Palladium-promoted sulfur atom migration on carboranes: facile B(4)−S bond formation from mononuclear Pd-B(4) complexes

Synthesis and characterization of complex 3 ([(Py)₂Pd₂(4-S-1-PhCN-1,2-C₂B₁₀H₁₀)])

Complex 2a ([(Py)₂Pd₂(4-S-1-PhCN-1,2-C₂B₁₀H₁₀)]) was prepared from precursor tetranuclear complex 1 ([Pd₄(1-PhCNS-1,2-C₂B₁₀H₁₀)₄]), which was obtained in high yield (90%) with excellent regioselectivity. Complex 2a was treated in different solvents and at different temperatures to screen the optimal reaction conditions. When complex 2a was heated in toluene at 110°C for 3 min, the solution changed from colorless to yellow. The isolated yield was 70% (Scheme 1).

Scheme 1:

Preparation of B(4)-S dimer complexes.

The isolated product, obtained via column chromatography, was characterized using spectroscopic data and X-ray crystal structure determinations. In the ¹H NMR spectrum of complex 3, the signal at 4.29 ppm is characteristic of a C_cage-H resonance, which suggested the presence of a single B–S bond in the dimer structure. In the IR spectrum, the strong band at 2575 cm⁻¹ is characteristic of a B-H stretching mode. In the normal ¹¹B{¹H} NMR spectrum, there are three broad signals at –2.2, –11.7 and –18.7 ppm, in a ratio of 4: 14: 2, respectively, indicating the closed structure of the carborane cage. The ¹¹B{¹H} NMR spectrum of 3 is significantly different from that of its parent complex 2a [16]. According to this ratio, the shift at –18.66 ppm is obviously a cage_B-S resonance, by comparison with its parent complex. The ESI mass spectrum of 3 was consistent with the theoretical isotope distribution and revealed the formation of a dimer complex (See Supplementary Information, Figure 3).

The crystal structure further confirmed complex 3 to be a dimer complex, with S atoms bridging two Pd atoms (Figure 1). In the structure, the S atom has clearly migrated, leading to the formation of a complex with a B(4)–S bond. The coordination mode changed from B(4)–Pd–S–C–C_cage to B(4)–S–Pd–C–C_cage. The C=N distance is 1.270(7) Å in the structure of complex 3, which is almost identical with its parent complex (1.271(3) Å). The length of the Pd–C bond is ca. 2.00 Å and the length of the B–S bond is ca. 1.88 Å. The formation of the B(4)–S–Pd–C–C_cage five-membered ring may be more favourable than forming the B(4)–S–Pd–N–C–C_cage six-membered ring, the former inducing one C atom to bind with the palladium metal center.

Figure 1:

Molecular structure of complex 3: H atoms are omitted for clarity. Color code: C, black; N, blue; Pd, pink; B4, bright green; other B, gray; S, yellow. Selected bond lengths (Å) and angles (deg): Pd(1)–C(3) 1.994(6); Pd(1)–S(1) 2.3402(16); Pd(1)–S(2) 2.4245(15); S(1)–B(4) 1.883(7); C(1)–B(4) 1.708(9); N(1)–C(3) 1.270(7); C(3)–Pd(1)–S(1) 88.63(17); B(4)–S(1)–Pd(1) 96.37(2); Pd(1)–S(1)–Pd(2) 81.09(5); C(1)–B(4)–S(1) 110.3(4).

Synthesis and characterization of complex 4 ([(PhCH₂N≡C)₂Pd₂(4-S-1-PhCN-1,2-C₂B₁₀H₁₀)]) and complex 5 ([(PPh₃)₂Pd₂(4-S-1-PhCN-1,2-C₂B₁₀H₁₀)])

Based on the simple method for obtaining product 3, the reactivity of similar complexes containing other ligands (L=benzyl isocyanide, PPh₃) was also examined similar to that of complex 2a (Scheme 1). The palladium starting materials 2b and 2c were prepared via a similar route to that used to prepare complex 2a, by mixing the ligands with tetranuclear complex 1 in CH₂Cl₂. The crystal structure of the palladium starting material 2b is shown in the Supplementary Information (Figure 2). The structures of complex 4 and 5 were also confirmed fully by NMR spectroscopy and single-crystal X-ray crystallography. In the ¹H NMR spectra of complexes 4 and 5, singlets at δ 3.49 and 4.50 ppm are characteristic of C_cage–H resonances, respectively, which also suggests the formation of single B(4)–S bonds in their dimer complexes. The ESI mass spectrum of complex 5 is also consistent with the theoretical isotope distribution (In Supplementary Information, Figure 4). The molecular structures confirmed that complexes 4 and 5 have the same coordination mode, containing a five-membered ring as in complex 3, in which the S atoms had migrated to the B(4) sites (Figures 2 and 3).

Figure 2:

Molecular structure of complex 4: H atoms are omitted for clarity. Color code: C, black; N, blue; Pd, pink; B4, bright green; other B: gray; S, yellow. Selected bond lengths (Å) and angles (deg): Pd(1)–C(19) 1.954(6); Pd(1)–C(3) 2.036(5); Pd(1)–S(1) 2.3630(13); Pd(1)–S(2) 2.4193(12); S(1)–B(4) 1.884(6); N(3)–C(3) 1.255(6); C(1)–B(4) 1.689(8); C(19)–Pd(1)–C(3) 93.9(2); C(3)–Pd(1)–S(1) 90.58(14); B(4)–S(1)–Pd(1); 97.3(2); C(1)–B(4)–S(1) 110.7(4); C(1)–C(3)–Pd(1) 111.4(3).

Figure 3:

Molecular structure of complex 5: H atoms are omitted for clarity except for those involving hydrogen bonds. Color code: C, black; N, blue; Pd, pink; B4, bright green; other B, gray; P, green; S, yellow. Selected bond lengths (Å) and angles (deg): Pd(1)–C(3) 2.032(5); Pd(1)–S(1) 2.3796(13); Pd(1)–S(2) 2.4166(13); S(1)–B(4) 1.867(6); N(1)–C(3) 1.246(7); B(4)–C(1) 1.735(7); C(3)–Pd(1)–S(1) 86.44(14); B(4)–S(1)–Pd(1) 96.59(18); C(1)–B(4)–S(1) 109.5(4).

A plausible mechanism for the S-atom migration process

To gain insight into the S-atom migration process, Dppe (1, 2-bis(diphenylphosphino)ethane) – a bidentate ligand and poor leaving group for palladium centers – was used to prepare palladium starting material complex 6 (Scheme 2). The molecular structure of complex 6 is shown in the Supplementary Information (Figure 1). The structures of the palladium starting materials indicate that they exhibit coordinated mode paralleling to each other [16]. The complexes differ by the leaving ability of the ligand, which is in the following order: pyridine>benzyl isocyanide>triphenylphosphine>Dppe. However, when complex 6 was treated in the same way as other palladium starting materials, no reaction was observed through monitoring by TLC, despite prolonged reaction time. This suggested that PPh₃ or pyridine leaves in the first step, and may trigger the reaction process. Only when there is an unoccupied coordination site at the palladium center can the sulfur atom migration reaction be induced, forming a B(4)–S bond.

Scheme 2:

Use of Dppe as ligand to demonstrate the S-atom migration reaction.

The completion time (as monitored by TLC) is in the order: pyridine<benzyl isocyanide<triphenylphosphine<Dppe, which is the inverse of the order of the leaving ability of the ligand. In other words, the easier the ligand can leave, the less time the reaction needs. It seems the leaving ability of the leaving group determines the reaction rate under the heating conditions. All of these sulfur-atom migration reactions involve intramolecular carbon-sulfur bond activation, leading to new derivatives containing B(4)–S bond. The cleavage of carbon-sulfur bonds has been extensively reported in the literature [12].

As shown in Scheme 3, a plausible mechanism has been proposed to illustrate the sulfur atom migration process under optimal heating conditions. The loss of pyridine or PPh₃, which is fundamentally important in the reaction, leads to a vacant coordination site. After that, a transition state containing two three-membered rings is formed, in which the Pd center activates the C–S bond. Finally, the rearrangement product is produced. This migration of S atoms to o-carborane B(4) sites, and formation of B(4)-S complexes, is unprecedented.

Scheme 3:

A plausible mechanism of S-atom migration.

General procedures

All reactions were carried out under an atmosphere of dry, oxygen-free nitrogen using standard Schlenk techniques, with subsequent manipulations under ambient conditions. All solvents were dried and distilled under N₂ immediately prior to use. CH₂Cl₂ was dried over CaH₂; toluene and n-hexane were dried over Na. The palladium starting materials 2a and 2c were prepared according to literature procedures [16]. ¹H NMR (400 MHz) spectra were measured with an AVANCE III HD spectrometer. ¹¹B NMR (160 MHz) spectra were recorded with a Bruker DMX-500 spectrometer. IR (KBr) spectra were measured with a Nicolet FT-IR spectrophotometer. Elemental analysis was performed on a Vario EL Elemental Analyzer. ESI-MS spectra were recorded on a Bruker micrOTOF11 using electrospray ionization. Diffraction data of all the above were collected on a Bruker Smart APEX CCD diffractometer with graphite-monochromated Mo K_α radiation (λ=0.71073 Å).

X-ray crystal structure determinations

Single crystals of 3, 4, 5, 6, 2b suitable for X-ray diffraction study were obtained at room temperature. X-ray intensity data of 3, 4, 6, 2b were collected at 173 K on a CCD-Bruker SMART APEX system. X-ray intensity data of 5 was collected at 296 K on a CCD-Bruker SMART APEX system. In these data, the disordered solvent molecules which could not be restrained properly were removed using the SQUEEZE route.

In the asymmetric unit of complex 4, 5 ISOR and 5 DFIX instructions were used to restrain ligands and solvent molecules so that there were 35 restraints in the data. The hydrogen atoms of chloroform molecules could not be found and others were placed in calculated positions.

In asymmetric unit of complex 5, there were disordered solvents (one dichloromethane and one diethyl ether molecules) which could not be restrained properly. Therefore, SQUEEZE algorithm was used to omit them. One phenyl group was disordered and it was divided into two parts (69:31). 9 ISOR, 1 FLAT and 8 DFIX instructions were used to restrain the disordered phenyl group so that there were 68 restraints in the data. H₂ and H₁₁ were found in difference Fourier map and others were put in calculated positions.

In the asymmetric unit of complex 6, one phenyl group was disordered and it was divided into two parts (81:19). 15 ISOR and 3 DFIX instructions were used to restrain the phenyl groups and carborane cages so that there were 93 restraints in the data.

These structures were solved by direct methods, using Fourier techniques, and refined on F² by a full-matrix least-squares method. All calculations were carried out with the SHELXTL program. A summary of the crystallographic data and selected experimental information are shown in Supplementary Information Table 1 and Table 2.

CCDC-1559018 (3), 1559019 (4), 1559020 (5), 1559021 (6) and 1559022 (2b) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Synthesis of complex 3 [(Py)₂Pd₂(4-S-1-PhCN-1,2-C₂B₁₀H₁₀)]

The palladium starting material ([(Py)₂Pd(1-PhCNS-1,2-C₂B₁₀H₁₀)], 54 mg, 0.10 mmol) and distilled toluene (10 mL) were added to a Schlenk flask under nitrogen atmosphere. After the reaction mixture was stirred at 110°C for ca. 3 min, the colorless solution turned bright yellow. The solvent was then removed under vacuum. The crude solid was purified by column chromatography on silica gel. Elution with petroleum ether/CH₂Cl₂ (1/1) gave the pure complex 3 (64 mg, 70%) as a yellow crystalline solid. ¹H NMR (400 MHz, CDCl₃): δ/ppm: 1.30–2.75 (m, 18H, B-H); 4.29 (s, 1H, C_cage-H); 8.08, 8.07, 7.01, 6.99, 6.97, 6.96, 6.94 (10H, pyridine H); 7.51, 7.49, 7.47, 6.92, 6.90, 6.85, 6.84 (10H, ArH). ¹³C NMR (100 MHz, CDCl₃): δ/ppm: 52.67, 92.37 (cage C); 181.31 (−N=C−), 152.09, 128.30, 124.62 (pyridine C); 149.81, 137.02, 123.65, 120.56 (ArC). ¹¹B NMR (160 MHz, CDCl₃): δ/ppm: –2.24 (4B), –11.71 14B), –18.66 (2B). IR (KBr): ν (cm⁻¹) 1604 (C=N), 2575 (B−H), 1448 (C=N, pyridine). EI-MS: m/z calcd for (3 – 2 pyridine + H)⁺: 768.20. Found: 768.20. Elemental analysis: calculated for C₂₈H₄₀B₂₀N₄Pd₂S₂: C, 36.32; H, 4.35; N, 6.05. Found: C, 36.42; H, 4.44; N, 6.08.

Synthesis of complex 4 [(PhCH₂N≡C)₂Pd₂(4-S-1-PhCN-1,2-C₂B₁₀H₁₀)]

The palladium starting material [(PhCH₂N≡C)₂Pd(1-PhCNS-1,2-C₂B₁₀H₁₀), 63 mg, 0.10 mmol] and distilled toluene (10 mL) were added to a Schlenk flask under nitrogen atmosphere. After the reaction mixture was stirred at 110°C for ca. 20 min, the colorless solution turned yellow green. The solvent was then removed under vacuum. The crude solid was purified by column chromatography on silica gel. Elution with petroleum ether/CH₂Cl₂ (1/1) gave the pure complex 4 (48 mg, 50%) as a yellow crystalline solid. ¹H NMR (400 MHz, CDCl₃): δ/ppm: 1.60–3.26 (m, 18H, B-H), 4.29 (s, 2H, C_cage–H); 4.18 (s, 4H, −CH₂−); 7.37, 7.36, 7.28, 7.24, 7.10, 7.08, 7.05, 7.04 (20H, ArH). ¹³C NMR (100 MHz, CDCl₃): δ/ppm: 54.86, 94.19 (cage C), 47.38 (−CH₂−); 175.43 (−N=C−); 151.71 (−N≡C−); 130.32, 129.12, 128.99, 128.88, 127.36, 125.16, 121.91 (ArC). Elemental analysis: calculated for C₃₂H₄₄B₂₀N₄Pd₂S₂: C, 40.76; H, 4.43; N, 5.59. Found: C, 40.88; H, 4.52; N, 5.61.

Synthesis of complex 5 [(PPh₃)₂Pd₂(4-S-1-PhCN-1,2-C₂B₁₀H₁₀)]

The palladium starting material ([(PPh₃)₂Pd(1-PhCNS-1,2-C₂B₁₀H₁₀)], 91 mg, 0.10 mmol) and distilled toluene (10 mL) were added to a Schlenk flask under a nitrogen atmosphere. After the reaction mixture was stirred at 110°C for ca. 2 h, the light yellow solution turned bright yellow. The solvent was then removed under vacuum. The crude solid was purified by column chromatography on silica gel. Elution with petroleum ether/CH₂Cl₂ (1/1) gave the pure complex 5 (39 mg, 30%) as a yellow crystalline solid. ¹H NMR (400 MHz, CDCl₃): δ/ppm: 1.26–2.85 (m, 18H, B-H), 3.49 (s, 2H, C_cage–H); 8.44, 7.67, 7.48, 7.46, 7.38, 7.08, 6.84, 6.56 (40H, ArH). ¹³C NMR (100 MHz, CDCl₃): δ/ppm: 51.05, 96.22 (cage C), 175.91 (−N=C−), 148.26, 138.01, 134.77, 134.48, 134.35, 133.29, 132.19, 131.15, 128.75, 128.63, 128.27, 128.05, 125.12, 121.30 (ArC). ¹¹B NMR (160 MHz, CDCl₃): δ/ppm: –3.33 (6B), –9.55 (6B), –14.07 (8B). IR (KBr): ν (cm⁻¹) 1608 (C=N), 2589 (B−H). Elemental analysis: calculated for C₅₄H₆₀B₂₀N₂P₂Pd₂S₂: C, 50.21; H, 4.70; N, 2.28. Found: C, 50.19; H, 4.68; N, 2.17.

Synthesis of complex 6 [(Dppe)₂Pd(1-PhCNS-1,2-C₂B₁₀H₁₀)]

Tetranuclear palladium complex 1 (Pd₄(1-PhCNS-1,2-C₂B₁₀H₁₀)₄, 77 mg, 0.05 mmol) was dissolved in 5 mL of CH₂Cl₂, into which 1,2-bis(diphenylphosphino)ethane (Dppe) (80 mg, 0.2 mmol) was added. After ca. 4 h, the solvent was removed and the residue was washed a few times with n-hexane, producing a white solid. Complex 6 was obtained quantitatively. ¹H NMR (400 MHz, CDCl₃): δ/ppm: 1.60–3.20 (m, 9H, B-H); 4.22 (s, 1H, C_cage–H); 2.46 (m, 4H, −CH₂−); 7.72, 7.70, 7.69, 7.67, 7.62, 7.58, 7.55, 7.51, 7.48, 7.46, 7.41, 7.39, 7.37, 7.35, 7.28, 7.24, 7.08, 7.06, 7.02, 7.01, 6.99 (m, 25H, ArH). ¹³C NMR (100 MHz, CDCl₃): δ/ppm: 62.49, 84.84 (cage C); 150.13 (−N=C−); 134.18, 134.11, 133.99, 133.13, 133.11, 133.00, 132.99, 131.34, 131.24, 130.81, 129,11, 129.07, 129.01, 128.97, 128.71, 128.42 (ArC). IR (KBr): ν (cm⁻¹) 1581 (C=N), 2585 (B−H). Elemental analysis: calculated for C₃₅H₃₉B₁₀NP₂PdS: C, 53.80.75; H, 5.15; N, 1.83. Found: C, 53.74; H, 5.03; N, 1.79.

Synthesis of complex 2b [(PhCH₂N≡C)₂Pd(1-PhCNS-1,2-C₂B₁₀H₁₀)]

Tetranuclear palladium complex 1 (Pd₄(1-PhCNS-1,2-C₂B₁₀H₁₀)₄, 77 mg, 0.05 mmol) was dissolved in 5 mL of CH₂Cl₂, into which benzyl isocyanide (0.05 mL, excess) was added. About 10 min later, the solvent was removed and the residue washed a few times with n-hexane, producing a white solid. Complex 2b was obtained quantitatively. ¹H NMR (400 MHz, CDCl₃): δ/ppm: 1.59–3.27 (m, 9H, B–H), 4.22 (s, 1H, C_cage–H); 4.83, 4.67 (s, 4H, −CH₂−); 7.41, 7.40, 7.37, 7.36, 7.35, 7.32, 7.31, 7.29, 7.24, 7.23 (15H, ArH). ¹³C NMR (100 MHz, CDCl₃): δ/ppm: 62.64, 83.73 (cage C), 150.36 (−N=C−), 171.84 (−N≡C−), 48.24 (−CH₂−); 129.33, 129.21 129.16, 128.82, 127.54, 127.02, 123.92, 120.86 (ArC). IR (KBr): ν (cm⁻¹) 1569 (C=N), 2582 (B−H). Elemental analysis: calculated for C₂₅H₂₉B₁₀N₄PdS: C, 47.55; H, 4.71; N, 8.90. Found: C, 47.50; H, 4.62; N, 8.86.