Bis-Porphyrin Racks with Space-Separated Co-Planar Porphyrin Rings

A porphyrin appended norbornenyl building block 8 has been isolated and coupled, using a 1,3-dipolar ACE reaction, to yield bis-porphyrin compounds in which the porphyrin moietes are angled upward relative to the norbornane backbone.


Introduction
Following the elucidation of the strict geometric alignment of chromophores within purple photosynthetic bacteria [1], numerous reports have appeared in the literature regarding approaches to the construction of multiporphyrinic arrays, with a view to mimicking the natural system as a light harvesting machine. Initially, these reports involved the covalent attachment of chromophores [2], however recent efforts in this field have increasingly taken advantage of self-assembly strategies for array construction [3].
The design of new molecular architectures for bis-porphyrins and their use in a fundamental study of self-assembly processes has been an active goal of our recent research [4,5]. In earlier papers, we concentrated on the synthesis of V-and U-shaped cavities with the porphyrin macrocycle attached in a fixed geometry by two bonds to the termini of fused carbocyclic frames. Two elements dominate the shape of such systems: a) the topology of the carbocyclic frame and b) the angle of attachment of the porphyrin ring to the frame. In a preliminary communication on this work [6], we enunciated the use of angled norbornene blocks which carry the porphyrin ring and showed how the topology of the frame could be used to achieve specific geometric outcomes. Implicit in the success of the approach, was the ability to make the angled blocks and to construct the molecular frame with predefined stereochemistry. We found that 1,3-dipolar ACE coupling [7] of cyclobutene epoxides with norbornenes has satisfied the latter requirement and the same reaction protocol is exploited in the present study. The norbornene blocks have been functionalised by condensation of norbornene-containing alicyclic α-diones with the Crossley porphyrin diamine 5 [8]. The αdiones 1 [4,6], 2 [6] and 3 [9] used in our earlier work have a geometry in which the α-dione component bears a downward relationship to the norbornene frame that is accentuated following condensation with the porphyrin. Such a geometry is well suited for the construction of U-and V-shaped bis-porphyrins as the molecular frame is curved downwards and helps position the porphyrins on the underside of the frame ("southern" systems; substituents on these systems have been designated either "northern" or "southern" depending on their relative orientation to the methano-bridges of the polynorbornane backbone [10]).
In this paper we discuss the preparation and use of a porphyrin block derived from the upwarddirected α-dione 4. Our new objective has been to use this block to prepare "northern" V-shaped systems by coupling with frames of linear geometry and co-planar bis-porphyrins by reaction with curved [n]polynorbornane frames.

Results and Discussion
The required α-dione 4 is a known compound and was prepared following the method reported by Martin et al. [11] (Scheme 2a). Condensation of 4 with the Crossley diamine 5 [8] produced the porphyrin block 8 (Scheme 2b) that was characterised by spectroscopy and high resolution electrospray mass spectrometry (HR-ESMS) (found m/z 1307.8294). The 1 H-NMR spectrum of 8 ( Figure 1  The porphyrin resonances, because of the Cs symmetry of 8, occur as a two doublets for Hg and Hg' (δ 9.04, 8.96) and singlets for Hi (δ 8.78) and the tetrazaanthrancene protons Hf (δ 8.62). The aryl protons resonate as two sets (ratio 2:1) reflecting the 3,5-substitution pattern of the aryl ring (δ 8.10 -7.81). The inner NH protons resonate at δ -2.39 as expected due to the large paramagnetic ring current present in the porphyrin macrocycle. The tert-butyl groups present in 8 resonate at δ 1.50 -1.53 and, in chloroform solution, these resonances overlap with the methano-bridge proton Ha resonance. The presence of the Ha proton resonance was detected by a COSY experiment. The resonance for the vinyl protons (He) occurred as a singlet at δ 6.49 and its presence was used to monitor the progress of the ACE coupling reaction. Typically, protons Hd (allylic) and Hc (benzylic) occur downfield owing to the strong electron-withdrawing effect of the azaheterocycle, accounting for the lower field shift of Hc relative to Hd. The broadening of the latter signal is due to coupling with the methano-bridge protons. Molecular modeling [12] of 8 was carried out at the semi-empirical level of theory (AM1) and revealed the overall shape of the block (Figure 2). The porphyrin macrocyclic ring is angled upward from the norbornene skeleton at an angle of 28˚ and thereby contrasts with the previously prepared blocks 1 -3. Several coupling reactions were investigated utilising block 8 to produce bis-porphyrin compounds. The coupling reactions were based on the ACE coupling protocol which utilises a 1,3-dipolar cycloaddition strategy in which the dipole is generated in situ by ring opening of an epoxide and trapped by the norbornene. In this work, bis-epoxide molecules 9, 11 and 13 were used to yield products of differing geometries due to the different spatial requirements of the bis-epoxide spacer backbone.
In the first instance, bis-epoxide 9 was heated in a sealed tube with block 8 (Scheme 3) at 140˚C for three days to yield the bis-porphyrin 10, which was isolated by chromatographic separation. Once again bis-porphyrin 10 was characterised by 1 H NMR spectroscopy ( Figure 3) and HR-ESMS with proton resonances being assigned using a combination of 2D COSY and NOESY experiments (in the NOESY spectrum of 10, in addition to the expected intramolecular signals, several additional cross peaks were observed and which were found to be concentration dependant, however, the 1 H-NMR spectrum of 10 showed no concentration dependant shifts and we are currently investigating this phenomena; a referee is thanked for bringing this to the author's attention).
The proton resonances for the porphyrin moiety are little changed from those observed in 8 except for the alicyclic backbone. The olefinic proton resonance (He) present in block 8 is lost upon coupling, being shifted upfield to δ 2.28 in line with the change in hybridisation.   Further, the new resonances for methylene protons Ha and Hb are diagnostic (δ 2.40 and 0.72) since Ha is significantly downfield shifted being influenced by the adjacent 7-oxanorbornane system. Such a shift reinforces the linear nature of the coupled product. HR-ESMS yielded a doubly charged molecular ion at the required position (m/z 1605.896). Molecular modeling (AM1) reveals the almost coplanarity of the porphyrin macrocycles within 10, with each porphyrin plane approximately 2.5˚ from the horizontal, with an interporphyrin centre-to-centre distance of 40 Å.

Scheme 4: Synthetic scheme outlining the isolation of bis-porphyrin 12.
In a similar manner to 10, the 1 H-NMR resonances for protons attached to the porphyrin moiety are little affected by the coupling process, yet major changes are observed for the alicyclic backbone proton Ha, Hb, and He resonances. The Ha resonance is influenced by the adjacent 7-oxanorbornane ring and is shifted downfield to δ 2.28, whereas Hb is shifted upfield to δ 0.71. The resonance for He is shifted upfield concomitant with the loss of olefinic character as a result of the coupling reaction. The resonances at δ 2.04 and δ 2.28 represent overlapping signals from several protons and this complication prevents the exact identification of the proton resonances ( Figure 4). HR-ESMS again supported the identity of 12 with a doubly charged molecular ion being observed (m/z 1512.889).
Molecular modeling (AM1) of 12 (Scheme 4) again reveals the near coplanarity of the porphyrin macrocycles within 12, with each porphyrin plane inclined only 4˚ from the horizontal. The interporphyrin centre-to-centre distance, in this case, was calculated to be 37 Å.   Characteristic 1 H-NMR resonances were again observed for Ha (δ 2.32) and Hb (δ 0.72) and supported the linear geometry for the coupling protocol. The structure of 14 was further supported by the presence of a doubly charged molecular ion observed at m/z 1602.9084 in the HR-ESMS.
Molecular modeling (AM1) of 14 (Scheme 5) revealed that each porphyrin plane is tilted upwards from coplananity by 17˚. Compound 14 represents the first example of a bis-porphyrin cavity in which the porphyrin macrocycle is directed upwards relative to the connecting alicyclic backbone. The interporphyrin centre-to-centre separation within 14 was calculated to be 39 Å.

Conclusions
Having successfully isolated 10, 11 and 12, current attention is being directed at the examination of the coordination chemistry of their metallated porphyrin derivatives and their ability to coordinate bidentate ligands. In particular, the assembly of the bis-porphyrin molecules to create larger supramolecular arrays will be targeted in an attempt to mimic the arrangement of photochromic pigments found in the natural photosynthetic apparatus. The results of these studies will be reported in due course.

Acknowledgements
The author is grateful to CQU for the provision of a Research Advancement Award 1999-2001 and the CMA for providing research facilities and a professional research environment in which to work.

General
All solvents were used as supplied with the exception of THF (distilled from Na/Benzophenone) and pyridine (distilled from KOH and stored over molecular sieves). Melting points were determined using GALLENKAMP melting point apparatus and are uncorrected. Thin layer chromatography was performed on pre-coated aluminium sheets on Merck silica gel 60 F 254 and visualised using visible or UV light (254 and 365nm). Column chromatography was carried out under a positive pressure of air using Merck silica gel (230-400 mesh).
NMR spectra were acquired on either a Bruker AMX300 (300MHz) or Bruker DRX400 (400MHz) using standard Bruker pulse programs. Unless otherwise stated, spectra were recorded at 303K using deuterated CHCl 3 as the solvent with tetramethylsilane (TMS) as an internal standard. Chemical shifts (δ) are reported as parts per million (ppm) with respect to TMS. Abbreviations used in assigning spectra include: s, singlet; bs, broad singlet; d, doublet; t, triplet; q, quartet; and m, multiplet. Mass spectra (MS) were analysed on Micromass Mass Spectrometer using electrospray (ES) techniques. Bisepoxides 9[13], 11 [14], and 13 [15] were kindly supplied by Dr Davor Margetic (CMA). Bis-porphyrin adduct 10: Porphyrin block 8 (105 mg, 80 µmol) was dissolved in THF (1 mL) along with bis-epoxide 9 (23 mg, 38 µmol) and the solution heated in sealed tube at 140˚C for 3 days. The material was taken to dryness and purified by column chromatography (silica) eluting with CHCl 3 which gave 10 as a purple solid (112 mg, quantitative yieldbased on block consumed Bis-porphyrin adduct 12: Porphyrin block 8 (68 mg, 52 µmol) along with bis-epoxide 11 (10 mg, 25 µmol) were dissolved in THF (1 mL) and the solution heated in a sealed tube in the dark at 140˚C for 2 days. The solution was taken to dryness and the resulting material purified by column chromatography (silica) eluting with DCM/petroleum spirit (1:1) to give recovered 8 (19 mg) followed by CHCl 3 /THF