A terminal and four-coordinate titanium alkylidene prepared by oxidatively induced α-hydrogen abstraction

Article abstract of DOI:10.1021/ja034786n

One-electron oxidation of the β-diketiminate titanium(III) bis-neopentyl complex (Nacnac)Ti(CH₂^tBu)₂ (Nacnac = [Ar]NC(Me)CHC(Me)N[Ar], Ar = 2,6-(CHMe₂)₂C₆H₃) promotes α-abstraction to afford the rare and terminal four-coordinate neopentylidene (Nacnac)Ti=CH^tBu(OTf), which was structurally characterized. Alkylidene (Nacnac)TiCH^tBu(OTf) reacts cleanly with benzophenone and the imine functionality of the Nacnac ligand to afford the corresponding Wittig-type products. Copyright

Full text of DOI:10.1021/ja034786n

Published on Web 04/24/2003
A Terminal and Four-Coordinate Titanium Alkylidene Prepared by Oxidatively
Induced r-Hydrogen Abstraction
Falguni Basuli, Brad C. Bailey, John Tomaszewski, John C. Huffman, and Daniel J. Mindiola*
Department of Chemistry and Molecular Structure Center, Indiana UniVersity, Bloomington, Indiana 47405
Received February 20, 2003; E-mail: mindiola@indiana.edu
Early-transition-metal alkylidenes play an important role in
synthetic transformations such as cross-metathesis, ring-closing
metathesis, ring-opening metathesis, ring-opening metathesis po-
lymerization, acyclic diene metathesis polymerization, acetylene
polymerization, and Wittig-type reactions. A series of reviews1 and
highlights2 in the literature exemplify the importance and need for
metal alkylidene complexes. High oxidation state metal alkylidenes
were reported over 25 years ago and are prepared commonly via
R-abstraction or R-deprotonation reactions from the corresponding
metal alkyl complex.1 To favor R-deprotonation in such systems,
one needs to prepare metal alkyl functionalities lacking â-hydro-
gens. In general, R-hydrogen abstraction to yield high-oxidation
state metal alkylidenes is induced thermally, photochemically, or
with Lewis bases to promote steric crowding.1a-d,3,4 In contrast to
groups 5 and 6, group 4 terminal metal alkylidene complexes, in
particular, Zr and Hf, are exceedingly rare,5 and the majority of
the few examples are titanium based with coordination numbers
g5.1c,3,4,6
Figure 1. Molecular structures of 3 and 5 showing the atom-labeling
scheme with thermal ellipsoids at the 50% probability level. H-atoms with
the exception of the R-hydrogens and aryl groups with the exception of the
ipso-carbons on the â-diketiminate or imido nitrogens have been omitted
for clarity.
Herein, we report a synthetic methodology to access the first
four-coordinate titanium complex containing a terminal metal-
carbon double bond. Preliminary Wittig-type reactions of the
titanium neopentylidene complex with ketone and imine function-
alities are also described.
the neopentyl ligands. In addition, R-abstraction often takes place
in five-coordinate d0 species.10 A cyclic voltammogram of a solution
of 2 (THF/TBAH) showed one irreversible oxidation wave at -0.90
V (referenced vs FeCp2/FeCp2+) for the Ti(III)/Ti(IV) couple.8
Chemically, it was found that treatment of pentane solutions of 2
with AgOTf caused a rapid color change from green to red-brown
concomitant with precipitation of a Ag0 mirror and quantitative
formation of the alkylidene complex (Nacnac)TidCHtBu(OTf) (3)
as evidenced by 1H and 13C NMR spectroscopic methods (Scheme
1). Complex 3 is likely formed from the putative five-coordinate
Our opening approach to preparing a low-coordinate and terminal
titanium alkylidene complex involved an adaptation for the synthesis
of the precursor (Nacnac)TiCl2 (Nacnac- ) [Ar]NC(Me)CHC(Me)-
N[Ar], Ar ) 2,6-(CHMe2)2C6H3) complex reported by Budzelaar
and co-workers.7 Following Budzelaar’s procedure, we recrystal-
lized the THF base adduct (Nacnac)TiCl2(THF) (1) from toluene
in 70% yield as dark-green blocks (Scheme 1). The isolation
of 1 avoids lower yields as well as additional steps to the THF-
t
intermediate (Nacnac)Ti(CH2 Bu)2(OTf). On the NMR time scale,
compound 3 displays two methine resonances, four diastereotopic
methyl groups on the isopropyls, as well as one methyl environment
for the â-carbons of the Nacnac backbone, all consistent with the
molecule having Cs symmetry. A CR resonance centered at δ 271
ppm with a JCH coupling constant of 95 Hz is diagnostic of 3 having
a terminal alkylidene functionality.3,4 The JCH coupling from the
13C NMR spectral data suggests an R-hydrogen agostic interaction
with the metal center.1c The 1H NMR CHR resonance was located
at 5.23 ppm and differentiated unambiguously from the CHγ
resonance for the Nacnac backbone (4.79 ppm) using HMQC NMR
methods.8,11 Large red-brown blocks of 3 were grown from pentane
at -35 °C (89% yield), and the single-crystal structure revealed a
four-coordinate titanium complex having Cs symmetry and having
the shortest TidC bond length reported12 (Ti(1)-C(33) ) 1.830(3)
Å, Figure 1).8 The tert-butyl group is along the σ-plane bisecting
N-Ti-N and oriented syn with respect to the triflato ligand. The
CRHR (H87) hydrogen was located in the Fourier electron map and
refined isotropically (Ti(1)-H(87) ) 1.92(3) Å). An R-agostic
interaction is also substantiated by the large Ti(1)-C(33)-C(34)
angle of 163.9(3)°.
1
free complex (Nacnac)TiCl2. Complex 1 was characterized by H
NMR spectroscopic methods, elemental analysis, and by single-
crystal X-ray diffraction.8 Ethereal solutions of 1 react rapidly with
t
2 equiv of LiCH2 Bu9 to afford emerald-green solutions of (Nacnac)-
t
Ti(CH2 Bu)2 (2), which was isolated as dark-green blocks in over
75% yield (Scheme 1). Complex 2 was fully characterized,8 and
the molecular structure shows no R-agostic interactions or remark-
able features similar to those reported for the bis-methyl complex
studied by Budzelaar.7
Scheme 1. Synthesis of Titanium Alkylidene 3
Because complex 2 does not bind Lewis bases such as THF or
PMe3, we reasoned that one-electron oxidation might enhance Lewis
acidity and promote deprotonation of the R-hydrogen of one of
Complex 3 reacts rapidly with 1 equiv of benzophenone at room
13
1
temperature to afford the olefin HtBuCdCPh2 and /2 equiv of
9
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J. AM. CHEM. SOC. 2003, 125, 6052-6053
10.1021/ja034786n CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
the titanium-oxo dimer [(Nacnac)Ti(µ2-O)(µ2-OTf)]2 (4)14 in quan-
titative yield (Scheme 2).8 Bridged titanium-oxo complex 4 and
the corresponding olefin were isolated in 90% and 91% yield,
respectively.6 The reactivity observed between 3 and benzophenone
follows well-established “Wittig-type” reagents studied in organic
synthesis.15
Acknowledgment. This work is dedicated to the memory of
Professor Richard Koerner. For financial support of this work, we
thank Indiana University-Bloomington, the Camille and Henry
Dreyfus Foundation, and the Ford Foundation. D.J.M. would like
to thank Mr. X. Hu, Dr. R. Isaacson, and Professors K. Meyer, K.
G. Caulton, J. M. Zaleski, and C. C. Cummins for insightful
discussions.
Scheme 2. Reactivity of Titanium Alkylidene 3
Supporting Information Available: Experimental preparation and
crystallographic data for compounds 1-6 (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
References
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(8) See the Supporting Information for complete experimental, spectral, and
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(12) A search of the Cambridge Crystallographic Database for titanium-
alkylidene bond lengths indicated values to be g1.884(4) Å.
(13) The formation of the alkene was confirmed by spectroscopic comparison
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J.-C. J. Am. Chem. Soc. 1972, 94, 2000.
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Although stable as a solid, complex 3 decomposes gradually in
solution (room temperature, >2 days) to a new product as evidenced
by 1H and 13C NMR spectroscopy. In fact, heating toluene or
benzene solutions of 3 to 60 °C for 2 h affords the titanium imido-
triflato complex supported by the chelating amido-diene ligand (η2-
HtBuCdC(Me)CHC(Me)N[Ar])TidNAr(µ2-OTf) (5) (65% isolated
yield, Scheme 2).8 Complex 5 is likely formed by a Wittig-type
reaction between the titanium neopentylidene and the imine-aryl
functionality of the Nacnac ligand. The molecular structure of 5 is
shown in Figure 1 and displays one of the diastereomers,8 and the
1H NMR spectrum of 5 also indicates one diastereomer being
present in solution at 25 °C. The chelate ligand in 5 exhibits a
resonance indicative of an amido-diene because addition of a Lewis
base such as Et2O affords crystals in 80% yield of the adduct (η1-
HtBuCdC(Me)CHC(Me)N[Ar])TidNAr(OTf)(Et2O) (6), in which
the olefinic pendant arm of the amide-diene ligand has been
displaced by the Lewis base (Scheme 2). Mild heating of 6 under
reduced pressure (50 °C, 2 h) regenerates 5 (Scheme 2). The
molecular structure of the etherate adduct 68 reveals a rare example
of a four-coordinate titanium imido.16
In summary, we have shown that one-electron oxidation of a
Ti(III) bis-neopentyl complex supported by a Nacnac ligand affords
a terminal, and four-coordinate, titanium neopentylidene and
titanium imido. In contrast to thermolytic reactions, an oxidatively
induced R-abstraction procedure17 can create a low-coordinate
titanium alkylidene complex containing a labile group (e.g., triflate).
We are currently exploring the mechanism behind the carbene-
imine Wittig reaction to make 5, as well as the chemical reactivity
of 3 with olefins.
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