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, 10–5kf = 2.34 ± 0.09 dm3 mol–1 s–1, and the RRS isomer reacts to give an intermediate bis(bipy) complex with 10–5kf = 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+ (10–5kf = 1.4 ± 0.1 dm3 mol–1 s–1) is followed by the much slower formation of [Fe(dmf)2(bipy)2]2+ (kf = 181 ± 7 dm3 mol–1 s–1) and [Fe(bipy)3]2+ (kf = 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, t2g6 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 10–3K3/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.