Mechanism of Ethylene Addition to Nickel Bis (oxothiolene) and Nickel

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The Mechanism of Ethylene Addition to Nickel Bis(oxothiolene) and Nickel Bis(dioxolene) Complexes Dusan N Sredojevic, Rajesh K. Raju, Salvador Moncho, and Edward N. Brothers J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b07742 • Publication Date (Web): 30 Aug 2016 Downloaded from http://pubs.acs.org on August 31, 2016

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The Journal of Physical Chemistry

The Mechanism of Ethylene Addition to Nickel bis(oxothiolene) and Nickel bis(dioxolene) Complexes Dusan N. Sredojevic,‡ Rajesh K. Raju,‡ Salvador Moncho,‡ and Edward N. Brothers‡ ‡

Science Program, Texas A&M University at Qatar, Education City, Doha, Qatar. E-mail: [email protected]

Abstract The electrochemically reversible binding of olefins by nickel bis(dithiolene) has been extensively studied, both theoretically and computationally. In order to optimize a catalyst for this process, we have investigated all possible reaction pathways of ethylene addition onto the related complex nickel bis(dioxolene), and the two isomers (cis and trans) of nickel bis(oxothiolene). Modern DFT calculations predict that the nickel bis(dioxolene) complex has limited practical use due to high barriers to binding. However, each of the two isomers of the nickel bis(oxothiolene) complexes display enhanced properties versus the original nickel bis(dithiolene) complex. Specifically, in nickel bis(dithiolene), the intraligand binding of olefins leads to decomposition, while interligand binding is required for reversibility; the two nickel bis(oxothiolene) complexes have greater selectivity toward the formation of the desired interligand adducts. For the full reaction pathways, the new complexes’ binding mechanisms are contrasted with the mechanism of the original catalyst.

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Introduction Olefins are present in raw petrochemicals, and have been widely used industrially as precursors in the synthesis of polymers and other products.1 The traditional industrial separation method, cryogenic distillation, is quite energy intensive and thus expensive.2 Some alternative approaches which utilize metal salts such as copper and silver salts are more efficient, but these metal salts are usually deactivated by common olefin impurities, such as CO, H2S, and acetylene.3 Previous studies have shown that metal bis(dithiolene) complexes exhibit reversible reactivity in cycloaddition reactions with many strained and cyclic alkenes, and that these reactions may be electrochemically controlled.4-16 In these reactions the carbon-carbon double bond donates two electrons to the complex, and binds to the sulfur atoms, as has been shown by X-ray crystallographic analysis.14,18 Based on this, an olefin separation using [Ni(S2C2R2)2] (R = CF3, CN) complexes was proposed by Wang and Stiefel.17 In this scheme, the nickel bis(dithiolene) complex would bind an olefin, producing a cisinterligand adduct, which would subsequently release the olefin upon electrochemical reduction. However, subsequent experimental investigations by Fekl and co-workers complicated this picture with two new and significant results. First, the main products in the reaction of [Ni(tfd)2] (tfd = S2C2(CF3)2) with an olefin were a series of metal decomposition species, as a result of intraligand addition. Second, the desired interligand adduct, which releases the olefin upon reduction, was only formed in the presence of a reduced nickel dithiolene complex.18 In other words, olefin plus [Ni(tfd)2] leads to decomposition, while olefin plus [Ni(tfd)2] plus [Ni(tfd)2]− leads to the desired product which can release the olefin. Theoretical investigations have supplemented experiment to elucidate this reaction. The first proposed mechanism appeared in 2002, positing that in the first step of the reaction, there is a twisted intermediate with a pseudotetrahedral nickel.19 This intermediate avoids constraints imposed by orbital symmetry for direct addition of ethylene, and could rearrange to the thermodynamically more stable interligand product (square-planar nickel).19 This predates the discovery of the role of the anion; following that discovery, a more complete mechanism has been proposed based on a combined experimental (kinetic measurements) and computational (DFT) study. This work invoked a Ni(tfd)2/Ni(tfd)2‾ dimer system in order to explain the selectivity of adduct formation in the presence of the reduced metal complex.20,21 The first step in the binding mechanism is the dimerization of a neutral and anionic Ni(tfd)2, which binds the olefin and then loses the Ni(tfd)2‾. This leaves a short-lived complex where the olefin is bound along a nickel-sulfur bond, which rearranges into the more stable interligand adduct. Without the anion and the subsequent dimer, the low energy path is the direct formation of the intraligand product, which then decomposes. In order to optimize the catalyst for olefin separation, recent theoretical investigations were carried out on cobalt and copper bis(dithiolenes).22 These calculations showed that the neutral cobalt bis(dithiolene) was best versus the variously charged Co and Cu complexes tested, as well as the original nickel complex, owing to favorable thermodynamics/kinetics, while effectively blocking the decomposition route. In a previous study, some of the authors have also examined the reactivity of platinum bis(dithiolene) with butadiene23 as a model for conjugated alkenes, in order to explain recent experimental findings.24 The results were in accordance with experimental results, demonstrating theory’s power to both explain and optimize this catalyst. The ultimate goal of this study is to optimize a metal complex for the reversible binding of olefins, with an eye toward developing new technologies in olefin separation from petrochemical


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feedstock. The optimized complex should bind olefins while not reacting with common contaminants such as acetylene, water, carbon monoxide, etc. It should also avoid decomposition, by imposing high energy requirements for the formation of the intraligand adduct, and the binding of olefin must be electrochemically reversible. Further, this "superior" complex should reversibly bind olefins via low barriers, preferably avoiding the requirement of the anion as a co-catalyst. There are a few different strategies on how to alter the reactivity of the original nickel bis(dithiolene) complexes toward the olefins by structural alteration. Optimization could be achieved by varying the metal atoms12 or by subtle electronic structure tuning through modifying the substituents on the chelated dithiolenes. Another strategy is to use other related redox-active ligands such as those examined by Grapperhaus et al.25 In this work we have altered the reaction by changing the donor atoms of the ligand from sulfur to oxygen, both partially and fully, which can be considered a variant of the Grapperhaus et al. approach. This modification has led to a significantly better catalyst for separation, one that fulfills several of the criteria detailed above.

Computational Methods All calculations have been performed using the development version of the Gaussian suite of programs.26 Following our previous benchmarking work on simplified Ni(S2C2H2)2 model systems,27 we used the ωB97XD28 density functional throughout; this functional includes both long-range exchange and empirical dispersion corrections. As previously,27 we used the all-electron 6-31++G** basis set29 for H, C, O, N, S and Ni atoms, with the WachtersHay30 modification on Ni. Geometries of all relevant species were optimized in the gas phase. The nature of the various species was confirmed via vibrational analysis, as reactants and products have no imaginary frequencies and transition states have exactly one. The intrinsic reaction coordinate (IRC)31 procedure has been also applied when needed to confirm that the transition states connect respective reactants, transition states and products. We also performed calculations in order to check the SCF stability of relevant species.32,33 In some cases, stable solutions were achieved by successive reoptimizations of the unrestricted orbitals. For those species where we could not find the stable solution, the Yamaguchi’s broken spin symmetry (BS) procedure34,35 was used to obtain the energy of the spin-purified low spin state, according to the following formula:

(1) (All Yamaguchi cases are labeled in the figures.) Solvation effects in chloroform were included via the SMD36 solvation model. The solution-phase free energies (∆Gsol) we cite in this work have been calculated by adding solvation energies to the gas phase relative free energies (∆Ggas). All 3D molecular structures were created in CYLview.37

Results and Discussion In this study we have examined three novel nickel complexes’ reactivity with ethylene. Changing various sulfur atoms of the original nickel dithiolene into oxygens made the complexes we study here (Figure 1). Replacing all four sulfurs with oxygens created the first complex to be tested (dioxolene; 1OO). The other two complexes, which represent two isomers (oxothiolene; 1OS-cis and 1OS-trans), were created by replacing one sulfur with oxygen in each ligand, with the isomerism coming from the relative locations of the oxygen (Figure 1). We then compared their reaction with ethylene to the original complex’s behavior. Because the original complex shows very similar behavior with electron withdrawing groups such as CN



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and CF3, we examined nickel bis(dioxolene) and nickel bis(oxothiolene) complexes with CN substituents.20,21,38

Figure 1. The analyzed nickel complexes: dioxolene (1OO), and two isomers of oxothiolene (1OS-cis, and 1OS-trans). Table 1. Calculated free energies of binding for the inter- and intraligand ethylene adducts of different nickel complexes. Energies are given in kcal/mol, and relative to infinitely separated metal complex and ethylene. Distortion energies (Edistor; kcal/mol) as defined in the text are also included. The numbering of different adducts is explained in Figure 2.

Adduct

∆G

Edistor

2SS 2OO 2O(S)-cis (SS-bound) 2(O)S-cis (OO-bound) 2OS-trans 3SS 3OO 3OS-cis 3OS-trans

-18.4 2.2 -19.1 -4.3 -7.5 -15.8 25.6 3.9 5.2

14.7 26.7 11.3 27.3 26.4 37.7 66.1 52.1 51.0

Calculated thermodynamic results for various interligand (2) and intraligand (3) adducts for all complexes relative to the unbound complex/ethylene couple are summarized in Table 1. For all three new complexes, the intraligand adducts are less stable than the interligand adduct (Figure 2). Table 1 shows that the presence of oxygen as a donor atom leads to considerable destabilization of intraligand adducts compare to the original complex, making their formation highly endergonic. This is desirable as the destabilization of the intraligand adduct suppresses the decomposition pathway. The destabilization of the intraligand adduct is largest for the dioxolene complex (25.6 kcal/mol). The same trend occurs for the interligand adducts (Figure 2) but this effect is somewhat less pronounced (Table 1). Binding at the oxygen atom is less favorable than binding at sulfur. There is a complex with a lower energy than the original nickel bis(dithiolene) which has ethylene bound only to sulfur atoms (Figure 2, 2O(S)-cis). Thus, the relative stability of an adduct depends on the number of the oxygen atoms included in ethylene binding (Table 1). The most stable is 2O(S)-cis, with two S-Cethylene bonds, whereas the least stable is 2OO, with two O-Cethylene bonds. One hypothesis was that frontier orbital effects caused the stability change; we thus calculated the energy gap between the HOMO orbital of ethylene and the LUMO orbitals of these three complexes (Table S2 in the SI). The variations in energy gaps among the complexes are not large and do not explain the variation.


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This can also be explained by considering distortion energies, which are calculated by optimizing the bound complex, then removing the ethylene and calculating the single point energy of the distorted complex (Table 1; Edistor); this is the energetic penalty of geometric change in the metal complex. The energy cost for distortion drastically rises for oxygen-containing complexes compared to the dithiolene complex. The distortion necessary to form the interligand complex is also less than the distortion needed for the intraligand complex. Combined, this results in larger energy differences between the intraligand and interligand complexes as the number of oxygens increase, and it also leads to endergonic formation of the intraligand product for the new complexes.

Figure 2. Optimized geometries for species in Figure 1 and Table 1. Values in regular and italic text are Mulliken and natural (NBO) atomic charges, respectively.

Table 2. Relative stabilities (kcal/mol) of anionic species of the interligand adduct, versus infinitely separated ethylene and the bare anionic complex.

Anion ‾

[2SS] [2OO]‾ [2O(S)-cis]‾ [2(O)S-cis]‾ [2OS-trans]‾

∆Gsol 20.5 10.5 -0.6 16.1 4.7

To be useful, it is necessary that the complex releases ethylene upon reduction, thus the relative stability of the anionic complexes is critical. This is summarized in Table 2, which shows that the binding



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energies of the reduced interligand adducts are unfavorable; most reduced species are sufficiently destabilized and should release the ethylene upon reduction. The lone exception is [2O(S)-cis]‾ that is still slightly favorable to binding (-0.6 kcal/mol). This mean that [2O(S)-cis]‾/[1O(S)-cis]‾ will be in equilibrium, and the concentration of ethylene will govern the ratio of bound and unbound species. Note these calculations were done assuming chloroform solvent and this might be overcome using a less polar solvent. For example, this anion is additionally destabilized in hexane solution by 2.5 kcal/mol (∆Gsol = 1.9 kcal/mol), and would thus dissociate the ethylene. We have also calculated the standard redox potentials for these complexes in solution following a procedure based on the Born-Haber cycle.39 The calculated E0 values for the 11 + e¯ → 21¯ and 12 + e¯ → 22¯ half-reactions are presented in the Supporting Information (Table 3S). The reduction of all examined adducts (2y_Ni_OO, 2_Ni_(O)S_cis, 2_Ni_O(S)_cis, and 2_Ni_OS_trans) is lower in energy than that of 2_Ni_SS. In addition, the oxidation of all anions (1_Ni_OO¯ and both forms of 1_Ni_OS¯), which is the necessary step to regenerate the catalyst, would also be a less energy intensive process, compared to the original dithiolene anion (1_Ni_SS¯). It should thus be noted that electrochemical steps for all three new complexes in this work would require less energy than when considering the nickel bis(dithiolene) system. The next step is to evaluate the kinetics and to discuss the reaction mechanism. The “direct” binding route which leads to the formation of the interligand (via TS2y) and intraligand (via TS3) adducts, previously found for nickel20,21 and cobalt22 bis(dithiolene) complexes, was explored for the nickel bis(dioxolene) and nickel bis(oxothiolene) complexes (Figure 3). However, the initial singlet transition states had significant instability. The energy was thus calculated using the contaminated singlet transition state geometry using Yamaguchi’s broken-spin-symmetry procedure (BS).34,35 After applying this, the free energy barriers for the dioxolene (1OO) complex via TS2y (52.6 kcal/mol) and TS3 (49.4 kcal/mol) appeared to be effectively insurmountable (Figure S1). For the two isomers of the oxothiolene complex, these transition states are lower in energy but still too high to be the competitive mechanisms. There are two ways to form the interligand adduct with the cis isomer: across the oxygens (TS2y(O)), and across the sulfurs (TS2y(S)), with barriers of 45.1 and 28.7 kcal/mol, respectively (Figure 4). The corrected free energy barrier for TS3 is 29.9 kcal/mol for the cis isomer. Only the sulfur side addition is similar to the Fan-Hall mechanism,19 which implies the formation of 2O(S)-cis through a twisted-intermediate (2y). The reaction route that operates along the oxygen side ends up with 2(O)S-cis, which has a pseudotetrahedral nickel geometry and no route to further rearrangement (Figure 4). For the trans isomer, there is only one possible TS2y, and it is isoenergetic with TS3 (28.2 kcal/mol) (Figure 3). In this case TS2y leads directly to the formation of interligand adduct 2OS-trans, avoiding the formation of twisted intermediate. While these are included for completeness, none of these routes are low enough in energy to be practical. For the original nickel bis(dithiolene), there is an alternative “indirect” mechanism21 which begins with direct ethylene coordination to the nickel, or even a direct transition state that leads to the formation ethylene bound along the nickel-sulfur bond, called 4.38 This can then convert to interligand adduct 2, or intraligand adduct 3.21 Related reaction mechanisms for 1OO, 1OS-trans and 1OS-cis with ethylene have thus been investigated. It turns out that there are direct routes to the formation of Ni – S or Ni – O ethylene bound species, via an appropriate transition state TS14. As shown in Figures 3 and 4, when the ethylene “swings” from along the nickel-oxygen or nickel-sulfur bonds, the barriers that lead to the 2 are lower than those that lead to 3. Turning first to the reaction profile for 1OO, the high energy barriers and thermodynamic instability of 2OO renders it a bad candidate for further examination, as it does not outperform the original complex (Figure S1; SI). In contrast, the nickel bis(oxothiolene) complexes (1OS-trans and 1OS-cis) show promise. The


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initial barriers for the direct route are too high, but binding across the Ni – X (X=O, S) bonds, which is the route to the formation of 4, are more favorable.

Figure 3. Energy profile for ethylene approach across the Ni-S and Ni-O bonds (black lines), together with the reaction routes for direct ethylene approach toward the donor atoms (red lines), for 1OS-trans complex. Relative free energies in solvent (CHCl3) are shown.

Figure 3 shows that the ethylene approach across the Ni – S bond is favorable to the competitive approach across Ni – O bond due to the lower barrier heights and relative stability of intermediate 4O(S)trans for the trans complex. The system overcomes a barrier of 7.9 kcal/mol to reach the stable intermediate 4O(S)-trans, which then can isomerize into either 2OS-trans or 3OS-trans. The free energy of activation connected to the formation of adduct 2OS-trans (TS42) is 10.5 kcal/mol lower than one connected to the formation of adduct 3OS-trans (TS43). This is of great importance, as even without an anionic co-catalyst, the low energy path (by a significant margin) is to 2 with a rate-determining barrier of 23.1 kcal/mol. Alternatively, on the oxygen side, system should overcome a barrier of 22.2 kcal/mol to produce 4(O)S-trans, followed by larger barriers to reach the oxygen side products of 2OS-trans and 3OS-trans. This ignores the pathway that would occur in the presence of the anion, which is the mechanism found for nickel bis(dithiolene), and thus an analogous process must be considered for completeness. The calculated anion assisted pathway, for the trans-isomer, reveals that the corresponding dimer species (D0¯ and D1¯) are fairly stable, and this binding would impede their dissociation, which is a necessary step for the proposed reaction course (Figure S2; S3). In addition, the indirect pathway is already intrinsically favored compare to the direct one (which has very similar selectivity for adducts 2 and 3), due to the much lower barriers (Figure 4); together this indicates that the indirect pathway is favored route. There are a few advantages of 1OS-trans versus the original Ni(tfd)2 complex, regarding its selectivity toward the interligand product 2. First, this complex has no need of an anionic co-catalyst to proceed via the “indirect” mechanism to product 2. Second, the kinetic selectivity of 2 in this pathway is greater, because the difference in the barriers associated with converting the intermediate 4 to 2 and 3 are



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larger. (The difference in this case is 10.5 kcal/mol, compared to the original complex with a difference of 7.7 kcal/mol). Third, the thermodynamic selectivity toward 2 is higher, because the reaction generating 3OS-trans is endothermic (by 5.2 kcal/mol) and the difference in thermodynamic stability between adduct 2 and 3 is larger for this complex than for the original nickel bis(dithiolene) (12.7 vs 2.6 kcal/mol). All these findings lead to the conclusion that 1OS-trans is a better candidate for olefin separation than the original nickel bis(dithiolene) complex. The corresponding reaction scheme for 1OS-cis is presented in Figure 4. Here again there are two possible routes and initial binding along the Ni – S bond is more favorable than binding along the Ni – O bond, both kinetically (∆TS14 = 13.4 kcal/mol), as well as thermodynamically (4O(S)-cis > 4(O)S-cis and 2O(S)cis > 2(O)S-cis).

Figure 4. The energy profile for ethylene approaching along the Ni-S and Ni-O bonds (black lines), together with reaction routes for direct ethylene approaches at the donor atoms (red lines), for the 1OS-cis complex. Relative free energies in solvent (CHCl3) are shown.

A stable 4O(S)-cis intermediate can be formed via direct addition of ethylene across the Ni–S bond of 1OS-cis by overcoming a barrier of 9.0 kcal/mol. This intermediate can isomerize to 2O(S)-cis through a barrier of 13.1 kcal/mol. Considering that the free energy of activation connected to 3OS-cis (TS43) is 18.2 kcal/mol higher, it can be concluded that there is kinetic selectivity for 2O(S)-cis. On the other side, the formation of the less stable 4(O)S-cis requires 13.4 kcal/mol more energy, and has an opposite selectivity for further rearrangements, which is of interest but not of consequence, as the most favorable pathway leads to the formation of interligand adduct 2O(S)-cis. Overall, this favored reaction is exergonic by 19.1 kcal/mol. Although 1OS-cis has a slightly higher barrier to form intermediate (4) compared to 1OS-trans (9.0 vs 7.9 kcal/mol), this complex has much higher selectivity toward the formation of interligand adduct 2. This can be seen by comparing the difference between the relative free energies of TS42 and TS43, which is in


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case of 1OS-cis as twice as large (Figures 3 and 4), i.e. 1OS-cis is significantly more selective. Furthermore, the barrier that leads to the formation of 2O(S)-cis is 9.0 kcal/mol lower than the barrier that leads to 2OS-trans. It seems that 1OS-cis may be even better candidate for olefin separation process than 1OS-trans, and certainly improves on the properties of the original nickel bis(dithiolene) complex. Here again, the dimer mechanism that would occur in the presence of the anion must be considered. There would be no benefit from the inclusion of the anion as a co-catalyst (Figure S2; S3), because the initial small stabilization due to the dimers formation would in turn increase the overall barriers of the reaction. In addition to the previous pathways (direct and indirect), another mechanism for ethylene addition was found for the oxygen-containing complexes that were not found in the original sulfur complexes (Figure 5). This mechanism involves the stepwise formation of the two C–S or C–O bonds. In the first step, only one carbon of ethylene links to a donor atom to form a monocoordinated-ethylene intermediate, 5. In a subsequent step, the ethylene swings over to form the second carbon-donor atom bond. Its biradical character characterizes the intermediate. The Mulliken atomic spin densities show that one electron of α spin excess located on the C2 atom of the ethylene (the final C atom), and β spin is delocalized throughout the whole molecule but mostly onto the opposite chelate ring (Figure 6). Owing to the distribution of the spin density that “neutralizes” the radical in the ethylene, this pathway leads exclusively to the formation of interligand adducts. Figure 5 shows the reaction pathways for the biradical mechanism, for both the 1OS-trans and 1OS-cis complexes. (The mechanism for 1OO, very similar to 1OS-cis, is provided in Supporting Information, Figure S4). Formally, each isomer can follow two different pathways with the biradical mechanism, starting by the formation of a C−S bond or a C−O bond. However, it was impossible to find a transition state closing the ring where the second step was C2–S bond formation (only pathways with C2–O bond formation were obtained). This considers the approach of the ethylene only toward the oxygen side of the molecule in the case of 1OS-cis, and only through 5O(S)-trans intermediate in the case of 1OS-trans. Alternatively, the approach of the ethylene toward the sulfur side of 1OS-cis can produce the 5O(S)-cis intermediate, but without further constitution of 2O(S)-cis. Similarly, the 5(O)S-trans intermediate, which was also located, is unable to form 2OS-trans. Our calculations to locate a transition state for the formation of the C2−S bond ended up with species related with the coordination to the Ni atom, suggesting that the C−S formation is blocked by the C−Ni formation due to the proximity of the nickel. On the other hand, C−O formation is more active, and we managed to find a TS for the second step. However, it must be noticed that for the trans product both pathways were fully determined and forming the C−Ni bond has a very low relative barrier TS54 (0.9 kcal/mol) (Figure 7). This could be due to the ability of sulfur to stabilize radicals, known for the organic compounds.40 This is supported for the spin distribution, with half of the spin density in the chelating ring located on the S atom for all the 5 intermediates. The incompatibility of this pathway with a second C−S formation can be the reason why a biradical mechanism was not found for the original nickel bis(dithiolene) complex.



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Figure 5. Energy profiles of biradical mechanism for the reactions of 1OS-cis and 1OS-trans with ethylene, to form the interligand adduct (2(O)S-cis and 2OS-trans). The relative free energies in solvent (CHCl3) are shown.

As can be seen from the reaction profile, the initial transition state TS15 and intermediate 5 are much lower in energy for 1OS-trans than for the 1OS-cis (Figure 5). In the case of trans isomer, 5O(S)-trans the intermediate is formed by overcoming a free energy of activation of only 15.7 kcal/mol (TS15). The ratedetermining step, the formation of 2OS-trans (TS52), has a barrier of 21.0 kcal/mol, which is lower than the rate-determining step obtained with the “classical” indirect mechanism (23.1 kcal/mol). On the other side, this mechanism is not favorable for cis isomer; the system has to overcome a barrier of 30.5 kcal/mol (TS15) to afford the 5(O)S-cis intermediate. Despite improving the indirect and direct formation of 2(O)S-cis (rate-determining barriers are 37.2 and 45.1 kcal/mol respectively), this pathway is kinetically uncompetitive with the S-binding route, with rate-determining step barrier of 15.0 kcal/mol.


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Figure 6. The optimized geometries for the species involved in Figure 5. Values for Mulliken atomic spin densities are presented.

Therefore, only the biradical pathway on 1OS-trans is competitive with the previously described mechanisms. Both competitive mechanisms of ethylene addition to 1OS-trans are presented in Figure 7. The initial barrier for the binding along the nickel-sulfur bond (7.9 kcal/mol) is much lower than the initial step in the biradical mechanism (15.7 kcal/mol), suggesting that the formation of 4 is favored. However, the overall rate determining steps are, in both cases, related with the second step of the reaction, and they are very similar in energy; the biradical is 1.8 kcal/mol lower. It can be concluded that two mechanisms, due to similarity in energy, are competitive, but they both lead to the same desired product, 2OS-trans.



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Figure 7. Energy profile for two competitive mechanisms for the reactions of 1OS-trans with ethylene, which lead to the formation of adducts 2OS-trans and 3OS-trans. Relative free energies in solvent (CHCl3) in kcal/mol are shown.

While to this point we have regarded 1OS-trans and 1OS-cis separately, it is likely that mixture would occur and be useful. Because they are very similar in energy (∆∆G=0.2 kcal/mol), they would probably be formed as mixture, although there are no experimental precedents for the synthesis of these complexes to our knowledge. (Also, we were unable to find a transition state for interconversion.) Both meet the important requirements described at the beginning of the work, namely that the compounds bind ethylene as the stable products rather than decomposing, and that they become unstable upon reduction to release the ethylene. In the case of 1OS-trans, we have found no route to decomposition and two nearly isoenergetic paths to the desired product, namely the biradical mechanism and the mechanism that starts with binding along the nickel-sulfur bond. For 1OS-cis, the biradical mechanism is energetically unfavorable as is the decomposition, leaving just the binding along the nickel-sulfur bond. As both isomers work, a mixture of two isomers of nickel bis(oxothiolene) complex would be effective, with the added benefit in preparing the catalyst that there is no need to separate isomers after its synthesis.

Conclusion In summary, we have investigated the possible reaction mechanisms of ethylene addition to three novel nickel complexes closely related to nickel bis(dithiolene). The three new complexes show somewhat different reactivity, but for all of them, three mechanisms have been found: the first with direct ethylene approach toward the donor atoms of the same or opposite chelates, the second involving initial binding along the nickel-donor atom bonds, and the third through the series of biradical species. It turns out that the 1OO complex is not a good candidate as a potential catalyst for the olefin separation process, due to high barriers and the relative thermodynamic instability of the interligand adduct. In contrast, the two isomers of the nickel complexes with hybrid ligands (1OS-trans and 1OS-cis) showed more promise, and they may be regarded as potential catalysts for olefin separation. In particular, compared to the original complex, the bis(oxothiolene) complexes are more selective toward the formation of the stable, interligand adduct without the addition of anionic co-catalyst.


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Acknowledgements This publication was made possible by NPRP grant no. 5-318-1-063 from the Qatar National Research Fund (a member of Qatar Foundation). The IT Research Computing group in Texas A&M University at Qatar provided the computational resources and services used in this work.

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Mechanism of Ethylene Addition to Nickel Bis (oxothiolene) and Nickel

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