Synthesis and X-ray Diffraction of Cyclopalladated Compounds Derived from Imine Ligands ^†

Abstract

The crystal structures of mononuclear cyclopalladated compounds with phosphine ligands are investigated. The reactions of the five-membered cyclopalladated dinuclear complexes [Pd(L)(µ-Cl)]₂ with the monophosphine ligand (PPh₃) and diphosphine ligand (dppm) in the molar ratio of 1:2, and ammonium hexafluoride in the case of compound b, result in the mononuclear complexes [Pd{2,3,4-(CHO)C₆H₃C(H)=NCy}{PPh₃}[Cl] (1a) and [Pd{2,3,4-(CHO)C₆H₃C(H)=NCy}{Ph₂PCH₂PPh₂-P,P}][PF6] (1b).

1. Introduction

The possible application of palladium compounds in medicine has become a particularly active and attractive study issue in bioinorganic and biological chemistry [1]. The use of chelating ligands in the development of physiologically active palladium compounds with improved kinetic stability is a well-established design principle [2]. Since the existence of a strong Pd–C bond in the [C, N] palladacycle enhances the stability of the organometallic complex, orthometallated N-donor ligands, such as imines, have been successfully employed for this purpose [3]. The nitrogen-donor ligands, palladacycles, are gaining popularity due to their numerous applications in organic synthesis, antitumoral drug synthesis, asymmetric synthesis, intermolecular aromatic C–H bond activation, synthesis and reactivity of organometallic complexes with biologically important ligands, and drug delivery [3]. Therefore, we report herein the synthesis and characterization of cyclopalladated compounds of the general formula [Pd{2,3,4-(MeO)₃C₆HC(H)=N-R}{R = Cy, 2,4,6-MeC₆H₂}(X = Cl, Br)] with PPh₃ and dppm ligands.

2. Result and Discussion

The treatment of the halogen-bridged ligand compound a with the PPh₃ in the molar ratio of 1:2 produced a monomer palladium(II) compound with PPh₃ ligand, and the treatment of compound b with the diphosphine dppm and NH₄PF₆ in a 1:2 molar ratio gave a monomer palladium(II) compound with phosphine chelated ligand (Scheme 1). The compounds were characterized by using ³¹P{¹H} and ¹H NMR spectroscopy. In the ¹H NMR, the proton H(5) for compounds 1a and 1b appears as a doublet by coupling to ³¹P. A doublet resonance of HC=N proton is coupled to ³¹P nucleus trans to nitrogen for compound 1a at 8.26 ppm [⁴J(PHi) = 9.1 Hz] and for compound 1b at 8.20 ppm [⁴J(PHi) = 7.6 Hz]. The OMe(C4) NMR resonance for compounds 1a and 1b is shifted to a lower frequency due to the shielding effect of the phosphine’s phenyl ring. The two inequivalent OMe(C4) groups have two different resonances in an antiparallel configuration, as one of them is not impacted by the phosphine’s phenyl ring. In the ³¹P-{¹H} NMR, a singlet ascribed to the coupling of compound 1a to the ³¹P nucleus is shifted to a lower field ca. 43 ppm, which is consistent with a phosphorus trans to nitrogen arrangement. In contrast, for compound 1b, the two inequivalent phosphorus nuclei are represented by two doublets at −4.33 [d, J = 62.9 Hz] and −27.53 [d, J = 62.9 Hz]. The phosphorus nucleus trans to the phenyl carbon C(6) has the lower-frequency doublet, while the phosphorus nucleus trans to the imine nitrogen has the higher-frequency doublet. This is predicated on the notion that a ligand with a higher trans influence shifts the phosphorus nucleus trans ³¹P resonance to a lower frequency.

3. X-ray Diffraction

The mononuclear molecules (one molecule per asymmetric unit) are present in 1a (Figure 1) and 1b, and a hexafluorophosphate anion is present in the case of the crystal structure 1b (Figure 2). The coordination sphere enclosing the palladium atom in the crystal structures 1a and 1b is formed by a nitrogen atom from the imine group, an ortho carbon atom from the phenyl ring (C1), a phosphor atom from a PPh₃, a chlorine atom in the case of the crystal structure of 1a, and two phosphorus atoms from a chelating dppm in the case of the crystal structure of 1b. The Pd1–C1, which is 2.027(5) Å for 1a and 2.036(3) Å for 1b, is in agreement with the partial multiple-bond character of the Pd–C bond [4]. The Pd(1)-N(1) bond length, which is 2.112(5) Å for 1a and 2.096(2) Å for 1b, is longer than the single bond predicted value of 2.011, which has an impact on the phosphine ligand’s trans effect [5]. It can be noticed in that there is an intermolecular interaction for compound 1b, resulting in a C_sp3···H···C weak interaction. The bond and angel interaction C38···H10···C10 are 2.838 Å and 113.36^◦, respectively, and the C38···C10 bond interaction is 3.331 Å (Figure 3). Table S1 lists specifics regarding the structure’s refinement and the final reliability factors.

4. Experimental Part

Compounds a and b were prepared in the same manner [6].

4.1. Synthesis of [Pd{2,3,4-(MeO)₃C₆HC(H)=N-C₆H₂}{PPh₃}]. (1a)

A total of (25 mg, 0.029 mmol) of compound a was added to acetone (10 cm³). The required quantity of triphenylphosphine was added (in a 1:2 molar ratio) and the mixture was agitated for 3 h at room temperature. The solution was reduced to a low volume, and the solid was recrystallized from dichloromethane/n-Hexane and dried in vacuo. The yield was 50%. IR = ν(C=N) 1569 cm⁻¹, ν(Pd–Cl) 298 cm⁻¹. NMR ¹H (400 MHz, CDCl₃)) δ 8.26 (d, ⁴J(PHi) = 9.1 Hz, 1H, Hi), 7.67 (t, ³J(HH) = 7.6 Hz, 6H, PPh₃), 7.35 (t, ³J(HH) = 7.6 Hz, 3H, PPh₃), 7.29 (d, ³J(HH) = 7.6 Hz, 6H, PPh₃), 5.65 (d, ⁴J(H5P) = 6.4 Hz, 1H, H5), 4.33 (m, ³J(HH) = 11.1 Hz, 1H, N-CH-Cy), 3.86 (s, 3H, OMe), 3.61 (s, 3H, OMe), 2.72 (s, 3H, OMe), 2.17–0.79 (m, 10H, Cy) (Figure S1). ³¹P NMR (δ ppm, CDCl₃) δ 42.86.

4.2. Synthesis of [Pd{2,3,4-(MeO)₃C₆HC(H)=N-2,4,6-Me₃C₆H₂}{Ph₂PCH₂PPh₂-P,P}](PF₆). (1b)

A total of (25 mg, 0.025 mmol) of compound b was added to acetone (10 cm³). The appropriate amounts of dppm and NH₄PF₆ were added in a molar ratio of (1:2), and the mixture was stirred for 3 h at room temperature. The orange precipitate formed was filtered off, recrystallized from dichloromethane/n-Hexane, and dried in vacuo. The yield was 85%. IR = ν(C=N) 1565 cm⁻¹. NMR ¹H (400 MHz, CDCl₃) δ 8.20 (d, ⁴J(PHi) = 7.6 Hz, 1H, Hi), 8.06–6.99 (m, 20H, PPh₂), 6.68 (s, 2H, Ha, Ha’), 6.03 (dd, ⁴J(H5P_trans) = 10.4 Hz, ⁴J (H5P_cis) = 7.6 Hz, 1H, H5), 4.27 (dd, ²J(HP) = 12.0, 8.0 Hz, 2H, PCH₂P), 3.99 (s, 3H, OMe), 3.79 (s, 3H, OMe), 3.17 (s, 3H, OMe), 2.25 (s, 3H, Me), 2.18 (s, 6H, Me^*) (Figure S2). ³¹P-{¹H} NMR (CDCl₃, δ ppm) −6.0 [d, J = 66.5], −30 [d, J = 66.5], −141 [h, PF₆⁻].