Reactivity of boranes with a titanium(iv) amine tris(phenolate) alkoxide complex; Formation of a Ti(iv) tetrahydroborate complex, a Ti(iii) dimer and a Ti(iv) hydroxide Lewis acid adduct

Article abstract of DOI:10.1039/b708378e

Treatment of the titanium(iv) alkoxide complex [Ti(OⁱPr)(OC ₆Me₂H₂CH₂)₃N] (2) with BH₃·THF, as part of a study into the utility and reactivity of (2) in the metal mediated borane reduction of acetophenone, results in alkoxide-hydride exchange and formation of the structurally characterised titanium(iv) tetrahydroborate complex [Ti{BH₄}(OC₆Me ₂H₂CH₂)₃N] (3). Complex (3) readily undergoes reduction to form the isolable titanium(iii) species [Ti(OC ₆Me₂H₂CH₂)₃N] ₂ (4). Reaction of (2) with B(C₆F₅)₃ results in formation of the Lewis acid adduct [Ti(OC₆Me ₂H₂CH₂)₃N][HO·B(C ₆F₅)₃] (5). In comparison, treatment of the less sterically encumbered alkoxide Ti(OⁱPr)₄ with B(C₆F₅)₃ results in alkoxide-aryl exchange and formation of the organometallic titanium complex [Ti(OⁱPr) ₃(C₆F₅)]₂ (6). The molecular structures of 3, 4, 5 and 6 have been determined by X-ray diffraction. The Royal Society of Chemistry.

Full text of DOI:10.1039/b708378e

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Reactivity of boranes with a titanium(IV) amine tris(phenolate) alkoxide
complex; formation of a Ti(IV) tetrahydroborate complex, a Ti(III) dimer
and a Ti(IV) hydroxide Lewis acid adduct†‡
Andrew L. Johnson,* Matthew G. Davidson* and Mary F. Mahon
Received 5th June 2007, Accepted 14th September 2007
First published as an Advance Article on the web 27th September 2007
DOI: 10.1039/b708378e
Treatment of the titanium(IV) alkoxide complex [Ti(OiPr)(OC6Me2H2CH2)3N] (2) with BH3·THF, as
part of a study into the utility and reactivity of (2) in the metal mediated borane reduction of
acetophenone, results in alkoxide–hydride exchange and formation of the structurally characterised
titanium(IV) tetrahydroborate complex [Ti{BH4}(OC6Me2H2CH2)3N] (3). Complex (3) readily
undergoes reduction to form the isolable titanium(III) species [Ti(OC6Me2H2CH2)3N]2 (4). Reaction of
(2) with B(C6F5)3 results in formation of the Lewis acid adduct [Ti(OC6Me2H2CH2)3N][HO·B(C6F5)3]
(5). In comparison, treatment of the less sterically encumbered alkoxide Ti(OiPr)4 with B(C6F5)3 results
in alkoxide–aryl exchange and formation of the organometallic titanium complex [Ti(OiPr)3(C6F5)]2 (6).
The molecular structures of 3, 4, 5 and 6 have been determined by X-ray diffraction.
production of stable titanium(IV) complexes and their utility in
catalytic applications.14 Here we report the reactivity of the amine
Introduction
The first reported reaction of the simple borane BH3·THF,
with the group(IV) alkoxide complex, Ti(OtBu)4, to form the
tetrahydroborate complex Ti(BH4)2(OtBu), was published in 1966
by James and Wallbridge.1 Since then, the reaction of boranes
such as BH3·L and B2H6 with metal alkoxides has provided a
method for the synthesis of metal tetrahydroborate systems.2 The
variable coordination modes of [BH4]− (g , g and g ) have led to
descriptions of tetrahydroborate as a ‘gate-keeper’ ligand,3 with
exploitation of the variable bonding to moderate the coordinative
saturation at the metal. As such, many compounds containing
tetrahydroborate ligands (and other B–H groups) have been, and
continue to be, investigated for their potential catalytic activity in
areas such as olefin polymerisation,3 ring-opening polymerisation
of cyclic esters,4 hydrogenation,5 hydroboration6 and reduction
reactions.7
tris(phenolate) titanium isopropoxide, 2, with BH3·THF, B(C6F5)3
and BPh3 respectively and describe the differing reactivity of these
boranes towards the titanium species. As part of this study we also
describe the reactivity of Ti(OiPr)4 with B(C6F5)3 and formation
of alkoxide–aryl exchange products.
1
2
3
Unfortunately, reactions between transition metal alkoxides and
boranes often result in reduction of the metal centre to a lower
oxidation state; however, in the very few cases that this method
has been used successfully, the physical and chemical data for such
products is sparse.1,2,8
More recently the reactive chemistry of the strongly Lewis
acidic borane B(C6F5)3, which has been extensively studied for
its application in catalytic processes,9 has attracted considerable
interest, and reactions with metal-oxo,10 metal-acyl,11 metal-
alkoxide12 and metal-carbonyl13 sytems have all been reported.
We have recently been interested in the utility of the tris-
(phenolamine) ligand (LH3), 1 (Fig. 1), as scaffolds for the
Fig. 1 Tris(phenolamine) ligand (LH3), 1.
Results and discussion
Titanium alkoxide compounds have been known for some time to
mediate reactions such as hydroboration6 and carbonyl reduction7
in the presence of boranes, and while many of these reactions are
complicated, metal hydrides and metal borohydrides have been
suggested as potentially reactive species.3–7 For this reason we
have been interested in the utility of the metal alkoxide complex,
2, as a catalyst in the reduction of carbonyl species, and the
general reactivity of such alkoxide complexes with other borane
systems.
It has previously been shown that the uncatalysed room
temperature reaction of borohydrides with acetophenone requires
ca. 24 h for complete reduction; the addition of Ti(OiPr)4 reduces
this reaction time to 30 min with a 93% yield.7b Preliminary
screening experiments showed that 2 is a moderately effective
precatalyst for the reduction of acetophenone at room temperature
Department of Chemistry, University of Bath, Claverton Down, Bath, UK
BA2 7AY. E-mail: A.L.Johnson@bath.ac.uk, M.G.Davidson@bath.ac.uk,
M.F.Mahon@bath.ac.uk; Fax: +44 (0)1225 8262312; Tel: +44 (0)1225
826443
† Dedicated to Prof. Ken Wade, a friend and mentor, on the occasion of
his 75th birthday.
‡ CCDC reference numbers 648697–648700. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b708378e
This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 5405–5411 | 5405
©

View Article Online
(2 h, 94% isolated yield) (Scheme 1). However, due the inherent
racemic nature of 2, no enantioselectivity was observed.14a
Scheme 1 Reduction of acetophenone by BH3·THF in the presence of
metal alkoxide catalyst, 2.
Addition of BH3·THF (or BH3·SMe2) to a THF solution of 2,
in the presence of substrate (1 : 10.1 : 10) results in an immediate
colour change of the solution from pale yellow to red. However, in
the absence of ketone the reaction mixture slowly (several hours)
undergoes further colour changes at room temperature from red–
orange, through green, to a dark blue–purple solution, indicating
the production of a Ti(III) species, the production of which is
enhanced at elevated temperatures.
Fig. 2 ORTEP (50% probability ellipsoids) diagram of 3. Selected
bond lengths (A): Ti(1)–O(1) 1.823(2), Ti(1)–N(1) 2.393(4), Ti(1)–B(1)
˚
2.230(7), Ti(1)–H(2B) 2.05(5), O(1)–C(1) 1.367(4); Bond angles (◦):
Ti(1)–O(1)–C(1) 137.8(2), N(1)–Ti(1)–O(1) 81.29(7), O(1)–Ti(1)–B(1)
98.71(7), H(1B)–B(1)–H(2B) 114.0(2). Toluene of crystallisation and
non-{BH} hydrogen atoms have been omitted for clarity.
(i) Reaction of 2 with BH3
In an attempt to elucidate the role of the metal alkoxide complex
2 in the borane reduction, the reaction between the metal alkoxide
and BH3 was investigated. The reaction of 2, in toluene, with excess
BH3·THF at −20 ◦C results in an immediate colour change from
pale yellow to deep red. On standing overnight, at −60 ◦C, highly
air-sensitive crystals of the titanium(IV) tetrahydroborate complex,
(L)Ti{BH4} 3, were formed, the identity of which was established
spectroscopically and confirmed by single crystal X-ray diffraction
(Fig. 2).
The monomeric, C3-symmetric complex contains an approxi-
mately trigonal bipyramidal Ti centre, which sits slightly out of
the ‘pocket’ of the equatorial phenolate-O atoms compared to
214a [distance of Ti atom above the plane of the three phenolate-
11
CH2 protons of the tripodal ligand. The 1H{ B} NMR spectrum
of compound 3 reveals the presence of a single resonance for
the {BH4} ligand which is consistent with the rapid exchange of
terminal and bridging hydrogen atoms at ambient temperatures,
resulting in magnetic degeneracy. 11B NMR spectroscopy confirms
this fluxional behaviour with the appearance of a quintet at d
16.3 ppm (JB–H = 86 Hz) in the 11B NMR spectrum at room
temperature.2b
To our knowledge, there exist only seven other examples of
crystallographically characterised titanium(IV) tetrahydroborate
complexes.15 Therefore complex 3 represents a rare example of
a structurally characterised titanium(IV) tetrahydroborate deriva-
tive. Moreover, 3 represents the first structurally characterised
product of an alkoxide–hydride exchange reaction, as the more
common synthetic route to tetrahydroborate derivatives is via
the reaction of metal halides with Li[BH4].2 Interestingly, a
common feature of 3 and other crystallographically characterised
tetrahydroborate complexes is the use of bulky ligand systems
which presumably lend a significant kinetic stability to the system.
The mode of formation (Scheme 2) of the complex can be
inferred from in situ 11B NMR studies performed on the reaction
mixture. Addition of excess BH3·THF to 2 results in the immediate
formation of 3 (d = 16.3 ppm (JB–H = 86 Hz)), along with
HB(OiPr)2 (d = 27.3 ppm)16 which presumably arises from a
rapid exchange process between two molecules of the unobserved
˚
˚
O atoms: 0.251(1), 0.247(1) A for 2 and 0.276(2) A for 3]. The
axial sites around the metal are occupied by the neutral nitrogen
atom of the ligand, and the {BH4} unit which is bound to the
3
central metal in a g -fashion (via three hydride bridges) [Ti(1)–
B(1) 2.230(7) A.]. A superficial comparison between 3 and the
˚
parent titanium complex 2, shows that despite replacement of the
terminal alkoxide ligand for a {BH4} group there is little change in
the coordination of the amine tris(phenolate) ligand to titanium
˚
˚
˚
˚
[average Ti–O(phenolate) distance: 1.850 A for 2 and 1.823(2) A
for 3] [Ti–N distances: 2.303(2), 2.295(2) A for 2 and 2.393(4) A
for 3]. As for 2, complex 3 shows ‘propeller’ chirality, evidenced
by the P and M enantiomers inherent in the crystallographic
classification, and the two resonances at d 2.84 and 3.91 ppm
observed in the 1H NMR spectra assigned to the diastereotopic N-
Scheme 2 Reaction schemes for the production of the titanium tetrahydroborate 3 from the metal alkoxide 2, showing the proposed intermediates.
5406 | Dalton Trans., 2007, 5405–5411 This journal is The Royal Society of Chemistry 2007
©

View Article Online
H2BOiPr. This observation is entirely consistent with other reports
on the relative exchange rates in alkoxyboron hydride species.17
Similar reactions between BH3·THF and metal chloride complexes
have been recorded and NMR studies have shown conclusively that
reactions proceed with the loss of BH2Cl.18 We therefore suggest
that the formation of 3 proceeds in an analogous manner. In
particular, product formation is initiated by BH3 coordination
to the Lewis basic alkoxide group, followed by elimination of
mono-alkoxyborane and the formation of a transient titanium
hydride species. This short-lived metal hydride has the potential to
react with a suitable substrate (as in Scheme 1), or alternatively, is
trapped by reaction with excess BH3 and results in the formation
and isolation of the metal borahydride complex 3 (Scheme 2).
Indeed, comparable exchange reactions between boron trihalides
e.g. BCl3 with metal alkoxides and metal amides have been known
for some time.19 It is interesting to note that although 2 reacts
rapidly with both BH3·THF and BH3·SMe2, the system was
found to be inactive when catecholborane and pinacolborane were
used. Similarly, when BH3·N(H)Me2 was used the reaction was
found to be much slower. We believe these observations to be
consistent with the formation of a metal alkoxide–borane adduct,
with the less Lewis acidic of the boranes (either through slow
adduct dissociation or by virtue of electron donation from ligands)
resulting in slower or impaired reactions.
Titanium(IV) hydride species are a highly reactive class of
compounds that have been implicated in a number of catalytic
reaction processes,2b but despite their importance only a few
examples are known.20 In many cases metal tetrahydroborate
complexes have been viewed as ‘protected’ metal hydride systems
in which the first step in the activation pathway is the loss of BH3
and the formation of a transient metal hydride species. In an effort
to isolate the proposed titanium(IV) hydride complex (Scheme 2),
a toluene solution of 3 was treated with two molar equivalents
of PMe3, at −78 ◦C in the expectation that either the hydride
species, or a PMe3 adduct, (Scheme 3) could be isolated in an
analogous reaction to those previously reported by No¨th involving
(ArO)3Ti{BH4} (ArO = 2,6-di-iso-propylphenyl).21 The resulting
immediate colour change from red through green to purple–blue
indicated that any hydride intermediate that might be forming
in the reaction rapidly decomposes to yield a highly reactive
Ti(III) species. Crystals of 4 were grown from a toluene solution at
−20 ◦C, in order to undertake structural characterisation.
Fig.
3
ORTEP (50% probability ellipsoids) diagram of 4. Se-
˚
lected bond lengths (A): Ti(1)–O(1) 1.8724(13), Ti(1)–O(2) 1.8724(13),
Ti(1)–O(3) 1.9801(12), Ti(1)–O(3A) 2.0361(13), Ti(1)–N(1) 2.2412(15),
O(1)–C(1) 1.352(2), O(2)–C(21) 1.349(2), O(3)–C(31) 1.379(2); Bond
angles (◦): Ti(1)–O(1)–C(1) 135.61(12), Ti(1)–O(2)–C(21) 139.61(12),
Ti(1)–O(3)–C(31) 135.98(11), N(1)–Ti(1)–O(3a) 176.08(5). Toluene of
crystallisation and hydrogen atoms have been omitted for clarity. The
symmetry operator (1 − x, 1 − y, 1 − z) generates the equivalent atoms
denoted with ‘A’.
˚
the nitrogen atom [Ti(1)–N(1): 2.241(2) A] and the phenolic
oxygen of the neighbouring Ti(III) unit occupying axial positions
[Ti(1)–O(3A): 2.0361(13) A]. As expected, the Ti–O bridging
bond is slightly elongated compared to the other equatorial Ti–
O contacts [Ti–O distances: 1.872(2), 1.873(2) A (non bridging)
and 1.980(2) A (bridging)]. The relatively long Ti–Ti distance
of 2.6616(6) A rules out the possibility of metal–metal bonding
interactions, and is consistent with the paramagnetic nature of
4. Although the precise mechanism of Ti(IV) tetrahydroborate
decomposition is not clearly understood, is most likely proceeds
via loss of BH3 (as BH3·PMe3) and formation of a Ti(IV) hydride
that, in the absence of substrate with which to react, is unstable
towards loss of H2 and reduction of the metal centre to Ti(III)
(Fig. 3).21
˚
˚
˚
˚
(ii) Reaction of 2 with BPh3 and B(C6F5)3
As previously stated, initial reaction between 2 and BH3·THF
proceeded with a significant excess of borane present in the
reaction mixture. Attempts to isolate the proposed borane adduct
(Scheme 2) by stoichiometric reactions between 2 and BH3·THF
were unsuccessful, yielding an equimolar mixture of compounds
2 and 3. It was hoped that treatment of 2 with a stronger, less
labile Lewis acid such as BPh3 or B(C6F5)3, where there would be
no possibility of alkoxide–hydride exchange, might yield isolable
adducts which would (in turn) serve as models for the proposed
borane adduct.
Scheme 3 Proposed reaction scheme for the synthesis of Ti(IV) hydride
species.
An X-ray crystallographic study of 4 reveals it to be a dimeric
complex, with two inversion related {(L)Ti(III)} units bridged by
a phenoxide unit of the tris(phenolate) ligand (Fig. 3), which is
reminiscent of the related Al(III) tris(phenolate) complex {(L)Al}2,
recently reported by Verkade.22 The geometry about each titanium
atom is best described as distorted trigonal bipyramidal, with
Reaction of complex 2 with B(C6F5)3 resulted in an instant
colour change from yellow to red and the isolation of red
This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 5405–5411 | 5407
©

View Article Online
crystals upon standing overnight, which were characterised
and identified as the titanium(IV) hydroxy–Lewis acid adduct
(L)Ti{(H)O·B(C6F5)3} (5).
147(3)◦], which may well account for its absence in the 1H NMR
spectrum.
Previous studies have shown that reaction of B(C6F5)3 with
alkoxides have resulted in aryl–alkoxide exchange,12 which is
presumably prohibited in the case of 2 by the steric bulk of
the amine tris(phenolate) ligand. Indeed reaction of Ti(OiPr)4
with B(C6F5)3, which was conducted as part of this study, results
in the formation of the dimeric organometallic complex {Ti(l-
OiPr)(OiPr)2(C6F5)}2 6, as shown in Fig. 5, the synthesis of
which, by reaction of TiCl(OiPr)3 with LiC6F5, has previously
been reported.24 The molecular structure (Fig. 5) establishes that
6 is a dimeric complex with five coordinate distorted trigonal
bipyramidal geometry at the metal centres, with each l2-OiPr
group occupying an axial equatorial site on adjacent metal
centres (Fig. 5), such that the ligands bridge the two metal
centres asymmetrically, with the equatorial Ti–O bonds [Ti(1)–
An X-ray crystallographic study of 5 reveals it to be a rare
example of a titanium(IV) hydroxide complex in which the titanium
hydroxide moiety is coordinated to the B(C6F5)3 via the oxygen of
the hydroxide group (Fig. 4).
˚
O(1), 1.9436(15) A] being shorter than the axial [Ti(1A)–O(1),
˚
2.0852(14) A]. The C-ipso atom of the perfluorophenyl group
˚
occupies a terminal equatorial site [Ti(1)–C(1) 2.177(2) A] and
both ligands are arranged transoid to each other such that the
molecule has crystallographic Ci symmetry. The terminal Ti–
i
˚
O Pr bond lengths [Ti(1)–O(2), 1.7595(14) A (axial); Ti(1)–O(3),
˚
1.7656(16) A (equatorial)] are unsurprisingly shorter than the
bridging Ti–O bonds, with very little variation between the axial
and equatorial positions.
Fig.
4
ORTEP (50% probability ellipsoids) diagram of 5. Se-
˚
lected bond lengths (A): Ti(1)–O(1) 1.9436(16), Ti(1)–O(2) 1.819(3),
Ti(1)–O(3) 1.804(3), Ti(1)–O(4) 2.034(3), Ti(1)–N(1) 2.227(3), B(1)–O(4)
1.528(5), B(1)–C(41) 1.640(6), B(1)–C(51) 1.655(6), B(1)–C(61) 1.653(5),
O(4)–F(62) 2.691(4), O(4)–H(4) 0.82(3), H(4)–F(62) 1.97(3), O(1)–C(11)
1.388(4), O(2)–C(21) 1.378(5), O(3)–C(31) 1.386(5); Bond angles (◦):
Ti(1)–O(4)–B(1) 143.5(3), Ti(1)–O(1)–C(11) 141.1(2), Ti(1)–O(2)–C(21)
140.9(2), Ti(1)–O(3)–C(31) 141.6(2), N(1)–Ti(1)–O(4) 170.98(13),
O(4)–H(4)–F(62) 147(3). Toluene of crystallisation and hydrogen atoms
[except for H(4)] have been omitted for clarity.
The molecular structure of 5 shows a similar coordination mode
of the tris(phenolate) ligand to that observed in complexes 2 and 3,
excepting replacement by a {(H)O·B(C6F5)3} unit in an axial site
of the pseudo trigonal bipyramidal titanium centre. Coordination
Fig. 5 ORTEP (50% probability ellipsoids) diagram of 6. Selected bond
lengths (A): Ti(1)–O(1A) 2.0852(14), Ti(1)–O(2) 1.7595(14), Ti(1)–C(1)
˚
2.177(2), Ti(1)–O(1) 1.9436(15), Ti(1)–O(3) 1.7656(16); Bond angles (◦):
O(1A)–Ti(1)–O(2) 167.75(7), O(1)–Ti(1)–C(1) 127.07(8), O(1)–Ti(1)–O(3)
118.87(7), C(1)–Ti(1)–O(3) 108.65(8). Hydrogen atoms have been omitted
for clarity. The symmetry operator (1 − x, −y, −z) generates the equivalent
atoms denoted with ‘A’.
˚
of the hydroxyl group by the {B(C6F5)3} [B(1)–O(4): 1.528(5) A]
fragment results in a lengthening of the Ti–O(H) bond and a
concomitant shortening of the axial Ti–N bond in comparison
to the tert-butyl substituted tris(phenolamine) Ti–OH complex
reported by Brown et al.23 [Ti(1)–O(4): 2.034(2) A, Ti(1)–N(1):
˚
˚
˚
2.227(3) A (cf. Ti–O: 1.810(3) and Ti–N: 2.364(3) A)].
In comparison with the reaction of B(C6F5)3, treatment of 2
with one molar equivalent of BPh3 in toluene resulted in a colour
change from pale yellow to orange. Allowing the solution to stand
The hydrogen atom of the hydroxide group, which was not
1
observed in the H NMR spectrum, was located by the X-ray
◦
structure determination and shown to be involved in a short
O–H · · · F hydrogen bonding interaction with one of the ortho-
F atoms on an adjacent {C6F5} ring of the {B(C6F5)3} unit
at −25 C overnight resulted in the isolation of a crop of yellow
micro-crystals, which were identified by NMR, elemental analysis
and mass spectrometry as the previously reported oxo-bridged
species (L)Ti–O–Ti(L) (7).25
˚
˚
[O(4)–F(62) 2.691(4) A; H(4)–F(62) 1.97(3) A; O(4)–H(4)–F(62)
5408 | Dalton Trans., 2007, 5405–5411
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
The reactions of BPh3 and B(C6F5)3 with 2 are consistent with
the formation of intermediate alkoxide–borane adducts. In both
cases we propose that coordination of the borane results in a
Lewis acid promoted loss of alkene (specifically propene in the
case of 4 and 5) rather than aryl-exchange which is presumably
inhibited by the steric bulk of the amine tris(phenolate) ligand. In
the case of the more strongly bound B(C6F5)3, the steric bulk of the
borane adduct inhibits further reaction, facilitating isolation of the
reactive hydroxy system 5. For the more weakly coordinated BPh3
containing system, the lability of the borane allows the hydroxy
complex to react further with a second molecule of alkoxide 2,
to form the bridged oxo complex 7 and free isopropanol. The
absence of H2O in the 1H NMR spectrum of the reaction between
BPh3 and 2, suggests that formation of the oxo complex 7 by
a condensation of two molecules of the hydroxide complex is
less likely. While we cannot completely exclude the possibility of
adventitious hydrolysis to yield 5 and 7, reproducible high yields
of these compounds under rigorous anhydrous conditions, in our
opinion, suggest alkene elimination from the alkoxide as a more
likely source of hydroxyl and oxo functionalities.
for 3 h at −20 ◦C. The solid was collected by filtration, washed with
cold hexane, and dried in vacuo. Yield: 1.59 g, 83%. Anal. Calcd
for C27H34B1N1O3Ti (C7H8)1: C, 71.5; H, 7.41; N, 2.45: Found:
C, 71.1; H, 7.94; N, 2.4; H NMR (300 MHz, 23 ◦C), CD2Cl2
1
(ppm): d 1.84 (d, 4H, BH4) 2.11 (s, 9H, L(Me/Me)), 2.14 (s, 9H,
L(Me/Me)), 2.97 (m, 3H, CH2 (AB system)), 3.91 (m, 3H, CH2
(BA system)), 6.68 (s, 3H, CH arom), 6.81 (s, 3H, CH arom); 11
B
NMR (96 MHz, 23 ◦C) −16.3 (quint, 1J(B–H) = 84 Hz); 13C{1H}
NMR (100 MHz) d 16.3, 21.2, 59.0, 123.7, 124.7, 128.0, 131.6,
132.0, 161.5.3.
[Ti(OC6Me2H2CH2)3N]2 4. To a stirred solution of 3 (1 mmol,
0.5 g) in toluene (15 ml), 2 ml of a 1.0 M solution of PMe3 in
THF (2 mmol) was added dropwise by syringe, at −15 ◦C. The
reaction mixture was allowed to warm to room temperature,
with stirring, during which time the solution changed colour
from orange to blue. The mixture was allowed to stir at room
temperature for 2 h, after which, removal of solvent under reduced
pressure resulted in the precipitation of a blue residue, which
was re-dissolved in a minimum of fresh toluene (10 ml), with
gentle heating. The solution has filtered warm to remove insoluble
residues. A blue–purple crystalline solid was obtained on standing
at −20 ◦C. The solid was collected by filtration, washed with cold
hexane, and dried in vacuo. Yield: 0.34 g, 74%. Anal. Calcd for
C54H60N2O6Ti2(C7H8)3: C, 74.74; H, 7.02; N, 2.23: Found: C, 74.6;
Conclusions
We have demonstrated the reactivity of the boranes BH3·THF,
BPh3 and B(C6F5)3 with {L}TiOiPr, and provided insight into
the reactive pathways and catalytic role of the metal alkoxide
species in borane reductions. Coordination of the borane to the
metal alkoxide facilitates exchange between the borane and the
metal alkoxide, where sterics permit. In instances where exchange
is prohibited by steric bulk, activation of the metal alkoxide by
coordination of the borane results in reduction of the alkoxide
group, by loss of alkene, and the formation of metal-hydroxy and
-oxo species.
H, 7.9; N, 2.2. FAB MS: m/z 928 [M2 ], 464 [M+], 416 [M+ − Ti]
+
[Ti(OC6Me2H2CH2)3N][HO·B(C6F5)3] 5. To a stirred solution
of 2 (2 mmol, 1 g) in toluene (15 ml), 2 mmol of B(C6F5)3 (1 g) in
toluene (5 ml) was added dropwise at −75 ◦C. The reaction mixture
was allowed to warm to −20 ◦C, during which time the solution
changed colour from orange to a deep red colour. The reaction
was allowed to warm to room temperature after which removal of
solvent under reduced pressure resulted in the precipitation of a
red residue, which was re-dissolved in a minimum of fresh toluene
(10 ml), with gentle heating. The solution has filtered warm to
remove insoluble residues. A red crystalline solid was obtained on
standing for 3 h at −20 ◦C. The solid was collected by filtration,
washed with cold hexane, and dried in vacuo. Yield: 1.43 g, 72%.
Anal. Calcd for C45H31B1F15N1O4Ti (C7H8)1.5: C, 58.91; H, 3.83;
N, 1.24: Found: C, 58.29; H, 3.81; N, 1.25; 1H NMR (300 MHz,
23 ◦C), CD2Cl2 (ppm): d 1.73 (s, 9H, L(Me/Me)), 2.17 (s, 9H,
L(Me/Me)), 3.09 (m, 3H, CH2 (AB system)), 4.01 (m, 3H, CH2
(BA system)), 6.74 (s, 3◦H, CH arom), 6.79 (s, 3H, CH arom);
11B NMR (96 MHz, 23 C) −1.17 (s, B(C6F6)3); 13C{1H} NMR
(75.5 MHz) d 15.7, 21.1, 59.2, 123.4, 124.1, 128.2, 132.2, 133.3,
160.9; 19F NMR (376 MHz) d −134.6 (m), −158.8 (m), −164.6
(m).
Experimental
All manipulations were carried out under an atmosphere of dry
argon using standard Schlenk and glove-box techniques. Solvents
were purified by conventional procedures and distilled prior to use.
BPh3, B(C6F5)3, BH3·THF and Ti(OiPr)4 were purchased from
Aldrich and used as received, without further purification. The
complex 2 was prepared using literature procedure.14a
Solution 1H and 13C NMR experiments were performed at
1
ambient temperature using a Bruker Avance-300. H NMR data
are referenced to residual non-deuterated solvent.
Syntheses of complexes
[Ti{BH4}(OC6Me2H2CH2)3N] 3. To a stirred solution of 2
(4 mmol, 2 g) in toluene (15 ml), 8 ml of a 1.0 M solution of
BH3·THF in THF (8 mmol) was added dropwise by syringe,
at 0 ◦C. The reaction mixture was allowed to warm to room
temperature, with stirring, during which time the solution changed
colour from orange to a deep red colour. The mixture was allowed
to stir at room temperature for 2 h, after which, removal of
solvent under reduced pressure resulted in the precipitation of a
red residue, which was re-dissolved in a minimum of fresh toluene
(10 ml), with gentle heating. The solution has filtered hot to remove
insoluble residues. A red crystalline solid was obtained on standing
[Ti(OiPr)3(C6F5)]2 6. To a stirred solution of Ti(OiPr)4
(1 mmol, 0.3 g) in toluene (15 ml), 1 mmol of B(C6F5)3 (0.5 g)
in toluene (5 ml) was added dropwise at −75 ◦C. The reaction
was allowed to warm to room temperature, and on standing a
colourless crystalline solid was obtained. The solid was collected
by filtration, washed with cold hexane, and dried in vacuo. A
second crop of crystals was obtained from the filtrate by cooling
◦
to −20 C Yield: 0.14 g, 61%. Anal. Calcd for C30H42F10O6Ti2 :
C, ◦45.93; H, 5.40; Found: C, 46.31; H, 5.98; 1H NMR (300 MHz,
23 C), CD2Cl2 (ppm): d1.18 (d, 3J = 12 Hz, 36H, CH(CH3)2) 4.77
(br m, 6H, CH(CH3)2) 13C{1H} NMR (75.5 MHz) d 23.9, 84.8.
This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 5405–5411 | 5409
©

View Article Online
Table 1 Crystal data and structure refinement for compounds 3, 4, 5 and 6
Compound
3
4
5
6
Formula
Formula weight
T/K
C34H42BNO3Ti
571.40
150(2)
Trigonal
¯
R3
C75H84N2O6Ti2
1205.24
150(2)
Triclinic
¯
P1
10.1340(2)
13.3940(2)
13.7670(3)
66.7420(10)
70.8950(10)
80.5040(10)
1620.87(5)
1
C55.5H42.5BF15NO4Ti
1131.1
150(2)
Triclinic
¯
P1
11.8900(5)
14.5430(4)
16.4190(7)
108.3880(10)
100.7370(10)
99.759(3)
2566.42(17)
2
1.464
C30H42F10O6Ti2
784.44
150(2)
Monoclinic
P21/n
8.8980(3)
16.8170(5)
12.0480(5)
90
96.5380(10)
90
1791.11(11)
2
1.455
Crystal system
Space group
˚
a/A
14.1580(2)
˚
b/A
˚
c/A
26.3930(6)
90
90
120
4581.66(14)
6
1.243
a/◦
b/◦
c /◦
3
˚
V/A
Z
Dc/g cm−3
1.235
0.300
l/mm−1
0.314
0.269
0.536
F(000)
1824
639
1154
808
Crystal size/mm
0.175 × 0.125 × 0.50
3.69 to 27.46
23 103
0.28 × 0.25 × 0.15
3.76 to 27.12
30 318
0.3 × 0.25 × 0.17
3.55 to 27.26
48 049
0.35 × 0.3 × 0.22
3.87 to 27.48
31 754
h range/◦
Reflections collected
Independent reflections
Reflections [I>2r((I)]
Data/restraints/parameters
Goodness-of-fit on F2
Final R1, wR2 [I>2r(I)]
Final R1, wR2 (all data)
2311 [R(int) = 0.0547]
7110 [R(int) = 0.0560]
11 290 [R(int) = 0.0932]
4084 [R(int) = 0.0942]
1803
5402
6259
2765
2311/16/137
1.047
0.0481, 0.1232
0.0670, 0.1343
0.330, −0.366
7110/0/433
1.017
0.0461, 0.1102
0.0700, 0.1215
0.496, −0.481
11 290/7/695
1.022
0.0733, 0.1727
0.148, 0.2095
0.755, −0.532
4084/0/224
1.058
0.0446, 0.0879
0.0866, 0.1037
0.258, −0.389
−3
˚
Max, min diff./e A
[{Ti(OC6Me2H2CH2)3N}(l2-O)] 7. To a stirred solution of of
2 (2 mmol, 1 g) in toluene (15 ml), 2 mmol of BPh3 (0.48 g) in
toluene (5 ml) was added dropwise at −75 ◦C. The reaction mixture
was allowed to warm to room temperature after which removal of
solvent under reduced pressure resulted in the precipitation of a
yellow–orange residue, which was re-dissolved in a minimum of
fresh toluene (10 ml), with gentle heating. The solution has filtered
warm to remove insoluble residues. An orange micro-crystalline
solid was obtained on standing overnight at −20 ◦C. The solid
was collected by filtration, washed with cold hexane, and dried
in vacuo. Yield: 0.74 g, 84%. Data for 7 is consistent with that
previously reported.25
region reveals a resemblance to toluene. However, no solvent
hydrogens were included in the refinement. In complex 4, the
asymmetric unit consists of one half of the dimeric species. One
and half molecules of disordered toluene was also seen to present
in the ASU. One molecule of toluene was partitioned over two sites
such that C(101)–C(107) and C(201)–C(207) and their associated
hydrogens show a 60 : 40 occupancy respectively. The residual
electron density was modelled as one half of a molecule of toluene,
with a disordered methyl group. As well as one molecule of the
titanium complex, the ASU of 5 also contains one and a half
molecules of toluene. The half molecule of toluene (a solvent
fragment at 50% occupancy) is modelled as a rigid hexagon and
refined isotropically with hydrogens in calculated positions using
a riding model.
X-Ray crystallography‡
CCDC reference numbers 648697–648700.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b708378e
Crystallographic data for compounds 3, 4, 5 and 6 are sum-
marised in Table 1. All data collections were implemented on a
Nonius KappaCCD diffractometer, using Mo-Ka radiation (k =
˚
0.71073 A). Data were corrected for Lorentz and polarization
Acknowledgements
effects, and structure refinement was by full-matrix least squares
on F2. Structure solution and refinement was performed using
SHELX-8626 and SHELXL-9727 software, respectively. Data were
uniformly corrected for Lorentz and polarisation. Hydrogen
atoms involved in hydrogen bonding and M–H bonding were
detected in the penultimate difference Fourier maps and freely
refined. All other hydrogen atoms were included at calculated
positions throughout, and refined using a riding model.
We thank the Nuffield Foundation for a Newly Appointed Lec-
turers Award (ALJ), the EPSRC and Johnson Matthey Catalysts
Ltd. (MGD) and the University of Bath for support.
References
1 B. D. James and M. G. H. Wallbridge, J. Inorg. Nucl. Chem., 1966, 28,
2456.
In 3, in addition to 1/3 of the complex molecule, the asymmet-
ric unit also contains a fraction of disordered solvent. Maximum
convergence was attained by treating this latter volume of the
electron density map as 5 carbon atoms bearing 0.5 occupancy.
Consideration of the crystallographic symmetry in the solvent
2 (a) H. Noth, M. Thomann, M. Bremer and G. Wagner, in Topics in the
Chemistry of Boron, ed. G. W. Kabalka, RSC, Cambridge, UK, 1994,
p. 337; (b) T. J. Marks and J. R. Kolb, Chem. Rev., 1977, 77, 263, and
references therein.
3 (a) M. Tamm, B. Dressel, V. Urban and T. Lugger, Inorg. Chem.
Commun., 2002, 5, 837; (b) T. Koch, S. Blaurock, E. Hey-Hawkins,
5410 | Dalton Trans., 2007, 5405–5411
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
M. Galan-Fereres, D. Plat and M. S. Eisen, J. Organomet. Chem., 2000,
595, 126.
4 (a) S. M. Guillaume, M. Schappacher and A. Soum, Macromolecules,
2003, 36, 54; (b) H. Noss, M. Oberthur, C. Fischer, W. P. Kretschmer
and R. Kempe, Eur. J. Inorg. Chem., 1999, 12, 2283; (c) F. Bonnet, A. R.
Cowley and P. Mountford, Inorg. Chem., 2005, 44, 9046.
13 For examples see: (a) R. Choukroun, C. Lorber and B. Donnadieu,
Organometallics, 2004, 23, 1434; (b) R. Choukroun, C. Lorber, C.
Lepetit and B. Donnadieu, Organometallics, 2003, 22, 1995; (c) R.
Choukroun, C. Lorber and L. Vendier, Eur. J. Inorg. Chem., 2004, 2,
317.
14 (a) S. D. Bull, M. G. Davidson, A. L. Johnson, D. E. J. E. Robinson
and M. F. Mahon, Chem. Commun., 2003, 1971; (b) K. Youngjo, G. K.
Jnaneshware and J. G. Verkade, Inorg. Chem., 2003, 42, 1437; (c) S.
Gendler, S. Segal, I. Goldberg, Z. Goldschidt and M. Kol, Inorg. Chem.,
2006, 45, 4783; (d) C. Redshaw, M. A. Rowan, D. M. Homden, S. H.
Dale, M. R. J. Elsegood, S. Matsui and S. Matsuura, Chem. Commun.,
2006, 3329; (e) P. Axe, S. D. Bull, M. G. Davidson, C. J. Gilfillan, M. D.
Jones, D. E. J. E. Robinson, L. E. Turner and W. L. Mitchell, Org. Lett.,
2007, 9, 223.
5 (a) A. I. Sizov, I. V. Molodnitskaya, B. M. Bulychev, B. M. Belskii and
G. L. Soloveichik, J. Organomet. Chem., 1988, 344, 185; (b) H. M. Lee,
C. Yang and G. Jai, J. Organomet. Chem., 2000, 601, 330; (c) J. B. Letts,
T. J. Mazanec and D. W. Meek, J. Am. Chem. Soc., 1982, 104, 3898.
6 (a) K. Burgess and M. J. Ohlmeyer, Chem. Rev., 1991, 91, 1171; (b) K.
Burgess and W. A. van der Donk, J. Am. Chem. Soc., 1994, 116, 6561;
K. Burgess and W. A. van der Donk, Organometallics, 1994, 13, 3616;
(c) C. N. Muhoro, X. He and J. F. Hartwig, J. Am. Chem. Soc., 1999,
121, 5033.
7 (a) K. S. Ravikumar, S. sinha and S. Chandrasekan, J. Org. Chem.,
1999, 64, 5841; (b) C. W. Lindsley and M. DiMare, Tetrahedron Lett.,
1994, 35, 5141; (c) G. Giffels, C. Dreisbach, U. Kragl, M. Weigerding,
H. Waldmann and C. Wandrey, Angew. Chem., Int. Ed. Engl., 1995, 34,
2005; (d) T. Yanagi, K. Kikuchi, H. Takeuchi, T. Ishikawa, T. Nishimura
and T. Kamijo, Chem. Lett., 1999, 1203; (e) I. Sarvary, F. Almqvist and
T. Frejd, Chem.–Eur. J., 2001, 7, 2158; (f) P. Hegarty, R. Lau and W. B.
Motherwell, Tetrahedron Lett., 2003, 44, 1851; (g) I. Sarvary, P.-O.
Norrby and T. Frejd, Chem.–Eur. J., 2004, 10, 128.
8 B. D. James and M. G. H. Wallbridge, Prog. Inorg. Chem., 1970, 11, 99.
9 (a) F. Focante, P. Mercandelli, A. Sironi and L. Resconi, Coord. Chem.
Rev., 2006, 250, 170; (b) G. Erker, Dalton Trans., 2005, 1883; (c) W. E.
Piers and T. Chivers, Chem. Soc. Rev., 1997, 26, 345.
10 For examples see: (a) U. Kilgore, F. Basili, J. C. Huffman and D. J.
Mindiola, Inorg. Chem., 2006, 45, 487; (b) D. Neculai, H. W. Roesky,
A. M. Neculai, J. Magull, B. Walfort and D. Stalke, Angew. Chem.,
Int. Ed., 2002, 41, 4294; (c) G. Barrado, L. Doerrer, M. L. H. Green
and M. A. Leech, J. Chem. Soc., Dalton Trans., 1999, 1061; (d) L. H.
Doerrer, J. R. Galsworthy, M. L. H. Green, M. A. Leech and M. Muller,
J. Chem. Soc., Dalton Trans., 1998, 3191; (e) J. R. Galsworthy, M. L. H.
Green, M. Muller and K. Prout, J. Chem. Soc., Dalton Trans., 1997,
1309.
15 Cambridge Structural Database ref. codes are: JUWSID, JUWSOJ,
KILYIN, PALROK, PALSEB, PALSOL and ZISQUIN.
16 H. C. Brown, B. Nazer and J. A. Sikorski, Organometallics, 1983, 2,
634.
17 D. J. Pasto, V. Balasubramaniyan and P. W. Wojtkowski, Inorg. Chem.,
1969, 8, 594.
18 (a) T. P. Fehlner, in Contemporary Boron Chemistry, ed. M. G. David-
son, A. K. Hughes, T. B. Marder, and K. Wade, RSC, Cambridge, UK,
2000, 255; (b) T. P. Fehlner, Organometallics, 2000, 19, 2643.
19 M. F. Lappert, P. P. Power, A. R. Sanger and R. C. Sirivastava, Metal
and Metalloid Amides: Syntheses, Structures and Physical and Chemical
Properties, Ellis Horwood, Chichester, 1980.
20 A search of the Cambridge Structural Database reveals seven Ti(IV) hy-
dride complexes. CSD ref. codes are: CAXVED, GAFKII, HOMJEY,
RUGCEB, BUGCEB01, ZANLIJ and ZISRIC.
21 H. No¨th and M. Schmidt, Inorg. Chem., 1995, 14, 4601.
22 W. Su, Y. Kim, A. Ellern, I. A. Guzei and J. G. Verkade, J. Am. Chem.
Soc., 2006, 128, 13727.
23 V. Ugrinova, G. A. Ellis and S. N. Brown, Chem. Commun., 2004,
468.
24 M. D. Rausch and H. B. Gordon, J. Organomet. Chem., 1974, 74,
85.
25 (a) M. Kol, M. Shamis, I. Goldberg, Z. Goldscmidt, S. Alfi and E.
Hayut-Salant, Inorg. Chem. Commun., 2001, 4, 177; (b) A. J. Nielson,
C. Shen and J. M. Waters, Polyhedron, 2006, 25, 2039.
26 G. M. Sheldrick, SHELX-86, Computer Program for Crystal Structure
Determination, University of Go¨ttingen, Germany, 1986.
27 G. M. Sheldrick, SHELX-97, Computer Program for Crystal Structure
Refinement, University of Go¨ttingen, Germany, 1997.
11 For examples see: (a) S. A. Llewellyn, M. L. H. Green and A. R. Cowley,
Dalton Trans., 2006, 1776; (b) G. D. Anderson, O. J. Boys, A. R. Cowley,
J. C. Green, M. L. H. Green, S. A. Llewellyn, C. M. von Beckh, S. I.
Pascu and I. C. Vei, J. Organomet. Chem., 2004, 689, 4407.
12 F. Wolff, R. Choukroun, C. Lorber and B. Donnadieu, Eur. J. Inorg.
Chem., 2003, 628.
This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 5405–5411 | 5411
©