A discrete ortho-lithiated acetophenone imine derivative: Isolation, characterization, and synthesis of group IV metal complexes

Article abstract of DOI:10.1021/om700530z

The reaction of a 3′,4′-methylenedioxyacetophenone imine (L-H; 1) with ⁿBuLi allows for the synthesis and isolation of a discrete ortho-lithiated imine (L-Li; 2). Through the use of salt metathesis pathways, this monoanionic, chelating N～C ligand has been employed to produce a series of titanium and zirconium complexes, including L₂MX₂ (M = Ti, X-Cl (5); M = Zr, X= Cl (3), CH₂SiMe₃ (4)) and L ₂Ti=N^tBu (6).

Full text of DOI:10.1021/om700530z

4094
Organometallics 2007, 26, 4094-4097
A Discrete Ortho-Lithiated Acetophenone Imine Derivative:
Isolation, Characterization, and Synthesis of Group IV Metal
Complexes
Tamam I. Baiz and Joseph A. R. Schmidt*
Department of Chemistry, The UniVersity of Toledo, 2801 West Bancroft Street, MS 602,
Toledo, Ohio 43606-3390
ReceiVed May 29, 2007
Scheme 1. Early-Metal Ortho-Metalated Imine Complexes
Produced by Cyano Insertion Reactionsa
Summary: The reaction of a 3′,4′-methylenedioxyacetophenone
imine (L-H; 1) with nBuLi allows for the synthesis and isolation
of a discrete ortho-lithiated imine (L-Li; 2). Through the use of
salt metathesis pathways, this monoanionic, chelating N∼C
ligand has been employed to produce a series of titanium and
zirconium complexes, including L2MX2 (M ) Ti, X ) Cl (5);
M ) Zr, X ) Cl (3), CH2SiMe3 (4)) and L2TidNtBu (6).
a M ) Ti, Zr, Ta; R ) alkyl, aryl, amino, phosphino; HX ) weak
acid, such as alcohol, amine, thiol.
The use of ortho-metalated ketones in late-metal catalysis has
become quite common in recent years. Notably, Crabtree has
shown that iridium hydrides supported by ortho-metalated
acetophenone ligands function as excellent catalysts for the
hydroalkoxylation and hydroamination of alkynes.1,2 In addition
to the use of ortho-metalated ketones, it has been shown that
the related imines also support a wide variety of late-metal
chemistry.3-15 Recently, Lai and co-workers reported the use
of ortho-metalated imines in iridium-mediated hydroamination
of alkenes and alkynes.15 On the basis of these results, we
hypothesized that ortho-metalated imines could likewise function
as useful ancillary ligands for early-metal organometallic
complexes.
Although late-metal complexes have been well investigated,
ortho-metalated imine ligands have seen only scant use with
highly Lewis acidic early transition metals. In fact, to date there
is no direct route for the synthesis of early-metal complexes
employing these ligands. In the cases where late metals are used,
ortho-metalated imine complexes are readily produced by
oxidative addition of the aryl C-H bond following ligation of
the imine nitrogen. This direct synthetic route is unavailable
for early transition metals, as traditional early-metal starting
materials typically contain metal centers in their maximum
oxidation states (eliminating the possibility for oxidative addition
of the aryl C-H bond). Additionally, due to the redox properties
of early metals, the use of starting materials with lower oxidation
states generally results in reductive processes, rather than the
desired C-H activation of these species.16,17 To date, the only
reported route for the synthesis of early-metal ortho-metalated
imine complexes entails the insertion of a cyano group into a
metal-benzyne bond, yielding dianionic versions of these
ligands; this is a method which has been employed for
nitriles,18-22 cyanophosphines,23-25 and cyanamides26 (Scheme
1). One major limitation of this synthetic route is that the nitro-
gen (anionic following cyano insertion) can only be converted
into the parent unsubstituted imine (RCdN- or RCdNH),
eliminating steric and electronic modification of this donor atom.
As a part of our research effort toward the design of new
unsymmetrical, bidentate ligand-supported complexes for use
in catalytic bond-forming reactions, we present the synthesis
and isolation of a newly developed bidentate, monoanionic,
ortho-lithiated imine ligand with straightforward steric and
electronic tunability. Herein, we detail the synthesis of several
group IV metal complexes derived from a 3′,4′-methylene-
dioxyacetophenone aryl imine. Ultimately, we speculate that
* To whom correspondence should be addressed. E-mail:
Joseph.Schmidt@utoledo.edu.
(1) Li, X. W.; Chen, P.; Faller, J. W.; Crabtree, R. H. Organometallics
2005, 24, 4810.
(2) Li, X. W.; Chianese, A. R.; Vogel, T.; Crabtree, R. H. Org. Lett.
2005, 7, 5437.
(3) Basu, S.; Dutta, S.; Drew, M. G. B.; Bhattacharya, S. J. Organomet.
Chem. 2006, 691, 3581.
(4) Davies, D. L.; Al-Duaij, O.; Fawcett, J.; Giardiello, M.; Hilton, S.
T.; Russell, D. R. Dalton Trans. 2003, 4132.
(5) de Pater, B. C.; Zijp, E. J.; Fruhauf, H. W.; Ernsting, J. M.; Elsevier,
C. J.; Vrieze, K. Organometallics 2004, 23, 269.
(6) Dowerah, D.; Radonovich, L. J.; Woolsey, N. F.; Heeg, M. J.
Organometallics 1990, 9, 614.
(7) Ezhova, M. B.; Patrick, B. O.; James, B. R. Organometallics 2005,
24, 3753.
(8) Hoogervorst, W. J.; Goubitz, K.; Fraanje, J.; Lutz, M.; Spek, A. L.;
Ernsting, J. M.; Elsevier, C. J. Organometallics 2004, 23, 4550.
(9) Huang, L. Y.; Aulwurm, U. R.; Heinemann, F.; Knock, F.; Kisch,
H. Chem. Eur. J. 1998, 4, 1641.
(10) Kawamoto, T. Inorg. Chim. Acta 2000, 300, 512.
(11) Krug, C.; Hartwig, J. F. Organometallics 2004, 23, 4594.
(12) Krug, C.; Hartwig, J. F. J. Am. Chem. Soc. 2004, 126, 2694.
(13) Marcazzan, P.; Patrick, B. O.; James, B. R. Organometallics 2005,
24, 1445.
(16) de Boer, E. J. M.; Teuben, J. H. J. Organomet. Chem. 1978, 153,
53.
(17) Scholz, J.; Hadi, G. A.; Thiele, K. H.; Gorls, H.; Weimann, R.;
Schumann, H.; Sieler, J. J. Organomet. Chem. 2001, 626, 243.
(18) Hsu, D. P.; Lucas, E. A.; Buchwald, S. L. Tetrahedron Lett. 1990,
31, 5563.
(19) Barr, K. J.; Watson, B. T.; Buchwald, S. L. Tetrahedron Lett. 1991,
32, 5465.
(20) Mashima, K.; Tanaka, Y.; Nakamura, A. Organometallics 1995,
14, 5642.
(21) Lee, H.; Bridgewater, B. M.; Parkin, G. Dalton Trans. 2000, 4490.
(22) Campora, J.; Buchwald, S. L. Organometallics 1993, 12, 4182.
(23) Zablocka, M.; Owsianik, K.; Igau, A.; Skowronska, A.; Majoral, J.
P. Eur. J. Inorg. Chem. 2002, 801.
(24) Owsianik, K.; Zablocka, M.; Donnadieu, B.; Majoral, J. P. Angew.
Chem., Int. Ed. 2003, 42, 2176.
(14) Martin, M.; Sola, E.; Tejero, S.; Andres, J. L.; Oro, L. A. Chem.
Eur. J. 2006, 12, 4043.
(15) Lai, R. Y.; Surekha, K.; Hayashi, A.; Ozawa, F.; Liu, Y. H.; Peng,
S. M.; Liu, S. T. Organometallics 2007, 26, 1062.
(25) Cadierno, V.; Zablocka, M.; Donnadieu, B.; Igau, A.; Majoral, J.
P. Organometallics 2002, 21, 3215.
(26) Owsianik, K.; Zablocka, M.; Igau, A.; Majoral, J. P.; Skowronska,
A. Eur. J. Inorg. Chem. 2001, 1693.
10.1021/om700530z CCC: $37.00 © 2007 American Chemical Society
Publication on Web 07/12/2007

Communications
Organometallics, Vol. 26, No. 17, 2007 4095
Scheme 2. Synthesis and Lithiation of the Acetophenone
Imine Derivative (Ar ) 2,6-Et2C6H3)
Scheme 3. Salt Metathesis of the Ortho-Lithiated Imine 2
with ZrCl4 Leading to L2ZrCl2 (3) as an Equilibrium of Two
Structural Isomers: 3a and 3ba
1
Figure 1. Experimental Keq values and representative H NMR
spectra of the -OCH2O- region at various temperatures for the
equilibrium between 3a and 3b. A small amount of ligand 1 (L-H)
is present.
slowly warmed to ambient temperature overnight. The product
3 is insoluble in pentane and diethyl ether and sparingly soluble
in toluene, while slowly decomposing in THF and CH2Cl2,
preventing efficient isolation from the byproduct LiCl. Liberal
washing with pentane was used to remove excess ligand, and
drying under vacuum yielded 3 in 85% yield as an orange solid.
All attempts to recrystallize this material were unsuccessful,
giving only a microcrystalline solid.
a Ar ) 2,6-Et2C6H3.
this ligand platform will allow access to a wide array of
complexes yielding new catalytic and stoichiometric reactivity.
The acetophenone imine ligand (L-H; 1) was readily syn-
thesized by the Schiff-base condensation of 3′,4′-methylene-
dioxyacetophenone with excess 2,6-diethylaniline in the pres-
ence of catalytic p-toluenesulfonic acid.27 The reaction mixture
was brought to reflux for 2 days, using a Dean-Stark trap to
collect the byproduct water upon formation. After organic
extraction, 1 was obtained by vacuum distillation, providing the
ligand in 85% yield (Scheme 2). The product could be readily
identified by NMR spectroscopy. Notably, a sharp singlet was
observed at δ 5.28 in the 1H NMR spectrum of 1, representative
of the -OCH2O- moiety on the phenyl ring.
1
The H NMR spectrum of 3 indicated the presence of two
highly symmetrical species in solution, most readily identified
by the distinctive peaks for the -OCH2O- resonances, appear-
ing as two weakly coupled doublets for each isomer. The two
distinct sets of peaks observed for the two isomers showed
varying ratios, depending on solution temperature (Figure 1).
Thus, the presence of two compounds in solution is inferred;
there is not just one species with inequivalent ligands. On the
basis of the observed symmetry, the empirical formula, and the
propensity for six-coordinate zirconium complexes to have cis-
chloride ligands, we suggest that the two observed species are
the C2-symmetric L2ZrCl2 isomers 3a,b (Scheme 3). The
observed C2 symmetry necessitates that either the carbons or
the nitrogens of the two ligands be arranged in a trans
configuration. Variable-temperature (VT) 1H NMR spectroscopy
revealed that one isomer (denoted 3a) was more thermodynami-
cally stable than the other (3b) in the temperature range 253-
333 K,28 and the thermodynamic parameters of the equilibrium
were investigated. Equilibrium constants (Keq) were calculated
throughout the temperature range, according to the equation
Keq) [3a]/[3b], where the relative ratios of the two species were
obtained from the integration of the -OCH2O- proton reso-
nances. The standard enthalpy (6.6 kJ mol-1) and entropy
changes (28 J mol-1 K-1) were estimated from a van’t Hoff
plot (Figure 2). These small isomerization enthalpy and entropy
values support near-equal concentrations of the two isomers at
approximately -40 °C.
n
The reaction of 1 with a stoichiometric amount of BuLi in
pentane resulted in deprotonation at the 2′-position to yield 1
equiv of butane, with concomitant formation of the ortho-
lithiated imine ligand (L-Li; 2) (Scheme 2). As the reaction was
very rapid and exothermic, cooling in an ice bath was used to
minimize byproduct formation. Following the reaction, the
mixture was slowly warmed to room temperature. The product,
an air- and moisture-sensitive yellow solid, was washed liberally
with pentane and dried under vacuum, affording 2 in excellent
yield. Quenching of the lithiated species with D2O confirmed
the regioselective lithiation, with quantitative recovery of 2′-
d-1. The lithiated ligand 2 could also be readily characterized
by its NMR spectroscopic data, with the most notable peak again
being the -OCH2O- resonance, now appearing as a singlet at
δ 4.81.
In order to demonstrate the utility of ortho-metalated imines
as ancillary ligands in early-metal organometallic chemistry, a
series of group IV metal complexes were targeted, with salt
metathesis reactions being the preferred method of synthesis.
Two equivalents of 2 were found to react readily with ZrCl4 to
form the complex L2ZrCl2 (3), where two halide ligands were
replaced by two bidentate ortho-metalated imines (Scheme 3).
The reaction was carried out at 0 °C and then the mixture was
Compound 3 was further functionalized by reaction with 2
equiv of LiCH2SiMe3 to produce the dialkylated species L2Zr-
(CH2SiMe3)2 (4) (Scheme 4) in moderate yield.29 Notably, the
1H NMR spectrum of 4 showed the presence of only one isomer
in solution, as evidenced by the equivalence of the SiMe3
(27) An earlier report details the synthesis of a related aldimine species
(3′,4′-methylenedioxybenzaldehyde cyclohexyl imine) and its in situ lithia-
tion, as determined by quenching experiments: Ziegler, F. E.; Fowler, K.
W. J. Org. Chem. 1976, 41, 1564.
(28) The C2 isomer with ligand carbon donor atoms in a trans orientation
was ascribed to the more stable isomer on the basis of the crystal structure
of the dialkylated species 4, detailed below.

4096 Organometallics, Vol. 26, No. 17, 2007
Communications
Figure 2. van’t Hoff plot for thermodynamic analysis of the
equilibrium of 3a and 3b.
Scheme 4. Reaction of 3 with LiCH2SiMe3 Leading to
L2Zr(CH2SiMe3)2 (4)a
Figure 3. ORTEP diagram (50% probability) of L2Zr(CH2SiMe3)2
(4). Disordered methyl groups, hydrogen atoms, and cocrystallized
ether have been removed for clarity. Bond lengths (Å) and angles
(deg): Zr1-C1 ) 2.228(3), Zr1-C5 ) 2.223(3), Zr1-C9 ) 2.297-
(3), Zr1-C28 ) 2.295(3), Zr1-N1 ) 2.540(3), Zr1-N2 ) 2.510-
(2); C1-Zr1-C5 ) 90.2(1), C1-Zr1-C9 ) 99.9(1), C1-Zr1-
C28 ) 107.3(1), C1-Zr1-N1 ) 87.5(1), C1-Zr1-N2 ) 174.8(1),
N1-Zr1-N2 ) 95.7(1).
a Ar ) 2,6-Et2C6H3.
in a wide range of organic solvents, making its complete
isolation and characterization untenable. In addition to poor
solubility, 5 exhibited little or no reactivity with a diverse array
of lithium reagents, including MeLi, LiCH2SiMe3, LiNEt2, and
LiNH(2,6-i-Pr2C6H3). The reactions were invariably slow and
proceeded to poor conversion, even with heat and in a variety
of solvents. This was somewhat surprising, as titanium was
expected to be more reactive than its heavier congener zirco-
nium. Perhaps, since the titanium is smaller, the steric bulk of
the ortho-metalated imine ligands is more pronounced in 5,
hindering access to the titanium center. It is also possible that
liberated lithium chloride is retained within the metal complex,
forming an “ate” salt, which would serve to further reduce the
reactivity of these species.35 Numerous examples in the literature
have shown that small alkali-metal salts are often trapped in
the coordination sphere of early transition metals, forming “ate”
salts of this type.36-39
In order to circumvent these solubility issues, we investigated
a titanium imido precursor as an entry point for this chemistry.
Two equivalents of 2 were found to react readily with (py)2Ti-
(dNtBu)Cl240 to form L2TidNtBu (6) (Scheme 5). The red solid
that was obtained showed moderate thermal stability (the solid
decomposes at 183 °C) and was soluble in most nonpolar
solvents. Additionally, it showed no evidence of coordination
by a variety of coordinating solvents, supporting its formulation
as a five-coordinate titanium imido species. The NMR spectra
of 6 indicated the presence of a single, highly symmetrical
resonances (δ 0.22) as well as the presence of only two weakly
coupled doublets for the diastereotopic -OCH2O- protons. The
lack of isomerization in this species can be attributed to the
steric demands of the large alkyl groups employed, forcing the
complex to adopt the more sterically favored isomer. X-ray-
quality crystals of 4 were obtained from diethyl ether at -25
°C, and the crystal structure reveals that the carbons of the two
ligands are in a trans orientation, while the two ligand nitrogen
atoms and the methylene carbons of the (trimethylsilyl)methyl
groups are each oriented in a cis arrangement (Figure 3).30 The
observed Zr-C distances are very similar to those observed in
other Zr-CH2SiMe3 complexes.31-33 Additionally, a significant
intramolecular π-stacking interaction is observed between the
ortho-metalated imine ligands. The N-Ar and 3′,4′-methylene-
dioxyphenyl planes are nearly parallel (average deviation ∼20°),
and the distance between the offset phenyl rings is approximately
3.4 Å, a value indicative of strong π-stacking interactions.34
This contributes to the observed stability of 4 and is also likely
an influence on the isomerization process observed for 3.
Titanium complexes with compositions similar to those of
the zirconium species discussed herein were also synthesized.
Two equivalents of the lithiated ligand 2 reacted readily with
TiCl4 to yield L2TiCl2 (5). Compound 5 was virtually insoluble
(29) The analogous L2ZrMe2 species was synthesized similarly from
reaction of 3 with 2 equiv of MeLi. Its solubility was virtually identical
with that of 3, preventing its isolation from unreacted 3. It could be readily
observed spectroscopically: 1H NMR (C6D6, 400 MHz) δ 7.14-7.01 (m,
6H), 6.90 (d, 2H, 8 Hz), 6.58 (d, 2H, 8 Hz), 5.32 (s, 4H), 2.59 (s, 6H),
2.56-2.38 (m, 8H), 1.75 (s, 6H), 1.15 (t, 12H, 8 Hz).
(30) Crystal data: C46H62N2O4Si2Zr‚1/2C4H10O, triclinic, P1h, a )
11.4685(6) Å, b ) 12.6056(6) Å, c ) 18.3425(9) Å, R ) 81.983(1)°, â )
77.081(1)°, γ ) 65.461(1)°, T ) -138 °C, R1 ) 0.068, wR2 ) 0.199 (all
data).
(31) Hoyt, H. M.; Michael, F. E.; Bergman, R. G. J. Am. Chem. Soc.
2004, 126, 1018.
(32) Cayias, J. Z.; Babaian, E. A.; Hrncir, D. C.; Bott, S. G.; Atwood,
J. L. J. Chem. Soc., Dalton Trans. 1986, 2743.
(35) A flame test indicated the presence of lithium in the sample, but
this may be representative of incomplete removal of LiCl from the material
on workup. A 7Li NMR showed only a single resonance at δ 0.00 ppm,
representative of LiCl. This still does not preclude the presence of “ate”
salts within the material, as it has only very low solubility in the NMR
solvent (C6D6).
(36) Lubben, T. V.; Wolczanski, P. T.; Vanduyne, G. D. Organometallics
1984, 3, 977.
(37) Firth, A. V.; Witt, E.; Stephan, D. W. Organometallics 1998, 17,
3716.
(38) Wood, D.; Yap, G. P. A.; Richeson, D. S. Inorg. Chem. 1999, 38,
5788.
(39) Witt, E.; Stephan, D. W. Inorg. Chem. 2001, 40, 3824.
(40) Blake, A. J.; Collier, P. E.; Dunn, S. C.; Li, W. S.; Mountford, P.;
Shishkin, O. V. J. Chem. Soc., Dalton Trans. 1997, 1549.
(33) Jeffery, J.; Lappert, M. F.; Luongthi, N. T.; Webb, M.; Atwood, J.
L.; Hunter, W. E. J. Chem. Soc., Dalton Trans. 1981, 1593.
(34) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112,
5525.

Communications
Organometallics, Vol. 26, No. 17, 2007 4097
Scheme 5. Synthesis of the Titanium Imido Derivative 6
and Its Subsequent Reactivity with Aldehydes To Form the
Analogous Oxo Product 7a
a metal-oxo product (Scheme 5). The metathesis reactions
between metal imido complexes and carbonyl groups have been
well documented, generally occurring via a cycloaddition
reaction to yield an imine and an oxo product.46
In summary, the synthesis of a bidentate, monoanionic, ortho-
metalated imine ligand, which can be readily synthesized using
n
a Schiff base condensation and easily activated with BuLi to
form the corresponding ortho-lithiated compound, has been
reported herein. The coordination chemistry of this new ligand
with titanium and zirconium precursors has been explored,
resulting in the synthesis of unprecedented ortho-metalated
titanium and zirconium complexes through salt metathesis
routes. We are currently investigating the use of these group
IV metal complexes in a broad range of stoichiometric and
catalytic applications. Additionally, we will continue to explore
the utility of these ortho-metalated imines as supporting ligands
for other early-metal centers.
t
a Ar ) 2,6-Et2C6H3; R ) Bu; R′ ) 3,4-methylenedioxyphenyl.
species, most likely a square-pyramidal complex with the two
bidentate ligands in trans basal positions and the imido group
occupying the electronically favored axial position, where it
functions as a six-electron-donor ligand. Titanium complexes
employing bulky bidentate ligands with tBu-imido groups are
commonly found with five-coordinate pseudo-square-pyramidal
coordination geometry,41-44 whereas bridging imido ligands
have generally been limited to lower coordinate titanium species
or those with less sterically bulky ligands.43,45
Acknowledgment. We gratefully acknowledge The Uni-
versity of Toledo for generous start-up funding, as well as a
University of Toledo Summer Research Fellowship (J.A.R.S.).
Additionally, we thank Dr. Kristin Kirschbaum and Dr. Allen
Oliver for insightful discussions.
Supporting Information Available: Text and a table giving
experimental procedures and characterization data for 1-7, ex-
perimental conditions and Keq values for 3 at various temperatures,
and crystallographic procedures and crystal data for 4 and a CIF
file giving additional crystallographic data, including bond lengths
and angles of compound 4. This material is available free of charge
via the Internet at http://pubs.acs.org.
As expected, 6 was shown to undergo Wittig-like reactivity
by an NMR-scale reaction with an aldehyde (piperonal) to form
(41) Hsu, S. H.; Chang, J. C.; Lai, C. L.; Hu, C. H.; Lee, H. M.; Lee, G.
H.; Peng, S. M.; Huang, J. H. Inorg. Chem. 2004, 43, 6786.
(42) Hagadorn, J. R.; Arnold, J. Organometallics 1998, 17, 1355.
(43) Coles, M. P.; Hitchcock, P. B. Organometallics 2003, 22, 5201.
(44) Muller, E.; Muller, J.; Olbrich, F.; Bruser, W.; Knapp, W.; Abeln,
D.; Edelmann, F. T. Eur. J. Inorg. Chem. 1998, 87.
OM700530Z
(45) Thorn, D. L.; Nugent, W. A.; Harlow, R. L. J. Am. Chem. Soc.
1981, 103, 357.
(46) Guiducci, A. E.; Boyd, C. L.; Mountford, P. Organometallics 2006,
25, 1167.