A Stable Coordination Complex of Rh(IV) in an N,O-Donor Environment

Abstract

We describe facial and meridional isomers of [Rh(III)(pyalk)3], as well as meridional [Rh(IV)(pyalk)3](+) {pyalk =2-(2-pyridyl)-2-propanoate}, the first coordination complex in an N,O-donor environment to show a clean, reversible Rh(III/IV) redox couple and to have a stable Rh(IV) form, which we characterize by EPR and UV-visible spectroscopy as well as X-ray crystallography. The unprecedented stability of the Rh(IV) species is ascribed to the exceptional donor strength of the ligands, their oxidation resistance, and the meridional coordination geometry.

Cationic (η5-C5Me4R)RhIII Complexes with Metalated Aryl Phosphines Featuring η4-Phosphorus plus Pseudo-Allylic Coordination

“…These results are in agreement with the lower accessibility of the M­(V) oxidation state for rhodium, although some stable Rh­(V) complexes are known . Recently, an unusual Rh­(IV) coordination compound was reported by Crabtree and co-workers . The computed energy barrier is in good agreement with the NMR measurements described above.…”

A full set of iridium(iv) pyridine-alkoxide stereoisomers: highly geometry-dependent redox properties

“…Electrochemistry revealed that the two isomers have a striking 340 mV difference in the potential of the Ir IV/III couple. Subsequent work 23 with rhodium echoed these findings – Rh( iii ) analogues of 1 and 2 shared similar thermodynamic and kinetic properties. The much rarer and elusive Rh( iv ) oxidation state was reached by oxidizing the Rh analogue of 1 , resulting in what may be the most stable and best-characterized Rh( iv ) complex in the literature.…”

“…For example, our Rh analogue of 1 gave a spectrum spanning 35 mT, roughly five times less than that of 1 . 23 This anisotropy can impact the observation of a signal when the t 2g orbitals are close in energy; therefore, lack of EPR activity should not be considered conclusive evidence for the absence of a mononuclear Ir( iv ) species.…”

N,N,O Pincer Ligand with a Deprotonatable Site That Promotes Redox‐Leveling, High Mn Oxidation States, and a Mn2O2 Dimer Competent for Catalytic Oxygen Evolution

“…With these bio‐inspired principles in mind, we report a new di‐µ‐oxo dimanganese complex of the pincer 2‐([2,2′‐bipyridin]‐6‐yl)propan‐2‐ol (bipyalkH), which acts as a strong π‐donor upon deprotonation of the alcohol. This ligand incorporates the pyridine alkoxide “pyalk” motif that we previously showed can stabilize high oxidation states in Rh and Ir and promote water‐oxidation catalysis, both with chemical oxidants and electrochemically, with Ir and Cu . We also have previously reported oxygen evolution from our low‐valent precatalyst Mn(bipyalkH)Cl 2 ( 1 , shown in Figure ) driven by KHSO 5 with retention of activity for > 160 h in unbuffered solutions.…”