{"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":"2011-08-10T19:55:14","modified_gmt":"2011-08-10T19:55:14","slug":"a-stable-borylene-an-exercise-in-lateral-thinking","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4719","title":{"rendered":"A stable borylene. An exercise in lateral thinking."},"content":{"rendered":"
\n

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.<\/p><\/div>\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

\"\"

A crystalline borylene. Click for 3D.<\/p><\/div>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
\"\"

HOMO molecular orbital for the borylene. Click for 3D.<\/p><\/div><\/td>\n

\"\"

HOMO-2 orbital, showing B-H donating character. Click for 3D.<\/p><\/div><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n<\/div> ","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":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"","footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[4],"tags":[651,373],"ppma_author":[2661],"class_list":["post-4719","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-basic-boron","tag-tutorial-material"],"yoast_head":"\nA stable borylene. An exercise in lateral thinking. - Henry Rzepa's Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4719\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"A stable borylene. An exercise in lateral thinking. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"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, […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4719\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2011-08-07T10:31:31+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2011-08-10T19:55:14+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/08\/borylene.jpg\" \/>\n<meta name=\"author\" content=\"Henry Rzepa\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Henry Rzepa\" \/>\n\t<meta name=\"twitter:label2\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"A stable borylene. An exercise in lateral thinking. - Henry Rzepa's Blog","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4719","og_locale":"en_GB","og_type":"article","og_title":"A stable borylene. An exercise in lateral thinking. - Henry Rzepa's Blog","og_description":"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?“. 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I refer to a chemistry lab from the 1800s I was recently taken to, where famous french chemists such as Joseph Gay-Lussac, Michel Chevreul and Edmond Fremy were professors. Although not used for chemistry any more, it is an incredible treasure trove of objects. Here\u2026","rel":"","context":"In "Historical"","block_context":{"text":"Historical","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=565"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":19828,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=19828","url_meta":{"origin":4719,"position":1},"title":"The “White City Trio” – The formation of an amide from an acid and an amine in non-polar solution (updated).","author":"Henry Rzepa","date":"August 8, 2018","format":false,"excerpt":"White City is a small area in west london created as an exhibition site in 1908, morphing over the years into an Olympic games venue, a greyhound track, the home nearby of the BBC (British Broadcasting Corporation) and most recently the new western campus for Imperial College London.\u2663 The first\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2018\/08\/reactants-1024x719.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":20440,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=20440","url_meta":{"origin":4719,"position":2},"title":"Free energy relationships and their linearity: a test example.","author":"Henry Rzepa","date":"January 13, 2019","format":false,"excerpt":"Linear free energy relationships (LFER) are associated with the dawn of physical organic chemistry in the late 1930s and its objectives in understanding chemical reactivity as measured by reaction rates and equilibria. The Hammett equation is the best known of the LFERs, albeit derived \"intuitively\". It is normally applied to\u2026","rel":"","context":"In "Chemical IT"","block_context":{"text":"Chemical IT","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":17633,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17633","url_meta":{"origin":4719,"position":3},"title":"George Olah and the norbornyl cation.","author":"Henry Rzepa","date":"March 10, 2017","format":false,"excerpt":"George Olah passed away on March 8th. He was part of the generation of scientists in the post-war 1950s who had access to chemical instrumentation that truly revolutionised chemistry. In particular he showed how the then newly available NMR spectroscopy illuminated structures of cations in solvents such \"Magic acid\". The\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":18091,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18091","url_meta":{"origin":4719,"position":4},"title":"The conformation of carboxylic acids revealed.","author":"Henry Rzepa","date":"April 11, 2017","format":false,"excerpt":"Following my conformational exploration of enols, here is one about a much more common molecule, a carboxylic acid. The components of the search are shown as four queries below, which will be combined in various Boolean senses (DOI:\u00a010.14469\/hpc\/2462). Query one defines the carboxylic acid, with 3-coordinate carbon specified at the\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/04\/051-1024x676.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":2423,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=2423","url_meta":{"origin":4719,"position":5},"title":"The oldest reaction mechanism: updated!","author":"Henry Rzepa","date":"September 14, 2010","format":false,"excerpt":"Unravelling reaction mechanisms is thought to be a 20th century phenomenon, coincident more or less with the development of electronic theories of chemistry. Hence electronic\u00a0arrow pushing as a term. But here I argue that the true origin of this immensely powerful technique in chemistry goes back to the 19th century.\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/09\/wheland.jpg?resize=350%2C200","width":350,"height":200},"classes":[]}],"jetpack_likes_enabled":false,"authors":[{"term_id":2661,"user_id":1,"is_guest":0,"slug":"admin","display_name":"Henry Rzepa","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","0":null,"1":"","2":"","3":"","4":"","5":"","6":"","7":"","8":""}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4719","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=4719"}],"version-history":[{"count":0,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4719\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4719"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4719"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4719"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=4719"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}