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Spin-State Dichotomy of On-Cycle Cobalt(II) Organometallics Informs Polar Cobalt(0/II) Versus Radical
Kavita Choudhary1, Bhaswati Paul1, L Reginald Mills1
1Department of Chemistry, University of Houston, Houston, Texas, USA.
None:
A well-defined, single-component precatalyst N,N,N',N'-tetramethylethylenediamine (TMEDA)cobalt(II) dibromide catalyzed Negishi arylation reactions of alkyl bromide, N-hydroxyphthalimide ester, and (hetero)aryl halide electrophiles, exhibiting both C(sp2)─C(sp2) and C(sp2)─C(sp3) bond-formation. Organometallic reactions of relevant weak field ligand cobalt(II) dihalide complexes with arylzinc reagents demonstrated monoarylation, yielding a series of isolable high spin (S = 3/2) cobalt(II)-monoaryl bromide compounds supported by TMEDA, 3,5-lutidine, or bis(oxazoline) (BOX) ligands. Relevant to C(sp2)-C(sp2) cross-coupling, cobalt(II)-monoaryl bromide complexes reacted with arylzinc nucleophiles to yield TMEDA-supported and bpy-supported low-spin (S = 1/2) cobalt(II)-diaryl complexes, which underwent reductive elimination in acetonitrile to provide direct evidence for cobalt(0/II) cycles. In C(sp2)─C(sp3) Negishi cross-coupling reactions, the isolated TMEDA-supported and BOX-supported cobalt(II)-monoaryl complexes were determined to be catalyst resting states by 1H and 19F NMR spectroscopies in acetonitrile-d3. Stoichiometric reactions demonstrated high spin (TMEDA)cobalt(II)-monoaryl to be competent for alkyl bromine atom abstraction and for alkyl radical capture, yielding C(sp2)─C(sp3) product by reductive elimination at (TMEDA)cobalt(III). These data demonstrated the versatile reactivity of high-spin (S = 3/2) cobalt(II)-monoaryl and low-spin cobalt(II)-diaryl (S = 1/2) complexes, engaging productive closed-shell cobalt(0/II) or open-shell cobalt(I/III) cross-coupling mechanisms depending on the aryl or alkyl electrophile substrate.
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