The cyclometalated nickel complex [(Phbpy)NiBr] (Phbpy- = 2,2′-bipyridine-6-phen-2-yl) - Synthesis, spectroscopic and electrochemical studies

Article abstract of DOI:10.1016/j.jorganchem.2014.10.013

The new organometallic nickel complex [(Phbpy)NiBr] containing the anionic tridentate N,N,C ligand 6-(phen-2-yl)-2,2′-bipyridine (Phbpy^-) was synthesised from PhbpyBr and [Ni(COD)₂] in excellent yields and was fully characterised (MS

Full text of DOI:10.1016/j.jorganchem.2014.10.013

Journal of Organometallic Chemistry 774 (2014) 86e93
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
The cyclometalated nickel complex [(Phbpy)NiBr]
ꢀ
0
(
Phbpy ¼ 2,2 -bipyridine-6-phen-2-yl) e Synthesis,
spectroscopic and electrochemical studies
*
Axel Klein , Benjamin Rausch, Andre Kaiser, Nicolas Vogt, Alexander Krest
Institut für Anorganische Chemie, Universit a€ t zu K o€ ln, Department für Chemie, Greinstraße 6, D-50939 K o€ ln, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 August 2014
Received in revised form
The new organometallic nickel complex [(Phbpy)NiBr] containing the anionic tridentate N,N,C ligand 6-
0
ꢀ
(
phen-2-yl)-2,2 -bipyridine (Phbpy ) was synthesised from PhbpyBr and [Ni(COD)
and was fully characterised (MS, NMR, single crystal XRD). [(Phbpy)NiBr] was reacted with (CF
giving the CF complex [(Phbpy)Ni(CF )]. Both new complexes were studied in detail by electrochemical
CV) and spectroelectrochemical (UV/vis/NIR absorption) methods. For both complexes the first one-
electron reduction and the first oxidation of the Br complex occur fully reversible in the CV, while
spectroelectrochemistry shows decomposition reactions. The CF complex is unstable in solution pro-
ducing 2-CF -Phbpy. Reactivity patterns for these reactions were discussed, also in comparison with the
non-cyclometalated complex [(bpy)Ni(Mes)Br].
2
] in excellent yields
3
)SiMe
3
29 September 2014
3
3
Accepted 7 October 2014
Available online 16 October 2014
(
3
Keywords:
Nickel
Cyclometalated
XRD
3
© 2014 Elsevier B.V. All rights reserved.
Spectroelectrochemistry
CeC coupling
Introduction
Ni(II) pre-catalysts were reduced by one-electron steps to reactive
Ni(II) species containing a reduced diimine ligand rather than
having the character of a monovalent Ni(I) species [8,10]. Related
diimine nickel complexes have also been applied in i) homoge-
neous electromediated reduction (HEMR) of olefins, ketons or alkyl
halides [11]; ii) electrochemical carboxylation of bromostyrenes
[12] or aziridines [13]; iii) electroreductive carbonylation of organic
halides [14]; or iv) electroreductive coupling of olefins and polyhalo
compounds [15].
Organonickel(II) complexes with terpyridine ligands have
recently entered the stage of nickel-catalysed CeC cross-coupling
reactions [1e4]. Detailed mechanistic studies on reactions under
Negishi-like conditions revealed that the terpy ligand contributes
through its
p-accepting ability to the stability or reactivity of
reduced species. Thus, the catalytically crucial reduced species
were better described as Ni(II) complexes bearing a reduced
ꢀ
0
0
00
(
radical anionic) terpy ligand [Ni(II)(terpy )(R)] (terpy ¼ 2,2 :6 ,2 -
terpyridine; R ¼ alkyl or aryl) than as Ni(I) species [Ni(I)(terpy)(R)]
3b,3e,4,5], while for the reduced complex [(terpy)NiBr] evidence
To substantiate our previous findings on organometallic Ni
terpy complexes we stepped on the idea to use the tridentate
0
ꢀ
[
anionic N,N,C coordinating 2,2 -bipyridine-6-phen-2-yl (Phbpy )
ligand instead of the neutral N,N,N coordinating terpy ligand.
Corresponding Ni(II) complexes of the composition [(N^N^C)Ni(R)]
(R ¼ alkyl, aryl or halido) are neutral, compared with the cationic
for a metal centred [Ni(I)] description has been found [3b].
Quite similar findings have been obtained recently by us on
organometallic complexes of the type [(N^N)Ni(R)Br] containing
þ
bidentate
a
-diimine ligands (N^N; R ¼ aryl), which were able to
[(N^N^N)Ni(R)] derivatives. This should largely change the elec-
perform electrocatalytic CeC or CeP coupling reactions with
various aryl-, vinyl- or alkyl halides ReX [6e8]. Mechanistic studies
on these systems, but also on the Cr/Ni catalysed vinylation of al-
dehydes (NozakieHiyamaeKishi reaction) [9] showed that the
tronic and catalytic properties. Also, from the expected coplanar
arrangement of the phenyl moiety with the bpypart of the ligand
these complexes should differ markedly from the previously
thoroughly investigated complexes [(N^N)Ni(aryl)Br] [8,16]. To our
ꢀ
knowledge nickel complexes containing the Phbpy ligand or
derivatives have not been reported yet, in contrast to numerous
Pd(II) [17,18] and Pt(II) [18e20] complexes of this ligand. Pt com-
plexes of Phbpy represent a class of interesting luminescent
materials [20].
ꢀ
*
Corresponding author.
E-mail address: axel.klein@uni-koeln.de (A. Klein).
URL: http://www.klein.uni-koeln.de/
http://dx.doi.org/10.1016/j.jorganchem.2014.10.013
022-328X/© 2014 Elsevier B.V. All rights reserved.
0

A. Klein et al. / Journal of Organometallic Chemistry 774 (2014) 86e93
87
Experimental
Table 1
Details of the crystal structure determination and selected structural data of
a
[
(Phbpy)NiBr] .
General
Formula/weight [g/mol]
Crystal system/space group
Cell [Å]
C
16
H
11Br
1
N
2
Ni
1
/369.89
/n (no. 14)
Monoclinic/P2
1
All preparations and measurement were carried out in a dry
argon atmosphere using Schlenk techniques. Solvents (CH
toluene, diethyl ether and MeCN) were dried using an MBRAUN MB
SPS-800 solvent purification system.
a ¼ 8.7252(7), b ¼ 16.4594(10),
2
Cl
2
, THF,
¼ 95.560(4)ꢂ
c ¼ 9.1139(5)
1302.70(15)/4
1.886/4.544
b
3
V [Å] /Z
3
ꢀ1
]
r
calc [g/cm ]/
m
[mm
Limiting indices
ꢀ11 < h < 11, ꢀ21 < k < 20,
ꢀ
11 < l < 11
Instruments
Refl. coll./uniq./Rint
8086/2820/0.0585
2820/0/181
Data/restraints/param.
Goof. on F2
1.078
NMR spectra were recorded using a Bruker Avance II 300 MHz
1
13
spectrometer ( H: 300.13 MHz, C: 75.47 MHz) using a triple
Final R
1
, wR
2
[I > 2
s(I)]
0.0501/0.1416
0.0685/0.1501
ꢀ1.011/1.442
resonance 1H,19F,BB inverse probehead. The unambiguous assign-
R1, wR2 (all data)
Drmin/max [10 e/pm3]
ꢀ6
1
13
1
1
ment of the H and C resonances was obtained from H TOCSY, H
Distances [Å]
NieN1, NieBr
1
1
13
COSY, H NOESY, gradient selected H, C HSQC and HMBC exper-
iments. All 2D NMR experiments were performed using standard
pulse sequences from the Bruker pulse program library. Chemical
1.848(5), 2.300(1)
1.969(5), 1.947(5)
1.475(7), 1.472(8)
NieN2, NieC12,
C10eC11, C1eC2
ꢂ
1
13
Angles [ ]
shifts were relative toTMS for H and C. The spectra analyses were
performed by the Bruker TopSpin 2 software. Elemental analyses
were carried out using a Hekatech CHNS EuroEA 3000 Analyzer. EI-
MS spectra were measured with a Finnigan MAT 900 S. Simulations
were performed using ISOPRO 3.0. Electrochemical experiments
N1eNieN2, N1eNieC12
N1eNieBr, N2eNieC12
BreNieC12, BreNieN2
Sum of angles around Ni
82.6(2), 82.8(2)
178.7(1), 165.4(2)
96.9(1), 97.7(1)
360.0(2)
a
Measured at 100(2) K at
l
¼ 0.71073 Å.
4 6
were carried out in 0.1 M nBu NPF solutions using a three-
permanganate solution in acetone was added drop wise until the
colour remained slightly purple. The formed dark solid was filtered
off and the solvent of the filtrate was evaporated. The received
brown oil was purified by column chromatography (silica gel,
diethyl ether/petroleum 2/1) which led to a yellowish powder. After
recrystallisation out of pentane the product was received as a col-
electrode configuration (glassy carbon electrode, Pt counter elec-
trode, Ag/AgCl reference) and an Autolab PGSTAT30 potentiostat
and function generator. Data were processed using GPES 4.9
(
General Electrochemical System Version 4.9). The ferrocene/fer-
þ
rocenium couple (FeCp
2
/FeCp
2
) served as internal reference.
Spectroelectrochemical investigations (UV/vis/NIR) were per-
formed at ambient temperature using an OTTLE (optical trans-
parent electrochemical) cell designed by J. Fiedler, Prague [21]. UV/
vis/NIR absorption spectra were recorded using Varian Cary50 Scan
or Cary05E photospectrometers.
12 2
ourless powder. Yield: 2.79 g (12 mmol, 40%). Anal.: C16H N
(
M
w
¼ 232.28 g/mol): C, 82.73; H 5.21; N, 12.06%. Found: C, 82.04;
1
H, 5.75; N, 10.94%. H NMR (300 MHz, CDCl
8
3
):
d
¼ 8.71 (d, 1H, H6),
0
.66 (d, 1H, H3), 8.40 (d, 1H, H3 ), 8.16 (d, 2H, Ha/He), 7.90 (m, 1H,
0
0
H4 ), 7.84 (m, 1H, H4), 7.78 (m, 1H, H5 ), 7.54 (m, 1H, Hb), 7.49 (m,
H, Hc), 7.45 (m, 1H, Hd), 7.35 (t, 1H).
1
Crystal structure determination
The data collection was performed at T ¼ 100(2) K on a STOE
Synthesis of PhbpyBr (adapted from Ref. [26])
IPDS I diffractometer with MoK
a
radiation (
l
¼ 0.71073 Å)
0
employing
ue2
q
scan technique. The structure was solved by direct
36 mg (2.3 mmol) 6-Phenyl-2,2 -bipyridine, 497 mg (2.8 mmol)
methods using the SHELXTL package [22] and refinement was
carried out with SHELXL97 employing full-matrix least-squares
N-bromosuccinimide and 27 mg (5 mmol %) Pd(OAc)
2
were sus-
ꢂ
pended in 25 mL of acetonitrile and heated for 14 h at 120 C in an
autoclave. After cooling to room temperature the yellow reaction
mixture was filtered over celite and the solvent was evaporated.
The yellow solid was extracted three times with 25 mL of hexane.
The solvent was evaporated again to receive a yellow oil as the
methods on F2 [23] with F
2
ꢁ 2
2
0
0
s(F ) with the results shown in
Table 1 (and Supplementary data). For the sake of a clear distinction
between the Ni coordinated N2 and C12 atoms, the site occupancy
factor for both atoms was freely refined and yielded unity. All non-
hydrogen atoms were treated anisotropically; hydrogen atoms
were included by using appropriate riding models.
product. Yield: 660 mg (2.6 mmol, 93%). Anal.
(M
16 11 2
C H N Br
w
¼ 311.18 g/mol): C, 61.76%; H 3.56%; N, 9.00%. Found: C, 60.92%;
1
H, 3.67%; N, 8.91%. H NMR (300 MHz, CDCl
3
):
d
¼ 8.72 (d, 1H, H6),
0
0
Reagents
8.55 (d, 1H, H3), 8.44 (d, 1H, H3 ), 7.92 (t, 1H, H4 ), 7.83 (dt, 1H, H4),
0
7
.77 (m, 1H, Hd), 7.75 (m, 1H, H5 ), 7.70 (m, 1H, Ha), 7.46 (t, 1H, Hb),
The complex [Ni(COD)
2
] [24] was synthesised according to a
7.33 (t, 2H, H5/Hc).
literature procedure. All other chemicals were purchased by com-
mercial suppliers and were used without further purification.
Synthesis of [(Phbpy)NiBr] (adapted from Ref. [27])
Synthesis of PhbpyH (adapted from Ref. [25])
2
400 mg (1.4 mmol) [Ni(COD) ] and 450 mg (1.4 mmol) PhbpyBr
were dissolved in dry THF under argon atmosphere. The mixture
immediately turned purple and after a short while intensive red.
After 14 h of stirring at room temperature the solvent was evapo-
rated in vacuum and the red residue was washed three times with
15 mL of pentane and once with 20 mL of heptane. The remaining
red product was dried in vacuum. Yield: 430 mg (1.12 mmol, 80%).
0
To a solution of 4.68 g (30 mmol) 2,2 -bipyridine in dry diethyl
ether were drop wise added 18 mL (36 mmol) of a 2 M phenyl
lithium solution in diethyl ether at 0 C. After 2 h the deep red
mixture was quenched with 50 mL of water whereupon the colour
changed to green. After the aqueous layer was extracted with
diethyl ether the combined organic layers were dried over mag-
nesium sulphate. The solvent was evaporated and the remaining oil
was diluted with 70 mL of acetone. A saturated potassium
Anal. C16
H
11
N
2
NiBr (M
W
¼ 369.87 g/mol): C, 51.96; H, 3.00; N,
1
7.57%. Found: C, 52.44; H, 3.17; N, 7.42%. H NMR (300 MHz,
0
CD
2
Cl
2
):
d
¼ 9.20 (s, 1H, H6), 7.98 (m, 1H, H4), 7.89 (m, 1H, H3 ), 7.84

88
A. Klein et al. / Journal of Organometallic Chemistry 774 (2014) 86e93
Scheme 1. Preparation of the title complex.
0
(
(
m,1H, H3), 7.77 (m,1H, Hd), 7.53 (m,1H, H4 ), 7.48 (m,1H, H5), 7.42
2
Upon mixing of PhbpyBr and [Ni(COD) ] in solution a deep purple
0
m, 1H, H5 ), 7.29 (m, 1H, Hb), 7.05 (m, 1H, Ha), 7.01 (m, 1H, Hc). EI-
colour was observed, which is due to the intermediate nickel(0)
complex [(PhbpyBr)Ni(COD)] [10] (Scheme 1). During the next few
minutes the colour of the mixture turn to bright red, which rep-
resents the cyclometalated product [(Phbpy)NiBr], formed through
an oxidative addition (Scheme 1).
þ
þ
þ
MS: m/z ¼ 370 [M] , 324 [[(Phbpy)NiCl]] , 289 [M ꢀ Br] , 232
þ
[M ꢀ Ni,Br þ H] .
3
Synthesis of [(Phbpy)Ni(CF )] (adapted from Ref. [28])
The complex [(Phbpy)NiBr] seem to be a very suitable starting
n
3
00 mg (0.8 mmol) [(Phbpy)NiBr] and 456 mg (3.0 mmol) CsF
material for further complexes [(Phbpy)Ni(L)] (L ¼ any ligand, n
were dissolved in 20 mL of dry THF. 411 L (3.0 mmol) of trime-
m
depends on the charge of L and is 0 for anionic ligands). As a first
example for such derivatives, the bromido complex was success-
fully reacted with trimethyl-(trifluormethyl)silane in the presence
thyl(trifluormethyl)silane were added drop wise yielding a deep
red solution that was stirred for 18 h at room temperature. After-
wards the solvent was evaporated which led to a brown solid that
of CsF to yield the CF
3 3
complex [(Phbpy)Ni(CF )].
2 2
was dissolved in a mixture of pentane and CH Cl 1:1 and filtrated.
The brown filtrate was dried and the received red solid was washed
with 15 mL of pentane and 25 mL of heptane and dried in vacuum.
The constitution of the new compounds was established
through elemental analyses, EI-MS and H (and F) NMR spec-
troscopy (see Experimental Section). The two nickel complexes are
1
19
Yield: 110 mg (0.3 mmol, 38%). Anal. C17
mol): C, 56.88; H, 3.09; N, 7.08%. Found: C, 50.17; H, 2.92; N, 5.32%.
H
11
F
3
N
2
Ni
1
(M
W
¼ 757.8 g/
well soluble in common organic solvents (toluene, CH
acetonitrile). While the Br complex is stable in these solutions (and
in the solid state), the CF complex turns out to be unstable in so-
lution (although degassed and dry) but can be stored for longer
times in the solid.
2 2
Cl , THF,
1
H NMR (300 MHz, CD
2
Cl
2
):
d
¼ 8.83 (s, 1H, H6), 7.93 (m, 1H, Ha),
3
0
7
.92 (m, 1H, H3 ), 7.79 (m, 1H, H4), 7.78 (m, 1H, Hc), 7.51 (m, 1H,
0
0
H4 ), 7.49 (m, 1H, H5 ), 7.48 (m, 1H, H3), 7.43 (m, 1H, H5), 7.43 (m,
1
1
9
H, Hb), 7.43 (m, 1H, Hd). F NMR (300 MHz, CD
2
Cl
2
)
d
¼ ꢀ28
þ
þ
(
[
NieCF
M ꢀ Ni] , 289 [M ꢀ CF
3
). EI-MS: m/z ¼ 358 [M] , 324 [[(Phbpy)NiCl]] , 305, 300
Crystal and molecular structures
þ
þ
þ
3
] , 232 [M ꢀ Ni, CF
3
þ H] .
Single crystals of [(Phbpy)NiBr] were obtained from THF solu-
tions and the crystal and molecular structure (Fig. 1) was deter-
mined with the results summarised in Table 1.
Results and discussion
Preparations and analytical characterisation
In the crystal structure [(Phbpy)NiBr] does not exhibit any pro-
nounced intermolecular interaction other than the coplanar stack-
ing visible in Fig. 1. The shortest interplanar distances are about
3.3 Å and the tilt angles are almost zero [29], quite similar to what
has been observed for the related complex [(terpy)NiBr] [3b] (see
Supplementary data for details). The molecular structure shows a
0
The 6-phenyl-2,2 -bipyridine precursor PhbpyH was prepared
through a variant of an established procedure [25]. The bromina-
tion of the 2 position in the phenyl ring was successfully carried out
using N-bromosuccinimide, also following an established method.
Fig. 1. Crystal structure of [(Phbpy)NiBr] (left; viewed along the crystallographic a axis) and molecular structure (right; at 50% probability level (with numbering); protons were
omitted for clarity).

A. Klein et al. / Journal of Organometallic Chemistry 774 (2014) 86e93
89
distorted square planar surrounding of the nickel atom with a short
central NieN1 bond of 1.848(5) Å and markedly longer peripheral
NieN2 (1.969(5) Å) and NieC12 (1.947(5) Å). Importantly, the
structure solution in monoclinic P2 /n allowed to unequivocally
1
assign the bonding N2 and C12 atoms in contrast to the Pd deriv-
Table 2
Redox potentials of Phbpy Ni complexes and related bpy Ni complexes .
a
EOx
Ered1
Ered2
Ered3
DEOxꢀEred1
[
(Phbpy)NiBr]
0.08 rev
ꢀ1.90 rev ꢀ2.52 rev ꢀ2.70 rev 1.98
ꢀ2.04 rev ꢀ2.71 irr ꢀ2.97 irr 1.96
ꢀ1.92 irr ꢀ2.08 rev ꢀ2.91 irr 2.13
b
[(Phbpy)Ni(CF)3] ꢀ0.08 prev
c
ative [(Phbpy)PdCl] (space group C2/c) [18b]. For the Pt complex
[(bpy)Ni(Mes)Br] 0.21 irr
c
[(bpy)Ni(Mes)
2
]
ꢀ0.14 irr
ꢀ2.19 rev ꢀ2.97 rev
NPF
e
2.05
[
(Phbpy)Pt(NCMe)][BF
markedly shorter than the MeN bond (7.8%) [18b], while in
(Phbpy)NiBr] this bond is only 1.1% shorter. For the Ni derivative
4
] (space group Pcnb) the MeC bond is
a
From cyclic voltammetry in THF/nBu
4
6
. Potentials (in V) referenced vs.
þ/0
2
. half-wave potentials for reversible pro-
þ
FeCp
2
/FeCp
2
. Potentials in V vs. FeCp
[
cesses (rev), cathodic peak potentials for irreversible reductions (irr).
the N1eNieC12 angle, which defines the distortion from a real
b
Partly reversible.
From Ref. [8b].
ꢂ
square-type coordination, is highest with 165.4(2) compared with
c
ꢂ
ꢂ
the Pd (160.6(1) ) and Pt (160.2(9) ) derivatives. Interestingly, all
three Phbpy complexes exhibit perfect planarity around the metal
centre.
the first reduction wave accounts for exactly one electron, the
When comparing [(Phbpy)NiBr] with nickel complexes of
second and third wave are slightly (2nd) or markedly (3rd) smaller.
0
0
00
2
,2 :6 ,2 -terpyridine (terpy) it must be first noted that only the
five-coordinated complex [(terpy)NiBr exists for bromido-
coordinated nickel(II) [3a,30]. For [(terpy)NiBr ] no structure has
been reported so far, instead structural data of the very related
For the CF derivative the same processes were observed, however,
3
2
]
only the first reduction is reversible, the oxidation is only partly
reversible and the 2nd and 3rd reductions are irreversible. Gener-
2
ally, the potentials for the CF complex are more negative compared
3
0
0
complex [(Rterpy)NiBr
2
] with Rterpy ¼ 6,6 -{(2,6-i-Pr
2
C
6
H
3
)N]
with the Br derivative, which probably accounts for the irrevers-
ibility of the 2nd and 3rd reduction. As outlined above, the complex
CH} -terpy [31] show a distorted trigonal bipyramidal coordination
2
with a relatively long central NieN1 bond and very long peripheral
NieN bonds (comparative data is provided in the Supplementary
data, Table S6). To obtain a square-planar coordination for terpy
nickel complexes the relatively weak coligand Br has to be replaced
by one strong alkyl or aryl coligand, which leads to cationic com-
[(Phbpy)Ni(CF )] decomposes upon standing in solution, a process
which is accelerated when exposed to air. The rather irreversible
oxidation wave is in line with this observation and points to an
oxidatively initiated decomposition reaction, which does not
happen for the Br derivative.
When comparing the behaviour of the cyclometalated bromide
complex with that of the thoroughly studied non-cyclometalated
complex [(bpy)Ni(Mes)Br] (Mes ¼ 2,4,6-trimethylphenyl ¼ mesi-
tyl) (Table 2) it is remarkable, that both reduction and oxidation
occur reversible for the cyclometalated complex, while for the
complex [(bpy)Ni(Mes)Br] both processes are irreversible. For the
reduction, which occurs at almost the same potential it has been
established, that the cleavage of bromide after the first reduction is
responsible for the irreversible behaviour (EC mechanism) [8]. This
is in line with the reversible character of the reduction of the
3
þ
plexes, such as [(terpy)Ni(Mes)] (Mes ¼ 2,4,6-trimethyl-phenyl)
[5]. Here, the central NieN bond is shortened due efficient back-
bonding of the electron density donated by the aryl coligand [5].
This effect can be also seen in [(Phbpy)NiBr] confirming the pre-
vious assumptions. Also in the one-electron reduced complexes
[(terpy)Ni(Me)] [3b,3e,32] and [(terpy)NiBr] [3b], square planar
coordination is found. While for the first no qualitatively satisfying
crystal structure could be obtained [3e], the latter shows generally
far longer NieN bonds, with the central NieN bond only slightly
shorter than the peripheral ones. Based on this structure, quantum
chemical calculations and spectroscopy it was concluded, that this
complex is best described as a Ni(I) complex with a neutral terpy
ligand, in contrast to the corresponding complexes [(terpy)Ni(R)]
with R ¼ methyl [3b,3e] or aryl [5], which are best described as
bis(mesityl) complex [(bpy)Ni(Mes) ]. It thus seems that for
2
[(Phbpy)NiBr] the cleavage of the Br ligand after reduction is
hampered.
For the oxidation a mainly nickel-centred process [Ni(II)/Ni(III)]
can be assumed from previous studies [8]. It seems, that the
cyclometalated complex [(Phbpy)NiBr] is largely stabilised in the
oxidised form (assumed trivalent nickel). A marked difference in
the electronic structure of both complexes can be expected from
the co-planar orientation or the aryl group for the cyclometalated
complex in contrast to the non-cyclometalated derivative in which
Ni(II) complex bearing a reduced (radical anionic) terpy ligand
ꢀ
[
Ni(II)(terpy )(R)].
Electrochemical experiments
The bromido complex [(Phbpy)NiBr] exhibits three reversible
reduction waves and one reversible oxidation wave (Fig. 2). While
ꢂ
the aryl group displays a tilt angle of about 75 (XRD [16e,f] and
3 4 6
Fig. 2. Cyclic voltammogramms of [(Phbpy)NiBr] (left) and [(Phbpy)Ni(CF )] (right) in THF/nBu NPF at 298 K, 200 mV/s scan rate.

90
A. Klein et al. / Journal of Organometallic Chemistry 774 (2014) 86e93
4 6
Fig. 3. Left: Absorption spectra recorded during cathodic electrolysis (spectroelectrochemistry) of [(Phbpy)NiBr] in THF/nBu NPF solutions; right: spectra of the parent complex as
solid line and the final reduced product dotted.
ꢀ
calculated [16d]). Previous calculations for [(bpy)Ni(Mes)Br] and
(bpy)Ni(Mes) ] have revealed marked contributions of the Br and
the Phbpy ligand. They do not appear in the non-cyclometalated
[
2
complexes [(bpy)Ni(Mes)Br] and [(bpy)Ni(Mes)
very high energy at 282 nm are identical for both complexes can be
unequivocally assigned to intraligand ( *) transitions ( ).
2
]. The bands at
Mes coligands to the largely nickel-centred HOMO (highest mo-
lecular orbital) of the complexes. Maybe, the co-planar binding of
p
ep
l
1
e
the Ph group including an extended delocalisation in the Phbpy
Upon spectroelectrochemical reduction (Fig. 3) the same
behaviour for both Phbpy complexes was observed. During the first
step from 0 to ꢀ2.4 V (compare Fig. 2) both long-wavelength bands
undergo red shifts, and the 350 bands lose markedly in intensity.
The intraligand bands at 282 nm remain almost unchanged.
Importantly, the conversion from the parent species to the so-
called intermediate complexes does not show isosbestic points,
thus no simple reaction occurs. These first reduction processes are
partially reversible; approximately 50% of the original spectra could
be recovered upon reverse sweep. Further reduction from ꢀ2.4
to ꢀ2.7 V goes along with the observation of three marked, partially
structures bands with maxima at 376, 593 and 1006 nm for Br and
ligand leads to a markedly increased contribution of the Phbpy
ligand to the HOMO with a high delocalisation of the odd electron
in the oxidised state over the nickel atom and the Phbpy ligand.
Spectroelectrochemical experiments (see next paragraph) and
quantum chemical calculations (in the future) might help to clarify
this.
UV/vis absorption spectroscopy and spectroelectrochemistry
The parent complex [(Phbpy)NiBr] (Fig. 3) exhibits a long-
ꢀ1
wavelength band at 505 nm (19,800 cm ) lying between the
ꢀ
1
377, 572 and 946 nm for the CF derivative. These processes seem to
maxima observed for [(bpy)Ni(Mes)Br] at 464 nm (21,550 cm
)
3
ꢀ
1
be fully reversible. Comparison with the spectrum of [(bpy)
and [(bpy)Ni(Mes)
Assuming the same mixed MLCT/L'LCT transition for these bands
16c,d], we can conclude that the Phbpy anion exerts a stronger
donation effect (lifting the HOMO energy) than (bpy) þ (Mes) but
smaller than bpy þ (Mes) . The complex [(Phbpy)Ni(CF )] exhibits
markedly higher absorption energy (468 nm or 21,370 cm ) in line
with the electron-withdrawing effect of CF [28]. The second MLCT/
L'LCT transitions, which are also observed in [(bpy)NiMes) ] occur
at around 390 nm ( ). The intense bands around 350 nm ( ) are
*) transitions of
2 4
] at 564 nm (17,730 cm ) (l in Table 3).
ꢀ
Ni(Mes)
2
]
and the spectral features of reduced bpy (Table 3)
clearly shows that these final species contain a reduced bpy-
moiety.
For the related complex [(bpy)Ni(Mes)Br] quite similar behav-
iour has been observed and was explained by an ECE mechanism
[
s-
2
3
ꢀ1
[
8]. ECE means that the parent complex is one-electron reduced (E),
3
then undergoes bromide cleavage (C) and the product can be
further reduced (E). It can be assumed, that the two Phbpy com-
plexes feature very similar mechanisms. However, the chemical
2
l
3
l
2
partly structured, which points to intraligand (pep
Table 3
UV/vis absorption spectral data of Phbpy nickel complexes and corresponding reduced and oxidised speciesa,b.
l
1
(ε)
l
2
(ε)
l
3
(ε)
l
4
(ε)
5
l (ε)
[
[
[
[
[
[
[
[
[
[
[
(Phbpy)NiBr]n
(Phbpy)NiBr]
n ¼ 0
282 (58.2)
276 (56.8)
296 (51.3)
e
355 (13.9)
352 (8.8)
376 (11.8)
315 (17.4)
347 (12.7)
346 (11.8)
377 (12.9)
308 (15.1)
e
388 (4.1)
399 (4.0)
505 (3.1)
547 (2.4)
593 (6.0)
e
e
e
n
n ¼ ꢀ1 intermediate
n ¼ ꢀ1 final
n ¼ þ1
(Phbpy)NiBr]n
(Phbpy)NiBr]n
(Phbpy)Ni(CF)
(Phbpy)Ni(CF)
(Phbpy)Ni(CF)
(Phbpy)Ni(CF)
e
1006 (1.3)
e
e
n
n
n
n
3
3
3
3
]
]
]
]
n ¼ 0
281 (22.7)
279 (23.0)
297 (22.1)
e
394 (4.6)
397 (5.0)
468 (2.7)
542 (2.1)
572 (5.8)
e
e
n ¼ ꢀ1 intermediate
n ¼ ꢀ1 final
n ¼ þ1
e
e
946 (1.4)
e
e
nc
(bpy)Ni(Mes)Br]
n ¼ 0
310 (17.2)
298 (15.1)
e
e
464 (2.3)
564 (3.1)
530 (2.6)
585
nc
(bpy)Ni(Mes)
2
]
]
n ¼ 0
e
367 (3.2)
e
e
nc
(bpy)Ni(Mes)
2
n ¼ ꢀ1
355 (14.9)
398
940 (1.7)
1230
ꢀ
bpy
e
e
a
ꢀ1
ꢀ1
Wavelengths of absorption maxima
Generated by in situ electrolysis in THF/0.1 M nBu NPF at ambient temperatures.
4 6
From Ref. [8b].
l
in nm; molar absorption coefficient ε in 1000 Mol cm .
b
c

A. Klein et al. / Journal of Organometallic Chemistry 774 (2014) 86e93
91
3
reactions (Br and CF cleavage, respectively) seem to be markedly
slower, allowing the observation of reversible redox waves in the
CV. Furthermore, the mechanisms for both complexes might be
ꢀ
different, since, while the generation of a Br anion in solution is
ꢀ
likely to occur in such a reaction, the generation of a CF
3
anion is
·
probably less preferred and the cleavage of a CF radical is more
3
likely [4].
The UV/vis absorption data for both intermediate complexes are
quite similar (spectra of [(Phbpy)Ni(CF ] in the Supplementary
3
data), and the formation of Phbpy Ni complexes can therefore be
assumed. On the other hand, the absorption bands of the final
reduced species for the Br and CF
3
complexes are different. The
3
Scheme 2. Proposed mechanism for a CF arylation reaction based on [(Phbpy)Ni].
overall features in these spectra are the same and are very similar to
reduced bpy. The product from the Br complex exhibits markedly
lower energies for the corresponding absorption bands l , l and l
2 4 5
in Table 3 pointing to a different substitution pattern of a reduced
Phbpy ligand. We therefore propose that the chemical reaction in
the ECE sequence of these complexes does not (only) comprise the
spectroelectrochemical studies (bulk electrolysis with isolation of
products or trapping of intermediates) will clarify the underlying
mechanisms. These experiments will be carried out in the near
future.
cleavage of the Br or CF
transfer to the 2-phenyl- moiety as radicals or in a kind of reductive
elimination leading to coordinated Phbpy-Br or Phbpy-CF ligands.
Bulk electrolysis studies on the complexes will help to clarify this in
the future.
3
coligands to the solution but (also) their
Decomposition of the CF
3
complex
3
The CF complex [(Phbpy)Ni(CF )] decomposes quite rapidly in
3
3
1
19
solution. This was studied by H and F NMR spectroscopy, UV/vis
absorption spectroscopy and EI-MS. The NMR and MS results
Upon spectroelectrochemical oxidation (Fig. 4) of the two
complexes almost all bands disappear and only one marked band at
around 310 nm characterises the oxidised products. This time the
process shows clear isosbestic points, however, the reversibility is
only about 50e60% (Figures in the Supplementary data).
Thus, for both the first reduction of both Phbpy complexes and
the oxidation of [(Phbpy)NiBr] the CV experiments show complete
reversibility, while in the slower spectroelectrochemical experi-
ments only partial reversibility is observed. Compared with the
very similar but non-cyclometalated system [(bpy)Ni(Mes)Br], in
which both oxidation and reduction occur completely irreversible
under the same CV conditions, the Phbpy systems seem to be more
stable after oxidation and reduction. For the non-cyclometalated
system [(bpy)Ni(Mes)Br] reversibility for the first reduction pro-
clearly show the generation of the CF
3
alkylated Phbpy ligand (6-(2-
0
(trifluoromethyl)phenyl)-2,2 -bipyridine) probably formed through
3
an internal CF transfer. UV/vis absorption and NMR spectroscopy
reveal that the process requires about two to 3 h and involve several
steps, e.g. intermediate nickel species were observed. For two
reasons we will study this process in more detail in the future: i) we
want to establish the underlying mechanisms which are probably
strongly linked to the behaviour under electrolysis (concerted
reductive elimination or radical reactions) and ii) we want to asses,
if in a related process the CF group could be transferred to other
3
substrates and not to the Phbpy ligand, preferably in a catalytic
fashion as depicted in Scheme 2.
ꢀ
ꢂ
cess, thus, hampering of the Br cleavage is only observed at ꢀ60 ,
Conclusions and outlook
while the oxidation remains irreversible even at very low tem-
ꢀ
peratures. Either the tridentate Phbpy ligand provides some kind
The new cyclometalated complex [(Phbpy)NiBr] (Phbpy ¼ 6-
0
of electronic stabilisation of the mono-reduced or oxidised species
or the decomposition reactions of these species differ markedly
from the species generated from the non-cyclometalated complex
(phen-2-yl)-2,2 -bipyridine anion) can be easily prepared from the
Br-Phbpy precursor and [Ni(COD)
2
]. The molecular structure from
ꢀ
single crystal XRD shows the tridentate N,N,C binding of the Phbpy
ligand. The bromido complex is probably a very suitable precursor
for further Phbpy Ni complexes. In a first example, the complex was
reacted with trimethyl(trifluormethyl)silane in the presence of CsF
[(bpy)Ni(Mes)Br]. Comparative quantum chemical calculations will
give insight to the electronic situation, while further
3
and gave [(Phbpy)Ni(CF )] in good yields. UV/vis absorption, elec-
trochemical and spectroelectrochemical investigations of [(Phbpy)
NiBr] show interesting differences to the non-cyclometalated but
otherwise quite related complex [(bpy)Ni(Mes)Br] (Mes ¼ 2,4,6-
trimethylphenyl). On the timescale of the cyclic voltammetry ex-
periments both oxidations and reductions were found to exhibit a
high degree of reversibility for the Phbpy systems at ambient
temperature, while for the non-cyclometalated derivative irre-
versible processes were observed throughout. While the Br com-
plex is completely stable in the solid and in solution, the CF
complex undergoes cleavage of 2-(CF )Phbpy in solution. The
3
3
product is in agreement with an overall reductive elimination.
However, a closer look showed that the underlying mechanism is
more complicate and will be subject to further studies. These
studies seek to clarify the reactivity details of the mono-reduced or
mono-oxidised species (anion cleavage or reductive elimination or
radical mechanisms) and will link them to what is observed for the
parent complexes. Also, it is intended to establish the bromido
Fig. 4. Absorption spectra of [(Phbpy)NiBr] during anodic scan from 0 V to þ0.8 V
(
spectroelectrochemistry) in THF/nBu
4
NPF
6
solutions.
complex as “platform” to allow stoichiometric or catalytic CF
3

92
A. Klein et al. / Journal of Organometallic Chemistry 774 (2014) 86e93
alkylations of various substrates. Finally, quantum chemical calcu-
lations will be carried out to assess the electronic structure of the
cyclometalated complexes and shed light on the “stabilising” effect
of the ligand Phbpy on the products of one-electron oxidation and
reduction.
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Acknowledgement
5
We are indebted to Dr. J o€ rg Neud o€ rfl (University of Cologne) for
single crystal XRD measurements and Dr. Ingo Pantenburg (Uni-
versity of Cologne) for fruitful discussions. We are also grateful for
financial support by Deutsche Forschungsgemeinschaft projects KL
3
(
(
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CCDC 1017365 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
(2000) 844e861.
[
[
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