A dinuclear, triple-stranded helicate with a diamide-bridged catechol/benzenedithiol ligand

Article abstract of DOI:10.1002/anie.200460188

Three of a kind: The directional O-O/S-S heteroclonor ligand H ₄-1 reacts with Ti^IV to give the triple-stranded dinuclear tetra-anion [Ti₂(1)₃]^4- (see picture, acac = 2,4-pentanedione). The reaction y

Full text of DOI:10.1002/anie.200460188

Angewandte
Chemie
Helical Structures
A Dinuclear, Triple-Stranded Helicate with a
Diamide-Bridged Catechol/Benzenedithiol
Ligand**
F. Ekkehardt Hahn,* Christian Schulze Isfort, and
Tania Pape
The spontaneous self-organization of coordination com-
pounds has been studied intensively over the last few
years.[1] One area of particular interest are the triple-stranded
helicates with catecholato donor groups.[2] Much less is known
about helicates with thiolato donor groups, only bis(benze-
nedithiol) ligands[3] and their dinuclear nickel complexes have
been reported.[4] Herein we present the synthesis of the first
catechol/benzenedithiol (O–O/S–S) ligand H4-1 and the
molecular structure of the triple-stranded L,L-helicate
Na(PNP)3[Ti2(1)3] (PNP+ =bis(triphenylphosphoranylidene)-
ammonium).
Triple-stranded helicates containing donor atoms from the
second period (O, N) contain two metal centers coordinated
in an octahedral fashion. However, under certain conditions,
macrobicyclic tris(catecholato) ligands can enforce a trigonal-
prismatic coordination environment in mononuclear com-
plexes.[5] In contrast, tris(benzenedithiolato) complexes can
be pseudo-octahedral (e.g. [WV(bdt)3]ꢀ, bdt = benzene-1,2-
dithiolate)[6] or trigonal-prismatic (e.g. [WVI(bdt)3]),[7]
depending on the oxidation state of the coordinated metal
center. The connection of a catechol and an o-benzenedithiol
donor group in the same ligand and the preparation of triple-
stranded dinuclear complexes with such ligands was of
interest to us. A parallel orientation of the ligands would
[*] Prof. Dr. F. E. Hahn, Dipl.-Chem. C. Schulze Isfort, T. Pape
Institut für Anorganische und Analytische Chemie
Westfälische Wilhelms-Universität Münster
Wilhelm-Klemm-Strasse 8, 48149 Münster (Germany)
Fax: (+49)251-833-3293
E-mail: fehahn@uni-muenster.de
[**] Financial support for this work was provided by the Deutsche
Forschungsgemeinschaft (GRK 673). We thank Dr. Klaus Bergander
for recording of the NMR spectra.
Angew. Chem. Int. Ed. 2004, 43, 4807 –4810
DOI: 10.1002/anie.200460188
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4807

Communications
allow the coordination geometry of the o-benzenedithiolato-
coordinated metal center and thus the helicity of the complex
to be switched by varying the oxidation state of this {MS6}
metal center.
The preparation of ligand H4-1 starts from 2,3-di(isopro-
pylsulfanyl) benzoic acid (2, Scheme 1).[8] This acid was
converted into the acid chloride, followed by the reaction with
Figure 1. 1H NMR spectra (in [D ]DMF, =DMF resonances) of H -
*
7
4
1 (top) and Na(PNP)3[Ti2(1)3] (bottom).
Scheme 1. Synthesis of ligand H4-4.
directional phenylalanine-bridged di(catechol) ligand with
TiIV gives a mixture of products containing a total of seven
double-stranded dinuclear stereoisomeric (L, D) and regioi-
someric (parallel and antiparallel) complexes.[13b,c]
the single tert-butoxycarbonyl (Boc) protected diamine[9] and
subsequent removal of the Boc protecting group. The primary
amine 4 reacts with 2,3-di(benzyloxy)benzoic acid chloride[10]
to give the ligand precursor 5. All the protecting groups were
simultaneously removed with Na/naphthalene in THF.[4,8]
After protonation with H2O/HCl ligand H4-1 was obtained
in about 59% yield (relative to 2) as a light yellow solid.
The 1H NMR spectrum of H4-1 (Figure 1, top) shows two
Ligand H4-1 shows directionality based on the two
different donor groups (O–O/S–S) and on the asymmetric
CMe2CH2 spacer between them. The reaction of three
equivalents of H4-1 with two equivalents of [TiO(acac)2] in
the presence of Na2CO3 in methanol gives a red-brownish
complex which dissolves well in methanol. Addition of PNPCl
and diffusion of diethyl ether into a saturated methanol
solution of the product gives the dinuclear complex
Na(PNP)3[Ti2[(1)3] as red-brown crystals. The NMR spectra
of Na(PNP)3[Ti2[(1)3] show a surprisingly small number of
signals (Figure 1, bottom). The aromatic protons of the
benzenedithiolato and the catecholato groups are detected
as one triplet (d = 6.65(Hg) and 6.32(Hc) ppm) and two
doublets (d = 7.08(Hf), 6.98(Hh) ppm, and d = 7.15(Hb),
6.21(Hd) ppm) for each ring. This result demonstrates that
the three ligands are oriented in an identical, parallel manner.
ꢀ
resonance signals for the N H protons (Ha, t, 9.10 ppm and
He, s, 8.25 ppm), which based on their multiplicity were
assigned to the catechol- and o-benzenedithiol-bound amide
groups. In addition, six well resolved resonance signals were
observed for the protons of the aromatic rings.
Only a few helical complexes with asymmetrical donor
groups derived from functionalized benzene are known.[11]
Albrecht et al. reported a tetradentate, directional ligand
containing an aminophenol and a catechol unit.[12] In addition,
directional ligands are known which have identical donor
groups but an asymmetric spacer between them.[13] The
orientation of directional ligands in dinuclear complexes
(parallel or antiparallel[14]) is of special interest since not only
complexes with stereoisomeric metal centers (L, D) but also
as regioisomeric complexes can form.
ꢀ
The downfield shift of the triplet Ha for the N H proton of
the catechol-bound amide (Dd = 0.70 ppm, Figure 1) is
remarkable. This shift indicates the formation of a planar
ꢀ
six-membered ring and a strong N H···O hydrogen bond
The directional catechol/aminophenol ligand[12] reacts
with GaIII or TiIV to give a dinuclear complex with parallel
orientation of the ligands, while the reaction with a mixture of
GaIII and TiIV (1:1) gives a dinuclear complex with an
antiparallel orientation of the ligands. The reaction of a
between the amide proton Ha and the o-catecholato oxygen
atom (Figure 1). This type of hydrogen bond has been
observed for helicates with amide-bridged catecholato
donor groups[15] and for other catechoylamide complexes
and has often been shown to be structure determining.[5,16]
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Angewandte
Chemie
The chemical shift for the amide protons next to the
benzenedithiolato group changes only marginally upon com-
plex formation indicating the presence of no or only very
ꢀ
weakN H···S hydrogen bonds.
All the NMR data indicate the presence of the parallel
regioisomer and only one pair of enantiomers in solution.
However, the NMR data do not allow the determination
which pair of enantiomers L,D/D,L or L,L/D,D is present.[17]
Slow diffusion of diethyl ether into a methanol solution of the
complex gave single crystals of Na(PNP)3[Ti2(1)3]·CH3OH·
H2O·Et2O suitable for an X-ray diffraction analysis.[18]
Figure 3. Section of the crystal structure of Na(PNP)3[Ti2(1)3]·CH3OH·
H2O·Et2O, for clarity the PNP cations and the diethyl ether molecule
are not shown.
The structure analysis revealed, that indeed the triple-
stranded helicate [Ti2(1)3]4ꢀ had formed (Figure 2). The three
ligands are oriented in a parallel manner, as expected. One
A similar spontaneous resolution of stereoisomers upon
crystallization was first described for triple-stranded NiII
complexes with oligobipyridine ligands.[21] Since the NMR
spectra show the formation of only one pair of enantiomers,
the formation of helicates of the L,D and D,L type, can be
excluded.
Currently we can only speculate about the driving force
for the formation of the parallel triple-stranded helical
complex [Ti2(1)3]4ꢀ. With the related directional ligand
containing catechol/aminophenol donor groups reported by
Albrecht et al. only the antiparallel ligand orientation was
observed for homobinuclear complexes,[12] which leads to the
smallest possible charge separation in the complex.[22] Ligand
H4-1 was expected to behave similarly. However, H4-1 is
directional with respect the donor groups and with respect the
spacer between them. Upon formation of the triple-stranded
helicate a distorted {TiS6} octahedron and a smaller {TiO6}
octahedron are obtained. It is reasonable to expect, that the
sterically more demanding (CMe2) part of the spacer
CMe2CH2 is oriented towards the larger {TiS6} octahedron
thereby causing the parallel orientation of the ligands. This
Figure 2. Molecular structure of the tetraanion [Ti2(1)3]4ꢀ in a side-
view (left) and along the Ti–Ti axis (right). Selected bond lengths [Š]
and angles [8]: Ti-S 2.381(3)–2.436(3), Ti-O 1.904(6)–2.010(6), in one
donor group S-Ti-S 81.64(9)–81.94(9), O-Ti-O 78.9(2)–80.9(3); the
ꢀ
ꢀ
Ti S or Ti O bonds to the donor atoms in 2-position at the aromatic
ring are always slightly longer than the bonds to donor atoms at the
3-positions at the aromatic rings.
ꢀ
orientation allows the formation of three stable N H···O
hydrogen bridges (length H···O 1.824–2.039 Š) and of three
ꢀ
titanium atom is coordinated in a distorted octahedral fashion
almost planar N H···O-C-C-C(O) rings within the catechoy-
lamide groups at the {TiO6} octahedron (Figure 2). Similarly
by the six oxygen atoms of three catechoylamide groups. The
IV
ꢀ
ꢀ
Ti O bond lengths fall in the range reported for other Ti
strong N H···S hydrogen bonds were not observed (distance
tris(catecholato) complexes.[19] The second titanium atom is
surrounded by six sulfur atoms from three benzenedithiolato
H···S 2.444–2.701 Š) and the benzenedithiolato groups are
not coplanar with their amide groups. We therefore propose a
structure-determining influence of the sterically unsymmet-
rical CMe2CH2 spacer for the formation of the parallel
regioisomer. This postulate is corroborated by the observa-
tion that a ligand isomeric to H4-1 with a symmetrical
CH2CH2 spacer forms dinuclear TiIV complexes of the known
structural type[13b,c] [Ti2(L)2(OMe)2]2ꢀ with an antiparallel
orientation of the ligands.[23]
The structure-determining influence of the unsymmetrical
spacer in ligand H4-1 will be more prominent in the synthesis
of heterobimetallic triple-stranded helicates, where the pref-
erences of the donor groups for different metal ions will be a
factor. We expect that the reaction of H4-1 with a mixture
(1:1) of TiIV and WV will yield a triple-stranded helicate with
parallel ligand orientation containing {TiO6} and {WS6}
octahedra. Such investigations are currently underway as
well as attempts to remove the helicity in such heterobime-
tallic complexes by oxidation of the {WVS6] octahedron to
give the trigonal-prismatic {WVIS6} coordination polyhedron.
ꢀ
groups in a strongly distorted octahedral fashion. The Ti S
bond lengths vary only slightly and are comparable to bond
lengths reported for tris(o-benzenedtihiolato) TiIV com-
plexes.[20] The helical twist between two titanium centers
measures 65.68.
The unit cell contains one [Ti2(1)3]4ꢀ ion, three PNP
cations, one sodium cation, and one molecule each of water,
methanol, and (disordered) diethyl ether. The PNP cations
maintain no contacts to the [Ti2(1)3]4ꢀ ion whereas the Na+ ion
acts as a bridge between the [Ti2(1)3]4ꢀ ions and this leads to
indefinite chains in the crystal lattice. The Na+ ion completes
its coordination sphere by the coordination of one molecule
each of methanol and water (Figure 3).
Both metal centers in [Ti2(1)3]4ꢀ in the crystal under
investigation assume the same configuration, namely L. Since
the complex crystallized in the acentric space group P1 and
the CD spectrum of the reaction product shows no Cotton
effect, crystals of the D,D enantiomer must also have formed.
Angew. Chem. Int. Ed. 2004, 43, 4807 –4810
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Communications
[7] H. V. Huynh, T. Lügger, F. E. Hahn, Eur. J. Inorg. Chem. 2002,
3007 – 3009.
Experimental Section
All synthetic manipulations were carried out under argon in Schlenk
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1661.
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Chem. 1999, 111, 1353 – 1355; Angew. Chem. Int. Ed. 1999, 38,
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Eur. J. 2001, 7, 3966 – 3975.
flasks. Solvents were dried, distilled, and stored under argon. Correct
elemental analyses (C, H, N, S) were obtained for all compounds.
H4-1: Yield 59% relative to 2. 1H NMR (300 MHz, [D7]DMF,
assignment of signals given in Figure 1): d = 10.44 (s, br, 2H, OH),
9.10 (t, 1H, Ha), 8.25 (s, 1H, He), 7.60 (d, 3J = 7.8 Hz, 1H, Hb), 7.49 (d,
3J = 8.2 Hz, 1H, Hf), 7.35 (d, 3J = 7.8 Hz, 1H, Hd), 7.12 (t, 3J = 7.8 Hz,
3
3
1H, Hc), 7.05 (d, J = 8.2 Hz, 1H, Hh), 6.78 (t, J = 8.2 Hz, 1H, Hg),
5.59 (s, br, 2H, SH), 3.82 (d, 2H, CH2), 1.56 ppm (s, 6H, CH3);
13
=
=
C NMR (75 MHz, [D7]DMF): d = 171.40 (C O), 169.93 (C O),
150.58, 147.51, 137.56, 133.98, 131.59, 130.63, 126.09, 125.63, 119.55,
118.83, 118.03, 116.07 (CAr), 56.12 (CH2C(CH3)2), 48.39
(CH2C(CH3)2), 24.87 ppm (CH2C(CH3)2); MALDI-MS (positive
ions): m/z (%): 393 (100) [M + H]+.
Na(PNP)3[Ti2[(1)3] was prepared by stirring ligand H4-1 (104 mg,
0.26 mmol), [TiO(acac)2] (45 mg, 0.17 mmol), and Na2CO3 (18 mg,
0.17 mol) in methanol at room temperature for 72 h. Subsequently,
the solvent was removed and the solid obtained was redissolved in
methanol. After addition of bis(triphenylphosphoranylidene)ammo-
nium chloride (PNPCl) (98 mg, 0.17 mmol) of the reaction mixture
was filtered. Slow diffusion of diethyl ether into the filtrate yielded
60 mg (0.06 mmol, 23%) of red-brown crystals of (PNP)3Na[Ti2(1)3]·
CH3OH·H2O·Et2O. 1H NMR (500 MHz, [D7]DMF, solvent-free com-
pound, assignment of signals given in Figure 1): d = 9.80 (t, br, 3H,
[14] E. C. Constable, F. Heirtzler, M. Neuburger, Z. Zehnder, J. Am.
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Kersting, M. Meyer, R. E. Powers, K. N. Raymond, J. Am. Chem.
Soc. 1996, 118, 7221 – 7222; c) C. Brückner, R. E. Powers, K. N.
Raymond, Angew. Chem. 1998, 110, 1937 – 1940; Angew. Chem.
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[17] The term meso-helicate, which is often used in the literature, is
not applicable here owing to the two different stereocenters
{TiS6} and {TiO6} present.
3
Ha), 8.14 (s, br, 3H, He), 7.80–7.60 (m, 90H, HAr, PNP), 7.15 (d, J =
7.8 Hz, 3H, Hb), 7.08 (d, 3J = 7.5 Hz, 3H, Hf), 6.98 (d, 3J = 7.5 Hz, 3H,
Hh), 6.65 (t, 3J = 7.5 Hz, 3H, Hg), 6.32 (t, 3J = 7.8 Hz, 3H, Hc), 6.21 (d,
3J = 7.8 Hz, 3H, Hd), 4.81 (s, br, 6H, CH2), 1.38 ppm (s, br, 18H, CH3);
13C NMR (125 MHz, [D7]DMF, solvent-free compound): d = 169.41
=
=
(C O), 167.44 (C O), 161.81, 161.40, 155.24, 152.05, 135.63 (CAr, 1),
134.34, 133.08, 133.03, 132.98, 130.24, 130.19, 130.11, 128.48, 128.46,
127.62 (CAr, PNP), 128.22, 123.03, 121.85, 117.86, 116.47, 115.56,
112.29 (CAr, 1), 54.98 (CH2C(CH3)2), 49.46 (CH2C(CH3)2), 25.16 ppm
(br, CH2C(CH3)2); MS (ESI, negative ions): m/z (%): 630.7 (100)
[18] X-ray diffraction study: Crystals of Na(PNP)3[Ti2(1)3]·CH3OH·
H2O·Et2O were obtained by diffusion of diethyl ether into a
methanol solution of Na(PNP)3[Ti2(1)3], C167H154N9NaO15P6-
S6Ti2, Mr = 3023.96, red crystal, 0.10 ” 0.07 ” 0.04 mm3, P1, a =
13.594(3), b = 17.190(4), c = 18.900(4) Š, a = 66.722(4), b =
[Ti2(1)3 + 2H]2ꢀ
.
Received: April 1, 2004
79.188(4),
g = 82.523(4)8,
V= 3977.6(15) Š3,
1calcd =
Keywords: chirality · helical structures · O,S ligands ·
1.262 gcmꢀ3, m = 0.306 mmꢀ1, semi-empirical absorption correc-
tion (0.9701 ꢁ Tꢁ 0.9879), w- and f-scans, 24856 measured
intensities (2.48 ꢁ 2q ꢁ 45.28), 20191 independent (Rint = 0.0583)
and 13406 observed (I ꢂ 2s(I)) intensities, l = 0.71073 Š, T=
123(2) K, Z = 1, R = 0.0771, wR2 = 0.1645 (refinement against
hF2i with H atoms on calculated positions). The diethyl ether
molecule in the unit cell is disordered, positional parameters for
the hydrogen atoms of the water and methanol OH protons were
not determined. CCDC 234748 contains the supplementary
crystallographic data for this paper. These 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: (+ 44)1223-336-033; or
deposit@ccdc.cam.ac.uk).
.
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Angew. Chem. Int. Ed. 2004, 43, 4807 –4810