Crystallographic characterisation of Ti(iv) piperazine complexes and their exploitation for the ring opening polymerisation of rac-lactide

Article abstract of DOI:10.1039/c0dt01542c

In this paper a series of eight Ti(iv) piperazine based complexes have been prepared and fully characterised in the solid-state by X-ray crystallography and in solution via NMR spectroscopy. In the solid-state either Ti ₂(L)(OⁱPr)

Full text of DOI:10.1039/c0dt01542c

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PAPER
Crystallographic characterisation of Ti(IV) piperazine complexes and their
exploitation for the ring opening polymerisation of rac-lactide
Stuart L. Hancock, Mary F. Mahon and Matthew D. Jones*
Received 5th November 2010, Accepted 21st December 2010
DOI: 10.1039/c0dt01542c
In this paper a series of eight Ti(IV) piperazine based complexes have been prepared and fully
characterised in the solid-state by X-ray crystallography and in solution via NMR spectroscopy. In the
solid-state either Ti2(L)(OiPr)6 or Ti2(L)2(OiPr)4 were observed depending upon the nature of the
starting ligand. For complexes with less sterically demanding ligands (1H2 and 2H2) an equilibrium was
observed: 2 Ti2(L)(OiPr)6 ¤ Ti2(L)2(OiPr)4 + 2 Ti(OiPr)4. The thermodynamic properties (DG, DH and
DS) have been investigated via variable temperature NMR spectroscopy. With more sterically
demanding ligands (3–8H2) the Ti2(L)(OiPr)6 form was the most prevalent in the solid-state and in
solution. These complexes have been tested for the production of polylactide under melt and solution
conditions with high conversions being obtained.
bimetallic structures have been prepared.13 To the best of our
Introduction
knowledge there are no reported examples of crystallographi-
The preparation of homogeneous Lewis acidic complexes for
cally characterised piperazine-phenolate complexes with Ti(IV),
the ring opening polymerisation (ROP) of cyclic esters (such as
although the analogous 1,4-bis(2-aminobenzyl)piperazine ligand
rac-lactide) has received considerable attention in recent years.1
Examples of initiators for this process include groups 1–3,2
Al(III),3 In(III),4 Zn(II),2b,5 lanthanides6 and pertinent to this study
group 4 metal centres.7 The polymers themselves have found
many uses from commodity plastics to high value biomedical
applications.8 The use of amine bis(phenolate) ligands in such
chemistry is ubiquitous and there are numerous examples of
such initiators in the literature.3c,7a,7d–7f However, the use of
the more conformationally strained piperazine derived ligands
for this polymerisation remains limited. A notable example of
work in this area is that by Yao and co-workers who have
recently prepared a Yb–Li bimetallic piperazidine complex.9
This was shown to act as a promising initiator for the ROP
of L-lactide. The same group have also very recently prepared
lanthanide complexes for L- and rac-lactide polymerisation based
on piperazine ligands.10 It has been found that N-substituted
piperazine complexes are very versatile and can bind to either one
or two metal centres.9 Previous crystallographically characterised
piperazine-phenolate complexes include Al(III),11 Pd(II),12 Zn(II)13
and Cu(II).14 For example, a series of Al(III) complexes with
1,4-bis(2-hydroxy-3,5-di-tert-butyl)piperazine have been prepared
and in this case either monometallic or bimetallic complexes
were formed in the solid-state.11 With the same ligand Zn(II)
has been used by Mountford to prepare Ti(IV) imido complexes.15
Also to the best of our knowledge there are no crystallographically
characterised complexes of the methyl substituted piperazine ring
systems. Crystallographically characterised complexes with the
homopiperazine ligand (7 membered ring) remain limited to
Fe(III),16 Cu(II),17 Ni(II)17 and one example of a Ti(IV) oxo complex
has been previously published.18
In this paper we report the preparation and characterisation of
new piperazine and homopiperazine ligands. These ligands were
complexed to Ti(IV) and either complexes of the form Ti2(L)(OiPr)6
or Ti2(L)2(OiPr)4 have been isolated in the solid-state. Interestingly,
in some cases a complex equilibrium was observed in solution
between these two dimers and Ti(OiPr)4. The complexes have been
tested for the ROP of rac-lactide with high conversions.
Results and discussion
Synthesis of ligands and complexes
The ligands were prepared via a modified Mannich reaction
1
as shown in Scheme 1.11 All ligands were characterised via H
1
and 13C{ H} NMR spectroscopy and HR-MS, 1H2 was also
characterised using single crystal X-ray diffraction. The complexes
were prepared by the reaction of 1 equivalent of ligand with 2
equivalents of Ti(OiPr)4 in CH2Cl2.7a,7d,7e It was observed that
the same products were also isolated in the solid-state with 1
equivalent of Ti(OiPr)4. For complexes with ligands 1H2 and 2H2
the solid-state structures are shown in Fig. 1 and selected bond
lengths and angles are in Table 1.
Department of Chemistry, University of Bath, Claverton Down, Bath, BA2
7AY, U.K. E-mail: mj205@bath.ac.uk; Fax: +44 (0)1225 386231; Tel: +44
(0)1225 384908
† Electronic supplementary information (ESI) available: Full characteri-
sation data and selected NMR data. CCDC reference numbers 800217–
800225. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0dt01542c
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◦
i
i
˚
Table 1 Selected bond lengths (A) and bond angles ( ) for complexes Ti2(1)2(O Pr)4 and Ti2(2–8)(O Pr)6
1
2
3
4
5
6
7
8
Ti1–O1
Ti1–O2
Ti1–O3
Ti1–O4
Ti1–N1
Ti1–N3
O1–Ti1–N1
O4–Ti1–N1
O1–Ti1–N3
O4–Ti1–N3
O1–Ti1–O2
1.859(2)
1.857(2)
1.817(2)
1.775(2)
—
2.353(3)
—
1.7813(18)
1.8080(18)
1.8360(18)
1.8883(17)
2.3499(18)
—
175.03(9)
79.86(7)
—
1.7758(17)
1.8131(17)
1.8317(17)
1.8735(17)
2.3547(19)
—
177.20(8)
81.07(7)
—
1.7835(17)
1.8001(17)
1.834(2)
1.8809(18)
2.351(2)
—
173.01(8)
79.27(7)
—
1.7803(17)
1.8127(17)
1.8308(17)
1.8649(16)
2.3567(18)
—
176.55(7)
80.49(6)
—
1.790(3)
1.817(3)
1.817(3)
1.869(3)
2.357(3)
—
177.19(13)
81.68(11)
—
1.796(4)
1.809(3)
1.8334(18)
1.8689(16
2.357218)
—
165.4(7)
79.78(7)
—
1.7798(19)
1.814(2)
1.832(2)
1.8624(19)
2.378(2)
—
173.78(9)
80.26(7)
—
—
81.87(9)
178.59(10)
117.50(10)
—
100.21(9)
—
99.64(8)
—
—
100.54(8)
—
99.02(15)
—
107.(10)
—
101.85(10)
100.53(10)
Scheme 1 Ligands and complexes prepared in this study.
For 1H2 the observed solid-state product was the dimeric species
Ti2(1)2(OiPr)4 with each Ti centre being trigonal pyramidal in
geometry. This is exemplified by analysis of the N(2)–Ti(2)–
O(8) angle of 177.15(12)◦ and N(2)–Ti(2)–O(5) of 81.29(9)◦. This
motif is similar to a structure recently reported by Tshuva and
co-workers, in which 1,4-bis(2-hydroxy-3,5-di-tert-butyl-benzyl)-
imidazolidine produced a dimer with Ti(IV).19 Each metal cen-
tre is coordinated to the phenoxide oxygen and nitrogen of
one ligand, a phenoxide centre from another ligand and two
isopropoxide moieties complete the coordination sphere. This
leaves one nitrogen per ligand uncoordinated. The metrics of each
titanium centre are analogues to Ti(IV) bisphenolates reported in
the literature.7a,7d–7f,19 For Ti2(1)2(OiPr)4 only small crystals were
isolated (0.08 ¥ 0.05 ¥ 0.02 mm) and it was necessary to use
synchrotron radiation to determine the structure. Analysis of the
bulk solid from a stoichiometric reaction via elemental analysis
was consistent with the major product being Ti2(1)(OiPr)6 but this
was not the crystalline product. For ligands 2H2–8H2 complexes
of the form Ti2(L)(OiPr)6 were observed in the solid-state. The
titanium centres are trigonal bipyramidal with one phenoxide, a
nitrogen and three isopropoxide moieties coordinating to each
metal centre.
Fig. 1 Top: Solid-state structure of Ti2(1)2(OiPr)4. Bottom: Solid-state
structure of Ti2(2)(OiPr)6. See Table 1 for selected bond lengths and angles.
The atoms labelled with the suffix A are related by the -x+1,-y,-z+1
symmetry operation.
2H2 with Ti(OiPr)4 that there are multiple species present in
solution. On careful examination of the spectra and VT NMR
spectroscopic studies it was concluded that they are consistent
with a dynamic equilibrium in solution: 2 Ti2(L)(OiPr)6
¤
Ti2(L)2(OiPr)4 + 2 Ti(OiPr)4 (Scheme 1). Analogous equilibria
have been observed by Sharpless and Boyle in tartrate and
binaphtholate Ti(IV) complexes respectively.20 Significantly, in this
work we have solid-state evidence for both species being formed.
The 1H NMR spectrum from the product with 1H2 has two
isopropoxide resonances at 4.89 and 4.47 ppm respectively. From
1
It is clear from analysis of the room temperature H NMR
spectra for the products isolated from the reaction of 1H2 and
2034 | Dalton Trans., 2011, 40, 2033–2037
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Table 2 Solution polymerisation data
DOSY NMR spectroscopic measurements and from comparison
with neat Ti(OiPr)4 it is clear that the resonance at 4.47 ppm arises
from Ti(OiPr)4 as this has a significantly higher diffusion rate
than the other two species. The resonance at 4.89 ppm originates
from Ti2(L)(OiPr)6 and Ti2(L)2(OiPr)4 present in solution. The
equilibrium has been further investigated by studying the variation
of K (the equilibrium constant) with temperature, see Fig. 2 for
the van’t Hoff plot for the complexes isolated with 1–2H2, in
CDCl3. For the complexes formed with 3–5H2 the solution-state
NMR spectra are consistent with one major species present in
solution, which is the Ti2(L)(OiPr)6 form. In these examples it
is hypothesised that the bulky group (tBu or amyl) in the ortho
position (R1) hinders the formation of the Ti2L2(OiPr)4 species.
It must be noted that the methylene region of the 1H NMR
spectra of the titanium dimers are non trivial due to a multitude
of peaks, indicative of a species with low-symmetry coupled
with fluctionality on the NMR timescale. Extensive variable
temperature NMR spectroscopic investigations were attempted,
which showed complex diastereomeric coupling in the methylene
region.† However, from analysis of the aromatic and isopropoxide
regions it is clear that the Ti2(L)(OiPr)6 form is prevalent in
solution.† Elemental analysis supports the formation of the
Ti2(L)(OiPr)6 species in the solid-state.
b
c
d
Ligand
Conv.a
Mn
Mn
PDIc
Pm
1
96
95
96
95
96
95
94
91
96
94
7000
20600
7000
6900
20800
6900
6850
6600
7000
6850
1800
12950
1500
5900
11300
5600
4200
1200
3700
1200
1.72
1.38
1.51
1.37
1.48
1.37
1.34
2.21
1.28
2.31
0.5
0.55
0.5
0.55
0.55
0.6
1e
2
3
3e
4
5
6
7
8
0.6
0.55
0.55
0.5
Conditions: Monomer:complex ratio 100 : 1 solvent toluene, T = 80 ◦C,
time = 24 h.a determined from 1H NMR analysis; b Calculated Mn
=
(Conv/100 ¥ 144 ¥ 50) + 60. c determined from GPC analysis using THF
as the solvent and reference to polystyrene standards; d determined from
the analysis of the methine region of the 1H homonuclear decoupled NMR
spectrum. e Monomer:complex ratio 300 : 1.
Table 3 Melt polymerisation data
b
Ligand
Conv.a
Mn
Mn
PDIc
Pm
c
d
1
96
97
97
95
97
99
95
96
95
99
20800
62900
21000
20600
62900
21450
20600
20800
20600
21450
15900
55450
23900
20900
56750
23700
13200
13600
20400
14200
1.86
1.56
1.53
2.01
1.57
2.01
1.56
1.53
1.63
2.03
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.55
0.55
0.5
1e
2
3
3e
4
5
6
7
8
Conditions: Monomer:complex ratio 300 : 1,
T
=
130 ◦C, time
=
=
30 min.a determined from 1H NMR analysis; b Calculated Mn
(Conv/100 ¥ 144 ¥ 150) + 60. c determined from GPC analysis using
THF as the solvent and reference to polystyrene standards; d determined
from the analysis of the methine region of the 1H homonuclear decoupled
NMR spectrum. e Monomer:complex ratio 900 : 1.
for the reduction in molecular weights compared to the theoretical
values. From VT NMR spectroscopic studies (C6D5CD3 at 80 ◦C)
for complexes Ti2(3–8)(OiPr)6 the 2Ti:1 L dimer was the major
moiety and this is presumably the catalytically active species. For
complexes with ligands 1H2–2H2 this is not the case from VT
NMR the major form is Ti2(L)2(OiPr)4. The presence of multiple
species in solution could also be responsible for the broad range of
molecular weights. Low molecular weights were formed, however,
if the monomer:complex ratio was increased higher molecular
weights could be produced.
Attempts then turned to the polymerisation under melt con-
ditions, where a much closer correlation between calculated and
experimental molecular weights was seen. Complexes were seen
to be incredibly active under these conditions and quantitative
conversions were observed after only 30 min. In this case
reasonably high molecular weights were obtained and again the
PDIs were relatively large. Interestingly, these complexes are more
active than traditional amine bis(phenolates) based on Ti(IV) –
for example those with a -MeN(CH)2NMe- back bone afforded
a 70% yield after 2 h.21 This is compared to a near quantitative
conversion after 2 h in this work, under analogous conditions.
Fig. 2 van’t Hoff plots for the equilibrium described in the text and
Scheme 1. Squares ligand = 1H2 (concentration 17.4 mg ml-1) DG =
5.1 kJ mol-1, DH = 37.3 kJ mol-1 and DS = 108 J K-1 mol-1 Triangles
ligand = 2H2 (concentration = 19.5 mg ml-1) DG = 10.9 kJ mol-1, DH =
30.7 kJ mol-1 and DS = 66.2 J K-1 mol-1. Solvent = CDCl3.
For ligands (6–7H2) where R3 = Me, two resonances for
the isopropoxides are observed in the 1H NMR spectrum, as
expected due to the asymmetry in this ligand system. For the
homopiperazine complex Ti2(8)(OiPr)6 was again the dominant
species observed in the solution-state.
Polymerisation results
The complexes were tested for the ROP of rac-lactide under both
solution (Table 2) and melt conditions (Table 3). Under solution
conditions high conversions were observed after 24 h. However,
relatively low molecular weight material was formed with a broad
PDI and a very slight isotactic bias. It is probable that more than
one polymer chain is growing per metal centre and this is the cause
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Table 4 X-ray crystallographic parameters
Ti2(1)2(OiPr)4 Ti2(2)(OiPr)6
Complex
Empirical formula C59H91N4O8Ti2 C46H82N2O8Ti2 C52H94N2O8Ti2 C52H96N2O8Ti2 C62H116N2O8Ti2 C53H96N2O8Ti2 C47H83N2O8Ti2 C56H103N2O8Ti2
Ti2(3)(OiPr)6 Ti2(4)(OiPr)6
Ti2(5)(OiPr)6
Ti2(6)(OiPr)6
Ti2(7)(OiPr)6
Ti2(8)(OiPr)6
Formula weight
Crystal system
Space group
1080.16
Monoclinic
P21/c
23.224(6)
16.478(5)
31.844(9)
90
96.782(3)
90
12101(6)
8
886.94
Monoclinic
P21/c
9.5960(5)
23.1890(14)
11.4210(8)
90
91.398(3)
90
2540.7(3)
2
971.09
Monoclinic
P21/n
973.11
1113.37
Triclinic
P1
985.12
Monoclinic
P21/n
449.98
1028.20
Triclinic
P1
Triclinic
Triclinic
¯
¯
¯
¯
P1
P1
˚
a/A
10.8260(6)
18.8930(9)
14.5330(10)
90
104.050(2)
90
2883.6(3)
2
1.118
0.324
42193
3.54–24.00
4506. 0.1040
1.064
9.4410(3)
10.0110(3)
15.1580(5)
79.509(1)
84.676(1)
87.797(1)
1402.28(8)
1
11.0450(10)
13.2550(11)
13.9140(14)
115.990(5)
109.683(5)
91.805(5)
1685.0(3)
1
1.097
0.285
33515
4.09–27.52
7641, 0.0614
1.017
9.2460(1)
24.3060(3)
26.2840(3)
90
91.5990(10)
90
5904.59(12)
4
1.108
0.318
67951
3.53–24.99
9.4140(4)
9.9250(4)
13.9710(12)
92.550(2)
94.4870(10)
92.879(3)
1298.18(13)
1
14.2220(3)
15.3740(4)
17.4080(4)
69.6250(10)
89.943(2)
62.866(1)
3118.09(13)
2
1.095
0.303
53817
3.75–27.50
14138, 0.0624
1.006
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
Volume/A
Z
Dcalc/g cm-3
m/mm-1
1.186
0.317
102329
2.83–25.17
1.159
0.362
27064
3.68–27.48
1.152
0.334
19702
3.72–27.48
6423, 0.0458
1.025
1.151
0.355
18880
4.51–27.48
Refns collected
q range/◦
Indep. refns (Rint) 23430, 0.0710 5764, 0.0931
10306, 0.0726 5909, 0.0247
1.049 1.137
Goodness-of-fit
1.061
1.049
R1, wR2 [I > 2s(I)] 0.0727, 0.1800 0.0572, 0.1373 0.0434, 0.0873 0.0599, 0.1525 0.0590, 0.1504 0.0851, 0.2225 0.0604, 0.1663 0.0607, 0.1579
R1, wR2 [all data] 0.1017, 0.2005 0.0852, 0.1525 0.0682, 0.0985 0.0796, 0.1701 0.0903, 0.1713 0.10550.2403 0.0677, 0.1747 0.0976, 0.1847
Max, min
difference/e A
0.644, -0.472 0.575, -0.539 0.233, -0.354 1.352, -0.653 0.761, -0.626
0.887, -0.504 0.942, -0.723 0.680, -0.630
-3
˚
(3 ¥ 50 ml) to remove any unreacted monomer. For solution
polymerisations a monomer:complex ratio of 100 : 1 was used. In
all cases 1 g of lactide and the appropriate amount of initiator
were dissolved in toluene (10 ml) these were placed in a pre-
heated oil bath and heated for the desired amount of time. The
reaction was quenched by the addition of methanol (20 ml).
1H NMR spectroscopy (CDCl3) and GPC (THF) were used to
determine tacticity and molecular weights (Mn and Mw) of the
polymers produced; Pr/m (the probability of heterotactic/isotactic
linkages) were determined by analysis of the methine region of
the homonuclear decoupled 1H NMR spectra. The equations
used to calculate Pr/m are given by Coates.2b Gel Permeation
Chromatography (GPC) analyses were performed on a Polymer
Laboratories PL-GPC 50 integrated system using a PLgel 5 mm
MIXED-D 300 ¥ 7.5 mm column at 35 ◦C, THF solvent (flow
rate 1.0 ml min-1). The polydispersity index (PDI) was determined
from Mw/Mn where Mn is the number average molecular weight
and Mw the weight average molecular weight. The polymers were
referenced to polystyrene standards.
Conclusion
In conclusion eight new piperazine Ti(IV) complexes have been
prepared and characterised via single crystal X-ray diffraction.
Two dimeric species have been isolated in the solid-state. In
solution three species have been observed, however increasing the
steric bulk facilitates the formation of one species in solution.
The complexes have also been shown to be active for the ROP of
rac-lactide with high molecular weights being produced under the
industrially preferred melt conditions.
Experimental
For the preparation and characterisation of metal complexes, all
reactions and manipulations were performed under an inert atmo-
sphere of argon using standard Schlenk or glovebox techniques.
Ti(OiPr)4 (97% Aldrich) was purified by vacuum distillation prior.
rac-LA (Aldrich) was recrystallised from toluene and sublimed
twice prior to use. All other chemicals were purchased from
Aldrich. All solvents used in the preparation of metal complexes
and polymerisation reactions were dry and obtained via SPS
Single crystal diffraction
(solvent purification system). 1H and 13C{ H} NMR spectra were
1
All data {except those for Ti2(1)2(OiPr)4 which were recorded
recorded on a Bruker 250, 300 or 400 MHz instrument and
referenced to residual solvent peaks. Coupling constants are given
in Hertz. Elemental analyses were performed by Mr. A. K. Carver
at the Department of Chemistry, University of Bath. The ligands
were prepared according to standard literature procedures and the
˚
using Synchrotron radiation, l = 0.68890 A} were collected on
a Nonius kappa CCD diffractometer with Mo-Ka radiation,
˚
l = 0.71073 A, see Table 4. T = 150(2) K throughout and all
structures were solved by direct methods and refined on F2 data
using the SHELXL-97 suite of programs.22 Hydrogen atoms, were
included in idealised positions and refined using the riding model.
Refinements were generally straightforward with the following
exceptions and points of note. In certain cases disorder was rife and
in those cases it was deemed prudent to leave disordered groups
isotropic on merit, in all cases the structures are unambiguously
purity confirmed via 1H/13C{ H} NMR and HR-MS prior to use.
1
Polymerisation procedure:
For solvent-free polymerisations the monomer: complex ratio
employed was 300 : 1 at a temperature of 130 ◦C, in all cases
1 g of rac-lactide was used. After the reaction time methanol
(20 ml) was added to quench the reaction and the resulting
solid was dissolved in dichloromethane. The solvents were re-
moved in-vacuo and the resulting solid washed with methanol
t
determined. In Ti2(2)(OiPr)6 one Bu group was disordered in a
60 : 40 ratio; Ti2(4)(OiPr)6 contained two isopropoxide groups that
were disordered in ratios of 50 : 50 and 60 : 40; these were treated
anisotropically but their ADPs were less than ideal; Ti2(6)(OiPr)6
2036 | Dalton Trans., 2011, 40, 2033–2037
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the isopropoxide groups adorning Ti2 are disordered over two
positions in a 55 : 45 ratio. The methyl groups of two Bu (C20,
Commun., 2009, 4115–4117; (e) J. E. Kasperczyk, Macromolecules,
1995, 28, 3937–3939; (f) B. T. Ko and C. C. Lin, J. Am. Chem. Soc.,
2001, 123, 7973–7977.
t
C50) functionalities are disordered over two positions again in a
55 : 45 ratio. In addition the methyl group of the piperazine ring is
disordered on three (C26–28) of the ring carbons in a 60 : 20 : 20
ratio. The disordered moieties have been refined isotropically. Only
one H-atom has been added to each carbon C26–28; Ti2(7)(OiPr)6
two isopropoxide groups were disordered in ratios of 70 : 30 and
80 : 20, these were refined isotropically. Lastly, in Ti2(8)(OiPr)6 two
isopropoxide groups were disordered in a 60 : 40 ratio.
3 (a) M. Bouyahyi, T. Roisnel and J. F. Carpentier, Organometallics, 2010,
29, 491–500; (b) M. H. Chisholm, J. C. Gallucci, K. T. Quisenberry and
Z. P. Zhou, Inorg. Chem., 2008, 47, 2613–2624; (c) A. L. Johnson,
M. G. Davidson, Y. Perez, M. D. Jones, N. Merle, P. R. Raithby and
S. P. Richards, Dalton Trans., 2009, 5551–5558; (d) B. Lian, H. Y. Ma,
T. P. Spaniol and J. Okuda, Dalton Trans., 2009, 9033–9042; (e) N.
Nomura, R. Ishii, Y. Yamamoto and T. Kondo, Chem.–Eur. J., 2007,
13, 4433–4451; (f) M. Shen, W. J. Zhang, K. Nomura and W. H. Sun,
Dalton Trans., 2009, 9000–9009.
4 (a) J. C. Buffet, J. Okuda and P. L. Arnold, Inorg. Chem., 2010, 49,
419–426; (b) A. Pietrangelo, M. A. Hillmyer and W. B. Tolman, Chem.
Commun., 2009, 2736–2737; (c) A. Pietrangelo, S. C. Knight, A. K.
Gupta, L. J. Yao, M. A. Hillmyer and W. B. Tolman, J. Am. Chem.
Soc., 2010, 132, 11649–11657.
5 (a) C. K. Williams, L. E. Breyfogle, S. K. Choi, W. Nam, V. G. Young,
M. A. Hillmyer and W. B. Tolman, J. Am. Chem. Soc., 2003, 125, 11350–
11359; (b) M. D. Jones, M. G. Davidson, C. G. Keir, L. M. Hughes,
M. F. Mahon and D. C. Apperley, Eur. J. Inorg. Chem., 2009, 635–642.
6 (a) A. Buchard, R. H. Platel, A. Auffrant, X. F. Le Goff, P. Le Floch and
C. K. Williams, Organometallics, 2010, 29, 2892–2900; (b) N. Ajellal,
D. M. Lyubov, M. A. Sinenkov, G. K. Fukin, A. V. Cherkasov, C. M.
Thomas, J. F. Carpentier and A. A. Trifonov, Chem.–Eur. J., 2008, 14,
5440–5448; (c) P. L. Arnold, J. C. Buffet, R. P. Blaudeck, S. Sujecki,
A. J. Blake and C. Wilson, Angew. Chem., Int. Ed., 2008, 47, 6033–6036.
7 (a) S. Gendler, S. Segal, I. Goldberg, Z. Goldschmidt and M. Kol,
Inorg. Chem., 2006, 45, 4783–4790; (b) Y. Kim, G. K. Jnaneshwara and
J. G. Verkade, Inorg. Chem., 2003, 42, 1437–1447; (c) E. Sergeeva, J.
Kopilov, I. Goldberg and M. Kol, Inorg. Chem., 2010, 49, 3977–3979;
(d) E. L. Whitelaw, M. D. Jones and M. F. Mahon, Inorg. Chem., 2010,
49, 7176–7181; (e) E. L. Whitelaw, M. D. Jones, M. F. Mahon and
G. Kociok-Kohn, Dalton Trans., 2009, 9020–9025; (f) A. L. Zelikoff,
J. Kopilov, I. Goldberg, G. W. Coates and M. Kol, Chem. Commun.,
2009, 6804–6806.
8 (a) R. A. Jain, Biomaterials, 2000, 21, 2475–2490; (b) J. Panyam and V.
Labhasetwar, Adv. Drug Delivery Rev., 2003, 55, 329–347.
9 Z. J. Zhang, X. P. Xu, S. Sun, Y. M. Yao, Y. Zhang and Q. Shen, Chem.
Commun., 2009, 7414–7416.
10 Y. J. Luo, W. Y. Li, D. Lin, Y. M. Yao, Y. Zhang and Q. Shen,
Organometallics, 2010, 29, 3507–3514.
11 N. C. Johnstone, E. S. Aazam, P. B. Hitchcock and J. R. Fulton,
J. Organomet. Chem., 2010, 695, 170–176.
12 S. Mohanty, D. Suresh, M. S. Balakrishna and J. T. Mague,
J. Organomet. Chem., 2009, 694, 2114–2121.
13 J. D. Farwell, P. B. Hitchcock, M. F. Lappert, G. A. Luinstra, A. V.
Protchenko and X. H. Wei, J. Organomet. Chem., 2008, 693, 1861–
1869.
14 (a) S. Mukhopadhyay, D. Mandal, P. B. Chatterjee, C. Desplanches,
J. P. Sutter, R. J. Butcher and M. Chaudhury, Inorg. Chem., 2004,
43, 8501–8509; (b) N. Sengottuvelan, D. Saravanakumar, S. Sridevi, V.
Narayanan and M. Kandaswamy, Supramol. Chem., 2004, 16, 129–136;
(c) K. Kubono, C. Noshita, K. Tani and K. Yokoi, Acta Crystallogr.,
Sect. E: Struct. Rep. Online, 2009, 65, m1685–U1593.
15 J. Lloyd, S. Z. Vatsadze, D. A. Robson, A. J. Blake and P. Mountford,
J. Organomet. Chem., 1999, 591, 114–126.
16 (a) R. Mayilmurugan, M. Sankaralingam, E. Suresh and M. Palanian-
davar, Dalton Trans., 2010, 39, 9611–9625; (b) R. Mayilmurugan, H.
Stoeckli-Evans, E. Suresh and M. Palaniandavar, Dalton Trans., 2009,
5101–5114.
Ligand and complex preparation
Typical procedures are as follows, see supporting information for
the characterisation of ligands 2H2–8H2 and their complexes.
1H2 2,4-di-methylphenol (8.50 g, 69.6 mmol), piperazine an-
hydrous (3.00 g, 34.8 mmol), and formaldehyde (38% in H2O)
(5.78 ml, 2.35 g, 78.1 mmol) were refluxed in MeOH (40 ml) for
24 h. During which time a white precipitate was observed this
was filtered and washed with cold MeOH and dried to yield a
1
white solid (5.64 g, 15.9 mmol, 46%). H NMR (CDCl3) 2.22
(6H, s, CH3), 2.23 (6H, s, CH3), 2.30–3.20 (8H, br, CH2), 3.67
(4H, s, CH2), 6.65 (2H, d, J = 1.5 Hz, ArH), 6.85 (2H, d, J =
1
1.5 Hz, ArH), 10.54 (2H, br, OH). 13C{ H} NMR (CDCl3) 15.7
(CH3), 20.5 (CH3), 52.4 (CH2), 61.4 (CH2), 119.9 (Ar), 124.7 (Ar),
126.9 (Ar–H), 127.9 (Ar), 138.8 (Ar–H), 153.4 (Ar–O). Calc. m/z
[C22H30N2O2 + H]+ 355.2385. Found 355.2488.
Ti2(1)(OiPr)6 1H2 (0.50 g, 1.41 mmol) and Ti(OiPr)4 (0.85 ml,
2.87 mmol) were dissolved in CH2Cl2 (30 ml) and stirred (16 h). The
solvent was removed in-vacuo and recrystallised from hot hexane
(40 ml) to yield a pale yellow crystals (0.51 g, 0.64 mmol, 45%).
The NMR was a mixture of Ti2(1)(OiPr)6 and Ti2(1)2(OiPr)4 and
Ti(OiPr)4 as discussed in the text. The NMRs for the individual
components are: Ti2(1)(OiPr)6 1H NMR (CDCl3) (233 K) 1.26
(36H, d, J = 6.0 Hz, CH3), 2.13 (6H, s, CH3), 2.15 (3H, s, CH3),
2.16 (3H, s, CH3), 2.30–3.90 (8H, br, CH2), 4.09 (4H, m, N–CH2–
Ar), 4.89 (6H, br, CH), 6.69 (2H, s Ar–H), 6.83 (2H, s, ArH).
Ti2(1)2(OiPr)4 1H NMR (233 K) 0.94 (3H, d, J = 6.0 Hz, CH3),
0.97 (3H, d, J = 6.0 Hz, CH3), 1.16 (6H, d, J = 6.0 Hz, Me), 1.20–
1.40 (12H, br, CH3), 2.08 (3H, s, Me), 2.10–2.25 (18H, br, CH3),
2.28 (3H, s, CH3), 2.30–3.90 (16H, br, CH2), 4.09 (8H, m, CH2),
4.89 (4H, b, CH), 6.57 (2H, s, Ar–H), 6.76 (2H, s, Ar–H), 6.84
(2H, s, Ar–H), 6.92 (2H, s, Ar–H). Ti(OiPr)4 1H NMR (CDCl3)
(233 K) 1.26 (24H, d, J = 6.0 Hz, CH3), 4.47 (4H, sept, J = 6.0 Hz,
1
CH). 13C{ H} NMR (CDCl3) 16.8 (CH3), 20.5 (CH3), 26.7 (CH3),
52.0 (CH2), 77.8 (CH), 122.7 (Ar), 124.8 (Ar), 127.0 (Ar), 127.7
(Ar–H), 130.8 (Ar–H), 158.2 (Ar–O). Calc.(%) for C40H70N2O8Ti2:
C 59.85, H 8.79, N 3.49. Found (%); C 59.3, H 8.80, N 3.72.
17 M. Du, X. J. Zhao, J. H. Guo, X. H. Bu and J. Ribas, Eur. J. Inorg.
Chem., 2005, 294–304.
18 J. Baldamus and E. Hecht, Z. Anorg. Allg. Chem., 2003, 629, 188–
191.
Notes and references
19 C. M. Manna, M. Shavit and E. Y. Tshuva, J. Organomet. Chem., 2008,
1 (a) O. Dechy-Cabaret, B. Martin-Vaca and D. Bourissou, Chem. Rev.,
2004, 104, 6147–6176; (b) R. H. Platel, L. M. Hodgson and C. K.
Williams, Polym. Rev., 2008, 48, 11–63; (c) N. Ajellal, J. F. Carpentier,
C. Guillaume, S. M. Guillaume, M. Helou, V. Poirier, Y. Sarazin and
A. Trifonov, Dalton Trans., 2010, 39, 8363–8376.
2 (a) C. N. Ayala, M. H. Chisholm, J. C. Gallucci and C. Krempner,
Dalton Trans., 2009, 9237–9245; (b) T. M. Ovitt and G. W. Coates,
J. Am. Chem. Soc., 2002, 124, 1316–1326; (c) L. M. Hodgson, R. H.
Platel, A. J. P. White and C. K. Williams, Macromolecules, 2008, 41,
8603–8607; (d) R. H. Platel, A. J. P. White and C. K. Williams, Chem.
693, 3947–3950.
20 (a) M. G. Finn and K. B. Sharpless, J. Am. Chem. Soc., 1991, 113,
113–126; (b) T. J. Boyle, D. L. Barnes, J. A. Heppert, L. Morales, F.
Takusagawa and J. W. Connolly, Organometallics, 1992, 11, 1112–1126.
21 A. J. Chmura, M. G. Davidson, M. D. Jones, M. D. Lunn, M. F.
Mahon, A. F. Johnson, P. Khunkamchoo, S. L. Roberts and S. S. F.
Wong, Macromolecules, 2006, 39, 7250–7257.
22 G. M. Sheldrick, SHELXL-97, Program for refinement of crystal
structures, University of Go¨ttingen, Germany, 1997.
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