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Inorganic Chemistry

Boron-nitrogen compound is close cousin of benzene

The latest ‘inorganic benzene’ has pronounced aromaticity

by Mark Peplow
February 16, 2020 | APPEARED IN VOLUME 98, ISSUE 7

09807-scicon7-aromatic.jpg

A cunningly modified ring of boron and nitrogen atoms is the closest inorganic analog of benzene yet created, its creators say (Angew. Chem., Int. Ed. 2020, DOI: 10.1002/anie.201915790). The first inorganic version of benzene was reported almost a century ago. Borazine (B3N3H6) contains a flat six-membered ring and a delocalized cloud of six π electrons but is only moderately aromatic. Since borazine’s discovery, a handful of other inorganic benzenes have been created, none of which seem to have much aromaticity. Rei Kinjo and Kei Ota at Nanyang Technological University have now created a 1,4,2,3,5,6-diazatetraborinine derivative that is much more benzene-like than its predecessors. The blue solid is a valence isomer of benzene, with each ring atom bearing only one substituent. Unlike borazine’s alternating pattern of elements, diazatetraborinine’s ring includes four boron atoms that sit in pairs and bond to either chlorine or trimethylphosphine. X-ray crystallography shows that the molecule is flat and has a perfectly hexagonal central ring, while nuclear magnetic resonance measurements demonstrate the magnetic deshielding effects typical of aromatic rings. The researchers’ theoretical calculations show that the molecule’s six π electrons are highly delocalized and that its aromaticity lies between that of benzene and borazine. “By modulating the position of inorganic elements, we have demonstrated that even inorganic benzenes can exhibit a pronounced aromatic nature,” Kinjo says.

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Comments
Dr. Paul C. Li (February 22, 2020 8:40 PM)
Dear Honorable Editor @cen.acs.org.:
My training in vibrational spectroscopy both IR and Raman might be useful in probing the aromaticity of the (B3N3)H6 . Regretfully, it is not mentioned in the precious article by Mr. Peplow.
The following three equations were applied for estimating the so called ring breathing mode of benzene-like molecule.
Richard M. Badger’s empirical force constant equations

k = [ 1.86 * 10 to the 5th powers /( delta X ) to the third powers]. ———-(1)

Definition of delta X is modified to be the extension of the bond w.r.t. original bond length

Hooke’s law of vibration borrowed to be used in molecular spectroscopy with respect.

( nu or v bar / 1307) square = k/u(mu). ———-(2)

The last mathematical formula is also a borrowed form in a text by Francis Hildebrand

T = 2.17 A * rho * f(squared). ———-(3)
where T, tension of the ring or the drum; rho, the density of the ring structure; f , the vibrational frequency and A, the area of the ring or the drum, according to section 9.9 of page 446 in “Advanced Calculus for Applications” Prentice-Hall 1963.

A verification remains to be seen. Submitted with highest degree of sincerity and honest by a life long chemist in belief of good chemistry is ready for a better life and life itself is chemistry.


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