Chiral 1,8-naphthyridine based ligands: Syntheses and characterization of Di- and tetranuclear copper (I) and silver (I) complexes

Article abstract of DOI:10.1016/j.ica.2018.11.014

Oxazoline and camphor-pyrazole units are introduced on the 1,8-naphthyridine scaffold to access chiral ligands L¹, L² and L³. Metalation of these chiral ligands with Cu(I) and Ag(I) precursors afforded di- and tetranuclear complexes [Cu₄I₄(L¹)₂] (1), [Cu₄I₄(L²)₂] (2), [Cu₂I₂(L³)] (3), [Cu₂I(L²)₂](OTf) (4), [Ag₂(L¹)₂](OTf)₂ (5) and [Ag₄(L²)₄Br](OTf)₃ (6), containing [M₄X_n] (n = 1,4 and X = Br, I) or [M₂X_n] (n = 0, 1, 2 and X = I) core. All complexes are structurally characterized. Naphthyridine-derived ligands reveal bridge-chelate coordination motif and hold two metal centers in close proximity. The tetranuclear complexes are dimer of dinuclear complexes bridged by the halides. Electronic absorption and emission spectra of copper complexes are reported. Catalytic utility of all complexes are examined for asymmetric transformations but they showed poor activity probably due to limited solubility and coordinative saturation at the metal centers. The best results are obtained with [L³/Cu salt] combination for cyclopropanation of styrene, N–H bond insertion and nitroaldol (Henry) reactions with very low enantioselectivity.

Full text of DOI:10.1016/j.ica.2018.11.014

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Research paper
Chiral 1,8-Naphthyridine Based Ligands: Syntheses and Characterization of Di-
and Tetranuclear Copper (I) and Silver (I) Complexes
Mithun Sarkar, Pragati Pandey, Jitendra K. Bera
PII:
S0020-1693(18)31516-0
https://doi.org/10.1016/j.ica.2018.11.014
ICA 18629
DOI:
Reference:
Inorganica Chimica Acta
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Please cite this article as: M. Sarkar, P. Pandey, J.K. Bera, Chiral 1,8-Naphthyridine Based Ligands: Syntheses and
Characterization of Di- and Tetranuclear Copper (I) and Silver (I) Complexes, Inorganica Chimica Acta (2018),
doi: https://doi.org/10.1016/j.ica.2018.11.014
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Chiral 1,8-Naphthyridine Based Ligands: Syntheses and
Characterization of Di- and Tetranuclear Copper (I) and
Silver (I) Complexes
Mithun Sarkar,a # Pragati Pandeya and Jitendra K. Beraa,b * $
a Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India.
b Center for Environmental Sciences and Engineering, Indian Institute of Technology Kanpur,
Kanpur 208016 India.
E‒mail: jbera@iitk.ac.in
# Present address: Department of Chemistry, Government General Degree College, Kaliganj
741137 India.
* To whom correspondence should be addressed.
$ Dedicated to Prof. Rabindranath Mukherjee.
1

Highlights
 Naphthyridine-functionalized oxazoline and camphor-pyrazole chiral ligands are
synthesized
 Di- and tetranuclear Cu(I) and Ag(I) complexes containing chiral ligands are
structurally characterized
 Each ligand displays bridge-chelate coordination motif holding two metal centers
in close proximity
 A combination of Cu salt and chiral camphor-pyrazole ligand affords appreciable
yields for asymmetric transformations but with very low enantioselectivity
Graphical Abstract
2

Abstract
Oxazoline and camphor-pyrazole units are introduced on the 1,8-naphthyridine scaffold
to access chiral ligands L1, L2 and L3. Metalation of these chiral ligands with Cu(I) and
Ag(I) precursors afforded di- and tetranuclear complexes [Cu4I4(L1)2] (1), [Cu4I4(L2)2] (2),
[Cu2I2(L3)] (3), [Cu2I(L2)2](OTf) (4), [Ag2(L1)2](OTf)2 (5) and [Ag4(L2)4Br](OTf)3 (6),
containing [M4Xn] (n = 1,4 and X = Br, I) or [M2Xn] (n = 0, 1, 2 and X = I) core. All
complexes are structurally characterized. Naphthyridine-derived ligands reveal bridge-
chelate coordination motif and hold two metal centers in close proximity. The
tetranuclear complexes are dimer of dinuclear complexes bridged by the halides.
Electronic absorption and emission spectra of copper complexes are reported. Catalytic
utility of all complexes are examined for asymmetric transformations but they showed
poor activity probably due to limited solubility and coordinative saturation at the metal
centers. The best results are obtained with [L3/Cu salt] combination for cyclopropanation
of styrene, N-H bond insertion and nitroaldol (Henry) reactions with very low
enantioselectivity.
Keywords
1,8-naphthyridine, Chiral ligand, Oxazoline, Camphor-pyrazole, Copper and Silver
Complexes
3

Introduction
1,8-naphthyridine (NP) is a useful scaffold with wide applications in coordination
chemistry,
supramolecular
architectures,
organometallics
and
biomedical
sciences.[1],[2],[3],[4] The syn, syn coordination motif of NP skeleton holds two metal
centers in close proximity. Functionalization at 2 and 7 position of NP has afforded
ligands with varied stereo-electronic properties capable of supporting a range of
unusual bimetallic constructs. Uyeda group has reported a low-valent Ni(I)−Ni(I)
complex [(NDI)Ni2(C6H6)] stabilized by redox active naphthyridine−diimine (NDI) pincer
ligand and studied different organic transformation reactions, including selective
cyclotrimerization, hydrosilyation of terminal alkynes, strain-induced ring-opening
reactions on dinickel platform.[5] Lippard and coworkers employed functionalized NP
ligands to construct several bimetallic complexes involving 3d metals for mimicking
metalloenzymes.[6]
Diiron
complexes
supported
by
2,7-bis{bis[2-(2-
pyridyl)ethyl]aminomethyl}-1,8-naphthyridine (BPEAN) and its derivatives were used for
studying O2 activation.[7] Tilley group designed dinucleating 2,7-bis(di(2-
pyridyl)fluoromethyl)-1,8-naphthyridine (DPFN) for the synthesis of several 3d metal
complexes.[8] Further modification led to 2,7-bis(1,1-dipyridylethyl)-1,8-naphthyridine
(dpen) which afforded an unusual  acetonitrile-bridged dicopper complex that
exhibits a three-center two-electron bonding interaction involving acetonitrile.[9] Suitably
modified NP ligands have been used for making different dimetal and metal-chain
complexes, studying inter-ligand electron transfers and a multitude of other
purposes.[10]
4

We have developed a range of NP functionalized ligands over the years that are
employed in coordination chemistry and catalysis.[11] Introduction of ferrocene unit to
NP afforded ligands, which upon metalation with divalent transition metal ions provided
an array of mixed-metal complexes.[12] A dipalladium(I) complex is accessed by amide-
linked naphthyridine-ferrocene hybrid ligands featuring short metal–metal distance.[13]
The presence of free nitrogen, offered by suitably designed NP scaffold, in the vicinity of
a metal ion is exploited for water activation.[14] By using a pyridine-functionalized NP
ligand (py-NP), the catalytic utility of [RuH(CO)(py-NP)(PPh3)2]Cl for direct conversion of
alcohols to the corresponding acids with alkaline water is studied (Scheme 1a).[15]
Exploiting the same strategy, water-mediated oxidation of metal-coordinated olefin to
carbonyl on an iridium center is reported by us.[16] An unsupported diruthenium (I,I)
complex incorporating N-heterocyclic carbene (NHC) functionalized NP ligands is used
for carbene transfer reaction at site trans to the metal-metal single bond (Scheme
1b).[17] A fused imidazo[1,2-a][1,8]naphthyridine based mesoionic NHC (MIC) ligand
was designed that upon metalation afforded [Ru(MIC)(COD)Br2], which showed an
excellent catalytic activity for selective C=C bond scission of olefins to aldehydes
(Scheme 1c).[18] Recently, a ruthenium catalyst incorporating pyrazole-functionalized
NP ligand is shown to catalyze oxidant-free and acceptor-less selective double
dehydrogenation of primary amines to nitriles.[19]
ACS Catal. 2017
Organometallics, 2011
JACS. 2014
Carbene transfer reaction
Oxidative cleavage of alkene
Water activation
Scheme 1. Selected complexes containing 1,8-naphthyridine functionalized ligand
5
scaffolds employed in catalysis.

Despite an impressive diversity of the NP ligands, chiral adaptions are rarely reported.
Although pyridine derivatives containing chiral oxazoline[20], imidazoline[21] and
camphor-pyrazole[22] are extensively used in asymmetric catalysis, only a handful of
NP variants are known in the literature.[23] In our quest to expand the scope of NP
ligands in deriving new structures and in organometallic catalysis, we have introduced
oxazoline and camphor-pyrazole chiral auxiliaries to NP. Herein, we report the
syntheses of naphthyridine based chiral ligands (L1–L3, Scheme 2). Use of appropriate
stoichiometry of the ligands with different copper and silver precursors have afforded
several di- and tetranuclear complexes. Catalytic asymmetric transformations are
attempted using these newly synthesized complexes.
Scheme 2. Chiral oxazoline and camphor-pyrazole based NP
ligands employed in this work.
Results and discussion
Syntheses of ligands L1–L3
Oxazoline unit was introduced to NP following a multi-step synthesis illustrated in
Scheme 3. Several procedures are available in the literature to construct the oxazoline
ring.[24] Coupling between appropriate carboxylic acid and optically active amino
alcohol appears to be more general and convenient. At first, 1,8-napthyridine-2-
carboxylic acid was synthesized following the literature procedure.19,[25] It was then
6

converted to the corresponding carbonyl chloride by treatment with oxalyl chloride.
Subsequent coupling with optically active amino alcohols produced hydroxy amide
derivative. Chirality of the amino alcohol was incorporated to the ligand in this step. The
hydroxy group of the amide derivative was then converted to a good chloro leaving
group by treatment with SOCl2. Base induced cyclization of chloro-amide in the final
step gave the desired oxazoline ligands L1 and L2 in 22% overall yield.
1
Both ligands were characterized by different spectroscopic techniques. The H NMR
spectrum of L1 (Fig. S8) shows five naphthyridine protons at the downfield region
ranging from  8.24–9.21 ppm. Five phenyl protons appear together at  7.34–7.39
ppm. The proton at the chiral center appears at  5.51 ppm, whereas peaks at  4.97
and 4.47 ppm correspond to two diastereototopic CH2 protons. The ESI–MS spectrum
shows a signal at m/z 276.1136 that is assigned for [M+H]+.
2
1
A similar H NMR spectrum is observed for L with additional diastereotopic benzylic
CH2 protons at  3.28 and 2.81 ppm (Fig. S9). The ESI–MS spectrum at m/z 290.1290
corresponds to the [M+H]+ ion.
1
2
7
Scheme 3. Synthesis of oxazoline based ligands L and L .

Synthesis of L3 involves coupling between 2,7-dichloro-1,8-naphthyridine[26] with two
equivalents of camphor-pyrazole [27] (Scheme 4). At first, commercially available
optically active (+)-camphor was converted to 1,3-dicarbonyl compound, which remains
predominantly in the enol form, by treatment with excess methyl formate and KH. In the
next step, (1R,4S)-3-Hydroxymethylenecamphor was treated with hydrazine hydrate to
form camphor-pyrazole ring (L). In a separate synthesis, treatment of 2,6-
diaminopyridine with (dl)-malic acid in conc. H2SO4 produced 2-amino-7-hydroxy-1,8-
naphthyridine which after diazotization by NaNO2 in conc. H2SO4 gave 2,7-dihydroxy-
1,8-naphthyridine. It was then converted to 2,7-dichloro-1,8-naphthyridine by treatment
with PCl5 and POCl3 in refluxing condition. Finally, two equivalents of L was refluxed
with one equivalent of 2,7-dichloro-1,8-naphthyridine in THF in presence of two
equivalents of KOH and 10 mol% of tetrabutylammonium bromide (TBABr) as catalyst
to produce ligand L3 in 35% overall yield.
1H NMR spectrum in CDCl3 discloses symmetric structure for the ligand L3 (Fig. S10).
One camphor-pyrazole proton and two naphthyridine protons of the symmetric half
appear as two singlets respectively at  = 8.46 and 8.09 ppm in a ratio 1:2. This is in
contrast to two separate doublets exhibited by similar naphthyridine based ligands. In
CDCl3, the naphthyridine and pyrazole units remain trans to each other (as in X-ray
structure), thereby allowing intramolecular H-bonding interaction between pyrazole
nitrogen and naphthyridine hydrogen (see Fig. S10). As a result, the meta-hydrogen is
deshielded and appears together with para-hydrogen as a singlet. However, this
8

degeneracy is removed when the spectrum is recorded in a mixed solvent 1:1
CD3OD/CDCl3. The ligand adapts a cis configuration supported by intermolecular
hydrogen bond interactions with methanol molecules (Fig. S11). Consequently, two
naphthyridine protons are observed as two doublets. The aliphatic region contains three
distinct singlets for three methyl groups, one multiplet for CH proton at the downfield
region and four multiplets for four diastereotopic CH2 protons. The ESI–MS spectrum at
m/z 479.2925 corresponds to the [M+H]+ ion.
Scheme 4. Synthesis of L3.
Ligand L3 was also characterized by X-ray crystallography. The molecular structure of
L3 (Fig.1) reveals anti-planar conformation of the naphthyridine and camphor-pyrazole
rings. It reveals the possibility of hydrogen bonding interaction between N and H atoms
involving both the naphthyridine and camphor-pyrazole rings (N1···H9, N4···H2,
N2···H20, N6···H7).
9

Fig. 1. Molecular structure of L3 with important atoms labeled.
Hydrogen atoms have been omitted for the sake of clarity. Thermal
ellipsoids are drawn at the 40% probability level.
Cu and Ag complexes 1–6
Treatment of L1–L3 separately with CuI in acetonitrile in a molar ratio 1:2 at room
temperature afforded complexes [Cu4I4(L1)2] (1), [Cu4I4(L2)2] (2) and [Cu2I2(L3)] (3),
within 30 minutes as dark red solids in high yields of 90%, 82% and 86%, respectively
(Scheme 5). Use of [Cu(CH3CN)4](OTf) with L2 in a ratio 1:1 and subsequent anion
exchange with tetrabutylammonium iodide resulted in the formation of discrete dinuclear
iodide-bridged complex [Cu2(L2)2(I)](OTf) (4) in 84% yield (Scheme 6). Similar types of
iodide-bridged di- and tetranuclear copper complexes with chiral Pybox ligands are
reported in literature.[28] Treatment of L1 and L2 ligands with Ag(OTf) in a molar ratio
1:1 produced disilver complex [Ag2(L1)2](OTf)2 (5) (80% yield) and bromide-bridged
tetrasilver complex [Ag4(L2)4(Br)](OTf)3 (6) (54% yield) respectively (Scheme 7). Same
reaction with L3 ligand resulted in the formation of insoluble polymeric metal complex
which could not be characterized.
10

Scheme 5. Syntheses of copper complexes 1–3.
Scheme 6. Synthesis of copper complex 4.
Scheme 7. Syntheses of silver complexes 5 and 6.
11

Complexes 1–6 were characterized by single crystal X-ray crystallography which
shows tetranuclear structures for complexes 1, 2 and 6, and discrete dinuclear
structures for complexes 3, 4 and 5 (Fig. 2) Molecular structures of complexes 1 and 2,
depicted in Fig. 2, contain [Cu4I4] core, but the disposition of the ligands in two
complexes are opposite. Complex 1 crystallizes in an orthorhombic chiral space group
P21212, whereas complex 2 crystallizes in triclinic P1 space group. Careful examination
of the crystal structure of 1 reveals a crystallographically imposed C2 axis which passes
through the midpoint between two central copper atoms. As a result, only half of the
molecule of 1 appears in the asymmetric unit, whereas complex 2 does not retain any
symmetry element and therefore the asymmetric unit contains the full molecule. Both
complexes show distorted ‘step-type’ geometry of [Cu4I4] unit (Scheme 8b) that is most
common among other geometries exhibited by various copper complexes having [Cu4I4]
core (Scheme 8).[29] The copper atoms in both complexes show different coordination
environment. The terminal copper atoms are coordinated to two bridging iodide ions and
two nitrogen atoms (proximal nitrogen of naphthyridine and nitrogen in the oxazoline
ring) of the ligand, while the central copper atoms are bonded to three bridging iodides
and a distal nitrogen atom of the naphthyridine unit. The structures also contain two
different types of bridging iodides (two iodides are doubly (μ2) bridged and the other two
are triply (μ3) bridged). The short Cu···Cu distances in both complexes range between
2.38(1)–2.593(4) Å, which are less than the sum of the van der Waals radii (2.80 Å)[30]
indicating closed shell interaction between the two copper centers. Molecular structure
of complex 3 shows the butterfly-shaped discrete [Cu2I2][31] core with Cu···Cu distance
2.558(2) Å.
12

The molecular structure of 4 (Fig. 2) consists of two copper centers spanned by two
naphthyridine ligands and one bridging iodide. One triflate anion was located in the
asymmetric unit. The two copper centers reside in distorted tetrahedral environment
with a Cu···Cu separation of 2.6189(9) Å. It crystallizes in an orthorhombic P212121
chiral space group. The Cu1–I1–Cu2, N1–Cu1–N5, N2–Cu2–N4 and N1–Cu1–I1 angles
are 59.35(2), 132.82(17), 129.46(17) and 115.17(12)° respectively.
Scheme 8. Some common geometries of [Cu4I4] unit.
13

2
1
3
Cu1···Cu2
2.5381(8),
Cu1···Cu1′
Cu1···Cu2 2.574(4), Cu3···Cu4 2.593(4), Cu1···Cu3
2.724(2), I1–Cu1–I2 101.02(11), I1–Cu2–I2 109.44(15), I3–
Cu3–I4 99.78(11), I3–Cu4–I4 107.53(14), Cu2–I2–Cu1–I3
21.6(2)
Cu1···Cu2 2.558(2), Cu1–I1–Cu2 58.46(5),
2.9463(11). I1–Cu1–I2 99.851(19), I1–Cu2–
I2 113.74(2), Cu2–I1–Cu1′–I2′ 10.6(1)
Cu1–I2–Cu2 61.79(5),
Cu1–N1–C1–N3
0.9(1), Cu2–N2–C8–N5 4.4(1)
5
6
4
Cu1···Cu2 2.6189(9), Cu1–I1–Cu2 59.35(2),
Ag···Ag 2.8024(11) Å, Ag1–Br1 2.8139(16), Ag2–Br1
2.7202(15), Ag2–N2 2.543(8), Ag2–N3 2.288(9), Ag2–
Br1–Ag1 60.82(3), Ag4–Br1–Ag3 58.96(3)
Ag1···Ag2
2.8238(12),
N1–Ag1–N5
N1–Cu1–N5
132.82(17),
N2–Cu2–N4
161.1(3), N2–Ag2–N4 169.6(3), N1–Ag1–
Ag2–N2 1.0(1), N5–Ag1–Ag2–N4 5.3(2)
129.46(17), N1–Cu1–I1 115.17(12), N2–Cu2–I1
106.70(12), N1–Cu1–Cu2–N2 6.3(1)
Fig. 2. Molecular structure of 1-6 with important atoms labeled. All the hydrogen atoms are omitted for the sake of clarity. Thermal ellipsoids
are drawn at the 40% probability level. Important bond distances (Å) and angles (deg) are given below each structure.
14

Molecular structure of 5 consists of two silver centers bridged by two naphthyridine
moieties that are trans to each other. Two silver centers reside in a distorted square
planar arrangement of ligands with Ag···Ag distance of 2.8238(12) Å.[32] Compound 5
crystallizes in monoclinic P21 chiral space group. The important bond and dihedral
angles are: N1–Ag1–N5 161.1(3)°, N2–Ag2–N4 169.6(3)°; N1–Ag1-Ag2-N2 1.0(1)° and
N5-Ag1-Ag2-N4 5.3(2)°. The crystal structure of tetrasilver(I) complex 6 exhibits two
dimeric silver units that are bridged by a central bromide ion. Each disilver unit is
bridged by two naphthyridine ligands trans to each other in a distorted tetrahedral
arrangement around the two silver centers. The Ag···Ag distance is 2.8024(11) Å.
Due to poor solubility in common organic solvents, NMR spectroscopic data of
1
13
complexes 1 and 2 could not be obtained. However, other complexes show H and C
NMR spectra very similar to the corresponding free ligands (see experimental section).
ESI–MS spectra of complexes show signals at m/z = 802.9774 for [Cu2I(L1)2]+ in 1,
830.9941 for [Cu2I(L2)2]+ in 2, 731.0501 for [M–I]+ in 3, 831.0107 for [M–OTf]+ in 4,
914.9477 for [M–OTf]+ in 5 and 941.0102 for [Ag2(L2)2(OTf)]+ in 6, respectively (Fig.
S16).
Electronic spectra
Electronic absorption spectra of ligands L1–L3 and complexes 1–6 were recorded in
dichloromethane at room temperature in the range 200–800 nm. The λmax with the
corresponding ε values are summarized in Table 1.
15

Table 1. Electronic absorption data for ligands L1–L3,
and complexes 1–6 in CH2Cl2 at room temperature.
Absorption Data
Compounds

max[nm] (10–3[mol–1dm3cm–1])
L1
L2
L3
1
255 (15.1), 314 (14.9), 319 (14.8)
262 (11.5), 316 (12.8), 327 (11.6)
274 (92.8), 360 (55.1), 378 (83.2)
305 (16.6), 312 (16.7), 445 (1.6)
314 (13.1), 396 (2.1)
2
3
264 (50.1), 293(35.0), 368 (30.1), 386
(42.9)
4
5
300 (24.8), 472 (4.6)
238 (20.4), 268 (9.9), 312 (10.0), 321
(11.2)
6
248 (7.8), 281 (6.2), 322 (15.0)
Ligands L1 and L2 exhibit multiple → transitions between = 255–327 nm (Fig.
S17). The absorptions observed in the UV region ranging from 238–322 nm for
complexes 1, 2, 4, 5 and 6 are ascribed to ligand-centered → transitions (Fig. 3). No
absorption peak is observed for complexes 5 and 6 in the visible region, whereas
complexes 1 and 2 show absorption tails ranging from 386–512 nm. These low energy
absorptions are assigned to metal to ligand charge transfer (MLCT) transitions mixed
with halogen to ligand charge transfer (XLCT) transitions, and are therefore assigned to
(X+M)LCT states.[33] Complex 4 shows absorption peak around 472 nm with high ε
value. This corresponds to halide to metal charge transfer transition and it overshadows
the MLCT and XLCT transitions observed for complexes 1 and 2. Ligand L3 shows
→ transitions at 274, 360 and 378 nm with high ε values (Fig. S17). These high ε
values stem probably from the extensive -delocalization between naphthyridine and
16

camphor-pyrazole rings. Complex 3 exhibits similar type of absorption spectrum (Fig. 3)
of free L3 ligand.
Photoluminescence properties of copper complexes 1-4 were studied. When a
dichloromethane solution of 1-4 is excited at 340 nm at room temperature, emission
maxima at 404, 400, 380 and 381 nm were observed, respectively. The tetranuclear
copper complexes 1 and 2 with [Cu4I4] core showed emission spectra at lower energy
compared to dinuclear analogs 3 and 4 (Fig. S18).
(c)
(b)
(a)
(d)
(f)
(e)
Fig. 3. UV–Vis spectra of complexes (a) 1, (b) 2, (c) 3, (d) 4, (e) 5 and (f) 6.
17

Attempted catalytic studies
Copper and silver salts in combination with variety of chiral ligands have been employed
in asymmetric transformation reactions.[34] At first, we examined the catalytic activity of
di- and tetranuclear complexes 1-6 for enantioselective addition of phenylacetylene to
N-benzylideneaniline to form propargylamine (Scheme 9, Table S2)[35]. Unfortunately
none of these complexes had shown satisfactory results.
Scheme 9. Enantioselective synthesis of (1,3-Diphenyl-2-propynyl) aniline.
Further, copper complexes 1-4 are found catalytically sluggish for cyclopropanation –
a most commonly performed reaction for Cu catalysts.[36] The low solubility of the
complexes and coordinative saturation at the metal centers are likely responsible for the
inactivity.28b A viable remedy is in situ generation of a coordinatively unsaturated and
catalytically active species. Accordingly, cyclopropanation reaction was attempted with
combination of copper salts and chiral ligands (L1, L2 and L3) (Table S3). The best
result was obtained with L3/[Cu(OTf)] for cyclopropanation of styrene with ethyl
diazoacetate (EDA) to give 70% yield after 24 h (Scheme 10, Table S3, entry 7).
Scheme 10. Cyclopropanation of styrene.
18

Similarly, the L3/[Cu(CH3CN)4](OTf) (1:2) (5 mol%) combination afforded N-H insertion
product in moderate yield of 66% for the reaction of 4-methoxy aniline (0.5 mmol) with
methyl phenyldiazoacetate (0.5 mmol) in DCM at room temperature after 12 h (Scheme
11). However, poor enantioselectivity was observed (10-18% e.e).
Scheme 11. N-H insertion reaction catalyzed by L3/ [Cu(CH3CN)4](OTf).
In view of the catalytic activity of the L3/Cu combination, nitroaldol (Henry) reaction[37]
was examined with complex 3. Reaction of p-nitro benzalehyde (1 mmol) with
nitromethane (10 mmol) catalyzed by 3 (5 mol%) and NaOAc (10 mol%) in dry EtOH (3
mL) at room temperature for 6 h gave the corresponding 2-nitro-1-(4-nitrophenyl)ethanol
in 85% yield. However, the enantioselectivity again was very low (Scheme 12). In
absence of NaOAc, no product formation was observed even after 24 h of stirring. It is
reasonable to assume that the additive NaOAc creates unsaturation at the copper
centers and thus allows the reaction to proceed.
Scheme 12. Nitroaldol (Henry) reaction catalyzed by 3.
19

Summary and Concluding Remarks
Oxazoline and camphor-pyrazole derived chiral 1,8-naphthyridine ligands are
synthesized and characterized by spectroscopic techniques. Several di- and
tetranuclear complexes are accessed by treating Cu(I) or Ag(I) salts with these ligands.
All complexes are fully characterized including X-ray crystallography. Each
naphthyridine-derived ligand displays bridge-chelate coordination behavior and holds
two metal centers in close proximity. Use of metal-halide precursors or addition of
external halides fulfills the metal coordination sites and thus allows easy isolation of the
metal complexes. These complexes are found to be catalytically inactive for a number
of organometallic transformations. Coordinative saturation at the metal centers blocks
the substrate approach and thus impedes the activity. A combination of Cu salt/ligand
was found to catalyze cyclopropanation of styrene, N-H insertion and nitroaldol (Henry)
reactions affording good yields. However, chiral side arms on the naphthyridine scaffold
failed to afford noticeable enantioselectivity. Clearly, the choice of metal precursors,
ligand geometry and reaction conditions need to be probed carefully to achieve better
activity and enantioselectivity. The purpose of this report is to draw attention on
naphthyridine based chiral ligands for asymmetric transformations on a dimetal
construct.
Experimental Section
General Procedures
All reactions were carried out under nitrogen atmosphere with the use of standard
Schlenk-line techniques unless stated otherwise and at room temperature (30 °C).
20

Glass wares were flame–dried under vacuum prior to use. Solvents were dried by
1
13
conventional methods prior to use. H and C NMR spectra were obtained on a JEOL
JNM–LA 500 MHz and JEOL JNM–LA 400 MHz spectrometer. Chemical shift values
were referenced to the residual signals of the deuterated solvents. ESI–MS were
recorded on a Waters Micro mass Quattro Micro triple-quadruple mass spectrometer.
Infrared spectra were recorded on a Bruker Vertex 70 FTIR spectrophotometer in the
ranges from 400 to 4000 cm-1. UV–Vis spectra were recorded using a JASCO V–670
UV–Vis absorption spectrophotometer. Emission spectra were recorded using a
Fluorolog FL3-21(Horiba Jobin Yvon) spectrofluorometer equipped with a xenon flash
lamp and also using a PTI Quanta Master Model QM-4 scanning spectrofluorometer
equipped with a 75-watt xenon lamp, emission and excitation monochromators,
excitation correction unit, and a PMT detector for both visible and NIR regions. The GC-
MS experiments were performed on an Agilent 7890A GC and 5975C MS. HPLC
analyses were performed on an Agilent 1200 series HPLC system using a Daicel chiral
column.
Materials
2-Aminonicotinaldehyde25 and optically active amino alcohols such as (S)-
phenylglycinol, (S)-phenylalaninol were prepared according to the literature
procedures.[38] 2,6-Diaminopyridine, (dl)-malic acid, KH (30% suspension in mineral
oil), SeO2, TBABr, TBAI, NaNO2, PCl5, hydrazine hydrate were purchased from Sigma-
Aldrich. Metal precursors, such as [Cu(CH3CN)4](OTf)[39], Ag(OTf)[40] were prepared
following the literature procedure reported earlier and CuI was purchased from Sigma-
21

Aldrich. Oxalyl chloride, SOCl2, POCl3, and methyl formate were purchased from local
supplier and distilled before use.
Syntheses of oxazoline based ligands L1 and L2
1,8-Naphthyridine-2-carbonyl chloride. To a suspension of 1,8-naphthyridine-2-
carboxylic acid (2 g, 0.011 mol) in dry benzene (50 mL), was added oxalyl chloride (1.4
mL, 0.017 mol) drop wisely at 0 ºC. Then few drops of DMF were added and the
reaction began immediately. It was then slowly brought to room temperature and heated
at 65ºC for 3 h until the gas evolution subsided. The solvent and excess oxalyl chloride
was removed under reduced pressure to afford a light yellow residue of 1,8-
Naphthyridine-2-carbonyl chloride in almost quantitative yield which was directly used
for the next step without further purification.
(S)-N-(2-hydroxy-1-phenylethyl)-1,8-naphthyridine-2-carboxamide.
(S)-
Phenylglycinol (1.51 g, 0.011 mol) was dissolved in dry THF (100 mL) and the solution
was chilled to 0°C. Then, triethylamine (3.9 mL, 0.028 mol) was added to the solution.
Subsequently, a suspension of 1,8-naphthyridine-2-carbonyl chloride (prepared in the
previous step) in dry THF was added portion wise during 30 minutes. Then the mixture
was slowly brought to room temperature and stirred at room temperature for additional
24 h. After that it was evaporated, taken in dichloromethane and extracted with
saturated aqueous sodium bicarbonate solution. After further extraction of the aqueous
phase with dichloromethane, the combined organic phases were dried over anhydrous
Na2SO4 and evaporated again to get a brown residue. The residue was then purified by
silica gel column chromatography using ethyl acetate/petroleum ether as eluent to
obtain (S)-N-(2-hydroxy-1-phenylethyl)-1,8-naphthyridine-2-carboxamide as colorless
22

powder. Yield: 2.76 g (82 %). 1H NMR (500 MHz, CDCl3):  = 9.17 (br, 1H), 9.04 (d, J =
6.3 Hz, 1H), 8.35 (br, 2H), 8.29 (d, J = 7.2 Hz, 1H), 7.58–7.61 (m, 1H), 7.42–7.45 (m,
2H), 7.30–7.33 (m, 2H), 7.23–7.26 (m, 1H), 5.33–5.37 (m, 1H), 4.06–4.12 ppm (m, 2H);
13C NMR (126 MHz, CDCl3):  = 164.1, 154.3, 153.8, 152.9, 139.0, 138.9, 137.8, 128.9,
127.9, 127.1, 124.3, 123.4, 120.6, 66.3, 56.7 ppm; IR (KBr):  = 3375 (s), 3293 (br, m),
3057 (w), 3024 (w), 2925 (w), 1675 (vs), 1601 (s), 1531 (vs), 1491 (vs), 1446 (m), 1423
(m), 1336 (w), 1295 (w), 1249 (w), 1162 (w), 1063 (s), 848 (m), 792 cm–1 (s); MS (ESI;
CH3CN): m/z 294.1247 [M+H]+, 316.1075 [M+Na]+.
(S)-N-(1-hydroxy-3-phenylpropan-2-yl)-1,8-naphthyridine-2-carboxamide. Synthetic
procedure was same as for (S)-N-(2-hydroxy-1-phenylethyl)-1,8-naphthyridine-2-
carboxamide with 1,8-naphthyridine-2-carboxylic acid (2 g, 0.011 mol) and (S)-2-amino-
1
3-phenylpropan-1-ol (1.66 g, 0.011 mol). Yield: 2.80 g (80 %). H NMR (500 MHz,
CDCl3):  = 9.18 (d, J = 3.1 Hz, 1H), 8.68 (d, J = 8.3 Hz, 1H), 8.38 (d, J = 8.3 Hz, 1H),
8.35 (d, J = 8.3 Hz, 1H), 8.30 (d, J = 7.4 Hz, 1H), 7.60–7.62 (m, 1H), 7.24–7.31 (m, 4H),
7.16–7.18 (m, 1H), 4.42–4.49 (m, 1H), 3.87 (dd, J = 11.6 Hz, 3.7 Hz, 1H), 3.77 (dd, J =
13
11.3 Hz, 5.5 Hz, 1H), 3.02–3.10 ppm (m, 2H); C NMR (126 MHz, CDCl3):  = 163.9,
154.1, 153.6, 153.1, 139.0, 138.1, 138.0, 129.4, 128.6, 126.6, 124.3, 123.4, 120.6, 63.8,
53.8, 37.3 ppm; IR (KBr):  = 3373 (s), 3299 (br, m), 3060 (w), 3027 (w), 2937 (w), 1662
(vs), 1603 (s), 1526 (vs), 1492 (vs), 1454 (m), 1421 (m), 1368 (w), 1296 (w), 1253 (w),
1162 (w), 1046 (s), 859 (m), 796 cm–1 (s); MS (ESI, CH3CN): m/z 308.1393 [M+H]+,
330.1212 [M+Na]+.
(S)-N-(2-chloro-1-phenylethyl)-1,8-naphthyridine-2-carboxamide. To a solution of
(S)-N-(2-hydroxy-1-phenylethyl)-1,8-naphthyridine-2-carboxamide (2 g, 6.82 mmol) in
23

dry 1,2-dichloroethane (50 mL), freshly distilled SOCl2 (5 mL, 68.2 mmol) was added
drop wise at room temperature and the resulting mixture was refluxed for 3 h until the
gas evolution subsided. Then, it was cooled to room temperature and solvent and
excess SOCl2 were removed under reduced pressure. The residue was taken in
dichloromethane and extracted cautiously with saturated aqueous solution of Na2CO3.
The combined organic phase was dried over anhydrous Na2SO4 and evaporated to
obtain a brown residue which was purified by silica gel column chromatography using
50% ethyl acetate/petroleum ether as eluent. A colorless crystalline product was
1
obtained. Yield: 1.62 g (76 %). H NMR (500 MHz, CDCl3):  = 9.20–9.22 (m, 1H), 8.90
(d, J = 7.7 Hz, 1H), 8.45 (dd, J = 8.3 Hz, 2Hz, 1H), 8.40 (dd, J = 8.3 Hz, 2.3 Hz, 1H),
8.30 (d, J = 8 Hz, 1H), 7.59–7.62 (m, 1H), 7.45–7.47 (m, 2H), 7.36–7.39 (m, 2H), 7.30–
7.33 (m, 1H), 5.54–5.58 (m, 1H), 3.97–3.99 ppm (m, 2H); 13C NMR (126 MHz, CDCl3): 
= 163.5, 154.6, 154.2, 152.4, 139.1, 138.3, 137.2, 128.9, 128.1, 126.9, 124.4, 123.1,
120.4, 54.9, 46.8 ppm; IR (KBr):  = 3371 (s), 3056 (w), 3026 (w), 2958 (w), 1670 (vs),
1603 (s), 1520 (vs), 1492 (vs), 1450 (m), 1373 (w), 1346 (w), 1293 (w), 1232 (w), 1130
(w), 862 (m), 793 (m), 700 cm–1 (s); MS (ESI, CH3CN): m/z 312.0909 [M+H]+.
(S)-N-(1-chloro-3-phenylpropan-2-yl)-1,8-naphthyridine-2-carboxamide. Same as
for (S)-N-(2-chloro-1-phenylethyl)-1,8-naphthyridine-2-carboxamide with (S)-N-(1-
hydroxy-3-phenylpropan-2-yl)-1,8-naphthyridine-2-carboxamide (2 g, 6.51 mmol) and
SOCl2 (4.7 mL, 65.1 mmol). Yield: 1.6 g (75%). 1H NMR (500 MHz, CDCl3):  = 9.23 (d,
J = 4.6 Hz, 1H), 8.60 (d, J = 8.9 Hz, 1H), 8.44 (dd, J = 7.9 Hz, 2.8 Hz, 1H), 8.40 (dd, J =
8.5 Hz, 3 Hz, 1H), 8.34 (d, J = 8 Hz, 1H), 7.63–7.65 (m, 1H), 7.28–7.32 (m, 4H), 7.20–
7.23 (m, 1H), 4.67–4.72 (m, 1H), 3.77 (dd, J = 11.3 Hz, 4 Hz, 1H), 3.62 (dd, J = 11.3 Hz,
24

4.9 Hz, 1H), 3.06–3.16 ppm (m, 2H); 13C NMR (126 MHz, CDCl3):  = 163.4, 154.2,
153.7, 152.8, 139.1, 138.1, 137.0, 129.4, 128.8, 126.9, 124.5, 123.5, 120.6, 51.9, 46.3,
37.8 ppm; IR (KBr):  = 3373 (s), 3060 (w), 3028 (w), 2958 (w), 1678 (vs), 1603 (s),
1520 (vs), 1491 (vs), 1448 (m), 1420 (w), 1345 (w), 1293 (w), 1235 (w), 1157 (w), 1128
(w), 860 (m), 792 (s), 701 cm–1 (s); MS (ESI, CH3CN); m/z 326.1068 [M+H]+.
(S)-2-(1,8-naphthyridin-2-yl)-4-phenyl-4,5-dihydrooxazole (L1). To a solution of (S)-
N-(2-chloro-1-phenylethyl)-1,8-naphthyridine-2-carboxamide (1.6 g, 5.13 mmol) in dry
ethanol (50 mL), 5.6 mL of 1(N) ethanolic NaOH (5.6 mmol) solution was added drop
wise and the solution was refluxed for 3 h under N2 atmosphere. Then the solvent was
removed by rotary evaporation and the residue was passed through a column packed
with silica gel using 3% MeOH/DCM as eluent. A nice white powder was obtained.
Yield: 1.2 g ,(85 %). 1H NMR (500 MHz, CDCl3):  = 9.21 (dd, J = 4.3 Hz, 2 Hz, 1H; NP),
8.40 (d, J = 8.3 Hz, 1H; NP), 8.29 (d, J = 8.3 Hz, 1H; NP), 8.24 (dd, J = 8.1 Hz, 2 Hz,
1H; NP), 7.54–7.57 (m, 1H; NP), 7.34–7.39 (m, 5H; Ph), 5.51 (dd, J = 10.3 Hz, 8.6 Hz,
13
1H; CH), 4.97 (t, 8.6 Hz, 1H; CH2), 4.47 ppm (t, J = 8.6 Hz, 1H; CH2); C NMR (126
MHz, CDCl3):  = 164.2, 155.5, 154.5, 149.9, 141.7, 138.1, 136.8, 129.0, 128.0, 126.9,
123.8, 123.3, 122.2, 75.6, 70.5 ppm; IR (KBr):  = 3440 (br), 3043 (w), 3023 (w), 3010
(w), 2960 (w), 2924 (w), 2896 (w), 2856 (w), 1638 (s), 1600 (s), 1549 (m), 1496 (m),
1448 (m), 1368 (s), 1334 (m), 1260 (m), 1124 (s), 1097 (vs), 1034 (m), 967 (m), 865 (s),
804 cm–1 (s); MS (ESI, CH3CN): m/z 276.1136 [M+H]+; [α]D25 = –175.4 (c 0.33, CHCl3).
(S)-4-Benzyl-2-(1,8-naphthyridin-2-yl)-4,5-dihydrooxazole (L2). Same as for (S)-N-(2-
chloro-1-phenylethyl)-1,8-naphthyridine-2-carboxamide
with
(S)-N-(1-chloro-3-
phenylpropan-2-yl)-1,8-naphthyridine-2-carboxamide (1.6 g, 4.91 mmol) and 5.4 mL of
25

1
1(N) ethanolic NaOH (5.4 mmol) solution. Yield: 1.2 g ,(84 %). H NMR (500 MHz,
CDCl3):  = 9.18 (dd, J = 4.3 Hz, 2 Hz, 1H; NP), 8.30 (d, J = 8.3 Hz, 1H; NP), 8.27 (d, J
= 8.3 Hz, 1H; NP), 8.22 (dd, J = 8.3 Hz, 2 Hz, 1H; NP), 7.52–7.54 (m, 1H; NP), 7.21–
7.32 (m, 5H; Ph), 4.67 – 4.74 (m, 1H, CH), 4.52 (t, J = 8.6 Hz, 1H, CH2), 4.30 (t, J = 8.6
Hz, 1H; CH2), 3.28 (dd, J = 13.7 Hz, 5.4 Hz, 1H; CH2), 2.81 ppm (dd, J = 13.7 Hz, 8.9
Hz, 1H; CH2); 13C NMR (126 MHz, CDCl3):  = 163.6, 155.5, 154.6, 150.0, 138.0, 137.8,
136.8, 129.4, 128.7, 126.8, 123.7, 123.2, 122.0, 72.8, 68.4, 41.8 ppm; IR (KBr):  =
3406 (br), 3044 (w), 3024 (w), 3010 (w), 2956 (w), 2922 (w), 2910 (w), 1637 (vs), 1600
(vs), 1552 (m), 1495 (s), 1451 (m), 1370 (s), 1295 (m), 1260 (m), 1232 (m), 1119 (s),
1098 (s), 1089 (s), 1065 (m), 1037 (m), 968 (s), 863 (s), 771 cm–1 (s); MS (ESI,
CH3CN); m/z 290.1290 [M+H]+; [α]D25 = –61.8 (c 0.33, CHCl3).
Synthesis of L3
(1R,4S)-3-Hydroxymethylenecamphor and (4S,7R)-7,8,8-trimethyl-4,5,6,7-tetrahydro-
4,7-methano-2-indazole were prepared according to the literature procedure starting
from
(+)-camphor.27
2-Amino-7-hydroxy-1,8-naphthyridine,
2,7-dihydroxy-1,8-
naphthyridine and 2,7-dichloro-1,8-naphthyridine were also prepared according to the
literature procedures known but with little modification.26
2-Amino-7-hydroxy-1,8-naphthyridine. 2,6-Diaminopyridine (20 g, 0.18 mol) and (dl)-
malic acid (27 g, 0.2 mol) were placed into a 500 mL three-neck round bottomed flask
fitted with an addition funnel, thermometer, and mechanical stirrer. The stirred mixture
was cooled in an ice-water bath and concentrated sulfuric acid (100 mL) was added
drop-wise. The rate of addition was controlled to maintain an internal temperature ≤ 45
°C during the addition. Next, the addition funnel was replaced by a reflux condenser and
26

the slurry was heated to an internal temperature of 110°C for 3 h. The solution was
transferred to a 1L beaker, cooled to 0°C by an ice-water bath and was made basic (pH
= 8) by careful addition of NH4OHaq (300 mL). The crude brown solid was collected by
vacuum filtration and was washed on the filter-paper with water (2 L). The solid was
triturated with 1:9 (v:v) water: methanol and again collected by vacuum filtration,
1
affording 7-amino-2-hydroxy-1,8-naphthyridine as a tan powder. Yield: 25 g (86%). H
NMR (500 MHz, DMSO-d6): δ = 11.91 (br s, 1H), 7.65 (d, J = 9.5, 1H), 7.65 (d, J = 8,
1H), 7.03 (br s, 2H), 6.34 (d, J = 8.5, 1H), 6.12 ppm (d, J = 9.2, 1H); 13C NMR (126
MHz; DMSO-d6): δ = 163.8, 160.6, 150.4, 139.8, 137.4, 114.8, 105.3, 105.0 ppm; MS
(ESI; CH3OH): m/z 162.0664 ([M+H]+).
2,7-Dihydroxy-1,8-naphthyridine. 2-Amino-7-hydroxy-l,8-naphthyridine (10 g, 62
mmol) was ground to a fine powder and added to concentrated sulfuric acid (80 mL),
and then sodium nitrite (5 g, 74mmol) was added. The mixture was allowed to stand for
10 min, poured over crushed ice, and allowed to stand for 10 min. Excess acid was
neutralized with sodium carbonate, and then the solution was acidified with glacial
acetic acid (pH = 3), giving 2,7-dihydroxy-1,8-naphthyridine as a brown powder. Yield =
8 g (80%). 1H NMR (500 MHz, DMSO-d6): δ = 7.54 (d, J = 8.9 Hz, 2H), 5.98 (d, J = 9.1
13
Hz, 2H), 3.31 ppm (brs, 2H); C NMR (126 MHz; DMSO-d6): δ = 166.2, 149.8, 139.4,
112.6, 102.0 ppm; MS (ESI; CH3OH): m/z 163.05 ([M+H]+).
2,7-Dichloro-1,8-naphthyridine. A mixture of 2,7-dihydroxy-l,8-naphthyridine (6 g, 37
mmol), phosphorus pentachloride (15.4 g, 74 mmol) and freshly distilled phosphorus
oxychloride (100 mL) were refluxed at 100 ºC for 2 h. After that, excess POCl3 was
removed by distillation and the remaining concentrated solution (5 mL) was poured into
27

a beaker containing crushed ice. Then, the solution was made alkaline (pH = 9) with
saturated aqueous solution of sodium carbonate. A brown precipitate was appeared and
it was collected by vacuum filtration and dried under high vacuum. This was then
extracted with chloroform (150 mL) using soxhlet extractor apparatus for two days.
Finally, the solvents were removed by rotary evaporation and dried under vacuum to
1
give a light yellow powder. Yield 5 g (68%). H NMR (500 MHz, DMSO-d6): δ = 8.58 (d,
J = 8.5 Hz, 2H), 7.76 ppm (d, J = 8.5 Hz, 2H); 13C NMR (126 MHz; DMSO-d6): δ =
170.2, 150.6, 139.8, 112.6, 102.4 ppm; MS (ESI; CH3OH): m/z 198.9834 ([M+H]+).
2,7-Bis{(4S,7R)-7,8,8-trimethyl-4,5,6,7-tetrahydro-4,7-methano-2-indazol-1-yl}-1,8-
naphthyridine (L3). (4S,7R)-7,8,8-Trimethyl-4,5,6,7-tetrahydro-4,7-methano-2-indazole
(3.7 g, 21 mmol), KOH (1.2 g, 21.4 mmol) and tetrabutyl ammonium bromide (320 mg, 1
mmol) were taken in a 250 mL RB and dissolved in dry tetrahydrofuran (100 mL) by
stirring at room temperature during 1h. Then, 2,7-dichloro-1,8-naphthyridine (2 g, 10
mmol) was added in one portion into the solution and it was refluxed under nitrogen for
12 h. The initially formed brown solution started precipitating out as the reaction
progressed. After the completion of reaction, it was cooled to room temperature and
evaporated completely by rotary evaporation. Then, it was extracted with CH2Cl2/H2O
and the organic layer was collected over anhydrous Na2SO4. Finally, the
dichloromethane layer was again evaporated to brown mass and purified by column
chromatography (Silica gel, 10% EtOAc/hexane as eluent) to obtain white powder. Yield
3.5 g (74%). 1H NMR (500 MHz, CDCl3): δ = 8.46 (s, 2H; CHPz), 8.09 (s, 4H; NP), 2.87
(d, J = 4 Hz, 2H; CH), 2.10–2.16 (m, 2H; CH2), 1.89–1.95 (m, 2H; CH2), 1.42–1.48 (m,
2H; CH2), 1.35 (s, 6H; Me), 1.27–1.32 (m, 2H; CH2), 1.00 (s, 6H, Me), 0.72 (s, 6H, Me);
28

1H NMR (400 MHz, CD3OD+CDCl3): δ = 8.29 (s, 2H; CHPz), 8.22 (d, J = 9.8 Hz, 2H;
NP), 7.97 (d, J = 8.7 Hz, 2H; NP), 2.82 (d, J = 3.6 Hz, 2H; CH), 2.09–2.17 (m, 2H; CH2),
1.88–1.95 (m, 2H; CH2), 1.33–1.39 (m, 2H; CH2), 1.28 (s, 6H; Me), 1.19–1.25 (m, 2H;
CH2), 0.96 (s, 6H; Me), 0.66 (s, 6H; Me); 13C NMR (126 MHz, CDCl3): δ = 171.2, 153.8,
138.4, 130.8, 119.9, 118.1, 111.9, 59.8, 50.6, 46.7, 33.5, 27.4, 20.7, 18.7, 10.7 ppm;13C
NMR (100 MHz, CD3OD+CDCl3): δ = 173.8, 156.5, 142.0, 133.3, 121.8, 121.0, 113.9,
62.2, 52.8, 49.5, 35.9, 29.6, 22.4, 20.6, 12.2 ppm; IR (KBr) ν = 3452 (br), 2959 (w),
2869 (w), 1598 (m), 1368 (s), 1288 (m), 932 cm–1 (m); MS (ESI; CHCl3) m/z 479.2925
([M+H]+); [α]D25 = +139.1 (c 0.33, CHCl3).
Syntheses of metal complexes
[Cu4I4(L1)2] (1)
CuI (50 mg, 0.263 mmol) was dissolved in 10 mL dry acetonitrile and 36 mg of (S)-2-
(1,8-naphthyridin-2-yl)-4-phenyl-4,5-dihydrooxazole (L1; 0.131 mmol) was added directly
to the solution at room temperature. Immediately after the addition, the color of the
solution became red and the compound started precipitating and completed within 30
minutes. Then, it was washed with diethyl ether (10 mL×3) and dried under vacuum to
afford 1 as dark red microcrystalline solid. Yield: 78 mg (90%, based on Cu). Block
shaped X-ray-quality red crystals of 1 were grown by layering ethereal solution of L1
onto an acetonitrile solution of CuI inside an 8 mm o.d. vacuum-sealed glass tube. IR
(KBr):  = 3424 (br), 3050 (w), 2924 (w), 2897 (w), 1633 (w), 1597 (s), 1553 (m), 1503
(m), 1496 (m), 1468 (m), 1449 (s), 1424 (s), 1382 (s), 1329 (m), 1266 (m), 1192 (m),
1159 (s), 1131 (m), 1049 (m), 970 (m), 942 (m), 926 (m), 859 (m), 781 (s), 698 (s). MS
(ESI; CH3CN): m/z 1182.6453 for [Cu4I3(L1)2]+, 802.9774 for [Cu2I(L1)2]+.
29