Hydroboration of Terminal Olefins with Pinacolborane Catalyzed by New Mono(2-Iminopyrrolyl) Cobalt(II) Complexes
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 -C H (1a), 2,4,6- Pr -C H (1b), 2,4,6-Ph -C H (1c; reported in this work), anthracen-9-yl (1d), CPh (1e; reported in this work)) were treated with K[N(SiMe ) ] in t...
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| Published in: | Inorganic chemistry Vol. 57; no. 14; p. 8146 |
|---|---|
| Main Authors: | , , , , |
| Format: | Journal Article |
| Language: | English |
| Published: |
United States
16.07.2018
|
| ISSN: | 1520-510X, 1520-510X |
| Online Access: | Get more information |
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| Summary: | 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
-C
H
(1a), 2,4,6-
Pr
-C
H
(1b), 2,4,6-Ph
-C
H
(1c; reported in this work), anthracen-9-yl (1d), CPh
(1e; reported in this work)) were treated with K[N(SiMe
)
] 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{κ
N,N'-5-R-NC
H
-2-C(H)═N(2,6-
Pr
-C
H
)}(Py)Cl] (3a-e; Py = pyridine) were prepared by salt metathesis of CoCl
(Py)
with the respective potassium salts 2a-e in moderate to good yields. When the CoCl
(THF)
precursor was combined with the in situ prepared sodium salt of ligand precursor 1b, the trinuclear complex [Co{κ
N, N'-5-(2,4,6-
Pr
-C
H
)-NC
H
-2-C(H)═N(2,6-
Pr
-C
H
)}(μ-Cl)]
[(μ-Cl)
Co(THF)
] (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(HBEt
), catalyzed the hydroboration of several α-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-Me
-C
H
) to 0.75:1 (3c, R = 2,4,6-Ph
-C
H
). Preliminary mechanistic studies indicate that the activation by K(HBEt
) gave rise to a Co(I) species, the catalyst system likely following an oxidative addition pathway. |
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| Bibliography: | ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
| ISSN: | 1520-510X 1520-510X |
| DOI: | 10.1021/acs.inorgchem.8b00568 |