Aminotroponiminates as tunable, redox-active ligands: Reversible single electron transfer and reductive dimerisation

Article abstract of DOI:10.1039/c6cc05762d

Aminotroponiminates (atis) are shown to be redox-active ligands. Under strongly reducing conditions, the result of electron transfer can be controlled by the choice of the metal bound to the ati ligand. Either reversible electron transfer or a reductively induced dimerisation is observed. The latter reaction is (regio- and diastereo-) selective and chemically reversible.

Full text of DOI:10.1039/c6cc05762d

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Aminotroponiminates as tunable, redox-active
ligands: reversible single electron transfer and
reductive dimerisation†
Cite this:DOI: 10.1039/c6cc05762d
Received 13th July 2016,
Accepted 20th July 2016
C. Lichtenberg* and I. Krummenacher
DOI: 10.1039/c6cc05762d
www.rsc.org/chemcomm
Aminotroponiminates (atis) are shown to be redox-active ligands.
Under strongly reducing conditions, the result of electron transfer
can be controlled by the choice of the metal bound to the ati ligand.
Either reversible electron transfer or a reductively induced dimerisation
is observed. The latter reaction is (regio- and diastereo-) selective
and chemically reversible.
Scheme 1 Related diazadiene (dad), aminotroponiminate (ati), and
o-phenylenediamido (pda) ligands; depicted ligand motifs are neutral,
monoanionic, and dianionic, respectively.
Once the domain of coordination chemists, redox-active ligands
have recently been turned into powerful tools in catalysis, materials
science, and main group chemistry.1 By providing multiple acces-
sible oxidation states, they may participate in electron transfer steps
of catalytic cycles, act as key components in electronically functional
materials, and confer redox-activity to compounds based on metals
with one predominant oxidation state. Diazadienes (dads) and ortho-
phenylenediamines (pdas) are archetypical examples of redox-active
ligands (Scheme 1).2 Each can adopt three different redox states, in
which ꢁthe ligand is neutral ([dad]0/[pda]0), radical monoanionic
([dad]ꢀ /[pda]ꢀꢁ), or dianionic ([dad]2ꢁ/[pda]2ꢁ). Bearing (formally)
one amino and one imino functionality, aminotroponimines (H-atis)
can be regarded as a hybrid of dad and pda ligands (Scheme 1).
A wide range of compounds based on ati ligands have been
investigated focusing on their coordination chemistry3 or catalytic
applications such as hydroamination,4 alkyne oligomerisation,5 and
propylene oxide polymerisation.5b The rigid C7 backbone is an
important structural feature of the ati ligand family. However, its
potential to play an active role as a binding site in coordination chemi-
stry has only recently been recognised.6,7 Although spin delocalisation
through the ligand backbone as a result of d(M) - p*(L) backbonding
has been studied in detail,8 its direct participation in well-defined
redox events has not been reported to date.
well-investigated class of ligands, showcasing potential for new
modes of action in synthesis, catalysis, and materials science.
The electrochemical behaviour of the ati ligand [H-atiPh/iPr
]
(1)4 was investigated by cyclic voltammetry in THF/0.1 M
[N(nBu)4][PF6] at 23 1C (Fig. 1). In addition, its rhodium
derivative [Rh(atiPh/iPr)(cod)] (2)7 and its sodium derivative
[Na(atiPh/iPr)(thf)] (3)7 were studied under identical conditions
as examples for ati ligands in the coordination sphere of an
electron rich and an electropositive metal centre, respectively
(Fig. 1). As previously reported for related tropocoronands,10
compounds 1–3 show an irreversible oxidation event at potentials
in the range of +0.2 to +0.4 V vs. Fc/Fc+ (ESI†). Early work on the
in situ generated, cycloheptatriene-based radical [Na2C7H7]ꢀ sug-
gests that the reduction of ati compounds might be feasible.9
Indeed, compound 1 shows a partially reversible redox event at
strongly negative potentials of ꢁ2.65 V, which becomes chemi-
cally reversible at high scan rates of 5 V sꢁ1. The rhodium
compound 2 shows an analogous redox event at ꢁ2.69 V, which
is reversible as shown by a linear correlation between the square
root of the scan rate and the peꢁak currents (ESI†). DFT calcula-
tions on the [Rh(atiPh/iPr)(cod)]ꢀ radical monoanion suggest a
description as a mainly ligand-centred radical to be most
suitable (ESI†). Compounds 1 and 2 are the first examples of
ati compounds with a confirmed ligand-centred redox chemistry
under reducing conditions.10 The sodium compound 3 shows a
reduction wave at a similar potential (ꢁ2.67 V) with a corres-
ponding oxidation wave at ꢁ0.69 V. The large peak separation of
This contribution aims at the investigation of the redox
properties of ati ligands. A well-documented, ligand-centred
redox chemistry would add a new facet to this otherwise
¨
¨
¨
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg,
¨
Am Hubland, 97074 Wurzburg, Germany. E-mail: crispin.lichtenberg@uni-wuerzburg.de
† Electronic supplementary information (ESI) available: Experimental and com-
putational details. CCDC 1455872. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c6cc05762d
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Scheme 2 Reductive dimerisation of 3 via proposed radical intermediate
5 to give the dimer 4. Oxidation of in situ generated 4 leads back to 3.
One resonance structure is shown for 3, 4, and 5.
The proton of this CH unit shows strong cross signals to one
proton of the C7 ring (1H,1H-COSY NMR) and to the iPr group
(1H,1H-NOESY NMR), indicating its localisation in a-position to
the CNiPr group (Scheme 2). Compound 4 was investigated by
single crystal X-ray analysis. It crystallised in the monoclinic space
group P2/c with Z = 4 (Fig. 2). Pairs of atoms in one formula unit of
4 are symmetry equivalent due to a C2 axis that runs parallel to the
crystallographic b-axis and crosses the C7–C70 bond. In addition,
there is a Ci symmetry element located at the centre of gravity
of the unit cell. Thus, a racemic mixture of (R,R)-4 and (S,S)-4
was formed without the meso compound (R,S)-4 being observed;
Fig. 1 Drawings (top) and cyclic voltammograms (bottom) of H-atiPh/iPr
(1), [Rh(atiPh/iPr)(cod)] (2), and [Na(atiPh/iPr)(thf)] (3) in THF/0.1 M [N(nBu)4][PF6]
at 23 1C and a scan rate of 250 mV sꢁ1
.
2.0 V indicates that the electron transfer is followed by a
chemical transformation in case of the reduction and the oxida-
tion. The similar absolute values of the peak currents suggest
chemical reversibility of these processes.
Well-defined, reversible covalent bond formations that can
be induced by electron transfer events are relatively rare and
often based on transition metal compounds.11,12 Such processes
have been discussed in the context of smart materials, molecular
batteries, and catalytic applications.11c–e Only in a few cases has
the experimental detection of intermediates been reported,11b,c
contributing to the mechanistic understanding of such
transformations.
In order to investigate the unexpected reactivity of 3 in
more detail, this compound was reduced with sodium sand.
The colour of the reaction mixture slowly changed from yellow
(lmax = 271, 367, 399, 432, 468 nm) to deep red (lmax = 272, 355,
467 nm). Within 19 h, the selective conversion of 3 to its dimer
[Na4(di-atiPh/iPr)(thf)n] (4) was observed with a selectivity of
Z95% as shown by NMR spectroscopy (Scheme 2). Oxidation
of the in situ generated 4 with two equivalents of AgBF4
re-formed compound 3 with a selectivity of Z95% as shown
by NMR spectroscopy. Thus, the sodium compounds 3/4
are susceptible to highly selective, reductively induced C–C
bond formation and oxidatively induced C–C bond cleavage,
respectively.13
Fig. 2 Molecular structure of [Na4(di-atiPh/iPr)(thf)6] (4) in the solid state.
Displacement ellipsoids are shown at the 50% probability level. Hydrogen
atoms and carbon atoms of THF ligands are omitted for clarity. The
asymmetric unit of 4 contains two chemically equivalent, but crystallo-
graphically independent half-formula-units of 4 only one of which is
discussed in the main text. Selected bond lengths (Å) and angles (1):
Na1–N1, 2.315(3); Na1–N2, 2.602(3); Na1–O1, 2.328(3); Na1–O2, 2.296(3);
Na2–N1, 2.403(3); Na2–N2, 2.370(3); Na2–N20, 2.406(3); Na2–O3, 2.249(3);
C1–N1, 1.344(4); C2–N2, 1.448(4); C1–C2, 1.423(5); C2–C3, 1.413(5);
C3–C4, 1.384(5); C4–C5, 1.413(5); C5–C6, 1.344(5); C6–C7, 1.473(5);
C1–C7, 1.557(4); C7–C70, 1.514(6); N1–Na1–N2, 67.73(10); O1–Na1–N2,
126.26(10); N1–Na2–N2, 70.37(10); N1–Na2–O3, 125.22(11).
The tetra-sodium species 4 could be isolated as a red solid.14
One set of resonances for both monomeric subunits in the
1H and 13C NMR spectra of 4 indicates the structure of a
symmetrical dimer. The increased aliphatic character of the
CH unit directly linked to the second C7 ring is mirrored by an
up-field shift of the corresponding resonance by 2.78 ppm (1H) and
69.9 ppm (13C), respectively, compared to the starting material 3.7
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In agreement with experimental findings, DFT calculations predict
the spin density in [Na2(atiPh/iPr)(thf)4]ꢀ to be delocalised over the
ati ligand backbone and the NPh unit with negligible spin density
at the NiPr group (Fig. 3b). In view of the highly delocalised
character of the radical intermediate, selective formation of 4
out of 30 possible dimerisation products is remarkable.16 The
calculations confirm the exothermic nature of the reaction
sequence 2Na + 2[Na(atiPh/iPr)(thf )3] - 2[Na2(atiPh/iPr)(thf )3]ꢀ -
[Na4(di-atiPh/iPr)(thf )6] with DH = 2 ꢂ (ꢁ17) kcal molꢁ1 (first step)
and ꢁ29 kcal molꢁ1 (second step).17 This rules out a monomer/
dimer equilibrium as opposed to recently reported related
reactions.18
Preliminary reactivity studies suggest that compounds based on
ati ligands can act as electron transfer catalysts in (cross-) coupling
reactions of aryl Grignard reagents with aryl bromides (ESI†).
In conclusion, this work shows that aminotroponiminates
can act as redox-active ligands operating at highly negative
potentials. Their properties can be tuned by choice of the metal
centre. The rhodium compound [Rh(atiPh/iPr)(cod)] (2) shows
reversible electron transfer. Reduction of the sodium compound
[Na(atiPh/iPr)(thf)] (3) yields the persistent ligand-centred radical
[Na2(atiPh/iPr)(thf)n]ꢀ (5), which dimerises with a diastereoselec-
tive C–C bond formation to give [Na4(di-atiPh/iPr)(thf)n] (4). Oxida-
tion of 4 cleanly regenerates monomer 3, indicating that the
reaction is chemically reversible. It is anticipated that this work
will stimulate research on the redox chemistry of the large ligand
family of aminotroponiminates with possible implications for
materials science and catalysis.
Fig. 3 (a) Experimental (black) and simulated (red) EPR spectrum of in situ
generated [Na2(atiPh/iPr)(thf)n]ꢀ (5) at 298 K in DME (for details see ESI†);
(b) calculated structure of [Na2(atiPh/iPr)(thf)4]ꢀ with spin density at an isovalue
of 0.0025 (a-spin: green, b-spin: orange). Hydrogen atoms and carbon atoms
of THF ligands are omitted for clarity.
C. L. thanks Prof. Dr Holger Braunschweig for his support,
Dr Rian Dewhurst for helpful discussions, the Alexander von
Humboldt Foundation a Feodor Lynen Return Fellowship, and
the Fonds der Chemischen Industrie for a Liebig Fellowship.
i.e. the formation of 4 is diastereoselective. The sodium atoms
show distorted tetrahedral coordination geometries with two
OTHF and two Nati atoms (Na1) or one OTHF and three Nati atoms
(Na2) in the first coordination sphere.15 Whereas the N atoms
of the NiPr groups (N1, N10) bridge two metal centres each,
those of the NPh groups (N2, N20) bridge three metal centres
each. This results in an elongation of the N2–Na1 bond (2.60 Å)
by ca. 0.2 Å compared to the N2–Na2/Na20 bonds. Furthermore,
the N2–C2 distance (1.45 Å) is also elongated compared to the
C1–N1 (1.34 Å) distance. The distances between the sp2 hybri-
dised carbon atoms C1–C6 of the C7 ring amount to 1.34–1.42 Å.
Altogether, this suggests significant electron delocalisation
between C1–C6 and N1.
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monitored by EPR spectroscopy (Fig. 3a). An isotropic signal
with giso = 2.003 was observed and tentatively assigned to the
radical species [Na2(atiPh/iPr)(L)n]ꢀ (5; L = thf, dme). The hyperfine
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shows significant coupling with the unpaired spin (a(N) = 10 MHz).
¨
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