Hafnium complex

The fact that the oxidation process occurs at a potential value slightly more positive than that exhibited by (Cp)Ti(Fc)3, despite the presence of the more electron-donating pentamethylcyclopentadienyl ligand, suggests that the hafnium atom is more electron-withdrawing than the titanium atom. 

To our knowledge, the onyl vanadium-ferrocene complexes structurally characterized are Fe[ j5-C5H4-V( 5-C5H4Me)2]2 [24] and [Fe( j -C5H4Se)2]V(0)( -C5Me5) [25]. Their redox behavior is not known. 

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More recent studies, particularly with slower hafnium complexes, have provided more detailed mechanistic insight As a step polymerization, the reaction is "nonideal" in that inequivalent reactivities for different Si-H functional groups in the system are observed. For exaniple, disilanes tend to be more reactive than monosilanes. Beyond disilane formation, the preferred dehydrocoupling reaction appears to involve addition of one silicon at a time to the growing chain, via M-S1H2R intermediates (n = 1 above). The Si-Si bond-forming reactions are also reversible. 

Bochmann M, Lancaster SJ (1995) Cationic group IV metal alkyl complexes and their role as olefin polymerization catalysts The formation of ethyl-bridged dinuclear and heterodinuclear zirconium and hafnium complex. J Org Chem 494 55-59... 

In contrast, exposure of 14-VE (diene)MCp Cl complexes (M = Zr, Hf) to CO (1 atm) results in the formation of cyclopentadienes70. The mechanism proposed for this transformation was elucidated with a carbon labeled CO ( CO) as requiring an initial coordination of CO to generate a (diene)MCp (CO)Cl complex 153 (Scheme 37). For the hafnium complex, the intermediate 153 (M = Hf) was observed by infrared spectroscopy. Insertion of CO into the a2, jt diene generates a metallacyclohexenone, which undergoes reductive elimination to generate the dimeric metallaoxirane species 154. -Hydride elimination from 154 (M = Zr, Hf) followed by 1,2-elimination produces substituted cyclopentadienes and the polymeric metal-oxide 155. Treatment of (diene)TiCp Cl with CO leads to isolation of the metallaoxirane complex 154 (M = Ti). 

This complex, unlike the simple homoalkyls of uranium 133), is stable at room temperature but decomposes without melting at 85 °C. The NMR spectrum of the diamagnetic complex confirms the presence of a Th—C a bond coordinated through the methylene carbon [t, 8.90 ppm singlet (2) t, 3.60 ppm doublet (2) t, 3.15— 2.60 ppm multiplet (3)]. Both the NMR and infrared spectra are in accordance with those of the presumably isostructural tetrabenzylzirconium and hafnium complexes. Also reported was the thorium(III) complex (C6H5CH2)3Th-THF. 

In contrast, data on the characterization and the catalytic properties of hafnium surface complexes are very scarce [42, 43]. Yet, some papers describe the hafnium complexes as more active than the zirconium analog, in the polymerization of ethylene [44], more selective in the oligomerization of propene [45, 46] and faster... 

The supported hafnium complex is quite stable up to 150°C (by FT-IR and GC studies, only traces of neopentane and methane are observed in the gas phase) [43]. Above this temperature, a significant amount of isobutene is also observed. At 250 °C, decomposition is total as no alkyl ligand remains linked to hafnium. 

Recently, dithio-diolate ligands have been employed for construction of group 4 metal catalysts for the ROP of lactide. These metal dithiolate complexes form mononuclear species of the type [(OSSO)M(OR)2] with an octahedrally coordinated metal center. These fluxional compounds acted as highly active catalysts in the ROP of L- and rac-lactide. Hafnium complexes were also introduced as initiators for the ROP of L-lactide and rac-lactide (vide infra) in very limited cases. To our knowledge, the hafnium derivative 146 displayed the highest activity among the group 4 catalysts reported to date (complete conversion of 300 equiv. of... 

Fig. 27 Amino bis(phenolate)-supported zirconium and hafnium complexes and Al-fluorous complexes for isoselective lactide ROP...

Zirconium and hafnium complexes are highly fluxional. NMR studies indicate stereochemical nonrigidity, facile exchange of terminal and bridging ligands, and rapid intermolecular ligand exchange. 

Very little X-ray data is available for zirconium or hafnium complexes that contain oxo ligands. The only X-ray evidence for the existence of the zirconyl or hafnyl group, M=Oz+, comes from a structural study of K2Zr03.157 This compound contains chains of Zr05 square pyramids that share basal edges. The Zr—O distance to the apical oxygen atom (1.92 A) is appreciably shorter than the sum of ionic radii (2.20 A), the sum of the covalent radii (2.22 A)... 

Cation exchange, solvent extraction and Raman spectroscopic studies of highly acidic zirconium sulfate solutions suggest that these solutions contain a mixture of solution species having from one to four sulfate ligands per Zr atom.153,351 The sulfato-zirconium complexes appear to be somewhat more stable than the analogous hafnium complexes.351... 

Haber process, 116 Hafnium complexes alkyl alkoxy reactions, 358 phthalocyanines, 865 porphyrins, 823 pseudohalides, 228 Hafnium(IV) complexes cupferron, 512 Haloarsine compounds, 1004 Halogen complexes, 675-687 bridge systems, 677-684 asymmetry, 678 theoretical studies, 682 double bridged, 681... 

For tri-n-octylphosphine oxide, for example, the shift in the P=0 stretching frequency from its value for the free extractant is greater for the hafnium complex (85 cm-1) than for the zirconium complex (70 cm-1).297... 

The first disilene-group-4 metal complex, disilene-hafnium complex 240, is synthesized using a similar reaction to those shown in Eqs. (108) and (109) [Eq. (113)].153 The reaction of 240 with trimethylphosphine gives the phosphine adduct 241 [Eq. 

Bis(arene)hafnium complexes, characteristics, 4, 697 Bis(arene)iron dications, characteristics, 6, 173 Bis(arene)niobium complexes, characteristics, 5, 95 Bis(arene)titanium(0) complexes, characteristics, 4, 243-244 Bis(arene)tricarbonylchromium complex, synthesis, 5, 258... 

Hafnium complexes with bis(cyclopentadienyl)s, 4, 731 in carbometallations, overview, 10, 255 in C-C bond formation, 10, 424 in ethylene polymerization, 4, 1139 for isotactic polypropylene, 4, 1126 metal-metal bonds, 4, 755 mixed-valence compounds, 4, 755 with pyridyl amines, for olefin polymerization, 4, 1094 in Ru-Os heterodinuclear compounds, 6, 1046 with silicon, 3, 515... 

O-Glycosidation of phenols.3 O-Aryl glycosides can be obtained in high yield by reaction of glycosyl fluorides with phenols in the presence of 4-A MS and this activator system. This hafnium complex is superior to Cp2ZrCl2, which is more useful for glycosidation of alcohols. 

Hafnium complex 83 in the presence of hexachloroethane as mild chlorinating agent afforded a new triphospha Dewar benzene 84 in 49% yield as a pale-yellow powder (Equation 20) <1997CB1491>. 

These results explain previous observations of the greater stability of zirconium thiocyanate and selenocyanate complexes compared with their hafnium analogues, and the greater stability of zirconium and hafnium complexes in MeCN compared with DMF in terms of competition between the ligand and solvent molecules for co-ordination sites on the metal. Zirconium alkoxides have been prepared from ZrCl4 and aliphatic alcohols158 but with salicylaldehyde a Meerwein-Ponndorf... 

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