Non-cyclopentadienyl titanium(III)-magnesium hydride formation by reduction of an amidotitanium(IV) complex

Article abstract of DOI:10.1021/om034164a

The reduction of an arylamido-based titanium(IV) chloride by magnesium has resulted in the formation of a unique, non-cyclopentadienyl titanium(III)-magnesium hydride with the concomitant intramolecular metalation of the arylamido ligands.

Full text of DOI:10.1021/om034164a

Organometallics 2003, 22, 4387-4389
4387
Non -cyclop en ta d ien yl Tita n iu m (III)-Ma gn esiu m
Hyd r id e F or m a tion by Red u ction of a n
Am id otita n iu m (IV) Com p lex
Q. Folashade Mokuolu, Paul A. Duckmanton, Alexander J . Blake,
Claire Wilson, and J ason B. Love*
School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K.
Received September 10, 2003
Sch em e 1. Syn th etic Rou te to th e Ti(III) Hyd r id es
2a a n d 2b
Summary: The reduction of an arylamido-based tita-
nium(IV) chloride by magnesium has resulted in the
formation of a unique, non-cyclopentadienyl titanium-
(III)-magnesium hydride with the concomitant in-
tramolecular metalation of the arylamido ligands.
Cyclopentadienyl titanium(III) hydrides are impli-
cated in a variety of catalytic processes, including the
hydrogenation of alkenes and ketimines, the hydrosi-
lylation of CdX species (X ) C, N, O), and perhaps most
importantly the dehydrocoupling of silanes to form
polysilanes.1,2 Furthermore, related scandium(III) hy-
drides are involved in the remarkable catalytic dehy-
drosilylation of methane.3 This rich vein of reactivity,
coupled with the vast array of new amido ligand based
titanium complexes that have been reported,4 should
mean that non-cyclopentadienyl titanium(III) hydrides
are prominent in stoichiometric or catalytic transforma-
tions. In fact, non-Cp Ti(III) hydrides are extremely
rare, with the dimeric hydride [Ti(µ-H)(L)]2 (L ) (Me3-
Si)N{CH2CH2N(SiMe3)}2) being the sole isolated and
fully characterized example.5 We have been investigat-
ing the synthesis and reactivity of Ti-based mixed-metal
complexes with a view to promoting controlled, dual-
site catalytic chemistry6 and so began to scrutinize the
reactivity patterns of the monometallic, amido-based
The X-ray crystal structure of 1b was determined
(Figure 1) and reveals a unique example of a structur-
ally characterized, monomeric Ti(IV) dihalide supported
solely by nonchelating amido ligands. The geometry at
Ti1 is tetrahedral, although Cl1-Ti1-Cl2 is compressed
(100.55(2)°) and N1-Ti1-N2 enlarged (117.74(6)°) and
standard Ti-N and Ti-Cl bond lengths are observed.
Two interactions appear to enforce the observed C2
symmetry; thus, the π-stacking of the two aryl rings (C1‚
‚‚C16 ) 3.27 Å) is reinforced by hydrogen-bonding
interactions between the methine H’s and opposing
chlorides (H‚‚‚Cl ) 3.02 and 3.65 Å). This hydrogen-
metallo ligand TiCl2[N(CHBut )(C6H4PPh2-3)]2 (1a ). We
2
report here the outcome of the reduction of 1a by
magnesium, which leads to the formation of the unusual
TiIII-Mg hydride complex 2a , a product more reminis-
cent of cyclopentadienyl-titanium than amido-tita-
nium chemistry.
The di-tert-butylmethyl-substituted anilides are syn-
thesized directly as the lithium salts Li[L1] and Li[L2]
via Schiff-base condensation of trimethylacetaldehyde
with the respective anilines followed by ButLi addition
across the resultant imine CdN double bond.7 The salt
elimination reactions between Li[L] and TiCl4(THF)2 (2:
1) in toluene generate the Ti(IV) halides 1a and 1b as
dark red crystalline solids in high yield (Scheme 1).
1
bonding interaction is conserved in solution. In the H
NMR spectrum of 1b, the methine H resonates at 5.70
ppm and represents a downfield shift of 2.80 ppm as
compared to that seen in Li[L2] (2.90 ppm).
The reaction between the Ti(IV) halides 1a and 1b
and magnesium dust in THF results in the slow forma-
tion of dark brown solutions from which 2a and 2b were
isolated as dark brown solids. 1H NMR spectroscopy
indicates that these compounds are paramagnetic and
shows only broad resonances at ca. 3.5 and 1.7 ppm that
are consistent with THF molecules undergoing dynamic
exchange with the paramagnetic center. Furthermore,
the solution susceptibility measurement of 2a by the
Evans method at 298 K results in µeff ) 2.11 µB, which
is consistent with a d1 Ti(III) species. This value is
higher than the expected spin-only moment (µcalcd ) 1.73
µB), which may be due to an orbital contribution and/or
the loss of nonstoichiometric amounts of THF from 2a
prior to weighing. To establish the identity of 2a , the
solid-state structure was determined (Figure 2). It is
* To whom correspondence should be addressed. E-mail:
jason.love@nottingham.ac.uk. Fax: +44 115 9513563.
(1) Harrod, J . F. Coord. Chem. Rev. 2000, 206-207, 493.
(2) Hoskin, A. J .; Stephan, D. W. Coord. Chem. Rev. 2002, 233-
234, 107.
(3) Sadow, A.; Tilley, T. D. Angew. Chem., Int. Ed. 2003, 42, 803.
(4) Kempe, R. Angew. Chem., Int. Ed. 2000, 39, 468.
(5) Love, J . B.; Clark, H. C. S.; Cloke, F. G. N.; Green, J . C.;
Hitchcock, P. B. J . Am. Chem. Soc. 1999, 121, 6843.
(6) Mokuolu, Q. F.; Avent, A. G.; Hitchcock, P. B.; Love, J . B. J .
Chem. Soc., Dalton Trans. 2001, 2551.
(7) Mokuolu, Q. F.; Duckmanton, P. A.; Hitchcock, P. B.; Love, J .
B. Submitted for publication.
10.1021/om034164a CCC: $25.00 © 2003 American Chemical Society
Publication on Web 10/03/2003

4388 Organometallics, Vol. 22, No. 22, 2003
Communications
and Cu.10 Structurally characterized Ti-µ2-aryls are
rare and are known only for [Cp2Ti(µ2-CH2)(µ2-C6H4X)-
Rh(COD)] (X ) H, 4-NMe2, 2-OMe) and complexes of
the type [Cp2TiCl(µ2-η1:η6-C6H5)Cr(CO)3]11-13 Further-
more, while metalation of an activated arene by transi-
tion metals is commonplace, metalation of usually
robust transition-metal arylamido ligands is rare. A
titanium alkyl complex supported by the tridentate
diamido ligand, [(PriN-o-C6H4)2O]2-, was found to un-
dergo C-O bond cleavage and aryl metalation in the
presence of DMPE, significantly via a putative Ti(II)
intermediate (see later).14 The Mg1-Cl1 bond length
(2.517(2) Å) and Mg1-Cl1-Mg1A angle (73.10(10)°) are
similar to those reported for chloride-bridged dimagne-
sium motifs.15
The most intriguing aspect of the structure of 2a is
the presence of two µ3-hydrides that symmetrically
bridge the three metals. The hydrides were located in
the difference Fourier map, and evidence for their
presence is reinforced by the distorted-octahedral ge-
ometries thus observed at Ti1 (C6-Ti1-C6A ) 169.6-
(3)°, N1-Ti-N1A ) 121.1(2)°) and Mg1 (C6-Mg1-Cl1
) 161.91(16)°, O1-Mg1-O2 ) 92.71(16)°). However,
further corroboration of the presence of hydrides was
sought. Infrared spectroscopy proved inadequate, as the
region in which Ti(µ-H)Mg absorptions would appear
(ca. 1500-900 cm-1) is masked by ligand absorptions;
unfortunately, no deuterated version of the ligand is
available at present. Preliminary solution EPR spectra
were obtained for both 2a and 2b and show similar
hyperfine patterns that superficially appear consistent
with the presence of two hydrides and two amido
nitrogens. Unfortunately, neither spectrum could be
adequately simulated. Concrete evidence for the pres-
ence of hydrides came from D2O quenching experiments,
in which D2O was vacuum-transferred to degassed
solutions of 2a or 2b, the volatiles were vacuum-
F igu r e 1. Solid-state structure of 1b (R ) H).
F igu r e 2. Solid-state structure of 2a (50% ellipsoids). For
clarity, P-aryl, N-But, and THF carbons and all hydrogens
except the metal hydrides are omitted.
1
apparent that the reduction product is not a straight-
forward Ti(III) complex but instead features cyclometa-
lated µ2-aryl rings bridging Ti and Mg, incorporation of
a MgClMg bonding motif, and, most importantly, µ3-
hydrides bridging the Ti and the two Mg metals.
No counterions were observed crystallographically;
therefore, 2a is charge neutral. This formulation was
also confirmed by combustion analysis. The coordination
of the ligand to the metal is unusual and warrants closer
inspection. The Ti1-N1 (2.051(4) Å) and Ti1-C6 bond
distances (2.249(6) Å) are longer than other known
TiIII-N (1.94-1.98 Å) and TiIII-C (2.14-2.21 Å) bond
distances8 and are indicative of an η3 interaction. This
is further corroborated by the short Ti‚‚‚C1 interaction
(2.535 Å) and the shortened N1-C1 bond distance
(1.395(6) Å) and by comparison to the structure of Ti-
[N(But)(C6H3Me2-3,5)]3; in this case, two of the anilide
ligands adopt similar η3 bonding modes with analogous
bond length distortions.9 Unlike the above Ti(anilide)3,
the aryl ring has been substituted by Mg in the ortho
position, thus resulting in a Ti(µ2-aryl)Mg bonding motif.
The Mg1-C6 bond distance (2.314(5) Å) is similar to
that found in the cluster complex Cu4MgPh6 (Mg-C )
2.35(1) Å), which contains a µ2-aryl group between Mg
transferred into an NMR tube, and the H NMR spectra
were recorded. The volatiles sampled, albeit ineffi-
ciently, by this method clearly show the presence of HD
(4.54 ppm, J HD ) 42.6 Hz), which can only arise from
the reaction of a metal hydride with D2O. At present,
the structure of 2b remains ambiguous, as no crystals
suitable for X-ray diffraction are available and combus-
tion analyses have been repeatedly unsatisfactory.
However, the similarity of the EPR spectra of 2a and
2b, along with the generation of HD by quenching with
D2O, strongly suggests that 2a and 2b have similar
structures.
The formation of the Ti(III) hydrides 2a and 2b by
magnesium reduction is replicated in cyclopentadienyl-
based titanium chemistry; thus, the reaction between
titanocene dihalides and magnesium results in the
formation of a variety of Cp2TiIII-Mg hydrides.16,17
However, none of these compounds exhibit the same
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Communications
Organometallics, Vol. 22, No. 22, 2003 4389
triply bridging hydride and TiMg2Cl motifs as 2a . These
systems also show a propensity for intramolecular C-H
activation reactions,18 which implies that Ti(II) species
are mechanistic intermediates. By using bulky, silylated
cyclopentadienyls, C5Me4SiMe2R (Cp′), the isolation of
the parent titanocene Cp′2Ti was recently realized;19
reduction of Cp′2TiCl2 by equimolar amounts of mag-
nesium also resulted in titanocene synthesis.20 It is
therefore possible that 2a is formed via a highly reactive
Ti(II) intermediate. To assess this supposition, cyclic
voltametry (CV) was carried out in CH2Cl2 and reveals
a single reversible wave at E° ) -0.81 V (0.1 V s-1, nBu4-
NBF4, SCE) with no further reductions to - 2.0 V. By
comparison to the CV of Cp2TiCl2, this is consistent with
the formation of the monohalo species {TiCl(L1)}n via
one-electron uptake and halide loss.21-23 Additionally,
controlled-potential electrolysis at -0.86 V revealed that
a multielectron process (number of electrons 2.44)
leading to decomposition is operating. This may be a
reaction of the electrochemically generated species with
solvent or a mechanistic step in the formation of 2a .
Whatever the mechanism of formation of 2a , it is
apparent that reduction processes comparable to those
of titanocene dihalides are in effect, thus reinforcing the
similarity of this particular amido-based titanium com-
plex to cyclopentadienyl analogues. We are at present
investigating further the mechanism of formation of 2a ,
in particular the source of hydride, and are exploring
the reaction chemistry of these systems. Preliminary
results show that 2a is inert toward CO and also that,
unlike Cp2TiCl2 reduction chemistry, reaction of 1a with
magnesium under an atmosphere of CO does not result
in the formation of a Ti(II) carbonyl but instead forms
2a as the sole product.
Ack n ow led gm en t. We thank the Royal Society
(J .B.L., University Research Fellowship), the University
of Nottingham, and the EPSRC (Q.F.M., P.A.D.) for
support.
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Su p p or tin g In for m a tion Ava ila ble: Text giving full
synthetic details and characterization data for all new com-
pounds and tables giving X-ray crystallographic data for
compounds 1b and 2a ; crystallographic data are also available
as CIF files. This material is available free of charge via the
Internet at http://pubs.acs.org.
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