RESUMO
Density functional calculations have been carried out on the experimentally characterized Co(III) [Co(N4)(O2CO)]+ carbonate complexes containing a tripodal tetraamine ligand (N4 = tpa, Metpa, Me2tpa, Me3tpa, pmea, pmap, tepa) and also the model [Co(NH3)4(O2CO)]+ system. Calculations on the model species, performed using both gas-phase and solvent-corrected procedures, have revealed that the inclusion of a condensed-phase environment is necessary to obtain generally satisfactory results for the structural and bonding properties in these systems. Using the solvent-corrected approach, the observed trends in structural parameters for the metal-ligand bonds, 59Co chemical shifts, and changes in visible absorption wavelengths have been satisfactorily reproduced for the [Co(N4)(O2CO)]+ complexes. A time-dependent density functional analysis of the electronic excitations indicates that the overall composition and character of the relevant (d-d) transitions remain similar throughout the series, indicating that the changes in the Co-N interactions, associated with the structural variations occurring as the N-donor ligand identity and size change, appear most likely responsible for the particular spectroscopic features displayed by these species. These observations are further supported by molecular orbital and energy decomposition analyses. The results from the present calculations confirm recent findings that the inclusion of a treatment for solvent effects plays a critical role in the computational modelling of coordination complexes involving mixed (anionic and neutral) ligands.
RESUMO
The synthesis, characterization, X-ray crystal structures, and reactivity in aqueous acidic solution of the Co(III) carbonate complexes [Co(tpa)(O2CO)]ClO4.H2O, [Co(Me-tpa)(O2CO)]ClO4.0.5H2O, [Co(Me2-tpa)(O2CO)]ClO4.0.5H2O, and [Co(Me3-tpa)(O2CO)]ClO4 are reported (tpa = tris(2-pyridylmethyl)amine; Me-tpa, Me2-tpa, and Me3-tpa are derivatives of tpa containing one, two, and three 6-methylpyridyl rings, respectively). The complexes display very different spectroscopic and 59Co NMR properties, consistent with the decreasing ligand field strength of the tripodal amine ligands in the order tpa > Me-tpa > Me(2)-tpa > Me3-tpa. X-ray structural data show an increase in the average Co-N bond distances as the number of methyl groups on the tripodal amine ligand increases, and this is the result of steric interactions between the methyl groups and the carbonate ligand and between the methyl groups themselves. Rate data for the acid hydrolysis of [Co(tpa)(O2CO)]+ (I = 1.0 M (NaClO4), 25.0 degrees C) over the [HClO4] range of 0.10-1.0 M are consistent with a previously proposed mechanism involving protonation of the carbonate ligand prior to ring-opening, but the equilibrium constant for protonation is smaller in this case than those obtained previously, as is the equilibrium constant for proton transfer from the exo to the endo O atoms. Comparative rate data ([HCl] = 6.0 M, 25.0 degrees C) for the four complexes show that those containing methylated ligands undergo acid hydrolysis between 25 and 90 times more slowly than [Co(tpa)(O2CO)]+ under the same conditions, and it is proposed that this rate difference is a result of steric factors. Inspection of space-filling diagrams shows that one of the endo oxygen atoms is significantly sterically hindered by the methyl groups of the tripodal amine ligands, thus inhibiting protonation at this site and leading to slower observed rates of hydrolysis. The results obtained in this study are consistent with the endo oxygen atoms being the mechanistically important site of protonation in the acid hydrolysis of metal complexes containing chelated carbonate.
