X-ray crystal structure of [Ni(III)(cyclam)(NO3)2](ClO4) and electron transfer reactions of the Ni(III)/Ni(II) couple

Abstract

The synthesis and crystal structure of [Ni(cyclam)(NO3)2](ClO4) are described (P1, a = 8.347(4) Å, b = 15.960(7) Å, c = 7.656(3) Å, α = 90.33(6)°, β = 110.97(4)°, γ = 104.45(5)°, V = 917.0(7) Å3, R = 0.0425, Rw = 0.0384). The crystal contains two unique Ni(III) cations within the unit cell, with a network of hydrogen bonding interactions linking one of the cations into a stacked configuration while the other cation contains a strong internal hydrogen bond. This provides an interesting inorganic example of multi-site cooperative binding involving a metal complex cation. By use of electron transfer data from known outer-sphere redox reagents, measurement of the cross-reactions with [Ni(cyclam)]3+/2+ in 3 M nitrate media yielded a self-exchange rate constant of 1.4 × 105 M−1 s−1. The structural data have been utilized to obtain a calculated estimate of the self-exchange rate constant (1.0 × 108 M−1 s−1) for this couple. The relationship between these two values is explored. The lack of adiabaticity is reflected in a value ≈0.01 for κeI not only for this couple but also for other related cyclam complexes.