Pyrrophens: Pyrrole-Based Hexadentate Ligands Tailor-Made for Uranyl (UO22+) Coordination and Molecular Recognition

Article abstract of DOI:10.1021/acs.inorgchem.0c00439

Derivatives of a novel pyrrole-containing Schiff base ligand system (called "pyrrophen") are presented which feature substituted phenylene linkers (R1 = R2 = H (H2L1); R1 = R2 = CH3 (H2L2)) and a binding pocket modeled after macrocyclic species. These lig

Full text of DOI:10.1021/acs.inorgchem.0c00439

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Article
Pyrrophens: Pyrrole-Based Hexadentate Ligands Tailor-Made for
2
+
Uranyl (UO ) Coordination and Molecular Recognition
2
‡
‡
Jacob T. Mayhugh, Julie E. Niklas, Madeleine G. Forbes, John D. Gorden, and Anne E. V. Gorden*
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sı Supporting Information
ABSTRACT: Derivatives of a novel pyrrole-containing Schiff base ligand system (called “pyrrophen”) are presented which feature
1
2
1
1
2
2
substituted phenylene linkers (R = R = H (H L ); R = R = CH (H L )) and a binding pocket modeled after macrocyclic
2
3
2
species. These ligands bind neutral CH OH in the solid state through pyrrolic hydrogen-bonding. The interaction of the uranyl
3
2
+
1−2
2+
2+
cation (UO2 ) and H2L yields planar hexagonal bipyramdial uranyl complexes, while the Cu and Zn complexes were found to
self-assemble as dinuclear helicate complexes (M L ) with H L under identical conditions. The favorable binding of UO over
Zn provides insight into the molecular recognition of uranyl over other metal species. Structural features of these complexes are
examined with special attention to features of the UO2 coordination environment which distinguish them from other related
salophen and porphyrinoid complexes.
1
2+
2
2
2
2
2
+
2
+
INTRODUCTION
“pacman” Schiff base-expanded polypyrrole macrocycles have
been studied as well, as they support unusual oxo-ligand
behaviors that are not typically accessible in standard
■
(
In f-element coordination chemistry, the linear uranyl cation
2
+
UO2 ) is generally characterized as a hard acceptor based on
17−19
porphyrinoid complexes.
Pearson’s HSAB (hard−soft acid−base) principle, owing to its
highly Lewis acidic and oxophilic nature.
topics of interest include novel ligands that contain hard donor
heteroatoms for sensing or extraction of uranyl (UO2 ) from
aqueous environments; however, many of these systems are
indiscriminate toward other cations
industrially viable in environmental systems.
as salens, as well as other mixed N/O-donor systems, have
been targeted and more extensively studied for their potential
1
−3
The synthetic accessibility and modularity of salen-type
Schiff base ligands have made them attractive targets for
potential use in separations applications; however, their affinity
for a panoply of cations often precludes them from use when
Current research
2
+
4
,20
4
,5
selective coordination is desired.
Recently, a new class of
or would not be
6
−8
ligands has been reported which utilizes a similar synthetic
methodology but replaces the salicylaldehyde with an ester-
Ligands such
21
substituted pyrrol-2-yl analogue. These species are of interest
for understanding self-assembly and molecular recogni-
3
+
3+
to achieve actinide-selective extractions from An /Ln
22−25
9
tion.
Previous reports focus on changes in the system’s
mixtures, taking advantage of the relative softness and
10
architecture which result from altering the enediamine spacer
unit or the pyrrolic arms. Most notably, these ligands self-
covalency of the trivalent 5f ions. The strategy of employing
softer donor ligands for the selective binding, chemosensing, or
simply the study of coordination complexes of hexavalent
2
+
Received: February 12, 2020
UO2 , though, has been largely unexplored. One exception is
expanded-porphyrin macrocycles, which have advantageously
large molar absorptivities and have been shown to be suitable
hosts for uranyl and other actinyl cations, though they suffer
from unfavorable kinetics and can be synthetically challenging
1
1−16
to prepare.
Systems with greater flexibility, such as
©
XXXX American Chemical Society
https://dx.doi.org/10.1021/acs.inorgchem.0c00439
A
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Article
Scheme 1. Coordination Modes of Pyrrophen
assemble with Cu2+ and Zn2+ to form M L helicate structures
vacuum to remove residual solvent prior to analysis except for H
L1
2
2
2
2
and H L , which are unstable to heating and have associated solvent,
through the coordination of the N-donor atoms exclu-
2
2
1,26
leading to larger errors. Low hydrogen content is observed for
sively.
Previously, Mn complexes of salen- and salophen-
1
duplicate samples of Cu (L ) no cause has been identified. Values
2
2
type pyrrolic ligands lacking the bulky ester substituents have
been investigated, which adopt a helical architecture with the
more flexible ethylenediamine-derived backbone and mono-
nuclear species with the more rigid phenylenediamine
are inconsistent with the inclusion of residual solvent or associated
1
anions. UO (L ) has lowest errors for a moiety formula including one
2
additional hydrogen atom, though no such proton is observed by any
spectroscopic method. All chemicals and solvents were used as
received from commercial sources, unless otherwise stated. All
solution-phase absorbance spectra were collected from 1000 to 200
nm on a VARIAN Cary 50 WinUV Spectrometer with a xenon lamp
using a 1 cm path length quartz cuvette. Titration studies were
performed using Teflon capped cuvettes to minimize evaporation of
volatile solvent over the course of the experiment. Binding constants
2
7
backbone. Similar transition metal helicates of dipyrrin-type
2
8
meso-bridged iminopyrrole ligands have also been observed.
2
+
2+
Both the Cu and Zn cations have ionic radii and charge-to-
2+
29,30
radius ratios similar to that of uranyl (UO2 ),
but the
affinity of this ligand (H L) toward uranyl, to the best of our
2
knowledge, has not been investigated. Given the presence of
six suitable donor atoms in this pyrrolic salophen-type system
31
were calculated using the program Bindfit from supramolecular.org.
Benzyl 4-Acetyl-3,5-dimethyl-1H-pyrrole-2-carboxylate (1).
(
akin to expanded-porphyrin analogs), its utilization in uranyl
Compound 1 was synthesized using modifications of a known
coordination was of interest. As this system is macroacyclic, it
presents a unique opportunity to explore the bridging of the
salen-type and extended porphyrin-type systems to form a new
class of ligand that effectively coordinates the uranyl cation
while utilizing a softer donor to increase its relative
discrimination. The ability to tune the ligand by altering the
substitution presents opportunities for improved coordination
kinetics over those of macrocyclic species and for improved
binding with uranyl versus other metals. Here, we present two
32
procedure. A cold solution of sodium nitrite (8.7 g, 0.13 mol) in
water (12 mL) was added dropwise to a stirring solution of benzyl
acetoacetate (22.5 mL, 0.13 mol) in acetic acid (28 mL) at 0 °C.
Once added, the solution was allowed to warm to room temperature
and stirred overnight. Acetylacetone (13.0 mL, 0.13 mol) and
additional acetic acid (48 mL) were added to the solution, and the
flask was cooled in an ice bath while zinc powder (33 g, 0.50 mol) was
added slowly. The reaction flask was then equipped with a condenser
and heated to 100 °C for 1 h in an oil bath. The hot solution was
poured over ice−water to quench the reaction. The resulting
precipitate was collected via vacuum filtration and washed with
additional deionized water. The solid was dissolved in hot ethanol,
then filtered while hot to remove any remaining zinc. Crystallization
bench-stable benzyl ester derivatives of a previously reported
1
ethyl ester bis(pyrrole)phenylenediamine ligand system (H L
2
2
21
and H L ), which we have nicknamed “pyrrophen,” new
2
uranyl and transition metal complexes, and describe the
features that make this framework particularly suitable for
uranyl coordination (see Scheme 1).
1
from ethanol gave compound 1 as a white solid (25.2 g, 71%). H
NMR (400 MHz, CDCl ): δ 2.45 (s, 3H), 2.51 (s, 3H), 2.62 (s, 3H),
3
5
.33 (s, 2H), 7.45−7.33 (m, 5H), 9.04 (brs, 1H).
Benzyl 4-Ethyl-3,5-dimethyl-1H-pyrrole-2-carboxylate (2). Com-
32
EXPERIMENTAL METHODS
pound 2 was synthesized using modifications of a known procedure.
■
A reaction flask was charged with a magnetic stir bar, 1 (24.4 g, 0.09
mol), sodium borohydride (8.0 g, 0.21 mol), and THF (240 mL) and
stirred at room temperature. The flask was then equipped with a
dropping funnel containing boron trifluoride diethyl etherate (53 mL,
0.43 mol). The entire system was kept under a constant stream of
General Considerations. Caution! The uranium metal salt
UO (OAc) ·2H Oused in this study contained depleted uranium.
2
2
2
Standard precautions for handling radioactive materials or heavy metals
such as uranyl nitrate and lead sulfate were followed.
1
All H NMR spectra were recorded on a Bruker AV 400 MHz or
nitrogen. The BF ·OEt was added dropwise over the course of 1 h.
Bruker AV 600 MHz spectrometer, reported in parts per million (δ,
3
2
1
After the addition was completed, the mixture was stirred for an
additional hour. The reaction was then quenched with the slow
addition of 10% HCl in water (until neutral) and extracted three
times with chloroform. The organic layers were collected and
evaporated to dryness under reduced pressure. The crude solid was
then dissolved in 400 mL of 80% ethanol in water and stored in a
refrigerator overnight. Compound 2 is collected as a white, needle-like
ppm) and referenced to residual protio-solvent (CDCl , δ 7.27, H;
7
3
1
3
7.16, C) or tetramethylsilane (TMS, δ 0.00). Deuterated
chloroform (CDCl ) was obtained from Cambridge Isotopes
3
Laboratories, Inc., Andover, Massachusetts, and used without further
purification. High-resolution mass spectrometry (HRMS) was
performed using a quadrupole time-of-flight spectrometer (Q- TOF
Premier, Waters) with electrospray ionization (ESI). Elemental
analysis (C,H,N) was performed by Atlantic Microlab, Norcross,
1
crystalline precipitate from this solution (18.9 g, 82%). H NMR (400
GA, and was collected in duplicate and reported as an average with
MHz, CDCl ): δ 1.05 (t, J = 7.6 Hz, 3H), 2.20 (s, 3H), 2.30 (s, 3H),
3
2
the exception of H L , for which there was not a sufficient sample for
2.38 (q, J = 7.5 Hz, 2H), 5.30 (s, 2H), 7.45−7.31 (m, 5H), 8.52 (brs,
1H).
2
duplicate analyses. Samples were dried overnight at 60 °C under a
B
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Benzyl 4-Ethyl-5-formyl-3-methyl-1H-pyrrole-2-carboxylate (3).
Compound 3 was synthesized using modifications of a known
evaporator, then stored at 0 °C overnight. The complex was collected
as a bright orange crystalline powder by vacuum filtration (yield:
33
1
procedure. Compound 2 (3.0 g, 12 mmol) was dissolved in a
mixture of THF (125 mL), acetic acid (32 mL), and water (32 mL)
and vigorously stirred at room temperature. To this mixture,
cerium(IV) ammonium nitrate (28.2 g, 51 mmol) was added in a
single portion. The reaction allowed to stir for 20 min before being
poured into water (250 mL) and extracted three times with
chloroform. The organic layers were combined, washed with saturated
sodium bicarbonate, and passed through a pad of silica gel. The
solution was then concentrated by means of a rotary evaporator and
0.070 g, 63%). H NMR (400 MHz, CDCl ): δ 1.09 (t, 6H), 2.16 (s,
3
6H), 2.48 (m, 4H), 4.27 (m, 2H), 5.21 (m, 2H), 6.42 (m, 2H), 6.77
1
3
(m, 4H), 6.88 (m, 8H), 7.52 (s, 2H). C NMR (600 MHz, CDCl ):
3
δ 9.50, 15.69, 16.72, 63.78, 119.94, 125.72, 126.56, 127.04, 131.79,
134.73, 135.08, 135.25, 139.96, 151.42, 161.27. FT-IR (ATR): 1699
−
1
−1
cm (s), νCO; 1610 cm (m), νCN. Anal. calcd for
Zn : C, 67.31; H, 5.35. 8.26. Found: C, 67.48; H, 5.33;
N, 8.23. λmax: 367 nm (51 700 M cm /Zn
HRMS (ESI+) calculated for C76
1355.4135. Found: 1355.4153. CCDC 1983238.
C
H
76
N
72
O
8
8
2
−1
−11
73
−1
−1
; 25 800 M cm /Zn).
2
+
H
N
O
8
Zn
([M + H] ): m/z
8
2
triturated with hexanes to give 3 as an off-white powder which was
1
1
1
collected by filtration (2.6 g, 83%). H NMR (400 MHz, CDCl ): δ
Synthesis of Cu
21 mL of a 20:1 THF/MeOH (v/v) solution in a 50 mL round-
bottom flask at room temperature and stirred. Cu(OAc) ·H O (0.039
2 2 2
(L ) . H L (0.100 g, 0.164 mmol) was dissolved in
3
1
.20 (t, J = 7.6 Hz, 3H), 2.31 (s, 3H), 2.75 (q, J = 7.6 Hz, 2H), 5.34
(
s, 2H), 7.45−7.35 (m, 5H), 9.44 (brs, 1H), 9.77 (s, 1H).
2
2
(Dibenzyl 5,5′-((1E,1′E)-(1,2-Phenylene Bis(azanylylidene))bis-
g, 0.197 mmol, 1.2 equiv) was added as a solid, causing a color change
from bright yellow to deep orange-brown. The flask was loosely
capped and stirred at room temperature for 2 h before being reduced
to a concentrate using a rotary evaporator, then stored at 0 °C
overnight. The complex was collected as a bronze powder by vacuum
filtration (yield: 0.103 g, 93%). FT-IR (ATR): 1701 cm− (s), νC
O. Anal. calcd for C H N O Cu : C, 67.49; H, 5.37; 8.28. Found: C,
(
(
methanylylidene))bis(4-ethyl-3-methyl-1H-pyrrole-2-carboxylate))
Pyrrophen, H L ). Pyrrophen was synthesized using a modification
1
2
26
from a known procedure. To a solution containing compound 3
1.1 g, 4.1 mmol) dissolved in a minimum amount of methanol, o-
(
1
phenylenediamine (0.22 g, 2 mmol) was added as a solid. The flask
was loosely capped and the mixture stirred at room temperature. After
7
6
72
8
8
2
−
1
−1
2
h, the ligand precipitated as a yellow powder; however, the reaction
67.33; H, 5.26; N, 8.21. λmax: 394 nm (66 200 M cm /Cu ; 33 100
cm /Cu). HRMS (ESI+) calculated for C H N O Cu ([M +
76 73 8 8 2
H] ): m/z 1354.4116. Found: 1354.4113. CCDC 1936510.
2
−
1
−1
was allowed to stir overnight to go to completion. The product was
M
+
collected by vacuum filtration (1.00 g, 82%), washed with methanol
1
1
and ethanol, and used directly for metal complexation without further
Synthesis of UO
21 mL of a 20:1 THF/MeOH (v/v) solution in a 50 mL round-
bottom flask at room temperature and stirred. UO (OAc) ·2H O
2
2 2
(L ). H L (0.100 g, 0.164 mmol) was dissolved in
1
purification. H NMR (400 MHz, CDCl ): δ 1.08 (t, J = 7.5 Hz, 6H),
3
2
.32 (s, 6H), 2.59 (q, J = 7.6 Hz, 4H), 5.31 (s, 4H), 7.12−7.06 (m,
2
2
2
H), 7.26−7.21 (m, 2H), 7.43−7.28 (m, 10H), 8.30 (s, 2H), 9.71
(0.083 g, 0.197 mmol, 1.2 equiv) was added as a solid, causing a
gradual color change from bright yellow to deep red-brown. The flask
was loosely capped and stirred at room temperature for 3 h before
being reduced to a concentrate using a rotary evaporator. The
solution was then stored at 0 °C overnight. The complex was
(
brs, 2H). 13C NMR (600 MHz, CDCl ): δ 10.10, 16.44, 17.07,
3
6
1
6.07, 120.26, 122.31, 126.75, 127.05, 128.30, 128.42, 128.67, 129.33,
−
1
33.05, 136.30, 145.05, 148.98, 161.05. FT-IR (ATR): 1696 cm (s),
−
1
νCO; 1610 cm (m), νCN. Anal. calcd for C H N O ·
3
8
38
4
4
CH OH: C, 72.42; H, 6.52; N, 8.66. Found: C, 72.27; H, 6.48; N,
collected as an orange-brown crystalline powder by vacuum filtration
3
−
1
−1
1
8
.84. λmax: 328 nm (42 800 M cm ). HRMS (ESI) calcd for
(Yield: 0.098 g, 68%). H NMR (400 MHz, CDCl
): δ 1.38 (t, J =
3
+
7
7
.60 Hz, 6H), 2.52 (s, 6H), 2.93 (q, J = 7.63 Hz, 4H), 5.89 (s, 4H),
C H N O ([M + H] ): m/z 615.2971. Found: 615.2977. CCDC
38
39
4
4
1
3
.36−7.43 (m, 8H), 7.62 (m, 4H), 7.87 (m, 2H), 9.88 (s, 2H).
C
1
936495.
NMR (600 MHz, CDCl ): δ 10.09, 17.01, 18.61, 68.42, 117.19,
Synthesis of Dibenzyl 5,5′-((1E,1′E)-((4,5-Dimethyl-1,2-
3
phenylene)bis(azanylylidene))-bis(methanylylidene))bis(4-ethyl-3-
methyl-1H-pyrrole-2-carboxylate) (Me Pyrrophen, H L ). Dimethyl-
126.24, 127.82, 128.48, 128.63, 128.87, 135.85, 136.06, 137.67,
2
−1
143.57, 145.43, 154.81, 172.66. FT-IR (ATR): 1389 cm (m),
2
2
−
1
−
⃛
pyrrophen was synthesized using a modification from a known
νC O; 908 cm
(s), ν OUO (s). Anal. calcd for
3
3
procedure. To a solution containing compound 3 (0.171 g, 0.63
C H N O U: C, 51.64; H, 4.22; N, 6.34. Found: C, 51.84; H,
4.23; N, 6.34. λmax: 386 nm (41 600 M cm ). HRMS (ESI+)
3
8
37
4
6
−
1
−1
mmol) dissolved in a minimum amount of methanol, 4,5-dimethyl-o-
phenylenediamine (0.041 g, 0.3 mmol) was added as a solid, and the
mixture was stirred at room temperature. Once completely dissolved,
one drop of acetic acid was added and the flask closely capped. The
reaction was stirred for an additional 30 min, and the orange solution
was then left to sit capped and undisturbed at room temperature for
+
calculated for C38
H N O U ([M + H] ): m/z 883.2111. Found:
37 4 6
2
L2 (0.0500 g,
2
(OAc) ·2H O
2
2
2
(0.0330 mg, 0.0778 mmol) was added as a solid. The capped solution
was left to stir at room temperature overnight (approximately 12 h).
The solution was then concentrated by means of a rotary evaporator
and subsequently chilled in a freezer for 48 h to induce crystallization.
4
8 h during which time a large crop of orange-yellow crystals formed
and were collected by vacuum filtration using a short-stemmed pipet
and washed with methanol. Prior to filtration, a single crystal suitable
for X-ray diffraction was isolated from the solution (yield: 0.134 g,
The product was collected via vacuum filtration and washed in ether
1
1
6
9.4%). H NMR (600 MHz, CDCl ): δ 1.09 (t, J = 7.45 Hz, 6H),
(24 mg, 34%). H NMR (600 MHz, CDCl
3
): δ 1.37 (t, J = 6.66 Hz,
3
2
.30 (d, J = 1.72 Hz, 12H), 2.59 (q, J = 7.51 Hz), 3.46 (s, 2H), 5.28
6H), 2.43 (s, 6H), 2.51 (s, 6H), 2.92 (q, J = 6.43 Hz, 4H), 5.88 (s,
13
(
s, 4H), 6.90 (s, 2H), 7.26−7.41 (m, 10H), 8.31 (s, 2H), 10.25 (bs,
4H), 7.35−7.39 (m, 6H), 7.60 (m, 6H), 9.81 (s, 2H). C NMR (600
H). 13C NMR (600 MHz, CDCl ): δ 10.12, 16.43, 17.06, 19.58,
MHz, CDCl ): δ 10.12, 17.04, 18.61, 20.25, 68.31, 117.73, 126.14,
2
6
1
3
3
1
1
28.48, 128.57, 128.85, 135.44, 135.96, 136.83, 137.15, 143.27,
6.03, 121.53, 122.00, 127.12, 128.30, 128.43, 128.68, 129.51, 132.62,
−
1
−
1
−
⃛
43.71, 153.85, 172.59. FT-IR (ATR): 1390 cm (m), νC O; 918
35.25, 136.34, 142.59, 148.36, 161.01. FT-IR (ATR): 1690 cm (s),
−
1
−
1
cm (s), ν OUO (s). Anal. calcd for C H N O U: C, 52.75;
νCO; 1607 cm (m), νCN. Anal. calcd for C H N O ·
3
40 40
4
6
4
0
42
4
4
H, 4.43; N, 6.15. Found: C, 52.67; H, 4.47; N, 6.19. λ : 388 nm
CH OH: C, 72.97; H, 6.87; N, 8.30. Found: C, 73.03; H, 6.95; N,
max
3
−
1
−1
−1
−1
(55 000 M cm ) HRMS (ESI+) calculated for C H N O NaU
([M + Na] ): m/z 933.3353. Found: 933.3431. CCDC 1971927.
8
.45. λmax: 319 nm (49 000 M cm ). HRMS (ESI+) calculated for
40 40 4 6
+
+
C H N O ([M + H] ): m/z 643.3284. Found: 643.3288. CCDC
40
43
4
4
1
971893.
1
1
Synthesis of Metal Complexes. Synthesis of Zn (L ) . H L
RESULTS AND DISCUSSION
2
2
2
■
(
(
0.100 g, 0.164 mmol) was dissolved in 21 mL of a 20:1 THF/MeOH
v/v) solution in a 50 mL round-bottom flask at room temperature
1
2
The pyrrophen ligands H L and H L were prepared through
2
2
the condensation of the respective o-phenylenediamines with
benzyl 4-ethyl-5-formyl-3-methyl-1H-pyrrole-2-carboxylate (3)
(Scheme 2) in methanol at room temperature. The resulting
products were found to precipitate from solution as bright
and stirred. Zn(OAc) ·2H O (0.043 g, 0.197 mmol, 1.2 equiv) was
2
2
added as a solid, causing a color change from bright yellow to intense
orange. The flask was loosely capped and stirred at room temperature
for 2 h before being reduced to a concentrate using a rotary
C
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Article
a
Scheme 2. Synthesis of Pyrrophen Ligands
Table 1. Average Bond Lengths (Å) for Hydrogen Bonds
and Methanol for Pyrrophen Ligands and Adducts from
References 13 and 35
bond
H L1
H L2
ref 13
ref 35
2
2
HO···Nim
OH···Nim
O−H
2.937
(2.24)
0.84
2.918
(2.20)
0.84
2.891
(2.09)
0.95
nr
N···OH
NH···OH
N−H
2.844
(1.96)
0.91
2.827
(1.98)
0.86
2.853
(1.97)
0.89
3.155
(2.33)
0.90
1.29
H C−OH
3
1.43
1.41
1.41
a
Lengths with unabridged significant figures and ESD values can be
found in the Supporting Information. (nr) Not reported.
NMRthe pyrrole H atoms are observed downfield at 9.70
1
2
ppm (H L ) and 10.25 ppm (H L ) as broad resonances,
2
2
indicating considerable hydrogen delocalization (Figures S1
and S5). These similarities to macrocyclic species not only are
37
of note for their reported correlation in anion binding but
give the pyrrophen framework a prospective amphoteric
coordination module for insight into self-assembly and
molecular recognition of other species.
Complexes were prepared by adding 1.2 equiv of M(OAc)2
(
M = Cu, Zn, UO ) into a THF/MeOH (20:1, v/v) solution
2
1
2
of H L or H L . Upon addition of the metal acetate, each
2
2
yellow solids. The synthesis of compound 3 is achieved with
the regiospecific oxidation of a 5-methyl pyrrole using
solution featured a distinct color change from yellow to brown,
orange, or deep red for the copper, zinc, and uranyl complexes,
respectively. Intensely colored solids were isolated by filtration
after concentrating the solution under reduced pressure and
cooling to 0 °C. Single crystals of the copper, zinc, and uranyl
33,34
Single crystals of H2L1
cerium(IV) ammonium nitrate.
2
and H L suitable for X-ray diffraction were grown by slow
2
1
diffusion (CH Cl /CH OH). In the solid-state, both the H L
2
2
3
2
2
1
2
and H L free ligands were found to adopt slightly cupped
complexes of H L and the uranyl complex of H L were
2
2
2
conformations and form adducts with neutral solvent
molecules (methanol; Figure 1) through pyrrolic hydrogen-
grown from CH Cl /CH OH and characterized by X-ray
2 2 3
diffraction. Unsurprisingly, the copper and zinc pyrrophen
complexes form M2L2 helicate ensembles (Figure 2)
Figure 2. Structures of Zn (left) and Cu (right) viewed
(L1) (L1)
2 2 2 2
along planes of phenyl spacers. Ellispsoids at 50%. Hydrogen atoms
removed for clarity.
1
2
Figure 1. Left: structures of H L ·CH OH (top) and H L ·CH OH
2
3
2
3
(
bottom), grown from CH Cl /CH OH and CH OH, respectively.
2 2 3 3
1
2
Right: side-on views of H L ·CH OH and H L ·CH OH along
2
3
2
3
coordination plane (right) showing solvent interactions. H atoms and
substituents (right) partially removed for clarity. Ellipsoids at 50%.
comparable to those previously reported by Wu and
21,26
Yang,
whereas the uranyl complexes adopt the hexagonal
bonding. Such interactions have been observed previously in
bipyramidal geometry (Figure 3) seen in porphyrin-type
macrocyclic complexes (Scheme 1). To our knowledge,
35,36
38
macrocyclic calix[4]pyrrole species
as well as for Schiff
13
base expanded porphyrins. Both ligands were found to have
donor−acceptor distances that compare well with those of
their macrocyclic cousins, though they more strongly resemble
those of the Schiff base expanded porphyrins (Table 1). The
there is only one other reported structure in which the
equatorial plane of uranyl is satisfied completely by a single,
softer-donor, nonmacrocyclic liganda bipodal aroylthiourea-
39
substituted bipyridine ligand (N S ). Recently, chelation of
4
2
diffuse nature of the N H bond that predisposes it to these
uranyl by a harder, catecholamine-based ligand was also
pyr
1
40
1
interactions is also apparent in the solution state by H
reported, but no structures were presented. UO (L ) and
2
D
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1
2
2
Figure 3. Structures of UO (L ) (a) and UO (L ) (b) showing full structure, views down the OUO units, and the equatorial planes.
2
Hydrogen atoms and/or substituents removed for clarity. Ellipsoids at 50%. (a) θ , 60.96(5)°; θ , 61.35(6)°; θ , 60.71(5)°; θ , 61.91(6)°; θ ,
0.06(5)°; θ , 55.92(5)°; θ , 60.15(5)°; ∑θ = 360.1°. (b) θ , 60.19(8)°; θ , 61.41(8)°; θ , 60.25(7)°; θ , 62.05(8)°; θ , 60.04(7)°; θ , 56.20(7)°;
1
2
3
4
5
6
6
avg
1
2
3
4
5
6
θavg, 60.19(8)°; ∑θ = 360.14°.
Table 2. Selected Bond Lengths (Å) for Pyrrophen Ligands and Their Complexes
H2L1
H2L2
Zn2L21
Cu2L21
UO (L )
1
UO (L )
2
2
2
CN
CO
M−N
M−Npyr
M−O
1.2839(19)
1.2134(19)
1.2816(14)
1.2132(13)
1.3075(13)
1.2085(14)
2.0793(9)
1.9592(9)
1.299(3)
1.209(3)
2.048(2)
1.933(2)
1.302(2)
1.237(2)
2.6597(17)
2.4374(16)
2.5789(14)
1.7722(16)
1.302(4)
1.235(4)
2.664(2)
2.441(3)
2.673(2)
1.774(2)
im
ester
im
UOyl
2
UO (L ) represent unique complexes in this regard, and
corresponding average CO distance in the “free” ester
2 2 2 2
2
1
1
highlight the potential the pyrrophen system shows for
molecular recognition of uranyl by completely satisfying the
equatorial plane. Selected bond lengths for all complexes as
well as the free ligands are summarized in Table 2.
carbonyls for Cu (L ) and Zn (L ) is 1.209(3) Å and
1.2084(14). The average U−Nim lengths of both complexes
(Table 2) are approximately 0.15 Å longer than those usually
42
found for U(VI) salophen species and are on par with U−
Notably, the formation of the double-stranded dinuclear
copper and zinc complexes indicates that the binding pocket of
the pyrrophen system cannot satisfy the coordination sphere of
these late transition metals in a 1:1 binding motif, and instead
the ligand twists about the phenyl spacer to form a ditopic
system in the observed 2:2 fashion. The zinc and copper
centers adopt distorted tetrahedral geometries with average
Cu−N , Cu−N , Zn−N , and Zn−N bond lengths of
N lengths reported for expanded porphyrin complexes, as are
im
11
the average U−Npyr lengths, though these distances do differ
somewhat from those of a comparable macrocyclic uranyl
grandephyrin complex which features a slightly larger
13
pocketthe U−N and UNpyr lengths of the pyrrophen
im
species are nearly 0.20 and 0.10 Å shorter, respectively. It is
noted for the grandephryin complex that the uranyl cation is
shifted off-center away from the imine N atomsthe uranium
center lies 2.536 Å from the midpoint of the vector defined by
the imine N atoms and 2.265 Å from the vector defined by the
pyr
im
pyr
im
1
.993(2) Å, 2.048(2) Å, 1.9592(9) Å, and 2.0793(9) Å,
respectively. The metal centers lie 3.300 Å (Cu) and 3.667 Å
Zn) apart. The formation of this helicate can also be observed
for the zinc complex in solution via H NMR as the ligand’s
methylene protons (δ 5.29) split into two distinct diaster-
iotopic peaks (δ 5.20, d; 4.27, d), signifying axial chirality
Figures S2 and S3).
While Zn and Cu undergo self-assembly into 2:2 helicate
complexes in the presence of H L , the distinct steric demands
of the uranyl ion render complexes with a 1:1 configuration in
which L and L fully occupy the equatorial plane as
hexadentate ligands. This is evident in both solution and the
solid state. H NMR of the uranyl complexes shows single
enantiotopic signals for the benzylic methylene protons (ca.
(
13
meso-bridged pyrrole N atoms, whereas the pyrrophen
1
complexes are significantly more centered with the uranium
atom lying an average of 2.299 Å from the imine midpoint and
2.318 Å from the midpoint of the vector defined by the
carbonyl O atoms. The near linearity of the pyrrole donors for
2
1
(
2
+
2+
UO (L ) and UO (L ) (N −U−N = 175.61(6)o and
1
1
1
2
2
1
4
2
1
74.99(9)°, respectively) as well as the average angle between
donors and ∑θ values (60.15(6) , 360.91°, and 60.02(8)°,
60.14°, respectively) also reflect that the uranyl resides almost
o
1
2
3
1
perfectly in the center of the ligand’s pocket, similar to some
macrocyclic systems, as well as highlights the planarity of these
complexes, a feature which is uncommon among the typically
5
.8 ppm). The hexagonal bipyramidal uranyl complexes are
13−15
undoubtably U(VI) species, evidenced by average UO
ruffled macrocyclic complexes
(Figure 3). This illustrates
yl
1
bond lengths of 1.7722(16) Å (UO (L )) and 1.774(2) Å
the ability of uranyl to engage favorably in primarily covalent
interactions, as the deprotonated pyrrole N atoms possess low
ionic character. Additionally, the high planarity and subsequent
2
2
41
(
UO (L )). Notably, the average CO distances of
2
1
.236(3) Å unambiguously represent a coordinative interaction
1
of the carbonyl oxygen with uranium in both cases. The
retention of conjugation of L when complexed to uranyl allow
E
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1
1
Figure 4. (a) UV−vis spectra of H L and complexes in 9:1 THF/MeOH (v/v). Data shown are final traces of the titration of 20 μM H L with
2
2
2+
1
M
to form 1:1 or 2:2 complexes. (b) 500 μM (upper vials) and 50 μM (lower vials) colorimetric series of H L and complexes in 9:1 THF/
2
1
1
MeOH (v/v), (c) top-down views of UO (L ) (right) and Zn (L ) (left) solutions, 500 μM in CH CN. (d and e) Addition of 0.5 (d) and 1.0 (e)
2
2
2
3
equivalent UO22+ (per Zn ) to Zn (L ) , 10 μM in 9:1 THF/MeOH.
2+
1
2
2
for significant delocalization of charge throughout the π-
system, thereby further “softening” the pyrroles.
energy absorptions are more intense, leading to visible color
differences in solution (Figure 4a,b). The uranyl and zinc
While these species are interesting for their unique
structures in the solid-state, the solution state behavior of
pyrrophen is also worth investigation, as the structural features
of the uranyl complexes suggest that the pyrrophen system is
particularly well-suited for forming stable uranyl complexes.
Given the spontaneous assembly of each metal complex at
room temperature and without the addition of base, as well as
the marked color differences among them, there is the
potential of this system to be exploited in further studies for
complexes are the most similar in color when viewed side on;
1
however, when viewed from above, the UO (L ) solution is
2
distinctly red-orange in color (Figure 4b,c). While the visible
color differences alone are not sufficient to clearly distinguish
the uranyl solution from the others, this can be done so
1
spectroscopically, as UO (L ) is the only species with a sharp,
2
intense absorption at 386 nm; however, greater differentiation
both in terms of solution color and absorption profile are still
necessary. These features are not ideal, but they are a
promising starting point in terms of molecular recognition of
uranyl. Additionally, it should be noted that colorimetric
differences, though useful from an identification standpoint, do
not speak to the ability of this ligand to selectively bind uranyl
from a mixture. Thus, further qualitative studies to determine
the binding preferences of pyrrophen were conducted in which
2
+
molecular recognition of UO . Initial screening of a wide
2
2
+
2+
2+
2+
2+
3+
array of metal acetates (Mg , Ca , VO , Mn , Fe Fe ,
2
+
2+
2+
2+
3+
4+
3+
4+
2+
Co , Ni , Cu , Zn , Ce , Ce , Dy , Th , and UO ) was
2
conducted to determine the range of coordinating cations for
1
2+
H L . As determined by UV−vis spectroscopy, only Co ,
2
2
+
2+
2+
2+
Ni , Cu , Zn , and UO
were found to form metal
2
2
+
complexes under ambient conditions. No other ions were
the ability of UO2 to displace zinc or copper from the M2L2
complexes was investigated.
observed to induce a visible change to pyrrophen. The ligand
1
(
H L ) features an intense π → π* band (Soret-like) at 328
Replacement of cobalt and nickel was not pursued, as the
cobalt complex is observed to dissociate rather quickly after
2
−
1
−1
nm (ε = 42 800 M cm ) with a shoulder at 375 nm (ε ≈
2 000 M cm ) indicative of charge-transfer from a
−
1
−1
2+
2
formation, even in an excess of Co (Figure S26), and both
coordinating solvent molecule (as seen in the solid-state).
the cobalt and nickel complexes are colorimetrically distinct
2
Similar features are observed for H L (λ = 319 nm, ε = 49,
from the uranyl complex in solution (Figure 4a−c). On
2
max
−
1
−1
2+
2+
2+
1
1
00 M cm ; Figure S14). Coordination of to Co , Ni ,
Cu , Zn , or UO results in a substantial bathochromic shift
addition of UO to solutions of Zn (L ) , displacement of
2 2 2
2+
2+
2+
2
2+
Zn was observed using UV−vis spectroscopy via the
immediate growth of the characteristic UO (L ) absorption
2
+
1
of the π → π* band (Figure 4). Only coordination to UO2
2
results in retention of approximately the same ε, or an increase
features (Figure 4d,e). Even when 0.5 per-metal equivalents of
−
1
−1
1
2
2+
in ε (41 600 M cm for UO (L ); 55 000 for UO (L )) as
UO2 was introduced, this replacement was observed within
2
2
the free ligand, indicating the uranyl complexes are planar in
solution, whereas the transition metal complexes are not. This
behavior is also distinct from expanded porphyrin analogues,
which either show a marked increase or decrease in ε due to
minutes. The same experiments were conducted using
1
Cu (L ) , but formation of the uranyl complex was not
2
2
observed, indicating the copper helicate is more stable in
solution. From these studies, we have determined that the
6
,14
1
ruffling or bowing of the ligand on complexation.
UV−vis
stability of the monomeric UO (L ) complex is intermediate
2
1
1
titrations of metal acetates into a solutions of pyrrophen
to that of the ditopic Cu (L ) and Zn (L ) helicates, though
2
2
2
2
1
(
H L ) show clear isosbestic points and confirm 1:1 binding
this may not necessarily reflect the stabilities of exclusively 1:1
2
ratios for all complexes (1:1 or 2:2), but the presence of
multiple isosbestic points in the nickel titration data, and the
smaller bathochromic shift of the π → π* band suggest
species.
In an attempt to characterize this quantitatively, formation
constants were calculated for complexes of H L (Table S2)
1
2
3
1,43,44
structural features distinct from that of the other transition
using UV−vis titration data,
though it must be
1
metal species (Figure S19). Titrations of H L with acetic acid
acknowledged that the methods used are based on the
assumption that 1:1 complexes are being formed, not the 2:2
complexes which are known to self-assemble in the presence of
2
resulted in no spectroscopic changes, affirming complex
formation is not hindered by ligand−acetate interactions.
2
+
2+ 21,26
Most of the transition metal complexes feature a λ
in a
Zn or Cu .
Where higher order supramolecular self-
max
similar range (approximately 370−390 nm) as that of the
assembly is concerned, such studies are nontrivial and require a
clear mechanism of formation and independent treatment of
uranyl complex, except for nickel (λ = 352), but their lower-
max
F
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Article
4
3,45
intra- and intermolecular constants.
Approximations of the
most relevant in aqueous systems and would be best for
comparison with other systems. Despite the limitations we
have encountered in quantitatively describing the binding
preferences of this system, we are encouraged by the narrow
range of cations which pyrrophen can coordinate, as our
preliminary investigations demonstrate it has no affinity for
binding constants for 1:1 complexes in 9:1 THF/MeOH are a
reasonable benchmark for comparing the relative binding
affinity of H L for transition metal ions
be secondary to more detailed qualitative studies. These
1
31,44
and should always
2
2+
2+
approximated log β values (Co , 6.11 ± 0.71; Ni , 7.31 ±
1
1
2
+
2+
2+
2
3+
4+
0
.53; Cu , 6.02 ± 1.33; Zn , 5.27 ± 0.09; UO , 6.74 ±
Ln ions or Th under ambient conditionsexclusion of
these species is valuable with respect to applications for
environmental cleanup or nuclear waste remediation. Studies
focused on the selective coordination of uranyl are ongoing,
with modifications to improve binding kinetics, sensitivity,
selectivity, and colorimetric response being pursued. This
0
.80) suggest that the copper complex should be more stable
than the zinc complex. This is consistent with what is observed
experimentally for the prepared 2:2 complexes with respect to
replacement by uranyl; however, this is difficult to conclude
given the nontraditional solvent system. The cobalt complex is
observed to fall apart after formation, yet the calculated log β11
is higher than other stable complexes, and the log β for
39
(along with the work from Abram et al. ) warrants further
investigation into the potential use of this highly modular and
chromatic framework in selective actinyl recognition.
1
1
1
UO (L ) is calculated to be higher than that of the 1:1 copper
2
complex. Additionally, formation of the uranyl complex at
concentrations appropriate for UV−vis spectroscopy is
significantly slower than that of the transition metal species,
ASSOCIATED CONTENT
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00439.
■
*
which further complicates analysis (Figure S23), however the
2
presence of methyl groups on the backbone of H L appears to
2
improve the kinetics considerably (Figure S24). The formation
NMR, UV−vis, and other solution-state data including
additional commentary on binding constants, as well as
crystallographic information (PDF)
1
constant for UO (L ) cannot be reasonably compared to those
2
40,46
of other reported aqueous-soluble systems
but is greater
than those reported for macrocyclic systems in mixed organic
Accession Codes
CCDC 1936495, 1936509−1936510, 1971893, 1971927, and
983238 contain the supplementary crystallographic data for
solvents, reflecting the value of a well-sized and flexible binding
4
7,48
pocket.
(Further information is given in the Supporting
1
Information.)
this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
In conclusion, pyrrophen, a sal-porphyrin analogue, as well
as its dimethyl derivative have been prepared and characterized
in the solution and solid state via NMR and UV−vis
spectroscopy, and with single crystal X-ray diffraction, as
1
1
1
have the complexes Cu (L ) , Zn (L ) , UO (L ), and
2
2
2
2
2
2
UO (L ). Of note is the coordination of the free ligands to
2
AUTHOR INFORMATION
Corresponding Author
methanol, thereby showing the inherent lack of electro-
negativity on the pyrrole moieties and preferred pseudoplanar
■
37
orientation, as observed in calix[4]pyrrole systems. Structural
analyses of the UO22 complexes suggest this ligand is
especially well-suited for uranyl coordination and help to
further elucidate the covalent-type coordination behavior of
Anne E. V. Gorden − Auburn University, Department of
Chemistry and Biochemistry, Auburn, Alabama 36849, United
States; orcid.org/0000-0001-6623-9880;
Email: anne.gorden@auburn.edu
+
1
2+
the uranyl cation. H L was found to preferentially bind UO
2
2
2
+
2+
Authors
over Zn but not over Cu the ability of uranyl to displace
zinc from the helicate architecture is of note, as these are the
two most similarly colored complexes. From a colorimetric
identification perspective, this is valuable in that it reduces the
possibility of false-positive responses for uranyl. The selectivity
of this ligand system for uranyl still requires improvement in
Jacob T. Mayhugh − Auburn University, Department of
Chemistry and Biochemistry, Auburn, Alabama 36849, United
States
Julie E. Niklas − Auburn University, Department of Chemistry
and Biochemistry, Auburn, Alabama 36849, United States
Madeleine G. Forbes − Auburn University, Department of
Chemistry and Biochemistry, Auburn, Alabama 36849, United
States
2+
2+
order to avoid competition from ions such as Cu and Ni
and to be able to identify UO22 from a mixture of such ions,
but features such as its synthetic accessibility and tunability, as
well as its utilitarian hexadentate cavity, make it an ideal
framework for such endeavors. This ligand and its complexes
give insight into the potential of softer-donor acyclic systems as
suitable candidates for selective uranyl recognition or
extraction by taking advantage of uranyl’s proposed covalent
+
John D. Gorden − Auburn University, Department of Chemistry
and Biochemistry, Auburn, Alabama 36849, United States;
orcid.org/0000-0002-0304-1954
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00439
8
,49
f orbitals.
With uranyl forming the only planar complex of
Author Contributions
the cations and given the synthetic modularity of the ligand, it
is feasible to prepare a pyrrophen derivative that sterically
disfavors the formation of transition metal helicate com-
plexesthis is a potential route by which to more clearly study
the solution phase thermodynamics without needing to
consider the self-assembly of higher order complexes as a
complicating factor. Future work will also include the
preparation of water-soluble derivatives as such data are
‡
These authors contributed equally. The manuscript was
written through contributions of all authors. All authors have
given approval to the final version of the manuscript.
Funding
United States Department of Energy - Chemical Sciences,
Geosciences, and Biosciences (CSGB) subdivision for Heavy
Elements Chemistry
G
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Article
Notes
(17) Zheng, X.-J.; Bell, N. L.; Stevens, C. J.; Zhong, Y.-X.;
Schreckenbach, G.; Arnold, P. L.; Love, J. B.; Pan, Q.-J. Relativistic
DFT and experimental studies of mono- and bis-actinyl complexes of
an expanded Schiff-base polypyrrole macrocycle. Dalton Transactions
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
2
(
016, 45 (40), 15910−15921.
The authors would like to acknowledge that this work was
funded by the United States Department of Energy − Basic
Energy Sciences through the Chemical Sciences, Geosciences,
and Biosciences (CSGB) subdivision for Heavy Elements
Chemistry with Award DE-SC0019177 to Auburn University.
18) Arnold, P. L.; Jones, G. M.; Pan, Q.-J.; Schreckenbach, G.;
Love, J. B. Co-linear, double-uranyl coordination by an expanded
Schiff-base polypyrrole macrocycle. Dalton Transactions 2012, 41
(
22), 6595−6597.
(19) Love, J. B. A macrocyclic approach to transition metal and
uranyl Pacman complexes. Chem. Commun. 2009, No. 22, 3154−
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