Synthesis, structure and oxidation of new ytterbium(ii) bis(phenolate) compounds and their catalytic activity towards ε-caprolactone

Article abstract of DOI:10.1039/b613409b

Two ytterbium(ii) bis(phenolate) complexes, [L^RYb] where R = NMe₂ 1 and OMe 2, have been synthesized and characterized, with 1 being structurally defined to be a dimeric species with an unsymmetrical coordination of the bis(phenolate) ligand which is preserved in solution. Both 1 and 2 have been oxidized by a variety of oxidants (AgX, ROH) to form heteroleptic ytterbium(iii) bis(phenolate) complexes: [(L^NMe2) YbPF₆] (3), [(L^NMe2)YbOSO₂CF₃(thf)] (4), [(L^NMe2)YbOBu^t] (5), [(L^NMe2)YbOPh] (6), [(L^OMe)YbOPh] (7). Compound 4 has been structurally characterized as having a quasi-octahedral environment around ytterbium, with significant inter species hydrogen bonding between CH_x and triflate fluorine atoms. Ligand exchange between Yb(N(SiMe₃)₂)₃(thf) ₂ and H₂L^R yielded [(L^NMe2) YbN(SiMe₃)₂] (8) and [(L^OMe)YbN(SiMe ₃)₂] (9), while metathesis from YbI₂(thf) ₂ and K₂L^OMe reproducibly afforded the surprising oxidized product [(L^OMe)₂YbK(dme)₂] (10), which was structurally characterized as having a distorted octahedral environment around the ytterbium(iii) centre. Compounds 1-9 were used to polymerize ε-caprolactone at room temperature in toluene, with only compounds 1, 2, 8 and 9 exhibiting significant catalytic activities. The polycaprolactone formed in these reactions was generally of high molecular weight and polydispersities <1.90 in all but one case. The Royal Society of Chemistry.

Full text of DOI:10.1039/b613409b

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www.rsc.org/dalton | Dalton Transactions
Synthesis, structure and oxidation of new ytterbium(II) bis(phenolate)
compounds and their catalytic activity towards e-caprolactone†
Ewan E. Delbridge,*a Divine T. Dugah,a Chassidy R. Nelson,a Brian W. Skeltonb and Allan H. Whiteb
Received 15th September 2006, Accepted 20th October 2006
First published as an Advance Article on the web 7th November 2006
DOI: 10.1039/b613409b
Two ytterbium(II) bis(phenolate) complexes, [LRYb] where R = NMe2 1 and OMe 2, have been
synthesized and characterized, with 1 being structurally defined to be a dimeric species with an
unsymmetrical coordination of the bis(phenolate) ligand which is preserved in solution. Both 1 and 2
have been oxidized by a variety of oxidants (AgX, ROH) to form heteroleptic ytterbium(III)
NMe
NMe
2
t
bis(phenolate) complexes: [(LNMe )YbPF6] (3), [(L
)YbOSO2CF3(thf)] (4), [(L
)YbOBu ] (5),
2
2
[(LNMe )YbOPh] (6), [(LOMe)YbOPh] (7). Compound 4 has been structurally characterized as having a
quasi-octahedral environment around ytterbium, with significant inter species hydrogen bonding
between CHx and triflate fluorine atoms. Ligand exchange between Yb(N(SiMe3)2)3(thf)2 and H2LR
2
yielded [(LNMe )YbN(SiMe3)2] (8) and [(LOMe)YbN(SiMe3)2] (9), while metathesis from YbI2(thf)2 and
2
K2LOMe reproducibly afforded the surprising oxidized product [(LOMe)2YbK(dme)2] (10), which was
structurally characterized as having a distorted octahedral environment around the ytterbium(III)
centre. Compounds 1–9 were used to polymerize e-caprolactone at room temperature in toluene, with
only compounds 1, 2, 8 and 9 exhibiting significant catalytic activities. The polycaprolactone formed in
these reactions was generally of high molecular weight and polydispersities <1.90 in all but one case.
precursors have been synthesized by reaction of the appropriate
Introduction
trivalent homoleptic lanthanide amide with an equivalent LH2
ligand via a protiolytic ligand exchange (or transamination)
reaction (eqn (1), R = Me, Rꢀ = H or R, Rꢀ = Me).19–21,25
Divalent lanthanide compounds have been known for many years,
the most familiar example being samarium diiodide which has
found countless applications in organic synthesis.1–4 The ability of
divalent samarium and ytterbium compounds to reduce a number
of organic and inorganic substrates illustrates a unique versatility
which can provide simple and elegant synthetic pathways to a
number of heteroleptic trivalent lanthanide compounds.5–7 These
synthetic, redox pathways are superior to traditional metathesis
salt elimination pathways, since product separation is usually more
straightforward and salt inclusion compounds,8 common among
the lanthanides, are avoided. Furthermore, synthesis of an array
of heteroleptic trivalent lanthanide compounds can be readily
accessed from homoleptic divalent precursors by simple reaction
with one of a number of oxidizing agents.9–12 Simple synthetic
routes to heteroleptic lanthanide(III) compounds are of interest,
these species having demonstrated great utility in catalysis13–15 and
luminescence16,17 studies.
More recently there has been interest in ring-opening polymer-
ization reactions of cyclic esters such as lactide and e-caprolactone
by heteroleptic lanthanide compounds.18–25 To date, many of the
studies have involved the synthesis of the lanthanide compounds
which have the general formula LMX, where L = a dianionic
stationary ancillary ligand, M = a trivalent lanthanide metal
and X = a monoanionic initiating group. Generally these catalyst
[Ln(N(SiR2Rꢀ)2)3] + LH2
→ [LLnN(SiR2Rꢀ)2] + 2 HN(SiR2Rꢀ)2
(1)
Modification of the initiating group requires the synthesis of
a different homoleptic starting material. In addition to ligand
exchange pathways, Shen and co-workers recently reported the
successful synthesis of ytterbium(III) bis(phenolate) complexes,
free from salt inclusion complications, by metathesis.26 Their
work, however, represents an exception to the general rule that
metathetical synthesis of the lanthanide complexes is generally
problematic.
Presented here is an alternative synthetic pathway to an LMX
motif which arises from oxidation (by XY, where Y = Ag, H) of a
homoleptic divalent LYb species. This study focuses on the synthe-
sis and characterization of two new divalent ytterbium phenolate
compounds and their subsequent oxidation to trivalent ytterbium
heteroleptic compounds. Both divalent and trivalent ytterbium
species presented herein have been examined in their reactivity
towards the ring-opening polymerization of e-caprolactone.
Results and discussion
Synthesis of divalent ytterbium compounds
aChemistry Department, University of North Dakota, Grand Forks, ND,
58202-9024, USA. E-mail: edelbridge@chem.und.edu; Fax: +1 701 777
2331; Tel: +1 701 777 2495
The readily prepared H2LR ligands27,28 (R = NMe2, OMe) were
treated with [Yb(N(SiMe3)2)2(thf)2]29 in 1 : 1 molar ratio in hexanes
depositing brick red crystals of 1 and a yellow powder of 2
(Scheme 1). The dimeric nature of 1 was confirmed by X-ray
bChemistry M313, The University of Western Australia, Crawley, WA, 6009,
Australia
1
† The HTML version of this article has been enhanced with colour images.
crystallography (see below). H and 13C NMR spectra in C4D8O
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opening polymerization reactions of cyclic esters such as lactide
and e-caprolactone.
A number of different X groups have been attached to the metal
atom by oxidation of the ytterbium(II) centre using an equivalent
of the appropriate oxidizing agent (Scheme 2). As a result a
series of ytterbium(III) bis(phenolate) alkoxides (X = OR; 5, 6,
7) and cationic ytterbium(III) bis(phenolate) compounds (X =
PF6, O3SCF3; 3 and 4) were synthesized. It was not possible to
access the analogous amides via this route. Instead, protiolytic
ligand exchange (transamidation) of [Yb(N(SiMe3)2)3] with an
equivalent of the appropriate bis(phenol) (LH2) was required
(Scheme 3). It is not surprising that oxidation of 1 or 2 by
HN(SiMe3)2 is not observed since no oxidation was seen in
the initial exchange reaction between the bis(phenol), H2LR and
[Yb(N(SiMe3)2)2(thf)2] (Scheme 1) where 1 and 2 are generated in
the presence of the free amide by-product, HN(SiMe3)2.
Scheme 1 Synthesis of 1 and 2.
indicated a symmetrical arrangement of the bis(phenolate) ligands.
Recording the 1H NMR spectrum in C6D6 or C7D8 revealed
a loss of symmetry of the bis(phenolate) ligand such that four
But and aromatic hydrogen signals were observed, and consistent
with the dimeric structure obtained in the solid state. A variable-
1
temperature H NMR study of 1 in C7D8 was unsuccessful in
clearly resolving the NCH2CH2NMe2 protons in the aliphatic
region of the spectrum. This could suggest a fluxional process
where the NMe2 group reversibly coordinates to ytterbium in
solution. It was not possible to obtain a NMR spectrum of 2
in C6D6 due to its poor solubility in non-donor solvents. Indeed,
attempts to probe the dimeric nature of 2 in solution in more polar
non-donor solvents such as dried CD2Cl2 were unsuccessful since
immediate oxidation was observed and an unhelpful paramagnetic
spectrum was recorded. From the similar solubility properties of
1 and 2 it may be that 2 also has a dimeric structure in the solid
state and in non-donor solvents. Several attempts to grow crystals
of 2 from a variety of solvent systems were unsuccessful. It is
worthy of note that the aggregated dimers are readily broken
up by the introduction of C4D8O as indicated by the 1H and
13C NMR spectra of 1 and 2, where equivalent and symmetrical
bis(phenolate) ligands were observed (Scheme 1). The coordinated
solvent was, however, very labile since removal of the solvent in 1
followed by recording of the spectrum in C6D6 demonstrated the
return of the dimeric species.
Scheme 2 Oxidation of homoleptic 1 or 2 to give LMX compounds.
Scheme 3 Synthesis of 8 and 9 via ligand exchange.
2
The
cationic
species
[(LNMe )Yb][PF6]
3
and
[(LNMe )Yb][O3SCF3] 4 (Scheme 2) were simply generated in
moderate yield by reducing the appropriate silver salt (to silver
metal) in hexanes and thf respectively at room temperature. It is
worthy of note that a donor solvent was required in generating
4, as repeating the procedure in hexanes resulted in no reaction.
Redox transmetalation reactions such as these often require the
use of donor solvents to aid in the electron transfer process.9 The
paramagnetic nature of the ytterbium(III) compounds precluded
acquisition of any usefully meaningful NMR spectroscopic
data, but good quality elemental analysis data supports the
formulations of 3 and 4, along with melting point, and X-ray
crystallographic characterization of 4 (see below). In the case of
2
Oxidation of divalent ytterbium compounds
The successful synthesis of homoleptic (in the sense of one type
of ligand) compounds 1 and 2 has now provided a pathway to
a variety of different ytterbium(III) heteroleptic bis(phenolate)
complexes by simple reaction with the appropriate oxidant. Several
types of heteroleptic ytterbium(III) bis(phenolate) compounds
were examined in search of the general LMX formulation (see
Introduction), as these make ideal catalyst precursors for the ring-
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4, elemental analysis calculations included coordination of thf
and a dme molecule of solvation, since the same batch of crystals
were used for elemental analysis and the X-ray study. The impetus
behind the synthesis of compounds 3 and 4 was to investigate the
effect of poorly nucleophilic initiating groups on the propensity
towards ring-opening polymerization ability of cyclic esters,
compared to nucleophilic alkoxide and amide groups (see below
was achieved by treatment of trivalent [Yb(N(SiMe3)2)3] with
the appropriate H2LR (R = NMe2 or OMe respectively). It
is worthy of note that lanthanide ligand exchange reactions
involving the sterically crowded N(SiMe3)2 ligand have been
problematic, especially where the exchanging ligand is also ster-
ically bulky.31–33 This has led to other researchers using the less
encumbered [Ln(N(SiHMe2)2)3(thf)x] systems to achieve exchange
with bis(phenol) compounds.
In this study, difficulties were encountered in achieving complete
exchange in the attempted synthesis of the analogous lanthanum
complexes of 8 and 9, using [La(N(SiMe3)2)3]. It may have been
anticipated that the smaller ytterbium centre would have been less
likely to cleanly displace two N(SiMe3)2 ligands but this was not the
case. Possibly the smaller ytterbium results in such ligand crowding
that in solution there maybe asymmetry in the Yb–N bond
distances, such that one N(SiMe3)2 group becomes more labile and
susceptible to protiolytic ligand exchange and displacement by the
bis(phenol) group. As with the other ytterbium(III) compounds
presented here, the principal source of characterization was by
elemental analysis which gave good results for the predicted
formulation.
for polymerization results).
t
Compounds [(LNMe )YbOBu ] 5, [(LNMe )YbOPh]
6
and
2
2
[(LOMe)YbOPh] 7 were generated in a similar fashion to 4 and
5, by reduction of the appropriate anhydrous alcohol (Scheme 2).
However the oxidation reaction was very sluggish and required
heating (50 ◦C). Indeed, the reactivity of 1 and 2 towards oxidation
was both solvent and alcohol dependent, so that thf or dme
were required for reduction of these sterically bulky alcohols.
Surprisingly, heating 1 with ButOH in hexanes for several days
produced no visible signs of oxidation and only addition of thf (to
break up aggregation) slowly causes oxidation to colorless 5 after
24 h of heating at 50 ◦C. This lack of reactivity towards the alcohols
(especially in non-donor solvents), whilst not typical in these types
of reactions, is probably best explained by the combination of
inertness of the dimeric nature of 1 and 2 and the bulkiness of
the substrate alcohol. A general reactivity profile indicated that
reactivity mirrored both steric and electronic differences of the
alcohols such that MeOH > PhOH > PriOH > ButOH. (Note
that the corresponding isopropoxide and methoxide compounds
are not reported here as repeated attempts to obtain analytically
pure samples were unsuccessful.30) However, based on color
change alone of each reaction (which is indicative of Yb(II) →
Yb(III) oxidation), the reactivity order was readily established.
Again the resulting paramagnetic, trivalent ytterbium species
precluded any useful NMR spectroscopic data being obtained.
However, satisfactory elemental analyses were obtained for all
three compounds; note that for 5, good analysis was found for
the unsolvated form, whereas for 6 and 7 a molecule of dme
was included in the analysis. Attempts to prove the presence of
dme by way of obtaining a hydrolyzed NMR spectrum using
CD3CN/CF3CO2D to obtain a LR : dme ratio were unsuccessful
since there were many overlapping signals in the methyl and
methylene region. However, the assumption of the presence of
dme in 6 and 7 is reasonable for several reasons. Based on the
X-ray crystal structures of 1, 4 and 10, which have five-, seven-
and six-coordinate ytterbium respectively (see below), it is not
unreasonable for 6 and 7 to have a unidentate or bidentate dme,
which would give a six- or seven-coordinate ytterbium respectively
(assuming LR is four-coordinate). Further, in 6 and 7, the OPh
ligand offers less steric hindrance compared with the more bulky
OBut ligand found in 5. As such, coordination of thf in 5 is not
feasible but dme coordination is likely in 6 and 7 on steric and
other grounds.
Synthesis of a bimetallic ytterbium compound
As already mentioned in the introduction, metathetical synthesis
of lanthanide compounds is often troublesome and plagued with
salt inclusion or generation of lanthanide ‘ate’ complexes, this,
however, in spite of Shen and Yao et al.26 recently successfully
synthesizing LNMe YbX where X = NPh2, Me by a metathesis
2
route via in situ generation of Na2LNMe and YbCl3. Similar
2
attempts were employed in this study to synthesize compounds
1 and 2 by metathesis from ytterbium(II) iodide and the in situ
generated dipotassium salt of the appropriate bis(phenolate).
An unexpected potassium ytterbium(III) bis(phenolate) complex
[(LOMe)2YbK(dme)2] 10 formed, as identified by an X-ray study
(see below) and elemental analysis (Scheme 4). It is believed that
incomplete deprotonation of the bis(phenol) by potassium hydride
caused oxidation of the ytterbium(II) centre and generation of 10.
Indeed, addition of at least a five-fold excess of potassium hydride
to ensure complete deprotonation of the bis(phenol) yielded 1 and
2. Furthermore, 10 could be reproducibly isolated by addition
of H2LR to a reaction mixture of K2LR and YbI2 in dme (R =
OMe; NMe2). Since compounds 1 and 2 have moderate and
poor solubility respectively in non-donor solvents, metathesis
Synthesis of trivalent ytterbium compounds by ligand exchange
reactions
In order to establish a greater understanding of the role of
the initiating group (X) on the ring-opening polymerization
of cyclic esters by ytterbium compounds, [(LNMe )YbN(SiMe3)2]
2
(8) and [(LOMe)YbN(SiMe3)2] (9), which contained the amide
initiator, were synthesized and characterized (Scheme 3). This
Scheme 4 Synthesis of the novel potassium–ytterbium(III) bimetallic
compound 10.
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Table 1 The ytterbium environment, 1
was abandoned since product separation from KI would be
challenging. Characterization of 10 by elemental analysis gave
satisfactory results by including one or two dme molecules in the
calculated formulation. The X-ray crystal structure confirms two
dme molecules present per molecule; desolvation upon drying the
compound for elemental analysis can be accommodated by either
formulation.
Atom
N(1)
r
O(12)
79.6(2)
O(22)
81.1(2)
129.9(2) 101.7(2)
115.6(2)
N(3)
72.4(2) 134.3(2)
O(12ꢀ)
2.545(6)
2.297(5)
2.225(6)
2.569(7)
O(12)
O(22)
N(3)
71.6(2)
90.9(2)
147.1(2)
O(12ꢀ) 2.349(5)
In a related study, Shen and co-workers34 synthesized a similar
alkali-metal ytterbium(III) bimetallic series via a more traditional
˚
ꢀ
ꢀ
ꢀ
Yb · · · Yb is 3.5651(9); O(12) · · · O(12 ) is 2.717(7) A; Yb–O(12)–Yb is
100.2(2), Yb,Ybꢀ–O(12)–C(12) are 129.3(5), 122.7(4) and Yb–O(22)–C(22)
112.7(5)◦.
route involving metathesis with two equivalents of M2LNMe
2
where M = Li, Na, K, with an equivalent of YbCl3 to afford
[(LNMe )Yb2M(thf)x]. Indeed 10 has been synthesized deliberately
2
using a similar protocol to that of Shen’s.
X-Ray crystallography of 1, 4 and 10
The results of the single crystal X-ray structure determination of 1
are consistent with that formulation in terms of stoichiometry and
connectivity, as a dimer of Yb2(LNMe )2 stoichiometry, modelled
2
as solvated with a molecule (disordered) of hexane; one half of
the dimeric formula unit comprises the asymmetric unit of the
structure. As in all of the present determinations, the ligand in 1
(Fig. 1) is a dianion, coordinating quasi ‘mer’ in 4 and 10 (Fig. 2
and 3, respectively). The centrosymmetric complex is unusual
among lanthanide complexes of this ligand, not only in being
binuclear, but also in that the YbO2Yb core of the molecule is
bridged by phenoxide oxygen atoms, the remainder of the metal
atom coordination environment being made up of the other donors
(N,Nꢀ,O) of the ligand, so that it is five-coordinate N2YbO3 (Fig. 1;
Tables 1 and 2), the oxygen atoms being much more strongly bound
than the nitrogen atoms. The oxygen atoms, nevertheless, comprise
one face of the coordination polyhedron, the overall consequence
being a residual void in an unusually sparse coordination sphere,
partly compensated by an agostic approach from a tert-butyl
hydrogen atom from the ligand in the alternate half of the dimer
(Yb · · · C(133), H(133c) (1 − x, y, 1 − z) 3.271(9), 2.40 (est.)
Fig. 2 Projection of a molecule of 4 showing selected hydrogen bonds.
Selected carbon and hydrogen atoms have been omitted for clarity.
˚
A). The ligand disposition appears somewhat strained; the two
Fig. 3 Projection of a molecule of 10, with hydrogen and tert-butyl methyl
groups omitted for clarity.
aromatic planes lie at angles of 41.7(2), 43.1(2)◦ to the central
Yb2O2 ‘plane’ which is slightly folded (v2 = 3241), with the YbN2O2
chelate ring of segment 2 severely folded across the N · · · O line in
a ‘boat’ conformation, and an unusually small Yb–O–C angle at
O(22).
The bulk of the bis(phenolate) ligand stabilizes the some-
what low coordination number of ytterbium(II). While there
are countless examples of heteroleptic35–39 and homoleptic36,40–42
Fig. 1 Projection of a centrosymmetric dimer of 1, normal to the YbO2Yb
‘plane’.
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Table 3 [LYb(O-thf)X]: comparative geometries
X
O,Oꢀ-tfsa
Ph2Nb
Me (2 mols.)b
˚
Distances (A)
Yb–N(1)
Yb–O(12)
Yb–O(22)
Yb–N(3)
Yb–O(thf)
Yb–X(X)
2.444(6)
2.093(6)
2.087(6)
2.495(9)
2.366(7)
2.419(7) (O(101))
2.487(7) (O(102))
2.516(3)
2.115(2)
2.112(2)
2.509(3)
2.338(2)
2.256(3) (N)
2.518(3), 2.513(3)
2.119(2), 2.124(2)
2.131(2), 2.130(2)
2.536(3), 2.546(3)
2.325(3), 2.330(2)
2.440(4), 2.442(3) (C)
Angles (◦)
N(1)–Yb–O(12)
N(1)–Yb–O(22)
N(1)–Yb–N(3)
N(1)–Yb–O(thf)
N(1)–Yb–X(X)
81.8(2)
80.2(2)
72.7(2)
83.4(3)
148.0(3) (O(101))
153.5(3) (O(102))
160.4(2)
89.0(3)
84.9(3)
79.71(9)
79.34(9)
70.7(1)
89.5(1)
169.7(1) (N)
79.89(9), 81.14(9)
80.41(9), 79.21(9)
70.1(1), 70.2(1)
104.3(1), 108.2(1)
160.6(1), 159.0(1) (C)
O(12)–Yb–O(22)
O(12)–Yb–N(3)
O(12)–Yb–O(thf)
O(12)–Yb–X(X)
154.76(9)
89.1(1)
82.9(1)
154.4(1), 154.1(1)
96.8(1), 91.9(1)
85.8(1), 84.1(1)
105.3(3) (O(101))
101.7(3) (O(102))
92.9(3)
99.7(1) (N)
100.5(1), 101.9(1) (C)
O(22)–Yb–N(3)
O(22)–Yb–O(thf)
O(22)–Yb–X(X)
89.1(1)
82.9(1)
103.4(1) (N)
91.7(1), 97.2(1)
83.4(1), 86.1(1)
102.6(1), 102.6(1) (C)
85.5(3)
94.1(3) (O(101))
91.2(3) (O(102))
155.9(2)
76.3(3) (O(101))
133.2(2) (O(102))
127.8(3) (O(101))
70.9(2) (O(102))
56.9(3)
N(3)–Yb–O(thf)
N(3)–Yb–X(X)
159.7(1)
99.3(1)
173.2(1), 175.9(1)
90.6(1), 88.9(1)
O(thf)–Yb–X(X)
100.7(1)
95.0(2), 92.8(1)
O(101)–Yb–O(102)
a This work (4). b Ref. 26.
(i.e. identical anionic ligands) ytterbium(III) aryloxide compounds,
there are relatively few ytterbium(II) homoleptic derivatives.22,42–50
Indeed, most homoleptic aryloxy divalent ytterbium compounds
accommodate molecules of solvation such as thf or dme. Of
these, there is only one example which is dimeric and contains
only one type of ligand, namely [(ArO)Yb(l-OAr)]2 (OAr =
2,6-di-tert-butyl-4-methylphenolato) reported by Lappert and co-
workers.43,49 While the structure of 1 has a similar structural
motif to that of Lappert’s compound, it is the first example of
an ytterbium(II) bis(phenolate) compound which is solvent free.
Sun and co-workers22 reported a methylene bridged bis(phenolate)
ytterbium(II) compound which was also dimeric in nature, but,
because of poor solubility, they could only characterize it with
donor solvents present. Furthermore, the solid-state structure,
which is preserved in solution (see above for NMR discussion)
is the first example showing the unsymmetrical coordination of
this type of bis(phenolate) ligand LR to a metal centre.
to occupy a single coordination site, then the complex may be
considered as quasi-octahedral, as in the other two complexes,
although in all cases the distortions are large (Table 3). Of the
three donor types, the amide appears the most strongly bound; the
present O,Oꢀ-distances, being comparable with the Yb–C distance
of the methyl adduct, presumably should be considered jointly as
the next strongest donor, with the central amine nitrogen donor
of LN(1), trans to X in all cases. The methyl adduct Yb–N(1)
distance is comparable with that in the amide counterpart; that
˚
in the triflate counterpart is shorter, surprisingly, by ca. 0.07 A.
Surprisingly large influences are seen in the cis-bonds as well, all
those of the present adduct being more or less shorter than in the
others despite the bidentate nature of the X ligand – perhaps a
consequence of ionic, cf., presumably, more covalent interactions.
There are long fluorine–CHx intermolecular contacts (Fig. 2), for
˚
which the shortest have F(103) · · · C(1) (x − 1, y, z) 3.52(1) A.
A previous study34 has described the structural characterization
of L2YbK(thf)3; the determination of the structure of 10 shows it to
be similar, the three unidentate thf molecules being supplanted by
a pair of dme chelate groups, the symmetry of the array increasing
in the process to be quasi-2 in projection down the K · · · Yb vector
(Fig. 3; Table 4), with the potassium atom six-coordinate. One
formula unit devoid of crystallographic symmetry comprises the
asymmetric unit of the structure. The geometry of the potassium
environment is well-removed from octahedral, two of the trans
angles being 167.5(1), 168.1(2), with the third only 136.7(3)◦. The
geometries of the L2YbK cores of the two complexes – (dme)2
and (thf)3 adducts – are remarkably similar (Table 4), impacting
The results of the structure determination of 4 are consis-
ꢀ
tent with that formulation [LNMe Yb(O,O -tfs)(O-thf)], solvated
2
(interestingly) with an additional lattice molecule of dme, one
formula unit, devoid of crystallographic symmetry, comprising
the asymmetric unit of the structure (Fig. 2). Despite the disorder
in the peripheral components of the structure, the present de-
termination provides an opportunity for comparison with two
other diversely different [LNMe Yb(O-thf)X] systems previously
2
studied,26 wherein X = (a) CH3, (b) Ph2N, (c) here being an
oxyanion, which, in the circumstances, is found to coordinate as an
O,Oꢀ-symmetrically bidentate ligand, of small bite; if considered
148 | Dalton Trans., 2007, 143–153
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Table 4 Comparative metal atom environments, 10, and [LNMe 2YbK(thf)3] (values for the latter in italics)
2
(a) The ytterbium environment
Atom
N(1)
r
O(112)
78.7(1)
77.18(8)
O(122)
77.6(1)
77.97(9)
155.9(2)
154.74(8)
N(2)
O(212)
99.3(1)
99.53(9)
88.0(1)
87.01(8)
91.5(1)
92.69(9)
79.2(1)
78.90(9)
O(222)
2.528(4)
2.538(3)
2.174(4)
2.160(2)
2.116(3)
2.112(2)
2.508(4)
2.499(3)
2.171(2)
2.189(2)
2.132(3)
2.128(2)
177.2(1)
176.98(8)
98.9(1)
100.10(8)
104.7(1)
104.62(9)
103.7(1)
103.19(9)
94.1(1)
94.26(8)
95.7(1)
95.73(9)
77.7(1)
78.22(9)
156.9(1)
156.95(9)
O(112)
O(122)
N(2)
O(212)
O(222)
(b) The potassium environment
Atom
r
O(212)
63.51(9)
O(101)
96.2(1)
111.5(2)
O(102)
107.0(1)
167.5(1)
59.8(2)
O(201)
O(202)
168.1(2)
104.6(2)
87.7(2)
84.7(2)
61.1(2)
O(112)
O(212)
O(101)
O(102)
O(201)
O(202)
2.905(3)a
2.827(4)a
2.795(6)
2.804(7)
2.741(8)
2.813(5)
120.6(2)
105.2(2)
136.7(3)
86.4(2)
◦
a
˚
˚
The Yb · · · K distance is 4.000(1) A; Yb–O(112, 212)–K are 102.97(9), 105.5(1) . Counterpart distances in the (thf)3 complex are 2.733(2), 2.846(2) A
with an angle of 64.86(7)◦ between.34
noticeably only on the K–O(bridging) distances, the angle between
also remaining essentially similar. It is noteworthy that both
methoxide oxygen atoms remain pendant, unable to coordinate
to the ytterbium centre presumably because of occlusion by the
tert-butyl groups. As with 1, the Yb–O and Yb–N bond distances
Scheme 5 Polymerization of e-caprolactone by ytterbium compounds.
of 10 are typical of six-coordinate Yb(III), with terminal Yb–
O(122/212) being shorter than bridging Yb–O(112/222).
to drug delivery materials.18,51–53 Recently Shen and Yao and co-
workers26 have investigated the ring-opening polymerization of
Polymerization of e-caprolactone by various LMX complexes
e-caprolactone by three ytterbium(III) bis(phenolate) compounds.
The ytterbium compounds synthesized here present ideal can-
didates for studying the ring-opening polymerization of cyclic
esters such as e-caprolactone. This is because the compounds here
contain a highly electropositive metal (ytterbium) and a stationary
spectator ligand (LR), both of which can lead to rapid and efficient
polymerization of these monomers (Scheme 5). Polycaprolactone
is among a growing number of polyesters to receive attention
because of its numerous applications ranging from packaging
The current study expands on their findings by investigating the
effect of the initiating group (X) on polymerization activities. Com-
pounds 1–4, 6, 8 and 9 were used to polymerize e-caprolactone at
various [monomer : catalyst] ratios at room temperature in toluene
for various reaction times (Table 5). Of the catalysts investigated,
complexes 8 and 9 (entries 6 and 10) were the most effective
with high yields of polycaprolactone forming within 10 min (room
temperature at 200 : 1 [monomer : catalyst]). Compounds 1 and
Table 5 Results from the polymerization of e-caprolactonea
Entry
Initiator
Time/min
[M]0/[I]0
Yieldb (%)
Mn (104) calc.c
10−4Mn (obs.)d
PDI
1
2
3
4
5
6
7
8
9
1
1
1
1
2
8
8
8
8
9
10
20
20e
20
30
7
10
20
40
10
200
300
300
400
200
200
200
300
400
200
75
60
70
52
48
85
100
70
54
90
1.71
2.05
2.40
2.37
1.10
1.94
2.28
2.40
2.47
2.05
3.37
3.68
3.29
3.10
7.12
0.90
1.59
5.08
6.75
1.28
1.69
1.72
1.67
1.69
1.63
1.49
1.63
1.80
1.90
2.27
10
a Polymerization conditions: toluene solvent, T = RT, Vtol/Ve-CL = 1.86, b Yield: weight of dry polymer/weight ◦of monomer used. c MW of e-CL ×
[M]0/[I]0 × (% yield). d Determined by GPC analysis in toluene with calibration to polystyrene standards. e T = 70 C.
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2 (entries 1 and 5) were also effective in these polymerization
reactions but were less active than 8 or 9. Since 1 and 2 are
both ytterbium(II) species, it is presumed that an initial oxidation
(and concomitant monomer reduction) precedes the ring-opening
polymerization of e-caprolactone. The exact nature of these
intermediates was not determined, although reaction of 1 with
L-lactide and e-caprolactone in a 1 : 1 Yb(II) : cyclic ester yields
an instant color change from deep red to pale yellow which is
indicative of ytterbium oxidation,54 so that the reduced catalytic
activity of 1 and 2 vs. 8 and 9 may be attributable to a deactivation
process as a result of the initial redox chemistry.
The cationic compounds 3 and 4, as well as the ytterbium(III)
alkoxide compounds 5–7, exhibited no activity towards poly-
merization of e-caprolactone even with heating over extended
reaction times (i.e. 80 ◦C, 24 h). It is clear from these data
that initiating groups (i.e. the X in LMX) do certainly affect
polymerization activities of e-caprolactone. Most likely the initial
ring-opening is very slow for compounds which show no poly-
merization of e-caprolactone. Either lack of nucleophilicity of the
non-coordinating anions, [PF6] and [O3SCF3], or the large steric
bulk of the alkoxide groups (OR) make compounds 4–7 poor
initiating groups.
(OR) or a non-coordinating anion (PF6 or triflate) no ring-opening
polymerization was observed.
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 tech-
niques. Solvents were dried and purified by distillation under ni-
trogen from sodium or potassium metal mixed with benzophenone
while deuterated solvents were dried over sodium metal and pu-
rified by vacuum transfer. The reagents [Yb(N(SiMe3)2)2(thf)2],29
[Yb(N(SiMe3)2)3],55 and H2LR (R = NMe2; OMe)27,28 were syn-
thesized by the reported procedures indicated. The synthesis of
YbI2(thf)2 was based on the reported method56 (for lanthanide(III)
chlorides) from ytterbium metal and mercury(II) iodide in thf by
redox transmetalation. The anhydrous YbCl3 was purchased from
Cerac and used as received and stored in the glovebox. All other
reagents were purchased from Acros and used as received except
for e-caprolactone which was dried over CaH2 for 48 h and distilled
at 10−4 Torr. In addition, light-sensitive silver salts were stored in
the dark. 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 Series FTIR Spectrometer. The 1H and 13C NMR spectra
were recorded on a Bruker AVANCE-500 NMR spectrometer and
referenced to C6D5H or C4D7HO (1H) and C6D6 or C4D8O (13C).
Elementalanalyses (sealedampoules under inert atmosphere)were
performed by Midwest Microlab, Indianapolis IN. Lanthanide
analyses were determined by EDTA titration using xylenol orange
indicator and hexamine buffer.57
Increasing the monomer to catalyst ratio from 200 : 1 to 300 :
1 and 400 : 1 for catalysts 1 and 8 produced a steady decrease
in percentage yield of polymer, consistent with a slower rate of
polymerization due to lower catalyst concentration.
GPC analysis of the polymer samples indicated the rapid
formation of high-molecular weight polymer at room temperature.
Polydispersities, obtained for all but one (entry 10: 2.27), fell within
a range from 1.43–1.90, indicative of some transesterification,
with longer reaction times producing broader polydispersities.
The efficacy of compound 9, which produced polymer with a
significant larger polydispersity in contrast to that produced by
8, is most probably explained by the difference in side arm (OMe
vs. NMe2) coordination to the metal. If the OMe group in 9 was
not coordinated to the ytterbium centre as tightly as the NMe2
group in 8, then possibly a larger catalytic pocket exists in 9
which facilitates transesterification reactions. A somewhat unusual
finding was that some calculated molecular weights Mn were lower
than the observed values. This could be because of partial catalyst
deactivation resulting in fewer active sites and therefore higher
molecular weight polymer being formed. This was most noticeably
observed for the divalent ytterbium catalysts 1 and 2. Possibly
the redox chemistry involved in the first step of polymerization
leads to extensive catalyst deactivation, leaving a smaller number
of activation sites available for ring-opening polymerization and
hence higher Mn values in these systems.
[LNMe Yb]2 (1). A deep orange hexanes solution (10 mL) of
2
[Yb(N(SiMe3)2)2(thf)2] (0.64 g, 1.00 mmol) was treated with a
colorless hexanes solution (9 mL) of H2LNMe (0.52 g, 1.00 mmol)
2
at room temperature for 4 h. Upon addition, an immediate color
change to deep red was observed with deposition of 1·C6H14
(0.52 g, 70%) as dark red crystals. Mp 220–222 ◦C (Found: C,
59.6; H, 8.3; N, 3.8. C74H122N4O4Yb2 requires (1·C6H14) C, 60.1;
H, 8.3; N, 3.8%); m˜max/cm−1 2855s, 1601w, 1443s, 1415s, 1358m,
1304s, 1261s, 1203m, 1165m, 1107m, 1026s, 930m, 910m, 879m,
829m, 806s, 737m, 683w, 644w, 582w, 517w, 424w (Nujol); dH
(500.1 MHz, C4D8O, 298 K) 1.25 (18H, s, But), 1.46 (18H, s,
But), 1.78 (6H, s, NMe2), 1.80 (2H, s, NCH2CH2NMe2), 2.51
(2H, s, NCH2CH2NMe2), 2.89 (2H, d, J 11.7, ArCH2N), 4.14
(2H, d, J 11.7, ArCH2N), 6.83 (2H, s, Ar), 7.08 (2H, s, Ar),
dH (500.1 MHz, C6D6) 1.98–1.24 (36H, overlapping signals of
But and CH2CH2NMe2), 2.47 (12H, s, NMe2), 2.96 (2H, d, J
12.0, ArCH2N), 3.03 (2H, d, J 12.0, ArCH2N), 4.87 (2H, d,
J 12.0, ArCH2N), 5.75 (2H, d, J 12.0, ArCH2N), 7.12 (1H, s,
Ar), 7.21 (1H, s, Ar), 7.51 (1H, s, Ar), 7.53 (1H, s, Ar), dC{H}
(125.8 MHz, C4D8O, 298 K) 30.5 (CMe3), 32.6 (CMe3), 34.3
(CMe3), 35.9 (CMe3), 45.7 (NMe2), 47.9 (NCH2CH2NMe2),
60.3 (NCH2CH2NMe2), 65.4 (ArCH2N), 123.7 (arom-CH), 124.9
(arom-CBut), 127.2 (arom-CH), 129.1 (arom-CBut), 135.3 (arom-
CCH2N), 166.9 (arom-CO).
Conclusions
A number of ytterbium bis(phenolate) compounds have been
synthesized and characterized which have the general LMX
formulation by oxidation of the homoleptic divalent ytterbium
compounds 1 and 2. In three cases, 1, 4 and 10, the X-ray
characterizations performed support the proposed formulations
of these compounds. Alteration of the X group was shown to
have a dramatic effect on ring-opening polymerization reactions
of e-caprolactone. In cases where the X group was an alkoxide
150 | Dalton Trans., 2007, 143–153
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LOMeYb (2). 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 H2LOMe (0.51 g, 1.00 mmol) in hexanes (20 mL)
precipitated a fine yellow powder from a red solution. Isolation
of the powder from the red solution yielded 2 (0.46 g, 67%).
Mp 178–180 ◦C (Found: C, 57.7; H, 7.6; N, 2.0. C33H51NO3Yb
requires C, 58.05; H, 7.5; N, 2.05%); m˜max/cm−1 2853s, 1601w,
1440s, 1412m, 1362m, 1323m, 1300m, 1260s, 1200w, 1165w,
1088s, 1065s, 1030s, 930m, 879m, 802s, 737w, 687w, 609w, 517w
(Nujol); dH (500.1 MHz, C4D8O, 298 K) 1.24 (18H, s, But), 1.43
(18H, s, But), 2.58 (2H, t, J 5.0, NCH2CH2O), 2.81 (3H, s,
OMe), 2.97 (2H, d, J 12.0, ArCH2N), 3.02 (2H, t, J 5.0,
NCH2CH2O), 4.17 (2H, d, J 12.0, ArCH2N), 6.83 (2H, s, Ar),
7.08 (2H, s, Ar), dC{H} (125.8 MHz, C4D8O, 298 K) 22.7 (CMe3),
22.8 (CMe3), 31.6 (CMe3), 33.1 (CMe3), 46.2 (NCH2CH2O), 55.4
(NCH2CH2O), 62.0 (OMe), 70.5 (ArCH2N), 120.8 (arom-CH),
122.1 (arom-CBut), 124.3 (arom-CH), 129.1 (arom-CBut), 132.8
(arom-CCH2N), 163.9 (arom-CO).
N, 3.55%. YbC44H69N2O5 requires (for one dme) C, 60.1; H, 7.9;
N, 3.2%.); m˜max/cm−1 2854s, 1592w, 1335w, 1203w, 1161w, 1076s,
1022s, 924w, 875w, 802s, 760m, 694w, 579w, 528w, 451w (Nujol).
[(LOMe)YbOPh] (7). Using a similar procedure to that de-
scribed for the synthesis of 6, treatment of 2 (0.50 g, 0.73 mmol)
with phenol (0.10 g, 1.10 mmol) in dme (25 mL) with heating
at 50 ◦C for 11 h, afforded a green yellow solution. Solvent
removal and recrystallization from hexanes afforded crystals of
7 (0.48 g, 85%). Mp 171–173 ◦C (Found: C, 59.7; H, 7.55; N,
1.6. C39H56NO4Yb requires (for no dme) C, 60.4; H, 7.3; N, 1.8%
YbC43H66NO6 requires (for 1 dme) C, 59.6; H, 7.7; N, 1.6%);
m˜max/cm−1 2858s, 1770w, 1592m, 1296s, 1265s, 1203m, 1165m,
1076s, 1022s, 910w, 833m, 802m, 759m, 694m, 602w, 579w, 528w,
455w.
[(LNMe )YbN(SiMe3)2] (8). To
a
stirred solution of
2
[Yb(N(SiMe3)2)3] (0.65 g, 1.00 mmol) in hexanes (11 mL),
H2LNMe (0.52 g, 1.00 mmol) in hexanes (8 mL) was added at
2
−78 ◦C. The reaction mixture was slowly warmed to room
temperature and stirred for a further 48 h. The solvent was
removed in vacuo to yield 8 as a yellow powder which was washed
with cold hexanes (2 mL) to remove liberated amine (0.68 g,
[(LNMe )YbPF6] (3). A deep red hexanes (6 mL) solution of
2
1·C6H14 (0.74 g, 0.50 mmol) was treated with a hexanes (8 mL)
solution of AgPF6 (0.25 g, 1.00 mmol) at room temperature for
24 h. An immediate color change to pale yellow was accompanied
by formation of a black precipitate (silver metal). The black solids
were separated from the yellow solution and the hexanes removed
in vacuo to reveal a yellow solid which was recrystallized from
hexanes to afford a yellow powder of 3 (0.55 g, 62%). Mp 184–
186 ◦C (Found: C, 50.2; H, 7.0; N, 3.2. C37H61F6N2O2PYb requires
C, 50.3; H, 7.0; N, 3.2%); m˜max/cm−1 1414m, 1202w, 1166w, 1103s,
1024s, 926w, 876w, 842s, 805s, 723s (Nujol).
◦
79%). Mp 160–162 C (Found: C, 57.0; H, 8.6; N, 4.6; Yb, 20.6.
C40H72N3O2Si2Yb requires C, 56.1; H, 8.5; N, 4.9; Yb, 20.2%;
m˜max/cm−1 1257s, 1203m, 1169m, 1099m, 1022s, 984m, 926m,
876m, 930m, 837s, 741m, 675w, 526w (Nujol).
[(LOMe)YbN(SiMe3)2] (9). Complex 9 was prepared by a proce-
dure analogous to the procedure for 8, except that H2LOMe (0.52 g,
1.00 mmol) was used in place of H2LNMe . Yellow needle-like
2
[(LNMe )YbOSO2CF3(thf)] (4). To a brick red powder of
crystals of 9 were obtained from a concentrated hexanes solution
at room temperature (0.64 g, 76%). Mp 162–164 C (Found: C,
55.3; H, 8.1; N, 3.5; Yb, 20.8. C39H69N2O3Si2Yb requires C, 55.55;
H, 8.25; N, 3.3; Yb, 20.5%; m˜max/cm−1 1260s, 1209m, 1140w, 1100m,
1024s, 984m, 876m, 930w, 839s, 741m, 675w (Nujol).
2
◦
1·C6H14 (0.68 g, 0.46 mmol) and a white powder of AgOSO2CF3
(0.26 g, 1.00 mmol) thf (9 mL) was added at room temperature for
12 h. The resulting pale yellow solution was separated from the
black precipitate and concentrated to yield a yellow solid which
was recrystallized from hexanes to yield 4 (0.49 g, 58%). X-Ray
quality crystals were grown from a dme/hexanes mixture at room
temperature. Mp 180–182 ◦C (Found: C, 51.4; H, 7.5; N, 2.7.
C43H72F3N2O8SYb requires (4·dme) C, 51.3; H, 7.2; N, 2.8%);
m˜max/cm−1 1165m, 1103s, 1026s, 926w, 875m, 802s, 725w, 675w,
640w (Nujol).
[(LOMe)2YbK(dme)2] (10). A thf solution (10 mL) of YbI2(thf)2
(0.29 g, 0.51 mmol) was treated with a thf (8 mL) solution of
K2LOMe (0.29 g, 0.50 mmol) [generated in situ. from KH (1.0 mmol)
and K2LOMe (0.50 mmol)]. Initially a dark red solution and a white
precipitate of KI formed and subsequently lightened to yellow
after stirring for 2 h at room temperature. The solvent was removed
to yield a pale yellow solid which was extracted with hexanes from
precipitated KI. Removal of hexane and recrystallization from
dme afforded yellow crystals of 10 (0.43 g, 60%). Mp 225 ◦C
(Found: C, 63.4; H, 8.2; N, 2.05%. C70H112KN2O8Yb requires (for
one dme) C, 63.6; H, 8.5; N, 2.1%. C74H122KN2O10Yb (for two
dme’s) C, 62.95; H, 8.7; N, 2.0%; m˜max/cm−1 2850s, 2727w, 1605w,
1416m, 1300m, 1281m, 1261m, 1234m, 1203w, 1115w, 1084m,
1018w, 933w, 879w, 833m, 802m, 740w, 528w, 444w (Nujol).
t
[(LNMe )YbOBu ] (5). A thf (10 mL) solution of tert-butanol
2
(0.10 g, 1.35 mmol) was added to a thf (10 mL) solution of
1·C6H14 (0.40 g, 0.27 mmol) and heated at 50 ◦C for 24 h. It
took several hours of heating before any discernable color change
was observed. After 24 h the solvent was removed to afford a light
green yellow solid of 5 (0.33 g, 80%). Mp 175–178 ◦C (Found:
C, 59.4; H, 7.95; N, 3.3. C38H63N2O3Yb requires C, 59.35; H, 8.3;
N, 3.6%); m˜max/cm−1 2852s, 2360w, 1766w, 1604w, 1416s, 1362m,
1304s, 1261s, 1203m, 1165m, 1095s, 1025s, 914w, 875m, 833s, 802s,
744m, 675m, 528m, 447m (Nujol).
A typical polymerization procedure with e-caprolactone
[(LNMe )YbOPh] (6). A dme (10 mL) solution of phenol (0.26 g,
A toluene (1.000 mL) solution of the appropriate catalyst (see
Table 5) (0.0631 mmol for 1 : 100; 0.0315 mmol for 1 : 200;
0.0158 mmol for 1 : 300; 0.0079 mmol for 1 : 400), was added
to e-CL (0.700 mL; 6.317 mmol) and toluene (0.300 mL) at room
temperature in a glovebox. After an appropriate time (Table 5) the
vigorously stirred reaction mixture started to gel, at which time an
aliquot (ca. 0.2 mL) was taken from the reaction mixture, removed
2
2.77 mmol) was added to a stirred dme (15 mL) solution of 1·C6H14
(1.00 g, 0.67 mmol) and heated at 50 ◦C for 11 h to give a yellow
solution after which time the dme was removed to yield a yellow
solid. Light yellow crystals of 6 (0.95 g, 81%) were grown at room
temperature from a hexanes solution. Mp 173 ◦C (Found: C, 60.2;
H, 7.6; N, 2.9. C40H59N2O3Yb requires (for no dme) C, 60.9; H, 7.5;
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from the glovebox and quenched with 5% M HCl-MeOH solution.
The precipitated polymer was filtered off from the solution and was
thoroughly washed with MeOH and dried to yield a white powder
2) = 1.184 g cm−3. lMo = 1.29 mm−1; specimen: 0.35 × 0.35 ×
0.25 mm; ‘T’min/max = 0.77. 2hmax = 58◦; Nt = 36801, N = 19227
(Rint = 0.026), No = 16330; R = 0.043, Rw = 0.084 (nw = 14), T ca.
298 K.
1
which weighed to give a yield and was analyzed by H NMR
spectroscopy (CDCl3) (Table 5). The polymer was also analyzed
by GPC to determine molecular weight distribution data.
Variata: Data were measured on a capillary mounted specimen
at room-temperature; attempted measurement at low temperature
was accompanied by specimen destruction. tert-Butyl group 225
was modelled as disordered over two sites, occupancies set at 0.5.
X-Ray crystallography-structure determinations
Single crystals suitable for X-ray crystallography of 1·C6H14,
4·dme and 10 were grown (see above). Full spheres of CCD
area-detector diffractometer data were measured (Bruker AXS
instrument, x-scans; monochromatic Mo-Ka radiation; k =
Acknowledgements
The authors are grateful to Professor Malcolm H Chisholm and
Dr. Keith Quisenberry at The Ohio State University for supplying
GPC data. Financial support from ND EPScOR program and
REU NSF CHE 0243832 (for CRN) is gratefully acknowledged.
˚
0.71073 A) yielding Nt(otal) reflections, these merging to N unique
after ‘empirical/multiscan’ absorption correction (proprietary
software), No with F > 4r(F) considered ‘observed’ and used
in the full matrix/large block least squares refinements on F2,
refining anisotropic displacement parameter for the non-hydrogen
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Conventional residuals R, Rw are cited at convergence (weights:
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employed within the Xtal 3.7 program system.58 Pertinent results
are presented below and in the Tables and Figures; where metal
atom environments are shown in matrix format, r is the meta◦l–
˚
ligand atom distance (A), other entries being the angles ( )
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Individual variations in procedure are cited under ‘variata’.
CCDC reference numbers 272546, 272547 and 621007.
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1·C6H14. C74H122N4O4Yb2, M = 1478.1. Monoclinic, space
groupP21/c (C52h, no. 14), a = 14.049(5), b = 9.693(5), c = 26.772(8)
◦
3
−3
˚
˚
A, b = 91.352(5) , V = 3645 A . Dc (Z = 2 dimers) = 1.347 g cm .
lMo = 2.6 mm−1; specimen: 0.09 × 0.09 × 0.06 mm; ‘T’min/max
=
0.85. 2hmax = 58◦; Nt = 34030, N = 9089 (Rint = 0.063), No = 6814;
R = 0.063, Rw = 0.13 (nw = 7.2). T ca. 153 K.
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3
−3
˚
˚
31.033(4) A, V = 4849 A . Dc (Z = 4) = 1.380 g cm . lMo
=
2.03 mm−1; specimen: 0.45 × 0.18 × 0.16 mm; ‘T’min/max = 0.74.
2hmax = 65◦; Nt = 27152, N = 15915 (Rint = 0.031), No = 14028;
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was modelled as disordered over two sets of sites, as also was the
R/NMe2CH2 component of the pendant chelate, and tert-butyl
group 15, and the coordinated thf molecule, site occupancies of
all being set at 0.5 except the tert-butyl components which refined
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¯
10. C74H122KN2O10Yb, M = 1412.1. Triclinic, space group P1
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1
˚
(Ci , no. 2), a = 14.057(1), b = 14.137(1), c = 23.703(2) A, a =
◦
3
˚
99.451(1), b = 95.457(2), c = 119.492(1) , V = 3958 A . Dc (Z =
152 | Dalton Trans., 2007, 143–153
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