The versatile, efficient, and stereoselective self-assembly of transition-metal helicates by using hydrogen-bonds

Article abstract of DOI:10.1002/chem.200400485

A diverse range of dinuclear double-stranded helicates in which the ligand strand is built up by using hydrogen-bonding has been synthesized. The helicates, formulated as [Co₂(L)₂(L-H)₂X ₂], readily self-assemble from a mixture of a suitable pyridine-alcohol compound (L; for example, 6-methylpyridine-2-melhanol, 1), and a CoX₂ salt in the presence of base. Nine such helicates have been characterized by X-ray crystallography. For helicates derived from the same pyridine-alcohol precursor, a remarkable regularity was found for both the molecular structure and the crystal packing arrangements, regardless of the nature of the ancillary ligand (X). A notable exception was observed in the solid-state structure of [Co₂(1)₂(1-H)₂(NCS) ₂] for which intermolecular nonbonded contacts between the sulfur atoms (S...S = 3.21 A) lead to the formation of 1D chains. Helicates derived from (R)-6-methylpyridine-2-methanol (2) are soluble in solvents such as CH₃CN and CH₂Cl₂, and their self-assembly could be monitored in solution by ¹H NMR, UV/Vis, and CD titrations. No intermediate complexes were observed to form in a significant concentration at any point throughout these titrations. The global thermodynamic stability constant of [Co₂(2)₂-(2-H)₂(NO ₃)₂] was calculated from spectrophotometric data to be logβ = 8.9(8). The stereoisomerism of these helicates was studied in some detail and the self-assembly process was found to be highly stereoselective. The chirality of the ligand precursors can control the absolute configuration of the metal centers and thus the overall helicity of the dinuclear assemblies. Furthermore, the enantiomers of rac-6-methylpyridine-2-methanol (3) undergo a self-recognition process to form exclusively bomochiral helicates in which the four pyridine-alcohol units possess the same chirality.

Full text of DOI:10.1002/chem.200400485

FULL PAPER
Chem. Eur. J. 2005, 11, 57 – 68
DOI: 10.1002/chem.200400485
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
57

The Versatile, Efficient, and Stereoselective Self-Assembly of Transition-
Metal Helicates by Using Hydrogen-Bonds
[
a, c]
[a, b]
Shane G. Telfer*
and Reiko Kuroda*
Abstract: A diverse range of dinuclear
double-stranded helicates in which the
ligand strand is built up by using hy-
drogen-bonding has been synthesized.
the solid-state structure of [Co (1) (1-
these titrations. The global thermody-
2
2
H) (NCS) ] for which intermolecular
namic stability constant of [Co (2) -
2
2
2
2
nonbonded contacts between the sulfur
atoms (S···S=3.21 Š) lead to the for-
mation of 1D chains. Helicates derived
from (R)-6-methylpyridine-2-methanol
(2) are soluble in solvents such as
CH CN and CH Cl , and their self-as-
(2-H) (NO ) ] was calculated from spec-
2 3 2
trophotometric data to be logb=8.9(8).
The stereoisomerism of these helicates
was studied in some detail and the self-
assembly process was found to be
highly stereoselective. The chirality of
the ligand precursors can control the
absolute configuration of the metal
centers and thus the overall helicity of
the dinuclear assemblies. Furthermore,
the enantiomers of rac-6-methylpyri-
dine-2-methanol (3) undergo a self-rec-
ognition process to form exclusively
homochiral helicates in which the four
pyridine–alcohol units possess the same
chirality.
The
helicates,
formulated
as
[
Co (L) (L-H) X ], readily self-assem-
2
2
2
2
ble from a mixture of a suitable pyri-
dine–alcohol compound (L; for exam-
ple, 6-methylpyridine-2-methanol, 1),
and a CoX2 salt in the presence of
base. Nine such helicates have been
characterized by X-ray crystallography.
For helicates derived from the same
pyridine–alcohol precursor, a remark-
able regularity was found for both the
molecular structure and the crystal
packing arrangements, regardless of
the nature of the ancillary ligand (X).
A notable exception was observed in
3
2
2
sembly could be monitored in solution
1
by H NMR, UV/Vis, and CD titra-
tions. No intermediate complexes were
observed to form in a significant con-
centration at any point throughout
Keywords: chirality
· cobalt ·
helical structures · N,O ligands ·
self-assembly
chemistry
·
supramolecular
Introduction
ous investigations that have contributed significantly to our
[3]
understanding of metal-based self-assembly processes, and
to the stereoselective synthesis of metallo-supramolecular
Transition-metal helicates have served as a pillar for the de-
velopment of metallo-supramolecular chemistry over the
[4,5]
architectures.
The conventional synthetic route to heli-
[
1,2]
past decade.
Helicates have formed the basis for numer-
cates involves the mixing of a preformed ligand strand with
an appropriate metal ion. In such systems, the ligand strand
is almost always a covalent organic compound that has been
synthesized by conventional methods. In a few cases, metal
[
a] S. G. Telfer, R. Kuroda
JSTERATO Kuroda Chiromorphology Project, Park Bldg., 4–7–6
Komaba, Meguro-ku, Tokyo, 153-0041 (Japan)
Fax : (+81)3-5465-0104
E-mail: shane.telfer@chiromor2.erato.rcast.u-tokyo.ac.jp
ckuroda@mail.ecc.u-tokyo.ac.jp
[6]
ions have been used to build up the ligand strand.
We have recently presented a novel synthesis of transi-
tion-metal helicates that employs hydrogen bonding be-
tween the oxygen atoms of pyridyl-alcohol compounds (e.g.,
[
b] R. Kuroda
[7]
Graduate School of Arts and Sciences, University of Tokyo, Komaba,
Meguro-ku, Tokyo, 153–8902 (Japan)
Fax : (+81)3-5452-0599
1 to build up the ligand strands (Scheme 1). These heli-
[
c] S. G. Telfer
Current address:
Department of Chemistry, University of Montreal
CP 6128 succ. Centre-Ville, Montreal, QC, H3C 3J7 (Canada)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
58
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/chem.200400485
Chem. Eur. J. 2005, 11, 57 – 68

Stereoselective Self-Assembly of Transition-Metal Helicates
FULL PAPER
mixtures of crystalline products that were identified by X-
ray crystallography as [Co (1) (1-H) X ] and [Co (1-
2
2
2
2
4
H) X (MeOH) ]. The formation of the tetranuclear cluster
4
2
2
complexes, which will be detailed in a separate publication,
can be ascribed to the stronger basicity of both NaOH and
NEt as compared to acetate. To circumvent the formation
3
of these clusters, we supposed that compound 1 may be able
to act as the requisite base. Indeed, the reaction of a variety
of CoX2 salts with four equivalents of 1 in CH CN was
found to lead to the rapid crystallization of analytically pure
3
Scheme 1. The self-assembly of helicates using hydrogen bonding to build
up the ligand strands.
[
Co (1) (1-H) X ] helicates. The insolubility of these heli-
2 2 2 2
cates self-assemble from eight simple components—four
ions and four small molecules—and are stabilized by a vari-
ety of supramolecular interactions: coordination bonds, hy-
drogen bonds, and p–p interactions. This work compliments
cates greatly facilitates their isolation—they are easily sepa-
rated from [1-H] (which can be recovered if necessary)
and any side products by simple filtration and washing. This
synthetic route proved to be both general and straightfor-
ward. The yield of the helicates was high, ranging from 73%
+
[8]
other recent advances in the fields of catalysis, crystal en-
[9]
[10]
gineering, and other supramolecular assemblies,
which
for X=NO to 90% for X=Br. X-ray quality crystals of the
3
benefit from a fruitful combination of coordination and hy-
drogen bonding.
[Co (1) (1-H) X ] helicates were prepared by performing
2
2
2
2
the same reaction in MeOH, although the overall yields
were somewhat depressed using this solvent.
This approach has several notable advantages over the
conventional synthesis of transition-metal helicates. The
self-assembly step is a very rapid and straightforward “one-
pot” procedure that simply involves combining suitable pyri-
dine–alcohol precursors with cobalt(ii) salts. A wide variety
of such precursors is readily available, either commercially
or by simple synthetic methods. This latter point means that
the influence of the structure of the ligand strand precursor
on the self-assembly process may be systematically investi-
gated. Further diversity may potentially be introduced into
the dinuclear assemblies by changing the metal ion and/or
the ancillary ligand.
[11]
Using the general method of Ikariya et al.,
we were
able to prepare the chiral compound (R)-2 (henceforth re-
ferred to as 2) in 93% ee by the asymmetric hydrogenation
of 2-acetyl-6-methylpyridine. The reaction of 2 with CoX2
salts in the presence of base also leads to the formation of
dinuclear double-stranded helicates that can be formulated
as [Co (2) (2-H) X ]. We found that compound 2 is far less
2
2
2
2
disposed than 1 to the formation of high nuclearity cluster
complexes in the presence of strong bases such as NEt . T he
3
highest yielding synthetic procedure, however, involved the
combination of one equivalent each of CoX and Co(OAc)2
2
Herein, we present the full details of our recent investiga-
tions into the use of hydrogen-bonding as a construction ele-
ment for the ligand strands of transition-metal helicates. We
have focused on the reaction of precursors 1–3 with a wide
range of cobalt(ii) salts (Scheme 1). We have found that the
self-assembly process is both versatile and high-yielding, and
complete diastereoselectivity may be observed in both so-
lution and the solid state.
with two equivalents of 2. Upon concentration of the so-
lution, crimson-colored (X=Cl, Br) or rose-colored (X=
NO ) crystals of [Co (2) (2-H) X ] deposited in good yield.
Similar observations were made when compound 3 (the rac-
emic analogue of 2) was used to prepare the helicates.
All of the new compounds described herein are air-stable
in the solid state and may be stored over long periods with-
out noticeable decomposition. The solubilities of the heli-
cates derived from 1–3 in non-aqueous polar solvents are
noticeably different, decreasing in the order 2 (generally
highly soluble), 3 (somewhat soluble), 1 (virtually insoluble).
Where soluble, the helicates were found to fairly stable (at
3
2
2
2
2
Results
Synthesis of helicates derived from 1–3: The original syn-
thetic route that we reported for the hydrogen-bonded heli-
cates derived from 1 and 2 involved the reaction of these
precursors with CoCl ·6H O and Co(OAc) ·4H O in a
high concentrations, vide infra) in solvents such as CH OH,
3
CH CN, CH NO , and CH Cl , however they decomposed in
3
3
2
2
2
DMSO, probably due to rupture of the hydrogen bonds.
2
2
2
2
[7]
MeOH/dioxane solvent mixture. Whilst this method leads
Solid-state structures of helicates derived from 1: The solid-
state structures of a range of [Co (1) (1-H) X ] helicates
to a good yield of [Co (1) (1-H) Cl ], the yield of other
2
2
2
2
2
2
2
2
[
Co (1) (1-H) X ] helicates (e.g., X=Br, I, NO ) was much
(X=Cl, Br, I, NO , SCN) has been determined by X-ray
2
2
2
2
3
3
lower (<15%). The reason for these low yields is not clear,
though we suspect that the acetate ions may promote the
formation of cluster complexes.
A number of variations were introduced into the synthetic
scheme in an effort to improve these yields and to formulate
a more general synthetic route. The replacement of the ace-
crystallography (Table 1). The structures of the first four of
these helicates are remarkably similar, and will be discussed
first. The packing arrangement of the SCN-containing heli-
cate is particularly interesting and will be discussed subse-
quently.
The molecular structure of the hydrogen-bonded helicates
is exemplified by the structure of [Co (1) (1-H) Br ]
tate ions by the alternative bases NaOH and NEt led to
3
2
2
2
2
Chem. Eur. J. 2005, 11, 57 – 68
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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
59

S. G. Telfer and R. Kuroda
Table 1. Crystallographic data for the [Co
2
(1)
2
(1-H)
2
X
2
] helicates.
Br
X
Cl
I
NO
3
SCN
orthorhombic
Pca2
crystal system
space group
a [Š]
orthorhombic
Pbcn
orthorhombic
Pbcn
orthorhombic
Pbcn
orthorhombic
Pbcn
1
14.4508(7)
15.1817(8)
13.5843(7)
2980.2(3)
0.0495
14.498(1)
15.243(1)
13.483(1)
2979.9(3)
0.0336
14.6153(7)
15.2092(7)
13.7119(6)
3048.0(2)
0.0242
14.681(1)
15.643(1)
13.506(1)
3101.7(4)
0.0370
16.850(1)
13.664(1)
14.021(1)
3228.2(4)
0.0298
b [Š]
c [Š]
3
V [Š ]
R
wR
2
(all data)
0.1538
0.0861
0.0586
0.0880
0.0760
Selected distances [Š]
Co···Co
4.38
2.42
2.13, 2.15
1.99, 2.00
2.31, 2.34
10.80
4.37
2.43
2.13, 2.15
1.99, 2.00
2.47
4.34
2.41
2.13, 2.16
1.99, 2.00
2.67, 2.70
10.87
4.43
2.42
2.13, 2.16
1.99, 2.00
2.20, 2.21
11.21
4.36
2.42
py
py[a]
O ꢀO
CoꢀN
py
CoꢀO
CoꢀX
1.99
1.98
12.52
[
b]
CoꢀX···XꢀCo
10.87
Selected angles [8]
N-Co-N
167.9, 171.7
113.8, 117.9
173.3
167.2, 172.8
114.0, 118.7
179.4
166.9, 172.9
114.6, 118.5
175.5
167.6, 174.1
111.4, 116.9
170.0
174.0, 174.6
115.7, 116.2
177.2, 177.4
py
py
O -Co-O
py
py
O -H-O
py
[
a] O refers to the oxygen atoms of 1. [b] The CoꢀX···XꢀCo distance refers to intermolecular separation of the Co centers of the rows of helicates
aligned along the crystallographic b axis.
(
Figure 1). The ligand strands are composed of a pair of hy-
drogen-bonded molecules of 1, and this strand coordinates
to two cobalt(ii) atoms in a bis(bidentate) fashion through
2 2 2 2
Figure 1. X-ray crystal structure of the [Co (1) (1-H) Br ] helicate. Hy-
drogen atoms omitted for clarity. Co: orange; N: blue; O: red; Br: dark
red.
Figure 2. Overlay of the skeletons of the molecular structures of four
Co (1) (1-H) ] helicates. X=Cl: green; X=Br: yellow; X=I: pink;
X=NO : blue.
[
2
2
2 2
X
3
its pyridyl and alcohol donor groups. Two such ligand
strands wrap around an axis defined by the two cobalt(ii)
centers. When the ancillary ligand coordinates in a mono-
dentate fashion (X=Cl, Br, I), the geometry of these metal
centers is roughly trigonal bypyramidal with the N-Co-N
the distance between the hydrogen-bonding oxygen atoms
(Table 1). As expected, the bidentate coordination mode of
ꢀ
py
the NO ligand leads to a slight compression of the O -Co-
3
py
py
vector describing the psuedo-C axis. The nitrate ion was
O angles (O =coordinating oxygen atom of the pyridine–
3
found to adopt a bidentate coordination mode, and the ge-
alcohol ligand), and the Co···Co distance is slightly elongat-
ed as a consequence.
ometry of the metal centers in [Co (1) (1-H) (NO ) ] is best
2
2
2
3 2
described as distorted octahedral.
These four [Co (1) (1-H) X ] helicates (X=Cl, Br, I,
2
2
2
2
The structural similarity of the series of helicates derived
NO ) are virtually isostructural: they crystallize in the same
3
from compound 1 (X=Cl, Br, I and NO ) is highlighted by
space group (Pbcn) and display very similar unit cell dimen-
sions and packing arrangements. A packing diagram of
[Co (1) (1-H) XBr ] is shown as an example in Figure 3. It
can be seen that the helicates align in and ꢁend-to-endꢂ fash-
ion to form rows along the crystallographic b axis. These
3
Figure 2 which shows an overlay of the solid-state structures
of these four complexes. Many geometrical parameters are
observed to be fairly constant within this series, for example,
the metal–ligand bond lengths, the Co···Co separation, and
2
2
2
2
60
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Chem. Eur. J. 2005, 11, 57 – 68

Stereoselective Self-Assembly of Transition-Metal Helicates
FULL PAPER
ternating chirality are arranged in zigzag rows in the crystal-
lographic ab plane.
Solid-state structures of helicates derived from 2: The solid
state structures of a series of [Co (2) (2-H) X ] helicates
2
2
2
2
(
X=Cl, Br, NO ) were determined by X-ray crystallography
3
and selected crystallographic data are presented in Table 2.
Table 2. Crystallographic data for [Co
2
(2)
2
(2-H)
Br
monoclinic
P2
2 2
X ] helicates.
Ancillary ligand (X)
Cl
orthorhombic
P2
11.4404(2)
17.704(2)
9.4371(1)
N/A
NO
monoclinic
P2
3
crystal system
space group
a [Š]
b [Š]
c [Š]
1
2
1
2
1
1
12.2145(6)
11.2358(5)
12.8948(6)
98.258(1)
1751.3(1)
0.0260
12.263(2)
11.587(2)
12.980(2)
98.723(3)
1822.9(5)
0.0455
Figure 3. The [Co
2
(1)
2
(1-H)
2
Br
2
] helicates forms homochiral rows of heli-
cates along the crystallographic b axis. Hydrogen atoms omitted for clari-
ty. Co: orange; N: blue; O: red; Br: dark red.
b [8]
V [Š ]
3
1911.4(3)
0.0654
R
wR
2
(all data)
0.1415
0.0537
0.0939
rows are homochiral, that is each constituent helicate has
the same (P or M) helicity. Alternating rows of P and M
helicates build up the layer (the crystal is racemic overall).
The intermolecular Co···Co distance between the helicates
aligned along the b axis is 10.87 Š, and a similar distance is
observed for the other helicates in this series (Table 1).
Closer contact is prevented by intermolecular interactions
of the pyridyl groups of the helicates of neighboring rows.
Within each row, there is a significant gap between the an-
cillary bromo ligands of the individual helicates and a dis-
tinct cavity is formed in the vicinity of these ligands. The
presence of this cavity offers a plausible explanation for the
conservation of the packing arrangement in this series of
helicates: ancillary ligands of different sizes may occupy
sites near this cavity without disturbing the overall packing
Selected distances [Š]
Co···Co
OꢀO
4.325
4.384
4.507
2.428
2.137–2.152
1.973–2.216
2.216–2.253
2.424, 2.441
2.125, 2.146
1.964, 2.000
2.344
2.427, 2.422
2.138–2.147
1.967–1.984
2.473, 2.480
CoꢀN
CoꢀO
CoꢀX
Selected angles [8]
N-Co-N
O-Co-O
O-H-O
170.1
118.2
179.9
167.9, 169.79
116.48, 116.56
172.97, 177.01
169.3, 170.6
110.6, 112.2
170.0
The molecular structure of [Co (2) (2-H) (NO ) ] is shown
2
2
2
3 2
as a representative example in Figure 5. The molecular
structures of the chloride- and bromide-containing helicates
were found to be very similar. Similarities may also be
noted between the helicates derived from 2 and those de-
rived from 1. This is highlighted by Figure S1 in the Sup-
[12]
arrangement.
The X-ray crystal structure of the [Co (1) (1-H) (NCS) ]
2
2
2
2
helicate reveals that its molecular structure is nearly identi-
cal to the other helicates formed from precursor 1 (Table 1).
As expected, the SCN auxiliary ligand coordinates through
its nitrogen atom. Although this helicate crystallizes in a dif-
ferent space group, its overall packing arrangement is rough-
ly similar to the other members of this series of helicates. In
this case, however, the S atoms of the SCN ligands of neigh-
boring helicates are separated by a distance of 3.21 Š. This
is considerably shorter than sum of the van der Waals radii
[13]
of these atoms (3.60 Š),
and is indicative of attractive
contacts between these S atoms. These contacts link the hel-
icates into a 1D chain along the crystallographic a axis
(
Figure 4). These chains are homochiral, that is, contacts
exist only between helicates of the same left- or right-
handed chirality. Overall the crystal is racemic: chains of al-
Figure 4. Formation of a 1D chain along the a axis in the solid state struc-
ture of [Co (1) (1-H) (NCS) ]. Co: orange; N: blue; O: red; S: yellow.
2 2 2 2
Figure 5. X-ray crystal structure of [Co
N: blue; O: red.
2
(2)
2
(2-H)
2
(NO
3
)
2
]. Co: orange;
Chem. Eur. J. 2005, 11, 57 – 68
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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
61

S. G. Telfer and R. Kuroda
porting Information, which shows an overlay of the skele-
investigate the self assembly reaction leading to the
tons of [Co (1) (1-H) Br ] and [Co (2) (2-H) Br ].
[Co (2) (2-H) X ] helicates in solution. The reaction could
be monitored by UV/Vis, CD, and H NMR spectroscopies.
2
2
2
2
2
2
2
2
2
2
2
2
1
Only one diastereomer is observed in the solid state struc-
tures of the three helicates derived from compound 2. In all
cases, the chirality of the ligand precursor 2 is efficiently
transferred to the helicate structure: the cobalt(ii) centers
adopt the L absolute configuration and the ligand strands
wrap around the Co···Co axis in a right-handed (P) fashion.
Spectral changes were monitored as CoX salts were added
2
to a solution of 2 using either NEt3 (method 1 ) or
Co(OAc) (method 2) as the requisite base. The two meth-
2
ods produced similar results.
4
2 þ 2 NEt þ 2 CoCl ! ½Co ð2Þ ð2ꢀHÞ Cl Š þ 2 HNEt Cl
3
2
2
2
2
2
3
Solution behavior of [Co (2) (2-H) X ] helicates
2
2
2
2
ðmethod 1Þ
Spectroscopic characterization of [Co (2) (2-H) X ] (X=Cl,
4 2 þ CoCl
ðmethod 2Þ
2
þ CoðOAcÞ
2
! ½Co
2
ð2Þ
2
ð2ꢀHÞ
2
Cl
2
Š þ 2 HOAc
2
2
2
2
1
Br, NO ) helicates in solution: T he H NMR spectra of the
3
three [Co (2) (2-H) X ] helicates (X=Cl, Br, NO ) dissolved
2
2
2
2
3
in CD CN all display a set of six peaks paramagnetically
3
1
The results of the H NMR titration using Method 1 are
presented in Figure 7. As the titration progressed, the dia-
magnetic peaks of free (R)-2 were steadily replaced by a
single set of paramagnetically shifted peaks. These paramag-
shifted over a range of around 60 ppm. This suggests that
the structure observed by X-ray crystallography is main-
tained in solution and that the complexes have average D2
symmetry. Furthermore, the observation of just one set of
peaks in all cases demonstrates that the three [Co (2) (2-
netically shifted peaks can be assigned to the [Co (2) (2-
H) Cl ] helicate by comparison with the spectrum of an au-
2
2
2
2
H) X ] helicates are formed in solution with high (>95%)
2
2
2
2
thentic sample, and represent the only detectable product at
all stages of the titration. A small amount of free 2 remains
observable at a Co/2 ratio of 1/2, however the addition of
further cobalt(ii) led to severe broadening of the signals.
diastereoselectivity.
CD spectroscopy also reflects both the similarity of the
solid state and solution structures and the high diastereose-
lectivity of helicate formation. The CD spectra of
[
Co (2) (2-H) Cl ] recorded in CH CN solution and as a
The formation of [Co (2) (2-H) Cl ] could be monitored
2
2
2
2
3
2
2
2
2
KBr disc are presented in Figure 6. The spectra show a close
resemblance, displaying three Cotton effects in the range
by CD spectroscopy in the visible wavelength range (400–
800 nm) using titration method 1 (see Figure S3 in the Sup-
porting Information). A solution with a fairly high concen-
tration of 2 (ca. 0.014m) was employed due to the rather
weak intensity of the CD spectrum in this region. A steady
increase in signal intensity was observed up to a Co/2 ratio
of 1/2, with an isosbestic point appearing at 410 nm. The
similarity in the shape of all these spectra, and the presence
of the isosbestic point, strongly suggest that only one chiral
product is formed when CoCl , 2, and NEt are combined in
300–900 nm which can be assigned to d–d transitions of the
cobalt(ii) centers.
2
3
CH CN. The identity of this product—[Co (2) (2-H) Cl ]—
3
2
2
2
2
was confirmed by measuring the CD spectrum of an inde-
pendent sample.
Although only minor changes were observed in the high
energy (200–350 nm) region of the UV/Vis spectrum, the
formation of [Co (2) (2-H) (NO ) ] could be conveniently
2
2
2
3 2
monitored in the visible (380–900 nm) region. An absorb-
ance band around 500 nm, along with weaker bands at
lower energies, were observed to steadily rise in intensity as
the titration progressed (see Figure S2 in the Supporting In-
formation).
ꢀ
3
2 2 2
Figure 6. CD spectra of [Co (2)(2-H) Cl ] in solution (5.5”10 m,
3
CH CN, full line) and in the solid state (KBr disc, dotted line). The solid-
state spectrum has been normalized to the solution spectrum at 532 nm.
A series of careful spectrophotometric titrations were car-
ried out in order to determine the global stability constant
of [Co (2) (2-H) (NO ) ]. The data were analysed with the
The electrospray mass spectra of the three helicates were
2
2
2
3 2
[14]
measured in CH CN. Although the neutral helicates cannot
Specfit program,
and were successfully fitted using a
3
be observed directly, very intense peaks corresponding to
chemical model with [Co (2) (2-H) (NO ) ] and “free” co-
balt(ii) as the absorbing species. The inclusion of other ab-
2
2
2
3 2
+
[15]
the fragmentation products [Co (2)(2-H) X] were ob-
2
2
served.
sorbing species such as [Co(2) ] dramatically worsened the
2
fit to the data. The global stability constant calculated for
Monitoring the formation of [Co (2) (2-H) X ] in solution:
[Co (2) (2-H) (NO ) ] was logb =8.9(8) (Eq. (1)). The ab-
2
2
2
2
2
2
2
3
2
24
A series of titration experiments was performed in order to
sorption spectra predicted by the fitting process for
62
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Chem. Eur. J. 2005, 11, 57 – 68

Stereoselective Self-Assembly of Transition-Metal Helicates
FULL PAPER
of 0.12m, the addition of
Co(NO ) initially leads to a
3
near-linear rise in the concen-
tration
of
[Co (2) (2-H) -
2 2 2
(
NO ) ]. The formation of this
3
2
complex tails off at higher Co/2
ratios. Despite its high concen-
tration, only around 80% of 2
is coordinated at a Co/2 ratio of
1
/2. This is in full accord with
1
the H NMR titrations present-
ed above where signals corre-
sponding to free 2 were still ob-
servable at a Co/2 ratio of 1/2
(
Figure 7).
Behavior of the [Co (2) (2-
2
2
H) X ] helicates in dilute so-
2
2
lution: The behavior of the
Co (2) (2-H) Cl ] helicate in
[
2
2
2
2
dilute solution was probed by
1
CD, UV/Vis, and H NMR
spectroscopies. CD spectra of
this complex recorded in the
1
Figure 7. H NMR spectra monitoring the titration of R-2 with CoCl
2
and NEt
3
in CD
3
CN. The intense peaks concentration range 22.0–0.1”
+
ꢀ3
in the region 0–4 ppm are due to residual solvent and HNEt
H) Cl ]. [2]=ca. 0.1m.
3
2 2
and obscure one of the signals of [Co (2) (2-
1
0
m are presented in Figure 9.
2
2
The ordinate scale corresponds
[
Co (2) (2-H) (NO ) ] and free cobalt(ii) were in very good
2 2 2 3 2
agreement with independently measured spectra.
K24
II
þ
ð1Þ
2
Co þ 4 2 Ð ½Co ð2Þ ð2-HÞ Š þ 2 H
2
2
2
Speciation curves showing the distribution of free 2 and
Co (2) (2-H) (NO ) ] were simulated using the calculated
[
2
2
2
3 2
stability constant (Figure 8). For an initial concentration of 2
2 2 2 2
Figure 9. The inversion of the CD spectrum of [Co (2) (2-H) Cl ] as the
concentration is lowered. Spectra measured in CH CN with effective con-
3
ꢀ
3
centrations of 2 of 22.0, 4.0, 2.0, 1.0, 0.5, 0.2, and 0.1”10 m.
to De values based on the total concentration of 2. A dra-
matic change was observed in the CD spectra as the solution
was diluted: the positive Cotton effect centred around
2
70 nm inverted to give a negative Cotton effect. The
changes to the UV spectrum in this concentration range
were much more subtle with a slight (ca. 10%) rise in inten-
sity noted for the peak at 263 nm.
By comparison with an independent sample, the CD spec-
trum recorded at the lowest concentration can be attributed
Figure 8. Simulated speciation curve showing distribution of free 2 (full
line) and [Co
2/1 ratio) with Co(NO
logb24 value of 8.9, and with an initial concentration of 2 of 0.12m.
2
(2)
2
(2-H)
2
Cl
2
] (dotted line) during the titration of 2/NEt
3
(
3
)·6H
2
O in CH CN. Curves calculated using a
3
Chem. Eur. J. 2005, 11, 57 – 68
www.chemeurj.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
63

S. G. Telfer and R. Kuroda
1
to free 2. This is supported by corresponding H NMR ex-
periments where the paramagnetic peaks due to the
[
Co (2) (2-H) Cl ] helicate were gradually replaced by a set
2 2 2 2
of diamagnetic peaks corresponding to free 2 as the sample
was diluted (see Figure S4 in the Supporting Information).
Helicates derived from 3: Hydrogen-bonded helicates could
also be prepared from the racemic pyridine-ethanol precur-
sor 3. Crimson-colored prismatic crystals of [Co (3) (3-
2
2
H) Cl ] suitable for X-ray crystallography were obtained by
2
2
recrystallisation from hot CH NO . The helicate was found
3
2
to crystallize in the space group P2 /n, and although the
1
poor quality of the diffraction data preclude analysis of the
fine details of the structure, the overall geometry and pack-
ing arrangement of the complex is clear. Selected crystallo-
graphic data and geometrical parameters are listed in
Table 3, and by comparison with the geometrical parameters
of the enantiopure [Co (2) (2-H) X ] helicates (Table 2), it
2
2
2
2
can be seen that the molecular structures of the two types of
helicates are very similar.
Table 3. Crystallographic data for the [Co
2
(3)
2
(3-H)
2
Cl
2
] helicate.
monoclinic
P2 /n
crystal system
space group
a [Š]
b [Š]
c [Š]
Figure 10. The packing arrangement of L,L-[Co
2 2 2 2
((R)-3) ((R)-3-H) Cl ]
(blue) and D,D-[Co ((S)-3) ((S)-3-H) Cl ] (red) in the solid state. The
1
2
2
2
2
14.401(1)
16.311(1)
16.771(1)
90.187(1)
3941.9(5)
0.1489
double-headed white arrows indicate p-p interactions between the pyrid-
yl groups of neighboring helicates, and the dotted blue lines represent hy-
drogen bonds. Solvent molecules (CH
the helicates are omitted.
3 2
NO ) which fill the voids between
b [8]
V [Š ]
3
R
wR
2
(all data)
0.3478
65 ppm. Six peaks are expected if the D symmetry of the
2
helicates structure is maintained in solution, however it ap-
pears that one of the peaks is obscured by the signal due to
Selected distances [Š]
Co···Co
OꢀO
4.402
2.43
2.13–2.14
1.97–1.99
2.32–2.33
residual H O around 1.5 ppm. This spectrum is practically
2
CoꢀN
CoꢀO
CoꢀCl
identical to the spectrum of [Co (2)(2-H) Cl ] recorded in
2
2
2
CD Cl , with the same peaks observed at the same chemical
2
2
shifts (Æ0.3 ppm). No signals corresponding to any other
Selected angles [8]
paramagnetic complexes were detected.
N-Co-N
O-Co-O
169.7–170.0
114.9–116.5
The attempted synthesis of helicates using precursor 5: T he
pyridyl-alcohol ligand precursor 5, which has a bromo sub-
stituent in place of the usual methyl group, is easily pre-
pared via reduction of commercially available 2-bromo-6-
The crystal structure is composed of an enantiomeric pair
of helicates: L,L-[Co ((R)-3) ((R)-3-H) Cl ] and D,D-
acetylpyridine with NaBH . Upon mixing 5 with 0.25 equiv-
2
2
2
2
4
[
Co ((S)-3) ((S)-3-H) Cl ]. The ligand strands in these com-
alents each of CoCl and Co(OAc) , either a blue solution
2
2
2
2
2
2
plexes describe right- and left-handed helices (i.e., P and M
helices) respectively. Helicates of the same helicity pack in
an end-to-end fashion to form homochiral columns along
the crystallographic a axis. Columns of alternating chirality
are held together by p–p interactions between the pyridyl
groups (interplane distances=ca. 3.6 Š) of neighboring heli-
cates to form sheets in the ac crystallographic plane
(CH CN) or a practically colorless solution (CH OH) was
3 3
observed. Blue oils formed when these solutions were con-
centrated under reduced pressure. Crystalline products
could not be obtained from solutions of these oils in spite of
attempts at crystallization under a variety of conditions. The
1
H NMR spectra of these oils in CD CN did not reveal the
3
presence of any paramagnetically shifted peaks, in contrast
to analogous experiments performed with 2 or 3. Several
broad peaks were observed in the region 1–8 ppm which can
probably be ascribed to free ligand. The ES-MS spectra of
these solutions displayed a strong peak at m/z 231 which
(
Figure 10). This packing arrangement is reminiscent of that
observed for the racemic [Co (1) (1-H) X ] helicates.
2
2
2
2
1
H NMR spectroscopy shows that [Co (3)(3-H) Cl ] re-
2
2
2
mains intact in solution at relatively high concentrations. A
spectrum of dissolved crystals recorded in CD Cl displays a
2
+
corresponds to [Co(5)] , however no polynuclear com-
2
2
set of five peaks paramagnetically shifted over a range of
plexes were detectable.
64
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Chem. Eur. J. 2005, 11, 57 – 68

Stereoselective Self-Assembly of Transition-Metal Helicates
FULL PAPER
Discussion
compound 3 is racemic, there are two potential homochiral
diastereomers of this helicate (in which all the units of 3
have the same absolute configuration), and four potential
heterochiral diastereomers (in which both (R)-3 and (S)-3
The combination of the pyridyl-alcohol compounds 1–3 with
CoX salts and a suitable base leads to the self-assembly of
2
a range of structurally similar dinuclear double-stranded hel-
icates. This represents a straightforward, general and high-
yielding synthetic route to transition-metal helicates.
are present). The X-ray crystal structure of [Co (3)(3-
2
H) Cl ] reveals that a self-recognition process takes place
2
2
amongst the ligand precursors to generate exclusively the
two homochiral diastereomers (as a racemic mixture). Simi-
Whilst the desired self-assembly pathway appears to be
followed for 2 and 3 regardless of the nature of the base, it
was noted compound 1 has a propensity to form cubane-like
[
7]
lar behavior is exhibited by [Co (4)(4-H) Cl ].
The
2
2
2
1
H NMR spectrum of [Co (3)(3-H) Cl ] shows that this self-
2
2
2
[16]
clusters in the presence of strong bases such as NEt3. The
formation of cubane-like structures by pyridine–alcohol li-
sorting phenomenon also occurs in solution. It appears that
the homochiral diastereoisomers of [Co (3)(3-H) Cl ] are
2
2
2
[17]
gands is well documented in the literature.
thermodynamically more stable than their heterochiral
counterparts, and this can be rationalized on steric grounds.
Inspection of a molecular model clearly shows that the re-
placement of one units of 3 with its enantiomer will lead to
A wide variety of mono-anionic ancillary ligands (X) were
successfully introduced into the [Co (1) (1-H) X ] (X=Cl,
2
2
2
2
Br, I, NO , SCN) and [Co (2) (2-H) X ] (X=Cl, Br, NO )
3
2
2
2
2
3
helicates. Within each set of helicates, there is a remarkable
regularity in both the molecular structure and the packing
of the helicates in the crystal structure. We also investigated
an unfavorable interaction between the a-CH group and a
3
pyridyl group of the other ligand strand. The enantiomeric
self-recognition of ligands in transition-metal complexes has
[20]
[21]
the reaction of 2 with CoSO to see whether a similar heli-
previously been observed by ourselves and others, and
general self-sorting phenomena in supramolecular systems
4
1
cate may form with a di-anionic ancillary ligand. H NMR
[22]
spectroscopy indicated, however, that helicate formation
was unsuccessful. Instead, this reaction produced crystals of
a 1D coordination polymer, [Co(1)SO ] , in which the mon-
have been explored in detail by Isaacs et al.
A second noteworthy point concerning the diastereoselec-
tive assembly of these helicates is the interplay between the
point chirality of the ligand precursors, the absolute configu-
rations of the metal centers, and the helicity described by
4
8
omeric units are linked by hydrogen bonding between the
2ꢀ
[16]
alcohol moieties and the coordinated SO4 ions.
These
result indicate that the overall charge neutrality of the heli-
cates may be an important factor in their formation and/or
crystallization.
the ligand strands. Both metal centers in the [Co (2)(2-
2
H) Cl ] complex adopt the L configuration in both solution
2
2
and the solid state which demonstrates that the point chirali-
ty of the pyridyl-alcohol groups controls (or predeter-
A diverse array of substituents (R=H, CH , CH NO )
3
2
2
[5]
may be introduced at the a-carbon atom of the pyridine–al-
cohol precursor without disrupting the self-assembly process
or the general structure of the helicates. On the other hand,
helicate formation is rather sensitive to the nature of the
substituent at the 6-position of the pyridine ring. Helicates
were observed to form readily from compounds with a CH3
group at this position, however we saw no evidence for the
formation of such structures starting from compounds which
have a bromine (5) or hydrogen (6, 7) in this position. The
mines ) the absolute configuration of the metal centers.
Similarly, the cobalt(ii) centers in the helicates that contain
(R)-3 are L, whilst those associated with (S)-3 adopt the D
configuration. The control of the absolute configuration of
the metal centers in helicates by the chirality of the ligand
strands has previously been observed in conventional heli-
[1,4,5,23]
cates.
The steric effects discussed above in the context
of homochiral self-sorting may also be invoked to explain
the observation of the present case of stereoselectivity, and
indeed the phenomenon of self-sorting and chirality prede-
[13]
van der Waals radius of Br is similar to that of CH3, thus
the failure of 5 to produce a helicate structure is unlikely to
termination are closely related: The observed L,L-[Co ((R)-
2
be due solely to steric effects. As the pK of 5 is around
3)((R-)3-H) Cl ] diastereomer may be converted to D,D-
NH
2
2
[18]
[19]
0.87, while the pKNH of 1 is 4.56, these observations can
[Co ((R)-3)((R-)3-H) Cl ] by inverting the metal centers, or
2 2 2
probably be ascribed to the poor coordinating ability of 5.
We tentatively attribute the failure of 6 and 7 to form heli-
cates to the existence of competing reaction pathways, and
we are currently working to establish the exact nature of the
actual products. It is noteworthy that potentiometric titra-
it may be converted to the enantiomer of the latter helicate
(L,L-[Co ((S)-3)((S-)3-H) Cl ]) by replacing the (R)-3 units
2
2
2
with (S)-3. The overall helicity of the dinuclear assemblies is
intimately related to the absolute configurations of the
[1]
metal centers, thus the ligand strands of the D,D complexes
describe M (left-handed) helices, whereas those of the L,L
complexes describe P (right-handed) helices.
tions have shown that, although the pK
lower than that of 1, the formation of [Cu (6-H) ] is thermo-
dynamically more favored than the formation of [Cu (1-
of 6 (4.16) is
NH
II
2
II
The self assembly process leading to the [Co (2) (2-H) X ]
2
1
2
2
2
[19]
H) ] due to destabilizing steric effects in the latter.
helicates could be conveniently monitored by H NMR, UV/
Vis and CD spectroscopies. At all Co/2 ratios, there was no
spectroscopic evidence for the formation of any complexes
(other than the helicates) in significant concentration. In
accord with these observations, the spectrophotometric titra-
tion of 2 with Co(NO ) could be fitted with just [Co (2) (2-
2
The self-assembly of the hydrogen-bonded helicates de-
rived from 2 and 3 is remarkably stereoselective in both so-
lution and the solid state. One salient aspect of this diaster-
eoselectivity is the self-association of ligand precursors of
the same chirality in [Co (3)(3-H) Cl ]. As the precursor
2
2
2
3
2
2
2
Chem. Eur. J. 2005, 11, 57 – 68
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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
65

S. G. Telfer and R. Kuroda
H) ] (which serves as a model for [Co (2) (2-H) (NO ) ] as
2
2
2
2
3 2
we did not treat the anion explicitly) and free cobalt(ii) as
the absorbing species. Although our hypotheses concerning
the exact mechanism of the self-assembly process remain
speculative, we envisage that the helicates assemble by a hi-
[24]
erarchical process in which coordination of two pyridine–
alcohol units to a cobalt(ii) ion is followed by a deprotona-
tion/dimerisation step to give the final assembly [Eq. (2)].
This proposal is based on the fact that metal-ligand bonding
is significantly stronger than hydrogen bonding therefore
Figure 11. The variables in the self-assembly of hydrogen-bonded heli-
cates discussed herein.
the CoL building blocks are more likely to form prior to
2
any hydrogen-bonded species such as a putative preformed
ligand strand “[L(L-H)]”. Further, the alcohol moieties are
rendered more acidic by coordination to the metal ion, and
are well positioned for dimerization as two hydrogen bonds
may form in a concerted step. Hydrophobic interactions be-
tween pyridine rings of different ligand strands, as detected
by X-ray crystallography, may play a role in the final step by
stabilizing the helicate structure. The fact that mononuclear
complexes are not detected in significant quantities may in-
dicate that the self-assembly process is cooperative, although
the accurate assessment of relevant stepwise equilibrium
constants would be required to examine this point in more
helicity of overall structure. The nature of the substituent at
the 6-position of the pyridyl ring is of significance, however,
with precursors featuring H or Br substituents failing to pro-
duce hydrogen-bonded helicates.
The [Co (2) (2-H) X ] were found to be soluble and
2
2
2
2
stable in a certain polar solvents which enabled spectroscop-
ic characterization of the self-assembly process. These re-
sults showed that the helicates are the only complexes to be
formed; however, their self-assembly is only efficient at rela-
tively high concentrations. At lower concentrations, the heli-
cates dissociate to give free 2 and cobalt(ii).
[25]
detail.
The major remaining point at which diversity may be in-
troduced into these structure—the identity of the metal
ion—is the focus of current investigations in our laboratory.
We are also interested in the possible applications of these
helicates as building blocks for extended multidimensional
arrays, and the intriguing prospect that the helicate chirality
may be able to be controlled by chiral ancillary ligands.
K1
K2
þ
ð2Þ
2
Co þ 4 L Ð 2½CoL Š Ð ½Co L ðLꢀHÞ Š þ 2 H
2
2
2
2
The stability constant calculated for formation of
Co (2) (2-H) (NO ) ] indicates that these assemblies are
[
2
2
2
3 2
only formed in appreciable amounts at relatively high con-
centrations. This was verified experimentally by monitoring
1
the CD and H NMR spectra of [Co (2)(2-H) Cl ] upon dilu-
2
2
2
tion. These results indicate that dissociation of the helicates
is significant at concentrations below about 0.01m, and that
the equilibria in Equation (2) are driven to the far left-hand
side to produce free 2 (rather than any mononuclear com-
plexes).
Experimental Section
1
General: H NMR spectra were recorded on a JEOL Alpha spectrometer
at 500 MHz at 218C and referenced to the residual solvent peak
1
3
(
CD
were recorded at 125 MHz and were reference to the residual solvent
peak (CDCl , d=77.2 ppm). UV/Vis absorbance data were recorded by
using a Shimadzu UV-3150 spectrometer, and extinction coefficients are
3 2 2 3
CN, d=1.95 ppm; CD Cl , d=5.32; CDCl , d=7.26 ppm). C NMR
3
Conclusion
ꢀ1
ꢀ1
given in units of m cm . Solution CD spectra were recorded on a
JASCO J720 spectropolarimeter at 258C, and De values are given in
ꢀ
1
ꢀ1
A wide range of dinuclear double-stranded helicates in
which the ligand strands are built up using hydrogen bond-
ing self-assemble upon mixing precursors 1–3 with CoX2
salts. The straightforward synthesis of these helicates ena-
bled us to undertake a systematic investigation of the self-
assembly process. In the present report, we have focused on
the variables highlighted in Figure 11.
units of m cm . Solid state CD spectra were measured as KBr discs on
[
26]
the purpose-built JASCO J800-KCM spectrophotometer, and were cor-
rected for artifacts arising from linear dichroism and linear birefringence.
ES-MS spectra were recorded on a Applied Biosystems Mariner spec-
ꢀ1
trometer with a flow rate of 5 mLmin . Nozzle potentials and tempera-
ture (40–708C) were kept to a minimum to reduce fragmentation. Micro-
analyses were performed by Toray Research Centre, Eigiyoo, Tokyo.
Unless otherwise stated, chemicals were purchased from Wako, TCI, or
Aldrich and used as received. Dried solvents were used for all reactions
and measurements.
We have found that a range of mono-anionic ancillary li-
gands (X) can be incorporated into these complexes. Heli-
cate formation is also not particularly sensitive to the nature
of the R group connected to the a-carbon center; precursors
with R=H, CH , and CH NO all led to similar structures.
Preparation of R-(6-methylpyridn-2-yl)ethan-1-ol (2): Compound 2 was
prepared by the asymmetric hydrogenation of 2-acetyl-6-methylpyri-
dine according to the method reported by Ikariya et al. for similar
acetyl compounds. The compound was purified by column chromatogra-
[
27]
[11]
3
2
2
phy on SiO
determined to be 93% by HPLC analysis using a Diacel AD-H column
150 mm”4.6 mm, hexane/2-propanol/NEt 98/2/0.1, 358C, flow rate=
1.0 mLmin ). The retention times of both enantiomers were determined
2
using ethyl acetate/methanol (98/2) as the eluent. The ee was
For pyridyl-alcohol compounds where this carbon is a ster-
eogenic center, the chirality at this point can control the ab-
solute configuration of the metal centers and the sense of
(
3
ꢀ
1
66
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2005, 11, 57 – 68

Stereoselective Self-Assembly of Transition-Metal Helicates
FULL PAPER
[
28]
1
using
(
1
a
sample of
500 MHz, CDCl ): d=1.53 (m, 3H; CH
H; OH), 4.93 (q, 2H; CH), 7.11–7.15 (m, 2H), 7.66 ppm (t, 1H);
3
(S=7.27 min and R=8.03 min).
H NMR
Co(OAc)
2
·4H
2
O (85.3 mg, 0.343 mmol) to give a violet solution. The
3
3
), 2.60 (s, 3H; py-CH
3
), 4.81 (br,
solution was concentrated slowly under reduced pressure until crimson-
violet crystals began to form. Et O was layered on top of this solution
which was refrigerated for 48 h. The crystals were then filtered off,
2
1
3
C NMR (125 MHz, CDCl
3
): d=24.32, 24.44, 68.77, 117.31, 122.33,
37.92, 157.22, 162.46 ppm; ORD: [a]2 =19.6 (c=0.30 in CHCl
D
1
3
); UV
washed with a minimum amount of cold (ꢀ108C) MeOH then Et O.
6
2
(
CH
Preparation of rac-(6-bromopyridn-2yl)ethan-1-ol (5): 6-Bromo-2-acetyl-
pyridine (600 mg, 3.0 mmol) was dissolved in dry CH OH (8 mL) and
cooled in an ice-water bath. NaBH (340 mg, 9.0 mmol) was added in
portions with stirring, and the evolution of a gas was apparent. The reac-
tion mixture was warmed to room temperature and stirred for 2 h. The
solvent was then removed and the colorless residue partitioned between
3
CN): 265 nm (3350); CD (CH
3
CN): 267 nm (ꢀ1.54).
Yield: 180 mg (71%). Elemental analysis calcd (%) for [Co (2) (2-
2
2
H)
2
Cl
2
]·2(CH
3
OH) (C34
H
50
N
4
O
6
Co
2
Cl
CN, 0.01m): d=ꢀ27.65 (3H), ꢀ8.06
CN, 5.5”
10 m): e=628 nm (94.3), 525 nm (61.6), 264 nm (13200); CD (CH CN,
5.5”10 m): De=746 nm (1.16), 532 nm (1.49), 502 (sh, 1.23), 311 nm
2
): C 51.1, H 6.3, N 7.0; found: C
1
50.7, H 5.8, N 6.7; H NMR (CD
3
3
(1H), 21.14 (1H), 22.79 (1H), 34.92 ppm (3H); UV/Vis (CH
3
4
ꢀ
3
3
ꢀ
3
(ꢀ1.70), 269 nm (22.3); ES-MS (CH
3
CN, 0.01m): 703.0 ([Co
2
(2)
2
-
-
+
+
CH
2
Cl
2
and H
2
O. The organic layer was separated, dried over MgSO
and the solvent removed to give 5 as a colorless oil. Yield: 540 mg
4
(2-H)
(2-H)
2
Cl] , 2%), 562.0 ([Co
2
(2)(2-H)
2
Cl] , 100%), 332.1 ([Co 2
2
2
2
+
2
]
, 12%); IR (KBr disc): n˜ =3420 (br m), 2852 (m), 1605 (s), 1578
1
ꢀ1
(
4
(
89%). H NMR (CDCl
.87 (q, 1H; CH), 2.29 (d, 1H), 7.38 (d, 1H), 7.55 ppm (t, 1H); C NMR
CDCl ): d=24.24, 69.27, 118.70, 126.77, 139.39, 141.23, 165.35 ppm.
Complexes of 1
Co (1) (1-H) Cl
.627 mmol) were mixed in CH
tate which was filtered off and washed with MeOH and diethyl ether.
Yield 39.2 mg (73%). Elemental analysis calcd (%) for [Co (1) (1-
H) Cl ]·H Co Cl ): C 48.2, H 5.2, N 8.0; found: C 48.7, H
.4, N 7.7; IR (KBr): n˜ =3433 (br m), 1607 (s), 1578 (m), 1472 (s), 1375
m), 1171 (m), 1126 (br m), 1097 (m), 1015 (s), 949 (br s), 926 (m), 787
3
): d=1.50 (d, 3H; CH
3
), 3.35 (br s, 1H; OH),
(m), 1468 (s), 1362 (s), 1312 (m), 1190 (br s), 951 (br s), 793 (m) cm
Co (2) (2-H) Br
MeOH (4 mL) and CoBr
Co(OAc) ·4H O (30.6 mg, 0.123 mmol) were added to give a violet
solution which was concentrated slowly under reduced pressure until
crimson-violet crystals began to form. Et O was layered on top of this so-
.
1
3
[
2
2
2
2
]: Compound 2 (67.5 mg, 0.49 mmol) was dissolved in
·4H (40.2 mg, 0.123 mmol) and
3
2
O
2
2
2
[
2
2
2
2
]: CoCl
2
·6H
2
O (45.6 mg, 0.157 mmol) and 1 (77.2 mg,
CN (1.5 mL) to give a crimson precipi-
0
3
2
lution which was then refrigerated for 48 h. The crystals were then fil-
tered off and the filtrate put aside. The crystals were washed with a mini-
mum amount of cold (ꢀ108C) MeOH then Et
filtrate was layered with more Et O and refrigerated to give a second
crop of product. Total yield: 59 mg (58%). Elemental analysis calcd (%)
2
2
O (C28H
36
N
4
O
5
2
2
2
O, and air-dried. The first
2
2
2
5
2
(
(
ꢀ
1
m) cm
.
for [Co
2
(2)
2
(2-H)
2
Br
2
]·H
2
O (C32
H
44Br
2
Co
2
N
4
O
5
): C 45.63, H 5.26, N 6.65;
1
found: C 45.8, H 5.2, N 6.1; H NMR (CD
3
CN, 0.01m): d=ꢀ26.87 (3H),
[
2
Co (1)
2
(1-H)
2
Br
2
]: CoBr
2
·6H
2
O (46.4 mg, 0.142 mmol) was added to a
CN (1.5 mL). A crimson crys-
talline precipitate formed rapidly. The reaction mixture was refrigerated
overnight and the precipitate was isolated by filtration and washed with
ꢀ
7.14 (1H), 2.20 (obscured by solvent peak), 22.20 (1H), 24.25 (1H),
3.96 ppm (3H); UV/Vis (CH
solution of 1 (70.0 mg, 0.568 mmol) in CH
3
ꢀ3
3
3
CN, 4.6”10 m): e=745 nm (sh, 29.8),
6
33 nm (91.0), 604 nm (92.2), 533 nm (70.2), 263 nm (16000); CD
ꢀ3
(
3
CH CN, 4.6”10 m): De=747 nm (1.71), 533 nm (2.16), 502 nm (sh,
MeOH and Et
for C28 Co
.4; IR (KBr disc): n˜ =3423 (br m), 2851 (m), 1607 (s), 1578 (m), 1472
s), 1375 (m), 1279 (w), 1244 (m), 1171 (m), 1128 (br m), 1097 (m), 1015
2
O. Yield: 49.0 mg (90%). Elemental analysis calcd (%)
1
0
.91), 310 nm (ꢀ2.26), 271 nm (24.1), 252 nm (ꢀ2.90); ES-MS (CH
3
CN,
H
34Br
2
2 4 4
N O
: C 43.77, H 4.46, N 7.29; found: C 44.0, H 4.6, N
+
.01m): 606.0 ([Co
2
(2)(2-H)
2
Br] , 100%); IR (KBr disc): n˜ =3415 (br m),
6
(
(
2
968 (m), 2866 (m), 1605 (s), 1578 (m), 1468 (s), 1364 (m), 1312 (w), 1244
ꢀ
1
ꢀ1
(m), 1161 (m), 1128 (br m), 937 (br s), 872 (m), 793 (m), 752 (m) cm .
m), 949 (br s), 926 (m), 858 (m), 779 (m), 733 (w) cm
Co (1) (1-H) ]: CoI ·2H O (65.6 mg, 0.188 mmol) was added to a so-
CN (1.5 mL). The deposition of
a crimson-violet precipitate began almost immediately. The reaction mix-
ture was refrigerated overnight and the precipitate isolated by filtration
.
[
Co
in CH
35.8 mg, 0.123 mmol) and Co(OAc)
2
(2)
2
(2-H)
CN (1 mL) and was added to a solution of Co(NO
·4H O (30.6 mg, 0.123 mmol) in
2
(NO
3
)
2
]: Compound 2 (68.7 mg, 0.50 mmol) was dissolved
[
2
2
2
I
2
2
2
3
3
)
2
·4H O
2
lution of 1 (92.6 mg, 0.752 mmol) in CH
3
(
2
2
MeOH (150 mL) to give a violet solution which was concentrated slowly
and washed with MeOH and Et
analysis calcd (%) for C28
C 39.6, H 4.2, N 5.5; IR (KBr disc): n˜ =3423 (br m), 2848 (m), 1605 (m),
2
O. Yield: 72.9 mg (90%). Elemental
under reduced pressure. A rose-coloured crystalline precipitate deposit-
H
34Co
2
I
2
N
4
O
4
: C 39.00, H 3.97, N 6.50; found:
ed. The precipitate was suspended in a mixture of CH
and was isolated by filtration, washed with cold (ꢀ108C) CH
Et O, and air-dried. Yield: 57.0 mg (58%). Elemental analysis calcd (%)
for [Co
3
CN and Et
2
O (1/3)
3
CN then
1
1
578 (m), 1470 (s), 1375 (m), 1242 (m), 1169 (m), 1126 (br m), 1097 (m),
2
ꢀ
1
015 (m), 947 (br s), 923 (m), 854 (m), 789 (m), 731 (w) cm
.
2
(2)
2
(2-H)
2
(NO
3
)
2
]·CH
3
OH (C33
H
46Co
2
N
6
O
11): C 48.30, H 5.65, N,
CN, 0.01m): d=ꢀ41.86
1
1
(
0.24; found: C 48.6, H 5.7, N 9.7; H NMR (CD
3
[
Co
2
(1) (1-H) (NO ]: Co(NO ·6H O (45.6 mg, 0.157 mmol) was added
2
2
3
)
2
3
)
2
2
3H), ꢀ2.15 (1H), 1.13 (1H), 25.61 (1H), 26.11 (1H), 50.27 ppm (3H);
to a solution of 1 (77.2 mg, 0.627 mmol) in MeOH (400 mL). The deposi-
tion of a rose-coloured precipitate began almost immediately. The reac-
tion mixture was refrigerated overnight and the precipitate isolated by
ꢀ3
3
UV/Vis (CH CN, 4.6”10 m): e=514 nm (48.1), 265 nm (15 200). CD
ꢀ
3
(
2
(
2
1
CH
92 nm (ꢀ2.72), 269 nm (19.0); ES-MS (CH
[Co (2)(2-H) NO ] , 100%); IR (KBr disc): n˜ =3433 (br m), 2978 (m),
866 (m), 1607 (m), 1466 (m), 1385 (s), 1285 (m), 1165 (m), 1084 (m),
3
CN, 4.6”10 m): De=615 nm (0.58), 517 nm (0.99), 471 nm (0.71),
ꢀ
1
3
CN, 0.01 molL ): 589.1
filtration and washed with MeOH and Et
mental analysis calcd (%) for C28
found: C 46.0, H 4.8, N 11.0; IR (KBr disc): n˜ =3414 (br m), 2826 (m),
2
O. Yield: 42.0 mg (73%). Ele-
+
H
34Co N O10: C 45.91, H 4.68, N 11.47;
2 6
2
2
3
ꢀ
1
013 (m), 935 (w), 870(w), 806 (m), 754 (w), 681 (w) cm
.
1
1
605 (m), 1578 (m), 1466 (s), 1385 (s), 1294 (s), 1163 (m), 1114 (br m),
082 (m), 1013 (m), 957 (br m), 926 (w), 839 (w), 779 (m), 731 (w), 660
Complexes of 3
2
Co (3) (3-H) Cl ]: This helicate was prepared as described above for 2.
ꢀ1
(
m), 517 (m) cm
Co (1) (1-H) (SCN)
63.1 mg, 0.25 mmol) were combined in CH
.01 mmol) was added to give a crimson precipitate which was filtered
.
[
2
2
2
[
(
1
2
2
2
2
]: Co(SCN)
2
(44.4 mg, 0.25 mmol) and Co(OAc)
OH (2 mL) and 1 (124.8 mg,
2
X-ray quality crystals could be grown by recrystallisation from hot
1
3
CH NO . H NMR (CD Cl ): d=ꢀ27.94 (3H), ꢀ8.24 (1H), 21.21 (1H),
3
2
2
2
22.91 (1H), 35.11 ppm (3H).
off, washed with MeOH and diethyl ether, and air dried. Refrigeration of
the filtrate produced violet crystals which were suitable for X-ray crystal-
lography. Total yield: 110 mg (60%). Elemental analysis calcd for
Spectrophotometric titrations: All operations were performed at 258C in
a controlled-temperature environment using the UV/Vis instrumentation
ꢀ1
described above. A solution of Co(NO
trated into a solution of 2 and NEt
3
)
2
·6H
2
O (ca. 0.4 molL ) was ti-
C
30
H
34Co
2
N
6
O
4
S
2
: C 49.73, H 4.73, N 11.60; found: C 49.4, H 4.8, N 11.4;
ꢀ1
3
(2/1 ratio, [2]=ca. 0.04 molL ) to
IR (KBr): n˜ =3423 (br m), 2849 (m), 2073 (vs), 1607 (m), 1580 (m), 1472
give a final Co/2 ratio of 1/2. Spectra were recorded following the addi-
tion of each aliquot in the wavelength region 380–860 nm. A cell with a
(
(
s), 1375 (w), 1171 (m), 1124 (br m), 1096 (m), 1015 (m), 953 (br m), 926
ꢀ1
m), 868 (m), 779 (m), 735 (w) cm
.
1
cm pathlength was used. Data were analyzed with the Specfit pro-
Complexes of 2
Co (2) (2-H) Cl
[14,29]
gram,
and the titration was repeated in order to obtain three stability
[
2
2
2
2
]: Compound 2 (188 mg, 1.37 mmol) was dissolved in
constants which agreed with each other within the error limits estimated
by the software.
MeOH (4 mL) and CoCl
2
·4H
2
O
(81.5 mg, 0.343 mmol) and
Chem. Eur. J. 2005, 11, 57 – 68
www.chemeurj.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
67

S. G. Telfer and R. Kuroda
X-ray crystallography: Data were collected using a Bruker APEX system
with MoKa radiation, and were corrected for Lorentzian, polarisation,
and absorption. Structures were solved by direct methods, and refined
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2
against jFj using anisotropic thermal displacement parameters for all
non-hydrogen atoms. Hydrogen atoms were placed in calculated posi-
tions except for the alcohol protons involved in hydrogen bonding
which were located on the electron density difference map (except for
2 2 2 2 3 2
the structure of [Co (3) (3-H) Cl ]·(CH NO ) where these protons
could not be located). CCDC-244233–CCDC-244239 contain the supple-
mentary crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax:
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(
(
+44)1223-336-033. The structures of [Co
2
(1)
2
(1-H)
2
Cl
2
]·H
2
O, [Co
2
(2)
2
-
2 2 2 2
[12] In the case of [Co (1) (1-H) Cl ], this cavity is occupied by a water
[
7]
2-H) Cl ], [Co (4) (4-H) Cl ] have been published previously.
2
2
2
2
2
2
molecule. However, the cavity is empty in the other helicates.
13] L. Pauling, The Chemical Bond, Cornell University Press, New
York, 1967.
[
[
[
14] H. Gampp, M. Maeder, C. J. Meyer, A. D. Zuberbuehler, Talanta
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Acknowledgement
15] No attempt was made to distinguish between precursor 2 and its de-
protonated form. Also, the nitrate ions were not explicitly treated
by the chemical model, but were implicitly assumed to bind quanti-
tatively to the cobalt(ii) ions.
We are very grateful to Mr. T. Sato for assistance with the X-ray crystal-
lography and to Dr T. Harada for measuring the solid-state CD spectra.
We also extend our warm thanks to Prof. T. Ikariya for a sample of com-
pound 7.
[
[
16] S. G. Telfer, unpublished observations.
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[
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28] This compound was prepared by the reaction of 2-acetyl-6-methyl-
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[
[
4
Received: May 17, 2004
Published online: November 18, 2004
68
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2005, 11, 57 – 68