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Ü. YILMAZ AND H. KÜÇÜKBAY
After the determination of optimization conditions, it also investigated the activities
of all the ligands (1–9) for similar coupling reactions (Table 2). It was observed that
aryl bromides gave coupling reactions with better yields than aryl chloride. It has been
observed that aryl halides with electron-releasing substituents are converted into prod-
ucts with lower efficiency than aryl halides with electron-withdrawing substituents in
accordance with the literature (Table 2, entries 1–10, and 21–29).[44] The result showed
that urea-derived ligands containing chalcogen (1-4) exhibited good catalytic activities.
In particular, the tellurourea ligand (5) showed the best catalytic activity on the SM
coupling reaction (Table 2, entries 6, 16, 25, 34, and 43). These days, palladium com-
plexes derived from chalcogenoether ligands (S, Se, or Te) have been successfully used
in C–C coupling reactions.[53] Tellurium-based compounds can be formed or modified
by two types of reactions which are insertion and extraction reactions, for the tellurium
is the most electropositive element of chalcogens, with multiple valency states between
ꢁ 2 and þ 6.[54,55] Due to the electropositivity of the tellurium donor atom of ligand,
the palladium(II) complex is made electron-rich by ligand. This is why tellurourea
derivatives are thought to be more active as ligands. On the other hand, the carbodi-
thioates (6, 7) and carbimidothioates (8, 9) have lower activities (Table 2, entries 7–10,
17–20, 26–29, 35–38, 44–47) compared to urea-derived ligands similar to the literature.
The catalytic activities of systems bearing carbodithioates are low because they can
strongly coordinate with palladium. In a study,[37] Delaude and coworkers were
observed that the yields are low than carboxylate ligands when the carboxydithioates
used as ligands. In fact, in the experiment using an aryl bromide, it is seen that the
yield reached in 8 h is only 39%. In our study, the yields were observed to be higher
(58–68%) compared to the literature, due to the fact that reaction times were reduced
by means of the microwave. In other studies by Delaude et al.,[56,57] it has been
observed that metal complexes with dithiocarboxylate ligands show very weak catalytic
activity compared to complexes with carboxylate ligands again. Although this situation
was explained by the degradation of the complex in the previous article,[37] it was
observed that the dithiocarboxylate complexes were more stable than the carboxylate
complexes in the thermal stability tests carried out in later studies.
For example, in a study[57], ruthenium complexes with dithiocarboxylate and carb-
oxylate ligands were used as catalysts in the ROMP reaction, and it was noted that com-
plexes with dithiocarboxylate ligands show weaker activity because they cannot be a
carbene source due to their stability.
Also, in our study, as dithiocarboxylate and carbimidothioate ligands formed stable
complexes with four donor atoms, they could not show high catalytic activity.
In another study, it was observed that the yields were good in the coupling reactions
performed with the palladium complex prepared using the bulky thiourea ligand.[38]
In terms of its electronegativities, tellurium appears to be the best ligand. As the
atomic volume increases from sulfur to tellurium in the group, the tellurium will pre-
sent its electrons to the metal atom more easily. Electron-rich metal complexes
catalyze the reaction better. Therefore, tellurourea ligand is expected to be more active
than selenourea. As a matter of fact, when looking at the results (Table 2, entries
48–53), it is seen that tellurourea is a more active ligand than its selenium and sulfur
counterparts.