Synthesis of Imidazole-Based [30]Heptaphyrin and Stable Figure-Eight [60]Tetradecaphyrins via [5 + 2] Condensations in One Pot

Eight [60]Tetradecaphyrins via [5 + 2] Condensations in One Pot

Letter

Synthesis of Imidazole-Based [30]Heptaphyrin and Stable Figure-

Chengming Li, Zhenmei Huang, Yanmei Hu, Wenbo Liang, Rongchuan Su, Mao Chen, Linsen Zhou,

Di Wu, Ge Gao,* and Jingsong You*

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sı Supporting Information

ABSTRACT: Derived from a Pd-catalyzed oxidative C−H/C−H

coupling reaction, two giant imidazole-based [60]tetradecaphyrins

adopting stable ﬁgure-eight geometry together with one [30]-

heptaphyrin are obtained by [5 + 2] MacDonald condensations in

one pot. The directional imidazole is believed to play a vital role for

the diverse cyclization and conformation stabilization.

xpanded porphyrins have gained much attention in recent

years due to their unique optical and electronic properties

and promising applications in near-infrared dyes, nonlinear

optical materials, photodynamic therapy, and metal-ion bind-

ing. As substantial progress has been made for medium-sized

In view of this challenge and our continuous interest in

building imidazole-based functional molecules, we herein

report a one-pot synthesis of two giant imidazole-based

[60]tetradecaphyrins together with a [30]heptaphyrin via [5 +

2] MacDonald condensations. Both the pentaheteroaryl and

bithiophene synthons are derived from one Pd-catalyzed

oxidative C−H/C−H coupling reaction. The two isomeric

[60]tetradecaphyrins are separated by chromatography and

characterized by single crystal X-ray analysis, while the

[30]heptaphyrin stays in two inseparable conformers. Their

aromaticity, conformational dynamics, and spectral properties

are investigated.

porphyrinoids in regard to their syntheses, conformations,

aromaticity, coordination behaviors, and radical characters,

expanded porhyrins bearing over 10 heteroarenes are still rare

because of their synthetic challenges, more complicated

geometry, and lower stability along with the added heteroaryl

−8

subunits.

The groups of Setsune and Osuka have isolated

multisized macrocycles (from [18]porphyrin up to [96]-

tetracosaphyrin) in ring-closure reactions of bipyrrole or di/

tripyrrane derivatives. By the development of relatively rigid and

π-conjugated polyheteroaryl units, more targeting syntheses are

The You group previously reported a Pd-catalyzed oxidative

C−H/C−H cross-coupling reaction of thiophene-2-carbalde-

hyde with 1-methyl-1H-imidazole to deliver 2-thienylated

realized, providing [54]dodecaphyrin and [64]hexadeca-

imidazole. We successfully attained both double cross-coupled

phyrin.

,5-dithienylated imidazole 1 and homocoupled bithiophene

Except classical pyrrole, other heteroarenes including

thiophene, furan, and pyridine have been widely utilized to

construct novel expanded porphyrins to tune their properties. In

contrast, azoles are rarely employed. In 2006, Sessler et al.

reported the ﬁrst example of tetraaza[32]octaphyrins incorpo-

dicarbaldehyde 2 in one pot by a modiﬁed procedure. 1 and 2

were then subjected to the Grignard reaction followed by

reacting with excess pyrrole, respectively, providing triheteroaryl

diol 3, bithienyl diol 4, pentaheteroaryl 5, and tetraheteroaryl 6

as alternative synthons to synthesize novel imidazole-based

rating the biimidazole subunit. Pyrazole-based Siamese-twin

expanded porphyrins via the MacDonald condensation (Scheme

porphyrins, which formed a bimetallic complex with copper,

with two independent conjugation pathways were prepared by

). The initial attempt by the [4 + 3] condensation of 3 and 6

resulted in a complicated mixture (Scheme 2, Strategy I).

Bru

ckner, Meyer, and co-workers. However, expanded

However, the [5 + 2] condensation of 4 and 5 gave three

porphyrins containing unsymmetrical imidazole-based subunits

have not yet been developed. It is probably because (1) the

imidazole-based π-conjugated subunit (here as 2,5-dithienylated

imidazole) is diﬃcult to synthesize by traditional method-

distinct spots on the TLC plate, which were isolated and puriﬁed

by chromatography on an alumina column. The HRMS analysis

of the most polar compound showed an ion peak at 1061.3774,

ology; (2) the unsymmetry of the subunit may result in a

complicated and ineﬃcient reaction from the synthetic point of

view.

Received: April 3, 2021

Published: April 22, 2021

2021 American Chemical Society

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Org. Lett. 2021, 23, 3746−3750

Organic Letters

signals of 9 support its unsymmetrical structure (Figure 2 a,b).

Letter

Scheme 1. Synthetic Route to Alternative Synthons 3−6

Starting from One C−H/C−H Oxidative Coupling Reaction

Scheme 2. Strategic Approaches to Imidazole-Based

Expanded Porphyrins

Figure 1. Single crystal X-ray structures of 8 and 9: (a, b) top views; (c,

d) side views. Only (M,M)-forms are shown. Distances are in Å.

two sets of C−H···N hydrogen bonds in 8: two major ones

between the imidazole and thiophene rings (∠C13−H−N3 =

52° and ∠C81−H−N6 = 148°) and two minor ones between

the pyrrole and thiophene rings (∠C12−H−N1 = 117° and

∠

C80−H−N8 = 116°). However, 9 has three hydrogen bonds

and loses a major one.

The H NMR spectra of 8 and 9 were recorded in CDCl at

room temperature, and the peaks were assigned with the help of

D COSY and NOESY spectra (for details, see the Supporting

Information ). The number of signals in the aromatic and alkyl

region of 8 are consistent with its C symmetry, while the split

matching a [5 + 2] product [30]heptaphyrin (calcd [M + H]⁺:

061.3773). The MALDI-TOF MS analysis of the two less polar

species gave similar peaks at 2120.7446 and 2120.7491,

respectively, agreeing with a double [5 + 2] product [60]-

tetradecaphyrin (Scheme 2, Strategy II).

The single crystals of the three compounds were cultivated.

The [30]heptaphyrin did not crystallize in any tested solvent.

The single crystals of two isomeric [60]tetradecaphyrins were

luckily obtained and revealed by the X-ray diﬀraction analysis to

both adopt a doubly twisted ﬁgure-eight geometry (Figure 1).

Two pairs of (P,P)- and (M,M)-enantiomers were found in each

cell, and therefore, the crystals are racemized. The less polar one

is C -symmetrical 8 with two parallel imidazole rings on the

ribbon crossing, while the more polar one is unsymmetrical 9

with two antiparallel imidazole rings. The dihedral angles

between two adjacent heteroarenes are all smaller than 26°,

indicating that an extended π-conjugation along 14 heteroarenes

is preserved. A notable feature distinct from known ﬁgure-eight

Figure 2. Partial H NMR spectra of 8 (a), 9 (b), and 7 (c) in CDCl at

298 K.

Two pairs of signals at 9.82 and 9.40 ppm in 8 are assigned to the

protons of the inverted thiophene rings. Only three peaks are

recorded for 9 in this area, in line with the absence of one major

hydrogen bond as found in its single crystal structure. The

unbounded proton is seen at 7.92 ppm. [60]Tetradecaphyrins 8

and 9 should be antiaromatic according to the Hu

electron formulation. However, the anisotropy of the induced

a,c,16−18

macrocycles

is the large overlap of the rigid and

conjugated 2,5-dithienylated imidazole subunits with distances

of 3.338 and 3.434 Å in 8 and 9, respectively, which implies

eﬀective π−π interactions at the ribbon crossing. Both

bithiophene units pose oppositely in each molecule, and the

bond lengths between the two thiophene rings are around 1.38

Å, indicating a quinoidal form. It is worth noting that there are

ckel 4n π-

current density (ACID) plots of 8 and 9 showed no obvious

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Org. Lett. 2021, 23, 3746−3750

Organic Letters

negative nucleus-independent chemical shift (NICS(1))

Letter

current circuit along the macrocycles (Figure 3b). Moreover, the

Figure 3. Calculated ACID plots and NICS(1) values of 7a (a) and 8

b). The isosurfaces are 0.04 and 0.03 for 7a and 8, respectively. The

(

magnetic ﬁeld is along the Z axis (vertically pointing out of the paper),

and the blue arrows highlight the dominant diatropic ring current ﬂow.

Figure 4. VT H NMR spectra (aromatic region) of 7 (a) in 1,1,2,2-

tetrachloroethane-d and 8 (b) in toluene-d . The signals for the

values of the imidazole and neighboring thiophene rings

revealed their localized aromatic characters (Figures S29 and

S30 ). These calculations suggested the overall nonaromatic

character of macrocycles 8 and 9. The normal chemical shift of

the N-methyl protons at 3.94 ppm supported this conclusion.

aromatic protons of the mesityl groups are marked by ◊.

(

P,P)-8 were successfully separated by chiral HPLC on a

Chiralpak IA (DAICEL) column (Figure S11). To the best of

our knowledge, these [60]tetradecaphyrins containing 14

heteroarenes are the largest expanded porphyrins adopting a

stable ﬁgure-eight conformation to date. The high conforma-

tional stability should be attributed to the intramolecular

hydrogen bonding and π−π interactions at the ribbon crossing

as observed in the single crystals.

The seeming discrepancy with the Huckel rule mainly results

from the eﬀective π−π interactions at the ribbon crossing, which

oﬀset the ring currents of two heptaphyrin-like hemimacrocycles

in 8 and 9.

The H NMR spectrum of [30]heptaphyrin exhibited two sets

of similar signals in an integration ratio of ca. 1:0.85 (Figure 2c),

indicating the coexistence of two diﬀerent conformers. The

peripheral thiophene β-protons appear in the downﬁeld region

of 8.51−10.23 ppm, and the peaks between 1.14 and 1.35 ppm

relate to one inverted thiophene ring of the bithiophene

The UV−vis-NIR spectrum of 7 in dichloromethane (DCM)

showed characteristic Soret- and Q-like bands at λ = 552 and

max

819 nm (Figure 5 a). Time-dependent density functional theory

subunits. Therefore, [30]heptaphyrin is assigned to a mixture

of conformers 7a and 7b, and the unfavorable 7c was not

observed. The N-methyl protons are shielded inside the

macrocycles by the diatropic ring currents at −0.98 and −1.15

ppm, suggesting the macrocyclic aromaticity of 7. The ACID

plots of 7a exhibited a nearly planar geometry and a clockwise

diatropic ring current circuit along the whole macrocycle.

Accordingly, a 30π-electron aromatic pathway could be drawn

(

Figure 3a). The reduction of 7 to access a 32π-electron

antiaromatic macrocycle gave an unstable compound, which was

rapidly oxidized back to 7 upon exposure to air.

The conformational ﬂexibility of these macrocycles in the

solutions was then studied. The variable-temperature (VT) H

NMR spectra of 7 in 1,1,2,2-tetrachloroethane-d from 298 to

38 K showed no signiﬁcant change of the chemical shifts and

intensities of various protons (Figure 4a), suggesting that the

interconversion between 7a and 7b does not occur in this

temperature range.

Figure 5. UV−vis-NIR spectra of 7 and 8 and their stepwise protonated

species in DCM (a, b). Inset: photographs of the corresponding

solutions.

15,23

Interestingly, the H NMR spectrum of

in toluene-d at room temperature exhibited one broad and

ﬁve well-separated peaks for aromatic protons of the mesityl

groups, indicating a slow conversion between the (M,M) and

(TD DFT) calculations indicated that the Q-like band can be

assigned to a large contribution of HOMO → LUMO and a

small contribution of HOMO−1 → LUMO+1 electronic

transitions (oscillator strength f = 0.4170) (Table S3). The

Soret-like band is correlated to multiple transitions of HOMO−

n → LUMO+m (n = 0, 1; m = 0, 1, 2), which are distributed

along the whole π-conjugated skeleton. The UV−vis-NIR

2a,24,25

(P,P) conformers on the NMR time scale.

The conforma-

tional stability is quite high as only a small broadening without

the coalescence of these peaks was observed upon heating up to

58 K (Figure 4b). The (M,M)-to-(P,P) interconversion energy

barrier was estimated to be around 20.6 kcal/mol on the basis of

a presumed tub structure upon optimization at the B3LYP/6-

spectrum of 8 displayed two broad peaks at λ = 516 and

max

1G** level (see the Supporting Information). The (M,M)- and

661 nm and a tail extending to 1600 nm, which originates from

748

Org. Lett. 2021, 23, 3746−3750

Organic Letters

Sichuan University, Chengdu, Sichuan 610064, PR China;

Letter

the forbidden HOMO → LUMO transition (oscillator strength f

orcid.org/0000-0001-9980-7311; Email: gg2b@

scu.edu.cn

0.0803) (Figure 5b and Table S4). The strong near-IR

absorption of 8 is associated with the large π-conjugation despite

its doubly twisted geometry. It is also worth noting that 7−9

remained stable in solution in an ambient atmosphere for 20

days as detected by the UV−vis-NIR and H NMR measure-

ments (Figure S13 −S17 ).

Jingsong You − Key Laboratory of Green Chemistry and

Technology of Ministry of Education, College of Chemistry,

orcid.org/0000-0002-0493-2388; Email: jsyou@

scu.edu.cn

The stepwise protonation process of 7 upon triﬂuoroacetic

acid (TFA) titration could be well distinguished by the color

change of the solution, and the corresponding spectral variations

were recorded (Figures 5a, inset and S18). The bathochromi-

cally shifted and split Soret- and Q-like bands were observed for

Authors

Chengming Li − Key Laboratory of Green Chemistry and

Technology of Ministry of Education, College of Chemistry,

Sichuan University, Chengdu, Sichuan 610064, PR China;

Department of Cardiology, Laboratory of Heart Valve Disease,

West China Hospital, Sichuan University, Chengdu, Sichuan

610041, PR China

Zhenmei Huang − Key Laboratory of Green Chemistry and

Technology of Ministry of Education, College of Chemistry,

Sichuan University, Chengdu, Sichuan 610064, PR China

Yanmei Hu − Key Laboratory of Green Chemistry and

Technology of Ministry of Education, College of Chemistry,

Sichuan University, Chengdu, Sichuan 610064, PR China

Wenbo Liang − Key Laboratory of Green Chemistry and

Technology of Ministry of Education, College of Chemistry,

Sichuan University, Chengdu, Sichuan 610064, PR China

Rongchuan Su − Key Laboratory of Green Chemistry and

Technology of Ministry of Education, College of Chemistry,

Sichuan University, Chengdu, Sichuan 610064, PR China

Mao Chen − Department of Cardiology, Laboratory of Heart

Valve Disease, West China Hospital, Sichuan University,

Chengdu, Sichuan 610041, PR China

monoprotonated 7·H . Diprotonation induced a signiﬁcant

enhancement of the Soret-like band and a blue shift of the Q-like

band. Subsequent triprotonation diminished the Soret-like band

and broadened the Q-like band. This process could also be

clearly viewed on the H NMR spectra by drastic “downﬁeld →

upﬁeld → downﬁ eld ” shifts for the resonance of the shielded N-

methyl protons (Figure S12). On the contrary, the absorption

spectra of 8 upon TFA titration exhibited only two color changes

of the solution (Figure 5b, inset). It is worth noting that the

absorption intensity of 8 in the second near-IR region (NIR-II,

000−1700 nm) was signiﬁcantly enhanced upon multiple

protonations (Figure S19). Similar behaviors were recorded for

(Figure S20).

In summary, starting from one Pd-catalyzed oxidative C−H/

C−H coupling reaction of thiophene-2-carbaldehyde with 1-

methyl-1H-imidazole, two giant imidazole-based [60]-

tetradecaphyrins 8 and 9 together with a [30]heptaphyrin 7

are formed in one pot via a [5 + 2] condensation strategy. 7

exhibits macrocyclic aromaticity and stays as two inseparable

conformers, while 8 and 9 are nonaromatic and represent the

largest expanded porphyrins in stable ﬁgure-eight geometry to

date. Our work demonstrates the power of the C−H bond

activation reaction, the importance of the condensation strategy,

and the key role of hydrogen bonding and π−π interactions

induced by the directional imidazole for conformation

stabilization, all of which lead to a new type of imidazole-

based expanded porphyrins.

Linsen Zhou − State Key Laboratory of Rare Earth Resource

Utilization, Institute of Materials, Chinese Academy of

Engineering Physics, Jiangyou, Sichuan 621908, PR China

Di Wu − Key Laboratory of Green Chemistry and Technology of

Ministry of Education, College of Chemistry, Sichuan

University, Chengdu, Sichuan 610064, PR China

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.orglett.1c01156

Notes

ASSOCIATED CONTENT

sı Supporting Information

■

The authors declare no competing ﬁnancial interest.

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.orglett.1c01156.

ACKNOWLEDGMENTS

■

This work was ﬁnancially supported by the National NSF of

China (Nos. 21772134 and 21772133) and the Fundamental

Research Funds for the Central Universities (No.

0826041D4117). We are also thankful for the valuable

discussion with Prof. Jishan Wu at the National University of

Singapore.

Detailed information on the experimental procedures,

characterization data, computational calculations, crys-

tallographic and spectroscopic data, and X-ray crystal

structures 8 and 9 (PDF)

Accession Codes

CCDC 2021732 and 2021733 contain the supplementary

crystallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif , or by

emailing data_request@ccdc.cam.ac.uk , or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

■

Ge Gao − Key Laboratory of Green Chemistry and Technology

of Ministry of Education, College of Chemistry, Sichuan

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