Nickel Complexes of a Hybrid Cyclopentadienyl-Phosphine Ligand

A new unsymmetrical hybrid pincer-type ligand featuring cyclopentadienyl and phosphine donors bridged by a 2,6-dimethylenepyridyl linker has been synthesized. Upon double deprotonation and treatment with [NiCl2(NCCH3)2] in THF, this ligand forms a distorted 18-electron κ2-P,N-η5-Cp nickel(II) piano-stool complex. This complex undergoes clean aromatization upon protonation, highlighting the proton-responsive nature of the ligand framework. The same scaffold stabilizes a related reduced κ2-P,N-η5-Cp nickel(I) species─an uncommon example of a 19-electron nickel(I) compound─which was characterized by continuous-wave X-band EPR spectroscopy. Crystal structures of the protonated, deprotonated, and reduced complexes were all obtained using single-crystal X-ray diffraction. These results provide entry into a versatile ligand platform that is demonstrated to be both proton-responsive and capable of supporting both Ni(II) and Ni(I) oxidation states.