Substituents drive ligand rearrangements, giving dinuclear rather than mononuclear complexes, and tune CoII/III redox potential

摘要

Three new tetradentate imine ligands, H L ~(HBr) , H L ~(ClH) and H L ~(BrH) (H L ~(R1R2) ) were synthesised by 2?:?1 condensation of the appropriately n -halo substituted pyridine-2-carboxaldehyde (5-bromo- 4a , 6-bromo- 4b or 6-chloro- 4c ) with 1,3-diaminopropan-2-ol ( 5 ). Reactions of each of these three ligands with one equivalent of cobalt( II ) tetrafluoroborate resulted in the formation of three N _(4) O _(2) coordinated cobalt( II ) complexes: the anticipated mononuclear complex [Co ~(II) (H L ~(HBr) )(MeOH) _(2) ](BF _(4) ) _(2) ( 1 ), and two unexpected dinuclear complexes, [Co ~(II) _(2) ( L ~(BrH-BF2OMe) )] _(2) (BF _(4) ) _(2) ( 2 ) and [Co ~(II) _(2) ( L ~(ClH-BF2OMe) )] _(2) (BF _(4) ) _(2) ( 3 ). Dinuclear 2 and 3 result from complexation of cobalt( II ) to the ligands derived from the sterically demanding 6-halo substituted pyridine-2-carboxaldehydes ( 4b and 4c ) undergoing rearrangement, reacting with MeOH and a BF _(4) anion, resulting in a pair of borate ester bridges between the two cobalt( II ) centres. A similar type of rearrangement is proposed for the PF _(6) analogues. Cyclic voltammetry in acetonitrile reveals that cobalt( II ) complexes 1–3 undergo a quasi-reversible oxidation: E _(m) = 0.57, 0.38 and 0.29 V vs. 0.01 AgNO _(3) /Ag, respectively. The observed E _(m) value is tuned by the ligand, with the 6-chloro-substituent leading to the lowest E _(m) value being observed for the corresponding cobalt complex, 3 , rather than for either of the complexes of the n -bromo-substituted ligands ( n = 6 or 5), 2 and 1 .