Ground electronic states, symmetry point groups, vibrational frequencies (cm−1) and binding energies (kJ/mol) for the MO(CH3Cl) species in solid argona.
In this chapter, we presented a short review of past and present experimental and theoretical work on the reactions of the transition metal monoxide and dioxide molecules with monochloromethane in excess argon matrices. A series of infrared absorption spectra combining with density functional theoretical (DFT) calculation characterized that the transition metal monoxide molecules produced by laser-ablated higher oxides activated C─H and C─Cl bonds of CH3Cl to first form the weakly bound MO(CH3Cl) (M = Sc, Y, Nb, Ta, Ti, Zr, Mn, Fe) complexes, which further photoisomerized to the more stable chlorine-transfer (Cl-transfer) CH3OMCl (M = Sc, Y), CH3M(O)Cl (M = Ti, Zr), CH3MOCl (M = Mn, Fe), and agostic hydrogen-transfer (H-transfer) CH2ClMOH (M = Sc, Y, Nb, Ta) products upon limited light excitation. Transition metal dioxides reaction with CH3Cl also formed MO2(CH3Cl) (M = Ti, Zr, Nb, Ta) complexes, which were further rearranged to the more stable Cl-transfer CH3OM(O)Cl (M = Ti, Zr) and agostic H-transfer CH2ClM(O)OH (M = Nb, Ta) molecules between the metal center atom and the chlorine atom upon ultraviolet light irradiation. Their different reactivity was interpreted according to the different valence electrons of metal center.
- chlorine transfer
- hydrogen transfer
- transition metal oxides
- agostic interaction
Monochloromethane, as the one of the simplest halohydrocarbons, also called methyl chloride, plays an important role in the industrial, synthetic, materials chemistry. It is always regarded that monochloromethane is the largest natural source of ozone-depleting chlorine compounds and accounts for about 15% of the present atmospheric chlorine content as one kind of chlorinated volatile organic compounds (CVOCs). At present, monochloromethane is observed in the dry leaf with the content of 0.1–0.3 μg/g/h, and large emissions of monochloromethane are observed from some common certain types of ferns and dipterocarpaceae [1, 2]. Monochloromethane is also industrially produced by the oxidation and chlorination reaction of methane in the presence of metal chloride catalyst, and drying monochloromethane conversed to gasoline and olefins on the methanol to gasoline (MTG) and the methanol to olefins (MTO) catalysts [3, 4]. The conversion of methyl chloride to hydrocarbons has been investigated since the mid-1980s . The product distribution of methyl chloride to hydrocarbons is strikingly similar to methanol conversion over the same topology . Recently several ZSM-5 zeolites and SAPO sieves catalysts were reported the high performances on the catalytic conversion of monochloromethane to light olefins [7–9]. Modified SAPO-34 catalysts were also chosen to enhance its catalytic performance for the conversion of chloromethane to light olefins [10–13]. The oxidation addition of metal into carbon-halogen bonds is a key step in many stoichiometric and catalytic reactions. Activation of compounds containing C─X (X = Cl, Br, I) bonds attracts widespread interest due to the underactive organic functional group and the inherent chemical properties. Predominantly alkyl and aryl halides are extensively applied as electrophiles in the transition metal-catalyzed cross-coupling reactions [14–16]. It has a far-reaching significance on carbon-chlorine (C─Cl) bond catalytic oxidation on the conversion of monochloromethane to gasoline and olefins.
The reactions of transition metal centers with chloromethane may serve as the simplest model for understanding the intrinsic mechanism of the organic halides catalytic oxidation processes. The reactions on transition metal atoms with monochloromethane have been intensively studied in solid noble gas matrices. Investigations have reported that C─X bond of CH3X (X = F, Cl, Br, I) are activated by transition metal atoms [17–22]. The higher valence of group 6 metals can form the methylidyne complexes CH = MH2X (M = Mo, W, X = H, F, Cl, Br) [23–26]. In this chapter, the reactions of simple transition-metal oxide molecules with monochloromethane in solid argon were reviewed using matrix infrared absorption spectroscopy and density functional theoretical (DFT) calculations. As shown in Scheme 1, the ground-state transition metal monoxide molecules activated carbon-hydrogen (C─H) and C─Cl bond of CH3Cl upon a certain wavelength excitation in argon matrices. The weakly bound MO(CH3Cl) (x = 1, 2; M = Sc, Y, Nb, Ta, Ti, Zr, Mn, Fe) complexes were initially formed and then isomerized to the more stable Cl-transfer CH3OMCl (M = Sc, Y) and CH3M(O)Cl (M = Ti, Zr, Nb, Ta, Mn, Fe), and agostic H-transfer CH2ClMOH (M = Sc, Y, Nb, Ta) isomers upon limited visible light excitation. The MO2(CH3Cl) (M = Ti, Zr, Nb, Ta), which were formed from the reactions on MO2 with CH3Cl, were further rearranged to the more stable Cl-transfer CH3OM(O)Cl (M = Ti, Zr) and H-transfer CH2ClM(O)OH (M = Nb, Ta) molecules with agostic interactions between the chlorine and the metal center under ultraviolet light irradiation.
2. Experimental and computational methods
The experimental setup for pulsed laser-ablated and matrix isolation Fourier transform infrared (FTIR) spectroscopic technique has been previously described in detail . 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 metal oxide targets, which were prepared by sintered metal oxide powders. Laser-evaporation of bulk higher metal oxide targets has been proved to be an extensively available technique to prepare pure metal oxides in noble gas matrices [32–34]. Using standard manometric technique, the CH3Cl/Ar samples were mixed at a proper proportion in a stainless steel vacuum line. The CH3Cl sample was subjected to several freeze-pump-thaw cycles at 77 K before use. The laser-evaporated species were co-deposited with chloromethane in excess argon onto a CsI window cooled normally to 6 K by a closed-cycle helium refrigerator (ARS, 202N). The matrix samples were deposited at a rate of approximately 5 mmol/h for 1–2 h. Isotopic-labeled 13CH3Cl and CD3Cl (ISOTEC, 99%) were used without further purification. Infrared spectra between 450 and 4000 cm−1 were recorded on a Bruker IFS 66v/s spectrometer using HgCdTe (MCT) detector cooled by liquid N2 at 0.5 cm−1 resolution. Samples were annealed to different temperatures and cooled back to 6 K to acquire the spectra, and selected samples were subjected to visible or broadband irradiation using a 250 W high-pressure mercury arc lamp with selected wavelength glass filters.
Density functional theoretical calculations were performed by using Gaussian 03 programs  to identify the experimental assignments. The three-parameter hybrid functional, according to Becke with additional correlation corrections from Lee, Yang, and Parr (B3LYP), was utilized [36, 37] to optimize ground geometries, calculate frequencies, and derive the zero-point vibrational energies. Transition-state optimizations were performed with the Berny geometry optimization algorithm at the B3LYP level. The 6-311++G(d, p) basis set was used for the H, C, O, Cl, Sc, Ti, Mn, and Fe atoms [38, 39], DGDZVP basis set for Y, Zr, and Nb atoms [40, 41], and the scalar-relativistic SDD pseudopotential and basis set for Ta atom [42, 43]. In addition, the CCSD(T) method was also applied to accurately calculate the single-point energies of the B3LYP-optimized structures with the same basis sets .
3. Transition metal monoxides reaction with CH3Cl
Reaction of transition metal monoxides (ScO, YO, TiO, ZrO, NbO, TaO, MnO, FeO) with monochloromethane was investigated in solid argon by infrared absorption spectroscopy, combining with isotopic substituted experiments and theoretical calculations. The initial reaction step is the formation of the MO(CH3Cl) (M = Sc, Y, Ti, Zr, Nb, Ta, Mn, Fe) complex with metal atom bound with chlorine atom and/or oxygen atom with H atoms on annealing. Upon a certain wavelength photolysis, the MO(CH3Cl) complex was isomerized by the insertion of the M═O to C─H and/or C─Cl/Cl─C bond. Selected region of infrared spectra is illustrated in Figures 1–4.
In both the scandium and yttrium experiments, two MO(CH3Cl) (M = Sc, Y) complex isomers were formed spontaneously on annealing . These absorptions of MO(CH3Cl) (M = Sc, Y) complex are observed at 898.4 and 919.1 cm−1 for Sc, and 1050.9, 805.9, and 784.8 cm−1 for Y, as shown in Table 1, which are corresponding to the Sc─O and Y─O vibration frequencies. The CH3OMCl and CH2ClMOH (M = Sc, Y) molecules were produced from the weakly bound MO(CH3Cl) complexes through photoinduced isomerization reactions on 250–300 nm wavelength irradiation, as shown in Figure 1. The CH3OMCl (M = Sc, Y) isomer observed at 1171.5 and 565.6 cm−1 for Sc and 1149.2 and 490.9 cm−1 for Y can be regarded as being formed through the addition of the C─Cl bond to the O═M bond, whereas the CH2ClMOH (M = Sc, Y) isomer observed at 3775.0 and 738.4 for Sc, and 3774.2 and 627.6 for Y is formed through the addition of the C─H bond to the M═O bond. On the basis of DFT calculations, the MO(CH3Cl) (M = Sc, Y) complex with Cs structure is more stable than the complex with C3v structure by 25.5 (Sc) or 24.0 (Y) kJ/mol. Both CH3OMCl and CH2ClMOH (M = Sc, Y) molecules are more stable than the MO(CH3Cl) complex isomers. The CH3OMCl (M = Sc, Y) molecule was predicted to proceed through a transition state with an energy barrier of 17.7 for Sc and 8.4 kJ/mol for Y from the MO(CH3Cl) complex, whereas the CH2ClMOH isomer also proceeded through a transition state with a much higher energy barrier of 160.1 for Sc and 178.5 kJ/mol for Y from the MO(CH3Cl) complex. The CH3OMCl (M = Sc, Y) structure is about 173.0 for Sc and 180.6 kJ/mol for Y lower in energy than the CH2ClScOH and CH2ClYOH isomer. The CH2ClMOH (M = Sc, Y) molecule was also calculated to involve agostic interaction observed between the metal atom and the chlorine atom due to short bond distances of 2.598 Å for Sc─Cl and 2.821 Å for Y─Cl. 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 [17, 21, 45].
|Molecule||Ground state||Point group||Vibrational frequencyb||Binding energyc||Ref.|
|YO(CH3Cl)||2A′||C s||1050.9, 783.5||−37.0|||
For IVB metal monoxides, the ground-state MO(CH3Cl) (M = Ti, Zr) complexes correlate to the ground-state TiO (3Δ) and ZrO (1Σ−). The binding energies are predicted to be 20.5 (Ti) and 12.2 kcal/mol (Zr), which are larger than the corresponding values of TiO(CH4) and ZrO(CH4) [45, 46]. The MO(CH3Cl) (M = Ti, Zr) complexes can rearrange to the CH3M(O)Cl isomers by metal terminal insertion to C─Cl bond upon UV light irradiation (λ < 300 nm), which are observed at 999.5 and 526.2 cm−1 for Ti, and 915.2 and 488.3 cm−1 for Zr, as shown in Figure 2. Theoretical calculations also indicated that the electronic state crossings exist from the MO (M = Ti, Zr) + CH3Cl reaction to the more stable CH3M(O)Cl molecules through the MO(CH3Cl) complexes traverse their corresponding transition states. The CH3M(O)Cl (M = Ti, Zr) molecule was predicted to have a singlet ground state without symmetry. According to CCSD(T) single-point calculations on B3LYP optimization geometry, the singlet ground state is 44.3 kcal/mol for CH3Ti(O)Cl and 52.2 kcal/mol for CH3Zr(O)Cl lower in energy than its corresponding triplet state. The triplet MO(CH3Cl) (M = Ti, Zr) isomerized to the singlet CH3M(O)Cl (M = Ti, Zr) molecule through their corresponding transition states, which indicated that these reactions related to the spin crossing under UV light irradiation.
|Molecule||Ground state||Point group||Vibrational frequencyb||Binding energyc||Ref.|
|CH3OScCl||2A||C 1||1171.5, 565.6||−268.8|||
|CH2ClScOH||2A||C 1||3775.0, 738.4||−95.8|||
|CH3OYCl||2A||C 1||1149.2, 490.9||−300.3|||
|CH2ClYOH||2A||C 1||3774.2, 627.6||−119.7|||
|CH3Ti(O)Cl||1A||C 1||999.5, 526.2||−324.1|||
|CH3Zr(O)Cl||1A||C 1||915.2, 488.3||−349.0|||
|CH2ClNb(O)H||2A||C 1||1698.0, 985.0||−136.7|||
|CH2ClTa(O)H||2A||C 1||1760.0, 984.8||−182.7|||
|CH3MnOCl||6A||C 1||569.6, 542.2||−55.2|||
The ground-state NbO(CH3Cl) and TaO(CH3Cl) molecules are related to the ground-state NbO (4Σ) and TaO (2∆). The predicted binding energies of 29.5 (Nb) and 5.0 kJ/mol (Ta) are larger than the corresponding values of NbO(CH4) and TaO(CH4) complexes , which were predicted to be very weakly interaction with the metal atom being bound to three hydrogen atoms of CH4. The MO(CH3Cl) (M = Nb, Ta) complexes rearranged to the more stable doublet CH2ClM(O)H isomer upon visible light excitation, as shown in Table 2. Thus, some excited states may be involved during the reaction process. The CH2ClM(O)H molecules were predicted to involve agostic interactions between the chlorine atom and the metal center. It is quite interesting to note that the CH2ClM(O)H (M = Nb, Ta) molecules involve agostic interactions between the chlorine atom and the metal atom. It is notable that the group 5 metal methylidene complexes are more agostically distorted than the group 4 metal complexes. Taking CH2ClNb(O)H as an example, the ∠ClCNb was predicted to be only 80.4° with a Cl─Nb distance of 2.624 Å. Agostic distortion interaction is a universal phenomenon in the structures of the early transition metal alkylidene complexes and even more popular in the structures of the small methylidene complexes, in which agostic interactions are also observed between the group 4–6 transition metal atom and one of the R-hydrogen atoms.
The reactions of FeO and MnO with CH3Cl first formed the MO(CH3Cl) (M = Mn, Fe) complexes when annealing, which can isomerize to CH3MOCl (M = Mn, Fe) upon 300 < λ < 580 nm irradiation. The products were characterized by isotopic IR studies with CD3Cl and 13CH3Cl and density functional calculations, as shown in Figure 3. Based on theoretical calculations, the MO(CH3Cl) (M = Mn, Fe) complexes have 5A′ for Fe and 6A′ ground state for Mn with Cs symmetry, respectively, as listed in Table 1. The binding energies of MO(CH3Cl) (M = Mn, Fe) are 9.3 and 23.3 kcal/mol lower than MO + CH3Cl, which are higher in energy than MO(CH4) and MO(Ng) (Ng = Ar, Kr, Xe) at the same calculation level [46–48]. The accurate CCSD(T) single-point calculations illustrate the CH3MOCl isomerism are 13.8 and 3.1 kcal/mol lower in energy than the MO(CH3Cl) (M = Mn, Fe) complexes.
The different reactivity of metal monoxide with CH3Cl can be rationalized in terms of changes in valence electron structures accompanied by electronic spin state crossing. In the scandium and yttrium reactions, the ground ScO and YO molecules reacted with CH3Cl to form two isomeric MO(CH3Cl) (M = Sc, Y) complexes spontaneously on annealing. Broad-band irradiation produced either the addition of the C─Cl bond to the O═M (M = Sc, Y) bond to form the CH3OMCl (M = Sc, Y) molecules with +II oxidation state of center metal or the addition of the C─H bond to the M═O bond to give the CH2ClMOH isomer with the valence of metal remaining in +II oxidation state. The CH2ClMOH (M = Sc, Y) include one α-chlorine atom to form agostic molecules between chlorine atom and metal center atom with less than 90° of ∠ClCM and short Cl---M (M = Sc, Y) distances. No α-H and/or α-Cl atom for the MO(CH3Cl) complex exist, so no agostic interaction is observed. Sc and Y have only three valence electrons, and hence they are not able to form high oxidation state structures. However, Mn and Fe have five and six valence electrons. Because their d orbitals are fully half-filled and hence are not easily lost, upon 300 < k < 580 nm irradiation the MO(CH3Cl) (M = Mn, Fe) complexes triggered the addition of the C─Cl bond to the M═O bond to form the CH3MOCl molecules with +II valence state. The Ti and Zr metals have four valence electrons, and their oxidation states increase from +II to +IV during the addition of MO insertion into the C─Cl bond to the metal to form CH3M(O)Cl molecules. For Nb and Ta, visible light irradiation triggered the H-atom transfer of the MO(CH3Cl) complexes from CH3Cl to the metal center to form the more stable CH2ClM(O)H isomers with the oxidation states of the metal increasing from the +II to +IV. However, the Nb and Ta have five valence electrons, and they cannot form +V oxidation structures, but possessing one valence electron characteristic of the agnostic chlorine effect.
4. Transition metal dioxides reaction with CH3Cl
The ground-state MO2 (M = Ti, Zr, Nb, Ta) molecules react with CH3Cl to first form the weakly bound MO2(CH3Cl) complexes with O···H and M···Cl bonds. For Ti and Zr, the MO2(CH3Cl) complexes can isomerize to the more stable CH3OM(O)Cl molecules with the addition of the C─Cl bond of CH3Cl to one of the O═M bond of MO2 on annealing after broadband light irradiation (λ < 300 nm), as shown in Figures 2 and 4. And the reaction potential energy profile interpreted the chemical reaction mechanism of C─Cl activation by MO2 (M = Ti, Zr). The photoisomerization reaction of MO2(CH3Cl) (M = Nb, Ta) is quite different from those of MO2(CH3Cl) (M = Ti, Zr). The MO2(CH3Cl) (M = Nb, Ta) complexes were initiated H-transfer under ultraviolet light irradiation to isomerize the more stable CH2ClM(O)OH molecules. The CH2ClM(O)OH (M = Nb, Ta) molecules were predicted to involve agostic interactions between the chlorine atom and the metal center. During the photoisomerization process, no electronic spin state crossings were found, as shown in Table 3, different from the reaction of metal monoxides with CH3Cl.
|Molecule||Ground state||Point group||Vibrational frequencyb||Binding energyc||Ref.|
|TiO2(CH3Cl)||1A||C 1||940.3, 906.2||−95.7|||
|ZrO2(CH3Cl)||1A′||C s||874.1, 804.2||−84.4|||
|NbO2(CH3Cl)||2A||C 1||948.1, 890.9||−75.66|||
|TaO2(CH3Cl)||2A′||C s||948.4, 890.8||−52.9|||
|CH3OTi(O)Cl||1A||C 1||1173.0, 1102.6, 634.1||−326.0|||
|CH3OZr(O)Cl||1A||C 1||1153.1, 901.2||−331.9|||
|CH2ClNb(O)OH||2A||C 1||3678.4, 979.4, 712.8||−185.5|||
|CH2ClTa(O)OH||2A||C1||3690.8, 975.9, 605.8, 495.7||−171.8|||
5. Conclusion and outlook
C─Cl and/or C─H bond of monochloromethane activation by transition metal monoxide and dioxide molecules has been investigated using matrix infrared spectroscopy in excess argon and density functional theoretical calculations. The metal monoxide and dioxide molecules prepared by laser-ablated bulk higher oxide targets reacted with monochloromethane to form the weakly bound MO(CH3Cl) (x = 1, 2; M = Sc, Y, Nb, Ta, Ti, Zr, Mn, Fe) complexes, which isomerized to the more stable CH3OMCl (M = Sc, Y), agostic CH2ClMOH (M = Sc, Y, Nb, Ta) and CH3M(O)Cl (M = Ti, Zr, Nb, Ta, Mn, Fe) isomers upon limited visible light excitation. Metal dioxides also reacted with CH3Cl to form MO2(CH3Cl) (M = Ti, Zr, Nb, Ta), which was rearranged to the more stable CH3OM(O)Cl (M = Ti, Zr) and CH2ClM(O)OH (M = Nb, Ta) molecules under ultraviolet light irradiation. Agostic interactions were observed in CH2ClMOH (M = Sc, Y, Nb, Ta) and CH2ClM(O)OH (M = Nb, Ta) between the chlorine atom and the metal center atom.
We gratefully acknowledge the financial support from National Natural Science Foundation of China (Grants No. 21273202 and 21473162). Y. Zhao is grateful to the Project Grants 521 Talents Cultivation of Zhejiang Sci-Tech University and China Scholarship Council (CSC) Foundation. This work is also supported by Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology.