Characterization and Investigation of Benzylic Carbenium Ion and Carbanion Ion-Diradical Intermediates With 3,5-Disubstituted (pi)-Donors/Acceptors

Abstract

High-spin magnetic materials have been based on monomeric units that contain metals. Far less research has been done to develop and characterize alternative high-spin building blocks consisting of mostly organic subunits. To date, there exists a small class of known high-spin organomagnetic building blocks. These organic building blocks are comprised of neutral intermediates such as diradicals, nitrenes, and carbenes. The work presented in this text will show that a novel class of high-spin intermediates also exists that derives from ionic rather than neutral intermediates.

Previous density functional theory (DFT) computations by Winter and Falvey identified a number of ion-diradicals that have favorable triplet ground states. For instance, the 3,5-bis(dimethylamino)benzyl carbenium ion, and 2-(3,5-dinitrophenyl)-1,3-dithiane carbanion, have singlet-triplet energy gaps (ΔESTs) of +1.9 and +0.3 kcal/mol, respectively.

Ion-diradicals are based on a general framework whereby either an electron deficient or electron rich exocyclic center is attached to a benzene ring that contains two π-electron withdrawing or donating groups meta with respects to this site. The objective of chapter 2 is to identify the electronic spin state of the 3,5-bis(dimethylamino)benzyl carbenium ion. We have shown that this carbenium ion can be generated from C-O bond heterolysis of 3,5-bis(dimethylamino)benzyl esters and alcohols when photolyzed in polar protic solvents (e.g. methanol, 2-propanol, 2,2,2-trifluoroethanol). Our product studies show that indeed the 3,5-bis(dimethylamino)benzyl ethers are formed from this process, however an unexpected reduction product (3,5-bis(dimethylamino)toluene) is also observed. The reduction product here aptly demonstrates how an ion-diradical could be generated. Formal one electron transfer from a NMe2 group to the exocyclic benzylic carbenium ion center, creates a triplet cation diradical (ion-diradical) intermediate, which eventually leads to the formation of the 3,5-bis(dimethylamino)toluene product observed in all photolysis experiments.

Chapters 4-6 investigates whether ion-diradicals can be generated from anionic exocyclic centers that have meta π-acceptor groups (e.g. NO2, CN, CO2R). Computations, NMR, UV-Vis, chemical trapping, H/D exchange, and Evans method magnetic susceptibility experiments demonstrate that the 2-(3,5-dinitrophenyl)-1,3-dithiane carbanion is a persistent and paramagnetic species. Chapters 5 and 6 focuses primarily on characterizing the electronic spin-states to similar anionic systems such as the 3,5-dinitroanilide anion, and 3,5-dinitrobenzyl methoxy ether carbanion.