Metal-Ligand Cooperative Synthesis of Benzonitrile by Electrochemical Reduction and Photolytic Splitting of Dinitrogen

Article abstract of DOI:10.1002/anie.201812125

Thermal nitrogen fixation relies on strong reductants to overcome the extraordinarily large N?N bond energy. Photochemical strategies that drive N₂ fixation are scarcely developed. Here, the synthesis of a dinuclear N₂-bridged complex is presented upon reduction of a rhenium(III) pincer platform. Photochemical splitting into terminal nitride complexes is triggered by visible light. Clean nitrogen transfer with benzoyl chloride to free benzamide and benzonitrile is enabled by cooperative 2 H⁺/2 e^? transfer of the pincer ligand. A three-step cycle is demonstrated for N₂ to nitrile fixation that relies on electrochemical reduction, photochemical N₂-splitting and thermal nitrogen transfer.

Full text of DOI:10.1002/anie.201812125

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Accepted Article
Title: Metal-Ligand Cooperative Synthesis of Benzonitrile via
Electrochemical Reduction and Photolytic Splitting of Dinitrogen
Authors: Florian Schendzielorz, Markus Finger, Josh Abbenseth,
Christian Würtele, Vera Krewald, and Sven Schneider
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201812125
Angew. Chem. 10.1002/ange.201812125
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10.1002/anie.201812125
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Metal-Ligand Cooperative Synthesis of Benzonitrile via
Electrochemical Reduction and Photolytic Splitting of Dinitrogen
Florian Schendzielorz,[a] Markus Finger,[a] Josh Abbenseth,[a] Christian Würtele,[a] Vera Krewald,*[b] and
Sven Schneider*[a]
Dedicated to Prof. Dr. F. Tuczek on the occasion of his 60th birthday
Abstract: Thermal nitrogen fixation relies on strong reductants to
overcome the extraordinarily large N–N bond energy. Photochemical
strategies that drive N2 fixation are scarcely developed. Here, the
synthesis of a dinuclear N2-bridged complex is presented upon
reduction of a rhenium(III) pincer platform. Photochemical splitting
into terminal nitride complexes is triggered by visible light. Clean N-
transfer with benzoyl chloride to free benzamide and benzonitrile is
enabled by cooperative 2H+/2e– transfer of the pincer ligand. A
three-step cycle is demonstrated for N2 to nitrile fixation that relies
on electrochemical reduction, photochemical N2-splitting and thermal
N-transfer.
determining and gives the nitride complex [ReNCl(PNPtBu)] (3). A
simple electronic structure model for the Re–N–N–Re core of
key intermediate 2 (π10δ4-configuration; Scheme 1) provides a
starting point to tune the thermochemistry.15 We now report a
modified platform that splits N2 photolytically into more reactive
nitrides that can be transferred with benzoyl chloride. The pincer
ligand serves as 2e–/2H+ reservoir, enabling electrochemical N2
reduction, photochemical splitting and thermal transfer within a
three-step cycle.
The Haber-Bosch process consumes large amounts of energy,
for the generation of H2. Electrocatalytic N2 reduction has
therefore been targeted as an attractive alternative.1,2 Nitrogen
fixation at ambient conditions with molecular catalysts has seen
remarkable progress,3 in some cases also for electrochemical
ammonia generation.4 The direct synthesis of other compounds
than NH3 from N2 remains a formidable challenge. Catalytic
5
6 , 7 , 8
protocols are only known for trisilylamine. Nitriles,
isocyanates, 9 silylamines, 10 , 11 and borylamines, 12 have been
synthesized within stoichiometric, cyclic reaction sequences
that allow for evaluating strategies to offset the extremely strong
N≡N bond (225 kcal/mol), enable E–N (E = C, Si, B) bond
formation and deliver six reduction equivalents. All reported
‘synthetic cycles’ proceed through initial N2 cleavage into nitride
complexes. Subsequent N-transfer typically requires strong
electrophiles like alkyl triflates. The thermochemistry of N2
splitting must therefore be tuned to avoid nitride overstabilization
and enable functionalization with reagents that are more
compatible with reductive conditions.
Scheme 1. Schematic frontier orbital correlation diagram for the splitting of N2-
bridged complex 2 into two nitrides 3 (n.b. = non bonding).13b,15
We have examined N2 activation and splitting, i.e. triggered by
chemical or electrochemical reduction of pincer halide
complexes.13 , 14 The rhenium(III) precursor [ReCl2(PNPtBu)] (1;
PNPtBu = N(CH2CH2PtBu2)2) exhibits a complex mechanism via
rapid ReIII/ReII-reduction, N2-binding, halide loss, ReII/ReI-
reduction and ReI/ReIII-comproportionation.13b Splitting of the
resulting dinuclear complex [(µ-N2){ReCl(PNPtBu)}2] (2) is rate-
The ReIII complex [ReCl3(HPNPiPr)] (4) was obtained in
70 % isolated yield. 16 In contrast to 1, the amide complex
[ReCl2(PNPiPr)] could not be isolated in analytical purity. 4
displays strongly shifted NMR resonances, as exemplified by the
31P{1H}-NMR signal (δP = -1525.9 ppm). However, the sharp
signals exhibit well-resolved J-coupling. This observation can be
rationalized with a magnetic, energetically isolated ground state
that results from excited state admixture via spin-orbit coupling.
Contributions from temperature independent paramagnetism
(TIP) are well documented for 3rd row complexes with d4
(octahedral ReIII, OsIV) and d6 configuration (square-planar,
OsII). 17 Single-crystal X-ray diffraction confirmed meridional
coordination of the neutral diphosphinoamine ligand (Figure 1a).
[a]
M. Sc. F. Schendzielorz, Dr. Markus Finger, M. Sc. J. Abbenseth,
Dr. C. Würtele, Prof. Dr. S. Schneider
Georg-August-Universität
Institut für Anorganische Chemie
Tammannstrasse 4, 37077 Göttingen (Germany)
E-mail: sven.schneider@chemie.uni-goettingen.de
Dr. Vera Krewald
[b]
Department of Chemistry
University of Bath
Claverton Down, Bath BA2 7AY, United Kingdom
Supporting information for this article is given via a link at the end of
the document.
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is in accordance with experiment and contrasts with cleavage of
2 (ΔG‡
= 26.9 kcal mol–1).13b Interestingly, splitting of 5 into
298
6trans is almost thermoneutral (ΔGo
= 2.2 kcal mol–1), as
298
compared with strongly exergonic splitting of 2 into square-
pyramidal 3 (ΔGo = –40.3 kcal mol–1).13b The less favorable
298
thermochemistry for six-coordinate 6trans is attributed to the
nitride trans-influence as expressed by a long Re–NPNP bond
(2.53 Å) and distorted pincer binding. Accordingly, relaxation to
isomeric [Re(N)(cis-Cl2)(HPNPiPr)] (6) is strongly favorable
(ΔGo298 = -11.0 kcal mol–1 per molecule).
The striking thermal stability of 5 vs. 2 can be rationalized
by qualitative molecular orbital (MO) considerations. Thermal
cleavage of a linearly N2 bridged M2N2 core proceeds via
electron transfer into an MO with M–N–N–M σ-σ∗-σ-character
(Scheme 1) within a zig-zag shaped transition state (TS).15,13b,19
Ligands trans to the N2-bridge raise this MO in energy, thereby
disfavoring N2-splitting. A classic case for such geometry
controlled reactivity might be the trisanilides [(RArN)3Mo}2(N2)],20
which split into nitrides in contrast to analogous triamidoamine
complexes.3a
Scheme 2. Four step synthetic cycle for benzonitrile/benzamide synthesis.
Complex 4 was investigated by cyclic voltammetry (CV)
under argon. Quasireversible, scan rate dependent reduction is
observed at -1.84 V suggesting chloride loss on the CV time-
18
scale.16,
Chemical reduction with
1
equiv. CoCp*2 in
tetrahydrofuran (THF) under an N2 atmosphere (Scheme 2)
gives an intensely blue product (5) in around 60% isolated yield.
Complex 5 exhibits sharp and strongly shifted NMR signals
suggesting an even-electron compound with a TIP contribution.
Symmetric N2 binding was confirmed by a single signal in the
15N{1H}-NMR spectrum and
a
band at 1733 cm–1 (15N2-5:
1675 cm–1) in the resonance Raman spectrum.16 Two doublets
in the 31P{1H}-NMR spectrum (δP = -370.6/-380.4 ppm) with
mutual trans-coupling (2JPP = 237 Hz), eight 1H NMR signals
assignable to CH3-groups and one amine NH-signal are in
agreement with an N2 bridged, dinuclear compound with C2
symmetry, in analogy to 2.13b Structural assignment as [(µ-
N2){ReCl2(HPNPiPr)}2] was further substantiated by LIFDI mass
spectrometry.
The molecular structure was confirmed by single crystal X-
ray diffraction (Figure 1b).16 The asymmetric unit features two
octahedrally coordinated, linearly N2-bridged Re-ions. The N–N
bond length (1.169(5) Å) is slightly shorter compared with 2
(1.202(10) Å),13b indicating moderate N2 activation in agreement
with the Raman data. In contrast to 2, the pincer nitrogen atoms
are located trans to the N2 bridge. The two {ReCl2(HPNPiPr)}
fragments are twisted with respect to each other by 75.5° giving
rise to idealized C2 symmetry.
Figure 1. Molecular structures of 4 (a), 5 (b), 6 (c) and 7 (d) in the crystal
obtained by single crystal X-ray diffraction. Hydrogen atoms were omitted for
clarity except NH protons.16
To overcome kinetically hindered N2-splitting we sought to
populate N–N antibonding MOs by electronic excitation.9,21,22
Photolysis of 5 in THF with a Xe arc lamp (λ > 305 nm) led to
gradual color change to yellow over 2 h. 31P and 1H NMR
spectra revealed the formation of cis-dichloro nitride complex 6
(Scheme 2) in 95 % spectroscopic yield upon comparison with
an original sample.16 Use of 15N2-labelled 5 confirmed photolytic
splitting of the N2 ligand by 15N{1H} NMR spectroscopy. A
quantum yield below 1 % was estimated by actinometry.16 The
configuration of 6 requires isomerization before or after N2-
splitting. Photolysis was therefore carried out in the presence of
NHex4Cl (0-500 equiv.).16 The independence of the reaction rate
excludes isomerization by chloride (photo)dissociation prior to or
as the rate determining step.
Unlike 2, complex 5 shows remarkable thermal stability.
No decomposition was observed over several days, even upon
heating to 60 °C in THF. Thermal splitting was therefore
examined computationally. Density functional theory (DFT)
nicely reproduced the geometry of 5.16 In analogy to 2,13b an
electronic triplet configuration was obtained as the ground state.
While any spin-orbit coupling effects likely present in this dimer
are insufficiently described by DFT, the high kinetic barrier
(ΔG‡
= 41.8 kcal mol–1) computed for thermal cleavage of 5
298
into the trans-dichloro nitride [Re(N)(trans-Cl2)(HPNPiPr)] (6trans
)
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In the solid state (Figure 1c),16 6 retains octahedral
coordination with one significantly elongated Re–Cl bond
(2.4309(7) Å vs. 2.6712(7) Å), reflecting the nitride trans-
influence. In turn, the Re≡N bond (1.669(2)Å) is longer
compared
to
five-coordinate
[Re(N)Cl(HPNPtBu)]Cl
(1.642(4) Å).13a Addition of NaBArF (BArF = tetrakis-(3,5-
24
24
(trifluoromethyl)phenyl)borate) to 6 in THF results in two new 31
P
NMR signals that are assigned to two isomers of
[Re(N)Cl(HPNPiPr)]+.16 In turn, 6 is fully restored upon addition of
NHex4Cl. Hence, six-coordinate [Re(N)Cl2(HPNPiPr)] is the
dominant species in solution. Reduced steric shielding facilitates
chloride coordination and thereby weakening of metal nitride
bonding.
Heating of 6 with PhC(O)Cl (2 equiv.) at 80 °C in 1,4-
dioxane over 15 h gives a new rhenium species (7) in 71 % yield
(Scheme 2). Paramagnetically shifted and sharp NMR signals
support reduction to ReIII. Two chemically inequivalent P-atoms
2
1
(δP = -1592.6, -1615.5 ppm; JPP = 248 Hz) and the H and 13C
NMR signatures are in agreement with pincer oxidation to an
imine ligand.16 Formation of [ReCl3(P=NPiPr)] (P=NPiPr
=
N(CHCH2iPr2)(CH2CH2iPr2)) was confirmed by LIFDI-MS and
comparison with an authentic sample.16 Crystallographic
characterization (Figure 1d) evidences nitride transfer and pincer
dehydrogenation.16 Two products account for 94 % of the nitride
ligand, i.e. benzamide (30 %), and benzonitrile (64 % with
equimolar benzoic acid), respectively. PhCN and PhCO2H are
products from the reaction of benzamide with benzoyl chloride,23
supporting that PhC(O)NH2 is the immediate product of nitride
benzoylation and 2e–/2H+-transfer from the pincer ligand.
Related reactivity was observed for [Ru(N)(HPNPtBu)]+, which
gives [Ru(NH3)(O2CPh)(P=NPtBu)]+ upon reaction with para-
methoxy benzoic acid.24 However, in that case azide was used
as nitrogen source rather than N2.
Next, re-reduction of the imine pincer ligand was examined
to evaluate the amine/imine redox couple as cooperating ligand
reservoir for 2e–/2H+ proton coupled electron transfer (PCET). 7
does not react with H2 under thermal or photolytic conditions.
However, chemical reduction is possible with stepwise addition
of Li[HBEt3] and diphenylammonium chloride (Scheme 2). On
this route, 4 is obtained in 62 % spectroscopic yield due to the
formation of rhenium hydrides as byproducts, which reform 7
upon hydrolytic quenching.
We therefore turned to electrochemical regeneration of 4.
The CV of 7 exhibits quasi-reversible reduction at -1.70 V.
Titrating in benzoic acid (0-15 eq.; Figure 3a) results in a
pronounced increase of the cathodic current and buildup of a
second, quasi-reversible reductive feature at -1.84 V, which is
assigned to parent 4. The strong increase of the first wave is
indicative of multielectron reduction in the presence of acid. In
fact, the viable intermediate [ReCl3(PNPiPr)] (8) exhibits a quasi-
reversible reduction at -1.16 V,16 confirming a potential inversion
after the first e–/H+-transfer to 7. The electrochemical data in the
presence of acid is therefore rationalized with two subsequent
PCET steps at around –1.7 V that regenerate 4.
Figure 2. a) Comparison of the experimental (blue) and TDDFT-computed
(red; S = 1; see ESI for details) electronic absorption spectra. The productive
region for N2 splitting is highlighted in the computed spectrum (green) and
shown in the inset. b) Difference densities (loss in yellow, gain in red) of the
two transitions marked with an asterisk in the inset.
Complex 5 exhibits two maxima above 300 nm in the UV-
vis spectrum (λmax = 336, ε = 38027 M–1cm–1; 578 nm, ε = 12294
M–1cm–1; Figure 2a). Wavelength selective photolysis indicates
that the productive excitations for N2-splitting are centered at a
shoulder around 400 nm allowing for the use of a violet LED
(390 nm) with the same yield.16 TD-DFT reasonably well
reproduced the electronic absorption spectrum (Figure 2a) with
almost identical results for the open-shell singlet and triplet
electronic structure configurations.16 Computational evaluation of
the highest intensity, yet unproductive absorptions indicates
dominant contributions from transitions within the Re–N–N–Re
π-manifold, such as (π*-π-π*)→(π*-π*-π*) (calc: 278 nm, 4.46 eV)
and (π-π*-π)→(π*-π-π*) (calc: 460 nm, 2.70 eV). In contrast, two
transitions in the productive spectral region (Figure 2a, inset)
exhibit mainly (δ/δ*)→(π*-π*-π*) character (Figure 2b), implying
considerably stronger Re→N2 charge transfer. The weak
oscillator strengths (fosc ≤ 0.02) are in line with the low quantum
yield obtained for photolytic splitting of the N2 bridge.
While the excited state dynamics are not known at this
point, the Re→N2 MLCT character of these states is consistent
with N2 cleavage. Occupation of Re–N–N–Re (π*-π*-π*)-MOs
weakens N–N π-bonding and increases the flexibility of the
Re2N2-core, which could facilitate cleavage via a zig-zag TS. At
the same time, charge separation should enhance Re–N
bonding. The spectral features of 5 resemble the visible region
Controlled potential electrolysis (CPE) of 7 at the half peak
potential of the first reduction feature (E = -1.65 V) did not give
appreciable amounts of 4 in the presence of benzoic acid
(10 eq). 2,6-Dichlorophenol (DCP) was therefore employed,
of
[{Cp*(depf)Mo}2(η1:η1:µ2-N2)]
(depf
=
1,1’-
bisdi(ethylphosphino)ferrocene), which also splits into nitrides
upon photolysis at 580 > λ > 400 nm.22d
THF
which exhibits about same pKa as benzoic acid (pKa = 25.1)25
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yet a conjugate base that is less prone to metal coordination. In
fact, bulk electrolysis of 7 at E = -1.65 V in the presence of DCP
(10 eq.) for 7 h results in full consumption of 7 and formation of 4
as the only electroactive species. 1H NMR spectroscopy
confirmed a yield of 99 % and coulometry the transfer of 1.96 e–
per 7.
as a reservoir for nitrogen hydrogenolysis upon 2e–/2H+-PCET
and electrochemical re-hydrogenation. Our results demonstrate
how metal-ligand cooperativity and photo- and electrochemical
approaches can facilitate the design of platforms for N2-fixation.
Figure 3. Left: CV of 7 in presence of 0–15 eq. benzoic acid. Right: CV of 7
with 10 eq 2,6-dichlorophenol under N2 before CPE (orange), after 8 h CPE at
-1.65 V (pink) and after subsequent 5 h CPE at -1.85 V (blue).
Scheme 3. Optimized, three-step synthetic cycle.
Acknowledgements
The quantitative regeneration of
4
motivated the
examination of in situ electrochemical N2 activation. In a CPE
experiment, 7 was electrolyzed with DCP (10 eq.) under N2 for
8 h at E = -1.65 V, followed by 5 h at E = -1.85 V, i.e. the half
potential of the reduction of 4 (Figure 3b). Overall, 3.32 e– per
Re atom were transferred. Formation of complex 5 during the
second electrolysis step was indicated by the deep blue color
and confirmed by 1H and 31P{1H} NMR spectroscopy. UV/Vis
spectroscopic quantification gave a yield around 70 % (Scheme
3). In situ photolysis (390 nm LED) of this mixture revealed N2
splitting to nitride 6 as the only detectable product by 31P{1H} and
1H NMR spectroscopy. However, the yield in 6 dropped to 14 %
with respect to parent 7, i.e. considerably lower compared to
photolysis of isolated 5.
In summary, a three-step cycle for the generation of
benzamide/benzonitrile from N2 in overall 61 % yield (with
respect to N) was established that relies on electrochemical N2
activation, photochemical splitting into nitrides and thermal
nitrogen transfer (Scheme 3). This model reveals some basic
principles in comparison to our previously reported system.7,13
Use of a sterically less encumbered pincer ligand stabilizes
higher coordination numbers. In consequence, thermal N2
splitting becomes less favorable which can be overcome by
photolysis with visible light. Based on the experimental and
theoretical data, the photochemical reactivity is associated with
the population of a dissociative state with Re→N2 MLCT
character. The higher coordination number weakens nitride
bonding, thus enabling the use of a weaker electrophile than
alkyl triflates. Importantly, the cooperating pincer ligand serves
This work was supported by the European Research Council
(ERC Consolidator Grant Agreement 646747, grant holder
S.Sch.). V.K. acknowledges a 50th Anniversary Prize Fellowship
from the University of Bath and its Balena High Performance
Computing (HPC) Service. The authors thank Dr. C. Volkmann
for solving the crystal structure of complex 8.
Keywords: Nitrogen Fixation • Rhenium • Pincer Ligand •
Photochemistry • Electrochemistry
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Entry for the Table of Contents
COMMUNICATION
Cooperative Fixation: N2 fixation to
benzonitrile and benzamide is
reported within a three-step cycle that
utilizes electrochemical reduction,
photochemical N2 splitting and thermal
N-transfer with benzoylchloride.
Product formation is enabled by
cooperativity of the functional pincer
ligand, which serves as a 2H+/2e–
reservoir.
F. Schendzielorz, M. Finger, J.
Abbenseth, C. Würtele, V. Krewald,* S.
Schneider*
Page No. – Page No.
Metal-Ligand Cooperative Synthesis
of Benzonitrile via Electrochemical
Reduction and Photolytic Splitting of
Dinitrogen
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