FERROMAGNETIC VERSUS ANTIFERROMAGNETIC EXCHANGE IN 5 STRUCTURALLY ANALOGOUS CARBOXYLATE-BRIDGED TRINUCLEAR FERROUS COMPLEXES

The synthesis, structural characterization, and magnetic properties of linear, trinuclear complexes of general formula [Fe-3(O(2)CR)(6)L(2)] are reported. Addition of Fe(OAc)(2) to the bidentate nitrogen donor ligands bis(1-methyl-2-imidazolyl)phenylhydroxymethane (BIPhOH) and zo lyl)-1-(3,5-di-tert-butyl-4-hydroxyphenyl)ethane (BIDPhEH) afforded [F e-3(OAc)(6)(BIPhOH)(2)]. 2MeOH (3) and [Fe-3(OAc)(6)(BIDPhEH)(2)] (4), respectively. Complex 3 crystallizes in P ($) over bar 1, with a = 8. 985(2) Angstrom, b = 9.148(1) Angstrom, c = 15.761(2) Angstrom, alpha = 80.73(1)degrees, beta = 81.28(1)degrees, gamma = 101.91(1)degrees, V = 1228.0(4) Angstrom(3), and Z = 1 (R = 0.046, R(W) 0.056), and compl ex 4 crystallizes in P2(1)/n, with a = 12.809(5) Angstrom, b = 22.214( 5) Angstrom, c = 13.793(5) Angstrom, beta = 91.19(2)degrees, V = 3924( 2) Angstrom(3), and Z = 2 (R = 0.055, R(W) = 0.076). Addition of Fe(BF 4)(2) . 6H(2)O and sodium benzoate to the ligands is[2-((4S)-(1-methyl ethyl)-1,3-oxazolinyl)]methane ((i)PrOx) and i-tert-butyl-4-hydroxyphe nyl)butyl]ethylenediamine (PheMe(3)Eda) yielded compounds [Fe-3(O(2)CP h)(6)((i)PrOx)(2)] (5) and [Fe-3(O(2)CPh)(6)(PheMe(3)Eda)(2)] (6), res pectively. Compound 5 crystallizes in P2(1)2(1)2(1), with a = 14.677(3 ) Angstrom, b = 19.289(4) Angstrom, c = 23.066(6) Angstrom, V = 6530(4 ) Angstrom(3), and Z = 4 (R = 0.059, R(w) = 0.065), and compound 6 in P2(1)/n, with a = 10.111(1) Angstrom, b = 31.389(2) Angstrom, c = 14.2 43(2) Angstrom, beta = 100.76(5)degrees, V = 4441(1) Angstrom(3), and Z = 2 (R = 0.047, R(W) = 0.063). In all of these complexes, the iron a toms are linked by two bidentate and one unidentate bridging carboxyla te ligands. The coordination spheres of the terminal iron atoms are co mpleted by the bidentate nitrogen ligands and, in compounds 5 and 6, b y the second ''dangling'' oxygen atom of the unidentate bridging carbo xylate. We investigated the magnetic properties of these four compound s, as well as those of [Fe-3(OAc)(6)-(BIPhMe)(2)] (1), reported previo usly (Rardin, R. L.; Poganiuch, P.; Bino A.; Goldberg, D. P.; Tolman, W. B.; Liu, S.; Lippard, S. J. J. Am. Chem. Sec. 1992, 114, 5240-5249) . Three of the complexes (1, 3, and 4) exhibit intramolecular ferromag netic exchange coupling, J = -2 to -5 cm(-1) (H = JS(1) . S-2), result ing in high-spin S = 6 ground states, and the remaining two complexes are antiferromagnetically coupled, with S = 2 ground states. The magne tic properties thus correlate with the structural differences between the two classes of compounds. Both temperature-dependent magnetic susc eptibility and high-field magnetization measurements reveal this behav ior. The magnetic data were fit to a theoretical model incorporating e xchange coupling, single-ion zero-field splitting, and g tenser anisot ropy. A set of consensus magnetic parameters for each compound was obt ained, with weak magnetic exchange (/J/ less than or equal to 5 cm(-1) ) between nearest-neighbor iron atoms being a common feature for all f ive compounds. The X-band EPR spectra of 1, 3, and 4 at 4 K displayed broad, low-field (g(obs) approximate to 18) signals consistent with in teger-spin ground states. These spectra an remarkably similar to those of the reduced, diiron(II) centers found in the non-heme iron protein s methane monooxygenase (MMOH), hemerythrin (Hr), and the R2 protein o f ribonucleotide reductase. Theoretical calculations indicate that sev eral allowed EPR transitions from the resulting manifold of ground and low-lying excited energy levels arising from the integer spin (S = 6) state could be responsible for the broad low-field absorption feature .