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A molecular dynamics simulation study on the frustrated Lewis pairs in ionic liquids

Steric hindered frustrated Lewis pairs (FLPs) have been shown to activate hydrogen molecules, and their reactivity is strongly determined by the geometric parameters of the Lewis acid s and bases. A recent experimental study showed that ionic liquids (ILs) could largely improve the effective configuration of FLPs. However, the detailed mechanistic profile is still unclear. Herein, we performed a molecular dynamics (MD) simulations, aimi ng to reveal the effects of ILs on the structures of FLPs, and to present a rule for selecting more efficient reaction media. For this purpose, mixture systems were adopt consisting of the ILs [Cnmim][NTf2] (n= 6, 10, 14), and the typical FLP (tBu)3P/B(C6F5)3 . Radial distribution function (RDF) results show that toluene competes with (tBu)3P to interact with B(C6F5)3 , resulting in a relatively low effective (tBu)3P/B(C6F5)3 complex. [Cnmim][NTf2] is more intended to form a solvated shell surrounding the (tBu)3P/B(C6F5)3 , which increases the amount of effective FLPs. Spatial distribution function (SDF) results show that toluene formed a continuum solvation shell, which hinders the interactions of (tBu)3P and B(C6F5)3 , while [Cnmim][NTf2] leave a relatively large empty space, which is accessible by (tBu3)P molecules, resulting in a higher probability of Lewis acids and bases interactions. Lastly, we find that the longer alkyl chain length of[Cnmim] cations, the higher probability of effective FLPs.


Introduction
On the one hand, Lewis acids (LA) and Lewis bases (LB) with large steric hindrance cannot react with each other to form the thermodynamically stable classic Lewis adducts 1 . On the other hand, the secondary interactions (i.e. dispersion, and hydrogen bonds) between the substituents could result in a LA/LB complex. Such complex gains a name of the Frustrated Lewis pair, FLP, which was initially proposed by Stephan and his co-workers 2 . Interestingly, the first example of FLPs, Mes2P(C6F4)B(C6F5)2, was shown to activate hydrogen molecules (H2) reversibly 2 .
The author found that injection of H2 into the solution of Mes2P(C6F4)B(C6F5)2 at 25 °C generated a product of [Mes2PH] + (C6F4)[BH(C6F5)2] − , and heating the product to 150 °C led to regeneration of H2 and the phosphino-borane reactant. After that, a large number of inter-/intra-molecular FLPs have been explored [3][4][5] . Many important applications have been reported, such as activation of H2 6 , capture of CO2 7 , SO2 8 and NOx 9 , and hydrogenation reduction of several unsaturated chemical bonds, including C=C 10 , C=O 11 , C=N 12 , C≡C 13 , C=N 14 . Nowadays, the FLPs chemistry have become one of the emerge fields because of their special electronic structures and important applications. 15 The interesting application of FLPs towards H2 activation have drawn great attention from the theoretical and computational chemists as well [16][17][18] . Density functional theory (DFT) and molecular dynamics (MD) simulations studies conclude that the formation of FLPs and its reactivity towards H2 is strongly determined by the distance between LAs and LBs. Among the configurations of LA/LB complexes, only those with the correct orientation and suitable LA-LB distance can be considered as reactive pairs which activate small molecules 19 . According to the previous static DFT calculations on the H2 activation by a series of FLPs [20][21][22] , the distance between active centers of reactive FLPs ranges from 3 Å to 5 Å 23 . MD studies further expanded these distances to be no more than 6 Å by taking flexibility of FLPs into account 24-25 . However, MD simulations of (tBu)3P/B(C6F5)3 in toluene found that the amount of effective LA/LB pairs are rather low 25 . The author found that probability of finding pairs with LA-LB distance shorter than 6 Å is only ca. 2 % of the total simulated LA/LB pairs, which decreases to be 0.5 % if the distance is limited to 5 Å.
In a recent study, Brown et al. 26 found that the population of effective FLPs has been significantly increased when replacing the toluene solvent with ionic liquids (ILs). In that work, the authors investigated the structures and interactions of (tBu)3P/B(C6F5)3 in the [C10mim][NTf2] IL by 19 F and 31 P NMR spectroscopy, and they found that the amount of borane-phosphine pairs which have P-B interactions has been increased to be more than 20%. Herein, we performed MD simulations to reveal the mechanism of interactions between ILs and FLPs, and its effects on the structures of FLPs, which is still unclear to the best of our knowledge. The results show that ILs are more intended to form a solvated shell surrounding the LA/LB pairs, which increases the amount of effective FLPs. Moreover, we find that such effects are somehow related to the alkyl chain length of the cations.

Computational details
All MD simulations were performed for the FLP (B(C6F5)3/(tBu)3P), toluene and B(C6F5)3 molecules, previous force field parameters have been utilized without any modification, 26 and atomic interactions were represented by OPLS force fileds [29][30] . For toluene, the OPLS-AA force field [29][30] [31][32] For all systems, energy minimization was first performed. Then an isothermal-isochoric (NVT) equilibration was carried out at a temperature of 300K, and an isothermal-isobaric (NPT) equilibration was performed at a pressure of 1 bar and at a temperature of 300 K. In MD simulations, the integration step was set to be 2 fs. The production runs were performed for 50 ns in NPT ensemble. The equation of motion was integrated by leap-frog algorithms 33 . The temperature was kept constant using the Nose-Hoover thermostat 34 with the relaxation times of 0.2 ps, and the pressure was maintained by using the Parrinello-Rahman barostat 35 algorithm with the coupling time of 2 ps. Bond lengths involving hydrogen atoms were kept constant by the LINCS algorithms 36 . All intermolecular interactions between atoms in the simulated systems were calculated within the cut-off distance of 1.2 nm, and the long-range electrostatics interaction was calculated by Particle Mesh Ewald method 37 .
The radial distribution function (RDF) was obtained from the gmx-rdf module of GROMACS software. 27 The structures are presented by the visual molecular dynamics (VMD) package. 38

Results and Discussion
At first, the force fields parameters were validated by comparing the simulated density of ILs with the experimental data.  Fig. 2. In the case of toluene ( Fig. 2a), the firs peak of (tBu)3P (black line) appears around 5.5 Å, the second peaks appears around 6.5 Å, and the third one appears about 8.0 Å. Moreover, we found that the first peak of the toluene (red line) appear around 6.0 Å, which is very close to first peak of the (tBu)3P. Note that the height of such peak for toluene is almost four times larger than that of (tBu)3P. These findings indicate that in this region, toluene  Next, we analyzed the spatial distribution function (SDF) of the simulated systems, which gives the probability of atoms or molecules appearing in the 3D space around the central molecule. Visualization of SDF was performed using the gOpenMol 3. 00 software [43][44] , and the results are given in Fig. 3a and Subsequently, we selected two represented structures with P-B distance less than 4.8 Å for the solvents of toluene (Fig. 3c) and [C10mim][NTf2] (Fig. 3d) Fig. 2 and Fig.4)

Conclusions
In this work, we performed a series of molecular dynamics (