Article pubs.acs.org/Organometallics
Self-Assembled Cage Structures and Ethylene Polymerization Behavior of Palladium Alkyl Complexes That Contain PhosphineBis(arenesulfonate) Ligands Jia Wei, Zhongliang Shen, Alexander S. Filatov, Qian Liu, and Richard F. Jordan* Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637, United States S Supporting Information *
ABSTRACT: The phosphine-bis-arenesulfonate ligands PR2(2-SO3Li-5-R1-Ph)(2-SO3−-5-R1-Ph) (Li[OPO−]; Li[1a− e], a: R1 = Me, R2 = Ph, b: R1 = iPr, R2 = Ph, c: R1 = Cy, R2 = Ph, d: R1 = tBu, R2 = Ph, e: R1 = R2 = tBu, f: R1 = R2 = Cy) coordinate as κ2-P,O chelators in (Li-OPO)PdMeL complexes (4a−c,f: L = 4-(5-nonyl)pyridine = py′; 5b−d: L = py; 2e: L = py′). 4a−c,f and 5b−d self-assemble into tetrameric structures in which four (Li-OPO)PdMeL units are arranged around a cubic Li4S4O12 cage formed from the four non-Pd-bound ArSO3Li units and one oxygen from each of the Pd-bound ArSO3− units. The {(Li-OPO)PdMeL}4 assemblies are in equilibrium with monomeric (Li-OPO)PdMeL species in CD2Cl2 and CDCl2CDCl2 solution. Crystallization of 2e from a toluene/pentane mixture at −40 °C yields a trimeric C1-symmetric assembly (6e) in which three (Li-1e)PdMe(py′) units are held together by a Li3S3O9 core. Crystallization of 2e from toluene/pentane at room temperature results in partial loss of py′ and formation of a Pd6 assembly, {(Li-1e)PdMe(py′)}4{(Li-1e)PdMe}2 (7e). 4a−c,f, 5b−d, and 2e exist as monomeric (LiOPO)PdMeL complexes in CD3OD solution, and under these conditions exchange of the Pd-bound and non-Pd-bound ArSO3− units is much faster for 4a,c (ΔG⧧ = 12.3, 12.2 kcal/mol at −10 °C) than for 2e (ΔG⧧ = 19.2 kcal/mol at 24 °C). This difference in molecular rigidity may contribute to the differences in the self-assembly behavior of these compounds. 4a,f and 5b catalyze the polymerization of ethylene to linear polyethylene. The molecular weight and molecular weight distribution of the polymer are strongly influenced by the extent of dissociation of the Pd4 cage under the polymerization conditions.
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INTRODUCTION Palladium alkyl complexes that contain ortho-phosphinoarenesulfonate (PO−) ligands, (PO)PdMeL (Scheme 1),
four (Li-OPO)PdMe(py′) units linked by Li−oxygen bonds involving all three oxygens (O1−O3) of the non-Pd-bound sulfonate group and one oxygen (O4) of the Pd-bound sulfonate group, which form a central Li4S4O12 cage. The four (Li-OPO)PdMe(py′) units are equivalent and are spatially separated into two pairs with RR and SS configurations at the phosphorus centers and a Pd−Pd distance of 6.04 Å within each pair. This S4-symmetric structure is the only observed stereoisomer for 4a. The Pd−Me groups (C21) are positioned over the face of the py′ ring on the adjacent Pd center. The cage structure of 4a remains substantially intact in aprotic and weakly coordinating solvents such as CH2Cl2 at low temperature. However, above −10 °C in CD2Cl2, 4a partly dissociates into monomeric (Li-OPO)PdMe(py′) “Pd1” species (Scheme 2).2 The equilibrium constant Keq = [Pd1]4/[Pd4] is 1.2 × 10−7 M3 at 25 °C but much larger, 9.7 × 10−7 M3, at 80 °C in CDCl2CDCl2 (Scheme 2), as determined by 1H and 31P NMR spectroscopy. Compound 4a polymerizes ethylene to linear PE with a bimodal molecular weight (MW) distribution comprising a high-MW fraction (106 Da) and a low-MW fraction (104 Da) in
Scheme 1
polymerize ethylene to linear polyethylene (PE) and copolymerize ethylene with a variety of polar vinyl comonomers to linear copolymers.1 Analogous phosphino-bisarenesulfonate ligands form (κ2-P,O-Li-OPO)PdMeL complexes, which self-assemble into tetrameric “Pd4 cage” species that are held together by Li-sulfonate interactions (Scheme 1).2 As shown in Figure 1, the solid-state structure of {[κ2-P,OPPh(2-SO3-5-Me-Ph)(2-SO3Li-5-Me-Ph)]PdMe(py′)}4 (4a, py′ = 4-(5-nonyl)pyridine) comprises a cubic assembly of © XXXX American Chemical Society
Received: August 4, 2016
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DOI: 10.1021/acs.organomet.6b00629 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Scheme 3
Figure 1. Solid-state molecular structure of {[κ2-P,O-PPh(2-SO3-5Me-Ph)(2-SO3Li-5-Me-Ph)]PdMe(py′)}4 (4a, py′ = 4-(5-nonyl)pyridine) from ref 2. The H atoms and the 4-(5-nonyl) substituents on the py′ ligands are omitted.
[Ar′BO3PAr]4 cage products under dilute conditions and hexameric [Ar′BO3PAr]6 products (Scheme 3d) under concentrated conditions, and these species interconvert in solution.10 In addition to cage structures, arenesulfonate−metal interactions can generate a variety of other one-, two-, and three-dimensional oligomeric and polymeric structures.11 One of our goals in this area is to design Pd4 cage catalysts analogous to 4a that are resistant to dissociation to monomeric Pd1 species under olefin polymerization conditions. Such catalysts may enable the synthesis of high-MW ethylene/vinyl fluoride copolymers and simplify mechanistic studies aimed at understanding the origin of the high MW. In this paper we describe the synthesis of a series of new Li2[OPO] ligands that incorporate various alkyl substituents on the arenesulfonate groups and the phosphorus center and their conversion to the corresponding (Li-OPO)PdMe(L) complexes (L = py, py′). While this work has not led to Pd4 cage compounds that are significantly more robust than 4a, it has provided insights to how the OPO2− ligand substituents influence the self-assembly behavior of these complexes and led to the discovery of new trimeric and hexameric cage structures.
Scheme 2
hexane suspension at 80 °C or CH2Cl2 solution at 25 °C. Available evidence suggests that the intact Pd4 cage produces the high-MW fraction, while Pd1 species generated by cage dissociation produce the low-MW fraction.2 The close proximity of the growing polymer chains on adjacent Pd units and the steric blockage of one axial site of each Pd center by the Li4S4O12 cage in the putative [(Li-OPO)Pd(R)(CH2 CH2)]4 active species may disfavor chain transfer, leading to the observed high-MW polymer. 4a also copolymerizes ethylene with vinyl fluoride (VF) with significantly higher VF incorporation (up to 3.6 mol %) compared to analogous mononuclear (PO)PdMe(L) catalysts (
