Synthesis of Boryldiazenido Complexes from Tungsten Dinitrogen Complexes

Abstract

The anionic tungsten dinitrogen complex trans-[NBu4][W(NCS)(N2)(dppe)2] reacted with primary and secondary boranes to give the corresponding boryldiazenido complexes, among which trans-[W(NCS)(N=NBHCMe2CHMe2)(dppe)2] was structurally determined. A related boryldiazenido complex was also obtained by the reaction of trans-[W(N2)2(dppe)2] with 9-borabicyclo[3.3.1]nonyl trifluoromethanesulfonate. These reactions provide the first examples of boration of coordinated dinitrogen.

Group 6 Transition‐Metal/Boron Frustrated Lewis Pair Templates Activate N2and Allow its Facile Borylation and Silylation

“…The 31 PNMR spectra indicated clean conversion to new complexes 7 with phosphorus resonances at 66.0 (7 Mo )a nd 65.0 ppm (7 W ). [25] These reactions support the notion that dinitrogen complexes can serve as the Lewis base component of an FLP,thus providing abasis for the development of new functionalization methods of coordinated dinitrogen. The 29 Si HMQC NMR spectra of 7 Mo and 7 W give signals at 2.0 and À1.45 ppm for the Si atoms,respectively (vs.0.2 ppm for free Et 3 SiH), which suggests that the positive charge is not located at the Si atom ( Figure S21-25 and S27-31).…”

“…These observations account for the formation of square pyramidal, ionic complexes again stabilized by an agostic interaction in the vacant apical site.B ya nalogy with the B-H splitting experiments described above,w ec onclude that 3 Mo and 3 W are also able to perform the FLP-type splitting of Si À Hb onds between 2 and their N 2 ligands (Scheme 4). Its borylation and silylation have indeed already been described in the literature;w ith complexes 1 Mo and 1 W , [25] but also in other end-on dinitrogen complexes, [25,26] these reactions rely on the use of more electrophilic boron or silicon (pseudo)halides.Anotable exception is the related, anionic trans-[NBu 4 ][W(NCS)-(N 2 )(dppe) 2 ]c omplex, which reacted with primary or secondary hydroboranes.I n this case,b orylation of dinitrogen occurred without transfer of the hydride,w hich ended up in the ammonium hydroborate byproduct, [25a] analogously to the 3 ! It is worth mentioning that the IR spectra of 7 Mo and 7 W both present ab road, weak n BH band around 2400 cm À1 for the hydroborate as well as ab road one at 1664 and 1638 cm À1 , respectively,w hich indicate as tronger activation of the N 2 moiety compared to complexes 3,b ut weaker than in 5 ( Figure S26 and S32).…”

Group 6 Transition‐Metal/Boron Frustrated Lewis Pair Templates Activate N2and Allow its Facile Borylation and Silylation

“…Comparison of the bond lengths of the W-N-Ncore in 7 W with those of 3 W and 5 W points out agreater backdonation from Wtothe N 2 moiety [W1ÀN1 1.789(2) ,N1 À N2 1.232(3) ]than in 3 W ,but weaker than in 5 W ,asaresult of lower electronegativity of Si compared to Bi nt he B(C 6 F 5 ) 2 group.T he N À Si bond length falls within the range of those found in related complexes. [25] These reactions support the notion that dinitrogen complexes can serve as the Lewis base component of an FLP,thus providing abasis for the development of new functionalization methods of coordinated dinitrogen. Its borylation and silylation have indeed already been described in the literature;w ith complexes 1 Mo and 1 W , [25] but also in other end-on dinitrogen complexes, [25,26] these reactions rely on the use of more electrophilic boron or silicon (pseudo)halides.Anotable exception is the related, anionic trans-[NBu 4 ][W(NCS)-(N 2 )(dppe) 2 ]c omplex, which reacted with primary or secondary hydroboranes.I n this case,b orylation of dinitrogen occurred without transfer of the hydride,w hich ended up in the ammonium hydroborate byproduct, [25a] analogously to the 3 !…”

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“…We have reported elsewhere4 that the HOMO of the [Ta 2 (μ‐η 1 :η 2 ‐N 2 )] core of 1 has π‐bonding character between Ta1 and N1 (numbering as in the structure of 2 ), and therefore this reaction can be described as the hydroboration of a metal–dinitrogen π bond. To our knowledge, this B–H addition across a metal–dinitrogen unit is unique, although simple borylations of coordinated dinitrogen have been reported 7…”