Structure, magnetic properties and electronic structure of a nickel(II) complex with redox-active 6-(8-quinolylamino)-2,4-bis(tert-butyl)phenol

Article abstract of DOI:10.1016/j.poly.2015.06.010

We report the preparation and electronic structure of a nickel complex 2 containing ligand 6-(8-quinolylamino)-2,4-bis(tert-butyl)phenol 1. The complex is neutral, six-coordinate (pseudo-octahedral) and homoleptic containing two equivalents of 1 and no counteranions. The bond distances measured by single crystal X-ray diffraction studies suggest that the two equivalents of 1 are bound as mono-anion radicals (iminosemiquinone oxidation state). Variable temperature magnetic susceptibility data indicates a quintet (S = 2) ground state for this complex resulting from ferromagnetic coupling between Ni²⁺ and the coordinated radical anions. Cyclic voltammetry experiments and UV-Vis-NIR spectroscopy indicate the presence of coordinated iminosemiquinone radicals, and these experimental results are corroborated by density functional theory calculations, which support the proposed assignments. Complex 2 is easily oxidized in the presence of ferrocenium hexafluorophosphate or aerial dioxygen to produce a complex with closed shell ligands and spectroscopic features characteristic of ortho-iminoquinone ligands.

Full text of DOI:10.1016/j.poly.2015.06.010

Polyhedron xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Structure, magnetic properties and electronic structure of a nickel(II)
complex with redox-active 6-(8-quinolylamino)-2,4-bis
(tert-butyl)phenol
Molina A.L. Sheepwash a, Alan J. Lough b, Lorenzo Poggini c, Giordano Poneti c,d, Martin T. Lemaire a,
⇑
a Department of Chemistry, Brandon University, Brandon, Manitoba R7A 6A9, Canada
b Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
c Department of Chemistry ‘‘Ugo Schiff’’ and INSTM Research Unit of Florence, University of Florence, Sesto Fiorentino, Italy
d Department of Applied Science and Technology, ‘‘Guglielmo Marconi’’ University, via Plinio 44, 00193 Roma, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the preparation and electronic structure of a nickel complex 2 containing ligand 6-(8-quinoly-
lamino)-2,4-bis(tert-butyl)phenol 1. The complex is neutral, six-coordinate (pseudo-octahedral) and
homoleptic containing two equivalents of 1 and no counteranions. The bond distances measured by sin-
gle crystal X-ray diffraction studies suggest that the two equivalents of 1 are bound as mono-anion rad-
icals (iminosemiquinone oxidation state). Variable temperature magnetic susceptibility data indicates a
quintet (S = 2) ground state for this complex resulting from ferromagnetic coupling between Ni2+ and the
coordinated radical anions. Cyclic voltammetry experiments and UV–Vis–NIR spectroscopy indicate the
presence of coordinated iminosemiquinone radicals, and these experimental results are corroborated by
density functional theory calculations, which support the proposed assignments. Complex 2 is easily oxi-
dized in the presence of ferrocenium hexafluorophosphate or aerial dioxygen to produce a complex with
closed shell ligands and spectroscopic features characteristic of ortho-iminoquinone ligands.
Ó 2015 Elsevier Ltd. All rights reserved.
Received 3 April 2015
Accepted 8 June 2015
Available online xxxx
Keywords:
Nickel complexes
Iminosemiquinone ligands
Redox non-innocence
Magnetometry
Density functional theory
1. Introduction
variation of the amine in the synthesis [7–10]. In 2004
Wieghardt and co-workers described the preparation of 1 (Fig. 1)
Transition metal complexes containing redox non-innocent (or
redox active) ligands have received considerable recent attention
[1–3]. Often the redox active ligand can complicate oxidation state
assignment making it a considerable challenge to characterize the
electronic structure. These complexes are of interest as models for
active sites of metal containing enzymes in bio-inorganic chem-
istry [4] or as potential electron reservoirs serving as efficient
homogeneous catalysts [5]. We are also interested in the magnetic
properties of complexes containing redox active ligands, in partic-
ular, semiquinone or iminosemiquinone-based ligands, where
temperature dependent electron transfer reactions are often
observed with cobalt (in so-called valence tautomerism) [6].
There are fewer examples of iminosemiquinone complexes relative
to the parent semiquinone but iminosemiquinone systems are easy
to prepare and can present a wide variety of structural types by
by reaction of 8-aminoquinoline with 3,5-di-tert-butylcatechol in
the presence of triethylamine [11]. Also reported were the struc-
tural and variable temperature magnetic properties of several first
row transition metal complexes containing 1 in different oxidation
states, including Mn, Fe, and Co. Unusual redox activity was
observed, including the isolation of a Mn4+ complex containing
two equivalents of the dianion of 1, featuring an S = 3/2 ground
state. Absent from this report was any nickel coordination chem-
istry, but nickel complexes with related redox active ligands have
been shown to exhibit interesting electronic structures [12–15].
We hypothesized that reaction of Ni2+ with 1 in the presence of
air could afford a quintet (S = 2) complex, containing triplet Ni2+
ferromagnetically coupled to two doublet radical anions of 1 in
the iminosemiquinone oxidation state by strict orthogonality of
the magnetic orbitals imposed by the rigid chelate in analogy with
what was previously found for other nickel-semiquinonate adducts
[16]. Herein we report an alternative preparation of 1 as well as the
preparation of a nickel complex containing 1 (2), in which we
observe unusual electronic spectroscopic features, including
absorptions well into the NIR region of the spectrum. On the basis
⇑
Corresponding author at: Department of Chemistry, Brandon University, 270-
18th Street, Brandon, Manitoba R7A 6A9, Canada. Tel.: +1 (204) 727 7407; fax: +1
(204) 727 7234.
E-mail address: lemairem@brandonu.ca (M.T. Lemaire).
http://dx.doi.org/10.1016/j.poly.2015.06.010
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: M.A.L. Sheepwash et al., Polyhedron (2015), http://dx.doi.org/10.1016/j.poly.2015.06.010

2
M.A.L. Sheepwash et al. / Polyhedron xxx (2015) xxx–xxx
of a wide variety of physical techniques backed up with theoretical
results from density functional theory (DFT) calculations, we con-
clude that the electronic structure of complex 2 is best described
as of Ni2+ and two coordinated iminosemiquinone radicals to pro-
duce a quintet ground state through intramolecular ferromagnetic
nickel-iminosemiquinone exchange coupling. Somewhat surpris-
ingly, even in the solid state, 2 presents marked reactivity to aerial
dioxygen to produce a triplet Ni2+ complex with closed shell
ligands having spectroscopic features consistent with an imino-
quinone oxidation state.
were measured using a combination of u scans and x scans. The
data were processed using APEX2 and SAINT [23]. Absorption correc-
tions were carried out using SADABS [23]. The structure was solved
and refined using SHELXTL [24] for full-matrix least-squares refine-
ment that was based on F2. H atoms were included in calculated
positions and allowed to refine in riding-motion approximation
with Uꢁisoꢁtied to the carrier atom.
2.5. Synthesis
2.5.1. 6-(8-quinolylamino)-2,4-bis(tert-butyl)phenol (1)
3,5-di-tert-butylcatechol (1.5 g, 6.8 mmol) was dissolved in
CH3CN (15 mL) and combined with solid 8-aminoquinoline
(0.97 g, 6.8 mmol). The solution gradually clarified and turned
amber in color. The reaction was heated to reflux in air for 1.5 h,
cooled to room temperature and stirred at room temperature over-
night. The reaction mixture was evaporated to dryness and the
residue was dissolved in a minimum amount of hot methanol.
Cooling the hot solution in the refrigerator produced tan colored
crystals over a period of one week. Yield, 0.50 g (21%). 1H NMR
(300 MHz, CDCl3): d 8.85 (dd, 1H), 8.16 (dd, 1H), 7.48 (m, 2H),
7.30 (m, 3H), 7.16 (d, 1H), 6.68 (d, 1H), 6.43 (s, 1H), 1.51 (s, 9H),
1.31 (s, 9H). HR-MS (ESI+): Calc’d for C23H29N2O (MH+) m/z
2. Materials and methods
2.1. General procedures
All reagents were commercially available and used as received
unless otherwise stated. FT-IR spectra were recorded on
a
Shimadzu IR-Affinity spectrometer as KBr discs. LC–MS experi-
ments were recorded on an Agilent 1260 Infinity liquid chro-
matograph/6530 accurate mass Q-TOF in high resolution mode in
70:30 acetonitrile/water using positive mode electrospray ioniza-
tion (HPLC grade solvents). UV–Vis spectra were recorded in
CH2Cl2 solution on a Shimadzu 3600 UV–Vis–NIR spectrophotome-
ter using quartz cuvette cells. Cyclic voltammetry (CV) experi-
ments were performed with a Bioanalytical Systems Inc. (BASI)
Epsilon electrochemical workstation. Compounds 1 and 2 were dis-
solved in anhydrous solvent (CH2Cl2), and then deaerated by sparg-
ing with N2 gas for 10–15 min. Solution concentrations were
approximately 10ꢀ3 M in analyte containing approximately 0.1 M
supporting electrolyte (Bu4NPF6). A typical three-electrode set-up
was used including a platinum working electrode, Ag/AgCl refer-
ence electrode, and a platinum wire auxiliary electrode. The scan
rate for all CV experiments was 100 mV/s.
349.2279, found 349.2275. Anal. Calc. for (%)
C23H28N2O: C,
79.27; H, 8.10; N, 8.04. Found: C, 79.44; H, 8.41; N, 8.20%.
2.5.2. Ni(1)2 (2)
To a solution of Ni(NO3)2ꢂ6H2O (0.90 g, 0.31 mmol) in dry
CH3CN (15 mL) was added 1 (0.20 g, 0.57 mmol). Gradually, a clear,
pale green solution was observed. To this solution was added about
10 drops of triethylamine, which resulted in dark green colored
solution and the production of a crystalline, dark green, product.
Approximately 5 mL more of CH3CN was added and the reaction
was stirred for another 5 min at room temperature. The crystalline
solid was collected by vacuum filtration, washed with small por-
tions of very cold CH3CN, and dried. Yield, 180 mg (78%). Anal.
Calc. for NiC46H52N4O2: C, 73.51; H, 6.97; N, 7.45. Found: C,
73.95; H, 6.77; N, 7.34%. MS (ESI +): m/z 750.6 (M+, 100%). FT-IR
(KBr): 3057(w), 2951(s), 2906(w), 1587(m), 1514(w), 1467(s),
1437(s), 1383(m), 1363(m), 1313(w), 1265(w), 1201(w),
1166(w), 1117(w), 1097(w), 1024(vw), 910(w), 825(w), 785(w),
2.2. Variable temperature magnetic susceptibility measurements
Solid state variable temperature magnetic susceptibility mea-
surements were recorded on a superconducting quantum interfer-
ence device (SQUID) magnetometer (Quantum Design MPMS) with
a 5.5 T magnet (temperature range 1.8–400 K) in an external field
of 2000 Oe. A sample of 2 was carefully weighed into a gelatin cap-
sule, loaded into a plastic straw and attached to the sample trans-
port rod. The magnetization of the sample was scanned over a 2–
300 K temperature range. Diamagnetic corrections to the paramag-
netic susceptibilities were accomplished using Pascal’s constants.
752(w), 685(w), 646(vw), 608(w) cmꢀ1. UV–Vis (CH2Cl2): kmax
(e
(Mꢀ1 cmꢀ1)): 351 nm (2.5 ꢃ 104), 486 (2.3 ꢃ 104), 633 (1.1 ꢃ 104),
871 (8.1 ꢃ 103), 986 (8.2 ꢃ 103).
3. Results and discussion
2.3. Computational details
3.1. Synthesis, structural and electronic properties
All DFT calculations were performed using the Gaussian09
(Revision D.01) package [17] using the B3LYP hybrid functional
[18,19] and the 6-31G(d,p) basis set for the geometry optimization
and frequency calculation or the def2-tzvp basis set for single point
energy calculations. The converged wave functions were tested to
confirm that they correspond to the ground state surface. Tight SCF
convergence criteria were used for all calculations. The program
Chemissian [20] was used for determining atomic orbital composi-
tions employing Mulliken Population Analysis. The intensities of
the 50 lowest-energy electronic transitions were calculated by
TD-DFT [21,22] with the same functional/basis set combination
employed for the single-point calculations.
Ligand 1 is prepared by refluxing 8-aminoquinoline with 3,5-di-
tert-butylcatechol in acetonitrile solution. Solvent removal and dis-
solution of the remnant reddish oil in a minimum amount of
methanol produces crystals of 1 over a period of a week or more
in
a
refrigerator. Reacting two equivalents of
1
with
Ni(NO3)2ꢂ6H2O in acetonitrile/water mixture produces a dull green
2.4. X-ray crystallography
Data were collected on a Bruker Kappa APEX-DUO diffractome-
ter using monochromated Mo K
a
radiation (Bruker Triumph) and
Fig. 1. Structure of 6-(8-quinolylamino)-2,4-bis(tert-butyl)phenol 1.
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M.A.L. Sheepwash et al. / Polyhedron xxx (2015) xxx–xxx
3
Fig. 2. Displacement ellipsoid representation of the molecular structure of 2 (ellipsoids at 30% probability). CH3CN solvate molecules and H atoms are not shown for clarity.
Symmetry operation (ꢀx, y, ½ ꢀ z). Important bond distances (Å) and angles (°) with standard uncertainties in brackets: Ni(1)–N(2), 2.0037(12); Ni(1)–O(1), 2.0750(10);
Ni(1)–N(1), 2.0928(12); O(1)–C(15), 1.2855(16); N(2)–C(10), 1.3532(16); N(2)–C(8), 1.3799(16); C(10)–C(11), 1.4219(18); C(10)–C(15), 1.4642(18); C(11)–C(12), 1.3761(18);
C(12)–C(13), 1.4308(19); C(12)–C(16), 1.5344(19); C(13)–C(14), 1.3755(19); C(14)–C(15), 1.4496(18); C(14)–C(20), 1.5397(19). N(2a)–Ni(1)–N(2), 178.20(6); N(2)–Ni(1)–
O(1), 80.02(4); N(2)–Ni(1)–O(1a), 101.31(4); O(1a)–Ni(1)–O(1), 87.22(6); N(2a)–Ni(1)–N(1), 98.81(4); N(2)–Ni(1)–N(1), 79.90(4); O(1a)–Ni(1)–N(1), 94.81(4); O(1)–Ni(1)–
N(1), 159.82(4); N(1)–Ni(1)–N(1a), 90.16(6); C(15)–O(1)–Ni(1), 111.73(8); C(10)–N(2)–C(8), 128.71(11); C(10)–N(2)–Ni(1), 114.93(8); C(11)–C(10)–C(15), 119.94(11);
C(12)–C(11)–C(10), 120.66(12); C(11)–C(12)–C(13), 118.22(12); C(14)–C(13)–C(12), 124.56(12); C(13)–C(14)–C(15), 117.54(12); O(1)–C(15)–C(14), 122.60(12); O(1)–C(15)–
C(10), 119.54(11); C(14)–C(15)–C(10), 117.81(11).
solution that quickly darkens upon the addition of triethylamine.
Also, a dark green crystalline solid of 2 is rapidly produced and iso-
lated in moderate yield and these crystals were found to be suit-
able for single crystal X-ray diffraction. Microanalysis and ESI
mass spectrometry conclusively indicate the structure of 2 is neu-
tral and contains nickel and two equivalents of fully deprotonated
1; no counter-cations or anions were detected. The neutral nature
of complex 2 is indicative of a nickel(II) complex coordinated by
two equivalents of 1 in the iminosemiquinone radical-anion oxida-
tion state, or as the only other plausible structure given the micro-
analytical and mass spectrometry data, nickel(II) coordinated by
two equivalents of 1 in completely different oxidation states: ami-
docatecholate (dianion) and iminobenzoquinone (neutral).
However, the latter structure containing two different redox forms
of 1 should feature a very low energy IVCT transition (>2000 nm),
which is not detected in the electronic spectrum of 2 [25]. In the
molecular structure of 2 (Fig. 2, Table 1), a crystallographic twofold
axis renders equivalent the two coordinated radical anions of 1.
Coordinate bond distances are shortest to the amino N atom and
slightly longer to the quinolyl ring N and O donor atoms. The bond
distances within the iminosemiquinone ring are completely con-
sistent with the anion-radical oxidation state for 1 in the complex,
as had been established previously [11].
tautomerism (Figs. S1 and S2 in the Supporting Information).
Also, the addition of excess oxidant (ferrocenium hexafluorophos-
phate) results in the disappearance of the very low energy absorp-
tions and the appearance of a new intense absorption centered at
600 nm. Time dependent density functional theory (TD-DFT) has
been used to calculate the absorption spectrum of complex 2 (vide
infra) and lower energy transitions are primarily ligand centered,
while higher energy absorptions have some MLCT character.
Data from electrochemical (cyclic voltammetry, CV) experi-
ments on 1 and 2 clearly support the assignment of anion radical
iminosemiquinone oxidation state for each coordinated equivalent
of 1 in complex 2. In the CV of ligand 1 (Fig. S3 in the Supporting
The electronic spectrum of 2 in CH2Cl2 is shown in Fig. 3 and
features a series of absorptions across the visible region and into
the near infrared. These spectral features are not temperature
dependent in ethanol solution (at least not down to the lowest
measureable temperature of ꢀ70 °C). Other X-ray photoelectron
spectroscopy (XPS) experiments also indicated only the presence
of Ni2+ and no Ni3+, which supports a static, temperature indepen-
Fig. 3. UV–Visible spectra of 2 (blue trace) and 2 reacted with excess ferrocenium
dent electronic structure for
2
and the absence of valence
hexafluorphosphate (red trace) in CH2Cl2. (Colour online.)
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M.A.L. Sheepwash et al. / Polyhedron xxx (2015) xxx–xxx
Table 1
Crystallographic Data for 2.
2
Empirical formula
C50H58N6NiO2
fw
833.73
0.270 ꢃ 0.050 ꢃ 0.010
20.111(6)
14.373(4)
16.259(4)
monoclinic
90
Dimensions (mm)
a (Å)
b (Å)
c (Å)
Cryst syst
a
(°)
b (°)
101.988(6)
90
4597(2)
C2/c
c
(°)
V (Å3)
Space group
Z
l
4
(mmꢀ1
)
0.467
T (K)
147(2)
Independent reflections
Number of parameters
5249
274
R1 [I > 2
wR2 (all data)
r
(I)]
0.0306
0.0772
Fig. 5. Variable temperature magnetic susceptibility data for 2 (squares) at a field of
2000 Oe. The best fit to the experimental data is represented by the black line.
were unable to confirm the anticipated S = 2 ground state for the
complex (Fig. S4 in the Supporting Information). This result was
not unexpected as the combination of factors responsible for the
drop in
vmT at low temperature are dominant at 2 K. We could
fit the temperature dependence of the susceptibility using the pro-
gram PHI [26] with an isotropic spin Hamiltonian including two
exchange parameters. The best-fit parameters included two
exchange parameters, J1, which is between Ni2+ (g = 2.03) and the
coordinated radicals (g = 2.00) and J2, which is the exchange cou-
pling between the coordinated radicals through Ni2+. As antici-
pated, J1 is moderately ferromagnetic (+90 cmꢀ1), while J2 is very
weakly antiferromagnetic (ꢀ0.8 cmꢀ1). In order to fit the low tem-
perature data points, we had to include an antiferromagnetic inter-
molecular exchange coupling parameter (zJ0) of ꢀ1 K as well as a
5% component of an S = 1 impurity. We carried out broken symme-
try density functional theory (BS-DFT) calculations [27] on geome-
try optimized 2 in the quintet state. The calculated coordinate
bond distances agree within 0.03 Å of the experimental values in
all cases and the agreement among the bond distances within
the coordinated ligands is very good. The calculated spin density
distribution within 2 is shown in Fig. 6. Our calculations predict
a quintet ground state for 2 with an estimated Ni2+-radical
exchange coupling constant of 239 cmꢀ1. The calculated result is
in agreement with the sign of the exchange coupling
Fig. 4. CV of 2 in CH2Cl2 containing approximately 0.1 M Bu4NPF6. Scan rate
100 mV/s and y-axis (not shown) is in units of A.
l
Information) there are two irreversible anodic processes (and no
cathodic processes until very high negative potential, which is irre-
versible), which are assigned to sequential oxidation of 1 to first an
open-shell iminosemiquinone oxidation state and next to the fully
oxidized and closed shell iminoquinone. In complex 2 (Fig. 4) there
are two sets of two reversible anodic and quasi-reversible cathodic
processes observed in the CV. Each anodic wave represents a single
electron oxidation of each coordinated equivalent of 1 to the imi-
noquinone forms. These oxidative processes are centered at very
low positive potential versus Ag/Ag+ and account for the oxidative
instability of complex 2 even in the solid state. The cathodic waves
result from one electron reduction of the coordinated imi-
nosemiquinone form of 1 to the amidocatecholate dianion oxida-
tion state.
3.2. Variable temperature magnetic susceptibility and DFT calculations
The variable temperature magnetic susceptibility of 2 was
investigated via SQUID magnetometry and the results are plotted
in Fig. 5 in terms of
vmT versus T. At 300 K, the value of vmT is
2.29 cm3 molꢀ1 K, which is higher than the calculated value for
S = 1 Ni2+ (g = 2.03) and two uncoupled S = ½ (g = 2.00) coordinated
radicals (1.78 cm3 molꢀ1 K). As temperature is decreased there is a
rise in
v
mT, which plateaus to a value of 2.54 cm3 K molꢀ1 at 130 K.
These features indicate dominant intramolecular ferromagnetic
exchange coupling between Ni2+ and the coordinated radicals. At
low temperatures,
vmT values drop rapidly, which may result from
any or a combination of the following: intermolecular antiferro-
magnetic exchange, zero field splitting and/or antiferromagnetic
coupling between coordinated radicals. Unfortunately, low tem-
perature (2 K) magnetization versus field experiments (up to 5 T)
Fig. 6. Calculated spin-density distribution in the quintet state of 2. Isovalue at
0.0040.
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M.A.L. Sheepwash et al. / Polyhedron xxx (2015) xxx–xxx
5
Fig. 7. Calculated b-spin frontier orbitals: Top left, bHOMO (3% Ni), top right, bHOMOꢀ1 (5% Ni), bottom left, bLUMO (2% Ni), bottom right, bLUMO+1 (2% Ni). Isovalues at
0.020.
(ferromagnetic) that was determined by fitting the experimental
susceptibility data, but it is roughly nearly three times greater.
The ferromagnetic nature of the interaction is easily rationalized
for Ni2+ and
p-SOMOs of coordinated 2 in the IMSQ oxidation
state). Wieghardt and co-workers previously observed very strong
exchange coupling in Fe3+, Mn4+, and Co2+ complexes containing 2
in the IMSQ oxidation state, with temperature independent
on the basis of strict orthogonality of the magnetic orbitals (r-type
Fig. 8. Calculated
a
-spin frontier orbitals: Top left,
aHOMO (2% Ni), top right,
a
HOMOꢀ1(2% Ni), bottom left,
aLUMO (1% Ni), bottom right, aLUMO+1 (1% Ni).
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M.A.L. Sheepwash et al. / Polyhedron xxx (2015) xxx–xxx
shed light on the electronic structure of transition metal complexes
containing coordinated iminosemiquinone ligands. We are cur-
rently developing new ligands based on 1, which should be capable
of generating polynuclear complexes with interesting electronic
structures and unprecedented redox activity.
Acknowledgments
M.T.L. acknowledges funding from NSERC, CFI, the Canada
Research Chairs program, and Brandon University. We thank Dr.
Paul Dube (Brockhouse Institute for Materials Research) for vari-
able temperature magnetic susceptibility data. L.P. and G.P. grate-
fully acknowledge the financial support from Ente Cassa di
Risparmio di Firenze and Brunetto Cortigiani for helpful
discussions.
Fig. 9. Experimental absorption spectrum of 2 (curve) in CH2Cl2 along with the
calculated absorption spectrum (black bars, calculated at the B3LYP/def2-tzvp level
of theory).
Appendix A. Supplementary data
CCDC 1052673 contains the supplementary crystallographic
data for complex 2. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1016/j.poly.2015.06.010.
magnetic moments resulting from very strong (J > ꢀ200 cmꢀ1
)
antiferromagnetic coupling between the d magnetic orbitals of
p
the metal ions and p-SOMO of the radical anion of 2 [11].
Time-dependent density functional theory (TD-DFT) calcula-
tions were carried out to better understand the electronic structure
of 2 and the results are illustrated as a line spectrum in Fig. 9 over-
laid on the experimental spectrum. The calculated transitions
agree well with the experimental data with the exception of the
lowest energy band in the experimental spectrum (986 nm).
Three sets of intense transitions are calculated at energies that
agree with transitions observed experimentally. The lowest energy
calculated transitions are found at 835 and 826 nm and are largely
References
[1] P.J. Chirik, Inorg. Chem. 50 (2011) 9737.
[2] W. Kaim, Eur. J. Inorg. Chem. (2012) 343.
[3] W. Kaim, Inorg. Chem. 50 (2011) 9752.
ligand
centered,
resulting
from
transitions
between
[4] W. Kaim, B. Schwederski, Coord. Chem. Rev. 254 (2010) 1580.
[5] O.R. Luca, R.H. Crabtree, Chem. Soc. Rev. 42 (2013) 1440.
[6] T. Tezgerevska, K.G. Alley, C. Boskovic, Coord. Chem. Rev. 268 (2014) 23.
[7] K.S. Min, T. Weyhermüller, E. Bothe, K. Wieghardt, Inorg. Chem. 43 (2004)
2922.
[8] A.S. Roy, P. Saha, N.D. Adhikary, P. Ghosh, Inorg. Chem. 50 (2011) 2488.
[9] K.M. Clark, J. Bendix, A.F. Heyduk, J.W. Ziller, Inorg. Chem. 51 (2012) 7457.
[10] A. Rajput, A.K. Sharma, S.K. Barman, D. Koley, M. Steinert, R. Mukherjee, Inorg.
Chem. 53 (2014) 36.
[11] K.S. Min, T. Weyhermüller, K. Wieghardt, Dalton Trans. (2004) 178.
[12] Y. Shimazaki, F. Tani, K. Fukui, Y. Naruta, O. Yamauchi, J. Am. Chem. Soc. 125
(2003) 10512.
[13] L. Chiang, A. Kochem, O. Jarjayes, T.J. Dunn, H. Vezin, M. Sakaguchi, T. Ogura, M.
Orio, Y. Shimazaki, F. Thomas, T. Storr, Chem. Eur. J. 18 (2012) 14117.
[14] D. Herebian, E. Bothe, F. Neese, T. Weyhermüller, K. Wieghardt, J. Am. Chem.
Soc. 125 (2003) 9116.
[15] A. Paretzki, M. Bubrin, J. Fielder, S. Záliš, W. Kaim, Chem. Eur. J. 20 (2014) 5414.
[16] A. Bencini, C. Carbonera, A. Dei, M.G.F. Vaz, Dalton Trans. (2003) 1701.
[17] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M.
Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y.
Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro,
M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M.
Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C.
Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth,
P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, Ö. Farkas, J.B. Foresman,
J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision D.01, Gaussian, Inc.,
Wallingford CT, 2009.
bHOMOꢀ1/bHOMO and bLUMO/bLUMO+1 (Fig. 7) and the calcu-
lated transitions at 637 and 636 nm result from transition between
a
HOMO/
a
HOMOꢀ1 and
aLUMO/aLUMO+1 (Fig. 8). Higher energy
transitions calculated at 487 and 470 nm consist of a significant
MLCT component originating from orbitals with substantial Ni2+
character (bHOMOꢀ2, bHOMOꢀ3, bHOMOꢀ4 and bHOMOꢀ5) into
bLUMO or bLUMO+1.
Of note, crystalline 2 decomposes in solution over a period of
hours and in the solid state over a period of several days. The elec-
tronic spectrum of 2 after the solid has been sitting under ambient
conditions for several days is similar to the spectrum obtained by
oxidation of 2 with ferrocenium hexafluorophosphate (Fig. S5 in
the Supporting Information) including the loss of the low energy
absorption bands. Given the very low positive oxidation potential
observed for 2 this reactivity is not surprising. In fact, the FT-IR
spectrum (Fig. S6 in the Supporting Information) of the air exposed
crystals of 2 provides evidence for ligand oxidation to the imino-
quinone oxidation state by loss of one electron, with an absorp-
tions at energies consistent with quinone carbonyl (>1620 cmꢀ1).
Unfortunately the air-exposed crystals would no longer diffract,
so we could not obtain structural data from single crystal X-ray
diffraction for this decomposition product.
[18] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
4. Conclusions
[19] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[20] L.V. Skripnikov, Chemissian V. 4.01, Visualization Computer Program,
<www.chemissian.com>, 2014.
[21] M.E. Casida, In Recent Advances in Density Functional Methods; D.P. Chong
(Ed.), World Scientific: Singapore, 1995; p 155.
We have extended the coordination chemistry of an interesting
redox active ligand to include nickel. For the first time, a ferromag-
netic interaction is observed between ligand
1 in the imi-
[22] R.E. Stratmann, G.E. Scuseria, M.J. Frisch, J. Chem. Phys. 109 (1998) 8218.
nosemiquinone oxidation state and a transition metal ion. This is
an intriguing result and suggests the possibility for novel magnetic
materials by preparing polytopic derivatives of 1 and polynuclear
complexes featuring strong intramolecular exchange coupling.
We have also completed a computational study of 2, which has
[23] Bruker (2007). APEX2, SAINT
& SADABS Bruker AXS Inc., Madison, Wisconsin, USA.
[24] G.M. Sheldrick, Acta Crystallogr. A64 (2008) 112.
[25] D.M. Adams, L. Noodleman, D.N. Hendrickson, Inorg. Chem. 36 (1997) 3966.
[26] N.F. Chilton, R.P. Anderson, L.D. Turner, A. Soncini, K.S. Murray, J. Comput.
Chem. 34 (2013) 1164.
[27] L. Noodleman, J. Chem. Phys. 74 (1979) 5737.
Please cite this article in press as: M.A.L. Sheepwash et al., Polyhedron (2015), http://dx.doi.org/10.1016/j.poly.2015.06.010