Switchable Aromaticity in an Isostructural Mn Phthalocyanine Series Isolated in Five Separate Redox States

Article abstract of DOI:10.1021/jacs.8b12899

The synthesis and characterization of a new phthalocyanine (Pc) Mn-nitride complex, (^OEtPc)MnN (2; ^OEtPc = 1,4,8,11,15,18,22,25-octaethoxy-Pc), as well as its stable, readily accessible oxidized (2⁺ and 2²⁺) and

Full text of DOI:10.1021/jacs.8b12899

Article
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Switchable Aromaticity in an Isostructural Mn Phthalocyanine Series
Isolated in Five Separate Redox States
Camden Hunt,† Madeline Peterson,† Cassidy Anderson,† Tieyan Chang,‡ Guang Wu,† Steve Scheiner,§
,†
́
and Gabriel Menard*
†Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States
‡ChemMatCARS, University of Chicago, Argonne, Illinois 60493, United States
§Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, United States
S
* Supporting Information
ABSTRACT: The synthesis and characterization of a new phthalo-
cyanine (Pc) Mn-nitride complex, (OEtPc)MnN (2; OEtPc =
1,4,8,11,15,18,22,25-octaethoxy-Pc), as well as its stable, readily
accessible oxidized (2+ and 22+) and reduced (2−, 22−) congeners is
reported. This unique isostructural series displays switchable aromatic
character spanning the aromatic (2), nonaromatic (22+), and
antiaromatic (22−) triad, in addition to the open-shell radical states
(2+, 2−). All complexes were structurally characterized and displayed
significant structural distortions at the redox extrema (22+, 22−)
consistent with proposed [16 or 18]annulene π ring circuit models.
Spectroscopic and computational studies further support these models.
This isolated, fully characterized, isostructural series spanning five
redox states (22+, 2+, 2, 2−, 22−) is unique in both the Pc and related
macrocyclic (ex. porphyrinoids) literature and may offer direct insight into structural−electronic correlations driven by
switchable aromaticity.
for the development of next generation organic electronic
devices.12−15 In contrast, “switchable” aromatic character may
be used to design single-molecule transistors and other
molecular-scale electronic gates. A molecular macrocyclic
platform spanning separate aromatic statesand correlated
structural perturbationsmay guide the development of such
materials and their properties. While redox-switchable
aromaticity in macrocyclic platforms, such as porphyrinoids,
has been studied,1 with several isolated discrete antiaromatic or
nonaromatic species reported,16−26 to the best of our
knowledge, there are no known examples of a single stable,
isostructural complexeither porphyrin or Pc-based, or
otherisolated in multiple separate redox states, including
radical states, and spanning the aromatic, nonaromatic, and
antiaromatic triad. Previous work has created a backbone of
information regarding the electronic character of Pc complexes
in various oxidation states, and extensive effort has been
focused on determining the loci of redox events and how they
relate to the aromaticity of a given system, producing a large
body of useful knowledge for the field.18,20,27−31 However,
while many Pc complexes have been studied in each of the
states separately, it can be difficult to make direct comparisons
due to the many different substituents and redox-active metal
INTRODUCTION
ckel’s (4n + 2)π e− rule is conventionally used to predict
the aromatic character of cyclic, planar, π-conjugated
compounds. However, in extended aromatic systems this rule
often gives unsatisfactory predictions of in/stability, and
■
Hu
̈
deviations from the tenets of Huckel’s rule, such as the
̈
requirement for planarity, become more common.1−3 In
concert with this, macrocyclic platforms often adopt localized
internal π circuits within the extended π manifold as
exemplified by Vogel’s [18]annulene/18π e− model applied
to porphyrins4,5 and related phthalocyanines (Pc) (Figure
1a,b).6 An alternative but less common model involving the
dianionic ([16]annulene)2− substructure has also been
proposed as the primary aromatic circuit, particularly for
metalloporphyrins (Figure 1c).5,7 While the annulene model
aids in understanding the complexity of aromaticity in large
macrocyclic systems, it is not a complete description,
particularly regarding correlations between local and macro-
cyclic ring currents, both demonstrated to be important
considerations when describing total aromaticity.2,8−10
The inherent interplay of resonance circuits and electronic
properties has resulted in the development of several
structure/function relationships between aromaticity and
electronic behavior in large, conjugated systems.11 Notably,
the correlation between aromaticity and conductance has been
well described by many and may have important implications
Received: December 1, 2018
© XXXX American Chemical Society
A
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resonance circuit pathways as a function of redox state. This
study may provide valuable information on the interplay of
redox state and aromatic character in Pc and related
macrocycles, and may serve as an excellent platform in which
to study molecular material applications pertaining to tunable
aromaticity.
RESULTS AND DISCUSSION
■
Synthesis. The symmetrically substituted proligand,
EtOPcH2, was prepared by previously reported methods.32,33
Metalation of EtOPcH2 with MnCl2 under aerobic conditions in
refluxing dimethylformamide (DMF) for 5 h yielded a dark red
product (λmax = 825 nm) after purification (Note: all λmax
values reported correspond to the Q peaks) (Figure 2a, path
(i)).34 Single crystals suitable for XRD studies were grown by
layering benzene over a saturated fluorobenzene solution and
confirmed the structure as the Mn(III) species, EtOPcMnCl (1)
(Figure S27). Bond metrics are similar to other Mn(III)Cl
macrocyclic species, such as a similar porphyrin derivative.35
The high-spin, S = 2 state at Mn was confirmed by solution
magnetic moment determination using the Evans method,36
and resulted in paramagnetically broadened resonances in the
1H NMR spectrum.
Figure 1. (a) Depiction of porphine (solid lines), the parent molecule
to porphyrins, and the related phthalocyanine (solid + dashed lines)
coordinated to a generic metal (M). (b) The [18]annulene/18π e−
circuit, in bold. (c) The dianionic ([16]annulene)2−/18π e− circuit in
bold.
centers used across publications. It is therefore of interest to
use an isostructural Pc redox series with a redox-inactive metal
core in order to ensure that all comparisons are due primarily
to the change in oxidation state while minimizing the effect of
ring substitution or choice of metal.
Our interest in generating a terminal Mn-nitride stemmed
from our previous work.37 The nitride, EtOPcMnN (2), was
readily generated from 1 under oxidative conditions using
NaOCl and aqueous NH3 in methanol (Figure 2a, path
(ii)).37−39 The dark green (λmax = 767 nm) diamagnetic
complex was isolated in high yield (∼89%) and was
structurally characterized by XRD studies (Figure 2b). While
several Pc metal nitrides have been reported,40,41 including
with Mn,42−45 2 represents the first crystallographically
characterized terminal Pc-metal nitride complex, according to
the Cambridge Structural Database. The MnN in 2 is
1.555(9) Å, similar to other MnN bonds in comparable
symmetry and oxidation state.39,46,47 A more detailed structural
analysis is provided in the following section. The low-spin,
diamagnetic nature of 2 is further consistent with other triply
bonded Mn(V) nitrides, as is the MnN stretching frequency
located at 1030 cm−1, identified by comparison with the
isotopologue, 2-15N (Figures S18−S20).37,48
Herein, we demonstrate what we credit as the first such
clearly characterized example of a macrocyclic Pc complex that
can readily access aromatic (neutral), antiaromatic (dire-
duced), and nonaromatic (dioxidized) states, in addition to the
singly oxidized or reduced states. This was accomplished using
the new Mn-nitride, (O E t Pc)MnN (2, O E t Pc
=
1,4,8,11,15,18,22,25-octaethoxy-Pc), by chemical oxidation or
reduction. The redox behavior and electronic structure were
probed through a suite of structural, spectroscopic, electro-
chemical, and computational methods and revealed ligand-
borne redox events with the Pc ligand spanning from Pc(0) to
Pc(4-) formal oxidation states. Additionally, compelling
evidence for the operative local and macrocyclic resonance
circuits are provided. We consider this complex a rare class of
resonance circuit model compound due to the capability of
accessing all three principal states of aromatic behavior in a
stable and isolable manner, and apparent changing of
Phthalocyanines are well-known redox-active ligands capable
of spanning multiple oxidation states from Pc(0) to Pc(6-).34
In conjunction with a highly redox-active metal center (Mn),
Figure 2. (a) Synthesis of reported complexes following the general conditions: (i) MnCl2, DMF, 100 °C, 5 h, O2; (ii) NH4OH/NaOCl, MeOH,
r.t., 15 min; (iii) [(4-BrC6H4)3N][B(C6F5)4] (2 equiv), DCM, r.t., 5 min; (iv) [(4-BrC6H4)3N][B(C6F5)4] (1 equiv), DCM, r.t., 5 min; (v) KC8 (1
equiv), kryp (1 equiv), THF, r.t., 10 min; (vi) KC8 (5 equiv), kryp (2 equiv), THF, r.t., 10 min. (b) Solid state molecular structure of 2. H atoms
and cocrystallized solvent have been omitted for clarity. (c) CV of 2, taken in DCM. Conditions: 0.29 mM of 2, 0.1 M of [Bu4N][PF6], 3 mm
diameter glassy carbon working electrode, Pt wire counter electrode, and Pt wire pseudoreference electrode.
B
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we next proceeded to evaluate the redox behavior of 2.
Electrochemical analysis of 2 by cyclic voltammetry (CV) in
dichloromethane (DCM) revealed a total of 4 reversible redox
events (Figure 2c): 2 separate oxidation events at E1/2 = 0.02 V
and E1/2 = 0.45 V, and 2 reduction events at E1/2 = −1.38 V
and E1/2 = −1.75 V referenced to the ferrocene/ferrocenium
(Fc/Fc+) redox couple. Given the aromatic nature of 2, as well
as the relative scarcity of crystallographically characterized
PcM complexes in varying oxidation states,18,20,21,49−53 we next
proceeded to isolate each redox state to gain an understanding
of the changing aromaticity and concomitant structural
changes. Certainly, similar work has been done with other
systems. Leznoff has published a CrPc series that was
characterized across five oxidation states.53 The comparison
is interesting given the redox activity of the Cr center coupled
with the redox noninnocence of the Pc ring. However, there
are two distinct differences between the Leznoff work and the
work presented here. First, the CrPc series is not isostructural.
Second, as will be described below, the Mn(V) does not
appear to participate in the reported redox events which is not
true of the CrPc series. Here, the Mn(V) core (perhaps
surprisingly) appears to be electronically inert, with the nitride
functioning as a nonreactive “cap”, rather than a reactive motif,
allowing the redox behavior of the Pc ring to be studied in
isolation.
borne reduction event (Figure S16). Similarly, treatment of 2
with excess KC8 in the presence of Kryptofix-222 yielded a
dark blue, diamagnetic complex which, similar to 22+, displays
an upfield shifted 1H NMR spectrum relative to 2 (vide infra).
Single crystals suitable for XRD studies were grown from
benzene/isooctane by vapor diffusion and the solid-state
structure confirmed the composition as the dianion, [K-
(kryp)]2[EtOPcMnN] (22−) (Figures 2a, path (vi), S29).
Structural characteristics for both 2− and 22− are described
in more detail in the following section. We note that 22− likely
contains trace 2− in solution as observed by a resonance in the
EPR spectrum similar to 2− (Figure S17). This is a known
issue with compounds containing the highly reduced Pc(4-)
ligand21 and likely accounts for a small amount of
contamination in solution.
Structural Properties. While both metal and/or ligand-
based redox events are possible in this system, a closer look at
the MnN bond lengths for 22+ → 22− (Figure 3) suggests
The oxidized complexes were targeted using the tris(4-
bromophenyl)ammoniumyl tetrakis(pentafluorophenyl)borate
“magic blue” oxidant, [(4-BrC6H4)3N][B(C6F5)4] (E1/2 = 0.70
V vs Fc/Fc+).37,54,55 To isolate the mono-oxidized product, a
dark green diamagnetic solution of 2 in DCM was treated to 1
equiv of [(4-BrC6H4)3N][B(C6F5)4] resulting in a dark red
1
(λmax = 820 nm), H NMR-silent solution (Figure 2a, path
(iv)). Single crystals suitable for XRD studies of the product
were grown by layering benzene over a saturated fluoroben-
zene solution. The solid-state structure confirmed the
composition of the new product as [EtOPcMnN][B(C6F5)4]
(2+) (Figure S30). To probe the locus of oxidation in 2+, single
crystals were dissolved in DCM and analyzed by X-band EPR
spectroscopy at 100 K. An intense isotropic signal centered at g
= 1.995 indicative of an organic radical species is observed,
consistent with a 1 e− oxidation of the ligand π system (Figure
S14). In contrast, treatment of 2 with 2 equiv of [(4-
BrC6H4)3N][B(C6F5)4] resulted in a deep fuchsia (λmax = 832
nm) solution which, after workup, displayed an upfield shifted
diamagnetic 1H NMR spectrum relative to 2 (vide infra)
(Figure 2a, path (iii)). Single crystals suitable for XRD studies
were isolated by layering a concentrated solution of product in
fluorobenzene with benzene at room temperature. The identity
of the product was confirmed as the dication, [EtOPcMnN]-
[B(C6F5)4]2 (22+) (Figure S31). Structural characteristics for
both 2+ and 22+ are described in more detail in the following
section.
Figure 3. Solid-state structures of 22+, 2+, 2, 2−, and 22− illustrating
MnN bond lengths and core structural distortions. Hydrogen
atoms, peripheral substituted benzene groups, and cocrystallized
solvent molecules are omitted for clarity.
little to no participation of the Mn center in the observed
redox processes, consistent with the EPR spectroscopic results
above. Such participation would be expected to either render
the MnN bond reactive37,57 and/or alter its bond length. In
contrast, the observed bond lengths are all similar to other
Mn(V)N bonds in comparable symmetry and oxidation
states37,39,46−48 (it should be noted that while MN bond
length is a useful metric for determining the oxidation state of
the metal in nitride complexes, it cannot reliably serve as a
standalone measure for determining metal oxidation state).
The only notable change is with 22− where nitride
coordination to a K+ (from [K(kryp)]+) occurs (Figure
S29). However, this appears to have no impact on the Mn
For the anionic states of 2, KC8 was used as the reductant of
choice.56 Chemical reduction of 2 with 1 equiv of KC8 in the
presence of the cryptand, 4,7,13,16,21,24-hexaoxa-1,10-
diazabicyclo[8.8.8]hexacosane (Kryptofix-222 = kryp), resulted
1
in a dark teal, H NMR silent solution. Single crystals suitable
for XRD studies were grown from tetrahydrofuran (THF)/
isooctane by vapor diffusion and the solid-state structure
confirmed the composition as the monoanion, [K(kryp)]-
[
EtOPcMnN] (2−) (Figures 2a, path (v), S28). Similar to 2+,
the EPR spectrum of 2− dissolved in THF revealed an intense,
isotropic signal centered at g = 1.996 consistent with a ligand-
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N bond length (Figure 3) or the resulting diamagnetism of the
compound.58
Perhaps the most striking differences between all structures
are the pronounced Pc ring distortions. While PcM complexes
generally adopt planar geometries,16,18,34,49,59 axial metal
bonding (ex. M−Cl, MO)20,49,51,52,60 and/or heavy element
bonding in the pocket61,62 typically yields domed structures. In
contrast, ortho group incorporation on the isoindole rings63−66
can result in saddling of the Pc ring with adjacent isoindole
rings pointing in opposite directions relative to the N4 plane
(N4N2N4N6N8, Figure 4a).65 The degree of
Figure 4. (a) Depictions of the δ, γ, and isoindolic (I1−I4) planes, as
well as relevant atom labels. (b) Twist and tilt notation used in this
report where clockwise twist and upward tilt are given positive values.
doming or saddling is reported with respect to the dihedral
angles formed between the N4 and isoindole planes or between
opposing isoindole planes (ex. I1 vs I3, Figure 4a),
respectively.20,21,63−66 While the MnN bond vector sits
atop the N4 planes in our complexes (Figure 3) with Mn−N4
distances ranging from 0.40 to 0.48 Å (Table 1), none of the
structures are domed. Only 22+ is saddled and contains
dihedral angles of 35.7° and 43.3° (average 39.5°) for the I1−I3
and I2−I4 planes, respectively (Figure 4a), comparable to very
saddled structures.63 In contrast, 2+, 2, 2−, and 22− are best
described as adopting mixed nonplanar conformations.49 As
such, we find that a description of each structure’s distortions
applying the domed/saddled dihedral convention is insufficient
as each isoindole ring can be twisted and/or tilted relative to
the N4 plane. Therefore, we define secondary γ and δ planes
encompassing the Mn−N1−N4−N8 and Mn−N1−N2−N6
planes, respectively. The twist/tilt angles are then calculated
from the normal of an isoindole plane (Ii) relative to a given γ
or δ plane (Table 1) with a clockwise twist or an upward tilt
given as positive (Figure 4b). Using this approach provides a
more detailed look at Pc ring distortions, while also capturing
traditional saddled dihedral angles, such as for 22+, by instead
summing the I1−I3 and I2−I4 tilt angles (Table 1). As an
overall metric of distortion, we have summed the average twist
and tilt values and have found a clear trend in structural
distortions wherein both 22+ and 22− are heavily distorted
21.6° and 19.3°, respectivelyrelative to 2+ (6.9°), 2 (7.5°),
and 2− (9.4°) (Table 1).
A closer look at the bond lengths within the EtOPc structures
may provide insight as to the nature of the distortions. While
the Pc(2-) ligand is overall aromatic, this aromaticity is often
ascribed to localized internal neutral [18]annulene or dianionic
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([16]annulene)2− 18 π e− circuits encircling the central pocket,
as described above (Figure 1b,c).5,7,18,20,21 Assuming average
C−N single and double bond lengths of 1.47 and 1.27 Å,67
respectively, a closer look at the bond lengths in 2 suggests a
[16]annulene circuit is at play (Figures 1c, S33A). Specifically,
the average C−N bond length of the [16]annulene (C8N8)
circuit between pyrrolic nitrogen (Npyr) and Cα, as well as
between meso nitrogen (Nmeso) and Cα (Figure 4a) is 1.351 Å
(range: 1.307−1.408 Å). In contrast, the average Cα−Cβ bond
length is 1.459 Å (range: 1.436−1.473 Å), suggesting a
disconnection between the central C8N8, 18 π e− circuit from
each of the outer 6 π phenyl aromatic systems (Figure 1).
Furthermore, the C8N8 ring is nearly perfectly planar with an
average atom-to-plane displacement of only 0.032 Å (range:
0.002−0.095 Å) (Figure S32). Together, this is consistent with
a ([16]annulene)2−, 18 π e− model in 2 (Figure 1c).5,7
Similar to 2, the average Cα−Cβ bonds in 22+ are elongated
(average: 1.452 Å; range: 1.434−1.479 Å) relative to the C−N
bonds of the C8N8 core (average: 1.352 Å; range: 1.315−1.387
Å) (Figure S33A). This again suggests disconnected 6 π phenyl
aromatic fragments tethered to a 16 π nonaromatic C8N8 core
(vide infra). In contrast to 2, the change to a nonaromatic
system in 22+ is evidenced by the significant distortions from
planarity observed in the C8N8 core where the average atom-
to-plane displacement is now 0.206 Å (range: 0.003−0.292 Å)
(Figure S32). Such reported 16 π nonaromatic porphyrinoid
cores show similar degrees of distortions from planarity.22
A similarly distorted C8N8 core is observed in 22− where the
average atom-to-plane displacement is comparable to 22+ at
0.159 Å (range: 0.007−0.322 Å) (Figure S32). However, in
contrast to 2 and 22+, the Nmeso−Cα bonds (Figure 4a) display
a distinct short/long pattern with mean short and long bonds
of 1.298 Å (range: 1.295−1.301 Å) and 1.374 Å (range:
1.370−1.377 Å), respectively. Furthermore, this short/long
pattern propagates along a single axis containing two trans-
disposed isoindole units, connected to the C8N8 core by
shortened Cα−Cβ bonds (mean: 1.412 Å; range: 1.397−1.426
Å) relative to the perpendicular Cα−Cβ set (mean: 1.467 Å;
range: 1.460−1.474 Å), the latter being similar to those in 2
and 22+ (Figure S33A). Together, this data suggests a 20 π e−,
dianionic ([18]annulene)2− anti-aromatic framework (Figure
1b). The bond length patterns are also similar to previously
observed antiaromatic Pc(4-) systems.18,20,21
appear at 7.63 (CH), 4.97 (CH2), and 1.85 (CH3) ppm in
CD2Cl2 (Figure 5a). The corresponding resonances for 22+ in
Figure 5. 1H NMR spectra of (a) 2 in CD2Cl2; (b) 22+ in CD2Cl2; (c)
22− in C6D6, illustrating the shift in phenyl (H1), methylene (H2), and
methyl (H3) resonances as a function of oxidation state and associated
aromatic character. Kryptofix-222 (kryp) resonances are also shown in
c. Other peaks in a−c correspond to residual solvents.
the same solvent are only slightly upfield shifted (7.51, 4.60,
and 1.60 ppm) and not indicative of any major paratropic ring
current supporting a localized inner, nonaromatic [16]-
annulene ring (Figure 5b). In contrast, the resonances of 22−
are significantly upfield shifted (4.53, 2.53, 0.64 ppm) and
indicative of a strong, inner paratropic current. In addition, the
resonances attributed to the Kryptofix-222 protons are
broadened and significantly downfield shifted at (6.30, 5.79,
4.29 ppm) relative to published values (Figure 5c).74−76 This
may be the result of equilibrium coordination to the terminal
nitride forcing the [K(kryp)]+ cations to reside above the plane
of the paratropic current, as observed in the solid state
structure (Figure S29), and resulting in a downfield shift
similar to previous reports.17−19 We note that the spectrum of
22− was collected in C6D6 for stability reasons, and directly
compared to 2, also collected in C6D6, (Figure S5B), and
reveals that the shift is not merely from solvent effects. Lastly,
1
we note that the H NMR resonances of 22+ and 22− both
display C4 in solution, in contrast to the XRD data (vida
supra), and is likely a result of rapid interchange of orthogonal
tautomers consistent with other similar annulene structures.73
The UV−vis spectra of all five isolated redox states in the
series were measured. The Q-band for 2 occurs at 767 nm
(Figure S22) and becomes progressively red-shifted and
broadened upon oxidation (2+: 820 nm, 22+ 832 nm; Figures
S23−S24). These results do not follow predictive models
proposed by Gouterman and Lever for the correlation between
oxidation state and Q-band position in metallophthalocya-
nines.77 However, the apparent switching from an aromatic (2)
to nonaromatic (22+) core, and the high degree of structural
distortion adds complexity that is not necessarily represented
in the above predictive models. It should be noted that the
UV−vis spectrum of 22+ still displays strikingly low energy
transitions for our nonaromatic assignment, differing from
analogous 16 π e− porphyrin-based systems. However,
nonaromatic Pc-derivatives have been demonstrated to display
such UV−vis spectra, and may be the result of intramacrocycle
charge transfer transitions.78 Due to the surprisingly small shift
in the observed Q-band, we also performed time-dependent
In contrast to the redox extrema in 22+ and 22−, 2+ and 2−
show minimal net distortions relative to 2 (Table 1),
consistent with a delocalized radical state. The average Cα−
Cβ bond length for 2+ and 2− do not vary significantly from the
values of 2 (Figure S33B). Additionally, the MN bond
length in both 2+ (1.526(5) Å) and 2− (1.497(8) Å) relative to
2 (1.555(9) Å) seem to suggest little electronic participation of
the metal or apical nitrogen toward these events, corroborating
the solution-state spectra that suggest a highly delocalized
radical residing on the ring, a common state for mono-oxidized
and monoreduced macrocyclic systems.16,35,50−52
Spectroscopic Properties. NMR spectroscopy is one of
the most direct experimental methods for distinguishing
diatropic from paratropic ring currents commonly associated
with classic Huckel aromatic (4n + 2)π and antiaromatic (4n)π
̈
systems, respectively.1,68−70 Diatropic π electron ring currents
result in typical downfield shifted outer ring protons (ex.
benzene), whereas an opposite paratropic current emerges in
antiaromatic systems resulting in an opposite upfield shift of
outer ring protons.17−19,71,72 The H NMR resonances of 2
1
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Figure 6. (a−c) Proposed aromatic (blue), nonaromatic (gray), and antiaromatic (purple) circuits, as well as elongated single bonds (red) for 2
(a), 22+ (b), 22− (c). Circles represent NICS values, with the area of each circle directly proportional to the NICS value (blue = negative, purple =
positive), at 1 Å above the respective subring geometric center and normalized against the highest absolute value (see SI for full computational
details). (d−f) Corresponding HOMOs for 2 (d), 22+ (e), 22− (f).
UV−vis experiments to verify that 22+ did not decompose
within the time scale of data collection, and verified that it is
stable (Figure S25).
significant structural distortions observed in 22+ relative to 2
(Figure 3, Table 1), with the former adopting a central
[16]annulene 16 π e− core. Indeed, structural parameters, such
as atom-to-plane displacement patterns (vide supra, Figure
S32), largely mimic those reported for the solid state structure
of authentic [16]annulene.84
In contrast to the oxidation series, the reduction products
are quite air- and moisture-sensitive. This coupled with the
necessarily low concentration for collection led to increased
difficulty in data collection. Indeed, time-dependent UV−vis
spectral acquisition of putative 22− indicates decomposition,
with two initial weak blue-shifted absorption peaks, at 577 and
624 nm, in the expected regime for Pc(4-) complexes
decomposing into two new red-shifted bands, at 673 and
752 nm, similar to the peaks of 2 (Figure S22, Figure
S26).79−81 Lastly, attempted acquisition of the spectrum for 2−
reveal two bands similar to 2, again indicating likely
decomposition. Given the above considerations, the UV−vis
spectra of the reductive products remain unassigned.
More drastic changes are observed in the high positive NICS
values of 22− relative to 2, consistent with a strong paratropic
ring current extending along a dianionic ([18]annulene)2− 20
π e− antiaromatic core (Figure 6c). These NICS data are
consistent with the large upfield shifts observed for peripheral
Pc protons, and downfield shifts observed for Kryptofix-222
protons sitting atop the paratropic ring current (vide supra,
Figure 5c). This paratropic circuit, spanning a single
[18]annulene axis, is also supported by the structural data
indicating short vs long Cα−Cβ bond lengths in orthogonal
isoindole units (vide supra and Figure 6c). As in 22+, we
believe that the aromaticity switch between 2 (aromatic) and
22− (antiaromatic) is mostly responsible for the significant
structural distortions observed (vide supra, Figure 3, Table 1),
similar to those observed from planar [18]annulene85 to the
distorted ([18]annulene)2− dianion.86
DFT studies at the M06-2X level of theory with the 6-
31+G** basis set were also performed on the entire
isostructural series 22+, 2+, 2, 2−, and 22−, utilizing the XRD
coordinates as starting geometries (see SI for full computa-
tional details). The highest occupied molecular orbitals
(HOMOs) for 2, 22+, and 22− are shown in Figures 6d, 6e,
and 6f, respectively. For 2 and 22+, a central orbital density
contribution encompassing the proposed [16]annulene core,
encircled by a circular node, is observed and is flanked by
orbital densities on the outer benzene fragments (Figures
6a,b,d,e). Interestingly, the HOMO of 22− also reflects the
structural data and model proposed for the antiaromatic
[18]annulene2− model (Figures 6c,f). The orbital density
shown also corresponds to the short bonds described above.
Together, the frontier orbitals of all complexes are consistent
with exclusively ligand-borne redox events with little to no
contribution from the Mn center (Figures S35−S39). The only
Computational Results. To correlate our NMR data, we
performed Nucleus-Independent Chemical Shift (NICS)
calculations on 2, 22+, and 22− (Figure 6a−c). Developed by
Schleyer,82 this technique has been highly effective in
distinguishing diatropic (aromatic) from paratropic (antiar-
omatic) ring currents in macrocyclic compounds, such as
porphyrinoids and Pc.2,9,17,18,25,68,83 The NICS calculations
were performed at a point in space 1 Å above the plane of each
subring following the normal (see Figure 6a blue dots which
indicate the centers of each subring). For 2, negative NICS
values are observed throughout the entire ring system (Figure
6a), consistent with diatropic (aromatic) current. In contrast, a
substantial reduction in diatropic current is observed in 22+,
particularly in the central [16]annulene core and adjacent
pyrrole subrings, indicating a significant decrease in aromaticity
(Figure 6b). The outer phenyl fragments, however, are
significantly less affected by this loss in diatropic current,
supporting a localized aromaticity switch from aromatic (2) to
nonaromatic (22+) at the [16]annulene core. These NICS data
are also consistent with the observed modest upfield shift of
1
the H NMR resonances for 22+ relative to 2 (Figure 5a,b).
Together, we propose that the switch from aromatic (2) to
nonaromatic (22+) π systems is likely the root cause of the
F
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Journal of the American Chemical Society
Article
exception is in 22− where the HOMO displays a small fraction
of orbital density at the nitride, consistent with its coordination
to [K(kryp)]+ (Figures 6f, S39).
of NaHCO3 was slurried in 20 mL DMF in open air and heated to
100 °C for 5 h, resulting in a red/purple solution. After cooling, 150
mL of water was added, and the solution was stirred for 30 min. The
mixture was filtered and washed with 100 mL of water, followed by
100 mL of Et2O. The resulting red powder was dried under vacuum.
Single crystals suitable for XRD studies were obtained by layering
benzene over a saturated fluorobenzene solution of 1 at room
CONCLUSION
■
In conclusion, we have outlined the synthesis and character-
ization of a novel manganese nitride phthalocyanine (2), which
is the first clearly characterized example of a large macrocyclic
system that can access the triad of aromatic, nonaromatic, and
antiaromatic states through a series of reversible redox events.
Combined structural, spectroscopic, and computational studies
reveal that all redox events are ligand-borne centering on
specific annulene-like internal circuits. Altering the degree of
aromaticity and pathway of these circuits is proposed to be the
root cause of the observed structural distortions. Perhaps
surprisingly, the frontier orbitals are demonstrated to have
minimal contribution from the Mn(V) center or the apical
nitrogen, with the MnN motif functioning more as an inert
“cap” than an electronic participant. The synthetic accessibility,
stability, isolable nature, and scope of aromatic behavior make
this complex an attractive platform for studying switchable
aromaticity in broader contexts, such as in organic electronic
devices. Our current interests are in studying these stable
complexes as charge carriers for energy storage applications.
1
temperature. Yield: 1.35 g (61.4%). H NMR (400 MHz, CD2Cl2) δ
6.69 (bs), 1.74 (bs), −5.82 (bs). Anal. Calc. for
C48H48ClMnN8O8•CH2Cl2: C, 56.58; H, 4.84; N, 10.77. Found: C,
55.66; H, 4.47; N, 11.73. μeff (Evans method): 4.64 μB (S = 2).
λmax(Q-peak) = 825 nm.
EtOPcMnN (2). Compound 1 (1.05 g, 1.05 mmol, 1 equiv) was
added to 100 mL of MeOH, resulting in a purple/red solution.
Concentrated NH4OH (1.05 mL, 14.7 mmol, 15 equiv) was added
dropwise over the course of 5 min followed by 9.5 mL (∼6 equiv)
Clorox bleach over the course of 15 min, resulting in the production
of a white gas and a green solution. The solution was stirred for an
additional 5 min and then placed into an ice bath for 10 min. While
cooling, 75 mL of DCM was slowly added, followed by 50 mL of
H2O, and subsequently transferred to a separatory funnel. The organic
layer was washed 3 times with 50 mL of H2O and then reduced to
dryness under vacuum at 60 °C, yielding a dark green powder which
was then dissolved in DCM and recrystallized by layering with Et2O,
yielding dark green plate-like crystals. Yield: 0.908 g (88.5%). 1H
NMR (400 MHz, CD2Cl2) δ 7.63 (s, 8H, C6H2), 4.97 (m, 16H,
OCH2CH3), 1.85 (t, J = 7.0 Hz, 24H, OCH2CH3). 13C{1H} NMR δ
150.9, 148.3, 127.2, 117.6, 67.0, 15.44. Anal. Calc. for
C48H48MnN9O8: C, 61.73; H, 5.18; N, 13.50. Found: C, 61.59; H,
5.00; N, 13.45. λmax(Q-peak) = 767 nm.
EXPERIMENTAL SECTION
■
3,6-Diethoxyphthalonitrile. 3,6-Diethoxyphthalonitrile was syn-
thesized using a modified procedure previously reported by
Rauchfuss.33 A mixture of 10.00 g (0.062 mol, 1 equiv) of 2,3-
dicyanohydroquinone and 17.26 g (0.124 mol, 2 equiv) of K2CO3 in
125 mL of wet acetone was heated to reflux and sparged with N2 for
10 min. Ethyl iodide (29.02 g, 0.186 mol, 3 equiv) was then added
dropwise to the mixture. The yellow slurry was stirred under reflux for
24 h. After cooling, the yellow solid was filtered off and washed with
500 mL of water, 150 mL of Et2O, collected, and then dried under
[
EtOPcMnN][B(C6F5)4] (2+). In a glovebox, 0.020 g (0.0214 mmol)
of 2 was dissolved in ∼7.5 mL of DCM. [(4-BrC6H4)3N][B(C6F5)4]
(24.9 mg, 0.0214 mmol, 1 equiv) was dissolved in ∼2.5 mL of DCM,
which was added dropwise to the green DCM solution of 2 under
stirring, turning the solution wine red. The solution was stirred for 5
min, then reduced to dryness under reduced pressure. Benzene (15
mL) was added, and the solution was stirred for an additional 10 min.
The slurry was filtered through a glass wool Celite plug resulting in a
light green/yellow effluent and dark powder on the plug. The powder
was extracted with DCM, resulting in a wine-red solution. The
solution was pumped to dryness, resulting in a dark red powder that
was stored at −40 °C. Recrystallizations were performed by layering
in fluorobenzene with benzene at room temperature, resulting in small
trapezoidal red crystals. Yield: 0.031 g (90.9%). 1H NMR (400 MHz,
CD2Cl2): Silent. 11B{1H} NMR δ −14.7 (s). 19F{1H} NMR δ 131.2
(bs), 161.8 (t), 165.6 (bs). Anal. Calc. for C72H48BF20MnN9O8: C,
53.62; H, 3.00; N, 7.82. Found: C, 53.46; H, 2.67; N, 7.38. λmax(Q-
peak) = 820 nm.
1
vacuum to afford an off-white powder. Yield: 6.78 g (50.2%). H
NMR (400 MHz, CDCl3) δ 7.14 (s, 2H, C6H2.), 4.13 (q, J = 7.0 Hz,
4H, OCH2CH3), 1.47 (t, J = 7.0 Hz, 6H, OCH2CH3).
Lithium 1,4,8,11,15,18,22,25-Octaethoxyphthalocyanine
(
EtOPcHLi). This precursor was synthesized using a modified
procedure previously reported by Rauchfuss.33 A mixture of 6.00 g
(0.028 mol, 1 equiv) of 3−6-diethoxyphthalonitrile in 75 mL of wet
EtOH was heated to reflux and sparged with N2 for 10 min. Li pellets
(2.43 g, 0.347 mol, 12.5 equiv) were added over the course of 10 min,
resulting in a large quantity of white gas evolving as the mixture
turned dark green. After addition of the Li pellets, another 75 mL of
wet EtOH was added. The mixture was refluxed for 5 days. After
cooling, 100 mL of EtOH and 100 mL of H2O was added, and the
green solid was filtered off and washed with 100 mL of H2O, 100 mL
of EtOH, and 100 mL of Et2O. The resulting green powder was dried
[
EtOPcMnN][B(C6F5)4]2 (22+). In a glovebox, 0.020 g (0.0214
mmol) of 2 was dissolved in ∼7.5 mL of DCM. [(4-BrC6H4)3N]-
[B(C6F5)4] (49.7 mg, 0.0428 mmol, 2 equiv) was dissolved in ∼2.5
mL of DCM, which was added dropwise to the green DCM solution
of 2 under stirring, turning the solution deep fuchsia. The solution
was stirred for 5 min, then reduced to dryness under reduced
pressure. Benzene (15 mL) was added, and the solution was stirred
for an additional 10 min. The slurry was filtered through a glass wool
Celite plug resulting in a light blue effluent and dark powder on the
plug. The powder was extracted with DCM, resulting in a deep
fuchsia solution. The solution was pumped to dryness, resulting in
dark purple microcrystals that were stored at −40 °C. Recrystalliza-
tions were performed by layering in fluorobenzene with benzene at
room temperature, resulting in rectangular purple crystals. Yield:
0.031 g (64.0%). 1H NMR (400 MHz, CD2Cl2) δ 7.51 (s, 8H, C6H2),
4.60 (q, J = 7.0 Hz, 16H, OCH2CH3), 1.60 (t, J = 7.0 Hz, 24H,
OCH2CH3). 13C{1H} NMR (low signal/noise) δ 149.7, 147.2, 137.8,
135.5, 130.4, 128.7, 115.5, 67.3, 15.1. 11B{1H} NMR δ −16.7 (s).
19F{1H} NMR δ 133.2 (bs), 163.6 (t), 167.5 (bs). Anal. Calc. for
C96H48B2F40MnN9O8: C, 50.31; H, 2.11; N, 5.50. Found: C, 50.70;
H, 1.91; N, 5.24. λmax(Q-peak) = 832 nm.
1
under dynamic vacuum for 24 h. Yield: 4.64 g (76.6%). H NMR
(400 MHz, CDCl3) δ 14.74 (s, 1H, NH), 7.52−7.42 (m, 8H, C6H2),
7
4.95 (m, 16H, OCH2CH3), 1.82 (m, 24H, OCH2CH3). Li NMR
(CDCl3): Silent.
1,4,8,11,15,18,22,25-Octaethoxyphthalocyanine (EtOPcH2).
EtOPcHLi (4.00 g, 4.56 mmol, 1 equiv) was slurried in 100 mL of
H2O in a flask and heated to 60 °C. HCl (12.1 M, 4 mL, 10 equiv)
was added dropwise over the course of 15 min, changing the green
slurry to a purple solution. After 72 h of stirring, 19 g (0.1368 mol, 30
equiv) of K2CO3 was carefully added over the course of 15 min,
returning the purple solution to a green slurry. The slurry was filtered
and washed with 100 mL of H2O, and 100 mL of Et2O. The resulting
green powder was dried under vacuum. Yield: 2.81 g (71%). 1H NMR
(400 MHz, CDCl3) δ 7.61 (s, 8H, C6H2), 4.95 (q, J = 7.0 Hz, 16H,
OCH2CH3), 1.84 (t, J = 7.0 Hz, 24H, OCH2CH3), 0.21 (s, 2H, NH).
EtOPcMnCl (1). A mixture of 2.00 g (2.3 mmol, 1 equiv) of
EtOPcH2, 2.00 g (excess) of MnCl2, and 0.969 g (11.5 mmol, 5 equiv)
G
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Journal of the American Chemical Society
Article
[K(kryp)][EtOPcMnN] (2−). In a glovebox, 0.050 g (0.0535 mmol)
of 2 was dissolved in ∼7.5 mL of THF with 0.022 g (0.0588 mmol,
1.1 equiv) of Kryptofix 222. KC8 (0.0072 g, 0.0533 mmol, 1 equiv)
was slurried in THF and added dropwise to the green solution of 2
under stirring, turning the solution deep teal. The solution was stirred
for 10 min, then reduced to dryness under vacuum. Benzene (10 mL)
was added, and the solution stirred for 15 min, resulting in a dark
green/teal solution that was filtered through a glass wool Celite plug
resulting in a deep green/teal effluent and dark powder on the plug.
The product was extracted with THF, resulting in a dark teal effluent
which was then reduced to dryness to yield the microcrystalline
product. Recrystallizations were performed by layering in THF with
isooctane at −30 °C, resulting in small, trapezoidal purple/blue
crystals. Yield 0.010 g (13.9%). 1H NMR (400 MHz, THF-d8): Silent
with the exception of trace signals of 22− present. Anal. Calc. for
C66H84KMnN11O14: C, 58.74; H, 6.27; N, 11.42. Found: C, 58.87; H,
6.37; N, 11.33.
Foundation, under grant number NSF/CHE-1346572. Use of
the Advanced Photon Source, an Office of Science User
Facility operated for the U.S. Department of Energy (DOE)
Office of Science by Argonne National Laboratory, was
supported by the U.S. DOE under Contract No. DE-AC02-
06CH11357. Prof. Lior Sepunaru is thanked for helpful
discussions.
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́
̈
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b12899.
■
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NMR, EPR, UV−vis, FT-IR, DFT, electrochemical data
(PDF)
Crystallographic data of 1 (CIF)
Crystallographic data of 2 (CIF)
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Crystallographic data of 2− (CIF)
Crystallographic data of 22− (CIF)
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AUTHOR INFORMATION
Corresponding Author
*menard@chem.ucsb.edu
ORCID
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Gabriel Menard: 0000-0002-2801-0863
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ACKNOWLEDGMENTS
■
We thank the University of California, Santa Barbara and the
California NanoSystems Institute (CNSI) Challenge Grant
program for financial support. NSF’s ChemMatCARS Sector
15 is principally supported by the Divisions of Chemistry
(CHE) and Materials Research (DMR), National Science
H
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