Quite a few threads have developed in this series of posts, and following each leads in rather different directions. In this previous post the comment was made that coordinating a carbon dication to the face of a cyclopentadienyl anion resulted in a monocation which had a remarkably high proton affinity. So it is a simple progression to ask whether these systems may in turn harbour a large affinity for binding not so much a H+ as the next homologue He2+?
Uncompressed Monovalent Helium
October 3rd, 2009Pentavalent nitrogen and boron
October 3rd, 2009The previous posts have seen how a molecule containing a hypervalent carbon atom can be designed by making a series of logical chemical connections. Another logical step is to investigate whether the adjacent atoms in the periodic table may exhibit similar effects (C2+ ≡ B+ ≡ N3+ ≡ Be ≡ O4+). So here are reported some results (B3LYP/6-311G(d,p) ) for boron, beryllium and nitrogen, for the general tetramethyl substituted system shown below
Full circle with carbon hypervalencies
October 2nd, 2009The previous post talked about making links or connections. And part of the purpose for presenting this chemistry as a blog is to expose how these connections are made, or or less as it happens in real time (and not the chronologically sanitized version of discovery that most research papers are). So each post represents an evolution or mutation from the previous one. To recapitulate, we have seen how the idea of cyclopentadienyl anion as a ligand for a dipositive carbon atom has evolved. Let us move in yet another direction; the cyclobutadienyl dianion. This ligand has recently been shown to bind Mg2+ (DOI: 10.1002/ejic.200800066), so why not He2+? And picking up again the previous theme, we will then protonate the bound complex. The result now is a monocation, and it has the C4v-symmetric structure shown below (DOI: 10042/to-2438). This bears some resemblance to pyramidane, a neutral C5H4 compound with hemispherical carbon reported in 2001 (DOI: 10.1021/jp011642r) which is also a stable minimum in the potential energy surface.
It’s Hexa-coordinate carbon Spock – but not as we know it!
October 2nd, 2009Science is about making connections. And these can often be made between the most unlikely concepts. Thus in the posts I have made about pentavalent carbon, one can identify a series of conceptual connections. The first, by Matthias Bickelhaupt and co, resulted in the suggestion of a possible frozen SN2 transition state. They used astatine, and this enabled a connection to be made between another good nucleophile/nucleofuge, cyclopentadienyl anion. This too seems to lead to a frozen Sn2 transition state. The cyclopentadienyl theme then asks whether this anion can coordinate a much simpler unit, a C2+ dication (rather than Bickelhaupt’s suggestion of a (NC)3C+ cation/radical) and indeed that complex is also frozen, again with 5-coordinate carbon, and this time with five equal C-C bonds. So here, the perhaps inevitable progression of ideas moves on to examining the properties of this complex, the outcome being a quite counter-intuitive suggestion which moves us into new territory.
It’s penta-coordinate carbon Spock- but not as we know it!
September 30th, 2009In the previous two posts, I noted the recent suggestion of how a stable frozen SN2 transition state might be made. This is characterised by a central carbon with five coordinated ligands. The original suggestion included two astatine atoms as ligands (X=At), but in my post I suggested an alternative which would have five carbon ligands instead (X=cyclopentadienyl anion).
Capturing penta-coordinate carbon! (Part 2).
September 23rd, 2009In this follow-up to the previous post, I will try to address the question what is the nature of the bonds in penta-coordinate carbon?
Capturing penta-coordinate carbon! (Part 1).
September 22nd, 2009The bimolecular nucleophilic substitution reaction at saturated carbon is an icon of organic chemistry, and is better known by its mechanistic label, SN2. It is normally a slow reaction, with half lives often measured in hours. This implies a significant barrier to reaction (~15-20 kcal/mol) for the transition state, shown below (X is normally both a good nucleophile and a good nucleofuge/leaving group, such as halide, cyanide, etc. Y can have a wide variety of forms).
Spotting the unexpected: Anomeric effects
September 18th, 2009Chemistry can be very focussed nowadays. This especially applies to target-driven synthesis, where the objective is to make a specified molecule, in perhaps as an original manner as possible. A welcome, but not always essential aspect of such syntheses is the discovery of new chemistry. In this blog, I will suggest that the focus on the target can mean that interesting chemistry can get over-looked (or if observed, not fully exploited in subsequent publications). Taking a synthesis-oriented publication at (almost) random entitled Synthesis of 1-Oxadecalins from Anisole Promoted by Tungsten (DOI: 10.1021/ja803605m) which appeared in 2008, the following molecule appears as one of the (many) intermediates.
The Fragile Web
August 31st, 2009One of the many clever things that clever people can do with the Web is harvest it, aggregate it, classify it etc. Its not just Google that does this sort of thing! Egon Willighagen is one of those clever people. He runs the Chemical blogspace which does all sorts of amazing things with blogs.