A Chromium(II) Tetracarbene Complex Allows Unprecedented Oxidative Group Transfer

Abstract

Multiple distinct oxidative group transfer reactions to low valent chromium were examined. Six new chromium complexes were prepared from a highly electronically unsaturated Cr(II) square planar complex that was supported by a macrocyclic tetracarbene ligand. This complex's reactivity with MeNO and disparate azides was investigated. The reaction with MeNO generated a highly stable Cr(IV)-oxo complex. Less bulky organic azides such as p-tolyl and n-octyl azides gave rise to metallotetrazenes, while more sterically demanding mesityl and adamantyl azides generated Cr(IV)-imido complexes. The reaction of the square planar Cr(II) complex with TMS-azide yielded the first linearly bridging nitrido chromium species. Reductive group transfer was explored for a Cr(IV)-imido complex, and organic products, such as aziridines, were formed after addition. Cr(IV) imidos and oxos are quite rare, while tetrazenes and bridging nitridos are virtually unknown. This is the most detailed study on oxidative group transfer reactions using chromium based complexes on a single auxiliary ligand to date.

Actinide tetra-N-heterocyclic carbene ‘sandwiches’

“…The synthesis of the octa-NHC actinide complexes is extremely challenging. For transition metal systems, we have typically deprotonated the 16-atom ring macrocyclic ligand [H 4 ( BMe2,-Me TC H )]Br 2 with n BuLi at low temperature prior to adding a metal salt, [34][35][36][37][38][39] but this methodology is unsuccessful with 5f salts, possibly due to lithium adduct formation. 44 Other strong bases, such as NaCH 2 Ph in diethyl ether, led to degradation of [H 4 ( BMe 2 ,Me TC H )]Br 2 .…”

“…1F). [34][35][36][37][38][39] We considered that the increased ionic radii of actinides versus transition metals could lead to actinide bis(NHC macrocycle) 'sandwiches', reminiscent of porphyrins, phthalocyanines, or even cyclooctatetraene (COT) sandwiches (e.g. (COT) 2 M), and, more importantly, that the strong s-donor capacity of the NHC ligand should impart unusual electronic structures on these complexes.…”

Catalytic aziridination with alcoholic substrates via a chromium tetracarbene catalyst

“…34, 35 Notably, our recent research with a similar tetracarbene Cr(IV) imide complex demonstrates non-catalytic imide transfer. 36 We suspect that 1 is more reactive both due to the stronger donor NHCs and the key trans chloride ligand. The former explains why relatively weak oxidants like aryl azides are reactive as opposed to only stronger oxidants that have been previously employed (like PhIO).…”

Synthesis of Chromium(II) Complexes with Chelating Bis(alkoxide) Ligand and Their Reactions with Organoazides and Diazoalkanes

“…Molecules 2020, 24, x 2 of 16 chromium(IV)-imido to transfer nitrene to olefins or isocyanides to form aziridines and carbodiimides, respectively [29][30][31]. C-C and C-H bond activation reactions featuring reactive chromium(IV)-imido were also reported [32].…”

“…In contrast, early transition metals (particularly chromium) typically exhibit less reactive nitrene functionalities due to the stability of multiple metal-nitrogen bonds [22][23][24][25][26][27][28]. However, recent studies demonstrated the ability of coordinatively unsaturated chromium(IV)-imido to transfer nitrene to olefins or isocyanides to form aziridines and carbodiimides, respectively [29][30][31]. C-C and C-H bond activation reactions featuring reactive chromium(IV)-imido were also reported [32].…”