Synthesis, oxidation and unusual coordination chemistry of the diphosphete complex [(Cp′′′Co)2(μ,η4:4-(CH3CP)2)]†

Notwithstanding four decades of active research, very rare examples of diphosphabutadiene complexes arising from dimerisation of the phosphaalkyne CH₃–CP have been reported. Herein, we show that methyl phosphaalkyne dimerises in the coordination sphere of Cp′′′Co fragments to yield the unprecedented diphosphete complex [Cp′′′Co(η⁴-P₂C₂Me₂)] (1) and the triple-decker complex [{Cp′′′Co}₂(μ,η⁴:η⁴-P₂C₂Me₂)] (2). Compound 2 can be oxidised with Ag[Al{OC(CF₃)₃}₄] (Ag[TEF]) to give [{Cp′′′Co}₂(μ,η⁴:η⁴-P₂C₂Me₂)][TEF] (3). Additionally, it shows an unusual coordination behaviour towards copper halides and [W(CO)₄(nbd)] (nbd = norbornadiene). Its reaction with CuCl leads to the formation of the 1D polymer [{Cp′′′Co}₂(μ,η⁴:η²:η¹:η¹-P₂C₂Me₂){Cu(μ-Cl)}₂]_n (6) while its reaction with CuBr, CuI and [W(CO)₄(nbd)] results in the metallacyclic compounds [(Cp′′′Co){μ₃,η⁴:η¹:η¹-P₂C₂Me₂}(CH₃CN){Cu(μ-X)}₂]₂ (X = Br (7a), I (7b)) and [Cp′′′Co{μ₃,η⁴:η¹:η¹-P₂C₂Me₂}{W(CO)₄}]₃ (8). In 6, an isomerisation of the diphosphete ring to a diphosphacyclopentadienyl ring occurs. In 7 and 8, the loss of Cp′′′Co moieties from the former compound 2 is observed. The formation of 1,3-diphosphete rings in 1 and 2 instead of the more thermodynamically favoured 1,2-diphosphete rings are explained by DFT computations.