Dipyrrinphenol-Mn(iii) complex: Synthesis, electrochemistry, spectroscopic characterisation and reactivity

Article abstract of DOI:10.1039/c1dt10868a

Herein, we report the manganese complex with a novel trianionic ligand, the pentafluorophenyldipyrrinphenol ligand DPPH₃. The X-ray crystal structure reveals that the Mn^III complex exists in a dimeric form in the solid state. Electrochemical studies indicate two quasi-reversible one electron oxidation processes. EPR data on the one electron oxidised species in solution support the formation of a monuclear Mn complex with an S = 3/2 spin system. Preliminary studies towards epoxidation reactions were tested in the presence of iodosylbenzene (PhIO) and are in favour of an oxygen-atom-transfer (OAT) reaction catalyzed by the Mn^III complex.

Full text of DOI:10.1039/c1dt10868a

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COMMUNICATION
Dipyrrinphenol–Mn(III) complex: synthesis, electrochemistry, spectroscopic
characterisation and reactivity†
Sanae El Ghachtouli,a Karolina Wo´jcik,b Laurent Copey,b Florence Szydlo,b Eric Framery,b
Catherine Goux-Henry,b Laurianne Billon,a Marie-France Charlot,a Re´gis Guillot,a Bruno Andrioletti*b and
Ally Aukauloo*a,c
Received 9th May 2011, Accepted 5th July 2011
DOI: 10.1039/c1dt10868a
Herein, we report the manganese complex with a novel tri-
anionic ligand, the pentafluorophenyldipyrrinphenol ligand
DPPH3. The X-ray crystal structure reveals that the MnIII
complex exists in a dimeric form in the solid state. Electro-
chemical studies indicate two quasi-reversible one electron
oxidation processes. EPR data on the one electron oxidised
species in solution support the formation of a monuclear Mn
complex with an S = 3/2 spin system. Preliminary studies
towards epoxidation reactions were tested in the presence of
iodosylbenzene (PhIO) and are in favour of an oxygen-atom-
transfer (OAT) reaction catalyzed by the MnIII complex.
ligand: pentafluorophenyldipyrrinphenol (DPPH3). Preliminary
results concerning the epoxidation of cyclooctene using iodosyl-
benzene (PhIO) as terminal oxidant are also presented.6
The dipyrrinphenol ligand can be seen as a combination of
structural features from a porphyrin and a salen core. The synthesis
of the ligand DPPH3 was realised following the condensation of
2-orthoanisylpyrrole with pentaflurobenzaldehyde in acidic con-
ditions followed by classical deprotection of the methoxy groups
with BBr3 according to Nabeshima’s strategy.7 Insertion of man-
ganese in the free base DPPH3 was realised by refluxing with an
excess of Mn(OAc)2 in a toluene–EtOH mixture in the presence of
NEt3 as base under aerobic conditions (Scheme 1). Color changes
from blue to green and usual analytical techniques probe the
insertion of the metal ion in the N2O2 coordination environment.
The development of robust molecular catalysts to drive oxidation
reactions in environmentally benign conditions is an ongoing
challenge and a priority for chemists. Prototypal metal com-
plexes in this field are based on well-established porphyrin,1
phtalocyanine,2 corrole3 and salen4 derivatives. These molecular
systems act as monooxygenase catalysts upon activation with
oxygen containing oxidants to produce high valent metal–oxo
species. TetraAmidoMacrocyclic Ligands (TAML) developed by
Collins and collaborators are also known to give access to reactive
high-valent metal–oxo derivatives that lead to degradation of
persistent organic pollutants.5 The difference in reactivity between
these families of complexes bearing the same metal ion evidences
the primary importance of the ligand set surrounding the metal
ion that shapes the electronic structure of the active species. In
this communication, we describe the synthesis, spectroscopic and
structural characterisation of a manganese(III) derivative of a new
Scheme 1
Single crystals of MnDPP of suitable quality for X-ray analysis,
were produced by slow evaporation of acetonitrile in air.
The crystal structure shows that the complex exists as a
dimer of [MnDPP] in the solid state (see Fig. 1). The lack of
counter anions attests the presence of a manganese(III) ion in the
trianionic N2O2 coordinating site. Each manganese ion adopts
a square pyramidal geometry, with the DPP ligand as the basal
plane and the oxygen atom of a phenolato group bridging two
aInstitut de Chimie Mole´culaire et des Mate´riaux d’Orsay, UMR-CNRS
8182, Universite´ de Paris-Sud XI, F-91405, Orsay, France. E-mail: ally.
aukauloo@u-psud.fr; Fax: +33 (0) 169 154 756; Tel: +33 (0) 169 154 755
bInstitut de Chimie et Biochimie Mole´culaire et Supramole´culaire (Equipe
CASYEN), UMR-CNRS 5246, Universite´ Claude Bernard Lyon1,
Baˆtiment Curien (CPE) 43 Boulevard du 11 novembre 1918, 69622, Villeur-
banne Cedex, France. E-mail: bruno.andrioletti@univ-lyon1.fr; Fax: +33
(0) 472 448 160; Tel: +33 (0) 472 446 264
monomeric units.8 We note also a particularly short intradimer
9
˚
Mn–Mn distance at 3.146 A. The oxygen atom of the bridging
cCEA, iBiTec-S, Service de Bioe´nerge´tique Biologie Structurale et
Me´canismes (SB2SM), F-91191, Gif-sur-Yvette, France
phenol is tilted towards the opposite manganese ion and departs
˚
from the N2O2 mean plane by 0.773 A, while the Mn ion is found
˚
† Electronic supplementary information (ESI) available: Electronic sup-
plementary information (ESI) available: Synthesis, characterization of
DPPMn, Cyclic voltammetry, DFT Calculations. CCDC reference num-
bers 811013. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1dt10868a
at 0.337 A outside the basal plane.
Mass spectrometry of the isolated compound using ESI tech-
nique shows a major peak at 569 corresponding to [MnDPP +
Na+]+. A peak at 610 attributable to a monomeric manganese
9090 | Dalton Trans., 2011, 40, 9090–9093
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complex with an axial acetonitrile molecule to [MnDPP(CH3CN)
+ Na+]+ was also detected under our experimental conditions.
Typically, MnIII (d4, S = 2) is spectroscopically silent in the
conventional EPR experiment (X-Band, 9 GHz) but transitions
can be observed using parallel polarisation EPR. The parallel
mode EPR spectrum of [MnDPP(CH3CN)] in acetonitrile exhibits
six well-resolved hyperfine lines centred at g ª 8.05 that are split
by 39.2 G. The morphology and field position of these signals are
consistent with previously reported mononuclear MnIII complexes
in similar geometries.10 Although MnDPP was evidenced in the
monomeric form in solution by mass spectrometry and EPR
spectroscopy, attempts to crystallise the manganese monomer
were unfruitful to date.
less positive values while the second oxidation process becomes
irreversible. These effects on the electrochemical response in the
presence of water imply the participation of water molecules in the
coordination sphere of the manganese centre.11
Exhaustive bulk electrolysis was performed both in pure and
wet acetonitrile at 253 K, at + 0.8 V/SCE and coulometry
confirmed a one-electron oxidation process. Furthermore, the
chemical reversibility of the oxidation process was supported
by the identical electrochemical response on the electrolysed
solution in comparison with the initial one. This process is
associated with a colour change from green to intense dark green.
Monitoring this oxidation process by UV-Visible spectroscopy
shows among other changes the appearance of two absorption
bands at 870 and 970 nm (Fig. 3). These electronic features are
reminiscent to the formation of a metalloradical type species.
The electrochemical re-reduction of the oxidised species generated
gave a spectrum corresponding to that initially observed for
MnDPP, demonstrating the chemical reversibility of the one-
electron electrochemical process.
Fig. 1 Crystal structure of [MnDPP]2. Hydrogen atoms are omitted
˚
for clarity. Selected metric distances (A): Mn–N(1) 1.954(3), Mn–N(2)
1.942(3), Mn–O(1) 1.909(3), Mn–O(2) 1.854(3), Mn–O(1)* 2.206(3),
Mn–Mn* 3.146(13). The asterisk indicates the symmetry operation (1 - x,
-y, -z).
Fig. 3 UV-vis absorption spectra recorded in the course of the elec-
trochemical oxidation of MnDPP at 0.8 V vs. SCE, Conditions: 0.2 M
NBu4PF6, 1 mM solution in acetonitrile, T = 253 K, optical path: 0.5 mm.
Running the cyclic voltammogram of the manganese complex
in acetonitrile, (Fig. 2) we observed two quasi-reversible anodic
processes of equal intensity at 0.67 V and 1.23 V vs. SCE, that were
attributed to the MnIV/MnIII couple and to a putative MnV/MnIV
couple, respectively. Upon addition of water, we noticed a shift
of the redox potential of the MnIV/III couple by 60 mV to
Perpendicular mode X-band EPR data obtained at 5 K on
the electrolysed solution in acetonitrile in the presence of water,
indicate well-resolved (Fig. 4) signals with geff at 4.9, 3.0 and 1.8
compatible with the one electron oxidation of the MnIII complex
Fig. 4 Perpendicular mode EPR spectrum of a 2 mM solution of MnDPP
in acetonitrile containing 6% water (0.2 M NBu4PF6) recorded after
exhaustive reduction at 0.8 V vs. SCE; T = 253 K; working electrode:
glassy carbon crucible; auxiliary electrode: Pt grid separated from the rest
of the solution with a fritted glass. EPR recording conditions: microwave
frequency 9.38 GHz; modulation frequency 100 kHz; microwave power
2 mW; modulation amplitude 1 mT; T = 5 K; time constant 160 ms. * trace
of free Mn(II).
Fig. 2 Cyclic voltammetry of MnDPP (2 mM) at 0.1 V s-1, T = 253 K,
on a glassy carbon electrode in acetronitrile (0.2 M NBu4PF6), (green line)
and in acetonitrile containing 6% water (blue line). The corresponding
dotted lines indicate the reversible nature of the first oxidation wave.
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to a S = 3/2 spin system.10,12 Only Mn55 hyperfine coupling was
resolved for the g = 4.9 signal with an A value of 91 G.
Density functional theory (DFT) calculations were used to
support this finding. The geometry optimisation of the penta-
coordinated [Mn(DPP)(OH2)] monomer with a water molecule
In acetonitrile, at room temperature, MnDPP catalyzed effi-
ciently the oxidation of the alkene to the corresponding epoxide.
Indeed, in the presence of 10 equiv., the GC yield of the epoxide
was 97%. This observation pertains to the formation of a high
valent Mn(V)–oxo species. A recent report has indeed shown
the formation of a genuine manganese(V)–oxo species within the
same family of ligands upon treatment of the corresponding
Mn(III) complex with peroxynitrite ions (ONOO-).13 However no
reactivity towards organic substrates was reported.
These preliminary catalytic results clearly indicate that MnDPP
constitutes an efficient catalyst for the epoxidation of alkene.
Further work on the catalytic activities of this novel manganese
complex using other oxidising agents are currently ongoing in our
labs.
˚
in apical position places the water molecule at 2.27 A from the
manganese centre, while the Mulliken atomic spin on manganese
and the map of the spin density distribution are characteristic
of a high-spin MnIII centre (see SI†). Theoretical data on the
oxidised complex [Mn(DPP)(OH2)]+ with a total spin state
S = 3/2 lead to an optimised geometry with a Mn–OW distance
˚
of 2.24 A. Interestingly, we found that the Mulliken atomic spin
on manganese is 3.87 (a value close to four unpaired electrons)
and the map of the spin density distribution (Fig. 5) reveals a spin
density of essentially negative sign developed on the whole DPP
ligand. In such a configuration, the quartet state of the oxidised
complex results from an antiferromagnetic coupling between an
SMn = 2 MnIII ion and an SL = 1/2 ligand radical. Thus the electronic
structure is best described as [MnIII(DPP∑+)(OH2)]+.
Conclusion
We described here the manganese complex of the pentafluo-
rophenyldipyrrinphenol ligand, DPPH3. We have shown the ability
to transfer an oxygen atom to organic substrates. Collected
spectroscopic and theoretical data converge towards a structure–
reactivity relationship similar to those of Salen and Corrole. We
believe that the synthetic versatility of the DDPH3 core will allow
us to efficiently tune the reactivity of the corresponding metal
complexes. Furthermore, the participation of the ligand in the
redox properties of the metal complex holds the promise of an
intriguing non-innocent ligand.14 Work is underway in our labs to
develop the coordination chemistry of the dipyrrinphenol ligand.
This work was supported by the EU/Energy SOLAR-H2
project (FP7 contract 212508) and by the EU/PERG-GA-2009-
247924-ENCAT grant (FP7-PEOPLE-2009-RG) for Dr Florence
Szydlo.
Fig. 5 Map of the spin density distribution for the quartet states
of [Mn(DPP)(OH2)]+ (left) evidencing the antiferromagnetic coupling
between a high spin MnIII ion and a ligand DPP∑+ radical and of
[Mn(DPP)(OH)] (right) showing a high spin MnIV ion.
As deprotonation of the water ligand may occur in the oxidised
aqua complex under certain experimental conditions, a DFT
study was also performed on the oxidised hydroxo monomer
in the quartet state. A noticeable shrinking of the optimised
Notes and references
˚
Mn–OW distance was found at 1.79 A and the Mulliken atomic
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DPPMn
(mmol)
PhIO
(mmol)
Epoxide
(GC-yield) (%)
Entry
Temp.
1a
2a
3a
4
1
1
1
0
100
10
10
rt
rt
38
97
78
0
60 ◦
rt
C
100
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a Reaction conditions: 1 mmol of MnDPP, x mmol PhIO, 1000 mmol alkene,
2 mL solvent.
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