Electrochemistry in Nanostructered Inorganic Molecular Materials
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Electrochemistry in Nanostructered Inorganic Molecular Materials Mary Elizabeth Williams and Joseph T. Hupp Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 Abstract We have previously described the synthesis of a family of molecular ‘squares’ based on octahedral Re(I) coordination of difunctional bridging ligands. The size and chemical composition of the square cavity is highly tunable, and the inorganic cyclophanes are being actively studied in catalytic, separations, and sensing applications. Electrochemical techniques have been primary methods for characterization of the transport properties of thin films of the nanostructured materials. For example, cyclic voltammetry and rotating disk electrode voltammetry experiments have revealed size-selective permeation by redox probes, where the size cutoff is determined by the internal square dimensions. We have more recently begun to employ scanning electrochemical microscopy to spatially image micropatterned electrodes containing these thin film materials, simultaneously allowing us to obtain permeability data and topographical information. This paper describes data obtained by employing porphyrin-based molecular squares that feature chemically tailored cavities. Introduction We have actively been involved in the design and synthesis of molecular compounds capable of functioning in the solid state as microporous and mesoporous materials [1]. Among the applications, or potential applications, for such materials are membrane catalytic reactivity, environmental sensing of volatile hydrocarbons and heavy metals, and chemical separations [2]. To obtain the desired materials, we have prepared a family of inorganic complexes based on ciscoordination of ReI (CO)3Cl fragments, according to the synthetic strategy shown in Scheme 1. CO Cl OC Re OC
Re(CO)5Cl + THF, 48 hours
Cl OC Re OC CO
CO CO Re CO Cl
CO Re CO Cl
CO
Scheme 1. General synthetic scheme for the rhenium-based molecular squares. Although only one isomer, with respect to carbonyl and chloro ligand positions, is shown, it is believed that all four of the possible isomers are obtained. Using a series of difunctional pyridine-based bridging ligands, some of which are shown in Scheme 2, we have demonstrated that the size of the molecular square (the spacing between Re corners) may be varied from 7 to 24 Å [1]. All of the molecular squares possess Re(CO)3Cl Y1.5.1
N
N
N 10Å
N
H3C(H2C)3
N
12Å O
O
O
O
N
18Å
R
(CH2)3CH3
N N H3C(H2C)3 20Å
24Å
N Zn N N N
N Zn
N
N
N
7Å
N N (CH2)3CH3
N N
Zn
O
N O
t-Bu
t-Bu
N
N
R R=
23Å
N
O O Et
Scheme 2. Structures of the pyridine-based ligands: (A) pyrazine; (B) 4,4’-bipyridine; (C) 4,4’-bis(pyridyl) ethylene; (D) dipyridyl ; (E) 2,8,12,18-tetrabutyl-3,7,13,17- tetramethyl5,15-bis(4-pyridyl) ZnII porphyrin; (F) bis(3-tert-butyl-5-(4-pyridyl) salicylidene)-1,2phenylenediaminozinc(II); (G) 10,20-bis(4-ethoxy phenyl)-5,15-bis(ethynyl-4-pyridyl) ZnII porphyrin. corners and thus are neutral in charge, making them solub
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