Low-oxidation state cobalt–magnesium complexes: ion-pairing and reactivity

Abstract

Magnesium cobaltates (^Arnacnac)MgCo(COD)₂ (1–3) were synthesised by reacting (^Arnacnac)MgI(OEt₂) with K[Co(η⁴-COD)₂] (COD = 1,5-cyclooctadiene) [^Arnacnac = CH(ArNCMe)₂; Ar = 2,4,6-Me₃-C₆H₂ (Mes), 2,6-Et₂-C₆H₃ (Dep), 2,6-iPr₂-C₆H₃Mes (Dipp)]. Compounds 1–3 form contact ion-pairs in toluene, while solvent separated ion-pairs are formed in THF. The effect of ion-pairing on the reactivity is illustrated by reaction of 2 with tert-butylphosphaalkyne, which affords distinct 1,3-diphosphacyclobutadiene complexes. The heteroleptic sandwich complex [(^Depnacnac)MgCo(P₂C₂tBu₂)]₂ (4) is selectively formed in toluene, while the homoleptic bis(1,3-diphosphacyclobutadiene) complex [(^Depnacnac)Mg(THF)₃][Co(P₂C₂tBu₂)₂] (5) is obtained in THF. Complex 4 is a precursor to further unusual phosphaorganometallic compounds. Substitution of the labile COD ligand in 4 by white phosphorus (P₄) enabled the synthesis of the phosphorus-rich sandwich compound [(^Depnacnac)MgCoP₄(P₂C₂tBu₂)]₂ (6). The heterobimetallic complex (Cp*NiP₂C₂tBu₂)Co(COD) (7) was isolated after treatment of 4 with Cp*Ni(acac) (Cp* = C₅Me₅, acac = acetylacetonate).