An effective strategy for immobilizing a homogeneous palladium complex onto silica: Efficient and reusable catalyst for Suzuki-Miyaura reactions

Article abstract of DOI:10.1016/j.catcom.2013.06.039

A simple strategy has been adopted to immobilize a homogeneous palladium complex onto silica gel by introducing 4-pyridinecarbaldehyde into the coordination sphere of palladium. The material has been characterized by FTIR, BET measurements, XRD, SEM-EDX,

Full text of DOI:10.1016/j.catcom.2013.06.039

Catalysis Communications 41 (2013) 75–78
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Catalysis Communications
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Short Communication
An effective strategy for immobilizing a homogeneous palladium complex
onto silica: Efficient and reusable catalyst for Suzuki–Miyaura reactions
Chandan Sarmah , Debojeet Sahu , Pankaj Das a,b,⁎
a
a
a
Department of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India
Department of Chemistry, Rajiv Gandhi University, Doimukh, Itanagar 791112, Arunachal Pradesh, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple strategy has been adopted to immobilize a homogeneous palladium complex onto silica gel by in-
troducing 4-pyridinecarbaldehyde into the coordination sphere of palladium. The material has been charac-
terized by FTIR, BET measurements, XRD, SEM-EDX, and ICP-AES. The supported material has acted as an
efficient catalyst for the Suzuki–Miyaura reactions of aryl halides with low palladium loading (0.04 mol%)
in environment-friendly reaction media. In addition, the heterogeneous catalyst could retain the high activity
of the homogeneous counterpart catalyst, showed almost no leaching, and could be reused several times
without loss of activity.
Received 8 March 2013
Received in revised form 19 June 2013
Accepted 28 June 2013
Available online 9 July 2013
Keywords:
Heterogeneous catalyst
P,N-donor ligand
© 2013 Elsevier B.V. All rights reserved.
Suzuki–Miyaura reaction
Schiff-base condensation
Silica support
1
. Introduction
The Suzuki–Miyaura cross-coupling reaction of aryl halides with
ligand like, 4-pyridinecarboxaldehyde (PCA), into the coordination
sphere of palladium and investigated the potentiality of the heteroge-
neous material as catalyst for Suzuki–Miyaura reactions of aryl halides.
The inclusion of the PCA ligand would allow the homogeneous complex
to be anchored onto amine-functionalized support via Schiff-base con-
densation between the reactive aldehyde group of the homogeneous
part, and the amine group covalently linked with the support. It is
worth to note that the Schiff-base strategy had earlier been employed
for anchoring palladium catalyst onto various supports, however, in
most of the cases Schiff-bases were pre-attached with the support via
various post-grafting synthesis [11,15,18–20]. One major limitation of
this pre-attached strategy is that it often results different coordination
environments of palladium before and after metal immobilization,
which might be responsible for exhibiting different catalytic behavior
by homogeneous and heterogeneous counterparts.
arylboronic acids catalyzed by homogeneous palladium complexes is
one of the most simple and successful strategies for C–C bond formation
reaction in organic synthesis [1]. Typically, the reaction is catalyzed
by soluble palladium complexes bearing different phosphorous or
nitrogen-based ligands [2–4]. Although, homogeneous catalysts are
often very much active and selective producing excellent yields, from
economic and environmental standpoint heterogeneous catalysts are
much more advantageous. Thus, transforming an active homogeneous
catalyst into effective heterogeneous catalyst is an attractive area of re-
search in the field of catalysis [5–7]. Consequently, during the past few
years, several strategies had been adopted to heterogenize homoge-
neous palladium compounds into/onto various supports such as, silica
[8–13], MCM-41 [14–16], alumina [17], polymer [18] etc. While signifi-
cant progresses have been achieved with respect to development of
stable and reusable catalysts of high efficiency, but often such catalysts
showed lower activity compared to their soluble counterparts [6,7].
Thus, there is a need to develop improved and practical protocols for
synthesizing novel heterogeneous catalysts that can show comparable
results with their homogeneous analogues.
2. Experimental
2.1. Synthesis
2
1
2.1.1. Synthesis of [Pd(η -P,N-PPh
To a solution of the [PdCl
2
Py)(η -N-PCA)
2
](ClO
4
)
2
(2)
2
2
(η -P,N-PPh
2
Py)] (1) [21] (0.05 g,
In this work we have reported a simple strategy for immobilizing a
homogeneous palladium complex by introducing a carbonyl-containing
0.114 mmol) in 20 mL dichloromethane (DCM), a solution AgClO
4
(0.048 g, 0.228 mmol) in 10 mL DCM was added drop by drop. A so-
lution of the PCA ligand (0.024 g, 0.228 mmol) in 10 mL DCM was
added to the reaction mixture and the resultant solution was stirred
at room temperature for 3 h. A curdy white precipitate of AgCl deposited
at the bottom of the solution was filtered off and the yellow coloured
solution obtained was dried in vacuo. The solid compound obtained
⁎
Corresponding author at: Department of Chemistry, Rajiv Gandhi University,
Doimukh, Itanagar 791112, Arunachal Pradesh, India. Tel.: +91 360 2278573;
fax: +91 360 2277889.
E-mail address: pankajd29@yahoo.com (P. Das).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.06.039

76
C. Sarmah et al. / Catalysis Communications 41 (2013) 75–78
was washed with petroleum ether and hexane and dried to get a bright
yellow coloured compound of complex 2.
stretching of imine. Moreover, the disappearance of the carbonyl band
−1
at 1714 cm
of complex 2 and appearance of a weak band at
574 cm for pyridine ν(C = N) stretching further substantiate immo-
bilization of complex 2 on silica.
−1
1
2
2
.1.2. Immobilization of complex 2 onto silica gel: SiO @APTES-Pd
Commercially available silica gel (60–120 mesh) was reacted with
3-aminopropyltriethoxysilane (APTES) as per reported procedure [23] to
2
3.2.2. N adsorption–desorption isotherm
get APTES-functionalized silica gel, SiO
2
@APTES. 1.5 g of SiO
2
@APTES
Nitrogen adsorption–desorption isotherms of the silica-based
materials (Fig. 1), measured at −196 °C confirmed their mesoporous na-
ture. The isotherms of all the samples followed typical type IV patterns
according to IUPAC definitions of porosity [22]. A sharp capillary uptake
was added in small portions to a solution containing 0.063 mmol of com-
plex 2 in 30 mL ethanol. The reaction mixture was refluxed under stir-
ring for 24 h. The orange solids deposited were separated from the
solvent by filtration and washed repeatedly through Soxhlet extraction
with ethanol and acetone and dried overnight at 120 °C.
is observed for the adsorption of N
are of uniform size. The specific surface area (ABET) obtained by the BET
2
at P/Po ≈ 0.4 indicating mesopores
2
method for free SiO
with APTES reduced to 381.1 m /g (Table S.1: SI). Immobilization of
palladium complex onto SiO @APTES resulted a further decrease in the
BET value. In a similar manner, a significant decrease in pore volume
2
was 436.7 m /g which upon functionalization
2
2
.2. General procedure for the Suzuki–Miyaura reaction
2
A 50 mL round-bottomed flask was charged with aryl halide
A
3
(
1 mmol), arylboronic acid (1.1 mmol), K
2
CO
3
(3 mmol), catalyst
2
has been observed between parent SiO (0.45 cm /g) and complex
3
material (0.02 g), and solvent (6 mL). The mixture was refluxed under
stirring condition at 60 °C for the required time. After completion, the
catalyst was separated by filtration and washes the residual solid with
the same reaction solvent. The filtrate was diluted with water (20 mL)
and extracted with ether (3 × 20 mL). The combined extract was
immobilized silica (0.20 cm /g), further substantiating immobilization
of the complex 2 onto silica. However, the pore diameter was not affect-
ed by immobilization of the complex on silica.
3.2.3. XRD, SEM and ICP analysis
washed with brine (3 × 20 mL) and dried over Na
tion of the solvent under reduced pressure, the products were purified
by column chromatography (silica gel, ethyl acetate/hexane, 1:9).
2
SO
4
. After evapora-
The powder X-ray diffraction patterns (Fig. S.2: SI) of parent silica
gel exhibit a broad peak at 2θ = 22.403° which upon complex immobi-
lization remains almost unchanged (except slight decrease in intensity).
This result suggests that silica maintains its mesoporous structure even
after palladium loading. A very low intense peak is also observed at
3
. Result and discussion
2θ = 35.64° attributed to 111 plane of palladium [24,25]. The SEM
3
.1. Synthesis of the catalyst
image of the fresh catalyst (Fig. S.3: SI) shows silica particles are of dif-
ferent diameters. Although the palladium particles and their disper-
sions are not clearly visible from the SEM spectra, the EDX spectra
clearly shows the presence of palladium on silica (Fig. S.4: SI). The pal-
ladium content determined by ICP-AES is found to be 0.019 mmol/g of
silica gel, and this value is very close to that determined from nitrogen
elemental analysis (0.021 mmol/g of silica gel).
The heterogeneous catalyst has been prepared by a three-step
procedure. In the first step, a new homogeneous precursor complex,
2
1
2 2 4 2
[Pd(η -P,N-PPh Py)(η -N-PCA) ](ClO ) (2) has been prepared by
incorporating two PCA ligands into the coordination sphere of palla-
dium via abstraction of chlorides from a previously reported [21]
2
complex [PdCl
Scheme 1)
2
(η -P,N-PPh
2
Py)] (1) with two equivalents of AgClO
4
(
.
The complex 2 has been characterized by elemental analy-
3.3. Catalytic activity
1
13
31
sis, mass, FTIR, H, C and P NMR spectroscopy (supplementary infor-
mation, SI). In the second step, we have functionalized silica gel with
APTES to get the material, SiO
3.3.1. Suzuki–Miyaura activity of SiO
2
@APTES-Pd
@APTES-Pd material as catalyst
2
@APTES which has reactive amine groups.
To evaluate the effectiveness of SiO
2
Based on nitrogen elemental analysis, the amount of APTES loaded into
in the Suzuki–Miyaura reaction, initial reaction was performed between
4-bromoanisole and phenylboronic acid as a model reaction with 0.02 g
of catalyst (containing 0.04 mol% palladium). The reaction was per-
−
1
silica surface was calculated as 0.377 mmol g . Finally, the palladium
anchored heterogeneous catalyst, SiO @APTES-Pd, is prepared via
2
Schiff-base condensation between the amine groups of SiO
2
@APTES
2 3
formed at 60 °C in aqueous-isopropanol (1:1) with K CO as base, as
and aldehyde groups of the complex 2 (Scheme 2).
this condition was found to give the best results with our recently re-
ported silica-based catalyst [23]. We are delighted to see that the reac-
tion proceeded smoothly and 96% 4-methoxybiphenyl was isolated
after 6 h of reaction time (Table 1, entry 1). It is worth to mention
that there exists one report where APTES was directly used as support
for palladium catalyzed Suzuki–Miyaura reactions. However compared
to our present catalyst, the reported APTES-based catalyst required a
harsh reaction condition (DMA, T ≥ 95 °C, 1–2 mol% Pd) for getting
comparable yields [26]. On evaluating the effects of solvents in our
3
.2. Characterization of the heterogeneous catalyst
3
.2.1. FTIR spectra
FTIR spectra of the silica-based materials are shown in the supplemen-
tary information (Fig. S.1). Upon immobilization of the complex 2 on silica
−1
gel, the O–H band of parent silica centered at 1624 cm merges with a
−1
new broad band centered at 1647 cm
attributable to the C = N
P
OHC
OHC
N
N
P
Cl
PdCl2
2 AgClO , 2 PCA
4
Pd
(ClO )
4 2
Pd
1:1
CH Cl , rt, 3h
N
PPh2
2
2
N
Cl
N
Where
Complex 1
Complex 2
P
N
N
PPh2
Scheme 1. Synthesis of complex 2 via abstraction of halide from the complex 1.

C. Sarmah et al. / Catalysis Communications 41 (2013) 75–78
77
O
O
O
EtO
OH EtO
O
O
O
Si
Si
N
CH
OH
N
P
Toluene
Reflux,
N2
Complex 2
Si
Pd
(ClO )
4 2
+
Si
NH2
NH2
Ethanol,
Reflux
O
O
O
N
N CH
N
OH
EtO
SiO @APTES-Pd
SiO2
APTES
SiO @APTES
2
2
Scheme 2. Immobilization of the complex 2 onto silica gel through Schiff-base condensation.
i
model reaction we have noticed that besides PrOH-H
2
O our system can
O (entry 2)
O (entry 3). Noteworthy to highlight that under similar
experimental conditions neat alcohol as solvents gave much less yield
entry 6–8) and neat water remained virtually ineffective (entry 9).
3.3.3. A comparison between PCA and non-PCA-based catalytic system
In order to check the effectiveness of our protocol we have also
immobilized the complex 1 directly onto the APTES-functionalized
tolerate other aqueous-alcoholic solvents such as EtOH-H
or MeOH-H
2
2
SiO
is designated as SiO
ladium was used for both the grafting experiments, ICP-AES shows that
2
2
(0.063 mmol of complex 1 in 1.5 g SiO @APTES) and the material
(
2
@APTES-Pd(1). Although the same amount of pal-
much lower amount of palladium was anchored in SiO
(0.009 mmolg ) compared to SiO @APTES-Pd (0.019 mmolg ). For
2
2
@APTES-Pd(1)
−1
−1
3
.3.2. Homogeneous vs. heterogeneous
Encouraged by the results of the model reaction, we have tested
our silica-based catalyst SiO @APTES-Pd for a range of electronically
2
comparing their catalytic activities, we have performed the model reac-
tion using both the catalysts under similar experimental conditions. We
and sterically diverse aryl bromides/iodides as substrates (Table 2).
As expected, the reactions with aryl iodides were slightly faster than
corresponding aryl bromides (entry 1 vs.7 & entry 5 vs. 6). Both the
donor- and acceptor-substituted aryl bromides coupled effectively
with phenylboronic acid to give the corresponding biaryl compounds
in excellent yields (entry 1–4). Interestingly, under slightly extended re-
action time, sterically demanding substrates, such as, 2-bromoanisole
and 2-bromotoluene also showed good yields (entry 8 & 9). Unfortunate-
ly, our catalyst is not effective for aryl chloride as substrate and only neg-
ligible amount of product was obtained with 4-chloronitrobenzene even
after increasing the temperature to 100 °C and catalyst loading to
2
are gratified to see that the PCA-based catalyst, SiO @APTES-Pd, show
much superior activity (yield 96%) over non PCA-based catalyst, SiO @
2
APTES-Pd(1) (yield 62%).
3.3.4. Heterogeneity and reusability test
2
In order to check the heterogeneity of the catalyst SiO @APTES-Pd,
hot filtration test and solid-phase poisoning test were performed for
the model reaction. In the hot filtration test, the solid catalyst was fil-
tered off at 44% completion of the reaction (analyzed by GC) and the
filtrate was kept on stirring for further 24 h. No increase in product
concentration was noticed (Fig. 2), indicating heterogeneous nature
of our catalyst. In the solid-phase poisoning test, the commercially
1
mol% (entry 11). As mentioned earlier a common problem associated
with most of the heterogeneous catalysts is that their activity drops
after immobilization compared to their corresponding homogeneous
analogues. In order to check this property, we have also performed the
same set of catalytic reactions using homogeneous complex 2 as catalyst
by keeping the catalyst mol% same (Table 2). We are very much glad to
see that in all the cases, under slightly extended reaction time; our hetero-
geneous catalyst could exhibit comparable results with that of the homo-
geneous complex 2.
available 3-marcaptopropyl-functionalized silica (SH-SiO
as a scavenger for the leached palladium [26–28]. The coordination
of leached palladium by SH-SiO is expected to stop the reaction.
However, the kinetic profile diagram (Fig. S.5: SI) in absence and
presence of SH-SiO shows almost similar activities which is consistent
with the heterogeneous nature of the catalyst.
Since reusability is one of the important aspects of a heterogeneous
catalyst and to check reusability, the catalyst material was separated
2
) was used
2
2
from the model reaction by simple filtration, washed thoroughly with
i
2
PrOH-H O followed by DCM, dried and reused for subsequent runs.
SiO2
Almost negligible drop in activity is observed after using the solid cata-
lyst for six times (Fig. S.6, Table S.2: SI). The ICP-AES analysis of the six
time used catalyst showed almost no leaching (b2%). The SEM picture
3
2
2
1
1
00
50
00
50
00
SiO @APTES
2
SiO @APTES-Pd
2
SiO @APTES-Pd (6th use)
2
Table 1
Solvent optimization study in the Suzuki–Miyaura reaction of 4-bromoanisole with
a
2
phenylboronic acid using SiO @APTES-Pd.
Entry
Solvent
Time (h)
Yield (%)b,c
i
1
2
3
4
5
6
7
8
9
PrOH-H
2
O (1:1)
O (1:1)
O (1:1)
6
6
6
12
12
6
6
6
10
96
95
92
64
72
67
53
64
12
EtOH-H
MeOH-H
THF-H
2
5
0
2
2
O (1:1)
Toluene-H
2
O (1:1)
i
0
PrOH
0
.0
0.2
0.4
0.6
0.8
1.0
EtOH
MeOH
H
Relative pressure (P/P0)
2
O
a
Fig. 1.
N
2
adsorption–desorption isotherms of free silica gel (SiO
@APTES), complex
2
),
2
Reaction condition: 1.0 mmol 4-bromoanisole, 1.1 mmol phenylboronic acid, 3.0 mmol
base, solvent (6 mL), 0.02 g catalyst.
3
-aminopropyltriethoxysilane-functionalized silica gel (SiO
2
b
immobilized silica gel (SiO
recorded at −196 °C.
2
@APTES-Pd) and SiO
2
@APTES-Pd after 6 times reuse
Isolated yield.
Yields are of average of two runs.
c

78
C. Sarmah et al. / Catalysis Communications 41 (2013) 75–78
Table 2
clearly shows the presence of palladium suggesting that the catalyst
could be reused for further runs.
a
Suzuki–Miyaura reactions of aryl halides with phenylboronic acid using SiO
and comparison with homogeneous counterpart (complex 2).
2
@APTES-Pd
4
. Conclusion
In conclusion we have developed a simple strategy to immobilize a
homogeneous palladium complex onto silica gel. The supported catalyst
showed excellent activity and recyclability for the Suzuki–Miyaura re-
actions. Moreover, our heterogeneous catalyst could retain high activity
of the homogeneous counterpart catalyst.
Entry
R
X
SiO
2
@APTES-Pd
Complex 2
Yield (%)b,c
Time (h)
Yield (%)
Time (h)
1
2
3
4
5
6
7
8
9
4-CH
4-CH
4-NO
4-COCH
H
H
4-CH
2-OCH
2-CH
4-NO
4-NO
3
O
Br
Br
Br
Br
Br
I
6
6
6
6
5
3
4
8
8
96
95
96
97
98
97
96
82
84
trace
2
2
2
2
2
1
1
4
4
96
96
98
96
98
98
99
91
89
trace
3
Acknowledgements
2
3
The University Grants Commission (UGC), New Delhi and the
Council of Scientific and Industrial Research (CSIR), New Delhi, are
gratefully acknowledged for their financial supports with Grant
nos. 36-73/2008 and 02-0081/12 respectively.
3
O
I
3
Br
Br
Cl
Cl
3
1
1
0
1
2
2
12
24
12
24
d
d
12
16
Appendix A. Supplementary data
a
Reaction conditions: 1.0 mmol aryl halide, 1.1 mmol arylboronic acid, 3.0 mmol
base, solvent (6 mL).
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2013.06.039.
b
Isolated yield.
Yields are of average of two runs.
c
d
Reactions are carried out at 100 °C using 1.0 mol% catalyst.
References
[
[
[
1] N. Miyaura, A. Suzuki, Chem. Rev. 95 (1995) 2457–2483.
2] F.A. Littke, G.C. Fu, Angew. Chem. Int. Ed. 41 (2002) 4176–4211.
3] J.P. Wolfe, S.L. Buchwald, Angew. Chem. Int. Ed. 38 (1999) 2413–2416.
1
00
No filtration
Hot filtration
[4] N.T.S. Phan, M.V.D. Sluys, C.W. Jones, Adv. Synth. Catal. 348 (2006) 609–679.
[
[
5] M. Mora, C. Jimenez-Sanchidrian, J.R. Ruiz, Curr. Org. Chem. 16 (2012) 1128–1150.
6] A. Molnar, Chem. Rev. 111 (2011) 2251–2320.
80
60
40
20
0
[7] V. Polshettiwar, C. Len, A. Fihri, Coord. Chem. Rev. 253 (2009) 2599–2626.
[
8] R.B. Bedford, C.S.J. Cazin, M.B. Hursthouse, M.E. Light, K.J. Pike, S. Wimperis,
J. Organomet. Chem. 633 (2001) 173–181.
[
9] M. Lemay, V. Pandarus, M. Simard, O. Marion, L. Tremblay, F. Beland, Top. Catal. 53
(2010) 1059–1062.
[
[
[
10] W. Chen, P. Li, L. Wang, Tetrahedron 67 (2011) 318–325.
11] S. Paul, J.H. Clark, J. Mol. Catal. A: Chem. 215 (2004) 107–111.
12] L. Wan, C. Cai, Catal. Commun. 24 (2012) 105–108.
[13] A. Kumbhar, S. Kamble, S. Jadhab, G. Rashinkar, R. Salunkhe, Catal. Lett. 142
2012) 1388–1396.
(
[
[
14] H. Zhao, J. Peng, R. Xiao, M. Cai, J. Mol. Catal. A: Chem. 337 (2011) 56–60.
15] K. Dhara, K. Sarkar, D. Srimani, S.K. Saha, P. Chattopadhyay, A. Bhaumik, Dalton
Trans. 39 (2010) 6395–6402.
16] Q. Zhang, H. Su, J. Luo, Y. Wei, Tetrahedron 69 (2013) 447–454.
17] S.S. Soomro, F.L. Ansari, K. Chatziapostolou, K. Kohler, J. Catal. 273 (2010) 138–146.
18] Y. He, C. Cai, Catal. Commun. 12 (2011) 678–683.
[
[
[
[
19] M.K. Bhunia, S.K. Das, P. Pachfule, R. Banerjee, A. Bhaumik, Dalton Trans. 39
0
1 h
2 h
3 h
4 h
5 h
6 h
(2012) 1304–1311.
Time (h)
[20] E.B. Mubofu, J.H. Clark, D.J. Macquarrie, Green Chem. 3 (2001) 23–25.
[
[
21] J. Liu, C. Jakob, K.J. Sheridan, F. Al-Mosule, B.T. Heaton, J.A. Iggo, M. Matthews, J.
Pelletier, R. Whymann, J.F. Bickley, A. Steiner, Dalton Trans. 39 (2010) 7921–
Fig. 2. Kinetic profile diagram of the hot filtration test for the Suzuki coupling reaction
of the 4-bromoanisole with phenylboronic acid.
7935.
22] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T.
Siemieniewska, Pure Appl. Chem. 57 (1985) 603–619.
[
[
23] C. Sarmah, D. Sahu, P. Das, Catal. Today 198 (2012) 197–203.
24] A. Ramakrishnan, K. Dumbuya, J. Ofili, H.P. Steinruck, J.M. Gottfried, W. Schwieger,
Appl. Clay Sci. 51 (2011) 8–14.
of the used catalyst (Fig. S.7: SI) is very much similar to the fresh cata-
lyst, indicating that the morphology of the catalyst remained unaffected
even after repeated uses. This fact is further corroborated by BET analy-
sis (Table S.1: SI), which shows there is no significant change in the sur-
[25] L.M. Neal, M.L. Everett, G.B. Hoflund, H.E. Hagelin-Weaver, J. Mol. Catal. A: Chem.
35 (2011) 210–221.
3
[
[
26] L. Zhang, L. Wang, H. Li, P. Li, Synth. Commun. 38 (2008) 1498–1511.
27] J.M. Richardson, C.W. Jones, J. Catal. 251 (2007) 80–93.
2
face area of the six times used catalyst (201.1 m /g) and the fresh
2
catalyst (193.2 m /g). The EDX analysis (Fig. S.8: SI) of the used catalyst
[28] S. Bhunia, R. Sen, S. Koner, Inorg. Chim. Acta 363 (2010) 3993–3999.