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

In the last few years a number of research groups started significant activity in the exploration of the coordination and organometallic chemistry of the heavier alkaline-earth metals. [1] Most of this work is focused on calcium, [2] an inexpensive and non-toxic element present in significant amounts in the human body. Indeed, synthetic access to such large alkalineearth metal complexes is often limited by their kinetic lability with detrimental Schlenk-type equilibria. [3] Within this reserach not only the synthesis of new compounds but also the structures, [4] the reactivity, [5] and some catalytic applications were investigated. [6] Among these latter the ring-opening polymerization (ROP) of cyclic esters such as e-caprolactone (e-CL) and lactide (LA), allows access to polymers with controlled molar mass and molar mass distribution and especially, in some cases, controlled stereochemistry. [2a, 7] In contrast to this well-established chemistry, the application of structurally characterized strontium compounds as initiators in ROP remains very rare. Besides ill-defined amino isopropoxyl strontium, [8] phenoxide ligated cationic strontium complexes [9] and some phenoxide strontium amides [10] were only very recently used for the ROP of LA.Whereas in related rare-earth chemistry, borohydrides such as [Ln(BH 4 ) 3 (thf) 3 ] [11] or its derivatives have become well-established initiators for the ROP of cyclic esters and carbonates, [12] comparable chemistry of alkaline-earth elements was only first published recently by Cushion and Mountford (e.g. on calcium). [13] This is rather surprising because [Ca(BH 4 ) 2 (thf) 2 ] is commercially available for applications in materials science. [14] In contrast, such borohydride chemistry involving strontium is even less developed. Although, the synthesis and the solidstate structure of [Sr(BH 4 ) 2 (thf) 2 ] (1) were already reported in 1995, [15] there is to date, to the best of our knowledge, no structurally characterized coordination or organometallic strontium borohydride compound known.Herein, we describe the synthesis and structural characterization of the half-sandwich [Cp*Sr(BH 4 )(thf) 2 ] 2 (Cp* = h 5 -C 5 Me 5 ; 2) and the bis(phosphinimino)methanide [{(Me 3 SiNPPh 2 ) 2 CH}Sr-(BH 4 )(thf) 2 ] (3) complexes, as well as their application as initiators for the ROP of e-CL. The starting material 1 was prepared according to a literature procedure from [Sr(OEt) 2 ] and a solution of BH 3 in THF. [15] Reaction of 1 with KCp* and K[(Me 3 SiNPPh 2 ) 2 CH] [16] resulted in the corresponding heteroleptic monoborohydride derivatives 2 and 3, respectively (Scheme 1).The new complexes 2 and 3 have been characterized by standard analytical/spectroscopic techniques and the solidstate structures were established by single-crystal X-ray analysis. The 1 H NMR spectra show the expected resonance for the Cp* ligand of 2 at d = 1.98 ppm and a broad signal for the methine group of the bis(phosphinimino)methanide ligand of 3 at d = 1.72 ppm. The BH 4 À groups are observed as...

Keywords: Lutetium / N ligands / Potassium / Pyrrole / Yttrium

Bis(phosphinimino)methanide bisborohydride complexes of lanthanum, yttrium and lutetium, [{CH (PPh 2 NSiMe 3 ) 2 }La(BH 4 ) 2 (THF)] ( 1) and [{CH(PPh 2 NSiMe 3 ) 2 }Ln(BH 4 ) 2 ] (Ln ¼ Y (2), Lu (3)), have been investigated in the ring-opening polymerization (ROP) of trimethylene carbonate (TMC). All three initiators afforded linear poly(trimethylene carbonate)s (PTMCs) in toluene at 23 C. 1 H NMR analyses of the polycarbonates revealed the formation of a-hydroxy,u-formate telechelic PTMCs, as previously observed in the ROP of TMC initiated by [Sm(BH 4 ) 3 (THF) 3 ]. This suggested the nonreduction of the carbonyl of the carbonate group in the active species by the BH 3 moiety, as hinted by this same prior experimental work. Formation of a,u-dihydroxy PTMCs, resulting from the reduction of the carbonyl, is also likely and cannot be ruled out from experimental data. DFT investigations, focused on the initiation step, supported two energetically (thermodynamically and kinetically) favorable and similar reaction pathways leading to two distinct end-functionalized PTMCs. Depending on whether reduction of the carbonyl occurred or not, a,u-dihydroxy or a-hydroxy,u-formate telechelic PTMCs were predicted, respectively. Although these two calculated feasible approaches are very close in energy, the formation of the latter heterofunctionalized a-hydroxy,u-formate telechelic PTMCs is slightly preferred computationally. This first in silico study on the mechanism of the ROP of a cyclic carbonate revealed several features without any precedent in the ROP of a cyclic ester (3caprolactone or lactide). An easily accessible (no activation barrier) intermediate in which BH 3 is trapped by the intracyclic oxygen of a non-opened TMC ring has been located for the first time. The low activation barrier for the opening of the TMC ring that proceeds without B-H activation is predicted to be competitive, affording the thermodynamically less stable BH 3 adduct. Finally, trapping of BH 3 by the nitrogen of the {CH(PPh 2 NSiMe 3 ) 2 } À ligand, in agreement with previous findings, highlights again the valuable role of this bisphosphiniminomethanide ligand.

[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* = (η(5)-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.