Morphological Plasticity of LiCl Clusters Interacting with Grignard Reagent in Tetrahydrofuran

Ab initio molecular dynamics simulations are used to explore tetrahydrofuran (THF) solutions containing pure LiCl and LiCl with CH3MgCl, as model constituents of the turbo Grignard reagent. LiCl aggregates as Li4Cl4, which preferentially assumes compact cubane-like conformations. In particular, an open-edge pseudotetrahedral frame is promoted by solvent-assisted Li–Cl bond cleavage. Among the Grignard species involved in the Schlenk equilibrium, LiCl prefers to coordinate MgCl2 through μ2-Cl bridges. Using a 1:1 Li:Mg ratio, the plastic tetranuclear LiCl cluster decomposes to a highly solvated mixed LiCl·MgCl2 aggregate with prevalent Li–(μ2-Cl)2–Mg rings and linear LiCl entities. The MgCl2-assisted disaggregation of Li4Cl4 occurs through transient structures analogous to those detected for pure LiCl in THF, also corresponding to moieties observed in the solid state. This study identifies a synergistic role of LiCl for the determination of the compounds present in turbo Grignard solutions. LiCl shifts the Schlenk equilibrium promoting a higher concentration of dialkylmagnesium, while decomposing into smaller, more soluble, mixed Li:Mg:Cl clusters.

Computational methods S2

2.
Li4X4 (X = Cl and Br) structures observed experimentally and calculated (X = Cl) in this work S4

3.
Free energy profiles for Li4Cl4 as a function of the solvation S5

4.
Calculated structural parameters of LiCl species S6

5.
Free energy profiles for Grignard species interacting with Li4Cl4 S6

6.
Atomic charges of LiCl and associated Grignard·LiCl species S8

7.
Calculated structural parameters of selected clusters. S9

8.
Energy decomposition analysis of selected clusters. S10

AIMD Simulations
The initial model system, created using the PACKMOL package, 1 consists of a cubic periodic box of 20 Å length filled by THF molecules to reproduce the experimental density of THF at room temperature. 2 The followed protocol is analogous to what described in previous works. 3,4 The electronic problem is solved at the DFT level of theory, with the PBE approximation of the exchange-correlation functional, 5 and D3 Grimme correction for dispersion. 6 The choice of the PBE functional is based on benchmark calculations presented in ref. 4 All atoms are described with a DZVP basis set, using a molecularly optimized version for Li and Cl, and with GTH pseudopotentials. Auxiliary plane-waves basis set is cut-off to 250 Ry. AIMD simulations are run in the NVT ensemble using the Velocity Verlet algorithm with 0.25 fs timestep, fixing the temperature at 300K with a CSVR thermostat 7 of 50 ps time constant. AIMD trajectories are computed with the CP2K package 8 and analyzed with the VMD 1.9.4 software. 9 The initial coordinates are obtained following a 30 ps run at the target temperature to ensure equilibration of the system.

Free energy calculations
Free energy surfaces are computed by thermodynamic integration in the Blue Moon ensemble: 10 where l is a generic collective variable.
Li4Cl4: l = coordination number (CN) 11 between Li and O(THF), estimated by: where n = 12, m = 24, R0 = 2.5Å and dij is the distance between each Li and O.
Grignard + Li4Cl4: l = distance (d) between the two species: The constraint force Fc is collected over 10-30 ps runs, monitoring the running average as convergence criterium 12 (discarding the first 5 ps of the trajectory to ensure equilibration at the target value). All graphs report the standard deviation of the constraint forces collected over AIMD trajectories as error bars. The uncertainties on the free energy points are estimated by propagation of the error from the standard deviation of the forces.

Energy Decomposition Analysis
CH3MgCl•Li4Cl4(THF)5, MgCl2•Li4Cl4(THF)7, and Mg(CH3)2•Li4Cl4(THF)6 were optimized at the PBE-D3/TZ2P level, 5 using the COSMO model 13,14 for simulating bulk solvation in THF. These structures were used to perform a quantitative energy decomposition analysis (EDA) 15 as implemented in the ADF software package. 16,17 We selected the monomeric forms of the Grignard S3 reagent (CH3MgCl, MgCl2, and Mg(CH3)2) and the Li4Cl4 as fragments in order to explore the Mg- Quantitative EDA used in this work divides the interaction energy ΔE "%( into four physically meaningful terms: The classical electrostatic interaction ∆Velstat is the energy between the unperturbed charge distributions of the prepared fragments. The Pauli repulsion ∆EPauli, responsible for steric repulsion, arises from the destabilizing interactions between occupied orbitals of the fragments. The stabilizing orbital interaction term ∆Eorb accounts for charge transfer and polarization, and the dispersion energy ∆Edisp is a long-range electron correlation effect.