154
9 Carbon-Carbon Cross-Coupling Reactions
towards the phenyl boronic acid when breaking the carbon-boron bond, and then
flows back when forming the palladium-boron bond. In the reductive elimination
step, the carbon-carbon bond is formed and the product is released from the palladium
surface. This process involves charge flowing back from the binary complex to the
catalyst (Fig. 9.7, red arrow). Different steps of the catalytic cycle for the Suzuki
cross-coupling reaction require the catalyst to serve alternately as an efficient charge
donor or acceptor. The conducting nature of the graphene allows it to activate both
the donation and acceptance of charge by exchanging charge with the cluster.
Also examined, was the individual steps and overall reaction activation energies
for free Pd n clusters (n = 1, 4, 13, and 14), providing insight into the microscopic
process. The Pd atom [87, 88] and Pd 4 clusters were selected based on earlier studies
[89] where it was found that the presence of Pd 1 and Pd 4 fragments in solution were
critical for cross-coupling reactions. The results in Fig. 9.8a suggest that the activation
energies are cluster size-dependant. It was found that Pd with a low coordination tends
to have a lower activation energy than Pd with a higher coordination, although the
support and ligands also play a significant role [90].
The activation energy of Pd n clusters bound to a double atom vacancy sites in the
graphene sheet were calculated to understand the effect of the Pd-support interaction,
along with the activation energies for a Pd atom ligated by P(Me) 3 as a model of a
homogeneous catalyst. For Pd 4 , Pd 13 , and Pd 14 , the activation energies for clusters
supported on reduced graphene were all lower than those for the corresponding free
clusters, indicating strong Pd-support interaction that enhances Pd n cluster reactivity
in each step. Figure 9.8b shows the density of states of Pd 4 and Pd 4 /G at the transition
state for reductive elimination. The bonding and antibonding orbitals that control the
formation of the C–C bond are significantly stabilized by the presence of the defected
graphene. The stabilization is due to the positive charged state of the supported cluster
as the support makes the catalyst a superior charge acceptor. The full Suzuki reaction
pathway for Pd 4 and Pd 4 /G is shown in Fig. 9.8c. Strikingly, the activation energy for
all three steps are lower for the supported cluster than for the free cluster, illustrating
the support activates both electron donating and electron withdrawing steps in the
reaction cycle. These results strongly suggest that the supported Pd clusters are more
effective catalysts because of lower activation energies resulting from the electronic
interaction with the graphene support.
The charges on reagents and products were further analysed at each step for each
cluster (Fig. 9.9) to identify the physical origin of the reduced activation energies.
First let us consider, the trans-metalation step which involves both charge donation
and back donation. As seen in Fig. 9.9a, the electron-rich ligand of the Pd/P(Me) 3
catalyst actively enhances charge donation, but the back flow of the charge is hindered
which leads to a high activation energy. In contrast, graphene-supported Pd clusters, especially Pd 4 /G, have lower activation energies for trans-metalation due to the
conductive nature of graphene allowing it to accept charge. There is a clear inverse
relationship between the net charge flow and the activation energies demonstrating
that catalysts which effectively accept charge are more active for trans-metalation.
The relationship between charge flow and activation energy was also observed
in the oxidative addition and reductive elimination steps (Fig. 9.9b, c). In oxidative
9 Carbon-Carbon Cross-Coupling Reactions
towards the phenyl boronic acid when breaking the carbon-boron bond, and then
flows back when forming the palladium-boron bond. In the reductive elimination
step, the carbon-carbon bond is formed and the product is released from the palladium
surface. This process involves charge flowing back from the binary complex to the
catalyst (Fig. 9.7, red arrow). Different steps of the catalytic cycle for the Suzuki
cross-coupling reaction require the catalyst to serve alternately as an efficient charge
donor or acceptor. The conducting nature of the graphene allows it to activate both
the donation and acceptance of charge by exchanging charge with the cluster.
Also examined, was the individual steps and overall reaction activation energies
for free Pd n clusters (n = 1, 4, 13, and 14), providing insight into the microscopic
process. The Pd atom [87, 88] and Pd 4 clusters were selected based on earlier studies
[89] where it was found that the presence of Pd 1 and Pd 4 fragments in solution were
critical for cross-coupling reactions. The results in Fig. 9.8a suggest that the activation
energies are cluster size-dependant. It was found that Pd with a low coordination tends
to have a lower activation energy than Pd with a higher coordination, although the
support and ligands also play a significant role [90].
The activation energy of Pd n clusters bound to a double atom vacancy sites in the
graphene sheet were calculated to understand the effect of the Pd-support interaction,
along with the activation energies for a Pd atom ligated by P(Me) 3 as a model of a
homogeneous catalyst. For Pd 4 , Pd 13 , and Pd 14 , the activation energies for clusters
supported on reduced graphene were all lower than those for the corresponding free
clusters, indicating strong Pd-support interaction that enhances Pd n cluster reactivity
in each step. Figure 9.8b shows the density of states of Pd 4 and Pd 4 /G at the transition
state for reductive elimination. The bonding and antibonding orbitals that control the
formation of the C–C bond are significantly stabilized by the presence of the defected
graphene. The stabilization is due to the positive charged state of the supported cluster
as the support makes the catalyst a superior charge acceptor. The full Suzuki reaction
pathway for Pd 4 and Pd 4 /G is shown in Fig. 9.8c. Strikingly, the activation energy for
all three steps are lower for the supported cluster than for the free cluster, illustrating
the support activates both electron donating and electron withdrawing steps in the
reaction cycle. These results strongly suggest that the supported Pd clusters are more
effective catalysts because of lower activation energies resulting from the electronic
interaction with the graphene support.
The charges on reagents and products were further analysed at each step for each
cluster (Fig. 9.9) to identify the physical origin of the reduced activation energies.
First let us consider, the trans-metalation step which involves both charge donation
and back donation. As seen in Fig. 9.9a, the electron-rich ligand of the Pd/P(Me) 3
catalyst actively enhances charge donation, but the back flow of the charge is hindered
which leads to a high activation energy. In contrast, graphene-supported Pd clusters, especially Pd 4 /G, have lower activation energies for trans-metalation due to the
conductive nature of graphene allowing it to accept charge. There is a clear inverse
relationship between the net charge flow and the activation energies demonstrating
that catalysts which effectively accept charge are more active for trans-metalation.
The relationship between charge flow and activation energy was also observed
in the oxidative addition and reductive elimination steps (Fig. 9.9b, c). In oxidative