Quantitative estimates of steric effects: Intramolecular strain energy effects on the activation energy for aquation of some trans-dichlorotetraaminecobalt(III) complexes |
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| Institution: | 1. Department of Chemistry, M. V. Lomonosov Moscow State University, 1 Leninskie Gory, 119992 Moscow, Russian Federation;2. A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 ul. Vavilova, 119991 Moscow, Russian Federation;3. N.S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninsky Prosp. 31, 119991 Moscow, Russian Federation;4. Inorganic Chemistry Department, Peoples'' Friendship University of Russia, 6 Miklukho-Maklaya St., 117198 Moscow, Russian Federation;1. School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China;2. Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa;1. Unit of Biophysical Chemistry, Department of Physical Chemistry, Faculty of Chemistry, University of Lodz, Pomorska 165, Lodz 90-236, Poland;2. Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89B, 61-614 Poznan, Poland;1. College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018, PR China;2. Department of Physics, Beihang University, Beijing, 100191, PR China;1. Sylvana Research Associates, San Antonio, Texas;2. Isis Clinical Research, Austin, Texas;3. Kerville Research Associates, Kerrville, Texas;4. Biogenics Research Institute, San Antonio, Texas;5. Sirius Clinical Research, Austin, Texas |
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| Abstract: | In an effort to quantitatively estimate steric contributions to the aquation rates of a series of structurally related cobalt(III) tetraamine complexes, strain energy minimization calculations have been performed on the reactant and some plausible transition state structures. Free energies of activation ΔG*obs, are factored as: ΔG*obs, = ΔG*bb + ΔG*strain + ΔG*CF + ΔG*solvation + … where ΔG*bb is the free energy change associated with bond breaking, ΔG*solvation is the solvation free energy difference between the reactant and a proposed transition stare, ΔG*CF is the difference in crystal field stabilization between the reactant and a proposed transition state, and ΔG*strain is the strain energy difference between the reactant complex and a proposed transition state. The activation energy for the aquation of a hypothetical ‘strain free’ complex is defined as ΔG*int and reflects the energy required for the bond breaking step with all other terms. For the cations trans-(RR,SS)-dichloro-1,8- diamino-3,6-diazaoctanecobalt(III)(trans Co(2,2,2- tet)Cl2]+), trans-(RR,SS)- or trans-(RS)-dichloro-1.9- diamino-3,7-diazanonanecobalt(III)(trans Co(2,3,2- tet)Cl2]+ and trans-(RS)-dichloro-1,10-diamino-4,7- diazadecanecobalt(III)(transCo(3,2,3-tet)Cl2]+) ΔG*int is found to be a constant 123 kJ/mol. For the trans-dichlorocobalt(III) complexes with the ligands 1,4,7,10-tetraazacyclotridecane(13]-ane-N4), 1,4,8, 11-tetraazacyclotetradecane(14]-ane-N4), 1,4,8,12- tetraazacyclopentadecane(15]-ane-N4) and 1,5,9,13- tetraazacyclohexadecane(16]-ane-N4), ΔG*int lies in the range 133–139 kJ/mol. |
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