Tuning the Optoelectronic Properties of Stannoles by the Judicious Choice of the Organic Substituents

Borylated 2,3,4,5-Tetrachlorophthalimide and Their 2,3,4,5-Tetrachlorobenzamide Analogues: Synthesis, Their Glycosidase Inhibition and Anticancer Properties in View to Boron Neutron Capture Therapy

Tetrachlorinated phthalimide analogues bearing a boron-pinacolate ester group were synthesised via two synthetic routes and evaluated in their glycosidase modulating and anticancer properties, with a view to use them in boron neutron capture therapy (BNCT), a promising radiation type for cancer, as this therapy does little damage to biological tissue. An unexpected decarbonylation/decarboxylation to five 2,3,4,5-tetrachlorobenzamides was observed and confirmed by X-ray crystallography studies, thus, giving access to a family of borylated 2,3,4,5-tetrachlorobenzamides. Biological evaluation showed the benzamide drugs to possess good to weak potencies (74.7-870 μM) in the inhibition of glycosidases, and to have good to moderate selectivity in the inhibition of a panel of 18 glycosidases. Furthermore, in the inhibition of selected glycosidases, there is a core subset of three animal glycosidases, which is always inhibited (rat intestinal maltase α-glucosidase, bovine liver β-glucosidase and β-galactosidase). This could indicate the involvement of the boron atom in the binding. These glycosidases are targeted for the management of diabetes, viral infections (via a broad-spectrum approach) and lysosomal storage disorders. Assays against cancer cell lines revealed potency in growth inhibition for three molecules, and selectivity for one of these molecules, with the growth of the normal cell line MCF10A not being affected by this compound. One of these molecules showed both potency and selectivity; thus, it is a candidate for further study in this area. This paper provides numerous novel aspects, including expedited access to borylated 2,3,4,5-tetrachlorophthalimides and to 2,3,4,5-tetrachlorobenzamides. The latter constitutes a novel family of glycosidase modulating drugs. Furthermore, a greener synthetic access to such structures is described.

Latest News: Reactions of Group 4 Bis(trimethylsilyl)acetylene Metallocene Complexes and Applications of the Obtained Products

Recently published reactions of group 4 metallocene bis(trimethylsilyl)acetylene (btmsa) complexes from the last two years are reviewed. Complexes like Cp'2 Ti(η2 -Me3 SiC2 SiMe3 ) and Cp2 Zr(py)(η2 -Me3 SiC2 SiMe3 ) with Cp' as Cp (cyclopentadienyl) and Cp* (pentamethylcyclopentadienyl) have been considered (py=pyridine). These complexes can liberate a reactive low-valent titanium or zirconium center by dissociation of the ligands and act as ''masked'' MII complexes (M=Ti, Zr). They represent excellent sources for the clean generation of the reactive coordinatively and electronically unsaturated complex fragments [Cp'2 M]. This is the reason why they were used for many synthetic and catalytic reactions during the last years. As an update to several review articles on this topic, this contribution provides an update with recent examples of preparative organometallic and organic chemistry of these complexes, acting as reagents for a wide range of coordinating and coupling reactions. In addition, applications and investigations concerning reaction products derived from this chemistry are mentioned, too.

The influence of the formal replacement of thiophenes by stannoles in terthiophene and sexithiophene on the optoelectronic properties and electrochemical behavior

Polystannoles with thienyl co-monomers are in many ways similar to polythiophenes, but they display much reduced band gaps. However, their polymerization processes are not well researched. Thiophene can be oxidatively electropolymerized, but as stannoles are organometallic, the fundamental question arises whether their inclusion in a conjugated backbone can protect them sufficiently to be able to perform an oxidative electropolymerization. As well-defined oligothiophenes can be used as models to understand the optical and electronic properties of polythiophenes, we transposed this concept on stannole containing polymers; therefore we synthesized a monomeric 1 and dimeric thiophene-flanked stannole 2 and investigated their optoelectronic properties comparatively including polystannoles and the corresponding oligothiophenes in our analysis. With respect to monomer 1, a significantly redshifted absorption (λmax = 510 nm, Δ = 93 nm) and a small optical band gap (Eg,opt(2) = 2.13 eV), close to the bandgap of polymeric stannoles, was observed. In comparison to oligothiophenes, these thienyl-flanked stannoles exhibited a redshift in absorption and emission as well as a lower oxidation potential. Despite these differences, they showed an oligothiophene-like electrochemical behavior. Stannole 1 and the dimer 2 were subjected to an electropolymerization process. This process was investigated in detail by spectroelectrochemical methods which showed that radical cation species were formed in situ but readily decomposed. Nevertheless, under the milder multiscan cyclovoltammetric conditions, electropolymerization occurred as shown by cyclovoltammetry.

π-Conjugated stannole copolymers synthesised by a tin-selective Stille cross-coupling reaction

The synthesis of four well-defined conjugated polymers TStTT1-4 containing unusual heterocycle units in the main chain, namely stannole units as building blocks, is reported. The stannole-thiophenyl copolymers were generated by tin-selective Stille coupling reactions in nearly quantitative yields of 94% to 98%. NMR data show that the tin atoms in the rings remain unaffected. Weight-average molecular weights (M w) were high (4900-10 900 Da and 9600-21 900 Da); and molecular weight distributions (M w/M n) were between 1.9 and 2.3. The new materials are strongly absorbing and appear blue-black to purple-black. All iodothiophenyl-stannole monomers St1-4 and the resulting bisthiophenyl-stannole copolymers TStTT1-4 were investigated with respect to their optoelectronic properties. The absorption maxima of the polymers are strongly bathochromically shifted compared to their monomers by about 76 nm to 126 nm in chloroform. Density functional theory calculations support our experimental results of the single stannoles St1-4 showing small HOMO-LUMO energy gaps of 3.17-3.24 eV. The optical band gaps of the polymers are much more decreased and were determined to be only 1.61-1.79 eV. Furthermore, both the molecular structures of stannoles St2 and St3 from single crystal X-ray analyses and the results of the geometry optimisation by DFT confirm the high planarity of the molecules backbone leading to efficient conjugation within the molecule.

Experimental and Theoretical Studies of a Spirostannole and Formation of a Pentaorganostannate

A new spirostannole, 1,1',3,3'-tetrakis(5-methylthiophen-2-yl)-4,4',5,5',6,6',7,7'-octahydro-2,2'-spirobi[benzo[c]stannole] (4), is synthesised and the molecular structure is compared with the optimised geometry from DFT calculations. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are twice degenerated and show a small HOMO-LUMO energy gap of 3.2 eV. In addition, cyclic voltammetry measurements are conducted and three redox processes are observed. Absorption and emission spectra show maxima at λ_abs,max 436 nm and λ_em,max 533 nm, respectively. Spirostannole 4 is a strongly absorbing material, but an extremely weak emitter in solution at 295.15 K. However, when the solution is cooled from 280 to 80 K, the emission becomes visible. The reaction of spirostannole 4 with methyllithium is monitored by NMR spectroscopy at 238.15 K. The ¹¹⁹Sn{¹H} NMR signal shifts from -36.0 (4) to -211.0 ppm, which is indicative of the formation of the lithium pentaorganostannate 5. The complex is thermally instable at 295.15 K, but insights into the molecular structure and electronic behaviour are obtained by DFT and TD-DFT calculations.

Aggregation induced emission - emissive stannoles in the solid state

The optoelectronic and structural properties of six stannoles are reported. All revealed extremely weak emission in solution at 295 K, but intensive fluorescence in the solid state with quantum yields (ΦF) of up to 11.1% in the crystal, and of up to 24.4% (ΦF) in the thin film.

Negishi's Reagent Versus Rosenthal's Reagent in the Formation of Zirconacyclopentadienes

Zirconacyclopentadienes are versatile precursors for a large number of heteroles, which are accessible by Zr-element exchange reactions. The vast majority of reports describe their preparation by the use of Negishi's reagent, which is a species that is formed in situ. The zirconacyclopentadiene is then formed by the addition of one equivalent of a diyne or two equivalents of a monoyne moiety to this Negishi species. Another route involves Rosenthal's reagent (Cp2 Zr(py)Me3 SiC≡CSiMe3 ), which then reacts with a diyne or monoyne moiety. In this work, the efficiency of both routes was compared in terms of reaction time, stability of the product in the reaction mixture, and yield. The synthetic implications of using both routes are evaluated. Novel zirconacyclopentadienes were synthesized, characterized directly from the reaction mixture, and crystal structures could be obtained in most cases.

Advantages of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes as Metallocene Sources Towards Other Synthetically used Systems

Active species for synthetic and catalytic applications are formed from well defined complexes or mixtures of compounds. For group 4 metallocenes, three pathways for the formation of the reactive complex fragment [Cp'2M] are known: (i) reductive mixtures and well defined complexes which are able to form the metallocene fragments either by (ii) addition or (iii) substitution reactions. In this account for each of theses systems (i)-(iii) a prominent example will be discussed in detail, (i) the Negishi reagent Cp2ZrCl2/n-BuLi, (ii) bis(η5 : η1-pentafulvene) complexes and (iii) metallocene bis(trimethylsilyl)acetylene complexes, to show the advantages and the disadvantages for each of these methods for synthetic applications. This account summarizes some main advantages of group 4 metallocene bis(trimethylsilyl)acetylene complexes as metallocene generating agents over other synthetically used systems. For each of the special purposes, all described systems have advantages as well as disadvantages. The aim of this overview is to help synthetic chemists in selecting the most effective system on the basis of [Cp'2M] (M=Ti, Zr) for synthetic or catalytic puposes.

Synthesis of Stannole-Containing π-Conjugated Polymers by Post-Element Transformation of Organotitanium Polymer

The synthesis of stannole-2,5-diyl-containing π-conjugated polymers by the post-element transformation of a regioregular organotitanium polymer is described. For example, a 1,1-diphenylstannole-containing polymer is obtained in 83% yield by the reaction of a regioregular organotitanium polymer, which is prepared from 1,4-bis(2-ethylhexyloxy)-2,5-diethynylbenzene and a low-valent titanium complex with diphenyltin dichloride at -50 °C to ambient temperature. The number-average molecular weight and molecular weight distribution (M_n and M_w /M_n ) of the stannole-containing polymer are estimated as 4800 and 1.8, respectively. The obtained polymer is found to have the extended π-conjugated backbone and relatively low-lying lowest unoccupied molecular orbital (LUMO) energy level (-3.12 eV), which is supported by its UV-vis absorption spectrum and cyclic voltammetric (CV) analysis. In addition, the stannole-containing polymer is found to be applicable to a chemosensor for fluoride anion where the color and photoluminescence intensity of the polymer solution exhibits a distinct change in the presence of a fluoride anion.

Electronic Push-Pull Modulation by Peripheral Substituents in Pentaaryl Boroles

Establishing access to a bulky tetraaryl dilithiobutadiene (Ph*C)₄ Li₂ (Ph*=3,5-tBu₂ (C₆ H₃ )) allowed for the synthesis of five-membered heterocycles with incorporated main-group elements. Along with an amino borole, a set of substituted pentaaryl boroles (Ph*C)₄ BAr has been synthesized. The examination of their absorption spectra and computational studies by means of DFT granted insight into the influence of peripheral substituents on the electronic features of the parent pentaphenyl borole (PhC)₄ BPh. Introduction of the more electron-rich Ph* residue at the carbon atoms increases the HOMO energy, redshifting the visible π/π*-absorption bands compared with the parent pentaphenyl borole. The influence on the frontier orbitals of three different boron-bound aryls with electronically modulating substituents in the remote 3,5-positions Ar=3,5-R₂ -C₆ H₃ (R=Me, H, CF₃ ) was studied. The substituents were found to increase (+I effect, Me) or decrease (-I effect, CF₃ ) the LUMO energy, thus directly affecting the visible absorption spectra. This represents the first study on HOMO-LUMO-gap adjustments by a combined push-pull approach of a substituted pentaphenylborole.

Synthesis, Structure, Thermal Behavior and cis/trans Isomerization of 2,2'-(EMe₃)₂ (E = C, Si, Ge, Sn) Substituted Azobenzenes

The synthesis of a series of 2,2′-bis(trimethyltetrel) azobenzenes is reported, evaluating the different synthetic approaches that different group 14 element substituents individually require. The synthetic access to the carbon substituted congener is very different from the heavier tetrels, in that the key step is the formation of the N=N bond in azobenzene, rather than the azobenzene-C bond. Sn could be introduced with a cross-coupling route, whereas the Si and Ge congeners were prepared by a stannylation-lithiation-electrophilic quenching sequence. Iodo-lithium exchange was also a possible route to obtain the dilithiated species, which can be attributed to the chelating effect of the nitrogen atoms. However, the organo-lead species could not be obtained via these routes. The resulting structures were fully characterized (NMR, FTIR, HRMS and XRD). Furthermore, their thermal properties (TGA and DSC) and their photoswitching behavior in solution (UV-VIS & NMR experiments) were investigated and compared for the different tetrels (C, Si, Ge, Sn).

Grignard synthesis of fluorinated nanoporous element organic frameworks based on the heteroatoms P, B, Si, Sn and Ge

We present the synthesis and characterization of fluorinated polymers based on P, B, Si, Sn and Ge as heteroatoms via Grignard activation.