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

We report the synthesis of two fluoride bridged cobalt(II) dimers -[Co-2(II)(mu-F)(pnN(4)-PhCl)(2)(OH2)(MeCN)] (BF4)(3) (1) and [Co-2(II)(mu-F)(2)(pnN(4)-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 (mu-F) or di-bridged (mu-F)(2) - synthetically differentiated by the absence or presence of pyridine, respectively, during metalation. The synergistic roles of pyridine and the N-ClPh(4) 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 (ClPh)N4 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-2(II)(pnN(4))(3)](BF4)(4) (3). In magnetic studies, complexes 1 (mu(eff) = 6.24 mu(B), 298 K) and 2 (mu(eff) = 7.70 mu(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 [(sic)Co-F-Co = 159.47(11)degrees] results in very strong AFM coupling (J = -14.9 cm(-1)). In contrast, the more acute Co2F2 diamond core [(sic)Co-F-Co = 98.8(2)degrees, 99.1(2)degrees] 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.