Bis(imino)pyrazine-Supported Iron Complexes: Ligand-Based Redox Chemistry, Dearomatization, and Reversible C-C Bond Formation

Article abstract of DOI:10.1021/acs.inorgchem.9b03665

Iron complexes supported by novel π-acidic bis(imino)pyrazine (P^PzDI) ligands can be functionalized at the nonligated nitrogen atom, and this has a marked effect on the redox properties of the resulting complexes. Dearomatization is observed in the presence of cobaltocene, which reversibly reduces the pyrazine core and not the imine functionality, as observed in the case of the pyridinediimine-ligated iron analogues. The resulting ligand-based radical is prone to dimerization through the formation of a long carbon-carbon bond, which can be subsequently cleaved under mild oxidative conditions.

Full text of DOI:10.1021/acs.inorgchem.9b03665

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Bis(imino)pyrazine-Supported Iron Complexes: Ligand-Based Redox
Chemistry, Dearomatization, and Reversible C−C Bond Formation
Nicolas I. Regenauer, Simon Settele, Eckhard Bill, Hubert Wadepohl, and Dragos-̧ Adrian Rosç a*
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ABSTRACT: Iron complexes supported by novel π-acidic bis-
(imino)pyrazine (PPzDI) ligands can be functionalized at the
nonligated nitrogen atom, and this has a marked effect on the
redox properties of the resulting complexes. Dearomatization is
observed in the presence of cobaltocene, which reversibly reduces
the pyrazine core and not the imine functionality, as observed in
the case of the pyridinediimine-ligated iron analogues. The resulting ligand-based radical is prone to dimerization through the
formation of a long carbon−carbon bond, which can be subsequently cleaved under mild oxidative conditions.
Chart 1. Various Developments of Catalytically Relevant
PDI-Type Ligands
INTRODUCTION
■
Pyridinediimine ligands (PDI) represent a privileged class of
scaffolds, capable of stabilizing a wide range of transition
metals in a variety of oxidation states. Brookhart and Gibson
demonstrated their unique role in the chemistry of iron and
cobalt, where the corresponding complexes can act as catalysts
for ethylene polymerization.1 Since these original reports
published two decades ago, the interest in first-row-transition-
metal-based PDI ligands has substantially increased, prompted
by wide catalytic applications in a series of challenging
transformations, such as cycloadditions,2 hydroelementation
reactions of unsaturated C−C3 and C−O bonds,4 and coupling
of CO2 with ethylene.5 The reported reactivity competes with
and occasionally even surpasses that reported for noble-metal
catalysts.6 The key to success is closely related to the electronic
structure of the catalysts, which facilitates two-electron-redox
processes, even for metals where one-electron transfer is the
most accessible thermodynamic and kinetic pathway. PDI
ligands achieve this by acting as an electron reservoir for the
metal center, thereby being able to reversibly accept electrons,
depending on the specific requirements of the transformation.7
This feature enables these systems to stabilize key species, such
as dinitrogen, carbene, or imido fragments,8 and therefore have
been investigated spectroscopically and computationally in
detail by Chirik, Wieghardt, and others.9,10 As it has been
recognized that subtle electronic and steric changes can have
an important effect on reactivity and electronic structure,
synthetic work has also focused on modifying the initial ligand
scaffold, which has given rise to powerful new catalytic systems
for the targeted applications. For example, it has been shown
that increasing the steric bulk of R3 (A, Chart 1) can change
the ground state in iron dinitrogen complexes stabilized by
PDI ligands.10c Replacing the aromatic group on the imine
nitrogen atom with cyclohexyl switches the selectivity in the
hydroboration of terminal alkynes,11 while introducing a
pendant phosphine arm or amine on this position yields
−
efficient catalysts for ketone hydrosilylation and NO2
reduction (B, Chart 1).12 Chiral information could be
introduced in the PDI backbone by employing chiral
oxazolines, which gives rise to the well-established PyBox
ligand family (C, Chart 1).13 Furthermore, the imine fragment
could be tethered to the meta position of the heterocyclic core
in order to control the metal−imine dissociation rates (D,
Chart 1), with important consequences in cycloaddition
reactions and selective oligomerization of ethylene (Figure
1).14
As the catalytic properties and the electronic structure of
PDI-based metal complexes seem to be very sensitive to the
electronic density around the metal center,15 we have
envisaged that enhancing the π acidity of the N-heterocycle
Received: December 17, 2019
https://dx.doi.org/10.1021/acs.inorgchem.9b03665
© XXXX American Chemical Society
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a
Scheme 1
Figure 1. Solid-state characterization data for 8. (left) Molecular
structure obtained by single-crystal X-ray diffraction. Hydrogen atoms
57
̈
are omitted for clarity. (right) Zero-field Fe Mossbauer spectrum
recorded at 80 K. The red line represents a fit with a Lorentzian
quadrupole doublet with the following parameters: δ = 0.83 mm s−1,
|ΔEQ| = 2.42 mm s−1. The deviations indicate a 8% contamination
with an unknown species with the following fitted parameters: δ =
0.30 mm s−1, |ΔEQ| = 1.28 mm s−1.
a
Reagents and conditions: (a) tributyl(1-ethoxyvinyl)stannane (2.3
equiv), Pd(PPh3)2Cl2 (2.5 mol %), toluene, 120 °C, 76%; (b) (1-
ethoxyvinyl)zinc chloride (6 equiv), Ni(dppp)Cl2 (10 mol %), THF/
dioxane, 70 °C, 40%; (c) HCl (1 M aqueous), THF, 80%; (d)
DippNH2, PTSA·H2O (15 mol %), toluene, 140 °C, 77% (Dipp =
2,6-diisopropylphenyl, PTSA = p-toluenesulfonic acid); (e) H2SO4
(98% aqueous), pyruvic acid, (NH4)S2O8, AgNO3, H2O, 50 °C, 25%;
(f) DippNH2, MeOH, HOAc, reflux, 75%.
would facilitate the delocalization of electrons from the metal
onto the ligand. For this task, we have sought to replace the
pyridine core with pyrazine, which would bring the following
assets: (i) pyrazine is a strong π acceptor with the lowest π−π*
transitions documented to be about 0.95 eV smaller those of
than pyridine,16 while its basicity is reduced by about 5 orders
of magnitude (pKa 1.30 vs 5.20 in pyridine),17 (ii) the N atom
can stabilize transient species in various transformations via
intermolecular coordination, and (iii) the nitrogen atom in the
4-position can be further functionalized through alkylation,
protonation, or reactions with Lewis acids, which have an
important influence on the electron density around the metal
center. This functionality has been recently explored by Tilley,
Bergman, and others in bipyrazine- or bipyrimidine-based
palladium and platinum systems, where they have shown that a
coordination of a Lewis acid can alter the redox potential by
600 mV and enhance the rate of reductive elimination by up to
108 in comparison to control experiments.18
In order to evaluate the redox properties of 4 against a well-
established backdrop, we decided to initially turn our attention
to the iron chemistry of this system. Treating a THF solution
of FeCl2 with 4 at room temperature affords (PPzDI)FeCl2 (8)
as a green paramagnetic solid in very good yield. The solution
magnetic moment (μeff = 4.96 μB, Evans method) is
characteristic for a high-spin Fe(II) complex (S = 2) (Scheme
2). Despite the paramagnetic character, the 1H and 13C spectra
are well resolved (see the Supporting Information).
a
Scheme 2
RESULTS AND DISCUSSION
■
The synthesis of the ligand class type E (Chart 1) was achieved
via a three-step synthesis, starting from 2,6-dichloropyrazine
(1). A Stille coupling under low catalytic loading (2.5 mol %)
furnished the divinyl ether 2, which was hydrolyzed under mild
acidic conditions to the analogous diketone 3. Finally, an acid-
catalyzed condensation with 2,6-diisopropylaniline afforded the
target pyrazinediimine ligand (PPzDI). The reaction sequence
could be conducted on scale, furnishing 7 g of 4 in an overall
yield of 47%. A Ni-catalyzed Negishi coupling could also be
used to obtain the divinyl ether 2; however, this method
requires a relatively high catalytic loading and large excess of
the zinc nucleophile. Moreover, attempting to use this method
on scale (>3 g) results in a considerable drop in isolated yield.
The mild palladium-catalyzed route is orthogonal to the
other general synthetic pathways employed for the synthesis of
PDI-type ligands.19 The desired connectivity could also be
confirmed by single-crystal X-ray diffraction (see the
Supporting Information). We note, however, that a previously
reported synthesis for 4,20 which would be achieved via a
Minisci coupling between an acyl radical generated from
pyruvic acid and 2-acylpyrazine 5 followed by an acid-catalyzed
condensation with 2,6-diisopropylaniline, yielded the (wrong)
2,5-isomer 7 (Scheme 1), as established through our single-
crystal X-ray diffraction studies (see the Supporting Informa-
tion).21
a
Reagents and conditions: (a) FeCl2, THF, 92%; (b) Na/Hg, CO (1
atm), toluene, 40%; (c) (bda)Fe(CO)3, toluene, 110 °C, 70% (bda =
benzylideneacetone).
Single-crystal X-ray analysis (Figure 1, left) shows that,
under the crystallization conditions employed, complex 8 is
monomeric and the unligated pyrazine nitrogen atom is not
engaged in coordination with a neighboring molecule.22 The
geometry around the metal center is best described as a
distorted square pyramid (τ = 0.36),23 with Fe−Cl bond
distances essentially identical with those of (iPrPDI)FeCl2 (for
8, Fe−Cl1 2.2561(4) Å and Fe−Cl2 2.3041(4) Å; for
(iPrPDI)FeCl2,1a,b Fe−Cl1 2.266(2) Å and Fe−Cl2 2.311(2)
̈
Å). The other metrical data (vide infra) and Mossbauer
spectroscopy (Figure 1, right) further confirm the structure of
8 as a high-spin Fe(II) complex, bound to a neutral PPzDI
scaffold.
Subjecting 8 to 1 atm of CO under reducing conditions
(Na/Hg) affords (PPzDI)Fe(CO)2 (9) in moderate yield
(40%). The reaction byproduct was invariably proligand 4,
which displays solubility properties similar to those of 9,
ensuring a difficult separation from the reaction mixture. To
B
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circumvent this, we sought to obtain 9 directly by reacting 4
with an Fe(0) source such as (bda)Fe(CO)3 (bda =
benzylideneacetone), relying on the facile displacement of
bda under thermolytic conditions. This method allows access
to 9 on scale, in good isolated yields (Scheme 2), and avoids
the use of large quantities of mercury. The formally Fe(0)
complex is obtained as a green, diamagnetic solid, with a C2v
symmetry in solution, as observed by NMR spectroscopy. It
displays good solubility in all common organic solvents,
including pentane, from which the compound could be
recrystallized. The molecular structure (Figure 2, left) confirms
(vs ferrocene), shifted anodically by 260 mV in comparison to
the iPrPDI ligand (−2.62 V vs ferrocene) and very similar to
that of the 4-CF3-iPrPDI ligand (−2.35 V vs ferrocene) (Figure
3).19b Complex 9 displays one reversible oxidation event at a
Figure 3. Cyclic voltammetry (glassy-carbon working electrode, 0.1
M (nBu4N)[B(C6F5)4], scan rate 100 mV s−1 in THF at 295 K versus
ferrocene0/+): (left) overlay of iPrPDI (blue) and iPrPPzDI (red) (4);
(right) overlay of iPr(PDI)Fe(CO)2 (blue) and iPr(PPzDI)Fe(CO)2
(red) (9).
Figure 2. Solid-state characterization data for 9. (left) Molecular
structure obtained by single-crystal X-ray diffraction. Only one of the
two molecules present in the asymmetric unit is displayed. The
cocrystallized pentane molecule and hydrogen atoms are omitted for
peak potential of −0.23 V (vs ferrocene) and one reversible
reduction event at −2.28 V (vs ferrocene), both anodically
shifted by 260 and 180 mV, respectively, in comparison to
(iPrPDI)Fe(CO)2 (−0.49 and −2.46 V vs ferrocene). In
analogy to the studies reported by Chirik, we assign the
oxidation wave to be metal-based (Fe0 to FeI) and the
reduction wave to be ligand-based (L to L•−).25 The data
suggest that the magnitude of the HOMO−LUMO gaps are
similar, where the levels of the frontier orbitals in (PPzDI)Fe-
(CO)2 would be lower in comparison to the (PDI)Fe(CO)2
analogue, which was also confirmed by computing the
energetic levels of the frontier orbitals (vide infra). The
oxidation state of the formally Fe(0) carbonyl complexes
supported by potentially noninnocent iminopyridine-based
PNN and PDI systems has been previously discussed in great
depth and assessed through combined spectroscopic and
computational means.26 Metrical parameters obtained from
clarity. (right) Zero-field 57Fe−Mossbauer spectrum recorded at 80 K.
̈
The red line represents a fit with a Lorentzian quadrupole doublet
with the following parameters: δ = 0.03 mm s−1, |ΔEQ| = 1.29 mm s−1.
The deviations at ca. 0 and 2 mm s−1 indicate an 11% contamination
with an unknown Fe(II) species.
that the geometry around the metal center is idealized square
pyramidal (∠OC−Fe−CO = 94.63(9)°), while the N2−Cimine
,
Cimine−Cipso, and Cipso−N3 bond distances are nearly identical
with those of (iPrPDI)Fe(CO)2 (for 9, N2−Cimine 1.326(2) Å,
Cimine−Cipso 1.432(3) Å, and Cipso−N3 1.375(2) Å; for
(iPrPDI)Fe(CO)2,10b N2−Cimine 1.332(2) Å, Cimine−Cipso
1.428(3) Å, and Cipso−N3 1.379(2) Å).
The solid-state IR (ATR, Table 1) spectrum of 9 displays
two absorptions in a 0.9:1.1 intensity ratio, suggesting two
Table 1. 15N NMR Data and Infrared Stretches νCO for
Complexes 4, 9, [10]·I, 11, and (PDI)Fe(CO)2
single-crystal diffractometry, especially the N2−Cimine, Cimine
−
C
ipso, and Cipso−N3 bond distances, have been found to have
a
15N NMR
IR (νCO)
diagnostic value in distinguishing between Fe0 species bound
by a neutral ligand, LFe0(CO)2, and an FeII species
antiferromagnetically coupled to a reduced ligand diradical,
L•2−FeII(CO)2. In the case of 9, the N2−Cimine and Cipso−N3
b
complex
δN1
δN2
δN4
solid
soln
4
345.8
255.0
250.8
261.9
222.9
c
c
c
c
321.8
e
e
f
(PDI)Fe(CO)2
9
[10]·I
11
1946, 1888 1974, 1914
1967, 1904 1984, 1925
1999, 1938 2010, 1952
1955, 1889 1972, 1907
bond distances appear significantly elongated while the Cimine
−
d
Cipso bond is significantly contracted in compared to those in
the free ligand 4 and (PPzDI)FeCl2 (8) (Figure 4). The bond
distances are similar to those computed for the monoanionic
form of the α-iminopyridine of 1.34, 1.41, and 1.39 Å
respectively,9 while Wieghardt’s single structural parameter
Δexp = 0.0772 (Δcalc(DFT,B3LYP) = 0.0767; vide infra)
145.5
80.8
e
185.9
a
b
c
d
Determined by 1H−15N HMBC. IR-ATR,. δN1 = δN2
.
Not
e
f
observable. In pentane. In CH2Cl2.
nonequivalent CO ligands, as confirmed by single-crystal X-ray
diffraction. The νCO stretching frequencies of 9 (1967, 1904
cm−1) are slightly shifted to higher frequencies in comparison
to those in (iPrPDI)Fe(CO)2 (1946, 1888 cm−1),24 suggesting
a greater π acidity of pyrazine in comparison to pyridine,
therefore reducing back-bonding to the carbonyl ligands. The
increased π acidity of the pyrazine is also reflected in the redox
potentials of 4 and 9. Proligand 4 displays a reversible
reduction wave in THF solution at a peak potential of −2.36 V
suggests an important contribution from the resonance
27
structure consisting of direduced (PPzDI)•2−
.
In line with
this interpretation, 15N NMR of the imine nitrogen atom (N
C) also shows a significant upfield shift in complex 9 (δN
250.8), in comparison to that in the free ligand 4 (δN 345.8),
suggesting an accumulation of charge density on this site.
Nevertheless, this phenomenon could be also ascribed to
strong back-bonding from an electron-rich Fe(0) center to the
PPzDI ligand, and not necessarily to ligand reduction.
C
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Figure 5. Qualitative molecular orbital diagram comparing the energy
levels of complex 9 and the pyridine-based analogue (iPrPDI)Fe(CO)2
(closed shell singlet, B3LYP, def2-TZVP(-f)). Canonical molecular
orbitals are displayed.
Figure 4. Bond distance comparison among 4, 8, and 9. Mean values
are given for the two independent molecules of 4 and 9. Standard
uncertainties are those of individual values. Metric data were obtained
from single-crystal X-ray diffraction.
diamagnetic powder, insoluble in aliphatic hydrocarbons but
partially soluble in aromatic hydrocarbons (Scheme 3). Solid-
̈
Mossbauer spectroscopy (Figure 2, right) also favors an
important contribution from the LFe0(CO)2 resonance form,
suggesting a zero oxidation state on the iron center (δ = 0.03
mm s−1, |ΔEQ| = 1.29 mm s−1).
Scheme 3
In order to investigate the potential ligand redox non-
innocence, we examined the electronic structures of 8 and 9 by
DFT calculations. The structure of compound 8 was modeled
through spin-unrestricted DFT calculations at the B3LYP/
TZVP(-f) level of theory.28 Geometry optimizations produced
structural features which are in good agreement with the
state IR data display two absorptions in a 1:1 intensity,
suggesting that the geometry at the metal center would be
square pyramidal and thus not influenced by the additional
positive charge. The νCO stretching frequencies are shifted,
however, by ca. 30 cm−1 to higher frequencies (1999 and 1938
cm−1, Table 1), consistent with the reduced electron density
on the iron center, which reduces the back-bonding to the CO
ligands.32 For comparison, in a related recent example of the
pyridine-based (PCP)Ru(CO)2Cl, methylation of the nitrogen
atom exerts a more attenuated influence on the νCO value, from
2023 and 1945 cm−1 in iPr(PCP)Ru(CO)2Cl to 2034 and 1969
cm−1 in [MeiPr(PCP)Ru(CO)2Cl]OTf. In this case, back-
bonding from the metal to the methylated PCP ligand was
calculated to be negligible, while in the case of [10]·I it would
be significant.
̈
metric data obtained from X-ray crystallography. Mossbauer
parameters could also be reproduced (δcalcd = 0.75 mm s−1,
ΔEQ(calcd) = 2.23 mm s−1),29 supporting the description of
compound 8 as a hs-Fe(II) complex. The electronic structure
of 9 was analyzed through spin-unrestricted broken-symmetry
(BS)30 calculations at the B3LYP/TZVP(-f) level of theory,
where three possible formulations were taken into account: (i)
[(PPzDI)2−FeII], where a BS(2,2) (quintet) approach was used,
(ii) [(PPzDI)−FeI], where a BS(1,1) (triplet) approach was
used, and (iii) [(PPzDI)Fe0], where 9 was modeled as an
unrestricted singlet. All strategies converged to the same
BS(0,0) (i.e. closed-shell) solution, similar to case for the
(PDI)Fe(CO)2 analogue reported by Chirik.25 Additionally,
the overlap integrals (Sαβ) of the unrestricted corresponding
orbitals (UCOs) with values close to unity (for the highest
occupied orbitals) provided further evidence for a BS(0,0)
ground state.31 Therefore, the system was further computed as
a closed-shell singlet. The metric parameters of the closed-shell
solution are in good agreement with those measured by single-
15N NMR spectroscopy (Table 1) reveals that the δN value
of the CNimine group is slightly shifted downfield (for 9, δN
250.8; for [10]·I, δN 261.9), suggesting a decrease in electron
density on this site. This interpretation is corroborated by the
structural metric data (Figure 6, left) which reveal that, in the
̈
crystal X-ray diffraction, and the calculated Mossbauer
parameters (δcalcd = −0.03 mm s−1, ΔEQ(calcd) = 1.28 mm
s−1) reproduce well those determined experimentally. On the
basis of the experimental and computational data, we therefore
favor the LFe0(CO)2 formulation. A qualitative molecular
orbital diagram comparison between 9 and the pyridine-based
analogue (Figure 5) confirms the enhanced π-acceptor
character of the pyrazine-based ligand, which is also
corroborated by the cyclic voltammetry data (vide supra,
Figure 3).
As complex 9 possesses a nitrogen atom amenable for
further functionalization, we sought to study the effect of
methylation on the electronic properties and reactivity of
(PPzDI)Fe(CO)2 complexes. Reacting complex 9 with a mild
methylation reagent such as iodomethane affords the N-
methylated complex [10]·I, isolated as a dark brown
Figure 6. Solid-state characterization data for [10]·I. (left) Molecular
structure obtained by single-crystal X-ray diffraction. Most hydrogen
57
̈
atoms are omitted for clarity. (right) Zero-field Fe Mossbauer
spectrum recorded at 80 K. The red line represents a fit with a
Lorentzian quadrupole doublet with the following parameters: δ =
0.00 mm s−1, |ΔEQ| = 1.06 mm s−1.
D
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case of [10]·I, the N2−Cimine bond is slightly contracted in
comparison to its nonmethylated congener 9 (1.320(2) Å in
[10]·I, 1.326(2) Å in 9). The Fe−N3 bonds are also shorter
(decreases to 1.810(2) Å in [10]·I from 1.828(1) Å in 9),33
consistent with the ionic character of [10]·I, as well as
important back-bonding from Fe to the strongly π accepting
pyrazinium core in comparison to 9. The greater stabilization
of pyazinium cations through back-bonding from a metal
center could constitute the driving force in the facile
methylation reaction of 9 in the presence of iodomethane. In
the absence of the metal, mixing 4 with excess iodomethane
(10−20 equiv) for 3 days at 80 °C showed no signs of
methylation. The diamagnetic character of [10]·I is also in line
the Supporting Information for details). While we can
tentatively assign the latter reduction wave to the ligand-
based imine reduction (see the CV of 9 above for
comparison), we wondered whether the first wave at −1.53
V would correspond to a ligand core reduction. Adding
cobaltocene to a benzene suspension of [10]·I afforded an
instant color change from black to purple (Scheme 4). NMR
a
Scheme 4
̈
with the Mossbauer data (Figure 6, right) characteristic for a
Fe(0) species (δ = 0.00 mm s−1, |ΔEQ| = 1.06 mm s−1). N-
methylation therefore does not seem to alter the singlet ground
state observed in 9. This could also be corroborated through
BS-DFT calculations performed on [10]·I, which converged to
a BS(0,0) solution (i.e., closed shell). The calculated
̈
Mossbauer parameters based on this solution are also in
excellent agreement with the experimentally determined values
(δcalcd = −0.02 mm s−1, ΔEQ(calcd) = 1.07 mm s−1). A qualitative
molecular orbital diagram comparison between 9 and [10]·I
(Figure 7) reveals that N-methylation reduces significantly the
a
Reduction conditions: Cp2Co (1 equiv), benzene, room temper-
ature, 80% (isolated yield), X = I; Oxidation conditions: Fc[PF6] (1.4
equiv), C6D6, room temperature, quantitative (NMR yield), X = PF6
(substoichiometric amounts of Fc[PF6] were used in order to avoid
oxidation of the metal center). The numbering scheme is given for 11.
analysis reveals the formation of a diamagnetic compound
which exhibits a C1 symmetry in solution. The imine character
of the C−N bond on the side of the radical formation is lost, as
exhibited by δN2 185.9 (compared to δN1 222.9) and δC19 141.2
(compared to δC3 162.8), while the dearomatization of the
pyrazine ring is evident in the pronounced upfield shift of δN4
80.8 (compared to δN4 145.5 in [10]·I). Single crystal X-ray
diffraction (Figure 8, left) shows the formation of a dimeric
Figure 7. Qualitative molecular orbital diagram illustrating the effect
of N-methylation on the energy levels of the frontier orbitals in 10+, in
comparison to 9 (closed-shell singlet, B3LYP, def2-TZVP(-f)).
Canonical molecular orbitals are displayed.
energy of the frontier orbitals in the cationic complex (by 3.27
eV for the LUMO and 3.34 eV for the HOMO), further
suggesting an important alteration of the electronic properties
of [10]·I.
Figure 8. Solid-state characterization data for 11. (left) Molecular
structure obtained by single-crystal X-ray diffraction. Most hydrogen
57
̈
atoms are omitted for clarity. (right) Zero-field Fe Mossbauer
spectrum recorded at 80 K. The red line represents a fit with a
Lorentzian quadrupole doublet with the following parameters: δ =
0.02 mm s−1, |ΔEQ| = 0.99 mm s−1.
REDOX CHEMISTRY
■
Pyridine-based PDI systems coordinated to iron are known to
show versatile redox chemistry, where either the metal center
or the imine arm can be reduced. Although it is theoretically
possible that the heterocyclic core could also be reduced,
therefore allowing the PDI system to formally store three
electrons, to the best of our knowledge, this has not yet been
achieved experimentally. As N-methylated pyrazine systems are
considerately more electron poor in comparison to their
pyridine congeners, we were wondering if this would enable
the reduction of the nitrogen heterocycle in favor of the imine
arms. In order to study this, we initially conducted cyclic
voltammetry experiments on compound [10]·I, which shows
one quasi-reversible wave at a peak potential of −1.53 V
(oxidation −1.43 V) and one irreversible wave at −2.10 V (see
species, likely obtained through the generation of a radical at
the carbon atom α to the cationic nitrogen atom, which
subsequently dimerizes to give 11.34 The dearomatized
pyrazine ring deviates significantly from planarity, adopting a
distorted-boat conformation. The imine character on the side
of the radical formation is attenuated, illustrated through the
elongation of the N2−C19 bond (1.358(9) Å, in comparison
to N1−C3 1.313(8) Å). Interestingly, the bond created
through the radical dimerization appears to be unusually long
for a C−C σ bond (C17−C57 1.600(9) Å),35,36 possibly due
to the partial stabilization of the radical character of C17 and
C57 due to the conjugation with the π system, as well as spin
E
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̈
delocalization onto the metal. Nevertheless, Mossbauer
spectroscopy suggests a singlet Fe(0) structure, in line with
the diamagnetic character observed by NMR spectroscopy
(Figure 8, right).
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
While at least two resonance structures can be envisioned for
11, taking the spectroscopic and metrical data into account, we
suggest the resonance form B (Figure 9) to be the dominant
one.
AUTHOR INFORMATION
Corresponding Author
■
Dragos-̧ Adrian Rosç a − Anorganisch-Chemisches Institut,
̈
Universitat Heidelberg, 69120 Heidelberg, Germany;
orcid.org/0000-0002-0273-5495; Phone: +49 6221 54
8596; Email: dragos.rosca@uni-heidelberg.de
Authors
Nicolas I. Regenauer − Anorganisch-Chemisches Institut,
̈
Universitat Heidelberg, 69120 Heidelberg, Germany
Simon Settele − Anorganisch-Chemisches Institut, Universitat
̈
Heidelberg, 69120 Heidelberg, Germany
Figure 9. Possible resonance structures and metric data for one
dimeric unit of 11 (R = second dimeric unit of 11).
Eckhard Bill − Max-Planck-Institut fu
̈
r Chemische
Energiekonversion, 45470 Mulheim/Ruhr, Germany;
̈
orcid.org/0000-0001-9138-3964
The long C−C bond formed as a result of radical
dimerization prompted us to investigate whether it can be
homolytically cleaved under mild oxidizing conditions.
Treating a dichloromethane-d2 solution of 11 with a
substoichiometric amount of Fc[PF6] instantly affords [10]·
PF6 alongside ferrocene, as judged by NMR spectroscopy,
showcasing the weak character of the C−C bond as well as the
thermodynamic driving force for the rearomatization of the
pyrazine ring. Compound 11 represents a rare example of a
structurally characterized metal complex where a C−C bond
can be cleaved under relatively mild oxidative conditions.37
Hubert Wadepohl − Anorganisch-Chemisches Institut,
̈
Universitat Heidelberg, 69120 Heidelberg, Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b03665
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
CONCLUSIONS
■
ACKNOWLEDGMENTS
In conclusion, we have developed new types of redox-active
ligands, where the pyrazine core allows for further function-
alization of the unligated nitrogen atom. N-methylation of
(PPzDI)Fe(CO)2 proceeds under mild conditions, for which
the driving force is the back-bonding stabilization of the
resulting cationic species, which is evident in the pronounced
effect on the CO stretching frequencies. In contrast to the well-
documented pyridine-based PDI chemistry where the imine
functionality is most prone to reduction, in the case of N-
methylated PPzDI complexes, the pyrazinium core is more
easily reduced in comparison to the imine functionality,
through the generation of a ligand-based radical. This radical
readily dimerizes through the formation of a weak C−C bond
which can be subsequently cleaved under mild oxidative
conditions. The weak character of the C−C bond suggests
that, providing there is enough steric bulk to prevent
dimerization, the parent pyrazine-based radical species would
be isolable, an endeavor which we are pursuing at the moment.
■
Generous financial support from the Fonds der Chemischen
Industrie (FCI) through Liebig research fellowships (N.I.R.
and D.-A.R.) is gratefully acknowledged. The computational
work was supported by the bwHPC initiative and the bwHPC-
C5 project provided through the associated computing services
of the JUSTUS HPC facility located at the University of Ulm
(Grant INST 40/467-1). We thank Dr. Sorin-Claudiu Rosc
(Max-Planck-Institut fur Kohlenforschung) and Mr. Bernd
Mienert (Max-Planck-Institut fur Chemische Energiekonver-
̧ a
̈
̈
̈
sion) for assistance with the Mossbauer sample preparations
and measurements. Priv.-Doz. Joachim Ballmann and Dr. Felix
Anderl are acknowledged for helpful discussions, and Prof Lutz
H. Gade is acknowledged for continued interest in our work.
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03665.
Cartesian coordinates for the calculated structures
(XYZ)
Synthesis protocols, spectroscopic data, X-ray crystallo-
graphic data, and computational details (PDF)
Accession Codes
CCDC 1968498−1968502 and 1968504 contain the supple-
mentary crystallographic data for this paper. These data can be
F
https://dx.doi.org/10.1021/acs.inorgchem.9b03665
Inorg. Chem. XXXX, XXX, XXX−XXX

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state (see ref 22 for similar cases), the reported values in the main text
are from our in-house ATR solid-state measurement of the reported
complex. For compounds 9, [10]·I, and 11 we have also recorded the
IR spectra in solution, all of which show 11−25 cm−1 deviations to
higher frequencies in comparison to solid-state data (see Table 1).
H
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Inorganic Chemistry
pubs.acs.org/IC
Article
of Acridine-Based Pincer Complexes of 3d Metals and C−C Bond
Formation Organometallics, 2020, DOI: 10.1021/acs.organo-
met.9b00607.
(35) The standard C(sp3)−C(sp3) bond distance is 1.54 Å: CRC
Handbook of Chemistry and Physics, 93rd ed.; Haynes, W., Ed.; CRC
Press: Boca Raton, FL, 2012.
(36) We note that in certain organic radical dimers, C(sp3)−C(sp3)
bond lengths to up to 1.71 Å have been observed. See for example:
Schreiner, P. R.; Chernish, L. V.; Gunchenko, P. A.; Tikhonchuk, E.
Y.; Hausmann, H.; Serafin, M.; Schlecht, S.; Dahl, J. E. P.; Carlson, R.
M.; Fokin, A. A. Overcoming lability of extremely long carbon-carbon
bonds through dispersion forces. Nature 2011, 477, 308−311 and
references therein.
(37) For another example where a diradical could be cleaved
oxidatively, see: (a) Hsu, S. C. N.; Yeh, W.-Y.; Lee, G.-H.; Peng, S.-M.
Reductive Dimerization and Thermal Rearrangement of Biphenylene
Coordinated to Tricarbonylmanganese. J. Am. Chem. Soc. 1998, 120,
13250−13251. For examples where ligand-centered radicals are in
equilibrium with their dimeric analogues, see: (b) Nocton, G.;
Lukens, W. W.; Booth, C. H.; Rozenel, S. S.; Medling, S. A.; Maron,
L.; Andersen, R. A. Reversible Sigma CC Bond Formation Between
Phenanthroline Ligands Activated by (C5Me5)2Yb. J. Am. Chem. Soc.
2014, 136, 8626−8641. (c) Liu, B.; Yoshida, T.; Li, X.; Stepien, M.;
Shinokubo, H.; Chmielewski, P. J. Reversible Carbon-Carbon Bond
Breaking and Spin Equilibria in Bis(pyriminenorcorrole). Angew.
Chem., Int. Ed. 2016, 55, 13142−13146. (d) Andrez, J.; Guidal, V.;
Scopelliti, R.; Pecaut, J.; Gambarelli, S.; Mazzanti, M. Ligand and
Metal Based Multielecron Redox Chemistry of Cobalt Supported by
Tetradentate Schiff Bases. J. Am. Chem. Soc. 2017, 139, 8628−8638.
I
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