Absolute structure determination of (2R,1′S)-2-(1′-benzyl- 2′-hydroxyethylamino)-4-phenylbutanoic acid

Article abstract of DOI:10.1016/j.molstruc.2004.04.001

Crystallization-induced asymmetric transformation was employed to prepare a labile γ-oxo-α-aminoacid (3) with a new stereogenic center in high diastereomeric and enantiomeric purity. Compound 3 was reduced to a stable compound, (2R,1′S)-2-(1′-benzyl-2′-hydroxyethylamino)-4- phenylbutanoic acid (4), which crystallizes in space group P2₁. Upon deprotonation, 4 becomes a tridentate monoanionic chelating ligand, 4 ^-H, which reacts with copper nitrate yielding [Cu(4 ^-H)(H₂O)₂]NO₃·H₂O (5). Complex 5 crystallizes in space group P2₁ and has a rare composition among crystallographically characterized mononuclear five-coordinate transition metal complexes. The absolute structure determination of 5 allowed by inference the assignment of absolute configuration to compound 3.

Full text of DOI:10.1016/j.molstruc.2004.04.001

Journal of Molecular Structure 697 (2004) 101–107
www.elsevier.com/locate/molstruc
Absolute structure determination of (2R,10S)-2-(10-benzyl-
20-hydroxyethylamino)-4-phenylbutanoic acid
b
D. Berkes , A. Kolarovic , R.G. Raptis , P. Baran
b,
ˇa,1
ˇa
*
a
´
Department of Organic Chemistry, Slovak Technical University, Radlinskeho 9, 812 37 Bratislava, Slovakia
b
´
Department of Chemistry, University of Puerto Rico, Rıo Piedras Campus, P.O. Box 23346, San Juan, PR 00931-3346, USA
Received 29 January 2004; revised 23 March 2004; accepted 1 April 2004
Abstract
Crystallization-induced asymmetric transformation was employed to prepare a labile g-oxo-a-aminoacid (3) with a new stereogenic center
in high diastereomeric and enantiomeric purity. Compound 3 was reduced to a stable compound, (2R,10S)-2-(10-benzyl-20-hydro-
hydroxyethylamino)-4-phenylbutanoic acid (4), which crystallizes in space group P21. Upon deprotonation, 4 becomes a tridentate
monoanionic chelating ligand, 42H, which reacts with copper nitrate yielding [Cu(42H)(H2O)2]NO3·H2O (5). Complex 5 crystallizes in
space group P21 and has a rare composition among crystallographically characterized mononuclear five-coordinate transition metal
complexes. The absolute structure determination of 5 allowed by inference the assignment of absolute configuration to compound 3.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Unnatural amino acids; Absolute configuration; Crystallization-induced asymmetric transformation; Copper(II) complex
1. Introduction
epimerisation and decomposition. Fortunately, after
removal of the activating carbonyl group we were able to
prepare a stable amino acid, (2R,10S)-2-(10-benzyl-20-
hydroxy-ethylamino)-4-phenylbutanoic acid, (4) with neg-
ligible epimerisation. In order to determine its absolute
structure, crystals suitable for X-ray analysis have to be
grown. Crystallization of compound 4 is difficult and
usually yields very thin hair-like crystals. However, after
the complexation of 4 with copper(II), suitable crystals for
X-ray study were obtained and the absolute structure of the
formed complex was determined and by inference, the
absolute structure of 4. Later, crystals of compound 4 of
sufficient quality were obtained, too. Syntheses, crystal
structures and spectroscopic characterization of 4 and its
copper complex [Cu(42H)(H2O)2]NO3·H2O (5) are
included in the present study.
The synthesis of unnatural amino acids still remains a
challenge for organic chemists trying to prepare enantio-
and diastereomerically pure derivatives in the simplest and
most inexpensive ways. In this connection, crystallization-
induced asymmetric transformation (CIAT) is a promising
method [1]. The conjugate addition of enantiomerically
pure 1-phenylethylamines on the aroylacrylic acids using
CIAT conditions has been applied recently to the synthesis
of optically pure homophenylalanines [2], and derivatives of
syn-g-hydroxy-a-amino-g-phenylbutanoic acids [3]. These
results prompted us to study the stereoselectivity of addition
of easily accessible chiral amino alcohols to aroylacrylic
acids [4]. An example of (S)-phenylalaninol (2) is given in
Scheme 1. The absolute configuration of the newly built
stereogenic centre of the adduct (3) could not be assigned by
the usual spectroscopic methods. Moreover, the g-oxo-
a-aminoacid (3) is unstable both in alkaline and neutral
media and its crystallization leads to a significant
2. Experimental
2.1. General
*
Corresponding author. Tel.: þ1-787-764-0000x7665; fax: þ1-787-
756-8242.
Melting points were obtained using a Kofler hot plate and
are uncorrected. Optical rotations were measured with a
Perkin–Elmer 241 polarimeter with a water-jacketed 10 cm
E-mail addresses: baranp@adam.uprr.pr (P. Baran); dusan.berkes@
ˇ
stuba.sk (D. Berkes).
1
Tel.: þ421-2-52968560; fax: þ421-2-52968560.
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.04.001

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D. Berkes et al. / Journal of Molecular Structure 697 (2004) 101–107
102
protons. 13C NMR spectra were recorded on a Varian
VXR-300 (75.43 MHz) spectrometer. Chemical shifts are
quoted in ppm and are referenced to the central resonance of
CD3OD (dC ¼ 49:0 ppm for 75.43 MHz). Infrared spectra
were recorded on a Nicolet 750 FTIR spectrophotometers as
KBr pellets.
2.2. Preparation of (2R,10S)-2-(10-benzyl-20-
hydroxyethylamino)-4-phenylbutanoic acid (4)
Amino acid 3 (1.96 g; 6 mmol) was dissolved in
mixture of methanol (90 ml) and 1 M HCl (150 ml).
Thereafter the catalyst (10% Pd/C) was added (0.40 g;
20 mol %). The suspension was stirred under H2 (1.1 atm.)
for one day. The catalyst was filtered off and washed with
1% HCl (50 ml). The mixture was partially concentrated
under reduced pressure. The pH of the remaining solution is
adjusted to 5.0–5.5 with 2N NaOH. The precipitate was
filtered off, washed with Et2O and dried to afford 1.85 g
(98%) of amino acid 4 (d.r. . 95:5) as a white solid. An
analytical sample was obtained by recrystallization from
EtOH with several drops of water: m.p. 212–213 8C,
Scheme 1.
cell at the wavelength of sodium line D (l ¼ 589 nm).
Specific rotations are given in units of 1021 deg cm2 g21
and concentrations are given in g/100 ml. 1H-NMR spectra
were recorded on a Varian VXR-300 (299.94 MHz)
spectrometer. Chemical shifts ðdÞ are quoted in ppm and
are referenced to residual CH3OH (dH ¼ 3:34 ppm for
299.94 MHz), used as internal reference. Coupling con-
stants ðJÞ are recorded in Hertz. Abbreviations with
quotation marks mean that the signal frequency is
different from the theoretically predicted one. H-A, H-B
mean the signals of magnetically nonequivalent CH2
½aꢀ2D0 2 53 (THF/1 M HCl ¼ 4/1;
c
0.6). 1H-NMR
(300 MHz; CD3OD, DCl/TMS): 7.20–7.40 (m, 10H,
2 £ Ph), 4.08 (t, 1H, J ¼ 6:3; H2), 3.73 (dd, 1H, J3A;3B
¼
12:0; J3A;4 ¼ 3:3; H3A), 3.64 (dd, 1H, J3A;3B ¼ 12:0;
J3B;4 ¼ 5:4; H3B), 3.51 (m, 1H, H4), 3.16 (dd, 1H, J5A;5B
¼
13:5; J4;5A ¼ 5:1; H5A), 3.03 (dd, 1H, J5A;5B ¼ 13:5;
Table 1
Crystal data and structure refinement for 4 and 5
4
5
Empirical formula
Formula weight
Temperature (K)
Crystal system
C19H23NO3
313.38
C19H28CuN2O9
491.97
298(2)
302(2)
Monoclinic
P21
Monoclinic
P21
Space group
˚
˚
Unit cell dimensions
a ¼ 10:959ð2Þ A
a ¼ 6:190ð1Þ A
˚
˚
b ¼ 5:972ð1Þ A
b ¼ 9:046ð1Þ A
˚
˚
c ¼ 13:033ð2Þ A
c ¼ 20:515ð2Þ A
b ¼ 92:360ð2Þ8
852.2(2), 2
1.221
b ¼ 90:915ð2Þ8
1148.5(2), 2
1.423
3
Volume (A ), Z
˚
Density (calculated) (Mg m23
)
Absorption coefficient (mm21
)
0.082
1.001
Fð000Þ
336
514
Crystal size (mm3)
0.16 £ 0.07 £ 0.04
0.32 £ 0.14 £ 0.07
u range for data collection, deg
Limiting indices
1.86–23.258
212 # h # 11; 26 # k # 4; 213 # l # 14
3785
1.99–23.268
26 # h # 6; 29 # k # 10; 222 # l # 19
5022
Reflections measured
Independent reflections
Observed reflections ½I . 2sðIÞꢀ
Refinement method
1939 ½RðintÞ ¼ 0:0566ꢀ
1223 ½Rint ¼ 0:0813ꢀ
Full-matrix least-squares on F2
1939/1/209
3206 ½RðintÞ ¼ 0:0251ꢀ
3010 ½Rint ¼ 0:0287ꢀ
Full-matrix least-squares on F2
3206/1/283
Data/restraints/parameters
Goodness of fit on F2
0.882
1.033
Final R indices ½I . 2sðIÞꢀ
R indices (all data)
R1 ¼ 0:0451; wR2 ¼ 0:0796
R1 ¼ 0:0914; wR2 ¼ 0:0906
R1 ¼ 0:0326; wR2 ¼ 0:0869
R1 ¼ 0:0353; wR2 ¼ 0:0886
23
23
˚
0.121 and 20.143 e A
˚
0.245 and 20.183 e A
Largest diff. peak and hole

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D. Berkes et al. / Journal of Molecular Structure 697 (2004) 101–107
103
J4;5B ¼ 9:5; H5B), 2.82–2.95 (m, 1H, H13A), 2.65–2.80
(m, 1H, H13B), 2.30 (m, 2H, H12); 13C-NMR (75 MHz;
CD3OD, DCl/TMS): 171.3 (C1); 141.1, 137.2, 130.3, 130.0,
129.7, 129.5, 128.4, 127.5 (C–Ar); 62.6, 60.1, 59.0 (C2, C4,
C3); 34.6, 32.9, 32.1 (C12, C13, C5); IR (KBr disk):
3366(s), 3085(m), 3063(m), 3032(m), 3010(w), 2961(m),
2925(m), 2877(m), 2613(w), 2582(w), 1636(s), 1608(vs),
1587(w), 1570(vs), 1496(m), 1475(vw), 1456(m),
1447(vw), 1434(w), 1394(s), 1383(m), 1355(m), 1339(m),
1329(w), 1314(w), 1307(w), 1285(vw), 1272(w), 1235(w),
1221(m), 1205(vw), 1178(w), 1160(w), 1110(m), 1064(m),
1059(w), 1030(s), 1009(sh), 983(vw), 964(w), 940(w),
906(vw), 909(w), 873(w), 816(w), 783(w), 755(m),
740(m), 696(vs), 643(w), 629(vw), 608(m), 576(w),
535(m), 503(m), 453(w), 414(w).
on F2; SHELXTL-93 [8], incorporated in SHELXTL, Version
5.1 [9]. The initial E-maps yielded all non-hydrogen atom
positions. Hydrogen atoms were geometrically positioned
and left riding on their parent atoms during structure
refinement. All non-hydrogen atoms were refined aniso-
tropically. Table 1 summarizes the structural and refinement
parameters. A list of important distances and angles is given
in Table 2.
Table 2
˚
Selected bond lengths (A) and angles (8) for 5 and 4
Cu(1)–O(5)
Cu(1)–O(1)
Cu(1)–O(4)
Cu(1)–N(1)
Cu(1)–O(3)
O(1)–C(1)
O(2)–C(1)
O(3)–C(3)
N(1)–C(4)
N(1)–C(2)
C(1)–C(2)
C(2)–C(12)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(12)–C(13)
C(13)–C(14)
1.935(3)
1.962(2)
1.977(3)
2.014(3)
2.179(3)
1.292(4)
1.211(4)
1.432(5)
1.482(5)
1.497(4)
1.516(5)
1.533(5)
1.521(5)
1.547(5)
1.498(6)
1.527(5)
1.524(5)
2.3. Preparation of [Cu(42H)(H2O)2]NO3·H2O (5)
1.255(4)
1.255(5)
1.420(5)
1.515(4)
1.498(4)
1.535(5)
1.526(5)
1.518(5)
1.526(5)
1.507(5)
1.518(5)
1.504(5)
To a solution of Cu(NO3)2·2.5H2O (0.020 g; 0.09 mmol)
in methanol (2 ml) was added 4 (0.012 g; 0.02 mmol)
dissolved in 2 ml of THF/H2O ¼ 10:1 at ambient tempera-
ture. No colour change was observed. After three weeks,
pale blue needles of 5 suitable for X-ray experiment were
isolated. Yield: 0.014 g (74%). Elemental analysis, found
(calcd for C19H28CuN2O9): C, 45.53 (46.39); H, 5.75 (5.74);
N, 5.72 (5.69)%. IR (KBr disk): <3460(sh), 3278(m),
3170(m), 3086(m), 3062(m), 3026(m), 3000(w), 2978(m),
2949(m), 2933(m), 2922(m), 2864(m), 1682(s), 1653(m),
1628(s), 1603(m), 1585(m), 1541(w), 1522(vw), 1497(m),
1473(vw), 1456(m), 1431(m), 1407(sh), 1385(vs), 1360(m),
1342(m), 1335(m), 1325(m), 1271(vw), 1253(sh), 1230(w),
1209(vw), 1192(w), 1155(vw), 1120(w), 1097(vw),
1086(vw), 1068(m), 1041(m), 1026(m), 1005(w), 989(w),
953(vw), 922(vw), 906(vw), 889(vw), 850(vw), 823(w),
779(m), 746(s), 737(m), 698(s), 629(w), 602(w), 582(vw),
573(w), 501(w), 472(vw), 438(w), 405(vw).
O(5)–Cu(1)–O(1)
O(5)–Cu(1)–O(4)
O(1)–Cu(1)–O(4)
O(5)–Cu(1)–N(1)
O(1)–Cu(1)–N(1)
O(4)–Cu(1)–N(1)
O(5)–Cu(1)–O(3)
O(1)–Cu(1)–O(3)
O(4)–Cu(1)–O(3)
N(1)–Cu(1)–O(3)
C(1)–O(1)–Cu(1)
C(3)–O(3)–Cu(1)
C(4)–N(1)–Cu(1)
C(2)–N(1)–Cu(1)
C(4)–N(1)–C(2)
O(2)–C(1)–O(1)
O(2)–C(1)–C(2)
O(1)–C(1)–C(2)
N(1)–C(2)–C(1)
N(1)–C(2)–C(12)
C(1)–C(2)–C(12)
O(3)–C(3)–C(4)
N(1)–C(4)–C(3)
N(1)–C(4)–C(5)
C(3)–C(4)–C(5)
C(6)–C(5)–C(4)
C(11)–C(6)–C(5)
C(7)–C(6)–C(5)
C(13)–C(12)–C(2)
C(14)–C(13)–C(12)
C(19)–C(14)–C(13)
C(15)–C(14)–C(13)
93.37(12)
86.88(13)
159.32(15)
177.05(12)
83.69(10)
95.81(12)
96.16(12)
103.08(11)
97.44(15)
84.69(11)
114.0(2)
103.2(2)
108.8(2)
104.2(2)
114.2(3)
124.1(3)
120.1(3)
115.8(3)
108.2(3)
111.4(3)
107.6(3)
111.3(3)
109.2(3)
114.1(3)
108.6(3)
117.1(3)
121.4(4)
119.1(4)
113.8(3)
111.2(3)
121.3(4)
121.1(4)
2.4. Crystallography
Single crystals of 4 suitable for X-ray crystalloghraphic
studies were obtained as colourless needles from THF/H2O
(10:1) solution by slow evaporation. Crystals of 5 were
obtained directly from the reaction mixture. The crystals
were mounted on the tip of glass fiber with epoxy glue.
Single crystal analyses were carried out on a Bruker
SMART 1K CCD diffractometer. The frame data were
acquired with the SMART [5] software using Mo Ka
115.2(3)
126.9(4)
115.1(4)
118.0(4)
108.0(3)
110.8(3)
110.3(3)
113.2(3)
109.3(3)
110.2(3)
114.3(3)
112.3(3)
120.5(4)
121.2(5)
115.9(3)
111.4(3)
121.5(4)
121.2(4)
˚
radiation (l ¼ 0:71073 A). Final values of the cell par-
ameters were obtained from least-squares refinement of the
positions of 911 reflections. A total of 1271 45-s frames
were collected in three sets with 0.38 v-scan for both
structures. The frames were then processed using the SAINT
software [6] to give the hkl file corrected for Lorentz and
polarization effects. No absorption correction was applied.
The structures were solved by direct method using the
SHELX-90 [7] program and refined by least-squares method

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D. Berkes et al. / Journal of Molecular Structure 697 (2004) 101–107
104
3. Results and discussion
The title compound
4
was prepared in high
diastereomeric excess using an inexpensive and efficient
CIAT. The core principle of CIAT methodology is the
existence of dynamic equilibrium between the isomers in
solution. Thus the adduct 3 is sufficiently stable only in solid
state. In quest of the stabilization of the newly formed
stereogenic centre at C2, the further transformation to the
more stable derivate 4 has been realized. The NMR spectra
of the reduced derivate 4, taken in CD3OD/DCl, show some
extend of rigidity, owing to intramolecular hydrogen bonds.
The signals of the three CH2 groups at C3 and both benzylic
positions are magnetic non-equivalent and with well-
developed splitting (see experimental). However, neither
1
the measured splitting constants in H-NMR, nor the other
NMR experiments did allow interpretation of the configura-
tion on the newly formed stereogenic centre at C2.
The complete molecular structure of 4 was obtained from
the X-ray crystallographic study (Fig. 1). Moreover, it was
possible to determine the absolute configuration using the
modification of compound 4 to complex structure with a
heavier atom. We have decided to use 4 as an organic ligand
in a complexation reaction with a transition metal.
Compound 4 has four potential donor atoms: two carboxylic
and one hydroxide oxygens and one secondary amino
nitrogen. Copper(II) nitrate was chosen as a source of a
central atom. The choice of Cu(II) relies on its plasticity
towards different coordination environments [10], and the
affinity of copper(II) for O- as well as N-ligands. Nitrate was
used because of its poor complexation to Cu(II). An excess
Fig. 2. Molecular structure and atom labeling scheme for 5. Thermal
ellipsoids shown at 30% probability.
of Cu in the complexation reaction results in 1:1 metal to
ligand complex formation. Under the applied conditions,
ligand 4 was able to display its full complexation ability. In
the product 5, it acts as a tridentate chelating ligand using
nitrogen, hydroxide oxygen and one oxygen from the
carboxylic group to coordinate copper (Fig. 2) in a distorted
square-pyramidal manner. Compound 5 is chiral, as the
ligand 4 is. Since, the absolute structure of 5 was readily
determined from the X-ray study, the absolute structure of 4
was inferred.
All inter-atomic distances found in 4 are within typical
˚
ranges. The C–C bond lengths (1.504(5)–1.535(5) A) are
typical for single C–C bond and the shorter C–C contacts
˚
(1.350(8)–1.389(6) A) belong to conjugated benzene rings.
The carboxylic group is deprotonated with equal C–O bond
˚
lengths (1.255(4) and 1.255(5) A) indicating electron
delocalisation. The secondary amine N1 atom is protonated
˚
with C–N contacts (1.498(4) and 1.515(4) A), typical of
single bonds. The absolute configuration on the chiral atom
C2 is R while on C4 it is S. Three hydrogen bonds (Table 3),
in which secondary amine, hydroxyl and carboxyl groups
are involved, link molecules forming a linear polymeric
structure along the b-axis.
Table 3
Specified hydrogen bonds for 4
D–H H· · ·A D· · ·A
,(DHA) D–H· · ·A
0.82
0.90
0.90
2.05
1.99
1.87
2.778(4) 147.6
O3–H3· · ·O1 ðx; y þ 1; zÞ
2.838(4) 156.0
2.766(4) 173.9
N1–H1A· · ·O1 ð2x; y þ 1=2; 2zÞ
N1–H1B· · ·O2 ðx; y þ 1; zÞ
Fig. 1. Molecular structure and atom labeling scheme for 4. Thermal
ellipsoids shown at 30% probability.

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D. Berkes et al. / Journal of Molecular Structure 697 (2004) 101–107
105
Table 4
Specified hydrogen bonds for 5
D–H
H· · ·A
D· · ·A
,(DHA)
D–H· · ·A
0.77
0.82
0.83
0.82
0.83
0.78
0.83
0.70(4)
1.96
2.05
1.99
1.87
1.89
2.25
1.97
2.23(4)
2.717(4)
2.736(4)
2.774(4)
2.663(4)
2.700(4)
2.815(5)
2.761(5)
2.926(4)
166.3
140.5
157.3
162.7
167.6
129.4
159.5
172(4)
O3–H1O3· · ·O6 ð2x þ 1; y 2 1=2; 2z þ 1Þ
O4–H1O4· · ·O1 ðx þ 1; y; zÞ
O4–H2O4· · ·O7 ðx þ 1; y 2 1; zÞ
O5–H1O5· · ·O6 ðx; y 2 1; z 2 1Þ
O5–H2O5· · ·O7 ðx; y 2 1; zÞ
O6–H1O6· · ·O9 ð2x þ 1; y þ 1=2; 2z þ 1Þ
O6–H2O6· · ·O8 ðx þ 1; y; z þ 1Þ
N1–H1· · ·O2 ðx þ 1; y; zÞ
The molecular structure of coordinated compound 4 is
only slightly different from the free molecule. Interatomic
contacts are almost the same with minor differences in bond
lengths for donor atoms, but a different conformation has
been adopted to facilitate chelation to Cu. In the copper(II)
complex 5, compound 4 exists as a deprotonated mono-
anionic ligand (42H) loosing its Zwitterionic character. This
square-pyramidal environment, with the basal plane formed
by N1 and O1 of 42H and two coordinated water molecules.
The apical position is occupied by O3 from the chelating
˚
ligand. The copper atom is lifted 0.183(2) A above the
calculated best fit plane of the basal donor atoms, which is
indeed slightly tetrahedraly distorted, as evident from its
diagonal angles (N1–Cu1–O5, 177.05(12), O1–Cu1–O4,
˚
is reflected in the shortening of N1–C4 bond (1.482(5) A),
˚
159.32(15)8). The apical O3 atom is 2.348(3) A above the
(in spite the fact that the lone electron pair on N1 was
used towards coordination) and in C–O bond length
differentiation within the carboxylic group. The C1–O1
˚
distance becomes longer (1.292(4) A) and C1–O2
˚
shorter (1.211(4) A) indicating localization of the charge
on O1 atom. The hydroxylic group remains protonated
calculated ideal basal plane, deviating by 6.68 from the
perpendicular position with respect to Cu1. The complex
cation, [Cu(42H)(H2O)2]þ, counter-anion, NO23 and non-
coordinated interstitial water molecule are held together in
the crystal structure via a 3D hydrogen bond system,
involving all acidic hydrogen atoms (Table 4). Due to
carboxylic group presence, complex cations form 1D
polymeric chains along the crystallographic a axis via two
˚
but the C3–O3 distance is slightly elongated (1.432(5) A)
due to Cu1–O3 coordination. Copper(II) is in a distorted
Fig. 3. Packing diagram for compound 5 showing the 3D network of H-bonds. Hydrogen atoms not involved in H-bonding were omitted for clarity.

ˇ
D. Berkes et al. / Journal of Molecular Structure 697 (2004) 101–107
106
five-coordinate complexes were found, both are Cu(II)
complexes - one is polymeric [21], while the other one is
trinuclear [20]. In both complexes, tridentate ligands form
part of basal plane of distorted square pyramid unlike the
studied complex 5, in which 42H is in a fac-mode. Only few
more five-coordinate Cu(II) complexes with different
tridentate ONO ligands (H and J in Scheme 2) suitable for
comparison with 5 are known [27–32]. Both ligand types
coordinate in mer-mode and the complexes are square-
pyramidal. This only stresses the uniqueness of ligand 42H
,
inspite of its simple architecture.
4. Conclusions
The absolute structure of the title compound was
elucidated after the crystal structure of its copper(II)
complex was studied. Upon deprotonation, 4 becomes a
tridentate monoanionic chelating ligand, 42H, capable of
adopting a rare coordination motif. Complex [Cu(42H)-
(H2O)2]NO3·H2O (5) is indeed the first crystallographically
described mononuclear five-coordinate transition metal
complex of the type [M(ONO)LL0], where (ONO) is a
tridentate chelating ligand with O-carboxyl, N-amino, O-
hydroxo donor atoms and L, L0 any monodentate ligands.
Scheme 2.
H-bonds (N1–H1· · ·O2 and O4–H1O4· · ·O1). The chain is
stabilized by two additional H-bonds provided by O7 of
nitrate group (O4–H2O4· · ·O7 and O5–H2O5· · ·O7). The
nitrate anion and non-coordinated water molecule form four
more H-bonds, which interlink the above mentioned 1D
chains into a complicated 3D structure (Fig. 3).
Comparison of IR spectra for both compounds, ligand
4 and the Cu(II) complex 5, show important differences.
In 4, n(C–H), n(N–H), and n(O–H) vibrations are well
resolved, while in 5, those are superimposed on a broad
system of bands, which belong to coordinated and crystal
lattice water molecules. Coordination of carboxylic group
should cause shifts of both asymmetric and symmetric
infrared frequencies of COO group [11]. Strong absorp-
tions at 1394 and 1570 cm21 could be tentatively assigned
Acknowledgements
Financial support by the Slovak Grant Agency (No.
1/9250/02) is gratefully acknowledged.
Appendix A
to ns(CO22 ) and na(CO22) for 4, resulting in 176 cm21
D
CCDC 229987 and 229988 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK;
fax: þ44-1223-336033).
value [na(CO22)–ns(CO22)]. For 5, na(CO22 ) was shifted to
1682 and ns(CO22 ) to 1385 cm21, resulting in much
greater D value (297 cm21), indicative of the unidentate
coordination mode of carboxylate [11]. Frequencies in
other regions of the recorded infrared spectrum were
much less affected by the complexation of 4, and show
negligible shifts.
To the best of our knowledge and to our surprise, the
tridentate ligand 42H represents a rare motif among
tridentate ligands in crystallographically characterized
[12] transition metal complexes (A in Scheme 2). Upon
coordination, it forms two five-membered metallacycles
with central secondary amino N donor and side hydroxyl
and carboxyl O donor atoms. Only six other tridentate
ligands (and their derivatives) possessing similar skeletons
were found [13–26]. All six differ in their coordination
mode. Ligand B coordinates exclusively in fac-mode
[13–15], ligands E and F adopt both fac- and/or mer-type
of coordination [19–22], but ligands C, D, and G form only
mer-complexes [16–18,23–26]. Among them, only two
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