The Synthesis, Structure and Reactivity of B(C₆F₅)₃‐Stabilised Amide (MNH₂) Complexes of the Group 4 Metals

Abstract

Treatment of the homoleptic titanium amides [Ti(NR(2))(4)] (R=Me or Et) with the Brønsted acidic reagent H(3)NB(C(6)F(5))(3) results in the elimination of one molecule of amine and the formation of the four-coordinate amidoborate complexes [Ti(NR(2))(3){NH(2)B(C(6)F(5))(3)}], the identity of which was confirmed by X-ray crystallography. The reaction with [Zr(NMe(2))(4)] proceeds similarly but with retention of the amine ligand to give the trigonal-bipyramidal complex [Zr(NMe(2))(3){NH(2)B(C(6)F(5))(3)}(NMe(2)H)]. Cyclopentadienyl (Cp) amidoborate complexes, [MCp(NR(2))(2){NH(2)B(C(6)F(5))(3)}] (M=Ti, R=Me or Et; M=Zr, R=Me) can be prepared from [MCp(NR(2))(3)] and H(3)NB(C(6)F(5))(3), and exhibit greater thermal stability than the cyclopentadienyl-free compounds. H(3)NB(C(6)F(5))(3) reacts with nBuLi or LiN(SiMe(3))(2) to give LiNH(2)B(C(6)F(5))(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 H(3)NB(C(6)F(5))(3) was unsuccessful. However, LiNH(2)B(C(6)F(5))(3) does react with the highly electrophilic reagents [MCp(2)Me(mu-Me)B(C(6)F(5))(3)] to give [MCp(2)Me(mu-NH(2))B(C(6)F(5))(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)(NH(2))} fragment and the Lewis acid B(C(6)F(5))(3). Each of the structures has an intramolecular hydrogen-bonding arrangement in which one of the nitrogen-bonded hydrogen atoms participates in a bifurcated FHF interaction to ortho-F atoms.

Synthesis and Structure of Amido‐ and Imido(pentafluorophenyl)borane Zirconocene and Hafnocene Complexes: NH and BH Activation

“…Closer inspection of 8⋅ 4(thf) reveals that the structural parameters of the two amidoborane ligands are very different. The Hf1N11B12 angle is 122.05(17)°, which, while more obtuse than the tetrahedral ideal, is smaller than the value of 135.14(17)° found for [Cp 2 Hf(Me){NH 2 B(C 6 F 5 ) 3 }], in which the steric demands of the borane fragment is greater 17. In contrast, the Hf1N41B42 angle is only 90.98(15)°.…”

“…The structure of 3 b (Figure 3) is related to that of [([12]crown‐4)Li{NH 2 B(C 6 F 5 ) 3 }] 17. The reduced steric constraints lead to a closing of the LiNB angle from 121.35 to 114.32° and a shortening of the LiN bond length from 2.048 to 2.015 Å.…”

“…A similar pattern is found for the Hf⋅⋅⋅H distances of 2.637 and 2.840 Å; they are much greater than the agostic contact in 8⋅ 4(thf), 2.09 Å, but still significantly shorter than the Hf⋅⋅⋅H distance for the non‐agostic borane hydride 3.363 Å in 8⋅ 4(thf). At 2.236(6) and 2.226(5) Å, the HfN bond lengths are both shorter than that of the agostically bonded group 8⋅ 4(thf) 2.249(2) Å and the monoamidoborane complex [Cp 2 Hf(Me){NH 2 B(C 6 F 5 ) 3 }] 2.270(2) Å 17. Instead of a definitive case of one of the two amidoborane ligands engaging in an agostic interaction, the structure of 8⋅ PhMe appears to exhibit two weak interactions 33…”

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

“…[143] Related bridged anions of the type [( F RO) 3 Al(m-X)Al-(OR F ) 3 ] À (where X = Cl or OH) are also accessible,b ut are more prone to either exchange reactions and/or dissociation to yield the corresponding [XAl(OR F ) 3 ] À and Al(OR F ) 3 species. [154] Additionally, the bridged WCAs[ (C 6 F 5 ) 3 E-F-E(C 6 F 5 ) 3 ] À (where E = Al or Ga) were shown to be formed by the reaction of the corresponding Lewis acid E(C 6 F 5 ) 3 with an appropriate stoichiometry of FCPh 3 to form the [CPh 3 ] + salts.F urthermore,the [CPh 3 ] + salt of the doubly bridged dianionic WCA [{(C 6 F 5 ) 3 Al} 3 -(m-F) 2 ] 2À featuring an AlÀFÀAlÀFÀAl linkage was also reported and in conjunction with the aforementioned WCAsi nvestigated as co-catalysts for sterospecific olefin polymerization. Bochmann and co workers reported the cyanide-bridged [(F 5 C 6 ) 3 B-CN-B(C 6 F 5 ) 3 ] À as well as the related dianionic [M{CNBC 6 F 5 ) 3 } 4 ] 2À (M = Ni or Pd) species and demonstrated their ability to support extremely rapid ethene polymerization.…”

“…Such acids can be thought of as the conjugate acid of the WCA formed by the loss of ap roton, and can achieve high Brønsted acidity.T he strength of the underlying Lewis acid is integral to the formation of acorrespondingly strong Brønsted acid, therefore,t he use of the strong Lewis acids B(C 6 F 5 ) 3 and Al-(OR F ) 3 prevails in the literature. [154] Therelated aqua complex H 2 O!B(C 6 F 5 ) 3 has received attention in the literature [254,255] and has been shown to be capable of protonating (and hence oxidizing) metallocene M(Cp) 2 complexes (where M = Cr,F e, or Co). [248] Angewandte Chemie Reviews shown to be as trong enough Brønsted acid to protonate an amido group of M(NR 2 ) 4 (where M = Ti or Zr;R= Me or Et).…”