Bent metallocene

Abstract: The half-sandwich methylimido complexes [Nb(η-C5R5)(NMe)Cl2](R = H 1a, or Me 1b) have been prepared by treatment of [Nb(η-C5R5)Cl4] with NMe(SiMe3)2 in acetonitrile. The sterically hindered alkyl- and aryl-imido analogues [Nb(η-C5H5)(NR)Cl2](R = But1c, or C6H3Pri2-2,6 1d), [Nb(η-C5Me5)(N-C6H3Pri2-2,6)Cl2]1e and the tantalum compound [Ta(η-C5Me5)(N-C6H3Pri2-2,6)Cl2]1f are obtained by treatment of [M(η-C5R5)Cl4] with two equivalents of NHR(SiMe3) in chlorocarbon solvent. Crystal structures of 1a, 1c, 1d and 1f show that these complexes are mononuclear with quasi-linear imido ligands. The metal–nitrogen bond distances range from 1.744(3) to 1.780(5)A consistent with pseudo triple bonds. Compounds 1a–1e react with tertiary phosphines to give eighteen-electron adducts of the type [Nb(η-C5H5)(NR)Cl2(PR′3)]2a–2e. The crystal structure of [Nb(η-C5H5)(NMe)Cl2(PMe3)] shows a distorted four-legged piano-stool geometry in which the phosphine binds cis to the methylimido and cis to one of the chloride ligands. Alkoxide and aryloxide derivatives of the type [Nb(η-C5R5)(NR′)(OR″)2] may be prepared via reactions of the dichlorides 1 with 2 equivalents of LiOR″ in Et2O. Fenske–Hall quantum-chemical calculations indicate that there exists a close relationship between the Nb(η-C5H5)(NR) fragment and the bent metallocene moiety M(η-C5H5)2 for the Group 4 elements.

Abstract: The metallocene cation complex [Cp*2ZrCH3]+[B(C6F5)4]− inserts the phosphino-substituted alkyne Ph–C≡C–PPh2 into the [Zr]-CH3 bond to form the internally phosphane-stabilized cation [Cp*2Zr–C(═CMePh)PPh2]+ (10). Complex 10 adds alkyl isocyanides as well as pivalonitrile at a lateral site at the bent metallocene wedge with retention of the Zr–P bond. Complex 10 acts as a reactive frustrated Lewis pair toward heterocumulenes, undergoing Zr+/P addition reactions to the carbonyl groups of an alkyl isocyanate and of carbon dioxide to form the respective five-membered metallaheterocyclic adducts 13 and 14. With mesityl azide complex 10 undergoes a Zr+/P FLP N,N-addition reaction at the terminal azide nitrogen atom to form the four-membered FLP cycloadduct 15. The Zr+/P FLP is a reactive hydrogen activator. In a stoichiometric reaction it generates a hydridozirconocene cation that subsequently serves as a hydrogenation catalyst for various olefinic or acetylenic substrates. The Zr+/P pair 10 undergoes selective ...

Abstract: This paper reports on the potential of titanium compounds as building blocks for supramolecular polygons. Self-assembly reactions of low-valent titanocene units and N-heterocyclic bridging ligands lead to novel titanium-based supramolecular squares. Pyrazine (3), 4,4'-bipyridine (4), and tetrazine (5) were used as bridging ligands, and the acetylene complexes [Cp 2 Ti{η 2 -C 2 (SiMe 3 ) 2 )] (1) and [(tBuCp) 2 Ti{η 2 -C 2 (SiMe 3 ) 2 }] (2) as sources of titanocene fragments. Molecular rectangles can be synthesized by stepwise reduction of the titanocene dichlorides [Cp 2 TiCl 2 ] and [(tBuCp) 2 TiCl 2 ] and consecutive coordination of two different bridging ligands. The resulting complexes are the first examples of molecular rectangles containing bent metallocene corner units. Single-crystal X-ray analyses of the tetranuclear compounds revealed the geometric properties of the molecular polygons in the solid state. Comparison of bond lengths and angles in coordinated and free ligands reveals the reduced state of the bridging ligand in the low-valent titanium compounds. The syntheses and properties of these novel, highly air-and moisture-sensitive compounds are discussed.