Pyrazole ligation to cobalt(III) centers:: Syntheses, structures, and properties of cobalt(III) complexes of N,N′-bis[2-(1-pyrazolyl)ethyl]pyridine-2,6-dicarboxamide

Three cobalt(III) complexes of the ligand PyPz(2)PH(2) (1, PyPz(2)PH(2) = N,N'-bis[2-(1-pyrazolyl)ethyl]pyridine-2,6-dicarboxamide; Hs are the dissociable amide Hs), a ligand containing two peptide groups, one pyridine and two pyrazole rings, have been synthesized. They are [Co(PyPz(2)P)(H2O)]X (X = NO3-, 2a; ((H3O)(PF6)(2))(-), 2b), [Co(PyPz(2)P)(OH)] (3), and X[Co(PyPz(2)P)(2)] (X = Na+, 4a; 0.5[Co(H2O)(6)](2+), 4b). Complex 2b crystallizes in the triclinic space group P (1) over bar with a = 11.842(2) Angstrom, b = 11.920(3) Angstrom, c = 12.755(2) Angstrom, alpha = 111.21(2)degrees, beta = 103.866(13)degrees, gamma = 106.992(13)degrees, V = 1479.9(5) Angstrom(3), and Z = 2. The structure of complex 3 has been refined to R = 6.06% on the basis of 5417 I > 2 sigma(I) data, Complex 4b crystallizes in the triclinic space group P (1) over bar with a = 12.471(3) Angstrom, b = 14.070(3) Angstrom, c = 14.171(5) Angstrom, alpha = 71.58(2)degrees, beta = 68.08(2)degrees, gamma = 71.74(2)degrees, V = 2134.0(10) Angstrom(3), and Z = 2. The structure of complex 4b has been refined to R = 5.86% on the basis of 6679 I > 2 sigma(I) reflections. In complexes 2a,b and 3, the doubly deprotonated PyPz(2)P(2-) ligand binds one cobalt(III) center in a pentadentate fashion with five nitrogens of the two deprotonated amido nitrogens, two 2-pyrazole nitrogens, and one pyridine nitrogen. The octahedral coordination is completed by either a water molecule (complexes 2a and 2b) or a hydroxo group (complex 3) at the sixth site. Complex 2b is the first example of a structurally characterized cobalt(III) complex in which cobalt is bonded to 2-pyrazole nitrogens. In complexes 4a and 4b, two PyPz(2)P(2-) ligands are bonded to one cobalt(III) center to form a pseudoctahedral bis complex with two pyridine nitrogens and four deprotonated amido nitrogens serving as the donors. Addition of base to the aqua complex 2a (or 2b) affords the hydroxo complex 3. This transformation is reversible, and the pK(2), of the coordinated water is 7. Addition of 1 equiv of Na(2)PyPz(2)P, the disodium salt of PyPz(2)PH(2), to [Co(PyPz(2)P)(H2O)lf in methanol followed by warming results in quantitative formation of Na[Co(PyPz(2)P)(2)]. Large crystal field stabilization from the four coordinated amido nitrogens makes [Co(PyPz(2)P)(2)](-) thermodynamically more stable than [Co(PyPz(2)P)(H2O)](+).