Ligand mixed-valence and electrical conductivity in coordination complexes containing a redox-active phenalenol-substituted ligand

Article abstract of DOI:10.1039/c9dt01788g

A new redox-active hydrazone ligand bearing a phenalenol group is described (phpl), which produces neutral six-coordinate Fe and Co complexes (1 & 2) with the ligands identified in different oxidation states; an open-shell anion radical and closed-shell dianion. An intense and very low-energy intervalence charge transfer (IVCT) band is identified in solid-state and in solution in the complexes. Single crystals of 1 are semiconducting (at 300 K, σ = 3.05 × 10^-4 S cm^-1 with E_a = 245 meV).

Full text of DOI:10.1039/c9dt01788g

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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DOI: 10.1039/C9DT01788G
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Ligand mixed-valence and electrical conductivity in coordination
complexes containing a redox-active phenalenol-substituted
ligand
Received 00th January 20xx,
Accepted 00th January 20xx
Sarah Dalea, Nico M. Bonannoa, Mark Pelacciaa, Alan J. Loughb, Atsuhiro Miyawakic, Kazuyuki
Takahashic, and Martin T. Lemaire*a
DOI: 10.1039/x0xx00000x
A new redox-active hydrazone ligand bearing a phenalenol group is Coordinated arylazo ligands are easily reduced by one electron
described (phpl), which produces neutral six-coordinate Fe and Co to produce an open-shell anion radical state and coordination
complexes (1 & 2) with the ligands identified in different oxidation complexes featuring reduced arylazo ligands are known. Herein
states; an open-shell anion radical and closed-shell dianion. An we describe the synthesis of a new tridentate hydrazone ligand,
intense and very low-energy intervalence charge transfer (IVCT) 9-[2-(2-pyridyl)hydrazono]-1-phenalenol (phpl) (Figure 1), that
band is identified in solid-state and in solution in the complexes. when coordinated to iron and cobalt, adopts a unique
Single crystals of 1 are semiconducting (at 300 K,  = 3.05  10-4
cm-1 with Ea = 245 meV).
S
homoleptic structure featuring mixed-valent set of open- and
closed-shell ligands and displays a very low energy and intense
intervalence-charge transfer band. The open-shell oxidation
state of phpl, which is produced by aerial oxidation of the
deprotonated hydrazone in the presence of metal ions, is
shown via density functional theory calculations to resemble
the singly reduced arylazo ligand, including large acyclic N atom
spin densities. The solid-state structural properties and
electronic structure of the iron and cobalt complexes are
investigated in detail. Semiconducting behaviour is observed in
single crystals of the iron complex.
The synthesis of new redox-active ligands for the preparation of
unique coordination complexes and coordination materials
with unusual and potentially useful properties continues to be
an important focus for coordination chemists.1-13 With some
creative effort ligands can be prepared featuring a wide range
of interesting optical, electronic and/or magnetic properties
that can enhance the properties of the resulting complex when
coordinated to a metal ion. This is particularly true for magnetic
and conducting complexes containing open-shell ligands, where
strong magnetic coupling between ligand radicals and metal
ions can produce enhanced magnetic properties in single
molecule magnets (SMMs) or provide the driving force for the
transfer of an electron from a metal ion to create thermally
induced paramagnetic switching materials.14–24 Stacks of ligand
radicals can generate pathways for electrical conductivity
tethered to paramagnetic metals in multifunctional
materials.25–29 Recently reported electrically conducting metal-
organic frameworks highlight the promise of these efforts.30–34
Figure 1. Structure of 9-[2-(2-pyridyl)hydrazono]-1-phenalenol (phpl)
Ligand phpl is prepared in three steps as outlined in Scheme S1.
The first two steps to produce first 9-hydroxy-1-phenalenone
and then 9-methoxy-1-phenalenone are known.36,37 We found
9-methoxy-1-phenalenone would react cleanly with 2-
hydrazinopyridine in a nucleophilic substitution reaction as
described in the literature for a series of arylamines.33 The
product is isolated in one tautomeric form and our
spectroscopic data suggested the enol hydrazone structure is
the isolated form of the ligand, with an O-H resonance a little
One challenge is the design and synthesis of new ligands
capable of forming stable coordination complexes in open-shell
oxidation states. One class of ligand that has demonstrated
promise in this regard is the substituted arylazo ligand.35
1
aDepartment of Chemistry, Brock University, St. Catharines, Ontario, L2S 3A1.
bDepartment of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6
cDepartment of Chemistry, Graduate School of Science, Kobe University, Kobe, Japan
Electronic Supplementary Information (ESI) available: [synthetic, spectroscopic, X-
ray diffraction and computational data is available]. See DOI: 10.1039/x0xx00000x
beyond 13 ppm in the H NMR spectrum (Figures S1-S4). The
enol form is also the calculated optimized geometry using DFT
(Figure S5, top). The ligand is red and has an absorbance
maximum at 460 nm (Figure S3) and TD-DFT calculations on an
optimized structure confirm this transition as a predominately
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HOMO-LUMO absorption (Table S4) between the highly anion-radical and dianion (Figures S5 middleVie&w ArbticoletOtonlmine,
delocalized -frontier orbitals (Figure S6). Our interest in phpl respectively), and the trends in N-N and CD-OOIb: o10n.1d03d9is/Cta9nDcTe0s17t8h8aGt
is highlighted in Scheme 1, which describes the anticipated we have observed in 1 and 2 are also evident in the optimized
redox series for this ligand, which is similar to that observed uncoordinated structures. For example, the optimized phpl
with o-dioxolene ligands.
Double deprotonation of the anion-radical has shorter N-N and C-O bonds (1.298 & 1.248 Å,
hydrazone ligand produces the closed-shell dianion [L(2-)], which respectively) compared to the optimized phpl dianion (N-N and
could oxidize by loss of one electron to a radical anion [L(·-)
]
C-O bonds = 1.257 & 1.331 Å, respectively).
where the spin density could be distributed over the phenalenyl
rings, over the azo bond or localized on O. Subsequent one
electron oxidation of [L(·-)] produces the neutral closed shell
form [L(0)].
Scheme 1. Anticipated redox series for phpl highlighting potential dianionic [L(2-)], anion
radical [L(·-)] and neutral [L(0)] oxidation states of phpl
Coordination of phpl to Fe(NO3)3·9H2O or Co(ClO4)2·6H2O in
methanol (in air) produced dark solutions and dark precipitates.
The precipitated powders were dissolved in chloroform and
single crystals of both complexes were grown by pentane
diffusion. The molecular structure of complex 1 is shown in
Figure 2 (see Tables S5 & S6 for full listing of bond distances and
Figure 2. Left: Molecular structure of 1 (displacement ellipsoids at 30% probability). H
atoms and solvent molecules removed for clarity. In 1 and 2 one CHCl3 molecule is
refined at atomic precision but other disordered solvent was masked using
PLATON/SQUEEZE procedure. The crystals lose some solvent, which is reflected in the
elemental analysis results (ESI). Relevant bond distances (A) [with standard uncertainties
(su) in brackets]: Fe(1)-N(1), 1.876(2); Fe(1)-O(2), 1.8823(19); Fe(1)-O(1), 1.8952(18);
Fe(1)-N(4), 1.902(2); Fe(1)-N(3), 1.934(2); Fe(1)-N(6), 1.943(2); O(1)-C(1), 1.270(3); O(2)-
C(19), 1.303(3); N(1)-N(2), 1.338(3); N(1)-C(11), 1.387(3), N(2)-C(14), 1.378(3), N(4)-
C(29), 1.347(3); N(4)-N(5), 1.374(3); N(5)-C(32), 1.346(3). Right: Alternative view of 1
highlighting the structural differences between the two ligands.
angles and other crystallographic parameters).
The
isostructural complex 2 is shown in Figure S8 (Tables S9-S10).
Both structures were originally solved at 150 K. We also
collected data on a crystal of 1 at 300 K and the structure solved
at 300 K is nearly identical to the lower temperature structure
(Figure S7 and Tables S7-S8). In 1 and 2, the very short
coordinate bond distances [ranging from 1.876(2) to 1.943(2) in
1 and 1.882(2) to 1.909(2) in 2], with the shortest bonds to O
donors suggested Fe3+/Co3+ oxidation state assignments
because Fe4+/Co4+ were not anticipated to be stable given this
ligand environment. Since no nitrate anion (or perchlorate in 2)
was found in the structure the two ligands must be in different
oxidation states (anion-radical and dianion) for charge balance,
assuming trivalent metal ions. Indeed, this is clearly observed
in the crystal structures of 1 and 2 by comparing key bond
distances between the two coordinated ligands. The N-N and
C-O bond distances differ markedly; in one ligand the N-N and
C-O bonds are significantly longer [N(4)-N(5) = 1.374(3) in
1/1.390(3) in 2 and O(2)-C(19) = 1.303(3) in 1/1.314(3) Å in 2]
than in the other ligand [N(1)-N(2) = 1.338(3) in 1/1.336(3) in 2
and O(1)-C(1) = 1.270(3) in 1/1.278(3) Å in 2]. The ligand with
the shorter N-N bond assumes a planar conformation and the
other ligand with the longer N-N bond exhibits an observable
twisting when coordinated to the metal ion; DFT calculations
show that the planar ligand corresponds to an anion-radical
oxidation state and the twisted ligand is a closed-shell dianion.
Aerial oxidation of one ligand in each of 1 and 2 produces one
equivalent of the anion-radical. For comparison, we have
optimized the gas-phase structures of the uncoordinated phpl
The ligand mixed-valency in complexes 1 and 2 is also supported
by the presence of a very broad and intense intervalence charge
transfer (IVCT) band at very low energy identified in the mid-
infrared spectrum of single crystals of 1 (Figure 3, right & Figure
S13 for 2). In the FT-IR spectra of 1 and 2 the center of this band
is at approximately 3000 cm-1, which is significantly lower in
energy than the band found in cobalt dioxolene complexes
containing o-semiquinone/o-catecholate ligands (~4000 cm-
1).38 In the UV/VIS/NIR spectrum of 1 (Figure 3, left) an intense
absorption is noted in the visible region (18500 cm-1) and broad
transitions are also observed at lower energy (11360 & 5970
nm). In the UV/VIS/NIR spectrum of complex 2 (Figure S14), the
absorptions at lower energy are much weaker than those
observed in complex 1. These bands can be assigned as
combinations of inter- and intraligand and ligand-to-metal
charge transfer transitions.
2 | J. Name., 2012, 00, 1-3
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to approximately 50 K and are very close to the anticipated
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DOI: 10.1039/C9DT01788G
value for one S = ½ radical (0.375 cm K mol ). The gradual
decrease below 50 K can be assigned to weak intermolecular
antiferromagnetic magnetic exchange interactions. DFT
calculations on the doublet spin state of 2 indicate spin density
distributed over the coordinated anion-radical phpl ligand just
like in 1 (Figures S15-S16 & Table S12) but no spin density at the
metal ion, as anticipated for low-spin Co3+.
Figure 3. UV/VIS/NIR spectrum of 1 in CH2Cl2 at 298 K with the NIR region run at high
concentration to illustrate the low energy transitions (left). FT-IR (KBr) spectrum of 1
(right).
The electrochemical properties of 1 and 2 were investigated by
cyclic voltammetry (CV) in CH2Cl2 solution containing nNBu4PF6
supporting electrolyte (Figure S11 & S17). A series of very
closely spaced quasi-reversible waves were observed at low
anodic or cathodic potentials, highlighting the significant redox
activity of the complex. CV data from the uncoordinated and
protonated ligand were very poor; broad, unresolved and
irreversible processes were observed as the uncoordinated phpl
ligand is not very stable in solution and so we could not obtain
quality data for comparison to the electrochemical properties
of 1. In the solid-state single crystals of complex 1 are
diamagnetic and EPR silent, which indicates a low-spin Fe3+ ion
(S = ½) that is strongly antiferromagnetically coupled to a ligand
anion-radical. There is no temperature dependence of the
magnetic susceptibility values (which are negative over the
temperature range 300-5K) indicating essentially spin-pairing
between the d unpaired electron of the low-spin Fe3+ ion and
Figure 4. Left: Mulliken spin density in the broken-symmetry singlet state of 1
(B3LYP/def2-TZVP). Red and blue correspond to  and  spin densities, respectively.
Isovalue 0.004. Mid- & right: HOMO & LUMO in the broken symmetry singlet state
of 1 calculated at the B3LYP/def2-TZVP level of theory. Isovalue 0.03.
We measured the temperature dependence of the resistivity of
a suitable single crystal of 1 using the two-contact method. The
quality of crystals of 2 was too poor to be measured in this
manner. To our surprise, we observed semiconductor
properties in the neutral complex
1 (the temperature
dependence of the electrical conductivity of 1 is plotted in
Figure 5, left) with a room temperature conductivity value of
3.05  10-4 S cm-1. A low activation energy of 245 meV was
obtained from an Arrhenius-type plot (Figure 5, right). The
electrical conductivity present in complex 1 portends that the
phpl framework offers a promising scaffold to prepare single-
component multifunctional materials with potentially
interesting magnetic features (in, for example, new complexes
that exhibit spin transitions or in SMM materials) together with
the ability to transport electrical charge.
the spin of phpl radical-anion.
A broken-symmetry DFT
calculation using the coordinates from the X-ray structure
provided a lower energy open-shell singlet state and a large
magnetic exchange coupling constant of -243 cm-1. The
Mulliken spin density (Figure 4, left) is concentrated at Fe (0.796
 spin density) as well as on the ligand with the shorter N-N
bond distances and primarily distributed over the hydrazido N
atoms (0.46  spin density). The open-shell anion-radical ligand
oxidation state concentrates the spin density on the acyclic N
atoms, which is akin to a hydrazido anion-radical (Scheme 1,
centre) and bodes well for the use of this ligand to produce
strong magnetic interactions with paramagnetic metal ions.
The -spin frontier molecular orbitals of 1 (Figure 4, mid- &
right) are highly delocalized -orbitals with coefficients largely
concentrated on the ligands, with orbital density accumulated
on the electron rich dianionic ligand in the HOMO, but on the
anion-radical in the LUMO. TD-DFT calculations (Table S11) on
1 also predict very low energy transitions (2851 and 3255 cm-1)
that involve contributions from mainly the / spin frontier
orbitals, similar to the low energy transitions that have been
assigned as IVCT bands between catecholate and semiquinone
ligands in the low-spin state of cobalt o-dioxolene valence
tautomers.34 Complex 2 is paramagnetic with S = ½ originating
from one ligand anion-radical (Co3+ is low-spin and
diamagnetic), which was verified by variable temperature
magnetic susceptibility experiments and EPR spectroscopy
(Figures S18-S19). The values of mT (0.39 cm3 K mol-1) are
essentially temperature independent from room temperature
Figure 5. Temperature dependence of the electrical conductivity of 1 (left, single crystal,
2-point method). Arrhenius plot (right).
In summary, the design and synthesis of new functional ligands
remains an important challenge in coordination chemistry.
Based on the results presented herein we can conclude that the
new phpl ligand exhibits a number of intriguing features,
including different stable oxidation states upon metal ion
coordination, large spin densities on donor atoms in an open-
shell oxidation state and strong magnetic exchange coupling
with a paramagnetic transition metal ion. We have also
observed semiconductor properties in iron complex
1
containing phpl. The coordination chemistry of phpl and other
ligand derivatives produced by substitution at the phenalenol
ring, changing/altering the N-heterocycle and the overall ligand
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J. Name., 2013, 00, 1-3 | 3
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topicity should produce new multi-property coordination 16
materials. These efforts are underway and will be reported on
F. S. Guo and R. A. Layfield, Chem. Commun., V2ie0w17Ar,ti5cl3e,Online
3130–3133.
DOI: 10.1039/C9DT01788G
shortly.
This work was financially supported primarily with funding
obtained from the NSERC (RGPIN-2017-05230) and Brock 18
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Conflicts of interest
There are no conflicts to declare.
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Neutral Fe3+ and Co3+ complexes are reported featuring mixed-valent open- and closed-shell
ligands, low energy IVCT bands and electrical conductivity.