Synthesis and Thermal Properties of Novel NHC-Stabilized Cobalt Carbonyl Nitrosyl Complexes

The
synthesis of a series of cobalt NHC complexes of the types [Co­(NHC)2(CO)­(NO)] (NHC = iPr2Im (2), nPr2Im (3), Cy2Im (4), Me2Im (5), iPr2ImMe (6), Me2ImMe
(7), MeiPrIm (8), MetBuIm (9); R2Im = 1,3-dialkylimidazolin-2-ylidene) and [Co­(NHC)­(CO)2(NO)] (NHC = iPr2Im (13), nPr2Im (14), Me2Im (15), iPr2ImMe (16), Me2ImMe (17), MeiPrIm
(18), MetBuIm (19)) from
the reaction of the NHC with [Co­(CO)3(NO)] (1) is reported. These complexes have been characterized using elemental
analysis, IR spectroscopy, multinuclear NMR spectroscopy, and in many
cases by X-ray crystallography. Bulky NHCs tend to form the mono-NHC-substituted
complexes [Co­(NHC)­(CO)2(NO)], even from the reaction with
an stoichiometric excess of the NHC, as demonstrated by the synthesis
of [Co­(Dipp2Im)­(CO)2(NO)] (11),
[Co­(Mes2Im)­(CO)2(NO)] (12), and
[Co­(MecAAC)­(CO)2(NO)] (20). For tBu2Im a preferred coordination via the NHC backbone
(“abnormal” coordination at the 4-position) was observed
and the complex [Co­(tBu2aIm)­(CO)2(NO)] (10) was isolated. All of these complexes
are volatile, are stable upon sublimation and prolonged storage in
the gas phase, and readily decompose at higher temperatures. Furthermore,
DTA/TG analyses revealed that the complexes [Co­(NHC)2(CO)­(NO)]
are seemingly more stable toward thermal decomposition in comparison
to the complexes [Co­(NHC)­(CO)2(NO)]. We thus conclude that
the cobalt complexes of the type [Co­(NHC)­(CO)2(NO)] and
[Co­(NHC)2(CO)­(NO)] have potential for application as precursors
in the vapor deposition of thin cobalt films.