Synthesis and Comparison of Transition Metal Complexes of Abnormal and Normal Tetrazolylidenes: A Neglected Ligand Species

To date, only few examples of tetrazolylidene
carbenes coordinated to transition metal complexes have been described.
A direct comparison of “normal” tetrazolylidenes (1,4-substitution
pattern) and “abnormal” tetrazolylidenes (also referred
to as “mesoionic carbenes”; 1,3-substitution pattern)
has not been performed at all. In this work, we describe coordination
of both ligand types to a row of transition metals. For example, the
first examples of tetrazolylidene Mo, Ag, or Ir complexes have been
isolated and fully characterized. The mesoionic tetrazolylidene Ag
compound 2d is shown to be a viable carbene transfer
reagent. By means of NMR, IR, and Raman spectroscopy, as well as X-ray
crystallography and computational results, the pronounced differences
between the two ligand substitution patterns are highlighted. It is
shown that simply changing the position of a methyl group from N3
to N4 in the heterocycle can cause enormous differences in ligand
characteristics.