Organometallic color chemistry: Studies on [FcCH=CHC5H4NCH2C6H 4(tBu)]X (X = BPh4-, ClO4-)

Article abstract of DOI:10.1016/S0022-328X(00)00422-8

The title dyes, [FcCH=CHC₅H₄NCH₂C₆H ₄(^tBu)]X (X = BPh₄^-, ClO₄^-), developed for rapid screening of catalysts, are intensely colored, but bleach on exposure to visible light in solution. Detailed study shows that on irradiation these alkene dyes first undergo a trans-cis isomerization, then give an irreversible bleaching via decomposition. Both dyes show color dependence on the nature of the anion as a result of ion-pairing, which affects the intensity and position of the visible absorption. NMR, MS and EPR data are also reported.

Full text of DOI:10.1016/S0022-328X(00)00422-8

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Journal of Organometallic Chemistry 608 (2000) 146–152
Organometallic color chemistry: studies on
[FcCHꢀCHC5H4NCH2C6H4(tBu)]X (X=BPh4−, ClO4−)
Montserrat Die´guez a,*1, Marie-Noe¨lle Collomb b, Robert H. Crabtree a,*2
a Department of Chemistry, Yale Uni6ersity, PO Box 208107, New Ha6en, CT 06520-8107, USA
b Laboratoire d’Electrochimie Organique et de Photochimie Re´dox (CNRS UMR 5630), Uni6ersite´ Joseph Fourier Grenoble, BP 53X,
38041 Grenoble Cedex 9, France
Received 16 June 2000; received in revised form 28 June 2000
Abstract
The title dyes, [FcCHꢀCHC5H4NCH2C6H4(tBu)]X (X=BPh4−, ClO−4 ), developed for rapid screening of catalysts, are intensely
colored, but bleach on exposure to visible light in solution. Detailed study shows that on irradiation these alkene dyes first
undergo a trans–cis isomerization, then give an irreversible bleaching via decomposition. Both dyes show color dependence on the
nature of the anion as a result of ion-pairing, which affects the intensity and position of the visible absorption. NMR, MS and
EPR data are also reported. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Organometallic dyes; Trans–cis isomerization; Ion-pair
1. Introduction
2. Results and discussion
A dye molecule normally consists of an electron
donor (D) connected by an unsaturated link to an
electron acceptor (A), as in the diazo dyes of general
type DꢁNꢀNꢁA. Many organometallic functional
groups are good donors and many others are good
acceptors, so in principle numerous organometallic dyes
should be readily accessible. Organometallic functional
groups were essentially unknown during the classic
period of the development of color chemistry [1], and
so it is not surprising that few, if any, of the standard
commercially available organic dyes are organometallic
species. Recent developments in non linear optical
(NLO) materials have led to the very recent synthesis of
a number of interesting organometallic dyes [2].
2.1. Design of the dyes
We wanted to develop a color assay for homoge-
neous catalytic activity for use in rapid screening of
catalysts [3]. Bleaching of a dye on reduction of the
unsaturated link seemed appropriate, so we were led
to consider dyes of the type DꢁCHꢀCHꢁA and
DꢁNꢀCHꢁA, because catalytic reactions involving alke-
nes, and to a lesser extent, imines, are common. If the
CꢀC group acting as the unsaturated link were reduced,
the electronic connection between D and A would be
severed and the color would be lost. It was also impor-
tant to ensure that there were no functional groups
present that could bind to the catalyst and inhibit
catalytic activity. Almost all the organic D and A
groups have such functionalities, such as Me2N or MeO
for donors and ꢁNO2 for acceptors. The organometallic
donor ferrocenyl (Fc) (1) (Fig. 1) has no functionality
that would be expected to interfere with the catalysts
and so we have adopted this group as our donor. For
the acceptor, we considered a quaternized pyridinium
because there would again be no interfering functional
group and this choice would allow the dye precursor
pyridine derivatives to be covalently attached to
1 *Corresponding author. Present address: Departament de Qu´ım-
ica F´ısica i Inorga`nica, Universitat Rovira i Virgili, Pl. Imperial
Ta`rraco 1, 43005 Tarragona, Spain. Fax: +34-977-559563; e-mail:
dieguez@quimica.urv.es
2 *Corresponding author. Tel.: +1-203-4323925; fax: +1-203-
4326144; e-mail: robert.crabtree@yale.edu
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00422-8

M. Die´guez et al. / Journal of Organometallic Chemistry 608 (2000) 146–152
147
a better understanding of these molecules, we also
study the NMR, MS and EPR data. Although there are
a number of reports of stability studies for all-organic
NLO dyes, as far as we know, this paper represents the
first detailed study of the stabilities of organometallics
dyes.
2.3. Synthesis
The structures of key compounds referred to in this
study along with the labelling scheme are shown in Fig.
1. We previously showed [3] how the known
FcꢁCHꢀCHꢁC5H4N (2) [4] can be readily quaternized
(dimethylformamide, 25–80°C) with the commercially
available 4-BrCH2C6H5(tBu), to give the new dye as the
bromide salt 6. This particular benzyl group was chosen
to impart solubility. Anion exchange with AgX in
acetone (X=BPh4−, ClO4−, BF−4 ) gives the other salts
studied; we described the BPh4 (3) salt previously [3]
but the ClO−4 and BF−4 salts, 4 and 5, respectively, are
new.
Fig. 1.
chloromethyl groups of a solid support if required. The
counter-ion would also have to be non-coordinating,
such as BPh−4 . This line of reasoning led us to synthe-
size
the
organometallic
dye,
[FcꢁCHꢀCHꢁ
C5H4NR]BPh4. Initial experiments [3] suggested that
using 4-CH2C6H4(tBu) as the R group gave good solu-
bility for the resulting salt and so this became our first
dye, 3 (Fig. 1). In this paper we describe some of its key
properties. The major significant practical limitation
proves to be instability of the dye to light, in solution.
This study was carried out in the hope of suggesting
improvements for future dyes.
2.4. Detailed studies of the dyes
2.4.1. Electronic spectroscopy
2.2. Properties of the dyes
Unsubstituted ferrocene 1 is weakly colored and ex-
hibits two strong low energy bands at 326 and 440 nm
(Table 1) in the visible range of the electronic spectrum
The new dyes 3 and 4 (Fig. 1) proved suitable for
rapid screening for catalyst activity [3] but qualitative
studies showed that in tetrahydrofuran solution they
were not very stable to light, bleaching in tens of hours
under conventional laboratory fluorescent illumination.
Another problem was that the intensity of color seemed
to depend on the nature of the solvent. In the work
described below we show that trans–cis photoisomer-
ization occurs readily, followed by decomposition. Ex-
periments using dyes with different anions (Fig. 1) show
that ion pairing also influences the color. In pursuit of
1
which have been assigned [4,5] to A2g“1E2g and
1A1g“1E1g ligand-field (d–d) transitions.
The spectrum and visible color change dramatically
upon substitution of the cyclopentadienyl ring with
conjugated and/or acceptor groups. In such a case, a
strong increase of color intensity is observed as a result
of intramolecular charge transfer transitions between
the donor (ferrocene) and the acceptor [3,4,6]. Previous
studies on ferrocene derivatives containing a terminal
4-pyridyl (=4-py) group linked to the ferrocene by a
Table 1
Selected visible spectral data for compounds ferrocene (1), FcꢁCHꢀCH(4-py) (2), [FcꢁCHꢀCH(4-pyBz)]BPh4 (3), [FcꢁCHꢀCH(4-pyBz)]ClO4 (4),
[FcꢁCHꢀCH(4-pyBz)]BF4 (5), [FcꢁCHꢀCH(4-pyBz)]Br (6) recorded in THF and CH3CN
Compound
Solvent
Concentration (mM)
Color
umax (nm) (m dm3 mol−1 cm−1
)
1
2
3
4
5
6
THF
CH3CN
THF
CH3CN
THF
CH3CN
THF
CH3CN
THF a
CH3CN
THF a
CH3CN
1
1
Light yellow
Light yellow
Light yellow
Light yellow
Purple
Purple
Purple
Purple
Pink
326 (420)
325 (415)
374 (1640)
373 (1640)
370 (22 450)
366 (24 050)
365 (18 290)
366 (17 570)
361
440 (100)
440 (90)
0.073
0.073
0.049
0.037
0.035
0.037
0.037
0.037
0.0445
0.0445
466 (1360)
466 (1360)
565 (6130)
558 (6760)
554 (4860)
557 (6760)
549
555 (6490)
511
558 (5955)
Light purple
Light purple
Purple
362 (24 050)
361
361 (23 860)
a Poorly soluble in THF; some decomposition observed.

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M. Die´guez et al. / Journal of Organometallic Chemistry 608 (2000) 146–152
anion was the first indication that the formation of ion
pairs between the pyridinium cation and the anions
could affect the color both in tetrahydrofuran and
acetonitrile solvents.
As a test of this ion pairing idea, we added an excess
of ClO−4 (as Bu4N+ salt) to tetrahydrofuran or acetoni-
trile solutions containing the BPh−4 , BF4− or Br− pyri-
dinium compound. In all cases the resulting solutions
exhibited the same UV–vis spectra as that of a solution
of [FcꢁCHꢀCH(4ꢁpyBz)]ClO4. This indicates that the
initial ions pairs formed with BPh−4 , BF4− and Br− are
displaced in favour of ClO−4 when an excess of this ion
is present in either solvent. This proposal is consistent
with literature data: the formation of ion pairs between
1,1%-dimethyl-4,4%-bipyridinium (methyl viologen or
paraquat) PQ2+ with anions like BPh−4 , Br−, Cl−, I−,
Ph2C(OH)COO− has been demonstrated from the ob-
servation of charge transfer bands in the UV–vis spec-
trum [7–11].
2.4.2. Photolysis of compounds 3 and 4
Compounds 3 and 4 are stable in tetrahydrofuran
solution in the dark even over several months. Irradia-
tion of 3 (X=BPh4−) (3.7×10−5 M) in tetrahydro-
furan under argon (high pressure Hg lamp, 200 W)
produces a slow change of color. The solution turns
from deep purple to lighter purple after 30 min, then
red after 2 h, and finally light brown after 24 h of
continuous illumination.
The UV–vis absorption spectra shown in Fig. 2
indicate that in the time between 30 min and 2 h of
illumination, the initial absorption at 565 nm slowly
shifted to the blue (544 nm) and the band at 370 nm
progressively decreased in intensity. A new band at 511
nm appeared after 2 h. The new absorption peak at 511
nm as well as that at 544 nm then increaseed in
intensity with time while that at 370 nm decreased.
After 5 h of illumination no further changes in the
UV–vis spectrum were observed. Prolonged irradiation
(24 h) gave only decomposition of the sample (see
Sections 2.4.4 and 2.4.6).
The same behaviour has been observed in the case of
irradiation of compound 4, where the anion is ClO4−
(Fig. 3). Thus, after 2 h of irradiation, the initial
absorption at 554 nm shifted to the blue (535 nm), a
new band at 514 nm appeared and the signal at 365 nm
decreased in intensity. In this case, too, prolonged
irradiation (48 h) gave decomposition.
Spectral changes of the type observed in the first 5 h
are analogous to those previously reported for trans–
cis isomerization of styrylpyridines and stilbenes [12]
suggesting that trans–cis isomerization may be respon-
sible for the initial photochemical changes for our dyes.
The slight anion-dependent shifts in the absorption
maxima found for compounds 3 versus 4 after 5 h of
irradiation could indicate that the ion-pairs are still
present in the cis-isomer.
Fig. 2. UV–vis spectral changes during irradiation of a 3.7×10−5 M
solution of compound 3 in tetrahydrofuran. (a) t=0 s, (b) t=30 min,
(c) t=2 h, (d) t=3 h, (e) t=5 h.
CꢀC and/or a phenyl group show that the two lowest-
energy transitions of the visible region for these types of
substituted ferrocenes are substantially red-shifted with
respect to unsubstituted ferrocene with enhanced inten-
sities (1000BmB4600 l mol−1 cm−1 compared to mB
420 for ferrocene). Increased mixing of metal and
ligand orbitals give the d–d transistion a significant
ligand character and make them more ‘allowed’ [3,4,6].
Thus, for FcꢁCHꢀCH(4-py) (2), these bands appear at
374 and 466 nm (Table 1).
If the acceptor is a pyridinium group (compounds
[FcꢁCHꢀCH(4-pyBz)]X (3–6), X=BPh−4 , ClO4−, BF4−
and Br−, 4-pyBz=4-(N-{4-t-butylbenzyl}pyridinium),
we find that these effects are even more enhanced
(Table 1). Thus, for example, for complex
[FcꢁCHꢀCH(4ꢁpyBz)]BPh4 (3), the visible spectrum
shows the two lowest-energy transitions at 370 and 565
nm (m=22450 and 6130 l mol−1 cm−1, respectively).
The strong enhancement of color intensity and the red
shift seen in the pyridinium salts compared to the
parent pyridyl compound are ascribed to the stronger
acceptor character of the cationic pyridinium ring. In
moving from neutral compounds 1 and 2 to the ionic
compounds 3–6 the low energy band is more strongly
red-shifted than the high energy one. While the situa-
tion is very different, moving from 1 to 2, both neutral
compounds, the high energy band is red-shifted to a
greater extent [6]. The slight shifts of the two lowest
energy bands (B10 nm) observed on changing the

M. Die´guez et al. / Journal of Organometallic Chemistry 608 (2000) 146–152
149
for 3 and 48 h for 4. The peaks located at m/z 218
disappear and the mass spectra show new peaks at
lower m/z values that could be attributed to the degra-
dation of the compounds (see Sections 2.4.4 and 2.4.6).
2.4.4. NMR spectroscopy
1
The H-NMR spectra (Table 2) of compounds 3 and
4 were assigned using COSY and irradiation experi-
ments. The protons of the substituted cyclopentadienyl
groups resonate as two triplets at l 4.52 and 4.61 ppm
for compound 3 and at l 4.53 and 4.74 ppm for 4. The
five protons of the unsubstituted Cp ring appear as a
broad singlet at l 4.18 and 4.19 ppm for compounds 3
and 4, respectively. The two methylenic protons of CH2
group appear as a singlet at l 4.81 ppm for 3 while this
signal appears at much lower field in compound 4: l
5.69 ppm. The shift in the CH2 signal depends on the
anion involved and may be associated with the forma-
tion of ion pairs with contact between the pyridinium
groups and the anions.
1
The H-NMR spectra of the starting materials show
the olefinic protons of the double bond as two doublets
(Table 2) with a 3J(HꢁH) coupling constant around
15.6 Hz, consistent with a trans geometry for both
compounds 3 and 4 [3,4]. In the case of compound 3
the protons of the 4-pyBz and BPh−4 groups are very
close together (between l 6.80 and 7.60 ppm) and are
not sufficiently resolved to be assigned reliably.
Fig. 3. UV–vis spectral changes during irradiation of a 3.7×10−5 M
solution of compound 4 in tetrahydrofuran. (a) t=0 s, (b) t=2 h, (c)
t=5 h.
The use of optical filters shows that irradiation at 370
nm is chiefly responsible for the photochemical changes
observed.
In order to confirm the proposal of trans–cis isomer-
ization, GCMS and NMR studies for compounds 3 and
4 and photolysis of compound 2 have also been carried
out.
2.4.5. NMR beha6iour: photolysis of compounds 3 and
4
1
During the irradiation process the H-NMR shows
clear changes in the cyclopentadienyl and olefinic re-
gion as expected for the formation of the cis-isomer.
This is in good agreement with the different environ-
ment expected for these groups in the cis-isomer. Ex-
amination of models suggests that steric hindrance
should cause a strong deviation from planarity of the
4-pyridyl or ferrocenyl groups in order to avoid strong
repulsive interactions in the cis-isomer [13].
2.4.3. GCMS studies
A series of GCMS spectra obtained for 3 and 4
(3.7×10−5M) during the course of the irradiation in
tetrahydrofuran did not show any mass change, consis-
tent with trans–cis isomerization. Thus, the mass spec-
tra for 3 and 4 show the highest mass ions at m/z values
of 218 corresponding to loss of BPh−4 and ClO4− from
the half molecule, respectively. Different GCMS spectra
are obtained when the sample was irradiated for 24 h
Thus, in the case of complex 4, (Fig. 4) a new singlet
signal appears at l 4.18 ppm that increases in intensity
with time; this is attributed to the protons of the
Table 2
NMR spectroscopic data for complexes 3 and 4 a
Compound
ꢁCpSubstituted
ꢁCpUnsubstituted
ꢁCHꢀCHꢁ
CH2
Py
ꢁPh
CH3
3
4.52 (t)
4.61 (t)
4.18 (bs)
6.72 (d)
7.51 (d)
4.81 (s)
6.80–7.60 (m)
6.80–7.60 (m)
1.30(s)
JHꢁH=1.8
JHꢁH=15.7
4
4.53 (t)
4.74 (t)
4.19 (bs)
6.90 (d)
7.90 (d)
5.69 (s)
8.11 (d)
8.87 (d)
7.51 (m)
1.30 (s)
J
HꢁH=1.5
J
HꢁH=15.6
JHꢁH=6.7
a In THF-d8. Chemical shifts in ppm, coupling constants in Hz; room temperature. Abbreviations: s, singlet; bs, broad singlet; m, multiplet; d,
doublet; t, triplet.

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M. Die´guez et al. / Journal of Organometallic Chemistry 608 (2000) 146–152
Fig. 4. 1H-NMR spectral changes in the Cp region during irradiation of compound 4 in THF-d8 solution. (a) t=0 s, (b) t=2 h, (c) t=3 h.
Fig. 5. 1H-NMR spectral changes in the aromatic region during irradiation of compound 4 in THF-d8 solution. (a) t=0 s, (b) t=3 h.
Under prolonged irradiation for 24 (3) or 48 h (4), a
brown solution is formed as a result of decomposition
and the NMR signals are broadened due to the pres-
ence of paramagnetic Fe3+ (see Section 2.4.6).3
unsubstituted Cp for the cis-isomer. A new signal ap-
pears at l 4.83 ppm in the substituted Cp region, that
also increases in intensity in the same way with time,
while the signal at l 4.53 ppm remains constant, due to
an overlap of one of the signals from the substituted Cp
of the remaining trans-isomer.
2.4.6. EPR properties
The EPR spectra obtained for the initial tetrahydro-
furan solution of 3 (3.7×10−5 M), after 4 h of irradia-
tion (red solution) and after 24 h of irradiation are
shown in Fig. 6 (a) (b) and (c), respectively. The EPR
spectrum of the initial solution (Fig. 6 (a)) shows very
small signals at g=4.27 and 1.98 attributed to a highly
In the olefinic region two new doublets at l 7.28 and
l 7.72 ppm appear which correspond to the CHꢀCH
3
protons of the cis-isomer. In a key result, the J(HꢁH)
value of 11.8 Hz (Fig. 5) [13] is consistent with a cis
geometry.
A similar splitting has been observed in the Cp
region on irradiation of compound 3. Thus, the new
signals at l 4.70 and 4.12 ppm are attributed to substi-
tuted and unsubstituted Cp protons, respectively. For
the cis-complex the new signals from the olefinic pro-
tons overlap with the signals from the pyridium and
BPh−4 groups.
3 The irradiation process has been carried out under argon and at
0°C in order to avoid the formation of paramagnetic Fe3+ during the
maximum time. All attempts to obtain the NMR of the pure cis-
product have been unsuccessful. Prolonged irradiation for more than
3 h gave an NMR with broad signals due to the formation of
paramagnetic Fe3+ (see Section 2.4.6).

M. Die´guez et al. / Journal of Organometallic Chemistry 608 (2000) 146–152
151
rhombic high-spin Fe3+ ion and an organic radical
species, respectively, that we initially ascribed to impu-
rities or slight decomposition during sample handling
[14]. There is a slight increase in these peaks after
photolysis of the initial compound at 366 nm upon
irradiation for 4 h (Fig. 6 (b)). Irradiation of the sample
for 24 h yields a brown solution that shows much
enhanced Fe3+ and radical EPR signals (Fig. 6 (c)).
The yield of free Fe3+ generated upon photolysis is
about 15%, determined using a Fe(NO3)3 standard.
After 48 h of illumination, the yield of Fe3+ is as high
as 50%. The presence of a radical species may possibly
be attributed to the photolytic electron transfer within
the ion pair [7], but the absence of charge transfer band
in the ion pair between the anions and pyridium group
suggests another possible origin for these radicals. A
second, more plausible possibility is that the prolonged
irradiation could induce the transfer of one electron
from the donor to the acceptor group, generating Fe3+
and an organic radical species. It is well known that the
ferricinium [15] forms of this type of compounds are
not stable and tend to decompose rapidly, a similar
pathway could operate in the photochemical work.
2.4.7. Isomerization of FcꢁCHꢀCH(4-py) (2)
In order to confirm that compound 3 and 4 can
undergo to trans–cis isomerization process by photoly-
sis, the related neutral compound 2 was also studied.
Photolysis at 366 nm of 2 (3.7×10−5 M) in tetrahy-
drofuran induces a slow change of the UV–vis absorp-
tion spectrum with a shift of the absorption to the blue.
Thus, the absorption maxima at 466 and 374 nm are
shifted to 460 and 335 nm, respectively. These spectral
changes are in good agreement with literature data for
trans–cis isomerization of 4-stilbazone [16] and with
related non-ionic substituted ferrocenes compounds as
trans–cis-[1-ferrocenyl-2-(4-nitrophenyl)ethylene]
[6].
Slow decomposition did occur on long photolysis, but
only over ca. 3 weeks.
Work is in progress to synthesize new dyes that avoid
these problems. For example, the incorporation of the
double bond into a ring should lock the conformation
and prevent cis–trans isomerization process. This strat-
egy has previously been used with success for organic
NLO dyes [17].
2.4.8. Conclusion
Compounds 3 and 4, developed for rapid screening
of catalysts, are intensely colored but bleach in solution
on exposure to visible light. Detailed study shows that
the dyes first undergo a trans–cis isomerization, then
give an irreversible bleaching process via decomposi-
tion. The dyes show a color dependence as a result of
ion-pairing [18], which affects the intensity and position
of the visible absorption. NMR, MS and EPR data are
also reported.
3. Experimental
3.1. General methods
All operations were performed under an atmosphere
of nitrogen or argon. Solvents were dried and distilled
prior to use. 1H-NMR spectra were recorded on a
Variant Gemini 400 MHz spectrometer. The GCMS
experiments were recorded on a Hewlett-Packard
5890II gas chromatograph equipped with a Hewlett-
Packard 5971 mass spectrometer. Absorption spectra
were recorded on a Shimadsu UV-1203 spectrophoto-
meter. [FcꢁCHꢀCH(4-pyBz)]BPh4, [FcꢁCHꢀCH(4-
pyBz)]Br, [FcꢁCHꢀCH(4-pyBz)]Cl and FcꢁCHꢀCH-
(4-pyBz) were prepared by previously reported methods
[3,4].
Fig. 6. EPR spectral changes during irradiation of a solution of
compound 3 in tetrahydrofuran. (a) t=0 s, (b) t=4 h, (c) t=24 h.

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M. Die´guez et al. / Journal of Organometallic Chemistry 608 (2000) 146–152
3.2. Synthesis of [FcꢁCHꢀCH(4-pyBz)]ClO4 (4)
on the EPR work. We thank the US Department of
Energy for support. Dr M.N. Collomb thanks the
CNRS and NATO for financial support.
To an acetone solution (25 ml) of [FcꢁCHꢀCH(4-
pyBz)]Br (1 g, 1.9 mmol), AgClO4 (414 mg, 2 mmol)
was added. After 20 min, the AgBr precipitate was
filtered off. The addition of ether (50 ml) to the filtrate
afforded microcrystals of complex 4, which were filtered
off, washed with cold ether (3×10 ml) and vacuum
dried. (0.81 g, 80%). Anal. Found: C, 62.79; H, 5.70; N,
2.63. Calc. for C28H30ClFeNO4: C, 62.76; H, 5.64; N,
2.61%.
References
[1] H. Zollinger, Color Chemistry, VCH, Weinheim, 1987.
[2] (a) S. Barlow, H.E. Bunting, C. Ringham, J.C. Green, G.U.
Bublitz, S.G. Boxer, J.W. Perry, S.R. Marder, J. Am. Chem.
Soc. 121 (1999) 3715. (b) A. Mendiratta, S. Barlow, M.W. Day,
S.R. Marder, Organometallics 18 (1999) 454. (c) Z. Wu, R.
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3.3. Synthesis of [FcꢁCHꢀCH(4-pyBz)]BF4 (5)
Treatment of [FcꢁCHꢀCH(4-pyBz)]Br (1 g, 1.9
mmol) with AgBF4 (389 mg, 2 mmol) as described for
4 afforded complex 5. (0.92 g, 90%) Anal. Found: C,
63.95; H, 5.73; N, 2.79. Calc. for C28H30BF4FeN: C,
64.28; H, 5.78; N, 2.68%.
3.4. Photolysis
[5] Y.S. Sohn, D.N. Hendrickson, H.B. Gray, J. Am. Chem. Soc. 93
(1971) 3603.
Photolysis was carried out with an Oriel Corporation
high pressure mercury lamp, (200W). The photolysis
cell was an optical cuvette of 1 cm pathlength. The
solution and cell was purged with argon. Irradiation
was run in tetrahydrofuran. Photoisomerization was
investigated for compounds 2, 3 and 4 at a temperature
(B30°C) sufficiently low to stop thermal trans to cis
isomerization.
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Acknowledgements
We are very much indebted to Dr Lakshimi (Yale)
for her valuable assistance, discussions and suggestions
.