Head‐To‐Tail and Heteroleptic Pentanuclear Circular Helicates

Abstract

The rational design of polynuclear helicates is one of the major achievements of metallosupramolecular chemistry. [1] These linear structures form by self-assembly and consist of two or more multidentate ligand strands that are helically wrapped about a central array of metal cations.[1] Not only can polynuclear double-, triple-, and quadruple-stranded helicates now be made in a predictable fashion, [1f] they can also be programmed to express certain structural features of higherorder complexity. This goal may be achieved by elaborating on the basic design principles that govern helicate formation itself (i.e., careful consideration of ligand topology and metal stereoelectronic preference) and, amongst others, can entail: 1) directional control over ligand alignment, 2) selective incorporation of different metal cations, and 3) selective incorporation of different ligand strands, within the helical array.[1]The first two of these scenarios are intimately linked. They can be illustrated by considering a ditopic C s -symmetric Ndonor ligand with one bidentate and one tridentate donor site.[2] When combined with equal amounts of a fivecoordinate metal cation, a double-stranded dinuclear C 2 -symmetric head-to-tail (HT) helicate results, wherein each metal is coordinated by a "2+3" donor set comprising the tridentate head of one ligand and the bidentate tail of the other (Figure 1 a). Alternatively, combination of the same ligand with half an equivalent each of a four-and sixcoordinate cation gives a hetero-bimetallic head-to-head (HH) helicate. In this case, one metal is octahedrally (3+3) coordinated by the two tridentate heads whilst the other is tetrahedrally (2+2) coordinated by the two bidentate tails (Figure 1 b).[2] Importantly, the two ligands are now coaligned in a parallel fashion. The third scenario can likewise be realized by combining a five-coordinate cation with half an equivalent each of a C 2v -symmetric bis(bidentate) and bis(tridentate) ligand. The resulting structure is a C 2 -symmetric heteroleptic (mixed ligand) helicate, wherein both cations have the required bi-and tridentate (2+3) donor sets (Figure 1 c). [3] More subtle effects, such as interligand interactions [4] and cation or binding-site size mismatch, [5] can also be used to fine-tune structural complexity in linear helicates. However, neither these effects nor the more intuitive principles described above have ever been applied to helicates that have a higher nuclearity, that is, circular helicates. Herein we describe how some of the basic rules that govern linear helicate assembly can also be applied to generate the first heteroleptic and head-to-tail circular helicates.If the linear double-stranded helicates discussed above are denoted as [M 2 (L) 2 ], then circular helicates are cyclic oligomers that have the general formula [M n (L) n ] (n > 2) and retain the "over-and-under" ligand motif requisite of helical chirality.[6] Circular helicates can arise from intermolecular templating effects or intramolecular interactions that ...

Two-stage directed self-assembly of a cyclic [3]catenane

“…No combination of aniline and coordinating anion was found to generate the architecture of 1 where all iron(II) centres exist in a low-spin state, with insoluble precipitates being formed when higher ligand field strength anions (e.g. F − ) were employed.Five distinct building blocks thus self-assembled to form mixed-ligand circular helicate23 1 in solution. Crucially no homoleptic species of either ligand were detected by ESI-MS or 1 H NMR spectroscopy.…”

Dynamic Helicates Self‐Assembly from Homo‐ and Heterotopic Dynamic Covalent Ligand Strands

“…[16] In fact, by using a2 :2:1 Ligand:Cu II :Zn II mixture, ac omplex with the same ratio of metal cations was obtained. This is ar are example of ah exanuclear heterometallic (Cu 2 + and Zn 2 + )c ircular helicate, obtained by as elf-assembly approach( for hetero-self-assembling systems see also references [8] and [17] ). The structure of this speciesw as confirmed by Xray crystallography and is shown in Figure 2.…”

Progress in Helicates Directed by Metal Coordination

“…Rice 课题组 [96] 报道了一系列含有吡啶-噻唑的寡聚 物分子 51～53, 如图 [107] . 而 61 与 Ti 4＋ 配位后在二氯甲烷与正戊烷中扩散则形成三 核的环状螺旋结构, Ti 4＋ 分别与两个配体分子上的四个 酚基 O 原子及两个异丙氧基形成八面体型配位.…”