Functionalization of 9-(Dicyanomethylene)fluorene derivatives with substituted acetylenes

Article abstract of DOI:10.1002/ejoc.200390043

Treatment of (2,7-dibromo-9H-fluoren-9-ylidene)malononitrile (1) with excesses of substituted acetylenes RC ≡ CH [R = Ph, MeC₆H₄, (η⁵-C₅H₅)Fe(η⁵- C₅ H₄)] under Sonogas

Full text of DOI:10.1002/ejoc.200390043

FULL PAPER
Functionalization of 9-(Dicyanomethylene)fluorene Derivatives with Substituted
Acetylenes
Wai-Yeung Wong,*[a] Guo-Liang Lu,[a] Ka-Ho Choi,[a] and Zhenyang Lin[b]
Keywords: Alkynes / C-C coupling / Cyanides / Density functional calculations
Treatment of (2,7-dibromo-9H-fluoren-9-ylidene)malononitr- 1:2 molar ratio first have to be carried out, initially resulting
ile (1) with excesses of substituted acetylenes RCϵCH [R =
Ph, MeC6H4, (η5-C5H5)Fe(η5-C5H4)] under Sonogashira
coupling catalytic conditions afforded novel 9-[cyano(ethyn-
in the formation of compounds 5−7 in good yields. Upon sub-
sequent thermal treatment of 5−7 with malononitrile in
DMSO, compounds 8−10 can be isolated in high purity. All
yl)methylene]fluorene derivatives 2−4, respectively, in mod- attempts to obtain 8−10 from direct coupling of 1 and
erate yields. In these reactions, facile functionalization occurs
at the 9-methylene position in the fluorene, one of the cyano
groups being substituted by an acetylide moiety. In order to
prepare the 2,7-diethynyl-substituted 9-(dicyanomethylene)-
RCϵCH failed. Single-crystal X-ray structural analyses and
density functional calculations have been performed on 3, 4,
7, and 8.
fluorene compounds 8−10, palladium-catalyzed coupling re- ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
actions between 2,7-dibromofluoren-9-one and RCϵCH in a
Germany, 2003)
Introduction
with a donor moiety can efficiently sensitize the photocon-
ductivity of carbazole-containing polymers in the intramo-
lecular charge-transfer spectral region of the acceptors.[8a,8b]
A large number of polynitro-substituted fluoren-9-ones and
9-(dicyanomethylene)fluorenes have been synthesized and
studied, particularly by Perepichka’s group.[8] Very recently,
a comprehensive program investigating some organometal-
lic polyyne polymers incorporating fluorene-based auxiliar-
ies was initiated in our laboratory.[9] In the course of our
studies, the first example of a soluble, low-bandgap plat-
There is continuing research interest in the development
of carbon-rich organic and organometallic compounds con-
taining rigid, π-conjugated chains, due to their widespread
applications in the field of materials science.[1Ϫ3] In this
context, the use of fluorenyl chromophores and their deriv-
atives offers exciting perspectives for the design of new mo-
lecular and polymeric materials for various optoelectronic
applications.[4] It has been demonstrated that poly(2,7-fluo-
renediyl) derivatives are attractive as active components of
organic light-emitting diodes, thanks to their thermal and
chemical stability and their high emission quantum yields.[5]
Substituent derivatization at the C-9 position of the mono-
meric fluorenes offers the prospect of controlling polymer
properties such as solubility, emission wavelengths, pro-
cessability, and potential interchain interactions in films.[6]
A number of elegant synthetic means have been employed
to generate novel fluorene-containing materials with en-
hanced device performance.[7]
inum() polyyne with
a 9-(dicyanomethylene)fluorene
spacer has been reported.[9b] It has a bandgap located at
1.58 eV, the lowest of any of the metal polyynes known in
the literature. Modification of the polymer backbone with
electron-deficient dicyano-substituted electron acceptors in
the side chain was found to be effective for tuning the
bandgaps of this class of materials or of similar compounds
while maintaining their solubility and processability. In a
further step we have studied the coupling reactions between
(2,7-dibromo-9H-fluoren-9-ylidene)malononitrile (1) and
some terminal acetylenes. Although palladium-catalyzed
Sonogashira coupling between aryl halides and terminal
acetylenes has been shown to be a versatile method for in-
troduction of ethynyl groups into organic structures,[10] no
direct coupling products were obtained in these particular
cases. Instead, some unexpected molecules were formed,
one of the cyano groups being expelled by the incoming
acetylide moiety to afford unsymmetrical monocyano-sub-
stituted fluorene compounds. Attempts to produce the sym-
metrical coupling products require the utilization of the
corresponding fluoren-9-one derivatives in the first step in
On the other hand, electron acceptors of the fluorene
series have also aroused considerable attention in the study
of charge-transfer complexes with semiconducting and
photoconducting properties and as electron-transport mat-
erials.[8] Electron-deficient fluorene derivatives substituted
[a]
Department of Chemistry, Hong Kong Baptist University,
Waterloo Road, Kowloon Tong, Hong Kong, P. R. China
Fax: (internat.) ϩ 852/3411-7348
E-mail: rwywong@hkbu.edu.hk
[b]
Department of Chemistry, The Hong Kong University of
Science and Technology,
Clearwater Bay, Hong Kong, P. R. China
Eur. J. Org. Chem. 2003, 365Ϫ373  2003 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1434Ϫ193X/03/0102Ϫ0365 $ 20.00ϩ.50/0
365

W.-Y. Wong, G.-L. Lu, K.-H. Choi, Z. Lin
FULL PAPER
such a way that coupling reactions must take place before
the condensation reaction with malononitrile.
Results and Discussion
Synthesis
Scheme 1 summarizes the reaction steps affording the
new compounds in this study. Compound 1 was readily ac-
cessible in high yield by condensation between 2,7-dibrom-
ofluoren-9-one and malononitrile in DMSO at 110
°C.[8a,8d,8e,8g,9b] Treatment of 1 with slight excesses of substi-
tuted acetylenes RCϵCH [R
ϭ
Ph, MeC6H4, (η5-
C5H5)Fe(η5-C5H4)] in a Pd(OAc)2/PPh3/CuI/iPr2NH mix-
ture resulted in the production of new unsymmetrical
acetylide-functionalized compounds 2Ϫ4, respectively, in
moderate yields, accompanied by the elimination of HCN.
No substitution at the methylene position took place under
identical experimental conditions in the absence of the cata-
lysts, and only the hydrolyzed product 2,7-dibromofluoren-
9-one was isolated. However, the mechanistic pathway for
this reaction has not yet been established.[11] There was also
no sign of the formation of the expected direct coupling
products in each case. Compounds 2Ϫ4 were purified by
preparative TLC on silica with a mixture of hexane and
CH2Cl2 as eluent and obtained as solids in high purity. At-
tempted substitution of the remaining cyano group with
acetylenes by treatment of 2Ϫ4 with more RCϵCH was
unsuccessful[12] and again only hydrolysis took place to give
2,7-dibromofluoren-9-one as the sole product. In place of
the use of 1 as the starting precursor, Sonogashira coupling
reactions between 2,7-dibromofluoren-9-one and 1-alkynes
with a feed ratio of 1:2 provided, as anticipated, a high-
yield synthesis of bis(acetylene)-functionalized compounds
5Ϫ7 after TLC purification.[9,10] Subsequent treatment of
5Ϫ7 with malononitrile in DMSO was able to furnish the
desired dicyano-substituted products 8Ϫ10. All the new
compounds are generally soluble in common chlorinated
solvents such as CH2Cl2 and CHCl3, but we note that the
solubility is significantly lower for 8Ϫ10. Satisfactory ana-
lytical and spectroscopic (FTIR, NMR, and MS) data read-
ily establish the exact identity of each compound, and the
molecular structures of 3, 4, and 8 have been determined
unequivocally by single-crystal X-ray analyses.
Scheme 1. (i) RCϵCH, Pd(OAc)2, PPh3, CuI, iPr2NH; (ii)
CH2(CN)2, DMSO, 110 °C
tures, by comparison with data for previous 2,7-disubsti-
tuted fluorenes.[8,9] The proton signals due to the aromatic
1
and methyl groups are clearly evident from the H NMR
spectra. For 4 and 10, the expected ferrocenyl spectral pat-
tern can be observed, the unsubstituted C5H5 ring resonat-
ing as a strong singlet and the monosubstituted C5H4 ring
giving an unsymmetrical pair of ‘‘pseudo’’ triplets corres-
ponding to the A2B2 spectrum with J(adjacent) ഠ J(cross).
The spectral features of 5Ϫ10 confirm the symmetrical ar-
rangements of these molecules. In most cases the formulae
of new compounds were also established by the appearance
of intense molecular ion peaks in the respective positive
FAB mass spectra.
The molecular structures of 3, 4, and 8 were successfully
determined by X-ray crystallography and are illustrated in
Figures 1, 2, and 3, respectively. Pertinent bond lengths and
angles are collected in Tables 1 and 2. The crystal structure
of 7 was reported in our earlier paper.[13] The structure of
3 shows one of the cyano groups of the original precursor
replaced by a tolylacetylene fragment. This, together with
the remaining cyano unit, is bonded to C(14) at the 9-
methylene position of the fluorene nucleus.[12] Compound 4
is structurally similar to 3, except that a ferrocenylacetylene
group is bound to the methylene carbon atom at C(14) in-
stead. To the best of our knowledge, compounds 3 and 4
represent the first structurally characterized examples of
fluorene derivatives possessing σ-acetylene groups at their
9-methylene positions. The two structures do not show sig-
Spectroscopic and Structural Characterization
The solution IR spectra in each case reveal the character- nificant differences in the structural parameters of their cy-
istic ν(CϵC) vibrational bands, and the values at the side anofluorene moieties. The central fluorene ring is nearly
chain for 2Ϫ4 span the narrow 2186Ϫ2192 cmϪ1 range. For planar in each case (mean deviation ca. 0.0223 A in 3 and
˚
˚
the rigid-rod bis(acetylene) species, the CϵC stretching fre- 0.0067 A in 4) and makes a dihedral angle of 9.2° with the
quencies along the main chain are higher for the ferrocenyl C6H4 ring in 3. The exo double bond C(13)ϪC(14) is dis-
derivatives 7 and 10 (2210 cmϪ1) than for the analogous torted
from
planarity
by
6°
[defined
by
phenyl and tolyl congeners (ca. 2123Ϫ2144 cmϪ1). In addi- C(1)ϪC(13)ϪC(14)ϪC(15)] in 3, but the distortion is less
tion, strong absorptions due to ν(CϵN) and ν(CϭO) pronounced in 4. The C(13)ϪC(14) double bond length is
˚
stretching modes are apparent for the 9-(cyanomethylene)- typical at 1.361(8) (3) and 1.363(7) A (4), which compares
substituted fluorene and fluoren-9-one species, respectively. well with those in [(η5-C5H5)Fe{η5-C5H4CHϭ(Ar)}] [Ar ϭ
The 1H NMR spectra are entirely consistent with the struc- 2-formyl-9-fluorenyl: 1.354(6),[14] 2,7-diformyl-9-fluorenyl:
366
Eur. J. Org. Chem. 2003, 365Ϫ373

Functionalization of 9-(Dicyanomethylene)fluorene Derivatives with Substituted Acetylenes
[9c]
FULL PAPER
1.340(3),[14] 2,7-diethynyl-9-fluorenyl: 1.348(3) A], trans-
˚
[Ph(Et3P)2PtCϵCArCϵCPt(PEt3)2Ph] [Ar ϭ 9-(ferrocenyl-
methylene)fluorene-2,7-diyl: 1.37(2),[9c] 9-(dicyanomethy-
lene)fluorene-2,7-diyl: 1.348(9) A],[9b] 9-[cyano(dimethylam-
˚
[8e]
˚
ino)methylene]-2,4,5,7-tetranitrofluorene [1.388(4) A]
and other polynitro-substituted 9-(dicyanomethylene)fluor-
enes.[8d][8g] The exocyclic double bond is partially conjug-
ated with the acetylenic fragment, resulting in notable π-
bond delocalization along the C(13)ϪC(14)ϪC(16) frag-
Figure 3. A perspective drawing of compound 8
Structurally, the molecule of 8 consists of two phenylacet-
ylene end groups covalently appended to the central 9-(di-
cyanomethylene)fluorene moiety at the 2,7-positions
through C(8)ϪC(9) and C(18)ϪC(25) bonds. The 9-substi-
tuted fluorenediyl ring system is planar, with a mean devi-
˚
ment, with the C(14)ϪC(16) bond lengths being 1.444(8) A
˚
(3) and 1.432(8) A (4). The C(16)ϪC(17) bond lengths [3:
˚
1.189(8); 4: 1.187(8) A] are characteristic of acetylene σ-
bonding and the C(16)ϪC(17)ϪC(18) fragment does not
deviate significantly from linearity. For 4, the two cyclo-
pentadienyl rings are almost parallel, with a ring tilt angle
of 1.2°, and the ferrocenyl group is nearly eclipsed in the
solid state. The FeϪC(cyclopentadienyl) distances vary
˚
ation from the best plane of 0.0206 A, and almost coplanar
with the C(27) phenyl plane (dihedral angle 4.7°). The two
phenyl planes are significantly twisted with respect to each
other (dihedral angle 66.8°). The C(21)ϪC(22) bond is
˚
from 2.019(6) to 2.051(6) A and Fe(1) is at an average dis-
˚
1.360(3) A long, while the CϵC bond lengths are 1.198(2)
˚
tance of 1.6489 A from the centroids of the rings. In neither
˚
and 1.194(3) A. We anticipate that conjugation through the
case was any sign of formation of well-defined stacks in the
crystals detected.
molecule in the ground-state configuration should produce
a shortening of the C(6)ϪC(7), C(8)ϪC(9), C(18)ϪC(25),
˚
and C(26)ϪC(27) bonds [1.425(2)Ϫ1.434(2) A] and a
lengthening of the C(7)ϪC(8) and C(25)ϪC(26) bonds.[14]
Again, no plane-to-plane stacking between adjacent molec-
ules occurs in the crystal lattice of 8.
Absorption Properties
The solution electronic absorption spectra of all new
compounds were measured, and the data are collected in
Table 3. In general, the absorption spectra of the non-ferro-
cenyl compounds show intense, relatively high-energy bands
in the near UV and visible region, arising from the π Ǟ π*
transitions of the aromatic fragments. The electronic spectra
of the ferrocenyl species 4, 7, and 10 are essentially charac-
terized by the structureless π Ǟ π* transition bands of the
corresponding fluorene π skeleton together with metal-to-
ligand charge-transfer (MLCT) transitions in the low-en-
ergy region of the spectra, which was supported by molecu-
lar orbital calculations (vide infra). Within each particular
fluorene spacer, replacement of the R groups in the purely
organic compound with the ferrocenyl unit tends to red-
shift the positions of λmax. We note a bathochromic shift of
ca. 186 nm on going from 2 to 4 and of 135 nm on going
from 9 to 10. Comparison of the data for 5Ϫ10 shows that
the presence of the two π*-accepting cyano groups lowers
the optical gap values significantly, by as much as 0.50 eV,
since these groups can stabilize the LUMO to a greater ex-
tent than the carbonyl group in the fluoren-9-one counter-
parts. Moreover, the introduction of two terminal ethynyl
bridges at the 2,7-positions in 8Ϫ10 causes a narrowing of
the optical gaps relative to 1. One remarkable feature in this
study is the observation of a solvatochromic phenomenon
for the lowest-energy MLCT band in 4 (hexane Ǟ CH2Cl2,
red shift of 30 nm), consistent with a dipolar excited state
due to intramolecular charge-transfer (ICT) transition from
the electron-donating ferrocenyl group to the electron-with-
drawing cyano group through the conjugated system.[14]
Figure 1. A perspective drawing of compound 3
Figure 2. A perspective drawing of compound 4
Eur. J. Org. Chem. 2003, 365Ϫ373
367

W.-Y. Wong, G.-L. Lu, K.-H. Choi, Z. Lin
FULL PAPER
˚
Table 1. Selected bond lengths [A] and angles [°] for compounds 3 and 4
3
4
3
4
Br(1)ϪC(3)
C(13)ϪC(14)
C(15)ϪN(1)
C(16)ϪC(17)
Fe(1)ϪC(18)
Fe(1)ϪC(20)
Fe(1)ϪC(22)
Fe(1)ϪC(24)
Fe(1)ϪC(26)
1.920(6)
1.361(8)
1.155(8)
1.189(8)
1.894(6)
1.363(7)
1.134(7)
1.187(8)
2.019(6)
2.051(6)
2.030(6)
2.034(7)
2.045(8)
Br(2)ϪC(10)
1.891(6)
1.427(8)
1.444(8)
1.443(8)
1.894(6)
1.451(8)
1.432(8)
1.429(8)
2.039(6)
2.051(6)
2.046(8)
2.034(7)
2.025(7)
C(14)ϪC(15)
C(14)ϪC(16)
C(17)ϪC(18)
Fe(1)ϪC(19)
Fe(1)ϪC(21)
Fe(1)ϪC(23)
Fe(1)ϪC(25)
Fe(1)ϪC(27)
C(13)ϪC(14)ϪC(15)
C(14)ϪC(16)ϪC(17)
122.4(5)
176.9(6)
121.9(5)
176.4(7)
C(14)ϪC(15)ϪN(1)
C(16)ϪC(17)ϪC(18)
176.8(7)
175.0(6)
174.6(7)
173.9(7)
˚
cyclic voltammogram is characterized by a single quasi-re-
versible oxidation wave due to the presence of the ferrocenyl
electrophore in the compound. An anodic shift of the ferro-
cene/ferrocenium couple with respect to the ferrocene
standard is consistent with the unsaturation of the ethynyl
bridge, which makes the removal of an electron more diffi-
Table 2. Selected bond lengths [A] and angles [°] for compound 8
C(7)ϪC(8)
1.198(2) C(21)ϪC(22)
1.429(3) C(23)ϪN(1)
1.436(3) C(24)ϪN(2)
1.194(3)
1.360(3)
1.139(3)
1.143(3)
C(22)ϪC(23)
C(22)ϪC(24)
C(25)ϪC(26)
C(7)ϪC(8)ϪC(9)
175.6(2)
C(21)ϪC(22)ϪC(23) 123.01(17)
C(22)ϪC(23)ϪN(1) 178.6(2)
C(22)ϪC(24)ϪN(2) 177.4(2)
C(21)ϪC(22)ϪC(24) 123.29(17) cult than in pure ferrocene.[15] Clearly, the E1/2 value is more
C(18)ϪC(25)ϪC(26) 179.2(2)
anodic in 4 than in 7 and 10, owing to the closer attachment
of the strongly electronegative cyano group to the ferrocenyl
unit in the former. Analogously to 7 and to other, previ-
The presence of such an MLCT band in 4 accounts for the
deep blue color of its solution in CH2Cl2. In contrast, the
higher-energy absorptions of 4 are relatively insensitive to
variation in solvent polarities (Table 3). Electron acceptors
of the fluorene series exhibiting strong ICT bands have been
well documented,[8] although no evidence of significant sol-
vatochromic shifts in the electronic absorptions was ob-
tained for the other compounds.
ously reported, related bis(ferrocenyl) complexes, the two
ferrocenyl end groups exhibit only sparse electronic com-
munication in 10 and the oxidation wave only corresponds
to a single-step, two-electron oxidation involving concomit-
ant oxidation of the two terminal ferrocenyl moieties.[3f,13]
A single irreversible reduction wave, attributable to the
central 9-(dicyanomethylene)fluorene unit, is detected for
each of 8Ϫ10, occurring at potentials slightly more cathodic
than that of 1. The more negative value of this reduction
peak in 10 agrees with increased electron density in the vi-
cinity of the electron-donating ferrocenyl chromophore.
This is in contrast to that found for (2,7-diethynyl-9H-flu-
oren-9-ylidene)malononitrile and its gold() derivative, for
which two irreversible waves are observed.[16] However, no
reduction is associated with the cyano group in 2Ϫ4.
Electrochemistry
The electrochemical behavior of our compounds was
studied by cyclic voltammetry in CH2Cl2 at room temper-
ature, and the results for the electroactive compounds are
given in Table 3. For the ferrocene-containing species, each
Table 3. Electronic absorption and redox data for 1Ϫ10
Compound
λmax [nm] (ε ϫ 10Ϫ4 [Ϫ1·cmϪ1])[a]
Optical gap [eV][b]
Eox (∆Ep) [V][c]
Ered [V]
Ϫ1.01[e]
1
2
3
4
5
6
7
8
9
10
284, 316, 345, 361, 480[d]
2.13
2.89
2.81
1.74
2.45
2.57
2.13
1.94
2.11
1.62
260 (7.6), 269 (11.8), 283 (3.8), 386 (1.5), 406 (1.6)
273 (5.6), 283 (7.0), 394 (3.3), 415 (3.6)
270 (5.0), 284 (4.0), 387 sh (1.6), 404 (1.7), 592 (0.4)[f]
302 (6.7), 341 (4.2), 351 (4.7), 443 (0.4)
295 (1.4), 313 (1.3), 334 (1.0), 421 (0.05)
288 (8.7), 349 (4.7), 493 (0.8)[g]
320 (11.1), 352 sh (6.8), 544 (0.3)
286 (3.9), 314 (2.3), 345 (2.2), 360 (1.8), 488 (0.04)
320 (5.4), 461 sh (0.3), 623 (0.1)
0.27 (119)
0.14 (159)
0.14 (99)
Ϫ1.06[h]
Ϫ1.04[h]
Ϫ1.13[h]
[a]
[b]
[c]
All absorption spectra were recorded in CH2Cl2 at 293 K. Estimated from the absorption edge. All the potential values are with
reference to the external ferrocene standard. Eox ϭ (Epc ϩ Epa)/2 for reversible oxidation. ∆Ep in mV. Scan rate ϭ 100 mV·sϪ1 [d] Limited
.
[e]
[f]
solubility for accurate determination of ε. Reversible wave. Data [nm] (in ethanol): 267, 365, 400, 583; (in hexane): 257, 266, 274,
383, 400, 562. Ref.[13] [h] Irreversible wave.
[g]
368
Eur. J. Org. Chem. 2003, 365Ϫ373

Functionalization of 9-(Dicyanomethylene)fluorene Derivatives with Substituted Acetylenes
FULL PAPER
Theoretical Studies
orbitals. Each iron center in the complex is formally re-
garded as FeII. The two dz orbitals (of the two Fe centers)
2
To study the electronic structures of the unsymmetrical
monocyano complexes, we carried out molecular orbital
calculations (at the B3LYP level of density functional the-
ory) for 3 and 4, based on their experimental geometries
obtained from the crystallographic studies,[17] and the re-
sults were compared to those for 7 and 8. Examining the
characteristics of the highest occupied (HOMO) and the
lowest unoccupied (LUMO) molecular orbitals for 3 and 4,
the structures of which differ by only one substituent, we
found that the LUMOs of the two systems have almost the
same orbital characteristics as the π* antibonding in the
common conjugated unit. However, the HOMO in the
ferrocenyl system in 4 contains mainly the Fe dδ metal or-
bitals while the HOMO in 3 is principally composed of
atomic orbitals of the tolyl substituent (Figure 4). The cal-
culated HOMOϪLUMO gaps for 3 and 4 are 3.35 and
3.07 eV, respectively. The difference in the orbital character-
istics in the HOMO explains why compound 4 has a smaller
optical gap than 3, because the metal orbitals generally have
higher orbital energies than the π bonding orbitals of an
aromatic C6H4 group because of their ionization potentials.
are located in the lower energy region. The LUMOs corre-
spond to the π* orbitals of the organic aromatic entity. The
calculated HOMOϪLUMO gap for 7 (3.20 eV) is larger
than that found for 4 since the cyano group in the latter
molecule has a stabilizing effect on the LUMOs, resulting
in their remarkably low orbital energies. For 8, the HOMO
is π-bonding in nature and extensively delocalized in the
three aromatic rings and the two acetylenic units, while the
LUMO corresponds to the π*-antibonding of the central
fluorenyl unit with the participation of π* from the two
CN groups.
Figure 5. Spatial plots of the frontier molecular orbitals for 7
Figure 4. Spatial plots of the highest occupied (HOMO) and the
lowest unoccupied molecular orbitals (LUMO) for (a) 3 and (b) 4
Conclusions
Figure 5 shows contour plots[18] of the frontier molecular
In continuation of our recent studies on the chemistry
orbitals for the bis(ferrocenylethynyl) species 7. We found of 9-(dicyanomethylene)-substituted fluorene derivatives, a
that the HOMOs are very close to each other in orbital novel reactivity pattern Ϫ by which introduction of a σ-
energies and essentially consist of the four Fe dδ metal or- acetylene group at the 9-methylene position of the central
bitals together with small contributions from the highest π- fluorene system is feasible Ϫ has been demonstrated here
bonding orbitals of the fluorenone unit.[3f] The reason we and a series of 9-[cyano(ethynyl)methylene]fluorene com-
have four such dδ orbitals is because we have two Fe centers pounds have been prepared. The electronic absorption and
in the complex and each Fe metal center would be expected redox properties of these unsymmetrical molecules have
to have two. The δ-type is defined by viewing the orbitals been examined as a function of the electronic and geometric
along the CtϪFeϪCt axes (Ct: the center of each cyclo- natures of the 9-substituents of fluorene and are consistent
pentadienyl ligand). If the CtϪFeϪCt axis is defined as the with the theoretical calculations. Of particular interest is the
2
2
z axis, the two dδ orbitals correspond to the dxy and dx Ϫy
intramolecular charge-transfer transition encountered in
Eur. J. Org. Chem. 2003, 365Ϫ373
369

W.-Y. Wong, G.-L. Lu, K.-H. Choi, Z. Lin
FULL PAPER
(386.05): calcd. C 49.78, H 1.57, N 7.26; found C 49.50, H 1.38,
N 7.02.
the donor/acceptor-type compound 4, for which significant
solvatochromism is observed for the lowest-energy MLCT
band. The work here provides us with better understanding
in the molecular design of new C-9-functionalized fluorene-
based conjugated oligomers and copolymers for fine-tuning
of various physical properties.
Compound 2: A mixture of 1 (38.6 mg, 0.10 mmol) and phenylacet-
ylene (24.2 µL, 0.22 mmol) in iPr2NH (5 mL) was combined with
CuI (1.0 mg), Pd(OAc)2 (1.0 mg), and PPh3 (2.0 mg). The mixture
was heated at reflux for 18 h. After all the volatile components had
been removed under reduced pressure, the residue was taken up in
CH2Cl2 and the extract was filtered through a short silica gel col-
umn. The filtrate was concentrated and subjected to silica gel TLC
separation with hexane/CH2Cl2 (2:1, v/v) as eluent. The yellow
band (Rf ϭ 0.29) afforded an orange product, which was sub-
sequently recrystallized from CH2Cl2/hexane to give 2 in 22% yield
Experimental Section
General Procedures: All reactions were carried out under nitrogen
with the use of standard Schlenk techniques, but no special precau-
tions were taken to exclude oxygen during workup. Solvents were
predried and distilled from appropriate drying agents. All chemicals
were obtained from commercial sources, unless otherwise stated,
and used as received. Preparative TLC was performed on 0.7 mm
silica plates (Merck Kieselgel 60 GF254) prepared in our laboratory.
Ethynylferrocene[19] and compound 7[13] were prepared by literature
methods. IR spectra were recorded in CH2Cl2 with a PerkinϪElmer
Paragon 1000 PC or a Nicolet Magna 550 Series II FTIR spectro-
meter. Proton NMR spectra were measured in CDCl3 with a JEOL
EX270 or a Varian INOVA 400 MHz FT NMR spectrometer, with
1H NMR chemical shifts quoted relative to SiMe4. Fast atom bom-
bardment (FAB) mass spectra were recorded with a Finnigan MAT
SSQ710 mass spectrometer. Electronic absorption spectra were ob-
tained with a Hewlett Packard 8453 UV/Vis spectrometer. Cyclic
voltammetry experiments were performed with a Princeton Applied
Research (PAR) model 273A potentiostat. A conventional three-
electrode configuration consisting of a glassy-carbon working elec-
trode, a Pt wire counter-electrode and an Ag/AgNO3 reference elec-
trode (0.1  in acetonitrile) was used. The solvent in all measure-
ments was deoxygenated CH2Cl2 and the supporting electrolyte
was 0.1  [Bu4N]PF6. Ferrocene was added as a calibrant after
each set of measurements and all potentials reported are with refer-
ence to the ferrocene/ferrocenium couple. The number of electrons
transferred for compound 10 was estimated by comparison of the
peak height of the respective ferrocene oxidation wave with an
equal concentration of the ferrocene standard added in the same
system, in which one-electron oxidation was assumed. CAUTION:
The by-product HCN is extremely toxic, and all experimentation
in the preparations of 2Ϫ4 should be carried out in a well-vented
hood!
(10.0 mg). IR (CH2Cl2): ν˜ ϭ 2220 [ν(CϵN)], 2192 [ν(CϵC)] cmϪ1
1H NMR (CDCl3): δ ϭ 7.45Ϫ7.50 (m, 5 H, Ph), 7.55Ϫ7.58 (m, 2
H, 4-H, 5-H), 7.69Ϫ7.71 (m, 2 H, 3-H, 6-H), 8.57 (d, JH,H
.
ϭ
2.0 Hz, 1 H, 1-H or 8-H), 8.83 (d, JH,H ϭ 2.0 Hz, 1 H, 1-H or 8-
H) ppm. MS (FAB): m/z ϭ 461 [Mϩ]. C23H11Br2N (461.16): calcd.
C 59.90, H 2.40, N 3.04; found C 59.59, H 2.26, N 2.95.
Compound 3: Compound 3 was prepared under the conditions de-
scribed above for 2, but 4-ethynyltoluene (27.9 µL, 0.22 mmol) was
used instead, to afford 3 as a pale orange solid in 35% yield
(16.6 mg) after preparative TLC purification on silica (Rf ϭ 0.59)
with hexane/CH2Cl2 (2:1, v/v). IR (CH2Cl2): ν˜ ϭ 2219 [ν(CϵN)],
2190 [ν(CϵC)] cmϪ1 1H NMR (CDCl3): δ ϭ 2.44 (s, 3 H, Me),
.
7.30 (m, 2 H, C6H4), 7.47Ϫ7.50 (m, 2 H, 4-H, 5-H), 7.57Ϫ7.61 (m,
4 H, 3-H, 6-H ϩ C6H4), 8.60 (d, JH,H ϭ 2.0 Hz, 1 H, 1-H or 8-H),
8.87 (d, JH,H ϭ 2.0 Hz, 1 H, 1-H or 8-H) ppm. MS (FAB): m/z ϭ
475 [Mϩ]. C24H13Br2N (475.18): calcd. C 60.66, H 2.76, N 2.95;
found C 60.30, H 2.63, N 2.88.
Compound 4: Compound 4 was synthesized similarly, from ethynyl-
ferrocene (46.2 mg, 0.22 mmol), and was isolated as a deep blue
solid in 39% yield (22.0 mg) by TLC on silica (Rf ϭ 0.76) eluting
with hexane/CH2Cl2 (4:3, v/v). IR (CH2Cl2): ν˜ ϭ 2220 [ν(CϵN)],
1
2186 [ν(CϵC)] cmϪ1. H NMR (CDCl3): δ ϭ 4.34 (s, 5 H, C5H5),
4.49 (t, JH,H ϭ 2.0 Hz, 2 H, C5H4), 4.76 (t, JH,H ϭ 2.0 Hz, 2 H,
C5H4), 7.46Ϫ7.50 (m, 2 H, 4-H, 5-H), 7.55Ϫ7.60 (m, 2 H, 3-H, 6-
H), 8.57 (d, JH,H ϭ 1.6 Hz, 1 H, 1-H or 8-H), 8.82 (d, JH,H
ϭ
1.6 Hz, 1 H, 1-H or 8-H) ppm. MS (FAB): m/z ϭ 569 [Mϩ].
C27H15Br2FeN (569.08): calcd. C 56.99, H 2.66, N 2.46; found C
56.79, H 2.50, N 2.37.
Compound 5: 2,7-Dibromofluoren-9-one (101 mg, 0.30 mmol),
phenylacetylene (0.10 mL, 0.90 mmol), and iPr2NH (40 mL) were
mixed with catalytic amounts of CuI (2.0 mg), Pd(OAc)2 (2.0 mg),
and PPh3 (4.0 mg). The resulting mixture was stirred at 75 °C for
16 h. The volatile components were evaporated in vacuo, the res-
idue was taken up in CH2Cl2, and the solution was filtered through
a short silica column before being purified by TLC on silica (Rf ϭ
0.61), eluting with hexane/CH2Cl2 (3:2, v/v). Compound 5 was ob-
Computational Details: Density functional calculations at the
B3LYP level were carried out to study the electronic structures pre-
sented in the text. We were interested in knowing the properties of
the molecular orbitals in the frontier region for the molecules, so
single-point calculations were performed on the basis of their X-
ray crystal structures. The basis set used for C and H atoms was 6-
31G[20] while effective core potentials with a LanL2DZ basis set[21]
were employed for Br and Fe atoms. Polarization functions were tained as an orange solid in 56% yield (63.5 mg). IR (CH2Cl2):
1
added for Br atoms [ζd(Br) ϭ 0.389]. The Gaussian 98 program
was used for the calculations.[22] Molden 3.5 was used to generate
the contour plots of molecular orbitals.[18]
ν˜ ϭ 2123 [ν(CϵC)], 1723 [ν(CϭO)] cmϪ1. H NMR (CDCl3): δ ϭ
7.36Ϫ7.38 (m, 6 H, Ph), 7.52Ϫ7.56 (m, 6 H, 4-H, 5-H ϩ Ph), 7.67
(dd, JH,H ϭ 1.6, 7.6 Hz, 2 H, 3-H, 6-H), 7.82 (d, JH,H ϭ 1.6 Hz, 2
H, 1-H, 8-H) ppm. 13C NMR (CDCl3): δ ϭ 88.53, 91.43 (CϵC),
120.57, 122.71, 124.49, 127.43, 128.44, 128.72, 131.68, 134.42,
137.80, 143.19 (Ar), 192.29 (quat. C) ppm. MS (FAB): m/z ϭ 380
[Mϩ]. C29H16O (380.45): calcd. C 91.56, H 4.24; found C 91.40,
H 4.34.
Compound 1: A mixture of 2,7-dibromofluoren-9-one (203 mg,
0.60 mmol) and malononitrile (43.6 mg, 0.66 mmol) in DMSO
(3 mL) was heated at 110 °C for 5 h, during which the starting
yellow suspension turned dark red. The red powder was then fil-
tered off and washed with MeCN to give 1 in almost quantitative
yield (224 mg, 97%). IR (CH2Cl2): ν˜ ϭ 2227 [ν(CϵN)] cmϪ1
.
1H
Compound 6: A procedure similar to that used for 5 was employed,
NMR (CDCl3): δ ϭ 7.42 (d, JH,H ϭ 8.0 Hz, 2 H, 4-H, 5-H), 7.65
using 4-ethynyltoluene (0.11 mL, 0.90 mmol) to produce an analyt-
(dd, JH,H ϭ 1.2, 8.0 Hz, 2 H, 3-H, 6-H), 8.49 (d, JH,H ϭ 1.2 Hz, 2 ically pure sample of 6 as an orange solid in 78% yield (95.6 mg)
H, 1-H, 8-H) ppm. MS (FAB): m/z ϭ 386 [Mϩ]. C16H6Br2N2 after purification by TLC on silica (Rf ϭ 0.55), eluting with hexane/
370
Eur. J. Org. Chem. 2003, 365Ϫ373

Functionalization of 9-(Dicyanomethylene)fluorene Derivatives with Substituted Acetylenes
FULL PAPER
Table 4. Crystal data for compounds 3, 4, and 8
3
4
8
Empirical formula
Formula mass
Crystal system
Space group
C24H13Br2N
475.17
monoclinic
P21
3.9962(4)
25.246(3)
9.3067(10)
90
94.871(2)
90
C27H15Br2FeN
569.07
orthorhombic
Pbca
15.1338(12)
10.7557(9)
26.333(2)
90
90
90
4286.3(6)
8
C32H16N2
428.47
triclinic
¯
P1
˚
a [A]
7.7804(7)
9.1837(8)
16.2714(14)
93.397(2)
100.1110(10)
102.203(2)
1113.10(17)
2
˚
b [A]
˚
c [A]
α [°]
β [°]
γ [°]
3
˚
V [A ]
Z
935.53(17)
2
T [K]
293
4.341
293
4.448
293
0.075
µ(Mo-Kα) [mmϪ1
]
Reflections collected
Unique reflections
Rint
5494
3409
0.0229
2820
0.989
0.0389, 0.1001
0.0488, 0.1049
23960
4856
0.0890
2050
0.802
0.0416, 0.0988
0.1353, 0.1517
5542
3839
0.0306
3066
1.077
0.0585, 0.1729
0.0675, 0.1835
Observed reflections [I Ͼ 2.0σ(I)]
GOF on F2
R1, wR2 [I Ͼ 2σ(I)][a]
R1, wR2 (all data)[a]
2
2 2
2 2
R1 ϭ Σ||Fo| Ϫ |Fc||/Σ|Fo|; wR2 ϭ {Σ[w(Fo Ϫ Fc ) ]/Σ[w(Fo ) ]}1/2
.
CH2Cl2 (3:2, v/v). IR (CH2Cl2): ν˜ ϭ 2140 [ν(CϵC)], 1728 [ν(Cϭ
O)] cmϪ1. 1H NMR (CDCl3): δ ϭ 2.37 (s, 6 H, Me), 7.14 (d, JH,H ϭ 1H NMR (CDCl3): δ ϭ 4.28 (s, 10 H, C5H5), 4.30 (t, JH,H ϭ 2.0 Hz,
7.8 Hz, 4 H, C6H4), 7.38Ϫ7.43 (m, 6 H, 4-H, 5-H ϩ C6H4), 7.63 4 H, C5H4), 4.55 (t, JH,H ϭ 2.0 Hz, 4 H, C5H4), 7.49 (d, JH,H
(40.2 mg). IR (CH2Cl2): ν˜ ϭ 2229 [ν(CϵN)], 2209 [ν(CϵC)] cmϪ1
.
ϭ
(dd, JH,H ϭ 1.8, 8.0 Hz, 2 H, 3-H, 6-H), 7.77 (d, JH,H ϭ 1.8 Hz, 2 7.6 Hz, 2 H, 4-H, 5-H), 7.60 (dd, JH,H ϭ 1.2, 7.6 Hz, 2 H, 3-H, 6-
H, 1-H, 8-H) ppm. 13C NMR (CDCl3): δ ϭ 21.70 (Me), 73.43, H), 8.46 (d, JH,H ϭ 1.2 Hz, 2 H, 1-H, 8-H) ppm. MS (FAB): m/
81.51 (CϵC), 118.69, 121.74, 123.22, 127.74, 129.10, 132.26, z ϭ 644 [Mϩ]. C40H24Fe2N2 (644.34): calcd. C 74.56, H 3.75, N
135.16, 137.34, 139.36, 142.13 (Ar), 190.98 (quat. C) ppm. MS
(FAB): m/z ϭ 393 [Mϩ Ϫ Me]. C31H20O (408.50): calcd. C 91.15,
H 4.93; found C 91.10, H 4.88.
4.35; found C 74.30, H 3.68, N 4.24.
X-ray Crystallography: Yellow crystals of 3, 4, and 8 suitable for
X-ray diffraction studies were grown by slow concentration of their
solutions in hexane/CH2Cl2 at room temperature. Geometric and
intensity data were collected by use of graphite-monochromated
Compound 8: Compound 5 (30.4 mg, 0.08 mmol) and malononitrile
(5.30 mg, 0.08 mmol) were mixed in degassed DMSO (1 mL). The
yellow suspension was then heated to about 110 °C and stirred
for 5 h. After cooling and filtration, a dark brown precipitate was
obtained, and this was washed with a small amount of MeCN to
afford 8 in 94% yield (32.3 mg). IR (CH2Cl2): ν˜ ϭ 2228 [ν(CϵN)],
˚
Mo-Kα radiation (λ ϭ 0.71073 A) with a Bruker Axs SMART 1000
CCD area-detector. The collected frames were processed with
SAINT[23a] software and an absorption correction (SADABS[23b]
)
was applied to the collected reflections. The structures of these mo-
lecules were solved by direct methods (SHELXTL)[24] in conjunc-
tion with standard difference Fourier techniques and subsequently
refined by full-matrix, least-squares analyses on F2. All non-hydro-
gen atoms were assigned with anisotropic displacement parameters
and the hydrogen atoms were generated in their idealized positions
and allowed to ride on the respective carbon atoms. Crystal data
and other experimental details are summarized in Table 4. CCDC-
187006 to -187008 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
1
2144 [ν(CϵC)] cmϪ1. H NMR (CDCl3): δ ϭ 7.37Ϫ7.39 (m, 6 H,
Ph), 7.55Ϫ7.58 (m, 6 H, 4-H, 5-H ϩ Ph), 7.68 (dd, JH,H ϭ 1.2,
8.0 Hz, 2 H, 3-H, 6-H), 8.53 (d, JH,H ϭ 1.2 Hz, 2 H, 1-H, 8-H)
ppm. MS (FAB): m/z ϭ 428 [Mϩ]. C32H16N2 (428.49): calcd. C
89.70, H 3.76, N 6.54; found C 89.56, H 3.55, N 6.40.
Compound 9: The reaction between 6 (in place of 5; 102 mg,
0.25 mmol) and malononitrile (16.5 mg, 0.25 mmol) in degassed
DMSO (3 mL) at 110 °C resulted in the precipitation of the desired
product 9, which was worked up as above to give a reddish-orange
solid in 47% yield (53.6 mg). IR (CH2Cl2): ν˜ ϭ2227 [ν(CϵN)], 2138
[ν(CϵC)] cmϪ1. 1H NMR (CDCl3): δ ϭ 2.37 (s, 6 H, Me), 7.14 (d,
CB2 1EZ, UK; Fax: (internat.)
deposit@ccdc.cam.ac.uk].
ϩ 44-1223/336-033; E-mail:
J
H,H ϭ 8.1 Hz, 4 H, C6H4), 7.40Ϫ7.44 (m, 6 H, 4-H, 5-H ϩ C6H4),
7.66 (dd, JH,H ϭ 1.2, 8.1 Hz, 2 H, 3-H, 6-H), 8.50 (d, JH,H
ϭ
1.2 Hz, 2 H, 1-H, 8-H) ppm. MS (FAB): m/z ϭ 456 [Mϩ].
C34H20N2 (456.55): calcd. C 89.45, H 4.42, N 6.13; found C 89.20,
H 4.38, N 6.02.
Acknowledgments
Financial support from the Hong Kong Baptist University (FRG/
00-01/II-57 and FRG/01-02/II-48) is gratefully acknowledged.
Compound 10: This compound was prepared in a manner similar to
that used for 8, with compound 7 (47.7 mg, 0.08 mmol) as starting
material, giving the product as a greenish blue powder in 78% yield
Eur. J. Org. Chem. 2003, 365Ϫ373
371

W.-Y. Wong, G.-L. Lu, K.-H. Choi, Z. Lin
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