Amido rare-earth complexes supported by an ansa bis(amidinate) ligand with a rigid 1,8-naphthalene linker: Synthesis, structures and catalytic activity in rac-lactide polymerization and hydrophosphonylation of carbonyl compounds

Article abstract of DOI:10.1039/c4nj01698j

A synthetic approach to rare-earth amido complexes coordinated by an ansa bis(amidinate) ligand with a 1,8-naphthalene linker was developed and allowed for the synthesis of a series of complexes [1,8-C₁₀H₆{NC(tBu)N-2,6-Me₂

Full text of DOI:10.1039/c4nj01698j

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Amido rare-earth complexes supported by an ansa
bis(amidinate) ligand with a rigid 1,8-naphthalene
linker: synthesis, structures and catalytic activity in
rac-lactide polymerization and hydrophosphonylation
of carbonyl compounds†
Cite this:DOI: 10.1039/c4nj01698j
Marina V. Yakovenko,a Natalia Yu. Udilova,a Tatyana A. Glukhova,a
Anton V. Cherkasov,a Georgy K. Fukinab and Alexander A. Trifonov*abc
A synthetic approach to rare-earth amido complexes coordinated by an ansa bis(amidinate) ligand
with a 1,8-naphthalene linker was developed and allowed for the synthesis of a series of complexes
[1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]LnN(SiMe3)2(THF)n (Ln = Y, n = 0 (2); Ln = Sm, n = 1 (3); Ln = Nd,
n = 1 (4)) in reasonable yields. Complexes 2–4 initiate ring-opening polymerization (ROP) of rac-lactide
and enable complete conversion of 100–250 equiv. of monomer within 60–90 min at 25 1C. The
obtained polylactides feature atactic structures and moderate molecular-weight distributions (Mw/Mn
=
1.30–2.12). The experimental Mn values of the obtained polymers are found to be significantly higher
than the calculated ones due to a slow initiation stage. Effective immortal ROP of lactide with 3–5 equiv.
of isopropanol per metal center was performed using complexes 2–4 as the catalysts. The systems
2–4/iPrOH exhibit higher activities in ROP and allow for complete conversion of 100–300 equiv. of
rac-lactide to polymer within 30–60 min at 25 1C and provide a living polymerization mode and very narrow
polydispersities (Mw/Mn = 1.13–1.27). Complexes 2–4 as well as related borohydrides [1,8-C10H6{NC(tBu)N-
2,6-Me2C6H3}2]Ln(BH4)(m-BH4)Li(THF)2 (Ln = Sm, Nd) catalyze hydrophosphonylation of aldehydes at room
temperature with good reaction rates and hydrophosphonylation of benzylideneacetone at 65 1C.
Received (in Victoria, Australia)
30th September 2014,
Accepted 11th November 2014
DOI: 10.1039/c4nj01698j
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and isolation of a series of highly reactive species such as mono-,
bis(alkyl), cationic alkyl and hydrido complexes.1–3,9–16 Alkyl and
Introduction
The design and development of new ligand systems enabling hydrido rare-earth complexes coordinated by amidinate ligands
stabilization of monomeric metal complexes while provoking demonstrated a high catalytic activity in olefin polymerization,9–11
novel reactivity and providing control of metal mediated reactions isoprene polymerization,17,18 acetylene dimerization,10 olefin
remain one of the most intensely explored areas of organorare- hydroboration,19 hydrosilylation,20 and hydroamination.21 The
earth chemistry.1–3 This issue acquires particular significance in transition from bis(cyclopentadienyl) to a linked bis(cyclo-
the case of rare-earth metals possessing large ion sizes4 and having pentadienyl) ligation system22 led to the enhancement of the
a tendency for ligand redistribution reactions originating from catalytic activity of the alkyl and hydrido rare-earth species23–26
their electropositivity5 and predominantly ionic nature of metal– due to opening of the metal coordination sphere and subse-
ligand bonding. Monoanionic amidinate ligands proved to be a quent increase of its accessibility. Recently we developed and
suitable coordination platform6–8 which allowed for the synthesis successfully applied linked bis(amidinate) ligand systems with
rigid o-phenylene27,28 and 1,8-naphthalene linkers.29,30 At about the
same time rare-earth complexes coordinated by ansa bis(amidinate)
ligands with flexible linkers were reported.31–37 Comparison of
the reactivities and catalytic activities of the complexes sup-
ported by the linked bis(amidinate) systems with a similar
bulkiness of the substituents by the amidinate nitrogens and a
different length, flexibility, nature and structure of the linkers
creates a basis for the comparative analysis of the structure–
reactivity relationship. Moreover, estimation of the applicability
a G. A. Razuvaev Institute of Organometallic Chemistry of Russian Academy of Sciences,
Tropinina str. 49, 603950 Nizhny Novgorod, GSP-445, Russia.
E-mail: trif@iomc.ras.ru; Fax: +007-831-4627497
b Nizhny Novgorod State University, Gagarina av. 23, 603950, Nizhny Novgorod, Russia
c Institute of Organoelement Compounds of Russian Academy of Scienes,
Vavilova str. 28, 119334, Moscow, Russia
† Electronic supplementary information (ESI) available. CCDC 1022273 and
1022274. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c4nj01698j
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of a [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2] ligand system for the
synthesis of various classes of rare-earths presents the possibility
of catalytic use and was therefore the focus of our research. Herein
we report on the synthesis and structures of rare-earth amido
complexes coordinated by a [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]2ꢀ
ligand and their catalytic activity in ring-opening polymerization
(ROP) and hydrophosphonylation of carbonyl compounds.
Results and discussion
Synthesis and characterization of compounds 1–4
Lanthanide amides proved to be efficient initiators for the ring-
opening polymerization of cyclic esters,33,38–44 components of
catalyst systems for isoprene polymerization,38–44 olefin hydro-
amination,43,45–50 hydrophosphination,47 hydrophosphonylation of
carbonyl compounds,51–55 and aldimines.51,56 Initially, we intended
to prepare bis(amidinate) amido lanthanide complexes by the salt
metathesis reactions of [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]-
YCl(DME), [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]Nd(DME)(m-Cl)2-
Fig. 1 Molecular structure of 1, showing the atom numbering scheme.
Thermal ellipsoids drawn at the 30% probability level. Hydrogen atoms, tBu
and (Me)2C6H3 (Ar) groups of the amidinato ligand and the DME molecule
Li(DME), and [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]Sm(THF)(m-Cl)2-
Li(THF)2 with equimolar amounts of Li(Et2O)[N(SiMe3)2]57 are omitted for clarity.
29
and Na[N(SiMe3)2]58 (THF, 60 1C, 24 h). However this synthetic
approach turned out to be inefficient as the starting chloro
complexes were recovered from the reaction mixtures in nearly All our attempts to isolate the samarium containing product
quantitative yields (88–91%). Thereby for the synthesis of the prepared following the same scheme were unsuccessful. Complex 1
bis(amidinate) amido lanthanide complexes another synthetic was obtained as a yellow moisture- and air-sensitive crystalline solid
method was elaborated. Equimolar amounts of NdCl3 and soluble in THF, Et2O, toluene and insoluble in hexane. Complex 1
Li(Et2O)[N(SiMe3)2] were stirred in THF at 25 1C for 24 h. was characterized by X-ray diffraction analysis. Transparent yellow
Subsequently a solution of [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]- crystals of 1 suitable for X-ray single crystal structure determination
Na2(THF)n in situ prepared from 1,8-C10H6[NHC(tBu)QN(2,6- were obtained by slow concentration of the solution in Et2O at
Me2C6H3)][NQC(tBu)NH(2,6-Me2C6H3)] and NaN(SiMe3)2 (1: 2, ambient temperature. Complex 1 was isolated as a solvate 1ꢁ(Et2O).
toluene, 25 1C) was added and the reaction mixture was stirred at The molecular structure of 1 is depicted in Fig. 1, the relevant bond
60 1C for 24 h. Evaporation of the volatiles under vacuum, lengths and angles are compiled in Table 1, and the structure
extraction of the solid residue with toluene, treatment with DME refinement data are listed in Table 2. The X-ray study revealed
and recrystallization from Et2O afforded the bimetallic complex that 1 is an ate-complex containing both Nd3+ and Li+ ions and two
[1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]Nd[m-1,8-C10H6{NC(tBu)N-2,6- bis(amidinate) ligands. One bis(amidinate) ligand in 1 is terminal
Me2C6H3}2][Li(DME)] (1) in 43% yield (Scheme 1). Ate-complex 1 and chelates the neodymium center by both amidinate fragments
obviously results from the ligand redistribution reaction. Our which are located in the cis position with respect to the naphthalene
attempts to isolate other neodymium containing products failed. linker (the dihedral angle between the NNdN planes is 1091). In the
Scheme 1 Synthesis of complex [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]Nd[m-1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2][Li(DME)] (1).
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The distance from Nd3+ and the nitrogen atom of the bridging
amidinate ligand falls in the same interval (2.582(2) Å). The
average Nd–N bond length (2.55 Å) in 1 is slightly longer than
those in the related neodymium complexes coordinated by ansa
bis(amidinate) ligands.28–30 The NCN bond angle of the m-bridging
amidinate fragment (120.8(2)1) in 1 is essentially larger than
those of the terminal fragments (111.7(2)–112.3(2)1). The bond-
ing situation within the terminal and bridging NCN fragments of
complex 1 is similar and indicates negative charge delocalization
(1.327(2)–1.356(2) Å). The lithium atom in 1 is bound to one
nitrogen atom (Li(1)–N(4) 1.983(6) Å) and also accommodates a
DME molecule (Li(1)–O(1) 1.996(4) Å, Li(1)–O(2) 1.997(5) Å). In
addition, the Li+ ion exhibits short interactions with the p-system
of the adjacent aromatic ring of the naphthalene moiety (Li–C
2.510(5)–2.712(5) Å, Li–Arcentre 2.39 Å). The average Li–C distance
in 1 (2.59 Å) is much longer compared to that in complex
[Li(TMEDA)]2[C10H8] (2.370(2) Å).59 The similar coordination
mode was formerly described for Na+ in the samarium amidinate
ate-complex.60
Table 1 Selected bond lengths and angles for compound 1
Bond lengths (Å)
Bond angles (1)
Nd(1)–N(1)
Nd(1)–N(2)
Nd(1)–N(3)
Nd(1)–N(4)
Nd(1)–N(5)
Nd(1)–N(6)
Nd(1)–N(7)
Li(1)–N(8)
Li(1)–O(1)
Li(1)–O(2)
Li(1)–C(40)
Li(1)–C(39)
Li(1)–C(41)
N(1)–C(11)
N(3)–C(11)
N(5)–C(47)
N(7)–C(47)
N(4)–C(24)
N(2)–C(24)
N(6)–C(60)
N(8)–C(60)
2.473(2)
2.497(2)
2.626(2)
2.556(2)
2.460(2)
2.582(2)
2.632(2)
2.002(4)
1.996(4)
1.997(5)
2.510(5)
2.555(4)
2.712(5)
1.327(2)
1.342(2)
1.332(2)
1.344(3)
1.356(2)
1.320(3)
1.348(3)
1.335(3)
N(1)–C(11)–N(3)
N(2)–C(24)–N(4)
N(5)–C(47)–N(7)
N(8)–C(60)–N(6)
111.7(2)
112.3(2)
112.1(2)
120.8(2)
Sodium is less prone to the formation of ate-complexes.61
In order to suppress the ligand redistribution reaction and the
other ligand, just one amidinate fragment is chelating the Nd3+ while ate-complex formation we decided to avoid the presence of Li+
the second one is m-bridging: its different nitrogen atoms coordinate in the reaction mixture and to use NaN(SiMe3)2 instead of
different metal ions (Nd3+ or Li+). Unlike the previously published Li(Et2O)[N(SiMe3)2]. The proposed synthetic approach turned
related chloro complexes29 featuring rather symmetric coordination out fruitful and enabled us to synthesize of a series of new
of the amidinate ligands in 1 the Nd–N bond lengths for the bis(amidinate) amido lanthanide complexes. Equimolar amounts of
chelating amidinate fragments are different. The two amidinato LnCl3 and NaN(SiMe3)2 were reacted in THF at 25 1C for 24 h.
1,8-C10H6N nitrogens are situated noticeably closer to the metal Subsequently a solution of [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]-
center (2.473(2), 2.497(2), 2.460(2) Å), than the two nitrogen Na2(THF)n in situ prepared from 1,8-C10H6[NHC(tBu) = N(2,6-
atoms of the 2,6-Me2C6H3N groups (2.556(2), 2.626(2), 2.632(2) Å). Me2C6H3)][N
=
C(tBu)NH(2,6-Me2C6H3)] and NaN(SiMe3)2
Table 2 Crystallographic data and structure refinement details for 1 and 4
Complex
1
4
Empirical formula
Formula weight
T, K
C
80H104LiN8NdO3
C46H68N5NdOSi2
907.47
100(2)
1376.89
150(2)
Wavelength, Å
Crystal system
Space group
a [Å]
0.71073
Triclinic
P1
12.6452(6)
15.0817(7)
0.71073
Monoclinic
P2(1)/c
23.950(2)
%
b [Å]
19.426(1)
c [Å]
a [1]
20.7038(9)
101.662(1)
21.871(1)
90
b [1]
97.130(1)
106.030(1)
3648.0(3)
2
1.253
0.764
1454
116.878(1)
90
9076(1)
8
1.328
1.236
3800
g [1]
Volume, Å3
Z
r
calcd, g cmꢀ3
Absorption coefficient, mmꢀ1
F(000)
Crystal size, mm
y range for data collection, 1
Index ranges
Reflections collected
Independent reflections
Rint
0.26 ꢂ 0.24 ꢂ 0.13
0.35 ꢂ 0.26 ꢂ 0.23
1.97 to 27.00
0.95 to 25.00
ꢀ16 r h r 15, ꢀ13 r k r 19, ꢀ26 r l r 25
ꢀ28 r h r 28, ꢀ23 r k r 23, ꢀ26 r l r 26
23814
15794
0.0201
69897
15735
0.0415
Completeness to 2y, %
Goodness-of-fit on F2
Final R indices (I >2s(I))
R indices (all data)
Largest diff. peak and hole, e Åꢀ3
99.1
1.057
98.4
1.154
R1 = 0.0405, wR2 = 0.1023
R1 = 0.0507, wR2 = 0.1071
2.097/ꢀ0.597
R1 = 0.0770, wR2 = 0.1598
R1 = 0.0858, wR2 = 0.1628
2.409/ꢀ2.191
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Scheme 2 Synthesis of complexes [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]YN(SiMe3)2 (2), [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]SmN(SiMe3)2(THF) (3)
and [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]NdN(SiMe3)2(THF) (4).
(1 : 2, toluene, 25 1C) was added and the reaction mixture was
stirred at ambient temperature overnight. Evaporation of THF,
extraction of the solid residue with toluene, separation of NaCl
by filtration followed by recrystallization of the reaction pro-
duct from Et2O afforded bis(amidinate) amido lanthanide
complexes [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]YN(SiMe3)2 (2),
[1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]SmN(SiMe3)2(THF) (3) and
[1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]NdN(SiMe3)2(THF) (4) in
reasonable yields (2: 43%, 3: 52%, 4: 54%) (Scheme 2). Complexes
2–4 were obtained as pale-yellow crystalline moisture- and air-
sensitive solids. They are soluble in THF, Et2O, toluene and slightly
soluble in hexane. All complexes were fully characterized by
spectroscopic methods and elemental analyses.
The 1H and 13C{1H}NMR spectra of the diamagnetic yttrium
Fig. 2 Molecular structure of 4. Thermal ellipsoids drawn at the 30%
probability level. Hydrogen atoms are omitted for clarity.
derivative 2 in C6D6 at 20 1C show the expected set of signals
corresponding to the bis(amidinate) and amido ligands. A
singlet at ꢀ0.07 ppm corresponds to the methyl protons of
the N(SiMe3)2 group. The protons of the tBu groups give rise to
at ambient temperature. The X-ray studies revealed that 4
a singlet at 1.19 ppm and the protons of the Me groups appear
as a singlet at 2.49 ppm. The aromatic protons give a set of
crystallizes in the space group P2(1)/c. The crystals of 4 contain
two crystallographically independent molecules featuring very
similar geometrical parameters. Thus, only the structure of one
molecule will be discussed. The molecular structure of 4 is
shown in Fig. 2; the relevant bond lengths and angles are
reported in Table 3, and the structure refinement data are
listed in Table 2. The coordination sphere of the neodymium
atom in 4 is set up by four nitrogen atoms from two amidinate
fragments, one nitrogen atom from the terminal amido ligand
signals in the interval 6.78–7.43 ppm. Unlike 2, Sm(III) complex
3 has a larger ion size4 and according to the 1H and
13C{1H}NMR spectra the bis(amidinate) ligand and amido
1
group contain one coordinated THF molecule. In the H NMR
spectrum of the slightly paramagnetic Sm(III) complex 3 (meff
=
1.64 mB)62 the protons of the N(SiMe3)2 group give a broadened
singlet at 0.16 ppm. Singlets at 1.16 and 3.11 ppm correspond to the
protons of the tBu and Me groups respectively. Two broad singlets at
d = 1.53 and 3.32 ppm correspond to the b-CH2 and a-CH2 protons
of the THF molecule. The spectrum also exhibits a multiplet at 6.28–
7.72 ppm attributed to the aromatic protons. The IR spectra of
complexes 2–4 display characteristic bands corresponding to the
stretching vibrations of delocalized double C–N bonds of the
Table 3 Selected bond lengths and angles for compound 4
Bond lengths (Å)
Nd(1A)–N(1A)
Bond angles (1)
2.406(5)
2.413(5)
2.524(4)
2.568(5)
2.329(4)
2.503(4)
1.343(7)
1.331(8)
1.358(8)
1.318(7)
N(3A)–C(11A)–N(1A)
N(4A)–C(24A)–N(2A)
110.2(4)
112.4(4)
bis(amidinate) ligand (2: 1670, 1638; 3: 1670, 1637; 4: 1637 cmꢀ1). Nd(1A)–N(2A)
The amido group N(SiMe3)2 shows the absorption bands related
Nd(1A)–N(3A)
Nd(1A)–N(4A)
to the deformation vibrations of the C–H bonds of the methyl
Nd(1A)–N(5A)
groups (2 1246; 3 1250; 4 1247 cmꢀ1) as well as the stretching Nd(1A)–O(1A)
vibrations of Si–N bonds (2: 933; 3: 951; 4, 962 cmꢀ1).
N(1A)–C(11A)
N(3A)–C(11A)
N(2A)–C(24A)
Clear pale-yellow crystals of 4 suitable for X-ray diffraction
studies were obtained by slow concentration of Et2O solutions N(4A)–C(24A)
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Table 4 Polymerization of rac-lactide initiated by complexes 2–4a
t b (min)
Conversionc (%)
Mncalcd ꢂ 103e
Mnexp ꢂ 103c
Mw/Mn
Pr
d
Entry
Complex
ROH
[I]0/[iPrOH]0/[Mon]0
1
2
3
4
5
6
7
8
2
—
—
iPrOH
—
—
—
—
—
—
—
—
iPrOH
iPrOH
iPrOH
iPrOH
—
—
—
iPrOH
1 : 100
1 : 100
1 : 5 : 100
1 : 100
1 : 250
1 : 250
1 : 250
1 : 250
1 : 250
1 : 250
1 : 250
1 : 5 : 250
1 : 3 : 300
1 : 3 : 300
1 : 3 : 300
1 : 100
1 : 250
1 : 250
1 : 5 : 250
60
60
30
60
15
30
45
60
75
90
60
30
10
40
60
60
60
60
30
>99
65
>99
97
38
66
90
93
97
>99
71
>99
18
70
87
93
89
64
>99
14.25
9.36
2.85
25.33
79.78
3.23
1.37
1.83
1.26
2.12
1.30
1.35
1.53
1.57
1.41
1.53
1.97
1.23
1.15
1.13
1.26
1.30
1.41
1.49
1.27
0.55
0.55
0.52
0.53
0.54
0.55
0.54
0.54
0.54
0.53
0.56
0.53
0.53
0.54
0.53
0.55
0.53
0.53
0.53
2 f
2
3
3
3
3
3
3
13.97
13.68
23.76
32.40
33.48
34.92
35.64
2.56
24.91
21.80
43.18
51.46
57.14
57.14
59.69
67.83
5.06
2.59
9.52
12.47
13.39
16.83
42.35
3.16
9
10
11
12
13
14
15
16
17
18
19
3
3 f
3
7.13
2.59
3
3
3
4
10.08
12.57
13.39
32.04
23.04
7.13
4
4f
4
a
b
c
General conditions: toluene, [lactide] = 1.0 mol Lꢀ1, T = 25 1C. Reaction time was not necessarily optimized. Conversion of monomer, as
determined by 1H NMR spectroscopy. Experimental (corrected; see Experimental section) Mn and Mw/Mn values determined by GPC in THF vs.
d
e
polystyrene standards. Mn value calculated by assuming one polymer chain per metal center with the relationship: 144 ꢂ conversion ꢂ [Mon]/
f
[Ln]. Catalytic tests carried out in THF.
and one oxygen atom from the THF molecule. Unlike the related 3 metals and lanthanide derivatives,66,67–71 in particular amido,
chloro complex29 the amidinate fragments in 4 adopt a cis alkoxo and borohydrido complexes supported by N-containing
position with respect to the naphthalene linker (the dihedral ligands have been shown to be efficient.33,35,67–71
angle between the NNdN planes is 69.51) and the naphthalene
Amido compounds 2–4 were evaluated as initiators for the
linker is slightly distorted (maximum deviation of C atoms from ROP of racemic lactide. The polymerization tests were conducted
the plane C(1)–C(10) is 0.1 Å). The bonding situation in 4 similar either in THF or in toluene at room temperature at a concentration
to that previously described for the related lanthanide complexes of 1 mol Lꢀ1 rac-lactide. The results are compiled in Table 4.
coordinated by ansa bis(amidinate) ligands with o-phenylene28 Complexes 2–4 allowed 89–99% conversion of 100–250 equiv. of
and (–CH2–)3 32 linkers: the two amidinato 1,8-C10H6N nitrogens rac-lactide to polymer within 60–90 min at 25 1C in toluene. In THF
are situated noticeably closer to the metal center (2.406(5), these complexes showed lower activity than in toluene (Table 4,
2.413(5) Å), than the two nitrogen atoms of the 2,6-Me2C6H3N entries 2, 11, 18 and 1, 10, 17), presumably because of competitive
groups (2.524(4), 2.568(5) Å) and the average N–C bond lengths coordination to the catalytic centers between the monomer and
(1.34 Å, N–C bond lengths lie in the range 1.318(7)–1.358(8) Å) THF. A kinetic monitoring of the ROP reaction mediated by 3 in
are comparable to those reported for related bis(amidinate) toluene indicated a first-order dependence on lactide concentration,
complexes of neodymium.32 The bonding situation within the with an apparent first order rate constant kapp = 0.0478 minꢀ1
1
amidinate groups of complex 4 indicates delocalization of the (Fig. 3). The H NMR spectra of the polylactides formed showed
negative charge within the NCN fragments. The Nd–N(SiMe3)2 atactic microstructures. The Pr (probability of racemic linkage) values
bond length (2.239(4) Å) in 4 is considerably shorter compared to the
corresponding distances in the amido complexes L2NdN(SiMe3)2
(L = 3,5-tBu2-2-(O)-C6H2CHQN-2,6-iPr2C6H3, 2.340(4) Å; L = 3,5-tBu2-
2-(O)-C6H2CHQN-2,6-Me2C6H3, 2.433(5) Å).63,64
Catalytic activity of bis(amidinate) amido complexes in
ring-opening polymerization (ROP) of racemic lactide
Ring-opening polymerization (ROP) of cyclic esters, promoted
by single-site metal-based initiators/catalysts that operate via
coordination/insertion pathways, is definitely the most useful
route for preparing polyesters with good control over polymer
molecular weight, polydispersity, and microstructure.65 The
development over the past two decades of various types of
metal catalysts for the ROP of cyclic esters has demonstrated
Fig. 3 Plot of ln([Mon]0/[Mon]) as a function of time for the ROP of
the crucial influence that both the ancillary ligand and the
rac-lactide mediated by 3 (linear fit R2 = 0.9764) and 3/iPrOH (linear fit
initiating group exert on the polymerization performance.66
Among the large variety of investigated metal complexes, group
R
2 = 0.9876) at 25 1C in toluene, [Mon]0 = 1.0 mol Lꢀ1, [I]0/[Mon]0 = 1 : 250,
[I]0/[iPrOH]0/[Mon]0 = 1 : 3 : 300.
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Fig. 5 Mnexp, Mcalcd and Mw/Mn vs. conversion for ROP initiated by 3/
iPrOH. Toluene, 25 1C, [M]0 = 1.0 mol Lꢀ1, [I]0/[iPrOH]0/[Mon]0 = 1 : 3 : 300.
Fig. 4 Mn vs. conversion for ROP initiated by 3. Toluene, 25 1C, [Mon]0
1.0 mol Lꢀ1, [I]0/[Mon]0 = 1 : 250.
=
were observed under these conditions, as compared with those
obtained without the addition of alcohol, and the good match
between calculated and experimental molecular weights argue
for fast and reversible chain transfer. The Mnexp increased
linearly with the conversion of the monomer recorded and
the polydispersities kept in the range of 1.13–1.27 (Fig. 5),
indicating that it occurs in a living fashion. The catalytic
systems 2–4/iPrOH are not stereoselective since atactic PLAs
were recovered (Pr = 0.52–0.54).
Thus the systems 2–4/iPrOH provide much better control
over the ROP of rac-lactide compared to amido 2–4 and
borohydrido complexes [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]
Ln(BH4)2Li(THF)2 (Ln = Sm, Nd)13 and allow the polymerization
process to be carried out in the living mode producing polylactides
with defined molecular weight and a very narrow molecular weight
distribution.
are in the range of 0.53–0.56. The polylactides produced with
complexes 2–4 showed monomodal GPC traces with moderate
molecular-weight distributions in the range Mw/Mn = 1.30–2.12.
The experimental Mn values in most cases were found to be
significantly higher than the calculated ones (Fig. 4). It should
be noted that this general trend has often been observed for the
ROP of lactide initiated by rare-earth amido complexes. It is
associated with the lower nucleophilicity of the amido group
N(SiMe3)2, acting on the initiation stage as compared to the
nucleophilic alkoxide group –OR (R – the growing polymer
chain) resulting from opening of the lactide and the insertion
of the resulting fragments into the M–N(SiMe3)2 bond.72–76 It is
worthy to note that the previously obtained heterobimetallic
lanthanide borohydrides [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]-
Ln(BH4)2Li(THF)2 (Ln = Sm (5) Nd (6)) supported by the same
ligand system which acts as a single-site diinitiator, featured
lower activity both in toluene and in THF, enabling quantitative
conversions of 100–120 equiv. of rac-lactide at room tempera-
Hydrophosphonylation of carbonyl compounds catalyzed by
lanthanide amido and borohydrido complexes containing a
bis(amidinate) ligand with a rigid naphthalene linker
ture within a few hours. These initiators produced polylactides The catalytic hydrophosphonylation of aldehydes, ketones and
in a rather controlled fashion in terms of molecular weights imines is a straightforward and atom-economic method for
and relatively narrow unimodal molecular weight distributions the formation of P–C bonds, which allows for the synthesis of
(Mw/Mn = 1.4–1.7), but with only minor bias toward heterotactic a-amino and a-hydroxy phosphonic acids possessing important
sequences.30
biological activities.77–85
The possibility to achieve immortal polymerization with
The amido complexes 2–4 were tested as catalysts for
these systems, that is, to generate several PLA chains per metal hydrophosphonylation of aldehydes and ketones (Scheme 3).
center by introducing several equivalents of a chain transfer The reactions of aldehydes with diethyl phosphite (1 : 1 molar
agent, was explored with complexes 2–4 in the presence of iso- ratio) were carried out at 25 1C in the presence of 1 mol% of
propanol which acts as a chain transfer agent.
complexes 2–4. The results are summarized in Table 5. We
The polymerizations of rac-lactide with systems 2–4/iPrOH found that the reactions of diethyl phosphite with benzaldehyde
were conducted in the presence of 3 and 5 equiv. (vs. Ln) of and aliphatic aldehydes initiated by 2–4 (1 molar%, THF, 25 1C)
iPrOH. The systems 2–4/iPrOH allowed 87–99% conversion of afford the corresponding products 7d–7h in high yields (84–93%)
100–300 equiv. of rac-lactide to polymer within 30–60 min at within 24 hours (Table 5, entries 4–8, 13–17, 22–26). Introduction of
25 1C in toluene at a concentration of 1 mol Lꢀ1 rac-lactide. A kinetic any type of substituent (electron donating or electron withdrawing)
monitoring of the ROP reaction mediated by 3/iPrOH in toluene in the para-position of the phenyl ring of benzaldehyde results in
indicated a first-order dependence on lactide concentration, with an significant acceleration of the reactions. In these cases quantitative
apparent first order rate constant kapp = 0.0326 minꢀ1 (Fig. 3). The yields of the products 7a–7c were achieved in a very short time
ROP of rac-lactide with 2–4 proceeded in a much better controlled (15 minutes) (Table 5, entries 1–3, 10–12, 19–21). It should be noted
fashion when iPrOH was added in the reaction (Table 3, entries 3, that the LnIII ionic radius has no effect on the rate of the reactions
12–15, 19). The much narrower polydispersities (Mw/Mn = 1.13–1.27) of hydrophosphonylation of aldehydes.51,53–56
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Scheme 3
reactions of addition of diethyl phosphite to unactivated ketones
(BuMeC(O), Ph2C(O), MePhC(O)) and aldimines (PhNQCHPh,
PhNQCPh2, PhNQCH2) was performed at a catalyst concentration
of 5 mol% in THF. However, even after 48 hours at 65 1C only
starting substrates were isolated from the reaction mixtures. At
the same time the reactions of diethylphosphite with benzylidene-
acetone PhCHQCHC(O)Me (5 mol% of 2–4, 48 h, 65 1C) resulted in
the formation of a-hydroxy phosphonate PhCHQCHCMe(OH)-
P(O)(OEt)2 7i in 35–40% yields (Table 5, entries 9, 18, 27).
Table 5 Hydrophosphonylation of aldehydes and ketones catalyzed by
the complexes 2–6a
Entry Complex R
R0
Time Product Yieldb [%]
1
2
3
4
5
6
7
8
2
2
2
2
2
2
2
2
2c
3
3
3
3
3
3
3
3
3c
4
4
4
4
4
4
4
4
4c
5
5
5
5
5
5
5
5
5c
6
6
6
6
6
6
6
6
6c
4-Cl–C6H4
4-Me–C6H4
4-MeO–C6H4
Ph
Pen
Bu
iPr
Pr
Me
4-Cl–C6H4
4-Me–C6H4
4-MeO–C6H4
Ph
Pen
Bu
iPr
Pr
Me
4-Cl–C6H4
4-Me–C6H4
4-MeO–C6H4
Ph
Pen
Bu
iPr
Pr
Me
4-Cl–C6H4
4-Me–C6H4
4-MeO–C6H4
Ph
Pen
H
H
H
H
H
H
H
H
15 min 7a
15 min 7b
15 min 7c
99
99
99
92
88
87
85
86
35
99
99
99
93
92
89
85
84
38
99
99
99
92
89
87
86
86
40
99
99
99
93
93
86
88
82
40
99
99
99
92
92
91
83
81
43
24 h
24 h
24 h
24 h
24 h
7d
7e
7f
7g
7h
7i
The catalytic activity of lanthanide borohydrido complexes
in hydrophosphonylation of aldehydes and ketones remains less
investigated.28 Previously reported lanthanide borohydrides
[1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]Ln(BH4)(m-BH4)Li(THF)2 (Ln =
Sm (5), Nd (6))30 supported by the same ansa bis(amidinate) ligand
were tested as catalysts for hydrophosphonylation of carbonyl
compounds. The reactions of equimolar amounts of aldehyde
and diethyl phosphite (Table 5) were carried out in the presence
of 1 molar% of complexes 5 and 6 in THF at 25 1C. The
borohydrides 5 and 6 had catalytic activities similar to those of
amido derivatives 2–4. Thus, the addition of diethyl phosphite to
4-chloro-, 4-methyl- and 4-methoxybenzaldehyde gave the products
7a–7c in 99% yield in 15 min (Table 5, entries 28–30, 37–39). Under
the same reaction conditions, when benzaldehyde and aliphatic
aldehydes were used as the substrates the reaction was slower. The
products 7d–7h can be isolated in good to high yields in 24 hours
(Table 5, entries 31–35, 40–44). Benzylideneacetone was also applic-
able to this reaction. The reactions in the presence of complexes 5
and 6 (5 mol%) over 48 hours (THF, 65 1C) afforded a-hydroxy
phosphonate PhCHQCHCMe(OH)P(O)(OEt)2 (7i) in 40% and 43%
yields respectively (Table 5, entries 36, 45). It is noteworthy that
unlike the lanthanide amido complexes reported by Wang54
enabling 1,4-addition of diethyl phosphite to benzylideneacetone
at 40 1C complexes 2–6 at a high temperature afford exclusively
the product of 1,2-addition. This is obviously due to the steric
hindrance in the lanthanide coordination sphere caused by rigid
coordination of the ansa bis(amidinate) ligand system which
prevents 1,4-coordination of benzylideneacetone to the catalytic
center. Borohydrides 5 and 6 do not initiate the reactions of
unactivated ketones (BuMeC(O), Ph2C(O), MePhC(O)) and aldimines
(PhNQCHPh, PhNQCPh2, PhNQCH2) with diethylphosphite
(catalyst concentration 5 mol%, THF, 65 1C, 48 h).
9
PhCHQCH 48 h
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
H
H
H
H
H
H
H
H
15 min 7a
15 min 7b
15 min 7c
24 h
24 h
24 h
24 h
24 h
7d
7e
7f
7g
7h
7i
PhCHQCH 48 h
H
H
H
H
H
H
H
H
15 min 7a
15 min 7b
15 min 7c
24 h
24 h
24 h
24 h
24 h
7d
7e
7f
7g
7h
7i
PhCHQCH 48 h
H
H
H
H
H
H
H
H
15 min 7a
15 min 7b
15 min 7c
24 h
24 h
24 h
24 h
24 h
7d
7e
7f
7g
7h
7i
Bu
iPr
Pr
Me
PhCHQCH 48 h
4-Cl–C6H4
4-Me–C6H4
4-MeO–C6H4
Ph
Pen
Bu
iPr
Pr
H
H
H
H
H
H
H
H
15 min 7a
15 min 7b
15 min 7c
24 h
24 h
24 h
24 h
24 h
7d
7e
7f
7g
7h
7i
Me
PhCHQCH 48 h
a
Reaction conditions: diethyl phosphite (1.0 mmol), aldehyde (1.0 mmol),
solvent (2 mL), room temperature. Isolated yields. c The reactions were
b
carried out at 65 1C in the presence of 5 mol% of complexes.
It is known that hydrophosphorylation reactions of aldimines
and ketones require harsher conditions.51,54 The lanthanide com-
Experimental section
plexes with indolyl,51,55 pyrrole,54 methylene-linked pyrrolyl,56 and All experiments were performed in evacuated tubes, using
calix[4]-pyrrolyl53 ligands were found to be efficient initiators for this standard Schlenk-tube techniques, with the rigorous exclusion
transformation. Investigation of the catalytic activity of 2–4 in the of traces of moisture and air. After drying over KOH, THF and
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Et2O were purified by distillation from sodium/benzophenone solid residue was extracted with toluene (2 ꢂ 20 mL), and the
ketyl, hexane and toluene by distillation from sodium/triglyme extracts were filtered. Evaporation of toluene and recrystalliza-
benzophenone ketyl prior to use. Anhydrous YCl3, SmCl3, tion of the solid residue from Et2O afforded pale yellow crystals
58
NdCl3,86 Li(Et2O)[N(SiMe3)2],57 and NaN(SiMe3)2 were prepared of 1 (0.61 g, 43%). IR (KBr, Nujol, nmax cmꢀ1): 1653 (w), 1638 (w),
according to literature procedures. All other commercially available 1595 (w), 1564 (w), 1533 (m), 1510 (m), 1292 (m), 1250 (m), 1219
chemicals were used after the appropriate purification. NMR spectra (m), 1178 (m), 1113 (m), 1034 (m), 999 (m), 932 (m), 883 (m), 812
were recorded on Bruker Avance DRX-400 and DPX-200 spectro- (m), 768 (m), 741 (m), 658 (m), 588 (m). Anal. calcd for
meters in C6D6 at 20 1C, unless otherwise stated. Chemical shifts for [C80H104LiN8NdO3]: C, 69.78; H, 7.61; Nd, 10.48%; found: C,
1H and 13C spectra were referenced internally using the residual 69.39; H, 7.35; Nd, 10.63%.
solvent resonances and are reported relative to TMS in parts per
million (ppm). IR spectra were recorded as Nujol mulls on an ‘‘IR
Synthesis of [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]YN(SiMe3)2 (2)
Prestige-21 Shimadzu’’ instrument. Lanthanide metal analyses were A solution of NaN(SiMe3)2 (0.22 g, 1.18 mmol) in THF (20 mL)
carried out by complexometric titration.87
was added to a suspension of YCl3 (0.25 g, 1.18 mmol) in THF
The C, H, N elemental analyses were performed in the (30 mL) at 25 1C and the reaction mixture was stirred for 24 h.
microanalytical laboratory of the G. A. Razuvaev Institute of The resulting solution was added to a THF (20 mL) solution of
Organometallic Chemistry. GPC was carried out by using a [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]Na2(THF)n prepared in situ
chromatograph ‘‘Knauer Smartline’’ with Phenogel Pheno- from 1,8-C10H6{HNC(tBu)N-2,6-Me2C6H3}{NC(tBu)HN-2,6-Me2C6H3}
menex Columns 5u (300 ꢂ 7.8 mm) 104, 105 and a Security (0.63 g, 1.18 mmol) and NaN(SiMe3)2 (0.44 g, 2.37 mmol) in toluene
Guard Phenogel Column with RI and UV detectors (254 nm). (50 mL). The reaction mixture was stirred overnight, the volatiles
The mobile phase was THF and the flow rate was 2 mL minꢀ1
.
were removed in a vacuum, the solid residue was extracted with
The columns were calibrated by phenomenex medium- and toluene (2 ꢂ 20 mL), and the extracts were filtered. Evaporation of
high-molecular-weight polystyrene standard kits with peak toluene and recrystallization of the solid residue from Et2O afforded
molecular weights from 2700 to 2 570 000 Da. The number- pale yellow crystals of 2 (0.40 g, 43%). NMR (C6D6, 20 1C, 200 MHz; d
average molecular masses (Mn) and polydispersity indexes ppm) 1H: 7.43–6.78 (m, 12 H, C10H6, (CH3)2C6H3), 2.49 (s, 12 H, CH3),
(Mw/Mn) of the polymers were calculated with reference to a 1.19 (s, 18 H, C(CH3)3), ꢀ0.07 (s, 18 H, SiCH3). NMR (C6D6, 20 1C, 100
universal calibration vs. polystyrene standards. Mn values of the MHz; d ppm) 13C{1H}: 183.0, 153.7, 149.4, 148.0, 147.7, 142.9, 141.7,
polylactides were corrected with a Mark–Houwink factor of 137.2, 136.7, 136.3, 130.9, 130.7, 130.6, 129.6, 128.4, 125.0, 124.6,
0.58 to account for the difference in hydrodynamic volumes 123.5, 122.8, 120.1, 119.6, 119.1, 118.7 (C10H6, (CH3)2C6H3), 42.7
between polystyrene and polylactide.88 The microstructures of (s, C(CH3)3), 30.8 (s, C(CH3)3), 21.3 (s, CH3), 3.5 (s, SiCH3), 2.3
1
the polylactides were measured by homo-decoupling H NMR (s, SiCH3). IR (KBr, Nujol, nmax cmꢀ1): 1670 (w), 1638 (w), 1566 (w),
spectroscopy at 25 1C in CDCl3 with a Bruker Avance DRX-400 1417 (m), 1396 (m), 1361 (m), 1350 (m), 1331 (m), 1246 (m), 1215
spectroscope.89
(m), 1202 (m), 1175 (m), 1165 (m), 1093 (m), 1045 (m), 1016 (m), 982
X-ray diffraction intensity data for compounds 1 and 4 were (m), 933 (m), 870 (m), 837 (m), 812 (w), 760 (m), 677 (m), 652 (w).
collected on a Bruker Smart APEX diffractometer with graphite Anal. calcd for [C42H60N5Si2Y]: C, 64.67; H, 7.75; Y, 11.40%; found: C,
monochromated Mo-Ka radiation (l = 0.71073 Å) using o scans. 64.25; H, 7.39; Y, 11.68%.
The structures were solved by direct methods and were refined
on F2 using a SHELXTL90 package. All non-hydrogen atoms
Synthesis of [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]SmN(SiMe3)2-
(THF) (3)
were found from Fourier syntheses of electron density and were
refined anisotropically. All hydrogen atoms were placed in A solution of NaN(SiMe3)2 (0.20 g, 1.10 mmol) in THF (20 mL)
calculated positions and were refined in the riding model. was added to a suspension of SmCl3 (0.28 g, 1.10 mmol) in THF
SADABS91 were used to perform area-detector scaling and (30 mL) at 25 1C and the reaction mixture was stirred for 24 h.
absorption corrections. The details of crystallographic, collec- The resulting solution was added to a THF (20 mL) solution of
tion and refinement data are shown in Table 2. CCDC 1022273 [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]Na2(THF)n prepared in situ
(1) and 1022274 (4).
from 1,8-C10H6{HNC(tBu)N-2,6-Me2C6H3}{NC(tBu)HN-2,6-Me2C6H3}
(0.59 g, 1.10 mmol) and NaN(SiMe3)2 (0.40 g, 2.20 mmol) in toluene
(50 mL). The reaction mixture was stirred overnight, the volatiles
were removed in a vacuum, the solid residue was extracted with
Synthesis of [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]Nd-
[l-1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2][Li(DME)] (1)
A solution of Li(Et2O)[N(SiMe3)2] (0.24 g, 1.03 mmol) in THF toluene (2 ꢂ 20 mL), and the extracts were filtered. Evaporation of
(20 mL) was added to a suspension of NdCl3 (0.25 g, 1.03 mmol) toluene and recrystallization of the solid residue from Et2O afforded
in THF (30 mL) at 25 1C and the reaction mixture was stirred for pale yellow crystals of 3 (0.52 g, 52%). NMR (C6D6, 20 1C, 400 MHz; d
24 h. The resulting solution was added to a THF (20 mL) ppm) 1H: 7.72–6.28 (m, 12 H, C10H6, (CH3)2C6H3), 3.32 (br. s, 12 H,
solution of [1,8-C10H6-{NC(tBu)N-2,6-Me2C6H3}2]Na2(THF)n pre- THF, a-CH2), 3.11 (s, 12 H, CH3), 1.53 (br. s, 12 H, THF, b-CH2), 1.16
pared in situ from 1,8-C10H6{HNC(tBu)N-2,6-Me2C6H3}{NC(tBu)- (s, 18 H, C(CH3)3), 0.16 (s, 18 H, SiCH3). NMR (C6D6, 20 1C, 100 MHz;
HN-2,6-Me2C6H3} (0.55 g, 1.03 mmol) and NaN(SiMe3)2 (0.37 g, d ppm) 13C{1H}: 162.2, 147.5, 147.3, 146.5, 138.2, 135.7, 135.4, 129.7,
2.06 mmol) in toluene (50 mL) at 25 1C. The reaction mixture was 124.4, 121.9, 121.4, 120.4, 120.0, 118.2, 116.7, 116.0, 113.6, 98.1,
stirred overnight, the volatiles were removed in a vacuum, the (C10H6, (CH3)2C6H3) 71.89 (s, THF, a-CH2), 40.4 (s, C(CH3)3), 39.1
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(s, C(CH3)3), 29.93 (s, THF, b-CH2), 29.40 (s, C(CH3)3), 29.72 recrystallized from a THF–hexane mixture. The yield of product
(s, C(CH3)3), 16.22 (s, CH3), 1.05 (s, SiCH3). IR (KBr, Nujol, nmax was determined by weighing.
cmꢀ1): 1670 (w), 1637 (w), 1589 (w), 1556 (m), 1418 (s), 1396 (s), 1361
(s) 1350 (s), 1336 (m), 1294 (m), 1250 (m), 1213 (m), 1172 (m), 1117
(m), 1095 (m), 1065 (m), 1043 (m), 1034 (m), 951 (m), 932 (m), 894
(m), 841 (m), 826 (m), 825 (m), 787 (m), 760 (m), 690 (w), 617 (w), 602
Conclusions
(w). Anal. calcd for [C46H68N5OSi2Sm]: C, 60.47; H, 7.50; Sm, 16.46%; A series of rare-earth amido complexes [1,8-C10H6{NC(tBu)N-
found: C, 60.08; H, 7.33; Sm, 16.48%.
2,6-Me2C6H3}2]LnN(SiMe3)2(THF)n supported by ansa bis-
(amidinate) ligands was successfully synthesized by the salt
metathesis reactions starting from in situ generated [(Me3Si)2N]-
LnCl2(THF)2, while the attempts of substitution of a chloro
Synthesis of [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]NdN(SiMe3)2-
(THF) (4)
A solution of NaN(SiMe3)2 (0.22 g, 1.18 mmol) in THF (20 mL) ligand in [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]YCl(DME) and
was added to a suspension of NdCl3 (0.25 g, 1.18 mmol) in THF [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]Ln(L)(m-Cl)2Li(L)n (Ln = Sm,
(30 mL) at 25 1C and the reaction mixture was stirred for 24 h. L = DME; Ln = Nd, L = THF) by the [(Me3Si)2N]ꢀ group failed. The
The resulting solution was added to a THF (20 mL) solution of alkaline metal cation plays a crucial role in the outcome of
[1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]Na2(THF)n prepared in situ metathesis reaction: in the presence of Li+ the reaction pathway
from of 1,8-C10H6{HNC(tBu)N-2,6-Me2C6H3}{NC(tBu)HN-2,6- switches towards formation of amidinate ate-complex [1,8-C10H6-
Me2C6H3} (0.63 g, 1.18 mmol) with NaN(SiMe3)2 (0.44 g, {NC(tBu)N-2,6-Me2C6H3}2]Nd[m-1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]-
2.37 mmol) in toluene (50 mL). The reaction mixture was [Li(DME)].
stirred overnight, the solvent was evaporated in a vacuum,
Amido complexes 2–4 initiate ROP of rac-lactide under mild
the solid residue was extracted with toluene (20 mL), and conditions and convert 100–250 equiv. of monomer within
extracts were filtered. After recrystallization of the residue from 60–90 min into polylactides featuring atactic structures and
Et2O, pale yellow crystals of 4 were obtained in a yield of 0.58 g moderate molecular-weight distributions (Mw/Mn = 1.30–2.12).
(54%). IR (KBr, Nujol, nmax cmꢀ1): n~ = 1637 (w), 1589 (m), 1558 The experimental Mn values of the obtained polymers are found
(m), 1364 (m), 1335 (m), 1247 (m), 1213 (m), 1194 (m), 1173 (m), to be significantly higher than the calculated ones presumably
1117 (m), 1098 (m), 1080 (m), 1043 (m), 1032 (m), 962 (m), 889 due to the slow initiation stage. Amido complexes 2–4 turned
(m), 824 (m), 688 (w), 617 (w), 602 (w). Anal. calcd for out to be more efficient ROP catalysts compared to the related
[C46H68N5NdOSi2]: C, 60.88; H, 7.55; Nd, 15.89%; found: C, borohydrides [1,8-C10H6{NC(tBu)N-2,6-Me2C6H3}2]Ln(BH4)2Li(THF)2.
60.43; H, 7.45; Nd, 15.79%.
Effective immortal ROP of lactide with 3–5 equiv. of isopropanol per
metal center was performed using complexes 2–4 as the catalysts.
The systems 2–4/iPrOH exhibit higher activities in ROP and allow for
General experimental procedure for polymerization of rac-lactide
In a typical experiment (Table 4, entry 3), in a glovebox, a complete conversion of 100–300 equiv. of rac-lactide to polymer
Schlenk flask was charged with a solution of initiator 2 (16 mg, within 30–60 min at 25 1C and provide a living polymerization mode
0.0205 mmol) in toluene (1 mL). To this solution, rac-lactide and very narrow polydispersities (Mw/Mn = 1.13–1.27). Comparison of
(290 mg, 2.05 mmol, 100 equiv.) in toluene (1 mL) was added the results of the catalytic tests obtained with complexes coordinated
rapidly (isopropanol was added at this stage for experiments by linked bis(amidinate) systems [1,8-C10H6{NC(tBu)N-2,6-Me2-
conducted under immortal conditions). The mixture was C6H3}2]2ꢀ, [C6H4-1,2-{NC(tBu)N(2,6-Me2C6H3)}2]2ꢀ 28
[C6H4-1,2-
,
immediately stirred with a magnetic stir bar at 25 1C for 30 {NC(Ph)NSiMe3}2]2ꢀ 28 and [CH2{CH2NC(Ph)NSiMe3}2]2ꢀ 32
min. After an aliquot of the crude material was sampled by clearly demonstrate that a flexible –(CH2)3– linker provides a
pipet for determining monomer conversion by 1H NMR, the better degree of stereocontrol of ROP rac-lactide vs. rigid 1,8-
reaction was quenched with acidic methanol (ca. 1 mL of a
C
10H6 and C6H4-1,2.
1.2 M HCl solution in MeOH), and the polymer was precipitated
Complexes 2–4 as well as related borohydrides [1,8-C10H6-
with excess methanol (ca. 50 mL). The polymer was then {NC(tBu)N-2,6-Me2-C6H3}2]Ln(BH4)(m-BH4)Li(THF)2 (Ln = Sm,
filtered and dried under vacuum to constant weight. The Pr Nd) catalyze hydrophosphonylation of aldehydes at room tem-
values for obtained polylactides were calculated according to perature with good reaction rates and hydrophosphonylation of
the published protocol.92
benzylideneacetone at 65 1C.
General experimental procedure for hydrophosphonylation of
aldehydes, ketones and aldimines
Acknowledgements
HP(O)(OEt)2 (1 mmol, 0.138 g, 0.127 mL) was added to a
solution of aldehyde (ketone or aldimine) (1 mmol) and catalyst This work is supported by the Russian Foundation for Basic
(1 or 5 mol%) in toluene (2 mL). The reaction mixture was Research (Grant No. 14-03-31039 , 12-03-93109- ),
stirred at room temperature for 24 h and was subsequently and the Ministry of Education and Science of the Russian Federa-
hydrolyzed with water (1.0 mL), extracted with ethyl acetate tion (the agreement of August 27, 2013 No. 02.B.49.21.0003
(3 ꢂ 10.0 mL), dried with anhydrous Na2SO4 and filtered. After between The Ministry of Education and Science of the Russian
the solvent was removed under vacuum, the final product was Federation and Lobachevsky State University of Nizhny Novgorod).
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
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