The carbazole-based pincer ligand R(CbzPNP)H (R =
i
Pr,
t
Bu) has been used for the synthesis and characterization
of various low- and high-spin cobalt complexes. Upon treatment of
the high-spin complexes R(CbzPNP)CoCl (2
R
-Co
II
Cl) with NaHBEt3, the selective formation
of cobalt(II) hydride 3
i
Pr
-Co
II
H and T-shaped cobalt(I) complex 4
t
Bu
-Co
I
was observed, depending on the substituents
at the phosphorus atoms. For an unambiguous characterization of the
reaction products, a density functional theory (DFT) supported paramagnetic
NMR analysis was carried out, which established the electron configuration
and the oxidation states of the metal atoms, thus demonstrating the
significant impact of ligand substitution on the outcome of the reaction.
A distinct one-electron reactivity was found for 4
t
Bu
-Co
I
in the dehalogenation of
t
BuCl and cleavage of PhSSPh. On the other hand, the CoI species displayed two-electron redox behavior in the oxidative
addition of dihydrogen. The resulting dihydride complex 6
t
Bu
-Co
III
(H)
2
was found to display sluggish reactivity toward alkenes, whereas
the cobalt(II) hydride 3
i
Pr
-Co
II
H was successfully employed in the catalytic hydrogenation
of unhindered alkenes. The stoichiometric hydrogenolysis of 8
i
Pr
-Co
II
Bn at elevated
pressure (10 bar) led to a rapid cleavage of the Co–C bond
to yield hydride complex 3
i
Pr
-Co
II
H. On the other hand, treatment of 2
i
Pr
-Co
II
Cl with phenethylmagnesium
chloride directly resulted in the formation of 3
i
Pr
-Co
II
H, indicating facile β-H
elimination of the alkene insertion product (reversibly) generated
in the catalytic hydrogenation. On the basis of these observations,
a mechanistic pathway involving a key σ-bond metathesis step
of the CoII-alkyl species is proposed.
Cycloisomerisation reactions of (Z)-enynols can be conveniently performed in the eutectic mixture 1ChCl/2Gly under standard bench experimental conditions by using the novel Au(i) complex 3 as catalyst. Furthermore, one-pot tandem cyclosiomerization/Diels–Alder reactions are reported in this eutectic mixture.
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