Carbene Rearrangements in Three-Coordinate N‑Heterocyclic Carbene Complexes of Cobalt(II) Bis(trimethylsilyl)amide

The synthesis and molecular structures
of the cobalt­(II) N-heterocyclic carbene (NHC) complexes [(NHC)­Co­{N­(SiMe3)2}2], where NHC = 1,3-bis­(diisopropylphenyl)­imidazolylidene
(IPr) (6), 1,3-bis­(mesityl)­imidazolylidene (IMes) (7), and 1,3-bis­(tert-butyl)­imidazol-2-ylidene
(ItBu) (8), are reported.
Complexes 6–8 are rare examples of
three-coordinate cobalt NHC complexes. The steric congestion within
the coordination environments of the cobalt­(II) centers in 6 and 7 results in the longest Co–C­(NHC) distances
currently known. Investigating the thermal stability of 6–8, we have found that the steric congestion
in 6 is such that heating the complex to reflux in toluene
prompts a rearrangement from the normal, C2-bonding mode of the IPr
ligand to the corresponding “abnormal” or mesoionic
bonding mode. The rearrangement results in formation of [(aIPr)­Co­{N­(SiMe3)2}2] (9), which is the first cobalt complex of an abnormal NHC ligand.
The Co–C bond in 9 is 0.06 Å shorter than
the analogous bond in 6, suggesting that, although the
rearrangement occurs due to the spatial demands of the IPr ligand,
there is also a thermodynamic driving force in terms of the Co–C
bond. In contrast to the case for 6, complex 7 is stable with respect to the normal-to-abnormal rearrangement.
Refluxing complex 8 in toluene results in activation
of a tert-butyl substituent, which is eliminated
as isobutene, followed by formation of the 1-tert-butylimidazole complex [(tBuIm)­Co­{N­(SiMe3)2}2] (10).