Arylzinc alkoxides: [ArZnOCHPr2i]2 and Ar2Zn3(OCHPr2i)4 when Ar = C6H5, p-CF3C6H4, 2,4,6-Me3C6</

Article abstract of DOI:10.1021/ic048332i

From the reactions between diarylzinc compounds (Ar₂Zn) and the alcohol (Pr₂ⁱCHOH) in toluene, the compounds [ArZn(OCHPr₂ⁱ)]₂ (Ar = C₆H ₅, C₆F₅,

Full text of DOI:10.1021/ic048332i

Inorg. Chem. 2005, 44, 4777−4785
Arylzinc Alkoxides: [ArZnOCHPri ]2 and Ar2Zn3(OCHPri )4 When
2
2
Ar
)
C6H5, p-CF3C6H4, 2,4,6-Me3C6H2, and C6F5
Malcolm H. Chisholm,* Judith C. Gallucci, Hongfeng Yin, and Hongshi Zhen
Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue,
Columbus, Ohio 43210-1185
Received November 24, 2004
From the reactions between diarylzinc compounds (Ar2Zn) and the alcohol (Pri CHOH) in toluene, the compounds
2
[ArZn(OCHPri )]2 (Ar
) C6H5, C6F5, p-CF3C6H4, and 2,4,6-Me3C6H2) have been isolated and shown to exist in
2
equilibra with the trinuclear complexes Ar2Zn3(OCHPri )4 and Ar2Zn when Ar
) C6H5, C6F5, and p-CF3C6H4. The
2
trinuclear complexes have also been prepared from reactions of the Ar2Zn compounds with the alcohol, which
reveals that the ease of Zn−C(aryl) bond cleavage is sensitive to the nature of the Ar group: C6H5 > 4-CF3C6H4
> C6F5. The molecular structures of Ar2Zn3(OCHPri )4 where Ar
)
p-CF3C6H4 and C6F5 and [ArZn(OCHPri )]2 where
2
2
Ar
) C6F5, p-CF3C6H4, and 2,4,6-Me3C6H2 are reported based on single-crystal X-ray diffraction studies. The X-ray
structure of Zn(p-CF3C6H4)2 is also reported. The reactivity of these new compounds toward the polymerization of
propylene oxide (PO) and the copolymerization of PO and CO2 have been investigated along with related reactions
involving the partial hydrolysis of the Ar2Zn and R2Zn compounds, where R
) t-Bu, n-Bu, and n-Oct. These results
are compared with the previous studies employing Et2Zn as an organozinc precursor.
Introduction
where Ar ) C6H5, p-CF3C6H4, 2,4,6-Me3C6H2, and C6F5.
Also described are the reactions of these compounds toward
PO polymerization and copolymerization with CO2. Pertinent
to the present study is the recent pioneering work of
Darensbourg with zinc aryloxides and Coates’ studies of
â-diketonate zinc alkoxides in the copolymerization of
epoxides and carbon dioxide.3
Diethylzinc is commonly employed as a starting material
in the synthesis of alkeneoxide polymers. Most typically,
the diethylzinc is allowed to react with water or an alcohol
to form the active catalyst precursor system. The first report
of the formation of polypropylenecarbonate (PPC), which
is the regular alternating copolymer derived from propylene
oxide (PO) and carbon dioxide, appeared in 1969.1 This was
derived from the reaction between PO and CO2 on a ZnEt2/
H2O catalyst system. Discrete ethylzinc alkoxide clusters
were reported by Tsuruta and co-workers to be involved in
the polymerization of rac-PO to give isotactic polypropy-
leneoxide, poly R- and poly S-PO.2 In all of the polymeriza-
tions, the zinc-carbon bond is a spectator. This led us to
question whether related diarylzinc complexes might prove
useful as catalyst precursors since both steric and electronic
factors could now be brought to bear influence on the nature
of the active zinc site. In this paper, we describe our studies
of the reactions of diarylzinc compounds (Ar2Zn) with the
sterically demanding alcohol Pri2CHOH, which has led to
the formation of mixed aryl zinc alkoxides,((Ar)aZnb(OR)c)
Experimental Section
General. All reactions were carried out under an atmosphere of
N2 purified over 4 Å molecular sieves. Standard vacuum line,
Schlenk flasks, and N2-atmosphere drybox techniques were em-
ployed. Toluene, diethyl ether (Et2O), and tetrahydrofuran (THF)
were distilled from purple solutions of sodium/benzophenone.
2-Bromomesitylene (C9H11Br), 4-bromobenzotrifluoride (CF3C6H4-
Br), bromopentafluorobenzene (C6F5Br), 1,4-dioxane (C4H8O2), 2,4-
dimethyl-3-pentanol [HOCHPri ], and propylene oxide (C3H6O)
2
were bought from Aldrich and dried by 4 Å molecular sieves. Mg
(3) (a) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. Angew.
Chem., Int. Ed. 2002, 41, 2599-2602. (b) Cheng, M.; Lobkovsky, E.
B.; Coates, G. W. J. Am. Chem. Soc. 1998, 120 (42), 11018-11019.
(c) Allen, S. D.; Moore, D. R.; Lobkovsky, E. B.; Coates, G. W. J.
Am. Chem. Soc. 2002, 124 (48), 14284-14285. (d) Moore, D. R.;
Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2003,
125 (39), 11911-11924. (e) Byrne, C. M.; Allen, S. D.; Lobkovsky,
E. B. J. Am. Chem. Soc. 2004, 126 (37), 11404-11405. (f) Darens-
bourg, D. J.; Holtcamp, M. W.; Struck, G. E.; Zimmer, M. S.;
Niezgoda, S. A.; Rainey, P.; Robertson, J. B.; Draper, J. D.;
Reibenspies, J. H. J. Am. Chem. Soc. 1999, 121 (1), 107-116.
* Corresponding author. E-mail: chisholm@chemistry.ohio-state.edu.
(1) (a) Inoue, S.; Koinuma, H.; Tsuruta, T. J.Polym. Sci., Polym. Lett.
1969, 7, 287-292. (b) Inoue, S.; Koinuma, H.; Tsuruta, T. Makromol.
Chem. 1969, 130, 210-220.
(2) Ishimori, M.; Tsuruta, T. Kogyo Kagaku Zasshi 1966, 69 (12), 2310-
2314.
10.1021/ic048332i CCC: $30.25
Published on Web 05/25/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 13, 2005 4777

Chisholm et al.
0.93 g (8.1 mmol) of HOCHPr2i in 30 mL of toluene. The mixture
was stirred for 1 day, and volatile components were removed under
vacuum. The residue was re-dissolved by 20 mL of toluene and
0.5 g (4.3 mmol) of HOCHPr2i in 10 mL of toluene was added.
The solution was concentrated to saturation status and put in a
refrigerator at -15 °C. Fine crystals were obtained. Yield: 1.0 g,
pellets, ZnCl2, 1.9 M PhLi solution in cyclohexane, 1.0 M Et2Zn
solution in hexane, and 2.5 M n-BuLi solution in hexane were
purchased from Aldrich and used as received. Zn(2,4,6-Me3C6H2)2,
Zn(C6F5)2, Zn(C6H5)2, Zn[N(SiMe3)2]2, (n-Bu)2Zn, (t-Bu)2Zn, and
(n-Oct)2Zn were prepared according to literature procedures.4-9
Elemental analyses were performed by Atlantic Microlab Inc.
1H and 13C {H} NMR experiments were carried out with a Bruker
DPX-400 spectrometer equipped with a 5 mm broad-band probe.
The sample concentration for polymers were 50 mg/mL in CDCl3,
and the parameters used were as follows: number of scans, NS )
2000, number of data point, TD ) 65536. Gel permeation
chromatography measurements were carried out on a Waters 1525
binary HPLC pump and Waters 410 differential refractometer
equipped with styragel HR 2, HR 4, and HR 6 columns (100 and
10000 Å). The GPC was eluted with THF at 35 °C running at 1
mL/min and was calibrated using polystyrene standard. Matrix-
assisted laser desorption/ionization time-of-flight mass spectroscopy
(MALDI-TOF) was performed on a Bruker Reflex III mass
spectrometer operated in linear, positive ion mode with a N2 laser.
2,5-Dihydroxybenzoic acid (DHBA) was used as matrix.
Synthesis. (a) (p-CF3C6H4)2Zn. 5.0 g (22 mmol) of CF3C6H4-
Br in diethyl ether (Et2O, 40 mL) at -78 °C was slowly added
into 8.8 mL (2.5 M in hexane, 22 mmol) of n-BuLi dissolved in
40 mL of Et2O. The mixture was stirred for 1 h under -78 °C
before 1.5 g of ZnCl2 (11 mmol) in Et2O (100 mL) was added.
The orange solution was warmed slowly to room temperature,
stirred overnight, and then filtered to remove LiBr. Volatile solvent
was removed under a dynamic vacuum, and the solid was purified
by vacuum sublimation (110 °C/10-2 mbar), yielding a white solid,
2.6 g, 67%.
1
43%. H NMR (benzene-d6, 400 MHz): δ 0.95 (d, J ) 6.8, 24H,
CH(CH3)2), 1.05 (d, J ) 6.8, 24H, CH(CH3)2), 1.80 (m, 8H, CH-
(CH3)2), 3.43 (t, J ) 5.6, 4H, CH), 7.53 (d, J ) 7.5, 4H, CH), 7.66
(d, J ) 7.5, 4H, CH). 13C{H} (benzene-d6, 100 MHz): δ 18.91,
20.19, 33.05, 85.11, 124.26, 137.8, 154.23. Anal. Calcd for
C42H68O4F6Zn2: C, 53.26; H, 7.24. Found: C, 52.33; H, 7.04.
(d) [2,4,6-(CH3)3C6H2]2Zn2(OCHPri )2. In a solution of 1.26 g
2
(4.1 mmol) of [2,4,6-(CH3)3C6H2]2Zn in 50 mL of toluene was
added 0.44 g (3.8 mmol) of HOCHPri2 in 50 mL of toluene slowly.
The mixture was stirred overnight and filtered, and the filtrate was
evaporated under vacuum. Yield: 1.0 g, 80%. 1H NMR (benzene-
d6, 400 MHz): δ 0.78 (d, J ) 6.7, 12H, CH(CH3)2), 0.99 (d, J )
6.7, 12H, CH(CH3)2), 1.57 (m, 4H, CH(CH3)2), 2.23 (s, 6H, CH3),
2.55 (s, 12H, CH3), 3.16 (t, J ) 5.9, 2H, CH), 6.92 (s, 4H, CH).
13C{H} (benzene-d6, 100 MHz): δ 18.08, 20.30, 21.28, 28.45,
32.15, 84.80, 126.37, 137.13, 144.32, 146.42. Anal. Calcd for
C32H52O2Zn2: C, 64.11; H, 8.74. Found: C, 61.85; H, 8.64. EI-
HRMS(m/z) calcd for C32H52O2Zn2: 598.2519; found: 598.2538
(3.2 ppm).
(e) (C6F5)2Zn2(OCHPri )2. To the solution of 0.5 g (1.2 mmol)
2
of (C6F5)2Zn in 20 mL of toluene was added 0.13 g (1.1 mmol) of
HOCHPri2 in 20 mL of toluene. Mixture was stirred for 1 h, and
the volatile components were removed under vacuum. As deduced
by NMR spectroscopy, the reaction was complete. 1H NMR
(benzene-d6, 400 MHz): δ 0.74 (d, J ) 6.7, 12H, CH(CH3)2), 0.91
(d, J ) 6.7, 12H, CH(CH3)2), 1.56 (m, 4H, CH(CH3)2), 3.27 (t, J
) 5.8, 2H, CH). 13C{H} (benzene-d6, 100 MHz): δ 17.24, 20.04,
31.72, 86.08, 136, 138.5, 147.59, 150.07. Anal. Calcd for
C26H30O2F10Zn2: C, 44.92; H, 4.35; F, 27.33. Found: C, 44.05;
H, 4.28; F, 26.58.
A single crystal was obtained by sublimation in a sealed glass
tube (60-80 °C, 10-2 mbar). 1H NMR (CDCl3, 400 MHz): δ 7.63
(d, J ) 7.6, 4H, C6H4), 7.80 (d, J ) 7.6, 4H, C6H4). 13C{H} (CDCl3,
100 MHz): δ 124.18, 124.31, 130.56, 137.76, 151.33. Anal. Calcd
for C14H8F6Zn: C, 47.29; H, 2.27. Found: C, 45.53; H, 2.19. EI-
HRMS (m/z) calcd for C14H8F6Zn: 353.9822. Found: 353.9841
(5.4 ppm).
(f) (C6F5)2Zn3(OCHPri )4. To the solution of 0.44 g (1.2 mmol)
2
(b) (p-CF3C6H4)2Zn2(OCHPri )2. To a solution of 1.06 g (3.0
2
of (C6F5)2Zn in 10 mL of toluene was added 0.26 g (2.2 mmol) of
HOCHPri2 directly. The mixture was heated to 85 °C and kept for
20 h. The volatile components were removed under vacuum. NMR
spectroscopy showed that 95% of (C6F5)2Zn2(OCHPri2)2 was con-
verted to (C6F5)2Zn3(OCHPri2)4. 1H NMR (benzene-d6, 400 MHz):
δ 0.94 (d, J ) 6.8, 24H, CH(CH3)2), 1.04 (d, J ) 6.8, 24H, CH-
(CH3)2), 1.74 (m, 8H, CH(CH3)2), 3.43 (t, J ) 5.6, 4H, CH).
13C{H} (benzen-d6, 100 MHz): δ 18.58, 19.93, 33.12, 85.44. Anal.
Calcd for C40H60O4F10Zn3: C, 48.48; H, 6.10; F, 19.17. Found:
C, 45.63; H, 5.77; F, 19.18.
mmol) of (p-CF3C6H4)2Zn in toluene (20 mL) was slowly added
0.32 g (2.8 mmol) of HOCHPri2 dissolved in toluene (30 mL). The
solution was stirred for 1 h, and volatile components were removed
under vacuum, yielding a white powder. The reaction was complete
according to NMR spectroscopy. 1H NMR (benzene-d6, 400
MHz): δ 0.80 (d, J ) 6.7, 12H, CH(CH3)2), 0.89 (d, J ) 6.7, 12H,
CH(CH3)2), 1.61 (m, 4H, CH(CH3)2), 3.22 (t, J ) 5.8, 2H, CH),
7.50 (d, J ) 7.8, 4H, CH), 7.58 (d, J ) 7.6, 4H, CH). 13C{H}
(benzene-d6, 100 MHz): δ 17.49, 20.31, 32.07, 85.98, 123.98,
124.37, 126.68, 129.38, 129.63, 129.95, 130.27, 130.58, 138.69,
150.47. Anal. Calcd for C28H38O2F6Zn2: C, 51.63; H, 5.88.
Found: C, 50.21; H, 5.79. EI-HRMS (m/z) calcd for C28H38O2F5-
Zn2: 631.1344. Found: 631.1352 (1.3 ppm).
(g) (C6H5)2Zn2(OCHPri )2. To a solution of 1.05 g (4.5 mmol)
2
of (C6H5)2Zn in toluene (20 mL) was slowly added 0.50 g (4.3
mmol) of HOCHPri2 dissolved in toluene (30 mL). The solution
was stirred for 1 h, and volatile components were removed under
vacuum, yielding a white powder. The reaction was complete
according to NMR spectroscopy. 1H NMR (benzene-d6, 400
MHz): δ 0.84 (d, J ) 6.7, 12H, CH(CH3)2), 0.96 (d, J ) 6.7, 12H,
CH(CH3)2), 1.70 (m, 4H, CH(CH3)2), 3.34 (t, J ) 5.8, 2H, CH),
7.26 (m, 2H, C6H5), 7.34 (m, 4H, C6H5), 7.74 (m, 4H, C6H5).
13C{H} (benzene-d6, 100 MHz): δ 17.74, 20.48, 32.29, 85.93,
128.12, 128.36, 138.98, 145.69. Anal. Calcd for C26H40O2Zn2: C,
60.59; H, 7.82. Found: C, 57.45; H, 7.41. EI-HRMS(m/z) calcd
for C26H40O2Zn2: 514.1580; found: 514.1628 (9.3 ppm).
(c) (p-CF3C6H4)2Zn3(OCHPri )4. To a solution of 2.6 g (7.3
2
mmol) of (p-CF3C6H4)2Zn dissolved in toluene (20 mL) was added
(4) (a) Seidel, W.; Bureger, I. Z. Anorg. Allg. Chem. 1981, 473, 166-
170. (b) Cole, S. C.; Coles, M. P.; Hitchcock, P. B. J. Chem. Soc.,
Dalton. Trans. 2003, 3663-3664. (c) Sun, Y.; Piers, W. E.; Parvez,
M. Can. J. Chem. 1998, 76 (5), 513-517.
(5) Markies, P. R.; Schat, G.; Akkerman, O. S.; Bickelhaupt, F.; Smeets,
W. J. J.; Spek, A. L. Organometallics 1990, 9 (8), 2243-2247.
(6) Sheverdina, N. I.; Paleeva, I. E.; Zaitseva, N. A.; Kocheshkov, K. A.
Dokl. Akad. Nauk SSSR 1964, 155 (3), 623-625.
(h) (C6H5)2Zn3(OCHPri )4. To a solution of 0.53 g (2.4 mmol)
(7) Lehmkuhl, H.; Olbrysch, O. Justus Liebigs Ann. Chem. 1975, 1162-
2
1175.
of (C6H5)2Zn in toluene (20 mL) was added 0.56 g (4.8 mmol) of
HOCHPri2 dissolved in toluene (30 mL). The mixture was stirred
(8) Thiele, K. H.; Mueller, J. J. Prakt. Chem. 1966, 33 (5-6), 229-234.
(9) Blacker, A. J.; Fielden, J. M. European Patent EP 0946570, 1998.
4778 Inorganic Chemistry, Vol. 44, No. 13, 2005

Arylzinc Alkoxides
for 1 day, and volatile components were removed under vacuum,
yielding a white powder. NMR indicated 97% conversion. By
recrystallization 0.30 g of crystalline sample was obtained, 61%.
Results and Discussion
Synthesis. The diarylzinc precursors Ar2Zn, where Ar )
C6H5, C6F5, and 2,4,6-Me3C6H2, were known compounds and
the new compound (p-CF3C6H4)2Zn was prepared similarly.
This series was selected from considerations of their electron-
releasing properties and also the steric hindrance around the
zinc center. Most organozinc compounds of the form ZnR2
are linear including (C6F5)2Zn and (2,4,6-Me3C6H2)2Zn.4
However, diphenylzinc has a dimeric structure in the solid-
state as a result of weak Zn‚‚‚C(ipso) bonds of the type
schematically represented by 1.5 It was, therefore, of interest
to us to determine the influence of the p-CF3 group on the
solid-state molecular structure of (p-CF3C6H4)2Zn and that
is reported later in this work.
1H NMR (benzene-d6, 400 MHz): δ 0.97 (d, J ) 6.8, 24H, CH-
(CH3)2), 1.11 (d, J ) 9.4, 24H, CH(CH3)2), 1.90 (m, 8H, CH(CH3)2),
3.48 (t, J ) 5.6, 4H, CH), 7.22 (m, 2H, C6H5), 7.34 (t, J ) 7.5,
4H, C6H5). 7.77 (m, 4H, C6H5). 13C{H} (benzen-d6, 100 MHz): δ
19.10, 20.22, 33.14, 85.07, 127.57, 128.19, 137.92, 148.99. Anal.
Calcd for C40H70O4Zn3: C, 59.23; H, 8.70. Found: C, 56.23; H,
8.49. EI-HRMS(m/z) calcd for C33H55O3Zn3: 693.1994; found:
693.2002 (1.2 ppm).
Attempted Preparation of [Zn(OR)2]a. To pure 1.0 g (2.6
mmol) of Zn[N(SiMe3)2]2 was added 1.5 g (12.9 mmol) of
HOCHPri2 (no solvent was used). The resulting mixture was
stirred for 3 days, and the volatile components were removed under
1
vacuum, yielding a white solid, 0.6 g. H NMR (benzene-d6, 400
MHz): δ 1.05 (m, CH(CH3)2), 1.16 (m, CH(CH3)2), 1.89 (m,
CH(CH3)2), 1.98 (m, CH(CH3)2), 3.28 (t, 1H, OCH), 3.37 (t, 1H,
OCH), 3.42 (t, 1H, OCH), 3.50 (t, 1H, OCH). 13C{H} (benzen-d6,
100 MHz): 17.99, 19.10, 19.12, 19.27, 19.95, 20.73, 20.88, 21.92,
32.84, 33.55, 34.65, 35.11, 84.43, 84.74, 84.87. Anal. Calcd for
C14H30O2Zn: C, 56.85; H, 10.22. Found: C, 51.77; H, 9.50.
Copolymerization Studies. The compounds prepared above
were all evaluated for the copolymerization of propylene oxide and
carbon dioxide. As a comparison, the compounds (p-CF3C6H4)2-
Zn, Zn(2,4,6-Me3C6H2)2, Zn(C6F5)2, Zn(C6H5)2, (n-Bu)2Zn, (t-
Bu)2Zn, (n-Oct)2Zn, and Et2Zn were also each allowed to react with
H2O in the ratio of 1:1 in three different solvents: 1,4-dioxane,
diethyl ether, and tetrahydrofuran. The volatile components in those
reaction mixtures were removed under vacuum, and the residues
were then used as polymerization reagents. All the copolymerization
reactions were carried out by employing 50 mg of the solid catalysts
with 2 mL of propylene oxide under a pressure of carbon dioxide
(60 bar) at 60 °C in a 45-mL autoclave fitted with a magnetic stirrer
for 20 h. Yields of products were determined by 1H NMR
spectroscopy; the results are listed in Table 4.
[ArZn(OCHPri )]2. The reactions between Ar2Zn and 1
2
equiv of the alcohol in toluene at room temperature in all
cases yielded [ArZn(OCHPri )]2 compounds as white air-
2
sensitive, crystalline solids that were readily soluble in
benzene and toluene. The compound where Ar ) p-CF3C6H4
was unstable in solution and existed in equilibrium with
Ar2Zn and Ar2Zn3(OR)4 (eq 1):
Homopolymerization Studies. 50 mg of each complex [ArZn-
(OCHPri2)]2 and Ar2Zn3(OCHPri )4 was added to 2 mL of propy-
2
lene oxide at 60 °C in a 45-mL autoclave fitted with a magnetic
stirrer for 20 h. 50 mg of corresponding hydrolyzed Ar2Zn (with
ratio Ar2Zn: H2O ) 1:1) described previously were also prepared
in THF and used as heterogeneous catalysts with 2 mL of propylene
oxide in Schlenk flasks, stirring for 20 h. Yield of products were
Ar2Zn + Ar2Zn3(µ-OR)4 ) 2[Ar2Zn2(µ-OR)2]
(1)
1
estimated by H NMR spectroscopy.
For Ar ) CF3C6H4, the trinuclear compound was notably
less soluble in toluene which impedes the crystallization of
the dimeric compound Ar2Zn2(OR)2. Mixing pure Ar2Zn and
Ar2Zn3(OR)4 yields the dimeric compounds Ar2Zn2(OR)2, as
X-ray Crystallography. All the crystals are clear and colorless.
All work was done on a Nonius Kappa CCD diffractometer with
Mo KR radiation at either 150 or 200 K using an Oxford
Cryosystems Cryostream Cooler. A combination of φ and ω scans
with a frame width of 1.0 was used, and highly redundant data
sets were measured. Data integration was done with Denzo.10
Scaling and merging of the data were done with Scalepack.10 The
structures were solved by the Patterson method or by direct methods
in SHELXS-86.10 The rest of the non-hydrogen atoms were located
by standard Fourier methods. Full-matrix least-squares refinements
based on F2 were performed in SHELXL-93.10 Neutral atom
scattering factors were used and include terms for anomalous
dispersion.
1
judged by H NMR spectroscopy, when Ar ) p-CF3C6H4.
Ar2Zn3(OCHPr2i )4. The compounds Ar2Zn2(OCHPri)2
react further with the diisopropylmethanol in toluene in a
manner that reflects upon the electronic and steric properties
of the aryl group. For Ar ) C6H5, C6F5, and p-CF3C6H4, the
trinuclear complex is the kinetic product; its rate of formation
follows the order C6H5 > p-CF3C6H4 > C6F5. Indeed,
whereas the compound (C6H5)2Zn3(OCHPri )4 is formed in
2
toluene at room temperature within 1 day (>90% conver-
sion), the related pentafluorophenyl derivative requires
heating to 90 °C for an equivalent period. In the case of Ar
) 2,4,6-Me3C6H2, a trinuclear complex could not be isolated
because further alcoholysis occurred to give what we
(10) (a) Otwinowski, Z.; Minor, W. Macromolecular Crystallography;
Carter, C. W., Jr., Sweet, R. M., Eds.; Methods in Enzymology Series
276; Academic Press: San Diego, 1997; Part A, pp 307-326. (b)
Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467-473. (c) Sheldrick,
G. M. Universitat Gottingen, Germany, 1993.
Inorganic Chemistry, Vol. 44, No. 13, 2005 4779

Chisholm et al.
Table 1. Summary of Crystallographic Data
1
2
3
4
5
6
formula
fw
temp (K)
space group
a (Å)
b (Å)
c (Å)
C14H8F6Zn
355.57
C28H38F6O2Zn2
651.32
C32H52O2Zn2
599.48
C26H30F10O2Zn2
695.24
C42H68F6O4Zn3
947.07
C40H60F10O4Zn3
990.99
200
15
150
200
150
200
P21/n
P21/n
P21/c
P1h
P1h
P21/c
9.9007(1)
14.1319(2)
9.9475(2)
11.8764(1)
21.9804(3)
12.3577(2)
8.1374(1)
22.4401(3)
8.8604(1)
7.8035(1)
9.7433(1)
11.0728(1)
114.135(1)
95.441(1)
104.636(1)
724.403(14)
1
12.4100(1)
14.2090(1)
14.7423(1)
75.629(1)
69.088(1)
76.624(1)
2322.89(3)
2
19.3632(1)
15.4720(1)
16.4475(1)
R (deg)
â (deg)
γ (deg)
V (Å3)
106.245(1)
109.728(1)
92.628(1)
110.604(1)
1336.24(4)
4
3036.61(7)
4
1.425
1.639
1616.24(3)
2
1.232
1.508
4612.28(5)
4
1.427
1.624
Z
d (calcd) (g/cm3)
1.767
1.896
1.594
1.742
1.354
1.597
abs coeff (mm-1
crystal Size (mm3)
θ range
)
0.12 × 0.12 × 0.23 0.35 × 0.38 × 0.38 0.04 × 0.19 × 0.27 0.15 × 0.23 × 0.31 0.08 × 0.27 × 0.35 0.19 × 0.35 × 0.38
2.57-27.43
25180
3052
1.038
0.0589
2.04-27.48
58774
6948
1.041
0.0485
0.0736
2.47-27.49
32103
3705
1.042
0.0357
0.0664
2.42-27.50
18855
3322
1.044
0.0292
0.0627
2.08-27.51
59944
10640
1.031
0.0642
0.1141
2.25-27.49
91848
10580
1.048
0.0467
0.0877
reflns collected
independent reflns
GOF on F2
R1(F)a (all data)
wR2(F2)a (all data) 0.1165
2
2
2
a R1(F) ) ∑||Fo| - |Fc||/∑|Fo|; wR2(F2) ) {∑w(Fo - Fc )2/∑w(Fo )2}1/2
.
b For data collection.
Figure 1. Arrangement of Zn(p-CF3C6H4)2 in unit cell.
presume to be the ultimate thermodynamic product [Zn-
(OR)2]n. Further alcoholysis employing Pri CHOH and the
2
other trinuclear arylalkoxides is much slower.
The Ar2Zn3(µ-OCHPri )4 compounds are white air-sensi-
2
Figure 2. Intermolecular interactions of Zn(p-CF3C6H4)2.
tive crystalline solids and must be handled in dry solvents.
All of the compounds react with their respective Ar2Zn
compounds to produce the dimeric compounds Ar2Zn2(OR)2,
as predicted by eq 1.
low carbon elemental composition, we propose it is an oxo
alkoxide and exists as an aggregate.
Spectroscopic Characterization Data. The H and 13C
1
[Zn(OR)2]a Where R ) Pri CH. The ultimate product of
2
NMR and mass spectral data for the new compounds are
given in the Experimental Section. We note that none of the
new compounds give satisfactory carbon elemental analysis.
This is quite puzzling as in all other respects the samples
prepared by crystallization appear quite pure and even the
proton and fluorine EA data are acceptable. The carbon
analyses are 1-3% low. Yet the compounds show molecular
ions in the mass spectrometer.
alcoholysis would be a compound of formula Zn(OR)2, few
of which are known and none has been structurally charac-
terized.12 Because of the ambiguity in the ultimate product
in the reaction between dimesitylzinc and diisopropylmetha-
nol, we examined the reaction between solid Zn(NSi2Me6)2
and the neat alcohol Pri2CHOH. This gave a white solid that
we formulate as [Zn(OR)2]2.8 ZnO based on elemental
1
1
analysis, infrared, and H NMR spectroscopy. In the H
NMR, there are at least three types of OR groups. We can
say little more about the nature of this compound beyond
that it is nonvolatile but soluble in benzene. Because of the
Solid-State and Molecular Structures. A summary of
crystallographic data is given in Table 1.
Zn(p-CF3C6H4)2. The molecule contains a near linear
C-Zn-C central moiety, 178.50(9)°, with Zn-C ) 1.939(2)
Å. The molecules are arranged in rows in the solid-state as
shown in the stereo unit cell drawing in Figure 1. Within
each row, the intermolecular interactions involve aryl
CH‚‚‚aryl π bonding as shown in Figure 2. Thus, in contrast
(11) Chisholm, M. H.; Gallucci, J.; Phomphrai, K. Inorg. Chem. 2002, 41
(10), 2785-2794.
(12) Su, K.; Tilley, T. D.; Sailor, M. J. J. Am. Chem. Soc. 1996, 118, 3459-
3468.
4780 Inorganic Chemistry, Vol. 44, No. 13, 2005

Arylzinc Alkoxides
Figure 3. ORTEP drawing of complex [CF3C6H4Zn(OCHPri )]2 with thermal ellipsoids drawn at 50% probability level. Hydrogen atoms are omitted for
2
clarity.
Figure 4. ORTEP drawing of complex (C6F5)2Zn2(OCHPri2)2 with thermal ellipsoids drawn at 50% probability level. Hydrogen atoms are omitted for
clarity.
to Zn(C6H5)2, there are no short intermolecular Zn‚‚‚C
Table 2. Selected Bond Distances (Å) and Angles (deg) for
[ArZn(OCHPri )]2
interactions of the type shown in 1.5
2
[ArZn(OCHPri )]2 Where Ar ) p-CF3C6H4, C6F5, and
Complex [CF3C6H4 Zn(OCHPri )]2
2
2
Zn(1)-O(1)
1.923(1)
1.944(2)
80.77(5)
Zn(1)-O(2)
1.926(1)
2,4,6-Me3C6H2. These three compounds share a common
structure involving two three-coordinate zinc atoms united
by a pair of alkoxide bridges. In the solid-state when Ar )
C6F5 and 2,4,6-Me3C6H2, there is a crystallographically
imposed center of inversion. This is not the case for Ar )
p-CF3C6H4 whose molecular structure is shown in Figure 3.
Selected bond distances and angles for these related com-
pounds are given in Table 2. The Zn-C distances are all
very similar, 1.942(3) Å (avg) and are mostly slightly longer
than the Zn-O distances that fall in the range of 1.91-1.95
Å. These Zn-µ-O distances are notably longer than the
Zn-O (terminal) distance of 1.800(5) Å in the three
coordinate complex CH(CMeN-2,6-Pri2C6H3)2Zn(OtBu).11
The O-Zn-O angles ca. 81° are notably smaller than the
O-Zn-C angles, which range from 132 to 148°. Those
structures resemble the architecture of the dimer zinc and
Zn(1)-C(15)
O(1)-Zn(1)-O(2)
O(1)-Zn(1)-C(15)
C(15)-Zn(1)-Zn(2) 176.74(6)
142.92(7)
O(2)-Zn(1)-C(15) 136.30(7)
C(1)-O(1)-Zn(1)
132.13(10) Zn(1)-O(1)-Zn(2)
99.49(5)
Complex [C6F5Zn(OCHPri )]2
2
Zn-O*
1.911(1)
1.940(2)
81.65(4)
140.98(5)
131.19(9)
Zn-O
1.913(1)
Zn-C(1)
O*-Zn-O
O-Zn-C(1)
C(7)-O-Zn
O*-Zn-C(1)
C(1)-Zn-Zn*
Zn-O-Zn*
137.37(5)
178.14(5)
98.36(4)
Complex [2,4,6-Me3C6H2(OCHPri )]2
2
Zn-O
1.916(1)
1.955(1)
80.07(5)
131.97(6)
132.6(1)
Zn-C(1)
1.945(2)
Zn-O*
O*-Zn-O
O*-Zn-C(1)
C(10)-O-Zn
O-Zn-C(1)
C(1)-Zn-Zn*
Zn-O-Zn*
147.62(6)
170.78(5)
99.93(5)
cadmium bis-aryloxides reported by Darensbourg for Ar )
2,6-tBuC6H3.13
Inorganic Chemistry, Vol. 44, No. 13, 2005 4781

Chisholm et al.
Figure 5. ORTEP drawing of complex [2,4,6-(CH3)3C6H2]2Zn2(OCHPri2)2 with thermal ellipsoids drawn at 50% probability level. Hydrogen atoms are
omitted for clarity.
Figure 6. ORTEP drawing of complex (C6F5)2Zn3(OCHPri )4 with thermal ellipsoids drawn at 50% probability level. Hydrogen atoms are omitted for
2
clarity.
Ar2Zn3(OCHPri )4, Where Ar ) p-CF3C6H4 and C6F5.
Ph2Zn3(OR)4, the yield of PPC was barely detectable (<1%).
2
Each reaction produced cyclic propylene carbonate (PC) with
yields varying from a trace amount for (C6F5)2Zn2(OR)2 to
12% for Ph2Zn2(OR)4. Thus Ar2Znx(OR)y are not comparable
to Darensbourg’s Zn(OAr)2(THF)2 catalysts, which gave
1441 g of polymer/g of Zn in copolymerization of cyclo-
hexene oxide (CHO) and CO2, and 100% PC for PO and
CO2 reactions.3f,13
These two structures are very similar, and that of the
pentafluorophenyl complex is given in Figure 6. The central
zinc(2+) ion is four-coordinate in a pseudo-tetrahedral
environment, while the outer zinc ions are three-coordinate
and closely resemble those seen in the molecular structures
of the [ArZn(OR)]2 compounds just described. A summary
of bond distances and bond angles is given in Table 3.
Copolymerization of Propylene Oxide and Carbon
Dioxide. The arylzinc alkoxides are inactive in the copo-
lymerization of PO and CO2 at 60 °C and 20 h. For
(CF3C6H4)2Zn2(OR)2, (C6F5)2Zn2(OR)2, and (C6F5)2Zn3(OR)4
no polypropylenecarbonate (PPC) was obtained; for
(CF3C6H4)2Zn3(OR)4, (C9H11)2Zn2(OR)2, Ph2Zn2(OR)2, and
As a comparison, corresponding partially hydrolyzed
Ar2Zn (Ar2Zn:H2O ) 1:1) in three different solvents were
studied as catalyst precursors under similar reaction condi-
tions. Results are listed in Table 4. Hydrolyzed Ph2Zn, which
was prepared in THF, resulted in the highest PPC yield of
33% with 4% cyclic carbonate and 10% polyether linkage.
A similar catalyst precursor prepared in dioxane gave the
second highest PPC yield of 24%. Other hydrolyzed aryl
(13) Darensbourg, D. J.; Wildeson, J. R.; Lewis, S. J.; Yarbrough, J. C. J.
Am. Chem. Soc. 2002, 124 (24), 7075-7083.
4782 Inorganic Chemistry, Vol. 44, No. 13, 2005

Arylzinc Alkoxides
Figure 7. ORTEP drawing of complex (p-CF3C6H4)2Zn3(OCHPri2)4 with thermal ellipsoids drawn at 50% probability level. Hydrogen atoms are omitted
for clarity.
Figure 8. 13C {1H} NMR of PPO prepared with Ar2Zn + H2O (1:1) at CH and CH2 region. (a) By (C6H6)2Zn + H2O. (b) By (p-CF3C6H4)2Zn + H2O. (c)
By (C6F5)2Zn + H2O.
zinc compounds resulted in PPC in yields that ranged from
0% for (C6F5)2Zn + H2O prepared in THF to 16% for
(CF3C6H4)2Zn + H2O prepared in Et2O. By looking at the
reactions with the highest PPC yield for each Ar2Zn, it can
be concluded from the ratio of polyether versus polycarbon-
ate linkages that electron-donating aryl groups favor incor-
poration of CO2 and copolymer formation. There is, however,
no obvious relationship between solvents and the yields of
PPC. Results of copolymerization reactions catalyzed by R2-
Zn + H2O (R) Et, n-Bu, t-Bu, n-Oct) in three solvents, THF,
Et2O, dioxane, are also listed in Table 4.
The long-chain catalyst precursor, (n-Oct)2Zn + H2O,
resulted in the highest PPC yield of 11% when THF was
used, and the yield decreased with the solvent sequence THF
> Et2O > dioxane. In contrast (n-Bu)2Zn + H2O yielded
19% PPC in dioxane and only a trace in THF. (t-Bu)2Zn +
H2O and Et2Zn + H2O each gave similar PPC yields for all
three solvents.
The regiosequence of the PPC prepared by partially
hydrolyzed Ar2Zn or R2Zn precursors was determined by
looking at the 13C {1H} NMR spectra of the carbonate carbon
signals.14 It is surprising to see that the ratio of the TT:HT:
HH junction for all PPC samples are close to 1:5:1, and thus
ring-opening probability at the â-position, the methylene
carbon of PO during PPC formation, is calculated to be 0.8.
This fact suggests that for copolymerization, the active zinc
centers have similar steric and electronic structures when
they activate and ring open PO. In other words, for
hydrolyzed aryl or alkyl zinc systems, only certain sites are
active, and these are not significantly influenced by the initial
R or Ar group.
Homopolymerization of Propylene Oxide. Unlike the
copolymerization reactions noted above, the homopolymer-
(14) (a) Chisholm, M. H.; Navarro-Llobet, D.; Zhou, Z. Macromolecules
2002, 35 (17), 6494-6504. (b) Lednor, P. W.; Rol, N. C. J. Chem.
Soc., Chem. Commun. 1985, 9, 598-599.
Inorganic Chemistry, Vol. 44, No. 13, 2005 4783

Chisholm et al.
Table 3. Selected Bond Distances and Angles for Ar2Zn3(OCHPri )4
Table 5. Homopolymerization of Propylene Oxide by Ar2Znx(OR)y and
Partially Hydrolyzed Ar2Zn Compounds
2
Complex (CF3C6H4)2Zn3[(OCHPri )]4
2
catalyst
PPO (%)
Mn (× 10-3
)
PDI
Zn(1)-O(1)
1.923(2)
1.946(3)
1.955(2)
1.962(2)
1.928(2)
82.55(7)
136.6(1)
113.09(7)
137.55(8)
83.05(7)
97.75(8)
97.81(8)
Zn(1)-O(2)
1.937(2)
1.952(2)
1.956(2)
1.928(2)
1.940(2)
140.84(9)
81.33(7)
139.89(8)
113.75(8)
98.36(8)
97.58(8)
178.77(1)
Zn(1)-C(29)
Zn(2)-O(1)
a
Ph2Zn2(OR)2
Ph2Zn3(OR)4
39
39
23
20
48
41
10
10.9
16.4
10.5
20.4
8.80
8.03
1.17
1.85
1.89
1.72
1.72
1.62
1.56
3.00
Zn(2)-O(4)
Zn(2)-O(2)
a
Zn(2)-O(3)
Zn(3)-O(3)
a
a
(CF3C6H4)2Zn2(OR)2
(CF3C6H4)2Zn3(OR)4
Zn(3)-O(4)
Zn(3)-C(36)
O(1)-Zn(1)-O(2)
O(2)-Zn(1)-C(29)
O(1)-Zn(2)-O(4)
O(1)-Zn(2)-O(3)
O(3)-Zn(2)-O(4)
Zn(1)-O(2)-Zn(2)
Zn(3)-O(4)-Zn(2)
O(1)-Zn(1)-C(29)
O(1)-Zn(2)-O(2)
O(4)-Zn(2)-O(2)
O(2)-Zn(2)-O(3)
Zn(1)-O(1)-Zn(2)
Zn(3)-O(3)-Zn(2)
Zn(3)-Zn(2)-Zn(1)
a
(C6F5)2Zn2(OR)2
(C6F5)2Zn3(OR)4
a
a
(C9H11)2Zn2(OR)2
Ph2Zn + H2O (1:1)b
>95
25
23
0
12.5
28.1
11.8
nac
1.53
1.79
1.51
nac
(p-CF3C6H4)2Zn + H2O (1:1)b
(C6F5)2Zn+ H2O (1:1)b
(C9H11)2Zn + H2O (1:1)b
Complex (C6F5)2Zn3[(OCHPri )]4
2
a Reactions were carried out under similar conditions: catalyst, 50 mg;
PO, 2 mL (1.2 g); temperature, 60 °C; reaction time, 20 h. Yield estimated
from 1H NMR spectrum of product mixture, CDCl3 as solvent. b Catalysts
prepared in THF. Polymerization reaction parameters are the same in a
except that reaction temperature is 25 °C. c na, not applicable.
Zn(1)-O(1)
1.916(1)
1.950(2)
1.956(1)
1.954(1)
1.903(1)
84.12(6)
140.22(7)
135.54(7)
116.94(6)
140.22(6)
96.98(6)
97.09(6)
Zn(1)-O(2)
1.913(1)
1.957(1)
1.954(1)
1.919(1)
1.950(2)
135.61(7)
81.99(6)
109.57(6)
81.98(6)
96.78(6)
96.63(6)
172.88(1)
Zn(1)-C(1)
Zn(2)-O(1)
Zn(2)-O(4)
Zn(2)-O(2)
Zn(2)-O(3)
Zn(3)-O(3)
Zn(3)-O(4)
Zn(3)-C(7)
O(1)-Zn(1)-O(2)
O(1)-Zn(1)-C(1)
O(1)-Zn(2)-O(4)
O(1)-Zn(2)-O(3)
O(2)-Zn(2)-O(3)
Zn(1)-O(2)-Zn(2)
Zn(3)-O(4)-Zn(2)
O(2)-Zn(1)-C(1)
O(1)-Zn(2)-O(2)
O(4)-Zn(2)-O(2)
O(4)-Zn(2)-O(3)
Zn(1)-O(1)-Zn(2)
Zn(3)-O(3)-Zn(2)
Zn(3)-Zn(2)-Zn(1)
Further support for this proposal is seen in the following.
The complex (C6H6)2Zn2(OR)2 was mixed with 10 equiv of
PO in a sealed J-Young tube and heated at 60 °C for 40 h,
30% of PO was converted into PPO, and there was no
observable change in chemical shifts for complex (C6H6)2-
Zn2(OR)2. The 13C {1H} NMR revealed that the stereose-
quence of PPO that was catalyzed by Ar2Znx(OR)y have
identical patterns at CH and CH2 region as those catalyzed
by partially hydrolyzed Ar2Zn catalysts. So it is quite
reasonable to propose that the active Zn-OH center comes
from the hydrolysis of (C6H6)2Zn2(OR)2. The regioregularity
of PPO followed the order of C6H6 > p-CF3C6H4 > C6F5.
As shown in Figure 5, hydrolyzed (C6H5)2Zn opens PO to
give over 90% regioregular PPO (HTHTHT), with enriched
ii tetrads being seen in the CH region.15 However, the
hydrolyzed (C6F5)2Zn catalyst system ring opens PO almost
randomly to give regioirregular PPO (see Figure 5). This is
evidence of the influence of the aryl group on the active
zinc center.
The regiosequence difference in the PPO can be explained
in terms of the activation of PO upon complexation to the
metal as originally proposed by Kuran.16 Electron-withdraw-
ing groups such as C6F5 generate Lewis acidic metal centers
and promote the ring-opening of PO by an SN1 mechanism.
This leads to regioirregular PPO as the PO undergoes C-O
bond rupture and formation at both the methine and meth-
ylene carbon atoms. With less electrophilic metal centers as
with phenyl or ethyl zinc + H2O, the ring-opening is
regioselective by attack at the methylene carbon, and the
mechanism can by described as metal-promoted base cat-
alysis.
Table 4. Copolymerization of Carbon Dioxide with Propylene Oxidea
PPC
PC PPO
(%)
Mn
catalyst
solventb (%)
(%) (× 10-3
)
PDI
(CF3C6H4)2Zn2(OR)2 toluene
(CF3C6H4)2Zn3(OR)4 toluene
(C9H11)2Zn2(OR)2
(C6F5)2Zn2(OR)2
(C6F5)2Zn3(OR)4
Ph2Zn2(OR)2
0
1
trace trace
3
4
trace
5
toluene trace
toluene
toluene
0
0
trace 12
1
8
toluene trace
2
12
2
trace
2
8
Ph2Zn3(OR)4
(CF3C6H4)2Zn + H2O THF
toluene
1
10
16
1
9
4
4
0
10
2
33
1
44.8
69.9
3.92
4.97
Et2O
dioxane
THF
Et2O
dioxane
THF
Et2O
dioxane
THF
1
1
2
11
trace
2
(C9H11)2Zn + H2O
(C6F5)2Zn + H2O
Ph2Zn + H2O
65.0
39.8
86.9
3.03
3.50
3.97
2
2
1
1
0
9
5
7
27.2
16.0
66.5
trace
75.0
3.57
1.49
4.19
1
1
4
10
Et2O
trace
5
dioxane 24
6
3.44
(n-Bu)2Zn + H2O
(t-Bu)2Zn + H2O
(Oct)2Zn + H2O
Et2Zn + H2O
THF
Et2O
dioxane 19
THF
Et2O
dioxane
THF
Et2O
dioxane
THF
Et2O
trace trace trace
2
trace
2
68.2
123
4.24
2.86
3.45
5.05
4.87
2.96
1.41
6
2
3
2
4
2
4
1
3
trace
4
3
5
9
3
11
3
28.7
45.6
46.7
77.4
312
2
trace trace
12
9
4
2
4
4
2
4
127
135
73.5
3.32
2.17
3.86
dioxane 11
Concluding Remarks. It seems that none of the discrete
zinc alkoxides reported here is active in either the homopo-
lymerization of PO or in its copolymerization with CO2. This
lends credence to the view that, in the Coates and Darens-
bourg catalyst systems, it is the terminal OR or OAr ligands
that initiate and sustain polymerization. Bridging OR groups
are inactive.
a All reactions were carried under similar conditions: catalyst, 50 mg;
PO, 2 mL (1.2 g); temperature, 60 °C; PCO , 60 bar; reaction time, 20 h; no
2
solvent applied. Yield estimated from 1H NMR spectrum of product mixture,
CDCl3 as solvent. b Solvent applied in preparation of catalysts.
ization of PO by Ar2Znx(OR)y resulted in 10-48% conver-
sion to PPO at 60 °C in 20 h. However, only -OH-ended
chains were observed by MALDI-TOF mass spectroscopy,
which suggests that it is Zn-OH, not Zn-OR that functions
as the active initiator during the homopolymerization of PO.
(15) Chisholm, M. H.; Navarro-Llobet, D. Macromolecules 2002, 35 (6),
2389-2392.
(16) Kuran, W. Appl. Organomet. Chem. 1991, 5 (3), 191-194.
4784 Inorganic Chemistry, Vol. 44, No. 13, 2005

Arylzinc Alkoxides
(OCHPri )4, CCDC 203297; [2,4,6-(CH3)3C6H2]2Zn2(OCHPri )2,
Acknowledgment. We thank the Department of Energy,
Office of Basic Science, Chemistry Division, and Petroleum
Research Foundation for finacial support of this work. We
also thank the staff of the Campus Chemical Instrument
Center (CCIC) for running the samples in Mass spectrometer.
2
2
CCDC 203298; (p-CF3C6H4)2Zn, CCDC 203299; (p-CF3C6H4)2-
Zn2(OCHPri )2, CCDC 203300; (C6F5)2Zn2(OCHPri )2, CCDC
2
2
203301. This material is available free of charge via the Internet at
http://pubs.acs.org.
Supporting Information Available: Crystallographic informa-
tion on (p-CF3C6H4)2Zn3(OCHPri )4, CCDC 203296; (C6F5)2Zn3-
IC048332I
2
Inorganic Chemistry, Vol. 44, No. 13, 2005 4785