Arsenic–π Interactions Stabilize a Self‐Assembled As₂L₃ Supramolecular Complex

Abstract

The self-assembly of supramolecular coordination compounds has resulted in structures with a diverse range of properties, shapes, sizes, and stoichiometries, including helicates (M 2 L 2 , M 2 L 3 , etc.), triangles (M 3 L 3 ), squares (M 4 L 4 ), rings (e.g., M 6 L 6 ), and polyhedra (M 4 L 4 , M 4 L 6 , M 6 L 6 , M 8 L 12 , etc.).[1] These assemblies almost exclusively contain metals with square-planar, tetrahedral or octahedral coordination geometries. Metals that can exhibit more peculiar coordination geometries have typically been avoided as components for a supramolecular design strategy, [2] most likely because of their variable coordination spheres. Herein we describe a supramolecular design strategy for forming arsenic-based assemblies, which relies on the self-assembly of arsenic(iii) with thiol ligands-and hence the reversibility and lability of arsenic-sulfur bonds-and we report the first member of this class, [As 2 L 3 ] (H 2 L = a,a'-dimercapto-para-xylene). The design strategy incorporates the unusual, yet predictable trigonal-pyramidal coordination geometry of As III featuring a stereochemically active lone pair when coordinated by sulfurbased ligands (Figure 1 a). [3][4][5][6] We selected arsenic(iii) as a design component because of its unusual coordination geometry and a general lack of specific and powerful chelators for this highly toxic ion. [7] Arsenic compounds are legendary for their toxicity, and hydrated As III is recognized as a known human carcinogen. Arsenic is abundant in the earths crust and is all-toofrequently present as an environmental health hazard; [8][9][10] however, specific chelators for environmental and in vivo remediation and sensing applications are lacking. Furthermore, the stereochemically active lone pair of As III adds a novel feature to the targeted supramolecular assemblies: for instance, lone pairs directed into the cavity would provide a unique soft, Lewis-basic cavity environment (Figure 1 b). [11] Figure 1 illustrates the design strategy for forming C 3h -symmetric [As 2 L 3 ] assemblies, which is based on rigid, twofold symmetric dithiols capable of acting as bridging ligands. Dithiol ligand H 2 L[12] (Scheme 1) was the initial bridging ligand investigated, because it is easily prepared and thiolates show a well-known affinity for As III .[12] Using a ligand such as H 2 L allows for two possible orientations of the arsenic lone pairs: into or out of the cavity of the complex. Molecular models (CAChe, MM3) indicated that the lone pairs would be directed into the cavity with an AsÀAs distance of about 6 . [13] H 2 L, synthesized by a literature procedure, [12] forms a yellowish white solid in 52 % yield when treated with stoichiometric amounts of AsCl 3 and KOH in THF/methanol mixtures. The product has very simple 1 H and 13 C NMR spectra, indicative of formation of the desired high-symmetry C 3h complex, and the parent ion peak [H{As 2 L 3 }] + is present in the electrospray ionization mass spectrum. Interestingly, [As 2 L 3 ] also forms in the absence of any...

Synthesis and characterization of a rare arsenic trithiolate with an organic disulfide linkage and 2-chloro-benzo-1,3,2-dithiastibole

“…The As-S bonds in 1 range from 2.23 Å to 2.29 Å . There are no intramolecular As-p interactions, despite the fact that As-p interactions are a well known characteristic of As-aromatic systems [3].…”

“…Recent interest in arsenic thiolates as building blocks in self-assembled macrocycles [2,3] along with a lack of single crystal X-ray structural analysis of relatively simple arsenic thiolates has prompted our work on the synthesis and characterization of several arsenic dithiolates [4]. These are of the general formula RS 2 AsX where R is an organic group connecting the sulfurs and X = Cl or I.…”

The Importance of Stereochemically Active Lone Pairs For Influencing PbII and AsIII Protein Binding

“…[27, 34] More generally our study is a contribution to highlight and rationalize further subtle factors affecting the coordination geometry of heavy main group elements in proteins and supramolecular systems. Indeed, in the process of developing new supramolecular coordination compounds, Johnson and collaborators have already thoroughly explored the role played by weak attractive forces, such as As···π interactions and secondary bonding interactions, in trigonal-pyramidal As(III) complexes, [28b] demonstrating that such interactions can dramatically affect the coordination geometry of As(III) in thiolate complexes. [28a, 50] In particular, while exohedral coordination is generally observed in As(III) compounds, the intramolecular As···π interactions in As 2 (L) 3 and As 2 (L) 2 Cl 2 complexes in which L is an aromatic ligand was shown to induce endohedral directionality of the As(III) stereochemically active lone pair.…”

“…[24] The preference of trigonal-pyramidal thiolate structures by As(III) with small molecules [24e, 25] and in proteins [26] is further proven by a recent crystal structure of the designed three-stranded coiled coil As( CS L9C) 3 by Touw et al [26–27] Crystal structures of ArsD and ArsR have not yet been reported to explain the exact mode of As coordination, however, an exo-conformation (shown in scheme 1) 1 of AsS 3 was speculated based on the crystal structure of synthetic small molecules with aromatic rings or chelated alkyl dithiolates. [25] In contrast, Johnson and coworkers (using small molecules) [28] and Pecoraro and coworkers (using designed 3-strand coiled coil peptides) [27] discovered that the orientation of the lone pair could influence the As(III) to adopt an endo conformation. The As( CS L9C) 3 [PDB code 2JGO] was the first example that directly mimicked the coordination environment of As(III) in ArsR and ArsD.…”

Co-operation of π⋯π, Cu(II)⋯π, carbonyl⋯π and hydrogen-bonding forces leading to the formation of water cluster mimics observed in the reassessed crystal structure of [Cu(mal)(phen)(H2O)]2·3H2O (H2mal=malonic acid, phen=1,10-phenanthroline)

“…The complicacies and sophistication of self-assembly process is due to the presence of various possible interactions which can lead to many ways of assembling the building blocks. Usually the strength of these possible interactions also come in a wide range including strong covalent force, intermediate coordinative force [4][5][6][7] and various weak forces such as hydrogen bonding [8][9][10][11][12][13][14], pÁÁÁp interaction [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], C-HÁÁÁp interaction [31,32], metalÁÁÁp [33][34][35][36][37][38], anionÁÁÁp interaction [39] and lone pairÁÁÁp interaction [40][41][42][43][44][45][46][47][48]. The existence of phenomena like polymorphism [49][50]…”