(IP) _2Ga ~(III) complexes facilitate net two-electron redox transformations (IP = α-Iminopyridine)

摘要

Reaction of M ~+[(IP ~(2-)) _2Ga] ~- (IP = iminopyridine, M = Bu 4N, 1a; (DME) 3Na, 1b) with pyridine N-oxide affords two-electron-oxidized (IP ~-) _2Ga(OH) (2) in reactions where the product outcome is independent of the cation identity, M ~+. In a second example of net two-electron chemistry, outer sphere oxidation of M ~+[(IP ~(2-)) _2Ga] ~- using either 1 or 2 equiv of the one-electron oxidant ferrocenium afforded [(IP ~-) _2Ga] ~+ (3) in either 44 or 87% yield, respectively. Reaction with 1 equiv of TEMPO, a one-electron oxidant, afforded the two-electron-oxidized product (IP ~-) _2Ga(TEMPO) (4). Reduction of 2IP by 3Na and subsequent reaction with GaCl _3 yielded a 1:1 mixture of (IP ~-) _2GaCl and 1. Most remarkably, all of these reactions are overall two-electron processes and only the (IP ~-) _2GaX and [(IP ~(2-)) _2Ga] ~- oxidation states are thermodynamically accessible to us. Analogous aluminum chemistry previously afforded either one-electron or two-electron reactions and mixed-valent states. The thermodynamic accessibility of the mixed-valent states of (IP ~(2-))(IP ~-)E, where E = Al or Ga, can be compared using cyclic voltammetry measurements. These measurements indicated that the mixed-valent state [(IP ~(2-))(IP ~-)Ga] ~+ is not significantly stabilized with respect to disproportionation on the time scale of the electrochemistry experiment. The electrochemically observed differences in thermodynamic stability of the mixed-valent state [(IP ~(2-))(IP ~-)E] ~+ can be rationalized by the observation that the dihedral angle between the ligand planes containing the π-system of IP is roughly 5° larger in all gallium complexes compared with aluminum analogs. Presumably, a larger dihedral angle provides weaker electronic coupling between the π-systems of IP via the E-X σ* orbital. Alternatively, the observed difference may be a result of the "inert pair effect": a contracted Ga component in the E-X σ* orbital would also afford weaker electronic coupling.