The active-site centers of metalloenzymes containing polynuclear transition metal-ions (Fe, Mn, Ni, Cu) are known to activate small molecules (O2, H2O, N2, N2O, etc), stabilize high-valent metal-oxygen intermediates and cause oxidation catalysis. This has intrigued bioinorganic chemists to mimic and design industrially useful catalysts.1 The diiron and tetramanganese active-sites, bridged by carboxylate and supported by histidine ligands, have caught recent attention.2 Synthetic modelling studies using a combination of tripodal pyridylalkylamine and carboxylate or carboxylate-appended N-ligands, have served as excellent platforms to reproduce some of the active-site features of Fe and Mn centers. The analogous studies with cobalt or other metals are scarce and often produced 1D-polymers.3 Besides, a systematic study of carboxylate-appended N-ligands with variable alkyl chain length of carboxylate arm and the influence of anions of cobalt(II)-salts, on the structure, spectroscopy and O2 reactivity is sporadic.4
Here, we have synthesized a series of carboxylate-appended bispicolylamine ligands having methylene (L1), ethylene (L2) and propylene (L3) alkyl linkers between the carboxylate and tertiary amine-nitrogen and studied the influence of alkyl chain length and anions (Cl, N3, ClO4, BPh4, BArF24) of different cobalt(II)-salts on the structural diversity, spectroscopy and O2/H2O2 reactivity of Co(II) complexes. The alkyl chain length and anions controlled the metal topology and nuclearity from mono-, di- tri- and tetranuclear structures, as established by X-ray structure and a combination of spectroscopic techniques. O2 reactivity provided metal- and ligand-based oxidation products, different from a related study.5 In this presentation, results of these investigations, highlighting the impact of alkyl tether length, anions and solvent, on the structure, nuclearity (Con, n = 1-4), spectroscopy (FT-IR, UV-vis, 1H-NMR, ESI-MS) and magnetism of Co(II) complexes, and reactivity studies, will be presented.
1. Jasniewski, A. J.; Que, L., Jr., Chem. Rev. 2018, 2554.
2. Christou, G., Acc. Chem. Res. 1989, 328.
3. Lucas, N. T. et al., CrystEngComm, 2015, 2974.
4. McKenzie et al., Dalton Trans., 2011, 10698.
5. Anjana,S.S.; Varghese, B.; Murthy, N.N., Dalton Trans., 2020, 3187.