Reactions and Applications of
Titanium Imido Complexes
NILAY HAZARI AND PHILIP MOUNTFORD*
Chemistry Research Laboratory, University of Oxford,
Mansfield Road, Oxford OX1 3TA, United Kingdom
Received June 28, 2005
ABSTRACT
This Account highlights aspects of the reactions and applications
of titanium imido complexes. Over the past decade in particular,
the TidNR linkage has been shown to couple stoichiometrically
with a variety of unsaturated substrates including CO2, carbodi-
imides, isocyanates, isocyanides, acetonitrile, phosphaalkynes,
alkynes, alkenes, and allenes. Especially recently, there has been
much interest in using titanium imides as catalysts for hydroami-
nation and olefin polymerization. The advances in these areas are
also reviewed.
Introduction
For over 3 decades, transition-metal complexes containing
multiply bonded ligands have generated considerable
interest and considerable advances have been made in
understanding and developing their structures and reactiv-
ity.1-4 Such complexes may also play a vital role in certain
industrial and biological systems. In particular, their
involvement in catalysis and as reagents for synthesis has
highlighted their utility.
As a terminal ligand, the formally dianionic imido
(NR)2- group coordinates through a metal-nitrogen mul-
tiple bond.5 One of the main points of interest is the
reactivity of the unsaturated MdNR linkage itself. How-
ever, imido groups can also act as an ancillary or sup-
porting ligands, as is the case for certain ring opening
metathesis6 or Ziegler Natta7 olefin polymerization cata-
lysts. Imido compounds have also been employed in the
metal organic chemical vapor deposition (MOCVD) of
metal nitrides.8,9 Furthermore, they have been implicated
in the industrial ammoxidation of propylene10 and as
intermediates in the enzymatic fixation of nitrogen.11
Although many examples of MdNR group reactivity are
known,1,2 the most reactive metal-imido linkages occur
in Group 4.2,12-14 A previous review covered aspects of
titanium imido chemistry up to 1997.12 This Account
summarizes general routes to titanium imido compounds
and then focuses on recent aspects of their reactivity and
applications. There is an emphasis on our own work
because the space limitation of Accounts of Chemical
Research prevents a comprehensive review. Similarly, we
do not provide a detailed comparison with analogous
zirconium systems.14
Preparation of Titanium Imido Complexes
The first structurally authenticated titanium imido com-
plexes were described in 1990. Roesky et al.15 showed that
reaction of TiCl4 with (Me3Si)2NP(S)Ph2 and pyridine
formed 1. Rothwell found that the Ł2-azobenzene ligand
in Ti(Ł2-PhNNPh)(OAr)2(py′)2 (Ar ) 2,6-iPr2C6H3) was
cleaved upon thermolysis, forming the phenylimido com-
plex 2 (Figure 1).16
Besides reactions involving N-H or Si-N bond cleav-
age,2,8,12,17 titanium imido complexes can be synthesized
using several other methods. Several groups have reported
the preparation of imido complexes through the oxidation
of TiII complexes.18-21 For example, TiCl2(TMEDA)2 reacts
with Ph2N2, forming 3 (eq 1).18 Recently, Mindiola et al.22
demonstrated that a TiIV imido complex could be prepared
from a TiIII precursor through an oxidatively induced
R-hydrogen abstraction reaction. TiIV imido complexes
have also been prepared from TiCl3 and TiCl3(THF)3.23,24
Another route into titanium imido complexes was through
the reactions of Ti(L)Cl2 (L ) tetratolylporphyrinato25 or
dimethylcalix[4]arene dianion26) with 2 equiv of MNHR
(M ) Li or K, R ) alkyl or aryl). Transient imido complexes
such as 4 (eq 2) can be generated from the tris(amido)
species Ti(tBu3SiNH)3Cl.27
A useful advance toward a general route to titanium
imido compounds was the report that the tert-butylimido
complexes Ti(NtBu)Cl2(L)n [L ) py, n ) 2 (5) or 3 (6); L )
NC5H4tBu, n ) 2 (7)] could be readily obtained in multi-
gram quantities from TiCl4, tBuNH2, and the appropriate
pyridine (Scheme 1).28,29 Arylimido analogues could not
be obtained directly from ArNH2 and TiCl4, but arylamine/
tert-butylimide exchange reactions of 6 readily afforded
arylimido complexes of the type Ti(NAr)Cl2(py)3 [Ar ) 2,6-
Me2C6H3 (8), 2,6-iPr2C6H3 (9), Ph (10), Tol (11), or
* To whom correspondence should be addressed. Telephone: +44-
1865-285140. Fax: +44-1865-285141. E-mail: philip.mountford@
chem.ox.ac.uk.
Nilay Hazari earned a B.Sc. (honors) degree and an M.Sc. degree at the University
of Sydney (1999-2003). He is currently undertaking a D.Phil. degree at the
University of Oxford under the supervision of Dr. P. Mountford and Professor J.
C. Green.
Philip Mountford gained a D.Phil. degree at the University of Oxford under the
supervision of Professor M. L. H. Green. After a further period in Oxford as a
Junior Research Fellow, he was appointed to a lectureship at the University of
Nottingham. In 1998, he returned to Oxford, where he is a Reader in Inorganic
Chemistry. He is a past recipient of the Royal Society of Chemistry’s Sir Edward
Frankland Fellowship and was recently a visiting professor at the Universite´ Louis
Pasteur, Strasbourg, and the Universite´ Bordeaux I, France, and also at the
University of Heidelberg, Germany.
Acc. Chem. Res. 2005, 38, 839-849
10.1021/ar030244z CCC: $30.25  2005 American Chemical Society VOL. 38, NO. 11, 2005 / ACCOUNTS OF CHEMICAL RESEARCH 839
Published on Web 10/12/2005

4-NO2C6H4 (12)].29 Such exchange reactions are now a
widespread method for preparing titanium arylimido
complexes.30 Complexes of the type Ti(NR)Cl2(py)3 (R )
alkyl or aryl) have proven to be extremely convenient entry
points into titanium imido chemistry, with a wide variety
of ancillary ligands being introduced via metathesis reac-
tions (Figure 2 shows a selection).12,31,32
Another general synthetic advance was the reaction of
Ti(NMe2)2Cl2 with RNH2, which provides an efficient and
high-yielding route to the compounds Ti(NR)(NHMe2)2-
Cl2 (13) (R ) alkyl or aryl, eq 3).33 The major advantage of
this route is that it allows complexes with a wider variety
of imido N substituents to be synthesized. Terminal
diphenylhydrazido(2-) compounds can be prepared ana-
logously.34 Certain complexes Ti(NR)(NHMe2)2Cl2 (R )
alkyl, aryl, or NPh2) have been used to prepare imido
compounds with calix[4]arene, triazacyclononane, and
tris(pyrazolyl)methane coligands.26,34-36
A wide range of titanium complexes incorporating a
terminal imido ligand can be readily synthesized. Recent
efforts have mostly focused on their reactivity. An overview
of the stoichiometric and catalytic reactions and applica-
tions of titanium imido compounds follows.
Stoichiometric Reactions
C-H, H-H, and S-H Bond Activation. Several reports
of C-H activation by titanium imido species have ap-
peared. Wolczanski et al. showed that the transient species
Ti(NSitBu3)(NHSitBu3)2 (4) was capable of activating C-H
bonds in benzene.27 The related disiloxane transient
species Ti(NSitBu3)(OSitBu3)2 (14) was more reactive than
4,37 and these reactions have been studied computation-
ally.38 Interestingly, although 14 activated a range of alkyl
(including CH4) and aryl C-H bonds, it reacted with
alkynes to form metallacycles (Scheme 2).
The reaction between Cp2
/Ti(NPh) and HCtCR (R )
Ph or SiMe3) leads to alkynyl C-H bond activation giv-
ing the anilido-acetylide complexes Cp2
/Ti(NHPh)(Ct
CR) (15).19 The reaction proceeds without observable
metallacylic intermediates. In contrast, the reaction of
Cp2
/Ti(NPh) with acetylene itself gives the structur-
ally characterized azametallacycle Cp2
/Ti{N(Ph)CHdCH}
FIGURE 1. First crystallographically characterized six- (1)15 and five-
coordinate (2)16 titanium imido complexes.
Scheme 1
FIGURE 2. Examples of titanium imido species synthesized from Ti(NtBu)Cl2(py)3 (6).
Reactions and Applications of Titanium Imido Complexes Hazari and Mountford
840 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 38, NO. 11, 2005