Defect-Induced Acceleration of a Solid-State Chemical Reaction in Zinc Alkoxide Single Crystals

Article abstract of DOI:10.1021/ic030341o

A family of dinuclear (salicylaldimato)zinc(μ₂-alkoxide) complexes show unusual behavior, decomposing to olefins and the corresponding μ₂-hydroxide complexes under very mild conditions in the solid state. The reaction in homogeneous solution is, on the other hand, not observable, even at elevated temperatures over extended periods of time. The reaction in the solid state occurs preferentially on one crystallographic face and displays kinetics characteristic of polymorphic transformations in single crystals.

Full text of DOI:10.1021/ic030341o

Inorg. Chem. 2004, 43, 3164−3169
Defect-Induced Acceleration of a Solid-State Chemical Reaction in Zinc
Alkoxide Single Crystals
Eva Scho1n, Dietmar A. Plattner, and Peter Chen*
Laboratorium fu¨r Organische Chemie, Eidgeno¨ssische Technische Hochschule, Zu¨rich, Switzerland
Received December 19, 2003
A family of dinuclear (salicylaldimato)zinc(µ2-alkoxide) complexes show unusual behavior, decomposing to olefins
and the corresponding µ2-hydroxide complexes under very mild conditions in the solid state. The reaction in
homogeneous solution is, on the other hand, not observable, even at elevated temperatures over extended periods
of time. The reaction in the solid state occurs preferentially on one crystallographic face and displays kinetics
characteristic of polymorphic transformations in single crystals.
Introduction
solution, occurring visibly on the time scale of minutes even
at room temperature, as opposed to months at elevated
temperatures for the same complexes in solution. Moreover,
as can be observed macroscopically, in at least one poly-
morphic form of the crystal, the elimination occurs prefer-
entially on one crystallographic face. The combination of
unusual features makes this reaction a model by which the
general elimination reaction, and possible means by which
it may be manipulated to promote reaction under milder
conditions, can be studied.
Thermal decomposition of transition metal alkoxides is
widely used to prepare metal oxide films and materials.1 The
alkoxides are used because they are cost-effective and have,
in the best cases, a well-defined stoichiometry. Nevertheless,
their thermal decompositionsa reaction in which an olefin
is eliminatedsnormally requires high temperatures. We
report a novel acceleration of the olefin elimination reaction
in the solid state for a particular family of zinc alkoxide
complexes 1 which, depending on the polymorph, is triggered
by either mechanical damage to the surface of X-ray quality
single crystals or loss of an ordered solvent molecule.
Experimental Section
Methods and Materials. Unless otherwise specified, all syn-
theses and manipulations of Zn(II) complexes were carried out under
an argon atmosphere using standard Schlenk techniques or in an
argon filled glovebox. THF, benzene-d6, and toluene were distilled
from sodium prior to being used. The solvents for ligand syntheses
were purchased in p.A. quality from Fluka. Infrared spectra were
recorded on a Paragon1000 FT-IR spectrometer. The 1H NMR and
13C NMR spectra were recorded on Varian Gemini 300 and Varian
Mercury 300 instruments. Chemical shifts (δ) are given in ppm
relative to tetramethylsilane with residual solvent proton resonance
as internal standard and coupling constants in (J) in hertz. Elemental
analyses were carried out by the Mikrolabor of the Laboratorium
fu¨r Organische Chemie der ETH Zu¨rich.
General Procedure for the Preparation of the N-Isopropyl-
salicylaldimine Ligands. To a 1.1 M solution of N-isopropylamine
in dichloromethane was added the salicylaldehyde in an equimolar
ratio. After 1 h of stirring at room temperature the yellow reaction
mixture was dried with anhydrous MgSO4 and filtered. Evaporation
of the solvent gave the desired imine in excellent yield, which was
used without further purification.
General Procedure for [SalZnOtBu] 1a-e. To a solution of
zinc bis(trimethylsilyl)amide (0.76 mL, 1.909 mmol) in THF (8
mL) was added a solution of the 3-methylsalicylaldimine (338 mg,
Surprisingly, the solid-state reaction can proceed considerably
faster than the corresponding reaction in homogeneous
* Author to whom correspondence should be addressed. E-mail: chen@
org.chem.ethz.ch.
(1) Mehrotra, R. C.; Singh, A. Prog. Inorg. Chem. 1997, 46, 239.
Mehrotra, R. C. AdV. Inorg. Chem. Radiochem. 1983, 26, 269.
3164 Inorganic Chemistry, Vol. 43, No. 10, 2004
10.1021/ic030341o CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/17/2004

Solid-State Reaction in Zinc Alkoxide Single Crystals
1.909 mmol) in THF (2 mL). After 5 min of stirring, 172 of µL
t-BuOH (1.909 mmol) was added via a syringe to the yellow
solution. The resulting pale yellow-green solution was stirred for
an extra 90 min and then concentrated down to 1-2 mL.
[(3Ph-sal)Zn(µ-OtBu)]2 (1c). The complex 1c was prepared
according to the general procedure (77%). Anal. Calcd for (C20H25-
NO2Zn)2: C, 63.75; H, 6.69; N, 3.72. Found: C, 63.78; H, 6.64;
N, 3.91. 1H NMR (CD2Cl2, 300 MHz): δ 1.09 (s, 18H, C(CH3)3),
1.49 (d, J ) 6.54 Hz, 12H, CH(CH3)2), 3.72 (sept, J ) 6.54 Hz,
2H, CH(CH3)2), 6.64 (“t”, J ) 7.5 Hz, 2H, aryl-H), 7.12 (dd, J )
7.78, 1.87 Hz, 2H, aryl-H), 7.29-7.35 (m, 6H, Ph-H), 7.47 (dd, J
) 7.47, 1.87 Hz, 2H, aryl-H), 7.7-7.8 (m, 4H, Ph-H), 8.29 (s, 2H,
aldimine-H). 13C NMR (CD2Cl2, 75.45 MHz): δ 24.95, 33.41,
63.60, 71.42, 114.22, 118.95, 126.50, 127.86, 127.93, 130.05,
134.24, 135.37, 135.99, 140.09, 168.86, 169.52.
[(3Me-sal)Zn(µ-OtBu)]2 (1a). The resulting pale yellow-green
solution was stirred for an additional 90 min and then concentrated
down to 1-2 mL. Approximately 2 mL of hexane was added, and
the solution was then placed at -20 °C. Pale yellow crystals formed
after 2 days. The supernatant liquor was removed, and the crystals
were dried under vacuum and collected to yield 405 mg (68%).
Anal. Calcd for (C15H23NO2Zn)2: C, 57.24; H, 7.37; N, 4.45.
Found: C, 56.98; H, 7.23; N, 4.73. 1H NMR (C6D6, 300 MHz): δ
1.26 (d, J ) 6.54 Hz, 12H, CH(CH3)2), 1.41 (s, 18H, C(CH3)3),
2.50 (s, 6H, 3-CH3), 3.08 (sept, 2H, J ) 6.54 Hz, CH(CH3)2), 6.5-
6.65 (m, 2H, aryl-H), 6.8-6.9 (m, 2H, aryl-H), 7.2-7.3 (m, 2H,
aryl-H), 7.78 (s, 2H, aldimine-H). 13C NMR (C6D6, 75.45 MHz):
δ 17.18, 24.70, 33.50, 63.04, 71.50, 113.84, 116.74, 132.01, 133.82,
135.24, 169.27. X-ray analysis of the crystals revealed that the
complex exists as a µ-t-butoxide-bridged dimer in the solid state.
(3Me-sal)Zn(µ-OtBu)]2‚2Toluene (1a‚2Toluene). To a solution
of zinc bis(trimethylsilyl)amide (0.90 mL, 2.23 mmol) in toluene
(8 mL) was added a solution of the 3-methylsalicylaldimine 1 (395
mg, 2.23 mmol) in toluene (2 mL). After 5 min of stirring, 213 µL
of t-BuOH (2.32 mmol) was added via a syringe to the yellow
solution. The resulting pale yellow-green solution was stirred for
an additional 90 min and then concentrated down to 1-2 mL. The
solution was then placed at -20 °C. Colorless block-shaped crystals
formed after several days. The supernatant liquid was removed,
and the crystals were dried under an argon flow at -30 °C for 60
min (59%). Anal. Calcd for (C22H31NO2Zn)2: C, 64.94; H, 7.68;
[(3iPr-sal)Zn(µ-OtBu)]2‚thf (1d‚thf). The complex 1d‚thf was
prepared according to the general procedure (50%). Anal. Calcd
for (C34H54NO2Zn)2: C, 59.57; H, 7.94; N, 4.09. Found: C, 59.51;
H, 7.99; N, 4.01. 1H NMR (C6D6, 300 MHz): δ 1.25 (d, J ) 6.53
Hz, 12H, CH(CH3)2), 1.37 (d, J ) 6.84 Hz, 12H, NCH(CH3)2),
1.42 (s, 18H, C(CH3)3), 3.10 (sept, J ) 6.53 Hz, 2H, CH(CH3)2),
3.98 (sept, J ) 6.84 Hz, 2H, NCH(CH3)2), 6.67 (“t”, J ) 7.47 Hz,
2H, aryl-5H), 6.87 (dd, J ) 8.09, 1.87 Hz, 2H, aryl-4H), 7.34 (dd,
2H, J ) 7.16, 1.87 Hz, aryl-6H), 7.77 (s, 2H, aldimine-H). 13C
NMR (C6D6, 75.45 MHz): δ 23.06, 24.86, 27.07, 33.79, 63.05,
71.52, 114.17, 117.22, 131.18, 133.74, 142.39, 169.31. IR (KBr):
ν˜ ) 3425 (w), 2972, 1621, 1547, 1458, 1410 (s), 1368, 1326, 1288,
1259, 1231 (w), 1208, 1146 (m), 1104, 1052, 1019, 986 (w), 934
(m), 892, 854, 835, 807 (w), 750 (s), 712, 627, 590, 566, 529 (w)
cm-1. Block-shaped, slightly yellow crystals suitable for X-ray
analysis were obtained at -20 °C from THF/hexane after 2 days.
The X-ray analysis revealed that the complex exists as a µ-t-
butoxide-bridged dimer with one molecule of THF in the solid state.
[(5Nitrosal)Zn(µ-OtBu)]2‚Toluene (1e‚Toluene). The complex
1e‚toluene was prepared in toluene according to the general
procedure (77%). Anal. Calcd for (C14H20N2O4Zn)2‚(C7H8): C,
1
N, 3.44. Found: C, 60.97; H, 6.99; N, 3.92. H NMR (C6D6, 200
MHz): δ 1.25 (d, J ) 6.64 Hz, 12H, CH(CH3)2), 1.40 (s, 18H,
C(CH3)3), 2.11 (s, 3H, CH3-toluene), 2.50 (s, 6H, 3-CH3), 3.08 (sept,
J ) 6.64 Hz, 2H, CH(CH3)2), 6.58 (“t”, J ) 7.85 Hz, 2H, aryl-H),
6.86 (dd, J ) 7.85, 1.66 Hz, 2H, aryl-H), 6.95-7.2 (m, 5H, aryl-
H), 7.2-7.3 (m, 2H, aryl-H), 7.76 (s, 2H, aldimine-H). 13C NMR
(C6D6, 50.3 MHz): δ 17.40, 21.38, 24.84, 33.64, 63.17, 71.58,
113.84, 116.73, 125.68, 129.33, 132.00, 133.81, 135.20, 169.18,
170.48. IR (KBr, disk): ν˜ ) 3422 (w) [v(H-O-H)], 2966, 2896
(m), 2366, 1846, 1812, 1736 (w), 1614, 1550, 1465, 1415 (s), 1350,
1342 (w), 1326 (m), 1242, 1219, 1182, 1144, 1089 (m), 1040, 992,
1
53.65; H, 6.17; N, 7.15. Found: C, 53.37; H, 6.18; N, 7.23. H
NMR (C6D6, 200 MHz): δ 1.14 (d, J ) 6.53 Hz, 12H, CH(CH3)2),
1.24 (s, 18H, C(CH3)3), 2.11 (s, 3H, CH3-toluene), 3.02 (sept, J )
6.54 Hz, 2H, CH(CH3)2), 6.74 (d, J ) 9.65 Hz, 2H, Aryl-3H,),
7.0-7.2 (m, 5H, toluene-H), 7.26 (s, 2H, aldimine-H), 7.86 (d, J
) 3.11 Hz, 2H, aryl-6H), 7.98 (dd, J ) 9.65, 3.11 Hz, 2H, aryl-
4H). 13C NMR (C6D6, 50.28 MHz): δ 21.25, 24.36, 33.57, 62.99,
71.75, 116.15, 124.00, 125.71, 128.54, 129.33, 130.00, 136.69,
168.59, 175.74. IR (KBr): ν˜ ) 2968 (m), 2359 (w), 1625, 1604
(s), 1550 (m), 1508 (w), 1490 (m), 1438 (w), 1417, 1358 (m), 1317
(s), 1247, 1194, 1144, 1130, 1105 (m), 1033, 1000 (w), 948, 930
971 (w), 936 (s), 870 (w), 752 (s), 723, 628 (w), 594 (s) cm-1
.
X-ray analysis of the crystals revealed that the complex exits as a
µ-t-butoxide-bridged dimer with two toluene molecules per dimer
unit (one per metal atom) in the solid state.
(m), 842, 788, 762, 742, 690, 652, 590, 512 cm-1
.
(3Mesal)2Zn‚Toluene (2a‚Toluene). Zinc acetate dihydrate (436
mg, 2 mmol) was added to a solution of 3-methylsalicylaldimine
(709 mg, 4 mmol) in 20 mL of methanol and refluxed for 16 h.
The resulting yellow solution was cooled to 5 °C. The precipitate
formed was filtered and dried in a vacuum to give 660 mg (79%)
of yellow block-shaped crystals of the desired bis-salicylaldiminato
complex of zinc. Anal. Calcd for C22H28N2O2Zn: C, 63.24; H, 6.75;
[(5Me-sal)Zn(µ-OtBu)]2‚thf (1b‚thf). The complex 1b‚thf was
prepared according to the general procedure (75%). Anal. Calcd
for (C15H23NO2Zn)2: C, 57.24; H, 7.37; N, 4.45. Found: C, 56.77;
H, 7.00; N, 5.05. 1H NMR (C6D6, 300 MHz): δ 1.28 (d, J ) 6.64
Hz, 12H, CH(CH3)2), 1.41 (s, 18H, C(CH3)3), 2.14 (s, 6H, 5-CH3),
3.17 (sept, J ) 6.64 Hz, 2H, CH(CH3)2), 3.57 (traces THF), 6.65
(d, J ) 2.49 Hz, 2H, Aryl-3H), 7.01 (dd, J ) 8.72, 2.49 Hz, 2H,
Aryl-4H), 7.23 (d, J ) 8.72 Hz, 2H, Aryl-6H), 7.72 (s, 2H,
aldimine-H). 13C NMR (C6D6, 75.45 MHz): δ 20.24, 24.90, 25.83
(THF), 33.90, 62.97, 67.83 (THF), 71.53, 110.02, 117.30, 122.22,
124.19, 135.28, 136.83, 169.01, 170.45. IR (CH2CL2): ν˜ ) 3055
(s), 2975, 2360, 2305, 1758 (w), 1621 (s), 1534, 1458, 1474, 1421
(m), 1398, 1369, 1316, 1211, 1154, 1134, 1104 (m), 1063 (w),
896 (s), 831, 806 (w) cm-1. Block-shaped, slightly yellow crystals
suitable for X-ray analysis were obtained at -20 °C from THF/
hexane after 2 days. The X-ray analysis revealed that the complex
exists as a µ-t-butoxide-bridged dimer with one molecule of THF
in the solid state.
1
N, 6.70. Found: C, 63.20; H, 6.94; N, 6.63. H NMR (C6D6, 200
MHz): δ 0.82, 1.10 (2d, J ) 6.23 Hz, 12H, CH(CH3)2), 2.47 (s,
6H, 3-CH3), 2.91 (sept, J ) 6.23 Hz, 2H, CH(CH3)2), 6.58 (“t”,
2H, Aryl-H), 6.84 (dd, 2H, aryl-H), 7.26 (dd, 2H, aryl-H), 7.66 (s,
2H, aldimine-H). 13C NMR (C6D6, 75.45 MHz): δ 17.38, 23.38,
24.56, 62.87, 113.96, 117.34, 131.80, 133.99, 135.28, 169.26,
170.13. IR (KBr, disk): ν˜ ) 2962, 2915, 2359 (m), 1613, 1547,
1458, 1425, 1406 (s), 1368, 1316 (w), 1264, 1217, 11421, 1090
(m), 1033, 962 (w), 863 (w), 797, 750 (s), 722 (w) cm-1. X-ray
analysis of the crystals from toluene revealed that the complex exists
as a four coordinate monomer with distorted tetrahedral geometry
and one molecule of toluene in the solid state.
Inorganic Chemistry, Vol. 43, No. 10, 2004 3165

Scho1n et al.
(5Mesal)2Zn (2b). Zinc acetate dihydrate (280.97 mg, 1.28
mmol) was added to a solution of 5-methylsalicylaldimine (453
mg, 2.55 mmol) in 20 mL of methanol and refluxed for 16 h. The
resulting yellow solution was cooled to 5 °C. The precipitate formed
was filtered and dried in a vacuum to give 252 mg (77%) of yellow
crystals of the desired bis-salicylaldiminato complex of zinc. Anal.
Calcd for C22H28N2O2Zn: C, 63.24; H, 6.75; N, 6.70. Found: C,
63.17; H, 6.68; N, 6.60. 1H NMR (C6D 6, 200 MHz): δ 0.89, 1.15
(b, 12H, CH(CH3)2), 2.15 (s, 6H, 3-CH3), 2.96 (sept, 2H, CH(CH3)2),
6.63 (dd, 2H, aryl-H), 7.03 (dd, 2H, aryl-H), 7.21 (d, 2H, aryl-H),
7.61 (s, 2H, aldimine-H).
to the hydroxide; the NMR signals at the initial and at the final
stage were exactly the same.
Thermolysis of [(3Mesal)Zn(µ-OtBu)]2 (1a) to [(3Mesal)Zn-
(OH)]2 (1f). The crystals of the same batch used for X-ray analysis
were powdered for 1 min. The NMR of the obtained powder, taken
immediately, still showed the signals of the µ-bridged t-butoxide
1a; 10 mg of the powder was heated for 16 h (50 °C) to afford a
product that could be identified as the hydroxide by elemental
analysis and IR. Anal. Calcd for C22H30N2O4Zn2: C, 51.08; H, 5.85;
1
N, 5.42. Found: C, 51.34; H, 6.14; N, 5.24. H NMR (C6D6, 300
MHz): δ 0.81 (d, 3H, J ) 6.54 Hz, CH(CH3)2), 1.10 (d, 3H, J )
6.54 Hz, CH(CH3)2), 2.48 (s, 3H, 3-CH3), 2.90 (sept, 1H, J ) 6.54
Hz, CH(CH3)2), 6.59 (t, 1H, aryl-H), 6.83 (dd, 1H, J ) 7.89 Hz,
2.03 Hz), 7.27 (m, 1H, aryl-H), 7.64 (s, 1H, aldimine-H). IR (KBr,
disk): ν˜ ) 3396 (m, b) [v(O-H)], 2967, 2896 (m), 1618, 1550,
1455, 1429, 1409 (s), 1375 (w), 1317 (s), 1264 (w), 1216, 1145,
1090 (s), 1031 (m), 984, 968 (w), 869, 856 (m), 752 (s), 721, 626.
Thermolysis of (1a‚2Toluene) to [(3Me-sal)Zn(OH)]2 (1f). The
thermolysis of t-butoxide (1a‚2toluene) was carried out without
additional solvent in a sidearm flask with Young valve connected
with another sidearm flask. The zinc complex (75 mg) was held
for 16 h at room temperature in the closed flask, the valve was
then opened, and the volatile products were distilled for 6 h into
the connected cooled flask (-40 °C) filled with 1 mL of CDCl3.
The only detectable products were toluene and traces of condensed
606 (m), 544 (w) cm-1
.
Thermolysis of [(3Mesal)Zn(µ-OtBu)]2 (1a) to the 2:1 Com-
plex (3Mesal)2Zn (2a). Kinetic Study in Solution. In a drybox,
0.7 mL of toluene-d8, distilled trap-to-trap from molecular sieves
and then degassed by three freeze-pump-thaw cycles, and 80 mg
of the Zn complex 1a were placed in a NMR tube equipped with
a Young valve, the tube was closed, and the starting complex was
characterized by 1H NMR spectroscopy. The slightly yellow
solution was heated to 80 °C. The conversion was calculated by
the averaged ratio of intensities of the following signals of the 1:1
complex (δ 1.25 (d, 12H, CH(CH3)2), 2.38 (s, 6H, 3-CH3), 3.07
(sept, 2H, CH(CH3)2), 7.67 (s, 2H, aldimine-H)) to the correspond-
ing signals of the 2:1 complex (δ 0.80, 1.07 (2d, 12H, CH(CH3)2),
2.34 (s, 6H, 3-CH3), 2.89 (sept, 2H, CH(CH3)2), 7.68 (s, 2H,
aldimine-H)). After an initial immediate conversion of some 1a to
2a by traces of residual water, no further conversion could be seen
at 5, 13, and 81 days reaction time in the homogeneous solution.
A similar experiment with benzene-d6 at 50 °C gave the same
results.
Thermolysis of Crystals of [(3Mesal)Zn(µ-OtBu)]2‚2Toluene
(1a‚2Toluene) to the Hydroxide [(3Mesal)Zn(OH)]2 (1f). Kinetic
Study in the Solid State. Eight single crystals of 1a‚2toluene, all
of a similar size, were chosen directly from the toluene mother
liquor and placed in an empty Schlenk tube through which dry argon
was slowly flowed. Periodically, a crystal was removed and
dissolved in dry benzene-d6 (dried over molecular sieves, distilled,
and then filtered through activated Al2O3) and checked by 1H NMR.
This drying procedure works better than the sieves alone, and leaves
no significant trace water. The extent of conversion of 1a to 1f
(and subsequently to 2a in solution) was determined by comparing
the relative intensities of the peaks assigned to both the isopropyl
and methyl groups on the ligand. Moreover, the amount of toluene
remaining in the crystal could be determined by integration of peak
from its methyl protons. The results are displayed in Figure 4.
1
water. H NMR (CDCl3, 300 MHz): δ 1.55 (H2O), 2.36 (s, 3H,
CH3), 7.17 (m, 4H, aryl-CH(o/p)), 7.25 (m, 2H, aryl-CH(m)). The
solid, powdered residue could be identified as the 1:1 Zn complex
1
without toluene as solvent molecule. H NMR (C6D6, 300 MHz):
δ 1.26 (d, 6H, JHH ) 6.64 Hz, CH(CH3)2), 2.50 (s, 3H, 3-CH3),
3.08 (sept, 1H, J ) 6.64 Hz, CH(CH3)2), 6.58 (t, 1H, aryl-H), 6.86
(dd, 1H, J ) 7.89 Hz, 2.03 Hz), 7.26 (m, 1H, aryl-H), 7.76 (s, 1H,
aldimine-H). The solid residue (60 mg) was held for an extra 24 h
at ambient temperature, and the volatile products were distilled into
the connected, opened cooling trap (-40 °C) filled with 1.5 mL of
CDCl3 under an argon atmosphere. The detected, volatile products
1
were isobutene and traces of water. H NMR (CDCl3, 300 MHz):
δ 1.53 (H2O), 1.73 (t, 6H, J ) 1.25 Hz), 4.66 (sept, 2H, J ) 1.25
Hz). The powdered, slightly yellow product could by identified as
the hydroxide [(3Mesal)Zn(OH)]2, 1f, by elemental analysis and
IR. The NMR of the hydroxide 1f upon dissolution of the powder
in C6D6 showed the same signals as the bis-salicylaldiminato
complex of zinc 2a. Anal. Calcd for C22H30N2O4Zn2: C, 51.08; H,
5.85; N, 5.42. Found: C, 50.82; H, 5.95; N, 5.35. 1H NMR (C6D6,
300 MHz): δ 0.82 (d, 3H, J ) 6.54 Hz, CH(CH3)2), 1.10 (d, 3H,
J ) 6.54 Hz, CH(CH3)2), 2.47 (s, 3H, 3-CH3), 2.90 (sept, 1H, J )
6.54 Hz, CH(CH3)2), 6.59 (t, 1H, aryl-H), 6.83 (dd, 1H, J ) 7.89
Hz, 2.03 Hz), (m, 1H, aryl-H), 7.65 (s, 1H, aldimine-H). IR (KBr,
disk): ν˜ ) 3386 (s) [v(O-H)], 2968, 2896 (m), 1622, 1551, 1457,
1429, 1409 (s), 1375 (w), 1317 (s), 1240 (w), 1217, 1141 (s), 1090,
1034, 984, 968, 869, 856 (m), 752 (s), 721, 628 (w), 544 (m) cm-1
.
The transformation of a single crystal of (1a‚2toluene) to the
hydroxide at three different times is shown in Figure 3.
Attempted Thermolysis of Crystals of [(3Mesal)Zn(µ-OtBu)]2
(1a) to the Hydroxide [(3Mesal)Zn(OH)]2 (1f). X-ray Structural
Study. Single crystals suitable for X-ray analysis of 1a (without
ordered solvent) were obtained after 7 days from a concentrated
THF solution of the complex layered with hexane and maintained
at -20 °C. The colorless, block-shaped single crystal employed in
this experiment was picked out directly from the pale yellow mother
liquor. The temperature dependence of the thermolysis of 1a should
be determined in principle by heating gradually the single crystal
of 1a and solving the structure at different temperature. A phase
transformation of 1a could be monitored between 251 and 266 K,
but even after several days at 343 K no crystalline state transforma-
tion to the hydroxide or any other product proceeded. This result
was confirmed by NMR: three crystals of the dimeric zinc complex
1a from THF (15 mg) were dried for 30 min under an argon flow
at -20 °C and heated for 24 h at 50 °C. No conversion proceeded
Thermolysis of [(5Mesal)Zn(µ-OtBu)]2‚thf (1b‚thf) to [(5Me-
sal)Zn(OH)]2 (1g). The crystals of the t-butoxide (1b‚thf) were
heated for 16 h at ambient temperature under an argon atmosphere
to to afford a yellow powder object shaped as the initial crystal.
The product could be identified as the hydroxide 1g (100%) by
elemental analysis. The NMR of the hydroxide 1g after dissolution
of the powder in C6D6 showed the same signals as the bis-
salicylaldiminato complex of zinc 2b. Anal. Calcd for C22H30N2O4-
Zn2: C, 51.08; H, 5.85; N, 5.42. Found: C, 51.15; H, 6.00; N,
1
5.34. H NMR (C6D6, 200 MHz): δ 0.88, 1.15 (2d, 12H, CH-
(CH3)2), 2.15 (s, 6H, 5-CH3), 2.96 (sept, 2H, CH(CH3)2), 6.62 (d,
3166 Inorganic Chemistry, Vol. 43, No. 10, 2004

Solid-State Reaction in Zinc Alkoxide Single Crystals
2H, aryl-3H), 7.03 (dd, 2H, aryl-4H), 7.21 (d, 2H, aryl-6H), 7.61
(s, 2H, aldimine-H).
Thermolysis of [(3iPrsal)Zn(µ-OtBu)]2‚thf (1d‚thf) to [(3i
Prsal)Zn(OH)]2 (1h). The procedure is analogous to the one used
for hydroxide 1g. The obtained, slightly yellow powder could be
identified as [(3i Prsal)Zn(OH)]2 (1h) (100%) by elemental analysis
and IR. Anal. Calcd for C26H38N2O4Zn2: C, 54.46; H, 6.68; N,
1
4.89. Found: C, 54.54; H, 6.65; N, 4.74. H NMR (C6D6, 200
MHz): δ 0.84 (d, 3H, CH(CH3)2) 1.00 (d, 2H, CH(CH3)2) 1.10 (d,
4H, CH(CH3)2) 1.19 (d, 3H, CH(CH3)2), 1.37 (m, 12H, CH(CH3)2),
2.15 (s, 6H, 5-CH3), 2.92 (sept, 2H, CH(CH3)2), 3.95 (sept, 2H,
CH(CH3)2), 6.66 (m, 2H, aryl-H), 6.82 (m, 2H, aryl-H), 7.34 (dd,
2H, aryl-H), 7.68 (s, 2H, aldimine-H). IR (KBr, disk): ν˜ ) 3422
(m, b), 2966, 1624, 1547, 1460, 1429, 1409 (s), 1366, 1324, 1287,
1231 (w), 1208, 1146 (m), 1108, 1052, 1019 (w), 934, 893, 857,
Figure 1. Superposition of the molecular structures of 1a as found by
X-ray analysis. Crystals of (1a‚2toluene) grown from toluene and 1a in
solvent-free crystals grown from THF/hexane show similar structures.
837, 807 (w), 750 (s), 708, 669, 628, 606, 568, 521 (w) cm-1
.
with presumed coformation of Zn(OH)2 which would be
expected to be colloidal and hence difficult to see. One
assumes that 1f or 1g is the first-formed product in the solid
state or in solution, the subsequent ligand rearrangement to
form 2 in solution would be driven by the insolubility of
Zn(OH)2.
Thermolysis of [(5Nitro)Zn(µ-OtBu)]2‚Toluene (1e‚Toluene)
to [(5Nitro)Zn(OH)]2 (1i). The procedure is analogous to the one
used for hydroxide 1g. The obtained, slightly yellow powder could
be identified as [(5Nitro)Zn(OH)]2 (1i) (100%) by elemental analysis
and IR. Anal. Calcd for C20H24N4O8Zn2: C, 41.47; H, 4.18; N, 9.67.
Found: C, 41.73; H, 4.32; N, 9.35. IR (KBr, disk): ν˜ ) 3425 (m,b),
2963, 2925, 2868 (m), 2368 (w), 1736, 1686, 1654, 1637, 1604
(s), 1560, 1474, 1408 (m), 1319 (s), 1102 (m), 948, 906 (m), 835,
788, 755, 731, 688, 656 (w), 590.
While the elimination of isobutene is the expected reaction,
the observed rate of the reaction was initially extremely
confusing. Dissolved in solution, 1a was found to be stable
for up to nearly 12 weeks in both benzene and toluene at 50
°C and 80 °C, respectively. The time scale for decomposition,
at room temperature, of different X-ray-quality crystals of
1a and 1b depended on the origin of the particular crystal
and its handling, ranging from minutes to months. A striking
example of the intrinsic thermal stability of 1a is given by
a temperature-dependent X-ray structure study. A single
crystal of 1a, grown from THF/hexane, was selected and
carefully mounted (without cutting) under exclusion of
oxygen and moisture on the diffractometer. Starting at 191
K, the temperature was stepped up by increments of 5, 10,
or 20 K up to a final temperature of 343 K. The crystal was
held at each temperature for approximately 2 days, during
which a dataset was collected. Between 251 and 266 K, a
change in the cell dimensions of the unit cell was observed
(see Supporting Information), presumably corresponding to
phase transitions in the crystal, but the conformation of the
molecule structure, the basic composition, structure, and
visible appearance of the crystal remained unchanged
throughout the entire measurement period of eight weeks.
On the other hand, other identically grown crystals of 1a
could also be observed to react in the solid state within
minutes at room temperature with different handling.
With careful experimentation, reproducible conditions
were found by which the accelerated reaction could be
induced or inhibited in the solid state. These were tested
repeatedly with crystals from the same batch, as well as from
different batches. Crystals of 1a grown from THF/alkane and
containing no solvent in the unit cell are extremely thermally
stable until their surface is damaged, for example, by cutting
or scratching, upon which the crystal immediately begins to
decompose. The visible alteration of the crystal starts at the
site of damage and propagates rapidly throughout the entire
bulk, leaving a fragile, opaque object in the shape of the
original crystal. Crystals of 1a grown from toluene and
Results
While previous studies of salicylaldimato zinc complexes2
found only the 2:1 complexes 2, careful crystallization of
solutions containing zinc salts and salicylaldimines in the
proper stoichiometry produces the novel µ-alkoxy-bridged
1:1 complexes 1, the structures of which were confirmed by
single-crystal X-ray diffraction. The overall structural pattern,
with the doubly bridged dinuclear core, is a common motif
in zinc alkoxide complexes.3 In the present case, X-ray-
quality crystals of either 1 or (1‚solvent) can be reproducibly
prepared, depending on the crystallization solvent and
conditions. While the structure of 1a differs only slightly in
the crystals with or without toluene, Figure 1, the crystals
grown from toluene contain ordered solvent molecules, as
can be seen in the depictions of the unit cells in Figure 2.
As expected, the 1:1 complex 1 showed a single, clean,
quantitative thermal decomposition reaction: loss of isobutene,
converting the µ-alkoxy-bridged 1:1 complex into the µ-hy-
droxy-bridged analogue. The isobutene elimination reaction
was identified as the decomposition pathway in the solid state
by condensation of the released isobutene in a cold trap and
elemental analysis, as well as IR spectroscopy, of the
powdery material left by decomposition of the crystal. In
solution, the reaction can be followed by 1H NMR, by which
one sees conversion of the µ-alkoxy-bridged 1:1 complexes,
1a or 1b, into the corresponding 2:1 complexes 2a or 2b,
(2) Darensbourg, D. J.; Rainey, P.; Yarbrough, J. Inorg. Chem. 2001, 40,
986.
(3) Erxleben, A. Inorg. Chem. 2001, 40, 208. van der Schaaf, P. A.;
Wissing, E.; Boersma, J.; Smeets, W. J. J.; Spek, A. L.; van Koten,
G. Organometallics 1993, 12, 3624. Jastrzebski, J. T. B. H.; Boersma,
J.; van Koten, G.; Smeets, W. J. J.; Spek, A. L. Recl. TraV. Chim.
Pays-Bas 1988, 107, 263. Kageyama, H.; Kai, Y.; Kasai, N.; Suzuki,
C.; Yoshino, N.; Tsuruta, T. Makromol. Chem., Rapid Commun. 1984,
5, 89. Kageyama, H.; Miki, K.; Tanaka, N.; Kasai, N.; Ishimori, M.;
Heki, T.; Tsuruta, T. Makromol. Chem., Rapid Commun. 1982, 3, 947.
Inorganic Chemistry, Vol. 43, No. 10, 2004 3167

Scho1n et al.
Figure 2. Unit cells of crystals of 1a for crystals grown from toluene (left panel) versus those grown from THF/hexane (right panel). The former crystals
contain one ordered toluene molecule per zinc atom, while the latter are solvent-free.
Figure 3. Changes in a single crystal of (1a‚2toluene) as dry nitrogen is slowly flowed over it at 20 °C. The visible change corresponding to the conversion
of the single crystal into a powder aggregate in the shape of the original crystal proceeds selectively from the [100] face of the crystal.
containing one ordered molecule of toluene per metal atom
are stable at room temperature as long as they remain in the
mother liquor from which they were grown. If they are
removed from the mother liquor, they remain stable in a
closed Schlenk tube. If they are mechanically damaged, they
react quickly. If they are subjected to vacuum, or if a slow
stream of dry inert gas is blown over the crystals, they react
as well. The reaction of a single, well-faceted crystal of 1a‚
2toluene as dry nitrogen was gently blown over the crystal
at 20 °C was followed photographically over time. The
crystal at three different times is shown in Figure 3. The
reaction clearly occurs preferentially from one face of the
crystal, identified as the [100] face. A more quantitative
examination of the solid-state decomposition of single
crystals of (1a‚2toluene) is shown in Figure 4, where eight
Figure 4. Percentage of either 1a (blue) or toluene (red) remaining in
each one of eight single crystals of (1a‚2toluene) at the specified time as
dry argon is blown over the crystals at room temperature. The fit to the
toluene data is a single exponential; that for 1a is sigmoidal.
comparable single crystals were chosen, placed in a dry
Schlenk under flowing argon at room temperature, and then
one-by-one removed and analyzed for extent of conversion.
As is evident from Figure 4, approximately half of the toluene
escapes from the crystal rapidly, which triggers a somewhat
slower decomposition of 1a which is complete in a few hours
at room temperature.
electronic applications by thermolysis of metal alkoxides of
known structure and stoichiometry has led to a reexamination
of this long-known class of compounds. Particularly interest-
ing are metal alkoxides of low nuclearity because their
improved solubility properties4 make deposition of thin layers
more convenient. The vast majority of these alkoxides
require, however, high temperatures, typically >200°C, to
Discussion
The preparation of high-performance metal oxide materi-
als, thin films in particular, for mechanical, optical, and
3168 Inorganic Chemistry, Vol. 43, No. 10, 2004

Solid-State Reaction in Zinc Alkoxide Single Crystals
eliminate an olefinic fragment5 and thus begin transformation
to an oxide material.
polymorphic transformations.10 The putative role of physical
damage in initiating the present reaction finds precedent in
the theory put forth by Mnyukh,11 who proposed that phase
transitions in crystals initiate exclusively at defect sites. The
present solid-state reaction differs, however, from most
polymorphic transformations in that the specific process,
elimination of isobutene in this case, does not occur at a
perceptible rate in homogeneous solution even at tempera-
tures well in excess of those used in the solid-state reaction.
Nevertheless, the shape of the curve in Figure 4, as well as
the gross chemical behavior of crystals of 1, underlines the
fundamental similarity of the chemical process to those
generally regarded as polymorphic transformations.
The specialized zinc alkoxide complexes 1a-e exhibit the
same basic chemistry in that they undergo a clean, quantita-
tive elimination of isobutene upon thermolysis. The unsur-
prising reaction displays, however, surprising characteristics.
Whereas the solution-phase thermolysis indicates that the
temperature needed for a practical conversion is as high as
would be expected from the vast majority of analogous
complexes, the solid-state reaction can be much faster.
Moreover, the accelerated solid-state reaction requires some
kind of physical damage to the crystal surface for initiation.
Whether the physical damage is induced mechanically or
by evaporative loss of ordered solvent molecules from the
crystal, the reaction, once initiated, propagates rapidly
through the entire bulk of the crystal.
The observed phenomena are strongly reminiscent of
polymorphic transformations6 in organic crystals. Several
substances have well-documented solid-state transformations
corresponding to conformational changes in the molecule,
but which from the point-of-view of the crystal may be
treated as phase transitions.7,8 Most notable is the mecha-
nochromic behavior of dimethyl 3,6-dichloro-2,5-dihydroxy-
terephthalate,9 for which a phase transition in the solid state
may be induced by scratching the surface of the yellow
polymorph; the transformation to the white polymorph
propagates from the scratch along the [100] direction until
the entire crystal is transformed. Furthermore, the rate of
the solid-state transformation in the present work, depicted
in Figure 4, displays sigmoidal kinetics which is typical for
Conclusion
We have synthesized and characterized a new family of
zinc alkoxide complexes in which the stoichiometry of the
salicylaldimine ligand and the zinc is 1:1. The µ-bridging
alkoxide serves as a model for the investigation of the olefin
elimination from the more general metal alkoxide complex.
As we see for 1a and 1b, it is possible to induce a gigantic
rate acceleration, which potentially is of considerable practi-
cal value. While the experiment was performed on X-ray-
quality single crystals, mechanistic understanding could lead
to the same acceleration becoming available for films,
amorphous solids, or the polycrystalline materal. The mecha-
nistic studies are underway.
Acknowledgment. We acknowledge helpful discussions
with Professors Jack Dunitz and Reinhard Nesper at the ETH
Zu¨rich. We would like to thank Bernd Schweizer and Paul
Seiler for carrying out the X-ray analyses of complexes 1b-e
and 2a. Photographic monitoring by Christian Mensing is
also acknowledged. Support for the project came from the
Research Commission of the ETH and the Swiss National
Science Foundation.
(4) Geerts, R. L.; Huffman, J. C.; Caulton, K. G. Inorg. Chem. 1986, 25,
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Polyhedron 1994, 13, 1111. Lee, G. R.; Crayston, J. A. AdV. Mater.
1993, 5, 434. Lee, G. R.; Crayston, J. A. Chem. ReV. 1993, 93, 1205.
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Organomet. Chem. 2001, 630, 253. Ashby, E. C.; Willard, G. F.; Goel,
A. B. J. Org. Chem. 1979, 44, 1221.
(6) Caira, M. R. Top. Curr. Chem. 1998, 198, 163. Dunitz, J. D. Acta
Crystallogr. B 1995, 51, 619. Dunitz, J. D.; Bernstein, J. Acc. Chem.
Res. 1995, 28, 193.
(7) Kaftory, M.; Botoshansky, M.; Kapon, M.; Shteiman, V. Acta
Crystallogr. B 2001, 57, 791. Mataka, S.; Moriyama, H.; Sawada, T.;
Takahashi, K.; Sakashita, H.; Tashiro, M. Chem. Lett. 1996, 363.
Soltzberg, L. J.; Cannon, S. M.; Ho, Y. W.; Armstrong, E. C.;
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Supporting Information Available: Seven X-ray crystal-
lographic files, in CIF format. Details of X-ray crystallography,
table of X-ray crystallographic data, and figures depicting X-ray
crystal structures. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC030341O
(8) Sheridan, A. K.; Anwar, J. Chem. Mater. 1996, 8, 1042.
(9) Na¨ther, C.; Nagel, N.; Bock, H.; Seitz, W.; Havlas, Z. Acta Crystallogr.
B 1996, 52, 697. Richardson, M. F.; Yang, Q. C.; Novotny-Bregger,
E.; Dunitz, J. D. Acta Crystallogr. B 1990, 46, 653. Byrn, S. R.; Curtin,
D. Y.; Paul, I. C. J. Am. Chem. Soc. 1972, 94, 890.
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Inorganic Chemistry, Vol. 43, No. 10, 2004 3169