Exploring High Oxidation State Complexes and Oxidation Chemistry with Group 9 Metal

Out of the metals from group 9 of the periodic table, cobalt, rhodium and iridium have all been well studied and have found application in several catalytic processes but oxidation chemistry is less explored so far. We have therefore explored the possible utility of these metals in oxidation chemistry and now report novel complexes of rare high oxidation states for rhodium and iridium.;Development of mild and selective catalysts for the functionalization of C--H bonds remains a major challenge in modern chemistry and finds important applications in organic synthesis. During studies of the Co(II)--Oxone system for the selective oxidation of C--H bonds, we found an unexpected conversion of phenyl rings into carboxylic acid groups. This type of transformation makes the phenyl group a masked carboxylic acid equivalent and, therefore, has significance in synthetic organic chemistry. The oxidative conversion of --C6H4R into --COON groups using a Co(II)--Oxone mixture as the catalytic system proceeds under mild conditions in air and with water as solvent. In this study, we have demonstrated a selective C-1-I activation process, phenyl group oxidation, using the Co(II)Oxone system as catalyst, along with its broad substrate scope. Initial mechanistic studies to probe the role of cobalt have also been performed.;There is interest in high oxidation state transition metal complexes because of their involvement in various catalytic processes. The most common oxidation states encountered for rhodium are I, II and III with only a very few reports of stable Rh(IV) oxidation state compounds, namely the homoleptic fluorides and some bulk oxide materials. We describe facial and meridional isomers of [Rh"(pyalk)3], as well as meridional [Rh'v(pyalk)3]+ (pyalk =2-(2-pyridyl)-2-propanolate), the first stable Rh(IV) coordination complex in an N,0-donor environment to show a clean, reversible Rhlllnv redox couple. The Rh(IV) complex has been extensively characterized. An EPR spectrum has also been obtained which confirms the oxidation state assignment as Rh(IV), because the coupling between the unpaired electron from a d5 Rh(IV) and the nuclear spin of rhodium was observed. The unprecedented stability of the Rh(IV) species is ascribed to the exceptional donor strength of the ligand, their oxidation resistance, and the favorable coordination geometry.;Water oxidation is a kinetically and thermodynamically challenging reaction and synthesis of robust catalysts is a topic of current interest. An advance in the water oxidation field came with our group's report of the highly active iridium `blue solution' water oxidation catalyst obtained by oxidation of Cp*IrL(OH/C1) precursors (L = pyalk). This catalyst has eluded full characterization although Ir(IV,IV) and Ir(IV,V) oxo dimers have been proposed as key catalytic intermediates. We now report a route to a series of well-defined Ir(IV,IV) mono-micro-oxo dimers, which serve as model complexes for the `blue solution'. Chemical and electrochemical oxidation or reduction of the Ir(IV,IV) mono-microoxo dimers results in the formation of fully characterized Ir(V,V), Ir(IV,V) and Ir(III,III) complexes, the study of which sheds light on the properties of the `blue solution' and lays the ground work for development of further well characterized iridium water oxidation catalysts.