The effect of counter-ions on the supramolecular arrangement of (benzonitrile)[1,2-bis(diphenylphosphino)ethane](η5- cyclopentadienyl)iron(II) cations

Article abstract of DOI:10.1107/S0108270106038662

The title compound, [1,2-bis(diphenylphosphino)ethane]-(η⁵- cyclopentadienyl)(4-nitrobenzonitrile)iron(II) iodide, [Fe-(η⁵- C₅H₅)(C₇H₄N₂O ₂)(C₂₆H₄P₂)]I, crystallizes in the non-centrosymmetric space group Cc, which is a promising result for obtaining quadratic non-linear optical properties. However, the packing shows that the iodide counter-ion promotes the cancellation of almost all the dipoles, resulting in a supramolecular motif of cationic chains aligned in opposite directions making an angle of 35.2°. The use of PF₆^- as counter-ion induces the crystallization of the complex in a centrosymmetric space group. These results show that the introduction of different counter-ions, of different size and geometry, allows specific and directional intermolecular interactions that can determine the formation of a particular type of crystal packing.

Full text of DOI:10.1107/S0108270106038662

metal-organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
non-linearity and crystal structure is not yet completely
understood and experimentalists still do not have complete
control over the spatial arrangement of the molecules, the
mechanisms leading to large microscopic effects are well
understood and a large number of NLO molecules have been
synthesized and investigated (Goovaerts et al., 2001). There-
fore, after an appropriate selection of molecules with high ꢁ
(the molecular hyperpolarizability associated with second-
harmonic generation phenomena), much improvement is
possible if solid-state materials can be obtained with the
desired arrangement to display enhanced macroscopic prop-
erties.
Among the different strategies developed to overcome this
major obstacle, the creation of Coulombic interactions by
varying the counter-ions is one approach (Marder et al., 1991;
Dias, Garcia, Rodrigues et al., 1994). This effect can provide
the driving force to overcome the centrosymmetry originating
from dipolar interaction in organic and organometallic
compounds. Some additional factors may also contribute to a
better response in the solid state, these being mainly related to
an adequate alignment of the chromophore in the bulk
material, leading to optimal phase matching (Garcia, Rodri-
gues, Dias et al., 2001).
From our previous work on organometallic compounds
containing p-substituted benzonitriles coordinated to several
iron(II) and ruthenium(II) derivatives (Dias, Garcia, Mendes
et al., 1994; Garcia, Robalo et al., 2001; Wenseleers et al., 1998;
Garcia et al., 2003), we have identi®ed the metallic fragment
[Fe(ꢀ5-cyclopentadienyl)(dppe)]+ [dppe is 1,2-bis(diphenyl-
phosphino)ethane] as a good electron-donor group towards
the coordinated nitrile and, in particular, the cationic complex
[(ꢀ5-C5H5)(dppe)Fe(p-NCC6H4NO2)]+, due to its good stabi-
lity in solution and the solid state, seemed to be a good
candidate for future studies. In order to improve the under-
standing of the role of counter-ion variation in the bulk
material response, we have synthesized and characterized the
new title complex, [FeCp(p-NCC6H4NO2)(dppe)]I, (where Cp
is ꢀ5-C5H5), (I), and present its structural characterization here.
ISSN 0108-2701
The effect of counter-ions on the
supramolecular arrangement of
(benzonitrile)[1,2-bis(diphenyl-
phosphino)ethane](g5-cyclopenta-
dienyl)iron(II) cations
a
b
Â
M. Teresa Duarte, * M. Fatima M. Piedade, M. Paula
Robalo,c C. Jacoba and M. Helena Garciab
a
Â
Â
Centro de Quõmica Estrutural, Instituto Superior Tecnico, Avenida Rovisco Pais,
1049-001 Lisboa, Portugal, bDepartamento de Quõmica e Bioquõmica, Faculdade de
Â
Â
Ã
Ciencias da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal,
c
Â
and Departamento de Engenharia Quõmica, Instituto Superior de Engenharia de
Â
Lisboa, Rua Conselheiro Emõdio Navarro 1, 1959-007 Lisboa, Portugal
Correspondence e-mail: teresa.duarte@ist.utl.pt
Received 3 August 2006
Accepted 21 September 2006
Online 31 October 2006
The title compound, [1,2-bis(diphenylphosphino)ethane]-
(ꢀ5-cyclopentadienyl)(4-nitrobenzonitrile)iron(II) iodide, [Fe-
(ꢀ5-C5H5)(C7H4N2O2)(C26H4P2)]I, crystallizes in the non-
centrosymmetric space group Cc, which is a promising result
for obtaining quadratic non-linear optical properties.
However, the packing shows that the iodide counter-ion
promotes the cancellation of almost all the dipoles, resulting in
a supramolecular motif of cationic chains aligned in opposite
directions making an angle of 35.2ꢀ. The use of PF6 as
counter-ion induces the crystallization of the complex in a
centrosymmetric space group. These results show that the
introduction of different counter-ions, of different size and
geometry, allows speci®c and directional intermolecular
interactions that can determine the formation of a particular
type of crystal packing.
Comment
Molecular crystals, based on organic molecules or transition
metal coordination complexes which assemble in the solid
state as a consequence of non-covalent interactions, have been
the subject of important research during the last decade
(Aakeroy & Beatty, 2001). The interest in these materials
stems from the potential to manipulate their solid-state
properties by systematic variations of their molecular features
and crystal-packing motifs.
The design of organometallic materials for second-order
non-linear optical (NLO) applications is based on two steps,
namely the maximization of the molecular non-linearity by the
modi®cation of the structural features of the molecule, and
optimization of the crystal packing of the molecules in the
solid state. Although the relation between macroscopic
Complex (I) was obtained by the displacement of iodide by
the nitrile ligand in the precursor compound [FeCpI(dppe)]
(see scheme) with a slight excess of 4-nitrobenzonitrile in
dichloromethane at room temperature. The dark-red cationic
Acta Cryst. (2006). C62, m531±m534
DOI: 10.1107/S0108270106038662
# 2006 International Union of Crystallography m531

metal-organic compounds
complex which formed was naturally stabilized by the iodide
counter-ion. After work-up and recrystallization from
dichloromethane±diethyl ether, the complex was obtained as
dark-red crystals, fairly stable towards oxidation in air and
moisture in the solid state. The formulation is supported by
analytical data, and by IR and 1H, 13C and 31P NMR spectra.
Complex (I) shows the characteristic ꢂ(C N) stretching
band at 2215 cm 1, and two bands at 1513 and 1335 cm 1 for
the asymmetric and symmetric stretching of the NO2 group,
respectively. The ꢂ(C N) band shows a negative shift of
25 cm 1. This result is consistent with our earlier studies (Diaz
et al., 1993; Garcia, Robalo et al., 2001) of similar complexes,
where the negative shift for the ꢂ(C N) stretching vibration
upon coordination was related to a decrease in the CN bond
order, caused by the ꢃ back-bonding between the d orbitals of
the metal and the ꢃ* orbitals of the CN group.
Analysis of the NMR data shows that the coordination of
p-nitrobenzonitrile leads to a signi®cant shielding of the
aromatic ortho-H atoms compared with the corresponding
uncoordinated nitrile ligand (7.90 p.p.m. in CDCl3). These
data corroborate the metal±nitrile interaction based on the
metal!nitrile ꢃ back-donation contribution. Comparing
these results with the data obtained previously (Garcia,
Robalo et al., 2001) for the complex [FeCp(dppe)(p-
NCC6H4NO2)]PF6 (6.75 p.p.m. for the ortho H-atoms in
CDCl3), we observe a difference of approximately 0.3 p.p.m.
between the values found for these compounds with two
different counter-ions, consistent with our earlier studies for
similar ruthenium(II) complexes (Dias, Garcia, Rodrigues et
al., 1994).
from the Fe centre to the nitrile ligand. Comparison of this
value with the corresponding data for the complex
1
[FeCp(dppe)(p-NCC6H4NO2)]PF6 (461 nm; " = 460 M 1 cm
for the MLCT band) shows a more intense lower energy
MLCT for the iodide complex. Since such low-energy MLCT
bands are typically associated with large molecular quadratic
NLO responses (Garcia, Robalo et al., 2001; Garcia et al.,
2002), the title compound seems to be a promising candidate
for NLO properties. Furthermore, these results suggest that
the Coulombic interactions derived from counter-ion variation
may have an additional effect on the electronic interaction and
contribute to a more effective polarization in the chromo-
phore. This intramolecular effect should be more intense in
the solid state.
The molecular structure of (I) is shown in Fig. 1. The cation
has the typical pseudo-octahedral three-legged piano stool
geometry around the Fe, on the assumption that the Cp group
takes up three coordination sites. This geometry is supported
by the angles around the metal centre, which are all close to
90ꢀ (Table 1), as well as by the remaining angles P1ÐFe1ÐCp
[126.0 (12)ꢀ], P2ÐFe1ÐCp [131.9 (9)ꢀ] and N1ÐFe1ÐCp
[121.6 (6)ꢀ] (Cp is the centroid of the cyclopentalienyl ring),
which are all larger, as expected. This geometrical feature is
comparable with those found in the family of compounds
reported by our group (Garcia, Robalo et al., 2001; Garcia et
al., 2003).
Ê
The FeÐN bond length in (I) [1.875 (13) A] is similar to the
corresponding distance found in [FeCp(dppe)(p-NCC6-
The UV±vis absorption spectrum of complex (I), recorded
in chloroform solution, exhibits an intense broad absorption
band at 256 nm (" = 3920 M 1 cm 1) and a lower energy band
at 464 nm (" = 860 M 1 cm 1), this band being attributable to
a d±ꢃ* metal-to-ligand charge-transfer (MLCT) transition
Figure 1
Figure 2
A view of the cation of (I), showing the atom-numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level and H
atoms have been omitted for clarity.
(a) A view of the packing, showing the cationic chain along the [110]
direction. (b) A view of the crystal packing of (I) along a direction
perpendicular to the ac plane.
ꢁ
m532 Duarte et al. [Fe(C5H5)(C7H4N2O2)(C26H4P2)]I
Acta Cryst. (2006). C62, m531±m534

metal-organic compounds
H4NO2)]PF6 (Garcia, Robalo et al., 2001) (Table 1; Garcia,
Robalo et al., 2001). These X-ray data corroborate the spec-
troscopic IR and NMR data obtained for the complex. The
nitrile group shows an almost linear geometry, with FeÐN1Ð
C1 and N1ÐC1ÐC2 angles of 175.6 (11) and 177.9 (16)ꢀ,
respectively, indicating that the Fe atom and the benzonitrile
ligand are in the same plane. These features provide evidence
for the existence of ꢃ back-donation in the solid state. With
the aim of enhancing the molecular NLO properties in the
solid state and considering the different crystallization results
obtained using PF6 and I as counter-ions, which have
probably induced centrosymmetric and asymmetric crystal-
lization, respectively, we proposed to analyse the crystal
packing obtained in both cases and draw conclusions on the
role of the anion in the process.
the ac plane. The effect of the iodide anion is the quasi-
cancellation of the dipoles.
Ê
Finally, a ꢃ±ꢃ stacking interaction (3.32 A) between the
benzene group of the nitrile and a phosphine phenyl ring is
observed between cations, as can be seen in Fig. 2(b), along a
direction perpendicular to the ac plane, which strengthens the
relative position of the cations in two adjacent planes.
The crystal packing analysis of complex [FeCp(dppe)(p-
NCC6H4NO2)]PF6, whose molecular structure we reported
previously (Garcia, Robalo et al., 2001), shows the cancellation
of molecular dipoles resulting from the formation of a
hydrogen bond between the O atoms of the nitrile groups and
the H atoms of the CH2 group of the phosphines of two other
molecules, thus forming a tetramer, depicted in Fig. 3(a). This
tetramer promotes two dipole cancellations, the ®rst between
the molecules connected through the H atoms acting as
bridges, and the other between the two molecules to which the
H atoms belong. This geometry is strengthened by inter-
molecular ꢃ±ꢃ interactions between the benzene rings of two
nitrile moieties, as well as with a phenyl ring of a phosphine
(Fig. 3b).
Complex (I) has a non-centrosymmetric packing arrange-
ment. A short hydrogen-bond interaction, involving the O
atom of the nitrile moiety and a Cp H atom [C14Á Á ÁO1i =
i
i
Ê
Ê
3.51 (3) A, H14Á Á ÁO1 = 2.64 (3) A and C14ÐH14Á Á ÁO1 =
157 (1)ꢀ; symmetry code: (i)
+ x,
+ y, z] promotes the
2
1
2
1
formation of a cationic chain along the [110] direction (Fig. 2a).
Each cationic chain is aligned in a direction almost opposite to
that of the molecules in adjacent chains, making an angle of
35.2ꢀ, as they are connected by an iodide anion [C3ÐH3Á Á Á
In this structure, there are two types of PF6 molecules, each
with half occupancy; one interacts with four different cations,
Ê
with short contacts ranging from 3.150 to 3.355 A, while the
other only interacts with two cationic fragments, with
Ê
Ê
I1 = 3.03 (4) A and C7ÐH7Á Á ÁI1 = 3.06 (4) A], resulting in
Ê
distances in the range 3.174±3.124 A. As can be seen in
alternating chains of anions and cations along the diagonal of
Fig. 3(b), the cationic molecules form a column along a, while
the PF6 anions occupy the channels along b.
From these results, we can conclude that the introduction of
different counter-ions, of different size and geometry allowing
speci®c and directional intermolecular interactions, can be a
determining factor for the formation of a particular type of
crystal packing. Although interactions of the same type (short
contacts between the counter-ion and cationic molecules, ꢃ±ꢃ
stacking interactions and hydrogen bonding between the O
atoms of the terminal nitro group and neighbouring mol-
ecules) were found in both cases, they interact synergistically,
promoting different arrangments in the crystal packing.
Further studies are in progress to introduce new counter-
ions, and second-harmonic generation measurements on the
®nal materials are being carried out.
Experimental
To a solution of [Fe(ꢀ5-C5H5)I(dppe)] (Garcia, Robalo et al., 2001)
(0.3 mmol) in dichloromethane (20 ml) was added 4-nitrobenzonitrile
(0.33 mmol, 1.1 equivalents) at room temperature. The mixture was
stirred at room temperature for 22 h. A change in the colour of the
solution was observed from purple to dark red. The red solution was
®ltered, evaporated under vacuum to dryness and washed several
times with diethyl ether. The dark-red residue was further puri®ed by
vapour diffusion of diethyl ether into a concentrated dichloro-
methane solution, affording dark-red crystals of (I) [yield 167 mg,
70%; m.p. 433 K (decomposition)]. Analysis calculated for C38H33-
FeIN2O2P2: C 57.46, H 4.19, N 3.53%; found: C 56.91, H 4.27, N
Figure 3
1
3.49%. IR (KBr, ꢂ, cm 1): 2215 (C N), 1513 and 1335 (NO2); H
(a) A view of the tetramer motif formed in the crystal packing of the
complex [CpFe(dppe)(p-NCC6H4NO2)]PF6. (b) A view of the dipole
cancellation enhancing the ꢃ±ꢃ stacking interactions.
NMR (300 MHz, CDCl3): ꢄ 2.65 (m, 4H, CH2, dppe), 4.65 (s, 5H, ꢀ5-
C5H5), 7.02 (d, 2H, J = 8.4 Hz, H2, H6), 7.20±7.80 (m, 20H, C6H5,
ꢁ
Acta Cryst. (2006). C62, m531±m534
Duarte et al.
[Fe(C5H5)(C7H4N2O2)(C26H4P2)]I m533

metal-organic compounds
dppe), 7.97 (d, 2H, J = 8.4 Hz, H3, H5); 13C{1H} NMR (75 MHz,
CDCl3): ꢄ 28.48 (t, JCP = 20.4 Hz, CH2, dppe), 80.57 (ꢀ5-C5H5), 116.91
(C1), 123.68 (C3, C5), 129.28 (m, JCP = 4.4 Hz, C6H5-dppe), 129.64 (m,
C6H5-dppe), 130.69 (C2, C6), 131.04 (C6H5-dppe), 131.31 (NC),
133.05 (C6H5-dppe), 133.81 (C6H5-dppe), 136.41 (t, JCP = 20.9 Hz,
Data collection: CAD-4 EXPRESS (Enraf±Nonius, 1994); cell
re®nement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms &
Wocadlo, 1995); program(s) used to solve structure: SIR99 (Altomare
et al., 1999); program(s) used to re®ne structure: SHELXL97 (Shel-
drick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997); software
used to prepare material for publication: WinGX (Farrugia, 1999),
PLATON (Spek, 2003) and enCIFer (Allen et al., 2004).
C-ipso, C6H5-dppe), 149.04 (C4); 31P{1H} NMR (75 MHz, CDCl3):
ꢄ 97.64 (dppe); UV±vis (CHCl3, ꢅmax, nm): 256 (" = 3920 M 1 cm
)
1
1
and 464 (" = 860 M cm 1).
Ã
The authors thank the FundacËaÄo para a Ciencia e Tecno-
Crystal data
logia (FCT) and POCTI for ®nancial support (grant Nos.
POCTI/2000/CTM/35514 and POCT/QUI/48443/2002).
[Fe(C5H5)(C7H4N2O2)(C26H4P2)]I
Mr = 794.35
Monoclinic, Cc
Z = 4
Dx = 1.523 Mg m
Mo Kꢆ radiation
ꢇ = 1.46 mm
T = 293 (2) K
Prism, dark red
0.4 Â 0.2 Â 0.2 mm
3
1
Ê
a = 10.602 (3) A
Ê
b = 26.834 (7) A
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: HJ3019). Services for accessing these data are
described at the back of the journal.
Ê
c = 12.489 (3) A
ꢁ = 102.77 (2)ꢀ
V = 3465.2 (16) A
3
Ê
Data collection
Enraf±Nonius MACH3
diffractometer
!/2ꢈ scans
Absorption correction: scan
(North et al., 1968)
Tmin = 0.508, Tmax = 0.645
(expected range = 0.589±0.748)
3987 measured re¯ections
3987 independent re¯ections
2074 re¯ections with I > 2ꢉ(I)
Rint = 0
References
Aakeroy, C. B. & Beatty, A. M. (2001). Aust. J. Chem. 54, 409±421.
Allen, F. H., Johnson, O., Shields, G. P., Smith, B. R. & Towler, M. (2004). J.
Appl. Cryst. 37, 335±338.
Altomare, A., Burla, M. C., Camalli, M., Cascarano, G. L., Giacovazzo, C.,
Guagliardi, A., Moliterni, A. G. G., Polidori, G. & Spagna, R. (1999). J.
Appl. Cryst. 32, 115±119.
ꢈmax = 27.0ꢀ
3 standard re¯ections
every 400 re¯ections
intensity decay: none
Dias, A. R., Garcia, M. H., Mendes, P. J., Robalo, M. P. & Rodrigues, J. C.
(1994). Trends Organomet. Chem. 1, 335±343.
Dias, A. R., Garcia, M. H., Robalo, M. P., Green, M. L. H., Lai, K. K., Pulham,
A. J. & Klueber, S. M. (1993). J. Organomet. Chem. 453, 241±247.
Dias, A. R., Garcia, M. H., Rodrigues, J. C., Green, M. L. H. & Kuebler, S. M.
(1994). J. Organomet. Chem. 475, 241±245.
Enraf±Nonius (1994). CAD-4 EXPRESS. Version 5.1/1.2. Enraf±Nonius,
Delft, The Netherlands.
Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565.
Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837±838.
Flack, H. D. (1983). Acta Cryst. A39, 876±881.
Re®nement
Re®nement on F2
R[F2 > 2ꢉ(F2)] = 0.071
wR(F2) = 0.155
S = 0.95
3987 re¯ections
w = 1/[ꢉ2(Fo2) + (0.0682P)2]
where P = (Fo2 + 2Fc2)/3
(Á/ꢉ)max = 0.007
3
Ê
Áꢊmax = 0.66 e A
3
Ê
0.58 e A
Áꢊmin
=
415 parameters
H-atom parameters constrained
Absolute structure: Flack (1983),
with 120 Friedel pairs
Flack parameter: 0.03 (4)
Garcia, M. H., Robalo, M. P., Dias, A. R., Duarte, M. T., Wenseleers, W., Aerts,
G., Goovaerts, E., Cifuentes, M. P., Hurst, S., Humphrey, M. G., Samoc, M. &
Luther-Davies, B. (2002). Organometallics, 21, 2107±2118.
Table 1
Selected geometric parameters (A, ).
ꢀ
Ê
Â
Garcia, M. H., Robalo, M. P., Dias, A. R., Fatima, M., Piedade, M., GalvaÄo, A.,
Fe1ÐN1
Fe1ÐP1
Fe1ÐP2
N1ÐC1
C6ÐC7
C6ÐC5
C1ÐC2
1.875 (13)
2.209 (4)
2.211 (4)
1.171 (17)
1.335 (19)
1.388 (19)
1.390 (19)
C4ÐC3
C4ÐC5
C2ÐC7
C2ÐC3
C5ÐN2
O1ÐN2
N2ÐO2
1.34 (2)
Wenseleers, W. & Goovaerts, E. (2001). J. Organomet. Chem. 619, 252±264.
Garcia, M. H., Robalo, M. P., Duarte, M. T., Piedade, M. F. M., Dias, A. R.,
Goovaerts, E. & Wenseleers, W. (2003). Chem. Phys. Lett. 367, 390±397.
Garcia, M. H., Rodrigues, J. C., Dias, A. R., Piedade, M. F. M., Duarte, M. T.,
Robalo, M. P. & Lopes, N. (2001). J. Organomet. Chem. 632, 133±144.
Goovaerts, E., Wenseleers, W. E., Garcia, M. H. & Cross, G. H. (2001).
Handbook of Advanced Electronic and Photonic Materials, edited by H. S.
Nalwa, Vol. 9, ch. 3, pp. 127±191. San Diego: Academic Press.
Harms, K. & Wocadlo, S. (1995). XCAD4. University of Marburg, Germany.
Marder, S. R., Perry, J. W. & Tiemann, B. G. (1991). Organometallics, 10, 1896±
1901.
1.366 (19)
1.370 (18)
1.40 (2)
1.462 (18)
1.207 (19)
1.20 (2)
N1ÐFe1ÐP1
N1ÐFe1ÐP2
P1ÐFe1ÐP2
90.5 (3)
87.7 (3)
86.52 (14)
C1ÐN1ÐFe1
N1ÐC1ÐC2
175.6 (11)
178.0 (16)
North, A. C. T., Phillips, D. C. & Mathews, F. S. (1968). Acta Cryst. A24, 351±
359.
Sheldrick, G. M. (1997). SHELXS97 and SHELXL97. University of
All phenyl and cyclopentadienyl H atoms were positioned using an
È
Gottingen, Germany.
Spek, A. L. (2003). J. Appl. Cryst. 36, 7±13.
Ê
idealized aromatic geometry, with CÐH = 0.93 A and Uiso(H) =
1.2Ueq(C). Methylene H atoms were also placed in idealized posi-
Ê
tions, with CÐH = 0.97 A and Uiso(H) = 1.2Ueq(C). Poor crystal
quality precluded the acquisition of more accurate data.
Wenseleers, W., Gerbrandij, A. W., Goovaerts, E., Garcia, M. H., Robalo, M. P.,
Mendes, P. J., Rodrigues, J. C. & Dias, A. R. (1998). J. Mater. Chem. 8, 925±
930.
ꢁ
m534 Duarte et al. [Fe(C5H5)(C7H4N2O2)(C26H4P2)]I
Acta Cryst. (2006). C62, m531±m534