Structural characterization of a tetrametallic diamine-bis(phenolate) complex of lithium and synthesis of a related bismuth complex

Article abstract of DOI:10.1016/j.poly.2015.07.071

A novel lithium complex was prepared from the reaction of 1,4-bis(2-hydroxy-3,5-di-tert-butyl-benzyl)imidazolidine H₂[O₂N₂]^BuBuIm (L1H₂) with n-butyllithium to provide the corresponding tetralithium amine-bis(phenolate) complex {Li₂[L1]}₂·4THF, 1. Variable temperature ⁷Li NMR revealed that this complex is labile in solution, dissociating at elevated temperatures to afford two dilithium entities. Additionally, ⁷Li MAS NMR was performed on 1 to provide information regarding the lithium coordination environment in the bulk solid-state. The reactivity of 1 was assessed in the ring-expansion polymerization of ε-caprolactone (ε-CL), which was first order in ε-CL with an activation energy of 50.9 kJmol^-1. Reaction of 1 and a related Li complex (formed in situ) with BiCl₃ afforded hydrolytically unstable bismuth phenolate species, as evidenced by the isolation and structural characterization of [Bi₄(Cl)₃(μ-Cl)(μ-O)(O)₂{[O₂N₂]^BuBuPip}₂], 2, where [O₂N₂]^BuBuPip is the homopiperazine-containing analog of L1.

Full text of DOI:10.1016/j.poly.2015.07.071

Polyhedron 102 (2015) 60–68
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Structural characterization of a tetrametallic diamine-bis(phenolate)
complex of lithium and synthesis of a related bismuth complex
Marcus W. Drover a, Jennifer N. Murphy a, Jenna C. Flogeras a, Céline M. Schneider a,b, Louise N. Dawe a,c,1
,
Francesca M. Kerton a,
⇑
a Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland A1B 3X7, Canada
b NMR Laboratory, C-CART, Memorial University of Newfoundland, St. John’s, NL A1B 3X7, Canada
c C-CART X-ray Diffraction Laboratory, Memorial University of Newfoundland, St. John’s, Newfoundland A1B 3X7, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel lithium complex was prepared from the reaction of 1,4-bis(2-hydroxy-3,5-di-tert-butyl-ben-
zyl)imidazolidine H2[O2N2]BuBuIm (L1H2) with n-butyllithium to provide the corresponding tetralithium
amine-bis(phenolate) complex {Li2[L1]}2ꢀ4THF, 1. Variable temperature 7Li NMR revealed that this com-
plex is labile in solution, dissociating at elevated temperatures to afford two dilithium entities.
Additionally, 7Li MAS NMR was performed on 1 to provide information regarding the lithium coordina-
tion environment in the bulk solid-state. The reactivity of 1 was assessed in the ring-expansion polymer-
Received 5 June 2015
Accepted 29 July 2015
Available online 5 August 2015
Keywords:
Lithium
N, O ligands
Coordination compounds
NMR studies
Bismuth
ization of
Reaction of 1 and a related Li complex (formed in situ) with BiCl3 afforded hydrolytically unstable bis-
muth phenolate species, as evidenced by the isolation and structural characterization of [Bi4(Cl)3(
Cl)(
-O)(O)2{[O2N2]BuBuPip}2], 2, where [O2N2]BuBuPip is the homopiperazine-containing analog of L1.
e-caprolactone (e-CL), which was first order in e .
-CL with an activation energy of 50.9 kJmolꢁ1
l
-
l
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
the compounds contain two chelating ligands around a tetra-lithi-
ated core and adopt an elongated ladder-like conformation as
described by Clegg et al. in 2008 [4]. Typically, two Li atoms are
nominally 5-coordinate, resulting from two bridging oxygen
atoms, two nitrogen atoms, and an ipso-C from an adjacent ben-
zene ring. This motif is a standard structural feature of many
lithium-containing compounds. In the work of Huang and Chen,
solid-state structure analysis revealed a tetra-lithium complex,
which adopted a boat-like conformation, flanked on each side by
an amine-bis(phenol) ligand [3]. More recently, Kozak and co-
workers have reported the synthesis of analogous ladder com-
pounds using diamine-bis(phenol) ligands bearing THF pendant
arms [5], and showed that tridentate amine-bis(phenolate) adopt
ladder-like structures in the solid-state [6]. They also employed
6Li and 7Li NMR spectroscopy in solution and the solid-state to gain
insight into similarities and differences in the number of Li envi-
ronments for both phases [6]. However, they were unable to model
their solid-state NMR data to obtain quadrupolar-coupling
parameters.
Amine-phenolate ligands have been used to form complexes
with metals from around the periodic table for a diverse range of
applications from biomimetic modelling to catalysis [1]. The ease
by which these ligands are modified gives rise to a myriad of
potential ligand donor systems to stabilize both electron rich and
electron poor metal centres. They have been widely studied with
s-, d- and f-block metals but p-block compounds are less well rep-
resented than analogs from the left hand side of the periodic table.
In order to prepare many of these complexes, lithium adducts must
be prepared first, which can then be reacted with a metal halide to
give the desired product through a salt metathesis reaction.
Over the years, several amine-bis(phenolate) complexes of
lithium have been structurally characterized [2–4]. One of the first,
which we reported in 2006, showed that in the presence of 1,4-
dioxane, two isomeric amine-bis(phenolate ligands could yield
structurally different lithium complexes namely a polymeric com-
plex or a tetralithium species with 1,4-dioxane acting as the bridg-
ing group [2]. In many cases, when a bridging ether is not present,
Many lithium compounds of phenolate ligands have found
application as initiators in ring-opening polymerization (ROP)
reactions of cyclic esters including lactide and e-caprolactone (e-
⇑
Corresponding author. Tel.: +1 709 864 8089; fax: +1 709 864 3702.
E-mail address: fkerton@mun.ca (F.M. Kerton).
Current address: Department of Chemistry, Wilfrid Laurier University, Science
CL) [3,6–11]. In most cases, reactions can proceed in the absence
of a co-initiator (alcohol) to yield cyclic polymers or in the pres-
ence of alcohol to yield linear polyesters. In some cases, reactions
1
Building, 75 University Ave. W., Waterloo, Ontario N2L 3C5, Canada.
http://dx.doi.org/10.1016/j.poly.2015.07.071
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

M.W. Drover et al. / Polyhedron 102 (2015) 60–68
61
clearly proceed via an activated monomer mechanism [11],
whereas in other cases the mechanism is less clear but in the
absence of alcohol they likely proceed via a conventional coordina-
tion-insertion process.
external referencing standard. Analytical simulations of 7Li{1H}
SSNMR spectra were performed using DMFit.
2.2. Crystal structure determination
Given the rigid, heterocycle-containing backbone of the ligands
used in the current study (Fig. 1), we thought they might be well-
suited for coordination with a large p-block metal ion. In particular,
bismuth(III) salts have been widely implemented in organic syn-
thesis (including: Mukaiyama–aldol [12,13], Michael addition
[12], acylation reactions [14], and formation and deprotection of
acetals [15]) and therefore, we thought that amine-phenolate bis-
muth complexes may show interesting catalytic behaviour.
Despite the known catalytic activity of bismuth salts, well-charac-
terized coordination compounds of bismuth are limited in compar-
ison with many transition metals and light s/p-block elements. The
lack of structural data has prevented extensive studies of the cat-
alytic features and reactivity of many metallorganic bismuth com-
pounds. This may be because the synthesis of metallorganic
bismuth compounds is often plagued by the formation of multi-
metallic oxygen-bridged clusters [16–20]. This typically results
from the facile hydrolysis or oxidation of the bismuth-containing
metal precursor, affording complex aggregates with BixOy central
cores. Although cluster formation in bismuth-containing materials
is common, there are examples where this phenomenon has been
successfully avoided by employing an amine-phenol ligand [21].
Specifically, amine-tris(phenol) ligands were implemented as scaf-
folds in the synthesis of mononuclear compounds of bismuth and
antimony. With bismuth, this led to the formation of an arene
bridged inverted-sandwich complex.
Single crystals of suitable dimensions were used for data collec-
tion. Methods of crystal growth are outlined in the synthetic pro-
cedures below. Crystals of
1 and 2 were mounted on low
temperature diffraction loops. All measurements were made on a
Rigaku Saturn70 CCD diffractometer using graphite monochro-
mated Mo Ka radiation, equipped with a SHINE optic. For all struc-
tures, H-atoms were introduced in calculated positions and refined
on a riding model while all non-hydrogen atoms were refined
anisotropically. In the structure of 1, the carbon atoms of one
THF group, and one lattice solvent toluene molecule were disor-
dered over two positions (C38–C41: C38A–C41A with refined occu-
pancies 0.464(17): 0.536(17) and C42–C48: C42A–C48A with
refined occupancy 0.42(2): 0.58(2).) Distance and rigid bond
restraints were applied to both disorder groups. In the structure
of 2, a disordered toluene molecule was treated with similar aniso-
tropic displacement restraints and the corresponding H-atoms
were omitted from the model, but included in the formula for
the calculation of intensive properties.
Structural illustrations were created using Mercury software,
which is available free of charge from http://www.ccdc.cam.ac.
uk/products/mercury/. Crystallographic and structure refinement
data of compounds 1 and 2, and their CCDC reference numbers
are given in Table 1.
2.3. Synthesis of compounds
2.3.1. Synthesis of 1
2. Experimental
H2[O2N2]BuBuIm (2.01 g, 3.94 mmol) was dissolved in THF
(30.0 mL) and cooled to ꢁ78 °C. n-Butyllithium (1.6 M, 5.4 mL,
8.7 mmol) was slowly added to give a cloudy, yellow solution
which was warmed to room temperature and allowed to react
for an additional 72 h. After this period, volatiles were removed
under reduced pressure to afford an off-white powder (2.35 g,
98%). Clear, colorless crystals suitable for X-ray diffraction were
2.1. General and instrumental considerations
The proligands, H2[O2N2]BuBuIm and H2[O2N2]BuBuPip, were pre-
pared in water following literature procedures [22,23]. All other
manipulations were carried out under a nitrogen atmosphere using
either standard Schlenk techniques or a glovebox. n-Butyllithium
(1.6 M in hexanes) and BiCl3 were purchased from Alfa-Aesar and
used without further purification. Deuterated solvents were pur-
chased from Cambridge Isotope Laboratories Inc., purified and
dried before use.
obtained via slow evaporation of
a solution of 1 in a 1:1
toluene:pentane solution (at ꢁ35 °C) under an inert atmosphere.
1H NMR (300 MHz, C6D6, 298 K): d 7.59 (d, ArH, 2H, 4J = 2.5 Hz);
7.49 (d, ArH, 2H, 4J = 2.5 Hz); 7.07 (overlapping multiplet, ArH,
4H); 4.71 (d, CH, 2H, 2J = 11.2 Hz); 4.66 (d, CH, 2H, 2J = 11.3 Hz);
4.38 (d, CH, 2H, 2J = 11.0 Hz); 4.09 (d, CH, 2H, 2J = 5.8 Hz); 3.26
(m, CH2, THF, 22H); 2.84 (t, CH2; imid ring, 4H, 3J = 11.9 Hz); 2.55
(m, CH2, imid ring, 4H); 2.15 (m, CH2, imid ring, 4H); 1.73 (s,
C(CH3)3, 18H); 1.57 (s, C(CH3)3, 18H); 1.51 (s, C(CH3)3, 18H); 1.45
(s, C(CH3)3, 18H); 1.17 (m, CH2, THF, 22H). 13C{1H} NMR
(75.4 MHz, C6D6, 298 K): d 165.3 (ArCOH); 164.9 (ArCOH); 137.0
(ArC(CH3)3); 136.7 (ArC(CH3)3); 133.8 (ArC(CH3)3); 133.73
(ArC(CH3)3); 126.9 (ArCH); 126.2 (ArCH); 125.7 (ArCH); 125.1
(ArCH); 124.4 (ArCCH2N); 124.0 (ArCCH2N); 78.52 (ArCCH2N),
78.48 (ArCCH2N), 68.4 (CH2: THF); 61.7 (NCHN); 61.1 (NCHN);
52.5 (NCH2CH2N); 50.6 (NCH2CH2N); 36.0 (ArC(CH3)3); 35.9
(ArC(CH3)3); 34.5 (ArC(CH3)3); 34.4 (ArC(CH3)3); 32.9 (ArC(CH3)3);
32.8 (ArC(CH3)3); 31.3 (ArC(CH3)3); 31.1 (ArC(CH3)3); 25.6 (CH2:
MALDI-TOF mass spectrometry was performed on an Applied
Biosystems 4800 MALDI TOF/TOF Analyzer equipped with a reflec-
tron, delayed ion extraction and high performance nitrogen laser
(200 Hz operating at 355 nm). Anthracene was used as the matrix
[24,25]. Elemental analyses were performed at Guelph Chemical
Laboratories, Guelph, ON, Canada. 1H, 13C{1H} and 7Li{1H} NMR
spectra were recorded on a Bruker Avance 300 MHz spectrometer
at 25 °C (unless otherwise stated) and were referenced internally
using the residual 1H and 13C resonances of the solvent. 13C reso-
nances were assigned on the basis of DEPT and 2D-NMR experi-
ments. 7Li{1H} NMR spectra were referenced externally to
saturated solutions of LiCl. For solid-state NMR experiments, sam-
ples were finely ground and packed into a 4 mm zirconium oxide
rotor. 7Li{1H} SSNMR ( 0(7Li) = 98.2 MHz) spectra were recorded
t
THF). 7Li{1H} NMR (49.6 MHz, C7D8, 193 K): d 1.90,
(Li1); 1.55, 1/2 = 20.6 Hz (Li2); 0.70, 1/2 = 44.3 Hz (Li3); 0.23,
1/2 = 20.6 Hz. Anal. Calc. for C78H124Li4N4O7: C, 74.49; H, 9.94; N,
x1/2 = 51.5 Hz
on a Bruker Avance 600 MHz spectrometer at 25 °C. 7Li{1H} spectra
were collected using a solid-state MAS 1H/X probe. For MAS exper-
iments, spectra at three different spinning rates, ranging between
2.5 and 15 kHz, were acquired. The magic angle was adjusted using
x
x
x
4.46. Found: C, 74.77; H, 10.25; N, 4.52%. MS (MALDI-TOF) m/z
(%, ion): 1047.9 (18, Li4[L1]+2); 1040.9 (8, [Li4[L1]2ꢁLi]+).
KBr. A single 3 ls pulse was employed to excite the central and
satellite transitions (CT and ST); 5 s recycle delays were used.
The spectral widths were 400 kHz and 3958 complex data points
were acquired. Isotropic chemical shifts are reported with respect
to LiCl, whose 7Li{1H} spectrum was recorded and used as an
2.3.2. Synthesis of bismuth complexes
To a slurry of 1 (2.05 g, 3.92 mmol) in 15.0 mL THF, was added
dropwise (via cannula) to a suspension of BiCl3 (1.24 g, 3.94 mmol)

62
M.W. Drover et al. / Polyhedron 102 (2015) 60–68
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
O
N
O
t-Bu
O
t-Bu
t-Bu
O
N
N
N
L1 [O2N2]BuBuIm
L2 [O2N2]BuBuPip
Fig. 1. Amine-bis(phenolate) ligands used in this study.
Table 1
C, 54.12; H, 6.71; N, 3.30%. MS (MALDI-TOF) m/z (%, ion): 778.4
(4, Bi[L2]Cl+); 743.4 (100, Bi[L2]+). This compound decomposed in
NMR solvents. Reaction of Bi[O2N2]BuBuPipCl with adventitious
moisture provided a pale, yellow solid 2 (0.11 g, 9%), with the for-
Crystallographic and structure refinement data for 1 and 2.
1
2
CCDC
1403400
C96H148Li4N4O8
1514.01
163(2)
monoclinic
P21/c
11.5812(18)
17.424(3)
23.613(4)
90.00
101.386(7)
90.00
4671.1(13)
2
1.076
1403401
C108.5H152Bi4Cl4N4O6
2586.15
163(2)
monoclinic
P21/c
15.141(4)
21.244(5)
32.957(8)
90.00
96.526(3)
90.00
10532(5)
4
1.631
6.806
5116.0
mula, Bi4(l-Cl)(l-O)2[L2]2(Cl)3. 2 was characterized by single-crys-
Empirical formula
Formula weight
T (K)
Crystal system
Space group
a (Å)
tal X-ray diffraction, elemental microanalysis, solid-state NMR (it
was insoluble in NMR solvents) and MS. 13C{1H} NMR
(150.8 MHz, SS, 298 K): d 155.8(ArCO); 140.8 (ArC(CH3)3); 136.1
(ArC(CH3)3); 124.6 (ArCH); 75.5 (ArCH2N); 58.0 (CH2; homopip.
ring); 53.9 (CH2; homopip. ring); 34.3 (ArC(CH3)3); 32.0
(ArC(CH3)3). Anal. Calc. for C70H108Bi4Cl4N4O6: C, 40.43; H, 5.24;
N, 2.69. Found: C, 40.17; H, 5.12; N, 2.89%. MS (MALDI-TOF) m/z
(%, ion): 778.4 (7, Bi[L2]Cl+); 743.4 (100, Bi[L2]+).
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
2.3.3. Typical polymerization procedure
qcalc (g/cm3)
(mmꢁ1
)
0.066
1656.0
Reaction mixtures were prepared in a glove box and subsequent
operations were performed using standard Schlenk techniques. A
sealable Schlenk flask equipped with a stir bar was charged with
a solution of 1 (73.6 mg) in 2.0 mL toluene. A second Schlenk flask
l
F(000)
Crystal size (mm3)
Radiation
0.38 ꢃ 0.35 ꢃ 0.34
0.12 ꢃ 0.11 ꢃ 0.06
Mo K
a
(k = 0.71075)
Mo Ka (k = 0.71075)
2h range for data collection 5.98–53
(°)
5.16–53
was charged with a toluene (2.0 mL) solution of e-caprolactone
(0.401 g, 3.54 mmol, 50 equiv). The two flasks were then attached
to a Schlenk line and temperature equilibration was ensured in
both Schlenk flasks by stirring the solutions for 10 min in a temper-
ature controlled oil bath. The complex solution was transferred to
the rapidly stirring monomer solution and polymerization times
were measured from that point. At given time intervals, aliquots
of the reaction mixture were removed via pipette for determining
monomer conversion by 1H NMR spectroscopy. The reaction was
quenched with methanol once near-quantitative conversion had
been obtained. GPC data were collected on a Viscotek GPCMax
Index ranges
ꢁ14 6 h 6 14,
ꢁ19 6 h 6 19,
ꢁ26 6 k 6 26,
ꢁ41 6 l 6 41
110405
ꢁ20 6 k 6 21,
ꢁ29 6 l 6 29
Reflections collected
Independent reflections
42399
9668 (0.0442)
21788 (0.0883)
(Rint
Data/restraints/parameters 9668/62/620
)
21,788/24/1182
1.166
Goodness-of-fit (GOF)
1.039
on F2
Final R indexes [I P 2
r(I)]
R1 = 0.0666,
wR2 = 0.1802
R1 = 0.0894,
wR2 = 0.1945
0.42/ꢁ0.30
R1 = 0.0616,
wR2 = 0.1198
R1 = 0.0749,
wR2 = 0.1270
2.26/ꢁ1.87
Final R indexes [all data]
System equipped with a Refractive Index Detector (Phenogel 5
l
Largest diff.
linear/mixed bed 300 Å ꢂ 4.60 mm column in series with
a
peak/hole (e Åꢁ3
)
Phenogel 5
l
, 100 Å, 300 Å ꢂ 4.60 mm column). Samples were
run in chloroform at a concentration of 1 mg mLꢁ1 at 35 °C. The
instrument was calibrated against polystyrene standards
GPC
(Viscotek) to determine the molecular weights (Mn
and Mw)
in 15.0 mL THF at ꢁ78 °C. After 30 min, the resulting yellow solu-
tion was allowed to warm to room temperature, resulting in an
orange solution. After 24 h, volatiles were removed under reduced
pressure to afford an orange powder. The resulting solid was then
extracted with toluene (20.0 mL) and filtered through Celite.
Subsequent solvent removal gave a pale, orange solid (1.01 g,
40%, assuming product was Bi[O2N2]BuBuImCl). Attempts to grow
crystals were unsuccessful.
and the polydispersity index (Mw/Mn or Ð) of the polymers. The
GPC
Mn value was calculated according to Mn = 0.56Mn
.
3. Results and discussion
3.1. Molecular structure of 1
BuBuIm
In
a
similar fashion, attempts were made to prepare
The tetradentate ligand H2[O2N2
]
(L1H2) was prepared
Bi[O2N2]BuBuPipCl: Li2[O2N2]BuBuPip (1.86 mmol) was prepared
in situ in THF (15 mL). To this solution was added dropwise (via
cannula) to a suspension of BiCl3 (0.57 g, 1.86 mmol) in THF
(15 mL) at ꢁ78 °C. The reaction was allowed to proceed as above.
Solvent removal gave a pale, orange solid (0.48 g, 33%). Yellow
crystals were obtained via slow evaporation of a solution of
Bi[O2N2]BuBuPipCl in a 1:1 toluene:pentane solution (at ꢁ35 °C)
under an inert atmosphere. These crystals rapidly decomposed at
T > ꢁ35 °C. This material was partially characterized as follows:
Anal. Calc. for C35H54BiClN2O2: C, 53.95; H, 6.98; N, 3.59. Found:
from 2,4-di-tert-butylphenol, ethylenediamine and three equiv.
formaldehyde [22]. The ligand was dried, recrystallized and then
reacted with n-butyllithium in THF, Scheme 1. This afforded
{Li2[L1]}2ꢀ4THF, 1, in near quantitative yield.
The 1H NMR resonances for H2[L1] appear as sharp, well-de-
fined peaks which are easily assignable to each of their methyl
(CH3), methylene (CH2) and methine (CH) protons. Upon reaction
to yield 1, the resonances of L1 change significantly. In solution,
H2[L1] exhibits free rotation about its CAr–CH2–Nimid bonds, result-
ing in chemically equivalent, isochronous methylene protons and a

M.W. Drover et al. / Polyhedron 102 (2015) 60–68
63
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
OH
t-Bu
HO
N
N
N
N
O
O
O
O
4 eq. n-BuLi
O
2
t-Bu
t-Bu
O
O
Li
Li
Li
Li
THF
O
-78
C
N
N
t-Bu
t-Bu
t-Bu
t-Bu
Scheme 1. Synthesis of complex 1.
single resonance at 3.35 ppm. However, once lithiated, these pro-
tons become chemically inequivalent and non-isochronous, result-
ing in diastereotopicity. This gives rise to four sets of doublets each
of integration two. This downfield shift and change in multiplicity
is well known for other amine-phenolate systems, and is charac-
teristic of ligand metalation [2–5,7]. Coupling constants (2J) for
the diastereotopic protons of 1 are in close agreement to those
reported by others for related ligand systems [5].
Crystals suitable for single crystal X-ray diffraction were grown
by slow evaporation of a 1:1 toluene:pentane solution of 1 under
an inert atmosphere at ꢁ35 °C. The structure of 1 is shown in
Fig. 2. It is ladder-like in nature and is comprised of four Li atoms,
capped by two amine-bis(phenolate) ligands. At its core, 1 consists
of a 6-membered metallacycle, with two adjacent rhombic or
‘‘daisy-chain’’ Li2O2 units. For 1, each Li centre is tetracoordinate;
Li(1) is bound by two bridging phenolate-O atoms and two THFs,
while Li(2) is bound by the tertiary amine of the imidazolidine
backbone from two different ligands (N1 and N2) and the bridging
phenolate-O donor atoms. Bond lengths around the Li atoms are
provided in Table 2. The Li–OAr bond distances are in the range
1.901(4) to 1.938(4) Å, which is similar to those observed by others
for related complexes [3–6]. Although 1 is novel we note that, dur-
ing their studies on lanthanide coordination chemistry, Shen and
co-workers reported the synthesis of a tetrasodium complex of
L1 [26], which is structurally very similar to 1.
acquired were reproducible and afforded chemical shifts and peak
widths identical to those obtained in previous runs. This eliminates
the possibility of sample decomposition at high temperature or
differences between samples of 1. The shifts are consistent with
those reported for other Li-based phenolates, indicating multiple
unique environments around the Li atoms in each case [3,6]. This
observation suggests that for samples in solution, 1 contains
ligands arranged in a slightly different formal arrangement (e.g.
twisted) to those in the crystalline sample (which would give rise
to two 7Li environments if maintained in solution). It should also
be noted that four different 7Li environments are also seen in
solid-state NMR spectra obtained on powdered samples of 1
(Supplementary data).
The rate of exchange of 7Li nuclei can be extrapolated from the
7Li{1H} NMR data (Eq. (1)), and we propose that this Li–Li exchange
is facilitated via the postulated dilithium species at high tempera-
ture (Scheme 2). In modelling our spectral data, 193 K, the freezing
point of C7D8, was taken as the point at which the rate of exchange
was smallest. Similarly, intermediate exchange points (219–363 K;
extensive peak overlap) and the coalescence point (373 K; single,
sharp peak) were also modelled using exchange rate constants.
ꢀ
ꢁ
kexc
kBT
h
D
G ¼ ꢁRTln
¼
D
H ꢁ T
D
S
ð1Þ
3.2. NMR studies of 1
From crystallography, the molecular structure of 1 is highly
symmetric and if this was maintained in solution, one might
expect to see only one type of phenolate environment (i.e. one pair
of t-Bu environments). In practice, two ligand groups are observed
and this is likely due to twisting along the backbone of the ligand.
Two sets of aromatic protons, t-Bu, and imidazolidine resonances
are all seen in C6D6 at 298 K. The presence of two types of ligand
environment suggests that this tetra-lithium complex does not dis-
sociate upon dissolution in C7D8 or C6D6. Another possibility, if
conditions were serendipitous, would be the presence of a Li4
and Li2 equilibrium mixture with a ratio of 1:2 to yield resonances
of the correct integration in the spectrum. As described below, this
is not the case. To the best of our knowledge, this system repre-
sents one of the few lithium bis(phenolate) structures that does
not disaggregate in solution at room temperature. This observation
prompted variable-temperature 1H and 7Li{1H} NMR studies (Fig. 3
and Supplementary data). At 193 K, four distinct Li environments
are evident for 1 (d 1.90,
(Li2); 0.70, 1/2 = 44.3 Hz
20.6 Hz (Li4)). These signals coalesce to one Li environment (d
1.07,
1/2 = 48.0 Hz) at 373 K. Each of the 7Li{1H} NMR spectra
x
1/2 = 51.5 Hz (Li1); 1.55,
x1/2 = 20.6 Hz
x
(Li3); 0.23,
x1/2 =
Fig. 2. Molecular structure diagram of 1. Thermal ellipsoids shown at 50%
probability and H-atoms removed for clarity.
x

64
M.W. Drover et al. / Polyhedron 102 (2015) 60–68
Table 2
3.3. Reactivity studies of 1
3.3.1. Ring-opening polymerization of
Selected bond lengths (Å) and angles (°) for 1.
e
-caprolactone using 1
Li1–O1
Li1–O2
Li1–O3
Li1–O4
Li2–O1
Li2–O2
Li2–N1
Li2–N2
1.938(4)
1.901(4)
2.076(4)
1.994(4)
1.903(4)
1.917(4)
2.096(4)
2.095(4)
C38–O4–Li1
C41–O4–Li1
C7–N1–Li2
C8–N1–Li2
C17–O2–Li2
Li1–O2–Li2
C34–O3–Li1
C37–O3–Li1
126.2(6)
125.2(7)
96.4(1)
106.0(2)
120.0(2)
84.5(2)
C1–O1–Li1
C1–O1–Li2
Li1–O1–Li2
C17–O2–Li1
C10–N1–Li2
C9–N2–Li2
C10–N2–Li2
C11–N2–Li2
123.5(2)
122.8(2)
83.9(2)
133.9(2)
126.8(2)
106.5(2)
128.1(2)
94.4(2)
In the field of ring-opening polymerization (ROP) of cyclic
esters, Li-based initiators have advantages of being easily accessi-
ble, stable on storage and non-toxic in the quantities required of
initiators. Huang and Chen reported lithium complexes bearing
dianionic amine-bis(phenolate) ligands for the controlled ROP of
L-lactide [3]. Each of these tetra-lithiated complexes provided the
corresponding polymer in the absence or presence of benzyl alco-
hol co-initiator (30–70% yield after 30 min at 25.6 °C). Using a
dilithium amine-bis(phenolate) complex as an initiator in ROP of
rac-lactide, Dean et al. achieved 86–91% conversion in 60–
120 min at 26 °C in the absence of co-initiator to yield low molec-
ular weight polymers with broad dispersity (2.5–4.1) [6]. We have
also published a related article dealing with the polymerization of
120.7(2)
131.6(2)
This analysis allowed the determination of the enthalpy,
entropy and Gibbs free energy (at 298 K) associated with the Li-ex-
change process
(D , D
H = 4.80 kJmolꢁ1 S = 185.6 Jmolꢁ1Kꢁ1 and
D
G = ꢁ53.5 kJmolꢁ1) (Fig. 4). These data are in good agreement
with previous studies in the literature. Jackman and Smith studied
Li-exchange in the following three systems: 4-bromophenolate in
e-caprolactone (e-CL), initiated by lithium piperazinyl amine-phe-
THF
(
D
G = ꢁ28.9 kJmolꢁ1), 3,5-dimethylphenolate in pyridine
nolate complexes [10], which we proposed to proceed via a coordi-
nation-insertion mechanism, which is consistent with results
reported by others [3,8,28–30]. However, it should also be noted
that lithium phenolate complexes can also perform ROP reactions
via an activated monomer mechanism [11].
(
D
G = ꢁ46.0 kJmolꢁ1
)
and
2-isopropylphenolate
in
THF
(D
G = ꢁ52.9 kJmolꢁ1) [27]. For these molecules, similar to the
results reported herein, temperature-dependant Li4/Li2 equilibria
were proposed in order to explain the exchange phenomena. It is
not surprising that our data are most similar to the 2-isopropy-
lphenolate complexes, as both this and 1 contain ortho sub-
stituents on the phenolate group, which would strongly influence
the exchange process.
ROP of
in the absence of a co-initiator. A [
and the solvent employed was toluene. These are similar reaction
e-CL was performed using 1 as a homogeneous catalyst,
e-CL]:[Li] ratio of 50:1 was used,
Fig. 3. Variable temperature 7Li{1H} NMR spectra of 1 in C7D8.
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
O
t-Bu
t-Bu
N
N
N
N
N
N
O
O
O
O
G
O
O
O
O
Li
Li
Li
Li
O
Li
O
Li
O
t-Bu
t-Bu
Li2[O2N2]BuBuIm(THF)2
Scheme 2. Proposed tetralithium-dilithium equilibrium for 1.
1
t-Bu
t-Bu

M.W. Drover et al. / Polyhedron 102 (2015) 60–68
65
Fig. 4. Gibbs free energy of Li–Li exchange (
D
G) for 1.
Fig. 6. Molecular structure diagram of 2. Thermal ellipsoids shown at 50%
probability and H-atoms removed for clarity.
parameters to those we have used previously [10]. 1H NMR spec-
troscopy was used to follow the reaction, with aliquots being
extracted at appropriate intervals and % conversion values calcu-
lated. After 2.5 h, 95% conversion was obtained. Similar conversion
levels of 97% and 100% were obtained in our previous studies using
0
trimetallic amine-phenolate complexes {Li[ONN]RR
}
3
[R = Me,
R0 = t-Bu and R = R0 = t-Am] at 25 °C, after 30 and 40 min [10]. The
longer reaction time required (under identical conditions) for 1
to achieve similar conversions may reflect the tendency of this
complex to maintain its aggregated state in solution, as observed
in the NMR studies reported above. NMR analyses of the resulting
polymers show that they contain the expected polycaprolactone
methylene signals at 1.39, 1.64, 2.32 and 4.07 ppm in CDCl3 at
298 K, and are consistent with those previously reported [10,31].
Fig. 5. Semilogarithmic plot for the ring-expansion polymerization of
e-caprolactone initiated by 1.
t-Bu
N
t-Bu
N
N
N
Bi
Cl
O
O
(i) 2 eq. n-BuLi, THF, -78
C
HO
t-Bu
t-Bu
OH
t-Bu
t-Bu
(ii) BiCl3, THF, -78
C
t-Bu
H2[O2N2]BuBuPip
t-Bu
t-Bu
Hydrolytically unstable
t-Bu
t-Bu t-Bu
Cl
O
O
N
N
N
Bi
Bi
Bi
O
O
O
Cl
Adventitious
moisture
N
Bi
O
Cl
Cl
t-Bu t-Bu
t-Bu
t-Bu
2
Scheme 3. Synthesis of complex 2.

66
M.W. Drover et al. / Polyhedron 102 (2015) 60–68
Table 3
Selected bond lengths (Å) and angles (°) for 2.
Bi1–O1
Bi1–O2
Bi1–O5
Bi1–N1
Bi1–N2
Bi2–Cl1
Bi2–Cl2
Bi2–Cl3
2.366(4)
2.294(5)
2.060(5)
2.491(7)
2.450(6)
2.642(3)
2.570(2)
3.016(2)
Bi2–O1
Bi2–O4
Bi2–O5
Bi2–O6
Bi3–Cl3
Bi3–Cl4
Bi3–O2
Bi3–O3
2.482(5)
2.509(5)
2.461(4)
2.489(4)
2.783(2)
2.606(2)
2.654(6)
2.660(6)
Bi3–O5
Bi3–O6
Bi4–O3
Bi4–O4
Bi4–O6
Bi4–N3
Bi4–N4
2.175(5)
2.164(5)
2.294(6)
2.364(5)
2.061(5)
2.474(6)
2.483(7)
O1–Bi1–O2
O1–Bi1–O5
O1–Bi1–N1
O1–Bi1–N2
O2–Bi1–O5
O2–Bi1–N1
O2–Bi1–N2
O5–Bi1–N1
O5–Bi1–N2
N1–Bi1–N2
Cl1–Bi2–Cl2
Cl1–Bi2–Cl3
Cl1–Bi2–O1
Cl1–Bi2–O4
Cl1–Bi2–O5
Cl1–Bi2–O6
Cl2–Bi2–Cl3
Cl2–Bi2–O1
Cl2–Bi2–O4
Cl2–Bi2–O5
Cl2–Bi2–O6
Cl3–Bi2–O1
Cl3–Bi2–O4
Cl3–Bi2–O5
Cl3–Bi2–O6
126.7(s2)
74.5(2)
79.4(2)
142.3(2)
78.3(2)
144.8(2)
81.3(2)
88.9(2)
90.1(2)
65.9(2)
86.69(8)
125.94(7)
85.0(1)
86.5(1)
143.4(1)
146.6(1)
147.36(7)
107.4(1)
105.8(1)
81.7(1)
81.7(1)
78.8(1)
79.1(1)
71.6(1)
70.4(1)
O3–Bi4–N4
O4–Bi4–O6
O4–Bi4–N3
O4–Bi4–N4
O6–Bi4–N3
O6–Bi4–N4
N3–Bi4–N4
Bi2–Cl3–Bi3
Bi1–O1–Bi2
Bi1–O1–C1
Bi2–O1–C1
Bi1–O2–Bi3
Bi1–O2–C19
Bi3–O2–C19
Bi3–O3–Bi4
Bi3–O3–C36
Bi4–O3–C36
Bi2–O4–Bi4
Bi2–O4–C54
Bi4–O4–C54
Bi1–O5–Bi2
Bi1–O5–Bi3
Bi2–O5–Bi3
Bi2–O6–Bi3
Bi2–O6–Bi4
143.3(2)
76.7(2)
143.0(2)
78.9(2)
90.0(2)
88.2(2)
Bi3–O6–Bi4
Cl4–Bi3–O2
Cl4–Bi3–O3
Cl4–Bi3–O5
Cl4–Bi3–O6
Cl3–Bi3–Cl4
Cl3–Bi3–O2
Cl3–Bi3–O3
Cl3–Bi3–O5
Cl3–Bi3–O6
O3–Bi4–O4
O3–Bi4–O6
O2–Bi3–O5
O2–Bi3–O6
O3–Bi3–O5
O3–Bi3–O6
O5–Bi3–O6
O3–Bi4–N3
O2–Bi3–O3
O1–Bi2–O4
O1–Bi2–O5
O1–Bi2–O6
O4–Bi2–O5
O4–Bi2–O6
O5–Bi2–O6
117.5(2)
100.4(1)
102.1(1)
89.1(1)
88.3(1)
165.61(7)
85.1(1)
81.0(1)
80.4(1)
79.7(1)
129.2(2)
78.4(2)
68.7(2)
143.7(2)
142.9(2)
68.9(2)
76.3(2)
79.7(2)
140.7(2)
145.1(2)
65.9(2)
128.5(2)
130.0(2)
66.9(2)
65.5(2)
66.1(2)
75.61(5)
104.1(2)
109.9(4)
138.5(4)
93.8(2)
114.7(5)
144.0(5)
93.4(2)
141.8(5)
116.3(5)
102.5(2)
138.4(5)
111.3(5)
115.2(2)
117.5(2)
100.1(2)
99.6(2)
113.0(2)
Fig. 7. (a) Side View and (b) Top View for the Bi4(O)2(OAr)4(NR2)4 core structure of compound 2. Ellipsoids are shown at the 50% probability level.
No signals could be assigned to polymer end groups (e.g. CH2OH at
3.65 ppm), suggesting that the polymers obtained were cyclic
polycaprolactone. 13C{1H} also confirmed the absence of a CH2OH
concentration. Subsequent thermodynamic analysis employed
the Arrhenius equation to illustrate the effect of a change in tem-
perature on the rate of reaction (R2 = 0.9285). This plot gave
Ea = 50.94 kJmolꢁ1 and a pre-exponential factor, A of 6.42 ꢃ 105 -
minꢁ1. This activation energy is a little lower than those we
reported for trimetallic lithium amine-phenolate catalysts (56.8–
67.2 kJmolꢁ1) in the absence of a co-initiator [10].
peak. The molecular weight of the polymer was 19000 g molꢁ1
,
which is significantly higher than the theoretically expected value
based on the initial [ -CL]:[Li] ratio and this discrepancy is likely
e
due to transesterification reactions occurring. The dispersity of
the polymer was 1.62, which also suggests that the reaction was
not well-controlled and that side reactions occured.
In order to gain a better understanding of these reactions, a ser-
ies of reactions were conducted at varying temperatures; 25, 40, 60
and 80 °C and semilogarithmic plots of the form, ln[CL]0/[CL]t vs.
time (min) were constructed (Fig. 5). These graphs are linear in
3.3.2. Using 1 and Li2[O2N2]BuBuPip as a ligand transfer reagent
In our group, we are interested in the synthesis of p-block coor-
dination compounds for use as catalysts in transformations of car-
bon dioxide. Others have shown that bismuth is a promising metal
in this regard [32,33]. Given the similar ionic radius of Bi3+ with the
lanthanides, which have already been studied with this type of
nature suggesting
a first order dependence on monomer

M.W. Drover et al. / Polyhedron 102 (2015) 60–68
67
Table 4
Comparison of Bi–Clbridge bond lengths in 2 with others reported to date.
Compound
Bi–Clbridge (Å), Bi coordination No.
Ref.
2, Bi4(
[TmBiCl(
ButCalix-8(BiCl)4(LiCl)6(DME)6(THF)3
l
-Cl)(
l
-O)2[L2]2(Cl)3
2.783(2), 6
2.807(5), 6
2.8421(11), 6
2.9325(13), 6
2.859(1), 4
2.746(4), 5
2.729(4), 5
3.076(2), 7
3.009(5), 6
3.0266(12), 6
3.0020(12), 5
3.074(13), 4
2.763(3), 5
2.852(4), 5
this work
[39]
[35]
[40]
[41]
l-Cl)]2
[NMe4]2[Bi2(2,6-OC6H3Me2)6(
Mo(OMe)5(CH3CN)Bi2Cl7
[Ph2BiCl]1
l-Cl)2]
[42]
[43]
[{(Me3Si)2CHBiCl2}ꢀEt2O]1
ligand [26], we attempted to prepare a bismuth complex using 1 as
the ligand transfer reagent. Upon stirring 1 or a closely related Li
complex (where the imidazoline unit has been replaced by a
homopiperazine group) with BiCl3 in THF, a colour change occurs
and pale orange solids could be isolated (Scheme 3). Elemental
analysis of these powders was in agreement with the formulations
Bi[O2N2]BuBuImCl and Bi[O2N2]BuBuPipCl. Mass spectrometric analy-
sis of Bi[O2N2]BuBuPipCl also indicated that the ligand had been suc-
cessfully transferred to bismuth. However, attempts to obtain NMR
spectra of these complexes was thwarted due to their instability in
common deuterated solvents at room temperature. During
attempts to recrystallize these species, we were able to isolate yel-
low crystals of 2, which we propose forms via reaction with water.
The formation of such bismuth oxo-clusters is a well-known phe-
nomenon in the coordination chemistry of bismuth, as discussed
below. Mass spectrometric data for 2 was nearly identical to that
of the orange powder in that the highest mass ions observed
matched a formulation of Bi[O2N2]BuBuPipCl.
332.87° (l3-O(6)) and 330.12° (l3-O(5)). These oxo-ligands are
very closely trigonal planar in structure, suggesting involvement
of the oxygen lone pair in bonding to Bi. Distant toluene molecules
were found to reside in empty spaces made by the coordinated
ligands, but did not interact with any of the four Bi. Although the
Bi4(O)2(OAr)4 core is not planar, a single plane consisting of
Bi(1)–Bi(4) is present. As a result, least squares plane analysis
was performed on 2 with Bi(1)–Bi(4) defining the plane of interest.
Bi(1)–Bi(4) were found to be coplanar with an average mean of
0.0594 Å. All atom-plane distances are in good agreement
(0.633–0.730 Å). Atoms O(1), O(2), O(3) and O(4) coordinate above
the plane with atom-plane distances ranging from 0.633–0.730 Å
and O(5) and O(6) coordinate below the plane with atom-plane
distances of 0.691 and 0.725 Å respectively (Fig. 7(a)).
4. Conclusions
In summary, 1 a tetrametallic Li complex and 2 a tetrametallic
Bi cluster have been prepared and structurally characterized. 1
was also extensively studied via variable temperature solution
and solid state 7Li{1H} NMR spectroscopy, which revealed solu-
tion-phase dynamics and increased asymmetry in bulk samples
of 1 compared with the sample used for single-crystal X-ray
diffraction analysis. Interestingly, MALDI-TOF MS supports the
presence of a Li4 moiety in the gas phase. Together, the data
obtained on 1 infer a strong tendency to remain tetrametallic over
bimetallic in all states of matter. Due to the reluctance of 1 to dis-
3.3.2.1. Molecular structure of 2. The structure of 2 (Bi4(l2-Cl)(l3-
O)2[L2]2(Cl)3) is shown in Fig. 6, with selected bond distances
and angles in Table 3. This compound is a tetrametallic Bi-contain-
ing, oxo-bridged cluster consisting of two diamine-bis(phenolate)
ligands. Two bridging oxide moieties, O(5) and O(6) do not belong
to the ligand framework, but instead are likely to have originated
from adventitious moisture, as previously observed by others
[17]. The oxo-cluster reported herein consists of two 5-coordinate,
one 6-coordinate, and one 7-coordinate bismuth(III). Both Bi(1)
and Bi(3) possess highly distorted square-based pyramidal geome-
tries, while Bi(2) and Bi(3) possess irregular trigonal prismatic and
face-capped octahedral geometries, respectively. This coordination
framework results in seven fused rings, five of which arise from
sociate, it shows comparatively low reactivity towards ROP of e-CL
but is reactive in the absence of co-initiator and kinetic data for
this ring expansion polymerization were obtained. Use of lithium
amine-phenolate complexes including 1 to prepare molecular bis-
muth compounds was thwarted by the moisture-sensitive nature
of the resulting products. However, a bismuth-oxo cluster complex
2 was structurally characterized, which has bonding parameters
similar to related examples in the literature.
linking Bi2(l-O)2 units and two from conjoining Bi2(l-O)2 and
Bi2( -Cl) units. Overall, this compound features a Bi4O2(OAr)4 core.
l
This structural motif is quite common among Bi-containing clus-
ters, in particular of those containing oxygen-containing bridges
[16–18,20,34–37]. Indeed, 2 does not achieve complete planarity,
remaining buckled or pinched in a ‘‘boat-like’’ conformation
(Fig. 7). This results from a bridging chloride ligand, which tethers
the molecule together via a Bi(2)–Cl(3)–Bi(3) bridge. Similar Bi–Cl–
Bi bridge bonds have been reported elsewhere, and are summa-
rized in Table 4. In most cases, one Bi–Cl interaction is shorter than
the other with ranges of 2.746(4)–2.9325(13) Å for the shorter one
and 2.763(3)–3.076(2) Å for the longer. A correlation with the
coordination number of bismuth can also be seen with a longer
Bi–Cl bond consistent with a greater Bi coordination number. In
compound 2, Bi(3) is 6-coordinate and has a Bi–Clbridge distance
of 2.783(2) Å compared to Bi(2) which is 7-coordinate, with a
bridging distance of 3.076(2) Å. The Bi–OAr distances in 2 range
from 2.29 to 2.37 Å and are comparable with other bismuth arylox-
Acknowledgements
The Natural Sciences and Engineering Research Council (NSERC)
of Canada for funding in the form of research grants (Discovery and
Research Tools and Instruments) to FMK. The Dean of Science at
Memorial University for an Undergraduate Research Award to JNM.
Appendix A. Supplementary data
CCDC 1403400 and 1403401 contains the supplementary crys-
tallographic data for 1 and 2. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1016/j.poly.2015.07.071.
ide systems [20,21,36,38]. The geometries about the
are also very similar to Bi-compounds reported elsewhere [19].
The sum of the Bi–O –Bi angles around 3-O atoms in 2 are
l3-O atoms
l
l

68
M.W. Drover et al. / Polyhedron 102 (2015) 60–68
[22] K.L. Collins, L.J. Corbett, S.M. Butt, G. Madhurambal, F.M. Kerton, Green Chem.
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