The synthesis of half-sandwich bis(pentafluorophenyl)boryl-substituted cyclopentadienyl zirconium, niobium and tantalum complexes and the isolation and molecular structure of a zwitterionic niobocene

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Abstract
Reaction of B(C5H4SiMe3)(C6F5)2 with MCl5 (M = Nb, Ta) leads to the first group 5 borylcyclopentadienyl half-sandwich complexes MCl4{C5H4B(C6F5)2} (1 and 2). In contrast, the reaction with ZrCl4 gives the metallocene ZrCl2{C5H4B(C6F5)2}2 (3). The use of ZrCl4(SMe2)2 instead of ZrCl4 as starting material allows the isolation of the monocyclopentadienyl zirconium complex ZrCl3{C5H4B(C6F5)2(SMe2)} (4). The utility of LiCp′ as a general route to zirconocenes is demonstrated by the synthesis of ZrCl2(Cp){C5H4B (C6F5)2(SMe2)} (5) and ZrCl2(Ind){C5H4B(C6F5)2(SMe2)} (6) (Ind = η5-indenyl). Boron-substituted niobocenes are prepared through the dehalostannylation reaction between the half-sandwich complexes and tin-substituted cyclopentadienes. They adopt zwitterionic structures in which a chloride ligand is transferred to boron, for example Nb(+)Cl2(C5H4SiMe3){C5H4B(−)(Cl)(C6F5)2} (9). The crystal structure of 9 has been determined by X-ray crystallography. Reaction of the strong base, pyridine, with the borylcyclopentadienyl complexes TiCl3{C5H4B(C6F5)2}, TiCl2(Cp){C5H4B(C6F5)2} and 1–6 leads to the generation of a series of pyridine adducts (10–17) in which the pyridine is bound to boron. The solid-state structures of the four coordinate adducts TiCl2(Cp){C5H4B(C6F5)2(py)} (10) and TiCl3{C5H4B(C6F5)2(py)} (14) are described. The half-sandwich zirconium and niobium complexes 15 and 16 are shown by spectroscopic and structural methods to coordinate a further one (Nb) or two (Zr) equivalents of pyridine to attain an octahedral geometry at the metal centre. The zwitterionic complexes 8 and 9 do not react with pyridine.

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