Titanium and zirconium complexes of preorganized tripodal triaryloxide ligands

Article abstract of DOI:10.1021/ja053740m

Tri(2-oxy-3,5-di-tert-butylphenyl)methane, [O₃]^3- has been used to prepare titanium and zirconium complexes of the general formula [O₃]MX (M = Ti, X = NEt₂, Cl, CH₂Ph; M = Zr, X = CH₂Ph). The tripodal [O₃] ligand in titanium complexes adopt the syn- and the anti-conformation, while the syn complex of zirconium undergoes facile C-H activation to give a 5-carbametalatrane [O₃C]Zr(THF)₃. Copyright

Full text of DOI:10.1021/ja053740m

Published on Web 08/06/2005
Titanium and Zirconium Complexes of Preorganized Tripodal Triaryloxide
Ligands
Fumio Akagi, Tsukasa Matsuo, and Hiroyuki Kawaguchi*
Coordination Chemistry Laboratories, Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan
Received June 7, 2005; E-mail: hkawa@ims.ac.jp.
Scheme 1
Multidentate ligands play an important role in coordination
chemistry and catalyst design. An attractive multidentate ligand is
a trianionic tetradentate ligand of the tripodal [X3E] type (X ) N,
O, S; E ) N, P), which had led to atrane molecules with unique
structures and patterns of reactivity.1 The degree of interaction
between the metal center and the neutral E atom can exert a
profound influence on the reactivity of the resulting complexes. In
this context, a tri(2-oxyphenyl)methane-derived system appears
practically attractive ([O3]3- ) tri(2-oxy-3,5-di-tert-butylphenyl)-
methane). This ligand can coordinate to a metal in two forms, which
differ mainly as a result of the relative stereochemistry at the
methine carbon (syn and anti forms, Scheme 1). Furthermore,
intramolecular metalation of the somewhat acidic methine linkage
in the [O3] complexes is expected to occur quite readily, resulting
in formation of 5-carbametalatranes2 ([O3C] complexes). However,
metal complexes with [O3]-derived ligands are rare, and their
coordination chemistry is largely unexplored.3
Scheme 2
Following our interest in the chemistry of linearly linked
triaryloxide tridentate ligands,4 we set out to investigate the use of
[O3]3- as an auxiliary ligand. Here we report the synthesis of Ti
and Zr complexes supported by the [O3] ligand and the rearrange-
ment of syn- to anti-complexes. In addition, formation of the [O3C]
complex via C-H activation of the [syn-O3] ligand is described.
Amine elimination of Ti(NEt2)4 with H3[O3] in toluene proceeded
smoothly at room temperature to afford orange [syn-O3]Ti(NEt2)
(1a) in 76% isolated yield (Scheme 2). NMR data are useful
indicators of complex formation. The 1H and 13C NMR spectra of
1a indicate a molecule with 3-fold symmetry. An important feature
of the 1H NMR spectrum is the high-field shift of the proton
attached to the methine carbon giving rise to a singlet at δ 4.96
(free H3[O3], δ 5.94 in C6D6). In the 13C NMR spectrum, the
methine atom exhibits a signal at δ 45.88, which is comparable to
C(1) atom, as evidenced by the sum of the C-C(1)-C angles of
352.7° (342.1° for H3[O3]). The structural motif of 1a is similar to
that found in [(syn-O3)TaCl3]-.3d
Although 1a is stable in the solid-state, heating the syn-complex
in toluene resulted in the clean conversion to the anti-complex 1b
according to NMR spectroscopy. This suggests that 1a is thermo-
dynamically unstable with respect to 1b, and the syn-isomer is a
kinetically stabilized intermediate in the Ti(NEt2)4/H3[O3] reaction.
The transformation of 1a to 1b in C7D8 was determined to be first
order in titanium according to 1H NMR spectroscopy. Temperature
dependence studies from 64 to 100 °C for this transformation
allowed for extraction of the activation parameters ∆Sq ) -35(1)
cal/(mol K), ∆Hq ) 16.3(4) kcal/mol from the Arrhenius plot. The
negative entropy, indicative of ordering in the transition state, might
be due to ring strain and/or solvation effects.8
The chloride derivative [anti-O3]TiCl (2) was readily synthesized
by reactions of 1a and 1b with Me3SiCl in C6D6 according to NMR
spectroscopy. Since the [O3] ligand underwent facile rearrangement
during the 1a/Me3SiCl reaction, attempts to prepare the syn-isomer
of 2 have met with failure. Alkylation of 2 with PhCH2MgCl
proceeded with retention of the anti-conformation to afford [anti-
O3]Ti(CH2Ph) (3) in 74% yield.
1
that of H3[O3] (δ 43.04). However, the small value of the JCH
coupling constant of 91.1 Hz (126.8 Hz for H3[O3]) suggests some
of agostic interaction between the metal and this proton (vide infra).
A single-crystal X-ray diffraction analysis of 1a shows that the
[O3] ligand adopts a syn-conformation with approximate C3V
symmetry to give a distorted trigonal-bipyramid complex (Figure
1a).5 The three aryloxide functions occupy the equatorial sites (av
Ti-O ) 1.852 Å; O-Ti-O ) 108°), while the methine H(1) proton
as well as the amide N atom [Ti-N ) 1.864(2) Å] form the axial
set [N-Ti-H(1) ) 172.5(9) Å]. The Ti-H(1) distance of 1.66(2)
Å [Ti-H(1)-C(1) ) 168(2)°; C(1)-H(1) ) 1.19(2) Å] is
significantly shorter than those observed for common agostic
Ti‚‚‚H-C interactions6 and is comparable to those of titanium
hydride complexes.7 Thus, the H(1) atom strongly interacts with
1
the metal, which is evident from the H NMR spectrum. We also
The NMR spectra are consistent with 1b, 2, and 3 having three-
fold symmetry. The 1H and 13C NMR signals attributed to the
methine linkage of the ligand appear to be sensitive criteria for
distinguishing between the syn- and anti-forms. The methine proton
resonances in the anti-complexes (1b, δ 6.04; 2, δ 5.94; 3, δ 5.89)
note the short O‚‚‚H(1) distances ranging from 1.86 to 2.05 Å. This
structural feature may be ascribed to the conformationally con-
strained cage provided by the [syn-O3] ligand. The potential strain
of such an arrangement is alleviated by a flattening of the methine
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11936
J. AM. CHEM. SOC. 2005, 127, 11936-11937
10.1021/ja053740m CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. Molecular structures of (a) 1a, (b) 3, and (c) 5. Methyl groups of the tert-butyl substituents have been omitted for clarity. Complex 1a has
crystallographic Cs symmetry about a plane including the O(1) aryloxide ring. The asymmetric unit of 3 contains two crystallographically independent
molecules, and a view of one of them is given here.
are shifted to lower fields than that of the syn-complex 1a. In the
13C NMR spectra, the methine carbon exhibits a signal at δ 64.14
for 1b, δ 62.89 for 2, and δ 64.0 for 3, which represents a large
downfield shift. Noteworthy, the 1JCH coupling constants (1b, 122.3
Hz; 2, 122.6 Hz; 3, 121.1 Hz) are larger than that observed in 1a
and is similar to that of H3[O3].
In conclusion, the use of the [O3] ligand turns out to be a
convenient entry into intriguing metalatranes, which include an
agostic M‚‚‚H-C interaction and a M-C bond as a transannular
interaction. While the Ti syn-complex 1a underwent facile trans-
formation to the anti-complexes 1b and 2, the Zr complex with
the syn-structure 4 was easily converted into the 5-carbametalatrane
5 via intramolecular C-H activation. Reactivity studies with these
group 4 metal complexes are ongoing.
To establish the anti-structure, an X-ray diffraction study was
carried out on a single crystal of 3 (Figure 1b).5 The complex
exhibits approximate tetrahedral geometry about the Ti center (av
Ti-O ) 1.852 Å; O-Ti-O ) 104.9°). In contrast to the syn-
complex 1a with C3V symmetry, the [anti-O3] ligand in 3 displays
a propeller-like conformation [av O-Ti-C(1)-C torsion angles:
1.3° for 1a and 16.4 for 3], thereby relieving the strain in the eight-
membered [TiO2C5] chelate ring. The [anti-O3] ligand has a
flattened methine carbon (ΣC-C(1)-C ) 353°) similar to that
found in 1a. The benzyl ligand adopts an η1-coordination.
When Zr(CH2Ph)4 was used instead of Ti(NEt2)4, the reaction
with H3[O3] in toluene/THF gave a mixture of [syn-O3]Zr(CH2-
Ph)(THF) (4) and [O3C]Zr(THF)3 (5). In contrast to the stability
of the anti complex 3, the syn complex 4 underwent facile C-H
activation of the ligand to generate 5 concomitant with elimination
of toluene according to NMR spectra of the mixture. Intramolecular
metalation in 4 appears to be facilitated by the preorganization of
the ligand and the close proximity of the methine proton to the
metal center having the benzyl group in the syn-conformation. While
isolation in pure form was hampered by its instability, the identity
of 4 is strongly supported by NMR data. The [O3] ligand in 4 adopts
a syn-conformation, as shown by the upfield 1H NMR shift of the
Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research [Nos. 17350031 and 14078101 (Priority
Areas “Reaction Control of Dynamic Complexes”)].
Supporting Information Available: Experimental procedures and
CIF files for 1a, 3, and 5. This material is available free of charge via
the Internet at http://pubs.acs.org.
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1
methine proton (δ 5.79) and the small JCH value of 96.8 Hz for
the methine carbon (δ 40.5).
The formulation of 5 was inferred by a combination of H and
1
13C NMR spectra in addition to single-crystal X-ray diffraction data
(Figure 1c). Most impressively, intramolecular C-H activation took
place at the methine carbon of the ligand to form a 5-carbameta-
latrane structure. The Zr center is best described as capped
octahedral, with the C(1) atom capping the aryloxide O3 face [Zr-
(5) The methine protons in 1a and 3 were located from the different Fourier
map and isotropically refined.
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(8) To test the influence of solvent, the thermolysis was also performed in
1,2-dichlorobenzene-d4: the activation paramers were found to be almost
the same as those in C7D8. The rate and Arrhenius plots for this system
are available as Supporting Information.
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C(1) ) 2.309(3) Å; average Zr-OAr ) 2.043 Å, OAr-Zr-OAr
)
113.5°]. Compound 5 is a rare example of an η1-trityl complex.9
Compared to that in 1a and 3, the C(1) atom assumes a normal sp3
carbon geometry [av C-C(1)-C angle ) 111.4°]. To accommodate
adjacent tetrahedral [C(1)] and capped octahedral [Zr] geometries,
the phenyl rings in 5 adopt the propeller geometry with av O-Zr-
C(1)-C of 15.9°. The NMR spectra of 5 are consistent with the
solid structure, and the signal of the C(1) atom appears as a singlet
at δ 83.8 in the 13C NMR spectrum.
JA053740M
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J. AM. CHEM. SOC. VOL. 127, NO. 34, 2005 11937