Sequential coupling approach to the synthesis of nickel(II) complexes with N-aryl-2-amino phenolates

Article abstract of DOI:10.1021/co3000665

A sequential multicomponent coupling approach is a powerful method for the construction of combinatorial libraries because structurally complex and diverse molecules can be synthesized from simple materials in short steps. In this paper, an efficient synthesis of nickel(II) complexes with N-aryl-2-amino phenols via a sequential three-step coupling approach is described, for potential use in nonlinear optical materials, bioinspired catalytic systems, and near-infrared absorbing filters. Seventeen N-aryl-2-amino phenolates were successfully synthesized in high yields based on the coupling of 3,5-di-tert-butylbenzene-1,2-diol with a pivotal aromatic scaffold, 4-bromo-2-iodo-aniline, followed by sequential Suzuki-Miyaura coupling with aryl boronates. A total of 16 analytically pure nickel(II) complexes with N-aryl-2-amino phenolates were obtained from 17 complexation trials. The procedure allowed us to assemble 4 components in high yields without protection, deprotection, oxidation or reduction steps. Various building blocks that included electron-donating, electron-withdrawing, and basic were used, and readily available, nontoxic and environmentally benign substrates and reagents were employed with no generation of toxic compounds. No strict anhydrous or degassed conditions were required. Absorption spectroscopic measurement of the synthesized nickel(II) complexes revealed that the ortho-substituent Ar ¹ exerted more influence on the absorption wavelength of the complexes than the para-substituent Ar². On the other hand, both substituents Ar¹ and Ar² influenced the molar absorptivity values. These observations should be useful for the design of new and useful nickel(II) complexes as near-infrared chromophores.

Full text of DOI:10.1021/co3000665

Research Article
pubs.acs.org/acscombsci
Sequential Coupling Approach to the Synthesis of Nickel(II)
Complexes with N‑aryl-2-amino Phenolates
Shinichiro Fuse, Hiroaki Tago, Masato M. Maitani, Yuji Wada, and Takashi Takahashi*
Department of Applied Chemistry, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan
S
* Supporting Information
ABSTRACT: A sequential multicomponent coupling approach is a powerful method for the construction of combinatorial
libraries because structurally complex and diverse molecules can be synthesized from simple materials in short steps. In this
paper, an efficient synthesis of nickel(II) complexes with N-aryl-2-amino phenols via a sequential three-step coupling approach is
described, for potential use in nonlinear optical materials, bioinspired catalytic systems, and near-infrared absorbing filters.
Seventeen N-aryl-2-amino phenolates were successfully synthesized in high yields based on the coupling of 3,5-di-tert-
butylbenzene-1,2-diol with a pivotal aromatic scaffold, 4-bromo-2-iodo-aniline, followed by sequential Suzuki−Miyaura coupling
with aryl boronates. A total of 16 analytically pure nickel(II) complexes with N-aryl-2-amino phenolates were obtained from 17
complexation trials. The procedure allowed us to assemble 4 components in high yields without protection, deprotection,
oxidation or reduction steps. Various building blocks that included electron-donating, electron-withdrawing, and basic were used,
and readily available, nontoxic and environmentally benign substrates and reagents were employed with no generation of toxic
compounds. No strict anhydrous or degassed conditions were required. Absorption spectroscopic measurement of the
synthesized nickel(II) complexes revealed that the ortho-substituent Ar1 exerted more influence on the absorption wavelength of
the complexes than the para-substituent Ar2. On the other hand, both substituents Ar1 and Ar2 influenced the molar absorptivity
values. These observations should be useful for the design of new and useful nickel(II) complexes as near-infrared chromophores.
KEYWORDS: sequential multicomponent coupling, combinatorial libraries, Suzuki−Miyaura coupling, nickel(II) complex,
N-aryl-2-amino phenol, near IR absorption
INTRODUCTION
■
Multicomponent coupling approaches to functional molecules
should meet the following criteria: (1) that multiple components
be sequentially coupled without extra steps such as protection,
deprotection, oxidation and reduction; (2) that each of the
coupling steps be compatible with a variety of functional groups;
(3) that good yields are obtained without the inconvenience of
strictly anhydrous and/or degassed conditions; and, (4) that readily
available, nontoxic, and environmentally benign substrates and
reagents be employed with no generation of toxic compounds.
Figure 1. Structure of a nickel(II) complex with N-aryl-2-amino
From this point of view, sequential Suzuki−Miyaura coupling
reactions based on aromatic scaffolds containing iodo- and bromo-
groups are useful. Because the iodo-group can be chemoselectively
activated without affecting the bromo-group, such molecules engage
and give sequential coupling reactions with good control of
regioselectivity, high yields, and high functional group compatibility.
phenolates.
Received: June 18, 2012
Revised: August 19, 2012
Published: August 23, 2012
© 2012 American Chemical Society
545
dx.doi.org/10.1021/co3000665 | ACS Comb. Sci. 2012, 14, 545−550

ACS Combinatorial Science
Research Article
Scheme 1. Combinatorial Synthesis of Nickel(II) Complexes with N-aryl-2-amino Phenolates
Scheme 2. First Coupling Reaction of the Aromatic Scaffold,
4-Bromo-2-iodo-aniline (5) with 3,5-Di-tert-butylbenzene-
1,2-diol (4)
Scheme 3. Sequential Suzuki−Miyaura Coupling Reaction
Based on the Aromatic Scaffold 8
In addition, Suzuki−Miyaura coupling1,2 requires neither anhydrous
conditions nor a toxic substrate, and generates no toxic compounds.
Several excellent syntheses employing the sequential Suzuki−
Miyaura coupling reaction based on the aromatic scaffolds
containing iodo- and bromo-groups have been reported.3−8
In 2001, Wieghardt and co-workers first reported a square-
planar, neutral, nickel(II) complex with N-aryl-2-amino-
phenolate 1 and revealed its unique singlet diradical character
by theoretical calculation, chemical synthesis, and spectroscopic
analysis (Figure 1).9−11 Recently, a nickel(II) complex with
N-aryl-2-amino-phenolate has garnered attention because of its
potential use for nonlinear optical materials,12 bioinspired
catalytic systems,13 and near-infrared absorbing filter.14 The
development of a divergent, protection/deprotection-free, short,
synthetic route for the construction of nickel(II) complexes with
2-amino-phenolates would encourage further applications of
these structures.
546
dx.doi.org/10.1021/co3000665 | ACS Comb. Sci. 2012, 14, 545−550

ACS Combinatorial Science
Research Article
Scheme 4. Synthesis of 17 N-aryl-2-amino Phenols Using Our Developed Procedure
Table 1. Synthesis of 17 N-aryl-2-amino Phenols Using Our
Developed Procedure
Table 2. Nickel(II) Complexes of the Prepared N-aryl-2-
amino Phenols 3
a
a
b
c
yield
entry
substrate
product
yield
λmax (nm)
ε (M−1·cm−1
)
1
3{1,1}
3{2,2}
3{3,3}
3{4,4}
3{5,5}
3{1,2}
3{1,3}
3{1,4}
3{1,5}
3{2,1}
3{2,3}
3{3,1}
3{3,2}
3{4,1}
3{4,5}
3{5,1}
3{5,4}
2{1,1}
2{2,2}
2{3,3}
2{4,4}
2{5,5}
2{1,2}
2{1,3}
2{1,4}
2{1,5}
2{2,1}
2{2,3}
2{3,1}
2{3,2}
2{4,1}
2{4,5}
2{5,1}
2{5,4}
22%
42%
57%
47%
914
925
909
923
7.0 × 103
1.6 × 104
3.0 × 104
2.1 × 104
entry
building block
product
first coupling
second coupling
98%
2
1
6{1}
6{2}
6{3}
6{4}
6{5}
3{1,1}
3{2,2}
3{3,3}
3{4,4}
3 {5,5}
3{1,2}
3{1,3}
3{1,4}
3{1,5}
3{2,1}
3{2,3}
3{3,1}
3{3,2}
3{4,1}
3{4,5}
3{5,1}
3{5,4}
3
2
quant.
71%
4
3
5
4
97%
6
65%
13%
44%
67%
70%
17%
66%
71%
53%
61%
20%
16%
917
917
916
917
920
917
909
912
922
920
905
911
1.6 × 104
2.3 × 104
1.2 × 104
1.8 × 104
1.2 × 104
2.7 × 104
2.1 × 104
3.1 × 104
2.3 × 104
1.6 × 104
7.1 × 103
5
30%
7
6
6{1}
6{1}
6{1}
6{1}
6{2}
6{2}
6{3}
6{3}
6{4}
6{4}
6{5}
6{5}
7{2}
7{3}
7{4}
7{5}
7{1}
7{3}
7{1}
7{2}
7{1}
7{5}
7{1}
7{4}
95%
95%
95%
95%
83%
83%
81%
81%
71%
71%
93%
99%
quant.
90%
77%
69%
78%
68%
89%
8
7
9
8
10
11
12
13
14
15
16
17
9
10
11
12
13
14
15
16
17
56%
b
1.0 × 104
30% (2 steps)
36% (2 steps)
a
b
c
b
Isolated yield. Absorption maxima in dichloromethane. Molar
a
b
absorption coefficients in dichloromethane.
Isolated yield. The first coupling products could not be purified
because of their poor solubilities; therefore, the obtained products
were used for the second coupling reaction without isolation.
reported a sequential palladium-catalyzed cross-coupling
approach to the synthesis of rod-like liquid crystals16 and re-
sorcylic acid lactones19,20 based on aromatic scaffolds that
would retain multiple reactive sites. Here, we wish to report
the development of a highly efficient sequential coupling
We have developed a multicomponent coupling approach
for the synthesis of bioactive compounds and functional materials in
solution and in a solid phase for decades.15−21 Recently, we
Scheme 5. Complexation of N-aryl-2-amino Phenol 3 with Nickel(II), and the Absorption Spectrum of the Nickel(II) Complex
2{1,1} in Dichloromethane
547
dx.doi.org/10.1021/co3000665 | ACS Comb. Sci. 2012, 14, 545−550

ACS Combinatorial Science
Research Article
Figure 2. (a) λmax (nm) of the nickel complexes 2. Ar1 = 1 (blue), 2 (purple), 3 (green), 4 (yellow), 5 (red). (b) λmax (nm) of 2. Ar2 = 1 (blue), 2 (purple),
3 (green), 4 (yellow), 5 (red). (c) ε values (M−1·cm−1) of 2. Ar1 = 1 (blue), 2 (purple), 3 (green), 4 (yellow), 5 (red). (d) ε values (M−1·cm−1) of 2.
Ar2 = 1 (blue), 2 (purple), 3 (green), 4 (yellow), 5 (red).
approach based on the aromatic scaffold retention of 3 reactive
sites, that is, amino-, bromo-, and iodo-groups in the synthesis
of nickel(II) complexes with N-aryl-2-amino phenolates.
RESULTS AND DISCUSSION
■
The synthetic plan for the construction of a library of nickel(II)
complexes with N-aryl-2-amino phenolates is shown in Scheme 1.
548
dx.doi.org/10.1021/co3000665 | ACS Comb. Sci. 2012, 14, 545−550

ACS Combinatorial Science
Research Article
Structurally diverse nickel complexes 2 with various aryl
groups, Ar1 and Ar2, would be synthesized by the complexation of
N-aryl-2-amino phenols 3 with nickel(II).9 The essence of our
synthetic route was sequential coupling reactions of the
commercially available 4-bromo-2-iodoaniline (5) at amino-,
iodo- and bromo-groups with components 4, 6, and 7. In the first
coupling reaction, the amino group of scaffold 5 was required to
react with 3,5-di-tert-butylbenzene-1,2-diol (4) without affecting
the bromo- and iodo-groups. While the analogous coupling
reaction of 4-bromo-aniline with 4 was previously reported,22 the
bulky and reactive iodo group adjacent to the amino group
caused some concern regarding amine reactivity and elimination
of the iodo group. The second step, a Suzuki−Miyaura coupling
reaction requiring activation of the carbon−iodine bond of 8
without affecting the bromo group, is well-known for simple
aromatic scaffolds.8 In our case, the task was to achieve such
chemoselectivity with an adjacent bulky and electron-rich
arylamino group. After introducing the building block 6 in the
second coupling reaction, the remaining carbon−bromine bond
would be targeted for a third coupling reaction with building
block 7. This strategy would allow us to introduce the same or
different building blocks into the key aromatic scaffold 5. The
arylboronate building blocks chosen included a simple thiophene
{1}, a thiophene containing an electron-donating triarylamine
substituent {2}, an electron-rich triarylamine building block {3},
an electron-deficient thiophene bearing tert-butyl cyanoacrylate
{4}, and a para-pyridyl unit {5}.
The reaction of 4 and 5 was tested under different conditions
(Scheme 2). Heating at 140 °C in the presence of NEt3 as
previously reported22 gave no reaction and recovered starting
materials, whereas heating under acidic conditions afforded the
undesired deiodinated product 9. In contrast, the desired
coupling product 8 was obtained in a good yield after heating
without additional reagents or solvent.23
The key Suzuki−Miyaura coupling reaction of 8 (Scheme 3)
with a simple thiophene-2-boronic acid was found to proceed
smoothly to give 10{1} (95% yield) under mild reaction
conditions (Pd(PPh3)4, K2CO3 in THF/H2O) in spite of the
presence of the adjacent bulky arylamino group. A subsequent
Suzuki−Miyaura coupling of the aryl bromide 10{1} with
thiophene-2-boronic acid using the more active monophos-
phine-palladium complex of tris(tert-butyl)phosphine similarly
gave the desired product, 10{1,1}, in excellent yield (99%). It was
possible to perform both steps simultaneously with the second
catalyst with an excellent yield (98%). None of these steps
required strict anhydrous or degassed conditions.
With one exception, the desired nickel(II) complexes 2 were all
obtained in varying yields as a dark solids. A noticeable feature of
their IR spectra was the absence of the bands at 3446 cm−1
attributable to v(OH) and v(NH) of corresponding N-aryl-2-
amino phenols 3.13 The 1H NMR spectra of complexes 2 were
quite similar to those of the corresponding N-aryl-2-amino
phenols 3. Complexation of 3{5,5} gave an insoluble black
precipitate instead of the expected compound 2{5,5}, presum-
ably because of coordination polymerization involving the
pyridine groups. The near IR absorption peak (905−925 nm)
1
of all examples of 2 (Table 2), and the H NMR spectra
supported the assignment of the desired nickel(II) complex
structure. All complexes 2 were stable in air at room temperature.
Relationships between structure and photophysical properties
of UV-near-IR absorbing compounds have been previously
investigated.25,26 Absorption maxima (λmax) and molar absorp-
tion coefficients (ε) of the prepared nickel complexes 2 are
shown in panels (a)−(d) in Figure 2. Interestingly, the ortho-
substituent Ar1 exerted more influence on λmax of the complexes
than the para-substituent Ar2, as shown in panels (a) and (b).
The wavelength of the maximum absorption became longer in
the following order Ar1 = 5 < 3 < 1 < 2 < 4. On the other hand,
both substituents Ar1 and Ar2 influenced the ε values, as shown in
panels (c) and (d). The ε values increased in the following order
Ar1 = 5 < 1 < 2 < 4 < 3, Ar2 = 1 = 4 < 2 < 5 < 3. The use of building
block 3 increased the ε values of the complexes irrespective of the
substituent position. The influence of the other substituents
varied by position. For instance, building block 5 decreased the ε
values of the complexes when it was employed as Ar1, whereas
the same substituent increased the ε values when employed as
Ar2.
CONCLUSION
■
The solution-phase combinatorial synthesis of 17 N-aryl-2-
amino phenolates, and their complexation with nickel(II) was
successfully demonstrated. The key for the synthesis was a highly
efficient three-step coupling procedure involving the sequential
engagement of aromatic amino, iodo, and bromo substituents
without extra steps such as protection, deprotection, oxidation,
and reduction. A total of 16 analytically pure nickel(II)
complexes with N-aryl-2-amino phenolates were obtained from
17 complexation trials. Various building blocks including
electron-donating, electron-withdrawing, and basic components
were used, and readily available, non toxic, and environmentally
benign substrates and reagents were employed with no genera-
tion of toxic compounds. In addition, the developed procedure
required no strict anhydrous or degassed conditions. Absorption
spectra of the synthesized nickel(II) complexes revealed that the
ortho-substituent (Ar1) exerted more influence on the near-IR
absorption wavelength than the para-substituent (Ar2), whereas
both substituents influenced molar absorptivity. These observa-
tions should be useful for the further design of nickel(II)-based
near-infrared dyes.
The above conditions were applied to the synthesis of 17 N-
aryl-2-amino phenols (Scheme 4 and Table 1). All the products
were adequately characterized using 1H NMR, 13C NMR, IR, and
HRMS. Although slightly decreased yields were observed with
the use of a building block containing pyridine (entries 5, 15, 16,
and 17), other combinations afforded the desired products in
good to excellent yields regardless of the electronic factor
presented by building blocks 6 and 7. Thus, structurally diverse
N-aryl-2-amino phenols, some of which can be expected to retain
antiviral22 and antioxidative24 activities, can be synthesized in
only three steps from commercially available 4-bromo-2-
iodoaniline (5).
The complexation of N-aryl-2-amino phenols 3 with nickel(II)
was examined (Scheme 5 and Table 2) using a standard reaction pro-
cedure at 95 °C, in acetonitrile solvent containing triethylamine.
After cooling to 0 °C, the desired products were isolated as
precipitates and purified by recrystallization (CH2Cl2-MeOH).
ASSOCIATED CONTENT
■
S
* Supporting Information
Analytical data (1H, 13C NMR, and absorption spectra) for the
compounds are provided. This material is available free of charge
via the Internet at http://pubs.acs.org.
549
dx.doi.org/10.1021/co3000665 | ACS Comb. Sci. 2012, 14, 545−550

ACS Combinatorial Science
Research Article
catalyzed carbonylation on a polymer support. Angew. Chem., Int. Ed.
2003, 42, 5230.
(16) Doi, T.; Inoue, H.; Tokita, M.; Watanabe, J.; Takahashi, T.
Sequential palladium-catalyzed coupling reactions on solid-phase. J.
Comb. Chem. 2008, 10, 135.
AUTHOR INFORMATION
■
Corresponding Author
*Phone: +81-3-5734-2120. Fax: +81-3-5734-2884. E-mail:
ttak@apc.titech.ac.jp.
(17) Yoshida, M.; Doi, T.; Kang, S. M.; Watanabe, J.; Takahashi, T.
Solid-phase combinatorial synthesis of ester-type banana-shaped
molecules by sequential palladium-catalyzed carbonylation. Chem.
Commun. 2009, 2756.
Notes
The authors declare no competing financial interest.
(18) Serizawa, T.; Miyamoto, S.; Numajiri, Y.; Fuse, S.; Doi, T.;
Takahashi, T. Stereoselective one-pot three-component coupling
approach towards the synthesis of the AC ring system of taxanes.
Tetrahedron Lett. 2009, 50, 3408.
ACKNOWLEDGMENTS
■
We thank Dr. Susumu Kawauchi and Mr. Yoshihiro Hayashi,
Tokyo Institute of Technology, for their fruitful suggestions.
(19) Fuse, S.; Sugiyama, S.; Takahashi, T. Rapid assembly of resorcylic
acid lactone frameworks through sequential palladium-catalyzed
coupling reactions. Chem.Asian J. 2010, 5, 2459.
REFERENCES
■
(1) Miyaura, N.; Suzuki, A. Stereoselective synthesis of arylated (E)-
alkenes by the reaction of alk-1-enylboranes with aryl halides in the
presence of palladium catalyst. J. Chem. Soc., Chem. Commun. 1979, 866.
(2) Miyaura, N.; Yamada, K.; Suzuki, A. New stereospecific cross-
coupling by the palladium-catalyzed reaction of 1-alkenylboranes with
1-alkenyl or 1-alkyunyl halides. Tetrahedron Lett. 1979, 3437.
(3) Gray, G. W.; Hird, M.; Lacey, D.; Toyne, K. J. The synthesis and
transition-temperatures of some 4,4″-dialkyl-1,1′-4′,1″-terphenyl and
4,4″-alkoxyalkyl-1,1′-4′,1″-terphenyl with 2,3-difluoro or 2′,3′-difluoro
substituents and of their biphenyl analogs. J. Chem. Soc., Perkin Trans. 2
1989, 2041.
(4) Unrau, C. M.; Campbell, M. G.; Snieckus, V. Directed ortho-
metalation-Suzuki cross coupling connections - convenient regiospecific
routes to functionalized meta-tetraryls and para-teraryls and meta-
quinquearyls. Tetrahedron Lett. 1992, 33, 2773.
(5) Hird, M.; Lewis, R. A.; Toyne, K. J.; West, J. J.; Wilson, M. K. The
synthesis of novel highly substituted benzene derivatives for use in
palladium-catalysed cross-coupling reactions. J. Chem. Soc., Perkin Trans.
1 1998, 3479.
(20) Sugiyama, S.; Fuse, S.; Takahashi, T. Three-component coupling
approach toward the synthesis of a resorcylic acid lactone framework.
Tetrahedron 2011, 67, 6654.
(21) Fuse, S.; Masui, H.; Tannna, A.; Shimizu, F.; Takahashi, T.
Combinatorial synthesis and evaluation of α-iminocarboxamide-nickel-
(II) catalysts for the copolymerization of ethylene and a polar monomer.
ACS Comb. Sci. 2012, 14, 17.
(22) Shadyro, O. I.; Sorokin, V. L.; Ksendzova, G. A.; Polozov, G. I.;
Nikolaeva, S. N.; Pavlova, N. I.; Savinova, O. V.; Boreko, E. I. Synthesis
and antiviral activity of N-acyl derivatives of 4,6-di-(tert-butyl)-2-
aminophenol. Pharm. Chem. J. 2003, 37, 399.
(23) Kracker, H. Patent DE 580519, 1933.
(24) Maslovskaya, L. A.; Petrikevich, D. K.; Timoshchuk, V. A.;
Shadyro, O. I. Synthesis and antioxidative properties of certain
derivatives of alkylated pyrocatechin. Zh. Obshch. Khim. 1996, 66, 1893.
(25) Kim, E.; Koh, M.; Lim, B. J.; Park, S. B. Emission Wavelength
Prediction of a Full-Color-Tunable Fluorescent Core Skeleton, 9-Aryl-
1,2-dihydropyrrolo 3,4-b indolizin-3-one. J. Am. Chem. Soc. 2011, 133,
6642.
(26) Kim, E.; Koh, M.; Ryu, J.; Park, S. B. Combinatorial discovery of
full-color-tunable emissive fluorescent probes using a single core
skeleton, 1,2-dihydropyrrolo 3,4-beta indolizin-3-one. J. Am. Chem. Soc.
2008, 130, 12206.
(6) Bo, Z. S.; Schluter, A. D. Synthesis of low-generation, aryl-/alkyl-
type, nonpolar dendrons carrying protected hydroxyalkyl groups in the
periphery. J. Org. Chem. 2002, 67, 5327.
(7) Voisin-Chiret, A. S.; Muraglia, M.; Burzicki, G.; Perato, S.; Corbo,
F.; Santos, J. S. D.; Franchini, C.; Rault, S. Synthesis of new
phenylpyridyl scaffolds using the Garlanding approach. Tetrahedron
2010, 66, 8000.
(8) Rossi, R.; Bellina, F.; Lessi, M. Highly selective palladium-catalyzed
Suzuki-Miyaura monocoupling reactions of ethene and arene derivatives
bearing two or more electrophilic sites. Tetrahedron 2011, 67, 6969.
(9) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermuller, T.;
Wieghardt, K. Electronic structure of bis(O-iminobenzosemiquinonato)-
metal complexes (Cu, Ni, Pd). The art of establishing physical oxidation
states in transition-metal complexes containing radical ligands. J. Am. Chem.
Soc. 2001, 123, 2213.
(10) Bachler, V.; Olbrich, G.; Neese, F.; Wieghardt, K. Theoretical
evidence for the singlet diradical character of square planar nickel
complexes containing two omicron-semiquinonato type ligands. Inorg.
Chem. 2002, 41, 4179.
(11) Min, K. S.; Weyhermuller, T.; Bothe, E.; Wieghardt, K.
Tetradentate bis(O-iminobenzosemiquinonate(1-)) π radical ligands
and their O-aminophenolate(1-) derivatives in complexes of nickel(II),
palladium(II), and copper(II). Inorg. Chem. 2004, 43, 2922.
(12) Fukui, H.; Kishi, R.; Minami, T.; Nagai, H.; Takahashi, H.; Kubo,
T.; Kamada, K.; Ohta, K.; Champagne, B.; Botek, E.; Nakano, M.
Theoretical study on second hyperpolarizabilities of singlet diradical
square planar nickel complexes involving O-semiquinonato type ligands.
J. Phys. Chem. A 2008, 112, 8423.
(13) Mukherjee, C.; Weyhermuller, T.; Bothe, E.; Chaudhuri, P.
Targeted oxidase reactivity with a new redox-active ligand incorporating
N(2)O(2) donor atoms. complexes of Cu(II), Ni(II), Pd(II), Fe(III),
and V(V). Inorg. Chem. 2008, 47, 11620.
(14) Sato, W.; Saito, Y. Patent WO 2005/02213 A1, 2005.
(15) Takahashi, T.; Kusaka, S.; Doi, T.; Sunazuka, T.; Omura, S. A
combinatorial synthesis of a macrosphelide library utilizing a palladium-
550
dx.doi.org/10.1021/co3000665 | ACS Comb. Sci. 2012, 14, 545−550