Tris(guanidinato)complexes of iridium and rhodium in the oxidation states +III and +IV: synthesis, characterization, and reactivity

Mononuclear [Ir{ArNC(NR2)NAr}(C8H14)2] complexes (where R = Me or Et; Ar = Ph or 4-MeC6H4; and C8H14 = cis-cyclooctene) were synthesized from the neutral N,Ndialkyl-N′,N′′-diarylguanidines via deprotonation and transmetalation. As confirmed by single-crystal structure determination, the guanidinato(1−) ligands coordinate the lowvalent d Ir center in an N,N′-chelating binding mode, and the C−N distances of the CN3 core suggest that these ligands function as stronger donors than related monoanionic, bidentate nitrogen-based ligands. In the reaction of the complexes with O2, new [Ir{ArNC(NR2)NAr}]3 complexes were identified among the products, suggesting a new route for their synthesis. Tris(guanidinato) complexes of Ir, [Ir{ArNC(NR2)NAr}3] (R = Me or Et; Ar = Ph or 4-MeC6H4), were synthesized and, as revealed by electrochemical measurements, can be oxidized reversibly at unusually low potentials. These potentials are even lower than those of other Ir complexes with very electron-rich, trianonic ligand sets as, for example, in cyclometalated tris(2-pyridylphenyl), corrolato and tris(dithiocarbamato) complexes. Chemical oxidation by [FeCp2]PF6 afforded isolable, paramagnetic Ir compounds, [Ir{ArNC(NR2)NAr}3]PF6, which were characterized by elemental analysis, mass spectrometry, electronic absorption and EPR spectroscopy and X-ray crystallography. The reactivity of these Ir complexes toward mild inorganic and organic reductants and substrates with O−H bonds was investigated. Similar tris(guanidinato) complexes of Rh, [Rh{ArNC(NR2)NAr}3] (R = Me or Et; Ar = Ph, 4-MeC6H4 or 4-MeOC6H4), were synthesized. These complexes also were oxidized reversibly at low potentials, as compared to the corresponding tris(amidinato) and tris(triazenido) complexes. Chemical oxidation by [FeCp2]PF6 afforded thermally unstable, paramagnetic Rh compounds, [Rh{ArNC(NR2)NAr}3]PF6, which were characterized by mass spectrometry and electronic absorption spectroscopy.