Posts Tagged ‘Proton’

A Theoretical Method for Distinguishing X‐H Bond Activation Mechanisms.

Wednesday, July 25th, 2018

Consider the four reactions. The first two are taught in introductory organic chemistry as (a) a proton transfer, often abbreviated PT, from X to B (a base) and (b) a hydride transfer from X to A (an acid). The third example is taught as a hydrogen atom transfer or HAT from X to (in this example) O. Recently an article has appeared[1] citing an example of a fourth fundamental type (d), which is given the acronym cPCET which I will expand later. Here I explore this last type a bit further, in the context that X-H bond activations are currently a very active area of research.

To help understand these four types, I have colour-coded the electron pair constituting the X-H covalent bond in red.

  1. In mechanism (a), this electron pair stays with X, thus liberating a proton which is captured by the base.
  2. The hydride transfer (b) is so-called because in fact this electron pair travels together with the proton, hence the term hydride or H–.
  3. Hydrogen atom transfers as in (c) in effect transfer both a proton and one electron to another atom (oxygen in the example above), leaving behind one electron on X. The electron and the proton are said to travel together as a “true” hydrogen atom.
  4. The fourth mechanism (d) is fundamentally different from (c) in that whilst the electron and the proton travel in concert (at the same time), they do not travel together. In this example the proton travels to the oxygen, whilst the electron travels to the iron centre, in the process reducing its oxidation state. This mode is now called a concerted proton-coupled electron transfer, or cPCET as above.

The tool employed to distinguish between mechanisms (c) and (d) is the IBO or intrinsic bond orbital localisation scheme.[2] One practical advantage of such a scheme over better known localisation methods such as NBO (Natural bond orbitals) is that IBOs can be made to transform smoothly during the course of a reaction, as followed by say an IRC (Intrinsic reaction coordinate). NBOs may instead show discontinuous behaviour along a reaction IRC. Klein and Knizia have located transition states for examples of both (c) and (d) above and studied the IBOs along such IRCs. The two IBO reaction transformations are very different, as illustrated below (used, with permission, from the article itself). For the HAT type (X=C above), an α-spin IBO morphs from a C-H bond into a H-O bond, whilst the β-spin counterpart morphs from being located on the C-H bond into a carbon-centered radical. For the cPCET mode, the α-spin IBO morphs from C-H to a C-centered radical, but the β-spin region grows onto an iron d-orbital. It is in fact even more complex than the diagram above implies, since some reorganisation of the O-Fe region occurs and the H…:O region is still anti-bonding at the transition state.

We can see from this that mechanistic reaction analysis is starting to track the “curly arrows” we conventionally use to represent reactions in some detail, as well as informing us about the relative detailing timing of the various curly arrows used. Of course this latter aspect cannot be easily represented by conventional curly arrows. It seems timely to revisit the vast corpus of organic and organometallic “curly arrow pushing” to starting adding such information!

References

  1. J.E.M.N. Klein, and G. Knizia, "cPCET versus HAT: A Direct Theoretical Method for Distinguishing X–H Bond‐Activation Mechanisms", Angewandte Chemie International Edition, vol. 57, pp. 11913-11917, 2018. https://doi.org/10.1002/anie.201805511
  2. G. Knizia, "Intrinsic Atomic Orbitals: An Unbiased Bridge between Quantum Theory and Chemical Concepts", Journal of Chemical Theory and Computation, vol. 9, pp. 4834-4843, 2013. https://doi.org/10.1021/ct400687b

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Intermolecular atom-atom bonds in crystals? The O…O case.

Saturday, July 25th, 2015

I recently followed this bloggers trail; link1 → link2 to arrive at this delightful short commentary on atom-atom bonds in crystals[1] by Jack Dunitz. Here he discusses that age-old question (to chemists), what is a bond? Even almost 100 years after Gilbert Lewis’ famous analysis,[2] we continue to ponder this question. Indeed, quite a debate on this topic broke out in a recent post here. My eye was caught by one example in Jack’s article: “The close stacking of planar anions, as occurs in salts of croconic acid …far from producing a lowering of the crystal energy, this stacking interaction in itself leads to an increase by several thousand kJ mol−1 arising from Coulombic repulsion between the doubly negatively charged anions” I thought I might explore this point a bit further in this post.

A search query of the Cambridge structure database was defined as below. Two non-bonded oxygen atoms are each attached to one carbon, each oxygen was defined as having one bonded atom (to carbon) and each assigned one negative charge. Addition of the usual constraints of R < 0.05, no errors, no disorder and specifying an intermolecular search produced 103 hits with the distance distribution shown below.

OO-query

O-O

Firstly, you should be aware that the van der Waals radius for oxygen is ~1.5Å, and so any contacts less than 3.0Å become interesting. What becomes particularly exciting is the distinct cluster at ~2.5Å. Could these be ~30 examples of close encounters of the type noted by Dunitz? Well, a control search has to be done, this time for O-H-O motifs, with each OH distance plotted as below:

OHO

The hot-spot occurs when both OH distances are equal at ~1.22Å, or an O…O separation close to 2.45Å. Time to quote Dunitz again “This large destabilization is, of course, more than compensated in the overall energy balance by the large stabilization arising from Coulombic interactions of the croconate anions with the surrounding cations.” In this case of course, the cation is a proton, residing at the half way point between the two oxygens. So two oxygens can indeed approach ~0.5Å closer than the sum of the vdw radii if a proton sits in-between them.

What do we learn? Well, firstly that one should always have a reality check of the results of any crystal structure search. The search did specify that the oxygens be non-bonded but also that they should both carry a negative charge and that both should only have one bonded atom. That should in theory at least have excluded any C-O-H-O-C structures, so why were about 30 such examples found? I can only speculate here, but recollect that 50 years ago when the CSD was founded, hydrogen atoms were rarely identified from the electron density. They were instead placed or “idealised” to where they might be expected. Nowadays any contentious hydrogens are almost always located rather than idealised, but clearly their status as bona-fide atoms is not quite so strong as the rest of the periodic table. So in at least some of these 30 examples with short O…O contacts, we might expect there to lurk a (possibly unrecognised) proton. But one never knows, there may be some real examples of O…O contacts with no such proton intervening. Now these really would be interesting.

Postscript. F is isoelectronic with O(-); below is the same search as defined above, but for non-bonded CF…FC approaches. F---F

The vdw radius of F is 1.45Å hence any non-bonded contact <2.9Å is worth taking a look at. But notice the small cluster of about 10 compounds for which the value is ~2.15Å. The F-H-F plot shows a hot spot at ~2.3 for the F…F separation, but there are zero hits for CF-H-FC. So these ten hits are indeed tantalising.

References

  1. J.D. Dunitz, "Intermolecular atom–atom bonds in crystals?", IUCrJ, vol. 2, pp. 157-158, 2015. https://doi.org/10.1107/s2052252515002006
  2. G.N. Lewis, "THE ATOM AND THE MOLECULE.", Journal of the American Chemical Society, vol. 38, pp. 762-785, 1916. https://doi.org/10.1021/ja02261a002

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