Homopiperazine and piperazine complexes of ZrIV and Hf IV and their application to the ring-opening polymerisation of lactide

Article abstract of DOI:10.1002/ejic.201100589

In this paper we describe the preparation and characterisation, by single-crystal X-ray diffraction, of twelve Zr^IV/Hf^IV complexes based on piperazine or homopiperazine salan ligands. With the piperazine ligands, a mixture of species

Full text of DOI:10.1002/ejic.201100589

FULL PAPER
DOI: 10.1002/ejic.201100589
Homopiperazine and Piperazine Complexes of ZrIV and HfIV and Their
Application to the Ring-Opening Polymerisation of Lactide
Stuart L. Hancock,[a] Mary F. Mahon,[a] Gabriele Kociok-Ko˝hn,[a] and Matthew D. Jones*[a]
Keywords: Titanium / Zirconium / Hafnium / Ring-opening polymerization / Sustainable chemistry
In this paper we describe the preparation and characterisa-
tion, by single-crystal X-ray diffraction, of twelve ZrIV/HfIV
complexes based on piperazine or homopiperazine salan li-
gands. With the piperazine ligands, a mixture of species was
observed and various solid-state structures were isolated.
However, with homopiperazine salan ligands, 1:1 ligand-to-
metal complexes were observed both in solution and in the
solid state. Interestingly, for the homopiperazine complexes
the isopropoxide ligands are trans to one another in the solid
state, most likely because of the rigid nature of the homo-
piperazine backbone. All homopiperazine HfIV and ZrIV com-
plexes were tested for the ring-opening polymerisation
(ROP) of rac-lactide. The complexes are active and produce
polylactide with narrow polydispersity indices. The kinetics
and living characteristics of the polymerisation have also
been investigated.
butyrolactone.[9a] In the case of rac-LA, atactic PLA was
formed. It has also been shown that bis(imino)phenoxide
complexes of ZrIV show high activities in the polymerisa-
tion.[9g]
Introduction
In recent years there has been an explosion of interest in
the use of single-site homogeneous catalysts for the ring-
opening polymerisation (ROP) of rac-lactide (rac-LA) to
produce polylactide (PLA).[1] This process has been com-
mercialised by Purac and NatureWorks, and the current cat-
alyst used for this process is based on SnII.[2] There is cur-
rently a desire to replace tin in this system, and – as a conse-
quence – a significant amount of work has been performed
in catalyst development.[1] The polymers themselves have
found extensive utility from biomedical to commodity poly-
mer applications,[3] due to the biodegradability and biocom-
patibility of PLA and the fact that the monomer can be
sourced from sustainable raw materials. The catalyst can
have a dramatic effect on the physical properties and degra-
dation rates of the resultant PLA.[4] For example, catalysts
The chemistry of group 4 metal complexes with symmet-
rical amine–bis(phenolate) ligands is rich and diverse, and
many ligand–metal complexes are known with possible geo-
metries shown in Scheme 1.[12] The use of bis(phenolate)
ligands based on 2,2Ј-bipyrrolidine, N,NЈ-dimethyl-1,2-di-
aminobenzene, N,NЈ-dimethyl-1,2-ethylenediamine and
N,NЈ-dimethylcyclohexane-1,2-diamine backbones forms
the α-cis isomer both in the solid state and solution once
reacted with Zr(OtBu)4.[13] Interestingly, a search of the
Cambridge Structural Database (CSD) indicates that there
are no crystallographically characterised complexes of the
trans isomer of group 4 metal complexes with amine bis-
(phenolate) ligands, where X = alkoxide, with the α-cis and
β-cis forms being prevalent.[14] Intriguingly, Budzelaar has
shown, using computational methods, that the trans geome-
try (mer–mer, where the ligand occupies the equatorial posi-
tion) is the intermediate ion pair in olefin polymerisa-
tion.[15] The use of homopiperazine as a backbone for bis-
(phenolate) ligands remains limited with the only examples
characterised in the solid state involving copper, nickel and
iron.[16]
based on groups 1–3,[5] lanthanides,[6] ZnII,[1a,1b,1g] AlIII [7]
,
InIII[8] and, pertinent to this study, group 4 metals have all
been shown to have significant activity.[9] We have recently
shown that TiIV salan systems with a piperazine backbone
are very effective catalysts for the bulk polymerisation of
rac-LA.[10] Kol has shown that a ZrIV complex of a tetra-
dentate phenylenediamine bis(phenolate) affords heterotac-
tic PLA under melt conditions.[11] The same group has also
shown that dithiodiolate complexes with ZrIV are active for
the production of hetereotactic PLA.[9h] It has been shown
that dinuclear ZrIV and HfIV complexes of Jacobsen’s ligand
are active for the controlled ROP of both rac-LA and β-
[a] Department of Chemistry, University of Bath,
Claverton Down, Bath BA2 7AY, United Kingdom
Fax: +44-01225-386231
E-mail: mj205@bath.ac.uk
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100589.
Scheme 1. Possible isomers of ONNO ligands with ZrIV or HfIV.
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 4596–4602

Homopiperazine and Piperazine Complexes of ZrIV and HfIV
Results and Discussion
Synthesis and Characterisation of Ligands and Complexes
Our initial attempts focused on salan ligands (1H2–3H2)
based on a piperazine backbone (Scheme 2). The ligands
are readily prepared by a modified Mannich reaction.[10]
One equivalent of the ligand was treated with one equiva-
lent of either Zr(OiPr)4OHiPr or Hf(OiPr)4OHiPr. We have
previously shown that dimers can be formed in the solid
state with such ligands and TiIV centres.[10]
Using the piperazine ligand, three different structural
motifs were isolated in the solid state. From the reaction of
1H2 with ZrIV a tetramer was observed (Figure 1). There
are two crystallographically unique ZrIV centres, which are
both in pseudo-octahedral environments. The phenoxide
oxygen atom O1 bridges between Zr1 and Zr2 with N1
binding to Zr1. Zr2 has three terminal isopropoxide ligands
with Zr–O distances ranging from 1.944(3) to 1.953(3) Å.
A coordinated 2-propanol with O6 completes the coordina-
tion sphere of Zr2 with a Zr2–O6 distance of 2.287(3) Å.
There is an H-bonding interaction between the alcoholic
proton attached to O6 and an oxygen acceptor of a neigh-
bouring isopropoxide unit.
Attempts then turned to 2H2, which again has a methyl
group ortho to the phenoxide. In this case ZrIV crystals of
a dimeric complex were isolated, where there are two crys-
tallographically unique ZrIV centres; Zr1 is six-coordinate
and Zr2 seven-coordinate, Figure 1. Zr1 has three terminal
isopropoxide groups, two bridging isopropoxide groups and
one bridging phenoxide moiety, with a Zr1–O4 distance of
Figure 1. Solid-state structures of Zr4(1)(OiPr)14(HOiPr)2 (top) and
Zr2(2)(OiPr)6 (bottom). The methyl groups of the iPr moieties and
2.286(4) Å. Zr2 is bound to two bridging isopropoxides, all hydrogen atoms have been removed for clarity. The ellipsoids
two amine nitrogen atoms with distances Zr2–N1 2.481(5)
are shown at the 30% probability level.
and Zr2–N2 2.450(6) Å, and N1–Zr2–N2 is 59.44(18)°; a
terminal isopropoxide and phenoxide complete the coordi- have observed dimers with tris(phenolate) ligands[18] and
nation sphere of Zr2. ZrIV dimers with bis(phenoxide) salan Sun has observed dinuclear species with ONNO ligands.[9d]
ligands are rare and this is the first example of a tetrameric Upon reacting Hf(OiPr)4(OHiPr) with 1H2 and 3H2 a fur-
species.[9d,17] Examples with similar ligands include those of ther dimeric species was observed in the solid state (Fig-
Gibson who has observed dimers with ONN ligands,[17] we ure 2).
Scheme 2. Ligands and complexes prepared in this study.
Eur. J. Inorg. Chem. 2011, 4596–4602
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S. L. Hancock, M. F. Mahon, G. Kociok-Kohn, M. D. Jones
˝
FULL PAPER
to ligands 4H2–7H2 (Scheme 2) with the aim of producing
complexes in higher purities for further polymerisation
studies. The homopiperazine ligands were treated with
1 equiv. of either Zr(OiPr)4OHiPr or Hf(OiPr)4OHiPr in
CH2Cl2, and good yields of pure 1:1 monomeric materials
were obtained after recrystallisation, see Figure 3 and
Table 2 for selected bond lengths and angles. We hypo-
thesise that formation of monomers for 4H2–7H2 is a conse-
quence of the reduced rigidity in the homopiperazine ring
compared to the piperazine ring.
Figure 2. Solid-state structure of Hf2(1)(OiPr)6. The methyl groups
of the iPr moieties and all hydrogen atoms have been removed for
clarity. The ellipsoids are shown at the 30% probability level.
The motif in this case is based on each HfIV centre being
bound to two terminal isopropoxide species, a terminal
phenoxide, a nitrogen atom of the piperazine ring and two
bridging isopropoxide moieties. This structure is analogous
to that recently reported by Kol for TiIV and ZrIV phen-
ylenediamine bis(phenolate) species.[11] However, this is the
first example of such a structure with HfIV. Selected bond
lengths and angles for the HfIV complexes are shown in
Table 1.
Figure 3. Solid-state structure of Zr(5)(OiPr)2. The methyl groups
of the iPr moieties and all hydrogen atoms have been removed for
clarity. The ellipsoids are shown at the 30% probability level.
Table 2. Selected bond lengths (Å) and angles (°) for the homo-
piperazine solid-state structures described in this work.
Hf(4)(OiPr)2 Zr(5)(OiPr)2 Zr(5)(OiPr)2·1/2(C4H8O)·
1/2(C3H8O)
M1–O1
M1–O2
M1–O3
M1–O4
M1–O5
M1–N1
M1–N2
O1–M1–O2
O3–M1–O4
N1–M1–N2
Ph–Ph planes
1.982(3)
1.966(3)
2.037(3)
2.037(3)
–
2.340(4)
2.349(3)
176.36(13)
119.36(12)
67.27(12)
108.2
1.9689(14)
1.9665(14)
2.0476(15)
2.0567(14)
–
2.3958(18)
2.3876(18)
174.71(6)
121.40(6)
66.40(6)
104.4
1.9708(17)
1.9652(16)
2.0950(17)
2.1292(15)
2.469(10)
2.4831(19)
2.4945(18)
169.18(7)
137.96(6)
63.79(6)
Table 1. Selected bond lengths (Å) and angles (°) for Hf2(1)-
(OiPr)6 and Hf2(3)(OiPr)6.
Hf2(1)(OiPr)6
Hf2(3)(OiPr)6
Hf1–O1
Hf1–O2
Hf1–O3
Hf1–O4
Hf1–O5
Hf1–N1
N1–Hf1–O1
O5–Hf1–O4
1.908(9)
1.913(10)
1.995(9)
2.204(10)
2.122(9)
2.548(12)
173.9(4)
71.2(3)
1.917(7)
1.929(9)
2.009(6)
2.152(10)
2.180(11)
2.534(9)
169.6(3)
71.8(3)
104.7
Hf(5)(OiPr)2 Zr(6)(OiPr)2 Hf(6)(OiPr)2
M1–O1
M1–O2
M1–O3
M1–O4
M1–O5
1.960(4)
1.955(4)
2.030(4)
2.041(4)
–
1.968(4)
1.964(4)
2.041(2)
–
1.957(7)
1.993(6)
2.040(2)
–
For the ZrIV piperazine species, the crystallised yields of
these complexes were poor (ca. 5%) and NMR spectro-
scopic analysis of the crude reaction mixture before
–
–
recrystallisation implied that a plethora of species were M1–N1
2.357(5)
2.364(5)
174.83(16)
119.17(15)
66.65(16)
105.1
2.387(3)
–
2.363(3)
–
172.7(4)
M1–N2
present as well as unreacted starting materials. The rigidity
O1–M1–O2
173.5(2)
of the ligands and size of ZrIV implies that various combi-
nations and ratios of metal-to-ligand can be observed for
these piperazine complexes. This is in direct contrast to our
previously reported TiIV systems.[10] For the HfIV piperazine
systems, the complexes were highly soluble in hexane and
common organic solvents, therefore, obtaining pure mate-
O3–M1–O4
N1–M1–N2
Ph–Ph planes
119.57(13)[a] 117.34(13)[a]
66.70(13)
98.8
67.46(13)
98.9
[a] Due to symmetry, this angle is O3–M1–O3A.
The ligands chosen impart varying steric constraints
rial by recrystallisation proved troublesome. However, from around the metal centre. In these cases monomeric species
1
the solution H NMR spectra it would appear that the 1:2 are formed in the solid state with the ONNO atoms of the
solid-state forms are also the main product in solution. ligands occupying the equatorial plane and the isopropox-
Attempts to increase the yields/purities for both the ZrIV ide groups are trans to one another with an iPrO–M–OiPr
and HfIV systems (varying temperature, order and speed of angle of approximately 175°. As a consequence the phenox-
addition of the salan ligand) were unsuccessful in increasing ide groups are cis to each other as are the amine groups.
the purity of the final product. Therefore, attempts turned This is in stark contrast to all amine bis(phenolate) (and
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Eur. J. Inorg. Chem. 2011, 4596–4602

Homopiperazine and Piperazine Complexes of ZrIV and HfIV
OSSO) ZrIV or HfIV isopropoxide complexes in the Polymerisation of rac-LA
CSD.[14,19] However, such species have been observed for
The monomeric complexes were tested for the ROP of
TiIV in solution.[12] Noteworthy, in dichloro species it is
more common for the ligand to occupy the equatorial sites
and the chloro in the axial positions.[20] The formation of
these trans complexes must be related to the fact that the
ligands are not as flexible as the majority of salen or salan
complexes used to date. The ligand is considerably distorted
from planarity with the phenyl rings forming a bowl-like
arrangement around the metal centre with isopropoxide O2
encapsulated in the bowl (Figure 3). This can be quantified
by the angle between the planes formed by the two phenox-
ide rings of ca. 100° (Table 2). The O3–M–O4 angles are
approximately 120°, indicating that there is a significant op-
portunity for further coordination. Moreover, when the
crude product from the reaction of 5H2 with ZrIV is recrys-
tallised from THF and hexane, this vacant site becomes oc-
cupied (Figure 4). The ZrIV centre is now coordinated to a
molecule of either THF or 2-propanol, and it is conse-
quently seven-coordinate; these solvents are both half-occu-
pied in the solid-state structure. Due to this extra ligand
and increased coordination number the ZrIV–O/N distances
of the salan ligands become significantly elongated com-
pared to those of the six-coordinate complexes. As expected
O3–M–O4 is more obtuse at 137.96(6)°, as the ligand ad-
justs itself to accommodate the coordination of the solvent.
This bodes well for ROP as it is generally assumed that the
first step is the coordination of the monomer to the metal Entry
rac-LA under solvent-free (Table 3) and solution (Table 4)
conditions. In the melt, high molecular weight PLA is ob-
tained and in some cases there is a slight isotactic bias to
the resultant polymer, the highest being Pm = 0.63 for
Hf(5)(OiPr)2. If the theoretical molecular weight is com-
pared to the measured Mn for ZrIV/HfIV catalysts, for en-
tries 1, 2, 4, 5, 6 and 7 two polymer chains grow per metal
centre whereas for entries 3, 8, 9, 10 one chain grows per
metal centre. To achieve high conversion with complexes
based on 4H2 and 7H2 (ortho-tBu group) then longer times,
3 h, were required compared to ligands with ortho-Me
groups, which only required 30 min to achieve high conver-
sions. A plot of Mn/PDI vs. conversion for Hf(4)(OiPr)2
shows a linear increase in molecular weight with conversion
and the PDI remains relatively constant (Figure 5). When
the monomer-to-initiator ratio was increased to 900:1 (en-
tries 2 and 7, Table 3) then the molecular weight increased
in a predictable fashion. However, the conversions were sig-
nificantly lower, which could be caused by the increased
viscosity as the molecular weight increases.
Table 3. Melt polymerisation data for Zr/Hf(4–7)(OiPr)2 at a
monomer/initiator ratio of 300:1 (1 g of monomer was used in all
cases at 130 °C).
[b]
[c]
Time (h) Conversion[a] Mn
PDI[b] Pm
centre.[3a]
1
2
3
4
5
6
7
8
Zr(4)(OiPr)2
3
86
48
97
79
89
87
41
91
96
90
20900 1.22 0.49
31950 1.12 0.44
45000 1.39 0.56
15200 1.17 0.49
19325 1.31 0.53
26000 1.14 0.50
28825 1.15 0.44
47825 1.33 0.63
37150 1.28 0.61
35600 1.32 0.50
Zr(4)(OiPr)2 6[d]
Zr(5)(OiPr)2 0.5
Zr(6)(OiPr)2 0.5
Zr(7)(OiPr)2
Hf(4)(OiPr)2
3
3
Hf(4)(OiPr)2 6[d]
Hf(5)(OiPr)2 0.5
Hf(6)(OiPr)2 0.5
9
10
Hf(7)(OiPr)2
3
[a] Conversion determined by 1H NMR spectroscopy. [b] Deter-
mined from GPC analysis with THF as the solvent. [c] Determined
from 1H NMR homonuclear decoupled NMR spectroscopy. [d]
Monomer/initiator ratio 900:1.
Figure 4. Solid-state structure of Zr(5)(OiPr)2·1/2THF·1/2IPA. The
methyl groups of the iPr moieties, the half-occupied 2-propanol
and all hydrogen atoms have been removed for clarity. The ellip-
soids are shown at the 30% probability level.
Table 4. Solution polymerisation data for Zr/Hf(4–7)(OiPr)2 at a
monomer/initiator ratio of 100:1 (1 g of monomer was dissolved in
10 mL of toluene in all cases at 80 °C).
[b]
[c]
Entry
Time (h) Conversion[a] Mn
PDI[b] Pm
The room temperature 1H NMR spectra of these 1:1 spe-
cies are complex and broad resonances are observed for the
methylene CH2 moieties. Upon cooling, the spectra sharpen
and clearly defined resonances are seen for the methylene
groups, two isopropoxide groups are observed, one of
which is significantly upfield from typical Zr/Hf-OiPr reso-
nances, with a methyl resonance close to 0 ppm. This is pre-
sumably the isopropoxide group in the bowl, which is in
close proximity (ca. 2.8 Å from the phenyl ring centroid to
the CH3 hydrogen atoms) to the two electron rich phenyl
rings causing these to be more shielded. Analysis of both
1
2
3
4
5
6
7
8
9
10
Zr(4)(OiPr)2 24
Zr(5)(OiPr)2 24
Zr(5)(OiPr)2 24[d]
Zr(6)(OiPr)2 24
Zr(7)(OiPr)2 24
Hf(4)(OiPr)2 24
Hf(5)(OiPr)2 24
Hf(5)(OiPr)2 24[d]
79
96
97
93
26
12
95
94
83
22
7975
1.21 0.55
10025 1.69 0.62
45700 1.59 0.62
10550 1.55 0.58
4400
1650
12825 1.68 0.63
52100 1.20 0.65
9275
2550
1.22
1.05
–
–
Hf(6)(OiPr)2
6
1.11 0.63
1.26 0.48
Hf(7)(OiPr)2 24
[a] Conversion determined by 1H NMR spectroscopy. [b] Deter-
mined from GPC analysis using THF as the solvent. [c] Deter-
mined from 1H NMR homonuclear decoupled NMR spectroscopy.
[d] Monomer/initiator ratio 300:1.
1
the H and 13C{1H} NMR spectra indicate that the solid-
state structures are maintained in solution.
Eur. J. Inorg. Chem. 2011, 4596–4602
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. L. Hancock, M. F. Mahon, G. Kociok-Kohn, M. D. Jones
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FULL PAPER
Conclusions
A series of ZrIV and HfIV complexes based on piperazine
and homopiperazine ligands have been prepared and char-
acterised in the solid state to reveal that the isopropoxide
groups are in trans positions. From NMR spectroscopic
analysis, this structure is maintained in solution. The com-
plexes have been shown to catalyse the ROP of rac-LA to
afford PLA in a predictable fashion with low PDIs. Work
is currently ongoing to fully understand the steric effects on
the nature of the polymerisation.
Experimental Section
General Procedures: For the preparation and characterisation of
metal complexes, all reactions and manipulations were performed
under an inert atmosphere of argon using standard Schlenk or
glovebox techniques. Zr(OiPr)4iPrOH (99.9%, Aldrich) and
Hf(OiPr)4iPrOH (99.9%, Strem) were used without further purifi-
cation. rac-LA (Aldrich) was recrystallised from toluene and sub-
limed 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 from a solvent
1
purification system. H and 13C{1H} NMR spectra were recorded
with a Bruker 250, 300, 400 or 500 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 purity
Figure 5. Mn/PDI vs. time for melt polymerisation with Hf(4)-
(OiPr)2 (circles = Mn, diamonds = PDI) (top). Solution kinetics for
Zr(5)(OiPr)2 (squares, kapp = 0.025 min–1, R2 = 0.9928) and
Hf(5)(OiPr)2 (triangles, kapp = 0.008 min–1, R2 = 0.9956) (bottom).
1
confirmed by H/13C{1H} NMR and HRMS prior to use.[10]
X-ray Crystallography: Crystallographic data are summarised in
Table 5. All data were collected with a Nonius Kappa CCD area
detector diffractometer [except those for Hf(4)(OiPr)2, which were
collected with a Xcalibur, Atlas diffractometer] using Mo-Kα radia-
tion (λ = 0.71073 Å) at a temperature of 150(2) K, and all struc-
tures were solved by direct methods and refined on all F2 data using
SHELXL-97.[21] Refinement was straightforward with the follow-
ing noteworthy points: for Zr4(1)(OiPr)14(HOiPr)2 two isopropox-
ides [bound to O8 and O9] were disordered in a 50:50 and 65:35
The solution polymerisation of these ZrIV and HfIV
homopiperazine complexes are more controlled than with
TiIV homopiperazine complexes.[10] In certain cases under
solution conditions, a slight isotactic bias was observed for
the PLA (Table 4). Interestingly, the tacticity observed in
this study with these trans complexes is lower than that seen ratio. For Hf2(1)(OiPr)6 two isopropoxides [bound to O9 and O10]
previously with α-cis group 4 salan complexes.[1c] Analysis
were left isotropic. In Zr2(2)(OiPr)6 the methyl group of the piperaz-
ine ring C17 was disordered on two carbon atoms (C15 and C13)
of the molecular weight data implies that one chain grows
in a 50:50 ratio; the hydrogen atoms associated with C15 and C13
are not included in the model, and the carbon atoms of the isopro-
poxide O6 were more anisotropic than desirable but attempts to
model them proved fruitless. In Hf2(3)(OiPr)6 the methyl groups of
the isopropoxide containing O1 were disordered over two positions
in a 75:25 ratio and the methyl groups from one tBu moiety were
disordered over two positions in a 60:40 ratio and these were left
isotropic. The asymmetric unit for Hf(4)(OiPr)2 consisted of two
per metal centre. As observed with the melt conditions, the
presence on an ortho-tBu group appears to slow the poly-
merisation in solution dramatically with significantly lower
conversions after 24 h for complexes prepared from 4H2
and 7H2 (entries 1, 5, 6 and 10, Table 4) presumably due to
steric arguments. When the monomer-to-initiator ratio was
increased (entries 3 and 8, Table 4) the molecular weight
increased accordingly. MALDI-ToF mass spectrometry half molecules of hexane. Hydrogen atoms were placed in calcu-
lated positions and refined using a riding model, except for 3H2
and Zr4(1)(OiPr)14(HOiPr)2 where the OH hydrogen atoms were
freely refined. CCDC-829388 [for Zr4(1)(OiPr)14(HOiPr)2], -829389
[for Hf2(1)(OiPr)6], -829390 [for Zr2(2)(OiPr)6], -829391 [for Hf2(3)-
(OiPr)6], -829392 [for Hf(4)(OiPr)2], -829398 [for Zr(5)(OiPr)2-
1/2(C4H8O)1/2(C3H8O)], -829394 [for Zr(5)(OiPr)2], -829393 [for
Hf(5)(OiPr)2], -829395 [for Zr(6)(OiPr)2], -829396 [for Hf(6)-
(OiPr)2] and -829397 (for 3H2) contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
indicated the presence of H and OiPr end groups for the
solution polymerisations, as expected with the coordination
insertion mechanism. Pseudo first order solution kinetics
were investigated for Zr(5)(OiPr)2 (kapp = 0.025 min–1) and
Hf(5)(OiPr)2 (kapp = 0.008 min–1), and the ZrIV complex is
three times faster than the HfIV complex. This is in agree-
ment with previous work, which indicates that ZrIV initia-
tors tend to be faster acting than the analogous HfIV com-
plexes.[9c,9g] Also noteworthy is the fact that the polymerisa-
tion does not show any significant induction period.
4600
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Eur. J. Inorg. Chem. 2011, 4596–4602

Homopiperazine and Piperazine Complexes of ZrIV and HfIV
Table 5. Crystallographic details for Zr4(1)(OiPr)14(HOiPr)2, Hf2(1)(OiPr)6, Zr2(2)(OiPr)6, Hf2(3)(OiPr)6, Hf(4)(OiPr)2, Zr(5)(OiPr)2,
Hf(5)(OiPr)2, Zr(6)(OiPr)2, Hf(6)(OiPr)2 and 3H2.
Compound reference
Zr4(1)(OiPr)14(HOiPr)2 Hf2(1)(OiPr)6
Zr2(2)(OiPr)6
Hf2(3)(OiPr)6
Hf(4)(OiPr)2
Chemical formula
Formula mass
C76H156N2O18Zr4
1750.91
C80H140Hf4N4O16
2127.92
C47H84N2O8Zr2
987.60
C26H47HfNO4
616.14
C23.50H41Hf0.50NO2
458.82
Crystal system
a (Å)
b (Å)
c (Å)
monoclinic
21.8419(4)
15.0338(3)
29.6284(5)
90
104.829(1)
90
triclinic
orthorhombic
14.3530(6)
39.3260(15)
19.3250(7)
90
90
90
monoclinic
10.3720(5)
17.1010(9)
16.7140(7)
90
105.698(3)
90
triclinic
8.7915(3)
16.8368(6)
17.6504(6)
66.874(3)
79.532(3)
84.196(3)
12.8800(6)
17.1940(9)
21.3410(10)
108.912(2)
93.491(3)
93.019(3)
4449.6(4)
α (°)
β (°)
γ (°)
Unit cell volume (Å3)
Space group
9404.9(3)
C2/c
10907.9(7)
C2cb
2854.0(2)
Pn
2361.41(14)
¯
P1
¯
P1
Z
4
2
8
4
4
Reflections measured
42398
34810
64279
28088
23772
Independent reflections (Rint
GoF
)
7991 (0.0794)
1.046
13794 (0.1529)
1.023
9549 (0.0949)
1.098
10193 (0.0707)
1.047
10822 (0.0696)
0.979
R1, wR2 [IϾ2σ(I)]
R1, wR2 [all data]
0.0541, 0.1022
0.0916, 0.1170
0.508, –0.653
0.0655, 0.1120
0.1482, 0.1420
1.145, –1.315
0.0536, 0.1273
0.0889, 0.1513
0.665, –0.561
0.0414, 0.0812
0.0752, 0.0957
1.258, –0.748
0.0492, 0.0746
0.0792, 0.0831
1.101, –1.352
Max [min] difference (eÅ–3)
Compound reference
Zr(5)(OiPr)2
Hf(5)(OiPr)2
Zr(6)(OiPr)2
Hf(6)(OiPr)2
3H2
Chemical formula
Formula mass
C29H44N2O4Zr
575.88
C29H44HfN2O4
663.15
C35H56N2O4Zr
660.04
C17.50H26Hf0.50NO2 C35H56N2O2
371.64
536.82
Crystal system
a (Å)
b (Å)
c (Å)
monoclinic
8.5750(6)
16.5030(14)
20.7000(15)
90
99.220(5)
90
monoclinic
8.5680(3)
16.4690(7)
20.6830(7)
90
99.410(3)
90
monoclinic
11.7950(5)
24.3760(10)
7.6360(4)
90
124.476(2)
90
monoclinic
11.7460(2)
24.2790(5)
7.6000(2)
90
124.504(1)
90
triclinic
10.0750(2)
13.3950(3)
14.3470(3)
65.770(1)
88.236(1)
71.981(1)
1668.67(6)
α (°)
β (°)
γ (°)
Unit cell volume (Å3)
Space group
2891.5(4)
P21/n
2879.23(19)
P21/n
1809.86(14)
Cm
1786.10(7)
Cm
¯
P1
Z
4
4
2
7278
4
2
Reflections measured
45579
50883
17045
31837
Independent reflections (Rint
GoF
)
6603 (0.0584)
1.107
6592 (0.0760)
1.185
3018 (0.0452)
1.029
4042 (0.0338)
1.065
7600 (0.0632)
1.034
R1, wR2 [IϾ2σ(I)]
R1, wR2 [all data]
0.0349, 0.0789
0.0524, 0.0897
0.417, –0.700
0.0414, 0.0888
0.0614, 0.0977
1.963, 1.601
0.0360, 0.0799
0.0394, 0.0816
0.462, –0.579
0.0201, 0.0491
0.0201, 0.0491
0.789, –1.438
0.0540, 0.1341
0.0788, 0.1518
0.320, –0.334
Max, min difference (eÅ–3)
Synthesis of the Ligands: A typical ligand synthesis is as follows for
4H2: 2,4-di-tert-butylphenol (4.12 g, 20.0 mmol), homopiperazine
(1 g, 10.0 mmol) and formaldehyde (38% in H2O) (1.66 mL, 0.63 g,
21.0 mmol) were heated to reflux in MeOH (40 mL) for 24 h then
cooled (0 °C). During which time a white precipitate was observed,
which was collected by filtration, washed with cold MeOH and
dried to yield a white solid (1.29 g, 2.4 mmol, 24%). 1H NMR
1.79 (br, 1 H, CH2), 2.19 (br, 1 H, CH2), 2.44 (br, 2 H, CH2), 2.95
(d, J = 6.5 Hz, 2 H, CH2), 3.17 (d, J = 11.5 Hz, 2 H, CH2), 3.52
(d, J = 6.5 Hz, 2 H, CH2), 3.63 (br, 2 H, NCH2Ar), 3.63 (br, 1 H,
CH), 4.32 (d, J = 11.5 Hz, 2 H, NCH2Ar), 4.32 (br, 1 H, CH), 6.86
(s, 2 H, ArH), 7.24 (s, 2 H, ArH) ppm. 13C{1H} NMR (CDCl3,
258 K): δ = 22.6 (CH2), 25.8 (CH3), 27.5 (CH3), 30.0 (CH3), 31.8
(CH3), 34.0 (C), 35.1 (C), 54.5 (CH2), 56.3 (CH2), 63.7 (CH2), 68.0
(CDCl3): δ = 1.32 (s, 18 H, tBu), 1.47 (s, 18 H, tBu), 1.95 (quin, J (CH), 69.0 (CH), 122.6 (Ar), 124.5 (ArH), 124.6 (ArH), 136.3 (Ar),
= 6.1 Hz, 2 H, ring-CH2), 2.81 (s, 4 H, ring-CH2), 2.78 (t, J = 136.3 (Ar), 137.6 (Ar), 161.3 (ArO) ppm. C41H68HfN2O4 (831.49):
6.0 Hz, 4 H, ring-CH2), 3.81 (s, 4 H, NCH2Ar), 6.86 (d, J = 2.3 Hz,
2 H, ArH), 7.26 (d, J = 2.3 Hz, 2 H, ArH), 11.03 (br, 2 H, OH)
ppm. 13C{1H} NMR (CDCl3): δ = 26.9 (CH2), 29.8 (CH3), 31.8
(CH3), 34.3 (C), 35.0 (C), 53.2 (CH2), 54.6 (CH2), 62.6 (CH2), 121.4
(Ar), 123.1 (ArH), 123.6 (ArH), 135.8 (Ar), 140.7 (Ar), 154.4 (ArO)
ppm. Calc. m/z [C35H56N2O2 + H]+ 537.4420; found 537.4442.
calcd. C 59.22, H 8.24, N 3.37; found C 59.8, H 8.26, N 3.68.
Polymerisation Procedure: For solvent-free polymerisations the
monomer:initiator ratio employed was 300:1 (unless otherwise
stated) at a temperature of 130 °C, in all cases 1 g of rac-LA was
used. After the reaction time methanol (20 mL) was added to
quench the reaction and the resulting solid was dissolved in dichlo-
romethane. The solvents were removed in vacuo and the resulting
solid was washed with copious amounts of methanol to remove
Synthesis and Characterisation of the Complexes: A typical example
for the preparation of a complex is as follows for Hf(4)(OiPr)2: 4H2
(0.53 g, 0.99 mmol) and Hf(OiPr)4·iPrOH (0.47 g, 0.99 mmol) were any unreacted monomer. 1H NMR spectroscopy (CDCl3) and GPC
dissolved in CH2Cl2 (30 mL) and stirred for 16 h. The solvent was
removed in vacuo, and the residue was recrystallised from hot hex-
ane (40 mL) to yield colourless crystals (0.14 g, 0.17 mmol, 17%).
(THF) were used to determine tacticity and molecular weights (Mn
and Mw) of the polymers produced; Pm (the probability of isotactic
linkages) was determined by analysis of the methine region of the
1
1H NMR (CDCl3, 233 K): δ = –0.02 (d, J = 6.0 Hz, 6 H, Me), 1.18 homonuclear decoupled H NMR spectra. The equations used to
(d, J = 6.0 Hz, 6 H, tBu), 1.20 (s, 18 H, tBu), 1.40 (s, 18 H, tBu),
calculate Pr and Pm are given by Coates et al.[1a] Gel permeation
Eur. J. Inorg. Chem. 2011, 4596–4602
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4601

S. L. Hancock, M. F. Mahon, G. Kociok-Kohn, M. D. Jones
˝
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Acknowledgments
We gratefully acknowledge the Engineering and Physical Sciences
Research Council (EPSRC) for the National Mass Spectrometry
Service Centre Swansea and the University of Bath for funding.
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Received: June 13, 2011
Published Online: September 5, 2011
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Eur. J. Inorg. Chem. 2011, 4596–4602