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

Abstract

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.

(Iminophosphoranyl)(thiophosphoranyl)methane rare-earth borohydride complexes: synthesis, structures and polymerization catalysis

“…Both ligands have been extensively used in main group, 1-16 transition [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and f-element 5,13,15,19,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] chemistry. Both ligands have been extensively used in main group, 1-16 transition [16][17][18][19][20][21][22][23][24][25][26][27][28][29]…”

“…Calculation studies revealed that the negative charge is mainly localized on the carbene carbon atom of the ligand backbone and that the charge delocalization from carbon to nitrogen is less important. 6,14,16,20,[36][37][38][39][40][41][42][43][44][45][46][47][48][49] In this context, some very efficient rare-earth metal initiators ligated by {CH(PPh 2 NSiMe 3 ) 2 } − have been unveiled for polymerization reactions. Mostly six membered metallacycles (N-P-C-P-N-M) are formed by chelation of the two imine groups to the metal center, adopting a pseudo-boat conformation.…”

“…Mostly six membered metallacycles (N-P-C-P-N-M) are formed by chelation of the two imine groups to the metal center, adopting a pseudo-boat conformation. 37 The bisborohydride analogues, [{(CH(PPh 2 NSiMe 3 ) 2 }La(BH 4 ) 2 (THF)] and [{CH(PPh 2 NSiMe 3 ) 2 }Ln(BH 4 ) 2 ] (Ln = Y, Lu) were demonstrated as successful initiators for the ROP of CL 49 and trimethylene carbonate (TMC), 46 as well as for the polymerization of MMA. 6,14,16,20,[36][37][38][39][40][41][42][43][44][45][46][47][48][49] In this context, some very efficient rare-earth metal initiators ligated by {CH(PPh 2 NSiMe 3 ) 2 } − have been unveiled for polymerization reactions.…”

A Comparison of Gallium and Indium Alkoxide Complexes as Catalysts for Ring-Opening Polymerization of Lactide

“…Metal alkoxide species, M-OR, have been shown to be the true active species in numerous ROP processes. [1][2][3][4][5][6][7][8][9][10][11][12][13][18][19][20][21][22][23][24] For example, in 2004, Lewiński and co-workers reported the first structurally characterized aluminum ε-caprolactone complex, indicating that the Al-Me group has poor reactivity with CL. 77 Thus, in most ROP cases, an alcohol is used as an initiator and is essential in order to start the polymerization.…”

Redox Control of Aluminum Ring-Opening Polymerization: A Combined Experimental and DFT Investigation

“…On the other hand, based on previous reports, [5][6][7][8][9][10][11][12][13][18][19][20][21][22][23][24][25][26][27] after the insertion of the first equivalent of other monomers (CL, VL, and TMC), the rates of polymerization will not decrease remarkably, indicating that the insertion of the first monomers will not cause such a significant chelation effect like LA. Thus, in these cases, the energy of the initiation step can be used to estimate the energy of propagation.…”