| Abstract |
The 5-substituted 2-aryliminopyrrolyl ligand precursors of the type 5-R-2-[ N-(2,6-diisopropylphenyl)formimino]-1 H-pyrrole (R = 2,6-Me-2-C6H3 (1a), 2,4,6-Pr-i(3)-C6H2 (1b), 2,4,6-Ph3-C6H3 (1c; reported in this work), anthracen-9-yl (1d), CPh3 (1e; reported in this work)) were treated with K[N(SiMe3)(2)] in toluene to yield the respective 5-R-2-[ N-(2,6-diisopropylphenyl)formimino]pyrrolyl potassium salts 2a-e in high yields. The paramagnetic 15-electron Co(II) complexes of the type [Co\kappa(2) N,N -5-R-NC4H2-2-C(H)=N(2,6-Pr-i(2)-C6H3)\(Py)Cl] (3a-e; Py = pyridine) were prepared by salt metathesis of CoCl2(Py)(4) with the respective potassium salts 2a-e in moderate to good yields. When the CoCl2(THF)(1.5) precursor was combined with the in situ prepared sodium salt of ligand precursor 1b, the trinuclear complex [Co\kappa(2) N, N -5-(2,4,6-Pr-i(3)-C6H2)-NC4H2-2-C(H)=N(2,6-Pr-i(2)-C6H3)\(mu-Cl)](2) [(mu-Cl)(2)Co(THF)(2)] (4) was obtained in high yields. Complexes 3a-e have high-spin electronic configurations both in solution and in the solid state. X-ray diffraction studies of complexes 3a-e confirmed distorted tetrahedral coordination geometries. Complex 4, on the other hand, is a linear trinuclear Co(II)-Co(II)-Co(II) complex with two terminal distorted tetrahedral four-coordinate sites and a central octahedral six-coordinate site, all in the high-spin state, S = 3/2, as confirmed by the magnetization measurements and DFT calculations. Solid-state magnetic measurements in both complexes 3a and 4 point to paramagnetic behavior with a significant contribution of spin-orbit coupling. Additionally, intramolecular antiferromagnetic coupling of the adjacent cobalt atoms is observed in 4. The Co(II) family 3a-d, on activation with K(HBEt3), catalyzed the hydroboration of several alpha-olefins with pinacolborane, in good to high yields (50-80\%). This system almost exclusively yielded the anti-Markovnikov (a-Mk) addition product, except when styrene was used, where the selectivity in the Markovnikov (Mk) product increased with increasing steric bulkiness of the 5-R-2-iminopyrrolyl substituent, with the a-Mk:Mk molar ratio varying from 2.33:1 (3a, R = 2,6-M(e)2-C6H3) to 0.75:1 (3c, R = 2,4,6-Ph-3-C6H3). Preliminary mechanistic studies indicate that the activation by K(HBEt3) gave rise to a Co(I) species, the catalyst system likely following an oxidative addition pathway. |
