Group 4 Complexes of atert-Butylphosphine-Bridged Biphenolate Ligand

Abstract

The coordination chemistry of group 4 complexes supported by the tridentate, dianionic biphenolate phosphine ligand that carries a phosphorus-bound tert-butyl group, 2,2'-tert-butylphosphino-bis(4,6-di-tert-butylphenolate) ([(t)Bu-OPO](2-)), is described. Metathetical reactions of {[(t)Bu-OPO]Li(2)(DME)}(2) with 2 or 1 equiv of TiCl(4)(THF)(2) selectively produce [(t)Bu-OPO]TiCl(2)(THF) (1a) and Ti[(t)Bu-OPO](2) (2a), respectively. Protonolysis of Ti(O(i)Pr)(4) with 2 or 1 equiv of H(2)[(t)Bu-OPO] cleanly generates 2a and [(t)Bu-OPO]Ti(O(i)Pr)(2) (3a), respectively. Complex 1a can alternatively be prepared from comproportionation of 2a with 1 equiv of TiCl(4)(THF)(2). Treatment of 1a with 2 equiv of NaO(t)Bu affords [(t)Bu-OPO]Ti(O(t)Bu)(2) (4a). In contrast, reactions of {[(t)Bu-OPO]Li(2)(DME)}(2) with ZrCl(4)(THF)(2) or HfCl(4)(THF)(2), regardless of stoichiometry of the starting materials employed, selectively give bis-ligated M[(t)Bu-OPO](2) [M = Zr (2b), Hf (2c)]. Comproportionation of 2b,c with MCl(4)(THF)(2) (M = Zr, Hf) leads to the formation of [(t)Bu-OPO]MCl(2)(THF) [M = Zr (1b), Hf (1c)], which, upon being treated with 2 equiv of NaO(t)Bu, generates [(t)Bu-OPO]M(O(t)Bu)(2)(THF) (4b,c). These synthetic results are markedly different from those obtained from analogous reactions employing a biphenolate phosphine ligand bearing a phosphorus-bound phenyl group ([Ph-OPO](2-)), highlighting a profound phosphorus substituent effect on complex conformation. The alkoxide complexes 3a and 4a-c are all active initiators for catalytic ring-opening polymerization of ε-caprolactone. To assess the potential phosphorus substituent effect on catalysis, [Ph-OPO]Ti(O(i)Pr)(2) (5a) was prepared, and its reactivity was examined. Interestingly, polymers prepared from 3a are characterized by low polydispersities with molecular weights that are linearly dependent on the monomer-to-initiator ratio, thus featuring a living system. The polydispersitiy indexes of polymers prepared from 5a, however, are relatively larger, indicative of the significance of the phosphorus-bound tert-butyl group in 3a in view of discouraging the undesirable transesterification.

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

“…12 etc. In the presence of one equiv of e-CL, the 27 Al chemical shift of 2b moves upfield to 129 ppm (Dv 1/2 = 11157 Hz), consistent with an increase in coordination number for a neutral organo-aluminum complex. 75,80 The 1 H NMR spectrum of the same solution reveals the presence of an aluminum-bound methyl group instead of ketone methyl, indicating no ROP reaction occurs during these NMR acquisitions.…”

“…75,80 The 1 H NMR spectrum of the same solution reveals the presence of an aluminum-bound methyl group instead of ketone methyl, indicating no ROP reaction occurs during these NMR acquisitions. The upfield change of the observed 27 Al chemical shift of 2b upon addition of e-CL is thus ascribable to the coordination of the heterocyclic monomer to aluminum, presumably via carbonyl oxygen atom. This hypothesis is consistent with the X-ray structure of an aluminum e-CL complex supported by a biphenolate ligand as elucidated by Lewi½ski et al 81 On the basis of the fact that the ROP rates of 2a-c are governed by the N-substituents, we propose that the monomer coordination proceeds predominantly, though perhaps not exclusively, on the two X-N-O (X = C for 2a-c or X = alkoxide for propagating intermediates) faces of the pseudo-tetrahedral catalysts.…”

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

“…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.…”

Use of pyrazoles as ligands greatly enhances the catalytic activity of titanium iso-propoxide for the ring-opening polymerization of l-lactide: a cooperation effect

“…Because of the non-cytotoxic property and strong Lewis acidity of titanium, Ti complexes 22,23,[30][31][32][33] are commonly used catalysts in LA ROP. Numerous Ti complexes bearing various ligands such as calix [4] arene, 33,34 Schiff base, [35][36][37][38][39] salen, [40][41][42] salan, [43][44][45][46] phenolate, [47][48][49][50] aminophenolate, 51,52 benzotriazole phenolate, [53][54][55] phosphinophenolate, 56 thiophenolate, 57,58 bis-phenolate-N-heterocyclic carbene, 45 pyridonate, 59,60 and pyrrolide 61 have been reported to exhibit considerable catalytic activity or controllability, which is contributed by ligands. However, for most studies, focusing on materials is inconvenient, because the synthesis and purication of Ti catalysts are time-consuming processes.…”