• Koutsantonis, George
• Orlowski, G. A.
• Nealon, G. L.
• Skelton, Brian
• Kraatz, H-B.
• White, Allan
• Harrowfield, J. M.

Abstract

A number of new (sarcophagine) cobalt compounds containing oligopeptide substituents terminated by a mercaptoethylamide unit, [Co(R)(R')sar](2)(6+) (R = NH2, CH3; R' = NHpeptide-CONHCH2CH2S-), 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]Cl-4 center dot H2O, [Co(CH3)(CH2CO2H)(2)}sar]Cl-3 center dot 2H(2)O and [Co(CH3)(N(CH2CO2)(2))sar]Cl center dot 6.5H(2)O 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]Cl-4 center dot H2O, the cations are of the same absolute configuration, but this alternates from one sheet to the next. In the dihydrate, [Co(CH3)(CH2CO2H)(2)}sar]Cl-3 center dot 2H(2)O, the structure appears to be very similar. The sheet alternation is different from the structure of the monohydrate, [Co(CH3)[N(CH2CO2H)}sar]Cl-4 center dot H2O. The deprotonated imidodiacetate complex, [Co(CH3){N(CH2CO2)(2)}sar]Cl center dot 6.5H(2)O, 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(GlyNHCH(2)CH(2)S-)](2)(6+) 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 angstrom, consistent with the formation of a mono- rather than a multi-layer (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007.

Topics

• impedance spectroscopy
• surface
• compound
• x-ray diffraction
• experiment
• x-ray photoelectron spectroscopy
• gold
• ellipsometry
• forming
• cobalt
• electrodeposition
• cyclic voltammetry
• scanning auger microscopy
• pulse voltammetry