Bis(phosphinoselenoic amides) as versatile chelating ligands for alkaline earth metal (Mg, Ca, Sr and Ba) complexes: Syntheses, structure and ε-caprolactone polymerisation

Article abstract of DOI:10.1039/c4dt00669k

We report here a series of heavier alkaline earth metal complexes with N,N′-(ethane-1,2-diyl)bis(P,P-diphenylphosphinoselenoic amide) using two synthetic routes. In the first route, the heavier alkaline earth metal bis(trimethylsilyl)amides [M{N(SiMe₃)₂} ₂(THF)_n] (M = Ca, Sr, Ba), when treated with phosphinoselenoic amine [Ph₂P(Se)NHCH₂CH ₂NHPPh₂(Se)] (1), afforded the corresponding alkaline earth metal complexes of the composition [(THF)₃M{Ph ₂P(Se)NCH₂CH₂NPPh₂(Se)}] [M = Ca (2), Sr (3), Ba (4)]. The metal complexes 2-4 were also obtained from a one-pot reaction, where potassium phosphinoselenoic amide was generated in situ by the reaction of compound 1 and [KN(SiMe₃)₂], followed by the addition of the respective metal diiodides in THF at room temperature. The magnesium complex [(THF)₃Mg{Ph₂P(Se)NCH₂CH ₂NPPh₂(Se)}] (5) was also prepared. The solid-state structures of alkaline earth metal complexes 2-5 were established by single crystal X-ray diffraction analysis. In the solid state, all the metal complexes are monomeric but in complexes 2-4, ligand 1 is chelated in a tetra-dentate fashion to each metal ion but in complex 5, ligand 1 behaves as a bidentate ligand. Complexes 2-4 were tested as catalysts for the ring-opening polymerisation of ε-caprolactone and a high level of activity for the barium complex 4 was observed, with narrow polydispersity. We also report the synthesis and structure of the bis(amidophosphino borane) ligand [Ph ₂P(BH₃)NHCH₂CH₂NHPPh ₂(BH₃)] (6) and the corresponding barium complex [(THF)₂Ba{Ph₂P(BH₃)NCH₂CH ₂NPPh₂(BH₃)}]₂ (7).

Full text of DOI:10.1039/c4dt00669k

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Bis(phosphinoselenoic amides) as versatile
chelating ligands for alkaline earth metal
(Mg, Ca, Sr and Ba) complexes: syntheses,
structure and ε-caprolactone polymerisation†
Cite this: Dalton Trans., 2014, 43,
8757
Ravi K. Kottalanka,a Harinath Adimulam,a Jayeeta Bhattacharjee,a H. Vignesh Babub
and Tarun K. Panda*a
We report here a series of heavier alkaline earth metal complexes with N,N’-(ethane-1,2-diyl)bis(P,P-
diphenylphosphinoselenoic amide) using two synthetic routes. In the first route, the heavier alkaline earth
metal bis(trimethylsilyl)amides [M{N(SiMe3)2}2(THF)n] (M = Ca, Sr, Ba), when treated with phosphinosele-
noic amine [Ph2P(Se)NHCH2CH2NHPPh2(Se)] (1), afforded the corresponding alkaline earth metal com-
plexes of the composition [(THF)3M{Ph2P(Se)NCH2CH2NPPh2(Se)}] [M = Ca (2), Sr (3), Ba (4)]. The metal
complexes 2–4 were also obtained from a one-pot reaction, where potassium phosphinoselenoic amide
was generated in situ by the reaction of compound 1 and [KN(SiMe3)2], followed by the addition of the
respective metal diiodides in THF at room temperature. The magnesium complex [(THF)3Mg{Ph2P(Se)-
NCH2CH2NPPh2(Se)}] (5) was also prepared. The solid-state structures of alkaline earth metal complexes
2–5 were established by single crystal X-ray diffraction analysis. In the solid state, all the metal complexes
are monomeric but in complexes 2–4, ligand 1 is chelated in a tetra-dentate fashion to each metal ion
but in complex 5, ligand 1 behaves as a bidentate ligand. Complexes 2–4 were tested as catalysts for the
ring-opening polymerisation of ε-caprolactone and a high level of activity for the barium complex 4 was
observed, with narrow polydispersity. We also report the synthesis and structure of the bis(amidophos-
phino borane) ligand [Ph2P(BH3)NHCH2CH2NHPPh2(BH3)] (6) and the corresponding barium complex
[(THF)2Ba{Ph2P(BH3)NCH2CH2NPPh2(BH3)}]2 (7).
Received 5th March 2014,
Accepted 3rd April 2014
DOI: 10.1039/c4dt00669k
www.rsc.org/dalton
polydispersity, tacticity, and polymer chain ends.3,4 Ring-
opening polymerisation (ROP) of cyclic esters promoted, for
Introduction
Aliphatic polyesters are currently considered as alternatives to example, by metal initiators proved to be the most efficient
synthetic petrochemical-based polymers and are attractive to way for preparing polyesters with controlled molecular weight
researchers, since the starting materials for their synthesis can and
microstructure
and
narrow
molecular-weight
be obtained from annually renewable resources. Their bio- distribution.5–7 This leads to the design and synthesis of new,
degradable and biocompatible nature along with their mech- well-defined, single-site catalysts that exhibit good activity, pro-
anical and physical properties make them prospective ductivity and selectivity for cyclic ester polymerisation and
thermoplastics with broad commercial applications (e.g., allow crucial polymer architecture control.
single-use packaging materials, medical sutures and drug
Group 2 metal complexes have attracted considerable atten-
delivery systems).1,2 It is well established that polymer pro- tion as initiators for the ROP of cyclic esters, and some of
perties are highly dependent on their intrinsic structural para- them have demonstrated impressive results.8 Recently, we
meters, such as polymer composition, molecular weight, reported the synthesis of alkaline earth metal complexes with
iminopyrrolyl and amidopyrrolyl ligands and they proved to be
highly active for the ROP of ε-caprolactone, affording high-
molecular-weight poly(ε-caprolactone)s.9 We have continuously
aDepartment of Chemistry, Indian Institute of Technology Hyderabad, Ordnance
Factory Estate, Yeddumailaram 502205, Andhra Pradesh, India.
studied the complexation reaction of amidophosphine-chalco-
gen-based ligands to alkaline earth metal precursors, for
example synthesis of homoleptic alkaline earth metal com-
E-mail: tpanda@iith.ac.in; Fax: +91(40)2301 6032; Tel: +91(40)2301 6036
bSchool of Chemistry, University of Hyderabad, Hyderabad, India
†Electronic supplementary information (ESI) available: For crystallographic
plexes of the composition [M(THF)2{Ph2P(Se)N(CHPh2)}2] (M =
details in CIF. CCDC 987282–987289. For ESI and crystallographic data in CIF or
Ca, Sr, Ba).10 We also reported the amido-phosphine borane
other electronic format see DOI: 10.1039/c4dt00669k
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complexes of [M(THF)2{Ph2P(BH3)N(CHPh2)}2] (M = Ca, Sr,
Ba).11 All these ligands are monoanionic and form homoleptic
mono-nuclear complexes. To explore the chemistry of heavier
alkaline earth metals and their application in ε-caprolactone
polymerisation, we focused on the dianionic system N,N′-
(ethane-1,2-diyl)bis(P,P-diphenylphosphinoselenoic
amide)
[Ph2P(Se)NCH2CH2NPPh2(Se)}]2− which was recently prepared
by Woollins et al.12 However, their detailed study of alkaline
earth metal chemistry has not been available so far. In this
context, the synthetic and structural details of bis(phosphino-
selenoic amide) alkaline earth metal complexes with the com-
position [(THF)3M{Ph2P(Se)NCH2CH2NPPh2(Se)}] [M = Ca (2),
Sr (3), Ba (4), Mg (5)] are presented. We also report the ROP
study of ε-caprolactone using complexes 2–4 as catalysts. In
addition, we present the synthesis and structure of the bis-
(amidophosphino
borane)
ligand
[Ph2P(BH3)NHCH2-
CH2NHPPh2(BH3)}] and the corresponding dimeric barium
complex [(THF)2Ba{Ph2P(BH3)NCH2CH2NPPh2(BH3)}]2.
Results and discussion
Ligands
Fig. 1 Solid-state structures of compounds 1-cis and 1-trans, ellipsoids
are drawn to encompass 30 per cent probability. Selected bond lengths
[Å] and bond angles [°]: 1-cis: P1–N1 1.665(2), P1–Se1 2.1194(7), P1–C1
1.810(3), P1–C8 1.810(3), N1–C7 1.468(3), C7–C7i 1.512(5), C7i–C7–N1
112.9(2), N1–P1–C8 102.55(12), N1–P1–Se1 116.97(8), N1–P1–C1
104.95(12), P1–N1–C7 120.73(18), Se1–P1–C8 113.29(10), Se1–P1–C1
111.36(9). 1-trans: P1–N1 1.6660(18), P1–Se1 2.1057(5), P1–C2 1.817(2),
P1–C8 1.816(2), N1–C1 1.468(3), C1–C1i 1.507(4), C1i–C1–N1 109.4(2),
N1–P1–C8 101.65(9), N1–P1–Se1 118.10(7), N1–P1–C2 102.72(10),
P1–N1–C1 117.58(14), Se1–P1–C8 112.19(7), Se1–P1–C2 112.45(7).
N,N′-(Ethane-1,2-diyl)bis(P,P-diphenylphosphinoselenoic amide)
was prepared according to the method described by Woollins
et al. by the reaction of bis(diphenylphosphino)ethane-1,2-
diamine, [Ph2PNHCH2CH2NHPPh2] and elemental selenium in
a 1 : 2 molar ratio at room temperature in THF.12 The spectro-
scopic data for compound 1 were in full agreement with the
reported values. The solid-state structure of compound 1 was
established by single crystal X-ray diffraction analysis. When
compound 1 was re-crystallised from a mixture of THF–pentane
(1 : 2) at −35 °C, a trans product (1-trans) was obtained.
However, crystallisation from dichloromethane at room temp-
erature afforded a cis product (1-cis). This indicates that there is
equilibrium between cis and trans forms in solution (see
Scheme 1). 1-trans crystallises in the orthorhombic space group
Pbca, with one molecule each of 1-trans and THF in the asym-
metric unit. In contrast, 1-cis crystallises in the monoclinic
space group C2/c, with four isolated molecules in the unit cell.
The molecular structures of 1-cis and 1-trans are shown in
Fig. 1. The details of the structural parameters are given in
Table TS1 in the ESI.†
1.116(2) Å for 1-cis and 1-trans respectively are in the expected
range, as reported for other phosphinoselenoic amido com-
pounds in the literature.13 In the centro-symmetric 1-trans
form, C1–N1 and C1i–N1i bonds are trans to each other. Com-
pound 1-cis also possesses a centre of inversion i, the middle
of the C7–C7i bond. C7–N1 and C7i–N1i bonds are cis to each
other and a dihedral angle of 67.09° is formed by the planes
containing C7, N1, P1 and C7i, N1i, P1i atoms. Thus, planes
containing the N1, P1, Se1 and N1i, P1i, Se1i atoms are not
coplanar, but almost orthogonal (86.90°) to each other.
Alkaline earth metal complexes
The P–Se bond distances [2.1194(7) Å for 1-cis and 2.1057(5)
Å for 1-trans] are in the range similar to that of [Ph2P(Se)NH-
(CHPh2)] [2.1086(12) Å], [Ph2P(Se)NH(CPh3)] [2.1166(8) Å] and
[Ph2P(Se)NH(2,6-Me2C6H3)] [2.1019(8) Å], previously we
reported to consider the phosphorus–selenium bond as a
double bond.10,13b P1–N1 bond distances of 1.665(2) Å and
The title compounds [(THF)3M{Ph2P(Se)NCH2CH2NPPh2(Se)}]
[M = Ca (2), Sr (3), Ba (4)] can be obtained by two different syn-
thetic approaches. Both the approaches were used for the
calcium complex 2, whereas the other complexes were
obtained using a one-pot reaction only. In the first approach,
the well-established compound [(THF)2Ca{N(SiMe3)2}2]14,15
was made to react with compound 1 in a 1 : 1 molar ratio in
THF at room temperature to afford the corresponding bis-
(diphenylphosphinoselenoic amide) complex 2 via the elimi-
nation of volatile bis(trimethylsilyl)amine (see Scheme 2).
However, the most convenient approach for obtaining
complexes 2–4 is a one-pot reaction, in which the ligand,
Scheme 1 trans and cis forms of ligand 1.
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Paper
The calcium, strontium bis(diphenylphosphinoselenoic
amido) complexes 2 and 3 crystallise in the triclinic space
ˉ
group P1, with two molecules of 2 and 3 in the unit cell
respectively. The slightly larger barium compound 4 also crys-
ˉ
tallises in the triclinic space group P1, with two independent
molecules of complex 4 in the asymmetric unit. The details of
the structural parameters are given in Table TS1 in ESI.† The
solid-state structures of complexes 3 and 4 are shown in Fig. 2
and 3 respectively. Complexes 2–4 are isostructural to each
other due to the similar ionic radii of the metal ions (1.00 Å,
1.18 Å and 1.35 Å respectively) for a coordination number of
six.19
Scheme 2 Syntheses of alkaline earth metal phosphinoselenoic amide
In all three complexes, the coordination polyhedron is
formed by dianionic bis(diphenylphosphinoselenoic amide)
[Ph2P(Se)NCH2CH2NP(Se)Ph2]2− ligands, and three THF mole-
cules which are present as solvates to provide the metal ion
seven-fold coordination. The ligand 1 coordinates to the alka-
line earth metal ion via chelation of two amido nitrogen atoms
and two selenium atoms attached to the phosphorus atoms.
complexes.
[Ph2P(Se)-NHCH2CH2NHP(Se)Ph2], is made to react with anhy-
drous potassium bis(trimethylsilyl)amide in a 1 : 2 molar ratio
in THF to generate in situ potassium salt of ligand 1, followed
by the addition of anhydrous alkaline earth metal diiodide to
the reaction mixture (see Scheme 2).16
The corresponding magnesium complex [(THF)3Mg{Ph2P-
(Se)NCH2CH2NPPh2(Se)}] (5) was obtained by the second
route, using ligand 1 and [KN(SiMe3)2], followed by addition of
magnesium diiodide in THF solvent (see Scheme 2).
The new complexes were characterised using standard
analytical and spectroscopic techniques, and the solid-state
structures of all four alkaline earth metal complexes were
established by single crystal X-ray diffraction analysis. A strong
absorption band at 550 cm−1 (for complex 2), 552 cm−1 (for
complex 3), 555 cm−1 (for complex 4) and 551 cm−1 (for
complex 5) in FT-IR spectra indicates a PvSe bond in each
complex. The PvSe stretching frequencies are within the
range reported by us.10,13b The 1H NMR spectra of the diamag-
netic compounds 2–5 show a multiplet signal [δ 3.39 ppm
(complex 2), 2.87 ppm (complex 3), 3.14 ppm (complex 4) and
3.09 ppm (complex 5)] for the four methylene protons and this
is very close to the resonance signal (δ 3.16 ppm) of the analo-
gous methylene protons present in free ligand 1. Each of the
complexes 2–5 shows a sharp signal in the 31P{1H} NMR
spectra [δ 71.8 (complex 2), 71.9 (complex 3), 73.3 ppm
(complex 4) and 43.7 ppm (complex 5)], which is significantly
low field shifted for complexes 2–4 and high field shifted for
complex 5 to free ligand 1 (δ 59.6 ppm),12 showing that both
the phosphorous atoms in each complex are chemically equi-
valent in solution. All three complexes are coordinated to THF
molecules, as is evident from the typical multiplet signals at
3.65–3.55 ppm and 1.35–1.33 ppm observed in 1H NMR
spectra.
Fig. 2 Solid-state structure of compound 3, ellipsoids are drawn to
encompass 30 per cent probability, omitting hydrogen atoms for clarity.
Selected bond lengths [Å] and bond angles [°]: complex 2 (Ca): P1–N1
1.592(8), P1–Se1 2.148(3), P1–C9 1.824(10), P1–C3 1.832(10), N1–C1
1.450(13), P2–N2 1.595(8), P2–Se2 2.155(3), P2–C15 1.833(9), P2–C21
1.824(10), N2–C2 1.433(13), C1–C2 1.526(13), Ca1–N1 2.386(8), Ca1–Se1
3.252(2), Ca1–P1 3.359(3), Ca1–N2 2.418(8), Ca1–Se2 3.300(2), Ca1–P2
3.364(3), Ca1–O1 2.374(7), Ca1–O2 2.429(7), Ca1–O3 2.385(7), C2–C1–
N1 108.0(8), N1–P1–C9 112.3(4), N1–P1–Se1 108.4(3), N1–P1–C3
111.0(4), P1–N1–C1 123.0(7), Se1–P1–C9 110.3(3), Se1–P1–C3 111.3(3),
C1–C2–N2 109.2(8), N2–P2–C15 111.9(4), N2–P2–Se2 108.8(3),
N2–P2–C21 112.4(5), P2–N2–C2 123.6(7), Se2–P2–C15 111.8(3), Se2–
P2–C21 110.1(3), N1–Ca1–Se1 63.3(2), N1–Ca1–N2 68.6(3), Se1–Ca1–Se2
165.42(7), O1–Ca1–O2 79.6(3), O1–Ca1–O3 159.1(3), O2–Ca1–O3
80.4(3), P1–Se1–Ca1 73.76(8), P2–Se2–Ca1 72.76(8). Complex 3 (Sr):
P1–N1 1.592(5), P1–Se1 2.1548(17), P1–C9 1.823(7), P1–C3 1.826(7),
N1–C1 1.465(7), P2–N2 1.589(5), P2–Se2 2.1559(17), P2–C15 1.837(6),
P2–C21 1.819(6), N2–C2 1.463(7), C1–C2 1.524(8), Sr1–N1 2.517(5), Sr1–
Se1 3.2788(10), Sr1–P1 3.4456(18), Sr1–N2 2.540(5), Sr1–Se2 3.3259(10),
Sr1–P2 3.4521(16), Sr1–O1 2.568(5), Sr1–O2 2.529(5), Sr1–O3 2.537(5),
C2–C1–N1 108.4(5), N1–P1–C9 111.3(3), N1–P1–Se1 109.13(19), N1–
P1–C3 112.7(3), P1–N1–C1 122.7(4), Se1–P1–C9 110.3(2), Se1–P1–C3
109.1(2), C1–C2–N2 109.3(5), N2–P2–C15 112.2(3), N2–P2–Se2 108.72(19),
N2–P2–C21 112.6(3), P2–N2–C2 123.4(4), Se2–P2–C15 111.2(2), Se2–P2–
C21 109.8(2), N1–Sr1–Se1 62.35(11), N1–Sr1–N2 65.80(15), Se1–Sr1–Se2
170.54(2), O1–Sr1–O2 80.67(18), O1–Sr1–O3 80.37(17), O2–Sr1–O3
159.22(17), P1–Se1–Sr1 75.57(5), P2–Se2–Sr1 74.67(5).
Although there is ongoing interest in alkaline earth organo-
metallics17 and particularly in the cyclopentadienyl chemistry
of these elements,18 complexes 2–5 represent, to the best of
our knowledge, the first alkaline earth metal complexes con-
taining a bis(diphenylphosphinoselenoic amide) ligand having
two sets of three heteroatoms, N, P and Se, adjacent to each
other in the ligand. Therefore, their molecular structures in
the solid state were determined by X-ray diffraction analysis.
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as a tetra-dentate chelating ligand to form a five-membered
metallacycle M1–N1–C1–C2–N2, where two four-membered
metallacycles M1–Se1–P1–N1 and M1–Se2–P2–N2 are fused
together to construct a polymetallacyclic motif tricyclometalla
[5.2.0.01,4]nonane structure. To the best of our knowledge, this
is the first example of such a structural motif in alkaline earth
metal complexes, with three adjacent hetero donor atoms—
selenium, phosphorus and nitrogen. Among the three M–O
distances for each complex [M–O distance is 2.429(7) Å for
complex 2, 2.568(5) Å for complex 3, 2.812(5) Å for complex 4],
the THF molecule resides in the basal plane of the distorted
pentagonal bi-pyramidal structure and is slightly elongated
compared to the remaining M–O distances [2.374(7) Å and
2.385(7) Å for complex 2, 2.537(5) Å and 2.568(5) Å for complex
3, 2.716(5) Å and 2.721(5) Å for complex 4] measured for the
THF molecules placed in the apical position. This slight
elongation can be explained by the extensive electron release
from the two anionic basal nitrogen atoms opposite the THF
molecule in the metal complex.
The magnesium ion has the smallest ionic radii among the
Mg2+ to Ba2+ ions and different coordination behaviours can
be anticipated.21 As we have observed that ligand 1 is acting as
a tetra-dentate chelating ligand towards moderately larger ions
(Ca2+ to Ba2+) it would be interesting to study its solid-state
structure to learn more about its flexible nature. The mag-
nesium compound 5 was crystallised from the THF–pentane
mixture as a colourless solid. Compound 5 crystallises in the
Fig. 3 Solid-state structure of compound 4, ellipsoids are drawn to
encompass 30 per cent probability, omitting hydrogen atoms for clarity.
Selected bond lengths [Å] and bond angles [°]: (Molecule 1) P1–N1 1.585(6),
P1–Se1 2.1583(19), P1–C14 1.834(7), P1–C3 1.835(7), N1–C1 1.458(9),
P2–N2 1.591(6), P2–Se2 2.1490(18), P2–C15 1.833(7), P2–C21 1.837(7),
N2–C2 1.478(8), C1–C2 1.517(9), Ba1–N1 2.657(5), Ba1–Se1 3.4706(9),
Ba1–P1 3.6129(17), Ba1–N2 2.654(6), Ba1–Se2 3.4071(9), Ba1–P2
3.5929(18), Ba1–O1 2.716(5), Ba1–O2 2.812(5), Ba1–O3 2.721(5), C2–
C1–N1 110.6(6), N1–P1–C14 112.6(3), N1–P1–Se1 110.1(2), N1–P1–C3
111.6(3), P1–N1–C1 122.3(5), Se1–P1–C14 109.4(2), Se1–P1–C3 110.0(2),
C1–C2–N2 108.8(6), N2–P2–C15 111.8(3), N2–P2–Se2 109.5(2), N2–
P2–C21 112.3(3), P2–N2–C2 121.9(5), Se2–P2–C15 110.4(2), Se2–P2–
C21 110.0(3), N1–Ba1–Se1 58.71(12), N1–Ba1–N2 63.45(17), Se1–Ba1–
Se2 175.44(2), O1–Ba1–O2 74.26(18), O1–Ba1–O3 155.90(19), O2–Ba1–
O3 85.91(18), P1–Se1–Ba1 75.90(5), P2–Se2–Ba1 76.91(5).
ˉ
triclinic space group P1, with two molecules in the unit cell.
Table TS1 in ESI† contains details of the structural refinement
parameters for compound 5 and its solid-state structure is
shown in Fig. 4. In contrast to compounds 2–4, it is observed
that for complex 5, a five magnesa-metallacycle Mg1–N1–C1–
C2–N2 is formed by the chelation of two amido nitrogen
atoms of ligand 1. Two selenium atoms, which are coordinated
to Ca–Ba to make two four-membered rings in complexes 2–4,
are unable to interact with the smaller magnesium ion (Mg1–
Se1 3.661 Å and Mg1–Se2 3.769 Å). This is an instance of flexi-
bility of the chelating ligand 1, switching from tetra-dentate to
bi-dentate fashion depending upon the metal ion. As three
THF molecules are chelated to the magnesium ion, the geome-
try around it is best described as a distorted trigonal bi-pyra-
midal geometry, having the amido nitrogens and one THF at
the equatorial position and two THF molecules in the apical
position. As expected, the Mg–N bond distances [2.066(3) Å
and 2.083(3) Å] are among the shortest with respect to M–N
distances [2.386(8) and 2.418(8) Å for Ca, 2.517(5) and 2.540(5)
Å for Sr, and 2.657(5) and 2.654(6) Å for Ba]. Five-membered
magnesium metallacycles are reported in the literature.22
The phosphorus–metal distances (3.365 and 3.359 Å for
complex 2, 3.452 and 3.446 Å for complex 3, and 3.593 and
3.613 Å for complex 4) are significantly greater than the sum of
the covalent radii of the respective metal ion and phosphorus
atom (3.07 Å for complex 2, 3.25 Å for complex 3 and 3.34 Å for
complex 4). This indicates that the metal ion and phosphorous
have no interaction between themselves. Thus, in each case,
the central metal ion adopts a distorted pentagonal bi-pyrami-
dal geometry around it, with two selenium atoms, two nitrogen
atoms of ligand 1, along with one oxygen atom from the THF
molecule, which is in the basal plane, whereas two remaining
THF molecules occupy the apical positions. In complex 2, the
M–N distances [2.386(8) Å and 2.418(8) Å for complex 2,
2.517(5) Å and 2.540(5) Å for complex 3, and 2.657(5) Å and
2.654(6) Å for complex 4] and M–Se distances [3.252(2) Å and
3.300(2) Å for complex 2, 3.2788(1) Å and 3.3259(1) Å for
complex 3 and 3.4706(9) Å and 3.4071(9) Å for complex 4] indi-
cate a slight asymmetrical attachment of the tetra-dentate
ligand 1 to the alkaline earth metal ion. This is due to the
presence of four phenyl rings attached to two phosphorus
atoms. However, similar M–N distances and M–Se distances
were observed in our previously reported complexes [(THF)2M-
{Ph2P(Se)N(CHPh2)}2] (M = Ca, Sr, Ba)10 and heavier alkaline
earth metal complexes reported by other groups.20 Thus, we
observe that bis(diphenylphosphinoselenoic amide) 1 behaves
Bis(amidodiphenylphosphine borane) ligand
Recently we introduced an amidophosphine–borane adduct as
a ligand and exploited its chelating behaviour in alkali metal
and alkaline earth metal chemistry.11 The amidophosphine
borane [Ph2P(BH3)NHR] acts as a monoanionic ligand and
coordinates to the metal ions through amido nitrogen and
borane hydrogen. We intended to extend the idea of amido-
8760 | Dalton Trans., 2014, 43, 8757–8766
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ances for the two chemically equivalent borane (BH3) groups
attached to the phosphorus atoms appear as a broad signal at
1
δ 1.4 ppm. In the H NMR spectra, the resonances of the ami-
dophosphine moiety in ligand 6 are only slightly shifted in
comparison to the starting material with those reported for
the phosphine amines.23 The multiplet signals at 2.78 ppm
can be assigned to the four methylene protons of ligand 6 in
which both the hydrogen atoms are diastereotopic to each
other. This indicates that methylene signals are slightly high
field shifted compared to the selenium analogue 1 (3.16 ppm).
Another broad signal at 3.02 ppm corresponding to the two
NH protons of ligand 6 is observed and is also shifted to the
higher field (3.24 ppm) compared to 1. Ligand 6 shows a
doublet signal in the 31P{1H} NMR spectrum at 58.8 ppm with
a coupling constant of 67.9 Hz due to coupling with the adja-
cent boron atom. In 11B{1H} NMR spectrum, the signal at
−38.1 ppm can be assigned to the BH3 group attached with
phosphorus. This observation is in agreement with our pre-
viously reported values.11 In the FT-IR spectra, a characteristic
signal for P–B bond stretching at 606 cm−1 was observed along
with another characteristic signal at 2380 cm−1 assigned to the
B–H stretching frequency. These values are in agreement with
those reported in the literature.24
The molecular structure of ligand 6 was established using
single crystal X-ray diffraction analysis. It crystallises in the
monoclinic space group Cc, with four independent molecules
in the unit cell (see Fig. 5). The details of the structural para-
meters are given in Table TS1 in the ESI.† The P1–B1 bond dis-
tances in 6 [1.9091(2) Å and 1.916(1) Å] are almost similar and
in agreement with reported values—1.918(6) Å for [Ph2P(BH3)-
NH(CHPh2)], 2.1019(8) Å for [{Ph2P(BH3)}2CH2] and 1.921(3) Å
for [(CH2-o-CF3C6H4)-(Ph)P(BH3)C4H8P(BH3)(Ph)(CH2-o-CF3C6H4)]
Fig. 4 Solid-state structure of compound 5, ellipsoids are drawn to
encompass 30 per cent probability, omitting hydrogen atoms for clarity.
Selected bond lengths [Å] and bond angles [°]: P1–N1 1.606(3), P1–Se1
2.1308(10), P1–C3 1.831(3), P1–C9 1.825(3), N1–C1 1.477(4), P2–N2
1.612(3), P2–Se2 2.1240(10), P2–C15 1.833(3), P2–C21 1.834(3), N2–C2
1.482(4), C1–C2 1.524(4), Mg1–N1 2.083(3), Mg1–N2 2.066(3), Mg1–O1
2.124(3), Mg1–O2 2.137(2), Mg1–O3 2.106(2), C2–C1–N1 108.9(3), N1–
P1–C9 112.57(15), N1–P1–Se1 110.69(10), N1–P1–C3 111.46(15), P1–N1–
C1 121.0(2), Se1–P1–C9 109.18(11), Se1–P1–C3 111.65(11), C1–C2–N2
109.0(3), N2–P2–C15 111.99(15), N2–P2–Se2 111.98(11), N2–P2–C21
110.05(15), P2–N2–C2 118.6(2), Se2–P2–C15 109.14(11), Se2–P2–C21
110.70(11), N1–Mg1–N2 83.40(12), O1–Mg1–O2 80.49(10), O1–Mg1–O3
160.13(11), O2–Mg1–O3 80.11(10), N1–Mg1–O1 94.85(11), N2–Mg1–O1
99.30(11), N1–Mg1–O3 97.76(11), N2–Mg1–O2 134.70(12), N2–Mg1–O1
99.30(11), N2–Mg1–O3 97.36(11).
phosphine–borane to bis(amidodiphenylphosphine borane) by
introducing a spacer of the ethylene bridge. Thus, bis(amido-
diphenylphosphine borane) would form a dianion and act as a
tetra-dentate ligand towards metal ions. In this paper we
present the synthesis and structure of the bis(amidophosphine
borane) ligand [Ph2P(BH3)NHCH2CH2NHPPh2(BH3)] and the
corresponding dimeric barium complex [(THF)2Ba{Ph2P(BH3)-
NCH2CH2NPPh2(BH3)}]2 to demonstrate our concept of versati-
lity of the amidophosphine backbone. The bis(amidodiphenyl-
phosphine borane) ligand [Ph2P(BH3)NHCH2CH2NHPPh2-
(BH3)] (6) was isolated as a white precipitate from the reaction
between bis(phosphineamine) [Ph2PNHCH2CH2NHPPh2] and
the borane adduct [BH3·SMe2] at room temperature in a
1 : 2 molar ratio in toluene as the solvent (see Scheme 3).
The formation of the amidophosphine borane ligand 6
from [Ph2PNHCH2CH2NHPPh2] can easily be followed by 1H
NMR spectroscopy measured in CDCl3, since additional reson-
Fig. 5 Solid-state structure of compound 6, ellipsoids are drawn to
encompass 30 per cent probability, omitting aromatic ring hydrogen
atoms for clarity. Selected bond lengths [Å] and bond angles [°]: P1–N1
1.668(3), P1–B1 1.920(5), P1–C3 1.805(4), P1–C9 1.815(4), N1–C1
1.463(5), C1–C2 1.523(5), C2–N2 1.437(5), P2–N2 1.660(3), P2–B2
1.896(5), P2–C15 1.810(4), P2–C21 1.817(4), B1–H1c 0.9600, B2–H2c
0.9600, B1–P1–N1 110.0(2), C3–P1–B1 110.3(2), C9–P1–B1 117.1(2),
N1–P1–C3 106.20(19), N1–P1–C9 107.62(17), P1–N1–C1 121.4(3),
N1–C1–C2 110.3(3), C1–C2–N2 111.9(3), C2–N2–P2 128.8(3), B2–P2–
N2 106.7(2), B2–P2–C15 111.7(2), B2–P2–C21 116.8(2), N2–P2–C15
108.20(18), N2–P2–C21 107.85(17).
Scheme 3 Synthesis of the bis(amidodiphenylphosphine borane) ligand 6.
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to be considered as the phosphorus–boron dative bond
reported by us and others.19,25,26 The P1–N1 bond ranges from
1.659(1) Å to 1.660(9) Å and the C1–N1 bond distances of
1.443(1) Å and 1.480(1) are also similar to those reported by us
previously: P1–N1 1.673(6) Å and C1–N1 1.453(8) Å for
Ph2PNH(CHPh2) and P1–N1 1.638(3) Å and C1–N1 1.468(5) Å
for Ph2P(BH3)NH(CHPh2).11,13
Bis(amidodiphenylphosphine borane) barium complex
Ligand 6 was made to react with [K{N(SiMe3)2}] in THF at an
ambient temperature in a 1 : 2 molar ratio followed by addition
of barium diiodide to afford the dimeric barium bis(amidodi-
phenyl-phosphine borane) complex [(THF)2Ba{Ph2P(BH3)-
NCH2CH2N-PPh2(BH3)}]2 (7) through the elimination of KI and
volatile tetramethylsilane (see Scheme 4).
Fig. 6 Solid state structure of compound 7, ellipsoids are drawn to
encompass 30 per cent probability, omitting hydrogen atoms (except in
the borane groups) for clarity. Selected bond lengths [Å] and bond
angles [°]: P1–N1 1.595(5), P1–B1 1.938(7), C1–N1 1.468(7), C1–C2 1.531(8),
C2–N2 1.475(7), P2–N2 1.596(5), P2–B2 1.943(8), B1–Ba1i 3.245(7),
B1–H2aa 1.16(6), B1–H2bb 1.07(7), B1–H2cc 1.10(5), B2–H1aa 1.10(5),
B2–H1bb 1.12(6), B1–H1cc 1.11(6), Ba1–N1 2.671(5), Ba1–N2 2.647(5),
Ba1–B1 3.332(6), Ba1–B2 3.251(7), Ba1–P1 3.6169(16), Ba1–P2
3.5540(16), Ba1–O1 2.730(5), Ba1–O2 2.744(5), Ba1–B1i 3.245(7), Ba1–
H1bb 2.90(6), Ba1–H2bb 2.95(7), B1–P1–N1 107.2(3), P1–N1–C1
125.3(4), N1–Ba1–N2 62.32(15), O1–Ba1–O2 169.80(14), N1–C1–C2
109.1(5), C1–C2–N2 109.2(5), N2–P2–B2 108.1(3).
In the FT-IR spectra, the strong absorption band at
605 cm−1 is assigned to the P–B bond of complex 7, which is
1
in the range of that of ligand 6 (606 cm−1). The H NMR spec-
trum of complex 7 in C6D6 is very similar to the spectra
recorded for compound 6 and reveals time-averaged Cs-sym-
metry in solution. The four methylene protons in the ligand
backbone appear as a multiplet at 2.69 ppm. The resonances
of the three protons attached to the boron atom appear as a
multiplet at 1.38 ppm in the 1H NMR spectra. In the proton
decoupled 31P NMR spectra, complex 7 shows only one
doublet signal at 70.8 ppm and these values are significantly
low-field shifted compared to the value for compound 6
(58.8 ppm) upon the coordination of barium atoms to the bis-
(amidophosphine-borane) ligand. The phosphorus atoms
present in the [Ph2P(BH3)NCH2CH2NPPh2(BH3)]2− moieties
are chemically equivalent. A broad signal at −37.6 ppm was
observed in the 11B{1H} NMR spectra of complex 7.
barium ion is best described as distorted pentagonal bi-pyra-
midal. It is noteworthy that the P–B distances [1.938(7) and
1.943(8) Å] are slightly elongated compared to that of the
ligand 6 [1.920(5) and 1.896(5) Å] even after the coordination
of the BH3 group to the barium centre. The Ba–N [2.671(5) and
2.647(5) Å] and Ba1–O1 [2.730(5) and 2.744(5) Å] distances are
in the range similar to that of the reported complexes.11
Compound 7 was re-crystallised from THF and n-pentane
(1 : 2 ratio) and was found to crystallise in the triclinic space
Ring opening polymerization study
ˉ
group P1, which has two molecules in the unit cell. The solid-
Catalytic activities of the calcium, strontium and barium com-
plexes 2, 3 and 4 were performed (see Scheme 5). Polymeri-
sation studies were typically conducted in toluene, with various
monomer/catalyst ratios at 25 °C. Selected data obtained with
respect to complexes 2, 3 and 4 are given in Table 1.
The catalytic ability of the newly synthesised mono-nuclear
calcium complex 2 to promote the ROP of ε-CL was first evalu-
ated (Table 1, entries 1–4). Indeed, the sluggish reactivity of
the calcium complexes is very similar to that observed in some
previously reported studies using other calcium complexes for
ROP of ε-caprolactone.8 Since the larger strontium derivatives
have been reported to be more active than the calcium conge-
ners in ROP,27,28 we tested compound 3 as a catalyst and
state structure of complex 7 is given in Fig. 6. The details of
the structural parameters are given in Table TS1 in the ESI.†
Compound 7 is dimeric and two barium ions are coordinated
by four amido nitrogen atoms and four BH3 groups of two
ligands 6. Out of four borane groups, two are in mode to co-
ordinate to the two barium ions. Each of the borane (BH3)
group coordinates through the hydrogen atoms in a η1 fashion
and has a Ba1–B1 bond length of 3.332(6) and Ba1–B2 is 3.251(7)
Å. Thus, ligand 6 can be considered a tetra-dentate ligand,
similar to what was observed for ligand 1 in the Ca–Ba com-
plexes (see above). Additionally, two THF molecules are co-
ordinated to each barium ion and the geometry around each
Scheme 5 ROP of ε-CL in toluene with calcium, strontium and barium
complexes 2–4 respectively.
Scheme 4 Synthesis of barium complex 7.
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Table 1 Polymerization of ε-caprolactone initiated by alkaline earth metal complexes of type [MC2H4(NPh2PvSe)2(THF)3] (where M = Ca, Sr, Ba)a
[ε-CL]0/
[M]0
Reac. temp.
[°C]
Reac. timeb
[min]
Conv.c
[%]
Mn(theo)
Mn(GPC)
Mw(GPC)
d
e
e
f
Entry
[M]
Solvent
[g mol−1
]
[g mol−1
]
[g mol−1
]
Mw/Mn
1
2
3
4
5
6
7
8
Ca
Ca
Ca
Ca
Sr
Sr
Sr
Sr
Sr
Ba
Ba
Ba
Ba
200
300
400
500
100
200
300
400
500
200
300
400
500
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
25
25
25
25
25
25
25
25
25
25
25
25
25
10
15
20
20
10
10
10
10
15
5
75
96
89
64
99
98
95
72
80
82
94
85
70
15 000
28 872
35 689
32 080
17 759
35 159
51 125
28 872
40 100
16 441
28 270
34 085
35 087
17 065
31 027
27 238
33 618
28 789
50 353
60 384
29 925
31 590
24 947
29 965
35 033
43 215
11 658
17 769
19 883
24 744
17 338
44 198
53 266
22 051
22 461
18 278
20 262
25 045
34 650
1.473
1.746
1.370
1.359
1.660
1.139
1.134
1.357
1.406
1.365
1.479
1.399
1.247
9
10
11
12
13
5
5
5
a Results are representative of at least two experiments. b Reaction times were not necessarily optimized. c Monomer conversions were determined
by 1H NMR spectroscopy. d Theoretical molar mass values calculated from the relation: [monomer]0/[M]0 × monomer conversion where [M]0
8.76 × 10−3 mmol and monomer weight of ε-CL = 114 g mol−1 e Experimental molar masses were determined by GPC versus polyethylene glycol
standards. f Molar mass distributions were calculated from GPC.
=
.
observed an enhanced rate of polymerisation (Table 1, entries AVANCE III-400 spectrometer. BRUKER ALPHA FT-IR was used
5–8). In both cases, higher reactivity was observed for conver- for FT-IR measurement. Elemental analyses were performed
sion of ε-caprolactone to poly-caprolactone and up to 500 ε-CL on a BRUKER EURO EA at the Indian Institute of Technology
units were successfully converted in high yields (90 per cent Hyderabad. Metal iodides (MgI2, CaI2, SrI2 and BaI2), KN-
and 80 per cent) within 15 and 10 minutes respectively at 25 °C. (SiMe3)2, Me2S·BH3 and ε-caprolactone were purchased from
The control over the ROP process was rather good, affording Sigma Aldrich and used as such. The bis(phosphineamine)
PCLs, featuring a good match between the observed (as deter- [Ph2PNHCH2CH2NHPPh2] and bis(phosphinoselenoic amine)
mined by GPC) and calculated molar mass values, as well as [Ph2P(Se)NHCH2CH2NHPPh2(Se)] (1) were prepared according
moderate dispersity data (PDI = Mw/Mn < 1.80). The overall to procedure prescribed in the literature.12,23 The NMR solvent
efficiency of the calcium initiator 2 towards the ROP of ε-CL was C6D6 was purchased from Sigma Aldrich and dried under Na/K
weaker than that of the strontium analogue 3. Being the largest alloy prior to use.
ionic radius of the barium atom, it was anticipated that
complex 4 would show the highest reactivity among all the three
alkaline earth metal complexes.29,30 In reality we observed that
up to 500 ε-CL units were successfully converted in good yields
(70 per cent) within 5 minutes at 25 °C (Table 1, entries 10–13).
The poly-caprolactone produced by the use of the barium cata-
lyst was a good match between the observed and calculated
molar mass values, and we observed a relatively narrow poly-dis-
persity data (PDI up to 1.25, entry 13 in Table 1). Thus, among
three metal complexes 2, 3 and 4, the barium complex showed
the highest activity for ROP of ε-caprolactone.
Preparation of [(THF)3Ca{Ph2P(Se)NCH2CH2NPPh2(Se)}] (2)
In a 50 mL dry Schlenk flask ligand 1 (200 mg, 0.34 mmol),
KN(SiMe3)2 (136 mg, 0.68 mmol) and CaI2 (100 mg,
0.34 mmol) were mixed together in 10 mL of THF at an
ambient temperature and stirred for 14 hours. The precipitate
of KI was filtered using a filter dropper and the filtrate was
dried in vacuo. The resulting white compound was further
purified by washing with pentane and crystals suitable for
X-ray analysis were grown from a THF–pentane (1 : 2 ratio)
mixture solvent at −40 °C.
Yield 202.0 mg (70.6%).
1H NMR (400 MHz, C6D6): δ = 8.04–7.99 (m, 8H, ArH),
7.10–7.05 (m, 12H, ArH), 3.77–3.74 (m, 12H, THF), 3.39 (m,
4H, CH2), 1.36–1.33 (m, 12H, THF) ppm; 13C NMR (100 MHz,
C6D6): δ = 133.1 (P–ArC), 131.9 (o-ArC), 131.8 (m-ArC), 129.7
(p-ArC), 68.5 (THF), 42.9 (CH2), 25.3 (THF) ppm; 31P NMR
(161.9 MHz, C6D6): δ = 71.8 ppm; FT-IR (selected frequencies):
3052 (ArC–H), 2920 (C–H), 1435 (P–C), 969 (P–N), 550 (PvSe)
cm−1. (C38H48CaN2O3P2Se2) (840.74) Calc. C 54.29, H 5.75,
N 3.33; found C 53.83, H 5.39, N 2.98.
Experimental
General consideration
All manipulations of air-sensitive materials were performed
with the rigorous exclusion of oxygen and moisture in flame-
dried Schlenk-type glassware either on a dual manifold
Schlenk line, interfaced to a high vacuum (10−4 torr) line, or in
an argon-filled M. Braun glove box. THF was pre-dried over Na
wire and distilled under nitrogen from sodium and benzophe-
none ketyl prior to use. Hydrocarbon solvents (toluene and
n-pentane) were distilled under nitrogen from LiAlH4 and
Preparation of [(THF)3Sr{Ph2P(Se)NCH2CH2NPPh2(Se)}] (3)
stored in the glove box. 1H NMR (400 MHz), 13C{1H} and In a 50 mL dry Schlenk flask ligand 1 (130 mg, 0.23 mmol),
31P{1H} NMR (161.9 MHz) spectra were recorded on a BRUKER KN(SiMe3)2 (89 mg, 0.45 mmol) and SrI2 (100 mg, 0.23 mmol)
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were mixed together in 10 mL of THF at an ambient tempera- quencies): 3052 (ArC–H), 2917 (C–H), 1435 (P–C), 997 (P–N),
ture and stirred for 14 hours. The precipitate of KI was filtered 551 (PvSe) cm−1. (C38H48MgN2O3P2Se2) (824.95) Calc. C 55.32,
using a filter dropper and the filtrate was dried in vacuo. The H 5.86, N 3.40; found C 54.83, H 5.49, N 3.18.
resulting white compound was further purified by washing
Preparation of [Ph2P(BH3)NHCH2CH2NHPPh2(BH3)] (6)
with pentane and crystals suitable for X-ray analysis were
grown from a THF–pentane (1 : 2 ratio) mixture solvent at To
a stirring solution of ethylenediamine (325.2 mg,
−40 °C.
5.4 mmol) and triethylamine (1.09 g, 1.5 mL, 10.8 mmol) in a
Yield 176.5 mg (87.8%).
THF–CH2Cl2 mixture solvent (10 mL), was added a solution of
1H NMR (400 MHz, C6D6): δ = 8.05–7.99 (m, 8H, ArH), chlorodiphenylphosphine (2.39 g, 10.8 mmol) in THF (5 mL),
7.12–6.97 (m, 12H, ArH), 3.57–3.53 (m, 12H, THF), 2.87 (m, drop-wise, and stirred for 3 hours. The precipitate formed was
4H, CH2), 1.41–1.37 (m, 12H, THF) ppm; 13C NMR (100 MHz, filtered and the solvent was removed in vacuo. To this, 20 mL
C6D6): δ = 133.8 (P–ArC), 132.9 (P–ArC), 131.0 (o-ArC), 130.8 of dry toluene and two equivalents of borane-dimethyl sul-
(m-ArC), 130.2 (p-ArC), 66.4 (THF), 40.9 (CH2), 24.4 (THF) ppm; phide (822.2 mg, 10.8 mmol) were added and stirred for
31P NMR (161.9 MHz, C6D6): δ = 71.9 ppm; FT-IR (selected fre- another 12 hours. The title compound was formed as a white
quencies): 3052 (ARC–H), 2922 (C–H), 1435 (P–C), 998 (P–N), precipitate, which further purified by washing several times
552 (PvSe) cm−1. (C38H48N2O3P2Se2Sr) (888.26) Calc. C 51.38, with n-hexane. Crystals suitable for X-ray diffraction analysis
H 5.45, N 3.15; found C 50.65, H 5.08, N 3.01.
were obtained from THF–n-pentane combination in a 1 : 2
ratio.
Preparation of [(THF)3Ba{Ph2P(Se)NCH2CH2NPPh2(Se)}] (4)
Yield 58.7% (1.45 g).
In a 50 mL dry Schlenk flask ligand 1 (150 mg, 0.256 mmol),
1H NMR (400 MHz, CDCl3): δ = 7.66–7.57 (m, 8H, ArH),
KN(SiMe3)2 (102 mg, 0.512 mmol) and BaI2 (100 mg, 7.55–7.32 (m 12H, ArH), 3.03–3.01 (m, 2H, P(BH3)NH),
0.256 mmol) were mixed together in 10 mL of THF at an 2.81–2.75 (m, 4H, CH2), 1.87–0.95 (m, 6H, BH3) ppm; 13C NMR
ambient temperature and stirred for 14 hours. The precipitate (100 MHz, C6D6): δ = 134.3 (P–ArC), 134.2 (P–ArC), 132.9
of KI was filtered using a filter dropper and the filtrate was (o-ArC), 132.8 (o-ArC), 132.2 (p-ArC), 132.1 (p-ArC), 131.4
dried in vacuo. The resulting white compound was further (m-ArC), 131.2 (m-ArC), 44.2 (CH2) ppm; 31P–{1H} NMR
purified by washing with pentane and crystals suitable for (161.9 MHz, CDCl3): δ = 58.8 (d, JP–B = 67.9 Hz) ppm; 11B–{1H}
X-ray analysis were grown from a THF–pentane (1 : 2 ratio) NMR (128.4 MHz, CDCl3): δ −38.1 ppm. FT-IR (selected fre-
mixture solvent at −40 °C.
quencies): 3366 (N–H), 3056 (ArC–H), 2960 (C–H), 2380 (B–H),
Yield 210.0 mg (87.5%).
1436 (P–C), 935 (P–N), 606 (P–B) cm−1. (C26H32B2N2P2) (456.10)
1H NMR (400 MHz, C6D6): δ = 8.14–8.02 (m, 8H, ArH), Calc. C 68.46, H 7.07, N 6.14; found C 67.98, H 6.79, N 5.88.
7.11–7.03 (m, 12H, ArH), 3.57–3.53 (m, 12H, THF), 3.14 (m,
Preparation of [(THF)2Ba{Ph2P(BH3)NCH2CH2NPPh2(BH3)}]2 (7)
4H, CH2), 1.41–1.38 (m, 12H, THF) ppm; 13C NMR (100 MHz,
C6D6): δ = 133.1 (P–ArC), 132.8 (P–ArC), 131.5 (o-ArC), 131.3 In a 50 mL dry Schlenk flask, ligand 6 (116.8 mg, 0.256 mmol),
(m-ArC), 130.7 (p-ArC), 67.5 (THF), 48.5 (CH2), 25.5 (THF) ppm; KN(SiMe3)2 (102 mg, 0.512 mmol) and BaI2 (100 mg,
31P NMR (161.9 MHz, C6D6): δ = 73.3 ppm; FT-IR (selected fre- 0.256 mmol) were mixed together in 10 mL of THF at an
quencies): 3052 (ArC–H), 2951 (C–H), 1434 (P–C), 997 (P–N), ambient temperature and stirred for 14 hours. The precipitate
555 (PvSe) cm−1. (C38H48BaN2O3P2Se2) (937.97) Calc. C 48.66, of KI was filtered using a filter dropper and the filtrate was
H 5.16, N 2.99; found C 47.88, H 4.72, N 2.69.
dried in vacuo. The resulting white compound was further
purified by washing with pentane and crystals suitable for
X-ray analysis were grown from a THF–pentane (1 : 2 ratio)
Preparation of [(THF)3Mg{Ph2P(Se)NCH2CH2NPPh2(Se)}] (5)
In a 50 mL dry Schlenk flask ligand 1 (210 mg, 0.36 mmol), mixture solvent at −40 °C.
KN(SiMe3)2 (143 mg, 0.72 mmol) and MgI2 (100 mg,
0.36 mmol) were mixed together in 10 mL of THF at an
Yield 153.0 mg (85.0%).
1H NMR (400 MHz, C6D6): δ = 7.59–7.54 (m, 8H, ArH),
ambient temperature and stirred for 14 hours. The precipitate 7.15–7.01 (m, 12H, ArH), 2.71–2.67 (m, 4H, CH2), 1.96–0.80 (m,
of KI was filtered using a filter dropper and the filtrate was 6H, BH3) ppm; 13C NMR (100 MHz, C6D6): δ = 133.3 (P–ArC),
dried in vacuo. The resulting white compound was further 132.6 (P–ArC), 132.2 (o-ArC), 132.1 (o-ArC), 131.3 (p-ArC), 131.2
purified by washing with pentane and crystals suitable for (p-ArC), 128.8 (m-ArC), 128.7 (m-ArC), 44.4 (CH2) ppm;
X-ray analysis were grown from a THF–pentane (1 : 2 ratio) 31P NMR (161.9 MHz, C6D6): δ = 70.8 (d, JP–B = 62.4 MHz) ppm;
mixture solvent at −40 °C.
11B–{1H} NMR (128.4 MHz, C6D6): δ −37.6 ppm. FT-IR (selected
frequencies): 3056 (ArC–H), 2957 (C–H), 2375 (B–H), 1434
Yield 202.0 mg (70.6%).
1H NMR (400 MHz, C6D6): δ = 7.92–7.87 (m, 8H, ArH), (P–C), 996 (P–N), 605 (P–B) cm−1. (C34H46B2BaN2O2P2) (735.62)
7.01–6.90 (m, 12H, ArH), 3.61–3.58 (m, 12H, THF), 3.09 (m, Calc. C 55.51, H 6.30, N 3.81; found C 54.92, H 6.02, N 3.31.
4H, CH2), 1.32–1.26 (m, 12H, THF) ppm; 13C NMR (100 MHz,
Typical polymerisation experiment
C6D6): δ = 132.1 (P–ArC), 131.9 (P–ArC), 131.7 (o-ArC), 131.6
(m-ArC), 129.6 (p-ArC), 68.1 (THF), 29.9 (CH2), 25.4 (THF) ppm; In a glove box under an argon atmosphere, the catalyst was
31P NMR (161.1 MHz, C6D6): δ = 43.7 ppm; FT-IR (selected fre- dissolved in the appropriate amount (1.0 mL) of dry toluene.
8764 | Dalton Trans., 2014, 43, 8757–8766
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ε-Caprolactone in 1.0 mL of toluene was then added under vig- largest ionic radius, acts as the best catalyst among the three
orous stirring. The reaction mixture was stirred at room temp- analogous complexes.
erature for 5–20 minutes, after which the reaction mixture was
quenched by addition of a small amount of (1.0 mL) methanol
and then added acidified methanol in little excess. The
polymer was precipitated in excess methanol and it was fil-
Acknowledgements
tered and dried under vacuum. The final polymer was then This work was supported by the Department of Science and
analysed by NMR and GPC.
Technology India (DST) under the SERC Fast Track
Scheme (SR/FT/CS-74/2010) and start-up grant from IIT
Hyderabad. R. K. K., A. H. and J. B. thank the University
Grants Commission, India for their PhD fellowship. We thank
Dr K. Muralidharan of University of Hyderabad for providing
the facility to measure the polymers.
X-Ray crystallographic studies of 2–6
Single crystals of compounds 2–5 and 7 were grown from a
THF and pentane mixture at −40 °C under an inert atmos-
phere. The single crystals of 1-cis, 1-trans and 6 suitable for
X-ray measurement were grown at room temperature. For com-
pounds 2–5 and 7, a crystal of suitable dimensions was
mounted on a CryoLoop (Hampton Research Corp.) with a
layer of light mineral oil and placed in a nitrogen stream at
150(2) K. However, for compounds 1-cis, 1-trans and 6, the data
were collected at 293 K. All measurements were made on an
Oxford Supernova Xcalibur Eos CCD detector with graphite-
monochromatic Cu-Kα (1.54184 Å) radiation. Crystal data and
structure refinement parameters are summarised in Table TS1
in the ESI.† The structures were solved by direct methods
(SIR92)31 and refined on F2 by full-matrix least-squares
methods using SHELXL-97.32 Non-hydrogen atoms were aniso-
tropically refined. H atoms were included in the refinement in
calculated positions riding on their carrier atoms. No restraint
was made for all the compounds. For compounds 2 and 5, two
carbon atoms which are part of the two coordinated THF mole-
cules are slightly thermally disorder and treated anisotropi-
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8766 | Dalton Trans., 2014, 43, 8757–8766
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