Synthesis and Selectivity in the Formation of Cyclophosphazene-Derived 1,3-Cyclohexadienes from Reactions of RCpCo(COD) [R = MeOC(O)] with Alkynes and Alkenes

The first examples of mono and bisfluorophosphazene derived [η 5 -cyclopentadienyl] [η 4 -1,3-cyclohexadiene] cobalt complexes have been prepared along with the sandwich compound [η 5 -carbomethoxycyclopentadienyl] [η 4 -1,3-bis(pentafluorocyclotriphosphazenyl)-2,4-diphenylcyclobutadiene] cobalt and acetylene trimerized products from the reactions of [η 5 -MeOC(O)C 5 H 4 ]Co[COD], PhC≡CP 3 N 3 F 5 and phenylacetylene in the presence or absence of an additional cycloalkene or indene. Formation of these mono and bis fluorophosphazene derived cobalt cyclohexadiene complexes provide experimental evidence for a metallacyclopentadiene pathway for cyclohexadiene formation in CpCo catalyzed reactions. Selectivity is also observed in the formation of bisfluorophosphazene derived cyclohexadienes which stems from the fact that two P 3 N 3 F 5 units cannot be accommodated on vicinal carbon atoms of a carbacycle or metallacycle. Interestingly, reactions of (β-phenylethynyl)pentafluorobenzene with [η 5 -MeOC(O)C 5 H 4 ]Co[COD] in the presence and absence of external cycloalkene under identical reaction conditions yielded only the cis and trans isomers of the metallocene [η 5 -MeOC(O)C5H4]Co[η 4 -C4Ph2(C6F5)2] along with alkyne trimerized product indicating that the selectivity in cyclohexadiene formation is governed more by steric than electronic factors. All the new compounds were characterized by 1 H, 13 C, 31 P, and 19 F NMR as well as mass spectrometry and elemental analysis. Mono and bispentafluorocyclotriphosphazene derived [η 5 -cyclopentadienyl] [η 4 -1,3-cyclohexadiene] cobalt complexes and [η 5 -carbomethoxycyclopentadienyl] [η 4 -bis(1,3-pentafluorophenyl)-2,4-diphenylcyclobutadiene] cobalt have also been structurally characterized by single crystal X-ray analysis.