An iron(III) - Catechol complex as a mushroom pigment

Full text of DOI:10.1002/(SICI)1521-3773(19981217)37:23<3292::AID-ANIE3292>3.0.CO;2-N

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Keywords: alkynes
reactions ´ rearrangements
´ allenes ´ butadienes ´ pericyclic
An Iron(iii) ± Catechol Complex as a Mushroom
Pigment**
Franz von Nussbaum, Peter Spiteller, Matthias Rüth,
Wolfgang Steglich,* Gerhard Wanner, Brandy
Gamblin, Lorenzo Stievano, and Friedrich E. Wagner
[1] K. Banert, S. Groth, Angew. Chem. 1992, 104, 865 ± 867; Angew. Chem.
Int. Ed. Engl. 1992, 31, 866 ± 868.
[2] K. Banert, H. Hückstädt, K. Vrobel, Angew. Chem. 1992, 104, 72 ± 74;
Angew. Chem. Int. Ed. Engl. 1992, 31, 90 ± 92; K. Banert, S. Groth, H.
Hückstädt, K. Vrobel, Phosphorus Sulfur Silicon Relat. Elem. 1994,
95 ± 96, 323 ± 324.
Dedicated to Prof. Heinrich Nöth
on the occasion of his 70th birthday
[3] K. Banert, C. Toth, Angew. Chem. 1995, 107, 1776 ± 1778; Angew.
Chem. Int. Ed. Engl. 1995, 34, 1627 ± 1629.
Cortinarius violaceus [(L.: Fr.) S. F. Gray][1] is a spectacular
mushroom well known for its dark blue-violet color and its
cedar-wood-like smell. Fries[2] described this type species of
the genus Cortinarius as ªspecies nobilissima, pulcherrimaº.
Previous efforts to examine the chemical nature of the violet
pigment failed because of its instability and polarity. The dark
violet water extracts of fresh or freeze-dried fruit bodies
became dark brown within minutes. We were able to suppress
this decomposition by exhaustive extraction of the lyophilized
mushrooms with methanol prior to the water extraction. After
freeze-drying the resulting aqueous solution, the residue was
purified by chromatography on a Sephadex LH-20 column
with methanol/water (1/1). This procedure enabled us to
enrich the highly sensitive pigment. The pigmentꢁs NMR
spectra were inconclusive due to strong signal broadening by
paramagnetic iron, which can be detected both in the extract
and the mushrooms. Since the intensity of the violet color
correlates with the concentration of iron, the presence of an
iron complex as the coloring principle seems obvious.
[4] H. Priebe, Angew. Chem. 1984, 96, 728 ± 729; Angew. Chem. Int. Ed.
Engl. 1984, 23, 736 ± 737; K. Banert, Tetrahedron Lett. 1985, 26, 5261 ±
5264; Chem. Ber. 1987, 120, 1891 ± 1896; K. Banert, M. Hagedorn,
Angew. Chem. 1989, 101, 1710 ± 1711; Angew. Chem. Int. Ed. Engl.
1989, 28, 1675 ± 1676.
[5] B. Müller, Diplomarbeit, Technische Universität Chemnitz, 1997.
[6] A. Günther, Dissertation, Technische Universität Chemnitz, not yet
published.
[7] Y. Ishino, I. Nishiguchi, M. Kim, T. Hirashima, Synthesis 1982, 740 ±
742; A. Srikrishna, S. Nagaraju, J. Chem. Soc. Perkin Trans. 1 1992,
311 ± 312.
[8] H. Hopf, R. Kirsch, Tetrahedron Lett. 1985, 26, 3327 ± 3330.
[9] T. Pollok, H. Schmidbaur, Tetrahedron Lett. 1987, 28, 1085 ± 1088;
R. Y. Chen, B. Z. Cai, K. S. Feng, Chin. Chem. Lett. 1992, 3, 157 ± 158.
Â
[10] M. Huche, P. Cresson, Bull. Soc. Chim. Fr. 1975, 3 ± 4, 800 ± 804; M.
Â
Huche, P. Cresson, Tetrahedron Lett. 1973, 4291 ± 4292.
[11] G. Smith, C. J. M. Stirling, J. Chem. Soc. C 1971, 1530 ± 1535.
[12] S. Jeganathan, W. H. Okamura, Tetrahedron Lett. 1982, 23, 4763 ±
4764.
[13] a) R. Lespieau, Bull. Soc. Chim. Fr. 1928, 43, 199 ± 210; b) V. A.
Engelhardt, J. E. Castle, J. Am. Chem. Soc. 1953, 75, 1734; c) D. Miller,
J. Chem. Soc. C 1969, 12 ± 15; d) S. Holand, R. Epsztein, I. Marszak,
Bull. Soc. Chim. Fr. 1969, 3213 ± 3218; e) H. P. Figeys, M. Gelbcke,
Tetrahedron Lett. 1970, 5139 ± 5142; f) K. S. Jeong, P. Sjö, K. B.
Sharpless, Tetrahedron Lett. 1992, 33, 3833 ± 3836.
Energy dispersive X-ray analysis[3] (EDX), atomic absorp-
tion spectroscopy[4] (AAS), and the thiocyanate test indicated
that C. violaceus is capable of accumulating iron in a unique
manner. The amount of iron in the mushroom is 4.5 ±
[14] For the conditions of the flash vacuum pyrolysis, see Scheme 2 and
refs. [2, 3].
1
7.5 mg g (dry weight), which is about 100 times the average
[15] The geometrical isomers are assigned on the basis of 1H and 13C NMR
investigations (including homonuclear NOE difference spectra,
studies with lanthanide shift reagents, 3J(31P, 1H) data, comparisons
with ethene-1,2-diyl compounds analogously substituted). The con-
figurations of the C ± C double bonds can also be verified by
comparison of the rates of the Diels ± Alder reactions, with the rates
being higher for (Z)-12a, (Z)-14aa ± e, (E)-26, (E)-28, and (E)-31,
respectively. The X-ray crystal structure analysis of the unlike isomer
of 16ba was performed. We thank Prof. Dr. H.-J. Deiseroth and Dr. L.
Kienle, Universität-GH Siegen, for this investigation.
1
value of 0.06 mgg determined for other basidiomycetes.[5, 6]
The Mössbauer spectra[7] (Figures 1 and 2, Table 1) show that
the iron in the pigment is trivalent, and the ESR spectra[8]
indicate a high-spin FeIII complex.[9] The spectra of the freeze-
dried mushrooms and the water extracts are identical.
After addition of aqueous HCl to the enriched pigment (R)-
b-dopa ((R)-3) can be identified as the complex ligand.[10]
Chromatographic workup of the methanol extracts followed
by recrystallization yields the analytically pure amino acid.
The (R)-b-dopa amounts to 2% of the dry weight of the
mushroom. b-Dopa is a new natural product that has been
synthesized only as the racemate before.[11] We obtained
[16] H. Staudinger, J. Siegward, Helv. Chim. Acta 1920, 3, 824 ± 833.
[17] A. G. M. Barrett, D. Dhanak, G. G. Graboski, S. J. Taylor, Org. Synth.
Coll. Vol. 1993, 8, 550 ± 553.
[18] F. Kurzer, Org. Synth. Coll. Vol. 1963, 4, 937 ± 939.
[19] K. Banert, Chem. Ber. 1989, 122, 1963 ± 1967.
[20] F. Sondheimer, N. Stjernström, D. Rosenthal, J. Org. Chem. 1959, 24,
1280 ± 1284.
[21] Alternative preparation of 23: T. G. Back, E. K. Y. Lai, K. R.
Muralidharan, J. Org. Chem. 1990, 55, 4595 ± 4602.
[*] Prof. Dr. W. Steglich, Dipl.-Chem. F. von Nussbaum,
Dipl.-Chem. P. Spiteller, Dipl.-Chem. M. Rüth
Institut für Organische Chemie der Universität
Karlstrasse 23, D-80333 München (Germany)
Fax : (‡ 49)89-5902-604
[22] Synthesis from ethynylmagnesium bromide and 4-pentenal, 59%;
alternative preparation of 25a: D. Seebach, A. K. Beck, B. Schmidt,
Y. M. Wang, Tetrahedron 1994, 50, 4363 ± 4384.
E-mail: wos@org.chemie.uni-muenchen.de
Prof. Dr. G. Wanner
Botanisches Institut der Universität München (Germany)
B. Gamblin, Dipl.-Chem. Lorenzo Stievano, Prof. Dr. F. E. Wagner
Physik-Department E15 der Technischen Universität München,
Garching (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(SFB 369). We thank H. Hartl for the AAS measurements, G. Sturm
for ESR spectra, and Prof. Dr. M. Spiteller for ESI-MS and APCI-MS
investigations.
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Table 1. 57Fe Mössbauer parameters[21] for the quadrupole doublets at
room temperature and at 4.2 K. Quadrupole splitting QS; isomer shift IS;
line width LW.
Sample
T
[K]
QS
[mms
IS[a]
[mms
LW
Area[b]
[mms ] [%]
1
1
1
]
]
(a) freeze-dried
C. violaceus
(b) water extract
4.2 0.80(1) 0.38(1) 0.45(1)
294
4.2 0.74(1) 0.38(1) 0.53(1)
294 0.64(1) 0.40(1) 0.51(1) 100
4.2 0.89(1) 0.40(1) 0.33(1) 70(1)
0.94(2) 0.41(1) 0.85(3) 100
50(1)
0.64(1) 0.39(1) 0.62(2) 100
43(1)
(c) synthetic 1:2 FeIII
±
b-dopa complex (pH 6) 294
[a] The isomer shifts refer to a-Fe. [b] The spectra recorded at 4.2 K show
(in addition to the quadrupole doublet with the given fractional area) a
magnetically split component with a distribution of hyperfine fields
between 52.5(2) and 13.5(3) T, an effective quadrupole splitting of QS ˆ
1
0.02(2) mms 1 and an isomer shift of IS ˆ 0.38(2) mms
.
Figure 1. Mössbauer spectra at room temperature. a) Freeze-dried C.
violaceus; b) water extract; c) synthetic 1:2 iron(iii) ± b-dopa complex at
pH 6.2. Trel ˆ relative transmission.
Scheme 1. Synthesis of (R)-3. a) MeOH, cat. HCl, reflux; b) PhCH2COCl,
CH2Cl2, NEt3, cat. DMAP; c) LiOH, H2O/MeOH (1/1); d) penicillin
acylase, pH 7.5, phosphate buffer, 258C, 12 h; e) 48% HBr, 1458C, 4 h;
f) Dowex 50 WX8. DMAP ˆ 4-dimethylaminopyridine.
Since comparison of the specific optical rotations of the
natural and the synthetic products was ambiguous, the
absolute configuration and optical purity of the natural ligand
were determined by GC/MS.[14] For this purpose, natural b-
dopa was O-methylated with diazomethane and subsequently
acylated with Mosherꢁs (R)-acid chloride.[15] The derivatives
obtained from the natural and the synthetic amino acid were
identical.[16] Within the scope of this method the natural
product was enantiomerically pure.
The UV/Vis spectrum of the enriched pigment at pH 6.2,
the physiological pH value of the mushroom, exhibited a
ligand-to-metal charge-transfer (LMCT) absorption of lmax
ˆ
571 nm, which is typical for iron(iii) ± catechol complexes. At
pH 9.0 a hypsochromic shift to lmax ˆ 487 nm is observed. The
blue-violet solution changes to bordeaux and decomposes
quickly. Addition of KCN or ethylenediaminetetraacetic acid
(EDTA) to the original solution causes discoloration.
Figure 2. Mössbauer spectra at 4.2 K. a) Freeze-dried C. violaceus;
b) water extract; c) synthetic 1:2 iron(iii) ± b-dopa complex at pH 6.2.
(R)-3 from racemic (R,S)-3-amino-3-(3,4-dimethoxyphenyl)-
propionic acid (1), which is easily accessible by Rodionovꢁs
procedure.[12] After conversion of (R,S)-1 into the N-phenyl-
acetyl derivative 2, the action of penicillin acylase[13] yields
optically pure (R)-1. Ether cleavage with 48% HBr affords
(R)-b-dopa hydrobromide, which can be converted into the
corresponding hydrochloride by ion exchange (Scheme 1).
The behavior of the pigment is consistent with that of other
iron(iii) ± catechol complexes, in which the cation:ligand ratio
depends on the pH value. An increase in the pH value leads to
the typical color changes from green (pH 3, 1:1)[17] to blue-
violet (pH 6, 1:2) and finally bordeaux (pH 9, 1:3; Figure 3).
The UV/Vis data of iron(iii) ± dopamine[18] and iron(iii) ±
Angew. Chem. Int. Ed. 1998, 37, No. 23
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Keywords: amino acids ´ dyes ´ iron ´ Moessbauer spectro-
scopy ´ structure elucidation
[1] Cortinarius violaceus and C. hercynicus are synonymous; for the
taxonomy of this widely distributed species, see for example T. E.
Brandrud, Nord. J. Bot. 1983, 3, 577 ± 592.
[2] E. Fries, Monographia Hymenomycetum Suecicae, Vol. II (Ed.: C. A.
Leffler), Uppsala, 1863, p. 46.
[3] EDX: Slices of freeze-dried fruit bodies were mounted on conductive
carbon adhesive tabs (W. Plannet GmbH), coated with carbon, and
examined in a field emission scanning electron microscope (Hitachi
S-4100). The detection of the elements was carried out with an
ultrathin window EDX detector (Noran Instruments) either in the
spot or mapping mode.
Figure 3. UV/Vis spectra of the synthetic iron(iii) ± b-dopa complex in
water.
[4] AAS: The amount of Fe was detected with a Perkin-Elmer 1100B
instrument at 248.30 nm. A FeCl3 solution (Titrisol, Merck) was used
as standard.
catechol complexes[19, 20] at various pH values are in close
agreement with those of our pigment.
The experimental evidence indicates that the pigment from
C. violaceus is an iron ± ligand 1:2 complex; however, it cannot
be determined if it is mono- or binuclear (4 or 5, Scheme 2).
[5] G. Tyler, Trans. Br. Mycol. Soc. 1980, 74, 41 ± 49.
[6] The false chanterelle (Hygrophoropsis aurantiaca) and the bolete
1
Suillus variegatus are also rich in iron (1 ± 3 mgg dry weight): M.
Lupper, Dissertation, Universität Würzburg, 1988.
[7] The 57Fe Mössbauer spectra were recorded with a 57Co source in Rh.
Source and absorber were kept at the same temperature. The
spectrometer, equipped with a Kr proportional counter to detect the
14.4-keV g radiation used a sinusoidal velocity waveform.
[8] All ESR spectra were measured on solids and showed a characteristic
signal at g ˆ 4.26 (Dh ˆ 9.4 mT); see also H. Oubrahim, J.-M. Richard,
Â
D. Cantin-Esnault, F. Seigle-Murandi, F. Trecourt, J. Chromatogr. A
1997, 758, 145 ± 157.
[9] a) B. F. Anderson, D. A. Buckingham, G. B. Robertson, J. Webb,
Nature 1976, 262, 722 ± 724; b) L. Burlamacchi, A. Lai, M. Moduzzi, G.
Scheme 2. Proposals for the blue-violet 1:2 iron(iii) ± b-dopa complex
species present in C. violaceus at pH 6.2.
Saba, J. Magn. Reson. 1983, 55, 39 ± 50.
23
[10] (R)-3: M.p. 1948C; [a]
ˆ
12.1 (c ˆ 0.5 in 6n HCl); Rf: 0.45
589
The Mössbauer spectra[21] of the pigment are in good agree-
ment with those of a binuclear 1:2 iron(iii) ± catechol com-
plex,[22] whose structure has been established by X-ray
analysis.[22] Neither mass spectroscopic methods nor chirop-
tical measurements led to a more detailed insight into the
structure of the natural complex. The analogy of our pigment
to an iron ink is clear. To the best of our knowledge, pigments
of this type have not yet been discovered in nature.[23, 24]
(nBuOH/EtOH/AcOH/H2O 4/1/1/1); IR (KBr): nÄ ˆ 3400 (br), 3043
(s), 1608 (m), 1571 (s, CO2 ), 1531 (s), 1466 (s), 1392 (s), 1361 (m), 1281
(m), 1250 (s), 1197 (w), 1173 (m), 1126 (m), 1042 (w), 986 (w), 948 (w),
929 (w), 868 (w), 818 (w), 792 (w), 775 (w), 696 (w), 650 (w), 595 (w),
573 (w), 460 cm 1 (w); 1H NMR (600 MHz, [D6]DMSO): d ˆ 2.30 (dd,
J ˆ 16.1, 3.9 Hz, 1H; CHaHb), 2.41 (dd, J ˆ 16.0, 10.0 Hz, 1H; CHaHb),
4.14 (dd, J ˆ 9.6, 3.8 Hz, 1H; ArCHN), 6.65 (dd, J ˆ 8.0, 1.7 Hz, 1H;
6-H), 6.69 (d, J ˆ 8.0 Hz, 1H; 5-H), 6.83 (s, 1H; 2-H); 13C NMR
(151 MHz, [D6]DMSO): d ˆ 40.83 (CH2), 51.90 (ArCHN), 114.81 (C-
2), 115.75 (C-5), 117.94 (C-6), 130.27 (C-1), 145.59 (C-4), 145.61 (C-3),
173.68 (CO2H); EI-MS (decomposition, 70 eV): m/z (%): 180 (7)
‡
‡
[M NH3] , 136 (100) [M NH3 CO2] ; FAB-MS (glycerol): m/z
‡
(%): 198 (45) [M‡H] , 181 (56), 171 (100), 153 (58), 136 (35); ESI-
Experimental Section
‡
‡
MS: m/z (%): 220 (8) [M‡Na] , 198 (34) [M‡H] , 181 (100), 171 (5),
163 (4), 139 (61); ESI-HR-MS: calcd for C9H12NO4: 198.0766; found:
198.0767; elemental analysis calcd for C9H11NO4 ´ 1ꢂ3 H2O: C 53.20, H
5.79, N 6.89; found: C 53.29, H 5.74, N 7.02. Tris(trimethylsilyl)-(R)-b-
Isolation of (R)-3: Powdered, freeze-dried fruit bodies (12.7 g) of C.
violaceus (collected in September 1996 in a spruce forest near Gilching,
Oberbayern, Germany) were defatted with CH2Cl2 (200 mL) for 2 d and
then extracted exhaustively with MeOH. After concentration of the
extracts, the residue (5.1 g) was purified by chromatography on a Sephadex
LH-20 column (40 Â 8 cm) with MeOH. Fractional recrystallization of the
crude material from dry MeOH removed sugar impurities and yielded
analytically pure (R)-3 (41 mg, 0.3% of dry weight).
‡
dopa: GC: Ri ˆ 2105; GC/MS (70 eV): m/z (%): 413 (8) [M ], 398 (2)
‡
‡
[M CH3] , 396 (2) [M NH3] , 381 (1), 282 (100) [M
‡
CH2CO2Si(CH3)3] , 268 (5), 210 (6), 194 (5), 167 (3), 147 (3), 117
(3), 75 (20) [HOSi(CH3)2] , 73 (49) [Si(CH3)3] ; HR-GC/MS: calcd
for C18H35NSi3O4: 413.1874, found: 413.1889.
‡
‡
[11] T. Posner, Justus Liebigs Ann. Chem. 1912, 389, 1 ± 120.
[12] W. M. Rodionov, E. A. Postovskaja, J. Am. Chem. Soc. 1929, 51, 841 ±
847.
Isolation of (R)-3 ´ HCl: The MeOH extracts (see the isolation of (R)-3)
were purified by chromatography on a cation-exchange column (Dowex
‡
50 WX8, H form). After washing with water (300 mL) the ligand was
[13] a) V. A. Soloshonok, N. A. Fokina, A. V. Rybakova, I. P. Shishkina,
S. V. Galushko, A. E. Sorochinsky, V. P. Kukhar, M. V. Savchenko,
eluted with 4n HCl (100 mL). Further purification on Dowex 50 WX8
yielded (R)-3 ´ HCl as a hygroscopic, beige foam (0.27 g, 2% of dry weight).
Ï
V. K. Svedas, Tetrahedron: Asymmetry 1995, 7, 1601 ± 1610; b) G.
Cardillo, L. Gentilucci, A. Tolomelli, C. Tomasini, J. Org. Chem. 1998,
63, 2351 ± 2353.
Enrichment of the blue-violet pigment: After the treatment with MeOH,
the mushroom residues were extracted with H2O (200 mL) at 258C for
20 min under argon. The resulting deep blue-violet solution was freeze-
dried and purified by chromatography repeatedly on Sephadex LH-20
(H2O/MeOH/0.1% KOH 25/25/1). Yield of pigment 15 mg, 0.1% of dry
weight.
[14] For GC/MS
a
fused silica capillary column (DB-5ms, 30 m Â
0.25 mm  0.25 mm; J & W) was used. Injector temperature 2808C.
Temperature program: 3 min isothermic at 508C, 2 min at 1008C, then
heated to 3008C with a rate of 3 Kmin 1, and finally 10 min isothermic
at 3008C. An aliquot (0.5 ± 1.5 mL) of a 1 ± 2% solution (m/v) was
Â
injected in every case. The Kovats indices were determined by co-
Received: June 23, 1998 [Z12032IE]
injection of 0.2 mL of a mixture consisting of n-alkanes (C10 ± C36).
German version: Angew. Chem. 1998, 110, 3483 ± 3485
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components of molecular devices.[2] Although single-, double-
and triple-stranded helicates have been well-documented,[2, 4]
to our knowledge the only example of a quadruply stranded
helicate is the recent report of pentanuclear metal complexes
bridged by a pentadentate ligand.[5] These species contain
metal ± metal bonds with terminal ancillary ligands on octa-
hedral metal centers and, as such, can be classified as
unsaturated helicates.[2] It was predicted[2] that the synthesis
of a saturated quadruply stranded helicate might be achieved
by employing a combination of square-planar metal centers
with oligomonodentate bridging ligands. We now report the
successful realization of this challenge, with the synthesis and
X-ray crystal structure of a quadruply stranded helicate that
encapsulates a hexafluorophosphate anion.
[15] A. J. Dale, H. S. Mosher, J. Org. Chem. 1969, 34, 2543 ± 2549.
[16] Determination of the configuration by GC/MS: MeOH (1 mL) and a
5% solution of CH2N2 in diethyl ether (2 mL) were added to natural
(R)-b-dopa (1 mg) at 258C. After 1 h the solvent was removed under
argon, and Mosherꢁs (R)-acid chloride (10 mL) was added to the
residue. After an additional 12 h at 258C MeOH (50 mL) was added,
and the GC/MS analysis was carried out. Methyl 3-(3,4-dimethoxy-
phenyl)-3-(3,3,3-trifluoro-2-methoxy-2-phenylpropionylamino)propi-
onate: 2'S,3R derivative: GC: Ri ˆ 2684; GC/EI-MS (70 eV): m/z (%):
‡
455 (10) [M ], 382 (8), 238 (45), 223 (83), 189 (38), 181 (100), 164 (33),
105 (23), 91 (13), 77 (19). 2'S,3S derivative: GC: Ri ˆ 2709.
[17] U. El-Ayaan, E. Herlinger, R. F. Jameson, W. F. Linert, J. Chem. Soc.
Dalton Trans. 1997, 2813 ± 2818.
[18] C. Gerard, H. Chehhal, R. P. Hugel, Polyhedron 1994, 13, 591 ± 597.
[19] R. C. Hider, A. R. Mohd-Nor, J. Silver, I. E. G. Morrison, L. V. C.
Rees, J. Chem. Soc. Dalton Trans. 1981, 609 ± 622.
[20] A. Avdeef, S. R. Sofen, T. L. Bregante, K. N. Raymond, J. Am. Chem.
Soc. 1978, 100, 5362 ± 5370.
We have previously reported that reaction of 1,4-bis(2-
pyridyloxy)benzene with silver nitrate results in the self-
assembly of a M2L2 dimetalloparacyclophane with intimate
p ± p stacking of the central benzene rings,[6] whilst replace-
ment of the central p-phenylene ring with a 2,7-naphthylene
spacer results in the formation of a M2L4 molecular box.[7] In
order to change the topology of the ligand so as to enlarge the
cavity within such metallosupramolecular species we have
begun to examine the coordination chemistry of the 3-pyridyl
analogues. Thus, the ligand 1,4-bis(3-pyridyloxy)benzene (1)
was prepared (Scheme 1) by reaction of 3-hydroxypyridine
with 1,4-dibromobenzene in the presence of potassium
[21] At 4.2 K the Mössbauer spectra of the natural samples show, in
addition to the quadrupole doublet, a broad background caused by
magnetic hyperfine splitting. The magnetic component is assigned to
iron hydroxides, and it increases with the ageing of the mushrooms.
The strongly broadened Mössbauer spectra observed at room temper-
ature are caused by a superposition of the quadrupole doublet of the
iron hydroxides, which do not show magnetic order yet, and the
quadrupole doublet of the iron(iii) ± catechol complex. Differences in
the line width and quadrupole splitting of the doublet observed at
4.2 K are attributable to badly crystallized iron hydroxides, which are
not ordered even at 4.2 K. The higher Lamb ± Mössbauer factor of the
iron hydroxides causes an overrepresentation of the iron hydroxides in
comparison to the iron(iii) ± catechol complex, especially at room
temperature.
carbonate and copper bronze.[8] Reactions of
1 with
[22] V. A. Grillo, G. R. Hanson, D. Wang, T. W. Hambley, L. R. Gahan,
K. S. Murray, B. Moubaraki, C. J. Hawkins, Inorg. Chem. 1996, 35,
3568 ± 3576.
[PdCl2(PPh3)2] and [PdI2(py)2] (py ˆ pyridine), in the pres-
ence of silver triflate, gave what we believe to be the dimeric
complexes 2 and 3, respectively, the formulations of which are
[23] A brown anthranoid ± iron complex was isolated from lichens of the
genus Cladonia: L. Alagna, T. Prosperi, A. A. G. Tomlinson, H.
Kjùsen, F. Mo, Biochim. Biophys. Acta 1990, 1036, 71 ± 77.
[24] a) Iron transport and storage in microorganisms, plants and animals:
Metal ions in biological systems, Vol. 35 (Eds.: A. Sigel, H. Sigel),
Dekker, New York, 1998; b) B. F. Matzanke, G. Müller-Matzanke,
K. N. Raymond in Iron Carriers and Iron Proteins, Physical Bio-
inorganic Chemistry Series, Vol. 5 (Ed.: T. M. Loehr), VCH, Wein-
heim, 1989, pp. 1 ± 122.
1
supported by their elemental analyses, H NMR and FAB
mass spectra (see Experimental Section). Diffusion of diethyl
ether into an acetonitrile solution of 3 containing ammonium
hexafluorophosphate resulted in a reorganization of the
components and the assembly of a M2L4 species (4), which
deposited from the solution as a tetrakis(hexafluorophos-
phate) salt and as a bis(acetonitrile) solvate. This latter
species was prepared more efficiently, in 70% yield, when the
appropriate 2:1 ligand:metal stoichiometry was employed.
A single crystal X-ray structure determination was carried
out to determine the structure of this compound unambigu-
ously.[9] The compound crystallizes in the centrosymmetric
space group P21/n, the asymmetric unit of which contains a
full M2L4 helical cage (4) that has each square-planar
palladium atom coordinated to the four bridging ligands and
within which resides a well-ordered PF6 ion (Figure 1).
External to the cage are three other PF6 anions (two of
which are disordered) and two acetonitrile solvate molecules
(not shown). The dimensions of the cage are defined by the
Pd ´´´ Pd separation [8.8402(8) Š] and the distance between
the centroids of the cofacial benzene rings [8.849(7) and
8.925(7) Š].
The First Coordinatively Saturated, Quadruply
Stranded Helicate and Its Encapsulation of a
Hexafluorophosphate Anion.**
David A. McMorran* and Peter J. Steel*
The use of metal ions to control the self-assembly of
discrete supramolecular species has been an area of intense
interest for some years,[1] with helicates having received
particular attention.[2, 3] This interest has been driven, at least
in part, by the potential use of helicates as functional
Figure 2 shows the cage viewed down the Pd ± Pd axis,
which serves to emphasize its approximate D4 symmetry and
the helical disposition of the ligands. The helical pitch is
defined by the approximate 458 angle subtended by each
ligand about the helical axis. The planes of the pyridine rings
are all approximately orthogonal to the planes of the linking
benzene rings. This cage can be classified as a saturated,
[*] Dr. D. A. McMorran, Dr. P. J. Steel
Department of Chemistry, University of Canterbury
Private Bag 4800, Christchurch (New Zealand)
Fax: (‡64)3-364-2110
E-mail: p.steel@chem.canterbury.ac.nz
[**] We thank the New Zealand Foundation for Research, Science, and
Technology for funding this research through a postdoctoral fellow-
ship to D.A.M.
Angew. Chem. Int. Ed. 1998, 37, No. 23
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
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