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(Ox(Me2))(2)(C5H4); Bo(M)Cp) form the 18-electron piano-stool compounds {Bo(M)Cp}ML2 (L = C2H4, C8H12, CO) containing two noncoordinated oxazolines. Bromination of {Bo(M)Cp}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 {Bo(M)Cp}RhBr2. The oxazolines exchange rapidly on the H-1 NMR time scale at room temperature but slowly at -10 degrees C. In contrast, the fluorenyl derivative MeC(Ox(Me2))(2)(C13H8) (BoMFlu) forms 16-electron square-planar rhodium(I) and iridium(I) complexes containing bidentate {C,N-Bo(M)Flu}M coordination that features a M-C single bond (i.e., monohapto-fluorenyl bonding). {Bo(M)Flu}Rh(eta(4)-C8H12) undergoes two electrochemically and chemically reversible one-electron redox events with E-1/2 at -640 and 220 mV. One-electron chemical oxidation provides the long-lived rhodium(II) hydrocarbyl species [{Bo(M)Flu}Rh(C8H12)](+), which reacts to give the monovalent species [{Bo(M)Flu-H}Rh(eta(4)-C8H12)](+). Alternatively, one-electron oxidation of {BoMFlu}Ir(eta(4)-C8H12) provides a transient diamagnetic iridium hydride, detected by H-1 NMR spectroscopy, that ultimately rearranges into [{Bo(M)Flu-H}Ir(eta(4)-C8H12)](+). This process can be prevented for both congeners by employing the allylic H-free dibenzo[a,e]cyclooctatetraene (C16H12) ligand. Oxidation of {Bo(M)Flu}M(eta(4)-C16H12) (M = Rh, Ir) provides [{Bo(M)Flu}M(eta(4)-C16H12)](+) with lifetimes of greater than 1 day.