Chiral alkaline earth metal complexes with M–Se
direct bond (M ¼ Mg, Ca, Sr, Ba): syntheses,
structures and 3-caprolactone polymerisation†
Ravi K. Kottalanka, Adimulam Harinath and Tarun K. Panda
*
We report here a series of enantiomeric pure alkaline earth metal complexes, each with a metallic direct
bond of selenium, with {HN(R-*CHMePh)(P(Se)Ph
2
)} (1a) and {HN(S-*CHMePh)(P(Se)Ph
2
)} (1b),
synthesised using two routes. The first route involves a trans metalation reaction of enantiomeric pure
potassium phosphinoselenoic amide [K{N(R-*CHMePh)(Ph
2
P(Se))}{THF}
n
] (2a) or [K{N(S-
*CHMePh)(Ph
2
P(Se))}{THF}
n
](2b) prepared from the reaction between either 1a or 1b and [KN(SiMe
3
)
2
],
and the corresponding alkaline earth metal diiodies in THF at room temperature to afford the
enantiomeric pure complexes of composition [M{N(R-*CHMePh)P(Se)Ph
2
}
2
(THF)
n
] [M ¼ Mg (3a), n ¼ 1;
M ¼ Ca (4a), Sr (5a) and Ba (6a), n ¼ 2] and [M{N(S-*CHMePh)P(Se)Ph
2
}
2
(THF)
n
] [M ¼ Mg (3b), n ¼ 1;
M ¼ Ca (4b), Sr (5b) and Ba (6b), n ¼ 2]. The same heavier alkaline earth metal complexes (4a–6a and
4b–6b) can also be obtained through the silylamine elimination method using the corresponding
metal bis(trimethylsilyl)amides [M{N(SiMe
3
)
2
}
2
(THF)
n
] (M ¼ Ca, Sr, Ba) with phosphinoselenoic amine
ligands 1a and 1b in ambient conditions. The solid-state structures of the metal complexes 4a–6a and
4b–6b were established using single-crystal X-ray diffraction analysis. In the solid state, all the metal
complexes crystallise in the monoclinic P2
1
space group and each phosphinoselenoic amido ligand is
ligated to the metal ion in a bidentate fashion. We also report the syntheses and structures of chiral
amidophosphine-borane ligands {HN(R-*CHMePh)(P(BH
3
)Ph
2
)} (7a) and {HN(S-*CHMePh)(P(BH
3
)Ph
2
)}
(7b) and the corresponding homoleptic barium complexes of composition [Ba{N(R-*CHMePh)P(BH
3
)
Ph
2
}
2
(THF)
2
](8a) and [Ba{N(R-*CHMePh)P(BH
3
)Ph
2
}
2
(THF)
2
](8b). The molecular structures of 8a and
8b in the solid state confirm the attachment of chiral amidophosphine-borane ligands to the barium
ions. The complexes 5 and 6 were tested as catalysts for the ring-opening polymerisation of
3-caprolactone. High activity in relation to the barium complexes 6a and 6b is observed, with
moderate to narrow polydispersity index.
Introduction
Efficient synthesis of optically active compounds is one of the
most important tasks of synthetic organic chemistry. The most
promising methodology is catalytic asymmetric synthesis using
a chiral metal centre. Among many useful metal species, alka-
line earth metals have long been recognised as belonging to a
class of less toxic and less harmful metals.
1,2
However, besides
the potential high utility of the alkaline earth species as a
homogeneous catalyst for ring-opening polymerisation of
various cyclic esters,
3,4
polymerisation of styrene and dienes,
5
and hydroamination and hydrophosphination reactions of
alkenes and alkynes,
6
its use in synthetic organic chemistry,
especially in asymmetric synthesis as chiral catalyst, has been
quite limited when compared to transition metal catalysts.
1,2
Recently it was revealed that several catalytic asymmetric
carbon–carbon bond-forming and related reactions proceeded
smoothly in high enantioselectivites with the use of chiral Ca,
Sr, and Ba catalysts.
7–10
Their strong Brønsted basicity and mild
Lewis acidity are promising and attractive characteristics and
can inuence their catalytic activity as well as their chiral
modication capability in a positive manner.
A wide variety of chiral phosphorus ligands have been
prepared over the years, and their coordination chemistry with
various metal ions has been studied extensively.
11
In homoge-
neous catalyses, bidentate phosphine ligands, especially those
having C
2
symmetry, have usually been employed. In most cases
the stereogenic centres are chiral phosphorus atoms or phos-
phines with chiral hydrocarbon substituents as derivatives of
the chiral pool.
11a
The synthesis and limited use of heteroatom-
substituted phosphines and their transition metal complexes
Department of Chemistry, Indian Institute of Technology Hyderabad, Ordnance
Factory Estate, Yeddumailaram 502205, Telangana, India. E-mail: tpanda@iith.ac.
in; Fax: +91 40 2301 6032; +91 40 2301 6036
† Electronic supplementary information (ESI) available. CCDC 1053400–1053411.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5ra04495b
Cite this: RSC Adv. , 2015, 5, 37755
Received 14th March 2015
Accepted 20th April 2015
DOI: 10.1039/c5ra04495b
www.rsc.org/advances
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