trans-4-Stilbenecarboxylatoalkylpentaamminecobalt(III) perchlorates: Synthesis and intramolecular energy transfer

Article abstract of DOI:10.1135/cccc19960342

Three homologs of tran-4-stilbenecarboxylic acid, viz., trans-4-stilbeneacetic acid (18), β-(trans-4-stilbene)propionic acid (19), and γ-trans-4-stilbene)butyric acid (20) were synthesized and, together with the parent trans-4-stilbenecarboxylic acid, use

Full text of DOI:10.1135/cccc19960342

342
Parkanyi, Yeh Huang, Chu, Jeffries, III:
trans-4-STILBENECARBOXYLATOALKYLPENTAAMMINECOBALT(III)
PERCHLORATES: SYNTHESIS AND INTRAMOLECULAR ENERGY
TRANSFER*
a
b
c
Cyril PARKANYI , Lois Shiow-Chyn YEH HUANG , Sung Gun CHU
and Alfred T. JEFFRIES, IIId
a Department of Chemistry,
Florida Atlantic University, 777 Glades Road, P.O.Box 3091,
Boca Raton, Florida 33431-0991, U.S.A.
b Challenger Schools,
California Central Office, Campbell, California 95008, U.S.A.
c Research Center, Hercules, Inc., Wilmington, Delaware 19899, U.S.A.
d Organic Chemical Division,
Philip A. Hunt Chemical Corporation, Lincoln, Rhode Island 02865, U.S.A.
Received February 3, 1995
Accepted November 20, 1995
Dedicated to Professor Vaclav Horak.
Three homologs of trans-4-stilbenecarboxylic acid, viz., trans-4-stilbeneacetic acid (18), β-(trans-4-stil-
bene)propionic acid (19), and γ-(trans-4-stilbene)butyric acid (20) were synthesized and, together with the
parent trans-4-stilbenecarboxylic acid, used to obtain the corresponding [ω-(trans-4-stilbene)carboxylatoal-
kylpentaamminecobalt(III) complexes, [(PhCH=CHC6H4(CH2)nCOO)Co(NH3)5]2+ (1–4), where n = 0,
1, 2, or 3. Fluorescence studies suggest that an intramolecular excitation energy transfer occurs from
the first excited singlet state of the ligand to the metal giving rise to a charge-transfer triplet-excited state
of the complex, based on the analogy with previous work on complex 1. The efficiency of the energy trans-
fer seems to decrease with increasing number of the methylene groups in the ligand.
Key words: trans-4-Stilbenecarboxylatoalkylpentaamminecobalt(III) complexes; Energy transfer;
Electronic spectra; Fluorescence; LMCT.
The existence of intramolecular energy transfer and its importance in photochemistry
have been clearly documented in the chemical literature and the examples include or-
* Some preliminary initial results of this work were presented at the 29th Southwest Regional Meeting of the
American Chemical Society, El Paso, Texas, December 5–7, 1973. The experimental part of this work was
carried out at the University of Texas at El Paso, El Paso, Texas, during the tenure of one of the authors
there (C. P.) and was the topic of the M.S. Theses of L. S. C. Yeh Huang and S. G. Chu.
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

trans-4-Stilbenecarboxylatocobalt(III) Complexes
343
ganic, organometallic, and inorganic coordination compounds1–9. A typical example is
a molecule consisting of two aromatic or heteroaromatic groups, Ar1 and Ar2, separated
from each other by a flexible or a rigid chain, X, i.e., Ar1–X–Ar2. In such compounds,
the two groups Ar1 and Ar2 can be excited separately by choosing the appropriate exci-
tation wavelengths. The possible intramolecular energy transfer between the two units
Ar1 and Ar2 manifests itself in changes of the intensities of emission (fluorescence or
phosphorescence) that are characteristic of Ar1 and Ar2 (refs10–17). The flexibility or
rigidity of the connecting chain X plays an important role as it determines the effective
distance between the two units and the efficiency of the transfer depends on their mu-
tual distance.
It is well known that electronic excitation energy can be transferred over distances of
the order of 30 Å or more18,19. According to Förster, the transfer takes place via a
dipole–dipole resonance interaction between the energy donor and acceptor chromo-
phores20,21. If one considers an excitation energy transfer between two similar molecules in
a solution, the critical distance between molecules at which the transfer still occurs can
be evaluated from the absorption and emission spectra and the lifetimes of excited states of
the respective compounds. Förster has shown that, in the case of weak coupling,
the transfer rate constant is proportional to the inverse sixth power of the distance
between the groups, r–6 (refs20,21). The rate of energy transfer is a function of the
respective orientation of the groups, the refractive index of the solvent, and the
overlap of the emission spectrum of the energy donor and the absorption spectrum of
the energy acceptor.
An instructive example is a study of a series of peptides carried out by Stryer and
Haugland23. The compounds under study were the oligomers of poly-(S)-proline (n = 1 to 12)
as spacers of defined length separating the energy donor and acceptor by fixed distances
from 12 to 46 Å. The energy donor used was an α-naphthyl group at the carboxyl end of
the peptide and the energy acceptor was a dansyl (1-dimethylamino-5-naphthalenesulfonyl)
group at the imino end. The dependence of the efficiency of energy transfer on the
distance was in an excellent agreement with Förster’s r–6 relationship.
Another similar example involves compounds of the type C10H7(CH2)nC14H9 (C10H7 =
naphthyl; C14H9 = anthryl)10. The irradiation of the naphthyl group present in these
compounds leads to a fluorescent emission characteristic of the anthracene moiety. A
number of other similar cases are known11–17
.
While there are numerous studies available on intramolecular energy transfer in or-
ganic photochemistry, intramolecular electronic excitation energy transfer in coordina-
tion compounds has been studied to a lesser extent. If, within a complex system
containing a transition metal ion, one part of the molecule acts as a donor and another
part as an acceptor, an intramolecular transfer of excitation energy can occur. For in-
stance, an excitation energy transfer can take place between an organic ligand and a
central transition metal ion.
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

344
Parkanyi, Yeh Huang, Chu, Jeffries, III:
Some transition metal chelates and metal carbonyls have been reported as quenchers
of organic triplet excited states; however, at least in some cases this effect may be due
to the catalysis of the radiationless return of the energy donor to its ground state. A
very interesting example of intramolecular energy transfer has been reported in the case
of certain rare earth chelates23,24 where excitation in the region of ligand absorption
results in emission from rare earth ion levels. Some evidence supporting energy transfer
in the opposite direction has also been found25.
In the solid complex [Cr(urea)6][Cr(CN)6], the low-temperature irradiation of the
cation absorption band leads to emission which is characteristic of the anion26. At low
temperatures, but not in the rigid media, organic donors were found to sensitize the
characteristic phosphorescence of [Cr(NCS)6]3– (benzil was one of the reported sen-
sitizers)27.
In principle, excitation energy transfer can lead not only to spectroscopic situations
in which the transfer results in emission of radiation but to chemical reactions as well.
This latter possibility represents an extremely valuable approach in studies of photo-
chemical reactions of coordination compounds. A few typical examples of photochemi-
cal reactions occurring as a result of an intermolecular or intramolecular excitation
energy transfer will be given.
It has been determined that organic senzitizers induce the photoaquation of aqueous
Reinecke sals, trans-[Cr(NH3)2(NCS)4]–, and [Cr(NH3)5(NCS)]2+ (ref.28). Intermolecu-
lar energy transfer has been shown to occur in redox photodecomposition of cobalt(III)
ammines with organic compounds known to have relatively stable triplet excited states
(benzil, trans-4-stilbenecarboxylic acid, benzophenone, biacetyl)29. It is assumed that
an energy transfer takes place between the lowest triplet state of the donor and a charge
transfer triplet state of the complex. Similar results were obtained by two other groups
of workers30,31
.
Also, the biacetyl-sensitized [Cr(CN)6]3– photoaquation was studied32 as well as the
naphthalene-sensitized redox photodecomposition of [Co(NH3)6]3+ (ref.33). Other
examples are available. In all the above-mentioned cases an intermolecular excitation
energy transfer occurs. Even more interesting are, of course, situations where an intra-
molecular excitation energy transfer leads to a photochemical reaction. Such an intra-
molecular excitation energy transfer was observed in aqueous solutions of the
(trans-4-stilbene)carboxylatopentaamminecobalt(III) ion, [Co(NH3)5(TSC)]2+ (where
TSC = trans-4-stilbenecarboxylate) (1; ref.34).
It is likely that the energy transfer generates a charge-transfer triplet-excited state of
the complex which subsequently undergoes redox decomposition. The suggested pro-
cess is intersystem crossing from the first excited singlet state of the organic ligand.
The goal of the present study was to synthesize a series of homologs of the above
complex 1 studied by Adamson, Vogler, and Lantzke34 (with a varying number of
methylene groups between the stilbene unit and the carboxylate group, 2–4), and to
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

trans-4-Stilbenecarboxylatocobalt(III) Complexes
345
explore intramolecular energy transfer in these complex salts, as a function of distance
of the stilbene unit from the cobalt ion.
2
O
CH CH
(H3N)5Co O
C
(CH2)n
3, n = 2
4,
1, n = 0
2, n = 1
n = 3
The geometry of the complexes is shown below (1–4, R = an organic group consti-
tuting the remainder of the respective carboxylic acid).
2
NH3
H3N
NH2
H
Co
H3N
O
O
C
R
NH3
1-4
EXPERIMENTAL
Aquapentaamminecobalt(III) chloride was reagent grade and was obtained from Alfa Products, Ward
Hill, MA, U.S.A. All other chemicals used in the work were commercial products as well.
Aquapentaamminecobalt(III) perchlorate was prepared according to the literature35. Benzaldehyde was
purified by distillation under nitrogen at reduced pressure. All solvents were purified and/or dried using
the standard procedures described in the literature.
Infrared spectra were recorded on a Perkin–Elmer model 710A spectrophotometer in pressed po-
~
tassium bromide disks (ν in cm–1). Electronic absorption spectra were measured on a Beckman DK-25
spectrophotometer in methanol–water (1 : 1, v/v) or ethanol–water (1 : 1, v/v). Fluorescence emission
spectra were obtained with an Aminco–Bowman 4-8911 spectrophotofluorometer in the same sol-
vents. Photochemical reactions were carried out in an immersion-well type photochemical reactor
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

346
Parkanyi, Yeh Huang, Chu, Jeffries, III:
(Ace Glass) equipped with a medium-pressure mercury lamp (450 W) and a Rayonet model RPR-208
preparative photochemical reactor, at 300 nm.
Melting points were determined on a Thomas–Hoover capillary melting point apparatus and are
uncorrected. Elemental microanalyses were performed by the microanalytical laboratory of the
Université de Droit, d’Économie et des Sciences d’Aix-Marseille, Saint-Jérôme, Marseille, France.
Synthesis of p-Chloromethylphenyl Substituted Carboxylic Acids 8–10
p-Chloromethylphenylacetic acid (8) (method A)36. Formalin (40% aqueous formaldehyde, 165 g,
2.2 mol) was added to a mixture of phenylacetic acid (5; 48 g, 0.35 mol) and zinc chloride (18 g,
0.13 mol) and hydrogen gas was passed through the solution at 75–80 °C for 16 h. The usual work-
up of the reaction mixture and recrystallization of the crude product from chloroform–carbon tetra-
chloride (1 : 1, v/v) yielded 19.2 g (30%) of p-chloromethylphenylacetic acid (8), m.p. 152–153 °C
(ref.36 gives m.p. 152–153 °C).
β-(p-Chloromethylphenyl)propionic acid (9) (method A). This compound was prepared from β-
phenylpropionic acid (6) in the same manner as described for the preparation of 8. Yield 28%; m.p.
117–118 °C (from carbon tetrachloride; ref.36 gives m.p. 118–119 °C).
γ-(p-Chloromethylphenyl)butyric acid (10) (method A). This acid was obtained from γ-phenyl-
butyric acid (7) in the same fashion as described for 8. Yield 27%; m.p. 118–120 °C (from carbon
tetrachloride; ref.36 gives m.p. 121–122 °C).
The following characteristic frequencies were observed in the infrared spectra of the above acids,
8–10 (KBr disk): 3 050–2 950 (OH stretching), 1 700–1 675 (C=O stretching), 920–895 (OH defor-
mation), 740–720 (C–Cl stretching).
p-Chloromethylphenylacetic acid (8) (method B)37. A mixture of dried phenylacetic acid (5; 1.36 g, 0.01
mol) and chloromethyl methyl ether (11; 2 ml, 2.12 g, 0.026 mol) was added dropwise during 0.5 h to
stannic chloride (0.8 ml, 1.78 g, 0.0068 mol) cooled in an ice bath. The reaction mixture was stirred
during the addition and then for an additional 1.5 h in an ice bath. The solution was poured into ice
water (20 ml) and the white precipitate was filtered off, dried under reduced pressure, and recrystal-
lized from chloroform to give 0.78 g (42%) of p-chloromethylphenylacetic acid (8), identical with the
product obtained by method A. Additional recrystallization decreased the yield to 35%.
β-(p-Chloromethylphenyl)propionic acid (9) (yield 56%) and γ-(p-chloromethylphenyl)butyric acid
(10) (yield 65%) were obtained by using method B with β-phenylpropionic acid (6) and γ-phenylbu-
tyric acid (7), respectively, as the starting materials. After purification, the respective yields of the
acids 9 and 10 were 45% and 52%.
Preparation of Phosphonium Salts 12–14
p-(Carboxymethyl)benzyltriphenylphosphoniumchloride (12). To a mixture of p-chloromethylphenyl-
acetic acid (8; 26.3 g, 0.142 mol) and triphenylphosphine (37.3 g, 0.142 mol), dry benzene (30 ml)
was added and the mixture was heated with stirring at 77–78 °C overnight. The precipitate was
washed several times with dry benzene and dried over phosphorus pentoxide under reduced pressure
at 90°C; yield 50.5 g (80%), m.p. 245–247 °C.
p-(β-Carboxyethyl)benzyltriphenylphosphonium chloride (13). This compound was obtained in the
same manner from 9 as the starting material. Yield 99%, m.p. 252–254 °C.
p-(γ-Carboxypropyl)benzyltriphenylphosphonium chloride (14). This phosphonium salt was ob-
tained from 10 in the same fashion as described for 12. Yield 75%, m.p. 209–213 °C.
The following characteristic frequencies were observed in the infrared spectra of the above phos-
phonium salts, 12–14 (KBr disk): 3 000–2 900 (OH stretching), 1 710–1 700 (C=O stretching), 910–
860 (OH deformation), 750–740 (P–CH2, benzyl, stretching).
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trans-4-Stilbenecarboxylatocobalt(III) Complexes
347
Homologs of trans-Stilbenecarboxylic Acid 18, 19 by Wittig Reaction38–40
trans-4-Stilbeneacetic acid (18). To a well-stirred suspension of 12 (15 g, 0.034 mol) in anhydrous
dimethylformamide (34 ml), an excess of sodium methoxide (4 g, 0.077 mol) suspended in anhydrous
dimethylformamide (22 ml) was added at 0–5 °C under nitrogen. The phosphorane intermediate (tan
to dark orange) formed almost instantaneously. The mixture was stirred for another 30 min and benz-
aldehyde (3.4 ml, 3.54 g, 0.033 mol) dissolved in anhydrous dimethylformamide (10 ml) was added
dropwise so that the temperature of the reaction mixture did not exceed 30 °C. After a few minutes,
the cooling bath was removed and the mixture was stirred for another 4 h at room temperature. Then
it was poured into ice water (50 ml), the unreacted benzaldehyde and triphenylphosphine oxide were
extracted into ether, and the aqueous layer was acidified with 3 mol/dm3 HCl, extracted with ether,
and the ether solution was washed with water, dried over anhydrous magnesium sulfate, and evapo-
rated to dryness. Recrystallization from carbon tetrachloride and then from ethanol yielded 0.61 g
(8%) of 15; m.p. 184–187 °C. This compound was identical in all respects with another sample of 18
prepared by the procedure described below (Wadsworth–Horner reaction).
trans-4-Stilbenepropionic acid (19). This compound was obtained in the same manner as above,
with 13 as the starting material; yield 6%, m.p. 202–204 °C. An alternate method of preparation used
phenyllithium instead of sodium methoxide, with ether as the solvent; the yield was 3%. This acid
(19) was identical in all respects with a sample of the acid prepared by the procedure outlined below
(Wadsworth–Horner reaction).
Preparation of Phosphonates 15–17 by Wadsworth–Horner Reaction41–43
Diethyl p-(carboxymethyl)benzylphosphonate (15). A mixture of (p-chloromethyl)phenylacetic acid
(8; 2.7 g, 0.015 mol) and triethyl phosphite (2.6 ml, 2.5 g, 0.015 mol) was refluxed at 170–180 °C for
1 h and then cooled. Unreacted triethylphosphite was distilled off at 50 °C/1 333 Pa. The colorless
liquid product was 15; yield 4.18 g (97%), n2D3 1.4921.
Diethyl p-(β-carboxyethyl)benzylphosphonate (16). This phosphonate was obtained similarly from 9;
yield 96%, n2D3 1.4920.
Diethyl p-(γ-carboxypropyl)benzylphosphonate (17). This compound was synthesized in the same
manner from 10; yield 95%, n2D3 1.4952.
The synthesized phosphonates 15–17 displayed the following characteristic frequencies in their in-
frared spectra (KBr disk): 3 005–2 995 (OH stretching), 1 730 (C=O stretching), 890–840 (OH de-
formation), 1 270–1 240 (P=O, free), 1 180–1 160 (P–OEt), 890–840 (OH deformation).
Homologs of trans-Stilbenecarboxylic Acid 18–20
trans-4-Stilbeneacetic acid (18). A solution of benzaldehyde (1.52 ml, 1.58 g, 0.015 mol) and 15
(4.32 g, 0.015 mol) in anhydrous dimethylformamide (5 ml) was added dropwise to a stirred suspen-
sion of sodium methoxide (2.5 g, 0.048 mol) in anhydrous dimethylformamide (10 ml) at 0–5 °C
within 10–15 min. The cooling bath was removed and stirring was continued at room temperature for
5 h. A similar work-up of the reaction mixture as in the above-described preparation of 18 gave 0.33 g
(9%) of the product; m.p. 187–188 °C (from benzene, refs44–46 gives m.p. 186–187 °C). IR spectrum
(KBr disk): 3 020 (OH stretching), 1 680 (C=O stretching), 1 505–1 395 (Ph), 950 (trans-CH=CH),
900 (OH deformation). UV spectrum (50% aq. MeOH), λmax nm (log ε): 228 (4.00), 298 (4.31), 311
(4.32), 323 sh (4.12). For C16H14O2 (283.3) calculated: 80.65% C, 5.92% H; found: 80.39% C, 6.04% H.
β-(trans-4-Stilbene)propionic acid (19). This compound was obtained in a similar manner as
above. Yield 20%, m.p. 209–211 °C (from benzene). IR spectrum (KBr disk): 3 050 (OH stretching),
1 705 (C=O stretching), 1 515–1 420 (Ph), 965 (trans-CH=CH), 915 cm–1 (OH deformation). UV
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

348
Parkanyi, Yeh Huang, Chu, Jeffries, III:
spectrum (50% MeOH), λmax nm (log ε): 228 (4.05), 297 (4.31), 311 (4.33), 322 sh (4.11). For
C17H16O2 (252.3) calculated: 80.92% C, 6.39% H; found: 81.63% C, 6.60% H.
γ-(trans-4-Stilbene)butyric acid47 (20). This acid was obtained in the same manner as described
above. Yield 22%, m.p. 162–164 °C (from carbon tetrachloride with a small amount of methanol). IR
spectrum (KBr disk): 3 030 (OH stretching), 1 690 (C=O stretching), 1 510–1 410 (Ph), 960 (trans-
CH=CH), 910 (OH deformation). UV spectrum (50% aq. MeOH), λmax nm (log ε): 227 (3.99), 297
(4.30), 310 (4.31), 323 sh (4.11). For C18H18O2 (266.3) calculated: 81.17% C, 6.81% H; found:
81.12% C, 7.00% H.
trans→cis Photoisomerization of the Acids
A solution of trans-4-stilbeneacetic acid (18; 0.028 g, 0.001 mol) in dioxane (10 ml) was irradiated
at 300 nm in dioxane (10 ml) for 20 h. Work-up of the reaction mixture and separation of the two
isomers in the form of their sodium salts (sodium salt of the trans-acid precipitates out in 1 mol/dm3
aqueous sodium hydroxide, the salt of the cis-isomer remains in the solution) gave crude cis-4-
stilbeneacetic acid (21) (68%). UV spectrum (50% MeOH), λmax nm (log ε): 223 (4.06), 308 (4.20).
The cis-isomers of 19 and 20 were obtained in the same way. UV spectrum (50% aq. MeOH),
λmax nm (log ε): β-(cis-4-stilbene)propionic acid (22): 223 (4.06), 309 (4.23); γ-(cis-4-stilbene)butyric
acid (23): 221 (4.04), 308 (4.22).
Cobalt(III) Complexes 1–4
(trans-4-Stilbene)carboxylatopentaamminecobalt(III) perchlorate (1). This compound was ob-
tained from the sodium salt of trans-4-stilbenecarboxylic acid and aquapentaamminecobalt(III) per-
chlorate as described in the litarature34. An adaptation of this procedure was also used to obtain the
three new cobalt(III) complexes, 2–4.
(trans-4-Stilbene)acetatopentaamminecobalt(III) perchlorate monohydrate (2). A mixture of the
sodium salt of 18 (0.119 g, 0.00046 mol) and of aquapentaamminecobalt(III) perchlorate (0.701 g,
0.00016 mol) was dissolved in dimethylacetamide (5 ml) and dimethylformamide (5 ml) and the mix-
ture was heated on a water bath at 80 °C with stirring for 3 h in the absence of light. The mixture
was poured into diluted perchloric acid (7%, approximately 30–40 ml), and the precipitated product
was recrystallized from hot water with added sodium perchlorate. The solution was filtered, and 2
was washed twice with cold water, once with ethanol, and several times with ethyl ether, and dried
in the dark over anhydrous magnesium perchlorate. The entire crystallization procedure was repeated
to remove traces of uncomplexed 18. The yield of 2 was 0.063 g (25%). IR spectrum (KBr disk): 3 250
(bonded NH), 1 560 (COO– asym. stretching), 1 390 (COO– sym. stretching), 1 305, 1 110, 965
(trans-stilbene), 840; UV-VIS spectrum (50% aq. MeOH), λmax nm (log ε): 228 (4.26), 298 (4.37),
311 (4.39), 323 sh (4.23), 500 (1.95). For C16H30Cl2CoN5O11 (598.3) calculated: 32.12% C, 5.05% H;
found: 31.98% C, 4.51% H.
β-(trans-4-Stilbene)propionatopentaamminecobalt(III) perchlorate (3). The perchlorate salt 3 was
prepared in the same manner as 2; yield 26%. IR spectrum (KBr disk): 3 250 (bonded NH), 1 560
(COO– asym. stretching), 1 370 (COO– sym. stretching), 1 290, 1 070, 945 (trans-stilbene), 825 cm–1.
UV-VIS spectrum (50% aq. MeOH), λmax nm (log ε): 228 (4.29), 297 (4.40), 311 (4.44), 322 sh
(4.25), 500 (2.11). For C17H30Cl2CoN5O10 (608.3) calculated: 33.56% C, 4.97% H, 11.51% N; found:
34.14% C, 5.08% H, 12.00% N.
γ-(trans-4-Stilbene)butyratopentaamminecobalt(III) perchlorate (4). This complex was obtained in
the same fashion as 2; yield 28%. IR spectrum (KBr disk): 3 275 (bonded NH), 1 580 (COO– asym.
stretching), 1 390 (COO– sym. stretching), 1 300, 1 090, 965 (trans-stilbene), 830. UV-VIS spectrum
(50% aq. MeOH), λmax nm (log ε): 227 (4.41), 297 (4.41), 310 (4.42), 323 sh (4.22), 500 (1.88). For
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

trans-4-Stilbenecarboxylatocobalt(III) Complexes
349
C18H32Cl2CoN5O10 (622.4) calculated: 34.73% C, 5.18% H, 11.25% N; found: 34.62% C, 5.10% H,
10.51% N.
RESULTS AND DISCUSSION
In order to obtain the desired series of carboxylatopentaamminecobalt(III) complexes,
it was necessary to syntesize the respective homologs of trans-4-stilbenecarboxylic
acid, viz. trans-4-stilbeneacetic acid (18), β-(trans-4-stilbene)propionic acid (19), and
γ-(trans-4-stilbene)butyric acid (20).
Two different methods were employed to obtain the desired homologs of trans-4-
stilbenecarboxylic acid, with p-chloromethyl substituted carboxylic acids used as the
starting materials in both cases. These acids, 8–10, were obtained by direct chloro-
methylation of the respective phenylalkylcarboxylic acids, i.e., phenylacetic acid (5),
β-phenylpropionic acid (6), and γ-phenylbutyric acid (7) which are commercially avail-
able. In method A, according to Bogdanov36, the acids were synthesized by passing
gaseous hydrogen chloride through a heated mixture of the respective starting acid and
zinc chloride in 40% formalin. The yields of the products were 27–30%. Method B
involved a reaction of the starting acid with chloromethyl methyl ether (11) in the
presence of stannic chloride in a cooled bath37. The yields in this case were better,
42–52%, and thus method B is preferable to method A. The two methods are outlined
in Scheme 1.
Two different approaches were used to convert the p-chloromethylphenylalkyl-
carboxylic acids into the desired stilbene derivatives. In the first approach, the Wittig
reaction involving intermediate phosphonium carbanions (ylides) was employed38–40
.
The chloromethylphenyl acids 8–10 were heated with triphenylphosphine in dry benzene
and converted into the corresponding carboxyalkylbenzyltriphenylphosphonium
chlorides 12–14 in a 75–99% yield (Scheme 1). Subsequent reaction of the phosphonium
salt with benzaldehyde in dimethylformamide in the presence of sodium methoxide or
phenyllithium (in diethyl ether) yielded the homologs of trans-4-stilbenecarboxylic
acid, 18, 19, in a low yield (3–8%) (Scheme 1).
The second, preferable approach utilized the Wadsworth–Horner modification of the
Wittig reaction, with phosphonates as the intermediates41–43. p-Chloromethylphenyl-
alkylcarboxylic acids 8–10 were converted into the corresponding phosphonates by re-
fluxing with triethyl phosphite in a 95–97% yield. The subsequent reaction of the
phosphonates 15–17 with benzaldehyde in dimethylformamide in the presence of sodium
methoxide in an ice-cooled bath gave the respective trans-4-stilbenealkylcarboxylic
acids 18–20 in a 9–22% yield (Scheme 1).
The structures of the resulting acids, 18–20, as well as of the intermediates in both
synthetic sequences were confirmed on the basis of their IR and UV spectra and, where
necessary, elemental analyses. The entire synthetic procedure is summarized in Scheme 1.
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

350
Parkanyi, Yeh Huang, Chu, Jeffries, III
n
n
csi
n
n
h
n
tarns
n
n
n
n
n
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

trans-4-Stilbenecarboxylatocobalt(III) Complexes
351
The red cobalt(III) complexes 1–4 were obtained from the sodium salts of the above
acids and aquapentaamminecobalt(III) perchlorate in N,N-dimethylacetamide34.
The electronic absorption spectral data of the homologs of trans-4-stilbenecarboxylic
acid 18–20 are given in the Experimental. In general, their spectra possess four strong
absorption bands (log ε ≥ 4.00) at 227–228, 297–298, 310–311, and 322–323 (sh) nm.
The spectra are analogous to the spectrum of the parent compound, trans-4-stilbenecar-
boxylic acid, with absorption maxima (in ethanol) at 230 (log ε 4.16), 310 (sh) (4.53),
319 (4.58), and 327 (sh) nm (4.42) (ref.48).
All trans-acids undergo facile photoisomerization when irradiated at 300 nm in dioxane,
resulting in the formation of the cis-isomers, with a shift of the absorption maxima in
their electronic absorption spectra (λmax at 221–223 and 308–309 nm; see Experimen-
tal). For comparison, the UV spectrum of cis-4-stilbenecarboxylic acid has absorption
maxima at 233 (log ε 4.32) and 292 nm (4.13) (refs48,49).
When comparing the electronic absorption spectra of the above free ligands with
those of the corresponding carboxylatopentaamminecobalt(III) complexes 1–4 (see Ex-
perimental), one can see that the spectra in the UV region are practically identical. The
difference is in the visible region where the cobalt complexes exhibit an additional
absorption band at 500 nm. As an example, the absorption curves of the complexes 2
and 3 are shown in Fig. 1. At 500 nm, the complexes are practically insensitive to
light34. Also, they are only slightly photosensitive in the ligand excitation region34. The
first ligand field bands of aqueous cobalt(III) ammine complexes of the type
10 000
ε
1 000
100
1
2
FIG. 1
Electronic absorption spectra of 1 (trans-4-stilbene)-
acetatopentaamminecobalt(III) perchlorate (2; n = 1)
and 2 [β-(trans-4-stilbene)propionato]pentaammine-
cobalt(III) perchlorate (3; n = 2) (in 50% aqueous
ethanol)
10
200
300
400
500
600
λ, nm
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352
Parkanyi, Yeh Huang, Chu, Jeffries, III:
1
1
1
1
[Co(NH3)5X]2+ correspond to the transition A1g → T1g and A1g → T2g in Oh sym-
metry (L1 and L2 bands, respectively). However, the complexes are sensitive to light
between 340 and 385 nm (internal ligand excitation region)34.
While detailed photochemical studies of the complexes were not planned in this
work and could be the topic of a subsequent investigation, we have studied emission
spectra of the free ligands (also in the form of their sodium salts, i.e., the corresponding
anions) and the respective cobalt(III) complexes. The free ligands, their anions, and the
cobalt complexes show the expected fluorescent emission which is characteristic of the
first excited singlet state of the organic ligand50. Stilbenes are not expected to give
phosphorescence in solution at room temperature51.
The fluorescence measurements were carried out with 312 nm exciting wavelength.
The spectra were obtained in 50% aqueous ethanol, for the organic ligands alone (sodium
salt), for solutions containing the ligand and a five-fold excess of aquapentaammine-
cobalt(III) perchlorate, and, finally, the corresponding carboxylatopentaammine-
cobalt(III) perchlorates. A fluorescence emission was observed in each case, with a
maximum at 363 nm, characteristic of the first excited singlet state of the ligand. Fluo-
rescence of the free ligand is decreased in the presence of aquapentaamminecobalt(III)
perchlorate, however, the intensity of fluorescence is even more diminished in the re-
spective carboxylatopentaamminecobalt(III) complexes. The relative intensities of fluo-
rescence in the case of 19 and its pentaamminecobalt(III) complex are shown in Fig. 2.
A comparison of the quenching effect for the four systems under study indicates that
this effect decreases with an increasing number of methylene groups in the organic
ligand, i.e., with increasing distance between the presumed energy donor (the stilbene
moiety) and the presumed energy acceptor (cobalt). While additional detailed studies of
this observation are needed, by analogy with the work of Adamson and co-workers34 on
(trans-4-stilbene)carboxylatopentaamminecobalt(III) perchlorate (1), an intramolecular
1
100
I
rel.%
2
80
60
FIG. 2
40
Fluorescence emission (312 nm exciting wave-
length, 50% aqueous ethanol):
3
1 0.000116
mol/dm3 β-(trans-4-stilbene)propionate ion (19);
2 0.000116 mol/dm3 19 + 0.00056 mol/dm3
[Co(NH3)5(H2O)]3+; 3 0.000116 mol/dm3 [β-(trans-
4-stilbene)propionato]pentaamminecobalt(III) per-
chlorate (3)
20
0
200
400
600
λ, nm
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

trans-4-Stilbenecarboxylatocobalt(III) Complexes
353
energy transfer (LMCT, ligand to metal charge transfer) is a distinct possibility. This
means that, in such a case, the energy transfer would occur from the first excited singlet
state of the organic ligand to the triplet LMCT state of the complex34. The number and
the accuracy of the data are not sufficient to establish the relationship between the
efficiency of energy transfer and the distance at this time.
In conclusion, the results presented here describe a series of cobalt(III) complexes
with an aromatic ligand which can be used to study intramolecular energy transfer from
the organic ligand to the metal. This work represents a follow-up on the work on the
first compound in the series, 1, as described by Adamson and co-workers34,52
.
The authors wish to thank Professor A. W. Adamson for his encouragement and his interest in this
work. The financial support of this work by the R. A. Welch Foundation, Houston, TX (work done in
Texas at the University of Texas at El Paso) is gratefully acknowledged.
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