C−H Bond Activation by an Imido Cobalt(III) and the Resulting Amido Cobalt(II) Complex

Abstract

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.

Imido Cobalt Complexes with Imidyl Character

“…This is in approximate agreement with susceptibility measurements on K{18c6} [4] in solid state (μeff = 4.64 μB at 300 K) revealing a linear 1/ vs T slope with no apparent spin-crossover (Figure 4a). The magnetic susceptibility at room temperature is higher than that of the related [Co(N t Bu)(N(SiMe3)2)2] -(3.52 μB) [36] , an intermediate spin (S = 1) imido cobalt(III) complex, as well as of any other paramagnetic imido cobalt(III) complex. [22,37,41] It is slightly lower than the spin only value of 4.92 μB of a quintet state, which might be indicative of a cobalt(II) ion weakly ferromagnetically coupled to an organic radical.…”

“…The Co-Namide bond lengths of ≈1.93 Å are similar to the related cobalt(III) imide [Co(N t Bu)(N(SiMe3)2)2] -(≈1.94 Å). [36] K{18c6} [2] turned out to be highly reactive and could not be isolated in pure form as it facilitates intramolecular C-H bond activation by H atom abstraction of one of the Meipr groups. This led to the alkyl/amido cobalt(III) complex K{18c6} [3], which co-crystallized in varying amounts, even when keeping the reaction time and temperature as short and low as possible.…”

“…However, no scrambling of the Me3Si function could be observed over a prolonged time by means of X-Ray diffraction analysis and 1 H NMR spectroscopy. Given the comparably long C-Nimide bonds found for complexes [2] -, [4] -, and [5]of around 1.75 Å as well as the previously reported B (Co-N t Bu: 1.707(1) Å) [36] , we wanted to elaborate if this might be directly or indirectly a result of the anionic character of the imido complexes. Together with a low-spin trisphosphinoborate imido iron(II) complex by Brown et al [46] this is a unique feature compared to all other structurally characterized transition metal imido complexes which are either neutral or cationic.…”

“…As a side-note, these computational results also validate in hindsight the previously made assignment of [Co(N t Bu)(N(SiMe3)2)2] -(B) as an intermediate spin system, judging from its Co-Nimide bond length (1.707(6) Å). [36] For the amide complex [6] − the high-spin state is also largely favoured (see ESI). Given the complexity of the system, the bond order is a valuable tool for considering the contributions made by occupied levels and understanding the bond character.…”

“…Recently, we described a unique trigonal alkylimido cobalt(III) complex [Co(N t Bu)(N(SiMe3)2)2] -(B), that exhibits one of the longest reported imido cobalt bonds (1.7067(12) Å) and a very high H atom abstraction capability (Figure 1). [36] The structural features and its remarkable reactivity were attributed to its intermediate spin state, unprecedented for an imido cobalt complex. Bearing in mind that the few known imidyl 3d-metal complexes contain aromatic N-substituents we were naturally interested to know how the installation of an aromatic imido substituent would influence the electronic spin state and electronic structure of the imido cobalt unit.…”