Alkali Metal and Alkaline Earth Metal Complexes with the Bis(borane-diphenylphosphanyl)amido Ligand - Synthesis, Structures, and Catalysis for Ring-Opening Polymerization of ε-Caprolactone

Article abstract of DOI:10.1002/zaac.201500593

The syntheses of lithium and alkaline earth metal complexes with the bis(borane-diphenylphosphanyl)amido ligand (1-H) of molecular formulas [{κ²-N(PPh₂(BH₃))₂}Li(THF)₂] (2) and [{κ³-N(PPh

Full text of DOI:10.1002/zaac.201500593

Journal of Inorganic and General Chemistry
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ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
DOI: 10.1002/zaac.201500593
Alkali Metal and Alkaline Earth Metal Complexes with the Bis(borane-
diphenylphosphanyl)amido Ligand – Synthesis, Structures, and Catalysis for
Ring-Opening Polymerization of ε-Caprolactone
Jayeeta Bhattacharjee,[a] Suman Das,[a] T. Dhileep N. Reddy,[a] Hari Pada Nayek,[b]
Bhabani S. Mallik,*[a] and Tarun K. Panda*[a]
Dedicated to Professor Kazushi Mashima on the Occasion of His 59th Birthday
Keywords: Lithium; Calcium; Barium; Boranes; Metallacycles; Chelating ligands
Abstract. The syntheses of lithium and alkaline earth metal
single-crystal X-ray diffraction analysis. In the solid-state structure of
complexes with the bis(borane-diphenylphosphanyl)amido ligand
2, the lithium ion is ligated by amido nitrogen atoms and hydrogen
(1-H) of molecular formulas [{κ2-N(PPh2(BH3))2}Li(THF)2] (2) and atoms of the BH3 group in κ2-coordination of the ligand 1 resulting in
[{κ3-N(PPh2(BH3))2}2M(THF)2] [(M = Ca (3), Sr (4), Ba (5)] are re-
ported. The lithium complex 2 was obtained by treatment of bis(bor- complex 5, κ3-coordination of the ligand 1 was observed, and the bar-
ane-diphenylphosphanyl)amine (1-H) with lithium bis(trimethylsilyl)- ium ion adopted a distorted octahedral arrangement. The metal com-
a distorted tetrahedral geometry around the lithium ion. However, in
amide in a 1:1 molar ratio via the silylamine elimination method. The plex 5 was tested as catalyst for the ring opening polymerization of ε-
corresponding homoleptic alkaline earth metal complexes 3–5 were caprolactone. High activity for the barium complex 5 towards ring
prepared by two synthetic routes – first, the treatment of metal opening polymerization (ROP) of ε-caprolactone with a narrow poly-
bis(trimethylsilyl)amide and protio ligand 1-H via the elimination of dispersity index was observed. Additionally, first-principle calculations
silylamine, and second, through salt metathesis reaction involving re-
to investigate the structure and coordination properties of alkaline earth
spective metal diiodides and lithium salt 2. The molecular structures metal complexes 3–5 as a comparative study between the experimental
of lithium complex 2 and barium complex 5 were established by
and theoretical findings were described.
suggested that a greater ring strain developed in dppa deriva-
tives as against the corresponding dppm ligand makes it more
attractive towards synthetic coordination chemistry.[3] Moodley
and co-workers[4] reported the synthesis of [Ru3(CO)10(μ-
Ph2PN(Et)PPh2)] containing a related ligand and prepared by
photochemical activation of [Ru3(CO)12], CoPd2 and Pd4 com-
plexes of dppa and other related ligands. These μ-bis(dialkyl-
Introduction
The ligand bis(diphenylphosphinoamine) (Ph2PNHPPh2,
dppa) has been attracted by a number of research groups due
to its ability to form short-bite angles in central metal atoms
present in bi- or polynuclear metal complexes owing to its
isoelectronic relationship with the more popular bis(diphenyl-
phosphinomethane) ligand (Ph2PCH2PPh2, dppm).[1] The in-
creased reactivity of dppa ligand can be explained by the en-
hanced acidity of N–H group as compared to that of the CH2
unit present in dppm.[2] The greater acidity of the NH proton
as compared to the CH2 protons may facilitate subsequent
functionalization reactions that would require very drastic con-
ditions in the case of dppm. Besides, some theoretical studies
phosphanyl)-alkane]bis[pentacarbonyl-metal(0)]
complexes
(CO)5MR2P(NH)-PR2M(CO)5 (M = Cr, Mo, W) were synthe-
sized from the reaction of M(CO)5(CH3CN) with the dppa li-
gand.[2,5–7] The chalcogen (sulfur and selenium) functionalized
dppa derivatives were also well defined to stabilize metal com-
plexes by the formation of chalcogen metal bonds.[8,9,10] These
unique ligands are potentially capable of coordinating through
the hard nitrogen and phosphorus donor atoms along with the
soft chalcogen donor atom to provide the highest coordination
sphere of the metal ion required for its stabilization.
The tetra-phenyldithioimidodiphosphine (SPPh2NHPPh2S)
acts as an activating agent. It can also act as a neutral or, more
commonly, anionic bidentate (SPPh2NPPh2S)– ligand. Its high
structural flexibility makes this ligand very versatile in binding
metal ions and thus forming square planar, tetrahedral, or octa-
hedral complexes through the sulfur atoms.[11,12]
* Dr. T. K. Panda
Fax: + 91-40-23016032
E-Mail: tpanda@iith.ac.in
* Dr. B. S. Mallik
E-Mail: bhabani@iith.ac.in
[a] Department of Chemistry
Indian Institute of Technology Hyderabad
Kandi – 502 285, Sangareddy, Telangana, India
[b] Department of Applied Chemistry
Indian School of Mines
Dhanbad, 826004, Jharkhand, India
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201500593 or from the au-
thor.
The di-chalcogenoimido-diphosphinto bismuth compounds,
Bi[(EPR2)2N]3 (E = S, Se; R = Ph, iPr), were found to be ideal
Z. Anorg. Allg. Chem. 2016, 642, (2), 118–127
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for use as single-source precursors as they are air-stable and 1984.[40] Herein, a similar procedure to the preparation of
readily obtainable in good synthetic yields.[13] The diselenoim- bis(diphenylphosphine)amine (dppa) was used. Initially, one
ido-diphosphinato ligand can also stabilize a wide range of equivalent of hexamethyldisilazane and two equivalents of
metal coordination spheres including alkali metals,[14–15] group chlorodiphenylphosphine, both heated to 60 °C were mixed.
12,[16] group 13,[17,18] group 14,[19,20,16] group 15,[21] group To this in situ prepared dppa, two equivalents of Me2S·BH3
16,[22,23] transition metals (V and Cr,[24] Mn[25,26] and Re,[27] were added dropwise at 80 °C (Scheme 1). The ligand 1-H
Ru, Rh, Ir,[15,28,29] Os,[30] Co,[31] group 10: Ni,[32] Pd,[32,33–35] was re-crystallized from hexane and was fully characterized
Pt,[14,30,34] and group 11:[35], and rare earth metals.[36,37] This by using spectroscopic and analytical techniques. The 1H,
could be due to the flexible nature of the ligand moieties, 31P{1H}, and 11B{1H} NMR spectra showed one set of signals
which adopt to several metallacyclic rings depending upon the for 1-H. The resonances for the BH3 protons appeared as a
coordination to the central metal atom. We had recently re- broad signal at δ = 1.86 ppm. The resonance of the NH proton
ported that the missing group 2 metal complexes of di-seleno- could be observed as a singlet at δ = 4.46 ppm. In the 31P{1H}
imidodiphosphinato ligand by synthesizing strontium and bar- spectrum of 1-H, a doublet signal at δ = 65.6 ppm was ob-
ium complexes via two synthetic routes - using diselenoimido- served. The boron atoms present in 1-H showed a doublet
diphosphinato ligand with either silylamide or the salt metathe- resonance centered at δ –35.9 ppm in 11B{1H} spectrum.
sis route). We observed that the strontium complex adopted a
distorted octahedral arrangement, whereas the environment of
the larger barium atom in the complex could best be described
as distorted pentagonal bipyramidal with M–Se bond lengths
increasing from Sr–Se to Ba–Se bond.[38]
In continuation of our studies on the heavier group 2 metal
Scheme 1. Synthesis of ligand 1-H from dppa.
with various chalcogen and borane derivatives of amidophos-
phines,[39] we observed that, if the phosphorus lone pairs in
aminophosphines were blocked by BH3 group by adduct for-
mation, the N–H became more acidic in comparison
with parent aminophosphines (RNHPPh2). Further, the
hydrogen atoms of the borane unit appeared to be preferred as
donors by the divalent alkaline-earth cations and other metal
complexes chemistry. This concept was supported by
Lithium Complex 2
The lithium complex of molecular composition [{κ2-
N(Ph2P(BH3))2}Li(THF)2] (2) was readily obtained as a color-
less compound by the treatment of bis(borane-diphenylphos-
phanyl)amine (1-H) with lithium bis(trimethylsilyl)-amide
[Li{N(SiMe3)2}] in a 1:1 molar ratio, in THF, and at ambient
temperature (Scheme 2). Complex 2 was fully characterized
alkaline
earth
metal
derivatives
such
as
[(THF)nM{(Ph2CH)NPPh2(BH3)}2(THF)n] (M = Ca, Sr, Ba)
and [(THF)2Ba{Ph2P(BH3)NCH2CH2NPPh2(BH3)}]2 com-
plexes as reported by us previously.[39f] In the dppa system,
1
by combustion analysis and H, 31P{1H}, and 11B{1H} NMR
spectroscopic technique. The molecular structure in the solid
state of the complex 2 was determined by single-crystal X-ray
diffraction analysis.
the
bis(borane-diphenylphosphanyl)amido
ligand
[(BH3)PPh2N-PPh2(BH3)] had been reported by Nöth and
Fluck in 1984.[40] Their coordination chemistry with metals
have however not been explored till date. In our ongoing pro-
ject in alkaline earth metal complexes with dppa based ligand
system, we were interested to develop the coordination chem-
istry of alkali metal and heavier alkaline earth metal complexes
with bis(borane-diphenylphosphanyl)amido ligand.
Herein, we report the synthetic details of bis(borane-di-
phenylphosphanyl)amido
calcium, strontium, and barium of molecular formulas
[{κ2-N(PPh2(BH3))2}Li(THF)2] [{κ3-
(2) and
complexes
of
lithium,
Scheme 2. Synthesis of lithium complex 2.
N(PPh2(BH3))2}2M(THF)2] [(M = Ca (3), Sr (4), Ba (5)]. The
molecular structures of complexes 2 and 5 are described along
with the ROP activity of ε-caprolactone using complex 5 as
catalyst. In addition, the first-principle calculations to investi-
gate the structure and coordination properties of alkaline earth
metal complexes 3–5 are also presented.
In 1H NMR spectrum of lithium compound 2, the broad
signal observed centered at δ = 1.63 ppm, represented the reso-
nance of three hydrogen atoms present in borane (BH3) group
attached to the phosphorus atom. This is slightly high-field
shifted as compared to that of a neutral ligand 1-H (δ =
1.86 ppm). Additionally, the resonances of protons for coordi-
nated THF molecules appeared at δ = 3.57 and 1.33 ppm as
multiplets, and these values are in the expected range. The
proton decoupled 31P{1H} NMR spectrum showed only one
Results and Discussion
Bis(borane-diphenylphosphanyl)amine (1-H)
The bis(borane-diphenylphosphanyl)amine was first synthe- doublet at δ = 55.9 ppm which was up-field shifted compared
sized and structurally characterized by Nöth and Fluck in to the corresponding value in 1-H (δ = 65.6 ppm). A doublet
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at δ = –35.1 ppm was observed in the 11B{1H} NMR spectrum edge, the lithium complex 2 is the first example of alkali metal
indicating a coupling with the adjacent phosphorus atom. complexes having bis(borane-diphenylphosphanyl)amide in
However, only one set of signals in 1H, 31P{1H}, and 11B{1H} the coordination sphere.
NMR spectra was observed due to chemical and magnetic
Table 1. Crystallographic details of details of complexes [{κ2-
equivalence as a consequence of an operative dynamic equilib-
rium in solution. The 31P{1H} and 11B{1H} chemical shifts
were found to be in good agreement with amidophosphine
chalcogen and borane complexes of lithium as reported by
us.[39]
N(PPh2(BH3))2}Li(THF)2] (2) and [{κ3-N(PPh2(BH3))2}2Ba(THF)2]
(5).
2
5·THF
Formula
C32H42B2LiNO2P2
563.17
orthorhombic
Pca21
C60H76B4BaN2O3P4
1177.68
monoclinic
P21/c
Formula weight
Crystal system
Space group
The lithium complex 2 was re-crystallized from THF/pent-
ane (1:2) and crystallized in orthorhombic space group Pca21
having four molecules in the unit cell. The solid-state structure a /Å
17.8254(10)
10.6111(7)
17.0236(12)
90
90
90
3220.0(4)
4
1.162
18.7605(17)
14.7581(4)
23.1357(12)
90
105.896(7)
90
6160.6(7)
4
1.270
b /Å
of complex 2 is given in Figure 1 along with the selected bond
angles and bond lengths. The details of the structural param-
c /Å
α /°
β /°
eters are given in Table 1.The solid-state structure of 2 con-
firmed a κ2-N, (BH3) coordination mode for ligand 1. The co-
γ /°
V /Å3
Z
ordination polyhedron of the lithium complex 2 was formed
by the ligation of one amido nitrogen atom and one borane
Density /Mg·m–3
hydrogen atom each of the ligand moiety and in a κ2 fashion
along with coordination from two THF molecules. Due to
smaller ionic radii, no interaction between lithium and phos-
phorus atoms was observed. Li–N bond length 2.08(2) Å was
within the range of [1.97(3) Å] as previously reported by our
group for [{κ2-Ph2CHNP(BH3)Ph2}Li(THF)2] complex, and
also similar to literature reports.[24] The Li–O distances of
1.93(2) and 1.91(2) Å were within the range of reported val-
ues. The lithium atom adopted a distorted tetrahedral arrange-
ment due to the ligation of amido atom, borane hydrogen atom,
and two THF molecules. A five-membered metallocycle Li1–
N1–P2–B1–H was formed by Li1, N1, P2, B1 and hydrogen
atom with the distance between lithium and boron atoms being
2.58(3) Å. The P1–N1–P2 bond angle 140.5(6)° was slightly
larger than that of the P–N–P bond angle 131.6° observed in
the neutral ligand upon coordination to the lithium ion – this
clearly indicated that amide nitrogen atom has strong interac-
tions with the central metal atom. To the best of our knowl-
T /K
Radiation
150(2)
150(2)
Cu-Kα (λ =
1.54184 Å)
1.433
Mo-Kα (λ =
0.71073 Å)
0.792
2440
Multi-scan
36574
μ /mm–1
F(000)
1200
Absorption correction Multi-scan
Reflections collected 6785
Unique reflections
Completeness to θ
GOF
4292 [Rint = 0.0699] 14217 [Rint = 0.0303]
98.3%
1.031
Full-matrix least-
squares on F2
0.0864; 0.2441
99.8%
1.051
Full-matrix least-
squares on F2
0.0517; 0.0886
Refinement method
R1; wR2
Alkaline Earth Metal Complexes 3–5
Currently, the organometallic chemistry of heavier alkaline
earth metals calcium, strontium, and barium is experiencing a
remarkable renaissance. While, for decades, this chemistry was
mainly focused on cyclic π-perimeter hydrocarbons like cyclo-
pentadienide anions,[41] recent years have seen exciting devel-
opments in the field of aryl,[42] allyl,[43] and benzyl deriva-
tives.[44] We, and others working in groups have been engaged
in determining the structure and reactivity of alkaline earth
metal species with reference to the design and development of
efficient catalysts.[39,45] The heavier alkaline earth metal com-
plexes [{κ3-N(PPh2(BH3))2}2M(THF)2] [(M = Ca (3), Sr (4),
Ba (5)] were readily prepared in high yield by two synthetic
routes. The first route was through the reaction between the
lithium salt (2) and corresponding alkaline earth metal diiod-
ides in THF solvent at ambient temperature (Scheme 3). In the
second method, the protonated ligand 1-H was treated with
metal bis(trimethylsilyl)amide to obtain complexes 3–5 via the
elimination of volatile bis(trimethylsilyl)amine (Scheme 3).
The pure complexes could be obtained from re-crystallization
of the crude compound from THF/pentane mixture. All the
complexes were air- and moisture sensitive, and sparingly sol-
uble in aromatic solvents such as benzene and toluene. All
three compounds were characterized by combustion analysis
and spectroscopic techniques; the solid-state structure of bar-
Figure 1. Molecular structure of lithium complex 2. Hydrogen atoms
are omitted for clarity.
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ium complex 5 was established through single-crystal X-ray was however established. The barium bis(borane-diphenyl-
diffraction analysis.
phosphanyl)amide complex 5 crystallized in the monoclinic
space group P21/c, having, in the unit cell, four molecules of
5 and one THF molecule. The details of the structural param-
eters of complex 5 are given in Table 1. The molecular struc-
ture of complex 5 is given in Figure 2.
Scheme 3. Syntheses of alkaline earth metal complexes 3–5.
A strong absorption band in each complex at 695 cm–1 (3),
699 cm–1 (4), and 695 cm–1 (5) in FT-IR spectra indicated the
evidence of P–B bond in the metal complexes. In the 1H NMR
spectra of complexes 3–5 measured in C6D6, the NH proton
of the ligand (1-H), which appeared at δ = 4.46 ppm, was ab-
sent due to deprotonation of the protonated ligand. A very
broad signal for each complex centered at δ = 2.1 ppm (for 3),
1.9 ppm (for 4), and 1.7 ppm (for 5) respectively with integra-
tion of six could be assigned to the hydrogen atoms of two
BH3 groups present in the ligand 1. The multiplet signals at δ
= 3.55 and 1.43 ppm (3), 3.59 and 1.40 ppm (4), and 3.57 and
1.44 ppm (5) could be assigned to the resonance of protons
from solvated THF molecules coordinated to the central metal
atom. One set of resonance signals for the phenyl protons were
also observed, which were in the same region as that of ligand
(1-H), thus indicating no significant effect of metal atoms onto
the phenyl groups due to complex formation. In 31P{1H} NMR
spectra of complexes 3–5, both the phosphorus atoms present
in the two bis(borane-diphenylphosphanyl)amide moiety were
chemically equivalent and showed only one doublet signal at
δ = 52.1 ppm (3) 46.6 ppm (4), and 53.1 ppm (5) respectively.
These values were significantly up-field shifted to that of com-
pound (1-H) (δ = 65.6 ppm) upon coordination of calcium,
strontium, and barium ions onto the ligand (1-H). These obser-
Figure 2. ORTEP drawing of complex 5 showing the atom labeling
scheme; ellipsoids are drawn to encompass 30% probability. Hydrogen
atoms are omitted (except BH3) for clarity. Selected bond lengths /
Å and bond angles /°: Ba1–N1 2.837(2), Ba1–N2 2.769(2), Ba1–O1
2.7225(19), Ba1–O2 2.794(2), Ba1–H1a 2.82(3), Ba1–H2b 2.87(3),
Ba1–H3a 2.88(3), Ba1–H4b 2.96(3), P1–N1 1.623(2), P2–N1 1.625(2),
P1–B1 1.919(3), P2–B2 1.924(3), P3–N2 1.627(2), P4–N2 1.631(2),
P3–B3 1.921(3), P4–B4 1.923(3), O1–Ba1–N2 130.49(6) O1–Ba1–O2
81.24(8), N1–Ba1–N2 129.16(6), H4b–Ba1–H2b 69.6(9), H4b–Ba1–
H3a 120.2(8), H4b–Ba1–H1a 137.9(8), H4b–Ba1–O1 72.6(6), H4b–
Ba1–O2 101.5(6), H4b–Ba1–N2 62.9(6), N2–Ba1–H3a 66.9(6), H2b–
Ba1–N1 66.2(7), H1a–Ba1–N1 66.0(6), P1–N1–P2 141.52(14), P3–
N2–P4 137.81(13).
The solid-state structure of 5 confirmed a κ3-coordination
vations were similar to our previous studies (δ = 46.9 ppm for mode for 1. The coordination polyhedron was formed by two
Ca, 46.3 ppm for Sr, and 46.9 ppm for Ba), where an alkaline {(P(BH3)Ph2)2N}– ligands and two THF molecules. The
earth metal ion was bound to a amidophosphine-borane ligand Ba–N distances [2.837(2) Å and 2.769(2) Å] were found to be
(δ = 56.8 ppm).[39d] In the 11B{1H} NMR experiment for each slightly elongated in comparison to the barium-nitrogen
of the complexes 3–5, a broad doublet signal centered at δ = covalent bond lengths of 2.720(4) and 2.706(4) Å as re-
–35.5 ppm (3), –31.6 (for 4), and –30.1 ppm (5) ppm, respec- ported for [Ba((Dipp)2DAD)(μ-I)(THF)2]2 [Dipp2DAD =
tively, were observed; these values were in the same range bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene] in the
(–35.4 ppm) as that of the starting material 1-H.
literature.[47] These Ba–N distances were also slightly
Although there has been continued interest in alkaline earth elongated compared to the previously reported barium
organometallics[46a] and particularly in the cyclopentadienyl complexes [Ba(THF)2{Ph2P(BH3)N(CHPh2)}2] [2.733(6) Å]
chemistry of these elements,[46b,c] complexes 3–5 represent, to and
[(THF)2Ba{Ph2P(BH3)NCH2CH2NPPh2(BH3)}]2
the best of our knowledge, the first characterized alkaline earth [2.671(5) Å].[39f] It can be noted that in the analogous stron-
metal complexes of the bis(borane-diphenylphosphanyl)amide tium and barium complexes [{η2-N(PPh2Se)2}2Sr(THF)2] and
ligand. Molecular structure determinations of complexes 3–5 [{η2-N(PPh2Se)2}2Ba(THF)3] κ2-coordination of the diseleno-
in the solid state were attempted using X-ray diffraction analy- imidodiphosphinato ligand [N(PPh2Se)2]– alone was observed,
sis. X-ray quality single crystals were obtained from the mix- leaving the amido nitrogen uncoordinated.[38] The Ba–O
ture of THF and n-pentane for each of the complexes. The [2.723(19) and 2.794(2) Å] distances were in agreement with
calcium and strontium complexes 3 and 4, respectively, were literature reports.[39c,39d,39f] The Ba–P [3.6875(7) and
found to diffract very weakly under X-ray. Due to this fact, 3.7115(7) Å] distances were considerably elongated in barium
data obtained was of poor quality and could therefore not be bis(borane-diphenylphosohino)amide complex (5) and were
published. The molecular structure of the barium complex 5 greater than the sum of the covalent radii of barium and phos-
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phorus (3.34 Å), thereby indicating that barium and phos- and coordination properties of Ca, Sr, and Ba metal ions with
phorus had no interaction between themselves. Thus, in com- ligands containing BH3 and Se. The chemical nature of bonds
plex 5, the central atom barium adopted a distorted-snub-dis- between metal ion and ligands, as well as metal ion and sol-
phenoidal arrangement having 12 faces due to the ligation vent, were investigated to understand the coordinating behav-
from two ligand (1) moieties and two THF molecules (see Fig- ior of metal ions. All the model structures were drawn in
ure 3). This high distortion can be primarily attributed to the GaussView[48] and optimized at B3LYP/LANL2DZ level[49] of
coordination of adjacent atoms present in the ligand and as electronic structure calculations using the Gaussian09 (Rev
indicated by the large angles N1–Ba1–H1a 66.0(6)° and N1– B.01) software.[50] The chosen method, combining hybrid den-
Ba1–H2b 66.2(7)°, and N2–Ba1–H3a 66.9(6)° and N2–Ba1– sity functional and potential for the metals studied herein has
H4a 62.9(6)° as observed in the solid-state structure of com- been used in earlier studies involving alkaline earth metal com-
pound (5). The borane (BH3) groups were coordinating plexation.[51]
through the hydrogen atoms in η1-fashion. The Ba1–B bond
The computed optimized structures of the complexes are
lengths were hence found to be in the range of [3.271(3)– shown in Figure 4. The coordinating environments of alkaline
3.353(3) Å]. In the barium compound 5, due to ligation from earth metal ions are very similar to the experimentally obtained
two bis(borane-diphenylphospheno)amide moieties, each li- complex structures for ions, wherever available (for 5). Within
gand formed an eight-membered metallocycle which was a re- the approximations of adopted computational methods, the cal-
sult of two fused five-membered metallacyclic rings involving culated structural properties are a little overestimated; the error
Ba1, H1a, B1, P1, N1, P2, B2, H2b or Ba1, H3a, B3, P3, N2, is however within 1%. A comparison of computed bonds and
P4, B4, H4a atoms. The two ligand moieties were oriented in
such a manner that both ligands were almost perpendicular to
each other. This was clearly indicated by the dihedral angle of
87.27° found between the planes containing N1, B1, B2, Ba1
and N2, B3, B4, Ba1 atoms.
Figure 3. Distorted snub-disphenoid arrangement observed in barium
complex 5. All phenyl groups attached to the phosphorus atoms, and
all carbon atoms, are omitted for clarity.
Computational Analysis
Due to the weak diffracting nature of crystals from com-
plexes 3 and 4, the solid-state structures of calcium and stron-
tium complexes remained elusive. Given this, coordinating be-
havior of the metal ions towards the bis(diphenylphosphino-
borane)amido ligand (1) were not obtained. We were therefore
interested in modeling the calcium (3) and strontium (4) metal
complexes, whose crystals were weakly diffracted in the X-
ray. In addition, by optimizing their structure computationally,
we wanted more insight into the structure and bonding of the
barium complex 5 along with analogous complexes [{η2-
N(PPh2Se)2}2Ca(THF)2] (6) [{η2-N(PPh2Se)2}2Sr(THF)2] (7),
Figure 4. Computed structures of complexes of bis(diphenylphos-
and [{η2-N(PPh2Se)2}2Ba(THF)3] (8), and the diselenoimido-
phino-borane)amido ligand (1) (A, B, and C), and diselenoimidodipho-
diphosphinato ligand (1Ј) that was reported by us earlier.[38]
sphinato ligand (1Ј) (D, E, and F), respectively. The top, middle and
bottom panels represent respectively the complexes of Ca, Sr, and Ba
For a better comparison, we have studied six metal com-
metal ions. All the hydrogen atoms except those of borane are omitted
for visualization clarity. The coordinating bonds around the metals ions
ciples calculations were performed to investigate the structure are shown in Å units.
plexes through computational methods in this paper. First-prin-
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angles, and available experimental data, is given in the Sup- persive forces and adopted computational methodology. Al-
porting Information. The distances and angles are found to be though Ca and Sr show similar interaction with solvent THF
close to the crystal structure data obtained using X-ray diffrac- molecules, the Sr–O bond is larger than the Ca–O bond. The
tion analysis. For example, couple of calculated distances like Ba–O bond length is the highest among these three. We ob-
Ba–O and Ba–N distances are 2.82 and 2.84 Å, whereas the served a clear trend of increase in M–Se, M–O, M–N, and M–
corresponding experimental values are 2.794 and 2.722 Å H bond lengths with the increase in ionic radii of metal ions.
respectively, for complex 5 of bis(diphenylphosphino-borane)- Although experimental data is not available for Ca–Se, Ca–
amido ligand 1. The calculated Ba–Se and Ba–O distances in H(BH2), and Sr–H(BH2) complexes, our computational calcu-
complex 8 of diselenoimidodiphosphinato ligand (1Ј) are 3.58 lations reveal that calcium can make the shortest Ca–H(BH2)
and 2.77 Å, whereas the experimental values are 3.353 and contact among the alkaline earth metal ions. The shortest M–
2.708 Å, respectively.[38]
H distances are 2.68, 2.96, and 2.99 Å for Ca, Sr, and Ba,
Table 2 and Table 3 represent the comparison of calculated respectively. For a particular metal ion, the hydride interactions
and experimentally obtained values corresponding to signifi- with four BH2 groups are similar. The adopted level of calcula-
cant coordinating distances around the ions for both types of tion can describe the B–H···M interaction satisfactorily. For a
ligands. The computed M–X (X = O/N/Se) distances of a par- particular metal ion, the hydride interactions with two BH2
ticular complex are very similar, thus indicating a symmetric groups are similar.
nature of bonds. The variation in ionic radii for alkaline earth
metal ions plays a major role in the interaction of these metal
ions with ligands and solvents. Due to similar ionic radii, the
Ring Opening Polymerization Study
arrangements around the Ca and Sr ions are similar. The two
Sr–O distances are symmetric whereas, of the three Ba–O
bonds; only two are symmetric (1Ј). Three Sr–Se bonds and
The catalytic activity of the metal bis(borane-diphenylphos-
phanyl)amide complex 5 was performed (Scheme 4). As ob-
served by us earlier, the calcium and strontium complexes
were less active as compared to barium complexes.[39f,39h] Po-
lymerization studies with various monomer/catalyst ratios
were conducted at 25 °C in toluene. In addition, the catalytic
activity of corresponding barium complex of selenium ana-
logue 8 was compared with results obtained from complex 5.
Selected data with respect to complex 5 is given in Table 2.
two Sr–Se bonds have the same distances. The calculated Ba–
Se distances in barium complexes are symmetric, and the dif-
ferences are within 0.2 Å as compared to the measured dis-
tances. This deviation may be due to combined effects of dis-
Table 2. Comparison of computed M–O, M–N, and M–H distances
with experimental values (where applicable) for the calcium, stron-
tium, and barium complexes with ligand 1. The M···H distances for
two hydrogen atoms of BH3 orientated towards metal ion are shown.
Ca
Sr
Ba
Distance /Å
B3LYP
B3LYP
B3LYP
Expt
M···O
2.52
2.49
2.63
2.68
2.82
2.81
2.794
2.722
Scheme 4. ROP of ε-CL in toluene with barium complexes 5 and 8.
M···N
M···H
2.56
2.57
2.70
2.68
2.84
2.86
2.769
2.838
The catalytic efficiency of the newly synthesized barium
bis(borane-diphenylphosphanyl)amide complex 5 to promote
the ROP of ε-CL was evaluated (Table 4, entries 1–5). Having
the largest ionic radius of the barium atom, it was anticipated
that complex 5 would show the highest reactivity towards the
ROP of ε-CL, which is very similar to that observed in some
previously reported studies using other barium complexes.[52]
The control over the ROP process was rather good, affording
PCLs, and featuring a considerable match between the ob-
served (as determined by GPC) and calculated molar mass val-
ues, as well as narrow dispersity data (PDI up to 1.18, entry 4
in Table 4). In reality, we observed that as much as 700 ε-CL
units were converted in good yields (90–98%) within 5 min
and at 25 °C (Table 4, entries 1–5). The results obtained by
using complex selenium analogue 8 as catalyst (entry 6–10,
Table 4) displayed a lower yield (70%, entry 1) of PCLs as
compared to those obtained from complex 5 (98%, entry 1).
The control over the ROP process was also rather lower using
complex 8 as catalyst (entry 6–10, Table 4) than those obtained
from complex 5. Thus, the newly synthesized barium bis-
(borane-diphenylphosphanyl)amido complex 5 proved to be
2.89
2.68
2.73
2.72
2.96
3.53
3.28
3.24
2.96
3.23
3.03
2.83
2.86
2.90
3.06
3.14
3.09
3.18
3.14
3.05
3.04
3.23
3.09
2.99
2.828
3.130
3.052
2.879
2.968
2.982
2.969
2.872
Table 3. Comparison of computed M–O and M–Se distances with ex-
perimental values (where applicable) for the calcium, strontium, and
barium complexes with ligand 1Ј.
Ca
Sr
Ba
Distance /Å B3LYP
B3LYP Expt
B3LYP Expt
M···O
2.39
2.39
2.54
2.54
2.523
2.523
2.77
2.76
2.77
2.708
2.796
2.71
M···Se
3.17
3.17
3.17
3.17
3.30
3.31
3.30
3.31
3.101
3.126
3.101
3.101
3.58
3.58
3.58
3.56
3.384
3.384
3.353
3.352
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Table 4. Polymerization of ε-caprolactone initiated by barium metal complexes 5 and 8.a)
Entry
Cat.
[ε-CL]0/[M]0 Reac. timeb)
Conv.c) /%
Mn (theo)d)
/g·mol–1
Mn (GPC)e)
/g·mol–1
Mw (GPC)e)
/g·mol–1
Mw/Mn (PDI)f)
/min
1
2
3
4
5
6
7
8
5
5
5
5
5
8
8
8
100
200
300
400
700
100
300
500
4
4
4
4
4
5
5
5
98
90
85
80
83
70
63
30
9802
10102
10412
25921
88483
56681
4993
18796
14746
43150
104251
106413
8963
1.86
1.42
1.66
1.18
1.87
1.79
1.64
1.63
18003
25505
32007
43509
7000
18900
15000
5373
5420
8830
8854
a) Results are representative of at least two experiments. b) Reaction times were not necessarily optimized. c) Monomer conversions were
1
determined by H NMR spectroscopy. d) Theoretical molar mass values calculated from the relation: [monomer]0/[M]0 ϫmonomer conversion,
where [M]0 = 8.76ϫ10–3 mmol and monomer weight of ε-CL = 114 g·mol–1. e) Experimental molar masses were determined by GPC vs.
polyethylene glycol standards. f) Molar mass distributions were calculated from GPC.
Synthesis of [{Ph2P(BH3)}2NH] (1-H):[40] In a 100 mL Schlenk flask,
and efficient catalyst for ROP of ε-caprolactone rather than
one equivalent of hexamethyldisilazane (1.2 mL, 5.57 mmol) and two
equivalents of chlorodiphenyl phosphine (2 mL, 11.4 mmol) were
mixed together with toluene (20 mL) and heated over 80 °C for 12 h.
diselenoimidodiphosphinato barium complex 8.
After that, 2 equivalents of Me2S·BH3 (1.22 mL) were added dropwise
onto it with constant stirring at room temperature. The mixture was
Conclusions
kept whilst stirring for another 20 h. The solution was evaporated un-
Metal complexes of bis(borane-diphenylphopshino)amide li-
der reduced pressure. The colorless residue was further purified by
gand by synthesizing lithium and alkaline earth metal com-
washing with n-pentane or n-hexane. Yield: 2 g (90%). 1H NMR
plexes via either silylamide method or salt metathesis route are
(400 MHz, CDCl3): δ = 7.63–7.72 (m, 4 H, ArH), 6.89 –7.04 (m, 6 H,
ArH), 1.86 (br., 3 H, BH3), 4.46(s, 1 H, NH) ppm. 13C{1H} NMR
reported herein. The lithium complex 2 displays κ2-coordina-
tion of the ligand 1 to the lithium ion in the solid state; κ3-
(100 MHz, CDCl3): δ = 132.8 (P attached ArC), 132.7 (P attached o-
coordination mode was observed in complex 5, where the cen-
ArC), 131.7 (P attached o-ArC), 128.6 (P attached p-ArC), 128.5 (P
attached m-ArC), 127.3 (P attached m-ArC). 31P{1H} NMR
(161.9 MHz, CDCl3): δ = 65.7 ppm (d, J = 85.8 Hz). 11B{1H} NMR
(128.4 MHz, CDCl3): δ = –35.4 (d, J = 21 Hz) ppm. FT-IR (selected
frequencies): ν˜ = 2382 (B–H), 1435 (P–C), 931 (P–N), 694 (P–B)
cm–1. C24H27B2NP2: calcd. C 69.79, H 6.59, N 3.39%; found C 69.59
H 6.41, N 3.19%.
tral metal atom was a barium ion. The arrangement around the
barium complex 5 is best described as distorted snub-disphe-
noidal. The computational studies were in well agreement with
the experimental observations. In addition, the calculated cal-
cium and strontium structures were also in agreement with the
higher analogue Ba complex as well as the corresponding dis-
elenoimidodiphosphinato strontium and barium complexes.
Additionally, the catalytic efficiency of complex 5 could be
demonstrated by studying the ROP of ε-caprolactone. The po-
lymerization process was good and very narrow polydispersity
(PDI up to 1.14) values were observed for barium complex.
Synthesis of [{κ2-N(PPh2(BH3))2}Li(THF)2] (2): In a 10 mL sample
vial, one equivalent of 1-H (50 mg, 0.12 mmol) and one equivalent
of lithium bis(trimethylsilyl) amide (21 mg, 0.12 mmol) were mixed
together with THF (5 mL). After 6 h, a small amount of THF (2 mL)
and n-pentane (2 mL) were added onto it and kept at –35 °C. Colorless
crystals of 2 were obtained after one day. Yield: 120 mg (90%). 1H
NMR (400 MHz, C6D6): δ = 7.63–7.74 (m, 4 H, ArH), 6.97–7.04 (m,
6 H, ArH), 1.63 (br., 3 H, BH3), 3.57 (m, THF), 1.33 (m, THF) ppm.
13C{1H} NMR (100 MHz, C6D6): δ = 132.8 (P attached ArC), 132.7
(P attached o-ArC), 131.7 (P attached o-ArC), 128.6 (P attached p-
ArC), 128.5 (P attached m-ArC), 127.8 (P attached m-ArC), 68.0
(THF), 25.6 (THF) ppm. 31P{1H} NMR (161.9 MHz, C6D6): δ =
55.9 ppm (d). 11B{1H} NMR (128.4 MHz, C6D6): δ = –35.08 (d, J =
14 Hz) ppm. FT-IR (selected frequencies): ν˜ = 2382 (B–H), 1435 (P–
C), 931 (P–N), 694 (P–B) cm–1. C32H42B2LiNO2P2: calcd. C 68.24, H
7.52, N 2.49%; found C 67.89 H 7.31, N 2.32%.
Experimental Section
All manipulations involving air- and moisture-sensitive organometallic
compounds were carried out in an argon atmosphere using the standard
Schlenk technique or argon-filled glove box. Hydrocarbon solvents (n-
pentane, toluene) were distilled under nitrogen from LiAlH4 and stored
in the glove box. THF was dried and deoxygenated by distillation over
sodium benzophenone ketyl under argon and distilled and dried with
CaH2 prior to storing in the glove box. C6D6 was dried with Na/K,
distilled and stored in the glove box. 1H NMR (400 MHz), 31P{1H}
(161.9 MHz), 13C{1H} NMR (100 MHz), and 11B{1H} (128.3 MHz) General Preparation of [{κ3-N(PPh2(BH3))2}2M(THF)2] [(M = Ca
spectra were measured with a Bruker Avance III-400 spectrometer. (3), Sr (4), Ba (5)]: Route 1: In a 10 mL sample vial, two equivalents
Elemental analyses were performed on a Bruker Euro EA at the Indian of 1-H (100 mg, 0.24 mmol) and one equivalent of corresponding
Institute of Technology Hyderabad. Ph2PNHPPh2 (dppa) was prepared metal precursor [M{N(SiMe3)2}2(THF)2] (M = Ca, Sr, Ba) were mixed
by the reaction of hexamethyldisilazen and chloro-diphenylphosphine together with THF (5 mL). After 6 h of stirring at room temperature
described in the literature.[53] [Li{N(SiMe3)2], MI2 were purchased another 2 mL of THF and n-pentane (2 mL) were added onto it and
from Alfa Asear India. [M{N(SiMe3)2}2(THF)2] (M = Ca, Sr, Ba)[54] the resulting mixture was kept at –35 °C. Colorless crystals were ob-
were prepared according to the literature procedures by using the re-
spective metal diiodides and [KN(SiMe3)2].
tained after 1 d. Route 2: In a 25 mL pre-dried Schlenk flask, lithium
salt 2 (100 mg, 0.177mmol) was mixed with the respective metal di-
Z. Anorg. Allg. Chem. 2016, 118–127
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iodides (0.0885 mmol) in THF solvent (10 mL) at ambient temperature Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK.
and the reaction mixture was stirring for 12 h. The white precipitate Copies of the data can be obtained free of charge on quoting the de-
of LiI was filtered off and the filtrate was evaporated under reduced pository numbers CCDC-1403873 (2) and CCDC-1403874 (5)
pressure. The resulting white compound was further purified by wash-
(Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://
ing with pentane and was re-crystallized from THF/pentane (1:2 ratio) www.ccdc.cam.ac.uk).
mixture at –35 °C. Yield: 125 mg (90%).
Supporting Information (see footnote on the first page of this article):
Spectral data for 1-H, complexes 2–5, and the computational details
of the first-principle calculations.
M = Ca (3): Yield:125 mg (90%). 1H NMR (400 MHz, C6D6): δ =
7.66 –7.71 (m, 16 H, ArH), 6.89 –7.07 (m, 24 H, ArH), 3.5 (br., 3 H,
BH3), 3.55 (m, THF), 1.43 (m, THF) ppm. 13C{1H} NMR (100 MHz,
C6D6): δ = 135.1 (P attached ArC),132.8 (P attached o-ArC), 131.9 (P
attached o-ArC), 128.9 (P attached p-ArC), 124.5 (P attached m-ArC),
122.1 (P attached m-ArC), 67.9 (THF), 25.7 (THF) ppm. 31P{1H}
NMR (161.9 MHz, C6D6): δ = 52.1 (d) ppm. 11B{1H} NMR
(128.4 MHz, C6D6): δ = –35.5 (d) ppm. FT-IR (selected frequencies):
Acknowledgements
This work is supported by the Ministry of New and Renewable Energy
(MNRE), India under project no. 103/209/2013-NT, dated: 29th Sep-
tember, 2014. The instrumental facilities were provided by the Indian
Institute of Technology Hyderabad (IITH) and part of the simulations
was performed in HPC facility, Indian Institute of Technology Hydera-
bad, India. J. B. and Th. D. N. R. thank the University Grants Commis-
sion (UGC), India, for their PhD fellowship. We thank Dr. Sanjib K.
Patra, Indian Institute of Technology, Kharagpur, for his help to mea-
sure the polymers. Generous support from Prof. K. Mashima, Osaka
University, Japan, is also gratefully acknowledged.
ν˜
=
2383 (B–H), 1434 (P–C), 938 (P–N), 695 (P–B) cm–1.
C52H60B4CaN2OP4: calcd. C 66.71, H 6.46, N 2.99%; found C 66.33
H 6.39, N 2.66%.
M = Sr (4): Yield:125 mg (90%). 1H NMR (400 MHz, C6D6): δ =
7.66 –7.85 (m, 16 H, ArH), 6.94–7.16 (m, 24 H, ArH), 1.90 (br., 3 H,
BH3), 3.59 (m, THF), 1.40 (m, THF) ppm. 13C{1H} NMR (100 MHz,
C6D6): δ = 132.9(P attached ArC), 131.7 (P attached o-ArC), 131.3
(P attached o-ArC), 129.3 (P attached p-ArC), 128.3 (P attached m-
ArC),127.8 (P attached m-ArC), 68.0 (THF), 25.7 (THF) ppm. 31P{1H}
NMR (161.9 MHz, C6D6): δ = 46.5 ppm. 11B{1H} NMR (128.4 MHz,
C6D6): δ = –31.6 (d, 30 Hz) ppm. FT-IR (selected frequencies): ν˜ =
2381 (B–H), 1437 (P–C), 948 (P–N), 699 (P–B) cm–1.
C52H60B4SrN2OP4: calcd. C 63.48, H 6.15, N 2.85%; found C 63.17
H 5.98, N 2.69%.
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2 2
2
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2
2
2
2 2
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4
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Z. Anorg. Allg. Chem. 2016, 118–127
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Zeitschrift für anorganische und allgemeine Chemie
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