Synthesis, Characterization and Catalytic Activity of a Tungsten(VI) Amino Triphenolate Complex

Article abstract of DOI:10.1007/s10562-017-2144-z

Abstract: Synthesis, characterization and catalytic activity of a novel tungsten(VI) amino triphenolate complex have been investigated. In particular, a tungsten(VI) amino triphenolate complex has been synthesized and tested in the oxidation of sulfides and cyclooctene, using hydrogen peroxide as terminal oxidant. The catalyst has proved to be air and water tolerant, also showing a good efficiency in terms of yields and selectivity. Moreover, an upgrade of our previous ligand synthesis is herein reported. The new developed procedure allows to obtain the triphenolamine in large scale without the use of chromatography for the intermediates purification. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2144-z

Catal Lett
DOI 10.1007/s10562-017-2144-z
Synthesis, Characterization and Catalytic Activity
of a Tungsten(VI) Amino Triphenolate Complex
Elena Badetti1 · Alessandro Bonetto1 · Francesco Romano1 · Luciano Marchiò2 ·
Cristiano Zonta1 · Giulia Licini1
Received: 16 May 2017 / Accepted: 13 July 2017
© Springer Science+Business Media, LLC 2017
Abstract Synthesis, characterization and catalytic activ-
ity of a novel tungsten(VI) amino triphenolate complex
have been investigated. In particular, a tungsten(VI) amino
triphenolate complex has been synthesized and tested in the
oxidation of sulfides and cyclooctene, using hydrogen per-
oxide as terminal oxidant. The catalyst has proved to be air
and water tolerant, also showing a good efficiency in terms
of yields and selectivity. Moreover, an upgrade of our pre-
vious ligand synthesis is herein reported. The new devel-
oped procedure allows to obtain the triphenolamine in large
scale without the use of chromatography for the intermedi-
ates purification.
Graphical Abstract
Keywords Tungsten · Epoxidation · Sulfoxidation ·
Polydentate ligand · Hydrogen Peroxide
1 Introduction
The use of W(VI) based catalysts for the epoxidations of ole-
fin dates back to the late 1940s and since that time has proved
to be useful for the oxidation of a variety of carbon–carbon
double bonds [1–4]. Heterogeneous W(VI) compounds,
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-017-2144-z) contains supplementary
material, which is available to authorized users.
2−
W-oxido cluster complexes, and tungstate WO4 deriva-
* Cristiano Zonta
tives gained significant interest in oxidation catalysis, mainly
with H2O2 as oxidizing agent [5–8]. In this specific case, it is
assumed that the actual oxidant specie is a η2-peroxotungsten
complex. Pertungstic acid and pertungstates are known
to give highly stable aqueous solutions, and the tungstate
ion itself has been shown to be quite superior to molybdate
and vanadate in epoxidation reactions with H2O2, since the
cristiano.zonta@unipd.it
* Giulia Licini
giulia.licini@unipd.it
1
Dipartimento di Scienze Chimiche and CIRCC, Università di
Padova, via Marzolo 1, 35131 Padua, Italy
2
Dipartimento di Chimica, Università degli Studi di Parma,
Viale delle Scienze 17/A, 43123 Parma, Italy
Vol.:(0123456789)
1 3

E. Badetti et al.
‘transition metal ion-induced decomposition’ of hydrogen
peroxide is much slower. Moreover, many Mo(VI)-based
catalysts reported in the literature have shown to be inac-
tive in the activation of H2O2 [9, 10]. To date, tungsten(VI)
complexes are probably the best transition-metal catalysts
for epoxidation reactions of alkenes with hydrogen peroxide,
even if sometimes the epoxides formed tend to hydrolyze to
the corresponding diols.
investigated using tris-(2-hydroxy-3,5-di-t-butylbenzyl)
amine as ligand [47]. This ligand was chosen in order to
avoid a possible halogenation of the ligand aromatic ring
positions during the haloperoxidation process. However,
the required “protection” of the para position resulted in a
complex characterized by high molecular weight and by a
low solubility in polar solvents. For these reasons, in order
to explore the oxidation of other substrates, we decided to
use a catalyst bearing a ligand with a single ortho-t-butyl
substituent on each aromatic ring.
One of the most known system for epoxidation, using
W(VI) and H2O2, was reported by Venturello [11]. His
work showed that a mixture of tungstate and phosphate, in
the presence of a tetraalkylammonium salt as phase transfer
agent, catalyzed olefin epoxidation with H2O2 in a bipha-
sic 1,2-dichloroethane/water medium. Since its discovery
in 1983, several evolutions of tungsten catalysts have been
reported [12–22]. Indeed, the literature regarding homogene-
ous mononuclear tungsten(VI) complexes as oxidation cata-
lysts is constantly increasing. Among the different molecu-
lar catalysts, metal complexes bearing amine and diamine
bis-phenolate ligands have attracted considerable attention
over past decades in olefin polymerization and ring-open-
ing polymerization of lactones and lactides [23, 24]. There
are few reports on tungsten complexes with this family of
ligands, and their application in epoxidation catalysis has
been only recently investigated [25].
Ligand 5 was prepared using the synthetic strategy
developed in our group, based on a threefold reductive ami-
nation of the corresponding substituted salicylic aldehyde
2 [48]. The reductive amination needs to be performed on
the protected aldehyde 2, which is obtained from the cor-
responding phenol 1 in good yield after ortho-formylation
and O-benzylation. This synthetic route, which has been
used successfully in the past years, offers the possibility
to access to a large class of functionalized ligand in high
yields. However, this method requires flash chromato-
graphic purification steps and therefore the overall process
can be carried out on only a few grams’ scale. To overcome
this problem, we improved the purification steps of the
intermediates in order to avoid the use of chromatography
(Scheme 1). The most critical purification step is the iso-
lation of the aldehyde 2, which is obtained as an oil after
O-benzylation. To overcome the chromatographic purifica-
tion, aldehyde 2 was transformed into the corresponding
bisulfite adduct 3, by mixing it with a solution of NaHSO3
and ethanol. The bisulfite adduct 3 is water soluble and can
be easily separated from the reaction mixture. Basification
of the aqueous phase allows the re-obtaining of aldehyde 2
which was back-extracted with CH2Cl2. Via this method, it
was possible to obtain 74 g (0.29 mol, 60% yield) of alde-
hyde 2, which was then used for the subsequent two steps.
The following steps, reductive amination and deprotection
of phenol group via Pd/C catalysed hydrogenolysis, can
be carried out in multigram scale without problem and the
product purifications can be performed by crystallization.
In our group there is a longstanding interest in the use of
polydentate ligands such as trialkanolamines, [26–31] tri-
picolylamines (TMPA) [32–37] and triphenolamines (TPA)
[38–49]. The latter have been extensively used by us with d0
metals. In detail, we have been interested in the use of Ti(IV),
V(V) and Mo(VI) complexes in oxidation reactions. More
recently, we have extended the study also to W(VI) for the
study of haloperoxidation reactivity [47]. The W(VI) com-
plex showed a remarkable stability under the harsh reaction
conditions required for the haloperoxidation, namely large
excess of hydrogen peroxide and acid. Here we report the
synthesis of a similar complex, which was tested in sulfoxi-
dation and epoxidation reactions, which affords significant
results using hydrogen peroxide as terminal oxidant. Within
the course of this study, we also optimize our previously
reported [48] synthetic methodology for the triphenolamine
synthesis via reductive amination, avoiding chromatographic
purifications within the different steps. This novel procedure
has allowed the preparation of tris-(2-hydroxy-3-t-butylben-
zyl)amine 5 in multigram scale.
2.2 Synthesis of the Tungsten Complex
Tungsten(VI) complex 6 can be synthesized by reaction
of WOCl4 with tris-(2-hydroxy-3-tert-butylbenzyl)amine
5 (Scheme 2). The methodology, originally developed by
Lehtonen for similar systems, [49–52] was slightly modi-
fied using an excess of triethylamine as base for deproto-
nation of the three phenol moieties. For this purpose, in a
glove-box, the solid WOCl4 was dissolved in methanol in
the presence of stoichiometric amounts of 5 and five equiv-
alents of triethylamine. The resulting mixture was stirred
at room temperature overnight obtaining an intense yel-
low colored solution. After addition of 4 N HCl solution,
2 Results and Discussion
2.1 Gram scale Synthesis of the Ligand
In our previous study, the haloperoxidation reactivity
catalyzed by a W(VI) aminotriphenolate complex, was
1 3

Synthesis, Characterization and Catalytic Activity of a Tungsten(VI) Amino Triphenolate…
Scheme 1 Synthesis and purifi-
cation of 5 via bisulfite adduct 3
1) (CH2O)n,MgCl2
THF
2) BnBr,K2CO3
CH3CN
t-Bu
t-Bu
t-Bu
NaHSO3
OBn
SO3
OH
OBn
OH
-
O
1
2
3
NH4OAc
NaBHOAc3
THF
t-Bu
t-Bu
H2
Pd/C
OH
OBn
N
N
THF
3
5
3
4
O
t-Bu
t-Bu
WOCl4
NEt3
W OH
OH
O
N
N
MeOH
3
3
5
6
Scheme 2 Synthesis of complex 6 starting from ligand 5
extraction with dichloromethane, and purification by flash
chromatography (dichloromethane/methanol) gave complex
6 as a yellow orange solid (65% yield). The formation of
the mononuclear six coordinated complex 6, was confirmed
1
by ESI-MS and H NMR studies. The ESI-MS spectrum,
in negative mode, shows an isotopic pattern consistent
with a W-dioxido complex bearing an amino triphenolate
ligand (m/z=716). Tungsten complex 6 1H NMR spectrum
is coherent with an octahedral, six coordinated species. In
fact, three different set of signals can be detected for the
diastereotopic benzylic protons: two doublets and one sin-
glet, as expected for an octahedral geometry.
Fig. 1 Molecular structure of 6 together with crystallization solvents
(H2O and CH3CN). Thermal ellipsoids reported at the 30% probabil-
ity level
in the equatorial plane are in the narrow range among
1.888(3)–1.923(3) Å. As expected, the W–Ooxido bond
distance is the shortest (1.707(3) Å) and it is consistent
with a W=O double bond. The longest distance is the
W–N (2.438(4) Å) in agreement with the trans influence
exerted by the oxido group [53]. Indeed, the W atom lies
out of the equatorial plane by 0.31 Å and it is directed
toward the oxido group.
2.3 X‑ray Crystal Structure of Complex 6
Suitable crystals for X-ray diffraction were obtained by
crystallization of complex 6 in acetonitrile. The molecu-
lar structure is reported in Fig. 1. The metal center exhib-
its a distorted octahedral geometry achieved by the oxy-
gen atoms of three phenoxy groups, a hydroxyl group, the
central nitrogen atom of the ligand and an oxido moiety.
The bond distances between the metal and oxygen atoms
The presence of water molecules of crystallization
favors the formation of a supramolecular dimer in the
solid state, due to the presence of four hydrogen bonds
between the hydroxido groups and the water molecules
(Fig. 2).
1 3

E. Badetti et al.
0.10
0.08
0.06
0.04
0.02
0.00
Fig. 2 Supramolecular dimer formed by two 6 complexes and two
water molecules of crystallization. Only the hydrogen atoms involved
into the HBs are shown. Symmetry code = 1−x; −y; 1−z
0
5
10
15
Time (hours)
2.4 Catalytic Activity of 6: Sulfoxidation
and Epoxidation
Fig. 3 Oxidation of methyl-p-tolyl sulfide with H2O2 (filled cir-
cle) and TBHP (filled square). Reaction conditions: 25°C;
[sulfide]0 = [oxidant]0 = 0.1 M; [6] =0.001 M; solvent=CD3CN
(with H2O2) or CD3Cl (with TBHP)
The use of multidentate amino triphenolate ligands has
already shown to lead high stable metal complexes, allow-
ing lower catalyst concentrations (without losing catalyst
integrity) and high TONs and TOFs in the oxidation of
different substrates. Starting from this background, W(VI)
complex 6 was tested as catalysts for oxidation. Even
though no significant catalytic activity was observed in the
oxidation of secondary and tertiary amines, alcohols, and
hydroxylation of ethyl benzene, very good results were
achieved in the oxidation of sulfides using H2O2 as the ter-
minal oxidant. In fact, complex 6 can activate hydrogen per-
oxide already at room temperature, affording quantitative
yields of the corresponding sulfoxides with catalyst loading
down to 1% (Table 1). This behavior is rather diverse from
what we reported for the corresponding Mo(VI) based sys-
tem, for which a reasonable reactivity was observed only at
60°C [42]. In Fig. 3a comparison between the activity of
complex 6 (1 mol%) with tert-butyl hydroperoxide (TBHP)
and hydrogen peroxide toward oxidation of methyl-p-tolyl
sulfide (sulfoxide formation) is reported. Higher yields in
much shorter reaction time were obtained using hydrogen
peroxide, as reported already with other W-based catalysts
[54–58].
of cyclooctene oxide was observed but much faster and
higher chemical yields were obtained using acetonitrile as
solvent and hydrogen peroxide as primary oxidant.
While substrate conversion up to 45% was obtained
using TBHP, with hydrogen peroxide in acetonitrile epox-
ide yields up to 94% were obtained using a two-fold excess
of oxidant. However, under these reaction conditions, we
observed discoloration of the reaction mixture at the end
of the reaction, that we associated to catalyst decomposi-
tion. Catalyst instability, under turn over conditions, could
derive by a partial oxidation of the ligand followed by
hydrolytic/oxidative decomposition of the metal complex
itself.
3 Conclusions
In this communication, we have reported the synthesis of a
novel W(VI) aminotriphenolate complex and its characteri-
zation. Catalytic studies showed that W(VI) TPA complex
6 can act as effective catalyst for oxygen transfer reactions
using hydrogen peroxide as terminal oxidant. In particu-
lar, high yields and selectivities were observed for sul-
foxidation of a series of sulfides and also cis-cyclooctene
was oxidized to the corresponding epoxide with yields up
to 94%. Further studies are needed to elucidate the nature
of the active species in the oxidation process and the real
origin of the catalyst decomposition in order to increase its
performances.
The scope of the reaction was explored with a series of
sulfides and data are reported in Table 1. High yields and
high sulfoxide/sulfone selectivity were obtained for all the
tested substrates bearing either aromatic or aliphatic sub-
stituents. The sulfoxides have been obtained as main prod-
ucts with very high sulfoxide/sulfone (SO:SO2) selectivity
in the majority of the cases (Table 1).
The epoxidation reaction using cis-cyclooctene 8 with
hydrogen peroxide/TBHP as oxidant in different solvents
was also explored (Table 2). In all the cases, the formation
1 3

Synthesis, Characterization and Catalytic Activity of a Tungsten(VI) Amino Triphenolate…
Table 1 Oxidation of sulfides 7a–g by 35% aqueous hydrogen perox-
ide catalysed by 6 (1 mol%)
3.1 Experimental Section
3.1.1 X-ray Crystallography
Yield (%)a SO:SO2
a
Substrate
A summary of data collection and structure refinement for
6 is reported in the Supporting Info. Single crystal data
were collected with a Bruker Smart Breeze area detector
diffractometer, Mo Kα: λ=0.71073 Å. The unit cell param-
eters were obtained using 60 ω-frames of 0.5° width and
scanned from three different zone of reciprocal lattice. The
intensity data were integrated from several series of expo-
sures frames (0.3° width) covering the sphere of reciprocal
space. CCDC 1531503 contain the supplementary crystal-
lographic data for this paper. A crystallographic table sum-
marizing the structural refinement is reported in the sup-
porting information.
1
98
99:1
2
3
4
99
89
72
86:14
99:1
99:1
Synthesis of Tungsten(VI) complex (6). In a glove-box
under nitrogen atmosphere, WOCl4 (136 mg, 0.40 mmol),
NEt3 (260 μl, 2 mmol) and the ligand precursor 5 (200 mg,
0.39 mmol) were mixed in dry and degassed methanol
(50 mL). The solution was left stirring at room tempera-
ture for 18 h and the resulting intense orange–red solution
was filtered through a short pad of silica (or in a very short
silica column) using dichloromethane/methanol (95:5) as
eluent. The fractions collected were evaporated, 20 mL
of HCl (1 N) were added and the mixture was stirred at
room temperature for 20 min. An extraction with 20 mL of
CH2Cl2 was then performed and the organic fraction was
evaporated to afford complex 6 as an intense orange–yel-
low solid; yield: 65%. Crystals suitable for X-ray diffraction
were obtained by recrystallization of the solid in CH3CN.
1H NMR (300 MHz, CD3CN) δ 7.17 (m, 3 H, ArH), 6.97
(d, J=7.42 Hz, 2 H, ArH), 6.86 (d, J=7.43 Hz, 1 H, ArH),
6.64 (t, J=7.59 Hz, 2 H, ArH), 6.53 (t, J=7.58 Hz, 1 H,
ArH), 3.97 (d, J=13.3 Hz, 2 H, NCH2), 3.67 (s, 2 H,
NCH2), 3.57 (d, J=13.3 Hz, 2 H, NCH2), 1.50 (s, 9 H),
1.28 (s, 18 H). 13C NMR (62.9 MHz, CDCl3): δ=158.85
(C), 141.57 (C), 140.56 (CH), 128.67 (CH), 128.14 (C),
126.87 (CH), 60.65 (CH) 35.66 (CH), 35.20 (CH), 30.60
(C), 30.31 (C); ESI-MS: m/z=716.2 (M−); elemental
analysis (CHN): calculated for C33H43NO5W+CH3CN:
C=55.42%, H=6.11%, N=3.69%; found: C 55.18%, H
6.43%, N 3.55%.
5
6
99
99
90:10
75:25
Reaction conditions: 25°C; [sulfide]0 =[H2O2]0
= 0.1 M [6]
=0.001 M in CD3OD
aTotal yield determined by H NMR analysis on the crude reaction
1
mixture after complete oxidant consumption (iodometric test)
Table 2 Oxidation of cis-cyclooctene 8 at 60°C catalysed by 6
(5 mol%)
ROOH
[ROOH] M
Solvent
t1/2 (min)
9 Yield (%)a
3.1.2 General Procedure for Catalytic Oxidations
TBHP
H2O2
H2O2
H2O2
0.1
0.1
0.1
0.2
CDCl3
–
45
57
63
94
CD3OD
CD3CN
CD3CN
240
33
15
In a screw-cap NMR tube, a solution of catalyst 6 (1 or
5 mol % in CDCl3, CD3OD or CD3CN), internal standard
(1,2-dichloroethane, DCE, 0.1 M), substrate (0.1 M) and
oxidant (35% aqueous H2O2, or 80% tert-butyl hydrop-
eroxide in water, 0.1 M) were mixed up to a final volume
of 0.6 mL. The experimental conditions of each cata-
lytic oxidation are reported in Fig. 3 and Tables 1 and 2.
Effect of the oxidant and solvent, Reaction conditions: 60°C;
[8]=0.1 M; [6] =0.005 M
aDetermined by 1H NMR analysis on the crude reaction mixture after
complete oxidant consumption (iodometric test)
1 3

E. Badetti et al.
29. Licini G, Bonchio M, Modena G, Nugent WA (1999) Pure Appl
Chem 71:463
Concentrations of reagents and products, were monitored
1
by integration of H NMR resonances in respect of the
30. Favretto L, Nugent WA, Licini G (2002) Tetrahedron Lett
43:2581
internal standard DCE (3.78 ppm).
31. Santoni G, Mba M, Bonchio M, Nugent WA, Zonta C, Licini G
(2010) Chem Eur J 16:645
Acknowledgements This research was supported by Università di
Padova (PRAT- CPDA153122) and Cariparo Foundation (F.R. fellow-
ship). The project has been carried out in the frame of Cost Action
CM1402—Crystallize.
32. Scaramuzzo FA, Licini G, Zonta C (2013) Chem Eur J 19:16809
33. Badetti E, Wurst K, Licini G, Zonta C (2016) Chem Eur J
22:6515
34. Berardozzi R, Badetti E, Carmo Dos Santos NA, Wurst K, Licini
G, Pescitelli G, Zonta C, Di Bari L (2016) Chem Comm 52:8428
35. Natali M, Badetti E, Deponti E, Gamberoni M, Scaramuzzo FA,
Sartorel A, Zonta C (2016) Dalton Trans 45:14764
36. Scaramuzzo FA, Badetti E, Licini G, Zonta C (2017) Eur J Org
Chem 11:1438
References
1. Mugdan M, Young DP (1949) J Chem Soc 2988
2. Prandi J, Kagan HB, Mimoun H (1986) Tetrahedron Lett
23:2617
37. Bravin C, Badetti E, Scaramuzzo FA, Licini G, Zonta C (2017) J
Am Chem Soc 139:6456
3. Venturello C, Alneri E, Ricci M (1983) J Org Chem 48:3831
4. Kirshenbaum KS, Sharpless KB (1985) J Org Chem 50:1979
5. De Vos DE, Sels BF, Jacobs PA (2002) Adv Catal 46:1
6. Weinstock IA, Barbuzzi EMG, Wemple MW, Cowan JJ, Reiner
RS, Sonnen DM, Heintz RA, Bond JS, Hill CL (2001) Nature
414:191
38. Mba M, Pontini M, Lovat S, Zonta C, Bernardinelli G, Kündig
EP, Licini GM (2008) Inorg Chem 47:8616
39. Zonta C, Cazzola E, Mba M, Licini G (2008) Adv Synth Catal
350:2503
40. Licini G, Mba M, Zonta C (2009) Dalton Trans. doi:10.1039/
b822653a
7. Ichihara J (2001) Tetrahedron Lett 42:695
41. Mba M, Prins LJ, Zonta C, Cametti M, Valkonen A, Rissanen K,
Licini G (2010) Dalton Trans 39:7384
8. Hoegaerts D, Sels BF, de Vos DE, Verpoort F, Jacobs PA (2000)
Catal Today 60:209
42. Romano F, Linden A, Mba M, Zonta C, Licini G (2010) Adv
Synth Catal 352:2937
9. Thiel WR, Eppinger J (1997) Chem Eur J, 696
10. White MC, Doyle AG, Jacobsen EN (2001) J Am Chem Soc
123:7194
43. Zonta C, Kolarovic A, Mba M, Pontini M, Kündig EP, Licini G
(2011) Chirality 23:796
11. Venturello G, Alneri E, Ricci M (1983) J Org Chem 48:3831
12. Sato K, Aoki M, Ogawa M, Hashimoto T, Noyori R (1996) J Org
Chem 61:8310
44. Whiteoak CJ, Gjoka B, Martin E, Martínez Belmonte M, Escu-
dero-Adán EC, Zonta C, Licini G, Kleij AW (2012) Inorg Chem
51:10639
13. Sato K, Aoki M, Ogawa M, Hashimoto T, Panyella D, Noyori R
(1997) Bull Chem Soc Jpn 70:905
45. Zonta C, Licini G (2013) Chem Eur J 19:9438
46. Badetti E, Gjoka B, Nagy EM, Bernardinelli G, Kündig PE,
Zonta C, Licini G (2015) Eur J Inorg Chem 21:3478
47. Badetti E, Romano F, Marchiò L, Taşkesenlioǧlu S, Daştan A,
Zonta C, Licini G (2016) Dalton Trans 45:14603
48. Badetti E, Lloveras V, Romano F, Di Lorenzo R, Veciana J,
Vidal-Gancedo J, Zonta C, Licini G (2016) Eur J Inorg Chem.
doi:10.1002/ejic.201670311
14. Sato K, Aoki M, Noyori R (1998) Science 281:1646
15. Sato K, Aoki M, Ogowa M, Hasimoto T, Noyori R (1996) J Org
Chem 61:8310
16. Sato K, Aoki M, Ogowa M, Hashimoto T, Panyela D, Noyori R
(1997) Bull Chem Soc Jpn 70:905
17. Kamata K, Yonehara K, Sumida Y, Yamaguchi M, Hikichi S,
Mizuno N (2003) Science 300:964
49. Miceli C, Rintjema J, Martin E, Escudero-Adán EC, Zonta C,
Licini G, Kleij AW (2017) ACS Catal 7:2367
18. Aminia M, Mehdi Haghdoost M, Bagherzadeh M (2014) Coord
Chem Rev 268:83
50. Prins LJ, Mba Blázquez M, Kolarović A, Licini G (2006) Tetra-
hedron Lett 47:2735
19. Dinoi C, Ciclosi M, Manoury E, Maron L, Perrin L, Poli R
(2010) Chem Eur J 16:9572
51. Hänninen MM, Sillanpää R, Kiveläb H, Lehtonen A (2011) Dal-
ton Trans 40:2868
20. Dinoi C, Poli R, Perrin L, Maron L (2012) Dalton Trans 41:1131
21. Vrdoljak V, Pisk J, Agustin D, Novak P, Parlov Vuković J,
Matković-Čalogović D (2014) New J Chem 38:6176
22. Vrdoljak V, Pisk J, Prugovečki B, Agustin D, Novak P,
Matković-Čalogović D (2016) RSC Adv 6:36384
23. Chmura AJ, Davidson MG, Frankis CJ, Jones MD, Lunn MD
(2008) Chem Commun. doi:10.1039/b718678a
24. Gendler S, Segal S, Goldberg I, Goldschmidt Z, Kol M (2006)
Inorg Chem 45:4783
52. Lehtonen A, Sillanpää R (2005) Organometallics 24:2795
53. Lehtonen A, Sillanpää R (2004) Inorg Chem 43:6501
54. Dupé A, Hossain K, Schachner JA, Belaj F, Lehtonen A, Nord-
lander E, Mösch-Zanetti NC (2015) Eur J Inorg Chem 21:3572
55. Ueyama N, Oku H, Kondo M, Okamura T, Yoshinaga N, Naka-
mura A (1996) Inorg Chem 35:643
56. Sözen-Aktaş P, Manoury E, Demirhan F, Poli R (2013) Eur J
Inorg Chem 15:2728
25. Peuronen A, Lehtonen A (2016) Top Catal 59:1132
26. Di Furia F, Licini G, Modena G, Motterle R (1996) J Org Chem
61:5175
57. Amarante TR, Antunes MM, Valente AA, Almeida Paz FA, Pill-
inger M, Gonçalves IS (2015) Inorg Chem 54:9690
58. Bhaumik C, Manoury E, Daran JC, Sözen-Aktaş P, Demirhan F,
Poli R (2014) J Organomet Chem 760:115
27. Bonchio M, Licini G, Modena G, Moro S, Bortolini O, Traldi P,
Nugent WA (1997) Chem Commun. doi:10.1039/A701227F
28. Bonchio M, Calloni S, Di Furia F, Licini G, Modena G, Moro S,
Nugent WA (1997) J Am Chem Soc 119:6935
1 3