Role of the X Coligands in Cyclometalated [Ni(Phbpy)X] Complexes (HPhbpy = 6-Phenyl-2,2′-bipyridine)

Article abstract of DOI:10.1021/acs.organomet.1c00237

The coligand X was varied in the organonickel complexes [Ni(Phbpy)X] (X = F, Cl, Br, I, C6F5) carrying the anionic tridentate CNN ligand 6-(phen-2-ide)-2,2′-bipyridine (Phbpy-) to study its effect on electronic structures of these complexes and their activity in Negishi-like C-C cross-coupling catalysis. The complexes were synthesized from the precursor [Ni(COD)2] (COD = 1,5-cyclooctadiene) by chelate-assisted oxidative addition into the phenyl C-X bond of the protoligand 6-(2-halidophenyl)-2,2′-bipyridine) and were obtained as red powders. Protoligands X-Phbpy carrying the halide surrogates X = OMe, OTf (triflate) failed in this reaction. Single-crystal XRD allowed us to add the structures of [Ni(Phbpy)Cl] and [Ni(Phbpy)I] to the previously reported Br derivative. Cyclic voltammetry showed reversible reductions for X = C6F5, F, Cl, while for Br and I the reversibility is reduced through rapid splitting of X- after reduction (EC mechanism). UV-vis spectroelectrochemistry confirmed the decreasing degree of reversibility along the series C6F5 > F > Cl ? Br > I, which parallels the "leaving group character"of the X coligands. This method also revealed mainly bpy centered reduction and essentially Ni(II)/Ni(III) oxidations, as corroborated by DFT calculations. The rather X-invariant long-wavelength UV-vis absorptions and excited states were analyzed in detail using TD-DFT and were consistent with predominant metal to ligand charge transfer (MLCT) character. Initial catalytic tests under Negishi-like conditions showed the complexes to be active as catalysts in C-C cross-coupling reactions but did not display marked differences along the series from Ni-F to Ni-I.

Full text of DOI:10.1021/acs.organomet.1c00237

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Article
Role of the X Coligands in Cyclometalated [Ni(Phbpy)X] Complexes
(HPhbpy = 6‑Phenyl-2,2′-bipyridine)
Nicolas Vogt, Aaron Sandleben, Lukas Kletsch, Sascha Schäfer, Mason T. Chin, David A. Vicic,
Gerald Hörner,* and Axel Klein*
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ABSTRACT: The coligand X was varied in the organonickel complexes
[Ni(Phbpy)X] (X = F, Cl, Br, I, C6F5) carrying the anionic tridentate C∧N∧N
ligand 6-(phen-2-ide)-2,2′-bipyridine (Phbpy−) to study its effect on electronic
structures of these complexes and their activity in Negishi-like C−C cross-coupling
catalysis. The complexes were synthesized from the precursor [Ni(COD)2] (COD
= 1,5-cyclooctadiene) by chelate-assisted oxidative addition into the phenyl C−X
bond of the protoligand 6-(2-halidophenyl)-2,2′-bipyridine) and were obtained as
red powders. Protoligands X−Phbpy carrying the halide surrogates X = OMe, OTf
(triflate) failed in this reaction. Single-crystal XRD allowed us to add the structures
of [Ni(Phbpy)Cl] and [Ni(Phbpy)I] to the previously reported Br derivative.
Cyclic voltammetry showed reversible reductions for X = C6F5, F, Cl, while for Br
and I the reversibility is reduced through rapid splitting of X− after reduction (EC mechanism). UV−vis spectroelectrochemistry
confirmed the decreasing degree of reversibility along the series C6F5 > F > Cl ≫ Br > I, which parallels the “leaving group
character” of the X coligands. This method also revealed mainly bpy centered reduction and essentially Ni(II)/Ni(III) oxidations, as
corroborated by DFT calculations. The rather X-invariant long-wavelength UV−vis absorptions and excited states were analyzed in
detail using TD-DFT and were consistent with predominant metal to ligand charge transfer (MLCT) character. Initial catalytic tests
under Negishi-like conditions showed the complexes to be active as catalysts in C−C cross-coupling reactions but did not display
marked differences along the series from Ni−F to Ni−I.
yields by introduction of a bromo substituent to the 2-position
of the phenyl unit and follow-up treatment with [Ni(COD)2]
(COD = 1,5-cyclooctadiene), affording the desired complex by a
chelation-mediated oxidative addition.25 This reaction has been
successfully carried out also for derivatives of Phbpy.26 The
[Ni(Phbpy)(CF3)] complex was synthesized from [Ni(Phbpy)-
Br] through a transmetalation from CF3(SiMe3).25 More
recently we have obtained the parent complex [Ni(Phbpy)Br]
through an alternative pathway involving base-assisted direct
C−H nickelation starting from HPhbpy and NiBr2.27 In this
context we also isolated the precoordinated species [Ni(κ2-N,N-
HPhbpy)Br2]2.27 In addition, the NCN derivatives [Ni-
(PyPhPy)X] (X = Cl, Br, I, OAc) were obtained recently in
good yields through similar methods.28
INTRODUCTION
■
Cyclometalated complexes for the noble transition metals Ru,
Rh, Ir, Pd, Pt, and Au carrying the anionic tridentate (Phbpy−)
ligand have been synthesized and investigated since the early
1990s.1−24 Most of them were accessed through a chelate-
assisted C−H activation reaction, presumably after N∧N
precoordination. Corresponding complexes of the base metal
nickel were first reported in 2014.24−26 Initially, the parent
[Ni(Phbpy)Br] complex (Chart 1, top) was generated in high
Chart 1. Synthesis of Cyclometalated Ni(II) Complexes
In this contribution we synthetically widen the [Ni(Phbpy)X]
motif by introducing the coligands F, Cl, I, and C6F5. With the
(2-halidophenyl)-2,2′-bipyridines XPhbpy (X = F, Cl, Br, I,
Received: April 13, 2021
Published: June 1, 2021
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OMe, OTf) and [Ni(COD)2] at hand, we successfully adapted
the oxidative addition method with [Ni(COD)2] (Chart 1, top).
Alternatively, a transmetalation reaction with XMgBr opens up a
general diversification pathway to [Ni(Phbpy)X], as exemplified
here with X = C6F5 (Chart 1, bottom).
[Ni(Phbpy)Br] in an excellent yield (98%). The new complexes
were red solids, as was the parent [Ni(Phbpy)Br]. Standing for
longer times in protic solvents leads to hydrolysis and formation
of green paramagnetic solutions and materials which indicate
nonorganometallic octahedral Ni(II) species.37 The C6F5
complex undergoes rapid decomposition reactions in solution.
The main reaction is a reductive elimination, yielding C6F5−
Phbpy and Ni(0) species (see below).
Single-Crystal XRD. In addition to the already well-known
crystal structure of [Ni(Phbpy)Br],25 single crystals suitable for
X-ray diffraction were obtained from a saturated CH2Cl2
solution of [Ni(Phbpy)Cl] and from a THF solution of
[Ni(Phbpy)I].
The complex [Ni(Phbpy)Cl] crystallizes in the monoclinic
space group P21/n (isotypic with [Ni(Phbpy)Br])25 containing
four asymmetric units per unit cell and allowing the
discrimination between N2 and C1 on the periphery of the
Phbpy ligand. The structure of [Ni(Phbpy)I] was initially solved
in the monoclinic space group C2/c containing a rotation axis in
the middle of the molecule. N2 and C1 were refined assuming a
1/1 disorder. The failure to discriminate between C and N can
be caused by the diffuse electron density of heavier atoms such as
iodine, which has already been reported for an analogous Pd(II)
complex.23 To be able to define N2 and C1, we thus embarked
With this series of complexes we cover a broad range in
electronegativity (EN) of the X coligands and expect marked
differences in their contributions to the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO). We probed the frontier orbitals using UV−vis
absorption spectroscopy and (spectro)electrochemical experi-
ments combined with quantum chemical calculations based on
density functional theory. Ready and variable access to the
complex family [Ni(Phbpy)X] allowed the study of electronic
effects of X substitution on the catalytic activity in nickel-
dependent C−C coupling. Both terpy Ni(II) complexes29−33
(terpy = 2,2′;6′,2′′-terpyridine) and complexes of the type
[Ni(II)(N∧N)(aryl)Br] (N∧N = α-diimine)34−37 have been
well-studied in the context of C−C coupling reactions. Here we
investigate the electronic properties and catalytic activities of
nickel complexes bearing the carbanionic Phbpy− for compar-
ison. In particular, we expected that, alongside decreasing EN,
the leaving-group character of the X coligands should also
markedly increase. Consequently, the complexes with X = Br, I
might be better precatalysts in Negishi-type C−C coupling
reactions over the X = Cl, F, C6F5 derivatives.
̅
on solving the structure in the triclinic space group P1 (details
are given in the Supporting Information), while no solution in
the isotypic monoclinic P21/n (found for X = Cl, Br) was
obtained from the data set.
RESULTS AND DISCUSSION
■
The crystal-packing structure (Figure 1, left) and the
molecular structure (Figure 1, right) of [Ni(Phbpy)Cl] are
Preparation and Characterization of Ligands and
Complexes. The protoligands X−Phbpy are readily accessible
via the Kroehnke pyridine synthesis26,27,38 and have been
synthesized from 2-acetylpyridine and 2′-halidoacetophenones.
X−Phbpy derivatives were characterized using NMR spectros-
copy and mass spectrometry, which both confirmed the
successful preparation of the products and their purity (details
are given in the Supporting Information). The complexes
[Ni(Phbpy)X] were prepared from the protoligands X−Phbpy
and [Ni(COD)2] by performing a chelation-assisted oxidative
addition into the C−X bond. For X = Cl, Br the red products
were immediately observed, while the blue or violet
intermediates observed for X = F, I likely correspond to
[Ni(X−Phbpy)(COD)].25,26 The latter is counterintuitive,
since the C−I bond should be the most reactive for oxidative
addition in the halide series. These Ni(0) intermediates were
also observed for X = OMe, OTf, where the oxidative addition
reaction leading to the target [Ni(Phbpy)X] complexes could
not be initiated, even when the reaction mixture was heated
under reflux (THF). This is remarkable in view of the recent
reports of the successful nickelation of two closely related 4-
(tBu)-2-(1,10-phenanthrolin-2-yl)phenol derivatives RO−
(tBu)Phphen (R = triflate, pivalyl) using the same method at
ambient temperature.39,40 Presumably, the more rigid phenan-
throline vs bipyridine precoordination and the tBu group which
probably stratifies the phenol group to the coordination plane in
conjunction with its electron-releasing property are responsible
for this success.
Figure 1. Crystal structure of [Ni(Phbpy)Cl] viewed along the
crystallographic a axis (left) and molecular structure (right) with
thermal ellipsoids at 50% probability. Protons are omitted for clarity.
very similar to those of the previously reported [Ni(Phbpy)Br].
Now having the series Cl−Br−I in hand, we found that in the
crystal structures the completely planar complexes with X = Cl,
Br, I all show head to tail stacking (Figure 1, left; more figures are
given in the Supporting Information).
The Ni−X distances in the molecules increased as expected
from the increasing covalent radii of the X atoms (Table 1).42
The Ni−N1 bond trans to the coligands X is rather invariant; no
trans influence was found. All three complexes were square-
planar with the sum of cis angles amounting to 360 1° (Table
1).
This is in keeping with the recently reported nickelation of a
phenolate-based semirigid N∧C∧N∧O ligand using Ni(OAc)2
under reflux in HOAc/HCCl3 (9/1).41
MS, elemental analysis, and single-crystal XRD confirmed the
successful preparation of [Ni(Phbpy)X] with X = F (82%), Cl
(87%), Br (74%), I (58% yield). [Ni(Phbpy)(C6F5)] was
synthesized by a transmetalation reaction25 of (C6F5)MgBr with
In comparison with the N∧C∧N-coordinated complexes
[Ni(PyPhPy)X],28,41 the central Ni−N1 bond of the Phbpy
(C∧N∧N coordinated) is not much longer than the central Ni−
C bond (+0.01−0.02 Å) in the PyPhPy derivatives, as the
stronger σ-donating power of the carbanionic phenide moiety vs
pyridine would suggest. Also the Ni−X bond lengths were only
about 4% longer for the PyPhPy derivatives.28 Obviously, the
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a
Table 1. Selected Atom Distances, Angles, and Dihedral
Angles of [Ni(Phbpy)X] (X = Cl, Br, I) from Single-Crystal
XRD
Table 2. Electrochemical Data of [Ni(Phbpy)X]
X
E1/2(Ox1)
E
1/2(Red1)
E
1/2(Red2)
ΔE(Ox1 − Red1)
a
b
C6F5
F
Cl
Br
I
0.32 irr
0.07 irr
0.04
0.08
0.15 irr
−1.90
−1.92
−1.93
−1.90
−1.93 irr
−2.59
−2.52
−2.58
−2.52
−2.62
2.22
1.99
1.97
1.98
2.08
b
X = Cl
X = Br
X = I
Distances/Å
1.917(3)
1.838(2)
1.973(2)
2.169(1)
Angles/deg
83.2(1)
82.3(1)
97.3(1)
97.3(1)
165.5(1)
179.2(1)
Ni1−C1
Ni1−N1
Ni1−N2
Ni1−X1
1.947(5)
1.848(5)
1.969(5)
2.301(1)
1.967(3)
1.847(3)
1.964(3)
2.490(1)
a
Potentials in V vs ferrocene/ferrocenium, recorded in 0.1 M
nBu4NPF6/THF, half-wave potentials E1/2 for reversible waves, Epa
= anodic peak potential or Epc = cathodic peak potential for
b
irreversible (irr) waves, scan rate 100 mV/s. Further oxidation at Epa
C1−Ni1−N1
N1−Ni1−N2
C1−Ni1−X1
N2−Ni1−X1
C1−Ni1−N2
N1−Ni1−X1
82.8(2)
82.6(2)
96.9(1)
97.7(1)
165.4(2)
178.7(1)
82.4(1)
82.7(1)
97.6(1)
97.3(1)
165.1(1)
180.0(1)
= 0.64 V.
By a decrease in the scan rate from the standard 100 to 20
mV/s, the reversibility of the Cl and the Br complexes also
decreased while the waves for the F and C6F5 complexes
remained fully reversible. This is in line with the EC mechanism
already reported for the [Ni(Phbpy)Br] complex.25,26 After the
electrochemical one-electron reduction (E) the bromide ligand
is cleaved (C). Within our series of complexes, the rate of the
cleavage goes with the trend of the EN and the “leaving group
character” of X: C6F5 ≈ F < Cl ≪ Br ≪ I.
Akin to previous work on the Ni(II) complexes of the
metalated N∧C∧N congener,28 the coligand X in [Ni(Phbpy)X]
has only a very mild effect on the reduction process, both
quantitatively and qualitatively: in particular the potentials
crowd at E(Red1) = −1.92 V, pointing to a ligand-centered
reduction process.
Dihedral Angles/deg
0.41(1)
C1−C6−C20−N1
N1−C24−C30−N2
0.49(1)
0.17(1)
0.9(4)
1.4(1)
0.21(1)
Sums of Angles/deg
360.1(1)
360.0(1)
360(1)
a
b
Structure solution for X = Cl, Br in P21/n and for X = I in P1
̅
. From
ref 25.
perfect match to all three donor atoms of both tridentate ligands
partially overrules the stronger trans influence of the carbanionic
phenide in comparison with pyridine.
Due to the high tendency of [Ni(Phbpy)(C6F5)] to
decompose via reductive eliminations in solution at ambient
temperature, no single crystals could be obtained. Attempts to
crystallize this complex at low temperatures were not successful.
From a batch containing [Ni(Phbpy)(C6F5)] and unconverted
[Ni(Phbpy)Br] (from NMR analysis), we obtained single
crystals of reasonable quality of the compound [Ni(κ2-N,N-
C6F5Phbpy)Br2]·THF containing a κ2-N,N-bound C6F5−Phbpy
ligand (data and figures are given in the Supporting
Information). This ligand is the expected product of the
reductive elimination together with zerovalent Ni. Indeed, we
found Ni nanoparticles alongside this C6F5−Phbpy ligand as the
main components in decomposed solutions of [Ni(Phbpy)-
(C6F5)]. The peculiar product [NiII(κ2-N,N-C6F5Phbpy)Br2]
was only obtained in the presence of the [Ni(Phbpy)Br]
component in this crystallization batch, explaining the Ni +II
oxidation state and the bromide ligands in this compound.
Electrochemical Experiments and Frontier MO Pat-
tern. Akin to the parent complex [Ni(Phbpy)Br],25 in THF
solutions the F, Cl, and C6F5 derivatives show a fully reversible
first reduction slightly positive of −2 V, while for [Ni(Phbpy)I]
an irreversible wave is observed (Figure 2 and Table 2; further
plots are given in the Supporting Information).
Density functional theory methods corroborate this con-
clusion (Figure 3; computational details are given in the
Figure 3. DFT-derived frontier molecular orbitals of [Ni(Phbpy)X] (X
= Cl, Br, I): blue, occupied; red, virtual. Plots for X = F, C6F5 are given in
Figures S20 and S24 in the Supporting Information.
Experimental Section). Computed metrics have been calibrated
with experimental data, and an excellent match is generally
observed (Table S6 in the Supporting Information). The
elongated Ni−N bond located trans to the Ni−C bond is an
artifact of the computation, as recently shown.40 Structures
constrained to the experimental metrics are isoenergetic. The π-
ligand-borne character of the virtual frontier MOs is conserved
along the series [Ni(Phbpy)X], as is the energy of the essentially
bpy-localized lowest unoccupied molecular orbital (LUMO).
Very similar results have been recently also received with Ni
complexes of related phenanthroline-derived C∧N∧N ligands.40
An excellent linear correlation of computed LUMO energies and
experimental reduction potentials exists across a series of 17
Figure 2. Cyclic voltammograms of [Ni(Phbpy)Cl (left) and
[Ni(Phbpy)I] (right) in 0.1 M nBu4NPF6/THF. Scan rate: 100 mV/s.
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planar Ni(II) complexes (Figure S28 in the Supporting
Information; additional data are from refs 28 and 40).
For the complexes with X = F, Cl, C6F5 further reversible
reduction waves were observed at around −2.6 V. For the
substitutionalyl labile Br and I complexes, intermediate
reduction waves were observed at around −2.3 V at slow
scans for X = Br and normal scans for X = I (Figure 2), likely
representing the reduction of the halide-stripped radical
complex [Ni(Phbpy)(THF)]•.25,26
In an allusion to the MO diagrams in Figure 3 (plots for X = F,
C6F5 are given in Figures S20 and S24 in the Supporting
Information), the three highest occupied frontier MOs are
clearly metal-borne (if they are superimposed with π-interacting
contributions of X and/or phenide). While our DFT settings
2
identify the nonbonding dz as the highest occupied molecular
orbitals (HOMOs) in all cases, this assignment may well be
method dependent, given the narrow clustering of the HOMO
to HOMO-2 levels within <100 meV. The contribution of π*-
antibonding X-ligand p orbitals to HOMO-2 becomes
increasingly sizable along the series F < Cl < Br < I (Figures
S20−S23 in the Supporting Information). Mulliken spin
densities of doublet cations [Ni(Phbpy)X]+ obtained from
preliminary DFT calculations indeed indicated an ever-growing
localization of spin density on X along the series F ≪ Cl < Br < I.
Nevertheless, the first oxidation event in CV must be associated
with the Ni(II/III) couple. Furthermore, the energies of the
HOMO vary only slightly with a variation of X so that conserved
oxidation potentials should prevail. Accordingly, [Ni(Phbpy)-
Cl] showed a reversible one-electron-oxidation wave at around 0
V, as was likewise observed for the parent bromido complex and
assigned to a Ni(II/III) couple.25,26 For X = F, C6F5, I,
irreversible oxidation waves were observed. For C6F5 we assume
that oxidation induces the notorious reductive elimination, as
was also found for the CF3 derivative previously.25 For X = F we
have evidence from EI-MS for a similar oxidatively induced
reductive elimination forming F−Phbpy (Figure S5 in the
Supporting Information), in line with our failure to obtain single
crystals of this complex. For X = iodide we believe that the iodide
ligand is the final “hole acceptor” (electron donor),43 after initial
oxidation of the Ni(II) center.
The electrochemical band gap is conserved at about 2 V for X
= F, Cl, Br, I, in keeping with the small variations of the LUMO
and HOMO energies almost compensating each other (Figure
3). The slightly higher value of 2.2 V for X = C6F5 very probably
reflects the essentially covalent Ni−C bond of the C6F5 coligand
in comparison with the ionic Ni−X bonds.
UV−Vis Absorption Spectroscopy and TD-DFT Calcu-
lations. The nature and energy span of the frontier MOs of the
entire complex series suggests the existence of optical transitions
with MLCT character in the visible spectral range. Indeed, the
optical spectra of the complexes all feature significant absorption
bands across the visible range up to 600 nm (THF solution;
Figure 4 and Table 3).
Figure 4. Experimental UV−vis absorption spectra of [Ni(Phbpy)X]
(X = C6F5, F, Cl, Br, I) in THF (top). TD-DFT derived convoluted and
line spectra of [Ni(Phbpy)X] (bottom).
In order to receive insights into the character of the
underlying individual optical transitions, the spectra were
modeled with TD-DFT methods. A very satisfying match of
the experimental band energies and intensities was obtained
with TPSSh/QZVP settings (bottom in Figure 4), which are
well suited to model planar Ni(II) complexes.28,39,40,44,45
With a view to the intensity (ε ≫ 1000 L mol−1 cm−1) an
assignment of the leading transitions as ligand field (d−d)
absorption bands can be ruled out. While the three expected d−
d transitions of planar d8 nickel(II), that is, 1A1g → 1B1g, 1A1g →
46
1
1
1B3g, and A1g → B1g, certainly contribute, TD-DFT results
for [Ni(Phbpy)Br] and derivatives with substituted Phbpy26 had
rather pointed to an interpretation in terms of metal to ligand
charge transfer bands (MLCT; dNi−π*bpy).
In agreement with the largely X invariant redox chemistry and
the conserved MO pattern, the five complexes show very similar
spectral features. Four sets of absorption bands are found in all
spectra, peaking at around 520 nm (λ1), 410 nm (λ2), 360 nm
(λ3), and 280 nm (λ4). The partially structured long-wavelength
envelope between 450 and 600 nm (2000 < εmax (L mol−1 cm−1)
< 5000) is in line with the visually observed red color. While the
energies of the main maxima are almost identical, the intensities
vary markedly. The same is true for the bands at around 400 nm
and the two UV bands at 350 and 280 nm (Table 3).
In keeping with this, the present TD-DFT difference densities
derived of diagnostic transitions of [Ni(Phbpy)Br] reveal
predominant MLCT character for the low-energy band (Figure
5; plots for for X = F, Cl, I are given in Figures S25−S27 in the
Supporting Information). It is noted that for X = Cl, Br, I the
HOMO → LUMO transition has vanishing intensity; this is a
recurrent finding for this class of compounds. The energies of
the leading vis transitions decrease along the series C6F5 > F > Cl
= Br > I, which is consistent with the EN of the halides. Toward
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Table 3. Absorption Maxima of the [Ni(Phbpy)X] (X = C6F5, F, Cl, Br, I) Complexes
X = C6F5
X = F
X = Cl
X = Br
X = I
a
a
a
a
b
c
λ1 (ε)
λ2 (ε)
λ3 (ε)
λ4 (ε)
496 (1.60)
504 (2.10)
506 (3.47)
391 (2.67)
354 (15.3)
281 (26.7)
2.45
509 (2.81)
392 (3.28)
355 (11.0)
282 (20.2)
513 (3.11)
401 (2.68)
352 (10.3)
276 (34.0)
2.42
402 (2.10)
351 (7.45)
277 (14.7)
2.50
401 (4.03)
353 (12.0)
277 (23.9)
2.46
d
spectral onset
2.44
a
b
Measured in THF, with absorption maxima λ in nm and molar absorption coefficients ε in 1000 L mol−1 cm−1. Long-wavelength shoulder at 560
c
d
nm. The band extends up to 540 nm. In eV.
2,4,6-trimethylphenyl).25,26,34−36 Upon further reduction, the
band system at 700−1000 nm shifts to 650−800 nm, while the
480−600 nm band system undergoes a slight red shift and the
380 nm band massively intensifies concomitant with a red shift
to 420 nm. This is typical for 2-fold-reduced Ni(II) complexes of
bpy.25,35 In a return to the initial potential after the first
reduction, the original spectrum could not be restored. Although
in the CV experiment the reduction is reversible, the far slower
SEC experiment reveals chemical irreversibility, most probably
due to the Ni−Cl scission after reduction (EC), which was
discussed above.
The same behavior was observed for the [Ni(Phbpy)F] and
[Ni(Phbpy)(C6F5)] derivatives (Figure S16 and S17 in the
Supporting Information). During oxidation, the three long-
wavelength bands are markedly bleached, while a band at about
320 nm grows in (Figure S19 in the Supporting Information).
The disappearance or marked blue shift of the long-wavelength
bands is in line with the MLCT character of these bands.
Removal of an electron from a metal-borne orbital necessarily
results in stabilization of this orbital (blue shift) and a loss of
intensity. At the same time, in the EI-MS(+) spectra of
[Ni(Phbpy)X] the [M]+ signature of the C−C coupled ligand
2,2′-bis((2,2′-bipyridin)-6-yl)-1,1′-biphenyl (C32H22N4) is ob-
served alongside the [Ni(Phbpy)]+ and [Phbpy]+ fragments
(see the Experimental Section). This C−C coupling is
consistent with a mechanism that involves removal of one
electron from the Ni−C bond, leaving oxidized Ni(III) and a
bpyPh• radical intermediate. For the X = F, C6F5 derivatives a
very similar bleaching of the long-wavelength band was observed
upon oxidation (Figures S16 and S17 in the Supporting
Information). The remaining bands lie markedly below 300
nm, indicating the presence of the protoligands XPhbpy (X = F,
C6F5). They were presumably produced by a reductive
elimination after oxidation. In contrast to this, after exhaustive
oxidation of the chlorido complex [Ni(Phbpy)Cl] intense UV
bands at 320 nm very probably represent the C−C coupled
bpyPh−Phbpy (Figure S18).
Figure 5. TD-DFT derived difference densities of diagnostic vis and
near-UV transitions: orange, source; blue, sink.
the UV regime the transitions are of mixed MLCT/L′LCT
character (πPh−π*bpy with minor contributions of X; Figure 5).
UV−Vis Spectroelectrochemistry (SEC). SEC measure-
ments were carried out on the complex [Ni(Phbpy)Cl] (Figure
6) to probe for the corresponding orbitals and the overall
Negishi-Type C−C Cross-Coupling Catalysis. Mechanis-
tically, the reduction of the Ni(II) precatalyst and the cleavage of
halide coligands play an important role in Negishi-type C−C
cross-coupling reactions.29−35 The four halogenido complexes
[Ni(Phbpy)X] (X = F, Cl, Br. I) were submitted to two typical
test reactions (Scheme 1) to probe for the effect of the X
coligand.
After a 12 h reaction time, the mixtures were studied using
GC-MS with hexamethylbenzene as an internal standard. In
addition to the starting materials, the targeted C−C cross
products 4-pentyltoluene and 4-(2-(1,3-dioxan-2-yl)ethyl)-
toluene were observed alongside the homocoupling products
n-decane (homoalkyl coupling), 4,4′-bis-toluene, and bis((1,3-
dioxan-2-yl)ethan-1yl) (homoaryl coupling). The cumulative
Figure 6. UV−vis absorption spectra recorded during electrochemical
reductions of [Ni(Phbpy)Cl] in in 0.1 M nBu4NPF6/THF, in 0.1 V
steps.
reversibility of the redox processes. During the first reduction,
structured long-wavelength bands appear in the ranges 700−
1000 and 480−600 nm. Together with the intense band at 380
nm, they are typical for a bpy-centered reduction, as was
expected on the basis of the CV discussion and the MO pattern
(see above).
Similar spectra were observed previously in the parent
complex [Ni(Phbpy)Br] and also in the noncyclometalated
complexes [Ni(bpy)(Mes)Br] and [Ni(bpy)Mes)2] (Mes =
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tuted 6-phenyl-2,2′-bipyridine; Figure 8). For reaction I the
Scheme 1. C−C Coupling Reactions under Negishi
a
accumulated yields range from 25 to 56%, thus being somewhat
Conditions
a
Conditions: 5 mol % of [Ni(Phbpy)X] precatalysts (X = F, Cl, Br, I),
Figure 8. Yields of the C−C cross coupling, homo aryl and homo alkyl
coupling, and accumulated yields for reaction I (left) and II (right)
using the complexes [Ni(R-Phbpy)Br] as precatalysts.
12 h, stirring at ambient T, in THF.
yields increased from 47 to 61% along the series X = I < Br < Cl <
F for reaction I (Figure 7).
lower in comparison with that for the unsubstituted [Ni-
(Phbpy)Br] (57%). For reaction II the yields are markedly
higher, ranging from 38 to 72%. The best catalyst for both
reactions in terms of selectivity is the complex [Ni(Ph(4′-(3-
MeOPh)bpy)Br] with a 3-MeO-substituted 4′-bpy ligand, while
the complex [Ni((3-MeOPh)bpy)Br] shows the highest total
yields (Tables S9 and S10). Unfortunately, the small number of
samples in view of the largely varying yields and selectivities
preclude a structure−activity correlation at this stage.
Figure 7. Yields of the C−C cross coupling, homo aryl and homo alkyl
coupling, and accumulated yields for reaction I (left) and reaction II
(right) using the complexes [Ni(Phbpy)X] (X = F, Cl, Br, I) as
precatalysts.
CONCLUSIONS
■
Nickel-driven C−C coupling schemes imply redox-dependent
labilization of the coordination sphere. Herein we have
addressed this hypothesis through the synthetic variation of
potentially labile, anionic coligands X in planar nickel(II)
platforms, [Ni(Phbpy)X] (with X = F, Cl, Br, I, C6F5). The
complexes with X = F, Cl, Br, I were synthesized in good (58%)
to excellent (87%) yields from [Ni(COD)2] (COD = 1,5-
cyclooctadiene) by chelate-assisted oxidative addition into the
phenyl C−X bond of the protoligand 6-(2-halidophenyl)-2,2′-
bipyridine, whereas the [Ni(Phbpy)(C6F5)] derivative was
accessible in excellent yield using the (C6F5)MgBr Grignard
reagent. Their use in hetero-C−C coupling revealed the entire
series of complexes to be good catalysts under Negishi-type
conditions. A comparison of precatalysts with varying X
coligands and with R substituents on the Phbpy ligand indeed
indicated a selectivity-steering influence of X, I ≫ Br > Cl ≈ F,
which appears to scale inversely with the overall turnover. In
contrast to this, the derivatives [Ni(R-Phbpy)Br] were highly
sensitive to the nature of the R substituent on the phenyl core.
An interpretation in terms of a combined effect of varied
electronegativity and leaving-group character of the coligand X
is corroborated by DFT-augmented (spectro-)electrochemical
investigations. That is, the reversibility of the reduction and
oxidation waves of the Phbpy complexes is massively altered
with varying X coligands, while the potentials are rather
conserved. Variable mixing of Ph and X contributions to the
essentially Ni centered highest occupied molecular orbitals
(HOMO) compensates for the variation in X, in complete
agreement with the invariant electrochemical data, E1/2(Ox1),
and the likewise X-invariant long-wavelength UV−vis absorp-
tions. The corresponding excitations were analyzed in detail
using TD-DFT and were consistent with metal to ligand charge
transfer (MLCT) character. The scan-rate-dependent electro-
chemical reduction of [Ni(Phbpy)X] increasingly gained
irreversible character for X = Br, I, whereas reduction remained
fully reversible for X = F, C6F5, even at the lowest scan rates. The
redox-dependent lability of the Ni−X bond clearly shows up in
To exclude the presence of Pd particles in the precatalyst
material, ICP-OES measurements with a detection limit of 3 ppb
were carried out on a number of [Ni(R-Phbpy)Br] derivatives,
including the parent complex [Ni(Phbpy)Br], and on the Ni
precursor [Ni(acac)2] for the [Ni(COD)2]. In none of the
materials did we find Pd.
The yields of the targeted C−C cross product 4-pentyltoluene
vary only slightly from 40 to 45% with no correlation with the
EN or leaving-group character of the X coligand. In addition, the
product selectivity, measured as the heterocoupling/homocou-
pling product ratio, is very similar for the complexes with X = F,
Cl, Br (ratio 2.3−3.4, Table S7 in the Supporting Information).
Strikingly, however, the iodido complex exhibited a markedly
higher ratio (11/1). Quite clearly, the selectivity in the hetero/
homo ratios is inversely coupled to the cumulative yields, but
even if the ratio is corrected through the cumulated yields, the
iodide complex is still markedly more selective.
For reaction II the cumulative yields are in the same range as
for reaction I with, once more, the iodido complex showing the
lowest yield. Generally, the product selectivity for reaction II was
markedly lower than for reaction I (Table S8). In keeping with
the leaving-group hypothesis, however, the complexes with X =
Br, I still reach a hetero/homo ratio of about 1, while for X = Cl,
F the homocoupling products are prevalent.
We tentatively ascribe the huge difference in terms of
selectivity for reaction I vs reaction II to the possibility for Zn
to form a chelate binding using the dioxane function in reaction
II, thus tuning its reactivity. Similar phenomena have been
reported previously.47−49 Further, the role of the different
halides might go beyond that of being a more or less suitable
leaving group. Salt byproducts are known to play a critical role in
Negishi cross-coupling reactions.50,51
For comparison we tested some of the previously reported
complexes [Ni((R)Phbpy)Br] (H−(R)Ph(R′)bpy = substi-
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complex the 70−90 lowest optical electronic transitions (Figure 5 and
Figures S25−S27 in the Supporting Information) were assessed with
ORCA implemented TD-DFT methods within the Tamm−Dancoff
approximation.
Materials. Commercially available chemicals were purchased from
Sigma-Aldrich, Acros, ABCR, or Fisher-Scientific and were used
without further purification. Dry THF was obtained from distillation
over sodium/potassium alloy.
the lack of reversibility in spectroelectrochemical reduction−
reoxidation cycles. Reduction potentials could be linearly
correlated with DFT-derived LUMO energies. This assignment
allows us to make a clear distinction between complexes derived
from isomeric metalating ligands: that is, C∧N∧N and N∧C∧N.
When the C∧N∧N-coordinated [Ni(Phbpy)X] complexes are
compared with the N∧C∧N-coordinated derivatives [Ni-
(PyPhPy)X], the different location of the carbanionic phenide
group causes a great difference in the long-wavelength
absorption energies (−0.4 eV) and the reduction potentials
(−0.4 V), which clearly shows the superior π-accepting capacity
of the bpy moiety in Phbpy in comparison with the two
electronically “isolated” pyridines in PyPhPy. Interestingly, the
bonding of X coligands is much less affected by this location. We
will embark on further catalytic tests to assess this parameter.
Preparation of the XPhbpy (X = F, Cl, Br, I, OH, OMe, OTf)
Protoligands and [Ni(COD)2]. The preparation is described in the
Supporting Information.
Preparation of the Complexes [Ni(Phbpy)X]: General
Procedure. Under an inert atmosphere the corresponding 6-(2-
halidophenyl)-2,2′-bipyridine derivative (1.0 mmol) was dissolved in 8
mL of dry THF. A 2 mL portion of a freshly prepared 0.5 M suspension
of [Ni(COD)2] in dry THF was added to the solution. The mixture
turned dark red, and a red precipitate formed. After 22 h of stirring at
ambient temperature the solution was decanted. The solid was washed
with 10 mL of dry pentane and dried under vacuum.
[Ni(Phbpy)F]. Yield: 3 g (13.2 mmol, 82%). Anal. Found (calcd) for
C16H11N2NiF (M = 308.97 g mol−1): C, 62.09 (62.20); H, 3.55 (3.59);
N, 9.02 (9.07). 1H NMR (600 MHz, CD2Cl2): δ 8.74 (s, 1H, H6’), 8.03
(m, 1H, H4′), 7.85 (m, 1H, H3′), 7.79 (m, 1H, H3), 7.59 (m, 1H, H4),
7.42−7.37 (m, 2H, Ha, H5′), 7.32 (m, 1H, H5′), 7.13−7.06 (m, 3H,
Hc, Hd, Hb) ppm. 19F NMR (282 MHz, CDCl3): δ −180.7 ppm. EI-
MS(+): m/z 308 [M]+, 289 [M − F]+, 250 [FPhbpy]+, 462 [bpyPh−
Phbpy]+. In solution, this complex readily undergoes a decomposition
reaction. In one of them, a reductive elimination yields F−Phbpy
comparable to what has been observed for the CF3 complex
[Ni(Phbpy)(CF3)].25 Data for F−Phbpy are given in the Supporting
Information.
EXPERIMENTAL SECTION
■
Instrumentation. 1H, 13C, and 19F NMR spectra were recorded on
a Bruker Avance II 300 MHz (1H, 300 MHz; 13C, 75 MHz; 19F, 282
MHz) double-resonance (BBFO) 5 mm observation probehead with a
z-gradient coil spectrometer. Chemical shifts were relative to TMS.
UV−vis absorption spectra were recorded on a Varian Cary 05E
spectrophotometer. Elemental analyses were obtained using a
HEKAtech CHNS EuroEA 3000 analyzer. EI-MS(+) spectra were
measured with a Finnigan MAT 95 instrument. Simulations were
performed using ISOPRO 3.0. Electrochemical measurements were
carried out in 0.1 M nBu4NPF6 solution in THF using a three-electrode
configuration (glassy-carbon electrode, Pt counter electrode, Ag/AgCl
reference) and a Metrohm Autolab PGSTAT30 potentiostat and
function generator. The potentials were referenced against the
ferrocene/ferrocenium redox couple as an internal standard. UV−vis
spectroelectrochemical measurements (in 0.1 M nBu4NPF6 solution in
THF) were performed using an optically transparent thin-layer
electrode (OTTLE) cell52,53 at room temperature. For single-crystal
structure analyses (XRD), crystals were obtained from CH2Cl2
solutions layered carefully with dry hexane. Measurements were
performed at 170(2) K using an IPDS IIT (STOE and Cie)
diffractometer, all with Mo Kα radiation (λ = 0.71073 Å) employing
the ω−φ−2θ scan technique. The structure was solved by direct
[Ni(Phbpy)Cl]. Yield: 3.7 (13.9 mmol, 87%). Anal. Found (calcd) for
C16H11N2NiCl (M = 325.42 g mol−1): C, 59.02 (59.05); H, 3.45 (3.41);
1
N, 8.66 (8.61). H NMR (600 MHz, CD2Cl2): δ 8.99 (d, 1H, H6′),
8.03 (m, 1H, H4′), 7.90 (m, 1H, H3′), 7.87 (m, 1H, H3), 7.56 (m, 1H,
H4), 7.53−7.50 (m, 3H, Ha, H5, H5′), 7.33 (m, 1H, Hc), 7.09 (m, 2H,
Hd, Hb) ppm. EI-MS(+): m/z 324 (100%), 326 (72%) [M]+, 289 [M
− Cl]+, 462 [bpyPh−Phbpy]+,266 [ClPhbpy]+.
[Ni(Phbpy)Br]. Yield: 3.7 g (11.8 mmol, 74%). Anal. Found (calcd)
for C16H11N2NiBr (M = 369.87 g mol−1): C, 51.92 (51.96); H, 3.01
(3.00); N, 7.61 (7.57). 1H NMR (300 MHz, CD2Cl2): δ 9.20 (s, 1H,
H6′), 7.98 (m, 1H, H4′), 7.89 (m, 1H, H3), 7.84 (m, 1H, H3′), 7.77
(m, 1H, Ha), 7.53 (m, 1H, H4), 7.48 (m, 1H, H5′), 7.42 (m, 1H, H5),
7.29 (m, 1H, Hc), 7.05 (m, 1H, Hd), 7.01 (m, 1H, Hb) ppm. EI-
MS(+): m/z 370 (100%), 368 (72%) [M]+, 289 [M − Br]+.
[Ni(Phbpy)I]. Yield: 3.7 g (9.3 mmol, 58%). Anal. Found (calcd) for
C16H11N2NiI (M = 416.87 g mol−1): C, 46.12 (46.10); H, 2.71 (2.66);
N, 6.74 (6.72). 1H NMR (300 MHz, CDCl3): δ 8.69 (dd, J = 4.8, 0.8
Hz, 1H), 8.51 (d, J = 8.0 Hz, 1H), 8.41 (dd, J = 7.9, 0.8 Hz, 1H), 7.95−
7.63 (m, 4H), 7.53−7.48 (m, 1H), 7.43−7.27 (m, 3H) ppm. EI-MS(+):
m/z 370 (100%), 368 (72%) [M]+, 289 [M − I]+, 358 [IPhbpy]+, 462
[bpyPh−Phbpy]+.
Preparation of [Ni(Phbpy)(C6F5)]. Under an inert atmosphere
185 mg of [Ni(Phbpy)Br] (0.5 mmol) was dissolved in THF. At 0 °C
0.5 mmol of freshly prepared C6F5MgBr solution in THF was added
dropwise to the solution. After 18 h of stirring at room temperature, the
mixture was filtered under an argon atmosphere before the solvent was
removed under reduced pressure. The resulting dark red solid was
washed with 10 mL of dry pentane. Yield: 224 mg (0.49 mmol, 98%).
Anal. Found (calcd) for C22H11F5N2Ni (M = 457.02 g mol−1): C, 57.72
(57.82); H, 2.42 (2.43); N, 6.14 (6.13). 1H NMR (300 MHz, CD2Cl2):
δ 7.95 (dt, 1H, J = 7.70, 1.54 Hz), 7.86 (t, 2H, J = 7.86 Hz), 7.76 (d, 1H,
J = 5.31 Hz), 7.57 (d, 2H, J = 11.98 Hz), 7.54 (d, 2H, J = 12.16 Hz), 7.34
(dt, 2H, J = 10.25, 1.37 Hz), 7.00 (dt, 1H, J = 7.42, 1.23 Hz), 6.88 (dt,
1H, J = 7.41, 1.43 Hz), 6.31 (d, 1H, J = 7.35 Hz) ppm. 19F NMR (282
MHz, CD2Cl2): δ −164.3 (m, 2F, 3,5-F), −162.2 (tt, 1F, J = 19.57, 2.21
Hz, 4-F), −116.0 (m 2F, 2,6-F) ppm. EI-MS(+): m/z 456 [M]+, 289
[M − C6F5]+. In solution, this complex readily undergoes reductive
elimination, yielding F5C6−Phbpy data (1H NMR (300 MHz, CD2Cl2)
δ 8.62 (d, 1H, J = 4.11 Hz), 8.35 (d, 1H, J = 7.86 Hz), 7.98 (d, 1H, J =
methods using SIR 2014,54 and refinement was carried out with
2
SHELXL 2016 by employing full-matrix least-squares methods on Fo
≥
55
2
2σ(Fo ). The numerical absorption corrections (X-RED; Stoe and
Cie, 2006) were performed after optimizing the crystal shapes using X-
SHAPE (Stoe and Cie, 2006).56 The non-hydrogen atoms were refined
with anisotropic displacement parameters without any constraints. The
hydrogen atoms were included by using appropriate riding models.
Data of the structure solutions and refinements for [Ni(Phbpy)Cl]
(CCDC 1484145), [Ni(Phbpy)I] (1821672), and [Ni(F5C6Phbpy)-
Br2]·THF (1484538) can be obtained free of charge at https://
summary.ccdc.cam.ac.uk/structure-summary-form or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2
1EZ UK (fax + 44 1223 336033 or e-mail deposit@ccdc.cam.ac.uk).
Computational Calculations (DFT). Electronic structure calcu-
lations on the complexes have been performed through density
functional theory (DFT) methods using the ORCA program package.57
For all optimizations triple-ξ-valence TZVP58 basis sets were used with
the generalized gradient approximated functional BP86.59 Pertinent
metrical data are arranged in Table S6 in the Supporting Information
alongside experimental data, showing good agreement of calculated and
experimental metrics. Molecular orbitals and electronic properties were
extracted from single-point calculations in the optimized positions with
the global hybrid functional TPSSh60 and quadruple-ξ-valence QZVP
basis sets. Grimme’s third-generation D3 correction of dispersion was
used;61,62 medium effects were approximated in a dielectric continuum
approach (COSMO), parametrized for THF.63 Coordinates of the
computed structures are assembled in the COORDINATES file in the
Supporting Information; frontier orbital landscapes are shown in Figure
3 and Figures S20−S24 in the Supporting Information. For each
1782
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8.01 Hz), 7.84 (dd, 1H, J = 7.47 Hz), 7.75 (dt, 1H, J = 1.67, 7.74 Hz),
7.66 (dt, 1H, J = 7.51, 1.37 Hz), 7.59 (dt, 1H, J = 1.38, 7.47 Hz), 7.47
(dd 1H, J = 6.39 Hz), 7.30 (dd, 1H, J = 12.25, 0.93 Hz), 7.27 (dd, 1H, J
= 2.49, 0.99 Hz) ppm; 19F NMR (282 MHz, CD2Cl2) δ −164.3 (m, 2F,
3,5-F), −157.49 (t, 1F, J = 20.78 Hz, 4-F), −141.72 (m, 2F, J = 23.23,
7.96 Hz, 2,6-F) ppm; EI-MS(+) m/z 398) comparable to those
observed for the CF3 complex [Ni(Phbpy)(CF3)].25
controlled Reactivity of Metal Complexes”) is acknowledged
for funding of this project. We also thank the Regional
Computing Center of the University of Cologne (RRZK) for
providing computing time on the DFG-funded High Perform-
ance Computing (HPC) system CHEOPS as well as for the
support. Prof. Dr. Andreas Terfort, Goethe-University Frank-
furt, is acknowledged for ICP-OES measurements on selected
Ni samples. We also thank Leo Payen (MSc. Chem.), University
of Cologne, for assistance with some syntheses. D.A.V. thanks
the Office of Basic Energy Sciences of the U.S. Department of
Energy (DE-SC0009363) for support of this work.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00237.
Details on the synthesis of the new complexes and the C−
H activation reactions, NMR spectra, further pictures of
the crystals, and molecular structures with tables of all
parameters (PDF)
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AUTHOR INFORMATION
Corresponding Authors
̈
Gerald Hörner − Institut fur Chemie, Anorganische Chemie IV,
Universität Bayreuth, D-95440 Bayreuth, Germany;
orcid.org/0000-0002-3883-2879;
(4) Evstigneev, M. P.; Lantushenko, A. O.; Yakovleva, Y. A.;
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6161−6175.
Email: gerald.hoerner@uni-bayreuth.de
Axel Klein − Department fur Chemie, Institut fur Anorganische
̈ ̈
Chemie, Universität zu Köln, D-50939 Köln, Germany;
orcid.org/0000-0003-0093-9619; Email: axel.klein@uni-
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Nicolas Vogt − Department fur Chemie, Institut fur
̈ ̈
Anorganische Chemie, Universität zu Köln, D-50939 Köln,
Germany; orcid.org/0000-0002-3437-4882
̈ ̈
Aaron Sandleben − Department fur Chemie, Institut fur
Anorganische Chemie, Universität zu Köln, D-50939 Köln,
Germany
Lukas Kletsch − Department fur Chemie, Institut fur
̈ ̈
Anorganische Chemie, Universität zu Köln, D-50939 Köln,
Germany
̈ ̈
Sascha Schäfer − Department fur Chemie, Institut fur
Anorganische Chemie, Universität zu Köln, D-50939 Köln,
Germany
Mason T. Chin − Department of Chemistry, Lehigh University,
Bethlehem, Pennsylvania 18015, United States
David A. Vicic − Department of Chemistry, Lehigh University,
Bethlehem, Pennsylvania 18015, United States; orcid.org/
0000-0002-4990-0355
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00237
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The Deutsche Forschungsgemeinschaft DFG KL1194/15-1 and
KL1194/16-1 [(DFG Priority Programme 2102 “Light-
■
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