The 3d-metal mediated nitrene transfer is under intense scrutiny due to its potential as an atom economic and ecologically benign way for the directed amination of (un)functionalised CÀH bonds. Here we present the isolation and characterisation of a rare, trigonal imido cobalt(III) complex, which bears a rather long cobalt-imido bond. It can cleanly cleave strong C À H bonds with a bond dissociation energy of up to 92 kcal mol À1 in an intermolecular fashion, unprecedented for imido cobalt complexes. This resulted in the amido cobalt(II) complex [Co(hmds) 2 (NH t Bu)] À . Kinetic studies on this reaction revealed an H atom transfer mechanism. Remarkably, the cobalt(II) amide itself is capable of mediating H atom abstraction or stepwise proton/electron transfer depending on the substrate. A cobalt-mediated catalytic application for substrate dehydrogenation using an organo azide is presented.
In this report, we
present intricate pathways for the synthesis
of linear nickel(I) silylamide K{m}[Ni(NR2)2] (NR2 = −N(SiMe3)2). This
is achieved first via the reduction of nickel(II) trisamide Li(donor)4[Ni(NR2)3] (Li(thf)
x
[1]) with KC8 in the presence of 18-crown-6
or crypt.222. In due course, the behavior of Li(donor)4[Ni(NR2)3] as a source of masked two-coordinate
nickel(II) hexamethyldisilazanide is explored, leading to the formation
of nickel(I) and nickel(II) N-donor adducts, as well as metal–metal-bonded
dinickel(I) trisamide K(toluene)[Ni2(NR2)3] (K(toluene)[5]). Finally, a convenient and
reliable synthesis of K{m}[Ni(NR2)2] by ligand
exchange of phosphines in [Ni(NR2)(PPh3)2] with K{m}(NR2) is presented. This allows for
the comprehensive analysis of its electronic properties which reveals
a fluxional behavior in solution with tight anion/cation interactions.
A pair of trigonal imido iron complexes ([Fe(NMes)L2]0,−) in two oxidation states is reported. The anionic complex K{crypt.222}[Fe(NMes)L2] is best described as an iron(ii) imide.
We report on the synthesis of a variety of trigonal imido cobalt complexes [Co(NAryl)L 2 ] À , (L= N-(Dipp)SiMe 3 ), Dipp = 2,6-diisopropylphenyl) with very long CoÀN Aryl bonds of around 1.75 . Their electronic structure was interrogated using a variety of physical and spectroscopic methods such as EPR or X-Ray absorption spectroscopy which leads to their description as highly unusual imidyl cobalt complexes. Computational analyses corroborate these findings and further reveal that the high-spin state is responsible for the imidyl character. Exchange of the Dipp substituent on the imide by the smaller mesityl function (2,4,6-trimethylphenyl) effectuates the unexpected Me 3 Si shift from the ancillary ligand set to the imidyl nitrogen, revealing a highly reactive, nucleophilic character of the imidyl unit.
The synthesis of a T‐shaped imido nickel complex is reported, obtained by the reaction of phenyl azide with the linear nickel(I) silylamide complex K{18‐crown‐6}[NiL2] (L=−N(Dipp)SiMe3; Dipp=2,6‐diisopropylphenyl). In addition, an unusual shift of a SiMe3 unit from an ancillary ligand to a putative [Ni=NPh] complex is observed. Examination of the resulting [Ni=NDipp] complex for its electronic properties leads to its description as a low‐spin nickel(III) imide and revealed only limited activity with respect to H‐atom abstraction from C−H bonds or nitrene. Attempts to obtain information about the general features of trigonal nickel(II) amide products, that would result from such a H‐atom abstraction reaction, via reaction of linear nickel(II) silylamide [NiL2] with alkali metal salts of primary amides revealed the reduction to the linear nickel(I) complex K{18‐crown‐6}[NiL2]. The same is observed for alkoxides, secondary amides or benzyl. Reaction of the organic salts with the anionic nickel(II) complex NBu4[NiBrL2] under salt elimination also give the linear nickel(I) complex. Partial formation of an otherwise stable T‐shaped nickel(II) can be observed only for the electron‐poor diphenyl amide. This implicates that reduction of the starting nickel(II) complexes by different organic alkali metal salts likely does not occur via a homolytic Ni−R bond cleavage but a direct SET process from the unligated substrate to the nickel(II) ion.
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