Amino-substituted butatrienes : Unusual η¹ ligands formed by an unusual reaction

Abstract

The reaction of the bis(amino)allenylidene complex [(CO)₅W═C═C═C(NMe₂)₂] with diethyldiazomethane yields two products, a cyclic carbene complex (2) by 1,2-addition of Et₂C−N₂ to the C_α−C_β bond of the allenylidene ligand and the η¹-butatriene complex [(CO)₅W{C[C(NMe₂)₂]═C═CEt₂}] (3). Complex 2 slowly eliminates N₂ and rearranges into 3. In contrast, only η¹-butatriene complexes, [(CO)₅M{C[C(NMe₂)XR]═C═C(R′)₂}], are isolated from the reaction of diazoalkanes (R′)₂C−N₂ (R′ = Me, Et, ⁿPr) with alkoxy(amino)allenylidene complexes [(CO)₅M═C═C═C(NMe₂)OR] (M = Cr, W; R = Me, Et, (−)-menthyl) or with the (alkylthio)(amino)allenylidene complex [(CO)₅W═C═C═C(NMe₂)SⁿPr]. These new η¹-butatriene complexes are related to C_α phosphine adducts of allenylidene complexes such as [(CO)₅W{C(PMe₃)═C═C(NMe₂)Ph}] (17) and might be regarded as C_α carbene adducts. However, the PMe₃ substituent in 17 is not replaced when 17 is treated with an N-heterocyclic carbene. Vice versa, the “carbene” substituent “C(NMe₂)OEt” in the η¹-butatriene complex [(CO)₅Cr{C[C(NMe₂)OEt]═C═CEt₂}] is not replaceable by PMe₃. Free N-heterocyclic carbenes do not add to the C_α atom of the allenylidene complex [(CO)₅W═C═C═C(C₆H₄NMe₂-p)₂] but instead to the C_γ atom, giving the dipolar neutral alkynyl complexes [(CO)₅W−C≡CC(C₆H₄NMe₂-p)₂L] (L = SIMe, SIMes). DFT calculations on the reaction mechanism indicate that a cyclic carbene complex and two isomeric η²-butatriene complexes are intermediates in the reaction pathway to form η¹-butatriene complexes. The structure of two representative examples of η¹-butatriene complexes and of one C_γ carbene adduct has been established by X-ray structure analyses.