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 Cα−Cβ 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 Cα phosphine adducts of allenylidene complexes such as [(CO)5W{C(PMe3)═C═C(NMe2)Ph}] (17) and might be regarded as Cα 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 Cα atom of the allenylidene complex [(CO)5W═C═C═C(C6H4NMe2-p)2] but instead to the Cγ 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 Cγ carbene adduct has been established by X-ray structure analyses.