Mechanochemical assembly of hydrogen bonded organic-organometallic solid compounds

Article abstract of DOI:10.1039/b209861j

Solvent-free reactions with molecular systems have been exploited to prepare hybrid organic-organometallic solids: grinding of the complex [Fe(η⁵-C₅H₄COOH)₂] with solid bases B generates quantitatively the corre

Full text of DOI:10.1039/b209861j

Mechanochemical assembly of hydrogen bonded organic-organometallic
solid compounds
Dario Braga,*a Lucia Maini,a Marco Polito,a Laurent Miroloa and Fabrizia Grepioni*b
a Dipartimento di Chimica G. Ciamician, Università di Bologna, Via Selmi 2, 40126 Bologna, Italy.
E-mail: dbraga@ciam.unibo.it
b Dipartimento di Chimica, Università di Sassari, Via Vienna 2, 07100 Sassari, Italy.
E-mail: grepioni@ssmain.uniss.it
Received (in Cambridge, UK) 9th October 2002, Accepted 5th November 2002
First published as an Advance Article on the web 18th November 2002
Solvent-free reactions with molecular systems have been
exploited to prepare hybrid organic–organometallic solids:
grinding of the complex [Fe(h5-C5H4COOH)2] with solid
bases B generates quantitatively the corresponding hydro-
gen bonded salts [Fe(h5-C5H4COOH)(h5-C5H4COO)][HB]
(B = 1,4-diazabicyclo[2.2.2]octane, 1,4-phenylenediamine);
gas–solid reactions are also possible with volatile bases.
As for the reverse process, thermogravimetric measurements
indicate that, on heating at ca. 235 °C, solid 3 loses 1 mole of 2.
No decomposition is observed and the nature of the final
product, i.e. 1, has again been confirmed by powder diffraction.
Co-grinding of the two solids in stoichiometric ratios other than
1+1 (e.g. 1+2 and 2+1) leads to formation of solid mixtures with
diffraction peaks corresponding to the unreacted excess reagent
in addition to the peaks of 3. It is also worth noting that
The continuing expansion of the supramolecular perception of
chemical reactivity brings about the possibility of exploring less
conventional synthetic procedures for the preparation of
molecular aggregates.1 This applies also to the fields of
molecular crystal engineering2 and solid state chemistry.3
Mechanochemistry is an established branch of chemical
sciences whereby reactants are mixed together in the solid state
without the intermediacy of solvents.4 However, mechanically
activated reactions have been exploited mainly with inorganic
solids, while less has been done with molecular (and mainly
organic) systems.5 Solvent free molecular reactions are attrac-
tive under both the environmental and topochemical view-
points,6 often offering alternative preparative routes to novel
chemicals and materials.7,8
Fig. 1 Reactivity scheme. (i) Mechanochemical treatment of solid 1 and 2
to give 3; (ii) uptake of 2 vapour by finely ground 1; (iii) removal of 2 from
solid 3 by thermal treatment. (a) The hydrogen bonded dimer in monoclinic
1; (b) the hydrogen bonded chain in solid 3. HCH atoms omitted for
clarity.
We have begun to utilize organometallic building blocks to
prepare novel molecular crystalline materials. Interesting
results are being obtained, in particular in the cases of solid–gas
reactions.9
In this communication we describe the preparation of novel
mixed organic–organometallic compounds by co-grinding of
solid reactants and the full structural characterization of the
products from single crystals obtained via seeding.
5
Moderate manual grinding of solid [Fe(h -C5H4COOH)2], 1,
monoclinic form,10 together with solid 1,4-diazabicy-
clo[2.2.2]octane (C6H12N2, DABCO), 2, and 1,4-phenyl-
enediamine, 4, in 1+1 ratio, leads to quantitative formation of
5
5
the corresponding compounds [Fe(h -C5H4COOH)(h -
5
5
C5H4COO)][HC6H12N2], 3, and [Fe(h -C5H4COOH)(h -
C5H4COO)][C6H4(NH2)(NH3)], 5, respectively.† No formation
of intermediate liquid phases7 has been observed.
Interestingly, 3 can also be obtained from the (hetero-phase)
uptake of vapour of 2 by solid 1, although the kinetics is
expectedly much slower than in the case of the co-grinding.†
The reaction is reversible since 1 can be fully regenerated if 2 is
sublimed off solid 3 by thermal treatment at ca. 235 °C. The
overall process is depicted in Fig. 1.
The problem of the structural characterization of powder
materials yielded by mechanochemical methods11a can be
circumvented by using a portion of the polycrystalline material
to seed11b from suitable solvents the growth of single crystals
for X-ray diffraction.‡ Comparison of the powder diffraction
patterns calculated on the basis of the single crystal structure
with that obtained from the ground polycrystalline material
allows us to determine whether the products have the same
chemical and structural composition.§ Fig. 2 shows that this is
the case for the reaction between monoclinic 110 and 2. Even
though the crystallinity of 3 obtained via grinding is not high, all
significant peaks can be easily recognised, while peaks
attributable to the starting materials 1 and 2 are absent.
Fig. 2 From top to bottom: comparison of X-ray powder diffractograms for
3 (calculated and experimental), 2 (experimental) and 1 (experimental).
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20 mg (72 mmol) of 1 and 8.1 mg (72 mmol) of 2 were manually ground in
an agate mortar for 5 min. Mechanical preparation of 5: 20 mg (72 mmol)
of 1 and 7.8 mg (72 mmol) of 4 were manually ground in an agate mortar
for 5 min. More prolonged grinding did not result in any appreciable change
in the diffraction patterns. For the reaction in hetero-phase 20 mg of 1 were
exposed at room temperature to vapours of 2 obtained by producing a mild
vacuum (water pump, ca. 30–40 mmHg) in the reaction apparatus; after 30
days the diffraction pattern showed peaks of the adduct together with
residual peaks due to the starting diacid.
‡ Single crystals of 3 and 5 were obtained by recrystallization in methanol
of the powders resulting from the grinding processes. X-Ray data collected
at 293 K on a Nonius-CAD4 diffractometer; MoKa radiation (l = 0.71073
¯
Å). Crystal data for 3: triclinic, P1, a = 7.452(2), b = 10.804(2), c =
11.319(2) Å, a = 104.83(2), b = 98.96(2), g = 105.33(2)°, V = 824.9(3)
Å3, Z = 2, 2894 independent reflections (3052 measured), wR2 = 0.1084,
¯
R1 = 0.0476. For 5: triclinic, P1, a = 7.228(7), b = 10.216(4), c =
11.803(2) Å, a = 103.74(3), b = 105.57(2), g = 96.88 (2)°, V = 799.7(9)
Å3, Z = 2, 4646 independent reflections (4852 measured), wR2 = 0.1794,
R1 = 0.0593. SHELX9713a and SCHAKAL9913b were used for structure
solution and graphical representations. CCDC 191507 and 191508. See
http://www.rsc.org/suppdata/cc/b2/b209861j/ for crystallographic data in
CIF format.
§ Powder data for all samples were collected on a Philips PW-1100
automated diffractometer with Cu-Ka radiation, graphite monochromator,
using quartz sample holders. For the pure reagents 25 mg of substance were
employed. The program PowderCell13c was used for calculation of X-ray
powder patterns.
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Fig. 3 Comparison of calculated (top) and measured (bottom) X-ray powder
diffractograms for 5, together with a representation of the crystal packing;
black spheres represent the C6H4(NH2) moiety bound to the –NH3 unit.
+
HCH atoms omitted for clarity.
4 J. F. Fernandez-Bertran, Pure Appl. Chem., 1999, 71, 581; V. V.
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mechanical grinding, vapour uptake and crystallization from
solution all lead to formation of the same product.
Analogously to 3, grinding of 1 with 1,4-phenylenediamine,
5
4, in 1+1 ratio leads to quantitative formation of [Fe(h -
5
C5H4COOH)(h -C5H4COO)][C6H4(NH2)(NH3)], 5.† As in the
case of 3, recrystallization from solution of the ground sample
allowed us to grow single crystals‡ of the same species obtained
via the solid–solid process (Fig. 3).
In summary we have shown that solvent-free mechanochem-
ical reactions with molecular systems can be exploited to
prepare new hybrid organic–organometallic materials. Solid–
solid reactions between molecular solids can be regarded as
supramolecular reactions between periodical (solid) super-
molecules.12 In the reaction between molecular solid reactants
to form a new molecular solid product the covalent bonding is
not affected while non-covalent van der Waals or hydrogen
bonding interactions are broken and formed.
The mechanochemical reaction generating 3 and 5 is quite
general. Reaction products have also been observed, for
instance, by co-grinding 1 with solid guanidine carbonate and
piperazine. These compounds will be described in a future
report together with the optimisation of the experimental
conditions for all processes described herein.
We thank MIUR (projects Supramolecular Devices and Solid
Supermolecules), the Universities of Bologna (project In-
novative Materials) and Sassari for financial support.
Notes and references
† 1,4-Diazabicyclo[2.2.2]octane, 2, 1,4-phenylenediamine, 4, and [Fe(h -
C5H4COOH)2] were purchased from Aldrich. Mechanical preparation of 3:
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