Shape-Selective Ligation to Dendrimer−Metalloporphyrins

Over the past decade, there has been remarkable progress in the synthesis of dendritic polymers.1 To generate functional dendritic polymers, there has been an growing interest in incorporation of specific functional moieties either on the surface or in the interior of dendrimers. For example, incorporation of redox metal centers on the exterior of a silane dendrimer led to catalytic oxidation of alkenes2a and electrochemical reduction of CO2 by phosphine dendrimers.2b Of particular interest has been the encapsulation of metalloporphyrins into the interior core of a dendritic polymer; such systems have shown interesting photophysical,3 electrochemical,4 oxygen binding,5 and regioselective6 catalysis. We have previously shown steric effects in dendrimermetalloporphyrins in access of alkene substrates to the metal during catalytic oxidation.6 We have now demonstrated dramatic control over ligation of Lewis bases by shape selectivity induced by the dendrimer substituents (dendrons). Shape control of ligation to functionalized metalloporphyrins and dendrimer-metalloporphyrins has been largely unexplored. While steric control for kinetic processes (e.g., epoxidation of alkenes, hydroxylation of alkanes7) has been extensively utilized, modification of equilibrium processes (e.g., axial base ligation) has been less examined. The notable exceptions are the detailed studies of steric control in CO versus O2 binding8 and a more limited report of a dendrimer-metalloporphyrin binding a dendrimer ligand.9 We report here the synthesis, characterization, and shape-selective ligation of dendrimer-metalloporphyrins derived from 2,6-diand 3,5-di-dendron-substituted meso-tetraphenylporphinatozinc(II) complexes. A series of polyphenyl ester dendrimers (G1 and G2 in Chart 1) and an amide dendrimer (G1A) were synthesized by convergent approach.6,10 The ester dendrimers, G1 and G2, were appended at all eight m-phenyl positions of ZnT(3′,5′-DHP)P, and amide dendrimer, G1A, at all eight of o-phenyl positions of the ZnT(2′,6′-DHP)P using a DCC/DPTS coupling reaction6,10 (Chart 1). ZnT(3′,5′-GnP)P (n ) 1 and 2) complexes were highly soluble in a variety of organic solvents including alkanes, while ZnT(2′,6′-G1AP)P showed good solubility in polar solvents but only limited solubility in cyclohexane or hexanes. The synthesized dendrimer metalloporphyrins were characterized by UV-visible, 1H NMR, and MALDI-TOF MS spectroscopic techniques. Homogeneity was demonstrated by HPLC and MALDI-MS. Complete synthesis and characterization details are available as Supporting Information. The shape selectivities of the binding sites of the Zn dendrimer-porphyrins were probed using the axial ligation of various nitrogenous bases of different shapes and sizes in toluene (Zn porphyrins were chosen because they generally bind only a single axial ligand). On ligation of bases, the visible absorption spectra of Zn dendrimer-porphyrins are red-shifted and show an increase in the extinction coefficient of both the Soret (B) and visible (Q) bands, just as with ZnTPP.11 As an example, data for titration of ZnT(3′,5′-G1P)P with 4-phenylpyridine are shown in Figure 1. For a series of nitrogenous bases of different sizes and shapes, equilibrium constants (Keq) were evaluated using standard procedures12 for the Zn dendrimer-porphyrins and ZnTPP (Figure 2). The Keq values of the ortho-substituted ZnT(2′,6′-G1AP)P were exquisitely sensitive to the shape and size of the substrates. ZnT(2′,6′-G1AP)P binds linear amines (4-phenylpyridine or dodecylamine) as well as the more open pocketed ZnT(3′,5′-GnP)P dendrimer-porphyrins and better than ZnTPP itself. In the case of nonlinear amines, however, differences in Keq of 103 to 105 were observed among the dendrimer-metalloporphyrins. Surprisingly, the meta-substituted ZnT(3′,5′-GnP)P (n ) 1, 2) showed a † Indian Institute of Technology. ‡ University of Illinois. (1) (a) Tomalia, D. A. AdV. Mater. 1994, 6, 529. (b) Frechet, J. M. J. Science 1994, 263, 1710. (c) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (d) Zeng, F.; Zimmerman, S. C. Chem. ReV. 1997, 97, 1681. (2) (a) Knapen, J. W. J.; van der Made, A. W.; de Wilde, J. C.; van Leeuwen, P. W. N. M.; Wijkens, P.; Grove, D. M.; van Koten, G. Nature 1994, 372, 659. (b) Roberts, J. C.; Adams, Y. E.; Tomalia, D.; Mercer-Smith, J. A.; Lavalle, D. K. Bioconjugate Chem. 1990, 1, 305. (c) Meidaner, A.; Curtis, C. 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(f) Bhyrappa, P.; Krishnan, V.; Nethaji, M. J. Chem. Soc., Dalton. Trans. 1993, 1901. (12) (a) Collman, J. P.; Brauman, J. I.; Doxsee, K. M.; Halbert, T. R.; Hayes, S. E.; Suslick, K. S. J. Am. Chem. Soc. 1978, 100, 2761. (b) Suslick, K. S.; Fox, M. M.; Reinert, T. J. J. Am. Chem. Soc. 1984, 106, 4522. Chart 1. Chemical Structures of Dendrimer-Metalloporphyrins and Dendrons 262 J. Am. Chem. Soc. 1999, 121, 262-263