Pyrazole Ligation to Cobalt(III) Centers:  Syntheses, Structures, and Properties of Cobalt(III) Complexes ofN,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)(H 2 O)]X (X = NO 3 -, 2a; ((H 3 O)(PF 6 ) 2 ) -, 2b ), [Co(PyPz 2 P)(OH)] ( 3 ), and X[Co(PyPz 2 P) 2 ] (X = Na +, 4a; 0.5[Co(H 2 O) 6 ] 2+, 4b ). Complex 2b crystallizes in the triclinic space group P 1̄ with a = 11.842(2) Å, b = 11.920(3) Å, c = 12.755(2) Å, α = 111.21(2)°, β = 103.866(13)°, γ = 106.992(13)°, V = 1479.9(5) Å 3, and Z = 2. The structure of complex 3 has been refined to R = 6.06% on the basis of 5417 I > 2σ( I ) data. Complex 4b crystallizes in the triclinic space group P 1̄ with a = 12.471(3) Å, b = 14.070(3) Å, c = 14.171(5) Å, α = 71.58(2)°, β = 68.08(2)°, γ = 71.74(2)°, V = 2134.0(10) Å 3, and Z = 2. The structure of complex 4b has been refined to R = 5.86% on the basis of 6679 I > 2σ( 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 p K a 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)(H 2 O)] + 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)(H 2 O)] + .