A nickel(II) thiosemicarbazonato complex: synthesis, structure, electrochemistry, and application in catalytic coupling of terminal alkynes with arylboronic acids

Article abstract of DOI:10.1007/s11243-017-0162-8

Abstract: A simple route for the synthesis of a new square-planar nickel(II) complex of 2-((3-methylthiophen-2-yl)methylene)hydrazinecarbothioamide is described. Elemental analysis, solution conductivity, and spectral measurements were used to authenticate the composition of the complex. In addition, the X-ray crystal structure of the complex was obtained. Each of the two ligands of the centrosymmetric structure is coordinated to the metal center via the azomethine N and the thioimidate S atoms, forming five-membered chelate rings. Infrared and NMR spectroscopic features of the complex are consistent with its molecular structure. The complex displays a quasi-reversible reduction with E_1/2 of ?0.83?V (ΔE_p 160?mV), indicating that the ligand system can stabilize the Ni(I) oxidation state. The complex acts as an active catalyst for the C–C coupling of aryl acetylenes with arylboronic acids containing a variety of substituents. The influence of solvent, base, reaction temperature, and catalyst loading on the C–C coupling reaction was evaluated. Graphical abstract: A new square-planar nickel(II) complex bearing 2-((3-methylthiophen-2-yl)methylene)-hydrazinecarbothioamide was synthesized and characterized by elemental analysis, FTIR, NMR, and single-crystal X-ray crystallographic technique. Cyclic voltammetric study indicates stabilization of Ni(I) oxidation state. The complex acts as an effective catalyst for the carbon–carbon cross-coupling reaction of substituted phenylacetylenes with various arylboronic acids under optimized conditions.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s11243-017-0162-8

Transit Met Chem
DOI 10.1007/s11243-017-0162-8
A nickel(II) thiosemicarbazonato complex: synthesis, structure,
electrochemistry, and application in catalytic coupling of terminal
alkynes with arylboronic acids
Rupesh Narayana Prabhu1 Jagadeesan Lakshmipraba2,3
•
Received: 14 April 2017 / Accepted: 3 July 2017
Ó Springer International Publishing AG 2017
Abstract A simple route for the synthesis of a new square-
planar nickel(II) complex of 2-((3-methylthiophen-2-
yl)methylene)hydrazinecarbothioamide is described. Ele-
mental analysis, solution conductivity, and spectral mea-
surements were used to authenticate the composition of the
complex. In addition, the X-ray crystal structure of the
complex was obtained. Each of the two ligands of the
centrosymmetric structure is coordinated to the metal
center via the azomethine N and the thioimidate S atoms,
forming five-membered chelate rings. Infrared and NMR
spectroscopic features of the complex are consistent with
its molecular structure. The complex displays a quasi-re-
versible reduction with E1/2 of -0.83 V (DEp 160 mV),
indicating that the ligand system can stabilize the
Ni(I) oxidation state. The complex acts as an active catalyst
for the C–C coupling of aryl acetylenes with arylboronic
acids containing a variety of substituents. The influence of
solvent, base, reaction temperature, and catalyst loading on
the C–C coupling reaction was evaluated.
Introduction
The diaryl acetylene unit is a prevalent motif in numerous
natural products, pharmaceuticals, conjugated oligomers,
polymers, dendrimers, optical materials, etc. [1–4]. The
traditional palladium-catalyzed Sonogashira reaction of
substituted phenylacetylene with aryl halides in the pres-
ence of CuI as co-catalyst has emerged as a powerful and
straightforward method for the construction of arylated
internal alkyne compounds [5–9]. However, copper salts
can induce homo-coupling of terminal alkynes to yield
diynes, which are difficult to separate from the desired
products due to their similar chromatographic mobility.
Moreover, potentially explosive copper acetylides can also
be formed. Hence, the development of new catalytic sys-
tems and modifications of the Sonogashira protocol has
received considerable attention. Transition-metal-catalyzed
oxidative coupling of terminal alkynes with arylboronic
acids has emerged as an alternative for the traditional
Sonogashira reaction. This C–C cross-coupling reaction
offers many advantages, such as mild reaction conditions,
commercial availability or ease of preparation of a large
number of air- and water-stable boronic acid derivatives,
easy handling, easy removal of boron residues, low toxicity
of reagents and by-products, and high functional group
tolerance, thus explaining the increasing interest in this
methodology. In recent years, Pd-, Cu-, Au-, Fe-, and Ni-
mediated [10–21] coupling of arylboronic acids with ter-
minal acetylenes to yield diaryl acetylenes has emerged as
an alternative approach for the Sonogashira reaction.
Although interesting results have been obtained, there is
still a need for the development of milder protocols for this
transformation.
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-017-0162-8) contains supplementary
material, which is available to authorized users.
& Rupesh Narayana Prabhu
rupesh@andavancollege.ac.in
1
Department of Chemistry, Srimad Andavan Arts and Science
College, Tiruchirappalli 620 005, Tamil Nadu, India
2
School of Chemistry, University of Hyderabad,
Hyderabad 500 046, Telangana, India
3
Thiosemicarbazones are strongly chelating nitrogen and
sulfur donor ligands for transition metals. They are
Present Address: Department of Chemistry, Bishop Heber
College, Tiruchirappalli 620 017, Tamil Nadu, India
123

Transit Met Chem
versatile ligands with mixed hard–soft donor character, can
be functionalized in various ways, possess synthetic flexi-
bility, and exhibit diverse coordination modes in their
metal complexes [22–28]. At present, there are only a few
reports on the synthetic utility of Ni(II) thiosemicar-
bazonato complexes as catalysts for various cross-coupling
reactions [29–34]. Moreover, to the best of our knowledge,
the use of Ni(II) thiosemicarbazonato complexes as cata-
lysts for the coupling of terminal alkynes with arylboronic
acids has not been explored so far. Herein, a new square-
planar thiosemicarbazonato nickel(II) complex of 2-((3-
methylthiophen-2-yl)methylene)hydrazinecarbothioamide
(HL) was prepared and evaluated as a homogeneous cata-
lyst in the synthesis of substituted diaryl acetylenes by the
oxidative coupling of phenylacetylene with arylboronic
acids containing a variety of substituents. The effects of
solvent, base, reaction temperature, and catalyst loading on
the yields were also investigated.
reaction mixture was filtered, and the filtrate was allowed
to stand for 3–5 days at room temperature. The dark brown
crystals so formed were filtered off, washed with diethyl
ether, and dried under vacuum.
[Ni(L)2] (1): color: brown; yield: 39 mg, 86%; Anal.
Calc. for C14H16N6NiS4: C, 36.93; H, 3.54; N, 18.46; S,
28.17%. Found: C, 36.83; H, 3.59; N, 18.56; S, 28.11%.
Selected IR data (KBr, cm-1): 3385 m(NHR), 1590
1
m(C=N), 1173 m(N–N), 743 m(C–S). H NMR (400 MHz,
DMSO-d6) (d ppm): 9.63 (s, 4H, NH2), 8.92 (s, 2H,
CH=N), 7.55 (d, 2H, J = 4.4 Hz, aromatic), 6.94 (d, 2H,
J = 4.8 Hz, aromatic), 2.27 (s, 6H, CH3). 13C NMR
(100 MHz, DMSO-d6) (d ppm): 167.91 (C–S), 152.34
(C=N), 147.42, 137.59, 134.35, 131.79, 14.18.
X-ray crystallography
Single crystals of the complex, suitable for X-ray diffrac-
tion analysis, were obtained by slow evaporation of the
reaction mixture (ethanol-dichloromethane) at room tem-
perature. The data collection was carried out using a Bru-
ker AXS Kappa APEX II single-crystal X-ray
diffractometer with monochromated Mo Ka radiation
Experimental
Materials and methods
˚
(k = 0.71073 A) at 293 K. The unit cell parameters were
Ni(OAc)2Á4H2O was purchased from Loba Chemie Pvt.
Ltd, and used as received. 3-Methyl-thiophene-2-carbox-
aldehyde, thiosemicarbazide and the substrates for the
coupling reactions were purchased from Sigma-Aldrich
and used as received. All other chemicals and solvents
were of commercial reagent grade and used as supplied.
Microanalyses (C, H, N, S) were obtained with a
Thermo Finnigan Flash EA1112 series elemental analyzer.
A Digisun DI-909 conductivity meter was used to measure
the solution electrical conductivity. Infrared spectra were
recorded in KBr pellets with a Perkin-Elmer 597 spec-
trophotometer in the range 4000–400 cm-1. NMR spectra
were recorded with a Bruker 400 MHz spectrometer, and
the chemical shifts are given in ppm referenced to the
deuterated solvent. Cyclic voltammetric experiments with
dimethylformamide solutions of the complexes (10-3 M)
containing 0.1 M tetrabutylammonium perchlorate (TBAP)
as the supporting electrolyte were performed using a CH
Instruments model 620A electrochemical analyzer. The
three-electrode measurements were carried out at 298 K
under a dinitrogen atmosphere with a glassy carbon
working electrode, a platinum wire auxiliary electrode, and
an Ag/AgCl reference electrode.
determined by the method of difference vectors using
reflections scanned from three different zones of the
reciprocal lattice. The intensity data were measured using
x and u scan with a frame width of 0.5°. Frame integration
and data reduction were performed using the Bruker
SAINT–Plus (version 7.06a) software [35]. The absorption
corrections were performed by multi-scan methods using
SADABS software [36]. Corrections were made for Lor-
entz and polarization effects. The structure was solved by
SIR92 [37] and refined by full-matrix least squares on F2
using SHELXL 97 [38]. All non-hydrogen atoms were
refined anisotropically, and the hydrogen atoms were
located from the difference Fourier map and constrained to
the ideal positions in the refinement procedure. The Platon
[39] and the Mercury [40] packages were used for
molecular graphics. The summary of the data collection
and refinement parameters is given in Table 1.
Procedure for the oxidative coupling reaction
In an oven-dried round-bottom flask, a mixture of aryl-
boronic acid (1.0 mmol), phenylacetylene (1.2 mmol),
complex 1 (5.0 mol%), and 1,8-diazabicycloundec-7-ene
(DBU) (2.0 mmol) in DMF (5 mL) was taken. The reaction
mixture was stirred at 100°C in air for 5 h. At the end of
this time, the reaction mixture was diluted with EtOAc
(20 mL) and washed with water (3 9 10 mL). The organic
layer was dried over anhydrous Na2SO4, filtered and the
solvent was stripped off under reduced pressure. The
Synthesis of complex 1
To
a
warm solution of Ni(OAc)2Á4H2O (25 mg,
0.10 mmol) in ethanol (10 mL) was added a solution of HL
(39.8 mg, 0.20 mmol) in dichloromethane (15 mL). The
123

Transit Met Chem
Table 1 Selected crystal data and structure refinement summary of 1
Chemical formula
Formula weight
Crystal system
Space group
C14H16N6NiS4
455.28
Monoclinic
P21/c
˚
a (A)
8.1529 (17)
7.4129 (16)
15.575 (3)
90.00
˚
b (A)
˚
c (A)
Scheme 1 Synthesis of the thiosemicarbazonato complex
a (°)
b (°)
c (°)
99.860 (3)
90.00
producing intense brown solutions. The presence of a
methyl group on the ligand is expected to slightly enhance
the solubility of the complex in organic solvents. The
complex is electrically non-conducting in solution, which
is consistent with its formulation as a neutral species. It is
non-hygroscopic and air stable in both the solid and the
liquid states at room temperature.
3
˚
Volume (A )
927.4 (3)
2
Z
q (g cm-3
l (mm-1
)
1.630
)
1.507
Reflections collected
Reflections unique
9022
1825
Reflections [I C 2r(I)]
Data/restraints/parameters
R1, wR2 [I C 2r(I)]
Goodness–of–fit on F2
1476
1825/0/116
0.0469, 0.1157
1.091
Spectroscopic characterization
The mode of coordination of L- to the nickel(II) center in
the complex was ascertained using FTIR spectroscopy. The
IR spectrum of free HL displays a m(C=N) absorption at
1605 cm-1, which is observed at a lower frequency for
complex 1 (1590 cm-1), suggesting the coordination of
azomethine N to the metal. This is further supported by an
increase in the stretching frequency of the N–N group in
complex 1 (1173 cm-1) when compared to free HL
-3
˚
Max./Min. peaks (e A
)
0.928/-0.240
residue was subjected to column chromatography on silica
gel using ethyl acetate and n-hexane mixture to afford the
1
desired product. The products were characterized by H
and 13C NMR analysis.
(1124 cm-1). The m(C=S) (847 cm-1
(3205 cm-1
frequencies of the N–NH–C=S group,
) and m(N–H)
)
observed for free HL, are absent from the spectrum of the
complex. This observation may be attributed to the eno-
lization of N–NH–C=S and subsequent coordination to the
metal through the thioimidate S [33, 34].
Results and discussion
Synthesis and properties of the complex
The coordination of L- to the metal center in the
1
complex was further probed by H NMR and 13C NMR
The Schiff base, 2-((3-methylthiophen-2-yl)methylene)hy-
drazinecarbothioamide (HL, where H represents the dis-
sociable proton), was prepared in *90% yield via
condensation of one mole equivalent each of thiosemicar-
bazide and 3-methyl-thiophene-2-carboxaldehyde, accord-
1
studies. The H NMR spectrum shows a downfield shift of
the azomethine protons for complex 1 (d 8.92 ppm) when
compared to free HL (d 8.20 ppm), suggesting deshielding
of the –CH=N– proton on coordination of the azomethine
N to Ni(II). The singlet from the N–NH–C=S proton of free
HL (d 11.30 ppm) is absent from the spectrum of complex
1, supporting enolization and coordination of the thioimi-
date S to the metal center [33, 34]. The chemical shifts of
the other protons of the coordinated ligand are present in
the expected regions. The 13C NMR spectrum of complex 1
displayed a downfield shift of the azomethine C signal (d
152.34 ppm) relative to that of the free HL (d 140.43 ppm),
indicating coordination of the azomethine N to the metal.
The signal due to the thioamide C in free HL (d
178.32 ppm) moves upfield in 1 (d 167.91 ppm), substan-
tiating enolization and subsequent coordination of the
ing to the literature procedure [33]. IR, H NMR, and 13C
1
NMR spectra were used to authenticate its identity. The
neutral square-planar Ni(II) complex, [Ni(L)2] (1), was
synthesized in *86% yield by the reaction of Ni(OAc)2-
4H2O with HL in 1:2 ratio in EtOH/CH2Cl2 (Scheme 1).
The observed elemental analysis of complex 1 is consistent
with its formulation, and it appears that each Schiff base
coordinates to the nickel(II) in a monobasic bidentate
fashion. The complex has moderate solubility in methanol,
ethanol, chloroform, dichloromethane, and acetonitrile, but
is readily soluble in polar solvents such as DMF, DMSO,
dimethylacetamide,
N-methyl-2-pyrrolidone,
etc.,
123

Transit Met Chem
Fig. 1 ORTEP plot of complex 1 with ellipsoids at 50% probability
˚
Table 2 Metal-centered bond lengths (A) and bond angles (8) in
complex 1
˚
Bond lengths (A)
Bond angles (8)
Ni–N1
Ni–S2
1.903 (3)
N1–Ni–N1a
S2–Ni–S2a
N1–Ni–S2
N2–N1–Ni
C7–N2–N1
N2–C7–S2
N1a–Ni–S2a
N1–Ni–S2a
N1a–Ni–S2
180.0 (2)
180.00 (3)
85.74 (9)
120.4 (2)
111.6 (3)
123.2 (2)
85.74 (9)
94.26 (9)
94.26 (9)
2.1703 (10)
1724 (4)
S2–C7
N2–C7
N1–N2
Ni–N1a
Ni–S2a
1.323 (5)
1.380 (4)
1.903 (3)
2.1703 (10)
ESD in parenthesis
Fig. 2 Cyclic voltammogram of complex 1
thioimidate S [23, 28]. The signals due to the other carbons
of the coordinated ligand appear in the expected regions.
ring. The ligands are in trans position with respect to the
azomethine N atoms, and the complex is centrosymmetric
around the Ni(II) center. The N(2)–C(7) and S(2)–C(7)
Single-crystal X-ray diffraction studies
˚
bond lengths are 1.323(5) and 1.724(4) A, respectively,
In order to confirm the structure of complex 1 by X-ray
crystallography, single crystals were grown. An ORTEP
view of complex 1 is shown in Fig. 1, and selected bond
parameters are listed in Table 2. The complex crystallizes
in the P21/c space group. The nickel(II) center has a
square-planar geometry, being chelated by two ligand
molecules. Each ligand coordinates via the azomethine N
and the thioimidate S, forming a five-membered chelate
consistent with deprotonation of the thioamide functional-
ity in the coordinated thiosemicarbazonate ligand
[33, 34, 41, 42].
Electrochemical study
Electrochemical studies were carried out for both free HL
and complex 1 in DMF solution under an atmosphere of N2
123

Transit Met Chem
Table 3 Optimization of reaction conditionsa
Complex 1
+
(HO)2B
Solvent, base
Entry
Solvent
Base
Temp. (°C)
Yieldb (%)
1
DMSO
NMP
DMA
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DBU
DBU
DBU
DBU
100
100
100
100
100
100
100
100
100
100
80
92
2
84
3
76
4
97
5
Piperidine
Et3N
90
6
81
7
CH3COOK
K2CO3
KHCO3
K3PO4
DBU
75
8
36
9
45
10
11
12
13
14
15
a
48
83
DBU
50
43
DBU
30
Trace
N.R.c
N.R.d
DBU
100
100
–
Phenylboronic acid (1.0 mmol), phenylacetylene (1.2 mmol), complex 1 (5 mol%), solvent (5 mL), base (2.0 mmol) in air for 5 h
b
c
d
Isolated yield after column chromatography (average of two independent runs)
Without complex 1. N.R. = No reaction
Without base
Catalytic studies
in the potential range of ?1.5 to –1.5 V. No redox wave
was observed within these potential limits for free HL, and
hence the response observed for complex 1 can be attrib-
uted to the metal center only. The complex showed a quasi-
reversible reduction (NiII ? NiI) at a scan rate of
100 mV s-1 with E1/2 of -0.83 V (Epc = -0.91 V;
Epa = -0.75 V, Fig. 2). This process is characterized by a
rather large peak-to-peak separation (DEp) of 160 mV. The
redox potentials are virtually independent of scan rates,
indicating quasi-reversibility. No oxidation curve was
observed in the potential range of 0 to ?1.5 V. Hence, the
electrochemical study indicates that the thiosemicarbazone
ligand is suitable for stabilizing the lower oxidation state of
Ni(II) but not its higher oxidation state.
A survey of the literature on the transition-metal-mediated
oxidative coupling of arylboronic acids with terminal
alkynes suggested that there is no clear-cut rule that a
specific solvent or base can be used to acquire highest
efficiency in this coupling reaction, and hence various
reaction conditions should be optimized first. Hence, the
oxidative coupling between phenylacetylene and phenyl-
boronic acid to furnish 1,2-diphenylethyne in the presence
of complex 1 as catalyst was first chosen as a model
reaction, and the solvent, base, and temperature were var-
ied sequentially to determine the optimum conditions
(Table 3). At the outset, various solvents were screened
Table 4 Effect of loading of the catalysta
Complex 1
DMF, DBU
+
(HO)2B
Entry
Complex 1 (mol%)
Yieldb (%)
1
2
3
4
a
5.0
2.0
1.0
0.5
97
90
68
39
Phenylboronic acid (1.0 mmol), phenylacetylene (1.2 mmol), complex 1 (0.5–5.0 mol%), DMF (5 mL), DBU (2.0 mmol) at 100 °C in air for 5 h
Isolated yield after column chromatography (average of two independent runs)
b
123

Transit Met Chem
Table 5 C–C coupling reactions of arylboronic acids with pheny-
lacetylenea
Table 6 C–C coupling reactions of phenylboronic acid with substi-
tuted phenylacetylenea
Complex 1
Complex 1
(HO)2B
DMF, DBU
DMF, DBU
100 oC, 5 h
+
(HO)2B
+
100 oC, 5 h
R
R
R
R
Entry
1
Product
Yieldb (%)
Entry
Product
Yieldb (%)
97
97
90
96
92
93
96
86
1
2
3
4
5
H
H
2
3
4
5
6
7
92
91
89
85
OCH3
H3CO
H3C
OHC
CH3
Cl
CHO
COCH3
H3CO
6
76
OCH3
OCH3
a
Phenylboronic acid (1.0 mmol), arylacetylene (1.2 mmol), complex
1 (5.0 mol %), DMF (5 mL), DBU (2.0 mmol) at 100°C in air for 5 h
Isolated yield after column chromatography (average of two inde-
pendent runs)
8
9
90
78
81
b
CHO
H3CO
OHC
loadings in the range of 5.0–0.5 mol% (Table 4). In the
presence of 5.0 mol% of complex 1, the desired alkyne was
obtained in 97% isolated yield after 5 h (entry 1). How-
ever, decreasing the catalyst loading from 2.0 to 0.5 mol%
(entries 2–4) considerably reduced the yield.
10
a
Arylboronic acid (1.0 mmol), phenylacetylene (1.2 mmol), com-
plex 1 (5.0 mol%), DMF (5 mL), DBU (2.0 mmol) at 100°C in air for
5 h
Under the optimal reaction conditions (DMF, DBU,
100°C, 5 h), the oxidative coupling of arylboronic acids
containing substituents of varying electronic properties
with phenylacetylene catalyzed by complex 1 was inves-
tigated in order to examine the scope of the reaction
(Table 5). The substituted arylboronic acids coupled
smoothly with phenylacetylene to give the corresponding
internal alkynes in good-to-excellent yields, when both
electron-donating (4-methoxy, 4-methyl) and electron-
withdrawing (4-chloro, 4-formyl, 4-acetyl) substituents
were present on the arylboronic acids. Arylboronic acids
with methoxy or formyl substituents on the meta position
(entries 7, 8) coupled efficiently. The presence of these
substituents at the ortho position (entries 9, 10) resulted in
slightly reduced reactivities than for the corresponding para
derivatives (entries 1, 5) which may be due to steric effects.
In addition, under the given conditions, the homo-coupling
by-products of neither the terminal alkyne nor the aryl-
boronic acid were detected. Additional coupling reactions
were then effectuated in an inverted manner: i.e., with
substituted phenylacetylenes and phenylboronic acid
(Table 6). All of these reactions also gave the desired
products in good-to-excellent isolated yields.
b
Isolated yield after column chromatography (average of two inde-
pendent runs)
(entries 1–4). The best reactivity was observed in the case
of DMF (entry 4). Notable sensitivity to base used was also
noted in DMF. After screening a variety of bases (entries
4–10), 1,8-diazabicycloundec-7-ene (DBU) was found to
give the best results. In DMF–DBU, a good isolated yield
of the desired product was obtained at 100°C (entry 4).
Lower temperatures resulted in reduced yields of the pro-
duct (entries 11–13). Hence, subsequent experiments were
carried out using DMF as solvent and DBU as base at
100°C. Further, control experiments demonstrated that the
reaction fails in the absence of complex 1 (entry 14) or the
base (entry 15), indicating that both are necessary for this
transformation. In addition, no precipitate was formed
during the course of the catalysis, arguing against the
formation of any heterogeneous species or polymerized
product of the thiophene moiety.
In order to determine the competency of complex 1 at
lower concentrations, the oxidative coupling was also
performed under the above reaction conditions at catalyst
123

Transit Met Chem
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Conclusion
In conclusion, the synthesis and characterization of a
nickel(II) thiosemicarbazonato complex have been descri-
bed. Both thiosemicarbazonate ligands coordinate through
the azomethine N and the thioimidate S atoms, giving a
distorted square-planar geometry around the Ni(II) center.
The electrochemical study indicates that the thiosemicar-
bazone ligands are able to stabilize the Ni(I) oxidation
state. Preliminary studies indicate that this complex can act
as an effective catalyst for the construction of internal
alkynes by the C–C coupling of various arylboronic acids
with substituted phenylacetylenes, giving good-to-excel-
lent isolated yields of the desired products. The scope,
mechanism, and synthetic applications of this catalytic
reaction are currently under further investigation in our
laboratory.
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Acknowledgements The use of the single-crystal diffractometer and
NMR facilities at the School of Chemistry, University of Hyderabad,
Telangana, India, is gratefully acknowledged.
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Appendix A. Supplementary data
Crystallographic data for the structural analysis have been
deposited with Cambridge crystallographic center, CCDC
No. 1502455. Copies of this information may be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: ?44 1223 336 033; or e-mail: deposit@ccdc.-
cam.ac.uk. Preparation and characterization data of HL; 1H
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1
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the coupling products. Supplementary data to this article
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