Chiral bis(amino alcohol) silver complex derived from (+)-camphor

Article abstract of DOI:10.1080/00958972.2015.1012071

A chiral silver(I) complex was synthesized from the reaction of a chiral amino alcohol derived from (+)-camphor and silver nitrate, with the structure unambiguously disclosed by X-ray analysis and 2-D-NMR experiments. In this complex, the amino alcohol molecule is a monodentate ligand through the amino nitrogen, requiring two amino alcohol units which are coordinated to the silver and located orthogonally through the NH-Ag-NH axis.

Full text of DOI:10.1080/00958972.2015.1012071

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Chiral bis(amino alcohol) silver complex
derived from (+)-camphor
Jaime Gonzáleza, Marisol Reyes-Lezamaa, David Coronaa & Erick
Cuevas-Yañeza
a Centro Conjunto de Investigación en Química Sustentable UAEM-
UNAM, Toluca, Mexico
Accepted author version posted online: 28 Jan 2015.Published
online: 11 Feb 2015.
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To cite this article: Jaime González, Marisol Reyes-Lezama, David Corona & Erick Cuevas-Yañez
(2015) Chiral bis(amino alcohol) silver complex derived from (+)-camphor, Journal of Coordination
Chemistry, 68:5, 863-874, DOI: 10.1080/00958972.2015.1012071
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Journal of Coordination Chemistry, 2015
Vol. 68, No. 5, 863–874, http://dx.doi.org/10.1080/00958972.2015.1012071
Chiral bis(amino alcohol) silver complex derived from
(+)-camphor
JAIME GONZÁLEZ, MARISOL REYES-LEZAMA, DAVID CORONA and
ERICK CUEVAS-YAÑEZ*
Centro Conjunto de Investigación en Química Sustentable UAEM-UNAM, Toluca, Mexico
(Received 3 April 2013; accepted 12 January 2015)
A chiral silver(I) complex was synthesized from the reaction of a chiral amino alcohol derived from
(+)-camphor and silver nitrate, with the structure unambiguously disclosed by X-ray analysis and
2-D-NMR experiments. In this complex, the amino alcohol molecule is a monodentate ligand
through the amino nitrogen, requiring two amino alcohol units which are coordinated to the silver
and located orthogonally through the NH–Ag–NH axis.
Keywords: Silver complex; Amino alcohol; (+)-Camphor
1. Introduction
Interest in amino alcohol metal complexes has increased as these compounds have diverse
applications as catalysts [1], inhibitors [2], ion exchangers [3], and dyes [4] as well as other
interesting applications. Some research groups have explored metals to modulate some
chemical properties in this class of complexes [5–10]. In spite of these examples, the
number of reports about the coordination of amino alcohols to metals is limited. In the case
*Corresponding author. Email: ecuevasy@uaemex.mx
© 2015 Taylor & Francis

864
J. González et al.
of silver, a few reports describe only formation and stability constants, while other physical
properties or crystallographic studies have not been reported for this kind of compounds
[11–14].
In connection with other projects, we initiated studies about the synthesis of chiral amino
alcohol metal complexes and applications in asymmetric catalysis. Several works about chi-
ral silver complexes have been described in the literature [15–20], but there are no reports
about chiral silver amino alcohol complexes. Herein, is described the preparation of the first
chiral silver complex using amino alcohols as ligands.
2. Experimental
2.1. General remarks
The starting materials were purchased from Aldrich Chemical Co. and used without purifi-
cation. Solvents were distilled before use. Silica plates of 0.20 mm thickness were used for
thin layer chromatography. Melting points were determined with a Fisher–Johns melting
point apparatus and they are uncorrected.
2.2. Physical measurements
1H and 13C NMR spectra were recorded using a Varian 500; the chemical shifts (δ) are
given in ppm relative to TMS as internal standard (0.00). For analytical purposes mass
spectra were recorded on a Shimadzu GCMS-QP2010 Plus and on a JEOL JMS-5X 10217
in the EI mode, 70 eV, 200 °C via direct inlet probe. Only the molecular and parent ions
(m/z) are reported. IR spectra were recorded on a Bruker TENSOR 27 FT instrument.
Suitable crystals of 2 (C22H42AgN3O5) were obtained from slow evaporation of a dilute
EtOH solution over several days. Single crystals were mounted on a Bruker APEX DUO
diffractometer equipped with an Apex II CCD detector, Mo-Kα radiation (λ = 0.71075 Å)
at 293 K. Frames were collected using omega scans and integrated with SAINT [21].
Multi-scan absorption correction (SADABS) [21] was applied. The structures were solved
by direct methods (SHELXS) [22] and refined using full-matrix least-squares on F2 with
SHELXL-97 [22] using SHELXLE GUI [23]. Weighted R factors, Rw, and all goodness-
of-fit indicators are based on F2. All non-hydrogen atoms were refined anisotropically.
Hydrogens of the C–H bonds were placed in idealized positions, whereas hydrogens of OH
groups were localized from the difference electron density map and their position was
refined with Uiso tied to the parent atom with distance restraints. The NOꢀ3 was disordered
over two positions in the structure (56.21/43.79%). Refining was carried out using geometry
(SADI, SAME) and Uij restraints were (SIMU, DELU) implemented in SHELXL-97. The
OH distances in 2 were restrained using DFIX. Three standard reflections for every 97
reflections were used to monitor the crystal stability.
2.3. Synthesis of (1R, 2R, 4R-2-amino-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl)methanol (1)
According to a modification of the procedure described by Hoyer [24] and Maki [25], a
mixture of (+)-camphor (9.78 g, 64.24 mM), KCN (16.87 g, 259.10 mM), and (NH4)2CO3
(30.0 g, 312.10 mM) in MeOH (50 mL) was poured into a Parr-type reactor. The reaction

Chiral bis(amino alcohol) silver
865
mixture was stirred at 50–60 °C for 24 h with a pressure of 220 psi of CO2 gas, and then
allowed to reach 120–130 °C and stirred for 10 h. The reaction mixture was cooled to room
temperature. Addition of a 1 : 1 ethanol–water mixture (200 mL) was kept at 0 °C until pre-
cipitation of white solid which was filtered and crystallized from methanol to afford the
camphor-2-spirohydantoin in 50% yield. The camphor-2-spirohydantoin (8.95 mM) was
added to a mixture of NaOH (3.224 g, 80.61 mM) in H2O (9 mL). The reaction mixture
was heated to 150 °C for 10 h in a tubular autoclave. The mixture flowed from the auto-
clave through a valved outlet to a condenser, where it was cooled below 90 °C. As the mix-
ture flowed from the reaction system, it was treated with activated carbon, filtered through
Celite, and the filtrate was neutralized with acetic acid to precipitate the product. The prod-
uct was removed by filtration, washed with water, and dried to give 2-amino-camphor-2-
carboxylic acid in 75% yield. Using a modification of the method by Wurtz and co-workers
̈
[26], 2-amino-camphor-2-carboxylic acid (10.0 g, 50.43 mM) was added to a vigorously
stirred dispersion of NaBH4 (6.20 g, 163.89 mM) in dry THF (180 mL) and the reaction
mixture was cooled to 0 °C. A solution of iodine (14.08 g, 55.47 mM) in THF (70 mL)
was added dropwise to the reaction mixture at 0 °C. Vigorous evolution of hydrogen
occurred and the mixture was stirred at reflux for 18 h. The mixture was cooled to room
temperature and MeOH was added slowly until gas evolution ceased. Solvent was removed
in vacuo, the residue was dissolved in aqueous 20% KOH solution (180 mL), and was stir-
red at reflux temperature for 3 h. The mixture was cooled to room temperature and
extracted with CH2Cl2 (4 × 150 mL). The combined organic layers were joined and dried
over Na2SO4 and the solvent was removed in vacuo to afford 1 as white solid (87% yield),
20
which was used without further purification. M.p. 108–110 °C; ½aꢁD = −35.5 (c 0.0012 in
1
DMSO). H NMR (300 MHz, CDCl3): δ 3.38 (q, J = 5.9 Hz, 2H), 1.90 (s, broad, 3H), 1.73
(d, J = 2.5 Hz (apparent), 2H), 1.68 (t, J = 3.5 Hz (apparent), 2H), 1.45 (d, J = 6.4 Hz
(apparent), 2H), 1.36 (d, J = 12.8 Hz (apparent), 1H), 1.07 (s, 3H), 0.88 (s, 3H), 0.86 (s,
3H). 13C NMR (75 MHz, CDCl3): δ. 69.0 (CH2), 61.5 (C), 50.9 (C), 49.9 (C), 44.9 (CH2),
44.6 (CH), 30.6 (CH2), 27.2 (CH2), 21.9 (CH3), 21.0 (CH3), 12.0 (CH3). IR (KBr) νmax
3400, 3124, 2991, 2938, 1700, 1592 cm−1. MS [EI+] m/z (%): 183 [M]+ (73). Anal. Calcd
for C11H21NO (%): C, 72.08; H, 11.55; N, 7.64. Found: C, 72.05; H, 11.50; N, 7.67.
2.4. Synthesis of bis(1R, 2R, 4R-2-amino-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl)methanol
silver complex (2)
To a solution of 1 (0.090 g, 0.535 mM) in methanol (5 mL), AgNO3 was added (0.0417 g,
0.246 mM). The resulting suspension was stirred at room temperature for 1 h. The resulting
pale-brown solution was filtered through Celite to remove AgNO3 and any black decompo-
sition product. The filtrate was evaporated in vacuo. Purification by crystallization in EtOH
20
afforded 2 as white solid (0.084 g, 72% yield), m.p. 77–82 °C; ½aꢁD = −37.5 (c 0.0028 in
1
DMSO). H NMR (500 MHz, CDCl3): δ 4.84 (s, broad 1H, OH), 3.62 (dd, J = 5.9 Hz, 2H,
assigned to H8, scheme 3), 3.51 (s, broad, 2H, NH2), 1.83 (dt, J = 12.8, 6.5 Hz, 1H,
assigned to H3β, scheme 3), 1.74 (m, 3H, assigned to H5 and H4, scheme 3), 1.61 (ddd,
J = 13.5, 9.2, 2.9 Hz, 1H, assigned to H6α, scheme 3), 1.52 (ddd, J = 13.7, 11.4, 3.0 Hz,
1H, assigned to H6β, scheme 3), 1.27 (d, J = 13.2 Hz, 1H, assigned to H3α, scheme 3),
1.05 (s, 3H, assigned to H9, scheme 3), 1.03 (s, 3H, assigned to H11, scheme 3), 0.88 (s,
3H, assigned to H10, scheme 3). 13C NMR (125 MHz, CDCl3): δ 66.7 (CH2), 60.9 (C),
48.8 (C), 47.3 (CH2), 42.1 (C), 41.7 (CH), 28.8 (CH2), 24.1 (CH2), 18.9 (CH3), 18.0

866
J. González et al.
(CH3), 9.9 (CH3). IR (KBr) νmax 3406, 3367, 3248, 29,952, 2874, 1714, 1596 cm−1. HRMS
(ESI+): for C22H42AgN2O2 Calcd 473.2297, found 473.2292. Anal. Calcd for
C22H42AgN2O2 (%): C, 55.69; H, 8.92; N, 6.74. Found: C, 55.71; H, 8.97; N, 6.68.
3. Results and discussion
3.1. Synthesis of ligand and silver complex
The first experiments were to prepare the chiral amino alcohol ligand. We considered that
commercially available enantiopure (+)-camphor is an ideal starting point for the synthesis of
the desired ligand. According to appropriate modifications in previous reports by Hoyer [24],
Maki [25], and Wurtz et al. [26], (+)-camphor was successfully converted to amino alchohol
̈
1 after 3 reaction steps in 33% overall yield from camphor (scheme 1). The straightforward
mixing of 1 with silver nitrate using methanol as solvent afforded the chiral silver complex 2
as a moisture-stable solid without adding any base to adjust the pH (scheme 2).
3.2. Crystal structure
Suitable crystals of 2 were obtained from ethanol upon slow evaporation over several days.
A single-crystal X-ray diffraction analysis was performed on the silver complex confirming
the spectroscopic results. An ORTEP and atom labeling diagram for 2 is presented in figure
1. Crystallographic data and structural refinement parameters of 2 are summarized in table 1
and relevant bond length and angle data are collected in supplementary material. From these
studies, 2 has a 1-D network and crystallizes in the orthorhombic space group P 21 21 21,
and the asymmetric unit contains four molecules. Two chiral amino alcohol ligands bind
Figure 1. ORTEP diagram and atom labeling system for 2. The NOꢀ3 is disordered and occupies two sites. Only
one site is depicted (40% probability ellipsoids).

Chiral bis(amino alcohol) silver
867
Table 1. Crystal data and structure refinement for 2, Ag
[C20H32(CH2)2(OH)2(NH2)2]NO3.
Crystal data
2
Identification code
Empirical formula
Formula weight
Temperature
Complex 2
C22H42AgN3O5
536.46
293(2) K
Wavelength
0.71073 Å
Crystal system
Space group
Orthorhombic
P2(1)2(1)2(1)
Unit cell dimensions
a = 7.3336(10) Å, α = 90°
b = 12.1467(16) Å, β = 90°
c = 28.836(4) Å, γ = 90°
2568.7(6) Å3
Volume
Z
4
Density (calculated)
Absorption coefficient
F (0 0 0)
Crystal size
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta = 25.11°
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
Absolute structure parameter
Largest diff. peak and hole
1.387 mg m−3
0.819 mm−1
1128
0.36 × 0.25 × 0.15 mm3
1.82°–25.11°
−8 ≤ h ≤ 8, −14 ≤ k ≤ 14, −34 ≤ l ≤ 34
36,189
4583 [R(int) = 0.0787]
99.8%
Semi-empirical from equivalents
0.8870 and 0.7592
Full-matrix least-squares on F2
4583/149/340
1.033
R1 = 0.0357a, wR2 = 0.0812b
R1 = 0.0405a, wR2 = 0.0840b
0.06(3)
0.509 and −0.701 (e Å−3
)
[a] = R1 = Σ||Fo| – |Fc||/Σ|Fo|, [b] = wR2 = [Σw(Fo2 – Fc2)2/Σ(Fo2)2]1/2
.
with one Ag(I) in the common linear N–Ag–N coordination (angle 173.83°) described by
other groups [17, 27–30] with weak interactions with the nitrate. Both Ag–N2 and Ag–N1
distances (2.130 and 2.136 Å) in 2 are standard in N–Ag coordination complexes with simi-
lar interactions (within experimental error) [31–35]. For example, Fromm and Sagué
observed Ag–N distances of 2.136 and 2.143 Å for a polymer with N–Ag–N coordination
[36]. The two organic ligands lie opposite each other at both sides of the plane bisecting the
Ag center. In contrast, oxygens in amino alcohol ligands are not coordinated to silver; there-
fore, the ligands and the metal center in 2 do not achieve metallocycles and only the linear
coordination is observed. As a consequence, the structure of 2 in solid state does not contain
a crystallographically imposed C2-symmetry axis, keeping only natural C1-symmetry axis in
this chiral complex. In addition, no structural differences between the coordinated and the
free ligand were found. Oxygens of the NOꢀ3 interact with both OH and NH2 groups of
ligand through hydrogen bridges, while the other oxygen interacts with the metal center (O
(4)–Ag(1) 2.93 Å), stabilizing 2 through a network of weak H-bond interactions.
The nitrate interacts through two oxygens, one establishes a hydrogen bridge with a NH,
while the other binds to the metal center, forming a six-membered metallacycle with bond
distances O(5)–H(2′) = 2.367 Å and O(4)–Ag(1) = 2.93 Å (table 2 and figure 2). A similar
O–H bond distance (2.25 Å) was identified by Kim and co-workers [39]. The metallacycle

868
J. González et al.
d(H···A)
Table 2. Hydrogen bonds for 2 (Å, °).a
D–H···A
d(D–H)
d(D···A)
\(DHA)
O(1)–H(1A)···O(3)i
O(1)–H(1A)···O(3A)i
O(2)–H(2A)···O(3)
O(2)–H(2A)···O(3A)
N(1)–H(1)···O(5A)i
N(1)–H(1)···O(5)i
N(2)–H(2′)···O(5A)ii
N(2)–H(2′)···O(5)ii
N(2)–H(2)···O(4)
0.82
0.82
2.10
2.02
2.89(2)
2.83(2)
2.81(2)
2.76(2)
3.30(2)
2.95(2)
3.12(2)
2.98(2)
3.07(1)
3.16(2)
163.8
174.8
0.83(1)
0.83(1)
0.83(2)
0.83(2)
0.83(2)
0.83(2)
0.82(2)
0.82(2)
2.21(4)
2.12(4)
2.50(3)
2.17(3)
2.44(3)
2.37(3)
2.44(3)
2.43(3)
140(5)
134(4)
162(4)
157(4)
139(3)
131(3)
134(3)
148(4)
N(2)–H(2)···O(4A)
aD = donor; A = acceptor.
Symmetry codes: (i) x + 1, y, z; (ii) x + 1/2, −y + 1/2, −z.
Figure 2. View of six-membered metallacycle in 2.
stemming from NOꢀ3 and Ag–NH2 fragments is well documented in the literature [37–40].
In addition, supramolecular interactions are achieved from these hydrogen bonds which
reveal the presence of a 1-D zigzag chain according to figure 3.
3.3. NMR spectra
1
The H NMR spectrum for 2 using CDCl3 as solvent is presented in figure 4. At low field
in this spectrum, a broad singlet is observed at 3.51 ppm, corresponding to hydrogens from

Chiral bis(amino alcohol) silver
869
Figure 3. View of 1-D chain in 2. View of the zigzag interaction.
the nitrogen. Although amine protons tend to appear over a wide range in the NMR
spectrum, it was possible to make a comparison between the monomer molecule 1, whose
amine proton signals are assigned at 1.90 ppm, and the corresponding coordination complex
2 showing a change in the amine protons shift, due to nitrogen coordination.
At 3.62 ppm, a doublet of doublets is present for the protons of the hydroxymethyl
group, this multiplicity owing to the magnetic and chemical difference between the hydro-
gens in this methylene, giving a geminal coupling (J = 12 Hz). The methylene hydrogens
assigned to C-3 (scheme 3) have different shifts in the spectrum, the proton H-3β is at
1.83 ppm, however, the proton H-3α is at 1.27 ppm. On the other hand, the H-3β is a dou-
blet of doublet of doublets signal, while the H-3α is only a doublet system. The methylene
group on C-6 presents two multiplets for their protons in 1.61 and 1.53 ppm. In the region

870
J. González et al.
Figure 4. 1H NMR spectrum for 2.
Figure 5. COSY spectrum for 2.

Chiral bis(amino alcohol) silver
871
1.76–1.71 ppm, a multiple signal for three hydrogens assigned to the methylene on C-5 and
the methyne on C-4 is presented. Finally, the shifts for the methyl groups were assigned as
follows: H-9 at 1.05, H-11 at 1.03, and H-10 at 0.877 ppm.
The correct assignation for the hydrogens was made using 2-D RMN COSY and NOESY
experiments. Through the COSY experiment (figure 5), the proton shifts for the three differ-
ent methylenes were identified through cross-peak correlations; for instance, the C-3 methy-
lene shows two different shifts for their protons, due to the presence of the amine and
hydroxymethyl groups which cause a paramagnetic shield on the β-hydrogen (1.81 ppm),
whereas the α-hydrogen chemical shift (1.26 ppm) is not affected. For the methylene pro-
tons on C-6, two signals at 1.53 and 1.61 ppm are observed and both hydrogens from the
methylene on C-5 are present at 1.75 ppm forming a multiplet. The chemical shift assign-
ment of these protons was important to substantiate the (R) configuration of the stereogenic
center in the molecule.
Analysis of the NOESY spectrum displayed some interesting features. For instance, the
C-8 methylene protons at 3.64 ppm present a through-space interaction at 1.03 ppm
assigned to the methyl group C-11, another interaction with the doublet at 1.26 ppm corre-
sponding to the α-hydrogen in C-3, and also an interaction with the doublet of doublets sys-
tem at 1.61 ppm corresponding to the α-hydrogen in C-6 (scheme 3 and figure 6). In
addition, the hydrogen on C-4 at 1.74 ppm has an interaction through space with a signal at
1.05 ppm assigned to the methyl C-9 which in turn has a cross-peak with the signal at
0.87 ppm corresponding to the methyl C-10. The hydrogen on C-10 exhibits a
through-space interaction with two signals; one of these signals at 1.61 ppm is assigned to
Figure 6. NOESY spectrum for 2.

872
J. González et al.
a-c
NH2
OH
O
1
Scheme 1. Reagents and conditions: (a) KCN, (NH4)2CO3, MeOH, 220 psi, 120–130 °C, and 10 h. (b) NaOH,
150 °C, and 10 h. (c) NaBH4, I2, THF, reflux, and 18 h.
H
HO
AgNO3/MeOH
r. t. /30 min
NH2
OH
N
Ag
H
H
N
H
OH
1
2
Scheme 2. Preparation of 2 from amino alcohol (1).
nOe
nOe
nOe
9
10
H3C
CH3
11
7
CH3
H
3β
Ag
6α
nOe
H
HO
CH2
H
H
5β
H
8
1
nOe
4
3
3α
NH2
5
NH2
2
6
5α
6β
5β
H
H
H
7
H
H
1
3β
6β
H3C
CH3
6
H
10
9
3
2
H C
5
11
4
3
8
H
H2C
H
3α
6α
5α
OH
H
nOe
nOe
Scheme 3. nOe interactions for 2.
the β-hydrogen on C-6, and other interaction with the β-hydrogen on C-5 at 1.75 ppm. All
these nOe interactions are displayed in scheme 3 and the spectrum is presented in figure 6.
4. Conclusion
A chiral silver complex was prepared from silver nitrate and an amino alcohol derived from
(+)-camphor through a mild and efficient method with a simple workup affording excellent
yields. This is the first report about the use of chiral amino alcohols for the synthesis chiral
silver complexes. Selectivity of NH2 and silver ion was observed. This coordination of
amino alcohol 1 indicates a versatile ligand which will be investigated in future research.
The simplicity of the method suggests that amino alcohol 1 and silver complex 2 will enjoy
widespread applications.

Chiral bis(amino alcohol) silver
873
Supplementary material
Crystallographic data for the structure reported in this article have been deposited with the
Cambridge Crystallographic Data Center (CCDC No. 911160). Copies of available materi-
als can be obtained free of charge on application to the Director, CCDC, 12 Union Road,
Cambridge CB2 IEZ, UK (Facsimile: +(44) 01223 336033; E-mail: deposit@ccdc.ac.uk).
Acknowledgements
Financial support from CONACYT (Project No. 135053) is gratefully acknowledged. The
authors would like to thank Signa S.A. de C. V. for some graciously donated solvents and
reagents and to N. Zavala, A. Nuñez, and L. Triana for the technical support.
Supplemental data
Supplemental data for this article can be accessed here [http://dx.doi.org/10.1080/00958972.2015.1012071].
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