Properties and Reactivity of Metallocarboxylates. Characterization of Aquobis(ethylenediamine)(hydroxycarbonyl)cobalt(III) (2+) and Its Ethyl Ester, trans-PF6

Article abstract of DOI:10.1021/ja00199a018

The title metallocarboxylate, first described by Vaudo et al. (J.Am.Chem.Soc. 1972, 94, 6655) has been further investigated, and its ethyl ester, prepared.Crystals of the ester (ethoxycarbonyl) PF6 are monoclinic: space group P2₁/c with a = 9.077(1) Angstroem, b = 16.375(2) Angstroem, c = 12.919(2) Angstroem, β = 92.15(1)deg, V = 1918.9(7) Angstroem³, Z = 4, and refined to a final R value of 0.079.The ethylenediamine ligands are trans to one another (Co-N ave, 1.944(13) Angstroem) with axial ligands CF3CO2(1-)(Co-O,2.046(7) Angstroem) and -C(O)OC2H5(1-)(Co-C, 1.922(9) Angstroem; C=O 1.196(9) Angstroem, ν_C=O 1645 cm^-1).The aquo hydroxycarbonyl complex (UV-vis: λ_max, nm <ε, M^-1 cm ^-1> 410 <125>, 314 sh <580>, 262 <5200>) is a dibasic acid with pK₁ = 2.5 +/- 0.5 and pK₂ = 3.7 +/- 0.5 at 15 deg C and 0.5 M ionic strength and, at pH > 5, is present as the neutral hydroxy-oxycarbonyl complex Co(en)2(OH)(CO2) (UV-vis: 450 <100>, 357 <245>, 318 <280>, 266 <9000>).The aquo ethoxycarbonyl complex (UV-vis: 424 <113>, 320 sh <210>, 264 <5100>) is a monobasic acid with pK_a = 8.7 +/- 0.2 at 25 deg C and 0.5 M ionic strength and, at high pH, is converted to Co(en)2(OH)(CO2Et)(1+) (UV-vis: 430 <159>, 329 sh <260>).The ethoxy function is hydrolyzed very slowly in the latter complexes (pH, k_obsd: 1, 2.5 * 1E-6 s^-1; 7, <2 * 1E-7 s^-1; 13, 3 * 1E-6 s^-1) to yield ethanol and the hydroxy- or oxycarbonyl, depending upon the pH.At pH > 0 the oxy- or hydroxycarbonyl complex undergoes decomposition through the singly protonated Co(en)2(H2O)(CO2)(1+), exhibiting a rate maximum (k_obsd ca. 3 * 1E-3 s^-1, 25 deg C) at ca. pH 3.2.