Substituent effects of N4 Schiff base ligands on the formation of fluoride-bridged dicobalt(ii) complexes via B–F abstraction: structures and magnetism

Abstract

We report the synthesis of two fluoride bridged cobalt(II) dimers – [Co^II₂(μ-F)(pnN₄-PhCl)₂(OH₂)(MeCN)](BF₄)₃ (1) and [Co^II₂(μ-F)₂(pnN₄-PhCl)₂](BF₄)₂ (2) – and related complexes derived from propyl-bridged N4 Schiff base plus pyridine ligands. Notably, the bridging fluoride ion(s) emanate from B–F abstraction processes on the BF₄ anions in the starting salt, [Co(H₂O)₆](BF₄)₂. Two types of bridging motifs are generated – mono-bridged (μ-F) or di-bridged (μ-F)₂ – synthetically differentiated by the absence or presence of pyridine, respectively, during metalation. The synergistic roles of pyridine and the ^ClPhN4 ligand in promoting B–F abstraction were clarified by the isolation and crystallization of the simple tetrakis-pyridine monomeric complex [Co(py)₄(MeCN)₂](BF₄)₂ (4) [no B–F abstraction]; subsequent addition of the ^ClPhN4 ligand to 4 resulted in formation of the dimeric, di-bridged complex 2. Omission of pyridine during metalation resulted in formation of the mono-bridged dimer 1. The bulky chlorophenyl substituents were obligate for B–F abstraction, as metalation of the unsubstituted N4 ligand resulted in the non-fluoride-bridged dimer, [Co^II₂(pnN₄)₃](BF₄)₄ (3). In magnetic studies, complexes 1 (μ_eff = 6.24μ_B, 298 K) and 2 (μ_eff = 7.70μ_B, 298 K) both exhibit antiferromagnetic (AFM) coupling, but to different extents. Temperature-dependent magnetic susceptibility measurements (SQUID, 2 → 300 K) reveal that the linearity of the mono-fluoride bridge in 1 [∠Co–F–Co = 159.47(11)°] results in very strong AFM coupling (J = −14.9 cm⁻¹). In contrast, the more acute Co₂F₂ diamond core [∠Co–F–Co = 98.8(2)°, 99.1(2)°] results in a smaller extent of AFM coupling (J = −2.97 cm⁻¹). Overall, the results indicate the ‘non-innocence’ of the BF₄ counterion in cobalt(II) chemistry, and dimers 1 and 2 affirm the effect of the geometry of the bridging fluoride ion(s) in determining the extent of AFM coupling.