Carbenes, including N-heterocyclic carbene (NHC) ligands, are used extensively to stabilize openshell transition metal complexes and organic radicals. Yet, it remains unknown, which carbene stabilizes a radical well and, thus, how to design radical-stabilizing C-donor ligands. With the large variety of C-donor ligands experimentally investigated and their electronic properties established, we report herein their radicalstabilizing effect. We show that radical stabilization can be understood by a captodative frontier orbital description involving π-donation to-and π-donation from the carbenes. This picture sheds a new perspective on NHC chemistry, where π-donor effects usually are assumed to be negligible. Further, it allows for the intuitive prediction of the thermodynamic stability of covalent radicals of main group-and transition metal carbene complexes, and the quantification of redox non-innocence.
Whereas
triplet-nitrene complexes of the late transition metals
are isolable and key intermediates in catalysis, singlet-nitrene ligands
remain elusive. Herein we communicate three such palladium terminal
imido complexes with singlet ground states. UV–vis–NIR
electronic spectroscopy with broad bands up to 1400 nm as well as
high-level computations (DFT, STEOM-CCSD, CASSCF/NEVPT2, EOS analysis)
and reactivity studies suggest significant palladium(0) singlet-nitrene
character. Although the aliphatic nitrene complexes proved to be too
reactive for isolation in analytically pure form as a result of elimination
of isobutylene, the aryl congener could be characterized by SC-XRD,
elemental analysis, IR-, NMR spectroscopy, and HRMS. The complexes’
distinguished ambiphilicity allows them to activate hexafluorobenzene,
triphenylphosphine, and pinacol borane, catalytically dehydrogenate
cyclohexene, and aminate ethylene via nitrene transfer at or below
room temperature.
The title oxastannaborininol compound, [Sn(C4H9)2(C10H7BO2)], has been synthesized and crystallized. While heterocycles containing a C–O–B group are common, heterocycles containing an E–O–B unit, where E is an element of the carbon group except for carbon, are rare. In fact, while heterocycles containing Si–O–B units are occasionally reported (although without crystal structures), there are no reports for the corresponding germanium, tin or lead analogues. Herein, the first synthesis and crystal structure of a heterocycle containing an Sn–O–B unit is described. The asymmetric unit contains one molecule showing a notable disorder of the tin atom and the butyl groups. They occupy two sets of positions with site-occupancy factors of 0.295 (6) and 0.705 (6).
Carbenes, including N‐heterocyclic carbene (NHC) ligands, are used extensively to stabilize open‐shell transition metal complexes and organic radicals. Yet, it remains unknown, which carbene stabilizes a radical well and, thus, how to design radical‐stabilizing C‐donor ligands. With the large variety of C‐donor ligands experimentally investigated and their electronic properties established, we report herein their radical‐stabilizing effect. We show that radical stabilization can be understood by a captodative frontier orbital description involving π‐donation to‐ and π‐donation from the carbenes. This picture sheds a new perspective on NHC chemistry, where π‐donor effects usually are assumed to be negligible. Further, it allows for the intuitive prediction of the thermodynamic stability of covalent radicals of main group‐ and transition metal carbene complexes, and the quantification of redox non‐innocence.
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