Catalysis of Methyl Transfer Reactions by Oriented External Electric

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Catalysis of Methyl Transfer Reactions by Oriented External Electric Fields: Are Gold-Thiolate Linkers Innocent? Rajeev Ramanan, David Danovich, Debasish Mandal, and Sason Shaik J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b00192 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Journal of the American Chemical Society

Catalysis of Methyl Transfer Reactions by Oriented External Electric Fields: Are Gold-Thiolate Linkers Innocent? Rajeev Ramanan,a David Danovich,a Debasish Mandal,b Sason Shaika* a.

Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel

b

School of Chemistry and Biochemistry, Thapar Institute of Engineering and Technology, Patiala 147 004 Punjab, India.

Abstract: Oriented external-electric-fields (OEEFs) are potent effectors of chemical change and control. We show that the Menshutkin reaction, between substituted pyridines and methyl iodide, can be catalyzed/inhibited at will, by just flipping the orientation of the EEF (F Z ) along the “reaction axis” (Z), N---C---I. A theoretical analysis shows that catalysis/inhibition obey the Bell-Evans-Polanyi Principle. Significant catalysis is predicted also for EEFs oriented off the reaction axis. Hence, the observation of catalysis can be up-scaled and may not require orienting the reactants vis-à-vis the field. It is further predicted that EEFs can also catalyze the front-side nucleophilic displacement reaction, thus violating the Walden-inversion paradigm. Finally, we considered the impact of gold-thiolate linkers, used experimentally to deliver the EEF stimuli, on the Menshutkin reaction. A few linkers were tested and proved not innocent. In the presence of F Z , the linkers participate in the electronic reorganization of the molecular system. In so doing, these linkers induce local-electric-fields, which map the effects of the EEF and induce catalysis/inhibition at will, as in the pristine reaction. However, as the EEF becomes more

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negative than -0.1 V/Å, an excited charge transfer state (CTS), which involves one-electron transfer from the 5p lone-pair of iodine to an antibonding orbital of the gold cluster, crosses below the closed-shell state of the Menshutkin reaction and causes a mechanistic crossover. This CTS catalyzes nucleophilic displacement of iodine radical from the CH 3 I•+ radical-cation. The above predictions and others discussed in the text are testable.

1. Introduction Catalysis is one of the most important areas in synthetic and mechanistic chemistry. Mainstream homogeneous/heterogeneous catalysis utilizes a molecule or a metal surface as catalysts, which enhance the rate of chemical reactions. A potential alternative to these chemical methods is the usage of oriented external electric fields (OEEFs) that function as “invisible catalysts” or co-catalysts to enhance/inhibit the reaction rate and control its selectivity at will.1 This approach has gained experimental support recently by the demonstration2 that the DielsAlder reaction, which involves simultaneous two C-C bond making can indeed be catalyzed by an OEEF stimulus, as predicted theoretically.3 Furthermore, theory showed1,3 that OEEFs can either catalyze or inhibit the reaction, as well as control its exo/endo selectivity. Similar predictions were made for other non-redox reactions, such as hydrocarbon hydroxylation and epoxidation by oxoiron complexes, as those used by cytochrome P450 and nonheme enzymes.1,4 Thus, theory reveals that reactivity and regioselectivity are in principle controllable by applying OEEFs along the “reaction axis”, which is the direction along which the electrons are reorganized during the conversion of reactants to products.1 On the other hand, stereoselectivity – like the exo/endo selectivity in the Diels-Alder reaction1,3 – may require orientations of the OEEF in a direction perpendicular to the reaction axis.


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The impact of external electric fields5-8 (EEFs) or designed electric fields (DEFs)8c,d on structure and reactivity is wide-ranging. There are by now quite a few experimental manifestations where EEF and DEF stimuli affect chemical reactivity and selectivity.8 Establishment of the features of these “invisible catalysts” is therefore timely and important. The present study reveals an important new feature concerning the interplay of the experimental set up and the target reaction. Thus, using the Menshutkin reaction, in Scheme 1,9 the present study explores the role of OEEFs in controlling the nucleophilic methyl transfer processes, with a dual goal in mind:

Scheme 1: Schematic representations of the transition state for the Menshutkin reaction of substituted pyridines with methyl iodide. The conventions (in GUASSIAN) for a positively Z-oriented vector of an electric field (F Z ) and the correspondingly stabilizing molecular dipole moment (µ) are depicted underneath the structure.

Firstly, we wish to explore the impact of OEEFs on reactivity patterns of the Menshutkin reaction, which is a widely used method for alkyl transfer.10 During the reaction, the nucleophile


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such as the substituted pyridines shown in Scheme 1 displaces the halogen from an alkyl halide. The gas phase Menshutkin reaction is generally endothermic and sluggish. Its rate may be increased in a variety of ways, by use of electron releasing substituents on the pyridine, a good leaving group on the alkyl derivative, polar solvents11 (e.g., acetonitrile), and a rather high temperature.12-14 Our first major goal then, is to explore the catalytic/inhibitory power of OEEFs along the reaction axis, and to establish structure-reactivity relations in the Menshutkin reaction series. While so doing, we intend also to elucidate the manner by which the OEEF governs a reaction and alters its rate. Does the OEEF change completely the reactivity patterns or does it exert effects, which follow fundamental concepts in physical organic chemistry like the Bell-Evans Polanyi Principle,15 or the Leffler-Hammond Postulate?16 Can the OEEF change the stereochemistry of the reaction from a Walden inversion to a front-side substitution with retention of configuration? In a pioneering study of the Menshutkin reaction, by Bertran and coworkers,17 it was concluded that a uniform electric field replicates the effect of solvent. We shall demonstrate, however that directionality matters, and the OEEF can also be used to inhibit the methylation by simply reversing the field’s direction. Inhibition is a control measure, which is a as important some times as catalysis. Our second major goal is to explore the interplay between OEEF effects on the target Menshutkin reaction, on the one hand, and on the other, on the thiolate-gold ((CH 2 ) n S-Au m ) linkers of the type used in the novel experimental set up employed by Aragonés et al,2 to achieve chemical catalysis. Thus, as shown in Scheme 2a, Aragonés et al,2 used a diene and a dienophilie that are oriented in the direction of the OEEF by means of thiol linker, which are connected to an STM tip and a gold surface. To mimic these conditions, even if only very approximately, we are


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going to investigate a Menshutkin reaction oriented by one and two thiol linkers connected to Au 5 gold clusters (recommended in ref. 2 as minimal presenters of the STM tip and the gold surface), as shown in Scheme 2b. As argued later, the effect calculated for Au 5 is reproduced with different geometries of the cluster and different number of atoms in Au n (n= 3,5).

Scheme 2: (a) A schematic description of the catalysis of a Diels-Alder reaction effected by a bias voltage between the STM tip (gold tip) and the gold surface. The figure was adapted with permission from ref. 2, Nature Publishing Group. (b) Models A and B used to study the interplay of OEEF- and gold-thiol linkers - effects on the Menshutkin reaction. Au 5 clusters mimic the gold species (see later also with Au 3 ).

As will be seen, the gold-thiolate linkers are not innocent and undergo electronic structure changes due to the applied OEEFs. The result is that the systems in Scheme 2b develop local electric fields (LEFs), which actually ‘copy’ the OEEF onto the molecular frame, and induce catalysis/inhibition which match the observations for the bare Menshutkin reaction under


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OEEF. However, increase of the OEEFs to F Z ~ -0.257 V/Å and beyond (in the negative direction), lead to an intriguing mechanistic switchover, due to a crossover of charge-transfer states of the systems. The latter state leads to nucleophilic displacement of iodine radical from a methyl-iodide radical cation, CH 3 I•+.

2. Methods Gaussian 09 was used to perform the calculations.18 The B3LYP functional,19 with the 6311+G(d,p) basis set (henceforth, G),20 were used for all atoms except iodine and gold. The core electrons of iodine and gold were described by the quasi-relativistic pseudopotential, MWB46,21 and the Stuttgart-Dresden basis set were used for valence orbitals. Models A and B with the Au 5 thiolate linkers (Scheme 2b) were studied with UB3LYP level of theory with broken symmetry approach. The performance of the B3LYP functional to reproduce experimental kinetic data was tested for the Menshutkin reactions in Scheme 1. The calculated barriers for pyridine and substituted pyridines with methyl iodide showed a small deviation of less than ±1.0 kcal/mol visà-vis experimental14 barriers. These values are summarized in Table S7 in SI document. All transition states were characterized by having a single imaginary frequency, and were also verified by intrinsic reaction coordinate (IRC) following.22 The nucleophile and leaving group in the pre-reactant complex were kept in a linear arrangement by constrained geometry optimization. Constraints in the optimization sometimes gave small imaginary frequencies, which are less than 20.0 cm-1 and were ignored. The computational details are summarized succinctly in the Supporting Information (SI) document, while the text contains key information.


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It was reported that the Menshutkin reaction is carried out in a range of solvents from highly polar ones down to nonpolar solvents (e.g., benzene, toluene etc.).23 As such, in order to mimic low dielectric media (e.g., mesitylene in Ref. 2) we simply used gas phase modeling in the presence of OEEFs. Notably, weak polar solvents normally do not screen the effect of external electric fields.24 For the pristine Menshutkin systems (without linkers), the OEEF was used in the range ±0.006 au (±0.3084 V/Å). This OEEF range gave rise to a “standard” closed-shell S N 2 TS. The usage of UB3LYP with broken-symmetry did not reveal any open-shell singlet character even at the extreme values of these fields. Details of the Mulliken charges for all transition states of the Menshutkin reactions are summarized in Tables S1 to S6 in the SI document. Models A and B (Scheme 2b) were studied also at the UB3LYP level of theory with the broken-symmetry approach. In the presence of the extreme OEEF values, the solutions revealed open-shell singlet transition states (TSs), which involve electron transfer from the iodine to the gold clusters. Spin densities at F Z = -0.006 au are given in Table S8 in the SI document. The open-shell singlet electronic structures of Models A and B (Scheme 2b), at the extreme end of a negatively oriented F Z , were characterized by the spin natural orbitals (SNOs) of the singlyoccupied shell of these species.25 The origins of the states, which gave rise to these open-shell singlet, were ascertained by time dependent DFT (TDDFT) calculations26 of Models A and B (Scheme 2b). Subsequently, the computational findings for A and B were further tested using model A with Au 5 clusters of two different geometries and an Au 3 cluster. Succinct summary of these computational results is provided in the SI document (Figure S5 and S7-S10).


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3. Results and Discussion 3.1. OEEF Effects on Pristine Menshutkin Systems The gas phase energy barriers for the series of pyridines reacting with methyl iodide, under field-free conditions, are displayed in Figure 1a. The lowest barrier is for the 3,5-dimethylpyridine, while the highest is for 4-cyano- and 3,5-dichloro-pyridine.

Figure 1. (a) The substituted pyridine nucleophiles used for studying the Menshutkin reaction with methyl iodide. The numbers underneath the nucleophiles are B3LYP/G barriers for the OEEF-free reaction. Parts (b) and (c) show the computed activation barriers (in kcal/mol) for the reaction as a function of the applied OEEF. The OEEF ranges from +0.006 au to -0.006 au. The Gaussian 09 convention for a positive field vector (+F Z ), stabilizing molecular dipole moment


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(µ), and reference coordinate axes are depicted in the inset between (b) and (c). (b) The reaction barriers for pyridine, as a nucleophile, at variable OEEFs along the X,Y and Z- directions. (c) Similar plots (as in (b)) for all other substituted pyridines, as a function of OEEFs along the Zaxis.

Let us proceed now to consider the influence of OEEFs along the three axes in positive and negative directions, firstly without the thiol-gold linkers. The reaction axis is Z that passes along the transition state (TS) moiety N---C---I. Figure 1b shows that F Z > 0 catalyzes the reaction while F Z < 0 inhibits it. The changes in the barrier are large, which is expected for an OEEF along the reaction axis.1 Note that along the reaction axis catalysis/inhibition sets in already at rather weak OEEFs of ±0.001 au (0.0514 V/Å). The observation of a clear catalysis/inhibition dichotomy requires fixing the reactants.27 Nevertheless, even in the X and Y directions in both positive and negative field orientations the reaction barrier decreases by up to 2.2 kcal/mol. The reason for these X- and Y-directed effects is that the fields along these axes deform the TS away from linearity. Thus, whereas the NCI angle of the TS is 180° in the Z-field, it changes to 167° in the X- and Y-fields. As such, the X- and Yfields have small projections along the reaction axis and this is the source of the small catalytic effect. This observation means that the reaction may be catalyzed also by applying an OEEF without strictly fixing the orientation of the reactants. In fact, a recent report on radical stability and reactivity showed that a significant component of the external electric field arises due to polarization and is largely independent of the orientation of the electric field.28 These polarization effects are maximum at the transition state due to larger delocalization of electron density.1,3,4


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Furthermore, a sufficiently strong OEEF may preferentially orient the molecules to react along the fastest path, in which the OEEF points along the “reaction axis” in the positive directions (of X, Y, or Z). Thus, catalysis is expected to manifest without orienting the molecules. On the other hand, inhibition would require orienting/fixing the molecules. Figure 1c summarizes the influence of Z-oriented OEEFs on barriers for all other substituted pyridine nucleophiles reacting with methyl iodide. It is seen that the different pyridines generate parallel family lines, while conserving the relative reactivity as in the fieldfree situation. At the same time, due to the significant catalytic effect, the least reactive substituted pyridines (3,5-dihcloro and 4-cynao), which possess high-energy barriers in the absence of an OEEF, become significantly reactive under properly oriented Z-field. Figure 2 further shows that for F Z > 0 the intrinsic dipole moment (µ) of the TS and the OEEF vector are oppositely aligned such that the OEEF stabilizes the TS, lowers the barrier and catalyzes the reaction. In contrast, for F Z < 0 both vectors are aligned in the same direction and the resultant repulsion destabilizes the TS, and inhibits the reaction.

Figure 2. The Menshutkin TS for pyridine reacting with methyl iodide. The F Z > 0 direction is drawn beneath the structure along with the molecular dipole moment (µ) in the catalytic region


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(F Z > 0). On the right hand side, are B3LYP/G calculated changes in the N-C and C-I distances (in Å) of this TS for the reaction as a function of F Z .

Apart from catalyzing the reaction, the OEEF confers remarkable changes on the transition state geometries. A complete list of geometrical changes for all substituted pyridines under the OEEFs is given in Table S9 of the SI document, while Figure 2 summarizes the TS’s N-C and C-I distances of the “reaction axis” at different OEEF values. It is seen that as we move from negative to larger and larger positive F Z values, the N-C distances gradually increases. Thus, in the catalytic region (F Z > 0), the OEEF creates a reactant like TS, while in the inhibitory region (F Z < 0) the OEEF creates a product like TS. At the same time, the C-I distance shrinks in the catalytic region (F Z > 0) and becomes reactant like. All in all, a catalytic OEEF creates reactant like TS, while an inhibitory OEEF creates a product like TS.

3.2. Understanding OEEF Effects on the Pristine Menshutkin Reaction To reason the root cause of the changes in barriers and in the geometry of TSs under an external electric field, we use the valence bond state correlation diagram (VBSCD) model.29 The VBSCD for the Menshutkin reaction is depicted in Figure 3a.


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a)

b)

Figure 3. (a) VBSCD for a generic Menshutkin reaction in different OEEF values. For the sake of clarity, the avoided crossing is shown only for the two black curves, i.e. for F Z = 0. The arches which connect the electrons in RC* and PD* signify spin pairing of these electrons to singlet pairs. (b) Intrinsic Reaction Coordinate (IRC) plots for F Z = 0 (black curve), F Z > 0 (red curve), and F Z < 0 (blue) values.


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The VBSCD correlates states of reactants and products to promoted excited states, thus generating two-state curves (shown in black in Figure 3a), which intersect along the reaction coordinate and avoid the crossing by quantum mechanical mixing. These promoted states are noted as RC* and PD* in Figure 3a. RC* is a charge-transfer state that has an electronic configuration of the product state (PD) but possessing the exact geometry of reactant state (RC). In other words, we are correlating in Figure 3a a vertical ionic charge-transfer state of the reactants with the ionic-ground state of the products. Similarly, the promoted state of product (PD*) possesses an electronic configuration of the reactant (RC) in the geometry of the product (PD). As such, while RC* is a charge transfer state, PD* is not ionic since it has the same electronic structure as the reactant ground state (RC). The two state curves in the black color that are labeled in Figure 3a with F Z = 0 represent these correlations. The final potential energy profile of the reaction arises from the avoid crossing of these two curves, and creates thereby a TS that lies below the crossing point. This final state for the reaction profile is represented by the bold black curve in Figure 3a. When the OEEF is applied in the positive direction, F Z > 0, the promoted state RC* which is ionic gets stabilized vis-à-vis the situation at F Z = 0. This modified state curve that correlates RC* with PD is depicted in a red color in Figure 3a, and is shown to be stabilized in energy relative to the black state curve for F Z = 0. At the same time the state curve which connects RC and PD* largely remains put under F Z > 0, since this curve involves basically a nonionic electronic state. As such, the stabilization of the state curve that connects RC* and PD will shift the crossing point towards the reactant side, thus lowering the barrier and resulting in an earlier TS. In the same vein of logic, an applied field in the opposite direction, F Z < 0, destabilizes both product and promoted state of the reactants, as shown by the blue colored line


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in Figure 3a. The new crossing point then move towards the right side of the reaction coordinate, thus shifting the TS position to a late geometry, and raising the barrier. The selective stabilization/destabilization of the state curve that connects the ionic RC* and PD configurations, is seen to manifest in the IRC plots in Figure 3b in accord with the VBSCD predictions. Firstly, on the product side, both IRC plots under F Z > 0 (red) and F Z < 0 (blue) are energetically well separated from the middle black curve for F Z = 0; the red curve is stabilized, while the blue one is destabilized. However, at the reactant side, which is dominated by the nonionic state curve, all the three IRC curves are less affected by the field and come relatively closer to one another. The resulting changes are also in accordance with Bell-Evans-Polanyi principle15 and the Loffler-Hammond postulate,16 which state that a more exothermic reaction will possess a lower barrier and a more reactant-like TS, and vice versa for an endothermic reaction. As such, the OEEF effects follow the fundamental principles of physical organic chemistry: catalysis at F Z > 0 is attended by a more exothermic reaction and an earlier TS, while inhibition at F Z < 0 is attended by an endothermic reaction and a late TS. Furthermore, the catalysis of the Menshutkin reaction by OEEFs follows the “reaction axis rule” derived before1:“An [electric field] oriented along the axis, wherein the electronic reorganization from reactants to products occurs, will catalyze the reaction by enhancing the stability of the ionic structures that contribute to the TS and manipulating thereby the barriers”. The magnitude of rate enhancement is higher along the +Z direction, while rate retardation is attained in –Z direction. In the sense that inhibition is sometimes equally important to catalysis, the use of OEEF offers both possibilities by simply flipping the field’s direction.


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Along the ±X and ±Y direction, the reaction exhibits rate enhancement as shown in Figure 1b. In many reactions, if a weak EEF is applied to the process in solution, without orienting the reactants, the reaction may not show significant rate enhancement due to the free rotation of molecules that averages to zero the effect of the applied field (there will remain an effect due to polarization in the TS). However, in the Menshutkin reaction we have a particular case where the rate retardation transpires only in one direction (F Z < 0), while all other five directions show more or less rate acceleration. As such, an OEEF-induced rate enhancement of Menshutkin reactions can be effectively achieved in nonpolar solvents without orienting the molecules. The catalytic effect can be probed using the two-components Menshutkin reactions reported by Rudine et al.13 This reaction mixture contains only pyridine and dichloromethane and is devoid of any other solvent. During the reaction, pyridine substitutes chlorine ions from dichloromethane in successive steps where the first substitution is the rate-determining step. This reaction can form a simple test of the electric field effect.

3.3. Can a Menshutkin Reaction Proceed with a Front side Attack? After studying the backside reaction, let us consider the possibility of nucleophiles approaching through the front-side of the C-I bond. A front-side attack of the nucleophile is “forbidden” since the avoided crossing is small due to symmetry-mismatch of the frontier orbitals (HOMO and LUMO). As such, the front-side TS would encounter mostly Pauli repulsion due to the interaction of the occupied orbitals of the reactants, without significant compensation due to the stabilizing filled-vacant orbital interactions. Indeed, in the field-free


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situation, the corresponding TS is 27.4 kcal/mol higher in energy than the TS for the backside trajectory. Nevertheless, in the presence of a favorable OEEF orientation the barrier for the frontside attack is lowered. For example, in F X = +0.006 au, the barrier is lowered by 11.9 kcal/mol, which is highly significant. It is seen that the X-field removes forbiddance of the orbital mixing to a considerable extent. However, since the electric field catalyzes also the backside pathway, the usage of OEEF cannot bring about a mechanistic crossover from backside to front-side trajectory for free tumbling reactants. However, pre-fixation of the reactants in a trajectory that favors a front-side approach should show a significant catalytic effect under X-oriented OEEF. A plot of barriers for front-side approach under all directions of applied fields along with barriers of S N 2 reaction and geometry of the TS are shown in Figure S1 to S2 in the SI document. As shown recently, there are nucleophilic substitution reactions, which proceed through front-side attack when there are some favorable orbital controls or stabilizing interactions for the front side approach.30

3.4. What Roles Do Gold Clusters and Thiolate Linkers Play in Experimental Setups? The experimental verification of electrostatic catalysis in Diels-Alder reaction was demonstrated with the scanning tunneling microscopy (STM) based method.2 As shown above in Scheme 2a, in this method the reactant molecules were connected to a gold-based STM tip and to a gold surface through thiol ((CH 2 ) n S) linkers that were subject to a voltage bias. Since the gold cluster (or even the STM tip) and gold surface are molecular species, we reasoned that application of an OEEF should in principle affect the gold tip, the linkers, and the gold surface. It


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is very important to elucidate the interplay of the reactants and the gold-linker entities in such a set up. Metallic clusters can be incredibly complex and dynamic, as may be seen from recent investigations of clusters as models for heterogeneous catalysis.31 We took a pragmatic approach and followed Ref. 2 using a model gold cluster of five atoms, as shown in Scheme 2b. In one model (A) we connected only the pyridine to a linker that carriers a gold cluster, (CH 2 ) 2 S-Au 5 , while allowing the methyl iodide to approach the so fixed pyridine. In the second model (B), we connected both the pyridine and the methyl iodide reactants to the (CH 2 ) 2 S-Au 5 and (CH 2 ) 3 SAu 5 moieties, respectively, and allowed the methyl iodide and pyridine to approach one another. To get some idea about the effect of the number of gold atoms and the geometry of the clusters, we later investigated additionally the (CH 2 ) 2 S-Au 3 and (CH 2 ) 2 S-Au 5 (planar) clusters. These additional Models, C and D, are discussed later. To assess the effects of these linkers on the properties of the reacting Menshutkin species and the respective reaction barriers, we re-optimized the reactant clusters and the corresponding TS structures with the attached linkers in a field-free situation. Constraints were applied on the Au 5 clusters during optimization of the RC and TS. The TS geometries of the two models are shown in Figure 4 along with group charges and dipole moments in the field-free situation and upon application of ±Z OEEFs. The OEEF influence is calculated using single point calculations on these geometries.


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Model A

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Model B

Figure 4. Transition state geometries (bond lengths in Å and angles in degrees), Mulliken group charges and dipole moments (Debye) for the TSs of models A and B, in the presence of (CH 2 ) n S-Au 5 links, in field free and F Z = ± 0.006 au. The charge and dipole values are colorcoded: without a field (in black), for F Z > 0 (in red), and for F Z < 0 (in blue). For clarity we show in the inset between (a) and (b) the convention for a +F Z OEEF which is parallel to N-C bond and the direction of the molecular dipole moment (µ) that interacts favorably with the +F Z field: (a) Model A: a single (CH 2 ) 2 S-Au 5 linker. (b) Model B: (CH 2 ) 2 S-Au 5 and (CH 2 ) 3 S-Au 5 linkers.

Inspection of both models A and B in Figure 4 reveals that, even in the field-free situation (charge and dipole values noted in black), the attachment of the (CH 2 ) n S-Au 5 linkers to the Menshutkin TS, subjects the target reaction to local dipoles of e.g., the Au-S bonds.32 It


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stands to reason therefore, the (CH 2 ) n S-Au 5 moieties may not remain innocent when OEEFs are applied on the system. Indeed, when ±F Z are applied to Model A there occur significant changes. Thus, as can be seen in Figure 4a, the +F Z (red values) enhances the ionicity of the C-I bond in the TS, and increase thereby the dipole moment (from -15.0 D without a field) to -26.0 D in the presence of the field (in red). On the other hand, flipping the field to –F Z strongly depletes the electron density (blue values) of the TS moiety, and transfers electron density mostly to the gold cluster, which becomes negative. This charge transfer flips the dipole moment of the entire system (Figure 4a) that changes from -26.0 D to +23.7 D. Natural charges obtained by Natural Bond Orbital (NBO) analysis (implemented in Gaussian-0918b) show the same trend. The charges are summarized in Figure S3 in the SI document. Model B in Figure 4b, with the (CH 2 ) 2 S-Au 5 and (CH 2 ) 3 S-Au 5 linkers, exhibits changes that occur mostly in the two gold clusters and the iodine. With +F Z the bottom Au 5 cluster, which is linked to the pyridine, becomes significantly more positive, Au 5 +0.7, while iodine more negative, I–0.64, and the upper gold cluster becomes slightly negative, Au 5 –0.11. This creates a huge dipole moment of -50.1 D. Just flipping the field to –F Z flips and augments the charges of the two Au 5 moieties, depletes the charge density on the iodine (to I–0.19), and flips the dipole moment to +58.9 D. It is apparent that the local electric fields (LEFs), which are generated due to the presence of the (CH 2 ) n S-Au 5 moieties, in response to the application of OEEF, will play their roles in the reactivity of the Menshutkin reaction. Figure 5 shows plots of the barriers as a function of F Z for the two models, A and B. The barrier plots are seen to have a normal region (in blue), which resembles the pristine reaction in Figure 1b. As seen from the blue region in Figure 5, the OEEF


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effects are more pronounced in Model B as indicated by the slopes of curve in blue regions; for Model A, the slope is 51.4 (kcal/VÅ-1) while for Model B the slope is 62.6 (kcal/VÅ-1). Beyond the blue region, the curves bend either abruptly or gradually (Fig. 5a vs. 5b) at some negative – F Z values, and these bent portions are marked by a red color.

Figure 5. Plots of Menshutkin reaction barriers as a function of Z-OEEFs (F Z ), for models A and B where the reactants are attached to (CH 2 ) n S-Au 5 . The conventions of +F Z and a stabilizing µ z are shown in the inset in between (a) and (b). At selected OEEF values, we note magnitudes of dipole moments (µ z ) alongside the depicted F Z vectors: (a) Barriers as a function of F Z when only the pyridine is linked to (CH 2 ) 2 S-Au 5 (Model A). (b) Barriers as a function of F Z when both reactants are linked to (CH 2 ) n S-Au 5 moieties (Model B). The change of color of the barriers curves from blue to red indicates a change in mechanism.

Inspection of Figures 5a and 5b reveals that the barrier variation in Figure 5 exhibits two regions. In the blue regions the barriers behave in a similar manner as we found above for the


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pristine Menshutkin reaction under OEEFs (see Figures 1b,c). Thus, the F Z > 0 range leads to catalysis, and the barriers go down as the OEEF’s intensity increases. Similarly, in part of the F Z < 0 range, barriers go up and lead to inhibition. This is a regular behavior, which is observed already in Figure 1b, and which was explained in Figure 3a using the VBSCD model.29 Thus, in the normal region of the blue curves the OEEF is ‘copied’ onto the molecular system, and induces catalysis/inhibition as in the pristine Menshutkin reaction. However, as F Z is made increasingly more negative, the barrier curves bend and the red parts of the curves exhibit again catalysis. What happens as we flip the OEEF to the negative direction (F Z < 0)? Inspection of Figure 5 shows that initially there is a normal behavior since the application of a negatively oriented F Z decreases the molecular dipole and raises the barrier. However, at some critical –F Z value the molecular dipole flips its direction at the turning point of the barrier curves, such that the negatively oriented F Z stabilizes now the molecular dipole. While the flip in the dipole at F Z < 0 explains why there is suddenly catalysis, still, these changes require understanding based on clear electronic structure principles. What is the electronic origin of the re-emergence of catalysis in critically negative values of F Z ? A detailed analysis of electronic structures reveals that the increase of electric fields in the Fz < 0 direction changes eventually the electronic structure of the target molecular system. At the more negative values of F Z , e.g. F Z < –(0.003-0.005 au), the ground electronic structure becomes an open-shell singlet state. Figure 6 shows as an example the singly occupied spinnatural orbitals (SNOs)25 for model A at F Z = -0.006 au (Figure S4 in the SI shows also the SNOs for Model B). The first SNO (Figure 6a) is localized on the 5p(π) orbital of iodine, and is perpendicular to the C-I bond, while the second SNO (Figure 6b) is primarily an antibonding


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σ*(Au-Au) orbital of the Au 5 cluster. Each SNO has an occupation close to single electron (0.80); one with spin α and the other with spin β, thus forming a singlet pair.25

Figure 6: Spin natural orbitals (SNOs) of Model A at F Z = -0.006 au: (a) the 5p(π) SNO on iodine, and (b) the σ*(Au-Au) SNO of the Au 5 cluster. The occupation numbers are +0.80 and 0.80, respectively, thus characterizing a singlet-pair, which resides in these SNOs.

It is apparent therefore, that at the extreme negative field value, there occurs a charge transfer from the iodine to the gold cluster, thus creating an open-shell singlet (OSS) state. To trace the origins of this state, we computed the first excited state of the TS in Model A at F Z = 0, using time dependent DFT (TDDFT).26 Details of the group charges and spin density of the first excited state and ground state is given in Figure S5 of the Supporting Information. As seen in Figure 7, this excited state is a charge-transfer state (CTS), which is the first excited state of the composite system, and it arises from the ground state of the TS at F Z = 0, by the transfer of a β electron from the iodine lone-pair to an antibonding orbital of Au 5 cluster. The spin density of the closed shell ground state [TS(CS)], which is Au 5 0.0 ••• I0.0 changes and becomes Au 5 –1.1 ••• I+1.1 in CTS(OSS). The Z-component of dipole moment of the CTS(OSS) (+52.8 D) is opposite to that of the closed-shell ground state (-15.0 D). Thus, the F Z < 0 field will destabilize


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the closed-shell TS (TS(CS)) and stabilize the open-shell singlet CTS(OSS). Due to these opposite effects, at some negative field close to F Z = -0.006 au, the CTS(OSS) state is highly stabilized while the TS(CS) state is destabilized. Consequently, the CTS(OSS) drops below the closed-shell state and defines the new open-shell singlet TS (TS(OSS)), as shown in Figure 7. At the crossing point and beyond, the molecular dipole moment also changes directions. Thus, the state-crossing scenario serves as a physical model for the flip of the molecular dipole and the break from inhibition to catalysis as shown above in Figure 5.

Figure 7: A schematic representation of the state-crossing diagram in Model A induced by negatively oriented F Z . The closed shell state TS(CS) represents the corresponding TS at F Z = 0, while the open-shell singlet state is a charge-transfer state (CTS(OSS)), which arises by a single electron excitation from 5p(I) to σ*(AuAu). Note that the dipoles of molecules of these two states are opposite. As such, as the field becomes more negative, CTS(OSS) is increasingly stabilized, while TS(CS) undergoes increasing destabilization. At some point, the two states cross, and CTS(OSS) gives rise to the open-shell singlet TS, [(TS(OSS)].


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To test the uniqueness of the model in Figure 7, we searched whether other spin states exist, which are even lower in energy than the OSS state (with singlet spin, S =0). It was found that, for the TS(OSS) in Model A, the triplet (S=1) state is 4.3 kcal/mol higher in energy than TS(OSS) at Fz= -0.006 au. Similarly, the relative energies of the spin states, S = 0, 1 and 2 for the reactant cluster in Model A, are in the order 0.0, 8.8 and 50.8 kcal/mol respectively, at F Z = 0.0 au. Thus, the reactive state is the singlet (S=0) OSS state that exhibits the break in the inhibitory region in Figures 5, due to crossover of the corresponding charge-transfer state (CTS), as in Figure 7. As a final check, we tested the wave function of the reaction product of Model A at F Z = -0.006 au, by keeping the iodine 5Å away from the N-methyl-pyridine moiety. The results revealed the same SNOs as in Figure 6 above, with single electron occupancies. See Figure S6 in the SI document for the IRC plots. As such, the mechanism switches over from a nucleophilic displacement of iodide (I–) to a displacement of iodine atom (I•) by nucleophilic attack of pyridine on the CH 3 -I•+ radical cation. Thus the link to S-Au-clusters can further manipulate the ionic mixing29b and reactivity in an unexpected manner by harnessing an excited charge-transfer state of the system. The role of CT mixing in modifying the bonding of ions and π systems (coronenes) was discussed recently.8m 3.4.1. Testing the Non-innocent Role of Other Gold Clusters? As we mentioned above, general modeling of metal clusters31 or surfaces33 can be incredibly complex. In this respect, the used Models A and B of the gold clusters constitute an extreme simplification of the problem. Nevertheless, these Models suggest a physical mechanism (Figure 7) in which the noninnocent behavior of the gold tip originates in the availability of low lying virtual orbitals, which can accept an electron from the reacting system if the latter has a low ionization potential.


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We have therefore decided to try other simple models and proceeded to test the presence of CTSs for other gold clusters. It is known that odd-membered (Au) n clusters (n = 1, 3, 5...) form stable gold-thiolate (Au—SR) bonds.33a,34 On the other hand, even-membered clusters (n = 2, 4, 6…) lead to the formation of a charge-transferred thiol-gold bonds (Au—SH-R), which are weaker. 33a,34 Such weaker linkers may dissociate under the influence of an OEEF. Hence, we refrained from using even-numbered gold clusters-thiols as linkers, and selected the Au 3 cluster, and an Au 5 cluster, which has a different geometry than the one used above in Model A (Figure 4). The optimized geometries of TSs with Au 3 (Model C) and Au 5 cluster in a different conformation (Model D) and their response to OEEF, are summarized Figure S7-S9 of the SI document. Firstly, the SNOs in Figure S9 reveal the same findings as in Figure 6, namely, that an electron from the lone pair of Iodine is transferred to the Au clusters (of Models C and D) to form open-shell singlet states (OSS). Furthermore, the analysis of the TDDFT results of Model C at the TS geometry, as shown in Figure S10 of the SI, reveals the presence of excited CTSs with open shell-singlet characters. Similarly, the excited CTS in Models C exhibit oppositely oriented dipole moments relative to those of the corresponding ground states (e.g., for Model C, the dipole moment of the ground state is -15.8 D while the excited state is 47.0 D). The plots of energy barrier vs. F Z in Figures S7 and S8 show that Models C and D respond to the OEEF in a virtually identical manner, as found in Figure 5. Thus, while applying a –F Z OEEF, the barriers initially rise (inhibition), and this is followed by a break in the curve resulting in catalysis. After the breaks in the curves, the dipole moment of the TSs flips from being negative to very large positive values which characterize the excited CTSs. These observations are identical to those observed in the original Au 5 -S(CH 2 ) 2 - linker in Figure 5.


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These results show that Au n -S(CH 2 ) 2 - clusters possess low-lying vacant Au-Au orbitals,35 which generate excited CTS states that will arise by accepting an electron from the iodine lone pair. This feature may well be generalized to other gold clusters. Our prediction of mechanistic crossover may ultimately become testable, if the working OEEF in the STM-BJ technique2 reaches the negative values employed in the present calculation. Furthermore, since the lone pair of chlorine has a higher ionization energy than that of iodine, one may further predict that the respective CTS will be higher in energy than in the studied cases here for methyl iodide, and the state crossover (Figure 7), will occur only in a more negative F Z value, which may not be accessible in the STM-BJ technique. In such an event, the catalysis/inhibition pattern as observed for the pristine Menshutkin reaction will be observed also in the presence of the Au n -thiolate linkers.

4. Conclusion The present study demonstrates that oriented-external electric fields (OEEFs) will act as catalysts of the Menshutkin reaction and will even bring about a mechanistic crossover. Thus, when the OEEF is oriented along the N---C---I reaction axis (Z) one can catalyze or inhibit the reaction at will, by just flipping the orientation of the field (from F Z > 0 to F Z < 0). A VB theoretical analysis29,36,37 shows that the location of transition state (TS) along the reaction coordinate is altered depending on the direction and magnitude of F Z . Thus, by just flipping the direction of F Z , from positive to negative, shifts the TS from being early to becoming late. Reorienting the field off the reaction axis shows that the reaction is in fact catalyzed along five out of six directions (±X, ±Y and +Z) of the applied field. Thus the external electric


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field, with the exception of F Z < 0, seems to act as a pseudo solvent17 for reactions that contain ionic or polar intermediates. As such, catalysis may be achieved even without strict orientation of the molecule with respect to the applied field. This opens up an opportunity to upscale the reaction to bulk quantity by using EEF. Two-component Menshutkin reactions13 may form good test cases for this prediction. Our intriguing findings concern the impact of gold-thiolate linkers (as the used in the STM-BJ experiment in Ref. 2) on reactivity patterns of the Menshutkin reaction. In the presence of F Z along the N---C---I reaction axis, we found that the linkers participate in the electronic reorganization of the molecular system, primarily because the gold clusters (Au 5 and Au 3 ) accumulate or release charge density easily in the presence of the field. In so doing, these linkers induce a local electric fields (LEF),8c,d which ‘copy’ the effects of the external field, F Z , and induces catalysis/inhibition at will by merely flipping the direction of F Z along the reaction axis. However, as the external field becomes more negative (F Z < -0.1 V/Å) there occurs a sudden flip in the direction of the local electric field. Our analysis shows that an excited charge transfer state (CTS), which involves one electron transfer from the 5p(π) lone-pair of iodine to an antibonding orbital of the gold cluster, crosses below the closed-shell state of the Menshutkin reaction and causes a mechanistic crossover. This CTS catalyzes nucleophilic displacement of an iodine atom from the CH 3 I•+ radical cation. The state-crossing diagram in Figure 7 serves as a physical model for the flip of the local electric field due to the molecular dipole, for the reemergence of catalysis, and for the mechanistic switchover. The STM-BJ set up can be restored if the iodine radical accepts the excess electron of the gold cluster, Au n –. In such an event the gold thiolate linkers would act as co-catalysts, which participate in a catalytic cycle of a charge-transfer catalysis.


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Using the state-crossing diagram in Figure 7, it is possible to predict that this mechanistic switch will occur at a more negative F Z by using CH 3 -Cl as substrate (and even more negative for CH 3 -F). In such a case, the barrier variation as a function of F Z will resemble the one found for the pristine system in Figure 1b,c. The predictions that the thiolate-gold linkers are not innocent are in principle testable.

Supporting Information: The Supporting Information is available free of charge via the internet at http://pubs.acs.org. The supporting information contains potential energy profiles, orbital pictures, optimized geometries, TDFT calculations of excited charge transfer states, Mulliken & NBO spin and charges, IRC plots, full citation of Gaussian 09 and Cartesian coordinates of the optimized geometries.

Corresponding Author: E-mail: [email protected] Acknowledgement: We acknowledge discussions with S. Ciampi. The work is supported by Israel-Science-Foundation funds.

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Catalysis of Methyl Transfer Reactions by Oriented External Electric

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