M. Hu et al. / Polyhedron 102 (2015) 308–312
309
O
O
O
N
O
Zr
O
Zr
O
OH
N
O
O
N
Zr(OiPr)4·iPrOH
N
O
O
Zr
Zr
O
OH
O
MeOH
O
N
O
N
1
Scheme 1. The synthesis of complex 1.
Elemental analyses were performed with an Elementar Vario EL
III elemental analyzer. Molecular weight and molecular weight
distribution were determined by gel permeation chromatography
(PL-GPC 220). The GPC column was eluted with THF at 40 °C at a
rate of 1 mL/min and the data were calibrated with polystyrene
standards. The unsymmetric N2O2-ligand (H2L) was prepared
according to a literature procedure [30] (Scheme S1).
in the freshly distilled toluene, followed recrystalling gave a novel
zirconium complex [Zr4L2( 3-O)2(
-OMe)2 (OiPr)6] (1) in small
l
l
yield, which is an oxo-, methoxy- and phenoxy-bridged tetranu-
clear zirconium complex. It is proposed that the methoxy bridge
in the tetranuclear complex 1 comes from the small amount of
methanol that remained in the synthesis of the ligand. Because
some multinuclear complexes employing multiple active centers
successfully catalyze high efficient and selective reaction, as metal-
loenzymes having multiple metal centers activate a high active and
selective reaction in nature, such multinuclear complexes attracted
researcher’s interesting recent decade [34,35]. However, multinu-
clear Zr complexes with N,O multidentate ligand is relatively rare.
Until now, Only a few structurally characterized multinuclear Zr
complexes bearing Schiff base ligands are found in literature [36].
We are interested in the tetranuclear Zr complex and try to
improve the yield of 1 by addition of methanol to the reaction
2.2. Preparation of the complexes
i
To a solution of Zr(OiPr)4Á PrOH (1.939 g, 5.0 mmol) in toluene
(30 mL), a solution of H2L (1.045 g, 2.5 mmol) in toluene (20 ml)
containing 1 equiv of MeOH (100 lL, 2.5 mmol) was added. The
solution was stirred for 6 h at room temperature. After removal
of solvent, the crude product was recrystallized with toluene/
hexane to give complex 1 as an off-white solid. Yield: 0.721 g
(35%). Anal. Calcd for C74H112N4O14Zr4: C, 53.98; H, 6.86; N, 3.40.
Found: C, 53.87; H, 6.90; N, 3.37%. The crystal of 1 suitable for
X-ray analysis was obtained from toluene/hexane.
i
solution. The reaction of H2L and Zr(OiPr)4Á PrOH in the presence
of 1 equiv of methanol in toluene afforded tetranuclear complex
1 in 35% yield (Scheme 1). The single crystal of 1 suitable for
X-ray study was obtained from toluene. Complex 1 was identified
by elemental analysis, and X-ray analysis.
2.3. X-ray structural determination
A single crystal of 1 suitable for X-ray crystallographic analysis
was selected following examination under a microscope. X-ray
diffraction measurements were performed on a Bruker Smart Apex
3.2. X-ray structure
The structure of complex 1 is solved in the monoclinic space
group P2(1)/c and details of the crystal data collection and
refinement parameters for 1 are listed in Table S1. Complex 1 is
a centrosymmetric tetranuclear structure composed of four Zr
atoms and each asymmetric unit has a half molecule (Fig. S1).
The Molecular structure of the tetranuclear zirconium complex 1
is shown in Fig. 1. Selected bond lengths and angles are listed in
Table S2. The X-ray analysis shows that complex 1 is an unusual
tetranuclear zirconium complex with two oxo-, two methoxy-
II CCD diffractometer with graphite monochromated Mo K
a radia-
tion (k = 0.71073 Å) at 298 K using the –2h scan mode. Data
x
processing was accomplished with the SAINT processing program
[31]. Intensity data were corrected for absorption by the SADABS
program [32]. The structures were solved by direct methods and
refined on F2 against full-matrix least-squares methods using the
SHELX-97 program package [33].
2.4. Typical procedure for polymerization of rac-LA
and two phenoxy-bridges. Each of the two bridging
was found coordinating to three metal centers, leading to the
formation of tetranuclear species containing two 3-oxo bridges.
The 3-oxo bridged complex 1 is formed because of hydrolyzes
l3-oxo atoms
The toluene solution (10 mL) of the racemic initiator 1 (0.083 g,
0.05 mmol) and rac-LA (1.44 g, 10 mmol) was stirred at prescribed
temperature for the prescribed time. The reaction was terminated
by cooling the tube quickly with an ice-water bath. A sample was
taken from the resulting solution for determination of the conver-
sion of rac-LA by 1H NMR spectroscopy. The volatiles were
removed in vacuo and the residue was dissolved in a minimum
amount of CH2Cl2. The polymer was precipitated with addition of
cold methanol. The resulting solid was filtered and washed with
ethanol (10 mL) twice and was then dried under high vacuum.
l
l
in water that is introduced by workup. Usually Zr4+ ion strongly
hydrolyzes in water due to the high charge/radius ratio [37].
There are two molecules in unit cell of 1 (Fig. 2). The N atom of
the tertiary amine in L2À becomes a chiral center because of the
restriction on the conformation inversion of the unsymmetric ter-
tiary amine upon coordination. It is notable that each molecule of
tetranuclear Zr complex 1 possesses two heterochiral N atoms
(N1(R), N1A(S)). However, homochiral complexes were formed in
our previous work and in preparation of other transition metal
complexes containing tertiary amine ligands because of chiral
induction upon coordination [30,38].
3. Results and discussion
3.1. Synthesis
The four zirconium atoms of 1 are unequivalently coordinated.
Two Zr centers (Zr1 and Zr1A) are 7-coordinate in a distorted pen-
tagonal bipyramidal geometry, and each of them is coordinated by
The unsymmetric N2O2-ligand (H2L) was prepared by reported
methods [30]. The reaction of H2L with zirconium(IV) isopropoxide
an isopropoxy O, a methoxy O and a bridging l3-oxo atom, as well