A stable open-shell redox active ditopic ligand

Article abstract of DOI:10.1039/c5dt04061b

Herein we describe the synthesis, structure and electronic properties of an unusual redox-active ditopic ligand with a stable open-shell configuration. This stable phenoxyl radical features intense and very low energy electronic transitions in the near in

Full text of DOI:10.1039/c5dt04061b

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A stable open-shell redox active ditopic ligand
Cite this: DOI: 10.1039/x0xx00000x
N. M. Bonanno,a,d A. J. Loughb, K. E. Prosserc, C. J. Walsbyc, P. K. Poddutoorid, and
*M. T. Lemairea,d
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
BAQP is synthesized in five steps starting from commercially
available 4-tert-butylphenol (Scheme 1). Iodination of 4-tert-
butylphenol followed by protection with methyl iodide produced
2,6-di-iodo-4-tert-butylanisole13, which was cross-coupled to excess
8-aminoquinoline using Buchwald-Hartwig C-N bond formation
chemistry14 to produce 1. We anticipated that deprotection of 1 with
BBr3 would yield phenol 2, however, we were unable to acquire a 1H
NMR spectrum of the deprotection product at room temperature.
High-resolution mass spectrometry (ESI and EI) and elemental
analysis indicated a molecular formula of this product consistent
with the structure of 2 as shown in Scheme 1. However, in the
liquid chromatogram of 2, we quickly see one peak change to two
peaks; the new peak that grows in has the same m/z as the original
peak and so is likely an isomer of 2. Upon cooling a d8-toluene
Herein we describe the synthesis, structure and electronic
properties of an unusual redox-active ditopic ligand with a
stable open-shell configuration. This stable phenoxyl radical
features intense and very low energy electronic transitions
in the near infrared (NIR) part of the spectrum and is
structurally set up to strongly spin couple coordinated
transition metal ions in [2x2] grid-type structures.
Redox-active bridging ligands that feature stable open-shell
configurations have been shown to be very effective spin coupling
units for the production of high-spin ground state complexes.1-3
Often and under the right conditions, complexes containing
paramagnetic bridging ligands produce excellent single molecule
magnets (SMMs) with high blocking temperatures and display
significantly higher thermal hysteresis temperatures.4-6 The very
strong spin coupling in these complexes serves to “isolate” the spin
ground state from excited states resulting in higher hysteresis
temperatures. So far the synthetically accessible paramagnetic
bridging ligands have allowed for the production of mostly
bimetallic complexes. In order to generate higher spin states there is
a need to design and synthesize open-shell ligands that are capable
of binding multiple metal ions in large coordination arrays.
Polytopic ligands7-9 are an obvious target in this regard and a large
number of these have been synthesized, but very few feature a stable
open-shell structure.10-12 While fascinating polynuclear grid-type
complexes have been reported with very large arrays of metal ions
included in the structure, the magnetic exchange coupling between
paramagnetic metal ions is always very weak superexchange through
the diamagnetic bridge. We have identified a synthetic approach to
produce polytopic paramagnetic bridging ligands and herein report
the synthesis and properties of 3 (BAQP). BAQP, a uniquely stable
phenoxyl radical, features two, contiguous NNO donor sites and
should be capable of binding two metal ions bridged by the central
spin-density rich O atom in [2x2] grid-type complexes. BAQP
represents the first crystallized phenoxyl radical lacking tert-butyl
substituents flanking the C-O bond.
1
solution of 2 down to 235 K, an H NMR spectrum of 2 interpreted
with the phenol structure described in Scheme 1 is acquired (Figure
S13†), which disappears if the solution is warmed back to room
temperature. In the 235 K spectrum, the OH resonance is observed
at 10.03 ppm and all 14 aromatic, 2 NH and 9 tert-butyl protons are
accounted for (at this temperature no coupling could be resolved but
the correct number of absorptions with appropriate integration are
observed). These data suggest that an equilibrium exists between
compound 2, in the form of the diamagnetic phenol product, with an
unknown paramagnetic species; the diamagnetic phenol is the stable
product at low temperature, and the paramagnetic isomer is
dominant at elevated temperatures. All of our spectroscopic data
indicate that this paramagnetic species bears the same molecular
formula as phenol 2 and is not a product from the aerial oxidation of
2, which we originally hypothesized. The UV-vis-NIR spectrum of
2 at room temperature (Figure S10†) is markedly different from the
spectrum of the oxidized product, 3 BAQP (Figure 4, described
later). Further support for the presence of this unusual equilibrium
was provided by EPR spectroscopy. An EPR spectrum was obtained
in toluene solution for 2 at room temperature, which disappears
when the solution is frozen but reappears on warming (Figure S14†).
We are currently attempting to acquire single crystal X-ray data to
identify the structure of the paramagnetic component of this
equilibrium.
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angles(°) [with standard uncertainties (su) in brackets]: O(1)-C(1), 1.256(2); N(1)-
C(2), 1.374(2); N(1)-C(7), 1.387(2); N(2)-C(6), 1.374(2); N(2)-C(16), 1.383(2); C(1)-
C(6), 1.457(3); C(1)-C(2), 1.457(3); C(2)-C(3), 1.380(3); C(3)-C(4), 1.400(3); C(4)-
We anticipated facile oxidation of 2 (which is more precisely an
equilibrium mixture of 2 with a paramagnetic product bearing the
same molecular formula as 2) and expected a three-electron
oxidation (including each NH and the OH) to produce an “aminyl”
type radical as shown in Figure 1. Oxidation with NaIO4 in
methanol immediately produced a forest green precipitate, which
was isolated and characterized. However, the high-resolution mass
spectrum indicated a product with a mass-to-charge ratio two units
higher than anticipated and features present in the FT-IR spectrum
were consistent with an NH stretch.
C(5), 1.405(3); C(5)-C(6), 1.377(3); C(4)-C(25), 1.533(3).
C(2)-N(1)-C(7),
131.28(17); C(6)-N(2)-C(16), 132.29(17); O(1)-C(1)-C(6), 121.64(16); O(1)-C(1)-
C(2), 116.80(16); C(2)-C(3)-C(1), 120.33(17); C(2)-C(3)-C(4), 121.04(17); C(3)-C(4)-
C(5), 120.15(17); C(3)-C(4)-C(25), 120.41(16); C(5)-C(4)-C(25), 119.23(17); C(6)-
C(5)-C(4), 120.84(17); N(2)-C(6)-C(5), 127.25(17); N(2)-C(6)-C(1), 112.19(16); C(5)-
C(6)-C(1), 120.56(16).
Relevant crystal data and structure refinement
parameters are provided in the Supporting Information (Table S1†).
In the molecular structure the metrical parameters of the flanking
aminoquinoline rings are unremarkable and typical. However, the
C-C bond distances around the central ring clearly show its non-
benzenoid nature; C(1)-C(6) and C(1)-C(2) distances are on the
order of single bonds and C(2)-C(3), C(3)-C(4), C(4)-C(5), C(5)-
C(6) distances are typical benzenoid length. The C(1)-O(1) distance
is between that typically observed for C-O single or double bonds
and in line with the distance measured in the two other known
monomeric phenoxyl radical crystals.15,16 The molecular packing of
3 is shown in Figure S1†. Very short π-contacts are evident between
head-over-tail stacked molecules in the ac plane. The distance
between O(1) and C(4) among adjacent molecules is 2.838 Å.
N
i. KI/H2O2
ii.CH3I, K2CO3
NH2
N
H
N
H
Buchwald-Hartwig
I
conditions
N
OCH3
1
N
I
OCH3
OH
BBr3/CH2Cl2
Other evidence for the open-shell nature of 3 is provided by electron
paramagnetic resonance (EPR) spectroscopy (Figure 3). Powder and
solution (CH2Cl2) EPR spectra are similar for 3 and display a broad
signal centered at around 3345 G (g = 2.004). No hyperfine
coupling was resolved in the solution spectrum, which was
deoxygenated and dilute. The solution EPR data were recorded over
a range of temperatures (Figure S4†), which provided very similar
spectra down to 20 K where a large increase in signal intensity was
observed. This signal intensity increase at 20 K is unusual and is a
feature of 3 that we are continuing to investigate.
NaIO4
MeOH/H2O
N
N
H
N
H
N
H
H
N
O
N
N
OH
N
3, baqp
2
Scheme 1. Synthesis of 3 (BAQP). Note that 2 is actually an equilibrium mixture
of 2 with a paramagnetic isomer that is not yet structurally characterized.
N
N
N
O
N
Fig.1. Anticipated “aminyl” radical following three-electron oxidation of 2.
From a THF solution we were fortunate to obtain X-ray diffraction
quality crystals of 3 by slow evaporation after about 3 months. We
note that the spectroscopic properties of the single crystals are
identical with the precipitated powder. The molecular structure of 3
is shown in Figure 2 and indeed indicates the presence of intact NH
groups. The central OH group is the only fragment touched in the
oxidation and the structure is that of a substituted phenoxyl radical.
3 is a very rare example of a structurally characterized phenoxyl
radical.15,16
Fig. 3. Powder (top) and solution (CH2Cl2, 1 mM) EPR spectra of 3 at 298 K.
Frequency = 9.38 GHz, microwave power = 2 mW, modulation amplitude = 0.5 G.
The UV-visible-NIR spectrum of 3 was recorded in CH2Cl2 and is
shown in Figure 3. The higher energy portion of the spectrum is
very different from that of 2 and a broad and intense absorption
centered at unusually low energy (1129 nm) was also observed. This
low energy band is assigned to the βHOMO-to-βLUMO transition in
3 from time-dependent density functional theory calculations (TD-
DFT). The energy of this transition is much lower than in other
reported phenoxyl radicals, which typically exhibit absorptions
between 600-800 nm.17 The large flanking aminoquinoline
substituents result in extensive π-delocalization, which is reflected in
the HOMO/LUMO composition (Figure S7 and S8†) and the
dramatic red shifted absorption energy. A low potential (-0.4 V vs
Ag/AgCl in DMF) reversible cathodic process was observed in the
cyclic voltammogram of 3, which we assign to the reduction of the
radical (Figures S5 and S6†). Anodic scans feature low potential
quasi-reversible waves centered at about 0.4 V (vs Ag/AgCl) and
higher potential irreversible processes. The lower potential wave is
Fig.2. Molecular structure of BAQP 3 (displacement ellipsoids at 30% probability)
with THF solvent molecule excluded for clarity. Relevant bond distances (Å) and
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5DT04061B
likely oxidation of one NH to produce an orthoquinone-type
structure.
clusters. The synthetic methodology to 3 can also be adapted to
larger polytopic ligands that we are currently synthesizing and
will report on shortly.
12000
10000
8000
6000
4000
2000
0
1
Acknowledgements
0.75
0.5
0.25
0
MTL acknowledges NSERC (DG and USRA to NMB), CFI,
Canada Research Chairs, Brandon University and Brock
University for funding of this research. We thank Prof. Art van
der Est (Brock University) for EPR data on 2 and Razvan
Simionescu (Brock) for running variable temperature NMR
experiments.
300
500
700
900 1100 1300 1500
Wavelength /nm
Notes and references
aDepartment of Chemistry, Brandon University, Brandon, MB, R7A 6A9,
Fig. 4. UV-vis-NIR spectrum of 3 (green trace) in THF (298 K). Black bars are
TDDFT calculated transitions (UB3LYP/def2-tzvp).
CANADA.
bDepartment of Chemistry, University of Toronto, Toronto, ON, M5S
3H6, CANADA.
cDepartment of Chemistry, Simon Fraser University, Burnaby, BC, V5A
To calculate the spin-density distribution in 3 we optimized the
geometry [UB3LYP/6-31G(d,p)] of the radical starting from the
coordinates obtained from X-ray crystallography. The calculated
structure is in good agreement to the X-ray structure. A single point
energy calculation (UB3LYP/def2-tzvp) was carried out on the
optimized structure and the resulting spin-density distribution is
shown in Figure 5. The calculated positive (α) spin-density is
concentrated at O(1) as well on the ortho [C(6) and C(2)], para
[C(4)] and ipso [C(1)] carbon atoms of the central ring, with smaller
components on N(2), N(1) and very small quantities on the flanking
quinoline rings. The spin density on these N atoms did not result in
resolved hyperfine splitting in the EPR spectrum of 3 in solution.
Smaller amounts of negative (β) spin-density are localized on meta
carbon atoms C(5) and C(3). This is the first example of a
crystallized phenoxyl radical with substituents other than tert-butyl
groups at C(6) and C(2). The large spin densities on N, N, O donor
atoms is promising for the observation of strong magnetic exchange
coupling in polynuclear complexes.
1S6
dCurrent address: Department of Chemistry, Brock University, St.
Catharines, ON, L2S 3A1, CANADA. E-mail: mlemaire@brocku.ca
†Electronic Supplementary Information (ESI) available: Synthetic details,
crystal data & cif, EPR, electrochemical and computational information
for 2 and 3. See DOI: 10.1039/c000000x/
1
2
3
4
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(BAQP).
This represents only the third structurally
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substituents at C2 and C6. This radical absorbs strongly in the
NIR region of the spectrum and holds great promise as a
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17 "Stable Radicals: Fundamentals and Applied Aspects of Odd-
Electron Compounds", R. G. Hicks, ed., John Wiley & Sons, Ltd.:
Wiltshire, 2010.
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