Crystal forms of hexafluorophosphate organometallic salts and the importance of charge-assisted C-H - F hydrogen bonds

Article abstract of DOI:10.1021/om9706626

Architecture, stability, and behavior with temperature of the hexafluorophosphate salts [(C₅H₅)₂Co][PF₆] (1), [(C₅H₅)₂Fe][PF₆] (2), [(C₆H₆)₂Cr][PF₆] (3), and [(C₆H₅Me)₂Cr][PF₆] (4) have been investigated. Crystals 1 and 2 have been measured by variable-temperature X-ray diffraction and differential scanning calorimetry. The phase-transitional behavior of [(C₅H₅)₂Co][PF₆], which undergoes two fully reversible crystal-to-crystal phase transitions, has been compared with the known behavior of [(C₅H₅)₂Fe][PF₆]. The room- and low-temperature phases (form I and form II) of [(C₅H₅)₂Fe][PF₆] and [(C⁵H₅)₂Co][PF₆] are isostructural and isomorphous. The basic packing features of crystalline 1 and 2 are maintained on substituting the metallocene system with the bisbenzenechromium cation in 3. The architecture common to the three salts is otherwise disrupted by substituting toluene for benzene in 4. It has been shown that, when assisted by the difference in charge between anions and cations, the C-H - F interactions play a significant role. The structural parameters obtained for C-H - F interactions in 1-4 have been compared with data retrieved from the Cambridge Crystallographic Database on interactions of the X-H - F^(δ-) type (X - C, N, O) in organometallic salts of the PF₆^- anion.

Full text of DOI:10.1021/om9706626

296
Organometallics 1998, 17, 296-307
Articles
Cr ysta l F or m s of Hexa flu or op h osp h a te Or ga n om eta llic
Sa lts a n d th e Im p or ta n ce of Ch a r ge-Assisted C-H---F
Hyd r ogen Bon d s
Fabrizia Grepioni,*,† Gianna Cojazzi,‡ Sylvia M. Draper,§ Noelle Scully,† and
Dario Braga*,†
Dipartimento di Chimica G. Ciamician and Centro CNR per lo Studio della Fisica delle
Macromolecole, c/ o Dipartimento di Chimica G. Ciamician,
Universita` di Bologna, Via Selmi 2, 40126 Bologna, Italy,
and Chemistry Department, Trinity College, Dublin 2, Ireland
Received J uly 31, 1997X
Architecture, stability, and behavior with temperature of the hexafluorophosphate salts
[(C5H5)2Co][PF6] (1), [(C5H5)2Fe][PF6] (2), [(C6H6)2Cr][PF6] (3), and [(C6H5Me)2Cr][PF6] (4)
have been investigated. Crystals 1 and 2 have been measured by variable-temperature X-ray
diffraction and differential scanning calorimetry. The phase-transitional behavior of
[(C5H5)2Co][PF6], which undergoes two fully reversible crystal-to-crystal phase transitions,
has been compared with the known behavior of [(C5H5)2Fe][PF6]. The room- and low-
temperature phases (form I and form II) of [(C5H5)2Fe][PF6] and [(C5H5)2Co][PF6] are
isostructural and isomorphous. The basic packing features of crystalline 1 and 2 are
maintained on substituting the metallocene system with the bisbenzenechromium cation in
3. The architecture common to the three salts is otherwise disrupted by substituting toluene
for benzene in 4. It has been shown that, when assisted by the difference in charge between
anions and cations, the C-H---F interactions play a significant role. The structural
parameters obtained for C-H---F interactions in 1-4 have been compared with data retrieved
from the Cambridge Crystallographic Database on interactions of the X-H---F(δ-) type (X )
-
C, N, O) in organometallic salts of the PF6 anion.
In tr od u ction
tallic solids. To this task we have devoted much of the
research efforts carried out in Bologna over the past few
years. Our attention has been focused in particular on
establishing analogies and differences between inter-
molecular or interionic interactions at work in organo-
metallic and organic crystals. In collaboration with
others, we have shown, for example, that while all
strong hydrogen-bond-donor and -acceptor groups be-
have essentially in the same way whether belonging to
organic or organometallic molecules, this is not so when
weaker interactions are involved. Besides the fascinat-
ing consequences of the direct participation of the metal
atoms in intermolecular interactions, we have shown
that hydrogen bonds such as C-H---O may have a
crucial effect on crystal cohesion when reinforced by the
different polarity of donors and acceptors, i.e., when
these weak electrostatic interactions are “charge as-
sisted”.4
Modern crystal engineering, namely the rational
design of solid materials with predefined arrangements
of the component molecules or ions, is a most attractive
field of research.1 Though born in the field of organic
chemistry, crystal engineering now represents an ex-
tremely promising source of new discoveries as well as
a fundamental scientific challenge in the neighboring
areas of inorganic and organometallic chemistry. The
implications of utilizing the structural and electronic
features of metal atoms in preparing novel materials
are enormous.2 In this respect, it is important to be able
to understand the factors responsible for cohesion and
behavior with temperature of inorganic and organome-
† Dipartimento di Chimica G. Ciamician.
‡ Centro CNR per lo Studio della Fisica delle Macromelecole.
§ Trinity College.
X Abstract published in Advance ACS Abstracts, December 15, 1997.
(1) (a) Desiraju, G. R. Organic Solid State Chemistry; Elsevier:
Amsterdam, 1987. (b) Desiraju, G. R. Crystal Engineering: The Design
of Organic Solids; Elsevier: Amsterdam, 1989.
In many cases there is no unique solution to the
problem of crystal cohesion as manifested by the phe-
(2) (a) Braga, D.; Grepioni, F. Acc. Chem. Res. 1994, 27, 51. (b)
Burrows, A. D.; Chan, C.-W.; Chowdry, M. M.; McGrady, J . E.; Mingos,
D. M. P. Chem. Soc. Rev. 1995, 329. (c) Subramanian, S.; Zaworotko,
M. J . Coord. Chem. Rev. 1994, 137, 357. (d) Braga, D.; Grepioni, F. J .
Chem. Soc., Chem. Commun. 1996, 571. (e) Braga D., Grepioni, F.,
Desiraju, G. R. Chem. Rev., submitted.
(3) (a) Braga, D.; Grepioni, F.; Sabatino, P.; Desiraju, G. R. Orga-
nometallics 1994, 13, 3532. (b) Braga, D.; Grepioni, F.; Tedesco, E.;
Biradha, K.; Desiraju, G. R. Organometallics 1997, 16, 1846 and
references therein.
S0276-7333(97)00662-6 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 01/07/1998

Crystal Forms of Hexafluorophosphate Organometallic Salts
Organometallics, Vol. 17, No. 3, 1998 297
nomenon of crystal polymorphism.5 In the presence of
polymorphism, the relationship between intermolecular
interactions and crystal stability becomes intriguing.
Polymorphism is related to the phase-transitional be-
havior of the crystalline material depending on whether
the polymorphic modifications belong to enantiotropic
systems (i.e., the solid-solid transition between poly-
morphs is below the solid-liquid transition and the
polymorphs interconvert before melting) or to monotro-
pic systems (i.e., the polymorphs melt before the solid-
solid transition can occur). In a supramolecular ap-
proach, the change in crystal structure associated with
a phase transition, in which intermolecular noncovalent
bonds are broken and formed, is the solid-state equiva-
lent of an isomerization at the molecular level. Since
chemical and physical properties of the crystalline
material can change dramatically with the solid-state
transformations, it would be highly desirable to be able
to understand the relationship between noncovalent
interactions and cohesion of the polymorphic phases.
[(C5H5)2Co][PF6] has been the subject of a preliminary
communication.8
The discussion will be organized as follows:
(i) The relationship between the three phases of
crystalline [(C5H5)2Co][PF6] (1) (form I at room temper-
ature, form II at 243 K, form III at 323 K) will be
discussed. New information concerning the high-tem-
perature phase will be added to those previously com-
municated.8
(ii) The crystal-packing and phase-transition behavior
of [(C5H5)2Co][PF6] will be compared with that of
[(C5H5)2Fe][PF6] (2). The room-temperature structure
of 2 has been determined previously and independently
by two groups7a,b and is isomorphous with that of 1. The
crystal structure of the low-temperature phase of crys-
talline [(C5H5)2Fe][PF6], not known before, has been
determined and will be used in the comparison. The
high-temperature phase transition of [(C5H5)2Fe][PF6]
has been measured by DSC, and data will be compared
with those available in the literature.7c
(iii) Crystals of the PF6- salts of [(C6H6)2Cr]+ (3) and
of [(C6H5Me)2Cr]+ (4) have also been obtained and
characterized. Analogies and differences with crystals
1 and 2 will be discussed.
(iv) Finally, the role played by C-H---F interactions
will be discussed. The results obtained from the com-
parative analysis of the hexafluorophosphate salts 1-4
will be put in the broader perspective of a Cambridge
Crystallographic Database investigation of the occur-
rence of X-H---F bonds (X ) C, N, O) involving the
hexafluorophosphate anion.
Many crystals of globular organometallic molecules
have been shown to undergo phase transitions, and, in
some cases, the presence of plastic phases is known.
These are characterized by short-range orientational
disorder and long-range order. For instance, the sub-
stituted ferrocene derivatives [(C5H5)(C5H4CHO)Fe]6a
and [(C5H5)(C5H4CMeO)Fe]6b as well as salts of the type
[(FC6H5)(C5H5)Fe][counterion] (counterion ) AsF6, PF6,
SbF6, and BF4)6a,c are all known to undergo order-
disorder phase transitions. The transition from the
ordered phase to a disordered one is usually associated
with the onset of reorientational motion of the molecules
or ions.6
Exp er im en ta l Section
Syn th esis of Cr ysta llin e [(C5H5)2Co][P F 6]. [(C5H5)2Co]
purchased from Aldrich (100 mg, 0.529 mmol) was dissolved
in acetonitrile and treated with HCl(aq). From the yellow
solution crystals of [(C5H5)2Co][PF6] were precipitated almost
quantitatively by addition of [NH4][PF6] (90 mg, 0.552 mmol).
The yellow crystals are stable in the air and in the mother
liquor.
Syn th esis of [(C6H6)2Cr ][P F6]. Brown crystalline [(C6H6)2-
Cr] purchased from Strem (48 mg, 0.231 mmol) was added to
a flask containing distilled water (5 mL). The mixture was
stirred until complete dissolution accompanied by the appear-
ance of a bright yellow color indicating complete oxidation to
[(C6H6)2Cr]+. To this solution was added [NH4][PF6] (40 mg,
0.245 mmol), and a yellow precipitate was obtained. After
filtration, the solid residue was dissolved in acetonitrile. Slow
evaporation of the solvent afforded crystals suitable for X-ray
diffraction.
In this paper, we will discuss crystal structure and
phase transitional behavior of the metallocene com-
plexes [(C5H5)2M][PF6] (M ) Co, 1; Fe, 2] and of the bis-
arene complexes [(arene)2Cr][PF6] (arene ) benzene, 3;
toluene, 4] that have all been crystallized as hexafluo-
rophosphate salts. Crystalline [(C5H5)2Fe][PF6]7a,b
2
was known to undergo a phase transition from a
monoclinic crystal to a plastic cubic phase at 347 K.7b
Both phases were characterized by single-crystal X-ray
diffraction and investigated by 2H NMR and 57Fe
Mo¨ssbauer techniques. Calorimetric measurements had
also indicated that a second phase transition occurs on
cooling the crystal below 210 K,7c but the crystal
structure of the low-temperature phase was not deter-
mined. The phase-transitional behavior of crystalline
Syn th esis of [(C6H5Me)2Cr ][P F 6]. [(C6H5Me)2Cr] pre-
pared according to the literature9 (60 mg 0.254 mmol) was
dissolved in distilled water (5 mL) and oxidized by oxygen
bubbled through the solution. To this solution was added
(4) (a) Braga, D.; Scaccianoce, L.; Grepioni, F.; Draper, S. M.
Organometallics 1996, 15, 4675. (b) Braga, D.; Grepioni, F.; Byrne, J .
J .; Wolf, A. J . Chem. Soc., Chem. Commun. 1995, 1023. (c) Braga, D.;
Costa, A. L.; Grepioni, F.; Scaccianoce, L.; Tagliavini, E. Organome-
tallics 1996, 15, 1084.
(5) Gavezzotti, A. Acc. Chem. Res. 1994, 27, 309. Dunitz, J .;
Bernstein, J . Acc. Chem. Res. 1995, 28, 193 and references therein.
(6) (a) Daniel, M. F.; Leadbetter, A. J .; Meads, R. E.; Parker, W. G.
J . Chem. Soc., Faraday Trans. 1978, 74, 456. (b) Sato, K., Katada, M.;
Sano, H.; Konno, M. Bull. Chem. Soc. J pn. 1984, 57, 2361. (c)
Fitzsimmons, B. W.; Sayer, I. J . Chem. Soc., Dalton Trans. 1991, 2907.
(7) (a) Martinez, R.; Tiripicchio, A. Acta Crystallogr. Sect. C 1990,
C46, 202. (b) Webb, R. J .; Lowery, M. D.; Shiomi, Y.; Sorai, M.;
Wittebort, R. J .; Hendrickson D. N. Inorg. Chem. 1992, 31, 5211. (c)
Sorai, M.; Shiomi, Y. Thermochimica Acta 1986, 109, 29.
(8) (a) Braga, D.; Scaccianoce, L.; Grepioni, F.; Draper, S. M.
Organometallics 1996, 15, 4675. (b) Some readers may be concerned
by the quality of the diffraction data obtained for form II of 2 and for
form III of 1. The overall quality of X-ray data collected on organo-
metallic crystals that undergo two-phase transitions within a small
temperature interval cannot be compared with those obtainable with
normal, well-behaving, crystals. Crystalline 1 and 2 are not polymorphs
that can be separately treated: the only way to collect data on the
low- and high-temperature phases is by having the crystals undergo
phase transition on the diffractometer starting from the room tem-
perature phases. Though reversible, the phase transitions cannot be
expected to leave the crystals unaffected.
(9) Calderazzo, F.; Invernizzi, R.; Marchetti, F.; Masi, F.; Moalli,
A.; Pampaloni, G.; Rocchi. L. Gazz. Chim. Ital. 1993, 123, 53.

298 Organometallics, Vol. 17, No. 3, 1998
Ta ble 1. Th er m od yn a m ic Da ta for th e P h a se Tr a n sition s of Cr ysta llin e 1 a n d 2
Grepioni et al.
T (K)
form I S form II
T (K)
form I S form III
∆H (kJ ‚mol-1
)
∆H (kJ ‚mol-1
)
form I S form III
form I S form II
mp (K)
417.15
1
2
251.77a
239.09b
313.86a
309.31b
347.10a
340.62b
346.94c
1.27a
1.34b
3.06a
3.05b
4.50a,b
4.84c
1.95c
213.05c
DSC, heating. DSC, cooling. c Calorimetric studies.7c
a
b
[NH4][PF6] (42 mg, 0.258 mmol), and a yellow precipitate was
obtained. After filtration, the solid residue was dissolved in
THF. The yellow THF solution was then placed in a test tube,
and toluene was slowly added dropwise; crystals suitable for
X-ray diffraction were obtained via slow diffusion of the two
solvents.
independent reflections) for 316 parameters; wR(F2 all) )
0.158, R1(1729 reflections with I > 2σ(I)) ) 0.048, S ) 1.065;
crystal size 0.14 × 0.15 × 0.08 mm, F(000) ) 708; θ-range 3.0-
30.0°. Absolute structure parameter ) 0.10(11).
4: C14H16CrF6P, M ) 381.24, monoclinic, space group P21/
n, a ) 6.793(2) Å, b ) 11.056(2) Å, c ) 10.740(5) Å, V ) 779.2(5)
Å3, Z ) 2, T ) 273(2) K, dc ) 1.625 g cm-1, µ ) 0.891 mm-1
,
Differ en tia l Sca n n in g Ca lor im etr y (DSC). DSC ther-
mograms of compounds 1 and 2 were measured on a Perkin-
Elmer DSC-7 in sealed Al-pans. 1: 5.150 mg of crystalline
material was taken from the same crystal batch used for the
diffraction experiments. Repeated cycles of cooling and heat-
ing (scanning rate 5.0 deg/min) had hystereses of 12.7 and 4.5
K for the form I S form II and the form I S form III
transitions, respectively. 2: 8.030 mg of crystalline material
was taken from the same crystal batch used for the diffraction
experiments. Repeated cycles of cooling and heating (scanning
rate 5.0 deg/min) had hysteresis of 6.48 °C for the form I S
form III transition. The low-temperature phase transition
could not be observed, as 213 K was the minimum temperature
reachable with the instrument used for the measurements.
Cr ysta l Str u ctu r e Deter m in a tion . Crystals of compound
3 and 4 are rapidly decomposed if exposed to air under the
X-ray beam. Data sets at low temperature for these species
were thus collected from crystal specimens covered with inert
glue (3) or sealed in a glass capillary together with some THF
solution (4). Diffraction intensities for compounds 1-4 were
collected on an Enraf-Nonius CAD-4 diffractometer equipped
with a graphite monochromator (Mo KR radiation, λ ) 0.710 69
Å). The Oxford Cryosystem device was used for the low- and
high-temperature data collections. All refinements have been
carried out on F2 using all reflections. Conventional R values
based on an I > 2σ(I) cutoff are also reported for sake of
comparison.
Cr ysta l Da ta . 1. form III, monoclinic data collection:
C10H10CoF6P, M ) 334.08, monoclinic, space group P21/c, a )
13.426(5) Å, b ) 9.518(5) Å, c ) 9.505(5) Å, V ) 1215(1) Å3, Z
) 4, T ) 323(2) K, dc ) 1.827 g cm-1, µ ) 1.597 mm-1, 2534
reflections measured, refinement on F2 (1550 independent
reflections) for 142 parameters; wR(F2 all) ) 0.382, R1(466
reflections with I > 2σ(I)) ) 0.096, S ) 0.938; crystal size 0.16
× 0.14 × 0.14 mm, F(000) ) 664; θ-range 3.0-23.0°. Crystal
data concerning the room-temperature (form I), low-temper-
ature (form II), and form III data collection in the cubic setting
are available in ref 8. Altogether, the experiments on crystals
of 1 were repeated seven times on four different crystals to
check reproducibility and to obtain the best possible set of
diffraction data for publication.8b
1167 reflections measured, refinement on F2 (1028 indepen-
dent reflections) for 93 parameters; wR(F2 all) ) 0.139, R1(672
reflections with I > 2σ(I)) ) 0.045, S ) 1.087; crystal size 0.16
× 0.12 × 0.10 mm, F(000) ) 772; σ-range 3.0-25.0°.
The computer programs SHELX8610a and SHELXL9210b
were used for structure solution and refinement. All atoms
except the hydrogen atoms and the C atoms in 1 (form III)
were treated anisotropically. Hydrogen atoms were added in
calculated positions and refined riding on their respective C
atoms. No attempt to model H-atom positions in the high
temperature form III was made. For all molecular representa-
tions the graphic program SCHAKAL9210c was used. The
program PLATON10d was used to calculate the hydrogen-
bonding interactions of the C-H---F type. For these calcula-
tions all C-H distances were normalized to the neutron-
derived value (1.08 Å). Crystal data and details of meas-
urement, tables of atomic coordinates, tables of anisotropic
thermal parameters, and tables of bond lengths and angles
are all available as Supporting Information and have been
deposited with the Cambridge Crystallographic Data Centre.
In t er m olecu la r Sea r ch es of X-H---F (X ) C, N, O)
In ter a ction s. Searches were made with version 5.13 of the
CSD10e (April 1997), by using the program QUEST3D,10f on
organometallic salts containing at least one transition metal
-
atom and one PF6 anion. Structures affected by disorder or
for which no coordinates of the H atoms were available were
excluded. Intermolecular contacts of the X-H---F type (X )
C, N, O) with H---F distances less than 2.6 Å and X-H---F
angles greater than 90° were accepted and subsequently
examined with the VISTA package.10f X-H distances were
all normalized to the neutron-derived values (1.080 Å for C-H,
1.009 Å for N-H, 0.983 Å for O-H).
Resu lts a n d Discu ssion
P h a se Tr a n sition s in [(C5H5)2Co][P F 6] (1) a n d in
[(C5H5)2F e][P F 6] (2). In view of the strict analogies
between structure and behavior with temperature of
crystalline 1 and 2, the two systems will be discussed
together. Both systems undergo two phase transitions,
indicated as form I S form II and form I S form III,
form I being the room-temperature structure, within
narrow temperature intervals. The thermodynamic
2. Form II: C10H10FeF6P, M ) 331.00, monoclinic, space
group P21/c, a ) 13.431(10) Å, b ) 9.268(4) Å, c ) 9.328(7) Å,
V ) 1153(1) Å3, Z ) 4, T ) 143(2) K, dc ) 1.907 g cm-1, µ )
1.503 mm-1, 1457 reflections measured, refinement on F2 (1198
independent reflections) for 148 parameters; wR(F2 all) )
0.386, R1 (642 reflections with I > 2σ(I)) ) 0.124, S ) 1.140;
crystal size 0.12 × 0.10 × 0.10 mm, F(000) ) 660; θ-range 3.0-
28.0°.8b Structural data for form I and form III for 2 are
obtained from ref 7b.
3: C12H12CrF6P, M ) 354.18, orthorhombic, space group
Pna21, a ) 9.587(6) Å, b ) 9.661(4) Å, c ) 27.510(10) Å, V )
2548(2) Å3, Z ) 8, T ) 143(2) K, dc ) 1.841 g cm-1, µ ) 1.081
mm-1, 4190 reflections measured, refinement on F2 (3753
(10) (a) SHELX86: Sheldrick, G. M. Acta Crystallogr. 1990, A46,
467. (b) SHELX92: Sheldrick, G. M. Program for Crystal Structure
Determination; University of Go¨ttingen, Go¨ttingen, Germany, 1992.
(c) SCHAKAL92: Keller, E. Graphical Representation of Molecular
Models; University of Freiburg: Germany, 1992. (d) PLATON: Spek,
A. L. Acta Crystallogr. Sect. A 1990, A46, C31. (e) Allen, F. H.; Davies,
J . E.; Galloy, J . J .; J ohnson, O.; Kennard, O.; Macrae C. F.; Mitchell,
E. M., Mitchell G. F.; Smith, J . M.; Watson, D. G. J . Chem. Inf. Comput.
Sci. 1991, 31, 187. (f) CSD User Manuals; Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.

Crystal Forms of Hexafluorophosphate Organometallic Salts
Organometallics, Vol. 17, No. 3, 1998 299
a
b
F igu r e 1. DSC thermograms for 1 (a) and 2 (b) showing the endothermic peaks for the orm I S form II of 1 and for the
form II S form III transitions of both 1 and 2. The form I S form II transition of 2 could not be measured for instrumental
limitations (see the Experimental Section).
data are summarized in Table 1. In the case of 1, the
DSC thermograms show two endothermic peaks at 252
and at 314 K (see Figure 1a). The enthalpy differences
associated with the two transitions are 1.30 and 3.05
kJ ‚mol-1 for the form I S form II and form I S form III
phase transitions, respectively. In the case of 2, the
form I S form II transition occurs at 213 K,7c while the
form I S form III transition occurs at 347 K (see Figure
1b). The enthalpy difference associated with this latter
The value given by Sorai and Shiomi7c in their previous
thermodynamic study (see Table 1) has been used as a
reference to perform the low-temperature diffraction
experiment on 2. However, the low-temperature form
I S form II phase transition was observed to occur only
at ca. 140 K on the single-crystal diffractometer, prob-
ably due to severe supercooling.
In the case of 1, the three crystal phases have been
fully characterized by single-crystal X-ray diffraction at
variable temperature carried out on the same crystal
specimen.8b In the case of 2 only the low-temperature
crystal structure (not determined before) has been
transition is 4.50 kJ ‚mol-1
. The enthalpy values of
these transitions fall within the range observed for most
polymorphic organic crystals.11 The DSC measurement
of the low-temperature phase transition in the case of
2 was not possible because of instrumental limitations.
(11) Gavezzotti, A.; Filippini, G. J . Am. Chem. Soc. 1995, 117, 12299.

300 Organometallics, Vol. 17, No. 3, 1998
Ta ble 2. Cell P a r a m eter s, Z, a n d P a ck in g Coefficien t Va lu es for Com p ou n d s 1-3
Grepioni et al.
species
form
a (Å)
b (Å)
c (Å)
â (deg)
Vcell (Å3)
Z
p.c.a
T (K)
1
1
1
II
I
13.419(7)
13.355(3)
6.72(1)
13.426(5)
13.431(9)
13.408(6)
6.806(4)
9.587(6)
9.667(8)
9.286(5)
9.441(8)
9.343(6)
9.427(6)
96.17(6)
92.87(4)
1157(1)
1187(1)
304(1)
1215(1)
1153(1)
1209(1)
315.3(3)
2548(2)
2604(3)
4
4
1
4
4
4
1
8
8
0.69
0.67
243
293
323
323
143
299
360
203
293
III cubic
III monoclinic
II
I
III
9.518(5)
9.268(4)
9.530(2)
9.505(5)
9.328(6)
9.482(2)
90.17(4)
96.91(6)
93.17(3)
0.65
0.70
0.67
0.64
0.70
0.69
2
2 (ref. 7b)
2 (ref. 7b)
3
9.661(4)
9.712(4)
27.510(10)
27.74(2)
3b
a
Packing coefficient was evaluated by the expression: p.c. ) [[Vanion + Vcation](Z/Vcell)], where Vanion + Vcation has been calculated with
the integration method.13 Cell parameters at room temperature were also evaluated for compound 3.
b
characterized, after the room temperature cell was
determined and compared to the literature values,7a,b
and the phase transition was observed on the diffrac-
tometer.
The room temperature form I of 1 and 2 is monoclinic
-
(P21/c) with the unit cell containing one PF6 anion in
a general position and two independent half-cations
located on the crystallographic centers of inversion. On
cooling both 1 and 2, the form I S form II phase
transition occurs, leading from the room-temperature
P21/c monoclinic cell to another P21/c monoclinic cell
with a different angle [92.87(4) and 96.17(1)°; 93.17(3)
and 96.91(6)° for form I and form II of 1 and 2,
respectively] and slightly different unit cell axes (see
Table 2). On passing through the phase transition the
b- and c-axes shorten with respect to the room temper-
ature cell, while the a-axis lengthens. The form I S
form II transformation is accompanied by a change in
cell volume from 1187 to 1157 Å3 and from 1209 to 1153
Å3 for 1 and 2, respectively. In summary, the form I S
form II phase transitions of 1 and 2 lead in both cases
from an ordered monoclinic cell to another ordered cell,
the only important difference being the temperature of
the transition, which is 26 K lower (on cooling) in the
case of 2 with respect to 1. The form I S form II phase
transition is fully reversible.8 The relationship between
the two crystal structures is depicted in Figure 2.
Cations of type A (empty atom spheres), with the Cp-
M-Cp molecular axes almost parallel to the a-axis, and
of type B (filled atom spheres), with the molecular axes
almost perpendicular to the a-axis (see Figure 2), stack
along the cell a-axis in an A/B/A/B sequence. On going
from form I to form II, the Cp-M-Cp molecular axes
that are eclipsed (in projection) at room temperature
become staggered at low temperature.
As shown by the calorimetric measurements (Figure
1), both 1 and 2 undergo a second phase transition at
314 and 347 K, respectively (on heating), to the crystal-
line form III. In the case of 1, diffraction data of form
III measured at 323 K showed fully ordered anions and
orientationally disordered cations in the cubic system
Pm-3.8 In the case of 2, the high-temperature plastic
phase has also been described in the cubic system Pm-
3, but with both anions and cations orientationally
disordered.7b As in the case of the form I S form II
transformation, the form I S form III transition has
been demonstrated, at least in the case of 1, to be fully
reversible on the same crystal sample as the form I S
form II transformation.8a
F igu r e 2. Schematic representation of the form I S form
II phase transition: the Cp-M-Cp molecular axes that
at room temperature are eclipsed (in projection) (a) become
staggered at low temperature (b).
monoclinic system. The purpose of this new experiment
on 1 was that of gaining insight into the phase-
transition mechanism by checking whether form III
could be described in a system of lower symmetry that
retained the two independent crystallographic sites for
the two cations. This idea arises from the observation
In the case of 1, we have been able to correlate the
cubic structure to the original monoclinic one, and a new
data collection has been carried out for form III in the

Crystal Forms of Hexafluorophosphate Organometallic Salts
Organometallics, Vol. 17, No. 3, 1998 301
F igu r e 3. Space-filling representation in the ac-plane of
the form III disorder monoclinic structure (a) and of the
ordered form II structure of 1. Note how the centers of mass
of the cations and the anions do not “move” upon crystal
transformation, while the Cp-ligands appear to rotate
around the metal centers to the disordered arrangement.
F igu r e 4. Space-filling projections of form III in the
monoclinic (this paper) and cubic (ref 8) settings. Note the
relationship between the monoclinic (corners occupied by
PF6 anions) and the cubic cells (corners occupied by
disordered cobalticinium cations).
-
The cubic cell, then, results from the average over
space of the two cobalticinium cation sites. A way to
depict it is as follows: as the temperature increases the
librational motion of the ligands requires more space;
that the transformation from low temperature to high
temperature is accompanied by a progressive change of
the monoclinic cell with the monoclinic angle “clicking”
from 96.17(6) to 92.87(4) to 90.17(4)° as the transforma-
tion proceeds via the form II f form I and form I f
form III transitions (see Table 2). This latter “click” is
associated with the onset of what appears to be an
almost random disordering of the Cp-ligands around the
cobalt centers, which do not change position. A space-
filling representation in the ac-plane of the disorder in
form III of 1 is shown in Figure 3a and compared with
a space-filling view of form II in the same orientation
(Figure 3b).
Unfortunately (though expectedly8b), the high-tem-
perature data sets are affected by a severe loss of
diffraction power. Only few hundred reflections are
measurable at 323 K, and the limited number of
reflections reduces the resolution, which is a serious
drawback in the presence of disorder. Nonetheless, we
have been able to locate peaks in the Fourier maps that
correspond to the fraction of carbon atoms with site
occupancy factors ranging from 0.60(5) to 0.10(5). Al-
together, the fractions of carbon atoms account satis-
factorily for as much as 98.4% of the expected electron
density. Space-filling projections of form III in the
monoclinic (this paper) and cubic (ref 8) settings are
compared in Figure 4.
-
hence, the “box” (defined by the PF6 anions) in which
the cobalticinium cation is encapsulated becomes larger
and, on average, more isotropic (i.e., closer to the cubic
symmetry). There is no way to tell with confidence from
the high-temperature diffraction data whether the
dynamic disordering of the Cp ligands makes the two
sites identical also over time. One of the questions
behind the observation of such a fully reversible order-
to-disorder phase transition is that related to the
degeneracy of the process. One has to (or, at least, has
to try to) explain how possibly the (apparent) degeneracy
of the high-temperature phase with metallocenium
cations in random orientations in both 1 and 2 disap-
pears on cooling. The diffraction experiments show that
(even after a long heating, ca. 24 h, at 323 K) the
ordered monoclinic structure is fully restored though
with a loss of diffraction power. These observations may
suggest that form III is cubic only on average over space
but remains monoclinic on average over time. Within
this hypothesis the orientational disorderings of the two
independent cations in the high-temperature phase are
not degenerate, allowing the Cp ligands to revert, on
cooling, to the original sites of lower energy. In view of
the paucity of available experimental data, this conclu-

302 Organometallics, Vol. 17, No. 3, 1998
Grepioni et al.
sion is admittedly speculative. Some support, however,
derives from the observation that when treated in the
monoclinic setting at high temperature the cell of 1
refines to slightly different values of the b- and c-axes
(see Table 2) and that the monoclinic â angle is slightly
different from 90°.
Cr ystal Str u ctu r e of th e P F6- Salts of [(C6H6)2Cr ]+
(3) a n d of [(C6H5Me)2Cr ]+ (4). On changing the 5-fold
symmetry of the metallocene cations of 1 and 2 to the
6-fold symmetry of the benzene ligands of 3, the crystal
architecture is essentially retained unchanged. The
relationship between the monoclinic unit cells of 1 and
2 and that of 3 is clear to see (Table 2): a (1, 2
monoclinic) ≈ b (3) , b (1, 2 monoclinic) ≈ c (3), c (1, 2
monoclinic) ≈ 2a (3).
The fact that the three crystals are quasi-isomor-
phous, in spite of the structural differences between the
organometallic cations, lends further support to the idea
that objects of similar shape tend to pack in a similar
manner, provided that the type of noncovalent interac-
tions at work are not significantly altered.12 A space-
filling representation of the crystal structure of 3 is
compared with the corresponding orientation of the
crystal structure of 1 (form II) in Figure 5.
In addition to this structural analogy there are some
important features that need to be discussed. Although
constructed of centrosymmetric bis-benzenechromium
cations and of centrosymmetric hexafluorophosphate
anions, compound 3 crystallizes in the polar space group
Pna21, with two independent [(C6H6)2Co]+ cations and
-
PF6 anions in the asymmetric unit. The benzene
ligands are in eclipsed conformation in both cations, as
usually observed for bis-benzene transition metal com-
plexes. The two independent cations in 3 have the same
relative arrangement observed in the low-temperature
phases of 1 and 2 (form II); i.e., the cation molecular
axes are staggered in projection (see Figure 6). How-
ever, the stacking sequence now is not A/B/A but A/B/
A′/B′/A/B, where the orientation of the cations A′ and
B′ with respect to that those of type A and B is related
by the crystallographic 21-axis. This decreases the
periodicity and doubles the cell c-axis with respect to
the cell a-axes of 1 and 2 (see Table 2).
Table 2 allows a comparison of the packing coefficients
(p.c.),13 i.e., of the efficiency of volume occupation in the
various forms of 1, and 2 with crystalline 3. It is worth
noting that the p.c. values vary from 0.69 to 0.65 and
from 0.70 to 0.64 on warming crystals 1 and 2, respec-
tively. These changes reflect a substantial decrease of
the crystal density as the temperature is raised in
keeping with the “plasticity” of the systems. Impor-
tantly, crystal 3, which retains the same packing
arrangement on cooling, undergoes a much smaller p.c.
change with temperature (from 0.69 to 0.70).
F igu r e 5. Comparison of the space-filling representation
of 3 with the corresponding orientation of 1 in its form II.
Note how the cations belonging to different layers have
similar relative orientations although the stacking peri-
odicity is A/B/A/B in 1 and A/B/A′/B′/A/B in 3.
methyl groups; this is the conformation observed in most
bis-toluene complexes, probably because it minimizes
the steric repulsions between the methyl ligands. The
presence of a methyl on the arene ligands disrupts the
6-fold symmetry of the cation and prevents a packing
arrangement similar to the one observed for the bis-
benzene analogue. The crystal is constituted of columns
of cations extending along the a-direction. The columns
of cations are in close contact; the anions also form
columns along the same direction, occupying the inter-
stices among the cation, as shown in Figure 7.
Da t a b a se An a lysis a n d t h e Im p or t a n ce of
Ch a r ge- Assisted C-H---F In ter a ction s. Since the
peripheral atoms in systems 1-4 are only of two types,
namely H and F, and the H-atoms are bound only to
carbon, these crystals can be used to test the role of
C-H---F(-P) interactions in controlling crystal stability
and cohesion. Interactions of this kind are expected to
be weak because of the low acidity of the C-H system.
However, the number of potential donors and acceptors
is very large (compared with compounds carrying con-
ventional strong donors such as -COOH, -OH, etc.)
and may well result in an important collective contribu-
tion to cohesion.
The compound [(C6H5Me)2Cr][PF6] (4) crystallizes in
the monoclinic space group P21/n. Both cation and
anion lie on crystallographic centers of inversion. The
two toluene ligands are staggered with respect to the
(12) (a) Braga, D.; Grepioni, F. Organometallics 1991, 10, 1254. (b)
Braga, D.; Grepioni, F. Organometallics 1991, 10, 2563.
(13) Packing coefficients are obtained from the ratio p.c. ) [[Vanion
+ Vcation](Z/Vcell)] where Z is the number of formulas in the unit cell,
Vcell is the cell volume, and Vanion and Vcation are the volumes of the
anions and cations evaluated by the integration method; see: Gavez-
zotti, A. J . Am. Chem. Soc. 1983, 95, 5220.
Furthermore, weak hydrogen bonds, such as
C-H---O bonds, can be strengthened via charge as-

Crystal Forms of Hexafluorophosphate Organometallic Salts
Organometallics, Vol. 17, No. 3, 1998 303
In order to put the analysis in a broad perspective,
we have analyzed the data available in the CSD and
searched for intermolecular H-bonds of the X-H---F(δ-)
type (X ) C, N, O see the Experimental Section). For
X ) C, two separate searches were carried out. In the
first, only C-H fragments for which the C atom is
directly bound to the metal atom were considered (149
compounds), in order to compare the result of the search
directly with the hydrogen-bonding parameters calcu-
lated for our compounds 1-4. In the second one no
restrictions were applied to the C atom (831 com-
pounds). Interactions of the N-H---F(δ-)(P) and
O-H---F(δ-)(P) types were also screened (80 and 15
compounds, respectively).
Our attention to the problem of X-H---F(δ-) interac-
tions was also attracted by the recent study of the
participation of organic fluorine in intermolecular hy-
drogen bonds reported by Dunitz and Taylor.16 These
authors have convincingly demonstrated, by means of
a database analysis and of theoretical calculations, that
fluorine when covalently bound to carbon does not
frequently form hydrogen bonds with conventional
hydrogen bond donors, including O-H, N-H, and C-H
groups.
F igu r e 6. Schematic representation of the cation arrange-
ment in 3. Note how the Bz-Cr-Bz axes are staggered in
projection as in the low-temperature phase of 1 and 2.
The results of our analysis, which is confined to
inorganic fluorine bound to phosphorous in PF6- anions,
are summarized in the scattergrams shown below in
Figures 8 and 12, where X-H---F(δ-)(P) angles (X ) C,
N, O) are plotted against the (X)H---F(δ-)(P) distances.
Although the sample population is very different in the
four cases, all types of interactions follow the normal
trend observed for hydrogen bonds, i.e., as the distance
between hydrogen and acceptor atoms decreases, the
X-H---F vectors becomes straighter and the bond ap-
proaches linearity.17 Besides this general behavior,
there is a large number of bonds that fall below the van
der Waals cutoff distances and are clearly indicative of
specific and directional interactions involving donors
and acceptors. Some key examples extracted from the
three searches will be discussed below.
The two scattergrams of (C)H---F(δ-)(P) interactions
(Figure 8a,b) show no appreciable difference; i.e., there
is no appreciable effect of the direct bonding to transi-
tion metal atoms on the geometry of the interaction.
This is confirmed by the histograms of (C)H---F(δ-)(P)
separations (Figure 8c,d), which show that, in spite of
the much larger number of salts in the general search
(831 versus 149 compounds), the two populations have
the same distribution. The percentage of contacts fal-
ling, for example, in the range 2.00-2.32 Å is 10% in
both cases. It can be observed that, as the H---F
distance moves toward shorter values, the distribution
of C-H---F(δ-)(P) angles, which covers the range 110-
180°, concentrates in an area of higher angular values.
Most short (C)H---F(δ-)(P) interactions, on the other
hand, involve metal-coordinated cyclopentadienyl and
arene ligands, which are also the most common donors
F igu r e 7. Molecular packing of compound 4: space-filling
representation of the cation and anion columns extending
parallel to the a-axis. H atoms are omitted for clarity.
sistance.14 Stronger C-H---O bonds are, of course, more
interesting in crystal engineering applications. Charge
assistance is “naturally” provided in organometallic
salts if the donor is charged positively and the acceptor
is charged negatively. This is, for instance, the case of
the organic-organometallic supramolecular Cr and Co
aggregates with cyclohexanedione and organic acids
recently described.14,15 The stability of these crystalline
materials is largely due to interactions of the
O-H---O(-) type between the organic moieties and of the
C-H(δ+)---O(δ-) type between the organometallic cations
and the anionic organic frameworks. In approaching
C-H---F interactions in this study we wondered if a
similar role could be ascribed to interactions of the
C-H(δ+)---F(δ-) type between organometallic cations and
the fluorine atoms of the PF6- anions. The PF6- anion
is one of the most common anions used to precipitate
and crystallize organometallic and inorganic cations.
(16) Dunitz, J .; Taylor, R. Chem. Eur. J . 1997, 3, 89.
(17) (a) Bernstein, J .; Davis, R. E.; Shimoni, L.; Chang, N.-L. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1555. (b) Bernstein, J .; Etter, M. C.;
Leiserowitz, L. In Structure Correlation; Bu¨rgi, H.-B., Dunitz, J . D.,
Eds.; VCH: Weinheim, 1994; p 431. (c) Taylor, R.; Kennard, O. J . Am.
Chem. Soc. 1982, 104, 5063. (d) Allen, F. H.; Kennard, O.; Taylor, R.
Acc. Chem. Res. 1983, 16, 146. (e) J effrey, G. A.; Saenger, W. Hydrogen
Bonding in Biological Structures; Springer-Verlag: Berlin, 1991.
(14) (a) Braga, D.; Costa, A. L.; Grepioni, F.; Scaccianoce, L.;
Tagliavini, E. 1997, 16, 2070. (b) Braga, D.; Angeloni, A.; Grepioni,
F.; Tagliavini, E. J . Chem. Soc., Chem. Commun. 1997, 1447.
(15) Braga, D.; Angeloni, A.; Grepioni, F.; Tagliavini, E. Organo-
metallics, in press.

304 Organometallics, Vol. 17, No. 3, 1998
Grepioni et al.
F igu r e 8. Scattergrams of C-H---F(δ-)(P) angles versus (C)H---F(δ-)(P) distances and histograms of (C)H---F(δ-)(P) distances
for C-atoms directly bound to transition metal atoms (a, c) and for “any” C-H (b, d).
in hydrogen bonds involving CO ligand acceptors.18 The
fact that the two types of interactions, namely (C)H---
O(C) and (C)H---F(δ-)(P) interactions, span the same
numerous C-H---F distances are present in 3, which
does not undergo a phase change, and in 2, for which
the stability of the intermediate phase is much larger
than in 1. A perspective view of the pattern of
C-H---F interactions along the c-axis is shown in Figure
9.
-
distance range suggests that fluorine in PF6 has a
basicity very similar to that of CO. The similarity
between the CO ligand and the PF fragment as accep-
tors is also in the tendency to multiple interactions as
will be shown below.
We have chosen to show two examples taken from the
restricted search: the first is compound [(C6H5-
Me)W(H)2(PMe3)3][PF6]19a (A in Figure 8a), which rep-
resents the shortest interaction; the second is the one
showing, in the interval of distances below 2.20 Å, the
angular value closest to 180°, [(C6H5F)2W(H)][PF6]19b (B
in Figure 8a). In crystalline [(C6H5Me)W(H)2(PMe3)3]-
[PF6], a fluorine atom is involved in a bifurcated
interaction with two hydrogen atoms belonging to the
toluene ligands of two neighboring tungsten cations (see
Figure 10). [(C6H5F)2W(H)][PF6] is relevant for our
discussion because its molecular formula is quite similar
to those of the compounds reported in this paper. One
The information on (C)H---F(δ-)(P) interactions in
crystalline 1-4, at low and room temperatures, is
summarized in Table 3. In both form I and form II of
1 and 2 there is a large number of C-H---F interactions
shorter than 2.6 Å between the C5H5 ligands and the
PF6- anions. There is good agreement with the distri-
bution shown in Figure 8a. In particular, it is note-
worthy that there are, respectively, two and five
C-H---F distances between 2.3 and 2.4 Å in form I and
form II of 2, while all links are above 2.42 Å in crystals
of 1. Distances as short as 2.3 are otherwise observed
in crystalline 3. Whether these differences reflect the
effect on the C-H acidity of the metal atoms (Co in 1,
Fe in 2, Cr, in 3) is difficult to state with confidence.
One may speculate, however, that shorter and more
-
fluorine atom of the PF6 anion participates in a
bifurcated interaction with two H atoms of the aromatic
(19) (a) Green, M. L. H.; Hughes, A. K.; Lincoln, P.; Martin-Polo, J .
J .; Mountford, P.; Sella, A.; Wong, L.-L.; Bandy, J . A.; Banks, T. W.;
Prout, K.; Watkin D. J . J . Chem. Soc., Dalton Trans. 1992, 2063. (b)
Prout, K.; Gourdon, A.; Couldwell, C.; Meunier, B.; Miao, F. M.;
Woolcock, J . Acta Crystallogr. Sect. B 1982, B38, 456.
(18) (a) Braga, D.; Grepioni, F.; Biradha, K.; Pedireddi, V. R.;
Desiraju, G. R. J . Am. Chem. Soc. 1995, 117, 3156. (b) Braga, D.;
Grepioni, F. Acc. Chem. Res. 1997, 30, 81.

Crystal Forms of Hexafluorophosphate Organometallic Salts
Organometallics, Vol. 17, No. 3, 1998 305
Ta ble 3. Hyd r ogen -Bon d in g P a r a m eter s for
Com p ou n d s 1-4
C(H)---F(δ-)(P)
(Å)
(C)H---F(δ-)(P)
(Å)
C-H---F(δ-)(P)
(°)
compd
1 (form I)
3.245
3.427
3.399
3.422
3.437
3.339
3.266
3.310
3.488
3.355
3.436
3.235
3.626
3.182
3.371
3.185
3.542
3.112
3.222
3.404
3.352
3.253
3.394
3.291
3.384
3.414
3.234
3.178
3.639
3.079
3.435
3.233
3.322
3.567
3.539
3.357
3.417
3.380
3.137
3.472
3.353
3.406
3.327
3.371
3.401
3.303
2.441
2.425
2.548
2.536
2.573
2.584
2.439
2.450
2.450
2.451
2.523
2.448
2.582
2.451
2.372
2.390
2.535
2.576
2.489
2.405
2.520
2.396
2.373
2.363
2.579
2.488
2.501
2.399
2.593
2.329
2.548
2.320
2.455
2.519
2.480
2.489
2.404
2.301
2.420
2.542
2.303
2.543
2.497
2.399
2.534
2.519
130.3
153.8
135.0
138.6
136.4
126.4
132.4
135.8
160.7
140.5
141.6
128.7
162.5
123.9
153.2
129.2
154.9
109.8
124.2
153.2
133.1
135.3
157.2
143.1
130.7
143.2
124.3
127.9
162.9
125.0
138.9
141.2
136.4
163.3
166.4
136.6
155.7
177.5
122.6
143.7
163.4
136.2
132.8
149.0
136.6
128.6
1 (form II)
2 (form I)
2 (form II)
F igu r e 9. Perspective view along the c-axis of the pattern
of (C)H---F(δ-)(P) interactions in compound 3.
3
4
F igu r e 10. (C)H---F(δ-)(P) interactions in crystalline
[(C6H5Me)W(H)2(PMe3)3][PF6] (A in Figure 8a): a fluorine
atom is involved in a bifurcated interaction with two
hydrogen atoms belonging to the toluene ligands of two
neighboring tungsten cations.
rings of a neighboring cation, while four other fluorine
atoms are involved in short pairs of interactions with
two additional cations related by translational sym-
metry (see Figure 11). It is interesting to note that the
shortest separation between the fluorine atoms co-
valently bound to the C6 rings and the hydrogen atoms
on an adjacent cation are quite long (2.764 and 2.597
Å). This observation is in keeping with the observation
discussed by Dunitz and Taylor17 that organic fluorine,
covalently bound to carbon, does not commonly form
hydrogen bonds. Importantly, however, the same au-
thors pointed out that when the fluorine atom is part
of an anion like, for example, the fluorobenzoate anion,
it can become “unusually electron rich” and a good
acceptor in hydrogen bonding. Clearly, this is the case
results are shown in Figure 12a, for O-H and N-H
donors, respectively. Both scattergrams resemble quite
closely the ones obtained for the carbon atom (see Figure
8), although in the case of oxygen the number of hits is
scarce and constituted mainly of compounds in which
the O-H fragment belongs to water molecules, present
as crystallization solvent or as a metal-coordinated
ligand.
Two examples have been selected from these searches
and are briefly discussed below. The interaction net-
work in crystalline [(5-Me-4,6-diaza-2,8-diazonio-nona-
20a
2,4,7-triene-3,7-diyl)Fe(H)2(CNMe)4][PF6]2 (A in Fig-
ure 12a) is shown in Figure 13. The cation and the
anion form a dimeric pair via two short hydrogen bond
interactions involving two fluorine atoms of the same
anion and two hydrogen atoms belonging to an N-H
-
of the F-atom in PF6 anions.
As mentioned above, N-H---F(δ-)(P) and O-H---F(δ-)
-
(P) interactions show the same distribution as
C-H---F(δ-)(P) interactions. The two searches have
been carried out with no restriction on the geometry of
the N-H and O-H fragments; i.e., they were not
requested to be directly bound to the metal center. The
(20) (a) Rojo, T.; Cortes, R.; Lezama, L.; Mesa, J . L.; Via, J .;
Arriortua, M. I.; Inorg. Chim. Acta 1989, 165, 91. (b) Castro, J . M.;
Hope, H. Inorg. Chem. 1978, 17, 1444.

306 Organometallics, Vol. 17, No. 3, 1998
Grepioni et al.
F igu r e 11. (C)H---F(δ-)(P) interactions in crystalline
F igu r e 13. (a) Hydrogen bonds of the (N)H---F(δ-)(P) type
in crystalline [(5-Me-4,6-diaza-2,8-diazonio-nona-2,4,7-
triene-3,7-diyl)Fe(H)2(CNMe)4][PF6]2 (A in Figure 12a). The
cation and the anion form a dimeric pair via two short
(N)H---F(δ-)(P) hydrogen-bond interactions.
[(C6H5F)2W(H)][PF6] (B in Figure 8a): a total of five
-
fluorine atoms of the PF6 anions participate in multiple
interactions (broken filled bonds) that are shorter than
those involving the organic fluorine atoms carried by the
ligands (broken empty bonds).
F igu r e 14. Hydrogen bonds of the (O)H---F(δ-)(P) type in
crystalline [(terpyridine)Cu(H2O)(NCO)][PF6]. Two anions
and two copper cationic complexes form a dimeric unit held
together by short O-H---F interactions involving two
metal-coordinated water molecules.
of the terpyridine group and a second different fluorine
atom of the same anions.
Con clu sion s
We have attempted with this paper to bring together
different issues of solid state chemistry, dynamics, and
crystal engineering. The major outcomes of this study
can be summarized as follows:
(i) Crystalline [(C5H5)2M][PF6] (M ) Co and Fe) are
remarkable structural systems. Both crystals are able
to switch reversibly between three phases, two of which
are fully ordered. The stability of the intermediate room
temperature phase (form I) differs in the two salts by
ca. 75° in the case of Co and 134° in the case of Fe.
(ii) The high-temperature phase (form III) is, at least
in the case of cobalt, related to the room-temperature
phase, and the whole phase transitional behavior can
be seen as a progressive adjustment of the crystal
structure to the dynamical requirements of the globular
cations that occupy more space as the temperature
increases.
F igu r e 12. Scattergrams of (X)H---F(δ-)(P) distances plot-
ted against X-H---F(δ-)(P) angles: X ) N (a), X ) O (b).
(iii) Crystalline 1 is remarkably robust so that the
phase transitions can be followed on the single crystal
diffractometer and diffraction data of the different phase
can be collected on the same crystal specimen.
(iv) Form III of 1 can be described either as a semi-
plastic cubic crystal in which the cobalticinium cations
have lost long-range order and occupy the center of a
“box” defined by ordered PF6- anion or as a monoclinic
and a methyl group on the cation. The interaction
network in crystalline [(terpyridine)Cu(H2O)(NCO)]-
[PF6]20b (A in Figure 12b) is depicted in Figure 14. Two
anions and two copper cationic complexes form a dimeric
unit held together by short O-H---F interactions,
involving the water molecules coordinated to the Cu
atoms, and by C-H---F interactions involving H atoms

Crystal Forms of Hexafluorophosphate Organometallic Salts
Organometallics, Vol. 17, No. 3, 1998 307
crystal (with the b-angle close to 90°) in which the
cations show different extent of orientational disorder
over two crystallographic independent sites. This latter
model has the advantage of providing a path for return-
ing to the nondegenerate ordered distribution of form
II on cooling the crystal.
(iv) The chromium analogue 3, in spite of the differ-
ence in molecular structure, crystallizes in a manner
that is strictly related to that of the low-temperature
phases of 1 and 2 and does not undergo phase changes
on cooling. When benzene is replaced with toluene, the
architecture of 1-3 is no longer compatible with the
cation shape, and the crystal structure of 4 is based on
cationic columns.
(vii) Both our experimental results and the abundant
information available in the CSD concur to indicate that
the inorganic fluorine atom in PF6- forms weak hydro-
gen bonds with a variety of donors. With respect to
organic fluorine, the X-H---F(δ-)(P) interaction is “charge
assisted”, i.e., the weak interaction is seemingly rein-
forced by the different ionic charge carried by anion and
cations.
The hydrogen bond provides the most efficacious and
robust noncovalent infrastructure to complex architec-
tures and, for this reason, is being widely employed in
these days of supramolecular chemistry and crystal
engineering. The interaction is basically electrostatic
in nature; hence, all factors influencing the charge
distribution on the donor/acceptor system affect the
strength of the hydrogen bond. Owing to the relevance
of organometallic ions, the role of ionic charge in
organometallic and inorganic crystal engineering is
much more relevant than in organic chemistry.
(v) The four crystalline materials present a large
number of short C-H---F(δ-)(P) interactions between the
cyclopentadienyl (1 and 2) or the arene ligands (3 and
4) and the PF6- anions. The length and number of short
distances correlates with the position of the metal atom
in the Periodic Table; i.e, more numerous and shorter
C-H---F(δ-)(P) distances are in the order (C6H6)2Cr+
>
(C5H5)2Fe+ > (C5H5)2Co+, which is also the order of
increasing need of back-donation from the metal atoms
onto the ligands.
Ack n ow led gm en t. D.B. and S.D. gratefully ac-
knowledge CIBA-Geigy for an ACE fellowship and the
ERASMUS exchange program “Crystallography” (N.S.).
We thank anonymous reviewers for useful comments
and suggestions. Financial support from the University
of Bologna - Project “Intelligent Molecules and Molec-
ular Aggregates” (D.B. and F.G.) is acknowledged.
(vi) The Cambridge Crystallographic Database con-
tains 926 coordination and organometallic salts crystal-
lized with the PF6- anion with (X)H---F(δ-)(P) distances
shorter than 2.6 Å (831 for X ) C, 80 for X ) N, 15 for
X ) O). The distribution of distances and angles are in
all three cases indicative of hydrogen-bonding behavior.
There are many short interactions, and as the distance
decreases the X-H---F(δ-)(P)angle approaches linearity.
When the donor is carbon, the behavior recalls very
closely that previously found in C-H---OC interactions
involving the CO ligand.
Su p p or tin g In for m a tion Ava ila ble: Tables of anisotro-
pic thermal parameters and fractional atomic coordinates, a
complete list of bond lengths and angles, ORTEP drawings,
and input files for the CSD searches (26 pages). Ordering
information is given on any current masthead page.
OM9706626