Synthesis and structure of amido- and imido(pentafluorophenyl)borane zirconocene and hafnocene complexes: N-H and B-H activation

Jacobs, Elizabeth A., Fuller, Anna, Coles, Simon J., Jones, Garth A., Tizzard, Graham J., Wright, Joseph A. and Lancaster, Simon J. (2012) Synthesis and structure of amido- and imido(pentafluorophenyl)borane zirconocene and hafnocene complexes: N-H and B-H activation. Chemistry - A European Journal, 18 (28), 8647-8658. (doi:10.1002/chem.201200704).

Abstract

Treatment of Me₂S · B(C₆F₅)_nH_3-n (n=1 or 2) with ammonia yields the corresponding adducts. H₃N · B(C₆F₅)H₂ dimerises in the solid state through N-H · H-B dihydrogen interactions. The adducts can be deprotonated to give lithium amidoboranes Li[NH₂B(C₆F₅)nH_3-n]. Reaction of the n=2 reagent with [Cp₂ZrCl₂] leads to disubstitution, but [Cp₂Zr{NH₂B(C₆F₅)₂H}₂] is in equilibrium with the product of beta-hydride elimination [Cp₂Zr(H){NH₂B(C₆F₅)₂H}], which proves to be the major isolated solid. The analogous reaction with [Cp₂HfCl₂] gives a mixture of [Cp2Hf{NH2B(C6F5)2H}2] and the N-H activation product [Cp2Hf{NHB(C6F5)2H}]. [Cp2Zr{NH2B(C6F5)2H}2]·PhMe and [Cp2Hf{NH2B(C6F5)2H}2]· 4(thf) exhibit beta-B-agostic chelate bonding of one of the two amidoborane ligands in the solid state. The agostic hydride is invariably coordinated to the outside of the metallocene wedge. Exceptionally, [Cp2Hf{NH2B(C6F5)2H}2]·PhMe has a structure in which the two amidoborane ligands adopt an intermediate coordination mode, in which neither is definitively agostic. [Cp2Hf{NHB(C6F5)2H}] has a formally dianionic imidoborane ligand chelating through an agostic interaction, but the bond-length distribution suggests a contribution from a zwitterionic amidoborane resonance structure. Treatment of the zwitterions [Cp2MMe(mu-Me)B(C6F5)3] (M=Zr, Hf) with Li[NH2B(C6F5)nH3-n] (n=2) results in [Cp2MMe{NH2B(C6F5)2H}] complexes, for which the spectroscopic data, particularly 1J(B,H), again suggest beta-B-agostic interactions. The reactions proceed similarly for the structurally encumbered [Cp''2ZrMe(mu-Me)B(C6F5)3] precursor (Cp''=1,3-C5H3(SiMe3)2, n=1 or 2) to give [Cp''2ZrMe{NH2B(C6F5)nH3-n}], both of which have been structurally characterised and show chelating, agostic amidoborane coordination. In contrast, the analogous hafnium chemistry leads to the recovery of [Cp''2HfMe2] and the formation of Li[HB(C6F5)3] through hydride abstraction

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Contributors

Author: Simon J. Coles

Author: Graham J. Tizzard

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