Porphyrin-fullerene host-guest chemistry

J. Am. Chem. Soc. 2000, 122, 10704-10705 Communications to the Editor Porphyrin-Fullerene Host-Guest Chemistry Dayong Sun, Fook S. Tham, Christopher A. Reed,* Leila Chaker, † Michael Burgess, † and Peter D. W. Boyd* ,† Department of Chemistry UniVersity of California RiVerside, California 92521-0403 Department of Chemistry The UniVersity of Auckland, PriVate Bag 92019, Auckland, New Zealand ReceiVed June 21, 2000 Fullerenes are spontaneously attracted to porphyrins and metalloporphyrins. This new supramolecular recognition element was first discovered in cocrystallates of C 60 and C 70 with tetraarylporphyrins 1-3 and octaethylmetalloporphyrins. 4,5 A natural feature of these structures is the zigzag arrangement of porphyrins with fullerenes sandwiched in the clefts. It is reproduced in calculations, and the unexpectedly strong interaction between a curved π surface and a flat π surface (C fullerene -to-porphyrin plane distance ∼2.7 A) is largely van der Waals in origin. 2 With this knowledge we have used molecular modeling to design a jaw-like bis-porphyrin with a similar fullerene-sized bite. The palladium-linked m-pyridyl dimer shown in Figure 1 has a high calculated “gas phase” binding enthalpy for C 60 ( ∼57 kcal‚mol -1 ) suggestive of tight fullerene binding in solution. A “chelate” effect from bidentate complexation should also con- tribute. While this work was in progress, a doubly strapped bis- porphyrin was reported to form a 1:1 complex with C 60 . 6 Notably, the binding constant (7 × 10 5 M -1 ) exceeds those of traditional concave hosts such as calixarenes, cyclotriveratralenes, and resorcarenes. 7 The palladium-linked 8 bis-porphyrins 1 and 2 were prepared according to Scheme 1 and characterized by 1 H NMR spectros- copy and FAB/MALDI mass spectrometry (see Supporting Information). Addition of C 60 or C 70 to solutions of these bis- porphyrins gave complexes that were chromatographically distinct The University of Auckland, New Zealand. (1) Sun, Y.; Drovetskaya, T.; Bolskar, R. D.; Bau, R.; Boyd, P. D. W.; Reed, C. A. J. Org. Chem. 1997, 62, 3642-3649. (2) Boyd, P. D. W.; Hodgson, M. C.; Chaker, L.; Rickard, C. E. F.; Oliver, A. G.; Brothers, P. J.; Bolskar, R.; Tham, F. S.; Reed, C. A. J. Am. Chem. Soc. 1999, 121, 10487-10495. (3) Reed, C. A.; Fackler, N. L. P.; Kim, K.-C.; Stasko, D.; Evans, D. R.; Boyd, P. D. W.; Rickard, C. E. F. J. Am. Chem. Soc. 1999, 121, 6314-6315. (4) Olmstead, M. M.; Costa, D. A.; Maitra, K.; Noll, B. C.; Phillips, S. L.; Van Calcar, P. M.; Balch, A. L. J. Am. Chem. Soc. 1999, 121, 7090-7097. (5) Stevenson, S.; Rice, G.; Glass, T.; Harich, K.; Cromer, F.; Jordan, M. R.; Craft, J.; Hadju, E.; Bible, R.; Olmstead, M. M.; Maitra, K.; Fisher, A. J.; Balch, A. L.; Dorn, H. C. Nature 1999, 401, 55-57. (6) Tashiro, K.; Aida, T.; Zheng, J.-Y.; Kinbara, K.; Saigo, K.; Sakamoto, S.; Yamaguchi, K. J. Am. Chem. Soc. 1999, 121, 9477-9478. (7) Fox, O. D.; Drew, M. G. B.; Wilkinson, E. J. S.; Beer, P. D. Chem. Commun. 2000, 391-392 and references therein. (8) Yuan, H.; Thomas, L.; Woo, L. K. Inorg. Chem. 1996, 35, 2808- Figure 1. Calculated minimum energy structure of PdCl 2 -linked mono- 3-pyridyltriphenylporphyrinPd(II) bis complex of C 60 (Universal Force Field, ver. 1.02). The porphyrin planes subtend an angle of 41.5° and Pd‚‚‚C ) 2.78, 2.85 and 2.86 A. Figure 2. Low temperature (-90 °C) 13 C NMR spectrum of toluene solution of 2:1 mixture of C 60 and 2. The peak marked * is due to a small amount of Pd2‚C 60 . from their individual components on thin layers or gravity columns. This indication of tight binding was also seen in MALDI mass spectra where substantial abundances of positive ions corresponding to 1:C 60 , 1:C 70 , 2:C 60 , and 2:C 70 were observed. 1 H NMR spectroscopy of the porphyrin components and 13 C NMR spectroscopy of the fullerenes are very useful for revealing the structural details and the dynamics of complexation. When fullerenes are added to porphyrins 1 and 2, ring current effects from the fullerenes cause upfield shifts in the central N-H protons (up to 0.34 ppm). Conversely, ring current effects of the porphyrins cause upfield shifts in the 13 C NMR of the fullerenes. For C 60 , the shift is 2.7 ppm for complexation by 2 (see Figure 2). This is consistent with a prediction of 2.8 ppm based on a 2.7 A approach of a fullerene 6:6 ring junction to the center of the porphyrin. 2 For C 70 , the five different 13 C signals are shifted by 1.2-2.9 ppm. The details reveal orientation. The largest shift is found for the C e carbon atoms of the equatorial belt, decreasing systematically toward the poles (see Figure 3). This is compelling evidence for “side-on” rather than “end-on” binding of C 70 and can be understood in terms of maximizing van der Waals attraction. It is also consistent with the observation that C 70 displaces C 60 from 2 and provides a basis for selectivity of fullerene binding and the chromatographic behavior of fullerenes on porphyrin-appended silica stationary phases. 9 (9) Xiao, J.; Savina, M. R.; Martin, G. B.; Francis, A. H.; Meyerhoff, M. E. J. Am. Chem. Soc. 1994, 116, 9341-9342. 10.1021/ja002214m CCC: $19.00 © 2000 American Chemical Society Published on Web 10/12/2000