Page 139
4.3: Synthesis and Characterization of Square Pyramidal Fe(III)
Page 140
Figure 4.4: [A] and [B] ESI–mass spectrum of 13; [C] and [D] ESI–mass spectrum of 14; experimental and simulated (inset).
Electrospray ionization mass spectra (ESI–MS) of 13, and 14 were measured in CH3CN in positive mode. Complex 13, and complex 14 showed a ~ 30% molecular ion peak at m/z = 518.53[13], 630.72[14]; corresponded to [M]+ (M = molecular mass) and a 100%
molecular ion peak at m/z = 483.53[13], 595.72[14]; corresponded to [M – Cl]+, respectively.
[C]
[D]
[B]
TH-1360_11612213
Page 141
Isotope distribution pattern examinations of the experimental results revealed the composition C28H31ClFeN2O2 for 13 and C36H47ClFeN2O2 for 14 (Figure 4.4).
The Single crystal X–ray diffraction measurements for 13 and 14 were performed at room temperature (296 K). ORTEP views of the molecular structure with atom labeling scheme for complex FeLMixed(H)Cl; (13), and FeLMixed(tBu)Cl; (14) are shown in Figure 4.5.
Selected bond distances and bond angles are given in the Table 4.2.
Figure 4.5: ORTEP representation of [A] FeLMixed(H)Cl; (13), and [B] FeLMixed(tBu)Cl; (14). Hydrogen atoms were omitted for clarity.
[B]
[A]
TH-1360_11612213
Page 142
X–ray diffraction examination of the complexes revealed that complex 13 crystallized in the monoclinic space group P121/n1, while, complex 14 crystallized in the triclinic system space group P–1. From the molecular structure it has been found that both complexes were five–coordinate where the central Fe atom was surrounded by N–H, O–H deprotonated mixed ligand at the basal position, and a chlorine atom bound axially. The central Fe atom was situated towards the apical chloride atom from the basal plane at 0.557 Å [13]; and at 0.584 Å [14], respectively (parenthesis represents the complex). The average dihedral angles between the basal plane and the individual plane passing through 3,5–di–tert–butylamidophenolate, salicylidene unit, and aminobenzylamine linker are shown in Table 4.2. The N1–Fe1–O2 bond angle was 160.81(12) [13]; 157.75(14) [14] and the O1–Fe1–N2 bond angle was 134.56(12) [13]; 133.97(13) [14], respectively, and deviated significantly from the linearity. Therefore, the coordination nature around Fe center was distorted square pyramidal (5 = 0.44 [13]; 0.40 [14], respectively).5e The Fe1–N1 bond distance was 2.050(3) [13];
2.037(3) [14] Å while, Fe1–N2 bond distance was 2.082(3) [13]; 2.076(3) [14] Å respectively.5,6,8 The Fe1–O1 and Fe1–O2 bond distances were 1.946(2) [13]; 1.936(3) [14]
Å and 1.897(3) [13]; 1.904(3) [14] Å, respectively.5,6,8 Above mentioned bond distances strongly suggested high spin (HS) and +III state of Fe atom in these complexes.5,6 All the C–
C bond distances in all C6–aryl rings were not same and were not falling in the range of 1.390.01 Å, as it would be in case of fully reduced phenyl form. Alternating short and long C–C bond distances pattern were observed in tert–butyl groups–containing amidophenolate unit in both 13, and 14. The bond distances at C1–C2, C2–C3, C3–C4, C4–C5, C5–C6, C6–
C1 were {1.443(5) [13]; 1.438(5) [14] Å}, {1.426(5) [13]; 1.439(5) [14] Å}, {1.369(5) [13];
1.373(6) [14] Å}, {1.433(5) [13]; 1.421(6) [14] Å}, {1.362(5) [13]; 1.354(5) [14] Å}, and {1.421(5) [13]; 1.426(5) [14] Å}, respectively.5 In addition, C1–N1 bond distance was 1.343(4) [13]; 1.337(4) [14] Å and C2–O1 bond distance was 1.303(4) [13]; 1.302(4) [14] Å.
Both the bond distances (C1–N1 and C2–O1) neither represented their single bond character nor corresponded with their double bond character, rather, in between. Hence, it could be argued that the coordinating amidophenolate unit existed in its oneelectron oxidized iminosemiquinonate form (ISQ1–). Furthermore, C–C bond distances in the salen unit were also not in the range of 1.39±0.01 Å, rather, a quinoid–type distortion was also found in the salen unit. The bond distances at C15–C16, C16–C17, C17–C18, C18–C19, C19–C20, C20–
C15 were {1.411(6) [13]; 1.421(5) [14] Å}, {1.367(7) [13]; 1.368(5) [14] Å}, {1.364(7) [13];
1.416(5) [14] Å}, {1.395(6) [13]; 1.388(6) [14] Å}, {1.385(6) [13]; 1.420(5) [14] Å}, and {1.412(6) [13]; 1.424(5) [14] Å}, respectively. Additionally, the shortening of C20–O2, and TH-1360_11612213
Page 143
C14–C15 bond distances and the elongation of N2–C14 bond distance were observed.
Herein, these features indicated the delocalization of phenolate1– charge over the O2–C20–
C15–C14–N2 unit (Table 4.2) and supported by previous reports14c–f where salen–phenolate unit has been found in their fully reduced form.
Table 4.2: Selected bond distances (Å) and bond angles () are shown for the complexes 13 and 14.
13 14
Fe1–N1 2.050(3) 2.037(3)
Fe1–N2 2.082(3) 2.076(3)
Fe1–O1 1.946(2) 1.936(3)
Fe1–O2 1.897(3) 1.904(3)
Fe1–Cl1 2.2232(12) 2.2129(15)
C2–O1 1.303(4) 1.302(4)
C1–N1 1.343(4) 1.337(4)
C1–C2 1.443(5) 1.438(5)
C2–C3 1.426(5) 1.439(5)
C3–C4 1.369(5) 1.373(6)
C4–C5 1.433(5) 1.421(6)
C5–C6 1.362(5) 1.354(5)
C6–C1 1.421(5) 1.426(5)
C20–O2 1.323(5) 1.320(4)
C14–N2 1.292(5) 1.272(5)
C14–C15 1.431(6) 1.446(5)
C15–C16 1.411(6) 1.421(5)
C16–C17 1.367(7) 1.368(5)
C17–C18 1.364(7) 1.416(5)
C18–C19 1.395(6) 1.388(6)
C19–C20 1.385(6) 1.420(5)
C20–C15 1.412(6) 1.424(5)
O1–Fe1–N1 78.30(10) 78.17(12)
N1–Fe1–N2 85.59(13) 86.43(12)
N2–Fe1–O2 88.60(13) 86.82(11)
O2–Fe1–O1 93.05(11) 91.50(12)
O1–Fe1–N2 134.56(12) 133.97(13)
N1–Fe1–O2 160.81(12) 157.75(14)
Cl1–Fe1–O1 113.36(8) 117.21(11)
Cl1–Fe1–N1 97.74(9) 97.88(10)
Cl1–Fe1–N2 110.73(9) 107.67(9)
Cl1–Fe1–O2 101.41(10) 104.36(10)
Fe1–O1–C2 117.7(2) 117.9(2)
Fe1–N1–C1 115.0(2) 115.7(2)
Fe1–N1–C7 120.9(2) 120.9(2)
Fe1–N2–C13 120.5(3) 120.7(2)
Fe1–N2–C14 121.2(3) 121.7(2)
Fe1–O2–C20 126.5(3) 125.2(2)
TH-1360_11612213
Page 144
Table 4.3: Comparative structural studies between complex 13 and complex 14.
Plane A: O1–N1–N2–O2
Plane B: O1–C2–C3–C4–C5–C6–C1–N1
Plane C: N1–C7–C8–C9–C10–C11–C12–C13–N2 Plane D: N2–C14–C15–C16–C17–C18–C19–C20–O2
Distance (Å) 13 14
Plane A and Fe1 0.5574(6) 0.5844(5)
Dihedral angle () 13 14
Plane A and Plane B –15.570(72) –21.131(67) Plane A and Plane C 27.721(75) 25.708(78) Plane A and Plane D 12.329(79) 16.042(75)
(–) indicated below the basal plane A; (+) indicated above the basal plane B
Figure 4.6: Mössbauer spectrum of 14 at 80 K and at zero–field.
TH-1360_11612213
Page 145
It has been observed that both complexes were having almost same molecular geometry; therefore, zero–field Mössbauer measurement only on solid sample of 14 was performed. The zero–field Mössbauer spectrum of solid sample 14 with its computer simulation is shown in Figure 4.6. Simulation to the experimental results provides the isomer shift value = 0.44 mms–1 and quadrupole splitting ΔEQ = 1.912 mms–1. The large value of isomer shift value and low quadruple splitting value are strongly suggested for the presence of high spin ferric ion in the neutral monoradical–coordinated complex.5a,f,g,15
Figure 4.7: µeff vs T plots for 14.
Temperature dependent magnetic susceptibility measurement on solid sample of 14 was examined in the temperature range of 2–290 K using SQUID magnetometer at external field 0.5 T. The µeff vs T plot for complex 14 is shown in Figure 4.7. From the µeff vs T plot, a constant magnetic moment µeff = 4.85 µB in temperature range 10–290 K was found. Below 10 K there was a stiff decrease in µeff value which was due to zero–field splitting and/or intermolecular antiferromagnetic interactions. The µeff value in 10–290 K temperature range was consistent with a strongly antiferromagnetically coupled high spin (HS) Fe(III) center (3d5 electronic configuration; S = 5/2) and a ligand center –radical (S = ½).5a,c,15b,c The experimental resulted was simulated using the following parameters; gFe(III) = 2.015, gR = 2.00, J 190 cm–1, D = 5.85 cm–1.
TH-1360_11612213
Page 146
Figure 4.8: UV–vis/NIR spectra for 13, and 14 are shown from 300 nm to 1500 nm range.
Electronic absorption spectra for 13, and 14 were recorded in CH2Cl2 at ambient temperature and are shown in Figure 4.8. Both complexes showed a broad absorption band at
max = 950 nm ( = 2250 M–1cm–1) which could be due to intervalence ligand(phenolate) –to–
ligand(iminosemiquinone) charge transfer. The intraligand charge transfer due to the presence of center –radical appeared at max = 730 nm ( = 6850 M–1cm–1) for 13, and max
= 690 nm ( = 8350 M–1cm–1) for complex 14.5c,f,16 Absorption band at max = 530 nm was attributed for the ligand(phenolate)–to–metal charge transfer (LMCT) ( = 5400 M–1cm–1 for 13, and = 6200 M–1cm–1 for 14).5c,f,16
TH-1360_11612213
Page 147