Copper(ii)-hydroxide facilitated C-C bond formation: The carboxamido pyridine system: Versus the methylimino pyridine system

Article abstract of DOI:10.1039/d0dt02288h

A copper(ii)-hydroxide-induced carbon-carbon bond formation reaction is explored with the synthesis of an asymmetric carboxamido-methylimino pyridine Cu(i) complex of [CuI(py(N-CO)(NC-C)ph2Me2)2]- (12). Two imine-methyl groups are coupled to form a bridge

Full text of DOI:10.1039/d0dt02288h

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DOI: 10.1039/D0DT02288H
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Copper(II)-Hydroxide Facilitated C−C Bond Formation:
Carboxamido Pyridine System versus Methylimino Pyridine
System
Yinghua Li,a,§ Weibin Fan,a,b,§ Zilong Zhang,a Xingkun Xie,a Shiqun Xianga,b and Deguang Huanga,*
Received 00th January 20xx,
Accepted 00th January 20xx
A copper(II)-hydroxide induced carbon–carbon bond formation reaction is explored with the synthesis of
asymmetric carboxamido−methylimino pyridine Cu(I) complex of [CuI(py(N−C=O)(N=C−C)ph2Me2)2]− (12). Two
imine-methyl groups are coupled to form a bridged C−C bond (N=C−C−C−C=N) at the methyl positions with the
reduction of two Cu2+ center ions to Cu+. The reaction is checked with three dicarboxamido pyridine [CuII–OH]
complexes, with which dinuclear Cu(I) complexes of [Cu2(py(N−C=O)2ph2R2)2]2− (R = methyl (3), methyl & allyl
(6)) and trinuclear [CuII-CuI-CuII] complex of [Cu3(20-py(N−C=O)2ph2dienMe3)2]+ (9) are obtained. The reactivities
of [CuII–L] (L = DMF, OH−) complexes in dicarboxamido pyridine, carboxamido−methylimino pyridine and
dimethylimino pyridine systems are discussed in terms of electron delocalization properties of ligands. A
cooperative metal−ligand (Cu2+ and enamide ligand) interaction is proposed based on the characterization of
ligated Cu(II) intermediates with the techniques of X-ray crystallography, UV-Vis spectroscopy, cyclic
voltammogram, EPR spectroscopy and DFT calculations.
DOI: 10.1039/x0xx00000x
www.rsc.org
synergistic manner to have more energetically accessible levels
for catalysis. Copper compounds are well established catalysts
for various oxidation and reduction reactions.2a,11 Ligand
promoted and Cu-catalyzed coupling reactions have been
described to have accelerating effect on these reactions.12
Questions come from the understanding of the nature of
catalytically active compounds, and further studies aim at
exploring the particular metal-ligand interaction system which
can be tuned to specific needs.
Introduction
Transition metal-catalyzed C–C bond formation reactions are
of great importance for the synthesis of natural products,1
bioactive molecules,2 and organic functional materials.3 The
interests in the development of new catalysts for functional
group transformation keep growing,4 and a desire for more
selective and efficient method to prepare organic substances
has driven efforts to make soluble catalysts for homogeneous
catalysis. Meanwhile, the reactivity and selectivity of catalysts
have been found widely influenced by the choice of the central
metal, and most efficient catalysts are related to the precious
metals such as palladium,5 iridium,6 platinum7 and gold,8 which
often perform two electron oxidation-reduction to promote
bond cleavage and bond formation reactions.
The steric and electronic properties of ligands are used to
control the catalytic performance of active species. Pincer
ligands have long been recognized in coordination chemistry
because of the constraints of orbital symmetry and their
distinctive properties in controlling of coordination sphere.13
A
a)
b)
Base metals are often more earth-abundant and far less
expensive for use in manufacturing services than precious
metals. In general, base metals prefer one electron redox
changes and pose challenges for controlling reactivity and/or
providing unique reaction pattern.9 The interplay of base
metals and their surrounding ligands must be considered for
the design of more potent catalysts with improved reaction
scope.10 Thus, the metal and the ligand can cooperate in a
H3C
R
CH3
O
O
N
N
R
R
R
N
N
NH
R
HN
R
R
R
R2
R2
py(N=C-C)2ph2
Methylimino Pyridine
H2py(N-C=O)2ph2
Carboxamido Pyridine
1
c)
d)
O
O
O
CH3
N
N
R
R
R
R
MII
OH
NH
R
N
qSate Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China.
bUniversity of Chinese Academy of Sciences, Beijing 100049, China.
§These authors contributed equally to this work.
N
N
R
R
R
R2
{M[py(N-C=O)2ph2R2](OH)}1-
M = Cu, Ni or Pd
Hpy(N-C=O)(N=C-C)ph2
Carboxamido-Methylimino Pyridine
*To whom correspondence should be addressed. E-mail: dhuang@fjirsm.ac.cn
Electronic Supplementary Information (ESI) available: Experimental section, X-ray
structure determinations, Crystallographic Data of Complexes, EPR spectra
simulation and calculations, Quantum chemical calculations and CIFs of complexes
1, 3, 3a, 5, 6, 9, 10 and 12. CCDC 2008133-2008140. See DOI: 10.1039/x0xx00000x
Fig. 1 Structures and abbreviations of NNN-pincer ligands and complexes.
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number of pincer complexes show relatively excellent thermal [py(N−C=O)2ph2
stability which is particularly important for homogeneous this reactivity. Metal, ligand and the hydroxo group worked in
catalysis at high temperature.14 Much of work is derived from
collaborative manner to facilitate the chemical
the first-row transition metal complexes, which are transformations.
coordinated with various bis(imino)pyridine NNN-pincer To our knowledge, the reduction behavior of transition
N,N’-bis(2,6- metal NNN-pincer complexes [M(py(N−C=O)2ph2R2)(OH)]1− (M
]
R2 2− would be the other contributing factor for
a
R2
R2
ligands py(N=C−C)2ph2
(py(N=C−C)2ph2
=
disubstitutedphenyl)-2,6-di(methylimine)pyridine, Fig. 1a). = Cu or Ni) was rarely studied, and the activating abilities of
These ligands can be used as reservoirs of electrons for bond- metals in the bis(carboxamide)pyridine ligand system have not
making reactions involving oxidation/reduction of the ligand to been investigated. As part of ongoing study of this basic ligand
tune the electronic properties.10a,15 In contrast, the system, we report here a C–C bond coupling reaction
bis(arylcarboxamide)pyridine
NNN-pincer
ligands facilitated by the NNN-pincer [CuII-OH] intermediate
R2
R2
H2py(N−C=O)2ph2
(H2py(N−C=O)2ph2
=
N,N’-bis(2,6- [Cu(py(N−C=O)(N=C−C)ph2Me2)(OH)]1−
11a
(Hpy(N−C=O)-
Me2
disubstitutedphenyl)-2,6-dicarboxamidepyridine, Fig. 1b) have (N=C−C)ph2
=
6-acetyl-N,N’-(2,6-dimethylphenyl)-
less redox activity and their coordinated transition metal picolinamide, Fig. 2d), involving hydroxide-ligand induced
compounds were mostly presented in higher valence states reduction of central Cu(II) to Cu(I) and the extent of metal to
such as divalence16 and trivalence.17 Cooperative activation of ligand electron transfer. As a result, two coordinated ligands
substrates
by
the
metal
complexes
of are coupled to generate a new C−C bond at the enamide
bis(arylcarboxamide)pyridine ligands was much in relation to positions forming a bridged bis(arylimino)pyridine ligand.
the fourth coordination donor, for example, the terminal
hydroxo
group
in
the
complexes
of
[M(py(N−C=O)2ph2R2)(OH)]1− (M = Cu or Ni) (Fig. 1c),18-23 where
the hydroxide showed remarkable nucleophilic reactivity on
the activation of carbon dioxide,18 dihydroanthracene,19
tetrahydrofuran,20 phenols,21 2-phenylpropionaldehyde22 and
organonitriles.23 The highly basic nature of the dianionic ligand
Results and Discussion
The deprotonation of the NNN-pincer ligand
Me2
H2py(N−C=O)2ph2
with two equivalents of Et4NOH in the
presence of Cu(OTf)2 in DMF affords the solvent ligated
1
2
CuII
1+ / 0
1
O
O
N
N
O
N
O
O
N
O
O
R1
R1
O
O
N
N
O
CH3
R1
O
CH2
R1
N
O
CH3
R1
N
Et4NOH
DMF
H2N NH2
MeCN
N
N
Me4NOH
or NaOH
R1
R1
R1
R1
N
N
N
N
a)
N
R1
R1
CuI CuI
CuII
N
N
CuI
d)
e)
f)
CuII
DMF
1
CuII
OH
2
N
N
N
N
R1
R1
CuII
CuII
N
CuII
N
N
N
N
N
N
H2N
R1
R1
R1
R1
N
DMF
N
O
R1
R1
R1
O
NH2
O
O
OH
OH2
X
n
3a
R1
=
3
12
11a
11b
10
10a
)
X = DMF ( ), X = Cl (
2
)
2
(50% yield based on
(10% yield based on
)
10
(40% yield based on
)
2
1
NiII
1
O
N
N
1+
O
O
O
O
N
N
toluene
N
Me4NOH
DMF
O
N
CuI
O
N
CH2
b)
CuI
O
CH3
R1
O
CH3
O
CH2
N
CuII
CuII
N
O
N
N
N
N
N
N
OH
Et4NOH
DMF
N
NiII
N
N
R2
R2
R2
R2
O
NiII
NiII
NiII
N
N
DMF
OH
N
N
N
N
DMF/THF
R1
R1
R1
R1
R1
R1
R1
OH
O
4
5
DMF
OH
OH2
14b
n
6
14
5
1
(30% yield based on
)
R2
=
13
(59% yield based on
)
13
14a
N
N
1
X
CuII
N
O
O
O
O
N
N
N
O
[Cu(OTf)2]
DMF
Me4NOH
DMF
1
1+
c)
1+
CH3
N
CuII
CuII
N
O
O
N
N
N
N
N
N
O
N
H3C
R1
H3C
CuI
N
N
N
DMF
R3
R1
OH
R3
8
Me4NOH
R1
R1
O
N
N
CuII
R1
R1
N
N
N
R1
N
N
X
NiI
CuI
DMF/THF
R1
N
N
R1
TfO
7
R1
CuII
DMF
N
N
N
N
H3C
O
N
N
R3
=
N
N
N
16
9
8
)
(45% yield based on
17
15
Fig. 2 The examination of NNN-pincer complexes Cu(II) to Cu(I) via [CuII−OH] intermediates: a) The generation of NNN-pincer Cu(I) by mixing of [CuII−OH] complex and
ethylenediamine; b) The investigation of the transformation of Cu(II) to Cu(I) without the presence of ethylenediamine; c) The investigation of the reduction of free Cu2+ to Cu+ by
[CuII−OH] complex; d) The exploration of the C-C bond formation of carboxamido−methylimino pyridine ligand via [CuII−OH] intermediate; e) The exploration of the configuration
of carboxamido−enamide pyridine [MII−OH] intermediate by nickel; f) The investigation of the stability of [CuII−OH] complex in dimethylimino pyridine system.
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[CuII-DMF] complex [CuII(py(N−C=O)2ph2Me2)(DMF)] (1) (Fig. examine the redox reaction (Scheme S1). The allyl group is
DOI: 10.1039/D0DT02288H
3a),
which
is
easily
convert
to
complex designed to enable the coordination ability of ligand to Cu+ ion.
(Et4N)[CuII(py(N−C=O)2ph2Me2)(OH)] (2) by an addition of Layer diffusion of toluene into a solution of [CuII−OH] complex
Et4NOH. The stirring of 2 with ethylenediamine (EDA) in MeCN of (Me4N)[CuII(py(N−C=O)2ph2Me/Allyl)(OH)] (5) in DMF under N2
under dinitrogen atmosphere generates
a
mixture of produces
a
monovalent
dicopper(I)
complex
of
complexes (Et4N)2[CuI(py(N−C=O)2ph2Me2)]2 (3) (Fig. 2a & 3b) (Me4N)2[CuI(py(N−C=O)2ph2Me/Allyl)]2 (6) spontaneously (Fig. 2b
and [CuII(py(N−C=O)2ph2Me2)(EDA)] (3a) (Fig. S1a). The & 3d). The complex 6 presents a symmetric coordination
monovalent dicopper complex 3 is isolated unexpectly, in configuration with each of Cu+ ion coordinated by three
which the two Cu(I) centers are bonded in different modes. nitrogen donors and one vinyl group, displaying a distorted
One is two-coordinated with a linear geometry, and the other tetrahedron geometry. The above two reactions are checked
is four-coordinated with distorted tetrahedron geometry. The with
analogous
[CuII-Cl]
complexes
of
and/or
characters of bond lengths (Cu−N: 1.854(1)−2.092(1) Å) and (Et4N)[CuII(py(N−C=O)2ph2Me2)(Cl)]
bond angels (N−Cu−N: 108.5(1)−173.3(1)°) indicate the (Et4N)[CuII(py(N−C=O)2ph2Me/Allyl)(Cl)], respectively. No Cu(I)
formation of Cu(I) complex 3 with a short CuI···CuI distance of product is obtained with the recovery of [CuII-Cl] starting
2.805(1) Å. Interestingly, this reaction does not generate the materials. This result supports the participation of hydroxo
cyanomethide complex of [CuII-CH2CN] and/or its related group in the oxidation-reduction process, probably as a
Cu(II)/Cu(I) species,23 probably due to the quick substitution of nucleophilic reagent for the deprotonation of organic species,
OH− ligand with EDA, following by the release of OH− anion as well as regulating the basicity of solution in favour of
into the solution. Moreover, the recrystalization of 3a in MeCN reduction of Cu2+ to Cu+. The basic nature of the dianionic
does NOT generate the Cu(I) complex 3. In order to validate NNN-pincer ligand would help to improve the nucleophilic
this reaction, a dicarboxamido NNN-pincer ligand with two reactivity of hydroxo group on the deprotonation of organic
allyl groups located on the two phenyl rings is synthesized to
substrates. Beyond that, the hydroxo-group induced reduction
of free Cu2+ ion is also investigated by the analogous [CuII−OH]
compound.
A
solution
of
(Me4N)[CuII(20-
py(N−C=O)2ph2dienMe3)(OH)] (8) (Fig. S1b) in DMF is treated
with one equivalent of solid Cu(OTf)2. The reaction affords the
mixed-valence trinuclear [CuII-CuI-CuII] complex [Cu3(20-
py(N−C=O)2ph2dienMe3)2](OTf) (9) smoothly (Fig. 2c & 3e). Each
of the Cu(II) ion in the side core of 9 has a distorted pyramid
geometry, while the central Cu(I) ion holds a distorted linear
geometry with CuI−N bond lengths of 1.880(2)-1.882(2) Å and
CuI-O of 2.683(2)-2.743(1) Å. This result is consistent with the
conclusion presented above; the hydroxo group in the
[CuII−OH] compounds facilitates the reduction of Cu2+ to Cu+. It
is currently unclear as to the fate of the hydroxide ligand. By
comparison, free Cu2+ ion works better than the coordinated
Cu2+ ion in the reduction process according to the reaction
time and yields in our system (Free Cu2+ ion: complex 9, 15
mins, yield 45%; Coordinated Cu2+ ion: complexes 3 and 6, one
week, yields 10% and 30%, respectively). All the yields of Cu(I)
and [NiII-OH] are calculated in reference to reactants [CuII-OH],
[CuII-DMF]n and [NiII-DMF]n respectively (see Fig. 2).
The cooperative metal−ligand interaction is examined by
the
design
of
copper(II)
complexes
of
[CuII(py(N−C=O)(N=C−C)ph2Me2)(X)]+/0 (X = DMF, OH) (Fig. 2d).
The asymmetric mixed 2,6-disubstituted arylcarboxamido-
arylimino pyridine ligand was supposed to have the
characteristics of both strong base quality of
diarylcarboxamido pyridine (H2py(N−C=O)2ph2Me2) and good
electron delocalization property of diarylimino pyridine
(py(N=C−C)ph2Me2).24 The latter has been usually used as π-
accepting ligand to support first-row transition metal
complexes that exhibit novel reactivity with the ligand
Fig.
3
Crystal
structures
of
[CuII(py(N−C=O)2ph2Me2)(DMF)]
[CuII(py(N−C=O)2ph2Me/Allyl)(OH)]−
(1),
(5),
2−
[CuI(py(N−C=O)2ph2Me2)]2
(3),
[CuI(py(N−C=O)2ph2Me/Allyl)]2 (6) and [Cu3(20-py(N−C=O)2ph2dienMe3)2]+ (9) with all skeleton getting involved in chemical reactions.10a Two
non-hydrogen atoms shown as 40% probability ellipsoids.
2−
synthetic routes are developed for the preparation of ligand
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[Cu(py(N−C=O)(N−C=C)ph2Me2)(OH2)] (11b) (FigV.iew2Adrti)c.le OTnlhinee
−
DOI: 10.1039/D0DT02288H
substitution of solvated DMF with OH anion runs faster than
the deprotonation of methyl group, and the added Et4NOH
would be used preferentially for coordination. In literature, the
formation of enamide ligands from deprotonation of
diaryliminopyridines has been well-precedented among
complexes of Cr(II),15c Mn(II),15a Fe(II),15b Co(II)15e and Ni(II)15d
.
This is the first example of C−C bond formation of
methylimino pyridine through the reduction of Cu(II) to Cu(I),
which implies a formal elimination of one proton from the
methyl group of intermediate species of 11a. Unfortunately, all
the efforts to isolate the intermediates of 11a and/or 11b are
unsuccessful.
A
analogous
[NiII−OH]
complex
of
(Et4N)[NiII(py(N−C=O)(N=C−C)ph2Me2)(OH)] (14) is prepared for
the examination of the coordination pattern and electronic
structure of likely complex 11a and/or its de-protonated
complex 11b. The latter would be the key compound in the
reaction of formation of C−C bond (Fig. 2e). X-ray
crystallography analysis of 14 reveals that the nickel(II) ion has
a square planar geometry with three nitrogen donors from
ligand and one oxygen donor from the hydroxo group lying in
the equatorial plane (Fig. 4c). The imine-methyl group in the
[NiII-OH] complex of 14a is attacked to lose a proton by the
hydroxide ligand, forming a conjugated enamide group (N3−C7
Fig.
4
Crystal structures of {[CuII(py(N−C=O)(N=C−C)ph2Me2)(DMF)](OTf)}n (10),
[CuI(py(N−C=O)(N=C−C)ph2Me2)2]− (12), [NiII(py(N−C=O)(N=C−C)ph2Me2)(OH)]− (14) and
[CuII(py(N=C−C)2ph2Me2)(DMF)(OTf)]+ (16) with all non-hydrogen atoms shown as 40%
probability ellipsoids. The structures of complexes 14 and 16 were solved by fast-data
collections with the crystallographic data shown in Table S3.
1.374(3) Å, C7=C8 1.357(4) Å). The dianionic carboxamido-
Me2 −
enamide pyridine ligand [py(N−C=O)(N−C=C)ph2
]
is
Me2
Hpy(N−C=O)(N=C−C)ph2
equivalent of
(Scheme S2). Addition of one
to the complex of
energetically strong enough to support the transformation of
complex 14b to 14. In complex 11b, the cooperative
metal−ligand interaction would occur between Cu2+ and NNN-
pincer ligand, after deprotonation of imine-methyl group with
hydroxide ligand. With the aim to check the stability of imino
group for the Cu2+ ion under base condition, the diimino
Me4NOH
{[CuII(py(N−C=O)(N=C−C)ph2Me2)(DMF)](OTf)}n (10) (Fig. 4a) in
DMF results in a light brown solution immediately under N2
atmosphere. Crystallization of the resultant solid from
DMF/Et2O
affords
the
product
(Me4N)[CuI(py(N−C=O)(N=C−C)ph2Me2)2]·1.5Et2O·0.5H2O (12) as
light brown crystals (Fig. 2d & 4b). The structure of 12 shows
that the Cu(I) center has a strongly distorted tetrahedral
geometry defined by four nitrogen donors from two anionic
arylcarboxamidopyridine ligands. The two ligands are coupled
to form a new C−C bond (C24−C25 = 1.45(3) Å, C24’−C25’ = 1.49(4)
Å) at the imino-methyl position. No restraint of bond length is
applied to the C24−C25 bond in the solution of the structure of
complex 12. The distance of the new C−C bond is shorter than
those of analogous diaryliminopyridine compounds with the
alkylation of ligands on the peripheral methyl atoms of N=C−C
groups, in which the coupled C−C bonds showed distinct single
bond characteristics with bond lengths between 1.523 Å and
1.562 Å.15a,15e Strongly disordered structure of 12 is
perceived as contributing to the short C−C bond distance. HR-
MS characterization supports the formation of a C−C single
bond rather than a C=C double bond in complex 12 (Fig. S2).
The driving force of the carbon−carbon bond coupling reaction
in complex 12 is investigated by the reaction of analogous
complex of [CuII(py(N−C=O)(N=C−C)ph2Me2)(Cl)] (10a) with one
equivalent of Me4NOH. The starting material of complex 10a is
recovered from reaction without the generation of new
species, which implies that the reaction of complex 10 to 12 is
induced by the hydroxo group, probably via intermediates of
pyridine
[CuII−DMF]
complex
of
[CuII(py(N=C−C)2ph2Me2)(DMF)(OTf)](OTf) (16) is prepared and
the compound is treated with one equivalent of Me4NOH or
Et4NOH. No product is obtained with the colour of solution
changing to light yellow-green (Fig. 2f). It seems that the
diimino compound would suffer from decomposition in the
presence of hydroxide ligand. A certain amount of brown-black
precipitate is found deposited at the bottom. That is to say,
the carboxamido group in complex 11a helps to hold the
[CuII−OH] unit, which triggers the deprotonation reaction
thereafter C−C bond coupling reaction.
Considering the different oxidative ability and reactivity of
CuII complexes in our systems, we set out to analyze the
structural characteristics of carboxamido-methylimino ligands
with conjugated π-electron system. The examination of the
influence of ligand types on the activation of copper(II) pincer
complexes is carried out by the techniques of UV-vis
spectroscopy, cyclic voltammetry and EPR spectroscopy. The
absorption bands of d-d transition show blue shifting and
widening with the increase of absorption coefficient in order
of complexes 1 > 10 > 16 (Fig. 5), which implies that the
complex 1 possesses larger d-orbital splitting energy than
[Cu(py(N−C=O)(N=C−C)ph2Me2)(OH)]
(11a)
and
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necessary for the coordinated hydroxo groups in the [CuII−OH]
View Article Online
DOI: 10.1039/D0DT02288H
complexes of 2, 5 and 8 donating minor electron density to the
metal centers. As a result, the coordination interaction
between Cu2+ ion and the OH− group decreases, and the
hydroxo group would tend to show good nucleophilic
reactivity on the deprotonation of organic species.
The basic nature of the dianionic carboxamido-pyridine
ligands is also investigated by the study of reduction potentials
of Cu(II) complexes. The cyclic voltammograms of complexes 1,
10 and 16 in DMF (0.1 M NBu4PF6, 298 K) at a scan rate of 100
mVs-1 shows reduction processes at Epc = -1.10 V, -0.72 V and -
0.43 V versus Fc+/Fc, which are assigned to the CuII/CuI couple
of complexes (Fig. 6a), respectively. The reduction process of
complex 16 at -0.43 V is associated with an oxidation process
at -0.34 V. The examination of this reduction-oxidation process
at variable scan rates reveals that the reduction of complex 16
to Cu(I) species is a reversible process in solution at 298 K (Fig.
6b). It indicates that the imino group has superior structural
characteristic with conjugated π-electron system, which is
beneficial to electron delocalization and enhancement of
reduction response of Cu(II) complex.
Fig. 5 UV-vis spectra of the complexes 1, 10 and 16 in DMF solution.
The above result is consistent with the analysis of EPR
spectra of complexes 1, 10 and 16 in DMF solution. The fluid X-
band spectra of 1, 10 and 16 (Fig. 7a-7c) show similar EPR
signals with a major quadruplets that results from hyperfine
coupling to 63,65Cu nuclei (I = 3/2, 100% combined natural
abundance). The seven-line super-hyperfine coupling on the
high-filed feature is assigned to the hyperfine coupling
involving three equivalent 14N nuclei (2nI+1, I = 1). Simulation
of these spectra gives giso = 2.101 and |Aiso| = 71 × 10-4 cm-1 for
1, giso = 2.111 and |Aiso| = 67 × 10-4 cm-1 for 10, giso = 2.128,
|Aiso| = 65 × 10-4 cm-1 for 16, with a Gaussian line width Wiso
=
1G for all of them. The frozen solution X-band spectra of 1, 10
and 16 (Fig. 7d-7f) are approximately axial with 63,65Cu
hyperfine, and the simulated spin-Hamiltonian parameters are
given in Table 1. The quasi-axial EPR spectra of complexes are
consistent with the approximate square-planar structures of
complexes 1, 10 and 16 determined by X-ray crystallography.
Thus, gxx and gyy are assigned to orientations lying in the
equatorial CuN3O plane, with gzz (the largest g-value)
perpendicular to this plane. Crystal field theory predicts a 3dxy-
based SOMO in the above Cu(II) complexes consistent with the
observed g║ > g┴ > ge pattern. Analysis of the simulated g and
A-values of these three complexes gave α2 = 67.8 % for 1, 67.6 %
for 10 and 64.3 % for 16 (see Supporting Information) with
reference to the interpretation of spin Hamiltonian
parameters developed for the analogous Cu(II), Ag(II) and Au(II)
complexes.23b,25 Thus, the contributions of 3dxy metal orbital to
the SOMO are 67.8 % for complex 1, 67.6 % for complex 10
and 64.3 % for complex 16, which indicates the larger ligand
contributions to the SOMO for 16 than 1 and 10 in the diimino-
pyridine system. The complexes of 1 and 10 have similar ligand
contributions to the SOMO with one or two anionic
carboxamido group participated in the system.
Fig. 6 (a) Cyclic voltammogram of complexes 1, 10 and 16 in DMF (0.1 M NBu4PF6) at
298 K; (b) Examination of the reversible reduction process of complex 16 at variable
scan rates.
complex 10 and 16 associating with the electron-donating
ability of the ligands. The dianionic carboxamido-pyridine
ligand donates more electron density to the Cu(II) center than
the imino pyridine ligand, and for this reason, it is only
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Fig. 7 X-band fluid EPR spectra (a)-(c) and frozen EPR spectra (d)-(f) of complexes 1, 10 and 16 in DMF solution (top). Their simulations are presented at the bottom. The simulated
spin-Hamiltonian parameters are listed in Table 1.
Table 1. EPR parameters derived from simulations of experimental X-band spectraa
Fluid
1
10
16
Frozen
1
gxx
gyy
gzz
|Axx|(Cu)
13
|Ayy|(Cu)
100
100
97
|Ayy|(Cu)
15
|Azz|(Cu)
100
100
97
|Azz|(Cu)
182
|Axx|(N)
12
5
13
|Axx|(N)
14
|Ayy|(N)
15
16
13
|Ayy|(N)
15
|Azz|(N)
15
16
13
|Azz|(N)
16
Wiso (G)
1 G
1 G
1 G
Wyy
70
1.960
1.945
1.988
gxx
2.048
2.048
2.050
2.172
2.194
2.198
gyy
2.060
2.057
2.069
2.172
2.194
2.198
gzz
2.223
2.234
2.250
1
1
|Axx|(Cu)
10
12
Wxx
9
25
20
Wzz
40
20
20
10
16
12
17
175
162
13.5
15
13.5
15
13.5
15
11
17
17
a units of |A| are in 10-4 cm-1.
Since the attempts to isolate the [CuII-OH] intermediates
C] with the N C, C C and Cu−O(H2O) bond lengths shown
of 11a and 11b are failed, the structures of them are optimized as 1.374 Å, 1.360 Å and 2.069 Å, respectively. The conjugated
by density functional theory (DFT) calculations (Fig. 8). The enamide group is similar to that of the analogous [NiII-OH]
imine-methyl group in complex 11a is shown as [N=C−C] unit complex 14, which is obtained by deprotonation of methyl
with the N=C bond length of 1.291 Å and the C−C bond length group with hydroxo group from [NiII-OH] moiety. However, it is
of 1.483 Å. The Cu−O(OH) bond length of 1.864 Å is comparable still not clear to us how the carbon-carbon formation reaction
to those of bond lengths in the analogous complexes of 2, 5 works.
and 8. In the state of 11b, the [N=C−C] unit changes to [N
C
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Acknowledgement
This research was supported by the National Natural Science
Foundation of China (Grants No. 21371171), and the Strategic
Priority Research Program of the Chinese Academy of Sciences
(Grant No. XDB20000000).
Notes and references
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In summary, we have described a [CuII−OH] species
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A copper(II)-hydroxide induced C–C bond formation is explored via cooperative metal−ligand
interaction and oxidative coupling reaction.