Influence of Electron-Withdrawing Substituents on the Electronic Structure of Oxidized Ni and Cu Salen Complexes

Article abstract of DOI:10.1021/acs.inorgchem.5b00783

Nickel (Ni(Sal^CF3)) and copper (Cu(Sal^CF3)) complexes of an electron-poor salen ligand were prepared, and their one-electron oxidized counterparts were studied using an array of spectroscopic and theoretical methods. The electrochemi

Full text of DOI:10.1021/acs.inorgchem.5b00783

Article
pubs.acs.org/IC
Influence of Electron-Withdrawing Substituents on the Electronic
Structure of Oxidized Ni and Cu Salen Complexes
Linus Chiang,† Khrystyna Herasymchuk,† Fabrice Thomas,§ and Tim Storr*,†
†Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
§Dep
́ ́ ́
artement de Chimie Moleculaire, Chimie Inorganique Redox (CIRE), UMR-5250, Universite Grenoble Alpes, BP 53, 38041
Grenoble Cedex 9, France
S
* Supporting Information
ABSTRACT: Nickel (Ni(SalCF3)) and copper (Cu(SalCF3)) complexes of an electron-poor salen ligand were prepared, and
their one-electron oxidized counterparts were studied using an array of spectroscopic and theoretical methods. The
electrochemistry of both complexes exhibited quasi-reversible redox processes at higher potentials in comparison to the M(SalR)
(R = tBu, OMe, NMe2) analogues, in line with the electron-withdrawing nature of the para-CF3 substituent. Chemical oxidation,
monitored by ultraviolet−visible−near-infrared (UV−vis−NIR) spectroscopy, afforded their corresponding one-electron
oxidized products. Ligand-based oxidation was observed for [Ni(SalCF3)]+•, as evidenced by sharp NIR transitions in the UV−
vis−NIR spectrum and a broad isotropic signal at g = 2.067 by solution electron paramagnetic resonance (EPR) spectroscopy.
Such sharp NIR transitions observed for [Ni(SalCF3)]+• are indicative of a delocalized electronic structure, which is in good
agreement with electrochemical measurements and density functional theory (DFT) calculations. In addition, the increased
Lewis acidity of [Ni(SalCF3)]+•, evident from the EPR g-value and DFT calculations, was further quantified by the binding affinity
of axial ligands to [Ni(SalCF3)]+•. For [Cu(SalCF3)]+, an intense ligand-to-metal charge transfer band at 18 700 cm−1 in the UV−
vis−NIR spectrum was observed, which is diagnostic for the formation of a CuIII species [J. Am. Chem. Soc., 2008, 130, 15448−
15459]. The CuIII character for [Cu(SalCF3)]+ is further confirmed by 19F NMR analysis. Taken together, these results show that
the electron-deficient salen ligand H2SalCF3 increases the Lewis acidity of the coordinating metal center.
abbreviation for N2O2 bis-Schiff-base bis-phenolate li-
gands).2b,12 Salen ligands have been extensively studied due
to their modular synthesis, ability to stabilize many metals in
different oxidation states, and versatility as catalysts.13 As an
example, one-electron oxidized NiII(Sal) derivatives exist in the
ligand radical form [NiII(Sal•)]+ in solution and the solid state,
where ligand electronic tuning via para-ring substituent
variation influences the degree of delocalization of the ligand
radical.12c The addition of exogenous ligands such as
dimethylformamide (DMF) or pyridine to [NiII(Sal•)]+ results
in the shifting of the oxidation locus to the octahedral
[NiIII(Sal)(D)2]+ form (D = axial donor ligand).12g,n,q,14
Recent work on CoII(Sal) systems has shown that the relative
donating abilities of the axial ligands dictates the formation of
[CoII(Sal•)(D)x]+ or [CoIII(Sal)(D)x]+ (x = 1 or 2).12s,15
In this work, we investigate the electronic structure of
oxidized nickel (Ni(SalCF3)) and copper (Cu(SalCF3))
1. INTRODUCTION
Transition metal complexes incorporating redox-active ligands
are of significant current interest,1 drawing inspiration from
enzymatic systems such as galactose oxidase2 and cytochrome
P450.3 Reports have documented the pro-radical nature of
ligands such as dioxolenes,4 dithiolenes,5 phenolates,2b,6 o-
phenylenediamines,7 amidophenolates,8 1,2-diimines,9 and
salens.1a This research has greatly improved our understanding
of the interaction between transition metal ions and pro-radical
ligands and has stimulated transition metal catalyst develop-
ment incorporating redox-active ligands.1,8b,c,9i,10
The relative ordering of the metal and ligand frontier orbitals
dictates whether a metal complex (Mn+L) will become a ligand-
radical complex ([Mn+L•]+) or a high-valent metal complex
([M(n+1)L]+) upon oxidation. Minor changes to the system
through solvent/temperature variations or addition of
exogenous ligands is sufficient in shifting the locus of oxidation
in certain cases.4,11 One area of recent focus by our group and
others is the redox activity and electronic structure of
tetradentate salen (Sal) ligands (salen is a common
Received: April 8, 2015
© XXXX American Chemical Society
A
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Inorganic Chemistry
Article
dry ice/acetone bath. Solvent contraction was accounted for in all
variable-temperature studies. Affinity constants were obtained by
refinement of the UV−vis titration data of the complexes with pyridine
in CH2Cl2. Data fitting was completed using SPECFIT software.
Cyclic voltammetry (CV) was performed on a PAR-263A potenti-
ometer, equipped with a Ag wire reference electrode, a platinum disk
complexes with electron-withdrawing CF3 substituents in the
para position (Chart 1) to determine if electron-withdrawing
Chart 1. Nickel (Ni(SalCF3)) and Copper (Cu(SalCF3))
Complexes
n
working electrode, and a Pt counter electrode with Bu4NClO4 (0.1
M) solutions in CH2Cl2. Decamethylferrocene was used as an internal
standard.25 1H, 13C, and 19F NMR spectra were recorded on a Bruker
AV-500 instrument. Mass spectra (electrospray ionization (ESI)
positive ion or ESI negative ion) were obtained on an Agilent 6210
TOF ESI-MS instrument. Elemental analyses (C, H, N) were
performed by Mr. P. Mulyk at Simon Fraser University on a Carlo
Erba EA1110 CHN elemental analyzer. All electron paramagnetic
resonance (EPR) spectra were collected using a Bruker EMXplus
spectrometer operating with a premiumX X-band (∼9.5 GHz)
microwave bridge. Low-temperature measurements of frozen solutions
used a Bruker helium temperature-control system and a continuous-
flow cryostat. Samples for X-band measurements were placed in 4 mm
outer-diameter sample tubes with sample volumes of ∼300 μL. Spectra
at 195 K were collected in capillary tubes, which were placed inside a
standard 4 mm EPR tube.
2.2. X-ray Structure Determination. Single crystal X-ray
crystallographic analysis of Ni(SalCF3) and Cu(SalCF3) was performed
on a Bruker X8 APEX II diffractometer with graphite monochromated
Mo Kα radiation. An orange block (Ni(SalCF3)) or a dark purple prism
(Cu(SalCF3)) crystal was mounted on a glass fibre. The data were
collected at 150 0.1 K to a maximum 2θ value of 55.0°. Data were
collected in a series of ϕ and ω in 0.50° widths with 10.0 s exposures.
The crystal-to-detector distance was 50 mm. The structure was solved
by direct methods (SIR92)26 and refined by least-squares procedures
using CRYSTALS (v14.40b)27 or ShelXle.28 All non-hydrogen atoms
were refined anisotropically. All C−H hydrogen atoms were placed in
calculated positions but were not refined. All crystal structure plots
were produced using ORTEP-3 and rendered with POV-Ray (v.3.6.2).
A summary of the crystal data and experimental parameters for
structure determinations are given in Table 1.
moieties promote metal-based versus ligand-based oxidation.
Because of the geometric preferences of Ni and Cu, the factors
governing the locus of oxidation differ. For example, while NiIII
d7 complexes are usually stabilized in an octahedral environ-
ment, a square-planar ligand geometry is much preferred for
CuIII complexes. Previous research focusing on square-planar
NiII(SalR) complexes found that the use of electron-donating
t
para-ring substituents (i.e. Bu, SR, OMe, NMe2) promotes
ligand radical formation at low oxidation potentials.12c,o
Interestingly, oxidation of four-coordinate square-planar NiII
complexes employing diamido-diphenolate ligands,16 o-phenyl-
enedioxamidates and related ligands,15c,17 and dipeptides18
afford NiIII species likely due to the tetra-anionic nature of the
ligands. Will dianionic M(SalCF3) complexes also support a NiIII
oxidation state in a square-planar ligand environment?
For Cu, the majority of CuIII complexes reported employ
anionic ligands such as carboxylates, thiolates, deprotonated
amides, carbamates, and N-confused porphyrins. Recent
interest in the stabilization of CuIII complexes arises from the
isolation of CuIII intermediates in organocopper chemistry19
and from the synthesis and reactivity of bis(μ-oxo)dicopper-
(III) complexes.20 Oxidation of the CuII(SalR) system employ-
2.3. Oxidation Protocol. Under an inert atmosphere at 195 K,
500 μL of a CH2Cl2 solution of the metal complex (4.6 mM) was
added to 3.0 mL of CH2Cl2. Monitored by UV−vis−NIR spectros-
copy, a saturated solution of N(C6H3Br2)3SbF6 in CH2Cl2 was added
in 60 μL aliquots resulting in clean conversion to the respective one-
electron oxidized species.
t
ing Bu para-ring substituents affords a CuIII complex in the
solid state, yet in solution a reversible spin-equilibrium exists
between the ligand radical species [CuII(Sal•)]+ and high-valent
metal form [CuIII(Sal)]+.12f Employing OMe para-ring
substituents, however, affords a ligand radical species upon
oxidation under all conditions investigated.12q These studies
highlight the effect of salen ligand electronics in dictating the
electronic structure of their oxidized forms. Herein, we describe
the electronic structure of oxidized M(SalCF3) (M = Ni, Cu)
using experimental and theoretical methods. Oxidized [Ni-
(SalCF3)]+• is demonstrated to exist as a delocalized ligand
radical with considerable metal ion participation in the
singularly occupied molecular orbital (SOMO), while oxidized
[Cu(SalCF3)]+ exists as a CuIII complex in solution and in the
solid state.
Table 1. Selected Crystallographic Data for Cu(SalCF3
)
complex
formula
Cu(SalCF3
)
C30H34F6N2O2Cu
632.14
formula weight
space group
a (Å)
P21/n
8.3007(7)
17.9066(14)
19.9819(16)
90
b (Å)
c (Å)
α (deg)
β (deg)
98.9040(15)
90
γ (deg)
V [Å3]
Z, Dcalc [g/cm3]
2934.3(4)
4
2. EXPERIMENTAL SECTION
T (K)
150
ρcalcd (g cm−3
λ (Å)
)
1.431
2.1. Materials and Methods. All chemicals used were of the
highest grade available and were further purified whenever necessary.21
2-tert-butyl-4-trifluoromethyl phenol was prepared from commercially
available 4-trifluoromethyl phenol by reported procedures.22 The
tris(2,4-dibromophenyl)aminium hexafluoroantimonate radical chem-
ical oxidant N(C6H3Br2)3SbF6 (E1/2 = 1.14 V, MeCN)23 was
synthesized according to published protocols.14,24 Electronic spectra
were obtained on a Cary 5000 spectrophotometer with a custom-
designed immersion fiber-optic probe with variable path length (1 and
10 mm; Hellma, Inc.). Constant temperatures were maintained by a
0.710 73
0.812
μ (cm−1
)
a
R indices with I > 2σ(I) (data)
5351
wR2
0.1626
R1
0.0440
goodness-of-fit on F2
1.509
a
Goodness-of-fit on F.
B
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2.4. X-ray Photoelectron Spectroscopy. Solid samples of the
oxidized species were prepared under an inert atmosphere. The
oxidized samples were prepared by dissolving the neutral compounds
in CH2Cl2, then cooled to 233 K, when N(C6H3Br2)3SbF6 was added
in one portion. Solvent was immediately evacuated, where the color
between the solution and resultant solid was maintained. X-ray
photoelectron spectra were obtained using a Kratos Analytical Axis
ULTRA spectrometer containing a DLD detector. The solid samples
were loaded onto the carbon tape under inert atmosphere, and the
auto-z correction was done using F(1s) binding energy. The X-ray
excitation source was at 15 kV and 10 mA. All spectra were referenced
to the C(1s) peak (284.2 eV).
2.5. Calculations. Geometry optimizations were performed using
the Gaussian 09 program (Revision D.01),29 the B3LYP30 functional
with a polarized continuum model (PCM) for CH2Cl2 (dielectric ε =
8.94),31 and the 6-31G(d) basis set on all atoms. This combination of
functional and basis set has been used previously for structurally
similar salen complexes, providing good matches to experimental
metrical parameters.12b,d,e A symmetric structure was used as a starting
point for all geometry optimizations. Frequency calculations at the
same level of theory confirmed that the optimized structures were
located at a minimum on the potential energy surface. Single point
calculations for the Ni complexes were performed using the B3LYP30
functional with a PCM for CH2Cl2,31 and the TZVP basis set of
Ahlrichs on all atoms.32 The intensities of the 30 lowest-energy
electronic transitions for [Ni(SalCF3)]+• were calculated by time-
dependent density functional theory (TD-DFT)33 at the B3LYP/
TZVP level with a PCM for CH2Cl2. The above calculations were also
completed using the CAM-B3LYP34 functional for comparison. Single
point calculations for energetic analysis of the Cu complexes were
performed using the BLYP35 functional with a PCM for CH2Cl2,31 and
the TZVP basis set of Ahlrichs on all atoms.32 AOMix36 was used for
determining atomic orbital compositions employing Mulliken
Population Analysis.
4H2O (22 mg, 0.09 mmol) in MeOH (2 mL). NEt3 (25 μL, 0.18
mmol) was added, and the resulting solution was stirred at room
temperature overnight, during which time an orange precipitate
formed, which was collected by filtration and washed with cold
methanol. The crude material was recrystallized in 1:1 CH2Cl2/MeOH
to afford orange crystals of Ni(SalCF3). Yield: 22 mg (40%). 19F NMR
(CDCl3, 560 MHz): δ = −62.5. MS (ESI): m/z (%): 627.20 (100)
[Ni(SalCF3) + H]+. Anal. Cald (%) for C30H34N2O2F6Ni: C 57.44, H
5.46, N 4.47; Found (%): C 57.82, H 5.19, N 4.54.
2.6.4. (N,N′-Bis(3-tert-butyl-5-trifluoromethylsalicylidene))-trans-
1,2-cyclohexanediamine Copper(II) (Cu(SalCF3)). To a solution of
H2(SalCF3) (100 mg, 0.18 mmol) in MeOH (2 mL) was added
Cu(OAc)2·H2O (35 mg, 0.18 mmol) in MeOH (2 mL). The resulting
solution was stirred at room temperature overnight, during which time
a dark purple precipitate formed, which was collected by filtration and
washed with cold methanol. The crude material was recrystallized in
1:1 CH2Cl2/MeOH to afford dark purple crystals. Yield: 60 mg (54%).
19F NMR (CDCl3, 560 MHz): δ = −64.4. MS (ESI): m/z (%): 632.19
(100) [Cu(SalCF3) + H]+. Anal. Cald (%) for C30H34N2O2F6Cu: C
57.00, H 5.42, N 4.43; Found (%): C 57.09, H 5.39, N 4.64.
3. RESULTS AND ANALYSIS
3.1. Synthesis and Solid State Characterization of
Ligands and Complexes. The ligand H2SalCF3 was
synthesized by condensation of trans-1,2-cyclohexanediamine
in the presence of 2 equiv of 3-tert-butyl-5-trifluoromethyl-2-
hydroxybenzaldehyde, which was prepared by the tert-
butylation of the commercially available 4-trifluoromethylphe-
nol, followed by a Duff formylation reaction. Ni(SalCF3) and
Cu(SalCF3) were synthesized by reacting H2(SalCF3) with the
corresponding metal acetate salts (Ni(OAc)2·4H2O and
Cu(OAc)2·4H2O) under aerobic conditions (Scheme 1).
2.6. Synthesis. 2.6.1. 3-tert-Butyl-5-trifluoromethylsalicyl-alde-
hyde (1). To a solution of 2-tert-butyl-4-trifluoromethyl phenol (1.5 g,
6.9 mmol) in trifluoroacetic acid (30 mL) was added hexamethyle-
netetramine (1.06 g, 7.6 mmol). The reaction mixture was stirred at
reflux for 16 h; then it was cooled to room temperature, and water (30
mL) was added. This solution was cooled and extracted with CH2Cl2
(3 × 50 mL), the organic phase was dried over Na2SO4, and the filtrate
was concentrated in vacuo. The crude product was subject to flash
column chromatography using 4:1 CHCl3/hexanes as the eluent to
afford a pale yellow oil as 3-tert-butyl-5-trifluoromethylsalicylaldehyde.
a
Scheme 1. Synthetic Scheme for H2(SalCF3) and M(SalCF3
)
1
Yield: 430 mg, 25%. H NMR (CDCl3, 400 MHz): δ = 12.11 (s, 1H,
OH), 9.92 (s, 1H, CHO), 7.71−7.70 (m, 2H, Ar−H), 1.44 (s, 9H,
tBu); 13C NMR (CDCl3, 100 MHz): δ = 196.7, 163.7, 140.0, 130.5
a
t
(3J(C,F) = 3 Hz), 129.4 (3JC−F = 4 Hz), 124.0 (1J(C,F) = 271 Hz),
121.8 (2J(C,F) = 33 Hz), 119.9, 35.3, 29.1. MS (ESI negative mode):
m/z (%): 245.08 (100) [1+H]+.
(i) 9:1 BuOH/MeOH, H2SO4, 74%; (ii) hexamethylenetetramine,
CF3COOH, 25%; (iii) 0.5 equiv trans-1,2-cyclohexanediamine,
MeOH, 93%; (iv) M(OAc)2, MeOH, Ni: 40%, Cu: 54%.
2.6.2. (N,N′-Bis(3-tert-butyl-5-trifluoromethylsalicylidene))-trans-
1,2-cyclohexanediamine (H2(SalCF3)). To a solution of trans-1,2-
cyclohexanediamine (71 mg, 0.6 mmol) dissolved in MeOH (3 mL)
was added 3-tert-butyl-5-trifluoromethyl-2-hydroxybenzaldehyde (305
mg, 1.2 mmol) in MeOH (3 mL). The reaction mixture was stirred at
room temperature for 16 h, during which time a light yellow
precipitate formed. The solid was filtered and washed with cold
NEt3 (2 equiv) was added to all metallation reactions. X-ray
quality crystals of Ni(SalCF3) and Cu(SalCF3) were grown by
slow diffusion of MeOH into a concentrated CH2Cl2 solution
of the compounds.
3.2. X-ray Analysis of Ni(SalCF3) and Cu(SalCF3). The
molecular structures of Ni(SalCF3) and Cu(SalCF3) are
presented in Supporting Information, Figure S1 and Figure 1,
respectively, and select crystallographic data for Cu(SalCF3) are
shown in Table 1. A high R value (11.1%) for the X-ray data of
Ni(SalCF3) arises from a second disordered molecule of
Ni(SalCF3) in the unit cell, and thus bond lengths could not
be accurately determined. The solid-state structure for the two
compounds exhibits a slightly distorted square-planar geometry
with the expected N2O2 coordination sphere from the ligand,
with the distortion likely due to the sterically demanding
ortho-tBu substituents. The C−O bond length of the phenolate
is often used to evaluate the oxidation state of the ligand.
1
MeOH. Yield: 330 mg (93%). H NMR (CDCl3): δ = 8.30 (s, 2H,
NCH), 7.44−7.45 (d, 2H, Ar−H, J = 2.0 Hz), 7.25−7.26 (d, 2H, Ar−
H, J = 2.0 Hz), 3.35−3.42 (m, 2H, CH), 2.02−2.05 (m, 2H, CH2),
1.88−1.96 (m, 2H, CH2), 1.74−1.82 (m, 2H, CH2), 1.45−1.55 (m,
t
2H, CH2), 1.40 (s, 18H, Bu). 13C NMR (CDCl3, 100 MHz): δ =
165.1, 163.2, 138.7, 127.0, (3JC−F: 4 Hz), 126.2 (3JC−F: 3 Hz), 124.5
2
(CF3, JC−F: 271 Hz), 120.0, 117.8, 72.3, 35.1, 32.9, 29.1, 24.3; 19F
NMR (CDCl3, 560 MHz): δ = −62.8. MS (ESI): m/z (%): 571.27
(100) [H2(SalCF3) + H]+. Anal. Cald (%) for C30H36N2O2F6: C 63.15,
H 6.36, N 4.91; Found (%): C 63.50, H 6.53, N 5.12.
2.6.3. (N,N′-Bis(3-tert-butyl-5-trifluoromethylsalicylidene))-trans-
1,2-cyclohexanediamine Nickel(II) (Ni(SalCF3)). To a solution of
H2(SalCF3) (50 mg, 0.09 mmol) in Et2O (2 mL) was added Ni(OAc)2·
C
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The redox potentials for Ni(SalR) versus Fc+/Fc are in line
with the electron-donating abilities of the para-ring sub-
stituents,12c which is reflected in the plot between first
oxidation potential against Hammett constants (σpara).39
A
linear correlation (R2 = 0.974) demonstrates that the oxidation
potential is predominantly affected by the relative donating
ability of the para-ring substituents (Figure 2 inset). Both the
first and second redox processes for Ni(SalR) vary by ca. 1 V in
the series (Table 2). In addition, the difference between the
first and second oxidation potentials (ΔE1/2) and compropor-
tionation constant (Kc) of the one-electron oxidized complexes,
calculated using eqs 1−3, are also reported in Table 2, which
provides insight regarding the degree of electronic coupling
between the two redox-active phenolates in [Ni(SalCF3)]+•.40
Both ΔE1/2 and Kc values for [Ni(SalCF3)]+• suggest a
delocalized ligand-radical electronic structure, which is further
supported by theoretical analysis (vide supra).
Figure 1. ORTEP plot of Cu(SalCF3) (50% probability) using POV-
Ray, excluding hydrogen atoms. Selected interatomic distances [Å]
and angles [deg]: Cu(1)−N(1): 1.938, Cu(1)−N(2): 1.939, Cu(1)−
O(1): 1.901, Cu(1)−O(2): 1.895, C(1)−O(1): 1.295, C(7)−O(2):
1.297; N(1)−Cu(1)−N(2): 84.2, N(2)−Cu(1)−O(2): 93.2, O(1)−
Cu(1)−O(2): 90.6, N(1)−Cu(1)−O(1): 92.7, N(1)−Cu(1)−O(2):
173.4, O(1)−Cu(1)−N(2): 172.5.
••
•
[ML] + [ML ] ↔ 2[ML]
(1)
(2)
(3)
• 2
[ML]
Kc =
••
[ML][ML ]
Cu(SalCF3) displayed C−O bond distances consistent with a
phenolate moiety, in line with other Cu(SalR) complexes,12q,37
indicating the dianionic nature of the complex. Overall, the
coordination sphere bond distances for Cu(SalCF3) were found
to be slightly longer in comparison to other reported Cu(SalR)
derivatives, suggesting phenolates bearing the CF3 moiety have
lower donating ability in comparison to analogues with a
para-tBu group. This is further illustrated by the shorter
coordination bond distances for salen complexes with an
electron-rich OMe para-ring substituent, in comparison to the
tBu analogue.12q
⎛
⎜
⎝
⎞
⎟
⎠
ΔEoxF
RT
Kc = exp
Two quasi-reversible one-electron oxidation waves were
observed for Cu(SalCF3) (Table 3, Supporting Information,
Figure S2). The redox processes for Cu(SalCF3) occur at
considerably higher potentials in comparison to Cu(SaltBu) and
Cu(SalOMe),12q,37 as expected based on the trends observed for
Ni(SalR).12c Interestingly, it has been reported that the one-
electron oxidation of Cu(SaltBu) affords a CuIII species in the
solid state, which is in equilibrium with a CuII-phenoxyl radical
species in solution.12f The locus of oxidation is strongly
dependent on temperature, highlighting the similarity in energy
between the two electronic states. Conversely, the one-electron
3.3. Electrochemistry. Redox processes for M(SalCF3
)
were probed by cyclic voltammetry (CV) in CH2Cl2 by using
tetra-n-butylammonium perchlorate (nBu4NClO4) as the
supporting electrolyte. Two quasi-reversible one-electron
oxidation waves are observed for Ni(SalCF3) as previously
observed for other Ni(SalR) complexes (Figure 2, Table
2).12c,38
oxidation of the more electron-rich OMe derivative Cu(SalOMe
)
results in a CuII-phenoxyl radical species at all temperatures.12q
Thus, the electron-withdrawing properties of the CF3 moiety is
expected to lower the energy of the ligand-based highest
occupied molecular orbital, thereby stabilizing the formation of
a CuIII species (vide infra). In addition, an irreversible reduction
wave was observed at −2.06 V in the CV spectrum; similar
reduction processes have been observed for other CuII−Schiff
base complexes indicating reduction to their respective CuI
forms.41 Similar to the oxidation processes, the para-ring
substituents in such salen systems can also tune the CuII/CuI
reduction potential.41c
3.4. Electronic Spectroscopy. 3.4.1. Chemical Oxidation
of Ni(SalCF3). The electronic absorption spectrum of Ni(SalCF3
)
is typical of low-spin d8 square-planar NiII bis-phenolate salen
complexes (Figure 3).12c,q While no absorption was observed at
energies lower than 20 000 cm−1 for the neutral form, two
intense NIR transitions were observed upon oxidation. These
sharp and intense NIR bands for [Ni(SalCF3)]+• are observed at
energies similar to those of [Ni(SaltBu)]+• (tBu: ν = 4700 cm−1,
ε = 21 600 M−1 cm−1, ν = 9100 cm−1, ε = 7200 M−1 cm−1; CF3:
ν = 4900 cm−1, ε = 16 200 M−1 cm−1, ν = 8600 cm−1, ε =
13 100 M−1 cm−1). Indeed, the band shapes for both
[Ni(SaltBu)]+• and [Ni(SalCF3)]+• are quite similar, suggesting
that both oxidized species are Class III delocalized systems by
the Robin−Day classification.42 While the stability of the
oxidized species precluded its isolation for further crystallo-
Figure 2. Cyclic voltammogram of Ni(SalCF3). (inset) Hammett plot
of the oxidation potentials of Ni(SalR) vs σpara of the para-ring
n
substituents. Conditions: 1.5 mM complex, 0.1 M Bu4NClO4, scan
rate 100 mV s−1, CH2Cl2, 298 K.
D
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a
Table 2. Redox Potentials of Ni(SalR) Versus Fc+/Fc
1
2
2
compound
E1/2 (V)
E1/2 (V)
ΔEox (E1/2 − E1/21) (V)
Kc
b
Ni(SalNMe2
)
b
−0.24 (0.14)
0.22 (0.14)
0.37 (0.14)
0.78 (0.13)
−0.08 (0.14)
0.59 (0.14)
0.85 (0.14)
1.19 (0.17)
0.16
0.37
0.48
2.9 × 103
1.0 × 108
2.4 × 1010
1.7 × 108
Ni(SalOMe
)
b
Ni(SaltBu
Ni(SalCF3
)
c
b
)
0.38
a
Peak-to-peak differences in brackets (|Epa − Epc| in V). Peak-to-peak difference for the Fc+/Fc couple at 233 K is ca. 0.13 V, while at 298 K it is ca.
b
c
0.12 V. CV performed at 233 K, ref 12c. CV performed at 298 K.
Table 3. Redox Potentials of Cu(SalR) Complexes Versus
Figures S3 and S4), and the resultant data were fit according to
eq 4.
a
Fc+/Fc
[[NiIII(SalR)(py)2]+]
1
2
compound
E1/2 (V)
0.28
E1/2 (V)
0.44
log β2 =
b
Cu(SalOMe
)
[[Ni(SalR)]+•][py]2
(4)
c
Cu(SaltBu
)
0.45
0.65
At 198 K, [Ni(SaltBu)]+• and [Ni(SalCF3)]+• exhibited log β2
values of 7.7 0.2 M−2 and 12.4 1.1 M−2, respectively. The
higher binding constant for [Ni(SalCF3)]+• reflects the expected
increase in electrophilicity of the Ni center chelated by an
electron poor ligand. The formation of [Ni(SalCF3)]+• and
[NiIII(SalCF3)(py)2]+ is further evidenced by EPR spectroscopy
(vide infra).
d
Cu(SalCF3
)
0.74 (0.16)
1.28 (0.16)
a
Peak-to-peak differences in brackets (|Epa − Epc| in V). Peak-to-peak
difference for the Fc+/Fc couple at 233 K is ca. 0.13 V, while at 298 K
it is ca. 0.12 V. CV performed at 233 K, ref 12q. CV performed at
b
c
d
233 K, ref 37. CV performed at 298 K.
3.4.3. Chemical Oxidation of Cu(SalCF3). The electronic
absorption spectrum of Cu(SalCF3) is typical of low-spin d9
square-planar CuII bis(phenolate) salen complexes, with an
intense charge transfer transition at 28 000 cm−1 (ε = 12 500
M−1 cm−1) and a weak d−d transition at 17 600 cm−1 (ε = 600
M−1 cm−1; Figure 4).12f,q The oxidation of Cu(SalCF3) leads to
Figure 3. Electronic spectra of Ni(SalCF3) (black), [Ni(SalCF3)]+•
(red), and [NiIII(SalCF3)(py)2]+ (blue). Intermediate gray lines
measured during the oxidation titration with N(C6H3Br2)3SbF6 in
CH2Cl2 at 198 K. Green bars represent transitions predicted by TD-
DFT.
graphic characterization (t1/2 (298 K) ca. 80 min), the
delocalized electronic structure is supported by EPR data and
density functional theory (DFT) calculations (vide infra).
Interestingly, while [Ni(SaltBu)]+• and [Ni(SalCF3)]+• both
exhibit delocalized electronic structures, the relative intensity of
their respective NIR features differ, and this is further explored
by TD-DFT calculations (vide infra).
Figure 4. Electronic spectra of Cu(SalCF3) (black) and [Cu(SalCF3)]+
(red) in CH2Cl2 at 198 K. Intermediate gray lines measured during the
oxidation titration with N(C6H3Br2)3SbF6.
the appearance of two new bands at 18 700 cm−1 (ε = 17 300
M−1 cm−1) and 6300 cm−1 (ε = 1400 M−1 cm−1), which are in
line with a previous report.12f The emergence of the intense
band at ca. 18 000 cm−1 has been linked to the formation of a
CuIII species (LMCT transition), as observed for [Cu(SaltBu)]+
at low temperature (ε = 14 000 M−1 cm−1).12f In comparison,
no such band was observed for [Cu(SalOMe)]+, in which the
formation of a CuII-phenoxyl radical species is favored due to
the electron-donating ability of the para-OMe moiety.12q In
addition, the intensity of the band at ca. 18 000 cm−1 for
[Cu(SaltBu)]+ is reduced by 50% upon warming from 198 to
298 K, signifying a reversible temperature-dependent equili-
3.4.2. Pyridine Binding to [Ni(SalCF3)]+•. The addition of 30
equiv of pyridine to [Ni(SalCF3)]+• in solution results in the
loss of the NIR transitions and emergence of a new transition at
20 300 cm−1 (Blue spectrum, Figure 3). This spectral pattern
has been previously observed for the axial binding of two
pyridines to [Ni(SaltBu)]+• to form [NiIII(SaltBu)(py)2]+, with
an accompanying shift in the locus of oxidation from the ligand
to metal.12g Binding affinities of pyridine to [Ni(SalR)]+• (R =
tBu, CF3) were determined by incremental addition of pyridine
to a solution of the oxidized species (Supporting Information,
E
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brium between a CuIII species at low temperature and a CuII-
phenoxyl radical species at room temperature.12f This effect
could not be probed for [Cu(SalCF3)]+ above 233 K due to the
thermal instability at 273 K, as evident by the irreversible
decrease in the LMCT transition with warming (Supporting
Information, Figure S5).
3.5. Continuous Wave X-band Electron Paramagnetic
Resonance. The X-band EPR spectrum of [Ni(SalCF3)]+•
collected at 195 K in CH2Cl2 showed a broad isotropic signal at
the sample was frozen, a rhombic spectrum consisting of two
components with gav of ca. 2.18 was observed (20 K), indicating
the formation of two distinct NiIII species (Figure 5b). This
likely arises from the axial ligation of a donor species (D, most
likely adventitious H2O from sample preparation) in solution
upon freezing, as the gav suggests a shift in the locus of
-
oxidation from ligand to metal to generate a [NiIII(SalCF3
)
(D)x]+ (x = 1 or 2) species. In contrast, the EPR spectrum of
[Ni(SaltBu)]+• at 20 K, prepared under the same conditions,
exhibits a ligand radical g-value (g1 = 2.063, g2 = 2.013, g3 =
1.988, gav = 2.021), indicating that [Ni(SaltBu)]+• maintains
ligand radical character upon freezing.38 As expected, the
addition of 30 equiv of pyridine and subsequent freezing results
in a rhombic spectrum, which was observed previously in the
formation of [NiIII(SaltBu)(py)2]+ (Figure 5c).12g In addition,
hyperfine coupling to the two nitrogen nuclei from the pyridine
moieties was observed. The shift in g-value between frozen
[Ni(SalCF3)]+• and [NiIII(SalCF3)(py)2]+ suggests that it is
sensitive to the identity of the axial ligand. This was observed
previously for a DMF adduct of [Ni(SaltBu)]+•.12n
g
iso = 2.067 (Figure 5a and Table 4). A minor signal at g = 2.003
The X-band EPR spectrum of Cu(SalCF3) collected at 20 K
exhibited features consistent with a square-planar d9 CuII center
as observed for other Cu(SalR) complexes (Figure 6, simulation
Figure 5. X-band EPR spectra for: (a) [Ni(SalCF3)]+• at 195 K; (b)
[NiIII(SalCF3)(D)x]+ (x = 1 or 2) at 20 K and (c) [NiIII(SalCF3)(py)2]+
at 20 K in CH2Cl2. Experimental spectrum: black. Overall simulations:
red. Individual species: blue dotted. Conditions: Frequency: (a) 9.386
GHz, (b) 9.383 GHz, and (c) 9.380 GHz; Power: 2.0 mW;
modulation frequency: 100 kHz; amplitude 0.6 mT. See Experimental
Section for details.
Figure 6. X-band EPR spectra for concentration matched samples of:
(a) Cu(SalCF3) and (b) [Cu(SalCF3)]+ in CH2Cl2 at 20 K.
Experimental spectrum: black. Simulation: red. Conditions: Fre-
quency: (a) 9.383 GHz and (b) 9.382 GHz; Power: 2.0 mW;
modulation frequency: 100 kHz; amplitude 0.6 mT.
Table 4. X-Band EPR Simulation Parameters for
[Ni(SalCF3)]+• Complexes
a
compound
gx
gy
gz
gav
b
[Ni(SalCF3)]+•, 195 K
2.067
2.192
2.182
2.135
c
c
[Ni(SalCF3)]+•, 20 K, S1
[Ni(SalCF3)]+•, 20 K, S2
2.328
2.291
2.209
2.247
2.234
2.171
2.001
2.020
2.025
parameters: g∥ = 2.193, g⊥ = 2.046, ACu∥ = 575, AN∥ = 85, ACu⊥
= 30, AN⊥ = 40, A values in MHz).12q,37 Oxidation of
Cu(SalCF3) to [Cu(SalCF3)]+ results in a substantial decrease in
the EPR signal to <10% of the original intensity by spin
integration, which supports the formation of a CuIII species as
suggested by the UV−vis experiment. The weak EPR signal of
[Cu(SalCF3)]+ may arise from its thermal decomposition during
sample preparation or from remaining unoxidized Cu(SalCF3).
3.6. 19F Nuclear Magnetic Resonance. Complementing
the electrochemical measurements, UV−vis−NIR spectra, and
EPR data, 19F NMR was used as an additional tool for
evaluating the electronic structure of Ni(SalCF3), Cu(SalCF3),
and the one-electron oxidized forms. Referenced to CFCl3,
sharp peaks were observed for the ligands H2(SalCF3) and
c
[NiIII(SalCF3)(py)2]+, 20 K
a
b
S1 = species 1; S2 = species 2. Collected in a capillary tube.
c
Collected in a 4 mm outer diameter EPR tube.
is preliminarily assigned as an organic decomposition product,
accounting for 3% of the overall signal by spin integration.
Because of solvent effects, collection of this spectrum was
performed in a capillary tube with a much smaller sample
volume, resulting in decreased signal strength. The observed g-
value suggests increased metal contribution to the SOMO
when compared to other [Ni(SalR)]+• complexes,12c which is
further corroborated with DFT calculations (vide infra). When
F
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Ni(SalCF3) at −62.8 ppm and −62.5 ppm, respectively (Figure
7a,b, Supporting Information, Figure S6, Table 5). These
product. Note that the signal for the SbF6 counterion could
not be detected due to the quadrupolar moment of the Sb
center.44
−
3.7. X-ray Photoelectron Spectroscopy. Ni and Cu XPS
was used to evaluate the metal oxidation states in both the
neutral and oxidized complexes in the solid state. Referenced to
the C 1s binding energy, the metal 2p3/2 and 2p1/2 binding
energies (Table 6) for both neutral complexes indicate a
Table 6. Metal (2p) Binding Energies Versus C(1s) (284.8
eV)
binding energy (eV)
compound
Ni(SalCF3
[Ni(SalCF3)]+•
Cu(SalCF3
[Cu(SalCF3)]+
2p3/2
2p1/2
)
872.1
872.1
954.0
955.9
854.8
854.9
934.1
935.9
)
common 2+ oxidation state, as previously observed for
structurally similar Ni45 and Cu12f complexes. Upon oxidation,
the Ni 2p3/2 and 2p1/2 binding energies for [Ni(SalCF3)]+•
experience a small shift toward higher energies (Supporting
Information, Figure S10 and Table 6), supporting the
formation of a NiII-phenoxyl rather than a NiIII-phenolate
species in the solid state. Fluorine KLL peaks (Auger
transition) at 856.8 and 876.2 eV are also visible in both the
oxidized Ni and Cu complexes (Supporting Information, Figure
S11).46 However, a larger shift in binding energy (ca. 1.8 eV,
Supporting Information, Figure S12) was observed from
oxidation of Cu(SalCF3) to [Cu(SalCF3)]+. This indicates a
change in Cu oxidation state from 2+ to 3+, consistent with a
previous report on [Cu(SaltBu)]+, which showed a shift in
binding energy of ca. 1.8 eV in comparison to its neutral form.
3.8. Theoretical Analysis. 3.8.1. Ni(SalCF3). A symmetric
structure was predicted for Ni(SalCF3) using the B3LYP
functional with a PCM for CH2Cl2 (Supporting Information,
Table S1). A symmetric structure was also predicted for
[Ni(SalCF3)]+•, with a contraction in the coordination sphere in
comparison to Ni(SalCF3) (Table S1). A delocalized electronic
structure, in which the ligand radical is distributed over both
aromatic phenolates, was predicted for [Ni(SalCF3)]+• (Figure
8), in agreement with the sharp and intense NIR bands
observed in the UV−vis−NIR spectrum (Figure 3). The
predicted spin density on the nickel center for [Ni(SalCF3)]+•
(SDNi: 34%), and thus metal contribution to the SOMO, is
higher in comparison to other [Ni(SalR)]+• complexes
([Ni(SaltBu)]+•: SDNi: 22%; [Ni(SalOMe)]+•: SDNi: 12%;
[Ni(SalNMe2)]+•: SDNi: 5%). This observation follows the
Figure 7. 19F NMR spectra of (a) H2(SalCF3), (b) Ni(SalCF3), (c)
Cu(SalCF3), and (d) [Cu(SalCF3)]+ recorded in CH2Cl2 at 298 K.
Table 5. 19F NMR Chemical Shifts
a
compound
H2(SalCF3
Ni(SalCF3
[Ni(SalCF3)]+•
Cu(SalCF3
[Cu(SalCF3)]+
chemical Shift (ppm)
W1/2 (Hz)
)
−62.8
−62.5
3
3
)
)
−64.4
−63.2
−64.0
135
35
3
a
Chemical shift relative to CFCl3.
chemical shifts are in agreement with other diamagnetic
phenol-containing compounds bearing a CF3 moiety.22,43
Peak widths at half height (W1/2) for both H2(SalCF3) and
Ni(SalCF3) were 3 Hz, as expected for a diamagnetic compound
(Table 5). On the other hand, Cu(SalCF3) exhibited a broad
signal at −64.4 ppm with W1/2 = 135 Hz due to the
paramagnetic CuII center (Figure 7c). The 19F peak for
Ni(SalCF3) disappears upon oxidation to [Ni(SalCF3)]+•,
consistent with the formation of a ligand radical in close
proximity to the CF3 moieties (Supporting Information, Figure
S7). In contrast, loss of the broad 19F signal of Cu(SalCF3) was
accompanied by the appearance of two signals upon oxidation
to [Cu(SalCF3)]+ (Figure 7d), namely, a major signal at −63.2
ppm (W1/2 = 35 Hz) and a minor signal at −64.0 ppm (W1/2
=
3 Hz). The minor signal is attributed to a decomposition
product from the oxidation reaction and increases over time
(Table 5, Supporting Information, Figure S9).
The decomposition is consistent with the limited stability of
[Cu(SalCF3)]+ in solution (t1/2 = 150 min at 298 K). The
relatively sharp 19F peak for [Cu(SalCF3)]+ is assigned to a CuIII
species (Figure 7d); the broadness in the signal is likely due to
fast electron exchange with a paramagnetic decomposition
Figure 8. Spin density plot of [Ni(SalCF3)]+• (SDNi = 34%). See
Experimental Section for calculation details.
G
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trends established previously for other [Ni(SalR)]+• complexes,
where electron-donating para-ring substituents decrease the
metal contribution to the SOMO,12c,47 and is in agreement with
the increased g-value in the EPR spectrum of [Ni(SalCF3)]+•
(Figure 5). Using CAM-B3LYP as the functional with no
solvent model also predicted a symmetric structure for
[Ni(SalCF3)]+•, albeit with slightly different SDNi values.47
TD-DFT calculations33 were undertaken to gain insight into
the spectral features of [Ni(SalCF3)]+•. Two NIR transitions
were predicted for [Ni(SalCF3)]+• (Figure 3), and the natural
transition orbitals (NTOs)48 contributing to the transitions are
shown in Table 7.
The S = 0 solution for [Cu(SalCF3)]+ was predicted to be
lowest in energy by 3.3 kcal/mol, which is in agreement with
the experimental data. This is consistent with [Cu(SalCF3)]+
being a CuIII-salen species, in comparison to [Cu(SaltBu)]+
where an equilibrium between CuIII-salen and a CuII-phenoxyl
radical species was observed.
4. DISCUSSION AND SUMMARY
Chemical oxidation of Ni(SalCF3) monitored by UV−vis−NIR
showed the formation of a delocalized ligand-radical species,
evidenced by the appearance of sharp and intense NIR
transitions (Figure 3). This is supported by a negligible shift
in metal binding energy by XPS as well as the disappearance of
the 19F NMR signal upon oxidation. An isotropic signal (g =
2.067) indicative of a ligand radical was observed in the
solution-state EPR of [Ni(SalCF3)]+• , and this g-value is higher
in comparison to those of other [Ni(SalR)]+•, consistent with
an increase in metal character for the SOMO of [Ni(SalCF3)]+•
due to the electron-withdrawing CF3 substituents (Figure 5a).
Interestingly, a rhombic signal comprised of two species at gav ≈
2.18 was observed when the sample was frozen, signifying the
formation of a NiIII species (Figure 5b). Typically, a low-spin S
= 1/2 NiIII center will display such a rhombic EPR pattern with
gav values ca. 2.12−2.17.16,18,49 The EPR spectrum of
[Ni(SalCF3)]+• exhibits two similar g∥ (gx and gy) and g⊥ (gz)
features, a pattern that is consistent with previous reports of
five- or six-coordinate NiIII species.16,18,49,50 This is in contrast
to four-coordinate NiIII species, where a single g∥ and two
similar g⊥ components are observed.16 This suggests the shift in
oxidation locus of [Ni(SalCF3)]+• upon freezing is due to
adventitious axial ligand binding, a process that is favored in
comparison to [Ni(SaltBu)]+• , as a result of increased Lewis
acidity of the Ni center.
Table 7. Natural Transition Orbitals Representing the
a
Transitions Contributing to the Two NIR Bands of
[Ni(SalCF3)]+•
TD-DFT calculations further support a delocalized electronic
structure, correctly predicting the two intense NIR electronic
transitions for [Ni(SalCF3)]+•. Interestingly, the two low-energy
bands display a similar intensity (Figure 3), while for
[Ni(SaltBu)]+• (Supporting Information, Figure S15) the lowest
energy transition is threefold more intense. To gain further
insight into this difference we analyzed the TD-DFT transitions
employing NTOs.48 While the calculated oscillator strengths do
not correctly predict the experimental ratio observed for
[Ni(SalCF3)]+•, the change in predicted band intensities in
comparison to [Ni(SaltBu)]+• (Supporting Information, Figure
S15) offers insight into the nature of the electronic transitions
for these two ligand radical complexes. For [Ni(SalCF3)]+•, the
low-energy transition is predicted to have significant ILCT
character with donation to the imine nitrogens (Table 7), while
the NTOs for the higher energy transition are fully delocalized,
and the increased intensity reflects greater overlap between the
donor and acceptor orbitals.51 For [Ni(SaltBu)]+•, the higher
energy transition is predicted to have significant donation to the
phenolate oxygens (Table S2), while the NTOs for the lowest
energy transition are fully delocalized with significant overlap
between the donor and acceptor orbitals. These data suggest
that the para-ring electron-withdrawing group not only serves
to increase the charge density on the Ni center but also alters
the nature of the low-energy ligand-radical electronic
transitions.
a
See Experimental Section for calculation details.
The lowest energy transition predicted at 5700 cm−1 matches
well with the experimental energy and is predicted to be a
delocalized intraligand charge transfer (ILCT) transition, with
donation to the imine nitrogens. The higher energy band (9300
cm−1) is predicted to be more intense, and the NTOs involved
are fully delocalized over the ligand framework (Table 7). Such
intense NIR transitions have also been predicted for the
similarly delocalized species [Ni(SaltBu)]+• (Supporting In-
formation, Table S2).12c These calculations support the
experimental data obtained in these studies in the assignment
of [Ni(SalCF3)]+• as a Class III intervalence compound.
3.8.2. Cu(SalCF3). A symmetric structure was predicted for
Cu(SalCF3) within 0.01 Å of the experimental values using the
B3LYP functional with a PCM for CH2Cl2 (Table S1). The
predicted spin density of Cu(SalCF3) shows spin-covalency
between the Cu dx −y orbital and the coordinating atoms, as
expected for a d9 metal complex (Supporting Information,
Figure S12).
2
2
Oxidation of Cu(SalCF3) can afford one of three electronic
states: (i) a d8 CuIII-salen complex (S = 0), (ii) a CuII complex
antiferromagnetically coupled to a phenoxyl radical (broken
symmetry, S = 0), or (iii) a CuII complex ferromagnetically
coupled to a phenoxyl radical (S = 1). All three possibilities
were explored, where the broken symmetry solution converges
to the singlet solution, which has been observed previously for
calculations of [Cu(SaltBu)]+ using the same level of theory.12f
Upon oxidation, [Cu(SalCF3)]+ demonstrates the character-
istics of a diamagnetic species, with a significant decrease in the
EPR signal (Figure 6) as well as the sharpening of the 19F NMR
signal (Figure 7). This was further evidenced by the appearance
H
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of an intense transition at ca. 18 000 M−1cm−1 in the UV−vis−
NIR spectra, a characteristic LMCT for a CuIII-salen complex,
suggesting the locus of oxidation for [Cu(SalCF3)]+ is primarily
metal-based. XPS measurements further support [Cu(SalCF3)]+
as a CuIII species in the solid state, where the binding energy of
the metal center is shifted upwards of 1.8 eV upon oxidation
from its neutral counterpart. Interestingly, [Cu(SaltBu)]+ exists
in a reversible temperature-dependent equilibrium in solution
between a CuIII-phenolate and a CuII-phenoxyl species, which is
manifested in a reduction of its LMCT band by ca. 50% upon
an increase in temperature.12f This equilibrium could not be
probed fully for [Cu(SalCF3)]+ due to its thermal instability
above 233 K (t1/2 = 150 min at 298 K, Supporting Information,
Figure S5). However, spectra at 198 and 233 K do not show
appreciable differences (Supporting Information, Figure S5),
suggesting that in this temperature range the CuIII-phenolate
electronic state is stabilized. The significant decrease in the
electronic spectra at 273 K is attributed to decomposition. In
addition, the calculated electronic energies support the
formation of a CuIII-phenolate species, where this electronic
state is favored over a triplet CuII-phenoxyl radical species by
3.3 kcal/mol. Under the same calculation parameters, the S = 1
CuII-phenoxyl radical solution for [Cu(SaltBu)]+ is favored by
1.3 kcal/mol,12f further supporting that [Cu(SalCF3)]+ exhibits
greater CuIII character in comparison to [Cu(SaltBu)]+. Overall,
these results show that Ni and Cu complexes of the electron-
poor salen ligand H2SalCF3 afford a delocalized phenoxyl radical
complex for Ni and a high-valent metal complex in the case of
Cu.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Full citation for reference 28, cif file of Cu(SalCF3), Hammett
analysis of oxidative potentials of Ni(SalR), pyridine binding
titrations for Ni(SalCF3), variable-temperature UV−vis−NIR
spectra of [Cu(SalCF3)]+, XPS spectra, additional 19F NMR
plots, experimental and calculated coordination sphere metrical
parameters, calculated natural transition orbitals, calculated spin
density plot, calculated optimized coordinates, TD-DFT
excitation energies and oscillator strengths. The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acs.inorgchem.5b00783.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tim_storr@sfu.ca.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by an NSERC Discovery Grant (T.S.)
and a travel grant from the France-Canada Research Fund (T.S.
and F.T.). L.C. thanks NSERC for a Research Exchange Travel
Award. Compute Canada and Westgrid are acknowledged for
access to computational resources.
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