Phenoxy-Amidine Ligands: Toward Lactic Acid-Tolerant Catalysts for Lactide Ring-Opening Polymerization

Abstract

The replacement of the imine functionality in the ubiquitous phenoxy-imine (FI) ligands by a more robust and donor N,N,N′-trisubstituted amidine function was examined and gave rise to the synthesis of five new phenoxy-amidine (FA) ligands (L1− L5). The solid-state structure of four proligands has been determined by X-ray diffraction analysis and showed that the amidine moiety is in a trans-configuration. The reaction of the phenol-amidine proligands with AlMe 3 afforded mononuclear (L1−L5)AlMe 2 (1a−5a). A similar alkane elimination route was used from ZnEt 2 and led to dinuclear [(L1−L5)ZnEt] 2 complexes (1b−5b) or to homoleptic (L2/L4) 2 Zn complexes (2b′, 4b′) depending on the metal/ligand ratio used. The structure of these complexes has been determined by NMR spectroscopy ( 1 H, 13 C, HMBC, HSQC, DOSY, and NOESY experiments) and X-ray diffraction study for seven of them. The crystal structure of the Al complexes showed FA ligands coordinated in a chelate fashion via the O atom of the aryloxy group and the imino-N atom, indicating that the amidine function has undergone trans−cis isomerization upon coordination. A similar chelating coordination mode was observed for the FA ligands with Zn metal ions. These complexes were used as initiators for the ring-opening polymerization of rac-lactide. FA−Zn complexes gave the best performance, affording polylactic acid with a narrow molecular weight distribution and heterotactic bias (Pr up to 0.75). Remarkably, some of these complexes were able to tolerate the presence of a large amount of lactic acid to the point of using it as a co-initiator during the polymerization reaction.

Bis(salicylamidine) Ligands (FAlen): A Variant of Salen with “à la Carte” Denticity

“…Assuming that both amidine functions in the proligand L2H 2 are trans in solution, in agreement with the solid-state structure and the known greater thermodynamic stability of trans-amidine, this means that trans−cis isomerization of one of the two amidine functions occurred upon coordination. Surprisingly, the 1 H and 13 C NMR spectra of 2 in CD 2 Cl 2 at 298 K shows a set of signals consistent with a C s -symmetric structure. We hypothesized that a dynamical process must take place in solution and can explain this inconsistency.…”

“…As mentioned in the Introduction of this paper, we previously noticed that some (phenoxyamidine)zinc complexes were active for ROP of rac-LA contaminated with lactic acid up to 4 mol %. 13 This finding is particularly interesting given that there are very few catalysts that can maintain high activity and operate reliably when technical-grade LA (containing lactic acid) is used. In order to examine whether FAlen Al complexes can also resist lactic acid, we performed ROP of LA using complexes 1 and 2, and exogenous lactic acid (Table 1, entries 8 and 9).…”

“…1 H NMR (300 MHz, CDCl 3 ): δ 8.09 (dd, J = 7.7 and 1.7 Hz, 1H, H7), 7.61 (ddd, J = 8.6, 7.5, and 1.7 Hz, 1H, H5), 7.20 (pseudo td, J = 7.6 and 0.9 Hz, 1H, H6), 7.06 (d, J = 8.5 Hz, 1H, H4), 4.82 (m, 1H, N(CH 2 CH 3 ) 2 ), 4.45 (m, 1H, N(CH 2 CH 3 ) 2 ), 4.29 (m, 2H, N(CH 2 CH 3 ) 2 ), 3.94 (s, 3H, OMe), 1.66 (t, J = 7.4 Hz, 3H, N(CH 2 CH 3 ) 2 ), 1.46 (t, J = 7.3 Hz, 3H, N(CH 2 CH 3 ) 2 ). { 1 H} 13 C NMR (75 MHz, CDCl 3 ): δ 171.7 (C1), 154.6 (C3), 135.4 (C5), 129.6 (C7), 122.3 (C6), 120.4 (C2), 111.7 (C4), 56.5 (CH 3 + CH 2 overlapping, OMe, N(CH 2 CH 3 ) 2 ), 54.7 (N(CH 2 CH 3 ) 2 ), 13.1 (N(CH 2 CH 3 ) 2 ), 12.6 (N(CH 2 CH 3 ) 2 ).…”

“…1 H NMR (500 MHz, CD 2 Cl 2 ): δ 9.70 (s, 2H, OH), 9.59 (broad signal, 2H, NH), 7.60 (d, J = 8.3 Hz, 2H, H4), 7.39 (ddd, J = 8.5, 5.5, and 3.5 Hz, 2H, H5), 6.98−6.92 (m, 4H overlapping, H6 + H7), 4.13 (m, 2H, N(CH 2 CH 3 ) 2 ), 3.87 (m, 2H, N(CH 2 CH 3 ) 2 ), 3.38−3.22 (m, 6H, H8 + N(CH 2 CH 3 ) 2 ), 3.22−3.15 (m, 2H, H8), 1.44 (t, J = 7.2 Hz, 6H, N(CH 2 CH 3 ) 2 ), 1.12 (t, J = 7.2 Hz, 6H, N(CH 2 CH 3 ) 2 ). { 1 H} 13 C NMR (125 MHz, CD 2 Cl 2 ): δ 163.5 (C1), 154.4 (C3), 133.7 (C5), 126.9 (C7), 120.7 (C6), 118.5 (C4), 114.6 (C2), 48.2 (N(CH 2 CH 3 ) 2 ), 44.9 (N(CH 2 CH 3 ) 2 ), 44.7 (C8), 13.7 (N(CH 2 CH 3 ) 2 ), 12.3 (N(CH 2 CH 3 ) 2 ).…”

“…The first proof of concept was obtained with (phenoxyamidine)zinc complexes that can promote the ring-opening polymerization (ROP) of rac-LA contaminated with up to 4 mol % lactic acid (a protic impurity often present in technical-grade LA), while (phenoxyimine)zinc analogue catalysts are inactive under the same conditions. 13 On the basis of the same strategy, we sought to develop an amidine variant of Salen ligands. Herein, we report the synthesis of these ligands, which we named FAlen (Figure 1), their coordination chemistry with aluminum, and the preliminary results in ROP of rac-LA.…”