Modification of a pyridine-alkoxide ligand during the synthesis of coordination compounds

A pyridine-alkoxide ligand (pyalk = 2-(2-pyridinyl)-2-propanolate) promotes high oxidation states, water-oxidation catalysis and is resistant to oxidation under catalytic conditions. However, several minor byproducts of pyalk are formed during syntheses of Ir(III) and (IV) complexes. We have now characterized three of these, which, although only formed as minor products, have given unexpected results. Under the reaction conditions studied, we find that pyalkH can undergo various modifications: conversion to the N-oxide, degradation of the geminal methyl groups to give picolinate, oxidation of the arene ring CH group adjacent to nitrogen, and coupling to give a dimeric pincer ligand. One of these products has a Ir₂(µ-O)(µ-OOCH) core that provides a plausible structural analogue for our elusive ‘blue solution’ oxidation catalyst, formed from our Cp∗Ir(pyalk)Cl precursor under oxidative conditions. In another case, a mononuclear Ir(III) product contains a novel NCCN pincer having a metal-bound aliphatic alkene as the central unit. Alkene complexes of Ir(III) are rare but here the binding is constrained by the pincer.