Redox Series of Cyclometalated Nickel Complexes [Ni((R)Ph(R′)bpy)Br]+/0/-/2- (H-(R)Ph(R′)bpy = Substituted 6-Phenyl-2,2′-bipyridine)

Article abstract of DOI:10.1021/acs.organomet.8b00559

New organonickel complexes [Ni((R)Ph(R′)bpy)Br] carrying various substituted derivatives of the tridentate ^-C^∧N^∧N ligand 6-(phenyl-2-ide)-2,2′-bipyridine (^-Phbpy) were synthesized from the precursor [Ni(COD)₂] (COD = 1,5-cyclooctadiene) and the protoligands (ligand precursors) Br-(R)Ph(R′)bpy. Several synthetic routes for the protoligands were studied and compared. All new compounds have been analyzed and spectroscopically characterized. From several complexes crystal and molecular structures were obtained from XRD experiments. UV-vis absorption spectroscopy and detailed electrochemical measurements reveal the impact of the various substituents on the electronic structure of the complexes. Quantum chemical DFT calculations illustrate the composition of highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) and support the assignment of the single-electron reduction and oxidation products as bpy-localized ligand radical species and transient nickel(III) intermediates, respectively.

Full text of DOI:10.1021/acs.organomet.8b00559

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Redox Series of Cyclometalated Nickel Complexes
[Ni((R)Ph(R′)bpy)Br]+/0/−/2− (H−(R)Ph(R′)bpy = Substituted 6‑Phenyl-2,
2′-bipyridine)
,‡
,†
Aaron Sandleben,† Nicolas Vogt,† Gerald Horner,* and Axel Klein*
̈
†
̈
̈
̈
̈
̈
Universitat zu Koln, Department fur Chemie, Institut fur Anorganische Chemie, Greinstraße 6, D-50939 Koln, Germany
‡Institut fu
̈
r Chemie, Theoretische Chemie, Technische Universitat Berlin, Straße des 17, Juni 135, D-10623 Berlin, Germany
̈
S
* Supporting Information
ABSTRACT: New organonickel complexes [Ni((R)Ph(R′)bpy)Br] carrying various
substituted derivatives of the tridentate −C∧N∧N ligand 6-(phenyl-2-ide)-2,2′-bipyridine
(−Phbpy) were synthesized from the precursor [Ni(COD)2] (COD = 1,5-cyclooctadiene)
and the protoligands (ligand precursors) Br−(R)Ph(R′)bpy. Several synthetic routes for the
protoligands were studied and compared. All new compounds have been analyzed and
spectroscopically characterized. From several complexes crystal and molecular structures
were obtained from XRD experiments. UV−vis absorption spectroscopy and detailed electro-
chemical measurements reveal the impact of the various substituents on the electronic structure of the complexes. Quantum
chemical DFT calculations illustrate the composition of highest occupied molecular orbitals (HOMO) and lowest unoccupied
molecular orbitals (LUMO) and support the assignment of the single-electron reduction and oxidation products as bpy-localized
ligand radical species and transient nickel(III) intermediates, respectively.
−
through substitution on both the presumably σ-donating Ph
part and the potentially π-accepting bpy part (Chart 1).
INTRODUCTION
■
The first complex of the tridentate cyclometalating ligand
6-(phenyl-2-ide)-2,2′-bipyridine (−Phbpy) was the Pd(II) com-
plex [Pd(Phbpy)Cl] reported in 1990.1 In the following years
further complexes containing the platinum group metals Ir, Pt,
and Ru were obtained through orthometalation/C−H bond
activation of the H−Phbpy protoligand (ligand precursor) or
by transmetalation reactions.1−5 The resulting ortho-metalated
complexes have found applications as photocatalysts3 or triplet
emitting materials.5−10 Only in 2014, the first Phbpy complex of
the base metal nickel [Ni(II)(Phbpy)Br] could be obtained from
the Ni(0) source [Ni(COD)2] (COD = 1,5-cyclooctadiene)
through chelate supported oxidative addition of the Br−Phbpy
protoligand.6−8 Akin to the previous complexes of Ir, Pt, and
Ru the nickel(II) analogue revealed reversible one-electron oxi-
dation and reduction. UV−vis spectroelectrochemistry allowed
a preliminary assignment to Ni(II)/Ni(III) and (Phbpy)−/
(Phbpy)2− redox pairs.6 Prolonged electrolysis revealed the
cleavage of the bromide coligand resulting in an uncharged radi-
cal complex with nominally a vacant coordination site. Very
similar to what has been reported for related organometallic
nickel complexes carrying redox-active diimine ligands and
halide coligands,9−11 the highly plausible stabilization of the
resulting complexes by solvent molecules was postulated.10−12
As reduction-induced solvolysis represents a key step in reduc-
tively activated C−C cross-coupling reactions catalyzed by
Ni(diimine) systems (Negishi13 or Kumada14 type), incre-
mental control of the reactivity of the resulting species is of
special interest.15−20 Compared with the isoelectronic terpy6,21
(terpy = 2,2′:6′,2″-terpyridine) system the anionic −Phbpy
ligand should allow for a more flexible electronic fine-tuning
−
Chart 1. General Structure of terpy (left) and Phbpy
(right) with Possible Substitution Patterns
Herein we report novel −Phbpy derivatives carrying sub-
stituents at various positions of the ligand. To this end we
synthesized a variety of Br−(R)Ph(R′)bpy protoligands using
22
̈
Krohnke type reactions. Cyclometalated Ni(II) com-
plexes [Ni((R)Ph(R′)bpy)Br] (Chart 2) were obtained from
Chart 2. New [Ni((R)Ph(R′)bpy)Br] Complexes with
Substitution Patterns (numbers for NMR assignment)
these protoligands by the established chelate supported
oxidative addition using [Ni(COD)2]. In order to isolate
ligand-borne electronic properties, we also synthesized the
Received: August 3, 2018
© XXXX American Chemical Society
A
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H−(R)Ph(R′)bpy derivatives, representing the “H+-coordi-
nated” ligands. All new materials were analytically and spectro-
scopically characterized with respect to geometric and elec-
tronic structure, both experimentally (single-crystal X-ray structure
elucidation; UV−vis absorption spectroscopy) and theoretically
(DFT computation).
The impact of the substituent pattern on the frontier orbitals
of the complexes was studied by UV−vis absorption spectro-
scopy and detailed electrochemical measurements including
UV−vis and EPR spectroelectrochemistry.23,24 Experimental
data on the elusive one-electron and two-electron reduction
products and their one-electron oxidized congeners is supple-
mented by detailed quantum chemical (DFT) calculations.
The combination of experimental data from spectroelectro-
chemistry and DFT calculations on the redox series [Ni((R)-
Ph(R′)bpy)Br]+/0/−/2− will give valuable insight and allow
fine-tuning of the redox properties for future applications of
these cyclometalated complexes in C−C cross coupling
catalysis.
optimizing the crystal shapes using X-SHAPE V1.06 (Stoe and Cie,
1999). The non-hydrogen atoms were refined with anisotropic displace-
ment parameters without any constraints. The hydrogen atoms were
included by using appropriate riding models. Data of the structure
solutions and refinements (CCDC numbers 1488104, 1488106,
1563983, 1501886, 1582497, 1484146, 1488102, 1488108, 1575606,
and 1586570) can be obtained free of charge at https://summary.
ccdc.cam.ac.uk/structures or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge, CB2 1EZ UK (fax: + 44-
1223 336033 or e-mail: deposit@ccdc.cam.ac.uk). Powder XRD was
measured on a Guinier-X-ray-powder diffractometer from Huber of
the type G670 with a Ge(111)-monochromator. The measurements
were conducted with a Cu Kα-beam with a wavelength of 1,5406 Å.
Processing of data was done with the software WinXPOW29 and
GnuPlot.30 Computational Details. Ground state electronic structure
calculations on the complexes have been done on the basis of density-
functional theory (DFT) methods with resolution of identity coulomb
approximation31,32 using ORCA33 or the TURBOMOLE34 program
packages and TMoleX 4.235 user interface. The basis sets def-SV(P),36
def2-TZVP,37 def2-TZVPP,38 and aug-cc-pVQZ39 with Becke’s
gradient-corrected exchange-energy functional BP8640 or hybrid
functional B3LYP41−43 were used. For UV−vis calculations the
molecule structures were first geometry optimized using a Conductor-
like Screening Model (COSMO)44,45 implemented in TURBO-
MOLE/ORCA using the respective dielectric constant of the desired
solvent.46 EPR data were taken from single-point computations in
the relaxed positions at the B3LYP-D3/TZVP (COSMO:THF) level
of theory (for modeling of EPR spectra, relativistic effects were
EXPERIMENTAL SECTION
■
1
Instrumentation. H, 13C, and 19F NMR spectra were recorded
on a Bruker Avance II 300 MHz (1H: 300.13 MHz, 13C: 75.47 MHz) -
double resonance (BBFO) 5 mm observe probehead with z-gradient
coil, Bruker Avance 400 MHz (1H: 400.13 MHz, 13C: 100.61 MHz,
19F: 376.50 MHz) using a triple resonance H, 19F, BB inverse probe
1
accounted for with van Wu
̈
llen’s zeroth-order relativistic approxima-
head or Bruker Avance II 600 MHz spectrometer (1H: 600.13 MHz,
13C: 150.93 MHz) - triple resonance (TBI) 5 mm inverse probehead
with z-gradient coil using a triple resonance. The unambiguous assign-
tion.47
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.
1
1
ment of the H and 13C resonances was obtained from H NOESY,
1
1H COSY, and gradient selected H, 13C HSQC, and HMBC experi-
Synthesis and spectroscopic characterization of the protoligands
Br−(R)Ph(R′)bpy and H−(R)Ph(R′)bpy and the nickel(0)
precursor biscycloocta-1,5-dienenickel(0) [Ni(COD)2] is outlined
in the Supporting Information (SI).
ments. All 2D NMR experiments were performed using standard pulse
sequences from the Bruker pulse program library. Chemical shifts are
relative to TMS and CCl3F, respectively. UV−vis absorption spectra
were recorded with Varian Cary 05E or Cary 50 scan spectropho-
tometers. Elemental analyses were obtained using a HEKAtech
CHNS EuroEA 3000 analyzer. EI-MS spectra were measured with a
Finnigan MAT 95. Simulations were performed using ISOPRO 3.0.
IR spectra were measured in ATR mode using a PerkinElmer 400
FT-IR spectrometer. Electrochemical measurements were carried out
in 0.1 M nBu4NPF6/THF solution using a three-electrode configura-
tion (glassy carbon working electrode, Pt counter electrode, Ag/AgCl
reference electrode) and a Metrohm Autolab PGSTAT30 potentiostat
and function generator. The ferrocene/ferrocenium couple served as
internal reference. Spectroelectrochemical measurements (in 0.1 M
nBu4NPF6/THF solutions) were performed using optically trans-
parent thin-layer electrode (OTTLE)23,25 cells at ambient temper-
ature for UV−vis spectra and a two-electrode capillary for EPR studies.
EPR spectra were recorded in the X band on a Bruker System
ELEXSYS 500E, with a Bruker Variable Temperature Unit ER 4131VT
(500 to 100 K). g values were calibrated using a 2,2-diphenyl-1-
picrylhydrazyl (dpph, g = 2.0036) sample. XRD single crystal structure
Synthesis of Complexes [Ni((R)Ph(R′)bpy)Br]General
Procedure. To a solution of the Br−(R)Ph(R′)bpy protoligand
(1.0 mmol, 1 equiv) in 8 mL of dry THF was added 2 mL of a freshly
produced 0.5 M suspension of [Ni(COD)2] in dry THF. The solution
immediately turned dark red and a red precipitate formed. After 20 h
of stirring at rt the solution was removed carefully by filtration. The
collected solid was washed using 10 mL of dry n-pentane, and the
product was received as a red solid after drying in vacuum. The yields
strongly depend on the solubility of the complexes and the effectivity
of the precipitation process. Isolation of further portions of the
products from the filtrate yielded largely impure materials, presumably
from hydrolysis or reductive elimination reactions (see below).
Thus, yields range from 30% to 70%.6 Unfortunately, for some
products the poor solubility prevented characterization using NMR
spectroscopy.
[Ni((MeO)Phbpy)Br]. Elemental analysis found (calc. for
C17H13BrN2NiO): C 51.06 (51.06); H 3.27 (3.28); N 7.02 (7.01).
EI-MS: 400 [M+] m/z.
analysis for H−(MeO)Phbpy (P21), H−Ph(7-CF3)bpy (P1
Br−Ph(7-CF3)bpy (P21/n), Br−PhTriazPy (Pna21), Br−(MeO)-
PhTriazPy (P212121), [Ni((MeO)Phbpy)Br] (P1), [Ni(Ph(7-CF3)-
bpy)Br] (P1), [Ni(Ph(6-{2-CH3Ph})bpy)Br] (P21/c), [Ni(Ph(7-{3-
MeOPh})bpy)Br] (P21/c), and [Ni(Ph(6-{4-MeOPh})bpy)Br] (P1).
̅
),
1
[Ni(Ph(7-CF3)bpy)Br]. H NMR (600 MHz, CD2Cl2) δ = 9.50
3
3
(s, 1H), 8.26 (d, 1H, JHH = 8.2 Hz), 8.17 (d, 1H, JHH = 8.6 Hz),
̅
3
8.06 (t, 1H, JHH = 8.0 Hz), 7.93−7.56 (m, 3H), 7.32 (s, 1H),
̅
7.17−6.88 (m, 2H) ppm. Elemental analysis found (calc. for
C17H10BrF3N2Ni): C 46.38 (46.63); H 2.34 (2.30); N: 6.45 (6.40).
EI-MS: 438 [M+] m/z.
̅
Crystals were obtained from concentrated solutions in dry CH2Cl2 or
THF layered carefully with dry n-pentane or n-hexane. Measurements
of the complexes were performed at 170(2) K using an IPDS IIT.
Ligand crystals were measured at 293(2) K using the IPDS II or IPDS
IIT (STOE and Cie.) diffractometer, all with Mo Kα radiation (λ =
0.71073 Å) employing the ω−φ−2θ scan technique. The structures
were solved by direct methods using SIR 9226 or SIR2014,27 and
refinement was carried out with SHELXL 201628 employing full-matrix
1
[Ni(Ph(7-CH3)bpy)Br]. H NMR (300 MHz, CDCl3) δ = 9.30
3
(s, 1H), 7.96 (d, 1H, JHH = 8.1 Hz), 7.87 (m, 2H), 7.53 (d, 1H,
3JHH = 8.1 Hz), 7.42−7.31 (m, 1H), 7.08 (s, 1H), 6.99 (s, 2H),
2.65 (s, 3H) ppm. Elemental analysis found (calc. for C17H13BrN2Ni):
53.29 (53.19); H 3.37 (3.41); N: 7.30 (7.30). EI-MS: 384 [M+] m/z.
[Ni(Ph(7-{3-MeOPh})bpy)Br]. Elemental analysis found (calc. for
C23H17BrN2NiO): C 57.97 (58.04); H 3.58 (3.60); N 6.01 (5.89).
EI-MS: 476 [M+] m/z.
2
least-squares methods on F0 ≥ 2σ(F02). The numerical absorption
corrections (X-RED V1.22; Stoe and Cie, 2001) were performed after
B
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[Ni(Ph(7-{4-MeOPh})bpy)Br]. Elemental analysis found (calc. for
C23H17BrN2NiO): C 58.00 (58.04); H 3.60 (3.60); N 5.89 (5.89).
EI-MS: 476 [M+] m/z.
protected. Lacking access to α,β-unsaturated starting materials
interfered with the introduction of R′ = alkyl.
The introduction of substituents at the 6-position of the
central pyridine unit and, more importantly, at the 7-position is
possible through an alternative sequence involving triazine
intermediates (Scheme 2). The reaction of 3-(2-pyridyl)-5-(2-
bromophenyl)-1,2,4-triazine with an enolate via a Diels−Alder
reaction with these substrates having inverse electron demand
followed by a retro Diels−Alder reaction (inverse-retro D.-A.)
results in a mixture of the monosubstituted pyridine carrying
the substituent in either the 7- or 6-position,50 with the regio-
chemistry being governed by steric factors. Mixtures indeed
prevail for enolates carrying aromatic residues, MeOPh. Enolates
carrying small groups like methyl and trifluoromethyl give the
7-position substituted product exclusively (for details see the
Supporting Information (SI)).
1
[Ni(Ph(6-{2-CH3Ph})bpy)Br]. H NMR (300 MHz, CDCl3) δ =
9.28 (d, 1H, 3JHH = 4.6 Hz), 7.92 (t, 2H, 3JHH = 7.7 Hz), 7.79 (d, 1H,
3JHH = 7.7 Hz), 7.53−7.31 (m, 7H), 7.23 (s, 1H), 7.11−6.96 (m, 2H),
2.35 (s, 3H) ppm. Elemental analysis found (calc. for C23H17BrN2Ni):
C 59.80 (60.05); H 3.71 (3.73); N 6.07 (6.09). EI-MS: 460 [M+] m/z.
[Ni(Ph(6-{3-NO2Ph})bpy)Br]. Elemental analysis found (calc. for
C22H14BrN3NiO2): C 53.77 (53.82); H 2.89 (2.87); N 8.57 (8.56).
EI-MS: 491 [M+] m/z.
[Ni(Ph(6-{3-MeOPh})bpy)Br]. Elemental analysis found (calc. for
C23H17BrN2NiO): C 58.20 (58.04); H 3.68 (3.60); N 5.95 (5.89).
EI-MS: 476 [M+] m/z.
[Ni(Ph(6-{4-MeOPh})bpy)Br]. Elemental analysis found (calc. for
C23H17BrN2NiO): C 54.89 (58.04); H 3.58 (3.60); N 5.44 (5.89).
EI-MS: 476 [M+] m/z.
[Ni(Ph(dc)bpy)Br] (dc = 6,7-cyclo-(CH2)3). Elemental analysis
found (calc. for C19H15BrN2Ni): C 55.70 (55.67); H 3.71 (3.69);
N 6.83 (6.83). EI-MS: 410 [M+] m/z.
[Ni((MeO)Ph(7-CF3)bpy)Br]. Elemental analysis found (calc. for
C18H12BrF3N2NiO): C 46.35 (46.21); H 2.57 (2.59); N 5.96 (5.99).
EI-MS: 468 [M+] m/z.
The Br−(R)Ph(R′)bpy protoligands were reacted with
[Ni(COD)2] using an established procedure6 to give the cyclo-
metalated complexes [Ni((R)Ph(R′)bpy)Br] in good yields
(for details see Experimental Section). Elemental analyses,
NMR spectroscopy (if applicable), and MS corroborate the
formulation of the complexes as 1:1:1 adducts of metal, cyclo-
metalated ligand, and bromido ligand, as do the solid state
structures derived from single-crystal X-ray diffraction (see below).
While the complexes show good stability toward oxygen, the
problems virulent in the NMR measurements (see below)
indicate some tendency to hydrolysis. NMR signals crowd in
the spectral region typical of diamagnetic compounds, corrobo-
rating the expected square planar geometry with a diamagnetic
character9 of the organonickel(II) complexes [Ni((R)Ph(R′)-
bpy)Br]. However, some of the 1H NMR spectra show marked
line broadening, which interferes with signal assignment. The
absence of significant signal shifts from the diamagnetic region
rules out an interpretation in terms of an open-shell electronic
structure which would be indicative of tetrahedral coordina-
tion. Rather we ascribe the broad lines to the presence of traces
of paramagnetic octahedrally configured Ni(II) complexes result-
ing from spurious hydrolysis reactions51 or from Ni particles
resulting from reductive elimination. Our attempts to receive
undisturbed spectra in dry CD2Cl2 using CaCl2 or to remove
particles through filtration were not successful. In EI-MS
experiments the facile loss of the bromide coligand contrasts
the high stability of the [Ni((R)Ph(R′)bpy)] fragment even at
an ionization energy of 70 eV and T > 200 °C. Thermal
analyses (TG-DTA) confirm this stability (see SI). Powder
diffraction experiments support high purity of the samples and
a high degree of crystallinity for the materials (see SI).
[Ni((MeO)Ph(7-CH3)bpy)Br]. Elemental analysis found (calc. for
C18H15BrN2NiO): C 52.27 (52.23); H 3.69 (3.65); N 6.78 (6.77).
EI-MS: 414 [M+] m/z.
RESULTS AND DISCUSSION
■
Preparation and Analyses. Essentially two synthesis path-
ways provided flexible access to the Br−(R)Ph(R′)bpy and
H−(R)Ph(R′)bpy derivatives which are depicted in Scheme 1
and Scheme 2. The latter were synthesized for the sake of
Scheme 1. Access to the Proto-ligands Br-(R)Ph(6-R′)-bpy
22,49
̈
via Krohnke Pyridine Synthesis
Scheme 2. Ligand Synthesis via Triazine Intermediates50
Crystal and Molecular Structures in the Solid. Single
crystals of sufficient quality for X-ray analysis could be obtained
from the protoligands Br−Ph(7-CF3)bpy, H−Ph(7-CF3)bpy,
and H−(MeO)Phbpy, from the precursors Br−PhTriazPy and
Br−(MeO)PhTriazPy, and from the complexes [Ni((MeO)-
Phbpy)Br], [Ni(Ph(7-CF3)bpy)Br], [Ni(Ph(6-{2-CH3Ph})-
bpy)Br], [Ni(Ph(7-{3-MeOPh})bpy)Br], and [Ni(Ph(6-{4-
MeOPh})bpy)Br]. The crystal and molecular structures were
solved and refined from X-ray diffraction data (all data in
Tables S4−S6 in the SI). Crystal and molecular structures are
presented in the SI (full data is deposited at CCDC). The
packing of the ligands and the complexes in the solid state is
unexceptional. Supramolecular interactions can be referred to
as π−π and π−σ interactions between the aromatic ligand
systems with centroid distances of 3.5−3.7 Å and offset angles
comparison of the cyclometalated complexes [Ni((R)Ph(R′)-
bpy)Br] with the “H+-coordinated” species. First is the ligand
synthesis via the Krohnke pyridine synthesis.22,48 By this method
̈
it is possible to introduce substituents into both peripheric rings
(phenyl, terminal pyridine). While an otherwise unsubstituted
central pyridine unit is obtained using a Mannich-like starting
material, introduction of a substituent R′ at the 6-position of
the newly formed central pyridine unit is accomplished using
chalkones. Using this method, we synthesized ligands with R =
H and OCH3 and R′ = 2-CH3Ph and 3-NO2Ph. The intro-
duction of amine functions at the Ph moiety failed, even when
C
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of approximately 22°.52 However, these forces will probably
not have a marked influence on the molecular structures.
In the complexes the Ni atom lies at the center of a distorted
square-planar [N2CBr] coordination sphere (Figure 1).
All along the complex series [Ni((R)Ph(R′)bpy)Br] studied
herein, the color impression and the overall absorption pattern
in the visible range is conserved. The complexes of the type
[Ni((R)Ph(R′)bpy)Br] show similar absorption behavior with
intense absorption bands dominating the UV region at about
280 nm (intraligand π−π* transitions) and broad long-
wavelength absorptions at about 420 and 520 nm (Figure 2).
Figure 1. Molecular structures of [Ni(Ph(7-CF3)bpy)Br] (left) and
[Ni(Ph(6-{2-CH3Ph})bpy)Br] (right) (thermal ellipsoids at 50%
probability level, H atoms omitted for clarity).
Complex metrics deriving from XRD-analysis closely match
the data reported for parent [Ni(Phbpy)Br] with respect to
bond lengths and angles (pertinent metrical data are sum-
marized in Tables S7−S8). The largest distortion from a
perfect square (planar) coordination of the Ni atom is caused
by the two bite angles C1−Ni1−N1 and N1−Ni1−N2 of about
85° and 82°, respectively, resulting in a C1−Ni1−N2 angle of
about 166° compared with the ideal 180°. The bromide coligand
and central pyridine ring reveal almost perfect angles between
175° and 180°. The sum of angles around Ni1 of 360° under-
lines close-to-perfect planarity of the coordination sphere. Poor
quality of the crystals refutes an involved analysis of the
complex metrics in some cases. Qualitative inspection of the
coordination geometry of the complexes, however, is com-
pletely in line with planar coordination. Extended theoretical
modeling also of the experimentally elusive complexes with
DFT methods further supports this notion. Metrics obtained at
the BP86-D3/TZVP (COSMO:THF) and B3LYP-D3/TZVP
(COSMO:THF) levels of theory gave close agreement with
the experimentally available data, typically matching the bond
lengths within Δd(Ni−D) ≤ 2 pm (see Table S7, SI). A signifi-
cant exception is noted only for the terminal N2−Ni bond in
[Ni((MeO)Phbpy)Br], where experiment and the theoretical
model differ by about 10 pm. The reason for the divergence,
which could not be remediated by variation of the modeling
settings (functional, basis set, and medium) is unclear at pre-
sent. In general the Phbpy ligand scaffold coordinates to Ni in
an almost completely planar fashion (Figure 1). It is only in the
structure of [Ni(Ph(7-CF3)bpy)Br], both in experiment and
theory, that a significant tilt angle of 8.2(1)° (14.2° by DFT)
renders the two pyridine rings not coplanar. This deviation is a
recurrent feature of the rather bulky trifluoromethyl group and
points to steric hindrance effects. The 6-aryl substituents are
markedly tilted relative to the coordination plane (or the cen-
tral pyridine unit) due to steric restrictions. Only limited conju-
gative effects of these substituents on the electronic properties
may therefore be expected. Accordingly, both the UV−vis
response and the redox properties are only weakly affected by
the presence of pyridine-appended phenyl rings (see below).
Orbital Structure and Optical Transitions. The parent
complex of the series, [Ni(Phbpy)Br], has been previously
studied by UV−vis absorption spectroscopy.6 A moderately
intense broad absorption envelope peaking with log εmax ≈ 3.0
at 505 nm proved responsible for the red color impression.
Figure 2. UV−vis absorption spectrum of [Ni(Ph(7-CF3)bpy)Br] at
ambient temperature in THF. Inset: Zoom-in on the long-wavelength
absorption bands.
It is noted that the width of the low-energy envelope is similar
for all compounds but is significantly broader for the CF3
derivatives. All new derivatives feature a red-shifted absorption
maximum of the low-energy transition, but the peak absorption
wavelength is only slightly susceptible to substitution effects
(see Table 1 and Figure S19, SI).
The strongest effect is obtained for the nominally push−pull
substituted ligand (MeO)Ph(7-CF3)bpy and the electron
deficient ligands Ph(7-CF3)bpy (Figure 2) and Ph(6-{3-
NO2Ph})bpy, where the absorption maximum is shifted to
lower energies by 700−800 cm−1 (Table 1). By contrast, electron-
releasing substituents cause spectral red-shifts <300 cm−1.
Table 1. Long-Wavelength Absorption Maxima of the
Nickel Complexes
[Ni((R)Ph(R′)bpy)Br]
λmax in nm/cm−1/eV
ε/103 M−1 cm−1
(MeO)Ph(7-CF3)bpy
Ph(7-CF3)bpy
530/18,850/2.34
527/18,980/2.35
525/19,040/2.36
517/19,330/2.40
517/19,330/2.40
516/19,400/2.41
515/19,410/2.41
514/19,440/2.41
514/19,470/2.41
514/19,460/2.41
514/19,470/2.41
512/19,520/2.42
509/19,660/2.44
1.7
2.0
2.8
1.6
2.7
2.0
2.2
2.3
1.9
2.1
2.1
2.7
2.8
Ph(6-{3-NO2Ph})bpy
(MeO)Ph(7-CH3)bpy
Ph(6-{3-MeOPh})bpy
Ph(6-{4-MeOPh})bpy
Ph(7-{3-MeOPh})bpy
Ph(7-{4-MeOPh})bpy
(MeO)Phbpy
Ph(dc)bpy
Ph(7-CH3)bpy
Ph(6-{2-CH3Ph})bpy
a
Phbpy
a
From ref 6.
The underlying low-energy transitions have been previously
attributed to a metal-to-ligand charge transfer (MLCT).6 This
assignment and the origin of the spectral red shifts have been
D
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Article
addressed theoretically through TD-DFT spectral modeling
and analysis of the frontier orbitals. Modeling by TD-DFT
gave satisfying qualitative agreement with the experimental
spectra (Figure 3), whereas quantitative metrics deviate
Figure 4. Energy and nature of the frontier MOs of selected members
of the substitution series (B3LYP-D3/TZVP/COSMO(THF)).
component can be safely assigned to a transition from a mainly
Ph-located frontier orbital (significant contributions of
Ni-borne d-orbitals) to an entirely bpy-borne sink orbital in
all cases. Ni centered d-d transitions and MLCT indeed are
expected to contribute at low energy but their oscillator strength
is actually too small to adequately fit to the minor Gaussian
components. In addition, the experimental data feature complete
invariance of the energy spacing of the Gaussians. Irrespective
of substitution, the symmetric subpeak separation amounts to
Δν ≈ 1300 cm−1. This invariance in spectral appearance and
energy points to a common origin of the visible band structure.
Accordingly, this visible band can well be interpreted as one
single electronic transition which is fine-structured by vibra-
tional progressions. The energy spacing of 1300 cm−1 coin-
cides with the energy range of typical bending modes of the
bpy system and it fits vibrational progressions of the same fre-
quency recorded in the absorption spectra and emission spec-
tra (more commonly) of [Ru(bpy)3]2+ and related systems.53
Numerical frequency calculations of one-electron reduced
2[Ni(Phbpy)Br]•− (representing the complex series) as a model
of the LL′CT excited state indeed revealed several intense
bpy-borne bending modes in the relevant frequency range
(see Figure S20, SI).
Electrochemistry Alluding to the analysis of the frontier
orbitals (see above), a bpy-ligand-borne LUMO prevails through-
out the complex series. Accordingly, the reduction potentials are
expected to report on the electronic character of the substitu-
ents R and R′. Less predictable appears the effect of substitu-
tion on the oxidation potentials, because the HOMO is poorly
defined in some of the complexes. Nevertheless all complexes
show very similar electrochemical behavior, with respect to
both reduction and oxidation (Table 2, for further data see
Table S1 in the SI).
Figure 3. Upper: UV−vis absorption spectrum of [Ni(Phbpy)Br]
in THF (bars: TD-DFT derived optical transitions; dotted:
broadened computed spectrum). Lower: Gaussian components
(width Δν 950 cm−1) of the long-wavelength band (dotted) and
their combination (dashed).
significantly. In particular, DFT systematically underestimates
the energy of the features in the visible range in most cases.
According to TD-DFT modeling, the low-energy envelope fea-
tures several contributions for all complexes (see Table S9, SI).
Three probable candidates are identified as being of
MNiLbpyCT or mixed (MNi/L″Br)LbpyCT, (MNi/L′pheny)LbpyCT
character (details in the SI). Thereby the accepting LUMO
orbital is independent of the substitution and almost entirely
resides on the bpy moiety (Figure 4). The aryl-substituted
derivatives exhibit partial extension of the LUMO to the
substituent. LUMO energies sensitively respond to the char-
acter of the substituents; i.e., the electron withdrawing CF3
unit strongly stabilizes the LUMO. By contrast, the electron
releasing Ph-appended OMe unit destabilizes the mainly
Ph-borne ligand orbital, which clearly constitutes the HOMO.
In most of the compounds, however, the HOMO cannot be
reasonably assigned due to degeneracy of up-to three MOs of
quite different character. These three energetically very close
lying orbitals consist of strong contributions from the Ni 3d
+Br, Ni 3d+Phπ or almost pure Ni 3d. In keeping with this
assignment, the long-wavelength bands of all complexes (for an
example, see Figure 3) can be deconvoluted into three Gaussian
components which may refer to three transitions involving the
three close-lying frontier orbitals. Nevertheless, the dominating
Cyclovoltammetric measurements show a reversible oxida-
tion process at around 0.1 V vs ferrocene/ferrocenium. It had
been previously assigned to a Ni(II)/Ni(III) couple for the
parent complex [Ni(Phbpy)Br].6 The oxidation potentials vary
by up to 0.15 V with the substitution pattern. Marked stabi-
lization of the oxidized state (less positive potentials) prevails
for the electron-donating MeO group on the Ph unit (ΔE1/2
=
−0.08 V) or Me and 6,7-cyclo(CH2)3 (dc) on the bpy core
(ΔE1/2 = −0.04 V). Destabilization of the oxidized state (more
positive potentials) correlates with the electron-withdrawing
E
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Table 2. Electrochemical Data of [Ni((R)Ph(R′)bpy)Br]
a
Complexes
[Ni((R)Ph(R′)bpy)Br]
Ox. I
Red. I
Red. II
b
Phbpy
(MeO)Phbpy
0.10
0.02
0.11
0.13 (Epa)
0.14 (Epa)
0.09
0.09
0.07
0.10
0.17
0.06
0.06
−1.90
−1.94
−1.84
−2.58 (Epc)
−2.63 (Epc)
−2.47
−2.54 (Epc)
−2.49 (Epc)
−2.42
Ph(6-{2-CH3Ph})bpy
Ph(6-{3-NO2Ph})bpy
Ph(6-{4-MeOPh})bpy
Ph(6-{3-MeOPh})bpy
Ph(7-{4-MeOPh})bpy
Ph(7-{3-MeOPh})bpy
Ph(7-CH3)bpy
Ph(7-CF3)bpy
Ph(dc)bpy
(MeO)Ph(7-CH3)bpy
(MeO)Ph(7-CF3)bpy
c
−2.00
−1.84
−1.83
−1.89
−1.83
−1.88
−1.63
−1.95
−1.91
−1.63
−2.53
−2.51 (Epc)
−2.58 (Epc)
−2.37 (Epc)
−2.62 (Epc)
−2.62 (Epc)
−2.38 (Epc)
0.16
a
From cyclic voltammetry experiments in 0.1 M THF/nBu4NPF6 at
298 K and scan rate 100 mV/s. Potentials in V vs ferrocene/ferrocenium.
Half-wave potentials E1/2 for reversible processes, cathodic peak
potentials Epc or anodic peak potentials Epa for irreversible processes.
b
c
From ref 6. Further reduction at −1.63 V (E1/2).
nature of the bpy-borne CF3 group (ΔE1/2 = +0.07 V).
The consistency among the observed effects indicates a con-
served oxidized state nature throughout the series. Accordingly
the singly occupied molecular orbitals (SOMO; the former
HOMO) of the oxidized complexes [Ni((R)Ph(R′)bpy)Br]•+
derived for R = OMe and R′ = CF3 both reveal significant
contributions of the metal but are not pure metal d-states
(Figure S21). Admixture of bromide-borne (R′ = CF3) and
Ph-borne (R = OMe) contributions indicates noninnocent
ligand behavior also upon oxidation.
Figure 5. UV−vis absorption spectra of [Ni((MeO)Phbpy)Br]
during electrochemical oxidation (upper) and reduction (lower),
measured in 0.1 M nBu4NPF6/THF solution at rt. Potentials
referenced vs ferrocene/ferrocenium.
Furthermore, all complexes show multiple reduction waves
at very negative potentials (−1.63 V < E1/2 < −2.00 V and
−2.37 V < E1/2 < −2.63 V) from which at least the first one is
completely reversible (for plots see SI). The reduction potentials
span a range of 0.3 V. In all complexes the first reduction is
assigned to a process centered at the bipyridine moiety of the
ligand, in line with previous observations for the parent com-
plex [Ni(Phbpy)Br]6 and our DFT calculations. An exception
is noted for [Ni(Ph(6-{3-NO2Ph})bpy)Br], where the first
reduction wave is located at the NO2 moiety. As expected, the
reduction centered at the bpy unit is markedly facilitated for
ligands containing the electron withdrawing CF3 group, while
aromatic substituents lead to only marginal increase of the
potential (less negative). Complexes containing electron
donating groups on the Ph and/or bpy unit show the lowest
(most negative) reduction potentials fully in line with a bpy-
centered LUMO (lowest unoccupied molecular orbital).
The Redox Series [Ni((R)Ph(R′)bpy)X]n (with n = +1, 0,
−1, −2 for X = Br and n = 0, −1 for X = THF). Further
insight into the redox processes results from coupled spectro-
scopic/electrochemical measurements (SEC).23,24 Upon oxi-
dation of [Ni((MeO)Phbpy)Br] (Figure 5, top) the bands in
the visible region vanish almost completely. On the time scale
of this experiment the oxidation is not reversible, whereas the
faster CV experiments were in line with electrochemical revers-
ibility (compare CV). The resulting final absorption spectrum
exhibits prominent maxima at 307 and 243 nm and only minor
components with ε < 100 M−1 cm−1 all across the visible range,
reminiscent of Ni2+-borne ligand-field transitions. TD-DFT
modeling of the formal one-electron oxidation product
[Ni(III)((MeO)Phbpy)Br]•+ (computed UV−vis transitions
of elusive [Ni((MeO)Phbpy)Br]•+ are given in Figure S22 and
Table S10, SI) is entirely at odds with the experimental data.
If these species, best formulated as nickel(III) would accumu-
late in solution, intense CT-like transitions necessarily arise.
By contrast, the experimental spectrum shares similarity with
the absorption spectrum of the corresponding protoligand
H−(MeO)Phbpy. Compared to the absorption spectrum of
the ligand, however, the absorption bands of the “processed”
species are still red-shifted by about 2400 cm−1, indicating that
the metal is still coordinated to the ligand. Thus, upon
electrochemical oxidation, the anionic Ph-moiety of the ligand
is protonated. Reaction of Ni(BF4)2 and H−(MeO)Phbpy
in THF indeed yields a UV−vis absorption spectrum which
closely mimics the experimental observations (Figure S23, SI).
Quantum chemical calculations of the hypothetical species
[Ni(II)(η2-(MeO)Phbpy)Br(THF)n]+ all suggest strong near-
UV absorptions peaking around 350 and 320 nm, for n = 1
or 2, respectively.
Although the nature of the terminal species refers to a
ligand-borne relaxation route, the initial redox event appears to
be metal centered. EPR spectra recorded after rapid freezing of
solutions in acetonitrile after chemical oxidation of [Ni-
((MeO)Phbpy)Br], [Ni(Phbpy)Br], and [Ni(Ph(7-CF3)bpy)-
Br] reveal an overlay of several paramagnetic species. Although
we refrain from an assignment of the subspectra, large
anisotropy of the g values (Δg ≈ 0.3) is in line with metal-
centered open-shell character. EPR parameters computed for
F
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Article
[Ni(III)(η2-Phbpy)Br(THF)n]•+ with n = 0 and 2 are in rea-
sonable agreement with the experimental data. Thus, assuming
a nickel centered electrochemical oxidation from Ni(II) to
Ni(III),54 the irreversibility pertinent to spectroelectrochemis-
try may be due to a follow-up chemical reaction in a so-called
EC mechanism.12 Although we cannot draw, at present, a
congruent picture of the oxidation-initiated relaxation pro-
cesses, two alternatives may be discussed. The chemical reac-
tion might consist in the homolytic cleavage of the Caryl−Ni or
the Br−Ni bond, since both the aryl group and the Br−
coligand represent electron-donors. Intermolecular oxidation
of solvent or electrolyte through the Ni(III) center can be
ruled out from the fact that this reaction is independent of the
solvent. It is noted, however, that a spin-quartet state resides
only ca. +50 kJ mol−1 above the ground-state doublet. The
former species reveals a concentration of spin density on the
decoordinated Ph moiety, rendering this species nickel(II)
ferromagnetically coupled to a phenyl radical. Stabilization of
the phenyl radical site through H atom abstraction from the
solvent THF resulting in [Ni(II)((MeO)(H)Phbpy)Br]+ is
highly plausible. Alluding to the two-state reactivity principle of
Figure 6. Relaxed surface scans along the Ni−Br vector in
[Ni((MeO)Phbpy)Br]0/− on the B3LYP-D3/TZVP/COSMO(THF)
level of theory.
Schroder, Schwarz, and Shaik,55 the intermediate population of
̈
the quartet-state may well account for the observed terminal
reaction products. Another possibility would be a reductive
elimination Ni(III) to Ni(I) with the formation of a Br−Caryl
bond. A comparable F3C−Caryl bond formation has been
observed for the [Ni(Phbpy)(CF3)] derivative6 and details of
the CH3−Caryl forming reductive elimination reaction have
been recently worked out for the cyclometalated complex
[Ni(II)(η3-PhPy-O-PPh2)(CH3)] which yielded a transient
Ni(I) species [Ni(I)(η2-(CH3)PhPy-O-PPh2)(MeCN)2] upon
one-electron oxidation.56 However, if this was happening we
should be able to observe the diagnostic EPR-pattern of a (meta-)
stable Ni(I) species in the aged reaction mixtures. In stark con-
trast, the resulting mixtures are EPR silent.
been argued previously the cleavage of bromide from the
reduced complexes [Ni((R)Ph(R′)bpy)Br]•− gives the coor-
dinatively unsaturated radical species [Ni((R)Ph(R′)bpy)]•
and through solvent coordination the radical complex
[Ni((R)Ph(R′)bpy)(solvent)]•.11
Additional support of the solvolysis hypothesis derives from
comparison with the electrochemically fully reversible redox
17
couple [Ni(tpy)Br]+/0
.
Here the reduced species is neutral
that weakens Coulomb repulsion. Accordingly the relaxed Ni−
Br scan reveals a much more stable Ni−Br bond than in
anionic [Ni((MeO)Phbpy)Br]•−, which is in agreement with
its inertness. Direct experimental evidence for such redox-
initiated solvolysis cannot be provided, however. According to
(TD-)DFT modeling both species share merely indistinguish-
able UV−vis and EPR spectroscopic parameters.
X band EPR spectroscopy on the in situ generated reduced
species gave conclusive spectra for ligand radical anions (for
instance, H−(MeO)Phbpy•−, Figure 7, left) and complexes
Different from the above discussion of the oxidation-induced
follow-up chemistry, interpretation of both reduction events is
straightforward. During electrochemical reduction of the neu-
tral complexes multiple new and intense absorption bands
emerge throughout the visible range (λmax = 400 and 600 nm)
with broad structured extensions to the NIR regime (λmax
=
950 nm; Figure 5 (bottom) and Figures S24−S27, SI).
A second reduction step produces additional vis-NIR signals at
lower energies. Similar observations that have been previously
made on the related [Ni(bpy)(Mes)Br] and [Ni(bpy)(Mes)2]
complexes have been assigned to a reduction at the bpy unit
producing new π*−π* absorptions from the now singly
occupied molecular orbital (SOMO, formerly the LUMO) to
higher unoccupied bpy-centered orbitals.54,57 TD-DFT calcu-
2
Figure 7. X band EPR spectra of H−(MeO)Phbpy•− (left) and
[Ni(Ph(7-CF3)bpy)Br]•− (right) measured during electrochemical
reduction at room temperature (upper spectra) in 0.1 M nBu4NPF6/
lations of the singly and double reduced complexes [Ni((R)-
Ph(R′)bpy)Br/THF]−/0 and 1[Ni((R)Ph(R′)bpy)Br/
THF]2−/− broadly corroborate the assignment of the character-
istic absorption processes in the vis-NIR region. However, par-
tial irreversibility of the first electrochemical reduction event
puts into question the invariance of the coordination upon
(de)charging. Indeed, DFT-relaxed scans (Figure 6) reveal a
massively labilized Ni−Br bond of [Ni((MeO)Phbpy)Br]n
upon one-electron reduction from n = 0 to n = −1. Complex
THF solution. Simulations below, using H−(MeO)Phbpy•− 2× aH
=
4.5 G, 2× aH = 1.9 G, 2× aN = 2.55 G; [Ni(Ph(7-CF3)bpy)Br]•− aH =
4.1 G, aH = 3.8 G, aH = 2.2 G, aH = 1.1 G, aN = 3.45 G, aN = 3.1 G.
(for instance, [Ni(Ph(7-CF3)bpy)Br]•−; Figure 7, right).
Additional spectra are compiled in the SI. It is noted that
the lines of the complex signal are much broader than in the
signal of the reduced ligand. This is most likely caused by the
quadrupole moment of the bromide coligand resulting in faster
relaxation and thus broader line shape. Assuming rapid Br−
cleavage from the reduced complex, we expected marked
fragmentation along the Ni−Br bond ([LNiBr]0/− → [LNi]+/0
+
Br−) is much more favorable in the one-electron reduced
anionic state. The redox-dependent weakening of the Ni−Br
bond can be estimated as ΔΔENi−Br ≈ −100 kJ mol−1, ren-
dering the bromide ligand labile toward substitution. As has
G
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Article
line-sharpening on prolonged electrolysis but failed to see this
effect. We explain this by incomplete cleavage of Br− under
these conditions (high concentration) and line-broadening
through Br− exchange.
concomitant destabilization and stabilization of the ligand-
localized LUMO. Substitution effects on the diagnostic visible
absorption envelope (predominantly LL′CT character) are
only minor. Single-electron reduction is clearly ligand localized
in all cases, as can be read from the intense NIR transitions
that are highly reminiscent of the bpy radical anion. Coulomb-
driven solvolysis of the anionic reduced complex (Br− → THF)
is identified as the source of partial electrochemical irrevers-
ibility. In turn, single electron oxidation very probably is metal
centered but induces a sequence of follow-up relaxation steps,
finally yielding a nickel(II) species exhibiting N2 coordination
of the reprotonated ligand H−Phbpy. Future work will aim to
test the suitability of these complexes as catalysts in Negishi-
type C−C cross coupling reactions and explore the underlying
mechanistic steps (reductive activation, oxidative addition,
reductive elimination) in stoichiometric model reactions.
In general the reduced species give isotropic signals with a
width of about 30 G. giso values of 2.0032 (H−(MeO)Phbpy)
and 2.0031 ([Ni(Ph(7-CF3)bpy)Br]) both are in the range of
the g value of the free electron58 and represent typical values
for “organic” radicals with marginal contribution of heavy atom
orbitals to the unpaired spin.59 Accordingly DFT-modeling
(ZORA-B3LYP-D3/TZVP) of the EPR parameters of [Ni-
(Ph(7-CF3)bpy)Br]•− consistently reveals Ni-borne spin den-
sities of only ρNi ≈ 0.02−0.04, irrespective of substitution and
speciation of coligand (Br versus THF). Similar to the obser-
vations made for the radical complex [Ni(bpy)(Mes)2]•−,54
the signals show hyperfine splitting (HFS) to N and H atoms
of the bpy unit. The coupling results in a 11 line signal for
H−(MeO)Phbpy•− and 9 lines for the complex. The spectrum
of the reduced ligand was simulated using 2× aH = 4.5 G, 2×
aH = 1.9 G, 2× aN = 2.55 G HFS. The spectrum of the radical
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.organomet.8b00559.
complex was simulated using aH = 4.1 G, aH = 3.8 G, aH
=
2.2 G, aH = 1.1 G, aN = 3.45 G, aN = 3.1 G. The simulations are
in line with the coupling constants and g values which result
from quantum chemical calculations (see Table S11, SI).
In conclusion, the singly reduced species exhibit ligand-radical
character, as reflected also by significant double-bond character
of the bpy-borne C−C bridge in the optimized structures. The
second reduction step then gives, most probably accompanied
by solvolysis of the coligand, the Ni(II) coordinated doubly
reduced diamagnetic bpy2− ligand in complexes [Ni((R)Ph-
(R′)bpy)(THF)]− (Scheme 3).
JMol coordinates (XYZ)
Powder diffractograms, further absorption spectra and
electrochemical measurements, as well as further
pictures of the crystal and molecular structures with
tables of all parameters (PDF)
Accession Codes
CCDC 1484146, 1488102, 1488104, 1488106, 1488108,
1501886, 1563983, 1575606, 1582497, and 1586570 contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting
The Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Scheme 3. Redox-Variant Speciation
AUTHOR INFORMATION
Corresponding Author
*E-mail: axel.klein@uni-koeln.de.
*E-mail: gerald.hoerner@tu-berlin.de.
■
ORCID
Aaron Sandleben: 0000-0002-4141-7406
Nicolas Vogt: 0000-0002-3437-4882
Axel Klein: 0000-0003-0093-9619
CONCLUSIONS
■
Different from their Pd and Pt congeners, only few examples of
nickel(II) complexes with cyclometalated ligands have been
reported. In this work we set out to vary the theme of Phbpy
derived organonickel complexes [Ni((R)Ph(R′)bpy)Br] carry-
Notes
The authors declare no competing financial interest.
ing various substituted derivatives of the tridentate C∧N∧N
−
ACKNOWLEDGMENTS
■
ligand 6-(phenyl-2-ide)-2,2′-bipyridine (−Phbpy). Ligand syn-
The Deutsche Forschungsgemeinschaft DFG KL1194/15-1 is
acknowledged for funding of this project. G.H. thanks DFG for
financial support within the cluster of excellence UNICAT and
Prof. Martin Kaupp (TU Berlin) for valuable discussions. We
would also like to thank the Regional Computing Center of the
University of Cologne (RRZK) for providing computing time
on the DFG-funded High Performance Computing (HPC)
system CHEOPS as well as for the support.
̈
thesis along Krohnke type sequences and via inverse-retro
Diels−Alder reaction of triazine substrates provided ready access
to complementary substitution series. Thereby substituents of
greatly varying electronic properties and with variable steric
demand were selectively introduced at several sites of the Ph
and bpy ligand moieties. Solid-state X-ray crystallography and
extended DFT-modeling of NiBr2 derived complexes [Ni((R)-
Ph(R′)bpy)Br] gave the congruent picture of planar coordi-
1
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