Mononuclear iron(III)macrocyclic complexes derived from 4-methyl-2,6-di(formyl/benzoyl)phenol and diamines: Synthesis, spectral speciation and electrochemical behaviour

Article abstract of DOI:10.1016/S0277-5387(01)00786-0

Two series of macrocyclic iron(III) complexes of stoichiometry [Fe(L)Cl₂]Cl (1, 2) have been synthesised and characterised. Compounds belonging to series 1 are derived from 4-methyl-2,6-diformylphenol and diamines (H₂L), and those of

Full text of DOI:10.1016/S0277-5387(01)00786-0

Polyhedron 20 (2001) 2019–2025
www.elsevier.com/locate/poly
Mononuclear iron(III)macrocyclic complexes derived from
4-methyl-2,6-di(formyl/benzoyl)phenol and diamines: synthesis,
spectral speciation and electrochemical behaviour
Sushil K. Gupta *, Yogendra S. Kushwah
School of Studies in Chemistry, Jiwaji Uni6ersity, Gwalior 474011, India
Received 5 February 2001; accepted 27 March 2001
Dedicated to Professor B.L. Khandelwal on the occasion of his 64th birthday
Abstract
Two series of macrocyclic iron(III) complexes of stoichiometry [Fe(L)Cl2]Cl (1, 2) have been synthesised and characterised.
Compounds belonging to series 1 are derived from 4-methyl-2,6-diformylphenol and diamines (H2L), and those of 2 from
4-methyl-2,6-dibenzoylphenol and diamines. All the brown complexes have been characterised by physicochemical techniques. The
mass, infrared, electronic, ESR and Mo¨ssbauer spectroscopies, magnetic susceptibility data, molar conductance, X-ray diffraction
and cyclic voltammetric studies provide unambiguous evidence that 1 and 2 are high-spin iron(III) complexes in which the metal
has an octahedral geometry. The Mo¨ssbauer data are consistent with high-spin iron(III) and substantial covalency in the
Fe(III)–ligand bonds. Cyclic voltammetric studies in DMSO of the mononuclear iron(III) complexes show that they undergo
quasi-reversible reduction with E1/2 approximately −0.74 V versus SCE. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Iron(III) complexes; High-spin; Unsaturated macrocyclic ligand; ESR; Mo¨ssbauer; Powder XRD; Cyclic voltammetry
2. Experimental
1. Introduction
Binuclear metal complexes of the unsaturated te-
traaza diphenol (H2L) and its derivatives have been the
focus of extensive studies [1–3], but the mononuclear
Fe(III) complexes have received little attention [4].
Iron-containing systems have great industrial and bio-
logical importance [5]; e.g. iron macrocyclic complexes
have shown great potential recently as catalysts for the
hydroxylation of alkanes [6]. Earlier we reported some
mononuclear oxovanadium(IV) [7], Cu(II) [8] and
Ni(II) [9] complexes with unsaturated macrocycles
derived from the [2+2] template condensation of 4-
methyl-2,6-di(formyl/benzoyl) phenol with various di-
amines in the presence of metal ions. In continuation of
our studies, we report herein the synthesis, characterisa-
tion and electrochemical behaviour of the mononuclear
high-spin Fe(III) macrocyclic complexes.
2.1. Materials and methods
All the chemicals used were of reagent grade and
were used as received. Published methods [10,11] were
used to synthesise 4-methyl-2,6-diformylphenol and 4-
methyl-2,6-dibenzoylphenol. Ethylenediamine, 1,3-di-
aminopropane o-phenylenediamine and ferric chloride
were obtained from Aldrich and Merck. Solvents were
purified by standard methods [12].
2.2. Preparation of the macrocyclic ligands
The macrocyclic ligands (H2L) were synthesised by
the condensation of 4-methyl-2,6-di(formyl/benzoyl)
phenol with the appropriate diamines in 2:2 molar
ratios by a modified literature method [13]. They
showed satisfactory melting points and elemental
analyses.
* Corresponding author.
E-mail address: drskgcy@sancharnet.in (S.K. Gupta).
0277-5387/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(01)00786-0

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S.K. Gupta, Y.S. Kushwah / Polyhedron 20 (2001) 2019–2025
(cm−1): w(FeꢀCl) 312, w(C···O) 1551, w(CꢁN) 1629,
1656, w(NꢀH) 3080. veff (BM) at r.t.: 5.60. UV–Vis
(nm) (m, M−1 cm−1) (DMSO): 523(3240), 451(3.0×
103), 371(1.1×104), 328(8.4×103), 266(2.0×104). ESR
(DMSO, r.t.): g 2.014. Mo¨ssbauer (l, mm s−1; DEQ,
mm s−1, r.t.): 0.308; 0.722. CV data: E1/2 (V) −0.764;
DEp (mV) 208. \M (V−1cm2 mol−1) (10−3 M, DMSO,
25°C): 70.
1c. Anal. Found: C, 56.46; H, 3.75; Cl, 16.40; N,
8.61. Calc. for C30H24N4O2Cl3Fe: C, 56.76; H, 3.81; Cl,
16.75; N, 8.82%. FAB MS; m/z (%): 599(1.3) [M]+,
562(11) [M−Cl]+, 527(7.2) [M−2Cl]+, 154(100). IR
(cm−1): w(FeꢀCl) 305, w(C···O) 1535, w(CꢁN) 1621,
1650, w(NꢀH) 3072. veff (BM) at r.t.: 6.40. UV–Vis
(nm) (m, M−1 cm−1) (DMSO): 495(1510), 444(8.07×
103), 285(1.1×104), 266(1.6×104). ESR (DMSO, r.t.):
g 2.013. Mo¨ssbauer (l, mm s−1; DEQ, mm s−1, r.t.):
0.173; 0.722. CV data: E1/2 (V) −0.653; DEp (mV) 161.
\
M
(V−1cm2 mol−1) (10−3 M, DMSO, 25°C): 56.
2.2.2. Template synthesis of complexes [Fe(L%)Cl2]Cl
(2a–2c)
These complexes were prepared by a similar proce-
dure as described above, employing 4-methyl-2,6-diben-
zoyl phenol and diamines in the presence of FeCl3 in a
2:2:1 ratio in 75–80% yields.
2a. Anal. Found: C, 65.25; H, 4.70; Cl, 12.80; N,
6.51. Calc. for C46H40N4O2Cl3Fe: C, 65.53; H, 4.78; Cl,
12.61; N, 6.64%. IR (cm−1): w(FeꢀCl) 315, w(C···O)
1508, w(CꢁN) 1595, 1640, w(NꢀH) 3190. veff (BM): 6.30.
ESR (polycrystalline, r.t.): g 2.00. Mo¨ssbauer (l,
mm s−1; DEQ, mm s−1, r.t.): 0.291; 0.563.
2b. Anal. Found: C, 66.40; H, 4.98; Cl, 12.56; N,
6.67. Calc. for C48H44N4O2Cl3Fe: C, 66.18; H, 5.09; Cl,
12.20; N, 6.43%. FAB MS: m/z (%) 835(1) [M]+, 800(6)
[M−Cl]+, 763(5) [M−2Cl]+, 578(100). IR (cm−1):
w(FeꢀCl) 310, w(C···O) 1548, w(CꢁN) 1599, 1632,
w(NꢀH) 3109. veff (BM) at r.t.: 6.4. ESR (polycrys-
talline, r.t.): g 2.00. Mo¨ssbauer (l, mm s−1; DEQ,
mm s−1, r.t.): 0.290; 0.595.
2.2.1. Template synthesis of complexes [Fe(L)Cl2]Cl
(1a–1c)
To a boiling solution of FeCl3 (2 mmol) and 4-
methyl-2,6-diformyl phenol (0.45 g, 4 mmol) in ethanol
(20 cm3), an ethanolic solution of the diamine (4 mmol)
[ethylenediamine (a), 1,3-diaminopropane (b) and o-
phenylenediamine (c)] was added slowly with constant
stirring and the mixture was boiled under reflux for 4 h
when a brown compound separated out. The product
was filtered out, washed with ethanol and dried in air.
The yields were 70–78%.
The complexes were also prepared by heating a 1:2
mixture of FeCl3 and ligand H2L in EtOH.
1a. Anal. Found: C, 48.71; H, 4.40; Cl, 19.40; N,
10.59. Calc. for C22H24N4O2Cl3Fe: C, 49.05; H, 4.49;
Cl, 19.74; N, 10.4%. FAB MS; m/z (%): 503(1) [M]+,
469(5) [MH−Cl]+, 432(15) [MH−2Cl]+, 154(100).
IR (cm−1): w(FeꢀCl) 319, w(C···O) 1548, w(CꢁN) 1630,
1652, w(NꢀH) 3187. veff (BM) at room temperature
(r.t.): 5.94. UV–Vis (nm) (m, M−1 cm−1) (DMSO):
519(2620), 444(3.9×103), 375(1.04×104), 330(9.7×
103), 267(2.05×104). ESR (DMSO, r.t.): g 2.014. Mo¨ss-
bauer (l, mm s−1; DEQ, mm s−1, r.t.): 0.295; 0.665. CV
2c. Anal. Found: C, 68.82; H, 4.35; Cl, 11.68; N,
6.21. Calc. for C54H40N4O2Cl3Fe: C, 69.06; H, 4.29; Cl,
11.32; N, 5.96%. IR (cm−1): w(FeꢀCl) 318, w(C···O)
1532, w(CꢁN) 1606, 1648, w(NꢀH) 3147. veff (BM) at
r.t.: 6.01. UV–Vis (nm) (m, M−1 cm−1) (DMSO):
570(2050), 459(5.780×103), 348(2.8×104), 297(4.9×
104), 267(4.08×104). ESR (DMSO, r.t.): g 1.986. Mo¨ss-
bauer (l, mm s−1; DEQ, mm s−1, r.t.): 0.289; 0.570. CV
data: E1/2 (V) −0.792; DEp (mV) 213.
\
M
(V−1cm2 mol−1) (10−3 M, DMSO, 25°C): 55.
data: E1/2 (V) −0.768; DEp (mV) 208.
\
M
(V−1cm2 mol−1) (10−3 M, DMSO, 25°C): 59.
2.3. Physical measurements
1b. Anal. Found: C, 50.60; H, 4.80; Cl, 18.40; N,
10.21. Calc. for C24H28N4O2Cl3Fe: C, 50.86; H, 4.97;
Cl, 18.76; N, 9.88%. FAB MS; m/z (%): 531(1) [M]+,
496(5.3) [M−Cl]+, 459(12) [M−2Cl]+, 154(100). IR
The IR spectra (4000–200 cm−1) were recorded on a
Perkin–Elmer 883 spectrophotometer with KBr/CsI
pellets. The electronic spectra in DMSO solution were

S.K. Gupta, Y.S. Kushwah / Polyhedron 20 (2001) 2019–2025
2021
obtained with
a
Shimadzu UV-160A recording
analyses were obtained on a Carlo Erba 1100 elemental
analyser.
spectrophotometer. The ESR spectra in the poly-
crystalline state and in DMSO solution were recorded
on a Varian E-112 spectrometer in the X-band region at
r.t. Room temperature magnetic susceptibility measure-
ments were performed with an EG and G PAR155
vibrating sample magnetometer. The effective magnetic
moments were calculated using the formula veff=2.828
(MT)1/2, where M is the corrected molar susceptibility.
The diamagnetic contributions were calculated by using
literature tabulations [14]. The FAB masses in the
positive mode were recorded on a VG Autospec mass
spectrometer. m-Nitrobenzyl alcohol (m-NOBA) was
used as the matrix.
3. Results and disscusion
3.1. Synthesis
The template condensation of 4-methyl-2,6-di(-
formyl/benzoyl)phenol with various diamines in the
presence of FeCl3 in ethanol (Scheme 1) yields light and
dark brown complexes of composition [Fe(L)Cl2]Cl
(1,2). All the complexes are stable at room temperature,
soluble in MeOH, DMSO and DMF (except 2a and
2b), and melt with decomposition at 230–280°C. The
molar conductance measurements support the formula-
tion [Fe(L)Cl2]Cl as a 1:1 electrolyte [15]. The elemental
analyses were in the agreement with the above formula-
tions and rule out the possibility that the complexes
should be formulated as [Fe(L)Cl2][FeCl4]. Thin layer
chromatographic analysis confirmed the purity of the
complexes.
The X-ray powder diffraction patterns for the
complexes were recorded on a Rigaku ‘Rotaflex’ RU
200 rotating anode X-ray diffractometer using Cu Ka
,
radiation (u=1.5418 A) with angular range 10–90°.
Mo¨ssbauer spectral measurements were made on a
constant-acceleration spectrometer calibrated with
metallic iron at r.t. All isomer shifts are reported with
respect to the r.t. iron(0) transmission spectrum. The
observed spectra were computer-fitted using Lorentzian
lines and a least-squares minimisation technique.
3.2. Mass spectra
2.4. Electrochemical experiments
The FAB positive-mass spectra of the Fe(III) cationic
complexes (EI for 2b) obtained by using m-NOBA as
matrix [16–18] show molecular ion peaks [Fe(L)Cl2]+
in all the complexes. The other fragments exhibit fea-
tures due to [M−Cl]+, [M−2Cl]+. For ions contain-
ing Cl peaks, M−Cl35 and M−Cl37 were observed.
The other peaks at 154 and 136 are due to the NBA
matrix. The calculated and observed isotropic patterns
are in good agreement. The mass spectra of complexes
2a and 2c could not be recorded due to insufficient
volatility.
All the measurements were performed under a nitro-
gen atmosphere in DMSO at r.t. Solutions contained
10−4 mol dm−3 of the complex and 0.1 mol dm−3 of
TEAP (tetraethylammoniumperchlorate) as the sup-
porting electrolyte. Scan speeds of 50–200 mV s−1
were employed. Cyclic voltammograms were run on an
EG and G Versastat II system. The three-electrode
measurements were carried out with a platinum elec-
trode in conjunction with a Pt wire auxilliary electrode
and a saturated calomel electrode (SCE) as the refer-
ence electrode. The electrode performance was moni-
tored by observing the ferrocenium/ferrocene (Fc+/Fc)
couple in the above solvent system.
3.3. FT IR spectra
The IR spectra of the ligands show weak broad
bands at 2600–2850 cm−1. These bands can be as-
cribed to OꢀH stretching vibrations, which are known
to shift significantly to lower frequencies as a result of
OH···N intra-molecular bonding [19–21]. Very strong
bands at 1270 cm−1 were assigned to in-plane bending
(OꢀH) vibrations of the ligands. The absence of OꢀH
stretching and l(OꢀH) vibration in the spectra of the
complexes indicated deprotonation of the ꢀOH groups
[19]. The bands observed in the region 3072–3190
cm−1 in the complex are assigned to NꢀH stretching.
The low value of w(NꢀH) is attributed to hydrogen
bonding. The presence of CꢁN stretching vibrations in
the spectra of the complexes 1 around 1626 and 1650
cm−1 and in 2 around 1600 and 1640 cm−1suggests
that only one arm of the formyl/benzoyal group of the
phenolic unit is involved in coordination. The bands at
Constant-potential electrolyses were carried out on
the same equipment. Conductivity measurements were
done with a Global DCM 900 digital conductivity
meter with a solvent concentration of 1 mM. Elemental
Scheme 1.

2022
S.K. Gupta, Y.S. Kushwah / Polyhedron 20 (2001) 2019–2025
1626 in 1 and 1600 cm−1 in 2 are at a frequency lower
by approximately 11 and 15 cm−1, respectively, than
those in the free ligands. This can be ascribed to the
withdrawn of electron density from the nitrogen atom
owing to coordination. The band observed at approxi-
mately 1550 cm−1 has been assigned due to the
w(C···O) vibrations of the phenol [22] as a consequence
of the delocalisation of double bond in the chelate rings
that has increased the bond order of the phenolic CꢀO.
The absorption bands in the 305–319 cm−1 region are
assignable to w(FeꢀCl) vibrations characteristic of ter-
minal chlorine in trans configurations in an octahedral
arrangement of the metal ion [23]. Even in the cases of
1c and 2c, a trans configuration seems more likely
because of the similarity of the data to those of the a
and b series.
[25]. This fact also allows easy observation of the ESR
spectrum at room temperature.
3.7. Mo¨ssbauer spectroscopy
Iron-57 Mo¨ssbauer spectroscopy was utilised to
provide a direct probe of the electronic and chemical
environments of the mononuclear iron(III) sites in these
complexes. Zero-field Mo¨ssbauer spectra of 1 and 2 at
300 K are shown in Fig. 1. Each spectrum consists of a
single quadrupole-split doublet, the two lines being of
unequal intensity. The asymmetric line broadening may
be attributed to slow spin-lattice relaxation [26], but in
the absence of spectra at low temperatures it is impossi-
ble to confirm this. While comparing the values (DEQ)
of the quadrupole-splitting for these two types of com-
plexes, it appears that the iron site in complex 2 (ca.
0.576 mm s−1) is more symmetric than that in complex
1 (ca. 0.703 mm s−1). The values of the isomer shifts
(l) are in the range 0.173–0.308 mm s−1, less than the
values reported (0.40–0.70 mm s−1) for high-spin
iron(III) octahedral complexes [27]. This indicates a
higher s-electron density at iron possibly due to the
higher covalent character of the bond to iron and/or
reduced d-electron density at iron(III) resulting in
deshielding of the s-electrons [28].
3.4. Magnetic properties
The magnetic susceptibility data of all the Fe(III)
complexes were determined in the solid state at room
temperature. The magnetic moments (Section 2) for all
the Fe(III) complexes lie in the range 5.70–6.40 BM.
The experimental veff data match those for other d5
Fe(III) complexes. This is apparent from the ground
state of the Fe(III) ion, which in the weak octahedral
6
field gives a A1g ground state that has no orbital
3.8. X-ray diffraction
angular momentum.
The X-ray powder diffraction patterns of the com-
plexes (Table 1) show them to be crystalline. However,
efforts to grow single crystals did not succeed and this
precluded a single crystal X-ray determination.
Thus, following the analytical and spectral evidence,
complexes 1 and 2 can be assigned a monomeric octa-
hedral geometry around iron with FeN2O2 cores and
two chlorines in the trans position. The proposed struc-
ture of these complexes is shown in Fig. 1.
3.5. Electronic spectra
The d–d transitions in high-spin Fe(III) complexes
are spin forbidden and hence weak. The high-spin
octahedral Fe(III) complexes are expected to show four
absorption bands in the electronic spectra correspond-
ing to the transitions: A1g“4T1g; A1g“4T2g; A1g“
6
6
6
4
6
{4Eg, A1g}; A1g“4T2g(D) [24]. In the present
investigation the Fe(III) complexes in DMSO exhibit
intense absorptions in the visible region. These low
energy transitions are ascribed to ligand-to-metal
charge transfer because of their large extinction coeffi-
cients and the fact that the metal ion is more easily
reduced than oxidised. This is consistent with the cyclic
voltammetric studies (see Section 4). In addition, all the
complexes show very intense absorptions in the UV
region, possibly due to intra-ligand transitions as ob-
served in the free ligands at similar energies.
3.6. ESR spectra
The room temperature ESR spectra in DMSO of
1a–1c and 2c, and in the polycrystalline state of 2a, 2b
show transitions centred at g=2.00. This may be at-
tributed to the extremely small spin orbital coupling

S.K. Gupta, Y.S. Kushwah / Polyhedron 20 (2001) 2019–2025
2023
Fig. 1. Zero-field 57Fe Mo¨ssbauer spectra of complexes 1a, 1b and 2a–2c at 300 K, referenced to iron metal.
4. Electrochemistry
perature. Fig. 2 shows the cyclic voltammograms of
complexes 1a and 1b. All the complexes exhibit quasi-
reversible Fe(III)/Fe(II) reduction waves (E1/2= −
0.653 to −0.792 V) at a scan rate of 50 mV s−1 with a
peak-to-peak width much larger than that observed for
the one-electron couple Fc+/Fc ion in the same solvent
4.1. Cyclic 6oltammetry
The electrochemical behaviour of the unsaturated
macrocyclic complexes 1a–1c and 2c has been studied
by cyclic voltammetry (CV) in DMSO using a Pt
electrode under dry nitrogen atmosphere at room tem-
under the same experimental conditions (E1/2
=
0.5(Ep,c+Ep,a)=0.48 and DEp=Ep,c−Ep,a=80
V

2024
S.K. Gupta, Y.S. Kushwah / Polyhedron 20 (2001) 2019–2025
mV). The observed E1/2 values for these couples are in
good agreement with the reported value [29]. The E1/2
this can be ascribed to the electron-rich substituents at
the 2,6-positions. The plot of cathodic peak current (ipc)
versus square root of scan rate (w1/2) is nearly linear,
indicative of quasi-reversible behaviour [30]. The peak-
to-peak separation (DEp) increases at high scan rates,
showing the quasi-reversible nature of the electron
transfer process. Further, the DEp values are greater
than the Nernstian value (DEp$59 mV) for a one-elec-
tron reduction system. This indicates a considerable
reorganisation of the coordination sphere during elec-
tron transfer. Controlled potential electrolysis for com-
plex 1a in DMSO solution at −0.90 V involved the
passage of 0.97 equiv. of charge, indicating one-electron
reduction of the mononuclear iron species.
value for 2c is more negative than those for series 1;
.
Table 1
The main bands in the X-ray diffraction powder patterns of the
complexes
,
Complex
d lines a (A)
[Fe(La)Cl2]Cl
(1a)
5.68(6), 4.43(7), 3.47(100), 3.35(82), 3.26(53),
2.99(80), 2.96(51), 2.72(26), 2.63(39), 2.09(21),
1.88(11), 1.59(12)
[Fe(Lb)Cl2]Cl
(1b)
5.27(24), 4.30(23), 4.20(51), 3.79(47), 3.65(100),
3.33(38), 2.93(39), 2.82(26), 2.36(30), 1.95(14),
1.88(12), 1.72(12)
[Fe(Lc)Cl2]Cl
(1c)
8.11(73), 7.56(63), 6.79(41), 6.21(44), 5.91(42),
5.00(46), 4.63(58), 4.33(50), 3.72(52), 3.23(100),
2.03(25)
Acknowledgements
[Fe(La% )Cl2]Cl
(2a)
[Fe(Lb% )Cl2]Cl
(2b)
3.48(100), 3.35(61), 3.26(51), 3.00(80), 2.96(44),
2.64(41), 2.08(39), 1.88(14), 1.6(13), 1.59(17)
6.34(14), 5.26(12), 4.30(12), 4.19(71), 3.79(40),
3.61(100), 3.33(53), 2.93(53), 2.40(46), 2.11(57),
1.72(13), 1.50(19)
We thank the Department of Science and Technol-
ogy, New Delhi, India, and the Royal Society of Chem-
istry, UK, for financial assistance, Dr A. Abdul-Sada,
University of Sussex, UK, for recording mass spectra,
RSIC, IIT Madras, for recording ESR spectra and
magnetic susceptibility measurements, Professor A.
Gupta, Inter University Consortium, Indore, for
providing facilities for Powder XRD and Mo¨ssbauer
spectral measurements, and referees for constructive
comments.
[Fe(Lc%)Cl2]Cl
(2c)
8.46(100), 7.55(8), 6.95(5), 5.63(5), 4.55(6),
3.44(14), 3.38(12), 2.81(12), 2.43(4), 1.89(2)
a Intensities are given in parentheses.
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