Multiplying the electron storage capacity of a bis-tetrazine pincer ligand

Article abstract of DOI:10.1039/c4dt00341a

An unexpected doubling in redox storage emerging from a new pincer ligand upon bis-ligation of iron(ii) is described. When tetrazine arms are present at the two ortho positions of pyridine, the resulting bis-tetrazinyl pyridine (btzp) pincer ligand displays a single one-electron reduction at ca. -0.85 V vs. Ag/AgCl. Complexation to iron, giving the cation Fe(btzp)₂ ²⁺, shows no oxidation but four reduction waves in cyclic voltammetry instead of the two expected for the two constituent ligands. Mossbauer, X-ray diffraction and NMR studies show the iron species to contain low spin Fe(ii), but with evidence of back donation from iron to the pincer ligands. CV and UV-Vis spectroelectrochemistry, as well as titration studies as monitored by CV, electronic spectra and EPR reveal the chemical reversibility of forming the reduced species. DFT and EPR studies show varying degrees of delocalization of unpaired spin in different species, including that of a btzp^-1 radical anion, partnered with various cations. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00341a

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Multiplying the electron storage capacity of a
bis-tetrazine pincer ligand†
Cite this: Dalton Trans., 2014, 43,
6513
Christopher R. Benson,a Alice K. Hui,a Kumar Parimal,a Brian J. Cook,a
Chun-Hsing Chen,a Richard L. Lord,*b Amar H. Flood*a and Kenneth G. Caulton*a
An unexpected doubling in redox storage emerging from a new pincer ligand upon bis-ligation of iron(II)
is described. When tetrazine arms are present at the two ortho positions of pyridine, the resulting bis-
tetrazinyl pyridine (btzp) pincer ligand displays a single one-electron reduction at ca. −0.85 V vs. Ag/AgCl.
Complexation to iron, giving the cation Fe(btzp)22+, shows no oxidation but four reduction waves in cyclic
voltammetry instead of the two expected for the two constituent ligands. Mossbauer, X-ray diffraction
and NMR studies show the iron species to contain low spin Fe(II), but with evidence of back donation from
iron to the pincer ligands. CV and UV-Vis spectroelectrochemistry, as well as titration studies as monitored
by CV, electronic spectra and EPR reveal the chemical reversibility of forming the reduced species. DFT
and EPR studies show varying degrees of delocalization of unpaired spin in different species, including
that of a btzp−1 radical anion, partnered with various cations.
Received 31st January 2014,
Accepted 4th February 2014
DOI: 10.1039/c4dt00341a
www.rsc.org/dalton
Introduction
Redox equivalents delivered into the manifold of orbitals
defined within transition metal complexes have been utilized
in myriad strategies for chemical transformations.1–4 Recent
attention has turned from metal-centered redox processes to
those centered on the attendant ligands (Fig. 1). This interest
emerged out of studies seeking to clarify the ambiguity sur-
rounding the site of reduction or oxidation, in complexes
characterized by “redox non-innocent” chemistry that could be
ascribed to either the metal or the ligand, particularly when
using ligands derived from quinones.5–7 As a sign of the times,
this question has evolved to the point where the unusual
quality of a ligand’s non-innocence is now replaced by the
deliberate design8,9 of ligands that will predictably display
multiple redox states. The design of molecular switches10–12 or
electrocatalytic processes13–15 involving ligand-derived pro-
cesses benefits from this evolution in our understanding. We
have also reviewed recently the exceptional potential of poly-
azine aromatics to accept electrons and hence to serve as com-
ponents of a new class of redox active ligands.16 Thus,
Fig. 1 Some redox-active ligands: (a) benzene dithiolate,6 (b) pyridine
bisdiimine,1 (c) bispyridyltetrazine,17 and (d) diazinyl pyridine.8
opportunities exist to explore novel variations on known, and
unknown, motifs that could serve as the origin of a ligand’s
redox activity. To this end, we present the synthesis, electronic
properties and representative metal complexation character of
a C2-symmetric, pincer-type terdentate ligand where a central
pyridine is flanked by two redox-active and highly π-acidic
tetrazines.17 Unexpectedly, from one ligand with one
reduction, we add a coordinating metal ion and multiply
the redox activity twofold. This type of behavior, where the
ligand’s ability to accept electron doubles, is unprecedented in
the behavior of complexes of polyazine ligands.
aIndiana University, Department of Chemistry, 800 East Kirkwood Avenue,
Bloomington, IN, USA. E-mail: caulton@indiana.edu; Fax: +1 812-855-8300;
Tel: +1 812-855-4798
bGrand Valley State University, Department of Chemistry, Allendale, MI, USA
†Electronic supplementary information (ESI) available: Crystallographic, syn-
thetic, DFT, and spectroscopic data, as well as results from supporting experi-
ments. CCDC 955991 and 955992. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4dt00341a
In coordination chemistry, Kaim17 has explored the poten-
tial of 1,2,4,5 tetrazines (Fig. 1c) when located between two
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pyridines to define a redox-active ligand that can couple two to cationic metals, but these reduction products were not
metal centers together via its accessible lowest unoccupied further characterized.
molecular orbital (LUMO) giving rise to Class II through to
Herein we report the successful synthesis and iron(II)
Class III mixed-valent character.18 Recently, some of us have coordination chemistry of the redox-active bis-tetrazinyl pyri-
employed this ligand’s redox activity to drive molecular switch- dine ligand, “btzp” (Scheme 1). We verified that the highly
ing in pseudorotaxanes.10–12 The LUMO which gives rise to π-acidic character of the ligand originates from the low-lying
these properties is localized on the nitrogens of the central LUMO delocalized across the tetrazines and the redox pro-
tetrazine ring with negligible orbital amplitude at the bridge- perties of the ligand are explored. Complexation with iron(II)
head carbons of the π* orbitals of the two pyridines. Thus, we leads to multiple redox states becoming accessible with the
wondered if this orbital situation could be engineered in iron(II) oxidation state being highly stabilized in this environ-
reverse such that the pyridine serves as the bridge between two ment; the iron’s oxidation was shifted by more than ∼900 mV
tetrazines (btzp, Scheme 1), which also offers the possibility of to beyond +2.2 V vs. Ag/AgCl compared to an analogous bis-
redox states changing by two electrons. The symmetrical ter- terpyridine complex. This work summarizes our results to
dentate ligand we designed represents the most expedient for- establish the veracity of the novel doubling in redox storage
mulation of this idea.
displayed by a bis-tetrazine pincer ligand that shows a single
Tetrazine rings are synthesized from organonitriles (R–CN) electron process as a free ligand, and then gains access to a
in reaction with hydrazine, followed by oxidation, to form the second reduction upon complexation.
1,2,4,5-tetrazine core disubstituted at the 3 and 6 positions.
With few exceptions, these tetrazines are prepared as coordi-
nating ligands in a symmetrical manner. The recent emergence
Results
of metal-free click chemistry has seen an exploration19 of non-
Synthesis of the bis-tetrazine ligand
symmetrical tetrazines, which is the class of substitution
Generation of this pincer proceeds (Scheme 1) from the
addition of hydrazine to 2,6-dicyano pyridine, which can
couple with acetonitrile in the presence of sulfur.19 Under the
conditions of the reaction, we were able to minimize the gene-
ration of pyridyl-tetrazine polymers through the addition of a
large excess of acetonitrile. Residual side products were
removed by column chromatography with pyridyl-tetrazine
polymer binding to the column gel, and the highly volatile
dimethyl tetrazine subliming during the evaporation of solvent
under vacuum or during chromatography. This molecule has a
lilac or bright fuchsia color typical of tetrazines, due to an
n→π* transition, and showing the small HOMO–LUMO gap in
the molecule (contrast colorless 2,2′-terpyridyl). The molecule
shows a 1H NMR singlet for the ring methyls, together with an
AX2 pattern in the aromatic region for the symmetrically sub-
stituted pyridyl.
X-ray diffraction from a single crystal of the free ligand pro-
vided confirmation of the structure (Fig. SI-1†) in the solid
state. The three rings are co-planar with deviations out of
plane attributed to crystal packing forces. The solid-state
packing shows the formation of anti-parallel rows of the btzp
ligands with close intermolecular contacts made by virtue of
multiple CH⋯N hydrogen bonds. Each btzp ligand is also
engaged in π stacking and while the ligands are offset in a
slip-stack manner, they are located approximately above and
below each other consistent with stacking typical of the largely
π-deficient character of the tetrazine rings.21
needed for btzp. With the exception of one multistep synthesis
reported in the patent literature,20 these tetrazine-forming
reactions are statistical in nature. We reasoned that our
desired non-symmetrically substituted tetrazine could sub-
sequently be separated from symmetrical co-products given
the large differences in the structure between btzp and the
other two by-products: dimethyl tetrazine and a pyridyl-tetra-
zine polymer.
For comparison, a study of terpy analogs (Fig. 1d) of
M(pincer)22+ complexes (Fig. 1, M = Fe and Co), where the outer
arms contained 2 or 3 nitrogens, has been reported.8 This
revealed low spin behavior for iron, and Fe–N bond lengths
were interpreted as showing weaker metal binding to polyazine
rings than to pyridine. In marked contrast to our results with
btpz, the reported iron complexes all showed one oxidation
somewhere between +1.5 and +2.0 V vs. SCE. Multiple
reductions were observed for the iron complexes between −0.5
and −1.5 V vs. SCE, and attributed to the usual stabilizations
of ligand-centered radical anions that occur upon coordination
2+
Synthesis and characterization of Fe(btzp)2
Addition of [Fe(H2O)6](BF4)2 to btzp at a 1 : 2 mole ratio, each
in acetonitrile, occurs with color change to dark blue within
minutes at 25 °C to form a dark solid. This solid was purified
by re-precipitation from acetonitrile with ether or benzene
and was established to be [Fe(btzp)2](BF4)2 by a variety of
Scheme 1 Synthetic approach.
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characterization methods. Its 1H NMR spectrum shows a
The oxidation state and the spin state of the iron center
doublet (intensity 2) and a triplet (intensity 1) for the pyridyl within the solid state structure of the [Fe(btzp)2](BF4)2·MeCN
hydrogens, as well as an intensity 6 signal for equivalent tetra- complex were determined using Mössbauer spectroscopy22
zine methyls, all in a chemical shift range consistent with dia- (Fig. SI-3†). A reproducible Mössbauer spectrum of the bis-
magnetism and proving twofold symmetry of two equivalent ligand complex with an isomer shift of δ = 0.18 mm s−1 and
pincer ligands. Addition of free btzp to
a
solution of quadrupole splitting of ΔEQ = 1.32 (mm s−1) are typical of low
2+
Fe(btzp)2 shows separate 1H NMR signals for free and co- spin iron(II) complexes.23–25
ordinated ligand protons, showing integrity, both thermo-
The electronic structure of the ligand and its complex was
dynamic and kinetic, of the complex. The 19F NMR spectrum further investigated using UV-Vis spectroscopy (Fig. 3). The
shows only a singlet for free BF4−1, resolved into 1 : 4 intensi- free ligand shows a visible absorption at 536 nm consistent
ties due to 10,11B isotope effect on chemical shift. The positive with the characteristic pink color of tetrazine-containing com-
ion mass spectrum of this compound in MeCN shows pounds with similarly modest absorptivity, ε = 700 M−1 cm−1
.
2+
Fe(btzp)2 (m/z = 295) and also the ion pair Fe(btzp)2F+ (609, Addition of up to 0.5 equivalent of iron(II) salt in MeCN led to
from F− capture from BF4−), but also the monocation the appearance (Fig. 3) of two new absorptions at 584 nm
Fe(btzp)2 (590), a product of reduction during electrospray, (7000 M−1 cm−1) and 419 nm (4700 M−1 cm−1) that are tenta-
+
which speaks for the ready reduction of the pincer ligands in tively assigned to metal-to-ligand charge-transfer (MLCT) tran-
this dication. The dication is unaffected by dissolved oxygen, sitions largely in line with other Fe(II) complexes of
1
even to the point of no broadening of the H NMR spectrum, polypyridyl-derived ligands.26,27 The lowest energy electronic
and no EPR signal.
transition corresponds to a band gap of 2.1 eV. Assuming a
Crystals grown from MeCN–hexane were shown (Fig. 2) by formal MLCT state with modest mixing between the metal-cen-
single crystal X-ray diffraction to contain two pincer ligands tered and the ligand-centered HOMO and LUMO respectively,
per iron, with high crystallographic symmetry of the cation so this optical energy gap provides a reasonable estimate of the
that only one octant of the species is unique (i.e., Fe, one tetra- voltage difference between the first reduction and first oxi-
zine and half of one pyridyl). Bond lengths of iron to N are dation of the complex in solution.
longer by 0.04 Å to the tetrazine than to pyridyl, but all are
short, consistent with the low spin state and hence empty σF*eN
Solution characterization of the ligand and its iron(II) complex
orbitals of eg symmetry on an octahedron.
To better understand the mechanism of complex formation,
an H NMR titration (Fig. 4) was carried out in which aliquots
1
of iron(II) tetrafluoroborate hexahydrate were added to a 3 mM
solution of btzp in CD3CN. Titration with aliquots of iron(II)
2+
salt resulted in the complete formation of the Fe(btzp)2
complex after addition of 0.5 equivalent with no residual
peaks associated with the remnant ligand, matching a 2 : 1
ligand : metal ratio. The shifts in the pyridyl peaks by 0.59 and
0.45 ppm for H4 and H3,5 respectively, are consistent with
prior studies on Fe(II) coordination of terpyridine and its
analog,28,29 and indicative of proton deshielding on account of
Fig. 2 ORTEP drawing (50% probabilities) of the non-hydrogen atoms
−
of [Fe(btzp)2]2+ as its BF4 salt, showing selected atom labeling.
Unlabeled atoms are carbon or (shaded) nitrogen. The dication has an S4
2+
axis along the N1/Fe1 line. Selected structural parameters: Fe1–N1,
1.900(11) Å; Fe1–N4, 1.947(7); N4–Fe1–N4*, 91.38(6)°; N4–Fe1–N4’,
162.2(4); N4–Fe1–N1’, 98.9(2); N4–Fe1–N1, 81.1(2).
Fig. 3 UV-Vis titration to form Fe(btzp)2 in solution (purple trace)
from the free ligand (pink trace), demonstrating the emergence of both
n→π* and d→π* transitions in the complex.
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2+
Fig. 4 1H NMR titration to form Fe(btzp)2 in solution beginning from a solution of btzp. Simultaneous appearance of product peaks with dis-
appearance of ligand peaks is indicative of tight binding conditions with slow chemical exchange on the time scale of the experiment.
the strong σ-donor properties of the pyridine. Shifts of protons
in similar structures have been seen with methylene protons
in the terdentate ligand on 2,6-bis(1,2,3-triazol-4-yl)pyridine28
as well as on the terminal methyl protons of the Cu(I) complex
of 3,6-bis-pyridyl tetrazine.11
The simultaneous decrease in intensity of peaks for the free
ligand and growth of peaks associated with the complex indi-
cates tight binding conditions, resulting in a slow exchange
process in which the intermediary solutions are composed of
clearly defined species (either free ligands or ML2 complexes)
with no detectable population of intermediates (e.g., com-
plexes with metals on the exo-directed nitrogen atoms on the
tetrazine moieties). Consequently, the structure observed in
the solid state exists as the dominant species in CD3CN solu-
tion under these conditions. This case is further bolstered by a
CV taken of the titration solution at the end of the NMR experi-
ment (Fig. 5) which shows an identical number and relative
2+
intensity of peaks as does the CV of the synthesized Fe(btzp)2
complex (vide infra).
Electron transfer reactivity of btzp and its Fe(II) complex
Free btzp (Fig. 6) exhibits a single sharply-defined redox
process with E1/2 = –850 mV vs. Ag/AgCl in MeCN, a value con-
sistent with the redox chemistry of uncoordinated monomeric
tetrazines.17 Peak-to-peak potential separation (ΔE) was found
to be 108 mV demonstrating reversibility, and with comparable
ipc and ipa. Although there are two tetrazine moieties per btzp
ligand that could take up one electron each, coulometry (see
Fig. 5 CV of the 0.5 equivalent NMR end point solution (blue) showing
identical features to the voltammetry profile of synthesized Fe(btzp)2
2+
ESI†) shows that the reduction at −850 mV represents only a (inset, purple). Upper trace involves a different iron concentration.
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2+
Fig. 6 Cyclic voltammetry on (a) free btzp ligand and (b) Fe(btzp)2 complex, showing near-ideal reversibility. Black arrow indicates the direction
of sweep.
single electron process. No oxidation peaks were observed in allowing us to directly compare the peak current for the charac-
2+
the solvent window.
teristic one electron Fe(II)/Fe(III) couple in Fe(terpy)2 to the
Our previous work with tetrazine ligands10–12 would predict observed processes within the tetrazine complex. All peak cur-
that addition of an electropositive transition metal center rents appeared to be roughly equal, suggesting that each of the
would simply cause anodic shifts of the free ligand redox four waves in the Fe(btzp)22+ voltammogram is a single electron
2+
potential. Surprisingly, Fe(btzp)2 shows (Fig. 6) four revers- couple. Titration of Fe(BF4)2·6H2O into a solution of btzp, moni-
ible redox couples with half-wave potentials at E1/2 = −25 mV, tored by CV, tied all the solution characterization data together
−190 mV, and −335 mV and −850 mV with no other processes, (Fig. S9†), illustrating both the emergence of the four wave CV
oxidative or reductive, in the solvent window. This latter obser- profile of Fe(btzp)22+ and its facile formation in solution.
vation is consistent with the optical band gap of 2.1 eV, i.e.,
Spectra of the reduced complexes and reversibility of the redox
changes
that given the position of the first reduction of the complex
near 0 V, the Fe(II/III) oxidation process would be anticipated to
be more positive than 2.1 V. Recent work8 on similar disubsti-
tuted pyridine ligands also bolsters this claim, finding that
increased nitrogen character on the flanking azine hetero-
cycles drives the Fe(II)/Fe(III) couple towards increasingly
anodic potentials. In contrast, Fe(terpy)22+ shows the oxidation
to the ferric ion at ∼+1.3 V, consistent with the significantly
poorer σ donor/strong π acceptor properties of tetrazines and
illustrating their inability to stabilize the Fe(III) oxidation state.
UV-Vis spectroelectrochemistry (UV-Vis SEC) was carried out
2+
on a solution of Fe(btzp)2
(Fig. 7) to characterize the
The open circuit potential measured for
Fe(btzp)2 from isolated solid was +0.004 V, an observation
a sample of
2+
consistent with all peaks observed during the cathodic sweep
2+
being reductions from species Fe(btzp)2
.
2+
The complexity of the CV of Fe(btzp)2 was unexpected,
motivating additional experiments. To discount the possibility
that the observed results were the consequence of non-redox
equilibration processes (e.g., ligand substitution by solvent)
2+
intrinsic to Fe(btzp)2 in MeCN, variable scan rates between
5 V s−1 and 50 mV s−1 were recorded, revealing no unexpected
change in relative peak heights or peak positions.
2+
Fig. 7 UV/Vis SEC of Fe(btzp)2 showing the dication and its three
sequentially reduced products at potentials marked on the cathodic
sweep of the slow scan-rate CV (0.2 mV s−1) shown in the inset (black =
0 V, red = −0.24 V, blue = −0.39 V, purple = −0.65 V). SEC experiment
To estimate the number of electrons transferred with each
redox process, a CV was recorded of a solution containing an
2+
equimolar mixture of Fe(btzp)2 and its dimethyl-terpyridine
2+
was carried out on a 1.18 mM solution of Fe(bisTzP)2 in a 1 mm thin-
layer SEC cell with a platinum mesh working electrode, a platinum wire
counter electrode, and a Ag/AgCl pseudoreference electrode.
2+
homolog, Fe(Me2terpy)2 (Fig. S8†). These structurally similar
complexes should have similar diffusion coefficients, thereby
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electronic spectral features of the complex through the first EPR characterization of the location of the electron upon
three reduction peaks shown in the CV in Fig. 5. Upon reduction
reduction, peaks initially present at 418 and 580 nm showed a
Equimolar Cp2Co upon reduction reacts with btzp in MeCN in
red shift. Additionally, the intensity of the peak originating at
time of mixing to give Cp2Co+ (verified by 1H NMR spectro-
418 nm increased while the peak at 580 nm decreased. Titra-
scopy) but shows no 1H NMR signal for the radical anion
tion with progressive aliquots of cobaltocene (Cp2Co) (Fig. 8)
pincer btzp−1 in the chemical shift range +200 to −100 ppm.
revealed changes to visible peaks consistent with the data gen-
The resulting radical produced by cobaltocene reduction was
erated by UV-Vis SEC, with good correspondence between peak
shown to have an approximate half-life of 10 minutes in MeCN
positions using both methods. An additional very broad peak
and 30 minutes in a 0.1 M solution of TBABF4 in MeCN. The
EPR spectrum of this solution (Fig. 9) taken immediately after
production, shows a well resolved 9 line pattern with g =
2.0040 due to four nitrogens coupling with AN = 5.1 G; this
was observed (see ESI†) stretching across the NIR into the IR
band, growing in intensity with the addition of equivalents of
Cp2Co, although the nature of this transition remains
unknown. The solution itself underwent a progressive color
value is comparable to that observed in the radical anion of
change with the addition of equivalents of Cp2Co, going from
symmetrical 1,2,4,5 tetrazine, C4N2H2, where all four symmetry
a dull purple in the unreduced state to green, orange and ulti-
equivalent nitrogens couple equally.30 Additional coupling to
mately a brown or tan color at higher equivalents. A separate
three I = 1/2 particles with AH = 1.3 G is resolved in the spec-
trum of [Cp2Co][btzp] and can be accurately simulated. For
comparison, reduction of btzp in THF by Na, Na(anthracene),
titration with decamethylferrocene, which has only sufficient
reducing power to obtain the doubly reduced product Fe
0
0
(btzp)2 , duplicates the Fe(btzp)2 spectra collected using
UV-Vis SEC and Cp2Co.
K mirror, KC8, or Mg all show a structured pattern at essen-
tially the same g value and with the analogous hyperfine struc-
ture and AN values, but not as well resolved (broader lines) as
with the non-interacting Cp2Co+ cation. We attribute the
broadening to a combination of dynamic effects and possible
quadrupolar broadening by each of these cations coordinated
to the btzp radical anion. EPR simulations show that coupling
to eight nitrogens of two tetrazines would give line intensities
observably different from the experiment, so the assignment
of detectable coupling to four tetrazine nitrogens is most con-
sistent with observation. No four nitrogens in (all planar) btzp
are symmetry equivalent, so we did EPR simulations to estab-
lish how different the AN values could be and still have the
differences unresolved experimentally. This showed that a 15%
difference would observably change the spectra. Simulations
showed that additional coupling to a pair of nitrogens larger
than 0.8 G would be resolved in our spectra. Likewise, coupling
Each reduction product generated in the Cp2Co titration
was reoxidized (inset, Fig. 8) with an excess of an acetonitrile
solution of nitrosonium tetrafluoroborate to determine the
stability of the reduced products and the reversibility of the
chemical reduction processes. In addition to a visible return of
color to that of Fe(btzp)22+, the spectra for these samples were
collected for comparison. The original UV-Vis peak positions
are recovered in all reduction products up to ∼4 equivalents of
Cp2Co. Correcting peak heights for dilution, reoxidation of
each sample shows that the spectrum for each reductive
process returns approximately 90% of original peak intensity.
2+
This suggests that the reduction products of Fe(btzp)2 in any
of its reduced forms are persistent on the timescale of these
experiments (approximately 15 min to completion of the
spectra) and that reoxidation is reversible.
Fig. 9 Observed (red) and simulated (blue) X-band EPR spectrum of
[Cp2Co][btzp] in MeCN at 25 °C. EPR parameters, X-band; microwave
frequency, 9.855; microwave power 2.002 mW; modulation amplitude,
0.50 G; time constant, 20.960 ms; scan time, 42 s.
2+
Fig. 8 UV/Vis spectra after chemical reduction of Fe(btzp)2 with suc-
cessive equivalents of Cp2Co. Inset shows dilution-corrected spectra
after [NO]PF6 reoxidation of each of the four reduction experiments.
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to the unique pyridyl nitrogen larger than 0.8 G would be arene ring; given that the tetrazine is the reduced arene here,
resolved in our spectra. this would break the symmetry equivalence of the two arms in
The question unresolved by these observations is whether (cation)[btzp]. We cannot establish this structural feature
the four nitrogens are in one tetrazine arm or are two nitro- based on the available experimental evidence.
gens (only) in each of the two arms. This is central to learning
DFT characterization of btzp−1 and its interaction with
counterions
whether the spin is delocalized over two arms, or localized in
one. If our assignment of the smaller hyperfine component to
only one methyl group is correct, this argues against delocali- We first carried out a survey of possible structures of the iso-
zation over both tetrazines; such delocalization would show lated anion btzp−1 using DFT methods (Table 1). We find that
hyperfine to six methyl protons. To resolve this, we have the inclusion of implicit solvation strongly influences the most
employed the radical with only one arm and formed [Cp2Co]- stable structure of this anion.31 Without consideration of
[3-pyridyl 6-methyl tetrazine] (Fig. S10†), and recorded its EPR solvent effects, a delocalized (twofold symmetric) state is the
spectrum. This also shows a nine line pattern, each line of minimum; all attempts to converge to a localized state through
which shows coupling to three I = 1/2 nuclear spins. In short, initial geometries and wavefunction manipulation collapse
the EPR spectrum of [Cp2Co][3-pyridyl 6-methyl tetrazine] is back to the delocalized state with two half-reduced tetrazine
very similar to that of btzp−1, which would indicate spin local- arms. However, when the continuum solvent is included
ization in btzp−1. We additionally tested our assignment of the during the geometry optimization, the delocalized twofold
three I = 1/2 hyperfine coupling to one methyl group by symmetric structure is found to be a transition state (TS). Dis-
forming [Cp2Co][1,2,4,5-dimethyltetrazine, C4N2Me2], (Fig. 10). placement away from this TS along the mode with an imagin-
This highly structured spectrum is well simulated by coupling ary frequency in either direction leads to a state with a
to four 14N spins (AN = 5.1 G) and six methyl protons (AH = 1.5 localized SOMO on one of the two tetrazine arms (see Fig. 11).
G). Overall, the hyperfine assignments are strongly supported This localized radical anion is the lowest energy minimum we
by these comparison radical spectra, and the observed number could locate, but lies less than 1 kcal mol−1 lower in energy
of hyperfine lines strongly supports the conclusion that than the delocalized anion (a transition state). Such a low
[Cp2Co][btzp] radical is localized within one tetrazine arm, barrier would suggest rapid interconversion between the loca-
with no resolvable participation of the pyridyl nitrogen in the lized anions, in contrast to the multiplicity of the observed
SOMO.
nitrogen hyperfine structure. Thus, the cation–anion inter-
Having established that the spin in the radical anion in actions that are not included in these calculations are likely
numerous (cation)[btzp] species in the polar and Lewis basic significant. The SOMO of the localized anion radical shows
solvents THF or MeCN is localized in one tetrazine arm, the coplanarity of the three rings, but negligible contribution from
remaining question is whether localization originates in sym- the pyridyl and neutral tetrazine rings which is consistent with
metry-breaking of the structure by location of the cation, or the experimental EPR data. Fig. 11 also shows that the dianion
whether localization is intrinsic to the unperturbed anion. btzp2− is twofold symmetric, but the spin is still primarily at
While nesting of Na+, K+ or Mg2+ in the center of the three the tetrazines.
inwardly directed nitrogens of the pincer moiety is possible, it
DFT calculations were carried out to see whether alkali
should not break twofold symmetry equivalence of the two metal cations are consistent with the localization observed
tetrazine arms, and structures of many arene/alkali metals ion experimentally in bptz−1. Two basic binding motifs were tested
pairs show that the electrophilic cation often sits above the as starting geometries for the computational optimization of
Na(bztp) with a continuum solvation model: (i) with the metal
η6-bound to one of the tetrazine rings and (ii) with the metal
κ3-bound to two tetrazine and one pyridine nitrogens. For the
Na complex, geometry optimizations from different starting
geometries converged to two different minima. The localized
structure shown in Fig. S6 in ESI† has Na+ η2-bound to the
reduced tetrazine ring of bztp (Na–N = 2.32 and 2.44 Å). The
Na–Npyr separation is large at 3.97 Å, ruling out any interaction
between Na+ and the other rings. A second structure has
κ3 connectivity, but the metal ion binds to only one N of each
tetrazine. Expanding our study to Li and K revealed (Fig. 12)
minima for both η2 and κ3, and the κ3 was more stable by
4–10 kcal mol−1 for all three alkali metals. Significantly, all
three κ3 structures have a highly asymmetric unpaired spin
(Fig. 12), located in the tetrazine with the shorter M/N dis-
tances. For completeness, we also minimized a κ3 structure
(Fig. 13) for reduced species Mg(btzp)+, to show the effect of a
Fig. 10 X-band EPR spectrum of [Cp2Co][dimethyl-1,2,4,5-tetrazine] in
MeCN at 25 °C. EPR parameters, see Fig. 11.
small dication. This too has Mg2+ closer to the tetrazine which
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Table 1 Bond lengths (Å) and dihedral angles (°) for free bztp in the neutral to dianion redox sequence. For the doublet monoreduced bztp in solu-
tion, the reduced ring tetrazine values are listed on left and the neutral tetrazine values are listed on right
Gas-phase
Solution-phase
bztp0 singlet bztp1− doublet bztp2− singlet bztp2− triplet bztp0 singlet bztp1− doublet bztp2− singlet bztp2− triplet
N
py–Cpy
1.337
1.489
1.348
1.343
1.317
1.320
1.341
1.497
1.344
1.342
1.362
1.347
0.0
1.352
1.503
1.345
1.341
1.404
1.375
0.0
1.349
1.495
1.347
1.346
1.405
1.399
1.338
1.490
1.344
1.347
1.321
1.315
0.0
1.340–1.343
1.501–1.487
1.339–1.348
1.336–1.348
1.397–1.327
1.384–1.317
0.0–0.0
1.346
1.510
1.334
1.336
1.404
1.371
0.0
1.344
1.506
1.338
1.338
1.402
1.396
0.3
Cpy–Ctet
Ctet–N1
Ctet–N3
N1–N2
N3–N4
N1–Ctet–Cpy–Npy 13.5
19.4
Fig. 11 Spin density isosurface plots (0.002 au) of doublet bztp1− (left)
and triplet bztp2− (right).
Fig. 13 Optimized geometry with short Mg–N bond lengths in ang-
strom (left) and the spin density isosurface plot of [Mg(bztp)]+ (right,
0.002 au).
the bond distances in the calculated species, metal radii
increase in the order Li+ < Mg2+ < Na+ < K+.
In summary, we find, with DFT, a localized structure that
agrees with experiment for M(btzp)−1
.
Na[dimethyltetrazine] structure and spin density
How can chemically inequivalent nitrogens fail to show a resol-
vably distinct hyperfine coupling in btzp−1? Our EPR hyperfine
multiplicity establishes that Na[dimethyltetrazine] also has
equal coupling to all four ring nitrogens. DFT geometry
optimizations of neutral doublet Na[dimethyltetrazine] are
informative of structural preferences in the absence of a
second tetrazine and the donor pyridyl. In the gas-phase two
minima were found, one (Fig. 14 left) where the Na+ remains
Fig. 12 Optimized geometries with short M–N bond lengths (left) and
spin density isosurface plots (right, 0.002 au) for the [M(bztp)]0 species
(M = Li (top), Na (middle), K (bottom)).
carries the majority of the spin density, so is wholly consistent
with the picture of asymmetric (localized) radical character,
but κ3 connectivity.
Taken together, these results show that none of these
metals is “too large” to fit in the plane of the pincer rings
(note the distortion of the MNCCN rings at left in Fig. 12 as M
gets larger), so any localization of unpaired spin in a κ3 struc-
ture is due to intrinsic electronic preferences. Also based on
Fig. 14 Two minimum energy structures for Na[dimethyltetrazine] from
gas phase calculations.
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Fig. 15 Spin densities from solution phase minimum energy structure
of Na[dimethyltetrazine], showing equal spin over all four nitrogens (i.e.,
centrosymmetric), in spite of asymmetric location of the Na+.
+
bound to the face of the ring (which is highly ruffled – Na/N
distances of 2.42 twice and 2.60 Å twice) and the other (Fig. 14
right) where the Na+ is η2 bound to two adjacent nitrogens in
the ring. The ruffled structure is surely symptomatic of loss of
aromaticity upon reduction. The η2 bound form is more stable
by over 30 kcal mol−1. During solution-phase optimizations
(continuum model, so no explicit solvent molecules are
binding to sodium), both starting structures (η2 and η6) con-
verge to the η2 form. Fig. 15 shows the spin density of the solu-
tion-phase η2 structure, showing that all four nitrogens retain
nearly equal spin; in spite of symmetry-breaking by sodium,
the Mulliken charge on each of the four nitrogens is identical
at −0.4. These agree with EPR spectral evidence for delocaliza-
tion of spin to all four nitrogens in this species. Interestingly,
optimizations (from both starting geometries) of the cation
singlet species Na(btzp)+ yield only the η2 bound form so we
can confidently say sodium has a preference to bind to the
harder Lewis base, nitrogen, rather than the softer filled π
system of the arene ring (i.e., η6). A sodium cation here
behaves like a hard Lewis acid, even though it is also
known32,33 to interact with the π system of purely hydrocarbon
radicals and non-radicals when an alternative nitrogen donor
is absent.
Fig. 16 X-band EPR spectrum of Fe(btzp)2 in MeCN at 25 °C. EPR
parameters, see Fig. 11.
MeCN, and shows (Fig. 16) an EPR spectrum at RT in MeCN
with g = 1.998 and a multiplet pattern which is consistent with
coupling to 4 nitrogens with AN = 6.0 G. The UV-Vis spectrum
of the reduced solution was restored to peaks consistent with
2+
dicationic Fe(btzp)2 by treatment with excess NO[PF6] in
MeCN.
Following reduction of Fe(btzp)22+ with equimolar Cp2Co in
CH3CN, then vacuum removal of solvent, and washing of the
green solid with toluene to remove any residual Cp2Co, the H
NMR spectrum of Fe(btzp)2 was recorded in CD2Cl2. This
showed signals at +19.9 and −3.3 ppm, intensity ∼2 : 1,
together with the Cp2Co+ singlet at 5.75 ppm, in correct inten-
sity for 1 : 1 reaction stoichiometry Co : Fe; the third btzp
signal is presumably too broad and weak to be detected.
1
+
In three separate solutions, we reduced Fe(btzp)22+ with 1.0,
1.5 and 2.0 equivalents of outer sphere reductant Cp2Co in
MeCN and recorded UV/Vis spectra, EPR, and then UV/Vis
again (to confirm persistence of the product in solution). At
the UV/Vis positions described above, we observed the growth
(1 equiv.), and then progressive decrease in EPR intensity of
Bond lengths in the η2-bound form of Na[dimethyltetra-
zine] are C2v symmetric. In the solution-phase structure, the
Na–N bond lengths are 2.36 Å; the N–N for those bound to Na
is 1.40 Å while those on the opposite side of the ring have a
N–N distance of 1.40 Å. This would suggest that the presence
of a metal cation has relatively little impact on the ligand geo-
metry but the C–N bond lengths do show a difference of
0.01 Å. The C–N bonds on the side with Na bound are 1.340 vs.
1.331 Å for those on the opposite side of the ring.
+
the 9-line signal assigned to Fe(btzp)2 . In each case, the UV/
Vis absorption was unchanged after the EPR collection,
showing that Fe(btzp)2 persists under these conditions for at
+
least 2 h. Consistent with the DFT result that the neutral
complex has two unpaired electrons, titration of the second
electron into the dication diminishes the EPR intensity charac-
teristic of the singly reduced (S = 1/2) species. This follows
since triplet states generally relax fast and thus give no EPR
signal until exceptionally low temperatures, if at all. Addition
of oxidant [NO]PF6 to a reduced solution recovered Fe(btzp)2
as judged by both spectroscopies, consistent with no loss of
btzp from iron upon reduction. The g and AN values are both
consistent with ligand centered reduction to give a species of
2+
,
+
EPR characterization of Fe(btzp)2
Several approaches were employed to further investigate the
2+
redox properties of the Fe(btzp)2 and prove that products are
+
2+
formula Fe(btzp)2 .
independent of the reducing agent. A reduction of Fe(btzp)2
Separately, a product with this same UV-Vis signature was
produced by addition of increasing aliquots (up to 1 : 1) of
(C5Me5)2Fe to a MeCN solution of Fe(btzp)22+. In contrast to
the slower heterogeneous reduction with zinc powder, this
with excess zinc powder occurred over 48 hours at RT in MeCN
to give an olive green solution from which the product was iso-
lated by filtration and evaporation to dryness. The resulting
+
Fe(btzp)2 exhibited absorptions at 449 nm and 637 nm in
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n+
solution-based outer sphere electron transfer reaction appears
to proceed to completion within time of mixing, accompanied
by the growth of a modest absorption at ∼750 nm, due to
(C5Me5)2Fe+.34–36 Addition of a solution of NO[PF6] to the
reduced species largely returns the original UV-Vis spectrum of
Fe(btzp)22+, consistent with the reversibility of the reduction
and the retention of the structure of unaltered btzp in the
reduced product.
Table 2 Bond lengths (Å) for species Fe(btzp)2
Species
Fe–Npy
Fe–Ntet
Npy–Cpy
C
py–Ctet
Ctet–Ntet
2+, S = 0
1+, S = 1/2
0, S = 1
1.923
1.915
1.912
2.000
1.977
1.963
1.341
1.342
1.342
1.469
1.467
1.467
1.355
1.364
1.369
included, but a delocalized structure was obtained in both
cases. Uncharged species Fe(btzp)2 was optimized as both a
We wanted to evaluate whether a slender and polar mole-
cule like MeCN would be the cause of localization of spin in
0
triplet and singlet; the triplet was lower in energy by ∼10 kcal
mol−1. The additional electron is again spread over both
ligands (Fig. 17), which is evidenced by the highly delocalized
spin density for the neutral species. It is worth noting that the
larger spin density lobes in the neutral are indicative of
+
one tetrazine in Fe(btzp)2 . This monocation was therefore syn-
thesized using Cp2Co in MeCN, and then the resulting solid
pumped in vacuum for several hours, to remove all MeCN. A
sample of THF was then saturated with the resulting solid, to
yield a faintly colored solution. The EPR spectrum of this
sample at 25 °C showed a multiplet with the same g value and
AN value as in MeCN, although the lines were broader and
signal strength was inferior due to low solubility of the com-
pound in this solvent. We conclude that the spin localization
in Fe(btzp)2+ is the same in both MeCN and in THF.
two spins. Thus, DFT predicts
a redox sequence of
[FeII(bztp)2]2+,S=0 → [FeII(bztp−1/2)2]1+,S=1/2 → [FeII(bztp−1)2]0,S=1
.
The metal–nitrogen, nitrogen-bridging carbon, and inter-
ring C–C bond lengths are established38–40 to be quite diagno-
stic of redox loci in polypyridyl (and related) species. A
n+
summary of the most important bond lengths in Fe(btzp)2 is
included in Table 2.
Fe(btzp)2n+ characterization by DFT
Large Fe–N(tetrazine) bond length contractions are
observed with each reduction, presumably due to the electro-
static attraction between the cationic metal and anionic
ligands; this Fe/N bond strengthening supports the experi-
mental evidence that the −850 mV potential process observed
by CV (Fig. 6b) cannot be due to reduction of free btzp from
ligand loss. The smaller Fe/N decrease to pyridyl N than to
tetrazine N upon reduction also supports the development of
anionic charge selectively at tetrazines. The individual Fe/N
distances show that S4 symmetry holds for all three charge
states. The Npy–Cpy and Cpy–Ctet bond lengths are essentially
invariant, reinforcing the fact that the central pyridine plays
only a small role in the acceptor behavior of bztp. The Ctet–Ntet
bond elongates from 1.355 to 1.364 to 1.369 Å, consistent with
occupation of an orbital with tetrazine π* character. The rela-
tively modest elongation of 0.014 Å is because the one or two
electron reduction is distributed over all four tetrazine arms.
In summary, the calculations yield spin delocalized over both
pincer arms first in one, and then in the second btzp.
Gas-phase geometry optimizations were performed on Fe-
(btzp)2 with n = 2, 1 and 0 in order to determine where elec-
n+
trons are added. Singlet, triplet, and quintet states were opti-
mized for the 2+ species and the singlet was found to be
lowest in energy (using B3LYP, a density functional known for
overstabilizing high spin states),37 in agreement with the experi-
ment. The triplet and quintet were approximately 7 and
12 kcal mol−1 higher in energy, respectively. The ground state
singlet Fe(btzp)2 is best described as FeII with two neutral
2+
bztp ligands. The HOMO is mostly due to tetrazine lone pairs
pointing both towards and away from Fe, and the LUMO is
composed of tetrazine π* orbitals. The monocation was opti-
mized as a doublet starting from both the optimized dication
structure and a localized structure with elongated N–N bonds
in only one of the tetrazine arms. Both starting structures opti-
mized to the structure with delocalized spin across the four
tetrazines of both btzp ligands, as shown in Fig. 17. Attempts
were made to optimize these structures with implicit solvation
Discussion
We have focused our efforts on confirming the emergence and
assignments of extra redox peaks. The iron complex shows
four reductive steps, while the free ligand shows only one
reductive process (in spite of containing two redox-active tetra-
zines). Each ligand goes from having a single electron redox
process when free of a metal center to being capable of two
one electron processes in the complex. This we term redox
multiplication: distinctly different ligand behavior (capacity)
dependent upon the ligand coordination environment. Our
DFT results may serve to clarify the reason for this behavior:
upon complexation with iron, extensive remixing of orbitals
Fig. 17 Spin density isosurface plots (0.002 au) for Fe(btzp)21+ (left) and
Fe(btzp)2 (right) species.
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occurs, giving the ligand access to the previously unattainable it forms a tight 2 : 1 complex at 1 mM (ESI). Unlike the iron(II)
second reduction process at greatly reduced potentials.
complex the CV data show just two reduction peaks at −0.64
The question of localized vs. delocalized is clearly influ- and −0.97 V (Fig. S2†) demonstrating the typical8 types of
enced by subtle factors (e.g., solvent, counter ions and hydro- stabilization expected when reducible ligands are coupled to
gen bonding), as has already been established41–43 in the case cationic centers: there is a direct correspondence between the
q
of intervalence charge transfer in M2 species where q is an number of redox processes and the number of ligands. This
odd number. Hydrogen bonding to a solvent is indeed one observation unambiguously shows that the iron(II) is involved
mechanism which has been seriously evaluated for converting synergistically with the ligands to multiply the storage of the
between localized and delocalized ground states of mixed redox activity of the complex.
valence systems.44 In the present case, some factors that the
DFT calculations have not modeled are interactions with redox
innocent counter anions and specific solvation (rather than
dielectric effects) with any charge dense regions of the reduced
Conclusion
complex. The outwardly directed nitrogens are sterically avail-
The successful synthesis and redox chemistry of the btzp
able as is the π face of the tetrazines. Furthermore also con-
ligand and its iron(II) bis-ligand complex have been described.
sider the fact that, in contrast to a terpyridyl ligand, the inner
The highly π-acidic character of the ligand was verified and orig-
coordination sphere surrounding the metal ion center invol-
inates from the low-lying LUMO on the tetrazines. Facile com-
ving the btzp ligand is very exposed. First, the lone pairs on
plexation with iron(II) in solution leads to multiple redox states
the nitrogen α to the M–N, may display greater negative charge
becoming accessible at modest potentials. With the iron(II)
density than the other pendant nitrogens due to the polarizing
state being highly stabilized in this environment, (unobserved)
influence of the Fe(II) center, akin to the electrostatic effect of
oxidation of FeIII was shifted anodically by at least 900 mV to
the Na+ ion; this is another location for counter ion or solvent
beyond +2.2 V vs. Ag/AgCl compared to an analogous bis-ter-
effects to be felt. Second, where terpy CH bonds would be, the
pyridine complex. If, as some hope,45 we are able to confer
tetrazine α nitrogens provide steric access to the Fe(II) center28
electronic “nobility” onto first row transition metals, redox pro-
providing another site for specific counter ion or solvation
perties like those demonstrated here would be highly advan-
effects for localizing the redox orbitals.
tageous, as they make it possible to access multiple low-energy
Electrostatic potential (ESP) maps can be used to establish
reductions as observed in the CV profiles reported here.
where an anion or a nucleophile would be attracted following
n+
A currently unsolved question for Fe(btzp)2 is charge
reduction. An ESP map of the ligand in its neutral (ESI†) and
localization/delocalization in mixed valence species, which
singly reduced form (Fig. 18) was used to evaluate the charge
most generally are odd-electron species with two composition-
distributions. Consistent with the geometry-optimized prefer-
ally identical halves. What we have discovered here is that the
ence of the sodium cation to the complex with the reduced
answer may be dependent on surrounding environment, and
half of the btzp−1 ligand in an η2 fashion, the ESP shows its
that even DFT calculations will need to model explicit part-
largest negative value coincident with the location of the nitro-
ners, once these are identified with certainty.
gen lone pairs, rather than the π-orbital location of the
unpaired spins.
Returning to the observed redox doubling, we prepared
[Na(bztp)2]+, containing a redox inactive cation, to compare its
behavior to the iron(II) complex. H NMR titration shows that
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6524 | Dalton Trans., 2014, 43, 6513–6524
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