Reactions of Neutral Cobalt(II) Complexes of a Dianionic Tetrapodal Pentadentate Ligand: Cobalt(III) Amides from Imido Radicals

Abstract

Neutral cobalt(II) complexes of the dianionic tetrapodal pentadentate ligand BPzPy, in which borate linkers supply the anionic charges, are reported. Both the six-coordinate THF adduct 1-THF and the five-coordinate THF-free complex 1 are in a high-spin S = 3/2 configuration in the ground state and have been structurally characterized by X-ray crystallography. These two Co(II) starting materials react rapidly with aryl azides of moderate steric bulk. The thermodynamic products of these reactions are low-spin, diamagnetic, Co(III) amido complexes that are either monomeric, when an external hydrogen atom source such as 1,4-cyclohexadiene is present, or dimeric products formed via C-C coupling of the azide aryl group and internal transfer of H to the nitrogen. These products are fully characterized and are rare examples of octahedral Co amido compounds; structural determinations reveal significant pyramidalization of the amido nitrogens due to π-π repulsion wherein the amido ligand is primarily a σ donor. The amido products arise from highly reactive Co(III) imido radical intermediates that are the kinetic products of the reactions of 1 or 1-THF with the azide reagents. The imido radicals can be detected by X-band EPR spectroscopy and have been probed by density functional theory computations, which indicate that this doublet species is characterized by a high degree of spin localization on the imido ligand, accounting for the reactivity with hydrogen atom sources and dimerization chemistry observed. The high coordination number and the electron-rich nature of the dianionic BPzPy ligand framework render the imido ligand formed highly reactive.

Terminal Imido Complexes of the Groups 9–11: Electronic Structure and Developments in the Last Decade

“…For complex 38 , [72] Piers and co‐workers observed a similar EPR signal as had been found for 33 [68c] . Accordingly, it was suggested that complex 38 is the key intermediate for the formation of the octahedral cobalt(III) amido complex 39 from 37 (Scheme 12).…”

“…Indeed, cobalt(IV)‐ and cobalt(V) diimido complexes 17 and 17 + feature shorter Co−N bonds of 1.665(3) and 1.640(3) Å, respectively [59] . Moving down the group to tetracoordinated 4 d rhodium(III) complex 18 , which is isostructural with 12 Tol , reveals a larger Rh−N distance of 1.780(2) Å [72] . This effect is enhanced for the hexacoordinated nitrene complex 19 (2.303(4) Å) [78,79] .…”

Activation of ammonia and hydrazine by electron rich Fe(ii) complexes supported by a dianionic pentadentate ligand platform through a common terminal Fe(iii) amido intermediate

“…We therefore thought of using our tetrapodal dianionic pentadentate B 2 Pz 4 Py ligand platform, which has been successful at accessing a variety of first-row metal complexes. [37][38][39][40][41] The B 2 Pz 4 Py ligand features two borate moieties at the linkage position between the four equatorial pyrazole arms and the axial pyridine, rendering the ligand dianionic and slightly more rigid than the PY5 platform. 42 The charge modification from the PY5 to the B 2 Pz 4 Py platform renders complexes of M(II) ions neutral, and higher oxidation state complexes are generally more accessible with this more electron rich ligand system.…”

Synthesis, Characterization and Reactivity of Neutral Octahedral Alkyl-Cobalt(III) Complexes Bearing a Dianionic Pentadentate Ligand

“…at 80˚C, along with the known five coordinate (B2Pz4Py)Co(II) complex 3, identifiable via its characteristic 11 B NMR chemical shift of 75 ppm (Figures S1-S7). 21 Protio benzene was also detected in these reactions. This suggests that under these conditions, the R• formed via bond homolysis can be trapped when it is sterically large enough to escape the solvent cage; in the case of 2-Me, this does not occur.…”

“…Based on the well-known pentapyridyl (PY5) system introduced 20 years ago, 17,18 and later on the work by Gardinier who substituted the equatorial pyridine arms with pyrazole donors, 19 our group has reported the synthesis of a B2Pz4Py ligand which features two borate moieties. [20][21][22] The introduction of the borate linkers renders the ligand dianionic and therefore more electron-rich. This stabilizes higher oxidation state intermediates so they can be studied in more detail.…”