Lithium complexes of tridentate amine biphenolate ligands containing distinct N-alkyl substituents

Ring-opening polymerization of lactides and ε-caprolactone catalyzed by Zn(II) aryl carboxylate complexes supported by 4-pyridinyl schiff base ligands

Synthesis and catalytic studies of aryl carboxylate Zn (II) complexes is reported. Reaction of substituted (E)-N-phenyl-1-(pyridin-4-yl)methanimine with a methanolic solution of Zn(CH3COO)2 and substituted aryl carboxylate co-ligands gave heteroleptic Zn(II) complexes; [Zn(C6H5COO)2(L1)]2 (1), [Zn(C7H7COO)2(L1)]2 (2), [Zn (4-F-C6H4COO)2(L1)]2 (3), [Zn(C6H5COO)2(L2)]2 (4), [Zn(C7H7COO)2(L2)]2 (5), [Zn (4-F-C6H4COO)2(L2)]2 (6), [Zn(C6H5COO)2(L3)]2 (7), [Zn(C7H7COO)2(L3)]2 (8), [Zn (4-F-C6H4COO)2(L3)]2 (9). The molecular structures of complexes 1 and 4 are dinuclear with the zinc atom in complex 1 adopting a distorted trigonal bipyramidal geometry in a bi-metallacycle while complex 4 is square pyramidal where all four benzoate ligands bridge the zinc metals in a paddle wheel arrangement. All complexes successfully initiated mass/bulk ring-opening polymerization (ROP) of ϵ-caprolactone (ϵ-CL) and lactides (LAs) monomers with or without alcohol co-initiators at elevated temperatures. Complexes 1, 4 and 6 containing the unsubstituted benzoate co-ligands were the most active in their triad; with complex 4 being the most active (k app) of 0.3450 h-1. The physicochemical properties of the polymerization products of l-lactide and rac-lactide in toluene revealed melting temperatures (Tm) between 116.58 °C and 188.03 °C, and decomposition temperatures between 278.78 °C and 331.32 °C suggestive of an isotactic PLA with a metal capped end.

Lithium, sodium, potassium and calcium amine-bis(phenolate) complexes in the ring-opening polymerization of rac-lactide

Compounds of Li, Na, K and Ca of a tetradentate amino-bis(phenolato) ligand were prepared. Bimetallic compounds formulated as M₂[L](THF)_n (where M = Na, n = 1 (1·THF) or Li, n = 1 (2·THF)) were synthesized via the reaction of H₂[L] (where [L] = 2-pyridylmethylamino-N,N-bis(2-methylene-4-methoxy-6-tert-butylphenolato) with sodium hydride or n-butyllithium, respectively, in THF. Monometallic complexes MH[L](THF)_n (where M = Na, n = 1 (3·THF), Li, n = 0 (4) and K, n = 0 (5)) were obtained by reaction of H₂[L] with MN(SiMe₃)₂ where M = Na, Li, or K. Calcium complex Ca[L](THF) (6·THF) was synthesized in two ways; reaction of Na₂[L] with calcium iodide in THF, and reaction of Ca[N(SiMe₃)₂]₂ with H₂[L] in toluene. Compounds 1-6 exhibit activity for rac-lactide polymerization under melt and solution conditions. Moderate control of polymer molecular weights was achieved in toluene, whereas polydisperse polymer was obtained under solvent free conditions. MALDI-TOF MS analysis of the polymer end groups revealed a predominantly cyclic nature for the polylactides.

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

Aluminum complexes containing [RP(O)(2-O-3,5-tBu₂C₆H₂)₂]²⁻ [R = tBu (3a), Ph (3b)] have been synthesized, structurally characterized, and their reactivity studied in comparison with those of their [RP(2-O-3,5-tBu₂C₆H₂)₂]²⁻ [R = tBu (2a), Ph (2b)] analogs. Treating AlMe₃ with one equiv of H₂[3a-b] in THF at 0°C affords quantitatively [3a-b]AlMe, subsequent reactions of which with benzyl alcohol in THF at 25°C generate {[3a-b]Al(μ₂-OCH₂Ph)}₂. The methyl [3a-b]AlMe and the benzyloxide {[3a-b]Al(μ₂-OCH₂Ph)}₂ are all active for catalytic ring-opening polymerization (ROP) of ε-caprolactone and rac-lactide (rac-LA). Controlled experiments reveal that {[3a]Al(μ₂-OCH₂Ph)}₂ is competent in living polymerization. Kinetic studies indicate that [3a]AlMe, in the presence of benzyl alcohol, catalyzes ROP of rac-LA at a rate faster than [3b]AlMe and [2a]AlMe(THF) by a factor of 1.8 and 23.6, respectively, highlighting the profound reactivity enhancement in ROP catalysis by varying the P-substituents of these biphenolate complexes of aluminum.

Aluminum complexes containing biphenolate phosphine ligands: synthesis and living ring-opening polymerization catalysis

This report describes the synthesis, structure, and reactivity of aluminum complexes containing tridentate biphenolate phosphine ligands of the type [RP(2-O-3,5-C₆H₂tBu₂)₂]^2- (R = tBu (2a), Ph (2b)). Alkane elimination of AlMe₃ with one equiv. of H₂[2a] or H₂[2b] in THF at 0 °C cleanly affords colorless crystalline [2a]AlMe(THF) (3a) and [2b]AlMe(THF) (3b), respectively. An X-ray diffraction study of 3a showed it to be a five-coordinate THF-bound species, whose coordination geometry is best described as trigonal bipyramidal, having the phosphorus donor and THF at axial positions. Treatment of either in situ prepared or isolated methyl complexes 3a,b with one equiv. of benzyl alcohol in toluene or THF generated their corresponding benzyloxides {[2a,b]Al(μ₂-OCH₂Ph)}₂ (4a,b). An X-ray diffraction study of 4a revealed a dimeric structure, in which the coordination geometry of aluminum is also distorted trigonal bipyramidal with the tridentate 2a ligand being facially bound. In the presence of one equiv. of benzyl alcohol, complex 3a is a competent catalyst precursor for the living ring-opening polymerization of ε-caprolactone (ε-CL) and rac-lactide (rac-LA), producing poly(ε-caprolactone) and poly(rac-lactide), respectively, in a controlled manner. As such, well-defined block copolymers of ε-CL with rac-LA can also be prepared by catalytic 3a. Kinetic studies revealed that 3a catalyzes the polymerization of rac-lactide at a rate 2-fold faster than that of 3b, indicating the significance of the P-substituent effect on this catalysis. Interestingly, the polymerization rate of rac-lactide by 3a is 16.5 times faster than that of l-lactide under otherwise identical conditions.

Homo- and Heteropolynuclear Clusters of Phosphine Triphenolates

The synthesis and structural characterization of a series of homo- and heteropolynuclear clusters constructed with a potentially tetradentate phosphine triphenolate ligand are presented. Treatment of tris(3,5-di-tert-butyl-2-hydroxyphenyl)phosphine (H3[O3P]) with 3 equiv of nBuLi in diethyl ether at -35 °C affords hexanuclear Li6[O3P]2(OEt2)2 (1) as colorless crystals. In situ lithiation of H3[O3P] with 3 equiv of nBuLi in THF at -35 °C followed by metathetical reactions with MnCl2 or NiCl2(DME) gives crystals of forest green pentanuclear MnLi4[O3P]2(THF)3 (2) or dark brown tetranuclear Ni2Li2[O3P]2(THF)2 (3), respectively. Alkane elimination of ZnR2 (R = Me, Et) with H3[O3P] in THF at 25 °C generates high yields of colorless crystalline trinuclear Zn3[O3P]2(THF)2 (4). The cluster structures of 1-4 were all determined by single crystal X-ray diffraction studies. These molecules represent the first examples of metal complexes supported by phosphine triphenolate derivatives. The cluster 2 contains a paramagnetic core of high spin Mn(II) (S = 5/2) as indicated by solution and solid state magnetic susceptibility measurements.

Ring-opening polymerization of cyclic esters with lithium amine-bis(phenolate) complexes

Lithium compounds of tetradentate amino-bis(phenolato)-tetrahydrofuranyl ligands, Li(2)[L1] (1) and Li(2)[L2] (2) (where [L1] = 2-tetrahydrofuranyl-N,N-bis(2-methylene-4-methyl-6-tert-butylphenolate), and [L2] = 2-tetrahydrofuranyl-N,N-bis(2-methylene-4,6-tert-butylphenolate)) were characterized by multinuclear solution NMR and solid-state (6)Li and (7)Li NMR spectroscopy. The proligands, n-propylamino-N,N-bis(2-methylene-4-methyl-6-tert-butylphenol), (H(2)[L3]) and benzylamino-N,N-bis(2-methylene-4,6-di-tert-amylphenol), H(2)[L4] were reacted with n-butyllithium in THF to give the related dilithium compounds Li(2)[L3] (4) and Li(2)[L4] (5), respectively. The pyridine adduct of 1, (py)(2)Li(2)[L1] (3) and complexes 4 and 5 have been structurally characterized by single-crystal X-ray diffraction and NMR spectroscopy. The reactivity of these complexes for the ring-opening polymerization of rac-lactide, as well as the influences of monomer concentration, monomer/Li molar ratio, polymerization temperature and time, were studied.