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Radicals and Anions of Siloles and Germoles.

ABSTRACT: The synthesis of persistent sila- and germacyclopentadienyl (silolyl- and germolyl-) radicals by careful stoichiometric reduction of the corresponding halides with potassium is reported. The radicals were characterized by EPR spectroscopy and trapping reactions. The reduction of tris(trimethylsilyl)silyl-substituted halides was successful while smaller substituents (i. e., t-Butyl, Ph) gave the corresponding dimers. The EPR spectroscopic parameter of the synthesized tetrolyl radicals indicate only small spin delocalization to the butadiene unit due to cross-hyperconjugation. Silolyl- and germolyl anions are unavoidable byproducts and are isolated in the form of their potassium salts and characterized by X-ray crystallography. The comparison of the molecular structures of two closely related potassium silolides provided an example for different coordination of the potassium cation to the silolyl anion (η¹ vs. η⁵ coordination) that triggers the switch between delocalized and localized states.

SUBMITTER: Reinhold CRW

PROVIDER: S-EPMC8453960 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:This study shows that methyl 2-aminobenzoate (also known as methyl anthranilate, hereafter MA) undergoes direct photolysis under UVC and UVB irradiation and that its photodegradation is further accelerated in the presence of H₂O₂. Hydrogen peroxide acts as a source of hydroxyl radicals (·OH) under photochemical conditions and yields MA hydroxyderivatives. The trend of MA photodegradation rate vs. H₂O₂ concentration reaches a plateau because of the combined effects of H₂O₂ absorption saturation and ·OH scavenging by H₂O₂. The addition of chloride ions causes scavenging of ·OH, yielding Cl₂·- as the most likely reactive species, and it increases the MA photodegradation rate at high H₂O₂ concentration values. The reaction between Cl₂·- and MA, which has second-order rate constant k C l 2 • - + M A = (4.0 ± 0.3) × 10⁸ M-1·s-1 (determined by laser flash photolysis), appears to be more selective than the ·OH process in the presence of H₂O₂, because Cl₂·- undergoes more limited scavenging by H₂O₂ compared to ·OH. While the addition of carbonate causes ·OH scavenging to produce CO₃·- ( k C O 3 • - + M A = (3.1 ± 0.2) × 10⁸ M-1·s-1), carbonate considerably inhibits the photodegradation of MA. A possible explanation is that the elevated pH values of the carbonate solutions make H₂O₂ to partially occur as HO₂-, which reacts very quickly with either ·OH or CO₃·- to produce O₂·-. The superoxide anion could reduce partially oxidised MA back to the initial substrate, with consequent inhibition of MA photodegradation. Fast MA photodegradation is also observed in the presence of persulphate/UV, which yields SO₄·- that reacts effectively with MA ( k S O 4 • - + M A = (5.6 ± 0.4) × 10⁸ M-1·s-1). Irradiated H₂O₂ is effective in photodegrading MA, but the resulting MA hydroxyderivatives are predicted to be about as toxic as the parent compound for aquatic organisms (most notably, fish and crustaceans).

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Radicals and Anions of Siloles and Germoles.

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