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
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
First published on 13th February 2014
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.
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Fig. 1 Some redox-active ligands: (a) benzene dithiolate,6 (b) pyridine bisdiimine,1 (c) bispyridyltetrazine,17 and (d) diazinyl pyridine.8 |
In coordination chemistry, Kaim17 has explored the potential of 1,2,4,5 tetrazines (Fig. 1c) when located between two pyridines to define a redox-active ligand that can couple two metal centers together via its accessible lowest unoccupied molecular orbital (LUMO) giving rise to Class II through to Class III mixed-valent character.18 Recently, some of us have employed this ligand's redox activity to drive molecular switching in pseudorotaxanes.10–12 The LUMO which gives rise to these properties is localized on the nitrogens of the central tetrazine ring with negligible orbital amplitude at the bridgehead carbons of the π* orbitals of the two pyridines. Thus, we wondered if this orbital situation could be engineered in reverse such that the pyridine serves as the bridge between two tetrazines (btzp, Scheme 1), which also offers the possibility of redox states changing by two electrons. The symmetrical terdentate ligand we designed represents the most expedient formulation of this idea.
Tetrazine rings are synthesized from organonitriles (R–CN) in reaction with hydrazine, followed by oxidation, to form the 1,2,4,5-tetrazine core disubstituted at the 3 and 6 positions. With few exceptions, these tetrazines are prepared as coordinating ligands in a symmetrical manner. The recent emergence of metal-free click chemistry has seen an exploration19 of non-symmetrical tetrazines, which is the class of substitution 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 subsequently 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-tetrazine 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 to cationic metals, but these reduction products were not further characterized.
Herein we report the successful synthesis and iron(II) coordination chemistry of the redox-active bis-tetrazinyl pyridine ligand, “btzp” (Scheme 1). We verified that the highly π-acidic character of the ligand originates from the low-lying LUMO delocalized across the tetrazines and the redox properties of the ligand are explored. Complexation with iron(II) leads to multiple redox states becoming accessible with the iron(II) oxidation state being highly stabilized in this environment; the iron's oxidation was shifted by more than ∼900 mV to beyond +2.2 V vs. Ag/AgCl compared to an analogous bis-terpyridine complex. This work summarizes our results to establish the veracity of the novel doubling in redox storage displayed by a bis-tetrazine pincer ligand that shows a single electron process as a free ligand, and then gains access to a second reduction upon complexation.
X-ray diffraction from a single crystal of the free ligand provided 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
Crystals grown from MeCN–hexane were shown (Fig. 2) by single crystal X-ray diffraction to contain two pincer ligands per iron, with high crystallographic symmetry of the cation so that only one octant of the species is unique (i.e., Fe, one tetrazine and half of one pyridyl). Bond lengths of iron to N are longer by 0.04 Å to the tetrazine than to pyridyl, but all are short, consistent with the low spin state and hence empty σ*FeN orbitals of eg symmetry on an octahedron.
The oxidation state and the spin state of the iron center within the solid state structure of the [Fe(btzp)2](BF4)2·MeCN complex were determined using Mössbauer spectroscopy22 (Fig. SI-3†). A reproducible Mössbauer spectrum of the bis-ligand complex with an isomer shift of δ = 0.18 mm s−1 and quadrupole splitting of ΔEQ = 1.32 (mm s−1) are typical of low spin iron(II) complexes.23–25
The electronic structure of the ligand and its complex was further investigated using UV-Vis spectroscopy (Fig. 3). The free ligand shows a visible absorption at 536 nm consistent with the characteristic pink color of tetrazine-containing compounds with similarly modest absorptivity, ε = 700 M−1 cm−1. Addition of up to 0.5 equivalent of iron(II) salt in MeCN led to the appearance (Fig. 3) of two new absorptions at 584 nm (7000 M−1 cm−1) and 419 nm (4700 M−1 cm−1) that are tentatively assigned to metal-to-ligand charge-transfer (MLCT) transitions largely in line with other Fe(II) complexes of polypyridyl-derived ligands.26,27 The lowest energy electronic transition corresponds to a band gap of 2.1 eV. Assuming a formal MLCT state with modest mixing between the metal-centered and the ligand-centered HOMO and LUMO respectively, this optical energy gap provides a reasonable estimate of the voltage difference between the first reduction and first oxidation of the complex in solution.
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Fig. 3 UV-Vis titration to form Fe(btzp)22+ in solution (purple trace) from the free ligand (pink trace), demonstrating the emergence of both n→π* and d→π* transitions in the complex. |
The simultaneous decrease in intensity of peaks for the free ligand and growth of peaks associated with the complex indicates 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., complexes 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 solution under these conditions. This case is further bolstered by a CV taken of the titration solution at the end of the NMR experiment (Fig. 5) which shows an identical number and relative intensity of peaks as does the CV of the synthesized Fe(btzp)22+ complex (vide infra).
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Fig. 6 Cyclic voltammetry on (a) free btzp ligand and (b) Fe(btzp)22+ complex, showing near-ideal reversibility. Black arrow indicates the direction of sweep. |
Our previous work with tetrazine ligands10–12 would predict that addition of an electropositive transition metal center would simply cause anodic shifts of the free ligand redox potential. Surprisingly, Fe(btzp)22+ shows (Fig. 6) four reversible redox couples with half-wave potentials at E1/2 = −25 mV, −190 mV, and −335 mV and −850 mV with no other processes, oxidative or reductive, in the solvent window. This latter observation is consistent with the optical band gap of 2.1 eV, i.e., 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 disubstituted pyridine ligands also bolsters this claim, finding that increased nitrogen character on the flanking azine heterocycles 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. The open circuit potential measured for a sample of Fe(btzp)22+ from isolated solid was +0.004 V, an observation consistent with all peaks observed during the cathodic sweep being reductions from species Fe(btzp)22+.
The complexity of the CV of Fe(btzp)22+ 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) intrinsic to Fe(btzp)22+ 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.
To estimate the number of electrons transferred with each redox process, a CV was recorded of a solution containing an equimolar mixture of Fe(btzp)22+ and its dimethyl-terpyridine homolog, Fe(Me2terpy)22+ (Fig. S8†). These structurally similar complexes should have similar diffusion coefficients, thereby allowing us to directly compare the peak current for the characteristic one electron Fe(II)/Fe(III) couple in Fe(terpy)22+ to the observed processes within the tetrazine complex. All peak currents appeared to be roughly equal, suggesting that each of the four waves in the Fe(btzp)22+ voltammogram is a single electron couple. Titration of Fe(BF4)2·6H2O into a solution of btzp, monitored by CV, tied all the solution characterization data together (Fig. S9†), illustrating both the emergence of the four wave CV profile of Fe(btzp)22+ and its facile formation in solution.
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. This suggests that the reduction products of Fe(btzp)22+ 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.
The question unresolved by these observations is whether the four nitrogens are in one tetrazine arm or are two nitrogens (only) in each of the two arms. This is central to learning 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 delocalization over both tetrazines; such delocalization would show hyperfine to six methyl protons. To resolve this, we have employed the radical with only one arm and formed [Cp2Co][3-pyridyl 6-methyl tetrazine] (Fig. S10†), and recorded its EPR spectrum. This also shows a nine line pattern, each line of which shows coupling to three I = 1/2 nuclear spins. In short, the EPR spectrum of [Cp2Co][3-pyridyl 6-methyl tetrazine] is very similar to that of btzp−1, which would indicate spin localization in btzp−1. We additionally tested our assignment of the three I = 1/2 hyperfine coupling to one methyl group by forming [Cp2Co][1,2,4,5-dimethyltetrazine, C4N2Me2], (Fig. 10). This highly structured spectrum is well simulated by coupling to four 14N spins (AN = 5.1 G) and six methyl protons (AH = 1.5 G). Overall, the hyperfine assignments are strongly supported by these comparison radical spectra, and the observed number of hyperfine lines strongly supports the conclusion that [Cp2Co][btzp] radical is localized within one tetrazine arm, with no resolvable participation of the pyridyl nitrogen in the SOMO.
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Fig. 10 X-band EPR spectrum of [Cp2Co][dimethyl-1,2,4,5-tetrazine] in MeCN at 25 °C. EPR parameters, see Fig. 11. |
Having established that the spin in the radical anion in numerous (cation)[btzp] species in the polar and Lewis basic solvents THF or MeCN is localized in one tetrazine arm, the remaining question is whether localization originates in symmetry-breaking of the structure by location of the cation, or whether localization is intrinsic to the unperturbed anion. While nesting of Na+, K+ or Mg2+ in the center of the three inwardly directed nitrogens of the pincer moiety is possible, it should not break twofold symmetry equivalence of the two tetrazine arms, and structures of many arene/alkali metals ion pairs show that the electrophilic cation often sits above the arene ring; given that the tetrazine is the reduced arene here, this would break the symmetry equivalence of the two arms in (cation)[btzp]. We cannot establish this structural feature based on the available experimental evidence.
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Fig. 11 Spin density isosurface plots (0.002 au) of doublet bztp1− (left) and triplet bztp2− (right). |
Gas-phase | Solution-phase | |||||||
---|---|---|---|---|---|---|---|---|
bztp0 singlet | bztp1− doublet | bztp2− singlet | bztp2− triplet | bztp0 singlet | bztp1− doublet | bztp2− singlet | bztp2− triplet | |
Npy–Cpy | 1.337 | 1.341 | 1.352 | 1.349 | 1.338 | 1.340–1.343 | 1.346 | 1.344 |
Cpy–Ctet | 1.489 | 1.497 | 1.503 | 1.495 | 1.490 | 1.501–1.487 | 1.510 | 1.506 |
Ctet–N1 | 1.348 | 1.344 | 1.345 | 1.347 | 1.344 | 1.339–1.348 | 1.334 | 1.338 |
Ctet–N3 | 1.343 | 1.342 | 1.341 | 1.346 | 1.347 | 1.336–1.348 | 1.336 | 1.338 |
N1–N2 | 1.317 | 1.362 | 1.404 | 1.405 | 1.321 | 1.397–1.327 | 1.404 | 1.402 |
N3–N4 | 1.320 | 1.347 | 1.375 | 1.399 | 1.315 | 1.384–1.317 | 1.371 | 1.396 |
N1–Ctet–Cpy–Npy | 13.5 | 0.0 | 0.0 | 19.4 | 0.0 | 0.0–0.0 | 0.0 | 0.3 |
DFT calculations were carried out to see whether alkali metal cations are consistent with the localization observed experimentally in bptz−1. Two basic binding motifs were tested 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 distances. For completeness, we also minimized a κ3 structure (Fig. 13) for reduced species Mg(btzp)+, to show the effect of a small dication. This too has Mg2+ closer to the tetrazine which carries the majority of the spin density, so is wholly consistent with the picture of asymmetric (localized) radical character, but κ3 connectivity.
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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)). |
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Fig. 13 Optimized geometry with short Mg–N bond lengths in angstrom (left) and the spin density isosurface plot of [Mg(bztp)]+ (right, 0.002 au). |
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 structure is due to intrinsic electronic preferences. Also based on 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.
Bond lengths in the η2-bound form of Na[dimethyltetrazine] 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 geometry 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.
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Fig. 16 X-band EPR spectrum of Fe(btzp)2+ in MeCN at 25 °C. EPR parameters, see Fig. 11. |
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 1H 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 intensity for 1
:
1 reaction stoichiometry Co
:
Fe; the third btzp signal is presumably too broad and weak to be detected.
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 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 characteristic 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)22+, 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 formula 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 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.
We wanted to evaluate whether a slender and polar molecule like MeCN would be the cause of localization of spin in one tetrazine in Fe(btzp)2+. This monocation was therefore synthesized 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 compound in this solvent. We conclude that the spin localization in Fe(btzp)2+ is the same in both MeCN and in THF.
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Fig. 17 Spin density isosurface plots (0.002 au) for Fe(btzp)21+ (left) and Fe(btzp)2 (right) species. |
The metal–nitrogen, nitrogen-bridging carbon, and inter-ring C–C bond lengths are established38–40 to be quite diagnostic of redox loci in polypyridyl (and related) species. A summary of the most important bond lengths in Fe(btzp)2n+ is included in Table 2.
Species | Fe–Npy | Fe–Ntet | Npy–Cpy | Cpy–Ctet | Ctet–Ntet |
---|---|---|---|---|---|
2+, S = 0 | 1.923 | 2.000 | 1.341 | 1.469 | 1.355 |
1+, S = 1/2 | 1.915 | 1.977 | 1.342 | 1.467 | 1.364 |
0, S = 1 | 1.912 | 1.963 | 1.342 | 1.467 | 1.369 |
Large Fe–N(tetrazine) bond length contractions are observed with each reduction, presumably due to the electrostatic attraction between the cationic metal and anionic ligands; this Fe/N bond strengthening supports the experimental 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 relatively 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.
The question of localized vs. delocalized is clearly influenced by subtle factors (e.g., solvent, counter ions and hydrogen bonding), as has already been established41–43 in the case of intervalence charge transfer in M2q species where q is an odd number. Hydrogen bonding to a solvent is indeed one mechanism which has been seriously evaluated for converting between localized and delocalized ground states of mixed 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 complex. The outwardly directed nitrogens are sterically available as is the π face of the tetrazines. Furthermore also consider the fact that, in contrast to a terpyridyl ligand, the inner coordination sphere surrounding the metal ion center involving the btzp ligand is very exposed. First, the lone pairs on the nitrogen α to the M–N, may display greater negative charge density than the other pendant nitrogens due to the polarizing influence of the Fe(II) center, akin to the electrostatic effect of the Na+ ion; this is another location for counter ion or solvent effects to be felt. Second, where terpy CH bonds would be, the tetrazine α nitrogens provide steric access to the Fe(II) center28 providing another site for specific counter ion or solvation effects for localizing the redox orbitals.
Electrostatic potential (ESP) maps can be used to establish where an anion or a nucleophile would be attracted following reduction. An ESP map of the ligand in its neutral (ESI†) and singly reduced form (Fig. 18) was used to evaluate the charge distributions. Consistent with the geometry-optimized preference of the sodium cation to the complex with the reduced half of the btzp−1 ligand in an η2 fashion, the ESP shows its largest negative value coincident with the location of the nitrogen lone pairs, rather than the π-orbital location of the unpaired spins.
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Fig. 18 ESP plot for bztp− with the reduced ring shown on left (isovalue = 0.02 au, range: +0.31 au in green to −0.31 au in red) showing ESP values in the regions indicated. |
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. 1H NMR titration shows that it forms a tight 2:
1 complex at 1 mM (ESI). Unlike the iron(II) complex the CV data show just two reduction peaks at −0.64 and −0.97 V (Fig. S2†) demonstrating the typical8 types of stabilization expected when reducible ligands are coupled to cationic centers: there is a direct correspondence between the number of redox processes and the number of ligands. This observation unambiguously shows that the iron(II) is involved synergistically with the ligands to multiply the storage of the redox activity of the complex.
A currently unsolved question for Fe(btzp)2n+ is charge localization/delocalization in mixed valence species, which most generally are odd-electron species with two compositionally identical halves. What we have discovered here is that the answer may be dependent on surrounding environment, and that even DFT calculations will need to model explicit partners, once these are identified with certainty.
Footnote |
† Electronic supplementary information (ESI) available: Crystallographic, synthetic, DFT, and spectroscopic data, as well as results from supporting experiments. CCDC 955991 and 955992. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt00341a |
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