Extending the Library of Boron Bases: A Contribution from Theory Shahnaz Sultana Rohman, † Bikash Sarmah, † Bitupon Borthakur, † Geetha S. Remya, ‡ Cherumuttathu H. Suresh, ‡ and Ashwini K. Phukan* ,† † Department of Chemical Sciences, Tezpur University, Napaam 784028, Assam India ‡ Chemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum, Kerala 695 019, India * S Supporting Information ABSTRACT: In recent years, the isolation of nucleophilic boron bases led to a paradigm shift in boron chemistry which prompted us to perform computational studies on a series of bis(carbene) borylene complexes. The structure and electronic properties of these complexes have been studied, and all of them were found to possess strong donor ability as evident from high values of calculated pK a and proton affinities (244-300 kcal mol -1 ). The ability to form strong donor-acceptor bond between the boron and carbene center, together with the high thermodynamic stability of the complexes, render them as a promising class of ligands for use in transition metal based systems. The comparable bond dissociation energies of hitherto unknown acyclic (3 A -5 A , 8 A , 15 A , 18 A , 19 A , and 21 A ) and cyclic borylenes (2 C -5 C , 9 C , 10 C , and 15 C ) to that of the synthetically accessible borylenes (2 A , 7 A , 9 A , and 16 A ) indicate the likelihood of isolation of these hitherto unknown borylenes. 1. INTRODUCTION The chemistry of low-valent main group elements has been a subject of keen interest for quite some time. Their utility as Lewis bases has been restricted to elements of group 14-16, 1 while only the heavier group 13 bases 2 have undergone a thorough study until date. The pioneering contribution of Power and his co-workers is worth mentioning in this context. 3 Complexes of group 13 elements, particularly boron with one unoccupied valence p orbital were traditionally considered only as Lewis acids. In order to behave as a base, the valence electrons of the monovalent boron atom have to pair up. This leaves the boron atom with one lone pair and two empty valence orbitals. These kind of free borylenes were indeed available as highly labile reaction intermediates generally synthesized in photochemically and thermally induced borylene transfer reactions. 4 Previously, transition metal fragments were used to stabilize such reactive species. 5 The first ever nucleophilic anionic boryl complex was isolated by Yamashita et al. in 2006. Structural analysis confirmed that the boryllithium features an sp 2 -hybridized boron atom and was isoelectronic with N-heterocyclic carbenes. 6 Synthesis of this compound opened up new avenues for the isolation of a number of anionic boron centered nucleophiles, and in 2010, Braunschweig et al. reported the isolation of the first ever carbene stabilized π-nucleophilic boryl anion. 7 Following these seminal works, a number of boryl complexes of electropositive metals or metalloids were also synthesized. 8 Realizing the potential stabilizing effect of carbenes on subvalent boron derivatives, Bertrand and co-workers selected ambiphilic cyclic(alkyl)(amino)carbene (CAAC) 9 as ligands to bind with the anionic boron center bearing two electron-with- drawing nitrile substituents, 10 yet the isolation of a neutral tricoordinate boron base remained elusive until Bertrand’s group reported the synthesis of a CAAC stabilized borylene. 11 The reactivity of this bis(carbene) adduct I (Figure 1) toward electrophiles was hindered by steric congestion around the boron atom. However, being isoelectronic to amines and phosphines, it was predicted to serve as a strong π-donor toward transition metal species. In order to further tune the electronic properties of the boron center, Bertrand et al. tried to prepare two unsymmetrically substituted tricoordinated borylene derivatives II and III. 12 The upfield shift of the 11 B NMR signals of II and III as compared to that of I is an indication of a sufficiently electron-rich boron center which was in accordance with the weak π-accepting ability of the constituent carbene ligands. Kinjo and co-workers installed sterically less demanding oxazol-2-ylidenes substituents at the boron center so as to enhance the reactivity of tricoordinated nucleophilic borylenes toward electrophiles. The basicity of this bis(oxazol-2-ylidene)-phenylborylene, IV, was demon- strated by reaction with trifluoromethanesulfonic acid and transition metal complexes. 13 Its application in the activation of CO 2 and formation of adducts with main group elements revealed the soft Lewis basic nature of the central boron atom. 14 The investigation of the electrophilic properties of a CAAC-aminoborylene adduct isoelectronic with singlet carbenes led to yet another bis(carbene)borylene adduct, V. The boron center in V is stabilized to a considerable extent by the push-pull effect of the amino group and the CAAC ligand, Received: May 14, 2019 Article pubs.acs.org/Organometallics Cite This: Organometallics XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.organomet.9b00317 Organometallics XXXX, XXX, XXX-XXX Downloaded by NOTTINGHAM TRENT UNIV at 10:15:27:873 on June 28, 2019 from https://pubs.acs.org/doi/10.1021/acs.organomet.9b00317.