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

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 degrees C, the RRR isomer reacts with [Fe(dmf)(6)](2+) to give [FeL](2+) with a formation rate constant, 10(-5)k(f)=2.34+/-0.09 dm(3) mol(-1) s(-1), and the RRS isomer reacts to give an intermediate bis(bipy) complex with 10(-5)k(f)=1.04+/-0.01 dm(3) 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) x 10(5) dm(3) mol(-1). For the reaction of [Fe(dmf)(6)](2+) with bipy at 25 degrees C, rapid formation of [Fe(dmf)(4)(bipy)](2+) (10(-5)k(f)=1.4+/-0.1 dm(3) mol(-1) s(-1)) is followed by the much slower formation of [Fe(dmf)(2)(bipy)(2)](2+) (k(f)=181 +/- 7 dm(3) mol(-1) s(-1)) and [Fe(bipy)(3)](2+) (k(f)=24.3 +/- 0.9 dm(3) 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 degrees 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) x 10(-3) s(-1) and (1.4 +/- 0.1) x 10(-3) s(-1) respectively, showing that both are inert, low-spin, t(2g)(6) 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(-3)K(1)/dm(3) mol(-1) = 9.6+/-0.7 < 10(-5)K(2)/dm(3) mol(-1) = 1.3 +/- 0.7 > 10(-5)K(3)/dm(3) mol(-1) = 3.5 +/- 0.2 (in water K-1 > K-2 much less than K-3). Overall formation constants for [Fe(bipy)(3)](2+) in dmf (log(10) beta(3) = 12.6) and in dimethyl sulfoxide (log(10) beta(3) = 10.7) are significantly smaller than the value in water (log(10) beta(3) = 17.4). Rates of solvolysis of [M(bipy)(3)](2+) (M = Fe or Ni) were also investigated in dimethyl sulfoxide solution.