cyclo-Hexaphosphane versus cyclo-hexaarsane fragmentation on a sulphur-capped mixed-metal dicobalt–iron skeleton

Reaction of the sulphur-capped dicobalt–iron complex [Co2Fe(μ3-S)(CO)9] with cyclo-(PhAs)6 in toluene at 70 °C gives [Co2Fe(μ3-S){μ-cyclo-(PhAs)6}(CO)7] (1) as the only product in good yield. Conversely, treatment of [Co2Fe(μ3-S)(CO)9] with cyclo-(PhP)6 under the same reaction conditions gives two isomers [Co2Fe(μ3-S){μ-cyclo-(PhP)6}(CO)7] (2) and [Co2Fe(μ3-S){μ-cyclo-(PhP)6}(CO)7] (3) in ca. 1:1 ratio. Single crystal X-ray analysis shows 1 and 2 to be isostructural with the intact six-membered ring adopting a chair conformation and bridging a cobalt–cobalt edge via either two arsine (1) or two phosphorus (2) atoms in the 1,5 positions of the respective rings. Thermolysis of 2 or of 3 in toluene at 120 °C gives [Co2Fe(μ3-SPPh)(μ-η2:η2:η1-P5Ph5)(CO)5] (4) in moderate yield, while thermolysis of 1 under the same conditions leads to complete decomposition. A single crystal X-ray diffraction study performed on 4 reveals the cyclo-hexaphosphane rings in 2 or 3 to have cleaved to give a P5Ph5 chain and a PPh group, the latter of which has additionally undergone coupling with the sulphur cap.