Nitrogen-14 magnetic resonance study of the exchange of acetonitrile on copper(II) complexes

Abstract

The kinetic parameters, determined by ¹⁴N n.m.r., for the exchange of acetonitrile are: Cu(MeCN)₆²⁺, k_ex(228 K) 1·6 × 10⁷ s^–1; Cu(tren)(MeCN)²⁺, k_ex(228 K)=(5·1 ± 0·7)× 10³ s^–1, ΔH^‡= 45 ± 4 kJ mol^–1, and ΔS^‡= 26 ± 16 J mol^–1 K^–1; Cu(trenol)(MeCN)₂²⁺, k_ex(228 K)=(8·0 ± 2·0)× 10⁴ s^–1, ΔH^‡= 26 ± 8 kJ mol^–1 and ΔS^‡=–34 ± 32 J mol^–1K^–1; and Cu(Me₆tren)(MeCN)²⁺, k_ex(228 K) < 10² s^–1, where in all cases k_ex refers to the exchange at a single site. The kinetic data are discussed in terms of a model whereby the rapidity of exchange on Cu(MeCN)₆²⁺ is attributed at least partially to a dynamic Jahn–Teller effect, and the reduction in lability upon co-ordination of polydentate ligands is due to elimination of this exchange pathway.