Influence of second sphere hydrogen bonding interaction on a manganese(ii)-aquo complex

Article abstract of DOI:10.1039/c1dt11858g

We have developed a pentadentate N₄O ligand scaffold with a benzimidazole group placed in a rigid fashion to develop hydrogen bonding interaction with the ligand in the sixth position. The mononuclear Mn(ii) complex with a water molecule was isolated and characterized. We discuss the role of the outer sphere ligand in stabilising a Mn(ii)-aquo complex.

Full text of DOI:10.1039/c1dt11858g

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COMMUNICATION
Influence of second sphere hydrogen bonding interaction on a
manganese(II)-aquo complex†
Sanae El Ghachtouli,a Re´gis Guillot,a Pierre Dorlet,b,d Elodie Anxolabe´he`re-Mallartc and Ally Aukauloo*a,d
Received 1st October 2011, Accepted 31st October 2011
DOI: 10.1039/c1dt11858g
We have developed a pentadentate N4O ligand scaffold with a
benzimidazole group placed in a rigid fashion to develop hy-
drogen bonding interaction with the ligand in the sixth posi-
tion. The mononuclear Mn(II) complex with a water molecule
was isolated and characterized. We discuss the role of the
outer sphere ligand in stabilising a Mn(II)-aquo complex.
During these past few years, there has been a growing recognition
for the role of outer sphere ligands on the structure/reactivity re-
lationship of several active sites of metalloenzymes.1 For instance,
different amino acid residues often located in the surrounding
Scheme 1
of the metal ion provide additional control in the binding and
activation of small molecules and also offer ways to modulate the
chemical reactivity of the metal ion through hydrogen bonding.
Introducing such a degree of sophistication in synthetic models
is a challenging task for a synthetic chemist. Remarkable effects
have already been observed on the metal activation of dioxygen
and water through the judicious positioning of a hydrogen bonding
network in the second coordination sphere of both manganese and
iron based complexes.2 Collman and colleagues have been pioneers
in the design of hydrogen bonded cavities on the porphyrin core
to stabilise the dioxygen adduct.3
In the course of our study on manganese complexes, we
have shown that the pentadentate N4O monoanionic ligand LH,
(Scheme 1) leads to the formation of a mono-m-oxo dinuclear man-
ganese(III) complex [1]2+.4 All attempts to isolate the mononuclear
manganese species, either in the +II or +III oxidation states with
an axially bound water molecule, have proved unsuccessful. In a
more recent study, we have shown that adding a bulky tert-butyl
group in the ortho- and para-positions of the phenol (tBuLH,
Scheme 1), we could isolate a manganese(III) complex bearing
a hydroxo ligand in the available sixth coordination site on the
metal centre, complex [2a]+. Electrochemical study indicates a
III/II
reversible Mn process on a cyclic voltammogram scale at quite
a negative potential (E0
/
= -0.35 V vs. SCE) in
III
II
-
Mn OH+H
O
Mn OH +OH
2
2
the presence of water.5 It has been argued that hydrogen bonding
in the second coordination sphere can also help to prevent the
formation of the M–O–M motif and at the same time stabilise
highly oxidised Metal–Oxo units.6–8
With the aim to understand the stepwise activation of a man-
ganese bound water molecule, we reasoned that the N4O ligand
skeleton stands as a good candidate to implement a functional
group that could develop hydrogen bonding with a bound water
molecule. We report here on the synthesis of a novel derivative
of the N4O ligand covalently assembled with a benzimidazole
as a potential donor for hydrogen bonding. The crystallographic
structure of the mononuclear Mn(II) complex with a bound water
molecule evidenced a hydrogen bonding interaction between the
water and the protonated form of the benzimidazole unit. We also
report on the spectroscopic characterisation and electrochemical
properties of the analogous chloro Mn(II) derivative. Further
comparison with compounds [2a]+ and [2b]+ helps to account for
the contribution of the second coordination sphere.
The synthetic pathway leading to the target ligand (LH2)
is presented in Scheme 2. The benzimidazole fragment was
positioned in an ortho position of the phenol group (compound
(a)) and was obtained in good yield by reaction of one equivalent
of 1,2-diamino benzene with 5-tert-Butyl-2-hydroxybenzaldehyde
in the presence of benzofuroxane.9 The introduction of a formyl
group on (a), was realised by the Duff procedure, treatment with
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
bLaboratoire Stress Oxydant et De´toxication CNRS URA 2096 F-91191 Gif-
sur-Yvette, France and CEA, iBiTec-S, SB2SM, F-91191, Gif-sur-Yvette,
France
cLaboratoire d’Electrochimie Mole´culaire UMR 7591 Univ Paris Diderot,
Sorbonne Paris Cite´, 15 rue Jean-Antoine de Ba¨ıf, F-75205, Paris, Cedex
13, France
dCEA, iBiTec-S, Service de Bioe´nerge´tique Biologie Structurale et
Me´canismes (SB2SM), F-91191, Gif-sur-Yvette, France, and CEA, iBiTec-
S, SB2SM, F-91191, Gif-sur-Yvette, France
† Electronic supplementary information (ESI) available: Experimental
section, crystallographic data and additional EPR spectrum. CCDC
reference numbers 817802 and 828978. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1dt11858g
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The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 1675–1677 | 1675
©

View Article Online
presence of the protonated form of the benzimidazole. This stems
from the deprotonation of the phenol group upon metallation with
the manganese(II) ion performed in the absence of external base.
The manganese(II) ion is wrapped by the N4O ligand and a water
molecule completes the coordination sphere. The mean Mn–N
˚
˚
bond distance is 2.2 A while the water molecule is at 2.291 A from
the manganese.11
A noticeable feature of this structure is the presence of a
bifurcated hydrogen bond between the proton from the N+–H
group of the benzimidazolium fragment with the oxygen atom of
the phenolate group on one side and the oxygen of the metal bound
water molecule on the other. The former hydrogen bond being the
˚
major component, with the N–H ◊ ◊ ◊ O(1) distance of 2.53 A and
the latter as the minor component with the N–H ◊ ◊ ◊ O(2) distance
˚
of 3.23 A respectively. The presence of the hydrogen bond between
the metal bound water molecule and the hydrogen donor group,
the N+–H group of the benzimidazolium fragment leads to a
pronounced tilt of the water molecule towards hydrogen donor
component. This is clearly evidenced by the angle sustained at the
metal ion by the O(1) atom of the phenol and O(2) of the water
molecule found at 77.7◦ 12 while a value of 94.1◦ was found for the
angle of the Mn(III) derivative [2a]+ with an axial hydroxo group.5
The electrochemical behaviour of the manganese complexes
[3]2+ and [4]+ was studied in acetonitrile (Fig.2) and compared
to the corresponding derivative with a tert-butyl group ([2a]+ and
[2b]+).5,13 The cyclic voltammogram of compound [4]+ shows one
quasi-reversible anodic wave at 0.5 V vs. SCE that was assigned to
Scheme 2 (i) 1 eq of benzofuroxane/methanol/60 ◦C 4h; (ii) HMT/TFA
reflux 2 days; (iii) (c)/ethanol under argon, (iv) ethanol, Mn(ClO4)2·6H2O
for [3]2+, MnCl2·4H2O/NaPF6 for [4]+.
hexamethyltetramine in trifluoroacetic acid followed by hydrolysis
in basic medium gave (b).10 The condensation of the aldehyde
derivative (b) on the primary amine (c) in ethanol under argon
leads to ligand LH2, which was isolated as a brown oil. The
metallation was performed in ethanol upon treatment of the ligand
LH2 with the manganese(II) perchlorate salt yielding the complex
[3]2+ as a pale yellow solid. The same synthesis was also realised
with MnCl2 with the aim to have a chloride ligand in the sixth
position, complex [4]+ (see ESI†).
III/II
the Mn process.
X-band EPR spectra obtained on the isolated powders exhibit
broad resonances over a wide range of magnetic field, supporting
a Mn(II) complex and excluding a mononuclear manganese in
a +III oxidation state which would be EPR silent under these
conditions (vide infra and ESI†). The X-ray analysis of complex
[3](ClO4)2 (Fig. 1) points to several structural features that need to
be mentioned. Two perchlorate ions accompany each manganese
unit. Given the monoanionic nature of the N4O ligand, the
+2 charge of the manganese complex can be explained by the
Fig. 2 Cyclic voltammetry of (2 mM) solution in acetonitrile at 0.1 V s-1,
T = 273 K, on a glassy carbon electrode (0.1 M NBu4PF6), (A) [3](ClO4)2,
(B) [4](PF6).
A mere comparison with the redox potential observed for the
corresponding tert-butyl derivative ([2b]+) (Table 1) indicates a
shift of about 400 mV to more positive potentials. This can be
partly due to the strong hydrogen bonding between the proton
donor N+–H fragment of the benzimidazolium and the phenolate
oxygen atom, thereby diminishing the donating power of the
phenolate group.
Fig. 1 Crystal structure of [3]2+. Hydrogen atoms counter anions and
solvent molecules were omitted for clarity. Selected metric distances
˚
(A): Mn–N(1) 2.233(5), Mn–N(2) 2.363(5), Mn–N(3) 2.164(4), Mn–N(4)
2.190(4), Mn–O(1) 2.056(4), Mn–O(2) 2.292(4).
1676 | Dalton Trans., 2012, 41, 1675–1677
This journal is
The Royal Society of Chemistry 2012
©

View Article Online
a
Table 1 E1/2a values (V vs. SCE) and Ep (for irreversible processes) for
characteristic signal above 100 mT16 and Mn(III) complex detected
in parallel mode with a signal below 100 mT. We also note an
increase in free Mn(II) content. Taken together these results agree
with the formation of a Mn(III) complex upon electolysis that
further chemically reacts, likely by dismutation, as supported by
the irreversibility of the wave observed in the cyclic voltammetry.
We have reported here on the functionalisation of the second
coordination sphere of a manganese complex. We have shown
that the presence of an imidazolium group in hydrogen bonding
interaction with a manganese bound water molecule shifts the
redox potential to more positive values. The irreversible nature
of the one electron oxidised species leading to the Mn(III) species
was explained by a dismutation reaction. Further efforts to design
more robust manganese based systems introducing at the same
time a second coordination sphere to understand the initial steps
of water activation are going on in our laboratory.
complexes [3](ClO4)2 and [4](PF6) in acetonitrile (0.1 M Bu4NPF6) and
related complexes
III
II
IV
III
Mn /Mn
Mn /Mn
E
1/2(DEp/mV)
E1/2(DEp/mV)
[3]2+
1.1 (irr)
—
—
[4]+
0.50 (196)
[2a]+
[2b]+
[2aH]2+
-0.35 (117)a
0.10 (96)b
0.82 (117)a
1.1 (96)b
0.32 (~200)a,c
a ref. 5; b ref. 13, c [2aH]2+ stands for the aqua form of [2a]+ in which the
hydroxo ligand is protonated.
For compound [3]2+, we observed one irreversible anodic process
at ca. 1.1 V vs. SCE. This value is close to the one reported in the
case of [LN5Mn(II)(OH2)]2+ complex (Ep = 1. 4 V vs. SCE) where
a
LN5 stands for a pentadentate nitrogen containing ligand.14 In the
present case, the destabilisation of the Mn(III) oxidation state,
contrary to the previously reported Mn(III)-aqua complex [2c]+
agrees with the decrease of the donating power of the phenolate
group. The irreversibility of the process is characteristic of an EC
mechanism15 as reported for complex [LN5Mn(II)(OH2)]2+14 this
irreversibility is attributed to arise from the deprotonation of a
bound water molecule upon oxidation.
Acknowledgements
This work was supported by ANR Take Care and the EU/Energy
SOLAR-H2 project (FP7 contract 212508).
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Fig. 3 EPR spectra recorded in the perpendicular (left panel) and parallel
(right panel) mode for [3](ClO4)2 before (bottom spectra) and after (top
spectra) exhaustive oxidative electrolysis at 1.3 V vs. SCE at 273 K in
acetonitrile.
Before electrolysis, the perpendicular mode EPR spectrum
exhibits large resonances in the 0–300 mT range comparable to
what was observed in the powder (see ESI†) and attributable to
the monouclear Mn(II) complex (the 6-line pattern at g = 2 is due
to free Mn(II) impurities which was estimated to be about 12% of
the total amount of Mn present). Upon exhaustive oxidation, the
EPR signal of the initial Mn(II) complex disappears in favor of a
mixture of Mn(IV) complex detected in perpendicular mode with a
This journal is
The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 1675–1677 | 1677
©