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

Three new tetradentate imine ligands, HL^HBr, HL^ClH and HL^BrH (HL^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 N₄O₂ coordinated cobalt(II) complexes: the anticipated mononuclear complex [Co^II(HL^HBr)(MeOH)₂](BF₄)₂ (1), and two unexpected dinuclear complexes, [Co^II₂(L^BrH-BF2OMe)]₂(BF₄)₂ (2) and [Co^II₂(L^ClH-BF2OMe)]₂(BF₄)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 BF₄ anion, resulting in a pair of borate ester bridges between the two cobalt(II) centres. A similar type of rearrangement is proposed for the PF₆ 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 AgNO₃/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.