1,5,9-tri(Phenylethynyl)-4,8,12-trioxaphosphangulene

Abstract

4,8,12-Trioxaphosphangulene is a bowl-shaped phosphorus-containing π-conjugated molecule whose molecular geometry is highly sensitive to the substituent attached to the phosphorus atom. Herein, we report the synthesis of a new tungsten pentacarbonyl complex of a chiral 4,8,12-trioxaphosphangulene bearing three phenylethynyl groups. The complex was prepared by coordination of the phosphine center to an in situ generated W(CO)₅ fragment and was characterized by multinuclear NMR spectroscopy and elemental analysis. The NMR spectra revealed that the phosphangulene framework retains its threefold symmetry in solution. Comparison of the NMR parameters with those of a previously reported phosphangulene–tungsten complex indicates that incorporation of the phenylethynyl substituents has little effect on either the coordination environment around the phosphorus atom or the phosphine–tungsten interaction. These findings demonstrate that the characteristic bowl-shaped phosphangulene framework is preserved upon tungsten coordination despite π-extension of the molecular framework.

1. Introduction

4,8,12-Trioxaphosphangulene is a unique bowl-shaped molecule containing a central phosphorus atom that was first synthesized by Krebs and co-workers in 1997 [1,2,3,4]. We have previously reported the synthesis of chiral 4,8,12-trioxaphosphangulene derivatives bearing three phenylethynyl groups [5,6] (Figure 1) and demonstrated their potential applications as bowl-shaped host molecules [5,6,7,8], chiral liquid crystals [9], and platforms for controlling chiroptical properties [10]. It has also been shown that the substituent attached to the phosphorus atom significantly influences the geometry of the bowl-shaped framework [11]. In the present study, we synthesized a novel chiral 4,8,12-trioxaphosphangulene derivative in which the substituent on the phosphorus atom is a tungsten complex fragment.

2. Results and Discussion

The trioxaphosphangulene bearing three phenylethynyl groups was synthesized as a racemic mixture according to the reported procedure and subsequently reacted as a phosphine ligand with in situ generated W(CO)₅(THF), which was prepared by photoirradiation of W(CO)₆ using a 400 W mercury lamp (Figure 2). Purification by silica gel column chromatography afforded the corresponding phosphine–tungsten complex as a pale-yellow powder. The tungsten complex was characterized by various NMR spectroscopic techniques and elemental analysis. In the NMR spectra, the trioxaphosphangulene framework exhibited apparent C₃ symmetry, indicating that the bowl-shaped structure was retained in solution. The carbonyl carbon signals of the tungsten moiety appeared as two distinct resonances corresponding to four equivalent equatorial carbonyl ligands and a single axial carbonyl ligand, reflecting their nonequivalent chemical environments.

The ³¹P NMR chemical shift (−95.1 ppm) and the P–W coupling constant (J_P-W = 260 Hz) of 4 (

Table 1. NMR chemical shifts and coupling constants of 4.

Position	δP (J in Hz)	δH (J in Hz)	δC (J in Hz)
P	−95.1 (JP-W = 260 Hz)	-	-
CO (ap)	-	-	197.4 (JC-P = 27.7 Hz),
CO (eq)	-	-	193.9 (JC-P = 7.8 Hz)
1	-	-	111.7 (JC-P = 36.7 Hz)
2	-	7.57 (JH-H = 8.4)	122.8
3	-	7.28 (JH-H = 8.4, JH-P = 4.0)	116.0 (JC-P = 4.6 Hz)

) are nearly identical to those reported for the tungsten pentacarbonyl complex of the achiral phosphangulene lacking phenylethynyl substituents (−96.6 ppm, J_P-W = 263 Hz) [11]. Likewise, the ¹³C NMR chemical shift (197.4 ppm) and C–P coupling constant (J_C-P = 27.7 Hz) of the carbonyl carbon located trans to the phosphine ligand are almost the same as those of the unsubstituted phosphangulene–tungsten complex (197.9 ppm, J_C-P = 26.4 Hz) [11]. These results indicate that neither the local structure around the phosphorus atom nor the nature of the phosphine–tungsten bond is significantly affected by the presence of the phenylethynyl groups. Although suitable single crystals of 4 for X-ray crystallographic analysis could not be obtained, the spectroscopic data strongly suggest that 4 adopts a structure similar to that of the previously reported unsubstituted phosphangulene–tungsten complex [11]. In the previously reported phosphangulene–tungsten complex, the phosphine–tungsten bond is relatively weak, resulting in a smaller C–P–C bond angle around the phosphorus center and consequently a deeper bowl-shaped geometry.

3. Materials and Methods

W(CO)₆ was purchased from Sigma-Aldrich and used as reagent grade without further purification. THF was purchased from Kanto chemical corporation as a high-grade dried solvent. A reaction was performed under a nitrogen atmosphere. Chromatography was performed using SiO₂-60N (0.063–0.212 mm; Kanto). The ¹H, ¹³C, and ³¹P NMR spectra were recorded by Bruker AVANCE400 (400 MHz) spectrometer using tetramethylsilane (0 ppm) and CDCl₃ (77 ppm) as internal standards, and phosphoric acid (0 ppm) as an external standard. Deuterated solvents were purchased from Cambridge Isotope Laboratories or Aldrich and used as received. Elemental analysis was performed at the Department of Chemistry, University of Tsukuba. Photoirradiation was performed using a 400 W high-pressure mercury lamp of SEN Light Corporation.

Tungstenpentacarbonyl complex of 1,5,9-tri(phenylethynyl)-4,8,12-trioxaphosphangulene

A mixture of 3 (9.6 mg, 16 μmol) and W(CO)₆ (28.5 mg, 81 μmol) was stirred in THF (10 mL) during UV irradiation (mercury lamp, 400 W). After evaporation, the crude product was purified by silica-gel column chromatography (eluent: 2% methanol/chloroform) to yield a pale-yellow powder of 4 (7.3 mg, 50%).

4: pale-yellow powder; ¹H NMR (400 MHz, CDCl₃) δ 7.63 (dd, J = 8.0, 1.6, 6H), 7.57 (d, J = 8.4, 3H), 7.41-7.38 (m, 9H), 7.28 (dd, J = 8.4, 4.0, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 197.4 (CO, d, J = 27.7 Hz), 193.9 (CO, d, J = 7.8 Hz), 156.9 (d, J = 2.5 Hz), 156.8 (d, J = 2.2 Hz), 135.0 (s), 131.8 (s), 128.8.0 (s), 128.4 (s), 122.8 (s), 116.0 (d, J = 4.6 Hz), 112.1 (d, J = 4.8 Hz), 111.7 (CP, J = 36.7 Hz), 95.3 (s), 82.7 (s); ³¹P NMR (162 MHz, CDCl₃) δ -95.1 (s, J = 260 Hz (satellite)); Anal. Calcd for C₄₇H₂₁O₈PW·H₂O: C, 59.64; H, 2.45. Found C, 59.27; H, 2.83.

4. Conclusions

A new tungsten pentacarbonyl complex derived from a chiral phenylethynyl-substituted 4,8,12-trioxaphosphangulene was synthesized and fully characterized by multinuclear NMR spectroscopy and elemental analysis. Spectroscopic analysis showed that coordination of the phosphine moiety to the W(CO)₅ fragment proceeds without significantly altering the intrinsic structural features of the phosphangulene framework. The close agreement of the NMR parameters with those of the previously reported parent complex suggests that π-extension by phenylethynyl substituents has only a minor influence on the geometry around the phosphorus center and on the P–W bonding interaction. The present study provides additional insight into the coordination chemistry of bowl-shaped phosphangulenes and offers a useful reference for the design of functional phosphorus-containing π-conjugated molecules based on this unique molecular scaffold.