Electronic Structures
of Ruthenium and Osmium Complexes
of 9,10-Phenanthrenequinone
- Publication date
- Publisher
Abstract
The reaction of 9,10-phenanthrenequinone (PQ) with [M<sup>II</sup>(H)(CO)(X)(PPh<sub>3</sub>)<sub>3</sub>] in boiling toluene
leads
to the homolytic cleavage of the M<sup>II</sup>–H bond, affording
the paramagnetic <i>trans</i>-[M(PQ)(PPh<sub>3</sub>)<sub>2</sub>(CO)X] (M = Ru, X = Cl, <b>1</b>; M = Os, X = Br, <b>3</b>) and <i>cis</i>-[M(PQ)(PPh<sub>3</sub>)<sub>2</sub>(CO)X] (M = Ru, X = Cl, <b>2</b>; M = Os, X = Br, <b>4</b>) complexes. Single-crystal X-ray structure determinations of <b>1</b>, <b>2</b>·toluene, and <b>4·</b>CH<sub>2</sub>Cl<sub>2</sub>, EPR spectra, and density functional theory
(DFT) calculations have substantiated that <b>1</b>–<b>4</b> are 9,10-phenanthrenesemiquinone radical (PQ<sup>•–</sup>) complexes of ruthenium(II) and osmium(II) and are defined as <i>trans</i>-[Ru<sup>II</sup>(PQ<sup>•–</sup>)(PPh<sub>3</sub>)<sub>2</sub>(CO)Cl] (<b>1</b>), <i>cis</i>-[Ru<sup>II</sup>(PQ<sup>•–</sup>)(PPh<sub>3</sub>)<sub>2</sub>(CO)Cl] (<b>2</b>), <i>trans</i>-[Os<sup>II</sup>(PQ<sup>•–</sup>)(PPh<sub>3</sub>)<sub>2</sub>(CO)
Br] (<b>3</b>), and <i>cis</i>-[Os<sup>II</sup>(PQ<sup>•–</sup>)(PPh<sub>3</sub>)<sub>2</sub>(CO)Br] (<b>4</b>). Two comparatively longer C–O [average lengths: <b>1</b>, 1.291(3) Å; <b>2</b>·toluene, 1.281(5)
Å; <b>4</b>·CH<sub>2</sub>Cl<sub>2</sub>, 1.300(8)
Å] and shorter C–C lengths [<b>1</b>, 1.418(5) Å; <b>2</b>·toluene, 1.439(6) Å; <b>4</b>·CH<sub>2</sub>Cl<sub>2</sub>, 1.434(9) Å] of the OO chelates are consistent
with the presence of a reduced PQ<sup>•–</sup> ligand
in <b>1</b>–<b>4</b>. A minor contribution of the
alternate resonance form, <i>trans</i>- or <i>cis</i>-[M<sup>I</sup>(PQ)(PPh<sub>3</sub>)<sub>2</sub>(CO)X], of <b>1</b>–<b>4</b> has been predicted by the anisotropic
X- and Q-band electron paramagnetic resonance spectra of the frozen
glasses of the complexes at 25 K and unrestricted DFT calculations
on <b>1</b>, <i>trans</i>-[Ru(PQ)(PMe<sub>3</sub>)<sub>2</sub>(CO)Cl] (<b>5</b>), <i>cis</i>-[Ru(PQ)(PMe<sub>3</sub>)<sub>2</sub>(CO)Cl] (<b>6</b>), and <i>cis</i>-[Os(PQ)(PMe<sub>3</sub>)<sub>2</sub>(CO)Br] (<b>7</b>). However,
no thermodynamic equilibria between [M<sup>II</sup>(PQ<sup>•–</sup>)(PPh<sub>3</sub>)<sub>2</sub>(CO)X] and [M<sup>I</sup>(PQ)(PPh<sub>3</sub>)<sub>2</sub>(CO)X] tautomers have been detected. <b>1</b>–<b>4</b> undergo one-electron oxidation at −0.06,
−0.05, 0.03, and −0.03 V versus a ferrocenium/ferrocene,
Fc<sup>+</sup>/Fc, couple because of the formation of PQ complexes
as <i>trans</i>-[Ru<sup>II</sup>(PQ)(PPh<sub>3</sub>)<sub>2</sub>(CO)Cl]<sup>+</sup> (<b>1</b><sup><b>+</b></sup>), <i>cis</i>-[Ru<sup>II</sup>(PQ)(PPh<sub>3</sub>)<sub>2</sub>(CO)Cl]<sup>+</sup> (<b>2</b><sup><b>+</b></sup>), <i>trans</i>-[Os<sup>II</sup>(PQ)(PPh<sub>3</sub>)<sub>2</sub>(CO)Br]<sup>+</sup> (<b>3</b><sup><b>+</b></sup>), and <i>cis</i>-[Os<sup>II</sup>(PQ)(PPh<sub>3</sub>)<sub>2</sub>(CO)Br]<sup>+</sup> (<b>4</b><sup><b>+</b></sup>). The trans isomers <b>1</b> and <b>3</b> also undergo
one-electron reduction at −1.11 and −0.96 V, forming
PQ<sup>2–</sup> complexes <i>trans</i>-[Ru<sup>II</sup>(PQ<sup>2–</sup>)(PPh<sub>3</sub>)<sub>2</sub>(CO)Cl]<sup>−</sup> (<b>1</b><sup><b>–</b></sup>) and <i>trans</i>-[Os<sup>II</sup>(PQ<sup>2–</sup>)(PPh<sub>3</sub>)<sub>2</sub>(CO)Br]<sup>−</sup> (<b>3</b><sup><b>–</b></sup>). Oxidation of <b>1</b> by I<sub>2</sub> affords diamagnetic <b>1</b><sup><b>+</b></sup>I<sub>3</sub><sup>–</sup> in low yields. Bond parameters of <b>1</b><sup><b>+</b></sup>I<sub>3</sub><sup>–</sup> [C–O, 1.256(3) and 1.258(3) Å; C–C, 1.482(3)
Å] are consistent with ligand oxidation, yielding a coordinated
PQ ligand. Origins of UV–vis/near-IR absorption features of <b>1</b>–<b>4</b> and the electrogenerated species have
been investigated by spectroelectrochemical measurements and time-dependent
DFT calculations on <b>5</b>, <b>6</b>, <b>5</b><sup><b>+</b></sup>, and <b>5</b><sup><b>–</b></sup>
