Synthesis of enantiomeric pure zirconium and hafnium benzamidinate complexes

Techniques in the synthesis of organometallic compounds of Hafnium

Hafnium is a transition metal and it is the 45th most abundant transition element present on the earth. Hafnium has been successfully alloyed with several metals including titanium, iron, and niobium. Hafnium complexes are less active olefin polymerization catalysts. In the current review synthesis of hafnium complexes involving bonding through different linkages like “carbon, nitrogen, oxygen, carbon and oxygen, nitrogen and cobalt nitrogen and oxygen, nitrogen and phosphorus, nitrogen and sulfur, phosphorus and carbon, phosphorus and oxygen, sulfur carbon and oxygen, carbon–nitrogen and oxygen, carbon–nitrogen and phosphorus, carbon–nitrogen oxygen, sulfur and phosphorus, carbon–oxygen phosphorus and nitrogen”. The commonly used solvents for the synthesis of Hafnium complexes are tetrahydrofuran, n-hexane, and toluene, etc. These complexes were mostly reported at different temperatures ranges from −35 to 110 °C with continuous stirring, according to the nature of ligands. An overview of techniques in the synthesis of Hafnium complexes through various routes has been compiled.

Catalytic intramolecular hydroamination of aminoallenes using titanium and tantalum complexes of sterically encumbered chiral sulfonamides

Titanium and tantalum catalysts supported by readily prepared chiral sulfonamide ligands catalyze hydroamination of aminoallenes that lack N-protecting groups.

Catalytic intramolecular hydroamination of aminoallenes using titanium complexes of chiral, tridentate, dianionic imine-diol ligands

Alkylation of d- or l-phenylalanine or valine alkyl esters was carried out using methyl or phenyl Grignard reagents. Subsequent condensation with salicylaldehyde, 3,5-di-tert-butylsalicylaldehyde, or 5-fluorosalicylaldehyde formed tridentate, X2L type, Schiff base ligands. Chiral shift NMR confirmed retention of stereochemistry during synthesis. X-ray crystal structures of four of the ligands show either inter- or intramolecular hydrogen bonding interactions. The ligands coordinate to the titanium reagents Ti(NMe2)4 or TiCl(NMe2)3 by protonolysis and displacement of two equivalents of HNMe2. The crystal structure of one example of Ti(X2L)Cl(NMe2) was determined and the complex has a distorted square pyramidal geometry with an axial NMe2 ligand. The bis-dimethylamide complexes are active catalysts for the ring closing hydroamination of di- and trisubstituted aminoallenes. The reaction of hepta-4,5-dienylamine at 135 °C with 5 mol% catalyst gives a mixture of 6-ethyl-2,3,4,5-tetrahydropyridine (40-72%) and both Z- and E-2-propenyl-pyrrolidine (25-52%). The ring closing reaction of 6-methyl-hepta-4,5-dienylamine at 135 °C with 5 mol% catalyst gives exclusively 2-(2-methyl-propenyl)-pyrrolidine. The pyrrolidine products are obtained with enantiomeric excesses up to 17%.

Benzylene-linked [PNP] scaffolds and their cyclometalated zirconium and hafnium complexes

The benzylene-linked [PNP] scaffolds HN(CH₂-o-C₆H₄PPh₂)₂ ([A]H) and HN(C₆H₄-o-CH₂PPh₂)₂ ([B]H) have been used for the synthesis of zirconium and hafnium complexes. For both ligands, the dimethylamides [A]M(NMe₂)₃ ([A]1-M) and [B]M(NMe₂)₃ ([B]1-M) were prepared and converted to the iodides [A]MI₃ ([A]2-M) and [B]MI₃ ([B]2-M) (M = Zr, Hf). Starting from these iodides, the corresponding benzyl derivatives [A]MBn₃ ([A]3-M) and [B]MBn₃ ([B]3-M) (M = Zr, Hf) were obtained via reaction with Bn₂Mg(OEt₂)₂. For zirconium, the benzylic ligand positions in [A]3-Zr and [B]3-Zr were found to cyclometalate readily, which led to the corresponding κ⁴-[PCNP]ZrBn₂ complexes [A]4-Zr and [B]4-Zr. As these complexes failed to hydrogenate cleanly, cyclometalated derivatives with only one alkyl substituent were targeted and the mixed benzyl chlorides κ⁴-[PCNP]MBnCl ([B]5-M, M = Zr, Hf) were obtained in the case of ligand [B]. Upon hydrogenation of [B]5-Zr, the η⁶-tolyl complex [B]Zr(η⁶-C₇H₈)Cl ([B]6-Zr) was generated cleanly, but the corresponding hafnium complex [B]5-Hf was found to decompose unselectively in the presence of H₂. Using a closely related carbazole-based [PNP] ligand, Gade and co-workers have shown recently that zirconium η⁶-arene complexes similar to [B]6-Zr may serve as zirconium(ii) synthons, namely when reacted with 2,6-Dipp-NC (L) or pyridine (py). Both these substrates were shown to react cleanly with [B]6-Zr, which led to the formation of the bis-isocyanide complex [B]ZrCl(L)₂ ([B]7-Zr) and the 2,2'-bipyridine derivative [B]ZrCl(bipy) ([B]8-Zr), respectively. Upon reaction of [B]Zr(η⁶-C₇H₈)Cl ([B]6-Zr) with NaBEt₃H, the cyclometalated derivative κ⁴-[PCNP]Zr(η⁶-C₇H₈) ([B]9-Zr) was isolated. In an attempt to synthesise terminal hydrides, complexes [A]MI₃ ([A]2-M) were treated with KBEt₃H, which led to the isolation of the cyclometalated hydrido complexes κ⁴-[PCNP]M(H)(κ³-Et₃BH) ([A]10-M; M = Zr, Hf) featuring a κ³-bound triethyl borohydride moiety.

Amidinate group 4 complexes in the polymerization of olefins

The current perspective will present the use of amidinate group 4 complexes in α-olefin polymerizations.