Free methylidyne? CCC-NHC tantalum bis(imido) reactivity: protonation, rearrangement to a mixed unsymmetrical CCC-N-heterocyclic carbene/N-heterocyclic dicarbene (CCC-NHC/NHDC) pincer tantalum bis(imido) complex

Attempted synthesis of a CCC-NHC pincer Ta complex was found to contain a N-heterocyclic dicarbene (left) and reaction at high temperature produced a methylidyne complex (right). [Display omitted] The coordination sphere of the reported (1,3-bis(3-butylimidazol-1-yl-2-idene)-2-phenylene)(tert-butylimido) CCC-NHC Ta (V) bis(imido) pincer complex 2 (Helgert et al., 2016, doi: 10.1021/acs.organomet.6b00216) has been observed to spontaneously rearrange to yield X-ray quality crystals of a CCC-NHDC/NHC pincer Ta (V) bis(imido) complex (3). Preliminary computations were consistent with a step-wise mechanism for the formation of the N-heterocyclic dicarbene pincer complex. Over time the CCC-NHC Ta (V) bis(imido) pincer complex 2 reacted with adventitious protons sources to form CCC-NHC Ta(V)(tert-butylamido)(tert-butylimido)iodo complex (4). The reaction of bis(imido) 2 with excess lithium tert-butylamide at elevated temperatures, in an effort to find a direct synthesis of 3, yielded methylidyne species 5.