Tuning catalytic activity of dimolybdenum paddlewheel complexes by ligands: mechanism study on the radical addition reaction of CCl4 to 1-hexene

The detailed catalytic mechanism of a series of paddlewheel complexes [Mo2L4] featuring Mo-Mo quadruply-bond on radical addition of CCl4 to 1-hexene was studied using density functional theory. Different ligands of Mo-Mo bond are investigated to illustrate the ligand effect on the catalytic activity. The results show that the Mo-Mo quadruply-bond paddlewheel complexes have high catalytic activities on the title reaction. The whole reaction involves 4 steps. Firstly, the C-Cl bond of first CCl4 is activated by [Mo2L4] catalyst, and [Mo2L3Cl] and CH3COOCCl3 are obtained. Then the second CCl4 adds to [Mo2L3Cl] to produce [Mo2L3Cl2] and·CCl3 radical;·CCl3 radical interacts with 1-hexene to get an addition, the addition product which reacts with one Cl atom of [Mo2L3Cl2] to get the last product nBuCHClCH2CCl3 and regenerate [Mo2L3Cl]. The addition of the first CCl4 to [Mo2L4] catalyst is the rate-determining step of the whole reaction. Because this step is not in the catalytic cycle, the reaction would speed up after a certain period of time. The catalytic activity of dimolybdenum paddlewheel complex is depended on the natural population analysis (NPA) charge of Mo and the redox potential E(Mo24+/Mo25+). The higher NPA of Mo atom and higher E(Mo24+/Mo25+) of the catalyst, the higher catalytic activity it has. Our results provide an explanation for experimental observations and useful insights for further development of bimetallic catalysts in radical addition reactions.

The study of K. Mashima et al. showed that Mo 2 (OCOAr) 4 (Ar=2,4,6-i Pr 3 C 6 H 2 ) was used for a catalytic radical addition reaction of CCl 4 to 1-hexene in THF-d 8 at 80°C to give 1,1,1,3-tetrachloroheptane regioselectively in 84% yield. They also found that the redox properties of [Mo 2 ] complexes are changed by replacing the ligands, and the catalytic activity for the radical addition reaction strongly depends on the redox potential of the [Mo 2 ] complexes [4]. N. J. Patmore found that Ling Wang and Lixia Kang contributed equally to this work. the electronic structure of Mo-Mo quadruple bonds can be tuned through O/S substitution of N by lowering the HOMO energy of Mo 2 and reducing the Mo 2 4+/5+ oxidation potential [27].
In 2016, K. Mashima et al. synthesized a series of mixed ligated tris(amidinate) dimolybdenum complexes as catalysts for radical addition of CCl 4 to 1-hexene [24]. Their experimental investigation showed that the nature of the ligands was a crucial factor for initiating the catalytic reaction. Rational catalytic cycle of radical addition reaction catalysed by [Mo 2 (DAniF) 3 (OCO(CH 3 ))] (DAniF = CH 3 NCHNCH 3 ) is proposed (Scheme 1). The catalytic activities of series of mixed-ligated dimolybdenum complexes are higher than those of homoleptic Mo 2 complexes [24].
In this work, the addition reaction between CCl 4 and 1hexene catalysed by a series of quadruple-bonded dimolybdenum complexes are investigated based on density functional theory calculations. The aims of this work are (1) to illustrate the addition reaction mechanism of CCl 4 and 1hexene catalysed by the quadruple-bonded dimolybdenum complexes, (2) to determine how the ligand tunes the catalytic activity of Mo 2 L 4 , and (3) to screen out the ligands of Mo 2 L 4 with high activity. We seek to provide theoretical prediction for the catalytic activity of quadruple-bonded dimolybdenum paddlewheel complexes on radical addition reaction and to inspire the future applications in organic syntheses.

Computational details
All of the calculations were performed at PBEPBE-D3 [28,29]/Def2-SVP [30,31] level using Gaussian 09 package [32]. A functional, including dispersion correction [33,34], has been proven that it can provide accurate energies for transition metals [35]. The vibrational frequency calculations were calculated at the same level to confirm the stable structure has no and the transition state has only one imaginary frequency. Intrinsic reaction coordinate (IRC) [36,37] was calculated to confirm the linkage relationship between the transition states and stable points. In order to characterize the chemical bond changes in the reaction pathway, the Wiberg bond index and natural population charge were also obtained at the PBEPBE/ Def2-SVP level using NBO 3.1 programme [38].

(OCOCH 3 ) (denoted as CAT) on title reaction
Based on the catalytic cycle for addition reaction of CCl 4 to 1hexene catalysed by CAT (Scheme 1), the catalytic mechanism has been calculated and determined. This transformation involves 4 steps. Firstly, the C-Cl bond of first CCl 4 is activated by CAT, and [Mo 2 L 3 Cl](L=CH 3 NCHNCH 3 ) and CH 3 COOCCl 3 are obtained. Then the second CCl 4 adds to Mo 2 L 3 Cl to produce [Mo 2 L 3 Cl 2 ] and CCl 3 radical; CCl 3 radical interacts with 1-hexene to carry out another addition reaction, and Cl atom migrates from [Mo 2 L 3 Cl 2 ], and products 1,1,1,3-tetrachloroheptane n BuCHClCH 2 CCl 3 and [Mo 2 L 3 Cl] are regenerated. The optimized geometries of the catalyst, reactants, complexes (COM), transition states (TS), and products are shown in Fig. 1, with the main parameters labelled on the geometries. The optimized xyz coordinates for the stationary points are given in Table S1-S22 (Supporting Information). The potential energy surfaces of the C-Cl bond activation are shown in Fig. 2 and those of the addition reactions in Fig. 3. For convenience, the total energy of the reactants of each step is taken as the reference zero of energy. group and interact with each other to form CH 3 OOCCl 3 and [Mo 2 (CH 3 NCHNCH 3 ) 3 Cl] (PCl). In PCl, the Cl atom is above the Mo-Mo bond, and the two Mo-Cl bonds are just the equal.
The potential energy surface of this reaction step is shown in Fig. 2(1). The energy barrier (ΔG # ) of TS1 is 21.5 kcal/mol and that of TS2 is 21.7 kcal/mol, and the energy barriers are not high; thus, this step could happen at mild conditions. Our calculated ΔG # is slightly smaller than that determined by kinetic experimental results, 26.0 kcal/mol at 303K [24]. The experimental value is larger because it is determined in the presence of excess CCl 4 and pyridine; the coordination of pyridine suppresses the reaction, which has been found in former studies [2,8]. In this step, the changes of Mo-Mo bond are slight. In CAT, the bond length of Mo-Mo bond is 2.098 Å, and the Wiberg bond order is 3.18. As the reaction happens, the Mo-Mo bond length increases a little. In PCl, the Wiberg bond order of Mo-Mo bond (3.23) is slightly larger than that in CAT.
Second C-Cl bond activation The second step also begins at the Cl atom of CCl 4 attacking the Mo atom of PCl to form the COM4. Then, this Cl atom links to Mo atom and pushes the Cl atom in PC1 to another Mo atom through TS3 to form [Mo 2 (CH 3 NCHNCH 3 ) 3 Cl 2 ] (PCl2) and CCl 3 radical. As shown in Fig. 2(2), the ΔG # of TS3 is only 1.6 kcal/mol, and the energy of TS3 is lower than the sum energy of separated reactants PCl and CCl 4 , meaning this step can occur easily. In PCl2, the bond order of Mo-Mo decreases to 2.69.
CCl 3 addition to 1-hexene The dissociated CCl 3 radical attacks the C1 atom of 1-hexene with an energy barrier of 9.0 kcal/ mol (Fig. 3). In this reaction process, the spin electron density migrates from the C atom of CCl 3 radical to the C2 atom of 1hexene, COM7 forms. This step has been discussed in our former work [8].
Cl atom migration and [Mo 2 L 3 Cl] regeneration The last step of the reaction is the C2 atom of COM7 interacts with one Cl atom of PCl2, and the Cl atom migrates from Mo atom to C2 atom via TS5. After TS5, COM9 forms, and then it separates to Product (Pro) 1,1,1,3-tetrachloroheptane and PCl. The ΔG # of this step is 16.2 kcal/mol. In summary, the whole reaction pathway contains four steps. Among them, the first C-Cl bond activation process is the rate-determining step and it is not in the catalytic cycle. The catalytic cycle contains the second C-Cl bond activation, CCl 3 radical addition to 1-hexene, Cl atom migration, and [Mo 2 L 3 Cl] regeneration processes. The ΔG # of the first step are about 22.0 kcal/mol; those of other three steps are within 20.0 kcal/mol. Therefore, increasing the reaction temperature could accelerate the first C-Cl bond activation process, and the whole reaction would speed up after a certain period of time and not need the heating.

Influences of ligands on the catalytic activity
Based on the mechanism study, the first step is the ratedetermining step, and the experimental results show that the catalytic activity is correlated to the redox potential of Mo 2 4+ /Mo 2 5+ [4,27]. In order to screen out the high activity catalyst, the electronic structures of [Mo 2 ] complexes with different ligands are calculated. The relationship between the electronic structure and the ΔG # of the first step is found to determine how the ligand tunes the catalytic activity of Mo 2 L 4 . The following catalysts are considered (Scheme 2).
The Gibbs energy barriers (ΔG 1 # ), redox potentials (E(Mo 2 4+ /Mo 2 5+ )), and natural population analysis charges of Mo atom of different catalysts are listed in Table 1. In general, the Gibbs free energy change in solution (ΔG soln ) during the reduction is defined as the sum of the electron affinity (EA) and solvation energy (ΔΔG solv ) [39,40]. The EA equals to the Gibbs free energy change in gas (ΔG gas ) and ΔΔG solv is the Gibbs Fig. 3 Potential energy surfaces of the addition reactions energy changes from the neutral state to anionic state in the solvent states. That is, ΔG soln = EA(ΔG gas )+ΔΔG solv . In this work, the calculation of redox potential is per-formed using a slightly modified procedure following an established method [41]. It is calculated as follows: Geometries were optimized in gas phase followed by single-point energy calculations in solvent CHCl 3 . Solvation was modelled by the polarized continuum model (PCM). The Gibbs free energies of the redox reaction in CHCl 3 were calculated as follows: Then the calculated Gibbs free energies were converted to absolute electron redox potential according to Nernst equation: ) and NPA charge of the catalyst, the higher activity it has.

Conclusions
The detailed catalytic mechanisms of a series of paddlewheel complexes [Mo 2 L 4 ] featuring Mo-Mo quadruply-bond with different ligands on the radical addition of CCl 4 to 1-hexene were investigated. The following conclusions can be drawn: (1) The Mo-Mo quadruply-bond paddlewheel complexes are good catalysts on the radical addition reaction of CCl 4 to 1-hexene.  ) and NPA charge of the catalyst, the higher activity it has.
Author contribution The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.