Synthesis, characterization, and formation constant of hexacoordinate iron(III) complexes

Article abstract of DOI:10.1080/00958970903302681

Twelve iron(III) complexes [Fe^III(L^X)₂]ClO₄, where (L^X)^- is the deprotonated form of a series of asymmetric ligands containing pyridine and substituted phenol moieties (XPh(OH)-CH=N-(CH₂)_n-Py that X = H, 5-Br, 5-NO₂, 5-OMe, 4-OMe, 3-OMe, and n = 1, 2), were synthesized and characterized by ^H NMR, IR, UV-Vis spectroscopy, mass spectrometry, and elemental analysis. Formation constants were measured using UV-Vis spectrophotometric titration at constant ionic strength (0.10 M NaClO₄) at 25 (±0.1)°C. The trend of the complex formation of Fe(III) ion with a given tridentate ligand decreases as follows: 5-OMe > 5-H > 5-NO₂ > 5-Br 5-OMe > 4-OMe > 5-OMe.

Full text of DOI:10.1080/00958970903302681

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Synthesis, characterization, and
formation constant of hexacoordinate
iron(III) complexes
Mozaffar Asadi a , Bahram Hemmateenejad a & Maryam
Mohammadikish a
a Department of Chemistry , College of Sciences, Shiraz
University , Shiraz 71454, I. R. Iran
Published online: 08 Oct 2009.
To cite this article: Mozaffar Asadi , Bahram Hemmateenejad & Maryam Mohammadikish (2010)
Synthesis, characterization, and formation constant of hexacoordinate iron(III) complexes, Journal
of Coordination Chemistry, 63:1, 124-135, DOI: 10.1080/00958970903302681
To link to this article: http://dx.doi.org/10.1080/00958970903302681
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Journal of Coordination Chemistry
Vol. 63, No. 1, 10 January 2010, 124–135
Synthesis, characterization, and formation constant of
hexacoordinate iron(III) complexes
MOZAFFAR ASADI*, BAHRAM HEMMATEENEJAD and
MARYAM MOHAMMADIKISH
Department of Chemistry, College of Sciences,
Shiraz University, Shiraz 71454, I. R. Iran
(Received 19 January 2009; in final form 10 July 2009)
Twelve iron(III) complexes [FeIII(LX)2]ClO4, where (LX)ꢀ is the deprotonated form of a series
of asymmetric ligands containing pyridine and substituted phenol moieties (XPh(OH)–CH¼N–
(CH2)n–Py that X ¼ H, 5-Br, 5-NO2, 5-OMe, 4-OMe, 3-OMe, and n ¼ 1, 2), were synthesized
and characterized by 1H NMR, IR, UV-Vis spectroscopy, mass spectrometry, and elemental
analysis. Formation constants were measured using UV-Vis spectrophotometric titration at
constant ionic strength (0.10 M NaClO4) at 25 (ꢁ0.1)ꢂC. The trend of the complex formation of
Fe(III) ion with a given tridentate ligand decreases as follows:
5-OMe 4 5-H 4 5-NO2 4 5-Br
5-OMe 4 4-OMe 4 3-OMe
Keywords: Iron; Schiff base; Formation constant; Tridentate ligand
1. Introduction
Imino Schiff bases and their complexes have extensive literature on their use as catalysts
in important industrial processes [1–6] and play a major role in coordination
chemistry [7]. Thermochromism and photochromism of the ligands [8], preparation,
properties, and structural aspects of some Mn [9–11], Fe [12], Co [13–15], Cu [14–17],
Ni [14, 15], V [18], and Ga [12] complexes with these Schiff bases have been carried out.
Comparatively little work is available on complexation and formation constants of
Fe(III) compounds. Tridentate ligands, N-{pyridine-2-ylmethyl}-2-hydroxy-benzylide-
neamine, N-{pyridine-2-ylethyl}-2-hydroxy-benzylideneamine, and their derivatives,
coordinate to Fe(III) forming bis-ligated complexes in the solid state [12].
In continuation of our previous work [19–27], we are interested in chelated complexes
of tridentate ligands and Fe(III). This article describes the synthesis, characterization,
and formation constants of 1 : 2 complexes of Fe3þ with XPh(OH)–CH¼N–(CH2)nPy,
that X ¼ H, 5-Br, 5-NO2, 5-OMe, 4-OMe, 3-OMe, and n ¼ 1, 2 (scheme 1) to
*Corresponding author. Email: asadi@susc.ac.ir
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958970903302681

Hexacoordinate iron(III)
125
R3
5
R3
5
R2
R2
R1
6
4
3
6
4
3
R1
OH
OH
N
N
3
4
N
3
4
N
6
6
5
5
R1 = R2 = R3 = H
HL1
HL2
HL3
HL4
HL5
HL6
R1 = R2 = R3 = H
HL7
HL8
HL9
HL10
HL11
R1 = R2 = H, R3 = Br
R1 = R2 = H, R3 = NO2
R1 = R2 = H, R3 = OMe
R1 = R3 = H, R2 = OMe
R2 = R3 = H, R1 = OMe
R1 = R2 = H, R3 = Br
R1 = R2 = H, R3 = NO2
R1 = R2 = H, R3 = OMe
R1 = R3 = H, R2 = OMe
R2 = R3 = H, R1 = OMe
HL12
Scheme 1. Schematic representation of the ligands and labels.
understand the role played by electron-donating and -withdrawing substituents, and the
coordination modes of the ligands on formation constants.
2. Experimental
2.1. Materials and physical measurements
All solvents and chemicals were purchased from Merck, Fluka or Aldrich and used
without purification.
UV-Vis spectra were recorded on a Perkin-Elmer Lambda 2 instrument. FT-IR
spectra were run on a Shimadzu FTIR-8300 spectrophotometer. 1H NMR spectra were
recorded on a Bruker Avance DPX-250 spectrometer in CDCl3 or DMSO-d6 using
TMS as an internal standard at 250 MHz. Mass spectra were obtained with Shimadzu
LCMS-2010EV and Perkin-Elmer R MU-6E instruments. Elemental analysis was
carried out on a Thermo Finnigan-Flash-1200. The effective magnetic moment was
measured using a Gouy balance.
2.2. Continuous variations and molar ratio methods
For continuous variation [28] (Job’s method), a series of solutions were prepared by
mixing the ligand (5 ꢃ 10ꢀ5 M) and metal (5 ꢃ 10ꢀ5 M) solutions in varying proportions
while keeping the total molar concentration of the mixed solutions constant.
For the molar ratio method [29], a series of solutions were prepared in which ligand
concentration was kept constant (5 ꢃ 10ꢀ5 M), while that of metal ion was varied
(2.5 ꢃ 10ꢀ6–1 ꢃ 10ꢀ4 M) for molar ratios [M(III)]/[ligand] ¼ 0–2. The spectra of all
solutions were recorded from 200 to 800 nm.

126
M. Asadi et al.
2.3. Equilibrium measurements
Complexes were obtained from the reaction of the metal with the ligands according to
equation (1).
þ
þ
2HL þ Fe3þ
½FeL2ꢄ þ 2H
ð1Þ
ꢀꢀꢀꢀꢀꢀ*
)ꢀꢀꢀꢀꢀꢀ
Formation constant measurements were carried out by spectrophotometric titrations of
ligands with various concentrations of the metal ion at constant ionic strength (0.10 M
NaClO4) at 25.0 (ꢁ0.1)ꢂC. Interaction of NaClO4 with the ligands and metal ion in
methanol was negligible. In a typical measurement, 3 mL of ligand (5 ꢃ 10ꢀ5 M) in
methanol was titrated with methanolic solution of Fe(NO3)3 ꢅ 9H2O. Metal concentra-
tions were varied till two-fold excess. UV-Vis spectra were recorded in the range
200–800 nm about 8 min after each addition. The absorption measurements were
monitored at various wavelengths in the 500–800 nm regions where the difference in
absorption between the ligands and the product was the largest after equilibrium was
attained; ligand and Fe(NO3)3 have negligible absorptivity.
2.4. Preparation of ligands
The tridentate Schiff bases were obtained by the condensation of 2-(aminomethyl)pyr-
idine or 2-(aminoethyl)pyridine with appropriate aldehyde: A solution of the amine
(10 mmol) in MeOH (15 mL) was added to a warm solution of the appropriate aldehyde
(10 mmol) in MeOH (25 mL) and the mixture was refluxed for 3 h. The solvent was
removed and the product was washed with Et2O (2 ꢃ 5 mL) (the oily product was used
without further purification).
HL1: Red oil (60%). Elemental Anal. Calcd for C13H12N2O (%): C 73.57, H 5.70, N
1
13.20; Found (%): C 73.62, H 5.81, N 13.40. H NMR (CDCl3): ꢀ ¼ 13.28 (br, 1H,
phOH), 8.53 (d, 4.1 Hz, 1H, py-H6), 8.47 (s, 1H, N¼CH), 7.62 (dt, 7.7 and 1.8 Hz, 1H,
py-H4), 7.28 (m, 3H, py-H3,5, ph-H6), 7.14 (dt, 7.0 and 1.5 Hz, 1H, ph-H4), 6.94
(d, 7.0 Hz, 1H, ph-H3), 6.85 (dt, 7.0 and 1.5 Hz, 1H, ph-H5), 4.88 (s, 2H, CH2) ppm.
FT-IR (cmꢀ1): 3375 (O–H), 3016 (C–H), 1628 (C¼N), 1589–1473 (C¼C), 1215 (C–O).
MS: m/z ¼ 212, 195, 149, 93, 69. UV-Vis (MeOH, 25ꢂC): ꢁmax (nm) ¼ 212
(" (Mꢀ1 cmꢀ1) ¼ 23,933), 261 (19,121), 318 (2690).
HL2: Yellow crystal (60%), m.p. ¼ 79ꢂC. Elemental Anal. Calcd for C13H11N2OBr (%):
1
C 53.63, H 3.81, N 9.62; Found (%): C 53.98, H 3.70, N 9.89. H NMR (CDCl3):
ꢀ ¼ 13.30 (s, 1H, phOH), 8.57 (d, 4.7 Hz, 1H, py-H6), 8.45 (s, 1H, N¼CH), 7.68 (dt, 7.6
and 1.5 Hz, 1H, py-H4), 7.40–7.18 (m, 4H, py-H3,5, ph-H4,6), 6.85 (d, 8.5 Hz, 1H, ph-
H3), 4.93 (s, 2H, CH2) ppm. FT-IR (KBr, cmꢀ1): 3433 (O–H), 3018 (C–H), 1628
(C¼N), 1585–1473 (C¼C), 1203 (C–O), MS: m/z ¼ 291, 275, 149, 93, 65. UV-Vis
(MeOH, 25ꢂC): ꢁmax (nm) ¼ 223 (" (Mꢀ1 cmꢀ1) ¼ 38,543), 260 (17,831), 329 (4363).
HL3: Yellow crystal (51%), m.p. ¼ 102ꢂC. Elemental Anal. Calcd for C13H11N3O3 (%):
1
C 60.70, H 4.31, N 16.33; Found (%): C 60.62, H 4.49, N 16.26. H NMR (CDCl3):
ꢀ ¼ 14.57 (s, 1H, phOH), 8.60 (d, 6.6 Hz, 1H, py-H6), 8.58 (s, 1H, N¼CH), 8.29 (d, 2.8 Hz,
1H, ph-H6), 8.20 (dd, 9.2 and 2.8 Hz, 1H, ph-H4), 7.72 (dt, 7.7 and 1.8 Hz, 1H, py-H4),
7.34 (d, 7.7 Hz, 1H, py-H3), 7.25 (m, 1H, py-H5), 7.99 (d, 9.2 Hz, 1H, ph-H3), 4.98 (s, 2H,
CH2) ppm. FT-IR (KBr, cmꢀ1): 3433 (O–H), 3066 (C–H), 1643 (C¼N), 1539–1423

Hexacoordinate iron(III)
127
(C¼C), 1261 (C–O). MS: m/z ¼ 257, 149, 97, 73, 57. UV-Vis (MeOH, 25ꢂC): ꢁmax
(nm) ¼ 235 (" (Mꢀ1 cmꢀ1) ¼ 18,397), 260 (24,689), 330 (11,522), 392 (8680).
HL4: Red oil (63%). Elemental Anal. Calcd for C14H14N2O2 (%): C 69.41, H 5.82,
1
N 11.56; Found (%): C 69.35, H 5.90, N 11.66. H NMR (CDCl3): ꢀ ¼ 12.39 (s, 1H,
phOH), 8.36 (d, 6.0 Hz, 1H, py-H6), 8.26 (s, 1H, N¼CH), 7.45 (dt, 7.7 and 1.7 Hz, 1H,
py-H4), 7.14 (d, 7.7 Hz, 1H, py-H3), 6.98 (dd, 6.2 and 2.1 Hz, 1H, ph-H4), 6.75 (m, 2H,
py-H5 and ph-H6), 6.61 (d, 6.2 Hz, 1H, ph-H3), 4.72 (s, 2H, CH2), 3.51 (s, 3H, OCH3)
ppm. FT-IR (cmꢀ1): 3318 (O–H), 3016 (C–H), 1635 (C¼N), 1589–1488 (C¼C), 1218
(C–O). MS: m/z ¼ 242, 225, 144, 93, 57. UV-Vis (MeOH, 25ꢂC): ꢁmax (nm) ¼ 227
(" (Mꢀ1 cmꢀ1) ¼ 15,290), 261 (8273), 299 (2390), 346 (2250).
HL5: Yellow crystal (40%), m.p. (67ꢂC). Elemental Anal. Calcd for C14H14N2O2 (%):
1
C 69.41, H 5.82, N 11.56; Found (%): C 69.21, H 6.16, N 11.26. H NMR (CDCl3):
ꢀ ¼ 13.79 (s, 1H, phOH), 8.57 (d, 4.4 Hz, 1H, py-H6), 8.40 (s, 1H, N¼CH), 7.68 (dt, 7.7
and 1.6 Hz, 1H, py-H4), 7.34 (d, 7.7 Hz, 1H, py-H3), 7.19 (d, 8.2 Hz, 1H, ph-H5), 7.16
(d, 8.2 Hz, 1H, ph-H6), 6.44 (s, 1H, ph-H3), 6.41 (m, 1H, py-H5), 4.88 (s, 2H, CH2), 3.82
(s, 3H, OCH3) ppm. FT-IR (KBr, cmꢀ1): 3452 (O–H), 3025 (C–H), 1647 (C¼N), 1519–
1423 (C¼C), 1223 (C–O). MS: m/z ¼ 242, 137, 93, 65. UV-Vis (MeOH, 25ꢂC): ꢁmax
(nm) ¼ 217 (" (Mꢀ1 cmꢀ1) ¼ 26,945), 279 (19,370), 301 (21,437), 379 (7350).
HL6: Yellow crystal (71%), m.p. ¼ 101ꢂC. Elemental Aanal. Calcd for C14H14N2O2
1
(%): C 69.41, H 5.82, N 11.56; Found (%): C 69.73, H 5.76, N 11.82. H NMR
(CDCl3): ꢀ ¼ 13.77 (s, 1H, phOH), 8.55 (d, 4.9 Hz, 1H, py-H6), 8.53 (s, 1H, N¼CH),
7.66 (dt, 7.7 and 1.7 Hz, 1H, py-H4), 7.37 (d, 7.7 Hz, 1H, py-H3), 7.26 (d, not resolved,
1H, ph-H6), 7.18 (dd, 6.3 Hz, not resolved, 1H, ph-H4), 6.93 (dd, 6.3 and 1.9 Hz, 1H,
ph-H5), 6.82 (dd, 7.7 Hz, not resolved, 1H, py-H5), 4.95 (s, 2H, CH2), 3.87 (s, 3H,
OCH3) ppm. FT–IR (KBr, cmꢀ1): 3414 (O–H), 3009 (C–H), 1635 (C¼N), 1589–1466
(C¼C), 1254 (C–O). MS: m/z ¼ 242, 167, 149, 93, 57. UV-Vis (MeOH, 25ꢂC): ꢁmax
(nm) ¼ 221 (" (Mꢀ1 cmꢀ1) ¼ 21,614), 262 (14,758), 325 (2172).
HL7: Red oil (55%). Elemental Anal. Calcd for C14H14N2O (%): C 74.31, H 6.24, N
1
12.38; Found (%): C 74.23, H 6.36, N 12.19. H NMR (CDCl3): ꢀ ¼ 13.11 (br, 1H,
phOH), 8.56 (d, 4.8 Hz, 1H, py-H6), 8.28 (s, 1H, N¼CH), 7.59 (dt, 7.6 and 1.2 Hz, 1H,
py-H4), 7.30–7.10 (m, 4H, py-H3,5, ph-H4,6), 6.90 (d, 8.3 Hz, 1H, ph-H3), 6.84 (t, 7.4 Hz,
1H, ph-H5), 4.02 (t, 6.9 Hz, 2H, CH2N), 3.17 (t, 6.9 Hz, 2H, CH2) ppm. FT-IR (cmꢀ1):
3394 (O–H), 3009 (C–H), 1628 (C¼N), 1589–1496 (C¼C), 1211 (C–O). MS: m/z ¼ 226,
106, 51. UV-Vis (MeOH, 25ꢂC): ꢁmax (nm) ¼ 214 (" (Mꢀ1 cmꢀ1) ¼ 27,812), 260 (24,447),
319 (4574).
HL8: Yellow crystal (52%), m.p. (72ꢂC). Elemental Anal. Calcd for C14H13N2OBr (%):
1
C 55.10, H 4.29, N 9.18; Found (%): C 55.34, H 4.12, N 9.23. H NMR (CDCl3):
ꢀ ¼ 13.37 (s, 1H, phOH), 8.56 (d, 2.9 Hz, 1H, py-H6), 8.18 (s, 1H, N¼CH), 7.59–6.78 (m,
6H, py-H3,4,5, ph-H3,4,6), 4.03 (t, 6.8 Hz, 2H, CH2N), 3.15 (t, 6.8 Hz, 2H, CH2) ppm.
FT-IR (KBr, cmꢀ1): 3425 (O–H), 3023 (C–H), 1635 (C¼N), 1589–1473 (C¼C), 1280
(C–O). MS: m/z ¼ 305, 289, 225, 185, 149, 106, 51. UV-Vis (MeOH, 25ꢂC): ꢁmax
(nm) ¼ 222 (" (Mꢀ1 cmꢀ1) ¼ 43,617), 254 (16,418), 324 (4028).
HL9: Yellow crystal (86%), m.p. (158ꢂC). Elemental Anal. Calcd for C14H13N3O3 (%):
C 61.99, H 4.83, N 15.49; Found (%): C 62.23, H 4.72, N 15.59. 1H NMR (DMSO-d6):
ꢀ ¼ 14.17 (s, 1H, phOH), 8.70 (s, 1H, N¼CH), 8.52 (d, 4.8 Hz, 1H, py-H6), 8.35

128
M. Asadi et al.
(d, 3.1 Hz, 1H, ph-H6), 8.00 (dd, 9.6 and 3.1 Hz, 1H, ph-H4), 7.72 (dt, 7.7 and 1.8 Hz,
1H, py-H4), 7.30 (d, 7.7 Hz, 1H, py-H3), 7.24 (m, 1H, py-H5), 6.56 (d, 9.6 Hz, 1H,
ph-H3), 4.06 (t, 6.7 Hz, 2H, CH2N), 3.19 (t, 6.7 Hz, 2H, CH2) ppm. FT–IR (KBr, cmꢀ1):
3440 (O–H), 3047 (C–H), 1666 (C¼N), 1535–1435 (C¼C), 1218 (C–O). MS: m/z ¼ 271,
255, 106, 51. UV-Vis (MeOH, 25ꢂC): ꢁmax (nm) ¼ 235 (" (Mꢀ1 cmꢀ1) ¼ 17,398), 260
(29,925), 345 (14,023), 393 (12,396).
HL10: Red oil (59%). Elemental Anal. Calcd for C15H16N2O2 (%): C 70.29, H 6.29, N
1
10.93; Found (%): C 70.38, H 6.18, N 11.07. H NMR (CDCl3): ꢀ ¼ 12.36 (s, 1H,
phOH), 8.46 (d, 4.4 Hz, 1H, py-H6), 8.14 (s, 1H, N¼CH), 7.48 (m, 1H, py-H4), 7.04 (m,
2H, py-H3,5), 6.81 (m, 2H, ph-H4,6), 6.61 (d, 2.5 Hz, 1H, ph-H3), 3.91 (t, 6.5 Hz, 2H,
CH2N), 3.63 (s, 3H, OCH3), 3.06 (t, 6.5 Hz, 2H, CH2) ppm. FT–IR (cmꢀ1): 3394 (O–H),
3016 (C–H), 1635 (C¼N), 1589–1497 (C¼C), 1265 (C–O). MS: m/z ¼ 265, 151, 106, 51.
UV-Vis (MeOH, 25ꢂC): ꢁmax (nm) ¼ 229 (" (Mꢀ1 cmꢀ1) ¼ 58,496), 260 (28,624),
343 (10,367).
HL11: Yellow crystal (58%), m.p. (52ꢂC). Elemental Anal. Calcd for C15H16N2O2 (%):
1
C 70.29, H 6.29, N 10.93; Found (%): C 70.13, H 6.35, N 11.12. H NMR (CDCl3):
ꢀ ¼ 13.69 (s, 1H, phOH), 8.44 (d, 4.4 Hz, 1H, py-H6), 7.95 (s, 1H, N¼CH), 7.44 (dt, 7.6
and 1.5 Hz, 1H, py-H4), 7.02 (d, 7.6 Hz, 1H, py-H3), 6.98 (m, 1H, py-H5), 6.89
(d, 8.5 Hz, 1H, ph-H6), 6.27 (d, 2.2 Hz, 1H, ph-H3), 6.20 (dd, 8.5 and 2.2 Hz, 1H, ph-
H5), 3.81 (t, 6.9 Hz, 2H, CH2N), 3.62 (s, 3H, OCH3), 3.01 (t, 6.9 Hz, 2H, CH2) ppm.
FT–IR (cmꢀ1): 3433 (O–H), 3018 (C–H), 1623 (C¼N), 1508–1465 (C¼C), 1223 (C–O).
MS: m/z ¼ 256, 151, 106, 51. UV-Vis (MeOH, 25ꢂC): ꢁmax (nm) ¼ 218
(" (Mꢀ1 cmꢀ1) ¼ 25,064), 251 (15,790), 297 (21,620), 378 (9620).
HL12: Red oil (75%). Elemental Anal. Calcd for C15H16N2O2 (%): C 70.29, H 6.29, N
1
10.93; Found (%): C 70.22, H 6.41, N 10.78. H NMR (CDCl3): C¸ ¼ 13.70 (s, 1H,
phOH), 8.46 (d, 4.6 Hz, 1H, py-H6), 8.17 (s, 1H, N¼CH), 7.49 (dt, 7.6 and 1.7 Hz, 1H,
py-H4), 7.07 (m, 2H, py-H3,5), 6.80 (dd, 7.2 and 2.3 Hz, 1H, ph-H4), 6.71 (m, 2H, ph-
H5,6), 3.94 (t, 6.7 Hz, 2H, CH2N), 3.79 (s, 3H, OCH3), 3.08 (t, 6.7 Hz, 2H, CH2) ppm.
FT–IR (cmꢀ1): 3414 (O–H), 3009 (C–H), 1635 (C¼N), 1589–1466 (C¼C), 1254 (C–O).
MS: m/z ¼ 256, 137, 106, 57. UV-Vis (MeOH, 25ꢂC): ꢁmax (nm) ¼ 220
(" (Mꢀ1 cmꢀ1) ¼ 15,333), 261 (9758), 293 (4114), 334 (1523).
2.5. Preparation of [FeIII(LX)2]ClO4 complexes
A general synthetic route was used for X ¼ 1–12 in which a solution of Fe(NO3)3 ꢅ 9H2O
(1.0 mmol) in MeOH (10 mL) was added to a 30 mL MeOH solution containing the
appropriate ligand (2.0 mmol) and Et3N (2.0 mmol). The resulting dark solution was
refluxed for 2 h, the mixture was filtered while warm and excess NaClO4 was added to
exchange nitrate by perchlorate. After 24 h, dark microcrystalline precipitates were
filtered, washed with H2O, cold MeOH, and Et2O.
Caution: Although no difficulties were experienced, all complexes were isolated as their
perchlorate salts, and therefore, they should be handled as potentially explosive
compounds.
[Fe(L1)2]ClO4 MeOH. Yield: 45%. m.p. 4250 C. Elemental Anal. Calcd for
ꢂ
.
C27H26N4O7ClFe (%): C 53.18, H 4.30, N 9.19; Found (%): C 52.95, H 4.08,

Hexacoordinate iron(III)
129
N 8.91. FT-IR (KBr, cmꢀ1): 1616 (C¼N), 1539–1442 (C¼C), 1296 (C–O), 1095
(O–ClO3). MS (ESI(þ) in MeOH : H2O): m/z 478 for [Fe(L1)2]þ. UV-Vis (MeOH,
25ꢂC): ꢁmax (nm) ¼ 233 (" (Mꢀ1 cmꢀ1) ¼ 43,062), 261 (32,771), 323 (12,640), 510 (3443).
[Fe(L2)2]ClO4. Yield: 44%. m.p. ¼ 246ꢂC. Elemental Anal. Calcd for
C26H20N4O6Br2ClFe (%): C 42.45, H 2.74, N 7.62; Found (%): C 42.51, H 2.77, N
7.68. FT-IR (KBr, cmꢀ1): 1620 (C¼N), 1524–1454 (C¼C), 1280 (C–O), 1095 (O–ClO3).
MS (ESI(þ) in MeOH : H2O): m/z 636 for [Fe(L2)2]þ. UV-Vis (MeOH, 25ꢂC): ꢁmax
(nm) ¼ 218 (" (Mꢀ1 cmꢀ1) ¼ 49,452), 234 (47,463), 349 (9258), 525 (3138).
[Fe(L3)2]ClO4. Yield: 32%. m.p. 4 250ꢂC. Elemental Anal. Calcd for
C26H20N6O10ClFe (%): C 46.76, H 3.02, N 12.59; Found (%): C 46.98, H 3.35, N
12.33. FT-IR (KBr, cmꢀ1): 1624 (C¼N), 1547–1469 (C¼C), 1308 (C–O), 1095 (O–
ClO3). MS (ESI(þ) in MeOH : H2O): m/z 568 for [Fe(L3)2]þ. UV-Vis (MeOH, 25ꢂC):
ꢁmax (nm) ¼ 220 (" (Mꢀ1 cmꢀ1) ¼ 58,967), 241 (51,385), 340 (39,888), 520 (4010).
[Fe(L4)2]ClO4 2H2O. Yield: 38%. m.p. ¼ 180ꢂC. Elemental Anal. Calcd for
.
C28H30N4O10ClFe (%): C 50.21, H 4.17, N 8.39; Found (%): C 50.36, H 4.25, N
8.61. FT-IR (KBr, cmꢀ1): 1608 (C¼N), 1539–1466 (C¼C), 1288 (C–O), 1095 (O–ClO3).
MS (ESI(þ) in MeOH : H2O): m/z 538 for [Fe(L4)2]þ. UV-Vis (MeOH, 25ꢂC): ꢁmax
(nm) ¼ 214 (" (Mꢀ1 cmꢀ1) ¼ 94,580), 242 (89,534), 382 (16,610), 575 (7067).
[Fe(L5)2]ClO4 Et2O. Yield: 35%. m.p. ¼ 141ꢂC. Elemental Anal. Calcd for
.
C32H36N4O9ClFe (%): C 53.70, H 5.10, N 7.83; Found (%): C 53.67, H 4.99, N
7.56. FT-IR (KBr, cmꢀ1): 1601 (C¼N), 1527–1404 (C¼C), 1223 (C–O), 1095 (O–ClO3).
MS (ESI(þ) in MeOH : H2O): m/z 538 for [Fe(L5)2]þ. UV-Vis (MeOH, 25ꢂC): ꢁmax
(nm) ¼ 245 (" (Mꢀ1 cmꢀ1) ¼ 41,415), 283 (43,543), 343 (14,300), 525 (3935).
[Fe(L6)2]ClO4. Yield: 46%. m.p. 4 250ꢂC. Elemental Anal. Calcd for C28H26N4O8ClFe
(%): C 53.00, H 4.32, N 8.58; Found (%): C 53.00, H 4.65, N 8.78. FT-IR (KBr, cmꢀ1):
1608 (C¼N), 1551–1435 (C¼C), 1223 (C–O), 1095 (O–ClO3). MS (ESI(þ) in
MeOH : H2O): m/z 538 for [Fe(L6)2]þ. UV-Vis (MeOH, 25ꢂC): ꢁmax (nm) ¼ 226
(" (Mꢀ1 cmꢀ1) ¼ 58,431), 273 (39,254), 350 (11,390), 550 (3910).
[Fe(L7)2]ClO4 H2O. Yield: 48%. m.p. ¼ 227ꢂC. Elemental Anal. Calcd for
.
C28H28N4O7ClFe (%): C 53.91, H 4.52, N 8.98; Found (%): C 53.67, H 4.31, N
8.83. FT-IR (KBr, cmꢀ1): 1608 (C¼N), 1542–1446 (C¼C), 1299 (C–O), 1095 (O–ClO3).
MS (ESI(þ) in MeOH : H2O): m/z 506 for [Fe(L7)2]þ. UV-Vis (MeOH, 25ꢂC): ꢁmax
(nm) ¼ 210 (" (Mꢀ1 cmꢀ1) ¼ 53,524ꢀ1), 234 (45,546), 260 (37,180), 324 (11,341),
551 (3549).
[Fe(L8)2]ClO4 H2O. Yield: 64%. m.p. ¼ 236ꢂC. Elemental Anal. Calcd for
.
C28H26N4O6Br2ClFe (%): C 43.03, H 3.35, N 7.17; Found (%): C 43.27, H 3.24, N
7.02. FT-IR (KBr, cmꢀ1): 1608 (C¼N), 1531–1454 (C¼C), 1384 (C–O), 1095 (O–ClO3).
MS (ESI(þ) in MeOH : H2O): m/z 664 for [Fe(L8)2]þ. UV-Vis (MeOH, 25ꢂC): ꢁmax
(nm) ¼ 221 (" (Mꢀ1 cmꢀ1) ¼ 77,273), 330 (12,511), 543 (3160).
[Fe(L9)2]ClO4 MeOH. Yield: 58%. m.p. ¼ 244ꢂC. Elemental Anal. Calcd for
.
C29H28N6O11ClFe (%): C 47.85, H 3.88, N 11.55; Found (%): C 47.56, H 3.82, N
11.83. FT-IR (KBr, cmꢀ1): 1605(C¼N), 1558–1466 (C¼C), 1315 (C–O), 1095 (O–ClO3).
MS (ESI(þ) in MeOH : H2O): m/z 596 for [Fe(L9)2]þ. UV-Vis (MeOH, 25ꢂC): ꢁmax
(nm) ¼ 227 (" (Mꢀ1 cmꢀ1) ¼ 50,915), 244 (54,593), 343 (36,552), 480 (3728).

130
M. Asadi et al.
[Fe(L10)2]ClO4. Yield: 69%. m.p. ¼ 224ꢂC. Elemental Anal. Calcd for C30H30N4O8ClFe
(%): C 54.11, H 4.54, N 8.41; Found: C 54.21, H 4.72, N 8.75. FT-IR (KBr, cmꢀ1): 1605
(C¼N), 1546–1446 (C¼C), 1249 (C–O), 1095 (O–ClO3). MS (ESI(þ) in MeOH : H2O):
m/z 566 for [Fe(L10)2]þ. UV-Vis (MeOH, 25ꢂC): ꢁmax (nm) ¼ 210 (" (Mꢀ1 cmꢀ1) ¼
53,365), 241 (60,235), 348 (12,822), 631 (4942).
[Fe(L11)2]ClO4. Yield: 38%. m.p. ¼ 165ꢂC. Elemental Anal. Calcd for C30H30N4O8ClFe
(%): C 54.11, H 4.54, N 8.41; Found (%): C 53.85, H 4.65, N 8.37. FT-IR (KBr, cmꢀ1):
1597 (C¼N), 1531–1443 (C¼C), 1230 (C–O), 1095 (O–ClO3). MS (ESI(þ) in
MeOH : H2O): m/z 566 for [Fe(L11)2]þ. UV-Vis (MeOH, 25ꢂC): ꢁmax (nm) ¼ 246
(" (Mꢀ1 cmꢀ1) ¼ 63,858), 281 (64,638), 380 (11,159), 560 (5289).
[Fe(L12)2]ClO4 H2O. Yield: 81%. m.p. ¼ 233ꢂC. Elemental Anal. Calcd for
.
C30H32N4O9ClFe (%): C 52.69, H 4.72, N 8.20; Found (%): C 52.73, H 4.59, N
8.59. FT-IR (KBr, cmꢀ1): 1605 (C¼N), 1547–1446 (C¼C), 1249 (C–O), 1095 (O–ClO3).
MS (ESI(þ) in MeOH : H2O): m/z 566 for [Fe(L12)2]þ. UV-Vis (MeOH, 25ꢂC): ꢁmax
(nm) ¼ 230 (" (Mꢀ1 cmꢀ1) ¼ 47,320), 267 (49,764), 350 (7194), 600 (2895).
3. Results and discussion
3.1. Synthesis and characterization
Reaction of 2-(2-aminomethyl)pyridine or 2-(2-aminoethyl)pyridine with one molar
equivalent of substituted salicylaldehyde in refluxing MeOH afforded a yellow solution.
Evaporation of this solution to dryness yielded ligands in good yields and are
characterized by 1H NMR, FTIR, UV-Vis spectroscopy, mass spectrometry, and
elemental analysis.
Complexes were synthesized by treating the respective ligand with iron(III) nitrate
nonahydrate in dry methanol, followed by counterion metathesis with sodium
perchlorate (equations (2) and (3)).
MeOH
2HL þ FeðNO3Þ3 ꢅ 9H2O þ 2Et3N ꢀꢀꢀꢀ! ½FeðLÞ2ꢄNO3 þ 2ðEt3NHÞNO3 þ 9H2O
ð2Þ
½FeðLÞ2ꢄNO3 þ NaClO4ꢀꢀꢀꢀ! ½FeðLÞ2ꢄClO4 þ NaNO3
ð3Þ
The elemental analyses are in agreement with the calculated values for 1 : 2 metal to
ligand ratio.
Based on the crystal structures of related complexes [12, 30], one may conclude that
these reactions yielded pseudo-octahedral mononuclear species in which iron is
surrounded by two tridentate ligands. Imbert et al. [12] showed that iron(III) is
positioned in a distorted octahedral N2N’2O2 environment with high spin 3d5
configuration.
3.2. Infrared spectroscopy, mass spectrometry
Weak bands at 2800–3100 cmꢀ1 are related to (C–H) vibrations. The imine ligands show
a C¼N peak around 1630 cmꢀ1 that shifts to lower frequencies upon coordination,

Hexacoordinate iron(III)
131
indicating nitrogen of azomethine coordinates to iron. The C¼C bond was at 1450–
1600 cmꢀ1 and perchlorate counterions were found between 1116 and 1088 cmꢀ1
The ESI mass spectrometry in positive mode in 1 : 1 MeOH/H2O gave single, well-
.
defined peaks corresponding to m/z ¼ [MIII(L)2]þ for all complexes. A typical mass
spectrum of Fe(L11)2 (Mw ¼ 566) is provided in ‘‘Supplementary material’’.
þ
3.3. Electronic absorption spectra, magnetic moment, and stoichiometry of the
complexes formed in solution
The electronic spectra of the ligands and complexes were measured in methanol to
compare differences between the ligands and complexes. Figure 1 shows spectra for
comparisons. All ligands show interligand ꢂ ! ꢂ* bands around 220 and 260 nm
attributed to pyridine and phenolate moieties. Another band attributed to the n ! ꢂ*
groups is observed around 330 nm.
The electronic spectra of complexes show two absorptions around 340 and 600 nm.
Transitions pꢂphenolate ! dꢂ*iron(III) are associated to the band at 600 nm, whereas the
higher-energy absorption at 340 nm is attributed to a pꢂphenolate ! dꢃ*iron(III) LMCT
process [12].
Magnetic moment values, ꢄeff, were measured for 12 solid complexes by Gouy
balance (Supplementary material). The ꢄeff values (5.9 B.M.) indicate that the Fe3þ ion
is high spin [12], ruling out d–d transitions in the visible spectra.
To determine the stoichiometry of the different complexes formed in solution from
the interaction of Fe(III) with HL1–HL12, two different spectrophotometric methods,
continuous variation (CV) [28], and molar ratio (MR) of Yoe and Jones [29] have been
applied.
Changes in the absorbance spectra of HL11 at different molar ratio of the Fe(III)
added in methanol (molar ratio experiment) are shown in figure 2. The presence of two
isosbestic points suggests two equilibria with 1 : 1 and 1 : 2 species.
The resulting molar ratio and continuous variation plots for this system are shown in
figure 3. The relationship obtained for the MR method are characterized by one break
at molar ratio [M]/[L] equals to 0.5, indicating that the stoichiometry of the complexes
100,000
80,000
60,000
40,000
20,000
0
200
300
400
500
600
700
λ (nm)
Figure 1. UV-Vis spectra of HL10(––) (5.0 ꢃ 10ꢀ5 M), Fe(L10)2ClO4 separately synthesized (– – –)
(2.0 ꢃ 10ꢀ5 M), and the product at the end of titration (. . .) (M : L ¼ 1 : 2).

132
M. Asadi et al.
1
0.8
0.6
0.4
0.2
0
300
400
500
600
700
λ (nm)
Figure 2. Spectrophotometric titration of HL11 (5.0 ꢃ 10ꢀ5 M) with various amounts of Fe(NO3)3 ꢅ 9H2O
(2.5 ꢃ 10ꢀ6–1 ꢃ 10ꢀ4 M) in methanol (I ¼ 0.10 M at 25ꢂC).
(a)
1
0.8
0.6
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
[M] / [L]
0.6
0.5
0.4
0.3
0.2
0.1
0
(b)
0
0.2
0.4
0.6
0.8
1
1.2
[M] / [M] + [L]
Figure 3. Molar ratio (L ¼ 5.0 ꢃ 10ꢀ5 M and M ¼ 2.5 ꢃ 10ꢀ6–1 ꢃ 10ꢀ4 M) (a) and continuous variation
(L ¼ 5.0 ꢃ 10ꢀ5 M and M ¼ 5.0 ꢃ 10ꢀ5 M that vary proportions); (b) plots of HL11 with Fe(NO3)3 ꢅ 9H2O in
methanol at 25ꢂC at ꢁ ¼ 400 nm.

Hexacoordinate iron(III)
133
is 1 : 2 (M : L). Results of the CV method support the results of the MR method. Curves
plotted for CV method are characterized by one clear maximum at metal mole fraction
equal to ꢆ0.33, for most cases, or by a shoulder accompanied by a maximum located
also at the same metal mole fraction value, i.e. 0.33 and/or 0.5. This behavior clearly
indicates that the stoichiometry of the different complexes formed in the solution from
the reaction of the Fe3+ ions with ligands (HL1ꢀHL12) is 1 : 2 and/or 1 : 1 (M : L).
3.4. Evaluation of the formation constants and Gibbs free energy of the different
metal chelates
The data obtained from the molar ratio spectrophotometric method, used to establish
the stoichiometry of the metal complexes, were also utilized for the determination of the
formation constants.
The formation constants, K1 and K2, were calculated using the SQUAD computer
program [31, 32], designed to calculate the best value for the formation constants of the
proposed model (equation (1)) by employing a non-linear, least-squares approach.
Also, the free energy changes DG0 of the formed complexes were calculated from
DG0 ¼ –RT ln K at 25ꢂC.
The K1 and K2 values and Gibbs free energies obtained for the metal complexes under
investigation are given in table 1. These data show that there was more difference
between K1 and K2 values. By considering concentration distribution curves for Fe(III)
species in solution, it was clear that the formation of mono complexes would be
negligible due to the mono complexes converting to bis complexes immediately. The bis
complexes with two tridentate chelating ligands have octahedral geometry preferred
over the mono complex with one ligand and three monodentate solvent molecules.
The log K values for Fe(II) and Fe(III) complexes, carried out by other workers, show
vast differences due to the kind and number of ligands [33–37]. According to the
electronic and steric effects, K1 must be larger than K2 [33, 34], but sometimes K2 can be
larger than K1 because of the chelating effect, structural reason, and so on [35, 36].
Our formation constants and Gibbs free energies follow the sequence:
5-OMe 4 5-H 4 5-NO2 4 5-Br
Table 1. Formation constants (log K1 and log K2) and Gibbs free energy (DG10 and DG20) for the complexes
of tridentate ligands with Fe(III) in methanol (I ¼ 0.10 M at 25ꢂC).
log K1
log K2
DG01 (kJ molꢀ1
)
DG02 (kJ molꢀ1
)
HL1
HL2
HL3
HL4
HL5
HL6
HL7
HL8
HL9
HL10
HL11
HL12
3.81 (ꢁ0.04)
3.15 (ꢁ0.09)
3.47 (ꢁ0.07)
4.02 (ꢁ0.09)
3.83 (ꢁ0.05)
3.61 (ꢁ0.02)
3.91 (ꢁ0.08)
3.33 (ꢁ0.05)
3.54 (ꢁ0.09)
4.11 (ꢁ0.07)
3.99 (ꢁ0.08)
3.78 (ꢁ0.07)
9.62 (ꢁ0.07)
9.15 (ꢁ0.05)
9.47 (ꢁ0.06)
9.72 (ꢁ0.08)
9.43 (ꢁ0.06)
9.31 (ꢁ0.08)
9.77 (ꢁ0.09)
9.45 (ꢁ0.06)
9.60 (ꢁ0.07)
10.09 (ꢁ0.09)
9.8 (ꢁ0.1)
ꢀ21.7 (ꢁ0.1)
ꢀ18.0 (ꢁ0.2)
ꢀ19.8 (ꢁ0.2)
ꢀ22.9 (ꢁ0.2)
ꢀ21.8 (ꢁ0.1)
ꢀ54.9 (ꢁ0.2)
ꢀ52.2 (ꢁ0.1)
ꢀ54.0 (ꢁ0.1)
ꢀ55.4 (ꢁ0.2)
ꢀ53.8 (ꢁ0.1)
ꢀ53.1 (ꢁ0.2)
ꢀ55.7 (ꢁ0.2)
ꢀ53.9 (ꢁ0.1)
ꢀ54.7 (ꢁ0.2)
ꢀ57.6 (ꢁ0.2)
ꢀ56.0 (ꢁ0.2)
ꢀ55.2 (ꢁ0.2)
ꢀ20.59 (ꢁ0.05)
ꢀ22.3 (ꢁ0.2)
ꢀ19.0 (ꢁ0.1)
ꢀ20.2 (ꢁ0.2)
ꢀ23.4 (ꢁ0.2)
ꢀ22.7 (ꢁ0.2)
ꢀ21.6 (ꢁ0.2)
9.68 (ꢁ0.08)

134
M. Asadi et al.
In the para substituted Schiff-base ligands, the stability constants and Gibbs free
energies vary as expected from electronic effects of the substituents decreasing
according to the sequence OCH3 4 H 4 Br 4 NO2, i.e., in order of an increase in
both electron-withdrawing and ꢂ-acceptor qualities of the substituents and the decrease
in donor ability of the ligand groups (mainly the phenoxy groups). This is somewhat in
agreement with the cited sequence, except for 5-NO2 that was attributed to the
hydrogen bonding between NO2 group and N¼CH.
The log K values (table 1) show that electron-donating groups (such as OMe) in the
para position to OH of the Schiff-base complex bind more strongly with Fe(III).
Conversely, electron-withdrawing groups such as NO2 bind more weakly. This behavior
confirms the results obtained in the recent work [38, 39] related to adducts between
different substituted Schiff-base complexes of Ni(II) and Cu(II) with organotin(IV)
dichloride. Also, our previous work on thermodynamics of different types of Co(III)
Schiff-base complexes as acceptor with donors such as phosphites [20, 24, 40] and
amines [21, 26] agrees with our present work on Fe(III) Schiff-base complexes.
To study the effect of the position of substituents on the formation of the complexes,
3-OMe, 4-OMe, and 5-OMe were selected. The results show the following trend in the
complex formation between iron and the tridentate ligand
5-OMe 4 4-OMe 4 3-OMe
related to the steric effects of substituted ligands.
4. Conclusion
Our results confirm the results of the recently published work. The stability of the
bidentate and tridentate Schiff-base complexes depends on the metal, the type of Schiff
base, and the substituent groups. Some complexes play efficient catalytic roles in
organic reactions, specially those of coordinated Fe(III) complexes, and some with
heterocyclic compounds are essential to many biochemical processes.
Heterocyclic tridentate ligands (NNO) with different substituents coordinate with
Fe(III) cations mainly by 2 : 1 mole ratio. Variations of stability constant of the
complexes formed verify the electronic and the position of the substituents.
The free energy of formation of the 12 iron Schiff-base complexes show the electronic
effect of the substituent groups, i.e.,
5-OMe 4 5-H 4 5-NO2 4 5-Br
The position of the methoxy substituent can also affect the K and the DG0 values and
the following trend was observed:
5-OMe 4 4-OMe 4 3-OMe
Acknowledgement
We are grateful to Shiraz University Research Council for their financial support.

Hexacoordinate iron(III)
135
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