A new scorpionate ligand: Tris(4,4-dimethyl-2-oxazolinyl)borate and its zirconium(IV) complexes

Article abstract of DOI:10.1021/om800252p

The first example of a new class of oxazoline-based scorpionate ligand, tris(4,4-dimethyl-2-oxazolinyl)phenyl borate, [To^M]^-, is prepared by reaction of 2-lithio-4,4-dimethyl-2-oxazolide and 0.3 equiv of dichlorophenylborane. The steric bulk of this ligand is greater than that of tris(3,5-Me₂-pyrazolyl)borate (Tp*), as quantified by comparison of solid angles of crystallographically characterized zirconium(IV) complexes.

Full text of DOI:10.1021/om800252p

Organometallics 2008, 27, 2399–2401
2399
A New Scorpionate Ligand: Tris(4,4-dimethyl-2-oxazolinyl)borate
and Its Zirconium(IV) Complexes
James F. Dunne, Jiachun Su, Arkady Ellern, and Aaron D. Sadow*
Department of Chemistry, U.S. DOE Ames Laboratory, Iowa State UniVersity, Ames, Iowa 50011-3111
ReceiVed March 18, 2008
rate complexes.4 In highly reactive complexes such as [Zr(κ3-
Tp*)(CH2C6H5)2]+, ligand decomposition through B-N cleavage
hinders possible utility in catalysis.2 In fact, isomerization via
1,2-sigmatropic shifts and B-N bond cleavage are common
reaction pathways for pyrazolylborate ligands in a range of
transition metal complexes.1c,d,3,11
Summary: The first example of a new class of oxazoline-based
scorpionate ligand, tris(4,4-dimethyl-2-oxazolinyl)phenyl borate,
[ToM]-, is prepared by reaction of 2-lithio-4,4-dimethyl-2-
oxazolide and 0.3 equiV of dichlorophenylborane. The steric
bulk of this ligand is greater than that of tris(3,5-Me2-
pyrazolyl)borate (Tp*), as quantified by comparison of solid
angles of crystallographically characterized zirconium(IV)
complexes.
To avoid this decomposition mechanism, we designed the
tris(oxazolinyl)borate scorpionate ligand that contains B-C
linkages, proposed to have greater resistance to cleavage and
isomerization processes.12 The synthesis of tris(4,4-dimethyl-
2-oxazolinyl)phenyl borate, [ToM]-, the first example of this
new ligand class, and its coordination chemistry in new
zirconium (IV) complexes are reported here. Lithium tris(4,4-
dimethyl-2-oxazolin-2-yl)phenyl borate (Li[ToM]) is prepared
by deprotonation of 4,4-dimethyl-2-oxazoline with n-BuLi (1.05
equiv) at -78 °C followed by addition of 0.30 equiv of
dichlorophenylborane (eq 1).
Sterically encumbered tris(pyrazolyl)borate ligands, such as
Tp* (HB(3,5-Me2-pyrazolyl)3), TptBuMe (HB(3-tBu-5-Me-pyra-
zolyl)3), and TpMes (HB(3-mesitylpyrazolyl)3), have recently
captured attention as useful ancillary ligands in early transition
metal chemistry.1–4 Early transition metal complexes of these
bulky ligands as well as those of parent tris(pyrazolyl)borate
(Tp) show interesting stoichiometric chemistry2–8 and display
catalytic activity in reactions such as olefin polymerization.2,4,8a,9,10
However, unsymmetrically substituted Tp derivatives (e.g.,
3-mesitylpyrazolyl) are susceptible toward isomerization pro-
cesses, giving mixtures of 3- and 5-substituted tris(pyrazolyl)bo-
* Corresponding author. E-mail: sadow@iastate.edu.
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Quantitative deprotonation of the oxazoline is necessary, as
mixtures of 2H-oxazoline, 2-lithio-oxazolide, and dichlorophe-
nylborane produce Li[ToM] and several unidentified products.
Independent experiments show that interaction of 4,4-dimethyl-
2-oxazoline and PhBCl2 in THF accounts for two of the side
products. Also, a slight excess of 2-lithio-oxazolide (0.30 equiv
of PhBCl2) is necessary, as 0.33 equiv of PhBCl2 and shorter
reaction times (<26 h) afford inseparable mixtures of Li[ToM]
and a species assigned as bis(4,4-dimethyl-2-oxazolin-2-yl)phe-
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10.1021/om800252p CCC: $40.75
2008 American Chemical Society
Publication on Web 05/07/2008

2400 Organometallics, Vol. 27, No. 11, 2008
Communications
1
The H NMR spectra of Li[ToM] dissolved in benzene-d6,
chloroform-d1, or methylene chloride-d2 contained broad reso-
nances at 1.4 and 3.3 ppm as well as aryl resonances. In contrast,
the 4,4-dimethyl and methylene groups (at 1.19 and 3.60 ppm)
on equivalent oxazoline rings gave sharp singlet resonances in
acetonitrile-d3. The formation of the borate center is supported
by the integrated ratio of oxazoline methyl to phenyl resonances
(18:5 H) as well as the broad singlet observed at -16.9 ppm in
the 11B NMR spectrum.13
This salt elimination route to Li[ToM] is related to the
synthesis of bis(oxazolinyl)diphenylborate ligands (borabox).14
Borabox, which also is prepared via reaction of 2-lithium
oxazolide and a chloroborane electrophile, forms in 4-12 h. In
contrast to this short reaction time, preparation of the neutral
ligand 1,1,1-tris(oxazolinyl)ethane (tris-ox) requires heating THF
solutions of lithium bis(oxazoline) and 2-bromo-2-oxazoline at
reflux for 5 days.15 Interestingly, the reaction conditions needed
for synthesis of Li[ToM] fall between these two ligands.
The acidic pro-ligand H[ToM] is prepared by flash chroma-
tography of Li[ToM] on silica gel in C6H14/i-PrOH/NEt3 (15:
1:1), giving the product as an off-white solid (eq 2).14
Figure 1. ORTEP diagram of H[ToM] at 50% probability. Hydrogen
atoms are omitted for clarity.
giving a yellow solution of [Zr(κ3-ToM)Cl3] and suspended
lithium chloride (eq 3).
The H NMR spectrum of H[ToM] (benzene-d6) contained
1
The same compound is also obtained by heating a mixture
of Li[ToM] and ZrCl4 in toluene to reflux for 24 h. In fact,
[Zr(κ3-ToM)Cl3] is recovered without decomposition after
isolated material is redissolved and heated in toluene at reflux
for 7 days. C3V-Symmetric [Zr(κ3-ToM)Cl3] is characterized by
sharp singlet resonances at 1.02 (Me) and 3.55 ppm (CH2), in
contrast to the broad resonances observed for Li[ToM] in this
solvent. In acetonitrile-d3, the oxazoline peaks are shifted
downfield by 0.1 and 0.3 ppm (to 1.29 and 3.90 ppm) in
comparison to Li[ToM]. Crystals of H[ToM] are obtained from
the chromatography solvent mixture after slow evaporation.
Although the crystals diffracted weakly, single-crystal X-ray
diffraction confirms the constitution and connectivity of [ToM]-,
in particular the formation of a tetrahedral borate center
containing four B-C bonds (Figure 1).
1
equivalent oxazoline groups that are observed in the H NMR
spectrum (benzene-d6) as singlet resonances at 3.28 (CH2) and
1.36 ppm (Me). A comparison of the IR spectra of Li[ToM]
versus [Zr(κ3-ToM)Cl3] showed the stretching frequency of the
CdN double bond shifts from 1607 cm-1 to lower energy at
1545 cm-1
.
The symmetry of [ToM]- in the solid state is Cs, as two of
the oxazolines’ CdN are coplanar (N1dC7-C17dN3 dihedral
) 0°), oriented toward each other as if to form a bidentate
chelate, and related by a mirror plane. That mirror plane contains
the phenyl group and the third oxazoline. However, the 1H NMR
spectrum of H[ToM] (benzene-d6 or acetonitrile-d3) indicates
that all three oxazoline groups are equivalent in solution.
We have prepared [ToM]Zr(IV) complexes through salt
metathesis with Li[ToM] and ZrCl4 as well as via amine
elimination with H[ToM] and Zr(NMe2)4. A salt elimination
reaction occurs when a mixture of Li[ToM] and 1.05 equiv of
ZrCl4 suspended in methylene chloride is stirred for 4 days,
Amine elimination also proved to be a viable route for the
preparation of [ToM]Zr compounds.16 Thus, reaction of
Zr(NMe2)4 and H[ToM] in benzene yields [Zr(κ3-ToM)(NMe2)3]
as a white solid (73%, eq 4). A micromolar scale reaction was
1
monitored in situ by H NMR spectroscopy, revealing that 1
1
equiv of NHMe2 is rapidly formed (<10 min). The H NMR
spectrum (benzene-d6) of the C3V-symmetric product contained
one set of oxazoline resonances (3.41 and 1.09 ppm) and a
singlet corresponding to equivalent NMe2 groups (3.15 ppm).
(13) Kidd, R. G. In NMR of Newly Accessible Nuclei; Laszlo, P., Ed.;
Academic Press: New York, 1983; Vol. 2, pp 49-77.
(14) (a) Mazet, C.; Kohler, V.; Pfaltz, A. Angew. Chem., Int. Ed. 2005,
44, 4888–4891. (b) Mazet, C.; Roseblade, S.; Kohler, V.; Pfaltz, A. Org.
Lett. 2006, 8, 1879–1882.
(15) (a) Bellemin-Laponnaz, S.; Gade, L. H. J. Chem. Soc., Chem.
Commun. 2002, 1286–1287. (b) Bellemin-Laponnaz, S.; Gade, L. H. Angew.
Chem., Int. Ed. 2002, 41, 3473–3475. (c) Ward, B. D.; Bellemin-Laponnaz,
S.; Gade, L. H. Angew. Chem., Int. Ed. 2005, 44, 1668–1671. (d) Lukesova,
L.; Ward, B. D.; Bellemin-Laponnaz, S.; Wadepohl, H.; Gade, L. H.
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In contrast, salt metathesis of [Zr(κ3-ToM)Cl3] and 3 equiv
of LiNMe2 (benzene-d6, room temperature, 24 h) provides a
1
Cs-symmetric product. The H NMR spectrum of the crude
reaction mixture revealed a 1:1 ratio of [ToM]:NMe2 groups,

Communications
Organometallics, Vol. 27, No. 11, 2008 2401
providing support for its assignment as [Zr(κ3-ToM)(NMe2)Cl2].
Under more forcing conditions (6 equiv of LiNMe2, 24 h, 60
°C), Zr(NMe2)4 and Li[ToM] are obtained. Although substitution
of one chloride in [Zr(κ3-ToM)Cl3] by [NMe2]- occurs, forma-
tion of [Zr(κ3-ToM)(NMe2)3] via salt elimination has not been
observed.
To further explore chloride substitutions, a toluene solution
of [Zr(κ3-ToM)Cl3] and KO-t-Bu was stirred overnight at room
temperature to provide [Zr(κ3-ToM)(O-t-Bu)Cl2].
Figure 3. ORTEP diagram of [Zr(κ3-ToM)(O-t-Bu)Cl2] at 50%
probability. Hydrogen atoms are omitted for clarity.
to 43% of a sphere’s surface). For comparison, the solid angle
for Tp* is 3.62 steradians (130° corresponding to 30% of a
sphere’s surface, determined for Tp*Zr(CH2C6H5)3).2
The 1H NMR spectrum of [Zr(κ3-ToM)(O-t-Bu)Cl2] highlights
its Cs-symmetry. As a result of the mirror plane that includes
the tert-butoxide oxygen, zirconium center, and trans-oxazoline,
the -CMe2- and -CH2- groups on that ring are equivalent
and appear as singlet resonances (1.52 and 3.38, respectively,
labeled as trans in the Newman projection in Figure 2).
Although the other two oxazoline rings are related by the
mirror plane, the -CH2- within each ring are endo or exo with
respect to the tert-butoxide and were observed as two coupled
These values for solid angles show that the 4,4-dimethylox-
azoline substituents in [ToM]- encompass the zirconium center
to a greater extent than the 3,5-dimethylpyrazole groups in Tp*.
Thus, tris(4,4-dimethyl-2-oxazolinyl)borate may offer enhanced
steric protection for highly reactive metal centers, such as those
of cationic zirconium alkyl and hydride complexes.2 Addition-
ally, optically active C3-symmetric tris(oxazolinyl)borates are
readily accessible due to the availability of enantiopure 2H-
oxazolines.20 These chiral tris(oxazolinyl)borates will comple-
ment the chemistry of the related optically active tris(pyra-
zolyl)boratespioneeredbyTolman.21 Wearecurrentlyinvestigating
our achiral complexes as catalysts, as well as preparing chiral
analogues for stereoselective polymerizations.15
2
doublets (3.38 and 3.25 ppm, JHH ) 8.72 Hz). The methyl
substituents in the CMe2 groups of these oxazolines are also
inequivalent (but not coupled), resulting in two singlet reso-
nances (1.45 and 1.05 ppm are exo and endo, respectively,
assigned with a NOESY experiment). Cross-peaks in a 1H-15N
HMBC experiment verify that these two peaks are cis to the
tert-butoxide, as they correlate to a 15N resonance at 239 ppm
(N trans is at 241 ppm).11a,17
Acknowledgment. We thank the U.S. DOE office of Basic
Energy Sciences, through the Catalysis Science Grant No.
AL-03-380-011, the Roy J. Carver Charitable Trust, and Iowa
State University for financial support. We also thank Dr.
Bruce Fulton for assistance with 15N NMR measurements.
A solid-state structure of the product confirmed the N,N,N-
κ3 coordination of [ToM]- to zirconium, as well as the overall
connectivity of the molecule (Figure 3). The zirconium center’s
geometry is distorted octahedral with a relatively short Zr-O
bond (1.891(3) Å) for a six-coordinate zirconium tert-butoxide.
Interestingly, [Zr(κ3-ToM)(O-t-Bu)Cl2] is C1-symmetric in the
solid state due to inequivalent twist conformations of the three
oxazoline rings such that the methyl groups on the 4-position
are positioned equatorial (pointed toward zirconium) or axial.
Thus, all Zr · · · C (4-Me) distances are inequivalent, ranging from
3.76 to 4.23 Å, creating an unusual steric pocket around the
zirconium center. The size of this pocket is best described by
the solid angle for [ToM]-,18,19 determined from the crystal-
lographic coordinates to be 5.40 steradians (164° corresponding
Supporting Information Available: Experimental procedures,
characterization, and X-ray crystallographic data are available free
of charge at http://pubs.acs.org.
OM800252P
(16) (a) Diamond, G. M.; Rodewald, S.; Jordan, R. F. Organometallics
1995, 14, 5–7. (b) Diamond, G. M.; Jordan, R. F.; Petersen, J. L.
Organometallics 1996, 15, 4030–4037.
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The solid angles were calculated from X-ray coordinates of [Zr(κ3-
Tp*)(CH2C6H5)3] (ref 2) and [Zr(κ3-ToM)(O-t-Bu)Cl2] using the program
Steric running the algorithms described in ref 18a. (d) For comparison, the
cone angles were also calculated using Steric: [ToM]-, 242°; [Tp*]-, 246°.
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of the Witwatersrand: South Africa, 1995.
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Figure 2. Newman projection illustrating the Cs-symmetry of
[Zr(κ3-ToM)(O-t-Bu)Cl2]. The groups attached to N represent CH2
and CMe2 substituents on the oxazoline.
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1153–1154. (b) Kitajima, N.; Tolman, W. B. Prog. Inorg. Chem. 1995,
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