Coordination versatility of 1,3-bis[3-(2-pyridyl)pyrazol-1-yl]propane: Co(II) and Ni(II) complexes

The ligand 1,3-bis[3-(2-pyridyl)pyrazol-1-yl]propane (L8) has afforded six-coordinate monomeric and dimeric complexes [(L8)CoII(H2O)2][ClO4]2 (1), [(L8)NiII(MeCN)2][BPh4]2 (2), [(L8)NiII(O2CMe)][BPh4] (3), and [(L8)2Co2II(μ-O2CMe)2][BPh4]2(4). The crystal structures of 1, 2 · MeCN, 3, and 4 revealed that the ligand L8 is flexible enough to expand its coordinating ability by fine-tuning the angle between the chelating fragments and hence folds around cobalt(II)/nickel(II) centers to act as a tetradentate chelate, allowing additional coordination by two trans-H2O, cis  -MeCN, and a bidentate acetate affording examples of distorted octahedral CoIIN2(pyridyl)N2′(pyrazole)O2(water), NiIIN2(pyridyl)N2′(pyrazole)N2″(acetonitrile), and NiIIN2(pyridyl)N2′(pyrazole)O2′(acetate) coordination. The angles between the two CoN2/NiN2 planes span a wide range 23.539(1)° (1), 76.934(8)° (2), and 69.874(14)° (3). In contrast, complex 4 is a bis-μ-1,3-acetato-bridged (syn–anti coordination mode) dicobalt(II) complex [Co⋯Co separation: 4.797(8) Å] in which L8 provides terminal bidentate pyridylpyrazole coordination to each cobalt(II) center. To our knowledge, this report provides first examples of such a coordination versatility of L8. Absorption spectral studies (MeCN solution) have been done for all the complexes. Complexes 1–3 are uniformly high-spin. Temperature-dependent (2–300 K) magnetic studies on 4 reveal weak ferromagnetic exchange coupling (H^=-JS1·S2) between two cobalt(II) (S = 3/2) ions. The best-fit parameters obtained are: Δ (axial splitting parameter) = −765(5) cm−1, λ (spin-orbit coupling) = −120(3) cm−1, k (orbital reduction factor) = 0.93, and J (magnetic exchange coupling constant) = +1.60(2) m−1.

Graphical abstractIn the complexes, trans-[(L8)CoII(H2O)2][ClO4]2, cis-[(L8)NiII(MeCN)2][BPh4]2, and [(L8)NiII(O2CMe)][BPh4] the ligand L8 is flexible enough to expand its coordination ability by fine-tuning the angle between the chelating fragments. In [(L8)2CoII(μ-O2CMe)2]X2 (acetates are coordinated in syn–anti mode), it provides terminal bidentate pyridylpyrazole coordination to each cobalt(II) ion. Notably, dicobalt(II) complex is weakly ferromagnetically coupled (J = +1.6 cm−1).Figure optionsDownload full-size imageDownload as PowerPoint slide