Cobalt(I) Complexes of 5-Aryl-2-iminopyrrolyl Ligands: Synthesis, Spin Isomerism, and Application in Catalytic Hydroboration

Abstract

This work reports the first successful isolation and full characterization of cobalt(I) complexes of 5-aryl-2-iminopyrrolyl ligands. In one approach, when [Co{κ² N,N'-5-(2,4,6-R₃-C₆H₂)-NC₄H₂-2-C(H)═N(2,6-ⁱPr₂-C₆H₃)}(Py)Cl] (R = ⁱPr, 1a; R = Ph, 1b) were reacted with K(HBEt₃) or Na(Hg) in toluene, the Co(I) arene complexes [Co{κ² N,N'-5-(2,4,6-ⁱPr₃-C₆H₂)-NC₄H₂-2-C(H)═N(2,6-ⁱPr₂-C₆H₃)}(η⁶-C₆H₅CH₃)] (2a) and [ Co{κ² N,N'-5-[2'-(κ:η⁶- C₆H₅)-C₆H₂-4',6'-Ph₂]-NC₄H₂-2-C(H)═N(2,6-ⁱPr₂-C₆H₃)}] (2b) were formed. The reaction of complex 1a with KC₈ in Et₂O yielded the [Co{κ² N,N'-5-(2,4,6-ⁱPr₃-C₆H₂)-NC₄H₂-2-C(H)═N(2,6-ⁱPr₂-C₆H₃)}]₂ (3). On another approach, the metathesis of potassium 5-(2,4,6-triisopropylphenyl)-2-( N-2,6-diisopropylphenylformimino)pyrrolyl (KLa) with CoCl(PMe₃)₃ yielded the bis(trimethylphosphine) complex [Co{κ² N,N'-5-(2,4,6-ⁱPr₃-C₆H₂)-NC₄H₂-2-C(H)═N(2,6-ⁱPr₂-C₆H₃)}(PMe₃)₂] (4) in a good yield. Complexes 2a, 3, and 4 are paramagnetic, high-spin species, while 2b is a diamagnetic complex. Compound 2b exhibited a spin isomerism behavior ( S = 0 ↔ S = 1) as determined by variable-temperature ¹H NMR experiments (Δ H° = 7.7 kcal mol^-1), being also supported by computational studies (Δ E = 4.2 kcal mol^-1). All complexes were tested in the hydroboration of styrene with pinacolborane (HBPin), with complex 4 exclusively yielding the respective anti-Markovnikov addition product. Additionally, all complexes catalyzed the fast and quantitative hydroboration of benzaldehyde with HBPin.