Aluminum Complexes of Tridentate Amine Biphenolate Ligands Containing Distinct N‐alkyls: Synthesis and Catalytic Ring‐opening Polymerization

Abstract

A series of aluminum complexes of amine biphenolate ligands that contain distinct N-alkyl substituents have been prepared and employed for ring-opening polymerization (ROP) of e-caprolactone (e-CL). Alkane elimination of trimethylaluminum with2-; R = tBu (1a), iPr (1b), nPr (1c)) in toluene solutions at 25°C produces colorless crystalline [R-ONO]AlMe (2a-c). X-ray diffraction studies of these organoaluminum complexes showed them to be four-coordinate species having a distorted tetrahedral coordination core. In the presence of benzyl alcohol, complexes 2a-c are all active catalyst precursors for ROP of e-CL. The ROP rates of e-CL are clearly a function of the steric sizes of the N-alkyl substituents, following an increasing order 2a < 2b < 2c. Interestingly, both 2b and 2c exhibit controlled ROP catalysis whereas 2a gives low molecular weight oligomers under identical conditions. In ROP catalyzed with 2b or 2c, the e-CL disappearance rates follow pseudofirst order kinetics, with the latter being about 2.86(2) times faster than the former. Complex 2b outperforms 2c in view of maintaining controlled ROP catalysis in reactions with extremely high monomerto-catalyst ratios. The complex constitution-reactivity relationship is discussed.

Enhanced Reactivity of Aluminum Complexes Containing P-Bridged Biphenolate Ligands in Ring-Opening Polymerization Catalysis

“…Subsequent attempts to crystallographically characterize [ 3a ]AlMe have thus far been unsuccessful. Nevertheless, the structure of [ 3a ]AlMe•AlMe 3 confirms the 4-coordinate nature for the aluminum center of the [ 3a ]AlMe moiety that has a distorted tetrahedral coordination core similar to [ 1a–c ]AlMe (Liang et al, 2013b). The bond distances and angles of [ 3a ]AlMe•AlMe 3 are unexceptional.…”

“…Interestingly, these methyl complexes are not THF-bound as evidenced by their 1 H NMR spectra. This result is reminiscent of 4-coordinate [ 1a–c ]AlMe (Liang et al, 2013b) but in contrast to 5-coordinate [ 2a-b ]AlMe(THF) (Chang et al, 2016), ascribable to the hardness similarity of O (from phosphinoxide in 3 ) to N (from 1 ) rather than P (from 2 ) in consideration of the distinct preferences of these hard and soft donors to bind to a hard aluminum (Fryzuk et al, 1996, 1998; Liang et al, 2004, 2010; Lee and Liang, 2005, 2009; Su and Liang, 2018). As a result, the solution structures of [ 3a-b ]AlMe and [ 1a–c ]AlMe should be much alike.…”

“…These complexes are intriguing as their catalytic activities are finely tunable by judiciously varying the peripheral substituents on the two phenolate rings and/or the bridge in between. While most studies concentrate on metal complexes of tetradentate biphenolate ligands such as ONNO (Ovitt and Coates, 2000; Zhong et al, 2002; Hormnirun et al, 2006; Zelikoff et al, 2009; Chen et al, 2012; Gao et al, 2015; Jones et al, 2015; Kirk et al, 2016; MacDonald et al, 2016; McKeown et al, 2016; Robert et al, 2017; Pang et al, 2018), OSSO (Buffet and Okuda, 2011; Buffet et al, 2011), ONSO (Stopper et al, 2012), and ONOX (X = OR, NR 2 ) (Alcazar-Roman et al, 2003; Gendler et al, 2006; Tang and Gibson, 2007; Phomphrai et al, 2010; Wichmann et al, 2012) as exemplified in Figure 1, parallel research centered upon tridentate counterparts is relatively rare (Chmura et al, 2006; Chang and Liang, 2007; Hsu and Liang, 2010; Liang et al, 2011, 2013a,b,c,d,e; Huang et al, 2013; Klitzke et al, 2014a,b; Chang et al, 2016).…”

“…It has been documented that complexes of tridentate OOO (Huang et al, 2013), ONO ( 1a-c ) (Liang et al, 2013a,b,c,d,e), and OPO ( 2a-b ) (Chang and Liang, 2007; Hsu and Liang, 2010; Liang et al, 2011; Chang et al, 2016) are active catalyst precursors for ROP of ε-caprolactone (ε-CL) or lactides (LAs). Studies on complexes of 1 and 2 have revealed that substituents at the pnictogen donor have decisive impacts on ROP catalysis if living polymerization is the goal.…”

“…Of note are aluminum complexes of 2a that polymerize ε-CL and rac -LA in a living manner to produce well-defined poly(ε-caprolactone) (PCL) and poly( rac -lactide) (PLA), respectively (Chang et al, 2016). In contrast, analogous complexes of 1a give instead low molecular weight oligo(ε-caprolactone) or PCL with a somewhat broad molecular weight distribution (Liang et al, 2013b). Constitutionally, complexes of 1a and 2a are much alike as both are pnictogen biphenolate derivatives carrying a pnictogen-bound tert -butyl group.…”