3.1 Chemistry

In this study, four Pd(II) complexes with the dppe and the phosphorus ylides derived from the reaction between dppm and α-bromoketo derivatives, [(dppe)Pd(Ph₂PCH₂PPh₂C(H)C(O)PhX)](OTf)₂ (X = Br (C¹), Cl (C²), NO₂ (C³), OCH₃ (C⁴)), were synthesized and investigated by physicochemical techniques. The reaction between [(dppe)Pd(OTf)₂] and ylide (Y) in the 1:1 molar ratio readily occurred, leading to the new complexes [(dppe)Pd(Y)](OTf)₂ (Scheme 2).

3.2 Materials and physical measurements

The required chemicals were of analytical reagent grade and were purchased from Merck and Alderich. Melting points were measured on a SMPI apparatus and are reported without correction. Elemental analyses for C, H and N were performed using a Perkin–Elmer 2400 series analyzer. The analyses were repeated twice to check the accuracy of the analyzed data. Fourier transform IR spectra were recorded on a Shimadzu 435-U-04 spectrophotometer and samples were prepared as KBr pellets, in 200–4000 cm⁻¹ region. UV-Vis spectra were recorded on a JASCO, V-670 Spectrophotometer, Japan, in the range 190–2700 nm. Mass spectra (electrospray) were recorded from a JEOL JMS 700 B/ES spectrometer. The thermo gravimetric analyses (TGA) were carried out in dynamic nitrogen atmosphere (20 ml min⁻¹) with a heating rate of 10 °C min⁻¹ in a platinum crucible using Mettler TA-4000/TG-50. The experiments were carried out on samples with an average mass of 10 mg. All electrochemical measurements were performed with a computer-controlled potentiostat, an Autolab electrochemical analyzer model PGSTAT30 (Eco Chemie, Utrecht, The Netherlands) and a standard three electrode cell consisting of an Au working electrode, a platinum wire auxiliary electrode and an Ag/AgCl (3 M KCl) reference electrode. The X-ray powder diffraction pattern was recorded over 2θ = 2–90° range using X-ray diffractometer model APD 2000. Radiation was provided by the copper anode (K_α, λ = 1.54056 Å) operated at 40 kV and 25 MA. NMR Spectra were recorded on a 90 MHz Jeol spectrometer (¹⁹F at 84.2 MHz and ³¹P at 36.26 MHz) or 400 MHz Bruker spectrometer (¹H at 400.13 MHz) in CDCl₃ as solvent at 25 °C. The splitting of proton resonances in the ¹H and ³¹P NMR spectra are shown as: s = singlet, d = doublet, t = triplet and m = multiplet. Chemical shifts (ppm) are relative to internal TMS (δ = 0 (¹H)), CFCl₃ (δ = 0 (¹⁹F)) and external 85% phosphoric acid (δ = 0 (³¹P)).

3.3 Crystal structure determination and refinement

Suitable crystals for ligand Y² was obtained from dichloromethane solution by slow evaporation of the solvent and mounted in random orientation on glass fibers. The X-ray intensity data were measured at 298 K on a Bruker SMART APEX CCD-based three-circle X-ray diffractometer system using graphite monochromated Mo K_α (λ = 0.71073 Å) radiation. The crystal structures were solved by direct methods and refined by using SHELXS-97 and SHELXL-97 crystallographic software packages [37–40]. All non-hydrogen atoms were refined anisotropically using reflections I > 2σ(I). Hydrogen atoms were located in ideal positions.

3.4 Computational details

The geometries of the complex C², phosphorus ylide Y² and dppe ligand in the gas phase were fully optimized using SVP and TZVP basis sets [41] at BP86 [42,43] level of theory. All calculations were performed using by Gaussian 03 [44] set of programs. Vibrational frequency analysis, calculated at the same level of theory, indicates that optimized structures are at the stationary points corresponding to local minima without any imaginary frequency. The ¹H and ³¹P NMR shielding results were obtained at BP86 level of theory and using both the SVP and TZVP basis sets. Both the ¹H and ³¹P isotropic shielding constants of the optimized mononuclear Pd (II) complex were calculated by using the gauge independent atomic orbital (GIAO) method [45,46].

3.5 Synthesis of phosphorus ylides Y^1-4

General procedure [25]: Bis(diphenylphosphino)methane (dppm) (0.2 g, 0.52 mmol) was dissolved in 5 ml of chloroform and then a solution (5 ml, CHCl₃) of 4-bromophenacyl bromide (0.144 g, 0.52 mmol) was added dropwise. The resulting reaction mixture was stirred for 2 h at room temperature. Addition of 25 ml of diethyl ether, after the prescribed reaction time caused the precipitation of the phosphonium salts. The resulting phosphonium salts (0.3 g, 0.45 mmol) were treated with triethyl amine (0.5 ml) in toluene (15 ml). The triethyl amine hydrobromide produced was filtered off and the toluene layer concentrated to 5 ml which upon further addition of petroleum ether (25 ml) resulted in the precipitation of the ligands as free-flowing solids. The product was collected and dried under vacuum (Scheme 2).

3.6 Synthesis of [Pd(dppe)Cl₂] (M¹)

General procedure: to a solution of the Bis(diphenylphosphino)ethane (dppe) (1 g, 2.5 mmol) in 20 ml ethanol was added to a stirred suspension of finely divided K₂PdCl₄ (0.81 g, 2.5 mmol) in 10 ml concentrated hydrochloric acid. The mixture were refluxed with stirring for eight hours and cooled to give precipitates of the product (1.6 g, 90%) (Scheme 2). ¹H NMR (CDCl₃): δ (ppm) 7.3–7.8 (m, 20H, C₆H₅), 1.82 (m, 4H, CH₂). ³¹P NMR (CDCl₃): δ (ppm) 66.7 (s, PPh₂). Anal. Calc. for C₂₆H₂₄Cl₂P₂Pd: C, 54.24; H, 4.20, Found: C, 54.12; H, 4.11%.

3.7 Synthesis of [Pd(dppe)(OTf)₂] (M²)

To a solution of complex M¹ (0.57 g, 1 mmol) in CH₂Cl₂ (20 ml) was added AgOTf (0.51 g, 2 mmol). The reaction mixture was protected from room light with an aluminum foil and stirred for 2 h at room temperature. The solution was then filtered through dry Celite and the solvent was evaporated under reduced pressure. The residue was washed with diethylether (2 × 10 ml) and dried under vacuum (Scheme 2). Product [Pd(dppe)(OTf)₂] was obtained as a white powder (0.67 g, 82%). ¹H NMR (CDCl₃): δ (ppm) 7.3–7.8 (m, 20H, C₆H₅), 1.94 (m, 4H, CH₂). ³¹P NMR (CDCl₃): δ (ppm) 60.1 (s, PPh₂). ¹⁹F NMR (CDCl₃): δ (ppm) 67.5 (s, CF₃). UV–vis (λmax, nm): 262 (intra-ligand transition, phenyl group), 324 (MLCT). Anal. Calc. for C₂₈H₂₄F₆O₆P₂PdS₂: C, 41.88; H, 3.01. Found: C, 41.70; H, 3.11%.

3.8 Synthesis of Pd complexes

3.9 Biological studies

4.1 FT-IR and ¹⁹F NMR studies

The proclivity of the OTf ligand to be replaced by nucleophiles is further shown by some observations indicating that in M² the poorly coordinated OTf is continuously being changed with the phosphorus ylides. Fluorine NMR (measured in CDCl₃) shows a weak peak for the trifluoromethanesulfonate ion in near 67.5 ppm in case of the complex M² (Supplementary material, Fig. S1). ¹⁹F NMR spectroscopy in room temperature reveals a single resonance due to OTf at −77.9 ppm in complex C² (Fig. S2), which indicates that the trifluoromethanesulfonate is a counter ion [49–51].

The IR spectrum of the ligands (Y¹-Y⁴) and their complexes showed a band at ∼1500 cm⁻¹ assigned to the carbonyl group. The characteristic band at 850 − 900 cm⁻¹ of the P–CH group of the phosphorus ylide and its complexes are assigned to P–C vibration (Table 1). The ν (CO), which is sensitive to complexation, occurs around 1600 cm⁻¹ in the parent ylides [52,53]. Coordination of the ylide through carbon causes an increase in the ν (CO), while, when O-coordination occurs, a lowering in the value of the ν (CO) would be expected [54]. Thus the IR absorption bands for the complexes at higher frequencies indicate that C-coordination has occurred [33].

4.2 ³¹P NMR studies

In the ³¹P{¹H} NMR spectra of the Ph₂PCH₂PPh₂C(H)C(O)PhX (Y¹–Y⁴) the signal due to PCH (P_d) and PCH₂ (P_c) appears as a two doublet in ∼ 11 and -30 ppm, respectively (Fig. S3). Also, the ³¹P chemical shift of M¹ due to P_a and P_b appears as singlet in 66 ppm (Fig. S4). The ³¹P NMR spectrum shows an interesting pattern of signals which allows assigning definitely the structure of these complexes (C¹-C⁴). The four P atoms in these complexes are chemically and magnetically non-equivalent (Scheme 2); therefore, the ³¹P {¹H} NMR spectrum exhibits four batch-peak (Fig. 1). The upfield peak (δP ≈ 55–60, m), (Δδ ≈ −10 ppm) is assigned to the P atom (P_a) of the bidentate ligand (dppe), which is directly attached to the Pd centre in the front of CH group and thus shielded. The ³JP_bP_a coupling constant of ∼20 Hz lies within the typical values of similar systems [55–57]. Following, the ²JP_cP_a and ³JP_dP_a coupling constant are ∼60 and ∼13 Hz, respectively [34]. The upfield peak (δP ≈ 55–59, ddd), (Δδ ≈ −8 ppm) is assigned to the P atom (P_b) of the dppe, which is directly attached to the Pd centre in the front of PCH₂ group (P_c) and thus shielded (Table 1). The extremely strong coupling with the P atom (P_b) of the dppe ligand (²JP_bP_c ≈ 355 Hz) can only be rationalized in the context of a trans arrangement between P_b and P_c [56–64]. The ³JP_bP_d coupling constant is ∼12 Hz. Two down-field peaks are assigned to two P atoms of the chelate phosphorus ylide. The ylidic P atom (P_d) of the phosphorus ylide is not directly coordinated to the metal centre, thus coupling with Pc is weak (²JP_dP_c ≈ 24 Hz) as expected [34]. Coupling with P_b (³JP_dP_b ≈ 12) and Pa (³JP_dP_a ≈ 13 Hz) gives rise to a downfield ddd-peak (δP ≈ 37–41, ddd), (Δδ ≈ +30 ppm). The chemical shifts for the two non-equivalent P-atoms of the ambidentate P,CH-ligand are assigned on the basis of published NMR data for these complexes [25]. Finally, the downfield peak (δP ≈ 19–24, ddd), (Δδ ≈ +50 ppm) is assigned to PCH (P_c).

The significant downfield shift of the signal from that of the free ylide is in agreement with the C-bonding of the ylides. The coordination via the phosphine moiety is also clearly evidenced from the strong downfield shifts of the signal due to PPh₂ group when compared to that of the same signal in the free ylide [33,58].

4.3 ¹H NMR studies

The d⁸ distorted square planar palladium(II) complexes are all diamagnetic and give resolved NMR spectra which confirm the nature of the binding [65]. Regarding the ¹H NMR spectrum of these complexes, the phenyl protons of the dppe ligand and phosphorus ylide are observed in the aromatic region of the spectrum, between 6.5–8.0 ppm [25]. According to the published data, the ¹HNMR spectrum of the palladium(II) complexes was consistent with coordination of the phosphorus ylide to the metal through the CH and P_c atoms. In the ¹H NMR spectrum, the signal due to the methene proton for complex C¹ is broad [25]. Similar behavior was observed earlier in the case of ylide complexes derived from PtCl₂ [66], whereas the complexes C²-C⁴ showed a multiple peak in ∼4.9 ppm due to the methene proton. The expected lower shielding of the ¹H nuclei for the PC(H) group upon complexation in the case of C-coordination was observed in their corresponding spectra [67]. All complexes showed a multiple peak in near 2–4 ppm due to CH₂ groups of the dppe ligand (e.g. complex C⁴, Fig. S5). The CH₂ protons in all complexes of the phosphorus ylide appear as a multiplet (m) δ ≈ 4.5 ppm (Fig. S5). A singlet appeared at 3.67 ppm corresponding to methyl group of protons in complex C⁴.

4.4 ESI mass spectrometry

The positive ion electrospray-mass spectra (ESI⁺-MS) of the complexes C¹–C⁴ were recorded as very dilute THF solution. The molecular ion was observed in the ESI mass spectra of all complexes. The phosphorus ylide Y¹ has a molecular ion peak at m/z 583. The mass spectrum of complex C¹ was characterized by the appearance of the fragment at m/z 1085.1 owing to the removal of two OTf group and [(dppe)Pd-Y¹]²⁺ was formed. A peak at m/z 1235.06 related to the molecular ion corresponds to elimination of one OTf ions to give [(dppe)Pd-Y¹]⁺OTf. Similar fragmentation patterns are observed for complexes C¹–C⁴ and closely matched to what is expected and clearly confirmed the formation of these compounds. Modeling pattern of the complex C¹ and C⁴ are presented in Fig. 2 and Fig. S6, respectively.

4.5 Electronic absorption

All palladium complexes have been found to be diamagnetic indicating a +2 oxidation state for palladium. The electronic spectra of the complexes have been recorded in solid state and displayed three bands in the region around 200–500 nm. The electronic spectra of the phosphorus ylides as a ligand showed spectral bands due to π→π* and n→π* transitions (Fig. S7). On complexation these bands are shifted (Fig. S7). The strong charge transfer transitions may interfere and prevent the observation of all the expected bands [68–70]. The strong bands ∼400 nm is assignable to a combination of metal ligand charge transfer (MLCT) and d-d [68,71]. The broad band at 330–370 nm (28,700 cm⁻¹) is assignable to combination of phosphor/metal charge transfer (L_πMCT) and d-d bands. The broad band that appeared around 470 nm (21,250 cm⁻¹) has been reported as due to both distorted square planar and undistorted square planar Pd(II) complexes [72,73]. The bands that appeared around 250, 350 and 450 nm have been assigned to intra-ligand transition, [74,75] LMCT [74–76] and MLCT [77,78], respectively. The two bands between 230 and 260 nm (39,000–42,200 cm⁻¹) in the Pd(II) complexes are assigned to ligand centered transitions (π-π* and n-π*).

The band near 31,740 cm⁻¹ in complex C¹ is spin-allowed transition; a_1g(d_x²_−y²) − b_1g(d_x²_−y²) i.e. ¹A_1g → ¹B_1g. The broad bands between 340 and 360 nm (27,700–29,400 cm⁻¹) in the palladium complexes are assignable to a combination of MLCT and e_g(d_yz,d_xz) − b_1g(d_x²_−y²), i.e. ¹A_1g → ¹E_g bands. Therefore, the electronic spectra of Pd (II) complexes are indicative of distorted square planar geometry [79–81].

4.6 Electrochemical studies

Electrochemical studies of M², phosphorus ylides as ligand (Y¹-Y⁴) and Pd (II) complexes (C¹-C⁴) have been carried out by cyclic voltammetry (CV) in dichloromethane solution, containing 0.1 M tetrabutylammonium tetrafluoroborate (TBATFB) as supporting electrolyte using Au working electrode and Pt wire counter electrode. All the potentials were referenced to Ag/AgCl reference electrode.

Cyclic voltammograms of 1 mM of M², Y³ and C³ in dichloromethane, containing 0.1 M TBATFB at a scan rate of 100 mV/s is shown in Fig. 3. For the complex M² the cathodic potential peak corresponds to the redox pair of Pd(II)/Pd(I) appeared at −0.65 V vs. Ag/AgCl. That reduction was due to one electron transfer process. When the potential was scanned further in the cathodic direction, a second cathodic potential peak at −1.12 V was observed, which corresponds to the redox pair of Pd(I)/Pd(0). There were no anodic peaks observed on the reverse scan [82]. This behavior is similar to that reported previously in cyclic voltammetric study of various types of Pd(II) complexes in aprotic solvents [82–84]. This irreversibility may be due to the reaction of a Pd(0) complex with adventitious O₂ or other components of the solution. Another cathodic peak observed at −0.45 V is corresponding to the reduction of dppe ligand.

For Y³, the cyclic voltammogram displayed two irreversible and quasi reversible reduction peaks at −1.65 V and −1.10 V, respectively. The first peak could be assigned to a four-electron reduction of nitro group generating the hydroxylamine derivative according to:

The second cathodic peak assigned to the further reduction of the hydroxylamine to p-amino derivative [7,85].

In addition, the CV of the C³ exhibited an irreversible reduction peak at −1.60 V corresponding to the reduction of nitro group. The reduction process is shifted towards lower potential revealed an enhancement of the reduction process in the presence of palladium [7]. The disappearance of the second reduction peak of nitro group indicated the participation of the hydroxylamine group in the complex formation.

The quasi reversible reduction peaks at −0.82 V and −1.18 V can be assigned to the formation of Pd(I) from Pd(II) and formation of Pd(0) from Pd(I), respectively. The irreversible cathodic peak at −0.55 V is corresponding to the reduction of dppe ligand. The reduction process is shifted toward more negative potential in the presence of nitro group. The existence of the cathodic peaks of Pd, NO₂ and the dppe in the CV of the C³ complex, is an evidence for the synthesis of this complex. The cyclic voltammetric data of metal center for the other complexes are given in Table 2.

4.7 Thermal analyses

Thermal stability of the complexes was determined by TG/DTG studies from 25–1000 °C. The compounds were heated to 950–1000 °C, all relevant weight loss was completed by 800 °C. Discussing the simultaneous TG/DTG curves constructed for phosphorus ylide (Y¹), one can observe two endothermic mass loss stages (supplementary material, Fig. S8). The 1st decomposition step at 80–710 °C is accompanied by a mass loss of 80.72% (calcd. 86.72%) may be attributed to the liberation of ylide without bromine atom. The decomposition ended at 797 °C with bromine as a final residue (obs. = 17.49%, calc. = 13.74%). The TG/DTG curves of C¹ (Fig. S8) exhibit three decomposition steps maximized at 391, 615 and 797 °C. The first pronounced peak at 157–593 °C is accompanied by a mass loss amounting to 76.28% (calcd. 75.76%) may be attributed to the liberation of dppe + Y¹ + CF₃. The second decomposition stages at 593–707 °C involve the loss of O₂. The decomposition ended at 797 °C with Pd + SO + SO₃CF₃ as a final residue (obs. = 18.40%, calc. = 21.92%).

For ylide (Y³), the degradation starts at 25 °C with four decomposition stages centered at 116, 285, 366 and 797 °C (Fig. 4a). The 1st and 2rd decomposition stages are attributed to the degradation of nitro group. The 3rd degradation step is due to the releasing of dppm (obs. = 68.50%, calc. = 70.20%). Decomposition ended at 797 °C with acetophenone as a final residue (obs. = 20.83%, calc. = 21.94%). The TG/DTG curves of complex C³ (Fig. 4b) exhibited three decomposition steps maximized at 369, 461 and 797 °C. The 1st pronounced peak at 190–439 °C is accompanied by a mass loss amounting to 67.23% (calcd. 66.78%) may be attributed to the liberation of dppe and one ylide (Y³) without NO₂ group. The second thermal stage, centered at 439–652 °C, is due to the degradation of 3O₂ with overall mass loss amounting to 21.93% (calcd. 26.40%) leaving two HSOCF₃, one NO₂ group and Pd as a final residue.

4.8 X-ray crystallography

Table 3 summarizes crystal and refinement data for Y². The molecular structure is shown in Fig. 5. This compound crystallizes in a monoclinic space group P2₁/c. As shown, both the phosphine and ylidic components are devoid of steric restrictions and can interact with metal ions and reagents. In the molecule of the title ligand, Y², the geometry around the P1 atom is nearly tetrahedral and the O atom is oriented cis to the P atom. The C(27)–C(26)–P(2) [115.3 (3)°] bond angle indicates a distorted trigonal arrangement about C(26). The P(1)-O [2.9104 (16) Å] distance is significantly shorter than the sum of the van der Waals radii of P and O (3.3 A), [86] indicating a strong intermolecular interaction between P⁺ and O⁻charge centers, which leads to the cis orientation. The P(2)–C(26) [1.711 (5) Å], C(27)–O(1) [1.267 (5) Å] and C(26)–C(27) [1.396 (7) Å] bond lengths within the ylidic fragment are comparable to those observed for the other monoketo ylides [87,88]. Thus, no steric or electronic effects due to the presence of –CH₂PPh₂ are anticipated. Also, in this molecule the bond lengths and angles are generally within normal ranges [33]. In ligand Y² a combination of several weak and medium intermolecular interactions, determined the structural assembly in this compound (Fig. 6).

4.9 X-ray powder diffraction

Single crystals of the studied complexes could not be obtained, because the studied complexes are amorphous by nature. The X-ray powder diffraction patterns of the phosphorus ylide (Y³) and its complex as representative examples were recorded over 2θ = 5–90° in order to obtain an idea about the lattice dynamics of these compounds. A comparison of the obtained XRD patterns of Y³ and C³, throws light on the fact that each compound represents a definite compound with a distinct structure (Fig. S9). This identification of the compounds was done by the known method [89]. Such facts suggest that the prepared complexes are amorphous [90]. The X-ray powder diffraction pattern of Y³ showed a higher degree of crystallinity. The values of 2θ, interplanar spacing d (Å) and the intensities (%) of Y³ and its complex were tabulated in Table 4. Two peaks characterized to the Pd (II) moiety are also observed at 36.5° and 44.2° [91].

4.10 Theoretical studies

4.10.1 Geometry optimization

The optimized geometries of phosphorus ylide (Y²) and dppe ligand are depicted in Fig. S10 and the metal complex C² is presented in Fig. 7. Representative selected calculated bond parameters for bidentate ligands Y² and dppe along with the experimentally determined data are listed in Table 5. The calculated bond lengths and bond angles for both ligands are in good agreement with the corresponding experimental values. As can be seen in Fig. 7, the coordination sphere around the metallic center in complex C² is made up of CH_ylide, P_ylide and two P_dppe completing the distorted square planar geometry. The P(3)-Pd-P(4) and C(2)-Pd-P(5) bond angles are 168.3 and 174.6°, respectively, indicating that the four donor atoms are not coplanar (Table 6).

Table 5

Selected bond distances (Å) and angles (°) for phosphorus ylide Y² and dppe.

	Y2		dppe
	Exp.	BP86/SVP	BP86/TZVP		Exp. [92]	BP86/SVP	BP86/TZVP
Bond distances				Bond distances
P2-C11	1.853 (4)	1.907	1.896	P1-C2	1.828 (4)	1.893	1.881
P2-C17	1.81 (2)	1.860	1.847	P1-C7	1.820 (3)	1.862	1.850
P2-C29	1.827 (4)	1.874	1.860	P1-C8	1.818 (4)	1.854	1.842
P10-C11	1.821 (4)	1.852	1.842	C2-C3	1.516 (4)	1.530	1.528
P10-C20	1.799 (5)	1.847	1.832	C2-H29	0.88 (3)	1.112	1.102
P10-C31	1.818 (5)	1.840	1.829	C2-H30	0.96 (3)	1.112	1.102
P10-C1	1.711 (5)	1.742	1.728	C8-C19	1.370 (7)	1.417	1.409
C11-H43	0.989 (4)	1.110	1.101	C8-C23	1.362 (6)	1.415	1.407
C11-H44	0.991 (4)	1.110	1.100	C19-C20	1.375 (10)	1.404	1.396
C1-C3	1.396 (7)	1.418	1.413	C20-C21	1.328 (11)	1.407	1.399
C1-H38	0.89 (4)	1.098	1.088	C21-C22	1.346 (10)	1.405	1.397
C3-O36	1.267 (5)	1.270	1.266	C22-C23	1.382 (8)	1.406	1.399
C3-C4	1.505 (6)	1.513	1.509
C4-C9	1.385 (6)	1.412	1.404	Bond angles
C8-C9	1.385 (7)	1.401	1.394	C2-P1-C8	102.6 (3)	103.479	103.691
C7-C8	1.369 (8)	1.407	1.397	C2-P1-C7	100.1 (3)	101.545	101.864
C6-C7	1.378 (7)	1.407	1.397	C7-P1-C8	101.3 (3)	103.283	103.518
C5-C6	1.379 (8)	1.403	1.396	P1-C7-C28	119.0 (5)	125.123	125.217
C4-C5	1.394 (8)	1.413	1.405	P1-C7-C24	123.2 (5)	116.351	116.358
C7-Cl37	1.749 (5)	1.755	1.751	P1-C8-C23	116.7 (6)	121.940	121.877
				P1-C8-C19	125.6 (6)	119.423	119.522
Bond angles				P1-C2-C3	111.1 (4)	118.898	118.829
C17-P2-C29	102.6 (8)	101.466	101.758
C11-P2-C29	98.8 (2)	97.683	97.896
P2-C11-P10	107.8 (8)	116.659	117.127
C11-P10-C31	107.2 (2)	106.537	106.503
C11-P10-C20	106.7 (2)	105.185	105.532
C1-P10-C11	113.3 (2)	116.705	116.610
C1-P10-C31	113.3 (2)	104.958	105.045
P10-C1-H38	113 (3)	117.095	116.835
C3-C1-P10	115.3 (3)	116.926	117.816
C1-C3-O36	121.0 (4)	121.327	121.402
C4-C3-O36	117.9 (4)	118.453	118.528
C4-C9-H42	119.2 (5)	117.402	117.777
C4-C5-H39	120.0 (5)	120.260	120.230
C8-C7-Cl37	120.0 (4)	119.650	119.575
C6-C7-Cl37	118.7 (4)	119.525	119.442

Table 6

Selected bond distances (Å) and angles (°) for complex C².

Bond distances	BP86/SVP	BP86/TZVP	Bond angles	BP86/SVP	BP86/TZVP
Pd10-C2	2.178	2.197	C2-Pd1-P4	89.297	88.475
Pd1-P3	2.422	2.422	C2-Pd1-P3	90.334	91.296
Pd1-P4	2.373	2.363	C2-Pd1-P5	174.249	174.600
Pd1-P5	2.401	2.379	P3-Pd1-P5	83.960	83.611
P3-C17	1.885	1.870	P3-Pd1-P4	170.315	168.322
P3-C18	1.838	1.828	P4-Pd1-P5	96.449	96.876
P3-C47	1.841	1.830	Pd1-C2-C6	117.579	117.451
P5-C16	1.869	1.856	Pd1-C2-P14	117.536	117.694
P5-C58	1.839	1.827	Pd1-C2-H67	101.196	100.224
P5-C63	1.840	1.829	Pd1-P4-C15	106.967	107.406
P4-C15	1.884	1.871	Pd1-P4-C29	117.606	116.306
P4-C29	1.846	1.835	Pd1-P4-C41	119.085	119.959
P4-C41	1.839	1.828	Pd1-P3-C17	106.802	106.681
C2-C6	1.536	1.532	Pd1-P3-C18	113.699	112.977
C2-P14	1.840	1.822	Pd1-P3-C47	120.802	121.965
C2-H67	1.111	1.103	Pd1-P5-C16	105.434	105.449
P14-C15	1.838	1.828	Pd1-P5-C58	114.958	116.228
P14-C32	1.827	1.816	Pd1-P5-C63	121.344	120.729
P14-C49	1.822	1.808	P3-C17-C16	112.174	111.668
C2-P14	1.840	1.822	P5-C16-C17	109.265	108.765
C15-H72	1.109	1.099	C2-P14-C15	106.897	106.368
C15-H73	1.109	1.100	P4-C15-P14	112.278	112.623
C6-O66	1.237	1.234	C2-C6-O66	117.898	117.760
C6-C7	1.490	1.486	C2-C6-C7	121.260	121.516
C16-C17	1.526	1.524	C7-C6-O66	120.841	120.723
C17-H76	1.110	1.100	P14-C15-H72	111.966	112.210
C17-H77	1.109	1.099	P14-C15-H73	106.056	105.762
C16-H74	1.109	1.099	P3-C17-H76	105.834	106.259
C16-H75	1.111	1.101	P3-C17-H77	108.388	108.684
C10-Cl13	1.737	1.732

4.10.2 ¹H NMR calculations

In order to correctly describe the experimental data, quantum chemical calculations of shielding constants were performed for the optimized structures of Y², C² and dppe. The absolute isotropic magnetic shielding constants (σ_i) were used to obtain the chemical shifts (Eq. (1)) by referring to the standard compound tetramethylsilane (TMS) for H atoms. The referred compound TMS were calculated at the same levels of theory.

δi=σTMS−σi

(1)

The shielding constants are presented as chemical shifts and are compared with the corresponding experimental data (Table 7). The computed ¹H chemical shifts of all remarkable protons in these compounds are in good agreement with the corresponding experimental values. As can be seen in Fig. 8, there is a good correlation between the experimental and calculated chemical shifts using both the TZVP and SVP basis sets. Thus it seems that the optimized gas phase structures are close to the structures of these compounds in solution.

Table 7

The experimental and theoretical ¹H NMR chemical shifts (δ, ppm) from TMS for the studied compounds.

Compound	Exp	Calcda	Atom	Assignment
		BP86/SVP	BP86/TZVP
Dppe	2.1	1.7	1.7	H29	CH2
	2.1	1.7	1.7	H31
	2.1	1.8	1.8	H30
	2.1	1.8	1.8	H32
Y2	3.6	3.0	3.1	H43	CH2
	3.6	3.0	3.1	H44
	4.2	3.8	4.2	H38	CH
	6.9	6.6	6.8	H40	Phenyl-Cl
	6.9	6.8	7.1	H41
	7.1	6.9	7.1	H39
	7.1	6.9	8.1	H42
C2	2.66	1.9	1.5	H76	CH2
	2.85	2.7	2.3	H75
	2.66	1.9	1.5	H77
	2.85	2.7	2.3	H74
	4.6	3.9	3.7	H73	CH2
	4.6	3.9	3.7	H72
	4.8	5.0	4.9	H67	CH
	6.96	6.3	6.2	H69	Phenyl-Cl
	6.98	6.6	6.4	H70
	6.99	6.8	6.6	H71
	7.01	7.2	6.9	H68

^a Using GIAO method.

4.11 Biological activity

4.11.2 Antibacterial activity

All complexes showed antibacterial activities against bacteria tested especially on Gram positive ones. In contrast, Serratia marcescens (-) was the most resistant bacterium (Table 11). When antimicrobial activities of the tested samples were compared with those of reference antibiotics, the inhibitory potency of the tested complexes was found to be remarkable (Fig. 9). Although all bacteria tested are resistant to Penicillin, the antibacterial effects of all complexes were higher than those of Penicillin on these bacteria. As it is well known, S. aureus and Bacillus species, especially B. cereus, are agents of food poisoning [99].

Table 11

Antibacterial activities of the studied complexes at different concentrations mg/ml (μM/l).

Microorganism	Inhibition zone diameter (mm)
C1	C2	C3	C4	Solution	Antibiotic
1 (742)	0.1 (74.2)	0.01 (7.42)	1 (746)	0.1 (74.6)	0.01 (7.46)	1 (740)	0.1 (74.0)	0.01 (7.40)	1 (749)	0.1 (74.9)	0.01 (7.49)	Hexane	Gentamycin
P. vulgaris (−)	13a	11b	9c	15a	11b	8c	16a	14b	10c	14a	12b	8c	7	30
B. cereus (+)	11a	na	na	10a	8b	na	8a	na	na	9a	8b	na	8	25
S. aureus (+)	18a	11b	na	16a	12b	10c	20a	15b	11c	15a	11b	10c	9	35
S. marcescens (−)	na	na	na	na	na	na	na	na	na	na	na	na	na	27

However, today it is known that the synergistic or antagonistic effect of one compound in minor percentage of mixture has to be considered. These complexes showed more activity against some bacteria under indicial experimental conditions. This would suggest that the structure of complexes may reduce the polarity of the metal ion mainly. Also, we can consider that the coordination may facilitate the ability of a complex to cross the lipid layer of the bacterial cell membrane and in this way may be affect the mechanisms of growth and development of microorganisms [100,101]. As can be seen in Table 11, the presence of Br, Cl, NO2 and OCH3 groups exerts a number of changes on antibacterial activities of the tested complexes. So, in case of C³, the presence of NO₂ moiety induced an increase of their action against most of bacteria, while complex C⁴ was found to have low activity against bacteria. The above results indicate that the complexes studied may be used in treatment of diseases caused by bacteria tested. However, further studies are needed to evaluate the in vivo potential of these compounds in animal models.