Ruthenium, Rhodium, Osmium, and Iridium Complexes
of Osazones (Osazones = Bis-Arylhydrazones of Glyoxal): Radical versus
Nonradical States
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Abstract
Phenyl
osazone (L<sup>NHPh</sup>H<sub>2</sub>), phenyl osazone anion radical
(L<sup>NHPh</sup>H<sub>2</sub><sup>•–</sup>), benzoyl
osazone (L<sup>NHCOPh</sup>H<sub>2</sub>), benzoyl osazone anion radical
(L<sup>NHCOPh</sup>H<sub>2</sub><sup>•–</sup>), benzoyl
osazone monoanion (L<sup>NCOPh</sup>HMe<sup>–</sup>), and anilido
osazone (L<sup>NHCONHPh</sup>HMe) complexes of ruthenium, osmium,
rhodium, and iridium of the types <i>trans</i>-[Os(L<sup>NHPh</sup>H<sub>2</sub>)(PPh<sub>3</sub>)<sub>2</sub>Br<sub>2</sub>] (<b>3</b>), <i>trans</i>-[Ir(L<sup>NHPh</sup>H<sub>2</sub><sup>•–</sup>)(PPh<sub>3</sub>)<sub>2</sub>Cl<sub>2</sub>] (<b>4</b>), <i>trans</i>-[Ru(L<sup>NHCOPh</sup>H<sub>2</sub>)(PPh<sub>3</sub>)<sub>2</sub>Cl<sub>2</sub>] (<b>5</b>), <i>trans</i>-[Os(L<sup>NHCOPh</sup>H<sub>2</sub>)(PPh<sub>3</sub>)<sub>2</sub>Br<sub>2</sub>] (<b>6</b>), <i>trans</i>- [Rh(L<sup>NHCOPh</sup>H<sub>2</sub><sup>•–</sup>)(PPh<sub>3</sub>)<sub>2</sub>Cl<sub>2</sub>] (<b>7</b>), <i>trans</i>-[Rh(L<sup>NHCOPh</sup>HMe<sup>–</sup>)(PPh<sub>3</sub>)<sub>2</sub>Cl]PF<sub>6</sub> ([<b>8</b>]PF<sub>6</sub>), and <i>trans</i>-[Ru(L<sup>NHCONHPh</sup>HMe)(PPh<sub>3</sub>)<sub>2</sub>Cl]Cl ([<b>9</b>]Cl) have been isolated
and compared (osazones = bis-arylhydrazones of glyoxal). The complexes
have been characterized by elemental analyses and IR, mass, and <sup>1</sup>H NMR spectra; in addition, single-crystal X-ray structure
determinations of <b>5</b>, <b>6</b>, [<b>8</b>]PF<sub>6</sub>, and [<b>9</b>]Cl have been carried out. EPR spectra
of <b>4</b> and <b>7</b> reveal that in the solid state
they are osazone anion radical complexes (<b>4</b>, <i>g</i><sub>av</sub> = 1.989; <b>7</b>, 2.028 (Δ<i>g</i> = 0.103)), while in solution the contribution of the M(II)
ions is greater (<b>4</b>, <i>g</i><sub>av</sub> =
2.052 (Δ<i>g</i> = 0.189); <b>7</b>, <i>g</i><sub>av</sub> = 2.102 (Δ<i>g</i> = 0.238)).
Mulliken spin densities on L<sup>NHPh</sup>H<sub>2</sub> and L<sup>NHCOPh</sup>H<sub>2</sub> obtained from unrestricted density functional
theory (DFT) calculations on <i>trans</i>-[Ir(L<sup>NHPh</sup>H<sub>2</sub>)(PMe<sub>3</sub>)<sub>2</sub>Cl<sub>2</sub>] (<b>4</b><sup>Me</sup>) and <i>trans</i>-[Rh(L<sup>NHCOPh</sup>H<sub>2</sub>)(PMe<sub>3</sub>)<sub>2</sub>Cl<sub>2</sub>] (<b>7</b><sup>Me</sup>) in the gas phase with doublet spin states
authenticated the existence of L<sup>NHPh</sup>H<sub>2</sub><sup>•–</sup> and L<sup>NHCOPh</sup>H<sub>2</sub><sup>•–</sup> anion
radicals in <b>4</b> and <b>7</b> coordinated to iridium(III)
and rhodium(III) ions. DFT calculations on <i>trans</i>-[Os(L<sup>NHPh</sup>H<sub>2</sub>)(PMe<sub>3</sub>)<sub>2</sub>Br<sub>2</sub>] (<b>3</b><sup>Me</sup>), <i>trans</i>-[Os(L<sup>NHCOPh</sup>H<sub>2</sub>)(PMe<sub>3</sub>)<sub>2</sub>Br<sub>2</sub>] (<b>6</b><sup>Me</sup>), and <i>trans</i>-[Ru(L<sup>NHCONHPh</sup>HMe<sup>–</sup>)(PMe<sub>3</sub>)<sub>2</sub>Cl] [<b>9</b><sup>Me</sup>]<sup>+</sup> with singlet spin states
established that the closed-shell singlet state (CSS) solutions of <b>3</b>, <b>5</b>, <b>6</b>, and [<b>9</b>]Cl
are stable. The lower value of M<sup>III</sup>/M<sup>II</sup> reduction
potentials and lower energy absorption bands corroborate the higher
extent of mixing of d orbitals with the π* orbital in the case
of <b>3</b> and <b>6</b>. Time-dependent (TD) DFT calculations
elucidated the MLCT as the origin of the lower energy absorption bands
of <b>3</b>, <b>5</b>, and <b>6</b> and π
→ π* as the origin of transitions in <b>4</b> and <b>7</b>
