Synthesis and crystal structure of a meso-meso directly linked bisporphyrin

Article abstract of DOI:10.1515/znb-2011-0601

We report the synthesis and characterization of a meso-meso directly linked bisporphyrin 6 bearing both alkyl and aryl residues. Its oxidative fusing results in the corresponding meso-meso β-β,β-β- linked bisporphyrin. The first crystal structure analysis

Full text of DOI:10.1515/znb-2011-0601

Synthesis and Crystal Structure of a Meso-Meso Directly
Linked Bisporphyrin
Mathias O. Sengea,b, Monica Pinteaa, and Aoife A. Ryana
a School of Chemistry, SFI Tetrapyrrole Laboratory, Trinity College Dublin, Dublin 2, Ireland
b Medicinal Chemistry, Institute of Molecular Medicine, Trinity Centre for Health Sciences,
Trinity College Dublin, St James’s Hospital, Dublin 8, Ireland
Reprint requests to Prof. Dr. M. O. Senge. Fax: #353-1-896-8537. E-mail: sengem@tcd.ie
Z. Naturforsch. 2011, 66b, 553 – 558; received March 30, 2011
We report the synthesis and characterization of a meso-meso directly linked bisporphyrin 6 bearing
both alkyl and aryl residues. Its oxidative fusing results in the corresponding meso-meso β-β,β-β-
linked bisporphyrin. The first crystal structure analysis of a meso-meso directly linked porphyrin
dimer (6) has shown the inequivalency of the two porphyrin units with regard to the macrocycle
conformation. Quite distinct mixings of distortion modes were observed for the two aromatic macro-
cycles.
Key words: Porphyrins, Conformational Analysis, Bisporphyrins, Tetrapyrroles, Crystal Structure
Introduction
Directly linked oligoporphyrins 1 have attracted
considerable interest in recent years [1, 2]. This is
mainly related to the preparation of polymers and
of the ability of the meso-meso-linked dimers to be
oxidatively fused to yield meso-meso, and 2 × β-
β-fused systems 2 (linked in the 2, 5, and 7 posi-
tions) [3]. Synthetically the meso-meso linked bis-
porphyrins 1 (n = 1) can be prepared either through
treatment of porphyrins with free meso positions with
AgPF6 [3a], total synthesis [4], AgPF6 + I2 [5], electro-
chemically [6], or Ullmann homocoupling [7]. Our ap-
proach to these systems involved the reaction of a pre-
cursor porphyrin with RLi, in situ oxidation of the an-
ion to the radical anion, followed by radical dimeriza-
tion [8].
Surprisingly, despite the ongoing interest in these
systems, only a few crystal structures of meso-meso-
linked trisporphyrins have been reported [2b, 9]. This
Results and Discussion
Scheme 1 outlines our initial syntheses. In line with
is related to the low solubility of some of these com- earlier results 5,15-diphenylporphyrin [14] was treated
pounds. Likewise, only limited studies on the prepa- with 6 equiv. of n-hexyl lithium followed by direct
ration of unsymmetrical systems thereof or porphyrin addition of DDQ without a hydrolysis step and gave
arrays amenable to further functionalization have been the meso-meso-linked dimer 6 as the main product in
published. In the context of our interest in the prepara- 40 % yield. Formation of this compound was accompa-
tion of near-IR absorbers for applications in photody- nied by the generation of small amounts of the mono-
namic therapy (PDT) [10] we have begun to explore (4) [15] and dihexylated (5) side products. The dimer 6
the chemistry of these compounds with the goal in could easily be converted through standard reactions
mind to make unsymmetrical and amphiphilic systems into the zinc(II) complex 7. The UV/Vis spectrum of
for use in preclinical studies [11 – 13].
this dimer exhibits the characteristic split Soret band
c
0932–0776 / 11 / 0600–0553 $ 06.00 ꢀ 2011 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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M. O. Senge et al. · A Meso-Meso Directly Linked Bisporphyrin
Scheme 1. Synthesis of meso-meso-linked bisporphyrins. i) n-HexLi, DDQ, THF, −78 ◦C; ii) Zn(OAc)2, CH3OH, CH2Cl2,
20 ◦C; iii) Sc(OTf)3, DDQ, toluene, ∆; iv) ZnO, TFA, CH2Cl2; v) Sc(OTf)3, DDQ, toluene, ∆; vi) Sc(OTf)3, DDQ, toluene,
20 ◦C.
Fig. 1. View of the molecular structure
of 6 in the crystal. Displacement ellip-
soids are drawn at the 50 % probability
level. Hydrogen atoms have been omitted
for clarity.
(422 and 455 nm) and Q-bands at 568 and 608 nm in formation of small amounts of the demetallated com-
CH2Cl2. As shown by the crystal structure of the free pound 6. Similarly, an increase of Sc(OTf)3 to 7 equiv.
base (vide supra) the two porphyrin rings lie almost or- and heating to reflux resulted only in the formation of
thogonal to each other, which results in a weak ground the singly linked free base 7. As an alternative route
state interaction between the two chromophores [16].
Next we attempted the preparation of the respective
triply linked dimer 9. Reaction of 6 with 5 equiv. of
DDQ and 5 equiv. of Sc(OTf)3 at 50 ◦C gave no fused
product, and raising the temperature resulted in the
we prepared the zinc(II) complex 8 from 4. Reaction
with 5◦equiv. of Sc(OTf)3 and 5 equiv. DDQ in toluene
at 50 C gave only the meso-meso-linked dimer 7 in
20 % yield. Surprisingly, performing the same reaction
at room temperature for 18 h resulted in the formation
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M. O. Senge et al. · A Meso-Meso Directly Linked Bisporphyrin
555
of the desired triply linked bisporphyrin 9 in very good agram shown in Fig. 2. Note, that the twist an-
yield. Although identification proved to be difficult due gle is different from the one in a related meso-β-
to solubility and stability issues, its formation could linked porphyrin, 2-(5,10,15-triphenylporphyrin-20-
clearly be proven by mass spectrometry and UV/Vis yl)-5,10,15,20-tetraphenylporphyrin, where an angle
spectroscopy. The absorption spectrum is very similar of 106◦ was observed [19].
to those of related systems in the literature [17] and ex-
hibits two strong Soret-like bands at 426 and 576 nm solvate (Fig. 2). Interestingly the compound does not
The compound crystallized as a dichloromethane
and a long wavelength absorption at 1135 nm in THF.
For compound 6 we were able to grow crystals
suitable for X-ray crystallographic structure analy-
sis. The structure is shown in Fig. 1 and presents
the first structure of a meso-meso directly linked
bisporphyrin [9a]. The meso-meso single bond link-
age is clearly indicated by the C5–C25 bond length
crystallize with an inversion center. The first obvious
difference is the conformation of the two hexyl side
chains, one of which shows a more folded arrange-
ment. The packing of the molecules is characterized
by very weak π-π interactions between neighboring
˚
ring II units. The mean plane separation is 3.34 A, with
a slip angle of 47.3◦ and a center-to-center separation
˚
˚
of 4.924 A. The methylene chloride solvate molecules
of 1.503(3) A. The two tetrapyrrole rings are
twisted against each other by 69.1(2)◦. This value
is similar to that found for the phenyl twist angle
in various meso-tetraarylporphyrins [18]. The dihe-
are located in the voids between these units. The short-
˚
est intermolecular contact is Cl1S···H405 (2.903 A).
More striking differences between the two por-
dral twist is clearly illustrated in the packing di- phyrin units were found upon an analysis of their
conformation. The average deviation from planarity,
˚
i. e. ∆24, was found to be 0.094 A for macrocy-
˚
cle 1 and 0.047 A for macrocycle 2. A more detailed
inspection using a normal structural decomposition
(NDS) [20] analysis revealed significant differences in
the distortion modes and types of the two porphyrin
macrocycles. Porphyrin unit 1 has about equal contri-
butions from sad and ruf out-of-plane distortion and
also exhibits wav(x) distortion. In contrast, porphyrin
unit 2 has smaller contributions from these distortion
types but has a significant wav(y) contribution. With
regard to the in-plane distortions the main differences
are larger B2g contributions in unit 2 and larger A2g
contributions in unit 1 (Fig. 3). Such large difference
Fig. 3. Graphical representation of the displacements along
the lowest-frequency coordinates that best simulate the two
porphyrin units in molecule 6. Signs have been omitted for
ease of comparison.
Fig. 2. View of the crystal structure of 6. Hydrogen atoms
have been omitted for clarity.
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M. O. Senge et al. · A Meso-Meso Directly Linked Bisporphyrin
in the type and composition of the distortion modes 598 nm (4.12). – 1H NMR (400 MHz, CDCl3, TMS):
δ = −2.15 (s, 4H, NH), 0.98 (t, J = 7.6 Hz, 6H, CH2CH2
CH2CH2CH2CH3), 1.44 (m, 4H, CH2CH2CH2CH2
CH2CH3), 1.53 (m, 4H, CH2CH2CH2CH2CH2CH3), 1.88
(m, 4H, CH2CH2CH2CH2CH2CH3), 2.63 (m, 2H, CH2CH2
CH2CH2CH2CH3), 5.14 (t, J = 7.16 Hz, 4H, CH2CH2
CH2CH2CH2CH3), 7.70 (s, 12H, Ar-H), 8.05 (d, J = 3.51,
4H, Hβ ), 8.22 (s, 8H, Ar-H), 8.55 (d, J = 4.68 Hz, 4H, Hβ ),
8.99 (d, J = 4.67 Hz, 4H, Hβ ), 9.60 ppm (d, J = 5.26 Hz,
are rare in bisporphyrins and are probably due to an in-
terplay of packing forces and to the mix of aryl- and
alkyl-substituent types [21].
Experimental Section
For general conditions and instrumentation see refer-
ence [22].
4H, Hβ ). – 13C NMR (150 MHz, CDCl3): δ = 13.7, 22.6,
29.2, 30.1, 31.8, 35.5, 38.8, 52.8, 53.0, 53.4, 53.6, 116.5,
120.3, 121.8, 126.4, 127.4, 131.4, 134.2, 141.9 ppm. –
HRMS ((+)-ES): m/z = 1091.5489 (calcd. 1091.5472 for
C76H67N8, [M+H]+).
5,15-Dihexyl-10,20-diphenylporphyrin (5)
5,15-diphenylporphyrin 3 (150 mg, 0.324 mmol) was
dissolved in 80 mL of dry THF under an argon◦atmosphere,
and the reaction mixture was cooled to −78 C. n-HexLi
(1.4 mL, 3.245 mmol) was added dropwise over 15 min via
syringe. The cold bath was removed and stirring continued
for 1.5 h at r. t. Next 0.5 mL of H2O was added and stirring
continued for 15 min. Then DDQ (730 mg, 3.245 mmol) in
10 mL of THF was added and the reaction mixture stirred
for an additional hour. The reaction mixture was filtered
through silica gel, followed by evaporation of the solvent.
The residue was purified by column chromatography
(n-hexane : ethyl acetate = 100 : 1, v/v). The first fraction
was 5,15-dihexyl-10,20-diphenylporphyrin (5) (14 mg,
0.022 mmol, 7 %) as purple crystals, the second fraction
contained 5-hexyl-10,20-diphenylporphyrin (4) (17 mg,
0.031 mmol, 10 %) [15] as purple crystals, and the third
fraction the directly linked dimer 6 (70 mg, 0.064 mmol,
40 %) as purple crystals. 5: M. p. > 300 ◦C. Rf = 0.5
(n-hexane : ethyl acetate = 1 : 1, v/v). – UV/Vis (CH2Cl2):
Bis{(5-hexyl-10,20-diphenylporphyrin-15-ylato)zinc(II)} (7)
Bis(5-hexyl-10,20-diphenylporphyrin-15-yl) (6) (85 mg,
0.0788 mmol) and zinc acetate dihydrate (205 mg,
0.934 mmol) in 20 mL dried CH2Cl2 were reacted at r. t.
for 3 h. Purification of the product was carried out by
recrystallization from CH2Cl2/MeOH to afford the desired
compound (80 mg, 0.065 mmol, 77 %) as purple crystals.
M. p. > 300 ◦C. Rf = 0.55 (n-hexane : CH2Cl2 = 1 : 2, v/v). –
UV/Vis (CH2Cl2): λmax(logε) = 422 (5.19), 456 (5.21), 568
1
(4.51), 608 nm (4.01). – H NMR (400 MHz, CDCl3): δ =
0.99 (t, J = 7.60 Hz, 6H, CH2CH2CH2CH2CH2CH3), 1.49
(m, 4H, CH2CH2CH2CH2CH2CH3), 1.63 (m, 4H, CH2CH2
CH2CH2CH2CH3), 1.93 (m, 4H, CH2CH2CH2CH2
CH2CH3), 2.67 (m, 4H, CH2CH2CH2CH2CH2CH3), 5.16
(t, J = 7.60 Hz, 4H, CH2CH2CH2CH2CH2CH3), 7.70 (s,
12H, Ar-H), 8.1 (d, J = 3.51 Hz, 4H, Hβ ), 8.25 (m, 8H,
Ar-H), 8.64 (d, J = 3.51 Hz, 4H, Hβ ), 9.08 (d, J = 3.5 Hz,
λ
max(logε) = 419 (5.71), 518 (4.31), 554 (4.09), 598 (3.86),
1
656 nm (3.93). – H NMR (400 MHz, CDCl3): δ = −2.66
(s, 2H, NH), 0.95 (t, J = 7.01 Hz, 3H, CH2CH2CH2CH2
CH2CH3), 1.41 (m, 2H, CH2CH2CH2CH2CH2CH3), 1.51
(m, 2H, CH2CH2CH2CH2CH2CH3), 1.82 (m, 2H, CH2CH2
CH2CH2CH2CH3), 2.51 (m, 2H, CH2CH2CH2CH2
CH2CH3), 4.97 (t, J = 7.16 Hz, 2H, CH2CH2CH2CH2
CH2CH3), 7.81 (s, 6H, Ar-H), 8.20 (s, 4H, Ar-H), 8.89
(d, J = 5.26 Hz, 4H, Hβ ), 9.46 (d, J = 4.68 Hz, 2H, Hβ ),
4H, Hβ ), 9.69 ppm (d, J = 4.1 Hz, 4H, Hβ ). – 13C NMR
(150 MHz, CDCl3): δ = 13.7, 22.3, 29.2, 29.9, 31.5, 38.6,
120.8, 125.9, 126.9, 128.6, 131.4, 131.6, 133.2, 133.8,
142.3, 148.9, 149.6, 154.5 ppm. – HRMS ((+)-LD): m/z =
1214.3636 (calcd. 1214.3680 for C76H62N8Zn2, [M]+).
Alternatively, this compound could be prepared in
20 % yield via reaction of porphyrin 8 with 5 eq. of DDQ
and 5 eq. of Sc(OTf)3 in dry toluene at 50 ◦C for 2 h.
9.62 ppm (d, J = 4.68 Hz, 2H, Hβ ). – 13C NMR (150 MHz,
CDCl3): δ = 13.9, 22.5, 29.5, 30.0, 31.7, 35.1, 35.4, 38.5,
38.6, 111.1, 112.6, 119.9, 121.1, 121.2, 126.3, 126.4,
126.5, 126.6, 126.7, 127.6, 127.8, 134.3, 134.5, 141.1,
141.8 ppm. – HRMS ((+)-ES): m/z = 631.3801 (calcd.
631.3808 for C44H47N4, [M+H]+).
(5-Hexyl-10,20-diphenylporphyrinato)zinc(II) (8)
The free base porphyrin 5 (90 mg, 0.2 mmol) was
dissolved in 20 mL of dried dichloromethane and treated
with zinc oxide (141.2 mg, 1.2 mmol). After the addition of
4 drops of TFA, the reaction mixture first turned green, and
subsequent a color change back to red indicated completion
Bis(5-hexyl-10,20-diphenylporphyrin-15-yl) (6)
The compound was obtained from the reaction de- of the reaction. The product was separated from ZnO and
scribed for the preparation of 5 as purple crystals (70 mg, most of the TFA by filtration through a short silica column.
0.064 mmol, 40 %). M. p. > 300 ◦C. Rf = 0.37 (n- Remaining traces of TFA were removed by washing with
hexane : ethyl acetate = 1 : 1, v/v). – UV/Vis (CH2Cl2): water. After drying with sodium sulfate, the solvent was
λmax(logε) = 418 (5.28), 448 (5.33), 523 (4.64), 558 (4.13), removed in vacuo, and the product was purified by column
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M. O. Senge et al. · A Meso-Meso Directly Linked Bisporphyrin
557
chromatography (silica gel, dichloromethane), followed m/z = 1210.3315 (calcd. 1210.3367 for C76H58N8Zn2,
by recrystallization from CH2Cl2/MeOH to give purple [M+H]+).
◦
crystals (109 mg, 0.178 mmol, 89 %). M. p. > 300 C. Rf =
0.57 (CH2Cl2 : n-hexane = 2 : 1, v/v). – UV/Vis (CH2Cl2):
max(logε) = 414 (4.84), 452 (3.42), 543 nm (3.44). –
X-Ray structure determination of 6
λ
Growth and handling of crystals followed the concept
developed by Hope [23]. Intensity data for the compound
were collected with a Rigaku CCD system using graphite-
1H NMR (400 MHz, CDCl3): δ = 0.95 (t, J = 7.02 Hz,
3H, CH2CH2CH2CH2CH2CH3), 1.44 (m, 2H, CH2CH2
CH2CH2CH2CH3), 1.55 (m, 2H, CH2CH2CH2CH2
CH2CH3), 1.88 (m, 2H, CH2CH2CH2CH2CH2CH3), 2.62
(m, 2H, CH2CH2CH2CH2CH2CH3), 5.13 (t, J = 7.16 Hz,
2H, CH2CH2CH2CH2CH2CH3), 7.83 (s, 6H, Ar-H), 8.26
(m, 4H, Ar-H), 9.07 (d, J = 5.26 Hz, 2H, Hβ ), 9.37 (d, J=
4.68 Hz, 2H, Hβ ), 9.66 (d, J = 4.68 Hz, 2H, Hβ ), 10.19 ppm
˚
monochromated MoKα radiation (λ = 0.71073 A). The in-
tensities were corrected for Lorentz, polarization and ab-
sorption effects. Non-hydrogen atoms were refined with
anisotropic displacement parameters, and hydrogen atoms
were placed into geometrically calculated positions and re-
fined using a ridging model. The structure was solved with
Direct Methods using the SHELXTL PLUS program system
and refined against |F2| with the program XL from SHELX-
97 using all data [24].
(s, 1H, Hmeso). – 13C NMR (100 MHz, CDCl3, TMS): δ =
13.7, 22.3, 29.9, 31.5, 35.4, 38.7, 104.6, 119.4, 121.6, 126.1,
128.3, 130.9, 131.6, 132.0, 133.8, 134.0, 142.3, 149.1,
149.2 ppm. – HRMS ((+)-ES): m/z = 609.1997 (calcd.
609.2006 for C38H33N4Zn, [M+H]+).
Crystal structure data: C76H66N8·CH2Cl2, M = 1176.29,
¯
triclinic, space group P1, a = 11.448(2), b = 15.175(2), c ◦=
˚
18.459(2) A, α = 77.168(4), β = 86.858(5), γ = 82.364(4) ,
Bis{(5-hexyl-10,20-diphenylporphyrin-13,15,17-triylato)-
zinc(II)} (9)
3
−1
˚
V = 3098.0(7) A , Z = 2, T = 98 K, µ(MoKα ) = 0.1 cm
23512 reflections measured, 10810 unique reflections (Rint
;
=
0.0208), 9615 reflections with I ≥ 2.0 σ(I), 805 refined
parameters; refinement against |F2|, R1(I ≥ 2.0 σ(I)) =
0.0582, wR2 (all data) = 0.1521, S = 1.034, ∆ρfin (max) =
(5-Hexyl-10,20-diphenylporphyrinato)zinc(II) (8) (30.5
mg, 0.05 mmol) was dissolved in dry toluene (freshly
distilled over CaH2). The flask was degassed with argon
and heated to 50 ◦C. Then DDQ (56.75 mg, 0.25 mmol)
and Sc(OTf)3 (123.04 mg, 0.25 mmol) in dry toluene were
added, and stirring continued for 3 h. THF (15 mL) was
added, and the reaction mixture was stirred for 60 min.
The reaction mixture was filtered through silica gel and the
solvent removed under reduced pressure. The residue was
purified by column chromatography (CH2Cl2 : CH3OH =
50 : 1, v/v) and gave purple crystals (25 mg, 0.020 mmol,
82 %). M. p. > 300 ◦C. Rf = 0.4 (CH2Cl2 : CH3OH =
40 : 1, v/v). – UV/Vis (THF): λmax(logε) = 426 (3.88), 479
0.793 e A−3. Refinement: Hydrogen atoms at N25 and N27
˚
were located in difference maps and refined using the stan-
dard riding model. For N21, N22, N23, N24 hydrogen atoms
with 50 % occupancy were added using the riding model.
The methylene chloride of solvation is disordered, and the
two Cl atoms were refined as disordered over two positions
with free occupancy variables. Several atoms still show high
librational movement.
CCDC 815159 (6) contains the supplementary crystallo-
graphic 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.
1
(3.59), 576 (3.90), 995 (3.03), 1135 nm (3.30). – H NMR
(600 MHz, CDCl3): δ = 0.90 (t, J = 13.41 Hz, 3H, CH2CH2
CH2CH2CH2CH3), 1.58 (m, 2H, CH2CH2CH2CH2
CH2CH3), 1.84 (m, 2H, CH2CH2CH2CH2CH2CH3), 1.98
(m, 2H, CH2CH2CH2CH2CH2CH3), 2.07 (m, 2H, CH2CH2
CH2CH2CH2CH3), 3.98 (t, J = 6.06 Hz, 2H, CH2CH2
CH2CH2CH2CH3), 7.00 (s, 4H, Hβ ), 7.56 (m, 20H, Ar-H),
7.74 (m, 4H, Hβ ), 8.23 (m, 4H, Hβ ) ppm. – HRMS ((+)-ES):
Acknowledgement
This work was supported by grants from the Sci-
ence Foundation Ireland (SFI P.I. 09/IN.1/B2650) and the
Health Research Board (HRB Translational Research Award
2007 TRA/2007/11).
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