Acceptor Concepts for Developing Efficient Suzuki Cross

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Donor/Acceptor Concepts for Developing Efficient Suzuki Cross-Coupling Catalysts Using Graphene Supported Ni, Cu, Fe, Pd, and Bimetallic Pd-Ni Clusters Yuan Yang, Arthur C Reber, Stanley E. Gilliland, Carlos E. Castano, B. Frank Gupton, and Shiv N. Khanna J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07538 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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The Journal of Physical Chemistry

Donor/Acceptor Concepts for Developing Efficient Suzuki Cross-Coupling Catalysts using Graphene Supported Ni, Cu, Fe, Pd, and Bimetallic Pd-Ni Clusters Y. Yang,1 A.C. Reber,2 S.E. Gilliland III, 1 C.E. Castano,3 B.F. Gupton,1 S.N. Khanna2* 1Department

of Chemical and Life Science Engineering, Virginia Commonwealth University,

Richmond Box 843068, Richmond, VA 23284, USA 2Department

of Physics, Virginia Commonwealth University, Richmond, VA 23284, USA

3Department

of Mechanical and Nuclear Engineering, Virginia Commonwealth University,

Richmond Box 843068, Richmond, VA 23284, USA

ABSTRACT: First-principles theoretical studies on the electronic properties and activation energies for the three steps of the Suzuki cross-coupling reaction have been performed on 3d transition metal clusters and Pd/Ni bimetallic clusters supported on defected graphene. The

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ability of the clusters to effectively donate and accept charge is found to be critical to the activity of the catalysts, and graphene further enhances this ability. Nickel acts as the best replacement for palladium in cross-coupling catalysts for the oxidation steps but is not a good replacement in the transmetallation and reductive elimination steps which require the cluster to serve as a charge acceptor. Reducing the size of the cluster from Ni13 to Ni4 enhances the activity due to the cluster being more positively charged. Bimetallic Pd/Ni clusters were found to offer even lower activation energies for all three steps of the Suzuki reaction due to charge donation from the Ni atoms to the Pd atoms making the bimetallic cluster a highly active cocatalyst. This study reveals that the donor-acceptor concepts that explain the enhanced activity of Pd clusters on defected graphene can also be applied to explain lowered activation energies in bimetallic clusters acting as co-catalysts.

1. Introduction The escalating demands for inexpensive, efficient and environmentally benign chemical processes have stimulated the development of heterogeneous catalysts.1-3 Many of the drawbacks of homogeneous catalysts, such as the lack of recyclability, difficulties in product separation, and the metal contamination of the product can be overcome via the rational heterogenization of the homogeneous catalysts.3-5 Numerous heterogeneous catalysts6-12 have been developed for the palladium-catalyzed carbon-carbon cross-coupling reactions which have emerged as a widely used strategy for forming carbon-carbon bonds.13-16 However, the heterogenization of the catalyst cannot solve the problem associated with the high cost of the


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Pd. In fact, heterogeneous catalysts generally have a lower turnover number (TON) and consume a larger quantity of precious metal than homogeneous catalysts.13 To reduce the cost of the catalysts, substituting the expensive Pd with earth abundant and less expensive first-row transition metals is appealing for lowering the costs of the catalyst.17-19 The improvement of cost efficiency is significant: Ni is a commodity metal that costs roughly $1.20 per mole, whereas Pd costs $1,500 per mole.20 It is possible that Ni or other 3d transition metals may exhibit different activities from those of Pd. For example, due to the strong charge donating capability of Ni, it has been applied to activate strongly energetically hindered bonds such as C-Cl, C-F, C-OR, and C-NR more effectively.19-27 Although, some success has been found in implementing homogenous Ni, Fe and Cu catalysts for a variety of cross-coupling reactions, the development of the heterogeneous catalysts has proven to be challenging. Reactions catalyzed by heterogeneous first-row transition metal catalyst usually require excess ligand and coupling partner, harsh reaction conditions, and a stoichiometric amount of metal.28-32 Moreover, for the first-row transition metals, the metal clusters/nanoparticles are extremely easy to be oxidized which leads to loss of reactivity.33-35 Due to the high cost of palladium, as well as the contamination and separation complications associated with homogeneous catalysts, the development of efficient heterogeneous first-row transition metal cross-coupling catalysts is highly desirable. A series of recent studies have demonstrated that a defected graphene support actively enhances the reactivity of supported Pd catalysts in cross-coupling reactions.8-12, 36-38 These


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studies focused on a model Suzuki reaction using 4-bromobenzoic acid and phenylboronic acid as reagents. The reaction follows a three-step pathway of oxidative addition, transmetallation, and reductive elimination. The studies found an extremely high turnover frequency for Pd clusters supported on defected graphene. The high activity was found to be linked to the formation of defects that can be induced through microwave heating.11 Theoretical investigations explained the origin of the high activity of these catalysts as due to the unique nature of graphene in that it is a support that may act as both a charge donor and as a charge acceptor.11 This property is found to be critical because the catalyst must serve as a charge donor in the oxidative addition step of the reaction, and it must serve as a charge acceptor in the transmetallation and reductive elimination steps of the reaction. The transmetallation step also requires some charge donation, although previous studies have found that the activation energy correlates quite strongly with the cluster’s ability to accept charge.11 The graphene support serves as a solid-state ligand and a reservoir of charge that actively enhances the charge transfer between the supported Pd nanoparticles and reagents.11 As the conductive nature of graphene makes it a support that is uniquely suited in enhancing activity, it is therefore reasonable to investigate whether graphene support may activate other less expensive first-row transition metal clusters as it does for palladium.39 Thus, our first hypothesis is that the defected graphene will enhance the ability of the catalysts to accept and donate charge which will lower the activation energy for different steps in the Suzuki cross-coupling reaction in first-row transition metal clusters such as nickel, iron, and copper. Our second hypothesis is that by changing the composition and size of the clusters,


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the activity may also be changed, with bimetallic clusters forming complementary active sites due to charge transfer,40-43 and the size of the cluster affecting the electronic properties and reactivity.44-52 In particular, Pd-Ni clusters are interesting as they have promising applications as electrochemical catalysts, and their similar electronic structure make possible either site to be active towards cross-coupling catalysis.52-54 Thus the goal of this paper is to use theoretical methods to see if the donor-acceptor concepts that explain the high activity of Pd clusters on defected graphene are generalizable to other systems, especially less expensive 3d transition metals such as nickel, and bimetallic co-catalysts where charge transfer between the two metals may activate the catalyst. We have investigated the electronic properties of Ni, Fe and Cu clusters supported on defected graphene, as well as theoretically determined the activation energy as compared to Pd in the full reaction cycle of the Suzuki cross-coupling reaction. Our results reveal that the electronic properties including the donor-acceptor characteristics of the metal clusters are modified after supporting the clusters on graphene defects. The metal clusters and the defected graphene form a charge donor and acceptor pair that stabilizes the metal clusters and may reduce sintering and enhance recyclability. We investigated the charge donor and charge acceptor characteristics of the clusters by evaluating the work function, and bromine binding energy as a proxy to the clusters ability to act as a charge donor, and the phosphine binding energy as a proxy to investigate the clusters ability to act as a charge acceptor. We also investigated the d-band centers. The activation energies of the full reaction cycle of


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Suzuki reaction were also studied using 13-atom 3d transition metal clusters, and it was found that the graphene support lowered the activation energies through enhancing the charge transfer. Additional investigations revealed that the use of bimetallic Pd/Ni clusters further lowered activation energies for the Suzuki coupling reaction, because the bimetallic cluster could act as a co-catalyst in which the Ni sites donate charge to the Pd sites, and the Pd sites then act as improved charged donors, and the Ni sites act as improved charge acceptors. We also found that reducing the size of the cluster lowered the activation energy. These theoretical investigations offer two strategies for enhancing the activity of transition metal cross-coupling catalysts supported on graphene, one is to change the size of the clusters to enhance the clusters ability to accept charge, and the second is to use bimetallic clusters in which charge transfer between the metals lead to improved charge transfer with reactants and lowers activation energies. 2. Theoretical Methods The theoretical studies used the gradient corrected functional proposed by Perdew, Burke, and Ernzerhof (PBE).55 The VASP code was used, the Kohn–Sham orbitals were expanded using a plane wave basis set and the cutoff was set to 400 eV.56-57 The projector-augmented wave method was used to treat electron-ion interactions while the DFT-D2 was applied to include the van der Waals corrections.58 A dipole correction was incorporated along the zaxis of the slab, a Gamma point was used for Brillouin zone integration, and the difference between the Fermi energy and the energy level of the vacuum was used to calculate the


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work function.59 Since Fen, Cun, and Nin clusters carry magnetic moments, various spin states were investigated to determine the ground state. Bader charges were used to determine the charge of the individual atoms. The nudged elastic band method was applied to optimize the reaction pathway.60 3. Results and Discussion Pd13 on defected graphene has been previously shown to be highly active for catalyzing Suzuki cross-coupling reactions, so our first goal was to examine the electronic properties of a series of 13-atom clusters of different elements (3d transition metals) to identify alternative clusters with similar electronic properties as Pd13 on defected graphene.10-11 The structures of Cu13, Fe13, Ni13, and Pd13 supported on a graphene with a double vacancy site are shown in Figure 1. Figure 2 shows the calculated binding energies of different metal clusters to the defected graphene, as well as the net charge transfer from the cluster to the graphene. The key to the success of Pd13 on graphene towards catalyzing cross-coupling reactions is the cluster’s ability to act as both an effective charge donor and a charge acceptor. This means that for a cluster with a different composition to be as effective as Pd, one would expect it to have similar charge transfer characteristics. Here we have included Mo13, Ru13, and Au13 for comparison, as these 4d and 5d transition metals are also reported to catalyze cross-coupling reaction. Their structures are shown in Table S1. For the case of Au13 clusters, the binding energies of the metal clusters to graphene is 4.2 eV versus 7.4 eV for Pd13, and Au13 transfers 0.8 e- to the graphene surface versus 1.90 e- for Pd13. The lower binding energy suggests that


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the Au cluster is less stable and more prone to sintering, and it also indicates that the Au cluster has lower charge donating capability. In contrast, Ru13 and Mo13 are strongly bound to the graphene support and donate 3.6 e- and 2.2 e- to the graphene support respectively. This charge transfer is significantly higher than the 1.90 e- charge transfer for Pd13 which indicates that the clusters have already acted as strong charge donors. We next examined the binding and charge transfer characteristics of the 3d transition metal clusters. The Ni13 cluster on graphene was found to have similar charge transfer between the cluster and graphene, 2.09 e- for Ni versus 1.90 e- for Pd. The Fe13 cluster donates 2.85 e- while Cu13 donates 1.10 e-. The binding energies of Ni13, Fe13, and Cu13 are all closer in binding energy to Pd13 than Ru13 and Au13. To more accurately examine their charge transferability, we next considered the work function of the supported clusters.


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Figure 1. Structure, selected bond length, binding energies to graphene of supported Cu13 (A), Ni13 (B), Fe13 (C), and Pd13 (D) clusters. The binding energy of the cluster to graphene is calculated as EBinding = EPdn + E(graphene) - E(Pd+graphene).

Figure 2. The binding energies of Au13, Ni13, Fe13, Mo13, Cu13, Pd13 and Ru13 clusters supported on graphene and the charge transfer to the graphene support. To further understand the interaction between the metal clusters and the graphene substrate, we examined the work function, the metal d-band center, and the bromine and the phosphine binding energies for the free and supported clusters, as shown in Figure 3. The work function is an indication of the energy required to remove one electron from the supported metal cluster and depends on the position of the Fermi energy and the strength of the dipole moment induced by the charge transfer between the metal cluster and the surface. For reference, the work function of pure graphene is calculated to be 5.42 eV. All of the supported clusters have work functions that are less than that of pure graphene due to the


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surface dipole lowering the work function due to the clusters donating charge to the surface. Fe donates the most charge to the graphene support, so it has the lowest work function due to the strength of the induced dipole. The ionization potential of the pure cluster also has an effect, as it helps to set the Fermi energy of the cluster/substrate system. Because of the high work function of graphene, the work function of the supported clusters are larger than those of the free metal clusters. This indicates the charge donation capability of the supported clusters is diminished as compared to the free clusters. Moreover, after the deposition, the work function of the Ni13/graphene cluster is closest to the work function of the Pd13 cluster, while the work function of the Fe13/graphene cluster and the Cu13/graphene cluster are lower. We note that all of these work functions are in a range from 4.1 eV to 4.6 eV. Next we consider the d-band center model which has proven to be practical for analyzing trends in the reactivity in transition metals.61-63 The d-band center of the metal cluster is an indication of the catalytic activity, as the d-band center often correlates to the binding energy of the reactants. A lower metal d-band center relative to the Fermi energy corresponds to a higher occupation of antibonding energy levels which destabilizes the binding of adsorbates. Two materials with similar d-band centers might be expected to have similar catalytic activity.


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Figure 3. (A) The work function of free and supported Fe13, Cu13, Ni13 and Pd13 clusters. (B) dband center of free and supported Fe13, Cu13, Ni13 and Pd13 clusters. (C) Br binding energy of free and supported Fe13, Cu13, Ni13 and Pd13 clusters. (D) Phosphine binding energy of free and supported Fe13, Cu13, Ni13 and Pd13 clusters. As shown in Figure 3B, the d-band center of all the metals shifted deeper after deposition on graphene. The lowering of the d-band center in the clusters will generally weaken the adsorbate-metal bonding. Furthermore, the d-band center of supported Ni13 cluster approaches the d-band center of the supported Pd13 cluster. To further evaluate the effect of the support on the donor-acceptor characteristics of the supported clusters, we have calculated the adsorption energy of bromine, a charge acceptor, and trimethyl phosphine, a charge donor, on the free and supported metal clusters. If a supported cluster is a good charge donor, then the binding energy of a charge acceptor such as bromine will be enhanced, and if a supported cluster is a good charge acceptor, then the binding of a charge donor such as a phosphine will be enhanced.64-66 Aryl-halides are


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commonly used compounds in cross-coupling reactions, so the binding of halides is related to the binding energy of an intermediate in the cross-coupling reaction. Phosphines are electronrich molecules that often serve as a ligand in cross-coupling reactions, because it enhances the charge donating ability of the Pd, increases the solubility of the catalyst, and prevents agglomeration of the catalyst. As shown in Figure 3C for the Pd13 cluster, the Br binding energy increases significantly after the deposition on graphene, while for Ni13, the Br binding decreases significantly, and Fe13, Cu13 have negligible increases of less than 0.1 eV in the Br binding energy after being placed on support. The increase of the Br binding energy of more than 0.4 eV on Pd13 indicates that the cluster is a much better charge donor when supported on graphene, while the Br binding energy of the Ni13 cluster dropped significantly which suggests the charge donating capability has been reduced. This result predicts that the activation energy for oxidative addition on Pd13 clusters supported by graphene should decrease, while Ni13 on graphene should have a higher activation energy than the unsupported cluster. In contrast, the phosphine binding energies of the metals are all increased considerably after deposition due to the supported clusters becoming more positively charged after donating charge to the graphene support. The binding energy of the Pd13 cluster is significantly higher than the phosphine binding energies of the first-row transition metal clusters which indicates that the first-row transition metal clusters are poor charge acceptors as compared to Pd. However, after deposition, the phosphine binding energy of Ni13 cluster approaches the binding energy of free Pd13 cluster and becomes the second best electron acceptor of all the clusters studied here. The improved electron acceptor capabilities suggest these supported


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catalysts will all have lower activation energies in transmetallation and reductive elimination steps. To investigate the possibility of replacing Pd with a 3d transition metal, we calculated the activation energies for the free and supported Fe13, Ni13, and Cu13 clusters and compared them to the activation energies for the free and supported Pd13 clusters in the full reaction cycle of the Suzuki reaction in Figure 4. The Suzuki reaction proceeds via three steps: oxidative addition, transmetallation, and reductive elimination. In the charge donating oxidative addition step, the activation energies of the free first-row transition metal clusters are significantly lower than that of the free Pd13 cluster. This is consistent with the free clusters all having significantly higher Br binding energies, indicating that the Fe, Ni, and Cu clusters are stronger charge donors than Pd13. The Fe13 cluster exhibits extremely low activation energy confirming that the Fe cluster is the strongest charge donor, as seen by its lowest work function, and its highest Br binding energy. After deposition, the activation energy for the oxidative addition on the Ni13 cluster has significantly increased, while in contrast, supported Pd13 shows a lower activation energy than a free Pd13 cluster. These changes are consistent with what is predicted by the changes in the Br binding energy. For Fe13, there is a small increase in the activation energy after deposition, while the Br binding predicts a small decrease. We note that the activation energy for oxidative addition on both Fe clusters are extremely low, less than 0.1 eV. For Cu13, the activation energy increases after being supported on graphene, this is most likely due to the graphene lowering the d-band center of Cu, which will weaken the binding, especially for the alkyl group which also binds to the cluster during


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the oxidative addition step. We find that the change in activation energies of the free and supported Ni, Pd, and Cu clusters are highly correlated with the changes in Br adsorption energy, our measure of the charge donating ability of the cluster. This result is consistent with our hypothesis that the charge donating capability of a cluster significantly affects its activity towards oxidative addition.

Figure 4. (A) Activation energies of free and supported Fe, Cu, Ni and Pd clusters in full reaction cycle. (B) Reaction pathway of free and supported Ni13 cluster in Suzuki reaction. Activation Energies of free and supported Ni13, Fe13, Cu13 and Pd13 clusters as a function of work function of the corresponding metal cluster in the oxidative addition (C), transmetallation (D) and reductive elimination (E) steps. For the charge withdrawing transmetallation and reductive elimination steps, the activation energies of the free Fe13, Cu13, and Ni13 clusters are significantly higher than that of Pd13. This


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suggests that Suzuki reactions catalyzed by the first-row transition metals will be hindered in these steps where clusters act as charge acceptors. After the deposition, the activation energies of supported Fe13, Cu13 and Ni13 clusters have been considerably reduced. This is consistent with our charge acceptor hypothesis, as all of these clusters have higher phosphine binding energies after being supported on graphene. For the supported Ni13 cluster, the activation energy is slightly higher than the free Pd13 cluster and has the lowest activation energy of the non-Palladium clusters. A similar trend is seen in the reductive elimination step that also requires the cluster to act as a charge acceptor. Among the studied systems, the Pd13 cluster on graphene has the lowest activation energy, the free Pd13 cluster has the second lowest, and the Ni13 on graphene has the lowest activation energy for the non-Palladium clusters. This is consistent with Ni13 on graphene being the most effective charge acceptor of the 3d transition metal clusters, as it has the largest phosphine binding energy. As Ni13 on graphene has the lowest activation energy of the non-Pd clusters studied here, we have plotted the full reaction pathway for the Suzuki cross-coupling reaction for Ni13 with and without support in Figure 4B. While the activation energies of Ni13/graphene are higher than those of Pd13/graphene, we next considered two methods to further reduce the activation energy for Ni clusters. Figure 4 C-E show the correlation between the work functions of the metal clusters and activation energies in the charge donating and charge withdrawing steps. In the charge donating oxidative addition step, a low work function of the cluster is correlated with a low activation energy. In the charge withdrawing transmetallation and reductive elimination steps, the correlation is also observed with the higher work function clusters having lower activation


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energies, except for the Cu13 cluster. This is believed to be a result of the weaker bonding energies of Cu leading to larger activation energies, although the Cu cluster undergoes a large reconstruction during the reaction due to low internal binding energy of the free Cu13 cluster. Our first strategy was to lower the activation energy of Ni by changing the size of the Ni cluster. Ni4, a tetrahedral cluster, was immobilized to the double vacancy defect site on graphene. The calculation of the binding energies of bromine and phosphine to the free and supported Ni4 cluster shows that both the Br and phosphine binding energies are increased by 0.23 eV and 0.28 eV respectively after deposition on graphene defects. As shown in Figure 5A, the activation energy of supported Ni4 cluster decreases by 0.04 eV in the oxidative addition step. In the charge withdrawing transmetallation and reductive elimination step, the activation energies drop by 0.18 eV (35%) and surprisingly 0.32 eV (90%) respectively after deposition. This significant reduction in the activation energy with the reduction of cluster size is due to the Ni active sites being adjacent to the positively charged Ni anchor atom within the graphene vacancy. Thus the cluster size reduction implies a much stronger electron acceptor. We also noted that for Pd, lower coordination leads to lower activation energies, therefore a similar effect is likely to occur with Ni.10 This effect is most pronounced in the charge accepting transmetallation and reductive elimination steps in the Suzuki cross-coupling reaction.


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Figure 5. A) The reaction pathway of free and supported Ni4 clusters in the Suzuki reaction. The structure and selected charge of B) Pd12Ni, C) Ni12Pd, and D) Ni2Pd2 clusters. E) The cocatalysis Suzuki reaction pathway for the Pd12Ni cluster. Our second approach for reducing the activation energy of the supported Suzuki crosscoupling catalysts is to use bimetallic catalysts. Ni12Pd, Pd12Ni and Ni2Pd2 clusters were tested in the Suzuki reaction as shown in Figure 5B-D. The results show that the electronic properties of the individual Pd and Ni active sites are modified significantly in the bimetallic cluster. As seen in Figure 5B-D, the Ni sites of the bimetallic Pd/Ni clusters donate charge to the Pd sites,


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with the Pd sites becoming negatively charged, while the Ni sites become positively charged. This modifies the charge donating and withdrawing capability of the atoms, which may then lead to the modification of the activation energies. One would expect that the bimetallic Ni sites will then become even better charge accepting sites than pure Ni clusters and that the bimetallic Pd sites will be even better charge donating sites than pure Pd clusters. In Figure 6A-D, the activation energy for the free and supported 13-atom and 4-atom clusters are shown. For the free 13-atom clusters, Ni13 is most active for the oxidative addition step, and the Pd12Ni cluster where the Ni atom is the active site is most active in the transmetallation and reductive elimination steps. This supports the concept that the charge transfer from Ni to Pd results in a Ni site that is activated towards charge accepting steps such as transmetallation and reductive elimination. The activation energy then follows Ni < Pd-(Ni) < Ni+(Pd) < Pd for the steps where the cluster acts as a charge donor. Pd-(Ni) denotes the Pd site on a bimetallic cluster where Ni has donated charge to Pd, and Ni+(Pd) denotes the Ni site on a bimetallic cluster where the Ni has donated charge to Pd. In contrast, for steps where the cluster acts as a charge acceptor the activation energy follows the trend as Ni+(Pd) < Pd < Pd(Ni) < Ni for transmetallation, and Ni+(Pd) < Pd-(Ni) < Pd < Ni for reductive elimination. When different metal atoms are contained in the same cluster, it is now possible for different steps in the reaction to be carried out on different active sites. We note that this pathway may only occur when there are adjacent bimetallic surface atoms, this mechanism will not occur for core-shell bimetallic particles. This “co-catalysis” pathway is shown in Figure 5E. Depositing the bimetallic clusters on graphene results in a further decrease in the activation energy, as


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seen in Figure 6B. After deposition, the lowest activation energy pathway follows a co-catalysis process on different Ni and Pd sites. For all three steps, the activation energies of the supported Pd12Ni cluster are all lower than the supported Pd13 cluster. Decreasing the cluster size to four atoms further lowers the activation energy of the catalysis. The activation energy of the free and supported Ni2Pd2 clusters are shown in Figure 6C-D, and these are found to be even more active than the 13 atom clusters. These results are consistent with the concept that the Ni becomes more positively charged in the bimetallic cluster, and hence it becomes a better charge acceptor, while Pd sites become more negatively charged and are more active in charge donating steps. By combining the graphene support that enhances charge transfer with the small size of the cluster and the bimetallic charge transfer, we find that the supported Pd2Ni2 cluster is the most active cluster studied here towards the Suzuki cross-coupling reaction.


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Figure 6. The activation energy of A) free Pd12Ni and Ni12Pd clusters, B) supported Pd12Ni and Ni12Pd clusters, C) free Pd2Ni2 and Ni2Pd2 clusters, and D) supported Pd2Ni2 and Ni2Pd2 clusters. Pd12Ni C-C and Pd12Ni/G C-C denote co-catalysis on the bimetallic cluster where the oxidative addition is performed on the Pd site and the transmetallation and reductive elimination steps occur on the Ni site. 4. Conclusions We have performed a series of calculations to investigate the electronic properties and activation energies for the different steps in the Suzuki cross-coupling reaction in first row transition metal clusters (Fe, Ni, and Cu) to ultimately find a cost-effective replacement for Pd. We found that the ability of clusters to act as both charge donors and charge acceptors is critical in understanding the relative catalytic activity of the different clusters. The Suzuki


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reaction catalyzed by the first-row transition metal clusters is primarily hindered by the charge withdrawing transmetallation and reductive elimination steps. The defected graphene support is found to enhance the charge transfer capabilities of the clusters and decrease their overall reaction activation energy barrier. Investigations into bimetallic Palladium-Nickel clusters revealed they may act as co-catalysts to further reduce activation energies due to the charge transfer from Ni to Pd resulting in the Ni sites acting as improved charge acceptors, and the Pd sites acting as improved charge donors. These calculations also revealed that the activation energy of the supported Ni cluster can be further curtailed even lower than the activation energy of supported Pd cluster by reducing the size of the Ni cluster and mixing Pd atoms into the Ni cluster. Thus, our investigations reveal that bimetallic Pd/Ni clusters supported on defected graphene, and small Ni4 clusters supported on defected graphene are potential strategies towards improving catalysis and reducing metal costs in cross-coupling catalysts.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Supporting information includes the methods, isomers, structures of the bare and supported clusters, activation energies, work functions, d-band centers, binding energies, and structures of theintermediates and transition states. This data is found in the associated PDF file. AUTHOR INFORMATION


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Corresponding Author *[email protected] Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources U.S. Department of Energy (DOE) under the award Number DE-SC0006420. ACKNOWLEDGMENT This work was supported by a grant from U.S. Department of Energy (DOE) under the award Number DE-SC0006420. REFERENCES Grunes, J.; Zhu, J.; Somorjai, G. A., Catalysis and Nanoscience. Chemical Communications 2003, 2257-2260. 2. Liu, L.; Corma, A., Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles. Chemical Reviews 2018, 118, 4981-5079. 3. Astruc, D.; Lu, F.; Aranzaes, J. R., Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis. Angewandte Chemie International Edition 2005, 44, 7852-7872. 1.


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Acceptor Concepts for Developing Efficient Suzuki Cross

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