Titanium and zirconium benzofuranoxides. Crystal structures and catalytic properties

Article abstract of DOI:10.1039/b715048b

Reactions of Ti(OⁱPr)₄ or Zr(OEt)₄ with 4 equivalents of 2,3-dihydro-2,2-dimethyl-7-benzofuranol (ddbfoH) in toluene gave neutral complexes that in the solid state are dimers of [Ti(μ-ddbfo) ₂(ddbfo)₆] and [Zr(ddbfo)₃(EtOH)(μ-EtO)] ₂ composition. The former could also be conveniently synthesized in a direct reaction of TiCl₄ with ddbfoH. This air-stable aryloxo compound was found to initiate living ring-opening polymerization of lactides affording polyesters with narrow molecular weight distribution. It also catalyzed addition of terminal acetylenes to aryl aldehydes. The Royal Society of Chemistry.

Full text of DOI:10.1039/b715048b

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www.rsc.org/dalton | Dalton Transactions
Titanium and zirconium benzofuranoxides. Crystal structures and catalytic
properties†
Katarzyna Krauzy-Dziedzic, Jolanta Ejfler, Sławomir Szafert and Piotr Sobota*
Received 1st October 2007, Accepted 22nd February 2008
First published as an Advance Article on the web 25th March 2008
DOI: 10.1039/b715048b
Reactions of Ti(OiPr)4 or Zr(OEt)4 with 4 equivalents of 2,3-dihydro-2,2-dimethyl-7-benzofuranol
(ddbfoH) in toluene gave neutral complexes that in the solid state are dimers of [Ti(l-ddbfo)2(ddbfo)6]
and [Zr(ddbfo)3(EtOH)(l-EtO)]2 composition. The former could also be conveniently synthesized in a
direct reaction of TiCl4 with ddbfoH. This air-stable aryloxo compound was found to initiate living
ring-opening polymerization of lactides affording polyesters with narrow molecular weight distribution.
It also catalyzed addition of terminal acetylenes to aryl aldehydes.
inexpensive substrate for the synthesis of pesticides, proved useful
Introduction
as a chelating agent for different transition metals. It is also able
to mediate mixed metal aryloxides,12 that are potential “single-
source” precursors to technologically important ceramic oxides.
Transition metal aryloxides have been the subject of intense
research since the early 1950’s. Interest first focused on the p
bonding abilities of aryloxo ligands as well as on a correlation
between their structure and resulting agglomeration of a complex
compound.1 The early applications of metal aryloxides in industry
focused on their use as antioxidants inhibiting decomposition of
mineral oils, varnishes and lacquers.
Results and discussion
Syntheses
More up to date chemistry turns the attention to the use
of metal aryloxides in synthetic organic chemistry. Especially
interesting is their use in enantioselective synthesis and catalysis.
For instance, titanium aryloxides catalyze the alkene/alkyne cross
coupling reactions that yield functionalized cyclic dienes.2 They
were successfully used for the cyclization of 1,6- and 1,7-dienes,3
addition of acetylenes to aromatic aldehydes,4 and enantioselective
reduction of ketones with boranes.5 Instead, zirconium aryloxide
was utilized as a catalyst in the first enantioselective Mannich type
reaction of aldimines with silyl enolates.6 This and some other
uses of zirconium aryloxides were nicely described by Yudin and
co-workers in their review on modified BINOLates in asymmetric
catalysis.7
Preparation of benzofuranoxide Ti(ddbfo)4 (1, ddbfoH = 2,3-
dihydro-2,2-dimethyl-7-benzofuranol) paralleled a common pro-
cedure for the preparation of metal aryloxides via ligand
exchange.1,13 As shown in Scheme 1 the direct reaction of Ti(OiPr)4
with 4 equivalents of 2,3-dihydro-2,2-dimethyl-7-benzofuranol in
toluene gave red 1 in 75% yield after workup. Compound 1 can
also be obtained with comparable yield (72%) in the direct reaction
of 4 equivalents of ddbfoH with TiCl4.
Besides their utilization in synthetic organic chemistry, titanium
and zirconium aryloxides constitute a base for covalent metal–
organic networks.8 In host–guest chemistry, they form inter-
esting metallocalixarenes.9 Moreover, they are valuable precur-
sors and initiators for different polymerization processes. They
are extensively exploited in homogeneous ethylene and a-olefin
polymerization.10 There has also been extensive recent interest in
their utilization in the polymerization of lactides and lactones.11
Such high molecular weight polyesters are becoming more and
more essential as biodegradable and environmentally friendly
alternatives for many of the current commodity polymers.
In this paper we report the synthesis, structural characterization
and reactivity of titanium and zirconium 7-benzofuranoxides. This
two-coordinating ligand that is derived from 7-benzofuranol, an
Scheme 1 Synthesis of 1.
The compound was characterized by 1H NMR. It showed one
set of signals of coordinated ddbfo ligands in expected positions
proving complete exchange of the OiPr groups. Also the elemental
analysis was correct for the Ti(ddbfo)4 formulation. The data
suggested monomeric species, although rapid exchange of the
bridging/terminal aryloxo ligand with Ti–O bond cleavage could
not be ruled out. The possible agglomeration of 1 in solution
was further studied with variable-temperature 1H NMR. The
spectra were collected to −65 ◦C showing no substantial changes
compared to the RT data and supporting the assumption of 1
being a monomer in solution.
Department of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383,
Wrocław, Poland
† CCDC reference numbers 629476 and 629477. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b715048b
2620 | Dalton Trans., 2008, 2620–2626
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Although metal aryloxides are claimed to be highly sus-
ceptible to hydrolysis,13 compound 1 appeared to be quite
stable in air. Similar stability was underlined for heteroleptic
titanium catecholates10f and mentioned for homoleptic Ti(2,6-
1
iPr2C6H3O)4.14 The H NMR of 1 after four weeks of exposure
to the air and moisture remained unchanged. The compound
is soluble in halogenated organic solvents and fairly soluble in
hexanes and toluene. It melts with decomposition at 140 ◦C (TGA
analysis revealed a substantial mass loss that starts around that
temperature).
Scheme 2 Synthesis of 2.
only brief exposure to moisture. It melts with decomposition at
143–145 ◦C.
Although agglomeration of 1 was not unambiguously excluded,
the spectroscopic and analytical data suggested a low coordinated
metal center. Based on that, the coordination abilities of 1 were
tested. The reaction with an excess of PPh3 in toluene gave a
Crystal structures of 1 and 2
1
red phosphine adduct in 38% yield. The H NMR data clearly
Complexity of the structures of the metal aryloxides depends
greatly on the substituents on the aryl ring. For instance
Ti(OPh)4·HOPh in the solid state is a dimer,17 while more
bulky 2,6-Me2C6H3OH,18–19 2,6-iPr2C6H3OH,20 2-tBuC6H4OH14 or
2,3,5,6-Me4C6HOH14 form monomeric Ti(OAr)4 species.
The crystal structure of 1 was determined as outlined in Table 1
and described in the Experimental section. In the solid state
complex 1 is a centrosymmetric dimer as shown in Fig. 1.
A discrete molecule of 1 contains two pentacoordinated tita-
nium atoms in an arrangement that is between trigonal bypiramid
and square pyramid (see O–Ti–O bond angles in Table 2) with a
s parameter of 0.3.21 The TiO5 core has five different Ti–Oaryloxo
distances of 1.7599(14), 1.8105(14), 1.8305(13), 1.9992(13) and
suggested coordination of two molecules of PPh3 that was further
confirmed by elemental analysis.
In contrast to titanium, homoleptic monomeric zirconium
aryloxides have proven to be much more elusive. The lit-
erature mentions two examples, but for one of them no
crystallographic data were published.15 Lately the homoleptic
ionic [NH2(CH3)2]2[Zr(OC6H4-2-Cl)6]·2THF was presented by
Giolando and co-workers.16 In an attempt to obtain a homoleptic
zirconium analog of 1 a reaction of Zr(OEt)4 with 4 equivalents
of ddbfoH was carried out as shown in Scheme 2.
Workup gave somewhat expected analytically pure heteroleptic
2 in 56% yield as colorless crystals. Longer reaction times,
different stoichiometries as well as higher temperatures invariably
˚
2.0783(13) A. The last two values that characterize the aryloxo
bridge show the magnitude of the rhombohedron asymmetry,
which, due to the centrosymmetry of 1, is ideally flat.
The terminal Ti–O distances are similar to that found
in monomeric Ti(2,6-Me2C6H3O)4 (1.7853(17), 1.7841(18),
1
lead to the same product. Complex 2 was characterized by H
NMR spectroscopy, elemental analysis and X-ray crystallog-
raphy. It is soluble in toluene and dichloromethane and can
be stored under dinitrogen for extended periods but tolerate
Table 1 Crystallographic data for 1 and 2
Complex
1
2
¯
¯
Space group
Crystal system
Chemical formula
M
P1
P1
Triclinic
C80H88O16Ti2
1401.30
12.1696(7)
12.8552(6)
13.0040(7)
73.920(4)
81.944(4)
Triclinic
C68H88O16Zr2
1343.82
11.1465(5)
12.1096(7)
13.1966(8)
93.782(5)
111.460(5)
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
64.201(5)
1759.50(16)
92.187(4)
1650.45(16)
3
˚
V/A
Z
1
1
qcalc/g cm−3
1.322
0.296
1.352
0.382
l/mm−1
T/K
F(000)
100(2)
740
100(2)
704
Crystal size/mm
Range for data collection/◦
Index ranges (h, k, l)
Reflections collected
Independent reflections
Rint
0.3 × 0.3 × 0.3
0.4 × 0.3 × 0.04
3.09 to 28.47
3.18 to 28.50
−15 to 16; −16 to 16; −17 to 17
−14 to 14; −16 to 15; −17 to 16
21 465
19 807
8172
0.0406
6059
442
7632
0.0447
6169
411
Reflections [I > 2r(I)]
Parameters
R indices (all data)
GOF
Dr (max.; min.)/e A
R1 = 0.0519 (0.0836) wR2 = 0.0994 (0.1067)
R1 = 0.0457 (0.0686) wR2 = 0.0843 (0.0900)
1.175
1.081
3
˚
0.324; −0.457
0.418, −0.365
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◦
◦
˚
˚
Table 3 Bond lengths [A] and angles [ ] for 2
Table 2 Bond lengths [A] and angles [ ] for 1
Ti(1)–O(11)
1.9992(13) Ti(1)–O(31)
2.0783(13) Ti(1)–O(41)
1.8305(13)
1.8105(14)
1.7599(14)
Zr–O(11)
Zr–O(21)
1.9685(15) Zr–O(41)
2.0086(16) Zr–O(41ꢀ)
1.9637(16) Zr–O(51)
3.4734(5)
2.1310(15)
2.1785(16)
2.2774(19)
Ti(1)–O(11ꢀ)
Ti(1)–O(21)
Zr–O(31)
O(11)–Ti(1)–O(21)
O(11)–Ti(1)–O(31)
O(11)–Ti(1)–O(41)
O(11)–Ti(1)–O(11ꢀ)
O(21)–Ti(1)–O(31)
O(21)–Ti(1)–O(41)
O(21)–Ti(1)–O(11ꢀ)
O(31)–Ti(1)–O(41)
136.46(6)
O(31)–Ti(1)–O(11ꢀ) 154.25(6)
Zr–Zrꢀ
90.63(6)
108.02(6)
68.96(6)
98.10(6)
110.40(6)
87.31(6)
105.54(6)
O(41)–Ti(1)–O(11ꢀ)
95.92(6)
O(11)–Zr–O(21)
O(11)–Zr–O(31)
O(11)–Zr–O(41)
O(11)–Zr–O(41ꢀ)
O(11)–Zr–O(51)
O(21)–Zr–O(31)
O(21)–Zr–O(41)
O(21)–Zr–O(41)ꢀ
Zr–O(11)–C(11)
Zr–O(21)–C(21)
Zr–O(31)–C(31)
Zr–O(41)–C(41)
99.13(7)
97.11(7)
160.08(6)
92.59(6)
86.86(7)
100.38(7)
93.53(7)
O(21)–Zr–O(51)
168.91(7)
95.66(6)
166.46(6)
88.03(7)
72.59(7)
78.35(7)
83.07(7)
Ti(1)–O(11)–C(11) 126.74(11)
Ti(1)–O(21)–C(21) 151.95(13)
Ti(1)–O(31)–C(31) 159.32(13)
Ti(1)–O(41)–C(41) 167.77(13)
Ti(1)–O(11ꢀ)–C(11ꢀ) 121.96(11)
Ti(1)–O(11)–Ti(1ꢀ) 111.04(6)
O(31)–Zr–O(41)
O(31)–Zr–O(41)ꢀ
O(31)–Zr–O(51)
O(41)–Zr–O(41ꢀ)
O(41)–Zr–O(51)
O(41)ꢀ–Zr–O(51)
87.32(7)
160.0(15)
139.91(15)
160.79(15)
127.26(14)
Zr–O(41ꢀ)–C(41ꢀ)
122.91(13)
Symmetry transformations used to generate primed atoms: −x + 2, −y +
1, −z.
Zr–O(51)–C(511)–Zr 129.8(6)
Zr–O(51)–C(512)–Zr 129.5(7)
Zr–O(41)–Zrꢀ
107.41(7)
Symmetry transformations used to generate primed atoms: −x + 1, −y +
1, −z + 2.
Fig. 1 View of 1 (hydrogen atoms were omitted for clarity). Symmetry
operation for related atoms: −x + 2, −y + 1, −z.
18–19
Ti(2,6-iPr2C6H3O)4 (1.781(3)
˚
1.7979(18) and 1.7990(18) A),
Fig. 2 View of 2 (hydrogen atoms were omitted for clarity; both positions
of disordered terminal EtOH are shown). Symmetry operation for related
atoms: −x + 1, −y + 1, −z + 2.
20a
t
14
˚
˚
and 1.780(3) A), Ti(2- BuC6H4O)4 (1.779(3) A) or Ti(2,3,5,6-
Me4C6HOH)4 (1.78(2), 1.76(2), 1.76(2), and 1.79(2) A). Also
14
˚
the bridging Ti–O distances are similar to that found in
17
˚
Ti(OC6H5)4·HOC6H5 (2.045(11) A). Interestingly, no ether oxy-
ꢀ
˚
Zr–O(41) of 2.1310(15) and 2.1785(16) A, respectively. Due to the
gen from the furan rings is involved in coordination to the metal
center. The Ti–O–C bond angles for terminal aryloxo ligands are
within 151.95(13) to 167.77(13)◦ suggesting substantial p bonding
character of the Ti–Oterminal bonds. There is a nice correlation
between the Ti–O bond lengths and the Ti–O–C bond angles (see
Table 2). The longer bridging Ti–O distances are also characterized
by much smaller Ti–O–C bond lengths.
symmetry it is also ideally flat.
Catalytic properties of 1
Addition of acetylenes to aldehydes. Stability of 1 against
moisture seemed very attractive for its use as a potential reagent
in different catalytic processes. For instance, the most widely used
catalyst precursor Ti(OiPr)4 has to be distilled prior to use in
order to remove oxohomopolymetallic products of its degradation.
It should also be standardized for having trustworthy results.
Complex 1 would appear a very attractive substitute for Ti(OiPr)4
if it has similar activity. To reveal its potential it was tested as a
catalyst in the addition of terminal acetylenes to aldehydes and as
an initiator of lactide ring-opening polymerization.
Addition of terminal acetylenes to aldehydes is one of the
very important methods of C–C bond formation. By this method
secondary propargyl alcohols are obtained that are very important
building blocks for numerous organic compounds. Addition of chi-
ral ancillary ligands enables this process to run enantioselectively.23
They are often added along with titanium species that, via
The neutral heteroleptic 2 also possesses a dimeric nature as
presented in Fig. 2. This centrosymmetric molecule exhibits two
identically coordinated ZrO6 cores.
The key bond lengths and angles are summarized in Ta-
ble 3 and they are similar to those found in Zr(O-2,6-
C6H3Me2)2(Me2calix)·CH2Cl2,9c and in ionic {Me2NH2}{[(O-2,6-
22
=
C6H3Me2)Zr]2(l-OCH2CH CH2)3}. Similar to 1 there is a nice
correlation between the Zr–Oterminal bond lengths and bond angles
that evidences the p character of the Zr–O bond. The longest
Zr–O(21) bond length is trans to the terminal EtOH molecule
that results from the protonation of the ethoxo ligand by acidic
benzofuranol in the reaction course. The ethoxo bridge is much
more symmetric compared to the bridge in 1 with Zr–O(41) and
2622 | Dalton Trans., 2008, 2620–2626
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Table 4 Addition of phenylacetylene to benzaldehyde with 1 as catalysta
ester end groups of the growing polymer chains and ddbfo ligands
coordinated to titanium in equimolar ratio (Fig. 3).
Entry
1/mol%
T/◦C
Yieldb (%)
1
2
3
4
5
60
60
60
10
30
25
60
100
25
85
68
42
86
100
25
a Acetylene/Et2Zn/aldehyde/1 = 10 : 10 : 3 : 1. b Isolated yield.
coordination, activate carbonyl groups and significantly improves
the final yield.
Fig. 3 1H NMR spectrum of the living polymer obtained with initiator 1
([M]0/[I]0 = 20) in toluene at 70 ◦C.
First the reaction of the addition of phenylacetylene to ben-
zaldehyde in toluene with 1 as the catalyst was tried and the
process was optimized. Results are presented in Table 4 showing
the optimum conditions to be 25 ◦C and 30 mol% catalyst
concentration. The reaction time after which no further conversion
was observed was 4 h.
1
The polymerization process was monitored by H NMR. The
molecular weight of the polymer increased linearly with respect
to the conversion of monomer indicating the living nature of the
polymerization system Fig. 4.
Under these optimized reaction conditions 1 was employed to
induce addition of terminal acetylenes to different aryl aldehydes.
As shown in Table 5 with one exception they all gave rise to the
desired products with moderate to very good yields.
Lactide polymerization. The catalytic behavior of 1 in the ring-
opening polymerization of L- and rac-lactide (L-LA, rac-LA) was
studied. As noted above, 1 in solution is most likely a monomeric
species and is able to easily form adducts with electron donors.
Treatment of 1 in toluene with two equivalents of L-LA readily
generated monomeric complex Ti(ddbfo)4(L-LA)2 as evidenced by
1H NMR. The spectrum showed characteristic resonances at d =
3.87 and 2.72 ppm assigned to methine (CH) protons of lactide and
methylene protons (CH2) of ddbfo ligands in a 1 : 2 ratio. Next the
polymerization of L-LA was tried with a monomer to initiator ratio
([M]0/[I]0) of 20, 100 and 200. At room temperature 1 displayed
poor reactivity with only 10% conversion being observed after 6
d. Instead, at 70 ◦C, complex 1 initiated polymerization of L-LA
(with ([M]0/[I]0) = 200) in 90% conversion within 20 h to afford
PLA with Mn of 13 600 and PDI of 1.10. Interestingly, the degree
of polymerization was almost a half of the monomer to initiator
ratio suggesting the formation of two polymer chains from each
Fig. 4 Plot of Mn vs. conversion at 70 ◦C in toluene with [M]0/[I]0] = 35
using 1 as the initiator.
The parameters of PLA obtained with initiator 1 and equimolar
amounts of ethanol as an initiating group were similar (Mn
=
28 000, PDI = 1.18) but the polymerization reaction proceeded
faster to reach 100% conversion within 12 h. Examination of the 1H
NMR spectrum of the obtained PLA shows resonances attributed
to ethyl ester as well as the hydroxyl chain ends and exactly one
polymer chain was formed. These results suggest a coordination–
insertion mechanism occurring through the insertion of L-LA into
aryloxo titanium.
1
molecule of 1. Additionally the H NMR spectrum of the living
polymer obtained with [M]0/[I]0 = 20 showed resonances from
The polymerization of rac-LA was next investigated. It has
been reported that carbonyl and methine carbon atoms are the
stereo-sensitive groups leading respectively to hexad and tetrad
sequences.24 Fig. 5 shows the 13C NMR spectra of carbonyl and
methine groups in PLA prepared using 1 as initiator. The intensi-
ties of the corresponding hexad and tetrad stereo sequences were
calculated according to the literature.25 These values suggested that
the ROP of rac-LA initiated by 1 show a preference for heterotactic
addition.
Table 5 Addition of terminal acetylenes to aromatic aldehydes promoted
by 1
Entry
Aldehyde
Acetylene
Yield (%)
≡
1
2
3
4
5
6
7
8
Benzaldehyde
PhC CH
PhC CH
100
91
85
28
66
43
57
67
≡
4-Bromobenzaldehyde
4-Chlorobenzaldehyde
≡
PhC CH
≡
≡
4-(TMSC C)benzaldehyde
PhC CH
≡
3,5-Dimethoxybenzaldehyde PhC CH
≡
2-Nitrobenzaldehyde
Benzaldehyde
Benzaldehyde
PhC CH
In conclusion, the efficient syntheses of homoleptic 1 and
heteroleptic 2 were developed. Both compounds were character-
ized by X-ray analysis to show dimeric structures. Complex 1
≡
4-MeC6H4C CH
≡
TMSC CH
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4 h. After that time it was cooled and the solvent containing HOiPr
was removed under vacuum. 30 mL of toluene was added and the
clear solution was refluxed for 6 h. The solution was then cooled
and the solvent was removed under vacuum. Hexanes (20 mL)
were added and the suspension was placed in a freezer overnight.
The red powder containing portions of crystalline material was
then filtered off, washed with cold hexanes (3 × 5 mL) and dried
under vacuum to give pure 1 in 75% yield (2.23 g; 3.18 mmol).
Method B. A 250 mL Schlenk flask was charged with TiCl4
(0.55 mL; 0.95 g; 5.03 mmol) and toluene (30 mL) and 3.00 mL
(3.30 g; 20.11 mmol) of ddbfoH was added. The colorless solution
immediately turned red-orange and the evolution of HCl started.
The solution was stirred at room temperature for 24 h and then
◦
Fig. 5 Expanded region of methine atoms in 13C NMR of poly(rac-LA)
obtained with 1 as initiator; (A) isi and (B) iii + iis/sii + sis.
the temperature was raised to 60 C and stirring was continued
until the evolution of gas had ceased. The solution was cooled,
concentrated under vacuum and left at room temperature. After
12 h a red crystalline material had deposited which was filtered
off and the filtrate was placed in the refrigerator to give after
overnight standing another portion of 1. Overall yield 72% (2.53 g;
3.62 mmol). Elemental analysis calcd for C40H44O8Ti (monomer:
700.66) (%): C, 68.57; H, 6.33. Found: C, 66.71 (66.67); H, 6.48
(6.66). 1H NMR (d, C6D6, 297 K): 1.32 (s, 24H, CH3), 2.76 (s, 8H,
CH2), 6.67–7.00 (m, 18H, Ph of ddbfo). 13C NMR (d, C6D6): 27.9
(8CH3), 43.9 (4CH2), 89.9 (4C(CH3)2), 116.9, 117.1, 121.1, 126.3,
148.3, 151.6 (24C of Ph).
survives extended periods in air and appears to be a good initiator
for controlled ring-opening polymerization of L-LA and rac-LA
providing monodisperse PLA with higher degrees of heterotactic
addition. The reaction showed a first-order dependence on [LA]
consisted with a coordination–insertion mechanism. Complex 1
also catalyzes the addition of terminal acetylenes to aryl aldehydes
to give propargyl alcohol derivatives in good yields.
Experimental
Ti(ddbfo)4(PPh3)2 (1·2PPh3). A 100 mL Schlenk flask was
charged with 1 (0.10 g; 0.14 mmol) and toluene (20 mL) and PPh3
(0.74 g; 2.82 mmol) was added. The solution was stirred for 3 h
and the solvent was evaporated. The residue was dissolved in warm
hexanes and the clear solution was placed in a fridge. Overnight a
red powder precipitated to give 1·2PPh3 in 38% yield (0.07 g; 0.05
mmol) Elemental analysis calcd for C76H74O8P2Ti (1225.25) (%):
C, 74.50; H, 6.09. Found: C, 74.38; H, 6.12. H NMR (d, C6D6,
297 K): 1.26 (s, 24H, CH3), 2.71 (s, 8H, CH2), 6.72–6.95 (m, 12H,
Ph of ddbfo), 7.15–7.17 and 7.48–7.54 (m, 30H, Ph3 of PPh3).
General data
All reactions were conducted under
a N2 atmosphere.
Chemicals were treated as follows: toluene, distilled from
Na/benzophenone; hexanes, distilled from P2O5; MeOH, dis-
tilled from Mg; Et2O, distilled from Na/benzophenone; 2,3-
dihydro-2,2-dimethyl-7-benzofuran alcohol (ddbfoH, Aldrich),
distilled prior to use; (3S)-cis-3,6-dimethyl-1,4-dioxane-2,5-
dione (L-LA; 98% Aldrich) and 3,6-dimethyl-1,4-dioxane-2,5-
dione (rac-LA; Aldrich) sublimed prior to use; benzalde-
hyde, 4-bromobenzaldehyde, 4-chlorobenzaldehyde, 3,5- and 2,4-
1
Zr2(l-OEt)2(HOEt)2(ddbfo)6
(2). Zr(OEt)4
(1.86
g;
6.85 mmol), ddbfoH 2.04 mL (2.25 g; 13.70 mmol), and
toluene (30 mL) were combined in a procedure analogous to
that for 1. After being refluxed for the second time the cloudy
solution was filtered, concentrated to one third and left at room
temperature overnight. White crystals of 2 precipitated. They
were filtered off, washed with a small amount of cold hexanes
(∼5 mL) and dried under vacuum. Yield 2.58 g (1.92 mmol; 56%).
Elemental analysis calcd for [C34H44O8Zr]2 (dimer: 1343.82) (%):
dimethoxybenzaldehyde, 2-nitrobenzaldehyde (6 × POCh), used
26
≡
≡
as received; 4-(TMSC C)benzaldehyde; PhC CH (Aldrich),
≡
≡
distilled prior to use; 4-MeC6H4C CH, TMSC CH (2 × Aldrich)
used as received; Ti(OiPr)4 (1.0 M solution in hexane), ZnEt2
(1.0 M solution in hexane), TiCl4, Zr(OEt)4 (4 × Aldrich), PPh3
(Fluka), Na2SO4, NaCl, aqueous HCl (3 × POCh), and C6D6
(Cambridge Isotope Laboratories), used as received.
NMR spectra were obtained on a BRUKER ESP 300E spec-
trometer. The weights and number-average molecular weights of
the PLAs were determined by gel permeation chromatography
(GPC; HPLC-HP 1090 II with DAD-UV/vis and RI detector
HP 1047A) using polystyrene calibration. Microanalyses were
conducted using a Vario EL III instrument (in-house).
1
C, 60.81; H, 6.46. Found: C, 60.20 (59.97); H, 6.40 (6.26). H
NMR (d, C6D6, 297 K): 1.26 (s, 36H, CH3), 2.71 (s, 12H, CH2),
3.49 (br s, 8H, OCH2CH3), 4.68 (br s, 12H, OCH2CH3), 6.52–7.05
1
(m, 18H, Ph of ddbfo); 13C { H} NMR (partial: C6D6, 297 K):
d = 28.0 (2C, CH3), 43.8 (1C, CH2), 90.00 (1C, C(CH3)2), 115.2,
115.9, 117.0, 117.1.
Syntheses
Addition procedure
Ti(ddbfo)4 (1). Method (A). A 250 mL Schlenk flask was
charged with Ti(OiPr)4 (1.20 g; 4.22 mmol) and toluene (30 mL)
and 2.50 mL (2.77 g; 16.88 mmol) of ddbfoH was added. The
colorless solution immediately turned red-orange. It was slowly
warmed to 100 ◦C and after 2 h it was refluxed for an additional
1 (usually around 0.05 g; 0.07 mmol) was placed in a Schlenk flask
and toluene (10 mL) was added. ZnEt2 (1.0 M solution in hexanes;
10 equivalents) was introduced and the mixture was stirred at room
temperature for 2 h. Next acetylene (10 equivalents) was added and
2624 | Dalton Trans., 2008, 2620–2626
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the stirring continued for 1 h. The orange-red solution was cooled
to 0 ◦C and treated with aldehyde (3 equivalents). The mixture was
allowed to warm to room temperature and stirred for 24 h. After
the reaction was complete (TLC monitored), it was cooled to 0 ◦C
and quenched with aqueous HCl (5%). The mixture was extracted
with diethyl ether. The organic layer was washed with brine, dried
over Na2SO4, concentrated and purified by flash chromatography.
The residue was analyzed by GC-MS.
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Catalytic polymerization
In a typical experiment the monomer of lactide was placed in
a Schlenk flask and a solution of 1 in toluene was added. The
reaction mixture was warmed to 70 ◦C with stirring. At 6 h
time intervals ca. 1 mL aliquots were taken out, the solvent
was removed under vacuum and the conversion was determined
using 1H NMR. After the reaction was finished it was terminated
with methanol and the sample was evaporated to dryness. The
remaining residues were redissolved in CH2Cl2 and the polymer
was precipitated with an excess of cold methanol. Filtration and
drying under vacuum yielded a white polymer.
X-Ray crystallography
Crystal data collection and refinement are summarized in
Table 1. Preliminary examination and intensity data collections
were carried out on a KUMA KM4 j-axis diffractometer with
graphite-monochromated Mo-Ka radiation and with scintillation
counter or CCD camera. All data were corrected for Lorentz
and polarization effects. Data reduction and analysis were carried
out with the Kuma Diffraction programs.27,28 The structures were
solved by direct methods29 and refined by the full-matrix least-
squares method on all F2 data using the SHELXL97 software.30
Hydrogen atoms were included in calculated positions and refined
in the riding mode using SHELXL97 default parameters. All
non-hydrogen atoms were refined with anisotropic displacement
parameters. The terminal EtOH molecules in 2 are disordered.
The occupancy factor for the more represented EtOH (C511 and
C521) is 0.52.
12 (a) J. Utko, S. Szafert, L. B. Jerzykiewicz and P. Sobota, Inorg.
Chem., 2005, 44, 5194; (b) J. Utko, J. Ejfler, S. Szafert, Ł. John, L. B.
Jerzykiewicz and P. Sobota, Inorg. Chem., 2006, 45, 5302.
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Acknowledgements
We thank the State Committee for Scientific Research (Poland)
for support of this research (grant No PBZ-KBN-118/T09/19).
20 (a) L. D. Durfee, S. L. Latesky, I. P. Rothwell, J. C. Huffman and K.
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The Royal Society of Chemistry 2008
©