Ruthenium, Rhodium, Osmium, and Iridium Complexes of Osazones (Osazones = Bis-Arylhydrazones of Glyoxal): Radical versus Nonradical States

Phenyl
osazone (LNHPhH2), phenyl osazone anion radical
(LNHPhH2•–), benzoyl
osazone (LNHCOPhH2), benzoyl osazone anion radical
(LNHCOPhH2•–), benzoyl
osazone monoanion (LNCOPhHMe–), and anilido
osazone (LNHCONHPhHMe) complexes of ruthenium, osmium,
rhodium, and iridium of the types trans-[Os­(LNHPhH2)­(PPh3)2Br2] (3), trans-[Ir­(LNHPhH2•–)­(PPh3)2Cl2] (4), trans-[Ru­(LNHCOPhH2)­(PPh3)2Cl2] (5), trans-[Os­(LNHCOPhH2)­(PPh3)2Br2] (6), trans- [Rh­(LNHCOPhH2•–)­(PPh3)2Cl2] (7), trans-[Rh­(LNHCOPhHMe–)­(PPh3)2Cl]­PF6 ([8]­PF6), and trans-[Ru­(LNHCONHPhHMe)­(PPh3)2Cl]Cl ([9]­Cl) have been isolated
and compared (osazones = bis-arylhydrazones of glyoxal). The complexes
have been characterized by elemental analyses and IR, mass, and 1H NMR spectra; in addition, single-crystal X-ray structure
determinations of 5, 6, [8]­PF6, and [9]Cl have been carried out. EPR spectra
of 4 and 7 reveal that in the solid state
they are osazone anion radical complexes (4, gav = 1.989; 7, 2.028 (Δg = 0.103)), while in solution the contribution of the M­(II)
ions is greater (4, gav =
2.052 (Δg = 0.189); 7, gav = 2.102 (Δg = 0.238)).
Mulliken spin densities on LNHPhH2 and LNHCOPhH2 obtained from unrestricted density functional
theory (DFT) calculations on trans-[Ir­(LNHPhH2)­(PMe3)2Cl2] (4Me) and trans-[Rh­(LNHCOPhH2)­(PMe3)2Cl2] (7Me) in the gas phase with doublet spin states
authenticated the existence of LNHPhH2•– and LNHCOPhH2•– anion
radicals in 4 and 7 coordinated to iridium­(III)
and rhodium­(III) ions. DFT calculations on trans-[Os­(LNHPhH2)­(PMe3)2Br2] (3Me), trans-[Os­(LNHCOPhH2)­(PMe3)2Br2] (6Me), and trans-[Ru­(LNHCONHPhHMe–)­(PMe3)2Cl] [9Me]+ with singlet spin states
established that the closed-shell singlet state (CSS) solutions of 3, 5, 6, and [9]­Cl
are stable. The lower value of MIII/MII reduction
potentials and lower energy absorption bands corroborate the higher
extent of mixing of d orbitals with the π* orbital in the case
of 3 and 6. Time-dependent (TD) DFT calculations
elucidated the MLCT as the origin of the lower energy absorption bands
of 3, 5, and 6 and π
→ π* as the origin of transitions in 4 and 7.