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

Abstract

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 ligandbased 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. Article pubs.acs.org/Organometallics

C(sp3)−C(sp3) Cross‐Coupling of Alkyl Bromides and Ethers Mediated by Metal and Visible Light Photoredox Catalysis

“…A plausible mechanism for the reaction between alkyl halides and ethers is shown in Scheme . We postulate that the mechanism proceeds through metallophotoredox cycles, as previously reported for C( sp 3 )−C( sp 2 ) cross coupling . The alkyl bromide undergoes oxidative addition on the Ni (0) complex to give a Ni (II)‐alkyl halide complex ( II ).…”

“…The mechanistic proposal involves the generation of a bromine radical after Ni−Br bond homolysis of the excited‐state nickel complex, which is activated by an energy‐transfer pathway. The photoinduced halogen elimination from a Ni (III) trichloride complex, previously reported by Nocera, was elegantly explored by Doyle's group. They merged the ability of chlorine radicals to activate C( sp 3 )−H bonds with the ability use of nickel to perform alkyl cross‐couplings .…”

Direct C(sp3)–H Cross Coupling Enabled by Catalytic Generation of Chlorine Radicals

“…Chlorine radicals are known to form stabilized adducts with aromatic functional groups. 9,10 Therefore, we reasoned that if benzene were utilized as a solvent, we could promote the desired photoelimination and simultaneously prevent competitive solvent C(sp 3 )–H abstraction. We recognized that the ability to use an inert solvent and limiting C–H coupling partner would be highly desirable, facilitating the application of this manifold to a broad range of C–H coupling partners under general reaction conditions.…”

Photochemical Nickel-Catalyzed C–H Arylation: Synthetic Scope and Mechanistic Investigations

“…Nocera has observed an analogous Ni–Cl homolysis from an excited-state Ni(III) complex. 11 To probe this hypothesis, we treated 26 with more strongly oxidizing photocatalysts Ru(bpz) 3 ·2PF 6 (excited state E Red = +1.45 V vs SCE) 1a and (9-MesAcr)ClO 4 (excited state E Red = +2.06 V vs SCE). 12 To our surprise, no C–H cross-coupled product was observed when these catalysts were employed with visible light irradiation.…”