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 σ 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 [(Sb i Pr 2 Ph) 2 Co(I) 2 ] ( 1 ) reveals that the stibine complex 1 exhibits a higher magnitude D value ( D = |24.96| 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 [( E R 3 ) 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 X –Co– 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 (ζ = 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.