Ligand Tuning in Pyridine-Alkoxide Ligated Cp*Ir^III Oxidation Catalysts

Abstract

Six novel derivatives of pyridine-alkoxide ligated Cp*Ir III complexes, potent precursors for homogeneous water and C−H oxidation catalysts, have been synthesized, characterized, and analyzed spectroscopically and kinetically for ligand effects. Variation of alkoxide and pyridine substituents was found to affect their solution speciation, activation behavior, and oxidation kinetics. Application of these precursors to catalytic C−H oxidation of ethyl benzenesulfonate with aqueous sodium periodate showed that the ligand substitution pattern, solution pH, and solvent all have pronounced influences on initial rates and final conversion values. Correlation with O 2 evolution profiles during C−H oxidation catalysis showed these competing reactions to occur sequentially, and demonstrates how it is possible to tune the activity and selectivity of the active species through the N^O ligand structure.

Electrochemical and Kinetic Insights into Molecular Water Oxidation Catalysts Derived from Cp*Ir(pyridine‐alkoxide) Complexes

“…Extending the pyridine backbone to a quinoline system added another 30 mV. This is consistent with our previous finding of 2 and 7 acting as precursors to slower but more C−H selective oxidation catalysts compared to the alkyl‐substituted pyridine alkoxide complexes . Therefore, cyclic voltammetry (under appropriate conditions) provides a mean of quantifying these electronic ligand effects in the precursor complexes.…”

“…While at present we can't offer a rationale for this effect yet, it is clear that one the reason for the higher C−H oxidation efficiencies obtained in aqueous t BuOH is that the co‐solvent steers the C−H vs. O−H oxidation competition more towards C−H oxidation by channelling less oxidant into the O 2 evolution cycle with the more active catalysts. Catalysts which are inherently slower are less influenced by this solvent effect, as previously shown by 2 as the most efficient C−H oxidation catalyst of the series …”

“…Previously we have monitored O 2 evolution during NaIO 4 ‐driven catalytic C−H oxidation of ethylbenzene‐sulfonate (EBS) in t BuOH/H 2 O mixtures with 1 – 7 to establish a correlation between activity and selectivity of the different catalysts . Here we now report the kinetics of pure O 2 evolution activity with NaIO 4 in neat aqueous solution, measured with a Clarke‐type electrode in the liquid phase (Figure ).…”

“…We also briefly investigated the effect of adding an oxidation‐resistant organic co‐solvent as typically required for catalytic C−H oxidations with these catalysts . Previously we found adding 20 vol % tert ‐butanol to be most efficient for this purpose, so O 2 evolution assays with NaIO 4 were repeated in 4 : 1 H 2 O/ t BuOH (Figure S5) but without any organic substrate present. We were surprised to find significant rate reductions in O 2 evolution (3–4 times slower) for the faster catalysts caused by the presence of 20 % t BuOH (Table ).…”

“…We have recently reported the synthesis of a series of pyridine‐alkoxide and quinoline‐alkoxide (collectively abbreviated as ‘pyalk’) ligated Cp*Ir III complexes 1 – 7 (Figure ), and have shown how the ligand substitution pattern affected the solution speciation, pre‐catalytic activation, and catalytic C−H oxygenation with aqueous NaIO 4 ,. It was found that under typical reaction conditions (μM to mM [Ir] concentrations in neutral aqueous solution at room temperature), all complexes 1 – 7 readily dissociated the halide ligand to become available for oxidative activation by hydrated periodate, a process which was fast relative to C−H oxidation under catalytic conditions.…”