Selective functionalisation of Re6 cluster anionic units: From hexa-hydroxo [Re6Q8(OH)6]4- (Q = S, Se) to neutral trans- [Re6Q8L4L′2] hybrid building blocks

Article abstract of DOI:10.1039/b822105g

The reaction of [Re₆Qⁱ₈(OH) ^a₆]^4- (Q = S, Se) with p-tert-butylpyridine (TBP) in water leads to neutral trans-[Re₆Q₈(TBP) ₄(OH)₂] whose hydroxyl

Full text of DOI:10.1039/b822105g

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Selective functionalisation of Re6 cluster anionic units: from hexa-hydroxo
[Re6Q8(OH)6]4- (Q = S, Se) to neutral trans-[Re6Q8L4L¢2] hybrid
building blocks†
Fre´de´rick Dorson,a Yann Molard,*a Ste´phane Cordier,*a Bruno Fabre,a Olga Efremova,b David Rondeau,c
Yuri Mironov,b Viorel Cˆırcu,d Nikolay Naumovb and Christiane Perrina
Received 9th December 2008, Accepted 8th January 2009
First published as an Advance Article on the web 21st January 2009
DOI: 10.1039/b822105g
The reaction of [Re6Qi8(OH)a ]4- (Q = S, Se) with p-tert-
complexes. In this paper, we present a high yield and simple
preparation method as well as the characterisation of neutral trans-
[Re6Q8(TBP)4(OH)2] obtained from K4[Re6Qi8(OH)6]·8H2O. The
trans-[Re6Q8(TBP)4(OH)2] were obtained for Q = S (compound
1a) and Se (compound 1b) and were further characterised by
NMR and mass spectrometry, UV-vis spectroscopy as well as
by single-crystal X-ray diffraction. The reactivity of these new
building units with carboxylic acid derivatives has then been
investigated.
The K4[Re6Qi8(OH)6]·8H2O precursor was synthesised in high
yield by reaction between the polymeric Re6Q8Br2 compound and
KOH according to the reported procedure.9 Compounds 1a and 1b
were obtained by reaction of K4[Re6Qi8(OH)6]·8H2O with a large
excess of p-tert-butylpyridine (TBP) in water (Scheme 1) followed
by two successive recrystallisations in CH2Cl2–ether mixtures.†
1H NMR experiments in CDCl3 on both S and Se derivatives
showed very simple spectra indicating the generation of highly
symmetric compounds. The coordination of TBP onto the cluster
core induces a large downfield shift of the Hortho pyridine proton
signals from 8.34 to 9.37 ppm for 1a and 9.66 ppm for 1b.
In regards to the ability of high resolution mass spectrometry to
characterise functionalised octahedral rhenium clusters when soft
ionization methods are used,10 1a and 1b have been analysed by
FAB-HRMS (Fast Atom Bombardment-High resolution Mass
Spectrometry).† The observed isotopic patterns are similar to
the distributions calculated from the elemental composition of
a [Re6Se8(TBP)4OH]+ cation in the case of 1b for example and are
therefore consistent with the grafting of 4 TBP on the cluster core
leading to neutral compounds based on two possible isomeric units
(cis and/or trans).
6
butylpyridine (TBP) in water leads to neutral trans-
[Re6Q8(TBP)4(OH)2] whose hydroxyl reactivity with car-
boxylic acid and TBP exchange reaction with functional
pyridine have been investigated.
Ax[M6Qi8Xa ] (A = alkali, Q = chalcogen/halogen, X = halogen)
6
octahedral transition metal clusters compounds,1 obtained via
high temperature synthesis, are an attractive class of cluster build-
ing blocks precursors for the design of functional supramolec-
ular architecture, polymeric frameworks or nanomaterials.2,3
(M6Qi8Xa )x- units are characterised by a rigid (M6Qi8)m+ cluster
6
core with interesting structural, electronic and photochemical
properties related to the number of metallic electrons available for
metal–metal bonds (VEC).4 Moreover, they can be functionalised
by exchange in solution of halogen apical ligands by organic
or hybrids moieties. Many examples of functional hexasubsti-
tuted M6Qi8La6 units are reported in the literature (L = functional
ligand).3,5–7 Hitherto, in rhenium chemistry, CsBr·Cs4Re6Si8Bra
6
and Cs4Re6Se8I6 constituted the standard inorganic solid-state pre-
cursor for molecular and supramolecular Re6 solution chemistry.7,8
If the reactivity of Re6Qi8Xa hexahalides is now well estab-
6
lished, little is known about that of [Re6Qi8(OH)a ]4- hexahydroxo
6
aSciences Chimiques de Rennes UMR 6226 CNRS Universite´ de
Rennes 1, Avenue du Ge´ne´ral Leclerc, 35042, Rennes Cedex, France.
E-mail: yann.molard@univ-rennes1.fr, stephane.cordier@univ-rennes1.fr;
Fax: +33 (0)2323 6536; Tel: +33 (0)223 235 814
bNikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian
Academy of Sciences, 3 Acad. Lavrentiev pr., 630090, Novosibirsk, Russia
cCIMA CNRS 6200, Universite´ d’Angers, 2 Bd. Lavoisier, 49045, Angers,
France
The high symmetry emphasised by NMR experiments was
confirmed crystallographically.‡ Fig. 1a shows the structure of
[Re6Se8(TBP)4(OH)2]. The four TBP groups are bonded to the
dDepartment of Inorganic Chemistry, University of Bucharest, Dumbrava
Rosie 23, Bucharest, 020464, sector 2, Romania
˚
† Electronic supplementary information (ESI) available: Synthesis and
characterisation of compounds, CIF data for 1a,b and 2a, cyclic voltam-
mograms of 2a,b and 3a,b, and POM pictures of 3a,b. CCDC reference
numbers 695751–695753. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b822105g
cluster via N atoms (Re–N distances: 2.186(8) and 2.198(7) A
for (1a) and (1b), respectively) and are located in equatorial
position around the Re6 cluster. The aromatic groups as well as
the Re4 square of the Re6 cluster merge in a same plane. The two
Scheme 1
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compounds 3a,b after reaction with the corresponding cluster
precursor in the same conditions.† 2a,b and 3a,b were obtained
by washing the crude with appropriate solvent to eliminate the
excess of remaining acid. 1H NMR experiments in CDCl3 clearly
showed the presence of two carboxylate derivatives around the
cluster. A careful look at the aromatic region of the spectra
indicates the formation of less than 10% of the unexpected cis-
isomer that could be eliminated in the case of compounds 2a,b after
successive recrystallisations. Thus, single crystals suitable for X-ray
diffraction measurements could be obtained by slow gas diffusion
of diethyl ether in a concentrated solution of compound 2a in
CH2Cl2 and confirmed the trans-substitution of the remaining
hydroxyl groups (Fig. 1b). The carboxylate anions are linked via
˚
Re–O single bonds of 2.080 (5) A in length, a value very close to
the Re–O distance in 1a. Note that this compound co-crystalises
in a triclinic system with six molecules of CH2Cl2 and that the
same type of infinite cluster chains as previously found for 1a and
1b were observed.
Fig. 1 Representation of cluster units according to X-ray diffraction
analysis: (a) [Re6Q8(TBP)4(OH)2] in 1b and (b) [Re6Q8(TBP)4(L1)2] in 2b.
OH- groups are located on trans position (Re–O: 2.071(11) and
FT-IR spectroscopy was used to further characterise the coor-
dination of Li ligands onto the cluster core. Several changes on the
spectrum of the corresponding acids HLi have been observed upon
complexation wherein the most significant are the disappearance
of the broad and intense nO–H stretching band centred around
3000 cm-1 and the displacement of the nC stretching band from
˚
2.125(10) A for 1a and 1b, respectively).
1a and 1b contain large solvent accessible voids in which solvent
molecules are strongly disordered. As it was not possible to locate
them by X-ray analysis, a SQUEEZE procedure implemented
in the PLATON package was used to improve the structure
refinement of 1b. Within the solid, the molecules are linked to
each other via p-stacking interactions between pyridyl rings. Two
pyridyl rings of one unit interact with two other pyridyl rings of
the neighbouring cluster. Consequently, each cluster is surrounded
by two adjacent clusters to form infinite cluster chains along
the c axis of the unit cell. The distance between the centres of
two aromatic rings in interaction has been calculated by simple
=
O
1685 cm-1 for HL1 to 1614 cm-1 in 2a and 2b and from 1688 cm-1 for
HL2 to 1616 cm-1 for 3a and 3b. From the FAB-HRMS analysis of
3b,† the comparison of the theoretical isotopic distribution with
the observed experimental pattern confirms the coordination of
the L2 ligands onto the cluster core.
Cyclic voltammetry, performed on compounds 2a and 2b in
CH2Cl2, revealed one reversible oxidation step at E0¢ = 0.98 V vs.
SCE (DEp = 102 mV) for 2a and at E0¢ = 0.77 V vs. SCE (DEp =
79 mV) for 2b attributed to the 24e-/23e- oxidation process of the
cluster core.13,6,11,14 The same oxidation step is observed in the case
of 3a (E0¢ = 0.99 V vs. SCE; DEp = 88 mV) and 3b (E0¢ = 0.76 V
vs. SCE; DEp = 91 mV) showing that the clusters are in the same
environment and thus giving one more proof of the ligation of L2
ligands.
The UV-vis spectra of compounds 3a,b obtained in CH2Cl2 are
shown in Fig. 2 along with their respective luminescence spectrum.
They revealed a characteristic absorption signal of Re6 clusters
derivatives with high absorption in the UV region and a shoulder
˚
geometric consideration and is equal to 3.90 A for 1a and 1b. Our
attempt to functionalise K4[Re6Qi8(OH)6]·8H2O with other water
soluble pyridine derivatives like pyridine, 4-aminopyridine or
4-methylamino-pyridine or to exchange the TBP coordinated to
the cluster core in 1a,b with those same ligands in organic media
led to insoluble compounds. This can be explained by the lake
of bulky groups attached to the fourth position of the pyridine
moiety thus leading to the aggregation of the modified clusters
toward strong p-stacking interactions between pyridyl rings.
It has been shown previously in the Re6Qi8Xa 4-/PEt3 systems
6
that several halogen substitution levels can be obtained by varying
the reaction conditions.11 The separation of the different isomers
for one define X/PEt3 ratio relies either on solubility differ-
ences or chromatography techniques. In our case, the OH/TBP
substitution reaction occurs in water. After the reaction, the
tetrasubstituted compound lies on the surface of the colourless
aqueous phase and can be extracted with a chlorinated solvent.
Its solubility in organic media may be due to the absence of charge
on the unit. Note that the 1H NMR spectra of the crude product
revealed the formation of a small quantity of the cis isomer
(around 10%) that can be removed after two recrystallisations.
This straightforward procedure allows the easy access to a neutral
modified octahedral cluster that can be further functionalised on
the two remaining hydroxyl groups (Scheme 1).
Indeed, the reaction of 1a,b with an excess of commercially
available gallic acid derivative HL1 in chlorobenzene gives 2a,b
along with two molecules of water. HL2 carboxylic acid was
synthesised according to reported procedure12 and generates
Fig. 2 Electronic absorption (A) and normalised emission (B) (lexc
450 nm) spectra of 3a (dashed line) and 3b (plain line).
=
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around 425 nm ending at 550 nm. Luminescence experiments
(lexc = 450 nm) showed a large emission spectrum centred at
720 nm. A similar behaviour was observed for compounds 1a,b
and 2a,b. In the case of 3a and 3b, the two waxy solids were layered
between two glass slides and observed by optical microscopy under
either laser irradiation or cross-polarised light. Irradiation of the
samples at lexc = 405 nm induces a bright red luminescence in
both cases. Polarised optical microscopy (POM) is a powerful tool
to determine in a first attempt whether those compounds could
behave like mesomorphic material. In both cases, a promising
birefringent texture could be observed at 25 ◦C.† However,
unique, Rint = 0.068, R1 = 0.0565 (I > 2s(I)), wR2 = 0.1262 (all data),
GOF = 1.05 for 460 parameters.
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◦
both compounds decompose around 250 C before reaching the
clearing temperature.
We can therefore emphasise that neutral compounds 1a and 1b,
toward the directionality of their easy selective functionalisation,
constitute relevant neutral building blocks for the design of
new families of luminescent materials that may behave like
mesomorphic materials. Studies involving the grafting of other
type of mesogenic gallate-like derivatives on compounds 1a,b are
currently under way.
Acknowledgements
This work has been done in the frame of PECO-NEI no. 370
and ECO-NET 188452H programs. The authors thank ANR
(CLUSTSURF ANR-FI-071215–01-01). Y. Mironov thanks the
Russian Foundation for Basic Research and Fondation Langlios
(grant 08-03-00267).
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2005, 8, 1798.
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Notes and references
‡ Crystal data for [Re6S8(TBP)4(OH)2] (1a): M = 1948.51 g mol-1,
monoclinic, space group C2/m, a = 18.3921(5), b = 17.9940(6), c =
◦
3
˚
˚
9.1061(3) A, b = 105.9900(10) , V = 2897.04 (16) A , Z = 2, m(MoKa) =
12.803 mm-1, F(000) = 1784, T = 100(2) K, 18 854 total reflections, 3438
unique, Rint = 0.072, R1 = 0.0418 (I > 2s(I)), wR2 = 0.1035 (all data),
GOF = 1.154 for 125 parameters. Crystal data for [Re6Se8(TBP)4(OH)2]
12 L. Plasseraud, L. G. Cuervo, D. Guillon, G. Suess-Fink, R. De-
schenaux, D. W. Bruce and B. Donnio, J. Mater. Chem., 2002, 12,
2653.
(1b): M = 2296.5 g mol-1, monoclinic, space group◦C2/m, a = 18.3921(5),
13 E0¢ = (Epa + Epc)/2, where Epa and Epc are the anodic and cathodic
peak potentials, respectively. DEp = Epa - Epc.
3
˚
˚
b = 17.9940(6), c = 9.1061(3) A, b = 105.990(10) , V = 2897.04(16) A ,
Z = 2, m(MoKa) = 16.519 mm-1, F(000) = 2018, T = 100(2) K, 19 626
total reflections, 3691 unique, Rint = 0.071, R1 = 0.0381 (I > 2s(I)),
wR2 = 0.0867 (all data), GOF = 1.037 for 121 parameters. Crystal data
for [Re6S8(TBP)4(L1)2]·6CH2Cl2 (2a): M = 2846.43 g mol-1, triclinic, space
14 T. Yoshimura, K. Umakoshi, Y. Sasaki and A. G. Sykes, Inorg. Chem.,
1999, 38, 5557; T. Yoshimura, K. Umakoshi, Y. Sasaki, S. Ishizaka,
H.-B. Kim and N. Kitamura, Inorg. Chem., 2000, 39, 1765; A. Itasaka,
M. Abe, T. Yoshimura, K. Tsuge, M. Suzuki, T. Imamura and Y. Sasaki,
Angew. Chem., Int. Ed., 2002, 41, 463; L. F. Szczepura, M. K. Oh and
S. A. Knott, Chem. Commun., 2007, 4617.
¯
˚
group P1, a = 8.9903(5), b = 14.8782(7), c = 16.7235(8) A, a = 80.5000(2),
◦
3
˚
b = 109.340(2), g = 89.977(3) , V = 2163.48(19) A , Z = 1, m(MoKa) =
8.975 mm-1, F(000) = 1348, T = 100(2) K, 36 599 total reflections, 9829
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 1297–1299 | 1299
©