Activation Volumes for Acid Aquation of Iron(II) Complexes of Alkyl-Substituted Bipyridine and Methanol Solvolysis of Tris(phenanthroline)iron(II)

Abstract

The aquation of Fe(R₂C₁₀H₆N₂)₃²⁺ (R₂C₁₀H₆N₂ = disubstituted 2,2′-bipyridine) was studied in aqueous acid at a range of pressures between 1 and 1380 bar. Activation volumes (Δ V^‡) in 1.0 and 0.01 M HCl respectively were + 12.5(±0.7) and + 15.5(±0.5) cm³ mol^-1 for R₂ = 4,4′-dimethyl, + 12.7(±0.6) and + 16.9(±0.6) cm³ mol^-1 for R₂ = 4,4′-diethyl, and + 13.7(±0.5) and + 17.4(±0.8) cm³ mol^-1 for R₂ = 5,5′-dimethyl. The methanol solvolysis of Fe( phen )₃²⁺ ( phen = 1,10-phenanthroline) in the presence of 0.1 M HOSO₂CF₃ yielded Δ H^‡ 120(±2) kJ mol^-1, ΔS^‡ +72(±5) J K^-1 mol^-1 and Δ V^‡ + 14.1(±0.2) cm³ mol^-1. The Δ V^‡ values differ little for a range of unsubstituted and substituted bipyridine and phenanthroline complexes of iron(II), nor does the value for Fe( phen )₃²⁺ vary with solvent. These observations are further evidence for a mechanism involving a significant positive contribution to Δ V^‡ from a low-spin ↔ high-spin preequilibrium operating.