Co(III) complexes of the type [(L)Co(O 2CO)] + (L = tripodal tetraamine ligand): Synthesis, structure, DFT calculations and 59Co NMR

Abstract

The syntheses and characterisation of the Co(III) complexes [(L)Co(O 2CO)]ClO 4 (L = a tripodal tetraamine ligand = baep, abap, uns-penp, dppa, trpn) are reported. Geometric isomers are possible for all but the trpn complex, owing to the non-equivalence of the three arms on the tripodal ligand, and both NMR and X-ray crystallography are used to identify the single isomer formed. X-ray crystal structures of the complexes [(L)Co(O 2CO)]ClO 4 · xH 2O (L = baep, x = 0.5; L = abap, x = 0; L = uns-penp, x = 1; L = dppa, x = 0; L = trpn, x = 1) are reported; little variation is observed in the geometry of the carbonate chelate ring while significant lengthening of bonds and expansion of angles involving the cobalt ion occurs as the number of six-membered chelate rings in the complex cations increases. 59Co NMR chemical shift data for the complexes show the expected linear relationship between λ max, the wavelength of the lowest energy d– d transition, and γ, the magnetogyric ratio of the 59Co nucleus. An excellent correlation between Δ, the d orbital splitting parameter, and δ( 59Co) also exists for these complexes. Rate data for the acid hydrolysis of [(L)Co(O 2CO)] + (L = uns-penp, dppa) in 1.0 M HClO 4 differ by two orders of magnitude, and this is attributed to the differing steric accessibility of the endo O atoms in each complex. DFT calculations on the complexes reproduce the isomeric preferences, UV–Vis and 59Co NMR spectroscopic data well, provided that solvent effects are included.