LA POLEMICA DE KAUTSKY CON LOS OPORTUNISTAS

The reaction of the Bc2hda and tBu2hda ligands with FeCl3·6H2O yielded the

coordination compounds [Fe(Bc2hda)(H2O)(EtOH)]·3H2O (22) and

[Fe(tBu2hda)(H2O)2]·4H2O (23), respectively. These compounds crystallise in the

monoclinic crystal system in the C2/c space group. These coordination compounds

contain mononuclear FeIII complexes (Figure 7.2). The structure of 23 was previously reported54 but its supramolecular structure was further investigated as part of this study.

Within these mononuclear complexes the FeIII ions display similar binding modes, with two carboxylate O atoms, one phenolic O atom and one N atom from the organic ligand coordinating to the FeIII ion. The distorted coordination environments of the FeIII ions are completed by the presence of solvent molecules. In 22 one ethanol and one water molecule bind to the central FeIII ion whereas in 23 two water molecules complete the coordination sphere. This slight modification is a consequence of the different preparation methods employed in the formation of the complexes.

The FeIII ions in both complexes, display distorted octahedral coordination geometries with the O donor atoms of the organic ligands occupying meridional

Figure 7.2: The mononuclear complexes contained in a)22 and in b)23 (solvent molecules removed

for clarity).Fe green, C black, O red, N blue and H white.

173 position. The N atom of the carboxylate ligand and the O donor atoms of the solvent molecules also adopt a meridional arrangement. The bond angles that give rise to the

coordination geometries are presented in Tables 7.2 and 7.3. Table 7.2: Selected bond angles for 22

Bond Angle Bond Angle

O(1)-Fe(1)-O(2) 94.89(2)° O(2)-Fe(1)-N(1) 81.52(1)° O(1)-Fe(1)-O(4) 164.65(2)° O(4)-Fe(1)-O(6) 94.99(2)° O(1)-Fe(1)-O(6) 99.74(2)° O(4)-Fe(1)-O(7) 84.07(1)° O(1)-Fe(1)-O(7) 91.42(2)° O(4)-Fe(1)-N(1) 78.11(2)° O(1)-Fe(1)-N(1) 87.88(2)° O(6)-Fe(1)-O(7) 90.75(1)° O(2)-Fe(1)-O(4) 89.29(2)º O(6)-Fe(1)-N(1) 169.29(2)° O(2)-Fe(1)-O(6) 90.29(1)° O(6)-Fe(1)-N(1) 96.61(1)° O(2)-Fe(1)-O(7) 173.34(2)°

Table 7.3: Selected bond angles for 23

Bond Angle Bond Angle

O(1)-Fe(1)-O(2) 101.69(1)° O(2)-Fe(1)-N(1) 78.37(1)° O(1)-Fe(1)-O(4) 95.38(1)° O(4)-Fe(1)-O(6) 80.56(1)° O(1)-Fe(1)-O(6) 173.37(1)° O(4)-Fe(1)-O(7) 104.34(1)° O(1)-Fe(1)-O(7) 91.44(1)° O(4)-Fe(1)-N(1) 79.84(1)° O(1)-Fe(1)-N(1) 93.53(1)° O(6)-Fe(1)-O(7) 84.60(1)° O(2)-Fe(1)-O(4) 152.71(1)° O(6)-Fe(1)-N(1) 99.84(1)° O(2)-Fe(1)-O(6) 84.08(1)° O(6)-Fe(1)-N(1) 173.43(1)° O(2)-Fe(1)-O(7) 96.42(1)°

The Fe-Ocarboxylate bonds within the complexes range from 2.002(1) Å to

2.026(3) Å. The Fe-Ophenol bond distances in 22 and 23 are 1.843(3) Å and 1.901(1) Å,

and the Fe-N bond lengths are 2.197(2) Å and 2.180(1) Å, respectively. In 22 the Fe- O bonds involving the solvent molecules are 1.987(1) Å for the coordinating water molecule and 2.039(2) Å for the coordinating ethanol molecule. Within 23, the bond distances involving the FeIII _{ion and the coordination water molecules are 1.959(1) Å}

174 and 2.180(1) Å, respectively. These bond lengths are summarised in Tables 7.4 and 7.5.

The two FeIII compounds, 22 and 23, form layered lamellar arrangements with the hydrophilic part of the monomeric FeIII complexes situated in the bc-

crystallographic plane and the hydrophobic organic moieties pointing parallel to the a-

axis (Figure 7.3). In both compounds, hydrogen bonded constitution water molecules link the FeIII complexes into network structures. Both these networks encapsulate ordered H-bonded water networks.

In 22 the H-bonded water network (Figure 7.4a) can best be described as a tape of fused (H2O)10 rings that extends parallel to the crystallographic c-axis. Half of

the molecular entities of the (H2O)10 ring motif are generated by symmetry operations

with the inversion centre situated in the centre of the ring. The O-O distances in the hydrogen bonded water network vary between 2.64(2) Å and 2.89(2) Å and the angles between the O-atoms range between 88.1(1) º and 117.9(1) º.

Compound 23 contains a H-bonded water network (Figure 7.4b) which consists of water molecules organised into 1D chains. The structural motif in the 1D chain reveal similarities to the crystal structure of ice Ih, in the respect that the

Figure 7.3:a) Layered lamellar structure of 22 viewed in the direction of the crystallographic c-axis.

b) Layered lamellar structure of 23 viewed in the direction of the crystallographic c-axis. Fe green, C

black, O red, N blue and H white.

b) a)

175 network displays both boat and chair conformations. The O-O distances of the network range between 2.76(3) Å and 2.87(2) Å which are comparable to those seen in the modifications of hexagonal ice Ih. However the observed O-O-O angles

(74.5(1) º - 137.2(1) º) deviate significantly from the ideal tetrahedral angle of 109.5 º which is characteristic for the Ih ice modification.

Table 7.5: Selected bond lengths for 22

Bond Bond Length Bond Bond Length

Fe(1)-O(1) 1.863(3) Å Fe(1)-O(6) 1.987(1) Å

Fe(1)-O(2) 2.009(4) Å Fe(1)-O(7) 2.039(2) Å

Fe(1)-O(4) 2.026(3) Å Fe(1)-N(1) 2.197(2) Å

Table 7.6: Selected bond lengths for 23

Bond Bond Length Bond Bond Length

Fe(1)-O(1) 1.901(1) Å Fe(1)-O(6) 2.197(1) Å

Fe(1)-O(2) 2.020(1) Å Fe(1)-O(7) 1.959(1) Å

Fe(1)-O(4) 2.002(1) Å Fe(1)-N(1) 2.180(1) Å

Figure 7.4: a) Hydrogen bonded network of constitutional (red) and coordination water (brown)

molecules in 22 as viewed in the direction of the crystallographic b-axis. b) Hydrogen bonded network

of constitutional (red) and coordination water (brown) molecules in 23 as viewed in the direction of the

crystallographic b-axis.

b) a)

176 These compounds were also characterised by FT-IR spectroscopy. Both compounds show strong asymmetric carboxylate stretches at 1573 cm-1 and 1575 cm-1, respectively.164 They both show strong symmetric carboxylate stretches at 1362 cm-1 and at 1365 cm-1. The ∆ = [υasymmetric(CO2-) -υsymmetric(CO2-)] values of 211 cm-1

for 22 and 210 cm-1 for 23, underline the X-ray analysis and suggest monodentate binding modes.164,175 The UV-vis spectra of both complexes dissolved in methanol display a phenolate LMCT band at 590 nm for 22 and at 588 nm for 23.201 This transition is characteristic of this type of mononuclear FeIII complex.201 CHN microanalysis was also employed to verify the composition and purity of the bulk samples of 22 and 23.

177 Table 7.6: Crystal data and structure refinement for 22

Identification code 22

Empirical formula FeC31H46NO11

Formula weight 664.54 g mol-1

Temperature 150(2) K

Wavelength 0.71073 Å

Crystal system monoclinic

Space group C2/c

Unit cell dimensions a = 44.981(9) Å α= 90°

b = 9.206(4) Å β= 108.938(7)° c = 16.552(6) Å γ= 90° Volume 6485(4) Å3 Z 8 Density (calculated) 1.361 Mg/m3 Absorption coefficient 0.525 mm-1 F(000) 2824 Crystal size 0.1 x 0.1 x 0.08 mm3 Theta range for data collection 0.96 to 23.30°

Index ranges -47<=h<=50, -10<=k<=10, -18<=l<=18 Reflections collected 21323

Independent reflections 4667 [R(int) = 0.0224] Completeness to theta = 23.30° 99.6 %

Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 4667 / 0 / 581

Goodness-of-fit on F2 1.100

Final R indices [I>2sigma(I)] R1 = 0.0290, wR2 = 0.0804 R indices (all data) R1 = 0.0352, wR2 = 0.0902 Largest diff. peak and hole 0.381 and -0.396 e.Å-3

178

7.4 The synthesis and self-assembly of [Fe(p-C

12

Mehda)(H

2

O)

2

]·

nH

2

O (24)

An important feature of 22 and 23 is that they form layered lamellar structures of an amphiphilic nature, thus making these complexes ideal candidates for self- assembly approaches to create nanomaterials. We set out to increase the amphiphilicity of our reaction system by employing an extended ligandp-C12Mehda,

which contains a C12 alkyl chain para to the phenolic oxygen donor. The reaction of

this ligand with FeIII salts in water, resulted in the precipitation of the coordination compound, [Fe(p-C12Mehda)(H2O)2]·nH2O (24).

The formation of this compound was verified by the comparison of its UV-vis and FT-IR spectra with those of compound 23 (Figure 7.5a). The FT-IR spectra of 24 and 23 (Figure 7.5a) are similar and both present evidence of the asymmetric and symmetric carboxylate bond stretches.164 In 24 these bond vibrations occur at 1576 cm-1 and 1362cm-1.164 CHN analyses are also in agreement with the constitutional assignment.

Compounds 23 and 24 also have similar UV-vis spectra (Figure 7.5b). These both display a band at ca. 580 nm which is characteristic of a phenolate LMCT

transition.201 Transmission electron microscopy (Figure 7.6) was used to structurally characterise powered samples of 24. Although these samples are very sensitive to electron-beam induced damage, it can be clearly observed that 24 displays a highly ordered motif resulting from the layered lamellar packing arrangement. The experiments suggest that 24 adopts a similar layered structure to 22 and 23. The observed inter-layer distance in 24 is ca 3.5 nm and this is in good agreement with the

expected dimension of the alkylated FeIII _{complexes and corresponds well to the}

d-

spacing determined by X-ray powder diffraction (Figure 7.5c). Energy dispersive X- ray (EDX) analysis was also used to confirm the composition of 24 (Figure 7.5c).

179

b) a)

c)

Figure 7.5:a) The FT-IR spectrum of 24 (blue) as compared to that of 23 (black). b) The UV-vis

spectrum of 24 (black) as compared to that of 23 (red). [23] = 1 x 10-5 mol L-1 and [24] = 1 x 10-5 mol L-1. c) X-ray powder diffraction pattern of 24 reveals a signal at 2θ of 2.3° which corresponds to a d-spacing

180 In attempts to assemble 24 into nanostructures, a solid sample was dispersed in EtOH/H2O solution. This solution was then heated to 80 °C and drop-cast onto a Si

surface. The Si surface was then viewed using a scanning electron microscope (SEM) which revealed the formation of isolated nanosized spherical vesicles (Figure 7.7 a-f). The vesicles range in diameter from ca. 10 nm to ca. 500 nm. It was possible to obtain

higher yields and separate the vesicles from the EtOH/H2O solution by using a 0.2 µm

polycarbonate membrane. The membrane was then imaged by SEM which also showed the generation of nanosized vesicles (Figure 7.7 g-h). It is most likely that the complexes in these aggregates adopt a similar lamellar arrangement to those observed in the corresponding crystalline compounds 22 and 23. This was verified by transmission electron microscopy (TEM) (Figure 7.8) which showed that vesicles are composed of layers that also display a highly ordered layered, endohedral packing motif. The TEM images demonstrate that the inter-layer distance within the vesicles is

ca. 3.5 nm (Figure 7.8 g and h).

Figure 7.6: TEM micrographs suggest a layered, lamellar arrangement of the FeIII complexes in 24.

10 nm

181

Figure 7.7: SEM micrographs of the spherical vesicles formed from the self-assembly of 24. a-f)

Nanosized vesicles formed by drop-casting an EtOH/H2O solution of 24 onto a Si surface. g) and h) The

spherical vesicles on a polycarbonate filter.

1µm 10µm 200nm 5µm 5µm 1µm 1µm 2µm a) b) c) d) e) f) g) h)

182

Figure 7.8: TEM micrographs of the spherical vesicles showing the layered lamellar arrangement with an

inter-layer spacing of ca. 3.5 nm.

b)

70nm 70nm 20nm 500nm _500nm 500nm 500nm 250nm c) d) e) f) g) h) a) b)

183 When the spherical vesicles were imaged using a JEOL FX 2000 operating at 200 kV at the National Institute for Material Science in Japan, an interesting phenomenon was observed. The vesicles appeared to undergo decomposition. This led to the formation of endohedral onion-type carbon structures which can be identified in high yields on the TEM grid (Figure 7.10). These endohedral carbon structures have diameters that range between ca. 15 nm and ca. 30 nm and display inter-layer distance

of ca. 0.4 nm which is significantly smaller than that seen for the vesicles. It is likely,

that the formation of the endohedral carbon structures is caused by heating effects induced by the high voltage electron beam. It has previously been reported that the irradiating effects of high voltage electron beams can instigate structural changes within a variety of different nanomaterials.202,203 _{It is also possible that the Fe}

contained within the vesicles may have acted as a catalyst for the formation of the endohedral onion type carbon structures as the ability of Fe to catalyse the formation of related nanomaterials is well known in the literature.204,205

Figure 7.10: TEM micrographs of the endohedral carbon structures which result from the

decomposition of the spherical vesicles.

b)

a)

c)

b)

184

7.5 The synthesis and self-assembly of [Fe(o-C

12

Mehda)(H

2

O)

2

]·

nH

2

O (25)

We next set out to determine the effect of the ligand functionality on the assembly of related nanostructures. This involved the use of a similar ligand o- C12Mehda, with the C12 alkyl chain now ortho-positioned to the phenolate O donor.

This ligand was reacted with FeCl3·6H2O generating the compound [Fe(o-

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