Nitrogen-14 NMR Study on Solvent Exchange of the Octahedral Cobalt(II) Ion in Neat 1,3-Propanediamine and n-Propylamine at Various Temperatures and Pressures. Tetrahedral−Octahedral Equilibrium of the Solvated Cobalt(II) Ion in n-Propylamine As Studied by EXAFS and Electronic Absorption Spectroscopy

Abstract

Solvated cobalt(II) ions in neat 1,3-propanediamine (tn) and n-propylamine (pa) have been characterized by electronic absorption spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. The equilibrium between tetrahedral and octahedral geometry for cobalt(II) ion has been observed in a neat pa solution, but not in neat diamine solutions such as tn and ethylenediamine (en). The thermodynamic parameters and equilibrium constant at 298 K for the geometrical equilibrium in pa were determined to be DeltaH degrees = -36.1 +/- 2.3 kJ mol(-1), DeltaS degrees = -163 +/- 8 J mol(-1) K(-1), and K(298) = 6.0 x 10(-3) M(-2), where K = [Co(pa)(6)(2+)]/{[Co(pa)(4)(2+)][pa](2)}. The equilibrium is caused by the large entropy gain in formation of the tetrahedral cobalt(II) species. The solvent exchange of cobalt(II) ion with octahedral geometry in tn and pa solutions has been studied by the (14)N NMR line-broadening method. The activation parameters and rate constants at 298 K for the solvent exchange reactions are as follows: DeltaH() = 49.3 +/- 0.9 kJ mol(-1), DeltaS() = 25 +/- 3 J mol(-1) K(-1), DeltaV() = 6.6 +/- 0.3 cm(3) mol(-1) at 302.1 K, and k(298) = 2.9 x 10(5) s(-1) for the tn exchange, and DeltaH() = 36.2 +/- 1.2 kJ mol(-1), DeltaS() = 35 +/- 6 J mol(-1) K(-1), and k(298) = 2.0 x 10(8) s(-1) for the pa exchange. By comparison of the activation parameters with those for the en exchange of cobalt(II) ion, it has been confirmed that the kinetic chelate strain effect is attributed to the large activation enthalpy for the bidentate chelate opening and that the enthalpic effect is smaller in the case of the six-membered tn chelate compared with the five-membered en chelate.

Solvation Structures of Manganese(II), Iron(II), Cobalt(II), Nickel(II), Copper(II), Zinc(II), and Gallium(III) Ions in Methanol, Ethanol, Dimethyl Sulfoxide, and Trimethyl Phosphate As Studied by EXAFS and Electronic Spectroscopies

“…Because the distinction is difficult using EXAFS measurement at a single temperature, we carried out variable-temperature EXAFS measurements between 281 and 311 K in order to experimentally clarify the solvation structure. In the case of the Co(II) ion, with an ionic radius similar to that of the Zn(II) ion, the solvation equilibrium between tetrahedral and octahedral is familiar due to the d 7 electronic configuration, and the enthalpy change, ∆H°, from the octahedral species (CoS 6 2+ , S ) solvent) to the tetrahedral species (CoS 4 2+ ) is reported to be 36.1 kJ mol -1 in PA 7 and ca. 50 kJ mol -1 in DMA.…”

“…Interestingly, in the strong σ-donating solvent PA, the solvation equilibrium between octahedral and tetrahedral struc-tures is observed for the Co(II) ion with d 7 electronic configuration, in which the steric repulsion of bound PA molecules is not expected. 7 As also seen for the Co(II) (4-coordinate pseudotetrahedral) and the Ni(II) (5-coordinate square pyramidal) ions in TMU, 3 the d electron configration of the metal(II) ion sometimes plays a key role in the selection of the solvation structure. Furthermore, as we have recently pointed out, 11,12 the M-N bond length of the solvated metal ion in nitrogen-donating solvents becomes longer according to the following order: nitriles with sp hybridizing nitrogen < pyridines with sp 2 hybridizing nitrogen < aliphatic amines with sp 3 hybridizing nitrogen.…”

Structure of the Short-Lived Intermediate Formed during the Metal Substitution Reaction of the Mercury(II) Porphyrin Complex with Cobalt(II) Ion in Aqueous Solution Determined by the Stopped-Flow EXAFS Method

“…The Con-N and Con-0 bond lengths are 212(2) and 221(2) pm, respectively. The Cou-N bond length is slightly shorter than that within the octahedrally sol vated cobalt(II) ion in nitrogen-donor solvents, e. g., 214 pm in 3-methylpyridine, 216 pm in 4-methylpyridine [22] and 217 pm in 1,3-propanediamine and in n-propylamine [23].…”

On the electronic structure and spectroscopic properties of a pseudo-tetrahedral cationic cobalt(ii) tetraamine complex ? ([35]adamanzane)cobalt(ii)Electronic supplementary information (ESI) available: Table S1: Cartesian coordinates of the DFT optimized geometry of the [Co([35]adz)]2+ ion. Table S2: Results of experimental and calculated infrared absorption spectra for the [Co([35]adz)]2+ ion. See http://www.rsc.org/suppdata/dt/b3/b305712g/

“…5 Tetrahedral complexes of cobalt() with amine ligands are quite rare. The few examples found in the literature are tetraammincobalt() perrhenate, 6 the tetrahedral cobalt() species found in neat solution of some organic amines 7 and cobalt() complexes of the 16-membered ligands 1,5,9,13tetraazacyclohexadecane ( [16]aneN 4 ) 8 and 2,4,4,10,12,12-hexamethyl-1,5,9,13-tetraazacyclohexadecane (Me 6 [16]aneN 4 ). 9 The latter compound and the [Co( [3 5 ]adz)] 2ϩ ion are unique examples of tetrahedral Co() amine complexes, which resist oxidation to cobalt() in aqueous solution.…”