Palladium Cyclometallated Compounds: Evaluation of Their Catalytic Activity in Cross-Coupling Reactions

: Catalysts are substances that can increase the speed of a chemical reaction and are often used in the chemical industry. Palladium is one of the most widely used metal centers in metal-based catalysts, and a lot of palladium complexes have been extensively used in many reactions, particularly in cross-coupling reactions with a carbon − carbon bond formation. All their possible applications as catalysts, along with their uses in biological assays as anticancer agents, make these family of complexes very interesting and highly studied, allowing the modiﬁcation of the ligands around the metal and the extreme modulation of their properties. Herein we report the synthesis of several palladium cyclometallated compounds with thiosemicarbazone ligands and bis(diphenylphosphino)methane (dppm). Additionally, we evaluate their catalytic activity in a Suzuki − Miyaura cross-coupling reaction, using 4-bromoacetophenone and phenylboronic acid as reagents and following the reaction with 1 H-NMR spectroscopy. A ﬁnal comparison between the catalytic conversions and the complexes allows us to propose the best structure for a catalytic purpose in these conditions.


Introduction
The chemistry of transition metals has been extensively studied over the years [1,2]. The high number of different metals and the ligands that can coordinate around them make this kind of complex very extensive with different properties and applications in coordinative and organometallic chemistry.
Cyclometallated compounds with palladium, using thiosemicarbazone ligands, are reported in this research work [11][12][13][14][15]. We show the catalytic activity for these ligands with different metal complexes [16]. The catalytic activity of all species synthesized is discussed for the Suzuki−Miyaura's reaction.

Experimental
The reactions to obtain the thiosemicarbazone ligands, tetranuclear compounds with palladium and reaction of these compounds with dppm were carried out following the procedure we reported earlier [17].

Synthesis of Homodinuclear Compounds (10-12)
Compounds 7-9 (15 mg) and bis(benzonitrile)palladium (II) chloride (quantities shown in Table 1) were added under nitrogen in a deoxygenated solution of acetone (Scheme 1). After stirring for 24 h at room temperature, the solvent was removed under a reduced pressure, and the residue was treated with dichloromethane-hexane, centrifugated and dried under a vacuum.

Results and Discussion
The previous synthetic route is shown in Scheme A1 and NMR spectra are included in Figures A1 and A2. The general procedures and characterization data are listed in Appendix B.
The comparison of the 1 H NMR spectra between the dinuclear compounds (10-12) with the previous ones (7-9) does not show very significant changes. The most remarkable one is the high field shift of the PCH 2 P protons, due to the second metal coordination to the free phosphorus atom. This fact is supported with the 31 P-{ 1 H} NMR spectrum of 10 because two doublets appear downfield, caused by the coordination of the two phosphorus atoms to different palladium metal centers (as shown in Figure 1).

Catalytic Conversion
The Suzuki−Miyaura reaction was carried out using 4-bromoacetophenone and phenylboronic acid as reagents (see Scheme 2). Aliquots were taken during the reaction, and results were monitored with 1 H NMR spectroscopy as shown in Figure 2.  Conversion results are shown in Table 2 for all reactions. The results show that the dinuclear compounds are extremely good catalysts, probably due to the Pd−Cl bond. The bond lability allows these compounds to be very effective in these conditions. Compounds 4-9 show poor catalytic activity in these conditions, especially compared to their homodinuclear counterparts.
NMR spectra of compounds 10-12 confirm the product structure. 3.

4.
Catalytic results show that the dinuclear compounds are better catalysts for the Suzuki−Miyaura reaction.

Appendix B
Elemental analyses were performed with a Thermo Finnigan analyzer, model Flash 1112. IR spectra were recorded on a Jasco model FT/IR-4600 spectrophotometer equipped with an ATR model ATR-PRO ONE. 1 H NMR spectra and 31 P-{ 1 H} NMR spectra were recorded on a Varian Inova 400 spectrometer operating at 400.14 MHz