Synthesis and Magnetic Properties of Antimony-Ligated Co(II) Complexes: Stibines versus Phosphines

Herein, we test the hypothesis that neutral, heavy-atom stibine donors can increase the extent of spin-orbit coupling on light, 3d transition metal. To this end, we developed a novel synthetic route toward coordinating a paramagnetic 3d metal ion.cobalt(II).with neutral stibine ligands. Such complexes have not been reported in the literature due to the weak s donor strength of stibines and the hard-soft mismatch between a 3d metal and a 5p ligand.which herein has been overcome using alkylated Sb donors. Magnetometry of [((SbPr2Ph)-Pr-i)(2)Co(I)(2)] (1) reveals that the stibine complex 1 exhibits a higher magnitude D value (D = vertical bar 24.96 vertical bar cm(-1)) than the spectroscopically derived value for the corresponding phosphine complex 3 (D = -13.13 cm(-1)), indicative of large zero-field splitting. CASSCF/NEVPT2 calculations corroborate the experimental D values for 1 and 3, predicting D = -31.9 and -8.9 cm(-1), respectively. A re-examination of magnetic parameters across the entire series [(ER3)(2)Co(X)(2)] (E = P -> Sb; X = Cl -> I) reveals that (i) increasingly heavy pnictogens lead to an increased X-Co-X bond angle, which is correlated with larger magnitude D values, and (ii) for a given XCo-X bond angle, the D value is always higher in the presence of a heavy pnictogen as compared with a heavy halide. Ab initio ligand field theory calculations for 1 (stibine complex) and 3 (phosphine complex) reveal no substantial differences in spin-orbit coupling (zeta = 479.2, 480.2 cm(-1)) or Racah parameter (B = 947.5, 943.9 cm(-1)), an indicator of covalency. Thus, some heavy atom effect on the D value beyond geometric perturbation is operative, but its precise mechanism(s) of action remains obscure.

Automated summary

This study tests the hypothesis that neutral, heavy-atom stibine donors can enhance spin-orbit coupling on light 3d transition metals. Through a novel synthetic route, the researchers successfully coordinated a paramagnetic 3d metal ion cobalt(II) with neutral stibine ligands. The results reveal that heavy pnictogens can increase the D value, indicating a larger zero-field splitting. The precise mechanism of the heavy atom effect on the D value remains unclear.