The Reversible Insertion Reaction of Carbon Dioxide with the W(CO)5OH- Anion. Isolation and Characterization of the Resulting Bicarbonate Complex [PPN][W(CO)5O2COH]

Article abstract of DOI:10.1021/ic9516525

[Et₄N][W(CO)₅OH] (1) and [PPN][W(CO)₅O₂COH] (2) have been synthesized and characterized by ¹H and ¹³C NMR and IR spectroscopies, and the X-ray crystal structure of 2 has been determined. Complex 2 crystallizes in the triclinic space group P1? with unit cell parameters a = 12.208(2) ?, b = 13.497(2) ?, c = 13.681(2) ?, α = 101.06(2)°, β = 114.76(1)°, γ = 98.45(2)°, V = 1942.6(5) ?³, and Z = 2. The structure of the anion of complex 2 consists of a central W(0) bound to five carbonyl ligands, and the coordination around the metal is completed by a monodentate bound bicarbonate ligand located 2.19(1) ? away from the metal center. In the solid state, two anions are hydrogen bonded to one another via the bicarbonate ligands in the unit cell. Complex 1 inserts CO₂, COS, or CS₂ to rapidly afford the corresponding bicarbonate or thiocarbonate complexes. The lower limit for the rate constant for the carboxylation of complex 1 has been determined to be 4.2 × 10^-4 M^-1 s^-1 at -70.2 °C, and the lower limit for the rate constant for the decarboxylation of complex 2 has been found to be 2.5 × 10^-3 s^-1 at 20.0 °C. In addition, the rate constant for the decarbonylation of 2 was determined to be 7.60 × 10^-3 s^-1 at 36.0 °C, a value which is somewhat faster than anticipated on the basis of analogous data for a large variety of W(CO)₅O₂CR^- derivatives. This is attributed to a diminution of the electron-withdrawing ability of the OH substituent in O₂COH as a result of hydrogen bonding to solvent. Nevertheless, it is clear that the rate of decarboxylation of the anion from complex 2 is faster than the rate of CO dissociation. Concomitantly, carboxylation of complex 1 is faster than CO dissociation, since the W(CO)₅OH^- is inert toward ¹³CO exchange on the time scale of carboxylation at -70.2 °C.

Full text of DOI:10.1021/ic9516525

4406
Inorg. Chem. 1996, 35, 4406-4413
The Reversible Insertion Reaction of Carbon Dioxide with the W(CO)5OH- Anion.
Isolation and Characterization of the Resulting Bicarbonate Complex
[PPN][W(CO)5O2COH]
Donald J. Darensbourg,* Monica L. Meckfessel Jones, and Joseph H. Reibenspies
Department of Chemistry, Texas A&M University, College Station, Texas 77843
ReceiVed December 28, 1995X
[Et4N][W(CO)5OH] (1) and [PPN][W(CO)5O2COH] (2) have been synthesized and characterized by 1H and 13
C
NMR and IR spectroscopies, and the X-ray crystal structure of 2 has been determined. Complex 2 crystallizes
in the triclinic space group P1h with unit cell parameters a ) 12.208(2) Å, b ) 13.497(2) Å, c ) 13.681(2) Å, R
) 101.06(2)°, â ) 114.76(1)°, γ ) 98.45(2)°, V ) 1942.6(5) Å3, and Z ) 2. The structure of the anion of
complex 2 consists of a central W(0) bound to five carbonyl ligands, and the coordination around the metal is
completed by a monodentate bound bicarbonate ligand located 2.19(1) Å away from the metal center. In the
solid state, two anions are hydrogen bonded to one another Via the bicarbonate ligands in the unit cell. Complex
1 inserts CO2, COS, or CS2 to rapidly afford the corresponding bicarbonate or thiocarbonate complexes. The
lower limit for the rate constant for the carboxylation of complex 1 has been determined to be 4.2 × 10-4 M-1
s-1 at -70.2 °C, and the lower limit for the rate constant for the decarboxylation of complex 2 has been found
to be 2.5 × 10-3 s-1 at 20.0 °C. In addition, the rate constant for the decarbonylation of 2 was determined to be
7.60 × 10-3 s-1 at 36.0 °C, a value which is somewhat faster than anticipated on the basis of analogous data for
a large variety of W(CO)5O2CR- derivatives. This is attributed to a diminution of the electron-withdrawing
ability of the OH substituent in O2COH as a result of hydrogen bonding to solvent. Nevertheless, it is clear that
the rate of decarboxylation of the anion from complex 2 is faster than the rate of CO dissociation. Concomitantly,
carboxylation of complex 1 is faster than CO dissociation, since the W(CO)5OH- is inert toward 13CO exchange
on the time scale of carboxylation at -70.2 °C.
Introduction
both a structural and a reactivity model for carbonic anhydrase,
(tris(pyrazolyl)hydroborato)Zn(OH), has been fully character-
The reversible insertion reaction of metal-hydroxide com-
plexes with carbon dioxide remains a subject of much interest
since it is a pivotal step in the carbonic anhydrase catalyzed
hydration of CO2 to HCO3- (eq 1). Carbonic anhydrase (CA),
ized.7 This complex has been found to react immediately and
reversibly with CO2 to form the monodentate bicarbonate
complex, which has been identified by IR spectroscopy, and to
catalyze the exchange of oxygen between CO2 and H217O.8 This
exchange reaction has also been reported for carbonic anhy-
drase.9
CO2 + H2Oyk1z HCO3- + H+
(1)
k-1
a zinc enzyme found in both plants and animals, is very efficient,
with the most active form, human carbonic anhydrase II (CAII),
increasing the rate constant for the forward reaction (k1) from
(4) (a) Banci, L.; Bertini, I.; Luchinat, C.; Donaire, A.; Martinez, M.-J.;
Moratal Mascarell, J. M. Comments Inorg. Chem. 1990, 9, 245. (b)
Silverman, D. N.; Lindskog, S. Acc. Chem. Res. 1988, 21, 30. (c)
Wooley, P. Nature 1975, 258, 677. (d) Bertini, I.; Luchinat, C.;
Scozzafava, A. Struct. Bonding (Berlin) 1981, 48, 45. (e) Sigel, H.,
Ed. Metal Ions in Biological Systems. Zinc and its Role in Biology
and Nutrition (Vol. 15); Marcel Dekker: New York, 1983. (f)
Lindskog, S.; Coleman, J. E. Proc. Natl. Acad. Sci. U.S.A. 1973, 70,
2505. (g) Yachandra, V.; Powers, L.; Spiro, T. G. J. Am. Chem. Soc.
1983, 105, 6596. (h) Khalifah, R. G. J. Biol. Chem. 1971, 246, 2561.
(i) Khalifah, R. G. Proc. Natl. Acad. Sci. U.S.A. 1973, 70, 1986.
(5) (a) Merz, K. M., Jr.; Hoffmann, R.; Dewar, M. J. S. J. Am. Chem.
Soc. 1989, 111, 5636. (b) Jacob, O.; Cardenas, R.; Tapia, O. J. Am.
Chem. Soc. 1990, 112, 8692. (c) Allen, L. C. Ann. N.Y. Acad. Sci.
1981, 367, 383. (d) Pullman, A. Ann. N.Y. Acad. Sci. 1981, 367, 340.
(e) Liang, J.-Y.; Lipscomb, W. N. Biochemistry 1987, 26, 5293. (f)
Liang, J.-Y.; Lipscomb, W. N. Int. J. Quantum Chem. 1989, 36, 299.
(g) Cook, C. M.; Allen, L. C. Ann. N.Y. Acad. Sci. 1984, 429, 84.
(6) (a) Gorrell, I. B.; Looney, A.; Parkin, G. J. Chem. Soc., Chem.
Commun. 1990, 220. (b) Han, R.; Gorrell, I. B.; Looney, A. G.; Parkin,
G. J. Chem. Soc., Chem. Commun. 1991, 717. (c) Alsfasser, R.; Powell,
A. K.; Vahrenkamp, H. Angew. Chem., Int. Ed. Engl. 1990, 29, 898.
(d) Alsfasser, R.; Trofimenko, S.; Looney, A.; Parkin, G.; Vahrenkamp,
H. Inorg. Chem. 1991, 30, 4098. (e) Looney, A.; Parkin, G.; Alsfasser,
R.; Ruf, M.; Vahrenkamp, H. Angew. Chem., Int. Ed. Engl. 1992, 31,
92. (f) Looney, A.; Han, R.; McNeill, K.; Parkin, G. J. Am. Chem.
Soc. 1993, 115, 4690. (g) Kimura, E.; Shiota, T.; Koike, T.; Shiro,
M.; Kodama, M. J. Am. Chem. Soc. 1990, 112, 5805. (h) Xiaoping,
Z.; van Eldik, R.; Koike, T.; Kimura, E. Inorg. Chem. 1993, 32, 5749.
(7) Alsfasser, R.; Trofimenko, S.; Looney, A.; Parkin, G.; Vahrenkamp,
H. Inorg. Chem. 1991, 30, 4098.
3.5 × 10-2 sec-1 1 to 1.4 × 106 s-1 2
.
The CAII isozyme has
been refined at 2.2 Å resolution3a (and more recently 1.54 Å),3b
with the active site being shown to be a zinc atom bound to
three histidine residues and one water molecule.3 Experimental4
and theoretical5 investigations of the activity of the enzyme have
led to the proposal that the mechanism for the enzymatic reaction
involves a monodentate bicarbonate intermediate.
Several researchers have prepared zinc complexes which are
models for carbonic anhydrase.6 One model complex which is
X Abstract published in AdVance ACS Abstracts, July 1, 1996.
(1) Colman, J. E. In Zinc Enzymes; Bertini, I., Luchinat, C., Maret, W.,
Zeppezauer, M., Eds.; Birkha¨user: Boston, MA, 1986; p 49.
(2) Silverman, D. N.; Lindskog, S. Acc. Chem. Res. 1988, 21, 30.
(3) (a) Ericksson, E. A.; Jones, T. A.; Liljas, A. In Zinc Enzymes; Bertini,
I., Luchinat, C., Maret, W., Zeppezauer, M., Eds.; Birkha¨user: Boston,
MA, 1986; p 317. (b) Hakansson, K.; Carlsson, M.; Svensson, L. A.;
Liljas, A. J. Mol. Biol. 1992, 227, 1192. (c) For reviews of structure
and reactivity of carbonic anhyrases, see: Brown, R. S. In Enzymatic
and Model Carboxylation and Reduction Reactions for Carbon Dioxide
Utilization; Aresta, M., Schloss, J. U., Eds.; Kluwer: Dordrecht, The
Netherlands, 1990; p 145. Bertini, I.; Luchinat, C. In Bioinorganic
Chemistry; Bertini, I., Gray, H. B., Lippard, S. J., Valentine, J. S.,
Eds.; University Science Books: Mill Valley, CA, 1994; p 37.
S0020-1669(95)01652-1 CCC: $12.00 © 1996 American Chemical Society

Reversible Insertion Reaction of Carbon Dioxide
Inorganic Chemistry, Vol. 35, No. 15, 1996 4407
none ketyl (diethyl ether, hexane), magnesium iodide (methanol), or
phosphorus pentoxide followed by calcium hydride (acetonitrile).
HPLC grade acetone was dried by stirring over potassium carbonate
overnight and then distilling from fresh potassium carbonate under
nitrogen. Acetone and tetrahydrofuran were purchased from Fisher
Scientific, and all other solvents were purchased from E.M. Science.
PPN(Cl) was purchased from Aldrich. NaHCO3 was purchased from
Fisher Chemical Co. Carbon disulfide was also purchased from Fisher
Chemical and was purified by shaking first with mercury, then with
cold, saturated HgCl2 solution, and finally with cold, saturated KMnO4
solution. The carbon disulfide was then dried by distillation from P2O5
under nitrogen.19 W(CO)6 was purchased from Strem. The solution
of 25% Et4N(OH) in methanol was purchased from Sigma. CO2, COS,
and CO were purchased from Matheson. 13CO, 13CO2, 13CS2, and d6-
acetone were purchased from Cambridge Isotopes. Infrared spectra
were taken on either an IBM FTIR/32 or a Mattson Galaxy 6021
spectrometer. A standard NaCl solution cell with a 0.1 mm path length
was used for all spectra. 1H and 13C spectra were taken on either a
Varian XL-200 or Varian XL-200e superconducting high-resolution
spectrometer with an internal deuterium lock in 5 or 10 mm tubes. For
variable temperature NMR experiments, the temperature was reported
to (0.1 °C by a thermocouple attached inside the magnet.
Synthesis. [PPN][HCO3]. This salt was prepared from 30.8 g of
NaHCO3 (0.366 mol) and 5.07 g of PPN(Cl) (8.84 mmol) using the
method published previously.23 The yield was 5.32 g or 89% (based
on PPN(Cl)). Anal. Found: 70.76, C; 5.50, H; 392 ppm, Cl-. Calcd
for [PPN][HCO3]‚2H2O: 69.92, C; 5.55, H.
[PPN][NO3]. Similarly, this salt was prepared from 21.5 g of NaNO3
(253 mmol) and 4.8 g of PPN(Cl) (8.4 mmol) using the method
published previously.20 The yield was 4.4 g or 88% (based on PPN-
(Cl)). Anal. Found: 70.55, C; 4.47, H; 412 ppm, Cl-. Calcd for
[PPN][NO3]‚1/2(H2O): 70.93, C; 5.13, H.
For quite some time we have been interested in the organo-
metallic chemistry of carbon dioxide. Part of these efforts has
focused upon the mechanistic aspects of C-H, C-C, and C-O
bond forming reactions resulting from CO2 insertion into M-H,
M-C, and M-O bonds. In this regard we are interested in
examining the interactions of CO2 with W(CO)5OH-. This
anionic metal carbonyl derivative provides a reactant where the
coordination sphere about the metal center may be easily
controlled and monitored during the CO2 insertion reaction.
However, the preparation of low-valent mononuclear group 6
carbonyl-hydroxo complexes has proven to be most difficult.
This is due to the tendency of these species to aggregate. In
1959, Hieber and co-workers reported the preparation of K3-
[M2(CO)6(OH)3] for M ) W and Mo,10 and the crystal structure
of the tungsten complex was later determined.11 In 1985,
McNeese reported the preparation and structure of [Et4N]4[Cr4-
(CO)12(µ3-OH)4],12 and the structures of the tungsten and
molybdenum analogs were presented later.13 The formation of
these clusters can be attributed to the cis-labilizing ability of
the OH ligand.12,14 Similar group 6 alkoxide clusters are also
known.15,16 The monomers are more difficult to isolate, but
[PPN][W(CO)5(OCH3)] has been identified in solution by
infrared spectroscopy.17 In addition, [Et4N][W(CO)5(OPh)] has
been structurally characterized by X-ray crystallography.18 This
complex has been found to readily insert CO2, COS, and CS2
and to lose CO to form the stable cubane structure [Et4N]4-
[W(CO)3(µ-OPh)]4 in the absence of a CO atmosphere. The
reactivity patterns of the analogous hydroxide complex would
be anticipated to be quite similar.
The present investigation will focus on the synthesis of the
mononuclear complex [Et4N][W(CO)5OH] and its reaction
chemistry with carbon dioxide (eq 2). Subsequent reactivity
and structural studies of the thus-formed bicarbonate complex
will be presented.
[Et4N][W(CO)5OH]. In a typical synthesis, this complex was
prepared by photolyzing 0.20 g of W(CO)6 (0.57 mmol) and 80 mL of
dry methanol for 2 h. The solution was then added to 0.48 g (0.81
mmol) of a solution of 25% Et4N(OH) in methanol cooled to 0 °C.
The solution was immediately placed under an atmosphere of CO to
prevent aggregation. IR (MeOH): 2061 (w), 1917 (s), 1864 (m) cm-1
.
W(CO)5OH- + CO2 h W(CO)5O2COH-
Experimental Section
(2)
After a solution of the complex in methanol under an atmosphere of
13CO was stirred for 2.5 h at ambient temperature, the complex was
enriched in 13CO. 13C NMR: 199.8 (cis CO’s), 203.9 (trans CO) ppm.
[Et4N][W(CO)5S(O)COH]. The placement of a solution of the
hydroxo complex prepared as described above under an atmosphere of
COS gave the insertion product, as indicated by the infrared spectrum.
Because the product was extremely soluble in methanol, the solvent
was removed from the dark red solution in Vacuo to give an oily brown
solid. Reprecipitating the complex from acetonitrile with diethyl ether
gave a yield of 0.302 g or 12.6% (based on W(CO)6). IR (MeOH):
2064 (w), 1924 (s), 1873 (m) cm-1. IR (CH3CN): 2063 (w), 1919
Materials. All manipulations were carried out in an argon-filled
glovebox or on a double-manifold Schlenk line. With the exception
of acetone, solvents were dried by distillation from sodium-benzophe-
(8) Looney, A.; Han, R. H.; McNeill, K.; Parkin, G. J. Am. Chem. Soc.
1993, 115, 4690.
(9) (a) Silverman, D. N.; Tu, C. K. J. Am. Chem. Soc. 1975, 97, 2263.
(b) Koenig, S. H.; Brown, R. D., III; Bertini, I.; Luchinat, C. J. Biophys.
1983, 41, 179. (c) Silverman, D. N.; Tu, C. K. J. Am. Chem. Soc.
1976, 98, 978. (d) Silverman, D. N.; Tu, C. K. J. Biol. Chem. 1977,
252, 3332. (e) Silverman, D. N.; Tu, C. K.; Lindskog, S.; Wynns, G.
C. J. Am. Chem. Soc. 1979, 101, 6734. (f) Bertini, I.; Canti, G.;
Luchinat, C. Inorg. Chim. Acta 1981, 56, 1.
(10) (a) Hieber, V. W.; Englert, K.; Rieger, K. Z. Anorg. Allg. Chem. 1959,
300, 304. (b) Hieber, V. W.; Englert, K.; Rieger, K. Z. Anorg. Allg.
Chem. 1959, 300, 295.
(11) Albano, V. G.; Ciani, G.; Manassero, M. J. Organomet. Chem. 1970,
25, C55.
(12) McNeese, T. J.; Mueller, T. E.; Wierda, D. A.; Darensbourg, D. J.;
Delord, T. J. Inorg. Chem. 1985, 24, 3465.
(13) Lin, J. T.; Yeh, S. K.; Lee, G. H; Wang, Y. J. Organomet. Chem.
1989, 361, 89.
(14) (a) Brown, T. L.; Atwood, J. D. J. Am. Chem. Soc. 1976, 98, 3160.
(b) Lichtenberger, D. L.; Brown, T. L. J. Am. Chem. Soc. 1978, 100,
366.
(15) (a) McNeese, T. J.; Cohen, M. B.; Foxman, B. M. Organometallics
1984, 3, 552. (b) Darensbourg, D. J.; Sanchez, K. M.; Reibenspies, J.
H Inorg. Chem. 1988, 27, 3269. (c) Darensbourg, D. J.; Mueller, B.
L.; Bischoff, C. J.; Johnson, C. C.; Sanchez, K. M.; Reibenspies, J. H
Isr. J. Chem. 1990, 30, 369.
(16) Darensbourg, D. J.; Klausmeyer, K. K.; Reibenspies, J. H. Unpublished
results.
(s), 1859 (m) cm-1
ppm.
.
13C NMR: 197.4 (cis CO’s), 201.5 (trans CO)
[Et4N][W(CO)5S2COH]. Adding 0.08 mL of CS2 (1 mmol) to a
solution of the hydroxo complex prepared as above and stirring the
solution for 30 min gave the insertion product. Removal of the solvent
in Vacuo yielded 0.504 g of a yellow-orange solid (a mixture of the η1
and η2 products). IR (MeOH): 2063 (w), 1926 (s), 1880 (m) cm-1
.
13C NMR: 196.9 (cis CO’s), 201.1 (trans CO), 199.7 (inserted 13CS2)
ppm.
[Et4N][W(CO)5O2COH]. When a sample of [Et4N][W(CO)5OH]
was placed under an atmosphere of CO2, the insertion product was
obtained immediately. An IR spectrum shows peaks at 2067 (w), 1927
(s), and 1866 (m) cm-1. The CO2 insertion product shows CO2 stretches
at 1647 and 1312 cm-1, while the 13CO2 insertion product shows CO2
(19) Gordon, A. J.; Ford, R. A. The Chemist’s Companion; Wiley-
Interscience: New York, 1971; p 432.
(20) Martiusen, A.; Songstad, J. Acta. Chem. Scand., Ser. A 1977, A31,
645.
(21) Ibers, J. A., Hamilton, W. C., Eds. International Tables for X-Ray
Crystallography; Kynoch Press: Birmingham, England, 1974; Vol.
IV.
(22) Cihonski, J. L.; Levenson, R. A. Inorg. Chem. 1975, 14, 1717.
(23) Darensbourg, D. J.; Meckfessel Jones, M. L.; Reibenspies, J. H. Inorg.
Chem. 1993, 32, 4675.
(17) (a) Darensbourg, D. J.; Gray, R. L.; Ovalles, C.; Pala, M. J. Mol.
Catal. 1985, 29, 285. (b) Bates, A.; Muraoka, M. T.; Trautman, R. J.
Inorg. Chem. 1993, 32, 2651.
(18) Darensbourg, D. J.; Sanchez, K. M.; Reibenspies, J. H.; Rheingold,
A. L. J. Am. Chem. Soc. 1989, 111, 7094.

4408 Inorganic Chemistry, Vol. 35, No. 15, 1996
Darensbourg et al.
stretches at 1603 and 1285 cm-1
.
13C NMR: 199.3 (cis CO’s), 203.5
Table 1. Crystallographic Data and Data Collection Parameters for
[PPN][W(CO)5O2COH]
(trans CO), 160.4 (inserted 13CO2) ppm. This latter signal split into a
doublet with a carbon-hydrogen coupling constant of 15 Hz when the
proton decoupler was turned off.
formula
formula weight
C42H31NO8P2W
923.5
[PPN][W(CO)5O2COH]. In a typical synthesis, 0.435 g of W(CO)6
(1.24 mmol) and 150 mL of dry acetone were photolyzed for 2 h to
give W(CO)5(acetone) and a small amount of W(CO)4(acetone)2. This
solution was then added to 0.616 g of [PPN][HCO3] (1.03 mmol) under
a CO2 atmosphere Via cannula. After the solution was warmed gently
and stirred for 1 h, the complex was isolated as an oil upon separation
in diethyl ether and hexane. Washing the product with diethyl ether
provided a yellow-orange powder. The yield was 0.582 g or 61%
(based on PPN(HCO3)). Anal. Found: 53.19, C; 4.45, H. Calcd for
[PPN][W(CO)5HCO3]‚H2O: 53.58, C; 4.53, H. IR (acetone): 2063
(w), 1914 (s), 1848 (m) cm-1. 1H NMR: 11.9 ppm.
space group
a, Å
b, Å
c, Å
triclinic, P1h
12.208(2)
13.497(2)
13.681(2)
101.06(2)
114.760(10)
98.45(2)
1942.6(5)
2
1.579
6.804
1.54178
296
R, deg
â, deg
γ, deg
V, Å3
Z
density (calcd), g/cm3
abs coeff mm-1
λ (Å)
T, K
[PPN][W(CO)5CS2OH]. Dissolving 0.116 g of the bicarbonate
complex (0.126 mmol) in 2 mL of acetone under N2, adding 0.5 mL
of CS2 (8 mmol) immediately, and allowing the mixture to stir for 30
min gave a red-orange solution. Adding diethyl ether and hexane gave
an orange-brown powder. The powder was washed three times with
diethyl ether and dried in vacuo to give a yield of 0.076 g or 63%
(based on [PPN][W(CO)5O2COH]). IR (acetone): 2065 (w), 1927 (s),
1875 (m) cm-1. Anal. Found: 49.69, C; 3.40, H. Calcd for [PPN]-
[W(CO)5CS2OH]‚H2O: 50.06, C; 3.03, H. 1H NMR: 11.9 ppm.
[PPN][W(CO)5NO3]. A mixture of 0.3 g of W(CO)6 (0.9 mmol)
and 150 mL of THF was photolyzed for 30 min to give W(CO)5(THF)
and a small amount of W(CO)4(THF)2. This solution was then added
to 0.40 g of [PPN][NO3] (0.7 mmol) Via cannula. The solution turned
orange after a few minutes. After 2.5 h of stirring, the solution showed
an IR spectrum with peaks for both the η1 and η2 products [2010 (w),
1919 (s), 1875 (sh), 1863 (m), 1829 (w) cm].
normlzd transmn coeff
0.9999-0.5978
4.9
5.1
R,a %
Rw,a %
a R ) ∑||Fo| - |Fc||/∑Fo. b Rw ) {[∑w(Fo - Fc)2/∑w(Fo)2]}1/2
.
Exchange of [PPN][W(CO)5O2COH] with 13CO2. [PPN][W-
(CO)5O2COH] (0.045 g, 0.049 mmol) was placed in a 10 mm NMR
tube under 13CO2. Three vials of d6-acetone were also placed under
an atmosphere of 13CO2 and were added to the tube. The solution was
allowed to sit at room temperature for 24 min with occasional shaking.
The NMR was cooled to -78 °C, and the tube was inserted. After the
NMR tube sat for 10 min to allow the temperature of the solution to
equilibrate, a spectrum was taken. The tube was removed from the
instrument, and the solution was allowed to warm to room temperature
and sit for 10 min with occasional shaking. A second spectrum was
then taken by the same method as above.
Kinetic Measurements. Reaction of [PPN][W(CO)5O2COH] and
P(OMe)3. [PPN][W(CO)5O2COH] (0.233 g, 0.252 mmol) was dis-
solved in 10.0 mL of dry acetone under CO2, and the solution was
placed in a thermostated water bath at 36.0 ( 0.1 °C. An IR spectrum
of the solution was taken, and 0.50 mL of P(OMe)3 (>20 equiv) was
added. The reaction was monitored by following the disappearance of
the IR peak at 1915 cm-1 with spectra being taken as quickly as possible
(about every 1.5 min). The absorbance at 1915 cm-1 was corrected
by subtracting the absorbance at 2150 cm-1, where no peaks occur.
The reaction of [PPN][W(CO)5O2COH] and P(OMe)3 in CH3CN was
similarly studied.
The peak for inserted CO2 was integrated Versus the peak at 135
ppm for the para carbons of the [PPN]+ counterion. The integration
of the inserted CO2 peak did not increase relative to the counterion
peak in the second spectrum.
Reaction of [Et4N][W(CO)5OH] and 13CO2. A 30 mL solution of
[Et4N][W(CO)5OH] was prepared from 0.413 g of W(CO)6 (1.18 mmol)
and 0.53 g of a solution of 25% Et4N(OH) in methanol (0.90 mmol).
A small amount of the cold solution was added to a 10 mm NMR tube
containing 0.6 mL of d6-acetone under an atmosphere of CO. The
tube was put in the sample holder and cooled in a dry ice/acetone bath.
The instrument was cooled to -70.2 ( 1.0 °C. The NMR tube was
filled with 13CO2 and inserted into the instrument as quickly as possible.
The first spectrum, which shows inserted 13CO2, was complete after
12 min and 11 s from when the sample was prepared. A second
spectrum, which shows no change, was complete after 18 min. The
NMR tube was then emptied into a graduated cylinder to determine
the total volume of the sample, which was 5.4 mL.
Reaction of [PPN][W(CO)5O2CCH2CN] and P(OMe)3. [PPN][W-
(CO)5O2CCH2CN] (0.018 g, 0.033 mmol) was dissolved in 10.0 mL
of dry acetone, and the solution was placed in a thermostated water
bath at 36.0 ( 0.1 °C. An IR spectrum of the solution was taken, and
0.50 mL of P(OMe)3 (>20 equiv) was added. The reaction was
monitored by following the disappearance of the IR peak at 1916 cm-1
with spectra being taken as quickly as possible (about every 1.5 min).
The absorbance at 1916 cm-1 was corrected by subtracting the
absorbance at 2150 cm-1. The kinetics for this reaction in THF have
previously been reported.20
Additional Reactivity Studies. A solution of [Et4N][W(CO)5OH]
was prepared from 0.0595 g of W(CO)6 (0.169 mmol), 50 mL of
methanol, and 0.08 g of a solution of 25% Et4N(OH) in methanol (0.1
mmol) and cooled to 0 °C. H218O (30.0 µL, 1.66 mmol) was dissolved
in 1 mL of methanol and added to the cold solution. The mixture was
allowed to stir for 2 h at room temperature under CO, and the IR
spectrum did not change. The solution was placed under a slight
pressure of CO2 after a brief vacuum was subsequently pulled on the
flask, and the mixture was allowed to stir for 1 h. An IR spectrum
showed no change in the CO2 peak for the incorporation of 18O.
X-ray Crystal Structure Determination of [PPN][W(CO)5O2COH].
Crystals were grown by the slow diffusion of hexane into a concentrated
solution of the complex in acetone at -10 °C. Crystal data and details
of data collection are given in Table 1. A yellow needle (0.05 × 0.12
× 0.50 mm) was mounted on a glass fiber with epoxy cement at room
temperature. The crystal was identified as triclinic, with a space group
of P1h. The unit cell dimensions are a ) 12.208(2) Å, b ) 13.497(2)
Å, c ) 13.681(2) Å, R ) 101.06(2)°, â ) 114.760(10)°, γ ) 98.45-
(2)°, V ) 1942.580 Å3, Z ) 2, and Dcalc ) 1.579 g cm-3. Data were
measured using Cu KR radiation (λ ) 1.54 178 Å) on a Rigaku AFC5R
X-ray diffractometer equipped with an oriented graphite monochro-
mator. Data were collected for 5.0° e 2θ e 120.0°. A total of 6081
reflections were collected, and a total of 3926 unique reflections, with
|I| g 2.0σI, were used in further calculations. Lorentz and polarization
Reaction of [PPN][W(CO)5O2COH] and 13CS2. Using the inver-
sion-recovery method, T1 for CS2 was determined to be 30.103 s. The
optimal 90° pulse width was determined to be 17.5°.
In a typical run, 10 mg of [PPN][W(CO)5O2COH] (0.011 mmol)
was placed in a 10 mm NMR tube. Three grams of d6-acetone and
varying amounts of 4% 13C-enriched CS2, prepared from CS2 and a
small amount of 13CS2 and measured with a 50 µL syringe, were placed
in a small flask. This solution was then added to the NMR tube, and
the tube was shaken and placed in the instrument as quickly as possible.
The typical delay from preparation of the sample to starting the data
collection was 5 min.
The instrument was set to take 15 spectra 2 min apart, with each
transient spectrum taking 2 min due to the 110 s relaxation delay. The
instrument was set to observe only the range from 219 to 174 ppm.
The temperature was regulated to 20.2 ( 1.0 °C.
The peak at 193.1 ppm for free CS2 was integrated, with all spectra
in each run being integrated with the same vertical and integral scales
and the same range. Three to nine equivalents of CS2 could be used,
since less was too small to observe and more resulted in no change for
the free CS2 peak.

Reversible Insertion Reaction of Carbon Dioxide
Inorganic Chemistry, Vol. 35, No. 15, 1996 4409
Scheme 1
would have been dominated by the large crown ether, making
the identication of the OH ligand difficult.
As we have previously briefly reported, the monomeric
hydroxide complex 1 rapidly undergoes an insertion reaction
with CO2, COS, and CS2 to provide the bicarbonate, thiocar-
bonate, and dithiocarbonate complexes, respectively.23 The
reaction rate and product were not altered upon carrying out
the insertion reaction with the cumulenes under an atmosphere
of carbon monoxide. Hence, these observations eliminate the
necessity for a coordination site at the metal center or the
complete dissociation of the hydroxide ligand from the metal
center during the insertion process. The ν(CO) infrared three-
band pattern shifts to slightly higher frequencies upon insertion
of the cumulenes, with the average ν(CO) frequency increasing
in the order CO2 < COS < CS2. For example, formation of
the bicarbonate complex in methanol results in three ν(CO)
bands at 2064 (w), 1927 (s), and 1866 (m) cm-1. Concomi-
tantly, in the carboxylate region of the infrared spectrum the
ν(CO2)asym and ν(CO2)sym vibrations were observed at 1647 and
1312 cm-1, exhibiting isotopic shifts to 1602 and 1285 cm-1
upon 13CO2 insertion. The separation of ν(CO2)asym and
ν(CO2)sym (∆ of 335 cm-1) is consistent with unidentate binding
of the bicarbonate ligand to the tungsten center.24,25 Further-
more, the CO2 insertion product was characterized by 13C NMR
spectroscopy. That is, using 13CO-enriched [Et4N][W(CO)5-
OH] and 13CO2, a 13C{1H} NMR spectrum with carbonyl
resonances at 198 (cis) ppm and 203 (trans) ppm, along with a
doublet at 159 ppm (JC-H ) 15 Hz) assigned to the bound
bicarbonate ligand, was observed. The 1H NMR spectra
corrections were applied to the data. The structure was solved by direct
methods, and hydrogen atoms were placed in idealized positions with
isotropic thermal parameters fixed at 0.08 Å. All non-hydrogen atoms
were refined with anisotropic temperature factors: R ) 0.049, Rw
)
0.051; Rint ) 0.04. Neutral atom scattering factors and anomalous
scattering correction terms were taken from International Tables for
X-Ray Crystallography.21 Atomic coordinates and equivalent isotropic
displacement parameters are given in the supporting information.
Results and Discussion
Synthesis and Spectral Characterizations. The elusive
mononuclear [Et4N][W(CO)5OH] derivative 1 was synthesized
via the procedure outlined in Scheme 1. Also contained in
Scheme 1 are the spectroscopic data for the carbonyl ligands
in complex 1. It was necessary to immediately place the once-
formed complex 1 under an atmosphere of carbon monoxide to
avoid aggregation. In addition, the use of a strong hydrogen-
bonding solvent such as methanol greatly retards the rate of
CO dissociation in 1, hence inhibiting the formation of multi-
nuclear anionic clusters such as W2(CO)6(OH)33- or [W(CO)3-
OH]44-. This is a consequence of the OH ligand being a poorer
π-donor, a phenomenon which stabilizes the five-coordinate
intermediate resulting from CO loss, when its lone pairs are
involved in hydrogen bonding as skeletally depicted in A and
B. However, even in methanol [Et4N][W(CO)5OH] readily
-
displayed resonances for the OH group of the HOCO2 and
-
HOCS2 ligands at 11.9 ppm in methanol.
Although the insertion of CO2 into the WsOH- moiety is
readily reversible, such that the bicarbonate complex is only
stable in a CO2 atmosphere, the insertion reactions of COS and
CS2 are irreversible. This latter observation is due to the
enhanced stability of the WsS bond over the WsO bond and/
or to the decrease in strength of the CdS bond of CS2 relative
to the CdO bond of CO2. Hence, the complexes resulting from
the insertions of CS2 and COS can be isolated by removing the
solvent in Vacuo; however, the CS2 product partially decarbo-
nylates during this process to give a mixture of η1 and η2
products. A similar chelated derivative has been isolated and
characterized from the insertion of CS2 into the WsCH3 bond
- 26
of (CO)5WCH3
.
Closely related insertion reactions of CO2,
COS, and CS2 with the W(CO)5H- anion to form the corre-
sponding formates have been observed.27 The product of COS
insertion into the chromium hydride analog [Cr(CO)5SC(O)H]-
has been structurally defined, and the ligand was found to be
bound to the metal Via the sulfur atom.27b
Because it was imperative to these investigations to accurately
define the structure of the anion resulting from CO2 insertion
into (CO)5WOH- and due to the limitations herein described
prohibiting isolation of crystalline 1, we have resorted to an
alternative synthesis of this complex. This is of particular
importance since structurally characterized monodentate bicar-
bonate complexes are rare23,28 and chelated bicarbonates even
undergoes CO ligand exchange with 13CO to provide the 13CO-
enriched complex.
Unfortunately, due to the extreme solubility of 1 in methanol
it was not possible to isolate the complex in the solid state by
precipitation. Furthermore, the W(CO)5OH- anion readily
decarbonylates upon attempting to remove the methanol under
reduced pressure. However, in addition to its reactivity pattern
(Vide infra), the ν(CO) infrared spectrum of the complex was
similar enough to the spectra reported for [Et4N][W(CO)5OPh]18
and [PPN][W(CO)5OCH3]17 in hydrogen-bonding solvents to
be confident of its identity. For example, the ν(CO) infrared
spectrum of [Et4N][W(CO)5OPh] in THF of 2057 (w), 1904
(s), and 1852 (m) cm-1 shifts to 2062 (w), 1917 (s), and 1863
(m) cm-1 in methanol;18 values almost identical with those found
for complex 1 (Scheme 1). The preparation of [dibenzo-18-
crown-6-K][W(CO)5OH] has previously been reported, but the
ν(CO) infrared spectrum [peaks at 2064 (w), 1923 (s), and 1902
(m) cm-1 in CH2Cl2]22 is significantly different from that
observed for the complex reported herein as well as for the
closely related alkoxide derivatives, which creates doubt about
its authenticity. No reactivity patterns for the complex were
provided. In addition, the C/H elemental analysis reported
(24) Deacon, G. B.; Phillips, R. J. Coord. Chem. ReV. 1980, 33, 227.
(25) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 4th ed.; Wiley: New York, 1986; p 231.
(26) Darensbourg, D. J.; Wiegreffe, H. P.; Reibenspies, J. H. Organome-
tallics 1991, 10, 6.
(27) (a) Darensbourg, D. J.; Rokicki, A. J. Am. Chem. Soc. 1982, 104,
349. (b) Darensbourg, D. J.; Rokicki, A. Organometallics 1982, 1,
1685.
(28) (a) Ito, M.; Ebihara, M.; Kawamura, T. Inorg. Chim. Acta 1994, 218,
199. (b) Ganguly, S.; Mague, J. T.; Roundhill, D. M. Inorg. Chem.
1992, 31, 3831. (c) Crutchley, R. J.; Powell, J.; Faggiani, R.; Lock,
C. J. L. Inorg. Chim. Acta 1977, 24, L15.

4410 Inorganic Chemistry, Vol. 35, No. 15, 1996
Darensbourg et al.
Figure 2. Intermolecular hydrogen bonding between bicarbonate
ligands in the two anions of complex 2.
Table 2. Selected Bond Lengths (Å)a for [PPN][W(CO)5O2COH]
Figure 1. Thermal ellipsoid drawing of the anion of complex 2,
W(CO)5O2COH-. Ellipsoids are drawn with 50% probability bound-
aries.
W(1)-O(6)
W(1)-C(2)
W(1)-C(4)
O(1)-C(1)
O(3)-C(3)
O(5)-C(5)
O(7)-C(6)
2.19(1)
2.03(1)
2.02(1)
1.16(2)
1.16(1)
1.14(1)
1.25(1)
W(1)-C(1)
W(1)-C(3)
W(1)-C(5)
O(2)-C(2)
O(4)-C(4)
O(6)-C(6)
O(8)-C(6)
1.94(1)
2.03(1)
2.04(1)
1.13(1)
1.14(2)
1.26(1)
1.26(2)
more scarce.29,30 [PPN][W(CO)5O2COH] (2) was prepared from
the reaction of W(CO)5(acetone), photochemically generated
from W(CO)6 in acetone, and [PPN][HCO3] in acetone under
an atmosphere of carbon dioxide.31-33 The product was isolated
as a yellow-orange powder upon precipitation from the reaction
solution by the addition of hexane/ether. The complex was
stored under CO2 in order to prevent decarboxylation. The
ν(CO) infrared spectrum of 2 in acetone consists of three bands
at 2063 (w), 1914 (s), and 1848 (m) cm-1. Although these are
somewhat lower than the ν(CO) values observed for the complex
resulting from the insertion of carbon dioxide into W(CO)5OH-
in the better hydrogen-bonding solvent methanol, complex 2
exhibits ν(CO) vibrations in methanol identical to those of the
insertion product in that solvent. Correspondingly, the OH
proton resonance in 2 appears at 11.9 ppm.
X-ray Structure of [PPN][W(CO)5O2COH]. Crystals suit-
able for X-ray crystallography were obtained by the slow
diffusion of hexane into a concentrated solution of 2 in acetone
at -10 °C. Figure 1 illustrates a drawing of the anion and its
atomic numbering scheme, and selected bond lengths and angles
for the anion are listed in Tables 2 and 3, respectively. The
structure consists of a central W(0) bound to five carbonyl
ligands, and the coordination around the metal is completed by
a monodentate bound bicarbonate ligand located 2.19(1) Å away
from the metal center. The distal oxygen of the bicarbonate
ligand is 3.4 Å away from the tungsten atom, with the dihedral
angle between the W(1)O(6)C(1)C(2)C(4) plane (mean deviation
) 0.0308 Å) and the CO3 plane (mean deviation ) 0.0069 Å)
being 49.6°. In other words, the distal oxygen of the bicarbonate
ligand lies almost directly between C(3) and C(4) of the cis
carbonyl groups. Although the bicarbonate hydrogen atom was
not specifically located, its presence is confirmed by both the
single counterion and the hydrogen bonding between the two
anions in the unit cell as illustrated in Figure 2. Furthermore,
it was located in solution by 1H and 13C{1H} NMR spectroscopic
measurements.
a Estimated standard deviations are given in parentheses.
Table 3. Selected Bond Angles (deg)a for [PPN][W(CO)5O2COH]
O(6)-W(1)-C(1)
C(1)-W(1)-C(2)
C(1)-W(1)-C(3)
O(6)-W(1)-C(4)
C(2)-W(1)-C(4)
O(6)-W(1)-C(5)
C(2)-W(1)-C(5)
C(4)-W(1)-C(5)
W(1)-C(2)-O(2)
W(1)-C(4)-O(4)
O(6)-C(6)-O(7)
O(7)-C(6)-O(8)
177.4(3)
89.3(5)
88.3(5)
90.1(5)
177.9(3)
89.8(5)
90.8(5)
91.2(5)
176.0(1)
177.0(1)
124.0(1)
118.0(1)
O(6)-W(1)-C(2)
O(6)-W(1)-C(3)
C(2)-W(1)-C(3)
C(1)-W(1)-C(4)
C(3)-W(1)-C(4)
C(1)-W(1)-C(5)
C(3)-W(1)-C(5)
W(1)-O(6)-C(6)
W(1)-C(1)-O(1)
W(1)-C(3)-O(3)
W(1)-C(5)-O(5)
O(6)-C(6)-O(8)
89.5(4)
94.0(5)
88.9(4)
91.2(6)
89.2(5)
88.0(5)
176.2(6)
129.0(9)
177.4(9)
178.0(1)
176.0(1)
118.0(1)
a Estimated standard deviations are given in parentheses.
Table 4. Comparative Interatomic Distances and Angles in
Monodentate Bicarbonate Complexes
a
b
c
[M]
trans-PdMe(PEt3)2
trans-PtPh(PEt3)2
W(CO)5
a (Å)
b (Å)
b′ (Å)
O‚‚‚O
1 (deg)
2 (deg)
2′ (deg)
1.27(4)
1.28(3)
1.31(3)
2.60(2)
120(3)
119(2)
121(3)
1.264(7)
1.320(7)
1.249(7)
2.649(6)
119.9(6)
115.2(2)
125.0(6)
1.26(1)
1.26(2)
1.25(1)
2.568(5)
118.0(1)
118.0(1)
124.0(1)
a Reference 28c. b Reference 28a. c This work.
The C-O distances within the monodentate bicarbonate
ligand are all the same within the limits of the determination
(see Table 4). This was previously noted in the trans-PdMe-
(O2COH)(PEt3)2 complex and ascribed to the fact that each
oxygen atom is involved in bonding to either a proton, as a
result of hydrogen bonding, or to the metal center.28c On the
other hand, in the closely related trans-PtPh(O2COH)(PEt3)2
complex one of the C-O bonds was significantly longer than
the other two.28a The other reported monodentate structure
Pd(O2COH)2(dppe), which was not intermolecularly hydrogen
bonded, displays C-O bond distances for the oxygens bound
to palladium which are much longer than the other two C-O
distances.28b Comparative interatomic distances and angles, as
defined in structure C, in the three monodentate bicarbonate
complexes which are intermolecularly hydrogen bonded are
summarized in Table 4.
(29) Yoshida, T.; Thorn, D. L.; Okano, T.; Ibers, J. A.; Otsuka, S. J. Am.
Chem. Soc. 1979, 101, 4212.
(30) Baxter, K. E.; Hanton, L. R.; Simpson, J.; Vincent, B. R.; Blackman,
A. G. Inorg. Chem. 1995, 34, 2795.
(31) It should be noted parenthetically that attempts to prepare [PPN][OH]
by analogy to the published method for [PPN][OMe]32 were unsuc-
cessful.33
(32) Bates, A.; Muraoka, M. T.; Trautman, R. J. Inorg. Chem. 1993, 32,
2651.
(33) (a) Darensbourg, D. J.; Pala, M.; Rheingold, A. L. Inorg. Chem. 1986,
25, 125. (b) Darensbourg, D. J.; Pala, M.; Simmons, D.; Rheingold,
A. L. Inorg. Chem. 1986, 25, 3537.

Reversible Insertion Reaction of Carbon Dioxide
Inorganic Chemistry, Vol. 35, No. 15, 1996 4411
substituent of the -O2CR group goes from electron withdrawing
to highly electron releasing. Hence, the carbonate ligand is
anticipated to be very CO labilizing and to provide a stable
chelated tungsten carbonyl anion. Both expectations have been
realized in that [PPN]2[W(CO)4CO3] is the exclusive product
which results from the rapid reaction of [PPN]2[CO3] and
CO Ligand Substitution Process. The CO-labilizing ability
of the carboxylate ligand in W(CO)5O2CR- anions has been
attributed to an involvement of the distal oxygen atom of the
monodentate carboxylate in cis CO substitution reactions, with
more-electron-donating carboxylate ligands leading to more
labile CO groups.34,35 Furthermore, it was found that there was
a strong correlation between the propensity of the carboxylate
to chelate and the rate of cis CO dissociation. That is, chelation
of the carboxylate was observed in instances where the distal
oxygen is electron-rich (e.g., R ) tert-butyl, Me, and H) and
was not seen for the electron-withdrawing substituents R ) CH2-
CN or CF3. In an effort to ascertain the influence of the bicar-
bonate ligand on the substitution reaction (eq 3), we have
investigated this process under the conditions analogous to those
previously employed for the series of W(CO)5(carboxylate)-.35
2-
W(CO)5THF.18,37,38 The intermediacy of W(CO)5CO3 was
not observed as expected since it has a calculated rate constant
for CO dissociation of 4.15 × 10-1 s-1 at 40 °C. Hence, it is
likely that the O-H bond in the monodentate bound bicarbonate
ligand is lengthened in solution, possible by hydrogen bonding
to the solvent such that it somewhat resembles the carbonate
ligand and as a consequence is more CO labilizing.
For [PPN][W(CO)5HCO3], no η2 product is seen, even though
there is some W(CO)4(acetone)2 in the photolytic solution from
which it is prepared. By contrast, the reaction of W(CO)5(THF)
and W(CO)4(THF)2 with PPN(NO3) produces both the η1 and
η2 products. The bicarbonate and nitrate ligands are isoelec-
tronic, and bound nitrate has been used as a model for bound
bicarbonate;39 nevertheless, their reactivity is different in this
case. Although CO dissociation in the W(CO)5O2COH- anion
is faster than expected, it is still much slower than CO loss in
W(CO)5OR- (R ) H or alkyl/aryl group). For example, the
rate constant for CO dissociation in W(CO)5OPh- was measured
to be 2.15 × 10-2 s-1 at 5 °C in THF.40 Thus, the reactiVity of
the W(CO)5 unit with regard to CO dissociation is greatly
altered (decreased) upon inserting CO2 into the (CO)5W-OH-
bond to proVide the (CO)5WO2COH- complex. In this regard
it is important to note that the CO ligand substitution kinetics
for complex 2 were determined in a CO2-saturated solution
under an atmosphere of carbon dioxide to maintain the complex
in its carboxylated form.
W(CO)5O2COH- + excess P(OMe)3yk1z
k-1
cis-W(CO)4[P(OMe)3]O2COH- + CO (3)
Reaction 3 has been established to be first order in metal
complex and, when run in the presence of a large excess of
P(OMe)3, to be zero order in P(OMe)3.35 The forward rate
constant k1 was determined in acetone to be 7.60 × 10-3 s-1 at
36 °C. This value is an order of magnitude greater than that
predicted (7.88 × 10-4 s-1) from the previously determined
linear free energy relationship of log k1 Versus σ*, where σ* )
1.34 for R ) OH.35 Indeed, this value is slightly greater than
10 times larger than the value of 4.27 × 10-4 s-1 at 36 °C
reported for CO dissociation in [PPN][W(CO)5O2CCH2CN] in
THF solvent. That is, since CH2CN has a σ* parameter of 1.30,
the rate constants for CO dissociation from the W(CO)5O2CR-
anion (R ) CH2CN or OH) would be expected to be similar.
However, k1 for the bicarbonate complex is similar to those
reported for more-electron-releasing carboxylates, such as [PPN]-
[W(CO)5O2CC(CH3)3] (1.09 × 10-2 s-1 at 40 °C) and
[PPN][W(CO)5O2CCH3] (5.55 × 10-3 s-1 at 40 °C) for which
the ligands have Taft parameters36 of -0.30 and 0.00, respec-
tively.
One possible explanation for the larger than expected rate
constant is a solvent effect. Therefore, two kinetic studies were
undertaken to determine whether or not a solvent effect was at
work here. In the first run, the reaction of [PPN][W(CO)5O2-
CCH2CN] with P(OMe)3 was studied in acetone at 36 °C. The
determined rate constant was 4.23 × 10-4 s-1, which is in good
agreement with the value of 4.27 × 10-4 s-1 reported for the
complex at 36 °C in THF. In the second run, the bicarbonate
complex was studied in acetonitrile. The rate constant was
found to be 6.08 × 10-3 s-1. This value is only slightly smaller
than the corresponding value determined in acetone of 7.60 ×
10-3 s-1. The results of these two kinetic studies rule out the
faster than expected rate having been caused by a solvent effect.
An alternative explanation for the enhanced rate of CO
dissociation from the monodentate bound bicarbonate complex
2 lies in the expected difference in CO-labilizing abilities
between the bicarbonate ligand and its deprotonated congener.
In proceeding from -O2COH- to -O2CO2-, the Taft σ*
parameter decreases from 1.34 to -2.78. That is, the R
Rate of Decarboxylation of [PPN][W(CO)5O2COH]. An
estimate of the rate of decarboxylation of complex 2 was
obtained by an examination of the 13CO2 exchange reaction (eq
4). A sample of 2 in acetone was allowed to stand for 24 min
W(CO)5O2COH- + 13CO2 h W(CO)5O213COH- + CO2
(4)
under an atmosphere of 13CO2 at 20 °C, and its 13C NMR
spectrum was recorded at -78 °C. The intensity ratio of the
signal at 160.4 ppm for the inserted 13CO2 Versus that of the
para carbons of the PPN+ counterion (135 ppm) was deter-
mined. Reaction for an additional 10 min at 20 °C resulted in
no change in this intensity ratio, nor did the free to inserted
13CO2 intensity ratio change. Hence, the decarboxylation
reaction was deemed to be complete within the initial 24 min
reaction period. Experiments carried out with different [2]
provided similar estimates for the rate of decarboxylation,
indicative of first-order behavior in [2].
Since carbon dioxide insertion into the W-OH- bond in 2
is very fast (Vide infra), the rate-limiting step in the exchange
process defined in eq 4 is decarboxylation. This is in accord
with our previous observations on the 13CO2 exchange reactions
with metalloformate derivatives.41,42 From the fact that the
(37) The structure of the [Et4N]2[W(CO)4CO3] salt has been defined
crystallographically.18,38
(38) Darensbourg, D. J.; Sanchez, K. M.; Rheingold, A. L. J. Am. Chem.
Soc. 1987, 109, 290.
(39) (a) Han, R.; Parkin, G. J. Am. Chem. Soc. 1991, 113, 9707. (b) Looney,
A.; Saleh, A.; Zhang, Y.; Parkin, G. Inorg. Chem. 1994, 33, 158. (c)
Looney, A.; Parkin, G. Inorg. Chem. 1994, 33, 1234.
(40) Chojnacki, J. A. Ph.D. Dissertation, Texas A&M University, College
Station, TX, 1993, p 56.
(34) Darensbourg, D. J.; Wiegreffe, H. P. Inorg. Chem. 1990, 29, 592.
(35) Darensbourg, D. J.; Joyce, J. A.; Bischoff, C. J.; Reibenspies, J. H.
Inorg. Chem. 1991, 30, 1137.
(41) Darensbourg, D. J.; Wiegreffe, P.; Riordan, C. G J. Am. Chem. Soc.
1990, 112, 5759.
(42) Darensbourg, D. J.; Wiegreffe, H. P.; Wiegreffe, P. W. J. Am. Chem.
Soc. 1990, 112, 9252.
(36) Taft, R. J. Am. Chem. Soc. 1952, 74, 3120.

4412 Inorganic Chemistry, Vol. 35, No. 15, 1996
Darensbourg et al.
Table 5. Rate Constants for 13CS2 Exchange with 2 as a Function
of [CS2]a
103kobs, s-1
102[CS2],b M
0.971
1.89
2.20
0.905
1.81
2.32
a Reactions carried out in d6-acetone at 20.2 °C. b 4% 13C-enriched
CS2 utilized.
Scheme 2
actual rate constant could be as much as 50% higher than this
estimated value. Nevertheless, a rate constant of this magnitude
would be in agreement with its estimate obtained from the
reversible 13CO2 exchange study (Vide supra).
Rate of Carboxylation Reaction of [Et4N][W(CO)5OH]
with 13CO2. It is apparent from our qualitative observations
reported herein and elsewhere18,43 that the CO2 insertion reaction
into tungsten(0) alkoxide or aryloxide bonds is fast in the
absence of steric hindrance. We have attempted to better
quantify this “very fast” process. Specifically, the rate constant
for the insertion of CO2 into the (CO)5W-OH- bond is of
course highly relevant to the previously discussed decarboxy-
lation studies (Scheme 2), where the rate-limiting step was
assumed to be decarboxylation.
An estimate of the time scale for the reaction of [Et4N]-
[W(CO)5OH] with 13CO2 was determined by 13C NMR spec-
troscopy. Because the reaction was known to occur rapidly at
ambient temperature, the sample (0.039 M [Et4N][W(CO)5OH]
in methanol) was kept cold in a dry ice/acetone bath at all times,
except when it was in the spectrometer at -70.2 °C. After the
sample was put under an atmosphere of 13CO2, it was placed in
the spectrometer as quickly as possible, and the first spectrum
was completed after 12.2 min from time of mixing. A second
spectrum was completed after a reaction time of 18 min. An
intense resonance for the inserted 13CO2 at 159 ppm was seen
in the first spectrum, and its integrated intensity did not increase
in the second spectrum. Therefore, the reaction was over in
the time period required for the acquisition of the first spectrum
(12.2 min).
Figure 3. Disappearance of [CS2] as indicated by the integrated
intensity of the 13C resonance of free CS2 ln[It - I∞ ], with time for
reaction 5.
reaction is essentially complete in at least 24 min, it is possible
to conclude that the upper limit for the 13CO2 exchange process
is t1/2 ) 5.0 min at 20 °C. In an effort to better quantify this
reaction, we have repeated the study employing CS2, where the
reverse, deinsertion reaction does not readily occur.
The exchange reaction indicated in eq 5 was followed utilizing
13C-enriched carbon disulfide, where the decrease in CS2
concentration with time was monitored by 13C NMR spectros-
copy. Nevertheless, employing this method for measuring the
rate of reaction 5 required solving other problems. First of all,
k2
9
W(CO)5O2COH- + 13CO2
8 W(CO)5S213COH- + CO2
(5)
the long relaxation delay required to obtain reliable integration
data for the free CS2 peak limits the speed at which spectra can
be taken. This problem was solved by taking spectra which
consisted of only one transient. While this method undermines
the automatic correction for instrumental noise which is
performed after every four transients, the amount of noise
introduced into the spectra was small and remained constant
throughout a given run. However, in order to increase the signal
to noise ratio for the free CS2 peak to a reasonable value, 4%
13C-enriched CS2 was required. Finally, this method of fol-
lowing the reaction limits the amount of CS2 which can be used.
Normally, the reaction would have been followed under pseudo-
first-order conditions (i.e., using more than 20 equiv of CS2).
However, it was found that using more than 9 equiv of carbon
disulfide results in immeasurably small changes in the integra-
tions for the free CS2 peak.
Under these conditions, several kinetic runs were carried out
using various quantities of carbon disulfide. From the initial
data, the rate constants (k2) were determined from a plot of ln-
[It - I∞] Versus time, where It and I∞ equal the integrated
intensity of the free CS2 13C resonance at time ) t and time )
∞, respectively. These plots were linear (a representative plot
is illustrated in Figure 3), and the rate constants for CS2
exchange are listed in Table 5. A plot of the observed rate
constant as a function of carbon disulfide concentration exhibits
curvature indicative of saturation kinetics. This observation
would be consistent with decarboxylation of the W(CO)5O2COH-
anion being rate limiting. Extrapolation to greater than 20 equiv
of CS2 would indicate a minimum value of k2 at 20.2 °C of
about 2.5 × 10-3 s-1. However, it should be noted that the
The rate of the insertion reaction of W(CO)5OH- with CO2
is assumed to be first order in [CO2], since the analogous process
involving the W(CO)5R- anions (R ) H, CH3) is second order,
first order in [metal complex] and first order in [CO2].44 The
[CO2] concentration in methanol at -70.2 °C was estimated to
be 9.2 M on the basis of comparative measurements carried
out in methanol (0-25 °C)45 and acetone,46 where in the latter
solvent the [CO2] was accurately determined from 0 to -75
°C. From this estimated [CO2], the initial concentration of the
substrate [W(CO)5OH-]0, and the maximum reaction time, we
can establish that the lower limit for k-2 for the carboxylation
process in Scheme 2 is 4.2 × 10-4 M-1 s-1 at -70.2 °C. It
should be recognized, however, that the actual value of k-2 could
indeed greatly exceed this value. Although this rate constant
(43) Darensbourg, D. J.; Mueller, B. L.; Bischoff, C. J.; Chojnacki, S. S.;
Reibenspies, J. H. Inorg. Chem. 1991, 30, 2418.
(44) Darensbourg, D. J.; Hanckel, R. K.; Bauch, C. G.; Pala, M.; Simmons,
D.; White, J. N. J. Am. Chem. Soc. 1985, 107, 7463.
(45) Yogish, K. J. Chem. Eng. Jpn. 1991, 24, 135.
(46) Endre, B.; Bor, G.; Marta, M. S.; Gabor, M.; Bela, M.; Geza, S.
Veszpremi Vegyip. Egy. Kozl. 1957, 1, 63.

Reversible Insertion Reaction of Carbon Dioxide
Inorganic Chemistry, Vol. 35, No. 15, 1996 4413
Scheme 3
Scheme 4
would be expected to be much larger at ambient temperature,
it is unlikely that it would increase by more than 4 or 5 orders
of magnitude. If the rate of the reaction is assumed to increase
by a factor of 2 or 3 for every 10 °C increase in temperature,
the lower limit for the rate constant (k-2) would lie between
2.2 × 10-1 and 8.3 M-1 s-1 at 20 °C.
Scheme 5
In an experiment designed to examine whether W(CO)5OH-
catalyzes the exchange of oxygen atoms between H218O and
CO2, a sample of [Et4N][W(CO)5OH] in methanol, stabilized
by excess carbon monoxide, was stirred in excess H218O for
several hours. The resultant solution was placed under an
atmosphere of CO2 and agitated for over 1 h at ambient
temperature. The solution infrared spectrum of CO2 exhibited
no incorporation of oxygen-18, indicative either of very slow
H2O/OH- exchange or no W-O bond rupture during the
insertion process. Nevertheless, due to our inability to assess
the level of 18O incorporation in the W(CO)5OH- derivative,
this experiment is equivocal. An observation which indicates
the absence of metal-oxygen bond cleavage during the insertion
reaction has been reported for the carboxylation process
involving cobalt(III) amine hydroxide complexes (eq 6).47-49
Although the reaction of CO2 with the W-OH group of
W(CO)5OH- to provide the bicarbonate complex W(CO)5O2-
COH- is formally similar to the insertion reaction of W(CO)5-
OPh- with CO2 to afford the aryl carbonate, an important
mechanistic feature must be considered in the former process.
That is, in the intermediate formed by electrophilic addition of
CO2 at the nucleophilic hydroxide center two pathways are
open: a simple proton transfer (D) or CO2 insertion (E) (Scheme
4). Attempts to distinguish between these two scenarios were
not definitive due to our inability to assess the level of 18O
incorporation into the OH group of W(CO)5OH-. On the other
hand, the insertion process involving COS or CS2 where W-S
bond formation occurs clearly involves W-O bond rupture.
Contrary to this observation, the {η3-HB(3-But-5-Mepz)3}-
ZnOH complex has been shown to be both a structural and
functional model for carbonic anhydrase. That is, this mono-
meric zinc hydroxide complex serves as an effective catalyst
for the exchange of oxygen atoms between CO2 and H217O in
benzene solution.50 This observation clearly reflects the in-
volvement of the metal center in activating CO2 during its
reaction with the M-OH functionality. Indeed, it is this
polarization of the CO2 molecule by the metal center (Scheme
5) which is used to rationalize the much greater second-order
rate constant for the attack of CA upon CO2 (108 M-1 s-1) as
compared to the analogous rate constant for OH- attack (8.5 ×
103 M-1 s-1).51,52 However, it is clear that other factors, such
as indirect ligands, play a role in the catalytic efficiency of the
metalloenzyme system.53
L5Cos18OH2+ + CO2
9
8 L5Cos18OCO2H2+
(6)
Conclusions
In spite of the fact that the rate constant for carboxylation
(k-2) is so imprecisely defined at ambient temperature, it is
nevertheless clear that k-2 is several orders of magnitude larger
than k2 (the rate constant for decarboxylation). This observation
is consistent with the evidence that the equilibrium illustrated
in eq 1 lies far to the right in carbon dioxide saturated methanol.
It is important to note that the carboxylation reaction was
measured under an atmosphere of CO. Similar mechanistic
aspects for the insertion of CO2 into the W-OPh bond of
W(CO)5OPh- have been presented, although this latter process
is qualitatively slower. That is, the proposed mechanism
involves the prior interaction of the Lewis acid site of CO2 with
the lone pairs on the alkoxide oxygen followed by a concerted
transformation to a four-centered transition state (Scheme 3).
This transition state appears not to require an open coordination
site, as the rate of insertion is not slowed in the presence of
additional CO.43 Further studies on the sterically hindered
[W(CO)5O-2,6-Ph2C6H3]- complex found it to be unreactive
to CO2, although it did insert COS, where W-X interaction is
significant.
(47) Chaffee, E.; Dasgupta, T. P.; Harris, G. M. J. Am. Chem. Soc. 1973,
95, 4169.
Acknowledgment. The financial support of this research by
the National Science Foundation (Grant 91-19737) and the
Robert A. Welch Foundation is greatly appreciated.
(48) Palmer, D. A.; Harris, G. M. Inorg. Chem. 1974, 13, 965.
(49) Buckingham, D. A. In Biological Aspects of Inorganic Chemistry;
Dolphin, D., Ed.; John Wiley: New York, 1977; p 141.
(50) Looney, A.; Han, R.; McNeill, K.; Parkin, G. J. Am. Chem. Soc. 1993,
115, 4690.
(51) Brown, R. S. In Carbon Dioxide as a Source of Carbon, Biochemical
and Chemical Uses; Aresta, M., Forti, G., Eds.; Reidel Publishing
Co.: Dordrecht, The Netherlands, 1987; p 169.
(52) (a) Pocker, Z. Y.; Bjorkquist, D. W. J. Am. Chem. Soc. 1977, 99,
6537. (b) Paneth, P.; O’Leary, M. H. J. Am. Chem. Soc. 1985, 107,
7381.
Supporting Information Available: An ORTEP drawing of
[PPN]+ and tables of the atomic coordinates and equivalent isotropic
displacement coefficients, bond lengths and angles, and anisotropic
thermal parameters for [PPN][W(CO)5HCO3] (8 pages). Ordering
information is given on any current masthead page.
(53) Kiefer, L. L.; Paterno, S. A.; Fierke, C. A. J. Am. Chem. Soc. 1995,
117, 6831.
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