Synthesis of (POCOP)Co(Ph)(X) Pincer Complexes and Observation of Aryl–Aryl Reductive Elimination Involving the Pincer Aryl

• Published in: Organometallics 2018-11, Vol.37 (21), p.3803-3812
• Main Authors: Foley, Bryan J, Palit, Chandra Mouli, Timpa, Samuel D, Ozerov, Oleg V
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/acs.organomet.8b00511

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.