Redox Chemistry of Bis(oxazolinyl)cyclopentadienyl and -fluorenyl Rhodium and Iridium Organometallic Compounds

Rhodium and iridium
compounds supported by 1-cyclopentadienyl-1,1-bis­(4,4-dimethyl-2-oxazolinyl)­ethane
(MeC­(OxMe2)2(C5H4); BoMCp) form the 18-electron piano-stool compounds {BoMCp}­ML2 (L = C2H4, C8H12, CO) containing two noncoordinated oxazolines. Bromination
of {BoMCp}­Rh­(C2H4)2 gives
{BoMCp}­RhBr2. A single-crystal X-ray diffraction
study reveals that only one oxazoline is coordinated in the solid-state
structure of {BoMCp}­RhBr2. The oxazolines exchange
rapidly on the 1H NMR time scale at room temperature but
slowly at −10 °C. In contrast, the fluorenyl derivative
MeC­(OxMe2)2(C13H8) (BoMFlu) forms 16-electron square-planar rhodium­(I) and iridium­(I)
complexes containing bidentate {C,N-BoMFlu}­M coordination that features a M–C single
bond (i.e., monohapto-fluorenyl bonding). {BoMFlu}­Rh­(η4-C8H12) undergoes two electrochemically
and chemically reversible one-electron redox events with E1/2 at −640 and 220 mV. One-electron chemical oxidation
provides the long-lived rhodium­(II) hydrocarbyl species [{BoMFlu}­Rh­(C8H12)]+, which reacts to
give the monovalent species [{BoMFlu-H}­Rh­(η4-C8H12)]+. Alternatively, one-electron
oxidation of {BoMFlu}­Ir­(η4-C8H12) provides a transient diamagnetic iridium hydride,
detected by 1H NMR spectroscopy, that ultimately rearranges
into [{BoMFlu-H}­Ir­(η4-C8H12)]+. This process can be prevented for both congeners
by employing the allylic H-free dibenzo­[a,e]­cyclooctatetraene (C16H12) ligand.
Oxidation of {BoMFlu}­M­(η4-C16H12) (M = Rh, Ir) provides [{BoMFlu}­M­(η4-C16H12)]+ with lifetimes
of greater than 1 day.