Role of Oleylamine Revisited: An Original Disproportionation Route to Monodispersed Cobalt and Nickel Nanocrystals

Abstract

Herein is highlighted an original disproportionation mechanism for the formation of hexagonal closed packed cobalt nanoparticles starting from low-valent cobalt complexes. By coupling experimental and theoretical studies, we demonstrate that oleylamine promotes the disproportionation of the starting metal complexes and controls the growth of the nanocrystals, in contrast to the role of reducing reagents proposed elsewhere. We also decipher the sensitivity of the synthesis in regard to the nature of the phosphine and of the halogen ligands in the starting cobalt precursors. Understanding of the mechanism allows the development of a less expensive and less toxic synthesis of these nanoparticles by decreasing the amount of oleylamine in alkane solvents. Finally, an extension of this process is presented and allows reaching size-and shape-controlled nanosphere and nanorod nickel nanoparticles.

Triggering Electron Transfer in Co(I) Dimers: Computational Evidences for a Reversible Disproportionation Mechanism

“…We have recently described its energy profile and shown that the disproportionation is made thermodynamically possible by the appropriate choice of the ligands. [2] In agreement with the HSAB theory, phosphine ligands are optimal to stabilize the transient Co(0) complex whereas amine ligands stabilize the Co(II) product. Deciphering each step of the synthetic process would help in controlling size, shape and crystallinity of the obtained metallic Co NPs.…”

“…In this process, Electron Transfer (ET) between two monomeric Co(I) leads to stable Co(II) complexes in solution and transient Co(0) ones. We have recently described its energy profile and shown that the disproportionation is made thermodynamically possible by the appropriate choice of the ligands [2] . In agreement with the HSAB theory, phosphine ligands are optimal to stabilize the transient Co(0) complex whereas amine ligands stabilize the Co(II) product.…”

“…The observations suggest a complete replacement of phosphines by amines under reaction conditions in the Co(I) precursor. [2] The two remaining structures (N.IV and N.V) display short CoÀ Co distances (2.74 and 2.60 Å) and no bridging Cl. The first one is characterized by a Cl on each Co (N.IV) and the second one with both Cl on the same Co (N.V).…”

“…The monomer CoCl(PH 3 ) 3 ground state is reported to be in a triplet state. [2,7] Consequently, a Co 2 Cl 2 (PH 3 ) 6 dimer with no or little coupling between the Co centers is a quintet. In disproportionation products, the monomeric Co(PH 3 ) 4 fundamental spin state is a doublet whereas that of CoCl 2 (PH 3 ) 2 is a quadruplet.…”

“…However, the disproportionation is made possible by the stabilization of the molecular products through adaptation of coordination around Co(II) (using amine) and Co(0) (using phosphine), following HSAB principles as evidenced in equation 3. [2] To favor ET, the coordination sphere in Co(II)À Co(0) dimers should thus mix amine and phosphine ligands.…”