The isolation and structure of titanium bromide compounds containing 2,6-diphenylphenoxide ligation; terminal and bridging ligand effects on Ti-Ti interactions

Article abstract of DOI:10.1039/b212909d

The titanium aryloxy bromides [Ti(OC₆H₃Ph ₂-2,6)₂Br₂] 1 and [(2,6-Ph₂C ₆H₃O)₂Ti(μ-Br)₂Ti(OC ₆H₃Ph₂-2,6)₂] 2 have been isolated and structurally characterized. Compound 1 adopts a pseudo-tetrahedral environment about titanium while 2 contains an edge-shared bis(tetrahedral) coordination geometry. The Ti-Ti distance is 3.0308(8) A in 2. A comparison of these compounds with their chloride counterparts as well as cyclopentadiene analogues has been made. The Ti-Ti distance in the compounds [Cp′₂Ti(μ-X)₂ TiCp′₂] (Cp′) = methylcyclopentadienyl) increases as the bridging ligand X is changed from Cl to Br. In contrast this parameter is almost identical in the aryloxide compounds [(ArO)₂Ti(μ-X)₂Ti(OAr)₂] (ArO = 2,6-diphenylphenoxide), due to the presence of a titanium-titanium bonding interaction. The Royal Society of Chemistry 2003.

Full text of DOI:10.1039/b212909d

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The isolation and structure of titanium bromide compounds
containing 2,6-diphenylphenoxide ligation; terminal and bridging
ligand effects on Ti–Ti interactions
Gillian F. Eade, Phillip E. Fanwick and Ian P. Rothwell*
Purdue University, Department of Chemistry, 560 Oval Drive, West Lafayette, IN 47907-2038,
USA. E-mail: rothwell@purdue.edu
Received 3rd January 2003, Accepted 12th January 2003
First published as an Advance Article on the web 19th February 2003
The titanium aryloxy bromides [Ti(OC6H3Ph2-2,6)2Br2] 1 and [(2,6-Ph2C6H3O)2Ti(µ-Br)2Ti(OC6H3Ph2-2,6)2] 2 have
been isolated and structurally characterized. Compound 1 adopts a pseudo-tetrahedral environment about titanium
while 2 contains an edge-shared bis(tetrahedral) coordination geometry. The Ti–Ti distance is 3.0308(8) Å in 2.
A comparison of these compounds with their chloride counterparts as well as cyclopentadiene analogues has been
made. The Ti–Ti distance in the compounds [CpЈ2Ti(µ-X)2TiCpЈ2] (CpЈ = methylcyclopentadienyl) increases as the
bridging ligand X is changed from Cl to Br. In contrast this parameter is almost identical in the aryloxide compounds
[(ArO)2Ti(µ-X)2Ti(OAr)2] (ArO = 2,6-diphenylphenoxide), due to the presence of a titanium–titanium bonding
interaction.
Introduction
The structure and reactivity of both titanium() and titan-
ium() compounds are important aspects of modern inorganic
and organometallic chemistry.1 In the case of the d1-derivatives
discrete paramagnetic, mononuclear compounds with coordin-
ation numbers as low as three (bulky ligands) are common.2 In
the case of dinuclear compounds containing two Ti() metal
centers there are a spectrum of possible Ti–Ti interactions and
corresponding magnetic behaviour. To date there has been no
molecular species isolated containing an unsupported Ti–Ti
bond.3 Hence the nature of the associated bridging ligands is an
important variable in probing the nature of the underlying
Ti–Ti interaction. During our studies of the organometallic
chemistry associated with substrates [(ArO)2TiCl2] (ArO =
bulky aryloxide ligand)4,5 we have isolated and characterized
the d1–d1 dimer [(2,6-Ph2C6H3O)2Ti(µ-Cl)2Ti(OC6H3Ph2)2].6 In
contrast to their paramagnetic, cyclopentadienyl analogues,7,8
this diamagnetic compound contains a short Ti–Ti distance of
2.9827(7) Å. Recent work has also led to the isolation of the
“hybrid”, mixed cyclopentadienyl aryloxides which have inter-
mediate Ti–Ti distances.9,10 In this paper we report on the syn-
thesis and characterization of mixed bromo aryloxides of Ti()
and Ti(). This study focuses on the possible influence of both
bridging and terminal ligands upon the nature of Ti()–Ti()
interactions.
Fig. 1 Molecular structure of [(2,6-Ph2C6H3O)2TiBr2] 1.
tetrahedral titanium atom with
a
crystallographically
imposed C2-axis leading to equivalent bromide and aryloxide
ligands.
The sodium amalgam reduction of hydrocarbon solutions of
1 occurs smoothly over a number of hours to yield a dark,
highly air and moisture sensitive solution. Recrystallization by
layering a saturated toluene solution with pentane yielded pure
crystals of titanium() compound [(2,6-Ph2C6H3O)2Ti(µ-Br)2-
Ti(OC6H3Ph2-2,6)2] 2 containing 1/2 toluene per Ti in the unit
cell (Scheme 1). The solid state structure of 2 (Fig. 2, Table 2) is
isostructural to that of the previously isolated chloride, with
Results and discussion
Synthesis and characterization of compounds
The addition of 2,6-diphenylphenol (2 equiv per Ti) to a ben-
zene solution of [TiBr4] leads to the elimination of HBr
and formation of the bis(aryloxide) [Ti(OC6H3Ph2-2,6)2Br2]
1 as deep red crystals in good yield (Scheme 1). The solid-
state structure of 1 (Fig. 1, Table 1) consists of a pseudo-
Table 2 Selected bond distances (Å) and angles (Њ) for [(2,6-Ph2C6-
H3O)2Ti(µ-Br)]2 2
Ti(1)–O(2)
Ti(2)–O(1)
Ti(1)–Br(1)
Ti(2)–Br(1)
Ti(1)–Ti(2)
1.840(2)
1.818(2)
2.5155(7)
2.5180(8)
3.0308(8)
Ti(1)–O(3)
Ti(2)–O(4)
Ti(1)–Br(2)
Ti(2)–Br(2)
1.826(3)
1.833(3)
2.5171(7)
2.5205(7)
Table 1 Selected bond distances (Å) and angles (Њ) for [(2,6-Ph2C6-
H3O)2TiBr2] 1
Ti–Br
2.3719(4)
Ti–O(1)
1.746(2)
O(2)–Ti(1)–O(3)
Br(1)–Ti(1)–Br(2)
Ti(1)–O(2)–C(11)
Ti(2)–O(1)–C(21)
145.6(1)
106.05(2)
156.8(2)
156.8(2)
O(1)–Ti(2)–O(4)
Br(1)–Ti(2)–Br(2)
Ti(1)–O(3)–C(31)
Ti(1)–O(4)–C(41)
146.2(1)
105.87(2)
153.3(3)
153.1(3)
O(1)–Ti–O(1)
O(1)–Ti–Br
Ti–O(1)–C(1)
112.6(1)
111.38(5)
168.3(2)
Br–Ti–Br
O(1)–Ti–Br
108.48(3)
106.50(6)
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 1 0 6 1 – 1 0 6 4
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Scheme 1
Table 3 Structural parameters for [(L)(LЈ)TiX2]; L, LЈ = Cp or ArO; X
= Cl, Br
[Compound]ref.
Ti–X/Å
L–Ti–LЈ/Њ
X–Ti–X/Њ
[Cp2TiCl2]11
[Cp2TiBr2]15
2.367(2)
2.361(1)
2.489(2)
2.491(2)
131
131
115
94
95
[CpTi(OC6HPh4-2,3,5,6)Cl2]9c 2.255(1)
2.240(1)
102
[Ti(OC6H3Ph2-2,6)2Cl2]13
[Ti(OC6H3Ph2-2,6)2Br2]a
2.206(1)
2.3719(4)
2.3720(4)
113
113
109
108
a This work.
[(2,6-Me2C6H3)2Ti(µ-NBut)2Ti(OC6H3Me2-2,6)2] 2.7909(7) Å,17
[(cb)2ClTi(µ-CHSiMe3)2Ti(cb)2] 2.9504(8) Å,18 and [(Cy2N)2-
ClTi(µ-CH2)2Ti(NCy2)2] 2.934(2) Å.19
Fig. 2 Molecular structure of [(2,6-Ph2C6H3O)2Ti(µ-Br)]2 2.
In the case of di-titanium() compounds, there exists a great
deal of variation in Ti–Ti distances for compounds containing
the [Ti(µ-Cl)2Ti]. This is highlighted by the titanium() halides
containing cyclopentadienyl and aryloxide ligands shown in
Table 4. Very few compounds contain Ti–Ti distances <3 Å
for [Ti(µ-Cl)2Ti] central cores. An important class of com-
pounds are the edge-shared bis-octahedral formamidinate
complexes [Ti2{RNC(H)CNR}2{RNC(H)CNR}2(µ-Cl)2] iso-
lated by Cotton et al. (R = C6H5, C6H4Ph-4)20 and Gambarotta
et al. (R = c-C6H11).21 In these diamagnetic compounds the
Ti–Ti distances of 2.8890(8), 2.916(3) and 2.942(2) Å respect-
ively were taken as indicative of the presence of a metal–metal
single bond. The aryloxide [(2,6-Ph2C6H3O)2Ti(µ-Cl)]2 is there-
fore unusual. It can be seen for the chlorides in Table 4 that
the Ti–Ti distance shortens dramatically as cyclopentadienyl
ligands are replaced by aryloxides. It may be argued that this is
simply a consequence of the corresponding decrease in Ti–Cl
distances. Furthermore there is an increase in Cl–Ti–Cl angle
noticed for the Ti() compounds (Table 3). Both of these
effects will lead to the metal centers being pulled closer together
as we substitute ArO for Cp in the dinuclear, hailde bridged
species. However, the data obtained in this study for the
bromides indicate that the Ti–Ti interaction in the aryloxide
compounds is not simply a function of the bridging ligands. It
can be seen that as Cl is replaced by Br in the compounds
[CpЈ2Ti(µ-X)2TiCpЈ2] (CpЈ = methylcyclopentadienyl) the Ti–Ti
distance increases from 3.926(3) to 4.125(4) Å. This increase of
∼0.2 Å can be accounted for by the underlying increase in Ti–µ-
X distances, ∼0.16 Å. However, in the [(ArO)2Ti(µ-X)2Ti(OAr)2]
(ArO = 2,6-diphenylphenoxide) the increase in Ti–Ti distance
is only ∼0.04 Å. This is despite the expected increase in
Ti–µ-X distances of ∼0.15 Å. Instead of increasing the Ti–Ti
distance, the aryloxide compounds compensate by moving the
halide atoms further apart with a corresponding increase in
the X–Ti–X angle and closing of the Ti–X–Ti angle on
replacing Cl by Br (Table 4). The key structural changes for the
four [Ti(µ-X)2Ti] trapezoids are shown exaggerated in the radar
plots shown in Fig. 3. In the case of the methylcyclopenta-
dienyl (CpЈ) compounds it can be clearly that the trapezoid
two bromide ligands bridging two pseudo-tetrahedral titanium
atoms. In both compounds there are large Ti–O–Ar angles,
which are a common feature of early d-block metal aryloxides.
Previous work has shown that the magnitude of these angles is
not particularly informative.
The nature of the aryloxide ligands in 1 and 2 precludes any
significant structural information being obtained from their
1H NMR spectra. However, both 2,6-diphenylphenoxide
compounds exhibit a sharp set of overlapping multiplets in the
aromatic region of the spectrum. In the case of 2 this result
indicates that the molecule is diamagnetic in C6D6 solution
under ambient conditions.
Discussion of structural parameters
All of the molecules of interest in this study contain the
titanium metal centers in a pseudo-tetrahedral environment. In
Table 3 are contained some key structural parameters for a
series of titanium() halides containing cyclopentadienyl and
aryloxide ligands.11–15 For the three chloride compounds it can
be seen that replacement of cyclopentadiene by aryloxide leads
to a significant shortening of the Ti–Cl bond lengths. This can
be rationalized in terms of a corresponding increase in the
electron deficiency of the metal center. The Ti–Br bond dis-
tances are, as expected, longer than the corresponding Ti–Cl
bonds. Based upon organic structures one expects an increase
of ∼0.15 Å for E–Br over corresponding E–Cl distances. For
the titanocene compounds the increase is ∼0.13 Å while it is
∼0.16 Å for the two aryloxide compounds (Table 3). A signifi-
cant observation is that the X–Ti–X angle does not vary as
X changes from Cl to Br.
In the case of the dinuclear compounds, the molecular geom-
etry is best described as edge-shared bis(tetrahedral) with a
[Ti(µ-X)2Ti] core. This is a very common structural motif in
titanium chemistry. In titanium() chemistry the shortest Ti–Ti
distances occur when dianionic groups such as oxo, imido or
alkylidene ligands bridge the d0-metals. Examples include
[{C5H2(SiMe3)3}ClTi(µ-O)2TiCl{C5H2(SiMe3)3}] 2.7071(4) Å,16
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Table 4 Structural parameters for [(L)(LЈ)Ti(µ-X)2Ti(L)(LЈ)]; L, LЈ = Cp or ArO; X = Cl, Br
[Compound]ref.
L–Ti–LЈ/Њ
X–Ti–X/Њ
Ti–X–Ti/Њ
Ti–X/Å
Ti–Ti/Å
7
7
[(C5H4Me)2Ti(µ-Cl)]2
131
79
101
101
2.535(2)
2.566(2)
2.562(2)
2.526(2)
2.722(2)
2.705(3)
2.400(1)
2.406(1)
2.3660(9)
2.3761(9)
2.5155(7)
2.5180(8)
2.5171(7)
2.5205(7)
3.926(3)
[(C5H4Me)2Ti(µ-Br)]2
134
125
144
81
92
99
88
78
4.125(4)
3.336(1)
2.9827(7)
3.0308(8)
[Cp(2,4-But2-Np-6-C6H3O)Ti(µ-Cl)]2
9c
6
[(2,6-Ph2C6H3O)2Ti(µ-Cl)]2
102
a
[(2,6-Ph2C6H3O)2Ti(µ-Br)]2
147
146
106
106
74
74
Np = 1-naphthyl.a This work.
Table 5 Crystal data and data collection parameters
1
2ؒ1/2toluene
Formula
Formula weight
Space group
C36H26Br2O2Ti
698.33
P21212 (No. 18)
C75.50H56Br2O4Ti2
1282.90
P1 (No. 2)
¯
a/Å
b/Å
c/Å
α/Њ
β/Њ
γ/Њ
V/Å
10.4945(3)
15.3372(4)
9.5602(6)
90
90
90
1538.77(17)
2
1.507
14.6298(3)
15.9823(4)
16.0980(5)
115.1707(11)
92.5252(13)
115.7050(14)
2946.5(3)
2
Z
ρcalc/g cmϪ3
1.446
Temperature/K
Radiation (wavelength)
R
RW
150
150
Mo Kα (0.71073 Å)
0.032
0.061
Mo Kα (0.71073 Å)
0.055
0.139
Synthesis of [Ti(OC6H3Ph2-2,6)2Br2] (1)
A sample of [TiBr4] (5.92 g, 0.016 mol) was dissolved in
benzene (250 mL) and stirred until an orange/yellow solution
was obtained. To this solution, solid 2,6-diphenylphenol
(7.87 g, 0.032 mol) was added. The red/brown mixture was left
to stir for 2 hours. The mixture was then evacuated to dryness
under vacuum giving an orange solid which could be recrystal-
lised from toluene/pentane. Yield = 10.1 g (88%). Anal. Calcd.
for C36H26Br2O2Ti: C, 61.92; H, 3.75; Br, 22.89. Found: C,
61.93; H, 3.89; Br, 22.72%. 1H NMR (C6D6, 30 ЊC): δ 6.60–7.60
(aromatics).
Fig. 3 “Radar” plot of the distances of the titanium and halide atoms
about the centroid of the [Ti(µ-X)2Ti] (X = Cl, Br) core in the four
compounds [CpЈ2Ti(µ-X)2TiCpЈ2] (CpЈ = methylcyclopentadienyl) and
[(ArO)2Ti(µ-X)2Ti(OAr)2] (ArO = 2,6-diphenylphenoxide). The change
in atomic positions is exaggerated by using a plot scale of 1.0–2.2 Å
from the centroid.
expands evenly as Br replaces Cl. However, in the case of the
2,6-diphenylphenoxides, the central core accommodates the
change from Cl to Br by increasing the halide–halide distance.
The Ti–Ti distance does not increase proportionately. This
analysis leads to the conclusion that the short Ti–Ti interaction
in the aryloxide compounds is less sensitive to the nature of the
bridging groups and implies the existence of a Ti–Ti single
bond.
Synthesis of [(2,6-Ph2C6H3O)2Ti(ꢀ-Br)2Ti(OC6H3Ph2-2,6)2] (2)
A sample of [Ti(OC6H3Ph2-2,6)2Br2] 1 (1.00 g, 1.43 mmol)
was added to benzene (25 mL) above sodium (excess, 0.050 g,
2.14 mmol) as a mercury amalgam pool. The red solution was
stirred vigorously overnight. The resulting solution was
decanted from the mercury pool, filtered to remove sodium
salts and then evacuated to dryness, affording a dark solid. This
crude solid was recrystallised from toluene/pentane as crystals
containing 1/2 toluene molecule per Ti in the unit cell. Yield =
0.40 g (45%). Anal. Calcd. for C75H56Br2O4Ti2: C, 70.55; H,
4.42; Br, 12.52. Found: C, 70.92; H, 4.41; Br, 12.27%. 1H NMR
(C6D6, 30 ЊC): δ 6.50–7.50 (aromatics).
Experimental
General remarks
All manipulations were carried out using standard syringe,
Schlenk line, and glovebox techniques.22 Hydrocarbon solvents
were dried by distillation from sodium/benzophenone and
stored under dry nitrogen. All reagents were dried over 3 Å
molecular sieves prior to use. The 1H NMR spectra were
recorded on a Varian Associates Gemini 200 and an Inova
300 spectrometer and were referenced using protio impurities
of commercial benzene-d6 as an internal standard. Micro-
analytical data were obtained in-house at Purdue. The
X-ray diffraction studies were completed “in-house” at Purdue
University.
X-Ray data collection and reduction
Crystal data and data collection parameters are contained in
Table 5. A suitable crystal was mounted on a glass fiber in a
random orientation under a cold stream of dry nitrogen. Pre-
liminary examination and final data collection were performed
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applied.24 Intensities of equivalent reflections were averaged.
The structure was solved using the structure solution program
PATTY in DIRDIF92.25 The remaining atoms were located in
succeeding difference Fourier syntheses. Hydrogen atoms were
included in the refinement but restrained to ride on the atom to
which they are bonded. The structure was refined in full-matrix
least-squares where the function minimized was Σw(|Fo|2
Ϫ
|Fc|2)2 and the weight w is defined as w = 1/[σ2(Fo2) ϩ (0.0585P)2
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taken from the “International Tables for Crystallography”.26
Refinement was performed on a AlphaServer 2100 using
SHELX-97.27 Crystallographic drawings were done using
programs ORTEP.28
CCDC reference numbers 200530 and 200531.
See http://www.rsc.org/suppdata/dt/b2/b212909d/ for crystal-
lographic files in CIF or other electronic format.
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Acknowledgements
We thank the National Science Foundation (Grant
CHE-0078405) for financial support of this research.
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