Improving 1‑Hexene Incorporation of Highly Active Cp–Chromium-Based Ethylene Polymerization Catalysts

Single-site chromium catalysts for olefin polymerization with donor functionalized cyclopentadienyl (Cp) ligands have been modified in order to improve their incorporation ability for the comonomer 1-hexene into the polymer chain under maintenance of their very high catalytic activities. A trimethylsilyl substituent in combination with a fused thiophene ring at the Cp ligand has been identified as the best ligand so far, leading to a doubling in 1-hexene incorporation and polyethylene (PE) with up to 27% 1-hexene content (by weight) has been obtained. The complexes lead to PE with molecular weight in the range of 50 000 to 800 000 g mol^–1 when used in homogeneous solution, however after supporting the complex on silica ultrahigh molecular weight polyethylene (UHMW-PE) is formed with 9.9% of 1-hexene incorporated into the chain. Although other known catalysts incorporate even more 1-hexene, the presented system is different as it combines considerable α-olefin incorporation with very high polymer molecular weights and very high catalytic activity. These improved single-site chromium catalysts maintain their advantageous properties on silica as solid support which makes them good candidates for their application in industrial processes for the synthesis of polyethylene materials with advanced properties

[2 + 2] Cycloaddition Products of Zirconium and Hafnium Hydrazinediides with Allenes and Heteroallenes and Their Thermally Induced Rearrangements

Reactions of the hydrazinediido complexes [M­(N₂^TBSN_py)­(NNPh₂)­(py)] (M = Zr (<b>1a</b>), Hf (<b>1b</b>)) with (hetero)­allenes result in a variety of [2 + 2] cycloaddition products of the general type [M­(N₂^TBSN_py)­(κ²<i>N,E</i>-(E­(E′R)­NNPh₂)­(py)] (E = CH₂, S; E′ = CH, N; R = alkyl, aryl). The reaction of [Zr­(N₂^TBSN_py)­(NNPh₂)­(py)] (<b>1a</b>) with 1 molar equiv of phenyl or mesityl isothiocyanate at room temperature yields [Zr­(N₂^TBSN_py)­(κ²<i>N,S</i>-SC­(NAr)­NNPh₂)­(py)] (Ar = phenyl (<b>2a</b>), mesityl (<b>2b</b>)). Reacting the hydrazinediides [M­(N₂^TBSN_py)­(NNPh₂)­(py)] (M = Zr (<b>1a</b>), Hf (<b>1b</b>)) with allenes results in the formation of the metallaazacyclobutanes [M­(N₂^TBSN_py)­(κ²<i>N</i>,<i>C</i>-N­(NPh₂)­CH₂CCH­(R))­(py)] (M = Zr, R = Ph (<b>4a</b>), cyclohexyl (<b>5a</b>), methyl (<b>6</b>); M = Hf, R = phenyl (<b>4b</b>), cyclohexyl (<b>5b</b>)). Subsequent heating of the cycloaddition products revealed different reactivity patterns: the complex [Zr­(N₂^TBSN_py)­(κ²<i>N,S</i>-SC­(NAr)­NNPh₂)­(py)] (<b>2a</b>) forms the isomerization product [Zr­(N₂^TBSN_py)­(κ²<i>N,S-</i>SC­(NNPh₂))­NPh] (<b>3</b>), retaining the N–N bond of the hydrazide. In contrast, the metallacyclobutanes <b>4a</b>,<b>b</b> and <b>5a</b>,<b>b</b> show a tendency toward N–N bond cleavage, resulting in the formation of the C–N- and C–C-coupled product complexes [M­(κ⁴<i>N,N,N,N</i>-N₂^TBSN_pyNC­(Me)CHCy)­(NPh₂)] (M = Zr (<b>7a</b>), Hf (<b>7b</b>)), [Zr­(N₂^TBSN_py)­(κ²<i>N</i><i>,C-</i>(Ph)­NC₆H₄C­(Me)C­(Ph)­NH)] (<b>8</b>) and [Zr­(κ⁴<i>N,N,N,N</i>-N₂^TBSN_pyNC­(Me)=CHPh)­(NPh₂)] (<b>9</b>)

Synthesis, Characterization, and Thermal Rearrangement of Zirconium Tetraazadienyl and Pentaazadienyl Complexes

Reaction of the zirconium dichloro complex [Zr­(N₂^TBSN_py)­Cl₂] (<b>1</b>) with 1 molar equiv of ArNHLi (Ar = Mes, DIPP) yielded the zirconium imido complexes [Zr­(N₂^TBSN_py)­(N^DIPP)­(py)] (<b>2</b>; N₂^TBSN_py = [(2-C₅H₄N)­C­(CH₃)­{CH₂NSi­(CH₃)₂<i>t</i>Bu}₂]^2–, DIPP = 2,6-diisopropylphenyl) and [Zr­(N₂^TBSN_py)­(N^Mes)­(py)] (<b>3</b>; Mes = mesityl). The imido complexes are converted to the tetraazadienido complexes [Zr­(N₂^TBSN_py)­(N^DIPPN₂N^Ph)] (<b>4</b>) and Zr­(N₂^TBSN_py)­(N^MesN₂N^Ph)] (<b>5</b>) by addition of phenyl azide, whereas the reaction of <b>2</b> or <b>3</b> with mesityl azide gave the alternative product <b>7</b>, in which the azide is coupled with the CH activated ancillary tripod ligand. Reaction of 1 molar equiv of trimethylsilyl azide or 1-adamantyl azide with the previously reported hydrazinediido complex [Zr­(N₂^TBSN_py)­(NNPh₂)­(py)] (<b>9</b>) at ambient temperature resulted in the formation of the five-membered zirconaacacycles [Zr­(N₂^TBSN_py)­(N^TMSN₃NPh₂)] (<b>10</b>) and [Zr­(N₂^TBSN_py)­(N^AdN₃NPh₂)] (<b>11</b>). Complex <b>11</b> was thermally converted into the diazenido complex <b>12</b> via loss of 1 molar equiv of molecular N₂. The direct formation of the analogous side-on-bonded diazenido analogue <b>13</b> was observed upon reaction of <b>9</b> with 1 equiv of mesityl azide at ambient temperature. On the basis of ¹⁵N labeling and DFT modeling (DFT­(B3PW91/6-31 g­(d))) a mechanism for the reaction pathway leading to <b>12</b> and <b>13</b> is proposed

Zirconium Hydrazides as Metallanitrene Synthons: Release of Molecular N₂ from a Hydrazinediido Complex Induced by Oxidative N–N Bond Cleavage

The N–N bond in the zirconium hydrazinediido(2−) complex [Zr­(N₂^TBSN_py)­(NNPh₂)­(py)] (<b>1</b>) is readily cleaved by one-electron oxidation. Reacting [Zr­(N₂^TBSN_py)­(NNPh₂)­(py)] (<b>1</b>) with 0.5 molar equiv of iodine led to the release of molecular N₂ and yielded the mixed diphenylamido/iodo complex [Zr­(N₂^TBSN_pyNPh₂)­(I)] (<b>2</b>). Exposure of hydrazinediide <b>1</b> to an excess of iodine resulted in further oxidation of the diphenylamido ligand, yielding the diiodo complex <b>3</b> and tetraphenylhydrazine. Similar reactivity was observed in the reaction of <b>1</b> with diphenyl diselenide and diaryl disulfides, which reacted to give the corresponding diphenylamido/arylchalcogenido complexes [Zr­(N₂^TBSN_pyNPh₂)­(SePh)] (<b>4a</b>) and [Zr­(N₂^TBSN_py)­(NPh₂)­(SAr)] (Ar = Ph (<b>4b</b>), C₆F₅ (<b>4c</b>)) along with N₂. The reactions were also carried out on an NMR scale with a ¹⁵N_α-labeled hydrazido complex (<b>1-</b>^<b>15</b><b>N</b>). In all cases a single ¹⁵N NMR resonance at 310.16 ppm, assigned to ¹⁵N₂, indicated the formation of dinitrogen from the N_α atom in the hydrazide. A crossover labeling experiment employing a 1:1 mixture of <b>1</b> and ¹⁵N_α-labeled <b>1-</b>^<b>15</b><b>N</b> revealed that the isotope distribution is, as expected, statistical 1:2:1 (¹⁴N₂: ^14/15N₂: ¹⁵N₂), which is consistent with a reaction pathway involving a dinuclear intermediate in the dinitrogen-forming step. Complex <b>1</b> reacted with N₂O to give a mixture of two compounds, the bis­(diphenylamido) complex <b>6</b> and the doubly bridged μ-oxo complex <b>7</b>. In contrast, reaction of <b>1</b> with 1 molar equiv of pyridinium <i>N</i>-oxide only gave the doubly bridged μ-oxo complex <b>7</b> along with 2,2′-bipyridine and diphenylamine