Spectroscopic and magnetic properties of square-planar bis[(cyclooctane-1,5-diyl)bis(pyrazol-1-yl)borato]cobalt(II)

Abstract

Ligand-field spectra, EPR spectra and magnetic susceptibility of the title compound are described in terms of square-planar Co(II) in the low-spin state ( S= 1 2 ). The presence of two short, agostic, CoH contacts (2.16 Å) results in a very slow relaxation, allowing the observation of EPR signals even at room temperature. This relaxation behaviour is also responsible for the fact that solid state NMR spectra ( 13C and 1H) could not be observed for this compound. The magnetic moment of 2.05(3) BM ( g av = 2.18(3)) in the 77–300 K region, agrees with the EPR g values for the powder ( g∥ = 1.65(5) and g⊥ = 2.37(5); g av =2.13(5)). Dopes of the title compound in the corresponding Zn compound (not X-ray powder isomorphous with the Co compound) and the Ni compound (powder isomorphous with Co compound), on the other hand, show quite different, and beautifully resolved EPR spectra. In the Zn lattice a very sharp EPR signal, with inverted, g values at g∥ = 2.05 (with an A∥ value of 18 Gauss) and g⊥ = 2.00, indicating a Co III)O 2 − species, appears to be present. Unfortunately, this mixed compound could not be purified through recrystallization, since on each purification step some Co(II) was lost from the lattice. In the isomorphous Ni lattice, however, the Co hyperfine splitting nicely shows up in a rhombic signal with hyperfine splittings present on each of the three g values and superhyperfine on g z ; the parameters are g x =2.68(2) ( A x = 115(5) G), g y = 2.54(4) ( A y = 135(5) G) and g z = 2.01(2) ( A z = 165(5) G; A N = 18(1) G). These EPR values differ slightly from the frozen-solution data of the pure Co compound in toluene, for which the parameters g x = 2.70 (very broad, A x unresolved), g y = 2.30(2) ( A y = 65(5) G) and g z = 2.02(2) ( A z = 150(5) G) are derived. Ligand-field spectral data of the pure Co compound are in agreement with a square-planar CoN 4 chromophore. In solutions of organic solvents, like toluene, THF and CH 2Cl 2, the colour changes somewhat and the ligand-field maxima are shifted slightly; however, no tetrahedral geometry can be deduced from these spectra, in agreement with frozen-solution EPR spectra. The ligand-field spectrum of the Co-doped zinc compound, on the other hand, shows clear bands due to tetrahedral Co(II). Solid state 1H and 13C NMR spectra of the pure Ni compound show almost the same spectrum as the diamagnetic Zn compound, indicating that the Ni(II) is indeed low spin in the solid state. No signals were found indicating the presence of agostic MH interactions. On the other hand, IR spectra clearly show CH bands indicative for agostic MH interactions, i.e. at 2690 (Co), 2780 (Cu) and 2800 (Ni) cm −1, whereas the non-isomorphous Zn compound has the CH stretching vibration at 2820 cm −1.