Electrochemical, Spectroscopic, and Theoretical Studies
on Diethynyl Ligand Bridged Ruthenium Complexes with 1,3-Bis(2-pyridylimino)isoindolate
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Abstract
A series
of ruthenium acetylide complexes [Ru(BPI)(PPh<sub>3</sub>)<sub>2</sub>(CCR)] (BPI = 1,3-bis(2-pyridylimino)isoindolate;
R = −C<sub>6</sub>H<sub>5</sub> (<b>2</b>), −Cp<sub>2</sub>Fe (<b>3a</b>), −C<sub>6</sub>H<sub>4</sub>C<sub>6</sub>H<sub>4</sub>CCCp<sub>2</sub>Fe (<b>3b</b>))
and bis(acetylide)-linked binuclear ruthenium complexes [{Ru(BPI)(PPh<sub>3</sub>)<sub>2</sub>}<sub>2</sub>(CCRCC)] (R = none
(<b>4</b>), 1,4-benzenediyl (<b>5</b>), 1,4-naphthalenediyl
(<b>6</b>), 9,10-anthracenediyl (<b>7</b>)) were synthesized
and characterized by ESI-MS spectrometry, IR, <sup>1</sup>H and <sup>31</sup>P NMR, and UV–vis–near-IR spectroscopy, and
cyclic and differential pulse voltammetry. Oxidation of <b>3</b>–<b>7</b> with 1 equiv of ferrocenium perchlorate afforded
the corresponding one-electron-oxidized complexes <b>3</b><sup>+</sup>–<b>7</b><sup>+</sup>. In contrast to the case
for <b>3a</b><sup><b>+</b></sup>, where spin density is
localized at the Fe center due to moderate electronic communication
between Ru<sup>II</sup> and Fe<sup>III</sup> centers along the Ru–CC–Cp<sub>2</sub>Fe backbone, the spin density is primarily populated on Ru
for <b>3b</b><sup><b>+</b></sup> without an appreciable
electronic interaction between Ru<sup>III</sup> and Fe<sup>II</sup> across the quite long bridging system RuCCC<sub>6</sub>H<sub>4</sub>C<sub>6</sub>H<sub>4</sub>CCCp<sub>2</sub>Fe. For
bis(acetylide)-linked binuclear ruthenium complexes <b>4</b>–<b>7</b>, electrochemical, UV–vis–near-IR
spectral and TD-DFT computational studies reveal that electronic delocalization
along the bridging RuCCRCCRu backbone is highly dependent
on the R spacer. It is demonstrated that with the gradual increase
of a π-conjugated system in aromatic R spacer, the electronic
delocalization shows progressive enhancement along the Ru–CCRCC–Ru
backbone due to an increasing participation of the bridging ligand. <b>4</b><sup>+</sup> displays highly electronically delocalized behavior,
whereas <b>5</b><sup>+</sup>–<b>7</b><sup>+</sup> are on the borderline of electronic delocalization