Ferrocenoyl Amino Acids: A Synthetic and Structural Study 1

Article abstract of DOI:10.1021/ic961454t

A series of ester-protected amino acids were coupled to ferrocenecarboxylic acid (1) using the DCC/HOBt protocol to give ferrocenoyl N-amino acids (amino acid = Glu(OBz)₂ (2a). Gly(OEt) (2b), Pro(OBz) (2c). Cys(SBz)OMe (2d), Ala(OBz) (2e), Tyr(OBz) (2f), Phe(OBz) (2g)). All products were fully characterized. The intermediate hydroxybenzotriazole active ester FcCOOBt (3) was isolated and fully characterized. The solid state structure of 2a, 2d, and 3 were determined by single-crystal X-ray diffraction. 2a: monoclinic P2₁ with a = 11.8142(5) ?, b = 9.7560(5) ?, c = 22.9456(10) ?, β = 90.246(5)°, V = 2644.7(2) ?³ Z = 2, R = 0.046, 2d: orthorhombic P2₁2₁2₁, with a = 9.957(2) ?, b = 11.680(2) ?, c = 36.452(2) ?, V = 4239.5(13) ?³, Z = 4, R = 0.065. The solid state structures of 2a and 2d show extensive C=O···H-N hydrogen bonding. 3: triclinic P1? with a = 7.0391(5) ?, b = 10.7922(7) ?, c = 11.1690(7) ?, α = 108.071(5)°, β= 107.957(5)°, γ = 103.896(5)°, V = 712.5(2) ?³, Z = 2, R = 0.030. The long ester bond distance of 1.427(2) ? provides a rationale for its inherent reactivity toward primary and secondary amines.

Full text of DOI:10.1021/ic961454t

2400
Inorg. Chem. 1997, 36, 2400-2405
Ferrocenoyl Amino Acids: A Synthetic and Structural Study†
Heinz-Bernhard Kraatz,* Janusz Lusztyk, and Gary D. Enright
Chemical Biology Program, Steacie Institute for Molecular Sciences, National Research Council of
Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Canada
ReceiVed December 4, 1996X
A series of ester-protected amino acids were coupled to ferrocenecarboxylic acid (1) using the DCC/HOBt protocol
to give ferrocenoyl N-amino acids (amino acid ) Glu(OBz)2 (2a), Gly(OEt) (2b), Pro(OBz) (2c), Cys(SBz)OMe
(2d), Ala(OBz) (2e), Tyr(OBz) (2f), Phe(OBz) (2g)). All products were fully characterized. The intermediate
hydroxybenzotriazole active ester FcCOOBt (3) was isolated and fully characterized. The solid state structures
of 2a, 2d, and 3 were determined by single-crystal X-ray diffraction. 2a: monoclinic P21 with a ) 11.8142(5)
Å, b ) 9.7560(5) Å, c ) 22.9456(10) Å, â ) 90.246(5)°, V ) 2644.7(2) Å3, Z ) 2, R ) 0.046. 2d: orthorhombic
P212121 with a ) 9.957(2) Å, b ) 11.680(2) Å, c ) 36.452(2) Å, V ) 4239.5(13) Å3, Z ) 4, R ) 0.065. The
solid state structures of 2a and 2d show extensive CdO‚‚‚H-N hydrogen bonding. 3: triclinic P1h with a )
7.0391(5) Å, b ) 10.7922(7) Å, c ) 11.1690(7) Å, R ) 108.071(5)°, â ) 107.957(5)°, γ ) 103.896(5)°, V )
712.5(2) Å3, Z ) 2, R ) 0.030. The long ester bond distance of 1.427(2) Å provides a rationale for its inherent
reactivity toward primary and secondary amines.
Introduction
in a lower observed rate.3a-e Furthermore, it was recognized
that the rate is influenced by the electric field generated in helical
peptide assemblies, and hence by the direction of ET.3p
A transition metal can be intimately involved in electron
transfer (ET) processes either as an electron storage device, as
part of an electron transfer chain, or as the reaction center itself.1
Progress has been made in understanding ET processes by
making use of chemically modified metalloproteins,2 which has
provided much insight into the redox behavior of the active site.
It is generally believed that long range ET occurs by tunneling,
while short-range ET may proceed by a through-bond mecha-
nism. The particular redox characteristics of a metal center are
likely to depend most critically on (a) small deviations from
the standard geometry imposed on the metal center by the
protein backbone, (b) the extended chemical environment
imposed on the metal by the tertiary structure of the protein,
and (c) direct metal-protein interactions. The influence of the
protein backbone on the stabilization of one redox state of a
metal over another has been implied. However, the role of the
protein backbone in modifying ET rates and its mechanisms is
only poorly understood. ET rate measurements for systems
having two redox centers linked by a linear peptide chain have
appeared in the literature over the years.3 It was shown
generally that a high flexibility of the oligopeptide linker results
We are interested in the ET properties of rigid and flexible
peptide assemblies, addressing the role of the peptidic backbone
in ET processes. As part of our ongoing investigation, we hope
to be able to delineate through-bond and through-space interac-
tions and draw conclusions about electronic effects of each
amino acid residue. However, in order to draw meaningful
conclusions about electronic properties of a particular system,
structural and theoretical considerations must be taken into
account. In addition, it has been widely recognized that the
dynamic properties of the system are an important factor in the
electron transfer process. The size of natural systems precludes
a detailed investigation of structural reorganization processes
that may occur during ET.
Rather than having to cope with large redox proteins, we are
focusing our attention on small peptide assemblies covalently
linked to various redox probes. These systems can be structur-
ally characterized by NMR methods and X-ray crystallography.
In addition, their size does not preclude high-level calculations
(e.g., density functional calculations). In our approach, the
redox probe is coupled to an amino acid and/or peptides of
defined chain length and structure. Ferrocene moieties have
been widely used in molecular recognition studies as probe
molecules, sensitive to small changes in electronic structure
brought about by substrate binding.4 Binding is achieved by
* Corresponding author. E-mail: bernie@100sci.lan.nrc.ca.
† Issued as NRCC-39137.
X Abstract published in AdVance ACS Abstracts, April 1, 1997.
(1) Bertini, I.; Gray, H. B.; Lippard, S. J.; Valentine, J. S. Bioinorganic
Chemistry. University Science Books: Mill Valley, CA, 1994.
(2) (a) Meier, M.; van Eldik, R.; Chang, I.-L.; Mines, G. A.; Wuttke, D.
S.; Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc. 1994, 116, 1577-
1578. (b) Wuttke, D. S.; Gray, H. B.; Fisher, S. L.; Imperiali, B. J.
Am. Chem. Soc. 1993, 115, 8455-8456. (c) Chang, I.-J.; Gray, H.
B.; Winkler, J. R. J. Am. Chem. Soc. 1991, 113, 7056. (d) Wuttke, D.
S.; Bjerrum, M. J.; Winkler, J. R.; Gray, H. B. Science 1992, 256,
1007.
Am. Chem. Soc. 1990, 112, 5640-5642. (j) Basu, G.; Kubasik, M.;
Angelos, D.; Secor, B.; Kuki, A. J. Am. Chem. Soc. 1990, 112, 9410-
9411. (k) Mecklenburg, S. L.; Peek, B. M.; Erickson, B. W.; Meyer,
T. J. J. Am. Chem. Soc. 1991, 113, 8540-8542. (l) Mecklenburg, S.
L.; Peek, B. M.; Schoonover, J. R.; McCafferty, D. G.; Wall, C. G.;
Erickson, B. W.; Meyer, T. J. J. Am. Chem. Soc. 1993, 115, 5479-
5495. (m) Mishna, A. K.; Chandrasekar, R.; Faraggi, M.; Klapper,
M. H. J. Am. Chem. Soc. 1994, 116, 1414-1422. (n) Hayashi, T.;
Takimura, T.; Hitomi, Y.; Ohara, T.; Ogoshi, H. J. Chem. Soc. Chem.
Commun. 1995, 545-555. (o) Jones, G.; Feng, Z.; Oh, C. J. Phys.
Chem. 1995, 99, 3883-3888. (p) Galoppini, E.; Fox, M. A. J. Am.
Chem. Soc. 1996, 118, 2299-2300.
(3) (a) Isied, S. S.; Vassilin, A. J. Am. Chem. Soc. 1984, 106, 1726-
1732. (b) Isied, S. S.; Vassilin, A. J. Am. Chem. Soc. 1984, 106, 1736-
1739. (c) Isied, S. S.; Vassilin, A.; Magnuson, R. H.; Schwarz, H. A.
J. Am. Chem. Soc. 1985, 107, 7432-7438. (d) Vassilin, A.; Wishart,
J. F.; van Hemelryck, B.; Schwarz, H. A.; Isied, S. S. J. Am. Chem.
Soc. 1990, 112, 7258-7266. (e) For a review see: Isied, S. S. Chem.
ReV. 1992, 92, 381. (f) Schanze, K. S.; Sauer, K. J. Am. Chem. Soc.
1988, 110, 1180-1186. (g) Sisido, M.; Tanaka, R.; Inai, Y.; Imanishi,
Y. J. Am. Chem. Soc. 1989, 111, 6790-6796. (h) Farragi, M.;
DeFelippis, M. R.; Klapper, M. H. J. Am. Chem. Soc. 1989, 111,
5141-5145. (i) DeFilippis, M. R.; Faraggi, M.; Klapper, M. H. J.
(4) (a) For a recent survey see: Beer, P. D. J. Chem. Soc., Chem. Commun.
1996, 689-696. (b) Chen, Z.; Graydon, A. R.; Beer, P. D. J. Chem.
Soc., Faraday Trans. 1996, 92, 97-102. (c) Also see: Constable, E.
C. Angew. Chem., Int. Ed. Engl 1991, 30, 407-408.
S0020-1669(96)01454-1 CCC: $14.00 Published 1997 by the American Chemical Society

Ferrocenoyl Amino Acids
Inorganic Chemistry, Vol. 36, No. 11, 1997 2401
simple coupling reactions, such as esterification or amidation.5a-c
Other methods include the introduction of the ferrocenylmethyl
(Fem) group by catalytic reductive alkylation of amino acids5d,e
and the Pd-catalyzed coupling of iodoferrocenes with
2-amidoacrylates.5f Degani and Heller have coupled the fer-
rocene moiety to glucose oxidase via the H2N group of lysine
and used it as a redox relay.6 Coupling was achieved using a
water-soluble carbodiimide (1-(3-(dimethylamino)propyl)-3-
ethylcarbodiimide). There is no structural information about
the ferrocene-protein linkage and its arrangement with respect
to the rest of the protein.
We now wish to report our first study focusing on synthetic
results of ferrocenecarboxylic acid/amino acid coupling reac-
tions. The structural properties of two ferrocenoyl-amino acid
esters and of ferrocenoylhydroxybenzotriazole active ester are
described. A preliminary account of this work was presented
earlier.7
C30H29FeNO5: C, 66.80; H, 5.42; N, 2.60. Found: C, 67.1; H, 5.6; N,
3.0. MW for C30H29FeNO5: calc, 539.4; found, 540.2 [M + 1]+. E1/2
) 0.180 V (vs Fc/Fc+). IR (film; cm-1; NaCl plates): 3400 (ν(N-
H), vbr), 1734 (br) and 1627 (ν(CdO)). 1H-NMR (δ in ppm,
CDCl3): 7.36-7.25 (m, aromatic H of Ph), 6.63 (1H, br d, JHH ) 7.7
Hz, -NH), 5.21 (2H, s, PhCH2O), 5.10 (2H, s, PhCH2O), 4.77 (2H,
m, -NCH- and H ortho to carboxy group on Cp ring, both signals
overlapping), 4.64 (1H, s, H ortho to carboxy group on Cp ring), 4.34
(2H, br s, H meta to carboxy group on Cp ring), 4.21 (5H, s,
unsubstituted Cp ring), 2.49 (2H, m, -CH2CH2COOBz), 2.35 (2H, m,
-CH2CH2COOBz). 13C-NMR (δ in ppm, CDCl3): 173.2 (CdO),
171.9 (CdO), 170.6 (CdO), 135.6 (quaternary C of Ph), 135.2 (+ve
DEPT, Ph), 128.6 (+ve DEPT, Ph), 128.4 (+ve DEPT, Ph), 128.3 (+ve
DEPT, Ph), 74.9 (quaternary C of Cp), 70.6 (+ve DEPT, 2C meta to
carboxy group on Cp ring), 69.7 (+ve DEPT, all C of unsubstituted
Cp), 68.5 (+ve DEPT, C ortho to carboxy group on Cp ring), 67.8 (C
ortho to carboxy group on Cp ring), 67.3 (-ve DEPT, -OCH2Ph),
66.6 (-ve DEPT, -OCH2Ph), 51.8 (+ve DEPT, N(H)CH(R)C(O)),
30.4 (-ve DEPT, -CH2CH2), 26.9 (-ve DEPT, -CH2CH2).
General Preparation for 2b-g. The synthesis employed for the
preparation of 2b-g was identical to that for 2a. Ferrocenecarboxylic
acid (0.5 g, 2.17 mmol), solid dicyclohexylcarbodiimide (0.50 g, 2.4
mmol), and HOBt (0.34 g, 2.4 mmol) were allowed to react in CH2Cl2
(10 mL). Amino acid ester hydrochloric acid salt (exact amount: see
Vide infra) was treated with Et3N in CH2Cl2, and the mixture was added
to the stirring slurry. Stirring was continued overnight. The procedure
for the isolation of the products was identical to that for 2a.
Ferrocenoyl-N-Gly(OEt) (2b). H-Gly-OEt (0.35 g, 2.5 mmol)
was used. Yield: 0.52 g, 76.0%, yellow microcrystalline solid. Anal.
Calc for C15H17NO3Fe: C, 57.16; H, 5.44; N, 4.44. Found: C, 56.9;
H, 5.5; N, 5.0. MW for C15H17NO3Fe: calc, 315.2; found, 316.1 [M
+ 1]+. E1/2 ) 0.181 V (vs Fc/Fc+). 1H-NMR (δ in ppm, CDCl3, 20
°C): 6.22 (1H, br s, -NH), 4.72 (2H, t, JHH ) 1.8 Hz, H ortho to
carboxy group on Cp ring), 4.36 (2H, t, JHH ) 1.8 Hz, H meta to
carboxy group on Cp ring), 4.28 (5H, s, unsubstituted Cp ring), 4.26
(2H, q, JHH ) 7.2 Hz, -OCH2-), 4.15 (d, JNH ) 5.4 Hz, N(H)CH2C-
(O)), 1.36 (3H, t, JHH ) 7.2 Hz,-CH2CH3). 13C-NMR (δ in ppm,
CDCl3, 20 °C): 71.26 (s, +ve DEPT, C ortho to carboxy group on
Fc), 70.49 (s, +ve DEPT, C meta to carboxy group on Fc), 68.90 (s,
+ve DEPT, unsubstituted Cp ring of Fc), 62.17 (s, -ve DEPT, -OCH2-
CH3), 42.01 (s, -ve DEPT, -N(H)CH2-), 14.87 (s, +ve DEPT,
-OCH2CH3).
Ferrocenoyl-N-Pro(OBz) (2c). H-Pro-OBzl‚HCl (0.60 g, 2.48
mmol) was used. Yield: 0.77 g, 85.0%, brown viscous oil. MW for
C23H23NO3Fe: calc, 417.3; found, 418.1 [M + 1]+. E1/2 ) 0.160 V
(vs Fc/Fc+). 1H-NMR (δ in ppm, CDCl3): 7.36 (5H, br m, aromatic
H of Ph), 5.21 (2H, second-order m, OCH2Ph), 4.86 (1H, s, H ortho to
carboxy group on Cp ring), 4.70 (2H, m, overlapping signals due to
N(H)CH(R)C(O) and H ortho to carboxy group on Cp ring), 4.36 (2H,
s, H meta to carboxy group on Cp ring), 4.24 (5H, s, H of unsubstituted
Cp ring), 3.96 and 3.79 (2H, m, two signals due to the two diastereotopic
H of -NCH2-), 2.21 and 1.99 (2H, m, two signals due to the two
diastereotopic H adjacent to stereocenter -C(H)CH2CH2CH2N-,
overlapping with signal due to one H of the -NCH2CH2CH2- group),
2.09 and 1.99 (2H, m, two signals due to the two diastereotopic H of
-NCH2CH2CH2-). 13C-NMR (δ in ppm, CDCl3): 172.2 (CdO),
169.7 (CdO), 135.8 (s, quartenary C of Ph), 128.5 (s, Ph), 128.1 (br
s, Ph), 73.4 (s, ipso C of Cp), 70.8 (s, C on Cp), 70.2 (s, C on Cp),
70.0 (s, C on Cp), 69.6 (s, unsubstituted Cp ring of Fc), 66.6 (s, -ve
DEPT, OCH2Ph), 60.2 (s, +ve DEPT, N(H)CH2C(O)), 49.0 (s, -ve
DEPT, N(H)CH2-), 28.6 (s, -ve DEPT,-CH2-), 25.5 (s, -ve
DEPT,-CH2-).
Ferrocenoyl-N-Cys(SBz)OMe (2d). H-Cys(SBzl)-OMe‚HCl (0.74
g, 2.2 mmol) was used. The crude orange-yellow material (yield )
0.75 g, 79.0%) was recrystallized from CH2Cl2 and then from Et2O.
X-ray-quality crystals were obtained as thin yellow needles by slow
evaporation of Et2O at -20 °C. Anal. Calc for C22H23NO3SFe: C,
60.43; H, 5.30; N, 3.20. Found: C, 60.2; H, 5.4; N, 4.0. MW for
C22H23NO3SFe: calc, 437.3; found, 438.1 [M + 1]+. E1/2 ) 0.191 V
(vs Fc/Fc+). 1H-NMR (δ in ppm, CDCl3): 7.32 (5H, m, aromatic H
of SCH2Ph), 6.47 (1H, d, JNH ) 7.0 Hz, -NH), 4.94 (1H, m, N(H)-
CH(R)C(O)), 4.73 (1H, s, H ortho to carboxy group on Cp ring), 4.69
Experimental Section
General Procedures. Glutamic acid dibenzyl ester was prepared
according to the published procedure from L-glutamic acid (Aldrich)
and benzyl alcohol under acidic conditions.8 H-Gly(OEt)‚HCl, H-Pro-
(OBz)‚HCl, H-Cys(SBz)OMe‚HCl, ferrocenecarboxylic acid (FC),
dicyclohexylcarbodiimide (DCC), and Hydroxybenzotriazole (HOBt)
were used as received (Aldrich). H-Ala(OBz)‚Tos and H-Tyr(OBz)‚-
Tos (Nova-Biochem) were used as received. All solvents were used
as received without further treatment. 1H- and 13C-NMR spectra were
recorded at 200.132 and 50.323 MHz, respectively, on a Bruker AC
200 NMR spectrometer. 2D-COSY spectra were recorded at 500.14
MHz on a Bruker AMX 500 NMR spectrometer. All chemical shifts
1
(δ) are reported in ppm and coupling constants (J) in Hz. The H-
and 13C-NMR chemical shifts are relative to tetramethylsilane (δ ) 0
ppm), which was added as an internal standard. Assignments in the
1H- and 13C{1H}-NMR were made using 13C-DEPT-135 (distortionless
enhancement by polarization transfer) and by 2D-COSY. All measure-
ments were carried out at 293 K unless otherwise specified. Elemental
analyses were carried out at the NRC Institute for Biology, Ottawa,
Canada.
Preparation of Ferrocenyl Amino Acids. Ferrocenoyl-N-
Glutamic Acid Dibenzyl Ester (2a). To a solution of ferrocenecar-
boxylic acid (0.55 g, 2.4 mmol) in CH2Cl2 (10 mL) were added solid
dicyclohexylcarbodiimide (0.50 g, 2.4 mmol) and HOBt (0.34 g, 2.4
mmol). Freshly prepared glutamic acid dibenzyl ester hydrochloric
acid salt (1.57 g, 4.8 mmol) was treated with Et3N in CH2Cl2, and the
mixture was added to the stirring slurry. Dicylcohexylurea precipitated
almost immediately. Stirring was continued at room temperature for
2 days. The urea was filtered off, and the filtrate was washed with
distilled water (3 × 100 mL) and subsequently dried over MgSO4. The
solvent was removed in vacuo. The crude product (0.81 g, 62.7%)
was recrystallized from ethyl acetate and then from diethyl ether to
give 0.45 g of yellow needles (yield 34.6%), which were of sufficient
quality to be used in the X-ray crystallographic study. Anal. Calc for
(5) (a) See for example: Tecilla, P.; Dixon, R. P.; Slobodkin, G.; Alavi,
D. S.; Waldeck, D. H.; Hamilton, A. D. J. Am. Chem. Soc. 1990,
112, 9408-9410. (b) Medina, J. C.; Gay, I.; Chen, Z.; Echegoyen, I.;
Gockel, G. W. J. Am. Chem. Soc. 1991, 113, 365. (c) Medina, J. C.;
Li, C.; Bott, S. G.; Atwood, J. L.; Gockel, G. W. J. Am. Chem. Soc.
1991, 113, 366. (d) Eckert, H.; Seidel, C. Angew. Chem., Int. Ed. Engl.
1986, 25, 159-160. (e) Beer, P. D.; Chen, Z.; Drew, M. G. B.;
Kingston, J.; Ogden, M.; Spencer, P. J. Chem. Soc., Chem. Commun.
1993, 1046-1048. (f) Carlstro¨m, A.-S.; Frejd, T. J. Org. Chem. 1990,
55, 4175-4180.
(6) (a) Degani, Y.; Heller, A. J. Phys. Chem. 1987, 91, 1285-1289. (b)
Degani, Y.; Heller, A. J. Am. Chem. Soc. 1988, 110, 2615-2620. (c)
For a review of electrode-bound systems see: Heller, A. J. Chem.
Phys. 1992, 96, 3579-3587.
(7) Kraatz, H.-B. Paper presented at the 3rd European Bioinorganic
(EUROBIC3) conference in Noordwijkerhout, The Netherlands, Aug
4-10, 1996; see Abstract A20.
(8) Bodanszky, M.; Bodanszky, A. The Practice of Peptide Synthesis;
Springer Verlag: Berlin, 1984; p 39.

2402 Inorganic Chemistry, Vol. 36, No. 11, 1997
Kraatz et al.
Table 1. Crystal Data for FcGlu(OBz)2 (2a), FcCys(SBz)OMe (2d), and FcCOOBt (3)
2a
2d
3
empirical formula
fw
C30H29FeNO5
539.4
C22H23FeNO3S
437.3
C17H13FeN3O2
347.2
space group
a, Å
b, Å
P21
P212121
9.957(2)
11.680(2)
36.452(6)
P1h
11.8142(5)
9.7560(5)
22.9456(10)
7.0391(5)
10.7922(7)
11.1690(7)
108.071(5)
107.957(5)
103.896 (5)
712.5 (2)
2
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
90.246(5)
2644.7 (2)
2
1.355
4239.5 (13)
4
1.370
F
calc, g/cm3
cryst size, mm
1.618
0.30 × 0.30 × 0.12
0.30 × 0.08 × 0.05
0.20 × 0.15 × 0.10
µ(Mo KR), cm-1
6.10
6.80
1.07
radiation (monochromated in
incident beam)
Mo KR (λ ) 0.710 73 Å),
graphite monochromated
0.046, 0.061
Cu (λ ) 1.540 60 Å),
graphite monochromated
0.065, 0.157
Mo KR (λ ) 0.710 73 Å),
graphite monochromated
0.030, 0.036
a
R, Rw
2
a R ) ∑(Fo - Fc)/∑(Fo); Rw ) [∑(w(Fo - Fc)2)/Σ(wFo )]1/2
.
(1H, s, H ortho to carboxy group on Cp ring), 4.37 (2H, s, H meta to
carboxy group on Cp ring), 4.25 (5H, s, unsubstituted Cp ring), 3.77
(3H, s, -OCH3), 1.82 (2H, m, SCH2Ph).
in ppm, CDCl3): 7.35-7.32 (4H, m, aromatic H of N3C6H4), 5.21 (2H,
s, H ortho to carboxy group on Cp ring), 4.83 (2H, s, H meta to carboxy
group on Cp ring), 4.21 (5H, s, unsubstituted Cp ring).
Structural Studies. All pertinent crystallographic information is
summarized in Table 1.
Ferrocenoyl-N-Ala(OBz) (2e). H-Ala-OBzl‚Tos (0.70 g, 2.2
mmol) was used. Yield: 0.65 g, 76.6%, yellow microcrystalline solid.
MW for C21H21NO3Fe: calc, 391.2; found, 392.1 [M + 1]+. 1H-NMR
(δ in ppm, CDCl3): 7.37 (5H, m, aromatic H of CH2Ph), 6.23 (1H, d,
JNH ) 6.6 Hz, -NH), 5.22 (2H, second-order m, -OCH2Ph), 4.80 (1H,
m, N(H)CH(R)C(O)), 4.72 (1H, s, H ortho to carboxy group on Cp
ring), 4.66 (1H, s, H ortho to carboxy group on Cp ring), 4.36 (2H, s,
H meta to carboxy group on Cp ring), 4.21 (5H, s, unsubstituted Cp
ring), 1.50 (3H, d, JHH ) 7.2 Hz, -CH3).
Ferrocenoyl-N-Tyr(OBz) (2f). H-Tyr-OBzl‚HCl (0.9 g, 2.2
mmol) was used. Yield: 0.76 g, 72.5%, orange solid. MW for
C27H25NO4Fe: calc, 483.3; found, 484.0 [M + 1]+. 1H-NMR (δ in
ppm, CDCl3): 7.36 (5H, br s, aromatic H of CH2Ph), 6.85 (AB quartet
of phenolic residue of Tyr), 6.13 (1H, d, JNH ) 8.0 Hz, -NH), 5.19
(2H, second-order m, -OCH2Ph), 5.05 (1H, dm, JNH ) 7.9 Hz, N(H)-
CH(R)C(O)), 4.62 (1H, s, H ortho to carboxy group on Cp ring), 4.57
(1H, s, H ortho to carboxy group on Cp ring), 4.31 (2H, s, H meta to
carboxy group on Cp ring), 4.10 (5H, s, unsubstituted Cp ring), 3.10
(2H, t, JHH ) 6.3 Hz,-CH2Ph).
Crystals of 2a, 2d, and 3 were mounted on glass fibers using epoxy
resin. Data collection for 2a and 3 proceeded at -150 °C on a Siemens
SMART diffractometer equipped with a CCD detector (Mo KR). The
data for 2a (3) were collected up to a 2θ maximum of 49° (for 3: 57.5°)
using the ω-scan mode. Of 11 939 (for 3: 8405) reflections measured,
7933 (for 3: 3644) were unique (7532 with I > 2.5σ(I)) (for 3: 3090
with I > 2.5σ(I)). A total of 256 reflections with 2θ < 49° (for 3: 92
reflections with 2θ < 57.5°) were missed. An empirical absorption
correction was applied. The merging R value on significant intensities
was 0.019 (for 3: 0.015). The cell parameters were determined from
8192 (for 3: 5655) reflections. The refinement was carried out with
the NRCVAX11,12 system of programs. All non-hydrogen atoms were
refined anisotropically. The hydrogen atoms were placed in calculated
positions and allowed to ride on their parent atoms. Weights based on
statistics were used. The refinement (based on Fo) converged to R )
0.046, Rw ) 0.061 (for 3: R ) 0.030, Rw ) 0.036).
Data collection for 2d proceeded at room temperature on an ENRAF
Nonius CAD 4 diffractometer (Cu KR). Its unit cell was obtained from
20 carefully centered reflections, obtained by a random search.
Monitoring three standard reflections every 100 reflections indicated
no decay of the crystal in the X-ray beam. Data were collected at
variable scan speeds in the ω-2θ-scan mode in the range of 4.83 <
2θ < 80.00, giving a total of 2121 reflections (1153 with I > 2σ(I)).
The data were corrected for Lorentz and polarization effects. No
absorption correction was applied due to the low absorption coefficients
(µ ) 6.10 (2a), 6.80 (2d), and 1.07 (3)). The structure of 2d was
solved by direct methods and refined by difference Fourier methods
using the SHELXTL program package.9 Hydrogen atoms were
introduced at calculated positions. Scattering factors and corrections
for anomalous dispersion for all heavy atoms were those of ref 10.
The absolute conformations of 2a and 2d were confirmed as a matter
of course. For 2a, the chirality was confirmed using the Rogers method.
The Flack method was used for 2d.
Ferrocenoyl-N-Phe(OBz) (2g). H-Phe-OBzl‚HCl (0.63 g, 2.16
mmol) was used. Yield: 0.88 g, 87.2%, yellow microcristalline solid.
MW for C27H25NO3Fe: calc, 467.3; found, 468.0 [M + 1]+. 1H-NMR
(δ in ppm, CDCl3): 7.36 (5H, m, aromatic H), 7.30 (2H, m, aromatic
H ortho), 7.11 (3H, m, aromatic H meta and para), 6.07 (1H, d, JNH
)
7.8 Hz, -NH), 5.19 (2H, second-order m, -OCH2Ph), 5.04 (1H, dt,
JNH ) 7.6 Hz, JHH ) 5.5 Hz, -N(H)CH(R)C(O)), 4.62 (1H, s, H ortho
to carboxy group on Cp ring), 4.60 (1H, s, H ortho to carboxy group
on Cp ring), 4.32 (2H, s, H meta to carboxy group on Cp ring), 4.11
(5H, s, H of unsubstituted Cp ring), 3.19 (2H, dd, JHH ) 5.9 Hz, JNH
) 1.9 Hz,-CH2Ph adjacent to stereocenter). 13C-NMR (δ in ppm,
CDCl3, 20 °C): 171.7 (CdO), 170.0 (CdO), 135.9 (s, quartenary C
of Ph), 129.2 (s, Ph), 128.6 (br s, Ph), 127.1 (s, Ph), 124.6 (s, Ph),
120.4 (s, Ph), 73.4 (s, ipso C of Cp), 70.4 (s, 2Cs meta to substituent
on Cp), 69.7 (s, unsubstituted Cp ring), 68.2 (s, C ortho to substituent
on Cp), 68.0 (s, C ortho to substituent on Cp), 67.2 (s, -OCH2Ph),
54.7 (N(H)CH2C(O)), 38.0 (s, -CH2Ph).
Results and Discussion
Ferrocenoyl-OBt (3). Ferrocenecarboxylic acid (0.55 g, 2.4 mmol)
was dissolved in CH2Cl2 (10 mL), and solid dicyclohexylcarbodiimide
(0.50 g, 2.4 mmol) and HOBt (0.34 g, 2.4 mmol) were added to the
stirring solution. After 3 h at room temperature, dicyclohexylurea was
removed by filtration. The filtrate was washed with saturated NaHCO3
solution (50 mL), followed by distilled water (50 mL), and then dried
over MgSO4. The solvent was removed in vacuo. The crude product
(yield 0.79 g, 94.8%) was recrystallized from ethyl acetate and then
from diethyl ether to give 0.68 g (81.6%) of dark red crystalline product.
The product was recrystallized at -20 °C from EtAc/Et2O (1:1) giving
dark-red blocks, used in the crystallographic study (Vide infra). MW
for C17H13N3O2Fe: calc, 347.2; found, 348.0 [M + 1]+. 1H-NMR (δ
a. Synthesis of 2a-g and 3. The reaction of ferrocenecar-
boxylic acid (1) with C-protected amino acid salts under basic
conditions in the presence of dicylcohexylcarbodiimide (DCC)
and hydroxybenzotriazole (HOBt) cleanly led to the formation
(9) SHELXTL, Version 5; Siemens Energy & Automation, Inc., 1994.
(10) International Tables for Crystallography; Kynoch Press: Birmingham,
U.K., 1974; Vol. 4.
(11) Gabe, E. J.; Le Page, Y.; Charland, J.-P.; Lee, F. L.; White, P. S. J.
Appl. Crystallogr. 1989, 22, 384-387.
(12) Le Page, Y.; Gabe, E. J. J. Appl. Crystallogr. 1979, 12, 464-466.

Ferrocenoyl Amino Acids
Inorganic Chemistry, Vol. 36, No. 11, 1997 2403
Scheme 1. Synthesis of Ferrocenoyl Amino Acid Estersa
Table 3. Selected Bond Distances (Å) and Angles (deg) for 2a
molecule 1
molecule 2
(a) Bond Distances
av Fe(1)-C
av Fe(1)-C
O(1)-C(11)
C(1)-C(11)
C(11)-N(1)
2.041(5) (for CpR) av Fe(101)-C
2.047(5) (for Cp) av Fe(101)-C
2.022(7)
2.065(9)
1.222(6)
1.507(7)
1.342(7)
1.226(6)
1.499(7)
1.347(6)
O(101)-C(111)
C(101)-C(111)
C(111)-N(101)
(b) Bond Angles
O(101)-C(111)-N(101) 122.8(5)
O(1)-C(11)-N(1) 122.5(4)
regions (δ 4.86-4.62 and 4.57-4.70). The two diastereotopic
H atoms in meta positions with respect to the amide group on
the Cp ring appear as a slightly broadened singlet (5-8 Hz) in
the shift range of δ 4.31-4.37. The H atoms of the unsubsti-
tuted Cp ring are observed as singlets for all compounds. The
H atom of the amide NH is observed as a doublet due to
coupling with the adjacent chiral CH group, with coupling
constants ranging from 8.0 Hz (for 2f) to 6.6 (for 2e).
a Key: (i) CH2Cl2, DCC, HOBt, 3 h; (ii) CH2Cl2, DCC, HOBt, AA
ester (free base), 2 days.
1
Table 2. Selected H-NMR Spectroscopic Data for the Ferrocenoyl
Amino Acid Esters 2a-g (CDCl3, δ in ppm, Room Temperature)
compd
NH
CH
Cportho
Cpmeta
Cp
2a
2b
2c
2d
2e
2f
6.63 (d, 7.7)
6.22 (s)
4.77
4.15
4.70
4.94
4.80
5.05
5.04
4.77, 4.64
4.72
4.86, 4.70
4.73, 4.61
4.72, 4.66
4.62, 4.57
4.62, 4.60
4.34
4.36
4.36
4.37
4.36
4.31
4.32
4.21
4.28
4.24
4.25
4.21
4.10
4.11
13C-NMR spectroscopy is a sensitive tool to qualitatively
evaluate changes in electron density caused by substituent
influences of the different amino acids.14 Interestingly, the
measurements show that the shift of the Cp C atom carrying
the amide substitutent is almost invariant with respect to amino
acid substitution (between δ 75 and 73). Hence, the 13C-NMR
results seem to suggest that no significant substituent influence
exists. Likewise the signal due to the ortho and meta C atoms
of the substituted Cp ring and those of the unsubstituted Cp
ring exhibit signals which do not vary significantly from one
amino acid to another.
The cyclic voltammograms for 2a-d were measured in
acetonitrile solution, with ferrocene as an internal redox stand-
ard. As expected, 2a-d exhibit fully reversible one-electron
oxidations, attributed to a ferrocene-based oxidation, resulting
in the formation of a ferrocenium cation. There are some slight
variations in oxidation potential, with respect to the attached
amino acid. The half-wave potenial of 2c (172 mVvs Fc/Fc+)
is lower than those of 2a (180 mV vs Fc/Fc+), 2b (181 mV vs
Fc/Fc+), and 2d (191 mV vs Fc/Fc+). Currently, we are invest-
igating the redox properties in more detail and hope to be able
correlate them to the electronic structures of the compounds.
c. Structural Studies. Selected bond distances and angles
for 2a are given in Table 3. An ORTEP view of one of the
molecules of complex 2a is given in Figure 1. 2a crystallizes
in the monoclinic space group P21 with two independent
molecules per asymmetric unit. In each molecule, the Cp rings
are virtually parallel to each other (Cp-Fe-Cp angles: 2.1-
(3)° (for molecule 1) and 1.6(4)° (for molecule 2)). The
distances of Fe to the C atoms of the Cp rings was normal and
compare well with those of other ferrocene structures.15
Interestingly, the Cp internal C-C bond distances in the
substituted Cp rings (1.430(9) and 1.424(12) Å) are longer than
in the unsubstituted Cp ring (1.411(12) and 1.404(15) Å) for
6.47 (d, 7.0)
6.23 (d, 6.6)
6.13 (d, 8.0)
6.07 (d, 7.8)
2g
of orange to yellow coupling products Fc-AA (2) (AA ) Glu-
(OBz)2 (2a), Gly(OEt) (2b), Pro(OBz) (2c), Cys(SBz)OMe (2d),
Ala(OBz) (2e), Tyr(OBz) (2f), Phe(OBz) (2g)) (Scheme 1).
This reaction is related to that used by Heller to incoporate
ferrocene moieties into proteins.6a,b Very recently, a ferrocene
moiety was incorporated into the lysine side chain and the
product characterized by spectroscopic methods.13 The product
can be easily separated from organic byproducts (e.g., dicyclo-
hexylurea) and excess reagents (e.g., hydroxybenzotriazole and
unreacted starting materials) by filtration and washings with
aqueous saturated KHCO3 solution and 5% citric acid. All
ferrocenoyl amino acid esters are diethyl ether soluble and were
recrystallized from diethyl ether to give crystalline compounds
(2a-b, 2d-g). 2c is a viscous oil.
In the absence of C-protected amino acids, the ferrocenoyl
benzotriazole ester (3) was obtained as a dark red crystalline
material, which is air and moisture stable in common solvents.
This compound does not hydrolyze under aqueous extraction
conditions employed in the isolation and purification of ferro-
cenoyl amino acid ester (aqueous saturated KHCO3, 5% citric
acid). In the presence of amino acids (free base), 3 reacts as
expected by amidation to give the ferrocenoyl amides.
b. Identification of Compounds 2a-g and 3. All com-
pounds were fully characterized by a combination of electro-
spray-MS (2a-g, 3), NMR spectroscopies, (2a-g, 3), and X-ray
crystallography (2a, 2d, and 3; Vide infra). Electrospray-MS,
a very soft ionization method widely used for biological
molecules, confirmed the molecular weight of all the compounds
1
(14) (a) van de Kuil, L. A.; Luitges, H.; Grove, D. M.; Zwicker, J. W.;
van der Linden, J. G. M.; Roelofsen, A. M.; Jenneskens, L. W.; Drenth,
W.; van Koten, G. Organometallics 1994, 13, 468. (b) Weisman, A.;
Gozin, M.; Kraatz, H.-B.; Milstein, D. Inorg. Chem. 1996, 35, 1792-
1797.
(15) (a) Grossel, M. C.; Goldspink, M. R.; Hrijac, J. A.; Weston, S. C.
Organometallics 1991, 10, 851-860. (b) Hall, D. C.; Danks, I. P.;
Nyburg, S. C.; Parkins, A. W.; Sharpe, N. W. Organometallics 1990,
9, 1602-1607. (c) Wang, J.-T.; Yuan, Y.-F.; Xu, Y.-M.; Zhang, Y.-
W.; Wang, R.-J.; Wang, H.-G. J. Organomet. Chem. 1994, 481, 211-
216. (d) Seidelmann, O.; Beyer, L.; Richter, R. Z. Naturforsch. 1995,
50B, 1679-1684. (e) Seidelmann, O.; Beyer, L.; Zdobinsky, G.;
Kirmse, R.; Dietze, F.; Richter, R. Z. Anorg. Allg. Chem. 1996, 622,
692-700. (f) Wen, Z.; Li, F.-Z.; Liu, Q.-W.; Huang, X.-Y. Jiegou
Hauxue 1995, 14, 108-112.
described. Table 2 summarizes selected H-NMR spectro-
scopic data for 2a-g. All spectra were fully assigned using
DEPT and 2D-COSY techniques.
The presence of the chiral carbon center in the amino acid
part of the molecule effectively generates two diastereotopic
sides of the molecules. This renders the two H atoms adjacent
to the amide functionality on the Cp ring magnetically non-
equivalent, giving rise to two singlets in narrowly defined
(13) McCafferty, D. G.; Bishop, B. M.; Wall, C. G.; Hughes, S. G.;
Mecklenburg, S. L.; Meyer, T. J.; Eickson, B. W. Tetrahedron 1995,
51, 1093-1106.

2404 Inorganic Chemistry, Vol. 36, No. 11, 1997
Kraatz et al.
Figure 2. ORTEP drawing of 2d (30% probability level). Only one
of the molecules of the asymmetric unit is shown here. Hydrogen atoms
are omitted for clarity.
Figure 1. ORTEP plot of 2a (30% probability level) showing the
adopted numbering scheme. The second molecule has similar structural
features (see Supporting Information). Hydrogen atoms are omitted for
clarity.
both molecules. This has been documented before.15 The
distances of the carbonyl carbon to the Cp ring (C(1)-C(11)
1.499(7)Å and C(101)-C(111) 1.507(7) Å) are normal C-C
single bond distances. It is interesting to note that nonstrained
ferrocenoyl amides have C-C(Cp) bonds ranging from 1.471-
(4) to 1.477(7) Å. However, for the macrocyclic 1,1′-[1,4,10,-
13-tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)dicarbonyl]-
ferrocene, a long C-C(Cp) bond of 1.501(7) Å has been
observed.15a This and other distortions in that particular
structure have been attributed to minimizations of ring strain
in the macrocyclic part of the molecule, rather than to specific
intramolecular interactions. The amide C(O)-N group is planar
in both molecules. The carbonyl C-O distances compare well
with those of other ferrocenoyl amides. The amide C-N
distances (C(11)-N(1) 1.347(6) Å and C(111)-N(101) 1.342-
(7) Å exhibit significant single-bond character. The amide plane
is rotated out of the Cp ring plane by 14.1(2)° (for molecule 1)
and 18.4(3)° (for molecule 2).
Figure 3. ORTEP drawing of 3 (30% probability level).
Table 4. Selected Bond Distances (Å) and Angles (deg) for 2d
molecule 1
molecule 2
The nonbonding distances between the carbonyl group and
an amide NH of an adjacent molecule are about 2.8 Å (O(1)‚‚‚N-
(101) 2.81 Å and O(101)‚‚‚N(1) 2.80 Å), indicating the presence
of strong H-bonding between neighboring molecules in the solid
state. The IR spectrum shows a very broad band at 3400 cm-1
assigned to ν(N-H), confirming H-bonding. Analysis of the
unit cell reveals a linear chainlike structure of molecules linking
through NH‚‚‚OdC hydrogen bonding. NH to water oxygen16
and NH‚‚‚OdC16b,17 are well documented in the literature.
Figure 2 shows an ORTEP drawing of 2d. Pertinent bond
distances and angles are summarized in Table 4. 2d crystallizes
in the orthorhombic space group P212121 with two independent
molecules per asymmetric unit. The structural features are very
similar to those of 2a, with both molecules having parallel Cp
rings (Cp-Fe-Cp: molecule 1, 1.83°; molecule 2, 1.52°). The
amide group is slightly rotated out of the plane of the Cp ring
to which it is linked (molecule 1, 5.44°; molecule 2, 11.95°).
The S-C distances (range 1.77(2)-1.823(6) Å) and the C-S-C
angle of 102.1(7)° are normal for thioether linkages.18 The
individual molecules are linked by N-H‚‚‚OdC bonding; N‚‚‚O
(a) Bond Distances
2.03(2) (for CpR) av Fe(101)-C
2.04(2) (for Cp) av Fe(101)-C
av Fe(1)-C
av Fe(1)-C
O(1)-C(11)
C(1)-C(11)
C(11)-N(1)
S(1)-C(15)
S(1)-C(16)
2.02(2)
2.07(3)
1.27(2)
1.48(2)
1.39(2)
1.79(2)
1.80(1)
1.21(2)
1.43(2)
1.34(2)
1.77(2)
1.823(6)
O(101)-C(111)
C(101)-C(111)
C(111)-N(101)
S(1)-C(15)
S(1)-C(16)
(b) Bond Angles
O(1)-C(11)-N(1) 123.6(26)
C(15)-S(1)-C(16) 102.1(7)
O(101)-C(111)-N(101) 119.3(30)
C(115)-S(101)-C(116) 102.2(7)
Table 5. Selected Bond Distances (Å) and Angles (deg) for 3
(a) Bond Distances
av Fe(1)-C 2.050(2) (for CpR) av Fe(1)-C 2.053(2) (for Cp)
O(1)-C(11) 1.196(2)
C(11)-O(2) 1.427(2)
C(1)-C(11) 1.450(3)
(b) Bond Angle
121.3(2)
O(1)-C(11)-O(2)
2.84 Å), forming endless chains similar to those observed in
2a and other amino acid structures.16b,17c
(16) (a) Freeman, H. C.; Taylor, M. R. Acta Crystallogr. 1965, 18, 939-
952. (b) Freeman, H. C.; Robinson, G.; Schoone, J. C. Acta
Crystallogr. 1964, 17, 719-730.
The crystal structure of 3 is shown in Figure 3. Selected
bond distances and angles are given in Table 5. 3 crystallizes
in the triclinic space group P1h. The ferrocene moiety is in the
eclipsed conformation with the ester group being slightly rotated
out of the plane of the Cp ring to which it is attached (10.06-
(10)°). The dimensions within the ferrocene moiety are
normal.15 Interestingly, the benzotriazole (Bt) substituent is
(17) (a) Marsh, R. E.; Glusker, J. P. Acta Crystallogr. 1961, 14, 1110-
1116. (b) Leung, Y. C.; Marsh, R. E. Acta Crystallogr. 1958, 11, 17-
31. (c) Haas, D. J. Acta Crystallogr. 1965, 19, 860-861.
(18) (a) Handbook of Chemistry and Physics, 67th ed.; CRC Press, Inc.:
Boca Raton, FL, 1987; pp F160-F162. (b) Boorman, P. M.; Gao, X.;
Parvez, J. Chem. Soc., Chem. Commun. 1992, 1656-1658.

Ferrocenoyl Amino Acids
Inorganic Chemistry, Vol. 36, No. 11, 1997 2405
Summary and Outlook
rotated out of the ester plane by 96.56(6)°, allowing no efficient
interaction between the π-systems of the Cp ring and the
benzotriazole. A similar behavior has been observed for
benzoylferrocene,19 where the solid state structure shows the
phenyl ring rotated out of the Cp plane by 40.4°. Steric
interference does not allow coplanarity of the Cp and Ph
groups.19 Similar steric problems are most likely responsible
for the arrangement of the Bt substituent.
The C(11)-O(1) bond distance of 1.196(2) Å is significantly
shorter than those observed for ferrocenecarboxylic acid20
(1.261(15) Å) and ferrocenedicarboxylic acid (1.228(3) Å).21
It is also shorter than those of other ferrocenecarboxylic esters,
such as 2-(1-hydroxyethyl)-1-ferrocenecarboxylic acid methyl
ester (CdO 1.205(4) Å)22 and 3-(diphenylphosphino)-1-fer-
rocenecarboxylic acid methyl ester (CdO 1.204(3) Å).23 The
C(11)-O(2) bond distance is about 0.09 Å longer than those
of other esters (1.427(2) Å). This crystallographic information
fits well with the observed facile amidation of active esters in
the presence of primary and secondary amines. Being a long
ester bond, it is inherently weak and reactive and hence it will
rapidly react to form an amide bond.
Our studies described the facile synthesis and characterization
of novel ferrocenoyl amino acid esters. While 13C-NMR
spectroscopy does not provide any insight into the electron
density variation presumably caused by a variation of the amino
acid, CV measurements suggest that there is an influence. We
are currently investigating this in more detail. We are in the
process of carrying out high-level calculations based on density
functional theory to gain further insight into the electronic
structures of the complexes. This may allow us to rationalize
the changes in redox potential. Initial results show that it is
possible to carry out solution peptide syntheses using ferrocenoyl
amino acid esters, by a deprotection-coupling sequence. We
are planning a full structural (solution and solid state) charac-
terization of all products to properly evaluate structural changes.
Acknowledgment. We wish to express our gratitude to
Abdelaziz Houmam for electrochemical measurements. The
NRC Biological Institute generously provided access to its NMR
and mass spectroscopic facilities. H.-B.K. is the recipient of
an NRC research associateship (1996-1998).
(19) Butler, I. R.; Cullen, W. R.; Rettig, S. J.; Trotter, J. Acta Crystallogr.
1988, C44, 1666-1667.
Supporting Information Available: Structural reports, unit cell
and ORTEP diagrams, and tables of isotropic and anisotropic displace-
ment parameters, atomic positional parameters for H and non-H atoms,
and bond lengths and angles for 2a, 2d, and 3 (29 pages). Ordering
information is given on any current masthead page.
(20) Cotton, F. A.; Reid, A. H., Jr. Acta Crystallogr. 1985, C41, 686-
688.
(21) Palenik, G. J. Inorg. Chem. 1969, 8, 2744-2749.
(22) Luo, Y.; Barton, R. J.; Robertson, B. E. Acta Crystallogr. 1990, C46,
1388-1391.
(23) Podlaha, J.; Stepnicka, P.; Ludvik, J.; Cisarova, I. Organometallics
1996, 15, 543-550.
IC961454T