## 3 Results and Discussion
### Catalytic Activity
#### 3.8.1 Effect of PdAS Loading
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Catalyst** & **Conversion** & **Selectivity MCIN 1** & **t\% conversion 2** & **r\({}_{0}\)** & **TOF** \\  & **\%** & **\%** & **min** & \(\times\)**10\({}^{4}\) mol.L-1\({}^{-1}\)min-1** & **h\({}^{-1}\)** \\ \hline Pd(CH\({}_{3}\)CO\({}_{2}\))\({}_{2}\) & 100 & 100 & 0.2 & 1280 & 29554 \\ PdAS & 100 & 100 & 6.4 & 40 & 929 \\ PdAS(1)-MA & 58 & 100 & 43.7 & 2 & 137 \\ PdAS(2)-MA & 99 & 100 & 8.3 & 9 & 720 \\ PdAS(5)-MA & 99 & 100 & 2.2 & 67 & 2782 \\ PdAS(10)-MA & 100 & 100 & 1.0 & 265 & 6122 \\ \hline \hline \end{tabular}  
* evaluated at the maximum of conversion level; 2 calculated according exponential fitting of [I-Ph] decrease included in the Figure S2 in supplementary material.  
\end{table} 
Table 3: I-Ph conversion, time to reach a conversion level of 5%, initial reaction rate (ro) and turnover frequency (TOF) for Pd-based catalysts. However, the catalytic mechanism for the Pd-SALOPHEN catalysts could involve the formation of intermediates Pd(II)-Pd(0) [67; 68], or different reaction states based on the participation of Pd(II)-Pd(IV) intermediates [69; 70]. Recently, [PERSON] et al. proposed a modified mechanism for the Heck reaction, which involves the use of a Pd(II)-porphyrin tetradentate N\({}_{4}\) complex [71], as shown in Scheme 2. This mechanism suggests the formation of Pd(0)-(L\({}_{2}\)) (i) from Pd(II)-(L\({}_{4}\)), either by dissociation followed by reduction or by reduction followed by dissociation. The coordinately unsaturated species, i, then undergoes oxidative addition with I-Ph to give intermediate ii, which undergoes insertion to cinnamate iii followed by \(\beta\)-hydrogen elimination to form iv. The intermediary iv participates in reductive elimination to regenerate the active component of Pd(0)-(L\({}_{2}\)) (i). Accordingly, since Pd(0)-(L\({}_{2}\)) is considered the catalytically active form of i in the Heck reaction, mercury poisoning tests [4] were carried out to evaluate the catalytic mechanism of the Pd-SALOPHEN homogeneous catalysts, as shown in of Figure 7b. The efficient catalytic inhibition observed following the addition of a large excess of Hg to the reaction demonstrates that the catalytic cycle of the homogenous Pd-SALOPHEN follows the Pd(II)-(L\({}_{4}\))/Pd(0)-(L\({}_{2}\)) pathway. To confirm the mechanism proposed by [PERSON] et al., transmission electron microscopy (TEM) characterization of the catalytic medium was evaluated to determine the nature of the Pd-SALOPHEN catalysts post-reaction, as shown in Figure 8b. As no metallic Pd nanoparticles were detected after the I-Ph/MA coupling reaction, we propose that following the complete consumption of I-Ph, the Pd-SALOPHEN catalyst could be recovered to give the Pd(II)-(L\({}_{4}\)) form through the Pd(0)-(L\({}_{2}\)) \(\rightleftharpoons\) Pd(II)-(L\({}_{4}\)) equilibrium. Figure 8: TEM micrographs for the used Pd-based catalysts. (**a**) Pd(CH\({}_{3}\)CO\({}_{2}\))\({}_{2}\)), (**b**) Pd-SALOPHEN, (**c**) PdAS(1)-MA, (**d**) PdAS(2)-MA, (**e**) PdAS(5)-MA and (**f**) PdAS(10)-MA. **Scheme 2. ** Proposed mechanism of the Heck reaction. (**a**) Mechanism for Pd(II)/Pd(0) adapted from [66] and (**b**) mechanism for tetradentate Pd(II) catalyst adapted from palladium(II)-porphyrin complex [71] as model homogeneous catalyst. In the case of PdAS(1)-MA and PdAS(2)-MA, lower activities were observed for these catalysts compared with Pd-SALOPHEN due to the steric constraints provided by the resin influencing the geometry of the Pd-SALOPHEN complex, and consequently modifying the catalyst's electronic and redox properties [67]. TEM characterization was then carried out on the used PdAS(1)-MA and PdAS(2)-MA catalysts, as shown in Figure 8c,d. Neither catalyst exhibited Pd NP formation following the reaction cycle, was it expected that the immobilized Pd-SALOPHEN complex on the MA-based resins followed the same catalytic cycle as the homogeneous catalyst at PdAS contents \(<0.40\,\mathrm{mmol_{gold}^{-1}}\).