Encapsulated Electroactive Molecules Based upon an Inorganic Cluster Surrounded by Dendron Ligands

Abstract

Encapsulated electroactive molecules are of great importance in understanding biological electron transfer 1 and are of interest for their potential utility in molecular electronics devices (e.g., a molecular redox switch). [2][3][4] We sought to design a tailorable model system in which electron transfer to and from a redox center would be attenuated in a rational fashion by groups surrounding it. Such a system is a logical first step in creating a molecule in which the redox state could be kinetically trapped, potentially permitting molecules in different redox states to coexist in small (admittedly as yet undefined) regions of space without facile electron transfer between them. Here, we report the preparation, characterization, and electrochemical properties of a series of encapsulated electroactive molecules. These are composed of dendrons 5-7 tetrahedrally disposed around an electroactive iron-sulfur core of the form [Fe 4 S 4 (SR) 4 ] 2-. Symmetric disposition of dendrons around an inorganic core represents, to the best of our knowledge, a new molecular topology and thus is a new example of a hybrid organic/ inorganic architecture. When the generation (number of branch points) of the ligands around the core is varied, a simple method for varying the degree of encapsulation is realized.Molecules in which dendrons surround an electroactive group have been prepared by attaching dendrons around the periphery of a porphyrin core. 8-11 Electron transfer to/from porphyrin cores has been affected by the dendron due to its bulkiness and by the microenvironment it creates around the core. The ironsulfur core employed here has much higher (pseudotetrahedral) symmetry and thus enforces a different arrangement of the ligands around it. Using a high-symmetry core should result in a more symmetric placement of the dendrons around it and thus an optimal scheme for encapsulation of the core by the dendron ligands.Molecules G0-G4 ( Figure 1) were synthesized via a ligand exchange reaction described originally by Holm et al. 12 in which aromatic thiols displace aliphatic thiols in FeS clusters of the form [Fe 4 S 4 (SR) 4 ] 2-. In this reaction, 4.1 equiv of a thiol-substituted dendron 13 and 1 equiv of (n-Bu 4 N) 2 [Fe 4 S 4 (S-t-Bu) 4 ] 14 were dissolved in a minimum amount of dimethylformamide in an inert atmosphere and heated at 40°C under partial vacuum for 30 min followed by removal of the solvent at higher vacuum. The resulting black solid was dissolved in methylene chloride and precipitated with the addition of excess diethyl ether. After being filtered and dried, the products appeared as black, shiny, slightly air-sensitive solids. Molecules G1-G4 were soluble in tetrahydrofuran, methylene chloride, and dimethylformamide. Molecule G0 was similarly soluble, with the exception of tetrahydrofuran. The solubility differences are presumed to be the result of the differing ligands surrounding the core. Because of their slight air sensitivity, all characterization of these clusters was performed in an inert atmosphere. Th...

Synthesis of poly(pyridylthioether) dendrimers incorporating a Fe2(CO)6 cluster core

“…The synthetic route we have adopted to prepare a dendrimer-encapsulated Fe 2 (CO) 6 core unit consists of treating commercially available Fe 3 (CO) 12 with a dendron-functionalized aromatic thiol which was synthesized via convergent methodology. 9 The building block designed for the synthesis of all dendron generations, namely compound 12, is depicted in Scheme 2, along with the synthetic approach we have developed for its preparation.…”

“…12 The main object of Gorman's research group, however, was to understand the relationship between dendritic structure and electrochemical properties of encapsulated iron-sulfur clusters. 13 Because dendritic wedges bearing a thiol functionality at their focal point play a key role in the chemistry of both noncovalent and covalent dendrimer-encapsulated metal clusters we decided to explore the synthesis of a new series of thiol-functionalized pyridylthioether-based dendrons.…”

“…It has been previously shown that the reactions between Fe 3 (CO) 12 and bulky aromatic thiols afford compounds of the type [Fe 2 (CO) 6 (m-SR) 2 ] in good yields. 15 We focused our efforts on the preparation of analogous systems with dendritic topologies.…”

“…To this end we initially reacted Fe 3 (CO) 12 with thiol 14 in THF, using triethylamine as a base, followed by treatment with HgCl 2 in a one-pot procedure (Scheme 4), which has been suggested from the communication describing the reactions of alkyl-and arylmercuric halides with [Et 3 NH][((-CO)-((-RS)Fe 2 (CO) 6 ] complexes. 25 According to the above procedure, the desired cluster compound 19 was obtained in 74% yield after chromatographic purification.…”

“…25 According to the above procedure, the desired cluster compound 19 was obtained in 74% yield after chromatographic purification. The analogous reaction of Fe 3 (CO) 12 with dendron-functionalized thiols 16 and 18 afforded the iron cluster dendrimers 20 and 21 in 69 and 49% yield, respectively (Scheme 4).…”