The synthesis, structure and reactivity of B(C6F5)3-stabilised amide (M-NH2) complexes of the group 4 metals

Treatment of the homoleptic titanium amides [Ti(NR2)4] (R=Me or Et) with the Brønsted acidic reagent H3N⋅B(C6F5)3 results in the elimination of one molecule of amine and the formation of the four-coordinate amidoborate complexes [Ti(NR2)3{NH2B(C6F5)3}], the identity of which was confirmed by X-ray crystallography. The reaction with [Zr(NMe2)4] proceeds similarly but with retention of the amine ligand to give the trigonal-bipyramidal complex [Zr(NMe2)3{NH2B(C6F5)3}(NMe2H)]. Cyclopentadienyl (Cp) amidoborate complexes, [MCp(NR2)2{NH2B(C6F5)3}] (M=Ti, R=Me or Et; M=Zr, R=Me) can be prepared from [MCp(NR2)3] and H3N⋅B(C6F5)3, and exhibit greater thermal stability than the cyclopentadienyl-free compounds. H3N⋅B(C6F5)3 reacts with nBuLi or LiN(SiMe3)2 to give LiNH2B(C6F5)3, which complexes with strong Lewis acids to form ion pairs that contain weakly coordinating anions. The attempted synthesis of metallocene amidoborate complexes from dialkyl or diamide precursors and H3N⋅B(C6F5)3 was unsuccessful. However, LiNH2B(C6F5)3 does react with the highly electrophilic reagents [MCp2Me(μ-Me)B(C6F5)3] to give [MCp2Me(μ-NH2)B(C6F5)3] (M=Zr or Hf). Comparison of the molecular structures of the Group 4 amidoborate complexes reveals very similar BN, TiN and ZrN bond lengths, which are consistent with a description of the bonding as a dative interaction between an {M(L)n(NH2)} fragment and the Lewis acid B(C6F5)3. Each of the structures has an intramolecular hydrogen-bonding arrangement in which one of the nitrogen-bonded hydrogen atoms participates in a bifurcated F⋅⋅⋅H⋅⋅⋅F interaction to ortho-F atoms.