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{κ2N,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{κ2N,N′-5-(2,4,6-iPr3-C6H2)-NC4H2-2-C(H)═N(2,6-iPr2-C6H3)}(η6-C6H5CH3)] (2a) and [Co{κ2N,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{κ2N,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{κ2N,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 (ΔH° = 7.7 kcal mol–1), being also supported by computational studies (ΔE = 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.