X-Ray crystal structure of a heterobimetallic Al-Zn-oxide complex

Article abstract of DOI:10.1039/c0cc05599a

OH/R (R = H, Me, i-Bu) exchange was observed in reactions of [MesnacnacZn(μ-OH)]₂ (1) with metal complexes L_nMR, whereas DippnacnacAl(Me)OH reacts with MesnacnacZnH with elimination of H ₂ and formation of the heterobimeta

Full text of DOI:10.1039/c0cc05599a

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X-Ray crystal structure of a heterobimetallic Al–Zn-oxide complexw
¨ ¨
Stephan Schulz,* Jan Spielmann, Dieter Blaser and Christoph Wolper
Received 15th December 2010, Accepted 12th January 2011
DOI: 10.1039/c0cc05599a
OH/R (R = H, Me, i-Bu) exchange was observed in reactions
of [MesnacnacZn(l-OH)]2 (1) with metal complexes LnMR,
whereas DippnacnacAl(Me)OH reacts with MesnacnacZnH
with elimination of H2 and formation of the heterobimetallic
Al–Zn-oxide complex MesnacnacZnOAl(Me)Dippnacnac 2.
LZnOH 1, which we assumed to be a valuable precursor for
heterobimetallic oxide complexes, and its reactions with
alkyl-substituted main group and transition metal complexes.
In addition, the synthesis and X-ray crystal structure of
the m-O-bridged complex LZn–m-O–Al(Me)L0
2 =
(L0
[HC{C(Me)N(2,6-i-Pr2C6H3)}2]) is reported.
LZnOH 1 was obtained from reactions of LZnMe or LZnH
with one equivalent of water in THF (Scheme 1). The 1H NMR
spectrum of 1 showed a singlet at ꢀ0.20 ppm (OH group). Pulsed
gradient spin echo (PGSE) diffusion measurements of 1 in
benzene-d6 (T = 25 1C) yielded a hydrodynamic radius of
5.90(1) A, which is larger than those reported for monomeric
L0ZnH (5.2(1) A),14 LZnH (4.96(25) A),13b and LZnMe
(4.65(23) A) in toluene-d8,13a indicating 1 to be dimeric in solution.
Heterobimetallic oxides have received growing interest due to
their potential to serve as precursors in materials sciences, i.e.
the synthesis of spinels MIIAl2O4,1 and catalysis.2 They are
expected to show new reactivity patterns and physical proper-
ties due to cooperative effects,3 which are typical for biological
systems such as proteins and which can not be achieved in
monometallic systems,4 and may also serve as a model for the
fixation of catalysts on oxide surfaces.5 Consequently, new
synthetic strategies for well-defined heterobimetallic complexes
containing metal centers with entirely different chemical prop-
erties were developed. Oxo-bridged Al–O–M complexes (M =
Ti, Zr, Hf) were obtained from reactions of metal hydroxides
such as DippnacnacAl(Me)OH with alkyl-substituted metal
complexes, i.e. group 4 metallocene and half-metallocene
complexes.6 They exhibit high catalytic activities in olefin
polymerization reactions.7 These studies have been con-
sequently extended to other main group and transition metal
and lanthanide complexes,8 proving their general applicability.
In contrast, heterobimetallic complexes with group 12 and
group 13 metals are unknown. This is surprising since bimetallic
Zn complexes are living single-site catalysts for the ring-opening
polymerization (ROP) of lactides9 and the epoxide/CO2
copolymerization,10 whereas bimetallic Ca/Zn complexes
catalyze epoxide/anhydride/CO2 terpolymerization11 and
epoxide/CO2 copolymerization reactions, respectively.12
1 shows a strong IR absorption [n(O–H)] at 3667 cmꢀ1
which corresponds well with values reported for a pyrazolyl-
borate zinc hydroxide (3611 cmꢀ1 15
and dimeric zinc
hydroxides such as [(Me2PhSi)3CZnOH]2 (3680 cmꢀ1 16
and
,
)
)
NHC–Zn(OMes)OH (3680 cmꢀ1; NHC = 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene),17 whereas the absorption
band observed for [{(Me3Si)2CHZn}2(m-OH){m-N(CH2py)2}]
(3312 cmꢀ1 18
)
is shifted to a smaller wave number.
ꢀ
1 crystallizes in the triclinic space group P1 with one
molecule in the unit cell as a hydroxide-bridged centro-
symmetric dimer (Fig. 1). The Zn–O bond lengths in 1 are
almost identical (Zn1–O1 1.9623(10); Zn1–O1A 1.9890(11) A)
as was previously observed for zinc hydroxides,10c,16,17,19,20
whereas terminal OH groups show shorter Zn–O bond lengths
(1.85–1.90 A).21 The C3N2Zn ring in 1 is planar (0.109 A rms
deviation from mean plane, 0.0368 for mean plane of C and
N-atoms) with the Zn atoms slightly out of the mean plane of
the C- and N-atoms (0.4218(17) A). C–C, C–N and Zn–N
bond lengths within the ring are almost identical compared to
the starting complexes LZnH and LZnMe.13
Due to our general interest in organozinc complexes LZnX
(L = [HC{C(Me)N(2,4,6-Me3C6H2)}2], X = H, R, OR),13 we
began to study the synthesis of molecular zinc hydroxides and
report herein on the synthesis of a molecular zinc hydroxide
1 was reacted with Me-substituted complexes of main group
(LiMe, AlMe3) and transition metals (ZnMe2, Cp*TiMe3).
University of Duisburg-Essen, Universitatsstr. 5-7, S07 S03 C30,
¨
45117 Essen, Germany. E-mail: stephan.schulz@uni-due.de;
Fax: +49 201 1833830; Tel: +49 201 1834635
w Electronic supplementary information (ESI) available: Full details
on the experimental procedures and characterization of 1, and 2
including single crystal X-ray diffraction, reactivity studies of
[MesnacnacZnOH]2 towards MeLi, AlMe3, Al(i-Bu)3, HAl(i-Bu)2,
Cp*TiMe3, ZnMe2 and DippnacnacAlH2 and preliminary experiments
on the CO2/CHO copolymerization of 2. CCDC 804558 (1) and
804557 (2). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc05599a
Scheme 1 Synthesis of 1 via hydrolysis of the zinc methyl/hydride
complexes.
c
2676 Chem. Commun., 2011, 47, 2676–2678
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LZn–m-O–Al(Me)L0 2 in almost quantitative yield. An analogous
reaction with LZnMe proceeded much slower, giving 2 in less
yield, demonstrating LZnH to be the more reactive species.
1
In situ monitoring of the reaction with LZnH by H NMR
spectroscopy showed steadily decreasing Zn–H and Al–OH
resonances. The resonances due to the three organic substituents
(L, L0, Me) in 2 itself show the expected 1 : 1 : 1 relative
intensity. We also reacted L0Al(Me)OH with L0ZnH, but L0ZnH
was found to be less reactive than LZnH, yielding L0Zn–
m-O–Al(Me)L0 in low yield together with several by-products.24
These findings indicate a kinetically controlled reaction pathway,
with the less effectively shielded zinc hydride complex LZnH
being more reactive than L0ZnH and LZn–Me.
2 crystallizes in the monoclinic space group P21/n (Fig. 2).
The Zn atom adopts a distorted trigonal planar coordination
sphere with Zn1 deviating 0.0208(16) A from the N3/N4/O1
plane. The N–Zn–O angles are slightly larger than 1201
(128.28(12)1, 132.88(11)1). Zn1 is off the mean plane of the
C3N2 central part of the ligand (0.230(4) A) as was observed in
1. The Al atom is distorted tetrahedrally coordinated and Al1
deviates from the mean plane of the C3N2 unit by 0.413(3) A.
The Zn–O bond length (1.782(2) A) is elongated compared to
the Al–O bond length (1.686(2) A) due to the different
covalent radii of Zn and Al. Analogous structural trends were
observed for Zn–N (1.950(3), 1.960(3) A) and Al–N bond
lengths (1.938(3), 1.918(2) A). The N–Al–N bond angle
(95.05(11)1) is slightly smaller compared to the N–Zn–N bond
angle (98.80(12)1). The C–C–C backbone angles of the
b-diketiminato ligands were found to be slightly smaller for
the Al heterocycle (127.4(3)1) compared to the Zn heterocycle
(130.9(4)1). The Al–O–Zn bond angle of 144.78(15)1 is within
the range of 141–1761 observed for oxo-bridged bimetallic
complexes of the general type Al–O–TM (TM = transition
metal).25
Fig. 1 Solid state structure of 1 (light part generated via inversion);
non-H-atoms shown as thermal ellipsoids at 50% probability levels
(others at arbitrary radii); H atoms partially omitted for clarity.
Surprisingly, no gas evolution but only the formation of LZnMe
via a hydroxide/methyl exchange reaction occurred. This reaction
pattern remarkably contrasts that observed for 1,4-dioxane
stabilized EtZnOH, which was found to react with AlMe3 with
methane/ethane evolution.23 Moreover, when 1 was reacted with
Al(i-Bu)3 and L0AlH2, the formation of LZn(i-Bu) and LZnH,
1
respectively, was unambiguously proved by H NMR spectro-
scopy,22 while the reaction of 1 with i-Bu2AlH yielded a mixture
1
of LZnH and LZn(i-Bu) in a 5 : 1 molar ratio according to H
NMR spectroscopic studies (Scheme 2).
Obviously, the Brønsted acidity of the Zn–OH group in 1 is less
pronounced compared to that of L0Al(Me)OH, L0Ga(Me)OH,
L0GeOH, Cp2Zr(Me)OH and [L0Sr(thf)–m-OH]2, respectively.
Our studies demonstrate the distinguished nucleophilicity of
Zn–OH, which is generally accepted for hydrolytic zinc enzymes.
Nevertheless, it is remarkable that a Zn–OH unit was transformed
into a Zn–alkyl and even a Zn–H unit by reaction with a metal
alkyl or a metal hydride complex such as L0AlH2.
In an attempt to prove the general accessibility of bimetallic
zinc oxides, we reacted L0Al(Me)OH with LZnH (Scheme 3).
The reaction proceeded with H2-evolution and formation of
Scheme 2 Reactions of 1 with metal alkyl and hydride complexes.
Fig. 2 Solid state structure of 2; H atoms are omitted for clarity,
Scheme 3 Synthesis of 2.
thermal ellipsoids at 30% probability levels.
c
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The catalytic activity of 2 towards CO2/cyclohexene oxide
(CHO) copolymerization was investigated in neat CHO with
10 bar CO2 at 50 1C and a monomer/catalyst ratio of 1000/1.
For comparison, the catalytic activities of LZnH and
L0Al(Me)OH were tested as well, but both showed no catalytic
activity. This is not surprising for LZnH, since b-diketiminate
zinc complexes with methyl-substituents at the 2,6-aryl positions
were reported to be inactive in CO2/CHO copolymerization
reactions.10a,c In stoichiometric NMR-scale reactions,
L0Al(Me)OH also showed no reaction with CHO up to
100 1C, whereas the reaction with CO2 (1 atm) occurred with
decomposition of L0Al(Me)OH and considerable formation of
L0H. This is probably due to initial formation of the inter-
mediate L0Al(Me)CO3H, containing an acidic H-atom that
readily protonates the b-diketiminato ligand with subsequent
formation of L0H and Al(Me)CO3. A comparable reactivity is
known for the calcium complex [L0CaOH]2, also yielding L0H
and CaCO3.26 Unfortunately, LZnOAl(Me)L0 2 showed
no increased catalytic activity compared to LZnH and
L0Al(Me)OH. To investigate this inactivity in more detail,
reactions of 2 with CO2 and CHO were tested in NMR-scale
experiments in deuterated benzene. 2 showed no reaction with
CHO within the temperature range of 20–100 1C, whereas the
reaction with 1 atm CO2 resulted in a clean conversion of 2
into a product with a completely new set of 1H NMR signals,
indicating that incorporation of CO2 is feasible. The charac-
terization of this new complex is currently under investigation.
LZnOH 1, which was prepared by controlled hydrolysis of
LZnX (X = H, Me) with water in THF solution, reacts with
alkyl- and hydride-substituted main group and transition
metal complexes by OH/Y exchange reactions (Y = H, Me,
i-Bu) and subsequently forms LZnY, whereas the more
Brønsted acidic L0Al(Me)OH reacts with LZnH to form the
bimetallic complex L0Al(Me)OZnL 2.
8 S. Singh, V. Jancik, H. W. Roesky and R. Herbst-Irmer, Inorg.
Chem., 2006, 45, 949; S. Nembenna, H. W. Roesky, S. K. Mandal,
R. B. Oswald, A. Pal, R. Herbst-Irmer, M. Noltemeyer and
H.-G. Schmidt, J. Am. Chem. Soc., 2006, 128, 13056;
S. Nembenna, S. Singh, A. Jana, H. W. Roesky, Y. Yang,
H. Ye, H. Ott and D. Stalke, Inorg. Chem., 2009, 48, 2273;
Y. Yang, H. W. Roesky, P. G. Jones, C.-W. So, Z. Zhang,
R. Herbst-Irmer and H. Ye, Inorg. Chem., 2007, 46, 10860;
J. Chai, V. Jancik, S. Singh, H. Zhu, C. He, H. W. Roesky,
H.-G. Schmidt, M. Noltemeyer and N. S. Hosmane, J. Am. Chem.
Soc., 2005, 127, 7521.
9 M. H. Chisholm, J. C. Gallucci and K. Phomphrai, Inorg. Chem.,
2005, 44, 8004; A. P. Dove, V. C. Gibson, E. L. Marshall,
A. J. P. White and D. J. Williams, Dalton Trans., 2004, 570;
L. R. Rieth, D. R. Moore, E. B. Lobkovsky and G. W. Coates,
J. Am. Chem. Soc., 2002, 124, 15239; M. H. Chisholm,
J. C. Huffman and K. Phomphrai, J. Chem. Soc., Dalton Trans.,
2001, 222; B. J. O’Keefe, M. A. Hillmeyer and W. B. Tolman,
J. Chem. Soc., Dalton Trans., 2001, 2215; M. Cheng,
A. B. Attygalle, E. B. Lobkovsky and G. W. Coates, J. Am. Chem.
Soc., 1999, 121, 11583. For a review article see: J. Wu, T.-L. Yu,
C.-T. Chen and C.-C. Lin, Coord. Chem. Rev., 2006, 250, 602.
10 D. R. Moore, M. Cheng, E. B. Lobkovsky and G. W. Coates,
J. Am. Chem. Soc., 2003, 125, 11911; D. R. Moore, M. Cheng,
E. B. Lobkovsky and G. W. Coates, Angew. Chem., Int. Ed., 2002,
41, 2599; M. Cheng, D. R. Moore, J. J. Reczek, B. M.
Chamberlain, E. B. Lobkovsky and G. W. Coates, J. Am. Chem.
Soc., 2001, 123, 8738; B. Y. Liu, C. Y. Tian, L. Zhanq, W. D. Yan
and W. J. Zhanq, J. Polym. Sci., Part A: Polym. Chem., 2006, 44,
6243; M. Kroger, C. Folli, O. Walter and M. Doring, Adv. Synth.
Catal., 2006, 348, 1908.
11 R. C. Jeske, J. M. Rowley and G. W. Coates, Angew. Chem., Int.
Ed., 2008, 47, 6041.
12 D. F.-J. Piesik, S. Range and S. Harder, Organometallics, 2008, 27,
6178.
13 S. Schulz, T. Eisenmann, D. Blaser and R. Boese, Z. Anorg. Allg.
Chem., 2009, 635, 995; S. Schulz, T. Eisenmann, D. Schuchmann,
M. Bolte, M. Kirchner, R. Boese, J. Spielmann and S. Harder,
Z. Naturforsch., 2009, 64b, 1397.
14 J. Spielmann, D. Piesik, B. Wittkamp, G. Jansen and S. Harder,
Chem. Commun., 2009, 3455.
15 A. Looney, R. Han, I. B. Gorrell, M. Cornebise, K. Yoon,
G. Parkin and A. L. Rheingold, Organometallics, 1995, 14, 274.
16 S. S. Al-Juaid, N. H. Buttrus, C. Eaborn, P. B. Hitchcock,
A. T. L. Roberts, J. D. Smith and A. C. Sullivan, J. Chem. Soc.,
Chem. Commun., 1986, 908.
Notes and references
1 F. Meyer, R. Hempelmann, S. Mathur and M. Veith, J. Mater.
Chem., 1999, 9, 1755; R. C. Mehrotra, Coord. Chem. (IUPAC),
1981, 21, 113; V. G. Kessler, S. Gohil and S. Parola, Dalton Trans.,
2003, 4, 544.
2 H. W. Roesky, I. Haiduc and N. S. Hosmane, Chem. Rev., 2003,
103, 2579; T. Carofiglio, C. Floriani, M. Rosi, A. Chiesi-Villa and
C. Rizzoli, Inorg. Chem., 1991, 30, 3245; M. S. Rau, C. M.
Kretz, G. L. Geoffroy, A. L. Rheingold and B. S. Haggerty,
Organometallics, 1994, 13, 1624; H. Li, M. Eddaoudi, J. Plevert,
M. O’Keeffe and O. M. Yaghi, J. Am. Chem. Soc., 2000, 122, 12409.
3 B. G. Ueland, G. C. Lau, R. J. Cava, J. R. O’Brien and P. Schiffer,
Phys. Rev. Lett., 2006, 96, 027216.
17 G. Anantharaman and K. Elango, Organometallics, 2007, 26, 1089.
18 E. Jaime, A. N. Kneifel, M. Westerhausen and J. Weston,
J. Organomet. Chem., 2008, 693, 1027.
19 M. H. Chisholm, J. C. Gallucci and K. Phomphrai, Inorg. Chem.,
2002, 41, 2785; C. Amort, H. Kopacka, B. Bildstein, K. Wurst and
Z. Kristallogr, Z. Kristallogr. - New Cryst. Struct., 2004, 219, 331.
20 A. M. Arif, A. H. Cowley, R. A. Jones and S. U. Koschmieder,
J. Chem. Soc., Chem. Commun., 1987, 1319.
21 B. M. Bridgewater and G. Parkin, Inorg. Chem. Commun., 2001, 4,
126; R. Alsfasser, S. Trofimenko, A. Looney, G. Parkin and
H. Vahrenkamp, Inorg. Chem., 1991, 30, 4098; M. Ruf and
H. Vahrenkamp, Inorg. Chem., 1996, 35, 6571.
4 N. Wheatley and P. Kalck, Chem. Rev., 1999, 99, 3379; R.-J. Tao,
F.-A. Li, S.-Q. Zang, Y.-X. Cheng, Q.-L. Wang, J.-Y. Niu and
D.-Z. Liao, J. Coord. Chem., 2006, 59, 901.
5 C. Coperet, M. Chabanas, R. P. Saint-Arroman and J.-M. Basset,
Angew. Chem., Int. Ed., 2003, 42, 156.
22 The reactions were in situ monitored by 1H NMR spectroscopy
and the complexes identified by comparing the spectra with those
of the known complexes. LZn(i-Bu) was identified by its 1H NMR
spectrum, but we could not isolate a pure sample from the reaction
product mixture. Details are given in the ESIw.
6 G. Bai, S. Singh, H. W. Roesky, M. Noltemeyer and
H.-G. Schmidt, J. Am. Chem. Soc., 2005, 127, 3449; Y. Yang,
T. Schulz, M. John, Z. Yang, V. M. Jimenez-Perez, H. W. Roesky,
P. M. Gurubasavaraj, D. Stalke and H. Ye, Organometallics, 2008,
27, 769; P. M. Gurubasavaraj, S. K. Mandal, H. W. Roesky,
R. B. Oswald, A. Pal and M. Noltemeyer, Inorg. Chem., 2007, 46,
1056; P. M. Gurubasavaraj, H. W. Roesky, B. Nekoueishahraki,
A. Pal and R. Herbst-Irmer, Inorg. Chem., 2008, 47, 5324.
23 W. Kuran and M. Czernecka, J. Organomet. Chem., 1984, 263, 1.
24 The complex L0Zn–m-O–Al(Me)L0 couldn’t be isolated from the
reaction mixture due to similar solubilities of the reaction
products.
25 A CSD structural database search (version 5.31 incl. update
August 2010) on neutral complexes containing an oxo-bridged
Al–O–TM unit (TM = transition metal) revealed 21 entries
(mean value 157.31).
7 For
a
H. W. Roesky, Acc. Chem. Res., 2010, 43, 248.
very recent review article see: S. K. Mandal and
26 C. Ruspic, S. Nembenna, A. Hofmeister, J. Magull, S. Harder and
H. W. Roesky, J. Am. Chem. Soc., 2006, 128, 15000.
c
2678 Chem. Commun., 2011, 47, 2676–2678
This journal is The Royal Society of Chemistry 2011