Syntheses, spectroscopic studies, and crystal structures of chiral [Rh(aminocarboxylato)(η4-cod)] and Chiral [Rh(amino alcohol)(η4-cod)]-(acetate) complexes with an example of a spontaneous resolution of a racemic mixture into homochiral helix-enantiomers

Article abstract of DOI:10.1002/ejic.200501033

The dimeric complex acetato(η⁴-cycloocta-1,5-diene) rhodium(I), [Rh(O₂CMe)(η⁴-cod)]₂ (cod = cycloocta-1,5-diene), reacts with amino acids [HAA = L-alanine, (S)-2-amino-2-phenylacetic acid (L-phenylglycine), N-methylglycine, and N-phenylglycine] and with the amino alcohol (S)-2-amino-2-phenylethanol to afford the aminocarboxylato(η⁴-cycloocta-1,5-diene)rhodium(I) complexes [Rh(AA)(η⁴-cod)] (AA = deprotonated amino acid = aminocarboxylato ligand) and [(S)-2-amino-2-phenylethanol](η⁴- cycloocta-1,5-diene)-rhodium(I) acetate, [Rh{(S)-HOCH₂-CH(Ph)-NH ₂}(η⁴-cod)]-(O₂CMe) (V). The complexes are characterized by IR, UV/ Vis, ¹H/¹³C NMR and mass spectroscopy. The achiral N-phenylglycine ligand gives a chiral N-phenylglycinato complex [Rh(O₂C-CH₂-NHPh) (η⁴-cod)] (IV) with the amine nitrogen atom becoming the stereogenic center upon metal coordination. Complex IV crystallizes in the tetragonal, chiral space group P4₃ and the crystal structure reveals twofold spontaneous resolution of a racemic mixture into homochiral helix-enantiomers. The investigated crystal contained only one type of helix, namely (left-handed or M-) 4₃-helical chains. This is traced first to an intermolecular N-H?O hydrogen bonding from the stereogenic amino group to a neighboring unligated carboxyl oxygen atom that connects only molecules of the same (R)-configuration into (left-handed or M-) 4 ₃-helical chains. This intrachain homochirality is supplemented, secondly, by the interlocking of adjacent chains with their corrugated van der Waals surface to allow for an interchain transmission of the sense of helicity, building the single crystal from the same homochiral helix-enantiomer. The enantiomeric amino alcohol complex V crystallizes in the monoclinic, noncentrosymmetric (Sohncke) space group P2₁. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200501033

FULL PAPER
DOI: 10.1002/ejic.200501033
Syntheses, Spectroscopic Studies, and Crystal Structures of Chiral
[Rh(aminocarboxylato)(η4-cod)] and Chiral [Rh(amino alcohol)(η4-cod)]-
(acetate) Complexes with an Example of a Spontaneous Resolution of a
Racemic Mixture into Homochiral Helix-Enantiomers
Mohammed Enamullah,*[a] Aisha Sharmin,[a] Miki Hasegawa,[b] Toshihiko Hoshi,[b]
Anne-Christine Chamayou,[c] and Christoph Janiak*[c]
Keywords: Rhodium() / Amino acids / Chirality / Spontaneous resolution / Helix
The dimeric complex acetato(η4-cycloocta-1,5-diene)rhodi-
um(
), [Rh(O2CMe)(η4-cod)]2 (cod = cycloocta-1,5-diene), re-
acts with amino acids [HAA = -alanine, (S)-2-amino-2-phen-
ylacetic acid ( -phenylglycine), N-methylglycine, and N-
phenylglycine] and with the amino alcohol (S)-2-amino-2-
phenylethanol to afford the
aminocarboxylato(η4-
cycloocta-1,5-diene)rhodium(
) complexes [Rh(AA)(η4-cod)]
spontaneous resolution of a racemic mixture into homochiral
helix-enantiomers. The investigated crystal contained only
one type of helix, namely (left-handed or M-) 43-helical
chains. This is traced first to an intermolecular N–H···O hy-
drogen bonding from the stereogenic amino group to a
neighboring unligated carboxyl oxygen atom that connects
only molecules of the same (R)-configuration into (left-han-
ded or M-) 43-helical chains. This intrachain homochirality is
supplemented, secondly, by the interlocking of adjacent
chains with their corrugated van der Waals surface to allow
for an interchain transmission of the sense of helicity, build-
ing the single crystal from the same homochiral helix-enanti-
omer. The enantiomeric amino alcohol complex V crystallizes
in the monoclinic, noncentrosymmetric (Sohncke) space
group P21.
I
L
L
I
(AA = deprotonated amino acid = aminocarboxylato ligand)
and [(S)-2-amino-2-phenylethanol](η4-cycloocta-1,5-diene)-
rhodium(I
) acetate, [Rh{(S)-HOCH2–CH(Ph)-NH2}(η4-cod)]-
(O2CMe) (V). The complexes are characterized by IR, UV/
Vis, 1H/13C NMR and mass spectroscopy. The achiral N-
phenylglycine ligand gives a chiral N-phenylglycinato com-
plex [Rh(O2C–CH2–NHPh)(η4-cod)] (IV) with the amine ni-
trogen atom becoming the stereogenic center upon metal co-
ordination. Complex IV crystallizes in the tetragonal, chiral
space group P43 and the crystal structure reveals twofold
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
Introduction
syntheses and crystal structures of the [Rh(AA)(CO)(L)]
(L
=
PPh3, AsPh3)[4a] and [Rh(-aziridin-2-carboxyl-
Syntheses and reactivities of metal complexes with amino
acids or peptides are of considerable interest and some of
them show potential applications as chiral catalysts.[1] Singh
first reported the syntheses of [Rh(AA)(CO)2][2a,2b] and
[Rh(AA)(CO)(L)2] (AA = deprotonated amino acid = ami-
nocarboxylato ligand; L = PPh3 or AsPh3) complexes.[2c,2d]
Later, Marko synthesized the [Rh(AA)(CO)2] complexes
starting from the dimeric [Rh(O2CR)(η4-cod)]2 by synthesis
of the intermediate [Rh(AA)(η4-cod)] complexes (cod =
cycloocta-1,5-diene).[3] Accordingly, Beck has reported the
ato)(CO)2][4b] complexes and efforts to synthesize the analo-
gous RuII-aminocarboxylato complexes containing the bi-
dentate 2,7-dimethylocta-2,6-dien-1,8-diyl ligand.[4c] Mo-
lybdocene and titanocene compounds of the formulae
[Cp2MoIV(κN,κO-AA)]+Cl–,[5]
PF6–,[6] [Cp2TiIV(κO-AA)2]2+ [7,8]
[Cp2MoIV(κN,κO-AA)]+-
,
and the isoelectronic half-
sandwich molybdenum compounds [CpMoII(CO)2(κN,κO-
AA)][9]
and
[CpMoII(NO)(I){κN,κO--H2NCH(tBu)-
COO}][10] have been prepared and structurally elucidated
(Cp = η5-C5H5, η5-cyclopentadienyl). However, no detailed
studies on the syntheses, spectroscopy, and crystal struc-
tures of Rh(η4-cod) complexes containing chiral amino
acid/amino alcohol or N-amino acids have been reported so
far. It is envisioned that these coligands will exercise strong
influences on the stereochemistry as well as the stability and
reactivity of the Rh complexes and their uses as catalysts
for the enantioselective hydrogenation of olefins, unsatu-
rated carboxylates, amidocinnamic acids, and their ester de-
rivatives.
[a] Department of Chemistry, Jahangirnagar University,
Dhaka 1342, Bangladesh
E-mail: menam@juniv.edu
[b] Department of Chemistry, College of Science and Engineering,
Aoyama Gakuin University,
Sagamihara-shi, Kanagawa 229-8558, Japan
[c] Institut für Anorganische und Analytische Chemie, Universität
Freiburg,
Albertstr. 21, 79104 Freiburg, Germany
E-mail: janiak@uni-freiburg.de
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
2146
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Eur. J. Inorg. Chem. 2006, 2146–2154

Spontaneous Resolution of a Racemic Mixture into Homochiral Helix-Enantiomers
FULL PAPER
Recently, we attempted to synthesize and characterize the ric) around 1620 cm–1, which is typical for the N,O-coordi-
Rh(η4-cod/diphos/triphos) complexes containing chiral nation of the aminocarboxylato ligand to RhI, as is the
amino acids and amino alcohols as coligands[11] and to shifting and splitting of the νNHasy/νNHsy stretching vi-
study their catalytic behaviors in an asymmetric hydrogena- brations.[2–4,11] Absence of the νO–H stretching band
tion reaction. Here, we report the syntheses and spectro- (which is usually observed as a strong broad band at about
scopic studies of the complexes [Rh(AA)(η4-cod)] {AA = 3450 cm–1 for the free carboxylic group of the amino acid)
-alaninato (I), (S)-2-amino-2-phenylacetato (II), N-methyl- in I–IV indicates acid deprotonation and bond formation
–
glycinato (III), N-phenylglycinato (IV)} and [Rh{(S)- between RhI and O–C(O). The vibrational results strongly
HOCH2–CH(Ph)-NH2}(η4-cod)](O2CMe) (V) as well as the suggest that the amino acidato ligand is bound to Rh by its
crystal structures of IV and V.
nitrogen and oxygen atoms as a N,O-chelate, as depicted
in Scheme 1.[2–4,11] In contrast to these findings, van Koten
synthesized the [Rh(-alaninato)(η4-cod)]2 complex starting
from [Rh{C6H3(CH2NMe2)2-o,oЈ-C,N}(η4-cod)] and de-
Results and Discussion
Reaction of dimeric [Rh(O2CMe)(η4-cod)]2 (cod = 1,5- scribed it as an O,O-coordinated carboxylato-bridged di-
cyclooctadiene) with amino acids in toluene/MeOH yields mer.[14]
the monomeric complexes [Rh(AA)(η4-cod)] (AA = depro-
FAB-MS spectra are dominated by ions containing the
tonated amino acid
= aminocarboxylato) (I–IV in species M2, M2 – AA = M + Rh(cod), M + AAH2, M +
Scheme 1). Reaction of [Rh(O2CMe)(η4-cod)]2 with the H, M – CO2, and Rh(cod) (M = molecule, AA = amino-
amino alcohol (S)-2-amino-2-phenylethanol gives the com- carboxylato) for the aminocarboxylato complexes
plex [Rh{(S)-HOCH2–CH(Ph)-NH2}(η4-cod)](O2CMe) (V [Rh(AA)(η4-cod)] (I–IV). The appearance of dinuclear spe-
in Scheme 1).
cies may be due to intermolecular hydrogen bonding as evi-
UV/Vis absorption spectra of complexes I–IV, measured denced by the crystal structure of IV (see below).
in toluene at 25 °C, are identical with each other and dif-
The rhodium-coordinated 1,5-cyclooctadiene ligand
ferent from those of the dimeric [Rh(O2CMe)(η4-cod)]2 and shows three proton NMR signals corresponding to the exo-
[RhCl(η4-cod)]2 complexes and different from those of the and endo-methylene proton and to the methine pro-
amino alcohol compound V (see Figure S1, Table S1 in the ton.[11,15,16] The CH2codexo signal appears as doublet–
Supporting Information).[12]
doublet, while CH2codendo and CHcod exhibit a multiplet.
In the infrared spectrum the aminocarboxylato com- In V the methylene protons of the chiral amino alcohol li-
plexes I–IV show a strong carbonyl band (νCO2, asymmet- gand are diastereotopic and couple differently to the vicinal
Scheme 1. Synthetic route to and formula of the complexes I–V.
Eur. J. Inorg. Chem. 2006, 2146–2154
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M. Enamullah, A. Sharmin, M. Hasegawa, T. Hoshi, A.-C. Chamayou, C. Janiak
FULL PAPER
Scheme 2. Hydrolysis of complexes I–V from NMR studies.
methine proton, thereby showing two sets of doublet of
doublets at 3.63 ppm and 3.71 ppm.[13] Similarly in I and II
the amino protons of the chiral amino carboxylate ligands
are diastereotopic and appear as two multiplets that are
more than 0.7 ppm apart. 13C NMR spectra of complexes
I–IV show a singlet at about 30 ppm for CH2cod and broad
peaks at 72–80 ppm for CHcod in [D6]DMSO.[11,15,16] Com-
plex V shows two peaks of equal intensity around 30 ppm
for CH2cod, indicating the nonequivalency of the carbon
atoms at either side of the chiral amino alcohol ligand. In
contrast to the related chiral complexes I and II, the non-
equivalency of the CH2cod carbon atoms may be enhanced
in V by the acetate anion coordination through N–H···O
hydrogen bonding (see structural analysis below).
Figure 1. Molecular structure of IV.
Upon addition of acid (as 5% DCl/D2O in CD3CN) the
1H and 13C NMR spectra indicate that complexes I–V are
readily hydrolyzed with dissociation of the amino carboxyl-
ate or amino alcohol ligand to form the dimeric chloro
complex [RhCl(η4-cod)]2 and the free amino acid or amino
alcohol (Scheme 2).[11]
Complexes IV and V could be crystallized and subjected
to single-crystal X-ray diffraction studies. The single-crystal
X-ray analysis proved the suggested N,O-chelate formation
of the aminocarboxylato or amino alcohol ligand to the
rhodium atom of the Rh(η4-cod) fragment (Figure 1 and
Figure 2). Related Rh(η4-cod) complexes with a five-mem-
bered Rh-N,O-chelate ligand are structurally elucidated
with 8-hydroxyquinolinato,[17] tryptophan benzyl ester,[18]
4-methylpyridinium 2-pyridylcarbonylmethylide,[19] 1-(2Ј-
pyridyl)-3-(dimethylamino)-2-propenone,[20] imidazole-4,5-
carboxylato,[21] orotato,[22] and 3-oxo-1-(pyridin-2-yl)-
prop-1-en-1-olato.[16a] The five-membered N,O-chelate ring
with rhodium is planar within (largest deviation) 0.16 Å
in IV and 0.28 Å in V. Selected distances and angles are
compared in Table 1. Both complexes are highly similar in
their relevant bonding parameters. The Rh–O distance to
the carboxylate oxygen in IV and to the alcohol oxygen in
V is identical.
Figure 2. Molecular structure of V showing the hydrogen bonding
to the surrounding symmetry-related acetate anions. Hydrogen-
bonding interactions (dashed lines) as D–H, H···A, D···A, D–H···A
[Å (°)]: O1–H1···O3 0.89(5), 1.58(4), 2.429(4), 159(4); N–H2···O22
0.99(5), 2.14(5), 3.001(7), 145(4); N–H2···O32 0.99(5), 2.51(5),
3.278(7), 134(4); N–H3···O21 0.89(4), 2.00(4), 2.885(5), 173(3);
symmetry transformations 1 = 1 + x, y, z; 2 = 1 – x, y + 1/2,
2 – z.
Table 1. Selected bond lengths [Å] and angles [°] in IV and V.
IV
V
Rh–O1
Rh–N
Rh–C9
Rh–C10
Rh–C13
Rh–C14
O1–Rh–N
O1–Rh–C9
O1–Rh–C10
O1–Rh–C13
O1–Rh–C14
N–Rh–C9
N–Rh–C10
N–Rh–C13
N–Rh–C14
2.059(3)
2.145(3)
2.112(5)
2.131(4)
2.108(4)
2.125(4)
80.82(12)
90.82(16)
95.39(16)
160.63(15)
161.03(17)
158.73(18)
161.51(17)
95.59(16)
99.11(18)
2.058(2)
2.110(2)
2.144(6)
2.113(6)
2.093(6)
2.105(6)
79.94(8)
95.8(2)
The achiral (prochiral) N-phenylglycine ligand acquires
a stereogenic center at the nitrogen atom upon metal coor-
dination. Initially this metal–ligand coordination will give
a racemic mixture of (R)- and (S)-configured complexes.
The chiral N-phenylglycinato complex [Rh(O2C–CH2–
93.9(2)
162.5(2)
159.0(2)
164.3(2)
156.7(2)
95.97(18)
96.32(17)
NHPh)(η4-cod)] (IV) crystallizes in the tetragonal, chiral
[23,24]
space group P43
with spontaneous resolution within
the single crystal. The homochirality results from the solid-
state packing in that N–H···O hydrogen bonding from the
stereogenic amino group to a neighboring unligated car-
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 2146–2154

Spontaneous Resolution of a Racemic Mixture into Homochiral Helix-Enantiomers
FULL PAPER
Figure 3. 43-Helical chain in IV along c. Hydrogen-bonding interaction (dashed line) as D–H, H···A, D···A, D–H···A [Å (°)]: N–H···O22Ј
0.73(4), 2.19(4), 2.925(5), 176(5); symmetry transformations: 2 = 2 – y, x, 0.75 + z; 2Ј = 2 – y, x, –0.25 + z; 3 = 2 – x, 2 – y, 0.5 + z; 3Ј
= 2 – x, 2 – y, –0.5 + z; 4 = y, 2 – x, 0.25 + z.
boxyl oxygen atom (Figure 3 and Figure 4) connects and Cg(J) and the normal to the plane I Ͻ60°; for C–H···π,
assembles only molecules of the same configuration [here H···Cg Ͻ 3.0 Å and an angle between the vector H···Cg(J)
(R), see Figure 1 and Figure 3] into a (left-handed or M-) and the normal to the plane J Ͻ30°].[31,32] In IV the shortest
43-helical chain (in the crystal investigated) along c (Fig- perpendicular distance of H on a phenyl ring plane is 3.3 Å,
ure 3 and Figure 4).
about 0.4–0.5 Å longer than seen in typical C–H···π interac-
tions.[33]
The investigated crystal of IV contained only one type of
helix, namely (left-handed or M-) 43-helical chains, thereby
representing a case of spontaneous resolution of a racemic
mixture into homochiral helix-enantiomers. To account for
this fact we invoke an interlocking of adjacent chains with
their corrugated van der Waals surface in order to transmit
the sense of helicity upon crystallization (Figure 5). The
overall ensemble of the crystals in a batch of IV can be
expected to be racemic, that is, to contain crystals of space
groups P43 and P41 with left- and right-handed helices
based on (R)- and (S)-configured metal–ligand complexes,
respectively, in equal amounts.
Figure 4. Left-handed 43-helical chain in IV viewed along the chain
direction.
Yet, most homochiral helices that are formed from achi-
ral ligands and even from racemic mixtures of chiral build-
ing blocks almost always lead to racemic mixtures of P-
and M- (right- and left-handed) helices.[25–30] Of interest are
structures that spontaneously resolve to contain only one
type of helix (within a crystal), that is a single, homochiral
helix-enantiomer that requires a sufficient homochiral hel-
icating element in the molecular building block, for exam-
ple, the ligand. This is typically a stereogenic center of an
enantiomerically pure ligand in combination with strong in-
terhelix supramolecular interactions. However, in the struc-
ture of IV there are no classical hydrogen-bonding interac-
Figure 5. The interlocking of two neighboring left-handed (M-) 43-
helical chains in IV based on the corrugated van der Waals surface
without any strong supramolecular interactions; space-filling repre-
sentation of the chains, which are differentiated by light and dark
gray shading.
Spontaneous resolution of achiral or of racemic chiral
tions between neighboring chains. The exterior of the molecular materials within the single crystal, that is,
chains is covered by CH bonds (Figure 4). Also, there are no crystallization in noncentrosymmetric (polar) space groups,
noteworthy reasonable π–π stacking or C–H···π interactions can be dictated by supramolecular solid-state packing inter-
seen in IV (or V) [that is, for π–π with centroid–centroid actions. This phenomenon has been observed by us in the
distances Ͻ6.0 Å, a dihedral angle between the ring planes crystallization of achiral hydrotris(pyrazolyl)boratothalli-
Ͻ10° and an angle between the centroid vector Cg(I)··· um() (in P21),[34,35] dihydrobis(1,2,4-triazolyl)boratothalli-
Eur. J. Inorg. Chem. 2006, 2146–2154
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FULL PAPER
um()[35] and -potassium (in P212121),[36] hydrotris(indazo-
The enantiomeric amino alcohol complex V crystallizes
lyl)boratothallium() (in C2),[35,37] and hydrotris(1,2,4-tri- in the monoclinic, noncentrosymmetric (Sohncke) space
azolyl)boratosilver() (in polar Pna21).[38] The spontaneous group P21.[23,24] Hydrogen-bonding interactions between
resolution of a racemic chiral material within a noncentro- the amino and hydroxy donor groups in the metal complex
symmetric space group was seen in the crystallization of to the acetate anion acceptors link the building blocks into
[Ag+(NH3)2](BINOLAT–)(BINOL)(EtOH) (in P1).[39] The supramolecular two-dimensional nets co-planar to the ab
crystallization of a racemic chiral material within a non- plane (Figure 6). Along c the nets are separated by the
centrosymmetric polar space group was encountered in the phenyl groups and cod ligands without meaningful π–π-
crystallization of [Λ/∆-Fe3(DABP)3]2+[∆/Λ-Fe2(DABP)3ʚ stacking or C–H···π interactions (see above).
Λ/∆-Fe1(DABP)3(nitrophenolate)6]2– (in P31c).[40] The
spontaneous resolution of rac-2,3-dihydro-2,3-dipyridyl-
Conclusions
benzo[e]indole with Cd(ClO4)2·6H2O is a recent example of
symmetry breaking through supramolecular interactions
and solvent, where in the presence of either 2-butanol or
ethanol crystals of the opposite enantiomers were favored
(crystallizing in the enantiomorphous space groups P6122
and P6522).[41] Particularly intriguing is the formation of a
homochiral (helical) polymer from achiral components
through spontaneous enantiomer resolution. A homochiral
helix winding together with homochiral crystallization was
reported for the adduct of 5-(9-anthracenyl)pyrimidine with
Cd(NO3)2·H2O·EtOH. The chirality arises from a pyrimi-
dine-Cd2+ helical array and is preserved in each crystal by
homochiral interstrand water–nitrate hydrogen bonding.
All the crystals are of the same chirality as a result of single-
colony homochiral crystal growth (homochiral crystalli-
zation).[42] The achiral building blocks in 1ϱ{[Ni(PhCOO)2-
(4,4Ј-bipy)]·2MeOH} gave rise to a 41- or 43-helical polymer
with homochirality in the crystal. Different crystals contain
statistically either P- or M-helices.[43] Homochiral right-
handed (P-) 61 helices of Cd{B(OMe)4} are found in
3ϱ{[Cd(tcm){B(OMe)4}]·xMeOH} with neighboring helices
connected by the tricyanomethanide (tcm) ion.[44] Com-
pound 1ϱ{[Mn(hfac)2{ferrocenyl bis(nitronyl nitroxide)}]·
CH2Cl2} spontaneously resolves in enantiomorphous crys-
tals with either Λ- or ∆-configuration at Mn and an ad-
ditional six sources of chirality within the P- or M-helices,
respectively (hfac = hexafluoroacetylacetonate).[45] Varia-
tion of the proton content with KOH/HClaq allows for a
reversible interconversion between the achiral molecular
square of [Cu(HL)(H2O)0.5]4(ClO4)4 (high pH) and a spon-
taneously resolved homochiral double-chain motif of
1ϱ{[H3O]2[Cu3(L)2Cl](ClO4)2(Cl)} (low pH) {H2L = 1,5-
diazacyclooctane-1,5-bis(3-propionic acid)}.[46]
Aminocarboxylato(η4-cycloocta-1,5-diene)rhodium()
complexes [Rh(AA)(η4-cod)] could be easily prepared from
dimeric [Rh(O2CMe)(η4-cod)]2 with amino acids in toluene/
MeOH. The compounds [Rh(AA)(η4-cod)] (AA = depro-
tonated amino acid = aminocarboxylato) were found to be
monomeric. Solid-state structures of [Rh(O2C–CH2–
NHPh)(η4-cod)] (IV) and [Rh{(S)-HOCH2–CH(Ph)–
NH2}(η4-cod)](O2CMe) (V) reveal strong intermolecular
N–H···O hydrogen-bonding interactions to assemble the
molecular units into 1D chains (IV) or 2D nets (V). In IV
the achiral (prochiral) N-phenylglycine ligand acquires a
stereogenic center at nitrogen upon complexation and the
racemic compound undergoes spontaneous resolution upon
crystallization. Supramolecular N–H···O and van der Waals
interactions are responsible for collecting only one enanti-
omer along fourfold screw axes of the same handedness
within a single crystal. This could be called a twofold spon-
taneous resolution in two homochiral building units, na-
mely the molecular complex and the fourfold helix.
Experimental Section
All reactions were carried out under dry nitrogen using Schlenk
techniques. Solvents used were highly purified and distilled: toluene
and benzene over Na metal; petroleum ether (PE) (boiling range
40–60 °C) over CaH2; methanol and ethanol over CaO under nitro-
gen. The starting complex [Rh(O2CMe)(η4-cod)]2 was synthesized
from [RhCl(η4-cod)]2[47] according to the literature.[3a] UV/Vis spec-
tra were obtained with a Shimadzu UV 3150 spectrophotometer in
toluene at 25 °C. IR spectra were recorded as KBr disks with a
Bruker IFS 66 FTIR Spectrometer at ambient temperature. NMR
spectra were run on a Bruker AC DPX 200 spectrometer operating
at 200 MHz (1H) and a JEOL AC 500 at 500 MHz (1H), 125 MHz
(13C) at 25 °C. NMR grade solvents [D6]DMSO, CD3OD, CD3CN,
and DCl/D2O solution (20% v/v) were used as internal standard
1
and deoxygenated prior to use. H and 13C NMR chemical shifts
are expressed in ppm relative to SiMe4 (δ = 0 ppm). FAB-MS (posi-
tive mode): Finnigan MAT 8230 with data system SS 300, matrix:
m-nitrobenzyl alcohol (NBA). EI- and CI-MS: Thermo-Finnigan
TSQ 700, with NH3 as ionization gas for CI.
(L
-Alaninato)(η4-cycloocta-1,5-diene)rhodium(
I),
{Rh[(S)-O2C–
CHMe–NH2](η4-cod)} (I): -Alanine (166 mg, 1.86 mmol) was dis-
solved in methanol (5 mL) and this solution was poured into a
solution of [Rh(O2CMe)(η4-cod)]2 (503 mg, 0.93 mmol) in toluene
(20 mL). Stirring the solution for 10–12 h at room temperature led
to the formation of a yellow precipitate. After the solvent was evap-
Figure 6. Two-dimensional hydrogen-bonded network in V. Phenyl
groups and cod ligands are not shown for clarity but only indicated
as broken-off bonds. For details of the hydrogen bonds see caption
to Figure 2.
2150
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Spontaneous Resolution of a Racemic Mixture into Homochiral Helix-Enantiomers
FULL PAPER
orated in vacuo, toluene/MeOH (4:1, v/v) (10 mL) was added to
the dried precipitate and evaporated again. This procedure was re-
peated three times. Finally, the precipitate was dried in vacuo (0.1–
was repeated three times. Then the dried precipitate was again dis-
solved in toluene/MeOH (4:1) (10 mL) and the volume reduced to
50% in vacuo at 40 °C. PE (40/60) (10 mL) was added very slowly
0.2 mbar) at 40 °C to give a yellow powder (yield 417 mg, 75%). to this hot solution. The precipitate formed was filtered off, washed
1H NMR (200 MHz, CD3OD): δ = 1.45 (d, J = 7.0 Hz, 3 H, CH3),
1.88 (dd, J = 8.2 Hz, 4 H, CH2codexo), 2.46 (m, 4 H, CH2codendo),
3.43 (q, J = 7.0 Hz, 1 H, CH), 4.13 (m, 4 H, CHcod) ppm. 1H
NMR (500 MHz, [D6]DMSO): δ = 1.20 (d, J = 7.0 Hz, 3 H, CH3),
with PE (40/60) and dried in vacuo (0.1–0.2 mbar) at 40 °C to give
a yellow powder (yield 125 mg, 75%). 1H NMR (200 MHz, [D6]-
DMSO): δ = 1.71 (dd, J = 8.0 Hz, 4 H, CH2codexo), 2.14 (s, 2 H,
CH2), 2.30 (m, 4 H, CH2codendo), 2.74 (s, 3 H, CH3), 3.51 (m, 4 H,
1
1.72 (dd, J = 8.0 Hz, 4 H, CH2codexo), 2.32 (m, 4 H, CH2codendo), CHcod), 3.64 (m, 1 H, NH) ppm. H NMR (500 MHz, CD3CN +
3.09 (q, J = 7.0 Hz, H, CH), 3.68 (m, 1 H, NH), 3.92 (m, 4 H, 5% DCl/D2O): δ = 1.75 (dd, J = 8.0 Hz, 4 H, CH2codexo), 2.35 (m,
1
CHcod), 4.31 (m, 1 H, NH) ppm. H NMR (500 MHz, CD3CN + 4 H, CH2codendo), 2.70 (s, 3 H, CH3), 3.86 (s, 2 H, CH2), 4.19 (m,
5% DCl/D2O): δ = 1.50 (d, J = 8 Hz, 3 H, CH3), 1.77 (dd, J =
8.5 Hz, 4 H, CH2codexo), 2.36 (m, 4 H, CH2codendo), 4.02 (q, J =
4 H, CHcod) ppm. 13C NMR (125 MHz, CD3CN + 5% DCl/D2O):
δ = 30.6 (s, CH2cod), 33.0 (s, CH3), 48.7 (s, CH2), 80.3 (d, JC,Rh
=
8.5 Hz, 1 H, CH), 4.22 (m, 4 H, CHcod) ppm. 13C NMR 13 Hz, CHcod), 167.5 (s, CO ) ppm. IR (KBr): ν = 3197 m
˜
2
(125 MHz, [D6]DMSO): δ = 20.4 (s, CH3), 30.1 (s, CH2cod), 52.5
(s, CH), 72.1, 79.6 (br., CHcod), 183.2 (s, CO2) ppm. 13C NMR
(125 MHz, CD3CN + 5% DCl/D2O): δ = 15.1 (s, CH3), 30.2 (s,
(νNHasy), 3100 sh (νNHsy), 2931 s (νCH), 1623 vs (νCO2 asy),
1590 sh (δNH), 1484 s (δCH2), 1381 s (δCH3), 1365 s (νCO2 sy
)
cm–1. MS (FAB) m/z (%) = 600 (30) [M2 + H+ + H]+, 510 (50)
CH2cod), 48.5 (s, CH), 80.3 (d, JC,Rh = 13 Hz, CHcod), 171.1 (s, [M2 – AA– = M + Rh(cod)+]+, 391 (35) [M + AAH2 + H]+, 300
+
CO ) ppm. IR (KBr): ν = 3258 s, 3190 sh (νNHasy), 3134 s, 3109 s
(100) [M + H]+, 255 (35) [M – CO2]+, 211 (55) [Rh(cod)]+ (AA =
˜
2
(νNHsy), 2940 s (νCH), 1620 vs (νCO2 asy), 1585 sh (δNH), 1383 s aminocarboxylato). C11H18NO2Rh (299.18): calcd. C 44.16, H
(δCH3), 1363 s (νCO2 sy) cm–1. MS (FAB): m/z (%) = 601 (20) [M2 6.06, N 4.68; found C 43.85, H 5.89, N 4.53.
+ H+ + 2H]+, 510 (58) [M2 – AA– = M + Rh(cod)+]+, 392 (40) [M
(η4-Cycloocta-1,5-diene)(N-phenylglycinato)rhodium(
I), [Rh(O2C–
+ AAH2+ + 2H]+, 300 (100) [M + H]+, 255 (25) [M+· – CO2]+, 211
(68) [Rh(cod)]+ (AA = aminocarboxylato). MS (EI = 70 eV): 299
(7) [M]+, 297 (13) [M – 2H]+, 255 (40) [M – CO2]+, 210 (50)
[Rh(cod) – H]+, 208 (55) [Rh(cod) – 3H]+, 44 (100) [CO2]+.
C11H18NO2Rh (299.18): calcd. C 44.16, H 6.06, N 4.68; found C
44.26, H 6.38, N 4.69.
CH2–NHPh)(η4-cod)] (IV): Two equiv. of N-phenylglycine [2-
(phenylamino)acetic acid] (85.9 mg, 0.57 mmol) were dissolved in
MeOH (5 mL). This solution was poured into a solution of
[Rh(O2CMe)(η4-cod)]2 (150.4 mg, 0.28 mmol) in toluene (20 mL)
and stirred for 10–12 h at room temperature. The volume was re-
duced to 60 % in vacuo at 40 °C, PE (40/60) (10 mL) was very
slowly added to the hot solution, and the combined solution was
left for crystallization. Red-orange needle crystals, suitable for X-
ray measurement, were obtained after 3 days. The crystals were
filtered off and washed three times with PE (5 mL each). Finally,
the red-orange crystals were dried in vacuo (0.1–0.2 mbar) at 40 °C
(yield 150 mg, 75%). 1H NMR (500 MHz, [D6]DMSO): δ = 1.64
[(S)-2-Amino-2-phenylacetato](η4-cycloocta-1,5-diene)rhodium(
I
),
(η4-Cycloocta-1,5-diene)(
L-phenylglycinato)rhodium(I), {Rh[(S)-
O2C–CHPh–NH2](η4-cod)} (II): The compound was prepared fol-
lowing the same procedure as for I, using (S)-2-amino-2-phenylace-
tic acid (-phenylglycine). The complex was obtained as a yellow
powder (yield 470 mg, 70%). 1H NMR (200 MHz, CD3CN): δ =
1.84 (dd, J = 8.0 Hz, 4 H, CH2codexo), 2.44 (m, 4 H, CH2codendo), (dd, J = 8.0 Hz, 4 H, CH2codexo), 2.09 (s, 2 H, CH2), 2.25 (m, 4
2.81 (m, 1 H, NH), 3.57 (m, 1 H, NH), 4.05 (m, 4 H, CHcod), 4.36
(m, 1 H, CH), 7.42 (m, 3 H, Hp/m-Ar), 7.76 (d, J = 6.6, 2 H, Ho-
H, CH2codendo), 3.60 (m, 4 H, CHcod), 3.67 (m, 1 H, NH), 7.01
(t, J = 7.0 Hz, 2 H, Ho-Ar), 7.27 (t, J = 6.5 Hz, 1 H, Hp-Ar), 7.33 (t,
Ar) ppm. 1H NMR (200 MHz, CD3OD): δ = 1.92 (dd, J = 8.0 Hz, J = 7.5 Hz, 2 H, Hm-Ar) ppm. 13C NMR (125 MHz, [D6]DMSO): δ
4 H, CH2codexo), 2.48 (m, 4 H, CH2codendo), 4.16 (m, 4 H, CHcod), = 29.7 (s, CH2cod), 55.9 (s, CH2), 77.9 (br., CHcod), 119.5 (s, Co-
4.49 (m, 1 H, CH), 7.47 (m, 3 H, Hp/m-Ar), 7.76 (m, 2 H, Ho-Ar) Ar), 124.3 (s, Cp-Ar), 129.2 (s, Cm-Ar), 146.1 (s, NC-Ar), 179.2 (s,
ppm. 1H NMR (500 MHz, CD3CN + 5% DCl/D2O): δ = 1.76 (dd, CO2) ppm. 1H NMR (500 MHz, CD3CN + 5% DCl/D2O): δ =
J = 8.5 Hz, 4 H, CH2codexo), 2.36 (m, 4 H, CH2codendo), 4.13 (m,
1.76 (dd, J = 8.0 Hz, 4 H, CH2codexo), 2.33 (m, 4 H, CH2codendo),
1 H, NH), 4.21 (m, 4 H, CHcod), 5.07 (m, 1 H, CH), 7.46–7.75 4.19 (m, 3 H, CH2 + NH), 4.22 (m, 4 H, CHcod), 7.45–7.52 (m, 5
(m, 5 H, H-Ar) ppm. 13C NMR (125 MHz, CD3CN + 5% DCl/ H, H-Ar) ppm. 13C NMR (125 MHz, CD3CN + 5% DCl/D2O): δ
D2O): δ = 30.4 (s, CH2cod), 56.1 (s, CH), 80.5 (d, JC,Rh = 13 Hz, = 29.9 (s, CH2cod), 50.7 (s, CH2), 80.5 (d, JCRh = 13 Hz, CHcod),
CHcod), 128.2 (s, Cm-Ar), 128.7 (s, Cp-Ar), 129.3 (s, Co-Ar), 130.0
122.5 (s, Co-Ar), 129.6 (s, Cp-Ar), 130.0 (s, Cm-Ar), 134.4 (s, NC-
(s, C-Ar), 170.0 (s, CO ) ppm. IR (KBr): ν = 3259 m, 3213 m Ar), 167.3 (s, CO ) ppm. IR (KBr): ν = 3142 m (νNHasy), 3097 m
˜
˜
2
2
(νNHasy), 3136 sh, 3102 m (νNHsy), 3056 s (νH-Ar), 2935 s (νCH), (νNHsy), 3053 s (νHAr), 2943 s (νCH), 1616 vs (νCO2 asy), 1600 s
1621 vs (νCO2 asy), 1591 s (δNH), 1363 s (νCO2 sy) cm–1. MS (FAB): (δNH), 1491 s (δCH2), 1366 s (νCO2 sy) cm–1. MS (FAB) m/z (%)
m/z (%) = 1017 (20) [(RhAA)4 + 4H + H+]+, 722 (8) [M2]+, 572 = 723 (5) [M2 + H]+, 573 (100) [M2 + H+ – AA = M + H+
+
(30) [M2 – AA– = M + Rh(cod)+]+, 362 (100) [M + H]+, 317 (45) Rh(cod)]+, 363 (100) [M + H+ + H]+, 315 (100) [M+· – H2 – CO2]+,
[M+· – CO2]+, 211 (60) [Rh(cod)]+ (AA = aminocarboxylato).
C16H20NO2Rh (361.25): calcd. C 53.20, H 5.58, N 3.88; found C
54.04, H 6.57, N 3.98.
211 (100) [Rh(cod)]+ (AA = aminocarboxylato). C16H20NO2Rh
(361.25): calcd. C 53.20, H 5.58, N 3.88; found C 52.90, H 5.46, N
4.00.
(η4-Cycloocta-1,5-diene)(N-methylglycinato)rhodium(
CH2–NHMe)(η4-cod)] (III): Two equiv. of N-methylglycine [2- Acetate, [Rh{(S)-HOCH2–CH(Ph)-NH2}(η4-cod)](O2CMe) (V):
(methylamino)acetic acid] (50 mg, 0.56 mmol) were dissolved in Two equiv. of (S)-2-amino-2-phenylethanol (71 mg, 0.52 mmol)
MeOH (5 mL) and this solution was poured into a solution of were dissolved in MeOH (5 mL). This solution was poured into a
I I)
), [Rh(O2C– [(S)-2-Amino-2-phenylethanol](η4-cycloocta-1,5-diene)rhodium(
[Rh(O2CMe)(η4-cod)]2 (150.4 mg, 0.28 mmol) in toluene (20 mL).
A yellow precipitate was formed after stirring the solution for 10–
12 h at room temperature. After the solvent was evaporated in
vacuo at 40 °C, the dried precipitate was dissolved in toluene/
MeOH (4:1, v/v) (10 mL) and evaporated again. This procedure
solution of [Rh(O2CMe)(η4-cod)]2 (133.4 mg, 0.25 mmol) in ben-
zene (20 mL) and stirred for 4–5 h at room temperature. The vol-
ume was reduced to 60% in vacuo at 40 °C, PE (40/60) (10 mL)
was very slowly added to the hot solution, and the combined solu-
tion was left for crystallization. Bright-yellow crystals, suitable for
Eur. J. Inorg. Chem. 2006, 2146–2154
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2151

M. Enamullah, A. Sharmin, M. Hasegawa, T. Hoshi, A.-C. Chamayou, C. Janiak
FULL PAPER
X-ray measurement, were obtained after 3 days at room tempera-
ture. The crystals were filtered off and washed three times with PE
X-ray Crystallography: Data Collection: Bruker AXS with CCD
area-detector, temperature 203(2) K, Mo-Kα radiation (λ =
0.71073 Å), graphite monochromator, ω-scans, data collection and
(5 mL each). Finally, the bright-yellow crystals were dried in vacuo
1
(0.1–0.2 mbar) at 40 °C (yield 141 mg, 70%). H NMR (500 MHz, cell refinement with SMART,[48] data reduction with SAINT,[48] ex-
[D6]DMSO): δ = 1.65 (dd, J = 8.0 Hz, 4 H, CH2codexo), 1.73 (s, 3
perimental absorption correction with SADABS.[49] Structure
H, CH3), 2.25 (m, 4 H, CH2codendo), 3.55 (t, J = 9.0 Hz, 1 H, CH), Analysis and Refinement: The structure was solved by direct meth-
3.63 (dd, 3J = 10.5 Hz, 4J = 5.5 Hz, 1 H, CH2), 3.71 (dd, 3J =
8.5 Hz, J = 5.5 Hz, 1 H, CH2), 3.87 (m, 4 H, CHcod), 7.27 (t, J
ods (SHELXS-97);[50] refinement was done by full-matrix least-
squares on F2 using the SHELXL-97 program suite.[50] For IV
structure refinement has also been attempted in the enantiomor-
phic space group P41 but this gave a Flack parameter of 1 with the
message “absolute structure probably wrong – invert and repeat
refinement”. All non-hydrogen positions were found and refined
with anisotropic temperature factors. Hydrogen atoms on oxygen
(–OH) and nitrogen (–NH–Ph, –NH2) were found and refined with
Ueq(H) = 1.2Ueq(N) in IV and freely in V. Hydrogen atoms on C
(phenyl, CH, CH2, and CH3) were calculated with appropriate ri-
4
= 7.5 Hz, 1 H, Hp-Ar), 7.34 (t, J = 7.5 Hz, 2 H, Ho-Ar), 7.45 (d, J
= 7.5 Hz, 2 H, Hm-Ar) ppm. 13C NMR (125 MHz, [D6]DMSO): δ
= 23.6 (s, CH3), 30.0, 30.1 (d, CH2cod), 60.8 (s, CH), 69.1 (s, CH2),
75.8 (br., CHcod), 127.2 (s, Co-Ar), 127.3 (s, Cp-Ar), 128.1 (s, Cm-
Ar), 140.3 (s, C-Ar), 174.6 (s, CO2) ppm. 1H NMR (500 MHz,
CD3CN + 5% DCl/D2O): δ = 1.77 (dd, J = 8.0 Hz, 4 H, CH2cod-
exo), 1.93 (s, 3 H, CH3), 2.36 (m, 4 H, CH2codendo), 3.87 (dd, J =
7.0 Hz, 2 H, CH2), 4.21 (m, 4 H, CHcod), 4.45 (t, J = 6.0 Hz, 1 H,
CH), 7.27 (m, 3 H, Hp/o-Ar), 7.48 (d, J = 2.0 Hz, 2 H, Hm-Ar) ding models (AFIX 43, 13, 23, and 33, respectively) and Ueq(H) =
ppm. 13C NMR (125 MHz, CD3CN + 5% DCl/D2O): δ = 20.1 (s,
CH3), 30.6 (s, CH2cod), 57.1 (s, CH), 62.7 (s, CH2), 80.1 (d, JC,Rh
= 13.0 Hz, CHcod), 127.7 (s, Co-Ar), 129.0 (s, Cm-Ar), 129.2 (s, Cp-
1.2Ueq(C) (CH, CH2) or Ueq(H) = 1.5Ueq(C) (CH3). Details of the
X-ray structure determinations and refinements are provided in
Table 2. Graphics were drawn with DIAMOND (Version 3.0e).[51]
Ar), 134.1 (s, C-Ar), 173.0 (s, CO ) ppm. IR (KBr): ν = 3410 m Computations on the supramolecular interactions were carried out
˜
2
(νOH), 3193 m (νNHasy), 3083 s (νNHsy), 3055 s (νHAr), 2937 s with PLATON for Windows.[52]
(νCH), 1575 s (δNH), 1558 sh (νCO2 asy), 1486 s (δCH2), 1442 s
CCDC-297844 (for IV) and -297845 (for V) contain the supplemen-
(νCO2 sy), 1385 s (δCH3) cm–1. MS (EI = 70 eV): 210 (2) [Rh(cod) –
tary crystallographic data for this paper. These data can be ob-
H]+, 106 (100) [AA – CH2OH = Ph–CH–NH2]+. MS (CI, NH3):
tained free of charge from The Cambridge Crystallographic Data
348 (0.4) [M – acetate = Rh(AA)(cod)]+, 228 (1.5) [Rh(NH3)-
Centre via www.ccdc.cam.ac.uk/data_request/cif.
(cod)]+, 138 (100) [AA + H]+, 106 (45) [AA – CH2OH = Ph–CH–
NH2]+ (AA = amino alcohol). C18H26NO3Rh (407.31): calcd. C
53.08, H 6.43, N 3.44; found C 53.06, H 6.33, N 3.44.
Supporting Information (for details see the footnote on the first
page of this article): UV/Vis spectra and data.
Table 2. Crystal data and structure refinement for IV and V.
Compound
IV
V
Empirical formula
M [gmol–1]
Crystal size [mm]
θ range [°]
C16H20NO2Rh
361.24
0.42×0.10×0.09
4.26–58.00
C18H26NO3Rh
407.31
0.40×0.27×0.16
3.42–57.56
h; k; l range
Crystal system
Space group
a [Å]
b [Å]
c [Å]
–12, 12; –12, 12; –21, 21
tetragonal
P43
9.556(3)
9.556(3)
16.281(6)
90
–12, 12; –10, 10; –16, 17
monoclinic
P21
9.5506(15)
7.9810(13)
12.773(2)
90
α [°]
β [°]
90
90
1486.8(8)
4
1.614
736
1.149
111.524(2)
90
905.7(2)
2
1.493
420
γ [°]
V [Å3]
Z
Dcalcd [gcm–3]
F(000)
µ [mm–1]
0.956
Max./min. transmission
Reflections collected
Independent reflections
Observed reflections [I Ͼ 2σ(I)]
Parameters refined
Max./min. ∆ρ[a] [eÅ–3]
R1/wR2 [I Ͼ 2σ(I)][b]
0.9076/0.6440
13666
3671 (Rint = 0.0719)
2700
0.8620/0.7010
8286
4189 (Rint = 0.0187)
3800
184
200
0.566/–0.452
0.0372/0.0594
0.0581/0.0646
0.897
0.0217/0.0000
0.01(4)
0.891/–0.502
0.0265/0.0636
0.0295/0.0647
0.976
0.0370/0.0000
0.01(3)
[b]
R1/wR2 (all reflect.)
Goodness-of-fit on F2 [c]
Weight scheme w; a/b[d]
Absolute structure parameter (Flack value[53]
)
[a] Largest difference peak and hole. [b] R1 = [Σ(||Fo| – |Fc||)/Σ|Fo|]; wR2 = [Σ[w(Fo 2 – Fc ) ]/Σ[w(Fo2)2]]1/2. [c] Goodness-of-fit = [Σ[w(Fo
–
2
2 2
2
Fc ) ]/(n – p)]1/2. [d] w = 1/[σ2(Fo2) + (aP)2 + bP] where P = [max(Fo or 0) + 2Fc ]/3.
2 2
2
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Eur. J. Inorg. Chem. 2006, 2146–2154

Spontaneous Resolution of a Racemic Mixture into Homochiral Helix-Enantiomers
FULL PAPER
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Acknowledgments
[19]
[20]
We thank the Alexander von Humboldt Foundation (AvH), Bonn,
and the Matsumae International Foundation (MIF), Tokyo, for of-
fering a post-doctoral fellowship to M.E. Our sincere thanks go to
Dr. Inayoshi for collecting the mass spectra at the Department of
Chemistry, College of Science and Engineering, Aoyama Gekuin
University, Tokyo. We acknowledge the financial support under the
JU projects, Department of Chemistry, Jahangirnagar University,
Bangladesh, by the Deutsche Forschungsgemeinschaft through
grant JA-466/14-1, and the Fonds der Chemischen Industrie.
[21]
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