Stereocontrol in dinuclear triple lithium-bridged titanium(IV) complexes: Solving some stereochemical mysteries

Article abstract of DOI:10.1002/chem.201402370

Compounds 1 a-f-H₂ form "monomeric" triscatecholate titanium(IV) complexes [Ti(1 a-f)₃]^2-, which in the presence of Li cations are in equilibrium with the triple lithium-bridged "dimers" [Li₃(Ti(1 a-f)₃

Full text of DOI:10.1002/chem.201402370

DOI: 10.1002/chem.201402370
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&
Self-Assembly
Stereocontrol in Dinuclear Triple Lithium-Bridged Titanium(IV)
Complexes: Solving Some Stereochemical Mysteries
Markus Albrecht,*[a] Elisabeth Isaak,[a] Verena Moha,[a] Gerhard Raabe,[a] and
Roland Frçhlich[b]
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Abstract: Compounds 1a–f-H2 form “monomeric” triscate-
cholate titanium(IV) complexes [Ti(1a–f)3]2À, which in the
presence of Li cations are in equilibrium with the triple lithi-
um-bridged “dimers” [Li3(Ti(1a–f)3)2]À. The equilibrium
strongly depends on the donor ability of the solvent. Usually,
in solvents with high donor ability, the stereochemically
labile monomer is preferred, whereas in nondonor solvents,
the dimer is the major species. In the latter, the stereochem-
istry at the complex units is “locked”. The configuration at
the titanium(IV) triscatecholates is influenced by addition of
chiral ammonium countercations. In this case, the induced
stereochemical information at the monomer is transferred to
the dimer. Alternatively, the configuration at the metal com-
plexes can be controlled by enantiomerically pure ester side
chains. Due to the different orientation of the ester groups
in the monomer or dimer, opposite configurations of the
triscatecholates were observed by circular dichroism (CD)
spectroscopy for [Ti(1c–e)3]2À or [Li3(Ti(1c–e)3)2]À. A surpris-
ing exception was found for the dimer [Li3(Ti(1 f)3)2]À. Herein,
the dimer is the dominating species in weak donor (metha-
nol), as well as strong donor (DMSO), solvents. This is due to
the bulkiness of the ester substituent destabilizing the
monomer. Due to the size of the substituent in [Li3(Ti(1 f)3)2]À
the esters have to adopt an unusual conformation in the
dimer resulting in a stereocontrol of the small methyl group.
Following this, opposite stereocontrol mechanisms were
observed for the central metal-complex units of [Li3(Ti(1c–
e)3)2]À or [Li3(Ti(1 f)3)2]À.
Introduction
bridged helicate-type complexes (D, E) from carbonyl-substi-
tuted catecholates (D)[20] or hydroxyquinolinates (E).[21]
Self-assembly is an elegant approach towards chemical com-
plexity.[1] This is well demonstrated in, for example, the often
highly symmetric structures of viruses[2] or enzymes.[3] The
same principles can be used for artificial systems, which are of
high interest not only due to their aesthetic appeal but also to
specific material properties.[4] Thus, “self-assembly” is a tool for
the “bottom-up” approach to nanomaterials and nano-
technology.[5]
The assembly of the helicate-type coordination compounds
D or E follows a hierarchical approach, which is briefly de-
scribed for the example of titanium(IV) complexes (D): A mix-
ture of carbonyl-substituted catechols, a titanium(IV) source,
and lithium carbonate leads to the initial formation of a mono-
nuclear triscatecholate complex, which subsequently “dimer-
izes”[22] by bridging of three lithium ions between the salicylate
binding sites of two titanium(IV) complexes. In many cases, the
equilibrium can be observed in solution, whereas in the crystal
dimers are formed. Equilibration between the monomeric anti
and the syn, as well as the corresponding L and D isomer of
the monomer, is fast on the NMR time scale and results in only
one set of signals (Scheme 1 and Figure 2).[20]
Self-assembly is mainly based on noncovalent interactions
between molecular building blocks. In this respect, hydrogen
bonding is one of the most prominent interactions.[6] In artifi-
cial systems, metal coordination plays a vital role due to very
stable bonding between the metal ions and appropriate li-
gands, which nevertheless is kinetically labile.[7] Metal-directed
self-assembly delivers multinuclear assemblies, which often
show a high symmetry. Examples are helicates,[8] tetrahedra,[9]
squares,[10] grids,[11] spheres,[12] as well as many others.[13]
Just recently, subcomponent[14] and multicomponent self-as-
sembly was investigated to introduce higher complexity.[15] As
a pleasant side effect of this approach, the molecular building
blocks might become simpler or are even formed in situ from
easily available precursors. In subcomponent self-assembly,
labile covalent bonds (e.g., imines), as well as coordinative
bonds to metals, are established simultaneously.[16] Hereby, se-
lection processes follow the principles of dynamic combinatori-
al chemistry[17] and systems chemistry.[18]
In [D6]DMSO, Li2[Ti(1b)3] is the major species in the mono-
mer/dimer equilibrium. A dimerization constant Kdim =166mÀ1
can be determined from the NMR spectrum. The isopropyl sig-
nals of the dimer are shifted to high field compared to the
monomer due to the close proximity of this group to the aro-
matic units of the second triscatecholate complex. From the
NMR data (Figure 3), it can be seen that the monomeric com-
plex racemizes fast on the NMR time scale (only one signal for
the methyl groups), whereas in the dimer the stereochemistry
at the two complex units is locked, and diastereotopic behav-
ior was found. In the dimer, both titanium triscatecholate moi-
eties are in syn arrangement and possess the same configura-
tion (LL or DD).[23] Racemization of the dimer only occurs by
dissociation into the monomers.[20]
Examples for multicomponent self-assembly of helicate type
complexes A–E are shown in Figure 1.[19] For a while, we study
the multicomponent self-assembly of dinuclear triple lithium-
Due to the lower donor and consequently lower lithium sol-
vating ability of [D4]MeOH compared to DMSO, the dimer is
the only observable complex in this solvent (Figure 3).
Crystals of [Me4N][Li3(Ti(1b)3)2] suitable for X-ray diffraction
analysis were obtained from methanol/ether in the presence of
a tetramethyl ammonium salt (Figure 4). The dimer adopts the
typical structure of a triple lithium-bridged bis(titanium(IV) tris-
catecholate).[20] Notably, the tetramethylammonium cation is
located close to the terminus of the dimer, with three methyl
[a] Prof. Dr. M. Albrecht, Dr. E. Isaak, Dr. V. Moha, Prof. Dr. G. Raabe
Institut fꢀr Organische Chemie, RWTH Aachen
Landoltweg 1, 52074 Aachen (Germany)
E-mail: markus.albrecht@oc.rwth-aachen.de
[b] Dr. R. Frçhlich
Organisch-Chemisches Institut
Westfꢁlische Wilhelms-Universitꢁt Mꢀnster
Corrensstrasse 40, 48149 Mꢀnster (Germany)
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groups interacting with terminal
catecholate oxygen atoms by
nonclassical hydrogen bond-
ing[24] (Ocat···Cmethyl 2.33 ꢁ).
Based on those findings, we
now describe investigations to
control the helicity of the com-
plexes by benefitting from the
dynamic behavior of the mono-
mers and the stereochemical ri-
gidity of the dimer. We were
able to induce stereochemistry
at the complexes by addition of
chiral cations or by attaching
chiral ester side chains. In the
latter case, an unusual solvent
dependent inversion of stereo-
chemistry of the complexes has
been observed and is now re-
ported for additional deriva-
tives.[25] Furthermore, it has been
shown that the solvent-depen-
dent epimerization behavior of
the compounds is sensitive to
subtle variations of the substitu-
ents. Introduction of a bromide
in the periphery of the ester
group resulted in a highly stable
dimer possessing an unexpected
stereochemistry.
Figure 1. Helicate-type complexes A–E formed in hierarchical multicomponent self-assembly processes.
Results and Discussion
Chiral induction by
countercations
The observation of a weak inter-
action between the dimer and
an ammonium cation in the
crystal of [Me4N][Li3(Ti(1b)3)2]
prompted us to test whether
chiral induction from an enantio-
merically pure ammonium ion
onto stereochemically locked di-
Scheme 1. Hierarchical self-assembly of dinuclear triple lithium-bridged titanium(IV) helicates.
meric catecholate complexes is possible in solution.[26,27]
One equivalent of (R)-phenylethylammonium chloride was
added to a solution of Li2[Ti(1a)3] in THF. This solvent
favors the presence of the dimer Li[Li3(Ti(1a)3)2] (Kdim(THF)
740.000mÀ1). A circular dichroism (CD) spectrum (Figure 5)
showed typical transitions for titanium triscatecholates at l=
342 nm (positive Cotton effect) and 408 nm (negative Cotton
effect). According to earlier investigations on the correlation of
calculated and experimentally determined CD spectra of tris-
catecholate titanium(IV) complexes by CD spectroscopy,[28] the
DD isomer can be assigned to be the dominating stereo-
isomer.
Figure 2. Ligands discussed in this study.
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Figure 5. CD spectrum of DD-Li[Li3(Ti(1a)3)2] in THF in the presence of
(R)-phenylethylammonium chloride.
Figure 3. 1H NMR spectra of Li2[Ti(1b)3]/Li[Li3(Ti(1b)3)2] in [D6]DMSO (top) or
[D4]MeOH (bottom). In [D6]DMSO, the monomer is the dominating species,
whereas in [D4]MeOH, only dimer is observed (the doublet at d=1.4 ppm
corresponds to traces of free ligand; the even smaller signal at d=1.3 ppm
might be due to the monomer, but cannot be unambiguously assigned).
center to the metal-complex units. The a-chiral ester 1c-H2
gave enantiomerically and diastereomerically pure Li2[Ti(1c)3]/
Li[Li3(Ti(1c)3)2], which showed dimerization constants in
[D4]MeOH of Kdim 4.600 and 16mÀ1 in [D6]DMSO. Surprisingly,
CD spectroscopy in methanol (e 32.63) or DMSO (e 46.68) gave
spectra, which are nearly mirror images to each other.[29]
Reason for this behavior is opposite configuration at the titani-
um catecholate units, which is controlled by the chiral ester
substituents.[25]
Ester units of secondary alcohols adopt a conformation, in
which the carbonyl and the a-proton are arranged syn to each
other.[30] In the monomer, the carbonyl unit is oriented to the
“outside” of the metal triscatecholate, whereas in the dimer,
due to binding of the lithium cations, it has to point inwards.
The rotation positions the big phenyl groups in different direc-
tions in the monomer or dimer inducing an opposite twist
(Figure 6). The S configuration at the ligand gave a L configu-
ration of the complex units of the monomer and a DD config-
uration at the dimer.
Figure 4. Solid-state structure of [Me4N][Li3(Ti(1b)3)2].
Induction of stereochemistry at Li[Li3(Ti(1a)3)2] cannot occur
directly by interaction with the chiral ammonium salt. Herein,
the configuration is already locked. However, interaction of the
ammonium ions with the monomeric anions [Ti(1a)3]2À, which
are present in the equilibrium, favors the DR diastereoisomer.
The monomers of this adduct form the dimer leading to the
DD isomer (Scheme 2). Herein, a transfer of stereochemical in-
formation from the ammonium cation to the dimeric helicate
type complexes via a contact ion pair of the monomer occurs.
However, the observations did not allow an estimation of the
efficiency of this process.
Figure 6. Schematic presentation of the orientation of the chiral ester sub-
stituents in the monomer Li2[Ti(1c)3] and in the dimer Li[Li3(Ti(1c)3)2].
To investigate this effect in more detail, we prepared the
[31]
ligands 1d-f-H2
and obtained the metal complexes
Li2[Ti(1d-f)3]/Li[Li3(Ti(1d-f)3)2]. The complexes were character-
ized by standard techniques, such as NMR spectroscopy or ESI
mass spectrometry.
Compound Li[Li3(Ti(1d)3)2] showed a dimerization constant
in methanol of Kdim 5160mÀ1. In the case of Li[Li3(Ti(1e)3)2],
only dimer was observed. In DMSO, only the monomeric spe-
cies Li2[Ti(1d, e)3] were detected by NMR spectrometry. For the
Chiral induction by enantiomerically pure ester substituents
In a preliminary study, we found that the stereochemical in-
duction in the dimers is dependent on the distance of a stereo-
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Figure 6 compares the different orientations of the
ester group in the monomer and in the dimer, re-
spectively. In the monomer, this substituent is forced
into the vicinity of the metal-complex unit, resulting
in a close proximity of two of such groups in the
anti-isomer and of all three of them in the syn-
isomer. This builds up steric constraints, which desta-
bilize the monomer. In the dimer, the bulky groups
orientate away from each other, and the system is
less sterically congested. Thus, in the case of Li[Li3(Ti-
(1 f)3)2], the dimer is favored over the monomer due
to a destabilization of the latter.
In addition, the bulkiness of the ester group does
affect the structure of the dimer [Li3(Ti(1 f)3)2]À. Inter-
actions between the ester group and the helicate
backbone are minimized by a slight twisting of the
OÀCH bond resulting in a close to linear arrangement
of the catecholate and the bromophenyl of a ligand.
From Figure 8d (green ligand), it can be seen that
the orientation of the medium-sized methyl group
Scheme 2. Equilibrium in the Li2[Ti(1a)3]/(R)-phenylethylammonium system
leading to the preferred formation of the DD isomer of Li[Li3(Ti(1a)3)2].
two systems, Li2[Ti(1d)3]/Li[Li3(Ti(1d)3)2] and Li2[Ti(1e)3]/Li[Li3(Ti-
(1e)3)2], we found a similar CD spectroscopic behavior as for
Li2[Ti(1c)3]/Li[Li3(Ti(1c)3)2].[25] With the S-configurated ligand 1d,
the monomer adopted a L, and the dimer a DD configuration,
whereas R-configurated 1e gave the D monomer and the LL
dimer (Figure 7).
Crystals of the naphthyl-derivative Na[Li3(Ti(1d)3)2] enabled
a crystal structure analysis (Figure 8a shows a side view). In
Figure 8b, the carbon skeleton of one of the ligands of the
dimer is shown in green, revealing the influence of the bulky
aromatic moiety at the S-configurated unit on the twist of the
dimeric helicate-type structure resulting in a DD configuration.
Surprisingly, for compound Li[Li3(Ti(1 f)3)2], the dimeric form
is preferred in methanol (Kdim 40.000mÀ1), as well as in DMSO
(only dimer was observed). Furthermore, the stereochemical
features are in contrast to the earlier observations. In methanol
or DMSO, the metal-complex units possess the same configura-
tion, because only dimers are present. However, opposite to
the earlier examples the S-configurated ligand induces LL
configuration at the metal-complex units (Figure 9).
A crystal-structure analysis of Li[Li3(Ti(1 f)3)2] revealed the ge-
ometry of the dimeric complex in the solid state.[20] As has
been already observed in solution, the S-configurated ligand
gave the LL isomer of the dimeric complex (Figure 8c,d).
Those findings can be explained by considering the size of
the chiral OCH(Me)-o-(BrPh) substituent. Due to the ortho bro-
mide, the substituent loses its flexibility. The bromide locks be-
tween the methyl group and the hydrogen atom, prohibiting
rotation around the secondary carbon-aryl bond. This nonflexi-
bility increases the bulkiness of this substituent. Next question
to be answered is how this affects the stability and the stereo-
chemistry of the dimer.
Figure 7. CD spectra of Li2[Ti(1d)3]/Li[Li3(Ti(1d)3)2] (top) and Li2[Ti(1e)3]/Li-
[Li3(Ti(1e)3)2] (bottom) in methanol, as well as in DMSO. The (S)-configurated
ligand 1d induces L for the monomer and DD for the dimer. (R)-1e gave in
the D-monomer and the LL-dimer.
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more effective way to control
the configuration at the metal
complex units is the introduction
of chiral ester substituents. Due
to the different orientation of
the ester group, opposite config-
uration is induced at the com-
plexes in the monomer or dimer.
As an exception, in Li[Li3(Ti-
(1 f)3)2], the sterically demanding
ester side chain destabilizes the
monomer and at the same time
adopts a conformation, in which
the configuration at the metal
complex is inversed. This is due
to the takeover of stereocontrol
by the small methyl group.
Experimental Section
General
Figure 8. Dimeric unit [Li3(Ti(1d)3)2]À (a,b) and [Li3(Ti(1 f)3)2]À (c,d) as was found in the crystal. C: black, H: white,
O: red, Ti: yellow, Li: blue, Br: brown; in the space-filling models (b,d) one of the ligands is shown in green.
NMR spectra were recorded on
a Varian Mercury 300, Inova 400, or
Inova 600 NMR spectrometers.
FTIR spectra were recorded on
a Bruker IFS spectrometer. Mass
spectra were obtained on a Finnigan SSQ 700 or LCQ Orbitrap XL
mass spectrometer. Elemental analyses were measured with a Her-
aeus CHN-O-Rapid analyzer. Ligand 1a-H2 and the complex Li[Li3(Ti-
(1a)3)2] were prepared as described before.[20a] Crystal-structure
data sets were collected with a Nonius KappaCCD diffractometer,
in the case of Mo-radiation equipped with a rotating anode gener-
ator. Programs used: data collection COLLECT[32] , data reduction
Denzo-SMN,[33] absorption correction Denzo,[34] structure solution
SHELXS-97,[35] structure refinement SHELXL-97.[36] CCDC-986913
([Me4N][Li3(Ti(1b)3]2], -895405 (Na[Li3(Ti(1d)3]2), and -895404 (Li-
[Li3(Ti(1 f)3]2) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
General procedure for the preparation of ligands 1b–f-H2
Figure 9. CD spectra of the LL-dimer Li[Li3(Ti(1 f)3)2] in methanol, as well as
The ligands were prepared by a modified literature procedure.[31]
2,3-Dihydroxybenzoic acid (ca. 5 mmol, 1 equiv) was added to
chloroform (50 mL), and the suspension was degassed and flushed
with argon. Thionyl chloride (20 equiv) was added, and the mixture
was stirred at 908C for 3 h. Volatiles were removed under reduced
pressure, and the colorless residue was dissolved in dry dichloro-
methane. Tetraethylammonium (3 equiv) and the corresponding al-
cohol in dry dichloromethane (20 mL) were added at 08C. After
three days stirring at RT, the solution was washed with saturated
aqueous NaHCO3, and water and the organic phase were dried
over Na2SO4. Removal of the solvent under reduced pressure gave
the crude product, which was purified by column chromatography
on silica gel.
in DMSO.
controls the twisting of the helical dinuclear metal complex re-
sulting in the LL stereoisomer.
Thus, in compound Li[Li3(Ti(1 f)3)2], the bulky ester group de-
stabilizes the monomer and induces some distortion in the
ligand. Based on this, the relatively small methyl group be-
comes the stereocontrolling unit.
Conclusion
Herein, the control of the stereochemistry of metal complexes
in hierarchically assembled helicates was investigated. Chiral
ammonium counterions interact with the stereolabile mono-
mers, and the chiral information is transferred to the dimer. A
Data for 1b-H2
Yield: 342 mg (1.74 mmol, 35%) as a colorless oil; Rf 0.36 (n-pen-
tane/ethyl acetate 10:1); 1H NMR (400 MHz, CDCl3): d=11.11 (bs,
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1H), 7.37 (dd, J=1.5 Hz, 8.0 Hz, 1H), 7.10 (dd, J=1.5 Hz, 8.0 Hz,
1H), 6.78 (t, J=8.0 Hz, 1H), 5.83 (bs, 1H), 5.29 (sept, J=6.3 Hz, 1H),
1.39 ppm (d, J=6.3 Hz, 6H); 13C NMR (75 MHz, CDCl3): d=169.94
(C), 148.96 (C), 145.00 (C), 120.54 (CH), 119.62 (CH), 118.98 (CH),
112.96 (C), 69.47 (CH), 21.77 ppm (CH3); MS(CI): m/z 197.4 [M+H]+;
IR(KBr): n˜ =3471, 3129, 2984, 2936, 2740, 2463, 1668, 1612, 1469,
1369, 1306, 1267, 1162, 1103, 1069, 975, 902, 842, 754, 716, 641,
587, 477 cmÀ1; elemental analysis calcd for C10H12O4 (Mw 196.20): C
61.22, H 6.16; found: C 60.90, H 5.84.
overnight at RT. Solvent was removed under reduced pressure to
obtain the complexes in quantitative yield.
Data for Li2[Ti(1b)3]/Li[Li3(Ti(1b)3)2]
Yield: 89 mg (quant) as orange solid; 1H NMR (300 MHz, CD3OD):
dimer: d=7.12–7.07 (m, 6H), 6.58–6.54 (m, 12H), 4.06 (pseudo
sept, J=6.2 Hz, 6H), 1.03 (d, J=6.2 Hz, 18H), 0.89 ppm (d, J=
1
6.2 Hz, 18H); H NMR (300 MHz, [D6]DMSO): monomer (main com-
ponent): d=6.76 (dd, J=1.3 Hz, 7.7 Hz, 3H), 6.26 (t, J=Hz, 3H),
6.13 (dd, J=1.3 Hz, 7.7 Hz, 3H), 4.96 pseudo sept, J=6.2 Hz, 6H),
1.23 (d, J=6.2 Hz, 18H); monomer (minor component): d=6.95
(dd, J=1.5 Hz, 7.8 Hz, 6H), 6.49 (t, J=7.8 Hz, 6H), 6.39 (dd, J=
1.5 Hz, 7.8 Hz, 6H), 3.97 (pseudo sept, J=6.1 Hz, 6H), 0.99 (d, J=
6.1 Hz, 18H), 0.85 ppm (d, J=6.1 Hz, 18H); negative ESI FTMS
(MeOH): m/z 1281.3 Li3[(1b)6Ti2]À; IR(KBr): n˜ =3406, 3065, 2981,
2932, 2664, 2318, 2081, 1951, 1667, 1593, 1559, 1442, 1364, 1295,
1252, 1213, 1104, 1062, 990, 904, 853, 789, 742, 678 cmÀ1; elemen-
tal analysis calcd for C30H30Li2O12Ti·2H2O·DMF (753.43): C 52.61, H
5.49, N 1.86; found: C 52.22, H 5.40, N 1.44.
Data for (S)-1d-H2
Scale: 0.58 mmol; yield: 27 mg (0.087 mmol, 15%) as colorless
solid; m.p. 898C; Rf 0.26 (CHCl3/n-pentane 4:1); H NMR (300 MHz,
1
CDCl3): d=10.90 (bs, OH), 7.90–7.82 (m, 4H), 7.58–7.47 (m, 4H),
7.11 (dd, J=1.5, 7.9 Hz, 1H), 6.83 (t, J=7.9 Hz, 1H), 6.30 (q, J=
6.7 Hz, 1H), 5.61 (bs, OH), 1.78 ppm (d, J=6.7 Hz, 3H); 13C NMR
(75 MHz, CD3OD): d=169.63 (C), 138.60 (C), 133.26 (C), 133.17 (C),
128.11 (CH), 127.64 (CH), 127.26 (CH), 125.94 (CH), 125.80 (CH),
124.59 (CH), 123.41 (CH), 120.39 (CH), 119.84 (CH), 118.69 (CH),
112.69 (C), 73.74 (CH), 21.16 ppm (CH3); MS (DIP/EI): m/z 308.2
[M+]; IR (KBr): n˜ =3435, 3052, 2981, 2932, 2710, 2326, 2179, 2141,
2063, 2025, 1979, 1955, 1922, 1739, 1669, 1621, 1592, 1494, 1450,
1352, 1302, 1216, 1153, 1126, 1054, 989, 967, 897, 864, 843, 820,
X-ray crystal-structure analysis of [N(CH3)4]0.5[Li3(Ti(1b)3)2]
Formula C60H60Li3O24Ti2 ꢂ1= (CH3)4N, M=1318.77 gmolÀ1, red crystal
2
0.23ꢂ0.20ꢂ0.13 mm3, a=30.7219(12) ꢁ, V=28996(2) ꢁ3, 1calcd
=
753, 716, 660 cmÀ1; elemental analysis calcd for C19H16O4·1= MeOH
(324.12): C 72.21, H 5.59; found: C 72.50, H 5.21.
2
1.208 gcmÀ3
, , empirical absorption correction
m=0.291 mmÀ1
(0.936ꢀTꢀ0.963), Z=16, cubic, space group F4132 (No. 210), l=
0.71073 ꢁ, T=223(2) K, w and f scans, 3203 reflections collected
(Æh, Æk, Æl), [(sinq)/l]=0.59 ꢁÀ1, 2090 independent (Rint =0.025)
and 1538 observed reflections [Iꢁ2s(I)], 142 refined parameters,
R=0.098, wR2 =0.238, Flack parameter 0.04(15), max. (min.) residu-
al electron density 0.32 (À0.56) eꢁÀ3, one half-occupied cation
(probably lithium) and solvent molecules could not be localized
either refined in a chemically meaningful way, therefore, the
SQUEEZE routine from the PLATON suite[37] was used, the hydrogen
atoms were calculated and refined as riding atoms.
Data for (R)-1e-H2
Scale: 0.43 mmol; yield: 23 mg (0.068 mmol, 16%) as colorless oil;
Rf 0.46 (n-pentane/EtOAc 5:1); H NMR (300 MHz, CDCl3): d=10.75
1
(bs, OH), 7.46–7.41 (m, 2H), 7.36 (dd, J=1.5, 8.0 Hz, 1H), 7.25–7.20
(m, 2H), 7.04 (dd, J=1.5, 8.0 Hz, 1H), 6.74 (t, J=8.0 Hz, 1H), 6.00
(q, J=6.6 Hz, 1H), 5.56 (bs, OH), 1.60 ppm (d, J=6.6 Hz, 3H);
13C NMR (75 MHz, CDCl3): d=169.51 (C), 149.00 (C), 145.07 (C),
140.03 (C), 131.84 (CH), 127.71 (CH), 122,15 (C), 120.48 (CH), 119.92
(CH), 119.22 (CH), 73.13 (CH), 22.18 ppm (CH3); MS (DIP/EI): m/z
336.1 [M+]; IR (KBr): n˜ =3517, 3137, 2984, 2928, 2853, 2079, 1903,
1668, 1596, 1467, 1408, 1352, 1295, 1263, 1234, 1149, 1060, 1005,
965, 905, 821, 751, 711 cmÀ1; HRMS (C15H13BrO4Na; 360.16): calcd
358.98894, found 358.98865.
Data for Li2[Ti(1d)3]/Li[Li3(Ti(1d)3)2]
Yield: 50 mg (quant) as orange solid; 1H NMR (600 MHz, CD3OD):
dimer (major): d=7.35 (d, J=8.5 Hz, 6H), 7.19–7.09 (m, 30H), 6.94
(d, J=8.5 Hz, 6H), 6.74 (dd, J=1.6, 7.6 Hz, 6H), 6.71 (dd, J=1.6,
8.5 Hz, 6H), 6.67 (t, J=7.6 Hz, 6H), 4.43 (q, J=6.6 Hz, 6H),
1.12 ppm (d, J=6.6 Hz, 18H); monomer (minor): d=7.92–7.89 (m,
3H), 7.86 (d, J=8.5 Hz, 3H), 7.79–7.75 (m, 3H), 7.73–7.70 (m, 3H),
7.52–7.50 (m, 6H), 6.56 (dd, J=1.4, 7.8 Hz, 3H), 5.76 (t, J=7.8 Hz,
3H), 5.24 (q, J=6.4 Hz, 3H), 1.27 ppm (d, J=6.4 Hz, 9H), and some
signals are hidden under solvent or dimer peaks; 1H NMR
(400 MHz, [D6]DMSO): monomer: d=7.99–7.94 (m, 3H), 7.93–7.87
(m, 3H), 7.82–7.74 (m, 6H), 7.73–7.67 (m, 3H), 7.45–7.39 (m, 6H),
6.96–6.86 (m, 3H), 6.32 (t, J=6.8 Hz, 3H), 6.20 (d, J=6.8 Hz, 3H),
6.05 (q, J=6.0 Hz, 3H), 1.50 (d, J=6.0 Hz, 9H); negative ESI FTMS
(MeOH): m/z 1954.5 Li3[(1d)6Ti2]À, 1970.5 NaLi2[(1d)6Ti2]À, 1986.5
LiNa2[(1d)6Ti2]À; IR (KBr): n˜ =3644, 3354, 3058, 2980, 2928, 2853,
2662, 2285, 2110, 2014, 1912, 1677, 1595, 1560, 1517, 1442, 1351,
1291, 1253, 1214, 1190, 1155, 1059, 985, 952, 920, 892, 856, 808,
Data for (S)-1 f-H2
Scale: 1.62 mmol scale; yield: 273 mg (0.810 mmol, 49%) as color-
less solid; M.p. 778C; Rf: 0.49 (dichloromethane); H NMR (300 MHz,
1
CDCl3): d=10.83 (bs, OH), 7.58 (dd, J=1.4, 7.8 Hz, 1H), 7.53–7.49
(m, 2H), 7.34 (td, J=1.4, 7.8 Hz, 1H), 7.18 (td, J=1.4, 7.8 Hz, 1H),
7.15–7.11 (m, 1H), 6.84 (t, 8.0 Hz, 1H), 6.42 (q, J=6.5 Hz, 1H), 5.68
(bs, OH), 1.68 ppm (d, J=6.5 Hz, 3H); 13C NMR (75 MHz, CDCl3): d=
169.32 (C), 149.05 (C), 145.10 (C), 140.65 (C), 133.00 (CH), 129.42
(CH), 127.10 (CH), 126.5 (CH), 121.64 (C), 120.49 (CH), 119.92 (CH),
119.22 (CH), 112.45 (C), 72.98 (CH), 21.24 ppm (CH3); MS (DIP/EI):
m/z 338.0 [M+]; IR (KBr): n˜ =3521, 3385, 3067, 2980, 2928, 2863,
2322, 2110, 1918, 1803, 1722, 1666, 1571, 1465, 1401, 1367, 1332,
1300, 1261, 1230, 1152, 1064, 998, 961, 902, 874, 843, 800, 750,
720, 666 cmÀ1; elemental analysis calcd for C15H13BrO4 (337.17): C
53.43, H 3.89; found: C 53.58, H 3.74.
744,
713,
680 cmÀ1
;
elemental
analysis
calcd
for
C57H42Li2O12Ti·H2O·MeOH (1030.74): C 67.58, H 4.69; found: C 67.43,
H 4.47.
General procedure for the preparation of complexes
X-ray crystal-structure analysis of Na[Li3(Ti(1d)3)2](C3H7NO)2
2,3-Dihydroxybenzoic ester (3.0 equiv), TiO(acac)2 (1.0 equiv), and
Li2CO3 (1.0 equiv) were dissolved in methanol (40 mL) and stirred
Formula C114H84Li3NaO24Ti2 ꢂ2 C3H7NO, M=2123.61 gmolÀ1, orange
crystal 0.27ꢂ0.25ꢂ0.20 mm3, a=11.8562(1), b=28.9632(3), c=
Chem. Eur. J. 2014, 20, 6650 – 6658
www.chemeurj.org
6656
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
32.1503(3) ꢁ,
V=11040.20(18) ꢁ3,
1calcd =1.278 gcmÀ3
,
m=
Acknowledgements
0.223 mmÀ1, empirical absorption correction (0.942ꢀTꢀ0.957), Z=
4, orthorhombic, space group P212121 (No. 19), l=0.71073 ꢁ, T=
223(2) K, w and f scans, 84396 reflections collected (Æh, Æk, Æl),
[(sinq)/l]=0.66 ꢁÀ1, 26470 independent (Rint =0.061) and 20171
observed reflections [Iꢁ2s(I)], 1373 refined parameters, R=0.087,
wR2 =0.234, Flack parameter 0.07(3), max. (min.) residual electron
density 1.03 (À0.52) eꢁÀ3, the hydrogen atoms were calculated
and refined as riding atoms.
A scholarship of the international research training group of
the DFG SELECA to E.I. is gratefully acknowledged. We thank
David Vancraen for the determination of the dimerization con-
stant of Li2[Ti(1a)3]/Li[Li3(Ti(1a)3)2] in THF.
Keywords: circular dichroism · coordination chemistry · self-
assembly · stereochemistry · titanium
Data for Li2[Ti(1e)3]/Li[Li3(Ti(1e)3)2]
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Yield: 24 mg (quant.) as orange solid; 1H NMR (300 MHz, CD3OD):
dimer: d=7.22 (d, J=7.7 Hz), 6.68–6.64 (m, 14H), 6.76–6.70 (m,
1
22H), 4.01 (q, J=6.6 Hz, 6H), 1.17 ppm (d, J=6.6 Hz, 18H); H NMR
(300 MHz, [D6]DMSO): monomer: d=7.58 (d, J=7.5 Hz, 6H), 7.43–
7.29 (m, 6H), 6.85 (d, J=7.0 Hz, 3H), 6.33 (t, J=7.5 Hz, 3H), 6.21 (d,
J=7.0 Hz, 3H), 5.87 (q, J=6.5 Hz, 3H), 1.38 ppm (m, 9H); negative
ESI FTMS (MeOH): m/z 1054.9 H[(1e)3Ti]À, 2126.8 Li3[(1e)6Ti2]À; IR
(KBr): n˜ =3398, 3066, 2982, 2928, 2852, 2667, 2199, 2052, 1899,
1839, 1776, 1681, 1592, 1558, 1489, 1443, 1355, 1293, 1252, 1217,
1188, 1154, 1059, 1008, 980, 949, 860, 819, 774, 744, 715, 680 cmÀ1
calcd for C45H33Br3Li2O12Ti·3H2O (1121.24): C 48.20, H 3.51; found: C
;
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Data for Li2[Ti(1 f)3]/Li[Li3(Ti(1 f)3)2]
Yield: 42 mg (quant.) as orange solid; 1H NMR (300 MHz, CD3OD):
dimer (major): d=7.27 (dd, J=2.0, 7.6 Hz, 12H), 7.10–7.04 (m,
12H), 6.65 (dd, J=2.0, 7.6 Hz) and 6.61 (t, J=7.6 Hz, in total12H),
6.45 (td, J=2.0, 7.6 Hz, 6H), 4.97 (q, J=6.6 Hz, 6H), 0.94 ppm (d,
J=6.6 Hz, 18H); monomer (minor) d=7.63 (d, J=7.7 Hz, 3H),
7.60–7.48 (m, 9H), 6.84 (dd, J=1.3, 8.2 Hz, 3H), 5.78 (t, J=7.7 Hz,
3H), 5.25 (q, J=6.1 Hz, 3H), 1.17 ppm (d, J=6.1 Hz, 9H), and miss-
ing signals are hidden under the peaks of the dimer; 1H NMR
(300 MHz, [D6]DMSO): dimer: d=7.67 (d, J=8.0 Hz, 3H), 7.60 (t, J=
7.2 Hz, 3H), 4.00 (d, J=7.2 Hz, 3H), 7.35 (t, J=7.2 Hz, 3H), 6.68 (d,
J=8.0 Hz, 3H), 6.45 (d, J=7.2 Hz, 3H), 5.64 (t, J=8.0 Hz, 3H), 5.06
(q, J=6.1 Hz, 3H), 1.11 ppm (d, J=6.1 Hz, 9H); negative ESI FTMS
(MeOH): m/z 2126.8 Li3[(1 f)6Ti2]À; IR(KBr): n˜ =3872, 3646, 3363,
3066, 2981, 2929, 2854, 2684, 2323, 2175, 2110, 2017, 1918, 1800,
1679, 1594, 1559, 1442, 1360, 1293, 1252, 1213, 1188, 1153, 1125,
1065, 981, 888, 859, 818, 743, 682 cmÀ1; elemental analysis calcd
for C45H33Br3Li2O12Ti (1067.20): C 50.64, H 3.12; found: C 50.23, H
3.03.
X-ray crystal-structure analysis of Li[Li3(Ti(1 f)3)2]
Formula C90H66Br6Li4O24Ti2; M=2134.45 gmolÀ1, red crystal 0.30ꢂ
0.20ꢂ0.15 mm3, a=27.2142(5), c=14.3257(3) ꢁ, V=10609.8(4) ꢁ3,
1calcd =1.336 gcmÀ3, m=2.473 mmÀ1, empirical absorption correc-
tion (0.524ꢀTꢀ0.708), Z=4, tetragonal, space group P42212 (No.
94), l=0.71073 ꢁ, T=223(2) K, w and f scans, 36227 reflections
collected (Æh, Æk, Æl), [(sinq)/l]=0.59 ꢁÀ1, 9180 independent
(Rint =0.116) and 5045 observed reflections [Iꢁ2s(I)], 567 refined
parameters, R=0.096, wR2 =0.250, Flack parameter 0.08(2), max.
(min.) residual electron density 0.46 (À0.37) eꢁÀ3, one half-occu-
pied cation (probably lithium) and solvent molecules could not be
localized either refined in a chemically meaningful way; therefore,
the SQUEEZE routine from the PLATON suite[37] was used, the hy-
drogen atoms were calculated and refined as riding atoms.
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Received: February 26, 2014
Published online on April 29, 2014
Chem. Eur. J. 2014, 20, 6650 – 6658
www.chemeurj.org
6658
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