Sterically variable dizinc complexes bearing bis(iminopyridyl)phenolate ligands: Synthesis, structures and reactivity studies

Article abstract of DOI:10.1039/b709385c

A series of chiral dizinc complexes of the type [(2,6-{ArNC(Me)C ₅H₃N}₂C₆H₃O)Zn ₂(-Cl)Cl₂] [Ar = 2,6-i-Pr₂C₆H ₃ (1a), 2,6-Me₂C₆H₃ (1b), 2,4,6-Me₃-C₆H₂ (1c), 2,4-Me₂C ₆H₃ (1d)] can be conveniently prepared in good yield by the template reaction of 2,6-{OC(Me)C₅H₃N} ₂C₆H₃OH with an excess of the corresponding aniline and two equivalents of zinc dichloride in n-BuOH at elevated temperature. Alternatively, the pro-ligands, 2,6-{(ArNC(Me)C₅H ₃N}₂C₆H₃OH [Ar = 2,6-i-Pr ₂C₆H₃ (L1-H), 2,6-Me₂C ₆H₃ (L2-H), 2,4,6-Me₃C₆H₂ (L3-H), 2,4-Me₂C₆H₃ (L4-H)], can be isolated and then treated with two equivalents of zinc dichloride to afford 1a-1d. Interaction of 1a with two equivalents of NaOAc in the presence of TlBF ₄ gives the diacetate-bridged salt [(L1)Zn₂(-OAc) ₂](BF₄) (2) while with Nadbm (dbm = dibenzoylmethanato) the bis(dbm)-chelated salt [(L1)Zn₂(dbm)₂](BF₄) (3) is obtained. Hydrolysis occurs on reaction of 1a with TlOEt to furnish [(L1)Zn₂(-OH)Cl₂] (4) as the only isolable product. Conversely, reaction of 1a with Tlhp (hp = 2-pyridonate) affords the neutral bis(pyridonate)-bridged trimetallic complex [(L1)Zn₃(-hp) ₂Cl₃] (5) as the major product along with 4 as the minor one. Complex 4 and mixtures of 4/5 act as modest activators for the ring-opening polymerisation of ε-caprolactone. Single crystal X-ray diffraction studies have been performed on 1b, 1c, 2, 3, 4 and 5 reveal Zn...Zn separations in the range: 3.069(4)-4.649(6) A. The Royal Society of Chemistry.

Full text of DOI:10.1039/b709385c

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www.rsc.org/dalton | Dalton Transactions
Sterically variable dizinc complexes bearing bis(iminopyridyl)phenolate
ligands: synthesis, structures and reactivity studies†
Yohan D. M. Champouret, William J. Nodes, Jason A. Scrimshire, Kuldip Singh, Gregory A. Solan* and
Isla Young
Received 20th June 2007, Accepted 2nd August 2007
First published as an Advance Article on the web 15th August 2007
DOI: 10.1039/b709385c
=
A series of chiral dizinc complexes of the type [(2,6-{ArN C(Me)C5H3N}2C6H3O)Zn2(l-Cl)Cl2] [Ar =
2,6-i-Pr2C6H3 (1a), 2,6-Me2C6H3 (1b), 2,4,6-Me3-C6H2 (1c), 2,4-Me2C6H3 (1d)] can be conveniently
=
prepared in good yield by the template reaction of 2,6-{O C(Me)C5H3N}2C6H3OH with an excess of
the corresponding aniline and two equivalents of zinc dichloride in n-BuOH at elevated temperature.
=
Alternatively, the pro-ligands, 2,6-{(ArN C(Me)C5H3N}2C6H3OH [Ar = 2,6-i-Pr2C6H3 (L1-H),
2,6-Me2C6H3 (L2-H), 2,4,6-Me3C6H2 (L3-H), 2,4-Me2C6H3 (L4-H)], can be isolated and then treated
with two equivalents of zinc dichloride to afford 1a–1d. Interaction of 1a with two equivalents of
NaOAc in the presence of TlBF4 gives the diacetate-bridged salt [(L1)Zn2(l-OAc)2](BF4) (2) while with
Nadbm (dbm = dibenzoylmethanato) the bis(dbm)-chelated salt [(L1)Zn2(dbm)2](BF4) (3) is obtained.
Hydrolysis occurs on reaction of 1a with TlOEt to furnish [(L1)Zn2(l-OH)Cl2] (4) as the only isolable
product. Conversely, reaction of 1a with Tlhp (hp = 2-pyridonate) affords the neutral bis(pyridonate)-
bridged trimetallic complex [(L1)Zn3(l-hp)2Cl3] (5) as the major product along with 4 as the minor one.
Complex 4 and mixtures of 4/5 act as modest activators for the ring-opening polymerisation of
e-caprolactone. Single crystal X-ray diffraction studies have been performed on 1b, 1c, 2, 3, 4 and 5
˚
reveal Zn · · · Zn separations in the range: 3.069(4)–4.649(6) A.
1
Introduction
The rational design and synthesis of ligand frameworks that are
capable of housing two (or more) zinc centres in close proximity
˚
(between 3 and 5 A) have long been the subject of research activity
due, in the main, to the connection the resulting complexes1 have
to the binuclear metal sites found in a number of zinc hydrolytic
enzymes (e.g., metallo-b-lactamases,2,3 some aminopeptidases,4
phosphotriesterase5 and alkaline phosphatase6). More recently,
these bio-inspired families of compartmental ligands and related
frameworks have been used to develop dizinc complexes that
can be employed in other fields including as molecular sensors,7
and as catalysts for a variety of transformations including the
ring-opening polymerisation8 of cyclic esters (e.g., rac-lactide,
e-caprolactone), epoxide/CO2 copolymerisations9 and for C–C
bond forming reactions.10
As part of our programme we have been targeting new preorgan-
ised binucleating manifolds11–13 that are potentially amenable to
systematic variation in their steric and electronic properties14 with
a view to preparing bimetallic oligomerisation/polymerisation
catalysts. For example, we have shown that diiron(II), dicobalt(II)
and dinickel(II) complexes of the 2,6-diisopropylphenyl derivative
of the monoanionic bis(iminopyridyl)phenolate [Lx, Fig. 1] ligand,
L1, display, on treatment with methylaluminoxane, some activity
for the oligomerisation of ethylene.13 A preliminary attempt at
Fig. 1 Bis(iminopyridyl)phenolate, Lx.
preparing a dizinc(II) complex bearing L1 has also led to the
isolation of [(L1)Zn2(l-Cl)Cl2] (1a).13
Herein, we report a new route to 1a and expand the family
of complexes of type [(Lx)Zn2(l-Cl)Cl2] (1) to cover a range
of aryl group substitution patterns (viz. Ar = 2,6-Me2C6H3,
2,4,6-Me3C6H2, 2,4-Me2C6H3). In addition, the coordination
chemistry of trichloride 1a is probed by exploring its reactivity
towards acetate, dibenzoylmethanato, ethoxide and 2-pyridonate;
the capacity of some of the derivatives to act as initiators for the
ring opening polymerisation of e-caprolactone is also examined.
2
Results and discussion
(a) Synthesis of 1
=
The template reaction of 2,6-{O C(Me)C5H3N}2C6H3OH with
two equivalents of ZnCl2 in the presence of an excess of ArNH2
(Ar = 2,6-i-Pr2C6H3, 2,6-Me2C6H3, 2,4,6-Me3C6H2, 2,4-Me2C6H3)
in n-butanol at elevated temperature gave complexes [(Lx)Zn2(l-
Cl)Cl2] [Lx = L1 (1a), L2 (1b), L3 (1c), L4 (1d)] in good yield,
respectively (Scheme 1). Alternatively, 1b–1d can also be prepared
in a manner similar to that previously reported for 1a,13 by firstly
Department of Chemistry, University of Leicester, University Road, Leices-
ter, UK LE1 7RH. E-mail: gas8@leicester.ac.uk
† CCDC reference numbers 651128–651133. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b709385c
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Scheme 1 Reagents and conditions: (i) 10 ArNH2, 2 ZnCl2, n-BuOH, 120 ◦C, 2 days; (ii) xs. ArNH2, 160 C, cat. H+, 30 min.; (iii) 2 ZnCl2, n-BuOH,
100 ◦C, 1 h; (iv) TlBF4, 2 NaOAc, MeCN, RT, 12 h; (v) TlOEt, CH2Cl2, RT, 12 h; (vi) TlBF4, 2 Nadbm, MeCN, RT, 12 h; (vii) Tlhhp (made in-situ),
MeCN, RT, 12 h.
=
reacting 2,6-{O C(Me)C5H3N}2C6H3OH with the corresponding
=
aniline to form 2,6-{(ArN C(Me)C5H3N}2C6H3OH [Ar = 2,6-
i-Pr2C6H3 (L1-H), 2,6-Me2C6H3 (L2-H), 2,4,6-Me3C6H2 (L3-H),
2,4-Me2C6H3 (L4-H)] and then treating these pro-ligands with two
equivalents of zinc dichloride (Scheme 1). All new pro-ligands (L2-
H, L3-H, L4-H) and complexes (1b–1d) have been characterised
by 1H NMR, IR spectroscopy, mass spectrometry and gave
satisfactory microanalytical data (see Experimental section). In
addition, complexes 1b and 1c have been the subject of single
crystal X-ray diffraction studies.
Single crystals of 1b and 1c suitable for X-ray determination
were grown by slow cooling (to ambient temperature) of hot
concentrated acetonitrile solutions containing the corresponding
complex. Two independent molecules are apparent in the asym-
metric unit of both 1b and 1c with only minor variations (vide
infra) in bond parameters apparent between molecules; in the case
of 1c the second molecule (B) adopts the enantiomeric twin form
of molecule A. A view of 1b is depicted in Fig. 2; selected bond
distances and angles are listed for both molecules in 1b and 1c in
Table 1. The structures are similar and will be discussed together
using molecule A in each case as the representative example.
Fig. 2 Molecular structure of 1b (molecule A) with a partial atom labeling
scheme; all hydrogen atoms have been omitted for clarity.
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˚
Table 1 Selected bond distances (A) and angles ( ) for 1b and 1c
1b
1c
Molecule A
Molecule B
Molecule A
Molecule B
Zn(1)–O(1)
Zn(1)–Cl(1)
Zn(1)–Cl(2)
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1) · · · Zn(2)
Zn(2)–O(1)
Zn(2)–Cl(1)
Zn(2)–Cl(3)
Zn(2)–N(3)
Zn(2)–N(4)
C(9)–N(1)
2.017(3)
2.4610(16)
2.2372(16)
2.120(4)
2.107(4)
3.341(4)
2.064(3)
2.3690(16)
2.2413(17)
2.123(4)
2.169(4)
1.285(7)
1.281(6)
1.996(4)
2.4827(17)
2.2456(18)
2.110(5)
2.130(4)
3.280(4)
2.063(4)
2.3511(17)
2.2209(18)
2.135(5)
2.145(4)
1.271(7)
1.286(7)
2.031(4)
2.439(3)
2.246(2)
2.143(6)
2.121(5)
3.327(5)
2.075(4)
2.368(3)
2.241(3)
2.120(6)
2.177(6)
1.285(8)
1.283(8)
2.051(4)
2.429 (3)
2.238(2)
2.119(6)
2.118(5)
3.325(5)
2.056(4)
2.371(3)
2.242(2)
2.118(6)
2.169(5)
1.279(8)
1.273(8)
C(27)–N(4)
N(1)–Zn(1)–O(1)
N(1)–Zn(1)–N(2)
N(1)–Zn(1)–Cl(1)
N(1)–Zn(1)–Cl(2)
N(2)–Zn(1)–Cl(1)
N(2)–Zn(1)–Cl(2)
N(2)–Zn(1)–O(1)
Cl(2)–Zn(1)–Cl(1)
N(4)–Zn(2)–O(1)
N(4)–Zn(2)–Cl(1)
N(4)–Zn(2)–Cl(3)
N(3)–Zn(2)–Cl(3)
Cl(3)–Zn(2)–Cl(1)
Zn(1)–O(1)–Zn(2)
Zn(1)–Cl(1)–Zn(2)
140.66(16)
77.96(17)
92.29(12)
113.34(13)
140.93(12)
107.75(12)
83.90(16)
110.82(5)
157.12(15)
97.29(11)
101.94(12)
125.34(13)
112.56(6)
109.89(15)
87.50(5)
141.99(17)
78.10(18)
94.91(14)
110.70(14)
139.15(13)
113.16(13)
82.94(16)
106.99(7)
156.24(16)
99.18(13)
103.63(13)
129.18(13)
113.66(7)
107.84(17)
85.43(5)
141.0(2)
78.1(2)
139.76(19)
77.3(2)
91.99(18)
112.63(16)
140.20(15)
107.01(16)
83.51(19)
112.34(8)
157.0(2)
97.86(14)
103.18(16)
120.92(16)
115.59(11)
108.2(2)
92.58(17)
112.44(17)
141.30(15)
107.11(16)
83.72(19)
111.23(8)
156.96(19)
97.26(14)
102.73(15)
123.06(15)
115.09(10)
108.14(19)
87.69(7)
87.60(7)
The molecular structures comprise two zinc atoms supported
on monoanionic Lx [Lx = L2 (1b), L3 (1c)] with the phenolate
oxygen atom [O(1)] bridging the metals and the two separate
pyridyl-imine units acting as chelates. Each metal centre is further
bound by one terminal chloride and one bridging chloride so as
to complete two independent five-coordinate geometries. Some
differences in the coordination sphere at each metal centre are
observed with the geometry at Zn(1) being described as square
pyramidal [s = 0.01 (1b), 0.01 (1c)]15 while at Zn(2) the geometry is
more distorted and tending towards trigonal pyramidal [s = 0.53
(1b), 0.60 (1c)]. The imino-pyridyl chelating moieties in Lx are
almost planar [tors.: N(1)–C(9)–C(11)–N(2) 3.12◦ (1b), 4.49◦ (1c);
N(3)–C(26)–C(27)–N(4) 11.64◦ (1b), 11.78◦ (1c)] with the N-aryl
groups quasi-orthogonal to these planes [tors.: C(2)–C(1)–N(1)–
C(9) 101.84◦ (1b), 99.03◦ (1c); C(34)–C(29)–N(4)–C(27) 91.66◦
(1b), 88.39◦ (1c)]. These inclinations of the aryl groups have the
effect that one ortho-methyl substituent per aryl group points
in a similar direction as a terminal chloride. With regard to
the central phenolate plane, each pyridyl-imine unit is twisted
away from planarity [tors.: C(21)–C(20)–C(22)–N(3) 35.42◦ (1b),
35.94◦ (1c); N(2)–C(15)–C(16)–C(21) 21.98◦ (1b), 20.48◦ (1c)],
which compares to the more uneven variation in 1a (1.6 vs.
36.2◦).13 As would be expected, the Zn–Cl (bridging) distances
in 1b and 1c are longer than the Zn–Cl (terminal) distances
with some asymmetry apparent between the Zn–Cl (bridging)
˚
˚
[2.017(3) A (1b); 2.031(4) A (1c)] in this case being shorter than
˚
˚
the Zn(2)–O(1) bond length [2.064(3) A (1b); 2.075(4) A (1c)].
The intermetallic Zn · · · Zn distances for both molecules of 1c
˚
˚
are similar [3.327(5) A, 3.325(5) A], while in 1b there is a notable
variation in the Zn · · · Zn distance between independent molecules
˚
(ca. 0.061 A); the reported range for complexes containing the
8b,13
˚
Zn(l-Ophenolate)(l-Cl)Zn motif is 3.209–3.348 A.
This variation
in intermetallic separation in 1b is also reflected by the Zn–O–
Zn angles [109.89(15)◦ (molecule A), 107.84(17)◦ (molecule B)].
The N(1)–C(9) and N(4)–C(27) bond distances in 1b and 1c
[ca. 1.280 A] are consistent with C N bond character. No
intermolecular contacts of note are apparent.
˚
=
The FAB mass spectra of complexes 1b–d show fragmentation
peaks in each case corresponding to the loss of one or two halide
groups from their respective molecular ions. In their IR spectra,
the m(C N)imine bands are seen at ca. 1590 cm−1 and shifted
=
by ca. 40 cm−1 to lower wavenumber in comparison with the
1
corresponding pro-ligand (Lx-H). The H NMR spectrum of 1d
indicates that (as with 1a13) in solution the metal centres occupy
equivalent binding sites as exemplified by the presence of only
one type of imino-methyl group (at d 2.25). Complexes 1b and
1c were, however, insufficiently soluble in CDCl3 to allow NMR
characterisation. Interestingly, dissolution of 1a–1d occurs readily
in DMSO-d6 with the resulting NMR spectra revealing a reduction
in the symmetry, perhaps suggesting the coordination of DMSO
at one metal centre. However, attempts at growing crystals suitable
for a single crystal X-ray diffraction studies were unsuccessful; in
the case of 1a affording crystals of L1-H.13
˚
distances [Zn(1)–Cl(1) 2.4610(16) vs. Zn(2)–Cl(1) 2.3690(16) A
˚
(1b); 2.439(3) vs. 2.368(3) A (1c)]. A similar asymmetry is observed
for the Zn–O (bridging) distances with the Zn(1)–O(1) bond
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˚
(b) Syntheses of 2 and 3
Table 2 Selected bond distances (A) and angles ( ) for 2
Interaction of 1a with two equivalents of NaOAc in the presence
of TlBF4 gave the diacetate-bridged salt [(L1)Zn2(l-OAc)2](BF4)
(2), while with two equivalents of Nadbm (dbm = dibenzoyl-
methanato) the bis(dbm)-chelated salt [(L1)Zn2(dbm)2](BF4) (3)
was obtained in good yield. Both 2 and 3 have been characterised
Cation A
Cation B
Zn(1)–O(1)
Zn(1)–O(3)
Zn(1)–O(4)
Zn(1)–N(1)
Zn(1)–N(2)
Zn(2)–O(1)
Zn(2)–O(2)
Zn(2)–O(5)
Zn(2)–N(3)
Zn(2)–N(4)
Zn(1) · · · Zn(2)
C(31)–N(1)
C(13)–N(4)
B–F (anion)
2.017(3)
1.948(3)
1.991(3)
2.124(3)
2.068(4)
2.017(3)
2.003(3)
1.960(3)
2.079(3)
2.109(4)
3.127(4)
1.267(5)
1.270(5)
1.283(8)–1.448(9)
2.085(3)
1.953(3)
1.959(3)
2.162(4)
2.058(4)
1.988(3)
2.005(3)
1.980(3)
2.087(3)
2.092(3)
3.161(3)
1.278(5)
1.271(5)
1.303(8)–1.358(8)
1
by FAB mass spectrometry, IR spectroscopy and by H NMR
spectroscopy (see Experimental section). Both complexes have
additionally been the subject of single crystal X-ray diffraction
studies.
Crystals of 2 suitable for the X-ray determination were grown
by slow evaporation of a hot acetonitrile solution containing
the complex. Two independent cations (A and B) and two
tetrafluoroborate anions are apparent in the asymmetric unit for
2 with some noteworthy differences evident between A and B
(vide infra). A view of one of the independent cations (A) is
depicted in Fig. 3; selected bond distances and angles for both
A and B and the BF4 anions are listed in Table 2. The following
discussion is concerned with cation A while any variations between
A and B will be highlighted. As with 1, two zinc ions in 2 are
bridged by a phenolate oxygen atom and chelated by two pyridyl-
imine units from L1. In addition, a pair of syn–syn acetate groups
bridge the metal centres to afford a Zn(l-Ophenolate)(l-OAc)2Zn core.
The five-coordinate geometries displayed by the metal centres
can be best described as distorted square pyramidal [s = 0.41
(Zn(1)), 0.35 (Zn(2))]15 with O(2) and O(4) defining the apical sites.
Within L1, both pyridyl-imine units are nearly planar [tors.: N(1)–
C(30)–C(31)–N(2) 8.32◦ and N(3)–C(15)–C(13)–N(4) 12.35◦] but
are notably tilted with respect to the central phenolate group
[tors.: N(2)–C(24)–C(26)–C(25) 34.08◦; N(3)–C(19)–N(20)–C(25)
34.79◦]; this tilting is more uneven in cation B [tors.: N(2)–C(24)–
C(26)–C(25) 41.64◦ vs. N(3)–C(19)–N(20)–C(25) 14.15◦]. The two
O(1)–Zn(1)–N(1)
O(4)–Zn(1)–N(2)
O(3)–Zn(1)–O(4)
O(4)–Zn(1)–O(1)
O(1)–Zn(2)–N(4)
O(5)–Zn(2)–N(3)
O(2)–Zn(2)–O(5)
O(5)–Zn(2)–O(1)
Zn(1)–O(1)–Zn(2)
165.13(13)
99.25(14)
118.68(14)
95.79(12)
165.35(13)
144.01(14)
120.18(14)
94.71(13)
101.64(12)
162.80(13)
105.78(14)
114.97(14)
97.48(12)
162.20(13)
146.79(13)
109.55(13)
94.21(12)
101.76(13)
found in other dizinc cations containing the Zn(l-Ophenolate)(l-
OAc)2Zn core.16 Some variation is apparent in the four Zn–Oacetate
bond distances in A [range: 1.948(3)–2.003(3) A] with a similar
trend evident in B. The BF4 anions occupy the cavities created by
˚
˚
adjacent cations with the closest H · · · F contact being 2.445 A.
Suitable crystals of 3 for the X-ray determination were grown
from chloroform by slow diffusion of hexane at room temperature.
A perspective view of the cation in 3 is depicted in Fig. 4; selected
bond distances and angles are listed in Table 3. The structure
of 3 consists of a dizinc cation and a tetrafluoroborate anion
with a crystallographic plane of symmetry present within the
cationic unit which lies along the O(1)–C(1)–C(4) vector. Within
the cationic unit the two zinc atoms are bound by L1 in a fashion
similar to that seen in 1 and 2 with each zinc atom additionally
˚
Zn–O(1)phenolate bond lengths are equivalent [at 2.017(3) A] in cation
A while a noticeable asymmetry is present in B [2.085(3) vs.
˚
1.988(3) A]. This flexibility in L1 also affects the intermetallic
˚
separations with the Zn · · · .Zn distance in B [3.161(3) A] being
˚
marginally longer than in A [3.127(4) A] but falling in the range
Fig. 3 Molecular structure of the cationic unit in 2 (cation A) with a
partial atom labeling scheme; all hydrogen atoms have been omitted for
clarity.
Fig. 4 Molecular structure of the cationic unit in 3 with a partial atom
labeling scheme; all hydrogen atoms have been omitted for clarity.
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˚
Table 3 Selected bond distances (A) and angles ( ) for 3
Zn(1)–O(1)
Zn(1)–O(2)
Zn(1)–O(3)
Zn(1)–N(1)
2.042(2)
1.984(4)
1.968(4)
2.085(5)
Zn(1)–N(2)
2.099(5)
3.794(5)
1.289(7)
1.287(1)–1.468(10)
Zn(1) · · · Zn(1A)
C(10)–N(1)
B–F (anion)
N(1)–Zn(1)–O(1)
O(2)–Zn(1)–O(3)
O(1)–Zn(1)–N(1)
144.96(17)
93.85(17)
144.96(17)
N(1)–Zn(1)–N(2)
Zn(1)–O(1)–Zn(1A)
O(2)–Zn(1)–N(2)
76.7(2)
136.4(3)
161.44(19)
Atoms with suffix A are generated by symmetry (−x + 1,y, −z + 3/2).
bound by a chelating dbm ligand so as to complete a distorted
octahedral geometry [O(1)–Zn(1)–N(1) 144.96(17)◦, O(2)–Zn(1)–
N(2) 161.44(19)◦] at each metal centre. As with 1 and 2, the
central phenolate group is tilted with respect to the pyridyl-imine
planes [tors.: C(1)–C(2)–C(5)–N(2) 34.4◦]. The absence of a second
bridging ligand (cf., 1 and 2) results in the Zn–O(1)–Zn angle being
more open [136.4(3)◦] which is accompanied by a more elongated
˚
Zn · · · Zn separation [3.794(5) A]. The BF4 anions sit between
˚
adjacent cations with the closest contact [2.376 A] being between
a para-H on one of the 2,6-diisopropylphenyl groups and a BF4
fluoride atom.
In the FAB mass spectra of 2 and 3 fragmentation peaks
corresponding to the loss of BF4 ions along with acetate or diben-
zoylmethanato fragmentation from their respective molecular ions
=
are apparent. The m(C N)imine bands are unclear for both species in
their IR spectra and are likely masked by stretches corresponding
to bridging acetate (asymmetric stretch) or chelating dbm ligands
in 2 and 3, respectively. In their 1H NMR spectra the imino-methyl
groups (two per complex) are equivalent with singlet resonances
visible at ca. d 2.33; the CH3CO2 (2) or CH(CPhCO)2 (3) protons
also take the form of singlets at d 1.93 and d 5.79, respectively. In
addition, the 19F NMR spectra of 2 and 3 reveal singlets at ca. d
Fig. 5 Molecular structure of 4 with a partial atom labeling scheme; all
hydrogen atoms, apart from H2, have been omitted for clarity.
◦
˚
Table 4 Selected bond distances (A) and angles ( ) for 4
−
−153 consistent with the presence of the BF4 counter anions.
Zn(1)–O(1)
Zn(1)–O(2)
Zn(1)–Cl(1)
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1). . .Zn(2)
Zn(2)–O(1)
2.003(3)
1.998(3)
2.2547(17)
2.145(4)
2.134(4)
3.069(4)
2.148(3)
Zn(2)–O(2)
Zn(2)–Cl(2)
Zn(2)–N(3)
Zn(2)–N(4)
C(13)–N(1)
C(31)–N(4)
1.989(3)
2.214(2)
2.087(4)
2.155(4)
1.286(6)
1.288(5)
(c) Syntheses of 4 and 5
Given the ease of reactivity of 1a towards acetate and dibenzoyl-
methanato, we decided to probe the reactions of 1a with alkoxides
and pyridonates. Thus, reaction of 1a with TlOEt gave [(L1)Zn2(l-
OH)Cl2] (4) while with Tlhp (hp = 2-pyridonate), a mixture of 4
and [(L1)Zn3(l-hp)2Cl3] (5) were isolated in a 3 : 7 ratio (from 1H
NMR spectrum). Complexes 4 and 5 have been characterised by
IR and 1H NMR spectroscopy along with FAB mass spectrometry
(see Experimental section). In addition, both complexes have also
been the subject of single crystal X-ray diffraction studies.
Crystals of 4 suitable for the X-ray determination were grown
from chloroform by slow diffusion of diethyl ether at room temper-
ature. A view of 4 is shown in Fig. 5; selected bond distances and
angles are listed in Table 4. The molecular structure of 4 resembles
1a13 with a l-hydroxide ligand replacing the bridging chloride in
1a. Inspection of the geometric parameter (s)15 reveals both zinc
centres, as with 1a and 2, to adopt distorted square pyramidal
geometries [s = 0.06 (Zn(1)), 0.29 (Zn(2))] while the central
phenolate group is also tilted unevenly with respect to the pyridyl-
imine units [tors.: N(2)–C(19)–C(20)–C(25) 14.4◦; C(25)–C(24)–
C(26)–N(3) 37.4◦] in L1. The effect of the presence of the bridging
hydroxide is to reduce the Zn(1)–O(1)–Zn(2) angle [Zn(1)–O(1)–
N(1)–Zn(1)–O(1)
N(2)–Zn(1)–O(2)
O(1)–Zn(1)–O(2)
Cl(1)–Zn(1)–O(2)
N(4)–Zn(2)–O(1)
149.81(13)
139.65(13)
80.27(12)
116.18(10)
145.95(12)
N(3)–Zn(2)–O(2)
Cl(2)–Zn(2)–O(2)
O(1)–Zn(2)–O(2)
Zn(1)–O(1)–Zn(2)
Zn(1)–O(2)–Zn(2)
128.74(13)
117.67(9)
77.03(12)
95.30(12)
100.69(13)
Zn(2) 95.30(12)◦ vs. 101.63(13)◦ (1a)13] while also compressing
13
˚
˚
the Zn(1) · · · Zn(2) separation [3.069 A vs. 3.209(4) A (1a) ];
the range in intermetallic separations in complexes containing
Zn(l-Ophenolate)(l-OH)Zn cores is 3.042–3.139 A. The bridging
17
˚
hydroxide [O(2)] is almost symmetrically disposed across the metal
˚
˚
centres [Zn(1)–O(2) 1.998(3) A vs. Zn(2)–O(2) 1.989(3) A] while
˚
the phenolate bridge is more uneven [Zn(1)–O(1) 2.003(3) A vs.
Zn(2)–O(1) 2.148(3) A]. No intermolecular contacts of note are
˚
present.
Crystals of 5 suitable for the X-ray determination were grown
from dichloromethane by slow diffusion of hexane at room
temperature. A view of 5 is shown in Fig. 6; selected bond distances
and angles are listed in Table 5. The structure of 5 is based on a
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The Royal Society of Chemistry 2007
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Interestingly, a similar observation in 1a is only seen at low
temperature;13 it is tempting to ascribe this temperature difference
to hydrogen bonding effects in 4. The presence of the bridging
hydroxide is supported by a weak band at 3480 cm−1 in the IR
spectrum while a broad singlet is seen in its 1H NMR spectrum at
d 3.5. The FAB mass spectrum for 4 shows fragmentation peaks
corresponding to the loss of a hydroxide group from the molecular
ion in 4.
Despite repeated attempts at recrystallising 5, the presence of
complex 4 remained a constant side product and thus analytically
pure samples of 5 could not be obtained. Nevertheless, the
1H NMR spectrum showed, in addition to peaks for 4, peaks
consistent with the solid state structure with the inequivalent
imino-methyl resonances visible at d 2.30 and 2.31. In the FAB
mass spectrum, a fragmentation peak corresponding to the loss
of one pyridonate group from the molecular ion in 5 was
evident.
Fig. 6 Molecular structure of 5 with a partial atom labeling scheme; all
hydrogen atoms apart from H1 have been omitted for clarity.
◦
˚
Table 5 Selected bond distances (A) and angles ( ) for 5
The formation of a bridging hydroxide ligand in 4 during the
reaction of 1a with TlOEt was unexpected and the origin of the
hydrolysis reaction is uncertain. It is noteworthy that Hillmyer
et al. have also reported the difficulties in the reactions of related
dizinc trichlorides with thallium ethoxides presumably in this case
also forming hydroxide complexes.8b Indeed, the ready hydrolysis
reactions of zinc halides have been reported elsewhere.22 The
presence of 4 as a significant by-product in the reaction of 1 with
Tlhp is likely due to the incomplete formation of Tlhp (made in-situ
from TlOEt and Hhp) with the result that any unreacted TlOEt
would undergo the hydrolysis reaction detailed above with L1-
H. Notably, heating the mixture of 4 and 5 in air in acetonitrile did
not lead to an increased ratio of 4 suggesting that 5 is hydrolytically
stable under these conditions.
Zn(1)–O(1)
Zn(1)–O(2)
Zn(1)–Cl(1)
Zn(1)–N(1)
Zn(1)–N(2)
Zn(2)–O(1)
Zn(2)–O(3)
Zn(2)–Cl(2)
Zn(2)–N(5)
2.014(5)
2.008(6)
2.268(2)
2.131(7)
2.149(7)
2.004(5)
1.939(6)
2.217(3)
1.965(8)
Zn(1) · · · Zn(2)
Zn(3)–Cl(3)
Zn(3)–N(3)
Zn(3)–N(4)
Zn(3)–N(6)
Zn(2) · · · Zn(3)
C(13)–N(1)
C(31)–N(4)
3.268(5)
2.196(3)
2.012(7)
2.067(7)
2.000(7)
4.649(6)
1.276(10)
1.258(10)
N(1)–Zn(1)–O(1)
N(2)–Zn(1)–O(2)
Cl(1)–Zn(1)–O(1)
Cl(1)–Zn(1)–O(2)
N(5)–Zn(2)–O(1)
N(5)–Zn(2)–O(3)
140.8(2)
147.0(2)
103.16(18)
106.02(19)
99.3(3)
N(5)–Zn(2)–Cl(2)
Zn(1)–O(1)–Zn(2)
N(3)–Zn(3)–Cl(3)
N(3)–Zn(3)–N(4)
N(3)–Zn(3)–N(6)
121.2(2)
108.9(3)
121.41(19)
80.7(3)
123.7(3)
104.9(3)
By analogy with the ethoxide-based dizinc catalysts reported by
Tolman and Hillmyer,8a,8b it was envisioned that hydroxide 4 and
the mixture of 4/pyridonate 5 may also serve as catalysts for the
ring opening of cyclic esters. Hence both were screened as catalysts
for the ring-opening polymerisation of e-caprolactone. Reactions
were performed by mixing e-caprolactone with a toluene solution
of the initiator at a set temperature to provide an initial monomer
concentration. Conversion to polycaprolactone was determined by
1H NMR spectroscopy by removing aliquots at various intervals.
Both 4 and 4/5 gave no evidence for any conversion after 72 h at
neutral trimetallic zinc complex in which L1 binds to all three zinc
atoms with Zn(1) and Zn(2) 1,3-bridged by one pyridonate ligand
and Zn(2) and Zn(3) by the other. In addition, each metal centre
is bound by a terminal chloride ligand. As with 1a–4, one of the
zinc centres [Zn(1)] fills a tridentate N,N,O-cavity within L1 with
the oxygen atom of the phenolate group additionally acting as a
bridging ligand to Zn(2). Unlike 1a–4, the pyridyl-imine unit in
L1 no longer binds to Zn(2) but has twisted away [tors.: C(25)–
C(24)–C(26)–N(3) 129.1◦] so to act as a N,N-chelate to Zn(3). The
result is that both distorted tetrahedral [Zn(2), Zn(3)] and square
pyramidal geometries (s = 0.1)15 are present within the complex.
The pyridonate-bridged zinc atoms have considerably different in-
◦
room temperature. On running the reaction of 4 at 80 C, trace
conversion was observed after 72 h. On the other hand at 100 ◦C,
8% conversion could be achieved after 22 h while after 117 h a 55%
conversion was noted. A similar reaction profile was observed for
the mixture of 4/5 at 100 ◦C, albeit with slightly reduced levels of
conversion.
˚
termetallic distances with the Zn(1) · · · Zn(2) distance [3.268(5) A]
being slightly longer than that seen in 1a13 while the Zn(2) · · · Zn(3)
˚
distance is significantly more elongated [4.649(6) A]. Notably, the
It is uncertain why these systems display such low activities
for e-caprolactone polymerisation when compared to previously
reported ethoxide-based dizinc systems,8a,8b but may be due to
the relative nucleophilicity of a hydroxide/pyridonate nucleophile
in 4/5 over an ethoxide (during the carbonyl attack step).
Furthermore, the steric bulk of the aryl groups in 4 may also be
impeding approach by an incoming molecule of e-caprolactone.
1,3-bridging mode of the pyridonate ligands, while common place
for other first row transition metals,18 has not been reported for
polyzinc complexes with crystallographically reported examples
displaying a preference for the ligand to act as an N,O-chelate,19
a
1,1-bridging ligand20 or as a monodentate O-bound ligand.21 No
intermolecular contacts are apparent of note.
1
As with 1 the imino-methyl signals in the H NMR spectrum
of 4 are equivalent with a singlet evident at d 2.26. The presence
of four sharp doublets for the CHMe2 protons (in a ratio of 6 :
6 : 6 : 6) in 4 suggests that rotation of both N-aryl groups and
isopropyl substituents is severely restricted at room temperature.
3
Conclusions
In this study, we have shown that a family of chiral dizinc
trichloride complexes of type 1 can be readily generated in
4570 | Dalton Trans., 2007, 4565–4575
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which the steric properties of the imino-aryl groups can be syste-
matically varied. Derivatisation of 1a by introducing a range
of monoanionic ligands (viz. OAc, dbm, OH, hp) has been
successfully undertaken leading to bimetallic (2–4) and trimetallic
species (5) with intermetallic separations ranging from 3.069(4)–
4.649(6) A. Preliminary screening of 4 and 4/5 as initiators for
the ring-opening polymerisation of e-caprolactone reveals only
modest activities/conversions.
ylaniline (1.094 g, 9.027 mmol, 10 eq.), ZnCl2 (0.246 g, 1.805 mmol,
2 eq.) and n-butanol (40 ml) gave [(L2)Zn2(l-Cl)Cl2] (1b) as yellow
blocks. Yield: 45% (0.315 g, 0.401 mmol). Compound 1b: 1H NMR
(300 MHz, CDCl3): insoluble. IR (cm−1): 1699 (w), 1634 (m), 1590
=
(s, m(C N)), 1567 (m), 1450 (s), 1413 (m), 1367 (m), 1260 (s),
˚
1206 (s), 1110 (m), 1016 (m), 827 (m), 766 (s), 790 (s), 748 (s). MS
(FAB): m/z 739 [M − Cl]+. Anal. Calc. for C36H33N4OZn2Cl3: C,
55.79; H, 4.26; N, 7.23. Found: C, 55.71; H, 4.22; N, 7.35%.
(c) 1c. Using the procedure described for 1a with 2,6-
=
{O C(Me)C5H3N}2C6H3OH (0.300 g, 0.903 mmol), 2,4,6-
4
Experimental
trimethylaniline (1.220 g, 9.027 mmol, 10 eq.), ZnCl2 (0.246 g,
1.805 mmol, 2 eq.) and n-butanol (40 ml) gave [(L3)Zn2(l-
Cl)Cl2] (1c) as yellow blocks. Yield: 53% (0.384 g, 0.479 mmol).
Compound 1c: 1H NMR (300 MHz, CDCl3): insoluble. IR (cm−1):
1635 (m), 1590 (s), 1568 (m), 1477 (m), 1454 (s), 1410 (m), 1366 (m),
1340 (w), 1253 (m), 1213 (s), 1184 (m), 1148 (w), 1111 (w),
1012 (m), 852 (s), 824 (m), 789 (s), 747 (s), 686 (w). MS (FAB):
m/z 767 [M − Cl]+. Anal. Calc. for C38H37N4OZn2Cl3: C, 56.83;
H, 4.61; N, 6.98. Found: C, 56.51; H, 4.22; N, 7.35%.
4.1 General
All reactions, unless otherwise stated, were carried out under
an atmosphere of dry, oxygen-free nitrogen, using standard
Schlenk techniques or in a nitrogen purged glove box. Solvents
were distilled under nitrogen from appropriate drying agents
and degassed prior to use.23 The infrared spectra were recorded
on a Perkin-Elmer Spectrum One FT-IR spectrometer on solid
samples. The ESI (ElectroSpray) and the FAB (Fast atom bom-
bardment) mass spectra were recorded using a micromass Quattra
LC mass spectrometer and a Kratos Concept spectrometer with
(d) 1d. Using the procedure described for 1a with 2,6-
=
{O C(Me)C5H3N}2C6H3OH (0.150 g, 0.451 mmol), 2,4-dimeth-
ylaniline (0.547 g, 4.513 mmol, 10 eq.), ZnCl2 (0.135 g, 0.993 mmol,
2.2 eq.) and n-butanol (30 ml) gave [(L4)Zn2(l-Cl)Cl2] (1d) as a
yellow solid. Yield: 51% (0.178 g, 0.230 mmol). Compound 1d:
dichloromethane or NBA as the matrix, respectively. H and 13C
1
NMR spectra were recorded on a Bruker ARX spectrometer
(300 MHz) at ambient temperature unless otherwise stated;
chemical shifts (d) are referred to the residual protic solvent
peaks and chemical shifts are in Hertz (Hz). Elemental analyses
were performed at the Science Technical Support Unit, London
Metropolitan University.
1H NMR (300 MHz, CDCl3): d 2.03 (s, 6H, Ar–CH3), 2.19 (s,
3
=
6H, Ar–CH3), 2.25 (s, 6H, CH3C N), 6.85 (t, JHH 6.6 Hz, 1H,
3
Phenolate-Hp), 6.9–7.0 (m, 6H, Ar–H), 7.66 (d, JHH = 6.6 Hz,
2H, Phenolate-Ho), 7.77 (d, 3JHH 7.8 Hz, 2H, Py-H), 7.9–8.1 (m,
−1
=
4H, Py-H). IR (cm ): 1635 (m, m(C N)), 1591 (s), 1555 (m),
1494 (m), 1455 (s), 1369 (m), 1343 (m), 1308 (m), 1246 (m), 1220 (s),
1196 (m), 1148 (w), 1012 (m), 861 (m), 845 (s), 826 (m), 791 (s),
764 (s), 734 (m). MS (FAB): m/z 739 [M-Cl]+. Anal. Calc. for
C36H33N4OZn2Cl3: C, 55.79; H, 4.26; N, 7.23. Found: C, 55.74; H,
4.24; N, 7.17%.
The reagents, 2,6-diisopropylaniline, 2,6-dimethylaniline, 2,4,6-
trimethylaniline, 2,4-dimethylaniline, sodium acetate, thallium
tetrafluoroborate, dibenzoylmethane, thallium ethoxide, 2-
hydroxypyridine and zinc dichloride were purchased from
Aldrich Chemical Co. and used without further purification;
e-caprolactone was distilled prior to use. The compounds, 2,6-
{O C(Me)C5H3N}2C6H3OH13 and L1-H13 were prepared accord-
=
ing to previously reported procedures. Nadbm was prepared by
treating a methanolic solution of dibenzoylmethane (Hdbm) with
an equimolar ratio of aqueous sodium hydroxide followed by
drying of the product under reduced pressure. All other chemicals
were obtained commercially and used without further purification.
4.3 Synthesis of Lx–H
=
(a) L2-H. 2,6-{O C(Me)C5H3N}2C6H3OH (0.500 g, 1.504 mmol)
was suspended in 2,6-dimethylaniline (1.823 g, 15.044 mmol,
10 eq.) and stirred for 15 min at 160 ◦C until dissolution. A catalytic
amount of formic acid was added, and the reaction mixture was
stirred for an additional 30 min at this temperature. Following
removal of the excess 2,6-dimethylaniline under reduced pressure,
the resulting brown residue was stirred in ethanol before being
cooled to −30 ◦C to afford a yellow precipitate. The precipitate was
collected by filtration and washed with cold ethanol to afford 2,6-
4.2 Template synthesis of 1
(a) 1a. An oven-dried Schlenk flask equipped with a magnetic
stir bar was evacuated and backfilled with nitrogen. The flask
was charged with 2,6-{O C(Me)C5H3N}2C6H3OH (0.300 g,
=
=
0.903 mmol), 2,6-diisopropylaniline (1.599 g, 9.027 mmol, 10 eq.),
ZnCl2 (0.271 g, 1.987 mmol, 2.2 eq.) and n-butanol (40 ml) and
the contents stirred at 125 ◦C for 72 h. After cooling to room
temperature, the suspension was concentrated to half volume and
hexane introduced to complete the precipitation of the product.
Following filtration and washing with hexane, the product was
dried overnight under reduced pressure to afford [(L1)Zn2(l-
Cl)Cl2] (1a) as a yellow solid. Recrystallisation from acetonitrile
gave 1a as yellow blocks. Yield: 46% (0.368 g, 0.416 mmol).
Compound 1a: the spectroscopic data are as previously reported.13
(b) 1b. Using the procedure described for 1a with 2,6-
{(2,6-Me2C6H3)N C(Me)C5H3N}2C6H3OH (L2-H) as a yellow
1
solid. Yield: 45% (0.361 g, 0.670 mmol). Compound L2-H: H
NMR (300 MHz, CDCl3): d 1.98 (s, 12H, Ar–CH3), 2.19 (s, 6H,
CH3C N), 6.88 (m, 2H, Ar–H), 7.0–7.09 (m, 5H, Ar–H and
=
3
3
Phenol-H), 7.87 (t, JHH = 7.8, 2H, Py-H), 7.00 (d, JHH = 7.8,
2H, Phenol-H), 8.14 (d, 3JHH = 8.1 Hz, 4JHH = 0.9 Hz, 2H, Py-H),
8.26 (d, 3JHH = 8.1 Hz, 4JHH = 0.9 Hz, 2H, Py-H). 13C { H} NMR
1
=
(75 MHz, CDCl3): 16.8 (CH3C N), 18.0 (CH3Ar), 119.1 (CH),
119.4 (CH), 123.2 (CH), 123.7 (CH), 124.1, 125.4, 128.0 (CH),
=
130.5 (CH), 137.4 (CH), 148.6, 154.5, 155.7, 158.0, 166.4 (C N).
IR (cm−1): 1638 (m), 1592 (m), 1567 (m), 1470 (m), 1450 (m),
1362 (m), 1267 (m), 1207 (m), 1170 (w), 1138 (w), 1112 (m), 825 (w),
=
{O C(Me)C5H3N}2C6H3OH (0.300 g, 0.903 mmol), 2,6-dimeth-
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815 (w), 783 (s), 762 (s), 741 (s), 691 (w). MS (ESI): m/z 539 [M
+ H]+. Anal. Calc. for C36H34N4O: C, 80.30; H, 6.32; N 10.41%.
Found: C, 80.55; H, 6.48; N, 10.27%.
4.5 Synthesis of 2
An oven-dried Schlenk flask equipped with a magnetic stir bar was
evacuated and backfilled with nitrogen. The flask was charged with
1a (0.100 g, 0.113 mmol), NaOAc (0.019 g, 0.226 mmol, 2 eq.) and
suspended in acetonitrile (8 ml) and stirred for 15 min at room
temperature. TlBF4 (0.049 g, 0.169 mmol, 1.5 eq.) was added and
the reaction mixture allowed to stir for a further 24 h. The solution
was filtered through Celite and concentrated under reduced
pressure to give a yellow solid residue. The residue was crystallised
from acetonitrile solution to afford [(L1)Zn2(l-OAc)2](BF4) (2) as
yellow crystals. Yield: 45% (0.050 g, 0.051 mmol). Compound 2:
(b) L3-H. A similar procedure to that outlined for L2-H
=
was employed using 2,6-{O C(Me)C5H3N}2C6H3OH (0.500 g,
1.504 mmol) in 2,4,6-trimethylaniline (2.034 g, 15.044 mmol, 10
=
eq.) gave 2,6-{(2,4,6-Me3C6H2)N C(Me)C5H3N}2C6H3OH (L3-
H) as a yellow solid. Yield: 49% (0.415 g, 0.732 mmol). Compound
L3-H: 1H NMR (300 MHz, CDCl3): d 1.94 (s, 12H, Ar–CH3), 2.18
=
(s, 6H, Ar–CH3), 2.23 (s, 6H, CH3C N), 6.83 (s, 4H, Ar–H), 7.04
(t, 1H, 3JHH = 7.8 Hz, Phenol-H), 7.87 (t, 3JHH = 8.0 Hz, 2H, Py-
H), 7.97 (d, 3JHH = 7.8 Hz, 2H, Phenol-H), 8.13 (d, 3JHH = 8.4 Hz,
1H NMR (300 MHz, CD3CN): d 0.91 (d, JHH = 6.7 Hz, 12H,
3
4JHH = 0.9 Hz, 2H, Py-H), 8.25 (d, 3JHH = 7.8 Hz, 4JHH = 0.9 Hz,
3
CH(CH3)2), 0.98 (d, JHH = 6.7 Hz, 12H, CH(CH3)2), 1.93 (s,
1
2H, Py-H). 13C { H} NMR (75 MHz, CDCl3): 16.8 (CH3C N),
=
=
6H, CH3CO2), 2.36 (s, 6H, CH3C N), 2.69 (br, 4H, CH(CH3)2),
17.9 (CH3Ar), 20.8 (CH3Ar), 119.1 (CH), 119.4 (CH), 123.6 (CH),
6.9–7.2 (m, 7H, Ar–H and Phenolate-Hp), 7.62 (d, 3JHH = 7.9 Hz,
2H, Py-H), 7.8–7.9 (m, 2H, Py–H), 8.2–8.4 (m, 4H, Phenolate-Hm
and Py–H). 19F NMR (283.5 MHz, CDCl3): d 152.36 (s, BF4). IR
124.2, 125.3, 128.6 (CH), 130.5 (CH), 132.3, 137.4 (CH), 146.1,
−1
=
154.6, 155.7, 158.0, 166.6 (C N). IR (cm ): 1635 (m), 1567 (m),
1476 (w), 1449 (m), 1363 (m), 1264 (m), 1215 (m), 1138 (w),
1112 (w), 850 (m), 824 (w) and 783 (s), 742 (s). MS (ESI): m/z
567 [M + H]+. Anal. Calc. for C38H38N4O: C, 80.42; H, 6.71; N
9.89%. Found: C, 80.72; H, 6.76; N, 9.79%.
(cm−1): 2964 (w), 1636 (m), 1589 (s, m(COO)asym and m(C N)),
=
1442 (s), 1408 (s, m(COO)sym), 1369 (m), 1326 (w), 1255 (m),
1198 (m), 1048 (br, m(B–F)), 937 (m), 861 (m), 825 (m), 792 (s),
775(s) and 747 (s). MS (FAB): m/z 899 [M − BF4]+, 855 [M −
MeCO − BF4]+. Anal. Calc. for C48H55N4O5BF4Zn2: C, 58.50; H,
5.59; N, 5.69. Found: C, 58.71; H, 5.62; N, 5.44%.
(c) L4-H. A similar procedure to that outlined for L2-H
=
was employed using 2,6-{O C(Me)C5H3N}2C6H3OH (0.200 g,
0.602 mmol) in 2,4-dimethylaniline (0.729 g, 6.018 mmol, 10 eq.)
gave L4-H as a yellow solid. Yield: 40% (0.130 g, 0.241 mmol).
4.6 Synthesis of 3
1
Compound L4-H: H NMR (300 MHz, CDCl3): d 2.04 (s, 6H,
=
Ar–CH3)), 2.26 (s, 6H, Ar–CH3), 2.30 (s, 6H, CH3C N), 6.52 (m,
An oven-dried Schlenk flask equipped with a magnetic stir bar
was evacuated and backfilled with nitrogen. Complex 1a (0.100 g,
0.113 mmol) and Nadbm (0.056 g, 0.226 mmol, 2 eq.) were
introduced, dissolved in acetonitrile (8 ml) and stirred for 15 min at
room temperature. TlBF4 (0.049 g, 0.169 mmol, 1.5 eq.) was added
to the reaction mixture and the contents allowed to stir for 72 h.
Following filtration through Celite, the filtrate was concentrated
under reduced pressure and the yellow solid residue dissolved
in chloroform (10 ml) and layered with hexane (20 ml). After
standing for three days [(L1)Zn2(dbm)2](BF4) (3) was obtained as
pale yellow blocks. Yield: 71% (0.105 g, 0.08 mmol). Compound
2H, Ar–H), 6.93–7.04 (m, 5H, Ar–H and Phenol-H), 7.82–7.88
3
(m, 2H, Py-H), 7.97 (d, JHH = 8.1 Hz, 2H, Phenol-H), 8.13 (d,
3JHH = 8.1 Hz, 2H, Py-H), 8.19 (d, 3JHH = 7.8 Hz, 2H, Py-H). IR
−1
=
(cm ): 1640 (m, m(C N)), 1591 (m), 1564 (w), 1494 (m), 1448 (m),
1363 (w), 1262 (m), 1217 (w), 1079 (br), 786 (s), 745 (m). MS (ESI)
(m/z): 539 [M + H]+. Anal. Calc. for C36H34N4O: C, 80.30; H,
6.32; N 10.41%. Found: C, 80.46; H, 6.49; N, 10.19%.
4.4 Synthesis of 1 from Lx–H
1
3: H NMR (300 MHz, CDCl3): d 0.6–0.7 (m, 24H, CH(CH3)2),
(a) 1b from L2-H. An oven-dried Schlenk flask equipped with
a magnetic stir bar was evacuated and backfilled with nitrogen.
The flask was charged with ZnCl2 (0.127 g, 0.928 mmol) in n-
=
2.29 (s, 6H, CH3C N), 2.61 (br, 4H, CH(CH3)2), 5.79 (s, 2H,
CH(CPhCO)2)), 6.7–7.5 (m, 29H, Ar–H, Py–H or Phenolate-
Hp), 7.90 (m, 2H, Ar–H), 8.1–8.3 (m, 4H, Py-H or Phenolate-
Hm). 19F NMR (283.5 MHz, CDCl3): d 153.6 (s, BF4). IR (cm−1):
2965 (w), 1636 (w), 1592 (s), 1545 (s), 1519 (s), 1479 (m), 1452 (s),
1381 (s), 1303 (w), 1269 (m), 1198 (m), 1054 (br, m(B–F)), 937 (m),
852 (m), 827 (m), 792 (s), 754 (s) and 689 (s). MS (FAB): m/z
1226 [M − BF4]+, 1004 [M − dbm − BF4]+. Anal. Calc. for
C74H71N4O5BF4Zn2: C, 67.65; H, 5.41; N, 4.27. Found: C, 67.89;
H, 5.30; N, 4.19%.
◦
butanol (30 ml) and the contents heated to 110 C until the zinc
salt had completely dissolved. L2-H (0.250 g, 0.464 mmol, 0.5
eq.) was added and the mixture stirred and heated to 110 ◦C
overnight. After cooling to room temperature, the suspension
was concentrated and washed several times with hexane to afford
[(L2)Zn2(l-Cl)Cl2] (1b) as a yellow solid. Yield: 85% (0.305 g,
0.394 mmol). Compound 1b: the spectroscopic data were as
described in section 4.2(b).
(b) 1c from L3-H. Using a procedure based on that described
for 1b employing ZnCl2 (0.120 g, 0.822 mmol) and L3-H (0.250 g,
0.441 mmol, 0.5 eq.) gave [(L3)Zn2(l-Cl)Cl2] (1c) as a yellow solid.
Yield: 78% (0.275 g, 0.343 mmol). Compound 1c: the spectroscopic
data were as described in section 4.2(c).
(c) 1d from L4-H. Using a procedure based on that described
for 1b employing ZnCl2 (0.319 g, 2.343 mmol) and L4-H (0.631 g,
1.171 mmol, 0.5 eq.) afforded [(L4)Zn2(l-Cl)Cl2] (1d) as a yellow
solid. Yield: 29% (0.262 g, 0.338 mmol). Compound 1d: the
spectroscopic data were as described in section 4.2(d).
4.7 Synthesis of 4
An oven-dried Schlenk flask equipped with a magnetic stir bar
was evacuated and backfilled with nitrogen. The flask was charged
with 1a (0.100 g, 0.110 mmol) a◦nd dissolved in dichloromethane
(5 ml) before being cooled to 0 C. A solution of TlOEt (29 mg,
8.2 ll, 0.110 mmol, 1 eq.) in dichloromethane (5 ml) in a second
Schlenk flask was cannular filtered into the flask containing 1a
to give an instantaneous formation of a creamy precipitate. The
reaction mixture was stirred at room temperature for a further
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12 h. Additional dichloromethane (10 ml) was added and the
mixture cannular filtered to give a pale yellow filtrate. The filtrate
was concentrated under reduced pressure and the solid residue
taken up in chloroform (10 ml) and layered with diethyl ether
(30 ml). After standing for three days [(L1)Zn2(l-OH)Cl2] (4) was
obtained as pale yellow blocks. Yield 86% (0.084 g, 0.100 mmol).
(ca. 100 ll) was withdrawn, quenched with a drop of pentane and
dried under reduced pressure at room temperature. Conversion
was determined by observing the 1H NMR resonances of the
polymer and monomer by dissolving in CDCl3.
4.10 Crystallographic studies
1
3
Compound 4: H NMR (300 MHz, CDCl3): d 0.63 (d, JHH
=
6.4 Hz, 6H, CH(CH3)2), 0.79 (d, 3JHH = 6.7 Hz, 6H, CH(CH3)2)),
Data collection for 1b, 1c, 2, 3, 4 and 5 was carried out
on a Bruker APEX 2000 CCD diffractometer using graphite-
0.99 (d, 3JHH = 7.0 Hz, 6H, CH(CH3)2)), 1.18 (d, 3JHH = 6.4 Hz,
˚
3
monochromated Mo-Ka radiation (k = 0.71073 A). Details of the
=
6H, CH(CH3)2), 2.26 (s, 6H, CH3C N), 2.81 (sept, JHH = 6.4 Hz,
2H, CH(CH3)2), 3.01 (sept, 3JHH = 6.7 Hz, 2H, CH(CH3)2), 3.50
(s, br, 1H, O-H), 6.72 (t, 3JHH = 7.9 Hz, 1H, Phenolate-Hp), 6.93
data collection, refinement and crystal data are listed in Table 6.
The data were corrected for Lorentz and polarization effects
and empirical absorption corrections applied. Structure solution
by direct methods and structure refinement on F2 employed
SHELXTL version 6.10.24 Hydrogen atoms were included in
3
(t, JHH = 4.7 Hz, 2H, Ar-H), 7.12 (m, 4H, Ar-H), 7.71 (m, 4H,
Ar-H, Py-H), 7.98 (m, 4H, Ph-H, Py-H). IR (cm−1): 3480 (w, br,
=
m(OH)), 2964 (m), 1634 (w), 1589 (m, m(C N)), 1547 (w), 1456 (m),
˚
calculated positions (C–H = 0.96 A) riding on the bonded atom
1406 (w), 1383 (w), 1366 (m), 1325 (w), 1258 (s), 1193 (m), 1087 (s),
1015 (s), 935 (w), 865 (m), 794 (s), 688 (m). MS (FAB): m/z 851
[M − OH]+. Anal. Calc. for C44H50N4O2Cl2Zn2.CHCl3: C, 54.72;
H, 5.16; N, 5.67. Found: C, 54.55; H, 5.29; N, 5.23%.
with isotropic displacement parameters set to 1.5 Ueq(C) for methyl
H atoms and 1.2 Ueq(C) for all other H atoms. All non-H atoms
were refined with anisotropic displacement parameters. In the case
of 3, the BF4 anion was disordered over a two-fold axis. For 1b, 2,
4 and 5, the SQUEEZE option in PLATON25 was used to remove
the disorder solvent molecules.
4.8 Synthesis of 4 and 5
CCDC reference numbers 651128–651133.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b709385c
An oven-dried Schlenk flask equipped with a magnetic stir bar was
evacuated and backfilled with nitrogen. The flask was charged
with 2-hydroxypyridine (0.011 g, 0.11 mmol) and dissolved in
acetonitrile (10 ml). TlOEt (8.2 ll, 0.11 mmol) was introduced
into the flask to give an instantaneous formation of a creamy
precipitate and the suspension stirred for 15 min. Compound
1a (0.100 g, 0.11 mmol) was added and the reaction mixture
stirred at room temperature for a further 12 h. The volatiles
were removed under reduced pressure and the residue taken up
in dichloromethane (30 ml). Following cannular filtration, the
filtrate was concentrated to half volume and layered with hexane
(ratio of CH2Cl2 to hexane = 1 : 3). After standing for one week,
yellow crystals of both 4 and [(L1)Zn3(l-hp)2Cl3] (5) in a 3 : 7
ratio (by 1H NMR) could be obtained. Combined yield 65%.
Compound 4: the spectroscopic data were as described in section
Acknowledgements
We thank the University of Leicester and the Nuffield Foundation
for an Undergraduate Bursary (to I. Y.) for financial assistance.
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