Yttrium and Lithium Keto-β-Diketiminate Complexes [{2,6-Me2C6H3N=C(Me)}2CС(tert-Bu)=O]2Y(μ2-Cl)2Li(THF)2 and {[{2,6-Me2C6H3N=C(Me)}2CС(tert-Bu)=O]Li(THF)}n. Synthesis, Molecular Structures, and Catalytic Activity in ε-Caprolactone Polymerization

The reaction of lithium β-diketiminate [{2,6-Me2C6H3N=CMe}2CH]Li with benzophenone in toluene at 25°C affords the coordination complex [{2,6-Me2C6H3N=CMe}2CH]Li(Ph2C=O) (I). New keto-β-diketimine {2,6-Me2C6H3N=C(Me)}2CHC(tert-Bu)=O (II) is synthesized by the reaction of tert-Bu(C=O)Cl with [{2,6-Me2C6H3N=CMe}2CH]Li. The metallation of keto-β-diketimine II with n-butyllithium in THF at 0°C gives lithium keto-β-diketiminate {[{2,6-Me2C6H3N=C(Me)}2CС(tert-Bu)=O]Li(THF)}n (III). The exchange reaction of YCl3 with compound III (molar ratio 1 : 2, THF) affords the yttrium bis(keto-diketiminate) complex [{2,6-Me2C6H3N=C(Me)}2CС(tert-Bu)=O]2Y(μ2-Cl)2L-(THF)2 (IV). The molecular structures of complexes I, III, and IV are determined by X-ray diffraction analysis (CIF files CCDC nos. 2001131 (I), 2001132 (III), and 2001133 (IV)). Complex IV in the crystalline state exists as an ate complex with one LiCl molecule. Complexes I, III, and IV are catalysts of ring-opening polymerization of ε-caprolactone in toluene at 25°С.


INTRODUCTION
The N,N-and N,O-containing ligands differed in the number of donor groups and in the length and nature of the bridge between the coordination sites are presently among the most used classes of non-cyclopentadienyl ligands in the chemistry of rare-earth elements. The amide, amidinate, and ketiminate ligands with the variable denticity and steric properties were used in the chemistry of rare-earth element derivatives as the stabilizing coordination environment [1][2][3][4][5][6][7][8].
Interest in ligands of the non-cyclopentadienyl type is primary evoked by the fact that a series of reactive compounds of transition d metals and rare-earth metals (REM) were synthesized owing to the application of these ligands. The synthesized compounds were catalytically active in the polymerization of dienes and methyl methacrylate, ring-opening polymerization of rac-lactide and -caprolactone, hydrogenation and hydrosilylation of olefins, and copolymerization of epoxides with CO 2 [6][7][8][9][10][11][12][13][14]. In addition, the role of the ligand environment is very high in the case of electropositive REM with large ion radii that form predominantly ionic metal-ligand bonds and are prone to ligand exchange reactions (Schlenk equilibrium). The chelate ligand is responsible for the suppression of the ligand redistribution and provides the kinetic stability of the complex. Therefore, the synthesis of polydentate N,N-and N,O-ligands capable of forming labile coordination bonds with the metal ion along with the strong covalent bond is among of important tasks. The necessary saturation of the coordination sphere of the metal ion in the complex is achieved due to these coordination bonds.

EXPERIMENTAL
All procedures on the synthesis and isolation of the products were carried out in a vacuum apparatus using the standard Schlenk techniques. Tetrahydrofuran (THF) was dried with potassium hydroxide and distilled over sodium benzophenone ketyl. Hexane and toluene were dehydrated by reflux and distillation over metallic sodium. Deuterated pyridine (C 5 D 5 N) was dried with calcium hydride, degassed, and condensed in vacuo. Deuterated benzene (C 6 D 6 ) was dried over metallic sodium, degassed, and condensed in vacuo. Compounds {2,6-Me 2 C 6 H 3 N=CMe} 2 CH 2 [11] and YCl 3 [15] were synthesized according to published procedures. Benzophenone, ε-caprolactone, C 5 D 5 N, C 6 D 6 , CDCl 3 , and 2,6-dimethylaniline were commercial reagents (Acros). IR spectra were recorded on a Bruker-Vertex 70 instrument. Samples of the compounds were prepared in a dry argon atmosphere as suspensions in Nujol. 1 H, 13 C, 7 Li, and HSQC 1 H-13 C NMR spectra were detected on Bruker Avance III and Bruker DRX-200 instruments (25°С, C 5 D 5 N, C 6 D 6 , CDCl 3 ). Chemical shifts are presented in ppm with respect to the known shifts of residual protons of the deuterated solvents. Elemental analyses were carried out on a Perkin-Elmer Series II CHNS/O Analyser 2400 instrument. The yttrium content was determined by complexonometric titration (Trilon B) using xylenol orange as the indicator [16].

Synthesis of lithium bis[(2,6-dimethylphenyl)(3-(1-((2,6-dimethylphenyl)imino)ethyl)-5,5-dimethyl-4oxohex-2-en-2-yl)anilide]dichloroyttrate(III) ditetrahydrofuranate (IV).
A solution of complex III (0.402 g, 0.43 mmol) in THF (15 mL) was poured to a suspension of YCl 3 (0.084 g, 0.43 mmol) in THF (10 mL) at 25°C. The reaction mixture was stirred for 12 h, and THF was removed in vacuo. The reaction product was extracted with toluene (25 mL) and decanted from an insoluble precipitate. The solvent was removed, and the substance was dried in vacuo for 20 min and dissolved in THF (2 mL). White crystals of complex IV were obtained by the slow condensation of hexane in a concentrated solution of the complex in ТHF at 25°С. The crystals were washed with cold hexane and dried in vacuo at 25°С for 20 min. The yield of the white crystals of complex IV was 0.323 g (67% Polymerization of ε-caprolactone (general procedure). Complex I (5.0 mg, 0.01 mmol) was dissolved in toluene (2.5 mL) in an inert atmosphere of a glove box, and ε-caprolactone (0.285 g, 2.50 mmol) was added. The reaction mixture was stirred at 25°C for 5 min, and an aliquot was taken to determine the conversion of the monomer by the NMR method. Then a 1.2 M solution of HCl in ethanol (1 mL) was added to the reaction mixture, and the polymer was precipitated with ethanol excess (20 mL). The solid residue was separated and dried in vacuo to a constant weight. The yield was determined by gravimetry.
X-ray diffraction analyses (XRD) for compounds I, III, and IV were carried out on Bruker D8 Quest (I) and Rigaku OD Xcalibur (III, IV) diffractometers (МоK α radiation, ω scan mode, λ = 0.71073 Å, T = 100(2) K). Experimental sets of intensities were measured and integrated using the APEX2 [17] and Crys-Alis Pro [18] program packages. An absorption correction was applied and the structures were solved and refined using the ABSPack (CrysAlis Pro ), SADABS [19], and SHELX [20] program packages. The structures were solved by a direct method and refined by full-matrix least squares for in the anisotropic approximation for non-hydrogen atoms. All hydrogen atoms were placed in the geometrically calculated positions and refined isotropically with the fixed thermal parameters U(H) iso = 1.2U(C) equiv (U(H) iso = 1.5U(C) equiv for methyl groups). The crystallographic data and parameters of XRD experiments and structure refinement for compounds I, III, and IV are presented in Table 1. Selected bond lengths and bond angles are given in Table 2 [11] with n-butyllithium in toluene at 0°C (Scheme 1). It was found that no addition of lithium diketiminate at the С=О bond of benzophenone occurred and the reaction afforded the adduct with benzophenone [(2,6-Me 2 C 6 H 3 N= CMe) 2 CH]Li(Ph 2 C=O) (I) in which the latter acted as the neutral ligand coordinated to the lithium ion. The removal of the solvent in vacuo followed by the 2 hkl F   The reaction of benzophenone with lithium diketiminate [{2,6-Me 2 C 6 H 3 N=CMe} 2 CH]Li under more drastic conditions in a solution of toluene (10 h, 110°C) or THF (7 h, 70°C) gave no desirable result, and the same adduct I was isolated from the reaction mixture after the recrystallization of the product from hexane (Scheme 1).
Transparent red crystals of complex I were obtained by slow concentrating from a hexane solution at room temperature. According to the XRD data, compound I is a Li(I) complex in which the metal cation is bound to two nitrogen atoms of the diketiminate ligand and one oxygen atom of the benzophenone molecule. Thus, the κ 2 -N,N-coordination mode usual for ligands of this type is observed in complex I. The molecular structure of complex I is presented in Fig. 1a  The study of the 1 H and 13 С NMR spectra of compound II showed that the ligand existed in a solution as two prototropic tautomers: ketoenaminimine (69%) and ketodiimine (31%). It was found by the NMR method (2D HSQC 1 H-13 C NMR spectrum, CDCl 3 ) that the singlet at 5.42 ppm corresponded to the methine proton α-CH of the central fragment СНССС of ketodiimine (minor isomer) and the protons of the NH groups of ketoenaminimine appeared as a singlet at 13.12 ppm. It should be mentioned that the 13 С NMR spectrum also exhibits two sets of signals corresponding to the ketoenaminimine and ketodiimine tautomeric forms of compound II. The IR spectrum of compound II contains an intense absorption band at 1656 cm -1 corresponding to asymmetric vibrations of the C=N multiple bonds of the keto-βdiketiminate ligand and an intense absorption band at 1697 cm -1 assigned to stretching vibrations of the С=O bond of the keto group. It should be mentioned that the IR spectrum of compound II in СН 2 Cl 2 contains an absorption band in the range characteristic of vibrations of the N-H bond (3304 cm -1 ). Thus, the study of compound II by NMR ( 1 Н, 13 С) and IR spectroscopy suggests that the compound exists in a solution in the ketoenaminimine and ketodiimine forms.  Complex III was isolated as light yellow crystals in a yield of 74%. Keto-β-diketiminate III is sensitive to air moisture and oxygen and highly soluble in aromatic hydrocarbons and ethereal solvents.
In the 1 Н NMR spectrum of diamagnetic complex III, the protons of the tert-Bu substituents appear as a singlet at 1.57 ppm. Two broadened singlets at 1.98 and 2.06 ppm correspond to the protons of the methyl groups of the (2,6-Me 2 C 6 H 3 N=CMe) fragments. Aromatic protons appear in a weak field as triplet (7.00 ppm, 3 J H,H = 7.3 Hz) and doublet (7.12 ppm, 3 J H,H = 7.3 Hz). The coordinated THF molecules in the 1 H NMR spectrum of complex III give two multiplets at 1.64 and 3.67 ppm attributed to the βand α-methylenic protons. The 7 Li NMR spectrum of complex III exhibits the single signal at 2.7 ppm (155.5 MHz, 25°С, C 5 D 5 N).
Transparent light yellow crystals of complex III were obtained by the slow condensation of hexane in a concentrated solution of the compound in THF. The κ 2 -N,N-coordination mode of the ketodiketiminate ligand by the lithium cation occurs in complex III like in complex I. However, it is shown by XRD that each Li + in compound III is additionally bound to the oxygen atom of the C=O group of the keto-β-diketiminate ligand of the adjacent molecule. Thus, complex III represents the 1D coordination polymer {[{2,6-Me 2 C 6 H 3 N=C(Me)} 2 CС(tert-Bu)=O]Li(THF)} n in which each metal cation is linked to two nitrogen atoms of one ketodiketiminate ligand, the oxygen atom of the keto group of the second ketodiketiminate ligand, and the oxygen atom of the THF molecule. The fragment of the crystal structure of complex III is presented in Fig. 1b The REM complexes in the N,N-diketiminate and N,N,N-triketiminate ligand environment demonstrated a fairly high catalytic activity in the ring-opening polymerization of rac-lactide and ε-caprolactone [8,28]. To study the influence of the coordination environment on the catalytic activity of the metal complexes and possible coordination modes of the new chelate N,N,O-ligand by REM ions, we carried out the reaction of complex III with anhydrous YCl 3 in anhydrous THF at a reactant ratio of 2 : 1 for 12 h (Scheme 4). After the reaction product was extracted with toluene and recrystallized from a THF-hexane mixture, the [{2,6-Me 2 C 6 H 3 N= C(Me)} 2 CС(tert-Bu)=O] 2 Y(μ 2 -Cl) 2 Li(THF) 2 complex (IV) was isolated as colorless crystals in a yield of 67%. Complex IV was characterized by elemental analysis and NMR and IR spectroscopy and represents a compound sensitive to air moisture and oxygen and highly soluble in aromatic hydrocarbons and ethereal solvents. The crystals of complex IV were obtained by the slow cooling of a concentrated solution of the compound in a THF-hexane (1 : 4) mixture to -20°C. It is shown by XRD that compound IV represents a monomeric ate complex and crystallizes as the solvate [{2,6-Me 2 C 6 H 3 N=C(Me)} 2 CС(tert-Bu)=O] 2 YCl 2 Li-(THF) 2 ⋅ 1/2THF ⋅ 1/4Hex (the molecular structure of complex IV is shown in Fig. 1c).
Unlike complex III, complex IV exhibits the κ 2 -N,O-coordination mode of the keto-diketiminate ligand by the metal atom. The Y 3+ cation in complex IV is linked with two oxygen atoms and two nitrogen atoms of two ketodiketiminate ligands and two μ 2bridging chlorine ligands. Thus, the CN of the yttrium atom in complex IV is formally equal to six. Both potentially tridentate ketodiketiminate ligands in compound IV are coordinated by the metallocenter via the bidentate mode, whereas the third coordination site of each ligand is not involved in the interaction with the metal. In turn, the Li + cation is linked with two chlorine atoms and two oxygen atoms of two ТHF molecules.
The keto-β-diketiminate ligand in compound IV is nonsymmetrically coordinated by the yttrium cation.  [29]. The Li-Cl bond lengths are 2.218(7) and 2.344(7) Å. The electron density delocalization in the metallocycles is less pronounced for compound IV than that in complexes I and III. The  [30]. In spite of the fact that the C-C bond lengths lie in a wide range of 1.395(5)-1.507(5) Å, they characterize the delocalization of the negative charge in the ketoiminate fragment rather than the alternation of the C-С distances. The YNCCСO metallocycles are strongly distorted: the angles between the NYO and NCCCO planes are 135.3(2)°a nd 140.60(9)°.
Complexes I, III, and IV catalyze the ring-opening polymerization of ε-caprolactone under mild conditions (25°С, toluene). For catalysis by lithium complexes I and III, the complete conversion of the monomers (1000 equiv.) is achieved within 30 min. The polydispersion indices of the samples of the synthesized polymers are characterized by mean values (M w /M n = 1.4-2.3), and the molecular weights of the polymers range from 10500 to 43200 (Table 3, entries 1-8). It is found as a result of the performed series of experiments involving catalysts I and III that the values of are strongly underestimated (Fig. 2) at high loadings of the monomer (500/1, 1000/1), which is due, most likely, to the competitive transesterification reaction (Table 3, (Table 3, entries 7, 8, 11, 12). It should be mentioned that yttrium bis(keto-diketiminate) chloride complex IV demonstrated a substantially lower catalytic activity in the ring-opening polymerization of ε-caprolactone compared to lithium complexes I and III. For catalysis by yttrium complex IV, the complete conversion of the monomer (1000 equiv.) is achieved within 24 h. Along with a good agreement of the experimental and calculated values of M n (Fig. 2), the polylactone samples characterized by a fairly narrow molecular weight distribution M w /M n = 1.6-1.8 and the high molecular weight M n = 53300-99900 (Table 3, entries 15, 16) were obtained in the case of compound IV.  2 YCl 2 Li-(THF) 2 (IV), being an ate complex with one LiCl molecule, was synthesized by the exchange reaction of YCl 3 with lithium derivative III. It is found by the XRD method that the monoanionic ketodiketiminate ligand in yttrium complex IV acts as the bidentate one and coordinates to the metal ion only via the nitrogen and oxygen atoms, whereas the third coordination site is not involved in the interaction with the metal, which is due, most likely, to the electronic state of the ligand  120  100  80  60  40  20   1200   Calculated  I  III  IV  III, THF   1000  800  600  400  200  0 M/Cat M n × 10 -3 Table 3.  rather than the ion radius of the metal. Complexes I, III, and IV initiate the ring-opening polymerization of ε-caprolactone in toluene at 25°С.