Thiopyridazine-Based Copper Boratrane Complexes Demonstrating the Z‑type Nature of the Ligand

In Z-type ligands the electrons for
the coordination bond are formally provided by the metal. They represent
an important addition to the much more extensively used L- and X-type
σ-donor ligands for the development of transition metal complexes
with new reactivities. We report here a new boron Z-type ligand with
three tethering thiopyridazinyl donors forming exclusively complexes
that feature a metal boron bond. Rational substitution pattern in
the backbone of the pyridazinyl heterocycle led to a well-behaved
ligand system that allowed preparation of a series of copper boratrane
complexes in high yields. They are found to be more soluble in common
organic solvents allowing reactivity studies in contrast to previous
complexes with this type of ligand. Thus, copper complexes [Cu­{B­(PnMe,tBu)3}­X] with X = Cl, OTf, N3, and
κN-NCS are reported. Solution behavior was
explored, and the molecular structures were determined by single-crystal
X-ray diffraction analyses. The thiocyanate ligand is found to coordinate
via its nitrogen atom pointing to a high oxidation state of the copper.
Density functional theory calculations indicate a high positive charge
on copper and a strong copper–boron interaction. Thus, here
reported complexes deliver synthetic evidence for the Z-type nature
of the ligand. These findings are important for further dissemination
of these types of ligands in coordination chemistry.