Novel organometallic building blocks for molecular crystal engineering

Article abstract of DOI:10.1021/om0300373

The synthesis and structural characterization of the ferrocenyl diboronic acid complex [Fe(η⁵-C₅H₄- B(OH)₂)₂] (1) and of its products of monosubstitution, [Fe(η⁵-C₅H₄-4-C₅H₄N) (η⁵-C₅H₄-B(OH)₂)] (2 in three polymorphic modifications, 2a-c), and of disubstitution, [Fe(η⁵-C₅H₄-4-C₅H₄ N)₂] (4), [Fe(η⁵-C₅H₄- C₆H₄-4-C₅H₄N)₂] (6), and [Fe(η⁵-C₅H₄-5-C₄H₃ N₂)₂] (7), are reported together with an investigation of the mode of supramolecular bonding in the solid state. The competition between the hydrogen-bonding interactions of the (B)O-H...O(B) and (B)O-H...N types in the cases of crystalline 1 and 2 has been investigated. The B(OH)₂ group provides two hydrogen bonding donor groups and two acceptors, forming mainly cyclic hydrogen-bonded systems in topological analogy with a primary amido group. Compounds 4, 6, and 7 are examples of neutral disubstituted pyridyl and pyrimidyl ferrocenyl complexes with potentials as supramolecular ligands. The compounds [Fe(η⁵-C₅ H₄-4-C₅H₄NH)(η⁵-C₅ H₄B(OH)₂)] [NO₃] (3a), [Fe(η⁵-C₅H₄-4-C₅H₄NH) (η⁵-C₅H₄-B(OH)₂)] [SO₄]·3H₂O (3b), and [Fe(η⁵-C₅H₄-4-C₅H₄ NH)₂][Cl]₂·4H₂O (5) have been obtained by treatment with acids of compounds 2 and 4, respectively. The interionic hydrogen bonds have also been investigated.

Full text of DOI:10.1021/om0300373

2142
Organometallics 2003, 22, 2142-2150
Novel Or ga n om eta llic Bu ild in g Block s for Molecu la r
Cr ysta l En gin eer in g. 2. Syn th esis a n d Ch a r a cter iza tion
of P yr id yl a n d P yr im id yl Der iva tives of Dibor on ic Acid ,
[F e(η5-C5H4-B(OH)2)2], a n d of P yr id yl Bor on ic Acid ,
[F e(η5-C5H4-4-C5H4N)(η5-C5H4-B(OH)2)]
Dario Braga,*,† Marco Polito, Marco Bracaccini, Daniela D’Addario,
Emilio Tagliavini,* and Luigina Sturba
Dipartimento di Chimica G. Ciamician, Universita` di Bologna, 40126 Bologna, Italy
Fabrizia Grepioni*
Dipartimento di Chimica, Universita` di Sassari, Via Vienna 2, 07100, Sassari, Italy
Received J anuary 16, 2003
The synthesis and structural characterization of the ferrocenyl diboronic acid complex
[Fe(η5-C5H4-B(OH)2)2] (1) and of its products of monosubstitution, [Fe(η5-C5H4-4-C5H4N)(η5-
C5H4-B(OH)2)] (2 in three polymorphic modifications, 2a -c), and of disubstitution, [Fe(η5-
C5H4-4-C5H4N)2] (4), [Fe(η5-C5H4-C6H4-4-C5H4N)2] (6), and [Fe(η5-C5H4-5-C4H3N2)2] (7), are
reported together with an investigation of the mode of supramolecular bonding in the solid
state. The competition between the hydrogen-bonding interactions of the (B)O-H‚‚‚O(B)
and (B)O-H‚‚‚N types in the cases of crystalline 1 and 2 has been investigated. The B(OH)2
group provides two hydrogen bonding donor groups and two acceptors, forming mainly cyclic
hydrogen-bonded systems in topological analogy with a primary amido group. Compounds
4, 6, and 7 are examples of neutral disubstituted pyridyl and pyrimidyl ferrocenyl complexes
with potentials as supramolecular ligands. The compounds [Fe(η5-C5H4-4-C5H4NH)(η5-C5H4-
B(OH)2)][NO3] (3a ), [Fe(η5-C5H4-4-C5H4NH)(η5-C5H4-B(OH)2)][SO4]‚3H2O (3b), and [Fe(η5-
C5H4-4-C5H4NH)2][Cl]2‚4H2O (5) have been obtained by treatment with acids of compounds
2 and 4, respectively. The interionic hydrogen bonds have also been investigated.
In tr od u ction
ionic interactions are responsible for cohesion and solid-
state properties of molecular crystals.3
The investigation of the bonds between molecules
involves all areas of chemistry, in particular the thriving
areas of supramolecular and materials chemistry.1 The
goal is that of reaching an intelligent control of the
recognition and assembly processes that lead from
molecular or ionic components to superstructures: hence,
from individual to collective (chemical and physical)
properties. All these ideas also apply to molecular
crystal engineering,2 the area of supramolecular chem-
istry devoted to the design and bottom-up construction
of functional crystalline materials. As noncovalent
interactions are responsible for the existence and func-
tioning of supermolecules, intermolecular and/or inter-
A major issue in modern crystal engineering is that
of exploiting the topological, electronic, and spin proper-
ties of metal atoms to construct molecular materials in
which metal centers are joined by ligands4 and can
communicate via coordination bonds or noncovalent
interactions, such as hydrogen bonds.3b We are contrib-
uting,5 together with others,6 to the efforts to open an
organometallic avenue to molecular crystal engineering.
Organometallic molecules and ions combine the su-
pramolecular bonding capacity of organic molecules with
(3) (a) Braga, D.; Grepioni, F. Acc. Chem. Res. 2000, 33, 601. (b)
Braga, D.; Grepioni, F. Coord. Chem. Rev. 1999, 183, 19. (c) Desiraju,
G. R., Ed. The Crystal as a Supramolecular Entity: Perspectives in
Supramolecular Chemistry; Wiley: Chichester, U.K., 1996; Vol. 2.
(4) (a) Eddaouddi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke, T.
M.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319. (b)
Batten, S. R.; CrystEngComm. 2001, 28. (c) Holman, K. T.; Pivovar,
A. M.; Swift, J . A.; Ward, M. D., Acc. Chem. Res. 2001, 34, 107. (d)
Yaghi, O. M.; Li, G.; Li, H., Nature 2000, 37, 703. (e) Noro, S.-I.;
Kitagawa, S.; Kondo, M.; Seki, K. Angew. Chem., Int. Ed. 2000,39,
2082. (f) Batten, S. B.; Robson, R., Angew. Chem., Int. Ed. 1998, 37,
1460. (g) Fujita, M., Acc. Chem. Res. 1999, 32, 53.
(5) (a) Braga, D.; Maini, L.; Polito, M.; Rossini, M.; Grepioni, F.
Chem. Eur. J . 2000, 6, 4227. (b) Braga, D.; Maini, L.; Grepioni, F.;
DeCian, A.; Felix, O.; Fisher, J .; Hosseini, M. W. New J . Chem. 2000,
24, 547. (c) Braga, D.; Grepioni, F. J . Chem. Soc., Dalton Trans. 1999,
1. (d) Braga, D.; Maini, L.; Prodi, L.; Caneschi, A.; Sessoli, R.; Grepioni,
F. Chem. Eur. J . 2000, 6, 1310. (e) Braga, D.; Maini, L.; Polito, M.;
Grepioni, F. Organometallics 1999, 18, 2577.
† E-mail: dbraga@ciam.unibo.it.
(1) (a) Lehn, J . M. Supramolecular Chemistry: Concepts and
Perspectives; VCH: Weinheim, Germany, 1995. (b) Philip, D.; Stoddard,
J . F., Angew. Chem., Int. Ed. Engl. 1996, 35, 1154. (c) Lawrence, D.
S.; J ang, T.; Levett, M., Chem. Rev. 1995, 95, 2229. (d) Haiduc, I.;
Edelmann, F. T. Supramolecular Organometallic Chemistry; Wiley-
VCH: Weinheim, Germany, 1999. (e) Steed, J . W.; Atwood, J . L.
Supramolecular Chemistry; Wiley: New York, 2000.
(2) (a) Braga, D., Grepioni, F., Orpen, A. G., Eds. Crystal Engineer-
ing: from Molecules and Crystals to Materials; Kluwer Academic:
Dordrecht, The Netherlands, 1999. (b) Hollingsworth, M. D. Science
2002, 295, 2410. (c) Desiraju, G. R. Crystal Engineering: The Design
of Organic Solids; Elsevier: Amsterdam, 1989. (d) Aakero¨y, C. B.;
Borovik, A. S. Coord. Chem. Rev. 1999, 183, 1.
10.1021/om0300373 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 04/11/2003

Molecular Crystal Engineering
Organometallics, Vol. 22, No. 10, 2003 2143
the presence of metal atoms, which bring about an
almost combinatorial mixture of valence, spin, and
charge states together with coordination geometries.
While many organic compounds utilized by the crystal
engineer are commercially available and can be used
directly in the supramolecular assembly (cocrystalliza-
tion, charge transfer, inclusion, host-guest, etc.) this
is not so with organometallic species, which need, most
often, to be synthesized on purpose. With this idea in
mind, we have begun to prepare, also in collaboration
with others, novel organometallic building blocks with
adequate supramolecular bonding functionalities for the
construction of desired architectures.7 We have focused
our strategy on the possibility of adding hydrogen-
bonding donor/acceptor groups, such as -COOH, -OH,
and -CHO, to robust sandwich complexes. The ratio-
nale for this choice is that the hydrogen bond is the
strongest of the noncovalent interactions and best
combines strength and directionality.8 Strength is a
synonym for cohesion and stability, while directionality
implies topological control and selectivity, which are
fundamental prerequisites for successful control of the
aggregation processes.
In previous studies, we have shown that carboxylic
-COOH, -OH, primary -CONH2, and secondary
-CONHR amido groups form essentially the same type
of hydrogen-bonding interactions whether as part of
organic molecules or as metal-coordinated ligands.9 This
is not surprising, as hydrogen bonds formed by such
strong donor and acceptor groups are at least 1 order
of magnitude stronger than most noncovalent interac-
tions. Dicarboxylic acid molecules, for example, offer the
possibility of supramolecular networking because of the
twin hydrogen bonding function. We have extensively
exploited this feature in the construction and/or utiliza-
tion of the sandwich acids [Fe(η5-C5H4COOH)2],10 [Co-
(η5-C5H4COOH)2]+,5a and [Cr(η6-C6H5COOH)2].7 The
cobalt cationic complex, in particular, has proved to be
extremely versatile for the selective trapping of alkali-
metal ions and for use in solid-gas reactions with
vapors of acids and bases.11
efforts toward a different supramolecular bonding func-
tionality, namely the boronic acid group -B(OH)2, and
investigate the intermolecular hydrogen-bonding pat-
terns formed by the diboronic acid ferrocenyl complex
[Fe(η5-C5H4-B(OH)2)2] (1), whose solid-state structure
was unknown, and by the three polymorphic modifica-
tions of the monosubstituted pyridyl derivative [Fe(η5-
C5H4-4-C5H4N)(η5-C5H4-B(OH)2)] (2). Second, we exploit
the boronic acid group -B(OH)2 as a chemical interme-
diate in the synthesis of disubstituted pyridyl and
pyrimidyl derivatives through a modified Suzuki cou-
pling protocol (used also to obtain 2; see below). The bis-
(pyridyl) derivative [Fe(η5-C5H4-4-C5H4N)2] (4),
the bis(phenylpyridyl) derivative [Fe(η5-C5H4-C6H4-4-
C5H4N)2] (6), and the bis(pyrimidyl) complex [Fe(η5-
C5H4-5-C4H3N2)2] (7), as well as the monosubstituted
species 2, have been synthesized and structurally
characterized. While the interest in compounds 1 and
2 is mainly in the hydrogen-bonding capacity of the
diboronic acid unit(s), compounds 4, 6, and 7 are
interesting as sophisticated supramolecular ligands
toward late-transition-metal atoms in the formation of
larger complexes or coordination networks. A prelimi-
nary study of compound 4 has shown that it can be
successfully used to prepare heterobimetallic metalla
macrocycles with metals such as Ag(I), Cu(II), and Zn-
(II).12 For the sake of completeness, the salts [Fe(η5-
C5H4-4-C5H4NH)(η5-C5H4-B(OH)2)][NO3] (3a ), [Fe(η5-
C5H4-4-C5H4NH)(η5-C5H4-B(OH)2)][SO4]‚3H2O (3b), and
[Fe(η5-C5H4-4-C5H4NH)2][Cl]2‚4H2O (5) have also been
prepared by treatment with acids of compounds 2 and
4, respectively.
It is worth recalling that boronic acids, which are
known to form covalently bonded complexes with diols,
have been attracting much attention and are currently
studied as a tool for carbohydrate sensors and as
potential substitutes for commercial enzyme-based
sensors;13a-d molecules containing R-B(OH)2 groups
attached to a ferrocene unit have also been designed
and synthesized and their properties analyzed with
techniques such as circular dichroism13e and electro-
chemistry.13f,g
The aim of this paper is essentially 2-fold. First, we
will expand our chemistry and crystal engineering
Chart 1 shows how the boronic acid group can form
hydrogen-bonding interactions similar to those observed
with primary amides and carboxylic acids.14a With
respect to the carboxylic ring, both amido and boronic
acid groups have the possibility for lateral bonds in
(6) (a) See, for a recent general entry in the area of inorganic crystal
engineering: J . Chem. Soc., Dalton Trans. 2000, 3705-3998 (the whole
issue). (b) For a recent review on the utilization of hydrogen bonds
between coordination compounds, see also: Beatty, A. M. CrystEng-
Comm 2001, 51. (c) Brammer, L.; Mareque Rivas, J . C.; Atencio, R.;
Fang, S.; Pigge, F. C. J . Chem. Soc., Dalton Trans. 2000, 3855. (d)
Zakaria, C. M.; Ferguson, G.; Lough, A. J .; Glidewell, C. Acta
Crystallogr., Sect. B 2002, 58, 786. (e) Elschenbroich, C.; Lu, F.; Harms,
K. Organometallics 2002, 21, 5152. (f) Elschenbroich, C.; Schiemann,
O.; Burghaus, O.; Harms, K. J . Am. Chem. Soc. 1997, 119, 7452.
(7) Braga, D.; Maini, L.; Grepioni, F.; Elschenbroich, C.; Paganelli,
F.; Schiemann, O. Organometallics 2001, 20, 1875.
(8) (a) Prins, L. J .; Reinhoudt, D. N.; Timmerman, P., Angew. Chem.,
Int. Ed. 2001, 40, 2382. (b) Aakero¨y, C. B.; Seddon, K. R. Chem. Soc.
Rev. 1993, 397. (c) Gordon, M. S.; J ensen, J . H., Acc. Chem. Res. 1996,
29, 536. (d) Braga, D.; Grepioni, F. Acc. Chem. Res. 1997, 30, 81. (e)
Brammer, L.; Zhao, D.; Ladipo, F. T.; Braddock-Wilking, J . Acta
Crystallogr., Sect. B 1995, B51, 632. (f) Novoa, J . J .; Nobeli, I.; Grepioni,
F.; Braga, D. New J . Chem. 2000, 24, 5.
(9) Braga, D.; Grepioni, F.; Desiraju, G. R. Chem. Rev. 1998, 98,
1375.
(10) (a) Takusagawa, F.; Koetzle, T. F. Acta Crystallogr. 1979, B35,
2888. (b) Palenik, G. J . Inorg. Chem. 1969, 8, 2744.
(11) (a) Braga, D.; Cojazzi, G.; Emiliani, D.; Maini, L.; Grepioni, F.
Chem. Commun. 2001, 2272. (b) Braga, D.; Cojazzi, G.; Emiliani, D.;
Maini, L.; Grepioni, F. Organometallics 2002, 21, 1315. (c) Braga, D.;
Maini, L.; Mazzotti, M.; Rubini, K.; Masic, A.; Gobetto, R.; Grepioni,
F., Chem. Commun. 2002, 2296.
addition to the formation of octaatomic rings (R2 (8) in
2
graph-set terminology14b,c). Moreover, the boronic acid
group may present the advantage, over groups contain-
ing CdO, of an expectedly smaller tendency to coordi-
(12) Braga, D.; Polito, M.; Bracaccini, M.; D’Addario, D.; Tagliavini,
E.; Proserpio, D. M.; Grepioni, F. Chem. Commun. 2002, 10, 1080.
(13) (a) Arimori, S.; Bell, M. L.; Oh, C. S.; Frimat, K. A.; J ames, T.
D. J . Chem. Soc., Perkin Trans. 1 2002, 803. (b) Ward, C. J .; Patel, P.;
J ames, T. D. J . Chem. Soc., Perkin Trans. 1 2002, 462. (c) DiCesare,
N.; Lakowicz, J . R. Tetrahedron Lett. 2002, 43, 2615. (d) Cary, D. R.;
Zaitseva, N. P.; Gray, K.; O’Day, K. E.; Darrow, C. B.; Lane, S. M.;
Peyser, T. A.; Satcher, J . H.; Van Antwerp, W. P.; Nelson, A. J .;
Reynolds, J . G. Inorg. Chem. 2002, 41, 1662. (e) Takaeuchi, M.; Mizuno,
T.; Shinkai, S.; Shirakami, S.; Itoh, T. Tetrahedron: Asymmetry 2000,
11, 3311. (f) Norrild, J . C.; Søtofte, I. J . Chem. Soc., Perkin Trans. 2
2002, 303. (g) Moore, A. N. J .; Wayner, D. D. M. Can. J . Chem. 1999,
77, 681.
(14) (a) Fournier, J .-H.; Maris, T.; Wuest, J . D.; Guo, W.; Galoppini,
E. J . Am. Chem. Soc. 2003, 125, 1002. (b) Etter, M. C. Acc. Chem. Res.
1990, 23, 120. (c) Bernstein, J .; Davis, R. E.; Shimoni, L.; Chang, N.-
L. Angew. Chem., Int. Ed. Engl. 1995, 34, 1555.

2144 Organometallics, Vol. 22, No. 10, 2003
Braga et al.
Sch em e 1. Syn th esis of Com p lexes 1, 2, a n d 4
Ch a r t 1. Com p a r ison betw een Hyd r ogen -Bon d in g
Rin g Motifs In volvin g Ca r boxylic (Left), Am id o
(Mid d le), a n d Bor on ic (Righ t) Gr ou p s
nate to metal centers. The presence of two boronic acid
groups in 1 and of the pyridyl ligand and of one boronic
acid group in 2 and the convolution of these features
with the conformational freedom around the coordina-
tion axis of the Cp ligands in both compounds may have
interesting consequences on the way these molecules
can interact in the solid state.
of 1 and 4-bromopyridine, in the presence of 4 mol %
(ddpf)PdCl2 catalyst, afforded the bis(pyridine) deriva-
tive 3 in 53% yield after chromatographic purification.
On the other hand, from the same reaction mixture
irradiated for a short period (20 min) in a microwave
oven, the monosubstituted product 2a was obtained in
57% yield. Similar reaction procedures were used to
prepare compounds 6 and 7 (see Experimental Section).
All compounds have been characterized by single-crystal
X-ray diffraction, and their structural features and
supramolecular bonding are discussed in the following.
Bor on ic Acid Com p lexes [F e(η5-C5H4-B(OH)2)2]
(1) a n d [F e(η5-C5H4-4-C5H4N)(η5-C5H4-B(OH)2)] (2)
a n d t h e Nit r a t e (3a ) a n d Su lfa t e (3b) Sa lt s of 2.
Relevant intermolecular hydrogen bonding parameters
for all compounds discussed herein are reported in Table
1. Compound 1 shows all the potentials of the boronic
acid group as a hydrogen-bonding linker. The principal
motif is represented in Figure 1a as chains of hydrogen-
bonded ferrocenyl moieties in a transoid conformation.
We were also intrigued by the possible competition
between interactions of (B)O-H‚‚‚N hydrogen bonds
and N‚‚‚B intermolecular bonds in compound 2, which
carries both the boronic acid group and the pyridyl
group. The two ligands have different acid-base prop-
erties: while the pyridine N atom is a Lewis base, the
boronic acid group behaves as a Brønsted acid because
of the polarized -OH groups. Thus compound 2 can
form, at least in principle, both intermolecular hydrogen
bridges in the solid state via the boronic acid groups
and intermolecular N- - -B coordination bonds with the
pyridyl Lewis base, as in H3B‚‚‚NH3.15 On the other
hand, the fact that compound 2 has been characterized
as two anhydrous polymorphic modifications, 2a and 2c,
as well as the hydrated form [Fe(η5-C5H4-4-C5H4N)(η5-
C5H4-B(OH)2)]‚0.5H2O (2b) will provide information on
the competition between (B)O-H‚‚‚N and (B)O-
H‚‚‚O(B) intermolecular interactions in the solid state.
The study of the relationship between polymorphic and
pseudo-polymorphic modifications often offers valuable
insight into the factors controlling supramolecular
bonding.16
Both boronic acid groups form R2 (8) rings (in graph set
2
terminology) with adjacent molecules along the chains,14
which are reminiscent of the chains formed by
the dicarboxylic acid sandwich complex [CoIII(C5H4-
COOH)2]+ 5a and by the two forms of [CrI(C6H5COOH)2]+.7
The chains criss-cross in the crystal structure (Figure
1b) and establish hydrogen-bonding cross links with
Resu lts a n d Discu ssion
Ferrocene-1,1′-diboronic acid (1) was prepared by a
reported procedure17a from 1,1′-dilithioferrocene and tri-
n-butyl borate, followed by hydrolysis of the resulting
dibutylboronate. The mono(pyridyl) boronic acid 2a and
the 1,1′-bis(4-pypridyl)ferrocene (4) were obtained from
1 through a modified Suzuki coupling protocol,17b,c as
depicted in Scheme 1. Prolonged heating of a mixture
lateral protons, forming 16-atom rings (R6 (16)); the
6
O‚‚‚O separation within the R2 (8) ring is shorter than
2
in the larger R6 (16) ring (2.81(1) vs 2.89(1) Å). It is also
6
noteworthy that the R2 (8) distance is longer than in
2
the dicarboxylic acids [FeII(C5H4COOH)2] (O‚‚‚O )
2.621(1), 2.643(1) Å and 2.593(4), 2.635(4) Å in the
triclinic10a and monoclinic10b forms, respectively), [CoIII-
(C5H4COOH)2]+ (O‚‚‚O ) 2.600(2) Å),5e [Cr(C6H5COOH)2]
(O‚‚‚O ranging from 2.558(2) to 2.643(2) Å) and [CrI(C6H5-
COOH)2]+ (O‚‚‚O ranging from 2.618(2) to 2.634(2) Å);7
this difference probably arises from the much stronger
nucleophilicity of the CdO system with respect to the
B-O(H) one. As a matter of fact, the O‚‚‚O separations
between neutral molecules, listed in Table 1, are closer
to the values of intermolecular distances in solid alco-
hols and water than to those usually observed between
-COOH groups.18
(15) (a) Leopold, K. B.; Canagartna, M.; Phillips, J . A. Acc. Chem.
Res. 1997, 30, 57. (b) Klooster, W. T.; Koetzle, T. F.; Siegbahn, P. E.
M.; Richardson, T. B.; Crabtree, R. H. J . Am. Chem. Soc. 1999, 121,
6337.
(16) (a) Bernstein, J .; Davey, R. J .; Henck, J .-O. Angew. Chem., Int.
Ed. 1999, 38, 3440. (b) Braga, D.; Grepioni, F. Chem. Soc. Rev. 2000,
4, 229. (c) Bladgen, N. R.; Davey, J . Chem. Br. 1999, 35, 44. (d)
Bernstein, J . Polymorphism in Molecular Crystals; Oxford University
Press: Oxford, U.K., 2002.
(17) (a) Knapp, R.; Rehahn, M. J . Organomet. Chem. 1993, 452, 235.
(b) Boy, P.; Combellans, C.; Thie`bault, A.; Amatore, C.; J utand, A.
Tetrahedron Lett. 1992, 33, 491. (c) Endtner, J . M.; Effenberger, F.;
Hartschuh, A.; Port, H. J . Am. Chem. Soc. 2000, 122, 3037.

Molecular Crystal Engineering
Organometallics, Vol. 22, No. 10, 2003 2145
several “supramolecular bonding options” for this mol-
ecule, since the B(OH)2 unit is capable of multiple
hydrogen-bonding donations as seen above with 1, and
the pyridyl ligand can donate an electron pair to a
suitable Lewis acid or accept hydrogen bond formation
from a suitable proton donor. Compound 2 has been
isolated in three forms, the two anhydrous forms 2a and
2c and the monohydrated pseudo-polymorphic form
2b.16
The family of crystals 2 provides some insight on the
“options” mentioned above:
(i) Intermolecular (B)O-H‚‚‚N interactions are present
in all crystals; hence, the pyridyl N atom favors hydrogen-
bonding acceptance from the O-H group over electron
donation toward the empty orbital on the B atoms.
(ii) Although all three crystals show the simultaneous
occurrence of both (B)OH‚‚‚O(B) and (B)OH‚‚‚N hydro-
gen bonds, the patterns are different. This is a conse-
quence of the high conformational freedom of complex
2 around the Fe atom coordination axis, which generates
different relative orientations of the hydrogen-bonding
groups.
In crystalline 2a the molecules form dimers via (B)-
OH‚‚‚N bonds (O‚‚‚N ) 2.70(1) Å), as shown in Figure
2a. These dimers are then linked in a secondary pattern
by the (B)OH‚‚‚O(B) lateral bonds (O‚‚‚O ) 2.97(1) Å).
This arrangement leads to eclipsing of the B(OH)2 group
over the C5H4N group. In 2b, on the other hand, the
primary motif appears to be the boronic acid ring based
on (B)OH‚‚‚O(B) bonds (O‚‚‚O ) 2.782(5) Å), which is
reminiscent of the carboxylic ring (see Chart 1), while
dimers are formed by the lateral O-H groups with the
pyridyl acceptors ((B)OH‚‚‚N bonds, O‚‚‚N 2.745(9) Å).
The conformation of the two ligands is cisoid. In
crystalline 2c a third, almost intermediate, topology is
observed (see Figure 2c), even though the molecule
adopts the same conformation as in 2b. In 2b there are
boronic rings based on (B)OH‚‚‚O(B) bonds (O‚‚‚O )
2.764(5) Å) and lateral (B)OH‚‚‚N bonds (O‚‚‚N )
2.790(5) and 2.844(5) Å) but there are also lateral (B)-
OH‚‚‚O(B) bonds as in 2a (O‚‚‚O ) 2.855(5) Å) leading
to the complex pattern shown in Figure 2c.
F igu r e 1. Two views of compound 1: (a, top) chains of
hydrogen-bonded ferrocenyl moieties in a transoid confor-
mation, forming R22(8) rings; (b, bottom) criss-crossing of
the chains in the crystal structure and establishment of
hydrogen-bonding cross-links with lateral protons, forming
larger 16-atom rings (R66(16)). HCH atoms are not shown
for clarity; HOH atoms are not observed (see the Experi-
mental Section).
Ta ble 1. Releva n t Hyd r ogen Bon d in g Dista n ces
(Å) for Com p ou n d s 1, 2a -c, 3a ,b, a n d 5
compd
O(H)‚‚‚X
N(H)‚‚‚X
1
O(1)‚‚‚O(2)
2.81(1)
2.89(1)
2.97(1)
2.70(1)
2.782(5)
2.745(9)
2.855(5)
2.764(5)
2.844(5)
2.790(5)
2.83(1)
2.75(1)
O(1)‚‚‚O(2)
2a
2b
2c
O(1)‚‚‚O(2)
O(2)‚‚‚N(1)
O(1)‚‚‚O(2)
O(1)‚‚‚N(1)
O(2)‚‚‚O(4)
O(4)‚‚‚O(3)
O(1)‚‚‚N(1)
These differences and analogies concur to indicate
that the (B)OH‚‚‚N and the (B)OH‚‚‚O(B) donor-accep-
tor systems have comparable strength and do not lead
to a strong topological preference. This characteristic,
coupled with the structural nonrigidity of the ferrocenyl
complexes, could justify the observation of different
crystal forms obtained in relatively similar crystalliza-
tion conditions (see the Experimental Section).
O(3)‚‚‚N(2)
3a
3b
O(1)‚‚‚O(4)nitrate
O(2)‚‚‚O(3)nitrate
O(1)‚‚‚O(6)sulfate
O(2)‚‚‚O(7)sulfate
O(3)‚‚‚O(9)water
O(4)‚‚‚O(11)water
O(9)‚‚‚O(8)water
N(1)‚‚‚O(4)nitrate 2.74(1)
2.725(4) N(1)‚‚‚O(10)water 2.800(6)
2.718(5) N(1)‚‚‚O(10)water 2.921(6)
2.730(5) N(2)‚‚‚O(8)sulfate 2.654(5)
2.717(5)
2.944(5)
O(10)water‚‚‚O(7)sulfate 2.837(5)
O(10)water‚‚‚O(8)sulfate 2.903(5)
O(11)water‚‚‚O(5)sulfate 2.757(5)
O(11)water‚‚‚O(6)sulfate 2.772(5)
Compound 2b is a hydrated form. The water mol-
ecules do not seem to play any particular role in the
packing; the distances from the neighboring atoms are
fairly long (shortest O‚‚‚Owater ) 3.13(1) Å) and indica-
tive of the fact that the water molecules are loosely held
in the packing. On the other hand, the thermogravi-
metric (TGA) treatment of 2b indicates that water can
be fully removed only when the temperature reaches
183 °C. Importantly, powder X-ray diffraction measure-
ments before and after dehydration show an almost
unchanged pattern, which indicates that the crystal 2b
5
O(1)water‚‚‚O(4)water
O(2)water‚‚‚O(3)water
O(3)water‚‚‚O(4)water
O(1)water‚‚‚Cl(1)
O(1)water‚‚‚Cl(2)
O(2)water‚‚‚Cl(1)
O(2)water‚‚‚Cl(2)
O(4)water‚‚‚Cl(2)
2.69(2)
2.72(2)
2.72(2)
3.08(2)
3.12(2)
3.05(2)
3.15(1)
3.13(2)
N(2)‚‚‚O(3)water
N(1)‚‚‚Cl(1)
2.69(2)
3.01(2)
Compound 2 carries both a B(OH)2 unit and a C5H4N
unit. In terms of molecular structure, the complex
represents the intermediate in the formation of the bis-
(pyridine) complex 4. The simultaneous presence of one
boronic acid group and one pyridyl group generates
(18) J effrey, G. A.; Saenger, W. Hydrogen Bonding in Biological
Structures; Springer-Verlag: Berlin, 1991.

2146 Organometallics, Vol. 22, No. 10, 2003
Braga et al.
F igu r e 2. (a, top) Hydrogen-bonded dimers formed by (B)-
OH‚‚‚N interactions in crystalline 2a . Note how the dimers
are connected via (B)OH‚‚‚O(B) interactions. The H atom
bound to O(1) was not observed (see the Experimental
Section). (b, middle) Hydrogen-bonding pattern in crystal-
line 2b. Note how the ligand conformation has changed
from eclipsed in 2a to cisoid in 2b. (c) Hydrogen-bonding
pattern in crystalline 2c. Note the similarity of the ligand
conformation to that in 2b and the presence of lateral (B)-
OH‚‚‚O(B) interactions as in 2a . HCH atoms are not shown
for clarity.
F igu r e 3. Hydrogen-bonding patterns in the nitrate 3a
(a, top) and sulfate 3b (b. middle) salts of [Fe(η5-C5H4-4-
C5H4NH)(η5-C5H4-B(OH)2)]+ and (c, bottom) projection in
the bc plane of the van der Waals ferrocenyl moiety “wall”
in 3b. HCH atoms not shown for clarity.
C5H4-4-C5H4NH)(η5-C5H4-B(OH)2)]2[SO4]‚3H2O (3b ).
Two independent hydrogen bonds are observed between
the B(OH)2 group and the [SO4]2- anion (O‚‚‚O ) 2.725-
(4) and 2.718(5) Å) (see Figure 3c). The shortest, charge-
assisted intermolecular H bond is between the +N-H
group and the [SO4]2- anion (N‚‚‚O ) 2.654(5) Å). The
three molecules interact via hydrogen bonding with the
boronic acid, the +N-H group, and the sulfate anion
(see Table 1). As can be appreciated from Figure 3b, the
hydrogen-bonding functions are all segregated between
two “walls” of ferrocenyl moieties, which are in van der
Waals contact with adjacent rows along the direction
of the c axis. A projection of the van der Waals “walls”
in the bc plane is shown in Figure 3c.
retains the hydrogen-bonded network structure upon
water removal.
Protonation of 2 with HNO3 leads to formation of the
cationic salt [Fe(η5-C5H4-4-C5H4NH)(η5-C5H4-B(OH)2)]-
[NO3] (3a ). Three independent hydrogen bonds are
observed, all involving the [NO3]- anion: two O‚‚‚O
hydrogen bonds between the boronic acid group and O
atoms of the anion (2.83(1), 2.75(1) Å) and the charge-
assisted +N-H‚‚‚O- interaction (N‚‚‚O ) 2.741(1) Å).
No link between the cations is present; thus, the
preference appears to go to charge-assisted interactions
between ions of opposite charge. The hydrogen-bonding
interactions are shown in Figure 3a. Protonation with
H2SO4 leads to formation of the hydrated salt [Fe(η5-
Disu bstitu ted Der ivatives [Fe(η5-C5H4-4-C5H4N)2]
(4), [F e(η5-C5H4-C6H4-4-C5H4N)2] (6), a n d [F e(η5-
C5H4-5-C4H3N2)2] (7). Compound 4 is the first of a
series of bipyridine-type ferrocenyl molecules and has

Molecular Crystal Engineering
Organometallics, Vol. 22, No. 10, 2003 2147
via a network of O(H)water‚‚‚Owater and O(H)water‚‚‚Cl-
hydrogen bonding interactions (Owater‚‚‚Owater and
Owater‚‚‚Cl distances in the ranges 2.69(2)-2.72(2) and
3.05(2)-3.15(1) Å, respectively).
The synthetic route that leads to 4 has been utilized
to generate the bis(phenylpyridyl) complex 6. In contrast
to 4, which adopts an eclipsed conformation in the solid
state, compound 6 adopts a fully transoid structure (see
Figure 6). The C5H4-C6H4-4-C5H4N system is not flat,
but twisted: the phenyl ring and the pyridyl rings form
angles of ca. 14.1 and 5.3° with the Cp ligand, respec-
tively. The molecules form ring dimers in their crystals
via C-H‚‚‚N interactions (C‚‚‚N distance 3.457(5) Å),
and the dimers are arranged in infinite chains extending
along the b axis direction (see Figure 6).
The eclipsed conformation is found again in the
ferrocene pyrimidyl complex 7 (see Figure 7a). The
pyrimidyl group forms a twist angle of ca. 28.7° with
the Cp ligand. The crystals of 7 are affected by an
interesting type of structural disorder with the molec-
ular piles shifted in the crystals by a pitch of “half a
molecule” along the a axis. This is due to the fact that
the inter-ring separation is almost the same whether
the ligands are in van der Waals contacts or are bound
via the metal atoms, so that the average crystal
structure shows metal atoms with 50% occupancy
between every Cp ring along the molecular piles. It is
noteworthy that one of the hydrogen atoms of the
pyrimidyl ligands points toward the iron metal center
of the adjacent pile (Fe‚‚‚H ) 3.20 Å for a normalized
H atom position), as can be observed in Figure 7b.
F igu r e 4. CH‚‚‚π interaction ((C)H‚‚‚π ) 2.66 Å, C-H‚‚‚
π ) 146.3°) observed in crystalline 4. The broken line
represents the distance from the H atom to the plane of
the aromatic ring on the adjacent molecule.
Con clu sion s
F igu r e 5. Formation of hydrogen-bonding interactions of
the +N-H‚‚‚Owater and of the +N-H‚‚‚Cl- type by the N-H
donors on each complex in crystalline 5. The cations are
arranged in piles and are connected to cations on adjacent
piles via a network of O(H)water‚‚‚Owater and O(H)water‚‚‚Cl-
hydrogen-bonding interactions.
The main scope of the present study has been that of
synthesizing and characterizing a series of new orga-
nometallic molecules that could add to the storehouse
of building blocks available to the “organometallic
crystal engineer”. A second objective has been that of
exploring less common hydrogen-bonding functionalities
that could provide alternatives, in terms of robustness
and transferability, to the strong hydrogen-bonding
groups usually employed to link organometallic building
blocks in molecular crystals.
Polycarboxylic acids are being widely utilized for the
bottom-up preparation of supramolecular aggregates
and crystalline materials based on molecular or ionic
building blocks.20 The -COOH group is, in fact, capable
of forming strong intermolecular hydrogen bonds and
the presence of several -COOH groups with different
orientations in space allows construction of one-, two-,
and three-dimensional aggregates. This also applies to
organometallic acids, although they are much less
common than polycarboxylic organic acids, which often
are commercially available. The -COOH functional
group, however, can easily give coordination to metal
centers when employed together with coordination
complexes. In many instances, there is a competition
between metal atoms accepting electron donation from
been used already to synthesize heterobimetallic met-
alla macrocycles.12 In the absence of stronger hydrogen-
bonding interactions, crystal cohesion is guaranteed by
van der Waals interactions and by CH‚‚‚π interactions
((C)H‚‚‚π ) 2.66 Å, C-H‚‚‚π ) 146.3°) (see Figure 4).
To evaluate CH‚‚‚π interactions, C-H distances were
normalized to the neutron derived value of 1.08 Å; the
distance reported is taken from the H atom to the plane
of the aromatic ring.19 It is noteworthy that, although
the transoid structure ought to be preferred for steric
reasons, the complex adopts an eclipsed conformation
in the solid state, very likely as a consequence of packing
optimization.
When 4 is treated with HCl, the hydrated dichloride
salt [Fe(η5-C5H4-4-C5H4NH)2][Cl]2‚4H2O (5) is obtained.
The two N-H donors on each complex form two
types of hydrogen-bonding interactions, namely an
+N-H‚‚‚O bond with water (N‚‚‚O ) 2.69(2) Å) and
+N-H‚‚‚Cl- bond with the chloride anion (N‚‚‚Cl ) 3.01-
(2) Å), as shown in Figure 5. The system crystallizes in
the chiral space group P212121, with the dications
arranged in piles along the a axis. The organometallic
cations belonging to adjacent piles are all interconnected
(20) (a) Braga, D.; Grepioni, F. J . Chem. Soc., Dalton Trans. 1999,
1. (b) Subramanian, S.; Zaworotko, M. J . Coord. Chem. Rev. 1994, 137,
357. (c) Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311.
(d) Aakero¨y, C. B.; Seddon, K. R. Chem. Soc. Rev. 1993, 397. (e)
Aakero¨y, C. B.; Beatty, A. M.; Helfrich, B. A. J . Am. Chem. Soc. 2002,
124, 14425.
(19) Nishio, M.; Hirota, M.; Umezawa, Y., The CH/ π Interaction:
Evidence, Nature, and Consequences; Wiley-VCH: New York, 1998.

2148 Organometallics, Vol. 22, No. 10, 2003
Braga et al.
F igu r e 6. Transoid conformation of the two ligands in crystalline 6, allowing formation of infinite chains of ring dimers,
linked via C-H‚‚‚N interactions.
Compound 2 provides also insight into the competition
between the pyridyl nitrogen and the boronic oxygen
as hydrogen-bonding acceptors. The three crystals differ
in the patterns of hydrogen-bonding intermolecular
interactions. The comparison of the hydrogen bond
networks in the three forms of compound 2 indicates
that the (B)OH‚‚‚N and the (B)OH‚‚‚O(B) donor-accep-
tor systems have comparable strength and do not lead
to strong topological preferences.
In terms of chemical behavior, we have shown that
one or both boronic acid groups in 1 can be substituted
for pyridyl and other groups, opening up the route to
the preparation of a plethora of novel mono- and
bidentate ligands. The pyridyl boronic complex 2 is
potentially useful for the simultaneous use as a ligand
toward a metal center and as an intermolecular linker
via the remaining B(OH)2 unit. Preliminary results have
been extremely encouraging and will be the subject of
future reports.
Disubstitution of the -B(OH)2 unit from 1 has al-
lowed preparation of the bis(pyridyl), bis(phenylpyridyl),
and bis(pyrimidyl) derivatives 4, 6, and 7. Thanks to
the possibility of metal coordination via the N atom lone
pairs, these molecules can be exploited as sophisticated
supramolecular ligands toward late-transition-metal
atoms in the formation of larger complexes or of
coordination networks. While we have not yet succeeded
in the preparation of coordination networks based on
4, 6, and 7, we have been able to prepare heterobime-
tallic metalla-macrocycles adducts of compound 4 with
AgI, CuII, and ZnII, confirming the potentials of the
system.12
F igu r e 7. (a, top) Eclipsed conformation of the ferrocene
pyrimidyl complex 7. (b, bottom) Molecular piles along the
a axis. Only one of the two disordered positions of iron is
shown. Note how one of the pyrimidyl hydrogen atoms
points toward an iron metal center on the adjacent pile.
the lone pairs on the O atoms and the capacity of
forming intermolecular hydrogen-bonding interactions.
This competition may prevent formation of supramo-
lecular metal-containing networks based on hydrogen-
bonding linkers.
In this study we have explored the hydrogen-bonding
capacity of a less conventional acid, namely the boronic
acid group -B(OH)2. In terms of Brønsted acidity the
B(OH)2 group is weaker than the -COOH group, in
view of the reduced polarization of the O-H bond,
because the O atom is bound to the electron-deficient
boron atom rather than to an electron-rich carbon atom.
In terms of nucleophilicity, on the other hand, the O
atom of the -O-H groups is much weaker than the
-CdO system. Altogether the B(OH)2‚‚‚B(OH)2 interac-
tion is weaker than the OH‚‚‚O interaction between two
-COOH groups. This is also reflected in the O‚‚‚O
separations. On the basis of this parameter one could
say that the interaction recalls the OH‚‚‚O bond ob-
served in alcohols.18 On the other hand, the study of
compounds 1 and 2 shows that the possibility of “lateral”
hydrogen bonds, afforded by the additional -OH units,
can be exploited to extend the hydrogen bond network
by linking hydrogen-bonded chains together, as in
primary amides.
Exp er im en ta l Section
Gen er a l In for m a tion . All the starting materials were
purchased from Aldrich and used as such without further
purification. The starting material ferrocene-1,1′-diboronic
acid, [Fe(η5-C5H4-B(OH)2)2] (1), was prepared according to the
literature procedure.17 Compound 1 (10 mg, 0.036 mmol) was
dissolved in water-acetone (1:1) (7 mL). Suitable single
crystals for X-ray studies were obtained by slow evaporation
of the solution at room temperature.
Syn th esis of [Fe(η5-C5H4-4-C5H4N)(η5-C5H4-B(OH)2)] (2a).
An aqueous solution of Na2CO3 solution (1 M, 2.2 mL) was
added to a solution of 4-bromopyridium hydrochloride (0.088
g, 0.45 mmol) in dioxane (6.5 mL) under Ar, to generate
4-bromopyridine; the suspension was heated to 50 °C. The
catalyst PdCl2[1,1′-bis(diphenylphosphino)ferrocene] (0.012 g,
0.014 mmol) was added, followed by a mixture of boronic acid
(0.123 g, 0.45 mmol) and NaOH (3 M, 0.24 mL) in DME (3
mL). The reaction was refluxed for 20 min in a conventional
microwave (power 650 kW). After hydrolysis with ice-water

Molecular Crystal Engineering
Organometallics, Vol. 22, No. 10, 2003 2149
the mixture was extracted with ethyl acetate (3 × 10 mL). The
organic layer was washed with an NH4Cl solution followed by
water, dried on Na2SO4, and concentrated. The residue was
chromatographed on silica gel with CH2Cl2/EtOH (95:5) to give
0.083 g of the product as a bright red solid. The solid was
dissolved in acetone and filtered; the solution was evaporated
in a rotary evaporator, yielding 0.079 g of pure product (yield
57%). Yellow single crystals of 2a were obtained by slow
evaporation at +4 °C of a solution obtained by dissolution of
11 mg of compound 2 in 12 mL of a CH2Cl2-CH3OH (9:1)
mixture. Anal. Calcd for 2a : C, 58.70; H, 4.60; N, 4.56.
Found: C, 58.51; H, 4.46; N, 4.57.
Syn th esis of [F e(η5-C5H4-4-C5H4N)(η5-C5H4-B(OH)2)]‚
0.5H2O (2b). Single crystals of 2b were obtained by slow
evaporation at room temperature of a solution obtained by
dissolution of 13 mg of compound 2a in acetone (5 mL) followed
by addition of H2O (2 mL). Anal. Calcd for 2b: C, 57.02; H,
4.79; N, 4.43. Found: C, 57.18; H, 4.77; N, 4.44.
Syn th esis of [Fe(η5-C5H4-4-C5H4N)(η5-C5H4-B(OH)2)] (2c).
Single crystals of 2c were obtained by slow evaporation at 4
°C of a solution obtained by dissolution of 17 mg of compound
2a in acetone (7 mL) followed by addition of 2 mL of an
aqueous 0.1 M solution of NH3. Anal. Calcd for 2c: C, 58.70;
H, 4.60; N, 4.56. Found: C, 58.65; H, 4.47; N, 4.56.
Syn th esis of [F e(η5-C5H4-4-C5H4NH)(η5-C5H4-B(OH)2)]-
[NO3] (3a ). Single crystals of 3a were obtained by evaporation
of an aqueous solution of 2a (10 mg, 0.0325 mmol) in HNO3
(0.01 M, 3.5 mL). Anal. Calcd for 3a : C, 48.70; H, 4.09; N,
7.57. Found: C, 48.57; H, 4.08; N, 7.55.
Syn th esis of [F e(η5-C5H4-4-C5H4NH)(η5-C5H4-B(OH)2)]-
[SO4]‚3H2O (3b). Single crystals of 3b were obtained by
evaporation of an aqueous solution of 2a (10 mg, 0.0325 mmol)
in H2SO4 (0.01 M, 3.5 mL). Anal. Calcd for 3b: C, 47.04; H,
4.74; N, 3.66. Found: C, 47.20; H, 4.75; N, 3.65.
Syn t h esis of [F e(η5-C5H 4-4-C5H 4NH )2][Cl]2‚4H 2O (5).
Single crystals of 5 were obtained by slow evaporation at 25
°C of a solution of 4 (10 mg; 0.0294 mmol, prepared according
to the literature procedure12) in HCl (0.01 M, 5.8 mL). Anal.
Calcd for 5: C, 49.72; H, 5.01; N, 5.80. Found: C, 49.61; H,
5.00; N, 5.81.
Syn th esis of [F e(η5-C5H4-C6H4-4-C5H4N)2] (6). To a solu-
tion of 4-(p-bromophenyl)pyridine21 (0.213 g, 91 mmol) in
dioxane (6.5 mL) under Ar was added 1 M aqueous Na2CO3
(2.73 mL); the catalyst PdCl2[1,1′-bis(diphenylphosphino)-
ferrocene] (0.006 g, 0.007 mmol) was added, followed by a
mixture of boronic acid (0.100 g, 0.36 mmol) and 3 M aqueous
NaOH (0.24 mL) in DME (15 mL). The reaction was refluxed
for 3 days. After hydrolysis with ice-water, the mixture was
extracted with CH2Cl2 (3 × 10 mL). The organic layer was
washed with NH4Cl solution followed by water and dried on
Na2SO4 and concentrated. The residue was chromatographed
on silica gel with CH2Cl2-EtOH (95:5) to give 0.082 g of the
product as a bright red solid (yield 46%). Single crystals of 6
were obtained by slow evaporation at 4 °C of a solution
obtained by dissolution of 11 mg of compound 6 in 6 mL of
CH2Cl2. Anal. Calcd for 6: C, 78.06; H, 4.91; N, 5.69. Found:
C, 77.88; H, 4.89; N, 5.68.
Syn th esis of [F e(η5-C5H4-5-C4H3N2)2] (7). To a solution
of 5-bromopyrimidine (0.141 g 0.91 mmol) in DME (5 mL)
under Ar was added the catalyst PdCl2[1,1′-bis(diphenylphos-
phino)ferrocene] (0.006 g, 0.007 mmol), followed by a mixture
of boronic acid (0.100 g 0.36 mmol) and 3 M aqueous NaOH
(0.24 mL) in DME (20 mL). The reaction mixture was refluxed
for 2 days. After hydrolysis with ice-water, the mixture was
extracted with CH2Cl2 (3 × 13 mL). The organic layer was
washed with NH4Cl solution followed by water and dried on
Na2SO4 and concentrated. The residue was chromatographed
on silica gel with CH2Cl2-EtOH (95:5) to give 0.071 g of the
(21) Boy, P.; Combellas, C.; Thie´bault, A. Tetrahedron Lett. 1992,
33(4), 491.

2150 Organometallics, Vol. 22, No. 10, 2003
Braga et al.
product as a bright red solid (yield 61%). Single crystals of 7
were prepared by slow evaporation at 4 °C of a benzene
solution of the compound. Anal. Calcd for 7: C, 63.18; H, 4.12;
N, 16.37. Found: C, 63.30; H, 4.13; N, 16.32.
graphical representations. For the evaluation of hydrogen-
bonding parameters the program PLATON was used.22c In all
cases, correspondence between the structures determined by
single-crystal X-ray diffraction and those of the bulk materials
precipitated from solution was confirmed by comparing the
experimental powder diffractograms obtained from the bulk
material with those calculated on the basis of the single-crystal
structures.
Cr ysta llogr a p h y. Crystal data and details of measure-
ments for compounds 1, 2a -c, 3a ,b, and 5-7 are reported in
Table 2. The structure of 4 has been described previously12
(CCDC-177938). Common to all compounds were the follow-
ing: X-ray data collected at room temperature on a Nonius
CAD-4 diffractometer, graphite monochromator (Mo KR radia-
tion, λ ) 0.710 73 Å), ψ-scan absorption correction. All non-H
atoms were refined anisotropically. All HOH and HNH atoms
in 2b,c, 3a ,b, and 5-7 and one HOH atom in 2a were directly
located from Fourier maps and refined; the second HOH atom
in 2a and the two HOH atoms in 1 were not observed. H atoms
bound to C atoms were added in calculated positions. In the
case of 5, which crystallizes in the chiral space group P212121,
the absolute structure was attributed to the lowest Flack
parameter (0.34(19) vs 0.64(11)). The computer program
SHELXL9722a was used for structure solution and refinement.
The computer program SCHAKAL9922b was used for all
Ack n ow led gm en t. We thank the MIUR (projects
Solid Supermolecules 2000-2001 and Supramolecular
Devices, 2001-2003), the University of Bologna (project
Innovative Materials), and the University of Sassari
(F.G.) for financial support.
Su p p or tin g In for m a tion Ava ila ble: Details about the
X-ray crystal structures, including tables of crystal data and
structure refinement parameters, atomic coordinates, bond
lengths and angles, and anisotropic displacement parameters,
TGA for crystalline 2b, comparison of X-ray powder patterns
for crystalline 2b before and after TGA, and ORTEP figures
for all compounds described herein. This material is available
free of charge via the Internet at http://pubs.acs.org.
(22) (a) Sheldrick, G. M. SHELX-97, Program for Crystal Structure
Determination; University of Go¨ttingen, Go¨ttingen, Germany, 1997.
(b) Keller, E., SCHAKAL99 Graphical Representation of Molecular
Models; University of Freiburg, Freiburg, Germany, 1999. (c) Spek,
A. L. PLATON. Acta Crystallogr. 1990, A46, C31.
OM0300373