Nickel(0) complexes of polyunsaturated azamacrocyclic ligands

Article abstract of DOI:10.1021/om201271y

A series of nickel(0) complexes, 3, 4, and 5, containing unsaturated 15-membered azamacrocyclic ligands have been synthesized. Structural characterization of the complexes is based on NMR spectroscopy, X-ray diffraction analysis, and differential scanning calorimetry. In addition, the catalytic activity of the complexes in the Suzuki-Miyaura cross-coupling of arylboronic acids and aryl halides has been tested.

Full text of DOI:10.1021/om201271y

Article
pubs.acs.org/Organometallics
Nickel(0) Complexes of Polyunsaturated Azamacrocyclic Ligands
Sandra Brun, Anna Pla-Quintana, and Anna Roglans*
Department of Chemistry, University of Girona, Campus de Montilivi s/n. E-17071-Girona, Spain
Klaus-Richard Porschke* and Richard Goddard
̈
Max-Planck-Institut fur Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mulheim an der Ruhr, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A series of nickel(0) complexes, 3, 4, and 5, containing unsaturated 15-membered azamacrocyclic ligands have
been synthesized. Structural characterization of the complexes is based on NMR spectroscopy, X-ray diffraction analysis, and
differential scanning calorimetry. In addition, the catalytic activity of the complexes in the Suzuki−Miyaura cross-coupling of
arylboronic acids and aryl halides has been tested.
INTRODUCTION
■
About 50 years ago Wilke made the seminal discovery that
transition metal atoms can be stabilized by cyclic polyene
ligands to form homoleptic transition metal−alkene complexes
such as trigonal-planar Ni(t,t,t-cdt) (t,t,t-cdt = trans,trans,trans-
1,5,9-cyclododecatriene) and tetrahedral Ni(cod)2 (cod =
cis,cis-1,5-cyclooctadiene).1 Later, by reaction of Ni(t,t,t-cdt)
with ethylene, the parent Ni(C2H4)3 was also obtained.2 The
complexes have been termed “naked nickel”,3 owing to the ease
with which displacement of the alkene ligands in the course of
stoichiometric and catalytic reactions can occur. More recently,
Porschke and co-workers4 have systematically studied the
̈
coordination behavior of acyclic 1,5-dienes, 1,6-dienes, 1,6-
diynes, and 1,6,11-trienes at Ni(0), Pd(0), and Pt(0), leading
inter alia to compounds of type I (Figure 1).4h An important
finding of these investigations was that ligands having 1,6- or
1,6,11-sequences of the olefinic functions are much better
suited for coordination at the given trigonal-planar metal
centers than the corresponding acyclic or cyclic 1,5-dienes and
1,5,9-trienes. While the chair-type, formally Cs-symmetrical
conformation of the (1,6-diene)M(0) entity in trigonal-planar
complexes seems to be particularly stable, a few tetrahedral
(1,6-diene)Ni(0) complexes showing a twist-type, formally C2-
symmetrical conformation have also been reported.5 Supple-
menting previous studies, the ligand properties of c,c,c-1,5,9-
cyclododecatriene were analyzed in detail.6 Nickel is also
known to be stabilized inside a macrocyclic scaffold of three
alkyne moieties, such as the planar tribenzocyclyne and
heterocyclotriynes containing silicon, germanium, or titanium.7
In recent years, following the unexpected discovery of an
(E,E,E)-1,6,11-triazacyclopentadeca-3,8,13-triene−Pd(0) com-
plex,8 the Roglans group has worked intensively on the
Figure 1. Ni(0), Pd(0), and Pt(0) complexes with acyclic diazatriene
(I) and cyclic triazatriene (II) and mixed triazaalkene/alkyne ligands
(III, IV), having the ene/yne functional groups in 1,6,11-sequence.
synthesis of a series of azamacrocycles with various ring sizes
and numbers of CC and CC bonds.9 The ability of these
macrocycles to coordinate at Pd(0), Pt(0), and Ag(I) has been
studied, with particular success in the stabilization of
palladium(0) (structures II, III, and IV in Figure 1). The
resulting palladium(0) complexes are very stable and have been
thoroughly characterized, paying special attention to the
stereoisomerism arising from the different faces of the olefinic
double bonds complexed to the metal. While in principle
complexation of the metal at one of the two stereotopic faces of
each CC function present in the macrocycles can give rise to
Received: December 22, 2011
Published: February 24, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of the Ni(0) Complexes 3, 4, and 5
a series of isomers, only the energetically most favorable isomer
is actually formed, as shown by NMR and X-ray techniques. It
appears that the conformation of the six-membered chelate
rings determines the stability of a complex and, hence, whether
it is formed. In the case of the Pd(0) complexes II,9d the
observed isomers show two azapalladacyclohexanic rings in
chair conformation and the third ring in twist conformation,
whereas for Pd(0) complexes III and IV the observed isomers
show only chair-type conformations.9e Of particular interest is
the fact that the Pd complexes of the 15-membered trienic
macrocycle (type II) have been found to be recoverable and
reusable catalysts in C−C bond-forming reactions, such as
Suzuki−Miyaura cross-couplings and Heck couplings. This has
been particularly successful when using diazonium salts as the
electrophilic counterpart.10
In the present study we investigate the coordination
properties of azamacrocycles 18c and 29e (Scheme 1) bonded
to nickel(0), with a focus on the different kinds of unsaturation
and rigidity of the scaffold. The new Ni(0) complexes have
been characterized in solution and the solid state by
spectroscopic (NMR, X-ray diffraction) and thermal techniques
(DSC) and probed for their reactivity in C−C bond-forming
reactions (Suzuki−Miyaura cross-coupling).
phenomenon. Most of these compounds remained a glass,
although 1·Pd(0), interestingly, recrystallized when reheated.9f
This behavior appears not to be paralleled by the Ni(0)
analogue 3, which showed a broad endothermic effect between
167 and 195 °C, attributable to unspecified changes in the
lattice, and eventually sharp melting at 207 °C, but no change
when recooled and reheated (see Supporting Information, SI).
When in a similar reaction Ni(cod)2 was treated with 1 equiv
of (E,E)-1,6,11-tris(4-tolylsulfonyl)-1,6,11-triazacyclopentade-
ca-3,8-diene-13-yne (2),9e a mixture of colorless crystals of 4
and some well-formed orange crystals of 5 precipitated at 20 °C
in the course of 1−2 days (Scheme 1). Formation of 5 was
avoided by using a 10% excess of 2. This way, pure 4 was
isolated in 80% yield. Separation of 4 from 5 is possible by
recrystallization from THF, in which 5 is insoluble. The crystals
of 4 obtained from THF include five solute molecules, causing
substantial disorder (see also SI for structure determination).
The solute THF is largely removed when the crystals are dried
in a vacuum at ambient temperature. Complex 4 readily
dissolves in THF and methylene dichloride, where it appears to
be stable, but it is insoluble in diethyl ether and n-pentane.
For complex 5 elemental analysis revealed a variable content
of up to two Ni atoms per macrocyclic ligand, suggesting the
presence of a potentially dinuclear complex (C33H37N3O6S3)-
Ni1+n(C8H12)n (n ≤ 1) in which 4 is associated with n Ni(cod)
entities. In fact, reacting isolated 4 with a further equivalent of
Ni(cod)2 afforded orange 5 in 91% yield. An in situ reaction of
macrocycle 2 with 2 equiv of Ni(cod)2 is less suited for the
preparation of 5 since here, owing to the slowness of the two
reaction steps and insolubility of 5, the crystallizing 5
incorporates a larger amount of 4. Any 4 incorporated in
crystalline 5 cannot be removed by treatment with THF.
Complex 5 is insoluble in the common organic solvents even
when heated, which precludes its solution NMR character-
ization.
RESULTS AND DISCUSSION
■
Preparation of the Nickel(0) Complexes. Reacting
equimolar amounts of Ni(cod)2 and (E,E,E)-1,6,11-tris(4-
tolylsulfonyl)-1,6,11-triazacyclopentadeca-3,8,13-triene (1)8c in
THF solution at room temperature for 1 h resulted in ligand
exchange. Cooling the solution to 0 °C afforded yellow crystals
of 3 in 78% yield (Scheme 1). The crystals of 3 include one
molecule of THF solute. Complex 3 readily dissolves in THF,
but is insoluble in diethyl ether and n-pentane. It appears
unstable in methylene dichloride. Complex 3 represents the
lighter homologue of the known derivatives 1·Pd(0) and
1·Pt(0).8d While the crystal structures of these complexes and
also of the uncoordinated 18c are known, the crystals of 3
obtained from THF have so far proved to be unsuitable for an
X-ray structural investigation, which unfortunately precludes a
comparison of the three homologues. We expect that the
structure of 3 will be very similar to that of the 1·Pd(0) and
1·Pt(0) analogues.
When complex 4 was studied by DSC, an endothermic effect
occurred around 164 °C, which can be ascribed to loss of THF.
Release of THF has been confirmed by heating solid 4 to 170
°C for one hour, which leaves the solvent-free but otherwise
unchanged compound, as evidenced by 1H NMR. Compounds
4 and 5 suffered exothermic decomposition in DSC at 225 and
213 °C, respectively (see SI). All three nickel complexes proved
to be sensitive to air and moisture.
A DSC (differential scanning calorimetry) study on
crystalline 1·Pd(0) and related compounds revealed vitrifica-
tion after recooling from the melt as an order−disorder
NMR Spectra of 3. The uncoordinated 15-membered
1
cyclotriene 1 shows two broad H signals for the six olefinic
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Table 1. Selected 1H and 13C NMR Resonances of the Uncoordinated Cyclotriene 1, the Ni(0) Complex 3, and the Homologues
1·Pd(0) and 1·Pt(0)
1H NMR
a
b
assignation
18b
3
a
b
c
d
e
f
3.67 (br, 12H)
5.56 (br s, 6H)
4.52 (m, 2H), 2.84 (dd, J = 14.0 and 11.6 Hz, 2H)
2.00 (ddd, J = 14.0, 11.6, and 2.4 Hz, 2H)
3.38 (ddd, J = 14.0, 10.8, and 4.0 Hz, 2H)
4.52, 1.34 (each m, 2H)
4.52, 1.38 (each m, 2H)
3.22 (m, 2H)
13C NMR
a
b
a
a
assignation
17
c
3
1·Pd(0)9d
1·Pt(0)9b
a
b
c
d
e
f
51.3
45.9
75.9
72.5
50.1
48.7
72.3
45.1
82.8
78.7
49.5
48.2
78.5
44.3
69.0
63.2
48.5
47.2
62.7
127.8
a
b
Solvent CDCl3. Solvent CD2Cl2.
and 12 methylene protons and two 13C signals of equal
intensity for the six olefinic and six methylene C atoms (Table
1).8b The spectra not only prove equivalence of all methylene
and the olefinic moieties but also show that the geminal
methylene protons are enantiotopic on the time average.
Assuming that the ground-state structure of the cyclotriene
skeleton of 1 in solution is similar to that in the solid state8b
and can be conceived as (idealized) C2-symmetrical with the
C2-axis passing through the center of one CC bond and the
opposite N atom, the spectra are consistent with rotations of
the other CC moieties about their vinylic C−C bonds,
resulting in 60° jumps of the C2-axis. The time-averaged
symmetry in solution is D3h.
of enantiomers. The geometry is corroborated by the 1H NMR
spectrum, which exhibits different resonances for the olefinic
protons at the asymmetrical azanickelacyclohexanic rings in
chair conformation (δ(H) 3.38 and 3.22; c, f) and the markedly
upfield olefinic resonance for the equivalent olefinic protons of
the C2-symmetrical ring in twist conformation (δ(H) 2.00; b).
Regarding the methylene groups, for the rings in chair
conformation the protons in equatorial (δ(H) 4.52; d, e) and
axial positions (δ(H) 1.34 and 1.38; d, e) have substantially
different chemical shifts, while for the ring in twist
conformation the differentiation between the pseudoequatorial
and pseudoaxial protons (δ(H) 4.52 and 2.84; a) is smaller.
Comparing the 13C NMR resonances of 3 with those of
1·Pd(0) and 1·Pt(0) and uncoordinated 1 (Table 1), we note
that the upfield shift of the olefinic C atoms (from δ(C) 127.8
for 1) is least for 1·Pd(0) (δ(C) 78.5−82.8), intermediate for 3
(δ(C) 72.3−75.9), and largest for 1·Pt(0) (δ(C) 62.7−69.0),
consistent with an increasing metal−alkene back-donation in
this order.11 The upfield shift of the methylene C atoms (from
δ(C) 51.3 for 1) is much smaller, but for each C atom a, d, and
e it strictly follows the sequence 3 < 1·Pd(0) < 1·Pt(0),
apparently reflecting continuous conformational changes in the
macrocycle due to the increasing size of the metals.
In contrast, complex 3 shows for the cyclotriene ligand a total
1
of nine H resonances of equal intensity, as there are three
signals for olefinic protons (b, c, and f) and three pairs of
signals for different methylene groups (a, d, and e) whose
geminal protons are now diastereotopic (Table 1). The 13C
NMR spectrum furnishes six signals of equal intensity (a−f).
Complete resonance assignments were performed on the basis
of 2D COSY and HSQC spectra (see SI). Similar to what was
found previously for 1·Pd(0) and 1·Pt(0)8c (for additional
treatises of the structures and NMR spectra of these and related
complexes see refs 9a, b, d), the triazacyclotriene ligand in 3
forms three formal azanickelacyclohexanic rings with Ni(0) in a
chair−chair−twist conformational combination, resulting in an
overall rigid C2-symmetrical structure and the presence of a pair
NMR Spectra of 4. The uncoordinated (E,E)-1,6,11-
1
triazacyclopentadeca-3,8-diene-13-yne 2 gives rise to five H
and six 13C NMR signals for the 15-membered ring.9e The
methylene protons are enantiotopic, indicating rotation of the
CC moieties about the vinylic C−C bonds. Thus, 2 shows
dynamic C2v symmetry in solution.
As for 2·Pd(0),9e the Ni(0) analogue 4 exhibits eight 1H and
a full six 13C NMR signals for the macrocyclic ligand (Table 2).
Complete resonance assignments were performed from 2D
COSY and HSQC spectra (see SI). Thus, the Ni atom is
coordinated to the two CC bonds and the CC bond inside
the cavity of the macrocycle in a rigid structure, rendering the
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Table 2. Selected 1H and 13C NMR Chemical Shifts of Macrocycle 2, the Ni(0) Complex 4, and the Pd(0) Homologue 2·Pd(0)
1H NMR
a
a
b
assignation
29e
2·Pd(0)9e
4
a
3.58 (d, J = 5.7 Hz, 4H)
4.72
1.72
3.93
3.68
4.67
2.31
4.77
3.76
4.66 (dd, J = 13.2 and 3.2 Hz, 2H),
1.32 (dd, J = 13.2 and 10.8 Hz, 2H)
3.42 (ddd, J = 13.2, 10.8, and 3.2 Hz, 2H)
3.15 (ddd, J = 13.2, 10.8, and 2.4 Hz, 2H)
4.58 (dd, J = 13.2 and 2.4 Hz, 2H),
2.00 (dd, J = 13.2 and 10.8 Hz, 2H)
4.54 (d, J = 13.6 Hz, 2H),
b
c
5.44 (m, 2H)
5.60 (m, 2H)
d
3.69 (d, J = 6.6 Hz, 4H)
e
3.88 (s, 4H)
4.13 (d, J = 13.6 Hz, 2H)
13C NMR
a
a
b
assignation
29e
2·Pd(0)9e
4
a
b
c
d
e
f
50.9
48.9
79.0
80.9
46.8
35.7
82.8
49.2
71.3
74.4
46.6
36.5
97.4
131.5
127.2
50.1
37.8
79.3
a
b
Solvent CDCl3. Solvent CD2Cl2.
geminal protons of the three types of methylene groups
diastereotopic. While the total number of signals would also be
in agreement with C2 symmetry, Cs symmetry of the complex is
substantiated by the large chemical shift difference between the
equatorial (δ(H) 4.66) and axial (δ(H) 1.32) diastereotopic
protons of methylene group a. This is consistent with a chair
(as opposed to twist) conformation of the azanickelacyclohex-
anic ring involving one C atom of each of the two coordinated
alkenes,9d leading to a meso combination of the two chiral M−
E-RCHCHR substructures. The shift differences of the
diastereotopic methylene protons in the asymmetric six-
membered rings involving one alkene and the alkyne functions
are less pronounced (δ(H) 4.58, 2.00 (d) and δ(H) 4.54, 4.13
(e)).
forming the back rest. The molecule resides on a crystallo-
graphically imposed mirror plane, which passes through the
nickel atom, the midpoint of the triple bond and N2 of the ring
ligand, and S2, C7, C10, and C11 of one SO2-tolyl group.
There are two identical complexes of 4 in the unit cell, which
are related to one another by an inversion center. The
remainder of the unit cell is occupied by five disordered THF
solute molecules per complex unit.
The Ni atom is trigonal-planar coordinated by the CC
bond and the two trans-substituted CC bonds, with both
carbon atoms of the CC bond and the midpoints of the C
C bond lying in one plane. This results in the formation of
three six-membered rings, each containing Ni and N at the 1,4-
positions and four C atoms in between. The central
nickelaazacyclohexanic ring involving the ring section between
the two CC bonds and N2 adopts a chair conformation, with
the dihedral angles within the ring ranging between 55° and
69°. The other two equivalent pseudonickelaazacyclohexanic
rings involving ring sections between one CC bond and the
CC bond and N1 (N1*) adopt “half-chair” conformations,
with dihedral angles in one-half of the ring varying between 44°
and 71° and in the other merely 15−37°, due to the fact that
C2 is constrained to lie in the coordination plane of Ni.
In contrast to 4, the conformation of the 1,6,11-
triazacyclopentadeca-3,8-dien-13-yne ligand in the Pd(0)
homologue 1·Pd(0),9e which crystallized from CH2Cl2−ethyl
acetate−hexane solution with one CH2Cl2 solute molecule in
As compared to 2, the signals of the olefinic protons and
carbon atoms (b, c) are markedly shifted upfield upon metal
coordination, indicative of partial sp2 → sp3 rehybridization of
11
1
carbon. This (negative) H and 13C coordination shift is
distinctly larger for the Ni complex 4 than for 2·Pd(0). In
contrast, the alkyne carbon atoms (f) are shifted downfield due
to partial sp → sp2 rehybridization. While this (positive) shift is
smaller in magnitude, it is again larger for 4 than for 2·Pd(0),
consistent with stronger back-bonding of nickel(0).
Crystal and Molecular Structures of 4 and 5. The
1,6,11-triazacyclopentadeca-3,8-diene-13-yne−nickel(0) com-
plexes 4 and 5 were further characterized by means of X-ray
crystallography (Table 3). Complex 4 is mononuclear, whereas
5 is a dinuclear complex containing an additional cod ligand.
Monoclinic 4 crystallizes slowly from THF at ambient
temperature to give well-formed colorless cubes (space group
P21/m (No. 11)), containing THF solute. The structure of 4
(Figure 2) resembles a chair, with the 15-membered ring
providing the seat, two SO2-tolyl groups at NCH2CCCH2N
representing the front legs, and the other SO2-tolyl group
the unit cell (triclinic crystal system, space group P1 (No. 2)),
̅
is asymmetric and more flattened, presumably due to the larger
radius of Pd compared to Ni, though a direct comparison with
4 is hampered due to extensive disorder in the ring in 1·Pd(0).
The structure of complex 5 (Figure 3) can be thought of as 4
with an additional Ni(cod) moiety coordinated to the CC
bond. The complex crystallizes solute-free from THF in the
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Table 3. Crystal Data for 4 and 5
4
5
empirical formula
C33H37N3NiO6S3·5(C4H8O)
colorless prism
1087.07
C33H37N3NiO6S3·0.834(C8H12Ni)
yellow plate
865.77
color and morphology
fw (g mol−1
)
temp (K)
100
100
wavelength (Å)
cryst syst
space group
unit cell dimens
a (Å)
0.71073
0.71073
monoclinic
orthorhombic
Pnma (No. 62)
P21/m (No. 11)
11.5404(14)
17.8908(14)
13.864(3)
90.0
18.539(3)
18.881(3)
11.2747(15)
90.0
b (Å)
c (Å)
α (deg)
β (deg)
112.408(11)
90.0
90.0
γ (deg)
V (Å3)
90.0
2646.3(6)
2
3946.6(9)
4
Z
V/Z (Å3)
calcd density (Mg·m−3
1323.2
986.7
)
1.364
1.457
abs coeff (mm−1
)
0.547
1.086
F(000) (e)
1160
1814
cryst size (mm3)
0.13 × 0.12 × 0.08
2.78−29.98
−16 ≤ h ≤ 16
−25 ≤ k ≤ 25
−19 ≤ l ≤ 16
37 802
0.22 × 0.10 × 0.04
2.10−31.45
−27 ≤ h ≤ 27
−27 ≤ k ≤ 27
−16 ≤ l ≤ 16
110 518
θ range for data collecn (deg)
index ranges
no. of rflns collected
no. of indep rflns
no. of rflns with I > 2σ(I)
completeness (%)
abs correction
7920 (Rint = 0.0802)
5283
6679 (Rint = 0.1310)
4745
99.8 (θ = 29.98°)
Gaussian
0.92/0.88
F2
99.4 (θ = 31.45°)
Gaussian
0.96/0.83
F2
max./min. transmn
full-matrix least-squares
no. of data/restraints/params
goodness of fit on F2
final R indices (I > 2σ(I))
R1
7920/0/394
1.054
6679/0/260
1.163
0.0730
0.1567
0.0857
0.2024
wR2
R indices (all data)
R1
0.1228
0.1214
wR2
0.1810
0.2199
largest diff peak/hole (e Å−3
)
0.912/−0.829
1.917/−1.214 (Ni2)
orthorhombic crystal system (space group Pnma (No. 62)),
with four identical molecules in the unit cell.
relatively short for a 2-fold-coordinated CC bond, but this is
likely to be an artifact of the largely anisotropic atomic
displacements of these atoms.
Each Ni atom is in its zero oxidation state and is trigonal-
planar ( 0.1 Å) coordinated by two CC bonds and the
bridging CC bond. The coordination planes of the two
metals, which each contain the two carbon atoms of the CC
bond, make an angle of 92° to one another. Whereas the two
CC bonds of the macrocyclic ligand lie approximately in the
coordination plane of Ni1, the two CC bonds of the cod
ligand lie perpendicular to the coordination plane of Ni2. The
Ni1···Ni2 distance of 2.684(2) Å indicates a significant d10−d10
interaction, which is supported by the observation that the
midpoints of the CC bonds of cod ligand lie slightly out (ca.
0.2 Å) of the plane defined by Ni2, C1, and C1*, away from the
Ni···Ni contact. Bond lengths of all the coordinated CC
bonds of the macrocycle (C4−C5 1.388(7) Å) and the cod
ligand (C19−C20 1.355(13) Å) are as expected. The C1−C1*
distance of the Ni2(μ-CC) entity at 1.303(10) Å appears
The Ni(cod) moiety is present in the crystal with a refined
occupancy of 83.4(4)%, indicating the crystal is in fact a
mixture of 4 and 5. Several crystals were investigated, and all
those studied showed a reduced occupancy of the Ni(cod)
group (see SI for details of the structure solution). The
molecule resides on a crystallographic mirror plane, as was the
case with 4. The plane passes through both nickel atoms Ni1
and Ni2, the midpoint of the triple bond C1−C1*, and the
atoms N2, S2, C14, C17, and C18, thereby bisecting the
macrocyclic ligand and one SO2-tolyl group and the cod ligand.
The volume per formula unit of 5 at 986.7 Å3 is smaller than
that of 4 (1323.2 Å3), which contains the five THF solute
molecules.
One of the ethano bridges in the cod ligand is disordered
over two positions. The C21A−C21B bond distance at 1.54(3)
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present and results in an unrealistically short C22−C22* bond
distance of 1.45(2) Å. The cod ring is thus similarly puckered
to that in Ni(cod)2.12 Bond lengths of the coordinated bonds at
1.36(1) Å are slightly shorter than expected (Ni(cod)2:
1.391(2) Å, mean), but this may be a consequence of the
relatively high atomic displacement parameters of the C atoms
of the cod.
Dinuclear trigonal-planar Ni(0) complexes of type
(L2Ni)2(μ-RCCR′), in which the two Ni atoms are
complexed by a bridging alkyne ligand and additional donor
ligands L such as phosphines,13 amines,14 cod,15 and alkyne
(L2),14,16 or as a combination of these ligands, are quite
common. Typical Ni···Ni distances range between 2.45 and
2.65 Å, and a typical CC bond length falls in the range 1.30−
1.39 Å (cf. {(cod)2Ni}2(μ-PhCCPh): Ni···Ni = 2.617 Å, C−
C = 1.386 Å).15a The CC bonds of the cod ligands always lie
perpendicular to the corresponding CC−Ni plane. Complex
5, with its macrocyclic diene-yne ligand, is so far the only
structurally characterized (L2Ni)2(μ-RCCR′)-type complex
in which two CC bonds of an olefinic ligand lie in the C
C−Ni plane.
Chemical Properties of 3, 4, and 5. To test their
reactivity, the 16-electron complexes 3, 4, and 5 were treated
with unsaturated substrates such as neat butadiene (−10 °C)
and neat 1,3,5,7-cyclooctatetraene (20 °C). None of the
reactions took place, and in all cases the starting complexes
were recovered. Treating 3, 4, and 5 in THF solution with the
smaller CO led readily to displacement of all ligands and
formation of Ni(CO)4.
Figure 2. Molecular structure of 4 (at 50% probability level). Selected
bond distances (Å) and bond angles (deg): Ni1−C1 = 1.940(3), Ni1−
C4 = 2.049(3), Ni1−C5 = 2.054(3), C1−C1* = 1.251(7), C1−C2 =
1.479(5), C3−C4 = 1.500(4), C4−C5 = 1.381(4), C5−C6 =
1.509(4), C1*−C1−C2 = 147.5(2), C2−N1−C3 = 114.8(3), C3−
C4−C5 = 123.9(3), C4−C5−C6 = 120.0(3), C6−N2−C6* =
112.1(3).
In order to explore whether the new macrocyclic atoms
exhibited possible catalytic activity, we chose to study the
Suzuki−Miyaura cross-coupling17 of arylboronic acids with aryl
halides to give biphenyl derivatives as a test reaction. While
Suzuki−Miyaura cross-coupling is routinely performed with
palladium catalysts,18 in reactions involving aryl chlorides and
aryl bromides, as well as aryl tosylates and aryl mesylates, some
nickel catalysts proved to be even more active than palladium
complexes. As Ni(0) is more nucleophilic than Pd(0), nickel
complexes attack aryl halides more easily in the oxidative
addition step, thus favoring the cross-coupling process. An
additional advantage, in particular for industrial applications, is
that nickel is much cheaper than palladium. In their pioneering
study Percec et al.19 used (dppf)NiCl2/Zn (dppf = 1,l′-
bis(diphenylphosphino)ferrocene) in THF in the presence of
K3PO4 as the catalytic system for the cross-coupling between
arylboronic acids and aryl sulfonates or aryl mesylates.
Significant progress has since been made in the Ni-catalyzed
version of Suzuki−Miyaura aryl−aryl cross-coupling,20 although
most applications started with a Ni(II) source and added a
reducer for reduction to Ni(0).
Since complexes 3, 4, and 5 contain nickel already in the zero
oxidation state and nevertheless appear thermally rather stable,
we envisaged their use in Suzuki−Miyaura cross-couplings of
arylboronic acids and aryl halides with a base as the only
additive. After some experimentation, a 5% molar solution of 3,
4, or 5 with 3 equiv of K3PO4 in refluxing 1,4-dioxane was able
to cross-couple several organoboronic acids with aryl bromides
with different electronic demands in moderate to good yields
(Scheme 2). For example, coupling of phenylboronic acid (6a)
with 4-bromoanisole affords 4-methoxybiphenyl (10b) in 76%
yield. A full description of the optimization process and the
experimental details of the catalysis, including Tables S5 and
S6, are presented in the SI.
Figure 3. Molecular structure of 5 (at 50% probability level). The
(cod)Ni moiety has a partial occupancy of 83.4(4)% in the crystal
studied. Selected bond distances (Å) and bond angles (deg): Ni1−C1
= 2.006(5), Ni1−C4 = 2.040(4), Ni1−C5 = 2.035(5), Ni2−C1 =
1.943(7), Ni2−C19 = 2.080(7), Ni2−C20 = 2.093(7), C1−C1* =
1.30(1), C1−C2 = 1.496(7), C3−C4 = 1.501(7), C4−C5 = 1.388(7),
C5−C6 = 1.505(7), Ni1···Ni2 = 2.684(2), C1*−C1−C2 = 140.2(3),
C2−N1−C3 = 113.8(4), C3−C4−C5 = 123.3(5), C4−C5−C6 =
118.8(5), C6−N2−C6* = 113.2(5).
Å is as expected for a C−C single bond. The disorder in the
second ethano bridge could not be resolved, but is clearly
1988
dx.doi.org/10.1021/om201271y | Organometallics 2012, 31, 1983−1990

Organometallics
Article
mmol) in THF (15 mL) was combined with a solution of Ni(cod)2
(110 mg, 0.40 mmol) in THF (10 mL) at room temperature. After
standing for 1 h, the mixture was cooled to 0 °C to obtain yellow
crystals. These were isolated by filtration, washed with diethyl ether,
and dried under vacuum: yield 250 mg (78%). Mp: 210−212 °C
(dec). IR (ATR): ν 2848, 1333, 1160 cm−1. For NMR data of the
macrocycle, see Table 1. NMR data of the 4-tolylsulfonyl substituents:
δ(H) 2.37 (s, isochronous Me), 7.29, 7.59, 7.69 (phenyl); δ(C) 21.5
(isochronous Me), 127.4, 127.8, 130.0, 130.1, 133.8, 135.7, 143.9,
144.2 (phenyl). Anal. Calcd (%) for C33H39N3NiO6S3·C4H8O (800.7):
C, 55.50; H, 5.92; N, 5.25; Ni, 7.33. Found: C, 55.61; H, 6.01; N, 5.07;
Ni, 6.70.
(E,E)-1,6,11-Tris(4-tolylsulfonyl)-1,6,11-triazacyclopentade-
ca-3,8-dien-13-yne−nickel(0) (4). A solution of 2 (300 mg, 0.45
mmol) in THF (40 mL) was combined with a solution of Ni(cod)2
(110 mg, 0.40 mmol) in THF (10 mL) at room temperature. After
initial mixing the solution was left without agitation for 1−3 days. The
yellow color faded slowly, and somewhat spontaneously large, almost
colorless crystals separated. The crystals were isolated by filtration,
washed with diethyl ether, and dried under vacuum at 20 °C (1 h):
yield 255 mg (80%). Mp: 228−230 °C (dec). The complex readily
released solute THF under vacuum. For the X-ray structure
determination, crystals were directly taken from the mother liquor,
dipped in perfluoropolyether oil, and mounted on the diffractometer.
IR (ATR): ν 2839, 1344, 1163 cm−1. For NMR data of the
macrocycle, see Table 2. NMR data of the 4-tolylsulfonyl substituents:
δ(H) 2.35 (s, 6H), 2.36 (s, 3H), Me; 7.26 (4H), 7.28 (2H), 7.58 (2H),
7.62 (4H), phenyl; δ(C) 21.6 (isochronous Me), 127.7, 127.8, 130.0,
130.1, 133.9, 134.4, 144.2, 144.3. ESIpos-MS (CD2Cl2): m/e 725 [M
− THF]+. Anal. Calcd (%) for C33H37N3NiO6S3·C4H8O (798.7): C,
55.64; H, 5.68; N, 5.26; O, 14.02; Ni, 7.35; S, 12.04. Found: N, 5.19;
Ni, 7.62.
Scheme 2. Suzuki−Miyaura Reactions Catalyzed by Ni(0)
Complexes 3, 4, or 5
CONCLUSIONS
■
We have reported on the synthesis, structure, and properties of
Ni(0) complexes 3, 4, and 5 having macrocyclic (E,E,E)-1,6,11-
triazacyclopentadeca-3,8,13-triene (1) and (E,E)-1,6,11-triaza-
cyclopentadeca-3,8-dien-13-yne (2) ligands with 4-tolylsulfonyl
substituents at nitrogen. The high thermal stability of the
complexes can be attributed to the repetitive 1,6-sequences of
CC bonds or CC and CC bonds, allowing the
formation of six-membered azametallacyclohexanic substruc-
tures, which can optimally stabilize the metal−ene or metal−
yne interactions by adopting chair-type (as opposed to twist-
type) conformations.
In detail, in 3 the cyclotriene ligand adopts a chair−chair−
twist conformation of C2 symmetry, due to the geometric
constraints imposed by the three E-RHCCHR bonds, as
evidenced by NMR. In 4 the cyclodiene-yne ligand adopts a
chair−chair−chair-type conformation having Cs symmetry.
Here, the symmetrical azametallacyclohexanic ring formed
with one carbon of each of the two E-RHCCHR bonds
prefers to adopt a chair conformation, as do the two flattened
rings, each containing a carbon atom of one E-RHCCHR
unit and the CC bond. As a consequence, while in both 3
and 4 the ligands are flexible enough to ensure good
coordination to the metal, the coordination is at the same
time rigid enough to exert high stability to the complexes.
Complex 5 is derived from 4 by coordination of a Ni(cod)
entity to the CC bond. The structures of 4 and 5 have been
confirmed by single-crystal X-ray analysis.
(E,E)-1,6,11-Tris(4-tolylsulfonyl)-1,6,11-triazacyclopentade-
ca-3,8-dien-13-yne−dinickel(0)-1,5-cyclooctadiene (5). A solu-
tion of complex 4 (240 mg, 0.30 mmol) in THF (40 mL) was
combined with a solution of Ni(cod)2 (85 mg, 0.31 mmol) in THF
(10 mL) at room temperature. After initial mixing the solution was left
undisturbed, and in the course of 3 days complex 5 separated in the
form of small intergrown, dark orange crystals. The mother liquor was
removed, and the product was washed with diethyl ether and dried
under vacuum: yield 245 mg (91%). Mp: 225−227 °C (dec).
Interestingly, the crystals grew preferentially on the surface of a
Teflon-coated stirring bar and on scratched glass. IR (ATR): ν 2870,
1342, 1163 cm−1. Anal. Calcd (%) for C41H49N3Ni2O6S3 (893.4): C,
55.12; H, 5.53; N, 4.70; Ni, 13.14. Found: C, 55.87; H, 5.59; N, 4.61;
Complexes 3−5 proved to be catalysts for Suzuki−Miyaura
cross-coupling of aryl/heteroaryl boronic acids and functional-
ized aryl bromides, and reaction examples suggest a quite broad
scope of the process.
1
Ni, 11.19. Recordings of H and 13C NMR and ESIpos mass spectra
were prevented by insolubility of the compound.
Reaction of Complexes 3, 4, and 5 with CO. A solution (or
suspension) of complexes 3, 4, and 5 (about 30 mg) in 5 mL of THF
was stirred under an atmosphere of carbon monoxide. The initial
yellow color of the solution disappeared within about 3 min. The
volatile components were evaporated under vacuum. The colorless
condensate was oxidized with H2O2 and positively tested for Ni(II),
suggesting that Ni(CO)4 had been formed. The free macrocycle,
forming a white solid, remained in the reaction flask.
EXPERIMENTAL SECTION
■
THF was dried by distillation from NaAlEt4. 1,4-Dioxane was distilled
from sodium under nitrogen. Reaction mixtures were chromato-
graphed on a silica gel column (230−400 mesh). Ni(cod)2,1c
1
(C33H39N3O6S3, fw = 669.9),8c and 2 (C33H37N3O6S3, fw = 667.9)9e
were prepared as published. Microanalyses were performed by
Mikroanalytisches Labor Kolbe, Mulheim (Germany). EI mass spectra
̈
1
were recorded at 70 eV. H NMR spectra were measured at 300 and
400 MHz, and 13C NMR spectra at 75 and 100 MHz with Bruker
instruments. 1H and 13C chemical shifts (δ) were referenced to
internal solvent resonances and are reported relative to SiMe4. Melting
ASSOCIATED CONTENT
■
S
* Supporting Information
NMR spectra of complexes 3 and 4 (Figures S1−S10), DSC
curves for compounds 3−5 (Figures S11−S13), and additional
information on the X-ray structure determinations of 4 and 5
(Figures S14, S15 and Tables S1−S4). Description of the
Ni(0)-catalyzed Suzuki−Miyaura coupling reactions (Tables
S5, S6) and spectroscopic characterization of the products. CIF
files containing X-ray crystallographic data for 4 and 5. This
material is available free of charge via the Internet at http://
pubs.acs.org. Supplementary crystallographic data (4, CCDC
points were recorded with a Buchi 510 instrument. DSC experiments
̈
were performed using Mettler Toledo DSC820e and DSC822e
instruments under inert conditions (continuous flow of 40 mL/min of
high-purity N2). Aluminum crucibles (40 μL) were used; sample
masses were between 4.5 and 7 mg. All handlings of nickel(0)
complexes were done with Schlenk-type glassware under an argon
atmosphere. For a detailed description of the Ni(0)-catalyzed Suzuki−
Miyaura couplings, see the Supporting Information.
(E,E,E)-1,6,11-Tris(4-tolylsulfonyl)-1,6,11-triazacyclopenta-
deca-3,8,13-triene−nickel(0) (3). A solution of 1 (268 mg, 0.40
1989
dx.doi.org/10.1021/om201271y | Organometallics 2012, 31, 1983−1990

Organometallics
Article
2005, 11, 2689. (e) Pla-Quintana, A.; Torrent, A.; Dachs, A.; Roglans,
849420; 5, CCDC 849421) can also be obtained free of charge
via http://www.ccdc.cam.ac.uk/deposit.
A.; Pleixats, R.; Moreno-Manas, M.; Parella, T.; Benet-Buchholz, J.
̃
Organometallics 2006, 25, 5612. (f) Dachs, A.; Masllorens, J.; Pla-
Quintana, A.; Roglans, A.; Farjas, J.; Parella, T. Organometallics 2008,
27, 5768.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: anna.roglans@udg.edu; poerschke@kofo.mpg.de.
(10) (a) Masllorens, J.; Moreno-Manas, M.; Pla-Quintana, A.;
̃
Roglans, A. Org. Lett. 2003, 5, 1559. (b) Masllorens, J.; Gonzal
Roglans, A. Eur. J. Org. Chem. 2007, 158.
́
ez, I.;
Notes
(11) Jolly, P. W.; Mynott, R. Adv. Organomet. Chem. 1981, 19, 257.
(12) (a) Dierks, H.; Dietrich, H. Z. Kristallogr. 1965, 122, 1.
(b) Macchi, P.; Proserpio, D. M.; Sironi, A. J. Am. Chem. Soc. 1998,
120, 1447.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(13) (a) Porschke, K.-R. Angew. Chem., Int. Ed. Engl. 1987, 26, 1288.
̈
The project was funded by the program Acciones Integradas
Hispano-Alemanas between Germany (DAAD, BMBF) and
Spain (MICINN, Spanish Ministry of Science and Innovation,
Project AIB2010DE-00262). Financial support from the
MICINN (Projects CTQ2008-05409, CTQ2011-23121) and
the Generalitat de Catalunya (Project 2009SGR637) is also
gratefully acknowledged. S.B. thanks the University of Girona
for a predoctoral grant.
(b) Porschke, K.-R. J. Am. Chem. Soc. 1989, 111, 5691. (c) Barrios-
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Francisco, R.; Benítez-Pae
J. J. Chem. Asian J. 2011, 6, 842.
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