Comparison of the rates and mechanisms of formation and solvolysis of [Fe(bipy)3]2+ (bipy = 2,2′-bipyridine) and [FeL]2+ [L = 1,4,7-tris(2,2′-bipyridyl-5-ylmethyl)-1,4,7-triazacyclononane] and their stabilities in dimethylformamide solution†

Abstract

Multiwavelength stopped-flow spectrophotometry has been used to compare the rates and mechanisms of formation of [Fe(bipy)₃]²⁺ (bipy = 2,2′-bipyridine) and [FeL]²⁺ [L = 1,4,7-tris(2,2′-bipyridyl-5-ylmethyl)-1,4,7-triazacyclononane, containing three co-ordinating bipy groups], in dimethylformamide (dmf) solution. Molecular modelling of L, and kinetic studies of its reaction with [Fe(dmf)₆]²⁺, are consistent with the presence of two N-configurational isomers (RRR and RRS); at 25 °C, the RRR isomer reacts with [Fe(dmf)₆]²⁺ to give [FeL]²⁺ with a formation rate constant, 10^–5k_f = 2.34 ± 0.09 dm³ mol^–1 s^–1, and the RRS isomer reacts to give an intermediate bis(bipy) complex with 10^–5k_f = 1.04 ± 0.01 dm³ mol^–1 s^–1. This bis(bipy) intermediate rearranges to the tris(bipy) chelate with a first-order rate constant of 0.71 ± 0.02 s^–1, but is also involved in the formation of a small amount of an [Fe₂L₂]⁴⁺ dimer. The kinetically determined dimer formation constant is (1.2 ± 0.1) × 10⁵ dm³ mol^–1. For the reaction of [Fe(dmf)₆]²⁺ with bipy at 25 °C, rapid formation of [Fe(dmf)₄(bipy)]²⁺ (10^–5k_f = 1.4 ± 0.1 dm³ mol^–1 s^–1) is followed by the much slower formation of [Fe(dmf)₂(bipy)₂]²⁺ (k_f = 181 ± 7 dm³ mol^–1 s^–1) and [Fe(bipy)₃]²⁺ (k_f = 24.3 ± 0.9 dm³ mol^–1 s^–1). The complex [FeL]²⁺ is significantly more stable than [Fe(bipy)₃]²⁺, as shown by the ready solvolysis of [Fe(bipy)₃]²⁺ at low dilution in dmf, whilst [FeL]²⁺ is stable to solvolysis under the same conditions. At 25 °C first-order rate constants for the dissociation of bipy from [Fe(bipy)₃]²⁺ and [Fe(dmf)₂(bipy)₂]²⁺ are (7.0 ± 0.3) × 10^–3 s^–1 and (1.4 ± 0.1) × 10^–3 s^–1 respectively, showing that both are inert, low-spin, t_2g⁶ systems. This contrasts with the behaviour in water, since [Fe(OH₂)₂(bipy)₂]²⁺ is known to be high spin and labile. The kinetically determined stepwise formation constants in dmf confirm this conclusion; values vary in the order 10^–3K₁/dm³ mol^–1 = 9.6 ± 0.7 < 10^–5K₂/dm³ mol^–1 = 1.3 ± 0.7 > 10^–3K₃/dm³ mol^–1 = 3.5 ± 0.2 (in water K₁ > K₂  K₃). Overall formation constants for [Fe(bipy)₃]²⁺ in dmf (log₁₀ β₃ = 12.6) and in dimethyl sulfoxide (log₁₀ β₃ = 10.7) are significantly smaller than the value in water (log₁₀ β₃ = 17.4). Rates of solvolysis of [M(bipy)₃]²⁺ (M = Fe or Ni) were also investigated in dimethyl sulfoxide solution.