Proton-Coupled Electron Transfer (PCET) with 1,4-Bisguanidino-Benzene Derivatives: Comparative Study and Use in Acid-Initiated C-H Activation

Article abstract of DOI:10.1002/chem.202101539

Proton-coupled electron transfer (PCET) is of key importance in modern synthetic chemistry. Redox-active guanidines were established by our group as valuable alternatives to toxic high-potential benzoquinones in a variety of different PCET reactions. In t

Full text of DOI:10.1002/chem.202101539

Accepted Article
Accepted Article
Title: Proton-Coupled Electron Transfer (PCET) with 1,4-Bisguanidino-
Benzene Derivatives: Comparative Study and Use in Acid-
Initiated C-H Activation
Authors: Petra Walter, Olaf Hübner, Elisabeth Kaifer, and Hans-Jörg
Himmel
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To be cited as: Chem. Eur. J. 10.1002/chem.202101539
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01/2020

10.1002/chem.202101539
Chemistry - A European Journal
Proton-Coupled Electron Transfer (PCET) with 1,4-Bisguanidino-
Benzene Derivatives: Comparative Study and Use in Acid-Initiated C-
H Activation
Petra Walter, Olaf Hübner, Elisabeth Kaifer, Hans-Jörg Himmel *
Abstract. Proton-coupled electron transfer (PCET) is of key importance in modern
synthetic chemistry. Redox-active guanidines were established by our group as
valuable alternatives to toxic high-potential benzoquinones in a variety of different
PCET reactions. In this work, we evaluate the PCET reactivity of a series of 1,4-
bisguanidino-benzenes varying in their redox potentials and proton affinities. The
relevant redox and protonation states are fully characterized and the compounds
sorted with respect to their PCET reactivity by comparative PCET experiments
supplemented by quantum-chemical calculations. Depending on the studied reactions,
the driving force is either electron transfer or proton transfer; thereby the influence of
both processes on the overall reactivity could be assessed. Then, two of the PCET
reagents are applied in representative oxidative aryl-aryl coupling reactions, namely
the intramolecular coupling of 3,3’’-4,4’’-tetramethoxy-o-terphenyl to give the
corresponding triphenylene, the intermolecular coupling of N-ethylcarbazole to give
N,N’-diethyl-3,3’-bicarbazole, and in the oxidative lactonization of 2-[(4-
methoxyphenyl)methyl]-benzoic acid. Under mild conditions, the reactions proceed
fast and efficient. Only small amounts of acid are needed, in clear contrast to the
corresponding coupling reactions with traditional high-potential benzoquinones such
as DDQ or chloranil requiring a large excess of a strong acid.
* P. Walter, Dr. O. Hübner, Dr. E. Kaifer, Prof. H.-J. Himmel,
Inorganic Chemistry, Ruprecht-Karls University of Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Email: hans-jorg.himmel@aci.uni-heidelberg.de
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Introduction
Proton-coupled electron transfer (PCET) plays a central role in modern synthetic
chemistry,[1,2,3,4] as it allows e.g. the selective oxidation of organic compounds and
other useful reactions.[5-8] In biology, PCET is involved in essential processes such as
photosynthesis and a variety of redox enzymatic reactions.[9-13] The PCET chemistry
of coordination compounds, e.g. oxo complexes of metals in high oxidation states, that
are transformed in PCET reactions to hydroxy- or aqua-complexes,[1] of metal oxide
clusters such as the decatungstate anion [W10O36]4−14 and of organic PCET
reagents, e.g. quinones,[15-18] TEMPO and other aminoxyl radicals,[19-22] as well as
Hantzsch esters and dihydropyridine derivatives,[23,24] are well documented. By
separating the electron acceptor/donor site from the proton acceptor/donor site, the
two decisive parameters for the thermodynamics of PCET, the redox potential and the
acid constant (proton affinity), could be tuned separately (“bidirectional” PCET).[6-8,10]
In the last years, our group established redox-active guanidines, comprising guanidino-
functionalized aromatics (GFAs) and urea azines, as versatile organic PCET reagents
and redox catalysts.[25-30] The compound 1,2,4,5-tetrakis(tetramethylguanidino)-
benzene (ttmgb, Scheme 1) is the archeotypical example for a GFA,[31,32] being an
electron donor in its reduced, neutral state, but a remarkably potent oxidant in its
oxidized, dicationic state, if electron transfer is coupled with proton transfer. Here, the
high Brønsted basicity of the reduced, neutral state (pKa ca. 25.3 for (ttmgb+H)+ in
CH3CN) contributes significantly to the driving force. The scope of the PCET reactivity
of redox-active guanidines in organic synthesis could be greatly expanded in the
presence of an excess of a strong acid. Hence, redox-active guanidines allow efficient
aryl-aryl coupling reactions of aromatic compounds with relatively high redox potential
up to at least 1.2 V vs. ferrocenium/ferrocene (Fc+/Fc).[29,30] Scheme 1 shows as
example the conversion of 3,3’’,4,4’’-tetramethoxy-o-terphenyl (oxidation potential of
0.74 V vs. Fc+/Fc) with one equivalent of (ttmgb)(BF4)2 to the corresponding
triphenylene coupling product in almost quantitative yield, requiring a large excess of
a strong acid (HBF4·OEt2).[29]
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Scheme 1. a) Lewis structure of neutral ttmgb, twofold-oxidized green (ttmgb)2+ and
reduced, twofold-protonated (ttmgb+2H)2+. b) Example for an efficient and fast aryl-
aryl coupling reaction with a salt of (ttmgb)2+.
Recently we showed that oxidized bisguanidine 1,4-bis(tetramethylguanidino)-
benzene GFA12+ (see Lewis structure in Figure 1a) is a stronger oxidant in PCET
reactions than the oxidized tetrakisguanidine 1,2,4,5-tetrakis(tetramethylguanidino)-
benzene, (ttmgb)2+.[30] Therefore, it is attractive to study the PCET chemistry of redox-
active bisguanidines in more detail. In this work, the PCET reactivity of the five different
1,4-bisguanidino-benzene derivatives sketched in Figure 1 is compared. Of these,
GFA2, GFA4 and GFA5 are new, while GFA1 [33] and GFA3 [30] were previously
reported. In comparative (2e−, 2H+) PCET reactions, the oxidized GFA12+ is reacted
with the reduced, diprotonated form of another bisguanidine. Depending on the
guanidino groups, these reactions are either driven by electron-transfer (while proton
transfer is unfavorable) or by proton-transfer (while electron-transfer is unfavorable);
thereby, information about the reaction mechanism could be obtained by comparing
the PCET reactivity. Finally, GFA12+ and GFA42+ are applied as PCET reagents in
representative intra- and intermolecular aryl-aryl coupling reactions, and in an oxidative
lactonization. (The reasons for the choice of these two compounds is given below.) In
sharp contrast to high-potential benzoquinones and also ttmgb, only a small amount of
acid is required to initiate fast and efficient reactions.
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Figure 1. a) Conversion of the dicationic oxidized form, GFA12+, in a (2e−, 2H+) proton-
coupled electron transfer reaction to give the doubly-protonated, reduced form,
(GFA1+2H)2+. b) Selection of other redox-active guanidine ligands studied in this work.
Of these, GFA2, GFA4 and GFA5 are new.
Results and Discussion
Synthesis and characterization. The three new bisguanidines GFA2, GFA4 and
GFA5 were synthesized from the corresponding diamino precursors. Reaction of
2- methoxy-1,4-diaminobenzene [34] with 2-chloro-1,1,3,3-tetramethylformamidinium
chloride gave GFA2. For the preparation of GFA4 and GFA5, 1,4-diaminobenzene was
reacted with 2-chloro-1,3-dimethyl-1H-benzo[d]imidazol-3-ium tetrafluoroborate [35] or
2-chloro-1,3-dimethylperimidinium tetrafluoroborate [36] (Scheme 2).
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Scheme 2. Synthesis of the three new redox-active bisguanidines GFA2, GFA4 and
GFA5.
Crystals of sufficient quality for structural analysis with single-crystal X-ray diffraction
(XRD) were obtained for GFA2 and GFA4.[37] Figure 2 illustrates the solid-state
structures of the two new bisguanidines, and selected structural parameters are
compiled in Tables 1 and 2. In line with the structures of other guanidino-substituted
aromatics,[25] the central CN3 unit of each guanidino group is highly tilted with respect
to the aromatic plane. A detailed analysis of this issue for GFA1 was published
previously by our group,[33] showing that this conformation is, for electronic reasons,
highly favored energetically with respect to the conformation with co-planar CN3 and
aromatic planes.
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Figure 2. Illustration of the structures of the neutral bisguanidines GFA2 and GFA4 in
the solid state. Displacement ellipsoides drawn at the 50% probability level. Hydrogen
atoms omitted. Colour code: C grey, N blue, O red.
Table 1. Selected bond lengths (in Å) measured for GFA2 and (GFA2+2H)(PF6)2 in
the solid state.[37]
bond GFA2
(GFA2+2H)(PF6)2
1.408(2) 1.422(2)
1.295(2) 1.350(2)
a
b
c
1.378(2)/ 1.338(2)/
1.390(2) 1.330(2)
1.398(2) 1.386(2)
1.390(2) 1.385(3)
1.389(2) 1.389(2)
1.409(2) 1.398(2)
1.385(2) 1.389(2)
1.404(2) 1.404(2)
d
e
f
g
h
i
Table 2. Selected bond lengths (in Å) measured for GFA4, (GFA4+2H)(PF6)2 and
GFA4(PF6)2 in the solid state.[37]
bond GFA4
(GFA4+2H)(PF6)2 GFA4(PF6)2
a
b
c
1.412(2) 1.421(2) 1.302(2)
1.287(2) 1.339(2)
1.376(3)
1.342(2)/
1.345(2)
1.462(2)/
1.451(3)
1.339(3)
1.387(2)/ 1.351(2)/
1.391(2) 1.349(2)
1.397(2)/ 1.394(2)/
1.400(2) 1.392(2)
1.387(2) 1.386(2)
d
e
Redox chemistry.
Experimental results. Prior to chemical oxidation experiments, we carried out cyclic
voltammetry to obtain information about the redox potentials. Figure 3 compares the
cyclic voltammograms for the five GFAs studied in this work, and Table 3 summarizes
the E1/2 and Eox values derived from these measurements. The curves for GFA1 –
GFA4 were recorded in CH2Cl2 solutions. In the case of GFA5, the low solubility of the
oxidized form hampered the determination of the redox potential in CH2Cl2 solution
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(dashed curve in Figure 3). Therefore, cyclic voltammetry measurements were
repeated with the salt GFA5(PF6)2 of the oxidized form (see below for its synthesis and
characterization) in CH3CN solution (neutral GFA5 is poorly soluble in this solvent),
yielding sharp waves due to a quasi-reversible two-electron redox process when the
measurements started in reduction direction. The cyclic voltammograms of GFA1 and
GFA2 clearly show a single reversible two-electron redox event; the first and the
second electron are removed at equal potential. For GFA3, the waves are broader,
signalling splitting of the two-electron redox process into two one-electron events with
E1/2 values of approximately -0.31 V for the redox couple GFA3·+/GFA30 and
approximately -0.21 V for the redox couple GFA32+/GFA3·+. The potential separation
of the two one-electron redox events increases for GFA4, with clearly separated E1/2
values of -0.25 V for the redox couple GFA4·+/GFA40 and -0.06 V for the redox couple
GFA42+/GFA4·+. From these values, one could estimate the disproportionation of
GFA4·+ to GFA4 and GFA42+ to be endergonic by 18 kJ mol−1 with an equilibrium
constant Kdisp of 6.1·10−4 (using the formula G0
Kdisp = exp[−{F/(RT)}·E1/2]).
=
F·E1/2 and
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Figure 3. Comparison between the cyclic voltammograms in CH2Cl2 solution (Ag/AgCl
reference electrode, 0.1 M (nBu)4NPF6 as supporting electrolyte, scan speed
100 mV s−1) for the five redox-active guanidines discussed in this work. Potentials
given with respect to the ferrocenium/ferrocene (Fc+/Fc) redox couple. In the case of
GFA5, the dashed curve was obtained when measuring the CV for a CH2Cl2 solution
of GFA5 in direction of oxidation, and the solid curve for measuring a CH3CN solution
of GFA5(PF6)2 (synthesis see below) in the direction of reduction.
Table 3. E1/2 and Eox values (in V vs. the Fc+/Fc couple) from cyclic voltammetry
measurements in CH2Cl2 solution (Faraday constant F = 9.648456·104 C mol−1).
1st redox (GFA·+/GFA0) 2nd redox (GFA2+/GFA·+) -F [(E1/2(1st) + E1/2(2nd)]
Compound E1/2(1st) / Eox
E1/2(2nd) / Eox
-0.23 / -0.14
-0.30 / -0.22
-0.21 / -0.14
-0.06 / 0.00
-0.15 / -0.12 V
(GFA2+ + 2 e− → GFA)
+44.4 kJ mol−1
+57.9 kJ mol−1
+50.2 kJ mol−1
+29.9 kJ mol−1
+28.9 kJ mol−1
GFA1
GFA2
GFA3
GFA4
GFA5a
-0.23 / -0.14
-0.30 / -0.22
-0.31 / -0.27
-0.25 / -0.18
-0.15 / -0.12
a Measurements for GFA5(PF6)2 in CH3CN due to low solubility in CH2Cl2.
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Cyclic voltammetry measurements were also carried out in CH3CN solution. The
following redox potentials were obtained: E1/2 = -0.21 V (Eox = -0.18 V) for GFA1, E1/2
= -0.30 V (Eox = -0.27 V) for GFA2, E1/2 = -0.24 V (Eox = -0.20 V) for GFA3, and (as
already mentioned) E1/2 = -0.15 V (Eox = -0.12 V) for GFA5. In all these cases, two-
electron redox processes were found. However, in the case of GFA4, the low solubility
in CH3CN prohibited measurements in this solvent.
Two electrons and two protons are exchanged in PCET reactions between the
dicationic, oxidized form of one and the doubly-protonated, reduced form of the other
bisguanidine (see below). Therefore the value -F [(E1/2(1st) + E1/2(2nd)] was calculated
to compare the two-electron acceptor properties, leading to the order for increasing
“two-electron acceptor strength“ (decreasing -F [(E1/2(1st) + E1/2(2nd)] value): GFA2 <
GFA3 < GFA1 < GFA4 < GFA5.
Chemical oxidation was carried out with two equivalents of Fc(PF6). In all cases, the
products of two-electron oxidation were isolated as stable, storable solid compounds.
Figure 4 illustrates the solid-state structures of the new compounds GFA4(PF6)2 and
GFA5(PF6)2.[37] Selected bond lengths are included in Tables 2 and 4. As expected,
the N=C imino bond length of the neutral GFA increases significantly upon oxidation
(e.g. from 1.287(2) Å in GFA4 to 1.376(3) Å in GFA4(PF6)2). By contrast, the N-C bond
connecting the guanidino group to the aromatic ring decreases (from 1.412(2) Å in
GFA4 to 1.302(2) Å in GFA4(PF6)2). The large differences in the C-C bond lengths
within the C6 ring of the oxidized molecules signal loss of the aromatic system, in line
with the Lewis structure shown for GFA12+ in Figure 1.
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Figure 4. Illustration of the solid-state structures of the bisguanidine dications GFA42+
−
and GFA52+, together with two PF6 counterions, obtained upon oxidation of the neutral
compounds. Displacement ellipsoides drawn at the 50% probability level. Hydrogen
atoms omitted. Colour code: C grey, N blue, P red, F green.
Table 4. Selected bond lengths (in Å) measured for (GFA5+2H)(PF6)2 and GFA5(PF6)2
in the solid state.[37]
bond (GFA5+2H)(PF6)2 GFA5(PF6)2
a
b
c
1.429(2)
1.349(2)
1.346(2)/
1.347(2)
1.391(2)/
1.386(2)
1.389(2)
1.288(6)
1.366(6)
1.342(6)/
1.339(6)
1.460(6)/
1.470(7)
1.336(7)
d
e
Finally, we prepared 1:1 mixtures between a neutral bisguanidine and the salt of the
corresponding doubly-oxidized compound in CH3CN solution and recorded UV-vis as
well as EPR spectra. The appearance of an EPR signal and strong new absorptions in
the visible region of the UV-vis spectra indicated the presence of the radical
monocationic bisguanidine in significant amount, formed in a comproportionation
reaction (despite of the absence of any sign of one-electron redox processes in the
cyclic voltammograms). For example, Figure 5 displays the UV-vis spectrum of the
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green 1:1 mixture of GFA4 and GFA4(PF6)2, showing strong bands centered at 699 nm
(very broad) and 426 nm that are absent in the spectrum of GFA4 or GFA(PF6)2 alone.
Moreover, the inlet shows the EPR spectrum measured for this mixture (broad,
unstructured signal at g = 2.00). TD-DFT calculations (B3LYP/def2-TZVP) predicted
strong electronic transitions at 718 and 386 nm (Figure 6), in excellent agreement with
the experiment. Plots of the isodensity surfaces for the orbitals involved in the transition
at 718 nm are included in Figure 6, being -orbitals delocalized over the complete
molecule.
In the case of the radical monocation GFA3·+, prepared similarly by mixing equimolar
amounts of GFA3 and GFA3(PF6)2, bands at 575 nm (with a shoulder at 542 nm) and
404 nm appeared.[30] Here, TD-DFT calculations (B3LYP/def2-TZVP) predicted
electronic transitions at 543/501 and 365/346 nm, again in good agreement with the
experimental values (see Figure 6). The blue shift of the lowest-energy absorption of
GFA3·+ compared to GFA4·+ could be explained by the reduced size of the -electron
system.
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Figure 5. a) From left to right: Photos of cuvettes with CH3CN solutions of GFA4,
GFA4(PF6)2 and an equimolar mixture of GFA4 and GFA4(PF6)2. b) UV-vis spectra of
GFA4, GFA4(PF6)2 and an equimolar mixture of GFA4 and GFA4(PF6)2 in CH3CN
solutions. The EPR spectrum of the 1:1 mixture in CH2Cl2 is shown in the inlet.
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Figure 6. Comparison between the UV-vis spectra of 1:1 mixtures of GFA3 and
GFA3(PF6)2 (black curve) and of GFA4 and GFA4(PF6)2 (red curve) in CH3CN
solutions. The spectra were normalized to the maximum of absorption of the lowest-
energy transition. Simulations of spectra for GFA3·+ and GFA4·+ on the basis of TD-
DFT calculations (B3LYP/def2-TZVP) are reproduced below the experimental spectra.
In addition, isodensity plots for the two orbitals involved in the lowest-energy electronic
transition of GFA4·+ at 718 nm are plotted (SOMO to LUMO transition, 52au(beta) →
53ag(beta)).
All 1:1 mixtures between the neutral and dicationic redox states show EPR signals
assigned to the radical monocations. In the case of GFA1·+ and GFA5·+, a destinct
hyperfine coupling (HFC) to hydrogen and nitrogen nuclei is visible (Figure 7), with a
line spacing of 1.8 G. For the other radical monocations, the HFC is not clearly resolved
and hidden under broad, unstructured signals. The presence of more than 20 lines
indicates hyperfine coupling to the four equivalent aromatic H atoms and the two
equivalent N atoms directly attached to the C6 ring, producing in theory
(2·2·1+1)(2·4·1/2+1) = 25 lines.
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Figure 7. EPR spectra of the radical monocations GFA1·+ and GFA5·+, formed from
1:1 mixtures of GFA12+ and GFA1 and of GFA52+ and GFA5, respectively, in CH2Cl2
solutions (concentrations ca. 5∙10−3 mol∙l−1) at room temperature.
Quantum-chemical studies. Structures and electronic energies were calculated with
the B3LYP functional together with the def2-TZVP basis set, since this combination
was shown to give reliable results in previous studies with redox-active
guanidines.[27,29,30] Thermal contributions were calculated with the B3LYP functional
and SV(P) basis set. The solvent effect was considered with the conductor-like
screening model (COSMO), with a relative permittivity r = 37.5 (CH3CN).(See
Supporting Information for more details.) The calculations reproduced accurately the
experimental solid-state structures (see Supporting Information for details). The
thermodynamics calculated for the pure electron transfer reactions between the
oxidized form GFA1(PF6)2 and one of the neutral, reduced bisguanidines GFA2-GFA5
(the anions were included in the calculations [30]) are compiled in Table 5. For GFA2
and GFA3, the Gibbs free reaction energies are negative, meaning that they are
stronger two-electron donors than GFA1. This result is in line with the cyclic
voltammetry measurements for which the term −F·[(E1/2(1st) + E1/2(2nd)], being
approximately the standard Gibbs free energy change for the reaction GFA2+ + 2 e− →
GFA, is more positive for GFA2 and GFA3 than for GFA1. By contrast, the Gibbs free
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reaction energies are positive for GFA4 and GFA5, again in line with the results of the
cyclic voltammetry measurements, showing that GFA4 and GFA5 are weaker two-
electron donors than GFA1.
Table 5. Calculated electronic energy, enthalpy at 0 K and Gibbs free energy at 298 K
for pure electron transfer reactions (B3LYP functional in combination with the stated
basis set, solvent effect modelled with the conductor-like screening model (COSMO)).
Anions included in the calculations.
r = 1
r = 37.5
GFA1(PF6)2 + GFA2 → GFA1 SV(P)
TZVPa
TZVPa
+ GFA2(PF6)2
-65.7
-59.9
-53.1
-54.1
-48.2
-41.5
-28.4
-22.6
-15.9
rE
rH0
rG298
GFA1(PF6)2 + GFA3 → GFA1 SV(P)
TZVPa
TZVPa
+ GFA3(PF6)2
-44.1
-44.4
-45.6
-39.3
-39.5
-40.8
TZVPa
-15.4
-15.6
-16.9
TZVPa
rE
rH0
rG298
GFA1(PF6)2 + GFA4 → GFA1 SV(P)
+ GFA4(PF6)2
-10.9
-13.0
-6.7
-10.8
-12.8
-6.5
+11.8
+9.7
+16.0
rE
rH0
rG298
GFA1(PF6)2 + GFA5 → GFA1 SV(P)
TZVPa
TZVPa
+ GFA5(PF6)2
-1.6
-4.8
-0.6
-1.9
-5.1
-0.9
+24.6
+21.4
+25.6
rE
rH0
rG298
a Zero-point energy and thermal contributions calculated with the SV(P) basis set.
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Protonation
Experimental results. All guanidines could be doubly-protonated with two equivalents
of NH4PF6. The new salts (GFA2+2H)(PF6)2, (GFA4+2H)(PF6)2 and (GFA5+2H)(PF6)2
were crystallized from concentrated CH3CN solutions layered with Et2O. Figure 8
illustrates the XRD structures in the solid state;[37] selected bond lengths are included
in Tables 1, 2 and 4. As expected, protonation occurs exclusively at the imino N atoms
and leads to an elongation of the imino N=C bond lengths (e.g. from 1.295(2) Å to
1.350(2) Å upon protonation of GFA2, and from 1.287(2) Å to 1.339(2) Å upon
protonation of GFA4) that could be rationalized by charge delocalization through
- bonding over the three nitrogen atoms and the central carbon atom of the protonated
guanidino group.
Figure 8. Illustration of the structures of the doubly-protonated bisguanidines in the
solid state. Displacement ellipsoides drawn at the 50% probability level. Hydrogen
−
atoms omitted. Hydrogen bonding between the dications and the PF6 counterions is
highlighted by dashed lines.
Quantum-chemical studies. Unfortunately, the experimental determination of the
Brønsted basicity from titration experiments is hampered by protonation equilibria and
poorly resolved protonation steps in both the NMR and UV-vis spectra. Therefore, we
first calculated the absolute proton affinities (at r = 1) for the five bisguanidines. Here,
the order of increasing proton affinity is GFA5 < GFA4 < GFA3 < GFA1 < GFA2 (Table
6). In another calculation, the proton transfer between GFA1 and GFA2 was calculated
−
with inclusion of PF6 counterions (reaction (GFA1+H)PF6 + GFA2 → GFA1 +
−
(GFA2+H)PF6). Interestingly, the PF6 counterions had a considerable influence on the
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thermodynamics. While rG298 for the reaction (GFA1+H)+ + GFA2 → GFA1 +
(GFA2+H)+ is negative (-21.1 kJ mol−1), it is slightly positive (+3.5 kJ mol−1) for the
reaction (GFA1+H)PF6 + GFA2 → GFA1 + (GFA2+H)PF6. It should be noted that the
starting structures for the calculations were obtained by removal of one of the protons
−
and an adjacent PF6 counterion from the experimental solid state structures of the
doubly-protonated bisguanidines. A complete survey of the structures that follow from
−
different starting geometries (different positions of the PF6 counterion around the
monoprotonated bisguanidine) was not undertaken due to the immense computational
cost of such a survey. Therefore we could not completely exclude a considerable error
for calculations that include anions. On the other hand, the addition of the anions is
generally advisable in COSMO calculations.[27,29,30]
Table 6. Electronic energy, enthalpy at 0 K and Gibbs free energy at 298 K for
monoprotonation of the bisguanidines calculated with density functional calculations
with the B3LYP functional and the stated basis set. The table also includes the
−
thermodynamics for proton transfer from GFA1 to GFA2 in which the PF6 counterions
were included.
GFA1 + H+ → (GFA1+H)+
SV(P)
TZVPa
-1104.1
-1065.0
-1038.1
-1110.0
-1071.0
-1044.0
rE
rH0
rG298
GFA2 + H+ → (GFA2+H)+
SV(P)
TZVPa
-1123.6
-1084.2
-1059.2
SV(P)
-1129.5
-1090.1
-1065.1
TZVPa
rE
rH0
rG298
(GFA1+H)PF6 + GFA2 →
GFA1 + (GFA2+H)PF6
+3.8
+3.0
+4.3
+3.0
+2.2
+3.5
rE
rH0
rG298
GFA3 + H+ → (GFA3+H)+
SV(P)
TZVPa
-1093.7
-1056.9
-1032.6
-1098.5
-1061.7
-1037.4
rE
rH0
rG298
GFA4 + H+ → (GFA4+H)+
SV(P)
TZVPa
-1075.1
-1038.6
-1012.4
-1082.8
-1046.3
-1020.0
rE
rH0
rG298
GFA5 + H+ → (GFA5+H)+
SV(P)
TZVPa
-1064.0
-1027.7
-1001.2
-1074.5
-1038.2
-1011.7
rE
rH0
rG298
a Zero-point energy and thermal contributions calculated with the SV(P) basis set.
17
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We also calculated the thermodynamics of double-proton exchange between
(GFA1+2H)(PF6)2 and one of the other neutral bisguanidines; the values could be
obtained by subtracting the values for the proton-coupled electron transfer (PCET, see
Table 7 in the next section) from the values for pure electron transfer (Table 5). A
discussion follows in the next section.
Comparative PCET reactivity
Experimental results. Next we inspected PCET reactions between the salt GFA1(PF6)2,
containing the twofold-oxidized form GFA12+, with the reduced, doubly-protonated form
of another bisguanidine. An example, reaction between GFA12+ and (GFA4+2H)2+, is
shown in Scheme 3. In all cases, GFA12+ was reduced and twofold protonated to give
(GFA1+2H)2+, and the reduced, twofold-protonated reaction partner deprotonated and
oxidized to the dication. Hence, the results indicate that the salt GFA1(PF6)2 is the
strongest oxidant in PCET reactions.
Scheme 3. Example PCET reaction between GFA1(PF6)2 and (GFA4+2H)(PF6)2
studied to sort the five bisguanidines with respect to their PCET reactivity.
The conversion was followed by 1H NMR spectroscopy to gain (qualitative) information
about the reaction rates (see Figure 9). The reaction between GFA12+ and
(GFA5+2H)2+ was completed almost instantly; therefore it is not included in Figure 9.
Reaction between GFA12+ and (GFA4+2H)2+ required 20 min for 93% conversion
under the chosen conditions. Reaction between GFA12+ and (GFA3+2H)2+ was slower;
93% conversion was reached after 180 min,[30] and 99% after 300 min. The lowest
reaction rate was measured for the reaction between GFA12+ and (GFA2+2H)2+,
requiring more than 800 min for 70% conversion (see Supporting Information for a
18
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10.1002/chem.202101539
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complete curve). In all experiments, no reaction intermediate appeared in the NMR
spectra.
Figure 9. Comparison between the conversion vs. time plots for the reaction of
GFA1(PF6)2 with the doubly-protonated, reduced forms of a second GFA in CH3CN
solution at room temperature (c = 0.015 mol·l−1 for both compounds).
Moreover, we followed the reaction between GFA12+ and (GFA4+2H)2+ by UV-vis
spectroscopy. The dication GFA42+ formed in this reaction could easily be traced by its
characteristic electronic absorption at 495 nm (Figure 10). No bands due to reaction
intermediates were detected. Hence, the NMR and UV-vis spectroscopic
measurements clearly show that an intermediate arising from the transfer of only one
proton and/or electron is not formed in significant amount. In further UV-vis
experiments, we tried to achieve pseudo-first order conditions by applying an excess
of GFA1(PF6)2 (10, 20 and 40 eq.). Clearly, the rate constant increases with increasing
concentration of GFA1(PF6)2 (see Supporting Information for details). However, an
analysis showed that the kinetics does not follow a simple second-order rate law. The
two reactants are both dications. Therefore, the approach of the two reactants to give
19
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a hydrogen-bonded complex is opposed by a strong electrostatic barrier, and also a
barrier due to solvent reorganization, contributing to the overall reaction rate.
Figure 10. a) UV-vis spectra for the PCET reaction between GFA1(PF6)2 and
(GFA4+2H)(PF6)2 in CH3CN at room temperature, both applied in a concentration of
4·10−5 mol∙l−1. The reaction is completed within 80 min under the applied conditions.
Quantum-chemical calculations. Table 7 summarizes thermodynamic state functions
calculated for the considered PCET reactions. The rG values for the PCET reactions
between the oxidized, dicationic redox state GFA12+ and the reduced, diprotonated
form of another bisguanidine are included as red bares in Figure 11. In all cases,
negative rG values were obtained, meaning that the reaction in which the doubly-
protonated, reduced bisguanidine is oxidized to the dicationic, oxidized bisguanidine is
exergonic. This result is in full agreement with the experimental results showing that
all these reactions take place in the predicted direction. Figure 11 directly compares
the Gibbs free energy change of reactions in which only electron transfer occurs (blue
bars) with reactions involving both electron and proton transfer (both at r = 37.5). In
the case of GFA4 and GFA5, pure electron transfer leading to oxidation of the two
bisguanidines by GFA1(PF6)2 are endergonic. Nevertheless, oxidation of (GFA4+2H)2+
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as well as (GFA5+2H)2+ by GFA12+ via PCET are exergonic reactions, and the
experiments showed that these reactions are even particularly fast. Compounds GFA4
and GFA5 are considerably weaker Brønsted bases than GFA1. The lower Brønsted
basicity leads to an exergonic reaction if electron transfer is coupled with proton
transfer. The high reaction rate implies that the proton-transfer step decisively
contributes to the overall rate.
According to the experiments, the reaction between (GFA2+2H)(PF6)2 and GFA1(PF6)2
is the slowest reaction. It also is the reaction that exhibits the highest (least negative)
Gibbs free energy change. However, the experimentally observed order in the reaction
rates does not fully correlate with the calculated order for the thermodynamics of the
(2e−,2H+) PCET reactions. Hence, reaction with (GFA3+2H)2+ is slower than reaction
with (GFA4+2H)2+, in opposition to the trend in the Gibbs free energy changes. This
result again points to the importance of proton transfer for the reaction rate, since GFA4
is a weaker proton acceptor (see Table 6) but also a weaker electron donor (see Table
5) compared with GFA3.
In a previous study we showed that the reaction between GFA1 and 10-methyl-9,10-
dihydroacridane (AcrH2) follows a mechanism in which an electron transfer equilibrium
is followed by proton transfer.[30] Both the rate of the electron transfer step and that of
the proton transfer step enter into the overall rate equation, that could be formulated
as kH = kPket/(k-et + kP), where kP is the rate constant for proton transfer, and ket and
k−et are the rate constants for electron transfer and back electron transfer regenerating
the reactants, respectively. Similar results were also reported for the reaction between
tetrachloro-1,4-benzoquinone (chloranil) and AcrH2 in acetonitrile.[38] Most likely, the
PCET reactions between the bisguanidines studied herein follow a similar mechanism.
However, as mentioned previously, the equilibrium constant for the formation of the
initial hydrogen-bond complex also enters into the rate equation, since the approach
of the two dicationic reaction partners is opposed by a high barrier due to electrostatic
repulsion and solvent reorganization.
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Table 7. Electronic energy, enthalpy at 0 K and Gibbs free energy at 298 K for proton-
coupled electron transfer reactions calculated with density functional calculations with
the B3LYP functional.
r = 1
r =37.5
GFA1(PF6)2 + (GFA2+2H)(PF6)2 →
(GFA1+2H)(PF6)2 + GFA2(PF6)2
SV(P)
-41.4
-36.1
-28.5
TZVPa
-40.4
-35.1
-27.4
TZVPa
-28.0
-22.7
-15.0
rE
rH0
rG298
GFA1(PF6)2 + (GFA3+2H)(PF6)2 →
(GFA1+2H)(PF6)2 + GFA3(PF6)2
SV(P)
-82.9
-77.4
-66.7
TZVPa
-74.5
-69.0
-58.2
TZVPa
-58.0
-52.5
-41.8
rE
rH0
rG298
GFA1(PF6)2 + (GFA4+2H)(PF6)2 →
(GFA1+2H)(PF6)2 + GFA4(PF6)2
SV(P)
-58.8
-55.7
-44.3
TZVPa
-51.9
-48.8
-37.3
TZVPa
-48.6
-45.5
-34.1
rE
rH0
rG298
GFA1(PF6)2 + (GFA5+2H)(PF6)2 →
(GFA1+2H)(PF6)2 + GFA5(PF6)2
SV(P)
-54.9
-53.3
-49.4
TZVPa
-51.8
-50.2
-47.8
TZVPa
-49.4
-47.8
-43.4
rE
rH0
rG298
a Zero-point energy and thermal contributions calculated with the SV(P) basis set.
Figure 11. Comparison between the reaction Gibbs free energy rG for the oxidation
of the neutral, reduced bisguanidines by GFA1(PF6)2 (dicationic, oxidized form) in a
22
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pure electron transfer reaction (blue bares) and in a PCET reaction (red bares) from
B3LYP/def2-TZVP calculations with COSMO (r = 37.5).
In summary, the experimental and quantum-chemical results show that GFA1 is the
strongest oxidant in PCET reactions of the five considered bisguanidines. However, it
is not the strongest oxidant in pure electron transfer reactions, since GFA4 and GFA5
exhibit higher redox potentials.
Test reactions for applications in preparative chemistry
In the final section of this work, we demonstrate the use of the two oxidized
bisguanidines GFA12+ and GFA42+ as PCET reactants in typical intra- and
intermolecular aryl-aryl coupling reactions, and in an oxidative lactonization reaction.
We selected these two bisguanidines for the following reasons: (i) GFA12+ is the
strongest oxidant in PCET reactions of all five considered bisguanidines. (ii) GFA42+ is
a stronger oxidant than GFA12+ in pure electron-transfer reactions, but exhibits a lower
proton affinity. By comparing the reactivity of GFA12+ and GFA42+, the importance of
both factors on the PCET reactivity could be analysed. (iii) GFA42+ displays a
characteristic band in the visible region (at 495 nm) that could be used to follow the
conversion by UV-vis spectroscopy. The test reactions do not intend to demonstrate
the scope of reactivity of these GFAs (see previous papers on this issue [29,30]). The
aims are to complete the analysis of the reactivity differences between GFA1 and
GFA4, giving information on the reaction mechanism, and to answer the question about
the required amount of acid for reaction initiation, highlighthing the advantages of GFA
reagents in PCET reactions with respect to high-potential quinones. We used
1
HBF4·OEt2 as acid and CH3CN as solvent; the Et2O signals in the H NMR spectra
could be used to verify the amount of added acid. In the case of the two coupling
reactions, the conversion could not be followed easily by NMR spectroscopy due to
the intermediate formation of radical species. Therefore, the conversion/yield was
determined after work-up (see Supporting Information for details).
Intramolecular oxidative coupling of 3,3’’-4,4’’-tetramethoxy-o-terphenyl.
We already reported the reaction of GFA1(PF6)2 with 3,3’’-4,4’’-tetramethoxy-o-
terphenyl (oxidation potential of +0.74 V vs. Fc+/Fc) to give the corresponding
triphenylene (Table 8).[30] Herein, we compare the reactions with GFA1(PF6)2 and
GFA4(PF6)2 as PCET reagent, carried out in CH3CN solution at room temperature. In
23
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both cases, the addition of a strong acid is needed, but only 12.5 mol% HBF4·OEt2 are
sufficient for clean, efficient coupling. The results are summarized in the table
integrated in Table 8. The conversion was determined by NMR spectroscopy after a
work-up procedure yielding a mixture of the coupling product and the reduced,
deprotonated bisguanidine (see Supporting Information for detailed information).
Reaction with GFA1(PF6)2 was faster than with GFA4(PF6)2. With 0.13 equivalents of
HBF4·OEt2, 88% of the coupling product was obtained after 6 h at room temperature
for reaction with GFA1(PF6)2, but only 80% after 25 h at room temperature for reaction
with GFA4(PF6)2. The best results were obtained for reaction at 333 K with GFA1(PF6)2
and 1.3 equivalents of HBF4·OEt2 (96% after 10 min reaction time). For comparison,
reaction with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) requires a huge
amount of a strong acid (e.g. methanesulfonic acid in CH2Cl2 in a 1:9 v/v solvent
mixture).[39]
To show that the coupling product could be quantitatively separated from the
bisguanidine, the mixture was dissolved in Et2O and the bisguanidine protonated by
addition of HCl·OEt2 for two reactions with GFA1(PF6)2 and GFA4(PF6)2 (see
Supporting Information for details) After filtration over celite the NMR spectrum of the
filtrate showed the sole presence of the coupling product. The remaining residue
consists of the twofold-protonated bisguanidine, that could be dissolved in CH3CN (and
re-oxidized to the PCTE reagent if desired).
PCET reagent eq. of HBF4·OEt2 temperature/time conversion
GFA1(PF6)2
GFA1(PF6)2
GFA4(PF6)2
GFA4(PF6)2
GFA4(PF6)2
0.13
1.3
298 K, 6 h
88%
96%
63%
80%
86%
333 K, 10 min
298 K, 6 h
0.13
0.13
1.1
298 K, 25 h
298 K, 1.5 h
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Table 8. Intramolecular aryl-aryl coupling reaction of 3,3’’-4,4’’-tetramethoxy-o-
terphenyl. The stated conversion was determined after work-up, leading to a mixture
of the coupling product and the reduced, twofold-protonated GFA from which the
coupling product could be quantitatively separated by flitration over celite (see SI for
details).
Intermolecular oxidative coupling of N-ethylcarbazole. An E1/2 value of +1.12 V vs. SCE
(+0.66 V vs. Fc+/Fc) was measured for N-ethylcarbazole. Venkatakrishnan et al.
showed that reaction of DDQ or chloranil with N-ethylcarbazole gives quantitative
conversion (>99%) of the bicarbazole coupling product (reaction in Scheme 4) in very
short time when carried out in CH2Cl2 solution; however the addition of a large excess
of methanesulfonic acid was required (1:10 v/v methanesulfonic acid:CH2Cl2 solvent
mixture).[40] In the reactions with GFA1 and GFA4, addition of only 1.25 eq. of an acid
(HBF4·OEt2) was required. The reactions were carried out in CH3CN at room
temperature. After 30 min reaction time and a work-up procedure (see SI for details),
the bicarbazole coupling product was obtained in excellent conversion of 97% for
reaction with GFA1(PF6)2 and 98% for reaction with GFA4(PF6)2.
Scheme 4. Intermolecular coupling reaction of N-ethylcarbazole to the bicarbazole.
The stated yield was determined after work-up (see SI for details).
Oxidative lactonization of 2-[(4-methoxyphenyl)methyl]-benzoic acid. Finally, we tested
the oxidative lactonization of 2-[(4-methoxyphenyl)methyl]-benzoic acid to give 3-(4-
methoxyphenyl)-1(3H)-isobenzofuranone. This reaction was shown to proceed in high
(NMR) yield with tetrachloro-o-benzoquinone (QCl, Table 9) in the presence of a
hydrogen-bond donor or a redox catalyst. Nocera, Jacobsen et al. reported reaction
with 1.3 equivalents of the benzoquinone and several different hydrogen-bond donor
compounds in CH2Cl2 solution at 4 °C; the best results (98% NMR yield after 24 h)
25
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were obtained with 0.1 eq. of the hydrogen-bond donor HBD sketched in Table 9.[41]
Greb et al. used 1.3 equivalents of QCl in combination with 0.1 equivalents of the
special silicon redox catalyst Si(CatCl)(SQCl)2 (see Table 9) to obtain > 95% NMR yield,
again in CH2Cl2 at 4 °C after 24 h reaction time.[42] Moreover, a photo-catalytic reaction
in CH3CN solution at room temperature with 5 mol% DDQ and 50 mol% tert-butyl nitrite
was reported, using dioxygen from air as terminal oxidant.[43] The mixture was
irradiated for 24 h by a blue compact fluorescent lamp with  = 450 ± 50 nm light. The
conversion was high, but a mixture of the phthalide product together with the
hydroxylated phthalide was obtained.[43]
In our initial experiments, we used one equivalent of GFA1(PF6)2 or GFA4(PF6)2, and
also one equivalent of HBF4·OEt2 (Table 9). Reaction was carried out in CH3CN at
room temperature, giving 98% NMR yield for reaction with GFA1(PF6)2 after 40 min
reaction time and also 98% with GFA4(PF6)2, but only after 5 h reaction time. The
conversion, directly estimated from the NMR spectra of the reaction mixture, for the
two PCET reagents is compared in Figure 12, showing that reaction is much faster with
GFA1(PF6)2. Hence the results of the test reactions are in line with the results of the
comparative PCET reactions between two of the GFAs. Although GFA4 exibits the
higher redox potential compared with GFA1, it reacts slower in PCET reactions. As
detailed above, this result could be explained by the lower Brønsted basicity of GFA4.
Thereby, the experiments also confirm the importance of the rate for proton transfer
for the overall reaction rate.
Reagent
Conditions
Additive
NMR
yield
98%
Authors
CH2Cl2,
4 °C, 24 h
0.1 eq. HBD
Nocera,
Jacobsen
et al.,
ref. 41
1.3 eq. QCl
CH2Cl2,
4 °C, 24 h
0.1 eq. Si(CatCl)(SQCl)2
>95% Greb et
al., ref. 42
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1.0 eq.
GFA1(PF6)2
1.0 eq.
GFA1(PF6)2
1.3 eq.
GFA1(PF6)2
1.0 eq.
GFA4(PF6)2
1.0 eq.
GFA4(PF6)2
CH3CN, 25 °C,
40 min
CH3CN, 60 °C,
30 min
CH3CN, 25 °C,
30 min
CH3CN, 25 °C,
5 h
1.3 eq. HBF4·OEt2
1.1 eq. HBF4·OEt2
1.3 eq. HBF4·OEt2
1.0 eq. HBF4·OEt2
1.0 eq. HBF4·OEt2
98%
this work
>99% this work
>99% this work
98%
90%
this work
this work
CH3CN, 60 °C,
30 min
Table 9. Oxidative lactonization of 2-[(4-methoxyphenyl)methyl]-benzoic acid to give
3-(4-methoxyphenyl)-1(3H)-isobenzofuranone. The yield was determined by NMR
spectroscopy directly out of the reaction mixture.
As expected, an increase of the temperature to 60 °C led to a decrease of the reaction
time for quantitative conversion (see Table 9). Finally, the reaction was repeated with
1.3 equivalents of GFA1(PF6)2 (see Table 9 and Figure 12, green curve). The reaction
proceeded faster than with 1.0 equivalent (quantitative conversion after 30 min),
confirming that the concentration of the PCET reagent enters into the rate equation
(see the discussion above).
27
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Figure 12. Conversion vs. time plots for lactonization of 2-[(4-methoxyphenyl)methyl]-
benzoic acid with GFA1(PF6)2 or GFA4(PF6)2, from NMR spectroscopic
measurements. See Supporting Information for the complete curve with 1 eq.
GFA4(PF6)2.
Conclusions
Proton-coupled electron transfer (PCET) is of immense importance in redox-enzymatic
reactions in biology and also in synthetic chemistry. In this work we analyse the effect
of modifications at the guanidino groups and the aromatic core on the PCET properties
of redox-active bisguanidines. For this systematic study, a series of five 1,4-
bisguanidino-benzene derivatives was considered. First, the new molecules were
synthesized and characterized in all relevent charge and protonation states. The
analysis of the redox potentials and proton affinities showed that the driving force for
PCET reactions is either the electron transfer (redox) step or the proton transfer step.
Thereby, comparative PCET reactions between the five bisguanidines provided
(qualitative) information about the mechanisms of PCET reactions. Interestingly,
28
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comparative reactions, being endergonic in the case of pure electron transfer, turned
to be most exergonic with fastest conversion if electron transfer is accompanied with
proton transfer. The differences in the Brønsted basicities explain these results,
highlighting their decisive role for the thermodynamics and demonstrating the
important contribution of the proton transfer step for the overall reaction rates. The
results of our combined experimental and quantum-chemical analysis clearly decode
the importance of the two parameters, redox potential and Brønsted basicity, for the
PCET properties of redox-active bisguanidines, and depict the possibilities for their fine
tuning.
Finally, two bisguanidino-benzene compounds were applied in typical intra- and
intermolecular aryl-aryl coupling reactions and in an oxidative lactonization. All these
reactions proceeded almost quantitatively at mild conditions. The amount of acid that
was needed to initiate the reactions (one or even only 0.13 equivalents) was much
lower than in traditional procedures relying on high-potential quinones. Hence, the
results of this work demonstrate the huge potential of redox-active guanidines as
valuable alternatives to toxic high-potential quinones for use as PCET reagents.
Acknowledgements
The authors gratefully acknowledge contineous financial support by the German
research foundation (DFG). We thank M. Sc. Rezisha Maskey for giving us some 2-
[(4-methoxyphenyl)methyl]-benzoic acid.
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29
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