Aspectos Demográficos del distrito: 91

[Fe3(L11)2](ClO4)3.nCH3CN (n = 1 or 2). (C8)

The synthesis of [Fe3(L11)2](ClO4)3.nCH3CN followed the procedure of Masuda et al21

for the preparation of the Fe(III) complex of tris(6-pivalamido-2-pyridylmethyl)amine. Masuda used sodium benzoate to introduce a carboxylate ligand for the purpose of successfully modelling the active ferric site of soybean lipoxygenase-1.22We therefore adopted the same approach in the expectation of making an analogous mononuclear Fe(III) complex. As in the reported synthesis oxidation of Fe(II) to Fe(III) occurs spontaneously in the air. However in the case of H3L11 the greater affinity for oxygen

donors by the hard Fe(III) centre leads to coordination of all three hydroxymethyl arms as in the case of the Mn(II) complex above. Furthermore the increased acidity of the hydroxyl protons as a result of coordination to Fe(III) leads to deprotonation in the presence of benzoate (acting here as base) and formation of an alkoxide-bridged linear trinuclear complex via incorporation of an additional Fe(III) centre. This was further confirmed by the analytical presence of co-crystallised benzoic acid along with sodium

perchlorate in the initially analysed yellow-brown solid residue although vapour diffusion of diethyl ether into an acetonitrile solution of a recrystallised sample gave X-ray quality crystals solvated by two acetonitriles. The structure of [Fe3(L11)2](ClO4)3.2CH3CN, figure 2.4, consists of a six-coordinated Fe(III) bound

only to the bridging alkoxide groups flanked by two hepta-coordinated Fe(III) centres coordinated to the three bridging alkoxides and the four nitrogen donors of L11.

Selected bond lengths and angles are listed in table 2.5. The central hexa-coordinated iron(III) is distorted from octahedral towards trigonal antiprismatic geometry (range Fe-O = 1.99-2.10 Å; tight O-Fe-O = ~80o, open O-Fe-O ~100o). The two hepta- coordinated iron(III)’s are in a distorted capped trigonal prismatic geometry similar to that found in [Mn(H3L11)]Cl2with the bond to the tertiary amine distinctly longer (Fe-

N = 2.366 Å) than those to the pyridine nitrogens (av Fe-N = 2.13 Å) and capping the expanded triangular face of the trigonal prism formed by the latter. Crystal data for all four complexes reported in this chapter are shown in table 2.6.

Table 2.5: Selected bond lengths (Å) and angles (°) for [Fe3(L11)2](ClO4)3.2CH3CN (esds in parentheses). Fe(1)-O(29) 2.102(2) Fe(1)-O(9) 2.082(2) Fe(2)-O(9) 1.989(2) Fe(1)-O(19) 2.097(2) Fe(2)-O(19) 2.003(2) Fe(1)-N(4) 2.133(3) Fe(2)-O(29) 2.010(2) Fe(1)-N(24) 2.143(3) Fe(2)-Fe(1) 2.8343(5) Fe(1)-N(14) 2.115(2) Fe(1)-N(1) 2.366(3) O(29)-Fe(2)-O(29a) 180.000 O(19)-Fe(1)-N(4) 150.30(10) O(29)-Fe(2)-O(9) 79.62(9) O(9)-Fe(1)-N(4) 75.16(10) O(29a)-Fe(2)-O(9) 100.38(9) O(29)-Fe(1)-N(24) 74.69(9) O(9)-Fe(2)-O(9a) 180.000 O(19)-Fe(1)-N(24) 94.59(9) O(29)-Fe(2)-O(19a) 100.79(9) O(9)-Fe(1)-N(24) 150.05(9) O(19)-Fe(2)-O(29) 79.21(9) N(4)-Fe(1)-N(24) 109.33(10) O(9)-Fe(2)-O(19) 79.56(8) O(29)-Fe(1)-N(14) 150.20(9) O(9a)-Fe(2)-O(19) 100.44(8) O(19)-Fe(1)-N(14) 75.20(9) O(19a)-Fe(2)-O(19) 180.000 O(9)-Fe(1)-N(14) 95.37(9) O(29)-Fe(1)-O(9) 75.45(8) N(4)-Fe(1)-N(14) 111.16(10) O(29)-Fe(1)-O(19) 75.05(8) O(29)-Fe(1)-N(4) 94.09(9) ___________________________________________________________________________________

__________________________________________________________________________________________________________

[Cu(H3L11)Cl]Cl [Cu(H3L11)Br]Br [Mn(H3L11)]Cl2.3H2O [Fe3(L11)2](ClO4)3.2CH3CN

__________________________________________________________________________________________________________

Formula C21H24Cl2CuN4O3 C21H24Br2CuN4O3 C21H30Cl2MnN4O6 C46H42Cl3Fe3N10O18

M 514.88 603.80 560.33 1296.80

Crystal system Monoclinic Monoclinic Trigonal Monoclinic

Space Group P21/c P21/n R-3c P21/c a/Å 9.2202(14) 8.6000(13) 13.0839(4) 11.4502(11) b/Å 25.647(4) 8.2400(13) 13.0839(4) 20.814(2) c/Å 10.0745(16) 33.120(5) 54.374(2) 11.5548(11) /o 90 90 90 90 /o 113.014(4) 96.5 90 108.197(2) /o 90 90 120 90 V/Å3 2192.7(6) 2331.9(6) 8061.1(5) 2616.1(4) Z 4 4 12 2 /cm-1 Reflections collected 11331 14320 13811 14484

Rint(no. of equivalent reflections) 0.0549 0.0309 0.0406 0.0379

Observed reflections [I/(I) > 2] 3645 3942 1753 4894

Direct current magnetic susceptibility studies were performed on a powdered polycrystalline sample of [Fe3(L11)2](ClO4)3.2CH3CN in the 5 – 300 K range in an applied field of 0.1 T. The

results are plotted as the χMT product versusT in figure 2.5. The room temperatureχMT value

of approximately 11.4 cm3 K mol-1 is slightly lower than that expected for three non- interacting Fe(III) ions (13.125 cm3 K mol-1). Upon cooling, the value of χMT decreases

gradually to approximately 4 cm3 K mol-1 at 5 K. This behaviour is indicative of weak antiferromagnetic exchange between the metal centres with the low temperature maximum suggesting anS= 5/2 ground state.

Figure 2.5: Plot of χMTversus T for [Fe3(L11)2](ClO4)3.2CH3CN in the temperature range 5 -

(J1) between the central metal ion Fe(2) and the peripheral Fe ions (Fe(1) and symmetry equivalent) mediated by the µ-bridging alkoxide ligands. However, attempts to simulate the experimental data with this simple model failed, and so the 2J-model, depicted in figure 2.6, including the Fe(1)…Fe(1’) interaction (J2) was employed. Using the program MAGPACK23 and employing the Hamiltonian

Ĥ= -2J1(Ŝ1·Ŝ2+Ŝ2·Ŝ1’) – 2J2(Ŝ1·Ŝ1’)

allowed us to satisfactorily simulate the data with the parametersJ1= - 3.1 cm-1,J2= + 0.2 cm- 1

and g = 2.00(2). This results in a spin ground state ofS = 5/2 with the first S = 3/2 excited state 17.5 cm-1 higher in energy, and the second excited state, S = 7/2, 22.5 cm-1 higher in energy.

Figure 2.6:The exchange interaction model employed for the [Fe3(L11)2]3+core.

The magnetic properties of exchanged coupled dinuclear complexes have, for some time, been known to depend on the identity of the metal ions, the nature of the bridging ligands providing the super-exchange pathway, and the bridging geometry, i.e.the angles and distances. There have been a number of magneto-structural correlations published for the Fe- O-Fe moiety, which have attempted to describe the relationship between the strength of interaction (J) and the Fe…Fe distance, the Fe…O distance and the Fe-O-Fe bridging angle, with, in almost all cases, the central bridging ion being an oxide rather than a hydroxide or alkoxide. As yet, none have appeared universally correct. For example, Gerloch suggested a

Fe1

J

J

bridging angle of 180°.24Gorun and Lippard suggested the main contribution to the exchange was the average Fe-O distance25 whilst Güdel and Weihe concluded that both the Fe-(µ-O) distance and the Fe-(µ-O)-Fe angle were important.26 To our knowledge, the only attempted magneto-structural correlation for alkoxide-bridged diiron(III) species was published by Caneschi and co-workers in 1997 who investigated molecules of the type [Fe2(OR)2L12], Table

2.7, where L12is aβ-diketonate ligand.27Based on their experimental evidence they suggested

a linear dependence of J with the Fe-O-Fe bridging angle (α), expressed as J = 1.48α - 135, with the switch from antiferromagnetic to ferromagnetic occurring at α = 91°. DFT calculations later refined the expression to J= 5.0(1)α - 450(10).28[Fe3(L11)2]3+with Fe-O-Fe

bridging angles of 87.12, 87.42 and 88.21° (αav = 87.58°), appears not follow these

experimentally or theoretically derived trends. All three bridging angles in [Fe3(L11)2]3+ are

below that expected for a ferromagnetic interaction, and the predicted exchange of +5.4 cm-1 (+12 cm-1via DFT) is some way off the observed value of -3.1 cm-1. Given that there are only a few structurally related trinuclear iron complexes reported (see below), and almost none with reliable magnetic data, it is not possible at this stage to provide a deep understanding of the origins of these differences, but clearly [Fe3(L11)2]3+“bucks the trend”. Thus it appears that, as

with the oxo-bridged species before them, there may not be a simple correlation for alkoxo- bridged Fe clusters between J and any single structural parameter, and that any universal correlation must invoke other relationships.