2.1 Physical measurements and materials

All the reactions were carried out under an atmosphere of dry nitrogen. Methanol was distilled over magnesium powder and diethyl ether (Et₂O) over a mixture of sodium and benzophenone just before use. All the other solvents were reagent-grade and used without further purification. Melting points were measured on a SMP3 apparatus. IR spectra were recorded on a Shimadzu 435-U-04 spectrophotometer from KBr pellets. ¹H, ¹³C and ³¹P NMR spectra were recorded on 400 MHz Bruker and 90 MHz Jeol spectrometer in DMSO-d₆ or CDCl₃ as the solvent at 25 °C. Chemical shifts (ppm) are reported according to internal TMS and external 85% phosphoric acid. Coupling constants are given in Hz. Elemental analyses for C, H and N atoms were performed using a PerkinElmer 2400 series analyzer.

2.2 X-ray crystallography

The single-crystal X-ray diffraction data of suitable crystals of 5 were collected with an Oxford Diffraction single-crystal X-ray diffractometer using the mirror monochromated Mo Kα radiation (0.71073 Å) at 130 K (Table 3). Gaussian absorption corrections were carried out using a multifaceted crystal model using CrysAlisPro [35]. Both structures were solved by direct methods and refined by the full-matrix least-squares method on F² using the SHELXTL-97 crystallographic package [36,37]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were inserted at calculated positions using a riding model, with isotropic displacement parameters.

2.3 Sample preparation

2.4 General experimental procedure for Suzuki cross-coupling reactions

A mixture of an aryl halide (0.75 mmol), phenyl boronic acid (1 mmol), complex 9 (0.001 mol%), Cs₂CO₃ (1.5 mmol), and DMF (2 mL) was heated to 110 °C for specified time. The reactions were monitored by thin-layer chromatography (TLC). The reaction mixture was then cooled to room temperature. After completion of the reaction, the mixture was diluted with n-hexane (15 mL) and water (15 mL). The organic layer was washed with brine (15 mL), dried over CaCl₂. The solvent was evaporated and a crude product was obtained, which was analyzed by ¹H and ³¹C NMR. The liquid residues were purified by silica gel column chromatography (n-hexane:EtOAc, 80:20), whereas the solid ones were purified by re-crystallization from ethanol and water.

3.1 Synthesis

Diphosphine Ph₂P(CH₂)₂PPh₂ reacts with 2 equiv of the appropriate ketones, forming the corresponding phosphonium salts 1–3. Further treatment with sodium bis(trimethylsilyl)amide leads to two-fold elimination of HX (X = Cl, Br), giving the free ligands 4–6. Reaction of these ligands with Pd(II) chloride in 1:1 molar ratio form C,C-chelated complexes [RC(O)CHPPh₂(CH₂)₂PPh₂CHC(O)R]PdCl₂, where R = 2-naphtyl (7), 2,4-dichlorophenyl (8), 3-nitrophenyl (9) (Scheme 1). All complexes are moderately soluble in dichloromethane and insoluble in non-polar solvents, such as n-hexane.

3.2 Spectroscopy

In the IR spectra of diphosphonium salts, a strong stretching absorption due to carbonyl groups has been observed, which confirms the formation of salts 1–3 symmetrically. IR data confirm the complete formation of the phosphorus ylides with the vanishing of the phosphonium CO band between 1658 and 1684 cm⁻¹ for diphosphonium salts 1–3 and the appearance of a new strong CO band relative to carbonyl stabilized ylides 4–6 at 1517 to 1585 cm⁻¹ [17,20]. As we have noted earlier [38], the coordination of the phosphorus ylides through carbon or oxygen causes a significant increase or decrease, respectively, in the carbonyl stretching frequency. Thus, the infrared absorption bands observed for complexes 7–9 indicate that the coordination of the symmetric phosphorus ylides with the palladium center occurs through the ylidic carbon atoms. The two carbonyl groups of all complexes are non-equivalent and discriminable in IR spectra (Table 1).

The ³¹P NMR spectra of all the compounds show all the expected resonances for the proposed structures. The ³¹P NMR spectra of phosphonium salts exhibit a singlet around 23–27 ppm due to PPh₂ groups, which indicates that the two phosphorus atoms are equivalent. The ¹H NMR spectra show, in addition to the expected aromatic resonances, two peaks centered around 4 and 6 ppm, which are attributable to the P(CH₂)₂P and PCH₂CO protons, respectively. The ³¹P NMR spectrum of phosphorus ylides 4–6 shows a singlet around 15 ppm, which corresponds to the PPh₂ groups. The phosphonium atoms of these compounds show upfield shifts compared to that of the parent phosphonium salt, suggesting some electron density increase in the P–C bonds [17,39]. The ¹H NMR spectra show all the expected resonances of symmetric phosphorus ylides 4–6. Chemical shifts appear to be shifted upfield with respect to the parent phosphonium salts, indicating that synthesis of the ylides has occurred. The room temperature ³¹P NMR spectra of complexes 7–9 exhibit two doublets around 28 and 32 ppm, which are assigned to the P1 and P2 atoms, respectively. This chemical shift is at a lower field than that observed for the corresponding free ylide (around 15 ppm), indicating that the coordination of the ylides has occurred. As reported earlier [17,40], the downfield shift of the ylidic phosphorus signifies the coordination of the ylidic moiety. The coordination of the bis-ylides 4–6 to the palladium center creates two stereogenic centers of the same configuration, as indicated by the presence of two ³¹P NMR signals (doublets with a coupling constant ³J_P–P of 2–7 Hz). In the ¹H NMR spectra, the signals due to the methinic protons for all the complexes are broad and centered at 5–6 ppm. The ¹H chemical shift values for the complexes appear to be shifted downfield with respect to the parent ylide, indicating also that the coordination of the ylides has occurred [40,41]. Selected NMR chemical shifts for compounds 1–9 are reported in Table 2.

3.3 X-ray crystallography

Colorless crystals of 5 were obtained from a chloroform solution by slow evaporation of the solvent. Relevant parameters concerning data collection and refinement for 5 are given in Table 3. Selected bond distances and angles for the two independent but chemically identical molecules found in the unit cell of 5 are displayed in Table 4. The crystal structure (Fig. 1) shows that in this molecule the geometry around the phosphorus atoms is nearly tetrahedral, and that the O atoms are oriented cis with regard to the adjacent P atom. The P(1)–C(1) (1.7218(19)) and C(1)–C(2) (1.384(3)) bond lengths are shorter than the (P⁺–C (sp³) (1.800)) and (C–C (sp³) (1.511)) normal values [42], respectively. The crystals of this compound contain CHCl₃ as the solvent.

This bond shortening is due to the ylidic resonance and is halfway between the common values for single (P–C = 1.80) and double (P = C = 1.66) bonds. The CO bond lengths of 5 (1.267(2)) also are larger than the normal value (1.210). This bond distance suggests an electron delocalization in this molecule [17,42]. The P–C(H) (1.7218(19)) and C(1)–C(2) (1.384(3)) bond lengths are close to the corresponding distances P–C(H) (1.705(2)) and C–C (1.404(3) Å) found in similar phosphorus ylides, such as [Ph₂PCH₂PPh₂C(H)C(O)Ph] [43]. The packing structure of 5 has short contacts between the adjacent molecules. In this case, the interaction between the O₂ and H₄₄ atoms of the chloroform as the solvent (1.988 Å) have a great importance in the formation of the packing structure (Supplementary data, Fig. 1S). The packing also shows two hydrogen bonds between oxygen and chlorine atoms (Fig. 2).

3.4 Suzuki cross-coupling reactions of aryl halides

The palladacycle complex 9 as a catalyst was assessed for its activity in the Suzuki cross-coupling reaction initially by studying the coupling of 4-bromoacetophenone with phenylboronic acid to form 4-phenylacetophenone as the sole product. Various parameters including solvent, base, and catalyst loading were screened to optimize the reaction conditions. The proper combination of base and solvent is extremely important; we have examined several different bases and solvents for the Suzuki reaction (Table 5). Initially, many commonly available bases were used with DMF as the solvent. Cs₂CO₃, K₂CO₃, and Na₂CO₃ as a base, which produced higher yields in short reaction times (Table 5, entries 1–3), respectively. The use of NaOAc, NaF, and NEt₃ as bases gave lower yields (Table 5, entries 4–6), respectively. In the reaction, several commonly used solvents were also tested (Table 5). Solvents, such as THF and H₂O were less effective (Table 5, entries 11–12) for this reaction. Dioxane, NMP, methanol, and toluene were moderately effective (Table 5, entries 7–10), respectively. After an optimization process, we found that our complex could perform the coupling of 4-bromoacetophenone with phenylboronic acid using a 0.001 mol% catalyst loading with Cs₂CO₃ at 110 °C in good yield in a 90-minute procedure when using technical-grade DMF as the solvent.

In order to gauge the further potential of 9 towards the Suzuki coupling methodology, we chose various substituted aryl halides for the reaction with phenylboronic acid, as shown in Table 6. Aryl bromides with various functional groups efficiently reacted with phenyl boronic acid (Table 6, entries 1–11) using Cs₂CO₃ and 2 mL DMF at 110 °C in the presence of palladium(II) catalyst to give Suzuki products in high yields. In palladium-catalyzed carbon–carbon bond formation reactions, it is commonly believed that better yields are achieved for aryl halides with electron-withdrawing rather than electron-donating substituents [51]. The reaction of electron-deficient aryl bromides with phenylboronic acid afforded efficiently the coupling products in more than 90% yield in short reaction times (Table 6, entries 2–3, 5). The electron-poor, 4-bromobenzaldehyde and 4-bromoacetophenone, also afforded high yields (Table 6, entry 2, 90% and entry 5, 91%). The reaction of p-bromo nitrobenzene with phenylboronic acid showed also high yields (Table 6, entry 3, 90%). In the electron-rich or deactivated p-bromotoluene, the yields are good (Table 6, entry 4, 80%). The electronically neutral bromobenzene (Table 6, entry 1) produced good amounts of the desired product. As expected, very satisfactory yields were obtained with the 2-bromothiophene and the 1-bromonaphthalene of phenylboronic acid (Table 6, entry 6, 84% and entry 7, 86%). Next, we examined the reaction of aryl iodides with phenylboronic acid for the Suzuki reaction. And good yields of the corresponding products were obtained under the optimized conditions (Table 6, entries 8–9). Trying to apply aryl chloride as an efficient substrate was successful (Table 6, entries 10–11). Electron-deficient substrates, such as 4-chloroacetophenone (Table 6, entry 11) coupled with phenylboronic acid gave yields of the coupling product in the order of 82%. The main drawback of the Pd-mediated Suzuki cross-coupling reaction is that only aryl iodides and aryl bromides can be used efficiently. Recent progress with increasing the reaction rate of aryl chlorides permitted to overcome this problem [52–54]. The stronger C–Cl bond is, in fact, responsible for the slower reaction rate of aryl halides because the oxidative addition step was suggested to be the rate-determining step in cross-coupling catalytic cycles [55].

The homogeneous nature of the catalysis was checked by the classical mercury test [56]. Addition of a drop of mercury to the reaction mixture did not affect the conversion of the reaction, which suggests that the catalysis is homogeneous in nature, since heterogeneous catalysts would form an amalgam, thereby poisoning it. The fact that metallic Hg does not kill the catalyst is a good point in favour of the fact that no metallic Pd is being produced.

The catalytic activity of a seven-membered Pd complex (A, 9) was higher than that of the five-membered Pd complex (B) in all the reactions in our previous work [34]. This may be due to the fact that Pd(II) in palladacycles with five-membered rings is difficult to reduce to Pd(0). As it can be seen in Table 7, some of the aryl halides with an electron-withdrawing or an electron-donating substituent have been tested, among which the seven-membered Pd complex (A) has a higher efficiency as a catalyst in a shorter reaction time. Although several catalytic systems have been reported to support the Suzuki C–C coupling reaction, a catalyst of this type is novel for its seven-membered ring containing bisphosphine bis ylide as a ligand coordinated through the CH methinic bond. The high efficiency of this catalyst in low catalyst loading, under aerobic conditions and short reaction times, makes it valuable.