Synthesis and characterization of new divalent lanthanide complexes supported by amine bis(phenolate) ligands and their applications in the ring opening polymerization of cyclic esters

Article abstract of DOI:10.1039/b816916k

Treatment of an array of bis(phenol)s H₂L^x {L ^x = [(^-OC₆H₂(2,4-R)(6-CH ₂))₂ NCH₂CH₂X], where X = CH ₂NMe₂, NMe₂,

Full text of DOI:10.1039/b816916k

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis and characterization of new divalent lanthanide complexes
supported by amine bis(phenolate) ligands and their applications in the ring
opening polymerization of cyclic esters†
Divine T. Dugah,a Brian W. Skeltonb and Ewan E. Delbridge*c
Received 1st October 2008, Accepted 11th November 2008
First published as an Advance Article on the web 15th January 2009
DOI: 10.1039/b816916k
Treatment of an array of bis(phenol)s H2Lx {Lx = [(-OC6H2(2,4-R)(6-CH2))2 NCH2CH2X], where X =
CH2NMe2, NMe2, NEt2, OMe and R = But, Pet where Pet = C(CH3)2Et} with [Ln(N(SiMe3)2)2(THF)2]
(Ln = Yb, Sm) in a 1:1 molar ratio in hexanes afforded a multitude of new divalent lanthanide
bis(phenolate) complexes: [L2cYb]2 (X = NEt2, R = But) 1, [L2bYb]2 (X = CH2NMe2, R = But) 2,
[L3aYb]2 (X = NMe2, R = Pet) 3, [L3bYb]2 (X = OMe, R = Pet) 4, [L2cSm] (X = NEt2, R = But) 5,
[L3cYb] (X = NEt2, R = Pet) 6, [L3cSm] (X = NEt2, R = Pet) 7. X-Ray crystallographic analyses of
compounds 1 and 3 reveal dimeric, centrosymmetric structures with 5-coordinate ytterbium centers
arising from bridging and terminal phenolate groups. A selection of divalent compounds (1, 3, 4, 5, 6
and 7) were tested as catalyst precursors in the polymerization of e-caprolactone and/or L-lactide
which resulted in high molecular weight polymers with PDIs of 1.11–2.81 and 1.13–1.56 for
e-caprolactone and L-lactide respectively. All polymerization studies were performed in either toluene
or THF and at room temperature (e-caprolactone) or 70 ◦C (L-lactide) at a variety of catalyst:monomer
ratios (1:100–1:300). Kinetics analyses of the polymerization of L-lactide by compounds 1 and 5
indicated pseudo-first order response with respect to L-lactide.
Recently, interest in divalent lanthanides has progressed to
Introduction
include ligands with less organometallic Ln–C character19,20 and
more Ln–X (X = N, O, S etc.)21,22 heteroatom bonding character
The synthetic challenges, novel chemical properties and availabil-
ity of the lanthanide elements continue to drive significant research
efforts towards further understanding this often misunderstood
(e.g. amides,4,23 bis(phenolate)s,14,24 b-diketiminates25,26). The mo-
tivation for this progression away from organometallic ligand sys-
group of elements. One area to receive increasing research efforts
tems lies primarily in the ability of these ligands to impart greater
is the synthesis and stabilization of the ephemeral (and often
stability to the catalytic systems without loss of ligand versatility.
elusive) divalent lanthanide compounds.1–4 More recently, ground-
This becomes apparent where these divalent lanthanide systems
are used as catalyst precursors in polymerization reactions.27–29
Although divalent lanthanide compounds are highly reactive and
air- and moisture-sensitive, typically a pre-polymerization oxida-
tion of the lanthanide(II) compound occurs (with concomitant
reduction of an equivalent of the monomer)27–30 to afford the
catalytically active trivalent lanthanide compound which then is
available for the propagating polymerization reactions. The use
of multidentate O, N donor ligands (rather than Cp ligands)
imparts greater stability to the catalytically active trivalent systems
which have significantly reduced propensity towards hydrolysis—
inevitably arising from trace moisture/air contamination typically
found in monomer feedstocks.
Among the recently reported non-cyclopentadienyl lanthanide
compounds ligands, those containing bis(phenolate) ligands31–33
(Chart 1) have received interest due to their ease of prepara-
tion and chemical versatility (both electronic and steric). This
versatility has led to the generation of a number of lanthanide
bis(phenolate) compounds which have displayed impressive cat-
alytic activities towards polar monomers such as lactide,33–37
e-caprolactone14,15,36,38–40 and methylmethacrylate.41 Only a handful
of these examples14,15 contained divalent lanthanide precursors,
despite their conceptually similar synthesis to their trivalent coun-
terparts (Scheme 1). In addition to their catalytic properties, these
breaking research accomplishments have shattered the commonly
held belief that only samarium, ytterbium and europium possess
accessible divalent elements under standard laboratory conditions
(i.e. solution chemistry)‡.5–8 Key to the stabilization of these diva-
lent lanthanide compounds9–11 is the use of versatile, bulky, high-
coordination number ligands (e.g. the cyclopentadienyl family)
despite their hydrolytic sensitivity. In addition to the intrinsic in-
terest posed by divalent lanthanide compounds, many researchers
have capitalized on their one-electron reducing capabilities12–16
with continued focus on small molecule activation (e.g. CO2
and COS by [(C5Me5)2Sm(THF)2]17) and polymerization reactions
{[(C5Me5)Ln(CH(SiMe3)2)(C5Me5)K(THF)2]n (Ln = Eu, Yb) in
the polymerization of styrene and ethylene18}.
aChemistry Department, University of North Dakota, Grand Forks, ND
58202, USA. Tel: 701 777 0380/3222
bChemistry M313, School of Biomedical, Biomolecular and Chemical
Sciences, The University of Western Australia, Crawley, WA 6009, Australia
cThe Lubrizol Corporation, 29400 Lakeland Blvd, Wickliffe, OH 44092,
USA. Tel: 440 943 4200. E-mail: eadl@lubrizol.com
† CCDC reference numbers 704056 and 704057. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b816916k
‡ Indeed Tm, Nd, Ho etc. divalent compounds, derived from novel solution
chemistry, are all well established.5–8
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Scheme 1 Synthesis of divalent samarium and ytterbium bis(phenolate) compounds.
of cyclic esters (e-caprolactone and L-lactide) have also been
evaluated.
Results and discussion
Synthesis of bis(phenolate) ligands
The ligands (Chart 1) were prepared following literature
procedures31 as analytically pure white powders via a single-step
Mannich type condensation reaction of the commercially available
formaldehyde, the appropriate substituted phenols and primary
amines. Optimal balance between steric and electronic properties
in stabilizing these lanthanide complexes were considered in ligand
selection. Using similar protocols, ligands H2L3b and H2L3c, which
have not been previously reported, were also synthesized with their
identities being confirmed by typical spectroscopic techniques
including elemental analysis, melting point, high-resolution mass
spectrometry, and IR- and NMR-spectroscopy.
Chart 1 Matrix of bis(phenol) ligands synthesized.
divalent lanthanide bis(phenolate) compounds also demonstrate
superior pathways to heteroleptic trivalent lanthanide complexes
via oxidation reactions;14,15 compared to the traditional salt
metathesis reactions.39,42
Synthesis of divalent lanthanide bis(phenolate) complexes
Previously the authors reported the first solvent-
free Yb(II) bis(phenolate) dimer, {Yb[OC6H2(2,4-But)(6-
CH2)]2NCH2CH2NMe2}2 which displayed promising catalytic
activity towards e-caprolactone in addition to oxidative reactivity
towards a variety of anhydrous alcohols to afford a multitude
of ytterbium(III) bis(phenolate) alkoxide heteroleptics.14
Subsequently, three new divalent Yb and Eu bis(phenolate)
compounds, similar to that presented above, have been reported
by Shen and Yao et al.24 Reported herein is a sizable expansion
of the authors’ previous work, where syntheses involving several
bis(phenolate) ligands coordinated to ytterbium and samarium,
along with structural investigations (solution and solid-state), are
presented. In addition, catalytic properties of these lanthanide
compound towards ring opening polymerization (ROP) studies
Access to divalent ytterbium and samarium bis(phenolate)
compounds was readily achieved by reaction of the appropri-
ate [Ln(N(SiMe3)2)2(THF)2]4 amide with one of the various
bis(phenol) ligands displayed in Chart 1. A number of trends have
been observed which indicated that steric encumbrance of the
phenolate ligand along with metal selection play an important
role in influencing overall stability and ultimately the solid-
state and solution properties of the lanthanide bis(phenolate)
compounds.
Unstable ytterbium(II) bis(phenolates) where
R
=
Me
(i.e. ligands L1a-c). Initially the bis(phenol) ligands, H2L1a-c
(Chart 1) (the methyl substituted phenols) were treated with
[Ln(N(SiMe3)2)2(THF)2] (1:1 molar ratio) in hexanes in an attempt
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to isolate the corresponding divalent ytterbium phenolate com-
pound. However, upon reagent mixing, the initially formed brick-
red solution (typical of Yb(II)14,15) rapidly became pale-yellow
within 20 minutes and deposited a yellow precipitate indicative of
likely oxidation to Yb(III) (Scheme 1). Unfortunately the highly
paramagnetic nature of the trivalent ytterbium precluded any
meaningful NMR spectra to be obtained. Moreover, this provides
indirect evidence of trivalent ytterbium since an intelligible spec-
trum from any divalent (diamagnetic) ytterbium would be antici-
pated. Since these reaction systems were found to be reproducible,
oxidation by adventitious air/moisture can be eliminated; a more
likely explanation would attribute oxidation of sub-stoichiometric
quantities of the divalent ytterbium bis(phenolate) complex with
free bis(phenol) [i.e. H2L1a-c in a typical Ln(II) redox reaction
producing H2 as a by-product (Scheme 1)14,43,44]. These results infer
that the less sterically demanding bis(phenol)/(phenolate) ligands
possibly have greater access to the redox active lanthanide metal
center, thus facilitating these oxidation reactions.§
Monomers: ytterbium and samarium bis(phenolates) where R =
But, Pet (i.e. ligand, L3c).
Compounds 6,7. It was postulated that further increasing the
steric constraints exerted by the bis(phenolate) ligands may sup-
press the propensity of these lanthanide bis(phenolate) complexes
to exist as aggregates (or dimers) in the presence of non-donor
solvent systems (see discussion above). As such, treatment of
the appropriate [Ln(N(SiMe3)2)2(THF)2] with H2L3c (Chart 1),
(where the side-arm substituents in H2L3a,b have been modified
from X = NMe2 or OMe to the bulkier NEt2 group) in 1:1 molar
ratio in hexanes afforded compounds 6 [YbL3c] and 7 [SmL3c].
Unlike compounds 1–4, longer reaction times¶ (9 and 24 h for
compounds 6 and 7 respectively cf. 4 h for 1–4) were necessary
to achieve quantitative protiolytic ligand exchange. Repeated
attempts to obtain X-ray crystallographic quality crystals of com-
pounds 6 and 7 were thwarted by formation of microcrystalline
powders. However, the solution properties of both compounds
were investigated by 1H and 13C NMR spectroscopic studies and
revealed only one phenolate environment (as evidenced by only
two unique aromatic C–H groups) which is consistent with a
monomeric structural motif; akin to those described above and
reported previously for the analogous THF adduct reported by
Shen and coworkers.15 Moreover, NMR spectra of both 6 and
7 in C4D8O were almost identical to those recorded in C6D6,
further supporting the supposition of predominant monomeric
divalent lanthanide bis(phenolate) compounds. Not surprisingly,
monomeric compound 7 [SmL3c] exhibited greater air-sensitivity
than its ytterbium analogue, compound 6 [YbL3c], and both
displayed greater air-sensitivity than the aggregated compounds
1–4 as ascertained semi-quantitatively by relative oxidation rates
of the solids in air.
Dimers: ytterbium(II) bis(phenolate) where R = But, Pet (i.e.
ligands L2b,c, L3a,b).
Compounds 1–4. Substituting the methyl groups for the bulkier
tertiary butyl (L2b,c) or tertiary pentyl (L3a,b) groups in the
bis(phenol) ligands provided a convenient mechanism to accessing
the analytically pure divalent ytterbium phenolate compounds 1–
4 (Scheme 1) in good yields (65–83%). The proposed formulations
(discussed below) are supported by typical analytical techniques
(see Experimental) including IR- and NMR-spectroscopy, elemen-
tal analysis (which in some cases returned %C values which were
low, but within the commonly held acceptable range for extremely
air-sensitive divalent Ln compounds) and for compounds 1 and 3,
X-ray crystallography.
X-ray crystallographic analysis of compounds 1 [YbL2c]2 and 3
[YbL3a]2, of which the crystals were obtained from non-donor solu-
tions, revealed dimeric structures with a centrosymmetric YbO2Yb
core and bridging and terminal phenolate oxygen donor atoms
Samarium bis(phenolate) where R = But, (i.e. ligand L2c).
Compound 5. Several attempts to synthesize a number of diva-
lent samarium bis(phenolate) compounds analogous to the ytter-
bium compounds described above were unsuccessful as indicated
by typical auto-oxidation of the divalent samarium compound24
upon addition of the bis(phenol) reagent. Specifically, Shen noted
that SmL2a could not be synthesized from [Sm(N(SiMe3)2)2(THF)2]
and H2L2a. These observations are not surprising given the
relative reducing power of the Sm(II) vs. Yb(II)45 {e.g. (C5Me5)2Ln
1
(vide infra). Detailed H NMR spectroscopic studies indicated
that the solid-state structure is preserved in non-donor deuterated
solvents such as C6D6 or C7D8 but the bis(phenolate) ligands
become symmetrical and the ytterbium complex monomeric, upon
addition of donor solvents such as C4D8O (Scheme 1). Moreover,
this dynamic solution behavior from dimer to monomer is totally
reversible and has been noted by the authors previously for a
related ytterbium bis(phenolate) complex {Yb[OC6H2(2,4-But)(6-
CH2)]2NCH2CH2NMe2}2.14 Subsequent research efforts by Shen
and co-workers15 reported monomeric THF analogue of this
compound which support the suppositions inferred from the
aforementioned solution NMR spectroscopic studies.
Though it was not possible to obtain solid state structures
for 2 [YbL2b] and 4 [YbL3b], all spectroscopic and analytical
data not only supported the dimeric formulation ‘[YbL]2’ but
also the dynamic solution behavior in donor/non-donor solvents
such that both solid-state structures are considered dimeric i.e.
[YbL]2 since each were isolated as solids from toluene/hexanes
solutions.
∫
(Ln = Sm, Yb) reacts with PhC CH to afford the pure Sm(III)
46
= = =
complex [(C5Me5)2Sm]2(PhC C C CPh) vs. the mixed valent
47
∫
Yb complex [(C5Me5)2Yb(III)]2(m-C CPh)4Yb(II) }. In addition,
the larger ionic radius of Sm(II) vs. Yb(II)45 undoubtedly enhances
oxidation reactions by providing greater access of the bis(phenol)
ligand substrate to the metal center. Similar oxidation reactions
of Sm(II) involving less bulky bis(phenolate) ligands have been
observed previously.48 Given these constraints, the successful
isolation of a samarium(II) bis(phenolate) compound [SmL2c] 5,
is a remarkable achievement (Scheme 1, Chart 1) given the L2c
ligand is significantly less sterically demanding than that found
in compound 7, L3c (described above). In the absence of any
X-ray crystallographic analyses,ꢀ it is difficult to explain the
¶ Within 4 h only 60% of compound 6 formed as monitored by 1H NMR
spectroscopy.
§ Indeed a number of other systems involving less encumbered bis(phenol)
ligand reproducibly yielded oxidized products.14
ꢀ Several attempts to obtain a solid state structure of 5 were thwarted by
crystal twinning.
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◦
˚
stability of this system; however an intimate balance between
metal size and ligand bulk likely saturates the coordination
environment about the metal center, protecting the metal center
from further redox chemistry. The ‘[SmL2c]’ formulation proposed
for compound 5, which may be dimeric or monomeric, was
supported by elemental analysis,**49 IR spectroscopy and NMR
spectroscopy (see Experimental). Compound 5 was insoluble in
non-donor solvents such as C6D6 or C7D8 precluding probing the
degree of possible solution and solid-state aggregation. However,
given similarities between compounds 5 and 1 [LnL2c], similar
aggregation properties are likely wherein compound 1 was deemed
as dimeric in non-donor solvents (see aforementioned discussion).
For this reason, compound 5 is grouped with compounds 1–4 in
Scheme 1.
Table 1 Selected bond distances (A) and angles ( ) for 1
˚
Bond distances (A)
Yb1-O41
Yb1-O31
Yb1-O11
2.229(4)
2.324(4)
2.357(3)
2.537(4)
2.593(4)
Yb2-O21
Yb2-O11
Yb2-O31
Yb2-N1
2.245(4)
2.328(3)
2.371(4)
2.521(4)
2.582(5)
Yb1-N2
Yb1-N203
Yb2-N103
Angles (◦)
O41-Yb1-O31
O41-Yb1-O11
O31-Yb1-O11
O41-Yb1-N2
O31-Yb1-N2
O11-Yb1-N2
O41-Yb1-N203
O31-Yb1-N203
O11-Yb1-N203
N2-Yb1-N203
131.66(14)
89.56(13)
73.20(12)
81.07(14)
79.17(13)
132.83(13)
112.11(15)
103.08(14)
150.65(14)
72.37(14)
O21-Yb2-O11
O21-Yb2-O31
O11-Yb2-O31
O21-Yb2-N1
O11-Yb2-N1
O31-Yb2-N1
O21-Yb2-N103
O11-Yb2-N103
O31-Yb2-N103
N1-Yb1-N103
133.09(14)
90.87(13)
72.86(12)
82.07(14)
79.18(13)
133.76(13)
112.34(15)
102.22(15)
148.84(15)
72.39(15)
X-Ray crystallography of compounds 1 and 3
The results from single crystal X-ray structure determinations of
compounds 1 and 3 are consistent with the proposed formulations,
[YbL]2, in terms of stoichiometry and connectivity i.e. dimers with
bridging and terminal asymmetric phenolate groups (Fig. 1 and
2 and Tables 1 and 2 respectively). Compound 1 was solved in
¯
the triclinic space group P1 with one molecule of toluene, while
compound 3 crystallizes in the monoclinic space group P21/n.
Fig. 2 Projection of one molecule of 3, with hydrogen and tert-pentyl
methyl and ethyl groups omitted for clarity. Only one component of the
disordered atoms is shown.
5-coordinate ytterbium atoms gain additional electron density via
agostic interactions of the tert-butyl and tert-pentyl hydrogen
atoms in compounds 1 and 3 respectively. As expected, these
agostic interactions†† are more significant in 3 which is probably
due to the closer proximity of the tert-pentyl hydrogens to
ytterbium in 3 versus those found in the tert-butyl groups of 1.
Ignoring these Yb ◊ ◊ ◊ H interactions, the formal 5-coordination
environment about ytterbium does not readily fit either symmetri-
cal trigonal bipyramidal or square-based pyramid models, which
is probably attributed to the overall conformational rigidity of the
bis(phenolate) ligands. The YbO2Yb cores in both compounds
Fig. 1 Projection of one molecule of 1, with hydrogen, tert-butyl methyl
groups and toluene solvent molecule omitted for clarity.
4
Each compound can be described as having an h -bis(phenolate)
ligand coordinated to the ytterbium(II) metal center with coordi-
nation saturation being fulfilled through additional oxygen coor-
4
dination (via bridging) from a neighboring h -bis(phenolate) unit
to give an overall 5-coordinate, dimeric ytterbium complex. These
** Albeit 1.7% low in carbon. Given the extreme air-sensitivity of
lanthanide(II) toward oxidation, repeated attempts at obtaining “satisfac-
tory” elemental analyses were thwarted presumably by imperfect analyses
techniques.49
†† Which were not observed in solution, as identified by 1H NMR
spectroscopy.
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Table 3 Results from the polymerization of e-caprolactonea
Entry Initiator Time/min [M]0/[I]0 % Conv.b 10-4 Mn(obs.)c PDIc
Table 2 Selected bond distances (A) and angles ( ) for 3
˚
Bond distances (A)
Yb1-O41
Yb1-O31
Yb1-O11
Yb1-N2
2.208(5)
2.316(5)
2.326(4)
2.530(4)
2.580(5)
Yb2-O21
Yb2-O11
Yb2-O31
Yb2-N103
Yb2-N1
2.250(3)
2.298(5)
2.333(4)
2.532(5)
2.531(6)
1
2
3
4
5
6
7
8
1
10
15
3
15
7
8
13
45
200
300
100
100
277
100
200
100
99.6
99.8
99.6
99.7
99.1
98.9
98.4
97.6
1.59
3.82
3.72
4.67
3.26
1.92
1.96
1.86
1.22
1.99
2.81
2.62
1.11
2.14
1.85
1.42
4
5
5d
5
Yb1-N203
Angles (◦)
7
O41-Yb1-O31
O41-Yb1-O11
O31-Yb1-O11
O41-Yb1-N2
O31-Yb1-N2
O11-Yb1-N2
O41-Yb1-N203
O31-Yb1-N203
O11-Yb1-N203
N2-Yb1-N203
139.35(17)
95.26(17)
72.52(16)
79.74(16)
79.76(15)
130.20(16)
100.88(18)
105.73(17)
154.74(16)
72.33(15)
O21-Yb2-O11
O21-Yb2-O31
O11-Yb2-O31
O21-Yb2-N103
O11-Yb2-N103
O31-Yb2-N103
O21-Yb2-N1
O11-Yb2-N1
O31-Yb2-N1
N103-Yb2-N1
130.66(12)
89.23(14)
72.72(16)
110.37(17)
104.76(17)
153.12(16)
78.85(14)
79.73(18)
130.93(16)
72.78(16)
7
7d
a Polymerization conditions: Toluene as solvent (unless otherwise stated),
T = RT, Vtol/VCL = 1.86. b Determined by 1H NMR analysis. c Determined
by GPC analysis in toluene with calibration to polystyrene standards.
d Solvent = THF.
In general, samarium-based catalysts polymerized e-capro-
lactone more rapidly than their ytterbium counterparts which
is in accordance with the findings of other researchers51–53,59,60
where larger lanthanide metal containing catalysts (e.g. Nd vs.
Yb) are more active ROP catalysts than those containing smaller
lanthanide ions.
Replacing toluene for donor solvents such as THF in the ROP
reactions invariably suppressed polymerization rates which again
is consistent with previous studies50,52 that have attributed this to
probable competition between the THF molecules and monomer
substrates for coordination to the metal center (the necessary first
step in the polymerization reaction). This, in turn, slows down the
polymerization process.
have symmetrical bridging modes with almost equivalent Yb–
(m-O) bond lengths which were longer (by ca. 0.10 A) than
terminal Yb–O bond lengths. In addition, all anionic Yb–O bond
lengths are shorter than Yb–N coordinate bonds; all of which are
comparable to other bridged Yb(II) bis(phenolate)s including the
related {Yb[OC6H2(2,4-But)(6-CH2)]2NCH2CH2NMe2}2.14
˚
Polymerization of e-caprolactone and L-lactide
While there are a numerous examples of trivalent lanthanide
catalyst precursors for the ring opening polymerization (ROP) of
cyclic esters such as lactide33,34,36,50,51 and lactones,36,38–40,52,53 there
are surprisingly few instances where divalent lanthanide com-
pounds have been employed.14,15,30,54 The divalent ytterbium and
samarium bis(phenolate) compounds 1–7 (vide supra) represent
ideal candidates for preliminary exploration into the propensity
of these compounds to function as catalyst precursors in ROP of
L-lactide and/or e-caprolactone to produce polymers which have
numerous applications from bio-friendly packaging materials to
bio-assimilable drug-delivery agents.55–57 Reported herein are our
results from initial ROP studies employing a variety of divalent
lanthanide bis(phenolate) compounds. A number of trends were
observed, including: 1) Divalent lanthanide bis(phenolate)s effec-
tively polymerize both e-caprolactone and the more challenging
L-lactide quantitatively 2) Samarium bearing catalysts are more
active than comparable ytterbium analogues 3) Donor solvents
such as THF suppress polymerization rates.
While the molecular structure of the catalyst (i.e. monomer
vs. dimer) in solution will undoubtedly have a profound effect
on ROP rates, it is difficult to understand the subtleties of
these effects in the present study, since the influence of the
monomer substrate (i.e. the cyclic ester) on catalyst aggregation
disruption is unknown. However, the dimeric compound 1 has
a significantly lower catalytic activity compared with the anal-
ogous monomeric Yb complex {(THF)2Yb[OC6H2(2,4-But)(6-
15
CH2)]2NCH2CH2NMe2} but has comparable activity to other
dimeric systems, such as [L*Yb(THF)2]2 {L* = 2,2¢-methylene-
bis(6-butyl-4-methylphenoxo)}.54
L-Lactide polymerization. Although a number of ROP studies
of e-caprolactone investigating lanthanide (II) bis(phenolate)
systems have been reported14,15,54 the present study is the first to
report in any detail on the ROP of L-lactide utilizing divalent
lanthanide bis(phenolate) complexes. Compounds 1, 5 and 7 were
evaluated as catalysts in the ROP of L-lactide (Table 4) and the
e-Caprolactone polymerization. Compounds 1, 4, 5 and 7
polymerized e-caprolactone quantitatively at room temperature
in toluene under 45 minutes, in a variety of initiator to monomer
ratios (1:100–1:300) (Table 3). In many cases, the observed molecu-
lar weights (Mn), determined by GPC, were greater than predicted
by theory; likely caused by catalyst deactivation via the initial
redox chemistry between the divalent lanthanide precursor and the
monomer substrate.14 This should lead to lower concentrations of
active catalyst and consequently formation of higher molecular
weight polymers. In a some cases (entries 1 and 5; Table 3)
the polydispersity indices (PDIs) are quite low (1.22 and 1.11),
suggesting that these catalysts 1 and 5 (which only differ in Ln
metal) do not significantly promote side reactions (such as chain
transfer or transesterification reactions58).
Table 4 Results from the polymerization of L-lactidea
Entry Initiator Time/min [M]0/[I]0 % Conv.b 10-4 Mn(obs.)c PDIc
1
2
3
4
5
6
1e
5e
1
24 h
24 h
60
30
50
100
100
100
100
100
100
6.34
9.65
86.6
95.8
93.6
94.7
—
—
1.61
2.61
1.96
3.1
—
—
1.13
1.56
1.25
1.3
5
5d
7
45
a Polymerization conditions: toluene as solvent (except stated otherwise),
T = 70 ◦C. b Determined by 1H NMR analysis. c Dertemined by GPC
analysis in toluene with calibration to polystyrene standards. d Solvent =
THF. e T = 25 ◦C.
1440 | Dalton Trans., 2009, 1436–1445
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reactions required heating (70 ◦C) to achieve respectable polymer
conversions (> 85%). In general, GPC data revealed narrower
PDIs than those obtained in ROP of e-caprolactone (1.13–1.56),
which reflects superior single-site control by the catalysts and
may be a function of the slower polymerization rates observed
in ROP lactide reactions. These differences in substrate reactivity
have been previously observed61 although no explanation was
provided. Again samarium containing catalysts exhibited faster
polymerization rates than the analogous ytterbium compounds
(entries 3 and 4) and the polymerization reactions were faster in
toluene than those performed in THF.
Finally, pseudo-first order kinetics in lactide were observed from
ln{[LA0]/[LA]t} vs. time plots from systems employing catalysts 1
and 5 at initiators:monomer ratios of 1:50 and 1:100 respectively
(kobs(I) = 0.0416 min-1 and kobs(II) = 0.0321 min-1) (Fig. 3).
These data suggest a mechanism which requires one molecule
of lactide per active site of catalyst (i.e. one molecule LA per
monomeric lanthanide catalyst or two molecules per dimeric
lanthanide catalyst).
to polymerize. Finally, kinetic studies revealed that these reac-
tions propagate with pseudo-first order kinetics with respect to
L-lactide.
Experimental
General
Since all lanthanide complexes described herein are air- and
moisture-sensitive all manipulations were performed under ni-
trogen atmospheres using typical Schlenk line and glove-box
techniques. Solvents were dried and purified by distillation
under nitrogen from sodium or potassium metal mixed with
benzophenone while deuterated solvents (C4D8O, C6D6 and C7D8)
were dried over sodium metal and purified by vacuum transfer.
The lanthanide precursors [Ln(N(SiMe3)2)2(THF)2] (Ln = Sm,
Yb) were synthesized by the reported procedure.4 The starting
materials used for the synthesis of the ligands H2LR {LR
=
[-OC6H2(2,4-R)(6-CH2)]2NCH2CH2X} (R = Me, But, Pet)31,32
were purchased from Acros and used without further purification.
Melting points were obtained from sealed capillaries on a Mel-
Temp apparatus and are uncorrected. IR spectra (4000–450 cm-1)
were recorded as KBr Nujol mulls on an ATI Mattson Genesis
1
Series FTIR Spectrometer. The H and 13C NMR spectra were
recorded on a Bruker AVANCE-500 NMR spectrometer and
referenced to C6D5H or C4D7HO or CHCl3 (1H) and C6D6 or
C4D8O or CDCl3 (13C). Elemental analyses (sealed ampoules
under inert atmosphere) were performed by Midwest Microlab,
Indianapolis, IN.
Synthesis of H2L3b. A mixture of 2,4-di-tert-pentylphenol
(10.00 g, 42.7 mmol), 2-methoxyethylenediamine (2.00 mL,
21.3 mmol) and 37% aqueous formaldehyde (4.40 mL, 58.7 mmol)
in methanol (10 mL) was refluxed for 20 h. The mixture was
then cooled in the freezer for 4 h and the supernatant solution
was decanted. The residue was triturated with ice cold methanol,
filtered, washed thoroughly with cold methanol and dried in vacuo
to give the bis-adduct H2L3b (7.24 g, 60%) as a white powder. Mp
140–143 ◦C. (Found: C, 78.4; H, 10.9; N, 2.6. C37H61NO3 requires
Fig. 3 Pseudo-first order kinetics plot of L-lactide polymerization
catalyzed by compounds 1 and 5 in toluene at 70 ◦C. (I) [1]:[LA] = 1:50,
(II) [5]:[LA] = 1:100.
C, 78.3; H, 10.9; N, 2.6%); nmax/cm-1 3370 s, 2623 m, 2036w, 1600w,
˜
Conclusions
1299 s, 1261 m, 1211w, 1095 s, 1060 m, 1018 s, 872 m, 802 m, 725 m,
601w (Nujol); MS: m/z 568.5 [M + H]+. dH (500.1 MHz, CDCl3,
298 K) 0.63 (6H, t, J 7.5 Hz, CCH2CH3), 0.65 (6H, t, J 7.6 Hz,
CCH2CH3), 1.25 (12H, s, C(CH3)2), 1.33 (12H, s, C(CH3)2), 1.57
(4H, q, J 7.5 Hz, CCH2CH3), 1.92 (4H, q, J 7.6 Hz, CCH2CH3),
2.72 (2H, t, J 5.2 Hz, NCH2CH2O), 3.45 (3H, s, OCH3), 3.51 (2H,
t, J 5.2 Hz, NCH2CH2O), 3.78 (4H, s, ArCH2N), 6.83 (2H, s,
Ar), 7.13 (2H, s, Ar), 8.54 (2H, s, OH), dC, (125.8 MHz, CDCl3,
298K) 9.6 (CCH2CH3), 10.0 (CCH2CH3), 28.2 (C(CH3)2Et),
29.0 (C(CH3)2Et), 33.2 (C(CH3)2Et), 37.5 (CCH2CH3), 37.6
(CCH2CH3), 38.9 (C(CH3)2Et), 51.7 (NCH2CH2O), 58.5
(NCH2CH2O), 59.2 (OCH3), 71.8 (ArCH2N), 121.8 (arom-CH),
125.9 (arom-CPet), 126.0 (arom-CH), 134.6 (arom-CPet), 139.3
(arom-CCH2N), 153.1 (arom-CO).
A
number of novel divalent ytterbium and samarium
bis(phenolate) complexes have been synthesized by straight-
forward amine elimination reactions. The less sterically demanding
bis(phenolate) ligands were found to produce trivalent lanthanide
complexes. However, more bulky ligand systems produced isolable
divalent lanthanide species which exist as either dimers (Ln =
Yb) or monomers in solution (Ln = Yb, Sm); as determined
by the intimate relationship between metal-ion size and steric
demands of the ligands. X-Ray crystallographic characterization
of compounds 1 and 3 revealed dimeric ytterbium(II) compounds
with symmetrically-bridging bis(phenolate) ligands; supporting
the proposed formulations ascertained by solution NMR spectro-
scopic studies. A selection of these divalent lanthanide compounds
has been evaluated as catalysts in the ROP of e-caprolactone
and L-lactide cyclic esters. In most cases quantitative polymer
conversion was established, making these catalysts the first group
of divalent lanthanide bis(phenolate) complexes to display such
activity towards L-lactide, the more difficult of the cyclic esters
Synthesis of H2L3c. Following a similar procedure for the
synthesis H2L3b, a mixture of 2,4-di-tert-pentylphenol (10.00 g,
42.7 mmol), N,N-diethylethylenediamine (3.00 mL, 21.3 mmol)
and 37% aqueous formaldehyde (4.40 mL, 58.7 mmol) in methanol
(10 mL) yielded a white powder of H2L3c (7.58 g, 58%). Mp
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101–102 ◦C. (Found: C, 79.1; H, 11.1; N, 4.8. C40H68N2O2 requires
3.93 (2H, d, J 13.0 Hz, ArCH2N), 6.19 (2H, s, Ar), 7.08 (2H, s,
Ar), dC{H} (125.8 MHz, C4D8O, 298K) 20.1 (NCH2CH2CH2N),
29.1 (CMe3), 31.2 (CMe3), 32.9 (CMe3), 34.6 (CMe3), 44.6
(NMe2), 46.2 (NCH2CH2CH2N), 58.7 (NCH2CH2CH2N), 59.8
(ArCH2N), 121.9 (arom-CH), 125.6 (arom-CBut), 127.5 (arom-
CH), 129.9 (arom-CBut), 133.9 (arom-CCH2N), 165.5 (arom-
CO).
C, 78.9; H, 11.3; N, 4.6%); nmax/cm-1 3345 s, 2730w, 2356 m, 1600
˜
m, 1261 m, 1095 s, 1049 m, 1022 s, 872w, 802 s, 725 m, 609 w (Nu-
jol); MS: m/z 609.5 [M + H]+. dH (500.1 MHz, C4D8O, 298 K) 0.65
(6H, t, J 7.0 Hz, CCH2CH3), 0.70 (6H, t, J 8.1 Hz, CCH2CH3),
1.51 (6H, t, J 7.0 Hz, N(CH2CH3)2), 1.29 (12H, s, C(CH3)2),
1.41 (12H, s, C(CH3)2), 1.63 (4H, q, J 7.0 Hz CCH2CH3),
1.95 (4H, q, J 8.1 Hz, CCH2CH3), 2.68-2.63 (8H, overlapping
signals of NCH2CH2N and N(CH2CH3)2), 3.75 (4H, s, ArCH2N),
6.88 (2H, s, Ar), 7.12 (2H, s, Ar), 9.7 (2H, s, OH), dC{H}
(125.8 MHz, C4D8O, 298K) 9.6 (CCH2CH3), 10.0 (CCH2CH3),
28.14 (C(CH3)2Et), 29.0 (C(CH3)2Et), 33.2 (C(CH3)2Et),
34.0 (C(CH3)2Et), 37.6 (CCH2CH3), 38.9 (CCH2CH3), 45.7
(NCH2CH3), 48.1 (NCH2CH3), 49.2 (NCH2CH2N), 50.4
(NCH2CH2N), 57.1 (ArCH2N), 121.6 (arom-CH), 125.9 (arom-
CPet), 127.2 (arom-CH), 124.4 (arom-CPet), 138.6 (arom-
CCH2N), 155.5 (arom-CO).
Synthesis of [YbL3a]2 (3). Using
for the synthesis of 1, hexanes solution (10 mL) of
a similar procedure
a
[Yb(N(SiMe3)2)2(THF)2] (0.64 g, 1.00 mmol) and a colorless
hexanes solution (5 mL) of H2L3a (0.58 g, 1.00 mmol) yielded a
greenish-brown solution from which 3 (0.59 g, 78%) precipitated
as dark green needle-like crystals. These were isolated from the
solution and dried in vacuo. Mp 224–226 ◦C. (Found: C, 60.7;
H, 8.3; N, 3.7. C76H124N4O4Yb2 requires C, 61.3; H, 8.5; N,
3.9%); nmax/cm-1 1739 w, 1600 w, 1299 w, 1261 s, 1214 w, 1161
˜
w, 1095 s, 1022 s, 933 w, 872 m, 802 s, 725 m, 602 m (Nujol);
dH (500.1 MHz, C4D8O, 298 K) 0.61–0.64 (12H, m, overlapping
signals of o-C(CH3)2CH2CH3 and p-C(CH3)2CH2CH3), 1.20
(12H, s, C(CH3)2CH2CH3), 1.41 (12H, s, C(CH3)2CH2CH3),
1.55 (6H, s, NMe2), 1.72 (4H, q, J 2 Hz, C(CH3)2CH2CH3),
1.81 (4H, q, J 2 Hz, C(CH3)2CH2CH3), 2.34 (2H, t, J
5.5 Hz, NCH2CH2N), 2.87 (2H, t, J 5.5 Hz, NCH2CH2N),
2.89 (2H, d, J 12.1 Hz, ArCH2N), 4.14 (2H, d, J 12.1 Hz,
ArCH2N), 6.74 (2H, s, Ar), 6.99 (2H, s, Ar), dC{H} (125.8 MHz,
C4D8O, 298K) 6.9 (C(CH3)2CH2CH3), 7.5 (C(CH3)2CH2CH3),
25.4 (C(CH3)2CH2CH3), 25.9 (C(CH3)2CH2CH3), 26.9
(C(CH3)2CH2CH3), 30.5 (C(CH3)2CH2CH3), 34.8 (C(CH3)2-
CH2CH3), 36.5 (C(CH3)2CH2CH3), 42.9 (NMe2), 44.9
(NCH2CH2N), 57.6 (NCH2CH2N), 62.7 (ArCH2N), 123.3
(arom-CH), 125.3 (arom-CPet), 126.2 (arom-CH), 126.6
(arom-CPet), 130.8 (arom-CCH2N), 164.2 (arom-CO).
Synthesis of [YbL2c]2 (1). A deep orange hexanes solution
(10 mL) of [Yb(N(SiMe3)2)2(THF)2] (0.64 g, 1.00 mmol) was
treated with a colorless hexanes solution (9 mL) of H2L2c (0.55 g,
1.00 mmol) which resulted instantly in a coffee brown solution.
Stirring was continued for 4 h at room temperature and compound
1·C7H8 (0.96 g, 75%) was obtained as cubic dark brown crystals
from a hexanes/toluene mixture, which was isolated and dried
in vacuo. Mp 220–222 ◦C. (Found: C, 60.2; H, 7.9; N, 4.0.
C72H116N4O4Yb2 requires C, 59.7; H, 8.1; N, 3.9%); nmax/cm-1
˜
1738 w, 1655 m, 1260 s, 1623 m, 1260 s, 1091 s, 1022 s, 866 w,
800 s, 697 w, 570 w, 546 m (Nujol); dH (500.1 MHz, C4D8O,
298K) 0.66 (6H, t, J 7 Hz, N(CH2CH3)2), 1.24 (18H, s, But),
1.47 (18H, s, But), 2.05 (2H, t, J 6.9 Hz, NCH2CH2N), 2.24 (4H,
q, J 7 Hz, N(CH2CH3)2), 2.25 (2H, t, J 6.9 Hz, NCH2CH2N),
2.89 (2H, d, J 12.3 Hz, ArCH2N), 4.55 (2H, d, J 12.3 Hz,
ArCH2N), 6.87 (2H, s, Ar), 7.09 (2H, s, Ar), dH (500.1 MHz,
C6D6) 0.67 (6H, t, J 7.0 Hz, N(CH2CH3)2), 1.34 (6H, t, J
6.7 Hz, N(CH2CH3)2), 1.36–1.80 (80H, overlapping signals of
But and NCH2CH2N), 1.95 (4H, q, J 7.0 Hz, N(CH2CH3)2),
2.97 (4H, q, J 6.7 Hz, N(CH2CH3)2), 3.25 (2H, d, J 12.2 Hz,
ArCH2N), 3.47 (2H, d, J 12.2 Hz, ArCH2N), 4.88 (2H, d, J
12.2 Hz, ArCH2N), 5.68 (2H, d, J 12.2 Hz, ArCH2N), 7.03 (2H, s,
Ar), 7.14 (2H, s, Ar), 7.35 (2H, s, Ar), 7.54 (2H, s, Ar), dC{H}
(125.8 MHz, C4D8O, 298 K) 5.3 (N(CH2CH3)2), 27.7 (CMe3),
29.8 (CMe3), 33.2 (CMe3), 37.7 (CMe3), 41.0 ((N(CH2CH3)2),
44.7 (NCH2CH2N), 49.2 (NCH2CH2N), 62.3 (ArCH2), 120.9
(arom-CH), 122.1 (arom-CBut), 124.4 (arom-CH), 129.0 (arom-
CCH2N), 132.6 (arom-CBut), 164.2 (arom-CO).
Synthesis of [YbL3b]2 (4). Using a similar procedure to
that described for the synthesis of 1, a reaction between
[Yb(N(SiMe3)2)2(THF)2] (0.64 g, 1.00 mmol) and H2L3b (0.57 g,
1.00 mmol) in hexanes (25 mL) yielded a yellow microcrystalline
powder of 4 (0.61 g, 83%). Mp 222–223 ◦C. (Found: C, 60.1;
H, 8.1; N, 1.9. C74H118N2O6Yb2 requires C, 60.2; H, 8.3; N,
1.9%); nmax/cm-1 3370 s, 2623 m, 2036w, 1600w, 1299 s, 1261 m,
˜
1211w, 1095 s, 1060 m, 1018 s, 872 m, 802 m, 725 m, 601 w
(Nujol); dH (500.1 MHz, C4D8O, 298 K) 0.62–0.65 (12H, m, over-
lapping signals of o-C(CH3)2CH2CH3 and p-C(CH3)2CH2CH3),
1.22 (12H, s, C(CH3)2CH2CH3), 1.36 (12H, s, C(CH3)2CH2CH3),
1.56 (4H, q, J 5.1 Hz, C(CH3)2CH2CH3), 1.78 (4H, q, J 5.5 Hz,
C(CH3)2CH2CH3), 2.59 (2H, t, J 4.1 Hz, NCH2CH2O), 2.81 (3H,
s, OMe), 2.93 (2H, d, J 11.1 Hz, ArCH2N), 2.98 (2H, t, J 4.1 Hz,
NCH2CH2O), 6.7 (2H, s, Ar), 7.13 (2H, s, Ar), dC{H} (125.8 MHz,
C4D8O, 298K) 7.2 (C(CH3)2CH2CH3), 7.7 (C(CH3)2CH2CH3),
25.6 (C(CH3)2CH2CH3), 25.8 (C(CH3)2CH2CH3), 27.1 (C(CH3)2-
CH2CH3), 27.2 (C(CH3)2CH2CH3), 30.6 (C(CH3)2CH2CH3), 34.9
(C(CH3)2CH2CH3), 35.7 (NCH2CH2O), 38.8 (NCH2CH2O), 55.6
(OCH3), 62.3 (ArCH2N), 122.0 (arom-CPet), 123.3 (arom-CH),
125.4 (arom-CH), 126.9 (arom-CPet), 131.2 (arom-CCH2N),
164.1 (arom-CO).
Synthesis of [YbL2b]2 (2). To a hexanes solution (6 mL) of
[Yb(N(SiMe3)2)2(THF)2] (0.32 g, 0.50 mmol) was added a colorless
hexanes solution (5 mL) of H2L2b (0.27 g, 0.50 mmol) which
led to an instant red solution. The reaction mixture was stirred
for 4 h at room temperature to afford 2 (0.23 g, 65%) as an
orange precipitate which was◦separated from the supernatant and
dried in vacuo. Mp 241–243 C. (Found: C, 60.1; H, 9.1; N, 4.1.
C70H112N4O4Yb2 requires C, 59.2; H, 7.9; N, 3.9%); nmax/cm-1
˜
2370w, 2342w, 1603 s, 1314w, 1261 s, 1093 s, 1024 s, 871 m, 801 s,
680w, 580 m (Nujol); dH (500.1 MHz, C4D8O, 298 K) 1.28 (18H, s,
But), 1.41 (18H, s, But), 1.50 (2H, m, NCH2CH2CH2N), 1.79 (2H,
t, J 7.1 Hz, NCH2CH2CH2N), 1.84 (6H, s, NMe2), 2.32 (2H, t,
J 7.1 Hz, NCH2CH2CH2N), 2.75 (2H, d, J 13.0 Hz, ArCH2N),
Synthesis of [SmL2c] (5). Using a similar procedure to that
described for the synthesis of the divalent ytterbium complexes,
a purple hexanes solution (10 mL) of [Sm(N(SiMe3)2)2(THF)2]
1442 | Dalton Trans., 2009, 1436–1445
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(0.62 g, 1.00 mmol) was stirred with a colorless hexanes solution
(5 mL) of H2L2c (0.55 g, 1.00 mmol) to afford coffee-brown crystals
of 5 (0.58 g, 83%), which were isolated from the supernatant
C4D8O, 298K) 2.2 (C(CH3)2CH2CH3), 5.9 (C(CH3)2CH2CH3),
22.5 (C(CH3)2CH2CH3), 26.6 (C(CH3)2CH2CH3), 26.9 (C(CH3)2-
CH2CH3), 27.2 (C(CH3)2CH2CH3), 29.8 (C(CH3)2CH2CH3), 32.8
(C(CH3)2CH2CH3), 34.6 (N(CH2Me)2), 35.7, (N(CH2Me)2), 37.1
(NCH2CH2N), 37.7 (NCH2CH2N), 44.7 (ArCH2), 103.6 (arom-
CH), 120.6 (arom-CPet), 122.9 (arom-CH), 127.7 (arom-CPet),
133.7 (arom-CCH2N), 167.5 (arom-CO).
◦
and dried in vacuo. Mp 192 C. (Found: C, 60.0; H, 8.3; N, 4.0.
C36H58N2O2Sm requires C, 61.7; H, 8.3; N, 4.0%); nmax/cm-1 1775w,
˜
1607 m, 1442 m, 1350w, 1367w, 1323w, 1268 s, 1163w, 1098 s,
1027 s, 931w, 876 s, 803 s, 740 s, 645w (Nujol); dH (500.1 MHz,
C4D8O, 298K) 0.19 (18H, s, But), 0.33 (18H, s, But), 0.87 (6H,
t, J 10 Hz, N(CH2CH3)2), 1.21 (2H, d, J 11.5 Hz, ArCH2), 1.37
(4H, q, J 10.2 Hz, N(CH2CH3)2), 3.53 (4H, m, NCH2CH2N), 4.23
(2H, d, J 11.5 Hz, ArCH2), 6.3 (2H, s, Ar), 7.2 (2H, s, Ar), dC{H}
(125.8 MHz, C4D8O, 298K) 11.6 (N(CH2CH3)2), 30.4 (CMe3),
30.9 (CMe3), 32.4 (CMe3), 32.9 (CMe3), 35.8 (NCH2CH2N),
36.1 (NCH2CH2N), 40.5 ((N(CH2CH3)2), 56.3 (ArCH2N), 121.5
(arom-CH), 122.2 (arom-CBut), 122.4 (arom-CH), 123.6 (arom-
CCH2N), 125.0 (arom-CBut), 134.2 (arom-CO).
Crystal data†
Data collection was by means of a Bruker Apex I (for 1) or
Oxford Diffraction Gemini (3) diffractometer. Following multi-
scan absorption corrections the structures were solved by direct
methods and refined by full-matrix refinement on F2 using the
SHELXL 97 program.62 Hydrogen atoms were placed in calculated
positions and refined as part of riding models.
Synthesis of [YbL3c] (6). Using a similar procedure to that
described for the synthesis of 1, a hexanes solution (10 mL) of
[Yb(N(SiMe3)2)2(THF)2] (0.64 g, 1.00 mmol) and a colorless hex-
anes solution (5 mL) of H2L3c (0.61 g, 1.00 mmol) was stirred for 9 h
and all volatiles removed to yield a brown microcrystalline powder
of 6 (0.62 g, 80%). Mp 180–181 ◦C. (Found: C, 61.4; H, 8.6; N, 3.5.
1. C72H116N4O4Yb2, C7H8, M = 1539.9, triclinic, a =
˚
˚
˚
9.8103(10) A, b = 14.4406(10) A, c = 28.081(3) A, a = 85.744(2),
3
˚
3
b = 86.711(2), g = 79.599(2), V = 3898.0(6) A , T = 173(2) K,
-1
¯
space group P1, specimen: 0.45 ¥ 0.36 ¥ 0.25 mm , m = 2.432 mm ,
-3
˚
Dc (Z = 2) = 1.312 Mg m , l(Mo Ka) = 0.71073 A, Tmin/max
=
0.77, 2qmax = 56.5◦. A total of 44182 reflections was collected, of
which 17646 were unique (Rint = 0.023); R1 = 0.046 (I > 2s(I)),
C40H66N2O2Yb requires C, 61.6; H, 8.5; N, 3.6%); nmax/cm-1 1741w,
˜
-3
˚
wR2 = 0.125 (all data), S = 1.09; |rmax| = 2.84 e A . The methyl
1612w, 1301 m, 1261 s, 1220w, 1165w, 1100 s, 1024 s, 933w, 875 m,
812 s, 725 m, 603 m (Nujol); dH (500.1 MHz, C4D8O, 298 K) 0.62–
0.66 (12H, m, overlapping signals of o-C(CH3)2CH2CH3 and p-
C(CH3)2CH2CH3), 1.10 (4H, q, J 7.1 Hz, C(CH3)2CH2CH3), 1.21
(12H, s, C(CH3)2CH2CH3), 1.23 (6H, t, J 6.2 Hz, N(CH2CH3)2),
1.34 (12H, s, C(CH3)2CH2CH3), 1.72 (4H, q, J 7.1 Hz,
C(CH3)2CH2CH3), 1.91 (4H, q, J 6.2 Hz, N(CH2CH3)2), 2.26 (2H,
t, J 7.0 Hz, NCH2CH2N), 2.88 (2H, t, J 7.0 Hz, NCH2CH2N),
3.08 (2H, d, J 12.3 Hz, ArCH2N), 4.21 (2H, d, J 12.3 Hz,
ArCH2N), 6.66 (2H, s, Ar), 7.07 (2H, s, Ar), dC{H} (125.8 MHz,
C4D8O, 298K) 5.1 (C(CH3)2CH2CH3), 6.8 (C(CH3)2CH2CH3),
7.0 (C(CH3)2CH2CH3), 7.3 (C(CH3)2CH2CH3), 7.4 (C(CH3)2-
CH2CH3), 7.5 (C(CH3)2CH2CH3), 7.7 (C(CH3)2CH2CH3), 8.0
(C(CH3)2CH2CH3), 25.7 (N(CH2CH3)2), 43.4 (N(CH2CH3)2),
48.1 (NCH2CH2N), 49.4 (NCH2CH2N), 62.5 (ArCH2N), 119.9
(arom-CH), 123.4 (arom-CPet), 125.3 (arom-CH), 130.8 (arom-
CPet), 151.4 (arom-CCH2N), 164.3 (arom-CO).
groups of two tert-butyl methyl groups and the toluene molecule
are each disordered over two sites with site occupancy factors for
one tert-butyl methyl group (14n) and for that of the toluene set
as being identical (0.62(2) and 1–0.62(2)) after trial refinement
showed them to be not significantly different. The site occupancy
factors for the two components of the other disordered tert-butyl
methyl group (44n) refined to 0.76(1) and 1–0.76(1). Atoms of
the minor components of tert-butyl methyl (44n) and also those
of the solvent toluene were refined with isotropic displacement
parameters. The C–C bond lengths of disordered atoms were
restrained to ideal values and the carbon atom of one methyl
group (C146) was refined with isotropic displacement parameters.
Since the remaining atoms showed no unacceptable anisotropy, no
further disordering was attempted. The atoms of both components
of the solvent toluene were refined as rigid groups.
3. C76H124N4O4Yb2,
M
=
1503.87, monoclinic,
˚
a
=
Synthesis of [SmL3c] (7). Using a similar procedure to that
described for the synthesis of 1, a purple hexanes solution (10 mL)
of [Sm(N(SiMe3)2)2(THF)2] (0.62 g, 1.00 mmol) and a colorless
hexanes solution (5 mL) of H2L3c (0.61 g, 1.00 mmol) were
combined and this mixture stirred for 24 h to yield deep purple
crystals of 7 (0.61 g, 80%) that were isolated from solution and
15.7477(3) A, b = 32.4941(5) A, c = 16.6133(3) A, b = 117.374(2),
˚
˚
3
˚
V = 7549.2(3) A , T = 100(2) K, space group P21/n, specimen:
0.10 ¥ 0.04 ¥ 0.02 mm3, m = 4.81 mm-1, Dc (Z = 2) = 1.323 Mg m-3,
l(Cu Ka) = 1.54184 A, Tmin/max = 0.42, 2qmax = 135.2◦. A total
˚
of 92938 reflections was collected, of which 13445 were unique
(Rint = 0.115); R1 = 0.060 (I > 2s(I)), wR2 = 0.188 (all data), S =
◦
-3
˚
dried in vacuo. Mp 224–225 C. (Found: C, 63.7; H, 9.0; N, 3.8.
0.88; |rmax| = 1.29 e A . The carbon atoms of one phenol group
C40H66N2O2Sm requires C, 63.4; H, 8.8; N, 3.7%); nmax/cm-1 1604
(no. 2), including the tert-pentyl methyl groups, another tert-pentyl
methyl group (44n) and one coordinated NMe2 group, are each
disordered over two sites, with site occupancy factors set at 0.5
after trial refinement. Disordered carbon atoms and those of the
tert-pentyl methyl group (14n) and tert-butyl groups were refined
with isotropic displacement parameters and their geometries were
restrained to ideal values. The pendant atoms of ring 3 exhibited
somewhat elongated ellipsoids but a satisfactory disordered model
could not be obtained. These atoms were subsequently refined with
isotropic displacement parameters with geometries restrained to
ideal values. As a result of this disorder and subsequent restraints,
˜
m, 1413 w, 1380 m, 1362w, 1324 w, 1262 s, 1203 m, 1165 w,
1094 m, 1023 m, 933 w, 915 w, 880 s, 813 s, 7420 s, 653 w, 521 s
(Nujol); dH (500.1 MHz, C4D8O, 298 K) -6.21 (6H, t, J 5.3 Hz
C(CH3)2CH2CH3), -0.83 (6H, t, J 5.1 Hz, C(CH3)2CH2CH3),
-0.03 (12H, s, C(CH3)2CH2CH3), 0.16 (12H, s, C(CH3)2CH2CH3),
0.21 (6H, t, J 7.1 Hz, N(CH2CH3)2), 0.48 (8H, m, overlapping
signals of o-C(CH3)2CH2CH3 and p-C(CH3)2CH2CH3), 0.51
(4H, q, J 7.1 Hz, N(CH2CH3)2), 1.26 (4H, m, NCH2CH2N),
1.62 (2H, d, J 11.1 Hz, ArCH2N), 1.72 (2H, d, J 11.1 Hz,
ArCH2N), 5.22 (2H, s, Ar), 5.46 (2H, s, Ar), dC{H} (125.8 MHz,
This journal is
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Dalton Trans., 2009, 1436–1445 | 1443
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View Article Online
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1
time an aliquot which was analyzed by H NMR spectroscopy
(CDCl3) to establish polymer conversion. At the conclusion of the
polymerization the reaction mixture was quenched with 5% wt.
HCl-MeOH solution (ca. 1 ml) before being analyzed by GPC to
determine molecular weight distribution data.
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reaction a 5% wt. HCl-MeOH solution (ca. 1 mL) quenched the
reaction mixture and precipitated the polymer. The polymer was
filtered off and dried prior to GPC analyses.
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Kinetics procedure. Lactide (0.200 g; 1.388 mmol) was dis-
solved in toluene (10 mL) and heated to 70 ◦C. To this an appro-
priate amount of the catalyst was added (0.0201 g; 0.0278 mmol
of 1; Fig. 3) and aliquots of the polymerization reaction were
quenched immediately with hexanes at the appropriate intervals.
The volatiles were subsequently evaporated and the amount of
formed polymer was analyzed by 1H NMR spectroscopy.
Acknowledgements
The authors wish to express sincere gratitude to Professor Lothar
Stahl for numerous useful suggestions regarding this research. In
addition, the author gratefully acknowledge: Professor Edward
Kolodka and Mr Kirtipal Barse UND for assistance with GPC
analyses. Financial support from ND EPScOR program is grate-
fully acknowledged.
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