Halogen Photoelimination from Monomeric Nickel(III) Complexes Enabled by the Secondary Coordination Sphere

Endothermic
halogen elimination reactions, in which molecular halogen photoproducts
are generated in the absence of chemical traps, are rare. Inspired
by the proclivity of mononuclear Ni­(III) complexes to participate
in challenging bond-forming reactions in organometallic chemistry,
we targeted Ni­(III) trihalide complexes as platforms to explore halogen
photoelimination. A suite of Ni­(III) trihalide complexes supported
by bidentate phosphine ligands has been synthesized and characterized.
Multinuclear NMR, EPR, and electronic absorption spectroscopies, as
well as single-crystal X-ray diffraction, have been utilized to characterize
this suite of complexes as distorted square pyramidal, S = 1/2 mononuclear Ni­(III) complexes. All complexes participate in
clean halogen photoelimination in solution and in the solid state.
Evolved halogen has been characterized by mass spectrometry and quantified
chemically. Energy storage via halogen elimination was established
by solution-phase calorimetry measurements; in all cases, halogen
elimination is substantially endothermic. Time-resolved photochemical
experiments have revealed a relatively long-lived photointermediate,
which we assign to be a Ni­(II) complex in which the photoextruded
chlorine radical interacts with a ligand-based aryl group. Computational
studies suggest that the observed intermediate arises from a dissociative
LMCT excited state. The participation of secondary coordination sphere
interactions to suppress back-reactions is an attractive design element
in the development of energy-storing halogen photoelimination involving
first-row transition metal complexes.