Binding of propylene oxide to porphyrin- and salen-M(III) cations, where M = Al, Ga, Cr, and Co

Article abstract of DOI:10.1021/ic048597x

The binding of propylene oxide (PO) to a series of metal cations LM(III)⁺, where for L = tetraphenylporphyrin (TPP) M = Al, Ga, Cr, and Co, and for L = (R,R)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2- cyclohexenediamine (salen) M = Al and Cr, was studied in the gas phase by electrospray tandem mass spectroscopy, and the relative stabilities of LM(PO)₂⁺ and LM(PO)⁺ cations were determined. The chromium(III) and aluminum(III) cations most tenaciously bind PO, and for M = Al, coordination to the TPP ligated metal center was favored relative to salen. For (TPP)M(PO)₂⁺, the dissociation of PO followed the order M = Al > Cr, but for (TPP)M(PO)⁺ the dissociation was M = Cr > Al. The single-crystal structural determinations on (R,R-salen)AlOCHMe(S)CH₂Cl-0.5PO and (R,R-salen)AlO ₂CMe-1.5py grown in neat PO and pyridine, respectively, reveal five-coordinate aluminum(III) centers with the alkoxide/acetate ligands in the axial position of a square-based pyramid. These results are discussed in terms of the reactivity of these metal complexes in ring-opening polymerizations and copolymerizations with PO and CO₂, respectively.

Full text of DOI:10.1021/ic048597x

Inorg. Chem. 2005, 44, 2588−2595
Binding of Propylene Oxide to Porphyrin
Where M Al, Ga, Cr, and Co
−
and Salen
−M(III) Cations,
)
Peter Chen,*,† Malcolm H. Chisholm,*,‡ Judith C. Gallucci,‡ Xiangyang Zhang,† and Zhiping Zhou‡
Department of Chemistry, The Ohio State UniVersity, 100 W. 18th AVenue,
Columbus, Ohio 43210-1185, and Laboratorium fu¨r Organische Chemie,
Eidgeno¨ssiche Technische Hochschule (ETH), CH-8092 Zu¨rich, Switzerland
Received October 7, 2004
The binding of propylene oxide (PO) to a series of metal cations LM(III)+, where for L
)
tetraphenylporphyrin
(TPP) M
(salen) M
) Al, Ga, Cr, and Co, and for L ) (R,R)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexenediamine
)
Al and Cr, was studied in the gas phase by electrospray tandem mass spectroscopy, and the relative
+
stabilities of LM(PO)2 and LM(PO)+ cations were determined. The chromium(III) and aluminum(III) cations most
tenaciously bind PO, and for M
For (TPP)M(PO)2+, the dissociation of PO followed the order M
was M Cr > Al. The single-crystal structural determinations on (R,R-salen)AlOCHMe(S)CH2Cl
(R,R-salen)AlO2CMe 1.5py grown in neat PO and pyridine, respectively, reveal five-coordinate aluminum(III) centers
)
Al, coordination to the TPP ligated metal center was favored relative to salen.
Al > Cr, but for (TPP)M(PO)+ the dissociation
0.5PO and
)
)
‚
‚
with the alkoxide/acetate ligands in the axial position of a square-based pyramid. These results are discussed in
terms of the reactivity of these metal complexes in ring-opening polymerizations and copolymerizations with PO
and CO2, respectively.
Table 1. Homopolymerization of PO by TPP-Metal Complexesa
Introduction
reacn time
(h)
PPOb
(%)
Catalytic reactions proceed via a series of elementary
reactions, each of which is in principle reversible, and
collectively these determine the turnover-limiting frequency
that replaces what is commonly called a rate-limiting step
for a reaction. In the ring-opening polymerization of epoxides
by metal complexes and in the copolymerization of epoxides
and carbon dioxide to give poly(alkene oxides) and poly-
(alkene carbonates), respectively, the coordination of the
alkene oxide to the metal is one of the key steps.1-10
Coordination to the Lewis acidic metal center facilitates ring
opening by both an SN2 and an SN1 reaction pathway.11,12
c
entry
catal system
10-3Mn
PDIc
1
2
3
4
5
6
(TPP)CrCl
120
120
168
168
120
120
76
59
50
2
61
44
28
1.53
1.38
1.32
(TPP)CrCl/DMAPd
(TPP)AlCl
(TPP)AlCl/DMAP
(TPP)CoCl
(TPP)CoCl/DMAP
e
a The polymerization reactions were carried out in neat PO (2 mL) at 25
°C; [cat]:[PO] ) 1:1500; 1 equiv of DMAP added if needed. No PPO was
formed by using (TPP)GaCl and (salen)AlCl, and PPO could be formed by
(salen)CrCl only at high [cat] and elevated temperatures. b Determined by
1H NMR. c Mn and PDI determined by gel permeation chromatography.
d DMAP, 4-(dimethylamino)pyridine. e No or little formation.
* To whom correspondence should be addressed. E-mail: chisholm@
chemistry.ohio-state.edu (M.H.C.).
As is shown in Tables 1 and 2, various tetraphenylpor-
phyrin- (TPP) and salen-metal(III) complexes (Figure 1)
are effective in formation of poly(propylene oxide), PPO,
and poly(propylene carbonate), PPC. There are, however,
notable and quite remarkable differences between the activity
of the metals Al(III), Cr(III), and Co(III), all of which have
a very similar ionic radius, and between the porphyrin and
salen ligands. We describe here studies of the binding of
propylene oxide to the porphyrin and salen metal(III) cations,
† Eidgeno¨ssiche Technische Hochschule.
‡ The Ohio State University.
(1) Darensbourg, D. J.; Holtcamp, M. W. Coord. Chem. ReV. 1996, 153,
155-174.
(2) Super, M. S.; Beckman, E. J. Trends Polym. Sci. 1997, 5, 236-240.
(3) Kuran, W. Prog. Polym. Sci. 1998, 23, 919-992.
(4) Nozaki, K. J. Polym. Sci. A: Polym. Chem. 2003, 42, 215-221.
(5) Rokicki, A.; Kuran, W. J. Macromol. Sci., ReV. Macromol. Chem.
1981, C21, 135-186.
(6) Nakano, K.; Kosaka, N.; Hiyama, T.; Nozaki, K. Dalton Trans. 2003,
4039-4050.
(7) Sigwalt, P. Angew. Makromol. Chem. 1981, 94, 161-180.
(8) Tsuruta, T. Pure Appl. Chem. 1981, 53, 1745-1751.
(9) Inoue, S. J. Polym. Sci. A: Polym. Chem. 2000, 38, 2861-2871.
(10) Inoue, S. Prog. Polym. Sci. 1988, 13, 63-81.
(11) Parker, R. E.; Isaacs, N. S. Chem. ReV. 1959, 59, 737-799.
(12) March, J. AdVanced Organic Chemistry, 4th ed.; 1992.
2588 Inorganic Chemistry, Vol. 44, No. 8, 2005
10.1021/ic048597x CCC: $30.25
© 2005 American Chemical Society
Published on Web 01/05/2005

Porphyrin- and Salen-M(III) Cations
Scheme 1. Representation of the Modified Finnigan MAT TSQ-700 ESI Tandem Mass Spectrometer
Table 2. Copolymerization of PO/CO2 by TPP- and salen-Metal
Complexesa
reacn time PPCb PPOc PCd
e
entry
catal system
(TPP)CrCl
(TPP)CrCl/DMAP
(TPP)AlCl
(TPP)AlCl/DMAP
(TPP)CoCl
(TPP)CoCl/DMAP
(salen)CrCl
(h)
(%) (%) (%) 10-3Mn PDI
1
2
3
4
5
6
7
8
9
48
48
144
144
48
8
49
4
92
26
21
7
f
85
23
12
31
1.49
1.41
1.19
1.24
57
1
48
3
1
2
144
144
144
144
3
13
25
1
1
9
14
1.21
1.35
(salen)CrCl/DMAP
(salen)AlCl
10 (salen)AlCl/DMAP
11g (salen)CoOAc
35
8
1.57
a The polymerization reactions were carried out in neat PO (2 mL) at
25 °C under 50 bar CO2 pressure; [cat]:[PO] ) 1:1500; 1 equiv of DMAP
added if needed. b Carbonate linkages in the polymers obtained in the
reaction mixtures determined by 1H NMR. c Ether linkages in the resulting
polymers determined by 1H NMR. d Cyclic propylene carbonate in the
products determined by 1H NMR. e Mn and PDI determined by gel
permeation chromatography. f No or little formation. g Reference 30,
[cat]:[PO] ) 1:200.
Figure 2. ESI/MS from the reactions between TPP-/salen-metal cations
and propylene oxide in the gas phase.
+
resultant ions (TPP)M(PO)n , where n ) 1 or 2, are detected
and selected. As is shown in Figure 2, the cobalt cation has
a very low affinity for PO and is present almost exclusively
as (TPP)Co+ with just a detectable amount of (TPP)Co-
+
(PO)2 . This is a clear indication of the weaker Lewis acidity
of (TPP)Co+. Aluminum forms both (TPP)Al(PO)n+ cations
for n ) 1 and 2 with a slight preference for n ) 1, while for
+
chromium the (TPP)Cr(PO)2 cation is clearly much pre-
ferred. Also shown in Figure 2 are ions due to (TPP)Ga-
+
+
(PO)n (n ) 1, 2) and related (salen)M(PO)n , where M )
Al and Cr, and these will be discussed later. These PO
adducts formed in the gas phase are not ring-opening
derivatives as will be shown by their collision-induced
displacement experiments; vide infra.
Figure 1. Tetraphenylporphyrin- and salen-metal(III) complexes.
LM+, in the gas phase that clearly reveal the relative Lewis
acidities of these ligated metal ions and furthermore their
propensity to bind one or two neutral ligands. Finally, we
report the molecular structures of S-PO, (R,R-salen)AlOCHMe-
(S)CH2Cl, and (R,R-salen)AlO2CMe‚1.5py that are relevant
to the ring opening of PO by aluminum salen and porphyrin
complexes.
In the second reaction chamber the relative binding
affinities of the (TPP)M(PO)n ions are examined by
+
collision-induced dissociation (CID) experiments. A selected
ion (TPP)M(PO)n+ (n ) 1 or 2) at the same concentration is
allowed to collide with xenon (3 × 10-5 Torr) and the voltage
is increased,14 and the relative abundance of the ions (both
the parent and daughter) are detected as shown in Scheme
1.
Results and Discussion
From the graphical data presented in Figure 3 we can
conclude that for the ions containing one PO the ease of
Gas-Phase Studies. 1. Binding of Propylene Oxide. We
studied the binding of PO to the (TPP)M+ cations by
electrospray tandem mass spectrometry as depicted in
Scheme 1.13 In the first reaction chamber the (TPP)M+ ions
are allowed to react with PO (1 × 10-2 Torr), and the
(13) Chen, P. Angew. Chem., Int. Ed. 2003, 42, 2832-2847.
(14) The xenon pressure is relatively low to allow the single-collision
experiments.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2589

Chen et al.
Figure 5. Plots of LM(PO)+ abundance versus CID voltage from the
Figure 3. Plots of LM(PO)+ abundance versus CID voltage from the
dissociation reaction of LM(PO)+ with xenon in the CID chamber
([M]+ ) LM+).
+
dissociation reactions of LM(PO)2 with xenon in the CID chamber
([M]+ ) LM+).
tion. In Figure 5, we show the appearance of the mono PO
cations. Here we see the growth of (TPP)Al(PO)+ > (TPP)-
Cr(PO)+ > (salen)Cr(PO)+ with the latter being at a
maximum in the order of only 5%. From this we can
conclude that the binding of PO to the four metal ions is in
the order Al > Cr and that for chromium the TPP favors
coordination relative to salen. For the chromium salen cations
there is cooperative dissociation of PO. As shown in the
+
following equation, for (salen)Cr(PO)2 , k2 . k1 whereas,
+
for (TPP)Al(PO)2 , the initial loss of PO forms (TPP)Al-
(PO)+ predominantly, i.e., k2 < k1.
LM(PO)2 -PO8 LM(PO)+ -PO8 LM+
+
k1
k2
2. Ring Opening of Propylene Oxide in the Gas Phase.
Since the cationic mechanism for epoxide ring opening is
well-known for some metal complexes,15-19 we examined
the possibility of a cationic pathway for PO ring-opening
polymerization in the gas phase by the TPP and salen metal
cations. As shown in Figure 2, no metal ions with PO adduct
number >2 were detected. Furthermore, we examined the
possibility of PO ring opening with the assist of neutral
nucleophiles, such as diethylamine and pyridine. One cation,
+
Figure 4. Plots of LM(PO)2 abundance versus CID voltage from the
+
dissociation reactions of LM(PO)2 with xenon in the CID chamber
([M]+ ) LM+).
dissociation follows the order (TPP)Ga+ > (salen)Al+
>
(TPP)Al+, which is the inverse order of their Lewis acidities.
This makes good sense as gallium is notably softer than
aluminum and element-element bond strengths decrease
down a series in the main group. The difference in PO
binding to the salen and porphyrin Al+ cations is also
particularly noteworthy and reflects the fact that salen is a
significantly stronger donor ligand relative to TPP. The
(TPP)Al+ cation is more Lewis acidic than its (salen)Al+
counterpart. We could not examine (TPP)Cr(PO)+ ion as its
concentration was relatively too low. However, a comparison
is seen for the bis PO adducts.
either LM(PO)+ or LM(PO)2 , was selected and reacted with
+
the neucleophile (3 × 10-4 Torr) in the CID chamber.
Interestingly, ligand displacements were observed rather than
PO ring opening as shown in Figure 6 in the case of pyridine.
We should notice that the species in the gas phase are in an
extremely low concentration relative to the solution phase,
and these cations are not among the ones with strong Lewis
(15) Bochmann, M.; Lancaster, S. J.; Hannant, M. D.; Rodriguez, A.;
Schormann, M.; Walker, D. A.; Woodman, T. J. Pure Appl. Chem.
2003, 75, 1183-1195.
+
Figures 4 and 5 compare the (TPP)M(PO)2 (M ) Al,
Cr) and (salen)Cr(PO)2+ ions with respect to their dissocia-
tion of PO. Specifically in Figure 4 we can see the loss of
the bis PO cations in the order (TPP)Al(PO)2+ . (TPP)Cr-
(16) Entelis, S. G.; Korovina, G. Makromol. Chem. 1974, 175, 1253-1280.
(17) Munoz-Hernandez, M.-A.; McKee, M. L.; Keizer, T. S.; Yearwood,
B. C.; Atwood, D. A. J. Chem. Soc., Dalton Trans. 2002, 410-414.
(18) Jegier, J. A.; Munoz-Hernandez, M.-A.; Atwood, D. A. J. Chem. Soc.,
Dalton Trans. 1999, 2583-2588.
+
+
(PO)2 ∼ (salen)Cr(PO)2 . This is a clear indication of the
(19) Inoue, M.; Sugita, T.; Kiso, Y.; Ichikawa, K. Bull. Chem. Soc. Jpn.
1976, 49, 1063-1071.
high affinity of the chromium(III) ion to retain six coordina-
2590 Inorganic Chemistry, Vol. 44, No. 8, 2005

Porphyrin- and Salen-M(III) Cations
Figure 7. ORTEP view of (R,R-salen)AlOCHMe(S)CH2Cl.
+
Figure 6. ESI/MS from the reaction of (TPP)Al(PO)2 with pyridine in
the gas phase.
acidity. In fact, PO ring-opening polymerization was ob-
served when a bimetallic cation of [(salen)Cr]2 was used
in the experiment.20
2+
Figure 8. ORTEP view of S-propylene oxide. Selected bond distances
(Å) and angles (deg): O(1c)-C(1c) 1.428(4), O(1c)-C(2c) 1.430(5),
C(1c)-C(2c) 1.431(6), C(2c)-C(3c) 1.454(6); C(1c)-O(1c)-C(2c) 60.1(2),
O(1c)-C(1c)-C(2c) 60.0(2), O(1c)-C(2c)-C(1c) 59.9(2), O(1c)-C(2c)-
C(3c) 117.6(3), C(1c)-C(2c)-C(3c) 123.5(4).
Polymerization Reactions in the Solution Phase. As
shown in Table 1 only the TPP-metal chlorides of aluminum
and chromium are effective in polymerizing PO at room
temperature and of these chromium is the more active. The
addition of the strong nucleophile (strong Lewis base, 1
equiv) 4-(dimethylamino)pyridine, DMAP, suppresses the
polymerization and for aluminum essentially renders the
system inactive. The related cobalt complex with or without
DMAP is essentially inactive. The salen metal chlorides of
chromium and aluminum are also inactive at 25 °C with
respect to homopolymerization of PO although both will ring
open one molecule of PO.21,22 However, at 75 °C in neat
PO in a bomb the salen chromium becomes active.20
Table 3. Selected Bond Distances (Å) and Angles (deg) for
(R,R-salen)AlOCHMe(S)CH2Cl
Al(1a)-O(1a)
Al(1a)-O(2a)
Al(1a)-O(3a)
Al(1a)-N(1a)
Al(1a)-N(2a)
1.800(2)
1.793(2)
1.736 (2)
2.027(2)
2.007(2)
O(3a)-C(2a)
C(1a)-C(2a)
C(2a)-C(3a)
Cl(1a)-C(3a)
1.405(3)
1.540(5)
1.521(5)
1.806(3)
O(3a)-Al(1a)-O(2a) 112.2(1)
O(3a)-Al(1a)-O(1a) 108.1(1)
O(1a)-Al(1a)-N(1a)
N(2a)-Al(1a)-N(1a)
87.5(1)
78.7(1)
O(2a)-Al(1a)-O(1a)
90.5(1)
C(2a)-O(3a)-Al(1a) 132.2(2)
O(3a)-Al(1a)-N(2a) 102.4(1)
O(2a)-Al(1a)-N(2a) 87.8(1)
O(1a)-Al(1a)-N(2a) 147.6(1)
O(3a)-Al(1a)-N(1a) 96.0(1)
O(2a)-Al(1a)-N(1a) 150.8(1)
O(3a)-C(2a)-C(3a)
O(3a)-C(2a)-C(1a)
C(3a)-C(2a)-C(1a)
105.3(2)
110.1(3)
111.3(3)
C(2a)-C(3a)-Cl(1a) 111.7(2)
In Table 2 we present data pertaining to the copolymer-
ization of PO and CO2, where it is evident that the reactivity
order is M ) Cr > Al > Co and that the added DMAP
favors the formation of PPC.23,24 Indeed in its absence only
a trace of CO2 is incorporated into the polymer which is
largely PPO. Also we can see that the activity order on the
attendant ligands is TPP > salen. We should note that
Darensbourg and co-workers have extensively studied the
salen-chromium system with respect to both changes of the
ligand itself (backbone and aromatic ring) and Lewis base
additives, and they recently reported that (salen)GaIII com-
pounds were of little reactivity in the copolymerization of
CHO and CO2.25-29 Also Coates has reported that salen
cobalt acetate is active in the formation of PPC,30 and (salen)-
CoIII compounds have been extensively studied as catalysts
in the hydrolytic kinetic resolution of terminal epoxides.31-33
Solid-State and Molecular Structures. The ring opening
of S-PO by (R,R-salen)AlCl yields two products: (R,R-
salen)AlOCHMe(S)CH2Cl (1) as the major product; (R,R-
salen)AlOCH2CHMe(R)Cl as the minor product (10%). The
molecular structure of compound 1 was determined on a
crystal grown in neat S-PO and is particularly pertinent to
this study. In the solid state there are two five coordinate
aluminum(III) centers that are nestled together in a head-
tail salen packing motif as shown in Figure S1 in the
Supporting Information. There is also one molecule of PO
(20) Shoen, E.; Zhang, X.; Zhou, Z.; Chisholm, M. H.; Chen, P., Inorg.
Chem. 2004, 43, 7278-7280.
(21) Jacobsen, E. N. Acc. Chem. Res. 2000, 33, 421-431.
(22) Paddock, R. L.; Nguyen, S. T. J. Am. Chem. Soc. 2001, 123, 11498-
11499.
(23) Aida, T.; Ishikawa, M.; Inoue, S. Macromolecules 1986, 19, 8-13.
(24) Jung, J. H.; Ree, M.; Chang, T. J. Polym. Sci. A: Polym. Chem. 1999,
37, 3329-3336.
(25) Darensbourg, D. J.; Mackiewicz, R. M.; Phelps, A. L.; Billodeaux,
D. R. Acc. Chem. Res., ACS ASAP.
(28) Darensbourg, D. J.; Yarbrough, J. C.; Ortiz, C.; Fang, C. C. J. Am.
Chem. Soc. 2003, 125, 7586-7591.
(29) Darensbourg, D. J.; Billodeaux, D. R. C. R. Chim. 2004, 7, 755-761.
(30) Qin, Z.; Thomas, C. M.; Lee, S.; Coates, G. W. Angew. Chem., Int.
Ed. 2003, 42, 5484-5487.
(31) Tohma, H. Organomet. News 1995, 137.
(26) Darensbourg, D. J.; Mackiewicz, R. M.; Rodgers, J. L.; Phelps, A. L.
Inorg. Chem. 2004, 43, 1831-1833.
(27) Darensbourg, D. J.; Yarbrough, J. C. J. Am. Chem. Soc. 2002, 124,
6335-6342.
(32) Tokunaga, M.; Larrow, J. F.; Kakiuchi, F.; Jacobsen, E. N. Science
1997, 277, 936-938.
(33) Nielsen, L. P. C.; Stevenson, C. P.; Blackmond, D. G.; Jacobsen, E.
N. J. Am. Chem. Soc. 2004, 126, 1360-1362.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2591

Chen et al.
Figure 9. ORTEP view of (R,R-salen)AlOOCMe.
Table 4. Selected Bond Distances (Å) and Angles (deg) for
(R,R-salen)AlOOCMe
trapped in the lattice. The coordination about aluminum is
best described as a square-based pyramid with the Al-OR
(alkoxide) bond in the axial site. Figure 7 shows one of the
two Al molecules in the unit cell. Selected bond distances
are given in Table 3.
Al(1a)-O(1a)
Al(1a)-O(2a)
Al(1a)-O(3a)
Al(1a)-N(1a)
1.776(2)
Al(1a)-N(2a)
O(3a)-C(37a)
O(4a)-C(37a)
C(37a)-C(38a)
2.002(3)
1.295(4)
1.210(4)
1.493(5)
1.795(2)
1.772 (2)
1.998(3)
The molecule of S-PO is shown in Figure 8. This is the
first X-ray structural determination of the free (unligated)
PO molecule.34-36 Pertinent bond distances and angles are
given in the caption. What is particularly pertinent here is
that the aluminum center remains five-coordinate even when
PO is present as a solvent. For (salen)chromium azide
Jacobsen reported the structure of the ring-opening product
of cyclopentene oxide and found a solvent molecule of THF
in the axial position.21,37
O(3a)-Al(1a)-O(2a) 100.6(1)
O(3a)-Al(1a)-O(1a) 104.3(1)
O(2a)-Al(1a)-N(1a)
O(1a)-Al(1a)-N(1a)
N(2a)-Al(1a)-N(1a)
154.9(1)
89.4(1)
79.7(1)
O(2a)-Al(1a)-O(1a)
90.6(1)
O(3a)-Al(1a)-N(2a) 105.9(1)
C(37a)-O(3a)-Al(1a) 143.5(2)
O(3a)-C(37a)-C(38a) 114.3(3)
O(2a)-Al(1a)-N(2a)
87.8(1)
O(1a)-Al(1a)-N(2a) 149.5(1)
O(3a)-Al(1a)-N(1a) 103.6(1)
O(3a)-C(37a)-O(4a)
O(4a)-C(37a)-C(38a) 122.1(3)
123.5(3)
Concluding Remarks
For a series of related ions LM+, the Lewis acidity order
follows M ) Cr(III) ∼ Al(III) > Co(III). For the formation
of the mono PO adducts LM(PO)+, the metal-PO bond
strength follows the order Al > Cr. For the adducts LM-
An alkoxide ligand is known to have a higher trans-
influence than that of a carboxylate or alkyl carbonate
ligand.38 Consequently we investigated the ability of (salen)-
AlO2CMe to bind a stronger Lewis base, namely pyridine.
The molecular structure of this compound determined from
a crystal grown in neat pyridine is shown in Figure 9. The
aluminum is again five-coordinate, and the acetate ligand is
in the apical position of a square-based pyramid. Significant
note is the fact that the acetate is η1-coordinated. The long
Al-O(acetate) distance is 3.45 Å, and the Al-O(acetate)
bond of 1.77 Å is also longer than that in the related Al-
O(alkoxide) bond (1.74 Å) (Tables 3 and 4). In the lattice
there are two Al molecules and three noncoordinated pyridine
molecules per unit cell (Figure S2). This is a clear indication
that the Al(III) center is only weakly Lewis acidic. Of course,
Al(III) six-coordinate ions are well-known and Atwood has
shown that (salen)Al cations with weakly coordinating anions
+
(PO)2 , the loss of the first PO is in the order Al > Cr. These
notable differences in the metal porphyrin complexes can
be traced to the dn configuration effects of the M(III) ions,
d0 for Al, d3 for Cr, and d6 for Co.40 The binding of PO to
the (TPP)M+ ions was also noted to be stronger relative to
the respective (salen)M+ ions, which is taken to indicate the
better donor properties of salen relative to TPP. Finally, the
structural characterizations of the (salen)AlOCHMeCH2Cl
and (salen)AlO2CMe molecules in the presence of excess
PO and pyridine, respectively, provide further testimony to
the importance of five-coordination relative to six for the
(salen)Al cation.
It is also of interest here to note the recent report of
Darensbourg et al. on the binding of oxiranes to (Tp)CdO2-
CMe, where Tp ) hydrotris(3-phenylpyrazol-1-yl)borate.41
These authors noted that the enthalpy of adduct formation
varied little with variation of the oxiranes (PO, CHO,
limonene oxide, norbornene oxide, pinene oxide, etc.). In
no case, however, did the soft Cd(II) center cause ring
opening of the oxirane.
-
(OTs-, BPh4 , etc.) will readily pick up two molecules of
O-donor solvents, such as MeOH, THF, or H2O.17,18,39
(34) Darensbourg, D. J.; Holtcamp, M. W.; Khandelwal, B.; Klausmeyer,
K. K.; Reibenspies, J. H. J. Am. Chem. Soc. 1995, 117, 538-539.
(35) Beckwith, J. D.; Tschinkl, M.; Picot, A.; Tsunoda, M.; Bachman, R.;
Gabbaie, F. P. Organometallics 2001, 20, 3169-3174.
(36) Harder, S.; Boersma, J.; Brandsma, L.; Kanters, J. A.; Duisenberg,
A. J. M.; Van Lenthe, J. H. Organometallics 1990, 9, 511-516.
(37) Hansen, K. B.; Leighton, J. L.; Jacobsen, E. N. J. Am. Chem. Soc.
1996, 118, 10924-10925.
(38) Coe, B. J.; Glenwright, S. J. Coord. Chem. ReV. 2000, 203, 5-80.
(39) Sahasrabudhe, S.; Yearwood, B. C.; Atwood, D. A.; Scott, M. J. Inorg.
Synth. 2004, 34, 14-20.
(40) Kettle, S. F. A., Ed. Physical Inorganic Chemistry: Coordination
Chemistry Approach; 1996; pp 31-38.
(41) Darensbourg, D. J.; Billodeaux, D. R.; Perez, L. M. Organometallics
2004, 23, 5286-5290.
2592 Inorganic Chemistry, Vol. 44, No. 8, 2005

Porphyrin- and Salen-M(III) Cations
this work even harder desolvation conditions were used to get strong
signal, e.g., tube lens ∼ 150 V. The transfer 24-pole (O1) acts as
an ion guide to separate the ions from neutral molecules which are
pumped off by a turbo pump located underneath the octopole. O1
is fitted with an open cylindrical sheath around the rods into which,
depending on the setup used, a collision gas can be bled for
thermalization or reaction at pressures up to 100 mTorr. Modifica-
tion of normal octopole by a 37 cm radio frequency (rf) 24-pole is
based on requirements of the longer reaction time and bigger
pressure of reagent gas. The ions then enter the actual mass
spectrometer, which is at 10-6 Torr and 70 °C manifold temperature
during operation. The configuration is quadrupole/octopole/quad-
rupole (Q1/O2/Q2), with the two quadrupoles as mass selection
stages and the second octopole operating as a collision-induced
dissociation (CID) cell. Spectra can be recorded in different modes.
In the normal ESMS mode, only one quadrupole is operated (either
Q1 or Q2), and a mass spectrum of the electrosprayed ions is
recorded. This mode serves primarily to characterize the ions
produced by a given set of conditions. In the daughter-ion mode,
Q1 is used to mass-select ions of a single mass-to-charge-ratio from
among all of the ions produced in O1, which are then collided or
reacted with a target gas in O2 (CID cell) and finally mass-analyzed
by Q2. This mode is used to obtain structural information (by
analysis of the fragments) or the specific reactivity of a species of
a given mass. Therefore, collision-induced dissociation (CID) could
be done in either the radio frequency (rf) 24-pole ion guide by
collision with Xe (0.08 Torr) or in a gas-filled (0.1 mTorr of either
Xe or reagent gas) rf octopole ion guide. CID and ion-molecule
reactions were all performed at low collision energy, i.e., -10 to
-50 V in the laboratory frame.
X-ray Crystallographic Studies. The data collection crystals
for (salen)AlOCHMeCH2Cl and (salen)AlOOCMe were both yel-
low chunks, which had been cut from a large crystal. Examination
of the diffraction pattern on a Nonius Kappa CCD diffractometer
indicated a triclinic crystal system. All work was done at 200 K
using an Oxford Cryosystems cryostream cooler. The data collection
strategy was set up to measure a hemisphere of reciprocal space
with a redundancy factor of 3.1 for (salen)AlOCH2CHMeCl and
3.4 for (salen)AlOOCMe, which means that 90% of the reflections
were measured at least 3.1 and 3.4 times, respectively. A combina-
tion of φ and ω scans with a frame width of 1.0° was used. Data
integration was done with Denzo, and scaling and merging of the
data was done with Scalepack.42 Merging the data (but not the
Friedel pairs) resulted in an Rint value of 0.036.
With respect to this study and the homopolymerization of
PO, the weaker Lewis acidity of the Co(III) ion would seem
to be responsible for its lack of activity. The difference
between chromium and aluminum where the activity order
is Cr > Al may well reflect the greater affinity for
six-coordination which would allow for a bimolecular
pathway in [Cr], as has been shown by Jacobsen for (salen)-
CrN3 in asymmetric ring opening of epoxides.37 However,
different mechanisms may operate for different metal ions
with different ligand sets and further speculation is not
warranted. As noted in the Introduction, a catalytic cycle
involves several elementary reaction steps and collectively
these determine the turnover-limiting frequency. What we
have established is the relative affinity of the Lewis acidic
metal cationic centers toward propylene oxide.
Experimental Section
All syntheses and solvent manipulations were carried out under
a nitrogen atmosphere using standard Schlenk-line and drybox
techniques. Solvents were dried in the standard procedures. Pro-
pylene oxide (Alfa Aesar) was distilled from calcium hydride.
5,10,15,20-Tetraphenylporphine (Acros), diethylaluminum chloride
(1.0 M solution in hexane, Aldrich), trimethylaluminum (2.0 M
solution in hexane, Aldrich), 4-(dimethylamino)pyridine (Aldrich),
(R,R)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexenedi-
amine (Aldrich), ((R,R)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-
cyclohexenediamino)aluminum chloride (Aldrich), and ((R,R)-N,-
N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexenediamino)chromium
chloride (Aldrich) were used as received. Chromium(II) chloride,
cobalt(II) chloride, and gallium(III) chloride were purchased from
Aldrich. Deuterated solvents were stored over 4 Å molecular sieves
for 24 h prior to use.
1
NMR Experiments. H and 13C{1H} NMR experiments were
carried out with a Bruker DRX-500 (5 mm broad band probe)
spectrometers, operating at proton Larmor frequencies of 500 MHz.
The parameters used in 13C{1H} NMR experiments on the Bruker
DRX-500 spectrometer were number of data points, TD ) 65 536,
sweep width, SWH ) 1502 Hz, relaxation time, D1 ) 2 s, and
chemical shift range, 0-200 ppm. Their peak frequencies were
1
referenced against the solvent, chloroform-d at 7.24 ppm for H
and 77.0 ppm for 13C{1H} NMR.
Gel Permeation Chromatography. Gel permeation chromato-
graphic (GPC) analysis was performed at 35 °C on a Waters Breeze
system equipped with a Waters 410 refractive index detector and
a set of two columns, Waters Styragel HR-2 and HR-4 (Milford,
MA). THF (HPLC grade) was used as the mobile phase at 1.0 mL/
min. The sample concentration was 0.1%, and the injection volume
was 100 µL. The samples were centrifuged and filtered before
analysis. The calibration curve was made with six polystyrene
standards covering the molecular weight range from 580 to 460
000 Da.
The structure of (salen)AlOCHMeCH2Cl was solved in P1 by
the direct methods in SHELXS-97.43 The asymmetric unit contains
two molecules of the Al complex and one solvent molecule of
propylene oxide. The Al complexes are labeled as molecules A
and B. One of the tert-butyl groups on molecule B was rotationally
disordered and was modeled as two isotropic sets of atoms.
The structure of (salen)AlOOCMe was solved in P1 by the direct
methods in SHELXS-97. The asymmetric unit contains two
molecules of the Al complex and three molecules of pyridine. The
Al complexes are labeled as molecules A and B. Some of the tert-
butyl groups were rotationally disordered and were modeled as two
isotropic sets of atoms. For two of the three pyridine molecules
(labeled as C and E), the location of the nitrogen atom was found
by initially naming all of the atoms as carbon and then seeing which
atom had the smallest refined isotropic U value. This atom was
then changed to a nitrogen atom. For the third pyridine ring (labeled
Mass Spectrometry/Gas-Phase Studies. The complexes are
typically diluted to 10-5 M with CH2Cl2 and electrosprayed on a
modified Finnigan MAT TSQ-700 tandem mass spectrometer
(Scheme 1) at a flow rate of 7-15 mL min-1 and at a potential of
4-5 kV using nitrogen as sheath gas. The ions are then passed
through a heated capillary (typically at 150 °C) where they are
declustered and the remaining solvent molecules are removed by
high vacuum. The extent of desolvation and collisional activation
can further be controlled by a tube lens potential, which typically
ranges from 50 V (soft condition) to 120 V (hard condition). In
(42) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307-326.
(43) Sheldrick, G. M.; Schneider, T. R. Methods Enzymol. 1997, 277, 319-
343.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2593

Chen et al.
Table 5. Crystallographic Details for (salen)AlOCHMeCH2Cl·0.5PO and (salen)AlOOCMe‚1.5py
(salen)AlOCHMeCH2Cl‚0.5PO
(salen)AlOOCMe‚1.5py
empirical formula
fw
C39H58AlClN2O3 + 1/2C3H6O
C38H55AlN2O3 + 3/2C5H5N
694.34
200
749.47
200
temp (K)
wavelengthh (Å)
cryst system
space group
unit cell dimens
a (Å)
0.710 73
triclinic
P1
0.710 73
triclinic
P1
12.776(1)
13.096(1)
13.855(1)
63.234(3)
1990.7(3)
2
1.158
0.157
12.554(1)
12.887(1)
14.614(1)
85.652(3)
2151.1(3)
2
b (Å)
c (Å)
â (deg)
V (Å3)
Z
D
calc (mg m-3
)
1.157
0.092
abs coeff (mm-1
)
F(000)
752
810
cryst size (mm-3
)
0.27 × 0.31 × 0.42
0.27 × 0.35 × 0.35
θ range for data collcn (deg)
index ranges
reflcns collcd
1.71-27.46
2.07-25.04
-16 e h e 16, -16 e k e 16, -17 e l e 17
54 924
-14 e h e 14, -15 e k e 15, -17 e l e 17
49 416
indpdt reflcns
17 967 [Rint ) 0.036]
full-matrix least squares on F2
17 967/3/894
0.01(6)
1.029
R1 ) 0.0572, wR2 ) 0.1451
R1 ) 0.0768, wR2 ) 0.1569
0.588 and -0.592
14 712 [Rint ) 0.035]
full-matrix least squares on F2
14 712/66/985
0.03(15)
1.041
R1 ) 0.0526, wR2 ) 0.1324
R1 ) 0.0671, wR2 ) 0.1420
0.493 and -0.312
refinement method
data/restraints/params
Flack param
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
largest diff peak and hole (e/Å3)
as D), the location of the nitrogen atom was not obvious with this
method, and so all of the atoms were designated as carbon atoms.
Experimental data relating to both structure determinations are
displayed in Table 5. CCDC 243038 and CCDC 245186 contain
the supplementary crystallographic data for (R,R-salen)AlOCHMe-
CH2Cl and (R,R-salen)AlOOCMe, respectively. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif,
by e-mailing data_request@ccdc.cam.ac.uk.
water, it was dried under vacuum at 100 °C. The crude product
was purified by column chromatography with CHCl3 as the eluent
over aluminia column. After CHCl3 was removed, the product was
dried overnight under vacuum at 100 °C, yielding 1.95 g of product.
UV-vis spectroscopy was used to confirm the formation of (TPP)-
CrCl.46 ESI/MS data: m/z ) 664, (TPP)Cr+.
Synthesis of TPP-Cobalt Chloride. (TPP)CoCl was prepared
according to the literature.47,48 (TPP)H2 (1.5 g, 2.4 mmol) was added
to 200 mL of DMF. The mixture was stirred under reflux at 170
°C. After 10 min the (TPP)H2 was dissolved and an excess of CoCl2
(0.4 g, 3.1 mmol) was added to the refluxing solution. After 30
min, an aliquot was taken from the mixture for UV-vis. The
spectrum showed free (TPP)H2 still remained in solution. A further
addition of CoCl2 (0.4 g) was added to the solution, refluxing for
another 30 min. No free (TPP)H2 was found in the UV-vis
measurement. The reaction mixture was allowed to cool to room
temperature and poured into 300 mL of concentrated HCl/MeOH
solution overnight. After the removal of solvents and washing with
a large amount of water, a purple solid was obtained and dried
under vacuum at 100 °C. The crude product was purified by column
chromatography with CH2Cl2 (5% MeOH) as the eluent over silica
column. After solvents were removed, the product was dried
overnight under vacuum at 100 °C, yielding 1.25 g of product. UV-
vis spectroscopy was used to confirm the formation of (TPP)CoCl.49
1H NMR (CDCl3, δ, ppm): 7.70, 8.19, 8.70 (aromatic). ESI/MS
data: m/z ) 671, (TPP)Co+.
Synthesis of TPP-Aluminum Chloride. (TPP)AlCl was pre-
pared according to the literature.44,45 To a solution of (TPP)H2 (1.0
g, 1.6 mmol) in 30 mL of dichloromethane was added a solution
of diethylaluminum chloride (Et2AlCl 1.0 M solution in hexane,
1.8 mL, 1.8 mmol). The addition was done slowly at room
temperature. The resulting solution was left stirring 3 h after which
the volatile fractions were removed under vacuum to give a purple
1
powder in 85% yield. H NMR (CDCl3, δ, ppm): 7.7, 8.2, 9.0
(aromatic). 13C{1H} NMR (CDCl3, δ, ppm): 120.73, 126.95,
128.06, 132.39, 134.23, 141.14, 148.51 (aromatic). ESI/MS data:
m/z ) 639, (TPP)Al+.
Synthesis of TPP-Chromium Chloride. (TPP)CrCl was pre-
pared according to the literature.46,47 (TPP)H2 (2.38 g, 3.9 mmol)
was added to 200 mL of dimethylformamide (DMF). The mixture
was stirred under reflux at 170 °C. After 10 min the (TPP)H2 was
dissolved and an excess of CrCl2 (0.75 g, 6.1 mmol) was added to
the refluxing solution. After 20 min, an aliquot was taken from the
mixture for UV-vis. The spectrum showed free (TPP)H2 still
remained in solution. A further addition of CrCl2 (0.5 g) was added
to the solution, refluxing for another 20 min. No free (TPP)H2 was
found in UV-vis measurement. The reaction mixture was allowed
to cool to room temperature and poured into 400 mL of ice-cold
water. After the solid was filtered out and washed three times with
Synthesis of TPP-Gallium Chloride. (TPP)GaCl was prepared
according to the literature.50,51 (TPP)H2 (0.25 g, 0.4 mmol),
anhydrous sodium acetate (0.8 g, 9.6 mmol), and GaCl3 (0.2 g, 1.1
(48) Sakurai, T.; Yamamoto, K.; Naito, H.; Nakamoto, N. Bull. Chem. Soc.
Jpn. 1976, 49, 3042-3046.
(44) Aida, T.; Inoue, S. J. Am. Chem. Soc. 1983, 105, 1304-1309.
(45) Aida, T.; Wada, K.; Inoue, S. Macromolecules 1987, 20, 237-241.
(46) Summerville, D. A.; Jones, R. D.; Hoffman, B. M.; Basolo, F. J. Am.
Chem. Soc. 1977, 99, 8195-8202.
(47) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem.
1970, 32, 2443-2445.
(49) Sugimoto, H.; Ueda, N.; Mori, M. Bull. Chem. Soc. Jpn. 1981, 54,
3425-3432.
(50) Witowska-Jarosz, J.; Gorski, L.; Malinowska, E.; Jarosz, M. J. Mass
Spectrom. 2002, 37, 1236-1241.
(51) Kadish, K. M.; Cornillon, J. L.; Coutsolelos, A.; Guilard, R. Inorg.
Chem. 1987, 26, 4167-4173.
2594 Inorganic Chemistry, Vol. 44, No. 8, 2005

Porphyrin- and Salen-M(III) Cations
mmol) were added to 100 mL of acetic acid. The mixture was stirred
and refluxed overnight at 120 °C. The reaction mixture was allowed
to cool to room temperature. After the removal of acetic acid, the
product was extracted with CHCl3. The crude product was purified
by column chromatography with CHCl3 as the eluent over an
alumina column. After CHCl3 was removed, the product was dried
overnight under vacuum at 100 °C, yielding 0.15 g of product. UV-
vis spectroscopy was used to confirm the formation of (TPP)GaCl.51
1H NMR (CDCl3, δ, ppm): 7.76, 8.20, 9.07 (aromatic). ESI/MS
data: m/z ) 681, (TPP)Ga+.
AlOCH(S)MeCH2Cl for X-ray measurement were obtained by slow
evaporation of the solvent from a concentrated S-PO solution of
the complex.
Homopolymerization Reactions of PO. In a typical reaction,
0.02 mmol of catalyst was allowed to react in 2 mL of neat PO
(28.6 mmol) in a flask at room temperature, with addition of 2.4
mg of DMAP (0.02 mmol) if needed. After a certain time, the
resulting mixture was analyzed by 1H NMR. Then the reaction was
quenched by adding methanol/HCl solution, and the obtained
polymer was analyzed by gel permeation chromatography.
Synthesis of R,R-salen-Aluminum Acetate. To a solution of
(R,R-salen)H2 (2.0 g, 3.7 mmol) in 20 mL of dichloromethane was
added a solution of trimethylaluminum (AlMe3 2.0 M solution in
hexane, 2.0 mL, 4.0 mmol), and the mixture was stirred for 3 h at
room temperature. A yellow powder was obtained after the removal
of volatile fractions under vacuum. The powder was redissolved
in hexane, followed by the addition of 0.25 mL of acetic acid (4.4
mmol), and stirred for overnight. A yellow precipitate was obtained
as the product (1.7 g). Crystals of this complex for X-ray mea-
surement were obtained by slow evaporation of the solvent from a
Copolymerization Reactions of PO/CO2. In a typical reaction,
0.02 mmol of catalyst was allowed to react in 2 mL of neat PO
(28.6 mmol) in a stainless steel reaction vessel (Parr) under 50 bar
CO2 pressure at room temperature, with addition of 2.4 mg of
DMAP (0.02 mmol) if needed. After a certain time, the resulting
mixture was analyzed by 1H NMR. Then the reaction was quenched
by adding methanol/ HCl solution, and the obtained polymer was
analyzed by gel permeation chromatography.
Acknowledgment. We thank the U.S. Department of
Energy, Office of Basic Sciences, Chemistry Division, for
financial support of this work at OSU and the Swiss National
Science Foundation as well as the Research Commission at
the ETH Zu¨rich for support in Zu¨rich. Z.Z. thanks The Ohio
State University for an international travel grant.
1
concentrated pyridine solution of the complex. H NMR (CDCl3,
δ, ppm): 1.79 (s, CH3COO), 1.29 (d, C(CH3)), 1.50 (s, C(CH3)),
1.44, 2.05, 2.41, 2.56, 3.09, 4.01 (cyclohexyl), 7.02, 7.09, 7.51
(aromatic), 8.18, 8.36(HCdN). Anal. Calcd: C, 72.35; H, 8.79;
N, 4.44. Found: C, 72.21; H, 8.66; N, 4.35.
Reactions between (R,R-salen)AlCl and S-PO. A 0.5 g (0.8
mmol) amount of (R,R-salen)AlCl was allowed to react with 4 mL
(57 mmol) of neat S-PO in 10 mL of CH2Cl2 for 2 h at room
temperature. A yellow powder was obtained after the removal of
the excess of PO and solvent under vacuum. The products were
Supporting Information Available: Crystallographic data and
ORTEP views for (R,R-salen)AlOCHMe(S)CH2Cl‚(0.5PO) and
(R,R-salen)Al(O2CMe)‚(1.5py). This material is available free of
charge via the Internet at http://pubs.acs.org.
1
analyzed by H NMR. The major regioisomer was obtained by
recrystallization of the product in toluene. Crystals of (R,R-salen)-
IC048597X
Inorganic Chemistry, Vol. 44, No. 8, 2005 2595