Bis(phosphinimino)methanide borohydride complexes of the rare-earth elements as initiators for the polymerization of methyl methacrylate: combined experimental and computational investigations

International audienceRare earth borohydride complexes are known as efficient initiators for the polymerization of both apolar and polar monomers. Significant contribution of the phosphiniminomethanide ligand on the reactivity of group 3 derivatives was previously established in the polymerization of ε-caprolactone. Investigations of the capability of bis(phosphinimino)methanide rare earth metal bisborohydrides, [{CH(PPh2NSiMe3)2}La(BH4)2(THF)] (1) and [{CH(PPh2NSiMe3)2}Ln(BH4)2] (Ln = Y (2), Lu (3)) to polymerize methyl methacrylate (MMA) both experimentally and computationally, are reported here. All three metallic compounds allowed the preparation of PMMA at room temperature. However, the overall performances of 1-3 remain quite poor based on experimental observations. DFT investigations on the insertion of the first two MMA molecules revealed that the incoming of the first MMA molecule was the most important step regardless of the nature of the metal center. The nucleophilic attack of MMA leads to the formation of a first adduct B followed by the unprecedented trapping of the liberated BH3 group by the nitrogen of the phosphiniminomethanide ligand to afford the active enolate species C. The unique significant role played by the phosphiniminomethanide ligand has thus been clearly unveiled and evidenced computationally. This whole first insertion of the MMA process is both kinetically and thermodynamically favorable. Trapping of the BH3 by the ancillary ligand appeared to make the second MMA insertion more energetically favorable than the first one, especially for lanthanum. The ketoenolate thus formed, Prod., is thermodynamically and kinetically favorable. In the case of yttrium, steric considerations, in addition to energetically comparable first and second MMA insertions , support the experimentally observed difficulty to polymerize MMA. DFT calculations closely corroborate experimental findings

Organometallic Strontium Borohydrides: Synthesis, X-ray Structures, Catalytic Polymerization of ε-Caprolactone, and Density Functional Calculations

International audienceno abstrac

(Iminophosphoranyl)(thiophosphoranyl)methanide {CH(PPh₂NSiMe₃)(Ph₂PS)}⁻ as a Ligand in Rare-Earth-Element Chemistry

The (iminophosphoranyl)­(thiophosphoranyl)­methanide {CH­(PPh₂NSiMe₃)­(PPh₂S)}⁻ has been introduced as a ligand into the chemistry of yttrium and the lanthanides. First, the bimetallic potassium reagent [K­{CH­(PPh₂NSiMe₃)­(PPh₂S)}]₂ was synthesized by deprotonation of [CH₂(PPh₂NSiMe₃)­(PPh₂S)] with KH. [K­{CH­(PPh₂NSiMe₃)­(PPh₂S)}]₂ forms a dimeric structure in the solid state. The potassium atoms are bridged by the sulfur atom of the ligand. Moreover, an η⁶ coordination of one phenyl ring is observed. The salt metathesis of [K­{CH­(PPh₂NSiMe₃)­(PPh₂S)}]₂ with LnCl₃ led to the dichloro complexes [{CH­(PPh₂NSiMe₃)­(PPh₂S)}­LnCl₂(THF)] (Ln = Dy, Er). The bis­(amido) compounds [{CH­(PPh₂NSiMe₃)­(PPh₂S)}­Ln­{N­(SiHMe₂)₂}₂] (Ln = Y, Sm, Er, Lu) were obtained by amine elimination from [CH₂(PPh₂NSiMe₃)­(PPh₂S)] and [Ln­{N­(SiHMe₂)₂}₃(THF)₂]. The amido complex [{CH­(PPh₂NSiMe₃)­(PPh₂S)}­Er­{N­(SiHMe₂)₂}₂] could also be accessed by the reaction of [{CH­(PPh₂NSiMe₃)­(PPh₂S)}­ErCl₂(THF)] with KN­(SiHMe₂)₂

Organometallic strontium borohydrides - synthesis, catalytic studies, and DFT calculations

International audience[Ca(BH4)2(THF)2] (1a), a known compound, was easily prepared following a convenient new procedure from [Ca(OMe)2] and BH3*THF in THF. Reaction of 1a with KCp* (Cp* = (η5-C5Me5)) and K{(Me3SiNPPh2)2CH} in a 1 : 1 ratio in THF resulted in the corresponding dimeric heteroleptic mono-borohydride derivatives [Cp*Ca(BH4)(THF)n]2 (2a) and [{(Me3SiNPPh2)2CH}Ca(BH4)(THF)2] (3a), respectively. Both compounds were fully characterized and the solid-state structure of 3a was established by single crystal X-ray diffraction. Compounds 1a, 2a, and 3a, together with the earlier reported compounds [Sr(BH4)2(THF)2] (1b), [Cp*Sr(BH4)(THF)2]2 (2b), and [{(Me3SiNPPh2)2CH}Sr(BH4)(THF)2] (3b), were used as initiators for the ROP of polar monomers. The general performances of the complexes in the ROP of ε-caprolactone and L-lactide demonstrate a relatively good control of the polymerization under the operating conditions established. α,ω-Dihydroxytelechelic poly(ε-caprolactone)s (PCLs) and poly(lactide)s (PLAs) were thus synthesized. DFT calculations on the initiation step of the ROP of ε-CL were carried out. Gibbs free energy profiles were determined for the three calcium complexes highlighting slightly more active calcium complexes as compared to strontium analogues, in agreement with experimental findings

Chiral Rare Earth Borohydride Complexes Supported by Amidinate Ligands: Synthesis, Structure, and Catalytic Activity in the Ring-Opening Polymerization of rac-Lactide

International audienceThe monoamidinato bisborohydride rare earth complexes [Ln{(S)-PEBA}(BH4)2(THF)2] (Ln = Sc (1), La (2), Nd (3), Sm (4), Yb (5), Lu (6)) were isolated as crystalline materials upon treatment of potassium N,N′-bis((S)-1-phenylethyl)benzamidinate ((S)-KPEBA) with the homoleptic trisborohydrides [Sc(BH4)3(THF)2] and [Ln(BH4)3(THF)3] (Ln = La, Nd, Sm, Yb, Lu), respectively. Compounds 1-6 are unique examples of enantiopure borohydride complexes of the rare earth metals. Different ionic radii of the metal centers were selected to cover the whole range of these elements with respect to the extent of the coordination sphere. All new complexes were thoroughly characterized by 1H, 13C{1H}, 11B, and 15N NMR and IR spectroscopies, also including single-crystal X-ray diffraction structure determination of each compound. The scandium, lanthanum, samarium, and lutetium complexes 1, 2, 4, and 6 were found active in the ring-opening polymerization of rac-lactide under mild operating conditions, providing atactic α,ω-dihydroxytelechelic poly(lactic acid) (PLA; Mn,SEC up to 18 800 g*mol-1). Most of the polymerizations proceed with a certain degree of control that is directed by molar mass values and relatively narrow dispersities (1.10 < ĐM < 1.34), within a moderate monomer-to-initiator ratio

Bis(phosphinimino)methanide borohydride complexes of the rare-earth elements as initiators for the ring-opening polymerization of trimethylene carbonate: combined experimental and computational investigations

International audienc

Chiral Rare Earth Borohydride Complexes Supported by Amidinate Ligands: Synthesis, Structure, and Catalytic Activity in the Ring-Opening Polymerization of <i>rac</i>-Lactide

The monoamidinato bisborohydride rare earth complexes [Ln­{(<i>S</i>)-PEBA}­(BH₄)₂(THF)₂] (Ln = Sc (<b>1</b>), La (<b>2</b>), Nd (<b>3</b>), Sm (<b>4</b>), Yb (<b>5</b>), Lu (<b>6</b>)) were isolated as crystalline materials upon treatment of potassium <i>N</i>,<i>N</i>′-bis­((<i>S</i>)-1-phenylethyl)­benzamidinate ((<i>S</i>)-KPEBA) with the homoleptic trisborohydrides [Sc­(BH₄)₃(THF)₂] and [Ln­(BH₄)₃(THF)₃] (Ln = La, Nd, Sm, Yb, Lu), respectively. Compounds <b>1</b>–<b>6</b> are unique examples of enantiopure borohydride complexes of the rare earth metals. Different ionic radii of the metal centers were selected to cover the whole range of these elements with respect to the extent of the coordination sphere. All new complexes were thoroughly characterized by ¹H, ¹³C­{¹H}, ¹¹B, and ¹⁵N NMR and IR spectroscopies, also including single-crystal X-ray diffraction structure determination of each compound. The scandium, lanthanum, samarium, and lutetium complexes <b>1</b>, <b>2</b>, <b>4</b>, and <b>6</b> were found active in the ring-opening polymerization of <i>rac</i>-lactide under mild operating conditions, providing atactic α,ω-dihydroxytelechelic poly­(lactic acid) (PLA; <i>M</i>_n,SEC up to 18 800 g·mol^–1). Most of the polymerizations proceed with a certain degree of control that is directed by molar mass values and relatively narrow dispersities (1.10 < <i>Đ</i>_M < 1.34), within a moderate monomer-to-initiator ratio