) and TaO4(Ar) were also observed.34 These niobium and tantalum oxide species have previously been identified from the effects of isotopic substitution in their infrared spectra and from comparison to frequencies calculated by using density functional theory.33,34 The relative yields of monoxide and dioxide depend strongly on the evaporation laser energy. The metal dioxide molecules are favored with relatively low evaporation laser energy, whereas the metal monoxide molecules are favored with relatively high laser energy. The infrared spectra in selected regions from codeposition of niobium oxide species with 0.5% CH3Cl in argon using relatively high laser energy (∼7.5 mJ/pulse) are shown in Figure 1. Besides the strong CH3Cl absorptions, the NbO absorption dominates the spectrum in the Nb-O stretching frequency region after sample deposition. A weak absorption at 935.6 cm-1

Zhao et al.

Figure 2. Infrared spectra in 3700-3650, 1050-860, and 675-550 cm-1 regions from codeposition of laser-evaporated niobium oxides with 0.5% CH3Cl in argon using relatively low laser energy. (a) 1 h of sample deposition at 6 K, (b) after 25 K annealing, (c) after 15 min of full-arc photolysis (250 nm < λ < 380 nm), and (d) after 25 K annealing.

was also observed on sample deposition and increased on sample annealing to 25 K at the expense of the NbO absorption (Figure 1, trace b). The 935.6 cm-1 absorption was destroyed when the sample was subjected to broadband irradiation using the tungsten lamp with a 500 nm long-wavelength pass filter (Figure 1, trace c), during which two new absorptions at 1698.0 and 985.0 cm-1 were produced. Similar experiment with relatively low evaporation laser energy (∼5 mJ/pulse) was also performed, and the spectra in selected regions are shown in Figure 2. As can be seen, the NbO2(Ar)2 absorptions dominate the spectrum in the Nb-O stretching frequency region after sample deposition (Figure 2, trace a), whereas the NbO absorption is barely observed. When the sample was annealed to 25 K (Figure 2, trace b), two weak absorptions at 948.1 and 890.9 cm-1 were produced at the expense of the NbO2(Ar)2 absorptions. The 948.1 and 890.9 cm-1 absorptions almost disappeared, whereas a group of new absorptions at 3678.4, 979.4, and 712.8 cm-1 appeared when the sample was subjected to broadband irradiation with the unfiltered high-pressure mercury arc lamp (250 < λ < 380 nm, Figure 2, trace c). The 3678.4, 979.4, and 712.8 cm-1 absorptions slightly sharpened upon further sample annealing to 35 K (Figure 2, trace d). Similar experiments were also performed using the isotopiclabeled 13CH3Cl/Ar and CD3Cl/Ar samples. The resulting difference IR spectra in selected regions are shown in Figure 3. The band positions of the newly observed product absorptions are listed in Table 1. The infrared spectra in selected regions from codeposition of tantalum oxide species with 0.5% CH3Cl in argon using different laser energies are shown in Figures 4 and 5, respectively. The tantalum monoxide is the dominant species from relatively high laser energy evaporation. As shown in Figure 4, weak absorption at 991.5 cm-1 was produced upon sample annealing to 25 K (Figure 4, trace b). This absorption disappeared under visible (λ > 500 nm) light irradiation, whereas two new absorptions at 1760.0 and 984.8 cm-1 were produced (Figure 4, trace c). In the experiment with relatively high laser energy, the tantalum dioxide is the dominant species after sample deposition (Figure 5, trace a), and two new absorptions at 948.4 and 890.8 cm-1 were produced on 25 K annealing when the TaO2 absorptions decreased (Figure 5, trace b). When the sample was subjected to broadband irradiation with the unfiltered highpressure mercury arc lamp (250 < λ < 380 nm, Figure 5, trace

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Figure 3. Difference spectra in the 3750-3475, 2800-2650, and 1045-760 cm-1 regions from codeposition of laser-evaporated niobium oxides with isotopic-substituted monochloromethane in excess argon. (Spectrum taken after 15 min of full-arc photolysis minus spectrum taken after sample annealing at 25 K. (a) 0.5% CH3Cl, (b) 0.5% 13 CH3Cl, and (c) 0.5% CD3Cl.

TABLE 1: Infrared Absorptions (inverse centimeters) from the Reactions of NbOx (x ) 1, 2) with CH3Cl in Solid Argona CH3Cl molecule

13

CH3Cl

Figure 5. Infrared spectra in the 3700-3650, 1025-850, and 620-470 cm-1 regions from codeposition of laser-evaporated tantalum oxides with 0.5% CH3Cl in argon using relatively low laser energy. (a) 1 h of sample deposition at 6 K, (b) after 25 K annealing, (c) after 15 min of full-arc photolysis (250 nm < λ < 380 nm), and (d) after 25 K annealing.

CD3Cl

assignment obsd calcd obsd calcd obsd calcd

NbO(CH3Cl) CH2ClNb(O)H

ν(Nb-O) 935.6 941.7 935.6 941.7 935.6 941.6 ν(Nb-H) 1698.0 1722.0 1698.0 1722.0 1220.0 1225.2 ν(Nb-O) 985.0 982.7 984.7 982.5 985.5 982.8 NbO2(CH3Cl) ν(Nb-O) 948.1 928.7 948.1 928.7 948.0 928.6 ν(Nb-O) 890.9 888.7 890.9 888.6 890.9 888.5 CH2ClNb(O)OH ν(O-H) 3678.4 3892.7 3678.1 3892.6 2713.9 2834.9 ν(Nb-O) 979.4 969.2 979.1 969.0 977.8 970.1 ν(Nb-OH) 712.8 667.0 712.4 666.6 697.3 646.5

a Calculated vibrational frequencies at the B3LYP/6-311++G(3df,p)// DGDZVP level are also listed for comparison.

Figure 6. Difference spectra in the 3800-3600, 2750-2700, 1000-870, and 740-610 cm-1 regions from codeposition of laser-evaporated tantalum oxides with isotopic-substituted monochloromethane in excess argon (spectrum taken after 15 min of full-arc photolysis minus spectrum taken after sample annealing at 25 K). (a) 0.5% CH3Cl, (b) 0.5% 13CH3Cl, and (c) 0.5% CD3Cl.

Figure 4. Infrared spectra in 1800-1750 and 1035-960 cm-1 regions from codeposition of laser-evaporated tantalum oxides with 0.5% CH3Cl in argon using relatively high laser energy. (a) 1 h of sample deposition at 6 K, (b) after 25 K annealing, (c) after 15 min of visible light irradiation (λ > 500 nm), and (d) after 25 K annealing.

c), the 948.4 and 890.8 cm-1 absorptions were destroyed, and four new absorptions at 3690.8, 975.9, 605.8, and 495.7 cm-1 were produced. Similar experiments were performed with the isotopic-substituted 13CH3Cl and CD3Cl samples. The resulting difference IR spectra in selected regions are shown in Figure

6. The band positions of the newly observed product absorptions for tantalum are listed in Table 2. MO(CH3Cl). The 935.6 cm-1 absorption increased on annealing at the expense of the NbO absorption. This absorption is strong in the experiments with relatively high laser energy, in which NbO is the dominant evaporation species. It shows no shift with CD3Cl and 13CH3Cl. The band position is ∼28.3 cm-1 red-shifted from the major site of NbO absorption in solid argon, which suggests that the 935.6 cm-1 absorption is due to the NbdO stretching vibration of a complex of NbO. Therefore, we assign the 935.6 cm-1 absorption to the NbdO stretching mode of the NbO(CH3Cl) complex. Similar absorption at 991.5 cm-1 in the tantalum experiments is assigned to the TaO(CH3Cl) complex, which is ∼22.8 cm-1 red-shifted from the TaO absorption under solid argon.

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TABLE 2: Infrared Absorptions (inverse centimeters) from the Reactions of TaOx (x ) 1, 2) with CH3Cl in Solid Argona CH3Cl molecule

13

CH3Cl

CD3Cl

assignment obsd calcd obsd calcd obsd calcd

TaO(CH3Cl) (Cs) CH2ClTa(O)H

ν(Ta-O) 991.5 1005.0 991.3 1005.0 991.2 1004.3 ν(Ta-H) 1760.0 1803.9 1760.0 1803.9 1259.7 1280.0 ν(Ta-O) 984.8 978.8 984.8 978.7 984.7 977.5 948.4 949.0 948.4 949.0 948.4 948.8 TaO2(CH3Cl) (Cs) ν(Ta-O) ν(Ta-O) 890.8 904.3 890.8 904.3 890.8 904.2 CH2ClTa(O)OH ν(O-H) 3690.8 3909.2 3690.8 3909.2 2722.8 2847.6 ν(Ta-O) 975.9 963.8 975.9 963.7 975.9 963.9 ν(Ta-OH) 605.8 685.0 684.8 660.1 δ(TaOH) 495.7 527.5 526.3 414.4

a Calculated vibrational frequencies at the B3LYP/6-311++G(3df,p)/ SDD level are also listed for comparison.

Figure 8. Optimized structures (bond lengths in angstroms, bond angles in degrees) of the species involved in the tantalum system.

Figure 7. Optimized structures (bond lengths in angstroms, bond angles in degrees) of the species involved in the niobium system.

To validate the experimental assignment, density functional theory calculations were performed. We have performed geometry optimization on the NbO(CH3Cl) complex with various initial structures. As shown in Figure 7, the NbO(CH3Cl) complex was predicted to have a stable quartet ground state with Cs symmetry, in which the Nb atom of NbO is coordinated to the Cl atom of CH3Cl with a Cl · · · Nb distance of 2.848 Å. The NbdO bond length (1.734 Å) is ∼0.014 Å longer than that of free NbO (1.720 Å) calculated at the same level of theory. The TaO(CH3Cl) complex was calculated to have a doublet ground state without symmetry (Figure 8). In this complex, the Ta atom of TaO is coordinated to the Cl atom of CH3Cl with a Cl · · · Ta distance of 3.928 Å, and the O atom of TaO is also coordinated to one H atom of CH3Cl to form a weak hydrogen bond with an O · · · H bond distance of 2.469 Å. The TadO bond length is elongated by 0.007 Å upon CH3Cl coordination. The harmonic MdO stretching vibrational frequencies of the two MO(CH3Cl) complexes were predicted to be 941.7 and 1005.0 cm-1, respectively, which are about 33.3 and 9.5 cm-1 red-shifted from those of free MO calculated at the same level of theory. For both complexes, the MdO stretching vibrational mode was predicted to have the largest IR intensity (248 km/ mol for NbO(CH3Cl) and 96 km/mol for TaO(CH3Cl)); the other vibrational modes were predicted to have much lower IR intensities than the MdO stretching mode (Table 3) and therefore are too weak to be observed.

The ground-state NbO(CH3Cl) and TaO(CH3Cl) complexes correlate to the ground-state NbO (4Σ) and TaO (2∆). The binding energies were predicted to be 29.5 (Nb) and 5.0 kJ/ mol (Ta), which are larger than the corresponding values of NbO(CH4) and TaO(CH4). The NbO(CH4) and TaO(CH4) complexes were predicted to be very weakly bound with the metal atom being coordinated to three hydrogen atoms of CH4.17 CH2ClM(O)H. The absorptions at 1698.0 and 985.0 cm-1 appeared together under λ > 500 nm visible light irradiation at the expense of the NbO(CH3Cl) absorption. This observation suggests that the 1698.0 and 985.0 cm-1 absorptions are due to a structural isomer of the NbO(CH3Cl) complex. The 1698.0 cm-1 absorption showed no shift with the 13CH3Cl sample but red-shifted to 1220.0 cm-1 when the CD3Cl sample was used. The resulting H/D isotopic frequency ratio of 1.392 indicates that the 1698.0 cm-1 absorption is due to a Nb-H stretching vibration. The 985.0 cm-1 absorption exhibited very small shifts with both the CD3Cl and 13CH3Cl samples (Table 1). The band position and isotopic shifts indicate that the 985.0 cm-1 absorption is due to a terminal NbdO stretching vibration. Accordingly, the 1698.0 and 985.0 cm-1 absorptions are assigned to the CH2ClNb(O)H molecule. The same modes of the analogous CH3Nb(O)H molecule were observed at 1686.6 and 975.8 cm-1 in solid argon.17 Similar absorptions at 1760.0 and 984.8 cm-1 in the tantalum experiments are assigned to the Ta-H and TadO stretching vibrational modes of the CH2ClTa(O)H molecule. The assignment is supported by DFT frequency calculations. As shown in Figure 7, the CH2ClNb(O)H molecule was predicted to have a doublet ground state without symmetry. The Nb-H and NbdO stretching vibrational frequencies were computed at 1722.0 and 982.7 cm-1, which are very close to the experimentally observed values. As listed in Table 1, the calculated isotopic frequency shifts are also in good agreement with the experimental values. The Nb-C bond length of CH2ClNb(O)H was calculated to be 2.176 Å, comparable to those of the methyl niobium hydride and niobium methylidene complexes recently characterized.21,22,36,37 The Nb-C bond is basically a single bond. The CH2ClTa(O)H molecule also was calculated to have a doublet ground state without symmetry (Figure 8) with the predicted Ta-H and TadO stretching

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TABLE 3: Calculated (B3LYP/6-311++G(3df,p)/DGDZVP/SDD) Total Energies (in hartree, after Zero Point Energy Corrections), Vibrational Frequencies (inverse centimeters), and Intensities (kilometers per mole) of the Species Mentioned in the Texta molecule

energy

frequency (intensity)

CH3Cl ( A1, C3V) NbO (4Σ-) NbO(CH3Cl) (C1, 4A)

-500.120179 -3830.448806 -4330.580196

CH2ClNb(O)H (C1, 2A)

-4330.620841

NbO2 (2A1, C2V) NbO2(Ar)2 (2A1, C2V) NbO2(CH3Cl) (C1, 2A)

-3905.755973 -1262.595632 -4405.904898

CH2ClNb(O)OH (C1, 2A)

-4405.946540

TaO (2∆) TaO(CH3Cl) (Cs, 2A′′)

-132.219913 -632.342052

CH2ClTa(O)H (C1, 2A)

-632.409377

TaO2 (2A1, C2V) TaO2(CH3Cl) (Cs, 2A′)

-207.554664 -707.694897

CH2ClTa(O)OH (C1, 2A)

-707.739987

3166.2 (4 × 2), 3070.9 (24), 1479.5 (6 × 2), 1375.1 (12), 1026.6 (2 × 2), 715.6 (27) 975.0(188) 3194.3(1), 3190.1(1), 3082.9(16), 1471.0(17), 1469.6(20), 1370.7(1), 1030.2(3), 1027.5(0), 941.7(248), 669.9(42) 3191.7(0), 3097.6(1), 1722.0(397), 1401.5(3), 1066.7(5), 1017.6(3), 982.7(197), 582.2(24), 565.5(53), 526.4(16), 442.4(22) 954.1(134), 924.7(264) 923.3(64), 877.5(306) 3199.3(3), 3196.9(4), 3077.8(17), 1476.8(8), 1461.6(9), 1373.3(5), 1047.3(12), 1032.3(1), 928.7(107), 888.7(296), 634.3(35) 3892.7(171), 3193.3(0), 3102.3(2), 1402.8(4), 1070.9(2), 1022.7(1), 969.2(205), 667.0(136), 575.6(5), 566.1(29), 501.5(137), 430.8(37) 1014.5 (102) 3177.9(1), 3171.9(3), 3073.5(29), 1484.5(3), 1476.7(3), 1376.9(5), 1036.2(7), 1030.0(1), 1005.0(96), 699.0(29) 3183.0(0), 3093.8(1), 1803.9(294), 1407.5(4), 1085.9(1), 1012.5(2), 978.8(158), 624.1(34), 591.8(7), 555.7(13), 412.9(22) 975.1(62), 927.9(193) 3203.4(0), 3193.4(11), 3078.4(11), 1476.8(4), 1468.4(6), 1378.2(3), 1052.2(6), 1031.6(0), 949.0(96), 904.3(244), 634.1(43) 3909.2(192), 3185.7(0), 3102.1(2), 1409.0(5), 1087.9(0), 1015.1(1), 963.8(157), 685.0(88), 585.4(0), 569.8(28), 527.5(160), 413.8(46), 406.0(69)

1

a

Only the vibrations above 400 cm-1 are listed.

vibrational frequencies and isotopic frequency shifts in good agreement with the observed values (Table 2). MO2(CH3Cl). The 948.1 and 890.9 cm-1 absorptions were produced on annealing at the expense of the NbO2(Ar)2 absorptions. These absorptions are strong in the experiments with relatively low evaporation laser energy, in which NbO2 is the dominate evaporation species. Both absorptions exhibited very small shifts with either the CD3Cl or 13CH3Cl samples. The band positions are about 16.9 and 21.0 cm-1 blue-shifted from those of NbO2(Ar)2 in solid argon. Hence, the 948.4 and 890.8 cm-1 absorptions are most likely due to the symmetric and antisymmetric NbO2 stretching vibrations of a NbO2(CH3Cl) complex. The suggested assignment of NbO2(CH3Cl) complex was supported by DFT calculations. As shown in Figure 7, the NbO2(CH3Cl) complex was predicted to have a doublet ground state with the Nb atom and one O atom of the NbO2 fragment being coordinated to the Cl atom and one H atom of the CH3Cl fragment, respectively. The O · · · H and Cl · · · Nb distances were computed to be 2.513 and 2.642 Å. Similar absorptions at 948.4 and 890.8 cm-1 in the tantalum experiments are assigned to the symmetric and antisymmetric TaO2 stretching vibrations of the TaO2(CH3Cl) complex, which are 16.9 and 16.2 cm-1 red-shifted from those of TaO2 observed in solid argon. The TaO2(CH3Cl) complex was predicted to have a doublet ground state with Cs symmetry (Figure 8). The O · · · H and Cl · · · Ta distances were predicted to be 2.660 and 2.757 Å, slightly longer than the corresponding distances of the NbO2(CH3Cl) complex. The binding energies of the NbO2(CH3Cl) and TaO2(CH3Cl) complexes were calculated to be 75.6 and 52.9 kJ/mol, respectively, significantly larger than those of the monoxide complexes. Previous theoretical study on transition metal oxide-methane complexes found that the binding energies of high oxidative complexes are larger than those of low oxidative complexes.14 The calculated harmonic vibrational frequencies of the MO2(CH3Cl) complexes are listed in Table 3. For both complexes, the experimentally observed symmetric and antisymmetric MO2 stretching modes were predicted to have the

largest IR intensities. Note that the NbO2 molecule trapped in solid argon is coordinated by two argon atoms and should be regarded as the NbO2(Ar)2 complex, whereas the TaO2 molecule is not coordinated by argon atom(s).34 The symmetric and antisymmetric NbO2 stretching modes of the NbO2(CH3Cl) complex were predicted to be blue-shifted from those of the NbO2(Ar)2 complex, whereas those of the TaO2(CH3Cl) complex are red-shifted with respect to those of TaO2 calculated at the same level, which are consistent with the experimental observations. CH2ClM(O)OH. The absorptions at 3678.4, 979.4, and 712.8 cm-1 appeared together at the expense of the NbO2(CH3Cl) absorptions and are assigned to different vibrational modes of the CH2ClNbO(OH) molecule, a structural isomer of NbO2(CH3Cl). The 3678.4 cm-1 shifted to 2713.9 cm-1 in the experiment with CD3Cl. The band position and H/D isotopic frequency ratio (1.3546) imply that this absorption is due to an O-H stretching vibration. The 712.8 cm-1 absorption is the corresponding Nb-OH stretching mode. The same modes of various niobium hydroxide complexes were characterized to be located in these frequency ranges.17,35 The 979.4 cm-1 absorption exhibited very small carbon-13 and deuterium isotopic shifts and is due to the NbdO stretching vibration. The same modes of the analogous CH3Nb(O)OH molecule were observed at 3707.8 and 960.5 cm-1 in solid argon.17 The NbdO stretching frequency of CH2ClNb(O)OH is higher, whereas the O-H stretching frequency is lower than those of CH3Nb(O)OH, which suggests that the NbdO and Nb-OH bonds in CH2ClNb(O)OH are slightly stronger than those in CH3Nb(O)OH because of increased electrostatic interactions. The CH2ClNb(O)OH molecule was predicted to have a doublet ground state without symmetry (Figure 7). The Nb-C bond length was calculated to be 2.186 Å, slightly longer than that of CH2ClNb(O)H. The Nb-C bond also is a single bond. The O-H, NbdO, and Nb-OH stretching modes were predicted at 3892.7, 969.2, and 667.0 cm-1 with the isotopic frequency shifts (Table 1) in good agreement with the experimental values.

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Similar absorptions at 3690.8, 975.9, 605.8, and 495.7 cm-1 in the tantalum experiments are assigned to different vibrational modes of the CH2ClTa(O)OH molecule (Table 2). The 3690.8 cm-1 absorption is due to the O-H stretching vibration. The 975.9 cm-1 absorption showed a very small shift with both 13 CH3Cl and CD3Cl and is appropriate for the TadO stretching vibration. The 605.8 and 495.7 cm-1 absorptions are due to the Ta-OH stretching and bending vibrational modes. The CH2ClTa(O)OH molecule was predicted to have a doublet ground state with structure very similar to that of the niobium analog (Figure 8). It is quite interesting to note that the above characterized CH2ClM(O)H and CH2ClM(O)OH (M ) Nb, Ta) molecules all involve agostic interactions between the chlorine atom and the metal atom. As shown in Figures 7 and 8, the CH2Cl groups in the CH2ClM(O)H and CH2ClM(O)OH (M ) Nb, Ta) molecules all are distorted with the chlorine atom located close to the metal centers. Taking the CH2ClNb(O)H as an example, the ∠ClCNb was predicted to be only 80.4° with a Cl-Nb distance of 2.624 Å. Such interaction is quite similar to the agostic 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.38 Agostic distortion is common in the structures of alkylidene complexes of early transition metals and even more common in the structures of the small methylidene complexes of group 4-6 metals,24,39,40 in which agostic interactions are observed between the metal atom and one of the R-hydrogen atoms. The agostic interactions between the chlorine atom and the metal atom have recently been observed in the CH2ClScOH and CH2ClYOH molecules.25 Reaction Mechanism. The niobium and tantalum monoxide and dioxide molecules were generated via laser evaporation and were trapped in solid argon. Sample annealing allows the reactants to diffuse and react to form the MO(CH3Cl) and MO2(CH3Cl) complexes, reactions 1-4, which were predicted to be exothermic. The binding energy of NbO2(CH3Cl) is larger than that of NbO2(Ar)2 calculated at the same level of theory (30.2 kJ/mol); therefore, the two argon atoms in NbO2(Ar)2 can be replaced by CH3Cl in forming the NbO2(CH3Cl) on annealing.

Zhao et al.

NbO(CH3Cl) + hν f CH2ClNb(O)H

(5)

TaO(CH3Cl) + hν f CH2ClTa(O)H

(6)

The isomerization reactions proceeded only under visible light irradiation, indicating that the reactions require activation energy. The potential energy profiles for the MO + CH3Cl reactions were calculated and are shown in Figure 9. For Nb, the NbO(CH3Cl) complex was predicted to have a quartet ground state, whereas the CH2ClNb(O)H isomer has a doublet ground state. Therefore, the isomerization reaction involves spin crossing. In the case of Ta, the oxidative addition of the H-C bond to form the CH2ClTa(O)H molecule from TaO(CH3Cl) conserves spin and proceeds via a transition state with an energy barrier of 56.7 kJ/mol. The photoisomerization reactions of NbO(CH3Cl) and TaO(CH3Cl) are quite different from those of ScO(CH3Cl) and YO(CH3Cl). In the Sc and Y reactions, the MO(CH3Cl) complexes isomerized to the CH3OMCl and CH2ClMOH molecules with the valence of metal remains in +II oxidation state.25 For Nb and Ta, the oxidation state of metal increases from +II to +IV during the addition of the C-H bond to metal in forming the CH3M(O)H structure. The different reactivity can be rationalized in terms of changes in valence electron structures. Sc and Y have only three valence electrons and hence are not able to form high oxidative structures. A similar difference in reactivity between transition metals has previously been reported for some other reactants such as H2O, NH3, and CH4.20,35,41,42 The NbO2(CH3Cl) and TaO2(CH3Cl) complex absorptions decreased upon UV irradiation (250 < λ < 380 nm), during

NbO(4Σ-) + CH3Cl(1A1) f NbO(CH3Cl)(4A) ∆E ) -29.5 kJ/mol (1) TaO(2∆) + CH3Cl(1A1) f TaO(CH3Cl)(2A) ∆E ) -5.0 kJ/mol (2) NbO2(2A1) + CH3Cl(1A1) f NbO2(CH3Cl)(2A) ∆E ) -75.6 kJ/mol (3) TaO2(2Σ+) + CH3Cl(1A1) f TaO2(CH3Cl)(2A) ∆E ) -52.9 kJ/mol (4) The NbO(CH3Cl) and TaO(CH3Cl) complex absorptions decreased on visible light irradiation (λ >500 nm), during which the CH2ClM(O)H absorptions were produced. This observation suggests that the CH2ClM(O)H molecules were generated from the MO(CH3Cl) complexes via photoinduced isomerization reactions 5 and 6.

Figure 9. Potential energy profiles for the MO + CH3Cl reactions calculated at the B3LYP level of theory. (Values are given in kilojoules per mole.)

Agostic Bonding Molecules between Metal and Cl

J. Phys. Chem. A, Vol. 114, No. 18, 2010 5785 Acknowledgment. We gratefully acknowledge finical support from National Natural Science Foundation of China (grant nos. 20803066 and 20773030), Zhejiang Provincial Natural Science Foundation of China (grant no. Y4090161), and National Basic Research Program of China (2007CB815203 and 2010CB732306). Supporting Information Available: Calculated geometries, total energies, vibrational frequencies, and intensities of the transition states mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 10. Potential energy profiles for the MO2 + CH3Cl reactions (Nb, red; Ta, green) calculated at the B3LYP level of theory. (Values are given in kilojoules per mole.)

which the CH2ClM(O)OH absorptions were produced. This suggests that the CH2ClNb(O)OH and CH2ClTa(O)OH molecules were generated from the NbO2(CH3Cl) and TaO2(CH3Cl) complexes via reactions 7 and 8. The observation of CH2ClNb(O)OH and CH2ClTa(O)OH only under broadband irradiation indicates that the isomerization reactions of CH2ClM(O)OH from MO2(CH3Cl) require activation, and some excited states may be involved.

NbO2(CH3Cl) + hν f CH2ClNb(O)OH

(7)

TaO2(CH3Cl) + hν f CH2ClTa(O)OH

(8)

The ground-state CH2ClNb(O)OH and CH2ClTa(O)OH molecules were predicted to be 109.9 and 118.9 kJ/mol lower in energy than the NbO2(CH3Cl) and TaO2(CH3Cl) complexes, respectively. As shown in Figure 10, the isomerization reactions were predicted to proceed via a transition state with an energy barrier of 112.6 kJ/mol for Nb and 93.6 kJ/mol for Ta, indicating that the isomerization reactions require activation energy. Conclusions Reactions of niobium and tantalum monoxide and dioxide molecules with monochloromethane have been investigated using matrix isolation infrared absorption spectroscopy and density functional calculation. The metal monoxide and dioxide reactants were prepared via pulsed laser evaporation of bulk metal oxide targets. The results show that the ground-state metal monoxides and dioxides react with CH3Cl in forming the MO(CH3Cl) and MO2(CH3Cl) (M ) Nb, Ta) complexes spontaneously on annealing. The high oxidative MO2(CH3Cl) complexes were predicted to be more strongly bound than the corresponding low oxidative MO(CH3Cl) complexes. Visible light irradiation of the MO(CH3Cl) complexes initiated the H-atom transfer from CH3Cl to the metal center in forming the more stable CH2ClM(O)H isomers. In contrast, the MO2(CH3Cl) complexes isomerized to the more stable CH2ClM(O)OH molecules upon UV excitation. Both the CH2ClM(O)H and CH2ClM(O)OH molecules were predicted to involve interactions between the chlorine atom and the metal atom, which are quite similar to the agostic 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.

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