Bis[μ-bis(2,6-diisopropylphenyl) phosphato-κ2 O:O′]bis[(2,2′-bipyridine-κ2 N,N′)lithium] toluene disolvate and its catalytic activity in ring-opening polymerization of ∊-caprolactone and l-dilactide

The solvated centrosymmmtric title compound, [Li2(C24H34O4P)2(C10H8N2)2]·2C7H8, was formed in the reaction between {Li[(2,6-iPr2C6H3-O)2POO](MeOH)3}(MeOH) and 2,2′-bipyridine (bipy) in toluene. The diaryl phosphate ligand demonstrates a μ-κO:κO′-bridging coordination mode and the 2,2′-bipyridine ligand is chelating to the Li+ cation generating a distorted tetrahedral LiN2O2 coordination polyhedron. The complex exhibits a unique dimeric Li2O4P2 core. Catalytic systems based on the title complex and on the closely related complex {Li[(2,6-iPr2C6H3-O)2POO](MeOH)3}(MeOH) display activity in the ring-opening polymerization of ∊-caprolactone and l-dilactide.

On the other hand, it is known that diaryl-substituted phosphoric acids in the presence of 3-phenylpropan-1-ol as an initiator are capable of catalysing ring-opening polymerization (ROP) of "-caprolactone ("-CL) and l-lactide (LLA) into poly("-caprolactone) (PCL) and poly(l-lactide) (PLLA) at high temperatures (453 K, bulk sample; Liu et al., 2019). It might be noted that ROP of "-CL and LLA can also be carried out at lower temperatures: 353 K for "-CL (bulk sample, the same initiator and catalysts; Saito et al., 2015) and 383 K for dl-lactide [30% of toluene by volume, glycolic acid derivatives of bio-metals (Mg, Zn, Al) were used as catalysts; .
We have tested salts (I) and (II) as precatalysts for "-CL and LLA polymerization under two different condition sets: (1) 373 K, $30% of toluene by volume and (2) 453 K, bulk sample (Fig. 2), using benzyl alcohol as an initiator. The monomer/precatalyst molar ratio is taken as 25:1 (with respect to one lithium phosphate unit) in order to monitor the reaction mixtures and to study the resulting short oligomers by 1 H NMR spectroscopy.

Figure 1
Synthesis of {[(2,6-i Pr 2 C 6 H 3 O) 2 PO 2 ]Li(bipy)} 2 (C 7 H 8 ) 2 , (II). PCL obtained, there are two types of the RO terminal group, namely, MeO and PhCH 2 O. Based on NMR integral intensities, their sum corresponds to the amount of the -CH 2 -OH terminal group. The MeO/PhCH 2 O ratio decreases upon increasing the taken amount of PhCH 2 OH, and the corresponding ratio is 1.00/0.00 for entry 1, 0.73/0.27 for entry 2 and 0.58/0.42 for entry 3. Thus, compound (I) does not require an additional activator because of the presence of the internal one, namely, MeOH molecules. Unlike for (I), polymerization of "-CL by (II) without an activator does not occur (entry 4). Activation by benzyl alcohol does not lead to a noticeable yield of PCL having only the PhCH 2 O-and -CH 2 -OH terminal groups (entries 5 and 6). The addition of two equivalents of the PhCH 2 OH activator provides higher conversions in the cases of both complexes (I) and (II) (entries 3 and 6).
Unlike "-CL oligomerization, the ROP of LLA has failed under the same conditions. For example, conversion of LLA to PLLA at the [LLA]/[(II)]/[PhCH 2 OH] ratio of 25:0.5:2 is only 6%. Therefore, oligomerization of LLA and "-CL has been studied further at a higher temperature ( Table 2). Conversions of "-CL in the case of complex (II) (entry 2) is even higher than that for (I), but the polymerization degree is higher than expected P n = 25, which may be explained by the faster reaction rate of the catalyst with the monomer, compared to the activation rate. The ROP of LLA proceeds under these conditions, but providing a rather low conversion to PLLA and the formation of shorter oligomers than expected (entries 3 and 4).
In summary, catalytic tests have displayed that systems based on complexes (I) and (II) are capable of catalysing ROP of cyclic esters, using "-caprolactone and l-dilactide as model substrates, but the catalytic activity of the systems is rather poor. Complex (I) does not require an initiator for polymerization of "-CL.

Structural commentary
The title compound (II) crystallizes in the monoclinic space group (P2 1 /n). Its asymmetric unit (see Fig. S1 in the supporting information) contains one non-coordinating toluene molecule and half the complex {Li 2 (bipy) 2 [(2,6-i Pr 2 C 6 H 3 -O) 2 PO 2 ] 2 }, which is located on an inversion centre (Fig. 3)   The molecular structure of {Li 2 (bipy) 2 [(2,6-i Pr 2 C 6 H 3 -O) 2 PO 2 ] 2 }. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. The minor disorder component of one of the isopropyl groups is shown with open lines. Symmetry code: (i) Àx + 1, Ày + 1, Àz + 2. Table 1 Polymerization of "-CL under mild conditions. M n is the number average molar mass; Ð is the polydispersity index; P n is the polymerization degree; Conv. is conversion of "-CL into PCL and defined as [  complex has an unusual Li 2 P 2 O 4 core ( The [Li(bipy)] + cation in (II) is nearly flat with the highest deviations from the plane being 0.102 (2) Å for N2, 0.115 (2) Å for C31 and 0.133 (2) Å for C34. To be more precise, the coordinated bipy ligand is slightly twisted about the C29-C30 bond; the dihedral angle between two planes formed by the N1/C25-C29 and N2/C30-C34 atoms is 8.41 (12) . Selected bond distances are given in Table 3. The P-O Ar bond distances are longer by 0.13-0.14 Å than the other two P-O distances. The P and Li atoms adopt distorted tetrahedral environments with the bond angles ranging from 77.49 (14) for N1-Li1-N2 to 120.5 (2) for O1-Li1-O2 i and from 98.16 (8) for O3-P1-O4 to 120.32 (9) for O1-P1-O2. The smallest O-P-O angle corresponds to the O Ar -P-O Ar angle between the two bulky aryl ligands. These observations for the P-O distances and O-P-O bond angles are also seen for the closely related salt (I) (Minyaev et al., 2015), for rare-earth complexes bearing the same phosphate ligand (Minyaev et al., 2017(Minyaev et al., , 2018a and for bis(2,6diisopropylphenyl)phosphoric acid (with the exception of the P-OH bond-distance value; Gupta et al., 2018). An explanation for this has been given earlier (Minyaev et al., 2017).

Supramolecular features
The extended structure of (II), for which packing plots are shown in Figs. S3-S5 of the supporting information, features weak C-HÁ Á ÁO and C-HÁ Á Á interactions (Table 4). Any aromaticstacking must be extremely weak, as the shortest centroid-centroid separation of aromatic rings is 4.1743 (13) Å .

Synthesis and crystallization
4.1. General remarks All synthetic manipulations were performed under a purified argon atmosphere, using Schlenk glassware, dry box techniques and absolute solvents. Hexane was distilled from Na/K alloy, toluene was distilled from sodium/benzophenone ketyl, 2,2 0 -bipyridine was recrystallized from absolute toluene prior to use. The salt [(2,6-i Pr 2 C 6 H 3 O) 2 PO 2 Li(MeOH) 3 ]-(MeOH) was synthesized according to the literature procedure (Minyaev et al., 2015). (3S,6S)-3,6-Dimethyl-1,4-dioxane-2,5-dione (l-lactide, Sigma-Aldrich, 99%) was purified by double sublimation under dynamic vacuum. "-Caprolactone ("-CL) was distilled from CaH 2 under vacuum. CDCl 3 (Cambridge Isotope Laboratories, Inc., D 99.8%) was used as purchased for registering the NMR spectra of polymer samples, and was distilled from CaH 2 under argon prior to recording the NMR spectra of (II). The 1 H NMR spectra of polymers were recorded on a Bruker AVANCE 400 spectrometer at 297K, the 1 H and 31 P{ 1 H} NMR spectra of (II) were registered on a Bruker AV-600 instrument; chemical shifts are reported in ppm relative to the solvent residual peak. Sizeexclusion chromatography (SEC) analysis of polymer samples was performed at 323 K using an Agilent PL-GPC 220 gel permeation chromatograph equipped with a PLgel column, with DMF as eluents (1 ml min À1 ) and poly(ethylene oxide) standards.
of (II) may lead to a nearly complete loss of the non-coordinating toluene molecules. Colourless prisms of (II) were formed upon recrystallization of the obtained microcrystalline solid from a warm ($333 K) nearly saturated solution in toluene by slow cooling to $268 K.

Polymerization procedures
Method 1. In a dry box, complex (I) (0.1 mmol, 55 mg) or complex (II) (0.05 mmol, 67 mg), a monomer (2.5 mmol, 285 mg for "-CL or 360 mg for LLA), and toluene (0.15 ml) or a solution of PhCH 2 OH (11 or 22 mg) in toluene (0.15 ml) were placed at room temperature ($298 K) in a vial, which was then sealed and taken out of the box. The mixture was stirred for 3 h at 373 K. After that, a sample of the mixture was taken to register a 1 H NMR spectrum to determine the monomer conversion. The remaining mixture was quenched with methanol (tenfold volume) containing 5 equiv. of acetic acid (with respect to Li phosphate), washed with methanol, dried under vacuum, taken for SEC and 1 H NMR studies.
Method 2 was performed as Method 1 with the following exceptions: (1) toluene was not added, (2) the mixture was stirred for 1 h at 453 K.
The monomer conversion was determined by 1 H NMR (in CDCl 3 ) of reaction mixtures, basing on integration of the following resonance signals: CH 2 OC O at 4.22 ppm for "-CL and at 4.05 ppm for PCL, CH(CH 3 )OC O at 5.04 ppm for LLA and at 5.15 ppm for PLLA. The end-group analysis was based on the following resonance signals of terminal-groups: 3.67 ppm for CH 3 -O-CO-, 5.11 ppm for Ph-CH 2 -O-CO-, 3.63 ppm for -CH 2 -CH 2 -OH in PCL and 4.37 ppm for -CO-CHCH 3 -OH in PLLA.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. The positions of hydrogen atoms (with the exception of the disordered fragment) were found from a difference-Fourier map but positioned geometrically (C-H distance = 0.95 Å for aromatic, 0.98 Å for methyl and 1.00 Å for methine H atoms) and refined as riding atoms with relative isotropic displacement parameters U iso (H) = 1.5U eq (C) for methyl H atoms and 1.2U eq (C) otherwise. A rotating group model was applied for methyl groups. Reflection 110 was affected by the beam stop and was therefore omitted from the refinement.
One isopropyl group is disordered over two orientations (atoms C23A, C24A and C23B, C24B) with a corresponding disorder ratio of 0.621 (4):0.379 (4). Similarity displacement ellipsoid constraints were applied for these atoms. The C-C bond distances in the disordered isopropyl fragment were restrained to be equal within 0.002 Å .

Roitershtein and Ilya E. Nifant'ev Computing details
Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS (Sheldrick, 2008); program(s) used to refine structure: SHELXL2017 (Sheldrick, 2015a); molecular graphics: SHELXTL (Sheldrick, 2015b); software used to prepare material for publication: SHELXTL (Sheldrick, 2015b) and publCIF (Westrip, 2010). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.73 e Å −3 Δρ min = −0.73 e Å −3 Special details Experimental. moisture sensitive Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (