Download Preparation and structural characterization of group 1 metal

Inorganica Chimica Acta 360 (2007) 136–142
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Preparation and structural characterization of group 1 metal
complexes containing a chelating biphenolato phosphine ligand
Yu-Ning Chang, Lan-Chang Liang
*
Department of Chemistry and Center for Nanoscience and Nanotechnology, National Sun Yat-sen University, Kaohsiung 80424, Taiwan
Received 15 June 2006; received in revised form 18 July 2006; accepted 24 July 2006
Available online 29 July 2006
Inorganic Chemistry – The Next Generation.
Abstract
The coordination chemistry of a potentially tridentate, dianionic biphenolato phosphine ligand with respect to group 1 metals
is described. Deprotonation of bis-(3,5-di-tert-butyl-2-hydroxyphenyl)phenylphosphine (H2[OPO]) with two equivalents of n-BuLi,
NaH, or KH in dimethoxyethane (DME) solutions produces the corresponding dinuclear alkali metal complexes [OPO]M2(DME)2
(M = Li, Na, K). The X-ray structure of [OPO]Li2(DME)2 reveals that the two lithium atoms are bridged by both phenolato
oxygen donors with only one lithium being coordinated to the phosphorus donor. Consistently, variable-temperature 31P{1H}
and 7Li{1H} NMR spectroscopic studies elucidate the coordination of the phosphorus donor in [OPO]Li2(DME)2 to one of
the lithium atoms in solution. Interestingly, an X-ray diffraction study of the potassium complex indicates a dimeric structure with
S2 symmetry for this species in which the four potassium atoms are bridged by both phosphorus and oxygen donors of the biphenolato phosphine ligands. These alkali metal complexes are active initiators for catalytic ring-opening polymerization of ecaprolactone.
2006 Elsevier B.V. All rights reserved.
Keywords: Alkali metals; Catalysis; Hybrid ligands; Phenolato ligands; Phosphine ligands; Ring-opening polymerization
1. Introduction
Metal complexes of hybrid chelating ligands that contain both hard and soft donor atoms are currently receiving
extensive attention [1–7]. With the hybrid characteristic,
ligands of this type are anticipated to bind to both hard
and soft metals on the basis of the hard and soft acids
and bases theory [8]. The apparent mismatch of hard–soft
donor–acceptor pairs has often led to metal complexes that
exhibit unusual thermal stability and chemical reactivity
[9–12]. One remarkable example is the silyl-derived amido
phosphine ligands (1, Fig. 1) developed by Fryzuk and
co-workers [13–19].
*
Corresponding author. Tel.: +886 7 5252000x3945; fax: +886 7
5253908.
E-mail address: [email protected] (L.-C. Liang).
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.07.050
Inspired by Fryzuk’s pioneering work regarding 1, we
and others began to investigate reaction chemistry involving amido phosphine complexes that contain an o-phenylene ligand backbone (2 and 3 in Fig. 1) [4,20–25]. It has
been reasoned [20] and demonstrated that the rigidity
and robustness imposed by the o-phenylene backbone in
these amido phosphine ligands are beneficial in view of
the inhibited propensity of phosphine dissociation and
enhanced thermal stability of the resulting metal complexes
[20,26–29]. Encouraged by the prosperous chemistry that
had been developed thus far based on 2 and 3, we became
interested in chelating phenolato phosphine ligands 4.
These ligands are closely analogous to 3 given that both
possess potentially identical hapticity, formal anionic
charge, and possible p-donor characteristic. Ligands 4,
however, are anticipated to be somewhat less sterically hindered than 3 due to the lack of substituents at the anionic
Y.-N. Chang, L.-C. Liang / Inorganica Chimica Acta 360 (2007) 136–142
137
2.3. X-ray crystallography
Fig. 1. Representative examples for hybrid chelating ligands.
donor atoms for the former. To compensate this inherent
discrepancy between 3 and 4, we thus chose to examine
those bearing a sterically demanding substituent ortho to
the phenolato oxygen atoms. Compound bis-(3,5-di-tertbutyl-2-hydroxyphenyl)phenylphosphine (H2[OPO]) is a
suitable ligand precursor available from the literature
[30]. To begin with, we report in this contribution the preparation and structural characterization of [OPO]2 with
respect to group 1 metals as these alkali metal complexes
should be convenient starting materials for metathetical
reactions with other main-group and transition metal
halides. We note that although ligands of the type 4 are
known since 1980 [31], compounds described herein represent the first examples of well-defined group 1 metal complexes of 4 to date.
2. Experimental
2.1. General procedures
Unless otherwise specified, all experiments were performed under nitrogen using standard Schlenk or glove
box techniques. All solvents were reagent grade or better
and purified by standard methods. The NMR spectra were
recorded on Varian Unity or Bruker AV instruments.
Chemical shifts (d) are listed as parts per million downfield
from tetramethylsilane and coupling constants (J) in
Hertz. 1H NMR spectra are referenced using the residual
solvent peak at d 7.16 for C6D6 and d 2.09 for toluene-d8
(the most upfield resonance). 13C NMR spectra are referenced using the residual solvent peak at d 128.39 for
C6D6. The assignment of the carbon atoms is based on
the DEPT 13C NMR spectroscopy. 31P and 7Li NMR
spectra are referenced externally using 85% H3PO4 at d
0, and LiCl in D2O at d 0, respectively. Routine coupling
constants are not listed. All NMR spectra were recorded
at room temperature in specified solvents unless otherwise
noted. Elemental analysis was performed on a Heraeus
CHN-O Rapid analyzer.
2.2. Materials
Compound H2[OPO] was prepared according to the literature procedures [30]. All other chemicals were obtained
from commercial vendors and used as received.
Table 1 summarizes the crystallographic data for [OPO]Li2(DME)2 and {[OPO]K2(DME)2}2. Data were collected
on a Bruker-Nonius Kappa CCD diffractometer with
graphite monochromated Mo Ka radiation (k = 0.7107
Å). Structures were solved by direct methods and refined
by full-matrix least-squares procedures against F2 using
WINGX crystallographic software package. All full-weight
non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions. In [OPO]Li2(DME)2, two tert-butyl groups are disordered with the
methyl substituents being in the ratio of 69:31 and 39:61,
respectively, over two conformations. In [OPO]K2(DME)2,
one tert-butyl group is disordered with the methyl substituents being in the ratio of 60:40 over two conformations
and one methoxy group is disordered with the methyl substituent being in the ratio of 49:51 over two conformations.
2.4. Synthesis of [OPO]Li2(DME)2
Solid
bis-(3,5-di-tert-butyl-2-hydroxyphenyl)phenylphosphine (100 mg, 0.19 mmol) was dissolved in DME
(4 mL) and cooled to 35 C. To this was added n-BuLi
(0.24 mL, 1.6 M in hexane, Aldrich, 0.38 mmol, 2 equiv.)
dropwise. After being stirred at room temperature for
1 h, the reaction solution was filtered through a pad of Celite and concentrated under reduced pressure until the volume became ca. 1 mL. Cooling the concentrated solution
to 35 C overnight afforded the product as colorless crystals suitable for X-ray diffraction analysis; yield 81.3 mg
(59%). 1H NMR (C6D6, 80 C, 500 MHz) d 7.68 (br s, 2,
Ar), 7.55 (br s, 2, Ar), 7.47 (d, 2, Ar), 7.18 (br s, 2, Ar),
7.08 (m, 1, Ar), 3.06 (s, 8, OCH2), 2.87 (s, 12, OMe), 1.61
(s, 18, CMe3), 1.32 (s, 18, CMe3). 1H NMR (C6D6,
200 MHz) d 7.73 (t, 2, Ar), 7.65 (br s, 2, Ar), 7.51 (d, 2,
Ar), 7.16 (m, 2, Ar), 7.07 (m, 1, Ar), 2.94 (s, 8, OCH2),
2.76 (s, 12, OMe), 1.66 (s, 18, CMe3), 1.34 (s, 18, CMe3).
1
H NMR (toluene-d8, 500 MHz) d 7.67 (t, 2, Ar), 7.51
(br s, 2, Ar), 7.44 (d, 2, Ar), 7.32 (br s, 1, Ar), 7.16 (t, 2,
Ar), 3.19 (s, 8, OCH2), 3.02 (s, 12, OMe), 1.62 (s, 18,
CMe3), 1.31 (s, 18, CMe3). 31P{1H} NMR (C6D6, 80 C,
202.31 MHz) d 31.95 (Dm1/2 = 63.85 Hz). 31P{1H} NMR
(C6D6, 80.95 MHz) d 32.64. 31P{1H} NMR (toluene-d8,
202.46 MHz) d 33.80 (Dm1/2 = 68.03 Hz). 31P{1H} NMR
(toluene-d8, 73 C, 202.46 MHz) d 35.43 (1:1:1:1 q,
1
JPLi = 33). 31P{1H} NMR (DME, 80.95 MHz) d 34.14.
7
Li{1H} NMR (C6D6, 194 MHz) d 1.69 (Dm1/2 =
39.86 Hz). 7Li{1H} NMR (toluene-d8, 194 MHz) d 0.80
(br s, Dm1/2 = 100.07 Hz). 7Li{1H} NMR (toluene-d8,
73 C, 194 MHz) d 1.50 (d, 1, 1JPLi = 33), 0.80 (s, 1).
13
C{1H} NMR (C6D6, 80 C, 125.70 MHz) d 168.13 (d,
JCP = 21.75, C), 140.54 (s, C), 137.76 (s, C), 136.90 (s,
C), 133.62 (d, JCP = 13.95, CH), 128.67 (s, CH), 128.00
(s, CH), 127.05 (s, CH), 126.36 (d, JCP = 9.55, C), 125.95
(s, CH), 71.49 (s, OCH2), 59.11 (s, OMe), 36.11 (s,
CMe3), 34.80 (s, CMe3), 32.46 (s, CMe3), 32.25(s, CMe3).
138
Y.-N. Chang, L.-C. Liang / Inorganica Chimica Acta 360 (2007) 136–142
Table 1
Crystallographic data for [OPO]Li2(DME)2 and {[OPO]K2(DME)2}2
Compound
[OPO]Li2(DME)2
{[OPO]K2(DME)2}2
Formula
Fw
Crystal size (mm)
Dcalc (Mg/m3)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a ()
b ()
c ()
V (Å3)
Z
T (K)
Radiation,
Mo Ka, k (Å)
2h Range ()
h, k, l Range
C42H65Li2O6P
710.79
0.57 · 0.32 · 0.22
1.085
monoclinic
P21/n
11.3150(2)
19.4720(4)
20.2640(5)
90
102.8570(10)
90
4352.74(16)
4
200(2)
0.71073
C84H130K4O12P2
1550.22
0.45 · 0.35 · 0.25
1.139
monoclinic
P21/n
14.4187(3)
15.6910(3)
20.0298(5)
90
94.3160(10)
90
4518.77(17)
2
200(2)
0.71073
4.12–50.66
12 6h 6 13,
23 6 k 6 17,
24 6 l 6 24
26 575
4.08–52.04
17 6h 6 17,
18 6 k 6 19,
24 6 l 6 22
29 559
7915
8797
0.0820
0.104
0.0508
0.286
7915/0/474
8797/0/458
1.087
R1 = 0.0868,
wR2 = 0.2205
R1 = 0.1427,
wR2 = 0.2647
0.769 to 1.379
1.027
R1 = 0.0565,
wR2 = 0.1424
R1 = 0.0859,
wR2 = 0.1604
0.437 to 0.904
Total number of
reflections
Number of independent
reflections
Rint
Absorption
coefficient (mm 1)
Number of data/
restraints/parameters
Goodness-of-fit
Final R indices
[I > 2r(I)]
R indices (all data)
Residual density (e/Å3)
Anal. Calc. for C42H65Li2O6P: C, 70.97; H, 9.22. Found: C,
70.99; H, 9.25%.
2.5. Synthesis of [OPO]Na2(DME)2
DME (5 mL) was added to a solid mixture of bis-(3,5-ditert-butyl-2-hydroxyphenyl)phenylphosphine
(300 mg,
0.58 mmol) and NaH (41.7 mg, 1.74 mmol, 3 equiv.) at
room temperature. After being stirred at room temperature
for 1 h, the reaction solution was filtered through a pad of
Celite, which was further washed with DME (1 mL) and filtered. The filtrates were combined and concentrated under
reduced pressure until the volume became ca. 1 mL. Cooling the concentrated solution to 35 C overnight afforded
the product as a colorless crystalline solid; yield 230 mg
(53%). 1H NMR (C6D6, 500 MHz) d 7.61 (br s, 2, Ar),
7.49 (d, 2, Ar), 7.29 (br s, 2, Ar), 7.12 (t, 2, Ar), 7.01 (t,
1, Ar), 2.85 (s, 12, OMe), 2.84 (s, 8, OCH2), 1.63 (s, 18,
CMe3), 1.35 (s, 18, CMe3). 31P{1H} NMR (C6D6,
202.31 MHz) d 39.36 (Dm1/2 = 50.78 Hz). 31P{1H} NMR
(DME, 80.95 MHz) d 38.20. 13C{1H} NMR (C6D6,
125.70 MHz) d 170.61 (d, JCP = 17.85, C), 143.65 (s, C),
136.43 (s, C), 134.19 (d, JCP = 13.20, CH), 133.79 (s, C),
130.26 (s, CH), 127.74 (s, CH), 126.85 (s, CH), 125.17 (s,
CH), 124.03 (m, C), 71.34 (s, OCH2), 59.04 (s, OMe),
35.81 (s, CMe3), 34.70 (s, CMe3), 32.60 (s, CMe3), 30.80
(s, CMe3). Anal. Calc. for C42H65Na2O6P: C, 67.90; H,
8.82. Found: C, 67.58; H, 8.76%.
2.6. Synthesis of {[OPO]K2(DME)2}2
DME (10 mL) was added to a solid mixture of bis-(3,5-ditert-butyl-2-hydroxyphenyl)phenylphosphine
(200 mg,
0.39 mmol) and KH (61.9 mg, 1.54 mmol, 3.9 equiv.) at
room temperature. The reaction mixture was heated to
80 C with stirring for 15 h, cooled to room temperature,
and filtered through a pad of Celite. The Celite pad was further washed with DME (5 mL) and filtered. The filtrates were
combined and concentrated under reduced pressure until the
volume became ca. 2 mL. Cooling the concentrated solution
to 35 C overnight afforded the product as colorless crystals suitable for X-ray diffraction analysis; yield 200 mg
(67%). 1H NMR (toluene-d8, 500 MHz) d 7.50 (d, 4, Ar),
7.38 (s, 4, Ar), 7.13 (m, 4, Ar), 6.78 (br s, 4, Ar), 5.22 (br s,
2, Ar), 3.16 (s, 16, OCH2), 3.01 (s, 24, OMe), 1.59 (br s, 36,
CMe3), 1.24 (s, 36, CMe3). 1H NMR (toluene-d8, 80 C,
500 MHz) d 7.41 (t, 4, Ar), 7.34 (d, 4, Ar), 7.12 (t, 4, Ar),
6.71 (q, 4, Ar), 4.89 (br s, 2, Ar), 3.26 (s, 16, OCH2), 3.09
(s, 24, OMe), 1.51 (s, 36, CMe3), 1.20 (s, 36, CMe3). 1H
NMR (C6D6, 500 MHz) d 7.57 (br s, 4, Ar), 7.47 (br s, 4,
Ar), 7.07 (br s, 4, Ar), 6.88 (br s, 4, Ar), 5.18 (br s, 2, Ar),
3.16 (s, 16, OCH2), 3.00 (s, 24, OMe), 1.67 (br s,
Dm1/2 = 31 Hz, 36, CMe3), 1.27 (s, 36, CMe3). 31P{1H}
NMR (C6D6, 202.31 MHz) d 28.00 (Dm1/2 = 169 Hz).
31
P{1H} NMR (DME, 80.95 MHz) d 32.69. 13C{1H}
NMR (C6D6, 125.70 MHz) d 170.06 (br s, C), 142.98 (br s,
C), 136.71 (s, C), 134.43 (s, CH), 132.71 (s, C), 129.37 (s,
C), 127.78 (s, CH), 124.67 (s, CH), 72.17 (s, OCH2), 58.95
(s, OMe), 35.90 (s, CMe3), 34.58 (s, CMe3), 32.59 (s,
CMe3), 30.73 (s, CMe3). Anal. Calc. for (C42H65K2O6P)2:
C, 65.08; H, 8.45. Found: C, 63.61; H, 8.46%.
2.7. General procedures for ring-opening polymerization of ecaprolactone outlined in Table 2
To a stirred solution of [OPO]M2(DME)2 in toluene at
room temperature was added e-caprolactone. The solution
was heated to 80 C for 12 h and cooled to room temperature. The reaction was quenched with 1 mL of hydrochloric
acid solution in methanol (1 M). The solution was evaporated to dryness under reduced pressure. The solid residue
was dissolved in toluene (1 mL) and hexane (5 mL) was
added. The precipitate thus formed was isolated and dried
in vacuo.
3. Results and discussion
at
Deprotonation of H2[OPO] [30] with 2 equiv. of n-BuLi
35 C in a number of polar or non-polar solvents such
Y.-N. Chang, L.-C. Liang / Inorganica Chimica Acta 360 (2007) 136–142
Table 2
Characterization of poly(e-caprolactone) prepared with [OPO]M2(DME)2
initiators (M = Li, Na)a
Entry
M
Equivalent
of monomer
MW (calcd)b
10
3
1
2
3
4
Li
Li
Li
Na
100
200
300
100
11 414
22 828
34 242
11 414
18.7
28.5
58.9
33.6
Mnc
10
3
29.8
43.7
91.1
51.7
Mwc
PDIc
1.59
1.53
1.55
1.54
a
Reaction conditions: 1.41 mM of [OPO]Li2(DME)2 or 1.35 mM of
[OPO]Na2(DME)2 in 10 mL toluene, 12 h, 80 C. Reaction time and
temperature were not minimized. Yields were all quantitative.
b
Calculated on the basis of the number of monomers employed.
c
Determined by GPC in THF, relative to polystyrene standards.
as dimethoxyethane (DME), THF, Et2O, toluene, or pentane produced cleanly the dilithium complex as indicated
by 31P{1H} NMR spectroscopy. Upon lithiation, the 31P
chemical shift typically changes from 50 ppm to ca.
32 ppm regardless of the solvent employed. Coordinating
solvents such as THF and Et2O seem presumably labile as
complete removal of these solvent molecules may be feasible under dynamic vacuum. As a result, no well-defined
THF- or Et2O-adduct of the dilithium complex was
obtained thus far from these reactions. Attempts to isolate
solvent-free [OPO]Li2 from reactions conducted in toluene
or pentane solutions also led to ill-defined product, the 1H
NMR spectra of which are often featureless. Nevertheless,
the lithiation of H2[OPO] in these solvents is essentially
clean as metathetical reactions of in situ prepared [OPO]Li2
with other metal halides produced quantitatively the anticipated metal complexes of [OPO]2 [32].
The only well-defined dilithium complex of [OPO]2
available from our experiments thus far involves DME
(Scheme 1). Colorless crystals of [OPO]Li2(DME)2 suitable
for X-ray diffraction analysis were obtained in 59% yield by
direct concentration of the reaction mixture followed by
cooling the DME solution to 35 C. The 1H NMR spectroscopy of [OPO]Li2(DME)2 indicates the presence of
2 equiv. of coordinated DME molecules. The tert-butyl
substituents in [OPO]2 are observed as two sharp singlet
139
resonances at 1.61 and 1.32 ppm with equal intensity in
the 1H NMR spectrum, consistent with Cs symmetry for
this molecule in solution. The phosphorus donor appears
as a broad singlet resonance at 33.80 ppm with the
peak-width at half height (Dm1/2) being 68 Hz in toluened8 at room temperature. A variable-temperature 31P{1H}
NMR study revealed a 1:1:1:1 quartet resonance centered
at 35.43 ppm with 1JPLi of 33 Hz at 73 C, indicating
internuclear coupling of 31P with a quadrupolar 7Li atom
(I = 3/2, natural abundance 92.6%). Consistently, the
room-temperature 7Li{1H} NMR spectroscopy in toluene-d8 exhibits a broad (Dm1/2 = 100 Hz) singlet resonance
at 0.80 ppm, which resolves upon cooling to 73 C to give
equal intensity of one doublet resonance at 1.50 ppm
(1JPLi = 33 Hz) and one singlet resonance at 0.80 ppm.
These results indicate that the phosphorus donor in [OPO]Li2(DME)2 is bound to only one rather than two lithium
atoms at low temperatures. As the temperature increases,
a rapid exchange occurs, which likely involves phosphine
dissociation from the coordinated lithium atom followed
by exchange of the lithium ions and subsequent association
of the phosphorus donor. As a result, the two coordinated
DME molecules are chemically equivalent at room temperature on the NMR timescale. The magnitude of the coupling constant 1JPLi of 33 Hz in [OPO]Li2(DME)2 is
similar to those found for other lithium phosphine complexes such as Li[N(o-C6H4PPh2)2](THF)2 (34 Hz) [20,29]
and [Li(THF)2][o-(2,6-Me2C6H3N)C6H4PPh2] (34 Hz) [33]
and the lithium phosphide derivative [Li(tmeda)]2[1,2-C6H4(PPh)2] (35 Hz) [34], but slightly smaller than
those of [Li(THF)2][o-(2,6-iPr2C6H3N)C6H4PPh2] (38 Hz)
[35], [(2,4,6-Me3C6H2)NLi-2-(4-MeC6H3)]2PPh(dioxane)
(40 Hz) [25], Li[N(o-C6H4PiPr2)2](THF) (46 Hz) [29], and
Li[N(o-C6H4PCy2)2](THF) (48 Hz) [29].
The X-ray structure of [OPO]Li2(DME)2 is depicted in
Fig. 2. The crystallographic data are summarized in Table
1. Consistent with the variable-temperature solution NMR
data, the phosphorus donor is coordinated to only one of
the lithium atoms. The Li–P distance of 2.573(5) Å is
comparable to those found in [(2,4,6-Me3C6H2)NLi-2(4-MeC6H3)]2PPh(THF)2 (2.501(3) Å) [25], Li[N(o-C6H4-
Scheme 1.
140
Y.-N. Chang, L.-C. Liang / Inorganica Chimica Acta 360 (2007) 136–142
Fig. 2. Molecular structure of [OPO]Li2(DME)2 with thermal ellipsoids
drawn at the 35% probability level. The methyl groups in [OPO]2 are
omitted for clarity. Selected bond distances (Å) and angles (): O(1)–Li(1)
2.031(7), O(2)–Li(1) 1.971(6), O(3)–Li(2) 2.038(6), O(4)–Li(2) 2.155(6),
O(5)–Li(1) 1.897(6), O(5)–Li(2) 1.957(6), O(6)–Li(2) 1.967(6), O(6)–Li(1)
1.985(7), P(1)–Li(2) 2.573(5), Li(1)–Li(2) 2.545(9); and O(5)–Li(1)–O(2)
121.4(3), O(5)–Li(1)–O(6) 97.5(3), O(2)–Li(1)–O(6) 112.7(3), O(5)–Li(1)–
O(1) 118.2(3), O(2)–Li(1)–O(1) 81.0(3), O(6)–Li(1)–O(1) 127.9(3), O(5)–
Li(2)–O(6) 96.2(3), O(5)–Li(2)–O(3) 117.0(3), O(6)–Li(2)–O(3) 98.9(3),
O(5)–Li(2)–O(4) 104.0(3), O(6)–Li(2)–O(4) 158.4(3), O(3)–Li(2)–O(4)
78.9(2), O(5)–Li(2)–P(1) 77.71(19), O(6)–Li(2)–P(1) 79.09(19), O(3)–
Li(2)–P(1) 165.2(3), O(4)–Li(2)–P(1) 97.6(2).
PCy2)2](OEt2) (2.540 Å average) [29], [Li(THF)2][o-(2,6Me2C6H3N)C6H4PPh2] (2.620(5) Å) [33], and Li[N(oC6H4PPh2)2](THF)2 (2.802 Å average) [20]. The two
lithium atoms are bridged by both oxygen donors of the
[OPO]2 ligand. The Li(1)–O(5)–Li(2)–O(6) ring is deviated
from the mean plane by 0.1200 Å. The coordination of an
g2 chelating solvent molecule such as DME to both lithium
atoms seems beneficial, at least to some degree, for the isolation of a well-defined [OPO]Li2 complex (vide supra), a
phenomenon that highlights the advantage of chelate effect
of DME as compared to monodentate THF or Et2O. The
coordination of bidentate DME molecules in [OPO]Li2(DME)2 is somewhat consistent with what has been
observed for 3, a sterically more demanding analogue to
[OPO]2 . For instance, although the core structure of the
dilithium complex [(2,4,6-Me3C6H2)NLi-2-(4-MeC6H3)]2PPh(THF)2 [25] resembles that of [OPO]Li2(DME)2, the
former requires only one coordinated THF molecule for
each lithium atom as it is much better shielded sterically
than that in the latter . The geometry of Li(1) is distorted
tetrahedral with the dihedral angle of the two O–Li–O
planes being 98.1, whereas that of Li(2) is perhaps best
described as distorted square pyramidal with O(5) being
the apical atom. The Li(2) atom is displaced from the mean
square plane of the square pyramid by 0.3389 Å toward
O(5). The remaining parameters are unexceptional.
The reactions of H2[OPO] with an excess amount of
NaH or KH in DME afforded high yields of colorless crystalline [OPO]Na2(DME)2 and {[OPO]K2(DME)2}2, respec-
tively. Attempts to grow X-ray quality crystals of
[OPO]Na2(DME)2 were not successful; however, the solution NMR spectroscopic data of this molecule at room
temperature are all similar to what has been observed for
[OPO]Li2(DME)2, suggesting that the solution structures
of these compounds are presumably similar. The phosphorus donor in [OPO]Na2(DME)2 is observed as a broad singlet resonance at 39.36 ppm (Dm1/2 = 51 Hz). Reminiscent
to that of [OPO]Li2(DME)2, the 1H NMR spectroscopy of
[OPO]Na2(DME)2 exhibits two sharp singlet resonances
with equal intensity at 1.63 ppm and 1.35 ppm for the
tert-butyl substituents in the phenolato phosphine ligand.
The aromatic proton chemical shifts for [OPO]Li2(DME)2
and [OPO]Na2(DME)2 are all within the expect values. In
contrast, the tert-butyl hydrogen atoms in {[OPO]K2(DME)2}2 at room temperature appear as one broad
(Dm1/2 = 31 Hz) and one sharp singlet resonances with
equal intensity at 1.67 and 1.27 ppm, respectively, which
sharpen upon heating to temperatures higher than 80 C
but broaden even further upon cooling to 0 C and below
to give unresolved signals, indicating that a fluxional process occurs in solution. Surprisingly, the aromatic hydrogen atoms in {[OPO]K2(DME)2}2 are observed in the
region from ca. 5 to 7.5 ppm. The upfield chemical shift
of ca. 5 ppm is unusual. The reason for this observation
is not clear at this stage. We tentatively ascribe this phenomenon to either the shielding effect of ring current from
one of the aromatic rings in [OPO]2 or the p coordination,
possibly in an g2 fashion, of the non-substituted phenyl
ring to one of the potassium atoms. Though indeterminate
due to the facile fluxionality of this molecule in solution,
the solution structure of {[OPO]K2(DME)2}2 is likely
different from those of [OPO]Li2(DME)2 and [OPO]Na2(DME)2. The phosphorus donor in {[OPO]K2(DME)2}2 exhibits a broad signal at 28.00 ppm (Dm1/2
= 169 Hz) in the 31P{1H} NMR spectroscopy.
Colorless crystals of {[OPO]K2(DME)2}2 suitable for Xray diffraction analysis were grown from a concentrated
DME solution at 35 C. As depicted in Fig. 3, this compound is a dimeric, S2-symmetric species in the solid state
with four potassium atoms being bridged by both phosphorus and oxygen donors of the biphenolato phosphine
ligands. The observation of dimeric {[OPO]K2(DME)2}2
and monomeric [OPO]Li2(DME)2 is consistent with the
ionic sizes of these alkali metals. Other than being bound
to [OPO]2 , each potassium atom in {[OPO]K2(DME)2}2
is further coordinated to an g2-DME molecule. The inversion center is located at the central point of the P(1)–K(1)–
P(1A)–K(1A) square plane. The two O(1)–K(1)–O(2)–K(2)
rings are deviated from the corresponding mean plane by
0.1695 Å. Unlike those of [OPO]Li2(DME)2, the two phenolato oxygen atoms in the corresponding four-membered
M2O2 rings in {[OPO]K2(DME)2}2 are not connected from
the same [OPO]2 ligand backbone. Three close contacts of
K(1)–C(2) (3.126(3) Å), K(1)–C(22) (3.210(3) Å), and
K(2)–C(15) (3.198(3) Å) are notably within the sum of
the potassium and carbon van der Waals radii of 4.45 Å.
Y.-N. Chang, L.-C. Liang / Inorganica Chimica Acta 360 (2007) 136–142
141
Fig. 3. Molecular structure of {[OPO]K2(DME)2}2 with thermal ellipsoids drawn at the 35% probability level. The methyl groups in coordinated DME
and [OPO]2 in the dimeric structure (left) are omitted for clarity. The core structure (middle) highlights the heteroatoms only. A ChemDraw
representation (right) shows the same view as the ORTEP. Selected bond distances (Å) and angles (): K(1)–O(2) 2.673(2), K(1)–O(1) 2.6939(19), K(1)–
O(3) 2.743(2), K(1)–O(4) 2.837(3), K(1)–C(2) 3.126(3), K(1)–C(22) 3.210(3), K(1)–P(1A) 3.3317(9), K(1)–P(1) 3.3853(9), K(2)–O(1) 2.5516(18), K(2)–O(2)
2.6408(19), K(2)–O(6) 2.725(2), K(2)–O(5) 2.898(3), K(2)–C(15) 3.198(3), K(2)–P(1A) 3.5868(9); and O(2)–K(1)–O(1) 88.29(6), O(2)–K(1)–O(3) 107.41(7),
O(1)–K(1)–O(3) 115.52(7), O(2)–K(1)–O(4) 157.62(7), O(1)–K(1)–O(4) 113.70(6), O(3)–K(1)–O(4) 60.43(8), O(2)–K(1)–C(2) 111.83(6), O(1)–K(1)–C(2)
24.47(6), O(3)–K(1)–C(2) 111.85(7), O(4)–K(1)–C(2) 90.51(7), O(2)–K(1)–C(22) 23.41(6), O(1)–K(1)–C(22) 106.57(6), O(3)–K(1)–C(22) 86.15(7), O(4)–
K(1)–C(22) 135.61(7), C(2)–K(1)–C(22) 131.03(7), O(2)–K(1)–P(1A) 94.85(4), O(1)–K(1)–P(1A) 56.07(4), O(3)–K(1)–P(1A) 156.38(6), O(4)–K(1)–P(1A)
100.86(6), C(2)–K(1)–P(1A) 50.26(5), C(22)–K(1)–P(1A) 117.06(5), O(2)–K(1)–P(1) 58.74(4), O(1)–K(1)–P(1) 140.41(5), O(3)–K(1)–P(1) 96.31(5), O(4)–
K(1)–P(1) 102.01(6), C(2)–K(1)–P(1) 151.76(5), C(22)–K(1)–P(1) 50.21(5), P(1A)–K(1)–P(1) 102.08(2), O(1)–K(2)–O(2) 92.07(6), O(1)–K(2)–O(6)
148.18(7), O(2)–K(2)–O(6) 119.70(7), O(1)–K(2)–O(5) 101.41(7), O(2)–K(2)–O(5) 126.99(7), O(6)–K(2)–O(5) 59.36(7), O(1)–K(2)–C(15) 68.91(6), O(2)–
K(2)–C(15) 117.29(7), O(6)–K(2)–C(15) 95.34(8), O(5)–K(2)–C(15) 115.40(7), O(1)–K(2)–P(1A) 52.91(4), O(2)–K(2)–P(1A) 89.73(4), O(6)–K(2)–P(1A)
123.22(6), O(5)–K(2)–P(1A) 138.39(6), C(15)–K(2)–P(1A) 30.84(5).
As a result, the coordination number for K(1) is eight and
that of K(2) is six. The K–C distances found in
{[OPO]K2(DME)2}2 are comparable to those of [K(g5C5H5)]x (ranging from 2.955(5) to 3.140(6) Å) [36], K(g5C5H5)(18-crown-6) (ranging from 2.969(6) to 3.131(7) Å)
[37], [K4(p-tert-butylcalix[4]arene-4H)(THF)5]2 (ranging
from 3.005(4) to 3.180(4) Å) [38], and [(THF)3K][(lOC6H3Me2-2,6)2Y(OC6H3Me2-2,6)2(THF)2] (ranging from
3.288(9) to 3.400(10) Å) [39]. It is worth noting that crystallographically characterized potassium complexes of
tri(hydrocarbyl)phosphine are extremely rare [40–42].
Complex {[OPO]K2(DME)2}2 represents the fourth example reported to date. In contrast to the coordination polymers found for the preceding potassium phosphine
complexes, {[OPO]K2(DME)2}2 is the first discrete molecule of this type. The K–P distances of 3.3853(9) Å,
3.5868(9) Å, and 3.3317(9) Å are comparable to those
found in the potassium phosphide complexes such as
[KPPh2]x (ranging from 3.2062(13) to 3.4949(16) Å) [43]
and {K(PPh2)[NMe(CH2CH2NMe2)2]}x (ranging from
3.3025(14) to 3.4208(15) Å) [44], and the potassium phosphine derivatives such as [(THF)K(C5H4CH2CH2PPh2)]x
(3.320(2) Å) [40] and {K[(Me3Si)2CP(C6H4-o-CH2OMe)2]}x (3.6836(15) Å) [42].
Several phenolato complexes are known as efficient initiators for catalytic ring-opening polymerization of heterocyclic molecules [45,46]. The catalytic activity of
[OPO]M2(DME)2 with respect to ring-opening polyermization of e-caprolactone was investigated. The preliminary
polymerization results and the characterization of poly(ecaprolactone) by gel permeation chromatography are summarized in Table 2. Regardless of the initiators and the
number of the monomers employed, the isolated yields of
poly(e-caprolactone) were all quantitative and the molecular weight distributions appeared to be consistently ca. 1.5
under the conditions examined. The molecular weight of
poly(e-caprolactone) prepared with [OPO]Na2(DME)2 is
higher than that with [OPO]Li2(DME)2 (entry 4 versus 1).
In summary, we have prepared a series of group 1 metal
complexes of [OPO]2 and established the solution and
solid-state structures of these molecules by multi-nuclear
NMR spectroscopy and X-ray crystallography. The
X-ray studies revealed a monomeric structure for [OPO]Li2(DME)2 but a dimeric {[OPO]K2(DME)2}2. These compounds represent thus far the first examples of well-defined
alkali metal complexes supported by chelating phenolato
phosphine ligands. Of particular note is the structurally
characterized {[OPO]K2(DME)2}2, which represents the
first discrete molecule of rarely encountered tri(hydrocarbyl)phosphine complexes of potassium. These compounds
are effective initiators for catalytic ring-opening polymerization of e-caprolactone. Studies involving the preparation
of transition metal complexes of [OPO]2 and the reaction
chemistry thereafter are currently under way.
142
Y.-N. Chang, L.-C. Liang / Inorganica Chimica Acta 360 (2007) 136–142
Acknowledgements
We thank the National Science Council of Taiwan for
financial support (NSC 94-2113-M-110-004) of this work
and Mr. Ting-Shen Kuo (National Taiwan Normal University) for crystallographic assistance. Acknowledgment
is also made to the National Center for High-performance
Computing (NCHC) for the access to chemical databases.
Appendix A. Supplementary data
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic Data
Centre, CCDC nos. 610580–610581. Copies of this information may be obtained free of charge from the Director,
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK, fax:
+44 1223 336 033, e-mail: [email protected]
www: http://www.ccdc.cam.ac.uk. Supplementary data
associated with this article can be found, in the online version, at doi:10.1016/j.ica.2006.07.050.
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