Snapshots of an oxidatively induced α-hydrogen abstraction reaction to prepare a terminal and four-coordinate titanium imide

Article abstract of DOI:10.1039/b304633h

One electron oxidation of the bis-anilido titanium(III) complex (Nacnac)Ti(NHAr)₂ (Nacnac^- = ArNC(CH₃)CHC(CH₃)NAr, Ar = 2,6-(CHMe₂)₂CaH₃) with AgOTf affords the cation [(Nacnac)T

Full text of DOI:10.1039/b304633h

Snapshots of an oxidatively induced a-hydrogen abstraction reaction to
prepare a terminal and four-coordinate titanium imide†
Falguni Basuli, Brad C. Bailey, John C. Huffman and Daniel J. Mindiola*
Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, Indiana 47405,
USA. E-mail: mindiola@indiana.edu; Fax: 001 812 855 8300; Tel: 001 812 855 2399
Received (in Purdue, IN, USA) 26th April 2003, Accepted 13th May 2003
First published as an Advance Article on the web 2nd June 2003
One electron oxidation of the bis-anilido titanium(III
)
A cyclic voltammogram of a solution of 1 (THF–TBAH)
indicated a reversible oxidation wave at 20.89 V (referenced
complex (Nacnac)Ti(NHAr)2
(Nacnac2
=
+
ArNC(CH3)CHC(CH3)NAr, Ar = 2,6-(CHMe2)2C6H3) with
AgOTf affords the cation [(Nacnac)Ti(NHAr)2][OTf] which
is isolated and shown to gradually transform, by a-hydrogen
abstraction, to the terminal and four-coordinate titanium
imide (Nacnac)TiNNAr(OTf).
vs. FeCp2/FeCp2 ) for the Ti(III)/Ti(IV) couple.† Chemically, it
was determined that treatment of a thawing THF solution of 1
with AgOTf caused a color change from green to orange–red
concomitant with formation of a Ag0 mirror. Immediate workup
after 5 minutes led to isolation of the bis-anilido triflato salt
[(Nacnac)Ti(NHAr)2][OTf] (2) as dark-red crystals in 71%
yield (Scheme 2).† Complex 2 decomposes gradually in
solution, and the yield of this complex is highly dependent on
both time and temperature. Complex 2 was characterized by 1H,
19F, 13C and IR spectroscopies.† Diagnostic features for 2
include two NH resonances centered at 11.1 and 10.4 ppm and
one methyl environment for the b-carbon of the Nacnac
backbone consistent with the molecule retaining Cs symmetry
in solution. Complex 2 is insoluble and stable in most common
organic solvents such as hexane, C6H6 and Et2O, but soluble in
THF and CH2Cl2 upon which it converts to a new complex and
free aniline (vide infra). The insolubility of 2 in organic solvents
suggests this complex to be a discrete salt where the OTf anion
interacts weakly with the metal center.
Realizing the importance of the triflato anion in the stability
of 2 we prepared a more stable salt of 2 using the weakly
coordinating B(C6F5)4 anion. Accordingly, oxidation of 1 with
[FeCp*2][B(C6F5)4]9 in Et2O at 235 °C affords in 92% isolated
yield the bis-anilido salt [(Nacnac)Ti(NHAr)2][B(C6F5)4] (3) as
brick-red microcrystals (Scheme 2).† Complex 3 shows nearly
identical spectroscopic features to 2 in solution (d for NH
protons are 11.4 and 10.9 in the 1H NMR spectrum of 3).† The
molecular structure of 3 was determined by single crystal X-ray
diffraction studies and exhibits a four-coordinate titanium
cation confined in a tetrahedral environment (Fig. 1).‡ Im-
portant crystallographic features for 3 include short Ti–Nanilido
bonds lengths of Ti(1)–N(33), 1.863(2) Å and Ti(1)–N(46),
1.875(2) Å, which is consistent with metal–ligand multiple
bond character.1 Most notably, the Ti–Nanilido bond length is
considerably shorter when compared to the neutral and d1
complex 1 (vide supra). Other important crystallographic
features for 3 include significantly different Ti(1)–N(33)–C(34)
and Ti(1)–N(46)–C(47) angles of 132.8(9)° and 157.5(9)°,
respectively. The a-hydrogens on the anilido nitrogens were
located in the E-map and are indicative of agostic interactions
with the Ti(IV) center.†
Low-coordinate transition metal imido complexes have experi-
enced a remarkable growth in the last 30 years and constitute a
landmark in inorganic chemistry.1 Wolczanski2 and Bergman3
have prepared perhaps the most reactive imido complexes of the
early-transition metals. This inherent reactivity can lead to the
activation of relatively inert bonds such as primary and
secondary2,3 C–H bonds in alkanes and can be tailored to the
unsaturated or electrophilic nature of the metal center. Parallel
to the reactivity of early-transition metal imido systems,
electron-rich late-transition metal imido complexes can also
engage in C–H activation processes,4 hence research in this area
continues to draw great attention. Our recent report of the
synthesis of a four-coordinate titanium alkylidene complex5 and
tertiary C–H bond activation reactions stemming from this
complex6 stimulated the pursuit of an analogous titanium imide
complex. Inspired by the work of Wolczanski we attempted to
prepare low-coordinate titanium imido complexes by an
oxidatively induced a-abstraction reaction. Herein we report the
conversion of a Ti(III) bis-anilido complex (Nacnac)Ti(NHAr)2,
(Nacnac2
=
ArNC(CH3)CHC(CH3)NAr,
Ar
=
2,6-(CHMe2)2C6H3),7 to the corresponding cation [(Nacnac)-
Ti(NHAr)2]+ and subsequent a-hydrogen abstraction to form a
stable and terminal four-coordinate titanium imide complex.
Our approach to preparing a low-coordinate and terminal
titanium imido complex involved a similar strategy for the
synthesis of the analogous alkylidene complex (Nacnac)-
TiNCHtBu(OTf) (Scheme 1).5 Using Budzelaar’s dichloride
precursor (Nacnac)TiCl2(THF)5,8 and 2 equiv of LiNHAr we
prepared the bis-anilido titanium(III) complex (Nacnac)Ti(N-
HAr)2 (1) in 92% yield as dark green blocks (Scheme 2).† In the
absence of moisture and oxygen complex 1 is indefinitely stable
in the solid state or in solution. X-band solution EPR spectra and
magnetic measurements of 1 are consistent with a d1 para-
magnetic species.† The molecular structure of 1† displays a
four-coordinate Ti(III) complex in a tetrahedral environment.
Salient features for the structure of 1 include titanium anilido
distances of Ti(1)–N(34), 1.961(2) Å and Ti(1)–N(47), 1.966(2)
Å.
Scheme 1 Synthesis of a four-coordinate titanium alkylidene via an
oxidatively induced a-hydrogen abstraction reaction.
† Electronic supplementary information (ESI) available: complete experi-
mental, spectroscopic, analytical, and crystallographic details for com-
plexes 1–4. See http://www.rsc.org/suppdata/cc/b3/b304633h/
Scheme 2 Synthesis of complexes 1–4 starting from Budzelaar’s precursor
(Nacnac)TiCl2(THF).
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CHEM. COMMUN., 2003, 1554–1555
This journal is © The Royal Society of Chemistry 2003

hydrogen abstraction and formation of a low-coordinate
titanium-imido complex.
For financial support of this research 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 and
K. G. Caulton for insightful discussions.
Notes and references
‡ Crystal data for 3·Et2O, C81H87BF20N4OTi: Monoclinic, P21/c, a =
13.8639(6), b = 36.266(6), c = 13.9675(6) Å, b = 93.2700(10)°, Z = 4,
m(Mo-Ka) = 0.210 mm21, V = 7591.3(6) Å3, Dc = 1.375 mg mm23, GoF
on F2 = 0.962, R(F) = 4.43% and R(wF) = 11.16%. Out of a total of
111677 reflections collected 17467 were independent and 9392 were
observed (Rint = 10.95 %) with I > 2sI (orange prism, 0.25 3 0.25 3 0.07
mm, 27.55° 4 Q 4 2.07°).
Fig. 1 Molecular structure of 3 (cation only) and 4 showing atom-labeling
scheme with thermal ellipsoids at the 50% probability level. H-atoms and
aryl groups with the exception of ipso-carbons on the nitrogen atoms the
have been omitted for clarity.§
§ CCDC 209461–209463. See http://www.rsc.org/suppdata/cc/b3/
Intrigued by the instability of 2 we reasoned that decomposi-
tion of this complex lead to a-hydrogen abstraction concomitant
with formation of a strong TiNN bond and free aniline. In fact,
if the reaction of 1 with AgOTf is allowed to proceed for 1 hour
or longer, subsequent work-up of the mixture affords the
titanium imido complex (Nacnac)TiNNAr(OTf) (4) as red
prisms in 67% yield (Scheme 2).† Hence, complex 2 is an
intermediate to 4 inasmuch as stirring solutions of isolated 2 at
room temperature in CH2Cl2 afford 4 and free aniline in
quantitative yield.† The role of the anion is important in the
stability of the cation since treatment of 3 with Tl(OTf)10 in
CH2Cl2 also promotes a-hydrogen abstraction to give 4 as
b304633h/ for crystallographic data in .cif or other electronic format.
¶ Crystal data for 4·pentane, C47H70F3N3O3STi: Triclinic, P1, a
¯
=
10.271(4), b = 12.965(7), c = 18.451(3) Å, a = 93.783(7)°, b =
106.097(9)°, g = 96.880°, Z = 2, m(Mo-Ka) = 0.282 mm21, V =
2330.9(6) Å3, Dc = 1.228 mg mm23, GoF on F2 = 0.933, R(F) = 4.00%
and R(wF) = 9.78%. Out of a total of 51577 reflections collected 10763
were independent and 7221 were observed (Rint = 8.34 %) with I > 2sI
(orange prism, 0.30 3 0.25 3 0.25 mm, 27.60° 4 Q 4 2.07°). The crystal
data shows a pseudo-inversion center and was merohedrally twinned
(domain ratio 57 : 43).
1 D. E. Wigley, Prog. Inorg. Chem., 1994, 113, 2985; W. A. Nugent and
J. M. Mayer, Metal-Ligand Multiple Bonds, John Wiley & Sons, New
York, 1988.
1
evidenced by the H NMR spectrum of the reaction mixture
2 C. C. Cummins, S. M. Baxter and P. T. Wolczanski, J. Am. Chem. Soc.,
1988, 110, 8731; C. C. Cummins, C. P. Schaller, G. D. Van Duyne, P.
T. Wolczanski, A. W. Chan and R. Hoffmann, J. Am. Chem. Soc., 1991,
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Am. Chem. Soc., 1998, 120, 4881.
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4 S. Thyagarajan, D. T. Shay, C. D. Incarvito, A. L. Rheingold and K. H.
Theopold, J. Am. Chem. Soc., 2003, 125, 4440.
5 F. Basuli, B. C. Bailey, J. Tomaszewski, J. C. Huffman and D. J.
Mindiola, J. Am. Chem. Soc., 2003, 125, 6052.
6 F. Basuli, B. C. Bailey, L. A. Watson, J. C. Huffman and D. J. Mindiola,
manuscript in preparation.
7 M. Stender, R. J. Wright, B. E. Eichler, J. Prust, M. M. Olmstead, H. W.
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For synthesis see ESI.
(Scheme 2).† The choice of solvent also plays an important role
in the a-abstraction process since THF appears to accelerate the
formation of 4, relative to CH2Cl2. Single crystals of 4 were
grown from pentane at 235 °C and the molecular structure is
depicted in Fig. 1.¶ The structure of complex 4 shows a rare
example of a four-coordinate titanium imido2,5 complex with a
short Ti(1)–N(33) bond length of 1.705(5) Å and a nearly linear
1
Ti–Nimido–Cipso linkage (170.5(1)°). H and 13C NMR spectra
are in accordance with 4 retaining Cs symmetry in solution,
which is also consistent with the molecular structure. Complex
4 is isostructural to the reported alkylidene derivative (Nac-
nac)TiNCHtBu(OTf).6
Our results suggest that both an electron deficient metal
center in addition to coordination of a fifth ligand (2OTf)
promotes a-hydrogen abstraction. Coordination of 2OTf likely
induces a-hydrogen abstraction by steric crowding of the
anilido ligands. Alternatively, Lewis bases such as THF appear
to enhance hydrogen abstraction, likely participating as proton
carriers. Schrock and co-workers have observed closely related
a-hydrogen abstraction reactions stemming from 5-coordinate
d0 molybdenum species.11 In contrast to alkyl groups on
titanium,5 the anilido lone pair of electrons greatly increases the
lifetime of the intermediate. This allows us to acquire a snapshot
of long-lived intermediates associated with the oxidatively
induced a-hydrogen abstraction process. The present work
defines the steps by which one electron oxidation can lead to a-
10 M. E. Woodhouse, F. D. Lewis and T. J. Marks, J. Am. Chem. Soc.,
1982, 104, 5586.
11 R. R. Schrock, Chem. Rev., 2002, 102, 145; R. Baumann, R. Stumpf, W.
M. Davis, L.-C. Liang and R. R. Schrock, J. Am. Chem. Soc., 1999, 121,
7823; R. R. Schrock, J. S. Murdzek, G. C. Bazan, J. Robbins, M. DiMare
and M. O’Reagan, J. Am. Chem. Soc., 1990, 112, 3875.
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