Esterification and isolation of the carboxylic acid with salicyl-bis-hydrazide via coordination of iron(III) 18-metallacrown-6 complex

Article abstract of DOI:10.1080/00958972.2010.520706

The trianionic heptadentate ligand, (Z)-3-(5- bromosalicylhydrazinocarbonyl) propenoic acid ((Z)-H₄bshcpa), has been synthesized in good yield and reacted with FeCl₃ 6H₂O to produce [Fe^III₆(C₁₂

Full text of DOI:10.1080/00958972.2010.520706

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Esterification and isolation of the
carboxylic acid with salicyl-bis-
hydrazide via coordination of iron(III)
18-metallacrown-6 complex
Chengzhi Jin a , Hui Yu a , Longfei Jin a , Lamei Wu a & Zhongqiang
Zhou a
a College of Chemistry and Material Science , South-Central
University for Nationalities , Wuhan 430074, P.R. China
Published online: 30 Sep 2010.
To cite this article: Chengzhi Jin , Hui Yu , Longfei Jin , Lamei Wu & Zhongqiang Zhou (2010)
Esterification and isolation of the carboxylic acid with salicyl-bis-hydrazide via coordination of
iron(III) 18-metallacrown-6 complex, Journal of Coordination Chemistry, 63:21, 3772-3782, DOI:
10.1080/00958972.2010.520706
To link to this article: http://dx.doi.org/10.1080/00958972.2010.520706
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Journal of Coordination Chemistry
Vol. 63, No. 21, 10 November 2010, 3772–3782
Esterification and isolation of the carboxylic acid
with salicyl-bis-hydrazide via coordination of
iron(III) 18-metallacrown-6 complex
CHENGZHI JIN, HUI YU, LONGFEI JIN*,
LAMEI WU and ZHONGQIANG ZHOU
College of Chemistry and Material Science, South-Central
University for Nationalities, Wuhan 430074, P.R. China
(Received 19 February 2010; in final form 16 July 2010)
The trianionic heptadentate ligand, (Z)-3-(50-bromosalicylhydrazinocarbonyl) propenoic acid
((Z)-H4bshcpa), has been synthesized in good yield and reacted with FeCl3 ꢀ 6H2O to produce
[FeIII6(C12H8N2O5Br)6(H2O)2(CH3OH)4] ꢀ 8H2O ꢀ 8CH3OH. The complex has been character-
ized by single-crystal X-ray diffraction. In the self-assembly process the ligand was esterified
and transferred into (Z)-methyl 3-(50-bromosalicylhydrazinocarbonyl) propenoate
((Z)-H3mbshcp). In the crystal structure, the neutral Fe(III) complex contains an 18-membered
metallacrown ring consisting of six Fe(III) and six trianionic ligands. The 18-membered
metallacrown ring is formed by six structural moieties of the type [Fe(III)–N–N]. Due to
the meridional coordination of the ligands to Fe3þ, the ligands enforce stereochemistry of the
Fe3þ ions as a propeller configuration with alternating Ã/D forms. The metallacrown can be
treated with SnCl2 to obtain purified ester. In addition, we have also obtained reduced esterified
ligand, methyl 3-(50-bromosalicylhydrazinocarbonyl) propanoate (H3mbshcp), with Zn powder
as reductant.
Keywords: Metallacrown; Iron complex; Crystal structure; Esterification
1. Introduction
Metallacrowns and analogues have attracted considerable attention [1, 2].
Metallacrowns, with Mn(III), Fe(III), Ni(II), Cu(II), Zn(II), Ga(III), and V(V)O,
[9-MC-3] [3, 4], [12-MC-4] [5–7], [15-MC-5] [8–10], [12-MC-6] [11], [16-MC-8] [12],
[18-MC-6] [13–16], [18-MC-8] [11], [30-MC-10] [17], [36-MC-12] [18], [40-MC-10] [19],
[60-MC-20] [20], stacking metallacrowns [21, 22] as well as a variety of dimers and fused
metallacrowns [23, 24] have been reported to date.
These complexes concentrate a large number of metal ions per unit volume, leading
to interesting behaviors, such as strong visible absorption spectra, magnetism and
bioactivity. Metallacrowns have already been related to sensors [25], molecular
recognition [26], and chiral building blocks [27, 28].
*Corresponding author. Email: longfei.jin@yahoo.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.520706

Iron(III) 18-metallacrown-6 complex
3773
OH
O
O
H
N
Br
N
H
OH
H
H
O
Scheme 1. Ligand (Z)-H4bshcpa.
Metallacrowns are typically prepared using salicylhydroxamic acids (H3shi) and
salicylhydrazides (H3shz). Taking the similarity between H3shz and carboxylic acids
with salicyl-bis-hydrazide into account, we have expanded the types of precursor ligand
with the intention of modifying chemical properties [29–31]. In this article, we report a
new potentially heptadentate ligand (Z)-3-(50-bromosalicylhydrazinocarbonyl) prope-
noic acid (1) ((Z)-H4bshcpa, scheme 1) and an iron(III) 18-metallacrown-6 complex
[FeIII6(C12H8N2O5Br)6(H2O)2(CH3OH)4] ꢀ 8H2O ꢀ 8CH3OH (2). In the self-assembly
process (Z)-3-(50-bromosalicylhydrazinocarbonyl) propenoic acid is esterified and
transferred into (Z)-methyl 3-(50-bromosalicylhydrazinocarbonyl) propenoate
((Z)-H3mbshcp) (3). The deprotonated (Z)-methyl 3-(50-bromosalicylhydrazinocarbo-
nyl) propenoate ((Z)-mbshcp3ꢁ) bridges neighboring ions through its hydrazide N–N
group. In addition, the metallacrown can be treated with SnCl2 and/or Zn powder to
obtain purified ester (Z)-H3mbshcp and methyl 3-(50-bromosalicylhydrazinocarbonyl)
propanoate (H3mbshcp) (4), respectively.
2. Experimental
2.1. Materials
Chemicals for the synthesis of the compounds were used as purchased. Methanol,
ethanol, chloroform, and ethyl acetate were used without purification. 5-Bromosalicylic
acid, sulfuric acid, hydrazine hydrate, maleic anhydride, pyridine, FeCl3 ꢀ 6H2O,
hydrochloric acid, Zn powder, and SnCl2 ꢀ 2H2O were purchased from China
Sinopharm Group Chemical Reagent Co., Ltd. All chemicals and solvents were
of reagent grade. 5-Bromosalicylhydrazide was prepared according to literature
procedures [32].
2.2. Characterization
1H and 13C NMR spectra were recorded on a Varian Inova 400 MHz NMR
spectrometer at 25ꢂC. Chemical shifts are referenced to residual solvent. Infrared
(IR) spectra were measured on a Thermo Nicolet Corporation NEXUS 470 FT-IR
Spectrometer as KBr pellets from 4000 to 400 cmꢁ1. UV-Vis spectra were recorded on a
Perkin-Elmer Lambda 35 UV/Vis Spectrometer. Thermogravimetry (TG) carried out
on Perkin-Elmer Diamond DSC TG-DTA 6300 thermal analyzer. C, H, N, and S
elemental analyses were performed on a Perkin-Elmer 2400 Series II CHNS/O
Analyzer. Fe was determined by atomic absorption spectroscopy on a Shimadzu
AA-6300 Spectrophotometer. Melting points were obtained using
a digital

3774
C. Jin et al.
melting-point apparatus, TP Micro Printer S-X6. Variable-temperature DC magnetic
susceptibility data were collected on polycrystalline samples from 2 to 300 K at 1 KOe
under zero field cooled conditions using a Quantum Design SQUID MPMS XL-7
magnetometer. Cyclic voltammetry (CV) was performed on an Eco Chemie Autolab
Eletrochemical analyzer. The experiments were carried out in a three-electrode cell with
platinum electrode and glassy-carbon electrode as the working electrodes, a Ag/AgCl
electrode saturated with KCl as the reference electrode. The supporting electrolyte is
tetrabutylammonium perchlorate (tbap) recrystallized twice from ethanol and dried
under vacuum. Pure nitrogen was insufflated in the solution of the compounds to
remove oxygen. All electrochemical measurements were carried out at room
temperature.
2.3. Synthesis of (Z)-3-(50-bromosalicylhydrazinocarbonyl) propenoic acid ((Z)-
H4bshcpa) (1)
A solution of maleic anhydride (5.88 g, 60 mmol) in chloroform (150 mL) was slowly
added to a solution of 5-bromosalicylhydrazide (11.55 g, 50 mmol) in chloroform
(300 mL) at 0ꢂC. The reaction mixture was warmed to boiling point and stirred for 5 h.
After the resulting mixture was concentrated under reduced pressure, the residue was
filtered and rinsed with ethanol. Yield: 89%; determination of the melting point of
(Z)-H4bshcpa shows that sublimation and oxidation occurred at 201ꢂC; Calcd for
C11H9N2O5Br (%): C, 40.14; H, 2.76; N, 8.51. Found (%): C, 39.90; H, 2.66; N, 8.42.
1H NMR (DMSO-d6), ꢀ ppm: 11.93 (bs, 1H; –PhOH), 10.83 (bs, 2H; –NH–NH–), 8.03
(s, 1H; o-PhCH), 7.61 (d, 1H; p-PhCH), 6.97 (d, 1H; m-PhCH), 6.38–6.47 (m, 2H;
–CH¼CH–); 13C NMR (DMSO-d6), ꢀ ppm: 167.67 (–COOH), 164.08 (–CO–PA),
162.47 (Ph–CO–), 161.18 (PhC–OH), 136.95 (–NHCO–CH¼), 133.57 (p-PhC), 131.99
(o-PhC), 127.29 (¼CH–COOH), 120.21 (PhC–CO–), 118.22 (m–PhC), 111.00 (PhC–Br);
IR (KBr pellet, cmꢁ1): ꢁ(O–H), 3174 s, broad; ꢁ(N–H), 3060 s, broad; ꢁ(C¼O), 1722 s;
ꢁ(C¼C), 1658 s; ꢁ(C¼N–C¼N), 1604 s.
2.4. Synthesis of [FeIII (C12H8N2O5Br)6(H2O)2(CH3OH)4]E 8H2OE 8CH3OH (2)
6
A mixture of FeCl3 ꢀ 6H2O (3.24 g, 12 mmol) in methanol (20 mL) and (Z)-H4bshcpa
(3.29 g, 10 mmol) in methanol (150 mL) was stirred for 20 min. Pyridine (10 mmol)
dissolved in methanol (10 mL) was then added. The resulting dark-brown solution was
stirred for 2 h at 40ꢂC, refluxed for 4 h, and filtered. After 7 days, black triclinic crystals
deposited from the mother liquor. In the complex, 1 has been esterified and transferred
into (Z)-methyl 3-(50-bromosalicylhydrazinocarbonyl) propenoate ((Z)-H3mbshcp) (3).
Yield: 52%; Calcd for C84H116N12O52Br6Fe6 (%): C, 34.31; H, 3.98; N, 5.72; Fe, 11.40.
Found (%): C, 34.08; H, 3.79; N, 5.48; Fe, 11.25. IR (KBr pellet, cmꢁ1): ꢁ(HO–H), 3439
vs, broad; ꢁ(OH), 3136 s, broad; ꢁ(C¼O), 1726 s; ꢁ(C¼C), 1649 s; ꢁ(C¼N–C¼N), 1591 s.
2.5. Preparation of (Z)-methyl 3-(50-bromosalicylhydrazinocarbonyl) propenoate
((Z)-H3mbshcp) (3)
To obtain 3, compound 2 is treated with SnCl2 ꢀ 2H2O. In the preparation, the resulting
dark-brown solution formed by the reaction of ligand with FeCl3 ꢀ 6H2O will be directly
concentrated under reduced pressure to obtain the residue 2.

Iron(III) 18-metallacrown-6 complex
3775
A solution of SnCl2 ꢀ 2H2O (6.77 g, 30 mmol) in 6 mol Lꢁ1 HCl (50 mL) was added to
the residue of 2 at 0ꢂC. The resultant mixture was slowly warmed to 40ꢂC, stirred for 4 h,
and filtered. The resulting white precipitate was rinsed with 6 mol Lꢁ1 HCl and dried
under vacuum to afford white solid. Yield: 88%; m.p. 172–174ꢂC, determination of the
melting point of (Z)-H3mbshcp shows sublimation occurred at 168ꢂC; Calcd for
C12H11N2O5Br (%): C, 42.00; H, 3.53; N, 8.17. Found (%): C, 41.89; H, 3.47; N, 8.08.
1H NMR (DMSO-d6), ꢀ ppm: 12.00 (s, 1H; ꢁPhOH), 10.98 (s, 1H; ꢁPhCONHꢁ), 10.77
(s, 1H; ꢁNHCOꢁCH2¼), 8.04 (s, 1H; o-PhCH), 7.62 (d, 1H; p-PhCH), 7.09 (d, 1H;
ꢁNHCOꢁCH¼), 6.54 (d, 1H; m-PhCH), 6.44 (d, 1H; ¼CHCOOꢁ), 3.68 (s, 3H; ꢁOCH3);
13C NMR (DMSO-d6), ꢀ ppm: 166.95 (ꢁCOOꢁ), 163.59 (ꢁNHCOꢁCH¼), 161.28
(PhCOꢁ), 157.20 (PhCꢁOH), 136.27 (p-PhC), 131.99 (ꢁNHCOꢁCH¼), 131.35
(¼CHꢁCOOꢁ), 127.23 (o-PhC), 119.61 (PhCꢁCOꢁ), 117.59 (m-PhC), 110.32
(m-PhCꢁBr), 51.69 (ꢁOCH3); IR (KBr pellet, cmꢁ1): ꢁ(O–H), 3230 s, broad; ꢁ(N–H),
3174 s, broad; ꢁ(C¼O), 1729 s; ꢁ(C¼C), 1640 s; ꢁ(C¼N–C¼N), 1596 vs.
2.6. Preparation of methyl 3-(50-bromosalicylhydrazinocarbonyl) propanoate
(H3mbshcp) (4)
H3mbshcp was synthesized similar to the synthesis procedure of 3. Zinc powder
(19.61 g, 300 mmol) was added to a solution of the residue of 2 in water (50 mL) at 0ꢂC,
and concentrated hydrochloric acid was added dropwise to the above solution with
stirring. The resultant mixture was slowly warmed to 30ꢂC and kept for 4 h. Compound
4 was extracted with ethyl acetate and the ethyl acetate extract was washed several times
with 1 mol Lꢁ1 HCl, and evaporated under reduced pressure. The residue was dried
under vacuum to afford white solid. Yield: 90%; m.p. 159–161ꢂC. Calcd for
C12H13N2O5Br (%): C, 41.75; H, 3.80; N, 8.12. Found (%): C, 41.63; H, 3.64; N,
8.00. 1H NMR (DMSO-d6), ꢀ ppm: 11.96 (s, 1H; ꢁPhOH), 10.59 (s, 1H; ꢁPhCONHꢁ),
10.40 (s, 1H; ꢁNHCOꢁCH2ꢁ), 8.03 (s, 1H; o-PhCH), 7.58 (d, 1H; p-PhCH), 6.94 (d, 1H;
m-PhCH), 3.61 (s, 3H; ꢁOCH3), 2.48ꢁ2.57 (m, 4H; ꢁCH2CH2ꢁ); 13C NMR (DMSO-
d6), ꢀ ppm: 178.02 (ꢁNHCOꢁCH2ꢁ), 174.89 (ꢁCOOꢁ), 170.09 (PhCOꢁ), 163.08
(PhCꢁOH), 141.73 (p-PhC), 136.42 (o-PhC), 125.05 (PhCꢁCOꢁ), 122.61 (m-PhC),
115.59 (m-PhC ꢁ Br), 56.84 (ꢁOCH3), 33.83 (ꢁNHCOꢁCH2ꢁ), 33.27 (ꢁCH2ꢁCOOꢁ); IR
(KBr pellet, cmꢁ1): ꢁ(O–H), 3290 s, broad; ꢁ(N–H), 3222 s, broad; ꢁ(C¼O), 1728 s;
ꢁ(C¼N–C¼N), 1601 vs.
2.7. X-ray crystal structure determination
A crystal of 2 with dimensions 0.40 mm ꢃ 0.10 mm ꢃ 0.06 mm was mounted in a glass
capillary with the mother liquor to prevent the loss of solvents during X-ray diffraction
data collection. Intensity data were collected with graphite-monochromated Mo-Ka
˚
radiation (ꢂ ¼ 0.71073 A) at 289(2) K on a Bruker Smart APEX diffractometer. From a
total of 30,898 reflections corrected by SADABS [33, 34] in the 1.47 ꢄ ꢃ ꢄ 26.00 range,
11,562 were independent with Rint ¼ 0.0548, of which 10,719 observed reflections with
I 4 2ꢄ(I) were used in the structural analysis. The structure was solved using direct
methods and standard difference map techniques and refined by full-matrix
least-squares on F2 with SHELXTL [34, 35]. All non-hydrogen atoms were refined
anisotropically. Hydrogens were included in calculated positions and refined using a

3776
C. Jin et al.
Table 1. Crystallographic data.
Empirical formula
Formula weight
Crystal system
Space group
C76H68Br6Fe6N12O36
2539.93
Triclinic
P-1
ꢂ
˚
Unit cell dimensions (A, )
a
b
c
ꢆ
ꢇ
ꢈ
13.551(2)
15.571(2)
16.423(3)
96.936(2)
107.235(3)
112.733(3)
2941.7(8), 1
1.434
2.830
1262
3
˚
Volume (A ), Z
Calculated density (mg cmꢁ3
)
Absorption coefficient (mmꢁ1
F(000)
)
Crystal size (mm3 )
ꢃ range for data collection (ꢂ)
Limiting indices
0.40 ꢃ 0.10 ꢃ 0.06
1.47–26.00
ꢁ16 ꢄ h ꢄ 15; ꢁ19 ꢄ k ꢄ 19;
0 ꢄ l ꢄ 20
Observed reflections
Independent reflections
R indices (all data)
30,898
11,562 [Rint ¼ 0.0548]
a
b
R1 ¼ 0.0534; wR2 ¼ 0.1584
Goodness-of-fit on F2
1.121
P
P
aR1 ¼ kFoj ꢁ jFck/ jFoj.
P
P
2
2
bwR2 ¼ [ wðFo2 ꢁ Fc2Þ / wðFo2Þ ]1/2
.
riding model. Problematic solvent disorder in 2 could not be modeled properly; thus,
the contribution of the solvent to the diffraction pattern was subtracted from the
observed data by the SQUEEZE method implemented in PLATON [36, 37]. The final
refinement converged at R1 ¼ 0.0534, wR2 ¼ 0.1584 (w ¼ 1/[ꢄ2ðFo2Þ þ (0.0767P)2 þ
2.5839P], where P ¼ ðFo2 þ 2Fc2Þ/3) (for 11,562 unique reflections), (D/ꢄ)max ¼ 0.013,
S ¼ 1.121, (Dꢅ)max ¼ 0.608 and (Dꢅ)min ¼ ꢁ0.655 e Aꢁ3. Crystal data and refinement
˚
details are presented in table 1.
3. Results and discussion
3.1. Synthesis and characterization
Iron metallacrown 2 was synthesized via the reaction of FeCl3 ꢀ 6H2O with
(Z)-3-(50-bromosalicylhydrazinocarbonyl) propenoic acid that has been prepared
previously through direct reaction between 5-bromosalicylhydrazide [32] and maleic
anhydride, in methanol þ pyridine solution (scheme 2). Complex 2 was a dark-brown
crystalline solid.
In 2, compound 1 has been esterified and transferred into (Z)-H3mbshcp (3, figure 1).
The synthesis system plays an important role in the formation of the new ligand.
Through the analysis of [FeIII6(C12H8N2O5Br)6(H2O)2(CH3OH)4] ꢀ 8H2O ꢀ 8CH3OH, we
deduce that the esterification of carboxylic acid with salicyl-bis-hydrazide is a new
internal esterification by the metallacrown itself, defined as an internal self-catalytic
reaction [38]. For the 18-MC-6 metallacrown [FeIII6(C12H8N2O5Br)6(H2O)2(CH3OH)4],
the reactant is [FeIII6(C10H5N2O3Br–COOH)6(H2O)m(CH3OH)n] (m ¼ 1–6, n ¼ 1–6),
which contains (Z)-H4bshcpa with a carboxylic acid group. The carboxylic acid group

Iron(III) 18-metallacrown-6 complex
3777
OH
O
H
O
H
6
+ 6FeCl 6H O+6C H N+18CH OH
H
3
2
5
5
3
N
Br
N
OH
H
O
CH OH
3
(2)+6C H N Cl+12HCl+32H O
5
5
2
Scheme 2. Synthesis of iron metallacrown 2.
OH
OH
O
H
O
H
O
H
H
N
N
OCH
3
Br
N
H
OCH
3
Br
N
H
O
O
O
3
4
Figure 1. (Z)-H3mbshcp (3) and H3mbshcp (4).
can be combined with iron, then reacted with a coordinated methanol to produce ester
(Z)-H3mbshcp. Eventually [FeIII6(C10H5N2O3Br–COOCH3)6(H2O)2(CH3OH)4] can be
obtained by the internal self-catalytic reaction. In the catalytic reaction, iron can be
identified as the catalytic center.
Metallacrown with esterified ligand was obtained by tuning the pH of reactant
solution to control hydrolysis of metal ion and deprotonation of the esterified ligand,
confirmed by the spectral characterization. In the IR spectra of 2, the absence of the
N–H stretching vibration is consistent with the deprotonation of the –NH–NH– group
and coordination to Fe(III), which may indicate that (Z)-H3mbshcp adopts the
complete deprotonated (Z)-mbshcp3ꢁ form. The C¼N–N¼C framework at l604 cmꢁ1
in the ligand shifted to 1591 cmꢁ1 upon coordination to Fe. So the basic binding sites
in 2 are similar to the previous reports [13], which is also consistent with the X-ray
structural analysis.
CV suggests that 2 is electrochemically inactive in methanol in the range þ1.5
to ꢁ1.5 V, consistent with the conclusion in the literature [10]. The absence of redox
activity of the metallacrown indicates redox stability. The ligand stabilizes three iron
ions in methanol.
3.2. Isolation of (Z)-H3mbshcp and H3mbshcp
To obtain (Z)-H3mbshcp in high yield, the metallacrown is concentrated under reduced
pressure. The residue is treated with SnCl2 in hydrochloric acid and results in purified
(Z)-H3mbshcp.
The esterified ligand 3 is an ester with bis-hydrazide, which cannot be synthesized via
a modular acid-catalyzed approach. The reaction between 1 and FeCl3 is a
self-assembly process, which yields an highly organized molecular architecture that
allows efficient electron transfer and subsequent esterification. The yield was calculated
on the basis of the starting ligand, giving metallacrown with ester in 98% yield.

3778
C. Jin et al.
Figure 2. Perspective view of 2, solvent molecules and all hydrogens have been omitted for clarity.
To obtain purified ester, the FeIII metallacrown was reduced according to equation (1).
ð2Þ þ SnCl2 þ HCl ! C12H11N2O5Br þ FeCl2 þ SnCl4
ð1Þ
The FeIII metallacrown can also be treated with Zn powder in a solution of
hydrochloric acid according to equation (2), but ester 4 is significantly different to 3 in
which the double bond C¼C in was reduced to single bond C–C (figure 1).
ð2Þ þ Zn þ HCl ! C12H13N2O5Br þ FeCl2 þ ZnCl2
ð2Þ
3.3. Crystal structure of 2
Complex 2 crystallizes in the triclinic system, space group P-1. A diagram of the crystal
structure of 2 is presented in figure 2. The bond lengths and angles in 2 show normal
values; important bond distances and angles are presented in table 2. The average
˚
˚
˚
iron–oxygen/nitrogen distance is Fe1 2.028 A, Fe2 2.020 A, and Fe3 2.024 A.
The structure exhibits a hexanuclear ring of irons linked by six hydrazide N–N
groups. The deprotonated (Z)-mbshcp3ꢁ is a trianionic pentadentate ligand, one

Iron(III) 18-metallacrown-6 complex
3779
ꢂ
˚
Table 2. Selected bond lengths (A) and angles ( ) in 2.
Fe1–O1
Fe1–O14
Fe1–N1
Fe1–O3
Fe1–O18
Fe1–N6
Fe2–O7
Fe2–O2
Fe2–N3
Fe2–O9
Fe2–O6
Fe2–N2
Fe3–O13
Fe3–O8
Fe3–N5
Fe3–O15
Fe3–N4
Fe3–O12
1.887(3) C2–O1
1.992(2) C7–N1
2.050(3) C7–O2
2.056(3) N1–N2
2.063(3) C8–O3
2.120(3) C8–N2
1.895(3) C11–O4
1.995(2) C11–O5
2.021(3) C15–O7
2.050(3) C20–N3
2.066(3) C20–O8
2.093(3) N3–N4
1.898(3) C21–N4
1.994(3) C21–O9
2.001(3) C24–O10
2.051(3) C24–O11
2.099(3) C32–O14
2.103(3) C36–O16
1.309(5)
1.337(4)
1.327(4)
1.424(4)
1.293(4)
1.307(5)
1.149(7)
1.298(7)
1.334(5)
1.294(5)
1.308(4)
1.443(4)
1.317(5)
1.254(4)
1.214(6)
1.319(6)
1.300(4)
1.238(7)
O1–Fe1–O14
O1–Fe1–N1
O14–Fe1–N1
O1–Fe1–O3
O14–Fe1–O3
N1–Fe1–O3
O1–Fe1–O18
O14–Fe1–O18
N1–Fe1–O18
O3–Fe1–O18
O1–Fe1–N6
O14–Fe1–N6
N1–Fe1–N6
O3–Fe1–N6
O18–Fe1–N6
O7–Fe2–O2
O7–Fe2–N3
O2–Fe2–N3
O7–Fe2–O9
O2–Fe2–O9
N3–Fe2–O9
O7–Fe2–O6
O2–Fe2–O6
N3–Fe2–O6
O9–Fe2–O6
O7–Fe2–N2
O2–Fe2–N2
N3–Fe2–N2
O9–Fe2–N2
O6–Fe2–N2
O13–Fe3–O8
O13–Fe3–N5
O8–Fe3–N5
103.57(11) O8–Fe3–N4
86.25(12) N5–Fe3–N4
170.17(11) O15–Fe3–N4
162.70(11) O13–Fe3–O12
93.73(11) O8–Fe3–O12
76.46(11) N5–Fe3–O12
94.76(15) O15–Fe3–O12
84.26(12) N4–Fe3–O12
95.67(13) C33–O15–Fe3
86.69(13) O1–C2–C1
94.16(13) O2–C7–N1
76.87(10) N1–C7–C1
102.11(12) C7–N1–N2
89.89(12) C8–N2–N1
160.57(13) O3–C8–N2
104.29(11) N2–C8–C9
85.99(11) C10–C9–C8
169.65(12) C9–C10–C11
162.11(11) O4–C11–O5
93.59(11) O4–C11–C10
76.12(11) C11–O5–C12
83.39(15) O7–C15–C14
92.23(12) N3–C20–O8
87.73(12) N3–C20–C14
95.34(13) C21–N4–N3
95.16(13) O9–C21–N4
76.41(11) N4–C21–C22
104.21(12) C23–C22–C21
89.69(12) C22–C23–C24
167.87(10) O10–C24–O11
101.14(12) O10–C24–C23
86.11(12) C24–O11–C25
172.72(13) O14–C32–C26
75.58(11)
104.75(12)
89.33(12)
93.65(15)
82.65(12)
96.30(13)
89.17(13)
157.84(13)
114.9(2)
125.0(3)
117.2(3)
122.6(3)
114.1(3)
109.7(3)
124.7(3)
116.3(3)
127.2(4)
131.2(5)
129.7(6)
119.5(5)
113.3(5)
122.9(3)
122.0(3)
119.9(3)
108.7(3)
124.3(3)
115.7(3)
129.9(4)
120.8(4)
123.2(5)
125.0(5)
117.2(4)
119.4(3)
119.2(3)
120.4(5)
128.0(5)
117.2(4)
O13–Fe3–O15 161.92(11) O15–C33–C34
O8–Fe3–O15
N5–Fe3–O15
O13–Fe3–N4
96.93(12) O16–C36–O17
75.83(11) O16–C36–C35
94.60(15) C36–O17–C37

3780
C. Jin et al.
phenolate oxygen, one carbonyl oxygen, and one hydrazide nitrogen bound to one
Fe3þ; the other carbonyl oxygen plus the other hydrazide nitrogen in the same ligand
are chelated to an adjacent Fe3þ. The specific connectivity of atoms forming the ring is
–Fe1–N1–N2–Fe2–N3–N4–Fe3–N5–N6A–Fe1A–N1A–N2A–Fe2A–N3A–N4A–Fe3A–
N5A–N6–. Therefore, the ligand is forcing all Fe3þ cations into a propeller configuration
with alternating Ã/D stereochemistry as ÃDÃ forms (figure 2). Two methanol and one
water coordinated at the metal centers with à configuration are found on one face of the
metallacrown, and the remaining two methanols and one water coordinated to the D
metal centers are found on the other face of the metallacrown. The six methyl acrylate
groups were also on both sides of the ring with alternating forms. The two faces of the
disc-shaped hexanuclear ring have opposite chiralities, resulting in the 18-membered
hexanuclear core ring system with an [Fe–N–N] repeat unit. There are no solvent
molecules in the ‘‘host’’ cavity of 2. It is also observed that all irons in 2 are in an
octahedral FeN2O4 environment, in which there is no Jahn–Teller distortion. The
˚
average neighboring Feꢀ ꢀ ꢀ Fe separation is 4.905 A. A packing diagram in a unit cell is
shown in figure S1. As can clearly be seen from figure S1, the molecules translating one
unit cell along the a direction are stacked.
3.4. Thermogravimetry
The TG curve of 2 exhibits four weight losses with increasing temperature. From 41ꢂC
to 210ꢂC, 2 loses 13.8% weight, assigned to loss of the outside solvents; from 210ꢂC to
384ꢂC, weight loss of 25.1% is ascribed to the loss of coordinated water and methanol
and sublimation of the residue. To this temperature, all of the solvent molecules and
coordinated small molecules are lost. From 384ꢂC to 592ꢂC, the weight loss is 38.9%
from composition of (Z)-mbshcp3ꢁ. From 592ꢂC to 800ꢂC, as the residue transformed
into Fe2O3, the TG curve shows almost no change in weight.
3.5. Magnetic properties
The magnetic properties of polycrystalline samples of 2 are shown in figure 3 between
2 K and 300 K. The molar effective magnetic moment (ꢉeff) shows the presence of an
antiferromagnetic coupling between the Fe(III) spin 5/2 centers. For 2, the effective
magnetic moment steadily decreased as the temperature was lowered from 300 K
(ꢉeff ¼ 13.18ꢉB) to 65 K (ꢉeff ¼ 9.75ꢉB) and then abruptly to reach 2.01ꢉB at 2.0 K. The
value of ꢉeff at 300 K is smaller than the expected value (14.49ꢉB) for six noninteracting
FeIII (S ¼ 5/2). This is characteristic of antiferromagnetic coupling, which is confirmed
by fitting to the Curie–Weiss law with ꢃ ¼ ꢁ22.4 K using data from 65 K to 300 K. With
lowering temperature, interactions among the iron cluster become obvious. At 2 K the
moment ꢉeff is 2.01ꢉB.
4. Conclusion
We have synthesized a macrocyclic hexanuclear iron(III) 18-metallacrown-6 complex,
[FeIII6(C12H8N2O5Br)6(H2O)2(CH3OH)4] ꢀ 8H2O ꢀ 8CH3OH, using
a
heptadentate

Iron(III) 18-metallacrown-6 complex
3781
Figure 3. Plots of ꢊꢁM1 (ꢂ), ꢉeff (h) vs. T and fitting line of ꢉeff (—) for 2.
ligand, (Z)-3-(50-bromosalicylhydrazinocarbonyl) propenoic acid. Single-crystal X-ray
structure analysis of 2 showed that the neutral Fe(III) complex contains an
18-membered metallacrown ring consisting of six [Fe(III)–N–N] units. Due to the
meridional coordination of the ligand to Fe3þ, the ligand enforces the stereochemistry
of Fe3þ as a propeller configuration with alternating Ã/D forms. The other
important structural features in 2 are that there is a vacant cavity in the center of
18-metallacrown-6 core ring and opposite chiralities on the two faces of the
metallacrown ring system. In the self-assembly all ligands were esterified. The
metallacrown can be treated with SnCl2 to obtain purified ester. In addition, we have
also obtained methyl 3-(50-bromosalicylhydrazinocarbonyl) propanoate, with Zn
powder.
Supplementary material
CCDC 679990 contains the supplementary crystallographic data for 2. The data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK;
Fax: þ44 1223 336 033; or Email: deposit@ccdc.cam.ac.uk).
Acknowledgments
This work was supported by the Natural Science Foundation of South-Central
University for Nationalities (YZZ07005), the Natural Science Foundation of Hubei
Province, China (2007ABA121), and the Key Project of Natural Science Foundation of
Hubei Province, China (2008CDA067).

3782
C. Jin et al.
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