The classic anomeric effect operates across a carbon atom attached to oxygens. One (of the two) lone pairs on the oxygen can donate into the σ* orbital of the C-O of the other oxygen (e.g. the red arrows) tending to weaken that bond whilst strengthening the donor oxygen C-O bond. Vice versa means e.g. the blue arrows weakening the other C-O bond. This effect tends to increase charge separation and the question then arises: how large can this effect get? To try to find out, we are going to do some crystal structure mining in this post! 
Archive for the ‘crystal_structure_mining’ Category
Two record breakers for the anomeric effect; one real, the other not.
Thursday, July 1st, 2021A reality-based suggestion for a molecule with a metal M⩸N quadruple bond.
Thursday, May 13th, 2021I noted in an earlier post the hypothesized example of (CO)3Fe⩸C[cite]10.1039/d0cp03436c[/cite] as exhibiting a carbon to iron quadruple bond and which might have precedent in known five-coordinate metal complexes where one of the ligands is a “carbide” or C ligand. I had previously mooted that the Fe⩸C combination might be replaceable by an isoelectronic Mn⩸N pair which could contain a quadruple bond to the nitrogen. An isoelectronic alternative to FeC could also be FeN+. Here I explore the possibility of realistic candidates for such bonded nitrogen.
Two new reality-based suggestions for molecules with a metal M⩸C quadruple bond.
Saturday, May 8th, 2021Following from much discussion over the last decade about the nature of C2, a diatomic molecule which some have suggested sustains a quadruple bond between the two carbon atoms, new ideas are now appearing for molecules in which such a bond may also exist between carbon and a transition metal atom. A suggested, albeit hypothetical example was C⩸Fe(CO)3[cite]10.1039/d0cp03436c[/cite]. Iron has a [Ar].3d6.4s2 electronic configuration and if we ionise to balance a C4- ligand, the iron becomes formally FeVI or [Ar].3d4. By adding 14 electrons deriving from the seven “bonds” to the 3d4, including a quadruple count from carbon, the Fe formally completes its 18-electron valence shell, as also found in e.g. Ferrocene.
The thermal reactions … took precisely the opposite stereochemical course to that which we had predicted
Wednesday, January 20th, 2021The quote of the post title comes from R. B. Woodward explaining the genesis of the discovery of what are now known as the Woodward-Hoffmann rules for pericyclic reactions.[cite]10.1021/ja01080a054[/cite] I first wrote about this in 2012, noting that “for (that) blog, I do not want to investigate the transition states”. Here I take a closer look at this aspect.
Fascinating stereoelectronic control in Metaldehyde and Chloral.
Tuesday, June 9th, 2020Metaldehyde is an insecticide used to control slugs. When we unsuccessfully tried to get some recently, I discovered it is now deprecated in the UK. So my immediate reaction was to look up its structure to see if that cast any light (below, R=CH3, shown as one stereoisomer).
The first ever curly arrows. Revisited with some crystal structure mining.
Wednesday, May 27th, 2020With the current global lockdown, and students along with everyone else staying at home, I have noticed some old posts of mine are getting more attention than normal. One of these is an analysis I did in 2012 of Robinson’s original curly arrow illustration.[cite]10.1002/jctb.5000435208[/cite] That and the fact that I am about to give a lecture on what I call my autobiographical journey discovering them, to our own students here (remotely of course), has prompted me to revisit my original discussion.
The strongest bond in the universe: A crystallographic reality check?
Monday, May 25th, 2020My previous two posts on the topic of strongest bonds have involved mono and diprotonating N2 and using quantum mechanics to predict the effect this has on the N-N bond via its length and vibrational stetching mode. Such species are very unlikely to be easily observed for verification. But how about a metal M+ instead of H+? It turns out that structures containing the fragment Ru-N≡N-Ru are a small but well studied class of organometallic. Here is a search of the CSD crystal database for this motif.
A molecular sponge for hydrogen storage- the future for road transport?
Sunday, April 19th, 2020In the news this week is a report of a molecule whose crystal lattice is capable of both storing and releasing large amounts of hydrogen gas at modest pressures and temperatures. Thus “NU-1501-Al” can absorb 14 weight% of hydrogen. To power a low-polluting car with a 500 km range, about 4-5 kg of hydrogen gas would be need to be stored and released safely. The molecule is of interest since it opens a systematic strategy of synthetically driven optimisation towards a viable ultra-porous storage material,[cite]10.1126/science.aaz8881[/cite] much like a lead drug compound can be optimised.
Choreographing a chemical ballet: a story of the mechanism of 1,4-Michael addition.
Monday, April 13th, 2020A reaction can be thought of as molecular dancers performing moves. A choreographer is needed to organise the performance into the ballet that is a reaction mechanism. Here I explore another facet of the Michael addition of a nucleophile to a conjugated carbonyl compound. The performers this time are p-toluene thiol playing the role of nucleophile, adding to but-2-enal (green) acting as the electrophile and with either water or ammonia serving the role of a catalytic base to help things along.†
Molecules of the year 2019: Hexagonal planar crystal structures.
Thursday, January 23rd, 2020Here is another selection from the Molecules-of-the-Year shortlist published by C&E News, in which hexagonal planar transition metal coordination is identified. This was a mode of metal coordination first mooted more than 100 years ago,[cite]10.1038/s41586-019-1616-2[/cite] but with the first examples only being discovered recently. The C&E News example comprises a central palladium atom surrounded by three hydride and three magnesium atoms, all seven atoms being in the same plane.