The cycloaddition reactions of small organic molecules involving alkane, alkene and alkyne have been a topic for intense investigation during the past decades as one of the most straightforward methods to produce useful carbocyclic systems [1-4]. Due to the thermodynamic stability and kinetic inertness of small organic molecules, the cycloaddition reactions for them are difficult to occur, which always need extremely high temperature or high pressure [5-8]. Therefore, the catalyst is essential for the cycloaddition reactions. Transition metal compounds are effective in catalyzing the cycloaddition reactions by changing its spin state due to the empty or half-filled d orbitals of transition metals, which have become the most abundant industrial catalysts for cycloaddition reactions [9-13].

In the past decades, the researchers applied different forms of transition metal catalysts including the bare transition metal atoms (Co, Ti, Y, Zr, Nb, Ni, Ru, Rh and Mo, etc.) [14-18], metal oxides (TiO₂, VO₂) [19-21], neutral metal clusters (Pd_n), ionic metal clusters (Fe_n⁺) [22] and so forth to study the reaction of acetylene cyclization through experiments and theoretical calculations [23-29]. The intermediate products M(η²⁻C₂H₂)⁺, M(η²-C₄H₄)⁺, M(η⁴-C₄H₄)⁺, M(η²-C₆H₆)⁺ and M(η⁶-C₆H₆)⁺ are generated. Their structures have been characterized in the gas phase experiments with theoretical calculations. M(η²⁻C₂H₂)⁺, M(η²-C₄H₄)⁺ and M((η²-C₆H₆)⁺ are metal-ligand ring structures which can be described by the Dewar-Chatt-Duncanson (DCD) complexation model [30-35]. According to this model, the interaction of metals and ligands can be described as σ donation and π back donation [30-35]. M(η⁴-C₄H₄)⁺ and M((η⁶-C₆H₆)⁺ with metal cation-π structure contain cyclobutadiene and benzene, which are well-known in organometallic chemistry, and even been employed in gas-phase ion chemistry [30-35].

In addition to the reactions to generate the cyclobutadiene ligand and benzene with even numbers of carbon atom, the cycloaddition reaction to form pentamethylcyclopentadienyl (Cp) ligand with odd numbers of carbon atom is also important type of reaction in the fields of organic synthesis, medicinal chemistry, pesticides, and chemical industry [36, 37]. As far as we know, the reaction to obtain a ring with odd numbers of carbon atom has not been obtained in the gas phase. Because the ground state of Ir⁺ is 5d⁷6s¹, which has the enough empty valence orbital (of suitable symmetry) for the cycloaddition reaction. The iridium and iridium complexes have been applied to mediate the different cyclization reaction of small organic molecules [38-41]. Therefore, in order to obtain the five-membered pentamethylcyclopentadienyl ring ligand, in this work, the cycloaddition reactions of methane and ethylene mediated by Ir⁺ have been designed and studied by the techniques of mass spectrometry in conjunction with theoretical calculations. We will clearly investigate the generation of the half-sandwich structure IrHCp⁺ (Cp = η⁵-C₅H₅) including pentamethylcyclopentadienyl ligand by high selectivity, the potential energy surfaces of consecutive reactions in different electronic states and the bond analysis of the products to further understand the special cycloaddition reaction and guide the rational design of new catalysts.

The experiments are performed by an ion trap mass spectrometer equipped with a laser vaporization-supersonic expansion ion source coupled with a flow tube reactor, which has been reported previously [42, 43]. For getting pentamethylcyclopentadienyl ligand, the cycloaddition reaction routes Ir⁺ → IrC₂H₂⁺ → IrC₄H₄⁺→ IrC₅H₆⁺ are designed. The metal ions Ir⁺ are generated by pulsed laser ablation of a rotating and translating metal Ir target. The 532 nm second harmonic of a Nd: YAG laser with an energy of 5–12 mJ/pulse is used. The nascent ablated plasma is entrained by the helium carrier gas (99.999%) expanded from a pulsed valve (General Valve, series 9) at a backing pressure of about 0.3–0.5 MPa. The generated ions are guided and mass-selected by the quadrupole, and then sent into a quadrupole linear ion trap. The mass spectra from the reactions of Ir⁺ ions with He, C₂H₄, C₂D₄ and ¹³C₂H₄ in the ion trap are shown in Fig. 1 (panels a1, b1, c1, d1). The results indicate that only one product with chemical formula of IrC₂H₂⁺ is generated by reaction 1.

Isotopic-labeling experiments by using the C₂D₄ and ¹³C₂H₄ (panels c1 and d1) confirm this result with the generation of IrC₂D₂⁺ and Ir¹³C₂H₂⁺. Similarly, the reaction of mass-selected IrC₂H₂⁺ with C₂H₄ is studied, and the mass spectra from the reactions with He, C₂H₄, C₂D₄, and ¹³C₂H₄ are shown in Fig. 1 (panels a2, b2, c2, d2). The observation of only product IrC₄H₄⁺ suggests that the reaction 2 takes place:

Isotopic-labeling experiments by using the C₂D₄ and ¹³C₂H₄ (panels c2 and d2) confirm that the generation of the IrC₂¹³C₂H₄⁺ and IrC₄H₂D₂⁺/IrC₄HD₃⁺/IrC₄D₄⁺.

For getting the five-membered ring ligand, the mass spectra from the reactions of IrC₄H₄⁺ with He, CH₄, CD₄, and ¹³CH₄ are studied and shown in Fig. 1 (panels a3, b3, c3, d3). One peak that can be assigned to the product ions with chemical formula of [IrC₅H₆]⁺ is observed in the spectra, reaction 3.

Isotopic-labeling experiments using the ¹³CH₄ sample confirm that the four carbon atoms in the [IrC₅H₆]⁺ product ion are originated from the C₂H₄ reactant and one carbon atom is originated from the CH₄ reactant. However, we point out that when Ar is added for cooling before the reaction gasses are inlet, no products are found for the reactions of [IrC₄H₄]⁺ with CH₄. For the reactions of IrC₂H₂⁺ or Ir⁺ with C₂H₄, no similar phenomenon is found.

In order to gain insight into the reaction mechanisms, the geometry optimization and frequency calculations for reactants, products, reaction intermediates (IMs) and transition states (TSs) were carried out by using the TPSS method and the Def2TZVP basis sets by Gaussian 09 software package [44-47]. All the stationary point structures were characterized on the Potential Energy Surface (PES) by performing vibrational frequencies analysis, which aimed at identifying the nature of the stationary point (minima or saddle point). In predicting the reaction pathways, the intrinsic reaction coordinate (IRC) [48, 49] calculations were performed to confirm the correctness of the transition states. All energies are reported by zero-point vibrational energy (ZPE) correction. The analyses of the quantum theory of atoms in molecules (QTAIM) [50, 51], charge decomposition analysis (CAD) [52] and orbital interaction diagram are generated using the Multiwfn package [53].

The various possible structures of dehydrogenation products IrC₂H₂⁺, IrC₄H₄⁺ and IrC₅H₆⁺ are obtained by calculations at the TPSS/def2-TZVP level and shown in Figs. S1-S3 (Supporting information). For IrC₂H₂⁺, the most stable structure is metal cation-π complex ³Ir(η²-C₂H₂)⁺. The corresponding singlet state structure is predicted to lie 0.09 eV higher in energy than the triplet state. The most stable structure of IrC₄H₄⁺ is metallacycle structure ¹Ir(η²-C₄H₄)⁺ of coupling by -C₂H₂ (from IrC₂H₂⁺) and -C₂H₂ (activated products of ethylene) which has the ground state ¹A₁. The ¹Ir(η⁴-C₄H₄)⁺ and corresponding triplet state structure ³Ir(η²-C₄H₄)⁺ are predicted to lie 0.35 eV and 0.58 eV higher in energy than the most stable structure. The most stable structure of IrC₅H₆⁺ is ¹IrH(η⁵-C₅H₅)⁺, which is half-sandwich structure with IrH moiety as the center and coordinated with Cp ligand that is obtained from the coupling by -C₂H₂, -C₂H₂ and -CH (activated products of methane). The isomers ¹Ir(η⁵-C₅H₆)⁺ and ¹IrH(η²-C₅H₅)⁺ are predicted to lie 0.43 eV and 0.45 eV higher in energy than the most stable structure.

The pathway for the first dehydrogenation reaction Ir⁺ + C₂H₄ starting with the quintet state as ground reactants is shown in Fig. 2 and the details are shown in Fig. S4 (Supporting information). An ethene adduct, ⁵I1 is formed upon initial approach. The efficient dehydrogenation at the thermal energies cannot occur along the quintet state surface because the transition state ⁵TS2 lies 0.68 eV above the reactants, so further calculations along the quintet state surface are not conducted. A surface crossing must occur from the quintet state to the singlet state or triplet state surface for dehydrogenation. Along the triplet state surface, the ethene adduct, ³I1, lies 3.82 eV lower in energy than the ground state reactants. From ³I1, the first oxidative addition process with the transfer of the first H atom from ethene to the Ir⁺ occurs through ³TS1 (−2.58 eV) resulting in the formation of the intermediate ³I2 (−3.24 eV). The second oxidative addition process, with C–H bond cleavage from -C₂H to the Ir⁺ center leads to generate ³I3 (−2.70 eV) by a transition state ³TS2 (−2.24 eV). Then, intermediate ³I4 (−2.95 eV) in which H₂ is adsorbed by Ir⁺ through weak interactions is formed with the decrease of H-H distance. The final triplet state product ³Ir(η²-C₂H₂)⁺ with metal cation-π structure is generated with the reductive elimination process of H₂. Along the singlet state surface, similar to the triplet state surface, this crossing takes place in the entrance channel because intermediate ¹I1 lies 3.28 eV below ⁵I1. Then the reaction occurs through the oxidative addition and reductive elimination processes of C-H bonds. It requires 0.09 eV more energy than the triplet state dehydrogenation process, but still lies 2.24 eV below ground state reactants. Therefore, an effective dehydrogenation reaction can not only occur along the triplet state surface but also along the singlet state surface, the singlet and triplet state products ¹Ir(η²-C₂H₂)⁺ and ³Ir(η²-C₂H₂)⁺ may be coexisting.

The pathway for the second dehydrogenation reaction, which begins with the singlet state or triplet state Ir(η²-C₂H₂)⁺ + C₂H₄ is shown in Fig. 3 and the details are shown in Fig. S5 (Supporting information). The single state ¹Ir(η²-C₂H₂)⁺ + C₂H₄ is determined as ground reactants for discussion. Along the singlet state surface, the adsorption complex, ¹I1 lies 2.85 eV lower in energy than the ground state reactants. The first oxidative addition process with the transfer of the first H atom from ethene to the Ir⁺ occurs through ¹TS1 (−2.08 eV) resulting in the formation of the intermediate ¹I2 (−2.70 eV). The second oxidative addition process, with C–H bond cleavage from -C₂H to the Ir⁺ center leads to intermediate ¹I3 (−2.83 eV) by a transition state ¹TS2 (−2.67 eV). The cycloaddition reaction with the coupling of -C₂H₂ from reactant Ir(η²-C₂H₂)⁺ and -C₂H₂ from the activated product of reactant C₂H₄ forms a metallacycle structure ¹I4 (−1.98 eV) through a transition state ¹TS3 (−0.60 eV). Then, intermediate ¹I5 (−2.60 eV) in which H₂ is adsorbed by the metal cation Ir⁺ through weak interactions is formed with the decrease of H-H distance. The final metallacycle structure ¹Ir(η²-C₄H₄)⁺ with singlet state is generated with the reductive elimination process of H₂. In addition, the ¹Ir((η⁴-C₄H₄)⁺ is also generated through the potential energy surface (¹I4–¹TS5–¹I6). The IRC calculations of the pathways ¹I3–¹TS3–¹I4 and ¹I4–¹TS5–¹I6 are performed and the results are shown in Figs. S6 and S7 (Supporting information). The reaction pathway to generate ³Ir(η²-C₄H₄)⁺ for the triplet surface is similar to that of the singlet state. It requires 0.58 eV more energy than the singlet state dehydrogenation process, but still lies 1.65 eV below ground state reactants, so the reaction can proceed along the singlet state and the triplet state surface, the ¹Ir(η⁴-C₄H₄)⁺, triplet and singlet products Ir(η²-C₄H₄)⁺ are generated. But the energy of ³TS4, ¹TS5 is higher than that of ¹TS4, and the energy of P4, P5 is higher than that of P3, we predict the main product of IrC₄H₄⁺ is ¹Ir(η²-C₄H₄)⁺.

The pathway for the third dehydrogenation reaction is shown in Fig. 4 and the details are shown in Figs. S8 and S9 (Supporting information). The reaction leading to the IrC₅H₆⁺ + H₂ products is predicted to proceed via oxidative addition, reductive elimination, ring-forming and dehydrogenation processes. If the reaction begins with the singlet state ¹Ir(η²-C₄H₄)⁺ + CH₄ or ¹Ir(η⁴-C₄H₄)⁺ + CH₄ as the ground reactants, the efficient dehydrogenation at the thermal energies cannot occur because the energy of transition state is higher than the reactants (Figs. S6 and S7), this is consistent of experimental results that no product is observed when ions are cooled by Ar gas. The experimental results can be explained that the part of the release energy in the reactions of IrC₂H₂⁺ or Ir⁺ with C₂H₄ is not transferred away and the product IrC₄H₄⁺ owns the extra energy to overcome the barrier. So the reaction can begin with the triplet state ³Ir(η²-C₄H₄)⁺ + CH₄ as ground reactants or singlet state with the extra energy [54]. Along the triplet state surface, the ethene adsorption complex, ³I1 lies 0.47 eV lower in energy than the ground state reactants. When the activation of the first C-H bond of CH₄ occurs, the second transfer of hydrogen atom leads to the intermediate ³I3 by the transition state ³TS2 which lies 0.05 eV above the reactants. Because of this, efficient dehydrogenation at thermal energies cannot occur along the triplet state surface, so further calculations along the triplet state surface are not conducted. The triplet state Ir(η²-C₄H₄)⁺ is directly coordinated to CH₄ for forming the ethene adduct, ¹I1, lies 1.13 eV lower in energy than the ground state reactants. From ¹I1, the first oxidative addition process with the transfer of the first H atom from CH₄ to the Ir⁺ occurs through ¹TS1 (−0.11 eV) resulting in the formation of the intermediate ¹I2 (−0.64 eV). The first reductive elimination process, with the transfer of H atom from Ir⁺ to adjacent C atom leads to intermediate ¹I3 (−1.74 eV) by a transition state ¹TS2 (−0.24 eV). The second oxidative addition process with the transfer of the H atom from -CH₃ to the Ir⁺ occurs through ¹TS3 (−0.89 eV) resulting in the formation of the intermediate ¹I4 (−1.36 eV). Then the first ring-forming reaction with the coupling of -C₄H₅ and -CH₂ from the activated product of reactant CH₄ forms a metallacycle structure ¹I5 (−1.54 eV) through a transition state ¹TS4 (−0.41 eV). The third oxidative addition process, with the transfer of H atom from -CH₂ (the activated product of reactant CH₄) to Ir⁺ atom leads to intermediate ¹I6 (−1.20 eV) by a transition state ¹TS5 (−0.87 eV). Then the H atom is transferred between the two C atoms to form intermediate ¹I7 (−1.65 eV) by a transition state ¹TS6 (−0.43 eV). The second ring-forming reaction with the coupling of two C atoms adjacent to Ir⁺ through ¹TS7 (−0.30 eV) results in the formation of the intermediate ¹I8 (−0.43 eV) with a like-half-sandwich structure that IrH₂⁺ connects to the five-membered ring -C₅H₆. The final oxidative addition process, with the transfer of the H atom from -CH₂ to the Ir⁺ center takes place to form the intermediate ¹I9, in which H₂ is adsorbed by the metal Ir through weak interactions. Finally, the half-sandwich structure IrHCp⁺ is generated with the reductive elimination process of H₂, which is consistent with the experimental results. The generated pathways of isomers Ir(η⁵-C₅H₆)⁺ and IrH(η²-C₅H₅)⁺ are exothermic processes which lie 0.98 eV and 0.96 eV below ground state reactants. But the energies of Ir(η⁵-C₅H₆)⁺ and IrH(η²-C₅H₅)⁺ are higher than the IrHCp⁺, Therefore, the IrHCp⁺ is major product, Ir (η⁵-C₅H₆)⁺ and IrH (η²-C₅H₅)⁺ may be also coexisting, which is similar with the isomers of IrC₄H₄⁺. The IRC calculations of the pathways ¹I3–¹TS3–¹I4 and ¹I4–¹TS5–¹I6 are performed and the results are shown in Figs. S10 and S11 (Supporting information).

In addition, the single-point energy of dispersion correction, the entropy and Gibbs free energy are calculated and the comparison diagrams are shown in Figs. S12–14 (Supporting information) and the values are given in Tables S1-S3 (Supporting information). The energy trends of dispersion correction and the entropy are consistent with the calculation of the TPSS function used. The single-point calculations using high-level CCSD(T) methods on the reactants, products, reaction intermediates and transition states are shown in Fig. 2, Fig. 3, Fig. 4. The reaction pathways are consistent with the TPSS function except for the ³TS3 of the cycloaddition reaction of Ir(η²-C₂H₂)⁺ with C₂H₄. The relative energy of transition states ³TS3 is higher 0.41 eV than the ground state reactants, but the energy singlet state ¹TS3 is lower than the reactant. So the cycloaddition reaction of Ir(η²-C₂H₂)⁺ with C₂H₄ might have a cross point (CP) in the singlet and triplet potential energy surfaces [55]. The reaction products are same as the results of TPSS function, so the calculation of TPSS function is accurate.

In order to explain the mechanism of the cyclization reaction, we analyze the localized molecular orbital (LMO) of the cyclization process and the results are shown in Fig. S15 (Supporting information). For the cyclization process of reaction ¹Ir(η²-C₂H₂)⁺ + C₂H₄, from ¹I3 to ¹I4 through the transition state ¹TS3, with the distance between the carbon atoms from ¹Ir(η²-C₂H₂)⁺ and C₂H₄ gradually decreases, the two π orbitals of -C₂H₂ from Ir(η²-C₂H₂)₂⁺ approach, and then overlap to form a multi-center bond (¹TS3), and further overlap to form a new C-C σ bond, the π orbitals of carbons in ring overlap result in forming the four-membered cyclobutadiene ring ligand C₄H₄. The LMO of the cyclization process above is the same to the that of the reaction ³Ir(η²-C₂H₂)⁺ + C₂H₄ to generate ³Ir(η⁴-C₄H₄)⁺. For the first ring formation process of reaction ¹Ir(η²-C₄H₄)⁺ + CH₄, the two overlapping π orbitals are from the -IrCH₂ with carbene structure and -C₂H₂ of ring η²-C₄H₄, which leads to form another new C-C σ bond. Finally, the π orbitals of carbons in ring overlap to form the pentamethylcyclopentadienyl ligand (Cp ring). Therefore, the mechanism of the cyclization reaction depends on the overlap of π orbitals which is consistent with the statement of previous paper [13].

In order to classify bond types, characterize bond nature, and distinguish bond strength of the special product IrH(Cp)⁺ containing pentamethylcyclopentadienyl ligand. The bond analysis is applied by the topological parameters at the bond critical point (BCP) between two atoms based on the quantum theory of atoms in molecules (QTAIM) theory [56−59]. From Table S4 (Supporting information), for the topological parameters the Ir-C bonds of IrH(Cp)⁺, ρ_b and ∇²ρ_b are positive, H_b is negative, and -G_b/V_b is greater than 0.5 and less than 1, which illustrate that IrH(Cp)⁺ is formed through the dative bond between IrH⁺ and Cp ligand [56-59]. For the topological parameters the C-C bond of Cp, ρ_b is positive, ∇²ρ_b and H_b are negative, and -G_b/V_b is less than 0.5, which illustrate the C-C bond of Cp is covalent bonds [56-59]. The analyses of orbital interaction diagram and charge decomposition analysis (CDA) [60, 61] are applied to understand how orbitals of fragments are mixed to form the dative bond of IrH(Cp)⁺ (Figs. S16 and S17 in Supporting information). For IrH(Cp)⁺, the LUMO is mainly composed of the d_xz orbital (44%) of ¹Ir⁺ and the π orbital (41%) of Cp, the all occupied molecular orbitals are formed by the s+d_z² orbital (68%) of IrH⁺ and the π orbital (20%) of Cp, the s+d_z² orbital (25%) of IrH⁺ and the π orbital (26%) of Cp, the d_xz orbital (34%) of ¹Ir⁺ and the π orbital (47%) of Cp, the d_xy orbital (34%) of ¹Ir⁺ and the π orbital (14%) of Cp, the s+d_z² orbital (16%) of IrH⁺ and the π orbital (64%) of Cp. The calculated data of CDA of IrH(Cp)⁺ indicate that the dative bond of IrH⁺ and Cp is formed between the donation from IrH⁺ to Cp and back-donation from Cp to IrH⁺ by above mixed orbitals. The numbers of donation electron and back-donation electron are 0.1164 and 0.1361 and the back-donation interaction is greater than donation.

Previous studies have shown that catalysts containing transition metals can effectively promote the reaction of acetylene cyclization to form cyclobutadiene ligand and benzene [23-29]. Cp is important ligand of sandwich structure compound whose importance has been shown in the fields of organic synthesis, catalysis, medicinal chemistry, pesticides, and chemical industry [36, 37]. Normally, due to the thermodynamic stability and kinetic inertness, the cycloaddition reaction of ethylene and methane to form pentamethylcyclopentadienyl ligand with the activation and forming of C-C bond and C-H bond does not happen at room temperature [5-8]. Besides, the ring with odd numbers of carbon atom is more difficult to generate than the ring with even numbers of carbon atom, because -C₂H₂ has the same π orbitals and is easier to overlap to form the ring with even numbers of carbon atom, while the ring with odd numbers of carbon atom have to overlap with carbene, which is a dehydrogenation product methane with different π orbital than that of the -C₂H₂. Therefore, despite a lot of work on synthesis of cyclobutadiene ligand and benzene, there is no work on the generation of Cp in the gas phase. In this paper, the cycloaddition reaction of two ethylene and methane by the "catalyst" Ir⁺ can happen to generate the Cp ring ligand by high selectivity, which proceeds by the activation/formation of three C-C bonds and seven C-H bonds and only one products IrC₂H₂⁺, IrC₄H₄⁺ and IrC₅H₆⁺ are found for Ir⁺/C₂H₄, IrC₂H₂⁺/C₂H₄ and IrC₄H₄⁺/CH₄ reactions in this process. The metal ions Ir⁺ plays a key role, which decreases the energy for C-H and C-C bond activation and promotes the overlap of different π orbital. In addition, our study shows that the products Ir(η²-C₂H₂)⁺, Ir(η²-C₄H₄)⁺, Ir(η⁴-C₄H₄)⁺ and IrH(Cp)⁺ are formed by the dative bonds. These may help to understand the reaction cycloaddition mechanism and guide the rational design of new half-sandwich and sandwich catalysts with tailored selectivity and increased efficiency.

In conclusion, the cycloaddition reactions of methane and ethylene mediated by Ir⁺ are designed, which have been studied by gas-phase experiments with theoretical calculations. Experimental results found the high selectivity reactions of Ir⁺/C₂H₄, IrC₂H₂⁺/C₂H₄ and IrC₄H₄⁺/CH_4, and only one products IrC₂H₂⁺, IrC₄H₄⁺ and IrC₅H₆⁺ are confirmed, respectively. Calculations have shown that Ir⁺ can mediate the cycloaddition reaction of CH₄ and two C₂H₄ to generate the half-sandwich structure IrHCp⁺ containing the pentamethylcyclopentadienyl ligand by continuous dehydrogenation reaction with the forming of three C-C bonds and seven C-H bonds. The orbital analysis indicates the mechanism of the cyclization reaction to generation of pentamethylcyclopentadienyl ligand depends on the overlap of -C₂H₂ and carbene π orbitals, which is more difficult than the overlap of same -C₂H₂ π orbitals to form cyclobutadiene ligand and benzene. The calculated QTAIM and CDA data of IrH(Cp)⁺ indicate that the dative bond of IrH⁺ and Cp is formed between the donation from IrH⁺ to Cp and back-donation from Cp to IrH⁺. The back-donation interaction is greater than donation. This study may help to understand the reaction mechanism and metal-mediated ability in cycloaddition reaction of organic compounds, which will be useful to guide the rational design of new catalysts with tailored selectivity and increased efficiency.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The work was supported by Beijing Natural Science Foundation (No. 2214064), the National Natural Science Foundation of China (Nos. 21603037, 21688102, 92161115, 21973016, 91545122), the Fundamental Research Funds for the Central Universities (Nos. JB2015RCY03, JB2019MS052) supported by the fund of North China Electric Power University.

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.02.002.