Siloxide and triflate gallium(III) complexes supported by the BDI Ligand

Full text of DOI:10.1021/ic010771j

6506
Inorg. Chem. 2001, 40, 6506-6508
and I.6 We describe here our findings for gallium. Since
the inception of this work, others have reported the synthesis
of (BDI)Ga(I),7 (BDI)GaX2 , where X ) Cl, I, and Me,6 and
(BDI)GaNAr′.8
Siloxide and Triflate Gallium(III) Complexes
Supported by the BDI Ligand
Malcolm H. Chisholm,* Diana Navarro-Llobet, and
Judith Gallucci
Experimental Section
Department of Chemistry, The Ohio State University,
Newman and Wolfrom Laboratories, 100 West 18th Avenue,
Columbus, Ohio 43210
All syntheses were done under an argon atmosphere using standard
Schlenk-line and drybox techniques. Toluene and ether were distilled
from sodium benzophenone ketyl under nitrogen. Benzene and hexanes
were distilled from potassium metal under nitrogen. GaCl3 (Acros),
potassium trimethyl-siloxide, and silver triflate (Aldrich) were used as
purchased. The ligand BDI-H (2-(2,6-diisopropyl-phenylamino)-4-(2,6-
diisopropylphenyl-imino)-2-pentene) was synthesized according to
literature procedures,9 as was Li(BDI)(THF). 9 (BDI)GaCl2 was prepared
from GaCl3 and Li(BDI)(THF) in toluene.6 PO was stirred over calcium
hydride for 24 h and distilled under nitrogen. Lactide (Aldrich) was
sublimed three times under nitrogen. Deuterated solvents were stored
over 4 Å molecular sieves for 24 h prior to use.
1H and 13C NMR experiments were carried out with Bruker DPX-
250 MHz and Bruker DPX-400 spectrometers operating at proton
Lamour frequencies of 250 and 400 MHz, respectively. Their peak
frequencies were referenced against benzene-d6 at 7.15 ppm. 19F NMR
experiments were carried out on a Bruker DPX-250 MHz spectrometer
and were referenced against 10% CFCl3 in acetone-d6 at 0 ppm. IR
spectra were recorded in a Perkin-Elmer Spectrum GX FT-IR System.
ReceiVed July 20, 2001
The ligand formed from deprotonation of 2-(2,6-diisopropyl-
phenylamino)-4-(2,6-diisopropylphenyl-imino)-2-pentene, com-
monly called BDI, has already found an important position in
single-site polymerization catalysis. For the later transition
metals, it has been used in olefin polymerization catalysis,1 and
bonded to zinc2 and magnesium,3 it has been employed in lactide
polymerization and the copolymerization of cyclohexene oxide.1
The Coates’ catalyst [(BDI)Zn(OiPr)]2, I, is remarkable in
showing stereoselective polymerization of rac-lactide to give
heterotactic PLA.4 This presumably arises from end-group
stereocontrol, for which the phenyl groups assist in creating a
wedge in which the entering lactide and growing chain interact
prior to the ring-opening event. This assumes that the reactive
intermediate is a four-coordinate Zn or Mg complex, as depicted
by II.
Synthesis of (BDI)GaCl(OSi(CH3)3). (BDI)GaCl2 (0.2 g, 0.36
mmol) was dissolved in benzene. This solution was added to solid
KOSi(CH3)3 (0.046 g, 0.36 mmol) at room temperature. The yellow
solution was left stirring overnight, after which time, the solvent was
evaporated under vacuum. The product was filtered from hexanes to
give a yellow, clear solution, which was concentrated and recrystallized
at -20 °C to give crystals suitable for X-ray diffraction. Yield: 0.18
g, 82%. E. A. Calcd: C, 62.80; H, 8.23; N, 4.58. Found: C, 62.68; H,
8.06; N, 4.53. IR (KBr, cm-1): 3050, 2975, 2960, 2900, 1600, 1510,
1
1480, 1455, 1400, 1200, 1150, 925, 825, 800, 740. H NMR (C6D6,
ppm): δ 7.14, 7.12 (m, aromatic), 4.71 (s, 1H, CH), 3.59, 3.29 (heptet,
4H, CH(CH3)2), 1.52 (s, 6H, (CH3)), 1.515 (d, 6 H, 3JH-H) 6.569 Hz,
3
CH(CH3)2), 1.45 (d, 6 H, JH-H) 7.074 Hz, CH(CH3)2), 1.17 (d, 6 H,
3
3JH-H) 7.074 Hz, CH(CH3)2), 1.065 (d, 6 H, JH-H) 7.075 Hz, CH-
(CH3)2), 0.11 (s, 9H, OSi(CH3)3). 13C NMR (C6D6, ppm): δ 170.8 (CH-
C)N), 145.4, 144.0, 139.1, 125.1, 124.0 (aromatic), 96.2 (CH-Cd
N), 28.9 (CH3-CdN), 28.1, 26.4, 24.8, 24.8 ((CH3)2CH), 24.2, 23.4
((CH3)2CH), 2.8 (OSi(CH3)3).
We were interested to see what influence the BDI ligand
might exert in ROP of lactides at a five coordinate metal center.
Prompted by these considerations, we turned our attention to
group 13 elements, Al and Ga, with the intention of preparing
(BDI)MX(OR) compounds, where X is an unreactive ancillary
ligand, such as Cl, and OR represents a reactive group with
respect to the ring-opening event. In our hands, we were
unsuccessful in preparing a compound of this type for aluminum,
although (BDI)AlX2 compounds are known for X ) Me,5 Cl,6
Synthesis of (BDI)GaCl(OSO2CF3). Ether was added to a flask
containing (BDI)GaCl2 (0.2 g, 0.36 mmol) and silver triflate (0.092 g,
0.36 mmol) at room temperature. The solution was left stirring
overnight, after which time, a gray precipitate formed. After filtering,
the resulting clear, colorless solution was evaporated under vacuum to
give a white solid. Yield: 0.20 g, 83%. E. A. Calcd: C, 53.63; H,
6.15; N, 4.17. Found: C, 52.79; H, 6.19; N, 4.16. IR (KBr, cm-1):
1
2964, 2871, 1530, 1465, 1438, 1375, 1204, 970, 810, 635. H NMR
* To whom correspondence should be addressed. chisholm@
chemistry.ohio-state.edu.
(1) Feldman, J.; McLain, S. J.; Parthasarathy, A.; Marshall, W. J.;
Calabrese, J. C.; Arthur, S. D. Organometallics, 1997, 16, 5114-
5116.
(2) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 1998,
120, 11018-11019.
(C6D6, ppm): δ 7.16, 7.13 (m, aromatic), 4.76 (s, 1H, CH), 3.32, 3.20
3
(heptet, 4H, CH(CH3)2), 1.46 (s, 6H, (CH3)), 1.485 (d, 6 H, JH-H
)
3
8.084 Hz, CH(CH3)2), 1.355 (d, 6 H, JH-H) 6.568 Hz, CH(CH3)2),
1.09 (d, 6 H, 3JH-H) 7.074 Hz, CH(CH3)2), 1.05 (d, 6 H, 3JH-H) 6.569
Hz, CH(CH3)2). 13C NMR (C6D6, ppm): δ 173.4 (CH-C)N), 144.6,
144.4, 137.1, 128.9, 125.1 (aromatic), 98.2 (CH-CdN), 28.8 (CH3-
CdN), 28.5, 25.5, 24.8, 24.7 ((CH3)2CH), 24.2, 23.5 ((CH3)2CH), OSO2-
CF3 unresolved. 19F NMR (C6D6, ppm): δ -75.5 (s, CF3).
(3) (a) Chisholm, M. H.; Huffman, J. H.; Phomphrai, K. J. Chem. Soc.,
Dalton Trans. 2001, 222. (b) Chamberlain, B. M.; Cheng, M.; Moore,
D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem.
Soc. 2001, 123, 3229-3238.
(4) (a) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W. J.
Am. Chem. Soc. 1998, 120, 11583-11584. (b) Coates, G. W.; Ovitt,
T. M. J. Am. Chem. Soc. 1999, 121, 4072-4073.
(7) Cui, C.; Roesky, H. W.; Schmidt, H.-G.; Noltenmeyer, M.; Hao, H.;
Cimpoesu, F. Angew. Chem., Int. Ed. 2000, 39, 4274.
(8) Hardman, N. G.; Cui, C.; Roesky, H. W., Fink, W. H.; Power, P. P.
Angew. Chem., Int. Ed. 2001, 40, 2172-2174.
(9) Budzelaar, P. H. M.; Bart van Oort, A.; Guy Orpen, A. Eur. J. Inorg.
Chem. 1998, 1485-1493.
(5) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1998,
120, 9384-9385.
(6) Stender, M.; Eichler, B. E.; Hardman, N. J.; Power, P. P.; Prust, J.;
Noltemeyer, M.; Roesky, H. W. Inorg. Chem. 2001, 40, 2794-2799.
10.1021/ic010771j CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/19/2001

Notes
Inorganic Chemistry, Vol. 40, No. 25, 2001 6507
Figure 1. Two views of the compound (BDI)GaCl(OSiMe3).
10
Table 1. Crystallographic Data for Compound (BDI)GaCl(OSiMe3)
Initially, we employed the dimeric compound ClGa(NMe2)2
in reactions with BDI-H and (BDI)Li(THF). In the former case,
we observed no reaction, and in the latter, mixtures were
obtained. Starting with GaCl3 and (BDI)Li(THF), we were able
to obtain (BDI)GaCl2 as a monomeric crystalline compound in
a manner similar to that recently reported.6 Attempted metathetic
reactions involving (BDI)GaCl2 and LiNMe2, LiNiPr2, KOtBu,
BDIGaCl(OSiMe3)
empirical formula
fw
C32H50ClGaN2OSi
612.00
P21/m
space group
a (Å)
b (Å)
9.1113(1)
20.1484(2)
10.1963(1)
90.00(0)
115.313(1)
90.00(0)
150
NaOtBu, or KOCH2 Bu in benzene, toluene, Et2O, and THF all
t
c (Å)
R (deg)
â (deg)
γ (deg)
Temp (K)
Z
failed to give single products of either mono- or disubstitution.
The reactions proceeded slowly and always gave mixtures of
products. Heating often displaced the BDI ligand as BDI-H.
However, analogous reactions with KOSiMe3 in benzene at
room temperature did yield the desired product, (BDI)GaCl-
(OSiMe3), as a white crystalline solid, which was soluble in
common organic solvents (benzene, toluene, hexanes, etc).
2
V (Å3)
1692.10(3)
1.201
0.71073
0.953
0.0373
0.0766
D
calc (mg/m3)
λ (Å)
µ (mm-1
R1 (F)
)
The reaction between AgOSO2CF3 and (BDI)GaCl2 in ether
at room temperature was successful in yielding the triflate (BDI)-
GaCl(OSO2CF3), which was also a white crystalline solid. The
solution NMR characterization of these compounds (Experi-
mental Section) clearly identified the four-coordinate metal
center as a compound of formula (BDI)GaX(Y). Specifically,
the iPr methyl signals appear as four sets of doublets. There is
restricted rotation about the aryl C-N bonds and no C2 axis of
Rw (F)
Polymerization Studies. Attempted polymerization reactions were
done as follows: (1) PO. BDIGaClX (X)Cl, OSiMe3) was added to
PO monomer. The solution was left stirring for 3 days, after which
time, the solvent was evaporated under vacuum. No polymerization
occurred. (2) L-Lactide. Lactide (0.14 g, 0.97 mmol) was dissolved in
benzene. The compound BDIGaClX (X ) Cl, OSiMe3, OSO2CF3, 0.020
mmol) was added to the solution. After stirring at 60 °C for 3 days,
the solvent was evaporated under vacuum. No polymerization occurred.
PO/(BDI)GaCl(OSO2CF3). 1 mL of PO was mixed with 10 mg of
catalyst and left stirring at room temperature. After 48 h, the excess
monomer was evaporated under vacuum to give PPO. 1H NMR (C6D6,
ppm): δ 1.10, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19 (m, CH3), 3.36, 3.39,
3.40, 3.53, 3.54, 3.55, 3.56, 3.57 (m, CH and CH2). 13C NMR (C6D6,
ppm): δ 16.8, 17.1, 17.7, 18.0, 18.3, 18.4, 18.8 (m, CH3), 73.5, 73.8,
74.2, 74.7, 75.0, 75.2, 75.6, 75.7, 75.8, 76.2 (m, CH and CH2). Yield:
40%.
i
symmetry, which causes the Pr groups to have diastereotopic
methyl groups.
Solid State and Molecular Structure of (BDI)GaCl-
(OSiMe3). A summary of crystallographic data is given in Table
1, and an ORTEP drawing of the molecule is given in Figure
1. A summary of selected bond distances and angles is given
in Table 2 , where a comparison is also made with the related
structures of (BDI)GaCl2 and (BDI)Sn(OiPr). The latter com-
pound involves a three-coordinate pyramidal Sn(II) center.11
Formally, a lone pair could be viewed to occupy the fourth
position at Sn(II). Its inclusion here is relevant, as this compound
has been shown to be active in initiating the ROP of lactides.11
The geometrical parameters for the two gallium compounds are
not surprisingly similar with respect to the GaN2Cl moiety. The
Ga-O distance of 1.78 Å is short. The Ga-O-Si angle is 136°,
Attempted Preparation of (BDI)Ga(X)(Y) Compounds. Equal
molar quantities of (BDI)GaCl2 or BDIGaCl(OSiMe3) were mixed with
one of LiNMe2, LiNiPr2, KOtBu, NaOtBu, or KOCH2 Bu. A solvent
t
such as toluene, ether, THF, or hexanes was added. The reaction was
left stirring overnight, after which time, the solvent was evaporated
under vacuum. The remaining solid was washed with hexanes.
Results and Discussion
(10) Noth, H.; Konrad, P. Z. Naturforsch. 1975, 30b, 681.
(11) Dove, A. P.; Gibson, V. C.; Marshall, E. L.; White, J. P.; Williams,
D. J. Chem. Commun. 2001, 283-284.
The challenge in the synthesis of a (BDI)GaX(OR) compound
rests in the way in which the ligands are introduced to Ga.

6508 Inorganic Chemistry, Vol. 40, No. 25, 2001
Notes
Table 2. Selected Bond Distances (Å) and Angles (deg) for
close to 120°, and it appears that the SiMe3 group is constrained
to lie over the Ga-Cl bond as a result of the steric interactions
(BDI)GaCl2,a (BDI)GaCl(OSiMe3), and (BDI)Sn(OiPr)b
i
(BDI)GaCl2 (BDI)GaCl (OSiMe3) (BDI)Sn (OiPr)
with the Pr groups of the BDI ligand (see Figure 1).
The comparison with the Sn(II) compound reveals the notably
larger radius of Sn(II) relative to Ga(III). Given the position of
the two elements in the periodic table, this would seem to solely
reflect the influence of charge on the metal center. The angles
subtended at Sn(II) are all closer to 90°, in part in a reflection
of the greater 4p element bonding. Conversely, in comparing
the angles at Ga(III) and Sn(II), Ga employs more sp3 character;
for Sn(II), the 4s orbital is less involved as a result of the inert
pair effect of the ns2 valence configuration.
Inspection of Figure 1 and Table 2 allows one to anticipate
the addition of a substrate to the Ga(III) center to form a five-
coordinate adduct. The BDI ligand, which has a constrained
N-Ga-N angle of less than 100°, would be expected to occupy
an axial-equatorial site of a trigonal bipyramid. Based on
considerations of trans influence and steric factors, we would
anticipate the OR ligand to be equatorial, as depicted in III.
M-N
1.926(3)
1.906(3)
2.228(1)
2.218(1)
1.920(1)
2.206(4)
2.208(4)
M-Cl
2.1781(6)
M-O
N-M-N
1.789(2)
98.34(8)
109.92(5)
2.000(5)
83.6(2)
100.2(1)
Cl-M-X 110.20(4)
N-M-X
X)O
112.36(5)
111.74(4)
94.1(2)
92.5(2)
X)Cl
113.21 (4)c
110.09 (4)
a From ref 5. b From ref 11. c From our data.
Although the negative findings could be explained in a
number of ways, we are inclined to the view that substrate
uptake is rate limiting, i.e., lactide fails to bind significantly to
this sterically demanding center.
From our previous work, we know that propylene oxide,
PO, is a better ligand than lactide and competes with THF in
binding to three coordinate Al(III) centers.12 We therefore
investigated the reactions of (BDI)GaCl2, (BDI)GaCl(OSiMe3),
and (BDI)GaCl(OSO2CF3) with PO (see Experimental Section).
Of those, only the triflate showed reactivity yielding highly
regioirregular polypropylene oxide typical of Lewis acidic
coordinate catalysis.12
Concluding Remarks
The position of the chloride and substrate could be either
axial or equatorial, depending upon a variety of factors. In any
event, the substrate and OR group would be cis, a requirement
for migratory ring opening.
Attempted Polymerization Reactions. Despite the consid-
erations above, we found that the (BDI)GaCl(OSiMe3) com-
pound was inactive for ROP of L-lactide in benzene, even at
60 °C. [The (BDI)GaCl2 and the triflate compound were
similarly inactive, but based on our earlier studies they were
not expected to be active].12
Although we were only successful in preparing one compound
of formula (BDI)GaCl(OR), it seems likely that if other
alkoxides are subsequently prepared they will be poor catalysts,
if indeed effective at all, in the ROP of lactides.
Acknowledgment is made to the donors of the Petroleum
Research Fund, administered by the ACS, for support of this
work.
Supporting Information Available: CIF data for compounds
(BDI)GaCl2 and (BDI)GaCl(OSiMe3) are available. This material is
available free of charge via the Internet at http://pubs.acs.org.
(12) Antelmann, B.; Chisholm, M. H.; Huffman, J. C.; Iyer, S. S.; Navarro-
Llobet, D.; Pagel, M.; Simonsick, W. J.; Zhong, W. Macromolecules
2001, 34, 3159-3175.
IC010771J