Templated formation of multi-porphyrin assemblies resembling a molecular universal joint Martin R. Johnston,a Maxwell J. Gunterb and Ronald N. Warrener*a a Centre for Molecular Architecture, Central Queensland University, Rockhampton, Queensland, 4702, Australia. E-mail: r.warrener@cqu.edu.au b Department of Chemistry, University of New England, Armidale, NSW, 2351, Australia Received (in Cambridge, UK) 21st September 1998, Accepted 13th November 1998 The rigid bis-porphyrinic host 2 forms 1:1 complexes with rigid 5,15-bis(4-pyridyl)porphyrins and both 1:1 and 2:1 complexes with tetrapyridylporphyrin 4, with the latter complex providing a new example of a self-assembled system containing an enclosed molecular environment.Strategies for self-assembly are receiving increasing attention for the construction of multichromophoric supramolecular arrays involving porphyrins.1 In particular, metal ion coordination utilising ligation of the metallated porphyrin nucleus has proven efficient for positioning photoactive components.2,3 Systems exerting co-operativity between two or more noncovalent interactions provide access to self-assembled arrays with increased stability compared to systems relying on a single interaction alone.3 The construction of such porphyrinic arrays is a key to the development of synthetic models which mimic the manner in which nature assembles the photoactive components within the photosynthetic apparatus and produces charge separation in high quantum yield.4 Here, we show that the rigid bis-porphyrin 2 can react with rigid dipyridyl porphyrins to form 1:1 complexes, and that the tetrapyridylporphyrin component 4 can form both 1:1 and 2:1 complexes;dagger; the latter porphyrin array 7 has a shape and mobility reminiscent of a universal joint.Dagger; While such a structure is novel and aesthetically intriguing, its value is conceptual; it may have interesting photophysical properties and its formation suggests similar methodologies for the assembly of other large covalent capsules.Molecular modelling (AM1) revealed that a self-assembled hostndash;guest arrangement between bis-porphyrin 2 and rigid dipyridyl rod 3 or tetrapyridyl rod 4 should be feasible, based on the following dimensions: a porphyrin centre-to-centre distance in 2 of 22 Aring; (Fig. 1); pyridine Nndash;N distances in 5,15-dipyridyl substituted porphyrins 3 and 4 of 15.7 and 15.5 Aring;, respectively; and an average zinc porphyrin to pyridyl bond distance of 2.2 Aring;.7 Mixing equimolar amounts of rigid bis-porphyrin 2 formed from 18 by treatment with Zn(OAc)2 in CH2Cl2ndash;MeOH and dipyridyl porphyrin rod 3sect; (CDCl3 or CD2Cl2) resulted in the formation of a 1:1 complex 5 (C2v).Examination by 1H NMR spectroscopy revealed sharp resonances for 3 complexed on the inside of 2 in which the resonances for 2 were slightly broadened. Such broadening has been observed previously in complexation studies involving 3,9 and in both cases these signals were sharpened by cooling the solution to 263 K or below.The a- and b-pyridyl resonances of 3 within 5 are drastically affected by the host porphyrin systemrsquo;s magnetic anisotropy, shifting upfield by 5.74 and 1.66 ppm respectively. In contrast, the proton resonances of the fused carbocyclic framework in 5 are only slightly affected by the guest porphyrin (ca. Dd 0.14). Such large pyridyl shifts are indicative of coordination of the bis-pyridyl porphyrin guest with the bis-zinc porphyrin host.3 UV spectroscopic titrations carried out in CH2Cl2 at 298 K (424 and 433 nm) yielded a Job plot with a maximum at x = 0.5, confirming the 1:1 stoichiometry of complexation in 5.Analysis of the titration data by non-linear least-squares revealed an empirical association constant of ca. 108 M21. This association constant is considerably stronger than that observed for pyridine itself with a zinc porphyrin precursor (104 M21) indicating strong chelation within 5.When the dipyridylporphyrin guest was replaced by the tetrapyridylporphyrin 4,10 the 1H NMR spectrum at 303 K for a 1:1 mixture of host 2 and guest 4 in CDCl3 revealed sharp Fig. 1 Molecular modelling of the skeletal backbone of 1 reveals the overall U-type geometry. Chem. Commun., 1998, 2739ndash;2740 2739N N N N N N N N N N N N R R Zn Zn Zn 1:1 2:1 3 4 Zn Zn 5 R = H 6 R = Pyridyl 7 Zn resonances for 2 and, surprisingly, only a single resonance for 4, the pyrrole NH (d 23.77, Dd 20.89 ppm).The resonances for the protons of the norbornyl backbone of cavity molecule 2 are again only slightly affected by the addition of 4 (ca. Dd 0.04 ppm), but the fact that those on the inside face of the backbone are most affected is consistent with the formation of an internal 1:1 complex. Cooling this solution to 233 K produced a new set of proton resonances where the resonances for a- and b-protons of 4 now reflect their new complexed environment.Two unique sets of resonances for the pyridyl ring protons were observed with the most shielded set occurring at d 3.19 (Dd 25.9 ppm) and 6.35 (Dd 21.8 ppm). The second set of pyridyl ring protons for 4 in the 1:1 complex 6 are less shielded (viz. a Dd 20.2 ppm, b Dd 20.49 ppm) from the resonances of uncomplexed 4 and this smaller shift is consistent with the uncoordinated pyridyl groups being in a complexed environment such as 6. Saturation transfer experiments revealed that the two sets of pyridyl ring proton resonances undergo rapid exchange.The above data support 4 being complexed within the cavity of 2 yet with an uncoordinated environment for the second set of pyridyl rings, as illustrated in Scheme 1. The formation of the 1:1 complex in solution is consistent with electrospray mass spectrometry of the complex, which yielded peaks for 6 (m/z 1837 M + 2H2+). We were intrigued by the possibility of encapsulating the tetratopic guest 4 within two of the ditopic host units 2. 1H NMR examination at 303 K of a solution containing a 2:1 ratio of 2 to 4 revealed a similar situation to the 1:1 case, i.e. complete absence of any resonances for 4 except for the pyrrole NH resonance (d 24.05 Dd 21.17 ppm) and small but significant changes for the resonances of 2 (ca. Dd 0.09 ppm). Cooling the solution to 233 K again yielded new resonances derived from guest 4, i.e. two sets of resonances for the pyridyl protons of 4, but now both sets of resonances indicate porphyrin coordination (Dd a 25.76, 26.17 and b 21.67, 22.99 ppm). Furthermore, two resonances were observed for the b-pyrrole resonances of 4 in 7 compared to a single resonance for uncomplexed 4.We interpret these data as follows. The two a- and b-pyridyl proton resonances are a result of the assymmetry enforced by the horizontal and rotationally rigid positioning of 4 within 7. The two b-pyrrole resonances can result from either of two effects, (a) the formation of a distorted complex (Scheme 1) where the complexation of the second equivalent of 2 is forced to adopt an eccentric position owing to the steric interactions between the tert-butyl substituents on the porphyrin subunitspara; and where the subunits are reciprocating slowly on the NMR timescale, or (b) the result of slow NH tautomerism in the central free-base porphyrin unit, with fast exchange between the two possible eccentric conformations of 7.middot; We also note that free-base 2 shows evidence of NH tautomerism at similar low temperatures.In either case, the result is an intriguing arrangement resembling a molecular scale mechanical universal joint. The concept of organising two host molecules around a central template has been successfully employed by others to create enclosed molecular environments.5,6 The ability of 2 to act as a host for other porphyrinic guests opens the way for noncovalent positioning of photoactive components and such studies are underway in our laboratories.Notes and references dagger; Crossley et al. have recently reported the first 2:1 complex involving the self-assembly of rigid bisporphyrins (2 equiv.) using a flexible tetratopic amine (ref. 5). Dagger; This complex also bears some resemblance to the topology of the Rebek lsquo;ball-likersquo; molecules, although the latter assemble independently of guest inclusion in contrast to the templated assembly described here (ref. 6). sect; The synthesis of the dipyridylporphyrin rod 3 was achieved by the acidcatalysed condensation of 3,3A-diethyl-4,4A-dimethyldipyrromethane and pyridine-4-carbaldehyde followed by oxidation with chloranil (ref. 9). para; Such an off-centre arrangement is also supported by molecular modelling. middot; We thank the referee for suggesting this alternative. Experiments are currently underway to differentiate between (a) and (b). 1 M. D. Ward, Chem. Soc. Rev., 1997, 365. 2 D.B. Amabilino and J. P. Sauvage, New J. Chem., 1998, 395 and references cited therein. 3 H. L. Anderson, C. A. Hunter and J. K. M. Sanders, J. Chem. Soc., Chem. Commun., 1989, 226; H. L. Anderson, S. Anderson and J. K. M. Sanders, J. Chem. Soc., Perkin Trans. 1, 1995, 2231; H. L. Anderson, Inorg. Chem., 1994, 33, 972; X. Chi, A. J. Guerin, R. A. Haycock, C. A. Hunter and L. D. Sarson, J. Chem. Soc., Chem. Commun., 1995, 2567; C. A. Hunter, R. K. and Hyde, Angew. Chem., Int. Ed.Engl., 1996, 35, 1936; E. Alessio, M. Macchi, S. Heath and L. G. Marzilli, Chem. Commun., 1996, 1411. 4 M. R. Wasielewski, Chem. Rev. 1992, 92, 435; V. Balzani and F. Scandola, Supramolecular Photochemistry, Ellis Horwood, Avon, 1991; H. Kurreck and M. Huber, Angew. Chem., Int. Ed. Engl., 1995, 34, 849. 5 J. N. Reek, A. P. H. L. Schenning, A. W. Bosman, E. W. Meijer and M. J. Crossley, Chem. Commun., 1998, 11. 6 J. P. Sauvage, New J. Chem., 1985, 9, 299; C. Valdes, U. P. Spitz, L. M. Toledo, S. W. Kubik and J. Rebek, J. Am. Chem. Soc., 1995, 117, 12733. 7 S. Anderson, H. L. Anderson, A. Bashall, M. McPartlin and J. K. M. Sanders, Angew. Chem., Int. Ed. Engl., 1995, 34, 1096. 8 R. N. Warrener, M. R. Johnston and M. J. Gunter, Synlett, 1998, 593. 9 H. L. Anderson, C. A. Hunter and J. K. M. Sanders, J. Chem. Soc., Chem. Commun., 1989, 226; H. L. Anderson, C. A. Hunter, M. N. Meah and J. K. M. Sanders, J. Am. Chem. Soc., 1990, 112, 5780. 10 F. R. Longo, M. G. Finarelli and J. B. Kim, J. Heterocycl. Chem., 1969, 6, 927; A. Adler, F. R. Longo, J. D. Finarelli, J. Goldmacher, J. Assour and L. Korsakoff, J. Org. Chem., 1967, 32, 476. Communication 8/07339B Scheme 1 2740 Chem. Commun., 1998, 2739ndash;2740
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