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

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journal contribution
posted on 03.03.2014, 00:00 by Sarat Chandra Patra, Thomas Weyhermüller, Prasanta Ghosh
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.