Guest Binding Drives Host Redistribution in Libraries of CoII 4 L4 Cages

Abstract

Two Co^II ₄ L₄ tetrahedral cages prepared from similar building blocks showed contrasting host-guest properties. One cage did not bind guests, whereas the second encapsulated a series of anions, due to electronic and geometric effects. When the building blocks of both cages were present during self-assembly, a library of five Co^II L^A _x L^B _4-x cages was formed in a statistical ratio in the absence of guests. Upon incorporation of anions able to interact preferentially with some library members, the products obtained were redistributed in favor of the best anion binders. To quantify the magnitudes of these templation effects, ESI-MS was used to gauge the effect of each template upon library redistribution.

Keywords: host-guest systems; mass spectrometry; metal-organic Cages; self-assembly; supramolecular library.

Figure 1

A and B self‐assembled with 2‐formylpyridine and Co^II to form 1 and 2, respectively. Three‐dimensional views are constructed from the X‐ray crystal structures of 1 and ClO₄ ⁻⊂2 (Co^II: orange, L^A: blue, L^B: red; orange lines indicate the closest metal–metal separations, to illustrate geometry). The insert gives the list of guests encapsulated by 2 but not 1 along with their volumes (ų).

Figure 2

a) ¹H NMR spectrum of Lib ${}_{\mathbf{N}\mathbf{T}\mathbf{f}{}_{\mathbf{2}}}$ with each of the imine signals attributed to one of the cage species in the insert. b) Low‐resolution ESI‐MS of Lib ${}_{\mathbf{N}\mathbf{T}\mathbf{f}{}_{\mathbf{2}}}$ . The +3 region of the mass spectra is expanded, showing the peak clusters corresponding to cages in the absence of a template. c) Normalized integrals of Lib ${}_{\mathbf{N}\mathbf{T}\mathbf{f}{}_{\mathbf{2}}}$ obtained by NMR and ESI‐MS compared to the statistical binomial distribution.

Figure 3

Low‐resolution ESI‐MS of Lib ${}_{\mathbf{B}\mathbf{F}{}_{\mathbf{4}}}$ obtained after addition of TBABF₄ (2 equiv) to Lib ${}_{\mathbf{N}\mathbf{T}\mathbf{f}{}_{\mathbf{2}}}$ . The +3 region of the mass spectra is expanded, showing the peak clusters corresponding to cages with no BF₄ ⁻ (green circle) and cages with one BF₄ ⁻ associated (yellow triangle).

Figure 4

a) Normalized ESI‐MS integrals of libraries with no guest (NTf₂ ⁻, gray) and after addition of guests (Br⁻, I⁻, BF₄ ⁻ and ClO₄ ⁻ in blue, yellow, red, and green, respectively). Each experiment was repeated three times and averaged across all charge states to obtain standard deviations, of which only positive values are shown as error bars for clarity. b) Factor of amplification or decrease of each congener generated in the libraries relative to the binomial distribution baseline with no amplification.

Figure 5

Formation of a library of Co^II L^A _x L^B _(4−x), 0≤x≤4. The library expressed a near‐statistical distribution of cage species in the absence of a guest. A bias towards structures incorporating more L^B was observed when guests were added.

Figure 6

Relative energies (kJ mol⁻¹) of each Co^II ₄ L^A _x L^B _4−x cage within the libraries. Each experiment was repeated three times and averaged across all charge states.