η6-Coordinated ruthenabenzenes from three-component assembly on a diruthenium μ-allenyl scaffold

The room temperature reactions with internal alkynes, RCCR, of the mu-allenyl acetonitrile complex [Ru2Cp2(CO)(2)(NCMe){mu-eta(1):eta(2) -(CH)-H-1=C-2=(CMe2)-Me-3}]BF4 (1-NCMe), freshly prepared from the tricarbonyl precursor [Ru2Cp2(CO)(3){mu-eta(1):eta(2) -(CH)-H-1=C-2=(CMe2)-Me-3}]BF4, 1, proceeded with alkyne insertion into ruthenium-allenyl bond and allenyl-CO coupling, affording compounds [Ru2Cp2(CO2{mu-eta(2):eta(5)-C(R)C(R)(CHC2)-H-1((CMe)-Me-3=CH2)C(OH))}]BF4 (R = Ph, 2; R = CO2Me, 3; R = CO2Et, 4) in 83-94% yields. Deprotonation of 2-4 by triethylamine gave [Ru2Cp2(CO)(2){mu-eta(2):eta(5)-C(R)C(R)CHC(CMe=CH2)C(O)}] (R = Ph, 5; R = CO2Me, 6; R = CO2Et, 7) in 75-88% yields, and 2-4 could be recovered upon HBF4 center dot Et2O addition to 5-7. All the products, 2-7, were fully characterized by elemental analysis, IR and multinuclear NMR spectroscopy. The structure of 2 was ascertained by single crystal X-ray diffraction and investigated by DFT calculations, revealing a six-membered ruthenacycle with Shannon aromaticity index in line with related compounds. The formation of ruthenium-coordinated ruthenabenzenes from a preexistent diruthenium scaffold is a versatile but underdeveloped approach exploiting cooperative effects typical of a dimetallic core.