Cages on Surfaces: Thiol Functionalisation of Co^III Sarcophagine Complexes

Abstract A number of new (sarcophagine)cobalt compounds containing oligopeptide substituents terminated by a mercaptoethylamide unit, [Co(R)(R′)sar]26+ (R = NH2, CH3; R′ = NHpeptideCONHCH2CH2S–), have been prepared utilising the carboxymethyl complexes [Co(R){N(CH2CO2H)}sar]3+ (R = NH2, CH3), and subsequently characterised. The amino acid residues, terminating with cystamine groups, were introduced into the carboxymethyl complexes using standard peptide coupling chemistry. The solid‐state structures of [Co(CH3){N(CH2CO2H)}sar]Cl4·H2O, [Co(CH3){N(CH2CO2H)2}sar]Cl3·2H2O and [Co(CH3){N(CH2CO2)2}sar]Cl·6.5H2O were determined by single‐crystal X‐ray diffraction experiments, the first two forming lattices containing homochiral sheets of cations in planes parallel to [0,0,1]. In the structure of [Co(CH3){N(CH2CO2H)}sar]Cl4·H2O, the cations are of the same absolute configuration, but this alternates from one sheet tothe next. In the dihydrate, [Co(CH3){N(CH2CO2H)2}sar]Cl3·2H2O, the structure appears to be very similar. The sheet alternation is different from the structure of the monohydrate, [Co(CH3){N(CH2CO2H)}sar]Cl4·H2O. The deprotonated imidodiacetate complex, [Co(CH3){N(CH2CO2)2}sar]Cl·6.5H2O, was prepared in an unrelated reaction; the structure displays hydogen‐bonding interactions involving the carboxylate groups and coordinated NH units. The solution electrochemistry of these compounds has been investigated using both cyclic voltammetry and differential pulse voltammetry. The compounds exhibited pH‐dependent, quasi‐reversible redox behaviour. A monolayer was prepared by the interaction of [Co(R)sar(GlyNHCH2CH2S–)]26+ and a gold microelectrode using both conventional “self assembly” (SAM) and electrodeposition (EDM) techniques. Surface cyclic voltammograms were measured, which, after a period of induction associated with surface reorganisation, were typical of an ordered monolayer. The monolayer was characterised by XPS, showing peaks assigned to the presence of S, Co, N, O and C. Ellipsometry gave a film thickness of 7 ± 1 Å, consistent with the formation of a mono‐ rather than a multi‐layer. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

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