A toluene guest in a poly(heteronuclear) complex host of ferrocenyl (CH2)2-bridged bis(pyrazole)

Article abstract of DOI:10.1021/om800704a

Reactions of ferrocenyl-(CH₂)_n-bridged bis(pyrazoles) with nickelocene afforded inter- and intramolecular dimeric poly(heteronuclear) cyclopentadienylnickel(II) complexes from which a novel toluene-embodied guest-host system was obtained by π-interaction. The present paper has demonstrated a new class of potentially useful organometallic building blocks for supramolecular assembly and the first example of a pyrazolato ligand exhibiting a charge transfer type π-interaction.

Full text of DOI:10.1021/om800704a

Organometallics 2008, 27, 4833–4836
4833
A Toluene Guest in a Poly(heteronuclear) Complex Host of
Ferrocenyl-(CH2)2-Bridged Bis(pyrazole)
Weiqiang Tan,† Zhengkun Yu,*,†,‡ Wei He,† Liandi Wang,† Jie Sun,‡ and Jinzhu Chen†
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road,
Dalian, Liaoning 116023, People’s Republic of China, and State Key Laboratory of Organometallic
Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road,
Shanghai 200032, People’s Republic of China
ReceiVed July 24, 2008
eneoxyacetyl-pyrazolyl)ferrocene,13a 1,1′-bis{3-(2-pyridyl)pyrazol-
5-yl)}ferrocene,13b Reports of 1,1′-bis{(pyrazol-1-yl)borate}ferro-
cene,14 bitopic ferrocenyl-linked bis(pyrazolyl)methane,15 and 1,1′-
bis{(pyrazol-1-yl)methyl}ferrocene and its ansa analogues16 are the
only known publications to date. On the basis of the structural
features of ferrocene-based bis(pyrazoles), it can be envisioned that
they could be potentially useful as organometallic building blocks
for molecular crystal engineering17 and the construction of
functional compounds.18 During our ongoing studies on transition-
metal complexes of pyrazolyl-based N-heterocyclic ligands,5a,b,19
we have become interested in 1,1′-bis{(pyrazol-4-yl)alkyl}ferrocenes,
which may be used as organometallic building blocks. Herein, we
report on the synthesis of ferrocenyl-(CH2)n-bridged bis(pyrazoles)
and a novel toluene-containing guest-host system formed by a
charge transfer type π-interaction of pyrazolato ligands for the first
time.
Summary: Reactions of ferrocenyl-(CH2)n-bridged bis(pyra-
zoles) with nickelocene afforded inter- and intramolecular
dimeric poly(heteronuclear) cyclopentadienylnickel(II) com-
plexes from which a noVel toluene-embodied guest-host system
was obtained by π-interaction. The present paper has demon-
strated a new class of potentially useful organometallic building
blocks for supramolecular assembly and the first example of a
pyrazolato ligand exhibiting a charge transfer type π-interaction.
Pyrazolato ligands have demonstrated rich coordination chem-
istry due to their diverse coordination modes to metals.1-4 Pyrazoles
and pyrazolato ligands are potentially useful in devising syntheti-
cally useful processes in which the presence of a hemiliable ligand
is required,5a,b as well as in constructing metal architectures for
chemical vapor depositon.5c Pyrazoles also play a unique role in
the design and synthesis of biologically active agents.6 In particular,
incorporation of a ferrocene unit into an organic molecule usually
results in unexpected biological activity7 or electrochemical proper-
ties8 for the newly formed compound. In this aspect, limited work
has been directed to the synthesis of ferrocenyl-substituted pyrazoles
(Fc-PzH) for construction of bioactive molecules and their func-
tional transition-metal complexes.8,9 Ferrocene-based mono(pyra-
zoles), e.g., ferrocenylmethyl pyrazoles,10 ferrocenylamido pyra-
zoles,11 and ferrocenyl-tris(pyrazol-1-yl)borate ligands,12 have
been reported. However, the synthesis of ferrocene-based bis(pyra-
zoles) remains a challenge due to the synthetic difficulty, and only
a few examples have been documented. 1,1′-Bis(1,3-phenyl-
1,1′-Bis{(1H-pyrazol-4-yl)methyl}ferrocenes (4) were syn-
thesized by a modified literature procedure starting from
ferrocene (1) instead of ferrocene-1,1′-dicarboxaldehyde (Scheme
(9) For selected recent papers, see: (a) Xie, Y.-S.; Pan, X.-H.; Zhao,
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Organometallics 2007, 26, 4663–4672. (b) Guo, S. L.; Peters, F.; de Biani,
F. F.; Bats, J. W.; Herdtweck, E.; Zanello, P.; Wagner, M. Inorg. Chem.
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M.; Zanello, P. Inorg. Chem. 1997, 36, 2103–2111.
† Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
‡ Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences.
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10.1021/om800704a CCC: $40.75
2008 American Chemical Society
Publication on Web 09/11/2008

4834 Organometallics, Vol. 27, No. 19, 2008
Communications
Scheme 1. Synthesis of 1,1′-Bis{(1H-pyrazol-4-yl)methyl}
Scheme 2. Synthesis of Ferrocenyl Bis(pyrazoles) 13 and 14a
ferrocenes (4)a
a
Legend: (i) n-BuLi, TMEDA, (CH2O)n, Et2O, 23 °C, 24 h; (ii) 1,3-
diketone, 40% aqueous HBF4, 23 °C, 30 min; (iii) 85% aqueous
hydrazine, EtOH, reflux, 4 h.
1).16,20 In a different fashion, 1,1′-bis{n-(1H-pyrazol-4-yl)ethyl
or -propyl}ferrocenes (13 and 14) were prepared (Scheme 2).
2-Alkylation of a 1,3-diketone by an organic halide, especially
a dihalide, easily leads to dialkylation and other side reactions
of its enolized isomer with the alkylating reagent. Thus, different
procedures were used to synthesize 4-substituted pyrazole
intermediates, i.e., 4-(n-bromo-(CH2)n)pyrazoles 9, which was
protected by 2-tetrahydropyranyl (THP) before it was further
transformed to the cyclopentadienyl-functionalized intermediate
11. Due to its thermal unstability, compound 11 was not further
purified and was directly applied in the next step, synthesis of
the THP-protected ferrocene-based bis(pyrazole) 12. Acid-
catalyzed deprotection of 12 afforded the desired bis(pyrazole)
product 13 or 14. All of the ferrocenyl bis(pyrazoles) were fully
characterized, including by X-ray single-crystal structural
determinations of 13a and 14 (see the Supporting Information).
These bis(pyrazoles) exhibit very similar NMR features in
solution, and their proton NMR spectra reveal that the ferrocenyl
(Fc) and pyrazole moieties are present in a 1:2 molar ratio in a
ferrocenyl bispyrazole molecule. The 1H and 13C NMR spectra
of 4 and 13 demonstrate one set of resonance signals for their
pyrazole groups, suggesting no detectable tautomerism of the
pyrazole moieties in solution. However, two different sets of
resonance signals were observed for the pyrazole groups in the
NMR spectra of 14, revealing 3- and 5-tautomerism of the
pyrazole rings in solution.
Reactions of the ferrocenyl bis(pyrazoles) with nickelocene
in a 1:2 molar ratio were carried out in toluene or CH2Cl2 at
ambient temperature. Treatment of bis(pyrazoles) 4b and 13a
with Cp2Ni afforded the intermolecular dimeric products 15
(38%) and 16 (80%), respectively (eq 1), while the reaction of
bis(pyrazole) 14 with Cp2Ni gave the intramolecular dimeric
product 17 in 82% yield (eq 2). The reactions of other ferrocenyl
bis(pyrazoles), i.e., 4a,c and 13b, with Cp2Ni were also pursued
under the same conditions, producing a mixture of complex
products from which no identified product was isolated. It has
been known that reactions of 4-halo- or 4-methyl-substituted
pyrazoles with Cp2Ni can form dimetallic [CpNi(µ-Pz)]2,
trimetallic [CpNi(µ-Pz)2]2Ni, or polymeric [Ni(µ-Pz)2]x com-
plexes (Pz ) simple substituted pyrazolato) under different
a
Legend: (i) 1,2-dibromoethane, K2CO3, DMSO, 23 °C, 12 h; (ii)
NH2NH2 · H2O, aqueous NH4Br (0.02 M), 23 °C, 24 h; (iii) 1,3-
bromochloropropane, K2CO3, KI, acetone, reflux, 72 h; (iv) NH2NH2 · H2O,
aqueous HCl, ethanol, reflux, 8 h; (v) PBr3, 1,2-dichloroethane, reflux,
3 h; (vi) 3,4-dihydro-2H-pyran, p-TsOH, CH3CN, reflux, 3 h; (vii)
NaC5H5(THF)0.27, THF, 23 °C, 24 h; (viii) n-BuLi, FeCl2, THF, 23-60
°C, 16 h; (ix) aqueous HCl, p-TsOH, methanol, 23 °C, 48 h.
conditions.24 In our cases, complexes 15-17 are formally
dimeric with respect to the coordination mode of the pyrazolatos
and NiCp moieties. Variation of the (CH2)n linkers and
substituents on the pyrazolato rings led to different types of
dimeric polynuclear complex products, which can be attributed
to the steric impact of the ferrocenyl bis(pyrazoles). Complexes
15-17 were formed as red precipitates during the reaction, and
after recrystallization from toluene or toluene/dichloromethane
at -20 °C they usually exist as deep red crystals incorporated
with the solvent molecules. These complexes exhibit similar
NMR features in solution, revealing the presence of NiCp rings
and ferrocenyl Cp′ moieties in a 1:1 molar ratio. The 1H NMR
resonance signals of the Cp groups in the complexes appear at
5.57-4.92 ppm in CDCl3, while those of the ferrocenyl Cp′
moieties are shown in the region of 3.99-3.72 ppm, respec-
tively. The proton NMR spectral analyses also reveal that
residual dichloromethane was incorporated in the crystals of
(21) Zefirov, N. S.; Kuznetsova, T. S.; Kozhushkov, S. I.; Surmina, L. S.;
Rashchupkina, Z. A. J. Org. Chem. USSR (Engl. Transl.) 1983, 19, 474–480.
(22) Meikrantz, S. B.; Smith, M. P.; Xia, X. B.; Natale, N. R. Synth.
Commun. 1994, 24, 399–407.
(23) Zefirov, N. S.; Kozhushkov, S. I.; Kuznetsova, T. S. Tetrahedron
1982, 38, 1693–1697.
(24) (a) Blake, A. B.; Ewing, D. F.; Hamlin, J. E.; Lockyer, J. M. J. Chem.
Soc., Dalton Trans. 1977, 19, 1897–1901. (b) Rettig, S. J.; Storr, A.; Summers,
D. A.; Thompson, R. C.; Trotter, J. Can. J. Chem. 1997, 75, 949–958. (c)
Storr, A.; Thompon, R. C. Can. J. Chem. 1998, 76, 1130–1137.

Communications
Organometallics, Vol. 27, No. 19, 2008 4835
complex 15, and toluene coalesced with complexes 16 and 17
in 2:1 and 1:2 molar ratios in their crystals, respectively.25
Figure 1. Perspective view of complex 15.
The ferrocenyl bis(pyrazoles) of type 4 (HPzCH2-Fc-
CH2PzH) may exist in a syn or anti configuration in the solid
state, depending on the substituents on the pyrazole rings.16
However, -(CH2)2-Fc-(CH2)2- bridged bis(pyrazole) 13a
exists in both syn and anti configurations in the solid state, while
the -(CH2)3-Fc-(CH2)3- bridged bis(pyrazole) 14 only exists
in an anti configuration (see the Supporting Information). In a
unit cell of the single crystals of 13a, one anti-configuration
molecule binds two syn-configuration molecules through the
intermolecular N-H · · · N hydrogen bonds between the pyrazole
moieties. Water is coalesced in the single crystals of 14, and
four molecules of the bis(pyrazole) are held together in a unit
cell by the hydrogen bonds (N-H · · · N and O-H · · · N) formed
between the bis(pyrazole) moieties and the incorporated water.
Complex 15 exhibits a twisted dimeric structure with the formula
{Fc(CH2-3,5-PhMePzNiCp)2}2, in which the pyrazolato moieties
demonstrate µ-η1:η1 coordination to the nickel atoms and two
ferrocene-based bis(pyrazolato) ligands are coordinated to the
four NiCp moieties through the pyrazolato nitrogen atoms
(Figure 1). The Fe1 · · · Fe1A and Ni1-Ni2 distances in 15 are
11.053 and 3.150 Å, and the Fe1 · · · Ni1, Fe1 · · · Ni2, Fe1A · · · Ni1,
and Fe1A · · · Ni2 distances are 7.118, 7.038, 7.736, and 7.981
Å, respectively. Three solvent CH2Cl2 molecules coalesce
outside the complex coordination cavity. Figure 2 also reveals
a dimeric structure for complex 16. Due to the extended (CH2)2
linkers and coplanarity of the pyrazolato and its two methyl
substituents, the dimeric molecule {Fc{(CH2)2-3,5-Me2PzNi-
Cp}2}2 (16) exhibits a rather symmetrical ring structure (Figure
2c). Unexpectedly, one solvent molecule, i.e., toluene, is
incorporated in the coordination cavity of 16, and a second
toluene molecule coalesces outside the cavity (Figure 2a).
Organic solvent molecules (relatively small molecules) can be
usually incorporated in the crystals of a target compound via
intermolecular van der Waals forces during recrystallization.
In general, only polycyclic aromatic molecules can be contained
within an organic supramolecular coordination cage.26 It has
been seldom documented for a simple complex coordination
cage of type 16 to trap a small aromatic molecule such as an
organic solvent molecule like toluene. The Fe1 · · · Fe1A,
Fe1 · · · Ni1, and Fe1 · · · Ni2 distances are 13.768, 9.150, and
8.673 Å, respectively, much longer than those in 15, and the
Ni1 · · · Ni2 distance is almost the same as that in 15 (3.160 Å).
The pyrazolato ring and its two methyl substituents and the two
coordinated nickel atoms are nearly coplanar (Figure 2c; see
the Supporting Information). The molecular plane of the
incorporated toluene (A) is almost parallel to the coplanes
Ni1-N2-C13-C12-C11-N1-Ni2 (B) and Ni1A-N2A-
C13A-C12A-C11A-N1A-Ni2A (B′) and perpendicular to
the coplanes composed of the other two pyrazolato moieties
and their coordinated nickel atoms, respectively. The distances
between the incorporated toluene plane (A) and its parallel
planes B and B′ are ca. 2.8 Å, suggesting an efficient sandwich-
type B-A-B′ π-interaction in the crystals of complex 16. This
structure is attributed to donor-acceptor charge transfer (CT)
interactions through the aromatic components (toluene and pyra-
zolatos). A pyrazolato moiety is electronically equivalent to a
cyclopentadienyl group (Cp), and the Ni(II)-coordinated pyrazolato
ligands are highly electron deficient due to such a coordination.
Thus, the incorporated electron-rich toluene molecule can efficiently
form a CT complex with 16 by π-stacking interactions with its
two neighboring parallel pyrazolato moieties. The distance between
the outside toluene plane and its neighboring pyrazolato plane is
longer than 3.0 Å, suggesting no obvious π-interaction between
them. Complex 17 exhibits a twisted intramolecular dimeric
structure with an incorporated toluene molecule outside the
coordination cavity (Figure 3). The Fe1 · · · Ni1, Fe1 · · · Ni2, and
Ni1 · · · Ni2 distances are 7.542, 7.546, and 3.165 Å, respectively.
Although one more CH2 group is introduced to the linker chain
between the ferrocenyl and pyrazolato moiety, the Fe · · · Ni
(25) (a) Crystal data for 15 · 3CH2Cl2: C88H79Cl6Fe2N8Ni4, monoclinic,
P21/n, a ) 12.7273(7) Å, b ) 18.5978(9) Å, c ) 17.2650(9) Å, R ) γ )
90°, ꢀ ) 90.6440(10)°, V ) 4086.4(4) Å3, Z ) 2, T ) 293(2) K, Dcalcd
)
1.469 g cm-3, R(F) ) 5.79% for 4856 observed reflections (2.62 e 2θ e
j
51.00°). (b) Crystal data for 16 · 2PhMe: C82H92Fe2N8Ni4, triclinic, P1, a
) 10.9417(8) Å, b ) 11.8878(9) Å, c ) 14.2357(11) Å, R ) 79.2390(10)°,
ꢀ ) 83.8729(10) °, γ ) 89.668(2)°, V ) 1808.5(2) Å3, Z ) 1, T ) 293(2)
K, Dcalcd ) 1.410 g cm-3, R(F) ) 4.55% for 4502 observed reflections
(4.12 e 2θ e 54.00°). (c) Crystal data for 17 · 0.5PhMe: C49. 50H54FeN4Ni2,
j
triclinic, P1, a ) 10.5506(9) Å, b ) 11.7665(10) Å, c ) 18.0307(16) Å, R
) 72.868(2)°, ꢀ ) 81.398(2)°, γ ) 76.811(2)°, V ) 2074.5(3) Å3, Z ) 2,
T ) 293(2) K, Dcalcd ) 1.406 g cm-3, R(F) ) 3.90% for 5918 observed
reflections (3.70 e 2θ e 54.00°).

4836 Organometallics, Vol. 27, No. 19, 2008
Communications
Figure 2. Perspective views of complex 16: (a) view with incorporated toluene molecules; (b) view showing π-stacking; (c) view without
the solvent molecules.
bis(pyrazoles) as organometallic building blocks in a supramo-
lecular assembly.
Typical Procedure for Synthesis of the Complexes: Syn-
thesis of Complex 16. A mixture of 13a (200 mg, 0.46 mmol)
and nickelocene (176 mg, 0.93 mmol) in 10 mL of CH2Cl2 was
stirred at ambient temperature for 24 h, forming a dark red solution.
All the volatiles were removed under reduced pressure, and the
resultant red residue was purified by flash silica gel column
chromatography with n-hexane/toluene (v/v, 1/1) as the eluent to
afford 16 as a red solid (250 mg, 80%). Single crystals suitable
for an X-ray crystallographic determination were grown from
dichloromethane/toluene (v/v, 1/10) at ambient temperature. Mp:
1
>120 °C dec. H NMR (CDCl3, 400 MHz, 23 °C): δ 7.37 and
7.30 (br each, 4:6 H, aromatic CH of 2 × PhMe), 5.57 (s, 20 H,
CH of 4 × CpNi), 3.99 and 3.72 (s and br each, 8:8 H, CH of
ferrocenyl), 2.47 and 2.42 (br each, 8:8 H, 4 × CH2Pz and 4 ×
CH2Fc), 2.32 (s, 24 H, 8 × CH3), 2.16 (br, 6 H, CH3 of toluene).
13C{1H} NMR (CDCl3): δ 153.4 (Cq, C-N of Pz), 137.7 (Cq,
i-C of PhMe), 129.2, 128.4, and 125.4 (s each, aromatic CH of
toluene outside the coordination cavity), 128.7, 127.8, and 124.9
(t each, aromatic CH of toluene inside the coordination cavity),
117.9 (Cq, C4 of Pz), 92.6 (CH of CpNi), 88.8 (Cq, i-C of
Figure 3. Perspective view of complex 17 showing the incorporated
toluene molecule.
ferrocenyl), 68.5 and 67.3 (CH of ferrocenyl), 30.4 and 25.3 (CH2Pz
and CH2Fc), 14.3 (CH3). Anal. Calcd for C68H76Fe2N8Ni4 · 2PhMe:
C, 64.11; H, 6.04; N, 7.29. Found: C, 64.21; H, 5.90; N, 7.22.
distances in 17 are shortened by 1.1-1.6 Å as compared to those
in 16 due to the twisted structure. Both complexes 15 and 17
demonstrate highly twisted molecular structures because methyl
and phenyl substituents are introduced to the pyrazolato rings,
resulting in no coplanarity for the pyrazolatos and their associated
substituents. Therefore, no solvent toluene molecule can be
embodied in their coordination cavities by π-interaction.
Acknowledgment. We are grateful to the National Natural
Science Foundation of China (Grant Nos. 20501018 and
20772124) for financial support of this research.
Supporting Information Available: Text, tables, figures, and
CIF files giving experimental procedures, analytical data, copies
of NMR spectra, and X-ray crystallographic data for 13a and 14-
17. This material is available free of charge via the Internet at
http://pubs.acs.org.
In conclusion, reactions of ferrocenyl-(CH2)n-bridged bis(pyra-
zoles) with nickelocene can be applied to synthesize complex
cavities which may form guest-host complexes by π-interactions.
The coplanarity of the pyrazolato moiety and its two associated
substituents and the (CH2)n-linker lengths are crucial to formation
of such a guest-host system. The present toluene-containing
guest-host system has demonstrated the first example of a
pyrazolato ligand exhibiting a charge transfer type π-interaction
and the potential application of ferrocene-based (CH2)n-linked
OM800704A
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