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Issue 34, 2018
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Substituents drive ligand rearrangements, giving dinuclear rather than mononuclear complexes, and tune CoII/III redox potential

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Abstract

Three new tetradentate imine ligands, HLHBr, HLClH and HLBrH (HLR1R2) 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 [CoII(HLHBr)(MeOH)2](BF4)2 (1), and two unexpected dinuclear complexes, [CoII2(LBrH-BF2OMe)]2(BF4)2 (2) and [CoII2(LClH-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: Em = 0.57, 0.38 and 0.29 V vs. 0.01 AgNO3/Ag, respectively. The observed Em value is tuned by the ligand, with the 6-chloro-substituent leading to the lowest Em 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.

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

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Publication details

The article was received on 17 Apr 2018, accepted on 16 May 2018 and first published on 16 May 2018


Article type: Paper
DOI: 10.1039/C8DT01502C
Citation: Dalton Trans., 2018,47, 11749-11759
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    Substituents drive ligand rearrangements, giving dinuclear rather than mononuclear complexes, and tune CoII/III redox potential

    F. N. H. Karabulut, H. L. C. Feltham and S. Brooker, Dalton Trans., 2018, 47, 11749
    DOI: 10.1039/C8DT01502C

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