Ferromagnetic versus Antiferromagnetic Exchange in Five Structurally Analogous Carboxylate-Bridged Trinuclear Ferrous Complexes

Article abstract of DOI:10.1021/ic00115a032

The synthesis, structural characterization, and magnetic properties of linear, trinuclear complexes of general formula [Fe3(O2CR)6L2] are reported. Addition of Fe(OAc)2 to the bidentate nitrogen donor ligands bis(1-methyl-2-imidazolyl)phenylhydroxymethane (BIPhOH) and 1,1-bis(1-methyl-2-imidazolyl)-1-(3,5-di-tert-butyl-4-hydroxyphenyl)ethane (BIDPhEH) afforded [Fe3(OAc)6(BIPhOH)2].2MeOH (3) and [Fe3(OAc)6(BIDPhEH)2] (4), respectively. CompIex 3 crystallizes in P1- , with a = 8.985(2) ?, b = 9.148(1) ?, c = 15.761(2) ?, α = 80.73(1)°, β = 81.28(1)°, γ = 101.91(1)°, V = 1228.0(4) ?**3, and Z = 1 (R = 0.046, Rw = 0.056), and complex 4 crystallizes in P21/n, with a = 12.809(5) ?, b = 22.214(5) ?, c = 13.793(5) ?, β= 91.19(2)°. V = 3924(2) ?**3, and Z = 2 (R = 0.055, Rw = 0.076). Addition of Fe(BF4)2.6H2O and sodium benzoate to the ligands bis(2-((4S)-(1-methylethyl)-1,3-oxazolinyl)]methane ((i)PrOx) and N,N,N'-trimethyl-N'-[4,4-dimethyl-4-(3,5-di-tert-butyl-4-hydroxyphenyl)butyl]ethylenediamine (PheMe3Eda) yielded compounds [Fe3(O2CPh)6((i)PrOx)2] (5) and [Fe2(O2CPh)6(PheMe3Eda)2] (6), respectively. Compound 5 crystallizes in P212121, with a = 14.677(3) ?, b = 19.289(4) ?, c = 23.066(6) ?, V = 6530(4) ?**3, and Z = 4 (R = 0.059, Rw = 0.065), and compound 6 in P21/n, with a = 10.111(1) ?, b = 31.389(2) ?, c = 14.243(2) ?, β = 100.76(5)°, V = 4441(1) ?**3, and Z = 2 (R = 0.047, Rw = 0.063). In all of these complexes, the iron atoms are linked by two bidentate and one unidentate bridging carboxylate ligands. The coordination spheres of the terminal iron atoms are compIeted by the bidentate nitrogen ligands and, in compounds 5 and 6, by the second dangling oxygen atom of the unidentate bridging carboxylate. We investigated the magnetic properties of these four compounds, as well as those of [Fe3(OAc)6(BIPhMe)2] (1), reported previously (Rardin, R. L.; Poganiuch,P.; Bino A.; Goldberg, D. P.; Tolman, W. B.; Liu, S.; Lippard, S. J. J.Am. Chem. Soc. 1992, 114, 5240-5249). Three of the complexes (1, 3, and4) exhibit intramolecular ferromagnetic exchange coupling, J = -2 to -5cm**-1 (.SCRIPTH. = JS1.S2), resulting in high-spin S = 6 ground states, and the remaining two complexes are antiferromagnetically coupled, with S = 2 ground states. The magnetic properties thus correlate with the structural differences between the two classes of compounds. Both temperature-dependent magnetic susceptibility and high-field magnetization measurements reveal this behavior. The magnetic data were fit to a theoretical model incorporating exchange coupling, single-ion zero-field ...