| Group 15 element radicals, as an important branch of main group radical chemistry, has become one of the emerging areas of research in recent years. Due to their high reactivities, most of radicals are readily subjected to oxidation, dimerization, polymerization and decomposition and can only be detected in the solution, which has prevented further investigation and practical applications. It is therefore essential to explore novel methods to effectively stabilize and isolate radicals, thereby laying a solid foundation for investigating their applications in organic synthesis, biomedical, and other functional materials. In this thesis, we are working on the isolation, characterization, and theoretical calculations of several types of Group 15 element radical cations stabilized by weakly coordinating anions.1. Three bis(triarylamine) dications 12+-32+ with the substituents of H, Me, and OMe, respectively, at para-positions of peripheral aryl rings, were stabilized and isolated by using weakly coordinating anions. Their geometric and electronic structures in the ground state were consequently investigated by UV-vis, EPR, Single crystal X-ray diffraction and superconducting quantum interference device (SQUID) measurements, in conjunction with DFT calculations. Our work demonstrated an interesting substituent dependence of the structures, energy gaps, diradical character, and spectroscopic as well as magnetic properties. The ground-state structures of these species were tunable with 12+ and 22+ as closed-shell singlets, while 32+ as an open-shell singlet diradicaloid in the solid state. Such a tuning was made by modification of triphenylamines with donor substituents at para-positions and the recovery of aromaticity of the quinoidal benzene rings in the forms of diradicaloids. EPR and SQUID measurements indicated the excited state of 32+is readily thermally accessible due to its smaller singlet-triplet energy gap. The work provides new and stable structural motifs with diradical character. It is expected more dications based on OMe-substituted systems featuring diradical character may be accessible by using weakly coordinating anions.2. A series of bis[N,N-di-(4-methoxylphenyl)amino]arene dications 42+-62+ with the bridging rings of phenyl, naphthyl, and anthyl, respectively, were synthesized and characterized. Their electronic structures in the ground state were investigated by various experiments assisted by theoretical calculations. The work demonstrated an interesting bridging dependence of the structures, energy gaps, diradical character, and spectroscopic as well as magnetic properties. The ground-state structures of these species were tunable with 42+ and 52+ as closed-shell singlets, while 62+ as a singlet diradicaloid with a moderate diradical character in the solid state. Such a tuning was made by modification of the bridge with benzene. The steric repulsion and recovery of aromaticity of bridging aryl rings cause a smaller singlet-triplet energy gap for 62+ and its excited triplet state is readily thermally accessible. The work provides a nitrogen analogue of Thiele’s hydrocarbon with considerable diradical character.3. A series of stable 4,4"-Di(bisarylamino)-p-terphenyl dications 72+ -92+ with the terminal substituents of H, Me, and OMe, respectively, were successfully isolated by using weakly coordinating anions, and their electronic structures and geometries in the ground state were systematically investigated by various experiments and DFT calculations. Our studies demonstrated an interesting substituent dependence of the structures, energy gaps, and diradical character as well as magnetic properties. The ground-state structures of these species were tunable with 72+ as a closed-shell quinoid, while 82+ and 92+ as singlet diradicaloids. Such an evolution was made by steric repulsion between the neighboring benzene rings in the bridging units and recovery of aromaticity, i.e. reduced BLAs and more negative NICSs, of the quinoidal benzene rings in the forms of diradicaloids. EPR and SQUID measurements indicated the excited states of 82+ and 92+ are readily thermally accessible due to their smaller singlet-triplet energy gaps. In addition, compared with homologous bis(triarylamine) dications, nitrogen analogues of Chichibabin’s hydrocarbons,72+ -92+ showed increasd singlet diradical character with the extension of conjugation length. The work provides nitrogen analogues of Muller’s hydrocarbon with significant diradical character.4. Two phosphorus-containing four-membered ring radical cations [P(N’Pr2)]4·+ (10·+) and [RP(μ-NSiMe3)]2·+(11·+, R= N(SiMe3)2) have been isolated and characterized by UV-Vis absorption spectroscopy and electron paramagnetic resonance (EPR), as well as single-crystal X-ray diffraction. Compared with their neutral molecules, the former shows elongated P-P bonds and more pyramidalized phosphorus atoms, while the latter exhibits the shortened P-Nring distances and larger angles around phosphorus centers. EPR studies indicate that for 10·+ spin density mainly resides on the exocyclic nitrogen atoms with very minor on endocyclic phosphorus atoms, while for 11·+ the situation is opposite. DFT calculations show such an inverse spin density distribution is controlled by the exocyclic substituents. The work suggests that isolation of radical cations of other phosphorus and heavier pnictogen (As, Sb, Bi) ring and cage systems is possible by using weakly coordinating anions. |
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