Nickel and Platinum PCP Pincer Complexes Incorporating an Acyclic Diaminoalkyl Central Moiety Connecting Imidazole or Pyrazole Rings

This work explores avenues toward
PCP pincer complexes with acyclic
diaminocarbene central moieties that are suitable for ligand-assisted
reactivity involving the carbene carbon. For this purpose, diphosphine
chelating ligands 1 and 2, with bis­(imidazolyl)­methyl
and bis­(pyrazolyl)­methyl backbones, respectively, were prepared in
two high-yield synthetic steps. Nickel­(II) and platinum­(II) dihalide
complexes 3 and 6, incorporating 1, were prepared and converted into PCP pincer ligands 5 and 7 upon deprotonation with KHMDS. Chelated nickel­(I)
complex 4 was obtained as a byproduct to 3. Although the solid-state structures of 5 and 7 presented geometric strain at the diaminomethyl carbon,
this moiety could not be converted into a diaminocarbene by proton
or hydride abstraction. 7 could be converted into platinum­(IV)
analog 8 by oxidation with PhICl2. Ligand 2 proved more versatile than 1, or less dependable
in its behavior, generating P,N-chelating cobalt­(II) dibromide complex 9. Easily accessible ligands 1 and 2 provide a new and versatile PCP pincer platform.