Allylnickel(II) and Allylpalladium(II) Derivatives of [(2-(Diphenylphosphino)ethyl)cyclopentadienyl]tricarbonylmetalates: Reactions with Free Radicals

Abstract

The emerging utility of the bridging (2-(diphenylphosphino)ethyl)cyclopentadienyl (CpPPh) ligand to kinetically stabilize early-late metal−metal bonds to facilitate novel reactions in heterobimetallics is further established by the syntheses of four-legged piano stool M′{M(η3-L)}(CO)3(μ-η5:η1-CpPPh) (M′ = Cr, Mo, W; M = Ni, Pd; L = allyl, 2-methylallyl, cyclohexenyl). Complexes 1−3 (M = Ni, L = allyl) and 4−6 (M = Ni, L = 2-methylallyl) are the first structurally characterized heterobimetallics with transition metal−Ni(η3-allyl) units, while 7−9 (M = Ni, L = cyclohexenyl) are the only structurally characterized transition metal−Ni(η3-cyclohexenyl) complexes. Like Pd(η3-allyl)Cl(PPh3), 1−3 and 10−12 (M = Pd, L = allyl) respectively provide 4,4,4-triphenyl-1-butene as the sole allyl ligand coupling product from competitive reactions of phenyl and trityl radicals. However, while phenyl radical attack at the Pd(II) of Pd(η3-allyl)Cl(PPh3) is proposed as the first step in the trityl radical−allyl ligand coupling reaction, direct trityl radical attack at η3-allyl is strongly suggested in 1−3 and 10−12, respectively. A modest heterobimetallic effect may render the chromium complexes 1 and 10 more reactive with trityl radical than the tungsten complexes 3 and 12.

Incorporation of Radicals during Ni(II)-Catalyzed Ethylene Insertion Polymerization

“…By contrast, the impact of free radicals on an insertion polymerization is a frequent recurring issue, which is poorly understood. A rich variety of metal-catalyzed transformations for small molecule synthesis involve radical pathways. − The role of free radicals in catalytic chain growth polymerizations has remained elusive to date, however.…”

(Di‐tert‐butylphosphanylethyl)cyclopentadienylnickel Chelates with Alkyl, Alkenyl, Aryl, and Alkynyl Ligands: Hints to Cumulenylidene Resonance Forms

“…Decomplexation of a phosphane tether is possible in many cases,, , and temporary decomplexation of the tether may create a vacant coordination site potentially useful for catalytic processes , . Remarkably, cyclopentadienyl nickel complexes with a phosphane tether have been explored to a much lesser extent with the first representatives having been reported by us some time ago . Herein, we report on the synthesis and characterization of a number of new alkyl and alkynyl chelate complexes and provide hints to the significance of cumulenylidene resonance forms.…”