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LatinXChem Twitter Poster Conference 2020

Atualizado: 24 de Set de 2020

The #LatinXChem Twitter Poster Conference was a huge success! Check out my contribution to the conference in the #LatinXChemTheo poster session!

Hi @LatinXChem, this is my work “The Fate of Cyanoborylene: Cyclotetramerization versus Diborene Formation from DFT and CASSCF/NEVPT2 Calculations - And Other Tales”. For more information, just follow the thread! #LatinXChem #LatinXChemTheo #Theo099 (1/32)

N-Heterocyclic Carbenes (NHCs) and Cyclic (Alkyl)(Amino) Carbenes (CAACs) are one of the most exciting classes of ligands for stabilizing reactive transition metal and main group species. CAACs are particularly fit for stabilizing radicals, biradicals and biradicaloids. (2/32)

CAACs and NHCs have also been used for a variety of applications, including the activation of small molecules and chemical bonds, as well as for the development of efficient and robust catalysts. (3/32) https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201410797

Differences in NHCs and CAACs are usually discussed in terms of σ-donating and π-accepting properties. Very few studies have closely examined the differences of CAACs and NHCs. A nice example is this paper by Holzmann, @ICCGroup2 and Frenking. (4/32) https://www.sciencedirect.com/science/article/abs/pii/S0022328X15001886

Here, we extend these investigations to novel experimentally-realized CAAC- and NHC-stabilized compounds, also exploring the consequences of interchanging the ligands and assessing the role of electronic and steric effects. (5/32)

The fantastic symbiosis between experimental and computational chemistry can be portrayed by imagining the experimental results and known molecular systems as islands surrounded by deep water. (6/32)

Although a comprehensive description of the islands gives a very nice overview of the landscape, there is a plethora of features and relevant information just underneath the surface of the water, down to the lowest layers of the ocean. As #compchems, we are the divers! (7/32)

The goal of this work is to investigate electrostructural differences between selected NHC- and CAAC-stabilized main group systems. For that, we performed calculations based on single- (SR) and multi-reference (MR) approaches, as summarized in the poster. (8/32)

Particularly, we are interested in understanding the features that drive biradicalic nature to the systems, and how to avoid/enhance it. A very nice review on biradicals and biradicaloids by Bonačić-Koutecký, Koutecký and Michl is here. (9/32) https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.198701701

The first tale of this poster is about the CAAC-stabilized AlH parent system. Our first publication related to this system was published in @J_A_C_S last year. (10/32) https://pubs.acs.org/doi/10.1021/jacs.9b09128

We found that, while (NHC)2AlH has a closed-shell singlet ground state (S-T gap of 34.8 kcal/mol from CASSCF/NEVPT2 calculations), the experimentally-realized (CAAC)2AlH is a singlet biradical, with a calcd. S-T gap of 5.0 kcal/mol and a biradical character of y0 = 0.36. (11/32)

The biradical character is also influenced by the L-Al-L torsional angles. In a following publication, we will explore these changes also comparing it to Bertrand's L2BH system, with no biradical character at the optimized structure. (12/32) https://science.sciencemag.org/content/333/6042/610

The second tale is about the Lewis Base-stabilized diboraanthracene. This work was recently published in @angew_chem as a #VIP paper. (13/32) https://onlinelibrary.wiley.com/doi/full/10.1002/anie.202008206 With increasing number of fused benzene rings, acenes gradually become organic p-type semiconductors, with high relevance for organic electronic applications. (14/32) https://pubs.acs.org/doi/10.1021/cr0501386

We show that, if boron atoms are incorporated into the acene backbone, open-shell singlet ground states are already accessible and energetically favorable for a smaller number of fused benzene rings. (15/32)

Indeed, by combining EPR, X-ray and QM calcs, we verified disjointed, open-shell singlet biradical ground states for CAAC-stabilized 9,10-diboraanthracenes, in contrast to the closed-shell singlet NHC-stabilized system synthesized by Harman. (16/32) https://pubs.acs.org/doi/abs/10.1021/jacs.7b06772

These results place diboron analogs of acenes as potential alternative and highly efficient materials for organic electronic applications. Boron chemistry is fantastic! :) (17/32)

The third tale is about C2R2-bridged diboron biradicals. Recently, CAAC-stabilized boron-based biradicals featuring unsaturated C2R2 bridges (R = Me, Et) between the boron centers and twisting of the bridge were prepared. (18/32) https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201813335

In contrast to their unbridged, triplet biradical counterparts, these species were found to have singlet biradical multiplicities, while all attempts to prepare their NHC analogs thus far have been unsuccessful. (19/32) https://www.nature.com/articles/s41467-018-02998-3

We show that sterics arising from the alkyl substituents in the bridge are fundamental for driving biradicalic character, as the CAAC-stabilized C2H2-bridged system has a closed-shell, planar backbone due to pi delocalization. (20/32)

As for the NHC-stabilized C2R2-bridged systems, they possess a helical structure and closed-shell singlet ground state, with their synthetic elusiveness being mostly attributed to kinetic effects as suggested by isodesmic reaction energy calculations. (21/32)

Finally, we come to the last tale of this work, the one that gives the title of this poster: the cyanoborylene system and its oligomeric structures. (22/32)

A Lewis base-stabilized diborene is a molecule of type LRB=BRL, which contains a boron-boron double bond and is formally the dimeric product of mono(Lewis base)-stabilized borylenes. For cyanoborylene, the R group is CN, and its CAAC-stabilized version is known. (23/32)

A fundamental question regarding borylenes is if, like carbenes, they also possess a Wanzlick-type equilibrium in solution. For carbenes, their propensity to dimerize or to stay as a naked species was thoroughly studied by Heinemann and Thiel (24/32). https://www.sciencedirect.com/science/article/pii/0009261493E1360S

Surprisingly, instead of forming a naked borylene, the reduction of (CAAC)BBr2(CN) generated a 12-membered ring cyanoborylene tetramer of butterfly shape. (25/32) https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.201608429

We started our computational investigation by comparing the NHC- and CAAC-stabilized borylenes. To our surprise, the trend in their S-T gaps is the opposite of the previous systems: CAAC is a closed-shell singlet; NHC is a triplet biradical. (26/32)

The ground-state multiplicity is a direct consequence of the CBCN arrangement. While the CAAC system has a linear CBCN motif, in the NHC version it is bent. We then investigated the distinct CAAC-stabilized (BCN)n rings to assess their electrostructural properties. (27/32)

The (BCN)2 ring has a biradicalic nature, which makes it less stable than its isomer, the closed-shell (CAAC)(CN)B=B(CN)(CAAC) dicyanodiborene species. The (BCN)3 ring is a planar closed-shell singlet, with considerable steric demands due to the diisopropylphenyl groups. (28/32)

Finally, the (BCN)4 ring has a beautiful butterfly structure, which properly attenuates destabilizing effects due to sterics. The BCN angles of the ring are more linear than in the previous cases. And its formation from 2 diborenes is very favorable (ΔG = –68.8 kcal/mol). (29/32)

From this work, we also suggest that diborene-stabilized cyanoborylenes are potential synthetic targets. We are currently working on obtaining these species experimentally. If succeeded, this will be a very nice case of synthesis driven by computational predictions. (30/32)

To summarize, CAACs and NHCs give rise to a fantastic set of novel main group systems, with electronic structures ranging from closed-shell to biradicals and distinct reactivity patterns. And S-T gaps greatly affect structure and reactivity of these very intriguing systems. (31/32)

Thank you very much for coming by! I hope you have enjoyed reading this. If you have any questions/comments/suggestions, do not hesitate to contact me! : (32/32)


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Julius-Maximilians-Universität Würzburg

Institute for Inorganic Chemistry

Institute for Sustainable Chemistry & Catalysis with Boron

Am Hubland, 97074, Würzburg, Germany.


Institute for Physical and Theoretical Chemistry

Emil-Fischer Strasse 42, 97074, Würzburg, Germany.

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E-mail: felipe.fantuzzi[at]uni-wuerzburg.de

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