Synthesis, isolation and structural investigation of Schiff-base alkoxytitanium complexes: Steric limitations of ligand coordination

Article abstract of DOI:10.1002/ejic.200600113

This paper reports the reaction of Ti(OiPr)₄ with a series of Schiff-base ligands. The Schiff-base proligands 1a-1 are synthesised by the condensation of salicylaldehyde with a range of primary alkylamine and aniline derivatives. The treatment of 1a-f with Ti(OiPr)₄ yields the octahedral bis(aryloxyimine)Ti(OiPr)₂ complexes 2a-f. X-ray crystal structure analysis of 2c, 2d and 2e reveals complexes with a trans-aldiminato oxygen atom and cis-N,cis-alkoxide ligand arrangement about the central metal atom. The reactions of Ti(OiPr)₄ with the ligands 1g and 1h result in a sterically induced ligand rearrangement to form the octahedral complexes 2g and 2h, also characterised by X-ray diffraction experiments, in which the nitrogen atoms of the O,N-chelate are now trans-orientated at the titanium centre. ¹H NMR analysis reveals significant deshielding of the isopropoxide methine proton, induced by this coordination mode. In contrast, reactions of 1i and 1j with Ti(OiPr)₄ result in the formation and isolation of the complexes 2i and 2j. X-ray crystal structure analysis shows complex 2j to have a previously unobserved ligand orientation, in which both ligands are trans-orientated, with respect to the aldiminato oxygen atoms, about the titanium centre, but steric bulk of the ligand inhibits the bidentate coordination of both O,N-ligands. Further increase in the steric bulk of the imine substituent results in a reduced reactivity for ligands 1k and 1l, such that Ti(OiPr)₄ reacts with 1k to form the dimeric mono(Schiff-base) complex 2k (characterised by X-ray analysis). No reaction is observed for 11. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200600113

FULL PAPER
DOI: 10.1002/ejic.200600113
Synthesis, Isolation and Structural Investigation of Schiff-Base Alkoxytitanium
Complexes: Steric Limitations of Ligand Coordination
Andrew L. Johnson,*[a] Matthew G. Davidson,*[a] Matthew D. Lunn,[b] and Mary F. Mahon[c]
Keywords: Titanium / Alkoxide / Steric effects / Schiff-base ligands / Coordination
This paper reports the reaction of Ti(OiPr)4 with a series of
Schiff-base ligands. The Schiff-base proligands 1a–l are syn-
thesised by the condensation of salicylaldehyde with a range
of primary alkylamine and aniline derivatives. The treatment
of 1a–f with Ti(OiPr)4 yields the octahedral bis(aryloxy–
duced by this coordination mode. In contrast, reactions of 1i
and 1j with Ti(OiPr)4 result in the formation and isolation of
the complexes 2i and 2j. X-ray crystal structure analysis
shows complex 2j to have a previously unobserved ligand
orientation, in which both ligands are trans-orientated, with
imine)Ti(OiPr)2 complexes 2a–f. X-ray crystal structure analy- respect to the aldiminato oxygen atoms, about the titanium
sis of 2c, 2d and 2e reveals complexes with a trans-aldiminato
oxygen atom and cis-N,cis-alkoxide ligand arrangement
about the central metal atom. The reactions of Ti(OiPr)4 with
the ligands 1g and 1h result in a sterically induced ligand
rearrangement to form the octahedral complexes 2g and 2h,
also characterised by X-ray diffraction experiments, in which
the nitrogen atoms of the O,N-chelate are now trans-orien-
tated at the titanium centre. 1H NMR analysis reveals signifi-
cant deshielding of the isopropoxide methine proton, in-
centre, but steric bulk of the ligand inhibits the bidentate
coordination of both O,N-ligands. Further increase in the
steric bulk of the imine substituent results in a reduced reac-
tivity for ligands 1k and 1l, such that Ti(OiPr)4 reacts with 1k
to form the dimeric mono(Schiff-base) complex 2k (charac-
terised by X-ray analysis). No reaction is observed for 1l.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
ford metal complexes possessing cis-located “active” sites, a
prerequisite for effective olefin polymerisation. Recently, a
family of group 4 transition-metal complexes, possessing
just such a cisoid arrangement of two aryloxy–imine li-
gands, have been shown to be highly active catalysts for
olefin polymerisation.[23–30] Considerable attention has also
focused on the use of group 4 metal alkoxide complexes in
the ring-opening polymerisations of cyclic esters such as
lactic acid and ε-caprolactone.[31–35]
As part of a more general study of the chemistry of tita-
nium alkoxides, we and others have been interested in the
use of similar complexes for a range of Lewis acid mediated
reactions and polymerisations.[31,32,36,37] Herein we report
the results of a synthetic and structural investigation of the
stereochemical control possible by steric variation of li-
gands, specifically at the imine substituent.
It has been shown previously that ortho substitution of
the aryloxy–imine ligands (R position) can have a marked
effect upon the structure and fluxional properties as well
as the activity of the Schiff-base metal complexes as olefin
polymerisation catalysts.[38,39]
Previous X-ray crystallographic studies have established
that of the five possible isomeric structures of (bidentate)2-
MX2 complexes (bidentate = aryloxy–imine ligand, X =
unidentate ligand) (Figure 1) the preferred structure is a
distorted octahedron with trans-O,cis-N,cis-X ligand ar-
rangement [Figure 1(i)],[14,23–30,40,41] a feature that has also
Introduction
In recent years the development of new and increasingly
active homogeneous catalysts for polymerisations has been
one of the driving forces behind the advances in organome-
tallic chemistry, the most significant progress having been
made in the area of metallocene chemistry.[1–5] Much of this
chemistry has focused on the development of new non-Cp
ligands for alkyl d0 metal compounds, which enhance α-
olefin polymerisation, in the presence of co-catalysts, such
as methylaluminoxane (MAO). A variety of ligands have
been explored in recent years, including, for example, alk-
oxides and chelating alkoxides such as hydroxybenzalde-
hydes,[6–9,10] N2O2 Schiff-base ligands,[11–18] and bidentate
N,O ligands such as 8-quinolinolato and pyridine–alk-
oxides.[19–22] Much of this work has focused on the forma-
tion of complexes that incorporate bidentate ligands to af-
[a] Department of Chemistry, University of Bath,
Claverton Down, Bath BA2 7AY, UK
Fax: +44-1225-8262312
E-mail: a.l.johnson@bath.ac.uk
m.g.davidson@bath.ac.uk
[b] Johnson Matthey Catalysts Ltd.,
P. O. Box 1, Belasis Avenue, Billingham, Cleveland TS23 1LB,
UK
[c] Bath Chemical Crystallography Unit, Department of Chemis-
try, University of Bath,
Claverton Down, Bath BA2 7AY, UK
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Schiff-Base Alkoxytitanium Complexes
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Scheme 1. Synthesis of ligands 1a–l and Ti complexes 2a–k.
been rationalised in terms of simple VSEPR concepts.[42]
DFT calculations have also shown this conformation to be
favoured in the gas phase.[23] Complexes with a trans-N,cis-
O,cis-X ligand arrangement have also been reported.[29]
Results and Discussion
The aryloxy–imine ligands 1a–l were readily prepared by
the condensation of salicylaldehyde with commercially
available primary amines or aniline derivatives in methanol
using standard procedures (Scheme 1)[43] and isolated as
sharp-melting white to yellow crystalline solids following
recrystallisation in 70–98% yields. The range of amine and
aniline derivatives used in this study were chosen because of
their potential to impart different steric demands at metal
centres.
Synthesis and Structural Characterisation of Schiff Base
Complexes
During the course of this study clear differences in the
stereochemistry of metal complexes isolated, facilitate the
Figure 1. Depiction of the conformational isomers possible for (bi-
dentate)2MX2 complexes.
Figure 2. Structural classes of Schiff-base complexes I–IV.
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the formation of diastereoisomers.[20] Variable-temperature
1H NMR spectra for the imine CH region are shown in
Figure 3. At room temperature (23 °C) the H NMR spec-
division of their discussion into four distinct categories:
bis(η2-trans-O,O) complexes, type I; bis(η2-trans-N,N) com-
plexes, type II; bis(η2,η1-trans-O,O) complexes, type III; and
mono(η1) complexes, type IV (Figure 2). 1H NMR spectro-
scopic data and X-ray data are discussed below. 13C NMR
spectroscopic data and elemental analyses are reported in
the Experimental Section and are consistent with the struc-
tures proposed.
1
trum of 2c shows one broad resonance each for the imine
CH, the methine proton of the isopropoxide ligand, and the
methine proton of the chiral imine substituent. Cooling of
the sample to –20 °C results in resolution of each of these
resonances into two peaks of intensity ratio 1:2.1, corre-
sponding to the two diastereoisomers possible for com-
plexes of type I shown in Figure 2. {It is likely that the two
major isomers observed in solution correspond to the two
diastereoisomers of the (trans,cis,cis) isomer [Figure 1(i)] as
non-bulky ligands prefer a trans-O,cis-N conformation.}
Cooling of the sample to –30 °C results in further changes
in the relative intensities of these signals until at –80 °C the
relative intensities are found in a 1:3.6 ratio. Although the
absolute stereochemistry of the major diastereoisomer ob-
served in the solution state cannot be determined unequivo-
cally, it may be inferred from the molecular structure deter-
mination of 2c. Compound 2c crystallises in a chiral space
group as only one diastereoisomer (see below), It is there-
fore likely that this is also the thermodynamically more
stable isomer observed in solution.
Type I Complexes, Bis(η2-trans-O,O) Coordination
The reactions of Ti(OiPr)4 with 2 equiv. of 1a–f proceed
rapidly at room temperature in toluene to afford HOiPr and
the bis(ligand) complexes [{RN=C(C6H4O)}2Ti(OiPr)2]
(2a–f) in high yield (Scheme 1). The complexes 2 were iso-
lated as analytically pure crystalline solids by recrystalli-
sation from concentrated toluene solutions.
1H NMR spectra of 2a–f at room temperature contain
one single set of sharp resonances, for both Schiff-base li-
gands and isopropoxide groups. Despite the variation in
imine substituents in the complexes 2a–f, signals due to the
isopropoxide ligands do not vary significantly (i.e., δ = 4.6–
4.9 ppm for the isopropoxide methine proton). It is well
documented that six-cordinate bis(chelate) complexes of the
structure types I and II (Figure 2) can undergo inversion of
configuration at the metal centre, exhibiting rapid ∆ h Λ
isomerisation and isopropoxide rotation and/or exchange at
ambient temperatures.[44–47] The observation of only one set
of resonances for each ligand type in the 1H and 13C NMR
spectra of 2a–f is consistent with a C2-symmetric structure.
In the specific case of the complexes 2c–e, single-crystal X-
ray diffraction studies confirm the complexes to be type I
(Figure 2) C2-symmetric complexes in the solid state.
The fluctional behaviour observed for 2c at low tempera-
tures is fully reversible, and warming of the sample to 60 °C
1
shows a slight sharpening of the signals in the H NMR
spectrum relative to the room-temperature spectrum, but
no evidence for metal-based isomerisation is observed.
The molecular structures of 2c, 2d, and 2e are shown
in Figures 4, 5 and 6, respectively, and key parameters are
summarised in Tables 1 and 5. All three complexes adopt a
distorted octahedral structure of type I, with a trans-aldimin-
ato,cis-alkoxide,cis-imine ligand arrangement.
In order to probe this fluxional behaviour further, low-
The asymmetric unit of 2c contains crystallographically
1
temperature H NMR spectra of 2c, which incorporates an independent molecules, both of which possess the same chi-
enantiomerically pure chiral ligand, are particularly in- rality but which differ marginally in the relative orientations
formative, because the presence of chiral ligands leads to of the isopropoxide groups. The coordination geometry
Figure 3. Variable-temperature 1H NMR spectra of 2c showing the imine CH region of the ligand (relative integrals shown in italics
above peaks).
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Schiff-Base Alkoxytitanium Complexes
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Figure 4. Molecular structures of the two independent molecules, A and B, in complex 2c. Structures are shown with 50% displacement
ellipsoids; hydrogen atoms have been omitted for clarity.
Figure 5. Molecular structure of complex 2d. The structure is
shown with 50% displacement ellipsoids. Hydrogen atoms have
been omitted for clarity. Disorder in the aryloxy unit (O1–N1) is
not shown.
around the central titanium atom is close to octahedral.
The axial O–Ti–O bond angle is distorted away from
linearity [O(1)–Ti(1)–O(4), 161.65(8)°; O(6)–Ti(2)–O(8),
Figure 6. (a) Molecular structure of complex 2e, shown with 50%
161.95(8)°] such that there is a displacement of the aldimin-
ato ring away from the isopropoxide ligands. The Ti–OiPr
moieties exhibit bond lengths of 1.777(2) Å [Ti(1)–O(2)],
1.817(2) Å [Ti(1)–O(3)], 1.826(2) Å [Ti(2)–O(7)], and
1.778(2) Å [Ti(2)–O(9)], with O–Ti–O angles of 99.85(9)°
and 101.04(8)° about Ti(1) and Ti(2), respectively. As ex-
pected, the Ti–N(imine) bonds are considerably longer:
displacement ellipsoids. Hydrogen atoms have been omitted for
clarity. (b) Supramolecular structure of 2e highlighting the
intermolecular C–H···F interactions C(26)···F(29A) 3.550(4) Å,
C(26)···F(30A) 3.406(4) Å, C(26)–H(26)···F(29A) 154.6°, C(26)–
H(26)···F(30A) 140.1°.
Previous work has highlighted the role of the imine C–
Ti(1)–N(1) 2.347(2), Ti(1)–N(2) 2.299(2), Ti(2)–N(3) H group in promoting weak intermolecular interactions in
2.321(2), Ti(2)–N(4) 2.300(2) Å. The coordination geome- the solid-state structures of Schiff-base complexes.[11] Of the
tries in complexes 2d and 2e are similar to that observed in titanium compounds described in this work, only 2e exhib-
2c, although subtle differences between the complexes are its significant intermolecular interactions involving this hy-
apparent (Table 1).
drogen-bond donor group: pairs of monomers are associ-
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Table 1. Selected bond lengths [Å] and angles [°] for complexes 2c
(molecules A and B), 2d and 2e.
2c
2d
2e
A
B
Ti–OAr
Ti–N
1.9262(18) 1.9417(18) 1.8945(14) 1.9190(16)
1.9312(18) 1.9181(18) 1.9042(13) 1.9327(18)
2.299(2)
2.347(2)
2.300(2)
2.321(2)
2.350(16)
2.3278(16)
2.328(2)
2.340(2)
Ti–OR
1.7767(19) 1.7778(18) 1.8126(14) 1.7791(19)
1.8170(18) 1.8257(18) 1.8186(14) 1.7979(17)
ArO–Ti–OAr 161.65(8)
161.95(8)
84.55(8)
81.14(7)
101.04(8)
165.35(7)
75.98(5)
81.06(6)
104.26(7)
156.71(9)
82.81(7)
79.95(7)
101.96(9)
N–Ti–N
83.72(8)
81.02(7)
99.85(9)
O–Ti–N
RO–Ti–OR
ated through mutual C–H···F interactions as shown in Fig-
ure 6(b). In related complexes, in which the imine group
is an ortho-F-substituted phenyl ring, C–H···F interactions
between the imine substituent and the β-H atoms of the
growing polymer chain have been cited as the reason for
the remarkable stability and catalytic behaviour of such
complexes.[48,49]
Figure 7. Molecular structure of 2h. Ellipsoids are shown with 50%
displacement probability. Hydrogen atoms have been omitted for
clarity. Equivalent atoms (*A) are generated by the symmetry
transformations –x+1, y, –z+1/2, and –x+1/2, –y+1/2, –z+1.
Type II Complexes, Bis(η2-trans-N,N) Coordination
Upon increasing the steric bulk of the imine substituent,
beyond that of o-methoxyaniline, significant changes in the
1H NMR spectra are observed, such that the isopropoxide
methine resonances shift from δ = 4.6–4.9 ppm in 2a–f to δ
= 3.4 and 3.7 ppm in 2g and 2h, respectively. We believe
this change is indicative of a transformation from a type I
to a type II structure. As with complexes 2a–f, NMR spec-
tra of 2g and 2h are consistent with C2-symmetric com-
plexes. A single-crystal X-ray structure of 2h confirms the
type II structure, in which the bidentate ligands adopt a
mutually trans-N,cis-O(Ar),cis-O(R) arrangement, as
shown in Figure 7; although there are no significant differ-
ences in Ti–N and Ti–O(Ar) bond lengths in 2h relative to
type I complexes 2c–e (Tables 1 and 2), there is movement
of the axial nitrogen atoms away from the isopropoxide li-
gands such that the N–Ti–N angle in 2h is 164.61(8)°. As
in the case of 2c–e, the geometry of the aryloxy–imine li-
gands forces the chelate angles around the Ti atom to devi-
ate from the ideal octahedral geometry, with a “bite” angle
[N(1)–Ti(1)–O(1)] of 82.33(10)°, which is comparable to the
average “bite” angles found in 2c–e [81.02(7)° (2cA),
81.14(7)° (2cB), 81.06(6)° (2d) and 79.95(7)° (2e)]. Erker et
al. have previously observed this unusual configuration in
Schiff-base Ti and Zr chloride complexes of the ligand
1h.[29]
Table 2. Selected bond lengths [Å] and angles [°] for complex 2h.
Ti(1)–O(1)
Ti(1)–O(2)
Ti(1)–N(1)
N(1)–Ti(1)–N(1*)
1.975(1)
1.805(1)
2.228(1)
164.61(8)
O(1)–Ti(1)–O(1*)
O(1)–Ti(1)–O(2)
O(2)–Ti(1)–O(2*)
O(1)–Ti(1)–N(1)
85.30(8)
171.27(6)
95.87(8)
82.33(6)
caused by reorientation of the ligands. This inference is sup-
ported by the solid-state structures of type I and type II
complexes, in which the isopropoxide methine proton is di-
rected towards an aryl ring in type II complexes but not in
type I. These orientations are apparent from Figures 5 and
7, type I and II complexes, respectively. Overall, the steric
environment surrounding the metal centre is altered signifi-
cantly because of the reorientation of the Schiff-base li-
gands as highlighted by the space-filling diagrams shown in
Figure 8. It would be surprising if these changes did not
influence the reactivity of the metal complexes, and it is
interesting to note that a recent report by Matsui et al. sug-
gests that very bulky imine substituents inhibit the activity
of Schiff-base Zr complexes as polyolefin precatalysts.[23]
Type III Complexes, Bis(η2η1-trans-O,O) Coordination
1
The significant shift observed in the H NMR spectra of
An additional increase in the steric bulk of the imine
the isopropoxide ligands on changing from type I to type II substituents (ligands 1i and 1j) leads to the formation of
complexes is not reflected in the Ti–O(isopropoxide) bond complexes 2i and 2j (Scheme 1). Characterisation of 2i and
1
lengths, which remain similar in both type I and II com- 2j by H NMR spectroscopy indicates the presence of the
plexes (see Tables 1 and 2). This suggests that the deshield- same ligand set as for 2a–h. However, for both complexes,
ing observed for type II complexes (2g and 2h) is due to the the isopropoxide methine proton is only slightly deshielded
proximity of the methine proton to an N-aryl group of the in comparison to the type I complexes 2a–f implying that
Schiff-base ligand rather than due to a differing trans effect the isopropoxide group no longer experiences the deshield-
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Schiff-Base Alkoxytitanium Complexes
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Figure 8. Complexes 2d [(a) and (c)] and 2h [(b) and (d)] highlighting orientation of the chelating ligands relative to the isopropoxide
methine protons [(a) and (b)] and the difference in the steric environment of the active site in type I and type II complexes [isopropoxide
groups have been omitted in (c) and (d) for clarity].
Figure 9. Molecular structure of complex 2j, shown with 50% displacement ellipsoids; hydrogen atoms have been omitted for clarity.
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ing effects observed in 2g and 2h, and that a further struc-
In the solid state, 2k has a dimeric structure (Figure 11,
tural change has taken place.
Table 4) consisting of two terminal alkoxide and two bridg-
X-ray structural analysis of complex 2j (Figure 9) reveals ing µ-OiPr groups, as well as one monodentate aryloxide
the nature of this change and represents a previously ligand coordinated to each Ti atom. This is structurally sim-
uncharacterised structure for disubstituted Schiff-base tita- ilar to the previously reported (alkoxide)(phenoxide)-
nium or zirconium complexes. Complex 2j exhibits a five- titanium
complex
[Ti2(O-2,4,6-Me3C6H2)2(OiPr)4(µ-
coordinate titanium centre, with a distorted square-pyrami- OiPr)2].[18,50] Each titanium centre in 2k possesses a trigo-
dal geometry, supporting two alkoxide ligands and two nal-bipyramidal geometry, with one µ2-OiPr group in an
aryloxide ligands, each exhibiting different bonding modes. axial and one in an equatorial site (Figure 12). The terminal
One ligand forms a chelate to the metal centre in the ex- alkoxy–imine ligand occupies an equatorial site, and within
pected η2 fashion, while the second binds in η1 mode solely
through the aryloxide group. The 2,4,6-triphenylphenyl
group of this monodentate ligand is orientated such that it
is remote from the metal centre, thus relieving steric conges-
tion. Selected bond lengths and angles for 2j are given in
Table 3. Complex 2j may be viewed as a structural model
for the proposed intermediate in the ∆ h Λ enantiomeris-
ation process of octahedral (AB)2MX2 complexes. The en-
antiomerisation process is reasoned to proceed via a five-
coordinate intermediate formed by the rupture of a metal–
ligand bond as shown in Figure 10.[19–21,44–47]
Table 3. Selected bond lengths [Å] and angles [°] for complex 2j.
Ti–O(1)
Ti–O(2)
Ti–O(3)
Ti–O(4)
Ti–N(2)
C(7)–N(1)
C(46)–N(2)
1.862(2)
1.942(2)
1.791(2)
1.774(2)
2.215(2)
1.276(3)
1.441(3)
O(2)–Ti–O(1)
O(3)–Ti–O(4)
O(3)–Ti–N(2)
O(4)–Ti–N(2)
O(2)–Ti–N(2)
164.72(8)
112.11(9)
140.21(8)
106.25(8)
80.27(7)
Figure 11. Molecular structure of complex 2k, shown with 50%
displacement ellipsoids; hydrogen atoms have been omitted for clar-
ity.
Figure 10. Proposed mechanism of racemisation for interchange of
∆ and Λ forms of the bis(aryloxy–imine)titanium complexes. Com-
plexes ∆Ј and ΛЈ represent the stable five-coordinate species, such
as 2j.[44–47]
Table 4. Selected bond lengths [Å] and angles [°] for complex 2k.
Ti(1)–Ti(2)
3.263(2)
Ti(1)–O(4)–Ti(2)
Ti(1)–O(5)–Ti(2)
O(4)–Ti(1)–O(5)
O(4)–Ti(2)–O(5)
107.05(9)
107.63(9)
72.30(8)
72.56(8)
Ti(1)–O(1)
Ti(1)–O(2)
Ti(1)–O(3)
Ti(1)–O(4)
Ti(1)–O(5)
C(7)–N(1)
1.882(2)
1.762(2)
1.778(2)
2.120(2)
1.929(2)
1.270(4)
Type IV Complexes, Mono(η1) Coordination
O(2)–Ti(1)–O(4)
O(1)–Ti(1)–O(5)
O(1)–Ti(1)–O(3)
O(1)–Ti(1)–O(5)
167.09(10)
125.83(9)
111.71(10)
125.83(9)
Further increases in the steric bulk of the imine substitu-
ent (ligand 1k, Scheme 1) inhibits formation of disubsti-
tuted complexes analogous to 2a–j, and results in only
monosubstitution of Ti(OiPr)4 to yield 2k, even in the pres-
ence of excess 1k and in refluxing toluene. Under similar
reaction conditions, 1l is recovered intact and no evidence
for even monosubstitution is observed.
Ti(2)–O(8)
Ti(2)–O(6)
Ti(2)–O(7)
Ti(2)–O(4)
Ti(2)–O(5)
C(87)–N(2)
1.883(2)
1.788(2)
1.766(2)
1.924(2)
2.112(2)
1.272(4)
O(5)–Ti(2)–O(7)
O(4)–Ti(2)–O(6)
O(4)–Ti(2)–O(8)
O(6)–Ti(2)–O(8)
170.29(9)
115.90(10)
122.29(9)
174.69(10)
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Schiff-Base Alkoxytitanium Complexes
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were purified by conventional procedures and distilled prior to use.
Ti(OiPr)4 was purchased from Aldrich and used as received, with-
out further purification. The ligands 1a–m were prepared using the
appropriate amine and salicylaldehyde, all purchased from Aldrich,
using standard literature techniques.[51] Solution 1H, and 13C NMR
experiments were performed at ambient temperature with a Bruker
Avance-300. 1H NMR spectroscopic data are referenced to residual
non-deuterated solvent.
the dimer both ligands are arranged trans to each other
such that the molecule has crystallographic Ci symmetry.
The bridging alkoxide ligands do so unsymmetrically, with
the equatorial Ti–O bonds [Ti(1)–O(5) 1.929(2) Å, Ti(2)–
O(4) 1.924(2) Å] being shorter than the axial [Ti(1)–O(4)
2.120(2) Å, Ti(2)–O(5) 2.112(2) Å] ones. Other bond lengths
and angles found in the structure of 2k are as expected.
Syntheses of Complexes
[Ti(OiPr)2{η2-OC6H4C(H)NtBu}2] (2a): To a stirred solution of 1a
(0.36 g, 2 mmol) in 20 mL of toluene was added Ti(OiPr)4 (0.3 mL,
1 mmol) dropwise by syringe, at 0 °C. The mixture was heated at
reflux for 2 h. The solution was cooled to room temperature before
removal of solvent under reduced pressure. The yellow residue was
dissolved in a minimum of fresh toluene (6 mL), warmed to ca.
40 °C, and filtered through a Celite pad into a Schlenk flask. The
filtrate was layered with 5 mL of dry pentane and allowed to stand
at 4 °C for 2 days after which the solid microcrystalline product
was collected by filtration, washed with 5 mL of cold hexane and
dried in vacuo. Yield: 0.36 g (70%). C28H42N2O4Ti (518.3): calcd.
1
C 64.86, H 8.10, N 5.40; found C 65.1, H 8.17, N 5.41. H NMR
(300 MHz, 23 °C, CDCl3): δ = 1.17 [br. s, 12 H, OCH(CH3)2], 1.36
3
[s, 18 H, NC(CH3)3], 4.62 [sept, JHH = 6 Hz, OCH(CH3)2, 2 H],
Figure 12. Structure of 2k, showing the distorted trigonal-bipyram-
idal geometry about the two titanium centres, with 50% displace-
ment ellipsoids; hydrogen atoms, methyl groups and the imine sub-
stituents have been omitted for clarity.
6.6–6.85 (br. m, 4 H, CHarom), 7.18–7.26 (br. m, 2 H, CHarom),
7.27–7.45 (br. m, 2 H, CHarom), 8.34 [s, 2 H, C(H)=N] ppm. 13C
NMR (75.5 MHz, 23 °C, CDCl3): δ = 25.8, 30.9, 61.4, 77.6, 118.5,
119.3, 123.8, 132.5, 133.2, 160.1, 166.5 ppm.
Complexes 2b–2k were prepared in a manner analogous to that of
2a.
Conclusions
[Ti(OiPr)2{η2-OC6H4C(H)N(C10H15)}2] (2b): Yield: 0.38 g (57%).
C40H54N2O4Ti (674.4): calcd. C 71.20, H 8.07, N 4.15; found C
71.60, H 8.01, N 4.09. 1H NMR (300 MHz, 23 °C, CDCl3): δ =
1.22 [br . s, 12 H, OCH(CH3)2], 1.53–2.09 (br. m, 30 H, NC10H15),
4.69 [br. m, 2 H, OCH(CH3)2], 6.67–6.82 (br. m, 4 H, CHarom),
7.14–7.28 (br. m, 2 H, CHarom), 7.32–7.47 (br. m, 2 H, CHarom),
8.32 [s, 2 H, C(H)=N] ppm. 13C NMR (75.5 MHz, 23 °C, CDCl3):
δ = 25.9, 30.2, 36.7, 43.3, 78.4, 117.8, 118.7, 119.4, 131.7, 132.5,
160.4, 164.9 ppm.
We have synthesised, isolated and structurally character-
ised Schiff-base complexes of titanium with the previously
observed C2-symmetric type I (2a–f) and type II (2g and
2h) structures. Type II complexes are characterised in their
1H NMR spectra by deshielding of the isopropoxide meth-
ine protons which highlights a significant change in the ste-
ric characteristics of the metal centre. A further increase in
the steric demands of the imine substituent results in the
isolation of complexes with the hitherto unknown five-co-
ordinate type III structure (2i and 2j), which can be viewed
as a structural model for the proposed intermediate in ∆
h Λ racemisation of C2-symmetric complexes of this type.
Increasing the steric bulk of the ligand still further induces
a decrease in reactivity of the ligands towards Ti(OiPr)4.
Thus, ligand 1k substitutes only one alkoxide to form the
bridged alkoxide complex 2k, and 1l is unreactive towards
Ti(OiPr)4.
The ability to control molecular structure and particu-
larly the environment about Lewis acidic metal centres, by
steric variation at a ligands periphery, described here,
clearly shows that, although somewhat distant from the
metal centre, the choice of imine substituent is critical to
the ligand coordination geometry about the central metal,
and points to possible the rational design of metallo-or-
ganic complexes for use in catalysis.
[Ti(OiPr)2{η2-OC6H4C(H)N[(R)-CHMePh]}2] (2c): Yield: 0.44 g
(72%). C36H42N2O4Ti (614.3): calcd. C 70.36, H 6.84, N 4.56;
found C 69.60, H 6.84, N 4.42. 1H NMR (300 MHz, 23 °C,
CDCl3): δ = 1.02 [br. s, 12 H, OCH(CH3)2], 1.45 [br. s, 6 H,
CH(CH3)Ph], 4.67 [sept, OCH(CH3)2, 3JHH = 6 Hz, 2 H], 5.29 (br.
m, 2 H, CHMePh) 6.58–6.66 (br. m, 2 H, CHarom), 6.76–6.82 (br.
3
4
m, 2 H, CHarom), 6.97 (dd, JHH = 7.5 Hz, JHH = 1.5 Hz, 2 H,
CHarom), 7.06–7.15 (br. m, 10 H, CHarom), 7.23 (ddd, 3JHH = 8 Hz,
3JHH = 7.2 Hz, 4JHH = 2.1 Hz, 2 H, CHarom), 7.88 [s, 2 H, C(H)=N]
ppm. 13C NMR (75.5 MHz, 23 °C, CDCl3): δ = 21.6, 25.7, 62.5,
78.6, 117.5, 119.4, 123.0, 126.9, 127.6, 128.8, 128.9, 134.18, 134.52,
163.7, 164.7 ppm.
[Ti(OiPr)2{η2-OC6H4C(H)NC6H5}2] (2d): Yield: 0.38 g (68%).
C32H34N2O4Ti (558.2): calcd. C 68.80, H 6.09, N 5.01; found C
68.50, H 6.13, N 4.98. 1H NMR (300 MHz, 23 °C, CDCl3): δ =
3
1.23 [br. s, 12 H, OCH(CH3)2], 4.9 [sept, JHH = 6 Hz, 2 H,
OCH(CH3)2], 6.11–6.17 (m, 2 H, CHarom), 6.49 (ddd, 3JHH = 9 Hz,
4
3JHH = 7.5 Hz, JHH = 1.2 Hz, 2 H, CHarom), 6.78–7.04 (m, 14 H,
CHarom), 7.87 [s, 2 H, C(H)=N] ppm. 13C NMR (75.5 MHz, 23 °C,
CDCl3): δ = 25.9, 79.0, 117.8, 119.8, 123.1, 126.2, 128.4, 134.3,
135.2, 154.3, 163.7, 166.3 ppm.
Experimental Section
General Remarks: All manipulations were carried out under dry
[Ti(OiPr)2{η2-OC6H4C(H)NC6F5}2] (2e): Yield: 0.6 g (81%).
argon using standard Schlenk and glove-box techniques. Solvents
C32H24F10N2O4Ti (738.1): calcd. C 52.03, H 3.25, N 3.79; found C
Eur. J. Inorg. Chem. 2006, 3088–3098
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3095

A. L. Johnson, M. G. Davidson, M. D. Lunn, M. F. Mahon
FULL PAPER
51.8, H 3.28, N 3.78. 1H NMR (400 MHz, 23 °C, CDCl3): δ = 1.11 found C 66.50, H 6.09, N 4.45. 1H NMR (300 MHz, 23 °C,
3
[br. s, 12 H, OCH(CH3)2], 4.77 [sept, JHH = 5.8 Hz, 2 H,
CDCl3): δ = 1.19 [br. s, 12 H, OCH(CH3)2], 3.38 (s, 6 H, OCH3),
3
OCH(CH3)2], 6.45–6.47 (m, 2 H, CHarom), 6.74–6.78 (m, 2 H, 4.88 [sept, JHH = 6 Hz, 2 H, OCH(CH3)2], 5.99–6.08 (m, 2 H,
CHarom), 7.20 (dd, 3JHH = 8 Hz, 4JHH = 1.6 Hz, 2 H, CHarom), 7.34 CHarom), 6.20–6.28 (m, 2 H, CHarom), 6.36–6.47 (m, 2 H, CHarom),
3
3
4
(ddd, JHH = 8.4 Hz, JHH = 6.8 Hz, JHH = 2 Hz, 2 H, CHarom), 6.62–6.97 (m, 8 H, CHarom), 7.43–7.56 (m, 2 H, CHarom), 7.89 [s, 2
8.07 [s, 2 H, C(H)=N] ppm. 13C NMR (100 MHz, 23 °C, CDCl3): H, C(H)=N] ppm. 13C NMR (75.5 MHz, 23 °C, CDCl3): δ = 26.8,
δ = 25.5, 80.7, 118.0, 118.9, 121.2, 135.0, 136.5, 156.5, 164.9,
171.8 ppm. 19F NMR (376 MHz, 23 °C, CDCl3): δ = –148.3 (m),
–160.1 (t, J = 21 Hz), –164.0 (m) ppm.
[Ti(OiPr)2{η2-OC6H4C(H)NC6H4-o-OCH3}2] (2f): Yield: 0.46 g (58%). C38H46N2O4Ti (642.3): calcd. C 71.03, H 7.17, N 4.36;
56.0, 79.5, 111.7, 117.7, 119.8, 120.6, 123.8, 126.9, 127.8, 134.5,
135.2, 143.6, 151.6, 136.9, 169.7 ppm.
[Ti(OiPr)2{η2-OC6H4C(H)NC6(CH3)3H2}2] (2g): Yield: 0.37 g
(74%). C34H38N2O6Ti (618.2): calcd. C 66.02, H 6.15, N 4.53;
found C 71.30, H 7.15, N 4.32. 1H NMR (300 MHz, 23 °C,
Table 5. Crystal data and structure refinement for compounds 2c, 2d, 2e, 2h, 2j and 2k.
Compound
2c
2d
2e
Empirical formula
Formula mass
T [K]
Crystal system
Space group
C36H42N2O4Ti
614.62
150(2)
monoclinic
P21
C32H34N2O4Ti
558.51
150(2)
monoclinic
P21/a
C32H24F10N2O4Ti
738.43
396(2)
monoclinic
P21/c
a [Å]
b [Å]
c [Å]
18.1430(3)
10.1730(2)
18.3320(4)
106.635(1)
3241.91(11)
4
16.7770(1)
9.9880(2)
17.2620(3)
90.143(1)
2892.56(8)
4
14.3970(2)
20.1450(4)
21.5010(4)
149.477(1)
3167.10(10)
4
β [°]
V [Å3]
Z
Dc [g cm–3]
1.259
0.305
1.283
0.334
1.549
0.370
µ [mm–1]
F(000)
Crystal size [mm]
θ range [°]
1304
1176
1496
0.28×0.08×0.08
3.52–26.38
57745
13136 [0.0516]
10091
13136/1/788
0.994
0.0423, 0.1024
0.0670, 0.1161
0.523/–0.308
–0.017(16)
0.22×0.15×0.10
3.54–27.48
41742
6582 [0.0380]
5305
6582/24/411
1.030
0.0470, 0.1218
0.0615, 0.1304
1.236/–0.756
–
0.25×0.17×0.15
3.60–27.49
10943
6613 [0.0326]
4509
6613/0/446
1.011
0.0466, 0.1121
0.0814, 0.1304
0.602/–0.525
–
Reflections collected
Independent reflections [R(int)]
Reflections [I Ͼ 2σ(I)]
Data/restraints/parameters
Goodness-of-fit on F2
Final R1, wR2 [I Ͼ 2σ(I)]
Final R1, wR2 (all data)
Max/min difference [e·Å–3]
Absolute structure parameter
Compound
2h
2j
2k
Empirical formula
Formula mass
T [K]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C51H65N2O4Ti
817.95
150(2)
monoclinic
C2/c
23.1450(4)
17.0660(4)
14.5820(4)
C75H66N2O4Ti
1107.20
150(2)
orthorhombic
Pbca
18.2330(2)
24.8400(2)
26.3420(3)
C75H118N2O8Ti2
1271.51
150(2) K
triclinic
¯
P1
15.3200(3)
15.6480(3)
18.1500(4)
75.569(1)
65.055(1)
85.119(1)
3819.69(13)
2
126.617(1)
γ [°]
V [Å3]
4623.03(18)
4
1.175
11930.5(2)
8
1.233
Z
Dc [g cm–3]
1.106
0.259
µ [mm–1]
0.230
0.197
F(000)
Crystal size [mm]
θ range [°]
1756
4672
1380
0.25×0.17×0.15
3.63–27.46
39356
5270 [0.1181]
3287
5270/0/275
1.027
0.0511, 0.1131
0.1035, 0.1332
0.346/–0.356
0.25×0.15×0.08
3.54–27.49.
26279
13649 [0.0620]
7735
13649/0/768
0.969
0.0552, 0.1351
0.1209, 0.1651
0.648/–0.482
0.175×0.175×0.075
3.91–24.98
37007
13339 [0.0752]
8557
13339/24/868
0.996
0.0556, 0.1355
0.1004, 0.1637
0.746/–0.373
Reflections collected
Independent reflections [R(int)]
Reflections [I Ͼ 2σ(I)]
Data/restraints/parameters
Goodness-of-fit on F2
Final R1, wR2 [I Ͼ 2σ(I)]
Final R1, wR2 (all data)
Max/min difference [e·Å–3]
3096
www.eurjic.org
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 3088–3098

Schiff-Base Alkoxytitanium Complexes
FULL PAPER
of one half of a molecule with the central titanium atom residing
3
CDCl3): δ = 0.38 [d, JHH = 12 Hz, 12 H, OCH(CH3)2], 2.21 (s, 3
3
H, p-CH3) 2.38 (s, 6 H, o-CH3), 3.45 [sept, JHH = 9 Hz, 2 H, on a twofold rotation axis, in addition to half of a toluene molecule
3
3
4
OCH(CH3)2], 6.52 (ddd, JHH = 7.6 Hz, JHH = 6.8 Hz, JHH
=
proximate to a crystallographic inversion centre. The methyl group
0.9 Hz, 2 H, CHarom) 6.59–6.62 (m, 2 H, CHarom), 6.83 (s, 4 H, in the solvent was disordered in a 1:1 ratio by virtue of straddling
3
4
CHarom), 7.08 (dd, JHH = 7.8 Hz, JHH = 1.8 Hz, 2 H, CHarom
)
this inversion centre. One full toluene molecule was also seen to be
present in the structure of 2j. In addition, one of the isopropyl
groups exhibited disorder whereby C(90)–(92) were disordered in a
1:1 ratio with C(90Ј)–(92Ј). Similarly, the asymmetric unit in 2k
was also observed to contain a solvent molecule (toluene) evenly
disordered over two sites. In particular, C(101)–(106) exhibited 1:1
disorder with C(201)–C(206). Both partial rings were refined as
rigid hexagons. Disordered toluene methyl groups were not in-
cluded in the model. Moreover, one tert-butyl substituent was also
found to be disordered in a 70:30 ratio, whereby C(95A)–(96C)
were refined at 70% occupancy and C(95D)–(95F) were refined at
30% occupancy. CCDC-239547 to -239552 contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
3
3
4
7.22 (ddd, JHH = 8.4 Hz, JHH = 7.2 Hz, JHH = 1.8 Hz, 2 H,
CHarom), 7.88 [s, 2 H, C(H)=N] ppm. 13C NMR (75.5 MHz, 23 °C,
CDCl3): δ = 19.8, 21.1, 24.9, 77.7, 115.7, 119.9, 121.7, 129.0, 131.5,
134.6, 135.4, 135.7, 151.7, 167.8, 169.2 ppm.
[Ti(OiPr)2{η2-OC6H4C(H)NC6[CH(CH3)2]2H3}2] (2h): Yield: 0.6 g
(83%). C44H58N2O4Ti (726.4): calcd. C 72.73, H 7.99, N 3.86;
found C 72.30, H 8.01, N 3.76. 1H NMR (300 MHz, 23 °C,
CDCl3): δ = 0.51 [br. s, 12 H, OCH(CH3)2], 1.25 [br. s, 24 H,
CCCH(CH3)2], 3.77 [sept, JHH = 7 Hz, 2 H, OCH(CH3)2], 3.87
[sept, JHH = 9.2 Hz, 2 H, CCH(CH3)2], 6.62–6.65 (m, 4 H,
CHarom), 7.19–7.27 (m, 8 H, CHarom), 7.35–7.39 (m, 2 H, CHarom),
8.05 [s, 2 H, C(H)=N] ppm. 13C NMR (75.5 MHz, 23 °C, CDCl3):
δ = 25.29, 27.46, 27.48, 77.8, 115.61, 120.0, 124.17, 124.17, 126.92,
134.9, 136.1, 142.2, 152.2, 167.5, 169 ppm.
3
3
[Ti(OiPr)2{η2-OC6H4C(H)NCH(Ph)2}2] (2i): Yield: 0.49 g (66%).
C46H46N2O4Ti (738.3): calcd. C 74.79, H 6.23, N 3.79; found C
74.70, H 6.20, N 3.78. 1H NMR (300 MHz, 23 °C, CDCl3): δ = 0.92
[d, JHH = 6.4 Hz, 12 H, OCH(CH3)2], 4.5 [sept, JHH = 8.9 Hz, 2
H, OCH(CH3)2], 6.57–6.60 (m, 2 H, CHarom), 6.72–6.73 (m, 2 H,
CHarom), 6.82–6.84 (m, 4 H, CHarom), 7.02–7.27 (m, 20 H, CHarom),
7.79 [s, 2 H, C(H)=N] ppm. 13C NMR (75.5 MHz, 23 °C, CDCl3):
δ = 24.3, 70.2, 77.8, 115.9, 118.2, 121.3, 125.8, 126.9, 127.7, 128.1,
133.1, 139.9, 162.6, 166.3 ppm.
Acknowledgments
3
3
We thank Johnson Matthey Catalysts Ltd. (A. L. J., M. G. D.,
M. D. L.) and the the EPSRC (M. G. D.) for support.
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[Ti(OiPr)2{η2-OC6H4C(H)N[C6H2(Ph)3]}{η1-OC6H4C(H)NC6H2-
(Ph)3}] (2j): Yield: 0.59 g (58%). C68H58N2O4Ti (1014.4): calcd. C
80.47, H 5.71, N 2.76; found C 80.40, H 5.73, N 2.73. 1H NMR
3
(300 MHz, 23 °C, CDCl3): δ = 0.69 [d, JHH = 6.4 Hz, 12 H,
3
OCH(CH3)2], 4.14 [sept, JHH = 9 Hz, 2 H, OCH(CH3)2], 6.37–
7.50 (m, 42 H, CHarom), 8.29 [s, 2 H, C(H)=N] ppm. 13C NMR
(75.5 MHz, 23 °C, CDCl3): δ = 25.6 80.6, 119.1, 120.2, 125.7, 127.2,
127.4, 127.5, 127.7, 128.3, 128.5, 129.3, 130.3, 130.5, 135.3, 138.3,
140.1, 141.1, 150.2, 164.3, 165.9 ppm.
[Ti(OiPr)3η1-OC6H4C(H)N{C6H2(tBu)3}]2 (2k): Yield: 0.48 g
(82%, based on Ti). C68H110N2O8Ti2 (1178.7): calcd. C 69.27, H
1
9.34, N 2.38; found C 69.02, H 9.18, N 2.39. H NMR (300 MHz,
3
23 °C, CDCl3): δ = 0.99 [d, JHH = 9 Hz, 36 H, OCH(CH3)2], 1.24
3
[s, 18 H, C(CH3)3], 1.27 [s, 36 H, C(CH3)3], 4.43 [sept, JHH
=
6.3 Hz, 6 H, OCH(CH3)2], 6.8–6.94 (m, 4 H, CHarom), 7.11–7.38
(m, 4 H, CHarom), 7.96–8.14 (m, 4 H, CHarom), 8.56 [s, 2 H,
C(H)=N] ppm. 13C NMR (75.5 MHz, 23 °C, CDCl3): δ = 26.6,
31.9, 32.5, 35.0, 36.4, 77.6, 120.9, 121.9, 122.5, 127.8, 132.6, 138.9,
141.2, 151.9, 158.4, 165.1, 168.2 ppm.
X-ray Crystallographic Study: Crystallographic data for com-
pounds 2c, 2d, 2e, 2h, 2j and 2k are summarised in Table 5. All
data collections were carried out with a Nonius KappaCCD dif-
fractometer. Structure solution and refinement were performed
using SHELX86[52] and SHELX97[53] software, respectively. Exper-
imental data relating to all structure determinations are summa-
rised in Table 5. Full-matrix anisotropic refinement was im-
plemented in the final least-squares cycles throughout. All data
were corrected for Lorentz and polarisation and, with the excep-
tions of 2d, 2e and 2j, for extinction effects. Hydrogen atoms were
included at calculated positions throughout. In 2d, one of the
phenyl groups was seen to exhibit positional disorder. In particular,
C(1)–C(6) were found to be disordered in a 1:1 ratio with C(1A)–
C(6A). All carbon–carbon distances therein were refined subject to
rigid-bond restraints. In complex 2h, the asymmetric unit consisted
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Published Online: June 7, 2006
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