{"id":4719,"date":"2011-08-07T10:31:31","date_gmt":"2011-08-07T10:31:31","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=4719"},"modified":"2011-08-07T10:31:31","modified_gmt":"2011-08-07T10:31:31","slug":"a-stable-borylene-an-exercise-in-lateral-thinking","status":"publish","type":"post","link":"https:\/\/rzepa.net\/blog\/2011\/08\/07\/a-stable-borylene-an-exercise-in-lateral-thinking\/","title":{"rendered":"A stable borylene. An exercise in lateral thinking."},"content":{"rendered":"

I have often heard the question posed “how much of chemistry has been discovered?<\/em>” Another might be “has most of chemistry, like low-hanging fruit, already been picked?<\/em>“. Well, time and time again, one comes across examples which are only a simple diagram or so away from what might be found in any introductory chemistry<\/a> text, and which would tend to indicate the answers to these questions is a resounding\u00a0no<\/strong>. Take for example the three reactions shown below.<\/p>\n

\"\"<\/a>
Simple acid-base chemistry in an iso-electronic series.<\/figcaption><\/figure>\n

The top row shows a quaternary ammonium salt behaving as an acid in loosing a proton to become an amine. The shared covalent N-H bond becomes a lone pair on the amine, concepts first introduced by G. N. Lewis<\/a>\u00a0almost 100 years ago, and still an absolute bedrock of introductory chemistry. A little later, students learn that hydrocarbons are much less acidic. The conjugate base, a carbanion<\/span>, is nevertheless a species very well known to synthetic chemists. You will also have noticed the iso-electronic progression in moving from nitrogen to carbon. Well, it’s a tiny lateral extrapolation to ask what might happen to boron, the next element in this iso-electronic series? The answer is shown as the bottom reaction (red)\u00a0in an article recently published by Bertrand and co-workers (DOI:\u00a010.1126\/science.1207573<\/a>).<\/p>\n

Why is this so newsworthy? Because in the Lewis model, boron has hitherto always acted in its known chemistry as a recipient<\/strong> of a lone pair, but never potentially as a donor<\/strong>! It is true however, that e.g.<\/em> a borohydride anion (BH4<\/sub>–<\/sup>) can donate not so much a lone pair as the covalent pair in one of the B-H bonds, in effect a hydride anion. So the fascinating species made by Bertrand and co-workers, R2<\/sub>H<\/span>B2-<\/sup>and\u00a0named a borylene<\/strong> now raises a new question. What does the borylene shown above prefer to do, donate a lone pair (green above) or instead donate a B-H covalent electron pair (blue above)? Does one enhance<\/a> the other? Extrapolating introductory textbook chemistry can so easily lead to new chemistry, so don’t give up hope of ever doing it yourself!<\/p>\n

I noted that hydrocarbons are very much less acidic than ammonium salts (n-butyl lithium, a typical unstabilized carbanion, is a very dangerous beast, although benzyl anion does form stable crystals<\/a>). One might imagine that an unstabilized borylene would be even more so. In fact, it was created as a (double) zwitterion, with the two R groups themselves each bearing a +ve charge to balance the two -ve charges on the boron. It is shown below, surrounded by inert alkyl groups to prevent it reacting.<\/p>\n

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A crystalline borylene. Click for 3D.<\/figcaption><\/figure>So, is it a lone pair or a hydride donor? According to calculation<\/a>, it’s the former<\/a>, since the HOMO (the most reactive pair of electrons in the molecule) is a \u03c0-type boron centred orbital, rather than a B-H \u03c3-type. Indeed, it is reported that the molecule does react with a proton to form a crystalline derivative (called a boronium cation<\/em> in the published article, although I think it could equally be called a borohydride<\/em>, with the +ve charge residing instead on one of the R groups!). \u00a0What remains to be established is whether it could ambidently also act as a hydride donor.<\/p>\n

Postscript.<\/strong> \u00a0The HOMO referred to above can be seen as a 3D model if you click on the graphic below. Next to it is the HOMO-2 orbital, which has \u00a0B-H character, showing that both types of reactivity could be present.<\/p>\n\n\n\n
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HOMO molecular orbital for the borylene. Click for 3D.<\/figcaption><\/figure><\/td>\n
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HOMO-2 orbital, showing B-H donating character. Click for 3D.<\/figcaption><\/figure><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n","protected":false},"excerpt":{"rendered":"

I have often heard the question posed “how much of chemistry has been discovered?” Another might be “has most of chemistry, like low-hanging fruit, already been picked?“. Well, time and time again, one comes across examples which are only a simple diagram or so away from what might be found in any introductory chemistry text, […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[11],"tags":[264,2489],"class_list":["post-4719","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-basic-boron","tag-tutorial-material"],"_links":{"self":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/4719","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/comments?post=4719"}],"version-history":[{"count":0,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/4719\/revisions"}],"wp:attachment":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/media?parent=4719"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/categories?post=4719"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/tags?post=4719"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}