Organometallic Derivatives of Orotic Acid. CO-Labilizing Ability of the Amido Group in Chromium and Tungsten Carbonyl Complexes

Article abstract of DOI:10.1021/ic980233f

Novel orotic acid and uracil derivatives of tungsten and chromium(0), [Et₄N]₂[Cr(CO)₄(orotate)] (1), [Et₄N]₂-[W(CO)₄(orotate)] (2), [Et₄N]₂[W(CO)₄(dihydroorotate)] (3), and [Et₄N][W(CO)₅(uracilate)] (4) [where orotate = (C₅H₂O₄N₂)^2-; dihydroorotate = (C₅H₄O₄N₂)^2-; uracilate = (C₄H₃O₂N₂)^-], have been synthesized via reaction of M(CO)₅THF with the tetraethylammonium salt of the corresponding acid or uracil. These complexes have been characterized in solution by IR and ¹³C NMR spectroscopy and in the solid state by X-ray crystallography. The geometry of the metal dianions in 1 and 2 is that of a distorted octahedron consisting of four carbonyl ligands and a nearly planar five-membered orotate chelate ring, bound through the N1 and one of its carboxylate oxygen atoms. The uracil ring, including the exocyclic oxygens, itself deviates from pianarity by only 0.009 A?. However, the structure of complex 3, which closely resembles that of complexes 1 and 2, has a puckered uracil ring. The structure of complex 4 consists of the uracilate ligand bound through the deprotonated N1 to a tungsten pentacarbonyl fragment. Although the orotate complexes are resistant to thermal decarboxylation, they readily undergo decarbonylation reactions. In this regard, quantitative investigations of the lability of the carbonyl ligands on complexes 1-4 have been carried out. All complexes exhibited a low energy barrier for CO dissociation as demonstrated by ¹³CO exchange studies. For example, the first-order rate constants for intermolecular CO exchange in complexes 2 and 3 were measured to be 6.05 × 10^-4 and 3.17 × 10^-3 s^-1 at 0 °C, respectively. This facile CO dissociation is attributed to competition of the metal center with the uracil ring for the π donation of electron density from the deprotonated N1 atom of the orotate ligand. As expected, this interaction is enhanced when the pseudoaromaticity of the uracil ring is disrupted in complex 3. The activation parameters for the intermolecular exchange of CO in complex 2 were determined to be ΔH^# = 63.2 ± 3.8 kJ/mol and ΔS^# = - 82.8 ± 13.0 J/mol·K, values consistent with a bond-making/bond-breaking (M?CO/M-N) mechanistic pathway. The rate of intermolecular CO exchange was similarly examined in complex 4. The uracilate ligand displayed a π donating capability comparable to that seen for chloride in the W(CO)₅Cl ^- anion but much less π donor character than the phenoxide ligand in W(CO)₅OPh ^-. The activation parameters of the CO exchange process in complex 4 were found to be ΔH? = 106.9 ± 4.3 kJ/mol and ΔS^# = 16.3 ± 13.7 J/mol·K.

Full text of DOI:10.1021/ic980233f

2538
Inorg. Chem. 1998, 37, 2538-2546
Organometallic Derivatives of Orotic Acid. CO-Labilizing Ability of the Amido Group in
Chromium and Tungsten Carbonyl Complexes
Donald J. Darensbourg,* Jennifer D. Draper, David L. Larkins, Brian J. Frost, and
Joseph H. Reibenspies
Department of Chemistry, Texas A&M University, P.O. Box 300012, College Station, Texas 77842
ReceiVed March 4, 1998
Novel orotic acid and uracil derivatives of tungsten and chromium(0), [Et4N]2[Cr(CO)4(orotate)] (1), [Et4N]2-
[W(CO)4(orotate)] (2), [Et4N]2[W(CO)4(dihydroorotate)] (3), and [Et4N][W(CO)5(uracilate)] (4) [where orotate
) (C5H2O4N2)2-; dihydroorotate ) (C5H4O4N2)2-; uracilate ) (C4H3O2N2)-], have been synthesized via reaction
of M(CO)5THF with the tetraethylammonium salt of the corresponding acid or uracil. These complexes have
been characterized in solution by IR and 13C NMR spectroscopy and in the solid state by X-ray crystallography.
The geometry of the metal dianions in 1 and 2 is that of a distorted octahedron consisting of four carbonyl
ligands and a nearly planar five-membered orotate chelate ring, bound through the N1 and one of its carboxylate
oxygen atoms. The uracil ring, including the exocyclic oxygens, itself deviates from planarity by only 0.009 Å.
However, the structure of complex 3, which closely resembles that of complexes 1 and 2, has a puckered uracil
ring. The structure of complex 4 consists of the uracilate ligand bound through the deprotonated N1 to a tungsten
pentacarbonyl fragment. Although the orotate complexes are resistant to thermal decarboxylation, they readily
undergo decarbonylation reactions. In this regard, quantitative investigations of the lability of the carbonyl ligands
on complexes 1-4 have been carried out. All complexes exhibited a low energy barrier for CO dissociation as
demonstrated by 13CO exchange studies. For example, the first-order rate constants for intermolecular CO exchange
in complexes 2 and 3 were measured to be 6.05 × 10-4 and 3.17 × 10-3 s-1 at 0 °C, respectively. This facile
CO dissociation is attributed to competition of the metal center with the uracil ring for the π donation of electron
density from the deprotonated N1 atom of the orotate ligand. As expected, this interaction is enhanced when the
pseudoaromaticity of the uracil ring is disrupted in complex 3. The activation parameters for the intermolecular
exchange of CO in complex 2 were determined to be ∆Hq ) 63.2 ( 3.8 kJ/mol and ∆Sq ) - 82.8 ( 13.0
C
J/mol‚K, values consistent with a bond-making/bond-breaking (M‚‚‚CO/M N) mechanistic pathway. The rate
s
of intermolecular CO exchange was similarly examined in complex 4. The uracilate ligand displayed a π donating
capability comparable to that seen for chloride in the W(CO)5Cl - anion but much less π donor character than the
phenoxide ligand in W(CO)5OPh -. The activation parameters of the CO exchange process in complex 4 were
found to be ∆H‡ ) 106.9 ( 4.3 kJ/mol and ∆Sq ) 16.3 ( 13.7 J/mol‚K.
Introduction
In closely related studies we have attributed the greatly
enhanced CO lability exhibited in selected amino acid deriva-
The superior capability of amido ligands to serve as π
donating ligands in low-valent organometallic complexes has
been substantially documented.1 For example, Caulton has
reported -NHPh to be a similar or better π donor ligand than
-OPh in low-valent, sixteen-electron derivatives of ruthenium-
(II).2 As a consequence of this property of amido ligands it is
expected that they would serve as CO-labilizing groups in metal
carbonyl derivatives. This phenomenon usually results from π
stabilization of the transition-state and subsequently of the
tives of tungsten hexacarbonyl, e.g., W(CO)4(O2CCH2NH2)-
and W(CO)4(O2CCH2NHMe)-, to the transient formation of a
substitutionally labile amido intermediate.4 This is reminiscent
of the well-accepted SN1CB mechanism for ligand substitution
involving the Co(NH3)5X2+ complex cation.5 To further
investigate this reaction process we have begun an examination
of organometallic derivatives of orotic acid, an amino acid where
N1 is generally deprotonated when it binds to a metal center.6
The numbering scheme for orotic acid and the product of its
decarboxylation, uracil, are shown below. Also provided is the
structure of 5,6-dihydroorotic acid in which the pseudoaroma-
ticity of the uracil ring is dissipated.
unsaturated intermediate arising from ligand dissociation, i.e.,
c
intramolecular RHN M π donation which leads to electronic
s
saturation. Indeed, we have recently shown that the coordina-
tively and electronically unsaturated amido anionic derivatives
of tungsten(0) carbonyl, [W(CO)3OC6H4NH]2- and
[W(CO)3NHC6H4NH]2-, are quite stable and even crystallo-
graphically characterizable as a consequence of the π donating
ability of the amido ligands.3
(3) Darensbourg, D. J.; Klausmeyer, K. K.; Reibenspies, J. H. Inorg. Chem.
1996, 35, 1535.
(4) (a) Darensbourg, D. J.; Atnip, E. V.; Klausmeyer, K. K.; Reibenspies,
J. H. Inorg. Chem. 1994, 33, 5230. (b) Darensbourg, D. J.; Draper, J.
D.; Reibenspies, J. H. Inorg. Chem. 1997, 36, 3648.
(5) Tobe, M. L. AdV. Inorg. Bioinorg. Mech. 1983, 2, 1.
(6) (a) Mutikainen, I. Ann. Acad. Sci. Fenn. Ser. A2, Chem. 1988, 217,
and references therein. (b) Ruf, M.; Weis, K.; Vahrenkamp, H. Inorg.
Chem. 1997, 36, 2130.
(1) Caulton, K. G. New J. Chem. 1994, 18, 25.
(2) Poulton, J. T.; Folting, K.; Streib, W. E.; Caulton, K. G. Inorg. Chem.
1992, 31, 3190.
S0020-1669(98)00233-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 05/02/1998

Organometallic Derivatives of Orotic Acid
Inorganic Chemistry, Vol. 37, No. 10, 1998 2539
via the deprotonated N1 atom. The W(CO)5(uracilate)- anion
is formally the product of decarboxylation of W(CO)4(orotate)2-
followed by subsequent protonation at C6 and addition of CO
to the metal center. Although there are no metals involved in
the enzyme system that catalyzes the decarboxylation of orotic
acid, there are other decarboxylation processes which are
catalyzed by metal complexes. Pertinent to this communication
we have previously demonstrated that the W(CO)5(O2CCH2CN)-
anion serves as an effective catalyst precursor for the decar-
boxylation of cyanoacetic acid.11
Experimental Section
Methods and Materials. All manipulations were performed on a
double manifold Schlenk vacuum line under an atmosphere of argon
or in an argon-filled glovebox. Solvents were dried and deoxygenated
by distillation from the appropriate reagent under a nitrogen atmosphere.
Photolysis experiments were performed using a mercury arc 450 W
UV immersion lamp purchased from Ace Glass Co. Infrared spectra
were recorded on a Matteson 6022 spectrometer with DTGS and MCT
detectors. Routine infrared spectra were collected using a 0.10-mm
CaF2 cell. 13C NMR spectra were obtained on a Varian Unity XL-300
spectrometer. 13CO was purchased from Cambridge Isotopes and used
as received. Cr(CO)6 and W(CO)6 were purchased from Strem
Chemicals, Inc., and used without further purification. Anhydrous
orotic acid was obtained from Lancaster Synthesis, Inc., and used as
received. Uracil, bis(triphenylphosphoranylidiene)ammonium chloride
(PPNCl), and L-dihydroorotic acid were purchased from Aldrich
Chemical and used without purification. Microanalyses were performed
by Canadian Microanalytical Service, Ltd. (Delta, B C, Canada).
Synthesis of [Et4N]2[C5H2N2O4] and [Et4N]2[C5H4N2O4] Salts.
The synthesis of these salts was accomplished by adding dropwise 2
equiv (2.8 mmol) of Et4NOH, 25% in MeOH, to 1.4 mmol of anhydrous
orotic acid or L-dihydroorotic acid stirring in MeOH, affording a cloudy
solution. The mixture was stirred for 1 h, after which the MeOH was
removed under vacuum overnight, leaving behind a white powder. Anal.
Calcd for [Et4N]2[C5H2N2O4], C21H42N4O4: C, 60.84; H, 10.21; N,
13.51. Found: C, 58.58; H, 10.21; N, 12.86.
Synthesis of [Et4N]2[M(CO)4(orotate)], 1 and 2. The synthesis
of [Et4N]2[M(CO)4(C5H2N2O4)] (where M ) Cr or W and orotate )
C5H2N2O42-) was accomplished in yields in excess of 85% by the
reaction of 1.4 mmol of M(CO)5THF (prepared by photolysis of 0.300
g of Cr(CO)6 or 0.500 g of W(CO)6 in THF for 1 h) with 1 equiv of
the deprotonated orotic acid. The M(CO)5THF intermediates are
identified by their ν(CO) infrared spectra: M ) Cr, 2073 (w), 1936
(s), 1894 (m); M ) W, 2073 (w), 1929 (s), 1890 (m) cm-1. Longer
photolysis times for W(CO)6 will lead to production of cis-W(CO)4-
(THF)2, ν(CO): 2013 (w), 1876 (vs), 1830 (m) cm-1. After 1 h, the
THF was removed under vacuum, yielding a light orange (when M )
W) or a dark orange (when M ) Cr) powder. The powders were
washed with hexanes to remove any residual M(CO)6, dissolved in CH3-
CN and filtered through Celite. Crystals were obtained of the [Et4N]2
[Cr(CO)4(orotate)] complex (1) by the slow diffusion of diethyl ether
into a concentrated CH3CN solution of the complex at -10 °C; crystals
of complex 2, [Et4N]2 [W(CO)4(orotate)],were similarly obtained at
room temperature. Anal. Calcd for [Et4N]2[Cr(CO)4(C5H2N2O4)] (1),
[C25H42N4O8Cr]: C, 51.89; H, 7.32; N, 9.68. Found: C, 51.85; H,
7.73; N, 9.68. Anal. Calcd for [Et4N]2[W(CO)4(C5H2N2O4)] (2),
[C25H42N4O8W]: C, 42.26; H, 5.96; N, 7.89. Found: C, 42.14; H,
6.16; N, 7.96.
Orotic acid is important in pyrimidine biosynthesis, i.e., orotic
acid is a precursor in the synthesis of the cytidine and thymidine
nucleotides, and it has been confirmed that metal ions are
necessary for this process.7 Relevant to this, there is much
current interest in the mechanistic aspects of the enzymatic
decarboxylation reaction of orotidylic acid to uridine-5′-
phosphate, the starting point for the synthesis of the cytidine
and thymidine nucleotides.8,9 The enzymatic catalyzed decar-
boxylation process displays a catalytic proficiency of 2.0 × 1023
M-1, the most enhanced rate for a known enzymatic reaction.
The nonenzymatic t1/2 value at 25 °C of 78 million years was
obtained from the decarboxylation of 1-methylorotic acid.8 Lee
and Houk have proposed on the basis of theoretical studies that
the greatly accelerated enzymatically catalyzed process occurs
by way of a resonance-stabilized carbene intermediate afforded
subsequently to proton transfer at O4 from a lysine group of
the enzyme.9
Furthermore, metal orotates are proposed as biological carriers
for a variety of metal ions.10 Hence, our interest in the bonding
modes exhibited by orotate to low oxidation state metal species,
coupled with the role of the amido ligand as a CO-labilizing
group, has prompted us to investigate group 6 metal carbonyl
derivatives of orotic acid. Specifically, the [Et4N]2[M(CO)4-
(orotate)] (M ) Cr, W) derivatives have been synthesized and
characterized both in solution and the solid-state, and the rates
of CO dissociation have been determined via 13CO exchange
studies. When the heterocyclic N1 atom is deprotonated, its
electron density is delocalized within the pseudoaromatic uracil
ring and the exocyclic carbonyl oxygen atoms. Hence, there is
expected to be competition of the ring system vs the unsaturated
metal center upon M-CO dissociation for the electron density
provided by the pseudoamido ligand. In turn, the effective
competitiveness of the unsaturated metal center for this electron
density is anticipated to be reflected in the rate constant for
CO dissociation. This concept is further examined by measuring
the rate of CO dissociation in the W(CO)4(dihydroorotate)2-
dianion.
For comparative purposes we have performed similar struc-
tural and reactivity studies on the [W(CO)5(uracilate)]- anion,
where the uracilate ligand is also bonded to the metal center
Synthesis of [Et4N]2[W(CO)4(L-dihydroorotate)], 3. The synthesis
of [Et4N]2[M(CO)4(C5H4N2O4)] (where L-dihydroorotate ) C5H4N2O42-
)
was accomplished in yields in excess of 85% by the reaction of 0.64
mmol of M(CO)5THF (prepared by photolysis of 0.223 g of W(CO)6
in THF for forty minutes) with one equivalent of the deprotonated
L-dihydroorotic acid [Et4N]2[C5H4N2O4]. After 1 h, the THF was
removed under vacuum, yielding a yellow powder. The powder was
washed in hexanes to remove any residual M(CO)6, dissolved in CH3-
(7) Victor, J.; Greenberg, L. B.; Sloan, D. L. J. Biol. Chem. 1979, 254,
2647.
(8) (a) Radzicka, A.; Wolfenden, R. Science 1995, 267, 90. (b) Beak, P.;
Siegel, B. J. Am. Chem. Soc. 1976, 98, 3601.
(9) Lee, J. K.; Houk, K. N. Science 1997, 276, 942.
(10) (a) Kumberger, O.; Reide, J.; Schmidbaur, H. Chem. Ber. 1991, 124,
2739. (b) Bach, I.; Kumberger, O.; Schmidbaur, H. Chem. Ber. 1990,
123, 2267.
(11) Darensbourg, D. J.; Chojnacki, J. A.; Atnip, E. V. J. Am. Chem. Soc.
1993, 115, 4675.

2540 Inorganic Chemistry, Vol. 37, No. 10, 1998
Darensbourg et al.
Table 1. Crystallographic Data for Complexes 1-4
1
2
3
4
empirical formula
fw
C28H46.50N5.50O8Cr
640.21
C2/c
7004(2)
8
C28H46.50N5.50O8W
772.06
C2/c
6961(2)
8
C27H46N5O8.5
759.53
P212121
6653.4(11)
8
W
C45H33N3O5P2W
973.58
P1h
space group
V, Å3
2064.3(7)
2
Z
d
calc, g/cm3
1.214
1.473
1.517
1.566
a, Å
b, Å
c, Å
16.552(3)
12.122(2)
34.929(7)
-
16.383(2)
12.121(1)
34.939(4)
-
13.488(2)
13.7357(11)
35.913(3)
-
10.173(2)
13.973(3)
14.959(3)
81.95(3)
88.90(3)
78.67(3)
193
2.924
0.710 73
7.66
R, deg
â, deg
91.99(3)
-
90.121(10)
-
-
γ, deg
-
193
3.525
0.710 73
5.35
T, K
193
3.131
1.547 18
7.95
17.02
193
3.369
0.710 73
6.65
14.40
µ(Mo KR), mm-1
wavelength, Å
RF,a%
R
wF,b%
12.07
9.81
a RF ) ∑|Fo - Fc|/∑Fo. RwF ) {[∑w(Fo - Fc2)2]/(∑wFo )}1/2
.
b
2
2
CN and filtered through Celite. Crystals of the [Et4N]2 [W(CO)4(L-
dihydroorotate)] complex (3) were obtained by the slow diffusion of
diethyl ether into a concentrated CH3CN solution of the complex at
room temperature. Anal. Calcd for [Et4N]2[W(CO)4(C5H4N2O4)] (3),
[C25H44N4O8W]: C, 42.14; H, 6.22; N, 7.86. Found: C, 43.86; H,
6.85; N, 7.95.
[Et4N]2[W(CO)4(dihydroorotate)], 3. Crystal data and details of
data collection are given in Table 1. An orange block of 3 was mounted
on a glass fiber with epoxy cement at room temperature and cooled in
a liquid nitrogen cold stream. Preliminary examination and data
collection were performed on a Siemens P4 X-ray diffractometer (Mo
KR, λ ) 0.710 73 Å radiation). Data collection methods and parameters
are identical to that described for complexes 1 and 2. Lorentz and
polarization corrections were applied to 6131 reflections for 3. A total
of 6051 unique reflections were used in further calculations. Aniso-
tropic refinement for all non-hydrogen atoms yielded R ) 0.0535, Rw
) 0.1207, and S ) 1.010.
[PPN][W(CO)5(uracilate)], 4. Crystal data and details of data
collection are given in Table 1. A yellow block of 4 was mounted on
a glass fiber with epoxy cement at room temperature and cooled in a
liquid nitrogen cold stream. Preliminary examination and data collec-
tion were performed on a Siemens P4 X-ray diffractometer (Mo KR,
λ ) 0.710 73 Å radiation). Data collection methods and parameters
are identical to that described for complexes 1 and 2. Lorentz and
polarization corrections were applied to 6671 reflections for 4. A total
of 6667 unique reflections were used in further calculations. Aniso-
tropic refinement for all non-hydrogen atoms yielded R ) 0.0766, Rw
) 0.1864, and S ) 1.037.
Kinetic Measurements. The rates of 13CO exchange in complexes
1-3 were determined by placing an acetonitrile solution of the
corresponding complex (0.6 mmol in 20 mL) under an atmosphere of
13CO. The kinetic data on complex 4 were obtained by placing an
atmosphere of 13CO over a THF solution of 4 (0.1 mmol in 30 mL of
THF). The solutions were situated in a thermostated water bath at 0.0
°C for 1 and 3 and at a variety of temperatures for 2 (0.0, 5.0, 20.0,
25.0, 30.0, and 35.0 °C) and 4 (30.0, 40.1, 49.9, and 58.0 °C), and
shaken periodically. Prior to obtaining a room-temperature IR of
complexes 1-3, the solutions were transferred via cannula to an
evacuated test tube maintained at the appropriate temperature and the
13CO atmosphere was removed by vacuum in order to quench the
exchange process. In all complexes the exchange process was
monitored by observing the decrease in absorbance of the highest
frequency ν(CO) band of the starting complex as a function of time.
Synthesis of [Et4N][W(CO)5(uracilate)], 4. The synthesis of [Et4N]-
[W(CO)5(uracilate)] was accomplished in yields greater than 85% by
reaction of 1.4 mmol of W(CO)5THF (prepared by photolysis of 0.500
g of W(CO)6 in 60 mL THF for 1 h), with 1.4 mmol of uracil, 1.4
mmol of NEt4OH, and 10 mL of MeOH. The solvent was removed
via vacuum after 1 h of stirring, leaving behind a yellow air-stable
powder (0.697 g, 85%). The powder was recrystallized from a THF
solution by layering with hexanes. The resulting powder was washed
2 × 20 mL hexanes and vacuum-dried, leaving an analytically pure
yellow powder (0.612 g, 75%). Anal. Calcd for [Et4N][W(CO)5-
(C4H3N2O2)] (4), [C17H23O7N3W]: C, 36.12; H, 4.10; N, 7.43; Found:
C, 35.53; H, 4.29; N, 7.45. The PPN salt was synthesized in a manner
similar to that of the NEt4 salt, 1.4 mmol of W(CO)5THF (prepared by
photolysis of 0.500 g of W(CO)6 in 60 mL of THF for 1 h) was reacted
with 1.4 mmol of uracil and excess NaH. PPNCl (1.4 mmol) was then
added to exchange the sodium cation for PPN+. Crystals of the PPN+
salt were obtained by layering a THF solution of [PPN][W(CO)5-
(C4H3N2O2)] with hexanes at ambient temperature.
X-ray Crystallography. [Et4N]2[M(CO)4(orotate)] Derivatives (1
and 2). Crystal data and details of data collection are given in Table
1. A yellow-orange block of 1 and a yellow block of 2 were mounted
on glass fibers with epoxy cement at room temperature and cooled in
a liquid nitrogen cold stream. Preliminary examination and data
collection were performed on a Rigaku AFC5R X-ray diffractometer
(Cu KR, λ ) 1.541 78 Å radiation) for 1 and on a Siemens P4 X-ray
diffractometer (Mo KR, λ ) 0.710 73 Å radiation) for 2. Cell
parameters were calculated from the least-squares fitting of the setting
angles for 24 reflections. ω scans for several intense reflections
indicated acceptable crystal quality. Data were collected for 4.0 e 2θ
e 50°. Three control reflections, collected every 97 reflections, showed
no significant trends. Background measurements by stationary-crystal
and stationary-counter techniques were taken at the beginning and end
of each scan for half the total scan time. Lorentz and polarization
corrections were applied to 5297 for 1 and 6075 reflections for 2. A
semiempirical absorption correction was applied to 1 and 2. A total
of 5103 unique reflections for 1 and 5856 for 2, with |I| g 2.0σ(I),
were used in further calculations. Both structures were solved by direct
methods [Sheldrick, G. M. SHELXS Program Package; (1993)]. Full-
matrix least-squares anisotropic refinement for all non-hydrogen atoms
yielded and R ) 0.0732, Rw ) 0.1375 and S ) 1.115 for 1 and R )
0.0603, Rw ) 0.1253 and S ) 1.147 for 2. Hydrogen atoms were placed
in idealized positions with isotropic thermal parameters fixed at 0.08.
Neutral-atom scattering factors and anomalous scattering correction
terms were taken from International Tables for X-ray Crystallography.
Results
Synthesis and Spectral and Structural Characterization.
[Et4N]2[M(CO)4(orotate)] (M ) Cr, W), [Et4N]2[W(CO)4-
(dihydroorotate)], and [Et4N][W(CO)5(uracilate)]. The zero
valent metal carbonyl derivatives of orotic acid, L-dihydroorotic
acid and uracil were synthesized in greater than 85% yield by
the labile ligand displacement reaction of M(CO)5THF, obtained
via photolysis of Cr(CO)6 or W(CO)6, with the doubly depro-
tonated tetraethylammonium salt of anhydrous orotic acid
[Et4N]2[C5H2N2O4] (1, 2) or dihydroorotic acid [Et4N]2-

Organometallic Derivatives of Orotic Acid
Inorganic Chemistry, Vol. 37, No. 10, 1998 2541
Table 2. Stretching Frequencies of the Carbonyl and Carboxylate Ligands in the M(CO)4(orotate)2- (1, 2), M(co)4(dihydroorotate)2- (3), and
W(CO)5(uracilate)- (4) Anionsa
ν(CdO), cm-1
ν(CtO), cm-1
ν(CdO), cm-1
complexa
asym
sym
[Cr(CO)4(orotate)2-
]
1990 (w)
1987 (w)
1980 (w)
2065 (w)
1854 (vs)
1840 (sh)
1829 (sh)
1830 (sh)
1858 (m)b
1797 (m)b
1792 (m)b
1782 (m)b
-
1653
1659
1647
-
1362c
1357c
1379c
-
1641
1617d
1643d
[W(CO)4(orotate)2-
]
1842 (vs)
1838 (vs)
1917 (vs)
1650
[W(CO)4(L-dihydroorotate)2-
[W(CO)5(uracilate)-]
]
1625d,e
1668
-
e
1639d
a As the tetraethylammonium salt. b Spectra determined in CH3CN. c Carboxylate stretches. d Organic carbonyl stretches from the uracil ring.
e One broad peak observed.
Table 3. 13C NMR Data for the Carbonyl and Carboxylate
Scheme 1
Ligands in M(CO)4(orotate)2- (1, 2), M(CO)4(dihydroorotate)2- (3),
and W(CO)5(uracilate)- (4) Anions
13C resonance, ppm
CO
CO
CO
complex
[Cr(CO)4(orotate)2-
(axial) (trans to N) (trans to O)
a,b
a,b
]
216.5
205.5
207.4
200.6
232.4
218.0
218.7
205.7
232.5c
218.1c
219.6
-
[W(CO)4(orotate)2-
]
[W(CO)4(L-dihydroorotate)2-
[W(CO)5(uracilate)-]a,d
]
a,b
a Spectra determined in acetonitrile-d3. b As the tetraethylammonium
salt. c The assignments of the 13C signals for CO groups trans to N vs
those trans to O are based on the general observation that the 13C
resonance for a CO group trans to N is upfield relative to a CO group
trans to O in W(CO)5 derivatives (see e.g.: Todd, L. J.; Wilkinson, J.
R. J. Organomet. Chem. 1974, 77, 1; Darensbourg, D. J.; Wiegreffe,
H. P. Inorg. Chem. 1990, 29, 592). Nevertheless, the signals are quite
close, and an absolute distinction is not needed at this time. d As the
PPN salt.
[C5H4N2O4] (3), or the monodeprotonated PPN+ or NEt4+ salts
of uracil (4). Scheme 1 uses orotic acid to summarize the
approach employed in the synthesis of complexes 1-3. The
reaction proceeds, as evident by the time-dependent ν(CO)
infrared spectrum, by way of the displacement of THF by the
carboxylic oxygen atom or the N1 nitrogen atom on the doubly
deprotonated orotate ligand, followed shortly by the chelation
to the metal center with concomitant loss of CO. Formation of
either pentacarbonyl intermediate, i.e., metal-carboxylate12 or
metal-amide,3 is anticipated to have labile CO ligands leading
readily to a chelated product.
Complexes 1-3 exhibit a four-band pattern in the carbonyl
region of the infrared spectrum consistent with a cis-disubstituted
tetracarbonylmetal center, while complex 4 displays a three-
band pattern typical of a monosubstituted pentacarbonyl deriva-
tive.13 These frequencies, as well as those corresponding to
the carboxylate stretches (1-3) and the organic carbonyls
(ketones), are provided in Table 2. Because the CO ligands
are such good probes of the electron-donating ability of the
ancillary ligands in metal carbonyl derivatives, it is of interest
to compare the ν(CO) values in the orotate complexes reported
upon herein with the corresponding values for the glycine
derivatives.4 The average ν(CO) frequency in both the chro-
mium and tungsten orotate complexes (1 and 2) is 11-12 cm-1
lower than those in the glycinate derivatives,4 indicative of a
significant enhancement of electron donating ability of the
orotate dianion vs the glycinate monoanion. Presumably this
greater donating ability of the orotate ligand is due to the
presence of the additional negative charge provided by the amido
group. Similarly, when the pseudoaromatic orotate ligand is
replaced by the dihydroorotate ligand in the W(CO)4 derivative,
as expected the average ν(CO) frequency decreases by an
Figure 1. 13C NMR spectrum of [Et4N]2[W(CO)4(orotate)] (2) in CD3-
CN at 75 MHz.
from the ν(CO) infrared data the 13C NMR chemical shifts for
the carbonyl ligands in the orotate derivatives are downfield of
the corresponding values in the glycine complexes, further
indication of a more electron-rich carbon center.16 Chemical
shift data for the M(CO)4 moieties reveal a slight electronic
difference between the CO ligand trans to oxygen versus the
CO ligand trans to nitrogen. The 13C NMR spectra of
complexes 1-3 display three signals for the carbonyl carbons
in an approximate 2:1:1 intensity ratio (see Figure 1). The more
intense resonance for the two carbonyl ligands cis to both the
nitrogen and oxygen atoms of the orotate or dihydroorotate
ligands is observed upfield from the two smaller signals assigned
additional 5 cm-1
. Likewise, the ν(CO) vibrations in the
W(CO)5(uracilate)- anion are on average shifted 13 cm-1 toward
lower frequency than those observed for a typical saturated
amine complex such as W(CO)5(piperidine).14,15
Table 3 lists the 13C NMR data for the complexes in both
the CO and carboxylate regions of the spectra. As anticipated
(14) The three ν(CO) frequencies in W(CO)5piperidine in acetonitrile
solution appear at 2071 (w), 1926 (s), and 1882 (m) cm-1. These are
significantly shifted, in particular the vibrational mode trans to the
amine, from the corresponding values determined in hexane solution.15.
(15) Dennenberg, R. J.; Darensbourg, D. J. Inorg. Chem. 1972, 11, 1.
(16) Mann, B. E.; Taylor, B. F. 13C NMR Data for Organometallic
Compounds; Academic Press: New York, 1981.
(12) Darensbourg, D. J.; Joyce, J. A.; Bischoff, C. J.; Reibenspies, J. H.
Inorg. Chem. 1991, 30, 1137.
(13) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: New York,
1975.

2542 Inorganic Chemistry, Vol. 37, No. 10, 1998
Darensbourg et al.
Figure 2. Thermal ellipsoid drawing of the anion of complex 2 with
Figure 3. Thermal ellipsoid drawing of the anion of complex 3 with
atomic numbering scheme.
atomic numbering scheme.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
[Et4N]2[[W(co)4(orotate)] (2)a
orotate residue that forms a five-membered, almost planar,17
chelate ring at the metal center through its deprotonated N1
atom and one of the carboxylate oxygen atoms. The slightly
distorted octahedral geometry at the metal center is achieved
by coordination of four carbonyl ligands. The metal centers in
1 and 2 deviate from the mean plane of the pyrimidine ring by
only 0.030 and 0.004 Å, respectively. Furthermore, the pyri-
midine rings, including the exocyclic oxygen atoms, do not
exhibit large deviations from planarity (0.009 Å). This in turn
is indicative of the N1 atom possessing essentially sp2 hybrid-
ization. All intraligand distances in the pyrimidine ring lie
within the range observed for a large variety of N1 coordinated
uracils.6
The orotate ligand forms bite angles of 76.3(2) and 72.2(3)°
with the metal atom in complexes 1 and 2, respectively. The
Cr-O bond length observed in 1 of 2.086(4) Å is significantly
shorter than that noted in other amino acid derivatives of
chromium(0); e.g., in Cr(CO)4(NH2CH2CO2)- the Cr-O bond
length is 2.22(1) Å.4b On the other hand, the Cr-N bond
distance (2.119(5) Å) in 1, where nitrogen is deprotonated, is
nearly identical to the Cr-N bond distance (2.15(1) Å) noted
in the other amino acid derivative of chromium where the
nitrogen is a neutral ligand. Unlike the chromium analogue,
the W-O bond length in complex 2 of 2.177(8) Å is similar to
that noted in other amino acid derivatives of tungsten(0); e.g.,
in W(CO)4(NH2CH2CO2)- the W-O bond length is 2.156(21)
Å.4a However, the W-N bond distance (2.258(9) Å) in 2 when
nitrogen is deprotonated is significantly shorter than the W-N
bond distance (2.328(22) Å) in the glycine derivative.
W(1)-C(1)
W(1)-C(2)
W(1)-C(3)
W(1)-C(4)
W(1)-O(5)
W(1)-N(1)
O(5)-C(5)
O(6)-C(5)
O(7)-C(8)
2.014(12)
2.051(12)
1.894(12)
1.905(13)
2.177(8)
O(8)-C(9)
N(1)-C(6)
N(1)-C(9)
N(2)-C(9)
N(2)-C(8)
C(5)-C(6)
C(6)-C(7)
C(7)-C(8)
1.253(14)
1.347(14)
1.39(2)
1.387(14)
1.39(2)
1.55(2)
1.35(2)
2.258(9)
1.255(13)
1.231(13)
1.234(14)
1.42(2)
C(3)-W(1)-C(4)
C(3)-W(1)-C(1)
C(4)-W(1)-C(1)
C(3)-W(1)-C(2)
C(4)-W(1)-C(2)
C(1)-W(1)-C(2)
C(3)-W(1)-O(5)
C(4)-W(1)-O(5)
C(1)-W(1)-O(5)
C(2)-W(1)-O(5)
C(3)-W(1)-N(1)
C(4)-W(1)-N(1)
C(1)-W(1)-N(1)
C(2)-W(1)-N(1)
O(5)-W(1)-N(1)
C(5)-O(5)-W(1)
C(6)-N(1)-C(9)
C(6)-N(1)-W(1)
C(9)-N(1)-W(1)
90.5(5)
89.1(4)
82.9(5)
86.9(5)
84.0(5)
166.2(5)
93.7(4)
175.6(4)
96.1(4)
97.3(5)
165.9(4)
103.6(4)
93.4(4)
93.7(5)
72.2(3)
123.0(7)
118.1(10)
116.5(7)
125.3(8)
C(9)-N(2)-C(8)
O(1)-C(1)-W(1)
O(2)-C(2)-W(1)
O(3)-C(3)-W(1)
O(4)-C(4)-W(1)
O(6)-C(5)-O(5)
O(6)-C(5)-C(6)
O(5)-C(5)-C(6)
N(1)-C(6)-C(7)
N(1)-C(6)-C(5)
C(7)-C(6)-C(5)
C(6)-C(7)-C(8)
O(7)-C(8)-N(2)
O(7)-C(8)-C(7)
N(2)-C(8)-C(7)
O(8)-C(9)-N(1)
O(8)-C(9)-N(2)
N(1)-C(9)-N(2)
127.5(11)
167.0(10)
169.4(13)
178.3(10)
178.6(9)
128.3(11)
117.4(10)
114.3(10)
125.3(11)
114.0(9)
120.7(11)
120.5(11)
119.4(12)
128.2(12)
112.4(11)
123.3(10)
120.5(11)
116.2(12)
a Estimated standard deviations are given in parentheses.
to the carbonyl ligands trans to the oxygen and nitrogen donors.
Each resonance for complex 2 reveals satellites due to coupling
with the 183W nucleus (JC-W ) 132.9 Hz for axial carbonyls
and an average of 111.1 Hz for the equatorial carbonyls). In
complex 3 the axial carbonyl carbons couple to tungsten-183
with a constant of 125.7 Hz while the tungsten satellites for
the peaks assigned to the equatorial carbons are weak and
overlapping, making an accurate determination of JC-W difficult.
The 13C NMR spectrum of 4 exhibits two signals for the
carbonyl carbons in an approximate 4:1 ratio. The intense
upfield peak (JW-C ) 130.1 Hz) is assigned to the four carbonyl
ligands cis to the nitrogen atom of the uracilate ligand while
the smaller downfield peak is attributed to the carbonyl ligand
trans to the uracilate ligand.
The metal-carbonyl bond distances in complexes 1 and 2
exhibit similar trends. In each the average M-Cax bonds are
longer than the average M-Ceq bonds, i.e., 1.864(7) vs 1.800-
(7) Å for 1 and 2.03(1) vs 1.90(1) Å for 2. The carboxylate
and amide groups exhibit a small trans effect, giving equatorial
Cr-C bond lengths of 1.777(6) (trans to nitrogen) and 1.822-
(8) Å (trans to oxygen) for 1 and W-C bond lengths of 1.89-
(1) (trans to nitrogen) and 1.91(1) Å (trans to oxygen) for 2.
The axial carbonyls ligands are somewhat bent, with M-C-O
bond angles of 169.5(7) for 1 and 168.2(2) for 2.
The structure of 3 determined by crystallographic analysis is
presented in Figure 3. Selected bond lengths and angles are
provided in Table 5. Although two molecules of the complex
exist in the unit cell, both exhibit similar bond distances and
bond angles. One molecule of water and one molecule of
methanol are present in the unit cell in addition to the
tetraethylammonium counterions. The dianion of complex 3
consists of an L-dihydroorotate residue bound in a five
The structures of complexes 1 and 2 were determined by
crystallographic analysis; thermal ellipsoid drawings of the
dianions are provided in Figures 1S and 2 and selected bond
lengths and angles in Table 4. The unit cell also contains two
tetraethylammonium counterions and an acetonitrile solvent
molecule. The dianions of complexes 1 and 2 consist of an
(17) The mean plane defined by the CO2 unit of the orotate ligand vs that
defined by the atoms of the uracil fragment is tilted by only 1.5°.

Organometallic Derivatives of Orotic Acid
Inorganic Chemistry, Vol. 37, No. 10, 1998 2543
Table 5. Bond Lengths (Å) and Angles (deg) for 3a
W(1A)-C(3A)
W(1A)-C(2A)
W(1A)-C(4A)
W(1A)-C(1A)
W(1A)-O(5A)
W(1A)-N(1A)
O(1A)-C(1A)
O(2A)-C(2A)
O(3A)-C(3A)
O(4A)-C(4A)
O(5A)-C(5A)
1.92(2)
1.97(2)
1.97(2)
2.00(2)
2.211(12)
2.231(13)
1.16(3)
1.20(3)
1.21(2)
1.13(2)
1.32(2)
O(6A)-C(5A)
O(7A)-C(8A)
O(8A)-C(9A)
N(1A)-C(9A)
N(1A)-C(6A)
N(2A)-C(8A)
N(2A)-C(9A)
C(5A)-C(6A)
C(6A)-C(7A)
C(7A)-C(8A)
1.20(3)
1.26(2)
1.22(2)
1.31(2)
1.47(2)
1.27(3)
1.43(2)
1.51(3)
1.53(3)
1.54(2)
C(3A)-W(1A)-C(2A) 85.2(11) C(8A)-N(2A)-C(9A) 127(2)
C(3A)-W(1A)-C(4A) 87.8(9) O(1A)-C(1A)-W(1A) 172(2)
C(2A)-W(1A)-C(4A) 84.6(9) O(2A)-C(2A)-W(1A) 171(2)
C(3A)-W(1A)-C(1A) 85.9(11) O(3A)-C(3A)-W(1A) 179(2)
C(2A)-W(1A)-C(1A) 168.2(10) O(4A)-C(4A)-W(1A) 175(2)
C(4A)-W(1A)-C(1A) 87.3(9) O(6A)-C(5A)-O(5A) 123(2)
C(3A)-W(1A)-O(5A) 97.0(8) O(6A)-C(5A)-C(6A) 122(2)
C(2A)-W(1A)-O(5A) 95.2(7) O(5A)-C(5A)-C(6A) 115(2)
C(4A)-W(1A)-O(5A) 175.2(7) N(1A)-C(6A)-C(5A) 112(2)
C(1A)-W(1A)-O(5A) 93.5(6) N(1A)-C(6A)-C(7A) 113(2)
C(3A)-W(1A)-N(1A) 169.9(8) C(5A)-C(6A)-C(7A) 110(2)
C(2A)-W(1A)-N(1A) 94.9(9) C(6A)-C(7A)-C(8A) 108(2)
C(4A)-W(1A)-N(1A) 102.3(7) O(7A)-C(8A)-N(2A) 127(2)
C(1A)-W(1A)-N(1A) 95.3(8) O(7A)-C(8A)-C(7A) 116(2)
O(5A)-W(1A)-N(1A) 73.0(5) N(2A)-C(8A)-C(7A) 117(2)
C(5A)-O(5A)-W(1A) 121.0(12) O(8A)-C(9A)-N(1A) 127(2)
C(9A)-N(1A)-C(6A) 117.5(14) O(8A)-C(9A)-N(2A) 115(2)
C(9A)-N(1A)-W(1A) 127.4(11) N(1A)-C(9A)-N(2A) 118(2)
C(6A)-N(1A)-W(1A) 114.0(11)
Figure 5. Thermal ellipsoid drawing of the anion of complex 4 with
atomic numbering scheme.
Table 6. Bond Lengths (Å) and Angles (deg) for 4a
W(1)-C(1)
W(1)-C(2)
W(1)-C(3)
W(1)-C(4)
W(1)-C(5)
W(1)-N(1)
O(1)-C(1)
O(2)-C(2)
O(3)-C(3)
O(4)-C(4)
2.07(2)
2.04(2)
1.960(12)
2.03(2)
2.026(13)
2.281(8)
1.14(2)
1.15(2)
1.171(14)
1.15(2)
O(5)-C(5)
O(7)-C(8)
O(8)-C(9)
N(1)-C(6)
N(1)-C(9)
N(2)-C(8)
N(2)-C(9)
C(6)-C(7)
C(7)-C(8)
1.14(2)
1.247(14)
1.215(14)
1.33(2)
1.38(2)
1.36(2)
1.399(14)
1.37(2)
1.42(2)
a Estimated standard deviations are given in parentheses.
C(1)-W(1)-N(1)
C(2)-W(1)-N(1)
C(3)-W(1)-N(1)
C(4)-W(1)-N(1)
C(3)-W(1)-C(5)
C(3)-W(1)-C(4)
C(5)-W(1)-C(4)
C(3)-W(1)-C(2)
C(5)-W(1)-C(2)
C(4)-W(1)-C(2)
C(3)-W(1)-C(1)
C(5)-W(1)-C(1)
C(4)-W(1)-C(1)
C(2)-W(1)-C(1)
C(5)-W(1)-N(1)
95.2(4)
92.4(4)
177.2(5)
90.9(4)
87.1(5)
87.3(5)
91.6(5)
89.6(5)
92.2(5)
174.9(5)
86.9(5)
173.2(5)
91.2(6)
84.7(6)
90.9(4)
C(6)-N(1)-C(9)
C(6)-N(1)-W(1)
C(9)-N(1)-W(1)
C(8)-N(2)-C(9)
O(1)-C(1)-W(1)
O(2)-C(2)-W(1)
O(3)-C(3)-W(1)
O(4)-C(4)-W(1)
O(5)-C(5)-W(1)
O(7)-C(8)-N(2)
O(8)-C(9)-N(2)
N(1)-C(6)-C(7)
N(1)-C(9)-N(2)
C(6)-C(7)-C(8)
119.4(9)
122.5(8)
118.0(7)
126.7(11)
172.2(12)
174.3(10)
178.8(10)
176.9(11)
176.1(11)
121.0(12)
119.3(10)
125.0(12)
115.6(11)
118.7(13)
Figure 4. Ball and stick representation of complexes 2 and 3 comparing
the planar orotate ring vs the puckered L-dihydroorotate ring.
membered chelate ring at the metal center. This is similar to
the structures observed for complexes 1 and 2; however,
complex 3 differs from the orotate complexes in that the
pyrimidine ring is no longer planar. This is readily observed
in Figure 4 where the L-dihydroorotate complex (3) and the
orotate complex (2) are shown from a side-on view with the
uracil ring perpendicular to the plane of the page. The metal
lies out of the chelate plane by 0.102 Å, and the pyrimidine
ring itself deviates from planarity by 0.155 Å. Hence, in this
instance the N1 atom of the heterocyclic ring exhibits more of
an sp3 type hybridization as compared with that in complex 3.18
The intraligand distances in the pyrimidine ring are similar to
those found in the uracil ring with the exception of the C(6)-
C(7) bond length. That is, an expected the bond distance
lengthens from 1.31 Å in the orotate complex 2 to 1.53 Å in
the dihydroorotate analogue, complex 3.19
a Estimated standard deviations are given in parentheses.
The dihydroorotate residue in complex 3 forms a bite angle
of 73.0(5)° with the metal center. The W-O bond length of
2.211(12) Å is longer than W-O bond length observed for either
the tungsten orotate or glycinate derivatives. The observed
W-N bond distance in complex 3 is 2.231(13) Å. While this
is significantly shorter than that of the glycine derivative, it is
similar to that observed in the orotate derivative where N1 is
also deprotonated. The metal-carbonyl bond distances in
complex 3 show the same general characteristics as those
observed in complexes 1 and 2. The average M-Ceq bonds
are slightly shorter than the average M-Cax bonds (1.985(2) Å
vs 1.945(2) Å), and the axial carbonyl ligands are only slightly
bent with M-C-O bond angles of 171(2) and 172(2)°.
The structure of 4 determined by crystallographic analysis is
presented in Figure 5. Selected bond lengths and angles are
provided in Table 6. Complex 4 consists of a uracilate ligand
bound to a tungsten pentacarbonyl fragment through the
deprotonated N1 atom with a W-N bond length of 2.281(8)
Å. This bond distance is slightly shorter than the W-N bond
length of 2.331(5) Å observed in W(CO)5piperidine,20 and
(18) A reviewer has suggested alternatively that the N1 atom could have
sp2 hybridization with a pure p orbital imperfectly aligned for donation
to the metal center.
(19) The methyl ester of dihydroorotic acid exhibits a C(6)-C(7) bond
length of 1.520 Å; see: Hambley, T. W.; Phillips, L.; Poiner, A. C.;
Christopherson, R. I. Acta Crystallogr., Sect. B (Struct. Sci.) 1993,
49, 130.

2544 Inorganic Chemistry, Vol. 37, No. 10, 1998
Darensbourg et al.
Table 7. First-Order Rate Constants for CO Dissociation in
Orotate and Dihydroorotate Metal Derivativesa,b
complex
temp (°C)
k (s-1) × 104
1
2
0
0
5
25
30
35
0
49.0
6.05
9.11
54.2
100
133
31.7
0.225
3
W(CO)4(glycinate)-
0
a In CH3CN. b All kinetic data were determined as the Et4N+ salts
of the dianions to avoid significant ion-pairing effects (Payne, M. W.;
Leussing, D. L.; Shore, S. G. Organometallics 1991, 10, 574).
process monitored (Figure 7), the first 13C resonance to appear
corresponds to the two axial CO ligands cis to both N and O
atoms (205 ppm). The 13C resonance due to the carbonyl ligand
trans to oxygen (cis to the pseudoamido ligand) appears next,
followed by the CO group trans to nitrogen. Hence, the
enhanced rate and stereoselectivity of CO exchange is consistent
with that expected for amido ligands being better cis-labilizing
ligands than carboxylates.
Figure 6. Spectral changes recorded for the process involving carbonyl
ligand substitution in 2 with 13CO at 30 °C. (Inset) Plot of ln(abs) vs
time (min) for this process, indicating first-order kinetics.
slightly longer than the analogous parameter found in
W(CO)5pyridine of 2.26(1) Å.21 The W-C bond length of the
carbonyl ligand trans to the N1 is shorter (1.96(1) Å) than those
cis to N1, with an average bond length of 2.04(2) Å. As
observed in the orotate derivatives, the metal center in complex
4 is only out of the mean plane (0.012 Å) defined by the
pyrimidine ring by 0.008 Å.
Carbon Monoxide Ligand Exchange Reactions. The
reactivity of complexes 1-4 toward dissociatiVe ligand ex-
change (e.g., as illustrated in eq 1 for the orotate derivatives)
was investigated using 13C-labeled carbon monoxide as an
incoming ligand (L). It has been previously well established
that these ligand substitution processes involving the weak
nucleophile carbon monoxide as an incoming ligand are
dissociative in nature.4 Furthermore, the reaction was demon-
strated by high-pressure NMR experiments not to involve chelate
ring opening with concomitant formation of a pentacarbonyl
transient that subsequently undergoes carbon monoxide loss.4
The first-order rate constants for CO dissociation in com-
plexes 1-3 including temperature-dependent data for complex
2, are indicated in Table 7. For comparative purposes analogous
kinetic data are provided for the W(CO)4(glycinate)- anion.4b
The following conclusions are readily apparent from the kinetic
data in Table 7. In all of these anionic metal carbonyl
derivatives there is a low barrier to CO dissociation, i.e., all
the processes are very fast. For example, the rate constant for
CO dissociation in W(CO)6 has been extrapolated to have a
value of 1.0 × 10-14 s-1 at 30 °C,22 which is some 10-12 times
smaller than the analogous value in complex 2. As expected
based on weaker metal-CO bonding, dissociation of the CO
ligand in the Cr(CO)4(orotate)2- derivative is faster than that
noted in the tungsten analogue.
Another revealing trend is the observation that CO dissocia-
tion in W(CO)4(glycinate)- is slower by an order of magnitude
than in W(CO)4(orotate)2-, which in turn is significantly slower
than CO dissociation in W(CO)4(dihydroorotate)2-. This is best
seen upon comparing the data in Table 7 at 0 °C. That is, as
anticipated, the pseudoamido group in 2 is more CO-labilizing
due to π donation to the metal center upon CO dissociation as
compared with the glycinate derivative where there is only a
transient formation of an amido intermediate. Furthermore, as
the aromaticity in the heterocyclic ring is interrupted as in
complex 3 the amido group becomes a better π donor and hence
a better cis CO-labilizing ligand.23,24 Nevertheless, the CO-
labilizing abilities of the “amido” groups in complexes 2 and 3
are much less than what is seen for an amido group which is
not stabilized via a neighboring carbonyl functionality. For
instance, the amido ligands in the anionic complexes,
W(CO)3[NHC6H4NH]2- and W(CO)3[NHC6H4O]2-, are able to
stabilize coordination and electronic unsaturation at the metal
center. Consistent with a mechanistic pathway which involves
both bond-breaking and bond-making, i.e., M-C bond-breaking
and M-N bond-making, the activation parameters for intermo-
lecular CO exchange in complex 2 were determined to be ∆Hq
) 63.2 ( 3.8 kJ/mol and ∆Sq) -82.8 ( 13.0 J/mol‚K.
M(CO)4(orotate)2-
+
13CO n ) 1-48
M(CO)4-n(13CO)n(orotate)2- + n12CO (1)
The ligand substitution reactions were carried out in an excess
of 13C-labeled CO and were followed by infrared spectroscopy
in the ν(CO) region. Specifically, the decrease in the absorbance
of the high-frequency, well-isolated, CO stretching vibration
in each respective complex was employed in monitoring the
reactions’ progress. Figure 6 depicts such spectral changes
recorded for the process involving carbonyl ligand substitution
in the W(CO)4(orotate)2- anion with 13CO, whereas the inset
in Figure 6 shows the decay of the parent dianion follows first-
order kinetics. Disappearance of the all 12CO species is
indicated in Figure 6 with initial appearance of the mono-13CO
substituted shown. Over an extended period of time all the 13
-
CO-substituted derivatives are observed. Similar kinetic data
were noted for the other complexes investigated.
The initial site of CO incorporation into the W(CO)4(orotate)2-
dianion was examined by 13C NMR spectroscopy. When an
atmosphere of 13CO was placed over a CD3CN solution of
complex 2 in an NMR tube at -15 °C and the CO exchange
(20) Moralejo, C.; Langford, C. H.; Bird, P. H. Can. J. Chem. 1991, 69,
2033.
(21) (a) Klement, U. Z. Kristallogr. 1993, 208, 111. (b) Tutt, L.; Zink, J.
I. J. Am. Chem. Soc. 1986, 108, 5830.
(22) (a) Angelici, R. J. Organomet. Chem. ReV. 1968, 3, 173. (b)
Darensbourg, D. J. AdV. Organomet. Chem. 1982, 21, 113.
(23) Lichtenberger, D. L.; Brown, T. L. J. Am. Chem. Soc. 1978, 100, 366.
(24) Davy, R. D.; Hall, M. B. Inorg. Chem. 1989, 28, 3524.

Organometallic Derivatives of Orotic Acid
Inorganic Chemistry, Vol. 37, No. 10, 1998 2545
Figure 7. 13C NMR of 13CO exchange in complex 2 at -15 °C, (a) 13 min, (b) 22 min, and (c) 40 min after addition of 13CO.
Table 8. Observed First-Order Rate Constants for the 13CO
Exchange of [NEt4][W(CO)5(uracilate)] as a Function of
Temperaturea
°C),25 and much faster than the CO exchange which occurs in
the parent hexacarbonyl.22 The corresponding enthalpies of
activation (∆Hq) for CO dissociation vary accordingly with
-OPh < uracilate < CO, specifically with values of 82.4 <
106.9 < 166 kJ/mol. Equation 2 should be distinguished from
reactions involving group 6 pentacarbonyl complexes containing
amine ligands. In these instances, the complexes undergo
dissociative amine loss rather than facile CO exchange. To cite
an example, W(CO)5piperidine under an atmosphere of CO
dissociates the amine ligand with a first-order rate constant of
1.30 × 10-5 s-1 at 60.3 °C yielding W(CO)6 with no CO
exchange occurring.26
temperature (°C)
kobs (s-1) × 104
30.0
40.1
49.9
58.0
0.183
0.617
2.667
6.850
a In THF.
Summary and Conclusions
Thermally stable orotic acid derivatives of chromium and
tungsten carbonyls were prepared in good yield from the
reactions of the M(CO)5THF adducts with the tetraethylammo-
nium salts of the corresponding acid.27 The X-ray structures
of the orotate derivatives of both metals revealed a distorted
octahedral geometry about the metal centers which were
associated with four carbonyl ligands and a planar orotate ring.
The orotate ligand was bound to the metal center through the
deprotonated nitrogen (1) atom and one of the carboxylate
oxygen atoms. 13C NMR and ν(CO) infrared spectroscopies
indicated the orotate dianion to possess significantly enhanced
electron donating ability as compared with the glycinate
monoanion similarly bound to M(CO)4 fragments. Furthermore,
the dihydroorotate dianion exhibited an even greater electron
donating ability toward the metal tetracarbonyl moiety. Al-
though the orotate metal carbonyl derivatives exhibited no
tendency to undergo thermal decarboxylation in solution at 50
°C, these complexes readily underwent decarbonylation at
temperatures as low as 0 °C. That is, carbon monoxide ligand
exchange reactions carried out in the presence of the 13CO
demonstrated the orotate metal carbonyl complexes to contain
very labile CO ligands, with first-order rate constants for CO
Figure 8. Eyring plot for intermolecular CO exchange in complex 4,
[Et4N][W(CO)5(C4H3N2O2)].
In addition, the weak but quite significant π donating ability
of the uracilate ligand is reflected in the kinetic parameters for
the dissociation of CO in the complex [Et4N][W(CO)5-
(uracilate)]. Table 8 contains the temperature-dependent first-
order rate constants for CO loss determined by the 13CO-
exchange process. The reaction as defined in eq 2 was
monitored by the disappearance for the high-frequency ν(CO)
band observed at 2061.8 cm-1 in THF. The Eyring plot, Figure
8, provided activation parameters ∆Hq ) 106.9 ( 4.3 kJ/mol
and ∆Sq ) 16.3 ( 13.7 J/mol‚K for the CO exchange process
of complex 4. The activation parameters and rates of CO
(25) (a) Darensbourg, D. J.; Klausmeyer, K. K.; Reibenspies, J. H. Inorg.
Chem. 1995, 34, 1933. (b) Darensbourg, D. J.; Klausmeyer, K. K.;
Draper, J. D.; Chojnacki, J. A.; Reibenspies, J. H. Inorg. Chim. Acta
1998, 270, 405.
(26) Darensbourg, D. J.; Ewen, J. A. Inorg. Chem. 1981, 20, 4168.
(27) There are a limited number of organometallic derivatives of amino
acids currently in the literature. For additional contributions by others,
see: (a) Meder, H.-J.; Beck, W. Z. Naturforsch. 1986, 41B, 1247. (b)
Petri, W.; Beck, W. Chem. Ber. 1984, 117, 3265. (c) Grotjahn, D. B.;
Groy, T. L. J. Am. Chem. Soc. 1994, 116, 6969. (d) Schubert, U.;
Tewinkel, S.; Mo¨ller, F. Inorg. Chem. 1995, 34, 995. (e) Severin, K.;
Mihan, S.; Beck, W. Chem. Ber. 1995, 128, 1127. (f) Grotjahn, D.
B.; Joubran, C.; Hubbard, J. L. Organometallics 1996, 15, 1230.
W(CO)5(uracilate)- + 13CO f
W(CO)5-n(13CO)n(uracilate)- + 12CO (2)
exchange for complex 4 can be compared to that of other cis-
labilizing ligands such as phenoxides and chloride.17 For
example, the CO exchange rate for 4 is much slower than that
for W(CO)5(OPh)- (k ) 2.15 × 10-2 s-1 at 5 °C),25 just slightly
slower than that for W(CO)5Cl- (k ) 1.01 × 10-4 s-1 at 5

2546 Inorganic Chemistry, Vol. 37, No. 10, 1998
Darensbourg et al.
dissociation having values ranging from 49 to 6.05 (×10-4) s-1
at 0 °C. Furthermore, it was illustrated that the tungsten orotate
derivative undergoes stereoselective CO exchange, with the CO
ligands cis to the pseudoamido group being exchanged at a faster
rate. These results clearly demonstrate that the unsaturated
metal center, resulting from CO dissociation, is competitive with
the pseudoaromatic uracil ring system for the electron density
provided by the amido ligand. NeVertheless, unlike the bis-
amido (NH-C6H4-NH)2- or amidophenoxide (NH-C6H4-
O)2- ligands which afford stable group 6 metal tricarbonyl
species, there is no eVidence other than kinetic data for the
intermediary of a “16-electron” complex containing the orotate
ligand. As anticipated, upon hydrogenation of the orotate ring
where it is no longer pseudoaromatic, the CO ligands in the
M(CO)4 unit are even more labile, indicative of the deprotonated
nitrogen (1) atom being more amide-like in character. A similar
CO-labilizing effect was seen in the W(CO)5(uracilate)- deriva-
tive where the CO ligands are highly labile relative to saturated
amine metal analogues.
Acknowledgment. The financial support of this research by
the National Science Foundation (Grant CHE96-15866) and the
Robert A. Welch Foundation is greatly appreciated.
Supporting Information Available: X-ray crystallographic files,
in CIF format, for complexes 1, 2, and 4 are available on the Internet
only. Access information is given on any current masthead page.
Note Added in Proof
Subsequent to the submission of this manuscript it has become
apparent that zinc is indeed present in the OMP decarboxylase
enzyme (Miller, B. G.; Traut, T. W.; Wolfenden, R. J. Am.
Chem. Soc. 1998, 120, 2666).
IC980233F