Elucidation of the Structure of a Thiol Functionalized Cu-tmpa Complex Anchored to Gold via a Self-Assembled Monolayer

Abstract

The structure of the copper complex of the 6-((1-butanethiol)oxy)-tris(2-pyridylmethyl)amine ligand (Cu-tmpa-O(CH₂)₄SH) anchored to a gold surface has been investigated. To enable covalent attachment of the complex to the gold surface, a heteromolecular self-assembled monolayer (SAM) of butanethiol and a thiol-substituted tmpa ligand was used. Subsequent formation of the immobilized copper complex by cyclic voltammetry in the presence of Cu(OTf)₂ resulted in the formation of the anchored Cu-tmpa-O(CH₂)₄SH system which, according to scanning electron microscopy and X-ray diffraction, did not contain any accumulated copper nanoparticles or crystalline copper material. Electrochemical investigation of the heterogenized system barely showed any redox activity and lacked the typical Cu^II/I redox couple in contrast to the homogeneous complex in solution. The difference between the heterogenized system and the homogeneous complex was confirmed by X-ray photoelectron spectroscopy; the XPS spectrum did not show any satellite features of a Cu^II species but instead showed the presence of a Cu^I ion in a ∼2:3 ratio to nitrogen and a ∼2:7 ratio to sulfur. The +I oxidation state of the copper species was confirmed by the edge position in the X-ray absorption near-edge structure (XANES) region of the X-ray absorption spectrum. These results show that upon immobilization of Cu-tmpa-O(CH₂)₄SH, the resulting structure is not identical to the homogeneous Cu^II-tmpa complex. Upon anchoring, a novel Cu^I species is formed instead. This illustrates the importance of a thorough characterization of heterogenized molecular systems before drawing any conclusions regarding the structure-function relationships.

Scheme 1. Synthetic Pathway toward the Thiol Ligand of Interest

Reagents and conditions: (a) NaBH(OAc)₃, THF, 3 Å molecular sieves, N₂ atm., r.t., overnight, 92%; (b) 4-((^tbutyldimethylsilyl)oxy)butan-1-ol, NaH, THF, N₂ atm., reflux, overnight; (c) TBAF, THF, 0 °C, 6 h, 68% over 2 steps; (d) SOCl₂, CHCl₃, r.t., overnight, 80%; (e) 1. thiourea, KI, EtOH, reflux, 2 days, 2. NaHCO₃, EtOH, reflux, overnight, 27%.

Scheme 2. Schematic Overview of the Steps Involved in the Electrode Modifications (Light Gray) with the Modified Electrodes Shown in Dark Gray

Figure 1

Cyclic voltammograms of an unmodified gold WE (gray lines) and modified WEs Au|mixed SAM (red line in a), ^CVAu|mixed SAM|Cu (blue line in a), ^EDTAAu|mixed SAM|Cu (green line in a), Au|butanethiol (red line in b), ^CVAu|butanethiol|Cu (blue line in b), and ^EDTAAu|butanethiol (green line in b) in 100 mM pH 7 HEPES buffer solution in 75 mM Na₂SO₄ electrolyte under an oxygen atmosphere at a scan rate of 100 mV s^–1. The modified WEs were measured in the dark. For clarity, only the third scan of all measurements is shown. The full measurements of 5 scans are shown in Figures S1 and S2.

Figure 2

Cyclic voltammograms of modified WEs Au|mixed SAM (a) and Au|butanethiol (b) in 0.3 mM Cu(OTf)₂ in 100 mM pH 7 HEPES buffer solution in 75 mM Na₂SO₄ electrolyte under an argon atmosphere at a scan rate of 100 mV s^–1. The first scan is depicted as a red line. The second, third, and fourth scans are depicted as gray lines, and the fifth scan is depicted as a blue line.

Figure 3

XPS spectra (black lines) of modified electrodes ^CVAu|mixed SAM|Cu, ^EDTAAu|mixed SAM|Cu, ^ORRAu|mixed SAM|Cu, and ^CVAu|butanethiol|Cu, and reference compounds [Cu(tmpa)(MeCN)](OTf)₂, Cu(OTf)₂, and 4. (a) Cu 2p region, (b) Cu L₃M_4,5M_4,5 Auger region, (c) N 1s region, and (d) S 2p region of the XPS spectra. The deconvolution of the Cu 2p_3/2, Cu L₃M_4,5M_4,5 Auger, N 1s, and S 2p regions is depicted in gray.

Figure 4

XANES region of the Cu K-edge XAS spectra measured for ^EDTAAu|mixed SAM|Cu (bold blue line) and [Cu(tmpa)(MeCN)](OTf)₂ (red line). Reference spectra from the literature of a distorted trigonal bipyramidal Cu^II complex (red dashed line, complex 1 in Figure S8), 13 Cu^I complexes⁻ (thin blue lines, complexes 2–14 in Figure S8), metallic copper (green line), and Cu⁰ nanoparticles (dashed green line) are depicted as well. The spectral data of the red dashed line was reprinted and adapted with permission from ref (36). Copyright 1987 American Chemical Society. The spectral data of the thin blue lines was reprinted and adapted with permission from refs (−40). Copyright 1987 American Chemical Society, 1996 John Wiley and Sons, 2013 American Chemical Society, 2013 Taylor & Francis, 2014 American Chemical Society, respectively. The spectral data of the green line was reprinted and adapted with permission from ref (46). Copyright 2013 Elsevier. The spectral data of the dashed green line was reprinted and adapted with permission from ref (47). Copyright 2018 John Wiley and Sons. The data was extracted using ScanIt. The shown spectrum of ^EDTAAu|mixed SAM|Cu is a smoothed fit of the raw data (Figure S6).

Figure 5

SEM image, quantitative elemental mapping (a), and EDX spectrum (b) of modified electrode ^EDTAAu|mixed SAM|Cu. Scale bars in a are 5 μm.

Figure 6

Schematic representation of the modification on the gold surface taking into account the elemental ratios and oxidation state assignment on the basis of XPS and XANES. In analogy with the structural arrangements of the Cu₄O₄ core of tetranuclear copper units into a tetramer cubic structure,⁻ a cubane type of arrangement of the bridging copper ions with water as the coordinating solvent and hydroxide as the coordinating counterion (L = H₂O and X = OH^–) seems likely.