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Chalcogen-based species are common ligands in transition-metal chemistry and display a variety of coordination modes. Like alkyl- and arylchalcogenolates, silylchalcogenolates are able to stabilize transition-metal complexes. Metal chalcogenolates LnM-ESiR3 with small organic residues R can serve as precursors for larger metal–chalcogenide clusters, which can be accessed by cleaving the E-Si bond. Furthermore, large silyl residues at the chalcogen atom serve to kinetically stabilize reactive systems. To explore the diverse chemistry of this class of compounds, a number of different silyl chalcogenolates were synthesized, including the sodium siloxide Ph2MeSiONa and the chalcogen derivatives of the extremely sterically hindered silyl residues tBu2PhSi- und tBu3Si-. The anionic silyl species tBu2PhSiNa and tBu3SiNa nucleophilically degrade elemental chalcogens (S, Se, and Te), thus producing the silyl chalcogenolates tBu2PhSiENa and tBu3SiENa (E = S, Se, Te). The chemical and structural properties of these compounds were studied. Protonolysis produces the corresponding chalcogenols tBu2RSiEH, while oxidation leads to the dichalcogenides tBu2RSiE-ESiRtBu2 (R = tBu, Ph; E = S, Se, Te). Oxidative addition of the dichalcogenides to metal centers in low oxidation states offers one route to chalcogenolate complexes. To investigate the realm of this approach, three oligochalcogen compounds R3SiE-E′n-ESiR3 were synthesized. The tetrasulfane tBu3SiS-S2-SSitBu3 and the chalcogen(II)dithiolates (tBu3SiS)2Se and (tBu3SiS)2Te were produced, and their stability was investigated. The direct comparison of isoelectronic species allows for a deeper understanding of their similarities and differences. The silanides R3Si– can be considered as anionic phosphane analogues in which a phosphorus atom has been formally replaced with a Si– unit. Phosphanylborhydrides R2BH3P– also belong to this isoelectronic series. The same analogy holds true for the chalcogen derivatives related to the phosphane chalcogenides R3P=E. With this in mind, complexes of the CpFe(CO)2 fragment with the different isoelectronic ligands were synthesized and compared. The silyl-based ligands were found to be the strongest donors of the two isoelectronic series. The differences in donor strength were roughly twice as large for the nonchalcogen species as for the chalcogen-based ligands. To further investigate the chemistry of transition-metal silyl chalcogenolate complexes, the coordination behavior of the chalcogenolates tBu2RSiE– (R = tBu, Ph; E = S, Se, Te) was studied. Salt metathesis of silyl thiolates with appropriate metal halides leads to the multinuclear complexes [Cu(SSitBu2Ph)]4 and [ZnCl(SSitBu3)(THF)]2. Metathesis products were identified in the reactions of BrMn(CO)5 with one or two equivalents of tBu3SiSNa(THF)2. Diproporationation of these compounds leads to dimeric Mn(I)Mn(II) complexes. The crystal structure of the dinuclear disproportionation product [(CO)3Mn(mu-SSitBu3)3Mn(SSitBu3)]– displays a terminal tBu3SiS– ligand, which coordinates with a Mn-S-Si angle of 180°. This geometry indicates that the thiolate can be considered as a six-electron donor (2 sigma e–, 4 pie–), analogous to the cyclopentadienyl ligand. Photoinduced oxidative addition of the dichalcogenides to Fe(CO)5 leads to the dimeric complexes [(CO)3Fe(ESitBu3)]2 (E = S, Se, Te). The tellurolate complex forms quantitatively within 8 h. The thiolate complex, on the other hand, is formed slowly over a period of six months. IR-spectroscopic investigation of the CO vibrations of the three homologous complexes indicates that the tellurolate is the strongest donor of the series.
The introduction of a trigonal boron atom into a polyaromatic hydrocarbon (PAH) core is an extremely powerful tool to provide organic scaffolds with optoelectronic properties as well as optimal packing in the solid state. However, boron-doped PAHs (B-PAHs) often display low processability due to their poor solubility. The distortion of the molecular scaffold provides a suitable strategy to enhance the solubility properties of B-PAHs while maintaining good stacking properties and sufficient electronic conjugation.
Extreme distortion of the molecular structure can be achieved in helical-shaped PAHs, namely helicenes which are screw-shaped inherently chiral polycycles, formed by ortho-fused aromatic or heteroaromatic rings. The presence of a helical structure in B-PAHs is expected to strongly influence their physico-chemical properties leading to compounds characterized by peculiar features promising for applications in next generation functional materials. Despite the great potential of this class of compounds, only few examples of borahelicenes have been reported in the literature and those mainly consist of carbohelicene-based structures. However, the considerable structural diversity achievable by introducing different boraheterocycles (oxaborine, borole, borepin) and other heteroaromatic rings (thiophene, furan, pyrrole) into the same helical scaffold, suggests that a large variety of compounds with intriguing features could be accessible via currently unexplored synthetic routes. The design, synthesis, and properties investigation of new boraheterohelicenes (BHHs) is therefore a relevant research topic and is the object of this PhD project, aimed to obtain several BHHs with structural diversity, as well as to study their reactivity, electrochemical and photophysical features for better understanding their potential as building blocks for material science.
The thesis work was carried out in part at the University of Milan in the laboratories of Prof. Emanuela Licandro and in part at the Goethe Universität Frankfurt am Main under the supervision of Prof. Dr. Matthias Wagner, within a co-tutelle programme. Owing to the long-standing expertise of Prof. Licandro group in the synthesis of tetrathia helicenes and that of Prof. Dr. Wagner group in the synthesis of boron-doped PAHs (e.g. boron helicene 4BH; Figure 1), I conceived this PhD project designing a series of thiahelicenes containing one or more B-O bond into the helical scaffold.Tetrathia helicenes, consisting of thiophene and benzene rings fused in an alternating fashion, are configurationally stable heterohelicenes which exist as pair of enantiomers.
This class of molecules is particularly interesting since it merges the properties of oligothiophenes with those of helicenes, giving rise to systems with peculiar electronic and chiroptical properties which make them appealing building blocks for applications in manifold fields of science, including optoelectronics, catalysis and biology.
The introduction of trigonal boron atom into a thiahelicene scaffold gives rise to a novel class of unexplored boron π-conjugated molecules with potentially interesting features.
The present Ph.D. thesis was therefore intended to provide a meaningful contribution in the development of innovative and versatile syntheses of BO-doped tetrathia helicenes as well as the study of their stereochemical and optoelectronic properties to identify potential applications of these systems in material science. The first selected structures containing one or two oxaborine rings in the helical scaffold are shown in figure 2. The presence of the bulky mesityl group at the boron atom is necessary to ensure stability to the molecule.
It is noteworthy that compound 2 is the skeletal isomer of 1, as the direction of the B-O bond is opposite in the two molecules. In the course of the research work, after the evaluation of the photophysical properties of 1, helicene 2 was designed to get information on the structure-property relationship and evaluate how the position of the BO-bond into the helical scaffold can influence the electronic properties of BO-doped thiahelicenes.