2.1 Synthesis of 2,5-bis(pyrrol-2-yl)phosphole derivatives

Scheme 1 illustrates the synthesis of 2,5-bis(pyrrol-2-yl)phosphole derivatives and 2,5-bis(5-phenylpyrrol-2-yl)phosphole derivatives. The starting materials, pyrrolyl-capped 1,6-heptadiynes 2a,b, were prepared by Sonogashira coupling of terminal-free 1,6-heptadiyne and the corresponding 1-Boc-2-iodopyrroles (Boc = t-butoxycarbonyl) 1a,b [12,13]. Reactions of 2a and 2b with (η²-propene)Ti(Oi-Pr)₂, generated in situ from Ti(Oi-Pr)₄ and 2 equiv of i-PrMgCl [6], followed by treatment with PhPCl₂ gave N-Boc-protected 2,5-bis(pyrrol-2-yl)phosphole 3a (43%) and 2,5-bis(5-phenylpyrrol-2-yl)phosphole 3b, respectively. Compound 3b could not be separated from unreacted 2b and was used as a mixture in the following N-Boc deprotection step. The N-Boc groups of 3a,b were successfully removed by the reaction with sodium methoxide in methanol to give σ³ derivatives 4a,b. The phenyl-capped derivative 4b was isolated as an air stable solid in 28% yield based on 2b, whereas the α-free derivative 4a was difficult to completely purify due to its instability in atmospheric environment. Therefore, 4a was used for subsequent transformations without isolation. On treatment with hydrogen peroxide or m-chloroperbenzoic acid (mCPBA), each σ³-phosphorus center of 4a,b was readily oxygenated, affording the corresponding P-oxides 5a,b. When treated with elemental sulfur, 4a,b were converted to P-sulfides 6a,b.

The pyrrole–phosphole–pyrrole hybrids 3–6 were characterized by conventional spectroscopic techniques (¹H, ¹³C, ³¹P NMR, high-resolution MS, IR) and elemental analysis. Although 4a gradually decomposed in solution, its structure was confirmed by NMR spectroscopy and MS spectrometry. The ³¹P peaks of the σ⁴ derivatives 6a,b (δ_P 63.4, 63.6) and 5a,b (δ_P 54.3, 55.6) were observed at downfield compared to those of 4a,b (δ_P 28.1, 28.5), which clearly reflects differences in the electronic character and the oxidation state of phosphorus. In the ¹H NMR spectra, 3–6 showed only one set of pyrrolic CH and NH protons, indicating that all of these compounds have C_s symmetry in solution. The NH protons of α,α′-free derivatives 4a, 5a, and 6a appeared at δ 8.01, 9.09, and 9.61, respectively. The same holds true for a series of the phenyl-capped derivatives 4b (δ 8.26), 5b (δ 9.28), and 6b (δ 9.96). Judging from the downfield appearance of the NH peaks of 5 and 6 relative to those of 4 and free pyrrole (δ 8.0), there may exist hydrogen-bonding interaction between the NH protons and the chalcogen atoms bound to phosphorus in 5 and 6. To get some insight into relative conformation between the pyrrole and phosphole rings in solution, we also measured NOESY spectra for 4b and 5b in CD₂Cl₂. In both cases, the NH proton and one of the β-CH protons of the pyrrole ring show nuclear Overhauser effects (NOE) toward allylic methylene protons of the trimethylene bridge fused to the phosphole ring. This result implies that the inter-ring C_α–C_α′ bonds between pyrrole and phosphole rings rotate rapidly in solution on the NMR timescale. In the IR spectra of 5a and 5b, PO stretching frequencies were observed at ν 1165 and 1180 cm⁻¹, respectively. The ν_PO value of 5a is somewhat smaller than that (ν_PO = 1178 cm⁻¹) of a similar trimethylene-fused 2,5-diphenylphosphole P–oxide [14].⁴

The crystal structure of 5b was elucidated by X-ray crystallography. The ORTEP diagram is shown in Fig. 1 with selected bond lengths and bond angles. The phosphorus center forms a tetrahedral geometry with average C–P–C and C–P–O angles of 101.9° and 116.4°, respectively. The C–C bond length alternation of the phosphole subunit differs considerably from that of two pyrrole subunits, which reflects the difference in aromaticity between these two kinds of heterole rings. The relatively narrow dihedral angles between the central phosphole and adjacent pyrrole rings (4.7–18.3°) indicate efficient π-conjugation through the pyrrole–phosphole–pyrrole linkage. As for the stereochemistry at the inter-ring bonds, one pyrrole ring adopts an S-trans (anti) conformation against the phosphole ring, whereas the other adopts an S-cis (syn) conformation. The N(2)–H group of the syn pyrrole ring is directed to the P–oxo group with N•••O distance of 3.20 Å, indicating that weak hydrogen-bonding interaction is present between the NH proton and the oxygen atom in 5b [15].⁵ The PO distance of 1.4871(11) Å is almost identical to that [1.489(1) Å] of Tanaka's 2,5-diphenyl-1-phenylphosphole P–oxide [16].

2.2 Optical and electrochemical properties of 2,5-bis(pyrrol-2-yl)phosphole derivatives

To investigate the optical and electrochemical properties of α,α′-free and α,α′-phenyl-capped pyrrole–phosphole–pyrrole π-systems, we measured UV-vis absorption and fluorescence spectra and redox potentials of 4–6 in CH₂Cl₂. The results are summarized in Table 1 and Fig. 2. In the absorption spectra, the α,α′-free derivatives 4a, 5a, and 6a display the lowest π–π* transitions at λ_ab 416, 458, and 454 nm, respectively. As typically observed for phosphole derivatives, chemical functionalizations at the phosphorus center from σ³ to σ⁴ induce red-shifts of λ_ab values (Δλ_ab = 38–42 nm). The same trend was observed for the phenyl-capped series, 4b, 5b, and 6b (Δλ_ab = 47–48 nm). The σ³ derivatives 4a,b are fluorescent, whereas the σ⁴ derivatives 5a,b and 6a,b are weakly fluorescent. Notably, 4b emits intense yellowish green fluorescence at λ_em 536 nm with fluorescence quantum yield (Φ_f) of 0.78. To our knowledge, this is the highest Φ_f value ever reported for type A 2,5-diarylphospholes.⁶ Nonradiative pathways from the S₁ state of 4b may be suppressed and/or a radiative pathway may be accelerated by the effective π conjugation through the pyrrole–phosphole–pyrrole π-linkage.⁷ The absorption/emission maxima of 4a, 5a, and 6a are appreciably red-shifted compared to those of the previously reported 2,5-diarylphosphole analogs. In other words, the introduction of the pyrrolyl groups narrows HOMO–LUMO gaps of π-conjugated phospholes of type A. For instance, the optical HOMO–LUMO gaps of the respective σ³-P and σ⁴-P–oxo derivatives are 2.69 eV (Ar = 2-pyrrolyl) vs. 2.72–3.02 eV (2-thienyl, phenyl, 2-pyridyl, measured in THF) [4d] and 2.39 eV (Ar = 2-pyrrolyl) vs. 2.51–2.83 eV (2-thienyl, phenyl, measured in THF) [4b, 8b, 14].

Redox potentials of 4b, 5a,b, and 6a,b were measured by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) with 0.1 M nBu₄NPF₆ as an electrolyte in CH₂Cl₂ (Table 1). Although reduction processes were not observed in the measurement window (−2.6 V to +1.0 V vs. Fc/Fc⁺), oxidation processes were observed for all compounds examined. The α,α′-free derivatives 5a and 6a showed irreversible voltammograms accompanied by deposition of insoluble substances, whereas the α,α′-phenyl-capped derivatives 4b, 5b, and 6b showed two reversible oxidation processes in CV measurements. In the cases of 5a and 6a, subsequent chemical reactions such as an electropolymerization may occur at the α-free pyrrole rings when electrochemically oxidized. The P-functionalizations (from 4b to 5b and 6b) make a definite impact on the first and second oxidation processes, both of which are shifted to lower potentials by 0.16–0.22 V. It is worthy to note that the electrochemical oxidations of 4, 5, and 6 occur at much lower potentials than those reported for related 2,5-diarylphospholes (aryl = 2-thienyl, phenyl, 2-pyridyl).⁸ For instance, the difference in the first oxidation potentials between 5a and 2,5-diphenylphosphole P–oxide [14] of the same trimethylene-fused type reaches 1 V. These experimental results demonstrate that the pyrrole components play a key role in fine-tuning of the redox properties, especially electrochemical oxidation properties, of type A phosphole derivatives.

To get more insight into electronic structures of the phosphole–pyrrole hybrid π-systems, we performed density functional theory (DFT) calculations on model compounds 4m and 5m in three forms, anti–anti, anti–syn, and syn–syn (Fig. 3). At the optimized structures, inter-ring torsion angles are small (10.1–21.7°) for all models. This indicates that the effective π-conjugation is attained over the three heterole rings. There is negligible difference in relative energies of the three conformers for 4m (ΔE < 0.3 kcal mol⁻¹), implying that all forms are equally conceivable. In contrast, anti–syn and syn–syn conformers of 5m are more stabilized by 3.5 and 6.1 kcal mol⁻¹, respectively, than anti–anti isomer. Considering the fact that the stabilization energy of the syn–syn conformer is about twice as that of the anti–syn one, the origin of this energetic stabilization is attributable mainly to the intramolecular NH•••O=P hydrogen-bonding interaction. Indeed, the N•••O distance calculated for 5m (anti–syn) (3.067 Å) is within the range of this type of hydrogen bonds [15]. In each model, HOMO consists of conjugated polyene-derived π-networks, whereas LUMO possesses a significant character of central phosphole units. The P-oxygenation from 4m to 5m lowers LUMO and HOMO levels by 0.62–0.68 eV and 0.25–0.29 eV, respectively. Note that LUMOs are more stabilized than HOMOs for all conformers. This is a typical trend for phosphole derivatives and has been rationalized by considering effective σ*(P–X)–π* orbital interaction including the dienic π-systems as depicted in Fig. 3b. The LUMO levels of the P–oxo models 5m are more stabilized in the order anti–anti (−1.93 eV) > anti–syn (−2.00 eV) > syn–syn (−2.06 eV), which displays a key contribution of the NH•••O=P hydrogen-bonds in the stabilization of LUMO. Time-dependent DFT (TD-DFT) calculation on 4m in the syn–syn form revealed that the lowest excitation band observed for the σ³-P derivatives 4 is based on a HOMO → LUMO π–π* transition.

2.3 Coordination behavior of 2,5-bis(5-phenylpyrrol-2-yl)phosphole

Recently, the structure–optical property relationship of metal-bridged phosphole-containing π-systems has received growing interest [17].⁹ To investigate the coordination behavior of 2,5-bis(pyrrol-2-yl)phosphole as a phosphine ligand, 4b was reacted with gold(I) and platinum(II) salts (Scheme 2). Treatment of 4b with one equiv of AuCl(SMe₂) in CH₂Cl₂ at room temperature yielded AuCl–phosphine complex 7 in 80% yield. When treated with a half equiv of PtCl₂ under the same conditions, PtCl₂–bis(phosphine) complex 8 was formed as a trans/cis mixture (ca. 1:1). Metal complexes 7 and 8 were characterized by spectroscopy and X-ray crystallography. The ³¹P chemical shifts of 7, trans-8, and cis-8 are δ_P 51.4, δ_P 51.6 (J_P–Pt = 2244 Hz), and δ_P 31.5 (J_P–Pt = 3177 Hz), respectively. The downfield appearance of the NH protons of 8 (δ 9.73–9.79) relative to those of 7 (δ 8.48) suggests that the pyrrolic NH groups spatially interact with the chlorine atoms in 8 (vide infra). High-resolution mass spectra of 7 and trans-8 showed respective parent ion peaks (M⁺) at m/z 714.1267 (calcd. 714.1266) and m/z 1229.2854 (calcd. 1229.2849).

The crystal structures of 7 and trans-8 were successfully determined by X-ray crystallography. The ORTEP diagrams are shown in Figs. 4 and 5 with selected bond lengths and bond angles. In each complex, the phosphorus center forms a tetrahedral geometry with average C–P–C and C–P–M angles of 101.9–103.3° and 115.2–114.7°, respectively. The C–P–C bond angles of 7 and 8 are almost identical to those of 5b. In complex 7, the gold(I) center adopts a linear geometry with P–Au–Cl bond angle of 176.53(8)°, and both the pyrrole rings adopt anti–anti conformation against the phosphole ring with small dihedral angles between the two adjacent heterole rings (6.2–9.9°). Judging from the packing structure, neither intramolecular nor intermolecular interaction between the pyrrolic NH protons and the chlorine atoms is present in 7.

In trans-8, the platinum(II) atom is the center of C_i symmetry and forms a square planar geometry (Σ_P–Pt–Cl = 180°). As shown in Fig. 5a, the Pt–P coordination bonds link two pyrrole–phosphole–pyrrole π-planes as slipped, parallel fashion, and each π system adopts anti–syn conformation with dihedral angles between the two adjacent heterole rings of 12.9–19.5°. Note that the syn pyrrole rings direct their NH groups toward the chlorine atoms bound to platinum with an intramolecular N•••Cl distance of 3.31 Å [18],¹⁰ indicating there exists cooperative hydrogen-bonding interaction between the NH protons and the chlorine atoms. Such a coordinating property cannot be produced by other conventional aryl substituents and shows the characteristic structural features of the pyrrole ligands. A relatively short distance (ca. 3.3 Å) between the center of anti-pyrrole ring and the nearest ortho-proton of the facing α-phenyl group implies that weak CH•••π interaction is also present at least in the solid state.

2.4 Optical properties of metal complexes of 2,5-bis(1-phenylpyrrol-2-yl)phosphole

Finally, we measured absorption and fluorescence spectra of the metal complexes, 7 and trans-8.¹¹ Fig. 2 clearly displays that these two complexes show different optical properties. The AuCl complexation (from 4b to 7) induces red-shifts of the absorption and fluorescence maxima (Δλ_ab = 45 nm; Δλ_em = 70 nm) and decreases of Φ_f value (from 0.78 to 0.09) and fluorescence lifetime (from 3.50 ns to 1.54 ns). A small difference in the observed Stokes shift (Δλ) between 7 (Δλ = 3.34 × 10³ cm⁻¹) and 4b (Δλ = 3.13 × 10³ cm⁻¹) may suggest that the attachment of the AuCl moiety does not affect the intrinsic character of the HOMO → LUMO transition significantly. In sharp contrast, the PtCl₂ complexation (from 4b to trans-8) varies optical properties of the free phosphine ligand dramatically. In the absorption spectrum of trans-8, two separated, broad bands were observed at λ_ab 450 and 533 nm with similar intensity. The high-energy band is slightly blue-shifted (Δλ_ab = −9 nm) relative to λ_ab of 4b (459 nm), whereas the low-energy band is largely red-shifted (Δλ_ab = 74 nm). It is likely that two slipped, parallel π-planes with C_i symmetry linked by the Pt–P and NH•••Cl bonds play a crucial role in the unusual splitting observed for trans-8. No fluorescence was observed for trans-8.

To get a deep insight into the character of the observed absorption bands, we performed TD-DFT calculations on models of Au^I and trans-Pt^II complexes (Fig. 6 and Table 2). The structures of 7m and 8m, where all the phenyl substituents were replaced by hydrogens, were optimized starting from the X-ray geometries observed for 7 and trans-8. As shown in Fig. 6a, HOMO and LUMO of 7m basically maintain the character of the respective orbitals of 4m (anti-anti) except for their relative energies; LUMO is more stabilized than HOMO by the AuCl complexation. The TD-DFT result of 7m (Table 2) implies that the intense absorption band observed for 7 is based on HOMO → LUMO excitation. Fig. 6b shows five frontier orbitals of 8m, HOMO − 1, HOMO, LUMO, LUMO + 1, and LUMO + 2, with their orbital energies. Obviously, HOMO and HOMO − 1, which are constructed by two original HOMOs of the free base, do not interact with each other. As a consequence, HOMO and HOMO − 1 have almost the same energies. On the other hand, LUMO and LUMO + 2 consist of the metal-derived d orbitals and the ligand-derived π*-orbitals. Importantly, the coordination to the platinum(II) center induces a significant split of the original LUMOs of the symmetrical π-conjugated ligands probably due to σ*(M–P)–π* orbital interaction. The TD-DFT results of 8m revealed that the two absorption bands observed for trans-8 are attributable mainly to HOMO → LUMO + 2 (2.95 eV) and HOMO − 1 → LUMO + 1 (2.64 eV) excitations. The finding that the lowest excitation state (HOMO → LUMO) is not the high-intensity transition for 8m (oscillator strength is less than 0.1) may also explain the nonfluorescent property of the Pt complex 8. These results show that the metal complexation is a promising approach to make a large impact on the optical properties of 2,5-bis(pyrrol-2-yl)phosphole derivatives.

In summary, we established a convenient method for the synthesis of new π-conjugated phospholes bearing two pyrrole substituents, and revealed their optical, electrochemical, and coordinating properties. The flat π-planes, low excitation energies, and small oxidation potentials observed for the σ³-P and σ⁴-P derivatives represent the efficient π-conjugation through the pyrrole–phosphole–pyrrole linkage as well as electron excessive nature of the pyrrole subunits. It is also noteworthy that the gold(I) and platinum(II) complexes coordinated by the phenyl-capped σ³-P derivative exhibit quite different structural and optical properties. It is of particular interest that trans-Pt^II–bisphosphine complex contains slipped, parallel π-planes linked by the Pt–P bonds and the NH–Cl hydrogen bonds. This cooperative interaction gives rise to split π–π* transitions at the long-wavelength region, which can be rationalized by considering the orbital interaction between the Pt–X bonds and the hybrid π-systems. The combination of phosphole and pyrrole has great synthetic potential, since these functionalizable heteroles are readily subject to new π-framework construction. Further studies on materials chemistry of phosphole–pyrrole π-conjugated systems are now in progress.

3.1 Synthesis of 2a,b

A mixture of 1a (3.58 g, 12.2 mmol), CuI (212 mg, 1.11 mmol), PdCl₂(PPh₃)₂ (390 mg, 0.55 mmol), 1,6-heptadiyne (0.64 mL, 5.6 mmol), triethylamine (12 mL), and 1,4-dioxane (8 mL) was stirred at room temperature. After 24 h, the reaction mixture was filtrated through a Celite bed, and the filtrates were concentrated under reduced pressure. The oily residue was subjected onto alumina column chromatography (hexane/CH₂Cl₂ = 1/1) to give 2a as a pale brown oil (R_f = 0.2, 2.0 g, 85%). ¹H NMR (CDCl₃, 300 MHz): δ 1.60 (s, 18H, t-Bu), 1.91 (quint, 2H, J = 7.0 Hz, CH₂CH₂CH₂), 2.62 (t, 4H, J = 7.0 Hz, CH₂CH₂CH₂), 6.11 (pseudo t, 2H, J = 3.3 Hz pyrrole-β), 6.43 (m, 2H, pyrrole-β), 7.21 (m, 2H, pyrrole-α); ¹³C{¹H} NMR (CDCl₃, 75 MHz): δ 18.8, 27.6, 27.7, 73.1, 83.6, 92.4, 110.5, 115.5, 119.7, 121.5, 148.2; HRMS (EI) Calculated for C₂₅H₃₀N₂O₄ (M⁺): 422.2206. Found: m/z 422.2203.

According to a similar procedure, compound 2b was prepared from 1b and 1,6-heptadiyne. Yield 81%. Pale brown oil. ¹H NMR (CDCl₃, 400 MHz): δ 1.34 (s, 18H, t-Bu), 1.94 (quint, 2H, J = 6.8 Hz, CH₂CH₂CH₂), 2.67 (t, 4H, J = 6.8 Hz, CH₂CH₂CH₂), 6.13 (d, 2H, J = 3.4 Hz, pyrrole-β), 6.45 (d, 2H, J = 3.4 Hz, pyrrole-β), 7.26–7.38 (m, 10H, Ph); ¹³C{¹H} NMR (CDCl₃, 100 MHz): δ 18.9, 27.17, 27.20, 27.7, 27.8, 73.0, 83.8, 93.1, 112.4, 117.4, 117.7, 127.0, 127.7, 128.1, 133.9, 135.6, 148.7; MS (EI): m/z 374 ([M − 2Boc]⁺).

3.2 Synthesis of 3a,b

To a mixture of 2a (6.3 g, 15 mmol), Ti(Oi-Pr)₄ (4.4 mL, 15 mmol), and Et₂O (210 mL) was slowly added an ether solution of i-PrMgCl (2.0 M × 15 mL, 30 mmol) at −78 °C, and the resulting mixture was stirred for 2 h at −50 °C. Dichloro(phenyl)phosphine (2.0 mL, 15 mmol) was then added to the mixture at this temperature, and the resulting suspension was allowed to warm to 0 °C and stirred for 1 h at this temperature. After stirring for an additional 2 h at room temperature, a saturated aq NH₄Cl solution was poured into the reaction mixture, and insoluble substances were filtrated off through a Celite bed. The Celite bed was washed with AcOEt repeatedly, and the filtrate was washed with brine. The aqueous phase was extracted with AcOEt, and the combined organic extracts were dried over Na₂SO₄ and evaporated under reduced pressure. The oily residue was subjected onto alumina column chromatography (hexane/CH₂Cl₂ = 2/1). The yellow fraction (R_f = 0.4) was collected and evaporated to give a yellow solid (3.4 g, 43%). Mp 46–47 °C; ¹H NMR (CDCl₃, 400 MHz): δ 1.52 (s, 18H, t-Bu), 2.13–2.22 (m, 2H, CH₂CH₂CH₂), 2.31–2.43 (m, 2H, CH₂CH₂CH₂), 2.50–2.69 (m, 2H, CH₂CH₂CH₂), 5.95 (broad s, 2H, pyrrole-β), 6.07 (pseudo t, 2H, J = 3.4 Hz, pyrrole-β), 7.22–7.25 (m, 2H, pyrrole-α), 7.15–7.25 (m, 3H, P-Ph-m,p), 7.34–7.40 (m, 2H, P-Ph-o); ¹³C{¹H} NMR (CDCl₃, 100 MHz): δ 27.9, 28.8, 29.0, 83.2, 110.6, 114.6 (d, J_P–C = 6.6 Hz), 122.5, 128.1 (d, J_P–C = 7.4 Hz), 128.7, 129.1, 129.7 (d, J_P–C = 24.0 Hz), 133.0 (d, J_P–C = 15.7 Hz), 133.5 (d, J_P–C = 19.0 Hz), 149.2, 155.5 (d, J_P–C = 11.6 Hz); ³¹P{¹H} NMR (CDCl₃, 162 MHz): δ + 45.9; UV-vis (CH₂Cl₂): λ_max (ɛ) 371 nm (10600); HRMS (FAB) calculated for C₃₁H₃₅N₂O₄P (M⁺): 530.2334. Found: m/z 530.2353.

According to a similar procedure, compound 3b was prepared from 2b. However, 3b could not be purified completely, as the remaining 2b was difficult to separate from the crude product. ¹H NMR (CD₂Cl₂, 400 MHz): δ 1.21 (s, 18H, t-Bu), 2.17–2.27 (m, 2H, CH₂CH₂CH₂), 2.38–2.50 (m, 2H, CH₂CH₂CH₂), 2.68–2.80 (m, 2H, CH₂CH₂CH₂), 6.01–6.13 (m, 4H, pyrrole-β), 7.20–7.40 (m, 15H, Ph); ³¹P{¹H} NMR (CDCl₃, 162 MHz): δ + 45.6; HRMS (FAB) calculated for C₄₃H₄₃N₂O₄P (M⁺): 682.2960. Found: m/z 682.2989.

3.3 Synthesis of 4a,b

To a THF solution (1.5 mL) of 3a (100 mg, 0.188 mmol) was added a MeOH solution (3 mL) of NaOMe (102 mg, 1.88 mmol) at room temperature. After stirring at 70 °C for 16 h, water (10 mL) and diethyl ether (10 mL) were added. The aqueous phase was extracted with diethyl ether and the combined organic extracts were dried over Na₂SO₄. After removal of solvents under reduced pressure, the resulting mixture was subjected to alumina column chromatography (hexane/CH₂Cl₂ = 2/1 to CH₂Cl₂/acetone = 6/1). The yellow fraction (R_f = 0.5; hexane/CH₂Cl₂ = 2/1) was collected, evaporated, and reprepicipated from CH₂Cl₂/hexane to give 4a (47 mg, 76%) as a brownish red solid. ¹H NMR (CDCl₃, 400 MHz): δ 2.33–2.49 (m, 2H, CH₂CH₂CH₂), 2.63–2.72 (m, 2H, CH₂CH₂CH₂), 2.78–2.87 (m, 2H, CH₂CH₂CH₂), 6.17–6.18 (m, 4H, pyrrole-β), 6.65–6.66 (m, 2H, pyrrole-α), 7.26–7.34 (m, 3H, P-Ph-m,p), 7.53–7.57 (m, 2H, P-Ph-o), 8.01 (broad s, 2H, NH); ¹³C{¹H} NMR (CDCl₃, 100 MHz): δ 28.6, 29.2, 107.3 (d, J_P–C = 5.0 Hz), 109.8, 118.1, 125.7, 129.1 (d, J_P–C = 7.4 Hz), 129.5 (d, J_P–C = 22.3 Hz), 130.0, 133.3 (d, J_P–C = 14.1 Hz), 133.6 (d, J_P–C = 19.8 Hz), 150.7 (d, J_P–C = 8.3 Hz); ³¹P{¹H} NMR (CDCl₃, 162 MHz): δ + 28.1; UV-vis (CH₂Cl₂): λ_max: 416 nm; HRMS (EI) calcd for C₂₁H₁₉N₂P (M⁺): 330.1286. Found: m/z 330.1292.

According to a similar procedure, compound 4b was prepared from 3b. Yield: 28% (based on 2b). Red solid. Mp 194–195 °C; ¹H NMR (CD₂Cl₂, 400 MHz): δ 2.40–2.55 (m, 2H, CH₂CH₂CH₂), 2.70–2.80 (m, 2H, CH₂CH₂CH₂), 2.85–2.94 (m, 2H, CH₂CH₂CH₂), 6.25 (pseudo t, 2H, J = 2.9 Hz, pyrrole-β), 6.47 (pseudo t, 2H, J = 2.9 Hz, pyrrole-β), 7.15–7.19 (m, 2H, P-Ph-m), 7.31–7.37 (m, 11H, Ph), 7.61–7.65 (m, 2H, P-Ph-o), 8.26 (broad s, 2H, NH); ¹³C{¹H} NMR (CDCl₃, 75 MHz): δ 28.6, 29.1, 107.8, 109.6 (d, J_P–C = 5.6 Hz), 118.2, 123.2, 125.5, 126.0, 128.8, 129.2 (d, J_P–C = 8.1 Hz), 130.2 (d, J_P–C = 1.9 Hz), 130.7 (d, J_P–C = 22.3 Hz), 132.02, 132.05, 133.4 (d, J_P–C = 14.3 Hz), 133.5 (d, J_P–C = 19.8 Hz), 151.1 (d, J_P–C = 9.3 Hz), One of the carbon atoms could not be detected clearly; ³¹P{¹H} NMR (CD₂Cl₂, 162 MHz): δ + 28.5; UV-vis (CH₂Cl₂): λ_max (ɛ) 330 (15200), 459 nm (34400); HRMS (EI) calcd for C₃₃H₂₇N₂P (M⁺): 482.1912. Found: m/z 482.1905. Anal. calculated for C₃₃H₂₇N₂P: C, 82.14; H, 5.64; N, 5.81. Found: C, 81.88; H, 5.63; N, 5.75.

3.4 Synthesis of 5a,b

To a THF solution (1.5 mL) of 3a (71.5 mg, 0.135 mmol) was added MeOH (3 mL) solution of NaOMe (72.8 mg, 1.35 mmol) at room temperature. After stirring at 70 °C for 16 h, water (10 mL) and diethyl ether (10 mL) were added. The aqueous phase was extracted with diethyl ether and the combined organic extracts were dried over Na₂SO₄ and evaporated under reduced pressure. The residue was subjected onto alumina column chromatography (hexane/CH₂Cl₂ = 2/1 to CH₂Cl₂/acetone = 6/1). The yellow fraction (R_f = 0.4; hexane/CH₂Cl₂ = 2/1) was collected. After removal of solvents under reduced pressure, the resulting mixture was resolved in CH₂Cl₂. To the resulting solution was added hydrogen peroxide (30% solution in H₂O, 0.1 mL). After stirring 1 h, the mixture was concentrated under reduced pressure, and the solid residue was washed with diethyl ether to give 5a as a red solid (37.1 mg, 79%). Mp 255 °C (dec); ¹H NMR (CDCl₃, 400 MHz): δ 2.00–2.10 (m, 1H, CH₂CH₂CH₂), 2.28–2.40 (m, 1H, CH₂CH₂CH₂), 2.66–2.80 (m, 2H, CH₂CH₂CH₂), 2.81–2.90 (m, 2H, CH₂CH₂CH₂), 6.25 (s, 2H, pyrrole-β), 6.32 (s, 2H, pyrrole-β), 6.80 (s, 2H, pyrrole-α), 7.30–7.45 (m, 3H, P-Ph-m,p), 7.65–7.78 (m, 2H, P-Ph-o), 9.09 (broad s, 2H, NH); ¹³C{¹H} NMR (CDCl₃, 100 MHz): δ 25.8, 29.3 (d, J_P–C = 11.6 Hz), 109.2 (d, J_P–C = 7.4 Hz), 110.1, 116.0, 117.0, 119.9, 126.2 (d, J_P–C = 11.6 Hz), 128.9 (d, J_P–C = 12.4 Hz), 130.4 (d, J_P–C = 10.7 Hz), 132.1 (d, J_P–C = 3.3 Hz), 149.6 (d, J_P–C = 24.8 Hz); ³¹P{¹H} NMR (CDCl₃, 162 MHz): δ + 54.3; IR (KBr): ν = 1165 (PO) cm⁻¹; UV-vis (CH₂Cl₂): λ_max (ɛ) 458 nm (16700); HRMS (EI) calculated for C₂₁H₁₉N₂OP (M⁺): 346.1235; Found: m/z 346.1241.

Compound 5b was prepared from 4b and mCPBA. Yield, 93%. Reddish brown solid. Mp 260 °C (dec); ¹H NMR (CDCl₃, 400 MHz): δ 2.00–2.15 (m, 1H, CH₂CH₂CH₂), 2.31–2.42 (m, 1H, CH₂CH₂CH₂), 2.69–2.80 (m, 2H, CH₂CH₂CH₂), 2.80–2.94 (m, 2H, CH₂CH₂CH₂), 6.38 (pseudo t, 2H, J = 2.9 Hz, pyrrole-β), 6.55 (s, 2H, pyrrole-β), 7.18–7.21 (m, 2H, P-Ph-m), 7.27–7.49 (m, 11H, Ph), 7.79–7.84 (m, 2H, P-Ph-o), 9.28 (broad s, 2H, NH); ¹³C{¹H} NMR (CDCl₃, 100 MHz): δ 25.8, 29.3 (d, J_P–C = 11.6 Hz), 108.2, 111.4 (d, J_P–C = 7.4 Hz), 115.8, 116.7, 123.8, 126.6, 127.6 (d, J_P–C = 12.4 Hz), 128.9, 129.1 (d, J_P–C = 12.4 Hz), 130.0, 130.3 (d, J_P–C = 10.7 Hz), 130.9, 131.7, 132.2 (d, J_P–C = 2.5 Hz), 133.8, 149.8 (d, J_P–C = 25.6 Hz); ³¹P{¹H} NMR (CDCl₃, 162 MHz): δ + 55.6; IR (KBr): ν = 1180 (PO) cm⁻¹; UV-vis (CH₂Cl₂): λ_max (ɛ) 322 (21000), 506 nm (24100); HRMS (EI) calculated for C₃₃H₂₇N₂OP (M⁺): 498.1861; Found: m/z 498.1858.

3.5 Synthesis of 6a,b

To a THF solution (3 mL) of 4a (198 mg, 0.373 mmol) was added MeOH solution (6 mL) of NaOMe (204 mg, 3.77 mmol) at room temperature. After stirring at 70 °C for 16 h, water (10 mL) and diethyl ether (10 mL) were added. The aqueous phase was extracted with diethyl ether and the combined organic extracts were dried over Na₂SO₄. After removal of solvents under reduced pressure, the resulting mixture was resolved in CH₂Cl₂. To the resulting solution was added elemental sulfur (60.3 mg, 1.88 mmol). After stirring 3 h, the mixture was concentrated under reduced pressure, and the solid residue was subjected to alumina column chromatography (hexane/CH₂Cl₂ = 2/1). The orange fraction (R_f = 0.2) was collected, evaporated, and reprecipitated from CH₂Cl₂/hexane to give 6a as an orange solid (117 mg, 86%). Mp 220 °C (dec); ¹H NMR (CDCl₃, 400 MHz): δ 2.13–2.25 (m, 1H, CH₂CH₂CH₂), 2.33–2.47 (m, 1H, CH₂CH₂CH₂), 3.20–3.40 (m, 4H, CH₂CH₂CH₂), 6.21–6.23 (m, 2H, pyrrole-β), 6.30 (s, 2H, pyrrole-β), 6.76–6.78 (m, 2H, pyrrole-α), 7.33–7.44 (m, 3H, P-Ph-m,p), 7.84–7.90 (m, 2H, P-Ph-o), 9.61 (broad s, 2H, NH); ¹³C{¹H} NMR (CDCl₃, 100 MHz): δ 26.9, 29.3 (d, J_P–C = 10.7 Hz), 109.7 (d, J_P–C = 8.3 Hz), 109.8, 117.8, 118.6, 119.6, 126.3 (d, J_P–C = 12.4 Hz), 128.9 (d, J _P–C = 12.4 Hz), 130.2 (d, J_P–C = 11.6 Hz), 132.0 (d, J_P–C = 3.3 Hz), 151.0 (d, J _P–C = 22.3 Hz); ³¹P{¹H} NMR (CDCl₃, 162 MHz): δ + 63.4; UV-vis (CH₂Cl₂): λ_max (ɛ) 454 nm (17,500); HRMS (EI) calculated for C₂₁H₁₉N₂PS (M⁺): 362.1007; Found: m/z 362.1011.

According to a similar procedure, compound 6b was prepared from 4b and elemental sulfur. Yield, 82%. Brown solid. Mp 286 °C (dec); ¹H NMR (CD₂Cl₂, 400 MHz): δ 2.20–2.35 (m, 1H, CH₂CH₂CH₂), 2.35–2.52 (m, 1H, CH₂CH₂CH₂), 2.75–2.90 (m, 4H, CH₂CH₂CH₂), 6.41 (s, 2H, pyrrole-β), 6.57 (s, 2H, pyrrole-β), 7.20–7.25 (m, 2H, P-Ph-m), 7.30–7.52 (m, 11H, Ph), 7.90–8.05 (m, 2H, P-Ph-o), 9.96 (broad s, 2H, NH); ¹³C{¹H} NMR (CDCl₃, 100 MHz): δ 27.0, 29.4 (d, J_P–C = 11.6 Hz), 107.7, 111.7 (d, J_P–C = 8.3 Hz), 117.4, 118.2, 123.8, 126.6, 127.6 (d, J_P–C = 12.4 Hz), 128.9, 129.0, 129.1 (d, J_P–C = 12.4 Hz), 130.2 (d, J_P–C = 11.6 Hz), 131.8, 132.1 (d, J_P–C = 3.3 Hz), 133.6, 151.3 (d, J_P–C = 21.5 Hz), Two of the carbon atoms could not be detected clearly; ³¹P{¹H} NMR (CD₂Cl₂, 162 MHz): δ + 63.6; UV-vis (CH₂Cl₂): λ_max (ɛ) 320 (20300), 507 nm (22700); HRMS (EI) calculated for C₃₃H₂₇N₂PS (M⁺): m/z 514.1633; Found: m/z 514.1630.

3.6 Synthesis of 7

A mixture of 4b (10 mg, 0.021 mmol), AuCl•SMe2 (6.1 mg, 0.021 mmol), and CH₂Cl₂ (1 mL) was stirred for 30 min at room temperature. The mixture was then concentrated under reduced pressure and subjected to silica gel column chromatography (CH₂Cl₂). The red fraction (R_f = 0.7) was collected, evaporated, and reprecipitated from CH₂Cl₂/hexane to give 7 as a dark brown solid (12.0 mg, 80%). Mp 215–216 °C; ¹H NMR (CD₂Cl₂, 400 MHz): δ 2.47–2.55 (m, 2H, CH₂CH₂CH₂), 2.88–3.02 (m, 4H, CH₂CH₂CH₂), 6.46 (broad s, 2H, pyrrole-β), 6.50 (broad s, 2H, pyrrole-β), 7.20–7.26 (m, 2H, P-Ph-m), 7.33–7.58 (m, 11H, Ph), 7.85–7.91 (m, 2H, P-Ph-o), 8.48 (broad s, 2H, NH); ³¹P{¹H} NMR (CD₂Cl₂, 162 MHz): δ + 51.4; UV-vis (CH₂Cl₂): λ_max (ɛ) 504 nm (29900); HRMS (FAB) calculated for C₃₃H₂₇AuClN₂P (M⁺): 714.1266; Found: m/z 714.1267.

3.7 Synthesis of 8

A mixture of 4b (10.8 mg, 0.022 mmol), PtCl₂ (3.0 mg, 0.010 mmol), and CH₂Cl₂/MeOH (2:1, 1.5 mL) was stirred at room temperature. After 3 h, the mixture was concentrated under reduced pressure and then filtered off through a Celite bed. The filtrate was reprecipitated from CH₂Cl₂/hexane to afford 8 (8.3 mg, 67%) as a trans/cis mixture (ca. 1:1). Recrystallization of trans-rich 8 from CH₂Cl₂–hexane by a slow diffusion method afforded single crystals of trans-isomer as a dark brown solid. trans-8: ¹H NMR (CD₂Cl₂, 400 MHz): δ 2.15–2.44 (m, 4H, CH₂CH₂CH₂), 2.71–2.74 (m, 8H, CH₂CH₂CH₂), 6.43 (broad s, 2H, pyrrole-β), 6.55 (broad s, 2H, pyrrole-β), 7.12–7.25 (m, 18H, Ph and P-Ph-m,p), 7.39–7.42 (m, 8H, Ph), 7.55–7.71 (m, 4H, P-Ph-o), 9.73 (broad s, 4H, NH); ³¹P{¹H} NMR (CD₂Cl₂, 162 MHz): δ + 51.6 (J_Pt–P = 2244 Hz); UV-vis (CH₂Cl₂): λ_max (ɛ) 450 (23000), 533 nm (25000); HRMS (FAB) calculated for C₆₆H₅₄N₄Cl₂P₂Pt (M⁺): 1229.2849. Found: m/z 1229.2854. cis-8 (cis/trans > 95/5): ¹H NMR (CDCl₃, 400 MHz): δ 2.05–2.30 (m, 4H, CH₂CH₂CH₂), 2.30–2.61 (m, 8H, CH₂CH₂CH₂), 6.19 (broad s, 2H, pyrrole-β), 6.31 (broad s, 2H, pyrrole-β), 7.05–7.34 (m, 18H, Ph and P-Ph-m,p), 7.40–7.50 (m, 8H, Ph), 7.61–7.75 (m, 4H, P-Ph-o), 9.79 (broad s, 4H, NH); ³¹P{¹H} NMR (CD₂Cl₂, 162 MHz): δ + 31.5 (J_Pt–P = 3177 Hz); UV-vis (CH₂Cl₂): λ_max (ɛ) 541 nm; HRMS (FAB) calculated for C₆₆H₅₄N₄Cl₂P₂Pt (M⁺): 1229.2849. Found: m/z 1229.2837.

3.8 X-ray structural determination

Single crystals of 5b, 7, and trans-8 were grown from CH₂Cl₂–MeOH (for 5b and 7) or CH₂Cl₂–hexane (for 8). All measurements were made on a Rigaku Mercury-8 CCD area detector with graphite monochromated Mo–Kα radiation at 143 K. The structures were solved by a direct method (SIR92) [19] and expanded using Fourier techniques [20]. Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined using the rigid model. All calculations were performed using CrystalStructure¹² crystallographic software package except for refinement, which was performed using SHELXL-97 [21]. Crystal data and structural refinement parameters are as follows. 5b: C₃₃H₂₇N₂OP, red block, 0.30 × 0.20 × 0.08 mm, MW = 498.54, monoclinic, space group = P2₁/n, a = 11.092(2) Å, b = 18.861(3) Å, c = 11.952(2) Å, β = 98.244(2)°, V = 2474.5(8) ų, Z = 4, D_c = 1.338 g cm⁻³, μ = 1.42 cm⁻¹, 28128 observed, 5653 unique, 335 variables, R_w = 0.1239, R = 0.0495 (I > 2.0σ(I)), GOF = 1.152. 7: C₃₃H₂₇AuClN₂P, red block, 0.25 × 0.20 × 0.08 mm, MW = 714.95, monoclinic, space group = P2₁/c, a = 11.116(3) Å, b = 19.045(5) Å, c = 13.303(4) Å, β = 97.365(5)°, V = 2793.1(14) ų, Z = 4, D_c = 1.700 g cm⁻³, μ = 54.64 cm⁻¹, 31812 observed, 6257 unique, 344 variables, R_w = 0.1311, R = 0.0533 (I > 2.0σ(I)), GOF = 1.175. trans-8: C₆₆H₅₄Cl₂N₄P₂Pt, dark brown block, 0.25 × 0.20 × 0.10 mm, MW = 1231.06, monoclinic, space group = P2₁/c, a = 15.424(9) Å, b = 11.803(6) Å, c = 18.046(10) Å, β = 114.382(8)°, V = 2992(3) ų, Z = 2, D_c = 1.366 g cm⁻³, μ = 25.30 cm⁻¹, 19757 observed, 6716 unique, 341 variables, R_w = 0.1761, R = 0.0653 (I > 2.0σ(I)), GOF = 1.087. CCDC-762249 (5b), CCDC-762250 (7), and CCDC-763570 (trans-8) 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.

3.9 Fluorescence lifetime measurements

The time profiles of fluorescence in CH₂Cl₂ at 21 °C were measured by a streak camera, using the second harmonic output (400 nm, 150 fs) of a Ti:Sapphire regenerative amplifier system as an excitation source. The fluorescence decays of 4b and 7 were well fitted by the single exponential component.

3.10 Computational details

The geometry optimization was performed by the B3LYP method [22] with basis sets of 6-31G* [23] for C, H, N, O, P, and Cl atoms and LANL2DZ [24] for Pt and Au atoms. X-ray structures are used as initial geometries for 7m and 8m. We carried out vibration analysis with the B3LYP method to ascertain that each optimized geometry was not in saddle but in equilibrium points. The excited states and oscillator strengths are evaluated by the TD-B3LYP method, where forty excited states were solved. In Table 2, the states whose oscillator strengths are less than 0.2 are not included. All calculations were carried out with the Gaussian 03 package [25].