Selection and Amplification of Mixed‐Metal Porphyrin Cages from Dynamic Combinatorial Libraries

Abstract

Mixed metallo-porphyrin cages were selected and amplified from dynamic combinatorial libraries (DCLs) by using appropriate templates. The cages are composed of two bisphosphine substituted zinc(II) porphyrins as ligand donors and two rhodium(III) or ruthenium(II) porphyrins as ligand acceptors, and are connected through metal-phosphorus coordination. Ru and Rh porphyrins that display a large structural diversity were employed. The templating was achieved by using 4,4'-bpy, 3,3'-dimethyl-4,4'-bipyridine and benzo[lmn]-3,8-phenanthroline, and acts through zinc-nitrogen coordination. The absolute amount of amplification from the DCLs is strongly dependent on the combination of the Ru/Rh porphyrin and the template; cages with sterically demanding porphyrins can only form with smaller templates. In the case of tert-butyl-substituted TPP (TPP=tetraphenylporphyrin), cages are not formed at all. The formation of the cages is usually complete within 24 h at an ambient temperature; in the case of the cage containing Rh(III)OEP (OEP=octaethylporphyrin) and bpy, the pseudo-first-order rate constant of cage formation was determined to be 2.1+/-0.1x10(-4) s(-1) (CDCl(3), 25 degrees C). Alternatively, heating the mixtures to 65 degrees C and cooling to room temperature yields the cages within minutes. The (1)H NMR chemical shifts of several characteristic protons show large differences upon changing the identity of the Ru/Rh porphyrin and the central metal; this is most likely to arise from variations in the geometry of the cages. The X-ray crystal structure of a cage, which contains Rh(III)OEP as a porphyrin acceptor and bpy as template, demonstrates that the cages can adopt severely distorted conformations to accommodate the relatively short templates. An extension to mixed DCLs showed that only limited selectivity is displayed by the various templates. Formation of mixed cages that contain two different rhodium porphyrins prevents effective selection, although the kinetic lability of the systems allows for some amplification. This lability, however, also prevents isolation of the individual cages. Removal of the template leads to re-equilibration, thus the templates act as scaffolds to keep the structures intact.

Dynamic Combinatorial Chemistry

“…-215 Cu(I),216 Cu(II),[217][218][219] Fe(II),20,25,26,215,[220][221][222] Ga(III),223 In(III),224 Ir-(II),225 Ni(II), 226 Pd(II),216,[227][228][229][230] Rh(II),231,232 Ru(II),116,225,232 Ti(IV),224,[233][234][235] and Zn(II)16,24,231,236-239 ions have been used to generate DCLs via metal-ligand exchange. The exact Scheme 20.…”

Structural Aspects of Porphyrins for Functional Materials Applications

“…Figure 18. Mixed metalloporphyrin cage, from data in [7,54]. Dark blue = Zn, dark green = Rh, orange = P. Cl is not shown.…”

Self-assembly using dynamic combinatorial chemistry

“…Inspection of some of the possible structures reveals an interesting cyclic tetraporphyrin cage; this appears to be the correct size and shape to allow coordination of 4,4 -bipyridyl within its cavity to the two Zn centres. Indeed, addition of this ligand does lead to quantitative formation of the tetraporphyrin cage containing the bipyridyl as a cavity-bound scaffold (Stulz et al . 2003).…”

“…Of the many different reversible chemical reactions that have been explored for DCC we shall look at examples of just three: metalloporphyrin-ligand coordination, (Stulz et al . 2003).…”