Kinetic Study of the Insertion and Deinsertion of Carbon Dioxide into fac-(CO)3(dppe)MnOR Derivatives

Article abstract of DOI:10.1021/om034087j

The insertion of carbon dioxide into the Mn - O bond of fac-(CO) ₃(dppe)MnOCH₃ (1) was observed to occur instantaneously at -78 °C by in situ infrared spectroscopy. The product of carboxylation of 1, fac-(CO)₃(dppe)MnOC(O)OCH₃ (2), underwent decar-boxylation with a first-order rate constant of 1.49 × 10 ^-4 CO₂ at 23 °C. The kinetic parameters for this process were determined by trapping the intermediate produced upon CO ₂ extrusion, complex 1, with COS to provide the very stable fac-(CO)₃(dppe)MnSC(O)OCH₃ (3) derivative. The structure of 3 was determined by single-crystal X-ray diffraction analysis, establishing the presence of the Mn - S bond.

Full text of DOI:10.1021/om034087j

Organometallics 2003, 22, 5585-5588
5585
Kin etic Stu d y of th e In ser tion a n d Dein ser tion of Ca r bon
Dioxid e in to fa c-(CO)3(d p p e)Mn OR Der iva tives
Donald J . Darensbourg,* Way-Zen Lee,† Andrea L. Phelps, and Erin Guidry
Department of Chemistry, Texas A&M University, College Station, Texas 77843
Received August 4, 2003
Summary: The insertion of carbon dioxide into the
Mn-O bond of fac-(CO)3(dppe)MnOCH3 (1) was ob-
served to occur instantaneously at -78 °C by in situ
infrared spectroscopy. The product of carboxylation of
1, fac-(CO)3(dppe)MnOC(O)OCH3 (2), underwent decar-
boxylation with a first-order rate constant of 1.49 × 10-4
s-1 at 23 °C. The kinetic parameters for this process were
determined by trapping the intermediate produced upon
CO2 extrusion, complex 1, with COS to provide the very
stable fac-(CO)3(dppe)MnSC(O)OCH3 (3) derivative. The
structure of 3 was determined by single-crystal X-ray
diffraction analysis, establishing the presence of the
Mn-S bond.
anionic group 6 metal carbonyl derivatives that this
insertion reaction does not require a coordination site
at the metal center. The reaction is proposed to involve
the interaction of the weakly electrophilic carbon center
of CO2 with the lone pair on the oxygen atom of the
alkoxide or aryloxide (OR groups).3 That is, a four-
centered transition state as depicted in A is suggested
In tr od u ction
A major focus of our current research is the catalytic
coupling of carbon dioxide and epoxides to produce
polycarbonates.1 We and others have employed a variety
of metal complexes as effective homogeneous catalysts
or catalyst precursors for this environmentally benign
synthesis of these biodegradable thermoplastics.2 Fun-
damental to understanding the intimate mechanistic
details of this process is a knowledge of the factors
influencing the insertion of carbon dioxide into metal-
oxygen bonds and its reverse deinsertion process (boxed
portion of eq 1). We have in the past established for
to occur along the insertion pathway. A similar conclu-
sion was reached for an analogous insertion reaction
involving CO2 and CS2 with neutral rhenium alkoxide
complexes by Simpson and Bergman.4 Consistent with
this interpretation, it was further demonstrated that the
insertion reaction was retarded in instances where there
was steric hindrance around the oxygen atom, whether
it be due to the steric bulk of R or an ancillary adjacent
ligand.5 However, there are few quantitative kinetic
measurements for the insertion or deinsertion of CO2
into M-OR bonds. In particular, it is of significance to
quantify the rate of the insertion reaction as a function
of the electronic nature of the R substituent on -OR.
In this communication we have chosen to investigate
well-characterized and well-behaved complexes of Mn-
(I), as illustrated in eq 2, where dppe ) bis(diphenyl-
phosphino)ethane.6 Although this system is not of
* To whom correspondence should be addressed. Fax: (979) 845-
0158. E-mail: djdarens@mail.chem.tamu.edu.
† Current address: Department of Chemistry, National Taiwan
Normal University, No. 88, Sec. 4, Ting-Chow Rd., Taipei, Taiwan 116,
ROC.
(1) For our most recent contribution to this subject, see: Darens-
bourg, D. J .; Rodgers, J . L.; Fang, C. C. Inorg. Chem. 2003, 42, 4498-
4500.
(2) (a) Darensbourg, D. J .; Holtcamp, M. W. Macromolecules 1995,
28, 7577-7579. (b) Super, M.; Berluche, E.; Costello, C.; Beckman, E.
Macromolecules 1997, 30, 368-372. (c) Super, M.; Beckman, E. J .
Macromol. Symp. 1998, 127, 89-108. (d) Cheng, M.; Lobkovsky, E.
B.; Coates, G. W. J . Am. Chem. Soc. 1998, 120, 11018-11019. (e)
Beckman, E. Science 1999, 283, 946-947. (f) Darensbourg, D. J .;
Holtcamp, M. W.; Struck, G. E.; Zimmer, M. S.; Niezgoda, S. A.; Rainey,
P.; Robertson, J . B.; Draper, J . D.; Reibenspies, J . H. J . Am. Chem.
Soc. 1999, 121, 107-116. (g) Darensbourg, D. J .; Wildeson, J . R.;
Yarbrough, J . C.; Reibenspies, J . H. J . Am. Chem. Soc. 2000, 122,
12487-12496. (h) Cheng, M.; Moore, D. R.; Reczek, J . J .; Chamberlain,
B. M.; Lobkovsky, B. E.; Coates, G. W. J . Am. Chem. Soc. 2001, 123,
8738-8749. (i) Cheng, M.; Darling, N. A.; Lobkovsky, E. B.; Coates,
G. W. Chem. Commun. 2000, 2007-2008. (j) Eberhardt, R.; Allmen-
dinger, M.; Luinstra, G. A.; Rieger, B. Organometallics 2003, 22, 211-
214. (k) Inoue, S. J . Polym. Sci.: A 2000, 38, 2861-2871. (l) Kruper,
W. J .; Dellar, D. V. J . Org. Chem. 1995, 60, 725-727. (m) Mang, S.;
Cooper, A. I.; Colclough, M. E.; Chauhan, N.; Holmes, A. Macromol-
ecules 2000, 33, 303-308. (n) Allen, S. D.; Moore, D. R.; Lobkovsky,
E. B.; Coates, G. W. J . Am. Chem. Soc. 2002, 124, 14284-14285. (o)
Darensbourg, D. J .; Lewis, S. J .; Rodgers, J . L.; Yarbrough, J . C. Inorg.
Chem. 2003, 42, 581-589. (p) Darensbourg, D. J .; Yarbrough, J . C. J .
Am. Chem. Soc. 2002, 124, 6335. (q) Eberhardt, R.; Allmendinger, M.;
Rieger, B. Macromol. Rapid Commun. 2003, 24, 194-196. (r) Darens-
bourg, D. J .; Yarbrough, J . C.; Ortiz, C.; Fang, C. C. J . Am. Chem.
Soc. 2003, 125, 7586-7591.
k1
fac-(CO)3(dppe)Mn(OCH3) + CO2 y
\
k z
2
fac-(CO)3(dppe)Mn-OC(O)OCH3 (2)
(3) Darensbourg, D. J .; Sanchez, K. M.; Reibenspies, J . H.; Rhein-
gold, A. L. J . Am. Chem. Soc. 1989, 111, 7094-7103.
(4) Simpson, R. D.; Bergman, R. G. Angew. Chem., Int. Ed. Engl.
1992, 31, 220.
(5) Darensbourg, D. J .; Mueller, B. L.; Bischoff, C. J .; Chojnacki, S.
S.; Reibenspies, J . H. Inorg. Chem. 1991, 30, 2418-2424.
10.1021/om034087j CCC: $25.00 © 2003 American Chemical Society
Publication on Web 11/15/2003

5586 Organometallics, Vol. 22, No. 26, 2003
Notes
insertion reaction, which was complete within 1 min,
an accurate value for [CO2] is not available. Neverthe-
less, it is certainly less than its concentration at
saturation of ∼16 M at -78 °C.7 Therefore, employing
a [CO2] value of 16 M and a t1/2 value for the reaction of
about 20 s allows us to calculate a lower limit for the
second-order rate constant (k1) of 2.0 × 10-3 M-1 s-1 at
-78 °C. This value for the rate constant is probably
grossly underestimated.
In a separate experiment a methylene chloride solu-
tion of complex 1 was stirred at ambient temperature
in the presence of an atmosphere of 13CO. Over an
extended period of time no CO exchange of the ligands
in complex 1 with free 13CO in solution occurred.
Furthermore, there was no indication of CO insertion
into the Mn-OCH3 bond under these conditions. This
observation is consistent with previous studies in this
area, which have demonstrated that CO2 insertion into
metal-oxygen bonds occurs in the absence of a coordina-
tion site for prior CO2 binding at the metal center.
Reaction of the less electron-rich trifluoroethoxy
derivative fac-(CO)3(dppe)MnOCH2CF3 with a saturated
methylene chloride solution of CO2 was very slow at
ambient temperature, whereas under these reaction
conditions no reaction of fac-(CO)3(dppe)MnO-2,6-di-
methylphenoxide was observed. This is to be contrasted
with the anionic tungsten carbonyl analogues, which
were shown to readily react with CO2 to afford the
corresponding alkyl and aryl carbonate derivatives.5
These observations add further credence to the premise
that a very nucleophilic metal-oxygen center is re-
quired for reaction with the poorly electrophilic carbon
dioxide molecule.
F igu r e 1. (A) Infrared traces in the ν(CO) region taken
every 1 min for complex 1 in methylene chloride at
-78 °C, before and after addition of CO2. Infrared bands
at 2007, 1928, and 1893 cm-1 are due to 1, and bands at
2026, 1955, and 1912 cm-1 are due to 2. (B) Absorbance of
the ν(CO) band at 2026 cm-1 due to the product of CO2
insertion, fac-(CO)3(dppe)MnOC(O)OCH3 (2), as a function
of time before and after addition of CO2.
catalytic importance, it allows for definitive assessment
of the kinetic parameters and factors affecting this class
of reactions. These observations should be highly trans-
ferable between various metal systems, including those
relevant to catalytic CO2/epoxide coupling reactions.
The kinetic parameters for the reverse process of
eq 2, decarboxylation of the fac-(CO)3(dppe)MnOC(O)-
OCH3 complex (2), can be readily obtained by monitor-
ing the reaction via two methods. The most obvious
methodology would be to measure the rate of the CO2
exchange process, as defined in eq 3, from either
Resu lts a n d Discu ssion
fac-(CO)3(dppe)MnO12C(O)OCH3 + 13CO2 h
fac-(CO)3(dppe)MnO13C(O)OCH3 + 12CO2 (3)
Mandal, Ho, and Orchin have previously reported that
fac-(CO)3(dppe)MnOCH3 (1) is very reactive toward CO2
to afford the corresponding methyl carbonate complex.6b
Indeed, complex 1 was found to absorb CO2 from the
air both in solution and in the solid state. Consistent
with these observations, we found that it was necessary
to rigorously exclude adventitious sources of carbon
dioxide during the preparation and isolation of complex
1 to avoid premature formation of fac-(CO)3(dppe)-
MnOC(O)OCH3 (2). In an attempt to quantify the rate
of carbon dioxide insertion into 1 in methylene chloride
at -78 °C using an in situ infrared probe (see the
Supporting Information), we were only able to establish
a lower limit for the insertion rate. That is, when a CH2-
Cl2 solution of 1 at -78 °C was exposed to an atmo-
sphere of bone-dry CO2, the instantaneous transforma-
tion 1 f 2 occurred. Figure 1 illustrates the infrared
traces in the ν(CO) region of complex 1 prior to the
addition of CO2 and of complex 2 immediately after
addition of CO2. This experiment was performed by
briefly bubbling CO2 into a cooled solution of complex 1
via a syringe needle. Because of the rapidity of the
direction. This procedure would require monitoring of
the ν(CO2) bands directly as shifted upon isotopic
substitution. These vibrational modes are less intense
than the ν(CO) stretches. Furthermore, there could be
complications from the reverse process: i.e., eq 3 does
not have a driving force. An alternative and more
sensitive approach is to take advantage of the fact that
COS extrusion from fac-(CO)3(dppe)MnSC(O)OCH3 (3)
is not a facile process. Hence, we have determined the
rate of CO2 extrusion from complex 2 by trapping the
decarboxylated complex 1 in the presence of a large
excess of COS (Scheme 1).
Complex 3 has been independently synthesized and
isolated in crystalline form for X-ray analysis. The
complex crystallized, along with one molecule of meth-
ylene chloride, in the space group P21/c, as did the
previously reported methoxy carbonate analogue.6b
A
thermal ellipsoid drawing of 3 is depicted in Figure 2.
The molecular parameters in 3 are much like those
(6) (a) Mandal, S. K.; Ho, D. M.; Orchin, M. Inorg. Chem. 1991, 30,
2244-2248. (b) Mandal, S. K.; Ho, D. M.; Orchin, M. Organometallics
1993, 12, 1714-1719.
(7) Endre, B.; Bor, G.; Marta, M. S.; Gabor, M.; Bela, M.; Geza, S.
Veszpremi Vegyip. Egy. Kozl. 1957, 1, 89-98.

Notes
Organometallics, Vol. 22, No. 26, 2003 5587
Sch em e 1
F igu r e 3. (A) Stack plot of infrared spectra as a function
of time for the reaction described in Scheme 1. Infrared
bands at 2026, 1955, and 1912 cm-1 are due to 2, whereas
bands at 2020, 1955, and 1918 cm-1 are due to 3. The band
for free COS is seen at 2043 cm-1. (B) Absorbance vs time
plot, as well as ln(absorbance) vs time plot for the disap-
pearance of complex 2.
Ta ble 1. Tem p er a tu r e-Dep en d en t Ra te Con sta n ts
a n d Activa tion P a r a m eter s for Deca r boxyla tion of
Com p lex 2a
F igu r e 2. Thermal ellipsoid representation of complex 3,
fac-(CO)3(dppe)Mn-SC(O)OCH3.
temp (K)
105k2 (s-1
)
∆Hq (kJ /mol)
∆Sq (J /(mol K))
273.1
282.9
290.9
296.2
299.8
0.185
1.16
6.92
14.9
33.5
130.0 ( 3.4
121.6 ( 11.9
reported for complex 2. That is, the average Mn-P and
Mn-COeq distances are 2.333(3) and 1.815(11) Å, re-
spectively, in 3, as compared with 2.34(1) and 1.82(1) Å
in 2.
a
Reactions carried out in methylene chloride solution.
Similarly, the Mn-COax distance of 1.763(11) Å is
shorter than the average Mn-COeq distance and is not
significantly affected by being located trans to S as
opposed to O in 2, where the corresponding distance was
determined to be 1.769(5) Å. The Mn-S bond length was
found to be 2.363(3) Å, which is slightly shorter than
the sum of the covalent radii (2.48 Å) of manganese and
sulfur. A more complete listing of bond distances and
bond angles in complex 3 may be found in the Support-
ing Information.
Figure 3 illustrates the three-dimensional stack plot
of the infrared spectra taken in the ν(CO) and ν(COS)
regions as a function of time, where the absorbance at
2043 cm-1 is due to COS. The carbonyl stretching
vibrations in complex 2 at 2026, 1955, and 1912 cm-1
shift to 2020, 1955, and 1918 cm-1 upon replacing CO2
with COS to afford 3. The peak at 2026 cm-1 in complex
2 was used to calculate the first-order rate constants
for 2 f 3 in the presence of a large excess of COS. Figure
3B depicts a typical trace for the disappearance of
complex 2 as a function of time at 23 °C, along with a
plot of its integrated form. The temperature-dependent
first-order rate constants (k2) and the derived enthalpy
and entropy of activation values are listed in Table 1.
As can be seen from these data, the release of CO2 from
complex 2 is accompanied by a large positive entropy
of activation (121.6 J /(mol K)), which is consistent with
the loss of a stable small molecule. An extrapolation,
with significant inaccuracy, of the data in Table 1 to
-78 °C yields a k2 value of 6.6 × 10-17 s-1. Combining
this k2 with the estimated k1 for the carboxylation
reaction provides an equilibrium constant for 1 +
CO2 / 2 of g3 × 1013 M-1 at -78 °C (or a ∆G value for
reaction 2 of <-50 kJ /mol.). Hence, CO2 insertion into
the Mn-OCH3 bond of complex 1 is kinetically and
thermodynamically a highly favored process. Neverthe-
less, the kinetics of this reaction in general are ex-
tremely sensitive to the electronic nature of the sub-
stituents on the oxygen donor.
Su m m a r y
It is possible to glean some lessons from these and
related studies relevant to the enchainment of CO2 in
the copolymerization of CO2 and epoxides to provide
polycarbonates. Important among these is more con-
vincing evidence that CO2 insertion into M-OR bonds
does not require a site for CO2 binding prior to insertion.

5588 Organometallics, Vol. 22, No. 26, 2003
Notes
Ta ble 2. X-r a y Cr ysta llogr a p h ic Da ta for
An even more pertinent observation is the dramatic
differences in relative rates of CO2 insertion into the
Mn-OR moiety as a function of the R substituent, i.e.,
R ) Me (very fast) > R ) CH2CF3 (very slow) > R )
O-2,6-Me2C6H3 (not at all). Although these absolute
rates will vary with the metal and its ancillary ligands,
the relative order of reactivity is not expected to be
altered. For example, Simpson and Bergman found fac-
(CO)3(PMe3)2ReOCH3 to undergo carboxylation with
CO2 at -40 °C in less than 5 min, whereas the cresolate
analogue was unreactive toward CO2.4 By way of
contrast, the anionic aryloxide and fluoroalkoxide de-
rivatives of tungsten carbonyl readily insert CO2 at
ambient temperature.5,8 These results strongly suggest
that epoxides bearing electron-withdrawing substitu-
ents in close proximity to the oxygen will be poor
substrates for completely alternating copolymerization
processes with carbon dioxide. Indeed, Rokicki and
Kuran have reported the order of increasing oxirane
reactivity in the copolymerization reaction with CO2
fa c-(CO)3(d p p e)Mn SC(O)OCH3 (3)
empirical formula
fw
temp (K)
wavelength (Å)
space group
a (Å)
C31H27MnO5P2S‚CH2Cl2
713.39
110(2)
0.710 73
P21/c
8.761(2)
14.905(3)
25.230(5)
94.05
3286.4(12)
4
1.442
0.764
8.89
b (Å)
c (Å)
â (deg)
V (Å)3
Z
Dcalcd (g cm-3
)
abs coeff (mm-1
)
Ra
Rw
b
15.40
1.028
goodness of fit on F2
R )∑|Fo| - |Fc|/∑|Fo|. Rw) {[∑w(Fo - Fc2)2]/[∑w(Fo2)2]}1/2
.
a
b
2
pin. The crystal was then placed in a cold nitrogen stream
maintained at 110 K. The X-ray data were obtained on a
Bruker CCD diffractometer and covered more than a hemi-
sphere of reciprocal space by a combination of three sets of
exposures; each exposure set had a different angle æ for the
crystal orientation, and each exposure covered 0.3° in ω. The
crystal-to-detector distance was 5.0 cm. Crystal decay was
monitored by repeating the data collection for initial frames
at the end of the data set and analyzing the duplicate
reflections; crystal decay was negligible. The space group was
determined on the basis of systematic absences and intensity
statistics.
Crystal data and details of data collection are provided in
Table 2. The structure was solved by direct methods (SHELXS,
SHELTXL-PLUS program package, Sheldrick (1997)). Full-
matrix least-squares anisotropic refinement for all non-
hydrogen atoms yielded R(F) and Rw(F2) values as indicated
in Table 2 at convergence. All H atoms were placed at idealized
positions and refined with fixed isotropic displacement pa-
rameters equal to 1.2 (1.5 for methyl protons) times the
equivalent isotropic displacement parameters of the atoms to
which they were attached. Neutral atom scattering factors and
anomalous scattering factors were taken from Volume C of the
International Tables for X-ray Crystallography.
employing zinc-based catalysts as follows: ClCH2
<
C6H5CH2 < C6H5 < n-C4H9OCH2 < H < CH3.9
Exp er im en ta l Section
All syntheses were carried out under argon using standard
Schlenk and glovebox techniques. Solvents were distilled from
the appropriate reagents before use. fac-(CO)3(dppe)MnOCH3
and fac-(CO)3(dppe)MnOC(O)OCH3 were prepared according
to the literature.6 Routine infrared spectra were measured on
a Mattson Galaxy 6021 FT-IR spectrometer using a 0.1 mm
CaF2 sealed cell. NMR spectra were recorded on a Varian
Unity Plus 300 MHz spectrometer. Kinetic measurements
were monitored on ASI’s ReactIR 1000 system equipped with
an MCT detector and 30 bounce SiCOMP in situ probe.
The kinetic experiments were performed over the temper-
ature range of 10-23 °C using 0.15 mmol of fac-(CO)3(dppe)-
MnOC(O)OCH3 in 15 mL of dichloromethane and a large
excess of COS. One spectrum was collected every 1 min for
5 h.
P r ep a r a tion of fa c-(CO)3(d p p e)Mn SC(O)OCH3 (3). A
sample of fac-(CO)3(dppe)MnOCH3 dissolved in methylene
chloride was stirred at ambient temperature over an atmo-
sphere of COS for approximately 30 min. The solvent was
removed under vacuum, and the yellow product was isolated.
Crystals suitable for X-ray diffraction were grown by slow
evaporation of a concentrated methylene chloride solution of
3 at ambient temperature. Anal. Calcd for C31H27MnO5P2S‚CH2-
Cl2 (713.43): C, 53.87; H, 4.10. Found: C, 54.01; H, 4.03.
X-r a y Cr ysta llogr a p h y. A crystal of complex 3 was coated
with a cryogenic protectant (i.e. Paratone) and mounted on a
glass fiber, which in turn was fashioned to a copper mounting
Ack n ow led gm en t. Financial support from the Na-
tional Science Foundation (Grant Nos. CHE 99-10342
and CHE 02-34860, and Grant No. CHE 98-07975 for
purchase of X-ray equipment) and the Robert A. Welch
Foundation is greatly appreciated.
Su p p or tin g In for m a tion Ava ila ble: Complete details of
the X-ray diffraction study of complex 3, along with a table of
selected bond distances and angles and a figure of the in situ
infrared probe setup for the carbon dioxide insertion reaction
study; crystallographic data are also available as CIF files.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(8) Darensbourg, D. J .; Mueller, B. L.; Reibenspies, J . H.; Bischoff,
C. J . Inorg. Chem. 1990, 29, 1789-1791.
(9) Rokicki, A.; Kuran, W. J . Macromol. Sci. Rev. Macromol. Chem.
1981, C21, 135-186.
OM034087J