Kinetics and Mechanism of Base Hydrolysis in Cobalt(III) Complexes – The Case of a Complex CoLCl2+ where L has the Novel Topology of a Square-Pyramidal NN4 Coordination Cap

Abstract

Multi‐wavelength stopped‐flow spectrophotometry was used to study the kinetics of base hydrolysis of the octahedral cobalt(III) complex CoLCl2+ (2), in which the tetrapodal pentadentate ligand L has an NN4 donor set and forms a square‐pyramidal coordination cap [L = 2,6‐bis(1′,3′‐diamino‐2′‐methylprop‐2′‐yl)pyridine, 1]. The kinetic investigation, carried out at different temperatures, pressures and ionic strengths I, led to second‐order kinetics, rate = kOH [2][OH−], with kOH = 0.139 ± 0.001 M−1s−1 (I = 0.1 M) and kOH = 0.0570 ± 0.0004 M−1s−1 (I = 1.0 M) at 298 K. The temperature and pressure dependence of kOH resulted in ΔH‡ = 119 ± 3 kJmol−1 (I = 0.1 M) and ΔH‡ = 123 ± 3 kJmol−1 (I = 1.0 M), ΔS‡ = + 130 ± 13 Jmol−1K−1 (I = 0.1 M) and ΔS‡ = + 151 ± 15 Jmol−1K−1 (I = 1.0 M) and ΔV‡ = + 27.6 ± 0.6 cm3mol−1 (I = 1.0 M). The kinetic results support the operation of the conjugate base mechanism, Dcb, with intermediate deprotonation occuring cis to the leaving chloride ion. The observed inertness of complex 2 towards base hydrolysis is discussed on the basis of the special structural features of the ligand L. The pKa of the coordinated water in the aqua species CoL(H2O)3+(3) was determined to be pKa = 6.0 ± 0.1 at I = 0.1 M. The X‐ray crystal structure analysis of the hydroxo complex [CoL(OH)](ClO4)2 (4) shows a mononuclear terminal (hydroxo)CoIII complex, in which the ligand provides a regular square‐pyramidal NN4 donor cap for the octahedrally six‐coordinate CoIII ion [4, monoclinic, space group C2/m, a = 16.841(6) Å, b = 8.541(3) Å, c = 15.112(5) Å, β = 109.13(1)°, Z = 4]. Coordination of L is accompanied by the formation of 6 six‐membered chelate rings, all of which adopt a boat conformation. In the crystal lattice, pairs of cations are associated via four (hydroxo)O···H–N(primary amine) hydrogen bonds and related through an inversion center. Full spectroscopic data for 4 (1H‐, 13C‐NMR, IR, MS) are presented.

A Structurally Characterised Pair of Dicobalt(III) Peroxo/Superoxo Complexes withC2-Symmetrical Tetrapodal Pentadentate Amine Ligands, and Some Reactivity en route

“…aminoethyl)ethane-1,2-diamine] [37] and, incidentally, the mononuclear hydroxo species [(1)Co(OH)](ClO 4 ) 2 . [6] This contrasts with a marked trans influence of the peroxo ligand (4) found in the structures of other CoN 5 complexes, where the CoϪN(trans) bond can be up to 3% longer than the other CoϪN bonds. [37,61] In the solid state, the central CoϪOϪOϪCo moiety of 5 is locked in a transoid conformation by four intramolecular hydrogen bonds indicated as dashed lines in Figure 1, defining the ''triangular'' arrangement N12ϪH12B⋅⋅⋅O1A⋅⋅⋅ H13AϪN13 and its symmetry-related counterpart [averaged distances and angles: d(N⋅⋅⋅O) ϭ 2.76 Å ; (NϪH⋅⋅⋅O) ϭ 126°].…”

“…[48] This would explain the moderate yields of 2, 4 and 6 obtained from these reactions (only about 50% of 1 is accounted for in the products); (ii) The chloride ion and water are unlikely reductants for Co III , on the grounds that the redox potentials of the half-reactions 2 Cl Ϫ Ǟ Cl 2 ϩ 2 e Ϫ and 2 H 2 O Ǟ O 2 ϩ 4 H ϩ ϩ 4 e Ϫ are too high for Co III to accomplish either oxidation and be reduced in the presence of amine ligand. While experimental evidence indicates that Na 3 6 ] 2ϩ ), it is unlikely that in the presence of the chelating amine ligand, cobalt(II) can originate from this reaction. We have no experimental data to support the notion that in our system of peroxo and superoxo complexes, the O 2 bridge derives from dioxygen generated internally, by a Co III /Co II redox reaction as indicated above.…”

“…Structural details of the square-pyramidally coordinated {(pyN 4 )Co} fragment have been described elsewhere, [1,6,7] and the corresponding bond lengths and angles in compounds 5, 7, 12 and 13 are given in Table 1 for reference. Parameters that quantify distortions (if any) of the ligand cap are listed in Table 2 [37] The dihedral angle Co1ϪO1ϪO1AϪ Co1A is 177.5(2)°.…”

“…Density functional theory-based calculations are currently being performed [67] in order to gain insight into the electronic structures of the adopted geometries. [43,65] [(1)CoBr]Br(PF 6 …”

“…Mononuclear cobalt(III) complexes of the parent ligand 1 that have been isolated so far have a chloro, [1] an OH, [6] or a CH 3 [7] ligand at the sixth coordination site. The kinetic inertness and the known reactivity of Co III amine complexes with respect to Mannich-type transformations of the coordinated ligands [8,9] led us to further explore this class of complexes.…”