Spin‐State Dichotomy of On‐Cycle Cobalt(II) Organometallics Informs Polar Cobalt(0/II) Versus Radical Cobalt(I/II/III) Cross‐Coupling Mechanisms

Abstract

ABSTRACT 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(sp 2 )─C(sp 2 ) and C(sp 2 )─C(sp 3 ) 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(sp 2 )–C(sp 2 ) 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(sp 2 )─C(sp 3 ) Negishi cross‐coupling reactions, the isolated TMEDA‐supported and BOX‐supported cobalt(II)–monoaryl complexes were determined to be catalyst resting states by 1 H and 19 F NMR spectroscopies in acetonitrile‐ d 3 . Stoichiometric reactions demonstrated high spin (TMEDA)cobalt(II)–monoaryl to be competent for alkyl bromine atom abstraction and for alkyl radical capture, yielding C(sp 2 )─C(sp 3 ) 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.