Borohydride Ion Pair Configurations

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A: Kinetics and Dynamics

Surprisingly Flexible Oxonium/Borohydride Ion Pair Configurations Mojgan Heshmat, and Timofei Privalov J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11851 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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The Journal of Physical Chemistry

Surprisingly Flexible Oxonium/Borohydride Ion Pair Configurations Mojgan Heshmat and Timofei Privalov* Department of Organic Chemistry, Stockholm University, Stockholm, 10691, Sweden E-mail: [email protected]

ABSTRACT We investigate geometry of the oxonium/borohydride ion pairs [ether-H(+)-ether][LA-H(-)] with dioxane, THF and Et2O as ethers and B(C6F5)3 as the Lewis acid (LA). The question is about possible location of the di-solvated proton, [ether-H(+)-ether], with respect to hydride of structurally complex [LA-H(-)] anion. Using the Born-Oppenheimer molecular dynamics and a comparison of the potential and free energies of the optimized configurations, we show that herein considered ion pairs are much more flexible geometrically than previously thought. Conformers with different location of cation with respect to anion are governed by a flat energy-landscape. We found a novel configuration in which oxonium is below [LA-H(-)], with respect to the direction of borane→hydride vector, and the proton-hydride distance is circa 6Å. With calculations of the vibrational spectra of [ether-H(+)-ether][(C6F5)3B-H(-)] for dioxane, THF and Et2O as ethers, we investigate manifestation of the SSLB-type (short, strong, low-barrier) hydrogen bonding in the OHO motif of oxonium cation.

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1. INTRODUCTION Hydrogen bonding and intermolecular ion pairing between a cation and an anion are important topics in chemical, biological and material sciences. On one hand, the hydrogen bond can take an asymmetric form with the “localized” proton – e.g., D-H…D′ where D and D′ are lone pair donor atoms such as oxygen, nitrogen etc. and distances D-H and H…D′ differ significantly (a short D-H and a long H…D′).1, 2, 3, 4 In enantioselective asymmetric reactions, C-H…O hydrogen bonds and configuration-specific ion pairing can be very significant mechanistically.5, 6, 7 But on the other hand, there are reported D-H-D′ cases where the proton cannot be specifically assigned to a one or the other donor atom – i.e., so-called short, strong, low-barrier (SSLB) hydrogen bonds.8, 9, 10 In the SSLB-type hydrogen bond, a short D…D′ distance permits a rather low-energy barrier between the possible asymmetric forms, D-H…D′ and D…H-D′, so that the location of the proton is effectively intermediate between the D/D′ pair of atoms (D-H ≈ H-D′). An important case of SSLB hydrogen bonding is the oxonium cation, [ether-H(+)-ether], corresponding to the proton di-solvated in ether solvent. 8, 9, 10 In this article, we are going to investigate structural isomerism (conformers) of ion pairs in which oxonium, [ether-H(+)-ether], is paired with borohydride anion, [(EWG)3B-H(-)]; here “EWG” denotes bulky electron withdrawing group such as C6F5. We will show surprising structural freedom of [ether-H(+)-ether]/[(EWG)3B-H(-)] pairing and interconversion between conformers with vast structural difference. Herein employed methodology combines the Born-Oppenheimer molecular dynamics at finite (non-zero) temperature, probing of the potential energy “landscape” and considerations of the calculated vibrational (IR) spectra. In addition to phenomena of hydrogen bonding and ion pairing, what makes the complex [ether-H(+)-ether][(EWG)3B-H(-)] particularly interesting is that it can be the product of the heterolytic H2-cleavage in ether solutions of borane Lewis acids (Scheme 1).

11, 12

Traditionally, transition-metal complexes played central role in activation of H2 and enabling hydrogenation reactions.13, 14, 15 Relatively recently, it had been demonstrated that the key step of H2-activation can be achieved using frustrated Lewis pairs (FLPs) with main group elements.16, 17, 18, 19, 20, 21, 22, 23, 24, 25 At present, there is continuing surge of interest in the metalfree activation of H2 by various types of FLPs and the so-called FLP-catalysis of the hydrogenations of a variety of substrates including those with the unsaturated N=C and O=C bonds.11, 12, 17, 20, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 Since Stephan’s seminal work,16 many

reports

describe

such

FLP

reactions

in

the

non-donor

hydrocarbon

or

chlorinated/brominated solvents (toluene, dichloromethane, bromobenzene etc.) and 2 ACS Paragon Plus Environment

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moderately strong donor-solvents (ethers) as well; in the interest of space we are able to mention only a small part of relevant literature. The metal-free activation of H2 in ether solutions of borane Lewis acids (LA) is of particular interest nowadays. 42, 11, 12 According to experiment, a typically used LA is B(C6F5)3, BCF henceforth, or its derivatives with various electron withdrawing groups (EWGs); and 1,4-dioxane, THF or diethyl ether (Et2O) are examples of donor-solvents with suitably strong Lewis basic character due to O-atom donor. For ether solutions of borane LAs, H2-activation was confirmed by 11B and 1H NMR.11, 27

Scheme 1. Schematics of the heterolytic H2-cleavage in oxygen bearing donor-solvent (ether) with borane LA such as BCF, B(C6F5)3, or its analogs.11, 12 The non-coordinated LA and a solvent molecule can be regarded as a so-called “thermally frustrated” LB/LA pair.43 The oxygen bearing solvent molecule is denoted as O(SM), for brevity, and EWG stands for electron withdrawing group.

Spectroscopy and X-ray crystallography data, reported for some salts of protonated solvents with O-atom donor, generally support the notion of di-solvation of the proton – i.e., [ether-H(+)-ether][anion] is valid molecular representation in general.9,

10, 44, 45, 46

For the

observed product of a reaction of H2 with ether solutions of BCF or its analogs, the notion of two ether molecules coordinated to the proton is consistent with reported observations.11, 12, 27 In part, persistent interest in knowing more about the manner of hydrogen bonding in an oxonium cation and specifics of the cation-anion interaction stems from notable effects in vibrational (IR) spectra.

8, 9

The vibrational OHO-contribution can shift from its typical

position at circa 3000 cm-1 for normal hydrogen bonding (O-H…O) to as low as 1000 cm-1 for SSLB hydrogen bonding (O-H-O) – e.g., the IR spectra of salts including oxonium cations. 8, 9, 10, 47

Better understanding of SSLB-type O-H(+)-O bonding – which is quite different from 3 ACS Paragon Plus Environment


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typical N-H…O or O-H…O bonding with asymmetric distances between the proton and hetero atoms – is highly desirable especially with account of molecular motion of cation perturbed by docking to a structurally complex anion. Differently from previously known salts composed of an oxonium cation and borate anion – e.g., Jutzi’s acid [(Et2O)2H(+)][(C6F5)4B(-)] 46

or other salts with borate anions [(EWG)4B(-)] having four substituents on boron

45

– an

(-)

anion of the type [(EWG)3B-H ] has only three boron-bound groups and a certain direction defined by B→H “vector” that is perpendicular to the “equatorial” plane encompassing the trio of the boron-bound C-atoms. This gives, in principle, possibility to two main configurations of [ether-H(+)-ether][LA-H(-)] – see Scheme 2 for details – but thus far only the face-to-face configurational isomer had been discussed in the literature. 30, 50 The question about conformations of [ether-H(+)-ether][LA-H(-)] is important because the complex is considered to be the proton/hydride donor in the mechanisms of recent nonmetal catalyzed hydrogenation reactions.

11, 12, 27, 28, 42, 48, 49, 50

Specifically for the process

outlined in Scheme 1 and its plausible place in the overall mechanism,11, 12 transition state (TS) studies

30, 50

reaffirmed mechanistic importance of [ether-H(+)-ether][LA-H(-)] and

elucidated the minimum energy path (MEP) featuring the optimized ion pair configuration with oxonium at the “face” of [LA-H(-)] and with H(+)…H(-) distance about 3Å to 4Å– this is what we will denote as the face-to-face configurational isomer henceforth (Scheme 2). According to considerations from the literature, the face-to-face configuration of [ether-H(+)ether][LA-H(-)] corresponds to the face-to-face configuration of [ether-H(+)-ketone][LA-H(-)] in which the activated C(carbonyl) is relatively close to the hydride – hence, the notion of facile hydride transfer in nowadays discussed mechanisms. (+)

30, 50

But a scope of possible

(-)

conformers of [ether-H -ether][LA-H ] has not been elucidated thus far.

Scheme 2. Schematic description of two main configurations for [ether-H(+)-ether][LA-H(-)]: A) the face-to-face configuration with oxonium at the “face” of [LA-H(-)] and with H(+)…H(-) distance about 30, 50 3Å to 4Å, already considered in the literature; B) a novel face-to back configuration with (-) (+)… (-) oxonium at the “back” of [LA-H ] and with H H ≈ 6 Å. O(SM) denotes solvent molecule with the O-atom donor (e.g., ether) and EWG denotes an electron withdrawing group (e.g., C6F5). 4 ACS Paragon Plus Environment

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Given all of the above and knowing recent advancements in NMR spectroscopy (e.g., the so-called NOE and diffusion NMR techniques) 51, we became motivated to investigate the scope of accessible molecular structures of [ether-H(+)-ether][BCF-H(-)] with molecular dynamics at finite (non-zero) temperature and the potential energy comparison of optimized isomers. Herein, ether is represented by dioxane, THF and Et2O. We are going to show that the thus far considered face-to-face isomer with H(+) facing H(-) at a distance of circa 3 Å to 4Å is not the only configuration of [ether-H(+)-ether][LA-H(-)] to be concerned about. The sum of cation-anion interactions in this system generates a flat potential energy landscape; thus, we found the face-to-back conformer with H(+)…H(-) distance about 6 Å due to oxonium cation docking “under” [LA-H(-)] anion, with respect to the direction of borane→hydride vector (Scheme 2). Also, we are going to elucidate correspondence between IR spectra and the manner of hydrogen bonding in the oxonium fragment and whether the latter belongs to the category of SSLB bonding with account of specifics of the cation-anion interaction. The calculated vibrational spectra provide us means to compare with experiment-based findings pertaining to known systems composed of the oxonium cation plus borate or other anion. The matter that we do not address herein is the dynamics-based “version” of MEP of H2-recombination/cleavage (Scheme 1) that involves relatively small subset of molecular structures [ether-H(+)-ether][BCF-H(-)] with H(+)…H(-) ⩽ 3.5 Å. For intermediates featured in Scheme 1, the potential energy profile is given in Table S1 in Supporting Information (SI). Although at different temperatures we have successfully simulated passage of the system along the said MEP (Scheme 1) starting from various optimized molecular structures [etherH(+)-ether][BCF-H(-)] with H(+)…H(-) ⩽ 3.5 Å, in the interest of space and clarity herein we can only address the configurational dynamics of [ether-H(+)-ether][BCF-H(-)] without going into details about H2-recombination/cleavage pathway as finite (non-zero) temperature. The latter is a rather complicated matter and we will report on that elsewhere in due time. In the interest of efficiency of BOMD simulations, the anion plus cation system is in gas phase without any additional solvent molecules in the molecular model. One can expect BOMD simulation in gas phase to be a sound approach because the central reason for the findings presented herein is weak ion pairing governed by the sum of the long-range and noncovalent interactions between the cationic [ether-H(+)-ether] and anionic [BCF-H(-)] fragments. Given conclusions derived for similar systems, 8, 9 it is clear that explicit solvation is not going to fundamentally alter the weak ion pairing especially since solvent is already a part of the cationic fragment including the proton. Furthermore, since explicit solute-solvent interactions are expected to add additional “layer” of complexity to quite complex “web” of 5 ACS Paragon Plus Environment


interactions that we deal with already and because the solvation shells are by no means static – thus, our results should give fair estimation of the behavior of the said ion pairs in solvent as well although presence of solvent might affect timescales of interconversion between conformers. With the cationic and anionic fragments, herein considered ion pairs are particularly suitable subjects for simulations using BOMD because the complete separation of reactants is ultimately prohibited by the electrostatic (Coulomb) interactions. 52 In comparison to meta-dynamics sampling techniques,

53, 54, 55, 56

herein used non-constrained BOMD

trajectory propagation has an important advantage that no foreknowledge of the so-called sampling order parameters is required. Still, due to unavoidable computational limitations, in the present article we are going to report more qualitative rather than fully quantitative understanding of the conformers and structural interconversion governed by the multidimensional potential and free energy surfaces describing interactions between oxonium cation and [BCF-H(-)]. Meta-dynamics approaches could be useful for the next stage of a more quantitative exploration which could involve the free energy mapping and explicit solvation. The main Section 3 of this article, Results and Discussion, is organized as follows: Section 3.1.1 presents dynamics of the conformational change from face-to-face to face-toback configuration; Section 3.1.2 and Section 3.1.3 deal with structural dynamics within faceto-face and face-to-back configurational spaces, respectively; Section 3.2 compares BOMDbased vibrational spectra with the sum of harmonic vibrational spectra of cation and anion; Section 3.3 deals with energy-comparison of the optimized BOMD snapshots and harmonic vibrational spectra calculated for complexes with different geometry of the oxonium cation.


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2. METHODS All herein reported Born-Oppenheimer molecular dynamics (BOMD) calculations were carried out using the commercially available package TeraChem (v. 1.9)

57, 58, 59, 60 61

at the

level of the dispersion-corrected density functional theory, DFT-D3,[66] using BLYP functional and the double-zeta basis set 6-31g** with added polarization functions on all atoms. This is sufficiently accurate and reasonably fast approach –methodological questions regarding density functionals, basis set superposition errors and accuracy of calculations involving non-covalent association of fully or partially charged and polarized fragments were already addressed by us previously in some depth.62 Using representative geometry-optimized face-to-face configuration (e.g. Figure 1A) as a starting point in the configurational space, we have computed in total fifty BOMD trajectories at different temperatures within the interval 300 K < T < 450 K. Each trajectory was propagated for 25 ps with 1 fs time step at BLYP-D3/6-31g** level of theory. For T ⩽ 400 K, the majority of 25 ps long trajectories have not featured H2 recombination. To be specific: two out of ten trajectories recombined H2 at T=300 K, four out of ten trajectories recombined H2 at T=350 K, two out of ten trajectories recombined H2 at T=400 K, six out of ten trajectories recombined H2 at T=425 K and nine out of ten trajectories recombined H2 at T=450 K. Dynamics of H2-recombination at finite (non-zero) will be reported elsewhere. Overall, purely conformational dynamics had been sampled for circa 0.75 ns for the case of dioxane plus for a few particularly interesting trajectories the propagation had been extended up to 100 ps. Additionally, a few BOMD trajectories were initiated from representative geometry-optimized face-to-back configuration (e.g. Figure 12B) and propagated up to 100 ps. Although we have mostly focused on dioxane, cases of THF and Et2O have been also simulated using BOMD in the same fashion. In analysis of BOMD trajectories, we include the root mean squared deviation (RMSD): ( , )=

∑ [ ( )

( )]

(Eq. 1)

In Eq.1, ( ) is the position-vector of atom i at time t along trajectory and the sum includes either all N atoms or a select group of particular interest; the reference structure corresponds to time t0. We put structures in maximal coincidence (via translation and rotation) prior to computing RMSD – this is done using RMSD calculation tool in VMD visualization suite.



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The BOMD-based vibrational spectra at finite temperature were computed using the Fourier transformation of the autocorrelation function of the time-dependent total dipole moment as follows: ( )~

< ( ) ( + )>

,

(Eq. 2)

where < ( ) ( + ) > is the autocorrelation of the total dipole moment d(t). 63, 64 Please note that BOMD-based vibrational spectra are free from the harmonic oscillator approximation and pertaining to fully moving and thus non-static system at finite temperature. Calculations which did not involve BOMD were carried out using the commercially available package Gaussian 65 with B3LYP density functional, fairly large split-valence triplezeta basis set 6-311g** with polarization functions on all atoms and the dispersion correction developed by Grimme et al.66,

67

The thermochemical calculations, i.e. the free energy

estimations in Table 1, we carried out with the ideal gas rigid rotor harmonic oscillator approximation using the fully optimized structures. Calculations of vibrational spectra for the optimized (local) structures involved harmonic approximation for all vibrational modes.


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3. RESULTS AND DISCUSSION 3.1. Dynamics of [ether-H(+)-ether][BCF-H(-)] at finite (non-zero) temperature. In order to probe structural evolution of [ether-H(+)-ether][BCF-H(-)] at finite temperature using BOMD, one needs a starting point in the configurational space. To obtain one, we subjected to geometry-optimization arbitrary initial structures with H(+)…H(-) ≈ 3.5 Å – for dioxane, THF and Et2O as ethers, results are shown in Figure 1. These structures are of the face-to-face kind that was considered in previous computational mechanistic studies of H2-cleavage in THF and Et2O solutions of borane Lewis acids (Scheme 1). 30, 50 In the face-toface molecular structures of [ether-H(+)-ether][BCF-H(-)], positioning and orientation of oxonium cation is such that its proton and the hydride of [BCF-H(-)] are facing each other with a relatively close distance (Figure 1). A)

H(-)BH(+) angle is 18o B)

H(-)BH(+) angle is 39o C)

H(-)BH(+) angle is 8o



Figure 1. Examples of the geometry-optimized face-to-face configurations for [dioxane-H(+)dioxane][BCF-H(-)], (A), [THF-H(+)-THF][BCF-H(-)], (B), and [Et2O-H(+)-OEt2][BCF-H(-)], (C). All distances are in Å. Below each optimized structure, we report the H(-)BH(+) angle.

For the most part of BOMD simulations, we focused on [dioxane-H(+)-dioxane][BCFH(-)] as representative example of [ether-H(+)-ether][BCF-H(-)] system. Using the face-to-face configuration (Figure 1A) as a starting point in the configurational space, we have computed in total fifty BOMD trajectories at different temperatures within the interval 300 K < T < 450 K. Each trajectory was propagated for 25 ps with 1 fs time step at BLYP-D3/6-31g** level of theory. For some trajectories, propagation had been extended to 50 ps in order to reveal more about possible structural dynamics. With BOMD simulations, our general goal had been twofold: (i) to elucidate whether at finite temperature dynamics follows or deviates from the minimum energy path (MEP) outlined by intermediates in Scheme 1, and (ii) to ascertain structural dynamics of [dioxane-H(+)-dioxane][BCF-H(-)] at finite temperature. We have achieved both goals but in this article we focus only on the matter of structural dynamics, simply in the interest of space and because it is more a novel matter, in our opinion, than the matter of dynamics-based refinement of the said MEP (Scheme 1). For T ⩽ 400 K, the majority of 25 ps long trajectories have not featured H2 recombination and revealed wealth of information about structural dynamics of [dioxane-H(+)-dioxane][BCF-H(-)] complex. 3.1.1.

Movement of oxonium cation to the back of BCF-H(-) at finite temperature.

One of our main results is that oxonium cation, [dioxane-H(+)-dioxane], had been able to move all the way to the back of [BCF-H(-)] in five BOMD trajectories initiated from the faceto-face configuration (Figure 1A) and propagated at T < 400 K. In all cases, the timescale of this conformational change had been about 5 ps. On the basis of an exemplary trajectory propagated at modest T ≈ 355 K, Figure 2A-C shows time-evolutions of the hydride-boron-proton angle, H(-)BH(+), the proton…hydride distance, H(+)…H(-), and the so-called root-mean-squared-deviation of atomic positions (commonly abbreviated as RMSD). We put structures in maximal coincidence (via translation and rotation) prior to computing RMSD according to Eq. 1. Figure S1 in SI shows timeevolution of the H(+)CCC dihedral angles which additionally describe the position of H(+) with respect to the plane of the boron-bound carbons. In the herein shown example, the transition from the face-to-face to the face-to-back configuration takes place during the time-interval 4.5 ps < t < 9.5 ps and this time-interval is highlighted in Figure 2A-C; after the conformational change, the system settles in and remains in the face-to-back configuration for the duration of trajectory-propagation. 10 ACS Paragon Plus Environment

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A) 180 Hydride B Proton angle

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3 2 1 0 0

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10 time (ps)

Figure 22. For [dioxaane-H(+)-diox xane][BCF-H H(-)] at T ≈ 35 55: the time-evolutions oof H(-)BH(+) angle a (A), (-)… (+)) H H distance (B B) and RMSD D (C). The sttarting point is the face-to-face configguration (Fig gure 1A). The trannsition from m the face-tto-face to tthe face-to-b back config guration takees place du uring the



highlighted interval 4.5 ps < t < 9.5 ps. Angles are in degrees, distances and RMSD are in Å and time (t) is in ps. All BOMD-snapshots are aligned in maximal coincidence prior to RMSD calculations.

In Figure 3A we show representative BOMD-snapshot at t ≈ 6.5 ps illustrating a transient configuration with oxonium cation at a side of [BCF-H(-)] and oxonium’s H(+) within the plane of the boron-bound carbons. In Figure 3B-C we show two representative BOMDsnapshots at t > 10 ps illustrating the face-to-back configuration of [dioxane-H(+)dioxane][BCF-H(-)] with oxonium below but somewhat on a side of [BCF-H(-)], H(-)BH(+) angle is circa 140o, and with oxonium directly below [BCF-H(-)], H(-)BH(+) angle is circa 180o. In Figure 4, the time-interval 4.5 ps < t < 9.5 ps of the configurational change from the faceto-face docking motif to the face-to-back motif is illustrated by an overlay of every 10th BOMD snapshot from the trajectory we use as an example herein. This is a direct evidence of the herein reported conformational change for [dioxane-H(+)-dioxane][BCF-H(-)] system at modest T ≈ 355 K; all BOMD-snapshots are aligned in maximal coincidence. As envisioned in Scheme 2 in the Introduction of this article, the difference between the face-to-face configuration (Figure 1A) and the face-to-back configuration (Figure 3B-C) is the position of oxonium cation, [dioxane-H(+)-dioxane], with respect to the direction defined by B→H vector. These configurations differ by circa 3.5Å in terms of RMSD and have significantly different H(+)…H(-) distances (circa 3Å versus circa 6Å, respectively). A)

B)


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C)

Figure 3. For illustration of the data presented in Figure 2, BOMD-snapshots of the configurationchange in [dioxane-H(+)-dioxane][BCF-H(-)] system at T ≈ 355 K: (A) a transient configuration with [dioxane-H(+)-dioxane] at the side of borohydride, H(+) is within the same plane as the boron-bound carbons and H(-)BH(+) angle is about 96o and; (B) and (C) snapshots of the face-to-back coordination motifs with [dioxane-H(+)-dioxane] located below [BCF-H(-)], H(-)BH(+) angle is about 140o and 180o.

To the best of our knowledge, this is the very first elucidation of the face-to-back configuration of [ether-H(+)-ether][BCF-H(-)].

Figure 4. An illustration of the transformation from the face-to-face to the face-to-back configuration described in Figure 2 – an overlay of every 10th snapshot of the period of time 4.5 ps < t < 9.5 ps; [dioxane-H(+)-dioxane][BCF-H(-)] system in gas phase at T ≈ 355 K. The color scale is based on the atomic masses: red color assigned to H atoms, the light and dark blue colors assigned to O and F atoms, respectively. All BOMD-snapshots are aligned in maximal coincidence. 13 ACS Paragon Plus Environment


3.1.2. Dynamics within the face-to-face configurational space. The finite duration of trajectory propagation and the finite number of trajectories are unavoidable limitations imposed by realities of dynamics-computation plus data analysis and this can limit the scope of the exposed conformational change. Satisfactorily enough, more than one BOMD trajectory has demonstrated the transition from the face-to-face to face-toback molecular structures within 25 ps or 50 ps long propagation. But starting from the faceto-face configuration (Figure 1A) at t = 0, a much more common behavior had been the motion “merely” within the basin of face-to-face coordination motif. This is what we would like to discuss in this section of the article in order to shed light on the somewhat surprising structural breadth of face-to-face coordination motif at finite temperature (T < 400 K). As mentioned earlier, both [dioxane-H(+)-dioxane] and [BCF-H(-)] are structurally complex and in our case it does not look like the cation/anion docking is described by a small set of geometrical parameters because it is governed by a sum of many long-range noncovalent interactions and involves orientation of oxonium with respect to flurophenyls and adjustable relative orientation of oxonium’s ethers. For the sake of clarity, we follow timeevolution of the most straightforward geometrical parameters. In Figure 5 we show the timeevolution of H(-)…H(+) distance and H(-)BH(+) angle on the basis of the extended (50 ps long) simulation of [dioxane-H(+)-dioxane][BCF-H(-)] system at T ≈ 350 K. Additionally, in Figure 6 we show the corresponding histograms for which one has the mean value = 3.6Å with a large standard deviation σHH = 1.1 Å and the mean value < H(-)BH(+) angle > ≈ 25o with also a large standard deviation σHBH = 30o. The dominant contribution in the sampled face-to-face coordination motif corresponds to the proton…hydride distance about 3 Å and the hydride-B-proton angle about 25o; however, that is accompanied by a sizable “tail” of structures with oxonium at a side of fluorophenyls in the “upper hemisphere” with respect to B→H(-) vector. In a few cases, further extension of trajectory propagation to even longer 100 ps range eventually produced a transformation to the face-to-back molecular structures alike that described in the previous section of this article. Apparently, both [dioxane-H(+)-dioxane] and [BCF-H(-)] are quite large and structurally complex and although our data leaves no doubt about flexible positioning of oxonium cation with respect to its anionic partner, 100 ps duration for all trajectories had not been deemed feasible. Therefore, we now indicate structural flexibility of the cation/anion pairing and accessibility of the face-to-back configuration in a qualitative rather than quantitative manner but we strongly suspect that given enough propagation time all


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trajectories initiated from the face-to-face coordination motif would eventually visit the faceto-back coordination motif and vice versa. A) Proton...Hydride distance (Å)

7.5 6.5 5.5 4.5 3.5 2.5 1.5 0

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50

time (ps)

B) Hydride-B-proton angle (degrees)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


140 120 100 80 60 40 20 0 0

5

10

15

20

25 time (ps)

Figure 5. Data from 50 ps long evolution of [dioxane-H(+)-dioxane][BCF-H(-)] system at T ≈ 355 K within the face-to-face configurational space: (A) time-evolution of the hydride…proton, H(-)…H(+), distance; (B) time-evolution of H(-)BH(+) angle. Time is in ps.



A)

B)

Figure 6. Statistical representation of the data shown in Figure 5 pertaining to a sub-set of trajectories showing motion within the basin of face-to-face configurations at T ≈ 355 K: (A) histogram of proton…hydride distance, H(-)…H(+); mean value is 3.6 Å with standard deviation 1.1 Å; (B) histogram of hydride boron proton, H(-)BH(+), angle; mean value is 50o with standard deviation 30o.

Taking stock now and with account of Figure 7, profound structural diversity of [dioxane-H(+)-dioxane][BCF-H(-)] system at finite temperature can be exposed in a meaningful manner with relatively a small number of BOMD-simulations (trajectories) propagated for moderately long durations of 25 ps or 50 ps. Starting from some optimized structure alike that shown in Figure 1A at t = 0, some of finite-duration trajectories show transition to the face-toback molecular configuration; some trajectories do not go that far and “only” show largeamplitude structural dynamics within the space of face-to-face molecular configurations. Over all, this concurs rather well with the notion of weak ion pairing permitting structural quasicontinuum, according to the literature,8 and the flat/even energy landscape perspective given further on in Section 3.3 (Table 1). All of this combined, the notion of structural quasicontinuum is a significant departure from previous considerations focused only on single (ad hoc, essentially) optimized face-to-face structure of [ether-H(+)-ether][BCF-H(-)] with closely positioned proton and hydride (i.e., the proton…hydride distance about 3 Å).30, 50 16 ACS Paragon Plus Environment

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Figure 7. Dynamic-based illustration of the extensiveness of the configurational space of the face-toface isomer of [dioxane-H(+)-dioxane][BCF-H(-)]; a view from the top. In addition to the data presented in Figures 5-6 on the basis of 50ps long trajectory calculated at T ≈ 355 K with 1 fs time step for [dioxane-H(+)-dioxane][BCF-H(-)] system, here we show an overlay of every 10th snapshot. All snapshots are aligned in maximal coincidence. The color scale is based on the atomic masses: red color assigned to H atoms, the light and dark blue colors assigned to O and F atoms, respectively.

3.1.3. Dynamics within the face-to-back configurational space. Since the face-to-back configuration of [ether-H(+)-ether][BCF-H(-)] system is a novelty, to the best of our knowledge, it can be instructive to take a closer look at it at finite temperature using dynamics. Now we discuss one of a few additional BOMD-trajectories initiated specifically from the fully optimized face-to-back isomer of [dioxane-H(+)dioxane][BCF-H(-)] system (e.g. Figure 12B in Section 3.3) and propagated at T ≈ 350 K for 40 ps with 1fs time step. For the duration of propagation, this trajectory and its analogs remained within the basin of the face-to-back isomer of [dioxane-H(+)-dioxane][BCF-H(-)] system.



A)

B)

C) Hydride B Proton angle (degrees)

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175 165 155 145 135 125 115 0

5

10

15

20

25

30

35

40

time (ps)

Figure 8. (A) and (B): statistical representation of a BOMD-trajectory initiated from the fully optimized face-to-back configuration of [dioxane-H(+)-dioxane][BCF-H(-)] and propagated for 40 ps at T ≈ 350 K (σT = 27 K) with 1fs time step. (C) hydride-boron-proton, H(-)BH(+), angle versus time in ps. For all of its duration, this trajectory (and its analogs) remained within the face-to-back basin.

Figure 8A/B shows statistical representation of a BOMD-trajectories initiated from the fully optimized face-to-back isomer and propagated for 40 ps at T ≈ 350 K (σT = 27 K) with 1fs time step. For the histogram of the proton…hydride distance, Figure 8A, the mean value is 18 ACS Paragon Plus Environment

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6.37 Å with standard deviation 0.31 Å; for the histogram of hydride-B-proton angle, Figure 8B, the mean value is 156o with the standard deviation 12o. To illustrate dynamics of [dioxane-H(+)-dioxane][BCF-H(-)] system in the apparently broad space of face-to-back configurations, in Figure 8C we show the hydride-boron-proton, H(-)BH(+), angle versus time. Values of H(-)BH(+) angle at circa 175o and circa 145o corresponds to two major local versions of the face-to-back isomer; timescale of conformational oscillations is in the range of 5 ps. With the said conformational oscillations taking place, [dioxane-H(+)-dioxane] remains docked to an area below [BCF-H(-)] – as illustration, in Figure 9 we show an overlay of every 10th snapshot from the herein discussed BOMD-trajectory initiated from optimized face-toback configuration and propagated for 40 ps at T ≈ 350 K (σT = 27 K) with 1 fs time step.

Figure 9. Dynamic-based portrayal of the of the extensiveness of the configurational space of the face-to-back isomer of [dioxane-H(+)-dioxane][BCF-H(-)]. In addition to the data presented in Figure 8 on the basis of 40ps long trajectory calculated at T ≈ 350 K with 1 fs time step for [dioxane-H(+)dioxane][BCF-H(-)] system, here we show an overlay of every 10th snapshot. All snapshots are aligned in maximal coincidence. The color scale is based on the atomic masses: red color assigned to H atoms, the light and dark blue colors assigned to O and F atoms, respectively.

The point here is that the face-to-back structural isomer of [dioxane-H(+)-dioxane][BCFH(-)] has an extensive configurational space likewise the face-to-face structural isomer. That and the data presented in Section 3.1.1 of this article indicate a facile connection between the face-to-back and the face-to-face motifs. For [ether-H(+)-ether][BCF-H(-)] with THF or Et2O, we have carried out 40 ps long BOMD simulations also starting from face-to-back molecular structures as described above in this section for [dioxane-H(+)-dioxane][BCF-H(-)]. Without going into repetitive details, for THF and Et2O obtained results were fully analogous to the



data presented in this section of the article; in Figure S2 we show THF and Et2O analogs of BOMD-snapshot overlays shown in Figure 9 for [dioxane-H(+)-dioxane][BCF-H(-)]. 3.2. Calculated vibrational spectra as indication of weak cation/anion pairing. It can be instructive to have a look at the calculated vibrational (IR) spectra for [etherH(+)-ether][BCF-H(-)] complexes and their cationic and anionic parts per se. For example, Figure 10A shows the calculated vibrational spectra for the optimized [BCF-H(-)] anion and [dioxane-H(+)-dioxane] cation per se (in blue and red, respectively) and the sum of these two spectra (in black). Figure S3A and Figure S4A in SI show the calculated vibrational spectra for the cases of THF and Et2O as ethers. These calculations we carried out within the harmonic oscillator approximation applied for each vibrational mode. In the optimized geometry of the isolated cation [ether-H(+)-ether], the O…O distance is 2.39 Å with dioxane or THF and 2.40 Å with EtO2 as an ether, respectively, at the B3LYP/6-311g** level of theory. Accordingly, the O-H distances are 1.19 Å and 1.202 Å. For [ether-H(+)-ether] cations per se (ether = dioxane, THF and Et2O), our calculated vibrational spectra agree with the literature regarding low-frequency shift of signature bands associated with OHO fragment due to SSLB-type bonding.8 For example, for [Et2O-H(+)-OEt2] cation (Figure S4A) we indeed find a broad band centered at 1000 cm-1 and relatively a sharper feature at 1500 cm-1, in agreement with results reported by Stoyanov et al. 8. A) IR activity, arb. units

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BCF-H Diox-H-Diox BCF-H + Diox-H-Diox

1.2 1 0.8 0.6 0.4 0.2 0 0

500

1000

1500

2000

wavenumber

(cm-1)


2500

3000

3500

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B)

Figure 10. (A) The calculated vibrational spectra of [BCF-H(-)] anion (in blue) and [dioxane-H(+)dioxane] cation (in red) together with the sum of the anion’s and cations spectra (in black); the harmonic oscillator approximation for each vibrational mode. (B) BOMD-based vibrational spectra computed using the Fourier transform of the autocorrelation function of the time-dependent total dipole moment according to (Eq. 2) at T ≈ 350 K. For the calculated spectra shown in (A), the full width at half maximum (FWHM) of the Lorentzian line shape convolution is 100 cm-1. Since BOMDbased vibrational spectrum in (B) pertains to the moving system at actually finite (non-zero) temperature, it is shown “as is” without additional convolution to a set line broadening.

The vibrational spectra can be computed on the basis of BOMD simulations using the Fourier transform of the autocorrelation function of the time-dependent total dipole moment according to (Eq. 2). For [dioxane-H(+)-dioxane][BCF-H(-)] at circa 350 K, the result is presented in Figure 10B; Figure S3B and Figure S4B in SI show the BOMD-based vibrational spectra for the cases of THF and Et2O as ethers. As one can see in Figure 10B, the BOMDbased vibrational spectra have the strongest bands at 1000 cm-1 and 1500 cm-1 and practically no significant contribution from OHO neither at 2000 cm-1 nor at 3000 cm-1. The BOMDbased vibrational spectra agree with the mere sum of the cation’s and anion’s vibrational spectra and this is well in line with the notion of weak ion pairing in BCF-H(-)/oxonium(+) systems with dioxane, THF and Et2O. For [dioxane-H(+)-dioxane][BCF-H(-)] at T ≈ 350 K, as an example, Figure 11 shows distributions of O…O distance and |Δ(O-H)| – that data concurs with geometrical characteristics typically associated with SSLB-type OHO bonding in a situation with weak ion pairing.8, 44 For [ether-H(+)-ether] cation per se, a small 0.2 Å elongation of O…O distance from its optimized equilibrium, ≈ 2.4 Å, corresponds to circa 2 kcal/mol increase of the 21 ACS Paragon Plus Environment


potential energy. Therefore, dynamics of [ether-H(+)-ether][BCF-H(-)] produces tight distribution of O…O distances (Figure 11A). For |Δ(O-H)|, the distribution (Figure 11B) is relatively broad because O-H bonding asymmetry can arise in OHO fragment without the O…O distance increase but from other perturbations of OHO motif in [ether-H(+)-ether] cation due to its interaction/docking to [BCF-H(-)] anion. The BOMD-based data presented in Figure 11B indeed indicates that predominantly a short O…O distance, ≈ 2.4 Å, permits a rather lowenergy barrier between the possible asymmetric forms O-H…O and O…H-O so that the location of the proton is effectively intermediate between the O-atoms in the oxonium cation. The possible asymmetric forms O-H…O and O…H-O are very close to each (Figure 11B). Therefore, the BOMD-based vibrational spectra at finite temperature firmly correspond to the expected manifestation of SSLB-type hydrogen bonding (Figure 10) – i.e., the cation/anion pairing is weak and thus the OHO motif of the oxonium cation in the complete cation plus anion system is close to the one in the free cation per se. A)

B)

Figure 11. Histograms of O…O distance (A) and Δ(O-H) (B) in the oxonium cation according to representative BOMD simulation of [dioxane-H(+)-dioxane][BCF-H(-)] at T ≈ 350 K.


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3.3. The energy differences – the face-to-face vs. face-to-back configurations. At last, we would like to ascertain the energy difference between the bonding motifs with the proton of [ether-H(+)-ether] facing the hydride of [BCF-H(-)] versus the bonding motif with [ether-H(+)-ether] at the back of [BCF-H(-)] – i.e., the face-to-face and face-to-back configurations described above on the basis of BOMD simulations. For comparison of relative energies of face-to-face and face-to-back configurations, a difficulty is that each of these two bonding motifs permits large variation of the optimized cation/anion pair. Thus, one has to deal with fluctuating energy of the optimized face-to-face and face-to-back configurations. Using BOMD snapshots as initial geometries, in Table 1 we report energy differences for seven fully optimized configurations corresponding to the face-to-face and face-to-back manner of oxonium docking to [BCF-H(-)]; structural details (Table S2) and the raw XYZ-data are provided in SI. Representing the face-to-face and face-to-back bonding motifs with a few but not too many, in the interest of space, optimized local configurations is a simple but reliable way of dealing with inherent structural flexibility of the system considered herein. Starting from different face-to-face and face-to-back BOMD snapshots as the initial structures, the process of geometry-optimization converges to different versions of either the face-to-face or face-to-back [ether-H(+)-ether][BCF-H(-)] configurations. Apparently, each of the two main configurations is an ensemble of many energetically close (local) PES minima describing different docking of oxonium cation to [BCF-H(-)] anion (Table 1); coordination of oxonium cation to a large [BCF-H(-)] anion does not appear to be governed by just one geometrical parameter or a “small set” of parameters because cation/anion interactions are governed by a sum of many long-range non-covalent interactions. For seven local configurations in each group reported in Table 1, standard deviation of RMSD is circa 0.6 Å, on average; additional geometrical details are provided in Table S2 in SI. In short, a set of characteristics which distinguish one local configuration from the other includes proton/hydride and proton/boron distances plus the corresponding angles, the orientation of oxonium cation as a whole and relative orientation of its ethers with respect to fluorophenyls.



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Table 1. Relative energies of seven face-to-face and face-to-back optimized molecular configurations representing the complex [ether-H(+)-ether][BCF-H(-)] with dioxane, THF and Et2O as ethers; all complexes are fully optimized and representative face-to-face and face-to-back BOMD snapshots served as initial structures. The total dipole moment, d, is in Debye. face-to-face isomer ΔE kcal/mol

face-to-back isomer

ΔG kcal/mol

d Debye

ΔE kcal/mol

ΔG kcal/mol

d Debye

Δ / Δ kcal/mol

Dioxane: 1.

2.75

0.95

15.02

0.90

0.04

14.93

2.

1.47

0.52

15.59

1.14

1.46

14.03

3.

2.94

1.95

13.61

0.00

0.00

14.73

4.

3.28

1.76

16.97

0.28

0.56

15.33

5.

1.73

-1.07

14.00

0.79

0.35

14.57

6.

1.41

-0.06

14.92

1.07

2.33

15.80

7.

0.28

0.27

13.81

1.00

1.95

15.74

1.98

0.62

0.74

0.96

1.

0.61

0.94

14.08

0.25

0.11

13.91

2.

1.47

0.97

14.82

0.97

0.30

14.71

3.

2.73

0.49

13.82

1.40

0.78

14.07

4.

5.19

3.02

16.59

0.10

0.00

13.21

5.

2.43

2.30

15.65

2.50

1.01

15.18

6.

0.51

1.30

14.36

0.00

0.31

13.72

7.

1.16

0.20

14.26

3.04

2.49

14.76

2.01

1.32

1.18

0.71

1.

2.90

5.35

13.63

0.84

2.22

14.71

2.

4.38

4.14

14.15

0.74

1.24

14.27

3.

0.94

1.46

13.41

4.85

5.27

17.05

4.

3.60

3.92

16.11

0.00

0.00

14.87

5.

0.78

0.10

14.89

0.70

1.34

15.47

6.

2.38

1.33

14.43

1.52

1.28

15.04

7.

4.31

5.26

15.65

2.08

1.52

15.97

2.75

3.08

1.53

1.84

1.24 / -0.34

THF:

0.83 / 0.60

Et2O:

1.22 / 1.24

For each of three ethers in Table 1, relative energies of face-to-face and face-to-back molecular structures are given with respect to the molecular configuration that has the lowest potential energy. For each face-to-face and face-to-back group in Table 1, molecular structures with the lowest relative potential energy are visualized in Figure 12. The face-toface molecular structures, Figures 12A/C/E, are of the kind that was considered in previously published computational mechanistic studies of H2-cleavage in THF and Et2O solutions of borane Lewis acids (Scheme 1).30, 50 In the face-to-face molecular structures of [ether-H(+)24 ACS Paragon Plus Environment

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ether][BCF-H(-)], positioning and orientation of oxonium cation is such that its proton and the hydride of [BCF-H(-)] are facing each other and the proton…hydride distance is about 3 Å. In the face-to-back molecular structures, Figures 12B/D/F, oxonium cation is at the “back” of [BCF-H(-)] anion with respect to the direction of B→H vector and consequently the proton…hydride distance is about 6 Å. A)

B)

H(-)BH(+) angle is 18o C)

H(-)BH(+) angle is 147o D)

H(-)BH(+) angle is 39o E)

H(-)BH(+) angle is 149o F)



Figure 12. Representative and fully optimized face-to-face and face-to-back configurations with the lowest relative potential energy (according to Table 1) for [dioxane-H(+)-dioxane][BCF-H(-)], (A) and (B), [THF-H(+)-THF][BCF-H(-)], (C) and (D), and [Et2O-H(+)-OEt2][BCF-H(-)], (E) and (F). All distances are in Å. Below each optimized structure, we report the H(-)BH(+) angle.



In terms of the averaged relative potential energy difference, in Table 1, the faceto-back configuration appears to be a bit more stable in comparison to the face-to-face configuration but the averaged potential energy difference, Δ, is rather small for each group of seven local configurations. For the estimated free energy, the difference between face-to-face and face-to-back configurations is fairly small as well and qualitatively it is on par with Boltzmann energy already at room temperature (kBT ≈ 0.6 kcal/mol at T ≈ 300 K). Perhaps, what can be a bit surprising at a first glance is that although relative positions of the cationic and anionic fragments are vastly different in face-to-face and face-to-back isomers, these isomers have fairly small overall energy difference (both, Δ and Δ) and similar magnitude of the total dipole moment (d). But this is to be expected, we think, considering that both the electrostatic interaction between the charged fragments (Coulomb interaction, etc.) and various non-covalent interactions (C-H…F interaction, etc.) combine into the effectively long-range force that governs overall interaction between oxonium cation and [BCF-H(-)] anion. Given that the dipole moment remains nearly the same in all herein calculated molecular structures, we do not expect polarity of solvent to drastically alter the energy “landscape” estimation provided by Table 1. Judging by relative energies of face-toface and face-to-back molecular structures (Table 1), lack of appreciable preference of one isomer over the other is comfortably in line with the notion of weak ion pairing between oxonium cation and its anionic counterpart of the type [(EWG)4B(-)].8, 9 For the question whether there is any preferable location of [ether-H(+)-ether] with respect to the hydride of [BCF-H(-)], the answer that our data thus far suggests is that there is none. Data presented in Table 1 herein and Table S2 in SI plus illustrations in Figure 12 and BOMD data described above indicate three-dimensional structural flexibility of oxonium docking to [BCF-H(-)] cation. This three-dimensional flexibility suggests that there can be thus far unaccounted entropic stabilization of [ether-H(+)-ether][BCF-H(-)] as compared to the free energy estimations50 on the basis of geometry optimized a stationary point plus thermochemical calculations using the rigid-rotor harmonic oscillator approximation. In other words, whereas previously the free energy profiles included some inherently local geometryoptimized [ether-H(+)-ether][BCF-H(-)] molecular configuration of the face-to-face type, our data herein indicates structural quasi-continuum for the complex [ether-H(+)-ether][BCF-H(-)]. According to published X-ray crystal analyses, the O…O distance for proton disolvates with O-atom solvent molecules is in the range 2.37 – 2.43 Å; this is considered to be meaningfully shorter than the average distance of circa 2.56Å corresponding to ice or protonated water clusters.8 Across all 42 optimized molecular structures, underlining Table 1, 26 ACS Paragon Plus Environment

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the averaged O…O distance is 2.43 Å and with standard deviation σOO = 0.03 Å. This is in line with BOMD data (Figure 11). For the vibrational (IR) spectra of proton solvates, [ether-H(+)-ether], it is significant whether the proton-oxygen bonds are close to normal unsymmetrical “short-long” motif, OH(+)…O, or the symmetrical “short-short” motif – i.e., whether the difference between O-H distances, |Δ(OH)|, is relatively large or small. The “normal” case of relatively large |Δ(OH)| corresponds to ice or protonated water clusters and there one finds the principal OHO stretch at its normal position near 3000 cm-1 in IR spectra. But for the symmetrical short-short hydrogen bonding |Δ(OH)| ≈ 0 and thus the OHO-band appears near 1000 cm-1 – this is measurable indication of short, strong, low-barrier (SSLB) hydrogen bonding as mentioned in the Introduction.8, 9 Across all optimized molecular structures in our case, averaged |Δ(O-H)| is circa 0.22 Å with σOH ≈ 0.1 Å (Table S2 provides geometrical details). Although the magnitude of |Δ(O-H)| is seemingly small the local molecular structures with relatively a larger and a smaller |Δ(O-H)| can produce distinctively different calculated IR-spectra. As an example, in Figure 13 we show the calculated vibrational spectra for two representative optimized [dioxane-H(+)-dioxane][BCF-H(-)] complexes featuring relatively a small, 0.07Å, and a large, 0.28Å, difference between OH distances, |Δ(O-H)|; these calculations use harmonic approximation for all vibrational modes. In SI, Figure S5 shows the calculated spectra for the cases of THF and Et2O as ethers. A seemingly small difference in |Δ(O-H)| is enough for a meaningful difference in the vibrational spectra of the optimized (local) complexes. The complexes with a more perturbed cation have lager |Δ(O-H)| and therefore a line at circa 2000 cm-1 in the harmonic vibrational spectrum. The calculated IR spectra do not distinguish the face-to-back and face-to-face configurations as long as they have fairly similar OH-asymmetry parameter |Δ(O-H)|. For the face-to-back and face-to-face configurations, geometry optimization gives complexes with fluctuating |Δ(O-H)| and it is |Δ(O-H)| that determines the position of OHO contribution in the calculated IR spectra of a given local complex. As Table S3 shows, B3LYP and BLYP produce acceptably similar results.


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IR activity, arb. units


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1

Diox. |Δ(O-H)|=0.07 Å

0.8

Diox. |Δ(O-H)|=0.28 Å

0.6 0.4 0.2 0 0

500

1000

1500

2000

wavenumber

2500

3000

3500

(cm-1)

Figure 13. The calculated vibrational spectra of two representative optimized [dioxane-H(+)dioxane][BCF-H(-)] complexes featuring relatively a small and a large difference between OH distances, |Δ(O-H)|; the harmonic oscillator approximation for each vibrational mode.

CONCLUSIONS We have investigated [ether-H(+)-ether][LA-H(-)] system using Born-Oppenheimer molecular dynamics (BOMD) trajectories at finite (non-zero) temperature and did energycomparison of a set of representative optimized structures. The system considered herein is mechanistically important for the field of hydrogen bonding and for FLP-catalyzed hydrogenation of ketones with H2 in ether solutions of borane Lewis acid.11,

12

The main

question that we address is about possible location of the di-solvated proton with respect to hydride in [ether-H(+)-ether][BCF-H(-)] system at finite (non-zero) temperature. On one hand, thus far made considerations in the literature only portray the di-solvated proton located in the general area of the hydride at about 3 Å distance.

30, 50

On the other hand, it had not been

investigated whether other configurations of [ether-H(+)-ether][BCF-H(-)] might be possible. For [ether-H(+)-ether][BCF-H(-)] we have uncovered a new configuration with the oxonium cation docked below [BCF-H(-)] with respect to the direction of B→H vector – i.e., the face-to-back configuration with the proton…hydride distance about 6 Å. Such surprising configuration is energetically at the same level as the previously considered structures with the proton/hydride proximity; and, with BOMD we have demonstrated interconversion between different configurations at finite temperature. This is in line with the notion of weak ion pairing in other known oxonium salts and with the observed face-to-back isomer of the ion pair composed of Cy3P-H(+) and (borane)LA-H(-) fragments; “Cy” abbreviates cyclohexane.8,

9, 18, 68

In previous mechanistic studies involving [ether-H(+)-ether][BCF-H(-)] 28 ACS Paragon Plus Environment

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with THF and Et2O ethers, only the face-to-face configuration with [ether-H(+)-ether] docked in the general area of hydride on top of [BCF-H(-)] had been considered. This have led to the notion of the oxocarbenium cation, [ether-H(+)-ketone], docked to [BCF-H(-)] in such a way that there could be a direct path for hydride transfer to the activated carbonyl carbon. 11, 12 Herein described face-to-back configuration indicates that previous considerations of the potential role of [ether-H(+)-ether][BCF-H(-)] as hydrogen donor have limited structural basis. Our results invite consideration of possible alternatives to the direct hydride-transfer path in ketone’s hydrogenation – one possibility could be the proton/acid catalyzed hydrogenation of ketones in Lewis basic solvent (ether) via splitting of hydrogen at the activated carbonyl carbon.69 Calculation of vibrational (IR) spectra gave insight into possible structure-property relations. At the level of geometry optimized and inherently local structures, for some optimized [ether-H(+)-ether][BCF-H(-)] molecular structures one has vibrational signature of SSLB-type OHO bonding with weak ion pairing but not for others all the while all of said molecular structures are generally located on a flat energy landscape but have different degree of anion influence on the geometry of hydrogen bonding in the “docked” oxonium cation. The BOMD-based vibrational spectra firmly concur with characteristics typically associated with SSLB-type OHO bonding in a situation with weak ion pairing. According to results presented herein, it might be insufficient to represent [ether-H(+)ether][LA-H(-)] system only with its face-to-face molecular structure in which [ether-H(+)ether] straightforwardly docks on top of [LA-H(-)]. Movement of the cationic [ether-H(+)ether] with respect to the anionic [LA-H(-)] rearranges the pattern of non-covalent interactions – this is highly flexible and weak ion pairing situation for which the long-range forces control interactions between the cationic and the anionic fragments. Different manners of docking of the oxonium cation to structurally complex [BCF-H(-)] anion do not correspond to significant potential (free) energy differences. Therefore, it is critical to account for distinctively different structural isomers which can be readily accessible via molecular motion at finite (non-zero) temperature – even the most simple energy landscape perspective includes variety of molecular structures within 3 kBT thermal energy interval. Dynamics of herein considered system strongly resembles roaming in the vast ensemble of loosely bound local minima in the configurational space. For [ether-H(+)-ether][LA-H(-)] system, herein we do not inquire into dynamics of H2-recombination and interaction with a substrate (ketone); we do that in the interest of space. For the same reason, herein we do not address explicitly solvated [etherH(+)-ether][LA-H(-)] intermediate. Therefore, we plan to continue dynamics-based 29 ACS Paragon Plus Environment


investigations of systems with oxonium and oxocarbenium cations with focus on effects which could originate from molecular motion at finite temperature under influence of longrange forces acting upon fragments of mechanistically significant ionic intermediates. That said, we hope that this article will be helpful for better understanding of the so-called SSLB (short, strong, low barrier) hydrogen bonding and, primarily, further development and mechanistic understanding of promising and actively researched non-metal but FLP-catalyzed hydrogenation of ketones with H2 in ether solutions of borane Lewis acid. Acknowledgements We thank Carl Tryggers Foundation for Scientific Research (PostDoc grant CTS 15:396) for financial support. Part of calculations reported in this article had been carried out using resources provided by computer-center HPC2N via the project SNIC 2017/1-338. Supporting information for this article includes Table S1 with the potential energy profiles including intermediates depicted in Scheme 1; Table S2 with a summary of essential geometrical characteristics of fully optimized complexes underlining Table 1; Table S3 with additional information pertaining to the calculated IR spectra of different configurations and comparison of results obtained with B3LYP and BLYP functionals, Figure S1 with additional information for data presented in Section 3.1.1; Figure S2 with additional illustrations of the face-to-back configurations with THF and Et2O; Figure S3-S5 with the calculated IR spectra for [ether-H(+)-ether][LA-H(-)] with THF and Et2O; XYZ data for all optimized molecular complexes and BOMD snapshots.

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TOC graphics:

TOC-entry (text):

The basic question that we address now is – where is the proton with respect to hydride in [ether-H(+)-ether][BCF-H(-)] system at finite (non-zero) temperature? Among all insights this article might give, the principle one is that the oxonium cation docks equally well to the back of [BCF-H(-)] – something that had not been taken into account previously in the literature.

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Borohydride Ion Pair Configurations

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