Oxidative cleavage of CH3 and CF3 radicals from BOXAM nickel complexes

Article abstract of DOI:10.1021/om300342r

Oxidation of the nickel complexes [(BOXAM)Ni(R)] (HBOXAM = bis((4-isopropyl-4,5-dihydrooxazol-2-yl)phenyl)amine) with R = CF₃, CH₃, Cl leads to the corresponding radical cationic complexes, which rapidly undergo homolytic Ni-R bond s

Full text of DOI:10.1021/om300342r

Article
pubs.acs.org/Organometallics
Oxidative Cleavage of CH3 and CF3 Radicals from BOXAM Nickel
Complexes
Axel Klein,*,† David A. Vicic,*,‡ Christian Biewer,† Iris Kieltsch,‡ Kathrin Stirnat,† and Claudia Hamacher†
†Institut fur Anorganische Chemie, Universitat zu Koln, Greinstraße 6, D-50939 Koln, Germany
̈
̈
̈
̈
‡Department of Chemistry, University of Hawaii, 2545 McCarthy Mall, Honolulu, Hawaii 96822, United States
S
* Supporting Information
ABSTRACT: Oxidation of the nickel complexes [(BOXAM)-
Ni(R)] (HBOXAM = bis((4-isopropyl-4,5-dihydrooxazol-2-yl)-
phenyl)amine) with R = CF3, CH3, Cl leads to the
corresponding radical cationic complexes, which rapidly undergo
homolytic Ni−R bond splitting. The corresponding radicals R•
can be generated by either anodic (electrochemical) or aerial
oxidation and were detected by spin trapping experiments using
N-tert-butyl-α-phenylnitrone (PBN). The splitting reaction was
also monitored by UV−vis−near-IR absorption spectroelec-
trochemistry, showing the formation of the “byproduct” complex
[(BOXAM)Ni(THF)]+ in THF solution. Additionally, the
undissociated complex radicals [(BOXAM)Ni(R)]•+ were studied by EPR and UV−vis−near-IR spectroelectrochemistry,
revealing inter alia largely ligand-centered oxidation. DFT calculations on ground and oxidized states support this finding.
we observed that the redox behavior of the three BOXAM
nickel complexes strongly depends on the coligand Cl, CF3, or
CH3, and the complexes display the same trends in oxidation
potentials observed for the corresponding phosphine com-
INTRODUCTION
■
Organonickel complexes have come into focus in recent years
for their application as catalysts in C−C coupling reactions.1−7
From two different points of view the use of the trifluoromethyl
group in such C−C coupling reactions is highly interesting and
challenging. While the introduction of the trifluoromethyl
group, the simplest perfluoroalkyl group, is of growing
importance in agrichemical, pharmaceutical, and materials
chemistry,8−12 the methods to introduce CF3 are still
underdeveloped; today mainly stoichiometric synthetic proce-
dures are applied to introduce the CF3 group (usually based on
Cu complexes),13−20 and only recently have the first catalytic
protocols been developed.21
In this context we have recently studied the complex
[(BOXAM)Ni(CF3)] (BOXAM = bis((4-isopropyl-4,5-dihy-
drooxazol-2-yl)phenyl)amine; Scheme 1), its CH3 derivative
(for comparison of CF3 vs CH3), and the precursor complex
[(BOXAM)NiCl].22 This last species might be a suitable
precatalyst for a catalytic CF3 alkylation process. In this study
p l e x e s [ ( d i p p e ) N i ( R ) 2 ] ( d i p p e
=
1 , 2 - b i s -
(diisopropylphosphino)ethane; R = CH3, CF3)22 in the sense
that the [(BOXAM)Ni(CF3)] complex was much more difficult
to oxidize than the [(BOXAM)Ni(CH3)] complex (−0.17 V
for R = CH3 vs +0.42 V for R = CF3). Furthermore, we found
that while the one-electron oxidation of the CH3 complex is
completely irreversible (also at lower temperature and higher
scan rates), the CF3 and Cl derivatives exhibit a certain degree
of reversibility. A detailed investigation of the CH3 complex
revealed that one-electron oxidation or treatment with oxygen
leads to the complex [(BOXAM)Ni(THF)]+ and a homolytic
cleavage of CH3 was assumed. Finally, from the similarity of the
oxidation potentials for the Cl and CF3 complexes we
concluded that the redox chemistry might be centered at the
BOXAM ligand, while the redox chemistry of the methyl
derivative resembles far more that of the phosphine complex
[(dippe)Ni(CH3)2] and might be more nickel-centered. To
support these assumptions, detailed spectroelectrochemical
(EPR and UV−vis−near-IR absorption) experiments have
been carried out (under anaerobic conditions) on the
complexes [(BOXAM)Ni(R)] (R = CH3, CF3, Cl) and
[(BOXAM)Ni(THF)](OAc) and are reported herein.
Scheme 1. Schematic Drawing of [(BOXAM)Ni(R)]
Complexes
Received: April 26, 2012
© XXXX American Chemical Society
A
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signal, a doublet of a triplet signal, simulated with aN = 14.1 G
and aH = 3.2 G represents the CH3−PBN adduct (sim I in
Figure 1), in line with earlier work reporting values of aN ≈ 14
G (NPBN) and aH ≈ 3.4 G (β-HPBN).23−25 The major signal
observed in our experiments was a simple narrow triplet with a
relatively small aN = 7.99 G and nonresolved (or missing) aH <
0.50 G (sim II in Figure 1). The underlying species could be
PhC(O)N(•O)tBu formed by oxidation of the trap, in line with
reported aN values of 7.5−8 G for this species.27,28 Also for the
Cl derivative the species PhC(O)N(•O)tBu could be observed
(aN = 8.00 G) together with a simple triplet with a larger aN of
15.28 G (see the figures in the Supporting Information), which
could not be assigned. It is well established that the Cl−PBN
adduct (aCl ≈ 4−6 G; aβ‑H ≈ 1 G)25,26 is highly unstable and
easily undergoes oxidation to PhC(O)N(•O)tBu.27,28
RESULTS AND DISCUSSION
■
The synthesis and analytical data of the BOXAM nickel
complexes [(BOXAM)Ni(R)] (R = CH3, CF3, Cl) and
[(BOXAM)Ni(THF)](OAc) were previously reported.22 In
addition, the electrochemistry of the four complexes has been
presented previously. To examine the role of THF as ligand in
the complex [(BOXAM)Ni(THF)]+, we investigated the
electrochemistry of the complex in various solvents. In all
solvents the one-electron-oxidation wave was not completely
reversible, comparable to the case for complexes carrying Cl or
CF3 coligands (for CVs see the Supporting Information).
EPR Spectroelectrochemistry. In order to establish the
character of the one-electron-oxidized complex species and the
character of the Ni−R bond scission (ionic or radical type), we
carried out a number of EPR experiments under anaerobic
conditions. In the first series we wanted to probe for the
products of the Ni−R splitting reaction by carrying out anodic
electrolysis on the three complexes (potentials 0 V for R =
CH3, 0.5 V for R = CF3, and 0.6 V for R = Cl) in the presence
(and absence) of the spin trap N-tert-butyl-α-phenylnitrone
(PBN; Scheme 2). Spectra are shown in Figure 1.
We can thus conclude that while •CF3 reacts readily with the
spin trap and the spin adduct is quite stable, •CH3 and •Cl also
perhaps react rapidly with PBN; however, the spin adducts are
markedly less stable. To test if the THF solvent reacts with the
spin adducts, we carried out the same experiments for the CF3
and the Cl derivatives in MeCN solution. For R = CF3 we
found exclusively the spin-trapped radical CF3−PBN (simu-
lation identical with that above), while for the Cl derivative
again the three-line signal for the oxidized adduct (aN = 8 G)
was observed, this time without further species. Although the
results were slightly different in MeCN, a direct involvement of
THF is unlikely, since no THF radical could be observed.29,30 It
is more likely that the nature of the solvent has a slight
influence on the stability of the spin adducts.
a
Scheme 2. Reaction of the Spin Trap PBN with a Radical
a
Nuclei in boldface represent possible coupling partners with the
When carrying out electrolysis experiments at low potentials
(−0.6 to +0.2 V for R = Me; 0 to +0.8 V for R = CF3, Cl) under
the same conditions without adding the spin trap, we did not
observe any EPR signal at 298 K. At 110 K in glassy frozen
solution very broad and unresolved EPR signals were observed
(Figure 2 and Supporting Information). When carrying out the
same electrolysis experiments at lower temperatures (278 K)
and freezing the solutions and recording spectra at an early
stage of the electrolysis (a few minutes), we also did not
observe signals at 278 K, while at 110 K the broad signals were
detectable. Thus, we conclude that the underlying species were
EPR silent in solution at ambient temperature (278−298 K).
Electrolysis at higher potentials (without PBN) led to the
observation of narrow resolved EPR signals centered at around
g = 2.006 (Figure 2 and Supporting Information). The narrow
unpaired electron. Note that nuclei of the R group might possibly also
couple.
For all three complexes narrow 1:1:1 triplet signals were
detected at g = 2.006 97 (CH3), 2.006 41 (CF3), and 2.006 16
(Cl). The general triplet character of the signals is due to
hyperfine splitting (hfs) of the unpaired electron to the isotope
14N (I = 1, 99.64% natural abundance). This and the observed g
values at around g = 2.006 indicate the formation of PBN-
bound radical species.23−26 The signal of the CF3 adduct shows
additional hfs to 19F (I = 1/2, 100% natural abundance), giving a
triplet of quintets with the simulated coupling constant aN
=
13.91 G, aH = 1.17 G (βH of PBN), and aF = 1.77 G (trapped
CF3 group), in line with reported values.23−25 For the CH3
derivative essentially two signals were observed. The minor
Figure 1. X-band EPR spectra obtained during anodic electrolysis of [(BOXAM)Ni(CF3)] (left) and [(BOXAM)Ni(CH3)] (right) in the presence
of PBN in THF/nBu4NPF6, recorded at 293 K with simulations (lower traces).
B
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Figure 2. X-band EPR spectra obtained during anodic electrolysis of [(BOXAM)Ni(CH3)] in THF/nBu4NPF6, electrolyzed at 298 K: (left)
spectrum recorded after electrolysis at −0.6 to +0.2 V and measured at 110 K in glassy frozen solution (with simulation below); (center) spectra
recorded after electrolysis at +0.2 to 1.0 V and measured at 110 K (broad signal) or at 298 K (narrow signal); (right) hf resolved spectrum recorded
at 298 K (narrow signal) with simulation (below) at g = 2.006, aH(3H) = 11.21 G, aH(2H) = 1.585 G, aN(1N) = 1.18 G, Gauss/Lorentzian = 1, and
line width 1.25 G.
a
Table 1. EPR Data of Nickel Complexes
complex
giso or gav
g3
g2
g1
Δg
ref
[(BOXAM)Ni(CH3)]•+
[(BOXAM)Ni(CF3)]•+
[(BOXAM)NiCl]•+
[Ni(TMPP-O)2]•+
[Ni(L1)]•−
2.073
2.077
2.073
2.20
2.12
2.08
2.02
0.09
this work
2.11
2.09
2.03
0.08
this work
2.10
2.09
2.03
0.07
this work
2.28
2.28
2.04
0.24
31
32
33
34
35
36
37
2.173
2.221
2.223
2.196
2.435
2.533
2.255
2.366
2.44
2.255
2.303
2.27
2.008
1.994
1.96
0.247
0.372
0.48
[Ni(L2)]•−
[Ni(emi)]−
[(NCN)NiCl2]
2.369
2.825
2.84
2.195
2.453
2.84
2.024
1.947
1.92
0.345
0.878
0.92
[(PPh3)2Ni(Fmes)Br]•+
[Ni(C6Cl5)4]·−
a
Definitions: gav = averaged g value = (g1 + g2 + g3)/3; Δg = g anisotropy = g1 − g3. Abbreviations: TMPP-O = monodemethylated form of tris(2,4,6-
trimethoxypheny1)phosphine (TMPP); H4L1 = N-[2-(oxalylamino)phenyl]oxalamic acid; NCN = C6H3(CH2NMe2)2-o,o′: H4L2 = 6,6-diethyl-
3,9,9,12,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradecane-2,5,7,10,13-pentaone; H4emi = N,N-ethylenebis(2-mercaptoisobutyramide); Fmes
=2,4,6-tris(trifluoromethyl)phenyl.
and resolved character of the signals suggests that they do not
represent nickel-containing radical species, and although
simulations were satisfactorily carried out, no unequivocal
information on the composition of the underlying species could
be extracted (for some details, see the Supporting Information).
Bulk electrolysis experiments and a detailed analysis of the
products will be carried out in the near future to assign these
species.
CH3, Cl). Such organometallic radicals would also be in line
with the relatively large g values, the small g anisotropy, and
their EPR silence at ambient temperature. Although in these
radicals the nickel atom (and the coligand R) has a marked
influence on the EPR spectroscopy, the total contribution of
nickel to the singly occupied molecular orbital (SOMO) is
probably small (see also DFT Calculations) and the description
of the radical complex is more like [(BOXAM•)NiII(R)]+ (A)
than [(BOXAM−)NiIII(R)]+ (B). The high similarity of the
corresponding EPR spectra (at 110 K) strongly supports the
resonance form A with the spin mainly centered on the
(neutral) BOXAM• radical ligand.
To distinguish between “real” Ni(III) complexes and Ni(II)
complexes of noninnocent ligands carrying largely the unpaired
electron on the ligand, both the g anisotropy (Δg) and the
averaged g values (gav) are indicative. Examples of essential
nickel contributions to the unpaired electron in a radical
complex in a sense of Ni(III) were the trimethoxyphenylphos-
phine complex [Ni(TMPP-O)2]•+ (Δg = 0.24),31 the oxamato
complex [Ni(L1)]•− (H4L1 = N-[2-(oxalylamino)phenyl]-
oxalamic acid; Δg = 0.247),32 the tetraamidonickel complex
[Ni(L2)]•− (H4L2 = 6,6-diethyl-3,9,9,12,12,14,14-octamethyl-
1,4,8,11-tetraazacyclotetradecane-2,5,7,10,13-pentaone; Δg =
0.372),33 and [Ni(emi)]•− (H4emi = N,N-ethylenebis(2-
mercaptoisobutyramide); Δg = 0.486).34
The broad signals at 110 K (Figure 2) are rhombic in nature.
The spectra have been simulated by assuming three g values,
and due to the ill-resolved character of the signals the accuracy
of the obtained g values is low and can be only provided at a
level 0.005. Nevertheless, the results confirm that the three
species are not identical (Table 1) and the g values lie in the
typical range of organometallic radicals containing nickel (thus
“organic” radicals coordinated to diamagnetic Ni(II)). The
latter assumption is supported by the relatively small g
anisotropy Δg of the three species (CH3, 0.09; CF3, 0.08; Cl,
0.07). We thus assign the signals observed at 110 K to the
oxidized complexes [(BOXAM)Ni(R)]•+ (R = CF3, CH3, Cl),
prior to dissociation. Importantly, when recording EPR spectra
on anodically electrolyzed solutions of [(BOXAM)Ni(THF)]+
in THF/nBu4NPF6, we failed at both 298 and 115 K to observe
reasonable signals. The missing signal at 115 K for the oxidized
THF complex (which should be [(BOXAM)Ni(THF)]•2+
)
strongly supports our assumption that the broad unresolved
spectra recorded for the oxidized complexes at 110 K are due to
the nondissociated species [(BOXAM)Ni(R)]•+ (R = CF3,
Interestingly, corresponding organonickel(III) complexes
(having a NiIII−C bond) such as the pincer complex
[(NCN)NiCl2]• (Δg = 0.345),35 the phosphine complex
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trans-[(PPh3)2Ni(Fmes)Br]•+ (Δg = 0.878),36 and the
homoleptic complex [Ni(C6Cl5)4]•− (Δg = 0.92),37 exhibit
even higher Δg values. As Table 1 shows, the gav values also
show the same trend.
contributions coming from the p orbitals of the amide nitrogen.
From this point of view it is easy to understand why the
electrochemical oxidation potentials are equal in magnitude,
because if the oxidation takes place on the pincer ligand, the
potentials are likely to be minimally influenced by the nature of
the coligands on the nickel (CH3 vs CF3 vs Cl).
An interesting insight was recently provided by Thomas et
al.38 They reported that the EPR spectroscopy of the oxidized
Ni complex [Ni(L3)]•+ containing the salen-type ligand (H2L3
= 6,6′-(2,2′)-bipyridine-6,6′-diyl)bis(2,4-di-tert-butylphenol)
depends on the absence or presence of axial ligands such as
pyridine. For the spectrum in the absence of pyridine
(distorted-square-planar complex) a rhombic signal with g3 =
2.06, g2 = 2.014, and g1 = 1.986 (gav = 2.02, Δg = 0.074) was
observed. In line with earlier observations,39 the spectral
features are assigned to the ligand-centered radical complex
[NiII(L3•)]+, which is a phenoxyl radical bound to diamagnetic
Ni(II). For the broad signal obtained in the presence of
pyridine, the g values g3 = 2.217, g2 = 2.179, and g1 = 2.032 were
obtained from a simulation (gav = 2.1426, Δg = 0.185). The
authors state that “these values are far from those expected for a
high-spin Ni(II) ion antiferromagnetically coupled to a
phenoxyl radical”40 and conclude that “the unpaired electron
Recent calculations on the different behavior of CF3 vs CH3
as ligands in transition metal complexes have shown that CF3 is
a strong σ-donor ligand, comparable to CH3.42−44 Additionally,
CF3 exhibits some degree of back-bonding (to σ*(C−F)),
while CH3 exhibits virtually none, which is, for example, the
reason M−CH3 bonds are frequently longer than M−CF3
bonds in complexes of electron-rich transition metals such as
Mn(I),43 Rh(I),43 Ru(II),44 Os(II),44 Ni(II),42 and Pt(II),43
while for early transition metals in high oxidation states such as
Ti(IV) or Hf(IV)44 the sequence of bond lengths is inverted.
This is completely in line with our calculated geometries (see
the Supporting Information), the calculated LUMOs depicted
in Figure 3 and the stabilization of the nickel-centered LUMO
energy. In contrast to the nickel-centered LUMO, the HOMOs
of the BOXAM complexes have mainly amide-ligand N p
orbital character and are thus less influenced by the nature of
the R coligand. This is fully in line with recent calculations on
the related bis (amino)amide nickel complexes [(L)Ni(R)] (L
= bis(2-(N,N-dimethylamino)phenyl)amine; R = CF3, Cl,
CH3), also exhibiting mainly amide-centered HOMOs.42
However, while in the latter system CF3 and Cl exhibit similar
HOMO energies with the CH3 derivative slightly destabilized,
in the BOXAM complexes the HOMO energy of the CF3
complex takes an intermediate position between the CH3
(higher energy) and Cl (lower energy) derivatives, which
might be explained by slightly higher nickel contributions to the
HOMO.
2
is mostly hosted by a metal dz orbital. Therefore the nickel ion
is at its (+III) oxidation state and resides within an elongated
octahedral geometry”.33,39−41 It is noticeable that the high-field
component is split into five lines of intensity 1:2:3:2:1, as
expected for the interaction of the electronic spin with two
equivalent nitrogen nuclei. The “pyridine adduct” of
[NiII(L3•)]+ thus represents the octahedral nickel(III) complex
[NiIIIL3(Py)2]+ involving two axially bound pyridines. Such
“switches” in character of a nickel radical upon changes in the
coordination sphere have been reported before.32,34,39
DFT Calculations. In order to help rationalize the
electrochemical and EPR data, DFT calculations were
performed on the parent complexes [(BOXAM)Ni(R)] and
[(BOXAM)Ni(THF)]+ and the oxidized species. Unrestricted
geometry optimizations (data are given in the Supporting
Information) were performed using the SV(P) basis set, and
analyses of the frontier molecular orbitals of the optimized
structures (Table 2) show that, in the gas phase, the HOMOs
and LUMOs of the three neutral BOXAM complexes are
similar in energy in comparison to the cationic complex
[(BOXAM)Ni(THF)]+.
The calculated spin density (Figure 4) of the radical cationic
complexes [(BOXAM)Ni(R)]•+ shows large contributions of
the diphenylamido group of the BOXAM ligand but also spin
density on the nickel atom and, for the Cl derivative (as
expected), on the Cl atom. Since we have no detailed EPR data
from these species, we can only state that qualitatively the
picture from the DFT calculations agrees with the EPR spectra,
from which a mainly ligand-centered character of the unpaired
electron was concluded.
UV−Vis−Near-IR Spectroelectrochemistry. Upon oxi-
dation of [(BOXAM)Ni(CH3)] in THF/nBu4NPF6 the long-
wavelength absorption band at 530 nm and the UV band at 350
nm bleach out, while a new band arises at 670 nm and the 440
nm band increases in intensity and shifts slightly to the blue
(Figure 5, left). Comparison with the spectrum of [(BOXAM)-
Ni(THF)]+ (Figure 6) reveals no similarities; thus, we can
assume that [(BOXAM)Ni(CH3)]•+ is formed, can be detected
by this method, and is not rapidly converted to [(BOXAM)-
Table 2. Calculated Frontier Orbital Energies
complex
orbital
orbital energy (eV)
[(BOXAM)Ni(CH3)]
124 (LUMO)
123 (HOMO)
136 (LUMO)
135 (HOMO)
128 (LUMO)
127 (HOMO)
139 (LUMO)
138 (HOMO)
−3.671
−2.310
−4.207
−2.626
−4.056
−2.871
−6.001
−7.172
[(BOXAM)Ni(CF3)]
[(BOXAM)NiCl]
•
Ni(THF)]+ by splitting of CH3.
[(BOXAM)Ni(THF)]+
Further oxidation (sweeping the voltage from 0.2 to 1.3 V,
Figure 5, right) leads to drastic changes in the spectra. A new
long-wavelength band at 630 nm grows in and quickly
dominates the spectrum. The bands with maxima at 440 and
320 nm vanish and leave a broad structured absorption band at
around 400 nm. Similar behavior is observed during the anodic
oxidation of the CF3 and Cl derivatives. However, here the two
processes cannot be completely resolved and the spectra for the
oxidized complexes [(BOXAM)Ni(R)]•+ (R = CF3, Cl) are
superimposed by the absorptions of the THF complex (spectra
in the Supporting Information). Nevertheless, the absorption
Closer inspection shows that the relative HOMO energies
for the three complexes agree qualitatively well with the
oxidation potential, putting the CF3 group alongside the Cl
ligand with their complexes exhibiting higher potential than the
CH3 derivative. Graphical representations of the HOMOs and
LUMOs for [(BOXAM)Ni(R)] (R = CF3, CH3, Cl) are shown
in Figure 3. Further analysis revealed that the HOMOs of all
three complexes were mainly ligand-centered, with the major
D
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Figure 3. Calculated HOMO (bottom) and LUMO (top) of [(BOXAM)Ni(R)] (R = CH3 (left), CF3 (center), Cl (right)). All lobes are shown at
0.02 isovalues.
Figure 4. Calculated spin density plot of [(BOXAM)Ni(R)]•+ (R = CH3 (left), CF3 (center), Cl (right)): isovalue 0.02 and density 0.04.
Figure 5. Absorption spectra taken during anodic electrolysis of [(BOXAM)Ni(CH3)] in THF/nBu4NPF6 solutions: (left) voltage range −0.6 to
+0.2 V; (right) voltage range +0.2 to +1.3 V.
E
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coligand in comparison to the strong electron donor coligands
CH3 and CF3 would explain the observed red shift, and from
this point of view the THF complex also fits this series.
Importantly, these assignments agree qualitatively very well
with the DFT calculated HOMOs and LUMOs of the
complexes, showing markedly higher HOMO−LUMO gaps
for the CH3 and CF3 complexes, in comparison to the Cl- and
THF-containing derivatives (Table 2).
If we look at the oxidative spectroelectrochemistry of
[(BOXAM)Ni(CH3)], the first oxidation (Figure 5) leads to
a spectrum of a species which is characterized also by three
characteristic bands (λ3−λ5) in the visible region. For the two
other derivatives [(BOXAM)Ni(R)] (R = CF3, Cl) similar
changes were observed, with one marked difference for R = Cl.
Here, the long-wavelength band λ5 for the oxidized species is
blue-shifted in comparison to the band for the parent complex
(Table 3). If the assignment of the long-wavelength absorption
bands of the parent complexes is correct, the oxidation process
[(BOXAM)Ni(R)] → [(BOXAM)Ni(R)]•+ should lead to a
stabilization of the ligand-centered HOMO and thus to a blue
shift of the corresponding LMCT bands with decreased
intensity (half empty). In this sense for the complexes with R
= CH3, CF3 the λ4 absorptions might be interpreted as the blue-
shifted long-wavelength LMCT bands (λ5 for the parent
species). The long-wavelength absorptions λ5 might then be
interpreted as intraligand (SOMO−LUMO) transitions. For R
= Cl the long-wavelength absorption λ5 is blue-shifted in
comparison to λ5 of the parent complex, in line with LMCT
character. For this complex, the SOMO to LUMO absorption,
proposed for the R = CH3, CF3 derivatives, was not observed.
This is probably due to the rapid conversion of the oxidized
complex [(BOXAM)NiCl]•+ to the THF complex [(BOXAM)-
Ni(THF)]+ and the oxidation of the latter (same potential).
Both the THF complex and its oxidation products (see the
Supporting Information) exhibit strong absorption bands
covering the range from 500 to 800 nm, thus probably
obscuring the weak long-wavelength absorption for the oxidized
R = Cl complex. To summarize, the results from UV−vis−near-
IR spectroelectrochemistry confirm qualitatively the assump-
tion of a largely ligand-centered oxidation [(BOXAM−)-
NiII(R)]/[(BOXAM•)NiII(R)]•+. In future work we will seek
detailed insight into the absorption spectroscopy of parent and
oxidized species by TD-DFT calculations.
Figure 6. Absorption spectra of [(BOXAM)Ni(CH3)] (solid line) and
[(BOXAM)Ni(THF)](OAc) (dashed line) (both in THF) and
anodically oxidized [(BOXAM)Ni(CH3)] (dotted line) (in
THF/nBu4NPF6 solution).
data could be extracted (Table 3). In line with the EPR
experiments, the absorption spectra observed upon electrolysis
at higher potentials are probably due to decomposition
products of unclear composition.
Table 3. Absorption Data of HBOXAM and BOXAM Nickel
a
Complexes
complex
λ1
λ2
λ3
λ4
λ5
[(BOXAM)Ni(CH3)]
[(BOXAM)Ni(CH3)]+
[(BOXAM)Ni(CF3)]
[(BOXAM)Ni(CF3)]+
[(BOXAM)NiCl]
[(BOXAM)Ni(Cl)]+
[(BOXAM)Ni(THF)]+
(HBOXAM)
224, 243
210, 242
239, 276
223
305
323
326
324
333
330
333
304
349
373
379
395
333
394
382
358
443
445
436
440
458
455
450
535
670
520
630
680
527
620
220, 283
280
226 sh, 283
255
a
Absorption maxima in nm, measured in THF/nBu4PF6 solutions.
Oxidized species were generated by in situ electrolysis in an OTTLE
cell.
A closer look at the collected absorption data in Table 3
shows that the absorption spectra of the parent complexes
[(BOXAM)Ni(R)] are characterized by several bands of
medium intensity in the visible to UV range (300−700 nm;
λ2−λ5 in Table 3) and intense absorptions in the UV (λ1). The
long-wavelength bands (λ5) for the CH3 and CF3 derivatives lie
in the same range (535 and 521 nm, respectively) and explain
the red color of the compounds, while the Cl derivative and the
cationic complex [(BOXAM)Ni(THF)]+ deviate markedly,
absorbing at 620 and 630 nm, respectively, and consequently
have a green color. The UV bands (λ1) can be assigned to
intraligand (π−π*) transitions in the BOXAM ligand, since
they also occur for the protonated ligand HBOXAM. The two
further bands λ2 and λ3 for HBOXAM can also be observed in
the nickel complexes and were assigned to further BOXAM-
centered intraligand transitions. The long-wavelength bands λ4
and λ5 both undergo red shifts along the series CF3 < CH3 <
THF < Cl. Both absorption bands are assigned to ligand
(BOXAM−) to metal (Ni(II)) charge transfer (LMCT)
transitions. Stabilization of the nickel LUMO by the Cl
CONCLUSIONS
■
The homolytical bond splitting and formation of R• radicals (R
= CF3, CH3, Cl) following one-electron oxidation of the nickel
complexes [(BOXAM)Ni(R)] (BOXAM = bis((4-isopropyl-
4,5-dihydrooxazol-2-yl)phenyl)amine) could be established by
spin trapping experiments using N-tert-butyl-α-phenylnitrone
(PBN). The splitting reaction was also monitored by UV−vis−
near-IR absorption spectroelectrochemistry, showing the
formation of the “byproduct” complex [(BOXAM)Ni(THF)]+
in THF solution from the radical cationic complexes
[(BOXAM)Ni(R)]•+. Furthermore, the character of the
undissociated complex radicals [(BOXAM)Ni(R)]•+ was
studied by EPR and UV−vis−near-IR spectroelectrochemistry,
revealing inter alia largely ligand-centered oxidation. DFT
calculations on ground and oxidized states confirm the largely
ligand-centered character of the radical complexes
[(BOXAM•)NiII(R)]•+. Furthermore, the DFT calculations
help to understand the conflicting observations that, while the
electrochemical oxidation potentials place the CF3 complex
F
dx.doi.org/10.1021/om300342r | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(2) Jones, G. D.; Martin, J. L.; McFarland, C.; Allen, O. R.; Hall, R.
E.; Haley, A. D.; Brandon, R. J.; Konovalova, T.; Desrochers, P. J.;
Pulay, P.; Vicic, D. A. J. Am. Chem. Soc. 2006, 128, 13175−13183.
(3) Jones, G. D.; McFarland, C.; Anderson, T. J.; Vicic, D. A. Chem.
Commun. 2005, 4211−4213.
alongside with the Cl derivative and the cationic THF complex
[(BOXAM)Ni(THF)]+, pointing to an electron-withdrawing
character of CF3, the long-wavelength absorption bands
(=HOMO−LUMO transition) of CF3 and CH3 are very
similar and are both far higher in energy, in comparison to
those for the Cl complex and the cationic THF complex. The
DFT calculated HOMO and LUMO energies are completely in
line with both findings and show that while the HOMO
energies (locus of the oxidation) are similar for CF3 and Cl, the
HOMO−LUMO gaps are similar for CH3 and CF3.
(4) Anderson, T. J.; Jones, G. D.; Vicic, D. A. J. Am. Chem. Soc. 2004,
126, 8100−8101; 2004, 126, 11113 (erratum).
(5) Gong, H.; Sinisi, R.; Gagne, M. R. J. Am. Chem. Soc. 2007, 129,
1908−1909.
(6) Terao, J.; Watanabe, H.; Ikumi, A.; Kuniyasu, H.; Kambe, N. J.
Am. Chem. Soc. 2002, 124, 4222−4223.
(7) Terao, J.; Todo, H.; Watanabe, H.; Ikumi, A.; Kambe, N. Angew.
Chem., Int. Ed. 2004, 43, 6180−6182.
EXPERIMENTAL SECTION
■
(8) Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V. Chem. Soc.
Rev. 2008, 37, 320−330.
General Considerations. All manipulations were performed using
standard Schlenk and high-vacuum techniques or in a nitrogen-filled
drybox, unless otherwise noted. Solvents were dried using a MBRAUN
MB SPS-800 solvent purification system. Electrochemical experiments
were carried out in 0.1 M nBu4NPF6 solutions using a three-electrode
configuration (glassy-carbon electrode, Pt counter electrode, Ag/AgCl
reference) and an Autolab PGSTAT30 potentiostat and function
generator. Data were processed using GPES 4.9 (General Electro-
chemical System Version 4.9). The ferrocene/ferrocenium couple
(9) Boehm, H.-J.; Banner, D.; Bendels, S.; Kansy, M.; Kuhn, B.;
Mueller, K.; Obst-Sander, U.; Stahl, M. ChemBioChem 2004, 5, 637−
643.
(10) Schlosser, M. Angew. Chem., Int. Ed. 2006, 45, 5432−5446.
(11) Zanda, M. New J. Chem. 2004, 28, 1401−1411.
(12) Jeschke, P. ChemBioChem 2004, 5, 570−589.
(13) Dubinina, G. G.; Furutachi, H.; Vicic, D. A. J. Am. Chem. Soc.
2008, 130, 8600−8601.
(14) Dubinina, G. G.; Ogikubo, J.; Vicic, D. A. Organometallics 2008,
27, 6233−6235.
(15) Cottet, F.; Schlosser, M. Eur. J. Org. Chem. 2002, 327−330.
(16) Urata, H.; Fuchikami, T. Tetrahedron Lett. 1991, 32, 91−94.
(17) Wiemers, D. M.; Burton, D. J. J. Am. Chem. Soc. 1986, 108,
832−834.
(18) (a) Kobayashi, Y.; Kumadaki, I. Tetrahedron Lett. 1969, 4095−
4096. (b) Kobayashi, Y.; Yamamoto, K.; Kumadaki, I. Tetrahedron Lett.
1979, 4071−4072.
(19) Molander, G. A.; Hoag, B. P. Organometallics 2003, 22, 3313−
3315.
+
(FeCp2/FeCp2 ) served as internal reference. Spectroelectrochemical
investigations (UV−vis−near-IR) were performed at ambient temper-
ature using an OTTLE (optical transparent electrochemical) cell
designed by J. Fiedler, Prague, Czech Republic.45 UV−vis−near-IR
absorption spectra were recorded using a Varian Cary50 Scan
photospectrometer. ESR spectra were recorded in the X-band on a
Bruker ELEXSYS 500E instrument equipped with a Bruker Variable
Temperature Unit ER 4131VT (500−100 K). g values were
determined using a dpph sample. Spectral simulation was performed
using Bruker SimFonia.
Quantum Chemical Calculations. All calculations were
performed using density functional theory as implemented in the
Turbomole6.346 package using the resolution of identity (RI)
approximation.47−49 The unconstrained geometry optimizations were
performed at the (RI-)BP86/SV(P) level. All minima were confirmed
as such by the absence of imaginary frequencies.
́
(20) (a) Bakhmutov, V. I.; Bozoglian, F.; Gomez, K.; Gonzalez, G.;
Grushin, V. V.; Macgregor, S. A.; Martin, E.; Miloserdov, F. M.;
Novikov, M. A.; Panetier, J. A.; Romashov, L. V. Organometallics 2012,
31, 1315−1328. (b) Grushin, V. V. Acc. Chem. Res. 2010, 43, 160−171.
(b) Grushin, V. V.; Marshall, W. J. J. Am. Chem. Soc. 2006, 128, 4632−
4641.
ASSOCIATED CONTENT
■
(21) See, for example: (a) Li, Y.; Chen, T.; Wang, H.; Zhang, R.; Jin,
K.; Wang, X.; Duan, C. Synlett 2011, 1713−1716. (b) Weng, Z.; Lee,
R.; Jia, W.; Yuan, Y.; Wang, W.; Feng, X.; Huang, K.-W.
Organometallics 2011, 30, 3229−3232. (c) Popov, I.; Lindeman, S.;
Daugulis, O. J. Am. Chem. Soc. 2011, 133, 9286−9289. (d) Knauber,
S
* Supporting Information
Figures, text, and a table giving cyclic voltammograms of
[(BOXAM)Ni(THF)](OAc), further EPR spectra, results from
DFT calculations, and UV−vis−near-IR absorption spectra
from spectroelectrochemical investigations. This material is
available free of charge via the Internet at http://pubs.acs.org.
T.; Arikan, F.; Roschenthaler, G.-V.; Gooßen, L. J. Chem. Eur. J. 2011,
̈
17, 2689−2697. (e) Oishi, M.; Kondo, H.; Amii, H. Chem. Commun.
2009, 1909−1911.
(22) Kieltsch, I.; Dubinina, G. G.; Hamacher, C.; Kaiser, A.; Torres-
Nieto, J.; Hutchinson, J. M.; Klein, A.; Budnikova, Y.; Vicic, D. A.
Organometallics 2010, 29, 1451−1456.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: axel.klein@uni-koeln.de (A.K.); vicic@hawaii.edu
(D.A.V.).
(23) Danilczuk, M.; Coms, F. D.; Schlick, S. FuelCells 2008, 8, 436−
452.
(24) Janzen, E. G.; Blackburn, B. J. J. Am. Chem. Soc. 1968, 90, 5909−
Notes
5910.
The authors declare no competing financial interest.
(25) Haire, D. L.; Oehler, U. M.; Krygsman, P. H.; Janzen, E. G. J.
Org. Chem. 1988, 53, 4535−4542.
(26) Gille, L.; Stoesser, R. J. Fluor. Chem. 1994, 69, 191−194.
(27) Davies, M. J.; Slater, T. F. Chem.-Biol. Interact. 1986, 58, 137−
147.
ACKNOWLEDGMENTS
■
C.H. and A.K. are grateful for support by the DFG (DFG KL
1194/4-1 and 5-1). D.A.V. thanks the Office of Basic Energy
Sciences of the U.S. Department of Energy (DE-FG02-
07ER15885) and the National Science Foundation (CHE-
0822523) for support of this work.
(28) (a) Eberson, L.; Persson, O. J. Chem. Soc., Perkin Trans. 2 1997,
893−898. (b) Eberson, L.; Persson, O. J. Chem. Soc., Perkin Trans. 2
1997, 1689−1696.
(29) Shiga, T.; Boukhors, A.; Douzou, P. J. Phys. Chem. 1967, 71,
4264−4273.
REFERENCES
(30) Dixon, W. T.; Norman, R. O. C. J. Chem. Soc. 1964, 4850−4856.
■
́ ́
(31) Dunbar, K. R.; Sun, J.-S.; Quillevere, A. Inorg. Chem. 1994, 33,
(1) Netherton, M. R.; Fu, G. C. Adv. Synth. Catal. 2004, 346, 1525−
1532.
3598−3601.
G
dx.doi.org/10.1021/om300342r | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(32) (a) Ottenwaelder, X.; Aukauloo, A.; Journaux, Y.; Carrasco, R.;
Cano, J.; Cervera, B.; Castro, I.; Curreli, S.; Munoz, M. C.; Rosello, A.
́
L.; Soto, B.; Ruiz-García, R. Dalton Trans. 2005, 2516−2526.
(b) Ottenwaelder, X.; Ruiz-Garcia, R.; Blondin, G.; Carasco, R.;
Cano, J.; Lexa, D.; Journaux, Y.; Aukauloo, A. Chem. Commun. 2004,
504−505.
(33) Collins, T. J.; Nichols, T. R.; Uffelman, E. S. J. Am. Chem. Soc.
1991, 113, 4708−4709.
(34) Kruger, H.-J.; Peng, G.; Holm, R. H. Inorg. Chem. 1991, 30,
̈
734−742.
(35) (a) Grove, D. M.; van Koten, G.; Mul, P.; Zoet, R.; van der
Linden, J. G. M.; Legters, J.; Schmitz, J. E. J.; Murrall, N. W.; Welch, A.
J. Inorg. Chem. 1988, 27, 2466−2473. (b) Kozhanov, K. A.; Bubnov,
M. P.; Cherkasov, V. K.; Fukin, G. K.; Vavilina, N. N.; Efremova, L.
Yu.; Abakumov, G. A. J. Magnet. Res. 2009, 197, 36−39.
(36) Klein, A.; Kaiser, A.; Sarkar, B.; Wanner, M.; Fiedler, J. Eur. J.
Inorg. Chem. 2007, 965−976.
́
́
(37) (a) Alonso, P. J.; Arauzo, A. B.; Garcıa-Monforte, M. A.; Martın,
A.; Menjon, B.; Rillo, C.; Tomas, M. Chem. Eur. J. 2009, 15, 11020−
́
́
11030. (b) Alonso, P. J.; Falvello, L. R.; Fornies, J.; Martin, A.;
Menjon, B.; Rodriguez, G. Chem. Commun. 1997, 503−504.
(38) Arora, H.; Philouze, C.; Jarjayes, O.; Thomas, F. Dalton Trans.
2010, 39, 10088−10098.
(39) (a) Rotthaus, O.; Labet, V.; Philouze, C.; Jarjayes, O.; Thomas,
F. Eur. J. Inorg. Chem. 2008, 4215−4224. (b) Rotthaus, O.; Jarjayes,
O.; Perez Del Valle, C.; Philouze, C.; Thomas, F. Chem. Commun.
2007, 4462−4464. (c) Rotthaus, O.; Jarjayes, O.; Thomas, F.;
Philouze, C.; Perez Del Valle, C.; Saint-Aman, E.; Pierre, J. L. Chem.
Eur. J. 2006, 12, 2293−2302. (d) Rotthaus, O.; Thomas, F.; Jarjayes,
O.; Philouze, C.; Saint-Aman, E.; Pierre, J. L. Chem. Eur. J. 2006, 12,
6953−6962.
(40) Shimazaki, Y.; Huth, S.; Karasawa, S.; Hirota, S.; Naruta, Y.;
Yamauchi, O. Inorg. Chem. 2004, 43, 7816−7822.
(41) (a) Xiao, Z.; Patrick, B. O.; Dolphin, D. Inorg. Chem. 2003, 42,
8125−8127. (b) Pinho, D.; Gomes, P.; Freire, C.; De Castro, B. Eur. J.
Inorg. Chem. 2001, 1483−1493. (c) Azevedo, F.; Carrondo, M. A.;
Castro, B.; Convery, M.; Domingues, D.; Freire, C.; Duarte, M. T.;
Nielsen, K.; Santos, I. C. Inorg. Chim. Acta 1994, 219, 43−54.
(d) Kruger, H.-J.; Holm, R. H. Inorg. Chem. 1987, 26, 3645−3647.
̈
(e) Lovecchio, F. V.; Gore, E. S.; Busch, D. H. J. Am. Chem. Soc. 1974,
96, 3109−3118. (f) Drago, R. S.; Baucom, E. I. Inorg. Chem. 1972, 11,
2064−2069.
(42) Madhira, V. N.; Ren, P.; Vechorkin, O.; Hu, X.; Vicic, D. A.
Dalton Trans. 2012, 41, 7915−7919.
(43) Algarra, A. G.; Grushin, V. V.; Macgregor, S. A. Organometallics
2012, 31, 1467−1476.
(44) Taw, F. L.; Clark, A. E.; Mueller, A. H.; Janicke, M. T.; Cantat,
T.; Scott, B. L.; Hay, P. J.; Hughes, R. P.; Kiplinger, J. L.
Organometallics 2012, 31, 1484−1499.
(45) (a) Kaim, W.; Fiedler, J. Chem. Soc. Rev. 2009, 38, 3373−3382.
(b) Krejcí
179−187.
̌ ̌
k, M.; Danek, M.; Hartl, F. J. Electroanal. Chem. 1991, 317,
(46) TURBOMOLE V6.3 2011, a development of University of
Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989−2007,
TURBOMOLE GmbH, since 2007; available from http://www.
turbomole.com.
(47) Ahlrichs, R.; Baer, M.; Haeser, M.; Horn, H.; Koelmel, C. Chem.
Phys. Lett. 1989, 162, 165−169.
(48) Deglmann, P.; Furche, F.; Ahlrichs, R. Chem. Phys. Lett. 2002,
362, 511−518.
(49) Weigend, F. Phys. Chem. Chem. Phys. 2006, 8, 1057−1065.
H
dx.doi.org/10.1021/om300342r | Organometallics XXXX, XXX, XXX−XXX