Synthesis of (POCOP)Co(Ph)(X) Pincer Complexes and Observation of Aryl-Aryl Reductive Elimination Involving the Pincer Aryl
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2018 American Chemical Society. A series of square-planar, low-spin Co(II) and Co(III) complexes supported by the POCOP pincer ligand have been prepared for the purpose of exploring the reactions potentially involved in aryl halide coupling catalysis (POCOP = 2,6-diisopropylphosphinoxyphenyl). The investigations determined that the Co(III)-aryl intermediates of the envisioned catalytic cycle are accessible, but the desired catalysis is derailed by a C-C reductive elimination reaction that involves the POCOP ligand. Metalation of the parent (POCOP)H ligand with CoCl2 and DMAP led to the formation of (POCOP)CoCl (A-1). Metathesis of A-1 with Me3SiX reagents allowed isolation of (POCOP)CoBr (A-2), (POCOP)CoI (A-3), and (POCOP)CoOTf (A-4). Reactions of A-1 with NaOtBu, MeLi, and PhLi led to (POCOP)CoOtBu (A-5), (POCOP)CoMe (A-6), and (POCOP)CoPh (A-7). Treatment of A-1 with PhSH surprisingly led to (POCOP)CoSPh (A-8) without the need for added base. A-7 could be oxidized to (POCOP)Co(Ph)(Cl) (B-1) using N-chlorosuccinimide; however, samples of B-1 produced in this fashion were unstable with respect to decomposition to A-1. Oxidation of A-7 to (POCOP)Co(Ph)(OAc) (B-2) was accomplished with PhI(OAc)2, and metathesis of B-2 with Me3SiX produced clean samples of B-1 and (POCOP)Co(Ph)(I) (B-3) that were thermally stable. Treatment of B-1 or B-3 with NaSPh resulted in the formation of (POCOP)Co(Ph)(SPh) (B-4). Complexes B-1-B-4 are low-spin Co(III) complexes. Thermolysis of B-4, instead of the expected C-S reductive elimination, resulted in the C-C elimination with the POCOP aryl, producing (POCOP)-Ph (D-1). Hydrolysis of D-1 yielded the known 2-phenylresorcinol (D-2). The solid-state structures of A-8, B-3, and B-4 were determined by X-ray crystallography.
