Catalytically Active N-Acylamidine–Zirconium Complexes: Synthesis, Structures, and Application in Ethylene Polymerization

Abstract

Three novel N-acylamidines 1a−c with varying steric bulk and substitution patterns were synthesized and thoroughly characterized by X-ray diffraction. Compounds 1a and 1b, which contain two additional binding sites located at two pyridine substituents, were treated with equimolar amounts of cyclopentadienylzirconium(IV) trichloride at room temperature. The X-ray data of the resulting coordination compounds 7a and 7b indicate the formation of five-membered metallacycles with one of the pyridine nitrogen atoms and the carbonyl oxygen atom acting as binding sites. For the complexation of ligand 1c, a different route was chosen: 1c was first deprotonated to yield the polymeric potassium compound 8 with a very complex substitution pattern based on O , N, and aromatic interactions with the potassium ions. Transmetalation of 8 with cyclopentadienylzirconium(IV) trichloride gave amidinate complex 9, which is dimeric in the solid state but exists in solution in equilibrium with monomeric species. After addition of methylaluminoxane (MAO), the three novel cyclopentadienylzirconium complexes 7a, 7b, and 9 gave active homogeneous single-site catalysts for the polymerization of ethylene. Of these three systems, 9/MAO turned out to be the most efficient one, showing activities 3−5 times higher than 7a/MAO and 7b/MAO, respectively, and producing polymers with a well-defined "monomodal" molecular weight distribution. An important feature of these materials is their broader distribution in molecular weight (PDI > 3), which is best seen in the products of reactions at 53°C, with a "monomodal" main fraction at higher molecular weight and only small fractions of low molecular weight. ■ INTRODUCTIONAfter more than 30 years, the development of new single-site catalysts for olefin polymerization based on early transition metals continues to be one the fields of highest interest for industry and academia. In recent years, research has been centered on robust and efficient precatalysts that can be synthesized in few steps from common (not exotic) materials with the purpose of making them commercially more attractive.Precatalysts based on bi-, tri-, and tetradentate ligands in which carbon, nitrogen, or oxygen atoms efficiently stabilize an early transition metal are worth mentioning (Scheme 1). The advantage of such ligands is the easy access and great control of the steric and electronic environment of the atoms that coordinate the metal, characteristics that are essential for having access to thermally more stable and efficient catalysts in olefin homo-and copolymerization reactions. Among others, reviews 1−4 and articles from the groups of Edelmann, 5 Sita, 6 Hagadorn, 7 Eisen, 8 Rausch, 9 Green, 10 Lappert, 11 Kretschmer and Kempe, 12 and Jin 13 describe the work in this challenging field and deserve special attention.A second strategy to achieve greater competitiveness of the systems based on early transition metals has focused on reducing or eliminating the dependence of these systems on the use of methylaluminoxane (MAO) as an alk...

Nickel Promoted Condensation of Acetamide and Benzonitrile for the Synthesis of an Imidoylamidine (N-NacNac) via Stable Imidoylamide Intermediate

“…Interestingly, even the axial interaction with the water molecule (Ni–O2, 2.096 Å) results in a shorter bond than the axial Ni-tertiary amine (N4) bond. The bond lengths of the IDD backbone are between 1.26–1.37 Å for the C–N bonds, falling within the range of precedent M-IDD complexes. , The shortest bond length is the C–O bond (1.262 Å). Close values of C–N and C–O bond lengths are indicative of charge delocalization in a singly deprotonated IDD ligand.…”

“…Close values of C–N and C–O bond lengths are indicative of charge delocalization in a singly deprotonated IDD ligand. In contrast, protonated IDD ligand free form or metal bound, such as the one found in copper­(II) pyridinophan- N -acetylamidine reported by Mirica et al, showed significant dissymmetry of the backbone, with distinguishable single (C–N, 1.37–1.39 Å) and double (C–O, 1.224 Å; terminal C–N, 1.282 Å) bonds …”

“…In an attempt to find a solution, we identified a bioinspired method that can allow for the controlled synthesis of unsymmetrical IDAs from 2 . Furthermore, the method can be adapted for the synthesis of IDDs, a class of ligands less commonly explored than IDAs, but that have been demonstrated to be useful in catalysis . Details on further reaction optimization for the synthesis of symmetrical and unsymmetrical IDAs and successful ligand liberation are forthcoming.…”

Compatibility of a Gaseous Dielectric with Al, Ag, and Cu and Gas‐Phase Synthesis of a New N‐Acylamidine Copper Complex

“…In the past 20 years N ‐acylamidines have been investigated mainly by the Würthwein group . The three heteroatoms (one oxygen and two nitrogen atoms) offer various coordination modes for transition metals.…”

“…Würthwein and co‐workers obtained N ‐acylamidine Cu complexes with coordination numbers four, five and six in square planar, tetrahedrally distorted planar, square pyramidal and octahedral coordination, respectively . They further extended the coordination possibilities either by the preparation of Bis( N ‐acylamidines) and Tris( N ‐acylamidines) which allow coordination to more than one metal center or by introduction of substituents (R 1 and R 2 in Figure ) with additional coordination sites like pyridines . However, without these additional binding sites the formation of six‐membered chelate rings, with metal coordination towards the oxygen of the acyl group and the amino nitrogen is most likely.…”