Reactions of TiCl4 with phosphines and alkylating reagents: An organometallic route to a titanium(II) cluster compound1

Article abstract of DOI:10.1016/S0022-328X(98)00655-X

Treatment of TiCl₄ with a stoichiometric amount of 1,2-bis(dimethylphosphino)ethane (dmpe) gives TiCl₄(dmpe) (1). This compound can be readily alkylated by 1 or 2 equivalents of PhCH₂MgCl affording TiCl₃(PhCH₂)(dmpe) (2) and TiCl₂(PhCH₂)₂(dmpe) (3), respectively. Syntheses and crystal structures of the three mononuclear titanium(IV) compounds 1-3 are presented. Decomposition of 'TiCl₃(Bu^t)(dmpe)' produces the dinuclear titanium(III) edge-sharing complex, Ti₂Cl₄(μ-Cl)₂(dmpe)₂ (4a). Its analogue, Ti₂Cl₄(μ-Cl)₂(depe)₂ (4b) has been obtained by direct reaction of TiCl₃(THF)₃ with the corresponding phosphine. Thermal decomposition of 'TiCl₂(Bu^t)₂(dmpe)' provides the first true, molecular metal atom cluster of titanium(II), Ti₃Cl₆(dmpe)₃ (5). The cluster molecule consists of an equilateral triangle of Ti^II atoms with three coplanar μ-Cl atoms bridging the edges. Each titanium atom is further coordinated by a chelating dmpe molecule and a chloride ion, with Ti-Ti distances of 2.872(3) A. The compound appears to be slightly paramagnetic but this is probably the result of a small amount of decomposition due to the difficulty of handling this pyrophoric complex. It is proposed that there are genuine Ti-Ti single bonds, and that this is a true metal atom cluster compound.

Full text of DOI:10.1016/S0022-328X(98)00655-X

Journal of Organometallic Chemistry 573 (1999) 78–86
Reactions of TiCl4 with phosphines and alkylating reagents: an
organometallic route to a titanium(II) cluster compound1
F. Albert Cotton a,*, Carlos A. Murillo a,b, Marina A. Petrukhina a
a Laboratory for Molecular Structure and Bonding, Department of Chemistry, Texas A&M Uni6ersity, and P.O. Box 300012, College Station,
TX 77842-3012, USA
b Department of Chemistry, Uni6ersidad de Costa Rica, Ciudad Uni6ersitaria, City??, Costa Rica
Received 12 March 1998
Abstract
Treatment of TiCl4 with a stoichiometric amount of 1,2-bis(dimethylphosphino)ethane (dmpe) gives TiCl4(dmpe) (1). This
compound can be readily alkylated by or equivalents of PhCH2MgCl affording TiCl3(PhCH2)(dmpe) (2) and
1
2
TiCl2(PhCH2)2(dmpe) (3), respectively. Syntheses and crystal structures of the three mononuclear titanium(IV) compounds 1-3 are
presented. Decomposition of ‘TiCl3(But)(dmpe)’ produces the dinuclear titanium(III) edge-sharing complex, Ti2Cl4(v-Cl)2(dmpe)2
(4a). Its analogue, Ti2Cl4(v-Cl)2(depe)2 (4b) has been obtained by direct reaction of TiCl3(THF)3 with the corresponding
phosphine. Thermal decomposition of ‘TiCl2(But)2(dmpe)’ provides the first true, molecular metal atom cluster of titanium(II),
Ti3Cl6(dmpe)3 (5). The cluster molecule consists of an equilateral triangle of TiII atoms with three coplanar v-Cl atoms bridging
the edges. Each titanium atom is further coordinated by a chelating dmpe molecule and a chloride ion, with Ti–Ti distances of
˚
2.872(3) A. The compound appears to be slightly paramagnetic but this is probably the result of a small amount of decomposition
due to the difficulty of handling this pyrophoric complex. It is proposed that there are genuine Ti–Ti single bonds, and that this
is a true metal atom cluster compound. © 1999 Published by Elsevier Science S.A. All rights reserved.
Keywords: Titanium complexes; Crystal structures; Ti–Ti bonding; Cluster; Bond distortions
though not entirely lacking. There is no significant
aqueous chemistry because TiII is too strong a reducing
agent. The complexes that have been reported are all
mononuclear, examples being TiCl2(py)4 [5] and
TiX2(dmpe)2 (X=Cl, CH3, BH4; dmpe=Me2PC2
H4PMe2) [6,7].
1. Introduction
Titanium tetrachloride readily forms adducts of the
type TiCl4Ln with donor ligands [1]. The coordination
numbers are generally five or six but higher coordina-
tion numbers also occur [2]. It is also known to react
with alkylating reagents to produce substitution com-
pounds of the type TiCl4−nRn [3].
In the presence of phosphines, reduction of TiCl4 to
produce TiIII species is sometimes observed [4], but
further reduction to TiII is difficult to accomplish.
Indeed, the chemistry of divalent titanium is exiguous
A study of the reactions of phosphine adducts of TiIV
with Grignard reagents has now yielded a series of
complexes with titanium in oxidation states varying
from II to IV.
2. Experimental section
* Corresponding author. Tel.: +1 409 8459351; fax: +1 409
8459351; e-mail: cotton@tamu.edu
2.1. General procedures
1 Dedicated to Professor Brian Johnson on the occasion of his 60th
birthday.
Manipulations during the preparation of all com-
0022-328X/99/$ - see front matter © 1999 Published by Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00655-X

F.A. Cotton et al. / Journal of Organometallic Chemistry 573 (1999) 78–86
79
pounds were carried out under an atmosphere of ar-
gon using standard Schlenk techniques. Chemicals
were purchased from the following commercial
sources and used as received: TiCl4, PhCH2MgCl,
ButMgCl (Aldrich), dmpe (Me2PC2H4PMe2) and depe
(Et2PC2H4PEt2) (Strem Chemicals), C6D6 (Cambridge
Isotope Laboratories). All solvents used were freshly
distilled under N2 from suitable drying agents. NMR
spectra were recorded at room temperature on a
UNITY-plus 300 multinuclear spectrometer operated
at 300 MHz for 1H, and at 121.4 MHz for 31P
(using 85% H3PO4 as an external standard). Elemen-
tal analyses were done on all thermally stable com-
pounds by Canadian Microanalytical Services; they
were satisfactory.
was filtered, and the filtrate was kept in the freezer.
Dark-brown crystals of 3 grew in a week. Yield: 0.33 g
(48.9%).
2.2.3.2. Method 2. To 10 ml of a 1 M solution of
PhCH2MgCl in diethyl ether, 0.5 ml of TiCl4 (4.5
mmol) was added at −40°C. The mixture was stirred
for 1 h, warmed to 0°C and then filtered. A stoichio-
metric amount of dmpe (4.5 mmol) in hexanes (7 ml)
was added to the filtrate resulting in a dark-brown
precipitate. It was filtered off, dried under vacuum, and
then it was dissolved in 8 ml of toluene. The solution
was kept in the freezer. Crystals of 3 grew after few
days. Yield: 0.76 g (37.6%). 31P{1H} NMR (C6D6,
1
20°C): l=18.6 (s). H NMR (300 MHz, C6D6): l=
7.40 (d, 2H, PhH-meta), 7.15 (m, 2H, PhH-ortho), 7.00
(t, H, PhH-para), 2.85 (t, 4H, CH2-Ph), 1.27 (d, 4H,
CH2), 0.90 (d, 12H, CH3).
2.2. Syntheses of compounds 1–5
2.2.1. TiCl4(dmpe) (1)
2.2.4. [TiCl2(v-Cl)(dmpe)]2 (4a)
A solution of 1 ml of TiCl4 (9 mmol) in 20 ml of
hexanes was cooled to −20°C; then a stoichiometric
amount of dmpe (1.5 ml, 9 mmol) was added. An
orange solid began to precipitate upon mixing. The
mixture was warmed to room temperature, and stirring
was continued over 1 h. The precipitate was isolated by
filtration, washed thoroughly with hexanes (3×15 ml)
and dried under vacuum. Yield: 2.88 g (94.1%).
Crystals suitable for X-ray crystallography were ob-
tained by slow crystallization from toluene at −20°C.
2.2.4.1. Method 1. A suspension containing 0.34 g (1.0
mmol) of 1 in 20 ml of toluene was cooled to −20°C.
Then, 0.6 ml of a 2 M solution (1.2 mmol) of ButMgCl
in diethyl ether was added. The color of the mixture
turned red-brown immediately. The mixture was
warmed to room temperature and stirring was contin-
ued 2–3 h. The suspension was then filtered and the red
filtrate was carefully layered with 20 ml of hexanes. Red
crystals of 4a formed in a few hours. Yield: 0.19 g
(61.3%).
31P{1H} NMR (C6D6, 20°C): l=26.08 (s). H NMR
(300 MHz, C6D6): l=0.95–0.99 (d, 4H, CH2), 1.10–
1.14 (d, 12H, CH3).
1
2.2.4.2. Method 2. To a suspension of 0.38 g (1 mmol)
of TiCl3(THF)3 in 25 ml of toluene was added 0.3 ml of
dmpe. After 20 min of stirring at room temperature a
red solid started to precipitate. It was filtered off,
washed with hexanes, and dried in vacuum. Yield: 0.25
g (79.4%). A few crystals of 4a were obtained by slow
diffusion of hexanes into the toluene filtrate.
2.2.2. TiCl3(PhCH2)(dmpe) (2)
To a cooled suspension (−20°C) containing 0.67 g
(1.96 mmol) of 1 in 20 ml of toluene were added 2.0 ml
of a 1 M solution of PhCH2MgCl in diethyl ether. A
red-purple color developed in a few minutes. The sus-
pension was stirred for 1.5 h at 0°C and then filtered.
The red-brown filtrate was placed in the freezer. Dark-
brown crystals of 2 formed in a few days. Yield: 0.25 g
(32.7%). 31P{1H} NMR (C6D6, 20°C): l=29.6 (d),
l=28.4 (d). 1H NMR (300 MHz, C6D6): l=7.45
(d, 2H, PhH-meta), 7.27 (t, 2H, PhH-ortho), 7.14
(m, H, PhH-para), 3.40 (d, 2H, CH2-Ph), 1.43 (d, 4H,
CH2), 1.20 (d, 12H, CH3).
2.2.5. [TiCl2(v-Cl)(depe)]2 (4b)
To a suspension containing 0.39 g (1.0 mmol) of
TiCl3(THF)3 in 15 ml of toluene, 0.25 ml (ca. 1.1 mmol)
of depe was added. The color of the mixture immedi-
ately turned brown-green. The mixture was stirred for 1
h at room temperature, and then filtered. The brown
filtrate was carefully layered with 15 ml of hexanes. Red
crystals of 4b grew within a few hours. Yield: 0.26 g
(68.1%).
2.2.3. TiCl2(PhCH2)2(dmpe) (3)
2.2.3.1. Method 1. A suspension of 0.51 g (1.5 mmol) of
1 in 25 ml of toluene was cooled to −20°C; then 3.2 ml
of a 1 M solution of PhCH2MgCl in diethyl ether were
added. The color turned dark-brown immediately. The
mixture was stirred for 2 h at ca. 0°C, and the volume
of the solution was then reduced by half. The mixture
2.2.6. [Ti(v-Cl)Cl(dmpe)]3 ·C7H8 (5 ·C7H8)
A suspension containing 0.51 g (1.5 mmol) of 1 in 20
ml of toluene was cooled to −20°C, then ButMgCl in
diethyl ether (1.6 ml of a 2 M solution, 3.2 mmol) was
added. The color of the mixture turned red-brown

80
F.A. Cotton et al. / Journal of Organometallic Chemistry 573 (1999) 78–86
Table 1
Crystallographic data for 1-5
1
2
3
4a
4b
5
Empirical formula
TiCl4P2C6H16
TiCl3P2C13H23
395.50
TiCl2P2C20H30
451.18
Ti2Cl6P4C12H32
608.76
Ti2Cl6P4C20H48
720.96
Ti3Cl6P6C25H56
898.92
Formula weight (g mol-1) 339.83
Crystal dimensions
Temperature (°C)
Crystal system
Space group
0.20×0.15×0.10 0.50×0.30×0.20 0.60×0.35×0.30 0.25×0.12×0.10 0.30×0.20×0.10 0.12×0.10×0.10
−60
−60
−60
−60
−60
−60
Orthorhombic
Pbca
Orthorhombic
Pbca
Tetragonal
I4/a
Monoclinic
P21/n
Orthorhombic
Pca21
Trigonal
R3c
˚
a (A)
16.234(2)
14.335(2)
24.504(2)
90
5702(1)
16
13.859(1)
11.811(1)
22.792(3)
90
3730.8(6)
8
19.202(4)
13.277(6)
24.337(2)
90
8973(3)
16
9.261(3)
16.610(2)
10.879(3)
95.43(2)
1331.7(8)
2
15.0307(8)
16.274(2)
˚
b (A)
˚
c (A)
13.5556(6)
90
3384.3(5)
4
27.731(7)
90
6360(1)
6
i (°)
V
Z
Dcalc. (g cm−3
)
1.583
1.535
1.408
1.046
1.336
0.764
1.518
1.440
1.415
1.145
1.408
1.173
Absorption coefficient
(mm−1
Radiation (u) (A)
)
˚
Mo–Kh (0.71073) Mo–Kh (0.71073) Mo–Kh (0.71073) Mo–Kh (0.71073) Mo–Kh (0.71073) Mo–Kh (0.71073)
Data/observed/parameters 3742/3085/224
2453/2205/215
0.0431/0.1016
0.0489/0.1108
1.04
2932/2685/346
0.0285/0.0665
0.0323/0.0714
1.09
1739/1537/98
0.0535/0.1301
0.0625/0.1389
1.10
4409/4220/280
0.0304/0.0711
0.0322/0.0739
1.10
1854/1680/120
0.0618/0.1213
0.0764/0.1395
1.10
Ra1 (wRb2) [I\2|(I)])
Ra1 (wRb2) (all data)
Goodness-of-fit
0.0514/0.1125
0.0671/0.1249
1.14
a R1=SꢀꢀFoꢀ−ꢀFcꢀꢀ/SꢀFoꢀ.
b wR2=[S[w(Fo2−Fc2)2]/S[w(F2o)2]]1/2
.
immediately. The mixture was warmed to room temper-
ature and the stirring was continued for 3 h. The
volume of the solvent was reduced by more than half
and the mixture was filtered. Tiny block-shaped crystals
of [Ti(v-Cl)Cl(dmpe)]3 ·C7H8 (5·C7H8) formed after the
dark-brown filtrate was kept in the refrigerator for
about a week. Yield: 0.07 g (15.4%).
difference Fourier maps in the SHELXL-93 program
[11]. Except for 3, all hydrogen atoms were included at
idealized positions for the structure factor calculations
but not refined. Details of data collection and structure
refinement for 1-5 are reported in Table 1.
3. Results and discussion
2.3. X-ray crystallographic procedures
3.1. Description of structures
Single crystals of 1–5 were obtained as described
above. In each case, a crystal of suitable quality was
affixed to the end of a quartz fiber with grease, and
then placed in a cold nitrogen stream (−60°C) on a
Nonius Fast diffractometer equipped with an area de-
tector and monochromated Mo–Kh radiation (u=
3.1.1. TiCl4(dmpe) (1)
This compound crystallizes in the orthorhombic
space group Pbca with sixteen molecules per unit cell.
The structure of 1 consists of two crystallographically
independent TiCl4(dmpe) molecules. A perspective view
of one of them is presented in Fig. 1; the structural data
are listed in Table 2. Each molecule consists of four
chlorine and two phosphorus atoms bound to the TiIV
center with average Ti–Cl distances of 2.289(2) A and
Ti–P distances of 2.581(2) A. The chelating dmpe
ligand is crystallographically disordered only in one
type of molecule. The polyhedron around the titanium
atom is a distorted octahedron with bond angles rang-
ing from 75.95(6) to 113.43(7)°.
˚
0.71073 A). Unit cell determinations and data
collections followed routine procedures and practices of
this laboratory [8]. Oscillation photographs of principal
axes were taken to confirm Laue class and axial lengths.
All data were corrected for Lorentz and polarization
effects. The intensities for 1 and 5 were corrected for
absorption anisotropy effects using a local adaptation
of the program SORTAV [9].
All calculations were done on a DEC Alpha running
VMS. The coordinates of titanium atoms for all of the
structures were found in direct methods E-maps using
the structure solution program SHELXTL [10]. The
positions of the remaining atoms were located by the
use of a combination of least squares refinements and
˚
˚
3.1.2. TiCl3(PhCH2)dmpe (2) and TiCl2(PhCH2)2dmpe
(3)
Crystals of 2 conform to the orthorhombic space
group Pbca with eight molecules per unit cell. Com-

F.A. Cotton et al. / Journal of Organometallic Chemistry 573 (1999) 78–86
81
Fig. 1. A drawing of one independent TiCl4(dmpe) molecule showing
the atom labeling scheme. Atoms are represented by their thermal
ellipsoids at 40% probability level. Hydrogen atoms are omitted for
clarity.
Fig. 2. Perspective drawing of the TiCl3(PhCH2)(dmpe) molecule
showing the atom labeling scheme. Atoms are represented by their
thermal ellipsoids at 40% probability level. Hydrogen atoms are
omitted for clarity.
Of most interest in structures 2 and 3 are the coordi-
nation characteristics of the benzyl groups. In 2, the
single benzyl group is bent towards the titanium atom
with a Ti–C(1)–C(2) angle of 81.9(2)°. This results in
the close proximity of the phenyl ring to the titanium
pound 3 crystallizes in the tetragonal space group I4/a
with sixteen molecules per unit cell. The complexes
have similar structures with a distorted octahedral ar-
rangement of ligands around the TiIV atom. Molecules
of 2 and 3 contain a chelating diphosphine ligand,
which is crystallographically disordered in both cases.
Six-coordination is achieved by one benzyl group and
three chloride ions in 2, and by two benzyl groups and
two chlorine atoms in 3. Perspective views of the molec-
ular structures of 2 and 3 are shown in Figs. 2 and 3,
respectively. Selected bond distances and angles are
listed in Table 2.
˚
atom (Fig. 4). The Ti–C(2) distance is 2.464(4) A;
˚
Ti–C(1) is 2.202(4) A.
In the crystal structure of 3 the two benzyl groups are
nonequivalent. The first one, bound through C(1), has
an acute Ti–C(1)–C(2) angle of 88.9(2)° and a Ti–C(2)
˚
distance of 2.605(2) A. This contrasts to the value of
124.0(2)° for the second benzyl group bound to TiIV
through C(8).
Table 2
˚
Selected bond distances (A) and angles (deg.) in 1–3
TiCl4(dmpe)*
TiCl3(PhCH2)(dmpe)
TiCl2(PhCH2)2(dmpe)
1
2
3
(a)
(b)
Ti–Cl(1)
Ti–Cl(2)
Ti–Cl(3)
Ti–Cl(4)
2.273(2)
2.288(2)
2.307(2)
2.293(2)
2.277(2)
2.280(2)
2.309(2)
2.285(2)
2.357(1)
2.329(1)
2.329(1)
2.3314(7)
2.3330(7)
Ti–P(1)
Ti–P(2)
P(1)–Ti–P(2)
2.582(2)
2.577(2)
75.95(6)
2.590(2)
2.574(2)
75.45(6)
2.583(1)
2.586(2)
76.83(5)
2.6312(9)
2.6486(9)
73.84(2)
Ti–C(1)
Ti–C(2)
Ti–C(8)
2.204(4)
2.464(4)
2.188(3)
2.605(2)
2.213(2)
Ti–C(1)–C(2)
Ti–C(8)–C(9)
81.9(2)
88.9(2)
124.9(2)
* Two independent molecules.

82
F.A. Cotton et al. / Journal of Organometallic Chemistry 573 (1999) 78–86
Fig. 5. Perspective drawing of the [Ti(v-Cl)2Cl2(dmpe)]2 molecule
showing the atom labeling scheme. Atoms are represented by their
thermal ellipsoids at 40% probability level. Hydrogen atoms are
omitted for clarity.
sist of discrete dinuclear chloride-bridged titanium units
of composition [TiCl2(v-Cl)(PP)]2 (PP=dmpe and
depe, respectively). The chelating diphosphines are crys-
tallographically disordered in 4a and 4b. The dinuclear
molecule 4a possesses a crystallographically imposed
inversion center, and closely approaches ideal D2h sym-
metry. In 4b the titanium atoms are crystallographically
independent. Both dinuclear compounds are composed
of two edge-sharing octahedra in which each TiIII cen-
ter has a distorted octahedral environment. The molec-
ular drawings of 4a and 4b are shown in Figs. 5 and 6,
respectively. Selected bond lengths and angles are listed
in Table 3 and compared to those reported in the
literature [12] for an analogous [TiCl2(v-Cl)(dippe)]2
compound (dippe=Pri2PC2H4PPr2i ). All three com-
plexes correspond to the 1,3,5,7-type of isomer [13] with
the diphosphine groups being equatorial.
Fig. 3. A drawing of the molecular structure of TiCl2(PhCH2)2(dmpe)
showing the atom labeling scheme. Atoms are represented by their
thermal ellipsoids at 40% probability level. Hydrogen atoms are
omitted for clarity.
3.1.3. [TiCl2(v-Cl)(dmpe)]2 (4a) and [TiCl2(v-Cl)(depe)]2
(4b)
Compound 4a crystallizes in the monoclinic space
group P21/n with two molecules in the unit cell, while
2
4b conforms to the orthorhombic space group Pca21
with four molecules per unit cell. Both complexes con-
Fig. 4. View of the TiCl3P2(CH2Ph) unit in 2. Hydrogen and carbon
atoms C(8)–C(18) are omitted for clarity.
2 An attempt to refine the structure in the centrosymmetric space
group Pbcm was unsuccessful; crystals were likely racemic twins as
the solution proceeded well using a TWIN refinement in the space
group Pca21 (the BASF parameter was ca. 0.5).
Fig. 6. Perspective drawing of the [Ti(v-Cl)2Cl2(depe)]2 molecule
showing the atom labeling scheme. Atoms are represented by their
thermal ellipsoids at 40% probability level. Hydrogen atoms are
omitted for clarity.

F.A. Cotton et al. / Journal of Organometallic Chemistry 573 (1999) 78–86
83
Table 3
˚
Selected bond distances (A) and angles (deg.) in Ti2(v-Cl)2Cl4(PP)2
compounds (PP=dmpe, depe, dippe)
dmpe
4a
depe*
4b
dippe
[12]
Ti· · ·Ti
Ti–Clbr
Ti–Clbr’
Ti–Clterm
Ti–Clterm’
Ti–P(1)
3.121(2)
2.437(2)
2.444(2)
2.311(2)
2.339(2)
2.581(2)
2.581(2)
78.71(6)
79.49(6)
100.51(6)
164.77(7)
3.2138(9)
2.439(1)
2.441(1)
2.326(1)
2.330(1)
2.604(1)
2.610(1)
78.37(4)
82.37(4)
97.63(4)
166.34(5)
3.438(2)
2.459(4)
2.462(4)
2.310(4)
2.313(4)
2.645(5)
2.658(5)
78.6(2)
Ti–P(2)
P(1)–Ti-P(2)
Ti–Clbr–Ti
Clbr–Ti–Clbr’
Clt–Ti–Clt
88.6(1)
91.4(1)
169.6(2)
* Averaged for two titanium octahedra.
3.1.4. [Ti(v-Cl)Cl(dmpe)]3 ·C7H8 (5 ·C7H8)
Compound 5·C7H8 crystallizes in the trigonal space
group R3c with six trinuclear molecules per unit cell;
the interstitial toluene molecule is crystallographically
disordered. The molecular structure of one enantiomor-
phic molecule of 5 is shown in Fig. 7. The molecule has
C3 symmetry and there are equal numbers of the two
enantiomorphs in the unit cell. Three TiII atoms are
Fig. 7. Perspective drawing of the trinuclear [Ti(v-Cl)Cl(dmpe)]3
cluster showing the atom labeling scheme. Atoms are represented by
their thermal ellipsoids at 40% probability level. Hydrogen atoms are
omitted for clarity.
Compounds (2) and (3) have been obtained in good
yields and have been crystallographically characterized
(6ide supra). Both are stable as solids at room tempera-
ture under an inert atmosphere but must be kept below
0°C while in solution; toluene solutions containing (2)
and (3) quickly decompose at room temperature.
Structures of both TiCl3(PhCH2)(dmpe) (2) and
TiCl2(PhCH2)2(dmpe) (3) provide interesting examples
of benzyl ligands bending toward the electron deficient
TiIV center. The Ti–C(1)–C(2) angle of 81.9(2) in 2 is
well below the ideal tetrahedral value 109.5° and signifi-
cantly smaller than the value of 88.9(2)° in 3 (an angle
Ti–C(8)–C(9) for the second benzyl group in 3 is
124.9(2)°). The driving force for this quasi-pi type
interaction is the tendency of the TiIV atom to acquire
a share in more than just the two electrons that would
constitute a simple Ti–C sigma bond. It is thus related
˚
bridged by three chloride ligands (Ti–Clbr 2.436(2) A),
forming a Ti3(v-Cl)3 core. Each titanium atom has a
chelating diphosphine ligand, dmpe, with Ti(1)–P(1)
˚
and Ti(1)–P(2) distances of 2.656(3) and 2.598(3) A
(Table 4), respectively, and an angle P(1)–Ti–P(2) of
76.87(9)°. In addition, there is one terminal chlorine
˚
˚
atom (Ti–Clterm 2.401(3) A) per titanium atom; the
Ti–Ti distance is 2.872(3) A.
3.2. Synthetic considerations
The dmpe adduct of TiCl4 was prepared in practi-
cally quantitative yield by stoichiometric addition of
phosphine to a solution of titanium tetrachloride in
hexanes. An excess of dmpe has been shown [2] to yield
the eight-coordinate complex TiCl4(dmpe)2.
We have found that the six-coordinate complex
TiCl4(dmpe) (1) can be easily alkylated by reaction with
a stoichiometric amount of the corresponding Grignard
reagents according to Eq. (1a) and Eq. (1b):
Table 4
˚
Selected bond distances (A) and angles (deg.) in 5
TiCl4(dmpe)+PhCH2MgCl
Ti(1)–Ti(1A)
Ti(1)–Cl(1)
Ti(1)–Cl(2)
Cl(1)–Ti(1)–Cl(2)
Cl(1A)–Ti(1)–Cl(2) 162.4(1)
Cl(1)–Ti(1)–Cl(1A) 92.2(1)
2.872(3)
2.436(3)
2.401(3)
103.99(9)
Ti(1)–P(1)
Ti(1)–P(2)
2.656(3)
2.598(3)
161.1(1)
161.1(1)
84.77(9)
88.22(8)
79.59(9)
86.3(1)
“TiCl3(PhCH2)(dmpe)+MgCl2
(2)
(1a)
(1b)
Cl(1)–Ti(1)–P(1)
Cl(1A)–Ti(1)–P(1)
Cl(1)–Ti(1)–P(2)
Cl(1A)–Ti(1)–P(2)
Cl(2)–Ti(1)–P(1)
Cl(2)–Ti(1)–P(2)
TiCl4(dmpe)+2 PhCH2MgCl
P(1)–Ti(1)–P(2)
Ti(1)–Cl(1)–Ti(1)
76.87(9)
72.27(9)
“TiCl2(PhCH2)2(dmpe)+2 MgCl2
(3)

84
F.A. Cotton et al. / Journal of Organometallic Chemistry 573 (1999) 78–86
be synthesized in higher yield by direct reaction of
TiCl3(THF)3 with dmpe in toluene according to the
procedure used for the preparation of dippe analogue
[12]. For comparison, we also prepared the depe ana-
logue 4b following the above procedure.
An X-ray crystal structure determination of 4a reveals
an octahedral geometry about each TiIII center and a
˚
Ti· · ·Ti distance of 3.121(2) A (Fig. 5). Corresponding
metal-metal distances for the analogous [TiCl2(v-
Cl)(depe)]2 (4b) and [TiCl2(v-Cl)(dippe)]2 [12] were found
˚
Scheme 1.
to be longer, namely 3.2138(9) and 3.438(2) A, respec-
tively. As expected for geometric reasons the Ti–Clbr–Ti
angles increase from 79.49(6) to 88.6(1)°, and the Clbr–
Ti–Clbr angles decrease from 100.51(6) to 91(4)° on going
from the dmpe to the dippe compound (Table 3).
Although the Ti· · ·Ti contact is substantially shorter
in 4a than those of other chloride-bridged TiIII dimers,
a magnetic moment of 1.04 vB per titanium center
indicates there is little or no direct titanium–titanium
bonding in 4a. It is worth noting that these data for three
structurally similar complexes [TiCl2(v-Cl)(PP)]2 (PP=
dmpe, depe, dippe) provides a good example of how the
structure is influenced by the ligand size: the more bulky
phosphine ligand results in the longer Ti· · ·Ti distance
to, though different from, the i–H interaction found
previously in TiCl3(C2H5)(dmpe) [14], where the Ti–C–
C angle is 86.3(6)°. The type of Ti–benzyl interaction
seen in 2 and 3 was found for the first time [15] in the
compounds M(PhCH2)4, M=Ti, Zr, Hf.
It may be noted that the enhanced interaction of a
benzyl group with an electron defficient metal atom is
actually known to occur in two related but distinct ways.
Even earlier than the observations on the M(PhCH2)4
molecules, it was found that in (p-CH3C6H4CH2)(p5-
C5H5)Mo(CO)2 [16] there is a structure of the type shown
as I in Scheme 1. Here the metal atom engages in the
formation of a p3-allyl bond and seriously affects the
bonding in the phenyl ring, so that there is a marked
alternation in the C–C bond lengths, which are in two
˚
(more than a 0.3 A difference between the dmpe and
dippe analogues).
When a TiCl4(dmpe) solution in toluene was treated
with two equivalents of ButMgCl a TiII compound,
[Ti(v-Cl)Cl(dmpe)]3 (5) was isolated; it is relatively stable
in crystalline form but solutions maintained at room
temperature quickly decompose as indicated by changing
features in the NMR spectra3. The reactivity towards air
is also very high, even in the solid state; crushed crystals
are pyrophoric and slowly decompose on standing in an
argon dry box at room temperature.
˚
ranges, 1.33–1.36 and 1.42–1.43 A. The relationship of
the Ti atom to the benzyl group in compounds 2 and 3,
as well as in the M(PhCH2)4 compounds is as shown in
structure II of Scheme 1. It can be characterized by a
strong Ti–C(1) | interaction together with secondary
interaction involving C(2) and probably C(3) and C(7)
part of y-system. As shown in Fig. 4 for complex 2 the
two distances Ti–C(3) and Ti–C(7) are nearly identical
˚
(3.068(4) and 3.076(4) A, respectively). In addition, the
The cluster molecule 5 consists of an equilateral
triangle of TiII atoms with three coplanar v-Cl atoms
bridging the edges. Each titanium atom is further coor-
dinated by a chelating dmpe and a chloride ion (Fig. 7).
C(2)–C(3) and C(2)–C(7) distances (averaged to 1.400(6)
˚
˚
A in 2, and to 1.405(4) A in 3) are just a bit longer than
other C-C bond lengths in aromatic rings (average
˚
˚
1.371(6) A in both 2 and 3). Other four- [17] and
The short Ti–Ti separation of 2.872(3) A in 5 is about
˚
five-coordinate [18] titanium-benzyl compounds have
also been shown to exhibit the same type of so-called
‘benzallyl’ distortions. To our knowledge, complexes 2
and 3 (formally 12 electron species) are the first examples
containing this distortion among six-coordinate TiIV
compounds. The value of 81.9(2)° for the Ti–C–C angle
in 2 is the smallest so far reported; even those in the
tetrahedral Ti(PhCH2)4 complex are larger (88(2)° for the
room temperature experiment ([15]b), and 92(1)° at
−40°C) [15]a) (88(2)°, . . . ([15]b), . . . ([15]a)).
In an effort to make tert-butyl analogues of com-
pounds 2 and 3, reactions analogous to Eq. (1a) and Eq.
(1b) were undertaken. When a toluene solution contain-
ing 1 was reacted with one equivalent of ButMgCl at
−20°C the only product isolated was a dinuclear TiIII
complex, [TiCl2(v-Cl)(dmpe)]2 (4a). This compound can
0.2 A greater than the sum of the Pauling R1 radii (2.648
˚
A). With the short Ti–Ti distances found in 5, the
molecule would be expected to be diamagnetic, but a
residual paramagnetism, even in the solid state, was
observed. Because of the extreme instability of the
compound, we presume this is the result of impurities
formed during the measurement process.
3 The 31P{1H} NMR spectrum of crystals 5·C7H7 in C6D6 (20°C,
121.4 MHz) exhibits a singlet at −47.8 ppm, which resembles that of
the free phosphine (−48.07 ppm). The 1H NMR spectrum at room
temperature in C6D6 confirms the presence of an interstitial toluene
molecule (2.09(5) ppm); it also shows that phosphine dissociation
occurs as part of some decomposition processes. Unfortunately, the
limited solubility of 5 in toluene as well as its instability in solutions
prevented the collection of low-temperature NMR data.

F.A. Cotton et al. / Journal of Organometallic Chemistry 573 (1999) 78–86
85
The only previously reported compound [19] that
4. Supplementary material available
bears any resemblance to 5 is Ti7Cl16 (and, to a lesser
extent, Ti7Br16). This mixed valence compound contains
an array of fused TiIVCl6 octahedra and TiI3ICl13 units in
a 1:2 ratio. Within the Ti3Cl13 units there are Ti–Ti
Crystallographic data for compounds 1–5 have been
deposited with the Cambridge Crystallographic Data
Centre (entities carrying the suffix x represent dummy
atoms used for modeling disordered atoms). Copies of
the data can be obtained free of charge on application
to The Director, CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (Fax: +44 1223 336033; e-mail:
deposit@chemcrys.cam.ac.uk).
˚
distances of about 2.954(2) A that may be presumed to
correspond to Ti–Ti single bonds. In this case the
Ti63+unit is encapsulated in a solid state environment
and also has a capping (v3-Cl) atom in addition to three
edge bridges.
Examples of ZrIII–ZrIII and HfIII–HfIII bonds are
well known. Representative compounds for zirconium
are Zr2Cl6(dppe)2, Zr2Cl6(PMe2Ph)4 and Zr2Cl6(PEt3)4,
in which the Zr–Zr distances range from 3.104(5) to
Acknowledgements
˚
˚
3.169(1) A [20], which are 0.10 to 0.16 A longer than
2×R1. Similarly, in the hafnium compounds
Hf2Cl6(dippe)2 [21] and Hf2Cl6(PMe2Ph)4 [22] the Hf–
We thank the Robert A. Welch Foundation support
through Grant No. A494 and the Vicerrector´ıa de
Investigacio´n U.C.R. (Project 115-87-516). We thank
Dr E.V. Dikarev for assistance with X-ray
crystallography.
˚
˚
Hf distances are 3.099(1) A and 3.0886(3) A, respec-
tively (2×R1=2.884). While there might have been
some hesitation in assigning Zr–Zr and Hf–Hf bonds
˚
because of these distances being 0.10 to 0.20 A greater
than the values of 2×R1 for the metal atoms, a theo-
retical study supported [23] such an assignment.
By contrast, as already noted, titanium compounds
of the same type, such as 4 have relatively longer
Ti· · ·Ti distances (3.121(2) in 4a versus 2×R1=2.448
References
[1] F.A. Cotton, R.A. Walton, Multiple Bonds between Metal
Atoms, 2nd ed., Oxford University Press, New York, 1993.
[2] F.A. Cotton, J.H. Matonic, C.A. Murillo, M.A. Petrukhina,
Inorg. Chim. Acta 267 (1998) 173; and references therein.
[3] M. Bochmann, in: E.W. Abel, F.G. Stone, G.W. Wilkinson
(Eds), Comprehensive Organometallic Chemistry II, vol. 4, chap-
ter 5, Pergamon, New York, 1995.
[4] G.W.A. Fowles, R.A. Walton, J. Chem. Soc. A (1967) 4330.
[5] M.A. Araya, F.A. Cotton, J.H. Matonic, C.A. Murillo, Inorg.
Chem. 34 (1995) 5424.
˚
˚
A, giving an excess of 0.67 A) and are paramagnetic.
Other examples of TiIII–TiIII bonds are scarce; the only
known examples are found [24] in complexes of the
type Ti2(RNC(H)NR)2(v-RNC(H)NR)2(v-Cl)2 with the
˚
Ti–Ti distances of ca. 2.9 A.
There are a few other cases where Ti· · ·Ti distances
[6] J.A. Jensen, S.R. Wilson, A.J. Schultz, G.S. Girolami, J. Am.
Chem. Soc. 109 (1987) 8094.
˚
˚
˚
of 3.110 A [25], 2.75–2.78 A [26] and 2.745 A [27] have
been reported, but these are of uncertain structure, of
very complex structure, or contain strongly constrain-
ing bridges, respectively.
[7] R. Morres, G.S. Girolami, Inorg. Chem. 29 (1990) 4169.
[8] (a) A. Bino, F.A. Cotton, P.E. Fanwick, Inorg. Chem. 18 (1979)
3558. (b) F.A. Cotton, B.A. Frenz, G. Deganello, A. Shaver, J.
Organomet. Chem. 50 (1973) 227. (c) F.A. Cotton, E.V.
Dikarev, X. Feng, Inorg. Chim. Acta 237 (1995) 19.
[9] R.H. Blessing, Acta Cryst. A 51 (1995) 33.
[10] SHELXTLV.5, Siemens Industrial Automation Inc., Madison,
WI, 1994.
[11] G.M. Sheldrick, in: H.D. Flack, L. Parkanyi, K. Simon (Eds),
Crystallographic Computing 6, Oxford University Press, Oxford,
UK, 1993, pp.111–122.
[12] A.R. Hermes, G.S. Girolami, Inorg. Chem. 29 (1990) 313.
[13] F.A. Cotton, E.V. Dikarev, M.A. Petrukhina, J. Am. Chem.
Soc. 119 (1997) 12541.
[14] Z. Dawoodi, M.L.H.Green, V.S.B. Mtetwa, K. Prout, A.J.
Schultz, J.M. Williams, T.F. Koetzle, J. Chem. Soc. Dalton
Trans. (1986) 1629.
[15] (a) G.R. Davies, J.A.J. Jarvis, J.B.T. Kilbourn, Chem. Soc.
Chem. Commun. (1971) 1511. (b) I.W. Bassi, G. Allegra, R.
Scordamaglia, J. Am. Chem. Soc. 93 (1971) 3787.
[16] F.A. Cotton, M.D. LaPrade, J. Am. Chem. Soc. 90 (1968) 5418.
[17] L.M. Alvaro, T. Cuenca, J.C. Flores, P. Royo, M.A. Pellinghelli,
A. Tiripicchio, Organometallics 11 (1992) 3301.
[18] H. Stoeckli-Evans, Helv. Chim. Acta. 58 (1975) 93.
[19] B. Krebs, G. Henkel, Z. anorg. allg. Chem. 474 (1981) 149.
[20] F.A. Cotton, M.P. Diebold, P.A. Kibala, Inorg. Chem. 27 (1988)
799.
It is noteworthy that in contrast to the benzyl-con-
taining TiIV complexes 2 and 3, tert-butyl-substituted
titanium analogues of the type ‘TiCl4-nButn(dmpe)’, n=
1, 2 have not been isolated under similar conditions.
They are thermally unstable and attempts to prepare
them lead instead to the formation of dinuclear and
trinuclear titanium chloride-phosphine molecules 4 and
5, respectively. The weak intramolecular interaction
between the aromatic ligand and TiIV found in 2 and 3
could account for the enhanced stability of the titanium
benzyls compared to that of the corresponding alkyls.
This interaction in 2 was found to be the most promi-
nent (as measured by the angle Ti–C(1)–C(2)) in the
titanium-benzyl compounds.
In summary, compound 5 is the first molecular tita-
nium cluster compound of any kind, the first example
of TiII–TiII bonding and a rare example of a relatively
thermally stable, though pyrophoric, TiII compound in
the solid state; however, in solution it quickly
decomposes.

86
F.A. Cotton et al. / Journal of Organometallic Chemistry 573 (1999) 78–86
[21] P.M. Morse, S.R. Wilson, G.S. Girolami, Inorg. Chem. 29 (1990)
3200.
(b) S. Hao, K. Feghali, S. Gambarotta, Inorg. Chem. 36 (1997)
1745.
[22] F.A. Cotton, P.A. Kibala, W.A. Wojtczak, Inorg. Chim. Acta 177
(1990) 1.
[23] M.-M. Rohmer, M. Be´nard, Organometallics 10 (1991) 157.
[24] (a) F.A. Cotton, W.A. Wojtczak, Gazz. Chim. Ital. 123 (1993) 499.
[25] P. Corradine, A. Sirigu, Inorg. Chem. 6 (1967) 601.
[26] S.I. Troyanov, V.A. Tafeenko, Zhur. Obsh. Khim. 60 (1989) 2416.
[27] F. Gauvin, J. Britten, E. Samuel, J.F. Harrod, J. Am. Chem. Soc.
114 (1992) 1489.
.