Phosphorus complexes of a triply-fused [24]pentaphyrin

Abstract

Treatment of an N-fused [24]pentaphyrin with POCl₃ in the presence of triethylamine gave a triply-fused pentaphyrin P=O complex through an N-fusion reaction and oxidative C–C bond formation between the pyrrolic β-positions. This complex displays strong antiaromatic character due to its 24π-electronic network as evinced by its ¹H NMR chemical shifts, absorption spectrum, and NICS calculation. The P=O complex was reduced with BH₃·SMe₂ to provide P–BH₃ complex with concurrent hydrogenation at the pyrrolic β-positions as a rare case of porphyrinoid P(III) complex.

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In recent years, there are increasing interests in the chemistry of expanded porphyrins because of their unique optical, electrochemical, and coordination properties, which are mostly derived from their large conjugated circuits and conformational flexibilities that are not shared by porphyrins.¹ Among these, N-fused reactions of expanded porphyrins are attractive because the resultant tricyclic ring system imposes a planar and rigid substructure, hence causing large structural and electronic perturbations.^2–4N-Fused structures were formed either by nucleophilic attack of a pyrrolic nitrogen atom to the neighboring β-carbon atom^2,3,4a,b or by nucleophilic substitution of ortho-halogen at a meso-aryl substituent with an adjacent pyrrolic nitrogen atom.^4c–g In particular, N-fusion strategy has been extensively used to create novel structures from N-confused porphyrinoids by Furuta et al.^2,3

meso-Aryl substituted pentaphyrins exist only as N-fused forms that leave a smaller cavity than those of porphyrins.^4a,b As a continuation of our research toward the exploration of novel porphyrinoids,^1h we examined phosphorus insertion into an N-fused pentaphyrin, since the phosphorus atom is smaller than the usual metal ions and is potentially a good fit for the small cavity of an N-fused [24]pentaphyrin. Additionally the phosphorus insertion has been recently shown to be effective in the creation of novel porphyrinoids such as triazacorroles,^5a,bN-fused porphyrins,^5cN-fused telluraporphyrins,^5d an expanded isophlorin,^5e and a Möbius antiaromatic molecule.^5f In this paper, we report that phosphorus insertion into N-fused [24]pentaphyrin 1 triggered further double fusion reactions to provide a phosphorus complex of a triply-fused structure, 2. This phosphoramide P=O complex 2 was transformed into a P=S complex 3 and P–BH₃ complex 4.

Results and discussion

Synthesis of phosphorus complex

First, we examined the reaction of meso-pentakis(pentafluorophenyl)-substituted N-fused [24]pentaphyrin 1 with POCl₃. After extensive optimizing experiments, treatment of 1 with POCl₃ (100 equiv) in the presence of triethylamine at 120 °C for 12 h gave the best result to yield phosphoramide P=O complex 2 in 66% yield after chromatographic separation over a silica gel column (Scheme 1).

Scheme 1 Synthesis of phosphorus complex 2.

The high-resolution electrospray ionization time-of-flight (HR-ESI-TOF) mass spectrum of 2 showed the parent negative ion peak at m/z = 1273.9612 (calcd for C₅₅H₆F₂₄N₅OPCl, [M + Cl]⁻: 1273.9621), thus indicating the insertion of a P=O moiety and the elimination of HF, and H₂. Fortunately, we obtained single crystals of 2 suitable for X-ray diffraction analysis by vapor diffusion of hexane into its chlorobenzene solution.‡ The unit cell contained two independent molecules in the unsymmetric unit and the crystal structure revealed the phosphorus insertion and a triply-fused structure (Fig. 1). The inserted phosphorus atom was bound to N(1), N(2) and one β-carbon atom, C(12), with distances of 1.689(5) and 1.697(6) Å for P–N(1), 1.670(5) and 1.670(6) Å for P–N(2), and 1.732(7) and 1.735(8) Å for P–C(12), respectively. Curiously, another N-fusion reaction occurred between N(5) and the pentafluorophenyl substituent at C(55), and C–C bond formation occurred between C(13) and C(23) to form an unprecedented triply-fused skeleton. The C(13)–C(23) bond length is 1.409(9) and 1.401(10) Å, being slightly longer than the typical C–C bond length in a benzene ring (1.39 Å). The ³¹P NMR spectrum of 2 displays a signal at δ = 7.12 ppm consistent with the phosphoramide moiety.⁵ The ¹H NMR spectrum of 2 exhibits five peaks due to the β-protons at δ = 6.35, 6.05, 4.59, 4.43, and 4.34 ppm (Fig. 2a). These large upfield shifts of the outer β-protons are similar to those of antiaromatic porphyrinoid, such as the doubly N-fused pentaphyrin and [28]hexaphyrin mono-Au^III complex (δ = 6.38–4.07 ppm), indicating a paratropic ring current and thus antiaromatic character.^3a,6a Consistent with this conjecture, the UV/Vis absorption spectrum shows a blue-shifted Soret-like band and almost no Q-band-like absorption band (Fig. 4). These features are typical of antiaromatic expanded porphyrins.^6b,c The antiaromaticity of 2 is derived from its rigid planar conformation forced by the P=O-incorporated and triply-fused structure. The formation of 2 may be accounted for in terms of initial electrophilic attack of the phosphoryl group by C(12) followed by nucleophilic attack of C(23) at C(13) and subsequent oxidation, and final N-fusion reaction between N(5) of pyrrole E and the meso-pentafluorophenyl substituent at C(55) (See Supporting Information†).

Fig. 1 X-Ray crystal structure of P=O complex 2: (a) top view and (b) side view. One of the two independent molecules in the unsymmetric unit cell is shown. Thermal ellipsoids represent 30% probability and meso-pentafluorophenyl substituents and solvent molecules are omitted for clarity.

Fig. 2 ¹H NMR spectra of (a) 2, (b) 3, (c) 4 and (d) 5 in CDCl₃ at 25 °C. Peaks marked with * are due to residual solvents and impurities.

Phosphorus(III) complex

Then, we attempted to obtain the corresponding P(III) complex, since phosphorus-incorporated porphyrinoids have been confined to P(V) species and no P(III) porphyrinoid has been reported in the literature. Trivalent phosphines are usually prepared from pentavalent phosphine oxides by reduction with silane reducing reagents.⁷ However, treatment of the P=O complex 2 with trichlorosilane or phenylsilane resulted in complete recovery of 2 without formation of the corresponding phosphorus(III) complex. Therefore, we planned out two strategies for the synthesis of the phosphorus(III) complex, (a) conversion of the complex 2 to the corresponding phosphine sulfide (P=S) complex followed by its desulfurization,⁸ or (b) reduction to the phosphine-borane (P–BH₃) complex followed by liberation of the desired P(III) complex with a strong base.⁹ First, the P=S complex 3 was synthesized by the reaction of 2 with Lawesson's reagent in 84% yield (Scheme 2).¹⁰ The HR-ESI-TOF mass spectrum of 3 displays the parent negative ion peak at m/z = 1289.9379 (calcd for C₅₅H₆F₂₄N₅PSCl, [M + Cl]⁻: 1289.9392). The ³¹P NMR spectrum of 3 shows a peak in a downfield region at δ = 58.55 ppm in accord with the formation of a P=S moiety.^10b The structure of 3 was also unambiguously revealed by X-ray diffraction analysis to be a triply-fused structure, which is essentially the same as that of 2 (Fig. 3a).‡ The ¹H NMR spectrum of 3 exhibits six signals due to β-protons at δ = 6.18, 5.88, 4.36, 4.26, 4.16, and 4.13 ppm, which suggests a paratropic ring current and hence antiaromaticity for 3 (Fig. 2b). The UV/Vis absorption spectrum of 3 is similar to that of 2. Desulfurization of 3 was attempted with P(nBu)₃, P(NMe₂)₃, and RANEY® nickel but these attempts failed, giving merely complicated mixtures.

Scheme 2 Attempts to synthesize the phosphorus(III) complex.

Fig. 3 X-Ray crystal structures of (a) P=S complex 3, (b) P–BH₃ complex 4 and (c) chlorin-type P=O complex 5: top view (left) and side view (right). Thermal ellipsoids represent (a, b) 50% and (c) 30% probability and meso-pentafluorophenyl substituents and solvent molecules are omitted for clarity.

In the next step, the P=O complex 2 was reduced with BH₃·SMe₂^9b to provide the P–BH₃ complex 4 in 54% yield (Scheme 2). The HR-ESI-TOF mass spectrum of 4 shows the parent negative ion peak at m/z = 1238.0370 (calcd for C₅₅H₁₀F₂₄N₅PB, [M–H]⁻: 1238.0398), in line with the assigned P–BH₃ structure that bears a hydrogenated pyrrole ring (pyrrole B) in Fig. 3b. X-Ray diffraction analysis of 4 reveals a C(7)–C(8) single bond with bond length of 1.544(5) Å, which is distinctly longer than those of C(2)–C(3) and C(17)–C(18) double bonds; 1.390(5) and 1.375(5) Å, respectively. In fact, two signals due to sp³-hybridized carbon atoms C(7) and C(8) are observed at δ = 27.0 and 24.5 ppm in the ¹³C NMR spectrum. The ¹H NMR spectrum of 4 exhibits four peaks due to the β-protons of the reduced pyrrole B at δ = 2.65, 2.53, 2.37, and 2.18 ppm, and four peaks due to the β-protons of the pyrroles A and D at δ = 7.22, 6.96, 6.33, and 5.57 ppm (Fig. 2c). These data indicate that the complex 4 is a nonaromatic macrocycle. In the ³¹P and ¹¹B NMR spectra, broad peaks at δ = 119.22 and −32 ppm were observed, respectively. The UV/Vis absorption spectrum of 4 shows weak and broad absorption at around 800 nm, which is similar to those of linear oligopyrroles (Fig. 4). Thus, it may be concluded that the macrocyclic conjugation becomes weak and almost vanished in 4 as a consequence of the hydrogenation at the pyrrole B.

Fig. 4 UV/Vis absorption spectra of 1 (black), 2 (red), and 4 (blue) in CH₂Cl₂.

Many phosphorus complexes of porphyrins and expanded porphyrins were prepared so far but the phosphorus atoms within porphyrinic cavities were confined to exist as a phosphorus(V) state.^11,12 Thus, we attempted to liberate the BH₃ moiety from 4 to generate a free phosphorus(III) species. Treatment of 4 with an excess amount of triethylamine gave 2 in 50% yield under ambient conditions. This result suggested that the detachment of BH₃ from a fused pentaphyrin P(III) complex was possible but the resultant phosphorus(III) was readily oxidized under aerobic conditions, along with concurrent dehydrogenation at the β-position. A different complex was obtained in 31% yield along with 2 when the reaction was run with freshly purified triethylamine and solvent under nitrogen atmosphere using Schlenk techniques (Scheme 2). This complex was characterized as a chlorin-type P=O complex, 5, on the basis of the X-ray diffraction analysis (Fig. 3c), which revealed a P=O moiety and a single bond between C(7) and C(8). The HR-ESI-TOF mass spectrum, ¹H-NMR spectrum, and ³¹P-NMR spectrum are all consistent with the structure of 5. Therefore, it is conceivable that a free P(III) species is also quite susceptible towards oxygenation. In this regard, it is worth noting that complex 4 can be regarded as a phosphorus(III) complex, which is rare in the chemistry of phosphorus porphyrinoids.

Electrochemistry

The electrochemical properties were studied by cyclic voltammetry in CH₂Cl₂versusferrocene/ferrocenium ion (Fc/Fc⁺) with tetrabutylammonium hexafluorophosphate as an electrolyte (See Supporting Information†). The P=O complex 2 underwent two reversible oxidations at 0.49 and 0.81 V, and two reversible reductions at −0.80 and −1.17 V, which allowed an estimation of the electrochemical HOMO–LUMO gap to be small, 1.29 eV. The electrochemical properties of 3 are similar to those of 2, and the HOMO–LUMO gap was estimated to be 1.27 eV. These data are in line with the assignments of 2 and 3 as antiaromatic species.^1h On the other hand, since the complexes 4 and 5 are less stable than the complexes 2 and 3 under electrochemical measurement, the voltammograms were obtained with a faster scan rate (0.1 V s⁻¹). The two reductions were observed at −1.02 and −1.54 V as reversible processes, but the two oxidations were observed as irreversible processes for 4 at 0.57 and 0.90 V. The complex 5 displayed two reversible reductions at −1.01 and −1.55 V, and one reversible oxidation at 0.60 V and one irreversible oxidation at 0.91 V. The instability of 4 and 5 under oxidative conditions suggests that the chlorin-like structures are prone to be oxidized under the electrochemical measurement conditions. The electrochemical HOMO–LUMO gaps have been estimated to be 1.59 and 1.61 eV for 4 and 5, respectively.

Theoretical calculations

We performed DFT calculations (B3LYP/6-31G(d)) for 2, 3, 4, and 5 (See Supporting Information†).¹³ The molecular orbitals of 2 and 3 revealed the non-degenerated HOMO/HOMO − 1 and LUMO/LUMO + 1, and the small HOMO–LUMO gaps (1.47 and 1.45 eV). The large positive nuclear independent chemical shift (NICS)¹⁴ values at the centroid of macrocycles 2 and 3 were calculated (+23.09 ppm for 2 and +22.80 ppm for 3). These computational results are consistent with the antiaromatic character of 2 and 3. In contrast, the NICS values of 4 and 5 were calculated to be +0.17 and +1.36 ppm, respectively, indicating their nonaromatic characters.

Conclusions

In summary, the treatment of 1 with POCl₃ gave the P=O complex 2 having a triply-fused macrocycle through the N-fusion reaction and C–C bond formation between the pyrrolic β-positions. The P=O complex 2 was converted to the P=S complex 3 by the action of Lawesson's reagent, while 2 was reduced to the P–BH₃ complex 4 with BH₃·SMe₂ with the concomitant hydrogenation at the pyrrolic β-position. The P–BH₃ complex 4 is, to the best of our knowledge, the first phosphorus(III) complex of porphyrinoids. The complexes 2 and 3 exhibit strong 24π antiaromatic characters, while the complexes 4 and 5 are nonaromatic macrocycles due to the weak macrocyclic π-conjugation. Exploration of other phosphorus complexes of expanded porphyrins are worthy of further study.

Acknowledgements

This work was supported by Grants-in-Aid (No. 22245006 (A), and 20108001 “pi-Space”) from MEXT, Japan. T.H. acknowledges a JSPS Fellowship for Young Scientists.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: General experimental methods, HR-ESI-TOF mass spectra, UV/Vis absorption spectra, NMR spectra, cyclic voltammograms, X-ray crystal structures and results of DFT calculations. CCDC reference numbers 827554-827556 and 842955. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1sc00653c

‡ Crystallographic data for 2: 4(C₅₅H₆F₂₄N₅OP)·13(C₆H₅Cl), M_r = 6421.63, monoclinic, space groupP2₁/a (No.14), a = 29.5322(5), b = 10.3705(2), c = 41.5808(8) Å, β = 99.1602(8)°, V = 12572.3(4) ų, T = 93(2)K, ρ_c = 1.696 g cm⁻³, Z = 2, 114624 reflections measured, 19582 unique (R_int = 0.1338), R₁ = 0.0993 [I > 2.0σ(I)], wR₂ = 0.2961 (all data), GOF = 0.988; Crystallographic data for 3: C₅₅H₆F₂₄N₅PS·3(C₇H₈), M_r = 1532.09, monoclinic, space groupP2₁/c (No.14), a = 19.4591(5), b = 8.6913(2), c = 37.1609(11) Å, β = 94.5998(18)°, V = 6264.6(3) ų, T = 93(2)K, ρ_c = 1.624 g cm⁻³, Z = 4, 60930 reflections measured, 11391 unique (R_int = 0.1115), R₁ = 0.0675 [I > 2.0σ(I)], wR₂ = 0.1500 (all data), GOF = 1.057; Crystallographic data for 4: 2(C₅₅H₁₁BF₂₄N₅P)·3(C₆H₁₄)·C₇H₈, M_r = 2829.59, monoclinic, space groupC2/c (No.15), a = 30.1073(6), b = 18.0820(3), c = 22.4608(4) Å, β = 106.2569(10)°, V = 11738.8(4) ų, T = 93(2)K, ρ_c = 1.601 g cm⁻³, Z = 4, 55183 reflections measured, 10540 unique (R_int = 0.1381), R₁ = 0.0759 [I > 2.0σ(I)], wR₂ = 0.1619 (all data), GOF = 1.001. Crystallographic data for 5: C₅₅H₈F₂₄N₅OP·C₂H₃N·0.75(CCl₄)·0.5(water), M_r = 1405.92, triclinic, space groupP1̄ (No.2), a = 10.1849(5), b = 14.9773(8), c = 17.6759(10) Å, α = 85.568(3), β = 88.271(3), γ = 76.766(3)°, V = 2616.7(2) ų, T = 93(2)K, ρ_c = 1.784 g cm⁻³, Z = 2, 21103 reflections measured, 7529 unique (R_int = 0.1076), R₁ = 0.1190 [I > 2.0σ(I)], wR₂ = 0.3747 (all data), GOF = 1.076.

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