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The “Decisive” Role for Secondary Coordination Sphere Nucleophiles on Hydrogen Atom Transfer (HAT) Reactions: Does It Exist

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The “Decisive” Role for Secondary Coordination Sphere Nucleophiles on Hydrogen Atom Transfer (HAT) Reactions: Does It Exist The “Decisive” Role for Secondary Coordination Sphere Nucleophiles on Hydrogen Atom Transfer (HAT) Reactions: Does it Exist and What is its Origin? Yumiao Ma*a and Yishan Li b a. BSJ Institute, Haidian, Beijing, People’s Republic of China, 100084 b. Department of Chemistry, Tsinghua University, Haidian, Beijing, People’s Republic of China, 100084 [email protected] Abstract: Although it has been reported that some radical reactions are possibly promoted by external ions, the origin of this phenomenon is unclear. In this work, several hydrogen atom transfer (HAT) reactions in the presence of anions were studied by density functional theory (DFT) calculations, electronic structure analysis and other methods, and it is concluded that both the electrostatic interaction and polarization of the transition state (TS) by the electric field generated by anions play a fundamental role in the TS stabilization effect, whereas the “charge shift bonding” that was previously presumed to be a major contributor is ruled out. Although the stabilization toward TSs in terms of electronic energy (and thus enthalpy) is significant, it should be noted that the effect is almost completely cancelled by entropy and solvation, and further cancelled by the formation of stable resting states. Thus there is still a long way for this effect to be used in actual catalysis. Introduction The “electrostatic catalysis” or “salt effect” is a long-standing and well-established concept. Early in 1990s, the catalytic effect of ions that seem inert at the first glance toward organic chemical transformations has been studied by Craig Wilcox1-3. Later on, the promotion of cobalt-carbon bond dissociation by a nearby charge was found in a biochemistry-related Vitamin B complex4. In the recent years, the catalytic effect of charged groups toward Diels-Alder reaction was studied by Michelle Coote5, 6, and Kendall Houk7. The catalytic effect of charged groups is believed to have an electrostatic nature, proceeding through interaction between the dipolar moment of transition states (TSs) and the electric field generated by nearby charges, and thus is closely relevant to the external electric field effect in chemical reactions, which has been documented extensively in many cases8- 13. It is noteworthy that hydrogen atom transfer (HAT) reactions have also been reported to be affected by metal ions and ligands14-17, which is believed to be a field-induced phenomenon (charge- induced catalysis). On the other hand, however, Thomas Cundari and coworkers reported that external anions provide “decisive” stabilization to the TS for the hydrogen atom transfer (HAT) reaction between methane and hydroperoxyl radical very recently18. The authors concluded that the interaction between anions and the HAT TS is due to “charge shift bonding19, 20” originating from a 2-center-3- electron interaction, which is a brand new explanation for the influence of external ions. Thus it is interesting how much role charge shift bonding plays in the reported reactions, and also in other examples that were previously believed to be field-originated. In this work, we conducted a more detailed investigation on the “salt effect” for HAT reactions, which will provide new understanding toward this long-standing concept. Computational Methods The geometry optimization of all structures were performed with the Gaussian 16 program21, at M11/6-311+G(d,p) level22-26, if not specially mentioned. DLPNO-CCSD(T) calculations were carried out with the ORCA 4.2 program27, 28, in combination with the aug-cc-pVTZ basis set29-31. All electronic structure analysis, including but not limited to bond critical point (BCP) properties, electron localization function (ELF), electron density Laplacian, were performed using the Multiwfn program32, based on the wavefunction obtained at M11/ma-def2-TZVPP level33. The GAMESS-US program34 was employed for LMO-EDA calculations35. It is found that the GAMESS-US program gave wrong results with the M11 functional, and thus M06-2X/6-311+G(d,p) level36 was selected to perform energy decomposition with the M11-optimized geometry. The SMD implicit solvation model37 was used for calculations with solvation effect, and the solvation free energies were obtained by G(M05-2X38/6-31G(d), with SMD(DMSO)) – G(M05- 2X/6-31G(d), gas phase). The final Gibbs free energies were obtained by the sum of DLPNO- CCSD(T) single point energy, M11/6-311+G(d,p) correction to free energy, and solvation free energy (the last term only for calculations in DMSO). Particularly, the solvation free energies for Cl-, Br- and proton are taken from experimental reports39, and those for F-, HO- and HS- were derived 40 41 42 from experimental pKa of HF, H2O and H2S in DMSO, which is 15.0 , 31.4 and 13.7 respectively. Results and Discussions Although the meta-GGA M06-L functional43 was employed in Cundari’s report, it was found from a benchmark study involving M06-L, B3LYP-D3BJ44, 45, M06-2X, wB97xD46 and M11 that the performances of density functionals parallel their Hartree-Fock (HF) components (Table S1), and thus the range-separated functional M11 with a large HF component was chosen to be the functional used for geometry optimization in this work. The TSs were located for the HAT reactions between methane and hydroperoxyl radical (Scheme 1a), in the presence of various anions X-. The energetics and optimized C-X bond lengths at DLPNO-CCSD(T)/aug-cc-pVTZ//M11/6-311+G(d,p) level are listed in Table 1. Note that at this stage only the TSs, but not preactivation complexes or any other resting states are discussed. The full Gibbs free energy surface will be discussed later. Scheme 1. The reactions studied in this work, and the definitions for some quantities discussed. Table 1. The C-X distance (angstrom), TS stabilization energy (E in kcal/mol) and free energy (G in kcal/mol), EA, electron density on the C-X bond critical point (BCP), spin population on X for each TS of reaction (a) in Scheme 1. a b X C-X distance E E G EA ρBCP(C-X) Spin population on X F- 2.4875 -14.8 -12.7 -6.4 -43.1 0.0226 0.073 HO- 2.5459 -18.6 -14.3 -4.9 -6.9 0.0249 0.405 HCOO- 2.7385 -8.6 -7.2 1.9 -55.8 0.0157 0.028 Cl- 2.9875 -10.0 -8.6 -2.2 -47.8 0.0137 0.057 Br- 3.2168 -8.8 -8.1 -1.9 -45.2 0.0124 0.076 HS- 3.0130 -12.6 -8.5 1.3 -18.6 0.0176 0.335 MeS- 2.9020 -16.5 -13.0 -3.2 0.33 0.0222 0.525 a. At M11/6-311+G(d,p) level. b. At DLPNO-CCSD(T)/aug-cc-pVTZ//M11/6-311+G(d,p) level. It is shown in Table 1 that all anions provide significant stabilization (i.e. negative E) to TS1 in terms of electronic energy, although largely cancelled by entropy. The second-row anions, fluoride and hydroxyl anion, are among the most stabilizing ones, whereas the HCOO- with delocalized negative charge exhibits much less stabilization. Interestingly, the heavier anions, Cl-, Br- and HS- provide similar stabilization energy at ~8.5 kcal/mol, while the E value becomes surprisingly much more negative upon replacement of the hydrogen in HS- with the methyl group. Overall, it is concluded that the combination of anions to TS1 is exothermic in most cases, and next we are about to figure out the reason. As suggested by Cundari’s work, we firstly examined the existence of charge shift bonding, which originates from the interplay between two resonance structures shown in Figure 1a. It has been proposed in the early research on the charge shift bonding in silicon-halogen bonds that resonance structures close in energy might lead to larger resonance energy47, and thus stronger charge shift bonding. Herein the energy difference between resonance structure 1 and 2 could simply be characterized by the difference in the vertical electron affinities (EA shown in Scheme 1) for the anion and the “bared” TS1(X=none). In addition, the magnitude of the involvement of 2 is reflected by the spin population on X. It is expected that a EA close to zero, as well as large spin population on X, should indicate strong charge shift interaction. The spin population values are listed in Table 1, and it is seen that for most anions the spin population on X- is negligible, except for OH-, HS- and MeS-, which also exhibit EAs closest to 0. Furthermore, the TS stabilization energy E correlates with both EA and spin population on X terribly. Thus it is questionable whether charge shift interaction could explain the observed energy change. Figure 1. (a) A schematic representation of the resonance structures contributing to “charge shift interaction”. (b) The correlation of E with spin population on the anion. (c) The correlation of E with EA. (d,e) The ELF (d) and electron density Laplacian (e) Contour for TS1(X=HO-). (f) The ELF contour for F2 molecule, a molecule with typical charge shift bonding. (g) The RDG isosurface for TS1(X=HO-), in which weak noncovalent interaction is shown in blue (stronger ones) and green (weaker ones). Traditionally the existence of charge shift bonding is characterized by the properties at bond critical point (BCP), such as the electron density, electron density Laplacian, and electron localization function (ELF). Typical charge shift bonds, such as that in F2 molecule, exhibit large electron density, positive Laplacian, and slightly accumulated ELF at BCP. The electron densities at the BCP located between the carbon atom and anion are recorded in Table 1, and all TSs exhibit negligible electron densities at BCPs.
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