Synthesis, crystal structures and reactivity of copper(i) amidate complexes with aryl halides: Insight into copper(i)-catalyzed Goldberg reaction

Article abstract of DOI:10.1039/c2dt00050d

In this paper, copper(i) amidate complexes (2-3), proposed intermediates in copper-catalyzed Goldberg reaction, have been prepared and characterized by elemental analysis, IR, ¹H NMR and X-ray crystallography. Ancillary ligand bis(diphenylphosp

Full text of DOI:10.1039/c2dt00050d

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PAPER
Synthesis, crystal structures and reactivity of copper(I) amidate complexes
with aryl halides: insight into copper(I)-catalyzed Goldberg reaction†
Xinfang Liu,a,b Songlin Zhanga and Yuqiang Ding*a
Received 8th January 2012, Accepted 7th March 2012
DOI: 10.1039/c2dt00050d
In this paper, copper(I) amidate complexes (2–3), proposed intermediates in copper-catalyzed Goldberg
reaction, have been prepared and characterized by elemental analysis, IR, 1H NMR and X-ray
crystallography. Ancillary ligand bis(diphenylphosphino)ferrocene (dppf) has contributed greatly to the
stability of the copper–amidate complexes due to its strong chelating ability and weak intermolecular
interactions. Thermal gravimetric analyses are carried out to determine the thermal competency of
complexes 2–3 as the intermediates of the high-temperature Goldberg reactions. Reaction of complexes 2
and 3 with aryl halides generates the N-arylation products 5–8, accompanied by the formation of a copper
(I) complex Cu(dppf)X (X = I or Br) 4, which has been determined by LC-MS analysis. These results
provide new evidence for the mechanism of copper(I)-catalyzed Goldberg reaction.
is 1 : 1.7 Kinetic and spectroscopic studies from Buchwald et al.
have shown that the diamine-ligated Cu(I) amidate complex is a
Introduction
Copper-mediated amide arylation, the Goldberg reaction,1 has
chemically and kinetically competent intermediate in copper-cat-
long been an efficient and friendly method for the construction
of aryl C–N bonds in organic synthesis, pharmaceuticals, and
polymer science.2 The synthetic scope of the traditional process
remained limited for many decades because it often required
high reaction temperatures and long reaction times.3 In recent
years, ancillary ligands such as 1,2-diamines,4 1,3-diketones,5
and others6 have been used in the copper-catalyzed Goldberg
reaction, and thus great improvements have been achieved in
developing mild reaction conditions and broadening the substrate
scope.
Although increased activity and substrate scope has been
acquired in the coupling of amides with aryl halides by conduct-
ing the reactions with added ancillary ligands, further improve-
ment of the coupling reactions require us to better understand the
reaction mechanism, and thus the synthesis, isolation and charac-
terization of copper amidate complexes (see Scheme 1) and their
arylated products. So far a few studies on the reactions of copper
amidate complexes with aryl halides have been reported. Yama-
moto et al. have shown that the precursor methylcopper(I) can
react with activated N–H bonds to obtain the Cu(I) amidates,
which can react with aryl halides when the Cu–nucleophile ratio
alyzed Goldberg reaction. In these studies, the Cu(I) amidate was
observed by NMR spectroscopy but not isolated.8 Later, Hartwig
et al. isolated and fully characterized two neutral, three-coordi-
nate copper amidate complexes from the reaction of copper tert-
butoxide with ancillary ligand and phthalimide; kinetic studies
and theoretical predictions defined the neutral copper amidate
complex as the active intermediate of the N-arylation process
and introduced the relationships between the structures and reac-
tivity of proposed intermediates.9 Finally, computational studies
on the mechanism of copper catalyzed N-arylation reaction by us
and Houk demonstrate that the neutral copper amidate complex
is the most active species and the oxidative addition of the
species to aryl halides is the rate-determining step of the catalytic
cycle.10,11
Although copper amide complexes have been implicated in
the copper-catalyzed Goldberg reaction, the preparation and iso-
lation of copper complexes with a terminal amide ligand is
aThe Key Laboratory of Food Colloids and Biotechnology, Ministry of
Education, School of Chemical and Material Engineering, Jiangnan
University, Wuxi, 214122, P.R. China. E-mail: yding@jiangnan.edu.cn;
Fax: +86-510-85917763; Tel: +86-510-85917763
bCollege of Chemistry and Chemical Engineering, Luoyang Normal
University, Luoyang, 471022, P.R. China
†Electronic supplementary information (ESI) available: 1H NMR
spectra for compounds 2–3 and 5–8, HL-MS-MS spectra of compound
4. CCDC 859530–859531. For ESI and crystallographic data in CIF or
other electronic format. See DOI: 10.1039/c2dt00050d
Scheme 1 The copper catalyzed Goldberg reaction.
Dalton Trans., 2012, 41, 5897–5902 | 5897
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rare,7,10 and only two three-coordinated copper amide complexes
with definite crystal structures are reported.10 Among few
reported arylation reactions between copper amide complexes
and aryl halides, the ancillary ligands and amides are limited to
chelating N,N-donor ligands, sometimes mono-phosphine
ligands, and cycloamides. More general acyclic amides are rarely
reported. Furthermore, the solutions of the reported precursors
and the corresponding amide intermediates are sensitive to light
or air and can decompose within hours, which may make it
difficult to isolate and characterize the complexes in pure form.
In order to obtain detailed information about the copper-cata-
lyzed Goldberg reaction, we prepared two new copper(I) amidate
complexes with acyclic amide ligands and bis(diphenylpho-
sphino)ferrocene (dppf) as an ancillary ligand. The chelating
ability of the dppf ligand could provide additional stability to the
resulting copper(I) complexes.12 The detailed synthesis and full
characterization including X-ray diffraction of the copper(I)
amide complexes are reported in this paper. Furthermore,
thermal properties and reactivity studies of these complexes with
aryl halides are presented to demonstrate their competence as
intermediates in the catalytic cycle of the copper(I)-catalyzed
Goldberg reaction.
1702 cm−1), respectively, which further demonstrates the coordi-
nation reactions between the amide anions and the copper(I)
1
centers. The H NMR spectra of complexes 2–3 in CDCl3 show
the proton signals of the cyclopentadienyl groups at
4.11–4.37 ppm and signals at 6.92–7.04 ppm for the aryl rings
of the amide anions, and complex 2 shows a methyl proton
signal at 2.28 ppm (Fig. S1 and S2†).
Crystal structures of complexes 2–3
To unambiguously elucidate the structures of complexes 2–3,
single crystal X-ray diffraction analyses were performed and are
shown in Fig. 1. Crystal data for complexes 2–3 are given in
Table 1 and the selected bond distances and bond angles are
shown in Table 2. Complex 2 is coordinated by one nitrogen
atom from the amide anion and chelated by two phosphine
atoms from the dppf ligand, forming a nearly ideal trigonal
planar geometry. The sum of the angles around the copper centre
is 358.7(6)° and the copper ion is 0.144 Å above the plane
P1–P2–N, which is responsible for the slight distortion from the
ideal trigonal planar environment. The bond distance of C41–N
(1.319(5) Å) is much shorter than that of C35–N (1.443(5) Å),
which indicates the partial double bond feature of C41–N from
the electron resonance between C41–N and CvO bonds.14 The
Cu–N distance is 1.979(3) Å, which is comparable to the
reported copper(I) complexes with nitrogen-containing ligands.15
The Cu–P distances are 2.24(1) Å and 2.26(1) Å, almost equal
to those copper(I) complexes bearing dppf, dppb, dtpb and other
bidentate phosphine ligands, indicating the similar chelating abil-
ities between these ligands.16
The molecular structure of complexes 3 is very similar to that
of complex 2. The bond lengths and angles of 3 differ slightly
from those of crystalline 2 (Table 1). It is noticeable that com-
plexes 2 (Fig. 2) and 3 display strong intermolecular π–π inter-
actions between the benzene rings of two adjacent molecules
with 3.469 Å (perpendicular distance), 3.771 Å (centroid dis-
tance) and 3.522 Å and 3.802 Å, respectively.17 In addition,
weak intermolecular hydrogen bonds between the adjacent
molecules in complex 2 (C28–H28⋯O, 3.326 Å, 154.4°;
C22–H22⋯O, 3.472 Å, 146.4°) and 3 (C32–H32⋯O, 3.295 Å,
148.3°; C22–H22⋯O, 3.475 Å, 148.1°; C16–H16⋯Cl, 3.638 Å,
139.3°) play an important role in stabilising the complexes.
Results and discussion
Synthesis and characterization of complexes 2–3
The synthesis of Cu(I)–amidate complexes 2–3 is outlined in
Scheme 2. Strong base such as CH3ONa is necessary for depro-
tonation of the weak acidic amidates to coordinate with the
copper(I) centre. However, strong bases are not good candidates
in copper-catalyzed Goldberg reactions because an excess of the
deprotonated amide may be formed and impede the desired aryl
amidation reaction via formation of an unreactive copper(I)
complex (Scheme 1).2a,13 The presumably unpleasant result has
been avoided by the careful selection of precursor 1 and dichlor-
omethane solvent. The reaction is carried out in dichloromethane
because CH3ONa dissolves slowly in this solvent, allowing the
base to be delivered gradually as the reaction proceeds, and thus
the rate of deprotonation of the amide matches the rate of the
amidation reaction.8 Meanwhile, the steric hindrance of the
bulky dppf ligand makes it difficult to further coordinate
between complexes 2–3 and excess amide anions. Another
benefit of the dppf ligand is that its redox-active group and che-
lating ability makes complexes 1–3 synthetically accessible and
isolable as stable compounds in air even for months.
Thermal properties
The IR spectra show a clear blueshift of 71 and 66 cm−1
between the carbonyl stretching frequency of complex 2–3
(1633 and 1636 cm−1) and those of the free amide (1704 and
The Goldberg reaction occurs mainly at 100–200 °C, so we were
interesting to see if the copper(I) intermediates were stable in the
high-temperature catalytic reactions. In order to determine the
Scheme 2 The synthesis route of complexes 2–3.
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5898 | Dalton Trans., 2012, 41, 5897–5902

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Fig. 1 ORTEP diagrams of complex 2 (a) and 3 (b) with thermal ellipsoids shown at the 30% probability level. Hydrogen atoms are omitted for
clarity.
Table 1 Crystal data and refinement parameters of complexes 2–3
Complexes
2
3
Empirical formula
Formula weight
Crystal system
Space group
a/Å
b/Å
c/Å
C43H35CuF3FeNOP2 C42H32ClCuF3FeNOP2
820.05
Monoclinic
P21/n
840.47
Monoclinic
P21/n
9.5178
22.194
17.301
92.38(3)
3651.4(1)
4
9.5154(2)
22.267(5)
17.178(3)
92.80(3)
3635.2(1)
4
β/°
V/Å3
Z
Dc/mg m−3
1.492
1.119
1.536
1.197
μ/mm−1
Fig. 2 The intermolecular π–π interactions in complex 2.
θ range
3.02–27.47
14 866/6136
3.02–27.48
20 645/6927
Reflections collected/
unique [R(int)]
F (000)
1680
1712
R1, wR2 [I > 2δ(I)]
GOF
0.0726, 0.1239
1.148
0.0717, 0.1169
1.168
Table 2 Selected bond distances (Å) and bond angles (°) for 2 and 3
Bond distances/angles
Complex 2
Complex 3
Cu–N
Cu–P1
1.979(3)
2.26(1)
1.981(3)
2.25(1)
Cu–P2
C35–N
N–C41
N–Cu–P2
N–Cu–P1
P2–Cu–P1
2.24(1)
2.23(1)
1.443(5)
1.319(5)
126.8(1)
119.3(1)
112.6(4)
1.429(4)
1.310(4)
126.1(8)
119.6(8)
113.0(4)
Fig. 3 TG curves of complexes 2–3.
thermal stabilities of complexes 2–3, pyrolysis experiments were
carried out by means of TG and the thermal curves were illus-
trated in Fig. 3. Complex 2 is stable from 50–230 °C indicating
its thermal competency to high-temperature arylation reactions.
Complex 2 gradually decomposes from 230–537 °C, and the
remainders are copper oxide and ferric oxide, which account for
22.10% (calcd 22.84%). The thermal behavior of complex 3 is
similar to 2; it decomposes from 240–542 °C, and the remain-
ders (22.52%, calcd 22.28%) are assigned to copper oxide and
ferric oxide. The TG experiments show that complexes 2–3 are
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Scheme 3 N-arylation reactions between complexes 2–3 and aryl halides.
MS (+): 617, Fig. S7†) with those of complexes containing the
[Cu(dppf)]+ fragment.18
Table 3 Reactions of complexes 2–3 with aryl halides
Entry Complex Aryl halide T (°C) Solvent
t/h Yielda (%)
Both copper(I) amidate complexes and the N-arylation pro-
ducts contain fluorine, so it is convenient to follow the reaction
process by 19F NMR. Herein we selected the reaction of
complex 2 with iodobenzene as a representative to investigate
the cycle in Scheme 1. The 19F NMR spectrum shows a single
signal at −66.21 ppm at the beginning of the reaction (Fig. S8†),
which is assigned to the fluorine atom in complex 2. As the reac-
tion occurs in 60 min, there appears a new signal at −69.65 ppm
(Fig. S8†), which is exactly the same value as that for the coup-
ling product 5. Additionally, this signal strengthens whereas the
original signal at −66.21 ppm decreases as the reaction proceeds.
All these results are consistent with the consumption of complex
2 and the formation of product 5, which supports the mechanistic
step of coupling of complex 2 with aryl iodide as shown in
Scheme 1.
1
2
3
4
5
6
7
8
3
3
3
3
3
3
3
2
2
3
3
3
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
p-CH3C6H5I 110
p-CH3C6H5I 110
C6H5Br
C6H5Cl
70
90
Diglyme 12
Diglyme 12
Diglyme 12 7 (52)
Diglyme 12 7 (54)
Dioxane 12 7 (62)
—
—
110
130
110
110
110
110
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
12 7 (63)
12 7 (68)
12 5 (69)
12 6 (72)
12 8 (67)
12 7 (15)
9
10
11
12
110
110
12
—
a Isolated yield.
These reactivity studies of complexes 2–3 and the characteriz-
ation of compound 4 as well as the 19F NMR signals of the reac-
tion system present evidence for the validity of the catalytic
cycle for the Goldberg reaction shown in Scheme 1.
thermally competent as intermediates in the copper-catalyzed
Goldberg reaction.
Reactivity of complexes 2–3 with aryl halides
To examine the kinetic competence of complexes 2 and 3, stoi-
chiometric reaction of complexes 2 and 3 with aryl halides have
been performed, as shown in Scheme 3. Initial optimization of
the reaction conditions of the arylation reaction was carried out
under various conditions as summarized in Table 3. From
Table 3, we found that the arylation reaction is highly dependent
on the reaction temperature. No product is obtained when temp-
erature is less than 110 °C (Table 3, entries 1–2), but the reaction
yields are not significantly enhanced when the temperature is
further increased (entries 3–4). DMF is identified as the optimal
solvent due to relatively low boiling point and the highest yield.
Under the optimized conditions (Table 3, entry 7–10), the
desired N-arylated amides 5–8 are obtained in moderate yields
Conclusions
In summary, we have prepared and fully characterized two
copper(I) amide complexes 2–3, which can be stable for months
due to the high chelating ability of the dppf ancillary ligand.
Thermal gravimetric analyses show that complexes 2–3 are ther-
mally stable until 200 °C and are competent for the high-temp-
erature Goldberg reactions. These complexes can react with aryl
halides to deliver the N-arylation products, accompanied by the
formation of a Cu(I)–X complex, which has been characterized
by LC-MS analysis. These results substantiate the catalytic cycle
shown in Scheme 1.
1
(Table 3, entries 7–10) and confirmed by H NMR (Fig. S4–
Experimental section
S7†). However, only 15% yield of product 7 was obtained when
complex 3 was treated with bromobenzene (Table 3, entry 11),
and no product is acquired when complex 3 reacts with chloro-
benzene (Table 3, entry 12).
Notably, in addition to the N-arylated products, another
copper(I) complex (dppf)CuI (4) has been isolated as a yellow
powder when the reaction system was concentrated and left to
stand overnight at room temperature. Compound 4 was identified
by LC-MS-MS analysis by comparison of its m/z value (ESI
Materials
CuCl, 1,1′-bis(diphenylphosphino)ferrocene (dppf) and other
chemicals were obtained from commercial sources and used
without further purification. All solvents were dried by common
methods and freshly distilled prior to use. N-(p-chlorophenyl)
trifluoroacetanilide and N-(p-methylphenyl) trifluoroacetanilide
were prepared according to the reported literature procedures.19
5900 | Dalton Trans., 2012, 41, 5897–5902
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The precursor complex [(dppf)Cu]2(μ-Cl)2 was synthesized via
as solvent under nitrogen atmosphere. The reaction mixture was
stirred at 110 °C for 12 hours. Periodically, a small portion of the
sample was taken out, and the extent of the reaction was checked
by 19F NMR. The reaction system was concentrated to about
2 mL and left to stand overnight at room temperature. Then a
yellow powder 4 was obtained by filtration and the N-arylated
amides were separated from the filtrate by TLC using ethyl
acetate–petroleum ether (1 : 10) as developing solvent.
Compound 4: yellow powder, yield (68%). ESI MS (+): 617
[Cu(dppf)]+.
the described procedure.20
Experimental details
The synthetic procedures involving the reaction of Cu(I) species
with CH3ONa (or NaH) and amide arylation reactions were
carried out under a nitrogen atmosphere using a standard
Schlenk flask, due to the oxidative stability of the copper(I) com-
plexes and the moisture-sensitive CH3ONa (or NaH) used in the
reactions.
Compound 5: yellow powder, yield (69%). IR (ν, cm−1): 1706
(vs), 1509(s), 1397(w), 1228(s), 1201(s), 1152(s), 1136(s), 1021
(w), 963(w), 811(m), 760(w), 731(m), 692(w), 630(w). 1H NMR
(400 MHz, DMSO) δ 7.59 (s, 1H), 7.45 (s, 5H), 7.34 (s, 1H),
7.25 (s, 2H), 2.30 (s, 3H). 19F NMR (377 MHz, DMSO) δ
−69.65 (s).
Syntheses of complexes 2–3
The heterodinuclear complexes 2–3 were prepared as outlined in
Scheme 2 and two synthesis methods of complex 2–3 are
described as follows.
Compound 6: yellow powder, yield (72%). IR (ν, cm−1): 1706
(vs), 1508(s), 1396(w), 1230(s), 1200(s), 1152(s), 1136(s), 1021
(w), 964(w), 810(m), 753(w), 731(m). 1H NMR (400 MHz,
DMSO): δ 7.45 (d, J = 6.4 Hz, 2H), 7.32 (d, J = 6.9 Hz, 2H),
7.24 (s, 4H), 2.30 (d, J = 2.5 Hz, 6H).
Method A: To a Schlenk flask containing amide (0.25 mmol)
and NaH (0.024 g, 1.0 mmol) were added THF solvent (20 mL)
under nitrogen atmosphere. The mixture was stirred at room
temperature overnight. The solvent was removed under reduced
pressure to leave a white solid. Then complex 1 (0.131g,
0.1 mmol) and CH2Cl2 (15 mL) were added and the mixture was
stirred for 24 hours at room temperature. The solution was
filtered over celite to remove resulted NaCl and the solvent was
removed by rotary evaporator at room temperature. Yellow block
crystals were obtained by recrystallization in diethyl ether. The
crystalline products 2–3 are stable in air for months.
Compound 7: yellow powder, yield (68%). IR (ν, cm−1): 1705
(vs), 1507(s), 1396(w), 1228(s), 1203(s), 1153(s), 1137(s), 1016
(m), 957(w), 820(m), 767(w), 748(w), 717(w). 1H NMR
(400 MHz, DMSO): δ 7.67 (s, 1H), 7.62 (s, 1H), 7.52 (s, 4H),
7.40 (s, 3H).
Compound 8: yellow powder, yield (67%). IR (ν, cm−1): 1701
(s), 1515(s), 1397(w), 1229(s), 1202(s), 1156(s), 1137(s), 1016
(w), 958(w), 810(m), 760(w), 731(w). 1H NMR (400 MHz,
DMSO): δ 7.64 (s, 1H), 7.50 (s, 4H), 7.36 (s, 1H), 7.26 (d, J =
5.3 Hz, 2H), 2.31 (s, 3H).
Method B: To a Schlenk flask containing complex 1 (0.131g,
0.1 mmol), amide (0.25 mmol) and CH3ONa (0.054 g,
1.0 mmol) were added CH2Cl2 (20 mL) under nitrogen atmos-
phere. The mixture was stirred at room temperature for 24 hours
and the subsequent route was identical with that of method A.
Complex 2: yellow block crystal, yield (92%).IR (ν, cm−1):
1633 ν(CvO), 1480 νPh(CvC), 1434 νCp(CvC), 1245 ν(C–
F), 1163 δPh(CH), 1098 ν(P–Ph), 1028 δCp(CH), 822
Characterization methods
Infrared (IR) spectra were recorded on thin films deposited from
a CH2Cl2 solution on an ATR accessory with a FTLA2000 spec-
1
trometer. H NMR (400 MHz) and 19F NMR (377 MHz) were
π
Cp(CH).1H NMR (400 MHz, CDCl3) δ 7.42–7.36 (m, 12H,
collected on a Bruker ACF-400 spectrometer. 1H NMR chemical
shifts are recorded with deuterated chloroform or DMSO as
solvent and tetramethylsilane as internal standard. 19F NMR
chemical shifts are recorded with deuterated DMSO as solvent
and CFCl3 as external standard. Elemental analyses (C, H, O
and N) were determined using a Bio-Rad elemental analysis
system. Thermal gravimetric analysis (TGA) was carried out
using an Exstar 6000 analyzer in nitrogen at a heating rate of
15 °C min−1 from 50 to 600 °C. Electron spray ion (ESI) mass
spectrum was measured using a Waters Platform ZMD4000 LC/
MS mass spectrometer (Waters, USA).
Ph), 7.29 (t, J = 7.4 Hz, 8H, Ph), 7.00 (d, J = 8.2 Hz, 2H, Ph of
amide), 6.92 (d, J = 8.0 Hz, 2H, Ph of amide), 4.37–4.32 (m,
4H, Cp), 4.11 (s, 4H, Cp), 2.28 (s, 3H, Me). 19F NMR
(377 MHz, DMSO)
δ −66.21 (ppm). Anal. Calcd for
C43H35CuF3FeNOP2: C, 62.98; H, 4.30; N, 1.71. Found: C,
63.21; H, 4.36; N, 1.78%.
Complex 3: orange prism crystal, yield (85%). The crystalline
product 3 is also stable in air for months. IR (ν, cm−1): 1636
ν(CvO), 1484 νPh(CvC), 1435 νCp(CvC), 1247 ν(C–F), 1165
δPh(CH), 1097 ν(P–Ph), 1031 δCp(CH), 821 πCp(CH), 738 ν(C–
1
1
Cl); H NMR (400 MHz, CDCl3) H NMR (400 MHz, CDCl3)
δ 7.43 (d, J = 6.9 Hz, 3H), 7.40 (d, J = 7.4 Hz, 3H), 7.37 (s,
2H), 7.35 (d, J = 5.0 Hz, 8H), 7.31 (d, J = 7.4 Hz, 4H), 7.04 (d,
J = 8.6 Hz, 2H), 6.99 (d, J = 8.6 Hz, 2H), 4.37–4.35 (m, 4H),
4.11 (s, 4H). Anal. Calcd for C42H32CuF3FeNOP2Cl: C, 60.02;
H, 3.84; N, 1.67. Found: C, 60.42; H, 3.91; N, 1.74%.
X-ray crystallography
X-ray data for complexes 2 and 3 were collected at 223 K using
a Bruker SMART APEX II CCD area detector diffractometer
(MoKa, λ = 0.71073 Å). The crystal structure was solved using
the SHELXTL program and refined using full matrix least
squares.21 The hydrogen atom positions were calculated theoreti-
cally and included in the final cycles of refinement in a riding
model along with attached carbon atoms.
Arylation reaction between complexes 2–3 and aryl halides
To a Schlenk flask containing complex 2 (0.084 g, 0.1 mmol),
iodobenzene (0.020 g, 0.1 mmol) was added with 25 mL DMF
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11 G. O. Jones, P. Liu, K. N. Houk and S. L. Buchwald, J. Am. Chem. Soc.,
2010, 132, 6205.
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The authors are grateful for the financial support from the
National Natural Science Foundation of China (No. 20971058),
Graduate Innovation and Research Foundation of Jiangsu Pro-
vince (No. CXZZ11_0468) and the Fundamental Research
Funds for the Central Universities (No. JUDCF11004).
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5902 | Dalton Trans., 2012, 41, 5897–5902
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