Nitroxyl as a ligand in ruthenium tetraammine systems: a density functional theory study

Abstract

The properties of the free nitroxyl molecule and the nitroxyl ligand in Ru(II) tetraammines (trans-[Ru(NH₃)₄(nitroxyl)ⁿ(L)]²⁺ⁿ (n = nitroxyl charge; L = NH₃, py, P(OEt)₃, H₂O, Cl⁻ and Br⁻)) were studied using density functional theory. According to the calculated conformational energies, HNO complexes are more stable than their deprotonated analogues, and the singlet configuration (trans-¹[Ru(NH₃)₄(L)HNO]²⁺) is lower in energy than the corresponding triplet (trans-³[Ru(NH₃)₄(L)HNO]²⁺). An evaluation of the σ and π components of the L–Ru–HNO bond suggest that the increased stability of these orbitals and the enhanced contributions from the HNO orbitals correlate to shorter Ru–N(H)O distances and higher ν_Ru–HNO stretching frequencies. The stability of the Ru–HNO bond was also evaluated through a theoretical kinetic study of HNO dissociation from trans-¹[Ru(NH₃)₄(L)HNO]²⁺. The order of the Ru–HNO bonding stability in trans-¹[Ru(NH₃)₄(L)HNO]²⁺ as a function of L was found to be as follows: H₂O > Cl⁻ ∼ Br⁻ > NH₃ > py > P(OEt)₃. This order parallels the order of the trans-effect and trans-influence series experimentally measured for L in octahedral complexes. The same trend was also observed using an explicit solvent model, considering the presence of both HNO and H₂O molecules in the transition state. For this series, the calculated bond dissociation enthalpies for the Ru–HNO bonds are in the range 23.8 to 45.7 kcal mol⁻¹. A good agreement was observed between the calculated ΔG^‡ values for the displacement of HNO by H₂O in trans-¹[Ru(NH₃)₄(P(OEt)₃HNO]²⁺ (23.4 kcal mol⁻¹) and the available experimental data for the substitution reactions of trans[Ru(NH₃)₄(POEt)₃(L_x)]²⁺ (19.4 to 24.0 kcal mol⁻¹ for L_x = isn and P(OET)₃, respectively).