Solvent and metal dependent 1H NMR hyperfine shifts in paramagnetic pentaamminemetal cyanide-bridged mixed-valence complexes

Abstract

¹ H NMR resonances, in several aprotic solvents, are reported for axial and equatorial ammonias coordinated to a single spin paramagnetic centre in the Robin-Day Class II cyanide-bridged mixed-valence cations [(OC)₅Cr(μ-CN)M(NH₃)₅]²⁺ (M = Ru, Os) as well as in the complex [(OC)₅Re(μ-CN)Ru(NH₃)₅]³⁺, whose synthesis and properties are reported herein. Using the appropriate isotropic hexaammine complex as a reference, the chemical shift difference between the ammonia protons, δ_ax − δ_eq, is found to be very sensitive to the paramagnetic metal (M), the remote diamagnetic metal (Cr or Re) and also to the donor properties of the solvent (as well as the counter-ion) as a result of hydrogen bonding interactions. The difference varies linearly with the MMCT energy, and in [(OC)₅Re(μ-CN)Ru(NH₃)₅]³⁺ can be tuned from positive (δ_ax > δ_eq) to negative (δ_ax < δ_eq) through zero (δ_ax = δ_eq) by the choice of solvent. This reflects the sign and magnitude of the axial ligand field parameter which is in turn a result of changes in the π-donor–acceptor interactions between the donor-cyanide bridging group and the pentaammine metal unit.