An organometallic approach to the synthesis of active site models for nickel-based catalysts

Abstract

Nickel complexes in sulfur-rich environments have been prepared to determine those factors which stabilize organonickel species and permit access to low valent nickel. The trans-(Ph2PCH2CH2S)2Ni complex was obtained by reaction of PPh2CH2CH2SH with Ni(acac)2 or Ni(COD)2. The reactivity of (Ph2PCH2CH2S)2Ni with organic or inorganic electrophiles was dominated by sulfur sites; for example, reaction with CH3I and BrCH2CH2CH2Br afford cationic thioethers, reaction with SO2 resulted in the formation of an SO2 adduct, and reaction with (NBD)Mo(CO)4 yielded the bimetallic NiP2S2Mo(CO)4. Nickel thioether complexes, [Ni(PSEt)2][BF4]2 and [Ni(PSSP)][BF4]2, showed two reduction waves ascribed to Ni[^II/I] and Ni[^1/0]. Ni^0 species can be prepared by chemical reduction with Na/Hg, which provided synthetic route to the desired organometallic complexes of the type [Ni(PSR)2L]+ with L = H, CH3, COCH3 . Acetyl products were produced either from oxidative addition of CH3COCI to Ni^0 or by CO insertion into Ni-CH3 at low temperature. The reversible CO insertion has been found in the reaction of CO with Ni-CH3 derivatives. The reaction of (Ph2PCH2CH2S)2Ni with Fe2(CO)9 at 70 °C resulted in ligand transfer and the isolation of an Fe2S2 complex which was characterized by x-ray crystallography. This complex displayed reactivity toward acid and afforded iron products with both terminal and bridged hydrides. Furthermore, chemical reduction of the Fe2S2 complex gave [Fe(CO)2 (PS)]2- and [Fe(CO)2(PS)]2[^2-] species, respectively. Several Ni/Fe complexes based on the N2S2Ni metalloligand, (bmedaco)Ni, have been synthesized. The tetrametallic [(bme-daco)NiFeCl2]2 has been characterized by x-ray crystallography. These Ni/Fe complexes may be useful for interpreting EXAFS and other spectroscopies data of enzymes.