Chemistry of cobalt complexes with 1,2-bis-(diethylphosphino)ethane: hydrides, carbon disulfide complexes, and C–H cleavage in activated alk-1-ynes. Crystal structure of [CoH(CCCO2Et)(Et2PCH2CH2PEt2)2][BPh4]

Abstract

The reaction of anhydrous CoCl₂ with depe [1,2-bis(diethylphosphino)ethane] yields [CoCl₂(depe)₂]1, which reacts with Na[BPh₄] in ethanol to give five-co-ordinate [CoCl(depe)₂][BPh₄]2. Both complexes have been studied by EPR spectroscopy. Complex 1 reacts with NaBH₄ in ethanol yielding [CoH₂(depe)₂]⁺, isolable as the tetraphenylborate salt 3. This compound is stereochemically non-rigid in solution, having a cis configuration at low temperatures. The spin–lattice relaxation times (T₁) for the hydrides of 3 suggest a classical formulation for this complex. The monohydride [CoH(depe)₂]4 is obtained by deprotonation of 3 using KOBu^t. Complex 3 reacts with CO and CS₂ with reductive elimination of H₂, yielding [Co(CO)(depe)₂][BPh₄]5 and [Co(η²-CS₂)(depe)₂][BPh₄]6, respectively, both stereochemically non-rigid in solution. The reaction of 6 with alkyl halides leads to CS₂ elimination, and formation of [CoX(depe)₂][BPh₄] derivatives (X = Br 7 or I 8). Complex 6 reacts with activated alk-1-ynes eliminating CS₂ and undergoing an oxidative addition of the alkyne, yielding the hydridoalkynyl complexes [CoH(CCR)(depe)₂][BPh₄](R = Ph 9 or CO₂Et 10). A mechanistic study of the reaction between 6 and HCCCO₂Et has been carried out, and the X-ray crystal structure of 10 determined.