Quintuply bonded [(H)L(iPr)Cr](2) reacts with alkynes RC≡CR (R = Me, Et, Ph, CF(3)) to form exclusively 1 : 1 adducts [(H)L(iPr)Cr](2)(RCCR). All products feature relatively short Cr-Cr distances (1.919-1.962 Å) and elongated C-C bonds (1.315-1.436 Å), consistent with [2+2] cycloaddition reactions. The hydrocarbon adducts are 4-membered metallacycles, the bridging alkynes of which are progressively skewed with respect to the Cr-Cr axis. In contrast, perfluoroalkyne adds across the metal ligand moiety.
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Reversible Cleavage/Formation of the Chromium-Chromium Quintuple Bond in the Highly Regioselective Alkyne Cyclotrimerization.
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Department of Chemistry and Frontier Research, Center on Fundamental and Applied Sciences and Matters, National Tsing Hua University, 101, Sec. 2, Guang-Fu Road, Hsinchu, 300, Taiwan.
Herein we report the employment of the quintuply bonded dichromium amidinates [Cr{κ -HC(N-2,6- Pr C H )(N-2,6-R C H )}] (R=iPr (1), Me (7)) as catalysts to mediate the [2+2+2] cyclotrimerization of terminal alkynes giving 1,3,5-trisubstituted benzenes. During the catalysis, the ultrashort Cr-Cr quintuple bond underwent reversible cleavage/formation, corroborated by the characterization of two inverted arene sandwich dichromium complexes (μ-η :η -1,3,5-(Me Si) C H )[Cr{κ -HC(N-2,6- Pr C H )(N-2,6-R C H )}] (R= Pr (5), Me (8)). In the presence of σ donors, such as THF and 2,4,6-Me C H CN, the bridging arene 1,3,5-(Me Si) C H in 5 and 8 was extruded and 1 and 7 were regenerated.
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∥Max Planck Institut für Chemische Energiekonversion (MPI-CEC), Stiftstraße 34-36, 45470 Mülheim an der Ruhr, Germany.
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