Dihalogeno(diphosphane)metal(ii) complexes (metal = Co, Ni, Pd) as pre-catalysts for the vinyl/addition polymerization of norbornene – elucidation of the activation process with B(C6F5)3/AlEt3 or Ag[closo-1-CB11H12] and evidence for the in situ formation of “naked” Pd2+ as a highly active species?

Dalton Transactions Pub Date: 2003-09-25 DOI: 10.1039/B302937A

Abstract

Dihalogenometal(II) complexes with bidentate phosphane ligands of the general type [M{Ph2P(CH2)nPPh2}X2] with n = 2 to 5, X = Cl or Br and M = Co, Ni or Pd have been utilized as catalysts for the vinyl/addition polymerization of norbornene. These complexes can be activated with the Lewis-acids methylalumoxane (MAO) or tris(pentafluorophenyl)borane, B(C6F5)3 in combination with triethylaluminium (AlEt3). The nickel(II) and palladium(II) complexes show very high polymerization activities up to 107 gpolymer molmetal?1 h?1. Yet, the complexes Pd(dppe)Cl2 (5, 1.9 × 107 gpolymer molPd?1 h?1) and Pd(dppp)Cl2 (6, 3.0 × 103 gpolymer molPd?1 h?1) demonstrated that small changes in the ligand structure could have great effects on the polymerization activity [dppe = 1,2-bis(diphenylphosphino)ethane, Ph2P(CH2)2PPh2; dppp = 1,3-bis(diphenylphosphino)propane, Ph2P(CH2)3PPh2]. The activation process of the pre-catalysts 5 and 6 in combination with B(C6F5)3/AlEt3 was followed by multinuclear (1H, 19F, and 31P) NMR investigations and by reaction with B(C6F5)3 and Ag[closo-1-CB11H12]. The reaction of B(C6F5)3 and AlEt3 leads to an aryl/alkyl group exchange resulting in the formation of AlEt3???n(C6F5)n and B(C6F5)3???nEtn with Al(C6F5)3 and BEt3 as main products for an about equimolar ratio. AlEt3???n(C6F5)n will then react with the pre-catalysts and abstract the chloride atoms to form [M{Ph2P(CH2)nPPh2}]2+ as the active species for the polymerization. The higher polymerization activity of 5/B(C6F5)3/AlEt3 compared to 6/B(C6F5)3/AlEt3 can be explained by a ligand redistribution reaction of unstable [PdII(dppe)]2+ to give inactive and isolable [PdII(dppe)2]2+ and highly active, “naked” Pd2+ cations together with the lower coordinating ability of the anionic adduct [Cl–Al(C6F5)3]? in comparison to [Cl–B(C6F5)3]?. The Lewis-acid Al(C6F5)3 is much more activating than B(C6F5)3. The [Pd(dppe)2]2+ cation from the ligand redistribution was isolated in the (X-ray) structurally elucidated compounds [PdII(dppe)2][ClB(C6F5)3]2·4CH2Cl2 and [PdII(dppe)2][CB11H11Cl]2·3CH2Cl2. The stable [Pd(dppp)]2+ cation from 6 could be crystallized as [PdII(dppp)(CB11H12)][CB11H12] (CB11H12 = mono-anionic carborane [closo-1-CB11H12]?).

Graphical abstract: Dihalogeno(diphosphane)metal(ii) complexes (metal = Co, Ni, Pd) as pre-catalysts for the vinyl/addition polymerization of norbornene – elucidation of the activation process with B(C6F5)3/AlEt3 or Ag[closo-1-CB11H12] and evidence for the in situ formation of “naked” Pd2+ as a highly active species
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