Sodium, magnesium and zinc complexes of mono(phenolate) heteroscorpionate ligands

Article abstract of DOI:10.1039/b813116c

The reaction of bis(3,5-dimethylpyrazolyl)methylphenol N₂O ^ArH (1) with NaH in THF formed dimeric [Na(κ²-N ₂O^Ar)(THF)]₂ (2), which contains a κ²(N,O)-bound bidentate N₂

Full text of DOI:10.1039/b813116c

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www.rsc.org/dalton | Dalton Transactions
Sodium, magnesium and zinc complexes of mono(phenolate)
heteroscorpionate ligands†
A. Daniel Schofield, Mariana Luna Barros, Michael G. Cushion, Andrew D. Schwarz and Philip Mountford*
Received 29th July 2008, Accepted 11th September 2008
First published as an Advance Article on the web 21st October 2008
DOI: 10.1039/b813116c
The reaction of bis(3,5-dimethylpyrazolyl)methylphenol N2OArH (1) with NaH in THF formed dimeric
2
2
[Na(k -N2OAr)(THF)]2 (2), which contains a k (N,O)-bound bidentate N2OAr ligand. The reaction of 1
with MgnBu2 gave the four-coordinate monomeric butyl compound Mg(N2OAr)nBu (3), whereas with
nBuMgCl, a mixture of products was formed, including the six-coordinate homoleptic species
Mg(N2OAr)2 (4). The reaction of [Na(k -N2OAr)(THF)]2 with nBuMgCl also gave 3, as did the
2
redistribution reaction of MgnBu2 with 4. The reaction of 1 with Mg{N(SiRMe2)2}2 afforded the
four-coordinate amide derivatives Mg(N2OAr){N(SiRMe2)2} (R = Me (6) or H (7)), together with 4.
The reactions of 1 with ZnMe2 or Zn{N(SiMe3)2}2 gave the monomeric compounds Zn(N2OAr)Me (8)
and Zn(N2OAr){N(SiMe3)2} (9), respectively. The reaction of 9 with HCl formed Zn(N2OAr)Cl (11), and
subsequent addition of LiN(SiHMe2)2 to 11 led to Zn(N2OAr){N(SiHMe2)2} (12). The reaction of 1
with either Zn{N(SiMe3)2}2 or 9 gave Zn(N2OAr)2. The compounds 2, 3, 4, 6, 8, 9 and 11 were
crystallographically characterized. Compound 7 was very active for the ring-opening polymerization
(ROP) of e-caprolactone (e-CL) but the process was very poorly controlled as judged by the Mn and
polydispersity index of the polymer. Compounds 3, 8, 9 and 12 gave poor conversions to poly(e-CL)
over extended periods. N2OArH = 2,4-di-tert-butyl-6-(bis(3,5-dimethylpyrazolyl)methyl)phenol.
and structural studies, more recent reports have also focused on
Introduction
catalytic applications in the field of ring-opening or Ziegler–Natta
polymerization catalysis, but mostly in the area of N3 donor ligand
sets.17–23
Pyrazole-based ligands, known collectively as poly(pyrazolyl)
ligands, are derived from two or more N-deprotonated pyra-
zole rings bound to a main group atom through one of
the ring nitrogen atoms. While tris(pyrazolyl) ligands based
on aluminium, indium, gallium,1 silicon2 and germanium3 are
known, the anionic tris(pyrazolyl)hydroborate ligands4–7 are the
most widespread. First introduced by Trofimenko in 1967,8
tris(pyrazolyl)hydroborates have become one of the most widely
exploited class of ligand in inorganic chemistry,6 and they have
been used to prepare complexes of most metals of the periodic
table. Tris(pyrazolyl)methanes are the neutral analogues of the
anionic tris(pyrazolyl)hydroborates and are formally derived by
We have recently been developing the synthesis and catalytic
applications of homo and heteroscorpionate complexes of the
main group and early transition metals.9,10,12,18,24–28 Although our
work has so far focused on N3 donor ligands, we recently became
interested in the bis(pyrazolyl)methylphenol (N2O-type donor)
protio-ligands shown in Fig. 1. A number of transition metal
phenolate substituted heteroscorpionates have been reported and
structurally characterized,19,29–39 including inone instancefor zinc40
in the context of modelling the binding sites of metalloproteins.
Catalytic applications of complexes of these ligands are sparse,
with reports for olefin polymerization for group 419 and the
attempted use of aluminium complexes for the ring-opening
polymerization (ROP) of e-caprolactone (e-CL).21 No group 1 or
group 2 complexes of any bis(pyrazolyl)methylphenolate ligand
have been reported. As alluded to above, Milione’s very recent
report on aluminium complexes derived from N2OArH (1, Fig. 1)
is the only one for main group metals with this ligand class.21 This
underdevelopment of heteroscorpionate-based phenolate ligands
-
replacing the apical {BH} anionic moiety with the isoelectronic
CR group. They (and their anionic methanide analogues9–12
)
have lately received renewed interest, and the synthesis and
coordination chemistry of these ligands has been reviewed.13–15
The tris(pyrazolyl)hydroborate and tris(pyrazolyl)methane/
methanide ligands are examples of homoscorpionate ligands in
which the three moieties appended to the apical group/atom
are the same. A rapidly emerging class of poly(pyrazolyl)alkane-
based ligands are the so-called heteroscorpionates in which one
of the pyrazolyl groups has been replaced by a different (usually)
anionic C, O, S or N donor.16 While much of the early work with
heteroscorpionate ligands was primarily concerned with synthetic
Department of Chemistry, Chemistry Research Laboratory, University of
Oxford, Mansfield Road, Oxford, UK OX1 3TA. E-mail: philip.mountford@
chem.ox.ac.uk
† CCDC reference numbers 696837–696843. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b813116c
Fig. 1 Previously reported bis(pyrazolyl)methylphenol protio-ligands.
Compound 1 is the one used in this contribution (abbreviation “N2OArH”).
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◦
2
Table 1 Selected distances (A) and angles ( ) for [Na(k -N2OAr)(THF)]2
(2). Atoms carrying the suffix ‘A’ are related to their counterparts by the
operator -x, 1 - y, 1 - z
˚
is in contrast to the position for main group, transition metal and
f element chelating phenolate ligands in general, which attract
widespread interest.41–49
In this contribution, we describe the synthesis and structures of
a series of new sodium, magnesium and zinc complexes derived
from N2OArH (1), and an investigation into their ROP capability.
Protio-ligand 1 was selected from those shown in Fig. 1 as it offered
the highest degree of steric substitution, both proximal and distal
to the site of metal complex binding.
Na(1)–O(1)
Na(1)–O(1A)
Na(1)–O(2)
O(1)–Na(1)–O(2)
O(1A)–Na(1)–O(2) 125.65(5)
O(1)–Na(1)–N(1)
Na(1)–O(1)–C(13)
Na(1)–O(1)–Na(1A) 89.71(5)
2.3261(13) Na(1)–N(1)
2.1785(13) Na(1) ◊ ◊ ◊ C(13)
2.2973(14)
2.4662(15)
2.7946(16)
126.33(5)
O(1A)–Na(1)–O(1)
90.29(5)
O(1A)–Na(1)–N(1) 124.16(5)
O(2)–Na(1)–N(1) 71.15(4)
Na(1A)–O(1)–C(13) 173.54(11)
93.94(5)
96.26(9)
Results and discussion
is shown in Fig. 2 and selected bond distances and angles are sum-
Synthesis of sodium and magnesium complexes of N2OAr
marized in Table 1. Molecules of 1 lie across crystallographic in-
2
Protio-ligand N2OArH (1) was prepared in ca. 60% yield from
bis(3,5-dimethylpyrazolyl)ketone following the general method
of The´ and Peterson.50 Although 1 itself has been mentioned
previously,19,21 its synthesis and characterizing data have not been
reported. These are given in the Experimental for completeness.
We first describe sodium and magnesium derivatives of 1. The new
compounds and their syntheses are summarized in Scheme 1.
Owing to the favourable Brønsted acidity of the ArOH group,
alkali metal phenolates are readily prepared by transmetallation
reactions with group 1 alkyls (typically nBuLi48) and/or hydrides
(for example, NaH51). In addition to being synthetically versatile
reagents, alkali metal phenolates show a range of interesting
solid-state structures.48,52–54 While certain sodium bis(pyrazolyl)-
methylphenolates have been generated in situ,19 they have not been
isolated or structurally characterized.
version centres and contain two Na(k -N2OAr)(THF) units linked
through asymmetrically bridging phenoxide moieties (Na(1)–
˚
˚
O(1) = 2.3261(13) A; Na(1)–O(1A) = 2.1785(13) A). To a first
approximation, each Na is four-coordinate, forming close contacts
to the aforementioned phenoxide oxygen atoms, the oxygen of a
coordinated THF molecule, and a nitrogen atom of one of the
pyrazolyl rings (N(1)). The other pyrazolyl ring (containing N(3),
N(4)) is not coordinated. There is a further contact to the ipso
˚
carbon of the phenyl group (Na(1) ◊ ◊ ◊ C(13) = 2.7946(16) A),
as is typically the case in alkali metal complexes of phenoxide
ligands,48,52–54 bringing the formal coordination number of each Na
to five. The central Na2(m-O)2 moiety in 1 is close to an ideal, planar
square arrangement (O(1)–Na(1)–O(1A) = 90.29(5)◦; Na(1)–
O(1)–Na(1A) = 89.71(5)◦).
From the point of view of alkali metal mono(phenoxide)
structural chemistry, several Na2(OAr)2(L)x systems have related
solid-state features.55–58 With respect to the coordination chem-
istry of N2OAr complexes (Fig. 1), the aluminium compound
N2OArH (1) reacts readily with NaH in THF to form the binu-
2
clear complex [Na(k -N2OAr)(THF)]2 (2) in reasonable yield after
recrystallization from THF (Scheme 1). The molecular structure
Scheme 1 Reactions of N2OArH (1) with sodium and magnesium reagents.
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2
Fig. 2 Displacement ellipsoid plots (25% probability) of [Na(k -N2OAr)(THF)]2 (2). H atoms omitted for clarity. Atoms carrying the suffix ‘A’ are related
to their counterparts by the symmetry operator -x, 1 - y, 1 - z. Left: approximately perpendicular to the Na2(m-O)2 plane. Right: approximately parallel
to the Na2(m-O)2 plane.
2
Al(k -N2OAr)Me2 is the only previously reported example of a
bidentate coordination mode for this ligand.21
At room temperature, the 1H and 13C NMR spectra for 1
show two equivalent Me2pz ring environments on the NMR
timescale, which is inconsistent with the solid-state structure.
Cooling a CD2Cl2 solution to 183 K led to partial decoalescence
of the Me2pz ring signals (four broad overlapping methyl and
two broad pyrazolyl ring methine resonances) consistent with the
solid state structure being maintained in solution, at least at low
temperatures.
2
Milione et al. reported that treatment of Al(k -N2OAr)R2
(R = Me or Et) with B(C6F5) generated the cationic com-
plexes [Al(N2OAr)R]+, which contain a tridentate bis(pyrazolyl)-
methylphenolate ligand according to the NMR data (these were
not structurally characterized).21 Scheme 1 summarizes our results
regarding the synthesis of neutral magnesium complexes of the
type Mg(N2OAr)X, isoelectronic (for X = alkyl or amide) with
Fig. 3 Displacement ellipsoid plot (20% probability) of Mg(N2OAr)nBu
(3). H atoms omitted for clarity.
the cationic aluminium systems. These are the first magnesium
complexes of any N2O-donor heteroscorpionate ligand.
The reaction of N2OArH (1) with MgnBu2 gave the “half-
sandwich” alkyl derivative Mg(N2OAr)nBu (3) in good yield. The
X-ray structure has also been determined (Fig. 3 and Table 2)
and the solution NMR and other data are consistent with this.
The solid state structure of 3 and all the new magnesium and zinc
complexes of N2OAr are compared and discussed below.
This reaction also produces small quantities of the homoleptic
(“sandwich”) complex Mg(O2NAr)2 (4, discussed below) even when
3 equiv. MgnBu2 are used. It is unlikely that 4 arises from a
ligand redistribution of 3 (as has been noted with less sterically
demanding tris(pyrazolyl)hydroborate magnesium alkyls59) since
heating a pure sample of 3 in C6D6 at 70 ◦C for 12 h gave no
change in the 1H NMR spectrum. In contrast, heating a mixture
of MgnBu2 and 4 in C6D6 formed the half-sandwich species 3,
establishing that this is the thermodynamically favoured product.
Therefore, the formation of 4 probably arises from the reaction of
first-formed 3 with protio-ligand 1 at a rate that is apparently
competitive with that between 1 and MgnBu2. This problem
was found to be particularly acute using the diamide reagents
Mg{N(SiRMe2)2}2 (R = Me or H, see below). Consistent with
this observation, the reaction of 3 with 1 on the NMR tube scale
quantitatively formed 4.
◦
Table 2 Selected distances (A) and angles ( ) for Mg(N2OAr)nBu (3) and
˚
Zn(N2OAr)Me (8)
Parameter
3 (M = Mg)
8 (M = Zn)
M(1)–O(1)
M(1)–N(1)
M(1)–N(3)
M(1)–C(26)
1.9058(19)
2.122(2)
2.142(2)
1.9416(14)
2.0954(19)
2.0935(18)
1.974(2)
2.124(3)
O(1)–M(1)–N(1)
O(1)–M(1)–N(3)
O(1)–M(1)–C(26)
N(1)–M(1)–N(3)
N(1)–M(1)–C(26)
N(3)–M(1)–C(26)
M(1)–O(1)–C(13)
96.55(8)
91.88(8)
126.27(11)
86.82(8)
122.08(11)
123.14(11)
137.75(16)
98.15(6)
92.29(6)
128.78(9)
85.89(7)
119.28(10)
121.95(9)
132.40(12)
A potential alternative synthesis of 3 was found on the NMR
2
tube scale by reaction of [Na(k -N2OAr)(THF)]2 (2) with nBuMgCl
in C6D6. Although quantitative formation of 3 appeared to occur
as judged by 1H NMR, this reaction could not be scaled up cleanly.
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ically characterized zinc analogue Zn(N2OAr)Cl (11, see below).
Attempts to isolate pure 5 from the MgCl2 led to decomposition.
The ready formation of insoluble (probably oligomeric)
scorpionate-type complexes [Mg(L)X]n (X = halide) has been
noted previously in the reactions of the corresponding protio-
ligands HL with Grignard reagents.12 Such reactions have also
been shown to give the corresponding homoleptic sandwich
derivatives Mg(L)2 via Schlenk-type equilibria.12,61 In principle, the
sandwich complex Mg(O2NAr)2 (4) could arise via a redistribution
reaction of first-formed 5, and/or by reaction of MgnBu2 (formed
from nBuMgCl via the Schlenk equilibrium) with N2OArH, which
would be present in comparable excess. In a control experiment,
the reaction of 1 (2 equiv.) with MgnBu2 afforded 4, quantitatively.
The reaction of N2OArH (1) with the diamide complexes
Mg{N(SiRMe2)2}2 (R = Me or H) gave modest isolated yields
of the half-sandwich derivatives Mg(N2OAr){N(SiMe3)2} (6, 11%)
and Mg(N2OAr){N(SiHMe2)2} (7, 21%), respectively, as white
solids. The NMR spectra of 6 and 7 are comparable to those
of the n-butyl homologue Mg(N2OAr)nBu (3). The X-ray structure
of 6 has been determined (Fig. 5 and Table 4). This is discussed
below and confirms that shown in Scheme 1. The low yields of 6
and 7 can be attributed to the facile formation of the sandwich
complex 4 in these reactions. A ca. 50% yield of 4 is obtained in
the reaction of 1 with Mg{N(SiMe3)2}2, for example. Attempts
◦
Table 3 Selected distances (A) and angles ( ) for Mg(N2OAr)2 (4)
˚
Mg(1)–O(1)
Mg(1)–N(1)
1.979(3)
2.206(4)
2.315(4)
Mg(1)–O(2)
Mg(1)–N(5)
Mg(1)–N(7)
1.964(3)
2.187(4)
Mg(1)–N(3)
2.312(4)
O(1)–Mg(1)–O(2)
O(2)–Mg(1)–N(1)
O(2)–Mg(1)–N(3)
O(1)–Mg(1)–N(5)
N(1)–Mg(1)–N(5)
O(1)–Mg(1)–N(7)
N(1)–Mg(1)–N(7)
N(5)–Mg(1)–N(7)
Mg(1)–O(2)–C(38)
98.49(14)
101.44(14)
168.96(15)
100.94(14)
167.49(15)
172.84(14)
89.30(14)
82.81(14)
144.7(3)
O(1)–Mg(1)–N(1)
O(1)–Mg(1)–N(3)
N(1)–Mg(1)–N(3)
O(2)–Mg(1)–N(5)
N(3)–Mg(1)–N(5)
O(2)–Mg(1)–N(7)
N(3)–Mg(1)–N(7)
Mg(1)–O(1)–C(13)
85.93(14)
91.81(14)
83.15(14)
87.98(14)
86.17(14)
87.68(14)
82.31(14)
139.7(3)
Difficulties in using this method for the formation of half-sandwich
magnesium alkyl complexes of tris(pyrazolyl)hydroborate ligands
have been noted previously by Parkin,60 whereas Otero has
reported success using certain N3 donor heteroscorpionates.20
The reaction of the protio-ligand 1 with nBuMgCl was expected
to provide the corresponding chloride species [Mg(N2OAr)Cl]n (5)
by analogy with the synthesis of 3. However, this reaction gave
both the six-coordinate sandwich complex Mg(O2NAr)2 (4) and an
insoluble material (Scheme 1). Compound 4 (obtained in 40% yield
based on 1) has been crystallographically characterized (Fig. 4 and
1
Table 3) and the molecular structure is discussed below. The H
and 13C NMR spectra of 4 show that this structure is maintained
in solution since they feature two Me2pz ring environments for the
groups cis and trans to the phenolate donors.
Fig. 5 Displacement ellipsoid plot (20% probability) of Mg(N2OAr)-
{N(SiMe3)2} (6). H atoms and toluene of crystallization omitted for clarity.
◦
Table 4 Selected distances (A) and angles ( ) for Mg(N2OAr){N(SiMe3)2}
˚
(6) and Zn(N2OAr){N(SiMe3)2} (9)
Fig. 4 Displacement ellipsoid plot (20% probability) of Mg(N2OAr)2 (4).
H atoms and diethyl ether of crystallization omitted for clarity.
Parameter
6 (M = Mg)
9 (M = Zn)
M(1)–O(1)
M(1)–N(1)
M(1)–N(3)
M(1)–N(5)
O(1)–M(1)–N(1)
O(1)–M(1)–N(3)
O(1)–M(1)–N(5)
N(1)–M(1)–N(3)
N(1)–M(1)–N(5)
N(3)–M(1)–N(5)
M(1)–O(1)–C(13)
1.903(2)
2.100(3)
2.124(3)
1.936(2)
2.061(2)
2.080(2)
1.923(3)
91.27(9)
96.78(9)
126.18(10)
91.55(9)
121.61(10)
120.80(11)
131.78(17)
The insoluble precipitate formed alongside 4 is believed to
contain a mixture of MgCl2 and [Mg(N2OAr)Cl]n (5, “n” is
undetermined), or even polymetallic aggregates of the two
components as has been observed previously for transition
metal bis(pyrazolyl)methylphenolate complexes.29–31,33–35,37–39 The
EI mass spectrum obtained from this solid gave peak envelopes
with the expected isotopic distributions for “[Mg(N2OAr)Cl]+”
(70% of the base peak’s intensity (namely [M - Me]+)). The solid
state IR spectrum was comparable to that of the crystallograph-
1.992(3)
91.13(10)
95.39(10)
127.76(11)
89.26(10)
122.10(11)
121.33(11)
136.54(19)
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to form a calcium analogue of 6 by reaction of 1 (1 equiv.)
with Ca{N(SiMe3)2}2(THF)262 gave a highly insoluble precipitate.
homoleptic sandwich complex Zn(N2OAr)2 (10, see below) was
found in this reaction. The NMR spectra for 8 were as expected
and consistent with the solid state structure (Fig. 6 and Table 2).
1
When followed by H NMR ca. half of the calcium compound
remained unreacted, indicating formation of a homoleptic com-
pound “Ca(N2OAr)2”. This reaction was not pursued.
Metal alkoxide and aryloxide complexes are known to be
efficient initiators for the ROP of cyclic esters.49,63–66 We therefore
attempted to prepare complexes of the type Mg(N2OAr)(OR) (R =
t
iPr, Bu or 2,6-C6H3Me2) by protonolysis reactions between the
n
corresponding ROH and Mg(N2OAr)X (X = Bu (3), N(SiMe3)2 (6)
or N(SiHMe2)2 (7)). Unfortunately, in all cases, unknown mixtures
of products were obtained although the 1H NMR spectra showed
successful elimination of the corresponding “HX”.
Synthesis of zinc complexes of N2OAr
Whereas no N2O donor heteroscorpionates of Mg had been re-
ported prior to our studies, certain zinc complexes of a homologue
of N2OArH (1) were described by Carrano in the context of
modelling the binding sites of zinc metalloproteins.40 However,
no ROP studies were carried out, nor were amide complexes
described. We therefore incorporated a study of zinc analogues
of the magnesium complexes Mg(N2OAr)X (3–7) into our study
of this chemistry. The new compounds and their syntheses are
summarized in Scheme 2 and (where available) their crystal
structures are compared with those of the magnesium homologues
in a later section.
Fig. 6 Displacement ellipsoid plot (25% probability) of Zn(N2OAr)Me
(8). H atoms omitted for clarity.
Heating a solution of N2OArH (1) and an excess of ZnMe2 at
N2OArH (1) reacted more readily with Zn{N(SiMe3)2}2 (3 h,
room temperature) to give Zn(N2OAr){N(SiMe3)2} (9) as a white
solid in 69% yield (Scheme 2). The good yield of 9 contrasts
sharply with that for the Mg homologue 6 (11%). The difference
is attributed to the negligible extent of formation of Zn(N2OAr)2
(10) and the low solubility of the latter, which allows for easy
◦
60 C for 14 h gave the monomethyl complex Zn(N2OAr)Me (8)
in modest yield. The lower yield and harsher conditions required
in this case, compared with those for the reaction with MgnBu2,
reflect the higher electronegativity of zinc compared to the group
2 metal. For the same reason, no evidence for formation of the
Scheme 2 Synthesis of zinc derivatives containing the N2OAr ligand.
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separation. The 1H and 13C NMR spectra of 9 are comparable to
those of 6 and are consistent with the solid state structure (Fig. 7)
discussed below. Although concomitant formation of Zn(N2OAr)2
(10) is not a problem in the synthesis of 9, the homoleptic
compound can be obtained from either Zn{N(SiMe3)2}2 and
2 equiv. of 1, or from pre-formed half-sandwich 9 and 1 equiv. of 1
(NMR tube scale only). In contrast to its magnesium homologue,
compound 10 is highly insoluble in non-reactive solvents (benzene,
THF, pyridine, CH2Cl2) and was characterized by elemental
analysis and IR spectroscopy.
Fig. 8 Displacement ellipsoid plot (25% probability) of Zn(N2OAr)Cl
(11). H atoms omitted for clarity.
use of more forcing conditions led to decomposition. The reactions
of Zn(N2OAr){N(SiMe3)2} (9) with ROH were more encouraging
on the NMR tube scale (cf. its reactions with N2OArH (1), HCl
and HN(SiHMe2)2 mentioned above). Regrettably, no well-defined
complex could be obtained from these reactions.
Fig. 7 Displacement ellipsoid plot (20% probability) of Zn(N2OAr)-
{N(SiMe3)2} (9). H atoms and toluene of crystallization omitted for clarity.
Structural studies of the magnesium and zinc complexes
2
Surprisingly, the reactions of [Na(k -N2OAr)(THF)]2 (2) with
As mentioned, we have structurally characterized a number of
the magnesium and zinc N2OAr complexes. We discuss them
systematically in turn here. Table 2 compares key distances and
angles for Mg(N2OAr)nBu (3, Fig. 3) and Zn(N2OAr)Me (8, Fig. 6).
Both complexes feature a severely distorted, tetrahedral metal
centre with C–M–O and C–M–N angles in the range ca. 119–
129◦ and N–M–N and N–M–O angles in the range ca. 86–98◦
ZnCl2 did not give access to Zn(N2OAr)Cl (11). This com-
pound was therefore prepared from the 1 : 1 reaction of
Zn(N2OAr){N(SiMe3)2} (9) with HCl (Et2O solution) in benzene
(69% yield). In contrast to [Mg(N2OAr)Cl]n (5), 11 has acceptable
solubility in halogenated solvents and appears to be indefinitely
stable in solution (no redistribution reaction to form homoleptic
10, for example). The molecular structure of 11 has been deter-
mined (Fig. 8 and Table 5) and is discussed below. Compound 11
also serves as an entry point to the less bulky amide derivative
Zn(N2OAr){N(SiHMe2)2} (12) upon reaction with LiN(SiHMe2)2
in benzene. The NMR spectra of 12 are comparable to those of
the magnesium analogue 7. Compound 12 can also be formed
by reaction of Zn(N2OAr){N(SiMe3)2} (9) with HN(SiHMe2)2 at
70 ◦C for 3 d (NMR tube scale in C6D6). No reaction was
observed between Zn(N2OAr)Me (8) and HN(SiHMe2)2 under
these conditions.
3
owing to the constraining k -N2OAr ligand. In general, the M–
C, M–N and M–O distances at each metal are within the ranges
previously reported for four-coordinate Mg and Zn.53,54 More
specifically the M–C values are also comparable to other “half-
sandwich” scorpionate and heteroscorpionate complexes reported
previously.20,22,40,60,67,68
Significant differences are found on comparing the trends in
metal–ligand distances for the different donor types. The four-
˚
coordinate effective ionic radii of Mg and Zn (0.71 and 0.74 A,
respectively69) are almost the same, with that of Mg being slightly
smaller. Although the valence AOs of Zn are the 4s and 4p (with
the 3d being semi core-like), the large increase in Zeff arising from
crossing the 3d series reduces the radius to nearly that of Mg (3s
As for the magnesium systems, we attempted to prepare zinc
alkoxide or aryloxide complexes of the type Zn(N2OAr)(OR) (R =
alkyl or aryl, see above). Limited reactivity was found between the
various ROH and Zn(N2OAr)Me (8) at room temperature, and the
and 3p valence AOs).70 The Mg–O distance in 3 is 0.036(2) A
˚
shorter than Zn–O in 8, whereas the Mg–C distance is 0.150(4)
A longer. The Mg–N distances are also systematically longer for
◦
Table 5 Selected distances (A) and angles ( ) for Zn(N2OAr)Cl (11)
˚
˚
Mg. These differences are attributable to the different bonding
characteristics of Mg and Zn. The former is more ionic in nature
and so the Mg–O bond will contain a higher ionic character,
apparently leading to a shorter bond length compared to Zn. Zinc,
with its higher Zeff, is more polarizing and covalent. Therefore,
the more polarizable alkyl (and to some extent pyrazolyl N)
Zn(1)–Cl(1)
Zn(1)–O(1)
O(1)–Zn(1)–N(1)
O(1)–Zn(1)–N(3)
O(1)–Zn(1)–Cl(1)
N(1)–Zn(1)–N(3)
2.1810(5)
1.9105(14)
93.81(6)
105.55(6)
123.28(4)
89.48(6)
Zn(1)–N(1)
Zn(1)–N(3)
N(1)–Zn(1)–Cl(1)
N(3)–Zn(1)–Cl(1)
Zn(1)–O(1)–C(13)
2.0494(17)
2.0505(16)
122.04(5)
116.02(5)
127.63(12)
90 | Dalton Trans., 2009, 85–96
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donors find a more favourable interaction with Zn than with Mg,
consistent with recent DFT-computed bond dissociation energies
in N3 donor Zn–Me and Mg–Me scorpionate complexes.12
Table 4 compares key distances and angles for the isomorphic
pair M(N2OAr){N(SiMe3)2} (M = Mg (6, Fig. 5) or Zn (9, Fig. 7)).
Again the M–N and M–O distances are within the usual ranges
and the overall geometries are comparable to those of 3 and 8. The
Mg–O distance is 0.033(3) shorter than Zn–O, whereas the Mg–N
bonds to the pyrazolyl and amide nitrogen atoms are all longer
than their Zn analogues. The M–Npz and M–O distances in 6 and
9 are systematically shorter than those in the alkyl homologues
3 and 8, despite the larger steric impact of the N(SiMe3)2 ligand
compared to Me or nBu. This is attributed to the better s-donor
ability of the alkyl ligands. The SiMe3 substituents in 6 and 9
are orientated approximately perpendicular to the plane defined
by N(1), N(3) and N(5). This avoids unfavourable interactions
between the pyrazolyl methyl groups of C(4) and C(9) and the
amide SiMe3 substituents.
as a test monomer (e-CL being more readily ring-opened via a
coordination–insertion mechanism than lactides for example).
In a typical experiment, using either THF or toluene solvent at
room temperature, a solution of the chosen initiator was treated
with 100 equiv. of e-CL and the ROP was monitored by taking
aliquots, which were evaluated by 1H NMR spectroscopy and gel
permeation chromatography (GPC).
Initial studies using the magnesium amide Mg(N2OAr){N-
(SiHMe3)2} (7) appeared encouraging. Addition of 100 equiv. of
e-CL to a toluene solution of 7 gave complete conversion to poly(e-
1
CL) immediately as judged by H NMR spectroscopy. However,
GPC analysis of the poly(e-CL) formed indicated rather poor
control of the polymerization process: the observed Mn value of
30 870 g mol-1 was considerably larger than that expected for
100% conversion (11 410 g mol-1) and the polydispersity index
(PDI, Mw/Mn) was 2.76. These data are consistent with slow
initiation of the polymerization in comparison with chain growth.
Disappointing results were also found in THF. In this case addition
of e-CL to a THF solution of 7 immediately gave a highly viscous
solution, which was impossible to stir. However, 1H NMR analysis
showed only 22% conversion. GPC analysis yielded experimental
Mn and PDI values of 36 390 and 2.64, respectively (the expected
Mn is only 2510 in this case).
Tables 3 and 5 summarize the bond distances and angles for
Mg(N2OAr)2 (4, Fig. 4) and Zn(N2OAr)Cl (11, Fig. 6). The Zn com-
plex has an analogous geometry to those of the alkyl and amide
homologues. The Zn–Npz and Zn–O distances in 11 are shorter
than those of either Zn(N2OAr)Me (8) or Zn(N2OAr){N(SiMe3)2}
(9), which is consistent with the poorer donor ability and higher
electronegativity of Cl.
The zinc amides Zn(N2OAr){N(SiRMe3)2} (R = Me (9) or
H (12)) were also assessed for the ROP of e-CL. Both were
very poor initiators with sluggish polymerization occurring over
25 h to give conversions of 21 and 10%, respectively. Over this
time, the solutions became progressively cloudy, suggesting some
sort of catalyst degradation. Although the agreement between
found and expected Mn values for the poly(e-CL) were better
than for 7 (found: 2640, 1850; expected: 2400, 1140 g mol-1)
the PDIs remained much higher than expected (2.30 and 2.60,
respectively). Finally the alkyl complexes Mg(N2OAr)nBu (3) and
Zn(N2OAr)Me (8) were evaluated. At best these showed less than
10% conversion of e-CL over time and again the solutions became
cloudy. The formation of these cloudy mixtures and very poor
conversions indicate that, unlike the corresponding N3 donor
scorpionate and heteroscorpionates, the N2OAr species undergo as-
yet undetermined degradation processes. Potentially, these involve
attack/insertion of the e-CL at the M–O bond to the N2OAr ligand
(despite the bulky tert-butyl substituent) as proposed by Milione
et al. for their cationic aluminium systems. Further detailed
experimental work would be required to uncover the exact reasons
for the poor activity of the compounds 3, 8, 9 and 12.
Mg(N2OAr)2 (4) contains a six-coordinate Mg atom in an
approximately octahedral geometry and overall molecular C2 sym-
metry. The phenolate donors occupy mutually cis coordination
sites with the ortho-tBu substituents orientated above and below
the equatorial plane defined by Mg(1), N(3), N(7), O(1) and O(2).
A number of homoleptic transition metal complexes of related
N2O donor heteroscorpionate ligands have been structurally
characterized.29,30,33 In contrast to 4, these have mutually trans
phenolate groups with approximately linear O–M–O linkages and
molecular C2h symmetry. The Mg–O and Mg–N distances in 4 are
significantly longer than in the four-coordinate complexes 3 and
6, as a consequence of its higher coordination number. The Mg–N
distances trans to the Mg–O bonds are significantly longer than the
˚
cis ones (D(Mg–N) = 0.109(6), 0.125(6) A) as a consequence of the
greater trans influence of the anionic oxygen donors. However, this
2
labilizing effect is not sufficient to force the k (N,O) coordination
2
2
found in [Na(k -N2OAr)(THF)]2 (2) or Milione’s Al(k -N2OAr)R2
(R = Me or Et).21
ROP of e-caprolactone using the magnesium and zinc complexes
Conclusions
While aliphatic polyesters were first made by step polymerization
(condensation), ring-opening polymerization (ROP) is the com-
mercial method of choice starting from cyclic esters as monomers.
Technologically, two of the most important esters are lactide
and e-caprolactone (e-CL).71–73 A number of group 2 and 12
complexes are known to be effective initiators for the ROP of
these cyclic esters.63–66 Among the systems evaluated are N3 donor
homoscorpionate68,74,75 and (more recently) heteroscorpionate20,22
Mg, Ca and Zn derivatives. Some of these show good activities
and control of the ROP. N2O-donor heteroscorpionates of groups
2 and 12 have not been studied and this represents a deficiency in
this area. We therefore carried out an assessment of the ROP
capability of the new alkyl and amide complexes using e-CL
We have reported the synthesis and solid state structures of a
range of new complexes of the heteroscorpionate ligand N2OAr.
Well-defined Na and Mg derivatives have been reported for the first
time and new zinc complexes of N2O donor heteroscorpionate ary-
loxide ligands have been prepared. The series of crystal structures
for the magnesium and zinc alkyl, amide, chloride and sandwich
compounds have allowed a systematic structural evaluation of
these systems as a function of the “X” ligand in compounds of
the type M(N2OAr)X. Despite the use of the bulky ortho tert-
butyl group in N2OAr, the sandwich compound formation is a
significant complication for the more ionic group 2 metals. ROP
of e-caprolactone by half-sandwich complexes M(N2OAr)X only
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Dalton Trans., 2009, 85–96 | 91
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proceeded to completion in the case of Mg(N2OAr){N(SiHMe3)2}
(7), but poor control of the polymerization process was inferred
from the measured poly(e-CL) molecular weights. The magnesium
and zinc alkyls, as well as the zinc amides, provided only very
poor e-CL conversions. Concomitant initiator degradation/side-
reactions may be the origin of the poor behaviour. Although metal
alkoxide derivatives M(N2OAr)OR (R = alkyl or aryl) would likely
make for better initiators, these were not accessible experimentally.
to RT. CH2Cl2 was added (100 mL) and the resulting blue
solution was washed with deionized H2O (3 ¥ 30 mL). The
organic phase was dried over MgSO4, filtered and the volatiles
removed under reduced pressure yielding N2OArH (1) as a white
solid, which was thoroughly dried in vacuo. Yield: 6.29 g (57%).
1H NMR (C6D6, 299.9 MHz, 293 K) d/ppm: 10.79 (1 H, s,
(Me2pz)2CHOArH), 7.69 (1 H, s, (Me2pz)2CHOArH), 7.58 (1 H,
d, 3J = 3 Hz, 4-C6H2 Bu2), 7.00 (1 H, d, 3J = 3 Hz, 6-
t
t
C6H2 Bu2), 5.59 (2 H, s, N2C3HMe2), 1.98 (6 H, s, N2C3HMe2),
1.97 (6 H, s, N2C3HMe2), 1.61 (9 H, s, 3-C6H2(CMe3)2),
Experimental
1
1.28 (9 H, s, 5-C6H2(CMe3)2) ppm. 13C-{ H} NMR (C6D6,
t
75.4 MHz, 293 K) d/ppm: 153.3 (2-C6H2 Bu2), 148.2 (3 or 5-
General methods and instrumentation
t
N2C3HMe2), 142.2 (5-C6H2 Bu2), 141.2 (3 or 5-N2C3HMe2), 140.0
t
t
t
All manipulations were carried out using standard Schlenk line or
dry-box techniques under an atmosphere of argon or dinitrogen.
Solvents were degassed by sparging with dinitrogen and dried
by passing through a column of the appropriate drying agent.76
Deuterated solvents were refluxed over the appropriate drying
agent, distilled and stored under dinitrogen in Teflon valve
ampoules. NMR samples were prepared under dinitrogen in 5 mm
(3-C6H2 Bu2), 125.3 (4-C6H2 Bu2), 125.1 (6-C6H2 Bu2), 124.7 (1-
C6H2(tBu)2), 106.9 (4-N2C3HMe2), 74.7 ((Me2pz)2CHOArH), 35.6
(3-C6H2(CMe3)2), 34.4 (5-C6H2(CMe3)2), 31.7 (5-C6H2(CMe3)2),
30.1 (3-C6H2(CMe3)2), 13.6 (N2C3HMe2), 11.2 (N2C3HMe2).
Anal. found (calcd for C25H36N4O): C 73.4 (73.5), H 8.8 (8.9),
N 13.7 (13.6)%. IR (KBr plates, Nujol mull, n/cm-1): 1556 (s),
1341 (m), 1312 (m), 1251 (w), 1231 (m), 1043 (w), 866 (w), 831 (w),
777 (m), 702 (w), 665 (s). EI-MS: m/z = 311 (100%) [M - Me2pz]+,
189 (40%) [M - Me–OAr]+.
1
Wilmad 507-PP tubes fitted with J. Young Teflon valves. H and
1
13C-{ H} NMR spectra were recorded on Varian Mercury-VX 300
and Varian Unity Plus 500 spectrometers and referenced internally
to residual protio-solvent (1H) or solvent (13C) resonances, and are
reported relative to tetramethylsilane (d = 0 ppm). Assignments
were confirmed as necessary with the use of DEPT-135, DEPT-
[Na(j2-N2OAr)(THF)]2 (2). To a slurry of NaH (0.028 g,
1.15 mmol) in THF (10 mL), N2OArH (1, 0.47 g, 1.15 mmol) in
THF (30 mL) at 0 ◦C was added, portion-wise. The mixture was
allowed to warm to RT and after 12 h volatiles were removed
under reduced pressure. The crude product was recrystallised
90, and two dimensional H–1H and 13C–1H NMR correlation
1
experiments. Chemical shifts are quoted in d (ppm) and coupling
constants in Hz. IR spectra were recorded on a Nicolet Magna
560 E.S.P. FTIR spectrometer. Samples were prepared in a
dry-box as Nujol mulls between KBr plates, and the data are
quoted in wavenumbers (cm-1). Mass spectra were recorded by the
mass spectrometry service of Oxford University’s Department of
Chemistry. Elemental analyses were carried out by the Elemental
Analysis Service at the London Metropolitan University.
Poly(e-CL) molecular weights (Mn, Mw were determined using
gel permeation chromatography (GCP). GPC analyses were
carried out using a Polymer Laboratories Plgel Mixed-D column
(300 mm length, 7.5 mm diameter) and a Polymer Laboratories
PL-GPC50 Plus instrument equipped with a refractive index
detector. THF (HPLC grade) was used as an eluent at 30 ◦C with
a rate of 1 mL min-1. Linear polystyrenes were used as primary
calibration standards, and Mark–Houwink corrections for poly(e-
CL) in THF77 were applied for the experimental samples.
◦
2
from THF at -30 C to give [Na(k -N2OAr)(THF)]2 (2) as a
white microcrystalline solid. Yield: 0.34 g (52%). 1H NMR (C6D6,
500.1 MHz, 293 K) d/ppm: 8.14 (1 H, br s, (Me2pz)2CHOAr),
t
7.00 (2 H, br overlapping s, 4 and 6-C6H2 Bu2), 5.86 (1 H,
br s, 4-N2C3HMe2), 5.72 (1 H, br s, 4-N2C3HMe2), 3.16 (4 H,
br m, 2-C4H8O), 2.11 (3 H, br s, N2C3HMe2), 1.96 (6 H, br
overlapping s, N2C3HMe2), 1.66 (3 H, br s, N2C3HMe2), 1.50
(4 H, br m, 3-C4H8O), 1.33 (9 H, s, 3-C6H2(CMe3)2), 1.08
1
(9 H, s, 5-C6H2(CMe3)2) ppm. 13C-{ H} NMR (C6D6, 75.4 MHz,
t
293 K) d/ppm: 165.5 (2-C6H2 Bu2), 147.8 (5-N2C3HMe2), 141.2
t
t
(3-N2C3HMe2), 137.5 (3-C6H2 Bu2), 131.6 (5-C6H2 Bu2), 125.6
t
t
t
(4-C6H2 Bu2), 124.5 (6-C6H2 Bu2), 122.7 (1-C6H2 Bu2), 106.7
(4-N2C3HMe2), 74.5 ((Me2pz)2CHOAr), 67.8 (2-C4H8O), 35.5 (3-
C6H2(CMe3)2), 34.1 (5-C6H2(CMe3)2), 32.1 (5-C6H2(CMe3)2), 30.6
(3-C6H2(CMe3)2), 25.6 (3-C4H8O), 13.4 (5-N2C3HMe2), 11.5 (3-
N2C3HMe2) ppm. Anal. found (calcd for C58H86N8Na2O4): C 69.4
(69.3), H 8.6 (8.6), N 11.1 (11.2)%. IR (KBr plates, Nujol mull,
n/cm-1): 1603 (m), 1556 (s), 1417 (s), 1359 (m), 1344 (m), 1304 (m),
1230 (m), 1197 (w), 1153 (w), 1123 (m), 1050 (s), 1026 (m), 987 (w),
887 (m), 860 (m), 829 (m), 785 (m), 745 (w), 732 (m), 703 (m),
682 (m), 665 (m), 636 (m), 627 (w).
Starting materials
Bis(3,5-dimethylpyrazolyl)ketone, Mg{N(SiRMe2)2}2 (R = Me or
H), Zn{N(SiMe3)2}2 and LiN(SiHMe2) were prepared according
to the literature methods.50,78–81 e-Caprolactone was dried over
freshly ground CaH2 and distilled before use. Other reagents were
purchased and used without further purification.
Mg(N2OAr)nBu (3). To a solution of MgnBu2 (1.57 mL,
1.57 mmol; 1 M in heptane,) in toluene at -78 ◦C (15 mL) was
added a solution of N2OArH (1, 0.21 g, 0.52 mmol) in toluene
(20 mL) dropwise at -78 ◦C. The pale yellow solution was left at
-78 ◦C for 2 h before being allowed to warm to RT. Volatiles
were removed under reduced pressure to yield Mg(N2OAr)nBu
(3) as a pale yellow solid, which was washed with pentane (2 ¥
10 mL) and dried in vacuo. The sample contained small amounts
N2OArH (1). Although N2OArH has been mentioned in the
literature,19 experimental details and characterization data have
not been reported. We therefore give them here for complete-
ness. A mixture of bis(3,5-dimethylpyrazolyl)ketone (5.88 g,
27.0 mmol), 3,5-di-tert-butlysalicylaldehyde (◦6.32 g, 27.0 mmol)
and CoCl2·6H2O (0.20 g) was heated to 100 C, stirring rapidly.
After 6 h, the blue melt became a blue solid and was cooled
1
of Mg(N2OAr)2 (4) by H NMR. Bulk separation of analytically
pure 3 was not possible, but a small number of diffraction-quality
92 | Dalton Trans., 2009, 85–96
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crystals were grown by slow evaporation of a solution in diethyl
ether. Yield: 0.25 g (90% based on 3).
1351 (m), 1323 (m), 1272 (m), 1249 (m), 1151 (w), 1035 (m),
847 (m), 809 (w), 786 (m), 712 (w), 665 (m). EI-MS: m/z = 839
(90%) [M]+, 744 (5%) [M - Me2pz]+, 646 (5%) [M - 2Me2pz]+, 526
(100%) [Mg(Me2pz)(N2OAr)]+.
Data for 5. IR (KBr plates, Nujol mull, n/cm-1): 1558 (m),
1363 (m), 1351 (m), 1319 (w), 1272 (m), 1248 (m), 1151 (w),
1054 (m), 1043 (m), 967 (w), 848 (m), 818 (m), 783 (w), 750 (w),
714 (m), 687 (w), 666 (m). EI-MS: m/z = 466 (70%) [M]+, 451
(100%) [M - Me]+, 355 (40%) [M - Me - Me2pz]+.
Alternative synthesis (NMR tube scale). A solution of
nBuMgCl (7 mL, 0.014 mmol; 2 M in diethyl ether) was added to
2
[Na(k -N2OAr)(THF)]2 (2, 7.8 mg, 0.07 mmol) in C6D6 (0.75 mL),
giving a colourless solution and a white precipitate. After 10 min,
1
the H NMR spectrum showed quantitative conversion to 3 and
free THF. Attempts to scale up this reaction were unsuccessful.
1H NMR (C6D6, 299.9 MHz, 293 K) d/ppm: 7.60 (1H, d,3J =
3.0 Hz, 4-C6H2 Bu2), 6.90 (1H, d, 3J = 3.0 Hz, 6-C6H2 Bu2), 6.67
(1H, s, (Me2pz)2CHOAr), 5.14 (2H, s, 4-N2C3HMe2), 2.36 (2H, m,
2-MgnBu), 2.13 (6H, s, 3-N2C3HMe2), 2.06 (2H, m, 3-MgnBu), 1.87
(3H, m, 4-MgnBu), 1.74 (9H, s, 3-C6H2(CMe3)2), 1.68 (6H, s, 5-
N2C3HMe2), 1.43 (9H, s, 5-C6H2(CMe3)2), 1.41 (2H, m, 1-MgnBu)
t
t
Mg(N2OAr){N(SiMe3)2} (6). To
a
solution of Mg{N-
(SiMe3)2}2 (0.449 g, 1.30 mmol) in toluene (20 mL) was added
a solution of N2OArH (1, 0.532 g, 1.30 mmol) in toluene
(30 mL) at RT dropwise over 30 min. After 12 h, a significant
amount of precipitate was produced (shown to be 4, ca. 50%
yield), which was filtered off and washed with toluene (3 ¥
10 mL). The combined filtrates were evaporated to dryness under
reduced pressure, washed with pentane (3 ¥ 10 mL) and dried
in vacuo yielding Mg(N2OAr){N(SiMe3)2} (6) as a white solid.
Yield: 0.085 g (11%). 1H NMR (C6D6, 299.9 MHz, 293 K)
ppm. 13C-{ H} NMR (C6D6, 75.4 MHz, 293 K) d/ppm: 164.0
1
t
t
(2-C6H2 Bu2), 149.5 (3-N2C3HMe2), 141.6 (3-C6H2 Bu2), 140.5
t
t
(5-N2C3HMe2), 133.2 (5-C6H2 Bu2), 127.0 (4-C6H2 Bu2), 126.5
t
t
(6-C6H2 Bu2), 121.1 (1-C6H2 Bu2), 106.3 (4-N2C3HMe2), 74.6
((Me2pz)2CHOAr), 36.1 (3-C6H2(CMe3)2), 33.9 (5-C6H2(CMe3)2),
33.0 (2-MgnBu), 32.0 (5-C6H2(CMe3)2), 31.5 (3-MgnBu), 29.7
(3-C6H2(CMe3)2), 15.0 (1-MgnBu), 13.0 (3-N2C3HMe2), 11.0 (5-
N2C3HMe2), 7.3 (4-MgnBu). An accurate elemental analysis was
not obtained. IR (KBr plates, Nujol mull, n/cm-1): 1560 (s),
1481 (s), 1363 (m), 1348 (m), 1322 (m), 1242 (m), 1148 (m),
1046 (m), 986 (w), 846 (m), 688 (m), 665 (m), 632 (m). EI-MS:
d/ppm: 7.54 (1H, d,3J = 2.7 Hz, 4-C6H2 Bu2), 6.78 (1H, d,
t
t
3J = 2.7 Hz, 6-C6H2 Bu2), 6.56 (1H, s, (Me2pz)2CHOAr), 5.15
(2H, s, 4-N2C3HMe2), 2.35 (6H, s, 3-N2C3HMe2), 1.70 (9H, s,
3-C6H2(CMe3)2), 1.60 (6H, s, 5-N2C3HMe2), 1.38 (9H, s, 5-
1
C6H2(CMe3)2), 0.55 (18H, s, Mg{N(SiMe3)2}). 13C-{ H} NMR
t
(C6D6, 75.4 MHz, 293 K) d/ppm: 163.4 (2-C6H2 Bu2), 150.1
n
m/z = 429 (5%) [M - Bu]+, 312 (90%) [(Me2pz)CHOAr]+, 81 (70%)
t
(3-N2C3HMe2), 141.8 (3-C6H2 Bu2), 141.0 (5-N2C3HMe2), 133.6
[MgnBu]+.
t
t
t
(5-C6H2 Bu2), 126.7 (4-C6H2 Bu2), 125.1 (6-C6H2 Bu2), 121.3 (1-
C6H2 Bu2), 106.7 (4-N2C3HMe2), 74.4 ((Me2pz)2CHOAr), 36.1
t
Reaction of N2OArH (1) with nBuMgCl: formation of Mg(N2OAr)2
(3-C6H2(CMe3)2), 33.8 (5-C6H2(CMe3)2), 32.0 (5-C6H2(CMe3)2),
30.1 (3-C6H2(CMe3)2), 14.0 (3-N2C3HMe2), 11.0 (5-N2C3HMe2),
6.4 (Mg{N(SiMe3)2}). Anal. found (calcd for C31H53N5OSi2Mg):
C 62.8 (62.9), H 9.1 (9.0), N 11.8 (11.8)%. IR (KBr plates,
Nujol mull, n/cm-1): 1561 (m), 1348 (m), 1007 (m), 888 (w),
845 (m), 665 (m). EI-MS: m/z = 311 (100%) [(Me2pz)CHOAr]+,
255 (70%) [Mg(OAr)(N)2]+, 241 (40%) [Mg(OAr)N]+, 190 (70%)
[Mg(pz)(NSi2)]+, 161 (10%) [N(SiMe3)2]+, 94 (90%) [Mg(NSi2)]+.
(4) and [Mg(N2OAr)Cl]n (5). To
a
solution of nBuMgCl
(0.69◦mmol, 0.35 mL; 2 M in diethyl ether) in benzene (20 mL)
at 5 C, N2OArH (1, 0.202 g, 0.50 mmol) in benzene (20 mL)
also at 5 ◦C was added dropwise. After 45 min, the solution was
warmed to RT and after 14 h a white precipitate and colourless
supernatant had formed. The solution was filtered off, and the
precipitate extracted with benzene (3 ¥ 10 mL). The combined
filtrates were evaporated to dryness and the residues washed with
pentane (3 ¥ 10 mL) and dried in vacuo to yield Mg(N2OAr)2 (4) as
a white powder. Yield: 0.083 g (40% based on 1). In an analogous
experiment using 0.48 mL nBuMgCl solution and 0.28 g 1, the first-
formed white precipitate (believed to contain [Mg(N2OAr)Cl]n (5)
and MgCl2) was isolated, washed thoroughly with benzene and
dried in vacuo. Yield: 0.20 g.
Mg(N2OAr){N(SiHMe2)2} (7).
A
solution of Mg{N-
(SiHMe2)2}2 (0.30 g, 1.05 mmol) in benzene (15 mL) was added
dropwise to a solution of N2OArH (1, 0.33 g, 0.81 mmol) in
benzene (15 mL). A light green solution was produced, which
was heated to 70 ◦C for 4 h, during which time a precipitate
was formed. This was filtered off and the filtrates evaporated to
dryness. The residues were washed with pentane (3 ¥ 20 mL)
and dried in vacuo yielding Mg(N2OAr){N(SiHMe2)2} (7) as a
Data for 4. 1H NMR (CD2Cl2, 299.9 MHz, 293 K) d/ppm: 7.26
(2H, d,3J = 2.7 Hz, 4-C6H2 Bu2), 7.04 (2H, s, (Me2pz)2CHOAr),
t
7.00 (2H, d, 3J = 2.7 Hz, 6-C6H2 Bu2), 5.84 (2H, s, 4-N2C3HMe2),
white solid. Yield: 0.09 g (21%). H NMR (C6D6, 299.9 MHz,
t
1
t
5.73 (2H, s, 4-N2C3HMe2), 2.49 (6H, s, 3-N2C3HMe2), 2.48
(6H, s, 5-N2C3HMe2), 2.02 (6H, s, 5-N2C3HMe2), 1.34 (18H, s,
3-C6H2(CMe3)2), 1.30 (18H, s, 5-C6H2(CMe3)2), 0.98 (6H, s, 3-
293 K) d/ppm: 7.56 (1H, d,3J = 2.7 Hz, 4-C6H2 Bu2), 6.82
3
t
(1H, d, J = 2.7 Hz, 6-C6H2 Bu2), 6.58 (1H, s, (Me2pz)2CHOAr),
5.50 (2H, m, Mg{N(SiHMe2)2}, 5.11 (2H, s, 4-N2C3HMe2),
2.31 (6H, s, 3-N2C3HMe2), 1.73 (9H, s, 3-C6H2(CMe3)2),
1.62 (6H, s, 5-N2C3HMe2), 1.40 (9H, s, 5-C6H2(CMe3)2), 0.65
1
N2C3HMe2). 13C-{ H} NMR (CD2Cl2, 75.4 MHz, 293 K) d/ppm:
t
164.7 (2-C6H2 Bu2), 150.0 (5-N2C3HMe2), 149.2 (3-N2C3HMe2),
140.1 (3-C6H2 Bu2), 139.2 (5-N2C3HMe2), 131.0 (5-C6H2 Bu2),
(12H, d, 3J = 3.3 Hz, Mg{N(SiHMe2)2}). 13C-{ H} NMR
t
t
1
t
t
t
t
127.0 (4-C6H2 Bu2), 125.5 (6-C6H2 Bu2), 121.8 (1-C6H2 Bu2), 106.8
(4-N2C3HMe2), 106.5 (4-N2C3HMe2), 74.7 ((Me2pz)2CHOAr), 36.1
(3-C6H2(CMe3)2), 33.8 (5-C6H2(CMe3)2), 31.9 (5-C6H2(CMe3)2),
31.8 (3-C6H2(CMe3)2), 14.3 (5-N2C3HMe2), 12.3 (3- and 5-
N2C3HMe2), 11.4 (3-N2C3HMe2) ppm. Anal. found (calcd for
C50H70N8O2Mg): C 71.2 (71.5), H 8.3 (8.4), N 13.5 (13.4)%. IR
(KBr plates, Nujol mull, n/cm-1): 1562 (m), 1478 (s), 1363 (m),
(C6D6, 75.4 MHz, 293 K) d/ppm: 163.5 (2-C6H2 Bu2), 150.0
t
(3-N2C3HMe2), 141.8 (3-C6H2 Bu2), 140.9 (5-N2C3HMe2), 133.6
t
t
t
(5-C6H2 Bu2), 126.8 (4-C6H2 Bu2), 125.0 (6-C6H2 Bu2), 121.2
t
(1-C6H2 Bu2), 106.7 (4-N2C3HMe2), 74.4 ((Me2pz)2CHOAr), 36.2
(3-C6H2(CMe3)2), 33.9 (5-C6H2(CMe3)2), 32.1 (5-C6H2(CMe3)2),
29.9 (3-C6H2(CMe3)2), 13.7 (3-N2C3HMe2), 11.0 (5-N2C3HMe2),
4.3 (Mg{N(SiHMe2)2}). Anal. found (calcd for C29H49N5OSi2Mg):
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 85–96 | 93
©

View Online
C 61.8 (61.7), H 8.7 (8.8), N 12.3 (12.4)%. IR (KBr plates, Nujol
mull) n/cm-1: 2050 (m), 1562 (m), 1349 (m), 1244 (m), 1050 (w),
931 (w), 899 (m), 845 (w), 808 (w), 667 (m). EI-MS: m/z =
311 (100%) [(Me2pz)CHOAr]+, 255 (60%) [Mg(OAr)(N)2]+, 241
(40%) [Mg(OAr)(N)]+, 190 (70%) [Mg(pz)(NSi2)]+, 133 (10%)
[N(SiHMe2)2]+, 94 (90%) [Mg(NSi2)]+.
filtered, and the solid residue was washed with hot toluene (70 ◦C,
3 ¥ 10 mL) and diethyl ether (3 ¥ 10 mL) and then dried in vacuo
to afford (N2OAr)2 (10) as a white powder. Yield: 0.08 g (39%).
Alternative NMR tube scale synthesis. Zn(N2OAr){N(SiMe3)2}
(9, 0.0085 g, 0.013 mmol) and N2OArH (1, 0.0055 g, 0.013 mmol)
were dissolved in C6D6 (0.75 mL), giving a pale yellow solution
1
and a precipitate. After 10 min, the H NMR spectrum showed
Zn(N2OAr)Me (8). A solution of N2OArH (1, 0.51 g, 1.26 mmol)
in toluene (30 mL) was added to ZnMe2 (1.9 mL, 3.78 mmol;
2 M in toluene). The mixture was heated to 60 ◦C for 14 h.
The volatiles were removed under reduced pressure to give a
yellow residue, which was washed with cold pentane (-35 ◦C, 3 ¥
20 mL) and dried in vacuo to afford Zn(N2OAr)Me (8) as a pale
HN(SiMe3)2 had been quantitatively produced and no 9 remained.
Compound 10 was highly insoluble and therefore NMR analysis
could not be carried out. Anal. found (calcd for C50H70N8O2Zn):
C 68.2 (68.2), H 8.1 (8.0), N 12.6 (12.7)%. IR (KBr plates, Nujol
mull) n/cm-1: 1603 (m), 1562 (s), 1360 (s), 1350 (s), 1332 (m),
1316 (m), 1272 (s), 1251 (s), 1150 (m), 1093 (m, br), 1038 (m),
982 (m), 969 (m), 896 (w), 852 (m), 834 (m), 801 (s), 783 (m),
744 (m), 714 (m), 685 (m), 665 (m), 633 (m), 608 (w).
1
yellow solid. Yield: 0.15 g (25%). H NMR (C6D6, 299.9 MHz,
293 K) d/ppm: 7.60 (1H, d,3J = 2.4 Hz, 4-C6H2 Bu2), 6.88
t
3
t
(1H, d, J = 2.7 Hz, 6-C6H2 Bu2), 6.65 (1H, s, (Me2pz)2CHOAr),
5.18 (2H, s, 4-N2C3HMe2), 2.06 (6H, s, 3-N2C3HMe2), 1.78
(9H, s, 3-C6H2(CMe3)2), 1.69 (6H, s, 5-N2C3HMe2), 1.43 (9H, s,
Zn(N2OAr)Cl (11). To a solution of Zn(N2OAr){N(SiMe3)2} (9,
0.68 g, 1.07 mmol) in benzene (25 mL), HCl (1.07 mL, 1.07 mmol;
1 M in diethyl ether) was added at RT. A white precipitate was
formed and after 8 h, the mixture was evaporated to dryness
under reduced pressure, washed with benzene (3 ¥ 10 mL) and
dried in vacuo to afford Zn(N2OAr)Cl (11) as a white solid. Yield:
5-C6H2(CMe3)2), 0.19 (3H, s, ZnMe). 13C-{ H} NMR (C6D6,
1
t
75.4 MHz, 293 K) d/ppm: 165.1 (2-C6H2 Bu2), 149.3 (3-
t
N2C3HMe2), 142.4 (3-C6H2 Bu2), 139.6 (5-N2C3HMe2), 132.7 (5-
C6H2 Bu2), 126.4 (4-C6H2 Bu2), 124.8 (6-C6H2 Bu2), 120.2 (1-
C6H2 Bu2), 106.1 (4-N2C3HMe2), 74.5 ((Me2pz)2CHOAr), 36.3 (3-
t
t
t
1
t
0.38 g (69%). H NMR (CD2Cl2, 299.9 MHz, 293 K) d/ppm:
7.27 (1H, d,3J = 3 Hz, 4-C6H2 Bu2), 6.95 (1H, s, (Me2pz)2CHOAr),
t
C6H2(CMe3)2), 33.9 (5-C6H2(CMe3)2), 32.1 (5-C6H2(CMe3)2), 30.0
(3-C6H2(CMe3)2), 12.9 (3-N2C3HMe2), 10.9 (5-N2C3HMe2), -15.8
(ZnMe) ppm. Anal. found (calcd for C26H38N4OZn): C 63.9 (64.0),
H 7.8 (7.9), N 11.4 (11.5)%. IR (KBr plates, Nujol mull) n/cm-1:
1558 (m), 1362 (s), 1349 (m), 1324 (m), 1249 (m), 1149 (w),
1046 (m), 845 (m), 834 (m), 783 (w), 710 (m), 689 (w), 665 (m).
EI-MS: m/z = 486 (100%) [M]+.
6.86 (1H, d, 3J = 3 Hz, 6-C6H2 Bu2), 6.01 (2H, s, 4-N2C3HMe2),
t
2.48 (6H, s, 5-N2C3HMe2), 2.42 (6H, s, 3-N2C3HMe2), 1.37 (9H, s,
1
3-C6H2(CMe3)2), 1.26 (9H, s, 5-C6H2(CMe3)2). 13C-{ H} NMR
t
(CD2Cl2, 75.4 MHz, 293 K) d/ppm: 163.2 (2-C6H2 Bu2), 151.1
t
(3-N2C3HMe2), 142.0 (3-C6H2 Bu2), 141.6 (5-N2C3HMe2), 134.9
t
t
t
(5-C6H2 Bu2), 127.0 (4-C6H2 Bu2), 125.7 (6-C6H2 Bu2), 119.8 (1-
t
C6H2 Bu2), 107.0 (4-N2C3HMe2), 74.3 ((Me2pz)2CHOAr), 36.0 (3-
Zn(N2OAr){N(SiMe3)2} (9). A solution of Zn{N(SiMe3)2}2
(0.26 g, 0.68 mmol) in toluene (10 mL) was added dropwise
to a solution of N2OArH (1, 0.21 g, 0.52 mmol) in toluene
(30 mL). After 3 h at RT, the mixture was filtered and the
filtrates evaporated to dryness under reduced pressure. The
residues were washed with pentane (3 ¥ 10 mL) and dried
in vacuo to give Zn(N2OAr){N(SiMe3)2} (9) as a white solid.
Yield: 0.23 g (69%). 1H NMR (C6D6, 299.9 MHz, 293 K)
C6H2(CMe3)2), 34.0 (5-C6H2(CMe3)2), 31.7 (5-C6H2(CMe3)2), 29.6
(3-C6H2(CMe3)2), 13.2 (3-N2C3HMe2), 11.7 (5-N2C3HMe2). Anal.
found (calcd for C25H35ClN4OZn): C 59.2 (59.1), H 6.9 (6.9),
N 10.9 (11.0)%. IR (KBr plates, Nujol mull) n/cm-1: 1556 (m),
1347 (m), 1315 (m), 1148 (w), 1054 (m), 844 (m), 813 (m), 781 (w),
708 (w), 690 (w), 676 (m), 666 (m). EI-MS: m/z = 506 (10%) [M]+.
Zn(N2OAr){N(SiHMe2)2} (12). To a slurry of Zn(N2OAr)Cl
(11, 0.29 g, 0.58 mmol) in benzene (15 mL), LiN(SiHMe2)2 (0.08 g,
0.58 mmol) in benzene (10 mL) was added at RT. After 24 h, the
solution was filtered and the filtrates were evaporated to dryness
under reduced pressure to afford Zn(N2OAr){N(SiHMe2)2} (12) as
a white solid, which was washed with benzene (3 ¥ 5 mL) and
dried in vacuo. Yield: 0.18 g (35%). 1H NMR (C6D6, 299.9 MHz,
d/ppm: 7.54 (1H, d,3J = 3.0 Hz, 4-C6H2 Bu2), 6.75 (1H, d,
t
3J = 2.7 Hz, 6-C6H2 Bu2), 6.52 (1H, s, (Me2pz)2CHOAr), 5.18
t
(2H, s, 4-N2C3HMe2), 2.36 (6H, s, 3-N2C3HMe2), 1.72 (9H, s,
3-C6H2(CMe3)2), 1.60 (6H, s, 5-N2C3HMe2), 1.38 (9H, s, 5-
1
C6H2(CMe3)2), 0.54 (18H, s, Zn{N(SiMe3)2}). 13C-{ H} NMR
t
(C6D6, 75.4 MHz, 293 K) d/ppm: 64.0 (2-C6H2 Bu2), 150.0 (3-
293 K) d/ppm: 7.56 (1H, d,3J = 2.7 Hz, 4-C6H2 Bu2), 6.77
t
t
N2C3HMe2), 142.8 (3-C6H2 Bu2), 140.3 (5-N2C3HMe2), 133.3 (5-
3
t
(1H, d, J = 2.7 Hz, 6-C6H2 Bu2), 6.52 (1H, s, (Me2pz)2CHOAr),
5.51 (2H, m, Zn{N(SiHMe2)2}), 5.12 (2H, s, 4-N2C3HMe2), 2.34
(6H, s, 3-N2C3HMe2), 1.74 (9H, s, 3-C6H2(CMe3)2), 1.60 (6H, s,
t
t
t
C6H2 Bu2), 126.7 (4-C6H2 Bu2), 125.2 (6-C6H2 Bu2), 120.3 (1-
C6H2 Bu2), 106.7 (4-N2C3HMe2), 74.4 ((Me2pz)2CHOAr), 36.3 (3-
t
C6H2(CMe3)2), 33.8 (5-C6H2(CMe3)2), 32.0 (5-C6H2(CMe3)2), 30.3
(3-C6H2(CMe3)2), 13.8 (3-N2C3HMe2), 10.9 (5-N2C3HMe2), 6.1
(Zn{N(SiMe3)2}). Anal. found (calcd for C31H53N5OSi2Zn): C 58.7
(58.8), H 8.5 (8.4), N 10.9 (11.1)%. IR (KBr plates, Nujol mull)
n/cm-1: 1560 (m), 1362 (m), 1344 (m), 1318 (m), 1245 (s), 1049 (m),
991 (s), 885 (s), 847 (s), 833 (s), 782 (w), 711 (w), 691 (w), 666 (m),
612 (m). EI-MS: m/z = 631 (80%) [M]+, 616 (20%) [M - Me]+,
472 (75%) [M - N(SiMe3)2]+, 225 (40%) [Zn{N(SiMe3)2}]+.
3
5-N2C3HMe2), 1.39 (9H, s, 5-C6H2(CMe3)2), 0.64 (12H, d, J =
3.3 Hz, Zn{N(SiHMe2)2}). 13C-{ H} NMR (C6D6, 75.4 MHz,
1
t
293 K) d/ppm: 164.1 (2-C6H2 Bu2), 149.9 (3-N2C3HMe2), 142.8
t
t
(3-C6H2 Bu2), 140.2 (5-N2C3HMe2), 133.5 (5-C6H2 Bu2), 126.7
t
t
t
(4-C6H2 Bu2), 125.2 (6-C6H2 Bu2), 120.2 (1-C6H2 Bu2), 106.6 (4-
N2C3HMe2), 74.4 ((Me2pz)2CHOAr), 36.4 (3-C6H2(CMe3)2), 33.8
(5-C6H2(CMe3)2), 32.0 (5-C6H2(CMe3)2), 30.1 (3-C6H2(CMe3)2),
13.5 (3-N2C3HMe2), 10.9 (5-N2C3HMe2), 3.8 (Zn{N(SiHMe2)2}).
Anal. found (calcd for C29H49N5OSi2Zn): C 57.6 (57.5), H 8.1
(8.2), N 11.5 (11.6)%. IR (KBr plates, Nujol mull, n/cm-1: 2084 (s),
2054 (s), 1605 (w), 1561 (m), 1347 (m), 1318 (m), 1244 (s), 1148 (m),
Zn(N2OAr)2 (10). Zn{N(SiMe3)2}2 (0.088 g, 0.23 mmol) was
added to a solution of N2OArH (1, 0.20 g 0.48 mmol) in CH2Cl2
(10 mL) at RT. After 14 h, the resultant orange suspension was
94 | Dalton Trans., 2009, 85–96
This journal is
The Royal Society of Chemistry 2009
©

View Online
2
Table 6 X-ray data collection and processing parameters for [Na(k -N2OAr)(THF)]2 (2), Mg(N2OAr)nBu (3), Mg(N2OAr)2·C4H10O (4·C4H10O),
Mg(N2OAr){N(SiMe3)2}·0.5(C7H8) (6·0.5(C7H8)), Zn(N2OAr)Me (8), Zn(N2OAr){N(SiMe3)2}·0.35(C7H8) (9·0.35(C7H8)) and Zn(N2OAr)Cl (11)
Parameter
2
3
4·C4H10O
6·0.5(C7H8)
8
9·0.35(C7H8)
11
Empirical formula
C58H86N8Na2O4 C29H44MgN4O C50H70MgN8O2· C31H53MgN5OSi2· C26H38N4OZn C31H53ZnN5OSi2· C25H35ClN4OZn
C4H10O
913.59
150
0.5(C7H8)
638.34
150
0.35(C7H8)
665.60
FW
1005.36
150
489.00
150
487.99
150
508.41
150
Temp./K
Wavelength/A
150
˚
0.71073
Triclinic
P1
0.71073
Orthorhombic Monoclinic
Pbca
0.71073
0.71073
Triclinic
P1
0.71073
Triclinic
P1
0.71073
Triclinic
P1
0.7107
Monoclinic
P21/c
Crystal system
¯
¯
¯
¯
Space group
P21/c
˚
a/A
10.9739(2)
11.2192(3)
11.9206(3)
77.4985(11)
79.4223(11)
88.4467(12)
1408.38(6)
11.2537(2)
21.1315(5)
23.8492(5)
90
90
90
5671.5(2)
10 777
5521
15.7496(4)
17.6620(5)
20.2696(5)
90
92.8818(17)
90
5631.3(3)
17 019
9832
11.5864(2)
12.7632(2)
14.2935(3)
105.6153(10)
104.8024(9)
95.2213(10)
1938.95(6)
15 046
10.1798(2)
11.0522(3)
12.7119(3)
69.3059(11)
85.6402(11)
79.8464(11)
1316.83(5)
10 580
11.6068(2)
12.6722(2)
14.2523(3)
105.2600(8)
104.7969(8)
94.9221(8)
1928.81(6)
15 008
9.81900(10)
23.2485(3)
11.11250(10)
90
94.5044(6)
90
2528.89(5)
11 508
5771
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Reflections measured 10 896
Unique reflections
6314
8798
5970
8788
Rint
0.026
1
1.185
0.088
0.053
8
1.145
0.090
0.055
4
1.078
0.77
0.034
2
1.093
0.139
0.031
2
1.231
0.956
0.031
2
1.146
0.729
0.023
4
1.335
1.101
Z
D
calcd/Mg m-3
Abs. coeff./mm-1
R indicesa
R1 = 0.0417
R1 = 0.0509
R1 = 0.0826
R1 = 0.0642
R1 = 0.0339
R1 = 0.0513
R1 = 0.0313
[I > 3s(I)]
Rw = 0.0430
Rw = 0.0594
Rw = 0.0879
Rw = 0.0704
Rw = 0.0335
Rw = 0.0542
Rw = 0.0317
√
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = ꢀFo| - |Fcꢀ/ |Fo|; Rw
=
{
w (|Fo| - |Fc|)2/ w|Fo|2}.
1050 (m), 1000 (s), 924 (m), 904 (s), 847 (m), 835 (m), 782 (m),
749 (w), 711 (m), 690 (w), 677 (m), 666 (m). EI-MS: m/z = 471
(70%) [M - N(SiHMe2)2]+.
General procedure for e-caprolactone polymerization experiments
In a dry-box, a Schlenk tube was charged with ca. 35 mmol of the
catalyst and a magnetic stirring bar. The tube was transferred to
a dual manifold Schlenk line and dissolved in toluene (1.5 mL).
A solution of e-caprolactone (100 equiv.) in toluene (2 mL) was
added via a steel cannula in one portion. The solutions were stirred
for up to 24 h, with aliquots typically being taken at 2, 4 and
6 h. These were quenched with non-dried toluene, evaporated
Crystal structure determinations of [Na(j2-N2OAr)(THF)]2 (2),
Mg(N2OAr)nBu (3), Mg(N2OAr)2·C4H10O (4·C4H10O), Mg(N2OAr)-
{N(SiMe3)2}·0.5(C7H8) (6·0.5(C7H8)), Zn(N2OAr)Me (8), Zn-
(N2OAr){N(SiMe3)2}·0.35(C7H8) (9·0.35(C7H8)) and Zn(N2OAr)Cl
(11). Crystal data collection and processing parameters are
given in Table 6. Crystals were mounted on glass fibers using
perfluoropolyether oil and cooled rapidly in a stream of cold
N2 using an Oxford Cryosystems Cryostream unit. Diffraction
data were measured using either an Enraf-Nonius KappaCCD
diffractometer. As appropriate, absorption and decay corrections
were applied to the data and equivalent reflections merged.82
The structures were solved by direct methods (SIR9283) and
further refinements and all other crystallographic calculations
were performed using the CRYSTALS program suite.84 Crystals
of 6 and 9 contain fractional occupancy toluene molecules of
crystallization disordered over crystallographic inversion centers.
These were refined isotropically with a common displacement
parameter for the carbon atoms. Geometric restraints were applied
to the C–C distances and C–C–C angles. The largest peak in the
final Fourier difference map was in the region of the disordered
toluene molecule. Crystals of 4 contained diethyl ether molecules
of crystallization. These were satisfactorily refined subject to
loose restraints applied to the displacement parameters. Further
details of the structure solution and refinements are given in
the ESI†. A full listing of atomic coordinates, bond lengths
and angles and displacement parameters for all the structures
have been deposited at the Cambridge Crystallographic Data
Centre.
1
to dryness and analysed by H NMR spectroscopy (CDCl3) and
GPC as appropriate. Where appropriate, the remaining solution
was pipetted into methanol to precipitate poly(CL), which was
washed and dried to a constant weight and analyzed by GPC.
Acknowledgements
This work was supported by the EPSRC, Oxford University’s
Clarendon Fund and Programme Alban, the European Union
Programme of High Level Scholarships for Latin America,
scholarship no. E05D059950MX.
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