Iron(III) Metallacryptand and Metallacryptate Assemblies Derived from Aroylbis(N,N-diethylthioureas)

Article abstract of DOI:10.1021/acs.inorgchem.7b01909

The reaction of isophthaloylbis(N,N-diethylthiourea), H₂L¹, with FeCl₃·6H₂O gives the dinuclear tris-complex [Fe₂(L¹)₃] (5), possessing a cryptand-like structure. A similar reaction with the ligand 2,6-dipicolinoylbis(N,N-diethylthiourea), H₂L², however, results in the formation of the anionic, mononuclear Fe(III) complex [Fe(L²)₂]^- (6), which could be isolated as its "Tl⁺ salt" by the subsequent addition of Tl(NO₃). A tighter view to the solid state structure of the obtained product, however, characterizes compound 6 as a one-dimensional coordination polymer, in which four-coordinate Tl⁺ ions connect the {[Fe(L²)₂]^-} units to infinite chains. When Fe³⁺ ions and Tl⁺ ions are added to H₂L² simultaneously in a one-pot reaction, a different product is obtained: a cationic trinuclear complex of the composition {M?[Fe₂(L²)₃]}⁺. It has been isolated as a PF₆^- salt and represents a {2}-metallacryptate with a nine-coordinate Tl⁺ ion in the central void. Structurally related products of the compositions {M?[Fe₂(L²)₃]}(PF₆) (M = Na⁺, K⁺, Rb⁺) (8(PF₆)) could be isolated from analogous reactions with alkaline salts instead of Tl(NO₃). {2}-Metallacryptates with larger central voids were synthesized with the ether-spaced aroylbis(N,N-diethylthiourea) H₂L³. The compounds {M?[Fe₂(L³)₃]}(PF₆) (M = K⁺, Rb⁺, Tl⁺ or Cs⁺) (9(PF₆)) were prepared by a similar protocol like those with H₂L² with the simultaneous addition of the metal ions to a solution of H₂L³. Due to the larger spacer between the aroylthiourea units, the coordination number of the central M⁺ ions is 12 by six carbonyl and six ether oxygen atoms. All products were characterized by elemental analysis, IR spectroscopy, and X-ray structure analysis. Cyclic voltammetric studies were carried out with the three representative complexes [Fe₂(L¹)₃], {K?[Fe₂(L²)₃]}(PF₆), and {K?[Fe₂(L³)₃]}(PF₆). The obtained voltammograms indicate the dependence of the redox properties of the oligonuclear systems on the conjugation in the organic backbones of the ligands.

Full text of DOI:10.1021/acs.inorgchem.7b01909

Article
pubs.acs.org/IC
Iron(III) Metallacryptand and Metallacryptate Assemblies Derived
from Aroylbis(N,N‑diethylthioureas)
Chien Thang Pham,†,§ Hung Huy Nguyen,‡ Adelheid Hagenbach,† and Ulrich Abram*,†
†Institute of Chemistry and Biochemistry, Freie Universitat Berlin, Fabeckstrasse 34-36, D-14195 Berlin, Germany
̈
‡Department of Inorganic Chemistry, VNU University of Science, 19 Le Thanh Tong, Hanoi, Vietnam
S
* Supporting Information
ABSTRACT: The reaction of isophthaloylbis(N,N-diethyl-
thiourea), H2L1, with FeCl3·6H2O gives the dinuclear tris-
complex [Fe2(L1)3] (5), possessing a cryptand-like structure. A
similar reaction with the ligand 2,6-dipicolinoylbis(N,N-
diethylthiourea), H2L2, however, results in the formation of
the anionic, mononuclear Fe(III) complex [Fe(L2)2]− (6),
which could be isolated as its “Tl+ salt” by the subsequent
addition of Tl(NO3). A tighter view to the solid state structure
of the obtained product, however, characterizes compound 6
as a one-dimensional coordination polymer, in which four-
coordinate Tl+ ions connect the {[Fe(L2)2]−} units to infinite
chains. When Fe3+ ions and Tl+ ions are added to H2L2 simultaneously in a one-pot reaction, a different product is obtained: a
−
cationic trinuclear complex of the composition {M⊂[Fe2(L2)3]}+. It has been isolated as a PF6 salt and represents a {2}-
metallacryptate with a nine-coordinate Tl+ ion in the central void. Structurally related products of the compositions
{M⊂[Fe2(L2)3]}(PF6) (M = Na+, K+, Rb+) (8(PF6)) could be isolated from analogous reactions with alkaline salts instead of
Tl(NO3). {2}-Metallacryptates with larger central voids were synthesized with the ether-spaced aroylbis(N,N-diethylthiourea)
H2L3. The compounds {M⊂[Fe2(L3)3]}(PF6) (M = K+, Rb+, Tl+ or Cs+) (9(PF6)) were prepared by a similar protocol like those
with H2L2 with the simultaneous addition of the metal ions to a solution of H2L3. Due to the larger spacer between the
aroylthiourea units, the coordination number of the central M+ ions is 12 by six carbonyl and six ether oxygen atoms. All products
were characterized by elemental analysis, IR spectroscopy, and X-ray structure analysis. Cyclic voltammetric studies were carried
out with the three representative complexes [Fe2(L1)3], {K⊂[Fe2(L2)3]}(PF6), and {K⊂[Fe2(L3)3]}(PF6). The obtained
voltammograms indicate the dependence of the redox properties of the oligonuclear systems on the conjugation in the organic
backbones of the ligands.
components for the construction of such systems will be
discussed.
INTRODUCTION
■
Metallacryptands (1) and metallacryptates (2) belong to one of
the most well-known classes of compounds, helicates, which
have been extensively studied in metallo-supramolecular
chemistry.1−15 In addition to the topological analogy between
the well-known cryptands and metallacryptands, these com-
pounds have a number of advantages over their organic
counterparts. Particularly, they are accessible in high yields in
one-pot reactions and exhibit spectroscopic, electronic, and
magnetic properties, which are inaccessible with their organic
analogues.16,17 Some strategies, namely, Raymond’s symmetry-
interaction method18 and Saalfrank and Albrecht’s template-
directed approach,19−22 have been developed and widely used
for rational syntheses of aesthetic metallacryptands/metal-
lacryptates.
N-Benzoyl(N′,N′-dialkylthioureas) are known as versatile
ligands, which form stable complexes with a large number of
transition metal ions.23 In the majority of structurally
characterized complexes, the ligands act as monoanionic,
bidentate S,O chelators. Such a coordination mode also plays
a vital role in binuclear complexes of the extended bipodal
ligand isophthaloylbis(N,N-diethylthiourea) H2L1 with divalent
transition metal ions such as Co2+, Ni2+, Cu2+, Pd2+, or Pt2+
(3),24−26 but also with In3+ (4) or the {ReO}2+ or {Re2O3}4+
cores.27,28
Recently, a number of heterometallic host−guest assemblies
of different topologies have been reported with 2,6-
dipicolinoylbis(N,N-diethylthiourea) H2L2, a ligand, which has
the same molecular skeleton as H2L1, but with a central
disubstituted pyridine ring instead of a phenylene ring.29 This
results in a completely different coordination chemistry. The
Due to the strict requirements of chemical information being
encoded in the subunits, however, the selection of appropriate
building blocks continues to be a challenge in the designing of
self-assembled metallacryptands. In this report, the potential
exploitation of aroylbis(N,N-dialkylthioureas) as molecular
Received: July 26, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01909
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Article
H2L3 in combination with other metal ions such as alkaline
metal ions or Tl+.
EXPERIMENTAL SECTION
■
Materials. All chemicals used in this study were reagent grade and
used without further purification. Solvents were dried and distilled.
N,N-Diethylthiourea was synthesized following the procedure reported
by Yokoyama et al.30 The preparation of 2,2′-[1,2-phenylenebis-
(oxy)]diacetic acid was adopted from the literature.31
Physical Measurements. IR spectra were measured from KBr
pellets on a Shimadzu FTIR 8300 spectrometer between 400 and 4000
cm−1. NMR spectra were recorded on a JEOL 400 MHz spectrometer.
ESI mass spectra were measured with an Agilent 6210 ESI-TOF
(Agilent Technology) mass spectrometer. Elemental analyses of
carbon, hydrogen, nitrogen, and sulfur were determined using a
Heraeus vario EL elemental analyzer. Reproductions of the IR, NMR,
and MS spectra are given as Supporting Information. Cyclic
voltammetry was performed at room temperature at a platinum
electrode in DMF solutions containing (n-Bu4N)(PF6) (0.15 M) and
the complex (0.2 mM), under an argon atmosphere. A three electrode
setup with a Gamry PCI4/300 potentiostat board and the PHE200
software was used. Redox potentials were internally referenced against
ferrocenium/ferrocence (Fc+/Fc).
Syntheses of the Ligands. H2L1. The ligand was synthesized
according to the method outlined by Douglas and Dains.32 Yield: 70%.
Elemental analysis calcd for C18H26N4O2S2: C, 54.80; H, 6.64; N,
14.20; S, 16.25%. Found: C, 55.16; H, 6.38; N, 14.36; S, 15.96%. IR
(KBr, cm−1): 3119 (m), 2974 (m), 2938 (m), 2876 (m), 1690 (s),
1537 (s), 1449 (s), 1431 (s), 1281 (m), 1215 (s), 1130 (s), 1076 (m),
1
887 (w), 725 (m). H NMR (400 MHz, CDCl3, ppm): 9.03 (s, 2H,
NH); 8.25 (s, 1H, Ph), 7.97 (dd, J = 7.6 Hz, 1.6 Hz, 2H, Ph), 7.53 (t, J
= 8.0 Hz, 1H, Ph); 4.01 (q, J = 7.0 Hz, 4H, CH2); 3.59 (q, J = 7.0 Hz,
4H, CH2), 1.35 (t, J = 7.2 Hz, 6H, CH3); 1.30 (t, J = 7.2 Hz, 6H,
CH3). 13C{1H} NMR (CDCl3, ppm): 179.5 (CO); 163.6 (CS);
133.0, 132.8, 129.5, 126.8 (Ph); 47.9, 47.6 (CH2); 13.4, 11.5 (CH3).
ESI+ MS (m/z): 811.2903 (calcd 811.2892), 81% [2M + Na]+;
433.1138 (calcd 433.1134), 52% [M + K]+; 417.1401 (calcd
417.1395), 100% [M + Na]+; 395.1574 (calcd 395.1575), 10% [M +
H]+.
reported complexes were formed by self-assembly, but the
obtained structural features can readily be understood following
basic concepts of inorganic and coordination chemistry, such as
Pearson’s concept of hard and soft acids and bases,13 the size or
preferred coordination polyhedra of the used metal ions. Thus,
“hard” and large metal ions, such as lanthanide or alkaline or
alkaline earth ions, are preferably directed to the central void
formed by two or three ligands {L2}2−, while octahedral
transition metal ions coordinate to the S,O chelating units.29
With regard to the obtained host−guest chemistry, it could be
interesting to increase the central void further in order to host
larger metal ions and/or to allow a controlled metal-exchange
behavior in this position. A potentially suitable ligand is the
novel catechol derivative H2L3, which not only increases the
size of the central void but also provides more donor atoms for
the coordination of large metal ions.
H2L2. The synthesis of H2L2 was performed by the procedure
published by Dixon and Taylor with some modifications.33 A mixture
of previously dried dipicolinic acid (3.34 g, 0.02 mol), SOCl2 (30 mL,
0.4 mol), and a few drops of DMF was heated on reflux under an
argon atmosphere until a clear solution was obtained. Subsequently,
the reaction mixture was stirred for two more hours and the residual
SOCl2 was removed under reduced pressure. This gives a colorless
solid of dipicolinoyl dichloride. This solid was dissolved in dry THF
(60 mL) and added dropwise at 0 °C (ice bath) to a mixture of N,N-
diethylthiourea (5.28 g, 0.04 mol) and Et3N (8.5 mL, 0.06 mol) in dry
THF (30 mL) under an argon atmosphere. The resulting mixture was
warmed up to 40−50 °C and stirred for 2 h. After cooling to room
temperature, the formed colorless precipitate of (Et3NH)Cl was
filtered off and the THF was removed under reduced pressure. The
resulting solid was washed thoroughly with MeOH, which finally gave
the ligand H2L2 as a colorless solid. Yield: 92% (7.27 g). Elemental
analysis calcd for C17H25N5O2S2: C, 51.62; H, 6.37; N, 17.71; S,
16.21%. Found: C, 51.70; H, 6.29; N, 17.79; S, 15.79%. IR (KBr,
cm−1): 3267 (m), 2974 (w), 2934 (w), 2874 (w), 1674 (s), 1520 (s),
1449 (s), 1418 (s), 1277 (m), 1225 (s), 1101 (w), 997 (w), 752 (m).
1H NMR (400 MHz, CDCl3, ppm): 9.82 (s, 2H, NH); 8.41 (d, J = 8.0
Hz, 2H, m-Py), 8.10 (t, J = 8.0 Hz, 1H, p-Py), 4.04 (q, br, J = 6.4 Hz,
4H, CH2); 3.67 (q, br, J = 6.4 Hz, 4H, CH2); 1.35 (s, br, 12H, CH3).
13C{1H} NMR (CDCl3, ppm): 178.3 (CO); 159.3 (CS); 148.0,
139.8, 126.9 (Py); 47.8, (CH2); 13.6, 11.5 (CH3). ESI+ MS (m/z):
813.2796 (calcd 813.2797), 85% [2M + Na]+; 434.1085 (calcd
434.1087), 10% [M + K]+; 418.1349 (calcd 418.1347), 100% [M +
Na]+.
H2L3. The ligand H2L3 was prepared by a similar procedure as
described for H2L2. A mixture of the dry dicarboxylic acid (4.52 g, 0.02
The present communication reports the coordination
chemistry of Fe(III) ions with the ligands H2L1, H2L2, and
B
DOI: 10.1021/acs.inorgchem.7b01909
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Article
mol) prepared from catechol and two equivalents of chloroacetic acid
and an excess of SOCl2 (30 mL, 0.4 mol) was heated to reflux under
an argon atmosphere for 3 h until a clear solution was obtained. The
resulting reaction mixture was stirred for 2 more hours, and the
residual SOCl2 was removed under reduced pressure. The remaining
solid was dissolved in dry THF (60 mL) and added dropwise at 0 °C
(ice bath) under an argon atmosphere to a mixture of N,N-
diethylthiourea (5.28 g, 0.04 mol) and Et3N (12.8 mL, 0.04 mol) in
dry THF (30 mL). After warming to 40−50 °C and stirring for 2 h,
the mixture was cooled to room temperature. A colorless precipitate of
(Et3NH)Cl was filtered off, and THF was removed under reduced
pressure. The resulting solid was washed thoroughly with MeOH to
give the ligand H2L3 as a colorless solid. Yield: 70% (6.36 g).
Elemental analysis calcd for C20H30N4O4S2: C, 52.84; H, 6.65; N,
12.32; S, 14.11%. Found: C, 52.72; H, 6.81; N, 12.05; S, 13.94%. IR
(KBr, cm−1): 3154 (m), 2982 (w), 2938 (w), 1667 (s), 1528 (s), 1456
(m), 1427 (m), 1279 (m), 1234 (s), 1200 (s), 1142 (m), 1061 (w),
vacuum. Single crystals suitable for X-ray analysis were obtained by
slow evaporation of solutions of the complexes in CH2Cl2/MeOH
mixtures.
{Rb⊂[Fe2(L2)3]}(PF6). Yield: 67% (49 mg). Elemental analysis calcd
for C51H69O6N15S6PF6Fe2Rb: C, 40.23; H, 4.57; N, 13.80; S, 12.63%.
Found: C, 40.54; H, 4.62; N, 13.48; S, 12.87%. IR (KBr, cm−1): 3427
(w), 2976 (w), 2934 (w), 1510 (s), 1433 (s), 1412 (s), 1356 (m),
1252 (m), 1150 (w), 839 (s), 746 (w), 557 (w). ESI+ MS (m/z):
1376.1625 (calcd 1376.1696), 39% {Rb⊂[Fe2(L2)3]}+; 1330.2156
(calcd 1330.2215), 100% {K⊂[Fe2(L2)3]}+.
{K⊂[Fe2(L2)3]}(PF6). Yield: 75% (65 mg). Elemental analysis calcd
for C51H69O6N15S6PF6Fe2K: C, 41.49; H, 4.71; N, 14.23; S, 13.03%.
Found: C, 41.52; H, 4.65; N, 14.25; S, 13.18%. IR (KBr, cm−1): 3441
(w), 2974 (w), 2934 (w), 1506 (s), 1435 (s), 1355 (m), 1254 (m),
1196 (w), 1076 (w), 841 (m). ESI+ MS (m/z): 1330.2180 (calcd
1330.2215), 100% {K⊂[Fe2(L2)3]}+.
{Na⊂[Fe2(L2)3]}(PF6). Yield: 90% (67 mg). Elemental analysis calcd
for C51H69O6N15S6PF6Fe2Na: C, 41.95; H, 4.76; N, 14.39; S, 13.18%.
Found: C, 41.72; H, 4.53; N, 14.14; S, 13.42%. IR (KBr, cm−1): 2974
(w), 2936 (w), 1541 (m), 1506 (s), 1437 (s), 1417 (s), 1355 (m),
1256 (m), 1152 (w), 1076 (w), 839 (m). ESI+ MS (m/z): 1330.2172
(calcd 1330.2215), 100% {K⊂[Fe2(L2)3]}+; 1314.2408 (calcd
1314.2476), 13% {Na⊂[Fe2(L2)3]}+.
1
961 (w), 754 (m). H NMR (400 MHz, CDCl3, ppm): 8.92 (s, 2H,
NH); 7.06−7.01 (m, 2H, Ph); 6.98−6.93 (m, 2H, Ph); 4.66 (s, 4H,
OCH2); 3.95 (q, J = 6.4 Hz, 4H, NCH2); 3.55 (q, J = 6.4 Hz, 4H,
NCH2); 1.31 (t, J = 6.4 Hz, 6H, CH3), 1.23 (t, J = 6.8 Hz, 6H, CH3).
13C{1H} NMR (CDCl3, ppm): 177.8 (CO); 165.1 (CS); 147.5,
123.7, 115.5 (Ph); 69.4 (OCH2); 47.8 (NCH2); 13.6, 11.5 (CH3).
ESI+ MS (m/z): 931.3332 (calcd 931.3315), 85% [2M + Na]+;
493.1349 (calcd 493.1345), 10% [M + K]+; 477.1617 (calcd
477.1606), 100% [M + Na]+; 455.1796 (calcd 455.1787), 5% [M +
H]+.
{Tl⊂[Fe2(L2)3]}(PF6). Yield: 56% (46 mg). Elemental analysis calcd
for {Tl⊂[Fe2(L2)3]}(PF6)·3H2O, C51H75O9N15S6PF6Fe2Tl: C, 36.13;
H, 4.46; N, 12.39; S, 11.34%. Found: C, 35.95; H, 4.37; N, 12.07; S,
11.62%. IR (KBr, cm−1): 3431 (w), 2976 (w), 2935 (w), 2872 (w),
1508 (s), 1431 (s), 1410 (s), 1356 (m), 1252 (m), 1150 (w), 839 (s),
746 (w), 557 (w). ESI+ MS (m/z): 1330.2259 (calcd 1330.2215),
100% {K⊂[Fe2(L2)3]}+.
Syntheses of the Complexes. [Fe2(L1)3]. H2L1 (59.2 mg, 0.15
mmol) was added to a solution of FeCl3·6H2O (27.0 mg, 0.1 mmol) in
1 mL of MeOH. The ligand dissolved quickly, and a dark red
precipitate deposited immediately from the solution after the addition
of 2 drops of Et3N. After stirring for 2 h at 50 °C, the product was
filtered off, washed with a small amount of MeOH, and dried in
vacuum. Single crystals suitable for X-ray analysis were obtained by
slow evaporation of a solution of the complex in DMF. Yield: 82% (53
mg). Elemental analysis calcd for C54H72O6N12S6Fe2: C, 50.30; H,
5.63; N, 13.04; S, 14.92%. Found: C, 50.15; H, 5.54; N, 13.09; S,
14.60%. IR (KBr, cm−1): 3429 (w), 2976 (w), 2932 (w), 2870 (w),
1504 (s), 1422 (vs), 1354 (m), 1256 (w), 1076 (w), 725 (w). ESI+ MS
(m/z): 1327.2321 (calcd 1327.2358), 12% [M + K]+; 1311.2590
(calcd 1311.2619), 33% [M + Na]+; 1289.2771 (calcd 1289.2799),
30% [M + H]+.
{M⊂[Fe2(L3)3]}(PF6) (M = Cs+, Rb+, K+, Tl+). H2L3 (68.2 mg, 0.15
mmol) was added to a solution of FeCl3·6H2O (27.0 mg, 0.1 mmol)
and 0.05 mmol of MCl (M = Cs+, Rb+ or K+) in 1 mL of MeOH and a
few drops of water. In the case of the Tl+ complex, Fe(NO3)3·9H2O
and TlNO3 were used as starting materials. The ligand dissolved
quickly, and the color of the solution changed immediately to dark red.
The mixture was stirred at room temperature for 30 min, and 3 drops
of Et3N were added. After stirring at room temperature for an
additional 30 min, (n-Bu4N)(PF6) (19.4 mg, 0.05 mmol) was added
and the temperature was increased to 50 °C and kept for 2 h. During
this process, a dark red precipitate deposited. The product was filtered
off, washed with a small amount of MeOH, and dried in vacuum.
Single crystals suitable for X-ray analysis were obtained by slow
evaporation of solutions of the complexes in CH2Cl2/MeOH mixtures.
{Cs⊂[Fe2(L3)3]}(PF6). Yield: 85% (74 mg). Elemental analysis calcd
for C60H84O12N12S6PF6Fe2Cs: C, 41.24; H, 4.85; N, 9.62; S, 11.01%.
Found: C, 40.94; H, 4.62; N, 9.37; S, 10.87%. IR (KBr, cm−1): 3427
(w), 2980 (w), 2936 (w), 1560 (m), 1533 (m), 1506 (vs), 1425 (vs),
1344 (m), 1248 (m), 1200 (m), 1126 (m), 841 (s), 745 (w), 557 (w).
ESI+ MS (m/z): 1601.2424 (calcd 1601.2409), 100%
{Cs⊂[Fe2(L3)3]}+; 1507.2968 (calcd 1507.2992), 16%
{K⊂[Fe2(L3)3]}+.
Tl[Fe(L2)2]. H2L2 (79.0 mg, 0.2 mmol) was added to a solution of
Fe(NO3)3·9H2O (40.4 mg, 0.1 mmol) in 1.5 mL of MeOH. The color
of the solution changed immediately to dark red. After stirring for 30
min at room temperature, 2 drops of Et3N and then TlNO3 (26.6 mg,
0.1 mmol) dissolved in H2O (0.5 mL) were added to the reaction
mixture. The temperature was increased to 50 °C and kept for 2 h.
Slow evaporation of the solvent at room temperature resulted in
precipitation of dark-red crystals, which were suitable for X-ray
analysis. The product was filtered off, washed with cold MeOH, and
dried in vacuum. Yield: 62% (65 mg). Elemental analysis calcd for
Tl[Fe(L2)2]·H2O, C34H48O5N10S4FeTl: C, 38.33; H, 4.54; N, 13.15; S,
12.04%. Found: C, 38.12; H, 4.42; N, 12.93; S, 11.80%. IR (KBr,
cm−1): 3422 (w), 2972 (w), 2932 (w), 2872 (w), 1601 (vs), 1499 (s),
1427 (s), 1360 (s), 1250 (s), 1099 (w), 957 (w), 756 (w). ESI− MS
{Rb⊂[Fe2(L3)3]}(PF6). Yield: 88% (75 mg). Elemental analysis calcd
for C60H84O12N12S6PF6Fe2Rb: C, 42.39; H, 4.98; N, 9.89; S, 11.32%.
Found: C, 42.42; H, 4.97; N, 9.75; S, 11.46%. IR (KBr, cm−1): 3441
(w), 2978 (w), 2932 (w), 1558 (m), 1539 (m), 1504 (vs), 1427 (vs),
1346 (m), 1254 (m), 1200 (w), 1126 (m), 841 (s), 744 (w), 559 (w).
ESI+ MS (m/z): 1553.2511 (calcd 1553.2473), 100%
{Rb⊂[Fe2(L3)3]}+; 1507.2972 (calcd 1507.2992), 13%
{K⊂[Fe2(L3)3]}+.
(m/z): 842.1966 (calcd 842.1936), 6% [Fe(L2)2]−, 416.1231 (calcd
−
416.1191), 25% [Na(L2)]
.
{M⊂[Fe2(L2)3]}(PF6) (M = Rb+, K+, Na+, Tl+). H2L2 (59.3 mg, 0.15
mmol) was added to a solution of FeCl3·6H2O (27.0 mg, 0.1 mmol)
and 0.05 mmol of MCl (M = Rb+, K+, or Na+) in 1 mL of MeOH and
a few drops of H2O were added. Due to the low solubility of TlCl in
H2O and MeOH in the case of Tl+, the nitrate salts, Fe(NO3)3·9H2O
and TlNO3, were used. The ligand dissolved rapidly, and the color of
the solution changed immediately to dark. The mixture was stirred for
30 min at room temperature, and 3 drops of Et3N were added. After
stirring for additional 30 min, (n-Bu4N)(PF6) (19.4 mg, 0.05 mmol)
was added, and the temperature was increased to 50 °C and kept for 2
h. During this time, a dark precipitate deposited. The product was
filtered off, washed with a small amount of MeOH, and dried in
{K⊂[Fe2(L3)3]}(PF6). Yield: 81% (65 mg). Elemental analysis calcd
for C60H84O12N12S6PF6Fe2K: C, 43.58; H, 5.12; N, 10.17; S, 11.64%.
Found: C, 43.58; H, 5.46; N, 10.05; S, 11.64%. IR (KBr, cm−1): 3429
(w), 2978 (w), 2932 (w), 1558 (m), 1539 (m), 1504 (vs), 1427 (vs),
1346 (m), 1254 (m), 1196 (w), 1126 (m), 841 (s), 741 (w), 559 (w).
ESI+ MS (m/z): 1507.3000 (calcd 1507.2992), 100%
{K⊂[Fe2(L3)3]}+.
{Tl⊂[Fe2(L3)3]}(PF6). Yield: 59% (54 mg). Elemental analysis calcd
for C60H84O12N12S6PF6Fe2Tl: C, 39.62; H, 4.66; N, 9.24; S, 10.58%.
Found: C, 39.82; H, 4.73; N, 9.28; S, 10.17%. IR (KBr, cm−1): 2978
C
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Scheme 1. Synthesis of the Ether-Spaced Ligand H2L3
corresponding [M + Na]+ and [M + K]+ ions in the ESI+ MS
spectrum are clear evidence for the formation of a neutral
complex with the composition of [Fe2(L1)3]. This is confirmed
by an X-ray diffraction analysis of single crystals obtained from
slow evaporation of a solution of the complex in DMF. Figure 1
depicts the molecular structure of the compound. Selected
bond lengths and torsion angles are given in Table S2.2.
(w), 2936 (w), 1557 (s), 1506, 1425 (s), 1342 (m), 1254 (m), 1198
(m), 1125 (m), 841 (s), 745 (w), 557 (w). ESI+ MS (m/z): 1673.3220
(calcd 1673.3100), 84% {Tl⊂[Fe2(L3)3]}+; 1507.3108 (calcd
1507.2992), 100% {K⊂[Fe2(L3)3]}+.
X-ray Crystallography. The intensities for the X-ray determi-
nations of [TlFe(L2)2]·0.5CH2Cl2, {Rb⊂[Fe2(L2)3]}(PF6)·0.5CH2Cl2,
{K⊂[Fe2(L2)3]}(PF6)·0.5CH2Cl2, {Cs⊂[Fe2(L3)3]}(PF6),
{Rb⊂[Fe2(L3)3]}(PF6), {K⊂[Fe2(L3)3]}(PF6), and {Tl⊂[Fe2(L3)3]}-
(PF6) were collected on a STOE IPDS 2T instrument at 200 K with
Mo Kα radiation (λ = 0.71073 Å) using a graphite monochromator.
The intensities for the X-ray determinations of [Fe2(L1)3],
{Na⊂[Fe2(L2)3]}(PF6)·CH2Cl2, and {Tl⊂[Fe2(L2)3]}(PF6)·CH2Cl2
were collected on a Bruker D8 Venture instrument at 100 K with
Mo Kα radiation (λ = 0.71073 Å) using a TRIUMPH mono-
chromator. Standard procedures were applied for data reduction and
absorption correction. Structure solution and refinement were
performed with the SHELXS 97, SHELXS 86, and SHELXL 2014/7
programs included in the WinGX program package.34−36 Hydrogen
atoms were calculated for idealized positions and treated with the
“riding model” option of SHELXL. Crystal data and structure
determination parameters are given in Table S2.1. Additional
information on the structure determinations has been deposited with
the Cambridge Crystallographic Data Centre.
RESULTS AND DISCUSSION
■
Figure 1. Molecular structure of [Fe2(L1)3] (5). Hydrogen atoms were
omitted for clarity.
Three aroylbis(N,N-dialkylthiourea) ligands, namely,
isophthaloylbis(N,N-diethylthiourea) H2L1, 2,6-dipicolinoylbis-
(N,N-diethylthiourea) H2L2, and the novel ether-spaced H2L3,
have been used for the syntheses of binuclear iron complexes.
Syntheses and characterization of the ligands H2L1 and H2L2
have been reported previously.28,33 The synthesis of H2L3 starts
with the preparation of a dicarboxylic acid derived from
catechol.31 Then, this acid is converted into its dichloride by
being refluxed in an excess of SOCl2. The reaction of the
dichloride with N,N-diethylthiourea in the presence of Et3N
gives the ligand H2L3 in 70% yield (Scheme 1).
The coordination environments of the metal ions are similar
to those in the In(III) complex, which has been recently
reported.27 In particular, the two iron ions coordinate with
three bis(bidentate) deprotonated ligands (L1)2− in distorted
octahedral coordination modes with a facial arrangement of the
donor atoms. The Fe−O and Fe−S bond lengths are in the
same ranges as those recently reported for an Fe(III) complex
of the simple bidentate benzoyl-N,N-diethylthiourea ligand.37
Despite considerable deviation of the six-membered chelate
rings from planarity, which is evident from the O−C−N−C
and C−N−C−S torsion angles (Table S2.2), the typical
extended π-systems for chelating aroylthiourea moieties are
indicated by partial double bond character of the C−O, C−N,
and C−S bonds.29 Additionally, slightly longer C(S)−Namide
bonds in comparison with the adjacent C(O)−Namide bonds
illustrate that electron density of the S,O chelates locates less on
the “soft” S donors than on the “hard” O donors, which are in
favor of the “hard” acceptor Fe3+. With its empty void, the
cryptand-like metallacycle [Fe2(L1)3] could be suitable as hosts
for metal ions. However, all attempts to produce such inclusion
complexes with small cations, even with Li+, have failed. This
result is probably due to the fact that the coordination capacity
of the central void is insufficient for cation binding, and more
seriously, the direction of three inner hydrogen atoms of the
phenylene spacers pointing toward the centers of the cavities
remarkably reduces their effective size. These complications will
be overcome, when the internal phenylene C−H group of H2L1
is replaced by a pyridine N atom as being done with the
synthesis of H2L2.
The IR spectrum of H2L3 is characterized by a medium broad
absorption at 3163 cm−1 and a strong absorption at 1659 cm−1,
which can be assigned to the stretching vibrations of the NH
1
and CO groups, respectively. The H NMR spectrum of
H2L3 in CDCl3 presents a broad signal at 8.92 ppm, which
confirms the presence of NH protons. Another broad signal at
4.66 ppm is assigned to the methylene protons of OCH2. Two
well-separated signal sets of two ethyl groups indicate the
hindered rotation around the C(S)−NEt2 bond, which is
normally observed for other aroyl-N,N-dialkylthioureas.23−29
When the ligand H2L1 reacts with iron(III) chloride in
MeOH, after the addition of the supporting base Et3N, a pure
product with the composition of [Fe2(L1)3] deposits directly
from the reaction mixture as a dark-red solid. In the IR
spectrum of the compound, no absorptions in the region above
3100 cm−1 corresponding to νNH stretches are observed. This is
a sign of the deprotonation of the ligand during complexation.
A bathochromic shift of the νCO band from 1690 cm−1 in
H2L1 to 1504 cm−1 in the complex strongly reflects the
formation of S,O chelates. The result of the elemental analysis
and the detection of the expected molecular ions as well as
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Scheme 2. Synthesis of the Fe/Tl Mixed-Metal Complexes with the Ligand H2L2
Assuming that the S,O coordination mode of the ligand
remains after the modification of the spacer, a neutral {2}-
metallacryptand with the composition of [Fe2(L2)3] should
result from the reaction of Fe3+ ions and H2L2 under the same
conditions used for the synthesis of the previous complex. But
all our attempts to isolate such a neutral iron(III) cryptand
failed. The formed dark red complex was very well soluble in
MeOH, which is a strong indicator for the formation of an ionic
compound. After efforts to precipitate such species with
different counterions, finally a crystalline product was isolated
after the addition of Tl(I) ions with a yield of 62%. The IR
spectrum of the compound exhibits a strong νCO band at
1601 cm−1, which corresponds to a bathochromic shift of
approximately 70 cm−1 with respect to the uncoordinated
ligand. Compared to the standard value of approximately 200
cm−1 reported for benzoylthioureato chelates,27,28,37 the shift is
quite moderate and might be caused by a smaller degree of
electron delocalization within the aroylthiourea moieties. The
absence of a νNH band in the region above 3100 cm−1 indicates
the expected deprotonation of the ligand. Slow evaporation of
the solution of the product in a CH2Cl2/MeOH mixture gave
single crystals of good quality. An X-ray diffraction analysis
exhibits a structure against the expectation of the neutral {2}-
metallacryptand [Fe2(L2)3]. The composition of the un-
expected compound can be described as an ion pair between
[Fe(L2)2]− anions (6) and Tl+ cations or the [TlFe(L2)2]∞
polymer (7) (Scheme 2). It is supported by elemental analysis
and the presence of a peak associated with the fragment
[Fe(L2)2]− in the ESI− MS spectrum. Figure 2a illustrates the
structure of the unusual mixed-metal complex. Selected bond
lengths are summarized in Table 1.
The Fe3+ ion is six-coordinate with a distorted octahedral
environment formed by two planar donor atom sets, (N,N,N)
Figure 2. (a) Molecular structure of [TlFe2(L2)2]∞ (7). (b) View
along the N01−Fe−N31 connecting line. Hydrogen atoms were
omitted for clarity.
and (O,N,N). Each of them belongs to one doubly
deprotonated pincer ligand (L2)2−. The former donor sets
consist of the pyridine N atom and two amide N atoms of the
first and second aroyl thiourea arms. Such a coordination mode
is found in the dinuclear Ni(II) complex of the ligand H2L2,29
and it is commonly observed in several bis-chelates of pincer-
type ligands based on pyridine-2,6-dicarboxamide.38−41 The
latter donor sets comprise the pyridine N atom of the other
ligand, the carbonyl O atom of its first arm, and the amide N
atom of the second arm. The fact that the C−O and C−S
bonds in the amide-coordinated arms are shorter than those in
the chelate rings of [Fe2(L1)3] implies the greater double bond
character of these bonds and, thus, the location of the negative
charge of the arms on the amide N atoms, which directly bind
to the Fe3+ ion. A similar charge location on the O40 atom can
be deduced from the remarkably elongated C40−O40 bond
and the shortened C40−N40 bond. The resulting hard Lewis
base {O40}− acts as a bridging ligand between the “hard” Lewis
acids Fe3+ and Tl+.
The Tl+ ions connect the “[Fe(L2)2]− anions” to 1D
polymers by establishing long-range bonds to two sulfur atoms
and the oxygen atom O40 and an oxygen atom of an adjacent
unit. The resulting coordination polyhedron of thallium can be
best described as a square pyramid or a distorted pseudo-
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Table 1. Selected Bond Lengths (Å) in [TlFe(L2)2]∞
Fe−N01
Fe−N10
Fe−N20
Fe−N31
Fe−O40
Fe−N50
Tl−S10
Tl−S40
Tl−O40
Tl−O50i
2.117(5)
2.139(5)
2.064(6)
2.074(5)
2.048(4)
2.132(5)
3.004(2)
2.967(2)
2.799(4)
2.599(5)
O10−C10
C10−N10
N10−C11
C11−N11
C11−S10
O20−C20
C20−N20
N20−C21
C21−N21
C21−S20
1.228(9)
1.356(8)
1.413(8)
1.308(9)
1.701(7)
1.245(8)
1.341(8)
1.415(9)
1.315(9)
1.682(7)
O40−C40
C40−N40
N40−C41
C41−N41
C41−S40
O50−C50
C50−N50
N50−C51
C51−N51
C51−S50
1.302(7)
1.292(8)
1.384(9)
1.323(1)
1.698(8)
1.236(8)
1.344(8)
1.417(8)
1.320(9)
1.659(8)
a
a
Symmetry operation used to generate equivalent atoms: i, x, 3/2 − y, z − 1/2.
triangular bipyramid taking into account a stereochemically
active 6s lone pair of the Tl+ ion as a fifth ligand. An analogous
coordination geometry has been recently reported for Pb2+ ions
in a polymeric complex with the ligand H2L1.27 The
intermolecular Tl−O interaction is responsible for the
formation of the 1D polymeric chain [TlFe(L2)2]∞ in the
solid state (Figure S2.2b) and can be considered as a driving
force for the separation of the anionic species [Fe(L2)2]− from
the reaction mixture by means of the large cation Tl+. Another
striking feature of the complex is the great deviation of the
−C(S)−NEt2 parts from the ligand planes (Figure 2b). Such
deviation prevents electron delocalization within aroylthiourea
moieties, even within the S,O chelate ring surrounding Tl+. This
is in agreement with the moderate bathochromic shift of the
νCO band observed in the IR spectrum and provides an
explanation for the lower degree of double bond character of
the C(S)−Namide bonds compared to the adjacent C−N bonds.
The failed synthesis of the {2}-metallacryptand [Fe(L2)3]
indicates that the “hard” acid Fe3+ favors a mixture of
“borderline” and “hard” base donor sets (N,N,N and O,N,N)
over a mixture of “soft” and “hard” base chelators (S,O). In
order to direct Fe3+ to the S,O-chelating aroylthiourea unit for
coordination, reactions of H2L2 with mixtures of Fe3+ and
“harder” metal ions were carried out. In the above-mentioned
reaction, Tl+ was added only in a second step of the reaction
and did not compete with the Fe3+ ions for the favored pyridine
nitrogen donors, and finally only acts as a “counterion”. When
Tl+ is added together with the Fe3+ ions to H2L2 before the
addition of the supporting base, the reaction gives a completely
different product (Scheme 2). The resulting ionic complex can
be separated as PF6− salt upon addition of (n-Bu4N)(PF6) with
a yield of 56%. The presence of the counteranion is verified by
a strong band in the IR spectrum of the isolated solid at 839
cm−1, which is characteristic of the νP−F vibrations.42 The
disappearance of the νNH band in the region above 3100 cm−1
and the strong bathochromic shift of the νCO band by about
170 cm−1 prove the deprotonation of the ligand and the
formation of S,O chelates with extended π-systems. This
information is a positive sign concerning the formation of the
expected {2}-ironcryptand [Fe2(L2)3]. An X-ray structure
analysis of the resulting complex confirms these conclusions
by revealing a host−guest compound with the composition of
{Tl⊂[Fe2(L2)3]}(PF6) (8a(PF6)). The cationic {2}-ironcryp-
tate presented in Figure 3a is most probably not simply
generated by the capture of the guest Tl+ ion into the void of
the {2}-ironcryptand [Fe2(L2)3] composed of two Fe3+ ions
and three pentadentate deprotonated ligands (L2)2−. The Tl+
ion plays a vital role in the formation of the {2}-ironcryptand
by acting as a template. In particular, the Tl+ ion occupies the
Figure 3. (a) Structure of the cationic Fe(III) {2}-metallacryptate
{Tl⊂[Fe2(L2)3]}+. Hydrogen atoms are omitted for clarity. (b) Axially
truncated trigonal bipyramidal coordination sphere surrounding the
guest cation Tl+.
central coordination positions of the {L2}2− ligands and, thus,
interferes with the potential formation of N,N,N or N,N,O
coordination of the Fe3+ ions as is observed in [TlFe(L2)2]∞. In
the {Tl⊂[Fe2(L2)3]}+ ion, each Fe atom is coordinated in a
distorted octahedral environment with a facial arrangement of
the sulfur atoms as found before for the tris-complexes of
In(III) and Fe(III) with the ligand H2L1. The Fe−S bond
lengths in the range 2.368(4)−2.451(9) Å are in good
agreement with those found in [Fe2(L1)3], while the Fe−O
bonds range between 2.003(3) Å and 2.045(3) Å and are
slightly longer than the corresponding bonds in [Fe2(L1)3]. The
elongation of the Fe−O bonds provides more space and more
electron density for the Tl+ ion in the central void. In the final
arrangement, the three metal ions show an almost linear
arrangement. The requirement of a negative charge to capture
the guest cation causes a partial location of π-electron density
on the oxygen atoms and a less degree of delocalization in the
chelate rings, which is pointed out by the clear distinction
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Scheme 3. Synthesis of the {M⊂[Fe2(L3)3]}(PF6) Complexes
between the adjacent C(O)−Namide and C(S)−Namide bond
lengths (Table S2.3). Adopting an axially truncated trigonal
bipyramidal geometry, the guest cation is nine-coordinate with
six oxygen donors and three pyridine nitrogen donors forming
the mutual base of the bipyramid (Figure 3b).
This coordination mode is hitherto unknown for Tl+ and
very rare for other hard monovalent metal ions such as alkali
metal ions, where only a few examples of potassium-containing
compounds have been reported.43−45 This lack of information
motivated us to undertake ongoing studies with the alkali metal
ions Li+, Na+, K+, Rb+, and Cs+ instead of Tl+.
Applying the same synthetic route as described for the
thallium product (one-pot reactions between FeCl3, alkali metal
chlorides, H2L2, and NEt3 in methanol with subsequent
addition of (n-Bu4N)(PF6)) gave pure products, which
precipitated directly from the reaction mixtures. The proposed
compositions {M⊂[Fe2(L2)3]}(PF6) (M = Rb+ (8b), K+ (8c),
or Na+ (8d)) of the isolated solids are supported by elemental
analyses, IR spectroscopy, and X-ray structural analyses. No
unusual features were found in the IR spectra or in the
molecular structures of the products The structures of these
compounds and selected bond lengths and angles are given in
Figure S2.4 and Table S2.4. Like the Tl+ ions in compound 8a,
the alkali metal ions in compounds 8b−8d possess coordina-
tion environments, which are best described by axially
truncated triangular bipyramids, which consist of three pyridine
nitrogen atoms being located at the vertices of the mutual
triangular base and six carbonyl oxygen atoms occupying the
remaining vertices of the two truncated triangular pyramids.
The ESI+ mass spectra of the inclusion compounds with alkali
metal ions strongly support the existence of the cationic
complex species by the presence of peaks of the fragments
[{M⊂[Fe2(L2)3]}]+ (M = Na+, K+, or Rb+). Meanwhile, only
the peak of the fragment [{K⊂[Fe2(L2)3]}]+, which results
from the exchange of the guest cation with K+ of the MS matrix,
appears in the mass spectrum of 8a.
delocalization of π-electrons, is proved by bathochromic shifts
of the νCO bands from 1660 cm−1 in H2L3 to the region
around 1560 cm−1. These shifts are remarkable, however not as
high as observed in other benzoylthiourea chelates, where these
bands usually appear between 1400 and 1500 cm−1. An
additional strong absorption band around 840 cm−1 is assigned
to the νP−F stretches. The existence of {M⊂[Fe2(L3)3]}+ (9)
species (M = Cs+, Rb+, K+, or Tl+) are also proven by the
detection of corresponding fragments in the ESI+ mass spectra
of the complexes. These observations and the elemental
analyses strongly suggest a general composition of
{M⊂[Fe2(L3)3]}(PF6) (M = Cs+, Rb+, K+, or Tl+) of the
metal−ligand assemblies. Single crystals suitable for X-ray
crystallography were produced from slow evaporation of
CH2Cl2/MeOH solutions.
It should be mentioned that inclusion complexes of the type
8 could not be isolated with Li+ or Cs+ guest ions. This might
be explained by the radii of these ions. While the radii of Na+,
K+, Rb+, and Tl+ are all in the range from 1.18 to 1.61 Å,46 the
size of the Li+ ion (0.92 Å) seems to be too small to be
stabilized in such a 9-coordinate environment. On the other
hand, the cavity formed by the {2}-ironcryptate with three
{L2}2− ligands seems to be too small to host the large Cs+ ions
(1.74 Å). This fact poses the question for the behavior of
similar compounds with the ether-spaced ligand H2L3, which
possesses two oxygen donor atoms instead of the central
pyridine nitrogen atom of H2L2. Thus, it should be able to
establish compounds with larger central cavities, provided it
also undergoes the expected, template-directed formation of
{2}-ironcryptands.
As a representative for the cationic host−guest complexes
{M⊂[Fe2(L3)3]}+, the structure of the corresponding potas-
sium compound is shown in Figure 4a. The structures of the
Rb+, Cs+, and Tl+ inclusion compounds are virtually identical
and shall, thus, not be shown in extra figures. Selected bonding
parameters of all compounds, however, are summarized in
Table 2. A structural analysis of the {M⊂[Fe2(L3)3]}+ cations
shows C3 symmetry for the {2}-metallacryptates. The three
metal ions are situated on a crystallographic 3-fold axis (Figure
4b). As expected, the metallacryptand “[Fe2(L3)3]” is composed
of two Fe(III) ions linked through three doubly deprotonated
ligands (L3)2−. As a consequence, each Fe3+ ion is octahedrally
coordinated to three S,O-chelating aroylthioureato moieties
with facial arrangement of the sulfur atoms. In analogy to the
Fe(III) metallacryptands derived from H2L2, the resulting
Fe(III) {2}-metallacryptand is capable to encapsulate a hard
monovalent cation in the central cavity. Alkaline metal ions like
K+, Rb+, and Cs+ or Tl+ ions are 12-coordinate by six carbonyl
and six ether-oxygen donors.
Despite the fact that the Fe−O and Fe−S bond lengths
(Table 2) are in the same ranges for those in Fe(III) tris-
complexes discussed in the preceding parts, the observed
planarity of chelate rings containing {L3}2− ligands is in direct
contrast to the considerable distortions observed in the related
complexes with {L1}2− or {L2}2−. This significant difference
arises from the higher flexibility of the backbone of the ligand
H2L3. In {L3}2−, free rotation of two aroylthiourea arms around
One-pot reactions of the ligand H2L3 with mixtures of FeCl3
and MCl (M = Cs+, Rb+, K+) or Fe(NO3)3 and TlNO3 in
MeOH result in dark-red solutions. The addition of Et3N and
subsequent workup with (n-Bu4N)(PF6) give dark-red solids,
which are readily soluble in organic solvents such as CH2Cl2 or
chloroform (Scheme 3).
In the IR spectra of products, there are no absorptions in the
region above 3100 cm−1 corresponding to νNH stretches, which
can be found in the spectrum of the uncoordinated ligand. This
is a strong hint for the existence of the doubly deprotonated
form (L3)2− of the ligand. The formation of chelate rings after
the coordination, which is normally accompanied by a large
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processes are summarized in Table 3. The voltammograms
display two consecutive one-electron transfer processes
corresponding to the quasi-reversible, stepwise reduction of
the two iron ions (Scheme 4).
Table 3. Cyclic Voltammetric Parameters of Consecutive
One-Electron Transfer Processes of the Iron(III)
Metallacryptates under Study
complex
[Fe2(L1)3]
Epc (V)
Epa (V)
E°1′ (V)
E°2′ (V)
−0.965
−0.782
−0.832
−0.756
−0.579
−0.663
−0.797
−0.616
−0.922
−0.737
{K⊂[Fe2(L2)3]}(PF6)
{K⊂[Fe2(L3)3]}(PF6)
Particularly, the cryptand-like complex [Fe2(L1)3] shows two
quasi-reversible reductions at −0.797 V (ΔEp = 82 mV) and
−0.922 V (ΔEp = 86 mV) that correspond to one-electron
transfer processes (Figure 5a). It is necessary to note that under
the same conditions the ΔEp value of the Fc+/Fc couple is 74
mV. In the case of the metallacryptate {K⊂[Fe2(L2)3]}(PF6), it
undergoes two quasi-reversible one-electron reductions at
−0.616 V (ΔEp = 80 mV) and −0.737 V (ΔEp = 82 mV)
(Figure 5b). The ΔEp values of the corresponding redox waves
are comparable to that of the Fc+/Fc couple (ΔEp = 78 mV). In
comparison with moderately resolved voltammograms of the
compounds [Fe2(L1)3] and {K⊂[Fe2(L2)3]}(PF6), the two
peaks corresponding to the redox processes of the Fe(III) ions
in {K⊂[Fe2(L3)3]}(PF6) overlap and become indistinguishable
(Figure 5c). Such low resolution of redox waves directly relates
to structural features of the corresponding complexes.
Specifically, since the separation of the two redox potentials
strongly depends on the degree of interactions between the two
redox centers of the intermediate mixed-valent compounds,47,48
the partially separated features of signals in the voltammograms
recorded for the complexes of H2L1 and H2L2 (Figures 5a and
5b) indicate slight delocalization of charge between the two
iron ions. This can be caused by the conjugation of the
chelating aroylthioureas with the aromatic spacers. In contrast,
the aliphatic O−CH2 bonds prevent a potential conjugation in
the backbone of (L3)2− ligands and, hence, produce electronic
isolation of two iron ions, which is responsible for the poor
resolution of the related redox waves (Figure 5c).
Figure 4. (a) Structure of the cationic Fe(III) {2}-metallacryptate
{K⊂[Fe2(L3)3]}+. Hydrogen atoms are omitted for clarity. Symmetry
operations used to generate equivalent atoms: i, −y, x − y, z; ii, −x + y,
−x, z. (b) View along the crystallographic 3-fold axis.
the O−CH2 bonds, which is reflected by the torsion angles
Car−O−CH2−C(O) (Table 2), is responsible for providing an
optimal coordination environment for metal ions. On the other
hand, in {L1}2− and {L2}2−, conjugation of two aroylthiourea
moieties with the aromatic spacers prevents such a free
rotation. Thus, the adaptation of the ligand skeletons for
chelating the metal ions is exclusively done by deviations of
donor atoms from the chelate planes, in other words by the
distortion of chelate rings.
CONCLUSIONS
■
A series of oligonuclear systems has been developed for three
aroylbis(N,N-dialkylthiourea) ligands. Reaction of H2L1 with
Fe3+ ions results in the binuclear complex [Fe2(L1)3] with a
cryptand-like structure, while one-pot reactions of H2L2 or
H2L3 with mixtures of Fe(III) ions and monovalent, “hard” ions
such as Cs+, Rb+, K+, Na+, or Tl+ give rise to {2}-
metallacryptates. These host−guest assemblies are afforded by
the encapsulation of one guest monocation M+ in the central
void of the {2}-metallacryptands {Fe2(L)3} (L = L2 or L3),
The utilization of Fe3+ ions in the metallamacrocycles
produced compounds with a redox behavior, which makes
them interesting for electrochemical studies. Thus, cyclic
voltammetry measurements of the representative compounds
[Fe2(L1)3], {K⊂[Fe2(L2)3]}(PF6), and {K⊂[Fe2(L3)3]}(PF6)
were undertaken in dry DMF under an atmosphere of dry
argon. Cyclic voltammetric parameters of electron transfer
Table 2. Selected Bond Lengths (Å) and Torsion Angles (deg) in {M⊂[Fe2(L3)3]}(PF6) Complexes
a
M
Fe−O
Fe−S
M−O
torsion angle
Cs+
Rb+
Tl+
K+
1.998(3)/2.011(3)
2.009(4)/2.014(4)
2.008(4)/2.012(4)
2.006(4)/2.007(4)
2.405(5)/2.413(6)
2.404(2)/2.415(8)
2.405(7)/2.416(5)
2.402(4)/2.415(8)
3.182(3)−3.251(3)
3.097(4)−3.185(4)
3.105(4)−3.219(4)
3.030(4)−3.155(4)
−179.00(5)/−73.44(9)
−175.22(6)/−72.64(2)
176.75(2)/71.90(1)
−173.10(2)/−72.28(7)
a
Torsion angle ∠{Car−O−CH2−C(O)}.
H
DOI: 10.1021/acs.inorgchem.7b01909
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Scheme 4. Stepwise Reduction of Fe(III) Centers in the Fe(III) Metallacryptands under Study
a
Guest cations and ligands are excluded for simplification.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01909.
Spectroscopic data, ellipsoid representations of the
oligonuclear complexes, and structural parameters
(PDF)
Accession Codes
CCDC 1545457, 1545460−1545463, and 1545465−1545469
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.ca-
m.ac.uk/data_request/cif, or by emailing data_request@ccdc.
cam.ac.uk, or by contacting The Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
+44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ulrich.abram@fu-berlin.de.
ORCID
Ulrich Abram: 0000-0002-1747-7927
Present Address
§Department of Inorganic Chemistry, VNU University of
Science, 19 Le Thanh Tong, Hanoi, Vietnam
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge a PhD scholarship for C.T.P. from
the German Academic Exchange Serice (DAAD). We also
express thanks to the Dahlem Research School (DRS, FU
Berlin) for financial support.
Figure 5. Cyclic voltammograms of the complexes (2 mM) in 0.15 M
(n-Bu4N)(PF6)−DMF at a scan rate of 150 mV/s: (a) [Fe2(L1)3], (b)
{K⊂[Fe2(L2)3]}(PF6), and (c) {K⊂[Fe2(L3)3]}(PF6).
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