Rate constants of the positronium oxidation reaction promoted by several caged and open CoIII complexes

Abstract

The rate constants, k_ox, of the oxidation reactions of positronium atoms, Ps, by the following caged (or encapsulated) and open Co^III complexes have been measured or remeasured with the aim of revealing a possible correlation between k_ox and the chemical constitution of the complexes: 1[Co(NH₃)₆]Cl₃, 2[Co(NH₃)₅H₂O]Cl₃, 3[Co(NH₃)₅F](NO₃)₂, 4[Co(NH₃)₅NO₂](NO₃)₂, 5[Co(en)₃]Cl₃, 6[Co(pn)₃]Cl₃, 7[Co(bipy)₃](ClO₄)₃, 8[Co(L₁)]Cl₃, 9[Co(L₂)]Cl₃, 10[Co(sep)]Cl₃, 11[Co(L₃)]Cl_3^˙{en = ethylenediamine; pn = propane-1,2-diamine; bipy = 2,2′-bipyridyl; L₁(and L₂)= dinitro (or diamine) 3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane; sep = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane; L₃= 1,1,1-tris(2′-aminoethyl-aminomethyl)ethane.}

The k_ox of the complexes are discussed together with those of the following complexes given in the literature: 12[Co(NH₃)₅OH]Cl₂, 13[Co(NH₃)₅CH₃COO]Cl₂, 14[Co(NH₃)₅Cl]Cl₂, 15[Co(NH₃)₅Br]Br₂ and 16 K[Co(edta)].

Depending on the ligand type, reduction of the open complexes may occur: (i) by electron hopping from Ps to the ligand and then to the empty Co^III orbitals (2, 4, 13, 16; k²⁹⁸_ox= 5–10 dm³ mol^–1 ns^–1); (ii) by electron tunnelling from the Ps atom to empty metal orbitals (1, 3, 5, 6, 11, 12; k²⁹⁸_ox= 0.7–2.8 dm³ mol^–1 ns^–1); (iii) by direct transfer of the Ps electron to the delocalized empty orbitals of Co^III(7, 14, 15; k²⁹⁸_ox= 15–30 dm³ mol^–1 ns^–1). It is hypothesized that the reduction of caged complexes (8, 9, 10; k²⁹⁸_ox= 6–15 dm³ mol^–1ns^–1) occurs by the third mechanism through the formation of a heptacoordinate adduct of the Co^III ion.

Steric hindrance by ligands also plays a role, k_ox of 1, 5, 6 and 11, belonging to the second group, are 2.8, 0.75, 1.12, 0.94 dm³ mol^–1 ns^–1, respectively, i.e. decreasing with increasing ligand size.

k _ox of the Ps reactions with caged and open Co^III complexes show the same trend as the rate constants, k_ex, for the self-exchange reactions between the Co^II and Co^III complexes, even through the k_ox values are several orders of magnitude larger than k_ex.