Studies On The Syntheses And Properties Of Diradical Species Containing Nitrogen,Arsenic And Boron

The stabilization of radical species with novel chemical/electronic structures is an important issue in radical chemistry,and favorable electronic effects and appropriate steric protection turn out to be two major strategies to achieve this target as reported in a series of relevant studies,more exactly,supporting with a long-range or widespread?-conjugated molecular skeleton is one of most common method for stabilizing unpaired electron spin in radical species.Herein,starting from analysis of the construction of stable radicals based on congeners or isoelectronic elements,we designed and synthesized stable radical species containing nitrogen,arsenic and boron,respectively,featuring with terphenyl,(indeno)fluorene and pyrene skeleton structurally in corresponding cases.Moreover,their chemical/electronic structures and macroscopic properties were well characterized and studied by means of various experimental methods in conjunction with theoretical calculations.Detailed research work and relevant results are listed as follows:1.By means of two-electron oxidations of five 4,4''-p/m-terphenyldiamine compounds 1-5 in the presence of weakly coordinating anions,we successfully stabilized and isolated the dications 1²⁺-3²⁺and carried out crystallographic structural analysis on their single crystals.For dications 1²⁺-5²⁺,a series of experimental procedures and DFT calculations were conducted to investigate into the electronic structures and relative energies of their ground states,as well as their magnetic and optical properties.This work reported first examples of terphenyldiamine diradical dications capable of stably exsiting in the solution phase and/or solid form.p-Terphenyldiamine dications 1²⁺-3²⁺are open-shell singlets in their ground states and their diradical character is dependent on the steric hindrance of alkyl substituents at the ortho-positions of the amino groups.Moreover,although the inherent high instability of dication 4²⁺precludes the determination of its ground state,the calculation result and observed linear Curie plot(via variable-temperature EPR measurements)for its(ⁱPr-substituted)analogue 5²⁺indicated that its ground state is triplet.2.By utilizing the fluorene and indenofluorene groups as the structural scaffolds,we synthesized three neutral(di)arsaalkene compounds 6-8,which underwent subsequent one/two-electron reductions to yield(di)radical(di)anions salts 6K,7K,7K₂ and 8K₂.Their structural features were uncovered by single crystal XRD analysis.Furthermore,their electronic structures,spin multiplicities for ground state and optical absorption properties were investigated by various experimental methods in conjunction with theoretical calculations.The studies demonstrated that the high stability of these radicals is most probably attributed to partial electron delocalization over the fluorene and indenofluorene moieties.Strikingly,7K₂ and 8K₂ are the first heavier Group 15 element-based diradicals featuring thermally accessible triplet states because of the small singlet-triplet energy gaps.3.In the pursuit of synthesizing novel diradical compounds containing double tetra-coordinate boron units,we reduced the Lewis pairs comprised of(tert-butyl substituted)pyrene-tetraone and tris(pentafluorophenyl)borane with alkali metal(KC₈),and unexpectedly isolated bis-borocyclic radical anion salt 9 and dimer-form-featured diamagnetic compound 10.Specifically,intra-and intermolecular C-F…K⁺interactions in certain degrees were observed in the solid-phase structure of 10 via detailed analysis of its crystallographic structure and packing modes.In view of the low yields for both of 9 and 10 as well as the high instability of one-electron reduction product of tris(pentafluorophenyl)borane,we proposed plausible mechanisms for the two reaction systems in question,wherein the formation of classic Lewis acid-base adduct(CLA),one-electron reduction by KC₈ and subsequent bond activation constitute the critical pathway to yield 9 or 10.Furthermore,various experimental methods in conjunction with DFT calculations were conducted to probe into electronic structure and properties of the radical anion 9,which disclosed its extraordinary stability is mostly attributed to widespread spin delocalization within the pyrene skeleton.In addition,in the comparative study of the interaction modes of aforementioned Lewis pairs under hydrogen atmosphere(without extra reducing agents),only the less basic Lewis base substrate,i.e.the non-alkylated pyrene-tetraone was observed to undergo obvious frustrated Lewis pair(FLP)hydrogenation process,and one of the final products turned out to be bis(pentafluorophenyl boronic)pyrene-tetraol ester(11)by XRD analysis,whose novel chemical structure suggests potential optical and electrical functions,yet separation and purification failed due to contamination of miscellaneous coproducts from concomitant reaction pathways.All in all,the alkali metal-mediated reactions between(tert-butyl substituted)pyrene-tetraone and tris(pentafluorophenyl)borane are inevitably competitively disturbed by the one-electron reduction of the latter and subsequent transmutation process,but still bring us a new approach for the synthesis of novel boron-containing species.