Chiral alkaline earth metal complexes with M – Se direct bond (M ¼ Mg, Ca, Sr, Ba): syntheses, structures and 3 -caprolactone polymerisation †

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 ﬁ rst route involves a trans metalation reaction of enantiomeric pure potassium phosphinoselenoic amide [K


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, alkaline earth metals have long been recognised as belonging to a class of less toxic and less harmful metals. 1,2However, 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][9][10] Their strong Brønsted basicity and mild Lewis acidity are promising and attractive characteristics and can inuence their catalytic activity as well as their chiral modication 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. 11In homogeneous 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 phosphines with chiral hydrocarbon substituents as derivatives of the chiral pool.11a The synthesis and limited use of heteroatomsubstituted phosphines and their transition metal complexes have received some attention lately as a result of the search for new structural diversity.11b However little has been published on the use of chiral amines as backbone for chiral phosphorus ligands.These chiral P, N ligands, which usually coordinate via the phosphorus atom to the centre metal, were basically used in transition metal and rare earth metal chemistry. 12ecently we introduced various amidophosphine chalcogenide and borane ligands with P, N, E (E ¼ O, S, Se, BH 3 ) as donor atoms, into alkaline earth metal chemistry to study their coordination properties. 13These unique ligands are potentially capable of coordinating through the hard nitrogen and phosphorus donor atoms as well as the so E donor atom.Bearing these characteristic features in mind, as well as our continuing interest in highly electropositive alkaline earth metals, catalytic activity and the vast potential of the eld in asymmetric synthesis, we proposed to synthesise various novel chiral alkaline earth metal complexes stabilised by chiral amidophosphine selenoids and boranes, to explore the chemistry of alkaline earth metals in asymmetric synthesis.To achieve our target compounds with high-purity and good yield, we chose chiral phosphineamines HN(R-*CHMePh)(PPh 2 ) and HN(S-*CHMePh)(PPh 2 ), which were originally introduced by Brunner into coordination chemistry of the late transition metals. 14Roesky et al. introduced the same ligands into zirconium chemistry, 15 group 3 and lanthanide chemistry. 16We synthesise the corresponding enantiomeric pure amidophosphine-selenoids [HN(R-*CHMePh)P(Se)Ph 2 ] (1a) and [HN(S-*CHMePh)P(Se)Ph 2 ] (1b) in order to introduce them into the alkaline earth metal chemistry.We envisage that these ligands potentially coordinate through the amido nitrogen and selenium atoms, thus forming a four-membered metallacycle with a centre metal ion.
The solid-state structures of 1a and 1b were conrmed using single-crystal X-ray diffraction analysis.The details of the structural parameters are given in Table TS1 in ESI.† The solidstate structures of both enantiomers and selected bond lengths and bond angles are shown in Fig. 1.From the molecular structure of two compounds, it is clear that both enantiomers are non-super imposable mirror images and crystallise in the triclinic space group P1, with one molecule in the unit cell.). 17The potassium complexes 2a and 2b were characterised using spectroscopic and analytical techniques.However, suitable crystals for X-ray diffraction analysis were not obtained due to high solubility of the compounds (2a,b) in the THF solvent.In FT-IR spectra, the compound (2a,b) showed a strong absorption band at 570 cm À1 which can be best assigned to characteristic P]Se bond stretching and it is in good agreement with our previously described potassium salts of phosphinoselenoic amides: 569 cm À1 for [{(THF) 2 KPh 2 P(Se)-N(CHPh 2 )} 2 ] and 570 cm À1 for [K(THF) 2 {Ph 2 P(Se)-N(CMe 3 )}] n .13c,g

31
P{ 1 H} NMR spectra of compound (2a,b) showed a sharp singlet resonance signal at d 48.6 ppm, which is up-eld shied (56.1 ppm) compared to that of the free ligand (1a,b), indicating clear evidence of the formation of potassium salt.Two multiplet signals in the region of 3.50-3.53ppm and 1.37-1.40ppm in 1 H spectra also conrm the presence of coordinated THF molecules in the complex 2a,b and using integration it was calculated that three THF molecules were coordinated.One set of signals was observed for the compound (2a,b) in the 1 H and 13 C{ 1 H} NMR spectra, similar to the free ligand, indicating a dynamic behaviour of the complexes in the solution state.
The catalytic ability of the newly synthesised enantiomeric pure mono-nuclear strontium complexes 5a or 5b to promote the ROP of 3-CL was rst evaluated (Table 1, entries 1-5).Indeed, the moderate reactivity of the strontium complexes is very similar to that observed in previously reported studies using other strontium complexes for ROP of 3-caprolactone. 29ince the larger ion radius barium complexes have been reported to be more active than the calcium and strontium congeners in ROP, 30,31 we tested compound 6a or 6b as a catalyst and observed an enhanced rate of polymerisation (Table 1, entries 6-10).In the case of strontium, higher reactivity was observed for conversion of 3-caprolactone to poly-caprolactone and up to 500 3-CL units were successfully converted in high yields (75-90 per cent), within 15 and 10 minutes respectively, at 25 C.The control over the ROP process was rather good, affording PCLs, featuring a considerable match between the observed (as determined by GPC) and calculated molar mass values, as well as moderate dispersity data (PDI ¼ M w /M n < 1.94).However, the overall efficiency of the strontium initiator 5a,b towards the ROP of 3-CL was weaker than that of the barium analogue 6a,b.Being the largest ionic radius of the barium atom, it was anticipated that complex 6a,b would show the highest reactivity among all the three alkaline earth metal complexes. 32,33In reality we observed that up to 500 3-CL units were successfully converted in good yields (80-98 per cent) within 10 minutes at 25 C (Table 1, entries 6-10).The polycaprolactone produced by the use of the barium catalyst was a considerable match between the observed and calculated molar mass values, and we observed a relatively narrow poly-dispersity data (PDI up to 1.55, entry 9 in Table 1).Thus, among strontium and barium metal complexes, the barium complexes 6a,b showed the highest activity for ROP of 3-caprolactone.

General consideration
All manipulations of air-sensitive materials were performed with the rigorous exclusion of oxygen and moisture in amedried Schlenk-type glassware either on a dual manifold Schlenk line, interfaced to a high vacuum (10 À4 Torr) line, or in an argon-lled M. Braun glove box.THF was pre-dried over Na wire and distilled under nitrogen from sodium and benzophenone ketyl prior to use.Hydrocarbon solvents (toluene and n-pentane) were distilled under nitrogen from LiAlH 4 and stored in the glove box. 1 H NMR (400 MHz), 13 C{ 1 H} and 31  selenium metal was ltered through a G4 frit to collect the yellow-coloured ltrate.Aer evaporation of the solvent from ltrate in vacuo, a light-yellow solid residue was obtained, which was further puried by washing with n-hexane.Compound 1a was re-crystallised from THF at room temperature.
Yield: 1.24 g (98%) (1a) and 1.25 g (99%) (1b).In a 50 ml pre-dried Schlenk ask, one equivalent (1.00 g, 2.60 mmol) of ligand 1a and one equivalent of potassium bis(trimethylsilyl)amide (520 mg, 2.60 mmol) were mixed together with 10 ml of dry THF.Aer 6 hours of stirring, the THF solvent was evaporated in vacuo and the dry compound was further puried by washing with n-pentane (5 ml) twice.The title compound 2a was obtained as a light orange powder.Compound 2b was also obtained by similar procedure.

Typical polymerisation experiment
In a glove box under argon atmosphere, the catalyst was dissolved in the appropriate amount (1.0 ml) of dry toluene.3-Caprolactone in 1.0 ml of toluene was then added along with vigorous stirring.The reaction mixture was stirred at room temperature for 5-20 minutes, aer which the reaction mixture was quenched by the addition of a small amount of (1.0 ml) methanol.Later, a small quantity of excess acidied methanol was added.The polymer was precipitated in excess methanol and it was ltered and dried under vacuum.The nal polymer was then analysed by NMR and GPC.

X-ray crystallographic studies of 1, 4-8
Single crystals of compounds 1a,b were grown from a concentrated solution of THF at room temperature.However, the single crystals of 4a,b-8a,b suitable for X-ray measurement were grown at À35 C under inert atmosphere.For compounds 4a,b-8a,b, (except 7a,b) 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 1a,b and 7a,b, the data were collected at 293 K.All measurements were made on an agillent Supernova X-calibur Eos CCD detector with graphite-monochromatic Cu-Ka (1.54184 Å) radiation.Crystal data and structure renement parameters are summarised in Table TS1 in the ESI.† The structures were solved by direct methods (SIR92) 34 and rened on F 2 by full-matrix least-squares methods; using SHELXL-97. 35on-hydrogen atoms were anisotropically rened.

Conclusion
We have demonstrated a series of alkaline earth metal complexes obtained in enantiomeric pure form with chiral phosphinoselenoic amides ligand through two routes of synthesis.In the solid-state structures of Ca-Ba complexes, the monoanionic ligand attached to the metal centre in k 2 fashion via the coordination of amido nitrogen and selenium atoms, conrming the bidentate chelation of chiral phosphinoselenoic amide.Thus, the enantiomeric pure compounds 4-6 are known to be a new class of alkaline earth metal complexes, and to the best of our knowledge, these are the rst examples of chiral alkaline earth metal complexes with a metal-selenium direct bond.We have also described the synthetic and structural features of chiral amidophosphine-borane ligands and the corresponding barium complex.It was found that the amidophosphine-borane ligand is coordinated through the amido nitrogen and BH 3 hydrogens (h 1 and h 2 ) to the barium ion.We have tested complexes 5-6 as catalysts for the ROP of 3-caprolactone and observed that the barium complex, having the largest ionic radius, acts as the best catalyst between the two analogous complexes.

Fig. 2
Fig. 2 Solid-state structures of calcium complexes 4a and 4b.Hydrogen atoms are omitted for clarity except methyl and methine hydrogen atoms.Selected bond lengths (Å) and bond angles ( ).

Fig. 3
Fig. 3 Solid-state structures of barium complexes 6a and 6b.Hydrogen atoms are omitted for clarity except methyl and methine hydrogen atoms.Selected bond lengths (Å) and bond angles ( ).
Scheme 6 Ring-opening polymerisation of 3-CL with strontium and barium complexes 5 and 6.

4 )
H atoms were included in the renement in calculated positions riding on their carrier atoms.No restraint was made with respect to any of the compounds.The function minimised was [ P w(F o 2 À F c 2 ) 2 ] (w ¼ 1/[s 2 (F o 2 ) + (aP) 2 + bP]), where P ¼ (max(F statistics.The function R1 and wR2 were ( P kF o | À |F c k)/ P |F o | and [ ] 1/2 , respectively.The Diamond-3 program was used to draw the molecule.†