Solid state π-π Stacking and higher order dimensional crystal packing, reactivity, and electrochemical behaviour of salphenazine actinide and transition metal complexes

Article abstract of DOI:10.1039/c6dt02389d

Condensation of a 2,3-diaminophenazine or 2,3-diamino-2-quinoxalinol with two equivalents of 3,5-ditertbutylsalicylaldehyde affords new Schiff base ligands. Here, we describe and compare the synthesis, UV-Vis, electrochemical, solution, and solid state behaviour of the free base, salphenazine ligand [L^I], and M[L^I] complexes, where M = UO₂(vi), Cu(ii), VO(iv), Zn(ii), Co(ii), and Ni(ii). The change in π-overlap and π-stacking between molecules and long-range ordering of the solid-state structure is vastly different depending on the size and electronic character of the metal. A sterically constrained μ-oxo Fe(iii) dimer complex is also reported.

Full text of DOI:10.1039/c6dt02389d

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Solid State π–π Stacking and Higher Order Dimensional Crystal
Packing, Reactivity, and Electrochemical Behaviour of
Salphenazine Actinide and Transition Metal Complexes
Received 00th January 20xx,
Accepted 00th January 20xx
E. E. Hardy,a M. A. Eddy,a B. A. Maynarda and A. E. V. Gordena
DOI: 10.1039/x0xx00000x
Condensation of
a
2,3-diaminophenazine or 2,3-diamino-2-quinoxalinol with two equivalents of 3,5-
www.rsc.org/
ditertbutylsalicylaldehyde affords new Schiff base ligands. Here, we describe and compare the synthesis, UV-Vis,
electrochemical, solution, and solid state behaviour of the free base, salphenazine ligand [LI], and M[LI] complexes, where
M = UO2(VI), Cu(II), VO(IV), Zn(II), Co(II), and Ni(II). The change in π-overlap and π-stacking between molecules and long-
range ordering of the solid-state structure is vastly different depending on the size and electronic character of the metal. A
sterically constrained μ-oxo Fe(III) dimer complex is also reported.
the uranyl dication. This, coupled with the imine nitrogen soft
donor, improves binding of actinides as compared to
lanthanides, which can tend to form polymeric aggregates.13,14
Introduction
Nuclear power and nuclear medicine are two potential
applications of actinide elements.1 The isolation of actinides
for nuclear fuels and accidental spills have introduced
actinides into the environment and raised concerns about
health effects.2 Rapid in-the-field identification and isolation
could greatly improve the ease of remediation.3 Schiff base-
type ligands with an O-N-N-O coordination core have been
investigated in numerous applications4 including: catalysis,5
antimicrobial assays,6 in organic light emitting diodes,7 and in
fluorescent or colorimetric sensors.8a-c Salen metal complexes
are of interest in these and other applications due to the
relative ease of introducing functionality while maintaining a
simple synthetic approach.8b Preparation of metal complexes
using metal templation reduces the potential of additional side
products while maintaining high yields.9
The salphenazine [L1] ligand features the further extended
phenazine backbone and a distinct hypsochromic shift with the
coordination of uranyl dication (UO22+). This is very distinct
from the bathochromic shift seen in the complexes with
transition metals.15,16 Here, we report the detailed
characterization of salphenazine [L1] and comparable salqu [LII]
metal complexes in solids by X-ray diffraction, in solution
spectroscopy, and electrochemistry.
Materials and Methods
General Synthetic Details
Caution! The uranium metal salts - UO2(NO3)2•6H2O - used in
this study contained depleted uranium, standard precautions
for handling radioactive materials or heavy metals, such as
uranyl nitrate and lead sulfate, were followed.
Actinides are present at low concentrations in soils and
wastewater streams.10 In comparison, transition metals are
present at much higher concentrations, and the transition
metals can compete with f-elements for coordination sites.11
This can complicate the rational design of ligands for actinide
selective coordination; in particular, the highly electropositive
copper has been found to yield a false positive result in ligands
designed to selectively isolate or detect uranyl (UO22+).12 The
O-N-N-O binding coordination pocket of the Salqu ligand [LII]
with an extended conjugated heterocycle (2-quinoxolinol)
incorporated into the backbone accommodates the
The reagents 1,5-difluoro-2,4-dinitrobenzene (97%, Matrix
Scientific), leucine methyl ester hydrochloride (TCI),
diisopropylethylamine (Aldrich), ammonium hydroxide (BDH),
ammonium formate (97%, Aldrich), palladium 5% on carbon,
dry, type 87L (Alpha Aesar), 3,5-di-tert-butylsalicyaldehyde
(98%, TCI), 2,3-diaminophenazine (98%, Aldrich), copper (II)
acetate hydrate (>98%, MC/B), copper (II) acetylacetonate
(98% STREM), vanadium (IV) bis(acetylacetonato)oxide (98%,
STREM), cobalt (II) acetate tetrahydrate (Mallinckrodt), nickel
(II) acetate tetrahydrate (Aldrich), zinc acetate dehydrate
(Aldrich) and tetra-n-butylammonium perchlorate (Alfa Aesar,
electrochemical grade) were used as received without further
preferential equatorial pentacoordinate binding geometry of
purification.
UO2(NO3)2•6H2O (98%, J. T. Baker) was
recrystallized from an aqueous nitric acid solution, stored
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under hexanes and washed and dried before use. THF (BDH), to [FeLI]+. HRMS (EI) m/z: (M+) calcd 696.3127, found
chloroform (BDH), ethyl acetate (BDH), hexanes (BDH), 696.3117.
acetone (BDH) and pyridine (anhydrous, Aldrich) were used as
received with no further purification.
Co[LI] 2,3-diaminophenazine (201 mg, 0.958 mmol)
dissolved in 35 mL of pyridine was added slowly to a round
UO2[LI] and Cu[LI] salphenazine metal complexes were bottom flask containing 3,5-ditertbutylsalicylaldehyde (541
prepared using metal templation methods.16 Synthesis of 6,7- mg, 2.31 mmol), cobalt (II) acetate hydrate (220. mg, 0.885
diamino-2-quinoxalinol and 3,5-ditertbutylsal-2-quinoxalinol mmol), 20 mL of pyridine, and a stir bar. The reaction vessel
(salqu) [H2LII]
templation.13a,17 Purity was confirmed by HRMS and NMR using a rotary evaporator, and the remaining solid was dried in
spectroscopy. a vacuum oven at 60 °C overnight. The resulting crude solid
,
UO2[LII], and Cu[LII] were prepared using metal was heated to 80 °C for 24 hours. The solvent was removed
VO[LI] 2,3-diaminophenazine (231 mg, 1.10 mmol) was was purified using column chromatography (4:3
dissolved in 35 mL of pyridine and added slowly to a round hexanes:acetone). Yield: 20.1 % HRMS (EI) m/z: (M+H) calcd
bottom flask containing 3,5-ditertbutylsalicylaldehyde (507 699.3109, found 699.3079.
mg, 2.17 mmol), vanadium (IV) bis(acetylacetonato)oxide (281
Ni[LI] 2,3-diaminophenazine (190. mg, 0.904 mmol) was
mg, 1.06 mmol), 20 mL of pyridine, and a stir bar. The reaction dissolved in 35 mL of pyridine and added slowly to a round
vessel was heated to 80 °C for 24 hours. The solvent was bottom flask containing 3,5-ditertbutylsalicylaldehyde (485
removed using a rotary evaporator, and the remaining solid mg, 2.07 mmol), nickel (II) acetate hydrate (193 mg, 0.778
dried in a vacuum oven at 60 °C overnight. This crude solid was mmol), 20 mL of pyridine, and a stir bar. The reaction vessel
purified using column chromatography (2:1 hexanes:ethyl was heated to 80 °C for 24 hours. The solvent was removed
acetate). Yield: 27.4 %; red crystals were grown from slow using a rotary evaporator, and the remaining solid was dried in
diffusion of methanol into a saturated solution in THF. CCDC: a vacuum oven at 60 °C overnight. The crude solid was
1452867. HRMS (EI) m/z (M+H) calcd 708.3166, found purified using column chromatography (2:1 hexanes:ethyl
708.3269.
acetate). The mobile phase was evaporated under reduced
Zn[LI] 2,3-diaminophenazine (197 mg, 0.940 mmol) was pressure to yield a dark red solid. Yield: 36.9 %. HRMS (EI) m/z:
dissolved in 35 mL of pyridine and subsequently added slowly (M+H) calcd 699.3209, found 699.3221.
[H2LI] (salphenazine) UO2[LI] (100 mg) was dissolved in 20
to
a
round
bottom
flask
containing
3,5-
mL of THF and added to 60 mL of chloroform. Nitric acid (80
mL, 1M) was added to the red organic phase in a separatory
funnel. The organic layer was washed a second time with
deionized water (80 mL). The yellow organic layer was
evaporated under reduced pressure and the ligand was
separated from the monosubstitued side product via column
chromatography (2:1 hexanes:ethyl acetate). Yield: 40%;
orange crystals were grown from slow evaporation of the
mobile phase. Products were confirmed via mass spec analysis
HRMS (EI) m/z: (M+H) calcd 643.3934, found 643.3942. CCDC:
1452415.
ditertbutylsalicylaldehyde (505 mg, 2.18 mmol), zinc acetate
dihydrate (237 mg, 1.04 mmol), 20 mL of pyridine, and a stir
bar. The reaction vessel was heated to 80 °C for 24 hours. The
solvent was removed using a rotary evaporator, and the
remaining solid was dried in a vacuum oven at 60 °C overnight.
This crude solid was purified using column chromatography
(2:1 hexanes:ethyl acetate). Yield: 24.2 %; red crystals were
grown from slow diffusion of methanol into a saturated
solution in THF. CCDC: 1452244. Metal complex formation was
confirmed via UV spectroscopy, λmax = 387 nm (ε = 1.79 x 104
cm-1 M-1) and 521 nm (ε = 1.51 x 104 cm-1 M-1). IR
spectroscopy, 3417.92(m), 2955.96(m), 1612.52(s), 1595.16(s),
1432.17(m), 1383.9(m), 1164.06(m), 1128.38(m), 1026.15(w),
983.71(w), 908.49(w), 870.88(w), 755.14(m), 677.99(w) cm-1.
HRMS (EI) m/z: (M+H) calcd 705.3069, found 643.3734. The
HRMS matrix was found to be acidic and stripped the zinc out
of the complex. The observed ion at 643.3734 m/z indicates
the presence of the protonated free base.
Crystallographic Details
Datasets were collected on a Bruker SMART APEX CCD X-ray
diffractometer unit using Mo Kα radiation, from crystals
mounted in Paratone-N oil on glass fibers. SMART (v 5.624)
was used for preliminary determination of cell constants and
data collection control. [H2LI], Zn[LI] and Fe[LI]-O-Fe[LI]
datasets were collected at 180K, VO[LI] dataset was collected
at 296K. Determination of integrated intensities and global cell
refinement were performed with the Bruker SAINT software
package using a narrow-frame integration algorithm. The
program suite SHELXL (v 5.1) was used for space group
determination, structure solution, and refinement.18
Refinement was performed against F2 by weighted full-matrix
least squares, and empirical absorption correction (SADABS)
was applied.19 The olex2.refinement package using Gauss-
Newton minimization was used for further refinement and to
generate a solvent mask to account for the 3 methanol
molecules in the unit cell of VO[LI]. and for 2 interstitial THF
Fe[LI]-O-Fe[LI] 2,3-diaminophenazine (211 mg, 1.01 mmol)
was dissolved in 35 mL of pyridine and subsequently added
slowly to
a
round bottom flask containing 3,5-
ditertbutylsalicylaldehyde (540. mg, 2.31 mmol), iron (III)
trichloride hexahydrate (277 mg, 1.03 mmol), 20 mL of
pyridine, and a stir bar. The reaction vessel was heated to 80
°C for 24 hours. The solvent was removed using a rotary
evaporator. The remaining solid was dried in a vacuum at 60 °C
oven overnight. The resulting crude solid was purified using
column chromatography (4:3 hexanes:acetone). Yield: 21.2 %;
red crystals were grown from slow evaporation of mobile
phase. CCDC: 1018600. The dimer dissociates in acidic solution
2 | J. Name., 2012, 00, 1-3
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molecules in the unit cell of Zn[LI].20 SQUEEZE was used to yl)phenol product via column chromatography.
remove 1.5 THF molecules from the unit cell of Fe[LI]-O-Fe[LI].
Projections were generated using the Olex2.1-1 graphics
program.20
P3
P2
Spectroscopic Details
P1
UV-Vis experiments were performed on a Varian Cary 50
WinUV Spectrophotometer. Solutions for each titration were
made individually and stirred for 5 minutes before data
collection.
Electrochemical Details
[H2LI]
Cyclic voltammetry experiments were performed using an
Figure 1. Salphenazine [H2LI] crystal structure. Carbon atoms shown in grey, nitrogen
shown in blue, oxygen shown in red, hydrogen atoms and interstitial ethyl acetate
removed for clarity. Solid-state π– π interactions are highlighted in this projection
AFCBP1 bipotentiostat driven by a PC with the Aftermath
software package. All experiments were completed with a Pt
disk working electrode, a Pt wire counter electrode, and an
Ag/AgCl reference electrode (BASi MF-2052). Solutions for
cyclic voltammetry experiments were 1.0 mM in the salqu
metal complex or ligand (M[LII] and [H2LII]), or 0.5 mM in the
salphenazine metal complex or ligand (M[LI] and [H2LI]). All
solutions were made in dichloromethane and contained 0.1 M
tetra-n-butylammonium perchlorate (TBAP). Platinum
electrodes were cleaned with piranha acid (H2SO4/H2O2),
polished in between experiments, and all electrodes were
stored according to factory suggestion. All experiments were
performed at room temperature.
The structures of the free-base form of ligand [H2LI] has
been characterized by single crystal X-ray diffraction.
Crystallographic details of ligands and metal series are listed in
Table 1 and important bond distances and angles are listed in
Table 2. The ligand [H2LI] crystallizes in the monoclinic space
group P21/n. Structural representations of [H2LI] is shown in
Figure 1. The π–π distance between the P1-P3 rings, also
depicted in Figure 1, is 3.471 Å. Of note is the non-planarity of
the tetradentate binding pocket. This is counterintuitive from
the highly conjugated system; however, the bond distances
are such that repulsion of the two oxygen atoms distorts the
planarity of the extended π-conjugated network. One phenol
oxygen atom is almost in plane with the phenazine backbone
while the second distorts from planarity significantly, distances
above the plane are O1-plane = 1.054(2) Å and O2-plane =
0.316(2) Å. The binding of a metal to the binding pocket causes
a significant change in π-overlap, which can be observed by
the change in the electronic spectra of both ligands. In
particular, the yellow solution of [H2LI] turns red in the
presence of common first row transition metals while it turns
an orange color in the presence of uranyl, the coordination of
which more greatly disturbs the planarity of conjugation.
When a metal is bound, the electronic repulsion of the phenol
oxygen is eliminated. The smaller ionic radii of transition
metals allows for more pi orbital overlap as compared to
uranyl, causing a bathochromic shift of the charge transfer
band. Complex VO[LI], shown in Figure 2, crystallizes in the
triclinic space group Pī . This complex has a distorted square
pyramidal geometry with the axial position being an oxo group
on the vanadyl cation (VO2+). The geometry of the metal in the
binding pocket is indicated by the bond angles of O1-V-N1, N1-
V-N2, N2-V-O2, O2-V-O1 of 86.94(8), 78.27(8), 87.31(8), and
86.90(7)° respectively, comparable to other vanadyl salen and
salophen complexes (N1-V1-N2 77.3-78.7° and O1-V-N1, N2-V-
O2, O2-V-O1 (86.3-88.8°).21 The VO[LI] complex is skewed
from square pyramidal geometry as compared to the
Zn[LI](CH3OH) and Fe[LI]-O-Fe[LI] complexes, both of which are
also square pyramidal geometries. The vanadium atom V-N1
Results and Discussion
Solid State
The condensation of 2,3-diaminophenazine with two
equivalents of 3,5-ditertbutylsalicylaldehyde was employed to
prepare are series of 1:1 metal:ligand complexes M[LI] in good
yields (20 - 36 %). The use of metal templation to prepare the
metal complexes limits the formation of an imidazole 2-(1H-
imidazo[4,5-b]phenazin-2-yl)phenol side product. This
templation synthetic scheme (Scheme 1) was used to prepare
the salphenzaine M[LI] complexes where M = UO2(VI), Cu(II),
VO(IV), Zn(II), Fe(III), Co(II), and Ni(II). The preparation of the
free-base form of [H2LI] in quantity both large enough and
pure enough for further studies required stripping the
coordinating metal from the complex using a nitric acid wash.
The free-base was dissolved in chloroform, and washed with 1
M HNO3. The organic phase was then washed three times with
water and then a brine solution, followed by evaporation
under reduced pressure. Finally, the free-base was separated
from the monosubstitued 2-(1H-imidazo[4,5-b]phenazin-2-
Scheme 1. Metal templated synthesis of salphenazine complexes M[LI].
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and V-N2 bond distances are 2.050(2), 2.054(2) Å, respectively. involving a dimer held together by π–π interactions in the solid
These bond lengths are comparable to state. These sheets are separated by 12.55 Å. The π–π distance
Table 1. Crystallographic data and details of data collection for [H2LI], UO2[LI](H2O), Cu[LI], VO[LI], Zn[LI](CH3OH), and Fe[LI]-O-Fe[LI].
others in the literature (2.05-2.07 Å) for salen type complexes, between the P1-P2 rings on phenazine of the dimer is 3.36 Å.
but the lack of symmetry in the angles is unusual.21 The twist These electron rich systems prefer off-center parallel π–π
in H2LI from the sterically hindered t-butyl groups causes a lack stacking. The M–P3 distance between the dimer phenazine
of symmetry in the binding pocket. The V-O1 and V-O2 bond rings is 3.235 Å, suggesting possible metal backbonding to the
distances of 1.929(2) and 1.930(2)
Å respectively are pi system of the ligand.
comparable to literature (1.91-1.95 Å) which have greater
variety than the vanadium nitrogen distances.21
Complex Zn[LI](CH3OH) crystallizes in the triclinic space
group Pī and is shown in Figure 2. This complex has a square
This complex forms pseudo two dimensional sheets pyramidal geometry around the zinc atom with a coordinating
neutral methanol molecule. This is observed by the close to
Table 2. Selected bond distances (Å) and bond angles (deg) for [H2LI], UO2[LI](H2O),
Cu[LI], VO[LI], Zn[LI](CH3OH), and Fe[LI]-O-Fe[LI].
90° bond angles of the zinc to the O-N-N-O ligand donor
atoms, O1-Zn-N1, N1-Zn-N2, N2-Zn-O2, O2-Zn-O1 are
89.90(11), 79.46(11), 89.55(10), 94.88(10)° respectively. These
bond angles are comparable to previously reported zinc
structures with coordinating methanol or pyridine solvent
(79.0-95.5°).22a,b The oxygen (O3) from the methanol is also
approximately 90° from the equatorially bound ligand with
O3-Zn-O1, O3-Zn-O2, O3-Zn-N1, O3-Zn-N2 angles of
101.02(11), 98.79(11), 95.01(10) and 105.18(11)° respectively.
This coordination solvent is quite labile, and the zinc metal
quickly dissociates from the ligand under acidic conditions. The
zinc atom binds less securely than the copper in the
tetradentate binding pocket with Zn-N1, Zn-N2, Zn-O1, Zn-O2
bond distances of 2.074(3), 2.076(3), 1.954(2), and 1.965(2) Å,
respectively. These bond lengths are within ± 0.03 Å of the
previously reported Zn(salphenzine)(DMSO) complex,23 and
within standard lengths of previously reported zinc salophen
structures(Zn-O 1.93-1.97 Å and Zn-N 2.04-2.09 Å).22a-c
Complex Fe[LI]-O-Fe[LI] crystallizes in the monoclinic space
4 | J. Name., 2012, 00, 1-3
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Zn[LI](CH3OH)
VO[LI]
Fe[LI]-O-Fe[LI]
Figure 2.Projections of VO[LI], Zn[LI](CH3OH), and Fe[LI]-O-Fe[LI] complexes. Solid-state dimers and pseudo 2-D sheets are also shown. Transition metal shown in purple,
carbon atoms shown in grey, nitrogen shown in blue, oxygen shown in red, hydrogen (when not omitted for clarity) are shown in white. All interstitial solvents were removed
for clarity.
group C2/c and is shown in Figure 2. The iron (III) atom is in a ligand causes a shift from a planar backbone and results in the
square pyramidal geometry with the axial position being uranium atom being 1.954(9) Å above the plane of the ligand
occupied by a bridging oxo group to a second, symmetry as defined by C4-C21-C22-C31. This puckering, highlighted in
related, iron-ligand complex. This phenomenon is well Figure 3, is the explanation of the spectroscopic distinction
documented in the literature, with 814 examples of mixed
nitrogen and oxygen donor ligands with μ-oxo geometries, and
28 examples of salen ligands with μ –oxo geometries reported
in the Cambridge Crystallographic Data Center (CCDC).24 The
iron atom binds in the tetradentate binding pocket, but has
the most distortion from square pyramidal as compared to the
vanadyl and zinc structures with O1-Fe-N1, N1-Fe-N2, N2-Fe-
O2, O2-Fe-O1 bond angles of 76.81(9), 87.14(10), 86.12(7), and
93.85(9)° respectively. These angles are within other examples
in the literature (76.3-94.5°).25a,b The Fe-N1, Fe-N2, Fe-O1, and
Fe-O2 bond distances are also the longest of the transition
metal structure in this M[LI] series with lengths of 2.1105(18),
2.100(3), 1.9216(17), and 1.906(2) Å, respectively. These
distances are comparable to other Fe-salen dimers and
elongated in comparison to some less sterically hindered
complexes.25a-e Of note is the lack of π-stacking in the dimer,
and the Fe1-O3-Fe1 angle of 161.50(16)° which is comparable
to other μ-oxo species25a,b but is well below the stereotypical
limits observed of 1.75-1.80°.25a This can be explained by steric
hindrance of the substituted aldehyde moieties. The π–π
distance between the P1-P2 rings on the external dimer
phenazine rings is 3.35 Å. The metal is 3.172 Å above the P3
ring.
Structurally, the uranyl metal complex UO2[LI] is most
similar to the ligand geometry. The axial positions of the
uranium coordination sphere are occupied by the two oxo “-
yl” groups of the uranyl subunit. The uranium atom is bound
Figure 3. Projections of M[LI] series highlighting the pucker of the tetradentate
further out in the pocket than the transition metals. These
binding pocket. Carbon atoms shown in grey, nitrogen shown in blue, oxygen
bond distances are consistently 0.5
Å longer than the
shown in red, transition metals shown in purple, uranium shown in green, and
hydrogen omitted for clarity.
transition metal analogues (Table 2). This puckering of the
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between uranyl and the transition metal analogues. The
planarity of the ligand, regardless of the steric bulk in the 3-
position of the aldehyde, for the transition metal complexes
was surprising. The planarity of the transition metal structures
can be observed most distinctly by the distance of the Cu-Lplane
distance of 0.178(3) Å. The VO[LI] structure is second only to
the uranyl structure in distance from metal to ligand plane
with V-Lplane distance of 0.967(7) Å. The Zn-Lplane distance of
0.837(2) Å above the ligand plane is accompanied by a twist in
the O-N-N-O pocket, explaining the lability observed in this
metal complex as compared to the other analogues. The iron
atom is closer to the plane of the ligand, however, than
Zn[LI](CH3OH) or VO[LI] structures, with a Fe-Lplane distance of
0.583(2) Å. The iron atom is closer to the ligand plane than the
other structures, while simultaneously having longer bond
lengths suggesting that the tert-butyl groups in the 3 and 3’
positions keep the pocket open wider than is strictly necessary
for this metal centre.
Table 3. UV-Visible data for M[LI]complexes and free base [H2LI], 20 µM in
pyridine solution.
λ (nm) ε (x 104 cm-1 M-1
)
[H2LI]
420
369
470
394
455
521
375
471
387
521
1.80
2.64
1.95
1.62
0.959
1.95
1.59
1.41
1.79
1.51
1.58
0.926
1.88
1.29
1.35
1.22
1.08
1.42
UO2[LI]
Cu[LI]
VO[LI]
Zn[LI]
Fe[LI]-O-Fe[LI] 408
511
Co[LI]
Ni[LI]
338
403
509
374
408
509
Spectroscopy
All UV measurements were done using
a Cary 50
spectrometer and with pyridine as a solvent due limitations of
solubility. The λmax values and molar absorptivities of the
charge transfer bands for M[LI] complexes are listed in Table 3
.
Ligand [H2LI] exhibits and absorption band at 424 nm. Upon 1:1
2+
binding of UO2 a 60 nm hypsochromic shift of the π-π*
excitation to 364 nm is observed. A charge transfer band with
a maximum absorbance at 470 nm (ε = 1.95 x 104 cm-1 M-1), a
separation of 48 nm from the original ligand peak, and a
shoulder at 520 nm are also observed in UO2[LI]. By
comparison, the Cu[LI] complex, exhibits a hypsochromic shift
of the ligand peak of 24 nm and a charge transfer band at 521
nm (ε = 1.95 x 104 cm-1 M-1) with a shoulder at 470 nm, a peak
separation of 98 nm.
Compound [H2LI] was complexed with other earth
abundant transition metals Cu2+, Zn2+, Fe3+, Co2+, Ni2+, and VO2+
as shown in Figure 4. The complex Zn[LI] exhibits a charge
transfer band at 520 nm (ε = 1.51 x 104 cm-1 M-1) with a
hypsochromic shift of the ligand peak by 37 nm. The Fe[LI]-O-
Fe[LI] complex has a charge transfer band at nm (9.26 x 103
cm-1 M-1) after the templation condensation reaction, but
shows no reaction when the free metal salt is introduced. The
presence of a Co2+ ion (Co[LI]) induced a charge transfer band
at 517 nm (ε = 1.35 x 104 cm-1 M-1), a peak separation of 93
nm, and a hypsochromic shift of the ligand peak by 12 nm.
Introduction of the Ni2+ ion (Ni[LI]) induced a charge transfer
band at 509 nm (ε = 1.42 x 104 cm-1 M-1), a peak separation of
85 nm, with a shoulder at 570 nm which was not present in
any of the other complexes. A hypsochromic shift of 10 nm,
along with an additional high energy peak was also observed.
Complex VO[LI], which is the least likely contaminant in waste
water samples, has a maximum at 471 nm (ε = 1.47 x 104 cm-1
M-1). The binding of vanadyl is much slower, over 24 hours,
than the other transition metals. The UV-Vis spectra of
compounds [H2LI] and M[LI] complexes are shown in Figure 4
.
The UO2[LI] complex has greater than 30 nm of separation
from all the transition metals, save VO[LI], and 50 nm
separation between the ligand and complex absorbance.
[H2LI]
UO2[LI]
Cu[LI]
VO[LI]
Zn[LI]
Fe[LI]-O-Fe[LI]
Co[LI]
Ni[LI]
0.5
0.4
0.3
0.2
0.1
0
Electrochemistry
Cyclic voltammagrams for the salphenazine compounds
[H2LI], UO2[LI], and Cu[LI] are shown in Figure 5a. The cyclic
voltammagrams of salqu compounds
(Scheme 2)
[H2LII],
UO2[LII], and Cu[LII] are compared in Figure 5b. The change in
stability of ligand oxidation was also investigated. All
compounds were studied at 1.0 mM concentration in a
370
420
470
520
570
620
dichloromethane solution with 0.100
M
of tetra-n-
Wavelength(nm)
butylammounium perchlorate electrolyte. Experiments were
run with a scan rate of 0.10-1.50 V s-1 on a platinum working
electrode vs. a Ag/AgCl reference electrode and a platinum
wire counter electrode.
Figure 4. UV-Vis spectral changes of M[LI] complexes at 20 µM concentration in
pyridine.
6 | J. Name., 2012, 00, 1-3
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[H2LI]
UO2[LI]
Cu[LI]
55
45
35
25
15
5
[H2LII]
M[LII]
-5
Scheme 2. Salqu [H2LII] and salqu complexes M[LII], synthesized according to
-15
literature procedure.
0.6 0.8
1
1.2 1.4 1.6 1.8
2
The salphenazine [H2LI] ligand shows four oxidation peaks,
at Epox,a = 1.21 V, Epox,b = 1.33 V, Epox,c = 1.56 V, and Epox,d = 1.77
V, none of which are reversible. This result suggests [H2LI] can
stabilize not only the cationic radical on the phenol groups, but
can further oxidize. Reduction peaks were not present in [H2LI].
The same is not true for salqu [H2LII]; two oxidation peaks at
E (V) vs Ag/AgCl
5a
55
45
35
25
15
5
[H2LII]
UO2[LII]
Cu[LII]
ox,a
ox,b
Ep =1.17 V and Ep = 1.61 V and two reduction peaks at
red,a
red,b
Ep
= -0.085 V and Ep
= -0.62 V which are all not
reversible are observed. The two oxidation peaks are indicative
of a phenoxyl radical formed from a single electron oxidation.
The two reduction peaks are indicative of reduction of the
imine nitrogen.
-5
-15
0.6 0.8
1
1.2 1.4 1.6 1.8
E (V) vs Ag/AgCl
2
Two of the oxidation peaks from [H2LI] are absent from the
ox,a
complex [H2LII] voltammagram. Two oxidation peaks at Ep
=
5b
1.19 V and Epox,b = 1.52 V remain and are now quasireversible
with separation of the anodic of cathodic peaks of 30 and 150
mV respectively shown in Figure 5b. One oxidation peak of
[H2LI] is not observed in the salphenazine complex Cu[LI] which
Figure 5a. Cyclic Voltammagram of [H2LI], UO2[LI], and Cu[LI]. Conditions: 0.5
mM ligand in dichloromethane, room temperature, tetra-n-butylammonium
perchlorate (0.1 M) as electrolyte with a 0.10 V s-1 scan rate. Figure 5b. Cyclic
Voltammagram of [H2LII], UO2[LII], and Cu[LII]. Conditions: 1.0 mM ligand in
dichloromethane, room temperature, tetra-n-butylammonium perchlorate (0.1
M) as electrolyte with a 0.10 V s-1 scan rate.
ox,a
ox,b
ox,c
has oxidation peaks at Ep = 1.17 V, Ep
= 1.30 V, Ep
=
1.72 V. The three remaining oxidation peaks do not shift in
oxidation potential from the ligand by more than E1/2 = 0.050 V peak Epred,c = 0.82 V present in complex Cu[LII] which may be
suggesting minimal stabilization of the phenoxyl radicals attributed to a Cu(III)-phenolate species that is present
formed upon oxidation by the copper ion. The oxidation peaks simultaneously with the Cu(II)-phenoxyl radical species.
in UO2[LI] become quasi reversible suggests that the phenoxyl
Complex UO2[LI] resulted in two reduction peaks, at Ep
=
red,a
red,b
radicals are stabilized by the uranyl ion. A new reduction peak -1.57 V and Ep
red
= -2.18 V, both of which are visibly
at Ep = 0.15 V is observed in the complex UO2[LI] that is not quasireversible in Figure 6. The UO2[LII] complex, has a
red,a
red,b
present in the free base, suggesting that the uranyl ion reduction peak at Ep
= -1.87 V and Ep
= -2.11 V. The
stabilizes the π–system enough to reduce the imine nitrogen reduction of U(VI) to U(V) in UO2[salophen] occurs at -1.67 V in
that did could occur in the free base [H2LI] (See supplemental pyridine and -1.55 in DMSO.27 Complex UO2[LII] has an ΔE=
information).
0.20 V and ΔE = 0.32 V for the U(VI)/U(V) redox couple,
The oxidation peaks in the salqu complexes UO2[LII] and revealing a better stabilization of the uranyl complex in the
Cu[LII] become quasi-reversible suggesting that the phenoxyl salqu ligand, even without the presence of a coordinating
radicals are stabilized by the metal ions. Complex Cu[LII] has solvent, than in previous salophen complexes. This
ox,a
ox,b
two single electron oxidation peaks at Ep = 1.06 V Ep
=
stabilization is not present in the UO2[LI] complex with a ΔE1/2
1.27 V with anodic and cathodic peak potential separation = 0.02 V. The second reduction peaks may be due to not only a
values in the range of 50 – 70 mV. Complex UO2[LII] has two reduction of uranium to the pentavalent uranyl cation, but
single electron oxidation peaks at Epox,a = 1.04 V Epox,b = 1.33 V subsequent disproportionation to U(IV) and U(VI) or
a
with anodic and cathodic peak potential separation values in rearrangement of the uranium geometry. If a coordinating
the range of 20 – 50 mV that seem to be quasi-reversible at solvent is removed from the coordination sphere, an
best. Both metal complexes show reversible ligand oxidation additional oxidation peak may present itself.
ox,a
peaks at Ep
and Epox,b, indicative of phenol type radical
oxidation of the ligand.26 The average shifts in the oxidation
potential from the free base [H2LII] of E1/2 = 0.12 and E1/2 = 0.
032 observed in complexes UO2[LII] and Cu[LII] suggest
moderate stabilization of the phenoxyl radical. A third cathodic
Conclusions
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Journal Name
20
0
2
3
4
Snow, M. S., Snyder, D. C., Clark, S. B., Kelley, M., Delmore, J.
E. Environ. Sci. Technol. 2015, 49, 2741.
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-20
-40
-60
-80
-100
-120
-140
5
6
7
UO2[LI]
UO2[LII]
-2.6 -2.4 -2.2 -2 -1.8 -1.6 -1.4 -1.2 -1
E (V) vs Fc/Fc+
Figure 6. Cyclic Voltammagram of UO2[LI] and UO2[LII]. Conditions: 0.5 mM
complex in dichloromethane, room temperature, tetra-n-butylammonium
perchlorate (0.1 M) as electrolyte with the ferrocene redox couple as an internal
standard.
Peng, Q., Xie, M., Huang, Y., Lu, Z., Cao, Y. Macromol. Chem.
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The salphenazine ligand, with the O-N-N-O salen type
bonding motif, has been synthesized and characterized in the
presence of various first row transition metals and uranium in
the most common oxidation state in aqueous solution (UO22+).
Complexes have been characterized by x-ray crystallography
and UV-visible spectroscopy, revealing the extended π-
conjugation of these ligands results in differentiation of a
uranyl signal as compared to these transition metal
contaminants. The solid-state characterization of this series
provides insight to the preferred coordination environment of
uranyl in these conjugated systems. The π–π stacking of all the
observed systems is within the confines of typical graphene
stacking distance of 3.21-3.50 Å. The metal centres often are
within 3.0-3.2 Å above a conjugated ring, suggesting possible
metal backbonding to the ring system. These insights can
guide further design for ligands with extended conjugation to
be used as on-site actinide chemosensors. The free bases,
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characterized by cyclic voltammetry. The extended π-
conjugation that is deemed responsible for differentiation
between metals in the spectroscopy does not necessarily
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couple, which may provide a secondary sensing method for
chemosensors of this type.
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Acknowledgements
This work was supported in part by the Defence Threat
Reduction Agency, Basic Research Award # HDTRA1-11-1-0044
to Auburn University. We gratefully acknowledge John Gorden
for helpful discussion and x-ray diffraction.
19 Bruker, Bruker AXS Inc.: Madison, Wisconsin, USA, 2001.
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