A Pair of Remarkably Stable Mononuclear Chromium(<scp>III</scp>) and Chromium(<scp>IV</scp>) Hydrides

Filippou, Alexander C.; Schneider, Sven; Schnakenburg, Gregor

doi:10.1002/ange.200352177

“…Indeed, the formation of binuclear bridging Cr hydride complexes has been previously reported, 24-27 while monomeric Cr hydrides are rather rare. 28 Sinema et al reported a C-H activation step at the Cp ligand on ansa-chromocene complexes in the presence of [H(OEt2)2BAr4], 29 and the related activation pathways were reported for other 3d metallocene-based complexes. 30,31 In analogy to Cr, di-nuclear bridging molybdocene-hydride complexes with activated Cp ligands are also known.…”

Section: Formation and Further Characterization Of Surface Cr Hydridesmentioning

confidence: 82%

Union Carbide Ethylene Polymerization Catalyst: From Uncovering Active Site Structures to Designing Molecularly-Defined Analogs

Trummer

¹

,

Nobile

²

,

Payard

³

et al. 2022

Preprint

0

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The Union Carbide(UC) catalyst, one of the first ethylene polymerization (EP) catalysts based on supported organometallic compounds, is based on chromocene dispersed on silica; it is distinct from the corresponding Phillips catalysts, based on supported chromium oxide, because it is responsive to H2 enabling tuning the resulting polymer. Yet, despite 50 years of research, the structure of its active sites has also been controversially discussed and remains elusive. Based on a combination of IR, EPR spectroscopies, labeling experiments, and modeling at the DFT level, we identified monomeric surface-supported Cr(III) hydrides, (≡SiO)Cr(Cp)-H, as the active sites of the UC catalyst. These active sites, with distinct EPR signatures, are exclusively formed at high Cr loadings because their formation requires a combination of congruent factors, namely the presence of grafted as well as adsorbed chromocene together with residual surface OH groups, and involves a C-H bond activation step of the Cp ring. These Cr hydrides readily initiate polymerization, yielding Cr(III) alkyl species that insert ethylene monomers through a Cossee Arlman-type mechanism, as evidenced by labeling and spectroscopic studies. This information inspired the design of a well-defined analog, that was prepared and characterized using SOMC by grafting CpCr(CH(SiMe3)2)2 on partially dehydroxylated silica. The resulting surface Cr sites show similar EPR signatures and produce similar polyethylene as the UC catalyst, further supporting the presence of similar active sites and opening the way to generating new classes of catalysts.

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The Binary Group 4 Azides [Ti(N₃)₄], [P(C₆H₅)₄][Ti(N₃)₅], and [P(C₆H₅)₄]₂[Ti(N₃)₆] and on Linear TiNNN Coordination

Haiges

¹

,

Boatz

²

,

Schneider

³

et al. 2004

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Whereas numerous partially azide-substituted titanium compounds had been reported, [1][2][3][4][5][6][7] no binary Group 4 azides were known. In a recent theoretical study, the Group 4 metal tetrazides [M(N 3 ) 4 ] (M = Ti, Zr, Hf, Th) were predicted [8] to be vibrationally stable, exhibiting tetrahedral structures with unique linear M À N À NN bond angles (see Figure 1). All the characterized covalent binary azide species possess bent MÀ NÀNN angles.[9] Herein, we report the synthesis, isolation, and characterization of the first binary Group 4 azide species Removal of the volatile products (CH 3 CN, (CH 3 ) 3 SiF, and excess (CH 3 ) 3 SiN 3 ) at ambient temperature results in the isolation of [Ti(N 3 ) 4 ] as an amorphous orange solid. All attempts to obtain single crystals by recrystallization or sublimation were unsuccessful.As expected for a highly endothermic, covalent polyazide species, [Ti(N 3 ) 4 ] is very shock sensitive and can explode violently when touched with a metal spatula or when exposed to a rapid change in temperature (e.g. freezing with liquid nitrogen). Its identity was established by the observed material balance, and vibrational and NMR spectroscopy. The presence of covalent azido ligands [10][11][12][13][14][15] is confirmed by the observed 14 N NMR shifts of d = À134 ppm (N b , Dñ1 = 2 = 26 Hz), À195 ppm (N g , Dñ1 = 2 = 39 Hz) and À255 ppm (N a , extremely broad) in DMSO solution at 25 8C.The observed Raman and IR spectra of solid [Ti(N 3 ) 4 ] are shown in Figure 2, and the frequencies and intensities are listed in the Experimental Section. The experimental vibrational spectra deviate significantly from those calculated for free [Ti(N 3 ) 4 ] at the B3LYP level of theory, [8,16] and resemble those of higher-coordinated compounds with bent MÀNÀNN bonds. Therefore, the predicted [8] tetrahedral structure with linear TiÀNÀNN bonds could not be confirmed. The discrepancy between the calculated and observed spectra arises from solid-state effects. The Ti atoms in [Ti(N 3 ) 4 ] are coordinatively unsaturated seeking higher coordination numbers by the formation of nitrogen bridges, and crystal-structure data will be required for a reliable determination of the precise arrangement of the azido ligands in solid [Ti(N 3 ) 4 ]. The growing of single crystals for such a study will be difficult because the compound is hard to recrystallize and does not sublime without decomposition. The structure determination of free monomeric [Ti(N 3 ) 4 ] and proof for its predicted tetrahedral structure with linear TiÀNÀNN bonds will be even more difficult.

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The Binary Group 4 Azides [Ti(N₃)₄], [P(C₆H₅)₄][Ti(N₃)₅], and [P(C₆H₅)₄]₂[Ti(N₃)₆] and on Linear TiNNN Coordination

Haiges

¹

,

Boatz

²

,

Schneider

³

et al. 2004

View full text Add to dashboard Cite

Whereas numerous partially azide-substituted titanium compounds had been reported, [1][2][3][4][5][6][7] no binary Group 4 azides were known. In a recent theoretical study, the Group 4 metal tetrazides [M(N 3 ) 4 ] (M = Ti, Zr, Hf, Th) were predicted [8] to be vibrationally stable, exhibiting tetrahedral structures with unique linear M À N À NN bond angles (see Figure 1). All the characterized covalent binary azide species possess bent MÀ NÀNN angles.[9] Herein, we report the synthesis, isolation, and characterization of the first binary Group 4 azide species Removal of the volatile products (CH 3 CN, (CH 3 ) 3 SiF, and excess (CH 3 ) 3 SiN 3 ) at ambient temperature results in the isolation of [Ti(N 3 ) 4 ] as an amorphous orange solid. All attempts to obtain single crystals by recrystallization or sublimation were unsuccessful.As expected for a highly endothermic, covalent polyazide species, [Ti(N 3 ) 4 ] is very shock sensitive and can explode violently when touched with a metal spatula or when exposed to a rapid change in temperature (e.g. freezing with liquid nitrogen). Its identity was established by the observed material balance, and vibrational and NMR spectroscopy. The presence of covalent azido ligands [10][11][12][13][14][15] is confirmed by the observed 14 N NMR shifts of d = À134 ppm (N b , Dñ1 = 2 = 26 Hz), À195 ppm (N g , Dñ1 = 2 = 39 Hz) and À255 ppm (N a , extremely broad) in DMSO solution at 25 8C.The observed Raman and IR spectra of solid [Ti(N 3 ) 4 ] are shown in Figure 2, and the frequencies and intensities are listed in the Experimental Section. The experimental vibrational spectra deviate significantly from those calculated for free [Ti(N 3 ) 4 ] at the B3LYP level of theory, [8,16] and resemble those of higher-coordinated compounds with bent MÀNÀNN bonds. Therefore, the predicted [8] tetrahedral structure with linear TiÀNÀNN bonds could not be confirmed. The discrepancy between the calculated and observed spectra arises from solid-state effects. The Ti atoms in [Ti(N 3 ) 4 ] are coordinatively unsaturated seeking higher coordination numbers by the formation of nitrogen bridges, and crystal-structure data will be required for a reliable determination of the precise arrangement of the azido ligands in solid [Ti(N 3 ) 4 ]. The growing of single crystals for such a study will be difficult because the compound is hard to recrystallize and does not sublime without decomposition. The structure determination of free monomeric [Ti(N 3 ) 4 ] and proof for its predicted tetrahedral structure with linear TiÀNÀNN bonds will be even more difficult.

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A Pair of Remarkably Stable Mononuclear Chromium(III) and Chromium(IV) Hydrides

Cited by 6 publications

References 25 publications

Union Carbide Ethylene Polymerization Catalyst: From Uncovering Active Site Structures to Designing Molecularly-Defined Analogs

Union Carbide Ethylene Polymerization Catalyst: From Uncovering Active Site Structures to Designing Molecularly-Defined Analogs

The Binary Group 4 Azides [Ti(N₃)₄], [P(C₆H₅)₄][Ti(N₃)₅], and [P(C₆H₅)₄]₂[Ti(N₃)₆] and on Linear TiNNN Coordination

The Binary Group 4 Azides [Ti(N₃)₄], [P(C₆H₅)₄][Ti(N₃)₅], and [P(C₆H₅)₄]₂[Ti(N₃)₆] and on Linear TiNNN Coordination

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A Pair of Remarkably Stable Mononuclear Chromium(III) and Chromium(IV) Hydrides

Cited by 6 publications

References 25 publications

Union Carbide Ethylene Polymerization Catalyst: From Uncovering Active Site Structures to Designing Molecularly-Defined Analogs

Union Carbide Ethylene Polymerization Catalyst: From Uncovering Active Site Structures to Designing Molecularly-Defined Analogs

The Binary Group 4 Azides [Ti(N3)4], [P(C6H5)4][Ti(N3)5], and [P(C6H5)4]2[Ti(N3)6] and on Linear TiNNN Coordination

The Binary Group 4 Azides [Ti(N3)4], [P(C6H5)4][Ti(N3)5], and [P(C6H5)4]2[Ti(N3)6] and on Linear TiNNN Coordination

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The Binary Group 4 Azides [Ti(N₃)₄], [P(C₆H₅)₄][Ti(N₃)₅], and [P(C₆H₅)₄]₂[Ti(N₃)₆] and on Linear TiNNN Coordination

The Binary Group 4 Azides [Ti(N₃)₄], [P(C₆H₅)₄][Ti(N₃)₅], and [P(C₆H₅)₄]₂[Ti(N₃)₆] and on Linear TiNNN Coordination