Palladium complexes are of interest because of their effective application in catalysis [1–5] and also in organic and organoelement synthesis [6–10]. Structural features of palladium coordination compounds are being intensively studied. As a rule, palladium-containing anions in numerous structurally characterized ionic complexes have the composition [PdHlg4]2– and, more rarely, [Pd2Hlg6]2– [11]. The effect of solvents on the design of anions in these complexes was considered [12]. The synthesis of complexes with the S-coordinated dimethyl sulfoxide molecule in mononuclear [PdCl3(DMSO)]– anions was reported [13–23]. We synthesized new palladium complexes and studied the solvent effect on the design of bromopalladate(II) anions.

structure 1

To obtain ionic phosphonium complexes of palladium, an aqueous solution of tetraorganylphosphonium bromide was added to a solution of palladium(II) bromide in hydrobromic acid. The formation of a brown precipitate was observed, which, after filtration and drying, was recrystallized from acetonitrile. After removal of the solvent, tetrabromopalladates of cyanomethyltriphenylphosphonium 1 and tetraphenylphosphonium 2 were obtained as brown crystals (1). Under similar conditions, in reaction (2) with the participation of cyclopentyltriphenylphosphonium bromide, the formation of a brown precipitate was also observed, during recrystallization of which from diethyl sulfoxide, bromine in the coordination sphere of the palladium atom was replaced by diethyl sulfoxide to form complex 3. Keeping a mixture of equimolar amounts of compound 2 and diamyl sulfoxide in acetonitrile did not lead to the replacement of the bromine atom in the coordination sphere of the metal by the n-donor ligand, whereas dimerization (3) of the anion occurred with the formation of the [Ph4P]2[Pd2Br6] complex (4).

Compounds 1–4 are brown crystalline substances resistant to moisture and atmospheric oxygen, readily soluble in acetonitrile, chloroform, and tetrahydrofuran, and insoluble in aliphatic hydrocarbons. In the IR spectra of complexes 1–4, intense absorption bands are observed in the regions 1439–1475 and 997–995 cm–1, which characterize stretching vibrations of the P–СPh bond. The cyano group in complex 1 manifests itself as a low-intensity narrow absorption band at 2255 cm–1 [24].

According to XRD data, the crystals of complexes 1–3 consist of tetrahedral organyltriphenylphosphonium cations and mononuclear square anions [PdBr4]2– or [PdBr3(Et2SO)]– (Table 1, Figs. 1–3). The crystal of complex 2 is formed by two types of crystallographically independent cations and anions.

Table 1. Crystallographic data, experimental parameters, and refinement of structures of complexes 1‒4
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Fig. 1.
figure 1

General view of the molecule of complex 1 in the crystal.

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Fig. 2.
figure 2

General view of the molecule of complex 2 in the crystal.

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Fig. 3.
figure 3

General view of the molecule of complex 3 in the crystal.

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Phosphorus atoms in the cations have slightly distorted tetrahedral coordination with close P–C bond lengths [1.780(3)–1.816(3), 1.791(5)–1.809(5), and 1.776(13)–1.812(8) Å, respectively] and with valence CPC angles of 106.69(15)–111.29(14) (1), 106.6(2)–113.5(2) (2), and 108.0(4)–111.1(4)° (3), which practically do not differ from the theoretical value. The P–CAlk bonds are longer than the P–CPh bonds (Table 2).

Table 2. Main bond lengths and bond angles in structures 1–4
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In the centrosymmetric square [PdBr4]2– anions, the Pd–Br bond lengths [2.4363(16), 2.4438(14) Å (1) and 2.4403(18)–2.466(3) Å (2)] are comparable with similar distances in the square asymmetric anion 3 [2.4337(19)–2.4517(19) Å]; the S–Pd distance [2.269(3) Å] is somewhat smaller than the sum of the covalent radii of palladium and sulfur atoms (2.34 Å [25]). In complex 3, the cis-angles BrPdBr [89.49(7) and 89.64(8)°] differ slightly from the theoretical value (90°); the trans-angles BrPdBr and SPdBr are 176.47(5) and 175.65(8)°, respectively. In the anions of complexes 1 and 2 the palladium atom is in the plane formed by four Br atoms, whereas in anion 3 the palladium atom deviates from the Br3S plane by 0.008 Å.

The crystal of complex 4 (Fig. 4) consists of tetrahedral tetraphenylphosphonium cations [CPC angles 108.50(15)°–110.61(15)°, P–C bonds 1.803(3)– 1.806(3) Å] and centrosymmetric binuclear anions [Pd2Br6]2–, in which the distances between the palladium atom and the terminal bromine atoms [2.4064(13) and 2.4141(11) Å] are much shorter than the bond lengths involving bridging bromine atoms [2.4541(14) and 2.4740(12) Å].

Fig. 4.
figure 4

General view of the molecule of complex 4 in the crystal.

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Packing of ions in the crystals of complexes 1–4 is caused by weak interionic contacts involving hydrogen atoms and Br, N, and O heteroatoms. Three types of contacts are observed in complex 1: C≡N∙∙∙H–C 2.58 Å, Pd–Br∙∙∙ H–Cmethylene 2.61, 3.01 Å, and Pd–Br∙∙∙H–CPh 3.00 Å. In crystals of complexes 2 and 4, tetraphenylphosphonium cations are bound to anions by Pd–Br∙∙∙H–C hydrogen bonds [2.93–3.02 (2) and 2.97 Å (4)]. The contacts in complex 3 are more diverse. Cations and anions are bound by hydrogen bonds (Pd–Br∙∙∙H–Ccyclopent 2.90, Pd–Br∙∙∙H–CPh 2.94, and S=O∙∙∙H–CPh 2.61, 2.65 Å ). The shortest contact (S=O∙∙∙H–Csulfoxide 2.43 Å) is less by 0.19 Å than the sum of the van der Waals radii of oxygen and hydrogen atoms (2.62 Å [26]) and is observed between diethyl sulfoxide anion ligands. The packing of ions and the system of interionic contacts in the crystal of complex 3 are shown in Fig. 5.

Fig. 5.
figure 5

System of interionic contacts in complex 3 (projection along the b crystallographic axis).

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Thus, palladium ionic complexes with mononuclear anions [Ph3PCH2CN][PdBr4], [Ph4P][PdBr4], and [Ph3PC5H9-cyclo][PdBr3(Et2SO)] were obtained from organyltriphenylphosphonium bromides and palladium dibromide in the presence of hydrobromic acid. Holding a mixture of equimolar amounts of [Ph4P][PdBr4] and diamyl sulfoxide in acetonitrile leads to the formation of the [Ph4P]2[Pd2Br6] complex. According to X-ray diffraction data, phosphonium complexes of palladium [Ph3PCH2CN][PdBr4], [Ph4P][PdBr4], and [Ph3PC5H9-cyclo][PdBr3(Et2SO)] contain square mononuclear [PdBr4]2– and [PdBr3(Et2SO)]– anions, and the [Ph4P]2[Pd2Br6] complex – planar binuclear [Pd2Br6]2 anions.

EXPERIMENTAL

The IR spectra of samples in KBr pellets were recorded on a Shimadzu IRAffinity-1S IR Fourier spectrometer in the range 4000–400 cm–1; elemental analysis was performed on a Euro EA3028-NT analyzer. X-ray diffraction analysis of crystals of complexes 1–4 was performed on a Bruker D8 QUEST automatic four-circle diffractometer (graphite monochromator) at 293 K. Data collection and primary processing, refinement of the unit cell parameters, absorption accounting, determination and refinement of structures were carried out according to programs [27–29]. The structures were determined by the direct method and refined by the least squares method in the anisotropic approximation for non-hydrogen atoms. Complete tables of atomic coordinates, bond lengths, and bond angles have been deposited at the Cambridge Crystallographic Data Center [CCDC 2115219 (1), 1898988 (2), 1908755 (3), 1898990 (4)].

Complex [Ph3PCH2CN]2[PdBr4] (1). Palladium dibromide (0.15 g, 0.56 mmol) was dissolved in 2 mL of 48% hydrobromic acid, and a solution of 0.43 g (1.12 mmol) of triphenylcyanomethylphosphonium bromide in 20 mL of hot water was added with stirring. The brown precipitate was filtered off, dried, and dissolved in 20 mL of acetonitrile. After evaporation of the solvent, 0.51 g (88%) of brown crystals were obtained, t. decomp. 215°C. IR spectrum, ν, cm–1: 3057, 3030, 2922, 2830, 2731, 2255, 1584, 1483, 1437, 1375, 1337, 1231, 1196, 1167, 1113, 995, 822, 743, 725, 5085, 509, 492, and 442. Found, %: С 46.54; H 3.38. C40H34Br4N2P2Pd. Calculated, %: С 46.61; H 3.33.

Complex [Ph4P]2[PdBr4] (2) was obtained similarly. Yield 93%, brown crystals, t. decomp. 248°C. IR spectrum, ν, cm–1: 3075, 3051, 3021, 3005, 2990, 1584, 1481, 1437, 1337, 1315, 1186, 1161, 1107, 1026, 995, 752, 723, 689, 615, 5627, and 432. Found, %: С 52.09; H 3.61. C48H40Br4P2Pd. Calculated, %: С 52.18; H 3.65.

Complex [Ph3PC5H9-cyclo][PdBr3(Et2SO)] (3) was obtained by a similar procedure followed by recrystallization of the precipitate from diethyl sulfoxide. Yield 72%, brown crystals, t. decomp. 149°C. IR spectrum, ν, cm–1: 3075, 3051, 3034, 2968, 2932, 2920, 2909, 2868, 1585, 1483, 1439, 1406, 1325, 1277, 1252, 1188, 1136, 1111, 995, 789, 762, 745, 725, 692, 525, and 449. Found, %: С 41.48; H 4.22. C27H32Br3OPPdS. Calculated, %: С 41.49; H 4.13.

Complex [Ph4P]2[Pd2Br6] (4) was obtained by a similar procedure followed by recrystallization of the precipitate from a diamyl sulfoxide–acetonitrile mixture. Yield 63%, brown crystals, t. decomp. 252°C. IR spectrum, ν, cm–1: 3075, 3051, 3005, 2988, 1583, 1481, 1437, 1339, 1315, 1186, 1161, 1107, 1026, 997, 752, 723, 689, 527, 461, and 434. Found, %: С 41.94; H 3.02. C48H40Br6PPd2. Calculated, %: С 42.05; H 2.94.