Structural characterization of several (CO)3(dppp)MnX derivatives, dppp = 1,3-bis(diphenylphosphino)propane and X = H, OTs, OC 2H5, Cl, Br, or N3. An assessment of their efficacy for catalyzing the coupling of carbon dioxide and epoxides

Article abstract of DOI:10.1021/om049352v

The X-ray structures of a series of (CO)₃(dppp)MnX complexes, X = H, OTs, OEt, Cl, Br, and N₃, and dppp = 1,3-bis(diphenylphosphino)propane, prepared by literature methods are reported. Several of these derivatives have been examined for their ability to serve as catalysts for the coupling of cyclohexene oxide and carbon dioxide. Although, these organometallic complexes do catalyze the formation of polycarbonate, their activity is not competitive with other very effective catalysts for this coupling reaction. Nevertheless, findings from these studies contribute significantly to our understanding of this important process. The complex containing the alkoxide ligand is the most active, yet it possesses a TON (mol epoxide consumed/mol Mn) of only 50 for a 24 h reaction. In this instance the (CO)₃(dppp)MnOEt complex exists as the carbonate species, (CO) ₃(dppp)MnOC(O)OEt, because of rapid CO₂ insertion into the Mn-OEt bond. The order of epoxide ring-opening by the X group (initiator) was found to be -OC(O)OR > Cl ≥ N₃ > Br. Furthermore, since these complexes are substitutionally inert, this process is best defined as occurring via an associative interchange process and is rate-limiting.

Full text of DOI:10.1021/om049352v

Organometallics 2004, 23, 6025-6030
6025
Structural Characterization of Several (CO)3(dppp)MnX
Derivatives, dppp ) 1,3-Bis(diphenylphosphino)propane
and X ) H, OTs, OC2H5, Cl, Br, or N3. An Assessment of
Their Efficacy for Catalyzing the Coupling of Carbon
Dioxide and Epoxides
Donald J. Darensbourg,* Poulomi Ganguly, and Damon R. Billodeaux
Department of Chemistry, Texas A&M University, College Station, Texas 77843
Received August 19, 2004
The X-ray structures of a series of (CO)3(dppp)MnX complexes, X ) H, OTs, OEt, Cl, Br,
and N3, and dppp ) 1,3-bis(diphenylphosphino)propane, prepared by literature methods are
reported. Several of these derivatives have been examined for their ability to serve as
catalysts for the coupling of cyclohexene oxide and carbon dioxide. Although, these
organometallic complexes do catalyze the formation of polycarbonate, their activity is not
competitive with other very effective catalysts for this coupling reaction. Nevertheless,
findings from these studies contribute significantly to our understanding of this important
process. The complex containing the alkoxide ligand is the most active, yet it possesses a
TON (mol epoxide consumed/mol Mn) of only 50 for a 24 h reaction. In this instance the
(CO)3(dppp)MnOEt complex exists as the carbonate species, (CO)3(dppp)MnOC(O)OEt,
because of rapid CO2 insertion into the Mn-OEt bond. The order of epoxide ring-opening
by the X group (initiator) was found to be -OC(O)OR > Cl g N3 > Br. Furthermore, since
these complexes are substitutionally inert, this process is best defined as occurring via an
associative interchange process and is rate-limiting.
Introduction
The catalytic coupling of CO2 and epoxides was first
reported by Inoue and co-workers in 1969, employing a
heterogeneous catalyst derived from Zn(CH2CH3)2 and
H2O.7 More recently, this process has received a great
deal of attention utilizing well-defined, more active,
homogeneous catalysts.8 As noted in eq 1, successive
ether enchainment leading to polyether linkages in the
copolymer is an undesirable reaction that accompanies
the completely alternating copolymerization process.
The coupling reaction of carbon dioxide and epoxides
to provide polycarbonates or cyclic organic carbonate
represents one of the most promising reactions for the
large-scale utilization of this low-cost C1 feedstock (eq
1).1 These CO2-derived products are extremely useful
materials, and this synthetic route affords an environ-
mentally benign alternative pathway to their production
than that commonly employed.2 That is, polycarbonates
are a class of thermoplastics highly regarded for their
optical clarity, exceptional impact resistance, and duc-
tility.2 Similarly, cyclic organic carbonates are of interest
for use in a variety of applications, including high-
boiling solvents,3 additives for hydraulic fluids,4 and the
curing of phenol-formaldehyde resins,5 plus many more.6
A fundamental step in the CO2/epoxide coupling
reaction is insertion of carbon dioxide into the metal-
alkoxide bond after the epoxide ring-opening process.
Indeed, in some instances this insertion reaction can
be rate-limiting.9 Hence, transition metal alkoxides are
of particular interest as models in this regard since they
* To whom correspondence should be addressed. Fax: (979) 845-
0158. E-mail: djdarens@mail.chem.tamu.edu.
(1) (a) Inoue, S. Carbon Dioxide as a Source of Carbon; Aresta, M.,
Forti, G., Eds.; Reidel Publishing Co.: Dordrecht, 1987; p 331. (b)
Darensbourg, D. J.; Holtcamp, M. W. Coord. Chem. Rev. 1996, 153,
155-174. (c) Rokicki, A.; Kuran, W. J. Macromol. Sci., Rev. Macromol.
Chem. 1981, C21, 135. (d) Super, M. S.; Beckman, E. J. Trends Polym.
Sci. 1997, 5, 236. (e) For a comprehensive recent review of the area
see: Coates, G. W.; Moore, D. R. Angew. Chem., Int. Ed. 2004, in press.
(2) Engineering Thermoplastics: Polycarbonates, Polyacetals, Poly-
esters, Cellulose Esters; Bottenbruch, L., Ed.; Hanson Publishing: New
York, 1996; p 112.
(7) Inoue, S.; Koinuma, H.; Tsuruta, T. J. Polym. Sci., Part B:
Polym. Phys. 1969, 7, 287.
(8) See very recent publications and references therein: (a) Darens-
bourg, D. J.; Yarbrough, J. C.; Ortiz, C.; Fang, C. C. J. Am. Chem.
Soc. 2003, 125, 7586-7591. (b) Darensbourg, D. J.; Mackiewicz, R. M.;
Rodgers, J. L.; Phelps, A. L. Inorg. Chem. 2004, 43, 1831-1833. (c)
Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. Angew.
Chem., Int. Ed. 2002, 41, 2599-2602. (d) Moore, D. R.; Cheng, M.;
Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2003, 115, 11911-
11924. (e) Eberhardt, R.; Allmendinger, M.; Rieger, M. Macromol.
Rapid Commun. 2003, 24, 194-196. (f) Lu, X.-B.; Wang, Y. Angew.
Chem., Int. Ed. 2004, 43, 3574-3577.
(3) Behr, A. Carbon Dioxide Activation by Metal Complexes; VCH:
Weinheim, 1988; p 7.
(4) Nankee, R. J.; Avery, J. R.; Schrems, J. E. Dow Chem. Patent
FR. 15,72,282, 1967; Chem. Abstr. 1970, 72, 69016p.
(5) Pizzi, H.; Stephanou, A. J. Appl. Polym. Sci. 1993, 49, 2157.
(6) Shaikh, A.-A. G.; Sivaram, S. Chem. Rev. 1996, 96, 951-976.
10.1021/om049352v CCC: $27.50 © 2004 American Chemical Society
Publication on Web 11/10/2004

6026 Organometallics, Vol. 23, No. 25, 2004
Darensbourg et al.
published procedure.15 6a (0.05 g, 0.1107 mmol) was added
with 30 mL of CH3CN to a suspension of excess NaN3 in CH3-
CN. The mixture was stirred overnight and filtered, and the
solvent was removed under reduced pressure. The spectro-
scopic data agreed with those previously reported.13 X-ray
quality crystals were grown by a slow diffusion of hexane to a
CH2Cl2 solution of 6.
undergo insertion of small molecules such as CO2 or CS2
into the metal-oxygen bond.10 For example, Orchin and
co-workers have reported the synthesis of octahedral
manganese alkoxide complexes of the general formula
(CO)3(dppp)MnOR and have examined their ability to
reversibily insert CO2 into the Mn-OR bond to give the
corresponding carbonate complexes.11 In situ infrared
spectroscopic kinetic studies by our group showed that
the insertion of carbon dioxide into the Mn-OR bond
occurred instantaneously at -78 °C via a concerted
mechanism.12 Because of the rapidity of this process,
only a lower limit for the second-order constant could
be established of 2.0 × 10-3 M-1 s-1 at -78 °C. This
finding prompted us to wonder whether these and other
(CO)3(dppp)MnX derivatives could serve as catalysts or
catalyst precursors for the CO2 epoxide coupling reac-
tion. In this report we will present our observations on
the subject along with mechanistic implications. In-
cluded in this study are the X-ray crystallographically
defined structures of these (CO)3(dppp)MnX (X ) H,
OTs, OC2H5, Cl, Br, and N3) derivatives.
X-ray Diffraction Studies. A Bausch and Lomb 10×
microscope was used to identify suitable crystals from a
representative sample of crystals of the same habit. Crystals
were coated with mineral oil, placed on a glass fiber, and
mounted on a Bruker SMART 1000 CCD diffractometer. X-ray
data were collected covering more than a hemisphere of
reciprocal space by a combination of three sets of exposures.
Each exposure had a different æ angle for the crystal orienta-
tion, and each exposure covered 0.3° in ω. The crystal to
detector distance was 4.9 cm. Decay was monitored by repeat-
ing collection of the initial 50 frames collected and analyzing
the duplicate reflections. Crystal decay was negligible. The
space group was determined on the basis of systematic
absences and intensity statistics. The structure was solved by
direct methods and refined by full-matrix least squares on F2.
All non-hydrogen atoms were refined with anisotropic dis-
placement parameters. All H atoms were placed in idealized
positions with fixed isotropic displacement parameters equal
to 1.5 times (1.2 for methyl protons) the equivalent isotropic
displacement parameters of the atom to which they are
attached.
The following programs were used: data collection and
cell refinement, SMART;16 data reduction, SAINTPLUS
(Bruker17); programs used to solve structures, SHELXS-97
(Sheldrick18); programs used to refine structures, SHELXL-
99 (Sheldrick19); molecular graphics and publication materials,
SHELXTL-Plus version 5.0 (Bruker20).
Copolymerization of CO2 and Epoxides. A typical
reaction was carried out using the following protocol. A 50 mg
sample of the catalyst was dissolved in 20 mL of cyclohexene
oxide and injected via inlet port into a Parr autoclave. The
reactor was subsequently charged to 500 psi with bone dry
CO2 and stirred at 80 °C for 24 h. After this time the autoclave
was cooled and the CO2 vented in a fume hood. The reactor
was opened, and the polymer was isolated by dissolution in
small amounts of methylene chloride followed by precipitation
from methanol.
Experimental Section
All syntheses were carried out under argon atmosphere
using standard Schlenk and glovebox techniques. Solvents
were distilled from appropriate reagents before use. All
reagents were commercially available and used without fur-
ther purification unless otherwise indicated. The synthetic
precursors Mn2(CO)10 and 1,3-bis(diphenylphosphino)propane
were purchased from Aldrich Chemical Co. Bone dry CO2 was
purchased from Scott Specialty Gases. Infrared spectroscopy
1
were recorded using a Mattson 6021 FTIR spectrometer. H
NMR spectra were recorded on a 300 MHz Varian Unity Plus
spectrometer.
Preparation of fac-(CO)3(dppp)MnH (1). The methodol-
ogy employed in this synthesis was identical to that reported
by Orchin and co-workers.11 X-ray quality crystals were grown
by the slow diffusion of hexane to a benzene solution of 1.
Preparation of fac-(CO)3(dppp)MnOTs (2). 2 was syn-
thesized by a previously published procedure.11 Yellow-orange
crystals were obtained from the diffusion of hexane to a
concentrated solution of 2 in CH2Cl2.
Preparation of fac-(CO)3(dppp)MnOC2H5 (3). A 1.0 g
(1.72 mmol) sample of fac-(CO)3(dppp)MnOCH3 (3a), the
synthesis of which has already been reported, was slurried
with 25 mL of dry ethanol and stirred for 2 h. After 2 h the
yellow solid was collected by filtration and washed with 5 mL
of hexane.11 X-ray quality crystals were obtained by slow
diffusion of hexane into a concentrated benzene solution of 3.
Preparation of fac-(CO)3(dppp)MnCl (4) and fac-(CO)3-
(dppp)MnBr (5). The syntheses of 4 and 5 have been reported
in the literature previously.13,14 Crystals were grown by slow
evaporation of a concentrated CH2Cl2 solution of the respective
complex into toluene.
Characterization was accomplished by 1H NMR and IR
spectroscopy. The amount of ether linkages was determined
via 1H NMR by integrating peaks corresponding to the methine
protons of the polyether at ∼3.45 ppm and the polycarbonate
at ∼4.6 ppm.
Results and Discussion
The syntheses of all of the (CO)3(dppp)MnX complexes
utilized in this study have been previously reported in
the literature.11,13-15 Most of these complexes originate
from the hydride derivative, (CO)3(dppp)MnH (1), which
is synthesized via the process depicted in eq 2. Reactions
3-6 indicate the routes to the other Mn(I) complexes of
interest. In all instances we were able to find suitable
Preparation of fac-(CO)3(dppp)MnN3 (6). fac-[(CO)3-
(dppp)Mn(OH2)]BF4 (6a) was synthesized by a previously
(9) Darensbourg, D. J.; Mackiewicz, R. M. Billodeaux, D. R. Orga-
nometallics 2004, in press.
(10) (a) Darensbourg, D. J.; Sanchez, K. M.; Reibenspies, J. H.;
Rheingold, A. L. J. Am. Chem. Soc. 1989, 111, 7094-7103. (b) Simpson,
R. D.; Bergman, R. G. Angew. Chem., Int. Ed. Engl. 1992, 31, 220. (c)
Darensbourg, D. J.; Mueller, B. L.; Bischoff, C. J.; Chojnacki, S. S.;
Reibenspies, J. H. Inorg. Chem. 1991, 30, 2418-2424.
(11) Mandel, S. K.; Ho, D. M.; Orchin, M. Organometallics 1993,
12, 1714-1719.
(15) Becker, T. M.; Orchin, M. Polyhedron 1999, 18, 2563-2571.
(16) SMART 1000 CCD; Bruker Analytical X-ray Systems: Madison,
WI, 1999.
(17) SAINT-Plus, version 6.02; Bruker: Madison, WI, 1999.
(18) Sheldrick, G. SHELXS-97: Program for Crystal Structure
Solution; Institut fur Anorganische Chemie der Universitat: Gottingen,
Germany, 1997.
(12) Darensbourg, D. J.; Lee, W.-Z.; Phelps, A. L.; Guidry, E.
Organometallics 2003, 22, 5585-5588.
(13) Li, G. Q.; Feldman, J. Polyhedron 1997, 16, 2041-2045.
(14) Riera; V. J. Organomet. Chem. 1981, 205, 371-379.
(19) Sheldrick, G. SHELXL-99: Program for Crystal Structure
Refinement; Institut fur Anorganische Chemie der Universitat: Got-
tingen, Germany, 1999.
(20) SHELXTL, version 5.0; Bruker: Madison, WI, 1999.

(CO)3(dppp)MnX Derivatives
Organometallics, Vol. 23, No. 25, 2004 6027
Table 1. Crystallographic Data and Data Collection Parameters for Complexes 1-6
1
2
3
4
5
6
empirical formula
cryst syst
space group
volume (Å3)
a (Å)
C
30H27MnO3P2
C
37H33MnO6P2S C32H31MnO4P2
C
30H26ClMnO3P2
C
30H26BrMnO3P2
C
30H26N3MnO3P2
triclinic
P1h
monoclinic
P2(1)/n
3385.7(17)
10.539(3)
14.793(4)
21.716(6)
90
triclinic
P1h
monoclinic
P2(1)/n
2696.8(8)
9.9382(18)
20.643(3)
13.666(3)
90
monoclinic
P2(1)/n
2739.4(19)
10.073(4)
20.612(8)
13.765(6)
90
monoclinic
P2(1)/c
3208.7(12)
12.826(3)
17.209(4)
15.102(3)
90
2634.3(15)
11.017(4)
15.451(5)
15.773(5)
95.888(6)
90.886(6)
99.3047(7)
110(2)
1.393
1443.7(12)
7.921(4)
10.031(5)
18.271(9)
94.634(8)
93.541(9)
91.340(9)
110(2)
b (Å)
c (Å)
R (deg)
â (deg)
90.534(5)
90
105.871(3)
90
106.551(7)
90
105.727(4)
90
γ (deg)
temperature (K)
dcalc (g/cm3)
Z
110(2)
110(2)
273(2)
110(2)
1.418
1.372
1.445
1.531
1.39
4
4
2
4
4
4
µ (mm-1
)
0.653
0.592
0.604
0.738
2.088
0.552
no. of reflns collected
no. of indep reflns
no. of params
11 907
7558
14 890
4885
6428
12 136
3879
12 140
3954
14 172
4611
4098
657
1.051
425
1.094
R1) 0.0382
wR2) 0.0437
353
334
0.968
R1) 0.0730
wR2)0.1430
334
1.159
R1) 0.0598
wR2) 0.0737
406
1.08
R1) 0.0623
wR2) 0.0875
goodness-of-fit on F2
1.043
final R indices [I>2σ(I)] R1) 0.0798
R1) 0.0726
wR2) 0.1017
R indices (all data)
wR2) 0.1078
Table 2. Selected Bond Lengths (Å) and Angles
conditions to obtain crystalline samples for carrying out
X-ray crystallographic studies of these derivatives.
(deg) for Complexes 1-6
Complex 1
Mn2(CO)10 + dppp C H OH8 (CO)3(dppp)MnH (2)
Mn(1A)-H(1A)
Mn(1A)-P(1A)
Mn(1A)-P(2A)
P(2A)-Mn-P(1A)
C(2A)-Mn-H(1A)
Mn(1B)-H(1B)
Mn(1B)-P(1B)
Mn(1B)-P(2B)
P(2B)-Mn-P(1B)
C(2B)-Mn-H(1B)
1.735(19)
2.2881(18)
2.2927(17)
89.42
Mn(1A)-C(1A)
Mn(1A)-C(2A)
Mn(1)-C(3A)
1.8120(6)
1.837(6)
1.814(7)
3
7
1
175.4(19)
1.71(6)
NaOCH3
Mn(1B)-C(1B)
Mn(1)-C(2B)
Mn(1)-C(3B)
1.814(7)
1.809(5)
1.812(6)
1 + p-TsOH f (CO)3(dppp)MnOTs
8
(3)
2.3047(16)
2.2828(18)
89.42
2
(CO)3(dppp)MnOCH3 C H OH8 (CO)3(dppp)MnOC2H5
2
5
175.4(19)
Complex 2
3a
3
Mn(1)-O(1)
O(1)-S(1)
S(1)-O(2)
S(1)-O(3)
S(1)-C(28)
2.092(2)
1.495(2)
1.439(2)
1.431(2)
1.769(3)
Mn(1)-C(27)
Mn(1)-C(26)
Mn(1)-C(36)
1.774(3)
1.833(3)
1.834(3)
1 + CCl4 f (CO)3(dppp)MnCl
(4)
4
C(26)-Mn(1)-O(1) 178.27(11) O(1)-S(1)-C(28) 102.78(12)
BrMn(CO)5 + dppp f (CO)3(dppp)MnBr (5)
O(3)-S(1)-O(2)
115.42(14)
5
Complex 3
Mn(1)-O(4)
2.037(3)
1.394(7)
1.534(8)
117.7(3)
90.54(6)
Mn(1)-C(1)
Mn(1)-C(2)
Mn(1)-C(3)
1.841(7)
1.832(6)
1.789(6)
1 + HBF4‚Et2O f
O(4)-C(31)
NaN3
C(31)-C(32)
C(31)-O(4)-Mn(1)
P(1)-Mn(1)-P(2)
[(CO)3(dppp)Mn(OH2)]BF4
8 (CO)3(dppp)MnN3
6a
6
(6)
Complex 4
2.379(2)
Mn(1)-Cl(1)
Mn(1)-C(1)
Mn(1)-C(2)
C(2)-Mn-Cl(1)
Mn(1)-C(3)
Mn(1)-C(3)
1.802(9)
1.821(6)
The structures of molecules 1-6 are indicative of a
manganese atom octahedrally coordinated to three
(facial) terminal carbonyl groups, a chelating 1,3-bis(di-
phenylphosphino)propane (dppp) ligand, and a nucleo-
phile (X). The latter group is trans to one carbonyl and
has a cis orientation to the remaining two carbonyls as
well as the chelating ligand, as has been observed by
Orchin and co-workers for one such derivative.11 Table
1 contains crystallographic and structure refinement
data for complexes 1-6. All thermal ellipsoid drawings
are at 50% probability, and in general the hydrogen
atoms have been omitted for clarity, with the notable
exception of the hydride in complex 1. The relevant bond
distances and bond angles based on the atomic number-
ing scheme in these figures are listed in Table 2. The
molecular structure of 1 is provided in Figure 1.
Complex 1 crystallized in the space group P1h with two
molecules in the asymmetric unit. The position of the
hydrogen atom was located by a difference map to define
the average Mn-H bond length of 1.72 Å.
1.814(9)
1.754(9)
174.9(3)
Complex 5
2.5189(11)
1.816(6)
1.783(6)
174.02(17)
Mn(1)-Br(1)
Mn(1)-C(1)
Mn(1)-C(2)
C(2)-Mn-Br(1)
Complex 6
Mn(1)-N(1)
2.126(3)
1.164(5)
1.200(6)
117.2(3)
175.4(4)
Mn(1)-C(1)
Mn(1)-C(2)
Mn(1)-C(3)
1.816(5)
1.832(5)
1.799(5)
N(1)-N(2)
N(2)-N(3)
N(2)-N(1)-Mn(1)
N(1)-N(2)-N(3)
Complex 2, (CO)3(dppp)MnOTs, crystallized in the
space group P21/n. A thermal ellipsoid representation
of 2 is shown in Figure 2. Selected bond distances and
bond angles are tabulated in Table 2. Notably, the Mn-
tosylate bond length was found to be 2.092(2) Å, with
the tosylate S(1)-O(1) bond distance of 1.495(2)Å being

6028 Organometallics, Vol. 23, No. 25, 2004
Darensbourg et al.
Figure 1. Thermal ellipsoid representation of fac-(CO)3-
(dppp)MnH, 1.
Figure 3. Thermal ellipsoid representation of fac-(CO)3-
(dppp)MnOC2H5, 3.
Figure 4. Thermal ellipsoid representation of fac-(CO)3-
(dppp)MnCl, 4.
Figure 2. Thermal ellipsoid representation of fac-(CO)3-
(dppp)MnOTs, 2.
slightly longer than the S(1)-O(2) and S(1)-O(3) dis-
tances, which average 1.435 Å. The sulfur-phenyl ring
(S(1)-C(28)) bond length was determined to be 1.769(3)
Å. As expected, the two types of carbonyl ligands have
different Mn-C distances, with the Mn-COaxial bond
length of 1.774(3) Å shorter than the Mn-COeq average
distance of 1.833(3) Å.
We were extremely fortunate to isolate in crystalline
form and characterize via X-ray crystallography the
(CO)3(dppp)MnX derivative where X ) ethoxide. In
general the alkoxide group in the absence of electron-
withdrawing substituents affords complexes that are
extremely reactive toward trace quantities of CO2 in the
atmosphere even in the solid state.12 Typically, these
derivatives are characterized in the solid state as their
CO2-inserted product. For example, the structure of the
methoxide analogue of 3 was reported as its alkoxycar-
bonate derivative, (CO)3(dppp)MnOC(O)OCH3.11 Com-
plex 3 crystallized in the triclinic space group P1h. The
Mn-O bond distance of 2.037(3) Å compares favorably
to the Mn-O bond distance of 2.029(3) Å observed in
the fac-(CO)3(dppp)MnOC(O)OCH3 derivative.11 As was
observed in complex 2, the Mn-COaxial bond length of
1.789(6) Å is significantly shorter than the Mn-COeq
distance of 1.841(7) Å. A thermal ellipsoid representa-
tion of 3 is provided in Figure 3.
Figure 5. Thermal ellipsoid representation of fac-(CO)3-
(dppp)MnBr, 5.
in the covalent radii of 0.15 Å of the two halogen atoms.
Unlike complex 3, these manganese derivatives are very
air stable. Figures 4 and 5 depict the thermal ellipsoid
representations of complexes 4 and 5.
In our studies involving the use of (salen)CrIIIX (X )
Cl or N3) complexes as catalysts for the effective
coupling of CO2 and epoxides to afford polycarbonates,
the azide derivatives were found to be better initiators
than their chloride analogues.8b,21 Hence, it was of
importance to obtain a well-characterized sample of
(CO)3(dppp)MnN3 for our current studies. This was
achieved via reaction of the aquo complex, [(CO)3(dppp)-
Mn(OH2)][BF4], with an aqueous solution of NaN3.
(CO)3(dppp)MnN3 (6) crystallized in the space group
P21/c as a benzene solvate. A thermal ellipsoid drawing
of complex 6 is found in Figure 6, along with the atomic
The halide derivatives, complexes 4 and 5, crystallized
in the monoclinic space group P21/n. The Mn-Cl bond
length in 4 of 2.379(2) Å is comparable to that seen in
the closely related derivative, fac-(CO)3(depe)MnCl, of
2.406(2) Å.11 The Mn-Br bond length in complex 5 of
2.518(9) Å is as expected on the basis of the difference
(21) Darensbourg, D. J.; Mackiewicz, R. M.; Rodgers, J. L.; Fang,
C. C.; Billodeaux, D. R.; Reibenspies, J. H. Inorg. Chem. 2004, 43,
6024-6034.

(CO)3(dppp)MnX Derivatives
Organometallics, Vol. 23, No. 25, 2004 6029
Figure 6. Thermal ellipsoid representation of fac-(CO)3-
(dppp)MnN3, 6.
Table 3. νCO Stretching Vibrations in
(CO)3(dppp)MnX Complexesa
Figure 7. Infrared spectra in the νCO and νCO regions
before and after CO2/cyclohexene oxide coupli2ng reac-
X
νCO, cm-1
average νCO, cm-1
tion. (A) νCO of (CO)3(dppp)MnOC2H5; (B) νCO and νCO of
2
H (1)
1998, 1926, 1902
2037, 1969, 1911
2009, 1936, 1891
2027, 1957, 1905
2028, 1963, 1908
2014, 1956, 1912
1942
1972
1945
1963
1966
1960
(*)(CO)3(dppp)MnOC(O)OC2H5, (1818, 1802) trans-cyclic
OTs (2)
OC2H5 (3)
Cl (4)b
carbonate and (1750 cm-1) poly(cyclohexylene)carbonate.
Scheme 1
Br (5)
N3 (6)
a Spectra determined in benzene solution. b Spectrum deter-
mined in dichloromethane.
numbering scheme for selected atoms. The Mn-azide
bond distance was found to be 2.126(3) Å, and the nearly
linear N3 unit (175.4(4)°) formed an angle with the Mn-
N(1) bond of 117.2(2)°. Bond distances within the azide
ligand are N(1)-N(2) ) 1.164(5) Å and N(2)-N(3) )
1.200(6) Å.
The complexes (CO)3(dppp)MnX (X ) OCH3, OC2H5
(3), Cl (4), Br (5), and N3 (6)) were examined for their
efficacy for catalyzing the coupling of cyclohexene oxide
and carbon dioxide (Scheme 1). Reaction conditions were
consistent with those we routinely employ for the very
effective (salen)CrX/cocatalyst systems, i.e., catalyst
loading 0.040 mol %, 500 psi CO2 pressure, and 80 °C.
After a polymer run the crude reaction mixture was
dissolved in methylene chloride and examined by in-
frared spectroscopy for product identification. Impor-
tantly, in these instances the highly informative metal
carbonyl absorbances in the infrared also provide a
probe of the resting state of the catalytically active
species. Figure 7 represents the infrared spectra in the
carbonyl region of the before and after reaction solutions
for the copolymerization process involving the most
effective catalyst examined here, complex 3. As is
evident in Figure 7, the major reaction product is the
poly(cyclohexylene)carbonate, with a minor quantity of
the trans-cyclohexyl carbonate also afforded. That is, the
An interesting feature of the MnI-azide linkage in
complex 6 as compared to that in (salen)CrIII azides is
the internal nitrogen-nitrogen bond distances.9 Con-
sidering the
resonance form for the free azide moiety, in the low-
valent manganese derivative the azide ligand binds via
the triply bonded terminal nitrogen, whereas in the
chromium(III) salen derivative it binds through the
terminal single-bonded nitrogen atom. That is, in the
six-coordinate chromium salen complex, N(1)-N(2) )
1.210(9) Å and N(2)-N(3) ) 1.144(8) Å. This difference
in binding modes might be anticipated on the basis of
hard/soft arguments. On the other hand, the νasym mode
for complex 6 is observed at 2056 cm-1, whereas that
of the chromium(III) azide complex is similarly found
at 2054 cm-1
.
νCO bond at 1750 cm-1 corresponds to the copolymer
2
Table 3 lists the νCO values for the manganese
derivatives described in this report. These νCO stretching
vibrations reflect the electron density available at the
metal center for back-bonding to the carbonyl ligands.
Therefore, it is possible from the average νCO frequency
for these six derivatives to order the electron-rich
character of the metal centers as 1 g 3 > 6 > 4 )
5 > 2. This will be useful information when considering
these complexes as catalysts for the coupling of CO2 and
epoxides.
carbonate function, with the weak absorbances at 1802
and 1818 cm-1 to that of the trans cyclic carbonate.22
The νCO absorbances at 2029, 1962, and 1903 cm-1 are
readily assigned to the CO stretching vibrations of the
(CO)3(dppp)Mn-OC(O)OR species, where R represents
the growing polymer chain. This assignment is greatly
aided by comparison with the fully characterized (CO)3-
(22) Darensbourg, D. J.; Lewis, S. J.; Rodgers, J. L.; Yarbrough, J.
C. Inorg. Chem. 2003, 42, 581-589.

6030 Organometallics, Vol. 23, No. 25, 2004
Darensbourg et al.
Table 4. Results of MnI Complexes-Catalyzed
Coupling Reactions of Cyclohexene Oxide and
Scheme 2
a
CO2
TONb
catalyst
copolymer cyclic carbonate % carbonate
(CO)3(dppp)MnOCH3
42
11
66
(CO)3(dppp)MnOC2H5 40 (48)c
10 (25)c
10 (28)c
10
70 (83)c
56 (72)c
47
(CO)3(dppp)MnCl
(CO)3(dppp)MnN3
(CO)3(dppp)MnBr
30 (44)c
24
12
8
25
a Reactions carried out for 24 h at 80 °C. b Mol of epoxide
consumed/mol of Mn. c Reactions carried out at 100 °C.
As the data in Table 4 indicate, the order of epoxide
ring-opening by the X group (initiator) is -OC(O)OR >
Cl g N3 > Br. This trend roughly parallels what is
observed in the (salen)CrX-catalyzed processes, as well
as the electron-donating abilities of the X groups as
revealed by their effect on the νCO values of the MnI
carbonyl derivatives. The observation that trans-cyclo-
hexyl carbonate is selectively produced is consistent
with it being formed by copolymer degradation.23 Con-
sequently, cyclic carbonate production should be inde-
pendent of the initiator, which is borne out by the data
in Table 4. The somewhat reduced cyclic carbonate
production in the case of X ) Br is undoubtedly due to
the significant reduction in copolymer production in this
instance. Furthermore, the enhanced production of
cyclic carbonate over copolymer at higher reaction
temperatures is consistent with a unimolecular degra-
dation of the growing polymer chain.8a In our previous
studies involving (salen)CrX derivatives as catalysts for
this process we have noted that at the onset of the
copolymerization process larger amounts of polyether
linkages are seen in the copolymer, which upon longer
reaction times afford copolymers with >99% carbonate
content. This observation would account for the large
degree of polyether linkages, i.e., the small degree of
carbonate linkages in the slowly initiating bromide
derivative.
(dppe)MnOC(O)OMe derivative previously reported.11,12
As expected, a similar spectrum in the νCO region is
observed at the end of the copolymerization reaction for
all of the manganese catalysts employed in Table 4.
Furthermore, this species would be expected to be the
resting state of the catalyst on the basis of the very facile
insertion of CO2 into the Mn-OMe bond of (CO)3(dppe)-
MnOMe.12
Table 4 summarizes the results of the CO2/cyclohex-
ene oxide coupling reaction catalyzed by the MnI deriva-
tives reported herein. Although these organometallic
complexes do serve as catalysts for this process, the
TONs for 24 h runs are not very impressive relative to
other catalysts for this reaction. For example, the best
catalysts found in this study were those containing
alkoxide initiators, yet these exhibited TONs only
around 50 mol epoxide consumed/mol Mn with 70%
carbonate linkages at 80 °C. Concomitantly, the selec-
tivity for copolymer production was only 80%; that is,
the minor product, trans-cyclohexyl carbonate, accounts
for 20% of the coupling product. As anticipated, an
increase in reaction temperature to 100 °C increased
the percentage of cyclic carbonate formation to about
34%; however, the higher temperature favorably en-
hanced both the TON and % carbonate linkages in the
copolymer.
Hence, these studies reinforce the proposed mecha-
nistic features of the more complex (salen)CrX/cocatalyst
systems.21 However, in this instance unlike in the
(salen)CrX/cocatalyst system,9 CO2 insertion into the
alkoxide intermediate under all reaction conditions is
faster than epoxide ring-opening by the transient car-
bonate moiety of the growing polymer chain (Scheme
2). That is, the rate-limiting step in the propagation
process of the copolymerization reaction is epoxide ring-
opening and not CO2 insertion.
Concluding Remarks
Herein we have structurally characterized a series of
closely related 18-electron derivatives of (CO)3(dppp)-
MnX, which differ only in the nature of the X group.
Although these complexes were found not to be very
effective catalysts for the coupling of CO2 and epoxides,
findings from these studies contribute significantly to
our understanding of this important process. Further-
more, these complexes have previously been shown via
13CO exchange studies to be thermally inert to ligand
dissociation.12 Hence, upon utilizing these derivatives
as catalysts for the CO2/epoxide coupling reaction, ring-
opening of the epoxide must be taking place via an
associative interchange mechanism. Because CO2 inser-
tion into the MnI-OR bond in the absence of electron-
withdrawing substituents on R has been shown to be
rapid even at -78 °C, in the presence of CO2 the
(CO)3(dppp)MnOR (R ) Me, Et) complexes will exist as
(CO)3(dppp)MnOC(O)OR (R ) Me, Et).12
Acknowledgment. Financial support from the Na-
tional Science Foundation (CHE 02-34860) and the
Robert A. Welch Foundation is greatly appreciated.
Supporting Information Available: Complete details for
the crystallographic study of complexes 1-6. This material is
available free of charge on the Internet at http://pubs.acs.org.
OM049352V
(23) Darensbourg, D. J.; Fang, C. C.; Rodgers, J. L. Organometallics
2004, 23, 924-927.