Installation of internal electric fields by non-redox active cations in transition metal complexes

Article abstract of DOI:10.1039/c9sc02870f

Local electric fields contribute to the high selectivity and catalytic activity in enzyme active sites and confined reaction centers in zeolites by modifying the relative energy of transition states, intermediates and/or products. Proximal charged functio

Full text of DOI:10.1039/c9sc02870f

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Installation of Internal Electric Fields by Non-Redox Active Cations
in Transition Metal Complexes
Kevin Kang,†a Jack Fuller III,†b Alexander H. Reath,a Joseph W. Ziller,a Anastassia N. Alexandrova,b,c*
Received 00th January 20xx,
Accepted 00th January 20xx
Jenny Y. Yanga*
DOI: 10.1039/x0xx00000x
Local electric fields contribute to the high selectivity and catalytic activity in enzyme active sites and confined reaction
centers in zeolites by modifying the relative energy of transition states, intermediates and/or products. Proximal charged
functionalities can generate equivalent internal electric fields in molecular systems but the magnitude of their effect and
impact on electronic structure has been minimally explored. To generate quantitative insight into installing internal fields in
synthetic systems, we report an experimental and computational study using transition metal (M1) Schiff base complexes
functionalized with a crown ether unit containing a mono- or dicationic alkali or alkaline earth metal ion (M2). The synthesis
and characterization of the complexes M1 = Ni(II) and M2 = Na+ or Ba2+ are reported. The electronic absorption spectra and
density functional theory (DFT) calculations establish that the cations generate a robust electric field at the metal, which
stabilizes the Ni-based molecular orbitals without significantly changing their relative energies. The stabilization is also
reflected in the experimental Ni(II/I) reduction potentials, which are shifted 0.12 V and 0.34 V positive for M2 = Na+ and Ba2+,
respectively, compared to a complex lacking a proximal cation. To compare with the cationic Ni complexes, we also
synthesized a series of Ni(salen) complexes modified in the 5’ position with electron-donating and -withdrawing
functionalities (-CF3, -Cl, -H, -tBu, and -OCH3). Data from this series of compounds provides further evidence that the
reduction potential shifts observed in the cationic complexes are not due to inductive ligand effects. DFT studies were also
performed on the previously reported monocationic and dicatonic Fe(II)(CH3CN) and Fe(III)Cl analogues of this system to
analyze the impact of an anionic chloride on the electrostatic potential and electronic structure of the Fe site.
www.rsc.org/
Introduction
Chart 1
Directional electric fields can play a critical role in directing the
rate and selectivity of chemical catalysis by manipulating activation
(OTf)n
N
O
N
O
[M1M2]n+
2
energies and the relative energies of catalytic intermediates.1,
M1
M2
Electrostatic fields are believed to be key contributors to the
accelerated reactivity or selectivity observed in a wide-range of
environments, from enzymatic active sites3, 4 to zeolite pores.5, 6
M1 : transition metal
M2 : alkali or
alkaline earth metal
O
O
O
O
Quantum mechanical studies have described harnessing
directional electric fields as a ‘smart reagent’ for mediating chemical
reactivity.7 Recent experimental studies have illustrated the utility of
applied (or external) electrostatic fields in mediating reactivity. For
example, Kanan et al. demonstrated the use of electric fields
generated at electrode surfaces to control metal oxide catalyzed
epoxide rearrangement8 and a carbene reaction.9 Additionally,
electric fields generated between an STM tip and surface can
accelerate localized isomerization reactions,10 a single molecule
Diels-Alder reaction,11 or cleavage of a C-O bond.12 Localized electric
fields from electrode nanostructures also plays a significant role in
electrocatalysis, with a prominent case revealing significantly
improved selectivity for CO2 reduction to hydrocarbons.13 These
experiments demonstrate the power of electric fields to accelerate
or induce desirable reactivity. However, the magnitude of an
external electric field generated at an electrode diminishes rapidly,
limiting its effect. Similarly, electric field generation at an STM tip is
limited to localized or single molecule reactivity. Additionally, higher
external fields can result in dielectric breakdown or deleterious
Faradaic processes.
a. Department of Chemistry, University of California, Irvine 92697
b. Department of Chemistry and Biochemistry, University of California, Los Angeles,
Los Angeles, CA, 90095, USA
c. California NanoSystems Institute, Los Angeles, CA, 90095, USA
† These authors contributed equally
Electronic Supplementary Information (ESI) available: Experimental methods,
synthetic procedures, 1H NMR, electronic absorption spectra, cyclic voltammetry,
infrared spectra, crystallographic data, computational data and coordinates. See
DOI: 10.1039/x0xx00000x
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In contrast, installation of an internal electric field in a molecular
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DOI: 10.1039/C9SC02870F
Scheme 1.
system can provide a predictable, directional, and consistent electric
field at each active site. Electric fields generated by internal dipoles
have been increasingly cited in selective or increased reactivity for
organic synthesis.14-30 Kanan et al. have demonstrated that the field
generated by ion pairing modifies the regioselectivity of Au(I)-
catalyzed hydroarylation of 3-substituted phenyl propargyl ethers.31
Coote et al. have also reported using pH switchable fields that rely
on protonation-deprotonation to vary radical stability.32
Our efforts to generate controllable and directional internal
electric fields have focused on installation of non-redox active
cations proximal to redox-active reaction sites. We first reported the
cobalt derivatives of the salen-crown ligand shown in Chart 1. The
crown moiety appended to the four-coordinate Schiff-base ligand
can accommodate a large range of cationic metal centers (M2). Alkali
redox potential and electronic properties. Computational methods
are also applied to the previously reported iron complexes in this
framework to understand the impact of anion coordination on the
electronic structure and redox potential. Additionally, we have
synthesized a series of Ni(salen) complexes with varying electron-
donating and -withdrawing functionalities in the 5’ position. We find
that the relationship between the spectroscopic properties and
reduction potential contrast with that of 2M, indicating minimal
inductive ligand effects with the latter. Taken together, this study
provides a detailed and quantitative examination of how positioned
cations can be utilized in synthetic systems to install electric field of
fixed magnitude and direction.
and alkaline earth metals were utilized because they are known to
34
generate significant electric fields in enzymes33,
and zeolite
cavities.35 A significant shift in the Co(II/I) redox potential was
observed upon installation of the redox-inactive cation. Electronic
absorption spectroscopy indicated the ligand field is not significantly
perturbed by M2. Instead, we suggested the shifts in redox potential
were likely due to the electrostatic field generated by M2, which was
supported by modelling M2 as a point charge.36 A subsequent study
on the iron analogues of the ligand in Chart 1 found the magnitude
of the Fe(III/II) redox potential also had a strong dependence on both
M2 and whether a counterion was coordinated to the Fe.37 However,
our initial studies lacked definitive evidence for the magnitude of the
electric field, its impact on the electronic structure of the redox-
active center, and the effect of inner-sphere anions. These details are
critical to effectively utilize cations to generate directional electric
fields to optimize reaction rates or selectivity.
Results and discussion
Synthesis. The 1M Schiff base ligands (Scheme 1) were
synthesized using a modified literature procedure.38 A dialdehyde
with an ether chain was templated with an alkali or alkaline earth
metal triflate salt, which subsequently underwent macrocyclization
via condensation with one equivalent of ethylene diamine. 1Na(OTf)
and 1Ba(OTf)2 were synthesized using the corresponding metal
triflate salts. They were purified via recrystallization by layering
diethyl ether on a methanol solution of the crude product (Scheme
1, M = Na+, Ba2+).
1M (M = Na+, Ba2+) ligand and an equivalent of Ni(OAc)2 were
refluxed in methanol to synthesize the heterobimetallic 2M Ni(II)
complexes (Scheme 1). Synthesis was followed by removal of
methanol and acetic acid in vacuo. The heterobimetallic complexes
were purified and recrystallized by vapor diffusion of diethyl ether
into the methanol solutions of each compound. The compound
In order to investigate the electric field effect in more detail, we
have synthesized the Ni(II) (M1) analogue of the ligand with Na+ and
Ba2+ cations as M2 (Scheme 1). As electric fields scale with charge, we
would expect dications to have a larger the effect on the magnitude
as monocations. The salen framework easily accommodates a
square-planar closed-shell Ni(II) coordination environment. In this
study, we detail the synthesis and spectroscopic properties of Ni(II)
complexes with Na+ or Ba2+ incorporated in the crown with
computational methods to more fully understand the changes in
Figure 1. Solid-state structures of 2Na(OTf) and 2Ba(OTf)2. Thermal ellipsoids are drawn to 50% probability.
Hydrogen atoms, outer-sphere anions, and solvent molecules have been omitted for clarity.
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Table 1. Summary of Structural, Spectroscopic, and Electrochemical Data for Ni(3’-OCH3-salen) and 2M Complexes. Electrochemical
DOI: 10.1039/C9SC02870F
and electronic absorption spectroscopy were taken in dimethylformamide.
Complex
aE1/2
Ni(II/I)
, V
bΔE,
cNi(II)
ν(C=N) λ(dd),
Calc.
λ(dd),
nm
λ(ππ*)e
, nm (ε,
Calc. λ
(ππ*)
, nm
λ
Calc. λ
(MLCT),
nm
NiM ,Å
V
ion, τ4
, cm-1
nm (ε,
(MLCT)e,
nm (ε, M-
1cm-1)
415
M-1cm-1)
M-1cm-1)
Ni(3’-OCH3-
salen)
2Na(OTf)
-2.11
-1.99
-1.77
--
--
0.0152d
0.0217
0.0882
1545
1553
1554
547
(111)
552
(113)
555
563
565
607
349
(8033)
345
(9800)
344
345
333
327
390
389
386
(5019)
406
0.12 3.4374(6)
0.34 3.7095(4)
(5695)
404
2Ba(OTf)2
(105)
(9358)
(5176)
c
aReduction potentials referenced to Fe(C5H5)2+/0. bDifference between the reduction potential of 2M and Ni(3’-OCH3-salen). τ4 value
describing the coordination geometry around the Ni(II) ions, where τ4 =1 for a tetrahedral geometry and τ4 = 0 for a square planar
geometry dfrom ref 40. eassignment based on ref. 42
identities were confirmed using high resolution mass spectrometry condensed with one equivalent of ethylenediamine to form the
and single crystal X-ray diffraction analysis. Purities were determined substituted salen ligand. Each 5’-R-salen ligand and an equivalent of
by elemental analysis. Ni(3’-OCH3-salen)) (3’-OCH3-salen = N,N’- Ni(OAc)2 were refluxed in methanol to form the Ni(5’-R-salen)
bis(3-methoxysalicylidene)-1,2-diaminoethane) was synthesized complexes. The products were isolated by removal of methanol and
according to a literature procedure39 to compare with the 2M acetic acid under reduced pressure. 1H NMR spectra for each Ni
complexes.
complex is shown in Figures S4-S8.
The compounds were characterized by 1H NMR, shown in Figures
Structural Studies. Single crystals for X-ray diffraction were grown
S2-S3. Compared to Ni(3’-OCH3-salen ), each 2M complex had a by diffusion of diethyl ether into methanol solutions of the 2M
downfield shift in the imine proton signal with a greater shift triflate salts; the solid-state structures are shown in Figure 1. In each
correlating with higher cationic charge. However, the imine proton heterobimetallic complex, the triflate counteranions are bound to
in 1M complexes was shifted even farther downfield than in the the M cation. In 2Na(OTf), the Na+ ion only appears bound to 5 of the
corresponding 2M complexes.
6 available oxygen donors. In 2Ba(OTf)2, two triflate anions are
coordinated to the Ba2+ and form a symmetric bridge to an adjacent
2Ba(OTf)2 complex. The mass spectrum does not contain peaks
consistent with a dimer, suggesting that the species only exists in the
solid-state form. A methanol molecule is also coordinated to each
Ba2+ cation.
The 5’-R salen ligands (R = CF3, Cl, tBu, OCH3) were synthesized
using a modified literature procedure.39 Two equivalents of 2-
hydroxy-5-R-benzaldehyde were dissolved in methanol and
Each complex contains a minimally distorted square planar nickel
ion with the M cation inhabiting the crown ether pocket. The 4
values describing the coordination around the Ni(II) ions are listed in
Table 1 and confirm similar coordination environments between the
compounds.40 The distances between the Ni(II) ion and M cations are
also listed in Table 1.
Electrochemistry. Cyclic voltammetry of Ni(3’-OCH3-salen) in
dimethylformamide versus Fe(C5H5)2+/0 reveals a reversible reduction
at -2.11 V that we assign to a Ni(II/I) redox event (Figure 2, top,
individual CVs are shown in Figures S26-28). The corresponding
Ni(II/I) reduction events in 2Na and 2Ba are also reversible and found
at -1.99 and -1.77 V, respectively. Compared to Ni(3’-OCH3-salen),
the redox potential for 2Na and 2Ba are shifted positive by 120 mV
and 340 mV, respectively. The increased difference in redox potential
correlates with increased cationic charge, which we have also
observed in similar complexes.36
Cyclic voltammetry of the Ni(5’-R-salen) (R = CF3, Cl, tBu, OCH3)
compounds are also reversible as shown in Figure 2 (bottom) and
summarized in Table 2. As expected, the appended inductive groups
result in changes to the Ni(II/I) reduction potential. Compared to
Ni(salen), addition of the trifluoromethyl and chloro electron-
withdrawing substituents resulted in positive shifts in redox potential
by 190 mV and 140 mV, respectively. Conversely, the addition of the
methoxy and tert-butyl electron-donating groups caused the redox
potential to shift negatively by 50 mV and 90 mV, respectively.
Figure 2. Cyclic voltammograms of the reversible Ni(I/II)
redox couples of (top) Ni(3’-OCH3-salen) and 2M (M = Na+,
Ba2+) and (bottom) Ni(5’-R-salen) where (R = CF3, Cl, tBu,
OCH3) in 0.1 M tetrabutylammonium hexafluorophosphate
in dimethylformamide under N2 at a scan rate of 1 V/s.
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Table 2. Summary of Structural, Spectroscopic, and Electrochemical Data for Ni(5’-R-salen). Electrochemical and electronic
absorption spectroscopy were taken in dimethylformamide.
Complex
Ni(5'-tBu-salen)
Ni(5'-OCH3-salen)
Ni(salen)
E1/2 Ni(II/I), Va
-2.17
ν(C=N), cm-1
1525
λ(d→d), nm (ε, M-1cm-1)b
538 (163)
λ(π-π*), nm (ε, M-1cm-1)b
335 (9328)
λMLCT, nm (ε, M-1cm-1)b
416 (7071)
-2.15
-2.12
1538
1533
551 (200)
532 (141)
344 (5947)
334 (8500)
434 (6932)
410 (6548)
Ni(5'-Cl-salen)
Ni(5'-CF3-salen)
-1.99
-1.91
1527
1541
534 (180)
528 (144)
337 (7738)
336 (6249)
417 (6205)
400 (5289)
aReduction potentials referenced to Fe(C5H5)2+/0. bassignment based on ref. 42
Electronic and Vibrational Spectroscopy. Solid state infrared
spectroscopy of the complexes was taken to compare the electronic
environment of the nickel center (Figures S29-S32X). As cationic
charge increases, the vibrational frequency of the imine C=N stretch
increases (Table 1). Ni(5’-R salen) displayed shifts in vibrational
frequency of the C=N stretch over a range of 14 cm-1 compared to
Ni(salen). These shifts did not correlate with electron-donating or -
withdrawing capability (Table 2).
The compounds were examined by electronic absorption and
infrared spectroscopy to further probe the electronic effects of the
Na+ and Ba2+ cation on the Ni(II) ligand field. The electronic
absorption spectra in dimethylformamide are shown in Figures S10-
S15 and are summarized in Table 1. The absorption spectrum of
Ni(3’-OCH3-salen) displays a π → π* transition at 349 nm and a d →
π* (MLCT) transition at 415 nm.41, 42 The 2M complexes exhibit a
slight blue shift in both their π → π* bands (≤5 nm) and MLCT bands
(≤11 nm) relative to Ni(3’-OCH3-salen). Additionally, Ni(3’-OCH3-
salen) has a d → d absorption band at 547 nm, which appears in 2M
with a small red shift (≤8 nm). Molar absorptivity for each transition
for 2M is comparable to Ni(3’-OCH3-salen). Changes in absorption in
2M compared to Ni(3’-OCH3-salen) indicate that the presence of M
has a small influence on the ligand field around the Ni(II) center.
The electronic absorption spectra of the monometallic compounds
are shown in Figures S16-S25 and are summarized in Table II.
Ni(salen) displays a d → d absorption band at 532 nm. Compared to
Figure 3. Effect of secondary metals on orbital energies in Ni
complexes. Orbitals involved in MLCT are bolded in the graph and
plotted on the right for the Ni(salen–OMe) complex.
Ni(salen), the same transition appears in the Ni(5’-R-salen)
complexes with a small red shift (≤9 nm) with the exception of Ni(5’-
OCH3-salen) which shifted this transition by 23 nm. As expected,
modifying the ligand with electron withdrawing functionalities shifts
the reduction potential positive. The inductive effect on the ligand
field is reflected in the increasing energy of the d-d band, which
correlates to changes in the HOMO-LUMO gap (although R = OCH3
does not fit the trend). In contrast, addition of the cations in 2M show
slightly smaller changes in their d-d band absorption despite larger
changes in reduction potential. The absorption wavelength for 2M
also shifts to slight lower energies as the reduction potential shifts
positive, or the opposite trend observed in the monomeric
complexes.
spectra with errors less than 0.2 eV (typical errors for TD-DFT being
0.1–0.5 eV)48; only small shifts in wavelengths were observed for
different secondary cations (Table 1). However, all the frontier
orbitals shift in energy by a similar magnitude with increasing
Table 3. Structural and redox properties from the
previously reported iron complexes in this ligand
framework.
b
E1/2
Fe−M
distance,
A°
Fe−Cl
distance,
A°
Compound
(M1-M2)
a
Computational Studies.
τ5
(Fe(III/II
), V
Nickel Complexes. To clarify the effect of the secondary cation,
density functional theory (DFT) calculations were performed using
the B97X-D functional.43 The def2-SVP basis set was used for
geometry optimizations and the def2-TZVP basis for final electronic
energies and spectra.44 Solvent was modeled using the SMD solvent
model45 with the corresponding parameters for dimethylformamide
or acetonitrile. Calculations were performed using Gaussian 0946 and
electrostatic potential maps were plotted using ChemCraft.47
Fe(II)(CH3CN)-K+
Fe(II)(CH3CN)-Ba2+
Fe(II)(salen)
-
-
-
-0.29
-0.09
-0.73
-0.69
-0.53
-0.76102
3.6592(4)
-
0.11
-
-
-
Fe(III)Cl-K+
3.6596(6)
3.8115(3)
0.36
0.13
2.218
2.228
Fe(III)ClBa2+
[Fe(III)(Cl)(salen)]1
-
0.20
2.238
02, 115
Electronic absorption spectra simulated using time-dependent
DFT (TD-DFT)48 showed fairly good agreement with experimental
aτ5 = 0 for a square pyramidal geometry and τ5 = 1 for a trigonal
bipyramidal geometry bReduction potentials in acetonitrile
referenced to ferrocene/ferrocenium.
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Figure 4. Effect of secondary metals on orbital energies in Fe(II)MeCN and Fe(II)Cl complexes. Geometries and HOMOs are depicted,
to show the lack of geometric distortion in Fe(II)MeCN and distortion in Fe(II)Cl: Cl- bends toward the secondary metal due to
electrostatic attraction, while Cl withdraws from the participation in the HOMO. All orbitals trend down in energy as a function of the
charge of the secondary metal, indicating the consistent effect of the intramolecular electric field.
cationic charge, as shown in Figure 3. The small relative changes in
DFT calculations revealed geometry distortions in the Fe(III)Cl
orbital energies is consistent with the modest differences observed complexes in the presence of the secondary metals. In the reduced
in absorption spectra. In addition, the energies of all the MOs near form of each complex, the chloride ligand was bent toward the
the HOMO-LUMO gap trend in response to the field in a nearly secondary metal, significantly distorting the square pyramidal ligand
parallel fashion. Hence, minimal spectral changes can be seen. Both field, and considerably reducing the ligand field splitting (notice a
the experimental and computational results indicate the major much more compressed MO manifold in this case, Figure 4). The
impact of the secondary cations is electrostatic as they do not distortion causes electronic changes to the redox active metal and its
significantly alter the ligand field at the redox site. Instead, as the coordination. The HOMO of Fe(II)salen, which is also the LUMO of
LUMO energy decreases from Ni(3’-OCH3-salen) to 2Na (by –0.27 eV) Fe(III)salen, is delocalized over the d-AO on Fe and the p-AO on Cl. As
and to 2Ba (by –0.42 eV), the reduction potential shifts positive.
the distortion occurs in the presence of the secondary metal, the
HOMO acquires a pure d-character. Due to symmetry breaking, the
nature of the HOMO in Fe(II)Cl changes from dxy in Fe(II)salen to dx2-
Iron Complexes. The synthesis, structure, and characterization of
several iron analogues were previously reported.37 Specifically,
complexes with M1 = Fe(II)(CH3CN) or Fe(III)Cl and M2 = K+ and Ba2+
were prepared. Both the Fe(II) and Fe(III) complexes displayed a
similar trend in redox potential to the Ni complexes (i.e. positive shift
was larger for M2 = Ba2+ than M2 = K+ and both were shifted relative
to the congener without any proximate cation as described in Table
3). The Fe(III/II) reduction potential of M1 = Fe(II)(CH3CN) were
shifted by 440 mV (M2 = K+) and 640 mV (M2 = Ba2+) more positive
than Fe(II)(salen). In contrast, when M1 = Fe(III)Cl, the equivalent
when the secondary metal is present. Nevertheless, the field
y2
exerted by the secondary metal shifts all the orbital energies in a
predictable downward trend with the increase of the charge of the
secondary metal. Furthermore, the trend lines for MO energies as a
function of the secondary metal charge have similar slopes for the
two Fe(II) complexes, indicating that the field experienced by Fe is
similar in strength in the two complexes.
Hence, the redox-inactive cation may cause not only the shift in
potential was only 70 mV (M2 = K+) and 230 mV (M2 = Ba2+) more orbital energies by exerting an electric field, but also a significant
positive compared to Fe(III)(salen)Cl. Both reductions are perturbation to geometric and electronic structures of the redox-
electrochemically reversible, indicating ligand loss upon electron active metal. In this case, the redox-active metal structurally distorts
transfer is not impacting the observed reduction potentials. Hence, as the anionic axial chloride ligand moves towards the cation. When
unlike in the Ni(II/I) case, the pure effect of the secondary metal gets the axial ligand is missing, as in the case of Ni, or does not hold a
entangled with the effect of the auxiliary ligand on Fe, suggesting negative charge, as in Fe(II)CH3CN, the isolated field effect of the
that the intramolecular field effects are affected. We sought to secondary metal is seen. Hence, the ligand environment must be
explain the difference between the Ni and Fe complexes using sufficiently rigid for the addition of a secondary metal to have a
computational techniques.
purely electrostatic effect. However, even accounting for geometric
effects the electrostatic effect is still robust and predictable.
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Another way to illustrate the presence or absence of significant
Even with some geometric distortion in the framework, it is clear
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distortions to the electronic structure of the redox site is the the cation exerts significant internal electric fields. Consistent with
DOI: 10.1039/C9SC02870F
electrostatic potential maps of the complexes (Figure 5). For Fe(II) an electrostatic effect, the impact of a dication is always larger than
complexes with acetonitrile, the maps for Fe(3’-OCH3-salen) and the monocation. Evidence for the electric field by changes in the
Fe(II)(CH3CN)-K+ look nearly indistinguishable at first sight, apart from redox potential is also notable because the measurements are made
the magnitude of the potential (seen in the scale bars), confirming a in solutions containing ~100-fold excess of electrolyte. Hence, the
clean intramolecular field effect of the secondary metal. In the case rigid positioning of the cation and molecular architecture results in
of the Fe(II)Cl analogues, the change of the magnitude of the an electric field that is not easily quenched by solvation or presence
electrostatic potential upon the addition of the secondary metal is of ions in high concentrations.
still seen, but smaller. However, the potential gains a significant
asymmetry, in accord with the changing geometry of the complex.
Electric fields have emerged as an important tool for accelerated
or selective reactivity.8-32 We have previously described examples of
how these types of cationic compounds disrupt traditional linear free
energy relationships.37, 49 Other potential applications of directional
fields include stabilizing polar transition states to improve reaction
rates. Additionally, directional fields can enable product selectivity
by selectively stabilizing intermediates that would otherwise be
comparable in energy in the absence of a field. Rate acceleration and
improved selectivity have been demonstrated using external electric
fields. This study demonstrates how the magnitude and direction of
internal electric fields can be controlled through synthetic design.
Incorporating a field into each reaction site does not require any
external distance-dependent field source and can now be scaled.
Acknowledgements
KK, AHR, and JYY would like to acknowledge support from NSF
CAREER Award (CHE-1554744). KK would also like to thank the UCI
Summer Undergraduate Research Program for support. JYY is
grateful for support as a Sloan Foundation Fellow, Camille Dreyfus
Teacher-Scholar, and CIFAR Azrieli Global Scholar in the Bio-inspired
Energy Program. JF(III) and ANA would like to acknowledge support
from NSF CAREER Award (CHE-1351968). Calculations were
performed on the UCLA IDRE shared computer cluster, Hoffman2.
Figure 5. Electrostatic potential maps. (top) Fe(II)(sal)CH3CN
and (bottom) Fe(II)(sal)Cl complexes, without (left) and with
(right) K+ in the secondary coordination sphere. The ligand for the
complexes is 3’-OCH3-salen (left) or salen-crown (right). In the
case of CH3CN, the metal coordination appears minimally
disturbed, apart from the global shift in the magnitude of the
electrostatic potential. In the case of Cl, the global shift is smaller,
and the metal coordination appears visibly perturbed by the
presence of K+.
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Our study demonstrates the use of proximal cations to install
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