Synthesis and study of the stability of phosphane palladacycles

: The chemistry of phosphane palladacycles[1] has been amply developed in last years. This kind of chemistry results interesting due to their applications in numerous fields such as organic synthesis[2], catalysis[3] or as potential biologically active materials[4]. Palladacycles bearing the diphosphine dppm have been described before[5] and we now report a new behavior shown by palladacycles of the type [Pd 2 (Ph 2 PCH 2 PPh 2 −P,P) 2 (C,N:C,N)]. In this case the coordination of dppm spontaneously shifts from chelate to bridging mode in solution giving a core A-frame conformation with two palladium atoms with a chlorido or bromido ligand between them.


Introduction
Palladium cyclometallated compounds and their derivatives have been widely studied in past years.There are many studies related to cyclometallated compounds derived from a wide variety of ligands and metals.In this work, a Schiff base was used as ligand for metallation; specifically the tetradentate [(C,N:C,N)] ligand derived from the condensation between 2,3,4-trimethoxybenzaldehyde and 4,4'-oxydianiline were chosen.
Diphosphines such as Ph2PCH2PPh2 (dppm) present different coordination modes towads metal centers.It usually acts as chelate due to the thermodynamic stability related to coordination rings but nevertheless it may also appear bridging two metal centers.This behavior depends on the relative stability between both conformations.
In this work we report a gradual shift from a chelating to a bridging mode of this diphosphine when the metal center is palladium.

Scheme 1. Synthesis reaction of the ligand
In a round bottom flask a 1:2 mixture of (p-NH2C6H4)2O and 2,3,4−(MeO)3C6H2CHO in ethanol (50 cm 3 ) was stirred at room temperature during 24 h.A white precipitate was formed, which was filtered off, washed with water and dried under vacuum.

Scheme 2. Synthesis reaction of the cyclometallated compound
In a round bottom flask the ligand (0.500 g) was dissolved in toluene (30 cm 3 ).Then, palladium(II) acetate (2 eq.) was added and the resulting mixture was stirred at 55° C for 24 h.After cooling to r. t. the red oil formed was separated, to afford the final product as a red solid.

Scheme 3. Synthesis reaction of the halide derivatives
In a round bottom flask a solution of the cyclometallated precursor (0.450 g) in 25 cm 3 of dichloromethane was treated with 35 cm 3 of an aqueous solution of NaX (0.05M) and the mixture was stirred at room temperature.After eight hours the organic layer was isolated and the solvent was removed under reduced pressure.Later, the residue was recrystallized ofrom a mixture of dichloromethane-hexane to give a yellow solid.

Synthesis of the phosphane palladacycles
Scheme 4. Synthesis reaction of the final products Ph2PCH2PPh2 (0.020 mmol) and ammonium hexafluorophosphate (0.024 mmol) were added to a solution of the cyclometallated compound (8.5 mg, 0.005 mmol) in acetone-d6 (0.6 cm 3 ) and left to stand until complete conversion.

Scheme 5. Solution shift
The resulting product shifts from chelate to a bridging coordination.The core consists of two Palladium atoms, two bridging dppm units and one bridging chlorido ligand.

Results
All the compounds were characterized by CHNS elemental analysis, proton and phosphorous nuclear magnetic resonance (NMR) in solution and single-crystal X-ray diffraction for the final products.

Elemental analysis
Elemental analyses were performed with a Fisons elemental analyzer, Model 1108; the results obtained are shown in Table 1 The results are consistent with the proposed structures.

Nuclear magnetic resonance
The NMR is a very useful technique for organic and inorganic chemistry.For this analysis it was used a Varian Inova 400 MHz spectrometer.

L = dppm
The time-dependent shift can be easely followed by close inspection of the 1 H and 31 P NMR spectra.

Figure 7 .
Figure 7. a) Thermal ellipsoid plot with chlorido for the asymmetric unit.Solvent molecules, hydrogen atoms and phosphane phenyls have been omitted for clarity.b) Thermal ellipsoid plot with bromido for the asymmetric unit.Solvent molecules, hydrogen atoms and phosphane phenyls have been omitted for clarity.

Table 1 .
. Results of the CHNS elemental analysis

Table 2 .
Crystal data and structure refinement