Bis(phosphinimino)methanide Borohydride Complexes of the Rare‐Earth Elements as Initiators for the Ring‐Opening Polymerization of ε‐Caprolactone: Combined Experimental and Computational Investigations

Abstract

Rare-earth-metal borohydrides are known to be efficient catalysts for the polymerization of apolar and polar monomers. The bis-borohydrides [{CH(PPh2 NSiMe3)2}La(BH4)2(THF)] and [{CH(PPh2NSiMe3)2}Ln(BH4)2] (Ln = Y, Lu) have been synthesized by two different synthetic routes. The lanthanum and the lutetium complexes were prepared from [Ln(BH4)3(THF)3] and K{CH(PPh2NSiMe3)2}, whereas the yttrium analogue was obtained from in situ prepared [{CH(PPh2NSiMe3)2}YCl2]2 and NaBH4. All new compounds were characterized by standard analytical/spectroscopic techniques, and the solid-state structures were established by single-crystal X-ray diffraction. The ring-opening polymerization (ROP) of ε-caprolactone initiated by [{CH(PPh2NSiMe3)2}La(BH4)2(THF)] and [{CH(PPh2NSiMe3)2}Ln(BH4)2] (Ln = Y, Lu) was studied. At 0 °C the molar mass distributions determined were the narrowest values (M(w)/M(n) = 1.06-1.11) ever obtained for the ROP of ε-caprolactone initiated by rare-earth-metal borohydride species. DFT investigations of the reaction mechanism indicate that this type of complex reacts in an unprecedented manner with the first B-H activation being achieved within two steps. This particularity has been attributed to the metallic fragment based on the natural bond order analysis.

Bis(phosphinimino)methanide borohydride complexes of neodymium and scandium

“…In the IR spectra of compound 1 and 2 two characteristic peaks for each complex (2204 cm À1 and 2418 cm À1 (1), and 2291 cm À1 and 2495 cm À1 (2)) were observed. The signals of compound 1 can be assigned to terminal tridentate Ln (h 3 eH 3 BeH) units [6] and are in agreement with other bis(phosphinimino)methanide complexes such as [{(Me 3 SiNPPh 2 ) 2 CH}La (BH 4 ) 2 (THF)] and [{(Me 3 SiNPPh 2 ) 2 CH}Ln(BH 4 ) 2 ] (Ln ¼ Y, Lu) [46]. We could not doubtless assign the coordination mode of the BH 4 À groups of compound 2 based on the IR data.…”

“…In addition, we have introduced the bis(phosphinimino)methanide {CH(PPh 2 NSiMe 3 ) 2 } À , which was previously used as a ligand in main group and transition metal chemistry by a number of research groups [42e45], into the borohydride chemistry of the rare earth elements [46]. In this context the bis(borohydrides) [{(Me 3 SiNPPh 2 ) 2 CH}La(BH 4 ) 2 (THF)] and [{(Me 3 SiNPPh 2 ) 2 CH}Ln (BH 4 ) 2 ] (Ln ¼ Y, Lu) were synthesized by two different synthetic routes.…”

Catalytic Behavior of Rare-Earth Borohydride Complexes in Polymerization of Polar Monomers

“…It is noteworthy that regarding polylactones and polylactides, the narrowest dispersity values (Mw=Mn < 1:06) were obtained for the ROP of CL initiated by the bisphosphiniminomethanide complexes [{CH (PPh 2 NTMS) 2 }La(BH 4 ) 2 (THF)] (20La) and [{CH(PPh 2 NTMS) 2 }R(BH 4 ) 2 ] (R ¼ Y (20Y), Lu (20Lu)), reported by Roesky, Guillaume, Maron, and coworkers (Jenter et al, 2010), or with the synthesis of PHB promoted by the diaminobis(phenoxide) complexes [{(O 2 N 2 ) 1 Nd(BH 4 ) 2 Li(THF)} 2 ] (31Nd) and [(O 2 N 2 ) 2 Nd(BH 4 ) 2 Li(THF) 2 ] (32Nd) reported by Carpentier, Trifonov, and coworkers (Sinenkov et al, 2011), or by the homoleptic rare-earth trisborohydride complexes [R(BH 4 ) 3 (THF) 3 ] (R¼ La (1La), Nd (1Nd), Sm (1Sm)) as found by Guillaume et al (2013), thus suggesting a well-controlled ROP process in these cases (Tables 1 and 2). The Mw=Mn values recorded for poly(lactide)s prepared from rare-earth borohydride catalysts were larger than the ones commonly obtained from a well-controlled ROP process involving the more common alkoxide initiators.…”

“…The ROP of CL proceeded smoothly at 20 C with complete conversion ([CL] 0 /[20Y/Lu] 0 150) being achieved within a few minutes in THF or toluene. The unique role played by the ligand itself in the ROP process was later substantiated by DFT insights by the same groups (Barros et al, 2008a;Jenter et al, 2010) (Table 6; vide infra). Indeed, at 0 C, the dispersity determined (1:06 < Mw=Mn < 1:11 for all three metal complexes) was lower than that from room temperature (1:16 < Mw=Mn < 1:49).…”

“…The lanthanum and lutetium complexes were prepared from [R(BH 4 ) 3 (THF) 3 ] (1R) and (Jenter et al, 2010). The lanthanum and lutetium complexes were prepared from [R(BH 4 ) 3 (THF) 3 ] (1R) and (Jenter et al, 2010).…”