From neutral Zn4O4 clusters to a cationic ZnO dimer in solution

Article abstract of DOI:10.1021/om061108b

The suitability of the tetrameric ZnO aggregates [(iPrO)ZnMe]₄ (1a) and [(Me₃SiO)ZnMe]₄ (1b) as potential sources for molecular models of low-coordinated zinc centers and active sites on zinc oxide catalysts is reported. The formation and fate of the resulting cationic Zn ₄O₄ degradation products by reaction of 1a and 1b with B(C₆F₅)₃ in the presence of different organic donor substrates have been studied by means of ESI mass spectrometry. While 1a affords the cationic monozinc complexes [MeZn(L)]⁺ (2a: L = THF, 2b: 15-c-5, 2c: DMAP), the cluster 1b furnishes in the presence of DMAP [MeZn(OSiMe₃)₂Zn(DMAP)₂]⁺ (3), the first dimeric ZnO aggregate cation in solution, and the [MeB(C₆F ₅)₃] anion. Additionally, the neutral dinuclear ZnO aggregate [(Me₃SiO)-Zn(C₆F₅)thf]₂ (4) results from Me/C₆F₅ exchange reactions as the final product, which has been characterized by NMR spectroscopy and a single-crystal X-ray diffraction analysis.

Full text of DOI:10.1021/om061108b

Organometallics 2007, 26, 2133-2136
2133
From Neutral Zn4O4 Clusters to a Cationic ZnO Dimer in Solution
Matthias Driess,*,† Klaus Merz,† and Robert Schoenen‡
Institute of Chemistry: Metalorganics and Inorganic Materials, Technische UniVersita¨t Berlin,
Strasse des 17. Juni 135, D-10623 Berlin, Germany, and Department of Chemistry, Ruhr-UniVersia¨t
Bochum, UniVersita¨tsstrasse 150, D-44801 Bochum, Germany
ReceiVed December 6, 2006
Summary: The suitability of the tetrameric ZnO aggregates
[(iPrO)ZnMe]4 (1a) and [(Me3SiO)ZnMe]4 (1b) as potential
sources for molecular models of low-coordinated zinc centers
and actiVe sites on zinc oxide catalysts is reported. The
formation and fate of the resulting cationic Zn4O4 degradation
products by reaction of 1a and 1b with B(C6F5)3 in the presence
of different organic donor substrates haVe been studied by
means of ESI mass spectrometry. While 1a affords the cationic
monozinc complexes [MeZn(L)]+ (2a: L ) THF, 2b: 15-c-5,
2c: DMAP), the cluster 1b furnishes in the presence of DMAP
[MeZn(OSiMe3)2Zn(DMAP)2]+ (3), the first dimeric ZnO ag-
gregate cation in solution, and the [MeB(C6F5)3] anion.
Additionally, the neutral dinuclear ZnO aggregate [(Me3SiO)-
Zn(C6F5)thf]2 (4) results from Me/C6F5 exchange reactions as
the final product, which has been characterized by NMR
spectroscopy and a single-crystal X-ray diffraction analysis.
allow changing specifically the environment of active zinc
centers at the molecular level. We have reported the synthesis
of a wide range of different molecular ZnnOm clusters9 that are
suitable single-source precursors for the synthesis of nanoscaled
zinc and zinc oxide particles,10 but they could also be promising
sources for molecular models of terminal, low-coordinated zinc
centers on ZnO surfaces. However, molecular compounds with
such highly electrophilic zinc atoms are expected to be extremely
reactive and therefore difficult to isolate. In fact, only a few
reports are known on the structural chemistry of mononuclear
cationic zinc complexes,11 but no polynuclear cationic ZnO
cluster could hitherto be isolated. One alternative approach to
generate polynuclear ZnO cluster cations is offered by mass
spectrometric (MS) methods. We have currently shown that one
can easily produce polynuclear ZnO cluster cations in the gas
phase under EI-MS conditions.10f The used ZnO cluster sources
are simply accessible by Bronsted acid-base reaction of
dialkylzinc compounds with silanoles or alcohols.9,10a,g The
nuclearity and the degree of aggregation of ZnO units is easily
tunable by changing the steric demand of the substituent at the
oxygen center. While methyl, trimethylsilyl, or isopropyl groups
lead to Zn4O4 clusters, the use of larger substituents such as
triphenylsilyl or tert-butyl favors the formation of spirocyclic
Zn3O4 or Zn6O10 clusters.9 How could one achieve the synthesis
and stabilization of cationic polynuclear ZnO clusters in the
condensed phase using the latter molecular ZnnOm clusters? This
could be achieved by replacing the alkyl group at the zinc atom
by a large and weakly coordinating anion such as B(C6F5)4- or
Introduction
Zinc oxide is an important heterogeneous catalyst for the
conversion of water gas (CO, CO2, H2) to methanol. Interest-
ingly, the catalytic activity of ZnO can be significantly increased
by the presence of promotors such as copper or alkali metals,
which apparently change the electronic properties of the active
sites at the ZnO surface.1-3 However, the efficiency of the
catalytic systems depends not only on their composition but
also on the size of the particles due to several structural defects.4
Therefore, the explanations for different activities of nondoped
and doped “real” ZnO catalysts have been a matter of
controversial discussions in the literature, and the complex
reaction mechanism could not be understood until recently.5,6
From results of different working groups, it is known that the
reaction on the ZnO surface takes place at low-coordinate, highly
electrophilic zinc centers.7,8 Another possibility to evaluate the
role of such zinc centers as active sites on ZnO surfaces is
offered by using the model catalyst approach. The latter may
-
Al(ORf)4 [Rf ) CH(CF3)2, C(CH3)(CF3)2, C(CF3)3].12 This
strategy has also been successfully used for the synthesis of
highly electrophilic, cationic zirconium complexes, which are
valuable homogeneous catalysts.13 The abstraction of a alkyl
group at the metal center can be achieved by reaction with strong
(9) (a) Driess, M.; Merz, K.; Rell, S. Eur. J. Inorg. Chem. 2000, 2517-
2522. (b) Driess, M.; Hu, L.; Merz, K. Eur. J. Inorg. Chem. 2003, 51-53.
(c) Schindler, F.; Schmidbaur, H. Angew. Chem. 1967, 16, 697-708.
(10) (a) Merz, K.; Schoenen, R.; Driess, M. J. Phys. IV Fr. 2001, 11,
467. (b) Driess, M.; Merz, K.; Schoenen, R.; Rabe, R.; Kruis, F. E.; Roy,
A.; Birkner, A. C. R.-Chim. 2003, 6 (3), 273. (c) Hambrock, J.; Rabe, S.;
Merz, K.; Wohlfarth, A.; Birkner, A.; Fischer, R. A.; Driess, M. J. Mater.
Chem. 2003, 13 (7), 1731. (d) Kurtz, M.; Bauer, N.; Buescher, C.; Wilmer,
H.; Hinrichsen, O.; Becker, R.; Rabe, S.; Merz, K.; Driess, M.; Fischer, R.
A.; Muhler, M. Catal. Lett. 2004, 92 (1-2), 49. (e) Polarz, S.; Roy, A.;
Merz, M.; Halm, S.; Schro¨der, D.; Schneider, L.; Bacher, G.; Kruis, F. E.;
Driess, M. Small 2005, 1, 2. (f) Schro¨der, D.; Schwarz, H.; Polarz, S.; Driess,
M. Phys. Chem. Chem. Phys. 2005, 7, 1049. (g) Merz, K.; Block, S.;
Schoenen, R.; Driess, M. Dalton Trans. 2003, 17, 3365.
* Corresponding author. E-mail: matthias.driess@tu-berlin.de.
† Technische Universita¨t Berlin.
‡ Ruhr-Universita¨t Bochum.
(1) Spencer, M. S. Catal. Lett. 1998, 50, 37-40.
(2) Chanchlani, K. G.; Hudgins, R. R.; Silveston, P. L. J. Catal. 1992,
136, 59-72.
(3) Rozovskii, A. Y.; Lin, G. I. Top. Catal. 2003, 22, 137-150.
(4) (a) Ischenko, V.; Polarz, S.; Grote, D.; Stavarache, V.; Fink, K.;
Driess, M. AdV. Funct. Mat. 2005, 15, 1945-1954. (b) Polarz, S.; Ischenko,
V.; Hinrichsen, O.; Driess, M. Angew. Chem., Int. Ed. 2006, 45, 2965-
2969.
(5) Millar, G. J.; Rochester, C. H.; Bailey, S.; Waugh, K. C. J. Chem.
Soc., Faraday Trans. 1993, 89 (7), 1109-1115.
(11) (a) Walker, D. A.; Woodman, T. J.; Hughes, D. L.; Bochmann, M.
Organometallics 2001, 20, 3772-3776. (b) Walker, D. A.; Woodman, T.
J.; Schormann, M.; Hughes, D. L.; Bochmann, M. Organometallics 2003,
22, 797-803. (c) Hannant, M. D.; Schormann, M.; Bochmann, M. Dalton
Trans. 2002, 4071-4073. (d) Sarazin, Y.; Schormann, M.; Bochmann, M.
Organometallics, 2004, 13, 3296-3302.
(6) (a) Fujita, S.; Usui, M.; Ito, H.; Takezawa, N. J. Catal. 1995, 157,
403-413. (b) Nakamura, J.; Choi, Y.; Fujitani, T. Top. Catal. 2003, 22,
277-285.
(7) French, S. A.; Sokol, A. A.; Bromley, S. T.; Catlow, C. R. A.;
Sherwood, P. Top. Catal. 2003, 24, 161-172
(8) Kurtz, M.; Strunk, J.; Hinrichsen, O.; Muhler, M.; Fink, K.; Meyer,
B.; Wo¨ll, C. Angew. Chem., Int. Ed. 2005, 44, 2790-2794.
(12) Krossing, I. Chem.-Eur. J. 2001, 7, 490-502.
(13) Brackemeyer, T.; Erker, G.; Fro¨hlich, R. Organometallics 1997, 16,
531-536.
10.1021/om061108b CCC: $37.00 © 2007 American Chemical Society
Publication on Web 03/16/2007

2134 Organometallics, Vol. 26, No. 8, 2007
Notes
Figure 2. ESI(+)-MS of the samples taken from the reaction
mixture of 1a and B(C6F5)3 in the presence of 15-c-5 in THF.
Scheme 1. Proposed Mechanism for the Degradation of
[(iPrO)ZnMe]4 in the Presence of B(C6F5)3 in THFa
Figure 1. ESI(+)- and ESI(-)-MS of the samples taken from the
reaction mixture of 1a and B(C6F5)3 in THF.
Lewis acids such as B(C6F5)3. Here we report our results on
the formation of the first cationic polynuclear ZnO clusters in
solution. In order to synthesize polynuclear ZnO aggregate
cations, we studied the reaction of the neutral Zn4O4 cluster
precursors 1a and 1b with B(C6F5)3 in the presence of different
organic donor substrates. A suitable method for monitoring the
reaction progress in the reaction mixtures is provided by the
ESI-MS technique (ESI ) electrospray ionization), because it
gently transfers preformed ions directly from solution to the
gas phase.14 It is an effective method to investigate reaction
pathways and to characterize reactive ionic intermediates.15,16
Thus, we studied the behavior of the starting materials
[(iPrO)ZnMe]4 (1a) and [(Me3SiO)ZnMe]4 (1b) under ESI-MS
conditions.
a
15-c-5 is 15-crown-5.
the re-formation of [(iPrO)ZnMe]4 (1a) were proven by 1H NMR
spectroscopy.
Solutions of 1a and 1b, respectively (0.09 mol in 10 mL of
THF), were stirred at room temperature in an argon atmosphere.
After 30 min, 100 µL of the solution was transferred into the
ESI source by a syringe pump at a flow rate of 3 µL min-1 for
the MS detection. In the beginning, a CapExit voltage of 98.5
V was applied. In both cases no signals of fragments of the
Zn4O4 aggregates in the cation or in the anion mode were
detected. When CapExit voltages ranging from 60 to 120 V
were applied, no obvious differences were observed. Thus we
fixed the standard procedure for MS detection. Subsequently,
we investigated the reaction of [(iPrO)ZnMe]4 (1a) with
B(C6F5)3 in THF without any additional stabilizing agent. A
solution of 1 (0.09 mol) in 10 mL of THF was treated with
B(C6F5)3 (0.09 mol) at room temperature in an argon atmo-
sphere. After the standard treatment, the signals of ions
[MeB(C6F5)3]- (principal ion m/z 527.1) and [MeZnthf]+
(principal ion m/z 151.7) and a weak signal with low intensity
of the ion [MeZn(thf)2]+ (principal ion m/z 223.5) were detected
(Figure 1). We interpret the presence of the [MeB(C6F5)3]- ion
as the result of the transfer of one methyl group at the zinc
center to the boron atom, and concomitantly, formation of the
tetranuclear ZnO cluster cation [(ZnOiPr)(iPrOZnMe)3]+ occurs.
Apparently, the initial tetranuclear aggregate cation is not stable
in solution and affords, under liberation of [Zn(OiPr)2] and
recombination of 1a, the monozinc complex [MeZn(thf)]+ (2a)
with a low-coordinated zinc center (Scheme 1).
In order to probe whether the initial tetranuclear ZnO cluster
cation can be stabilized in the presence of other donor molecules
than THF, we used the crown ether 15-c-5 because this ligand
is able to stabilize RMg+ and RZn+ cations (R ) Et, neopentyl)
appropriately.17 Treatment of a solution of 1a (0.09 mol) in 10
mL of THF with B(C6F5)3 (0.09 mol) in the presence of 15-c-5
(0.09 mol) at room temperature in an argon atmosphere leads
to a clear solution. Using the aforementioned standard measure-
ments for ESI-MS, the signals of the [Me(B(C6F5)3]- anion and
[MeZn(15-c-5)]+ cation 2b (principal ion m/z 299.5), respec-
tively, and a weak signal with low intensity that can be assigned
to the [C6F5Zn(15-c-5)]+ cation (principal ion m/z 431.5) were
detected (see Figure 2). The formation of the [C6F5Zn(15-c-
5)]+ cation indicates a methyl/C6F5 ligand exchange as side
reaction. Hence, the presence of 15-c-5 does not stabilize the
tetrameric ZnO aggregate cation but instead leads to similar
cationic degradation products as outlined in Scheme 1. Attempts
to use other strong donor molecules such as p-(dimethylamino)-
pyridine (DMAP) for the stabilization of the desired Zn4O4
cluster cation failed.
Since the steric and electronic nature of the substituent at
the oxygen atom should have a noticeable influence on the
stability of the primary polynuclear ZnO cluster cations, the
siloxy-substituted cluster 1b was probed. In analogy, solutions
of [(Me3SiO)ZnMe]4 (1b) (0.09 mol) in 10 mL of THF were
treated with B(C6F5)3 (0.09 mol) in the presence of several
organic donor substrates. However, only in the case of the use
of DMAP (0.09 mol) were we able to detect the novel dinuclear
ZnO cluster cation [MeZn(OSiMe3)2Zn(DMAP)2]+ (3) (principal
ion m/z 569.4), besides the counterion MeB(C6F5)3-, as preferred
The recombined [(iPrO)ZnMe]4 crystallized from the reaction
solution and was characterized by X-ray determination. Ad-
ditionally, the simultaneous formation of [Zn(OiPr)2] (2) and
(14) Fenn, J. B. Angew. Chem. 2003, 115, 3999-4024.
(15) Guo, H.; Qian, R.; Liao, Y.; Ma, S.; Guo, Y. J. Am. Chem. Soc.
2005, 127 (37), 13060-13064.
(16) Santos, L. S.; Pavam, C. H.; Almeida; W. P.; Coelho, F.; Eberlin,
M. N. Angew. Chem., Int. Ed. 2004, 43, 4330-4333.
(17) Tang, H.; Parvez, M.; Richey, H. G., Jr. Organometallics 2000,
19, 4810-4819.

Notes
Organometallics, Vol. 26, No. 8, 2007 2135
Figure 4. a. ESI(+)-MS (CapEx 113.5 V) of the samples taken
from the reaction mixture of 1b and B(C6F5)3 in the presence of
DMAP in THF.
Figure 3. (a) ESI(+)-MS (CapEx 98.5 V) of the samples taken
from the reaction mixture of 3 and B(C6F5)3 in the presence of
DMAP in THF. (b) Calculated (‚‚‚) and experimental pattern for
[MeZn(OSiMe3)2Zn(DMAP)2]+, 3.
Scheme 2. Proposed Mechanism for the Conversion of 1b
with B(C6F5)3 in THF in the Presence of DMAP
Figure 5. Molecular structure of [(Me3SiO)ZnC6F5(thf)]2, 4. The
hydrogen atoms are omitted for clarity. Selected distances [Å] and
angles [deg]: Zn(1)-O(1) 1.967(2), Zn(1)-O(2) 1.966(3), Zn(2)-
O(1) 1.963(3) Zn(2)-O(2) 1.966(2), Zn(1)-Zn(2) 2.8768(9) O(1)-
Zn(1)-O(2) 85.87(10) O(1)-Zn(2)-O(2) 85.9(1), Zn(1)-O(1)-
Zn(2) 94.1(1), Zn(1)-O(2)-Zn(2) 94.0(1).
methyl/C6F5 exchange between the zinc and boron atoms and
additional exchange of the DMAP ligand at zinc through THF
(Scheme 2).
The present investigation shows that the neutral tetrameric
ZnO aggregates 1a and 1b undergo a B(C6F5)3-assisted degrada-
tion, affording two types of highly electrophilic zinc complexes
as ionic products, which could be unequivocally identified only
by ESI-MS: (i) [MeZnL]+ (2a-2c) or [MeZnL2]+ (2d)
[MeB(C6F5)3]- and (ii) [MeZn(OSiMe3)2Zn(DMAP)2]+ (3)
[MeB(C6F5)3]-. However, due to Me/C6F5 exchange reactions,
the latter ion pair converts slowly into the neutral dimeric ZnO
compound 4, which can be isolated in crystalline form. The
simple access and the reasonable stability of the first dimeric
ZnO cluster cation 3 in THF solutions offers a promising and
versatile basis for model reactions of bonding activation and
chemisorption processes of small molecules such as H2, CO2,
and CO relevant in ZnO catalysis. Such studies are currently
underway.
ionic species (Scheme 2). Its composition is proven by pattern
calculation (Figure 3).
Additionally, the ESI-MS measurements were carried out
again, using different parameters with CapEx 113.5 V and
Skimmer 40 V. These led to the observation of additional signals
of the stabilized cationic MeZn species [MeZnDMAP]+ (2c)
(principal ion m/z 201.7) and [MeZn(DMAP)2]+ (2d) (principal
ion m/z 323.6). Clearly, the latter result implies a degradation
process similar to that observed for 1a. However, after 6 h the
formation of the neutral dimeric ZnO cluster 4 is complete,
which precipitates off in the reaction solution. Compound 4 can
be isolated in the form of colorless cubes in 22% yield. Its
constitution and composition are confirmed by NMR spectros-
copy and a single-crystal X-ray structure analysis (Figure 5).
The dimeric ZnO cluster 4 crystallizes in the monoclinic space
group P21/n and shows a nearly planar central four-membered
ring (Figure 5). The Zn atoms are tetrahedral, while the O atoms
are trigonal-planar coordinated. The values of the Zn-O
distances (1.97 Å) and the Zn‚‚‚Zn distances (2.88 Å) are in
the same region as observed for other dimeric ZnO clusters.4,10
Although the mechanism for the formation of 4 is hitherto
unknown, its formation can be rationalized by a complete
Experimental Details
All ESI-MS experimenets were performed on a Bruker Daltonics
Esquire 3000+. The vacuum was maintained by a turbomolecular
pump (ion source: 2 × 10-6 Torr). The ions were generated from
an external electrosray ionization source. Typically the electrospray
flow rate was 3 µL/min, which was maintained by a syringe pump.
The spray was directed into a heated glass capillary drying tube
with Pt coating in both ends, remaining at a temperature of about
500 K. Typically a high voltage of about 4000 V is applied between
the endplate and the spray needle.

2136 Organometallics, Vol. 26, No. 8, 2007
Notes
Preparation of 4. A solution of 50 mg (0.9 mmol) of 1b and
10 mg (0.9 mmol) of DMAP in 5 mL of THF was slowly treated
with a solution of 50 mg (0.9 mmol) of B(C6F5)3 in 5 mL of THF
at room temperature. Cooling of the clear solution at -20 °C affords
colorless crystals of 4. Yield: 30 mg (0.4 mmol; 22%); mp 134-
137 °C (dec). 1H NMR (C6D6): δ 3.7 (m, 4H, OCH2CH2), 1.8 (m,
4H, OCH2CH2), 0.2 ppm (s, 9H, Si(CH3)3). 13C{1H} NMR
(C6D6): δ 0.8 (s, Si(CH3)3), 37.7 (s, O-CH2), 17.6 (s, O-CH2-CH3),
independent, 5863 observed (Fo > 4σ(Fo)), 397 parameters; R1 )
0.0439, wR2 (all data) ) 0.1085. Structure solution was by direct
methods (SHELXS 97); refinement against F2 with all measured
reflections (SHELXTL 97). The positions of the H atoms were
calculated and considered isotropic according to a riding model.
Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publica-
tion no. CCDC 286904. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge CB2
1EZ,UK(fax: (+44)1223-336-033;e-mail: deposit@ccdc.cam.ac.uk).
1
1
137.4 (d, JC-F ) 246.5 Hz, C6F5), 141.5 (d, JC-F ) 208.1 Hz,
1
C6F5), 149.0 (d, JC-F ) 240.0 Hz, C6F5). Anal. Calcd for 4
(C26H34F10O4Si2Zn2): C 39.66, H 4.35. Found: C 39.45, H 4.30.
Crystal Structure Analysis of 4. A crystal of 4 was placed on
a glass capillary in perflourinated oil and measured in a cold gas
flow. The intensity data were measured with a Bruker axs area
detector (Mo KR radiation), λ ) 0.71073 Å (ω scan) at -60 °C;
monoclinic, P21/n, a ) 12.272(4) Å, b ) 14.036(5) Å, c ) 19.540-
(6) Å, â ) 97.91(1)°, V ) 3333(2)Å3, Z ) 4, µ ) 1.596 mm-1. A
total of 18 311 reflections were collected (2θmax ) 50°), 3907
Acknowledgment. We thank the Deutsche Forschungsge-
meinschaft (Collaborative Research Program 558) for financial
support.
OM061108B