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Journal article

New tripodal N-heterocyclic carbene chelators for small molecule activation

A review. In an effort to develop new tripodal N-heterocyclic carbene (NHC) ligands for small mol. activation, two new classes of tripodal NHC ligands TIMER and TIMENR were synthesized. The C-anchored tris(carbene) ligand system TIMER (R = Me, t-Bu) forms bi- or polynuclear metal complexes. While the Me deriv. exclusively forms trinuclear 3:2 [(TIMEMe)2M3]3+ with Group 11 metal ions, the tert-Bu deriv. yields a dinuclear 2:2 [(TIMEt-Bu)2Cu2]2+ with Cu(I). The latter complex shows both normal and abnormal carbene binding modes and accordingly, is best formulated as a bis(carbene)alkenyl complex. The N-anchored tris(carbene) ligands TIMENR (R = alkyl, aryl) bind to a variety of 1st-row transition metal ions in 1:1 stoichiometry, affording monomeric complexes with a protected reactivity cavity at the coordinated metal center. Complexes of TIMENR with Cu(I)/(II), Ni(0)/(I), and Co(I)/(II)/(III) were synthesized. The Co(I) complexes with the aryl-substituted TIMENR (R = mesityl, xylyl) ligands show great potential for small mol. activation. These complexes activate, e.g., dioxygen to form Co(III) peroxo complexes that, upon reaction with electrophilic org. substrates, transfer an O atom. The Co(I) complexes are also precursors for terminal Co(III) imido complexes. These imido complexes undergo unprecedented intramol. imido insertion reactions to form Co(II) imine species. The mol. and electronic structures of some representative metal NHC complexes as well as the nature of the metal-carbene bond of these metal NHC complexes was elucidated by x-ray and DFT computational methods and are discussed briefly. In contrast to the common assumption that NHCs are pure sigma-donors, the authors' studies revealed non-negligible and even significant pi-backbonding in electron-rich metal NHC complexes. [on SciFinder (R)]

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