Linked Supramolecular Building Blocks for Enhanced Cluster Formation

Abstract

Methylene-bridged calix[4]arenes have emerged as extremely versatile ligand supports in the formation of new polymetallic clusters possessing fascinating magnetic properties. Metal ion binding rules established for this building block allow one to partially rationalise the complex assembly process. The ability to covalently link calix[4]arenes at the methylene bridge provides significantly improved control over the introduction of different metal centres to resulting cluster motifs. Clusters assembled from bis-calix[4]arenes and transition metal ions or 3d-4f combinations display characteristic features of the analogous calix[4]arene supported clusters, thereby demonstrating an enhanced and rational approach towards the targeted synthesis of complex and challenging structures.

Bis‐Calix[4]arenes: From Ligand Design to the Directed Assembly of a Metal–Organic Trigonal Antiprism

“…We recently expanded our cluster‐forming chemistry with C[4] to focus on bis‐C[4], a building block comprising two C[4]s linked directly through a methylene bridge. In doing so we found that when the constituent C[4] cavities are proximal the molecule behaves closely to how one would expect with respect to cluster formation . To illustrate this, the analogous reaction used to form the [Mn III 2 Mn II 2 (C[4]) 2 ] cluster (Figure A) affords a new [Mn III 4 Mn II 4 (bis‐C[4]) 2 ] assembly (Figure C), which is essentially ‘double’ the original cage (Figure D).…”

“… Structures of [Mn III 2 Mn II 2 (C[4]) 2 ] (A), [Mn III 4 Ln III 4 (C[4]) 4 ] (B) and [Mn III 4 Mn II 4 (bis‐C[4]) 2 ] (C) clusters showing persistence of the [Mn(C[4])] − moiety. D) Cluster core topologies from A–C (rotated 90° for the Mn 8 cluster in C) to show structural capping behaviour of the [Mn(C[4])] − moiety and the relationship upon linking at the methylene bridge.…”

A New Family of 3d–4f Bis‐Calix[4]arene‐Supported Clusters

“…As is the case for [Mn III 2 Mn II 2 (OH) 2 (C[4]) 2 ], the Mn II/III ions in the bis‐C[4]‐supported cluster are linked through bridging phenolates and hydroxides. The second cluster to emerge from this exploratory chemistry was a [Mn III 4 Mn II 2 Gd III 2 (bis‐C[4]) 2 ] species that is a structural relative of the aforementioned Mn 8 cluster (compare Figure 1 D and E), the main difference being the interchange of two Mn II for Ln III ions; this behavior is a close parallel to that described above for C[4], further suggesting that methylene bridge linking is a viable route to the logical extension of metal ion binding rules/ behaviors for multi‐component systems. We recently showed that, as in C[4]‐supported cluster chemistry, variation in reaction stoichiometry has a dramatic effect on the assembly of bis‐C[4]‐supported clusters.…”

“…C) Tri‐capped trigonal prismatic Cu II 9 cluster, with the molecule shown along the C 3 axis of the prismatic core . D) Bis‐C[4]‐supported Mn III 4 Mn II 4 cluster showing two fused and distorted butterflies . E) Bis‐C[4]‐supported Mn III 6 Mn II 2 Ln III 2 cluster obtained via stoichiometric control .…”

“…This is of particular interest considering that, akin to C[4], bis‐C[4] would be conformationally flexible, but it could also act as a double capping species upon complexation with suitable metal ions . Initial investigations conducted with bis‐C[4] showed that the established binding rules were indeed maintained upon metal ion complexation, and the first cluster isolated was a [Mn III 4 Mn II 4 (bis‐C[4]) 2 ] species that can be visualized as two distorted and fused {Mn III 2 Mn II 2 } cores (compare Figure A and 1D) . Structural differences between these two related clusters are primarily due to methylene bridge linking, and in this case two Mn II ions occupy binding sites between the two C[4] moieties.…”