Many times have I been involved in projects in which we have synthesized sulfur-containing advanced organic materials, to pass on to physicists who in their labs stick them to gold surfaces. Self-assembly is one among many a good reason for doing these things. Our compounds have been either thiols or disulfides. My question is how this works – exactly how this works.

What is the true nature of gold-sulfur bonds? For thiols we often draw a straight single bond from the thiol sulfur (or thiolate, rather – more on that soon) down to a flat and arbitrary gold surface. As if it were an everyday covalent bond. But is it really so? First of all, in the case of thiols, a proton is obviously missing. I have no immediate objections to that; thiols are quite acidic, and the aqueous medium could easily act as a base and scavenge protons. But second, where is the negative charge going after the thiolate attacks the gold surface? And – are we redundantly sure that it is the thiolate, not a thiol, and that its proton is truly out of the equation?

When gold meets sulfur
Figure 1. Sulfur, meet gold. You will find each other attractive.

It gets worse. Disulfides bind as strong as thiols to gold surfaces. For the protocol, variations on higher chalcogens do too. But thiols and disulfides are not even in the same oxidation level! When disulfides are attached to gold surfaces, we illustrate this by leaving the internal disulfide bond intact, and attaching the not-very-specific dashed bond down to the arbitrary surface.

I have asked organic chemists. They don’t wanna talk about it. I have asked physicists. They say the bond is quite strong, at least 126 kJ/mol and that the reaction is practically irreversible. I haven’t bothered with analytical chemists. I need a physical organic chemist (I think?) to once and for all sort this mess out! Do we have one in the audience?

I repeat: What is the true nature of gold-sulfur bonds?

Covalent, ionic, metallic or something else? To which class of chemical bonds to they belong? Is something reduced or oxidized in the process? We know roughly how strong these bonds are. I wanna know what they are made of. Their innermost secrets. “Strong interaction” doesn’t quite cut it.

I want to know everything

Please?

 

9 Responses to What is the true nature of gold-sulfur bonds?

  1. bmcgrail says:

    Cross-post from here

    Speculatively, I’d say that the Au atoms on the surface reduce the disulfides to thiolates and the thiols attach directly, possibly facilitating the partial oxidation of the surface as well, especially if the samples are prepared in air. You may not see the oxidation at the surface because the electrons may come from the bulk, like they would in a reversible electrochemical reaction at a gold electrode. If this is a cluster, though, it’s likely that the atoms display semi-localized behavior, and this reasoning changes and you should be able to show local oxidation.

    The soft-soft interactions between S (or Se, or Te) and gold probably account for their stability vs. protonation or hydrolysis (water, oxide and hydroxide are terrible ligands for gold).

    There’s a ton of debate about this, though.

  2. Chemjobber says:

    What is ‘guld’? I have always known you were a Goa’uld!

  3. milkshake says:

    Au=S- bond is overlappishly chocovalent and its chaacter is relatively smelly because valence electrons in gold atom orbit the nucleus at supersonic speeds

  4. drpalms says:

    My general view on this matter is that all “bonds” are in truth a continuum of “interactions” that we oh-so desperately try to shuffle into one or the other conceptual man-made box. For instance, Lewis structures or bond hybrids, or even more fluffy stuff such as “hard/soft” (I guess “hard” would imply less fluffy, but you get my meaning). Sticking to the covalent v.s. ionic, there is first of all no true definition of ionic, since there is no true definition of atomic charge. Ignoring that, even most ionic bonds (such as NaCl) is partly covalent by most measures. Then of course there is the non-neglible “non-covalent” dispersion contributions… Truht is that, except maybe H2 and similar, bonding is a conceptual mess (unless of course you are a machine, and think in integrals and matrix elements). Thus, since the “nature of bonding” in general has not been solved, I unfortunately have to say your inquiry to the true nature of sulfur-gold bonds is a bit premature. The only unbiast and honest thing we can do is to estimate an interaction energy.

  5. csg says:

    Back then when doing my PhD I also made (fluorous) thiols and disulfides to stabilize gold nanoparticles (even thioacetates did the work). Other than doing elemental analysis and DSC to measure the surface coverage and stability, we haven’t really went after this problem what you brought up.

    If I can have a guess I would give my vote for the explanation in the first comment: Au is oxidized locally when RS- is attached.
    Disulfides are probably reduced as well, so the S-S bond is cleaved when getting in contact with the gold surface. This was actually seen when we compared the DSC of our Au-thiol and Au-disulfide particles.
    Protons from RSH are probably removed by the basic aqueous medium, as you mentioned. We never observed H2 evolution, so I can’t think of any other fates for the protons here.

    All in all, this problem can also go here: http://en.wikipedia.org/wiki/Unsolved_problems_in_chemistry

  6. CW says:

    Actually I liked your way of asking ;-) and even if it’s not the total answer (as research is going on) there is a nature review article on that topic from Hannu Häkkinen from 2012 on that topic
    http://www.nature.com/nchem/journal/v4/n6/pdf/nchem.1352.pdf
    I hope that will help you…

  7. piet wnm says:

    I am surprised that so little is known about this! Publications show that up to at least Au200 the compounds are molecular and have a large band gap. The interior could be regarded as metallic, but the electron follows certain rules, as if they were molecular compounds. The surface atoms, but not all, can be regarded as Au(I), but the ligand properties (NMR) show that they are rather electron rich, as a result of their interactions with the Au(0) core. Hard anions show both Au(I) and Au(0) in the XPS. Softer anions may show less resolution of the two, or none at all. We are working on a different type of soft anion on the surface and I will come back to this theme.

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